I&EC R E P O R T S DROP COALESCENCE The perpetually popular subject of drop coalescence has been investigated with variable success. Less popular but probably more useful are critical looks at the productivity of research programs, not in terms of financial return but in terms of useful results. G. B. Lawson [Chem. Process Eng. 48 ( 5 ) , 45 (1967)] has surveyed the complex field of research on drop coalescence and, not surprisingly, has concluded that there are gaps which require filling. This is particularly true of the “many drop s ystem .’’ The capacity of all forms of liquid extraction equipment is limited by the behavior of droplets. I n rotating disk contactors, for example, the droplets produced at the disks must retain their identity as they move through the equipment or a reduction of surface area of the droplets will decrease the mass transfer rates. However, at the interface, in the end of a typical column, rapid coalescence is desirable to minimize residence time. Similar problems exist in spray columns, pulse columns, mixer-settler units, etc. The inherent complexity of the problem has led to two simplified approaches in evaluating drop coalescence research. I n the first, the technique is to focus attention on the behavior of single drops at a twofluid interface; in the second, a more pragmatic and descriptive study of the operational performance of types of equipment. I n the simplest terms, the first approach may be called the mechanistic approach and the second the empirical approach. T h e mechanistic approach yields many numbers and great masses of mathematical expressions representing models. I t doesn’t seem to have yielded too much bona fide information. The empirical approach yields many data representing the performance of equipment. Because of the subtle, and usually unknown, differences between pilots and 12
actual operational units, it is virtually impossible to transpose information froin one unit to another. This is the traditional penalty of empiricism. I n summary, problems of coalescence are just about those of any other technical discipline-viz., there is a lot of research into the mechanisms with little coordination and even less useful result. The empiricists provide the most immediately useful information but it has little long term utility-at least with respect to providing designers and operators with performance criteria of yet to be built units. I t seems we have seen this all somewhere before.
ROCKETS FROM WASTE Like most other areas of the space business, propellant formulation and manufacture are reacting strongly to the cost consciousness of the Congress. I n speculating about the future development of rocket motors for the 62nd National Meeting of the A.I.Ch.E., J. C. Barr, of Lockheed Propulsion Co., noted that there are now no manufacturing facilities in the United States for the continuous production of solid propellants. Some plants employ semicontinuous or batch/continuous-cast methods but none have, apparently, even any plans for a truly continuous facility for packaged rocket motors. One of the more intriguing items in Barr’s discussion was the utilization of waste as a rocket fuel. By combining paper, human waste, food waste, powdered metal, and an oxidizer, it is possible to produce an auxiliary fuel. I t doesn’t provide high performance but it does get rid of the waste in a useful manner and adds to the overall weight/propulsion ratio of a motor. Two reactions to these ideas are incredulity at the state of the art of propellant production and wonder at the entremes that are apparently necessary in seeking efficiency of rocket performance. If it is possible to manufacture propellants with the
I N D U S T R t A L A N D ENGINEERING C H E M I S T R Y
same general approach one would apply to any other chemical product, then a liberal dose of chemical engineering seems in order for the rocket business. The economics of rocket production are now more strict than ever before, probably because of Congressional scrutiny and because the novelty of the business has worn off. O n the other hand, the nature of the product seems to make it desirable to produce in more isolated surroundings than one would expect in a paint factory. O n the other hand, conventional economics don’t appear applicable to the rocket business. Except for the fact that we all have to pay the bill, the only legitimate criterion for success is the approach to the limiting efficiency. This is truly a business of extremes where engineering is master.
CORRECTION T h e captions for the figures of the article beginning on page 106 of the May issue for 1967 (Vol. 59) should read as as follows: Figure 2. Fadeometer studies Figure 3. Comparison of esterification time with acid numbers for various formulations Figure 4. Comparison of burning rates of bromine-and chlorine-containing resins Figure 6. T h e effect of phosphorus on burning rate Figure 7. T h e effect of antimony oxide on the flammability of tetrabromophthalate resins Figure 8. Tunnel test results Figure 9. A S T M D-635-63 Figure 10. ASTAMD-757 apparatus Figure 11. A S T M D-757 burning rate us. Br content Figure 12. HLT-15 apparatus Figure 13. Method 2023.2 (LP-406b) test apparatus Figure 14. ASTM E-162-62T
I n addition the top curve in Figure 2, page 106 should be labeled B and the lower curve should be labeled A .
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