Membranes total reflux

Figure 19.6. Gas permeation equipment and performance, (a) Cutaway of a Monsanto Prism hollow fiber module for gas separation by permeation, (b) Flowsketch of a continuous column membrane gas separator, (c) Composition profiles of a mixture of C02 and Oz in a column 5 m long operated at total reflux [Thorman and Hwang in ( Turbak, Ed.), Synthetic Membranes II, American Chemical Society, Washington DC, 1981, pp. 259-279],...

The set of Equations 32 -r 37 has to be solved numerically. For the special case of total reflux, however, an analytical solution for the concentration profile along the membrane column can be derived if the pressure losses are neglected. In the case of total reflux, for every section of the column (Figure 6.21) ... 

Figure 6.21 Membrane column, material balance around a fiber end (total reflux).

Figure 6.23 Calculated concentration profile of a membrane column, total reflux.

Therefore, the permeate side gas mixture at equilibrium has a composition equal to that of the original feed gas mixture at f = 0 no separation has taken place in the closed system. This is why gas permeation devices in practice are operated as an open system any gas mixture produced hy permeation is immediately withdrawn from the device. Since the intrinsic membrane transport rates of the two species for unit partial pressure difference between the feed and the permeate are different, the gas mixture produced as a permeate has a composition different from that of the feed. The permeate mixture withdrawn from the device must have a composition different from that of the feed mixture for separation. See Sections 6.3.3.S, G.4.2.2, 7.2.1.1 and 8.1.9 for open systems. Continuous energy input will yield separation in a closed system - see Figures 8.1.4(b) for a system operated at total reflux. 

Usually in an enricher or the enriching section of the membrane column, the more permeable component is steadily concentrated from the feed inlet to the compressor. However, some of the results show that the shell-side and even the tube-side composition profiles can pass through a minimum. Note the experimental data in Figures 7 and 8. In these cases the feed flow is relatively slow and reflux action, rather than bulk flow, is predominant. Figure 8 Illustrates that a composition minimum can also occur dinring operation of the total column when the residue flow rate from the enriching section is too slow. 

In actual use, the number of stages obviously will be greater since reflux is not total. Available information indicates that U.S. gas diffusion plants for U23S production utilize 10,276 stages. Total membrane area for a given separation is given by21 ... 

In a given application, 100 mol/s of a saturated liquid containing 37 mol% ethanol and 63 mol% water must be separated to yield a product which is 99 mol% ethanol, and a residue containing 99 mol% water. The solution will be fed to a distillation column operating at atmospheric pressure, with a partial reboiler and a total condenser. The reflux ratio will be 1.5 times the minimum. The distillate will enter a membrane with parameters am = 70 and 0 = 0.6. The membrane boosts the concentration so that the permeate stream is the ethanol-rich product (xp = 0.99). The retentate stream is returned as a saturated liquid to the column to the tray at the nearest liquid concentration. 

Similar PVDF- -PSSA membranes were prepared by Flint and Slade [50] using large-scale EB under atmospheric conditions with a total dose of 150 kGy. The grafting reaction was conducted under reflux in a temperature range of 80-100°C and the subsequent sulfonation of grafted films was conducted using concentrated sulfuric acid (98%). 

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