Separations in flux

Separation processes often account for more than 50 percent of a chemical company's operating costs. Read how membranes and other non-thermal technologies poised to win a wider role.

By Mike Spear, editor at large

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Blue Membrane (BlueM), Wiesbaden, Germany, specializes in carbon/ceramic membranes and membrane reactors. CEO Soh<acute accent over e>éil Asgari says that both are resistant to harsh conditions such as temperatures up to 800ºC and the full range of pH from 1 to 14. The BlueM membranes are available as inorganic carbon/ceramic, composites or mixed matrix membranes with inorganic and organic textures. The company supplies membranes tailored to suit the particular project.

Compared to conventional polymeric membranes, he says the carbon/ceramic or composites can be tailored in selectivity and permeability for gas and liquid separation. They can also be produced in different configurations, such as “spacerless” honeycomb modules with cross-flow or linear-flow geometries. Tubular carbon[/ceramic elements have been developed for a wide range of applications, such as recovering high-value pharmaceutical precipitates, or for pre- and post-treatment stages with other separation technologies like RO, ion exchange or adsorption.

The carbon[/ceramic membranes are compatible with aggressive fluids like liquid sulfur and ammonia, can handle the recovery of catalysts in peroxide production, and offer the chemical inertness and temperature resistance the food industry demands to ensure sanitary operations.

Hybrid membranes could be said to be at the cutting edge of developments, but work is very much in progress on further developing conventional polymeric-based membranes. For instance, Membrane Technology and Research, Menlo Park, Calif., which since 1996 has offered VaporSep systems for recovery of propylene from petrochemical waste gases (Figure 1), recently introduced a new membrane for achieving high recovery of purified hydrogen. The first two commercial installations are expected to start up this year.

Gas separation

Figure 1. This polymeric-based-membrane unit at the SECCO petrochemical complex in Shanghai, China, recovers 1.5 tons/hr of 95%-pure propylene from vent streams.

Another company now beginning to see the fruits of earlier development is Membrane Extraction Technology (MET), London, U.K., a spin-off from the chemical engineering department of London’s Imperial College. MET has worked closely with the Grace Davison division of W.R. Grace, Columbia, Md., since 1999 and is the exclusive European supplier and application-developer for the fine chemicals and pharmaceuticals markets of that company’s Starmem range of nanofiltration membranes. Resistant to aromatic and aliphatic hydrocarbons, alcohols, ketones and esters, these membranes have had considerable success in organic solvent nanofiltration, including the recovery and reuse of phase-transfer catalysts, organometallic catalysts and chiral hydrogenation catalysts.

Starmem membranes also have been successfully used to swap solutes dissolved in high-boiling-point solvents to ones with a lower boiling point. MET’s business development manager Lina Christopolou gives the example of the phase-transfer catalyst tetraoctyl-ammonium bromide being swapped from 100% toluene solvent to 99.7% methanol, with less than 1% loss of catalyst. She says the benefits of this type of solvent swapping include its efficiency — producing less than one-third of the waste generated by distillation, its suitability for thermally sensitive materials, and insensitivity to azeotrope formation, for which it is “an excellent alternative to distillation.”

Pervaporation progress

One area where distillation has always had distinct drawbacks is in the handling of azeotropic mixtures. It is perhaps here that membrane technology — in the form of pervaporation — shows the most promise. As the name implies, pervaporation combines permeation and evaporation. Components of a liquid mixture are brought into contact with one side of a permeable nonporous membrane while a vacuum or gas purge is applied to the other side. The components have different affinities for the membrane and pass through it at different rates. The composition of the vapor produced on the other side of the membrane can differ dramatically from that evolved during a conventional distillation process.

According to Frank Seibert, technical manager of the separations research program at the University of Texas at Austin and director of the Separations Division of the American Institute of Chemical Engineers, “[Pervaporation] took off in Europe 10 to 15 years ago, when energy prices were high. Economically it was viable there, but not really in the U.S. when energy prices were a lot lower than they are now. But perhaps now it may start looking more attractive.”

Those early applications were mainly for the removal of water from organic solvents; the U.S. Environmental Protection Agency’s National Risk Management Research Laboratory at Cincinnati, Ohio, is now focusing part of its membrane development efforts in that area. Primary investigator Leland Vane reports the project aims to fabricate novel polymer/ceramic mixed membranes for the dehydration of alcohol, particularly isopropyl alcohol, which is used in many industries as a solvent and cleaning agent. One of the early goals is to develop a Pervaporation, Performance, Prediction Software and Database program to give potential users a means of assessing applications of pervaporation through computer modeling and simulation.

In the commercial world meanwhile, Sulzer Chemtech, Pasadena, Texas, continues to evolve its range of Pervap composite polymer membranes. These use an active PVA separating layer that preferentially permeates water. The crosslinking of this layer determines the way the membrane behaves in terms of permeate flux, selectivity and chemical resistance. Less-crosslinked membranes are now being used to separate methanol from other organics.

Real attractions

Just as membrane technologies are clearly gaining ground in sectors that were previously unaware of their potential, so too are other separation technologies such as simulated moving bed (SMB) chromatography. First developed in the 1960s for purification processes in the sugar industry and later taken up by refiners in the form of UOP’s Sorbex process for the purification of xylene isomers, SMB chromatography is now showing great promise in the pharmaceuticals and bioprocessing sectors, as reported in CP, September 2005, p. 38.

For instance, Ampac (formerly Aerojet) Fine Chemicals in Rancho Cordova, Calif., runs production-scale SMB units for a number of major pharmaceuticals producers, notes Olivier Dapremont, its director for chromatographic separations and co-author of the article. “We have a process here that we have been running for five years on the same separation,” he says, “with only the occasional break for maintenance or between production campaigns. It’s a very stable operation and is one of the reasons why the technology is starting to take off now.”

However, despite this growing interest in SMB, relatively few companies as yet offer the technology, whether on a custom-processing basis as at Ampac or for purchase. Dapremont acknowledges that this lack of choice could be a limiting factor. “If you want to use, say, crystallization for your separations, there may be a hundred or more companies out there to choose from — giving you much more flexibility and so perhaps less inclination to try SMB,” he observes. “But it’s the same with any new technology. You need to have a certain critical mass [of users] to keep the interest going.”

That’s not really a problem with long-established separation techniques like liquid/liquid extraction, but even here there are developments that are very much leading edge. One is the use of low-frequency high-intensity sound energy to improve mixing. Developer Resodyn of Butte, Mont., offers two variants of mixer technology: the ResonantAcoustics and the ResonantSonics (which operates in the audible spectrum), both of which are said to dramatically improve gas/liquid and liquid/liquid mixing over conventional agitator-driven techniques. Available as standalone RAM80 Resonant/Acoustic mixer units, they also are being developed as part of bioreactor units.

Despite distillation’s predominant position, it’s clear that choices in separations technology are in flux.

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