Pervaporation spiral-wound modules

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

Figure 4.10 Trichloroethane enrichment [ln(l — 1/E)] as a function of permeate flux Jv in pervaporation experiments with silicone rubber membranes in spiral-wound modules using solutions of 100 ppm trichloromethane in water [15]. Feed solution flow rates are shown...

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

Table 4.2 Enrichment factors measured for the pervaporation of VOCs from dilute solutions with silicone rubber spiral-wound modules...

Figure 9.14 Pervaporation separation factor, /9pervap> as a function of the VOC evaporation separation factor, /3evaP- Data obtained with laboratory-scale spiral-wound modules containing a composite silicone rubber membrane and in laboratory cells with thick membranes...

Treatment of contaminated wastewater by pervaporation is superficially attractive, however, only a few commercial installations have been built. Phenol, which azeotropes with water at around 10wt%, is typically recovered by extraction in large plants, where measures to eliminate extractant loss can be economically applied. Pervaporation is an attractive and cost competitive alternative for small plants. Organophilic membranes in spiral wound modules are used in these applications. 

Pervaporation membranes can be made in flat sheet (Fig. 17.4) or hollow fiber (Figs. 17.5 and 17.6). So far, flat-sheet membranes in plate-and-firame type of modules were the only type of membranes available for very few applications. There has been significant failure in spiral-wound modules and hollow-fiber modules due to membrane failure as well as lack of compatibility of epoxy and glue. Even though flat-sheet pervaporation membranes showed good performance in ethanol applicalions, there have been numerous failures in other applications. 

Pervaporation Most pervaporation systems are small so plate-and-frame systems were used in the first systems. Spiral-wound and capillary modules are being introduced. 

Figure 4.9 Derivation of the mass transfer coefficient by Wilson s method. Toluene/water enrichments are plotted as a function of feed solution superficial velocity in pervaporation experiments. Enrichments were measured at different feed solution superficial velocities with spiral-wound membrane modules [15]...

Cao B and Henson MA. Modeling of spiral wound pervaporation modules with application to the separation of styrene/ethylbenzene mixtures. J Membr Sci 2002 197 117-146. 

The way membranes (in various forms, i.e., cylindrical, coaxial, flat-sheet, spiral-wound, and hollow fiber, etc.) couple with the bioreactor depends on the role the membrane performs. As with catalytic and pervaporation membrane reactors, the simplest configuration consists of two separate but coupled units, one being the bioreactor the other the membrane module. The biocatalyst (e.g., enzymes, bacteria, yeasts, mammalian cells) could, in this case, be suspended in a solution and continuously circulated through the... 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

P. J. Hickey and C. H. Gooding, Modeling spiral wound membrane modules for pervaporative removal of volatile organic compounds from water. J. of Membrane Science, 88 (1994) 47-68. 

Hichey PJ, Gooding CH (1994) Mass transfer in spiral wound pervaporation modules. J Membr Sci 92(l) 59-74... 

The plate-and-frame module is highly effective in pervaporation applications, but is much less popular than spiral wound and hollow fiber modules for gas... 

---