Internal cake structure

The dry freeze-dried cake undoubtedly has a very peculiar structure since it has been carved under vacuum from a solid matrix. At the end of this process and when it is still under vacuum, its internal surface is quite clean and very reactive. It can be easily understood that its first contact with a foreign element, such as the gas used to rupture the vacuum, is determinant. This is, in fact, why the choice of this gas as well as the procedure to introduce it are critical. In that context the properties of the dry cake internal surface constitute a major element. 

Figure 2.15 Internal structure of a compressible filter cake...

Because of the assumptions underlying its derivation, the Kozeny-Carman equation is not valid at void fractions greater than 0.7 to 0.8 (Billings and Wilder, op. cit.). In addition, in situ measurement of the void fraction of a dust layer on a filter fabric is extremely difficult and has seldom even been attempted. The structure of the layer is dependent on the character of the fabric surface as well as on tfie characteristics of the dust, whereas the application of Eq. (17-12) implicitly assumes that K2 is dependent only on the properties of the dust. A smooth fabric surface permits the dust to become closely packed, leading to a relatively high value of K2. If the surface is napped or has numerous extended fibrils, the dust cake formed will be more porous and have a lower value of K2 [Billings and Wilder, op. cit. Snyder and Pring, Ind. Eng. Chem., 47, 960 (1955) and K. T. Semrau, unpublished data, SRI International, Menlo Park, Calif., 1952-1953]. 

The major feature of the reactor shown in Fig. 7.9 is its simple structure with few internal parts. This not only reduces the walls on which the solid particles may cake, but also favors increasing the time-averaged effective interface area for transfer because the droplets can essentially keep their fine-dispersed condition during the whole period of contact with the gas flow. For flue gas containing ashes it has the additional function of wet dust-removal. 

Fig. 18.6a,b. The omental cake sign neoplastic infiltration of the great omentum is visualized by ultrasound as uniformly thick, hyposonic band-shaped structure adjacent to the anterior and lateral walls of the abdomen, following the contour of the abdominal convexity and containing low-level non-structural internal echoes... 

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