Pervaporation unit

Dewatering of glycol is a difficult separation by distillation alone, so a hybrid process of the type shown in Figure 9.13(a) has been proposed. The product of the distillation step is approximately 90% glycol/10 /i- water. This mixture is then sent to a pervaporation unit to remove most of the water as a discharge-able product. The glycol concentrate produced by the pervaporation unit contains 1-2 wt% water and can be sent to an optional adsorption dryer if further dehydration is required. 

Figure 9.20(b) illustrates the use of pervaporation with two distillation columns to break a binary azeotrope such as benzene/cyclohexane. The feed is supplied at the azeotropic composition and is split into two streams by the pervaporation unit. The residue stream, rich in cyclohexane, is fed to a distillation column that produces a pure bottom product and an azeotropic top stream, which is recycled to the pervaporation unit. Similarly, the other distillation column treats the benzene-rich stream to produce a pure benzene product and an azeotropic mixture that is returned to the pervaporation unit. 

In membrane distillation, two liquids (usually two aqueous solutions) held at different temperatures are mechanically separated by a hydrophobic membrane. Vapors are transported via the membrane from the hot solution to the cold one. The most important (potential) applications of membrane distillation are in water desalination and water decontamination (77-79). Other possible fields of application include recovery of alcohols (e.g., ethanol, 2,3-butanediol) from fermentation broths (80), concentration of oil-water emulsions (81), and removal of water from azeotropic mixtures (82). Membrane (pervaporation) units can also be coupled with conventional distillation columns, for instance, in esterifications or in production of olefins, to split the azeotrope (83,84). 

The possibility of combining two different separation units into one, hybrid, process has not been considered in this chapter. Hybrid processes are quite novel and have only very recently been considered by industry and have, therefore, so far not made it into the standard textbooks. A hybrid process has the combined benefits of both of the component units and the benefits should theoretically outweigh the disadvantages. An example is a hybrid of a distillation column and a pervaporation unit for azeotropic separation, where the distillation unit alone is limited by the azeotropic point. Again, a lot of research is currently devoted to this type of operation and it is generally believed that it will become more widely used in the future. 

Figure 7.3. Experimental set-up for the fully automated continuous determination of phenols. FID — flame ionization deteotor, GC — gas chromatography, IV1 — HPLC injection valve, IV2 — low-pressure Injeotlon valve, M — membrane, PC — personal computer, PL — propagating liquid, PP — peristaltic pump, PU — pervaporation unit, RC — reaction chamber, S — sample, UP — ultrasonic probe, W — waste and WB — water bath. (Reproduoed with permission of Springer-Verlag, Ref. [31].)...

Shabtai Y, Chaimonitz S, Freeman A, Katchalski KE, Linder C, Nemas M, Perry M, and Kedem O. Continuous ethanol production by immobilized yeast reactor with membrane pervaporation unit. Biotech. Bioengg. 1991 38 869-876. 

FIGURE 13.10 Scheme of the MHS[FLM-PV] multimembrane hybrid system (1) cation-exchange membranes, (2) FLM, (5) vacuum system, (6) pervaporation unit, (7) pervaporation membranes, (8) contactor, (9) feed solution, (10) stripping solution. (From Wodzki, R. and Szczepanski, P., J. Mem. Sci., 197, 297, 2002. With permission.)... 

When the pervaporation unit is used for the continuous monitoring of a fermentation process with a view to determining analytes in samples containing suspended particles, or in slurries, the module is altered by increasing the diameter of both channels (inlet and outlet) of the donor chamber, which might otherwise be clogged. 

In multideterminations, and also when the target volatile analytes possess different boiling points, the pervaporation unit can be used at different temperatures to ensure selective separation of the species of interest. Use of a heating system allowing reproducible control of time and temperature is obviously mandatory here. 

Pervaporation/preconcentration of the analytes. First, the loop of the injection valve (IV1 in Fig. 4.24B) is filled with sample while IV2 and IV3 are in their injection and filling positions, respectively. In this way, the donor stream (H,0) is continuously circulated through the lower chamber of the pervaporation unit and the helium stream is continuously sent to the gas chromatograph meanwhile, a portion of the helium remains static in the acceptor chamber to collect the volatile species. Then, IV1 is switched to the injection position and the sample is transported by the HjO stream through the lower chamber of the pervaporation unit, which is immersed in a thermostatic bath at 80°C. 

Pervaporation occupies a special niche in the chemical industry—it is the only membrane process primarily used to purify chemicals. At the start of 2000, about 100 pervaporation units were operating worldwide, most of them dehydrating solvents, such as ethanol and isopropanol. Now that pervaporation has been proven in these end-of-pipe applications, attention is being focused on large-scale separations that are closer to the chemical reaction step—to be more critical for production and to bring much greater benefits. 

Hydrated solvent from a buffer tank is continuously circulated through the unit until the desired degree of dryness has been reached (Fig. 4). Such units are very flexible—different start and end water contents can be accommodated, and with multi-purpose membranes a range of solvents can be dehydrated. Note that batch pervaporation units are not suitable for feeds that may contain suspended solids or dissolved solids, which could precipitate as water is removed. 

Fig. 2 Continuous pervaporation unit. (View this art in color at www.dekker.com.)...

To debottleneck the system, reflux in the rectification column is reduced, giving more overhead product, but with a higher water content. The pervaporation unit is sized to remove enough water that the subsequent entrainer column is also unloaded. Both columns can then realize a significant capacity increase. 

The pervaporation unit required for debottlenecking is relatively small, because the driving force for water permeation is high. 

Online pervaporation (Fig. 20) replaces the distillation column altogether. Reaction mixture is continuously pumped through the pervaporation unit at a high rate, and a drier stream is returned to the reactor. 

Martin, N. Flexible and efficient. Batch pervaporation units for flexible dehydration. Process Eur. 2000, 1, 20-21. 

Ammonia Cigarettes Ultrasound-assisted bleaching UV—Vis 0.05 pg mL-1 Flow injection system with a pervaporation unit iterative change of flow direction for improving extraction Berthelot reaction [415]... 

Solid samples can also be analysed [275] by placing the sample at the bottom of the pervaporation unit and using the inlet stream as the digestion mixture. 

Diacetyl Beer Pervaporation UV—Vis 5.0 ng mL1 Flow injection system pervaporation unit thermostated at 90°C acceptor stream (alkaline a-naphthol + creatinesolution) halted during analyte collection [532]... 

The optimal feed concentration of the pervaporation unit depends on carrier flow rate, reflux ratio and number of theoretical trays of the extractive distillation. Retentate concentration and cut rate of the pervaporation stage follow from the requested product quality xlt + x31 <0.008. For the design of the pervaporation stage, the worst case has been assumed that only benzene and no furfural (3) will pervaporate. The major factor for the cost reduction is the much lower energy consumption of the hybrid process of 1.18 t/h heating steam against 1.7 t/h for the conventional process. 

According to Table 6.5, a further reduction of the specific separation costs is feasible if the effective membrane thickness can be reduced, for example, by employing optimized" asymmetric membranes.31 It should be noted, that the optimal feed concentration of the pervaporation unit is not only determined by the design of the extractive distillation but by the effective membrane thickness (or specific membrane costs) as well. With increasing permeate fluxes, or lower specific membrane costs, the optimal feed concentration is shifted to higher benzene concentrations. 

Combinations of distillation and pervaporation (membrane separation with a vapor permeate stream) have been applied to many separation problems. One example is the separation of ethanol-water mixtures. Ethanol and water form an azeotrope and therefore conventional distillation is not feasible. Using a combination including pervaporation, pure ethanol and water can be obtained as can be seen from Fig. 3.2-10. Water can be obtained from the bottom of the distillation column. The distillate stream consists of the azeotropic mixture which is fed into the pervaporation unit The highly selective polyvinyl alcohol membrane is capable of separating water from ethanol. Pure ethanol can be obtained from the retentate, and the permeate stream, which consists of ethanol and water, is recycled into the distillation column. 

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. 

Table 3.6-3 Solvents routinely dehydrated in vapor permeation/pervaporation units...

The organic membranes are tacky when in use and care is necessary when shutting a pervaporation unit down that the sheets of membrane do not come into contact with each other. 

Rapin JL (1988), The BETHENIVILLE pervaporation unit. The first large-scale productive plant for the dehydration of ethanol. Third Intern. Confer, on Pervaporation Processes in the Chem. Industry, Nancy, France, Sept. 19-22. 

Fig. 5.9 Configuration of a pervaporation reactor with an external pervaporation unit (1) and with an internal pervaporation unit (2), respectively [108].

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