There are various reasons why a filter may require to be dried, which include, but not limited to:

a)    Drying after integrity testing prior to filtration of a water-insoluble liquid.

b)    Drying of a hydrophobic filter such as Pall Emflon® PFR, prior to Water Intrusion Testing.

c)    Removal of water from a filter after integrity testing to allow the passage of moist heat (steam) for sterilization.

In principle, there are three possible scenarios for a drying process

  1. off-line oven drying
  2. in-line using pressurised gas
  3. applying a vacuum

The selection of the drying method is determined by the level of dryness required for the application. For example, for a filter that will be used for the sterilization of a water-insoluble product, a high level of dryness is required (oven drying).  If the purpose of the drying is allowing the passage of steam, a low to moderate level of drying is required (in-line using pressurized gas).

In order to determine if a capsule or cartridge filter has been completely dried, the filter can be weighed before wetting and after drying.  A weight within approximately 1% of the pre-wet weight will indicate sufficient drying depending on the application.

It is recommended that the drying procedure simulates the individual process and environmental conditions.  Therefore, it is suggested that this study be done on-site by the end user.

Oven Drying

1.1     Pre-treatment

To reduce the amount of fluid in the filter pack and thus speed up drying, the filter can be "blown down" with pressured gas prior to drying. A considerable amount of liquid will be expelled if the filter is subjected to the Forward Flow test pressure. Even more fluid will be expelled if a gas pressure above the bubble point is applied.

Note:  for safety reasons it is important to remain within the maximum operating conditions assigned to the filter capsule or housing.  Temporary excursions above this maximum pressure may be allowed – please contact a Pall representative for further guidance.

1.2      Equipment

We recommend the use of a fan-assisted oven to support the removal of moisture. A vacuum drying oven is a highly effective means of drying a filter in a short period.

1.3      Static Drying Parameters

The following table below shows suggested oven drying temperatures and times for Pall membranes (these conditions can be reduced if using a vacuum drying oven).

 

Membranes

Temperature

Time (hours)

Ultipor® N66 S grade* (NRPS)

65 °C

6-12

Posidyne®/Bio-Inert® (NFZ/ N*L) **

65 °C

12–16

Ultipor N66 (NF,NR,NA)

65 °C

14

Ultipor VF (DV*)

Fluorodyne® II (DBL,DFL,DJL)

Emflon® II (V002)

Emflon*** (PFR, HTPFR, PF, PFA)

90 °C

16

 

65 °C

14

Fluorodyne EX / Supor EX (UEDF,UEDT, UECV)

40 °C

36

SuporLife®/SuporFlow®

38 °C

48

Supor® (EBV, EKV, EAV, ECV)

40 °C

28

*Max Drying temperature is 96 °C for 6- 12 Hours

** Max Drying temperature is 96 °C for 12 Hours

*** Max Drying temperature is 96 °C for 16 Hours

NOTES:

·       Alcohol-wet filters should be flushed with water prior to drying

·       All temperatures are +/-2 °C

·       Capsules needs a prolonged drying time because the transport of moisture from the filter pack is hindered by the capsule shell.

Dry in-line using pressurized gas

Note: Compressed air or an inert gas should be used.  For safety reasons, reactive gases such as oxygen should never be used for filter drying.

Note:  for safety reasons it is important to remain within the maximum operating conditions assigned to the filter capsule or housing.  Temporary excursions above this maximum pressure may be allowed – please contact a Pall representative for further guidance.

2.1      "Blow-Down"

For this drying procedure it is necessary to apply a gas pressure above the bubble point of the filter with the respective wetting fluid.

2.2      Physical parameters influencing the drying process

The effectiveness of the drying cycle will be influenced by the following parameters:

  • Nature of the wetting fluid
  • Gas phase (dryness, temperature)
  • Gas flow rate

The following table below shows suggested gas flow rates and times for some Pall filter types to dry the filter of water to within 1% of the pre-wet weight.

 

Membranes

Flow Rate

(Nm3/hr)

Flow Rate (SCFM*)

Time (minutes)

SLK7002NRP

6.8

4

30

SLK7002DFLP

6.8

4

30

CFS92SPRRK

6.8

4

30

KA3EKVP1S

6.8

4

30

* Standard Cubic Feet per Minute

Longer times and higher flow rates will be required for larger area filters.

2.3 Drying using a pre-set pressure for a specified time is also a common method used for filter drying.  Generally, a starting pressure 25% above the minimum bubble point is recommended.  The pressure is expected to drop immediately as more pores of the membrane become evacuated of liquid (the resistance to the gas pressure decreases).

Drying times will be specific to the filter type, and the drying apparatus.  The following document offers guidance for drying times and pressures, and gives an overview of how a drying procedure should be evaluated:

Drying by applying a vacuum

This method is often used for the drying of vent filters in automated systems, such as freeze dryers. Applying a vacuum can considerably shorten the drying times so that a drying time below 1 hour becomes feasible.

For more information, please see Pall Publication , USTR2821 Application Note: Scientific and Technical Report – In Situ Drying of Pall Hydrophobic Air Filters Prior to Moist Heat Sterilization