Material and Engineering Considerations at Application Temperatures

The majority of gas separation applications use pressure difference as the driving force for the membrane separation. As such, the issues of sealing the ends of membrane elements and connecting the elements and the module or process piping are critical in providing gas-tight or essentially leakproof conditions. The seals and connections are necessary to prevent remixing of the permeate and the retentate streams. 

In most of the industrially important gas separation applications, the feed streams to be processed occur at high temperatures. It is very desirable not to ramp down the stream temperature and then ramp up again after the treatment It is exactly this reason that inorganic membranes are attractive due to their inherent thermal stabilities. Operation at high temperauires, however, not only confounds the above issues but also can affect the phases and microstructures of the membrane materials. All these factors have implications on the permeabilities and permselectivities. 

As will be introduced in the next chapter, a new class of reactors called membrane reactors combines two unit operations (membrane separation and catalytic reaction) into one compact operation. Many of the membrane reactors of potential interest use inorganic membranes and are operated in gas/vapor phases at high temperatures involving gas separation. Thus membrane separation of gases is often a critical part of a membrane reactor and the aforementioned material and engineering considerations apply. Therefore, discussions on these and other additional considerations will be treated in detail later in Chapters 9 through 11. 

A vastly growing number of investigations on inorganic membranes is witnessed today. Central to these activities is the development of membrane materials which under the applications conditions can offer high permselectivities with acceptably high permeabilities. The need for separation factors beyond those limited by Knudsen diffusion which is based on molecular mass is obvious. It appears that other transport mechanisms will be required to effect higher permselectivities. 

Molecular sieving and the interactions of gas molecules with the membrane are possible alternatives. As discussed in Chapter 4, if surface diffusion is operative on a gas but not the other, it can enhance the separation factor. Although surface diffusion contribution decreases with increasing temperature, it becomes more important as the pore diameter becomes smaller. Therefore, it is possible that as inorganic membranes with smaUer pore sizes become available their separation performance may increase not only due to molecular sieving effects but also surface diffusion or other transport mechanisms. 

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