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Vol. 23. No. 5
Evaporation of Water b y Hot, Dry Air’ R. H. Newton and T. C. Lloyd DEPARTMENT OF CHEMICAL ENGINEERING, YALEUNIVERSITY, NEW HAVEN,CONN.
The evaporation of water by hot, dry air has been tion from the heating wire, studied and the relative importance of water flow, air air evaporation to the passed up an unpacked galflow, and air temperature has been determined. concentration of soluvanized-sheet-iron tower, D, Water flow, including both form of spray and rate of tions on an industrial scale countercurrent to a spray of circulation, has been found to be the most important m e r s materially from t h e water which was continuously variable. analogous a p p l i c a t i o n s of c i r c u l a t e d by a pump, E. The fact that a high level of efficiency is obtainable humidification and dehumidiThe tower was 30.5 cm. (12 under widely differing conditions should encourage fication as commonly carried inches) in diameter and 122 the more frequent use of such a scheme of evaporation. out. P r e v i o u s s t u d i e s of cm. (4 feet) high. The lower end of the spray nozzle waa evaporation such as is here contemplated have been made chiefly to obtain data for the 28 cm. (11 inches) below the top of the tower. The water was drawn off a t the bottom of the tower and designing of Glover, Gilchrist, and Gaillard towers for concentration of sulfuric acid. I n these cases the temperatures at sprayed in a t the top through F . The rate of flow was conwhich evaporation is effected are higher than those of the pres- trolled by a valve in a by-pass around the pump and waa ent study and the vapor pressures of the acids differ consider- measured by a water meter, G. The temperature of the o u b going air was indicated by a thermometer, Tz, and that of the ably from that of water at the same temperature. circulating water by a thermometer, Ta,both of which were placed in wells of thin-walled copper tubing. The humidity of the outgoing air was determined by drawing off a measured volume of air which was dried by passing through calcium chloride tubes, the tubes being weighed before and after the experiment. I n no case did the second tube show any gain in weight, so in the latter experimenb the second tube was not weighed. The humidity of the ingoing air was measured by a sling psychrometer.
HE application of hot-
T
Determination of Heat Content of Dry Air
I n such a system, thermally insulated from its surroundings, if the heat supplied to the water by the pump is negligible, then by a simple equating of the heat in the entering air to the heat in the outgoing air, the temperature of the exit air
Figure 1-Apparatus for Study of Evaporation by Hot Air
I n this paper a study is made of the evaporation of water by hot, dry air, to determine by small-scale operation how nearly equilibrium is attained in such systems. The term “equilibrium” is used to denote that the air leaving the tower is at the temperature of the water being circulated and is saturated. As has been stated in a well-known text ( I ) , “In view of the simplicity of the apparatus and the ease of operation, i t is surprising that towers of this sort, fired directly with flue gases from a furnace, are not more widely used.” Apparatus
The apparatus is shown in Figure 1. An air stream, controlled by a throttling device on the fan inlet, was blown by a fan, A , measured by a calibrated pitot tube, B, and heated by passing over a number of electrical resistances, C, controlled by a rheostat. The air could thus be easily controlled from a zero to the maximum flow, and its temperature controlled more accurately than would be possible on a small scale with a fuel-fired furnace. The hot air, the temperature of which was measured by a thermometer, TI, placed in a well of thin-walled copper tubing far enough from the heater so that it was not affected by radia-
* Received March 6, 1931. This paper is based on an undergraduate thesis presented by the junior author, R . H. Newton.
Figure 2-Heat
Content of Dry and Saturated Air
may be determined for any inlet-air temperature aa shown in Figure 2. I n this figure air temperature is taken as abscissa, and heat content above 0” C. as calories per gram of dry air is taken as ordinate. Since no heat is lost within the system, the heat content of the inlet and exit air will be the same. The heat content of dry air a t the inlet temperature may be found on the curve labeled “dry air;” then, reading across
IXDUSTRIAL AND E,VGINEERING CHEMISTRY
lay, 1931
Figure 3-Nozzle
531
1. Air Flow Constant at 5.60 cm. per Second in Tower Figure 4-Nozzle 2 ' Air Flow Constant at 6.02 cm. per Second in Tower Grosvenor Humidity
WS.
Water Flow:
Hot-Air Temperature a s Parameter
I
IW
=&---+- ' WATER
now
IC
Figure 5-Nozzle
Prn so &r*
I
m
nin Ir
I PO
1: Hot-Air Temperature Constant at 132.2'
C.
Figure 6-Nozzle
Grosvenor Humidity V S . Water Flow:
horizontally-i. e., a t constant heat content-to the saturated line gives the temperature a t which this air will leave if saturated. Deviations from these equilibrium conditions must in practice be dependent on: (a) rate of water circulation, (b) form of water spray, ( c ) rate of air flow, and (d) temperature of hot air. The relative importance of these variables was studied experimentally. (a) and (b) obviously depend on the more fundamental variables-time and area of contact-but these quantities can neither be measured nor calculated, so that the directly measured values are of greater importance in the design of such towers.
2 : Hot-Air Temperature Constant at 143.3O C
Air Flow a s Parameter
The air flow was maintained constant a t 229 liters (8.1 cubic feet) per minute and the Grosvenor humidity of the exit air determined as a function of water flow a t a series of temperatures. These results are shown graphically in Figure 3 for nozzle 1 and in Figure 4 for nozzle 2. The hot-air temperature was then maintained constant and the Grosvenor humidity of the exit air determined as a function of water flow a t a series of different rates of air flow. The results are plotted in Figure 5 for nozzle 1 and in Figure 6 for nozzle 2.
Operation c '0°
The apparatus was run with hot-air temperature, hot-air flow, and water flow adjusted to the desired figure until 8. steady state was reached, as was indicated by unchanging temperature of the outgoing air. This required about 45 minutes. The humidity of the entering and outgoing air was then determined by the methods indicated. The conditions were then altered and other determinations made after a steady state had again been reached under the new conditions. Two types of spray nozzles were used as follows: Nozzle 1. Schutte Koerting, 4.5-mm. orifice, 1.25-cm. (S/8-inch standard) pipe connection, 45-degree spray. Capacity, 600 t o 1100 liters per hour (160 to 280 gallons per hour). Nozzle 2. Schutte Koerting, 2.5-mm. orifice, 0.9-cm. (I/,inch standard) pipe connection, 90 degree spray. Capacity, 60 to 230 liters per hour (15 to 60 gallons per hour).
A series of runs was made to determine the effect of each variable.
P
$
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90
L
$ 86 s
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2 50
'CENTIGRADE
Figure 7-Grosvenor Humidity vs. Hot-Air Temperature: Water Flow a s Parameter Nozzle 1: Air Flow Constant at 5 60 cm. per Second in Tower
The effect of hot-air temperature on the Grosvenor humidity of the exit air a t a number of different rates of water flow, air flow being constant, is shown in Figure 7 for nozzle 1. The Grosvenor humidity of the plots refers in all cases to the humidity a t the exit-air temperature. The weight of water absorbed from a certain volume of air measured a t the
INDUSTRIAL AND ENGINEERIYG CHEXISTRY
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Vol. 23, No. 5
Results
tion period, and thereafter rose very slowly. Since the power required to circulate the water increased more rapidly than the rate of circulation increased the optimum conditions for operation occur in the transition region, indicated in the graphs by the rapid change in curvature. Under these conditions the relative humidity was about 90 per cent.
It will readily be seen that water flow was of greater importance than the other variables. The humidity of the exit air increased very rapidly a t first, passed through a transi-
(1) Walker, Lewis, and McAdams, “Principles of Chemical Engineering,” p. 391, McGraw-Hill, 1927.
meter temperature was obtained by increase in weight of the calcium chloride tube. This volume of air was then converted to the exit-air temperature and the relative humidity (Grosvenor) thus obtained.
Literature Cited
Continuous Fermentation in the Production of Lactic Acid‘ E. 0. Whittier and L. A, Rogers RESEARCH LABORATORIES, BUREAU O F DAIRYINDUSTRY, DEPARTMEKT OF AGRICULTURE, WASHINGTON, D .
HE output of plants
c
A method for continuous lactic acid fermentation of s a u e r k r a u t a r e common the lactose of sweet whey has been devised and operated examples. The reaction inemploying i n d u s t r i a l fermentation processes on both the laboratory and the plant scale. This volved is, if we disregard the method with suitable modifications should be appliprobable formation of interis limited by the size of the cable to other industrial fermentations. mediate products, of the genfermentation v a t s a n d t h e era1 type, time required for the cycle of filling, sterilizing, actual fermentation, emptying, and cleanCe.Hi206 ----t 2CHs.CHOH.COOH ing, Since a complete cycle may in some cases be as long as ten days, the vat capacity in many such plants is necessarily When the better known organisms are the causative agents, made large. the proportion of side products is very small, consisting of Efforts have been made to reduce the length of the operat- volatile acids of low molecular weight, frequently acetic acid. ing cycle by the use of more active bacteria and special Of the large number of organisms known to produce lactreatments to accelerate the chemical action of the bac- tic acid to some extent, the best known are the common teria. These means have been found to be valuable, but to Streptococcus Eactis of souring milk and several of the lactobacilli such as L. bulqaricus and L. casei. The lactobacilli a limited degree. During the time required for cleaning, sterilizing, and have the advantage of being able to continue the fermenemptying the equipment and for the growth of the inoculat- tation until a pH value is reached approximately one unit ing organism to its maximum effective numbers, very little lower than is possible when S. lactis is used. This advantage actual fermentation takes place. If these time-consuming becomes of greater importance when the acid is partly neufactors could be eliminated and the actual fermentation tralized from time to time, as is done in the commercial process carried on continuonsly, the ratio of equipment cost fermentation. \F7ith the accumulation of lactates in the to production could be considerably reduced. solution, a limiting concentration of undissociated lactic acid When a sclitable medium is inoculated with a culture of becomes more definitely the factor inhibiting the fermenthe lactic type, growth and fermentation pass through a tation and thus the actual limiting pH value gradually indefinite series of phases. There is first the lag period, in creases (1, 2 ) . The higher the pH value in the fermenting which certain physiological changes take place in the cell solution, the greater the likelihood that contaminating orbut little or no multiplication occurs. This is followed by ganisms will succeed in utilizing part of the sugar and in cona period of active multiplication and fermentation until the verting it to undesired products, such as butyric acid. Hence normal population is reached and various factors come into the advantage of using an organism able to withstand high play to limit further activity of the cells. If transfers are acidity and high concentration of undissociated lactic acid, made into new medium a t the right stage of the growth curve, when practically complete utilization of sugar is desired. the lag period may be greatly reduced or entirely eliminated. A mycoderm in a culture of a lactobacillus has the associaThe method described in this paper of providing the cul- tive effect of accelerating the action of the lactobacillus and ture with a continuous flow of fresh materials is equivalent is frequently used for this purpose. to an infinitely rapid transfer and maintains the culture a t The abandonment of preliminary sterilization of equipment and media in industrial sugar fermentations is possible its most active phase. The authors have applied the idea of continuous fermen- only when other means can be used successfully t o prevent tation to the production of lactic acid from the lactose of loss of sugar and formation of undesired products. If the whey, with the hope that their results may be applied, not fermentation can be carried out within pH and temperature only in lactic acid production, but also, with suitable modifi- ranges unfavorable to foreign fermentations, i t is a very desirable situation. This is the case in the lactic fermencations, to other commercial fermentations. tation. At 44’ C. and between pH 5.0 and pH 5.8 the lacTheoretical tic fermentation may be kept clean of active contamination. Preservation of unsterilized sugar media may be accomThe fermentation of sugars to lactic acid is frequently plished by use of low temperatures, high acidity, or high encountered. The souring of milk and the production of alkalinity. Obviously, the means used to bring the preserved media to the proper condition for fermentation should not 1 Received January 2, 1931.
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