Homogeneous Cake

Quick cooling is often advantageous, e. g. for recombinant DNA-proteins, in order to avoid crystallization of salts and to obtain the best possible, homogeneous cake. 

In lyophilized preparations, mannitol (20-90% w/w) has been included as a carrier to produce a stiff, homogeneous cake that improves the appearance of the lyophilized plug in a vial. A pyrogen-free form is available specifically for this use. 

In a top-feed filter test, the filter cake will contain all of the solids, provided they are all emptied from the sample container. The danger in this type of test is that the solids will stratify, particularly if the c e formation time is prolonged. Close examination of the filter cake will indicate whether or not this has happened. If there has been significant stratification, the feed slurry should be modified by thickening and/or flocculation in order to increase the dry solids filtration rate and permit formation of a homogeneous cake. Another possibility, but not necessarily the best, is to use a thinner, but still dischargeable, cake to avoid stratification. 

The development so far is also based on constant a or on homogeneous cakes whose properties do not change with time or operating pressure. In fact, most cakes are compressible to some degree. As more fluid passes and more cake deposits, the older layers of solid are subject to frictional drag. The particles continue to move toward the filter surface and even to penetrate it, and the cake becomes denser in that direction. In other words, the cake is compressible, and the value of a varies. Significant cake compression is more likely in constant-rate filtration, where the pressure increases continuously. 

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