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Study of catalytic hydroconversion of pyrolysis bio-oils
Use of solvent and effluent recycling
Catarina Silva Lopes Cortez Coelho
Thesis to obtain the Master of Science Degree in
Chemical Engineering
Supervisors: Dr. Olivier Thinon
Prof. Ana Paula Vieira Soares Pereira Dias
Examination Committee
Chairperson: Prof. José Manuel Félix Madeira Lopes
Supervisor: Prof. Ana Paula Vieira Soares Pereira Dias
Members of the committee: Prof. Jaime Filipe Puna
October 2015
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Acknowledgements
I would like to express my deep gratitude to IFP Energies Nouvelles for this opportunity and to all
those who provided me the possibility to complete my internship. Special thanks should be given to
Olivier Thinon, my research supervisor, for his patient guidance, enthusiastic encouragement and
useful and constructive critiques during this six months internship. I would also like to thank my
supervisor from Portugal, Prof. Ana Paula Soares Dias, for her help in concluding and defending this
thesis.
Additionally, I wish to thank various people for their contribution to this project, Nadège Charon,
Serge Boivineau and Matthieu Ozagac, for their valuable technical support, which proved to be
essential for the success of this work. I would also like to extend my thanks to all the technicians of
the laboratory of Elbaite for their help, guidance and patience during my experimental work,
especially to Michel Chalençon and William Pelletant.
Finally, I would like to thank all the interns of IFPEN that contributed for my integration in this
institution and always supported me during this journey, especially to my dearest Portuguese friends
from Lyon for encouraging me and for making this a fantastic experience.
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Abstract
Facing the growing energy demand and the need to reduce fossil fuels dependence, the search for
renewable energy sources has become a subject of interest. Fast pyrolysis is a promising process to
convert biomass into liquid oil (bio-oil), which must be upgraded in order to remove oxygen and to
improve stability among other characteristics.
The catalytic upgrading of fast pyrolysis bio-oil was carried out in a batch unit by hydrodeoxygenation
(HDO) in the presence of a commercial catalyst. The reactor effluent is a partially upgraded bio-oil
(PUB) that should present an oxygen content between 5-15 wt.% and less than 5 wt.% of water.
The potential replacement of the start-up solvent by previously produced PUB was analyzed.
Different process parameters, that could affect the quality of the upgraded bio-oil, were studied:
reaction time, catalyst weight and activation, presence of diffusional limitations and the effect of
recycling only a distillation fraction of PUB. The results showed that higher catalyst weight had a
positive effect on the bio-oil HDO, whereas higher reaction times lead to no significant
improvements. Sulfided catalysts performed better than the reduced ones. Internal diffusional
limitations are present in the reference conditions, and the tests done with distillation fractions of
PUB showed less carbon residue.
The start-up of a continuous unit was simulated in a batch unit using successive PUB recycling. Data
from bio-oil composition and carbon residue showed that the steady state was reached after 3 to 4
PUB recycles.
Results pointed out that in order to recycle all the PUB and improve the quality of the upgraded bio-
oil, the conversion of the heavier fraction of the feedstock must be increased by improving the
catalyst performance. A promising option would be to recycle only a lighter fraction (lower
distillation temperature) of PUB.
Keywords
Bio-oil, catalytic upgrading, hydrodeoxygenation, recycling, pyrolysis, biomass
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Resumo
Devido ao consumo crescente de energia e à necessidade de reduzir a dependência de combustíveis
fósseis, a busca por fontes de energia renováveis tornou-se um assunto de interesse. A pirólise
rápida é um processo promissor para converter a biomassa em bio-óleo, que tem de ser estabilizado,
para reduzir o seu conteúdo em oxigénio e torná-lo num combustível seguro.
Estudou-se o upgrading catalítico de bio-óleos de pirólise rápida em modo descontínuo através da
hidrodesoxigenação na presença de um catalisador comercial. O efluente produzido é um bio-óleo
parcialmente estabilizado (PUB) que deverá apresentar uma composição de 5-15%m/m em oxigénio
e uma solubilidade em água menor que 5%m/m.
Foi estudada potencial substituição de um solvente estabilizante por PUB anteriormente produzido.
Para tal, diferentes parâmetros que poderiam influenciar a qualidade do efluente produzido, foram
estudados: massa e activação do catalisador, tempo de reacção, presença de limitações difusionais e
o efeito de reciclar somente uma fracção de destilação de PUB. Os resultados mostram que uma
maior massa de catalisador leva a um aumento significativo da HDO do bio-óleo, mas que maiores
tempos de reacção não levam a uma melhoria da qualidade do efluente produzido. Adicionalmente,
o catalisador sulfurado teve um melhor desempenho que o reduzido. Verificou-se também a
existência de limitações internas à transferência de massa nas condições de referência e que o
efluente apresenta um resíduo em carbono significativamente menor quando se recicla apenas uma
fracção de destilação de PUB.
O arranque de uma unidade contínua foi simulado numa unidade descontínua, fazendo a reciclagem
sucessiva de PUB. Os resultados mostram que após 3 a 4 reciclagens o estado estacionário foi
atingido, uma vez que a composição elementar do efluente e o seu resíduo em carbono se tornam
constantes.
Os resultados indicam que, para reciclar todo o PUB e melhorar a qualidade do bio-óleo estabilizado,
a conversão da fracção mais pesada da alimentação deve ser aumentada melhorando o desempenho
do catalisador. Uma opção promissora seria reciclar apenas a fração mais leve (menor temperatura
de destilação) de PUB.
Palavras-chave
Bio-óleo, upgrading catalítico, hidrodesoxigenação, reciclagem, pirólise, biomassa
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Table of contents
1. Context ............................................................................................................................................. 1
2. State of the art ................................................................................................................................. 2
2.1 Biofuel production from biomass .............................................................................................. 2
2.1.1 Biomass sources .................................................................................................................. 2
2.1.2 Biomass conversion into biofuel ......................................................................................... 6
2.1.3 Pyrolysis ............................................................................................................................... 7
2.2 Fast pyrolysis bio-oil ................................................................................................................. 11
2.2.1 Characteristics, applications and challenges ..................................................................... 11
2.2.2 Bio-oil Upgrading ............................................................................................................... 15
2.3 Bio-oil upgrading by catalytic hydrotreatment ........................................................................ 17
2.3.1 Reactions involved ............................................................................................................. 18
2.3.2 Use of solvent and effluent recycling ................................................................................ 20
3. Objective and Methodology .......................................................................................................... 23
4. Experimental work ......................................................................................................................... 25
4.1 Unit description ........................................................................................................................ 25
4.2 Bio-oil ....................................................................................................................................... 26
4.3 Catalyst activation .................................................................................................................... 26
4.4 Post treatment ......................................................................................................................... 27
4.5 Analysis ..................................................................................................................................... 28
4.5.1 Gaseous Phase ................................................................................................................... 28
4.5.2 Organic phase .................................................................................................................... 30
4.5.3 Aqueous phase .................................................................................................................. 34
4.5.4 Catalyst .............................................................................................................................. 35
5. Study the influence of operating conditions.................................................................................. 37
5.1 Experimental tests.................................................................................................................... 37
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5.2 PUB production and distillation fractions ................................................................................ 39
5.3 Results ...................................................................................................................................... 41
5.4 Composition of the aqueous phase ......................................................................................... 50
6. Study the evolution of the PUB quality after continuous recycling ............................................... 57
6.1 Experimental tests.................................................................................................................... 57
6.2 Results ...................................................................................................................................... 59
7. Conclusions .................................................................................................................................... 63
8. Future work .................................................................................................................................... 67
9. References ..................................................................................................................................... 69
Appendix A ............................................................................................................................................ 73
Appendix B ............................................................................................................................................ 75
Appendix C............................................................................................................................................. 76
xi
List of tables
Table 1- Comparison of four major Thermochemical Conversion Processes [19]. ................................. 7
Table 2 - Characteristics of different Pyrolysis Processes [23]. ............................................................... 9
Table 3- Composition of typical bio-oil [28]. ......................................................................................... 12
Table 4 –Comparison of typical properties of wood-derived bio-oil, gasoline and diesel [28]. ........... 13
Table 5- Comparison between different upgrading methods and technical feasibility of such
techniques [32]. ............................................................................................................................. 16
Table 6- Comparison between typical characteristics of fast pyrolysis bio-oils and PUB after the first
step of HDT [34]. ............................................................................................................................ 18
Table 7 - Properties of bio-oil. ............................................................................................................... 26
Table 8 - Yields of the bio-oil production, calculated on dry basis........................................................ 26
Table 9 – Operating conditions of experimental tests .......................................................................... 38
Table 10 - Production conditions of each set of test and the properties of the PUB produced. .......... 40
Table 11 - Properties of the bio-oil, PUB and the different feedstock used in the tests. ..................... 40
Table 12 - Properties of the distillation cuts ......................................................................................... 41
Table 13 - Results obtained for different operating conditions ............................................................ 43
Table 14 - Specific surface of the catalyst after some tests in comparison with the fresh reduced
catalyst .......................................................................................................................................... 43
Table 15 - Results obtained in tests of diffusional limitations .............................................................. 47
Table 16 - Results obtained in the study of the effect of feedstock ..................................................... 49
Table 17 - Substances present in the aqueous phase, determined by the GC/MS analysis ................. 52
Table 18 - Composition of the aqueous phase determined by GC-FID ................................................. 53
Table 19 - HPLC analysis results. ........................................................................................................... 53
Table 20 - Results of the ICP analysis .................................................................................................... 54
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Table 21 - Operating conditions of the first set of tests........................................................................ 57
Table 22 - Operating conditions of the second set of tests .................................................................. 58
Table 23 - Results of the first set of tests .............................................................................................. 59
Table 24 - Specific surface of the catalyst after the first set of tests in comparison with the fresh
reduced catalyst and the catalyst after the reference test. .......................................................... 60
Table 25 - Results of the second set of tests ......................................................................................... 61
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List of Figures
Figure 1- Routes for producing first generation biofuels (adapted from [7]). ........................................ 3
Figure 2- Biomass sources for first generation biofuels production in comparison with second
generation biofuels [9]. ................................................................................................................... 3
Figure 3- Lignocellulosic components (adapted from [12]). ................................................................... 5
Figure 4- Routes for producing second generation biofuels (adapted from [15]). ................................. 6
Figure 5- Simplified layout of a pyrolysis plant proposed by Dynamotive (adapted from [22]). ............ 8
Figure 7 -Applications for fast pyrolysis products [21]. ......................................................................... 14
Figure 8- Diagram of the transformation processes of biomass in order to produce upgraded bio-oils
by pyrolysis (adapted from [33]). .................................................................................................. 15
Figure 9- Diagram of the two-stage process to upgrade bio-oils developed by Dynamotive and
IFPEN/Axens [33]. .......................................................................................................................... 17
Figure 10- Reactions pathways in the hydrodeoxygenation process (adapted from [30][34]). ........... 19
Figure 11 - Schematic process diagram of a hydrotreatment unit with effluent recycling [40] ........... 21
Figure 12 - Two stage process to upgrade bio-oil ................................................................................. 23
Figure 13 - Schematic layout of the batch unit ..................................................................................... 25
Figure 14 - Effluent from the reactor. ................................................................................................... 27
Figure 15 - Aqueous phase before (left) and after (right) centrifugation. ............................................ 28
Figure 16 - Operation principle of GC [44]. ........................................................................................... 29
Figure 17 - Chromatogram obtained in the GC analysis of the reference test. .................................... 30
Figure 18 - Karl Fischer equipment Meter Toledo [47] (left) and its working principle [45] (right). .... 31
Figure 19 - Principle of backflush chromatography [48] ....................................................................... 32
Figure 20 - SEC operation principle. ...................................................................................................... 34
Figure 21 - Image of the distillation cuts of Partially Upgraded Bio-oil ................................................ 39
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Figure 22 - Gases produced during the tests with different operating conditions ............................... 44
Figure 23 - Gases produced during the tests of diffusional limitations ................................................ 46
Figure 24 - Gases produced during the tests done with destillation fractions of PUB ......................... 48
Figure 25 - SEC analysis: normalized RI signal on the signal maximum of 75 to 1000g/mol ................ 51
Figure 26 - SEC analysis: normalized UV 254nm signal on the signal maximum of 20 to 1000g/mol .. 51
Figure 27 - SEC analysis: normalized UV 280nm signal on the signal maximum of 20 to 1000g/mol .. 51
Figure 28 - Composition of inorganic compounds in the unreacted bio-oil ......................................... 54
Figure 29 - Gases produced during the first set of tests ....................................................................... 59
Figure 30 - Results of the SEC analysis (normalized UV 280nm signal on the signal maximum of 10 to
5030g/mol) .................................................................................................................................... 59
Figure 31 - Gases produced during the second set of tests .................................................................. 61
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Abbreviation list
BO Bio-oil
CCR Carbon Conradson Analysis
FID Flame ionization detector
GC Gaseous chromatography
HDO Hydrodeoxygenation
HDT Hydrotreatment
HPLC High performance liquid chromatography
ICP Inductively coupled plasma
IEA International Energy Agency
KF Karl Fischer analysis
MS Mass spectrometry
PUB Partially upgraded bio-oil
TCD Thermal conductivity detector
WHSV weight hourly space velocity
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1. Context
Current global energy supply is heavily based on fossil fuels, namely oil, coal and natural gas.
Moreover, its reserves are limited and the carbon dioxide emissions associated with the burn of fossil
fuels are contributing to the global warming. According to IEA, International Energy Agency [1], in
2012, 82% of the global energy supply relied on fossil fuels. Furthermore, preliminary estimations
indicate that the global CO2 emissions will rise 0,7% per year until 2035. This value is predicted to be
lower than the annual increase in energy demand of 1,2%, but is rather above the rate consistent
with the internationally agreed limits that restricted the rise in average global temperature to two
degrees Celsius [1].
Thus, facing world’s population growth and the development of technology, which will cause an
increase of energy consumption per capita, there is a clear urgency in finding a long-term alternative
energy source that can substitute fossil fuels. For this purpose, biomass presents itself as a promising
eco-friendly alternative, as it is recognized as a renewable source of energy and can be easily found
around the world, in a much more sustainable and well distributed way than fossil fuels [2]. Besides
this, biomass has a negligible content of sulphur, nitrogen and soot, when compared to conventional
fossils, which will reduce harmful gas emissions, such as nitrogen oxide, sulphur dioxide and soot [3].
Therefore, due to environmental concerns, biomass to energy production purposes is recently
receiving great attention.
In this field, IFP Energies Nouvelles (IFPEN) is a public-sector research center in the fields of energy,
transport and environment, and is currently developing technological solutions to reduce CO2
emissions, for instance, by producing fuels from biomass and using biomass for the production of
major petrochemical intermediates [4].
2
2. State of the art
2.1 Biofuel production from biomass
In view of the above, biofuel production from biomass has become a subject of interest and several
studies are being carried out in order to fully understand all the processes behind biofuel production
and its treatment/upgrading.
2.1.1 Biomass sources
Depending on the source, biomass can be converted to biofuel via different thermal or biological
processes.
For instance, sugar crops (such as sugar cane or sugar beet), or starch (corn or maize) can be
fermented to produce alcohols, such as ethanol, a liquid fuel, which can be used for transportation.
Ethanol has a high octane number and is a comparatively cleaner burning fuel, reducing the carbon
monoxide emission from vehicles. On those grounds, in Brazil it is standard practice the use of petrol
blended with 20–24% of ethanol [5].
Additionally, vegetable oils, as palm oil, soybean, or algae can be burned directly in a diesel engine or
a furnace, or can be chemically processed to produce fuels. Wood and its byproducts can also be
converted into liquid biofuels. In general, all plant biomass can simply be burned for heat and
electricity production proposes. Still, there is great potential in using this biomass to produce liquid
biofuels that can be eventually used in transportation and industry.
First generation biofuels include those produced from food crops (grains, sugar beet and oil seeds).
Figure 1 represents the routes used to produce first generation biofuels. Biochemically, ethanol can
be produced by fermenting the sugars present in food crops, as mentioned before. The other
possible route is to produce biofuels by the reaction of transesterification of triglycerides, in this
case, vegetable oils or animal fats, with an alcohol in the presence of a catalyst [6].
3
Figure 1- Routes for producing first generation biofuels (adapted from [7]).
In this matter, IFPEN [8] has been a pioneer in the development of first generation biodiesel and is
working on the hydrogenation of vegetable oils (Hydrotreated Vegetable Oil or HVO) for producing
biodiesel and biokerosene. The fuels produced using this technology offer excellent qualities for
diesel engines as they present high cetane number, absence of sulfur and aromatic hydrocarbons.
However, the sustainable production of first generation biofuels is being criticized due to the
competition with the food sector and environmental concerns about the impact of such use of crops
and edible material. Thus, the main disadvantage of using this source of carbon is the food-versus-
fuel debate, as an increase of production of these fuels can increase food prices. Therefore, a more
efficient alternative feedstock arose, see Figure 2.
Figure 2- Biomass sources for first generation biofuels production in comparison with second generation biofuels [9].
Second generation biofuels use agricultural and forest residues as raw material, also largely referred
as lignocellulosic feedstock. These represent plant’s cheap and abundant nonfood materials. The goal
of such processes is to extend the amount of biofuel that can be sustainably produced. Thus, non-
edible parts of crops, such as stems, leaves and husks, that have no use once the crop has been
4
extracted, as well as crops that cannot be used as food (like switch grass, grass, jatropha and whole
crop maize, amongst others), and also industry wastes (woodchips, skins and pulp from food
processing) are used to produce fuels. Currently, the production of ethanol from cellulosic materials
is the most developed second-generation process [5].
Besides these two categories, a third generation of biomass sources has recently arose. The term
third generation is used for biofuels that are produced from aquatic autotrophic organism, such as
algae. The main advantages of using algae for bio fuel production are their rapid growth rate, the fact
that they can accumulate the CO2 produced by photosynthesis as lipids, and they are able to produce
15-300 times more oil than traditional crops, on an area basis [10]. Also, similarly to second
generation sources, they do not compete with food crops. However, the economic and
environmental viability of the process of production of biofuels from microalgae presents a major
issue as it will be necessary to significantly reduce the energy consumed through the chain [8].
Therefore, there is still a lot of research work to be done in order to develop a large-scale production
process from third generation feedstock.
In the present work, wood is used as a second generation source of hydrocarbons to produce
biofuels. In general, wood is a lignocellulosic material mainly composed by cellulose (40-50% of the
dry wood weight), hemicelluloses (25-35%) and lignin (18-35%) [11]. These represent the structural
components of wood and their subunits are schematically indicated in Figure 3. Moreover, the
nonstructural components do not contribute to the cell wall structure and include polysaccharides of
starch, wood extractives, proteins, some water-soluble organic compounds and inorganic minerals,
as ash [11][6].
5
Figure 3- Lignocellulosic components (adapted from [12]).
Cellulose is a long chain polymer made of linked sugar molecules, making it a glucan polymer,
consisting of linear chains of 1,4--D bonded anhydroglucose units. Cellulose is insoluble in most
solvents, including strong alkaline solutions. Therefore, it is difficult to isolate it from wood, as it is
intimately associated with lignin and hemicelluloses [11][13].
Hemicelluloses are a mixture of polysaccharides synthesized in wood and composed of five subunits -
glucose, mannose, galactose, xylose and arabinose. Generally, hemicelluloses are of much lower
molecular weight than cellulose and some polymer chains can be branched. They can be cross linked
with lignin and, although they are not covalently bound to cellulose, the structure of polymers can
efficiently protect cellulose by creating a sheath. Therefore, lignin appears to contribute as a
structural component in the plant, strengthening lignocellulosic cell walls [11][12].
Lignin is a complex and high molecular weight branched polymer composed mainly of phenyl derived
alcohols. Its main subunits are guaiacyl, syringyl and p-hydroxyphenyl which are bounded by C-O or
C-C linkages [12].
6
2.1.2 Biomass conversion into biofuel
Regarding the conversion of biomass, there are two major routes to produce biofuels - biochemical
or thermochemical route.
Usually second generation biofuels require a first step before the final reaction that produces the
biofuel, which can be directly used or blended with gasoline/gasoil (Figure 4). Furthermore, the
thermochemical route is widely studied and there are several different processes that can be used to
produce the biofuel.
Unlike first generation bioethanol, lignocellulosic ethanol is produced in two steps. First, the
lignocellulosic biomass is hydrolyzed (using cellulose enzymes as a catalyst) in order to release the
sugar molecules. Then, fermentation of these sugars to ethanol is carried out by yeast or bacteria.
The by-product of this process is lignin, which can be burned to produce heat and power [14]. IFPEN
research is focused on the steps related to specific processes, such as pretreatment to release the
complex sugars and enzymatic hydrolysis to convert the complex sugars into simple and fermentable
sugars. Also, the optimization of the entire chain of the various steps, both on an environmental and
economic level, is studied in order to make this process competitive in an industrial scale [8].
Figure 4- Routes for producing second generation biofuels (adapted from [15]).
7
While biological processing is usually very selective and produces a small number of discrete
products in high yield using biological catalysts, thermochemical conversion of biomass often gives
multiple and complex products, in very short reaction times using inorganic catalysts [16].
Regarding the thermochemical processes to convert biomass, in which long chain hydrocarbons
(solid biomass) are broken into short chain organic compounds such as synthesis gas or oil, there are
basically three approaches - gasification, combustion or directly liquefy the biomass into
hydrocarbons. The latter includes hydrothermal liquefaction and pyrolysis. The choice of conversion
process depends on variables like quantity and type of the feedstock, project specific factors and the
requirements of its end use, among others. [17]. In Table 1 a comparison between the mentioned
biomass to energy conversion processes is represented. From all of these processes, pyrolysis has
attracted more interest in, due to its advantages in storage, transport and versatility, as it can be
used, for instance, to produce biofuels used in combustion engines, boilers and turbines [18].
As the biofuel used in this work was produced by fast pyrolysis, this description will be more focused
on pyrolysis.
Table 1- Comparison of four major Thermochemical Conversion Processes [19].
Process Temperature (°C) Pressure (MPa) Catalyst Drying
Liquefaction 250-330 5-20 Essential Not required
Pyrolysis 380-530 0.1-0.5 Not required Necessary
Gasification 500-1300 >0.1 Not essential Necessary
Combustion 700-1400 >0.1 Not required Not essential, but
may help
2.1.3 Pyrolysis
Pyrolysis is the thermochemical decomposition of biomass into a range of useful products, in total
absence of oxidizing agents (e.g. oxygen or air). It involves heating the biomass at a specific rate to a
maximum temperature (pyrolysis temperature), which is maintained for a specific period.
There are mainly three products derived from pyrolysis, namely char, bio-oil and fuel gas. Their
portion in the final bio fuel can be varied by adjusting the operation conditions. Lower process
temperatures and longer vapor residence times favor the production of charcoal, while high
temperatures and longer residence time increase the production of gases, and moderate
temperatures and short residence times favor the production of liquids [21].
8
Non-catalytic pyrolysis
Figure 5- Simplified layout of a pyrolysis plant proposed by Dynamotive (adapted from [22]).
The typical layout of a pyrolysis plant is schematically represented in Figure 5. The process begins
with biomass being fed into a pyrolysis chamber that contains hot solids and gases (fluidized bed)
which will heat the biomass up to pyrolysis temperature, at which decomposition starts. The
condensable and non-condensable vapors released from biomass leave the chamber and proceed to
a cyclone, which will separate the gases from the small particles of char that were dragged along with
it. The remaining char stays in the chamber. After the cyclone, the gas is subsequently cooled down
in a quenching system or similar. The condensable gases condense as bio-oil, while the non-
condensable vapors (mainly carbon dioxide, carbon monoxide, methane, ethane and ethylene) leave
the chamber as product gas that will be recycled or released and used for other purposes. As this gas
is free from oxygen, part of it may be recycled into the pyrolysis chamber as a heat carrier or
fluidizing medium. Similarly, the solid char may be collected as a commercial byproduct or burnt in a
separate chamber to produce the necessary heat for pyrolysis [23].
The produced liquid is a black tarry composed by complex hydrocarbons with large amounts of
oxygen, containing up to 25% of water [23]. It is mainly composed by phenolic components and,
compared with conventional fuel oils, presents half of their heating value (see Section 2.2.1 for
further information) [21]. Bio-oil is produced by rapidly depolymerizing and fragmenting biomass
structural components (cellulose, hemicellulose and lignin) which are subjected to a rapid increase in
temperature, followed by a sudden quenching to “freeze” the intermediate products of biomass. This
freezing step is crucial as it prevents further degradation, cleavage and second reactions to
9
occur [23]. Also, after the total condensation of the bio-oil occurs, it cannot be distilled due to its
instability, therefore, when fragmentation of the bio-oil is sought, this should be done during the
cooling step of the pyrolysis gases.
Based on the heating rate, pyrolysis may be broadly classified as slow or fast pyrolysis. If the heating
time is largely higher than the characteristic pyrolysis reaction time, then slow pyrolysis occurs.
Normally this includes heating rates of 5-7°C/min and, in this case, more char and less liquid and
gases are produced [17]. Moreover, the vapor residence time in the pyrolysis zone is on the order of
minutes or more. On the contrary, if the heating time is much slower than the reaction time, then
fast pyrolysis occurs and higher yields on liquid product are obtained. In fast pyrolysis the vapor
residence time is on the order of seconds or even milliseconds and heating rates range from 1 000 to
10 000 °C/s, but the peak temperature should be below 650°C, if bio-oil is the product of interest.
Otherwise, the peak temperature can be up to 1000°C/s, if the purpose is to produce gas. [23].
These two classifications can be branched into smaller categories according to the used residence
time and the medium in which pyrolysis is carried out. Table 2 presents a comparison between
different pyrolysis processes according to its characteristics.
Table 2 - Characteristics of different Pyrolysis Processes [23].
Pyrolysis Process Residence time Heating Rate Final Temperature (°C) Products
Carbonization Days Very low 400 Charcoal
Conventional 5-30 min Low 600 Char, bio-oil, gas
Fast <2 s Very high ~500 Bio-oil
Flash <1 s High <650 Bio-oil, chemical, gas
Ultra-rapid <0.5 s Very high ~1000 Chemicals, gas
Vacuum 2-30 s Medium 400 Bio-oil
Hydropyrolysis <10 s High <500 Bio-oil
Methano-pyrolysis <10 s High >700 Chemicals
The process of pyrolysis can be divided into four stages, according to the temperatures that are
achieved in each stage. Primarily, the drying occurs at temperatures of about 100°C, at which the
free moisture evaporates. This step is followed by the initial stage wherein the temperature raises up
to 300°C and the release of water and low molecular gases, such as carbon monoxide and carbon
dioxide, occurs. From about 200°C to 600°C the intermediate stage occurs and a primary pyrolysis
takes place. At this stage, most of the vapor is produced as large molecules of biomass particles
10
decompose into char, condensable and non-condensable gases. The final stage occurs between
300°C to 900°C with the secondary cracking of volatiles into char and non-condensable gases [23].
The choice of reactor plays an extremely important role in this process, although it usually represents
only 10-15% of the total capital cost of an integrated system [21]. Pyrolysis reactors include fixed
beds, moving beds, suspended and fluidized beds. Moreover, fast pyrolysis is successful with most of
fluidized bed reactors as it allows high heating rates, rapid de-volatilization and are easy to control,
facilitating the recovery of the desired product [17].
Catalytic pyrolysis
Typically, a bio-oil after pyolysis has to be upgraded in order to remove or modify the unwanted
compounds that make it unstable and unsuitable to use as a fuel. Its oxygen content, which ranges
from 20-35% (conf. Table 6 in Section 0) for a fast pyrolysis bio-oil, is responsible for most of the
unwanted characteristics of the bio-oil. To remove its oxygen, the bio-oil can be catalytically
upgraded by hydrodeoxygenation or catalytic cracking of the high molecular weight molecules (conf.
Section 0 for further information). In catalytic pyrolysis, these upgrading methods are directly applied
while pyrolysis is occurring and, unlike fast pyrolysis, the upgrading catalyst is put in contact with
biomass from the first stage [24].
From an economical point of view, this process is a promising alternative to the conventional fast
pyrolysis and posterior upgrading process, as the steps of condensation and re-evaporation for
upgrading of the bio-oil are avoided due to the fact that the desired changes occur before the initial
condensation. Additionally, a single reactor fulfills all process requirements to produce a quality
liquid fuel, without the need of a hydrogen high-pressure environment and additional complex
equipment [25][26]. However, the disadvantages associated with this technique include lower yields
on bio-oil, as many gases and solids are produced, and it is a process more difficult to decentralize
because of the catalyst and hydrogen supply.
Due to its potential in reducing the effective production cost of bio fuels, many consider catalytic
pyrolysis as the most promising way of producing a quality liquid fuel that can be used as a substitute
for petrochemical applications such as transportation fuel. Thus, many studies [24][25][26][27] have
been done in order to develop a process to produce a bio fuel with equivalent quality to the
upgraded bio fuel.
The choice of the catalyst presents the major challenge as, depending on the catalyst added to the
pyrolysis process, different reactions are enhanced and, thus, the produced bio fuel will have
11
different properties. When catalytic hydropyrolysis is sought, a conventional hydrodessulphurization
CoMo/Al2O3 catalyst can be used, as well as noble metal catalysts. The Gas Technology Institute
developed an Integrated Hydropyrolysis and Hydroconversion (IH2) process [27] that converts dried
biomass into a fuel that has a content reduced on oxygen and radicals, in only one step and in the
presence of a CoMo catalyst and low hydrogen pressure (<500psig). Typically, fast pyrolysis bio-oils
present a content on oxygen of about after this first step of hydropyrolysis the bio-oil presents 2,6
%wt and 0,5 %wt after the whole IH2 process. Furthermore, there is no need for additional hydrogen
supply to the process as a small methane reformer is used to convert the light gases produced into
renewable hydrogen [24][27].
In contrast, during catalytic cracking pyrolysis, a zeolite (typically HZSM-5) is used to perform the
deoxygenation of the bio-oil. In this case, no hydrogen is needed for the reaction, which occurs at
atmospheric pressure, and the bio-oil produced is rich in aromatic compounds, so it presents a high
cetane number. However, it is also stated that this bio-oil still has to undergo through an upgrading
step to remove the remaining oxygen as its quality is not yet the one obtained after a process of
upgrading. Depending on the process chosen, the oxygen content in the bio-oil ranges from 10-
20 %wt [24].
2.2 Fast pyrolysis bio-oil
2.2.1 Characteristics, applications and challenges
As mentioned before, the bio-oil obtained from fast pyrolysis is a dark brown liquid (see Figure 6)
with a distinctive odor, composed by a mixture of complex oxygenated hydrocarbons with an
appreciable proportion of water from both the original moisture and reaction product. In the best-
case scenario, a fast pyrolyzer can produce 75% of bio-oil, 12% of char and 13% of gas. The organic
liquid produced is highly oxygenated with up to 25% of water content [28]. Furthermore, typical
biofuels are also composed by lignin fragments, carboxylic acids and carbohydrates. Table 3 presents
the composition of typical bio-oils produced by fast pyrolysis. It is also important to refer that this
composition is heavily dependent on the source of biomass used to produce the biofuel.
Figure 6 – Bio-oil appearance
12
Table 3- Composition of typical bio-oil [28].
Major Group Compounds Mass (%)
Water Water 20-30
Lignin
fragments Insoluble pyrolytic lignin 15-30
Aldehydes Formaldehyde, acetaldehyde, hydroxyacetaldehyde, glyoxal, methylglyoxal 10-20
Carboxilic acids Formic, acetic, propionic, butyric, pentanoic, hexanoic, glycolic 10-15
Carbohydrates Cellobiosan, -D-levoglucosan, oligosaccharides, 1.6 anhydroglucofuranose 5-10
Phenols Phenol, cresols, guiacols, syringols 2-5
Furfurals Furfurals 1-4
Alcohols Methanol, ethanol 2-5
Ketones Acetol (1-hydroxy-2-propanone), cyclopentanone 1-5
As the liquid is formed by the rapid quenching of the gases produced in fast pyrolysis, it contains
many reactive species, making it relatively unstable when compared to conventional fossil fuels.
Several studies have been done in order to try to characterize pyrolysis bio fuels [28][30][31][32].
This liquid contains several hundred different chemicals in widely varying proportions, ranging from
formaldehyde and acetic acid to complex high molecular weight phenols, anhydrosugars and other
oligosaccharides. So, due to the presence of highly reactive components, bio-oil is rather unstable
and during its storage phase separation and solids formation can occur. Also, severe polymerization
will result in the formation of char. Although this liquid is commonly called as biofuel, it is not
miscible with conventional petroleum-derived fuels [21], which presents, obviously, a challenge for
its utilization as fuel.
Table 4 summarizes some important characteristics of fast pyrolysis biofuel and compares them with
conventional fossil fuels.
13
Table 4 –Comparison of typical properties of wood-derived bio-oil, gasoline and diesel [28].
Properties Bio-Oil Gasoline Diesel
Higher Heating Value (MJ/kg) 18-20 44 42
Density at 15°C (kg/m3) 1200 737 820-950
Flash Point (°C) 48-55 40 42
Pour Point (°C) -15 -60 -29
Viscosity at 40°C (cP) 40-100 0,37-0,44 2,4
pH 2,0-3,0
Solids (%wt) 0,2-1,0 0 0
Elemental Analysis (%wt)
C 42-47 84,9 87,4
H 6,0-8,0 14,76 12,1
O 46-51
N <0,1 0,08 392 ppm
S <0,02 1,39
From the comparison shown in Table 4, it can be seen that bio-oil has a heating value which is nearly
half of conventional fossil fuels, due to its high oxygen content. This means that it is necessary two
times more bio-oil to have the same energy released. However, bio-oils have comparable flash point
(lowest temperature at which it can vaporize to form an ignitable mixture), as well as pour point
(lowest temperature at which the bio-oil will begin to flow). Besides this, bio-oils also have the
following undesirable properties: higher water content, higher viscosity, higher ash content and
higher corrosiveness (due to higher acidity). Furthermore, it is stated that biofuels change with time,
more precisely, its viscosity increases while its volatility decreases and some deposition of
compounds may also occur due to polymerization, condensation, esterification and etherification
[21][28]. These undesired properties have so far limited the range of bio-oil application.
14
Figure 7 -Applications for fast pyrolysis products [21].
As stated before, applications for bio-oils (see Figure 7) include energy production, as it can be
burned in boilers or furnaces, chemical production (resins, fertilizers, agro-chemicals, acetic acid,
etc.) and it can also be used as transportation fuel. However, bio-oil has less hydrogen per carbon
atom than conventional fossil fuels and has a much higher oxygen content, is far less stable than
conventional fossil fuels and is also more corrosive [28]. Therefore, the presence of heteroatoms in
bio-oil, such as oxygen, is responsible for much of the challenges in direct integration of bio-oil into
petroleum refinery processing and end use infrastructure. To overcome this, bio-oil can be upgraded
in order to become more stable and used as a transportation fuel.
Figure 8 shows the transformation steps biomass has to go through in order to become a stable and
upgraded bio fuel.
FastPyrolysis
Gas
Liquid
Separation Chemicals
UpgradingConversion
Transport fuels, etc
Turbine
ElectricityEngine
Co-firing
Heat
Boiler
CharcoalCharcoal
applications
15
Figure 8- Diagram of the transformation processes of biomass in order to produce upgraded bio-oils by pyrolysis (adapted from [33]).
2.2.2 Bio-oil Upgrading
Bio-oil can be upgraded in a number of ways, either physically, chemically and/or catalytically.
Physical upgrading includes filtration to reduce its content on solid particles (reducing its viscosity
and also its content on alkali metals and inorganic compounds that are mostly present in char),
emulsification with diesel using surfactants in order to produce a transportation fuel or a fuel that
can be used for power generation, and addition of solvents (e.g. hydrocarbons such as tetralin or
decalin [3][30], or alcohols like ethanol or butanol [32][32]) to stabilize the oil and reduce its viscosity
[21][31]. Solvent addition presents a very important upgrading process because, besides stabilizing
the bio-oil and reducing its viscosity, minimizes the deterioration of bio-oil over time, reducing
secondary reactions’ rate. Furthermore, the solvent can provide thermal stability to the bio-oil,
reducing polymerization reactions and char formation during the step of hydrotreatment.
However, upgrading bio-oil to a conventional transportation fuel such as diesel, gasoline or kerosene
requires full deoxygenation by chemically and/or catalytically upgrading, which can be done either by
hydrotreating, catalytic vapor cracking or gasification to syngas followed by FT synthesis.
Table 5 presents the summary of the upgrading methods as well as its advantages and disadvantages.
16
Table 5- Comparison between different upgrading methods and technical feasibility of such techniques [32].
Upgrading methods
Treatment conditions/
requirement
Reaction mechanism/ process
description
Technique feasibility
Pros. Cons.
Hydrotreating/ hydrofining
Mild conditions (≈500 °C/ low pressure), chemical needed: H2/CO, catalyst (e.g. CoMo, HDS,
NiMo, HZSM-5)
Hydrogenation without simultaneous cracking
(eliminating N, O and S as NH3, H2O and H2S)
Cheaper route, commercialized
already
High coking (8-25%) and poor quality of fuels
obtained
Catalytic cracking/ Hydro-cracking/ hydrogenolysis
Severe conditions (> 350 °C, 100-2000 Psi), chemical needed: H2/CO
or H2 donor solvent, catalysts
(e.g. Ni/Al2O3-TiO2)
Hydrogenation with simultaneous cracking. Destructive (resulting in low molecular product)
Make large quantities of light
products
Need complicated equipment, excess cost,
catalyst deactivation,
reactor clogging
Sub-/super critical fluid
Mild conditions, organic solvents needed such as alcohol, acetone, ethyl
acetate, glycerol
Promotes the reaction by its unique transport properties: gas-like
diffusivity and liquid-like density, thus dissolved materials not soluble in either liquid or gaseous
phase solvent.
Higher oil yield, better fuel quality
(lower oxygen content, lower
viscosity)
Solvent is expensive
Solvent addition (direct add solvent or esterification of the oil with alcohol and acid catalysts)
Mild conditions, polar solvents needed such as
water, methanol, ethanol, and furfural
Reduces oil viscosity by three mechanisms: (1)Physical dilution
(2) Molecular dilution or by changing the oil
microstructure (3) chemical reactions like
esterification and acetalization
The most practical approach
(simplicity, the low cost of some
solvents and their beneficial effects on the oil properties)
Mechanisms involved in adding
solvent are not quite understand
yet
Emulsification/ emulsions
Mild conditions, need surfactant (e.g., CANMET)
Combines with diesel directly. Bio-oil is miscible with diesel fuels with the
aid of surfactants
Simple, less corrosive
Requires high energy for production
Steam reforming High temperature (800-900°C), need catalyst (e.g., Ni)
Catalytic steam reforming + water-gas shift
Produces H2 as a clean energy
resource
Complicated, requires steady,
dependable, fully developed
reactors
Chemical extracted from the bio-oils
Mild conditions Solvent extraction,
distillation, or chemical modification
Extract valuable chemicals
Low cost separation and
refining techniques still
needed
This work aims to study the catalytic hydrotreatment step of the bio-oil with the use of solvent
and/or recycling of the effluent to stabilize the bio-oil. Therefore, detailed information on this subject
will be given hereinafter.
17
2.3 Bio-oil upgrading by catalytic hydrotreatment
Currently, the most widely used hydrogenation processes for the conversion of petroleum and
petroleum products is hydrotreating [32]. The catalytic hydrotreatment (HDT) or hydroreforming of
bio-oils is performed at high temperatures (250-350°C) and high hydrogen pressure (100-300 bar)
[30][32]. Its purpose is to improve the quality of the product by reducing its content on oxygen,
which can vary between 30-45 wt.% and provides instability to the bio-oil, as reactions of
polymerization/condensation of the oxygenated species can occur. These reactions cause
degradation of the quality of the bio-oil and difficult its hydrotreatment.
For the purpose of upgrading bio-oil, Dynamotive and IFPEN/Axens developed a two stage process,
schematically represented in Figure 9 Erro! A origem da referência não foi encontrada..
Figure 9- Diagram of the two-stage process to upgrade bio-oils developed by Dynamotive and IFPEN/Axens [33].
The first step of the represented process (hydroreforming) aims to stabilize the bio-oil by
deoxygenating it, producing a Partially Upgraded Bio-oil (PUB), and occurs in the presence of
hydrogen and a typical catalyst used in hydroreforming. Three phases are produced in this step:
gaseous, organic and an aqueous phase. The latter two are usually separated by a physical process,
such as decantation or centrifugation. Typical characteristics of PUB are presented in Table 6.
18
Table 6- Comparison between typical characteristics of fast pyrolysis bio-oils and PUB after the first step of HDT [32].
Fast Pyrolysis Bio-oil PUB
Elemental Analysis
C 55-60% 75-80%
H 5-7% 9-11%
O 35-40% 7-15%
S <0,03% <0,03%
Density 1,2 <1
% H2O 25-30% <5%
Specific heat 16 35-40
Miscibility with hydrocarbons No Yes
Thermal stability Poor Good
Acidity Highly acid (pH=2-3) Moderate acidity
Moreover, the second step of the upgrading process of bio-oils allows the total removal of oxygen
from PUB via the addition of hydrogen and allows the conversion of high molecular weight
molecules, and is similar to the hydrotreatment/hydrocracking step used in refineries for petroleum
feedstock.
The main advantage of this two stage process is that it produces, from a non-treated pyrolysis bio-oil,
a stabilized bio fuel with a content on oxygen and water of less than 1%m/m and 0.1%m/m,
respectively, with lower consumption of hydrogen and without significant formation of coke or
polymers, which results in lower deactivation of the catalyst. Furthermore, once the pyrolysis bio-oil
does not need to undergo through any pretreatment step, the production cost is reduced [29].
Since this work is focused on the study of the first hydrotreatment step, further information on the
reactions involved, the catalyst that can be used, and possible process variations, such as the use of
solvent and effluent recycle, is going to be presented.
2.3.1 Reactions involved
Hydrodeoxygenation of bio-oils occurs at elevated temperatures (300-400°C) and in the presence of
hydrogen and a catalyst. The global reaction of hydrodeoxygenation of a hydrocarbon can be
generally represented as Equation 1. In this case, the coefficients x and y represent H/C and O/C
ratios, respectively. These values depend on the feedstock used for producing the bio-oil and the
operation conditions during its production. Representative values for dry based bio-oils are
(x,y)=(1.4 , 0.6) [30].
-(CHxOy)- + c H2 → -(CHx)- + H2O (l) + gas (H2O, CO2, CH4, CO, C2/C3…) Equation 1
19
However, there are a wide number of secondary reactions, presented in Figure 10, that can occur
during HDT, making the PUB a complex mixture of hydrocarbons, thus it is difficult to anticipate and
to model the reactions that occur in the upgrading process.
Figure 10- Reactions pathways in the hydrodeoxygenation process (adapted from [30][32]).
As represented in Figure 10 numerous reaction pathways can take place during hydrodeoxygenation.
For instance, in decarboxylation reactions the oxygen is released in the form of CO2 or CO. During
hydrogenation the hydrogen reacts with unsaturated molecules either forming saturated olefins,
converting aromatics to naphtenes, or forming alcohols. Cracking reactions are characterize by the
breakage of high hydrocarbon chain molecules into smaller alkanes and alkenes. Besides the
mentioned reactions, it can also occur equilibrium reactions between carbon dioxide and hydrogen
(inverse of the Water Gas Shift reaction and methanation, which produces methane). At high
temperatures (higher than 500°C) it can also occur the formation of hydrogen by the reaction of
steam reforming [30][32].
Along with these reactions it can also occur water separation due to changes in the molecular
structure of bio-oil, and reactions of dehydration in which water is produced from side reactions,
such as condensation polymerization.
Therefore, operation conditions and the catalyst play a crucial role in this process. The catalyst used
should enhance reactions of HDT and adsorb both hydrogen and the bio-oil. This can be done by a
wide range of catalysts with active phases such as molybdenum, noble metals, reduced/sulfided
metals, etc. and supports such as alumina, graphite and titanium or zirconium oxide [32]. Usually in
the process of hydrotreating of fossil based fuels sulfided CoMo or NiMo catalysts supported on
alumina or aluminosilicates are used. As stated by Wildschut [30], several tests were done in order to
20
try out these catalysts for HDO of bio-oils in a wide range of operation conditions, reactor types and
feedstock. Among these, CoMo/Al2O3 catalyst seemed to have higher deoxygenation activity
[35][36].
However, the catalyst appears to be deactivated after some hours of operation (Semolada et al [37]
indicated 100h) and severe deactivation of the catalyst is observed causing the formation of coking
and plugging the unit. This deactivation can be caused by difficulties in access to the catalyst pores
and/or active sites, irreversible poisoning of the catalyst (by nitrogen compounds), presence of water
(since the oxygen content is much higher than in conventional HDT) and sinterization of the catalyst,
among others [35][36].
Furthermore, the reduced form of the catalyst showed less activity than the sulfided form. This
implies that the addition of a sulphur source is advantageous when using these catalysts. However,
addition of sulfur is undesired because the product can be contaminated by sulfur-containing
species. Therefore, it has been studied the use of noble metals and other sulfur-free metals as HDO
catalysts [30][36].
2.3.2 Use of solvent and effluent recycling
The economic viability of the production of biofuels for energy applications depends on the discovery
of methods that allow its valorization into a liquid fuel of high quality, at a sufficiently low cost. As
mentioned before, catalytic hydrodeoxygenation has been a widely studied process to upgrade bio-
oil. Yet, high consumption of hydrogen, low liquid yield and the thermal instability of bio-oil, which
results in coke formation and deactivation of the catalyst, have made the economic feasibility of this
process, and thus its industrialization, difficult [38] [39] [29].
Therefore, new methods to convert the unstable and corrosive oxygenated compounds present in
the bio-oil are being developed. One of the possible ways to do so, is by using a solvent during the
upgrading of bio-oil, which will reduce the coke formation and enhance its HDO [39]. Many studies
have been done in order to study different solvents for this upgrading step. For instance, a hydrogen-
donor solvent, such as decalin or tetralin, can be used to reduce coke formation, however, due to
their hydrophobicity, it can be difficult to blend them. Alcohols, such as ethanol or butanol, are also
reported [29] to be used as solvents in the upgrading of bio-oils by esterification, however, the
products of such treatment are acidic, present a high water content and low specific heating value
and are said to be unstable and reactive, even after the upgrading process. More recent studies,
indicate supercritical solvents (typically methanol, ethanol and butanol) to enhance the mass and
heat transfer during the reaction [38][39].
21
Besides the use of solvent for bio-oil stabilization, other studies [29][40][41] indicate that the use of
effluent recycling is also a promising way to enhance the upgrading process. The effluent is
composed by already treated and stabilized compounds with a low oxygen content, which allows the
replacement of the solvent by an effluent stream. There are different process schemes to perform
the recycle of the effluent in an industrial scale, however, recent publications on this subject [40]
present a scheme of an industrial unit designed to recycle the effluent (Figure 11).
Figure 11 - Schematic process diagram of a hydrotreatment unit with effluent recycling [40]
This process is composed by a reactor section (18), two phase separation sections (24) and (46) and a
recycle column (38). In the first separation section (24) three phases are divided - the hydrogen
containing gases stream (30) is separated and can be reintroduced in the process as a recycling
stream or be directly introduced in the reactor as a promoter to the catalytic reaction; the aqueous
phase (26) is directed to a an aqueous-organic separation zone, which is not represented in the
scheme, and aims to further separate this stream into a water-rich stream and a phenolic-rich
stream, as it is mentioned in a second patent [41]; and the organic phase continues to the recycle
column (38) that separates the low molecular weight fraction of partially upgraded bio-oil from the
high. The high molecular weight fraction (42) is directed to a second hydroprocessing reactor system
that aims to produce a ultra-low oxygen pyrolysis bio-oil which will be used as a bio fuel. The low
molecular weight fraction (40) is separated from the organic volatile compounds (48) and the
remaining water (50) in a second phase separator and is recycled [40].
Therefore, in this invention, untreated pyrolysis bio-oil (12) is mixed, before entering the reactor,
with a heated low molecular weight fraction of partially upgraded bio-oil (14) that has a lower
22
content on reactive compounds (which can suffer second polymerization reactions, resulting in the
formation of solids and increase in the viscosity). However, the fact that stream (14) still presents
oxygenated compounds, although in low concentrations, makes these two streams miscible.
Moreover, it is also reported in this invention that incomplete conversion (i.e., leaving some oxygen
in the effluent) facilitates the separation of the low and high molecular weight fractions (streams 40
and 42, respectively), and yet the produced partially upgraded bio-oil can still proceed to a second
stage of hydroprocessing and produce a high quality biofuel [40].
23
3. Objective and Methodology
Within the framework of this project, several tests had been previously done [42] in order to study
the optimal operating conditions (e.g. hydrogen pressure, masse of feedstock and reaction time)
during the hydrotreatment of bio-oil, in the presence of a stabilizing solvent (an alcohol). However,
the solvent presents different properties and reactivity when compared to partially upgraded bio-oil
(PUB). These differences will probably influence the performance of the process and vary the optimal
conditions of the hydrotreatment, justifying the need to further study the operating conditions of the
process with a recycle of PUB, instead of solvent.
Figure 12 - Two stage process to upgrade bio-oil
Therefore, the present work is focused on the recycling of effluent in the first hydroreforming step of
the two stage process to upgrade bio-oils (Figure 12) and aims to study the influence that different
operating parameters have on the quality of the PUB. The main objective of the first hydrotreatment
step is to improve the quality of the bio-oil, so that the second treatment can be performed with a
conventional catalyst and under the same conditions used in the hydrotreatment/hydrocracking step
in the refining of petroleum products.
The quality of bio-oil can be influenced by several parameters, as it will be discussed further on, yet it
is expected that the first hydrotreatment step stabilizes the bio-oil, reduces its oxygen content to 5-
15 wt.%, decreases the water solubility to less than 5 wt.%, makes it miscible with hydrocarbons and
initiates the conversion of the macromolecules, which will lead to a reduction on heavier compounds
in the bio-oil (measured by the carbon Conradson analysis).
24
In a first part, the studied parameters include changes in operating conditions, such as reaction time,
mass of catalyst and activation of catalyst, and in the morphological properties of the catalyst in
order to verify whether diffusional limitations were present in the reaction. Lastly, it was also studied
the influence of recycling only a distillation fraction of PUB, instead of recycling all the effluent.
This work was followed-up by a second trial of tests, in which a continuous operation was simulated,
in order to estimate the evolution of the PUB after a series of continuous recycling. The first test of
each set of tests was performed in the presence of a solvent, simulating the start-up of the unit,
whereas the following tests were done by adding the organic phase produced in the previous test to
untreated bio-oil, in order to simulate a continuous recycling operation. The main objective of this
work is to study the start-up period of a continuous unit, before the establishment of the steady
state and to predict, if possible, the number of cycles necessary to achieve the steady state. These
tests will also give information on the quality of the effluent after each recycling cycle and once the
steady state is achieved, and provide clues on the modifications that may be considered on the
operating conditions to improve the final quality of the PUB.
In all the performed tests, the hydroreforming occurred in the presence of four distinct phases -
organic, aqueous, solid and gaseous. Therefore, a post treatment of the effluent had to be done, in
order to recover all the four different phases, analyze them and obtain their composition.
The next chapters will include a description of the unit that was used to perform the tests, along with
the post treatment that was conducted after each test, the analysis done to the different obtained
phases, as well as all the tests performed and the relevant conclusions that could be taken with the
presented results.
25
4. Experimental work
4.1 Unit description
The tests were performed in a small scale Autoclave unit, which works in batch mode, at high
temperature and pressure (up to 450°C and 170 bar), and has a total volume of 0.3L (Figure 13) [43].
The unit consists of a reactor, a ballast, a Grayel Bottle and all the instruments needed to control and
assure the proper functioning of the unit.
Figure 13 - Schematic layout of the batch unit
The reactor has a capacity of 355 mL and is made of Inconel alloy 718, a resistant material that can
stand high temperatures and pressures. This reactor has a supply circuit of hydrogen and nitrogen,
which can achieve pressures up to 170 bar. Usually hydrogen is used as a reactant whereas the
nitrogen is used either to inertize the reaction zone, or to perform preliminary tests which do not
involve catalytic reactions. The heating of the reactor is performed by one internal resistance along
with four heating rods and the stirring of the reactor is assured by a magnetic agitator with a Rushton
turbine. Additionally, the cooling of the reactor is done by a vortex of cold air [43].
The ballast is used to maintain the hydrogen pressure in the reactor, as in each test there is a
minimum targeted pressure in order to allow the reaction. In the cases of high consumption of
hydrogen a make-up of this gas is necessary. Thus, the ballast is essential because, by monitoring its
pressure, it is possible to estimate the quantity of hydrogen that was added. Finally, the Grayel Bottle
collects the gases from the reactor, which will be subsequently analyzed in a chromatograph.
26
The temperature and pressure in the reactor, ballast and Grayel Bottle are constantly monitored to
allow a complete knowledge of the gases that are introduced and removed from the reactor during
each test and, subsequently, perform a complete mass balance to the process.
4.2 Bio-oil
The bio-oil used in this experimental work was produced through fast pyrolysis of hardwood or
softwood without bark (i.e. sawdust).
The typical properties of the bio-oil used in the experimental work are shown in Table 7 and the
yields, on dry basis, of the bio-oil’s production process are presented in Table 8.
Table 7 - Properties of bio-oil.
KF (% H2O) Density (g/cm3)
Elemental Analysis (wt.%)
C H O CCR (wt.%)
Bio-oil 21,6% 1,234 41,6 7,3 50,0 20,2
Table 8 - Yields of the bio-oil production, calculated on dry basis.
Raw material Organics wt.% Water wt.% Gas wt.% Char wt.%
Sawdust 63 10 9 18
4.3 Catalyst activation
The catalyst used to perform the reaction is composed by an active phase of NiMo with a support of
nickel aluminate and, before each test, it had to be activated in order to form active sites in the
surface of the catalyst that will promote the reaction.
Typically, the catalyst was reduced with hydrogen, and the reduction occurred at 450°C for 2 hours
with a constant flow of hydrogen of 2 L/h/(g of catalyst). However, since the study of the activation
of the catalyst was also intended, a test was performed with a different activation method, in which
the catalyst was sulfided with H2S, in the same conditions. Since the procedure of sulphurization of
the catalyst requires specific formation and security measures, it was done by a technician in the
department of sulphurization of IFPEN.
27
After reduced/sulfided, the catalyst was placed in a basket made of stainless steel, which is put inside
the reactor (containing the liquid reactants), in a controlled inert atmosphere, to avoid its oxidation.
The function of the basket is to be a support to the catalyst and facilitate the separation and post
treatment.
4.4 Post treatment
Once the reaction was finished, the gases from the reactor were isolated in the Grayel Bottle (conf.
Figure 13) and collected in a gas bag, in order to be analyzed via gaseous chromatography.
The tests were performed in a batch unit, which means that at the end of each test the reactor was
opened (with the exception of the second trial of tests, see Section 0), the recipe (Figure 14) and
catalyst were collected and the reactor was cleaned. After this, the post treatment procedure, that
included cold filtration of the effluent, decantation and/or centrifugation, was carried out.
Figure 14 - Effluent from the reactor.
The humid catalyst used in the reaction was washed with acetone, in order to remove the remaining
effluent retained in the catalyst, in a specific equipment (Dionex ASE 150) that works at high pressure
with a flow of acetone. After the washing process, the catalyst was dried in an oven and could be
sent to analysis, if necessary.
The remaining effluent, composed by an aqueous and organic phase and the solids that were
produced during the reaction or/and from the catalyst, was then filtered in a vacuum system with a
microfiber filter with 1 µm of porosity that allowed the removal of most of the solids from the liquid
effluent. This step was performed in order to avoid contamination and subsequent deviations on the
analysis that were performed to the liquid phase. In the tests performed without basket it was found
that the content on solids in the liquid phase was much higher than in the other tests, and so a
28
centrifugation had to be additionally done to remove most of the solids before the filtration step.
The filtered solids suffered the same post treatment of the catalyst – washed with acetone and dried
in an oven – so that they could be weighted and quantified.
After the filtration of the liquid, the organic phase was separated from the aqueous phase by
decantation or, in those cases where this separation was difficult due to the proximity of their
densities, a centrifugation was performed. Figure 15 shows a comparison between the aqueous
phase that was obtained after decantation of the effluent and after decantation and centrifugation.
As it can be seen, in some cases there was a mixture of organic and aqueous phases even after the
step of decantation, probably due to the small difference of density and/or close polarity.
Figure 15 - Aqueous phase before (left) and after (right) centrifugation.
After separation of all the phases, it was necessary to analyze each one of them, in order to
determine the composition of both liquid phases, for a better understanding of this hydrotreatment
step.
4.5 Analysis
In order to determine the composition of each phase, several analyses were performed. Hereinafter,
the analysis performed to the gaseous, organic and aqueous phase, as well as the catalyst after
reaction will be detailed.
4.5.1 Gaseous Phase
Gaseous chromatography (GC)
The gaseous phase was analyzed by gaseous chromatography. Chromatography’s operation principle
is to separate the different constitutes of a mixture according to the different affinity that each
component presents between the mobile phase and stationary phase. Typically, the mobile phase is
a fluid, which is a gas in the case of GC, that carries out the mixture through the column (stationary
29
phase). The different elution rates of the components of the mixture cause their separation, and the
difference in the retention time allows the characterization and identification of each component of
the mixture.
Figure 16 - Operation principle of GC [44].
To distinguish each component of the mixture that is being eluted off the chromatography column, a
detector is used (see Figure 16). There are several detectors that can be used when performing a
chromatography. In the present case, one flame ionization detector (FID) and two thermal
conductivity detectors were used. The operation of the FID is based on the detection of ions formed
during combustion of organic compounds in a hydrogen flame, and this is typically used to detect
light hydrocarbons from C1 to C6. TCD detectors sense changes in the thermal conductivity of the
column effluent and compare it to a reference flow of carrier gas (hydrogen or helium). Unlike the
FID, TCD responds to all the substances and can be used to analyze CO, CO2, O2, N2, H2 and light
hydrocarbons [44]. However, for a greater accuracy in the results, the method used for the
quantification of the gases, is optimized in order to retain the value obtained by the detector that has
higher sensibility to each compound. Therefore, the FID1 will quantify the C1 to C6 hydrocarbons,
while TCD2 is responsible for analyzing CO2, N2 and CO and TCD3 quantifies the H2 (conf. Figure 17).
The analysis were performed in a Agilent Technologies 7890A GC System and a typical chromatogram
of the gases produced during the reaction is presented in Figure 17, which is related to the analysis
made to the gases of the reference test described in Section 5.1.
30
Figure 17 - Chromatogram obtained in the GC analysis of the reference test.
As it can be seen in the chromatogram, mainly hydrogen (which was initially injected in the reactor as
a precursor of the reaction), nitrogen, carbon dioxide, carbon monoxide and light hydrocarbon
molecules are present in the gas phase. The nitrogen presence is due to the necessity to increase the
pressure in the Grayel Bottle to allow the extraction of the gas from it. Therefore, it will not be taken
into account in the calculations and does not form part of the gases from the reaction. The
composition of the gaseous phase obtained in each test will be discussed after the presentation of
the results of each test.
4.5.2 Organic phase
Regarding the organic phase, as it is the most complex phase, several analyses were performed.
Faster analysis, such as the density and the content on water through Karl Fischer titration, were
performed. Nonetheless, the detailed composition of each organic phase was determined by more
complex analysis, such as backflush chromatography, elemental analysis and Carbon Conradson. The
next paragraphs include a more detailed description of each analysis.
31
Density
The density was measured by a digital density meter, which uses the oscillating tube technique and a
Peltier temperature control to perform its measurement. The technology is based on the principle of
inducing a vibration into a U-tube filled with the sample and measuring its oscillating frequency. The
higher the density of the sample inside the cell, the lower the oscillating frequency [45]. The
measurements were done in a Mettler Toledo DM 40 Density Meter equipment at 20,0°C.
Karl Fischer (KF)
The Karl Fischer method is a chemical analysis procedure to determine the moisture content in a sample, which
is based on the oxidation of sulfur dioxide by iodine in a methanolic hydroxide solution [46]. In principle, the
following chemical reaction takes place.
H2O + I2 + SO2 + CH3OH + 3RN → [RNH]SO4CH3 + 2[RNH]I
The titration can be performed volumetrically or coulometrically. For samples with high moisture
content, a volumetric titrimetry should be performed, whereas for those with water contents in the
ppm range, a coulometric procedure should be used [46][47]. Since high water contents are expected
in the analyzed organic phase (1-30 wt.%), a volumetric titration method was used. A Mettler Toledo
V20 Volumetric KF Titrator equipment (Figure 18) was used to do the analysis.
Figure 18 - Karl Fischer equipment Meter Toledo [47] (left) and its working principle [45] (right).
In this method, a KF solution containing iodine (in this case HYDRANAL®-Composite was used) serves
as titrant agent and is added until the first trace of excess iodine is present. The amount of iodine
converted and, consequently, the water content are determined from the volume of the burette of
the iodine-containing KF solution. The working medium is a solvent in which the sample is dissolved
and the KF reaction occurs. Usually, methanol is preferred as a solvent, as it assures a rapid and
stoichiometric course of reaction, most of the samples dissolve easily in methanol and it provides a
32
reliable indication of the end point [46][47]. All the analysis were performed with HYDRANAL® -
Methanol Rapid as a solvent of reaction.
Backflush gaseous chromatography (GC Backflush)
The composition of the organic phase was obtained by backflush chromatography. This analysis was
relatively important, as it allowed the estimation of the conversion of solvent during each reaction.
The equipment used is Agilent Technologies 7890B GC System.
This technique is used because the effluent of hydroreforming of biomass pyrolysis (PUB) is
consisted, in part, by low volatility compounds, so the use of a backflush system is needed to
maintain the integrity of the chromatographic column and avoid its pollution by the low volatile
compounds [48]. The principle of backflush system is detailed in the diagram below (Figure 19).
Figure 19 - Principle of backflush chromatography [48]
The sample is injected in a gas chromatograph equipped with an injector and a divider grafted
capillary column with an apolar stationary phase. The compounds are eluted with a carrier gas
(helium) according to their boiling point and detected with a flame ionization detector (FID). The
sensor’s signal is processed by a computer equipped with an acquisition and integration software
[48].
In the frontflush operation, the compounds are eluted through the pre-column and analytical column
and the auxiliary pressure controller assures the supply of extra carrier gas in the analytical column.
At a predetermined time, the carrier gas flow is reversed in the precolumn to backflush to the
injector the heavy compounds that were not eluted from the precolumn and, therefore, not send to
the detector as volatiles already eluted from the precolumn. The quantification of the compounds is
obtained by internal calibration [48].
33
Elemental analysis: CHSO
Additionally, an elemental analysis to the content on carbon, hydrogen and oxygen was performed to
the PUB and, in one particular test, it was also determined the content on sulphur. The analysis to
the carbon and hydrogen content were done using a thermal conductivity detector, while the oxygen
and sulphur content were obtained by infrared detection.
CHS analyses require high combustion temperature in an oxygen-rich environment. Its basic principle
is that, in a combustion chamber at high temperature (1150°C) and under a helium current, the
carbon will be converted into carbon dioxide, the hydrogen into water and the sulphur into sulphur
dioxide. Afterwards, the combustion products are passed through copper to remove the remaining
unconsumed oxygen and the gases continue through an absorbent trap that retains all the gases but
the carbon dioxide, water and sulphur dioxide. The detection of the gases can be done by a GC
separation, followed by a quantification using thermal conductivity detection (in the case of CH) or
infrared detection (in the case of S) [49][50].
The oxygen content is measured using a different technique, where the sample is placed in a graphite
crucible and inserted into a furnace. Then, a high current of nitrogen is passed through the crucible,
creating a temperature increase. The sample undergoes through pyrolysis and the generated gases
are directed to an infrared detector, where the oxygen content is measured as CO [50].
The CHO analysis was performed by an external laboratory, while the content on sulphur was done in
the analysis department of IFPEN.
Carbon Conradson
Furthermore, the carbon residue of the PUB was measured by the carbon Conradson (CCR) analysis,
done in the analysis department of IFPEN. This analysis consists in subjecting a weighted sample to a
destructive distillation for a fixed period of time, during which the residue undergoes through
cracking and coking reactions. Then, the remaining carbonaceous residue that was not volatized is
cooled down in a dessicator and weighted. The carbon Conradson residue is presented as a
percentage of the original sample.
Size-Exclusion Chromatography (SEC)
In Chapter 6.1 some tests performed with continuous recycling of the effluent are presented. Due to
the lack of quantity of effluent that remained after the recycling process, it was not possible to do a
CCR analysis to obtain its carbon residue. Therefore, a size-exclusion chromatography (SEC) was
34
performed, as it requires less product and it provides a molecular weight distribution of the sample.
The operation principle of SEC is based on the separation of the molecules by their size or molecular
weight (see Figure 20).
Figure 20 - SEC operation principle.
The analysis were performed in the IFPEN’s analysis department and the obtained molecular weight
distribution was relative to a reference of polystyrene within a range of 162-5030 g/mol.
4.5.3 Aqueous phase
The composition of the aqueous phase was obtained by several analyses. First, a size exclusion
chromatography (previously described) was performed in order to identify if there were high
molecular weight low volatile components that would justify a backflush configuration in the
remaining chromatographic analysis necessary to characterize the aqueous phase. According to the
SEC result, if the samples presented low volatile compounds, some of the following analysis (namely
gaseous chromatography/mass spectrometry, gaseous chromatography with a flame ionization
detector (already described in Section 4.5.1)) would be performed using a backflush configuration.
Besides these analyses, high performance liquid chromatography and inductively coupled plasma
mass spectrometry were also used to analyze the aqueous phase. A brief description of each analysis
is done in the following paragraphs.
Gaseous chromatography/Mass spectrometry (GC/MS)
GC-MS combines the features of gas-chromatography (GC) and mass spectrometry to identify the
different substances in the aqueous phase. The GC occurs normally and, as the molecules are being
eluted from the chromatographic column at different times, the mass spectrometer positioned
35
downstream will ionize the molecules, forming charged molecular ions and fragments that will be
separated according to their mass-to-charge ratio [51].
The advantage of using these two techniques together is that a much accurate identification of the
substances occurs. In fact, there are cases where the GC analysis alone cannot differentiate two
species, as an elution of two species can happen, meaning that they have the same retention time
and their curves in the chromatogram will be overlapped.
High Performance Liquid Chromatography (HPLC)
The operation principle of liquid chromatography is the same as the one of gaseous chromatography,
however, the mobile phase is a liquid, instead of a gas.
HPLC uses very small uniform packing particles, which results in much less band broadening, but
requires high pressure to force the mobile phase through the column. Therefore, a pump is needed
to force the mobile phase at a much higher velocity than normal liquid chromatography [51]. In this
case, a refractive index detector was used, which measures the refractive index of the effluent
passing through the flow cell and compares it with the mobile phase [52].
The main purpose of doing this analysis together with GC is to obtain a quantification of the lighter
compounds that can be dissolved in water, such as alcohols, acids or furfurals, that may not be
clearly quantifiable in a GC. This analysis was performed in the analysis department of IFPEN.
Elemental analysis: Inductively coupled plasma mass spectrometry (ICP-MS)
ICP-MS is a mass spectrometry analysis capable of quantifying inorganic compounds in a sample. The
determination is achieved by ionizing the sample with inductively coupled plasma and then using a mass
spectrometer to separate and quantify those ions [51]. In the present case, alkaline compounds and Ni (that
could come from the catalyst) were searched.
4.5.4 Catalyst
To understand the evolution of the catalyst during the tests, some analysis were done to a small
number of samples of used catalysts. In order to do so, the textural properties of the catalyst (namely
the specific surface area) was analyzed.
36
BET surface area by nitrogen adsorption
This technique is based on the principle that when a solid is cooled down to a constant temperature
in the presence of a gas (nitrogen in this case), part of the gas molecules will adsorb at the surface of
the solid, allowing a characterization of the surface of the solid.
The characterization of the specific surface area is done by the BET method, which assumes that the
adsorption occurs in well-defined sites and each site accepts only one adsorbed molecule. By
measuring the amount of gas that is adsorbed on the surface of the solid at a given temperature, it is
possible to obtain an adsorption isotherm that will allow the determination of the specific surface
area of the catalyst by the BET equation [53].
37
5. Study the influence of operating conditions
5.1 Experimental tests
Each experimental test was compared with a reference test in which the feedstock was composed by
bio-oil and PUB (50/50) and the operating conditions are considered to be in the reference state.
Moreover, the start-up of the unit, when no partially upgraded bio-oil is yet available, was also
simulated in a test using a reference solvent from the alcohols compounds family, instead of PUB.
The conditions of each test are listed in Table 9, where it is also highlighted (in grey) the conditions
that differ from the reference test.
In all the performed tests, the temperature and the initial hydrogen pressure in the reactor were the
same as in the reference test (Tref and pref ,
respectively), which cannot be revealed for confidential
reasons and are only specified in Appendix C. Furthermore, concerning the study of different
operating conditions, three tests were performed – one where the reaction time was doubled,
another where the mass of catalyst was increased by a factor of 2,3 and a compilation of the last
two. Additionally, the activation of the catalyst was also studied in an experiment where the NiMo
catalyst was sulfided with H2S instead of reduced with hydrogen.
Two additional tests were performed to determine if there are diffusional limitations in the reaction.
To conclude whether there were external diffusional limitations from the bulk of liquid to the surface
of the solid catalyst, a test without basked was done, as this can present an obstacle to the flow of
fluid inside the reactor and of its ability to reach the solid particles of catalyst. Internal limitations
were also studied using a catalyst without basket and in its crushed form to verify if there were
difficulties in the access to its inner pores.
Lastly, two tests were performed with two different cuts of distillation of PUB. Figure 21 shows the
different cuts obtained after distillation of a partially upgraded bio-oil. These tests aimed to verify
what was the effect of recycling only an intermediate fraction of PUB, as the heaviest fraction
(Fraction 5, which is a solid fraction that resembles bitumen and is composed by high molecular
weight hydrocarbons) and lighter fractions (Fractions 1 and 2, mainly composed by water and
unreacted solvent) were removed. Thus, the test done with Fraction 3 will be from now on referred
to as test with a recycling of an intermediate light fraction of PUB and, similarly, the test of recycling
of an intermediate heavy fraction of PUB was done with Fraction 4.
38
Table 9 – Operating conditions of experimental tests
Test Temperature Initial
hydrogen pressure
Duration Solvent m load:
B.O./solvent Catalyst m catalyst
With Solvent Tref
pref
timeref
Alcohol 80g (50/50) NiMo reduced mcata
ref
Reference Tref
pref
timeref
PUB 70g (50/50) NiMo reduced mcata
ref
Higher reaction time Tref
pref
2×timeref
PUB 70g (50/50) NiMo reduced mcata
ref
Higher catalyst mass Tref
pref
timeref
PUB 70g (50/50) NiMo reduced 2,3×mcata
ref
Higher catalyst mass + higher reaction time
Tref
pref
2×timeref
PUB 70g (50/50) NiMo reduced 2,3×mcata
ref
Activation of catalyst Tref
pref
timeref
PUB 70g (50/50) NiMo sulfided mcata
ref
External diffusional limitations
Tref
pref
timeref
PUB 70g (50/50) NiMo reduced without basket
mcata
ref
Internal diffusional limitations
Tref
pref
timeref
PUB 70g (50/50) NiMo reduced and
crushed without basket m
cata ref
Recycle of a light fraction of PUB
Tref
pref
timeref
Distillation cut 3
70g (50/50) NiMo reduced mcata
ref
Recycle of a heavy fraction of PUB
Tref
pref
timeref
Distillation cut 4
70g (50/50) NiMo reduced mcata
ref
39
Figure 21 - Image of the distillation cuts of Partially Upgraded Bio-oil
In the next chapter, a description of the production of PUB that was used to perform the tests and to
distillate, originating the different distillation cuts, is given.
5.2 PUB production and distillation fractions
The PUB used in the first experimental tests (Section 5) was produced before the beginning of this
internship, in a autoclave unit similar to the one described in Section 4.1-Unit description, only with a
higher volume of 0.5L. Three sets of tests were carried out in order to produce this PUB, and in each
set of test the catalyst used was always the same. After 4 to 5 successive tests, the reactor was
opened, the catalyst was replaced by a fresh one and a new set of test was started. The feedstock
used in each test was composed by a mixture of equal parts of bio-oil and an effluent that had also
been previously produced in a continuous unit from a feedstock made of bio-oil and solvent at T=Tref-
50°C and WHSVref.
Table 10 presents the operating conditions in each set of tests, the quantity of PUB obtained and its
properties. As before, the confidential parameters are indicated in Appendix C.
40
Table 10 - Production conditions of each set of test and the properties of the PUB produced.
Set of tests 1 Set of tests 2 Set of tests 3
Feedstock B.O. / Effluent B.O. / Effluent B.O. / Effluent
m load (g) 160 160 120
T (°C) Tref Tref Tref
Duration (h) timeref 1.2× timeref 1.3× timeref
Obtained effluent PUB 1 ~ 400g PUB 2 ~ 380g PUB 3 ~ 400g
Eau KF (%H2O) ~ 8% ~ 7% ~ 5 %
CCR (wt.%) 4.9 8.0 5.3
C (wt.%) 65.8 66.1 69.4
H (wt.%) 10.6 10.2 10.1
O (wt.%) 22.5 21.4 19.3
PUB 1 and 2 were considered to have lower quality than PUB 3, as they presented higher content on
oxygen. Therefore, these two PUB were assembled together and used to perform the distillation,
whereas PUB 3 was used as a feedstock in the experimental tests.
The properties of bio-oil and PUB 3 are presented in Table 11, as well as the ones of the feedstock
(bio-oil + PUB).
Table 11 - Properties of the bio-oil, PUB and the different feedstock used in the tests.
KF (% H2O) Density (g/cm3)
Solvent in effluent
Elemental Analysis (wt.%)*
C H O CCR (wt.%)*
Bio-oil 22% 1,234 0% 53,9 6,4 39,8 25,8
PUB 3 4% 0,967 22% 75,8 9,3 14,9 5,3
Feedstock: BO/PUB
10% 1,078 10% 72,7 7,8 19,5 14,5
* A correction was made to the values of CHO and CCR, thus they are presented without the contribution from
the solvent and water that were present in the organic phase. In Appendix A the gross values of CHO and CCR
are presented.
Although the amount of solvent in PUB 3 has been drastically reduced, it should be noticed that this
PUB does not represent rigorously the actual PUB obtained in a continuous operation in steady state,
but it is considered to be sufficiently similar to draw conclusions on the impact of recycling in the
process.
A preparative distillation was done to the mixture PUB 1 + PUB 2 in the analysis department of IFPEN
and the distillation cuts were divided in order to obtain the following sequence of fractions: two
light, light intermediate, heavy intermediate and heavy. For confidentiality reasons, the exact boiling
41
point of each cut is only presented in Appendix C, as it cannot be publicly revealed and, therefore,
the different distillation cuts will be referred to as distillation fraction 1, 2, 3, 4 and 5, according to
what is represented in Figure 21.
The first boiling point that comprises the light fraction was chosen so that all the water and solvent
present in the mixture of PUB were removed. For technical reasons imposed by the distillation
process, this first cut had to be divided into two fractions (Fraction 1 and 2) which together form part
of the light fraction.
The intermediate cuts were obtained in order to have a similar distribution of boiling points, whereas
the last one was imposed by technical reasons, as the heavier compounds started to be cracked
under these conditions.
The properties of each distillation fraction, namely the mass of each cut, its water content, density,
percentage of solvent that is present in each cut, the elemental analysis and the CCR value are
presented in Table 12.
Table 12 - Properties of the distillation cuts
Mass
obtained (g)
KF (% H2O)
Density (g/cm3)
Solvent in effluent (wt.%)
Elemental Analysis (wt.%)*
C H O
CCR
(wt.%)*
Distillation fraction 1
296 10,1% 0,853 62 76,0 9,3 14,8 N.D.
Distillation fraction 2
17 10,5% 0,882 43 53,9 6,4 39,8 N.D.
Distillation fraction 3
65 1,3% 0,938 5 64,1 11,6 24,3 N.D.
Distillation fraction 4
51 0,6% 1,108 0 66,4 10,4 23,3 N.D.
Distillation fraction 5
189 0% N.D. 0 70,7 10,1 19,2 44,8
* values presented on dry basis and without solvent and the gross values are presented in Appendix A.
5.3 Results
The objective of these tests is to study the impact that changes in the different operational
parameters (presented in Table 9) have on the quality of the effluent. The quality of the organic
phase (PUB) is determined by the conjunction of several parameters, namely its content on oxygen,
water solubility, and the Carbon Conradson value.
As mentioned in Section 2.2.2, the main objective of such treatment is to decrease the oxygen
content in the bio-oil, in order to make it more stable and miscible with hydrocarbons. Thus, the
lower the oxygen content on the PUB, the better the quality of the effluent. Besides this, low values
of CCR are also sought in order to have an effluent with less heavy residues that can potentially
42
impact negatively the second hydrotreatment stage (conf. Figure 12) and devalue the final
hydrocarbon product.
Besides the catalytic reactions that are responsible for the hydrotreatment of the bio-oil, thermic
reactions can also occur, which lead to an overproduction of CO2 and formation of coke and solids
that can deactivate the catalyst. In this way, this study also aims to study the operating conditions
that decrease the occurrence of thermic reactions and enhance the catalytic HDT.
Additionally, in order to avoid carbon loses, it is also seek that some of the removal of oxygen is done
in the form of water, instead of being release with gases by the form of CO or CO2. However, total
removal of the oxygen via the production of water leads to a higher consumption of hydrogen, when
compared with reactions of decarboxylation. On this subject, previews tests [42] were done in the
presence of nitrogen, instead of hydrogen, that indicated that the presence of water can enhance
steam reforming reactions, leading to in situ production of hydrogen. Since the economic feasibility
of the process is also a very important subject to address, an economic compromise between
hydrodeoxygenation with formation of water, which can increase the consumption of hydrogen but
can also enhance the in situ production of hydrogen, and decarboxylation with formation of CO and
CO2 must be achieved.
Operating conditions
Table 13 shows the results that were obtained in the tests done with the objective of studying the
impact of different operating conditions in the quality of the effluent (PUB). Here it is presented the
consumption of hydrogen in grams per gram of bio-oil introduced in the reactor, the ratio of phases
obtained in the end of each test, the content of water in the organic phase, the density of the organic
phase, the percentage of solvent that remained in the organic phase, the values of the elemental
analysis (on dry basis and without the contribution of the solvent), the values of CCR in the organic
phase, the overall hydrodeoxygenation that takes into account the amount of oxygen that was
initially in the feedstock and in the organic phase after reaction (the oxygen contained in the
compounds dissolved in the aqueous phase was not accounted in these calculations – see Appendix
B) and, finally, the Carbon yield that indicates the amount of carbon that remained in the organic
phase (calculations in Appendix B ). In Table 14 it is shown the values of the specific surface of the
catalyst after some tests and in Figure 22 it is also presented the mass of gases produced in each test.
43
Table 13 - Results obtained for different operating conditions
Test Cons. H2/ g
(B.O.) Ratio of phases
(org/aq/gas) KF
(%H2O) Density (g/cm3)
Solvent in effluent (wt.%)
Elemental Analysis* C(%) H(%) O(%) S(ppm)
CCR (wt.%)*
HDO global*
Carbon yield*
With solvent 1,55 (70/14/16) 8% 0,902 51% 75,2 8,7 16,1 N.D. 8,0 88% 53%
Reference 1,54 (71/22/7) 3,4% 1,001 13% 78,2 8,6 13,1 10 9,1 54% 59%
Higher reaction time
1,99 (55/31/14) 3,1% 1,014 7% 77,7 8,9 13,3 N.D. 9,8 58% 52%
Higher catalyst mass
2,72 (74/14/12) 2,7% 0,987 7% 78,9 9,5 11,6 N.D. 7,9 50% 72%
Higher catalyst mass +
higher reaction time
2,31 (59/27/14) 2,7% 0,993 4% 79,6 9,5 10,9 N.D. 9,3 61% 61%
Activation of catalyst
2,72 (45/30/25) 4% 0,996 1% 81,3 9,6 9,1 300 7,2 72% 52%
* values presented on dry basis and without solvent and the gross values are presented in Appendix A.
Table 14 - Specific surface of the catalyst after some tests in comparison with the fresh reduced catalyst
Catalyst Specific surface area (m2/g)
Catalyst reduced before test 132
Reference test 115
Higher reaction time 100
Higher catalyst mass + higher reaction time 99
44
Figure 22 - Gases produced during the tests with different operating conditions
In a primary phase, the reference test (bio-oil + PUB) was compared with the test performed with
solvent. It can be seen that there is a clear increase in the formation of gases in the test with solvent,
which may be due to its conversion into smaller molecules.
In the test with higher reaction time there is an increment in the consumption of hydrogen and in the
hydrodeoxygenation rate, however, there is not a clear improvement in the content of oxygen in the
organic phase and the CCR value is higher when compared with the reference test. In addition, more
gases (mainly CO2) are produced when the time of reaction is doubled, which can be due to the
occurrence of either more thermic reactions, more catalytic reactions of decarboxylation, or both.
Also, the specific surface area of the catalyst is lower, when compared with the reference test (Table
14), which may be due to higher coke deposition. Furthermore, we can see that when the reaction
time increases there is a clear increase in the amount of carbon loss (mainly in the form of CO2 – see
Figure 22), since the carbon yield is lower than in the reference test. All these facts show that an
increase in the reaction time causes an increase in the overall HDO, mainly due to an increase in
decarboxylation reactions, but may produce an effluent with lower quality, since its oxygen content
and CCR are higher than in the reference conditions.
Contrarily, when the mass of catalyst is increased, an increase in the hydrogen consumption and a
decrease in the oxygen content of the organic phase is shown, together with a slight decrease in the
CCR value. Additionally, although more gases C1-C4 are produced, less CO2 and less global carbon
losses are verified, since the carbon yield is higher than in the reference test. Despite the tendency
observed for the oxygen content and the hydrogen consumption, the HDO does not increase with
45
the mass of catalyst. The un-estimated quantity of organic compounds dissolved in the aqueous
phase induces an error in the HDO calculation and could explain this difference.
However, when the reaction time and mass of catalyst are increased, a decrease in the consumption
of hydrogen (when compared with the test with higher mass of catalyst) is obtained. This result is
unexpected and could be explained by a technical problem while measuring the hydrogen or in the
reduction of the catalyst, which would cause its deactivation and favor the occurrence of thermic
reactions, rather than catalytic. However, the specific surface area of the catalyst is almost the same
as in the test with higher reaction time (Table 14), which may indicate that the reduction of the
catalyst occurred as expected, since no significant higher coke deposition was verified. Additionally,
the results on the properties of the organic phase are similar to the test with higher catalyst mass,
besides the CCR value, which is higher, confirming once more that higher reaction times may
increase the production of heavier compounds. However, the overall HDO is higher than in all the
previous tests and it can also be seen that probably more hydrodeoxygenation with the production
of water occurred, since the ratio of water phase is higher, less CO2 was produced and a higher
carbon yield was obtained, when compared with the test of higher reaction time.
The last test of this set was done with a different activation of the catalyst. As mentioned before,
instead of being reduced with hydrogen, the catalyst was sulfided. This test was performed to study
the activation of the catalyst that provides better results in upgrading bio-oils, since, from an
economic point of view, the ideal is to treat bio-oils using the same commercial formulation of the
catalyst (with possible improvements in chemical and mechanical resistance) used in the
hydrotreatment of petroleum cuts in refineries. The results show that this test had the highest rate
of HDO, thus, the catalyst seems to be more active when sulfided. The same result was obtained by
Parapati, D. [54] when studying reduced and sulfided CoMo catalysts on hydroprocessing of
preteated bio-oil.
Furthermore, the content on oxygen and the CCR value are the lowest of this set of tests, making it
the best quality of effluent. However, a considerable increase in the amount of gases produced was
shown, possible due to the higher conversion of the solvent (in the end of the test there was only 1%
of solvent in the organic phase) and to higher production of CO2 that led to a low carbon yield. Also,
it occurred contamination of the bio-oil, as the amount of sulphur increased from 10 to 300 ppm.
This may present a problem, depending on the nature of sulfur compound, as the regulations on the
sulphur content of fuels are more and more strict and one of the advantages of using bio fuels is
precisely the fact that it does not contain almost any sulphur and, thus, there is no need to
desulphurize it. This point should be confirmed in the future, if sulfided catalyst are chosen for HDT.
46
Diffusional Limitations
As in the last case, the results of the analyses to the organic phase (Table 15)and the gases produced
during in the different reactions (Figure 23) are presented.
Figure 23 - Gases produced during the tests of diffusional limitations
The results shown in Table 15 support the conclusion that diffusional limitations are present in the
reaction in the reference conditions. Looking at the values of hydrogen consumption it can be seen
that in both tests (without basket and with crushed catalyst) there is a significant increase, when
compared with the reference test. This means that when the catalyst flows freely inside the reactor,
it is easier for hydrogen and other reactants to access it and, thus, the consumption of hydrogen
increases. However, at the end of the test without basket, the catalyst’s shape was eventually all
destroyed, which means that external limitations could not be individually studied, as changes in the
particle size will affect the final result. Thereby, in the reference conditions it is possible that there
are difficulties in the access of the liquid to the catalyst (external diffusional limitations), but there
are clear difficulties in mass transference inside the pores of the catalyst (internal diffusional
limitations).
Comparing both tests there is a significant difference in the oxygen content and, consequently, in the
global HDO values. This difference could be explained by a factor that could influence the activity of
the catalyst during the test. A priori, the destruction of the catalyst’s shape should not affect its
activity, but since no more evidences were found to justify this result, more analyses to the catalyst
should be done to try to explain this. When deactivation of the catalyst occurs, it is expected that less
catalytic reactions (less formation of CO2 water) and, consequently, more thermic reaction occur.
Thermic reactions lead to the formation of solids that could be derived from the
polymerization/polycondensation reactions of heavier compounds, therefore, the presence of heavy
compounds in the organic phase is reduced and the CCR value is lower.
47
Table 15 - Results obtained in tests of diffusional limitations
Test Cons. H2/ g (B.O.)
Ratio of phases (org/aq/gas)
KF (%H2O)
Density (g/cm3)
Solvent in effluent
(%)
Elemental Analysis (wt.%)*
C H O
CCR (wt.%)*
HDO global*
Carbon yield*
Reference 1,54 (71/22/7) 3% 1,001 13% 78,2 8,6 13,1 9,1 54% 59%
External diffusional limitations
2,22 (82/9/9) 6% 1,002 9% 78,4 9,2 12,4 3,2 43% 78%
Internal diffusional limitations
2,64 (40/34/26) 15% 1,007 6% 85,8 9,1 5,1 7,2 87% 45%
* values presented on dry basis and without solvent and the gross values are presented in Appendix A.
48
From the much lower carbon yield results in the test without basket and crushed catalyst, it can be
concluded that the additional deoxygenation was probably due to more decarboxylation reactions
and, therefore, an organic phase with the lowest content on oxygen (6 wt.%) was obtained in this
test. This also means that diffusional limitations are significant in these reactions.
Nevertheless, it should be also taken into account that the post treatment of this set of tests was
considerably more difficult than in the tests with baskets. The separation of the solids from the
effluent was difficult due to the tarry paste that was formed and, additionally, the density of the
obtained organic phase was very similar to water (conf.
Table 15), making it difficult to separate both liquid phases. This can, obviously, present a big
challenge in an industrial scale process.
Effect of feedstock
As mentioned before, the study of the effect of feedstock was done by recycling only a distillation
fraction of PUB instead of recycling the whole product. The tests were done with a light fraction
(Fraction 3) and heavy fraction (Fraction 4) of PUB. The results are shown in Table 16, followed by the
composition of the gases produced in the reaction, Figure 24.
Figure 24 - Gases produced during the tests done with destillation fractions of PUB
When only a fraction of PUB is recycled, the heavier compounds (Fraction 5) as well as most of water
and solvent (Fraction 1 and 2 from PUB) have already been removed from the mixture. Thus, the
content on heavier compounds in the effluent decreases and so does the CCR value.
0,0
0,5
1,0
1,5
2,0
2,5
Reference Recycle of a lightfraction of PUB
Recycle of a heavyfraction of PUB
Mas
s o
f ga
s (g
)
CO2 CH4 C2-C3 C4 CO
49
Table 16 - Results obtained in the study of the effect of feedstock
Test Cons. H2/ g
(B.O.) Ratio of phases
(org/aq/gas) KF (%H2O)
Density (g/cm3)
Solvent in effluent
(%)
Elemental Analysis (wt.%)*
C H O
CCR (wt.%)*
HDO global*
Carbon yield*
Reference 1,54 (71/22/7) 3,4% 1,001 13% 78,2 8,6 13,1 9,1 54% 59%
Recycle of a light fraction of PUB
1,99 (64/28/8) 4,0% 0,978 8% 75,6 9,2 15,2 5,4 72% 66%
Recycle of a heavy fraction of PUB
2,80 (75/22/3) 3,6% 1,020 2% 78,2 9,2 12,6 6,0 66% 91%
values presented on dry basis and without solvent and the gross values are presented in Appendix A.
50
In both tests it is verified a decrease in the production of CO2, which indicates that less
decarboxylation occurred. This also justifies the values of the carbon yield, which are higher than the
reference test for both tests. However, in the test where a light fraction was recycled it is verified a
very important increase in the production of CO, possibly due to the thermal decomposition of butyl
esters, which are heavily present in distillation fraction 3.
In this set of tests it is also verified that the concentration of solvent in both effluents is higher than
in distillation cuts 3 and 4 (see Table 12). Since the solvent should be converted during the reaction,
this observation is unexpected. After doing a mass balance to the solvent in the feed and in the PUB,
it was found that more solvent was present in the effluent than it was introduced in the reactor,
primarily in test where a light fraction was recycled. One possible explanation for this is the error
associated with the analysis that could cause some incertitude on the results, especially because
measured concentrations are very low. However, since this tendency is found in both tests, it is
possible that it occurred production of solvent during the tests by hydrolysis of butyl esters. In fact,
the GC analysis shows considerable concentrations of butyl esters in distillations cuts (mainly in
distillation fraction 3), which could suffer hydrolysis to form an alcohol (solvent). Due to lack of time
and information on the composition of the aqueous phase, it was not possible to perform a complete
mass balance to the process to determine if this hypothesis is valid, however, this should be done in
the future.
Looking at the values of hydrogen consumption, it can be seen that when recycling the heavy
fraction, more hydrogen consumption and less HDO occurs. This can be explained by the
hydrogenation of heavier compounds, such as aromatic compounds, that increases the consumption
of hydrogen, but does not influence the HDO.
5.4 Composition of the aqueous phase
To have an estimation of the composition of the aqueous phase produced in the hydrotreatment
step, three samples of aqueous phases of this set of tests were analysed – test with higher mass of
catalyst and reaction time, test with light distillation fraction and, finally, the test with heavy
distillation fraction. The determination of the quantity of major species present in these samples will
be of utmost importance, as it will provide clues on the possible treatment to the aqueous phase that
can be done in the process.
As already described in Section Analysis of the aqueous phase, an initial SEC analysis was done in
order to determine whether humic substances (from the degradation of sugars that are present in
the bio-oil) or other soluble substances of high molecular weight that could potentially distort the GC
51
analysis, were present in aqueous phase. The results of the SEC analysis are presented in Figure 25,
Figure 26 and Figure 27, with detection by refractive index (RI) or UV (at two different wavelengths).
Figure 25 - SEC analysis: normalized RI signal on the signal maximum of 75 to 1000g/mol
Figure 26 - SEC analysis: normalized UV 254nm signal on the signal maximum of 20 to 1000g/mol
Figure 27 - SEC analysis: normalized UV 280nm signal on the signal maximum of 20 to 1000g/mol
52
As it can be seen, all three different aqueous phases present a profile indicating that, a priori, the
majority of species have molecular weights below 350 g/mol. Therefore, no significant
concentrations of high molecular weight compounds, that could harm the functioning of the GC
equipment, are present in the aqueous phase and the following analysis can be carried.
Once the presence of high molecular weight species was determined by the SEC analysis, a GC/MS
analysis was done to provide qualitative information on the families of compounds present in the
aqueous phase. This information will also allow taking some conclusions on the major compounds
present in the aqueous phase, in addition with the results of the quantification of species provided
by the GC-FID and HPLC analysis. Table 17 presents the main families of compounds found in all the
analyzed aqueous phases, while Table 18 presents the composition of the aqueous determined by
GC-FID.
Table 17 - Substances present in the aqueous phase, determined by the GC/MS analysis
Family of compounds
Found substances
Water -
Alcohols Methanol, ethanol, propanol, butanol
Phenols Phenol, cresol
Ketones Acetone, butanone, pentanone, cyclopentanone, propyl-furanone
Acids Acetic, propanoic, butanoic, pentanoic
53
Table 18 - Composition of the aqueous phase determined by GC-FID
Compound (wt.%) Test higher mass of catalyst and reaction time
Test with light distillation
fraction
Test with heavy distillation
fraction
Methanol 1,67 2,94 3,90
Ethanol 0,37 0,82 0,66
Acetone 0,36 0,25 0,26
Isopropanol 0,03 0,04 0,04
1-Propanol 0,14 0,18 0,11
2-Butanone 0,05 0,06 0,07
Isobutanol 0,02 0,02 0,01
Acetic Acid 10,03 7,57 10,91
1-Butanol 0,78 1,00 0,52
Propanoic Acid 1,53 2,23 1,25
Dioxane
2,32
Butanoic Acid 0,82 1,63 0,44
Butyrolactone 0,14 1,26 0,49
Pentanoic Acid 0,07 0,13 0,06
Phenol 0,06 0,06
Cresol 0,01 0,03 0,03
Together with the GC analysis, a HPLC was also performed to obtain a quantification of the lighter
compounds that could be dissolved in water and could be difficult to quantify by GC-FID due to the
possible elution of peaks. The results of the HPLC analysis are present in Table 19.
Table 19 - HPLC analysis results.
Compound (g/L)
Test higher mass of catalyst and
reaction time
Test with light distillation
fraction
Test with heavy distillation
fraction
Acetic acid 74,6 52,0 55,0
Formic acid - 0,06 0,04
All the other tested substances in HPLC, namely lactic acid, succinic acid, pyruvic acid, levulinic acid,
furfural and 5-hydroxymethylfurfural, are not present in the aqueous phase.
54
As it can be seen from Table 18 and Table 19 there is a difference in the values of the concentration
of acetic acid. This difference is justified by the possible elution of compounds in the GC-FID analysis
and the difficult integration of large peaks, introducing an error in the GC-FID analysis of this
compound.
Finally, an ICP analysis provided an estimation of the inorganics present in this phase (Table 20).
Table 20 - Results of the ICP analysis
Inorganic Compound (mg/L)
Test higher mass of catalyst and
reaction time
Test with light distillation
fraction
Test with heavy distillation
fraction
Na 311 168 168
K 6,87 5,11 8,18
Mg 4,17 5,79 4,46
Ca 50,2 46,8 32,8
Fe 446 258 75,8
Ni 255 199 93,9
Si 5,5 7,43 11,7
This analysis shows that the inorganics predominantly present in the organic phase are iron, sodium,
calcium and nickel. To try to understand the origin of these inorganic compounds, it is also presented
in Figure 28 the results of an ICP analysis done to the bio-oil before reaction.
Figure 28 - Composition of inorganic compounds in the unreacted bio-oil
55
Additionally, a significant increase in the nickel concentration (that is not observed in the bio-oil)
occurs. This compound may come from the catalyst (NiMo on a nickel aluminate support) or from the
reactor, which is composed by a Inconel alloy containing Ni, due to corrosion (presence of acid).
As it can be seen, the aqueous phases seem to concentrate a certain number of inorganic
compounds such as sodium, iron and calcium. However, silicon, heavily present in the bio-oil, seems
to be present in much less concentration in the aqueous phase. Possibly, this compound remains
predominantly in the organic phase, however, more analysis to the organic phase should be done to
confirm this theory.
To conclude, alcohols and carboxylic acids are the major compounds dissolved in the aqueous phase
formed during the hydrotreatment of bio-oil when a recycling of PUB is used. A more complete study
should be done in order to access the possibility of recovering the major species, typically acetic acid
and methanol, to value the process. Another subject that should be further investigated is the
treatment of this phase, as the presence of heavier substances (e.g. phenols), may be harmful.
56
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57
6. Study the evolution of the PUB quality after continuous recycling
6.1 Experimental tests
These tests aim to study the evolution of the quality of the effluent, after a serious of batch tests,
using always the same catalyst. The idea is to simulate a continuous operation with a batch reactor,
by removing the effluent under inert atmosphere and without opening the reactor, and
reintroducing all the collected organic phase with unreacted bio-oil. In this way, a simulation of a
continuous recycling of the organic phase is performed, in order to study the evolution of the PUB
after several experiments and the possible deactivation of the catalyst.
The catalyst used in these tests is the same as in Chapter 5, NiMo with a support of nickel aluminate,
reduced with hydrogen. After a set of tests, when the reactor was opened and cleaned, the catalyst
suffered the same post treatment as in Chapter 5. The number of tests in each set of tests was not
predefined and should be set by the development of the experimental tests.
Two different sets of tests were performed. The first one was done in the same experimental unit
described in Chapter 4.1 (Figure 13), with the difference that only in the last test the reactor was
opened and the catalyst was washed and analyzed. For technical reasons, the second set of tests had
to be performed in a similar unit, but with a total volume of 0,5L.
First Set of Tests
The first set of tests was performed to understand what was the influence of varying the time of
reaction in order to keep constant the WHSVref used in the reference test in Chapter 5, with respect
to the feedstock (bio-oil + PUB). For this, it was assumed that 40 wt.% of solvent was present in the
PUB 1 and that a conversion of 20% of solvent was achieved in each test. Table 21 presents the
operating conditions of this set of tests. As previously, the confidential conditions are designated in
Appendix C.
Table 21 - Operating conditions of the first set of tests
Test Temperature Initial
hydrogen pressure
Duration Solvent m load:
B.O./solvent m catalyst
Produced effluent
1 Tref
pref
0.4×timeref
Alcohol 70g (50/50) 2.3×mcata
ref
PUB 1
2 Tref
pref
0.7×timeref
PUB 1 70g (50/50) 2.3×mcata
ref
PUB 2
3 Tref
pref
0.8×timeref
PUB 2 70g (50/50) 2.3×mcata
ref
PUB 3
58
As it can be seen in Table 21, only three tests were performed in the first set, due to the fact that
after the third test, a significant degradation of the effluent was observed, as the content on solids in
the organic increased substantially and almost no water phase was present. Therefore, it was
decided to stop the set of tests, open the reactor and proceed with the post treatment of all the
phases. To try to understand if significant deactivation of the catalyst was observed, a BET analysis
was performed.
Second Set of Tests
Contrarily to the first set of tests, the second set intended to study the evolution of the quality of the
PUB but holding constant the WHSVref with respect to the bio-oil only, which led to a constant time of
reaction (since the mass of the catalyst in the reactor is always the same). The operating conditions
are presented in Table 22 and the confidential information Appendix C.
Table 22 - Operating conditions of the second set of tests
Test Temperature Initial
hydrogen pressure
Duration Solvent m load:
B.O./solvent m catalyst
Produced effluent
1 Tref
pref
0.75×timeref
Alcohol 120g (50/50) 2.3×mcata
ref
PUB 1
2 Tref
pref
0.75×timeref
PUB 1 120g (50/50) 2.3×mcata
ref
PUB 2
3 Tref
pref
0.75×timeref
PUB 2 120g (50/50) 2.3×mcata
ref
PUB 3
4 Tref
pref
0.75×timeref
PUB 3 120g (50/50) 2.3×mcata
ref
PUB 4
5 Tref
pref
0.75×timeref
PUB 4 120g (50/50) 2.3×mcata
ref
PUB 5
In this case, the set was finished after five tests due to technical and logistical reasons. First, as it will
be discussed in the results section, the viscosity of the effluent increased significantly in the course of
this set of tests, making it very difficult to remove and introduce the effluent and load the reactor.
Secondly, due to the limited time available and the time required to perform the analysis, it was not
possible to continue the experimental work.
59
6.2 Results
First Set of Tests
Table 23 - Results of the first set of tests
Test
Cons. H2/
g (B.O.)
Ratio of phases
(org/aq/gas)
Mass of solids
(g)
KF (%H2O)
Density (g/cm3)
Solvent in effluent (wt.%)
Elemental Analysis (wt.%)*
C H O
CCR (wt.%)*
1 2,07 (64/17/18) 0,06 9 ND 52% 74,9 9,2 15,9 -
2 2,12 (60/23/16) 0,23 6 ND 27% 76,7 9,1 14,3 -
3 1,91 (69/7/22) 1,84 3 0,982 15% 77,1 9,0 13,9 6,7
* values presented on dry basis and without solvent and the gross values are presented in Appendix A.
Figure 29 - Gases produced during the first set of tests
Figure 30 - Results of the SEC analysis (normalized UV 280nm signal on the signal maximum of 10 to 5030g/mol)
60
Table 24 - Specific surface of the catalyst after the first set of tests in comparison with the fresh reduced catalyst and the catalyst after the reference test.
Catalyst Specific surface area (m2/g)
Catalyst reduced before test 132
Reference test 115
Catalyst after the first set of tests 117
As mentioned before, this set of tests was interrupted after 3 tests due to the significant degradation
of the effluent. After the third test was finished, a notable increase in the difficulty of removing the
effluent from the reactor was regarded, due to the much higher content of solids in the effluent.
Also, the water phase was substantially reduced, when compared with tests 1 and 2 (see Table 23).
Therefore, it was decided to stop the test and start a new one.
Since the amount of organic phase left to perform analysis was very little, as most of the organic
phase was used as a feedstock in the next test, it was not possible to perform density and CCR
analysis to the organic phase of Test 1 and 2. Instead, a SEC analysis (described in Section Analysis of
the Organic phase) was done to have an estimation of the qualitative distribution of the molecular
weight of the compounds in the mixture (Figure 30). From the results shown in Figure 30 it can be
seen that the steady state of the recycling process was probably not achieved in this set of tests, as
the distribution of the compounds of the three produced organic phases varies significantly. This can
also be seen in Figure 29, where it is shown that the composition of the gases is varying along the
different tests, and in Table 23, where changes in the water content in the organic phase indicate
that the composition of the mixture may be changing over the tests.
However, since the organic phase of the third test was not used in another test, it was possible to
perform a CCR analysis to that effluent. This result can be used to make a comparison with values
from other tests, which may provide additional clues to explain why a significant decrease in the
quality of the effluent was regarded. Additionally, by comparing this value with the CCR value of the
feedstock, it can be seen that the CCR of the organic phase of the third test is lower. This result is
unforeseen, since it is expected that, as the number of recycles increases, the CCR value also
increases due to more recycling of heavy fraction (see results of the second set of recycling tests).
One possible explanation is the increase of thermic reactions that, as mentioned before, lead to the
formation of solids that could be derived from the polymerization/polycondensation reactions of
heavier compounds. The increase of thermic reaction may be explained by the possible deactivation
of the catalyst. However, the value of the specific surface area of the catalyst after the test is higher
61
than the catalyst after the reference test, which suggests that no addition coke deposition occurred
(Table 24). A progressive leaching of Ni from the catalyst may also be possible. Further analysis are
needed on the catalyst to better understand what occurred during these tests.
Second Set of Tests
Table 25 - Results of the second set of tests
Test Cons. H2/ g (B.O.)
Ratio of phases (org/aq/gas)
Mass of solids (g)
KF (%H2O)
Solvent in effluent (wt.%)
Elemental Analysis (wt.%)*
C H O
CCR (wt.%)*
1 2,35 (93/0/6) 0,01 7% 44% 71,7 9,5 18,8 3,3
2 2,12 (51/43/4) 0,05 5% 33% 75,3 8,2 16,5 7,4
3 1,91 (55/38/6) 0,06 4% 12% 76,3 8,6 15,0 9,1
4 1,70 (55/38/5) 0,08 4% 5% 76,0 8,5 15,5 11,9
5 1,49 (55/38/6) 0,19 4% 1% 76,1 8,6 15,4 11,7
* values presented on dry basis and without solvent and the gross values are presented in Appendix A.
Figure 31 - Gases produced during the second set of tests
This set of tests was performed in a reactor with a higher volume and the operating conditions
included a WHSVref relative to the bio-oil constant and equal to the reference test from Chapter 5.
During the experimental work, a significant increase in the viscosity of the effluent was observed, but
no significant experimental problems arose from that.
Additionally, it should be noted that the value of the ratio of phases indicated for Test 1 might not
rigorously represent the composition of that effluent, since it was found that the agitation was
turned-off while the removal of that effluent occurred. This means that decantation of water in the
bottom of the reactor could have occurred and, since the tube responsible from removing the
effluent from the reactor was not placed until the bottom in this particular unit, some water may
62
have remained in the reactor. Still, this fact is not likely to influence the final results of the PUB, since
the water was removed in the following tests.
Looking at the results, namely the values presented in Table 25 and Figure 31, it can be seen that
from Test 3 the values become practically constant, with the exception of CCR value that only
becomes constant from Test 4 and the H2 consumption that continuously decreases. In addition, the
mass of solids filtrated from the effluent increases significantly in Test 5.
The CHO and CCR tendency seem to indicate that probably the steady-state was achieved after 3 to 4
recycles of PUB, but the decrease of hydrogen consumption from test 4 and the increase in the mass
of solids filtrated in the test 5 suggest a deactivation of the catalyst and an imminent degradation of
the quality of the effluent. The decrease of the H2 consumption from Test 1 to 4 could be also due to
the effect of recycling heavier fraction in the PUB, probably less reactive than bio-oil and the solvent.
The obtained final PUB presents an oxygen content of about 15 wt.%, 4 wt.% of water and 12% of
carbon residue (CCR). The oxygen and water content are in accordance with the target mentioned
above for the hydroreforming step, i.e. %O between 5 and 15 wt.% and water solubility lower than
5 wt.%, although a lower oxygen content would have been preferred. However, the high value of CCR
and the apparent high viscosity of the final PUB may cause technical problems for the second
hydrotreatment step with risks of reactor plugging (in case of fixed bed reactor) and deactivation of
catalyst. Nonetheless, the effluent proved to be soluble with hydrocarbons in a test done with
toluene, although it occurred the deposition of some solids, which may be due to the heavy CCR
value of this effluent or to the larger size of pores of the filter that was used in this test.
63
7. Conclusions
The objective of this work was to study the effluent recycling in the first hydrotreatment step to
upgrade bio-oils. Typically, bio-oils are composed by a mixture of highly reactive oxygenated species,
which make it unstable and, consequently, less suitable to be used as a fuel. Therefore, an upgrading
treatment is necessary before the bio-oil can become a suitable fuel for its end-use applications. For
this, a two-stage process is purposed, being the first a hydroreforming step that aims to improve the
quality of the bio-oil, so that the second hydrotreatment/hydrocracking step can be performed under
more severe conditions to produce a high quality fuel. Typically this upgrading process is performed
at high pressure and temperature, using a solvent and a catalyst to enhance hydrodeoxygenation
reactions in order to reduce the oxygen content in the bio-oils. The hydrodeoxygenation (HDO) can
occur by releasing the oxygen in the form of CO2 or CO (through decarboxylation reactions), or in the
form of water.
This study addresses the first hydroreforming step for upgrading bio-oils using the effluent recycling
as a stabilizing agent, instead of a solvent. The tests were done in a batch unit, in the presence of
hydrogen and of a NiMo catalyst with a nickel aluminate support, using as a feedstock a mixture of
bio-oil and a previously produced partially upgraded bio-oil (PUB). Different operating conditions
were tested and compared with a test in the reference conditions. The main objective was to
produce a stabilized PUB that presented an oxygen content of 5-15 wt.%, a water solubility of less
than 5 wt.% and a low carbon residue.
The results showed that in the reference conditions it is possible to produce a PUB within the set
limits, nonetheless a lower carbon residue and oxygen content would be preferred. For the different
studied operating conditions, it can be concluded that higher reaction times can lead to a higher
global HDO, but also to a lower carbon yield, probably due to an increase in decarboxylation
reactions. Also, the effluent presented higher oxygen content and carbon residue, when compared to
the reference test, which can be an indication that higher reaction times may not lead to a better
quality effluent. However, in the test with higher mass of catalyst it was seen an increase in carbon
yield of the PUB, producing also an effluent with better quality. These results seem to indicate that in
a continuous operation it would be desirable to maximize the volume of bio-oil in contact with
catalyst so that catalytic reactions are preferred to thermic reactions.
The activation of the catalyst was also a studied parameter in a test done with a sulfided catalyst,
instead of a reduced one. The results show that the catalyst presents higher activity when sulfided,
64
producing an effluent with the best quality (lowest oxygen content and carbon residue) but with a
lower carbon yield. Thus, a parametric study of operating conditions with a sulfided catalyst may be
necessary to see if it is possible to improve the carbon yield without compromising the quality of the
effluent. However, technical problems may arise when using a sulfided catalyst due to higher
pressure increases, especially when a solvent is used in the start-up of a continuous unit.
Furthermore, in this test, contamination of PUB with sulphur occurred, which may present a
hindrance to the use of sulfided catalysts, depending on the nature of the produced sulphur
compound, as one of the advantages of using bio-oils is, precisely, the fact that it contains only
residual amounts of sulphur and does not need to undergo through desulphurization. Thus,
application of sulfided catalysts in continuous operation units is still dependent on the chosen
solvent and the nature of the sulphured components present in the effluent.
The occurrence of diffusional limitations in the reference conditions was studied in a test with a free-
flowing catalyst, and another with a free-flowing, crushed catalyst. The results show that internal
limitations are present during the reaction in the reference conditions, since higher consumption of
hydrogen occurred and the PUB presented lower values of oxygen content and carbon residue. Also,
there are possible external diffusional limitations that should be further studied in a test with
different stirring speed. These mass transfer limitations had not been observed until today in the
tests performed with bio-oil and solvent. Thus, more tests should be done in order to determine if
recycling the whole PUB (including its heaviest fraction) has an impact in mass transfer, or if the
problem could be overcome by recycling only lighter fractions. Furthermore, in these tests the post-
treatment of the effluent was considerably more difficult than in the tests where the catalyst was
placed in a basket, which can be a crucial point in an industrial scale unit. This study also suggests
that an optimization of the structural properties of the catalyst and an adapted reactor technology
would be necessary to maximize the performance of the process by limiting the mass transfer
limitation.
Two additional tests were done with a different feedstock. Instead of using PUB, two distillation
fractions of PUB were used and mixed with unreacted bio-oil to make the feedstock. As expected, the
carbon residue of these two effluents was lower, since the heaviest fraction of PUB had been
removed in the distillation.
After the study of operating conditions, diffusional limitations and effect of feedstock, two more set
of tests were done to simulate a continuous operation recycle. For this, the start-up of the unit was
tested with a feedstock composed of bio-oil and solvent and the produced PUB was mixed with
unreacted bio-oil to form the feedstock of the next test. This procedure was successively repeated,
65
using always the same catalyst inside the reactor. The main objective of this set of tests was to study
the start-up period of a continuous unit, estimate PUB’s evolution after a series of continuous
recycling and predict the number of cycles necessary to reach the steady state.
In the first set of tests, the reaction time was increased from test to test, in order to maintain
constant the WHSV relative to the bio-oil and PUB. However, after three cycles of recycle a significant
degradation of the effluent was observed and the test had to be stopped. The results of this first set
of tests show a significant increase in the production of solids in the last test and that the steady-
state was not achieved.
In the second set of tests it was chosen to keep constant the time of reaction from test to test, so
that the WHSV relative to the bio-oil was constant. No significant degradation of the effluent was
regarded, however, the viscosity of the effluent clearly increased along the experiment. The results
show that, in the reference conditions, the steady state may be achieved after 3 to 4 cycles of
recycling, but a continuous decrease of hydrogen consumption and increase of solids formed suggest
a possible deactivation of the catalyst, which would impact the quality of the effluent by continuing
the recycling process. The produced effluent presents an oxygen content of about 15 wt.% and 4wt.%
of water solubility. However, this effluent presented a higher carbon residue than the reference test,
possibly due to the continuous recycle of unreactive heavier compounds present in the PUB.
Concluding, the properties of the produced PUB are within the desired limit, which means that it
could proceed to the second stage of hydrotreatment to produce a high quality biofuel. Nonetheless,
there are still some properties that could be improved, namely the carbon residue and oxygen
content, which could be further lowered.
Regarding the analysis of aqueous phases, the results of this study show that they are essentially
composed by carboxylic acid compounds and other oxygen-containing compounds from the family of
alcohols and ketones. Phenols are also identified, but in very small amounts.
In summary, it seems that in order to recycle all the PUB and improve the quality of the produced
effluent it is necessary to enhance the conversion of the heavier fraction of the feedstock BO/PUB by
adapting the operating conditions and/or improving the catalyst’s activity (e.g. by changing the active
phase, its activation or its morphology). A promising option would be to recycle only a lighter fraction
of PUB (boiling point to be determined in a posterior study) and send the heaviest cuts to a
converting step at more severe conditions, such as hydrocracking. So, more studies should be
performed in order to optimize the first hydroreforming step for upgrading bio-oils.
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8. Future work
Taking into account the obtained results, there are still some points that need to be further studied.
In addition, a brainstorming over the process can also be done, in order to discover new possible
improvements to the global process of upgrading bio-oils.
The formulation of the catalyst could be reviewed so that problems such as rapid deactivation and,
possibly, diffusional limitations can be avoided. In an industrial scale unit, the rapid deactivation of
the catalyst is a crucial point, since continuous supply of new catalyst results into an extremely
expensive process. Therefore, more studies should be done to adapt the morphology of the catalyst,
making it chemically stronger and enhance HDO reactions.
Due to the lack of time in the present study, only two sets of tests could be done to study the
continuous recycling of effluent. The results are satisfactory, however, more experiments should be
done in order to optimize the quality of the effluent produced in the steady state. The results of the
study done on the operating conditions, namely the increase in the mass of the catalyst and its
activation, should be used in the next set of continuous recycling tests.
The tests with distillation fractions of PUB show better results in terms of carbon residue and global
HDO of bio-oil. Therefore, it should also be studied the possibility of introducing a distillation column
in the process, so that the heaviest tarry fraction could be removed before recycling the PUB. In this
way, clogging problems and possible deposition of solids could be avoided in an industrial continuous
unit and, simultaneously, improve the quality of the effluent.
The recycling ratio of PUB can also be a subject of study, since it is possible that lower amounts of
PUB are sufficient to stabilize the bio-oil and, thus, less recycling of heavier compounds is needed. As
a matter of fact, some tests could also be performed to study the recycling of PUB together with fully
upgraded biofuel, which could also play an important role in stabilizing the unreacted bio-oil, if the
mixture is homogeneous.
The determination of the composition of different aqueous phases (including the aqueous phases
produced in the tests of continuous recycling of PUB) should also continue. There are mainly two
possible routes for the aqueous phase. In one way, some of the products can be recovered to value
the process. However, taking into account the obtained quantities, it is likely that the recovery may
be expensive by procedures such as membrane separation and/or liquid-liquid extraction.
Additionally, the presence of inorganic compounds in this type of separation process may cause
68
corrosion problems. Therefore, it seems more preferable to send the aqueous phase to biological
treatment, but the compatibility of the composition of these phases (containing many acid, but also
very harmful for the environment phenols) with processing unit is still necessary to be studied.
69
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73
Appendix A
Gross values of CHO and CCR
Table A 1 – Gross values of the properties of bio-oil, PUB and the different feedstock used in the tests
Elemental Analysis (wt.%) C H O
CCR (wt.%)
Bio-oil 41,6 7,3 49,1 20,6
PUB 3 69,0 10,1 19,3 5,4
Feedstock: BO/solvent 34,1 36,9 27,9 10,3
Feedstock: BO/PUB 64,1 8,6 26,2 14,5
Table A 2 – Gross values of the properties of distillation cuts
Elemental Analysis (wt.%)
C H O CCR (wt.%)
Distillation fraction 1 55,9 12,3 28,4 N.D.
Distillation fraction 2 57,1 11,5 28,9 N.D.
Distillation fraction 3 68,8 10,2 20,0 N.D.
Distillation fraction 4 74,7 9,3 16,1 N.D.
Distillation fraction 5 81,0 7,0 11,7 44,8
Table A 3 – Gross values of the properties of the effluents from the tests performed to study the influence of operating conditions
Test Elemental Analysis (wt.%)
C H O CCR (wt.%)
Test with solvent 41,5 7,2 49,1 10,3
Reference 73,1 9,3 16,7 7,7
Higher reaction time 73,7 9,3 16,2 8,7
Higher catalyst mass 75,1 9,7 14,3 7,2
Higher catalyst mass + higher reaction time 76,0 9,6 13,4 8,1
Activation of catalyst 77,0 9,6 12,2 6,9
External diffusional limitations 71,0 9,6 17,7 2,7
Internal diffusional limitations 69,6 9,5 19,0 6,5
Recycle of a light fraction of PUB 70,9 9,5 18,5 5,1
Recycle of a heavy fraction of PUB 75,1 9,4 15,4 5,7
74
Table A 4 – Gross values of the properties of the effluents produced during the first set of recycling tests
Test Elemental Analysis (wt.%)
C H O CCR (wt.%)
1 61,1 11,4 25,2 -
2 67,4 10,2 20,1 -
3 72,2 9,7 17,5 6,70
Table A 5 – Gross values of the properties of the effluents produced during the second set of recycling tests
Test Elemental Analysis (wt.%)
C H O CCR (wt.%)
1 62,1 11,3 24,5 3,5
2 66,9 10,1 21,7 7,5
3 70,5 9,2 18,9 9,2
4 72,1 8,9 19,0 11,9
5 73,4 8,7 18,1 11,7
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Appendix B
Calculations
The values presented of CHO and CCR were corrected so that the contribution from solvent and
water was not taken into account in the calculations of HDO and carbon yield. The corrections done
are indicated hereinafter.
𝑪𝐜𝐨𝐫𝐫𝐞𝐜𝐭𝐞𝐝 = 𝑪𝒈𝒓𝒐𝒔𝒔 − 𝑪𝒔𝒐𝒍𝒗𝒆𝒏𝒕
∑ 𝑪𝑯𝑶𝒈𝒓𝒐𝒔𝒔 − ∑ 𝑪𝑯𝑶𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − ∑ 𝑯𝑶𝒘𝒂𝒕𝒆𝒓
𝑯𝐜𝐨𝐫𝐫𝐞𝐜𝐭𝐞𝐝 = 𝑯𝒈𝒓𝒐𝒔𝒔 − 𝑯𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − 𝑯𝒘𝒂𝒕𝒆𝒓
∑ 𝑪𝑯𝑶𝒈𝒓𝒐𝒔𝒔 − ∑ 𝑪𝑯𝑶𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − ∑ 𝑯𝑶𝒘𝒂𝒕𝒆𝒓
𝑶𝐜𝐨𝐫𝐫𝐞𝐜𝐭𝐞𝐝 = 𝑶𝒈𝒓𝒐𝒔𝒔 − 𝑶𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − 𝑶𝒘𝒂𝒕𝒆𝒓
∑ 𝑪𝑯𝑶𝒈𝒓𝒐𝒔𝒔 − ∑ 𝑪𝑯𝑶𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − ∑ 𝑯𝑶𝒘𝒂𝒕𝒆𝒓
𝑪𝑪𝑹𝐜𝐨𝐫𝐫𝐞𝐜𝐭𝐞𝐝 = 𝑪𝑪𝑹𝒈𝒓𝒐𝒔𝒔
𝟏 − %𝒔𝒐𝒍𝒗𝒆𝒏𝒕 − %𝒘𝒂𝒕𝒆𝒓
The overall hydrodeoxygenation was calculated to obtain the amount of oxygen removed from the
organic phase. Therefore, it takes into account the amount of oxygen that was initially in the
feedstock and in the organic phase after reaction and does not account for the oxygen contained in
the compounds dissolved in the aqueous phase.
𝐇𝐃𝐎 = 𝟏 −𝒎𝑷𝑼𝑩 ∙ [(𝟏 − %𝑯𝟐𝑶 − %𝒔𝒐𝒍𝒗𝒆𝒏𝒕) ∙ %𝑶]𝑷𝑼𝑩
𝒎𝒂𝒔𝒔𝒇𝒆𝒆𝒅𝒔𝒕𝒐𝒄𝒌 ∙ [(𝟏 − %𝑯𝟐𝑶 − %𝒔𝒐𝒍𝒗𝒆𝒏𝒕) ∙ %𝑶]𝒇𝒆𝒆𝒅𝒔𝒕𝒐𝒄𝒌
The %O in each phase is given by the corrected values of the content on oxygen.
Using the same logic, the yield of carbon, which corresponds to the amount of carbon that remained
in the organic phase, was calculated with the values of the amount of carbon in the feedstock and in
the organic phase after reaction.
𝐂𝐚𝐫𝐛𝐨𝐧 𝐲𝐢𝐞𝐥𝐝 =𝒎𝒂𝒔𝒔𝑷𝑼𝑩 ∙ [(𝟏 − %𝑯𝟐𝑶 − %𝒔𝒐𝒍𝒗𝒆𝒏𝒕) ∙ %𝑪]𝑷𝑼𝑩
𝒎𝒂𝒔𝒔𝒇𝒆𝒆𝒅𝒔𝒕𝒐𝒄𝒌 ∙ [(𝟏 − %𝑯𝟐𝑶 − %𝒔𝒐𝒍𝒗𝒆𝒏𝒕) ∙ %𝑪]𝒇𝒆𝒆𝒅𝒕𝒐𝒄𝒌
76
Appendix C
Confidential Information
