Design of Composite Filtration Media Using Flow Porometry

Filtration media are required to have suitable pore size, pore size distribution, pore volume, pore surface area and permeability for performing satisfactorily in applications. It is difficult to encounter all the desired properties in a filtration medium. In this investigation, we have characterized filtration media by flow porometry, used these pre-characterized filtration media to design composite filtration media, and tested these composites by flow porometry. Specimen of the filtration medium to be tested is soaked in a wetting liquid and gas pressure on one side of the specimen is slowly increased to displace the liquid from the pores and increase gas flow. Gas pressure and flow rates through wet and dry samples are measured and analyzed to obtain the largest pore size, the pore size distribution, and permeability. All of these characteristics of filtration media were measured in this investigation. It was shown that composite filtration media possessing unique characteristics could be prepared by a logical selection of pre-characterized filtration media. INTRODUCTION Nonwoven and woven filtration media are widely used in industry for separating suspended particles from fluids. The largest particle that cannot pass through the filter is determined by the largest pore diameter in the filter. The mean flow pore diameter and pore size distribution, determine sizes of other particles, which may not pass through. Permeability determines the rate of the filtration process. Therefore, all these properties need to be controlled in the filter. Control of such parameters is often difficult. However, it may be easier to produce controlled characteristics in composite filtration media prepared from filters with known characteristics. In this investigation filtration media have been characterized. Composite filtration media have been prepared and characterized by flow porometry.. The results have been critically examined. THE TECHNIQUE Principle A sample of the material to be tested is soaked in a wetting liquid. These liquids are such that the liquid/solid(filter) interfacial free energy is less than the gas/solid(filter) interfacial free energy. Therefore, the wetting liquid fills the pores spontaneously, but cannot be spontaneously removed from the pores. Gas under pressure is applied to one side of the sample, so that the work done by the gas in displacing the liquid inside the pore can compensate for the increase in free energy caused by the increase in gas/solid surface due to the displacement of the liquid in the pore. This is illustrated in Figure 1. Figure 1. Displacement of liquid in a pore. Free energy balance may be used to derive the pressure required to displace the liquid at any location in the pore [1]. The relation is given below. p = γ cos θ (dS/dV) (1) where, p = differential pressure across the sample γ = surface tension of the liquid θ = contact angle dS = increase in gas/solid surface area in the pore dV = increase in volume of gas in the pore. For a cylindrical pore of diameter D, (dS/dV) = (4/D). It follows from this relation that the largest pore will be emptied at the lowest pressure. When the gas pressure is increased, at a certain value of the pressure the largest pore is emptied and gas starts flowing through the sample. With further increase in pressure, gas removes liquid from smaller pores and the gas flow through the sample is increased. The differential gas pressure and the flow rates through wet and dry samples are measured. These data are used to calculate all the pore characteristics. Equipment The sample chamber is shown in Figure 2. It is so designed that the gas is allowed to escape only through the sample. Figure 2. Sample chamber The instrument used in this investigation, was equipped with state-of-the-art devices and innovative design for accurate control and sensing of pressure and flow. The instrument was fully automated using windows based software. It requires very little operator time and produces highly reproducible data [2]. The instrument is shown in Figure 3. Figure 3. The Capillary Flow Porometer RESULTS AND DISCUSSION Pore diameter The flow rate-pressure data for a filter material are shown in Figure 4. The dry curve in this figure refers to data obtained with a dry sample and the wet curve corresponds to the sample whose pores at the beginning of the test were filled with the wetting liquid. Figure 4. Variation of flow rate with pressure The pore diameter is defined as the diameter D of a cylindrical opening such that (dS/dV) of the opening is the same as that of the pore at the location at which the gas displaces the liquid. For a cylindrical opening (dS/dV) is (4/D). Consequently, Equation 1 reduces to: D = (4 γ cos θ) / p (2) The wetting liquid Silwick was used. The surface tension of the liquid was 20.1 dynes/cm. Pore diameter was calculated taking cos θ to be one for this low surface tension wetting liquid [1]. The largest pore diameter (Bubble point) The largest pore diameter corresponded to the pressure at which flow started (Figure 4). The bubble point pressure is shown in Figure 4.Six filter materials designated as 1, 2, 3, 4, 5 & 6, were investigated. A number of samples taken from each material were tested. The largest pore diameters found in the filters are listed in Table I. The bubble point pore diameters in the investigated filters are in the range of about 25 and 120 microns. Table I. The largest pore diameter, mean flow pore diameter and permeability of six filter materials ----------------------------------------------------------------------------------------------------------------Filter #1 #2 #3 #4 #5 #6 ----------------------------------------------------------------------------------------------------------------The largest pore 120.6 25.17 24.29 86.54 61.53 96.07 diameter, microns ---------------------------------------------------------------------------------------------------------------Mean flow pore 40.45 11.73 11.77 43.67 23.20 44.77 diameter, microns --------------------------------------------------------------------------------------------------------------Permeability, darcies 10.44 0.520 0.515 0.628 0.723 1.68 -------------------------------------------------------------------------------------------------------------The mean flow pore diameter The mean flow pore diameter corresponds to the pressure at which the wet curve and the half dry curve shown in Figure 4 intersect. The half dry curve is calculated from the dry curve so that at a given pressure the half dry curve gives half of the flow through the dry curve. Consequently, half of the flow through the dry sample is through pores having diameters greater than the mean flow pore diameter. The mean flow pore diameters of the six filters are listed in Table I. The mean flow pore diameters are in the range of about 11 and 45 microns. Gas permeability Permeability, k is calculated from the gas flow rate using the following relation [3]. F = k [ A / (2 μ l ps)] [pi + po] [pi po] (3) where: F = flow rate in volume at STP per unit time A = area of the sample μ = viscosity of gas l = thickness of sample ps = standard pressure pi = inlet pressure po = outlet pressure The permeability of the filters are listed in Table i. Permeability varies in the wide range between about 10 and 0.5 darcies. Inhomogeneous structure of the filter material Scatter in the values of the largest pore diameter and mean flow pore diameter determined using samples from different parts of the same material give an indication of the extent of structural inhomogeneity of the filter material. The scatters are listed in Table II. Table II Scatter in the values of bubble point pore diameter and mean flow pore diameter of samples taken from different parts of material. -------------------------------------------------------------------------------------------Filter Scatter Bubble point pore diameter Mean flow pore diameter --------------------------------------------------------------------------------------------