Feed compression permeate side

Quasi-nondestmctive techniques include several transport measurements that are used to test specific membrane properties. They require sample mounting by compression sealing or glass solders that rarely leave the dehcate membrane surface intact. Gas transport properties of dense and microporous membranes are tested by measuring single gas ji as a function of and and by obtainmg fluxes and a/,from the stationary composition and flow rate of gas mixtures at the membrane feed and permeate side. To use the results of these measurements for comparison and optimized membrane designs, substantial... 

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. 

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. 

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. 

For both membrane systems investigated (Pd membrane reactor, and Pd membrane separator plus Pd membrane reactor), the design was carried out by operating at the same reactant molar ratio used in the industrial plant (Pd Reactor 1) as well as lower values (Pd Reactor 2). The feed stream is available in the plant at 16 atm (16.2 bar) and, therefore, no further compression of the feed was considered for the membrane units. Furthermore, no sweep gas or vacuum was applied at the permeate side, and the hydrogen coming from the membrane units was pure, without any need for separation devices downstream. However, the hydrogen pressure was too low to be directly used and, therefore, a compression step has been taken into account. 

The feed mixture is in the vapor phase, the partial vapor pressure of at least the critical (better permeating) component in the feed mixture is at or close to saturation, which may require compression of the feed mixture. The gradient in partial vapor pressure is maintained by a reduction of the permeate-side partial vapor pressure, too. The permeate leaves the membrane as a vapor and at least the critical (better permeating) component in the permeate can be condensed and removed as a liquid. Due to changes of saturation conditions (temperature or pressure) with changing composition of the feed mixture along its passage over the membrane some of the feed vapor may condense on the membrane surface and will be separated by pervaporation. 

Another membrane process that can be used in combination with a fermenter is pervaporation. The fermentation broth is supplied as feed to a pervaporation module, and a vacuum is applied to the permeate side of the membrane. When an ethanol-selective membrane is used, the bioreaction product ethanol passes through the membrane as permeate vapor, which is liquefied later by compression. Microorganisms, substrate, and nutrients are retained on the feed side. The product ethanol can therefore be purified, and since the membrane is ethanol selective, it can even be concentrated in the permeate. Corresponding to the feed ethanol concentration of 5.5 wt%, 30.6% of ethanol in the permeate can be achieved [279]. Thus, ethanol can be both purified and concentrated. Concentrated ethanol can further be dehydrated by pervaporation, using a water-selective membrane. 

The gas composition and pressure differential become very important when the more permeable gas in the feed has a low concentration. Since the partial pressure of the fast component on the permeate side cannot exceed its partial pressure on the feed side, high feed gas pressures and low permeate pressures are required to obtain efficient separations even with high separation factors. The differential pressure across the membrane relates directly to the membrane area required. Compression costs on the other hand are a function of pressure ratio. Therefore, operation at high pressure with a substantial pressure differential across the membrane but with a reasonably low pre.ssure ratio, is economically advantageous where recompression of the permeate is requited. 

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 ... 

The first limit is that the purified hydrogen is recovered at low pressure in the permeate side and requires compression in order to feed it back to reactors. As such, PSA (pressure swing adsorption) is a more attractive process, as the produced purified hydrogen is delivered directly at high pressure. 

---