ABSTRACT

The integration of an in-situation separation function within the reactor holds the promise of

increased conversion, higher selectivity and reduced capital investment. The introduction of an

in-situation separation function within the reaction zone leads to complex interactions between

vapour liquid equilibrium, vapour liquid mass transfer, intra-catalyst dilution (for

heterogeneously catalysed processes) and chemical kinetics. Such interactions have been shown

to lead to the phenomenon of multiple steady-states and complex dynamics, which have been

verified in experimental laboratory and pilot plant units. In this paper we have discussed the

Dynamics and Kinetics of four major separation processes namely Distillation; Extraction;

Absorption and Adsorption. Multi-functional reactor is discussed which is often used to embrace

reactive separations technology, which promises reduction in capital costs, increased conversion

and reduced by-product formation.

The design and operation issues for reactive separation systems are considerably more complex

than those involved for either conventional reactors or conventional separation columns. In

heterogeneous reactive separation the problem is exacerbated by the fact that these transfer

processes occur at length scales varying from 1 nm (pore diameter in micro gels) to say a few

meters (column dimensions).The time scales vary from 1 ms (dilution within gels) to say a few

hours (column dynamics). The phenomena at different scales interact with each other. Such

interactions, along with the strong nonlinearities introduced by the coupling between dilution and

chemical kinetics in counter-current contacting, have been shown to lead to the phenomenon of

multiple steady-states and complex dynamics, which have been verified in experimental

laboratory and pilot plant units.

The main underlying principle behind the application of reactive distillation for selectivity

enhancement is to facilitate the separation of selected components and favorably manipulate the

composition profiles in the reactive zone to get desired reaction. Reactive distillation today is a

well established operation that combines reaction and distillation in a single stage which offers

enhancement in the overall performance of the process. While there has been substantial

experimental & theoretical work reported on equilibrium related reactions, its practical

applications and its usage in selectivity engineering has not been well established. The basic

principle behind the application of this process for selectivity engineering is that one can

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favorably manipulate the composition profiles with the aid of distillation attributes to expedite

the desired reaction. A need for such progress has been emphasized here.

Survey has been done on the recent developments in reactive separation technology,

emphasising the available alternatives and pointing out obstacles in the way of successful

implementation of this technology.[1]

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Chapter 1: Introduction To Reactive Separation Technology

The traditional flow sheet of a chemical process consists of a reactor followed by a separation

unit to remove the unconverted reactants from the desired product and recycle these to the

reactor. This is done to maximize the conversion of reactants and improve selectivity to the

desired product, thereby reducing the costs associated with the separation step.

Strategies for arriving at the “ideal” reactor configuration have been discussed in the literature.

In recent years there has been considerable academic and industrial interest in the area of

reactive separations wherein the separation function is integrated within the reactor; a variety of

separation principles and concepts can be incorporated into the reactor. The term multi-

functional reactor is often used to embrace reactive separations technology, which promises

reduction in capital costs, increased conversion and reduced by-product formation .

When chemical reactions and physical separations have some overlapping operating conditions

the combination of these tasks in a single process unit can offer significant benefits. These

benefits could involve: avoidance of reaction equilibrium restrictions, higher conversion,

selectivity and yield, removal of side reactions and recycling streams, circumvention of non-

reactive azeotropes and last but not least reduction of number of units (investment cost) and

energy demands (heat integration).

Nowadays, the focus of the chemical and process industry has shifted towards the development

and application of integrated processes. This trend is motivated by benefits such as a reduction in

equipment and plant size and improvement of process efficiency and safety, and hence a better

process economy. Reactive distillation and Reactive Absorption is an important example of a

reactive separation process. Especially for equilibrium reactions like esterifications, ester

hydrolysis and etherification, the combination of reaction and separation within one zone is a

well-known alternative to conventional processes with sequential reaction and separation steps .

Chemical manufacturing companies produce materials based on chemical reactions between

selected feed stocks. In many cases the completion of the chemical reactions is limited by the

equilibrium between feed and product. The process must then include the separation of this

equilibrium mixture and recycling of the reactants. Usually reaction and separation stages are

carried out in discrete equipment units, and thus equipment and energy costs are added up from

these major steps. In recent decades, a combination of separation and reaction inside a single unit

has become more and more popular. This combination has been recognised by the chemical

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process industries for having favourable economics of carrying out reaction simultaneously with

separation for certain classes of reacting systems, and many new processes (called reactive

separations) have been invented based on this technology. The most important examples of

reactive separation processes (RSP) are reactive distillation (RD) and reactive absorption (RA.

Chemical process industries have shown increasing interest in the development of reactive

separation processes (RSP) combining reaction and separation mechanisms into a single,

integrated unit. Such processes bring several important advantages among which are increase of

reaction yield and selectivity, overcoming thermodynamic restrictions, e.g. azeotropes, and

considerable reduction in energy, water and solvent consumption. Important examples of

reactive separations are reactive distillation (RD) and reactive absorption (RA). Due to strong

interactions of chemical reaction and heat and mass transfer, the process behavior of RSP tends

to be quite complex.

When considering in-situ separation of product, it is important to stress that often removing only

one of the products of the reaction is sufficient to drive the equilibrium to the right or prevent

unwanted side reactions. There is usually a choice with respect to the product to separate from

the reaction zone.

Fig 1: (a) Conventional 5xed-bed reactor train, with inter-stage sulphur removal by condensation,

for Claus process. (b) Fixed bed reactor, with in-situ sorption of water by zeolite adsorbent.

Adapted from Agar (1999).[1]

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Fig 2:Various in-situ separation function integrated into the reactor[1]

5

Chapter 2: Introduction To Reactive Absorption Process

Many present-day commercial gas absorption processes involve systems in which chemical

reactions take place in the liquid phase. These reactions generally enhance the rate of absorption

and increase the capacity of the liquid solution to dissolve the solute, when compared with

physical absorption systems. A necessary prerequisite to understanding the subject of absorption

with chemical reaction is the development of a thorough understanding of the principles involved

in physical absorption. Reactive Absorption (RA) represents a process in which a selective

solution of gaseous species by a liquid solvent phase is combined with chemical reactions.

In RA reactions occur simultaneously with the component transport and absorptive separation.

These processes are predominantly used for the production of basic chemicals, e.g. sulphuric or

nitric acids and the removal of components from gas and liquid streams. This can be either the

cleanup of process gas streams or the removal of toxic or harmful substances in flue gases.

Absorbers or scrubbers where RA is performed are often considered as gas liquid reactors. If

more attention is paid to the mass transport, these apparatuses are rather treated as absorption

units .

The typical flow sheet of a RA process for gas cleaning includes an absorption column to

perform the removal of the compounds from the feed gas. The outlet gas leaves the column

nearly free of unwanted compounds (clean gas).Reactive Absorption is an important industrial

operation for production of some basic chemicals and for the removal of harmful substances

from gas streams. In recent decades, this process has become especially important for the

purification of gases to high purities. Reactive absorption is able to provide high throughput at

moderate partial pressures and without requiring large amounts of solvent. Most RA processes

are steady-state operations involving reactions in the liquid phase, although some applications

involve both liquid-phase and gas-phase reactions.

In reactive absorption, a fluid-fluid reaction takes place between a gas phase and a liquid phase.

At the same time, mass transfer from the gas to the liquid phase is also occurring. To be able to

completely understand reactive absorption, one must first have some understanding of the

kinetics of fluid-fluid reactions. RA is a complex rate-controlled process that occurs far from

thermodynamic equilibrium. Therefore, the equilibrium concept is often insufficient to describe

it, and instead accurate and reliable models involving the process kinetics are required. The

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effectiveness of online model based applications, such as process control and optimization,

depends strongly on the quality of the available model predictions. When considering in-situ

separation of product, it is important to stress that often removing only one of the products of the

reaction is sufficient to drive the equilibrium to the right or prevent unwanted side reactions.

There is usually a choice with respect to the product to separate from the reaction zone

Compared to reactive distillation, the absence of a reboiler and a condenser makes reactive

absorption a simpler process. However, the drawback is the small number of degrees of freedom

that makes it difficult to set the reactants feed ratio correctly and consequently to avoid

impurities in the products. Reactive absorption offers indeed significant advantages such as

minimal capital investment and operating costs, as well as no catalyst-related waste streams and

no soap formation. However, the controllability of the process is just as important as the capital

and operating savings. Therefore, it is important to note that reactive absorption has less degrees

of freedom and therefore more difficult to control than reactive distillation. Reactive Absorption

is usually dominated by the mass transport kinetics. Besides, in reactive processes, chemical

reactions must be taken into account.

Mass transfer in RA is explained using the two-film theory. The boundary between the gas phase

and the liquid phase is presumed to consist of a gas film adjacent to a liquid film. Flow in both of

these films is assumed to be laminar or stagnant. The main-body gas phase and liquid phase are

assumed to be completely mixed in turbulent flow so that no concentration gradient exists in the

main body of either phase. The solute concentration in the gas film at the interface is assumed to

be in equilibrium with the solute concentration in the liquid film at the interface. There is a solute

concentration gradient across both the gas film and the liquid film.

2.1 Characteristics Of Reactive Absorption.

There is no sharp dividing line between pure physical absorption and absorption controlled by

the rate of a chemical reaction. Most cases fall in an intermediate range in which the rate of

absorption is limited both by the resistance to diffusion and by the finite velocity of the reaction.

Consider an absorber with a reaction occurring in the liquid phase. Since the reaction is in the

liquid phase, the gas-phase rate coefficient KG is not affected. If the reaction is extremely fast

and irreversible, the rate of absorption may be governed completely by the resistance to diffusion

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in the gas phase. Therefore the absorption rate can be estimated by knowing the gas-phase rate

coefficient KG. The liquid-phase rate coefficient K6

+L is strongly affected by fast chemical reactions and generally increased with increasing

reaction rate.

The highest possible absorption rates will occur under conditions in which the liquid phase

resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. This

condition can be attained if KL is very large. Frequently, even though reaction consumes the

solute as it is dissolving, thereby enhancing both the mass transfer coefficient and the driving

force for absorption, the reaction rate is slow enough that the liquid-phase resistance must be

taken into account.

This may be due to an insufficient supply of a second reagent or to an inherently slow chemical

reaction. What this all boils down to is that the liquid-phase rate coefficient KL in the presence

of a chemical reaction normally is larger than the value found when only physical absorption

occurs. To account for the effects of chemical reaction, the liquid enhancement factor, E is

introduced.

E (rate of take up of solute for straight mass transfer)

(rate of takeup of solute whenreaction occurs) = k LkLo

< 1

Some of the fundamental characteristics of reactive absorption process are

shown below

RA represents a process in which a selective solution of gaseous species by a liquid

solvent phase is combined with chemical reactions.

As compared with purely physical absorption, RA does not necessarily require elevated

pressure and high solubility of absorbed components because of the chemical reaction ,

the equilibrium state can be shifted favorably resulting in enhanced solution capacity.

Most of the RA processes involve reactions in the liquid phase only, in some of them

both liquid and gas reactions occur.

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Usually the effect of chemical reactions in RA processes is advantageous only in the

region of low gas-phase concentrations due to limitations by the reaction stoichiometry or

equilibrium.

Most of the RA processes are steady-state operations, either homogeneously catalyzed or

auto-catalyzed.

2.2 Process Operation Of Reactive Absorption

The operation of reactive absorption is governed by complex rate-controlled parameters. Also

there are many constraints while using the combination of reaction and absorption operation

simultaneously. The general constraints are of Pressure; Temperature; Flow rate; Finding an

optimum zone feasible for both operations to occur simultaneously.

Pressure Constraints

Reactive absorption is best run at high pressures. The high pressure will force more gas into the

liquid phase and consequently a better reaction conversion will be attained. However, to operate

at high pressures requires the use of a compressor, which is usually the most expensive unit in a

plant design.

Temperature Considerations

In physical absorption it is best to operate at low temperatures to promote the gas to go to the

liquid phase and to avoid solvent losses to the vapor phase. In reactive absorption one would

want to operate at low enough temperatures to allow for absorption of the gas to the liquid, but if

the kinetics governing the reaction is not favorable at this temperature we have to weigh the

possibilities of operating at a higher. The burden is on the engineer to choose a solvent that reacts

with the solute at low enough temperatures.

Finding A Suitable Flowrate

Flooding issues, like those in physical absorption dominate this topic. We obviously want to

operate below the flooding velocity of the gas. However, in chemical absorption if the reaction

is essentially irreversible and the equilibrium partial pressure of the solute is zero, then the

countercurrent and co-current absorber require the same number of stages (McCabe). This is

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advantageous because co-current operation eliminates the flooding concerns. We can have very

high flow rates for both the gas and solvent streams and still attain very good separations.

2.3 Advantages And Disadvantages Of Reactive Absorption

Reactive absorption is typically used when physical absorption is not able to attain the proper

separation desired. Here are some pros and cons with reactive absorption:

2.3.1 ADVANTAGES OF USING REACTIVE ABSORPTION

1. Absence of the condenser and the reboiler compared to RD.

2. In terms of energy, heating and cooling requirements are about 30%.

3. Medium pressure steam can be used for the RA process as the operating temperatures are

lower than for the RD process.

4. Efficient use of raw materials: down to stoichiometric reactants ratio, high conversion as

equilibrium is shifted towards completion and no products are recycled as reflux or boil-

up vapours, and no thermal degradation of products due to the low temperature profile.

5. Effective use of the reactor volume leading to significantly high unit productivity, up to

6–10 higher than conventional processes.

6. Reduced CapEx and OpEx due to the integrated design with no reboiler or condenser.

7. Compared to similar reactive processes, about 20% savings on the total capital

investment and 30% less operating costs are possible.

8. A reaction in the liquid phase reduces the equilibrium partial pressure of the solute over

the solution, which greatly increases the driving force for mass transfer

9. Absorption plus reaction increases the mass transfer coefficient by introducing a greater

effective interfacial area since absorption can now take place in the nearly stagnant

regions as well as in the liquid holdup

10. Chemical absorption usually has a much more favorable equilibrium relationship than

physical absorption (solubility of most gases is very low).

2.3.2 DISADVANTAGES OF USING REACTIVE ABSORPTION

1. The Murphee plate efficiency is often quite low (10% is not unusual)

2. The suitable solvent stream may not be available in the current process or the solvent

stream may be very expensive

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3. Heat of reaction introduces non-isothermal conditions which may require cooling coils

(increase in capital costs)

4. Additional costly separation unit may be required.

5. RA is a complex rate-controlled process that occurs far from thermodynamic equilibrium.

Therefore, the equilibrium concept is often insufficient to describe it, and instead accurate

and reliable models involving the process kinetics are required.

6. Reactive Absorption has less degree’s of freedom which makes it difficult to set the

reactants feed ratio correctly and consequently avoid impurities in the products.

Therefore it is more difficult to control.

7. Further difficulties of RA applications may be caused by the reaction heat through

exothermic reactions and by relatively difficult solvent regeneration.

2.4 Application of Reactive absorption

The typical flow sheet of a RA process for gas cleaning includes an absorption column to

perform the removal of the compounds from the feed gas. The outlet gas leaves the column

nearly free of unwanted compounds (clean gas). This step is followed by a stripping column, in

which the solvent is recovered as shown in figure below. The industrial RA applications are

subdivided according to the captured components. In some cases, CO2 is the target component,

as in the fossil fuel combustion. In other processes, it is necessary to capture H2S selectively (e.g.

in the tail gas treating). However, in the most applications, both CO2 and H2S are captured along

with some other components, e.g. COS, HCN, CS2.

Other industrial applications include:-

i. CO2 Removal.

ii. H2S Removal.

iii. Removal of CO2 and/or H2S.

iv. Removal of CO2–H2S mixtures and other impurities.

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Chapter 3: Introduction To Reactive Distillation Process

RD, the combination of chemical reaction and distillation in a single column, is one of the most

important industrial applications of the multifunctional reactor concept. Recently, it has drawn

considerable attention because of its advantages, especially for equilibrium-limited reactions.

Several reviews have been published in the last decades which give an excellent introduction and

overview of RD

Reactive distillation is one of the most promising technologies of the integrated reaction-

separation concept because its design is simple and can increase the efficiency and profitability

of a process .

In RD, reaction and distillation take place within the same zone of a distillation column.

Reactants are converted to products with simultaneous separation of the products and recycle of

unused reactants. The RD process can be efficient both in size and cost of capital equipment as

well as in energy used to achieve complete conversion of reactants. Since reactor costs are often

less than 10% of the capital investment the combination of a relatively cheap reactor with a

distillation column offers great potential for overall savings. Among suitable RD processes are

etherifications, nitrations, esterifications, transesterifications, condensations and alcylations [12].

Conventional processes use one or more liquid-phase reactors with large excess of one reactant

in order to achieve high conversions of the other. This process requires a large capital

investment, high energy costs and a large inventory of solvents. In RD process the entire process

is carried out in a single column. The RD column represents an entire chemical plant and costs

one-fifth of the capital investment of the conventional process and consumes only one-fifth of

the energy

The successful commercialization of RD technology requires special attention to hardware

design that does not correspond to those for conventional (non-reactive) distillation.

The petroleum industry is a potential candidate for the use of RD technology. Co-current gas–

liquid downflow trickle bed reactors are widely applied for hydrodesulphurization (HDS) of

gasoil and heavier oils as shown in figure (a) below. The counter-current reactor shown in figure

(b) is essentially an RD column wherein the H2S is stripped from the liquid phase at the bottom

and carried to the top. The major bottleneck to the implementation of RD technology for counter-

current hydroprocessing in commercial practice relates to hardware limitations. There is a need

to develop improved hardware configurations that allow counter-current contacting of gas and

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liquid, in the presence of 1–1:5 mm catalyst particles, wherein the catalyst loadings are in the

region of 50–60%.

Figure 3:- Conventional flowsheet of a process consisting of a

reactor followed by a separation unit.

The successful commercialization of RD technology requires special attention to hardware

design that does not correspond to those for conventional (non-reactive) distillation.

The petroleum industry is a potential candidate for the use of RD technology. Co-current gas–

liquid downflow trickle bed reactors are widely applied for hydrodesulphurization (HDS) of

gasoil and heavier oils as shown in figure (a) below. The counter-current reactor shown in figure

(b) is essentially an RD column wherein the H2S is stripped from the liquid phase at the bottom

and carried to the top . The major bottleneck to the implementation of RD technology for

counter-current hydroprocessing in commercial practice relates to hardware limitations.

Figure 4: comparison between conventional and reactive methods

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3.1 Process Operation Of Reactive Distillation

• Improved conversion: By removing products from the reactive section, the chemical

equilibrium of equilibrium-limited reactions is shifted toward the product side, leading to

improved conversions.

• Circumventing/overcoming of azeotropes. For chemical systems that tend to form

azeotropes, reactive distillation avoids azeotropic mixtures by “reacting away”

participating components.

• Reduced side-product formation. Consecutive reactions are reduced by removing

products from the liquid reaction phase, thereby maintaining low product concentrations

in the reaction zone/phase.

• Direct heat integration and avoidance of hot-spots. For exothermic reactions, the heat of

the reaction can be directly used to evaporate components, reducing the amount of total

heat required and avoiding the occurrence of hot-spots.

• Capital savings: Removal of components due to reactions resulting in the simplified

downstream processing of reactants and products

• Decreased catalyst amount: Reduced amount of catalyst for a comparable conversion of

the reactants

3.2 Characteristics Of Reactive Absorption.

Reactive distillation is the integration of reaction and distillation within the same compartment of

space and time of one column. Several key factors ensure the successful application of this

technology in terms of lower operating and capital costs. Specifically, the process should

improve the conversion and selectivity of the reaction by removing products from the reactive

section and circumvent/overcome distillative separation boundaries, such as azeotropes .

Reactive distillation, in which chemical reaction and separation take place in one unit, offers

several advantages over conventional reactor–separator configurations, such as higher

conversion in equilibrium limited reversible reaction, a more profitable product distribution of

multiple reaction system, breaking of distillation azeotropes, significant capital saving, and so

14

on.

But even after several advantages it is not used in industries because of several reasons as listed

below:

1. Complexity resulting from non-ideality of components

2. Intricate hydrodynamics

3. Relying on empirical correlations for design and control

4. Difficulty in finding common operation range for T&P for both reaction and separation

5. Proper Boiling Point sequence

6. Optimum residence time

3.3 Advantages And Disadvantages Of Reactive Distillation

RD processes offer several advantages such as :-

i. Increased yield due to overcoming of chemical and thermodynamic equilibrium

limitations;

ii. Increased selectivity through suppression of undesired consecutive reactions;

iii. Reduced energy consumption via direct heat integra-tion in case of exothermic reactions;

iv. Avoidance of hot spots by simultaneous liquid evaporation;.

v. a reduction of the investment,vi. a simpler process,vii. the use of the heat of reaction (if present) in situ,viii. ease of control of the reaction temperature (evaporating system),

andix. the possibility of overcoming azeotropes.

These advantages result in reduced capital investment and operating costs.

However, our cognition to the reactive distillation process is far from enough due to its

complexity resulting from the non ideality of components and the interaction between them, and

the intricate hydrodynamics and transport phenomena in the column. Until now, the design,

control and simulation of the reactive distillation highly rely on empirical correlations for lack of

comprehensive understanding to the process .

The application of RD is somewhat limited by constraints, like:-

i. Common operation range for temperature and pressure for distillation and reaction.

ii. Proper boiling point sequence: the key component should be a top or a bottom product,

undesired side or consecutive products should be medium boiling components.

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iii. Difficulties in providing proper residence time characteristics.

3.4 APPLICATIONS:

Biotechnologies

Membrane-assisted Distillation

Organic Chemistry

Extension of Existing application areas.

Homogeneous catalysis

Bio-based processing

Retrofit

Flexible and multi-functional plants

Chap 4: Reactive extraction

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A Reactive Liquid Extraction process is a liquid-liquid extraction process that is intensified

through a mechanism involving a reversible reaction between the extracted chemical species and

a host chemical species constituting, or present in, the extractant. Since last few decades, there

has been a revitalization of attention towards new energy-efficient fermentation technology for

large production of fermentation chemicals due to sharp increase in petroleum costs.

Carboxylic acid has wide applications in various chemical, food, and pharmaceutical industries.

A growing demand for biodegradable polymers, substitutes for both conventional plastic

materials and new materials of specific uses such as controlled drug delivery or artificial

prostheses, draws attention the need for improvement of conventional processes for carboxylic

acids production. Reactive extraction is a multifunctional reactor having reaction and extraction

in a single unit. This intensified approach has the closed loop process with sustainability.

As per the recent trend of chemical industry, attention towards the production of fermentation

based chemicals has been increased due sharp increase in petroleum cost. It is the need of the

industry to develop the new energy efficient recovery process or substantial improvements in the

existing recovery technology. Carboxylic acids (Lactic, propionic, caproic, lactic, picolinic etc.)

can be produced by fermentation and are widely used in pharmaceutical, food and other allied

industries. Due to some specific applications it draws attention towards the development of new

recovery processes for carboxylic acids production. Many separation processes are available like,

stripping, adsorption, elctrodialysis, liquid-liquid extraction, pertraction, pervaporation, and

membrane solvent extraction. All these processes have their own advantages and disadvantages.

Reactive extraction is an alternative for the conventional process.

The basic philosophy of the process intensification methods is to choose the task in a manner

such that their combination leads to better overall performance. Since any chemical process

involves unit operations for reaction and separation, most of such task combinations fall under

the umbrella of reactive separation processes. The combination of reaction and separation is

effective when either the reaction substantially improves separation through enhanced mass

transfer rates or the separation drives the reaction to higher conversions or both. The fusion of

reaction and separation as one combined operation is also prized for its simplicity and novelty.

These operations are also coveted for the investment and operating cost savings garnered on

successful scale up to commercial operations.In recent years, reactive extraction processes are

gaining lot of importance in response to extreme economic pressure posed by industries as the

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result of emergence of new processes and decline of existing ones, demand of high purity

products with low cost and are environmentally safe . However, commercialization of reactive

separation processes is desired, which can be achieved by the mutual working of chemists and

engineers. Reactive extraction links chemical sources and sink to enhance reaction rates,

conversions and selectivity. Since most of chemical processes are equilibrium driven, removal of

product as soon as it is produced would lead to enhanced reaction rates, increased feed

conversions, reduce reaction severity and provide operation under milder conditions. Mass

transfer and reaction coupling improves catalyst life, since, high mass transfer forces lead to

better catalyst irrigation and surface renewals with transport of catalyst inhibitors away from

catalyst surfaces. Further, in reaction and separation operations, the duo would lead to high local

driving forces for separation, leading to reduction in equipment size, elimination of recyclable

streams and reductions in utility costs. Reactive separators also lead to safer equipments since it

reduces the working inventory of reactive chemicals in the equipment. Lower the hazardous

chemical, lower will be the chance of its leakage, spills and environmental release. Coupling of

reaction and separation also leads to suppression of byproduct reactions which are likely to

exhibit runaway behavior and the reactive separator design will increase the inherent safety in

the unit against severe process upsets. The combination also provides low cost equipment

through the consolidation of multiple pieces of process equipment into single piece and/or

through the elimination of process recycles streams

Chap 5:Case Studies

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5.1 Reactive distillation:

a. Hybrid Separation Processes – Combination of Reactive Distillation with Membrane

Separation

5.1.1 Summary

The analysis of hybrid separation processes combining membrane separation with conventional

distillation is described in (Kreis and Górak, 2006). In this experiment the combination of

reactive distillation along with reactive absorption is described. The reaction is as follows

C3H8O +C3H6O2 ⇔C6H12O2 + H2O

The esterification reaction is reversible; the equilibrium constant is a weak function of

temperature. The chemical system shows complex thermodynamic phase behavior with several

binary and ternary azeotropes as well as miscibility gaps. Boiling points of the pure components

and the azeotropic data of the system are shown in Table 1.

Table 1: Boiling points of pure components and azeotropic data at 1 atm

Due to two binary homogeneous azeotropes (1-propanol–water, propionic acid–water), one

binary heterogeneous (n-propyl propionate–water) as well as one ternary heterogeneous (n-

propyl propionate–1-propanol–water) temperature minimum azeotrope, a conventional process

requires a complex separation train downstream of the reactor.

One possible process alternative for n-propyl propionate synthesis in one apparatus is the

removal of the desired product (ProPro) at the bottom of the reactive distillation column while at

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the top, an almost azeotropic aqueous-organic mixture (POH/H2O) is obtained. A hydrophilic

membrane unit is located in the distillate stream to remove water out of the process. The water

depleted retentate is recycled back to the column. The coupling of the reactive distillation

column with a membrane module results in a hybrid process (Figure).

Fig 5: Reactive distillation column with a membrane separation located in the distillate stream.

Identification of the key aspects for the proper description of reactive

distillation

processes is as follows:

• The description of thermodynamic and physical properties of the

multicomponent System,

• The use of accurate reaction kinetics

• The hydraulic characteristics of the column internals in terms of

correlations for Hold-up and pressure drop and the validation of the

proposed process model with reliable experimental data.

The characteristics of the pilot plant are summarised in Table .

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Table 2: Reactive distillation - pilot plant characteristics

It should be emphasized that no side product (di-n-propyl ether, DPE) was detected by gas

chromatography and no parameter fitting is carried out. Due to an excess of 1-propanol and a

sufficient amount of catalyst, the propionic acid is nearly completely converted in the reactive

section N-propyl propionate leaves the column as a bottom product beside non-converted

propionic acid. At the top of the column, an almost binary mixture consisting of 1-propanol and

water near to azeotropic composition with a small amount of n-propyl propionate is obtained.

The simulation results are displayed with continuous lines. Both the composition profiles and the

temperature profile are predicted with high accuracy.

Figure 6: Liquid phase molar fraction (left) and temperature profile of the vapour phase (right.

The solid lines represent the simulation results, the symbols the experimental values

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Figure7: Pilot-scale experimental setup for vapour permeation.[8]

Feed, retentate and permeate samples are analysed with the aid of gas

chromatography to determine the mass fraction of the organic component.

The experimental data and operating conditions are summarised in Table.

Table 5: Experimental data for the operating conditions of the vapour

permeation experiments.

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It can be seen that the separation characteristics show a strong dependency on the feed

concentration

5.3Conclusion

The theoretical and experimental investigation shows that the heterogeneously catalysed

synthesis of n-propyl propionate via a hybrid process consisting of reactive distillation and

vapour permeation is feasible The analysis of the hybrid separation process shows high

complexity in comparison to the stand-alone unit-operations, reactive distillation and vapor

permeation. Thus, the presented validated process models for both units are linked together to

enable a theoretical analysis of the hybrid separation process. Both the influence of decisive

operational parameters, reboiler heat duty and reflux ratio, on the process performance in terms

of product purity and acid conversion and the effect of a variation of the recycle purity are

analyzed [8]

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5.2 Reactive absorption

a. Design of industrial reactive absorption processes in sour gas treatment

5.2.1 Summary

In the industry design of gas purification plants is still often done using empirical methods

and simple spreadsheet calculations. Thus, finding an efficient design for the units and

optimal operating parameters for the entire process is not straightforward. A literature review

revealed that in the past investigations focussed on absorption of CO2 in highly concentrated

solutions. Benson characterized the CO2 absorption with high potassium carbonate

concentrations (Benson et al., 1954). A rate-based model for the Benson Process based on

effective interfacial area to calculate the mass transfer was also developed by Cents .

Electrolyte System

The given reactive system of weak and strong electrolytes is described by considering the

following reactions with the dissociation constants Kj.

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Its contribution to the overall rate of reaction is therefore small and can be neglected .To

calculate the concentrations of molecular and ionic species in solution the chemical equilibria

are expressed with their dissociation constants Kj.

where ai is the activity of component i, ni,j is the stoichiometrical coefficient of component i

in reaction j and gi is the activity coefficient taking non-idealities into account.

Fig 8:One segment of the rate-based model including chemical reactions.

Since absorption behaviour in the process is mainly governed by the interplay of CO2 and H2S

with the alkaline potash solution, these components are considered in the validation of the partial

pressures. Concentrations of NH3 are considerably low in the entire process. Off-gas

specifications for HCN are usually met before the specifications for H2S are met. Indeed, for

higher concentrations of CO2 and H2S the accuracy is decreasing.. The pilot plant is described in

detail in Brettschneider et al.(2004) for the absorption mode.). The pilot plant is automated.

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Fig:9 Pilot plant flowsheet

Conclusion

For the design and the optimization of the process, a selective absorption and desorption-process,

a new model has been presented. The model was successfully validated by vapour–liquid

equilibrium data and reaction kinetics from the literature and by the conducted eight desorption

experiments and eight absorption experiments in the packed tower. The developed experimental

procedure included redundant measurements and a data reconciliation method to improve the

accuracy of the measurement data and also automatically detected gross errors. The model

uses a thermodynamically consistent approach for both the chemical and phase equilibria as well

as the reaction kinetics. It is now being used in industrial practice for the design of the individual

units and the overall process and can also be utilized online for model predictive control.

Furthermore, the industrial process was systematically optimized regarding annual costs using an

evolutionary strategy. This resulted in a decrease of 30% in operating costs still complying with

the restrictions for the gas outlet concentration. For a new process the heights of the absorber and

stripper were significantly reduced resulting also in a decrease in investment costs.[9]

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4.3 Reactive extraction

a.Direct production of biodiesel from rapeseed by reactive extraction/in

situ transesterification

Summary

Energy across the world is dwindling. The sources of energy are reducing day by day in the

past few years and alternative sources of such energy has been introduced. Consequently, the

development of sustainable and “greener” energy resources has taken centre stage. One of the

alternatives of renewable energy is

fuel derived from plant and animal matter. These so-called “biofuels” can be produced by

various processes, such as fermentation, transesterification, pyrolysis, gasification,

liquefaction, and hydro treatment. Among these processes, transesterification of vegetable oil

or animal fat to produce biodiesel has been successfully commercialized. Biodiesel is a

petroleum diesel substitute and can be readily used in most diesel engines without any

modification. Furthermore, engine performance using biodiesel has been shown to be

comparable to that of conventional diesel fuel. Other merits of biodiesel include reduced

emission of carbon monoxides, particulates and hydrocarbon from the engine and enhance

engine lubrication. transesterification can actually be performed directly from the oil-bearing

materials without prior extraction. This route which is often termed “reactive extraction” or

“in situ transesterification” has the advantages of simplifying the biodiesel production

process as well as potentially reducing production cost. In this study, the reactive extraction

of rapeseed with methanol has been characterized. The effects of process parameters on the

yield, conversion and reaction rate differ substantially from conventional transesterification

due to the dependence on both extraction and reaction. The rate of ester formation is mainly

affected by the catalyst concentration, temperature and particle size while the equilibrium

yield largely depends on the solvent to oil molar ratio.

It has been shown that methanol was not able to extract triglyceride fromrapeseed to any

considerable extent. However the presence of the alkaline catalyst allows significant

production of methyl ester from triglycerides. The mechanism of this process is still not

clear. Several studies have speculated on the mechanism although they have not supported

their theories with experimental result nor discussed it. the rate of extraction is very fast early

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in the reaction and decreases as the reaction progresses. Furthermore, from Table was

shown that the ester yield is low at low catalyst concentration, but the ester concentration in

the bulk methanol is similar at higher and lower concentrations. Hence, inadequate

conversion of triglycerides in the bulk methanol phase is not the cause of the lower extraction

yield. On the other hand, it is likely that at low catalyst concentration, there is insufficient

conversion of triglycerides to ester inside the seed, which reduces the yield of ester.

Table:

Conclusion

The maximum yield of reactive extraction of rapeseeds using methanol is determined

primarily by the methanol to oil molar ratio. Methanol to oil molar ratios greater than 400:1

were required to obtain ester yields higher than 80%. This is a much higher amount of

methanol than that needed for conventional transesterification. In order to make the process

economically viable, more research is needed to identify techniques that can reduce the

solvent consumption. The rate of the process, on the other hand, is mainly controlled by

the catalyst concentration and the seed particle size. Decreasing the catalyst concentration

and increasing the particle size reduces the overall rate of extraction. Temperatures within the

range of 30 to 60 °C only affect the initial rate of extraction, but the time to reach the

maximum yield is similar. It appears that the process is not significantly adversely affected

by water when the seed moisture content is less .Hence, the drying step could be removed if

the moisture content does not exceed this level, which should improve the process

economics.[10]

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