Depth centrifuge filters

To monitor both the copper and the polymer species during a PEEK/copper interface reaction, a PEEK overlayer thinner than 80 A, the approximate sampling depth of ESCA, was required. Centrifugal casting (3000 rpm for 15 sec) of an extract of PEEK in hexafluoroisopropanol (HFIP) onto copper disks proved successful for obtaining samples on which both PEEK and copper species could be detected by ESCA. The extract was obtained by passing a 20-30 wt. % suspension of PEEK in HFIP through a 0.5 pm Teflon filter. The solubility of a representative fraction of the PEEK in HFIP was verified by IR analysis. Comparison of the IR spectra from a PEEK/KBr pellet and the evaporated filtration product indicated identical spectra. 

This entry has presented a brief description of sedimenting and filtering centrifuges. The types of separation, separation duties, and modes of operation have been discussed. The basic common and distinct mechanical elements have also been dealt with. References are included for those readers seeking a more in-depth historical, theoretical, as well as practical background on the subject. 

Clarification, or harvest, is typically the first step in downstream processing. During clarification, the cells, cell debris, and other particulates are removed from the cell culture broth, which contains the product. Clarification can be accomplished by depth filtration or by centrifugation followed by some form of filtration. Tangential flow filtration (ITT), sometimes also referred to as cross-flow filtration (CFF) is also used for clarification. Tangential flow filters are designed either in a flat sheet mode or hollow fiber mode. 

Centrifugation is the preferred clarification method at large scales. Usually, a depth filter is used after the centrifuge to remove any smaller particulates that are not cleared by the centrifuge. In this case, however, a relatively small number of depth filters are needed as the centrifuge clears most of the cell debris. 

Cell-disrupt, centrifuge-by-settling, depth-filter, diafilter, ferment, ferment-continuously, microfilter, sterilize, transfer-through-sterilizer, ultrafilter... 

A pressure filter fed by a punqt other than a positive displacement type provides this kind of filtratian. The most common exanq>le is that of a centrifugal pump feeding a plate and firame pressure filter. Under these circumstances the dehveiy pressure of the punq> increases as the flow rate decreases. Ibe flow decreases as a consequence of increasing total resistance to filtration due to the increase in cake depth. The mathematical description of this process depends on knowledge of the pump characteristic, such as that shown in Figure 2.10. 

Centrifugation is often followed by depth filtration. Alternatively, several depth filtration steps can be employed for clarification instead of centrifugation, especially for bench-, pilot-, or smaller commercial-scale applications [133]. Within depth filters, the particles are trapped in the internal matrix of the packed, porous material having a void volume of up to 85%. They are typically composed of cellulose fibers, inorganic filter aids such as diatomaceous earth, inorganic charged resin binders, and synthetic polymers. As the retention mechanism of depth filters is not absolute, some particles will get through, and it is not possible to define a minimum particle size that will be held back. Therefore, an additional membrane... 

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