Columns reactive distillation

Ind. Eng. Chem. Res., 41, 2735, 2002 Ellenberger, J Krishna, R., Counter-current operation of structured catalytically packed distillation columns, Chem. Eng. Sci., 54,1339-1345, 1999 Krishna, R Hardware selection and design aspects for reactive-distillation columns, Reactive Distillation, eds Sundmacher K. and Kienle, A., Wiley-VCH, Weinheim, Germany, 169-189,... 

Figure 4.1 shows the one-column reactive distillation colunm flowsheet with stream information and equipment sizes. Specification products are produced at both ends of the column. Conversion is 95%, and product purities are 95mol%. Fresh feeds are 12.6mol/s. Note that the production rate of both products is 12.6mol/s with equal amounts of the two reactants lost in the two products. 

For the type II flowsheet, the product composition of water from the first column (reactive distillation column) is determined by the LL equilibrium, so no composition control is necessary. However, the reflux ratio of the reactive distillation column is fixed. The acetate product is withdrawn from the bottoms of the stripper and the composition is controlled by manipulating the vapor boilup as shown in Figure 13.1. 

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

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... 

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. 

In a distillation column reactor (DCR), reaction and distillation occur simultaneously. This technology is also referred to as reactive distillation, or, if a catalyst is involved, as catalytic distillation. DCRs offer distinct advantages of exploiting the exothermicity of reactions, such... 

Figure 33.5 shows the composition, temperature and reaction rate profiles in the reactive distillation column. The ester product with traces of methanol is the bottom product, whereas a mixture of water and fatty acid is the top product. This mixture is then separated in the additional distillation column and the acid is refluxed back to the RDC. The fatty ester is further purified in a small evaporator and methanol is recycled back to the RDC (Figures 33.3 and 33.4). 

The design of reactive distillation columns is complicated by the complex interactions between the reaction and separation processes. A comprehensive discussion of the process is given by Sundmacher and Kiene (2003). 

Continuous benzene alkylation was conducted in a reactive distillation column of the type illustrated in Figure 1. The process unit comprises the following principal elements a double column of solid catalyst 32, packing columns above and below the catalyst bed, a liquid reboiler 42 fitted with a liquid bottoms product takeoff 44, a condenser 21 fitted with a water collection and takeoff, and a feed inlet... 

Can equipment sets be combined (e.g., replacing reactive distillation with a separate reactor and multi-column fractionation train installing internal reboilers or heat exchangers) to reduce overall system volume ... 

Samant and Ng 159 Continuous Esterification, prepolycondensation Reactive distillation column... 

Many industrial processes involve mass transfer processes between a gas/vapour and a liquid. Usually, these transfer processes are described on the basis of Pick s law, but the Maxwell-Stefan theory finds increasing application. Especially for reactive distillation it can be anticipated that the Maxwell-Stefan theory should be used for describing the mass transfer processes. Moreover, with reactive distillation there is a need to take heat transfer and chemical reaction into account. The model developed in this study will be formulated on a generalized basis and as a consequence it can be used for many other gas-liquid and vapour-liquid transfer processes. However, reactive distillation has recently received considerable attention in literature. With reactive distillation reaction and separation are carried out simultaneously in one apparatus, usually a distillation column. This kind of processing can be advantageous for equilibrium reactions. By removing one of the products from the reactive zone by evaporation, the equilibrium is shifted to the product side and consequently higher conversions can be obtained. Commercial applications of reactive distillation are the production of methyl-... 

The variation of efficiencies is due to interaction phenomena caused by the simultaneous diffusional transport of several components. From a fundamental point of view one should therefore take these interaction phenomena explicitly into account in the description of the elementary processes (i.e. mass and heat transfer with chemical reaction). In literature this approach has been used within the non-equilibrium stage model (Sivasubramanian and Boston, 1990). Sawistowski (1983) and Sawistowski and Pilavakis (1979) have developed a model describing reactive distillation in a packed column. Their model incorporates a simple representation of the prevailing mass and heat transfer processes supplemented with a rate equation for chemical reaction, allowing chemical enhancement of mass transfer. They assumed elementary reaction kinetics, equal binary diffusion coefficients and equal molar latent heat of evaporation for each component. 

The higher boiling aqueous product fraction flows downwards through the lower distillation section, 10, to a reboiler, 15, where it is heated by an electrical heater. A portion of this higher-boiling aqueous product is withdrawn via an exit line, 15, as shown, and the remainder of the aqueous distillation reaction product is returned to the reactive distillation column, 10, by a reboiler return line. 

In the process described by Agrada. el al.. the concept of a countercurrent reactive distillation column (Fig. 6) is used. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

From a thermodynamics basis, the transesterification reaction favors the formation of methylphenyl carbonate (Equation 7.4), whilst its further disproportionation in a second-stage continuous reactive distillation column affords DPC with selectivity >99%. Although both reactions occur at a relatively high temperature ( 473 K), optimization of the reaction conditions and engineering design would allow a productivity that fitted with the economics [17, 27]. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Yoshida Y. Preparation of isophorone from acetone by using a reactive distillation column. JP 08245486, Daicel Chem, 1995. 

The most important examples of reactive separation processes (RSPs) are reactive distillation (RD), reactive absorption (RA), and reactive extraction (RE). In RD, 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 recycling of unused reactants. The RD process can be efficient in both 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 RD processes are etherifications, nitrations, esterifications, transesterifications, condensations, and alcylations (2). 

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