Oxygen permselective membranes

The methane combustion configuration is sketched in Figure 3.14 and consists of two sections [44]. Hydrogen permselective membranes are integrated in a fiuidized reform-ing/shift top section where ultra-pure H2 is extracted and the energy required for the SR is supplied via in situ methane oxidation in a separate fluidized bottom section, where oxygen is selectively fed to the methane/steam feed via oxygen permselective membranes. 

Oxygen permselective membranes. Oxygen selective membranes are used to supply the oxygen along the height of the reformer for the oxidative reforming reaction. 

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. 

Stancell et. al. ( 0) reported the possible use of ultrathin films deposited onto relatively permeable substrates as permselective membranes. Ultrathin and highly crosslinked coatings effectively distinguish between molecules of different sizes and increase the permselectivity of the substrate film. Chang et. al. ( ) demonstrated that the permeability coefficient of silicone rubber to oxygen decreased noticeably after depositing a plasma-polymerized ethylene film on the surface. Colter, et. al. (92.93) found similar effects of plasma polymerized films as diffusion barriers in controlled-released drug delivery systems. 

A trimethylsilyl or trifluoromethyl group containing poly(phenylacetylenes) for oxygen and ethanol permselective membranes had been prepared by WCle and a Rh complex [Rh(cyclooctadiens) (triphenyl-phosphine) 2] PFe (RhP). ... 

In the packed bed the catalyst is in a fixed configuration and in contact with a hydrogen permselective membrane. The most used packed bed configuration is the tubular one where the catalyst may be packed either inside the membrane tube or in the shell side, while the permeation stream is collected on the other side of the membrane (in case of hydrogen selective membranes) or one reactant is fed at the other side of the membrane (in case of oxygen selective membrane). 

Permselective membrane n. A thin film that will preferentially permit gases of different kinds to pass through the film at different rates. For common gases such as hydrogen, oxygen nitrogen, and carbon dioxide, silicon rubber is the most permeable polymer. 

In all experiments, periodical checking of the exit feed by gas chromatographic analysis on the N2 admixture revealed its absence within the limit of analysis sensitivity ( 0.1%). Hence, membrane retained its integrity and oxygen permselectivity in studied conditions. 

As can be seen, air instead of pure oxygen can be used directly as the oxidant in the dense ceramic membrane reactors because of the 100% oxygen permselectivity of the membrane. This may lead to a dramatic decrease in the operation cost of the membrane reactor. Furthermore, the oxygen can be introduced into the reaction side in a controllable mode. Thus, the excessive temperature excursion can be avoided, making operation of the reactor more safe. 

The CVI-experiments with silane as precursor did not show any increase in permselectivity of the membranes. The reaction temperature was in all cases 275°C and several oxygen pressures were tried. In each experiment, however, white powder was obtained on the membrane surface, indicating the decomposition of silane at the surface of the membrane. Reaction conditions could not be chosen in such a way that a highly separative layer was obtained. This was probably related to with the fact that the reactor temperature or the concentration of silane in the precursor gas was too low. Safety regulations, however, prohibited an increase of the silane concentration in the precursor flow. 

Metals such as Zr and Nb have also been proposed for H2 permselective dense membranes [1]. Dense silver membranes have been used for selective oxygen permeation [18],... 

Moreover, if this membrane is permselective for the added reactant, it is possible to use less expensive forms of this reactant (air instead oxygen), thus avoiding separation steps. Or, if explosive mixtures may be... 

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