Equilibrium-limited reactions

Membranes in catalysis can be used to improve selectivity and conversion of a chemical reaction, improve stability and lifetime of the catalyst, and improve the safety of operation. The most well-known example is in situ removal of products of an equilibrium-limited reaction. However, many more ways of application of a membrane can be thought of [1-3], such as using the membrane as a reactant distributor to control the reactant concentration levels in the reactor, or performing catalysis inside the membrane and having control over reactant feed and product removal. 

Membranes can be applied to catalysis in different ways. In most of the literature reports, the membrane is used on the reactor level (centimeter to meter scale) enclosing the reaction mixture (Figure 10.3). In most cases, the membrane is used as an inert permselective barrier in an equilibrium-limited reaction where at least one of the desired products is removed in situ to shift the extent of the reaction past the thermodynamic equilibrium. 

It is useful to combine reaction and separation for equilibrium-limited reactions and also for consecutive reactions, particularly when the desired intermediate products undergo faster undesirable reactions. The concept of extractive reactions for equilibrium-limited and consecutive reactions has been covered in Section 4.2.1. Distillation column reactors provide yet another strategy. 

SA conversion increases with increasing residence time, and with increasing MeOH SA to a maximum of about 98%. It appears that the maximum conversion increases to 99% for the highest MeOH SA studied (30), consistent with an equilibrium limited reaction. The esterification reaction rate was strongly suppressed by lowering MeOH/SA ratio below 10%. 

DME hydrolysis is an equilibrium-limited reaction and is considered as the rate-limiting step of overall DME steam reforming. The equilibrium conversion of hydration of DME is low at low temperatures (e.g. about 20% at 275 °C). However, when methanol formed in the first step is rapidly converted into H2 and CO2 by methanol steam reforming catalysts, high DME conversion is expected. Therefore, enhancement of DME hydrolysis is an important factor to obtain high reforming conversion. 

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous... 

Several important conclusions can be drawn from Figure 4.38. It appears that in general a simple catalytic reaction, which includes the dissociation of a diatomic molecule, will have this dissociation as the rate-determining step, when the reaction takes place under conditions close to equilibrium. This agrees well with the ammonia synthesis being dissociation rate-determined, as this process is the prototype of an equilibrium-limited reaction [128]. When the reaction is taking place far from equilibrium, the actual approach to equilibrium becomes unimportant, and the volcano plot very closely follows the volcano defined by the minimum value among the maximal possible rates for all reaction steps. 

Ca (continuous line) and Cb 0dotted line) in a batch reactor for the equilibrium limited reaction. Initial conditions are Cao = 1 molm-3 and Cbo = 0 molm-3... 

Finally, when chemical kinetics contrasts with equilibrium, the parallel scheme is not trivial, since one of the products can be favored in the early stages of the batch cycle by faster kinetics and hindered in the later stages by unfavorable equilibrium. Such a case is shown in Fig. 2.4 for parallel reactions of A to Pi via an equilibrium limited reaction and to P2 via an irreversible reaction. 

One of the most investigated fields is the pervaporation-assisted catalysis applied to equilibrium-limited reactions. 

By far the most studied reactions combined with pervaporation is esterification. It is a typical example of an equilibrium-limited reaction with industrial relevance. 

The semibatch can also be used for continuous product removal, such as vaporization of the primary product. This can increase yield in equilibrium limited reactions. 

Fig. 7.1. Principle of an adsorptive reactor for enhancing conversion of an equilibrium-limited reaction.

In the case of equilibrium-limited reactions, the combination of reaction and separation in one (multifunctional) apparatus allows equilibrium limitations to be overcome by removing one or more products from the phase in which the reaction takes place. If a catalyst is present in a process, in-situ removal of an inhibiting product, which strongly adsorbs onto the catalyst surface, will lead to a higher catalyst activity and lifetime. 

A tube-wall reactor, in which the catalyst is coated on the tube wall, is conceptually ideally suited for highly exothermic and equilibrium-limited reactions because the heat generated at the wall can be rapidly taken away by the coolant. Previous work (1) has numerically demonstrated that for highly exothermic selectivity reactions, the optimized tube-wall reactor is superior from both steady state production and dynamic points of view to the fixed-bed reactor. Also, the tube-wall reactor is being advanced as a possible reactor for carrying out methanation in coal gasification plants (2). From a reaction engineering point of view, it therefore seems appropriate to analyze the reactor for the analytically resolvable case of complex first-order isothermal reactions. 

If the intermediate stages are used to extract a limiting product in the case of equilibrium-limited reactions an example is the intermediate absorption of SO.-, before the last stage of the SO synthesis. 

A simultaneous countercurrent movement of solid and gaseous phases makes it possible to enhance the efficiency of an equilibrium limited reaction with only one product (Fig. 4(a)) [34]. A positive effect can be obtained for the reaction A B if the catalyst has a higher adsorption capacity for B than for A. In this case, the product B will be collected mainly in the upper part of the reactor, while some fraction of the reactant A will move down with the catalyst. Better performance is achieved when the reactants are fed at some side port of the column inert carrier gas comes to the bottom and the component B is stripped off the catalyst leaving the column (Fig 4(a)). The technique was verified experimentally for the hydrogenation of 1,3,5-trimethylbenzene to 1,3,5-trimethylcyclohexane over a supported platinum catalyst [34]. High purity product can be extracted after the catalytic reactor, and overequilibrium conversion can be obtained at certain operating conditions. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

An example of an equilibrium-limited reaction is the hydrogenation of aromatics in petroleum fractions. This is illustrated by Fig. 3, which shows that under the prevailing conditions, saturation of aromatics is kinetically limited at temperatures below about 370°C, but that above this temperature, thermodynamic limitation occurs. In cocurrent operation, the hydrogen partial pressure will be lowest at the reactor outlet, due to the... 

Conversion enhancement of equilibrium-limited reactions A5= B+C, A Catalytic " membrane Controlled addition of a technical-grade reactant B + inert. ( poisons.. .. 

The major features of and application opportunities for inorganic-membrane reactors have been described in some detail. We can conclude that inorganic-membrane reactors actually show promise for improving either conversion of equilibrium-limited reactions (e.g., dehydrogenations) or selectivity toward some intermediates of consecutive reaction pathways (e.g., partial oxidations). 

The removal of products to increase conversion in equilibrium-limited reactions is indeed one of the hrst applications that comes to mind when considering membranes in a reaction environment. To make beneficial the use of the membrane reactor, the membrane employed must be able to separate preferentially at least one of the products from the reaction mixture, at a reasonable rate and selectivity. If the selectivity of the membrane is insufficient, reactant loss will become significant and the overall conversion in the membrane reactor will be lower than in a conventional packed bed reactor. If the permeation rate is insufficient, the ratio between the membrane surface area required and the volume of the catalyst bed will become unrealistic. 

In standard chemical processes a reaction unit is followed by one or more separation steps. This enables a straightforward design of each unit operation and offers a high degree of freedom for process design. However, with equilibrium limited reactions the yield is limited and can only be enhanced by additional recycle streams. This increases the dimensions of all apparatuses and the investment costs, especially for low conversion rates. 

Mainly PV aided conversions have been studied and more in particular esterifications, a typical example of an equilibrium limited reaction with industrial relevance and well-known reaction mechanisms. " This hybrid process has already made it to several industrial applications. The thermodynamic equilibrium in such a reaction can be easily shifted and obtained in a shorter reaction time by removing one of the products. Pervaporation is especially interesting because it is not limited by relative volatility or azeotropes and energy consumption is generally low, because only the fraction that permeates undergoes the liquid/vapor phase change. It can also be operated at lower temperatures, which can better match the optimal conditions for reaction. 

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