Spiral Screen Packing for Efficient Laboratory Fractionating Columns HERBERT S. LECKY
411)
RAY \IOND 13. EWELL, Purdue Unitersity, Lafayette, Ind.
0
these split ivashers were spot-welded together t o form a loiig continuous spiral ( F , Figure 1). After some calculations concerning screen area, gradient of the spiral, and length of vapor path, it was decided to make the center hole about one third the diameter of the entire washer (except in the 1.25-cm., 0.5-inch, size), and to hare the turns spaced to give an average vapor path six to ten times the vertical height. Packings Lvere made in three sizes: 1.25em. (0.50-inch) outside diameter with 0.31-rm. (0.125-inch) hole, 1.875-cm. (0.75-inch) outside diameter with 0.625-cm. (0.25-inch) hole, and 3.75-cm. (1.5-inch) outside diameter with 1.25-cm. (0.5-inch) hole. A sealed glass tube or nickel rod was inserted in the center hole of the spiral to provide support and to prevent the vapors' rising directly up the center of the packing. The packing was then inserted into a close-fitting outer tube. This was easily done by screning a spiral of heavy wire into the spiral packing, inserting this assembly, and then unscrewing the wirfspiral. This ensures uniform spacing of the turns of the packing, iyhich is essential for most efficient operation. The packing was placed in a double heated jacket of a commoii type and fitted with a conventional type of stillhead for testing purposes. This and subsequent packings were tested at total reflux using the n-heptane-methylcyclohexane and benzeneethylene chloride binary test mixtures. A discussion of the use of these mixtures is given at the end of this paper.
K E of the principal rebearch tools in university and in-
dustrial chemical laboratories is the laboratory fractionating column, particularlv the packed column type. A number of types of packing are in common use, including chain packing, Raschig rings, Fenske single-turn helices of glass or metal, and Stedman conical wire gauze packing. All these and other, are described by V a r d (7'). The dtedman packing has been discussed in more detail by Bragg (2). who showed it to he the most efficipnt packing described u p t o that time.
The first trials gave a low efficiency (KO. 1, Table I), but it was found that the liquid was leaking between the packing and the outer glass tube a t some points. I n order to remedy this the whole glass tube was heated to softness and collapsed onto the packing. This improved the efficiency somewhat (Xo. 2, Table I), but there was still considerable leakage through the holes of the screen adjacent to the outer glass tube, apparently caused by the liquid's wetting the glass. This was remedied by crimping a narrow strip of metal foil around the outside edge of the spiral, sealing the rows of holes adjacent to t'he glass, and then collapsing the glass tube (No. 3, Table I). This increased the efficiency greatly (see Figure 2 ) . 34
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FIGURE 1. SPECIMENS OF SPIRAL SCREEX PACKIKG
26 24
22
! I & 20
.i. 0.75-inch cupped spiral B . Short section of A extended t o showed cupped edges C D ,E . 0 %inch cupped spirals F: Short section of 1.5-inch flat spiral with foil on edge.
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The efficiency of fractionating columns is determined largely b y the amount and thoroughness of contact between the descending liquid and the ascending vapor. Toward this end the idea of a continuous flat spiral of metal gauze enclosed between two concentric glass tubes was conceived. This type of packing should provide a long path for the liquid and vapor t o travel, liquid-vapor contact on both sides of the gauze spiral, small holdup, small pressure drop, and a large free space, making possible a large throughput.
I4O
100
200
?03
400
500
REFLUX RATE- ML PER HR
FIGURE 2. PERFORMASCE OF PACKING A . 0.75-inch flat spiral packing (No. 3, Table I) B. 0.75-inch Stedman packing
Figure 2 also shows a comparison with the Stedman packing using the data given by Bragg ( 2 ) . The reflux rates given by Bragg were multiplied by 39/45 to correct the two packings to the same cross-sectional area. The section used for the tests in Figure 2 was only 20 cm. (8 inches) long, but further
Packing of this type ~ a constructed 5 by punching washers from 60 X 60 or 80 X 80 mesh nickel gauze (the former was used for the most part, but there seemed to be little difference between the tTvo sizes). Earh n-ashrr way then cut along a radius and 54-4
and the collapsed tube so fragile (owing to strain) that an easier method of construction was sought. The next type of construction tried was that of a cupped spiral, the edges of the spiral being turned u p a t a 45' angle by stamping the washers in a die before spot-welding them. This construction caused a close fit with the glass because of the springiness of the gauze, and also the liquid tended to run down and over the gauze rather than through it, as i t did when the gauze met the glass at right angles. This type of coristruction proved to be the best com-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 12, KO. 9
butane) distilled under 143 mm. of mercury pressure in a 25plate section of 1.25-cm. (0.5-inch) cupped spiral packing. The corners are sharper and the flat sections more horizontal than in the distillation curves obtained from the usual types of analytical columns. The authors have settled on the cupped spiral design in 1.875-cm. (0.75-inch) and 1.25-cm. (0.5-inch) sizes for routine use in the research work in the Chemistry Department a t Purdue University. These have the turns spaced 40 to 70 per foot, since height efficiency is usually not a problem. The method of construction has been standardized, using steel punches and dies, so that the packing can be punched, spotwelded, and assembled by undergraduate student helpers.
22
40
60 80 ML. DISTILLED
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FIGURE 6. SAMPLE DISTILLATION CURVE CCRY PRESSURE
AT
120
140
143-MM. RIER-
Using 25-plate section of 0.5-inch cupped spiral packing. First plateau iu 2.2-dichlorobutsnr. second is 2,~-dichl~irobiitatir, third is 1,5-dlchlorobutane, fourth is 1,3-dichlorobutane. Sninple also contained mrnr 2chlorobutune. polychlorides, and possibly other impurities. Intrrinitt e n t %nil. take-off. Individual points not shown on account of density, b u t all la>. exactly on curve as drawn.
Binary Test Mixtures Ward (7‘) has recently given a discussion of binary test mixtures. The authors have used principally the system n-heptane-methylcyclohexane. The data of Beatty and Calingaert (1) shorn that this is an ideal solution with a volatility ratio of 1.07, which would not change appreciably in the small temperature intervals involved. A simple combination of Raoult’s and Dalton’s laws gives the ideal solution relation R Y1 = 1 ( R - 1)
-+ x 1
where XI and Yr are mole fractions of heptane in liquid and total-reflux intermittent take-off head, shown in Figure 7, was vapor in equilibrium, respectively, and R is the volatility ratio or relative volatility-i. e., the ratio of vapor pressures used. All the reflux passes through the U-tube and the bore of the stopcock, so that there is no dead liquid. The column of pure heptane and pure methylcyclohexane-at the paris allonred to come to equilibrium and the contents of the ticular distillation temperature. Compositions calculated U-tube (2 ml.) are drained. This can be done every 10 to 30 using this relation were combined with an equation for the minutes, depending on the mixture, the total number of theoindex of refraction as a function of composition retical plates, the diameter of the column, and the part of the N? = 1.4232 - 0.0410 XI 0.00555 X t distillation curve concerned. derived from the data of Bromiley and Quiggle (3) to give Figure 5 is a distillation curve of a mixture of approxithe curve in Figure 8. The vertical scale has no particular mately equal volumes of carbon tetrachloride and cyclohexane zero point, each division merely representing two theoretical distilled a t a pressure of 740 mm. of mercury in a SO-plate section plates; Any section of this curve of 1.875-cm: (0 75-inch) cupped should give the same number of spiral. These liquids boil 4” theoretical plates for a given apart, while the difference becolumn. tween the two nearly flat porThe system benzeneethylene tions of the curve i s about 3.6”. chloride was used in the earlier Calculations for this mixture uspart of this work. There are seving the vapor pressure data of eral sources of data on which the Scatchard, Wood, and Mochel(6) use of this mixture may be based, sliow that 58 theoretical plates and there is some divergence would be needed to distill a 95 among them. Following K a r d per cent carbon tetrachloride the authors have used the vapor vapor from a liquid containing pressure data of Rosanoff and 5 per cent of cnrbon tetrachloride. Easley ( 4 ) . “Apparent” volaAn ideal solution boiling a t these tility ratios calculated by the temperatures would require 49 relation given above do not agree theoretical plates for the same with the “ideal” volatility ratios separation. From these data i t given by Smith and Rlatheson (6) would seem that the column was from the vapor pressures of the operating at nearly its full effipure components. This is no reciency of 50 plates. The transiflection on either set of data, but tion portion of the curve between it simply means that benzenethe two horizontal sections is ethylene chloride is not an ideal about 20 ml., which is just equal solution, although it approaches to the holdup of the column a t i t fairly closely. The data of the reflux rate used, 50 plates X Rosanoff and Easley are in suf0.4 ml. per plate. ficiently close agreement with Figure 6 is a distillation curve U those of Zawidski (8) to justify of a mixture of dichlorobutanes, more confidence in the use of the 2-chlorobutane, polychlorides, FIGERE 7. DISTILLATION HEAD apparent volatility ratios for and othcr impurities (obtained With small constant holdup for operation at t o t a l reflux with calculating the number of the by chlorination of 2-chlorointermittent take-off
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ANALYTICAL EDITION
SEPTEMBER 15, 1940
547
INDEX OF REFRACTION
FIGURE 8. CURVEOF INDEX OF REFRACTION (at 20' C.) us. THEORETICAL PLATES Binary teat mixture n-heptane-methylc).clohexane. Vprticnl scale has n o particular zero point, each division merely representing t w o theoretical plates.
TABLE 11. COMPARATIVE DATAON BENZENE-ETHYLENE CLiLOltIDE BINARY TESTh l I X T U K E S Xcms
Apparent R ( R and E)
0 0 10 0 20 0 30 0 40 0.50 0 BO 0 70 0 80 0.90
1.15 1.14 1.13 1.12 1.11 1.11 1.11 1.12 1.15
1.00 Separation Interval, XcsHa 0.10-0 30 0.31)-0.50 0.50-0 70
0.70-0.90
Total, 0.10-0.90
...
...
Ideal R
(Sand M )
retical plates required to effect certain separations in this system. The ideal solution method used by Bragg leads to slightly higher theoretical plate values than the method using the equilibrium data of Rosanoff and Easley.
1.108
Acknowledgments
i:io9
The authors wish to express their thanks to P. E. Hardy, T. M. Burton, H. B. Hnss, R. F. Newton, K. S. Warren, and W. E. Fish for suggestions and assistance rendered. One of the authors (H. S. L.) wishes to acknowledge the financial assistance of the Purdue Research Foundation in the form of a research fellowship.
1:iio
1,'iii i.'iiz i',ii3
Number of Theoretical Plates Required Using R (Sand M ) Using R ( R and E) 11 13 8 7 8.5 8 12.5 12 42 38
theoretical plates required to effect a given separation than in the use of Smith and Matheson's ideal volatility ratios and the assumption of a n ideal solution. Bragg (2) has used this latter method. Table I1 shows a comparison of the v d a tility ratios from the two sources and of the number of theo-
Literature Cited (1) Beatty and Calingaert, ISD.E m . CREM.,26, 504, 904 (1934). (2) Bragg, Ibid., Anal. Ed., 11, 253 (1939). (3) Bromiley and Quigyle, IND.ENG.Camr., 25, 1136 (1933). (4) Rosanoff and Easley, J . Am. Chem. SOC.,31, 953 (1909). (5) Scatchard, Wood. and Mochel, Ihid., 61, 3206 (1939). (6) Smith and Matheson, J . Research Natl. Bur. Standards, 20, 641 (1938). (7) Ward, U. S. Bur. Mines, Tech. Paper 600 (1939). (8) Zawidski, 2. physik. Chem., 35, 129 (1900). PRESENTED before the DiviRion of Petroleum Chemistry a t t h e 99th Meeting of t h e American Chemical Society, Cincinnati, Ohio.
