tice. He presented his results recently to the American College of Allergy, meeting in Bal Harbour, Fla. About one of every 300,000 children given diphtheria-pertussistetanus (DPT) vaccine suffers brain damage, one symptom of which is seizures. About 40 such cases may occur per year in the U.S. A rising tide of liability suits against vaccine makers has raised liability insurance premiums. Two of the three U.S. makers of the vaccine, Connaught Laboratories and Wyeth Laboratories, ceased distribution in 1984,
leading to fears of a vaccine shortage. Lederle Laboratories is now the sole U.S. distributor. The cell surface markers that Garaghty identified belong to a group called human lymphocyte antigens (HLAs). A unique pattern of these antigens on cell surfaces enables the body's immune system to recognize and spare its own cells while attacking foreign antigens. The molecular similarity of some foreign antigens to HLAs is thought to cause the immune system to begin attacking the body's own cells, leading possibly to such diseases as
arthritis, diabetes, and multiple sclerosis. The markers implicated in Garaghty's work are HLA-A2/B18 and HLA-B5. A barrier to use of tests to screen children for these markers is their present $150 cost. Even if they should become as routine as that for HLA-B27 (90% of whose bearers develop the spinal bone disease ankylosing spondylitis), the cost would still be $50. One hope is that the cost may drop still further if such tests are included among those most states require on infants at birth. •
TECHNOLOGY
Gas separators offered for use in dehydration Monsanto is extending the scope of application for its Prism membrane gas separators. Following extensive development work, the separators are now being offered for use in dehydration of air and other streams in the vapor phase. This move quickly follows previous extensions of the technology to petrochemical cleanup, natural gas purification, and generation of carbon dioxide for recycle in enhanced oil recovery operations. The director of Monsanto's separations business group, Kamel Boustany, says that the use of the Prism separators to selectively remove water vapor will form the main thrust of the business in 1985. Most of this effort will be directed toward larger industrial installations, but there is great interest in some smaller units for conditioning instrument air and, possibly, the conditioning of air in computing centers. Boustany is also thinking about eventually building a small unit that could be used in residences as a possible alternative to the conventional electrically driven condensing unit. Prism separators use composite membranes in hollow-fiber form that permit high fluxes with large pressure drops. Through a process of adsorption, "dissolution," and desorption—sometimes termed activated diffusion—permselective membranes exhibit the ability to
transport one component more readily than another. The first Prism units became available in 1977. By the beginning of 1985, 90 units had been built or were on order, and the scope of applications has grown accordingly. Boustany estimates that the value of the typical Prism unit is $600,000. About 60% of the units have been sold overseas, and he expects this trend to continue. The original units were intended for hydrogen recovery in one form or another. A frequent use was, and still is, recovering hydrogen from t h e p u r g e streams in a m m o n i a plants. The hydrogen is recycled. Another application is for adjusting the h y d r o g e n / c a r b o n monoxide composition in synthesis gas streams rather than employing a shift reactor. The appeal of this application in the new wave of Q chemistry has been noted by a number of designers of syngas-based processes. Hydrogen recovery from ammonia purge streams is the biggest single application of Prism separators in terms of the number of units in operation. In terms of the volumes of gas treated, the biggest application probably will be the recovery of carbon dioxide for reinjection in enhanced oil recovery operations. The biggest single unit to date has been designed for this purpose in a Texas oil field and will handle 75 million standard cubic feet per day.
Boustany claims that, in this application, the membrane separator will recover up to 97% of the carbon dioxide at a purity of 95% or more. One of the more exotic applications is separation of nitrogen from oxygen in compressed air. The usual object is to recover 95% or more of the nitrogen for use in fire protection by mass blanketing of combustible materials. Such systems are commonly encountered on offshore oil and gas platforms and during the loading and unloading of hazardous materials from ships and other bulk carriers. Monsanto is projecting that the market for Prism separators will grow at a rate of about 6% per year for the next decade at least. The use of membrane separators in retrofitting old plants may be appealing to energy-conscious industrialists, but Boustany finds that most of the inquiries he is receiving relate to new installations. Most of the applications are large. Monsanto also is concerned about the general ignorance among designers and students of chemical engineering regarding membrane separation techniques. To help dispel that ignorance, Boustany is considering a small, portable demonstration unit that would be made widely available. In addition to increasing consciousness concerning this relatively new unit operation, the portable demonstrator also would help sell more Prism units, a pleasing prospect to Boustany. D February 18, 1985 C&EN
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