JUNE, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
2. Brines of low salt concentration favor the rapid formation of a relatively high amount of total titratable acid and the de-
velopment of relatively low brine pH. Brines of increasingly higher initial salt content favor correspondingly retarded rates of acid formation, lower total quantities of acids produced, and higher brine reaction or pH values. 3. For brines of any given salt concentration the higher acidity will be produced in the curing of cucumbers of the smaller sizes and lower acidity during the curing of the larger sizes.
Acknowledgment The writer wishes to acknowledge the assistance given b y Otto Veerhoff in the collection of a portion of the data presented. Valuable aid was also given by Chas. F. Cates and Sons, Inc., of Faison, N. C., in extending the facilities of t'heir plant and providing the green produce involved in these studies.
861
Literature Cited (1) Brown, C. W., J . Bact., 1, 104-5 (1916). (2) Campbell, C. H., Canner, 51, No. 14, 45-7 (1920). (3) Fabian, F. W., Bryan, C. S., and Etchells, J. L., Mich. State College Agr., Tech. Bull. 126 (1932). (4) Hasbrouck, F. F., Pure Products, 6, 509-14 (1910). (5) Heinae, B., 2. Untersuch. Nahr. Genussm., 6, 529, 577 (1903). (6) Kitahara, M., J . Agr. Chem. Soc. J a p a n , 12, 595-603 (1936). (7) Le Fevre, E., Canner, 48, No. 10, Pt. 2 (Conv. No.), 205 (1919). (8) Rahn, O., Canner Dried Fruit Pucker, 37, No. 20, 44-5, No. 21, 43-5 (1913). Paper 107 of t h e North Carolina Agricultural Experiment Station. I t is a report of a portion of t h e investigation in progress as a cooperative project between t h e D e p a r t m e n t of Horticulture of t h e experiment station and t h e Food Research Division, Bureau of .lericultural Chemistry and Engineering, United States D e p a r t m e n t of Agriculture.
TECHNICAL
Pressure Drop and Liquid Holdup in a Packed Tower EDGAR L. PIRET, CHARLES A. MANX, AND THOMAS W.4LL, JR. University of Minnesota, Minneapolis, Minn.
Data are presented on various operating characteristics of a large tower filled with large-diameter solid packing. The data show that Chilton and Colburn's equation for pressure drop can probably be applied with a fair degree of accuracy to large-size packings up to 13/4inches. A n increase in holdup is noted above but not below the loading point. The holdup data are in general agreement with those of others. It is shown that for low air rates and high liquid rates the pressure drop is materially different for countercurrent and parallel flow. The results of this investigation appear in a general way to confirm the many data obtained in small experimental towers.
A
NUMBER of investigations of the pressure drop,
liquid holdup, and flooding velocities in packed towers have been carried out for both solid and hollow packing shapes. However, the greater portion of these data have been obtained in towers ranging from 3 to 6 inches in diameter with packing correspondingly small to avoid excessive wall effects. The applicability of these data from small experimental size towers to commercial sizes is in doubt, and there is need for data on larger towers and larger diameter packing. For this investigation a moderately large tower (Figure 1) packed with 13/*-inch solid packing was available for study. Measurements of pressure drop were taken over a wide
range of air velocities and also for water rates varying from 0 to 2200 pounds/(hour)(square foot). The liquid holdupthat is, the amount of water in the tower under equilibrium conditions-was measured b y stopping the feed and allowing the tower to drain. This was done for various water and air rates.
Apparatus The packed tower was 21/2feet in diameter and contained 6 feet of packing. A detailed drawing of the apparatus is available in a previous publication (6). The distributor consisted of two rotating arms with five holes in each a r m so spaced that equal areas were covered by each spray. This was demonstrated by placing a series of beakers across the diameter and noting that they collected equal volumes. Three sizes of tubes (l/8, l/4, 3/g inch) were needed to cover the range of water flow. These distributors revolved at about 60 r. p. m. and caused no appreciable resistance to air flow. The stone packing was supported on an iron grill, made up of '/4 X 3/4 inch bars laid on edge on 1-inch centers. The air was supplied to the tank countercurrent to the water flow b y a volume fan through a 20-foot length of 71/2-inch pipe. The flow was determined from traverses taken with a pitot tubwccurately made according to National Physical Laboratory standard specifications (4). At low air flows a 3-inch steel tube was used, and traverses were made with a microPitot tube calibrated against the standard Pitot tube. The lowest flows were measured through a standard flange-connected orifice. It was found by experiment that these measuring devices checked each other with less than one per cent deviation. Pressure measurements were made with inclined-tube manometers or b y means of a Wahlen gage. The latter was sensitive to 0.0001 inch of alcohol (8). Pressure drop through the tower was first determined b y using the large piezometer
INDUSTRIAL AND ENGINEERING CHEMISTRY
862
rings installed on the circumference of the tower above and berow the packing. It was found, however, that at high liquid flows better results were obtained by using larger outlets which had been shielded from liquid and gas flow.
\-OL. 32. NO. 6
entire range of superficial velocity from 0.018 to 3.50 feet/ second. These data extend down to the range of air velocities encountered in trickling filters such as are used in sewage disposal (6). Chilton and Colburn developed a n equation for calculating the pressure drop through packed towers based on data for small size particles ( 1 ) . It has been recommended that the equation be used for packings less than 1 inch in diameter (6). The present results are only slightly above the best line drawn through the data available to these writers for their correlation. Thus it can be concluded that the equation of Chilton and Colburn applies to 13/4-in~h solid packing as well as it does to packings u p to 1 inch in diameter. The pressure drop through the tower when water is being distributed on i t increases with water rate (Table 11). ,4t 1 foot/second gas velocity, the pressure drop data lie on a straight line when plotted against liquid rate and follow the equation: AP,”
=
0.41
+ 0.00012 L
As observed before, when liquid is being distributed over the packing, a break occurs in the pressure drop-velocity plot a t high air velocities near 2 feet/second ( 2 , 3, 7 ) . This is known as a loading point and corresponds to the velocity at which spray clouds begin to appear in the outgoing air. Figure 3 shows that a t this point the liquid holdup begins to increase. Higher air velocities than those recorded could not be attained because of the rapidly increasing resistance of the tower above the loading point. This made i t impossible to attain a flooding velocity with the available fan.
FIGURE
1.
EXPERIMEXTAL
PACKED
TOWER
The liquid holdup was determined by collecting the water drained from the tower in 30 minutes after the feed was stopped. This time was chosen on the basis of experiments which showed that for all water rates u p to 2200 pounds/ (hour) (square foot), the rates of draining had become very low and identical after 30 minutes. If allowed to stand overnight, about 0.26 pound/cubic foot more water would drain from the tower. The water rate was measured through a flange-tapped calibrated orifice. The tower was packed with stones selected for roundness from gravel which had been retained on 11/2- and passed through a 2-inch square screen. Packing characteristics of the tower were as follows: No. packing units/cu. it. of tower Packing surface, sq. ft./cu. ft. (calcd. as spheres) Per cent dry void Mean volume of particle, cu. in. Mean diameter of particle, in. Mean dimensions of particle, in.
409 25.22 38.8 2.5s 1.68 2 . 2 7 X 1 . 6 3 X 1.16
The investigation of pressure drop and holdup was bmken u p into a series of separate experiments or runs. I n a single run, after equilibrium was attained, the air flow, water flow, and pressure drop through the tower were recorded. The amount of holdup was then determined by abruptly stopping both the flow of water and air and draining the tower.
Discussion of Results The data on pressure drop are given in Table I and Figure 2. Some of the very low velocity points are not shown on the plot but all the dry tower data lie on a straight line over the
FIGURE
2.
DROPAS A FUNCTION OF AIR A S D WATERRATER
PRESSURE
JUNE, 1940 TABLE
I S D U S T R I l L AND ENGINEERING CHEMISTRY
I.
CO
0
DROPI N
AP/N Lb./(Sq. Ft.) (Ft.)
Ft./&c. 3.50 2.98 2.59 2.377 1.612
ft.
PRESSURE
THE
DRYTOWERQ
TABLE11.
P
/'*
2750 2341 2036 1868 1267
14.90 13,32 13.84 14.32 16.32
1.107 0.865 0.836 0.705 0.701
0.536 0.291 0.323 0.195 0.2162
870 680 656 554 551
16.62 15.04 17.90 15.20 17.08
0.659 0.602 0.580 0.478 0.432
0.210 0.1562 0.1468 0,102 0.0914
518 473 432 375 340
18.72 16.72 18.82 17.28 19.00
0.379 0.3715 0.2873 0,248 0.2413
0,0688 0.0752 0.0403 0.0357 0.0339
298 292 226 195 189.7
18.53 21.10 18.94 22.48 22.56
0.222 0.0893 0.0852 0.0845 0,0833
0.0269 0.00559 0,00555 0.00527 0.00518
174.3 70.1 67.0 66.4 65.6
21.16 26.3 29.6 28.55 46.2
0.0743 0.0658 0.0477 0.0288 0.0180
0.00410 0.00343 0.00213 0.000643 0.000273
58.5 51.7 3i.5 22.6 14.1
28.64 30.75 36.22 30.0 32.6
Lb./(Hr.)(Sq. L, F t . ) Ft./Seo. Uo
Lb./(Sq. AP/.T Ft.') (Ft.)
I
0
&D Ft.
P
0.0581 0.412 0.964 1.542
0 0,1232 0,2695 0.426 0.536 0.95 1.994 2.806 3.298
0 0,01292 0.0452 0,0905 0.1378 0.430 1.59 3.53 5.42
0.308 0.3105
0 97 222 335 422 747 1597 2205 2595
317 578
1.020 1.016
0.407 0.451
0 .i i 4
802 799
973
0 0.2848 0.535 1.oo 1.502 2.91 2.463 2.54
0 O.OFl86 0.159 0.485 1.043 6.82 3.54 4.31
2200
(I).
H
Lb./C;. 0 0
0.332 1.000 1.500 2.000
46.7
Average air temperature = 78.0' F.; average air density = 0,070 Ib./cu.
* Modified friction fartor
DROPAND HOLDUPDATAI N THE WETTED TOWER"
PRESSURE
*cop
4.71 3.048 2.397 2.085 1.097
863
0
0 0
0
0 0,197 0.3072 0.3163 0.429 0.525 0.782 0.981 0.998 1.031 1.498 1.799 1.810 1.83 2.003 2.568 2.585
0 0,0494 0.222 0,0934 0.149 0.2133 0.411 0.612 0.706 0.712 1.432 2.30 2,097 2.18 3.037 7.26 7.46
0
0
0.291 0 0.2932
.. ..
..
1.063 1.097 1.082
..
1.060 1.244 1.66 1.669 1.665
..
1.662
.. .. .. 1 562
1.727
1.664 1,825
261 787 1180 1573
0 224 411 785 1182 2288 1940 2000
0 0 0 155 242 249 337 413 615 771 784 811 1178 1413 1422 1439 1579 2020 2033
AveFage water temp. = 70' F.: average air temp. = 7 8 O F.; average air density = 0.070 Ib./cu. ft.
N = height of tower, ft.
AP = pressure drop, lb./sq. ft. Uo = superficial air velocity, ft./sec. p = p =
Literature Cited
FIGURE3. LIQUIDHOLDUP AS A FUNCTIOS OF AIR ASD WATERRATES
Chilton, T. H., and Colburn, A. P., IND.ENG.CHEM.,23, 913
A second change in slope is observed a t a liquid rate of 2200 pounds/(hour)(square foot) and an air velocity of 0.4 foot/ second. A plausible explanation is that under these conditions of high liquid and low air flow the relative velocity of air to water becomes appreciably greater than t h a t of air to packing. The point is perhaps better illustrated by data taken a t a water rate of 1850 pounds/(hour)(square foot) when the pressure drops were measured for both countercurrent and parallel flow of water and air: --Countercurrent
CO
Ft./sec. 0 461 0 355
FlowAP/N
7-Parallel
Lb./(sq. ft.)(ft.)
Ft./sec.
0.1432 0.0975
0.460 0.350
CO
FlowAP/N Lb./(sq. ftJ
density of air, lb./cu. it. viscosity of air, lb./(ft.)(sec.).
(it.)
0,0762 0.0431
The effect of the direction of flow is evident, since for the rates employed the resistance in countercurrent flow is almost double t h a t in parallel flow. I n fact, the resistance in parallel flow is well below that in the dry tower.
Nomenclature D, = diameter of particle, ft. H = lb. of water/cu. ft. of packed volume L = liquor rate, lb./(hr.) (sq. ft. cross-sectional tower area)
(1931). Elgin, J. C., and Weiss, F. B., Ibid., 31, 435 (1939). Mach, E., Forsch. Gebiete Ingenieurw., 6,375 (1935). Ower, E., "iMeasurement of Air Flow", 2nd ed., London, Chapman and Hall, 1933. Perry, J. H., Chemical Engineers' Handbook, 1st ed., p. 744, New York, McGraw-Hill Book Co., 1934. Piret, E. L., Mann, C. A,, and Halvorson, H. O., IND.ENG. CHEM.,31, 706 (1939). White, A. M., Trans. Am. Inst. Chem. Engrs., 31, 391 (1935). Willard, A. C., Knatr, A. P., and Day, V. S., Univ. Ill. Exp. Sta., Bull. 120, 191 (1921).
