I
R. E. MANNING' and M. R. CANNON Department of Chemical Engineering, The Pennsylvania State University, Unirenity Park, Pa.
Distillation.Improvement
by Control.of Phase
Channeling in Packed Columns
Interdistributor screens provide on inexpensive way of improving efficiencies of packed towers
t
tillate compoSition of 90 mole $ ' 6, but that 1% of the liquid is channeling. This is eauivalent m &ration with a
moving fluid is given by the folloiving equation :
P,
=
lJ7
2gc
where P, = velocity pressure C' = fluid velocity p = fluid density fc = gravitational constant There is a fundamental and unique difference betlveen static pressure and velocity pressure. That difference is that static pressure is equal in all directions and velocity pressure exists only in the direction of flow. The proper application of Equation 1 leads to a simple equation for predicting the maximum rate a t Tvhich a distillation column can be operated under vacuum ( 6 ) , if its flood rate is knopvn a t atmospheric pressure. Other interesting applications have been briefly mentioned (7). Pitot tubes also operate on this principle. If a certain path through a packed bed offers less resistance to vapor flow than another path of equal cross-sectional area, a higher vapor velocity will occur in the path of lower resistance. The velocity pressure of the vapor in this path will tend to push the liquid flow to neighboring paths and thus the thrust of the rising vapor tends to cause liquid channeling because no packed bed is completely uniform and consequently differences in resistance to Ao\v \vi11 exist in various parts of the bed. Ryan (7) measured liquid channeling in a packed column, 1 foot in diameter. a t atmospheric pressure and Cusack (2) made similar measurements for vacuum operation. In their ivork it \vas shown that when rhe liquid is uniformly distributed over the rop of a bed of packing in the presence of a rising \*apor there is serious liquid channeling after the bed has been penetrated to a depth of 3 feet. Consequently, it was decided that an effort should be made to redistribute both phases at 3-foot intervals. The use of interdistributor plates \vas found to be effective in a column 1 foot in diameter. Interdistributor plates require flanges and the authors decided to try a simple device such as the interdistributor screens sho\vn in Figure 2. The theory of their operation is that vapor and liquid channeling will be broken up a t these screens and that the points of the packing filled cones \rill serve as liquid drip points and thus a t each such screen the two phases will receive some redistribution. The interdistributor screen is not difficult to fabricate. I t is made with an outside diameter slightly smaller than the inside diameter of the column in which it is to be used; the open cones are filled with packing and the screens are placed within the packed bed a t %foot intervals.
348
A
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51 6"
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1
7 0.9" I SECTION A-A Figure 2. Interdistributor screen for 5.75-inch diameter column made from 10mesh screen, wire diameter 0.025-inch
4.0-
-
l
l
l
l
~
l
l
i
l
~
I
1
l 1
l1
e
-~
-
P A C K E D HEIGHT 108 INCHES
2.8 z - z -
0
0.5
VAPOR Figure 3. screens
INDUSTRIAL AND ENGINEERING CHEMISTRY
VELOCITY
IN
1.0 FEET
PER
1.5 SECOND
Effect of vapor velocity on column efficiency without interdistributor
Column diameter 5.75 inches; packing 0.24 X 0 24 inch protruded t y p e 3 1 6 stainless steel
Test Equipment
The effectiveness of the interdistributor screens was measured in a column 5.75 inches in inside diameter (3). The test mixture employed was n-heptane and methylcyclohexane, which has a relative volatility of 1.070. All of the runs reported in this paper ‘were performed a t total reflux, although Manning’s thesis (5) from which these data were taken contains extensive data on finite reflux also. Individual points were obtained by preflooding for a few minutes and then dropping the vapor velocity to the test condition. For all tests a liquid distributor plate was used a t the top of the column ( 3 ) . The packing support consisted of a flat plate with 13 vapor riser tubes, 1 inch in diameter and 2 inches tall; one in the center, four on a radius of 1.16 inches, and eight on a radius of 2.17 inches. Each of these was capped with a conical screen. Twelve liquid return tubes of 3/&nch inside diameter and extending 0.5 inch below the plate were used.
A flat screen should never be used as a packing support for packings 0.24 inch or smaller. Such a flat screen packing support will cause a column to flood a t less than 50% of normal flood velocity and decrease the number of theoretical plates ( 5 ) .
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i
1
1
I
1
1
-+
2.8
a I- 2. w I
C *K E /t
HEIGHT
WITH SCREENS
2.0
VAPOR
VELOCITY
plate a t the top but no interdistributor screens or plates. Figure 4 shows the effect of putting one screen a t the mid-point of a 6-foot bed, and Figure 5 shows that in tall beds the effect is significantly good. Thus a 12-foot-tall column has an H.E.T.P. of 2.2 to 2.8 inches over a wide vapor velocity range with screens every
1
1
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2.2
INTERDISTRIBUTOR
ONE
I-A
SCREEN
L
w I
1.4
0 VAPOR
12 FEET
IN FEET PER
SECOND
3 feet, while a %foot bed has an H.E.T.P. of 2.4 to 3.6 inches over the same vapor velocity range in the absence of the interdistributor screens. Further work is being carried on to determine the effect of other designs of interdistributor screens and optimum spacing, together with their usefulness in larger diameter columns. Interesting data have been recently reported by Kiguchi ( 4 ) on a continuous operating column containing the same type of protruded stainless steel packing used in this work literature Cited
z n
I
tNS
NO INTERDISTRIBUTOR
i”-1i f
i
FEET
Column diameter, 5.75 inches
Figure 3 shows the efficiency of 0.24 X 0.24 inch protruded type 316 stainless steel packing in a column 5.75 inches in inside diameter a t packed heights of 3, 6, and 9 feet with a liquid distributor
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9
Figure 5. Effect of vapor velocity on column efficiency with and without interdistributor screens with 0.24 X 0.24 inch protruded stainless steel packing
Results
3.0-
PACKED HEIGHT
1
1
l
1
0.5 VELOCITY
1
1
1
1
I.o IN FEET
1
1
1
1
l
1
1.5 PER SECOND
Figure 4. Effect of vapor velocity on column efficiency with and without interdistributor screens at a packed height of 6 feet with 0.24 X 0.24 inch protruded stainless steel packing Column diameter, 5.75 inches
(1) Cannon, M. R., Oil Gas J. 51, 268-71, 318-20 (1952). (2) Cusack, J. H., .“Vacuum Fractionation,” Ph.D. thesis, Pennsylvania State Univ., University Park, Pa., June 1954. (3) Heinlein, A. C., Manning, R. E., Cannon, M. R., Chem. Eng. Progr. 47,344-6 (1951). (4) Kiguchi, S. T., TND. ENG. CHEM.46, 1363 (1954). ( 5 ) Manning, R. E., “Study of Factors Affecting Distillation Efficiency,” Ph.D. thesis, Pennsylvania StaG Univ., University Park, Pa., 1954. (6) Peters, M. S., Cannon, M. R., IND. IND.CHEM.44, 1452 (1952). ( 7 ) Ryan, J. F., “Factors Affecting Fractionation in Packed Columns,” Ph.D. thesis, Pennsylvania State Univ., University Park, Pa., 1953. RECEIVED for review April 6, 1956 ACCEPTEDAugust 20, 1956 Division of Petroleum Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, April 1955. VOL. 49, NO. 3
MARCH 1957
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