Mixed gas integrity testing of porous materials without permeate side access

Abstract

A method of integrity testing porous materials that is non-destructive to the material being tested. The inlet gas stream includes at least two gases, wherein one of the gases has a different permeability in liquid than the other, such as oxygen and nitrogen in water. The relative permeability of the gases is measured in the retentate stream, thereby avoiding accessing the permeate stream and potentially introducing contaminants to the material being tested. The integrity test is capable of detecting the presence of oversized pores or defects that can compromise the retention capability of the porous material.

Claims

1. A method of integrity testing a porous material, comprising: providing a porous material to be tested, said porous material having an upstream side and a filtrate side; providing a gas stream comprising at least first and second gases having different permeabilities in a liquid used to wet said porous material; introducing said gas stream to said upstream side of said porous material; causing said first and second gases to flow through said porous material; measuring the concentration of at least one of said first and second gases in a retentate stream exiting said upstream side of said porous material; and comparing the measured concentration to a predetermined concentration; wherein a difference between said measured concentration and said predetermined concentration is indicative of said porous material being non-integral.

2. The method of claim 1, wherein said first gas is oxygen and said second gas is nitrogen.

3. The method of claim 1, wherein said first gas is oxygen, and wherein said concentration of one of said gases is measured with an oxygen analyzer.

4. The method of claim 1, wherein said liquid comprises water.

5. The method of claim 1, wherein said porous material is a sterilizing grade filter.

6. The method of claim 1, wherein said porous material comprises a membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

(2) FIG. 1 is a schematic diagram of a flow configuration in a binary gas integrity test; and

(3) FIG. 2 is a schematic diagram of an integrity test arrangement in accordance with certain embodiments.

DETAILED DESCRIPTION

(4) Before describing the embodiments in further detail, a number of terms will be defined.

(5) As used herein, the singular forms a, an, and the include, plural referents unless the context clearly dictates otherwise.

(6) The expression integral as used herein when referring to porous materials such as a porous single layer or porous membrane, porous multilayers, or a plurality of porous membranes, means a non-defective porous material.

(7) The expression non-integral or as used herein when referring to porous materials such as a porous single layer or porous membrane, porous multilayers, and a plurality of porous membranes means a defective porous material. Non-limiting examples of defects in a porous layer or membrane include, but are not limited to, oversized pores, improper bonding (e.g., delamination or separation) between a plurality of porous layers or membranes that are bonded together to form a multilayer element, and defects on the porous layer or porous membrane.

(8) The expression porous material, as used herein, may include, but is not limited to, one or more porous membranes, sheets, rods, discs, tubes, layers, filters, filter elements, filtration media, containers, cylinders, cassettes, cartridges, columns, chips, beads, plates, monoliths, hollow fibers, and combinations thereof. The porous materials may be pleated, flat, spirally wound, and combinations thereof. It may be a single layered or multilayered membrane device. The membrane may be symmetric or asymmetric. The porous material may be contained in a housing, which may have an inlet and an outlet. It may be used for filtration of unwanted materials including contaminants such as infectious organisms and viruses, as well as environmental toxins and pollutants. The porous material may be comprised of any suitable material, including, but not limited to polyether sulfone, polyamide, e.g., nylon, cellulose, polytetrafluoroethylene, polysulfone, polyester, polyvinylidene fluoride, polypropylene, a fluorocarbon, e.g. poly (tetrafluoroethylene-co-perfluoro(alkyl vinyl ether)), poly carbonate, polyethylene, glass fiber, polycarbonate, ceramic, and metals.

(9) Embodiments disclosed herein include a method for integrity testing porous materials, including porous single layer materials, porous materials having a multilayered configuration, porous membranes and filters. The porous material may be in a housing providing a feed or inlet side and a permeate or outlet side.

(10) Turning now to FIG. 1, there is shown schematically a flow configuration in a binary gas integrity test. In a binary gas test, there is a feed (inlet) gas stream having at least two components in which one of the components permeates faster across the liquid filled membrane layer than a second component. The feed stream enters the upstream side of the membrane. The retentate stream, which is depleted of the faster permeating component, exits the upstream side of the membrane. The permeate stream, which is enriched in the faster permeating component, exits the downstream side of the membrane. In FIG. 1, Q.sub.F is the feed (inlet) flow rate, Q.sub.R is the retentate flow rate, Q.sub.p is the permeate flow rate, X.sub.f is the mole fraction of the faster permeating component in the feed stream, X.sub.r is the mole fraction of the faster permeating component in the retentate stream, and y.sub.p is the mole fraction of the faster permeating component in the permeate stream.

(11) Conventionally, the integrity of the membrane filter is assessed by measuring concentration of the permeate stream. For an integral filter, the concentration of the faster permeating gas should be within an expected range (determined either theoretically or empirically). A leak from the inlet side to the permeate side will result in a change in the permeate concentration and this deviation of the concentration of the faster permeating gas from the expected value is a signal for a leak or non-integral membrane filter.

(12) Since the binary gas test is a steady state process, however, in accordance with the embodiments disclosed herein, the concentration of the permeate gas can be determined from upstream measurements only. The composition of the permeate can be obtained by mass balance:
y.sub.p=(Q.sub.Fx.sub.fQ.sub.Rx.sub.r)/(Q.sub.FQ.sub.R)(1)

(13) Thus, by measuring only the upstream inlet and upstream outlet flow rates and concentrations, the integrity of the membrane filter can be assessed. The presence of a leak (indicative of a non-integral filter) will reduce the concentration of the faster permeating component in the permeate gas because the permeate gas will be diluted with inlet gas (permeate gas that is enriched with the faster permeating component mixed with inlet side gas). For a system in which the gas is perfectly mixed on both sides of the membrane, the gas composition on the permeate side of an integral membrane is given by:

(14) $\begin{array}{cc}\frac{y_p}{1-y_p}=\frac{\left(x_r-\mathrm{Pr}{y}_p\right)}{1-x_r-\mathrm{Pr}\left(1-y_p\right)}\mathrm{and}& \left(2\right)\\ y_p=\frac{x_f-\left(1-\right)x_r}{}& \left(3\right)\end{array}$

(15) where alpha is the ratio of the permeability of the fast gas to the permeability of the slow gas, Pr is the ratio of the permeate pressure to the feed pressure (P.sub.p/P.sub.f), and is the fraction of feed gas that has permeated the membrane (Q.sub.p/Q.sub.f or (Q.sub.fQ.sub.r)/Q.sub.f). Using the foregoing three equations, the expected compositions of the permeate and retentate streams in an integral filter can be calculated. If there is a leak from the inlet side of the filter to the permeate side of the filter, the permeate concentration will be reduced according to:

(16) $\begin{array}{cc}{y}_{p,\mathrm{ni}}=\frac{\left(Q_F-Q_R\right)y_p+Q_l{x}_f}{Q_F-Q_R+Q_l}& \left(4\right)\end{array}$

(17) where y.sub.p,ni is the non-integral permeate concentration, and Q.sub.l is the leak rate.

(18) In some embodiments, the predetermined concentration to which the measured concentrations are compared is the concentration expected from an integral porous material or device. The predetermined concentration may be the concentration of gas calculated to diffuse through an integral (i.e., non-defective), wetted porous material at a given temperature and pressure, or may be an actual concentration of gas that diffused through an integral wetted porous material at a given temperature and pressure.

(19) In certain embodiments, the porous material is wetted with a liquid (a wetting liquid) by saturating the porous material with the liquid. Suitable liquids include water, isopropyl alcohol and mixtures of isopropyl alcohol and water. Other liquids also could be used but may not be ideal due to cost and/or convenience.

(20) Suitable temperatures for carrying out the integrity test range from about 4 C. to about 40 C., preferably between about 22-24 C. Suitable feed pressures range from about 15 psia to about 100 psia, preferably about 40-70 psia.

(21) In certain embodiments, a plurality of gases, such as a low-cost binary gas pair, such as oxygen and nitrogen, available via compressed air, are used as the inlet gas to perform the test. Suitable amounts of each gas are not particularly limited. The gases should have different permeation rates through the liquid used to wet the porous material. The ratio of faster permeating gas to slower permeating gas in the gas mixture is influenced by a number of factors, including the ease of composition measurement, gas flow rate through the membrane, and economic considerations. In the case of air, the composition is fixed by ambient conditions. The composition of air on a dry basis is 20.95% 02, 78.09% N.sub.2, 0.93% Ar, 0.04% CO.sub.2, and trace levels of other gases.

Example 1

(22) An experimental setup for an upstream side binary gas test is shown in FIG. 2. The tested filters were sterilizing grade PVDF filters in 10-inch pleated cartridge format from MilliporeSigma (CVGL Durapore Cartridge Filter 10 in. 0.22 m). Air (consisting primarily of oxygen and nitrogen) is a convenient feed gas to use owing to its low cost, easy accessibility, and safe use. In this testing, a synthetic air mixture of 21.38% O.sub.2 and balance N.sub.2 was used. Oxygen permeates through water at about twice the rate of N.sub.2, so the permeate gas is expected to be oxygen enriched while the retentate gas is expected to be nitrogen enriched. The filter was wetted with water and the inlet gas was introduced to the filter at 40 psig. The permeate gas was vented to atmosphere. A metering valve was used to control the retentate flow rate, which was measured using a mass flow meter. The concentration of oxygen was measured with an oxygen analyzer (Servomex model 4100). For the retentate side testing, the retentate flow rate was held constant at about 88 sccm, which was suitable as a flow rate for the oxygen analyzer.

(23) Three devices that had previously been determined by the prior art binary gas test to be clearly integral (passing), marginal non-integral (marginal failure) and clearly non-integral (failing) were tested per the above description and FIG. 2. The integrity results are summarized in Table 1 below:

(24) TABLE-US-00001 TABLE 1 Integrity test results comparing prior art method to method of this invention. Prior art method (permeate side access) This invention (inlet side access only) y.sub.p (%) x.sub.p (%) Filter Expected Integrity Expected Integrity ID Integral Measured determination Integral Measured determination 77 30.7 30.72 Pass 20.07 20.08 Pass 17 30.7 30.53 Marginal fail 20.08 20.20 Marginal fail 14 30.7 28.35 Clear fail 19.53 20.12 Clear fail

(25) The method of the embodiments disclosed herein and the prior art method were in agreement for all three filters. In the prior art method, the expected integral concentration does not vary because is set to <<1%. As approaches zero, y.sub.p as a function approaches an asymptotic limit, and x.sub.r approaches x.sub.f. This can be demonstrated from Equations 2-3. For the prior art method, operating in a region where y.sub.p is not sensitive to is preferred since at low any fluctuations in (resulting from fluctuations in Q.sub.f, Q.sub.r, or Q.sub.p) will have minimal impact on the measurement of y.sub.p. For the inlet side test, it is Q.sub.r that is fixed (88 sccm in this example) and is allowed to fluctuate. Therefore, as varies, x.sub.r varies in accordance with equations 2-3. The expected integral concentrations were calculated using a=2.1, Pr=0.27 and 8 based on the measured values of Q.sub.f and Q.sub.r.

(26) After completion of the integrity tests, each of the cartridges listed in Table 1 was tested for bacterial retention using the method described in ASTM F 838-05, incorporated herein by reference. The retention performance of a filter can be quantified with the log reduction value (LRV) defined as:
LRV=Log(C.sub.i/C.sub.f)

(27) where C.sub.i and C.sub.f are the concentrations of bacteria in the inlet and filtrate streams, respectively. Filter 77 was completely retentive (no bacteria was found in the filtrate) and determined to have an LRV of >11.2. Filter 17 had an LRV of 7.8 and filter 14 had an LRV of 5.4 Thus, the bacterial retention tests were in alignment with the integrity test results.

(28) The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.