Method Finds Most Efficient Distillation

Purdue team also is developing methods for multi-component separation using membrane cascades.

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A newly developed screening technique for distillation column configurations promises to foster energy savings. The method, devised by Rakesh Agrawal, professor in the department of chemical engineering, and Vishesh Shah, a graduate student, at Purdue University, West Lafayette, Ind., generates all feasible configurations for non-azeotropic multi-component mixtures and rank orders them according to vapor-duty.

Myriad distillation-train configurations can separate a multi-component mixture into the desired number of products. For instance, five product streams generate 203 basic configurations and an additional 5,925 thermally coupled ones, notes Agrawal. While these use the same number of columns, reboilers and condensers, they can differ markedly in energy consumption.

Their technique may change industry practice, believes Agrawal. For instance, petroleum refineries generally have used the same distillation configuration for around 75 years, as it's recognized as the most-energy-efficient known to industry. However, the method generated 70 new sequences that reduce heat demand by 6% to 48%.

"This is important because improving efficiency by 10% at a refinery processing 250,000 bbl/d would save in excess of $12 million a year if oil were priced at $70 a barrel," he stresses. "And that's just a single refinery. For the U.S. petroleum industry as a whole, this is a huge potential savings."

One company and one national lab already are evaluating efficient crude distillation configurations identified by us, he adds.

"Our method is quite general and is applicable to all non-azeotropic multi-component distillations in a chemical or a petrochemical plant. Our method identifies several attractive candidate distillation configurations for these separations. Some of these distillation configurations are easy to retrofit to an existing configuration in a given chemical plant. However, due to existing constraints the full energy potential may not be realized… a new grassroots plant provides more freedom to choose distillation configurations that have highest energy savings," explains Agrawal. "In the future, we expect to be able to extend the method to mixtures that form azeotropes."

"We can extend the method to generate schemes for non-distillation and combined separation techniques. We are developing methods for multi-component separation using membrane cascades. However, we have not yet developed methods to rank-list alternatives for multi-component separations involving integrated membrane and distillation separation configurations," he adds.

The next steps in the development are to reduce the time taken by the program to perform the search for optimal configurations and make the program user friendly, says Agrawal.

The U.S. Department of Energy (DOE), which funded the research through its Industrial Technology Program, wants to enable industry to use the method, he notes. "Our plan is to make the program accessible to process engineers by mid-2010."

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