Reactive Distillation Process

Choice of the correct equipment is crucial for developing a reactive distillation process. The most important factors influencing the equipment are relative volatiHties and the reaction velocity. Some rules of thumb have been published in open Htera-ture (Gmehhng and Brehm, 1996). 

Whereas homogeneously catalyzed reactive distillation can be carried out in conventional tray columns (sometimes modified to ensure sufficient residence time of the reactants), a heterogeneous catalyst has to be fixed in the reactive section with the help of special internals. These internals have to combine good wetting characteristics to achieve a good contact between the Hquid and vapor phases with a large amount of catalyst that is readily accessible by the liquid in order to avoid macro-kinetic influences. 

The first type of internals is the so-called tea bag (Smith, 1984), which consists of wire-mesh bags that can be filled with the catalyst. These bags are stored on the trays of conventional distillation columns. The main drawback of this packing type is that transport Hmitations cannot be avoided, thus leading to smaller reaction rates. Furthermore the bags on the trays result in high additional pressure drops. 

Bags filled with catalyst do not necessarily have to be stored on the trays. Bags have also been stored in the downcomer, which is a region of liquid holdup (Garland, 

X Liquid phase mole fraction y Vapor phase mole frachon 

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The manufacture of high purity methyl acetate by a reactive distillation process has been accompHshed high conversion of one reactant can be achieved only with a large excess of the other reactant. Because the reaction is reversible, the rate of reaction ia the Hquid phase is iacreased by removing methyl acetate prefereatiaHy to the other components ia the reactioa mixture (100). 

Heat management is another important consideration in the implementation of a reactive distillation process. Conventional... 

Reactive distillation is a technique for combining a number of process operations in a single device. One company has developed a reactive distillation process for the manufacture of methyl acetate that reduces the number of distillation columns from eight to three, also eliminating an extraction column and a separate reactor (Agreda et al., 1990 Doherty and Buzad, 1992 Siirola, 1995). Inventory is reduced... 

A major challenge will be to develop new processes or step-up technologies that increase the yield and/or selectivity, use cheaper raw materials, decrease energy consumption, minimize the product separation and purification needs and lower capital investment. Iimoyative step-out technologies can still have a major impact on existing processes. An excellent example of such an accomplishment is the reactive distillation process developed by Eastman Chemicals for production of methyl acetate by via the reaction [2]... 

The conventional process consists of a reactor followed by eight distillation columns, one liquid-liquid extractor and a decantor. The reactive distillation process consists of one column that produces high-purity methyl acetate that does not require additional purification and there is no need to recover unconverted reactant. The reactive distillation process costs one fifth of the conventional process and consumes only one fifth of the energy. 

The Simulation and Control of the Reactive Distillation Process for Dimethylcarbonate Production... 

During the last decade many industrial processes shifted towards using solid acid catalysts (6). In contrast to liquid acids that possess well-defined acid properties, solid acids contain a variety of acid sites (7). Sohd acids are easily separated from the biodiesel product they need less equipment maintenance and form no polluting by-products. Therefore, to solve the problems associated with liquid catalysts, we propose their replacement with solid acids and develop a sustainable esterification process based on catalytic reactive distillation (8). The alternative of using solid acid catalysts in a reactive distillation process reduces the energy consumption and manufacturing pollution (i.e., less separation steps, no waste/salt streams). 

Figure 33.3. AspenPlus flowsheet of the catalytic reactive distillation process.

The modeling of RD processes is illustrated with the heterogenously catalyzed synthesis of methyl acetate and MTBE. The complex character of reactive distillation processes requires a detailed mathematical description of the interaction of mass transfer and chemical reaction and the dynamic column behavior. The most detailed model is based on a rigorous dynamic rate-based approach that takes into account diffusional interactions via the Maxwell-Stefan equations and overall reaction kinetics for the determination of the total conversion. All major influences of the column internals and the periphery can be considered by this approach. 

Luo HP, Xiao WD. A reactive distillation process for a cascade and azeotropic reacting system carbonylation of ethanol with dimethyl carbonate. Chem Eng Sci 2001 56 403 110. 

Muller D, Schafer JP, Leimkuhler HJ. The economic potential of reactive distillation processes exemplified by silane production. Proceedings of VDI-GVC, DECHEMA and EFCE Meeting, Section Thermal Separations, Bamberg, Germany, 2001. 

Similar to the reactive distillation process, at Da —> °° the composition space is again divided into two subspaces which have either pure THF or pure water as attractors (Fig. 4.30(c)). However, as a very important feature of the reactive membrane... 

A ternary system with a hyperbola-type PSPS is used to investigate the influence of membrane permeation (Fig. 4.32). The applied parameters (ct,A and Kg) and the corresponding eigenvalues of the matrix [A] are summarized in Tab. 4.3. For comparison, again the PSPS for the reactive distillation process is given in Fig. 4.32(a). The effect of a selective membrane with a diagonal [/e]-matrix is illustrated in Fig. 4.32(b, d). 

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. 

The determination of feasible products is very important for conceptual process design and for the evaluation of competing process variants. In this chapter, methods have been discussed to identify feasible products as singular points of residue curve maps (RCM). RCM-analysis is a tool which is well established for nonreactive and reactive distillation processes. Here, it is shown how RCM can also be used for reactive membrane separation processes. 

In this chapter, unifying concepts for analyzing and understanding the dynamics of integrated reaction separation processes with rapid chemical reactions are introduced. The text is based on some recent studies [11-13], and extends the concepts introduced earlier for reactive distillation processes [23] to other integrated reaction separation processes. The class of processes to be considered is rather broad. It includes reaction processes where simultaneous separation is used to enhance a reaction, for example, by shifting inherent equilibrium limitations. Various process examples of this kind are provided in this book. The chapter also includes separation processes with potentially reactive mixtures. In this case, a chemical reaction can be either an unwanted side effect or it can be used directly to achieve a certain separation, which is not possible under nonreactive conditions (see e.g. Ref. [10]). The latter represents a reaction-enhanced separation. 

Transformed concentration variables were first introduced by Doherty and co-workers [2, 41] for the steady-state design of reactive distillation processes. In the... 

Finally, it should be noted that the above treatment is only valid for constant flow rates. For processes without solvent (e.g., reactive distillation processes), this assumption is only valid for equimolar reactions. For equimolar reactions the definition of transformed concentration variables introduced by Ung and Doherty [41] reduces to the definition in Eq. (6). For processes with solvent, (e.g., reactive chromatographic processes), the assumption of constant flow rates is also valid in good approximation, if the concentration of the solvent is high compared to the other reacting species. This is also true if one of the reactants is used simultaneously as a solvent, as in many applications of reactive chromatography (see e.g. Refs. [1, 28]). 

In the scalar case (i.e., N = 1), wave solutions are easily constructed with the equilibrium diagram y(x) or Y(X). According to the above considerations, typical scalar problems are a binary nonreactive distillation process, a ternary reactive distillation process with a single chemical reaction, a reactive distillation process with Nc components and Nc - 2 chemical reactions, or a chromatographic reactor with Ns solutes... 

In this section the methods developed in the previous section will be applied to analyze the dynamic behavior of integrated reaction separation processes. Emphasis is placed on reactive distillation and reactive chromatography. Finally, possible applications to other integrated reaction separation processes including membrane reactors and sorption-enhanced reaction processes will be briefly discussed. More details about reactive distillation processes were provided in Ref. [39]. For chromatographic reactors the reader should refer to Chapter 6 of this book, for sorption-enhanced reaction processes to Chapter 7, and for membrane reactors to Chapter 12. 

Fig. 5.7. Transformed equilibrium function of a reactive distillation process with a reaction of type 2 A B + C and a mixture with constant relative volatilities (schematic)...

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

Wave models were successfully used for the design of a supervisory control system for automatic start-up of the coupled column system shown in Fig. 5.15 [19] and for model-based measurement and online optimization of distillation columns using nonlinear model predictive control [15], The approach was also extended to reactive distillation processes by using transformed concentration variables [22], However, in reactive - as in nonreactive - distillation, the approach applies only to processes with constant pattern waves, which must be checked first. 

A. Kienle, Nonlinear model reduction for nonreactive and reactive distillation processes using nonlinear wave propagation theory. 

Nonlinear dynamics and control of reactive distillation processes. In ... 

Note that the system (2.45) is a DAE system of nontrivial index, since z cannot be evaluated directly from the algebraic equations. A solution for the variables z must be obtained by differentiating the constraints k(x) = 0. For most chemical processes, such as reaction networks (Gerdtzen et al. 2004), reactive distillation processes (Vora 2000), and complex chemical plants (Kumar and Daoutidis 1999a), the z variables can be obtained after just one differentiation of the algebraic constraints ... 

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. 

Reactive Distillation processes combine the benefits of traditional unit operations with a substantial progress in reducing capital and operating costs and environmental impact (BP Review, 1997 Taylor and Krishna, 2000). 

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. 

Among the most important examples of RS processes are reactive distillation, reactive absorption, reactive stripping and reactive extraction. For instance, in reactive distillation, 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 reactive distillation process can be both efficient in size and cost of capital equipment and in energy used to achieve a 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 reactive distillation processes are etherifications, nitrations, esterifications, transesterifications, condensations and alcylations (Doherty and Buzad, 1992). 

Figure 3.24 Column setup and profiles for a reactive distillation process.

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