Special distillations reactive distillation

Reactor types modeled A, stoichiometric conversion B, equiUbrium/free-energy minimization, continuous stirred tank, and plug flow C, reactive distillation. Some vendors have special models for special reactions also, private company simulators usually have reactors of specific interest to their company. 

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. 

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. 

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 (non-reactive) distillation columns linked to the side reactors can be much smaller in diameter than the RD column and no specially designed trays (e. g., with higher weirs or additional sumps) or proprietary devices such as Katapak-S are necessary. The side-reactor concept is particularly attractive when the conversion requirements are not as stringent as assumed in the MeOAc case study above. 

Reactive distillation occurs in multiphase fluid systems, with an important role of the interfacial transport phenomena. It is an inherently multicomponent process with much more complexity than similar binary processes. Multi-component thermodynamic and diffusional coupling in the phases and at the interface is accompanied by complex hydrodynamics and chemical reactions [4, 42, 43]. As a consequence, an adequate process description has to be based on specially developed mathematical models. However, sophisticated RD models are hardly applicable for plant design, model-based control and online process optimization. For such cases, a reasonable model reduction should be applied [44],... 

Reactive distillation combines the unit operations reaction and distillation in a single apparatus. This fairly new technology has reached some importance in special fields of the process industry (Sundmacher and Kienle 2003). 

Special features of this Institute reflected fully in these Proceedings, are the treatments of biological reactions, facilitated transport, reactive distillation, solvent extraction of metals and some related aspects of coal utilisation. 

The starting point of this sequence is the reactive mixture xi on the chemical equilibrium line. This liquid mixture is in phase equilibrium with the vapor yl, which is totally condensed to x. Since this mixture is apart from the chemical equilibrium line, it reacts along the stoichiometric line to the equilibrium composition X2- As can be seen in figure 2.2, the difference of the slope between the stoichiometric and liquid-vapor equilibrium lines defines the orientation of the reactive distillation lines. This difference in behavior allows one to identify a point, at which both the phase equilibrium and stoichiometric lines are collinear and where liquid concentration remains unchanged. This special point (labelled A in figure 2.2) is conventionally referred to as reactive azeotrope and is surveyed in section 2.4. 

This chapter provides an exhaustive overview on current design methodologies with application to reactive distillation processing. By no means the presented material should be regarded as the ultimate overview, specially due to the dynamic developments lately experienced by RD research. The overview ends with a list of scientific literature... 

Catalytic Distillation Catalytic or reactive distillation is another example of the use of a hybrid reactor and combines catalysis and distillation in one column/reactor. Usually, we have a two-phase process with gas and liquid flowing in countercurrent mode. This requires special catalysts and packings, for example, monoliths, as in case of a fixed-bed flooding of the reactor would occur at high flow rates. In industry, catalytic distillation is already used for the production of MTBE (methyl tert-butyl ether), an important octane booster (Figure 4.10.77 DeGarmo, Parulekar, and Pinjala, 1992), cumene (DeGarmo, Parulekar, and Pinjala, 1992), and ethylbenzene (Podrebarac, Ng, and Rempel, 1997). 

A special type of catalytic reactor consists of reactive distillation columns, in which the reaction takes place on catalyst particles that are assembled into a column, while the products and reactants are continuously being separated by means of distillation. 

Figure ft-4 Semibatcb tcactcws. (a) Reactor suutup. reactive distillation. [Exccipted by special pcrmis-sirai from Chem. Eng.. 

For catalytic application where a transition metal catalyst is dissolved in the ionic liquid or the ionic liquid itself acts as the catalyst two additional aspects are of interest. Firstly, the special solubility properties of the ionic liquid enables a biphasic reaction mode in many cases. Exploitation of the miscibility gap between the ionic catalyst phase and the products allows, in this case, the catalyst to be isolated effectively from the product and reused many times. Secondly, the non-volatile nature of ionic liquids enables a more effective product isolation by distillation. Again, the possibility arises to reuse the isolated ionic catalyst phase. In both cases, the total reactivity of the applied catalysts is increased and catalyst consumption relative to the generated product is reduced. For example, all these advantages have been convincingly demonstrated for the transition metal catalysed hydroformylation [17]. 

The most basic raw petrochemical materials are liquefied petroleum gas, natural gas, gas from cracking operations, liquid distillate (C4 to C6), distillate from special cracking processes, and selected or isomerized cyclic fractions for aromatics. Mixtures are usually separated into their components at the petroleum refineries, then chemically converted into reactive precursors before being converted into salable chemicals within the plant. 

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