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Mar 1, 2006 (Vol. 26, No. 5)

Choosing & Scaling Up Right Filter Combo

Indicator, Verification, and Assurance Trials

  • Click Image To Enlarge +
    Fig.1: Re-usable device filter area for 47-mm disposable test filter. The diameter of the coloration is 41mm, which represents approximately 12.2cm squared.

    Filter train selection and scaling trials with small-scale flat-disc composites have become routine. Such trials are often used to evaluate the optimal filter combination, polymer, and required retentivity. However, trials performed with a low effective filtration area (EFA) and without a pleated configuration can only be used as indicator trials and not for final scaling or sizing purposes.

    Once the optimal combination has been indicated, pleated-device trials are required to verify the initial results, and verification trials (at mid scale, with pleated devices) and assurance trials (at nearly full scale) are necessary to size the filtration system.

    Most of the time, 47-mm disc composites are used for filterability trials. Comparability is not given unless the composites used are samples of the actual filter cartridge used in the final process. These composites are either installed in a 47-mm stainless steel holder or can be found welded into a plastic housing as a disposable device. However, in all cases the effective filtration area has to be verified when comparing the performance of these devices.

  • Trial Facts to Be Considered

    Click Image To Enlarge +
    Fig.2: Disposable device filter area

    The question of what effective filtration area to use during 47-mm disc trials often comes up (Figure 1). Most users have simply used a number between 12.5 and 13.5 cm2.

    The use of an incorrect effective filter area in comparison or scale-up calculations may lead to the wrong conclusions. The effective filtration area of common 47-mm discs within a stainless steel holder and disposable device were measured by measuring the diameter of the discolored portion of the filter disk. This provides a direct measurement of the effective filtration area, as only this area will be discolored.

    Recently filter manufacturers have begun to supply encapsulated disk devices to facilitate the performance of scale-down studies. These are marketed as scale-down devices specifically for such use. The devices themselves are sealed, preventing easy inspection of the filter membrane after use. This requires that the user refer to the published filtration area for the device for comparison or scale-up calculations.

    Severe nonlinearity experienced during scale-up studies led one investigator to perform destructive inspection of one such device. The diameter of the coloration within this device was measured to be 48 mm (Figure 2), which represents an effective filtration area of approximately 18 cm2.

    However, the literature for this device states an EFA of 13.8 cm2, introducing an error in excess of 30%. The effective filtration area of the disposable device is 36% larger than the 47-mm disc utilized in a stainless steel holder. The main reason for this discrepancy is the fact that the stainless-steel device requires an O-ring sealing. The O-ring diameter and seal will reduce the effective filtration area. The disposable device utilizes a heat welding, which is a thin ring around the membrane.

    In another instance, indicator trials performed at 47-mm scale were followed by verification trials utilizing 1,000-cm2 pleated capsule filters. The scale-up calculations also exhibited severe nonlinearity. This time it was found that a filter capsule marketed as a 1,000-cm2 device actually contained approximately 1,600-cm2 of effective filtration area (+60%). The reader is cautioned to verify the effective filtration area of any device used for indicator or verification trials.

  • The Right Filter Choice

    Click Image To Enlarge +
    Fig.3: Test results of 47-mm discs in different devices

    Filterability trials require thorough observation of the effective filtration area of such small-scale devices, especially when results of different devices are compared. Published data from manufacturers may not be tightly specified and may result in the use of incorrect scale factors and improper conclusions. The example in Figure 3 demonstrates the implications of comparing devices with incorrectly stated filtration areas.

    PES3* is a disposable device that has been used to measure total throughput, a typical test for filterability trials. The device shows higher throughput performance than the filter discs of other manufacturers. Yet, the other filter performances were tested with stainless steel holders, i.e., a 36% lower effective filtration area.

    Once the appropriate filter area values are utilized to calculate throughput per square meter, the performance is comparable to another PES. The first measured PES filter, previously disqualified from further consideration, now shows performance comparable to PES3#, which may be verified at a larger scale.

    If an incorrect EFA had been used for the comparison calculations, none of the other filters would have been chosen for additional filterability studies with pleated devices (verification and assurance trials). This would have eliminated the chance of a second vendor or supplementary tests, like unspecific adsorption or mechanical stability. The end user would have relied on only one filter, which limits flexibility and choice.

  • Summation

    Filterability trials require thorough observation of the effective filtration area of such small-scale devices, especially when results of different devices are compared. Published data from manufacturers may not be tightly specified and may result in the use of incorrect scale factors and improper conclusions. The example in Figure 3 demonstrates the implications of comparing devices with incorrectly stated filtration areas.

    PES3* is a disposable device that has been used to measure total throughput, a typical test for filterability trials. The device shows higher throughput performance than the filter discs of other manufacturers. Yet, the other filter performances were tested with stainless steel holders, i.e., a 36% lower effective filtration area.

    Once the appropriate filter area values are utilized to calculate throughput per square meter, the performance is comparable to another PES. The first measured PES filter, previously disqualified from further consideration, now shows performance comparable to PES3#, which may be verified at a larger scale.

    If an incorrect EFA had been used for the comparison calculations, none of the other filters would have been chosen for additional filterability studies with pleated devices (verification and assurance trials). This would have eliminated the chance of a second vendor or supplementary tests, like unspecific adsorption or mechanical stability. The end user would have relied on only one filter, which limits flexibility and choice.



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