Permselective reactor model

Permselective Reactor Model. This model was developed to size an experimental reactor system. Even though, some of our initial assun tions are being refined in our continuing efforts, some in ortant conclusions can be drawn from our early work. The first approach used was to idealize the permselectivity, and assume that only hydrogen diffuses through the membrane. This assun tion cannot be justified for the characteristics of the membranes studied experimentally. It does, however, permit one to place an upper limit on the expected performance of the system. 

Using a developed plug-flow membrane reactor model with the catalyst packed on the tube side, Mohan and Govind [1986] studied cyclohexane dehydrogenation. They concluded that, for a fixed length of the membrane reactor, the maximum conversion occurs at an optimum ratio of the permeation rate to the reaction rate. This effect will be discussed in more detail in Chapter 11. They also found that, as expected, a membrane with a highly permselective membrane for the product(s) over the reactant(s) results in a high conversion. 

As a building block for simulating more complex and practical membrane reactors, various membrane reactor models with simple geometries available from the literature have been reviewed. Four types of shell-and-tube membrane reactor models are presented packed-bed catalytic membrane reactors (a special case of which is catalytic membrane reactors), fluidized-bed catalytic membrane reactors, catalytic non-permselecdve membrane reactors with an opposing reactants geometry and catalytic non-permselective membrane multiphase reactors. Both dense and porous inorganic membranes have been considered. 

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

Figure 10.6 compares the model and experimental results of direct thermal decomposition of CO2 using a dense yttria-stabilized zirconia membrane shell-and-tube reactor [Itoh et. al., 1993]. The agreement for the reactor conversion is very good. At a CO2 feed rate of less than 20 cm /min and with a membrane thickness of 2,(XX) pm the conversion is significantly enhanced by the use of a permselective membrane for oxygen. Beyond a feed rate of 20 cm /min., however, the difference in conversion between a membrane and a conventional reactor. 

The direct benefit of having a permselective membrane in a fluidized-bed steam reformer can be evaluated by modeling the reactor with and without the membrane for otherwise identical reactor features and feed conditions. An example of the comparison... 

The membrane reactors and their models discussed so far utilize the permselective properties of the membranes. The membranes which can be catalytic or inert with respect to the reactions of interest benefit the reactor performances mostly by selectively removing a product or products to effect the equilibrium displacement. 

Catalytic Non-permselective Membrane Multiphase Reactor (CNMMR) Model - Laminar Flow Liquid Stream... 

Membranes have also been used in reactors where their permselective properties are not important. Instead their well-engineered porous matrix provides a well-controlled catalytic zone for those reactions requiring strict stoichiomeuic feed rates of reactants or a clear interface for multiphase reactions (e.g., a gas and a liquid reactant fed from opposing sides of the membrane). Functional models for these types of membrane reactors have also been developed. The conditions under which these reactors provide performance advantages have been identified. 

Several investigators have faced the problem of modeling of membrane reactors either to achieve a proper interpretation of their experimental data or to assess the role of the various operating parameters (temperature, membrane permeability and permselectivity, feed flow rates, and concentrations, etc.) on the performance of membrane reactors. In some other cases [61,138] modeling studies helped to point the way toward future experimental work concerning, e.g., the need for thinner or more permeable or more stable membranes to outperform conventional technologies for given applications. 

There are a number of membrane reactor systems, which have been studied experimentally, that fall outside the scope of this model, however, including reactors utilizing macroporous non-permselective membranes, multi-layer asymmetric membranes, etc. Models that have been developed to describe such reactors will be discussed throughout this chapter. In the membrane bioreactor literature, in particular, but also for some of the proposed large-scale catalytic membrane reactor systems (e.g., synthesis gas production) the experimental systems utilized are often very complex, in terms of their configuration, geometry, and, of course, reaction and transport characteristics. Completely effective models to describe these reactors have yet to be published, and the development of such models still remains an important technical challenge. 

As discussed in Chapter 2, in a number of membrane reactor applications the membrane is non-permselective, and it simply acts as a contactor device (when it is catalytic), or simply as a means to distribute one of the reactants in a more uniform manner (when it is inert). In modeling such reactors one must take into consideration, in addition to Knudsen diffusion, the presence of molecular diffusion and convective transport. The Dusty Gas Model... 

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