vo separation processes to raise the concentration of the product in solution from parts per million or several percent to the final concentrated form. Figure 18 illustrates the contribution of separation costs to the selling price of a biological product.*451
The most economical system design is achieved by the consideration of both capital and operating costs. When comparing the overall filter performance against a competing technology, care should be taken to ensure that the total cost or payback is based on the life-cycle rather than solely on the basis of initial capital or operating costs.
It is easier to compare two competing technologies or products on the basis of initial capital cost alone. However, this approach may be erroneous unless operating, maintenance and replacement costs are considered along with differences (or savings) in the value of product recovered or lost. For example, clarification of a fermentation broth with a cross-flow filter may cost up to 4 times higher compared to the capital cost of a pre-coat filter, but the operating cost may be only about 50% of that incurred with pre-coat filter.[121 The disposal cost of the filter aid will also add to the savings of CFF over pre-coat filtration.'461 The higher capital cost can be justified through cost savings yielding a reasonable payback (typically in the range of 3 to 4 years). Cross-flow filtration competes with many traditional separation and filtration technologies such as centrifuges, rotary pre-coat filters, cartridge filters, chemical treatment and settling and a filter press. The advantages and disadvantages of some of these alternatives were briefly discussed in Sec. 3.0. This section will highlight key items that make up the major portion of the capital and operating costs in cross-flow filtration.
The cross-flow filter accounts for a major portion of the capital cost. The relative percentage contribution to the total capital cost will vary from about 20% for small systems up to 50% for larger systems. Thus, replacement costs, when the CFF has a useful service life of only about a year, can be as much as 50% of the total system cost. Inorganic filters cost more than their polymer counter parts but can last about 5 to 10 years. Capital costs associated with feed pump and recirculation pump(s) represents anywhere from 5 to 15% of the total capital costs. The largest contribution to the operating cost in many cross-flow filtration systems is in the energy consumption for recirculation.'11 For example, in the production of common antibiotics such as penicillin or cephalosporin, high recirculation rates are maintained (corresponding to a cross-flow velocity in the range 5 to 8 m/s) to minimize concentration polarization. '21 Energy requirements under turbulent flow conditions are also significantly higher than under laminar flow situations, under otherwise similar conditions. In addition, total energy costs o>
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