Vapor permeation modules

FIGURE 6.27 General working principle of a pervaporation or vapor permeation module equipped with tubular ceramic membrane elements. 

Yamamura T, Kondo M, Abe J, Kita H, and Okamoto K. The vapor permeation module using zeohte NaA membrane for producing absolute ethanol from biomass ethanol. Proceedings of the Eight International Conference on Inorganic Membranes, Cincinnati, OH, July 18-22, 2004 599-603. 

Tubular pervaporation and vapor permeation modules are also under development to house polymer membranes and promise more predictable performance, easier membrane replacement, isothermal operation, and lower costs. 

Pressure losses at the feed side have to be reduced to a minimum in vapor permeation. Otherwise the process would no longer operate at constant pressure, but the feed vapor could reach a region where superheated conditions would exist. Consequently, pressure losses in vapor permeation modules have to be as low as several millibar only. In pervaporation the feed-side pressure losses are not that critical, but in multistage arrangements they will eventually limit the number of appHcable stages. 

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

Several companies have proposed vapor permeation modules to operate heavy hydrocarbon recovery from small natural gas flowstream since the 1990s MTR Inc., Borsig and GKSS. All the membranes in operation are based on sihcone selective layer which offer mixed-gas propane/methane separation selectivity comprised between 3 and 5 and butane/methane separation selectivity ranging from 5 to 10. 

Figure 4.19(b) shows an equivalent figure for a counter-flow module in which 5 % of the residue gas containing 100 ppm water vapor is expanded to 50 psia and introduced as a sweep gas. The water vapor concentration in the permeate gas at the end of the membrane then falls from 1900 ppm to 100 ppm, producing a dramatic increase in water vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module. 

Both Mitsui [26] and Sulzer [27] have commercialized these membranes for dehydration of alcohols by pervaporation or vapor/vapor permeation. The membranes are made in tubular form. Extraordinarily high selectivities have been reported for these membranes, and their ceramic nature allows operation at high temperatures, so fluxes are high. These advantages are, however, offset by the costs of the membrane modules, currently in excess of US 3000/m2 of membrane. 

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... 

The basic module types originally used for ultrafiltration and reverse osmosis in water treatment have been adapted for pervaporation and vapor permeation. 

Fig. 22 shows a reaction scheme enhanced by continuously removing water directly from the reactor. In this case, the water is removed from the vapor phase. A vapor stream is sparged from the reactor and circulated through a vapor permeation membrane module, where water is selectively permeated through the membranes. The membrane unit is sized, such that all the reaction water can be removed with the water/ alcohol ratio just below the azeotropic composition. 

Similarly to other traditional equipment used in separation processes, the main objectives when designing a vapor permeation or a pervaporation unit are the attainment of the highest possible mass-transfer surface to volume ratio, while maintaining adequate conditions to avoid detrimental mass-transport phenomena. These criteria, together with the need for simple operation and easy maintenance procedures, determine to a great extent the principles for module design. 

Indeed, in many cases, the membrane system cannot be used directly and often pretreatment is necessary to facilitate the membrane process. Pretreatment is important and necessary in MF, UF, and NF, while it is not that important for pervaporation (PV), vapor permeation (VP), or gas separation for which feed streams are usually much cleaner and do not contain many impurities. The cost of pretreatments can contribute appreciably to the overall costs. However, due to the intrinsic characteristics of modules and membrane material, ceramic membranes require less feed stream pretreatment and authorize very efficient cleaning and sanitizing procedures. 

The design of modules for pervaporation and vapor-permeation processes had been based on the experience gained in those for water treatment by membranes, like ultrafiltration and reverse osmosis. However, significant modifica-... 

Hollow fibers or capillary modules have not yet found an industrial application in pervaporation or vapor-permeation processes. A few data have been reported where organic capillary structures with an outside diameter of 0.5 to 1 mm have been coated with silicon and used in organophilic separation. With the flow on the shell side permeate pressure losses inside the bore of the fiber control the process. For specific organophilic applications, these pressure losses may be tolerable. For hydrophilic processes, however, the useful length of a module would be of the order of 20 to 30 cm only, even at an inner diameter of the capillary of 1 mm. Such a module, including housing and connection in any industrial application, is more costly than a plate module. So far no potting material is available that combines the necessary chemical and mechanical stability at the operation temperature and pressure of a dehydration plant. 

The criteria to choose between pervaporation or vapor permeation have been discussed in Section 3.2.6. In Fig. 3.10 the principal features of a standalone vapor-permeation plant are shown. The liquid feed stream from a storage tank is completely evaporated the composition of the vapor entering the membrane modules equals that of the feed entering the evaporator. If the feed is an azeotrope the composition of liquid feed, vapor, and evaporator content are identical. 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

The permeate pressure is 0.155 atm, and the vapor permeate streams from all of the modules flow to a single refrigerated condenser (with area 2.74 m and overall heat transfer coefficient of 730 kcal h m K ), which uses 62.5 kmol/h of 273 K refrigerant to condense the permeate vapor. Condenser heat duty is 108 kW. The total permeate flowrate is 8.69 kmol/h with a composition of 3.84 mol% ethanol, and this stream is fed back into the distillation column along with the fresh feed at Stage 19 (numbering from the top with the reflux drum as Stage 1). 

Figure 14.3 illustrates what happens inside the first three-cell module. Temperatures decrease from cell to cell. Permeate ethanol composition increases and permeate flowrate decreases from cell to cell. The fluxes from each of the three cells (with different temperatures and compositions) are mixed to give the total vapor permeate from the module. Each module has a total area of 300 m or 100 m per cell. 

An important dynamic parameter is the holdup in the pervaporation unit. Since the mass of the liquid retentate phase is much larger than that of the vapor permeate phase, it will dominate the dynamic response. The holdup depends on the volume. In this study, the data given in Geankoplis is used (328m /m ). Thus the 300 m module has a volume of 0.9 m that is split among the three cells in the module. 

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