I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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GAS VELOCIXY JBCC.,S.%WSEC/CM. FREE PAM
250
-0
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500
E XPOSURE PEROD, SECONDS
EFFECT OF COMPOSITION OF ABSORBING MATERIAL AT 1000° C.
FIQWRE 8.
curves for the fluorapatite and for calcium oxide. The transport resistance within the gas in evidence in the initial stages of the absorption with all four materials persisted throughout the tests with lime and apatite, whereas control by resistance within the liquid manifested itself in the absorption with rock and became predominant in the control of the absorption with matrix. The shape of the curves for rock phosphate and for matrix may be explained by assuming an increase in viscosity of the product resulting from the presence of impurities. Silica is probably the most important impurity in this respect, since experience in preparing highly siliceous metaphosphate melts in the laboratory has shown these compositions to be very viscous.
Conclusions 1. I n the initial stage of the absorption of phosphorus pentoxide by the rock phosphate, the absorption rate is governed by the rate of transfer through the gas surrounding the particles. I n this stage the absorption rate is directly proportional to the P205 concentration in the gas thmughout a range of concentrations corresponding to those which would result from burning elemental phosphorus with two to ninety times the theoretical proportion of air necessary for combustion. As the absorption progresses, the particles of rock become coated with a layer of liquid product. At temperatures below 900” C. and after a few seconds’ exposure to all the gases except those containing the lowest concentrations of PZOs,the rate of transfer through this liquid layer becomes the rate-determining step in the absorption. The absorption rate diminishes rapidly as absorption proceeds, and no proportionality is evident between P205 concentration and absorption rate, although the rate a t a high Pa06 concentration concentration. At temis generally greater than a t a low PZOS peratures of 1000” or 1100” C. also, provided the P206concentration is high, the resistance within the liquid apparently is predominant, but a t lower concentrations of the order of 20 mg. P20sper liter, S. T. P. (corresponding to the concentrations developed by burning phosphorus with twenty to thirty times the theoretical air), no decrease in absorption rate was observed within the duration of the experiments;
Vol. 33, No. 12
and the proportionality between the P2OS concentration and the absorption rate persisted. 2. The rate of absorption was found to be very dependent upon temperature, except a t the instant of initial exposure which is not of practical significance. After the rock is coated with a metaphosphate film, which is the condition of practical interest, it is estimated that the absorption rate a t 1100” C. is a t least f l t y times the rate a t 700” C. To maintain an absorption rate appropriate for full-scale operation, the temperature of the absorption zone of a metaphosphate unit should be not less than 1000” C. At lower temperatures, the absorption rate is greatly retarded as the layer of liquid product becomes thicker; furthermore, the product formed, particularly in the presence of high concentrations of phosphorus pentoxide vapor, is rich in Pz06and consequently hygroscopic. 3. The absorption rate is influenced by the gas velocity to a minor extent when the liquid resistance predominates. Even when the transport resistance within the gas was determinative, the absorption rate was found to vary only with the 0.15-0.25 power of the gas velocity over the range 0.05 to 0.6 linear foot per second, which is believed to be in the range of viscous flow. 4. The presence of moderate concentrations of water vapor has no appreciable influence on the absorption rate at
loooo c.
5. Although the effects of the different impurities in the rock phosphate have not been studied individually, the results show that the impurities, in general, decrease the absorption rate as absorption proceeds.
Acknowledgment The authors wish to express their appreciation t o R. L. Copson and J. W. H. Aldred for advice and criticism, and to other members of the TVA Chemical Engineering Staff for their cooperation during the progress of the work.
Literature Cited (1) Curtis, H. A., Copson, R. L., and Abrams. A. J., Chem. & Met. Eng., 44, 140-2 (1937). (2) Curtis, H. A., Copson, R. L., Abrams, A. J., and Junkins, J. N.. Ibid., 45, 318-22 (1938).
PRESENTED before the Division of Industrial and Engineering Chemistry st the lOlst Meeting of the American Chemical Society, St. Louis, &Io.
Phenolic Resins for Plywood-Correction It has been called to the attention of the author that the following statement in the article “Phenolic Resins for Plywood” [IND. ENG.CHEM.,33,976 (1941)lmight be misinterpreted: “In the latter part of the decade between 1920 and 1930, active development work took place in the field. There seems t o be little question that the products investigated initially were solutions or dispersions of phenolic resins. One company marketed for some time a dispersion of a phenolic resin in water. The process never became a great success chiefly because of the difficulty of controlling spread and adjusting moisture content at the time of gluing.” Reference is made to the fact that the first water dispersions brought out were entirely unsuccessful in their application for the reasons given in the article. However, it is true that very recently, within the last year or two, water dispersions and solutions of phenolic resins have been considerably improved, with the result that at least two companies are marketing successfully products of this general character for use as a plywood adhesive, particularly in the Douglas fir plywood industry. LOUISKLEIN
