Batch Reactive Distillation

Various acetals and ketals from the reaction of alcohols and diols with aldehydes and ketones can be advantageously obtained using ion-exchange resin catalysts. Batch reactive distillation or DCR can be employed to obtain these acetals at high selectivity at very high conversion levels. 

Also, 1,3-dioxolane was obtained from the reaction of ethylene glycol (EG) and aqueous formaldehyde in high yield using an ion-exchange resin catalyst. In a batch mode of operation, with 50% excess EG, the conversion of formaldehyde is limited to 50% due to equilibrium limitation, whereas in batch reactive distillation, formaldehyde conversion greater than 99%... 

Removal of formaldehyde from aqueous 2-butyne-l,4-diol, or a similar solution, which is relevant in the subsequent manufacture of c -2-butene-l,4-diol, by batch reactive distillation with methanol or ethylene glycol in the presence of Indion 130 as catalyst has also been reported 98% conversion of formaldehyde was obtained by reactive distillation with 7 times the stoichiometric quantity of methanol, compared to 15% conversion obtained in a closed system (Kolah and Sharma, 1995). 

Figure 4.33 illustrates the PSPS and bifurcation behavior of a simple batch reactive distillation process. Qualitatively, the surface of potential singular points is shaped in the form of a hyperbola due to the boiling sequence of the involved components. Along the left-hand part of the PSPS, the stable node branch and the saddle point branch 1 coming from the water vertex, meet each other at the kinetic tangent pinch point x = (0.0246, 0.7462) at the critical Damkohler number Da = 0.414. The right-hand part of the PSPS is the saddle point branch 2, which runs from pure THF to the binary azeotrope between THF and water. 

Cuille and Reklaitis (1986) and Albet et al. (1991) used similar model to simulate batch reactive distillation process. Egly et al. (1979), Reuter et al. (1989), Mujtaba (1989) and Mujtaba and Macchietto (1992, 1997) used a modified version of this model based on constant molar holdup in their studies. Sorensen and Skogestad (1996c), Sorensen et al. (1996b), Balasubramhanya and Doyle III (2000) used simple models for studying control strategies in batch reactive distillation. 

However, conventional batch distillation with chemical reaction (reaction and separation taking place in the same vessel and hence referred to as Batch REActive Distillation- BREAD) is particularly suitable when one of the reaction products has a lower boiling point than other products and reactants. The higher volatility of this product results in a decrease in its concentration in the liquid phase, therefore increasing the liquid temperature and hence reaction rate, in the case of irreversible reaction. With reversible reactions, elimination of products by distillation favours the forward reaction. In both cases higher conversion of the reactants is expected than by reaction alone. Therefore, in both cases, higher amount of distillate (proportional to the increase in conversion of the reactant) with desired purity is expected than by distillation alone (as in traditional approach) (Mujtaba and Macchietto, 1997). 

An extensive literature survey shows that very little attention has been given to modelling and simulation of batch reactive distillation, let alone optimisation of such process. The published literature deals with the mathematical modelling and numerical integration of the resulting dynamic equations systems, with few presenting computer simulation vs experimental results. Only few authors have discussed the design, control and optimal operational aspects of batch reactive distillation processes. 

Lehtonen et al. (1998) considered polyesterification of maleic acid with propylene glycol in an experimental batch reactive distillation system. There were two side reactions in addition to the main esterification reaction. The equipment consists of a 4000 ml batch reactor with a one theoretical plate distillation column and a condenser. The reactions took place in the liquid phase of the reactor. By removing the water by distillation, the reaction equilibrium was shifted to the production of more esters. The reaction temperatures were 150-190° C and the catalyst concentrations were varied between 0.01 and 0.1 mol%. The kinetic and mass transfer parameters were estimated via the experiments. These were then used to develop a full-scale dynamic process model for the system. 

Note that for a fixed operation time, t in Equation 9.1, the profit will increase with the increase in the distillate amount and a maximum profit optimisation problem will translate into a maximum distillate optimisation problem (Mujtaba and Macchietto, 1993 Diwekar, 1992). However, for any reaction scheme (some presented in Table 9.1) where one of the reaction products is the lightest in the mixture (and therefore suitable for distillation) the maximum conversion of the limiting reactant will always produce the highest achievable amount of distillate for a given purity and vice versa. This is true for reversible or irreversible reaction scheme and is already explained in the introduction section. Note for batch reactive distillation the maximum conversion problem and the maximum distillate problem can be interchangeably used in the maximum profit problem for fixed batch time. For non-reactive distillation system, of course, the maximum distillate problem has to be solved. 

Walsh et al. (1995) considered an industrial batch reactive distillation problem originally presented by Leversund et al. (1993) as a case study. A condensation polymerisation reaction between a dibasic aromatic acid (R1) and two glycols (R2, R3) was considered. The reaction products were a polymer product (P) and water... 

Bollyn, M. P. Wright, A. R., Development of a Process Model for a Batch Reactive Distillation—A case study. Computers chem. Engng. 1998,22 (SuppL), 587. 

Example 8.1 In the reaction system lA + 1C<- 3B and 15, we would like to determine whether it is possible to design a batch reactive distillation system that will produce acomposition of x esired = [0.02,0.93]. It has been predetermined that the feed to the system should be xf = [0.4,0.2],the relative volatilities to the systemaiea= [a, < c = [6,1,3], and the reaction rate is elementary. If the stem is indeed possible, produce the relevant R-RCM-M and comment on the system s behavior. 

Venimadhavan et. al. (1999) developed batch reactive distillation model for production of butyl acetate in presence of sulfuric acid catalyst. They also studied the kinetics of esterification of acetic acid with butanol and calculated the thermodynamic equilibrium constant in a temperature range of 373 K- 393 K. They found that the equilibrium constant did not vary strongly with temperature. 

G. Venimadhavan, M. F. Malone, and M. F. Doherty, A novel distillate policy for batch reactive distillation with appUcation to the production of butyl acetate, Ind. Eng. Chem. Res. 38, 714-722 (1999). 

MicroMENTOR is an educational package for solving distillation problems and includes MCCABE, PONCH, and BATCH for the MaCabe-Thiele, Ponchon-Savarit, and Batch binary distillations (11). The commercially available distillation software packages have been surveyed (15). Por reactive distillation, ASPEN software (16) is weU-known and widely adopted. 

The experiments of reactive distillation were carried out in a double-neck round-bottom flask working as a batch reactor. The reaction vessel was heated using a heating mantle and a magnetic stirrer was applied to create homogeneous slurry as reaction mixture. 

Fig. 4.1. Flash cascades [13] and batch processes (reactive condenser/reactive reboiler, [14]) being used to predict the top and bottom products of a countercurrent reactive distillation column.

Figure 1.3 shows a typical semi-batch (semi-continuous) distillation column. The operation of such columns is very similar to CBD columns except that a feed is introduced to the column in a continuous or semi-continuous mode. This type of column is suitable for extractive distillation, reactive distillation, etc. (Lang and coworkers, 1994, 1995 Mujtaba, 1999). Further details of semi-batch distillation in extractive mode of operation are provided in Chapter 10. 

Modelling and optimisation of batch reactive and extractive distillation processes... 

A reactive distillation (RD) process would bring evident technological and ecological advantages. An important feature is that the reactants can be fed in the stoichiometric ratio ensuring in this way the maximum efficiency of raw materials. Unlike a batch process, where the excess of alcohol is recovered by costly distillation, higher reaction rate can be achieved by internal alcohol recycle. However, the presence of water as a byproduct makes this wish much more difficult than it appears. 

UDP4-Sb The liquid-phase reaction 2A + B esC -I- D is carried out in a semi-batch reactor. Plot the conversion, volume, and species concentrations as a function of time. Reactive distillation is also considered in part (e). [2nd Ed. P4-27]... 

The equilibrium constant for this reaction is 5.2, so only moderate conversion could be obtained in a plug-flow or batch reactor. Purification of the product mixture would be very difficult, because there are two azeotropes, with boiling points close to that of methyl acetate. With reactive distillation, the higher boiling reactant, acetic acid, is fed near the top of the column, as shown in Figure 10.11, and methanol is fed near the bottom. This counter-... 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

The kinetics of a chemical reaction have a significant influence on the products that can be attained from a RD process. The attainable products of counterinfinite reflux ratio can be obtained as singular points of a reactive reboiler batch process (bottom product) or a reactive condenser batch process (distillate product). The compositions of both products are located on a unique singular point curve. This curve is independent of any special type of reaction kinetics. However, the locations of the top and bottom products on this curve depend on the structure of the rate equation and on the intensity of the reaction (Damkbhler number) in the considered reaction system. 

Due to the chemical equilibrium limitations, the reaction inside a batch stirred-tank reactor would lead to a conversion rate of DMC Xdmc = 0.45 and a selectivity of DMC towards DEC Sdmc.dec = 0.27. In particular, the low selectivity is caused by the unfavorable equilibrium constant of the second reaction step. Reactive distillation offers a possibility to increase this value through the removal of methanol from both reaction steps. 

Flowsheets for processes are sometimes generated without following the hierarchy of properties described previously. As an example, Siirola [20] proposed a reactive-distiUation solution to make methyl acetate. Unit operations that combine the property differences present abrupt departures from common methodologies. With the advent of various pieces of equipment, such as differential side-stream feed reactors (i.e., semicontinuously fed batch reactors), continuous evaporator-reactors (e.g., wiped-film evaporators), and reactive distillation columns, one can consider these unit operations in the development of conceptual designs. As an example, Doherty and Malone [21] have presented systematic methods for reactive distillation design. 

Deterministic optimization has been the common approach for batch distillation operation in previous studies. Since uncertainties exist, the results obtained by deterministic approaches may cause a high risk of constraint violations. In this work, we propose to use a stochastic optimization approach under chance constraints to address this problem. A new scheme for computing the probabilities and their gradients applicable to large scale nonlinear dynamic processes has been developed and applied to a semibatch reactive distillation process. The kinetic parameters and the tray efficiency are considered to be uncertain. The product purity specifications are to be ensured with chance constraints. The comparison of the stochastic results with the deterministic results is presented to indicate the robustness of the stochastic optimization. 

The recovery of acetic acid from its dilute aqueous solutions is a major problem in both petrochemical and fine chemical industries. Saha, et al. (2000) developed conventional methods of recovery of 30% acetic acid by reaction with n-butanol and isoamyl alcohol in a reactive distillation column using macroporous ion-exchange resin, Indion 30, as a catalyst bed, confined in stainless steel wire cages. They found that recovery of acetic acid was enhanced by reactive distillation compared to the batch operation. Hanika et al. (1999) studied the esterification butyl alcohol with acetic acid in a pilot plant using a reactive distillation column packed with commercial catalysts (KATPAK and CY). It was found that butyl acetate could be recovered in very high purity. This study had resulted in the development of a new technology for the manufacture of butylacetate. 

S. P. Chopade and M. M. Shaima, Reaction of ethanol and formaldehyde Use of versatile cation-exchange resins as catalyst in batch reactors and reactive distillation columns, Reactive and Functional Polymers. 32, 53-64 (1997). 

Chopade S. P. and M. M. Sharma, AcetaUzation of ethylene glycol with formaldehyde using cation-exchange resins as catalysts Batch versus reactive distillation. React. Fund. Polym. 34, 37-45 (1997). 

Xu Z. and M. P. Dudulrovic, Modeling and simulation of semi-batch photo reactive distillation, Chem. Eng. Sci. 54, 1397-1403 (1999). 

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