Pervaporation comes to age

Chemical Engineering Progress, Oct 2001 by Wynn, Nick

Reactions and Separations

This membrane process has found growing use for dehydrating organics. Uses keep increasing, mostly due to development of new, high-temperature membranes.

As a separation technique, pervaporation occupies a special niche in the chemical industry - it is the only membrane process primarily used to purify chemicals. Currently, about one hundred pervaporation units are 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 turning to separations closer to the chemical reaction step - more critical to production and promising much greater benefits.

This shift in focus is accelerating the development of new, more-robust membranes with better performance that can be used at higher temperatures. Over the next few years, pervaporation will be used increasingly to enhance reactor performance, either by purifying feeds or separating reaction products. Because pervaporation is a membrane process, these separations can be integrated with the reaction step, promising quantum improvements in reaction efficiencies, yields and process economics.

In pressure-driven membrane processes, such as ultra, micro and nanofiltration, the bulk component is purified by passing it through a porous membrane that holds back the minor component. The membrane acts rather like a filter or strainer. Reverse osmosis (RO) is similar, but uses nonporous membranes. In this case, the major component selectively permeates the membrane by preferential absorption, diffusion and desorption. The solute or minor component is held back.

Pervaporation and vapor permeation processes are used in the reverse situation, i.e., when the membrane is preferentially permeated by the minor component. The bulk fluid is held back by the membrane. To get a pure product stream in this situation, almost all of the permeating component has to pass through the membrane, so pervaporation and vapor permeation resort to application of vacuum to the permeate side of the membrane. Very high pressure ratios can be achieved, so the minor component can be almost completely removed without excessive pressure difference across the membrane. Undue mechanical stresses on the membrane and equipment are avoided.

Because substances that permeate nonporous membranes are reasonably volatile, application of vacuum always causes the permeate to be desorbed from the membrane in the vapor state. Hence, the term pervaporation is used if the feed to the membrane is liquid, since the contaminant appears to evaporate through the membrane. If the feed is vapor, or a gas/vapor mixture, the process is called vapor permeation.

The best-performing industrial membranes permeate water in preference to other components, so filtration and RO processes are used typically to purify water. In contrast, pervaporation and vapor permeation are commonly used to remove water from organics.

Wide-ranging applications

Membranes are selective either by pore size (porous membranes) or because of their chemical affinity for the permeating component. By far, the majority of pervaporation membranes in commercial use are hydrophilic. Most pervaporation membranes are therefore employed to dehydrate organics.

Although pervaporation and vapor permeation require significant driving forces to transport components through the membrane, the processes do not depend on particular vapor/liquid equilibria. Water is preferentially permeated from a stream irrespective of the other components present. In practice, pervaporation and vapor permeation are only competitive where distillation is difficult or costly.

Figure 1 shows where these processes are most usefully applied in dehydrating organics. The concentration and relative volatility of the organic are key variables.

Pervaporation is now coming of age as a separation process. Industrially, it has been introduced in end-of-pipe applications such as solvent recovery. As with other membrane processes, accumulated experience in both membrane fabrication, and design and operation of pervaporation units has matured the technology, such that it is now applied to separations that are integral to production. Although dehydrating the ethanol/water and isopropanol/water azeotropes account for the majority of all plants operating (roughly, four-fifths), most of the new plants being installed are in quite different applications.

Methanol and ethanol removal - Hydrophilic polymer membranes have also been developed that will permeate methanol, ethanol and, to some extent, isopropanol. (The lead photo shows a pervaporation plant for methanol treatment.)

These compounds can be removed from less polar organics, although membrane selectivity is not as high as when permeating water.

Methanol forms azeotropes with many substances, particularly esters, and often cannot be recovered from spent solvents or reaction mixtures by simple distillation. Pervaporation provides a simple way to break these azeotropes. Used alone or in combination with distillation, such units provide an economical and reliable route to recover or remove methanol.