The Proper Use of 47-mm Flat Disc Filters in Filter Sizing Studies - Why staining is crucial in flow decay studies. - BioPharm International


The Proper Use of 47-mm Flat Disc Filters in Filter Sizing Studies
Why staining is crucial in flow decay studies.

BioPharm International
Volume 22, Issue 9


When dealing with a "clean" liquid (i.e., a liquid devoid of suspended particles), the flow rate and the resulting throughput, per unit of time, at the applied differential pressure is straightforward to measure, based on the selected EFA. (Temperature must be kept constant, particularly because the liquid's viscosity is its reciprocal, and any increase in either factor will lead directly to a proportional rise in throughput.)

If the liquid contains suspended particulates, however, the filter's porosity will be decreased by particle retention, so additional EFA may be required to compensate for the filter area that is blocked or clogged by the particulates.4 In such cases, it will be necessary to conduct experiments to assess the EFA needed for batch processing.

Figure 2. Coomassie-blue stained 47-mm filter discs
Alternatively, it may be possible to restore lost throughput by using higher inlet pressures rather than increasing EFA. If filtration is already in progress, that would provide a more manageable alternative. The effectiveness of such a measure would depend, however, on the total suspended solids (TSS) in the solution. Higher differential pressures could increase the pressure drop by compacting any filter cake that may have formed. In contrast, a larger EFA would be less likely to produce such compaction under the same conditions, as there would be little or no filter cake build-up. High differential pressure also commonly leads to high flow rates, which can cause elevated fouling or adsorptive effects. Those effects, in turn, can lead to losses in product yield or the need for larger EFA.

If the preparation presented for filtration is relatively free from suspended particles, flat discs of even smaller diameters may suffice for filtration sizing tests because the EFA available for liquid permeation would not be diminished by particle deposits. An example of such an application would be filter sizing for deionized waters. These liquids contain so little suspended matter that the flow decline data can be secured fairly quickly from tests using a small filter. The fewer the solids in the suspension, the less demanding the mathematical extrapolation.

Nevertheless, a sizeable inaccuracy inheres to the extrapolation of results from a 47-mm disc's EFA of 1.49 in2 to the EFA expected from a 6 ft2 (or 3,864 in2) cartridge. At best, the results indicate only hypothetical, non-committal values; hence the large margins allowed for error. An assessment method that requires an EFA overdesign of as much as 1 or 1.5 times the extrapolated value does not merit endorsement.

It may well be that the extreme safety margins reported are exaggerations of the arithmetical uncertainties, reflecting the experimenter's strong fear of having to interrupt the filtration mid-stream to install new filters to allow processing of the batch to be completed. Indeed, the aseptic replacement of a filter is a risk-prone operation, so its avoidance is strongly recommended.

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