Scale-up and Differential Expansion

To illustrate the importance of scale and the coefficient of thermal expansion, let us analyze two membrane modules that are assembled at room temperature (nominally 25 °C) and then heated to 400 C for operation. One membrane mod- 

However, relaxation of gasket seals is not the only way that differential expansion can adversely affect the performance of membrane modules. It is important to remember that each membrane in the module is constrained by the module construction. If the membrane is in the form of a planar, flat sheet, then it is con- 

Strained in two dimensions (the % and y axes). If the membrane is a tube, fixed to a header at one end and free floating at the other, then it is also effectively constrained in two dimensions (the % and y axes). This is shown schematically in Fig. 5.3. Even worse would be a tubular membrane that is fixed at both ends to headers, resulting in a membrane that is constrained in three dimensions. Surprisingly, the latter is a design that is still often used in laboratory test-and-evalua-tion cells using tubular metal membranes (usually a thin permselective layer deposited onto a porous tube). 

Pd-40Cu was approximately matched by using Monel in the module components in direct contact with the membrane (the membrane support), whereas in the other case 304 stainless steel was used for the membrane support As shown in Table 5.1, the coefficient of thermal expansion of 304 stainless steel is substantially greater than the coefficient of thermal expansion of Pd-40Cu. Failure occurred after only a few cycles in the case of the 304 stainless steel components, whereas the Monel components demonstrated much improved durability. 

To demonstrate that flexible graphite gaskets can accommodate some difference in the degree of expansion between the membrane and module components, the experiment was repeated using feed-charmel and permeate-channel frames made of 304 stainless steel, while the membrane support remained Monel. Cyclic durability was very good, and the membranes remained devoid of visible wrinkles. 

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