I
LeROY A. BROMLEY, STANLEY
M. READ, and SARGIT S. BUPARA
Department of Chemical Engineering, University of California, Berkeley, Calif.
Falling Liquid Sheets A New Mass c
Transfer Device Thin vertical liquid sheets result in low pressure drop with high throughput, making the equipment particularly attractive for absorption processes
IN
THE search for improved equipment for mass transfer between gas and liquid phases a study was initiated on the movement of liquids down vertical single strings. I t was observed that the flow took the form of separate large drops. However, when two strings were brought into contact at one point, a liquid bridge or sheet formed between them. This sheet, although wavy and turbulent, seemed uniform in thickness, and, depending on flow rate, it could be maintained at a width up to 2 inches for about 1 foot. However, strings from 0.25 to 0.5 inch apart seemed to give the best sheets. Liquid was first fed into a slot through which the strings passed. However, a round hole from which the strings left a t an included angle up to 30" gave better starting characteristics. Smaller angles gave somewhat better starting than larger ones. An obstruction, needed to hold the strings apart, did not break the film as long a s ii was fairly small and S T R I N G S A R E A T T A C H E D A i T H E TOP O N E FOOT H I G H R E S E R V O I R
OF A
LlOUlD SHEET
S T R I N G S A R E ATTACHED AT T H E BOTTOM OF A FOUR FOOT T A L L F R A M E AND ARE PULLED TO FOUR POUNDS T E N S I O N
In column feed arrangement, monofilament nylon strings and neoprene 0 rings were used to create the falling liquid sheet
wetted by the liquid; O-rings were satisfactory and gave additional film area by diverting part of the flow to the space inside the ring. Strings of monofilament or braided nylon were most satisfactory. When water was the liquid, the strings had to be cleaned and soaked long enough to ensure wettability. All liquids tried gave satisfactory sheets. Besides water, these included salt water, sea water, dilute acid and base, toluene, and butyl acetate. Heights of gas film transfer units are comparable to those of packed columns, although the heights of liquid transfer units are somewhat poorer. The most important advantage of this liquid sheet column is its extremely high throughput of both liquid and gas with very low pressure drop.
length but nevertheless one that was self-healing. The break and complete rehealing took place in a matter of a second or two. I n attempting to determine the cause of the breaks, it was observed that the films could be completely destroyed by introducing air bubbles with the feed whereas plucking the strings (within limits) produced no effect. It thus ap-
Experimental On the basis of preliminary studies, a liquid sheet apparatus was built (see diagram). Thk-total liquid sheet width was 6 inches and the length 4 feet. The nylon monofilament strings were stretched to a tension of 4 pounds. At flow rates of water greater than 1.4 pounds per minute per inch of sheet width, a contirhous sheet was observed for about the first foot, below this an occasional break or hole would appear (see photographs) and a t about 4 feet approximately 60% of the width was covered with liquid films. The sheet could be made continuous with no holes down to nearly zero flow rate by the addition of a few parts per million of detergent forming, in effect, a soap film. High speed motion pictures (about 1000 frames per second) showed that the frequency of breaks in the film between the strings increased as the bottom was approached. Following a break, the lower boundary of the opening fell faster than the upper boundary thereby producing a hole of increasing
Front view shows dynamic sheet of water
Downward view shows breaks in liquid sheet which occurred more frequently as the bottom was approached VOL. 52, NO. 4
APRIL 1960
31 1
pears that the initial breaks are caused by air or gas bubbles bursting through the surface. Column Design
O n the basis of experience gained with the single sheet apparatus a column of 3 X 1.5 inch cross section and 4 feet long was built. The feed holes through which the nylon strings passed were ‘/8 inch in diameter spaced 0.5 inch apart with rows 0.25 inch apart drilled in 0.75inch-thick brass plate. The nylon monofilament (30-pound test, 0.26-inch diameter) strings were stretched to a tension of 4 pounds force and held in place at the bottom by taper pins 0.080 X 0.110 X 1.5 inches hand pressed into tapered holes placed in a square array 0.25 inch on centers in a 0.75-inch brass plate; 0 rings were again used to hold the strings separated. The top and bottom plates were supported by brass bars on either side of the string assembly, and the entire assembly was placed in 3-inch borosilicate glass pipe. A 3-inch borosilicate glass pipe cross a t the top just below the liquid entrance allowed removal of gas, and a tee at the bottom was used to admit the air and remove water. Above the feed
Table I.
I 600 AlaiNh,
400
1 800 1000 FLOW RATE
I
2000
- G , LES
3000
(hR)1FT21
Ammonia absorption experiments in string column showed heights of gas film transfer units comparable to those of packed columns
holes a 3-inch borosilicate glass tee was used to provide a “stilling” pond for the feed to allow escape of trapped air bubbles. The liquid head in this pond ranged from 4 to 16 inches above the feed holes. A m m o n i a Absorption
I n all ammonia absorption experiments the ammonia flow rate was held constant, and air rate varied. The resulting entering composition ranged up to 7y0 ammonia. The entering ammonia concentration was determined by orifice measurements of ammonia and
Absorption of Ammonia into W a t e r from an Air Stream in a String Column Under these operating condilions, liquid film resistance was not important
air and checked once by direct analysis. The amount unabsorbed was determined by direct analysis of the exit air stream as follows: Part of the exit air was bled through a dry, 1-liter flask until outlet gas composition was unchanged; the flask was then isolated and a known amount of 0.OliV hydrochloric acid added. Standard 0.01N sodium hydroxide was used to back titrate using a pH meter to determine the end point. The data and results are summarized in Table I and shown on the graph. Under the conditions of operation the liquid film resistance was of no importance. The entering air and water temperature were approximately 60’ F. Carbon Dioxide Absorption
Five runs were made to determine the magnitude of the liquid film resistance, I n the carbon dioxide runs the inlet composition was held between 10 and 12% for all runs. Analysis was made of the water leaving the column and of a \vater sample equilibrated with a small gas stream taken just before entrance to the column. Samples were analyzed by treating a known volume with barium chloride and sodium hydroxide and back titrating with 0.01.Y hydrochloric acid using phenolphthalein and finally a pH meter to determine the end point. The calculated results are given in Table 11. Pressure Drop
505 69,600 0.708 5.65 0.026 7.33 70,800 0.90 850 0,0497 4.45 4.23 69,100 1.01 982 3.96 3.70 0,0705 69,100 1.09 1090 3.67 3.22 0,0824 69,100 1.11 1260 3.61 0,0782 2.88 69,100 1.11 1260 0.0782 3.61 2.88 70,200 1.205 1402 3.32 0,0935 2.58 7 575 1580 73,700 3.47 1.15 0.0723 2.33 S 635 69,600 1.2 1580 3.34 0.0832 2.33 9 635 1660 69,600 3.34 1.20 0.0776 2.18 10 680 1980 69,600 3.19 1.25 0.0756 1.83 11 810 70,200 1.29 2140 3.09 0.0774 1.69 12 875 70,200 1.37 2324 0.0839 2.92 1.56 13 950 70,200 1.33 2519 0.0714 3.00 1.44 14 1030 1150 135,000 0.713 5.61 0.0112 3.052 15 470 135,000 0.752 1430 5.34 0.0117 70 2.45 16 590 1690 135,000 0,871 0.0211 4.59 2.09 70 17 695 1980 135,000 0.935 4.28 1.77 0.0245 70 18 820 Gas flow rate per unit cross-sectlon area of column. Ammonia flow rate = 14 8 cu f t /hr. c Water flow rate per unit cio~s-bectioiiarea of column. 202 350 400 460 515 515
1
2 3 4 5 6
Table 11.
36.3 36.9 36.0 36.0 36.0 36.0 36.6 38.4 36.3 36.3 36.3 36.6 36.6 36.6 70
Absorption of Carbon Dioxide into W a t e r from an Air Stream in a String Column
Acknowledgment
Inlet composition was between 10 and 1 2 % for all runs
Air, Run No. 1 2 3 4 5
COI Cone.,
Flow Rates COa,
mMoles/Liter G,“
Efflu-
cu.
cu.
HzO,
ent
Equi-
ft./hr.
ft./hr. 17.05 17.05 24.6 24.0 24.6
Ib./min.
H20 3.80 3.88 3.78 3.52 2.84
librium
NTUOL
5.68 5.74 5.34 5.32 5.26
1.104 1.12 1.304 1.08 0.817
138 138 220 195 240
36 36 36.3 27.3 70.8
a Gas flow rate per unit cross-section of column. tion area of column.
312
INDUSTRIAL A N D ENGINEERING CHEMISTRY
b
A manometer attached to the air inlet to the column indicated a maximum pressure drop of 0.8 inch of water through the 4-foot column at hizhest operating flow7 rate. The highest gas rates reported in the ammonia absorption were about 57, below flooding for the given liquid rate. Flooding is evidenced by a sudden buildup of water in the column usually either at top or bottom and the resultant sudden increase in pressure drop and carryover of liquid. L’nder all conditions of operation reported there was no visible carryover of liquid either in bulk or as spray. Any spray would have shown u p in the samples. Bulk carryover did occur, however, on flooding.
HTCOL, Lb./Hr. Ft. Sq. Ft. 345 3.62 345 3.57 345 3.07 497 3.70 600 4.90
Lb
Lb./Hr
Sq. Ft. 69,600 69,600 69,100 52,000 135,000 Water flow rate per unit cross-sec-
One of the authors, L. A. Bromley, expresses his thanks to the iMiller Institute of Basic Research in Science at the University of California for its financial contribution to this research. RECEIVED for review August 28, 1959 ACCEPTEDJanuary 12, 1960 Division of Industrial and Engineering Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
