Figure 1. In the horizontal fractionator, vapor flows straight through, countercurrent to the liquid which is pumped over a series o f porous bats which act as cross-flow contactors
MICHAEL MARKELS, J;.,l THOMAS B. DREW
and
Columbia University, New York, N. Y.
B E C A U S E VAPOR FLOW is restricted and a liquid seal is required, bubble-cap and sieve-tray columns operate a t high capacity with relatively high pressure drop. The use of packed columns to reduce pressure drop has not been successful in large sizes because of efficiencylosses from channeling. However, less channeling results if vapor flows through a horizontal duct containing a series of transverse grids or bats down which the liquid flows across the gas stream. Countercurrent movement of liquid and vapor can be had if the liquid drained from each bat is pumped to the top of the next bat upstream with respect to the vapor. The bats, made of porous material, break both liquid and vapor streams to promote contact. Each stage is of the cross-flow type while the over-all geometry gives the most favorable Lewis conditions (73). Pumps of the thermal siphon type should work well to move the liquid (74). A horizontal column was investigated (Figure 1) having individual stages of a cross-flow contactor-type which has received little attention in the literature.
Present address, Atlantic Corp., Alexandria, Va.
Research
Shower trays (72) used in the petroleum industry resemble this device without the bats; also, a wet gas scrubber has been reported (4,7), which uses a spray rather than a weir to introduce liquid to the stage. Therefore, to evaluate potentialities of the column design, operating characteristics of these cross-flow stages were studied. Apparatus and Procedure
Generally, the apparatus (Figure 2) consisted of a rectangular air duct containing a Lucite test zone in which a stage was carried, together with airand water-handling equipment and instrumentation. Vapor efficiency was measured by humidifying the air, and liquid phase efficiency was measured by oxygen desorption from water by air (3, 9, 70). Combining the two film efficiencies to obtain over-all efficiency The concept of a horizontal fractionator is not new; i t goes back to the horizontal partial condensers used in France at the beginning of the 19th century. Further development, however, was curtailed at that time b y invention of the modern bubblecap column which was so superior to other devices that further work on the horizontal column practically ceased ( 8 ) .
has been established both theoretically and experimentally (6, 76). The cross section of the Lucite duct, measured perpendicular to the air flow, was 24 inches high by 16 inches wide. The air loop included a 5-hp. fan equipped with suitable dampers so that the air could be recirculated. Before entering the test zone, air flowed successively through a heater, a measuring orifice, and past wet- and dry-bulb thermometers. I n the test zone it first passed through a dry, knitted stainlesssteel porous bat, 4 inches thick (Figure 3) which served as a radiation shield for upstream thermometers. Also, this bat set up a constant scale of turbulence and a standard velocity profile for the active stage which followed. The air then came in contact with the water stream on the active bat which could be set a t an angle of 5 5 O , 67", and 79' to the horizontal. This bat was held in the Lucite chamber with rubber sheeting a t the top, and to eliminate air bypassing, it was seated with a baffle into the sump water at the bottom. Pressure drop across this bat was measured by a differential water manometer. The air stream then passed thermometers which measured the outlet dry- and wetbulb temperatures and out through an entrainment separator bat into the fan intake. The entrainment separator was a wire-mesh bat like that a t the inlet but with provision for collecting the entrained water for measurement. VOL. 51, NO. 5
MAY 1959
619
The once-through water system consisted of an inlet block valve, an oxygen injection system for the liquid phase efficiency runs, and a constant-head tank where the water level was maintained by a float valve and temperature was controlled by injection of steam. From the tank, the water flowed through a measuring orifice, valves, a box with a notched distribution weir, and into the top of the active bat. Typically, in flowing down the bat, some of the water wept or fell from the upstream face, some was blown from the downstream face as spray and foam, and some remained within the bat volume. For any bat angle, the type of flow depended on the liquid and vapor flow rates and the pressure drop across the bat. The main stream of water flowed into a sump a t the bottom of the bat, into a large pan, and finally into the sewer. Any water that -sprayed or splashed over a baffle, which separated the entrainment sump from the active bat, was considered entrainment and therefore was added to that removed by the downstream, de-entrainment bat. Temperature of the water was measured behind the inlet weir and in the outlet sump. Samples for oxygen determinations were taken a t these same points. To avoid introducing moisture before the test section, the upstream wet-bulb temperature was taken in a small stream of air withdrawn from the upstream chamber. The downstream dry-bulb temperature is, in theory, that of the air between the droplets of entrained water. However, every device put into the air stream to remove the water droplets increased the humidity of the air and thus decreased the dry-bulb temperature. Therefore, a Lucite cyclone with a centrally located thermometer was devised with a cloth lining in the lower part which was kept wet with water at the wet-bulb temperature of the air. The cyclone which discharged to the room had a constant Murphree contacting efficiency of 0.20 over the operating range. By using this efficiency and the wet-bulb reading, the measured humidity leaving the cyclone could be corrected to get the actual humidity of the air between the water droplets as it left the active bat. This value was then used in calculating the vapor-phase Murphree efficiency. Oxygen content of the water into and out of the active bat and that in water saturated with air was measured with an Elecdropode. Fortunately, the corrections were such that these readings could be used directly in calculating the liquid phase efficiency. The central item in the apparatus, of course, was the porous bat itself. Bat No, 1 was the standard knitted design made by the Metal Textile Corp., Roselle, N. J. (Figure 3). Like all active bats it measured 16 X 25 X 4 inches
620
CONSTANT HEAD TANK
AIR HEATER
ZRRGL
A I R FLOW, AND STATIC
Ann
A I R DUCT
WATER I N
Too
I I
5 HP
BAT
-UPSTREAM
ENTMYMENT Dbl I
i S S U R E DROP, AND ZTlC PRESS ACTIVE STAGE
Figure 2. Measurements were made on a single active stage where water and air were brought into contact. Water flow was once through, but air could be recirculated Temperatures were read a t the point of sampling: t i and f o r water in and aut; T i and To, air in and out D and w, dry and wet bulb; oxygen samples, il and i?, in and out, respectively
and its construction was identical to the upstream and downstream deentrainment bats. It weighed 9.24 pounds and was made of knitted stainlesssteel lvire, 0.01 1 inch in diameter, which formed a matrix of about 9870 open volume. I t was supported by brass frames and was sealed at the top and bottom to prevent vapor bypassing. Bat No. 2, also mounted in a brass frame and made by the same company, was of knitted, 0.01 1-inch diameter stainless steel wire, and because of a looser knit, weighed only 6.6 pounds (Figure 4). In general construction, both of these bats are similar to Goodloe packing (5) except that they were made of larger wire which was not twisted into multiple filament strands; also, they were of more open construction, especially Bat No. 2. Bat No. 3, made by the Research Products Corp., Madison, Wis., weighed 2.25 pounds and conformed to the standard Coolpad design (Figure 5). The expanded form was made by slitting the aluminum sheet in alternating cuts 11/16 inch long and about 0.10 inch apart. Except for much finer construction, the resulting bat, also supported in a brass frame resembled Scofield packing (75). Bat No. 4, of 0.011-inch diameter galvanized-wire screen with the wires 0.085 inch apart and made by the American Air Filter Co., Louisville, Ky., is the company's standard HV-4 filter (Figure 6). The bat was made of flat and crimped layers, approximately 16 inches long and 4 inches wide, stacked 25 inches high. The crimped pieces were formed in a triangular pattern with 21 cycles across the 16-inch width. The crimping forms a regular 45 O saw-toothed pattern across each edge, with opposite edges exactly out of phase. Thus, two crimped pieces laid back to back with
INDUSTRIAL AND ENGINEERING CHEMISTRY
flat screen on each side, form an assembly which consists of open-based tetrahedrons 4 inches high. This gives a cellular structure of large filtering area and high porosity. The entire assembly was held in a frame constructed of a bent channel with openings at the top and bottom for entrance and exit of water. The theory used in calculating the results has been described (76). The Murphree plate efficiency for the vapor is defined by Equation 1 where actual absolute humidities can be used instead of concentrations as shown:
The liquid efficiency is defined by Equation 2 where the Elecdropode currents can be used for oxygen concentration:
Humidification runs were made with inlet wet-bulb temperature of the air equal to the inlet water temperature; this was effective in eliminating heat transfer in the liquid because the bat material was always covered with a film of water. The area for heat transfer was equal to the area for mass transfer as indicated by the constant water and wet-bulb temperatures measured across the stage. Pure vapor phase mass transfer was measured because the liquid phase did not contribute to the over-all mass transfer resistance. Because backmixing did not occur, local Murphree efficiency was equal for each local vapor path and therefore equal to over-all efficiency. Relating local efficiency to the number of gas phase transfer units gives EM"
=
EOG= 1
- e-No
(3)
FRACTIONATING DEVICE
Figure 3. Bat 1 was of stainless-steel wire knitted into 20 double sheets crimped alternately at an angle of 30"
Figure 4. Bat 2, more loosely knit, was of stainless-steel wire knitted into 22 crimped double sheets
Figure 5. Bat 3 was a standard Coolp a d design and contained 34 layers of expanded aluminum sheet
To avoid special calibrations and calculations, oxygen desorption runs were made with a water temperature close to 30" C. The oxygen released was so little compared to its concentration in air that the air concentration could be considered constant in traversing the stage. Air entered with a high relative humidity and at wet-bulb temperature close to the temperature of water; therefore, the vapor phase contributed almost no resistance. Because there was no back-mixing, the Murphree efficiency was equal for each local liquid path and therefore equal to the over-all efficiency:
right ordinate. Because of difficulty in precise control, liquid rates represented by each symbol vary slightly from graph to graph. Those values marked by superscript r have been rounded by as much as 0.5 g.p.m. per foot. Bat No. 1 operated best at an angle of 67" with the horizontal (Figure 7)-i.e., with the greatest proportion of the liquid within the bat volume for each combination of liquid and vapor flow rates. I n common with other bats tested? pressure drop increased with both vapor and liquid rate, but the curves tended to show a negative curvature in contrast to the positive curvature for the dry bat. Shape of the curves is best explained in terms of pressure balance across the bat. For a given liquid rate and vapor velocity, liquid weeps from the upstream side of the bat because of insufficient pressure drop or vapor velocity to hold it within the bat volume. Small increases in vapor flow rate, however, increase drag on the liquid drops; thus, both the proportion of liquid held within the bat volume and pressure drop increase rapidly. Further increase in vapor velocity decreases the average angle of descent and finally forces the liquid out of the back face as spray and foam. This reduces both the amount of liquid within the bat volume, and the rate a t which pressure drop is increased. The entrainment data, shown dotted, form a fan of curves having a positive slope and curvature which move up and to the left as the liquid rate is increased. The definition of entrainment is rather arbitrary and depends on the geometry of the system. However, the results show clearly the zones where the stage becomes overloaded and serve to compare the performance of the various bat designs. Bat No. 2 was tested a t angles of 55", 67 ", and 79 " which on the basis of experimental convenience, were chosen to cover the range of greatest interest
(Figure 8). At 55", the pressure drop, which holds water within the bat material, was even less than for bat No. l. Therefore, considerable weeping of water occurred at the inlet face and the pressure drop curves are steep. However, this small angle and low pressure drop tend to depress the amount of entrainment which becomes excessive only at high liquid and vapor rates. At 67" (Figure 8,B) the pressure drop curves are lower and flatten out while the entrainment curves become steeper at lower vapor velocities. Although the curves are similar to those in Figure 7 , the open weave of bat No. 2 gives about 5070 loiver pressure drop and less entrainment at the same operating conditions. At 79" (Figure 8.C), the presslire drop curves are still lower and the entrainment curves again steepen and move toward lower vapor velocity. At this angle, the fact that spray and foam are blown out of the back face of
E.,~L = EO== 1
- e-NL
(4)
Discussion The pressure drop and entrainment results are shown in a series of eight graphs (Figures 7 through l o ) , plotted with the same ordinate and abscissa to compare performance of the four bat designs. The pressure drop results are plotted against the left hand ordinate and the entrainment, dotted, against the
Figure 6. Bat 4, of 7 3 layers of galvanized-wire window screen, had a cellular structure when viewed in the direction of vapor flow
I 6
c Y $ 1 2 Y
Y *1
z
s
L
< 4
0 0
8
4 11
I2
/$e
6
c
"e Y
Y
c z
I Y
4
c e
*E
2
0 v
11 l l C C
Y f f /re:
Figure 10. The cellular structure of b a t 4 gave some important differences. Primes indicate pulsing I in g.p.m./ft.: 0 = 0 ; AP; IIIIII~IIIIIIIIIIIII e 0 = 10.6; 0 = 19.8'; A = 40.4'; 0 = 64'; Q, = 84.6'; A. At an angle of 55", pulsation i s more marked. 6. At 67", more water stayed in the bat because weeping did not occur. C. At 79", entrainment increased markedly and pressure drop decreased more 622
INDUSTRIAL A N D ENGINEERING CHEMISTRY
FRACTIONATING DEVICE
Figure 11. Bat No. 2. phree efficiency vs. Z
Mur-
Bat Angle
55' 6 7 ' 7 9 '
O
D
1, G.P.M./Ft.
10.6 20
*
O @ *
1
a
w
40
A V V 0 8 .
64
A.
In humidification runs, gas phase efficiency generally decreased linearly with increasing 2. 6. In oxygen desorption runs, liquid phase efficiency increased with increasing 2
I\
.50 0
.6c
B .501
1
IO
z
20 I 103
I
I
0
40
20
10
OO
water stayed in the bat because weeping, present when the 55" angle a t low air rates was used, is absent a t 67" (Figure 10,B). The steeper angle tended to reduce pulsing, but entrainment stayed about the same. Entrainment increased markedly and pressure drop decreased much more at 79" (Figure 10,C). Because the cells were almost horizontal, they no longer filled with water so that pulsing was absent; liquid was blown from the back face and operation was poor. T o compare the results of mass transfer measurements for bats Nos. 2 and 4, with literature values for other types of cross-flow contactors, a function was needed, which increased with increasing column load to indicate high capacity, and increased with decreasing pressure drop to indicate good multistage operation a t low pressure. A dimensionless group can be used for this purpose, which is the ratio of the vapor velocity
zx
= ( Z ) / A P
(5)
The ratio, 2, is proportional to the square of the F factor over the pressure drop, and equals the square of the Euler number used in steady, irrotational flow. The Murphree gas phase efficiency generally decreased linearly with increasing 2 for bat No. 2 (broad gray line, Figure 1I , A ) . However, because of excessive weeping from the front face of the bat, the low liquid-rate runs do not fall with the rest, especially at the smaller bat angles. This tended to increase 2 but decreased the efficiency. When plotted us. 2, the Murphree liquid phase efficiency data are generally more scattered but fall generally within 5% E M L for bat No. 2. The same drop in efficiency a t low liquid rates and low air rates is again present. T h e trend of
Figure 12. Bat No. 4. phree efficiency vs. Z
40
lo3
increasing efficiency with increasing 2 (wide shaded area, Figure 11,B) can be explained by noting the large influence of V" in the value of 2. Vapor phase efficiency decreases with increasing air velocity because of the increase in the amount of air to be contacted. O n the other hand, for liquid phase efficiency, increasing air velocity improves the contact of a constant amount of liquid and thus increases the efficiency. From the pressure drop and entrainment curves of bat No. 4, the tendency of this bat to hold liquid should give more regular efficiency results than bat No. 2. This is verified by the data which generally fall within the wide gray lines without the drastic drop in efficiency associated with weeping and blowing previously noted. The decrease in vapor phase efficiency with increasing Z is much less pronounced than for bat No. 2 (Figure 12,A) because of better liquid retention. The increase in liquid phase
head entering the stage to the stage pressure drop:
z
30
Mur-
Bat Angle
55' 67'
0
0
2 8
I, G.P.M./Ft. 10.6 19.8'
40.5'
I A Al
A.
In humidification runs, decrease in gas phase efficiency is less pronounced than for b a t 2. 6. In oxygen desorption runs, increase in liquid phase efficiency at lower Zvalues is followed b y a leveling at higher values
1
0
10
z
20 103
30
40
0
20
0
z VOL. 51, NO. 5
I:
33
1 40
103
MAY 1959
623
'01
I O
Figure 13.
Data for bat 4 is compared with that for bubble plate
FMV vs. 2. Values for Z were about 20 times greater than bubble plate at the same vapor phase efficiency and loading. Bubble plate data: 0 = (12); A = 13). B. FML VI. 2. Values for Z were about 50 times greater than those for bubble plate at comparable liquid phase efficiency and loading. Bubble plate data: 0 = ( 4 ) ; = (I).
A.
+
efficiency a t lower Z values is followed by a leveling a t higher values. Although some variation was found with operating conditions, the Murphree efficiency data for both bats was uniformly good. I n Figure 13,A, the log-linear scale is used to include the wide variation in Z of both bats and bubble-cap trays on one graph. The vapor phase efficiency increases with decreasing Z for both types of contactors because of the large effect of liquid velocity on the pressure drop which appears in the denominator. Most importantly, bat No. 4 provides Z values about 20 times the bubblecap data at the same efficiency and a t the same or higher loading (7, 2 ) . In Figure 13,B, bat No. 4 gives Z values about 50 times the bubble-cap values a t comparable loading and liquid phase efficiencies. Other data from sieve plate tests give Z values about 10% of the bat data, but a t considerably reduced efficiency ( 7 7 ) . The bats tested will operate satisfactorily within the shaded portions of the efficiency graphs. For example, bat No. 2 gives the highest efficiency and loading, but it is sensitive to operating conditions. Therefore, bats of this type would be useful under steady state operating conditions. Variable flows can best be handled by bats similar to No. 4. Variables such as bat width, height, thickness, angle, and spacing, which can be specified by the column designer, give as much latitude as is available in sieve and bubble-tray designs. For example, increasing the width to height ratio of the bat increases the liquidhandling capacity a t constant bat area. The optimum combination of bat design parameters will depend on economic factors and technical restraints. Conclusions This proposed horizontal distillation column has potential usefulness because low pressure drops were obtained a t high liquid and vapor loadings and with good efficiencies. Other bat materials may give even more favorable results.
624
Bat Angle
L, G.P.M./Ft
0 55;
10.6 10.6 20.0 20.0
@ 67"
mole fraction light component in vapor = dimensionless pressure drop ratio = density, lb./cu. ft. =
y
Z p
References Acknowledgment
Dale Babcock suggested the topic; materials were contributed by the Arnerican Air Filter Co., Metal Textile Corp., and Research Products Corp. Nomenclature
e
= portion of water flow entrained, e = J/j; 70
Murphree liquid efficiency for the stage EnrV = Murphree vapor efficiency for the stage EOG = Murphree vapor efficiency for a local vapor path (a function of bat thidkness) EoL = Murphree liquid efficiency for a local liquid path (a function of bat length) = water flow rate, g.p.m. f F = F factor, F = V ( P ~ ) ~ ' ~ g, = constant, 32.2 (lbm)(ft.)/(lbf) (secJ2 H = absolute humidity, lb. vapor/ lb. dry gas Subscripts 1 = Air entering active bat 2 = Air leaving active bat e = Saturated a t the wet bulb temperature Il = Electropode current reading, inlet water I 2 = Electropode current reading outlet water I, = Electropode current reading, on water saturated with air J = amount of water entrained, g.p.m. L = water flow rate, g.p.m./ft. of weir NG = number of gas phase transfer units; 0 = over-all NL = number of liquid phase transfer units, 0 = over-all AP = pressure drop across the bat, in. water V = air velocity through the bat, ft./sec. x = mole fraction light component in liquid EML
INDUSTRIAL A N D ENGINEERING CHEMISTRY
=
(1) Am. Inst. of Chem. Engrs., "Effect of Column Operating and Design Variables on Plate Efficiency," 2nd Annual Rept. University of Delaware, 1954. (2) Am. Inst. of Chem. Engrs. 3rd Annual Rept., University of Delaware, University of Michigan, and North Carolina State College, 1955. (3) Am. Inst. of Chem. Engrs. "Tray Efficiencies in Distillation Columns," 2nd Annual Rept., pp. 120-4, North Carolina State College, 1954. 4) Berly, E. M., First, M. W., Silverman, L., IND.ENG.CHEM.46, 1769-77 11954). (5) Bragg, L. B., Zbid., 49, 1062-6 (1957). (6) Deed, D. W., Shutz, P. W., Drew, T. B., Zbid., 39, 766-74 (1947). (7) First, M. W., Moschella, R., Silverman, L., Berly, E., Zbid., 43, 1363-70 (1951). (8) Forbes, R. J., "Short History of the Art of Distillation from the Beginning Up to the Death of Celier-Blumenthal," p. 285, E. J. Brill, Leiden, Holland, 1948. (9) Gerster, J. A., Bonnet, W. E., Hess, I., Trans. Am. Inst. Chem. Engr. 47, 532-37 (1951). (10) Gerster, J. A., Colburn, A. P., Bonnet, W. E., Carmody, T. W., Chem. Eng. Progr. 45, 716-24 (1949). (11) Jones, J. B., Pyle, C., Chem. Eng. Progr. 51, 424-8 (1955). (12) Kraft, W. W., IND.ENG. CHEM.40, 807-19 (1948). (13) Lewis, W. K., Jr., Zbid., 28, 399 (1936). (14) Markels, Michael, Jr., U. S. Patent 2,863,808 (Dec. 8, 1958). (15) Scofield, R . C., Chem. Eng. Progr. 46, 405-14 (1950); U. S. Patent 2,470,652 (May 17, 1949). (16) Sherwood, T. K., Pigford, R. L., "Absorption and Extraction," 2nd ed., McGraw-Hill, New York, 1952. RECEIVED for review March 17, 1958 ACCEPTED October 3, 1958 From a dissertation submitted in partial fulfillment of the degree of Doctor of Engineering Science at Columbia University. Work done with the help of the Du Pont Fellowship. Complete data and calculations are available through University Microfilms, Inc., Ann Arbor, Mich.
