Exact figures may be hard to come by, given the complexity of the chemical industry, but separation processes have been estimated as accounting for somewhere between 40% and 70% of its capital and operating costs. Add in petroleum refining and you have an industrial sector that requires 45% of its annual energy consumption simply to separate products from its main process streams. Not surprisingly, therefore, energy-intensive separation processes like distillation are facing increasing competition from a variety of alternative technologies.
When the Chemical Industry Vision2020 Technology Partnership was set up in 1996 (see CP, October 2005, p. 16), one of its aims was to establish a series of “roadmaps” that might guide the development of new processes and unit operations. When its “separations roadmap” was published in 2000, the technologies deserving priority for more concerted development efforts had been winnowed to six — adsorption, crystallization, distillation, extraction, membranes and separative reactors. The thinking was that, although some technologies have been around a lot longer than others, no separation process has really reached full maturity — at least in the sense that no further improvements are possible.
Times change, however, and much more effort — both academically and industrially — now appears to be going into developing robust and reliable alternatives to energy-thirsty thermal separation processes like distillation. Government, too, has added its backing for developing innovative separation technologies, even those of a high-risk nature. Charles Russomanno of the Office of Energy Efficiency and Renewable Energy at the U.S. Department of Energy (DOE), Washington, D.C., who fields applications for project funding under the Small Business Innovation Research Program, says that membrane technology, in particular, offers a viable alternative to conventional energy-intensive separations.
“Successful membrane applications today,” he reports, “include producing oxygen-enriched air for combustion, recovering and recycling hot wastewater, VOC [volatile organic compound] recovery, and hydrogen purification.” Combined with conventional techniques like distillation, membranes can help to deliver improved product purity at lower cost, says Russomanno. There are obstacles in the way, though. “Technical barriers include fouling, instability, low flux, low separation factors and poor durability,” he says. The DOE is looking for projects that will lead to new generations of organic, inorganic and ceramic membranes that will demonstrate greater thermal and chemical stability, better reliability, improved resistance to fouling and corrosion, and higher selectivity.
It might seem a tall order but a great deal of work has already been done, and a great deal more is in progress, as William Koros of the school of chemical and biomolecular engineering at the Georgia Institute of Technology, Atlanta, Ga., explains. “The realization that membranes require treatment as a cross-disciplinary specialty area,” he says, “has enabled movement of the technology from the lab into commercial reality. It’s critical to maintain this perspective to position membranes so they can economically handle more-aggressive feed streams.”
As he points out, the first large-scale commercially viable application of membrane technologies was the reverse osmosis (RO) purification of salt water. However, optimization of the membrane materials and structures for this application took place over 20 years, and only fairly recently has RO started taking over from thermal desalination plants. So, will it take as long for non-aqueous RO separations to find a niche? Koros thinks not: “There is no need to wait for the ‘ultimate’ membrane to be perfected before we can begin benefiting from the savings associated with membrane separations.” He says that reconfiguring existing thermal processes to produce vapor feeds to membrane units — based on existing gas-separation units — for targeted fractionations of valuable components could be an attractive evolutionary strategy.
Potentially, he says, membranes have all the characteristics of a “disruptive technology,” one that could bring about huge reductions in energy intensity in large-scale separations. “But they are not ready yet for aggressive hydrocarbon feeds,” a situation his own research work is set on remedying. “The key to the success of polymeric membranes,” he explains, “is that basically they’re so inexpensive. But they’re not very robust. We’re focusing on a type of bridge technology, hybrid materials with inorganics dispersed within a polymer. So you end up with some of the economics of polymerics and some of the strengths of inorganics.”
Current hybrids on the market typically include around 10% to 15% of inorganics — usually ceramic type materials — interspersed within the polymer structure. “But if you could move that ratio the other way, then that’s where the revolution would happen. Even if the cost was, say, 10 times that of a polymer membrane, with the extraordinary performance offered by inorganics then it, too, would become a disruptive technology — it changes everything,” says Koros.
His group is looking at zeolites or carbon as the inorganic component of hybrids. Carbon holds out more promise, as these membranes are easier to manufacture on a large scale. Basically, they can be made by pyrolyzing an asymmetric polymer membrane so that the thin (0.1-0.2 μm) surface layer stays in place while the body of the asymmetric structure is effectively transformed into a molecular sieve.
“You get extraordinary diffusion coefficients, much higher than with zeolites, so these carbons have the potential to be much more productive on an equivalent thickness basis,” he says.