Commercially-available membranes description

Thermal shock resistance. Temperature swing as part of the normal cycles of operation or regeneration of the membranes or membrane reactors can lead to deleterious thermal shock. The materials for the various components in a membrane reactor should be carefully selected to impart good thermal sh k resistance. This is particularly important for high temperature reactions. Also listed in Table 9.5 is a summary of various membrane materials along with qualitative description of their resistance to thermal shock. Again, the available data apply to dense materials. While various metal oxides have been made into commercial inorganic membranes, they tend to be affected by thermal shock much more than other ceramic materials. 

Most of the available commercial microporous membranes such as polysulfone, polyethersulfone, polyamide, cellulose, polyethylene, polypropylene, and polyvinylidene difluoride are prepared by phase inversion processes. The concept of phase inversion in membrane formation was introduced by Resting [75] and can be defined as follows a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer-rich phase forms the continuous membrane matrix and the polymer lean phase fills the pores. A detailed description of the phase inversion process is beyond the scope of this section as it was widely discussed in Chapters 1 and 2 nevertheless a short introduction of this process will be presented. 

Fig. 5 Commercially available ultrafiltration apparatus for pressures up to 6 bar. On the right-, the low molecular species accumulate together with the solvent in the Erlenmeyer flask, whereas the high molecular species are retained by the membrane. Via the tube on the upper side the cell can be set under pressure. On the left schematic description...

The most reliable method is probably the potentiometric titration procedure first reported by Dilley [101]. This procedure has the added advantage of avoiding the use of trichloromethane. The procedure for the manufacture of the membrane-indicating electrode has been modified and a simplified description is given below. Commercial variants are also becoming available. 

With the increased computational power of today s computers, more detailed simulations are possible. Thus, complex equations such as the Navier—Stokes equation can be solved in multiple dimensions, yielding accurate descriptions of such phenomena as heat and mass transfer and fluid and two-phase flow throughout the fuel cell. The type of models that do this analysis are based on a finite-element framework and are termed CFD models. CFD models are widely available through commercial packages, some of which include an electrochemistry module. As mentioned above, almost all of the CFD models are based on the Bernardi and Verbrugge model. That is to say that the incorporated electrochemical effects stem from their equations, such as their kinetic source terms in the catalyst layers and the use of Schlogl s equation for water transport in the membrane. 

Unfortunately, all the properties required for these calculations are not available in the open literature for the membranes that are used commercially. Therefore, some simplified descriptions and assumptions are employed here to show the trends expected of a membrane under given operating conditions. One can refine these calculations with more precise data, when made available by the membrane suppliers. 

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