Reactive distillation reversible reactions

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). 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Similar patterns of behavior can be observed in reactive distillation columns or other integrated reaction separation processes with fast reversible reactions [11, 23] as illustrated in Fig. 5.3 for a pure rectifying column with a ternary mixture and a reaction of type 2B A + C taking place in the liquid phase. However, due to the reaction equilibrium the profiles consist of a single concentration front, which is clearly different from the nonreactive problem illustrated in Fig. 5.2. 

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). 

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. 

The reversible reactions deserve particular attention. The in-situ removal of a product by reactive distillation, reactive extraction or by using selective membrane diffusion should be investigated. 

This case study investigates the possibility of applying reactive distillation to the synthesis of fatty-acid esters as a generic multiproduct process. As representative species we consider the lauric (dodecanoic) acid and some alcohols the series Q-C8, such as methanol, n-propanol and 2-ethyl-hexanol (isooctanol). The generic reversible chemical reaction is ... 

The advantage of bidimensional representation is evident if four reactive components are involved, as in the class of reversible reactions A + B <-> C + D. This situation covers an important number of industrial applications, as the esterification of acids with alcohols. Selecting C as the reference, the transformed variables are XA = xA + xc, XB = xB + xc and XD = xD- xc since v, = 0. The transformed variables sums to one, but only two are used as co-ordinates. Accordingly, the pure components may be placed in the corner of a square diagram, reactants or products on the same diagonal. Figure A. 5 displays the reactive distillation map traced as before for the relative volatilities 4/2/6/1 and the equilibrium constant K t = 5. 

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. 

The benefits of using reactive distillation are clearly proven in the production of fuel components (ethers) such as ferf-amyl methyl ether (TAME), methyl tert-butyl ether (MTBE) and methyl acetate. The latter is synthesized from acetic acid and methanol with a reversible liquid-phase reaction ... 

Reactive distillation is used with thermodynamically limited reversible liquid-phase reactions and is particularly attractive when one of the products has a tower boiling point than the reactants. For reversible reactions of this type. 

First, the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o Aj in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1]. 

For instantaneous chemical reactions a reactive distillation line represents the concentration profile of the liquid within a column. From its knowledge processes of reactive distillation can be designed (Stichlmair and Frey 1998) as is demonstrated at the example of the reversible instantaneous reaction a + b< c. 

A comparison of the processes shown in Figs. 11.5-1 and 11.5-2 demonstrates the high potential of reactive distillation for process simplification. This type of processes is generally applieable to systems with reversible chemical reactions, e.g., to esterification and etherification of alcohols, to alkylations, to dimerization of olefins, and to hydrogenation of aromatics (Sundmacher and Kienle 2003). 

Reactive distillation is used commonly when the chemical reaction is reversible, for example,... 

Gas-liquid plus catalytic solid Use when (i) the reaction occurs in the liquid phase (in the presence or not of homogeneous catalyst) or at the catalyst interface (ii) temperatures and pressures for reaction are consistent with distillation conditions (iii) reactions are reversible equilibrium not irreversible (iv) not for supercritical, gas phase reactions, or solid reactants or products, high temperatures or pressures. Minimizes catalyst poisoning, lower pressure than fixed bed. Used for hydrogenation reactions and MTBE and acrylamide production. For example, 90% conversion via reactive distillation contrasted with 70% conversion in fixed bed option. 

Reactive distillation columns incorporate both phase separation and chemical reaction in a single unit. In some systems, they have economic advantages over conventional reac-tor/separation/recycle flowsheets, particularly for reversible reactions in which chemical equilibrium constraints limit conversion in a conventional reactor. Because both reaction and separation occur in a single vessel operating at some pressure, the temperatures of reaction and separation are not independent. Therefore, reactive distillation is limited to systems in which the temperatures conducive for reaction are compatible with temperatures conducive for vapor-liquid separation. 

Reactive distillation is usually applied to systems in which the relative volatilities of the reactants and products are such that the products can be fairly easily removed from the reaction mixture while keeping the reactants inside the column. For example, consider the classical reactive distillation system with reactants A and B reacting to form products C and D in a reversible reaction... 

For isomer separations, extractive distillation usually fails, since the solvent has the same effect on both isomers. For example, Berg (1969) reported that the best entrainer for separating m- and p-xylene increased the relative volatility from 1.02 to 1.029. An alternative to normal extractive distillation is to use a solvent that preferentially and reversibly reacts with one of the isomers fPoherty and Malone. 20011 The process scheme will be similar to Figure 8-14. with the light isomer being product A and the heavy isomer product B. The forward reaction occurs in the first column, and the reaction product is fed to the second column. The reverse reaction occurs in column 2, and the reactive solvent is recycled to column 1. This procedure is quite similar to the combined reaction-distillation discussed in Section 8.8. 

Distillation columns are occasionally used as chemical reactors. The advantage of this approach is that distillation and reaction can take place simultaneously in the same vessel, and the products can be removed to drive the reversible reaction to conpletion. The most common industrial application is for the formation of esters from a carboxylic acid and an alcohol. For exanple, the manufacture of methyl acetate by reactive distillation was a major success that conventional processes could not conpete with fBiegler etal.. 1997T Reactive distillation was first patented by Backhaus in 1921 and has been the subject of many patents since then (see Doherty and Malone. 2001 Doherty et al.. 2008 and Siirola and Barnicld. 1997. for references). Reaction in a distillation column may also be undesirable when one of the desired products deconposes. 

Hybrid reactors reverse flow, reactive distillation, reactive extraction, reactive crystallization, chromatographic reactions, membrane reactions, fuel cells... 

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