Oxygen separation membrane reactors

Dense ceramic membranes allow oxygen separation with extremely high selectivity and can be incorporated into membrane reactors for a variety of oxygen-related reactions. The applications of dense ceramic MRs will bring many economic and environmental benefits, with improved selectivity and yields. However, in order to realize the potential benefits of MRs and commercialize them successfully, there are still many challenges that have to be faced not only from membrane materials but also from engineering aspects. 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

A special version of the membrane reactor using Pd was made for separating hydrogen and oxygen and their controlled reaction. 

GP 11] ]R 20] Investigations with a Pd membrane reactor relied on reaction of streams separated via a membrane (to prevent complete mixing of reactants, not to enhance conversion) [11]. A hydrogen/nitrogen stream is guided parallel to an oxygen stream, both separated by the membrane and water is thereby formed. The membranes, made by thin-film processes, can sustain a pressure up to 5 bar. 

Figure 9.4 Simplified sketch of the mixed conducting oxygen-separation membrane reactor part in the AZEP concept, after [20], Second combustor placed before the turbine improves efficiency, but also increases C02 emission.

A schematic of a palladium membrane reactor is shown in figure 1. The reversible reaction of 1-butene dehydrogenation occurs on the reaction side of the membrane in which the chrome-alumina catalyst is uniformly packed. The oxidation of hydrogen with oxygen in air occurs in the permeation or separation side on the palladium membrane surface. The... 

A number of research groups [83-86] have used a rather related concept for carrying out various selectivity limited reactions in a membrane reactor. The concept is illustrated schematically in the top part of Fig. 11.8 which is from a study by Harold and coworkers [84]. It applies to partial oxidation reactions where the desired reaction product can further react with oxygen to produce an imdesirable total oxidation product. In some instances it makes better sense (in terms of maximizing the reactor yield) to feed the two reactants separately on either side of the membrane rather that to co-feed them on either side. This is shown in the bottom part of Fig. 11.8 which shows the yield to the desired product as a fimction of the Thiele modulus as the degree of feed segregation... 

Oxidation of ligands can be avoided by the use of purely inorganic catalysts. If the catalysts are insoluble in the medium, as in zeolites or heteropolyacids, the workup is much simpler. If the oxidation can be run in the gas phase or in a melt, no solvent has to be separated. If the oxidant is oxygen, a membrane reactor can sometimes be used to keep the oxygen concentration low and allow the product to be separated continuously, thereby avoiding overoxidation. 

Another partial oxidation reaction that is attracting industrial attention for the application of reactive separations is the production of synthesis gas from methane [Stoukides, 2.127]. The earlier efforts made use of solid oxide solutions as electrolytes. Stoukides and coworkers (Eng and Stoukides [2.200, 2.126], Alqahtany et al. [2.201, 2.202]), for example, using a YSZ membrane in an electrochemical membrane reactor obtained a selectivity to CO and H2 of up to 86 %. They found that a Fe anodic electrode was as active as Ni in producing synthesis gas from methane (Alqahtany et al. [2.201, 2.202]), and that electro-chemically produced O was more effective in producing CO than gaseous oxygen (no ef-... 

---