Energy Efficiency

Don't give up on distillation

Opportunities for technology improvement exist, but there are still hurdles to overcome.

By Nick Basta

I'm not sure what equipment today's chemical engineering students work on in their introductory laboratory courses, but in my day we used distillation columns. The unit, if memory serves me correctly, was a squat, grime-encrusted apparatus in an otherwise clean and modern laboratory. We did fairly simple water-methanol fractionation, taking samples along the height of the column to monitor the progress of the separation and to compare distillation efficiency with McCabe-Thiele predictions.

It was good to focus on distillation, if only because it offered chemical engineering students a way to explain what they do that's different from other engineering students. But it also highlights the centrality of distillation to chemical engineering. It is, after all, one of the oldest unit operations (which some observers have traced back to the earliest archeological records for manufacturing alcoholic beverages) and also one of most widespread throughout the chemical industry.

The ubiquity of the technique means that distillation developments could have a major impact. However, the opportunities for improvement seem to remain just out of reach. In 2002, the Electric Power Research Institute (EPRI), Palo Alto, Calif., did a preliminary study and found that advanced distillation design techniques, combined with innovations in distillation equipment, potentially could save American industry $6 billion in energy expenses annually.

Even though the United States then was entering a period of heightened energy costs, which continues today, a larger study and demonstration project were not funded. "The high energy use by distillation in the process industries has been an issue for at least 20 years, more likely 40," says John Kunesh, former technical director of Fractionation Research Inc., an industry-supported group based in Stillwater, Okla., and head of AIChE's Separations Division. "It always boils down to incremental cost of energy versus cost of capital. Every time there is a big spike in energy costs there is renewed interest in various energy-saving schemes."

A perfect example of this can be found at the Office of Industrial Technologies of the U.S. Department of Energy (DOE), Washington, D.C. DOE, through its BestPractices program, cost-shares energy-efficiency studies. For instance, it worked with Chevron, San Francisco, to analyze operations at the company's Salt Lake City refinery. The study found that constructing a unit to separate saturates from unsaturates via distillation would allow Chevron to improve fluid catalytic cracker operation, produce more alkylate and market the saturated gas. Energy savings were estimated at 20 million Btu per year. But the projected $15 million cost of the unit would lead to a payout of 4.2 years, well above the three-year term that usually is the guideline for capital expenditures. Such an analysis is not so unusual , but for this example to be proposed as a "best practice" gives a sense of what industry is up against.

Sam Woinsky, an engineer now in private practice but who was formerly a design engineer at MW Kellogg and Foster-Wheeler, was one of the program managers who was trying to get the EPRI study off the ground. "Part of the problem was our timing," he says. "The industry was flat on its back then." But he also chides American companies for not supporting new technology development. "I'm convinced that what we think of as 'advanced distillation technology' today will be 'conventional' technology a few years from now," he says. "The problem is that most U.S. companies are too risk-averse to support this development; it will happen offshore."

Some might counter by arguing that distillation is a mature technology and therefore there simply might not be any more efficiency to extract. True, distillation systems have become highly refined through the efforts of many vendors and process developers. However, when you realize that, in most cases, the presence of a reflux stream represents a thermodynamic loss, significant opportunities for further efficiency improvements seem likely.

Is there a way to extract the risk from the distillation research "equation" to foster developments that might dramatically decrease the energy penalties paid for chemical production? Probably not. Nevertheless, industry should realize there are still opportunities.

By Nick Basta, editor at large