Permeation feed compression

Separators are sometimes arranged in series, as shown in Fig. 26,116. The frictional pressure drop on the feed side is usually small <1 atm), so two or three units can be put in series without having to recompress the feed. The permeate streams differ in purity and may be used for different purposes or they may all be combined. Another method of operation uses lower permeate pressures in successive units. The first unit produces permeate at moderate pressure so that the gas can be used directly without compression. The next unit operates at lower downstream pressure to compensate for decreased feed concentration, and the permeate is compressed for reuse. In a large plant a combined series-parallel arrangement could be used, with several pairs of permeators connected to a common source of feed. 

The estimation of the energy requirement has to be aehieved in a seeond step. For a single stage process, the evaluation is straightforward. Since the pressure ratio only plays a role in the analysis, feed compression or vacuum pumping at the permeate side can, in principle, be indifferently applied. For instanee, for a feed compression with atmospheric pressure at the permeate side (p" = 1) and no energy recovery system at the retentate (such as an expander), the energy requirement can be estimated as ... 

Compression If compression of either feed or permeate is required, it is highly likely that compression capital and operating costs will dominate the economics of the gas-separation process. In some applications, pressure is essentially free, such as when removing small quantities of CO2 from natural gas. The gas is often... 

Figure 1. Exploded view of RO cell (to scale). The various components of the cell fit together, are compressed by machine bolts, and are sealed with Viton O-rings. The membrane (effective diameter = 3.8 cm) is compressed against a porous steel plate (1/16 in., porosity = 25 pm) and flushed with feed solution. A certain amount of water penetrates the membrane and is collected as the permeate water (D). The feed solution enters the cell (A) and washes across the membrane (B) before being forced out of the cell (C).

In gas separation, a gas mixture at a pressure p0 is applied to the feed side of the membrane, while the permeate gas at a lower pressure (pt) is removed from the downstream side of the membrane. As before, the starting point for the derivation of the gas separation transport equation is to equate the chemical potentials on either side of the gas/membrane interface. This time, however, the chemical potential for the gas phase is given by Equation (2.8) for a compressible fluid, whereas Equation (2.7) for an incompressible medium is applied to the membrane phase. Substitution of these equations into Equation (2.20) at the gas/membrane feed interface yields3... 

The relationship between pressure ratio and selectivity is important because of the practical limitation to the pressure ratio achievable in gas separation systems. Compressing the feed stream to very high pressure or drawing a very hard vacuum on the permeate side of the membrane to achieve large pressure ratios both require large amounts of energy and expensive pumps. As a result, typical practical pressure ratios are in the range 5-20. 

Figure 8.20 shows another type of recycle design in which a recycle loop increases the concentration of the permeable component to the point at which it can be removed by a second process, most commonly condensation [38], The feed stream entering the recycle loop contains 1 % of the permeable component as in Figures 8.16-8.19. After compression to 20 atm, the feed gas passes through a condenser at 30 °C, but the VOC content is still below the condensation concentration at this temperature. The membrane unit separates the gas into a VOC-depleted residue stream and a vapor-enriched permeate stream, which is recirculated to the front of the compressor. Because the bulk of the vapor is recirculated, the concentration of vapor in the loop increases rapidly until the pressurized gas entering the condenser exceeds the vapor dew point of 6.1 %. At... 

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...

The largest current application of gas-separation membranes is separation of nitrogen (N2) from air. Capillary modules formed into bore-side feed modules are used almost exclusively in this application [10, 11]. The feed air is compressed to 6-10 bar and pumped through the membrane capillaries. Oxygen (02) permeates the membrane preferentially, leaving an oxygen-depleted, nitrogen-rich residue stream. The first membranes used for this application were based on poly(4-methyl-l-pentene) and ethyl cellulose, and had 02/N2 selectivities of about 4. Because of the modest... 

A commercial nitrogen enrichment system is illustrated in Fig. 17. Hollow-fiber membrane modules are connected to a compressed air feed at 70-150 psi. The feed in usually to the bore side of the hollow fibers. Oxygen (and water vapor that may be present) permeate out of the fiber into the shell and exit at low pressure. Dry, nitrogen-enriched air... 

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