Membrane distributor

Example 2 Reactant Dosing with Membranes ( Distributor )... 

There are multiple variations on the packaging and delivery systems employed by membrane distributors. Currently, AM is available in multiple forms, shapes, sizes, thicknesses, and cellular components. Some producers have also included additional components such as umbilical cord and other placenta derivatives. The selection of... 

Eig. 19. CME monopolar electrolyzer a, membrane b, cathode element c, half-cathode element d, current distributor e. Teflon tube f, CI2 + depleted brine manifold g, conductor rod h, CI2 + depleted brine outlet nozzle i, base frame j, recycled NaOH manifold k, recycled NaOH inlet nozzle 1, gasket (the gasket-to-element ratio is quite small) m, tie rod n, anode element o, H2 + NaOH manifold p, end plate, q, under cell bus bar (simplifies piping... 

Fig. 25. OxyTech MGC electroly2er a, membrane b, anode assembly c, manifold spacer d, anolyte outlet e, catholyte outlet f, bulkhead g, brine inlet h, NaOH inlet i, insulating channel j, bulkhead insulator k, interface material 1, cathode assembly m, interceU bus n, tie rod o, current distributor p,...

Fig. 13. A hoUow-fibet reverse osmosis membrane element. Courtesy of DuPont Permasep. In this twin design, the feedwater is fed under pressure into a central distributor tube where half the water is forced out tadiaUy through the first, ie, left-hand, fiber bundle and thus desalted. The remaining portion of the feedwater flows through the interconnector to an annular feed tube of the second, ie, right-hand, fiber bundle. As in the first bundle, the pressurized feedwater is forced out tadiaUy and desalted. The product water flows through the hoUow fibers, coUects at each end of the element, and exits there. The concentrated brine from both bundles flows through the concentric tube in the center of the second bundle and exits the element on the right.

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. 

Distributor The membrane is used to selectively feed components to a reaction mixture. 

Current is transferred to these meshes using distributors called spiders . The spiders have legs which distribute current to the mesh. The location of the spiders and the distribution of the anode and cathode spider legs has been developed via extensive calculation and trial to minimise the resistance of the spider/membrane/ mesh combination and to ensure that the resistance to current flow is equal across the whole membrane area, thus ensuring that there are no localised non-uniform current paths. The spiders are shown in Fig. 18.5 and the distribution pattern is shown in Fig. 18.6. 

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]. 

The performance of the membrane used was investigated in detail [33]. However, as it acts in this study simply as a distributor, and controlled convection is the essential transport mechanism, no specific knowledge is required to predict fluxes. A membrane suitable for the purpose described does not need to possess a special permselectivity. 

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. 

The Pd membrane tubes are positioned above a certain height from the gas distributor so that they are immersed in a gas mixture containing a considerable amount of hydrogen produced from the following reactions ... 

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. 

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