Membrane reactor nonisothermal

Yeung et al. [1994] extended the studies to a general case of a bed of catalyst pellets on the feed side of a membrane reactor where the membrane is catalytically inert for an arbitrary number of reactions with arbitrary kinetics under nonisothermal conditions. Their conclusions are similar to those for the case of pellets in a fixed bed reactor [Baratti et al., 1993]. It appears that the presence of a catalytically inert membrane and a permeate su-eam do not affect the nature of the optimal catalyst distribution but may... 

The Hrst type of generic model for shell-and-tube membrane reactors refers to a nonisothermal packed-bed catalytic membrane tubular reactor (PBCMTR) whose cross-sectional view is shown in Figure lO.l. Mathematical models for this type of membrane reactor have been reviewed quite extensively by Tsotsis et al. [1993b]. 

It is relevant at this point to stress the importance of preserving the nonisothermal condition of the reactor. Most of the modeling studies of membrane reactors assume an isothermal operation. However, as it has been demonstrated experimentally [Becker et al., 1993], this assumption is incorrect and, more often than not, a temperature profile exists along the membrane reactor length. 

Preferred flow patterns in nonisothermal membrane reactors. The discussions so far focus on flow patterns in an isothermal membrane reactor. In many situations, however, the membrane reactor is not operated under a uniform temperature. The choice between a plug flow (PFMR) and a perfect mixing membrane reactor (PMMR) depends on a number of factors. First of all, it depends on whether the reaction is endothermic or exothermic. 

IIJIJI Thermal Management of Nonisothermal Membrane Reactors... 

While isothermal condition is preferred to prevent undesirable reactions and their products from occurring and makes process control of the membrane reactors easier, it is sometimes difficult, if not impossible, to control the temperature of the reactor containing the reaction components uniformly across the reactor at a fixed level. Moreover, as will be pointed out later, certain nonisothermal conditions actually give better reactor performance than isothermal conditions. 

Effects of space time under nonisothermal conditions. The above discussions around the effects of space time on a membrane reactor performance are limited to isothermal conditions. The behavior of the reaction conversion in response to space time can be further complicated under nonisothermal conditions. 

First of all, the space time defined in Eq. (11-5) or (11-6) depends on the volume of the reactor and the total volumetric feed rate. Thus, for a given reactor volume, space time is inversely proportional to the total feed rate. Itoh et al. [1993] studied the use of a dense yttria-stabilized zirconia membrane reactor for thermal decomposition of carbon dioxide. The reactor temperature was not kept constant everywhere in the reactor but varying with the reactor length instead. The resulting temperature profile is parabolic with the maximum temperature at the midpoint of the reactor length. This nonisothermal... 

C.-Y. Tsai, Y.H. Ma, W.R. Moser and A.G. Dixon, Simulation of nonisothermal catalytic membrane reactor for methane partial oxidation to syngas, in Y.H. Ma (Ed.), Proceedings of the 3rd International Conference on Inorganic Membranes, Worcester, 1994, pp. 271-280. 

Dixon, A.G., 2001. Analysis of Intermediate Product Yield in Distributed-Feed Nonisothermal Tubular Membrane Reactors. Catalysis Today, 67(1-3) ... 

Based on the experimental data of the C02-selective facUitated transport membranes described earlier in this chapter, we proposed the concept of C02-selective WGS membrane reactor and developed a one-dimensional nonisothermal model to simulate the reaction and transport process (Huang et al., 2005a). The modeling results have shown that H2 enhancement and CO reduction to 10 ppm or lower are achievable for autothermal reforming synthesis gas via CO2 removal. With this model, we have elucidated the effects of system parameters on the membrane reactor performance. Using the membrane synthesized and the commercial WGS catalyst, we have obtained a CO concentration of less than 10 ppm in the H2 product in WGS membrane reactor experiments and verified the model developed (Huang et al., 2005b Zou et al., 2007). 

In the previous discussion of the one-dimensional nonisothermal simulation results it has been shown that for certain operating conditions the ethane conversion can be increased considerably in a PBMR compared to conventional fixed-bed reactors. The price, which had to be paid, was the higher local heat generation and insufficient heat removal. The problem is more pronounced in the large-scale apparatus. For illustration, the temperature profile in the PBMR calculated with the extended version of the a -model is depicted in Fig. 5.22. Accounting for the thermal resistance of the shell side and of the membrane, a temperature maximum of more than 20 K above the inlet and outer reactor wall temperature is predicted. For the sake of completeness it has to be noted that the thermal resistance of the shell side was calculated for an annulus filled with inert particles. This constructional modification is, compared to a reactor with an empty annulus, necessary, otherwise the reaction is becoming uncontrollable. Because of... 

The kinetics of reactions is specific for different reaction systems and processes and valid for isothermal and nonisothermal reactors. The effects of the kinetics on the conversion, selectivity, or yield depend on the reaction and may be quite pronounced. Liquid or gas phase reactions with high heat capacity can be performed in specific reactors, which operate isothermally or not. We will study the most common cases such as semibatch reactors, recycle reactors, fixed-bed reactors, and reactors with membranes. 

Caravella et al. (2008) PBMR - ID - Nonisothermal conditions - Plug flow - Membrane completely selective toward H2 permeation - Ergun s equation - Xu and Froment (1989a) The paper pointed out that the reactive/permeative stage distribution has to be considered an important reactor design parameter. 

Stream, that is, the H2 product, was <10 ppm (on the dry basis) for the various feed rates of the syngas at 20, 30, 40, 50, 60, and 70 cm /min (with 20% steam in the syngas). The nonisothermal model was modified for the flat-sheet membrane configuration of the reactor. As shown in this figure, the data agreed reasonably with the prediction by the model. 

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