I&EC REPORTS & COMMENTS Crystal physics takes a new turn Complexities of polyethytene combustion Higher selectivity in the fluid-fluid reactor
Crystallogenics The physics of most of the phenomena encountered in crystallization is fairly well understood. However, in the majority of situations, there are many variables and parameter interactions to be considered. Recognizing that detailed scientific insight and reliable engineering specifications were needed in this important area of materials preparation helped pioneer the crystallogenics approach. Crystallogenics means, literally, crystal genesis, or the birth and growth of crystals, and was selected as suitably descriptive for highly controlled crystallization. The word has now become synonymous with fundamental crystallization. W. A. Tiller, Stanford University, has amplified the basic idea of crystallogenics and, in a paper presented at the Second Research Applications Conference, sponsored by the USAFOAR, used the definition above as a basis for a plan to develop a "science-based" technology for crystals. Just how such a procedure is unique, other than involving the coining of a new word, viz., crystallogenics, is not apparent to all of us. It is difficult to imagine a technology being other than science-based, and one suspects that a bit of semantic fog, such as that befouling the information business, has crept onto the horizon of crystal physics.
Burning Polyethylene One of the disadvantages of polyethylene is its flammability. Flame retardant additives, such as antimony trioxide and highly chlorinated hydrocarbons, have been used to diminish this propensity in electrical sheathing where flammability is par-
ticularly undesirable. Nevertheless untreated polyethylene can burn, even if the rate is slow; a piece of polyethylene packaging film, e.g., will melt into a small bead and burn somewhat like a candle. The chemistry of the polyethylene combustion process is the subject of a recent study by S. J. Burge and C . F. H. Tipper of the University of Liverpool, England [Chem. Znd. (9), March 4, 19671. A low density polyethylene rod, 1 in. in diameter, was clamped vertically and shielded from drafts. After igniting the upper tip of the rod, a stable flame was obtained by using a high temperature glass mantle over the tip. A molten layer soon formed atop the rod to a depth of about 2 cm., and above the layer was a 1-mm. nonluminous gap. The flame itself was above the nonluminous gap and consisted of a cylindrical blue zone, about 4 mm. thick, topped with a yellow cone, 3 cm, high. This description is strikingly similar to that observed for the 3-stage flame of acetaldehyde in a vertical reactor with excess oxygen. The investigators measured temperature profiles through the flame, using a quartz-sheathed thermocouple, and concentration profiles, using a quartz sampling probe. Gas samples were analyzed with mass spectrometry. Temperature rapidly rose from about 200' C., 1 cm. below the surface of the melt, to a peak of 700' C., 2 cm. above this surface. At a point 0.1 mm. above the liquid surface, the nitrogen content was 75Y0m. and the oxygen concentration was lYom. Consequently, the flame was not a diffusion flame (one in which the rate of combustion is controlled by the rate of supply of reactants rather than by the rate of reaction) and it was deduced that oxygen entering the systemjust above
the liquid surface was consumed very rapidly, giving, as observed, large amounts of oxides of carbon and water. Moreover, since residence time for degradation products (hydrocarbons) in the 1-mm. thick zone above the melt was too short for the observed oxidation to take place there, it was concluded that this oxidation must take place in the liquid at about 400' C. As a result, the combustion process is viewed by Burge and Tipper as follows: Heat conduction from the flame melts the polymer and, as temperature increases, degradation due to alkanes and alkenes occurs. These hydrocarbons are partly oxidized in the liquid surface, the products being burned in the actual flame. In the blue region, reaction is slow-mass spectrometry figures showed that hydrocarbon content falls only slowly along the blue flame-and the hydrocarbons are not fully consumed until the temperature reaches 700" C.-Le., within the yellow cone, where they burn to give much carbon.
Fluid-Fluid Reactors A potential advantage of the fluidfluid reactor over the continuous stirred tank or homogeneous tubular reactor has been predicted from twofilm theory by J. Bridgwater of Cambridge University, England. Bridgwater's development [Chem. Eng. Sci.22,185 (1967)] is concerned with the general reaction scheme A+B+C in a gas-liquid system where both reactions are irreversible and first order. The sequence of events that occurs in such a system is diffusion of component A from the gas into the liquid VOL. 5 9
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where it reacts to form B, followed either by diffusion of B back into the gas or by conversion of B to C in the liquid. Application of the two-film theory enables a general expression for the yield of B (selectivity) to be developed. The most important dimensionless group that arises from this development, a,is determined in practice by the ratio of liquid volume to interfacial area. Bridgwater shows that, in general, where a is small high conversion to B is possible, particularly when both reactions are slow. Consequently, choice of a spray tower as the contacting device (low a ) would result in high selectivity for B. Conversely, if a is large, such as would be the case in a bubble-cap column at low gas rates, high conversion to C is possible. These predictions have implications for liquid phase oxidation of hydrocarbons by absorbed oxygen and other important commercial reactions.
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limiting Yields The Marasperses act to prevent, or minimize flocculation of insoluble particles in water suspensions. The addition of as little as .05% to 3.0% (based on weight of total solids) willa. stabilize aqueous suspensions of insoluble solids, b. change a viscous pasty mass to a free-flowing liquid, or c. decrease the amount of water required to fluidize a slurry Because of the potency of the Marasperses only the state of the mass i s changed. The volume is not appreciably affected. Marasperses are water-soluble, free-flowing powders used in a wide variety of industries. Use the coupon for additional information.
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The fact that many catalytic reactions do not proceed to thermodynamic equilibrium is usually explained by assuming that, as the reaction proceeds, the catalyst surface is gradually blocked off at a rate exceeding the rate of approach to equilibrium. The net result is a yield lower than that predicted from theory. G. D. Sakharov, however, suggests an alternate explanation which he believes more closely aligned with observation [Dokl. Akad. iVauk SSSR, 167 (4), 863 (19 6 6 ) 1. According to Sakharov, it seems probable that the catalyst may be able to influence the reaction only when the concentration of a reactant exceeds some minimal value, Cmin. As the concentration of the reactant diminishes below Cmin, the reaction ceases, exhibiting the limiting yield phenomenon. The alternate explanation is supported by experimental evidence.
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