Whether isolating a substance of commercial interest in the laboratory, purifying a drug or treating wastewater, separation technologies remain critical to the chemical process industry. The separations market continues to grow, driven by existing and new needs for wastewater, biotechnology, pharmaceutical and petrochemicals applications.
In fact, the market for all advanced novel separations techniques is expected to grow at an average annual rate of 23.4 percent until 2005, according to Business Communications Co., a Norwalk, Conn.-based market research firm. Membranes ," thin-film barriers that allow preferential passage of certain substances ," are leading this growth. These permeable barriers provide functional transport across the barrier and are characterized by properties of selectivity and flux.
This article focuses on membrane applications and takes a look at how these technologies are impacting chemical processes today, as well as how they might impact these processes tomorrow.
Taking center stage
The chemical industry does not bear witness to radical change often. The "next big thing" is seldom developed and implemented overnight. Many months and even years can lapse from the start of a research and development (R&D) project to widespread commercialization. Even then, industry perception can act as an additional barrier to technology adoption.
Take a look at membranes, for example. Membrane technology is not new. It has been commercially available for more than 30 years. However, membranes often have been relegated to secondary operations such as water purification upstream in the pharmaceutical industry, and to waste treatment or pollution control downstream. Today, however, they increasingly are being used or considered for mainstream operations.
The rising adoption of membrane separation technologies largely has been driven by the increasingly strict environmental regulations enacted over the last several decades, according to Membrane Separation Technologies to 2006, a new report from The Freedonia Group, Cleveland.
Some of these regulations include the U.S. Environmental Protection Agency's (EPA) Clean Water Act, Safe Drinking Water Act, Surface Water Treatment Rule and new arsenic rule, as well as food and drug purity rules from the U.S. Food and Drug Administration (FDA), according to Jennifer Mapes, the Freedonia industry analyst behind the report.
Today's markets for membranes all have one very specific requirement in common ," purity. These markets increasingly are turning to membranes because conventional filtration processes just do not provide the necessary level of purity, says Mapes.
However, Steve Matson, Ph.D., of the Worcester Polytechnic Institute (WPI) in Worcester, Mass., acknowledges that membranes alone are not "particularly good or efficient at achieving very high product stream purities." Instead, they are better suited to effecting bulk separations.
"As a result," he explains, "they can be paired with other types of separations in so-called hybrid' processes where a membrane accomplish[es] the bulk of the separation task and another separation process like adsorption accomplish[es] the last bit of the separation to achieve high product purity."
Although membranes are gaining acceptance in many industrial processing areas, they still have a ways to go in others.
"Membranes are not widely applied today in commodity industrial chemical processes for a variety of reasons," says Dr. Dickson E. Ozokwelu, lead technology manager (chemicals, petroleum and forest products) at the U.S. Department of Energy's Office of Industrial Technologies (OIT). "The foremost reasons are the cost of the membrane systems and the [long] lifetimes of [other] commercially viable technologies."
Membranes have been explored as an alternative to distillation because of their potential to dramatically reduce energy and manufacturing costs. Distillation remains the most widely used separation technology in the chemical industry, says Ozokwelu. However, this process heats the fluids to boiling, consuming a large amount of energy.
And the need for energy-efficient processes has never been greater, especially in petroleum refining. In 1994, the U.S. petroleum industry used 6.3 quadrillion British thermal units (Btu), making it the nation's most energy-intensive industry, according to OIT. Within the refining industry, separation processes ," primarily distillation ," account for nearly 40 percent of energy use.
A current OIT project is focusing on the use of membranes as a replacement for distillation in petroleum refining. Unlike distillation, membrane separation requires no heat. Instead, it relies on differences in the rate at which components pass through the membrane material.
The project, says OIT, will center on pervaporation separation of hydrocarbon mixtures and the use of reverse-selectivity membranes for hydrogen recovery. With pervaporation, the liquid phase contacts one side of the membrane, and the permeate side contacts a reduced-pressure gas phase. Because membranes undergo physical and chemical changes when exposed to organic liquids and elevated temperatures, researchers must find a way to develop pervaporation membranes that can withstand harsh operating conditions.
Two inorganic membranes are being considered for the project ," polymer membranes for the hydrocarbon separations and ceramic membranes for the hydrogen separations. Both types of materials have inherent shortcomings for membrane applications, says Ozokwelu, and a lot of hurdles remain.
"More progress has probably been made with the polymer membrane for hydrocarbon separations than with [the] ceramic membrane for hydrogen separations," he adds. "Hydrogen separations from mixtures using membranes have been a notorious challenge, and industrial investigations have been going on for 60 years. But the return for successful technology would be immense."