Distillation heterogeneous/homogeneous reactiv

Figure 3.6 Homogeneous (a) and heterogeneous (b) reactive distillation (c) structured packing (Katapak-S, from Sulzer) for reactive distillation, consisting of layers of metal wire gauze in which a catalyst is embedded and immobilized in pockets. ...

Heterogeneously catalyzed reactive distillation offers several advantages when compared to the homogeneously catalyzed process alternative. The size and location of the reactive section can be chosen independently of thermodynamic constraints, and an additional process step for separating the catalyst from the product can be avoided. In order to fix the catalyst in the reactive section of the column, heterogeneously catalyzed reactive distillation processes require special internals, which will be discussed in Section 3.2.4. 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

Taylor R. and Krishna R. (2003). Modelling of homogeneous and heterogeneous reactive distillation processes. In Reactive distillation, status and future directions (K. Sundmacher and A. Kienle, eds.), book chapter 9, (217-240), Wiley-VCH. 4.1, 4.3... 

The mathematical model comprises a set of partial differential equations of convective diffusion and heat conduction as well as the Navier-Stokes equations written for each phase separately. For the description of reactive separation processes (e.g. reactive absorption, reactive distillation), the reaction terms are introduced either as source terms in the convective diffusion and heat conduction equations or in the boundary condition at the channel wall, depending on whether the reaction is homogeneous or heterogeneous. The solution yields local concentration and temperature fields, which are used for calculation of the concentration and temperature profiles along the column. 

The productivity can be greatly improved by the implementation of continuous operations and the use of process-intensification techniques, such as reactive distillation. The replacement of a homogeneous catalyst by a heterogeneous one is highly desirable. These aspects will be discussed in greater detail in the next section. 

In reactive distillation, the type of the catalysis is important. Homogeneous catalysis is possible in most cases but needs a separation step to recycle the catalyst. This can be avoided in heterogeneous catalysis, but here special constructions are necessary to fix the catalyst in the reaction zone. If everything harmonizes, considerable advantages arise as can be seen with reference to the Eastman-Kodak Chemicals process for the production of methyl acetate. As can be seen in Figure 4 only one column is needed if reactive distillation is used as opposed to nine and a reactor if it is not used. 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

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. 

Modeling of Homogeneous and Heterogeneous Reactive Distillation Processes... 

Modeling of Homogeneous and Heterogeneous Reactive Distillation Processes 219 The E equations are the phase equilibrium relations... 

In reactive distillation, both the chemical reaction and the distillative separation of the product mixture are carried out simultaneously. This integrative strategy allows us to overcome chemical equilibrium limitations. For an exothermic reaction, the heat of reaction can be used directly for distillation. The term catalytic distillation is also used for such systems where a catalyst (homogeneous or heterogeneous) is used to accelerate the reaction. The synthesis of methyl acetate and MTBE (methyl tertiary butyl ether) are the two most prominent examples, where reactive distillation is used on an industrial scale (for MTBE see Section 4.10.8.1). It is beyond the scope of this textbook to discuss more details of this technology. Details can be found in the literature (Sundmacher and Kienle, 2002 Harmsen, 2007 Taylor and Krishna, 2000 Krishna, 2002 Stankiewicz, 2003). 

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