H. S. MYERS C. F. Braun & Co., Alhambra, Calif.
A Versatile Fractionating Column \
Improved operation at a fraction of the cost of the conventional column
A type of column with removable trays has been developed that can readily NEW
be adapted to all kinds of fractionation work. I t can be classified generally as a sieve-tray column without downcomers. Action is similar to that of a conventional perforated tray with downcomers, except that liquid intermittently drips downward through the holes that carry the vapor upward. This principle has been used extensively in large-diameter, commercial columns, but not in smallscale equipment. Equipment
The column is made of glass, and is vacuum-jacketed. The central tube is of precision-bore tubing, 1 inch in inside diameter and 36 inches long. Plates are of 16-gage stainless steel, threaded onto a central rod, and are easily removable (Figure 1). The tray assembly slides into the column and rests on a cone-shaped glass support built into the base of the column. This perforated cone can also serve as a packing support, if it is desired to replace the trays with packing. Tray spacing can be varied by screwing the plates closer together or farther apart on the central rod. O n later models a rodand-spacer arrangement was used instead of the threaded rod. Six different types of trays were tested. Hole diameters ranged from to 1/4 inch, with open areas from 19 to 35%.
tion range was used to compute tray efficiencies. This value has been reasonably well established (2, 3 ) . Analyses were made by refractive index. In addition, vacuum runs were made with the heptane-methylcyclohexane binary at 150 mm. of mercury. A relative volatility of 1.04 was used for the heptane-methylcyclohexane system at 150 mm. of mercury, which is probably conservative. Experimental data measured in a vapor-recirculating still (7) indicate a relative volatility of about 1.03. Vapor pressures, assuming ideal behavior, predict l .021. Hawkins and Brent (6) n-easured a value of 1.05 a t 300 mm. of mercury which, by extrapolation, would suggest about 1.02 at 150 mm. of mercury. Because the tray efficiencies reported here are based on a n alpha of 1.04, they may be somewhat low for the heptane-methylcyclohexane system at 150 mm. Equilibrium data for the system decane-trans-Decalin have been published
(5). The relative volatility a t 50 mm. of mercury is 1.19, and is constant over the entire composition range. Analyses were again made by refractive index. Results
Atmospheric Studies. Figure 2 shows the results for the trays with I / ~ E - , 7/64-, and I/k-inch holes at atmospheric pressure. Straight lines connect the data points for identification purposes only. No attempt has been made to smooth the results. Figure 2, A, also shows ranges of throughputs and efficiencies measured for a I-inch Oldershaw (8) column in the Braun laboratory under similar conditions as for the present tests. The Oldershaw column flooded at a throughput slightly above 2000 ml. of liquid per hour, which agrees with Oldershaw's data (8). The new 1-inch column is operable at three or four times this rate with comparable esciencies.
Procedure
Hole size; per cent open area, and tray spacing are the three major design variables that affect th'e performance of perforated trays without downcomers. All these variables were investigated. The tray-spacing variable was easy to isolate, but it was not always possible to separate the effect of hole diameter from that of open area. All the trays, except those with l/4-inch holes, were made from perforated sheet, and various hole sizes were not available at exactly the same open areas or vice versa. Therefore, the 1/4-inch holes were studied a t three open areas19, 25, and 3l%-and it was assumed that the effect of open area for the small holes and the '/*-inch holes is the same. Most of the runs were made a t atmospheric pressure with heptane-methylcyclohexane test mixture. A relative volatilip of 1.07 over the entire composi-
Figure 1.
The glass column is vacuum-jacketed, with easily removable plates Righf.
in operation at 2-inch tray spacing VOL. 50, NO. 11
NOVEMBER 1958
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7 1 6 4 ' HOLES, 35% OPEN HEPTANE MCH ATMOSPHEHIC PRESSURE
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ATNOSPHER,C PRESSURE
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HEPTANE- MCV e i N O S P H E S l G PRESSURE
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Figure 2. One-inch columns gave very s a t i s f a c t o r y performance at atmospheric pressure
5000 CC
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HOT L O U I O I ' I S
Pressure drop has also been plotted for the 50-mm. runs. The new trays show very low pressure drop, ranging from about 0.25 to 1.0 mm. of mercury per plate. Oldershaw columns could not be compared under vacuum, because they are not operable below about 250 mm. of mercury absolute. Capacity Tests with Water
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TROUGHPUT C C HOT LtOUIO/HR
Collins and Lantz ( 4 )describe a modified Oldershaw column that has 82 holes per tray instead of 42 as used by Oldershaw. The modified column was 28 mm. in diameter and had a maximum throughput of 3800 ml. per hour. This corresponds to about 3000 ml. per hour for a 1-inch column. Tray efficiencies were considerably lower than for the original design, averaging about 60%. Performance of this modified Oldershaw is also indicated on Figure 3, A. Throughputs have been adjusted from the 28-mm. column to a I-inch column by ratioing areas. The new-type trays have a sharply defined lower operating limit, called the load point. Below the load point there is essentially no liquid level on the trays and tray efficiencies are low. As boilup is increased to the load point, a liquid level of about '/2 inch suddenly builds up on the trays, and efficiencies increase accordingly, usually to their maximum value. The upper operating limit is the flood point, where the liquid no longer can
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flow down through the holes and is forced to build u p on the trays and into the condenser. Load points and flood points for all the trays tested at atmospheric pressure are listed in Table I. Vacuum Studies. Results of the vacuum studies are shown in Figure 3 and the load point and flood point data are also summarized in Table I. For heptane-methylcyclohexane at 150 mm. of mercury, tray efficiencies appear to be higher than at atmospheric pressure and may actually be even higher, as the relative volatility used in the computations is probably slightly high. For the decane-Decalin system at 50 mm. of mercury, it was necessary to go to 3-inch tray spacing to get any appreciable range between the load point and the flood point. But efficiencies under these conditions are very high, sometimes greater than 100%. While it is not impossible to exceed 100% tray efficiency, this was somewhat surprising, and the high-efficiency runs were repeatedly checked, each time with similar results.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Oldershaw-type columns with their 0.89-mm. holes do not operate satisfactorily with high surface tension liquids such as water. The first three hole sizes studied in this WOrk3/16-, "32-, and 7/G4-inch-were unsatisfactory for water. For this reason the trays with I/*-inch holes were conceived. They appear to operate very well with pure water, but efficiencies could not be measured. Tray action appeared to be excellent, and throughputs, based on capacity F factor, were about the same as for the hydrocarbon systems. Effecf of Variables Tray Spacing. As expected, increasing tray spacing increased both capacity and efficiency for all trays tested, undoubtedly because of decreased entrainment at the higher spacings. The load point was essentially independent of tray spacing over the range studied, although some of the trays showed a slight increase in load point at the higher spacings. This can also be explained by entrainment. Open Area. The ]/?-inch holes were studied with three, four, and five holes per tray, corresponding to open areas of 19,25, and 31%. Figure 4, A, shows the effect of open area on load point for the
FRACTIONATING COLUMN 90
Table
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Maximum and Minimum Loadings a t Atmospheric Pressure and under Vacuum no Flood Point 2-in. 3-in. z Il/z-in. Load spacing spacing z.70 Point" spacing HEPTANE-METWLCYCLOHEXANE, ATMO~PHERIC PRESSURE l/ls-Inch Holes, 30% Open Area
HOLES,
30%
OPEN
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HEPTtrNE 150mm Hp ABSOLUTE MCH
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Ml./hr. V,, ft./sec.
FP vhc
1400 0.51 0.23 0.95
3500 1.28 0.57 2.37
5500 2.01 0.90 3.72
inch Holes, 23% Open Area Ml./hr. vs,ft./sec. F. h
1800 0.66 0.30 1.58
2600 0.95 0.42 2.30
3500 1.28
7500 I 2.74 1.22 5.07
E
~
50
...
40 0
...
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I000
4wo
1000
inch Holes, 35% Open Area MlJhr. V,, ft./sec. F 8
Fh
3500 1.28 0.57 2.03
8500 3.10 1.39 4.92
1/4-Inch Holes, 19% Open Area, 3 Holes per Tray Ml./hr. V,, ft./sec. F, Fh
2000 0.73 0.33 2.14
2800 1.02 0.46 2.98
l/c-Inch Holes, 25% Open Area, 4 Holes per Tray Ml./hr. Vs, ft./sec. F* Fh
3000 1.10 0.49 2.44
4700 1.72 0.77 3.82
Ilc-Inch Holes, 31 % Open Area, 5 Holes per Ml./hr. Vs, ft./sec.
F, Fh
3800 1.39 0.63 2.49
6000 2.19 0.98 3.93
HEPTANE-METHYLCYCLOHEXANE, 150 MM.Hg ABSOLUTE I / d n c h Holes, 30% Open Area Ml./hr. V,, ft./sec. F a
Fh
8 50 1.44 0.31 1.24
...
...
... ...
0.54 2.18
inch Holes, 35% Open Area Ml./hr. 1500 2.53 V8, ft./sec. 0.54 1.87
F a
Fh
2500 5.91 1.27 4'. 36
5500 9.29 1.99 6.85
1.45 5.80
Fa Fh
600 2.62 0.36 1.40
... ...
... ...
720 3.15 0.43 1.68
... ...
0.69 3.22
F s Fh a
... ...
... ...
... ... ... ...
three I/d-inch trays. Load point is plotted in terms of a hole F factor, Fh = v h dp./sp. gr,llq,. There appears to be a slight increase in load point with increased open area, but the increase is not significant. I t might well be within experimental accuracy, as the load point is
I200
1400
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1900 8.31 1.15 4.44
1930 8.45 1.17 5.40
At 2-inch tray spacing; slightly higher at 3 inches, slightly lower at FA= V h G F h = Vhhl/ps/SP. g!'.liq:
11/% inches.
IO00
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l/d-lnch Holes, 25y0 Open Area Ml./hr. 1150 V,, ft./sec. 5.03
800
I20
DECANE-TRANS-DECALIN, 50 MM. Hg ABSOLUTE l/ls-Inch Holes, 30% Open Area Ml./hr. V,, ft./sec.
600
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THROUGHPUT, C C HOT LIQUIDIHR
Figure 3, One-inch columns under vacuum h a d high tray efficiencies
not always perfectly reproducible. The flood point, however, is definitely influenced by open area (Figure 4, B ) . AS the open area is increased, the flood point increases proportionately. On the other hand, efficiency decreases as open area is increased (Figure 4,C). For this
correlation an average efficiency a t a throughput approximately midway between the load point and the flood point wasused. Hole Size. I t was not always possible to separate the effect of hole size completely from that of open area. For the VOL. 50, NO. 1 1
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HOLE OiAMETER IN 32NDS
Figure 5. size I8
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% OPEN AREA
present comparison, it was assumed that the effect of open area is the same for the small holes as for I/4-inch holes. This may or may not be true, and the hole-size correlations given here should be considered only qualitative. Directionally, they are believed to be correct, but the magnitude of the effects may be in error. Load point increased with increasing hole size (Figure 5, A ) . Higher hole velocities, or actually higher hole F factors, are required to load the trays with the larger holes. The experimental load points have been adjusted to the load points that would be expected at 30% open area, by means of Figure 4, A. Flood point also increases with increasing hole size (Figure 5, @-the larger holes appear to have a higher capacity than the smaller holes. Tray efficiency seems to be affected only slightly by hole size (Figure 5, C). There appears to be a loss in tray efficiency with the larger hole sizes, although at the larger tray spacings the loss is minor. Conclusions
Fractionating columns of the type developed can be readily modified for
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4 Figure 4.
Tray efficiency is only slightly affected by hole Efficiency decreases as open area is increased
nearly any service. Some applications require a large number of trays without much concern for high capacity; here a low open-area tray at close tray spacing is suitable. Conversely, the separation may be simple, with prime interest in high capacity; here a high open-area tray at wide tray spacing is desirable. For pilot-plant work the chief concern is frequently that the column have a constant number of theoretical plates over a considerable throughput range. Again a wide tray spacing is favorable. There might be an application in dirty service, possibly a tendency toward polymer formation. Here the I/d-inch holes would be ideal, and the large holes would also be used for aqueous systems or other high surface tension liquids. Of the six types of trays tested, two are adequate for most applications-the 307, tray with l/le-inch holes, and the 25% tray with I/?-inch holes.
V, = vapor velocity through holes, feet per second Fs = V s d / p ,= capacity factor based on superficial velocity used to correlate flooding Fh = VI,.\/pV/sp, gr.iiq,= capacityfactor based on hole velocity used to correlate loading.
Nomenclature
RECEIVED for review March 24, 1958 ACCEPTED August 4, 1958 Division of Industrial and Engineering Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
p v = vapor density, pounds per cu. foot sp. gr.lLq,= specific gravity of hot liquid V, = superficial vapor velocity, feet per
INDUSTRIAL AND ENGINEERING CHEMISTRY
second
References
(1) Am. Petroleum Inst., Research Project 44, Natl. Bur. Standards, December 1952. (2) Beatty, H. A., Calingaert, G., IND. ENG.CHEM.26, 504 (1934). (3) Bromile E. C., Quiggle, D., Zhid., 25.1136 t933'r. (4) Collins,' F. C., Lantz, v., IND. ENG. CHEM.,ANAL.ED. 18, 673 (1946). (5) Fenske, M. R., Myers, €5. S., Quiggle, D., IND.ENC.CHEM.42, 649 (1950). (6) Hawkins, J. E., Brent. 3.' A,. Zhid.. . 43,2611 (1951). ' (7) Hipkin, H. G., Myers, H. S., Zhid., 46. 2524 11954). (8) Oldershaw, C. F., IND.ENG. CHEM., ANAL.ED. 13,265 (1941).
