**3. Reactive distillation**

Reactive distillation is a chemical unit operation in which chemical reaction and product separation occurs simultaneously in one unit. Reactive distillation column consists of a reactive section in the middle with non-reactive rectifying and stripping sections at the top and bottom.

Let us begin by considering a reversible reaction scheme where A and B react to give C and D. The boiling point of the components follows the sequence A, C, D and B. The traditional flow sheet for this process consists of a reactor followed by a sequence of distillation columns. The mixture of A and B is fed to the reactor, where the reaction takes place in the presence of a catalyst and reaches equilibrium. A distillation train is required to produce pure products C and D. The unreacted components, A and B, are recycled back to the reactor.

The stoichiometric equation for transesterification reaction [9] in general can be represented

**Sr.No Region/Country 2005 2006 2007 2008 2009**  1 North America 6.1 17.1 33.7 45.9 35.2 2 United States 5.9 16.3 32.0 44.1 32.9 3 Central and south America 0.5 2.2 15.2 38.6 57.9 4 Brazil 0.0 1.2 7.0 20.1 27.7 5 Europe 68.1 113.2 137.5 155.0 172.6 6 France 8.4 11.6 18.7 34.4 41.1 7 Germany 39.0 70.4 78.3 61.7 51.2 8 Italy 7.7 11.6 9.2 13.1 13.1 9 Eurasia 0.3 0.3 0.7 2.5 3.8 10 Lithuania 0.1 0.2 0.5 1.3 1.9 11 Asia and Oceania 2.2 9.1 15.8 28.8 38.5 12 China 0.8 4.0 6.0 8.0 8.0 13 India 0.2 0.4 0.2 0.2 0.4 14 Korea South 0.2 0.6 1.7 3.2 5.0 15 Malaysia 0.0 1.1 2.5 4.5 5.7 16 Thailand 0.4 0.4 1.2 7.7 10.5

WORLD 77.2 142.0 202.9 270.9 308.2

Source- U.S. Energy Information Administration, International Energy Statistics, Biofuels Production Table 1. World biodiesel productions by region and selected countries 2005-2009 (Thousand

Reactive distillation is a chemical unit operation in which chemical reaction and product separation occurs simultaneously in one unit. Reactive distillation column consists of a reactive section in the middle with non-reactive rectifying and stripping sections at the top

Let us begin by considering a reversible reaction scheme where A and B react to give C and D. The boiling point of the components follows the sequence A, C, D and B. The traditional flow sheet for this process consists of a reactor followed by a sequence of distillation columns. The mixture of A and B is fed to the reactor, where the reaction takes place in the presence of a catalyst and reaches equilibrium. A distillation train is required to produce pure products C and D. The unreacted components, A and B, are recycled back to the

as follows:

barrels per day)

and bottom.

reactor.

**3. Reactive distillation** 

**2. Biodiesel scenarios worldwide** 

The Reactive distillation technology offers many benefits as well as restrictions over the conventional process of reaction followed by distillation or other separation approaches. Reducing capital cost, higher conversion, improving selectivity, lower energy consumption, the reduction or elimination of solvents in the process and voidance of azeotropes are a few of the potential advantages offered by Reactive distillation. Conversion can be increased far beyond what is expected by the equilibrium due to the continuous removal of reaction products from the reactive zone. This helps to reduce capital and investment costs and may be important for sustainable development due to a lower consumption of resources.[7]

The fig.1 represents the general configuration of reactive distillation.

Fig. 1. The general configuration of Reactive Distillation

Based on the applied separation technology, reactive distillation, reactive extraction, reactive adsorption and other combined processes have been distinguished. The combined simultaneous performance of chemical reaction and a multi-component distillation process is an alternative, which has been increasingly used for the large-scale production of relevant chemicals. The use of reactive distillation process can have several advantages such as reduced downstream processing, utilization of heat of reaction for evaporation of liquid phase, simple temperature control of reactor, possibility of influencing chemical equilibria by removal of products and limitations imposed by azeotropic mixture. Several commercially important processes in reactive distillation have been identified in some recent reviews. [7]

Reactive distillation has been successfully applied for the etherification reaction to produce fuel ethers such as methyl tert-butyl ether (MTBE), tert-amyl methyl ether(TAME) and ethyl tertbutyl ether (ETBE). These have been the model reactions for the studies in reactive distillation in the last two decades. A small number of industrial applications of reactive distillation have been around for many decades. Low chemical equilibrium constants can be overcome and high conversions achieved by the removal of products from the location where the reaction is occurring. [6]

Transesterification by Reactive Distillation for Synthesis and Characterization of Biodiesel 293

Fig. 2. Standard process scheme for reversible reactions in which the conversion is limited

For reactive distillation to work, we should be able to remove the products from the reactants by distillation. This implies that the products should be lighter and/or heavier than the reactants. In terms of the relative volatilities of the four components, an ideal case is when one product is the lightest and the other product is the heaviest, with the reactants

α C > α D > α D The most obvious way to improve the reaction yield in an integrated unit is a continuous separation of one product out of the reaction zone. This allows for getting a 100% conversion

A+B ↔ C+D

C

Figure 4 presents the flow sheet of this ideal reactive distillation column. In this situation the lighter reactant A is fed into the lower section of the column but not at the very bottom. The heavier reactant B is fed into the upper section of the column but not at the very top. The middle of the column is the reactive section and contains number of reaction trays. The vapor flow rates through the reaction section change from tray to tray because of the heat of the reaction. As component A flows up the column, it reacts with descending B. Very light product C is quickly removed in the vapor phase from the reaction zone and flows up the column. Likewise, very heavy product D is quickly removed in the liquid phase and flows down the column. The section of the column above where the fresh feed of B is introduced (the rectifying section with NR trays) separates light product C from all of the heavier components, so a distillate is produced that is fairly

D

Fig. 3. Complete conversions of reactants in case of equilibrium reaction [7]

A, B Reaction +Product Separation

by the chemical equilibrium [9]

in case of reversible reactions [9].

pure product C.

being the intermediate boiling components.

It may be advantageous for liquid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recovery or by-product recycle scheme is complicated or made infeasible by azeotrope formation. Novel processes were proposed based on catalytic reactive distillation and reactive absorption to biodiesel production from esterification and transesterification reactions. The major benefits of this approach were: investment costs reducing about 45% energy savings compared to conventional reactive distillation, very high conversions, increased unit productivity, no excess of alcohol required and no catalyst neutralization step The advantage of reactive distillation can be summarized as follows [3]


#### **Biodiesel production by reactive distillation**

As the reaction and separation occurs simultaneously in the same unit in reactive distillation, it is attractive in those systems where certain chemical and phase equilibrium conditions exist. Because there are many types of reactions, there are many types of reactive distillation columns. In this section we describe the ideal classical situation, which will serve to outline the basics of reactive distillation. Consider the system in which the chemical reaction involves two reactants (A and B) producing two products (C and D). The reaction takes place in the liquid phase and is reversible.

#### A+B ↔ C+D

The number of the separation steps depends on the number of products, catalysts, solvents as well as reactants which are not converted. The main objective functions to increase process economics are selectivity as well as reaction yield what influences the reactor design.

Usually, each unit operation is typically performed in individual items of equipment, which, when arranged together in sequence, make up the complete process plant. As reaction and separation stages are carried out in discrete equipment units, equipment and energy costs are added up from these major steps. However, this historical view of plant design is now being challenged by seeking for combination of two or more unit operations into the one plant unit [4].

It may be advantageous for liquid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recovery or by-product recycle scheme is complicated or made infeasible by azeotrope formation. Novel processes were proposed based on catalytic reactive distillation and reactive absorption to biodiesel production from esterification and transesterification reactions. The major benefits of this approach were: investment costs reducing about 45% energy savings compared to conventional reactive distillation, very high conversions, increased unit productivity, no excess of alcohol required and no catalyst neutralization step

a. Simplification: From design view point the combinations of reaction system and

b. Improved conversion of reactant approaches 100%. This increase in conversion gives a

c. Improved selectivity: where, removing one of the products from the reaction mixture or maintaining a low concentration of one of the reagents can lead to reduction of the rates

e. Avoidance of azeotropes: RD is particularly advantageous when the reactor product is a mixture of species that can form several azeotropes with each other. RD conditions can

g. Heat integration benefits: If the reaction is exothermic, the heat of reaction can be used

h. Removal of the product from a system at equilibrium will cause more products to form. Therefore reactive distillation is capable to increase the conversion of equilibrium

As the reaction and separation occurs simultaneously in the same unit in reactive distillation, it is attractive in those systems where certain chemical and phase equilibrium conditions exist. Because there are many types of reactions, there are many types of reactive distillation columns. In this section we describe the ideal classical situation, which will serve to outline the basics of reactive distillation. Consider the system in which the chemical reaction involves two reactants (A and B) producing two products (C and D). The reaction

A+B ↔ C+D The number of the separation steps depends on the number of products, catalysts, solvents as well as reactants which are not converted. The main objective functions to increase process

Usually, each unit operation is typically performed in individual items of equipment, which, when arranged together in sequence, make up the complete process plant. As reaction and separation stages are carried out in discrete equipment units, equipment and energy costs are added up from these major steps. However, this historical view of plant design is now being challenged by seeking for combination of two or more unit operations into the one

economics are selectivity as well as reaction yield what influences the reactor design.

of side reactions and hence improved selectivity for the desired products. d. Significantly reduced catalyst requirement for the same degree of conversion.

The advantage of reactive distillation can be summarized as follows [3]

allow the azeotropes to be "reacted away" in a single vessel.

to provide the heat of vaporization and reduce the reboiler duty.

separation system can lead to significant capital saving.

benefit in reduced recycle costs.

f. There is a reduced by-product formation.

**Biodiesel production by reactive distillation** 

takes place in the liquid phase and is reversible.

limited reaction.

plant unit [4].

Fig. 2. Standard process scheme for reversible reactions in which the conversion is limited by the chemical equilibrium [9]

For reactive distillation to work, we should be able to remove the products from the reactants by distillation. This implies that the products should be lighter and/or heavier than the reactants. In terms of the relative volatilities of the four components, an ideal case is when one product is the lightest and the other product is the heaviest, with the reactants being the intermediate boiling components.

$$\mathsf{a}\mathsf{C} \succeq \mathsf{a}\mathsf{D} \succeq \mathsf{a}\mathsf{D}$$

The most obvious way to improve the reaction yield in an integrated unit is a continuous separation of one product out of the reaction zone. This allows for getting a 100% conversion in case of reversible reactions [9].

$$\text{A} + \text{B} \leftrightarrow \text{C} + \text{D}$$

Fig. 3. Complete conversions of reactants in case of equilibrium reaction [7]

Figure 4 presents the flow sheet of this ideal reactive distillation column. In this situation the lighter reactant A is fed into the lower section of the column but not at the very bottom. The heavier reactant B is fed into the upper section of the column but not at the very top. The middle of the column is the reactive section and contains number of reaction trays. The vapor flow rates through the reaction section change from tray to tray because of the heat of the reaction. As component A flows up the column, it reacts with descending B. Very light product C is quickly removed in the vapor phase from the reaction zone and flows up the column. Likewise, very heavy product D is quickly removed in the liquid phase and flows down the column. The section of the column above where the fresh feed of B is introduced (the rectifying section with NR trays) separates light product C from all of the heavier components, so a distillate is produced that is fairly pure product C.

Transesterification by Reactive Distillation for Synthesis and Characterization of Biodiesel 295

However, holdup is very important in reactive distillation because reaction rates directly depend on holdup (or the amount of catalyst) on each tray. This means that the holdup must be known before the column can be designed and before the column diameter is known. As a result, the design procedure for reactive distillation is iterative. A tray holdup is assumed and the column is designed to achieve the desired conversion and product purities. The diameter of the column is calculated from maximum vapor-loading correlations. Then the required height of liquid on the reactive trays to give the assumed tray holdup is calculated. Liquid heights greater than 10–15 cm are undesirable because of hydraulic pressure drop limitations. Thus, if the calculated liquid height is too large, a new and smaller tray holdup is assumed and the design calculations repeated. An alternative, which may be more expensive in terms of capital cost, is to make the column diameter larger than that required by vapor loading [9].

**4. Case study - Transesterification by reactive distillation for synthesis and** 

In this study, three commercially available feed stocks of vegetable oils are used .They are

Density (Kg/m3 at 288K)

Castor oil 115 (at 60oC) 938 229 -33 182 Coconut oil 24.85 (at 40 oC) 907 225 20 191.1 Cottonseed oil 35.42 (at 40 oC) 904 15 -15.5 192 Table 2. Physical Properties of Vegetable Oil Feed stocks Used For Transesterification

Methanol (Merck) of 99.5% purity (density: 0.785 g/mL at 30oC) was used in this

The two homogeneous basic catalysts (KOH & NaOH) used for reactive distillation were purchased from local Chemical store at Amravati. M.S.The heterogeneous catalyst used for transesterification Amberlyst BD15 was purchased from Dayo Scientific Laboratory, Nashik

Amberlyst 15 wet is a macro reticular, strongly acidic, polymeric catalyst. Its continuous open pore structure makes it an excellent heterogeneous acid catalyst for a wide variety of organic reactions. Amberlyst 15 is extremely resistant to mechanical and thermal shocks. It

Flash point oC

Pour point oC

Saponification value

**characterization of biodiesel** 

Sample Kinematic Viscosity,

cst (mm2/s)

1. Homogeneous base catalysts (KOH & NaOH) 2. Heterogeneous solid acid catalysts (Amberlyst 15)

**4.1 Materials and methods** 

a. Oil Feed stocks:

1. Castor seed oil 2. Cottonseed oil 3. Coconut oil

b. Methanol:

c. Catalyst:

**Amberlyst-15:** 

transesterification process.

Road, Nashik, M.S. India.

In this study the catalysts used are:

**Materials:** 

Fig. 4. Flow sheet of ideal reactive distillation column

The section of the column below where the fresh feed of A is introduced (the stripping section with NS trays) separates heavy product D from all of the lighter components, so a bottom is produced that is fairly pure product D. The reflux flow rate and the reboiler heat input can be manipulated to maintain these product purities. The specific numerical case has 30 total trays, consisting of 10 stripping trays, 10 reactive trays, and 10 rectifying trays. Trays are numbered from the bottom. Note that the concentrations of the reactants peak at their respective feed trays. The purities of the two products are both 95 mol%, with B the major impurity in the bottoms and A the major impurity in the distillate [7].

Reactive distillation column must be adjusted to achieve these specifications while optimizing some objective function such as total annual cost (TAC). These design degrees of freedom include pressure, reactive tray holdup, number of reactive trays, location of reactant feed streams, number of stripping trays, number of rectifying trays, reflux ratio, and reboiler heat input [9].

Tray holdup is another design aspect of reactive distillation that is different from conventional. Holdup has no effect on the steady-state design of a conventional column. It certainly affects dynamics but not steady-state design. Column diameter is determined from maximum vapor loading correlations after vapor rates have been determined that achieve the desired separation. Typical design specifications are the concentration of the heavy key component in the distillate and the concentration of the light key component in the bottoms.

However, holdup is very important in reactive distillation because reaction rates directly depend on holdup (or the amount of catalyst) on each tray. This means that the holdup must be known before the column can be designed and before the column diameter is known. As a result, the design procedure for reactive distillation is iterative. A tray holdup is assumed and the column is designed to achieve the desired conversion and product purities. The diameter of the column is calculated from maximum vapor-loading correlations. Then the required height of liquid on the reactive trays to give the assumed tray holdup is calculated. Liquid heights greater than 10–15 cm are undesirable because of hydraulic pressure drop limitations. Thus, if the calculated liquid height is too large, a new and smaller tray holdup is assumed and the design calculations repeated. An alternative, which may be more expensive in terms of capital cost, is to make the column diameter larger than that required by vapor loading [9].
