Permeate stream feed compression

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... 

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. 

Utilizing a single-membrane step to separate and concentrate the heavy hydrocarbons in the permeate stream [12]. The permeate stream is then compressed, followed by the condensa-tion/removal of heavy hydrocarbons, with the high-pressure methane residue stream recirculated to the feed stream reducing the methane loss. Only a few novel membranes have shown highly selective, stable performance, and the currently available rubbery m branes are likely to remain dominant however, the conventional refrigeration technology provides a stiff barrier for a broader membrane acceptance [14],... 

Thorman et al. (1975) have used two permeators of the type described in Example 2.2.4 in series to obtain O2-enriched air containing 0.310 mole fraction O2 from feed air (0.209 mole fraction O2). The feed air flow rate to the first permeator is 0.665 std. cm /s. The 02-enriched permeated stream containing 0.250 mole fraction O2 and having a flow rate of 0.365 std. cm /s is introduced as feed after compression into the second permeator, which has a permeate composition of 0.310 mole fraction O2. The permeate flow rate from the second permeator is 0.151 std. cm /s. The second permeator is somewhat smaller (silicone capillary length = 217 cm) than the first permeator (silicone capillary length = 691 cm). Obtain an estimate of the overall separation using 0 2, 9 and f. 

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 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... 

Vapor permeation (VP) and pervaporation (PV) are membrane separation processes whose only difference lies in the feed fluid being a vapor (VP) or a liquid (PV), respectively. This difference has impHcations for feed fluid handling as well as the nature of the transport phenomena occurring in the feed stream, as in VP the feed fluid is compressible whilst in PV it is effectively not however, this does not in any way affect the transport phenomena across and after the membrane barrier. For this reason, vapor permeation and pervaporation will be discussed simultaneously, with differences being expHcitly emphasized where necessary. 

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