Controlled Addition of Reactants

Consider the reaction A + B — C that is performed in a semibatch reactor as shown in Figure 5-19, where the reaction is first order and carried out by the controlled addition of reactant B. Assuming that the reaction is first order with respect to both A and B (i.e., second order overall), the rate of disappearance of components A and B is (—rA) = (—rB) - kiCACB. 

CMRs of this type (controlled addition of reactants) have been analyzed on the basis of reaction kinetics [73]. 

Equilibrium-restricted reactions (Section A9.3.3.1) have until now been the main field of research on CMRs. Other types of application, such as the controlled addition of reactants (Section A9.3.3.2) or the use of CMRs as active contactors (Section A9.3.3.3), seem however very promising, as they do not require permselective membranes and often operate at moderate temperatures. Especially attractive is the concept of active contactors where the membrane being the catalyst support becomes an active interface between two non-miscible reactants. Indeed this concept, initially developed for gas-liquid reaction [79] has been recently extended to aqueous-organic reactants [82], In both cases the contact between catalyst and limiting reactant which restricts the performance of conventional reactors is favored by the membrane. 

The concept of combining membranes and reactors is being explored in various configurations, which can be classified into three groups, related to the role of the membrane in the process. As shown in Figure 25.12, the membrane can act as (a) an extractor, where the removal of the product(s) increases the reaction conversion by shifting the reaction equilibrium (b) a distributor, where the controlled addition of reactant(s) limits side reactions and (c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone. In the first two cases, the membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

The combination of reaction and separation in one multifunctional membrane reactor is an interesting option. In such a reactor the membrane could be catalytically active itself, or it could serve only as a separation medium. There are several types of operation for such a reactor [33]. It could be used to separate the formed products from the reaction mixture. In this way it is possible to overcome equilibrium limitations or to improve the selectivity of the reaction. Another possibility is the controlled addition of reactant via the membrane, which might be of use in, for example, oxidation reactions or sequential reactions. The advantage of using zeolitic membranes in a membrane reactor is that they have a high thermal stability and exhibit a good selectivity. Moreover, they can be made catalytically active. 

This reaction is conducted in an isothermal semi-batch reactor. The desired product in this system is C. The objective is to convert as much as possible of reactant A by the controlled addition of reactant B, in a specified time //= 120 min. It is not appropriate to add all B initially because the second reaction will take place, increasing the concentration of the undesired by-product D. Therefore, to keep a low concentration of product D and at the same time increase the concentration of product C, the reactant B has to be fed in a stream with concentration = 0.2. A mechanistic model for this process can be found in [11]. 

FIGURE 9.29 Roles of the membrane in membrane reactors (a) Extractor the removal of produces) increases the reaction conversion by shifting the reaction equilibrium, (b) Distributor the controlled addition of reactant(s) limits side reactions, (c) and (d) Active contactors the controlled diffusion of reactant(s) to the catalytic membrane can lead to an engineered catalytic zone. 

The most generic distinction in the wide variety of membrane reactors can be made according to the possible functional roles of the membrane in the reactor, being controlled addition of reactants, separation of products from the reaction mixture and retention of the catalyst. Additionally, membrane processes can be divided based on the physical state of the retentate and permeate, respectively ... 

For gas-phase reactions, the controlled addition of reactants A and B can effectively be applied to systems with two competing reactions, e.g. partial oxidation of hydrocarbons ... 

In the past 2 decades, membrane reactors, as devices combining chemical reactions and a membrane separation in one unit, have attracted considerable attention. One of the major advantages of membrane reactors is that the equilibrium constraint of many reversible reactions can be overcome by removal of one or more product(s) through the membrane, thus further increasing the conversion. Controlled addition of reactants is also beneficial, and it not only decreases the undesired side reactions, but it also increases the safety. 

In this case, the membrane compartmentalizes the reactor and functions for separation but is not involved directly in the catalytic reaction (called an inert membrane ). The catalyst pellets are usually packed or fluidized on the inert membrane (Figure 1.12(a)), which acts as an extractor for fractionation of products and/or as a distributor for controlled addition of reactants. This incorporation of catalyst is most popular in practical use and can easily be operated. Since the catalyst is physically separated from the membrane, the separation fimction of the membrane and the activity of the catalyst can be modulated independently. Catalysts are generally placed on the separation layer... 

Distributor The controlled addition of reactant(s) limits side reactions to increase selectivity by optimizing the reactant concentration profile to mitigate the temperature rise in exothermic reactions Partial oxidation oxidative dehydrogenation of hydrocarbons oxidative coupling of methane... 

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