New Energy-Efficient Cocurrent Distillation Technology Is a Major Step Forward for Industry

Advances in Industrial Energy-Efficiency Technologies

Prepared for:
U.S. Department of Energy
Office of Industrial Technologies
Washington, D.C. 20585

Produced by:
National Renewable Energy Laboratory
Golden, Colorado 80401-3393

In conjunction with:
Energetics, Inc.
Columbia, Maryland 21046

DOE/CH10093-161
DE92016415
June 1993

Distillation technology has changed little since the 1800s

Distillation is a common but energy-intensive method of performing separations in the petroleum, chemical, food, pulp and paper, and pharmaceutical industries. In the chemical and petroleum industries alone, distillation is used to make 95% of all separations.

Photo: Testing of prototype cocurrent distillation columns is under way at several sites in Texas. The Trutna tray, shown here, is the first major innovation in distillation technology since the 1800s. (Photo courtesy of University of Texas at Austin) [provided in source document

In a distillation process, a liquid mixture is heated inside a cylindrical vessel or column. The component with the lowest boiling point begins to vaporize and exits from the top of the column, leaving the rest of the mixture behind. The process can be repeated as many times as needed to separate out all of the components of the mixture.

The thermal energy requirements of distillation are enormous. Distillation processes are thermodynamically less than 10% efficient and account for approximately 8% of the total energy use of the U.S. industrial sector. There have been no changes in distillation technology that have significantly increased its energy efficiency since its introduction. The technology used in the first distillation columns in the 1800s is basically the same as that used in some columns today. The U.S. Department of Energy's Office of Industrial Technologies (OIT) has targeted improved distillation as one of its major goals for saving energy in the industrial sector.

Cocurrent distillation saves energy and costs less than does conventional distillation

More than a decade ago, a Texas inventor named Williams Trutna patented a cocurrent contacting distillation process that is believed to represent the first major innovation in distillation technology. Mr. Trutna asked for and received funding support from OIT to develop the technology to the point of commercialization.

Photo: In cocurrent distillation, liquid is removed from the gas at the top of each stage before entering the next tray. [provided in source document]

Conventional distillation columns usually use metal objects such as trays to accelerate and control the concentration changes in the vapor and liquid. Cocurrent distillation utilizes a new tray design known as the Trutna tray. The novel concept underlying cocurrent distillation is to let the generated vapor carry the remaining the liquid upwards toward the next tray. A device for removing this entrained liquid-the collector-is located just below each tray. The collector consists of rows of channels that collect the liquid and transfer it to outlet channels (downcomers) along the side of the column.

In conventional distillation, the liquid is not carried upward to the next stage for collection. Conventional distillation relies on the force of gravity to separate the liquid from the liquid-gas mixture that collects on the trays.

Work to date indicates cocurrent distillation has both higher energy efficiency and better performance than conventional distillation. The technology provides increased mass transfer between the gas and liquid phases. This results in improved stage efficiencies and higher capacities in a given-size column.

The cost benefits of cocurrent distillation appear attractive compared with those of conventional distillation. For example, higher stage efficiencies are expected, and concurrent columns can be significantly narrower. In addition, the new columns can be shorter because less space is required between Trutna trays. All of these factors make a cocurrent distillation column cost as low as two-thirds that of a standard column.

The cocurrent distillation technology under development also saves energy. The energy input required for a cocurrent distillation column, based on the above benefits, is approximately one-third less than that required for a standard column. It is estimated that the adoption of cocurrent distillation will ultimately reduce the U.S. direct consumption of energy for distillation by 10%.

Large-scale testing is planned for several sites in Texas

A number of laboratory studies were recently completed to improve de-entrainment at high liquid flow rates and enhance mass transfer. The tests showed that altering the collector design enables the Trutna tray to meet a range of capacity requirements.

Because of its high capacity and low stage heights, cocurrent distillation is attractive for most gas/liquid contacting operations. In addition, retrofitting existing distillation columns is an attractive option because of the 30% to 60% increase in throughput rates possible over those of existing trays or packings.

Research and development on this project has taken place at the University of Texas in Austin. Researchers have completed first-phase testing of a six-stage, 18-in. (46-cm)-diameter prototype column. In addition, Giltsch, Inc., a cost-sharing partner, will perform testing of the Trutna tray in a 20-ft (6.1-m)-high, 3-ft (0.9-m)-diameter column at its test facilities in Dallas. This test is scheduled to begin by the end of 1993.

For More Information:

Separations Program
EE-233
Office of Industrial Technologies
U.S. Department of Energy
1000 Independence Ave., SW
Washington, DC 20585
(202) 586-0937