INTRODUCTION

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1.INTRODUCTION

1.1 GENERAL

As the petroleum resources are depleting and the crudes being

refined are becoming higher in density and have higher content of hetero

atoms (S,O,N etc) the need and demand for high-quality middle distillates

has grown significantly over the past decade and continues to grow in the

international market.

Stringent environment legislation regarding fuel specifications has

been imposed worldwide over the last decade in order to improve the

quality of diesel fuels with a view to reduce oxides of sulphur(SOX), oxides

of nitrogen(NOX) and particulate matter (PM) in the diesel engine exhaust

emissions which are major contributors to environmental pollution. In

order to meet the increased demand for on-road diesel, the refiners use

heavy distillates as blending components into the diesel pool. For the above

reasons, the refining industry today faces on the one hand tighter

specifications and on the other hand higher production demands for diesel.

As a result, refiners worldwide have started revamping or optimising the

existing middle-distillate hydrotreaters to achieve deep desulphurisation.

The tightening of diesel fuel specifications began in 1993 when the

USA lowered sulphur to 500ppmw maximum. The California Air

Resources Board (CARB) went one step further by mandating a maximum

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aromatic level as well as a minimum cetane level. Similar specifications

are being considered by the European Commission (EC), which requires

a maximum of 50ppmw sulphur as well as maximum cetane and

minimum polyaromatic levels. Germany is also considering tax incentives

to encourage refiners to produce and market diesel fuel with 10ppmw

sulphur.

In December 2000, the US Environmental Protection Agency

(EPA) introduced a new mandate to further reduce the level of sulphur in

on-road diesel by 97 percent (to 15ppmw maximum) starting in mid-2006.

In a typical refinery, diesel fuel is produced from one or more

blending components derived from crude distillation and conversion units.

The main blending components are heavy naphtha, straight run diesel,

hydrocracker diesel, Light Cycle Oil (LCO) from fluid catalytic cracker

(FCC), Coker diesel etc.

The desirability of these streams (for example LCO) would require

much more severe treating then others eg: Straight Run Light Gas Oil

(SRLGO).

The technologies for removing sulphur are not necessarily the same

as the technologies required to upgrade other diesel fuel qualities. In the

opinion of many refiners and process vendors, the technologies for

removing sulphur from diesel fuel are probably the least expensive and

easiest to implement compared to those required to upgrade other diesel

fuel qualities.

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At present, the most common diesel sulphur specifications

worldwide are around 500ppmw. However, the sulphur specifications are

being revised in many of the countries conforming to Euro-III/Euro-IV

specifications. In India, BUREAU OF INDIAN STANDARDS (BIS) has

laid down specification for auto fuels. Inline with global specification,

India has also reduced diesel fuel sulphur level from 1.0 wt%

(10000ppmw) down to 350ppmw (Euro-III or Bharat-III equivalent norms)

over the last decade and is likely to be reduced further to 50ppmw (Euro-

IV or Bharat-III equivalent norms) by April 1, 2010.

Initially, the diesel hydrodesulphurisation units in India were

designed to produce 500ppmw sulphur that consists of two reactors

operating in series. The typical operating conditions of the industrial units

are as follows: reactor temperature of 340oC,Reactor pressure of 40kg/cm2,

LHSV of 2.0h-1 and hydrogen-to-oil ratio of 200L/L

Current diesel specifications in India and proposed changer’s are

compared with Euro-III and Euro-IV specifications in table 1.1.

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Table 1.1 Existing and future specifications of diesel

Characteristics Bharat-III(current)

Bharat-IV

Euro- III Euro- IV

Density @ 15o C , kg/cm3

820-845 820-845 820-845 820-845

Kinematic viscosity @ 40o C,cSt

2.0-4.5 2.0-4.5 2.0-4.5 2.0-4.5

Flash point, o C 35 - 35 35Pour point , oC, max., WinterSummer

315

- - -

Cetane Number, min 51 51 51 51Cetane Index , min 46 - 46 48RCR on 10 % Residue max.

0.3 - 0.3 0.3

Total Sulphur, wt.% max.

0.035 0.005 0.035 0.005

Polycyclic Aromatic Hydrocarbon (PAH), wt.% , max.

11 11 11 11

Distillation , 95 vol.% Recovery@ oC, max.

370 360 360 360

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1.2. CRUDE ASSAY

Crude oil is defined as the mixture of naturally occurring

hydrocarbons that is refined in to diesel, gasoline, heating oil, jet

fuels, kerosene and literally thousands of other products called

petrochemicals. Crude oil are named according to their contents and

origins, and classified according to their per unit weight (specific

gravity). Heavier crudes yield more heat upon burning, but have

lower API gravity and market price in comparison to light (or sweet)

crudes.

1.2.1 COMPOSITION OF CRUDE OIL

The composition of crude oil varies according to where it was

obtained. This largely has to do with the type of ground in which

the oil was formed, and what contaminants were present and in what

relative concentrations. In addition to colour variations, there are also

various contaminant levels and various flowing properties. Variations

aside, crude oil, on average, has the composition shown below.

Carbon (C) 84-89

Hydrogen (H) 11-14

Sulfur (S) 0.1-4.5

Nitrogen (N) 0.01-0.8

Metals 0.0-0.01

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Crude oil is comprised of:

1. Paraffins

2. Oleffins

3. Naphthenes

4. Aromatics

1.2.2 TYPES OF CRUDE

The different  Types of Crude oil  have variations in viscosity

and appearance from one oil field to another. The variations range in

odour, color and in the basic properties and qualities. While all Types of

crude oil are basically hydrocarbons, there are differences in their

properties, especially in the variations in the molecular structure. The many

variations may also influence the suitability of the different types of crude

oil for particular products and the resulting quality of the products. 

The different Types of crude oil are classified based on the

American Petroleum Gravity (API) gravity and viscosity. The properties

may vary in terms of proportion of hydrocarbon elements, sulfur content

etc as it is extracted from different geographical locations all over the

world. If the API gravity of the crude oil is of 20 degrees or less, it is

graded as 'heavy', those with an API gravity of 40.1 degrees or greater than

that is known as 'light' and if the oil ranges between 20 and 40.1 degrees, it

is graded as 'intermediate'. Classifications are made based on the sulfur

content as well. Crude oil with low content of sulfur means 'sweet' and the

presence of high content sulfur is known as 'sour'. The purity of crude oil

increases or decreases based on the sulfur content as sulfur is an acidic

material.

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1.2.3.DISTILLATION COLUMN

Figure 1.2.3. distillation column

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1.2.4.DESCRIPTION OF DISTILLATION PROCESS

The diagram above is a schematic flow diagram of a typical oil

refinery that depicts the various unit processes and the flow of intermediate

product streams that occurs between the inlet crude oil feedstock and the

final end products. The diagram depicts only one of the literally hundreds

of different oil refinery configurations. The diagram also does not include

any of the usual refinery facilities providing utilities such as steam, cooling

water, and electric power as well as storage tanks for crude oil feedstock

and for intermediate products and end products.

The atmosphric distillation unit is the first processing unit in

virtually all petroleum refineries. It distills the incoming crude oil into

various fractions of different boiling ranges, each of which are then

processed further in the other refinery processing units.

The incoming crude oil is preheated by exchanging heat with some

of the hot, distilled fractions and other streams. It is then desalted to

remove inorganic salts (primarily sodium chloride).

Following the desalter, the crude oil is further heated by exchanging

heat with some of the hot, distilled fractions and other streams. It is then

heated in a fuel-fired furnace (fired heater) to a temperature of about

398 °C and routed into the bottom of the distillation unit.

The cooling and condensing of the distillation tower overhead is

provided partially by exchanging heat with the incoming crude oil and

partially by either an air-cooled or water-cooled condenser. Additional heat

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is removed from the distillation column by a pump around system as shown

in the diagram below.

As shown in the flow diagram, the overhead distillate fraction from

the distillation column is naphtha. The fractions removed from the side of

the distillation column at various points between the column top and

bottom are called sidecuts. Each of the sidecuts (i.e., the kerosene, light

gas oil and heavy gas oil) is cooled by exchanging heat with the incoming

crude oil. All of the fractions (i.e., the overhead naphtha, the sidecuts and

the bottom residue) are sent to intermediate storage tanks before being

processed further.

The reduced crude is sent to vacuum distillation , to obtain products

such as asphalt , wax, tar etc, The products obtained from distillation

column are processed in respective units.

1.2.5.MAJOR PRODUCTS

Petroleum products are usually grouped into three categories: light

distillates (LPG, gasoline, naphtha), middle distillates (kerosene, diesel),

heavy distillates and residuum (heavy fuel oil, lubricating oils, wax,

asphalt). This classification is based on the way crude oil is distilled and

separated into fractions (called distillates and residuum) as in the above

drawing.

Liquified petroleum gas (LPG)

Gasoline (also known as petrol)

Naphtha

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Kerosene and related jet aircraft fuels

Diesel fuel

Fuel oils

Lubricating oils

Paraffin wax

Asphalt and tar

Petroleum coke

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LITERATURE REVIEW

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2.LITERATURE REVIEW

2.1.GENERAL

Babich and Moulijin (2003) reported that organosulfur

compounds are usually present in almost all fractions of crude oil

distillation. Higher boiling point fractions contain relatively more sulfur

and the sulfur compounds are of higher molecular weight. The

reactivity of organosulfur compounds varies widely depending on their

structure and local sulphur atom environment. The low boiling crude oil

fraction contains mainly the aliphatic organosulfur compounds:

mercaptans, sulfides , and disulfides. They are very reactive in

conventional hydro treating processes and they can easily be completely

removed from the fuel. Other processes like merox can be applied to

extract mercaptans and disulfides from gasoline and light refinery

streams.

For higher boiling crude oil fractions such as heavy straight

run naphtha, straight run diesel and light FCC naphtha, the organosulfur

compounds predominantly contain thiophennic rings. These

compounds include thiophenes and benzothiophenes and their

alkylated derivatives. These thiophene containing compounds are more

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difficult than mercaptans and sulphides to convert via hydrotreating. The

heaviest fractions blended to the gasoline and diesel pools are bottom

FCC naphtha , coke naphtha , FCC and coker diesel, contains mainly

alkylated benzthiophenes, dibenzothiophenes (DBT) and

alkylbenzthiophenes, as well as polynuclear organic sulphur

compounds,i.e. the least sulphur compounds in the HDS rection.

The reactivity of sulphur compounds in the HDS follows this order

(from most rective to least reactive); thiophene>alkylated

thiophene>BT>alkylated BT>DBT and alkylated DBT without

substituents at the 4 and 6 positions>alkylated DBT with one substituent at

either the 4 or 6 position>alkylated DBT with alkyl substituents at the 4 or

6 position. Deep desulfurisation of the fuel implies that more and more of

the least reactive sulphur compounds must be converted.

2.1.LIETERATURE REVIEW ON HYDRODESULPHURISATION

PROCESS

The conventional HDS process is usually conducted over sulfided

Co-MO/Al2O3 and Ni-Mo/Al2O3cataylsts. Their performance in terms of

desulfurisation level, activity and selectivity depends on the properties of

the specific catalyst used , the reaction conditions, nature and

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concentrations of sulphur compounds present in the feed stream, and

reactor and process design.

Chunsang song (2003) reported that the key to ultra-deep

desulfurisation is the removal of refractory sulphur compounds from diesel

fuels. These compounds are higher molecular weight

dibenzothiophenes(DBTs) that contain substituents in positions adjacent

to the sulphur atom. In addition to straight-run gas oil , the light cycle oil

from fluid catalytic cracking of heavy oils is a major blend stock for diesel

fuels which tends to have the highest contents of refractory sulphur

compounds especially 4,6-dimethyldibenzothiophene (4,6- DMDBT), 4,6-

diehtylbenzothiophene(4,6-DEDBT), 4-methyldibenzothiophene (4-

MDBT) and dibenzothiophene(DBT).

Knudsen et al.(1999) reported that about four times more active

catalysts are required to reduce the diesel fuel sulfur content from 500 to

50ppmw compared to a typical Co-Mo catalyst at constant LHSV and the

corresponding temperature increament is about 38˚C.

Knudsen et al. (1999) reported that the effect of process variables

such as LHSV, temperature, hydrogen partial pressure, hydrogen sulfide

partial pressure and hydrogen to oil ratio on catalyst activity applied to

deep desulfurisation could be predicted by a suitable kinetic expression.

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They found that the equation 2.1 could be used to describe the kinetics of

Co-Mo and Ni-Mo catalysts for very deep desulfurisation of diesel. In the

expression for the rate of desulfurisation,the first term represents the direct

extraction route,which is enhanced by an increase of the hydrogen partial

pressure and inhibited by the presence of H2S. The second term represents

the hydrogenation route, which is also enhanced by an increase of

hydrogen partial pressure and inhibited by the presence of aromatic

compounds, and in particular heterocyclic compounds (denoted by F in

the equation)

2.2.LITERATURE REVIEW ON CATALYST

For Co-Mo catalysts, the second term can to a good approximation

be neglected, and the rate constant k, can be determined by integration of

the expression. The partial pressure of hydrogen sulfide, P H2S can be

expressed in terms of the sulfur concentration CS , which means that an

explicit expression can be obtained for k. The rate constant of catalyst

type, temperature and feedstock

For Ni-Mo catalyst both terms are important, and the removal of

the inhibitors has to be solves simultaneously in the rate equation.

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Lappas et al.(1999) carried out hydrodesulfurisation of LCO and

SRGO blend in a continuous flow, trickle bed hydrotreating pilot plant unit

using a commercial Co-Mo catalyst and investigated the effect of operating

parameters (temperature, pressure, weight hourly space velocity,

hydrogen-to –oil-ratio) on sulfur removal and aromatics saturation and

reported that by decreasing the space velocity and by increasing the

temperature, the pressure and the hydrogen-to -oil-ratio , the product

density and the aromatics and sulfur content of diesel decreased.

Selakovic and Jovanovic (2001) carried out hydrodesulfurisation of

middle distillates with various blends of LGO-LCO and LGO-LVGO using

Co-Mo catalyst of different producers with product sulfur specifications

and determined the required severity in achieving low sulfur levels.

Lamourelle and Nelson (2001) discussed various means to produce

ultralow sulfur diesel products using new generation Ni-Mo and Co-Mo

catalyst combinations. They studied various revamp options for making

ultralow sulphur diesel from existing hydro treating units, hydrogen

strategy , reaction pathways , catalyst options.

Bhaskar et al. (2004) developed a three phase non-isothermal

heterogeneous model to simulate the performance of pilot plant and

industrial trickle bed reactors applied to the hydrodesulfurisation of diesel

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fractions. The developed model was found to simulate the performance of

the industrial reactor adequately. The model was also applied to study the

influence of operating conditions on product quality.

Bhaskar et al. (2004) developed a three phase non- isothermal

heterogeneous model to stimulate the performance of pilot plant and

industrial trickle-bed reactors applied to the hydrodesulphurisation of diesel

fractions. The developed model was found to stimulate the performance of

the industrial reactor adequately. The model was applied to study the

influence of operating conditions on product quality.

The reactive reaction rates of various sulphur species are shown in

table 2.1. The more difficult sulphur species i.e. the substituted

dibenzothiophenes, have the highest boiling points and are more prevalent

in streams with high end points.

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Table 2.1. Relative reaction rates and boiling points of various sulphur

species

Sulphur species Relative Reaction

Rate

Boiling Point, oF

Thiophene 100 185

Benzothiophene 30 430

Dibenzothiophene 30 590

Methyl Dibenzothiophene 5 600-620

Dimethyl Dibenzothiophene 1 630-650

Trimethyl Dibenzothiophene 1 660-680

The basic reactions that take place during hydrodesulfurisaton are

as follows,

Mercaptans RSH + H2 RH +H2S

Sulfides R2SH + 2H2 2RH +H2S

Disulfides RSSR + 3H2 2RH +2H2S

Aromatics ArS +2H2 Aromatic + H2S ( excluding ring saturation)

Aromatic ring saturation is a reversible reaction that is controlled by

equilibrium.

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Salvatore and Michael (2004) reported that desulphurisation

reactions can follow a number of paths, but two routes are generally

favoured under typical hydro treating conditions. The most common route

for removing sulfur is the direct extraction, or hydrogenolysis, reaction

(carbon-sulfur bond breakage). This mechanism is predominant when the

sulfur requirement is above 350ppm. At this level most of the sulfur in the

more reactive compounds (up to and including non-substituted

dibenzothiophene) has been removed. Typically, conventional Cobalt-

molybdenum (Co-Mo) catalysts and low-to-moderate reactor pressure are

applied to favour this reaction mechanism.

Salvatore and Michael (2004) reported that removing sulfur from

most difficult compounds such as 4,6-dimethyldibenzothiophene, generally

follows the hydrogenation route. It is easier to break the sterically hindered

carbon-sulfur bond if one of the aromatic rings is first saturated. Saturating

the ring changes the molecule’s spatial configuration and makes the sulfur

atom more accessible to react with the catalyst’s active sites. After the

aromatic ring is hydrogenated, the sulfur atom can be removed via

hydrogenolysis. Nickel-molybdenum (Ni-Mo) catalysts and higher pressure

help promote the hydrogenation reaction mechanism.

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Bhaskar et al. (2004) reported that the hydrogenation route was

found to be slower than direct extraction for most of alkyl-substituted DBT

molecules but is much faster for sterically hindered DBT’S.

Knudsen et al. (1999) reported that Co-Mo catalysts desulfurise

primarily via the direct extraction route. Ni-Mo catalysts, exhibit a higher

hydrogenation activity, have a relatively higher selectivity for

desulfurisation via the hydrogenation route.

As compared with the Ni-Mo catalysts, the Co-Mo catalyst is not

as good for the removal of 4,6-DMDBT, but better than Ni-Mo catalyst for

removal of DBT and 4-MDBT.

Juarez et al. (1999) carried out hydrotreating of SRGO-LCO blends

to evaluate the effect of LCO on product quality at varied operating

conditions over a commercial Co-Mo catalyst and determined apparent

reaction orders and activation energies.

Bhaskar et al. (2000) reported that the revised specification of

diesel fuel can met by increasing the hydro treating capacity, operating the

existing units at high severity levels and using a modified higher activity

hydro desulphurisation catalysts .

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FEED USED

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3.DIESEL

Diesel is produced from the fractional distillation  of  crude oil

between 200 °C and 380 °C at atmospheric pressure , resulting in a

mixture of carbon chains that typically contain between 8 and 21 carbon

atoms per molecule. The best diesel fuels are “straight-run” Stocks,

derived from simple distillation of crude oil.

3.1.COMPOSITION

Diesel is composed of about

(i) 75% saturated hydrocarbons (primarily paraffins including  n, iso,

and cycloparaffins),

(ii) 25% aromatic hydrocarbons (including  naphthalenes

and alkylbenzenes).

The average chemical formula for common diesel fuel is C12H23, ranging

approximately from C10H20 to C15H28.

3.2TYPES OF DIESEL FUEL

In India we have two types of diesel fuels:

1. High speed diesel (HSD) used in automotive applications and

2. Light diesel oil (LDO) used in stationary applications

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3.3.DIESEL PROPERTIES

Sulphur content

Affects wear, depsits, and particulate emmissions. Diesel fuels

contain varying amounts of various sulphur compounds which increase oil

acidity. Legislation has reduced sulphur content of highway fuels to 0…5%

by weight. Off road fuel has an average of 0.29% sulphur by weight.

Cetane number

A measure of the starting and warm-up characteristics of a fuel. In

cold weather or in service with prolonged low loads, a higher cetane

number is desirable. Legislation dictates the Cetane Index shoud be 40 or

above.

Aromatic content

By definition, aromatic content is characterised by the presence of

the benzene family in hydrocarbon compounds that occur naturally in the

refining of diesel fuel. In the chemical make up of fuel, the heavier

aromatic compounds of toluene, xylene, and naphthalene are also present.

Limiting these aromatic compounds has the effect of reducing burning

temperature and thus NOX formation.

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Cloud & pour point

Affect low-temperature operation.  The cloud point of the fuel is the

temperature at which crystals of paraffin wax first appear. Crystals can be

detected by a cloudiness of the fuel. These crystals cause filters to plug.

API gravity

Related to heat content, affecting power and economy. Gravity is an

indication of the energy content of the fuel. A fuel with a high density (low

API gravity) contains more BTU's per gallon than a fuel with a low density

(higher API gravity).

API gravity = 141.5

specific gravity at 60oF60oF

− 131 .5

Ash

Measures inorganic residues - The small amount of non-

combustable metallic material found in almost all petroleum products is

commonly called ash. Ash content should not exceed 0.02 mass percent.

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Water sediment

Affect the life of fuel filters and injectors. The amount of water and

solid debris in the fuel is generally classified as water and sediment.  It is

good practice to filter fuel while it is being put into the fuel tank.  More

water vapor condenses in partially filled tanks due to tank breathing caused

by temperature changes.  Filter elements, fuel screens in the fill pump, and

fuel inlet connections on injectors must be cleaned or replaced when they

become dirty.  These screens and filters, in performing intended function,

will become clogged when using a poor or dirty fuel and will need to be

changed more often.  Water and sediments should not exceed 0.1 volume

percent.

Viscosity

Affects injector lubrication and atomization.  The injector system

works most effectively when the fuel has the proper "body" or viscosity. 

Fuels that meet the requirements of 1-D or 2-D diesel fuels are satisfactory

with Cummins fuel systems. 

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Carbon residue

Measures residue in fuel - can influence combustion.  The

tendency of a diesel fuel to form carbon deposits in an engine can be

estimated by various tests to determine the carbon residue after 90% of

the fuel has been evaporated .

3.4.INFERENCE

It is found that

1. Specific gravity, flash point, viscosity, sulfur content, and

carbon residue increase with increase in service severity.

2. The cetane number (which measures the fuel’s ignition

quality) decreases with increase in service severity. There is also a

decrease in volatility with increase in service severity.

3. Properties are, however, inter-related and it is difficult to

isolate the effect of any single variable

4. It is found that the self-ignition temperature of the normal

paraffins decreases as the length of the chain increases.

5. Since the cetane rating of the fuel is a measure of the

ignition characteristics of the fuel, it can be concluded that the

heavier members of the paraffin family have higher cetane ratings.

6. In fact, cetane, C16H34 (hexadecane) is the primary

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reference fuel in the cetane scale with an arbitrary cetane rating of

100 while other normal paraffins have cetane ratings that vary

almost linearly with the length of the chain.

3.5.Characteristics of Hydrocarbons as Diesel Fuel

S.N

O

GENERAL

FORMULA

HYDROCARBON CHARACTERISTICS

1. n-paraffins CnH2n+2

(Straight chain)

Low specific gravity & high

cetane number

2. Iso-paraffins CnH2n+2 (Branch

chain)

Same sp.gr as 1 but lower

cetane no. & B.P.

3. Cycloparaffins CnH2n Higher sp. Gr. Than 1 but

lower cetane number

4. Benzenes CnH2n-6 Higher sp.gr & B.P but lower

cetane number than 1-3

5. Naphthalenes CnH2n-12 Highest sp. Gr & B.P. &

lowest cetane no.

Important characteristics of diesel fuel

Knock characteristics-requires high cetane number

1. Starting characteristics-requires high volatility but will give lower

power

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2. Smoking and Odor-high volatility will give better mixing and lower

smoke but also lower power

3. Corrosion and Wear-due to presence of sulfur and ash

4. Ease of Handling-should have low pour point and viscosity for ease

of handling but high flash and fire point for safety and fire hazard.

5. Density, heat of combustion and cleanliness.

3.6.DIESEL FUEL SPECIFICATIONS

GENERAL FUEL CLASSIFICATION

ASTM TEST METHOD

GRADE LOW SULPHURNO. 1-D

GRADE HIGH SULPHUR NO. 2-D

Flash point0C Mm D 93 38 52Water and sediment, vol, max.

D 2709 0.05 0.05

Distillation temperature, 0C recorded

D 86

Min - 282 Max 288 338Viscosity, kinematic mm2/S at 400C Max 288 338

D 86 1.3-2.4 1.9-4.1

Ash, Max D 445 0.01 0.01Sulphur wt, max D 2622 0.05 0.05

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HYDROTREATING

4.1HYDROTREATING

30

Hydrotreating (HDT) is a catalytic process , which effectively

removes sulphur and other impurities like nitrogen, oxygen and metals

from crude oil and petroleum distillates.

A refining process for treating petroleum fractions from

atmospheric or vacuum distillation units (e.g., naphthas, middle

distillates, reformer feeds, residual fuel oil, and heavy gas oil) and other

petroleum (e.g., cat cracked naphtha, coker naphtha, gas oil, etc.) in the

presence of catalysts and substantial quantities of hydrogen. Hydrotreating

includes desulfurization, removal of substances (e.g., nitrogen compounds)

that deactivate catalysts, conversion of olefins to paraffins to reduce gum

formation in gasoline, and other processes to upgrade the quality of the

fractions.

Hydrotreating applications include Hydrodesulfurisation (HDS) of

kerosene, Hydrodesulfurisation (HDS) of naphtha, Hydrodesulfurisation

(HDS )of diesel, hydrofinishing of lube oil base stocks, hydrofinishing of

wax etc.

4.2HYDROTREATING REACTIONS

The chemical reactions that are of primary interest in hydrotreating

include

1. Hydrodesulfurisation (HDS)

2. Hydrodenitrogenation (HDN)

3. Hydrodeoxygenation (HDO)

4. Hydrogenation of aromatic compounds

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5. Saturation of olefins

6. Hydrodemetallation (HDM)

4.2.1.HYDRODESULPHURISATION

Hydrodesulfurization (HDS) is a catalytic chemical process widely

used to remove sulfur (S) from refined petroleum products such as

gasoline or petrol, jet fuel, kerosene, diesel fuel. The purpose of removing

the sulfur is to reduce the sulfur dioxide (SO2) emissions that result from

using those fuels in automotive vehicles, aircraft, railroad locomotives,

ships, gas or oil burning power plants, residential and industrial furnaces,

and other forms of fuel combustion.

R-S-R’ + 2H2 → RH + R’H +H2S

4.2.2.HYDRODENITROGENATION

The hydrogenolysis reaction is also used to reduce the nitrogen

content of a petroleum stream and removes as ammonia in a process

referred to as hydrodenitrogenation (HDN). The rate of HDN reaction

was little lower than the rate of HDS reaction.

R=N-R’+ 3H2 → RH + R’H

4.2.3.HYDRODEOXYGENATION

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During hydrotreating, oxygen compounds in the form of naphthenic

acids, phenols, alcohols, aldehydes etc were also removed as water vapour

from the feed.

R-O-R’ + 2 H2 → RH + R’H +H2O

4.2.4.HYDRODEMETALLATION

Metals like nickel, vanadium, iron, copper, sodium etc present in

traces get adsorbed over the catalyst

R-M + ½ H2 + A → RH + M-A

4.2.5.HYDROGENATION OF AROMATIC COMPOUNDS

Aromatics combined with hydrogen and get converted in to

napthenes.

C10 H8 + 2 H2 →C10 H12

4.2.6.SATURATION OF OLEFINS

Olefins combines with hydrogen and get saturated

R=R’ + H2 → HR-R’H

4.3.UNDESIRABLE REACTIONS

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The undesirable reactions that occur in hydrotreating are

1. Hydrocracking

2. Coking

These undesirable reactions reduce the yield of the product. At high

temperature and pressure of HDS reaction, Hydrocracking occurs as a side

reaction. Hydrocracking reaction has to be minimised, because it consumes

hydrogen, reduces the product yield and the hydrogen purity of the recycle

gas. It is limited by the selection of catalyst with low hydrocracking

capacity and working at low temperature. During HDS reaction the heavy

molecules are adsorbed on the acidic site of the catalyst and reduce the

catalytic activity

4.4.HYDROTREATING OPERATING VARIABLES

The principle operating variables affecting the performance of

hydrotreating reactions are as follows:

1. Reaction temperature

2. Hydrogen partial pressure

3. Liquid hourly space velocity and

4. Hydrogen-to-oil ratio

Reaction temperature :

34

Reactor temperature has strong influence on hydrotreating

reactions. Increasing reactor temperature increases conversion of sulphur,

nitrogen and oxygen compounds. But increasing the reactor temperature

also increases the side reactions namely, hydrocracking and coking.

Hydrogen partial pressure

Hydrogen partial pressure can be increased by increasing the

hydrogen recycle rate or hydrogen purity at a given feed rate. Increasing

hydrogen partial pressure reduces coke formation by suppressing

polymerization and condensation reactions. Adequate hydrogen-to-oil ratio

is to be maintained to have desirable cycle length.

Liquid hourly space velocity

Increasing Liquid Hourly Space Velocity(LHSV) increases feed

rate processed per unit volume of catalyst and hence reduces the severity of

hydrotreating reactions. However, lower LHSV will produce product with

low sulphur, nitrogen and oxygen. LHSV is fixed based on the design feed

rate of the unit.

Hydrogen-to-oil ratio

Hydrogen-to-oil ratio can be increased by increasing the hydrogen

recycle rate or hydrogen purity at a given feed rate and should be kept as

high as possible to decrease the rate of undesirable gases and to improve

the catalyst efficiency by avoiding coke formation.

35

CATALYST USED

5.1. HYDRO TREATING CATALYST

36

There are two types of catalyst available for hydrotreating purposes.

They are Cobalt-Molybdenum and Nickel- Molybdenum on alumina

support. Cobalt and Nickel are responsible for catalytic action whereas

Molybdenum acts as a promoter. Nickel- Molybdenum catalyst exhibits a

higher hydrogenation activity than Cobalt- Molybdenum catalyst and is

more suitable for treating cracked stocks. Very often the support material

also has catalytic action related to its chemical nature.

5.2.CATALYST OPTIONS FOR PRODUCING ULTRA LOW

SULPHUR DIESELS

Proper choice of catalyst is of paramount importance in the

production of Ultra Low Sulphur Diesel (ULSD). Recent advances in

hydrotreating catalyst technologies have significantly improved the sulphur

removal capability. The use of the most advanced hydrotreating catalyst or

catalyst system can significantly increase the desulphurisation capability of

the existing hydrotreaters.

New and improved HDS catalyst for ULSD production have been

developed and marketed by Akzo Nobel(KF 757, KF 848), Criterion

(Century and Centinel), Haldor-Topsoe(TK 573, TK 574), IFP, united

catalyst/Sud-Chemie, Advanced Refining, Exxon Mobil. Nippon Ketjen in

Japan and RIPP in China.

Improved level of desulphurisation can be obtained by using a more

active catalyst. Ther are both Co-Mo and Ni-Mo type of catalyst available

for the production of ULSD. But typically to reach ultra low sulphur levels

in the diesel product, a low space velocity and a high hydrogen partial

pressure will be needed if acceptable operating catalyst cycle is desired. Ni-

37

Mo catalyst will be a better choice for the production of ultra low sulphur

diesel at low space velocity and high hydrogen partial pressures. But for

existing units constrained by pressure and throughput, Co-Mo catalyst is a

better choice since it is better at high space velocities compared to Ni-Mo

catalyst.

In general, for low to moderate pressure and high-temperature

desulphurisation of distillate fuels, Co-Mo catalyst may be better than Ni-

Mo catalyst. For high-pressure and low-temperature conditions, Ni-Mo

catalyst performs better than Co-Mo catalyst.

The new generation catalyst exhibit 30-40% higher HDS activity

compared to previous generation catalyst (either Co-Mo or Ni-Mo). Use of

such high activity new generation catalyst will help in achieving ultra low

sulphur levels in existing units without shortening catalyst life.

About 15% more catalyst can be loaded using dense loading in

place of sock loading. This result in a 3-4oC reduction in start-of-run (SOR)

temperature for the same desulphurisation activity. Dense loading result in

a higher pressure-drop, which combined with a more uniform packing

normally obtained with dense loading will improve liquid distribution

within the catalyst bed.

38

PROCESS FLOWSHEET

39

FIGURE 5. HYDROTREATING PROCESS FLOW SHEET

40

PROCESS DESCRIPTION

41

6.PROCESS DESCRIPTION

The experiments have been carried out in a pilot plant that

was designed and assembled to perform hydrotreating operation . The

reactor of this pilot plant is a 500 cm3 vessel which can operate at

pressure of 90Kg/cm2 . the flow chart (demonstrates the schematic

process flow diagram (PFD) of this pilot plant . As it is shown in this

figure, the feed and hydrogen are mixed with a certain ratio before flowing

into the reactor. The mixed feed enters to the reactor with definite pressure

to achieve a predetermined temperature for performing the HDS reactions.

There are four thermocouples along the reactor for determining

the skin and the inside reactor temperature. The reactor product stream is

fed to a flash drum to separate gas and liquid products. The liquid product

is accumulated in a drum and the gas product is transferred for online

analysis to determine the H2S and H2 content of stream.

In addition to feed preparation, the catalyst should be presulfided

before using it with actual feed. Presulfiding of catalyst is done by

dimethyl disulfide (DMDS), which is added to the Isomax gas oil, and

circulated through the reactor bed.

This is performed to strengthen the activity and selectivity of

catalyst. To execute the procedure, an inert gas followed by a pure H2

stream is fed to the reactor. Then the reactor temperature is raised to a

certain value.

42

Presulfiding procedure was followed by a set of experiments which

was carried out to investigate the reproducibility of pilot plant setup and to

find out a suitable procedure for carrying out the main set of experiments.

The liquid samples were analyzed by Raney nickel method to

determine the total sulfur content. The results elucidated that for

improvement of reproducibility of experimental results, twelve hours

should pass to attain steady state of pilot plant system. By passing each six

hours the liquid sample is collected to analyze. The average total sulfur

content for each experiment is calculated by averaging the set of results

which are attained during the experiment. By passing one day the operating

condition can be changed to carry out another

experiment.

6.1.PROCESS OPTION FOR ULSD PRODUCTION

The main process option or types of improvements that may be

required to produce ULSD are listed and discussed below. Some of these

options can be readily implemented and may not have serious cost

implications while some other options require additional capital

investments. All these process options can improve product sulphur and

combination of them can be used to reduce sulphur significantly, but no

single option is adequate in producing ULSD.

43

Raising reactor temperature

This is obvious option to increase the desulphurisation activity. The

maximum reactor temperature is limited by design furnace outlet

temperature of the unit. Higher reactor temperature will also result in

shorter catalyst run lengths and severely restrict its usefulness.

Reducing throughputs

Reducing feed rates can decrease liquid hourly space velocities in

the reactor and thus increase the rates of hydrodesulphurisation and result

in lower product sulphur. The required throughput reduction is estimated to

be 8 times lower for the production of ULSD from a current level of 350

ppmw product sulphur making this option impractical.

Increasing hydrogen partial pressure

Increasing the hydrogen partial pressure in the reactor can reduce

the reactor SOR temperature and also reduces the rate of catalyst

deactivation. Hydrogen partial pressure can be increased by increasing the

purity of makeup hydrogen or purging the recycle gas and increasing its

purity.

Increasing the hydrogen purity by 10% of the recycle gas corresponds to

about 3oC decrease in average SOR temperature, and a 20-30% increase

cycle length. The effect of increasing hydrogen partial pressure by

increasing total pressure is less than by increasing hydrogen purity: a 10%

increase in hydrogen partial pressure corresponds to about 1oC decrease in

44

average reactor temperature. The reason for lower response is that H2S

partial pressure is also increased and total pressure is increased.

Increasing hydrogen sulphide partial pressure

Hydrogen sulphide strongly inhibits HDS reactions and its partial

pressure has great impact on hydrogen partial pressure. SOR temperature

should be raised to achieve the same HDS level when the recycle gas

contained large amount of hydrogen sulphide. This effect is lager at higher

total reactor pressure and more pronounced for Co-Mo catalysts than Ni-

Mo catalysts. Scrubbing the recycle gas to remove hydrogen sulphide will

decreased H2S partial pressure and increase hydrogen partial pressure. The

increased partial pressure of hydrogen can increase the catalyst life by

reducing coke formation.

Increasing hydrogen-to-oil ratio

Increasing hydrogen-to-oil ratio will reduce the inhibition effect of

H2S and ammonia and thereby increase the desulphurisation activity of

catalyst. This effect is rather small compared to the needs of achieving

ULSD. Increasing hydrogen-to-oil ratio by 50% may only gain 4-8% in

catalyst activity. Increasing the recycle gas-to-oil ratio also decreases the

reactor average hydrogen sulphide partial pressure and thus in turn

increases the apparent catalyst activity.

45

Adding reactor volumes

The catalyst volume can be increased either by adding new reactors

to existing units or by installing new dehydrosulphurisation units.

Generally doubling the catalyst volume results, in a 20oC decrease in

average temperature, if all other operating conditions are unchanged.

Increased catalyst volume decreases the deactivation rate of the

catalyst by reducing the start of run temperature in addition to the

availability of more deactivation temperature span. In addition, low LHSV

by itself reduces deactivation rate eve at the same SOR average reactor

temperature. Though this option requires new capital investment, it is the

best option in terms of good flexibility of feed stock and product quality.

Reactor internals

Reactor internals play a key role in improving the contact of the

reactant with the catalyst. Improper distribution of the reactants over the

catalyst will contribute to channelling through the catalyst bed, resulting in

an inefficient utilisation of the catalyst, development of hot spots and

premature catalyst deactivation due to coke formation. It was reported that

around one percent of bypassing of feed over catalyst an contribute 50 to

150 ppmw sulphur in the diesel product. Properly designed reactor internals

with good distribution of reactants over catalyst surface is necessary while

producing ultra low level of sulphur.

46

EXPERIMENTAL

DETAILS

47

7.EXPERIMENTAL DETAILS

Pilot plant studies were carried out to evaluate the performance of

DHDT catalyst.

The objective of the present work is to evaluate the DHDT catalyst

to study the effect of temperature on the performance of Diesel

hydrotreating catalyst.

The catalyst evaluation studies were carried out in a high pressure

catatest unit Procured from Vinci Technologies, France. The hydrotreating

reactions were carried out in co-current down flow mode of operation

without hydrogen recycle.

Feedstock Details

A straight run diesel sample collected from crude distillation

column was used in the pilot plant study. The characteristics of the straight

run diesel feed are presented in table 1.

Catalyst evaluated:

The DHDT catalyst is from procatalyse is NiO-MoO 3/Al2O3 type.

The characteristics of this catalyst are tabulated in table-2.

48

Table7.1.Characteristics of procatalyse hydrotreating catalyst.

Properties

Surface area , m2/gm 206

Pore volume ,ml/gm 0.55

Bulk density ,kg/L 0.737

Loss on ignition at 550o ,wt % 2.1

Single Grain Crushing Strength , DaN/mm

Average of 10 pellets

Max

Min

12.7

18.0

6.9

Bulk Crushing strength MPa 1.01

Ni O content ,wt % 4.2

Co O content ,wt % -

Mo O3 content ,wt % 18.0

49

Table 7.2.Properties of Diesel Products Hydrotreated in Pilot Plant

Reactor

Reactor pressure - 77.7 kg/cm2, Hydrogen / Oil Ratio – 150 lit/lit.

Properties Operating conditions

Reactor Temp,o C 310 320 330 340 350 360

LHSV , hr 0.7 0.7 0.7 0.7 0.7 0.7

Density @15o C

g/ml

0.8410 0.8392 0.8365 0.8345 0.8318 0.8289

API Gravity 36.8 37.1 37.7 38.1 38.7 39.3

Viscosity @ 40o

C ,cSt

4.47 4.34 4.32 4.21 4.19 4.13

Cetane Index 52.9 53.4 54.4 55.7 56.7 57.7

Sulfur , ppmw 254 137 88 67 55 42

Aniline Point, o C 76 76.5 78 79 81.5 83

ASTM D-86 Dist, Vol % ,o C

IBP 245 241 238 235 233 232

5 258 256 253 255 254 249

10 263 261 259 261 259 256

20 271 269 268 269 267 265

30 279 277 276 277 276 273

50 298 297 295 296 295 293

60 310 309 308 307 307 305

70 325 324 322 323 322 320

80 344 342 344 341 340 339

90 369 368 366 365 364 362

50

The experimental data obtained as a effect of temperature and

pressure, on the sulfur content of products hydrotreated over different

temperature is presented in the table.

The sulfur and nitrogen content of the hydrotreated products were

decreased due to the conversion of sulfur and nitrogen in the feed to

hydrogen sulfide and ammonia respectively. The aromatic content of the

products was found to decrease owing to the hydrogenation of aromatics to

naphthenes results in the decrease of density and specific gravity of

hydrotreated products. The decrease in density results in a higher API

gravity product. Saturation of olefins also increases the percentage of

saturates in the hydrotreated product. The increase in aniline point of the

hydrotreated products indicates that the product is highly paraffinic(more

saturated or less aromatic) than the feed. The decrease in aromatics reduces

the amount of carbonaceous residue in the product. The increase in aniline

point and API gravity results in a higher cetane number product , which in

turn indicates a lower ignition delay. The viscosity of the hydrotreated

products was also decreased which in turn relates to ease of starting of

diesel engine.

7.1.EFFECT OF TEMPERATURE

The effect of reactor temperature on product quality was studied

between 310o C and 360oc at 0.7 liquid hourly space velocity and

77.7 Kg/cm2.

The tables show the effect of reactor temperature on product

sulfur hydrotreated over catalyst NiO-MoO 3/Al2O3 at a hydrogen-to-oil

ratio of 150L/L.

51

With catalyst and 77.7kg/cm2 reactor pressure, the product with a

sulfur content of 254ppmw to 42 ppmw was obtained at a reactor

temperature of 360oC and a LHSV of 0.7h-1 indicating a conversion of

about 98.7 %. As expected ,highest conversion of sulfur compounds was

achieved at the severe most operating conditions employed.

From the table , it is shown that increase in temperature has

decreased the level of sulphur content in the feed. At a temperature of

310oC the sulfur content was 254 ppmw. As the temperature increases to

360oC sulphur content decreases to 42 ppmw.

It was found that the sulfur content of the product decreases with

increasing reactor temperature at constant LHSV. The effect of reactor

temperature on product sulfur was more pronounced at higher liquid hourly

space velocities and the sulfur conversion decrease at higher reactor

temperature.

7.2.EFFECT OF PRESSURE

The data show that the increase in the reactor pressure improved

product quality. The API gravity and cetane number of the hydrotreated

products increased and the sulfur and aromatic contents decreased with

increased reactor pressure. The rate of hydrodesulfurisation reaction is

faster at higher pressure. The reason for this effect of pressure is considered

to be that the higher the pressure better is the contact between hydrogen,

and hydrocarbons and the catalyst. In general, the effect is considerably

weaker at higher pressures i.e., sulfur conversion decreases with increasing

pressure. However, very high pressures reduce the activity of the catalyst,

owing to the adhesion of carbon to its surface.

52

At higher operating pressures, the effect of pressure on sulfur

conversion becomes insignificant. The partial pressure and the

concentration of hydrogen sulfide in the gas phase goes up as the

conversion of sulfur compounds increases. Beyond a certain operating

pressure, the concentration of hydrogen sulfide in the gas phase decreases

because of the increase in solubility of hydrogen sulfide in liquid phase.

In general, the hydrotreated products showed a decrease in density,

viscosity, sulfur, nitrogen and aromatic contents, and increase in API

gravity, aniline point, cetane number and saturates content when the

temperature is increased.

The polyaromatics content of the hydrotreated products was well

below 11wt% meeting Euro-III as well as Euro-IV specifications.

7.3.EFFECT OF ANILINE POINT

This is an approximate measure of the aromatic content of a

hydrocarbon fuel. It is defined as the lowest temperature at which a fuel oil

is completely miscible with an equal volume of aniline. Aniline is an

aromatic compound and aromatics are more miscible in aniline than are

paraffins. Hence, the lower the aniline point, the higher the aromatics

content in the fuel oil. The higher the aromatics content, the lower the

cetane number of the fuel. The aniline point can thus be used to indicate the

probable ignition behavior of a diesel fuel. aniline point increases from 76

to 83.

53

7.4.EFFECT OF IBP

According to American Society for Testing and Materials

petroleum- analysis distillation procedures, the recorded temperature when

the first drop of distilled vapor is liquefied and falls from the end of the

condenser.

The initial boiling point in the feed is actually higher than that of

the product. So when the initial boiling point is taken for 5 vol % it has a

certain higher temperature and finally if the temperature is noted for 90

vol % certainly it will have a temperature less than the 5 vol %.

7.5.OVERALL HDS KINETICS

The kinetics for sulfur removal from real feedstocks are complex

and depend on the distribution of sulfur compounds and the degree of

conversion. For industrial feedstocks, the structural differences between the

sulfur containing molecules make it impractical to produce a complex rate

equation to describe the HDS kinetics. Hence, in the present work the

following conventional power law kinetic expression was used (Juarez et

al.1999):

where S is the sulfur content in the feedstock (wt%), S the sulfur content in

the product (w%). n the apparent reaction order and k is the kinetic

constant.

54

The data at various space velocities, temperatures and product

sulfur contents were used for kinetic data analysis and to determine the

following apparent reaction orders and activation energies.

Diesel indx

The Diesel Index indicates the ignition quality of the fuel. It is

found to correlate, approximately, to the cetane number of commercial

fuels. It is obtained by the following equation

Diesel Index =

aniline po int (o F ) x Degrees API gravity (60 o F )100

Diesel Index and cetane number are usually about 50. Lower values will

result in smoky exhaust

Cetane improvement

Cetane number or CN is a measurement of the combustion quality

of diesel fuel during compression ignition. It is a significant expression of

the quality of a diesel fuel. Cetane has improved significantly . as the

temperature is increased the cetane number is increased 52.9 to 57.7.

Cetane improvers

55

These are compounds that readily decompose to give free radicals

and thus enhance the rate of chain initiation in diesel combustion. They

promote fast oxidation of fuels and thus improve their ignition

characteristics. Chemical compounds such as alkyl nitrates, ether nitrates,

dinitrates of polyethylene glycols and certain peroxides are well known

cetane improvers. In general, however, in view of their low cost and ease

of handling, most commercial significance has been attached to different

primary alkyl nitrates.

Cetane index

Cetane index is used as a substitute for the cetane number of diesel

fuel. The cetane index is calculated based on the fuel's density and

distillation range (ASTM D86). There are two methods used, ASTM D976

and D4737.

56

DESIGN OF FEED

VESSEL & SEPERATOR

OF A PILOT PLANT

57

8.1. DESIGN OF FEED VESSEL :

Design of feed vessel of hydro treater in a pilot plant

The feed vessel is a unfired pressure vessel . Pressure vessels used

in industry are leak-tight pressure containers, usually cylindrical or

spherical in shape, with different head configurations. They are usually

made from carbon or stainless steel and assembled by welding. Early

operation of pressure vessels and boilers resulted in numerous explosions,

causing loss of life and considerable property damage. In 1925 the

committee issued a set of rules for the design and construction of unfired

pressure vessels.

8.2.DESIGN CRITERIA

The Code design criteria consist of basic rules specifying the design

method, design load, allowable stress, acceptable material, and fabrication,

inspection certification requirements for vessel construction.The design

method known as "design by rule" uses design pressure, allowable stress,

and a design formula compatible with the geometry of the part to calculate

the minimum required thickness of the part. This procedure minimizes the

amount of analysis required to ensure that the vessel will not rupture or

undergo excessive distortion. In conjunction with specifying the vessel

thickness, the Code contains many construction details that must be

followed. Where vessels are subjected to complex loadings such as

58

cyclic, thermal, or localized loads, and where significant discontinuities

exist, the Code requires a more rigorous analysis to be performed. This

method is known as the "design by analysis" method.

8.3.SPECIFICATIONS

1. Here the operation is considered to be running for 12 hours .

2. Maximum liquid flow rate : 600 ml/hr.

(pumping capacity)

3. For a day, the flow rate is : 7.2 lt /day

4. Consider 30% excess for the N2 inert atmosphere.

5. So Z, we have to design the capacity of the vessel

approximately to be 10 litres.

6. Let the feed vessel considered to be cylindrical.

7. Volume = (Π/4) d2 L

8. Length of the cylindrical vessel = 320 mm

9. Diameter of the cylindrical vessel = 200m.

10. so, the volume = 10 litres

11. L/D ratio = 320/200 =1.6

MATERIAL OF CONSTRUCTION:

59

1. The material used here is SS316 .

2. It is stainless steel and the maximum allowable stress is

18100.

3. It can withstand temp of about 500 o C & pressure of about

150 Kg/cm2 .

TABLE8.4.Thickness calculation

DIMENSIONS UNIT VALUES

Design pressure Kg/cm2 3

Design temperature oC 150

Inside radius Mm 100

Maximum allowable stress PSI 18100

Joint efficiency (from table uw12) % 0.7

Circulated thickness (circular stress) Mm 3.42

Circulated thickness (longitudinal stress) Mm 1.71

Internal corrosion allowance Mm 0

External corrosion allowance Mm 1.25

Total thickness Mm 4.67

Desired thickness Mm 12

60

TABLE 8.5. Max Allowable stress in PSI

UPTO

100OF

UPTO

37.7OC

SS 316 SS 316L SS 304 MS

200 93.33 18800 14100 15700 11500

300 148.89 18400 12700 14100 11500

400 204.44 18100 11700 12900 11500

500 260.00 18000 10900 12100 11500

600 315.56 17000 10400 11400 11500

650 343.33 16700 10200 11200 11500

700 371.11 16300 10000 11100 11500

750 398.89 16100 9800 10800

800 426.67 15900 9600 10600

850 454.44 15700 9400 10400

900 482.22 15600 10200

950 510.00 15400 10000

8.6.CALCULATION OF CIRCULAR THICKNESS:

(i) (Circular stress)

61

((D.P *14.5) * (I.R/25.4) / ((M.A.S*J.E) - 0.6*D.P*14.5)) *25.4

((1 * 14.5) * (100/25.4) / ((18100*0.7) - 0.6*1*14.5))* 25.4 = 3.42

(ii) (Longitudinal stress):

((D.P*14.5)*(I.R/25.4) /((2*M.A.S*J.E) + (0.4*D.P*14.5))* 25.4

((1*14.5) * (100/25.4) / ((2*18100*0.7) +(0.4*1*14.5))*25.4 = 1.71

Total thickness :

(C.T.C+I.C.A+E.C.A)

(3.42+ 0 + 1.25 ) = 4.67

External pressure : 0

62

8.7.DESIGN OF GAS LIQUID SEPERATOR

Figure8.7. GAS LIQUID SEPERATOR

A vapor–liquid separator is a device used in several

industrial applications to separate a vapor–liquid mixture. For the common

variety, gravity is utilized in a vertical vessel to cause the liquid to settle to

the bottom of the vessel, where it is withdrawn. In low

gravity environments such as a space station, a common liquid separator

will not function because gravity is not usable as a separation mechanism.

In this case, centrifugal force needs to be utilized in a spinning centrifugal

separator to drive liquid towards the outer edge of the chamber for removal.

63

Gaseous components migrate towards the center. The gas outlet may itself

be surrounding by a spinning mesh screen or grating, so that any liquid that

does approach the outlet strikes the grating, is accelerated, and thrown

away from the outlet.

SPECIFICATIONS

1. Operation : Continous

2. Consider the seperator is occupied with 60% liquid & 40%

gas.

3. Volume : 1 lt

4. L/D ratio : 165/90 = 1.8

5. Length : 165 mm

6. Diameter : 90 mm

7. M.O.C : SS316.

8.8.Thickness calculation

DIMENSIONS UNIT VALUES

Design pressure Kg/cm2 77.7

Design temperature oC 400

Inside radius Mm 45

Maximum allowable stress PSI 18100

Joint efficiency (from table uw12) % 0.7

Circulated thickness (circular stress) Mm 1.6

Circulated thickness (longitudinal stress) Mm 1.6

Internal corrosion allowance Mm 0

64

External corrosion allowance Mm 1.25

Total thickness Mm 2.85

Desired thickness Mm 12

8.9.Calculation of circular thickness:

(i) (Circular stress)

((D.P *14.5) * (I.R/25.4) / ((M.A.S*J.E) 0.6*D.P*14.5))*25.4

(

(77.7 * 14.5) * (45/25.4) / ((18100*0.7) - 0.6*77.7*14.5))*25.4 = 1.6

(ii) (Longitudinal stress):

((D.P*14.5)*(I.R/25.4)/((2*M.A.S*J.E) + (0.4*D.P*14.5))*25.4

((77.7*14.5) * (45/25.4) / ((2*18100*0.7) +(0.4*77.7*14.5))*25.4 = 1.6

Total thickness :

(C.T.C+I.C.A+E.C.A)

(1.6+ 0 + 1.25 ) = 2.85

65

External pressure : 0

FIGURE 8.10 CETANE RESPONSE FOR VARIOUS CETANE

IMPROVERS

66

Scope of the project work

The scope of the project work includes the following:

1. To evaluate the performance of various activity new

generation hydrotreating catalyst in a pilot plant trickle-bed reactor

67

to determine the required severity level and different combination of

catalyst for the production of ultra low sulphur diesel.

2. To study the effect of reactor temperature and liquid hourly

space velocity on product quality at constant reactor pressure and

hydrogen-to-oil ratio using different hydrotreating catalysts.

Applications

1. Useful in the production of Ultra Low Sulfur Diesel

(ULSD) as per fuel quality regulations assigned by government.

2. We are able to minimize pollution in the environment by

reducing mainly sulfur & also other impurities present in the diesel

by hydro treating process.

68

RESULTS &

DISCUSSIONS

9.RESULTS & DISCUSSIONS

The hydrotreated products from pilot plant were characterized in

detail to study the extent Hydrodesulphurisation and cetane improvement.

69

1. Increase in temperature has increased the HDS activity as

evidenced by reduction in sulphur content progressively of 254

ppmw to 42ppmw.

2. Increase in temperature has beneficially increased the

important property namely Cetane Index also. The Cetane Index

increase observed was 4.5 to 10.5 units

CONCLUSIONS

The pilot plant studies help in concluding the demand less than

50ppm product sulphur can be achieved from high sulphur Raw Diesel

Sample. The Cetane improvement is remarkable from Diesel feed which

has high aromatic content.

APPENDIX

1) Determination of Density (ASTM D-4052)

The density, specific gravity and API volume (approximately

0.7ml) of diesel was introduced into an oscillating sample tube. The change

70

in oscillating frequency caused by the change in mass of the tube was

measured. Using the calibration data the density, specific gravity and API

gravity were determined.

2) Determination of Kinematic Viscosity (ASTM D-445)

Kinematic viscosity is the resistance to flow of fluid under gravity.

Kinematic Viscosity was determined at 40o C using Ubbelohde’s

Viscometer. Time was measured for a fixed volume of liquid to flow under

gravity through the capillary of a calibrated viscometer under a

reproducible driving head and at a closely controlled and known

temperature. The kinematic viscosity is the product of the measured flow

time and the calibration constant of the viscometer.

3) Determination of Pour Point (ASTM D-97)

The Pour Point is the lowest temperature, expressed as a multiple of

3o C at which the oil is observed to flow when cooled and examined under

prescribed conditions.

The Pour Point was determined using Pour Point apparatus. The

Pour Point is an index of the lowest temperature of its utility for certain

applications.

After preliminary heating, the sample was cooled at a specified rate

and examined at intervals of 3o C for its flow characteristics. The lowest

temperature at which movement of the sample occurred was observed and

recorded as the pour point.

71

4) Determination of Aniline Point (ASTM D-611)

Aniline Point is the minimum equilibrium solution temperature, in

degree Celsius, of a mixture of equal volumes of aniline and the sample

under test. The aniline point is used to estimate the aromatic content of

mixture.

Specified equal volumes of aniline and sample were placed in a

tube and mixed mechanically. The mixture was heated at a controlled rate

and the temperature at which two phases become miscible was noted. The

mixture was then cooled at a controlled rate and the temperature at which

two phases separate was recorded as the aniline point.

5) Determination of Flash Point (ASTM D-93)

Flash Pont is the minimum temperature at which the vapors from

test sample will give a momentary flash on application of a standard flame

under specific test conditions. The flash point is determined using Pensky-

Martens closed cup apparatus.

A brass test cup of specified dimensions, filled to the mark with test

sample and fitted with a cover of specified dimensions, was heated and the

sample stirred at specified rates. An ignition source was directed into the

test cup at regular intervals with simultaneous interruption of the stirring,

until a flash detected. The temperature at which flash was detected is

reported as flash point.

6) Determination of Ramsbottom Carbon Residue (ASTM D-524)

72

It is the amount of carbonaceous residue formed by evaporation and

thermal degradation (phyrolysis) of the sample.

The sample was weighed into a special glass bulb having a capillary

opening and placed in a metal furnace maintained at approximately 550o C.

The sample was quickly heated to the point at which all volatile matter is

evaporated out of the bulb with or without decomposition while the heavier

residue remaining in the bulb undergoes cracking and reactions. In the

latter portion of the heating period, the cole or carbon residue was

subjected to further decomposition or slight oxidation due to the possibility

of breathing air in to the bulb. After a specified heating period, the bulb

was removed from the bath, cooled in a dessicator, and the residue

remaining was again weighed and expressed as a percentage of the original

sample and reported as Ramsbottom Carbon Residue.

Provision is made for determining the proper operating

characteristics of the furnace with a controlled bulb containing a

thermocouple, which must give a specified time-temperature relationship.

7) Determination of Total Sulfur (ASTM D-2622)

The total sulfur content of diesel is determined by Energy

Dispersive S-ray Fluorescence spectrometry.

The diesel sample is placed in a beam emitted from the X-ray

source. The resultant excited characteristics X-radiation is measured and

the accumulated count is compared with counts from previously prepared

calibration samples to obtain the sulfur concentration in mass%.

8) Determination of Total Aromatics and Total Saturates (IP-391)

73

Diesel sample contains monoaromatic, diaromatic and polyaromatic

hydrocarbons.

A known mass of diesel sample was diluted in the mobile phase (n-

heptane) and a fixed volume of the solution was injected into a High

Performance Liquid Chromatography (HPLC) fitted with a polar column.

This column has little affinity for the non-aromatic hydrocarbons whilst

exhibiting a pronounced selectivity for aromatic hydrocarbons. As a result

of this selectivity the aromatic hydrocarbons are separated from the non-

aromatic hydrocarbons into distinct bands according to their ring structure

i.e., MAH, DAH, PAH compounds. At a pre-determined time, after the

elution of DAHs the column was back flused to elute the PHAs as a single

Sharp band.

The column was connected to a refractive index detector that

detects the components as they elute from the column. The electronic

signal from the detector was continually monitored by a data processor.

The amplitudes of the signal from the sample aromatics were compared

with those obtained from previously run calibration standards in order to

calculate % mass of MAHs, DAHs, PAHs in the diesel. The sum of MAHs,

DAHs and PAHs is reported as the total content of the aromatic contents in

the sample.

The Saturates content of the sample is obtained by subtracting the total

aromatics content from 100 percent.

9) Determination of Nitrogen (ASTM D-4629)

ANTEK analyzer as per ASTM D-4629 method determines the

Nitrogen content of the sample.

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10) Determination of ASTM Distillation Characteristics (ASTM D-

86)

The distillation characteristics of the sample were determined using

ASTM D-86 Distillation apparatus.

A 100 mL specimen of the sample was distilled under prescribed

conditions. The distillation was performed in a laboratory batch distillation

unit at ambient pressure under conditions that were designed to provide

approximately one theoretical plate fraction. Systematic observations of

temperature readings and volumes of condensate were made. The volume

of the residue and the losses were also recorded.

At the conclusion of the distillation the observed vapor

temperatures can be corrected for barometric pressure and the data were

examined for conformance to procedural requirements, such as distillation

rates. The test was repeated if any specified conditions have not been met.

Test results are commonly expressed as percentage recovered and

corresponding temperature in a table.

75

REFERENCES

References

1. Babich I.V. and Moulijn J.A. (2002), ‘ science and technology of

novel processes for deep sulfurisation of oil refinery streams : a review’,

fuel, vol.82,pp.607-631.

76

2. Bhaskar M., Valavarasu G., and Balaraman K.S (2000), ‘deep

desulfurization of a diesel blend in a pilot plant trickle bed reactor ’,

Petroleum Science and Technology, vol.18, No.7 & 8, pp.851-869.

3. Bhaskar M., Valavarasu G., Selvavathi V. and Sairam B.(2004),

‘Production of ultra Low Sulfur Diesel-Catalyst and process options’, XII

Refinery Technology Meet, Goa, India, September23-25,2004;pp.474-486.

4. Bhaskar M., Valavarasu G., Sairam B., Balaraman K.S and Balu

K.(2004),

‘Three Phase Reactor Model to Simulate the performance of pilot-plant

Trickle –Bed Reactors Sustaining Hydrotreating Reactions’, Ind.

Eng.Chem

Res., Vol.43,pp.6654-6669.

5. Chunshan Song (2003), ‘An overview of new approaches to deep

desulfurization for ultra-clean gasoline, diesel fuel and jet fuel’, Catalysis

today , Vol.86,pp.211-263.

6. Lamourelle A.P and Nelson D.E.(2001), ‘ Ultra low aromatic

diesel’, Petroleum Technology Quarterly , june 2001.

7. Perry’s chemical engineers’ handbook. — 7th ed. /

prepared by a staff of specialists under the editorial

direction of late editor Robert H.

Perry : editor, Don W. Green : associate editor, James

O’Hara Maloney.

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8. Salvatore T.JR. and Gunter P.M (2004), ‘ Fundamentals of ULSD

Production’, Petroleum Technology Quarterly – Summer 2004,pp.29-35.

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