Reactive Distillation, Extraction, Crystallization

Gas-liquid reaction with catalytic solid. Include the catalyst with structured packing in a distillation column. Related topic distillation. Section 4.2. 

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. 

Liquid with homogeneous catalyst etherification, esterification Liquid-liquid HIGEE for fast, very fast and highly exothermic liquid-liquid reactions such as nitrations, sulfonations and polymerizations. 

Equilibrium conversion 90%. Use a separate pre-reactor when the reaction rate at 80% conversion 0.5 initial rate. 

Use concentration profiles developed from either equilibrium or nonequUibrium reaction-separation to identify the reactive zone. The reflux ratio for reactive distillation is greater than for distillation. Use 1.2-1.4 X niinimum. Damkohleru = 1-20 and usually 1-10. 

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Reactive distillation/ extraction/ crystallization, HIGEE Reaction window = distillation windovs reaction equilibrium can be shifted by removing one or more of the species from the reaction space. 

Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

Reverse-flow reactors Reactive distillation Reactive extraction Reactive crystalization Chromatographic reactors Periodic separating reactors Membrane reactors Reactive extrusion Reactive comminution Fuel cells... 

The direct high yield synthesis of oxaspiropentanes from almost any type of aldehyde or ketone represents a particularly useful transformation because of the high reactivity of such compounds. This approach proves to be exceptionally simple. The DMSO reaction mixture can be directly extracted with pentane or hexane, the hydrocarbon solvent removed and the product isolated by distillation or crystallization. Since diphenyl sulfide is the only by-product extracted with the oxaspiropentane, the mixture can normally be used for most further synthetic transformations. Table 2 summarizes some of the oxaspiropentanes prepared by this method. 

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,... 

Reactive distillation Membrane-based reactive separations Reactive adsorption Reactive absorption Reactive extraction Reactive crystallization... 

James Douglas Let me add two more things. I think reactive crystallization and reactive extraction will become more important in the future, as has reactive distillation. Reactive distillation is receiving a lot of attention at present, but reactive separations should find more applications. 

Table 25.1 lists several combinations of reaction and separation. The sequencing of the two in the nomenclature of the different combinations clearly reveals their orientations. This chapter is primarily concerned with reactive extraction (also termed dissociation-extraction), extractive reaction, reactive distillation (or dissociation-extractive-distillation), and distillative reaction (or distillation column reactors). Crystallization is almost always used for separation and seldom for enhancing a reaction. A notable exception is when one of the reactants is a sparingly dissolving solid and the size of the crystallizing solid is less than the thickness of the film surrounding the reactant. Then the crystallizing microphase enhances the rate of dissolution and hence the rate of reaction, a situation that was considered in Chapter 23. 

Separate liquid mixtures using distillation, stripping, enhanced (extractive, azeotropic, reactive) distillation, liquid-liquid extraction, crystallization, and/or adsorption. The selection between these alternatives is considered in Chapter 7. 

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

Separation of heat-sensitive materials. High molar mass material is often heat sensitive and will decompose if distilled at high temperature. Low molar mass material can also be heat sensitive, particularly when its nature is highly reactive. Such material will normally be distilled under vacuum to reduce the boiling temperature. Crystallization and liquid-liquid extraction can be used as alternatives to the separation of high molar mass heat-sensitive materials. 

Today, RD is discussed as one part of the broader area of reactive separation, which comprises any combination of chemical reaction with separation such as distillation, stripping, absorption, extraction, adsorption, crystallization, and membrane separation. In the next decade, unifying approaches to reactive separators should be developed allowing the rigorous selection of the most suitable type of separation to be integrated into a chemical reactor. 

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