Reactive distillation principles

Process Applications The production of esters from alcohols and carboxylic acids illustrates many of the principles of reactive distillation as applied to equilibrium-limited systems. The equilibrium constants for esterification reactions are usually relatively close to unity. Large excesses of alcohols must be used to obtain acceptable yields with large recycles. In a reactive-distiUation scheme, the reac-... 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

The HI decomposition section flow sheets for both CEA and GA are heavily focused on efficient heat recovery. The basic principle for decomposition is the same for each. Reactive distillation of the HIX feed results in the production of hydrogen. The operating pressures in the distillation columns typically... 

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

Systems that have the most potential for reactive distillation are those where the reaction is reversible, heat of reaction is not excessively large, and the products have the correct volatilities in relation to the reactants. Those systems reach chemical equilibrium (i.e., reaction stops) unless the reactants are in large excess or the products are continuously removed. An example system has been reported in the literature by Eastman Chemical (Agreda et al., 1990) for the production of methyl acetate from methanol and acetic acid. The discussion about process operation and the control strategy shown in the paper certainlv adhere to the plantwide control principles we have outlined in this book. 

For consecutive reactions in which the desired product is formed in an intermediate step, excess reactant can be used to suppress additional series reactions by keeping the intermediate-species concentration low. A reactive distillation can achieve the same result by removing the desired intermediate from the reaction zone as it is formed. Similarly, if the equilibrium constant of a reversible reaction is small, high conversions of one reactant can be achieved by use of a large excess of the other reactant. Alternatively, by Le Chatelier s principle, the reaction can be driven to completion by removal of one or more of the products as they are formed. Typically, reactants can be kept much closer to stoichiometric proportions in a reactive distillation. 

Combining reaction and separation in the same device leads in principle to the most compact and economic design. For this reason Reactive Distillation (RD) has raised a high interest, both from industrial and scientific point of view. A synonym term is Catalytic Distillation, because only a catalyst can enhance the reaction rate at values compatible with the separation requirements. 

Stichlmair, J., J.R. Fair, J. L. Bravo, 1989, Separation of azeotropic mixture via enhanced distillation, Chem. Eng. Progress, 85(1), 63-69 Stichlmair, J., J. R. Herguijuela, 1992, Separation regions and processes of zeotropic and azeotropic ternary distillation, AIChEJ, 38, p. 1523-1535 Stichlmair, J. G., J. R. Fair, 1999, Distillation, Principles and Practice, Willey-VCH Strathmann, H., 1990, Membrane and Membrane Separation Processes, Ullmann s Encyclopaedia of Industrial Chemistry, vol. A16 Taylor, R., Krishna, R., 2000, Modelling reactive distillation, Chem. Eng. Sci., 52, 993-1005... 

It should be mentioned that the majority of the work presented here is graphically based simply because it is easier to grasp column into-actions and behavior when approached from this point of view. However, this need not be a limitation for the methods. The authors would also like to stress that it is not necessarily the specific material and problems presented in the book that are important, but more the tools that the reader should be equipped with. The concepts we present simply put tools into the designer s hand for him/her to create a unique column or separation structure that may solve his/her particular separation problem. For instance, both distributed feed and reactive distillation columns are discussed independently, although it is of course entirely possible to conceive of a reactive distributed feed system, which is not discussed. The tools in this book will thus first allow the reader understand, analyze, and design well-known and frequently encountered distillation problems. Second, the tools can be used to synthesize and develop new systems that peihaps have not even been thought of yet. This principle applies to virtually all the work in this book and the reader is urged to explore such concepts. 

The introduction of lead-free gasoline brought about a new technical process on a large scale reactive distillation (RD). Although the principle of this process had been known for many years [1], the need to produce huge quantities of ethers as antiknock enhancers caused rapid development of this technique more than 14 X 10 tonnes/year of ethers are produced. The catalysts for the production of methyl-t-butylether (MTBE), t-amylmethylether (TAME), or ethyl- butylether (ETBE), which are the main products for the fuel market, are acidic ion-exchange resins. The most important type is based on cross-linked polystyrene that is sulfo-nated to create the active acid sites. These resins are produced as beads of less than 3 mm in a suspension polymerization process. 

The concepts of equilibrium stages and of transfer units are, in principle, equivalent in case of parallel operating and equilibrium lines. The more the slopes of these lines differ the more superior are rate-based models to equilibrium-stage models. Further advantages of rate-based models are a better simulation of reactive distillation and absorption processes. At the present state of the art, however, equilibrium stage models are the standard tool for the simulation of distillation columns. 

There is no constant of integration due to the boundary condition that both AG/T and A(l/7 ) are zero at equilibrium. However, AH will be temperature-dependent most of the time. For example, in producing ammonia from hydrogen and nitrogen, the goal is to maximize the output of ammonia at the exit. An approximately constant AT between the optimal path and the equilibrium temperature provides the optimal temperature profile, which reduces the exergy loss by approximately 60% in the reactor. The equipartition of forces principle for multiple, independent rate-controlled reactions and multiphase and coupled phenomena, such as reactive distillations, may lead to the improved use of energy and reduced costs (Sauar et al., 1997). 

Reactive distillation (RD) is one of the most important reactive separations with potential industrial applications. Here, both reaction and distillation take place within the same zone of a distillation column. It facilitates the instantaneous removal of products in pure form by using distillation principle. New vapor phase of products can be created by either using the heat of reaction in case of exothermic reactions, or by supplying external heat in case of endothermic reactions. Thus reactive distillation provides effective utilization of heat of reaction for product separation and thereby leads to significant reduction in utility consumption. In addition, in-situ removal of products results in improved conversions and yields in case of equilibrium limited reactions, thereby contributing significantly to the overall intensification of the existing process. 

Multi-functional operations are discussed under two sections of reactive separations and hybrid separation platforms. Reactive separations of reactive distillation, reactive adsorption, and membrane reactors are presented in more detail including their principles, advantages and applicability to different systems. Hybrid separations incorporating different unit operations are discussed briefly along with their application and scope. 

The assumptions made in this work include 1) ideal vapor-hquid equilibrium, 2) equal molar feed (neat process), 3) the reactive holdup set by the column diameter, and 4) a sequential approach for optimization. It is interesting to note that the reactive zone can be placed at the upper section, lower section, middle, or both ends of the reactive distillation column, depending on the sequences of the relative volatilities. The principle is actually quite simple place the reactive zone where the reactants are most abundant and introduce the feeds to facilitate the reaction (considering the composition effect). 

Despite clear economic incentives for reactive distillation systems, there are relatively few articles that study the dynamics and control of reactive distillation columns. Al-Arfaj and Luyben give a review of the literature dealing with the closed-loop control of reactive distillation systems. Several control structures for an ideal two-product reactive distillation system and real chemical systems " have been proposed. One important principle in the control of reactive distillation is that we need to control one intermediate composition (or tray temperature) in order to maintain the stoichiometric balance between the two reactant components. ... 

Several hundred papers and patents have appeared in the area of reactive distillation, which are too numerous to discuss. A number of books have dealt with the subject such as (1) Distillation, Principles and Practice by Stichhnair and Fair, (2) Conceptual Design of Distillation Systems by Doherty and Malone," and (3) Reactive Distillation— Status and Future Directions by Sundmacher and Kienle. These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized, but there is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control stmcture development are not covered. Few quantitative eeonomic comparisons of conventional multiunit processes with reactive distillation are provided. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

This reaction, known as Fischer esterification, requires the presence of an acid catalyst. Because the carboxylic acid and the ester have similar reactivities, the reaction is useful only if a method can be found to drive the equilibrium in the direction of the desired product—the ester. In accord with Le Chatelier s principle, this is accomplished by using an excess of one of the reactants or by removing one of the products. An excess of the alcohol is used if it is readily available, as is the case for methanol or ethanol. Or water can be removed by azeotropic distillation with a solvent such as toluene. 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

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