Permeation sweep operation

Optimization of operating conditions is also made with reference to the final target The choice of whether to pressurize the feed stream or to use vacuum or a sweep gas on the permeate side, depends on assodated costs and on the specific case considered. For example, if the feed stream is already available in the plant at the pressure needed, the permeate can operate at atmospheric pressure. Otherwise, the energy required to apply vacuum or to pressurize the feed must be compared to the costs of the sweep gas as well as the costs related to the additional separation of the diluted permeate. 

Pervaporation. Vapor arbitrated pervaporation is used to remove ethanol from whiskey by selective passage of the alcohol through a membrane. Whiskey flows on one side of a membrane. A water-vapor stream flows on the other side and sweeps away the ethanol that permeates the membrane. Thus alcohol reduction and selective retention of flavor and aroma components can be achieved usiag membranes with a particular porosity. The ethanol can be recovered by condensing or scmbbiag the vapor stream. Pervaporation systems operate at or slightly above atmospheric pressure (Fig. 

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

An alternative carrier-gas system uses a condensable gas, such as steam, as the carrier sweep fluid. One variant of this system is illustrated in Figure 9.7(d). Low-grade steam is often available at low cost, and, if the permeate is immiscible with the condensed carrier, water, it can be recovered by decantation. The condensed water will contain some dissolved organic and can be recycled to the evaporator and then to the permeate side of the module. This operating mode is limited to water-immiscible permeates and to feed streams for which contamination of the feed liquid by water vapor permeating from the sweep gas is not a problem. This idea has been discovered, rediscovered, and patented a number of times, but never used commercially [37,38], If the permeate is soluble in the condensable... 

FIGURE 23 Pervaporation process concepts (a) partial vacuum operation and (b) permeate-side sweep stream operation. 

Introducing a significant quantity of the carrier (or sweep) gas to the permeate side of the membrane, however, has two major implications. One is that it may necessitate the need to separate the permeate from the carrier gas downstream of the membrane reactor operation if the permeate is a valuable product. The other is the cost associated with the sweep gas. If chemically compatible, air can be used as the least expensive sweep gas available. 

The permeation experiments have been carried out in a WK-type cel with generally at both sides atmospheric pressure, or higher pressures at the feed side. Subatmospheric pressures are obtained by dilution with helium, which is also used as sweep gas. The cell is placed in a GC oven which can operate from 200 K (by cooling with solid CO2) to 700 K. 

Complete membrane systems can be operated in a variety of modes with e.g. CO- or counter flow of feed (high pressure side) and permeate (low pressure side) streams and with membrane modules coupled in different ways. Permeation and separation in these complex engineering systems will not be treated in this chapter. Heat and mass transfer limitations on the gas-membrane surfaces or interfaces can be important with high fluxes and/or strongly adsorbing gases as well as in membrane reactors. These effects will not be treated explicitly but are introduced in experimental results, e.g., by variation of sweep rates of permeated gases. 

In most industrial installations, the first option has been proven to be the most effective and economical process. However, it is preferable to pass all the permeate through a vacuum pump and condense them at atmospheric pressure, if the permeate quantity is low or if it contains a very volatile organic. Regarding the second option, sweep gas operation is rarely used, because sweeping right up into the pores of composite membranes is difficult. 

Membrane processes operate in two basic modes. In Figure 9.3(a), the permeate stream is solely the components of the feed stream that transport across the membrane. Figure 9.3(b) illustrates the case where a sweep stream is introduced on the permeate side to collect the permeate. The sweep stream can operate cocurrent or countercurrent to the feed stream. One limiting case is when the streams on both sides of the membranes are perfectly mixed and there is no axial variation in solute concentration. These basic modes are then incorporated into various geometric configurations (Figure 9.4). 

The so-called carrier-gas method is the most complicated approach to gas permeation monitoring.19 Either pure-component or multicomponent permeation rates through Alms can be monitored using this device, shown in Fig. 20.3-8. A carrier gas such as helium, containing a desired partial pressure of the desired component or components. Hows past the upstressu face of the membrane. A downsttesm sweep gas picks up the permeated components and routes them to a gas chromatograph for analysis of the fluxes of each penetrant. An excellent discussion of such a system has been offered by Pye el al.and comparisons were made with manomeiric cells for pure gas permeation to prove that the results for both celts are essentially identical if care is taken in operation. 

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