Semicommercial Columns for Close Fractionation Triangular Pyramid-Type §tedman Packing L. B. BRAGG Foster Wheeler Corporation, Carteret, N. J.
A
TRIANGULAR pyramid-type packing was developed by D. F. Stedman in the laboratories of the National Research Council of Canada and patented in the United States ( I I ) , Canada (8), and elsewhere. Stedman has reported the efficiencies obtainable with triangular pyramidtype packing as determined by testing with mixtures of nhexane and cyclohexane (8, 7 , 9, IO). This packing is being further developed in the Foster Wheeler Corporation research laboratory. Three sizes of round triangular pyramidtype packing have been produced, the operating characteristics of which are presented below.
of the upper sheet. The construction is shown in Figure 1. The three sizes of packing on which tests are reported had diameters of 5.283, 7.823, and 15.443 cm. (2.080, 3.080, and 6.080 inches) and were called Nos. 128, 107, and 115, respectively. The packing is customarily fabricated from stainless-steel wire cloth, 15.75 X 23.6 mesh per cm. (40 X 60 mesh per inch), using wire 0.0229 cm. (0.009 inch) in diameter. It may, however, be made of any other material that can be drawn into wires and woven into cloth of the proper mesh, in accordance with the particular use to which the packing is to be put. Other mesh sizes will be satisfactory, as long as the Description of Packing packing has sufficient mechanical strength and the surface tension of the liquid is great enough to seal the openings of The packing is made of wire cloth which has been punched, the mesh, but certain sizes of mesh and wire diameter are blanked, and embossed to form circular sheets having a regubest ( 7 ) . lar pattern of raised triangular pyramids and an upturned lip around the circumference. The pyramids are located on In operation the liquid flows along the wire cloth and seals 3/s-inch equilateral-triangular centers. The sheets are also the openings of the mesh. ' The liquid flows down the sides perforated with 3/le-inch-diameter holes located on s/g-inch of the triangular pyramids, dividing into three portions as i t e q u i l a t e r a l - t ridoes so. Then it angular centers beflows along the tween the pyramesh at the base of mids. the pyramids until The pattern on it reaches t h e the circular sheets center of the sois placed so that called valley a t the by rotating adj u n c t i o n of t h e jacent sheets 120" e d g e s of t h r e e with respect to pyramids. Here it each other, the soflows through the called valleys at cloth to the peak of the junctions of the pyramid of the the bases of the next lower sheet of pyramids of the c l o t h w h i c h is upper sheet come welded to the u p immediatelyabove per sheet at this the apexes of the point. The liquid pyramids of the flows continuously lower sheet. The in this manner sheets are welded from the top to the together a t these bottom of a section points of contact. of packing, repeatThis placing of adedly dividing into jacent sheets three streams and causes the holes of then combining the lower sheet, three streams so as which serve as to give an effective v a p o r passagedistribution and ways, to be located m i x i n g of t h e below the apexes liquid. The tendof the pyramids FIGURE 1. THREESIZESOF STEDMAN ency of this action PACKING, TRIANGULAR PYRAMID TYPE 279
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 33, No. 2
the screen, and the irregularities of the woven screen cause turbulence in the liquid stream, so that the liquid is well exposed t o the vapors and the equivalent of an equilibrium contact is quickly attained.
AATER IN rTOP REFLUX SAMPLE
1 DISTRIBUTOR VAPOR JACKET PACKED COLUMN WITH APOR JACKET> PACKED WITH 3 6 INCHES O F TRIANGULAR PYRAMID TYPE STEDMAN PACKING
CONDENSATE LINE FOR VAPOR JACKET
Testing of Packing The packings were tested in steel columns equipped with vapor jackets. The jackets were connected to the still a t the lower end and through a needle valve t o the condenser a t the upper end. The packings fitted in the columns tightly enough so that they would not drop down, but the columns were provided with thin support plates with a circular cutout, slightly smaller than the bore of the column, to be certain that the packings would not slip down in the columns. I n all cases the column height was 122 cm. (48 inches) and the height of the packed section was 91.5 cm. (36 inches). The reflux to each column was given an initial partial distribution by simple distributors consisting of small pans; the bottoms of these pans had a number of 1.6-mm. (0.0625-inch) perforations through which passed several 12.7-mm. (0.50-inch) diameter vapor risers. The reflux was fed into this pan from the reflux collector beneath the condenser. The packed columns were connected t o the rest of the apparatus as shown in Figure 2. Binary mixtures of benzene-ethylene dichloride were used in all tests. The free volume inside the shell of the reboiler amounted to approximately 4 gallons, and 2-3 gallons of test mixture were used. All tests were made a t atmospheric pressure and with total reflux. The reflux rate was determined by turning the three-way plug cock below the column, so as to interrupt the flow of reflux back to the still, and noting the time required to collect a definite amount of reflux in the graduated glass bottle. The holdup of the column was determined by noting the liquid level in the reboiler with a calibrated level gage glass before starting the run, while the liquid was a t the boiling temperature, and then noting the level after equilibrium conditions had been attained. The initial level in the reboiler was checked at the end of the run after the column had been drained. The difference in levels represented the amount of the binary mixture in the column, condenser, and reflux collector both as liquid and
FIGTJRE 2. DIAGRAM OF COLUMN (r
1.40
40
1.30
36
1.20
36
1.10
34
1.00
32
030
18
0.20
16
0.10
14
0.00
12
W
to flow liquid toward the walls of the column is counterbalanced by the tendency of the upturned lip to flow liquid from the walls toward the center. The vapor flows upward through the vapor openings into the space below a pyramid. Here the vapor stream divides into three parts, and then each part flows upward through vapor openings in the next higher sheet of cloth together with vapor that in each case came from two other vapor openings. This flow is repeated until the vapor leaves the packing a t the top of the column, causing quite effective mixing of the vapor. There is a continual mixing and separation of both liquid and vapor so that channeling is not possible. The column in which the packing is inserted must fit the packing closely, so that any openings between the packing and the column are sealed by the liquid to prevent by-passing of the vapor. The underside of the liquid stream is exposed to the vapor as it flows along the mesh of
REFLUX RATE - GALS PER HOUR
FIGURE 3.
CHARACTERISTICS O F
5.583-CM, (2.080-Ih-CH) DIAMETER PACKING
February, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
vapor. This figure is, of course, not entirely correct as a representation of the holdup of the column. This method of determination, combined with. the difficulty of reading accurately a fluctuating level, gave somewhat erratic results at times, as the data indicate. The error is on the high side, and the figures obtained were considered sufficiently accurate for present purposes. The flood points were determined by increasing the distillation rates by substantially equal steps and noting the point a t which the bottom reflux rate decreased suddenlv. The points of complete flooding were obtiined by increasing the distillation rate until the pressure drop through the columns, fluctuations in the pressure drop readings, and a constantly decreasing level of the liquid in the reboiler indicated that the column was completely flooded. As a result of this method of determining the point of complete flooding, and since the level of the liquid in the column could not be observed, it was possible to have a condition where the reflux head of the column was partially flooded; this condition apparently existed a t the point of so-called complete flooding with the No. 128 packing. As was the case with the conical-type packing (9,S), it was found necessary to wet the packing thoroughly before the full efficiency could be developed. The packing was wet by running the column a t a rate above its capacity until it became completely flooded. The rate of distillation was next reduced until the flood subsided and was then set a t the point desired for the test being run The efficiency varied considerably with the rate of distillation and showed the same general shape of curve as the 25-mm. conicaltype Stedman packing (3). Below the flood point the efficiency was better, the lower the rate of distillation, until a peak was reached a t a rate below which the efficiency decreased rapidly. Above the flood point the efficiency improved with increasing rates of distillation, until a second peak was reached a t a rate above which the efficiency decreased rapidly. As with the conical-type packing (8, S), it was found necessary to continue the distillation for fairly long periods in order to reach a n equilibrium condition. Consequently, a procedure was adopted whereby a t the lower rates of distillation the columns were operated for 8 hours before the reflux samples were taken. At the higher rates of distillation the columns were operated 5 hours before the samples were,taken; then it was possible to change the reflux rateatolower a condition which was expected to give efficiency and to take a second set of samples 3 hours later. Occasional checks showed that this method of procedure made i t unnecessary to obtain check samples. The reflux samples removed from above and below the packing were analyzed by refractive index, and the number of
281
TABLE I. EFFICIENCY TESTS NO.
Reflux Rate Gal./Hr. 0.5 0.7 1.0 1.5 2.0 3.0 4.0 6.0 7.5 9.0
-nm-
Above packing Packing 1.4553 1.4551 1.4561 1.4549 1.4546 1.4560 1.4557 1.4647 1,4544 1.4549
Theoretical Total P1,ates Below Pressure Drop Holdup, In H. E. T. P., packina Mm. HR (In. H h Gal. Column Cm. (In.) 128, 5.283-Cm. (2.080-In.) Diameter: E m .~ p t y Column 1.7 54(21) 1.4538 1.4538 1.4550 1.0 91 (36) 1,4540 0.8 114(45) 1.4539 0.3i"(O.17) 0.7 131 (51) 0 . 3 7 (0.20 1.4554 1.4551 0.47 (0.251 1.4543 0.4 228 90) 1.4541 0 7 i ' ' 0' 40) 0.4 228 i90) 1.4545 0:93 {0:50)
.....
....
..... .....
.... .... ....
....
.
:::: .... .... ....
Packing 128, Packed Column 0.4 0.5 0.6 0.75 1.1 1.5 1.5 2.0 2.0 2.5 2.5
3.0
3.25 3.5 4.0 4.5 4.75 5.0 2.75 5.0
1.4676 1.4952 1.4924 1.4941 1.4909 1.4809 1.4828 1.4809 1.4789 1.4756 1.4710 1.4728 1.4657 1.4771 1.4715 1.4690 1.4710 1.4680
.... ....
1.4523 1.4517 1.4490 1.4516 1.4526 1.4498 1.4502 1.4508 1.4500 1.4504 1.4489 1.4500 1.4480 1,4510 1.4490 1.4480 1.4503
1.4605
. . ..
....
0.37 0 47 0156 0.93 2.3 4.7 4.7 9.3 9.3 15 15 22 24 26 37 49 58 66 18 65
0.08 0.05 0.08 0.05 0.11 0.16 0.11 0.24 0.18 0.28 0.26 0.37 0.40 0.32 0.40 0.45 0.53 1.06 0.21 1.06
(0.20) (0.25 (0.301 (0.50) (1.25) (2.5) (2.5) (5.0) 5.0)
k:!]
(11.5) (20.0)
I: ?:!
(35.0) (9.5) (35.0)
13.9 6.58(2.59) 38.3 2 . 4 9 0 94) 39.0 2.34!0:92) 36.7 2.49(0.98) 30.9 2.96(1.17) 26.9 3.40 1 3 4 ) 27.5 3.32[1:31) 25.1 3.64(1.44) 25.2 3.63(1.43) 22.2 4.12(1.62) 22.3 4.10(1.61) 21.0 4.35(1.71) 21.1 4.34 1 70) 22.2 4.12{1:62) 22.4 4.08 (1.61) 23.5 3.89 (1.53) 19.5 4.69 (1.85) 16.6 5.51,(2.17) Flood point Completely flooded
Packing 107, 7.823-Cm. (3.080-In.) Diameter: E m p t y Column 0.5 2.0 4.0
7.0 10.0 0.5 0.65 0.75 1.0 1.5 2.0 2.5 3.0 3.0 4.0 4.5 5.0 6.0 6.8 7.5 8.0 8.5 9.0 9.5
10.0 6.8 10.2
1.4544 1.4545 1.4538 1.4533 1.4526
1.4530 0.08 (0.04) 1.4532 1.4530 O.lQ'
