Permeate pressure

For rubbeiy membranes (hydrophobic), the degree of swelling has less effect on selectivity. Thus the permeate pressure is less critical to the separation, but it is critical to the driving force, thus flux, since the vapor pressure of the organic will be high compared to that of water. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

Permeate pressure = 1.013 x 10 N/m. The flowrate per module is bounded by the following minimum and maximum flows ... 

Equations (20-66) and (20-67) present single-pass formulas relating retentate solute concentration, retentate crossflow, permeate flow, and membrane area. For relevant low-feed-concentration applications, polarization is minimal and the flux is mainly a function of pressure. Spiral or hollow fiber modules with low feed channel and permeate pressure drops are preferred. 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Greenlaw et al. [18] have studied the effect of feed and permeate pressure on pervaporation flux in some detail some illustrative results are shown in... 

Figure 2.13 The effect of permeate pressure on the water flux through a silicone rubber pervaporation membrane. The arrows on the lower axis represent the saturation vapor pressures of the feed solution at the temperature of these experiments as predicted by Equation (2.79) [15]...

Figure 2.14 The effect of feed and permeate pressure on the flux of hexane through a rubbery pervaporation membrane. The flux is essentially independent of feed pressure up to 20 atm but is extremely sensitive to permeate pressure [18]. The explanation for this behavior is in the transport equation (2.79). Reprinted from J. Membr. Sci. 2, F.W. Greenlaw, W.D. Prince, R.A. Shelden and E.V. Thompson, Dependence of Diffusive Permeation Rates by Upstream and Downstream Pressures, p. 141, Copyright 1977, with permission from Elsevier...

A second method of determining the coefficient ( >,/5) and the intrinsic enrichment of the membrane Ea is to use Equation (4.11). The term ln(l — 1/E) is plotted against the permeate flux measured at constant feed solution flow rates but different permeate pressures or feed solution temperatures. This type of plot is shown in Figure 4.10 for data obtained with aqueous trichloroethane solutions in pervaporation experiments with silicone rubber membranes. 

Figure 4.19(a) shows the concentration of water vapor on the feed and permeate sides of the membrane module in the case of a simple counter-flow module. On the high-pressure side of the module, the water vapor concentration in the feed gas drops from 1000 ppm to about 310 ppm halfway through the module and to 100 ppm at the residue end. The graph directly below the module drawing shows the theoretical maximum concentration of water vapor on the permeate side of the membrane. This maximum is determined by the feed-to-permeate pressure ratio of 20 as described in the footnote to page 186. The actual calculated permeate-side concentration is also shown. The difference between these two lines is a measure of the driving force for water vapor transport across the membrane. At the feed end of the module, this difference is about 1000 ppm, but at the permeate end the difference is only about 100 ppm. 

Microfiltration cross-flow systems are often operated at a constant applied transmembrane pressure in the same way as the reverse osmosis and ultrafiltration systems described in Chapters 5 and 6. However, microfiltration membranes tend to foul and lose flux much more quickly than ultrafiltration and reverse osmosis membranes. The rapid decline in flux makes it difficult to control system operation. For this reason, microfiltration systems are often operated as constant flux systems, and the transmembrane pressure across the membrane is slowly increased to maintain the flow as the membrane fouls. Most commonly the feed pressure is fixed at some high value and the permeate pressure... 

The three factors that determine the performance of a membrane gas separation system are illustrated in Figure 8.12. The role of membrane selectivity is obvious not so obvious are the importance of the ratio of feed pressure (p ) to permeate pressure (pt) across the membrane, usually called the pressure ratio, [Pg.317]

This is called the membrane-selectivity-limited region, in which the membrane performance is determined only by the membrane selectivity and is independent of the pressure ratio. There is, of course, an intermediate region between these two limiting cases, in which both the pressure ratio and the membrane selectivity affect the membrane system performance. These three regions are illustrated in Figure 8.13, in which the calculated permeate concentration ( , ) is plotted versus pressure ratio pressure ratio of 1, feed pressure equal to the permeate pressure, no separation is achieved by the membrane. As the difference between the feed and permeate pressure increases,... 

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

The separation factor, /3pervap, contains contributions from the intrinsic permeation properties of the membrane, the composition and temperature of the feed liquid, and the permeate pressure of the membrane. The contributions of these factors are best understood if the pervaporation process is divided into two steps, as shown in Figure 9.3 [18]. The first step is evaporation of the feed liquid to form a saturated vapor in contact with the membrane the second step is diffusion of this vapor through the membrane to the low-pressure permeate side. This two-step description is only a conceptual representation in pervaporation no vapor phase actually contacts the membrane surface. Nonetheless, the representation of the process shown in Figure 9.3 is thermodynamically completely equivalent to the actual pervaporation process shown in Figure 9.F... 

The relationship between the three separation factors, /3pe ap, /3evap and /3mem, is illustrated in Figure 9.4. This type of plot was introduced by Shelden and Thompson [20] to illustrate the effect of permeate pressure on pervaporation separation... 

Figure 9.4 The effect of permeate pressure on the separation of ethanol/water mixtures with a polyfvinyl alcohol) membrane. The feed solution contains 20 wt% water and 80 wt% ethanol. The line drawn through the experimental data points is calculated from Equation (9.11)...

Concentration polarization plays a dominant role in the selection of membrane materials, operating conditions, and system design in the pervaporation of VOCs from water. Selection of the appropriate membrane thickness and permeate pressure is discussed in detail elsewhere [50], In general, concentration polarization effects are not a major problem for VOCs with separation factors less than 100-200. With solutions containing such VOCs, very high feed velocities through... 

The calculations shown in Figure 11.18 assume that a hard vacuum is maintained on the permeate side of the membrane. The operating and capital costs of vacuum and compression equipment prohibit these conditions in practical systems. More realistically, a carrier facilitated process would be operated either with a compressed gas feed and atmospheric pressure on the permeate side of the membrane, or with an ambient-pressure feed gas and a vacuum of about 0.1 atm on the permeate side. By substitution of specific values for the feed and permeate pressures into Equation (11.19), the optimum values of the equilibrium constant can be calculated. A plot illustrating this calculation for compression and vacuum operation is shown in Figure 11.19. 

Under the assumptions of this calculation, the optimum equilibrium constant is 0.3 atm-1 for compression operation (feed pressure, 10 atm permeate pressure,... 

Figure 11.19 Illustration of the effect of feed and permeate pressure on the optimum carrier equilibrium constant. Z>ra[R] (m)tot/ E vacuum operation, feed pressure 1 atm, permeate pressure 0.1 atm compression operation, feed pressure 10 atm, permeate pressure 1 atm...

Figure 11.21 Long-term performance of a composite solid polymer electrolyte membrane consisting of 80 wt% AgBF4 dissolved in a propylene oxide copolymer matrix. Feed gas, 70 vol% ethylene/30 vol% ethane at 50 psig permeate pressure, atmospheric [33,61]...

Figure 13.9 MSR reaction. CH4 equilibrium conversion for both traditional and membrane reactors. I is the ratio ofthe sweep flow rate to the CH4 feed flow rate. H20/CH4 feed molar ratio = 3, permeate pressure = 100 kPa.

At low permeate pressures typical of ideal pervaporation, the same form for membrane selectivity applies as for gas or vapor separation of components A and B, which will be discussed later. This simple selectivity equals the inverse ratio of the resistances of the membrane to permeation of components A and B [S2B / 2A from Eq. (1)]. This ratio can be shown to simply equal to the ratio of permeabilities of the two components in the material comprising the selective layer of the membrane. 

When pressure is the driving force of the process, the gaseous molecules will be transported from the high-pressure side to the low-pressure side of the membrane (see Figure 10.2 [18,19]). The permeation cell (Figure 10.2) is coupled with two (or more) pressure transducers to measure the reject pressure, Ph and the permeate pressure, P2, and a mass flow meter (/ ) to measure the flux (./) passing through the membrane [18,19],... 

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