Diffusive-flow test

Water-based tests [water intrusion tests (WITs)] [2.36] were developed a few years ago, but it is only recently that they have been promoted as a reliable alternative to the diffusion flow test. 

Fig. 8 Manual diffusive flow test set-up. (Reprinted from Technical Report No. 26, Sterilizing Filtration of Liquids 1998 by PDA.)...

In the diffusive-flow test, also called the forward flow test, the filter membrane or candle in its tightly closed case is set under continuous test gas pressure. The amount of air that diffuses through the filter membrane per time unit is measured downstream, on the sterile side of the filter. The pressure drop over the membrane should be constant during the test to prevent variations in diffusion rate. Collection and measurement of the air on the sterile side often require actions that may lead to contamination of the setup, and thus these actions should be performed aseptically. Furthermore, for small filter surface areas the volume of air that diffuses through the filter is small and therefore no accurate measurements are possible for these small filters. 

Polyethylene-based membranes are manufactured for use in hazardous waste landfills, lagoons, and similar applications. Two of these products have been tested to determine their effectiveness as barriers against radon diffusion. (In most cases, diffusive flow is considered of little or no significance as a mechanism of radon entry compared with convective flow). A 20-mil high-density polyethylene tested 99.9% effective in blocking radon diffusion under neutral pressure conditions. A 30-mil low-density polyethylene tested 98% effective in blocking radon diffusion under neutral pressure conditions. 

Another available product has two faces of aluminum foil over a core of glass scrim webbing it is coated with asphalt. The membrane is 0.012 in. thick. This material has not been tested as a barrier against diffusive flow of radon, but its performance should be similar to that of other foil-faced products. Seams are sealed with aluminum tape. 

Statistics. The results of 367 flow-through diffusion cell tests are presented. The standard deviation of the flux divided by the mean flux or the coefficient of variation between different flow-through cell types, pharmaceutical compounds, and different technicians are compared (4). The coefficients of variation are compared by analysis of variance (5) using the RSI (BBN Research Systems, Cambridge, MA) research statistics software. 

The pellet is saturated with CO2 throughout a test run, but C 02 is replaced gradually by C 02 by diffusion, first through the pores, or interstices between the crystals and binder, and then through the crystals themselves. This is analogous to the situation in a fixed-bed adsorption process, where the crystals are analogous to the porous pellets, except that the transfer process in the interstices is a diffusive flow rather than a bulk gas flow. Smith (5) considered complex detailed models for the various flow paths in a fixed-bed process, but as there is inevitably some approximation involved in describing the structure of the interstices, a much simpler approach has been used here. 

The integrity of sterilizing fillers is most often validated and routinely monitored by nondestructive methods. The U.S., European, and U.K. guidelines on sterile filtration refer to four methods of integrity measurement filtration flow rate, bubble point tests, diffusion (forward flow) tests, and pressure hold tests. Each of these has its uses in determining that routinely used filters are per-forming to the same standards as those validated for the particular products and processes. 

The importance of this equation is that it demonstrates that 7 is a linear function of the test pressure P, as long as the transition pressure between diffusive flow and capillary flow is not reached or exceeded. Other variables that must be controlled in diffusion testing include (a) the filter membrane area, because it defines the effective area of pores or void fraction (b) the temperature, because it defines the solubility of gas in liquid and (c) the composition of the liquid phase, because the presence of solutes affects the solubility coefficient. 

When a test gas (for example ambient air) is applied over a water moistened filter, just below the pressure level of the bubble point, test gas diffusion will occur through the water in the wetted membrane filter. This diffusion happens in all water filled pores, not only in the largest. This principle is the basis for two tests, which use different approaches to measure gas diffusion the pressure hold test and the diffusive-flow (forward flow) test. Other names for the same principle tests exist. These tests are performed at a pressure of about 80 % of the theoretical bubble point pressure of the filter. It is important that the largest pores are still filled with liquid. In this phase, diffusion occurs more or less linearly with the pressure drop over the... 

Proposed flux models for porous media invariably contain adjustable parameters whose values must be determined from suitably designed flow or diffusion measurements, and further measurements may be made to test the relative success of different models. This may involve extensive programs of experimentation, and the planning and interpretation of such work forms the topic of Chapter 10, However, there is in addition a relatively small number of experiments of historic importance which establish certain general features of flow and diffusion in porous media. These provide criteria which must be satisfied by any proposed flux model and are therefore of central importance in Che subject. They may be grouped into three classes. 

Petera, J., Nassehi, V. and Pittman, J.F.T., 1989. Petrov-Galerkiii methods on isoparametric bilinear and biquadratic elements tested for a scalar convection-diffusion problem. Ini.. J. Numer. Meth. Heat Fluid Flow 3, 205-222,... 

Whether surge is eaused by a deerease in flow veloeity or an inerease in rotational speeds, either the indueer or vaneless diffuser ean stall. Whieh stalls first is diffieult to determine, but eonsiderable testing has shown that for a low-pressure-ratio eompressor, the surge initiates in the diffuser seetion. For units with single-stage pressure ratios above 3 1, surge is probably initiated in the indueer. 

---