TECHNOLOGY
Membrane-Based Compressed Air Dryer A new kind of compressed air dryer, based on use of semipermeable membranes, is being commercialized by Permea Inc., a subsidiary of Monsanto. The combination of a proprietary spinning technique and a unique postspinning treatment of the membrane has yielded a material that permits high water vapor fluxes with small unit size. The dryers, called Prism Cactus, are the result of work by Permea development engineers Arthur W. Rice and M. Keith Murphy. They are an extension of the gas separation technology developed previously for the company's Prism and Prism Alpha membrane systems. A major difference is that the Prism Cactus dryers do not use a coated membrane—they use an uncoated membrane in which the upstream surface has been given the desired characteristics by thermal and /or solvent postspinning treatment. Gas passes through a membrane by a combination of diffusion through pores linking the surfaces of the membrane and permeation through the material of the membrane. Both mechanisms are operative in the Cactus dryers to provide the high water vapor flux and the simultaneous passage of enough "sweep air" to avoid water condensation on the downstream side of the membrane. Nonmembrane dryers require either condensation of the water vapor and subsequent purging of the liquid water or absorption in a dessicant material, which must be periodically recharged. Deliquescent materials also are in use. The high permeation rates in the Cactus dryers are the result of using a polysulfone spun in a hollow fiber membrane to give a high free volume and a graded-density skin. This is achieved by spinning from dopes containing Lewis acids and bases as solvents and media. The pore size of the membrane is then reduced by chemical and physical postspinning treatments to the required size and distribution. There
are also other proprietary steps in the manufacture of the membrane. According to Permea's director of business development and technology, Earl L. Beaver, the Cactus dryers are intended for most conventional compressed air service having upper temperature limits of 150 °Fe and pressure limits of 150 psig. The dryer is made in the "sheet-and-tube" configuration and can be inserted directly into compressed air lines. Wet air enters the interior of the hollow-fiber membranes at one end of the fiber bundle. As the air passes through, water vapor and a controlled amount of air permeate
There are no moving parts or heat loads from condensation and no electrical connections are needed through the membranes to the shell side, where they are vented to the atmosphere. It is this controlled "sweep air" that prevents the permeated water vapor from condensing. The sweep air and water vapor are continuously vented from the shell of the dryer. There is no water condensation and/or accumulation of condensate, or heat effect resulting from the condensation. The membrane doesn't foul from water films. Water vapor capacity of the Cactus units has been increased about 300% by the postspinning treatment and has made possible modules of considerable capacity and small dimensions. The largest module initially offered by Permea has a nominal diameter of 5 inches and a length of 25 inches. Nominal capacity of this unit is 8 standard cubic feet per minute, with a transmembrane pressure drop of less than 2 psig. These data assume that the feed is saturated with water vapor. Cactus dryers normally operate at
dew points down to —10 °F. However, Beaver says that units can be supplied that operate with dew points as low as —100 °F. Beaver expects the major initial interest in Cactus dryers to be in those compressed air applications in plants where corrosion and condensation are problems. Permea expects the dryers also to be in demand in plants and laboratories where dry instrument air is necessary. With the membrane dryers there are no moving parts, no heat loads from condensation, and no electrical connections required. Operating lifetimes are indefinite. The Cactus units can be used in remote and hazardous locations that do not lend themselves to easy or frequent maintenance. Beaver says that, since the dryers are modular, there is no real upper limit to size. For unusually dirty or contaminated air streams, it may be necessary to install prefilters to avoid membrane fouling. Although Permea is interested in providing compressed air dryers for plants and laboratories, the company is also looking to other areas of application. One is drying the exhaust from frost-free refrigerators without the necessity for vaporization heaters, exhaust fans, or condensate lines. Another is protection of produce and other perishable commodities in transit. In the course of premarket tests of the Cactus dryers, one success is regarded by Beaver to be the crucial test. Cactus dryers were used to dehumidify a ship's hold containing perishable produce. Although the voyage was prolonged over 30 days by a propulsion failure, the cargo arrived in good condition with no adverse quality problems. Another use might be in supplying dry air for hazardous locations, where maintenance is difficult to perform, and in confined living spaces where maintaining good air quality is essential to health. Joseph Haggin July 18, 1988 C&EN
35
Technology
New route to batch synthesis of peptides Chemists at Bio-Méga, Laval, Quebec, have developed a solid-phase process capable of 1-kg syntheses of peptides up to 50 amino acids long in 98% purity in a matter of weeks per batch. The company has used the process to make commercial quantities of such naturally occurring hormones as atrial natriuretic factor and growth hormone-releasing factor for human and veterinary drug research. Jean Gauthier, who is group leader for peptide synthesis and who invented the technique, says that 50 amino acids is the maximum number for which it is economical. Above that number, fermentation of genetically engineered microorganisms may be preferred. The Bio-Méga approach is to produce large batches of relatively pure, chemically protected subunits, which are then cleaved from the resin, purified, and reacted to make large batches of pure peptides. In this way, impurities resulting from incomplete or side reactions are minimized. Traditional solid-phase syntheses accumulate such impurities continuously as chemists couple amino acids from first to last without stopping. Products are then difficult to purify. The key to the success of the subunit approach was Gauthier's development of photochemically reactive spacer molecules to attach each first amino acid to the resin. These photoactive compounds allow separation of subunit peptides from resin in high yield. Chemists can thus detach subunits from and reattach them to resin as each synthetic strategy requires without significant loss. One such spacer compound is p-carboxymethyloxy-α- chloropropiophenone. The carboxyl group at one end forms an amide linkage to crosslinked aminopolystyrene. The chlorine at the other end reacts to esterify the carboxyl group of the first amino acid, whose amino group is protected. Gauthier stresses the importance of choosing subunits when plan36
July 18, 1988 C&EN
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ning syntheses. First, the Bio-Méga research workers often join subunits in solution rather than on resins, so they design subunits to be soluble compounds. Also, some sequences of amino acids are more susceptible to racemization than others and must be avoided in subunits. And chemists may want large amounts of subunit compounds that contain all but one amino acid of a peptide. Then they can assemble the subunits, inserting a different amino acid in the same position each time to make a variety of analogs. One example of strategic subunit design was the Bio-Méga synthesis of human atrial natriuretic factor(99-126). This hormone, secreted by the atrium of the heart, stimulates
sodium excretion and urine formation, relaxes peripheral blood vessel walls, and lowers aldosterone and renin concentrations. It thus shows promise as a drug against hypertension. Its peptide chain consists of amino acids 99 through 126, excised from a prohormone that is 126 amino acids long. A disulfide bond between two cysteine molecules closes the chain into a ring. The subunits made for the synthesis consisted not only of amino acid sequences 99 to 107, 108 to 114, 115 to 124, and 125 to 126, but also 121 to 126. By joining these in certain orders, the Bio-Méga chemists could make the hormone itself as well as analogs. Stephen Stinson
Kaolin filler improves nylon qualities A surface-treated kaolin filler for molding nylons that increases impact strength, tensile strength, and flexural modulus has been developed by Engelhard Corp. The company introduced the product late last month at the National Plastics Exposition in Chicago. Plasticizing modifiers that increase impact strength often cause losses in tensile strength and flexural modulus, and high loadings of fillers like talc and mica that increase structural strength often lead to embrittlement. The new reinforcement product will let compounders use up to 40% loadings of kaolin (which is cheaper than glass fiber) in conventional blending equipment to produce composites for metal-like automotive parts such as wheel covers, headlamp housings, and under-the-hood fan shrouds; electrical parts such as connectors; and appliance parts such as electric tool housings. Engelhard spokesmen estimate the worldwide market for nylon composites at 200 million lb per year.
Nylon 66 containing 40% by weight of the new filler has a Gardner impact strength of 68 inchlb, a tensile strength of 13,800 lb per sq in, and a flexural modulus of 800,000 lb per sq in. Engelhard spokesmen decline to reveal the nature of the kaolin modification, but it may consist of treatment first with about 0.2% by weight of triethanolamine, followed by 1% by weight of 7-aminopropyltriethoxysilane, and subsequent drying at 140 °C. Kaolin is a clay that occurs as layers of silica interleaved with layers of a hydrated alumina called gibbsite. Even after customary calcination at 1000 °C, edges and faces of the hexagonal plates bear hydroxyl groups to react with the silane to give resin-compatibilizing 7-aminopropyl groups on surfaces. Triethanolamine is a commonly used plasticizer in the manufacture of nylons. Engelhard has similar reinforcements in development for other resins. Stephen Stinson
