Reaction with Separation

Chemical equilibrium responds to a stress by moving to the side that relieves the effect of stress and returns the system to equilibrium, according to Le Chatlier s Principle. If we add heat to an exothermic reaction it will shift toward the reactants on the left. If we add mass, concentration, or pressure on the reactant side of the equilibrium, the system responds by shifting toward the products. If we can remove products from the reaction zone (the system or control volume), then we also shift the reaction equilibrium to the right. In fact, even if we remove just one of the products from a set of products, the system will shift to the right. 

In the case of the reaction class we are considering, that is, the dehydrogenation of alkanes to alkenes and hydrogen, continuous removal of either the alkene or hydrogen from the system will shift the conversion of the reactant alkane further to the products. This is the phenomenon we wish to examine. 

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Reactive distillation is the name given to the process where the chemical reaction and product separation are carried out simultaneously in one unit. Carrying out the reaction, with separation and purification of the product by distillation, gives the following advantages ... 

Figure 7. Fluorous tin reactions with separation by solid-liquid extraction...

Another multiphase reactor that achieves reaction with separation is catalytic distillation. In this reactor a catalyst is placed on the trays of a distillation column or packed into a distillation column, as shown in Figure 12-18. 

Harrison, D.J., Andersson, P.E., Li, P.C.H., Chiem, N., Tang, T., Smith, R., Szarka, R., Tran, T., Integration of biochemical and cellular reactions with separation on-chip. 19th International Symposium on Capillary Chromatography and Electrophoresis, May 18-22, 1997. 

Another group of forced unsteady-state processes combines chemical reaction with separation of products. For these processes a reactor with a circulated moving bed (Fig. 4(a)) can be employed. Circulation of the catalyst can be substituted by periodic changing of... 

Apparently, it is not difficult to imagine coupling a chemical reaction with separation or heat transfer to obtain a concept of multifunctional reactors which frequently result in higher productivity. 

Another group of FUSO combines chemical reaction with separation of products. These methods can employ a reactor with circulating bed of catalyst [3,4] or by periodic changes of feed and product ports in a reactor with several fixed beds, known as simulated moving bed reactor [5-7]. Reaction and separation can include periodic pressure changes using the known separation technique of pressure-swing adsorption. 

Steady-State Membrane Reaction with Separation... 

Equilibrium reaction with separation (distillation, extraction)... 

Type I. Parallel reactions with separate reactants and products A B (iti)... 

Type I. For parallel reactions with separate reactants, the analysis is easily handled, since the two steps are independent of each other. Indeed, the inclusion of Type 1 as a nearly complex scheme is not really necessary for the homogeneous batch reactions at constant volume treated here, since the yield and selectivity definitions are redundant in this case with rate and conversion. The system is important in heterogeneous systems, however, so we introduce it for later reference. For first-order, irreversible reactions ... 

Other methods of combining reaction with separations, such as extraction, crystallization, and adsorption, are being explored, but none have been used on a large scale. Using reactors with membranes that selectively remove a reaction product is a very promising development, but improvements in membrane permeability, selectivity, and high-temperature stability are needed for practical processes. 

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. 

Irrespective of whether reaction or separation is of primary concern, three types of reaction-separation techniques are commonly used reaction-extraction, reaction-distillation, and reaction-crystallization. Each of these can in theory be either reaction or separation oriented. Among other, less conventional, methods of combining reaction with separation are biphasing and the use of membrane reactors. These were considered in earlier chapters. 

Nisoli, A., Malone, M.F., Doherty, M.R, 1997. Attainable regions for reaction with separation. AICHE J. 43, 374—386. 

The strong collision master equation is readily solvable for the case of competing reactions with separate thresholds [81.V2] the result takes the form of generalisation of equation (5.17) and, neglecting the denominator as before, we have... 

Gas-flowing solids-fixed bed contactor was patented as early as 1948 [1]. The first industrial realization occurred in France in 1965, named the raining packed bed exchanger [31] for heat recuperation process. Since then, it has been efficiently exploited. Potential applications are gas purification, adsorption processes, drying, etc. Another interesting concept would be the use of gas-flowing solids-fixed bed catalytic reactors for the equilibrium reactions with separation of products in situ. 

The advantages of combining the water-gas shift reaction with separation of CO2 are as follows ... 

Our suggested method of numerical value analysis for the mechanisms of complex reactions is based on the use of the system of Hamiltonian equations for the mathematical modeling of reactions, with separation of the targeted functional that characterizes the quality of the selected chemical reaction. 

The connection between thermodynamics and chemical reaction engineering is very strong. First of all, we need to establish whether the conversions we desire at the temperatures and pressures involved are achievable—whether we reached the limits of thermodynamic equilibrium or are still left with room to maneuver. The second connection is the relation between the chemical and phase equilibria. If we are to design a reactor for a multiphase reaction, the phase equilibria become an immediate problem that we have to solve. The more novel connection comes later, when we intend to combine reaction with separation. A good a priori estimation in designing systans with a multitude of functionalities, such as a distillation column reactor or a monbrane reactor, requires the solution of the chemical reaction problems along with the phase equilibria and other defining constraints that come into play. 

Suitable spray reagents which produce coloured derivatives upon reaction with separated substances on a chromatoplate are not available for all the types of substances likely to occur in extractants. If an extractant contains a mixture of additives which have to be separated, and for some or all of which spray reagents are not available, then the following approach is recommended. 

In the reactions with separable rate, the second kind of changes of laws relate to the space function, there is no change of mechanism nor of mode, thus, the reactivity is not affected. 

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