hlal-, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
and (6) a decreased production of gaseous sulfur compounds, the last possibly because of an increased secondary reaction with the iron present. An experiment using sodium sulfide showed that this gave good catalytic activity under the conditions studied. These results are definitely encouraging and suggest a number of interesting phases for further study in relation to water-gas manufacture. It must be noted, however, that for routine commercial work in both producer-gas and water-gas making, sodium compounds possess a disadvantage because of the marked volatility of this metal a t the reaction temperatures. Another resides in the marked slagging effect of the alkaline compounds upon the fire-brick refractories ordinarily employed. While the latter may perhaps be overcome by the proper choice of an alkaline refractory, the loss of sodium due to the first suggests that the industry's attention might well be directed, in the present state of our knowledge, a t least, to such matters as the presence of naturally occurring alkali bases in oils and coals.
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LITERATURE CITED (1) Branson and Cobb, Gas J., 178, 901-5 (1927). (2) Cobb and Sutcliffe, Ibid., 178, 895-901 (1927). (3) Dashiell, Am. Gas dssoc. Proc., 1930, 886-97. (4) Fox and White, IND.EKG.CHEM.,23, 259 (1931). (5) Holtz, Dissertation, Johns Hopkins University, 1930; Huff and Holtz, IND. ENG.CHEII.,19, 1268 (1927); 22, 639 (1930). (6) Howard, Am. Gas Assoc. Monthly, 7, 579 (1925). (7) Klein, Ibid.,5, 183 (1923). (8) Marson and Cobb, Gas J., 175, 882-91 (1926). (9) Neuman, Kroger, and Fingas, Z . anorg. allgem. Chem., 197, 321-8 (1931). (10) Stewart, Am. Gas Assoc. Proc., 1924, 813; 1925, 1222. (11) Sutcliffe and Cobb, Gas J., 182, 946 (1928). (12) Taylor and Neville, J . Am. Chem. Soc., 43,2055-71 (1921). (13) Tessie du Motay and Marechal, British Patent 2548 (1867). (14) Watson and Ceaglske, IND. ENQ.CHEM.,Anal. Ed., 4, 70 (1932). RECEIVED November 8, 1933. Presented before the Division of Gas and Fuel Chemistry at the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 t o 31, 1933. The experimental material of this paper is abstracted from a dissertation presented to the Board of University Studies of the Johns Hopkins University by AI. A. Elliott in partial fulfilment of the requirements for the degree of doctor of philosophy June, 1933.
Entrainment in a Forced-Circulation Evaporator OREN C. CESSKA'. ~ N DWALTERL. BADGER, University of Michigan, Ann ilrbor, Mich.
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thrown obliquely is similar to HE purpose of this reThe entrainment in a laboratory forced-circuthe present problem; although search was to study the lation evaporator, containing thirty tubes, 0.5 he was mainly interested in the entrainment of liquor by inch 0.d. and 8 feet long, is determined by boiling point a t w h i c h e n t r a i n m e n t the vapor in an evaporator of the a salt solution and testing f o r sodium chloride in begins rather than the amount forced-circulation type, and to the condensed vapors. of entrainment, his treatment determine the relation between of the problem might easily be the quantity of liquor carried by An attempt is made to analyze the problem modified to apply to this case. the vapor and the conditions mathematically. The treatment, though incomHausbrand found the followmaintained within the evaporaplete and very approximate, indicates a type of ing relationship. : tor. function that should be a measure of the entrainThe small amount of work that ment. The results approximately confirm these has been done on the subject of entrainment has all pertained to deductions, although almost as good correlation D steam boilers. A large part of will be obtained by considering that entrainment G - I = this work has been done by Foulk is proportional to mass velocity. who has also made a rather comsin a - cos a tan gt plete summary of what has been done by others ( 2 ) . Vorkauf (4) studied the subject compre- where D = pressure exerted upon the drop by the vapor Q = projected plane surface of the drop hensively using small-scale laboratory apparatus designed t o Y e = density of the vapor give conditions similar to those in a steam boiler. u = relative velocity between vapor and drop The work referred to is of interest to this problem only inas! 3 = acceleration of gravity i c . = a numerical coefficient much as it gives some insight into the mechanism of entrainc = initial velocity of drop ment and advances some theories upon the formation and a = angle between path of the drop and the horizontal behavior of the droplets. The results obtained, however, t = time the drop is in vapor current before hitting the wall are not directly applicable because the forced-circulation G = diameter of drop il evaporator is quite different, both in design and in operation. More directly connected with evaporators is the study made by Hausbrand (5)in 1899. The discussion was entirely These equations when combined yield a relationship similar from a theoretical standpoint and mas not supplemented by to that derived below. any direct experimental data. It deals with the effect of currents of vapor upon droplets of liquor under various cirDISCUSSION OF EXTRAINMENT cumstances and gives equations expressing the relationship for each case. Entrainment is entirely a mechanical process. d small Of the various cases that Hausbrand considered, one in portion of the liquor in the body of the evaporator is carried which a vertical current of steam meets a drop of liquid upward in the form of droplets by the vapor, and passes into the condenser, resulting in a loss of the material con* Present address, Kelvinator Corporation, Detroit, Mich.
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I N D U STR I A L A N D E N G IN E E R I N G CH E M I STR Y
FIGURE 1. EXPERIMENT.4L EVAPORATOR
tained within the machine and the contamination of the overhead condensate. The process is, in general, made up of two parts-the formation of the droplets and the entraining of the droplets by the vapor. The manner in which these two steps take place depends to a great degree upon the type and design of the evaporator. The formation of droplets in a forced-circulation evaporator is different from that in a natural-circulation evaporator or a steam boiler. As a basis upon which to develop a theory for the process, consider the general principle upon which the forced-circulation type operates: I n Figure 1 the main portion of the liquor being evaporated is contained in the body, and surrounds but does not submerge the heating element, m. The circulating pump, p , draws the liquor from the body through the downtake, n, and forces it through the control valve, the distributing box, 0 , and the tubes, k. Emerging from the tubes a t a rather high velocity, the liquid strikes the deflector, j, which changes its direction and throws it obliquely downward against the wall in a sheet, called the “curtain.” Steam is supplied to the heating element and surrounds the tubes. Xearly all the vapor is formed in the upper part of the tubes, passes upward with the liquid, and is released below the deflector. All the vapor formed must pass through the curtain into the vapor space and then out into the condenser. Although small droplets will be ejected from the main body of the liquid by buhbles breaking, and the vapor leaving the tubes will undoubtedly carry a mist with it, the chance that these particles can pass through the curtain without being swept down is small. It is obvious, therefore, that any entrained particles appearing in the vapor space must have come from the curtain itself. The curtain probably is not a solid mass of liquid but consists of small drops varying in size, close together, and moving with considerable velocity. Assuming that the curtain remains unbroken, the vapor must pass between the particles and will carry some of them upward with it. The size of the particles picked up by the vapor will depend upon the direction and velocity in which
Vol. 26, No. 3
the particles are traveling when they leave the deflector and the velocity of the vapor through the curtain. Once the droplets of liquid have reached the vapor space, their further progress will depend upon their size and the velocity of the vapor a t this point. Only those particles that are sufficiently influenced by the vapor to overcome the force of gravity will proceed into the condenser; the remainder will fall back into the main body of the liquid. It is possible that, when the curtain has a very low velocity or is very thin, the conditions in the vapor space may control the amount of entrainment, but under the usual operating conditions it is more probable that the removal of droplets from the curtain controls throughout. This is because the velocity of the vapor through the curtain is usually less than in the space above, and that sufficient force is required to overcome the initial velocity of the drops as well ae .the effect of gravity. The force exerted upon small solid particles by a stream of fluid has been determined and has been utilized in the development of hydraulic settling and air separation. The derivation of the equation is generally known and depends upon the fact that the frictional resistance between the fluid and the particle must be equal to or greater than the force of gravity (1). The same line of reasoning can be applied in the development of the relationship between the factors controlling the amount of entrainment in a forced-circulation evaporator; that is, the force upon the droplets due to friction must be equal to that necessary to cause them to leave the curtain and travel upward. I n order to obtain this relationship it is necessary t
