ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT At
V p p p (p
=
temperature difference,
O
F.
= av. velocity over one cycle, ft./sec.
thermal coefficient of volumetric expansion, dynamic viscosity, lb./ft. sec. = density, lb./cu. f t . = function = =
O
F.-1
Literature Cited (1) Andreas, A., Ger. Patent 717,766 (Feb. 5, 1942). (2) Boelter, L. &I. K., personal communication, 1953. (3) Bosworth, R. C. L., “Heat Transfer Phenomena,” p. 101, Wiley, New York, 1952. (4) Ibid., p. 104. (5) Colburn, A. P., and Hougen, 0. A,, IXD. ENG.CHEM.,22, 522 (1930). (6) Fa;ber,‘E. A., and Scorah, R. L., Trans. Am. SOC.Mech. Engrs., 70, 369 (1948). (7) Lemlich, R., dissertation for Ph.D., University of Cincinnati, June 1954.
ULTRASONIC CHEMICAL EFFECTS VIRGINIA GRlFFlNG The Cafholic Universify o f America, Washington, D. C .
U
LTRASONIC chemical effects have been studied to determine why any chemical reactions occur when a system is irradiated with a high intensity ultrasonic beam rather than to concentrate on which reactions go and by what chemical kinetics. The experimental fact exists that no observable chemical reactions take place unless there is a permanent gas dissolved in the solution and the sound intensity is sufficiently high to make the liquid cavitate. Thus it is assumed that a gas dissolved in the solution will form minute bubbles in the sound field. The liquid serves as an efficient transducer which carries the energy of the-high intensity sound wave from the source to the interfaces between the small bubbles and the liquid. I n the adiabatic compression of a sound wave, the production of a high temperature is dependent on the existence of a large difference between the adiabatic and isothermal compressibility. As the sound wave of 5 to 10 watts per sq. cm. travels through the liquid, the temperature difference due to the adiabatic compression is of the order of a few degrees centigrade while a t these intensities a sniall bubble of gas may be compressed to half its volume, producing a temperature variation of several hundred degrees. A temperature gradient in the gas bubble is then set up, which produces periodic temperature variation in the liquid adjoining the gas bubble. If one assumes the model there are two possibilities for explaining the chemical reactions to be effected. The chemical reactions could be gas phase thermal reactions taking place in the gas bubble or the chemical reactions might take place a t the liquidgas interface, but still be due to the high gas temperature in the bubbles. These same thermal processes are responsible for the loss of acoustic energy. According to this idea, two thermal properties of the gas content of the bubble are important. First, y will determine the temperature reached in the compressed bubble. As the y of the gas approaches one the temperature inside the bubble becomes much lower. This explains why vapors such as ether inhibit these
1180
(8) Mcddams, W. H., “Heat Transmimion,” 2nd ed., p. 242. McGraw-Hill, Kew York, 1942. (9) Ibid., p. 243. (10) Martinelli, R. C., M.S.thesis in mechanical engineering, University of California, 1938. (11) Slartinelli, R. C., and Boelter, L. M. K., PTOC. 6th I n h . Congr. A p p l . Mech., 578 (1938). (12) Maschinenfabrik Oerliken, Brit. Patent 532,144 (Jan, 17, 1941). (13) Mason, W. E., personal communication, 1954. (14) Ormell, E. A. I., Ger. Patent 736,883 (May 20, 1943). (15) Robinson, R. S., U. S. Patent 2,514,797(July 11, 1950). (16) Schrey, A., French Patent 806,030 (Dec. 5, 1936). (17) West, F. B., and Taylor, A. T., Chem.. Eng. Progr., 48,39 (1952). (18) Worn, G. A , , and Rubin, F. L., U. S. Patent 2,664,274(December 29, 1953). RECEIVED for review December 20, 1954. ACCBPTEDApril 1 1 , 1955. From a dissertation presented b y R. Lemlioh, in partial fulfillment of the requirements for the doctor of philosophy degree at the University of Cincinnati, June 1954.
reactions even if the solution is saturated with air. Thus one would expect the rate of a chemical reaction to increase as the y of the dissolved gas increases whether the reaction occurs in the gas bubbles or in the liquid phase. Secondly, the thermal conductivity of the gas will determine how long the high temperature is maintained inside the bubble. If the reaction takes place inside the bubble the gas with the lower thermal conductivity will give a higher chemical yield than one with high conductivity. On the other hand, for any thermal reaction in the liquid phase, the thermal conductivity of the gas should have the opposite effect. A series of experiments was conducted to answer these questions. The chemical effects directly produced by ultrasonics are thermal gas phase reactions; they take place inside the gas bubbles. In water solutions, the reaction is the production of radicals by the thermal decomposition of water-probably OH radicals .4 similar reaction has not been obtained when water is not present even though a wide variety of organic substances has been tried. If another reactive gas or vapor is present in the bubble, the OH radical attacks the vapor forming other products within the bubble-e.g., carbon tetrachloride. These products then diffuse in the solution; various secondary reactions between these products and the species dissolved in the solution have been observed. Another secondary effect of the chemical reactions is luminescence ( 1 ) ; the intensity of luminescence shows the same general behavior as the chemical yields. The degradation of polymers, although due to cavitation, is not due to “hot spots” developed in and around the gas bubbles It is postulated that polymer degradation is caused by the same type of force that is so effective in mechanical dispersion and scrubbing action a t a low frequency of the order of 20 kc. Although the experiments have conclusively proved that the chemical reactions take place in the gas phase, it is impossible to rule out completely that the mechanism may be gaseous discharge inside the bubble rather than temperature Presentlv the data are being re-examined and new euperiments undertaken to settle this question.
literature Cited (1) Griffing, V., and Sette, D., J . Chem. P h y s . , 23, 503 (1955).
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 6
