REPORTS AND COMMENTS
KNOW YOUR PPM
Many instruments on the market measure the concentration of a vapor or gas in another gas. In these days of increasing intolerance of air pollution, monitors checking on the amount of sulfur dioxide and other noxious chemicals in the air have become common. For these instruments to be accurate, they naturally have to be calibrated using air which
greater these problem become, making production of large volumes of known-concentration gas impractical. Static methods, such as the batch method, are also inconvenient when, as is usually the case, several different concentrations are needed one after the other to calibrate an instrument-a different batch must be made up for each concentration. To avoid the problem inherent in static methods, it is necessary to adopt a dynamic, continuous production method. One that is both ac-
contains precisely known (but usually small) amounts of contaminants. Several problem have usually arisen when a "batch" of gas containing a known concentration of vaporized liquid (or another gas) has been prepared. Adsorption of the contaminant on the walls of the containing vessel or transfer lines results in a concentration that is lower than that computed on the basis of the amount of contaminant added to the gas. And the larger the vessel, the
curate and convenient has been developed by G. 0. Nelson and K. S. Griggs at the University of California's Lawrence Radiation Laboratory, and is described in Rev. Sci. Instr., 39, 927 (1968). This technique utilizes simple apparatus (see sketch) and can be used to produce concentrations in air (or another gas) in the range 2 to 2000 ppm for liquids and 0.05 to 2000 ppm for gases. Air or another gas is introduced into the apparatus via silica gel, soda
Calibration techique prouides flow of gas cmtaining known cmentration of vaporized liquid or second gas
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INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
lime, and charcoal filters and its flow rate is measured by one of a series of rotameters. If the contaminant to be introduced is a liquid, a 200-watt heater is used to bring up the air temperature so that vaporization of the liquid is enhanced. When a gas is to be dispersed in the air the heater is, of course, not needed. The liquid or gas is introduced into the airstream through a Teflon needle leading from a graduated syringe. The plunger of the syringe is advanced by means of a lead screw, which is in turn driven by a small synchronous motor through a variable ratio gear box with microaial adjustment. Use of syringes of different sizes and variations of the plunger advance rate enable the wide variety of contaminant concentrations to be achieved. When liquids are used, the warm air-vaporized liquid mixture is cooled before being transported to the instrument to be calibrated. The cooler chamber also acts as a mixer, to ensure a homogeneous dispersion. As a liquid is being introduced, a drop about two diameters wide forms on the end of the Teflon needle. When a new concentration is needed, only 20 sec for liquids (6 sec for gases) are takenforequilibrium conditions to be reached after the new plunger speed is set. Wall adsorption also reaches a new equilibrium, thus guaranteeing accurate concentration. Accuracy is claimed to be *l%, even at very low concentrations. If your problem are concerned not so much with putting a liquid into a gas in small amounts as with completely removing small amounts of gas from a liquid, you may be interested in reading about the degassing unit described by G. w. Preckshot in Int. J. Heaf Mass Tranrfcr, 11,1081 (1968).
THE CHALLENGE OF THE OCEAN The exploration and exploitation of the ocean will depend on development of economic technology The greatest challenge for the engineer is the development of technology that is econornically feasible. Unlike the race into space, the race into the ocean is based upon practicability in the ever-deepening probe for dollar-convertible fruits of the sea. During the space era, the engineer has been allowed to exercise his training in applying the integral sign to the development of technology. (This is part of the philosophy that an engineer must apply the integral sign and the dollar sign to the solution of his problems.) The development of aerospace technology received popular public support and, therefore, adequate federal financial support. The era of ocean technology is taking the route of economics, and any exploitation or development must either now or in the foreseeable future show a return on the investment. A symposium, “hlineral Resources of the W’orld Ocean,” at the U. S. Kava1 \Var College, Newport, R. I., brought together representatives of all those areas which must be brought to bear on the problem of undersea exploration : the scientific, the governmental, the business, the military, and the academic. These people addressed themselves to the economics of development of the ocean resources and offered well thought out insight and solutions, sometimes based on tedious research gleaned while spending months at sea. At times, the speakers reflected pessimism, but never was the idea of further exploiting the vast natural storehouses beneath the sea termed a n impossibility. Among those problems discussed were the shortcomings of current technology in undersea exploration, avoidance of legal or < < ocean”
Compact arc lamps, 30 to 30,000 watts.. . has it! sk us for 30 watts all AXenon-Mercury the way to Xenon, or Mercury. Lamps anything: u p 30,000.
for solar simulation, lasers, instrumentation,photochemistry or communications. Hanovia has it! You might want to operate at DC, AC, pulsed, simmer-flash or modulated modes. Ask us. Every lamp features high intensity, high brightness, full spectrum, long life, complete reliability, rapid start and no maintenance. Most have one universal starter. If you have trouble with compact arc lamps and their associated equipment, call Hanovia for help. That name again: HANOVIA. Made in America, too, which is a comforting thought. Write or call today for complete technical information,or special assistance on your own problems.
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
international conflicts resulting from boundary infractions, and friction stemming from inroads by undersea interests into the economic domains of land-mining groups. Dr. Vincent E. McKelvey, a senior economic geologist with the U. S. Geological Survey, added that there is a dire need for low cost marine mining systems and that successful exploitation of the ocean depths would not come early. H e said, (‘Mineral discovery and exploration have always been slow and difficult processes, requiring both imagination and sweat in prospecting, and faith, courage, and a willingness to gamble in exploration and development.” Dr. Thomas D. Barrow, Humble Oil & Refining Co., discussed the investment the oil companies have made in offshore ventures for oil, the gambles they must take, and the return they expect from their investment. Sen. Claiborne Pel1 (R. I.) offered his suggestions for the steps to be taken to solve the many variables that still separate us from the practical extraction of the sea’s mineral and other resources. One of these suggestions was that increasing use be made of the ingenious modern route to scientific and technologic advances known as the ((systems approach.” Although the phraseology may be fresh, the concept has its roots deeply implanted in science and engineering. I n probing and exploiting ocean resources, the systems approach is necessary to deal with the different parameters of ocean phenomena and their interaction. These parameters differ radically from those related to operations on land. Their nature and complexity in the sea environment require a systems approach to integrate all related activities within one entity. The effectiveness of a systems approach depends, of course, on such factors as cost, time, engineering design
REPORTS
and application, and information control and data processing. Every minute factor within this system has its effect in the final plan of how to use ocean resources economically.
SCRUBBING ICE CLEAN Pressure Juctuation technique helps keep impurities out of ice crystals as they are forming in dilute brine As the demand for pure water increases all over the world, so does interest in all methods of desalination and demineralization. These methods can be roughly divided into those that involve phase change (such as flash distillation and vacuum freezing) and those that do not (such as electrodialysis and reverse osmosis). Although the methods that do not involve change of phase, especially reverse osmosis, are most promising, their potential advantages-simplicity and low energy requirements-have never been fully realized in practice. I n fact, distillation and evaporation methods are the most widespread in use, and freezing is also common for small-scale application to low mineral content water. The freezing method depends on the tendency of water to exclude all foreign materials as it freezes to form ice, provided that conditions are “just right” for ice crystal growth. I n this case, “just right” means that the concentration of mineral in the water surrounding the crystal has to be small or trapping of impurities in the ice is unavoidable. This state of affairs makes freezing an impractical way to purify seawater and prompted a study of the way in which ice forms in a brine solution. The study was commissioned by the Office of Saline Water, U. S. Department of the Interior, and was carried out by R. F. Sekerka and R. A. Seidensticker and their associates in the Pittsburgh laboratories of Westinghouse.
The Westinghouse researchers found that, in the initial freezing stage, pure ice forms easily in a chilled, dilute brine solution, but that after a time, the impurities being pushed ahead of the advancing ice front become so concentrated that they disrupt the orderly process and start to form inclusions in the ice. Attempts to sweep away these impurities by agitating the brine were unsuccessful, because a stagnant layer of concentrated impurities tends to cling to the ice. However, it was eventually discovered that a decrease in pressure over the brine could cause dissolved air, inevitably present in the brine, to degas rapidly from the ice/brine interface, carrying impurities before it. T o impose a continuous vacuum on the brine,
would, of course, cause all the dissolved air to be freed in a very short time, so periodic “bursts” of vacuum were used to promote this “bubblescrubbing” action, interspersed by longer periods during which the pressure was atmospheric. I n this way, it proved possible to produce pure ice for an unlimited length of time, but even so, the rate of ice formation had to be kept down to about 1/2 in. per hr. Bubble scrubbing is ineffective when the salt concentration is high, as it is in seawater; the ice/brine interface cannot be maintained planar, and impurities are trapped in the ice crystal. Though this technique is obviously impractical for large-scale desalination, it does give valuable insight into the way in which water molecules are
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M o d e l 8170-EW
VOL. 6 0
NO.
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SEPTEMBER 1968
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REPORTS
Get a load of these. CH(OCH3)3 methyl orthoforma-3
CH (0CzH5)3 ethyl orthoformate
CH (0CioH21)3 decyl orthoformate
C&C( OC2H5)3 ethyl or thoace ta te
CH3CH2CH (OC2H5)3 ethy1 or thopropionate Think a bit about ortho esters and their reaction possibilities: these esters form a n ethoxymethylene group in condensation reactions; they form ketals and acetals; and they react by addition to a double bond. O u r ortho esters are used now as synthesis intermediates in the production of many important products; look promising in certain plastics and dyestuff applications; and are used profitably in water-scavenging operations.. Could be they can give you a better reaction route. For more information and/or our technical data bulletin on ortho esters or any of our chemicals write to us-on your company letterhead, please: Kay-Fries Chemicals, Inc., 360 Lexington Avenue, New York, New York 10017. A
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
able or unable to reject foreign molecules as they join together to form ice. An interesting observation made during the experiments was that under certain conditions, when an inclusion of liquid brine in the ice occurred, a naturally occurring temperature gradient forced the inclusion back out of the crystal. Readers interested in this phenomenon are referred to w. R. Wilcox’s review of inclusion removal by gradient techniques, published in the March 1968 issue of I&EC, p 12.
LIQUID EXTRACTION DILEMMA Should solvent or feed be the dispersed phase in extraction operation involving rotating-disk contactor? Whether solvent or feed should be made the continuous phase (the other being the dispersed phase) is an important question in operation of an extractor, and one which is usually taken care of by making the stream having the larger volumetric flow rate the dispersed phase. This customary decision is based on the fact that, for a given droplet size, this produces the largest interfacial area for mass transfer (see p 463 of “Liquid Extraction” by R. E. Treybal, McGraw-Hill &., 2nd ed., 1963). A recent article out of Czechoslovakia by T. Misek [Intern. Chem. Eng., 8, 439 (1968)] goes into this matter of choice in rather more detail. According to Misek, the choice of liquid €or the dispersed phase is dependent on whether the value of the distribution coefficient (ratio of solute concentration in the solvent to that in the feed) is greater or less than one, on the severity of operation-for which Misek gives a precise criterion-and on the incidence of axial mixing in the contactor. An interesting result of this analysis is that it may predict that the liquid with the lower flow rate should be the dispersed phase-a conclusion that flies in the face of tradition.
