ways, says Hagemeyer. First, a mill's alkaline white water can be recircu lated more often. Scale and corrosion problems, which in a rosin-alum sys tem result from the buildup of alu mina and sulfate ions, are sharply reduced. Thus the need for makeup water is cut in half, he points out, with a cor responding savings in the energy needed to heat the water to process temperatures. Effluent volume also is reduced, and what does come out has the optimum pH for clarification and treatment. Second, says Hagemeyer, a fiber web formed under alkaline conditions is 20 to 40% stronger than acid paper. Thus, without degrading the final products manufacturers can reduce the content of expensive fiber and add more of a much cheaper filler, used to give paper its whiteness and gloss. Furthermore, with alkaline papers papermakers can use calcium carbonate filler, which itself is cheaper than materials, like titanium dioxide, that are compatible with acid systems. Calcium carbonate has the added advantage of buffering the paper against acid city air. Of course, all these economies take hold only after a plant is converted, says Hagemeyer. The conversion it self is hectic. The crews must be re trained. Sludge left in the equipment from the acid process causes spotting on the paper. "Any mill that is back to full efficiency in less than a year is doing very well," Hagemeyer says. Still, several large companies have converted and are happy with the results. Π
Dual-membrane cell analyzes gas mixtures A new technique based on the prin ciple of counterdiffusion through an ordered pair of membranes provides a simple, reproducible, and accurate method of analyzing gas mixtures, according to its developers. The technique was devised by Bang Mo Kim of General Electric R&D Center, David J. Graves of Air Products & Chemicals, and John A. Quinn of the University of Pennsylvania. Kim de scribed it at the American Chemical Society meeting in Washington, D.C. Selectively permeable membranes have been used in various ways to analyze gases, Kim notes. Usually their function has been to separate gases from particulates or from a liq uid stream; thereafter, the gases have been analyzed by ordinary instru mental or chemical methods. Another technique makes use of the relative
permeation rates of gases through a membrane covering a constant vol ume chamber containing a reference gas. That technique hasn't been widely employed, Kim says, because it involves a nonsteady-state mea surement—rate of change of pres sure—and because the reference compartment has to be renewed for each test. The approach taken by Kim and his associates also makes use of pres sure changes caused by unequal per meation rates of gases through membranes. But in contrast to earlier methods, Kim says, their method measures pressure increase itself (a steady-state phenomenon), rather than rate of increase. The analyzer works something like this: A "test" membrane is in contact with the gas mixture to be analyzed; a reference membrane is in contact with a reference gas. The space be tween the membranes is isolated from the environment. If both gases are at the same "ambient" pressure, and if the permeability ratios for the two membranes are different, and if each gas first contacts the membrane through which it is more permeable, then the sum of the partial pressures in the intermembrane space will be
greater than the "ambient" pressure. This pressure increase is measured with an appropriate transducer and, with appropriate calculations, trans lated to a concentration measure ment. The team has built a prototype device employing a membrane pair consisting of a silicone polycarbonate copolymer and a commercial microporous membrane. With it, Kim says, the group has successfully analyzed mixtures such as nitrogen/oxygen, carbon dioxide/oxygen, nitrous oxide/oxygen, and helium/oxygen with an accuracy of ±1%, full-scale. With modifications of technique, one component of pseudobinary mixtures also could be measured. Compared to a thermal conduc tivity analyzer, Kim says, the dualmembrane analyzer offers the ad vantages of low cost, good linearity, simplicity and ruggedness, and no hot wire (with its attendant explosion hazard and corrosion problems). Its disadvantages include sensitivity to solvent vapors that might dissolve the membranes, a somewhat slower re sponse time (on the order of seconds), and an accuracy no better than that of the pressure transducer to which it is connected. D
Photoacoustics monitors auto soot emissions A photoacoustic technique is proving to be helpful for measuring soot emissions in auto engine exhausts. And Ford Motor Co. scientists have found that use of the technique can be broadened to include monitoring the effect of minor engine adjust ments while the engine is operating. Ford senior research scientist Steven Japar and his coworkers de scribed their work with the technique at the American Chemical Society meeting in Washington, D.C. General Motors scientists have been working with the method, too (C&EN, Aug. 13, page 18). The fast response time of the pho toacoustic technique makes it possi ble for the first time to analyze mass emissions as a function of the mode of engine operation, Japar says. Filtercollection techniques would be highly impractical. "The technique allows work that couldn't be done before," Japar adds. "It allows people to tinker and get instant readout." The Ford scientists had earlier determined that the technique could be used to measure particulates con tinuously over a driving cycle. And the results are immediately available at the end of the test, eliminating the delay that is inherent in the filtercollection method. Thus, Japar says, research programs on vehicle mass
emissions can be carried out more rapidly and more effectively. The technique makes use of the photoacoustic effect, discovered by Alexander Graham Bell but not of practical use until the development of lasers. If a light beam is directed into a cell containing particles, the parti cles absorb the light, heating up in the process. If the beam is rapidly mod ulated, the particles rapidly heat and cool, heating and cooling the sur rounding air. The resulting increases and decreases of pressure are picked up by a sensitive microphone and its associated electronics. In their work with dynamometer tests, the Ford scientists used a Mercedes diesel and a gasoline-pow ered Cougar Proco. They ran the cars through a portion of the federal test procedure that outlines a standard profile of acceleration, deceleration, idling, and the like. Exhausts were monitored with the photoacoustic device. Integration of the photoacoustic signals over the 8.5-minute test indi cates, Japar says, that the Mercedes produced about 48 times more soot than did the Cougar. However, the two curves are very similar, with the peaks correlating quite well with the acceleration modes in the test proce dure. D Oct. 1, 1979 C&EN
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