LLOYD J. SULLIVAN, THOMAS C. RUPPEL, and CHARLES
B.
WILLINGHAM
Mellon Institute, Pittsburgh 13, Pa.
Packed Thermal Diffusion Columns Effect of Changes in Annular Spacing and Packing Density
A 50 volume
% test mixture of the cis- and trans-isomers of
decahydronapthalene in packed thermal diffusion columns was used to study the effect of varying annular spacing and packing density for the batch separation of organic liquids in concentric tube columns packed with glass wool. There is a complex relation between percentage separation and annular spacing but a direct relation between percentage separation and packing density.
BATCH
apparatus for thermal diffusion are of two fundamental types. The first type and the oldest historically is the single-stage unit (4, 5), operated with a horizontal or vertical temperature gradient but without convection in the apparatus. The second type is the ClusiusDickel parallel wall unit (7, 3), usually with concentric walls, operated vertically with a horizontal temperature gradient. This unit is multistage with a countercurrent process in operation. Furry and Jones (2) have given an approximate theory for the separation of liquids in the Clusius-Dickel column. From this theory it was shown that the logarithm of the separation factor is inversely proportional to the fourth power of the annular spacing, where the separation factor for a binary pair is equal to the product of
work was to determine experimentally the effect on separation of changes in the annular spacing as a function of packing density. In order to make such an investigation, a test material must be used which
(Mole fraction of A/rnole fraction B),,, times (mole fraction of B/mole fraction A)bottom
d
In a previous report (6) the effective use of wider annular spacings by rotation of the inner wall of a concentric tube unit or by suitably packing the annulus was demonstrated. The packing material used was glass wool and the annular spacing 0.06 inch. This column would give essentially no separation as an open annulus apparatus. The purpose of this
1]0
separates normally by thermal diffusion but which under the operating conditions fails to separate completely at steady state. In this laboratory, it had been observed that mixtures of the cis- and transisomers of decahydronaphthalene ful-
1
70
O.lOg/ml.
-
.-5 g50-
n v) c
5 40E 30 0.05g/ml.
2ot /P
?
D
Figure 1.
for
INDUSTRIAL AND ENGINEERING CHEMISTRY
Separation in 0.03-inch annulus column at different packing densities
cis- and trans-decahydronaphthalene mixture
filled these requirements. A 50 volume ye mixture of cis- and trans-decahydronaphthalene was used as the principal test mixture in this work.
80
-
70 -
Apparatus and Experimental Procedure
The packed thermal diffusion columns used were made from stainless steel tubing and corresponded in details of construction with the one previously described (6),except that the ports were so spaced that 10% by volume fractions could be collected from the two top ports (fractions 1 and 2) and from the two bottom ports (fractions 6 and 7). The intermediate fractions were each 20% by volume. Variation in annular spacing was obtained by using inner tubes of different diameters; each tube required a new set of packing glands. The columns used in this study had a fractionating section of 60 inches. The inside diameter of the outer tube, the hot wall, was 1.37 inches. The outside diameters of the inner tubes, the cold wall, were 1.31, 1.25, and 1.I 3 inches, respectively. The tubes gave three columns of 0.03-, 0.06-, and 0.125-inch annular spacings with volumes of 100, 200, and 400 ml., respectively. To determine, with glass wool packing, the effect on separation as a function of packing density, steady state separations were obtained at three packing densities in the 0.03- and 0.06-inch and for four packing densities in the 0.125inch annulus columns. For comparison purposes a similar experimental curve was obtained for a 0.012-inch open annulus column. Individual runs were made on the cis- and trans-decahydronaphthalene test mixture for different lengths of time and the end fractions (1 0% by volume of charge) analyzed to determine the rate of approach to steady state. For comparison, the effect of packing density on separation a t constant time was studied also for the easily separable mixtures of n-hexadecane-decahydronaphthalene and n-hexadecane-methylnaphthalene in the same column. The mixture of isomeric methylnaphthalenes, approximately 20YOofthe pisomer, was used as received. The n-hexadecane and decahydronaphthalene were passed through silica gel until repeated processing gave constant refractive index values. The decahydronaphthalene was a 50 volume 70mixture of the cis and trans isomers. All experiments were carried out with a n average hot wall of 100' C. and an average cold wall of 20' C.-or a mean temperature difference of 80' C. The Pyrex glass wool, Corning No. 800, consists of fibers lying together in an oriented mat. This mat forms a layer which is rolled u p to form the roll of glass wool. The wool is unrolled to give
60
-
200
100
0
400
300
5
Time in Hours
Figure 2. Separation in 0.06-inch annulus column at different packing densities for cis- and frans-decahydronaphthalene mixture
0
50
100
Is0
200
250 300 Time in Hours
350
400
450
Figure 3. Separation in 0.1 25-inch annulus column at different packing densities for cis- and trans-decahydronaphthalene mixture
Figure A. Effect of packing density on separation for 0.03-,0.06-, and 0.1 25-inch packed thermal diffusion columns for cis- and trans-decahydronaphthalene mixture VOL. 49, NO. 1
JANUARY 1957
111
0
100
150
200
250 300 Time in Hours
350
400
450
After she column is reassembled, the Duco cement and the acetate thread are: removed by washing with acetone. To fill the column all ports are closed ; a container of the teat mixture is attached by neoprene pressure tubing 8.0 the bottom port and a vacuum pump to the top port. The top port i s opened to evacuate the column, then closed and the pump removed. To fill the column the bottom port is opened carefully so that no air is introduced into the column. The bottom port is then closed; the top port is opened and the outer tube brought up to operating temperature. When expansion of the material in the column i s completed, the top port i s closed and the filling is complete. To remove fractions, a sample container is fixed to the second port from the, top and a nitrogen source attached to the top port. The ports are opened and the fraction collected. If the material is too volatile for the use of nitrogen, gravity flow may be used. Using port 3 for the container and port 2 for nitrogen pressure, the second fraction is collected. This process is repeated down the column until all fractions have been collected.
J
Figure 5. Separation of cis- and brans-decahydronaphthalene mixture in openannulus and packed thermal diffusion columns a t nearly equai packing densities
a layer of the proper length. Knowing the weight of this length, the proper weight of glass wool is wrapped onto the inner tube of the apparatus so that all the fibers run with the central axis of the tube. The layers of wool are held to the tube with the seams and the ends coated lightly with Duco cement. For the higher density packings, the glass wool is bound to the tube by a tight winding of fine acetate thread. Since spacers are
used on the inner tube for centering it in the column, the glass wool located over the spacers is removed. The column is reassembled. If, upon reassembly, the packing tends to bunch, the column is repacked. The amount of packing which can be placed into the column is limited by this packing procedure. However, the fact that this procedure is adequate is shown later by the results.
/
0
0.05 0.10 Packing Density g./ml.
0.15
Figure 6. Effect of packing density on separation with 0.1 25-inch annular spacing for cis- and trans-decahydronaphthalene mixture
1 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
Results
In Figure 1 the percentage separation is plotted with respect to time in hours for the 0.03-inch packed column a t packing densities of 0.05, 0.08, and 0.10 gram per ml. The packing density is calculated by dividing the mass of glass wool
/ I
0.05
0
I
0.10
0. 5
Packing Density g./ml Figure 7. Effect of packing density on separation with 0.1 25-inch annular spacing for n-hexadecane and rnethylnaphthalene mixture
~
~
~~~
~
~~
Table 1.
Results for 1 00-Hour Separations of Comparatively Easily Separated Mixtures (Column capacity, 400 ml; annular space, 0.125 inch) Refractive Indices of Fractions Packing Density, G./Ml. 1 2 3 4 5 6 7 Separation, % n-Hexadecane and decahydronaphthalenes Run 1 0.06 1.4449 1.4470 1.4501 1.4530 1.4562 1.4588 1.4612 40 n-Hexadecane and methylnaphthalenes Run 2 0.06 1.4852 1.4949 1.5084 1.5242 1.5413 1.5518 1.5597 42 Run 3 0.09 1.4633 1.4737 1.4936 1.5231 1.5552 1.5702 1.5798 65 Run 4
by the total column volume. The 0.10 gram per ml. packing density is about the maximum that can be obtained with this annular spacing. The lowest percentage separation a t steady state for this annular spacing was given for the lowest packing density while the highest steady state value was given for highest packing density. However, the lowest packing density gave the fastest approach to steady state conditions. I n Figure 2 the percentage separation is again plotted as a function of time in hours for packing densities of 0.05, 0.09, and 0.14 gram per ml. in the 0.06-inch annulus column. The relationships observed for this column are the same as observed for the 0.03-inch annulus column with the exception that a higher maximum packing density and a higher percentage separation could be obtained. I n Figure 3 percentage separation obtained in the 0.125-inch annulus column is plotted against time in hours for packing densities of 0.06, 0.08, 0.09, and 0.12 gram per ml. The same relationships were found as for the two previous columns. However, the 0.12 gram per ml. packing density column, though not a maximum obtainable, seemed to offer such a small additional separation above that obtained for the 0.09 gram per ml. that it was taken as the practical limit of the packing density that would be used. The steady state data from Figures 1, 2, and 3 are used in Figure 4 where percentage separation is plotted with respect to packing density for the three annular spacings. For low packing densities the separation is inverse to annular spacing. However, the rate of change of separation with packing density is greatest for the 0.06-inch column to a packing density of approximately 0.10 gram per ml. where the curve breaks. Thus the separation obtained is a function of both the annular spacing and the packing density. Under the conditions of packing and with the test liquid used in this study the 200ml., 0.06-inch annulus column, a t high packingdensities, is a t least as good as the 100-mk., 0.03-inch annulus column, illustrating that a complex relationship is involved in the effects. I n Figure 5 the percentage separation is plotted with respect to time in hours for the 0.012-inch open annulus column,
0.125
1.4543
1.4661
1.4900
1.5278
and for the 0.03-, 0.06-, and 0.125-inch annulus columns. The volumes of the packed columns as stated previously are 100, 200, and 400 ml., respectively. The volume of an 0.012-inch open annulus column of equivalent diameter would be 45 ml. This figure gives a comparison of the separations obtained with the various columns in this study. For packing densities of 0.10 gram per ml. for the 0.03-inch, and 0.09 gram per ml. for the 0.06- and 0.125-inch annulus columns; the percentage separation is inverse to annular spacing. The 0.012-inch open annulus column gives slightly higher separations than any of the packed columns. However a t higher packing density (Figure 4) the 0.06-inch annulus column gives a greater separation than either the open annulus or the 0.03-inch packed annulus columns. The data for 500 hours for the 0.125inch annulus column (Figure 3) are repeated in Figure 6 in which percentage separation is plotted as a function of packing density. Over the range of packing densities, 0.06 to 0.09 gram per ml., a linear relation occurs. At higher packing weights the linear relation breaks down to produce an S-shaped curve. The results for 100-hour separations of n-hexadecane and decahydronaphthalene and n-hexadecane and methylnaphthalene in the 0.125-inch annulus column at three packing densities are given in Table I. The separations are regular and uniform over the length of the column. The highest degree of separation is given by the more highly packed column. The lowest degree of separation was from the column with the lowest packing density. The separation a t one fixed column packing is equivalent for the two test mixtures tried, showing that their ease of separation is also equivalent. The column with the smallest packing density gave separations of 42 and 40.5% for these test mixtures but, as shown in Figure 4, gave only 10% separation of the cis- and trans-decahydronaphthalenes test mixture, proving that it is a much more difficult mixture to separate and is, in this way, a good test mixture for column evaluation. Plotted in Figure 7 are the percentage separations of n-hexadecane from methylnaphthalene as a function of the packing density. For no packing the percentage separation is zero at this annular spacing.
1.5670
1.5849
1.5932
78
Increasing packing density results in a n increasing separation until at 0.12 gram per ml., 78% separation is obtained. Conclusions
Packing the annulus in a thermal diffusion column offers a method to attain some degree of separation with wide annular spacing. The percentage separation obtained a t steady state is a function of both the annular spacing and packing density. I n a column of fixed annular spacing, percentage separation increases with the packing density. However, the necessary time of operation is also increased. For difficult mixtures there appears to be a n optimum packing from the point of view of necessary operating time. Since there is as yet no adequate means for predicting batch separation of complex mixtures, a preliminary separation is helpful in making a choice of the column to be used. The 0.012-inch, 5- or 6-foot open annulus column with its small volume, 15 to 30 ml., and relatively high efficiency is excellent for this purpose. From the data obtained, the choice of a larger volume column can be made on the basis of the separation required for the end use of the separated material. High efficiency columns mean a longer steady state time; short time, large volume columns indicate a lower percentage separation. A separation a t 100 hours for the 0.03inch or 0.06-inch annulus columns would give essentially complete separation of the easily separable mixtures. Even the 0.125-inch annulus column, with a packing density of 0.12 gram per ml., gives 80y0separation for a charge of 400 ml. of material. literature Cited
(1) CIusius, K., Dickel, C., Naturwisscnschaften 27, 148, (1939). (21 \ , Furrv. W. H.. Jones. R. C.. Rev. Mbdern Phys.’ 18, No. 2, 151-224 (,1046 ’i. Jones, A. L., Milb CHE Ludw - I
sc
(1871)):
Sullivan, L. J., Ruppel, T. C., Willingham. C. B.. IND. ENG. CHEM. 47y208 (1955). ’ RECEIVED for review May 28, 1956 ACCEPTED July 5, 1956 VOL. 49, NO. 1
JANUARY 1958
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