Membrane reactors distributor

Figure 33.2 Typical scheme of a membrane reactor (distributor) for distributive feeding of a...

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

Clearly, the distributor type of membrane reactor is focussed on point (iv) above. Systematic theoretical studies explaining in the more detail the significance of reaction orders with regard to the selection of the component that should be dosed are available [53-55],... 

Several profound theoretical and experimental studies performed on the laboratory scale have been reported which focus on the use of various configurations of membrane reactors as a reactant distributor in order to improve selectivity-conversion performances. In particular, several industrially relevant partial oxidations have been investigated, including the oxidative coupling of methane [56], the oxidative dehydrogenations of propane [57], butane [58], methanol [59, 60], the epoxidation of ethylene [61], and the oxidation of butane to maleic anhydride [62]. 

Additional experimental data not presented here are summarized in Refs. [66, 67]. As was pointed out also in Ref. [64], these results highlight the important point that in membrane reactors, besides differences in local concentration profiles, different residence time distributions occur that lead to specific reactor behavior. Others [71] have also suggested that the flexibility of this type of distributor membrane reactors allows a certain target component to be produced efficiently within a complex reaction network. In the present example, there exist certain operating conditions under which the membrane reactor outperforms the conventional reactor in terms of the production of CO or CO2 (if these are considered as target products instead of ethylene). 

Several profound theoretical studies are available with regard to the distributor type of membrane reactor available. A thorough analysis of this situation has been presented in [72]. Later reports [53-55] deliver further instructive insight. 

Dosing profiles in distributor-type membrane reactors should be adjusted carefully in accordance with the reaction rates. 

FIGURE 6.28 Roles of the membrane in membrane reactors, (a) Extractor the removal of product(s) increases the reaction conversion by shifting the reaction equilibrium (b) distributor the controlled addition of reactant(s) Umits side reactions (c) and (d) active contactors the controlled diffusion of reactant(s) to the catal)dic membrane can lead to an engineered catal3dic zone. 

FIGURE 6.32 Schematic diagram of an integrated distributor/extractor membrane reactor based on the combination of dense ceramic oxygen and hydrogen transport membrane for syngas production. 

Zeolite-Membrane Reactors as Reactant Distributors for Selectivity Enhancement.301... 

Farrusseng D, Julbe A, and Guizard C. Evaluation of porous ceramic membranes as O2 distributors for the partial oxidation of alkanes in inert membrane reactors. Sep Purif Technol 2001 25 137-149. 

Tan and Li [5.78] and Li and Tan [5.79] developed theoretical models to describe the hollow-fiber membrane reactors ( Shanbhag et al [5.80, 5.81]), discussed in Chapter 2, which are applicable to water treatment processes, in which the membrane functions as an efficient reactant (e.g., ozone, hydrogen) distributor, and the chemical reaction takes place either in the hollow-fiber lumen or in the shellside. The model assumes a constant density... 

In this chapter, we will focus on the Oxygen-Ion Transport Membrane Reactors (OriMRs) as oxygen distributors for selective oxidation, as the open circuit operation mode and NEMCA phenomena are not of concern here. 

Due to the availability of the mentioned overviews it is not the goal of this chapter to consider the whole field of membrane reactors. Rather, the discussion below will be focused on presenting simplified and more detailed mathematical models capable of describing the performance of membrane reactors. Although there are several studies available for analyzing the combination of reaction and membrane separation (e.g. Salomon et al., 2000 Struis and Stucki, 2001 Wielandet al., 2002 Patil etal., 2005 Rohde etal., 2005) there is a need to analyze in more detail specific features of membrane reactors. The focus of this chapter will be the development and application of simplified and also more detailed mathematical models for packed-bed membrane reactors in which certain reactants are dosed over the reactor wall using nonselective membranes. This type of membrane reactor is sometimes also-called a distributor (Dalmon, 1997 Julbe et al, 2001). Despite this restricted focus of the work, most of the concepts considered should be applicable also in the analysis of other types of membrane reactors. 

In this chapter a specific type of membrane reactor, the so-called distributor was analyzed theoretically. In contrast to conventional tubular fixed-bed reactors (FBR), where all reactants are introduced together at the reactor inlet (cofeed mode), packed-bed membrane reactors (PBMR) allow dosing of one or several reactants via membranes over the reactor wall along the axial coordinate (distributed-feed mode). 

The distributor -type membrane reactor possesses different residence-time characteristics and different local concentration profiles compared to the conventional FBR. The additional degrees of freedom allow in complex networks of consecutive and parallel reactions, the selectivity and the yield to be enhanced with respect to a certain target product. The concept can be considered as an interesting option in the current attempts to improve and intensify reaction processes. 

Figure 7-21. Configuration and role of membranes in catalytic membrane reactors (a) extractor (b) distributor (c) contactor.

Figure 1.11 Principles of membrane reactors to enhance the reaction process (a,b) membrane as a product extractor (c,d) membrane as a reactant distributor (e,f) membrane as an active contactor.

In catalytic membrane reactors (CMRs), the reactions take place directly on the membrane and the membrane functions as both a catalyst and a separator/distributor.This requires that the membrane material has intrinsic catalytic activity or that it is modified by the addition of active components. Some of the commonly utilized inorganic (such as metal oxide and zeolite) and metal membranes are intrinsically catalytically active. In other cases, the catalysts can be integrated with the membrane into a single body by being coated on the membrane surface or deposited inside the membrane porous structure. In case the membrane does not participate in the reaction directly, but is used to add or remove certain species from the reactor, this is called an inert membrane reactor (IMR). 

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