The door opens for inorganic membranes

By By C. Kenna Amos,contributing editor

ChemicalProcessing.com

Keywords: Richard D. Noble, membrane developments, zeolites, inorganic membranes, polymeric materials, separation of gases, mixed-matrix, commercialization, chemical processing, chemical engineering and process engineering

Inorganic and hybrid membranes get nearer to commercialization

Zeolites and other inorganic-based thinfilm gas-separation membranes suffer from a perception problem, says Richard D. Noble, a professor in the chemical and biological engineering department of the University of Colorado, Boulder, Colo. The technical community generally believes “this is a research technology that has far too many problems and issues to be commercially successful,” he notes. However, Noble maintains, “Commercialization is closer than people think.”

Other academics agree. Commercialization will come in three to five years, predicts Yi Hua Ma, professor of chemical engineering at Worcester Polytechnic Institute, Worcester, Mass. That timeline also seems accurate to William J. Koros, professor of chemical engineering at the Georgia Institute of Technology’s School of Biochemical and Chemical Engineering, Atlanta.

Given such forecasts, it’s timely to look at membrane developments. Advances extend beyond zeolites to inorganic membranes such as palladium (Pd) and its alloys and mixed-matrix membranes such as zeolite-polymerics.

Potential benefits

One major attraction of the approximately 40 natural and more than 150 synthetic zeolites is their potential use in challenging environments that polymeric materials can’t survive. That means temperatures from approximately 120°C to more than 400°C, “typical reaction temperatures for petrochemicals,” notes Michael Tsapatsis, a professor in the chemical engineering and materials science department at the University of Minnesota, Minneapolis-St. Paul, Minn. “Because many reactions take place at high temperature, this is where most of the development is aiming.” The materials also function at pressures of 10 to 100 atmospheres, he adds.

Another feature of zeolites is their small pore sizes, typically less than 1 nm, and narrow pore-size distribution, adds Noble, who also serves as co-director of the National Science Foundation’s Center for Membrane Applied Science & Technology.

Zeolites’ properties can be tailored to get membranes for separation of different groups of gases and liquids, explains Jerry Y. S. Lin, professor of chemical engineering and director of the Materials for Separations Laboratory at Arizona State University, Tempe, Ariz. He calls zeolites the “hottest, most promising material” he’s examining now. “We need to improve performance and reduce cost. Then, we’ll find more applications,” emphasizes Lin, who works with “almost every inorganic chemical” and draws support from BP, Honda America, NGK and other firms.

 Membrane performance for separation of gases needs improvement in two areas, Lin says. One is temperature. “For example, [for] a gas mixture of CO2 over nitrogen, at low temperatures — between 100°C and room temperature — separation is OK. But at higher temperatures, performance decreases because efficiency is mixture-specific,” he notes. The other area is the number of non-zeolite pores in a membrane, which needs cutting to reduce selectivity interference, he adds.

The cost issue

Cost remains a major hurdle to commercialization. Zeolites/ inorganic membranes generally are “a lot more expensive than polymers,” notes Lin. “I think if you can produce a zeolite membrane at $1,000 per square meter, that’ll be [competitive],” he believes. Cost, of course, depends upon materials; metals such as palladium may be more expensive. “I would expect that within five to 10 years these issues will have been resolved — or at least improved,” he adds.

Developing manufacturing methods to create economical devices having minimal defects is critically important for producing commercially successful large-scale membrane systems, Koros stresses. With mass production, costs will come down, agrees Ma, who directs Worcester Polytechnic’s Center for Inorganic Membranes Studies.

Ma, with support from Shell over the past six or seven years, has been studying palladium alloys for hydrogen separation/production because pure palladium becomes brittle in the process. That occurs because up-and-down temperature swings cause constant expansion/contraction of the metal’s crystalline lattice.

Optimal alloy mixtures exist. It’s approximately 60-40 for Pd-Cu. “With gold, at about 5%-to-20% alloy, you get about 200% more hydrogen permeability than with just palladium,” Ma notes. A 23% silver alloy gives about 70% more hydrogen permeability compared to a pure Pd membrane, he adds.

Figure 1. Hydrogen diffuses through membrane and porous stainless steel support tube to its annular space. Source: Worcester Polytechnic.

Ma’s goal is to make the membranes as thin as possible, in the five-to-10-μ range, to allow more flux across them — and to lower costs. He notes that porous stainless steel serves as the substrate that mechanically supports the membrane and, unlike some other constructs, the membrane is on the outside of hollow tubes, to allow diffusion into the tube’s annular space (Figure 1).

Meanwhile, John Falconer, chair of Colorado’s chemical and biological engineering department, has shown that absorbed molecules can improve the performance of the zeolites such as MFI, whose pore size approximates the size of many organic molecules. “Recently, we’ve found that when zeolites absorb certain molecules in the pores, that makes the zeolite crystals swell.” This shrinks the pore size, dramatically raising selectivity. “We think it’s a big find,” he says. “If you look at branched C6s, a molecule that only goes through the defects — such as 2,2-dimethylbutane [C6H14] — is too large to go through the zeolites at significant rate.” However, by adding 1% to 2% of hexane to the gas, “we see the flux decrease through the defects by one to two orders of magnitude for some membranes.”