56
ANALYTICAL CHEMISTRY
[This point has been established experimentally ( I ) . ] However, satisfactory results also can be obtained b y other methods ( 4 ) . -4similar reversal is apparent in the order in which hydrocarbons are separated using extractive distillation with fluorochemicals as compared with the usual polar substances and the discussion concerning the use of fluorochemicals in azeotropic distillation appliee also t o their use in extractive distillation. T h e manner in which the change in structure of a fluorochemical affects its azeotrope-forming properties remains to be investigated. However, i t is probable t h a t the unique effect of fluorochemicals will be a t a maximum with perfluorinated compounds and will diminish as fluorine atoms in the molecule are replaced by those of other elements, ACKIVOWLEDG.MEIYT
T h e author desires to express his gratitude t o Ned C. Krouskop for supervision of the distillation operations and t o Michael D.
Domenick for assistance in the experimental work. Acknowledgment is made to Frederick D. Rossini for his advice and encouragement. LITERATURE CITED
Cines, AT. R., U. S.Patent 2,692,227 (Oct. 19, 1964). Hildebrand, J. H., and Scott, R, L., “Solubility of Nonelectrolytes,” Reinhold, Sew York, 1950. Mair, B. J., Glasgow, A. R., Jr., and Rossini. F. D., J . Research Xutl. Bur. Standmds 27, 39-63 (1941). Rossini, F. D., Mair, B. J., and Streiff, A. J., ”Hydrocarbons from Petroleum,” Reinhold, Sew York, 1953. RECEIVED for review J u n e 10, 1955. Accepted September 27, 1955. Division of Petroleum Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955. Investigation performed as part of the work of American Petroleum Institute Research Project 6 in the Petroleum Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
Fractionation of Hydrocarbons by Adsorption with Added Components BEVERIDGE 1. MAIR, MONTY J. MONTJAR, and FREDERICK D. ROSSlNl Carnegie lnstitute of Technology, Pittsburgh, Pa.
.4n improved method is described for separating hydrocarbons by adsorption, in which the fractionation takes place in the presence of two added components, one in the adsorbed phase and one in the liquid phase. Results are reported for experiments in which ethylene glycol monomethyl ether, or diethylene glycol monomethyl ether, constitutes the added component in the adsorbed phase, and heptacosafluorotributylamine, or perfluorocyclic ether constitutes the added component in the liquid phase. The method is particularly effective for the separation of branched paraffins from cycloparaffins.
As the fluorochemical passes downward over the pretreated adsorbent, a n interchange of hydrocarbon molecules takes place between the liquid phase composed largely of fluorochemical
165
S
ISCE 1934, the hmerican Petroleum Institute Research
I
Project 6 has used fractionation by regular adsorption extensively in its work on the separation and purification of hydrocarbons. The process is particularly effective for the separation of aromatics from olefins, paraffins, and cycloparaffins, and for the separation of olefins from the other types of hydrocarbons ( 3 , 6). This paper describes a new process for fractionating hydrocarbons by adsorption, in which the fractionation takes place in the presence of two added components, one in the adsorbed phase and one in the liquid phase, The process is an extension of the method of fractionation known as “partition chromatography,” which was first reported by Martin and Synge ( 4 ) and is now used extensively in many fields.
A.
B. C.
D.
E.
F.
G. H. I. J.
K. L.
.If. METHOD
s.
T h e adsorption column is packed with a n adsorbent, which has been pretreated t o contain the substance selected, to constitute the added component in the adsorbed phase. I n the present experiments, either ethylene glycol monomethyl ether (methyl Cellosolve) or diethylene glycol monomethyl ether (methyl Carbitol) was used, although many other polar organic substances are suitable for this purpose. The liquid mixture of hydrocarbons t o be separated is added to the adsorption column, and, after the mixture has completely entered the adsorbent, the material selected t o constitute the added component in the liquid phase is introduced a t the top of the column. I n the present experiments either heptacosafluorotributylamine ( CaF9)J ( Minnesota ?\lining and Manufacturiqg Co., S o . S-43),or perfluoiocyclic ether, C8FI8O(hlinnesota hlining and Manufacturing Co., 30. 0-i5, believed to consist of a fluorinated side chain attitchrd to a ring of five or six atoms, including oxygen) was used.
0. P.
R.
Figure 1. Glass adsorption column 1
Connection to source of inert gas pressure Standard spherical joint (18/9) Reservoir, 40 mm. in inside diameter Heavy ring seal Jacket sealed to rectifying section with inlet and outlet for passage of water Upper part of fractionating section, 30 mm. in inside diameter Three-pronged brace, slide fit and not sealed t o jacket, prong 5 mm. in diameter LIiddle part of fractionating section, 16 mm. in inside diameter Three-pronged brace, slide fit and not sealed to jacket, prong 5 mm. in diameter Lower part of fractionating section, 7 mm. in inside diameter Sintered-glass plate of medium porosity Heavy ring seal Stopcock, 2-mm. bore Jq-ith plug held firmly in place with spring clamp Standard taper ground joint (14135) Tip of column Receiver, graduated to 0.02 ml. in ranges 0 t o 1.2 ml. and 4.5 to 5.5 nil. Stopcock, 2-mm. bore
V O L U M E 2 8 , NO. 1, J A N U A R Y 1 9 5 6
57 eth? 1 alcohol, and the fluorocheniical, C,F,,O. give two phases a t -80" C., a phase largely fluorochemical and a phase largely alcohol plus hi-drocarbon. By washing the alcohol-hydrocarbon phase with water. 2,2,i-triniethylpentane,containing less than 0.2% of fluorochemical, can be recovered. 4PPARATIJS AND PROCEDURE
A. B.
Connection t o source of inert eas Dressure . Stopper Reservoir, 1000-nil. capacity D . Thermometer E . Metal weight. coiled lead sheet I". Glass cloth G. Asbestos 1/16 inch in thickness H. Electric resistance heating wire, asbestos covered I Corrugated sheet asbestos covered with aluniinurn foil. -4Ifol Pretreated adsorbent Glass wool Capillary tube. 2-mm. bore Tygon tubing Condenser C.
*
K L
3c
"7 it
I 90
=-I,
L
I
Figure 2. Glass adsorption column 2
plus some h j diocai bon and the adsorbed phase composed largely of the glycol ether plus some hydrocarbon. The fluorochemical reduces the escaping tendencv of the paraffin from the liquid phase relativelv more than that of the cj-cloparaffin, whereas the glycol ether reduces the escaping tendency of the cycloparaffin from the adsorbed phase more than that of the paraffin. Thus, the two added components enhance each other SO that the paraffinic component concentrates in the liquid phase, whereas the cycloparaffinic component concentrates in the adsorbed phase. As a result, the paraffin hydrocarbon issues from the column in advance of the cycloparaffin hydrocarbon. If the separation is reasonablj complete, a plot of refractive index (or other suitable physical property) of the filtrate with respect t o its volume shows two peaks. The first peak indicates the portion of filtrate containing the paraffinic component, whereas the second peak indicates the portion containing the cycloparaffin component. Various procedures have been used for the recovery of hydroh fluorochemical in the filcarbons fiom theii mixtures ~ i t the trate. \There the hydrocarbon and fluoi ochemical boil sufficiently fai a p t , they may be separated by distillation in a short colunin However, if hydrocarbon and fluorochemical boil relatively close together (within about GO" C.), a n azeotropic mixture is Ilkel) t o be formed ( 1 ) . llixtures of fluorochemicals and hydrocarbons can usually be resolved simply by cooling t o low tempeiatures and separating the two phases which are formed -4t -80" C., with 2,2,4trimethylpentane and the perfluorocyclic ether, the hydrocarbon phase contains abut 0.5% of the fluoi ochemical. Mixtures of 2 2,4-trimethvlpentane,
I t was found that the mixing of the adsorbent and added component was satisfactorily achieved in a Waring Blendor. The procedure for mixing was as follows: One milliliter of the selected solvent vias placed in the blender t o protect its bearings; a convenient quantity (70 grams) of adsorbent was then added; the blender was turned on, and the desired quantity of solvent added in 2- or 3-ml. portions. The amount of solvent added was the maximum volume the adsorbent could hold without becoming lumpy or sticky-that is, with the adsorbent remaining freeflowing. Mixing was continued until all lumps, which were temporarily produced, disappeared. Ordinarily, the time required for this operation was 5 minutes. The procedure causes a small reduction in particle size. I n addition t o its use in preparing an adsorbent for the fractionating experiments, this procedure was also used t o determine the volume of liquid a given weight of adsorbent could take up and still remain free-flowing. This was taken as the volume of liquid required to barely reach the point where the adsorbent changed from a free-flowing to a lumpy or wet state, and could usually be determined t o within 2 nil. (liquid) per 100 grams of adsorbent. The volume of the adsorbed phase per unit mass of adsorbent may also be obtained by determining the gain in weight resulting from the equilibration of a known amount of adsorbent with the saturated vapors of various liquids ( 2 ) or from the difference between the apparent and true specific volumes of the adsorbent. The method used in this investigation is less accurate than the two foregoing methods, but is adequate for the purpose of rapidly screening adsorbents. The two adsorption columns used in the work are shown in Figures 1 and 2. During the conrse of experiments with column 2, a portion of the pretreated adsorbent sometimes broke loose from the walls of the column, rose through the fluorochemical, and floated on its surface. T o avoid this, a layer of glass cloth was placed on top of the adsoi hent, and this, in turn, was held in position with a lead weight. -4part from this one modification, necessitated by the high density of the fluorochemicals, the procedure for packing the columns and conducting the experiments was similar t o that given in previous reports (6) Heptacosafluorotributylamine and perfluorocyclic ether were used as received. The ethylene glycol monomethyl ether was used as received. T h e diethylene glycol monomethyl ether was further purified by distillation. Although a number of adsorbents were tested for capacity, only one adsorbent, silica gel, 100 t o 200 mesh (Davison Chemical Corp.. Grade 70) was used in the present experiments. CAPACITY OF ADSORBENTS
The following results were obtained for the capacity of several adsorbents, in terms of the volume of liquid adsorbed (measured before adsorption) per unit mass of adsorbent: activated alumina (Aluminum Co. of Xmerica: Grade F-20) 0.23 ml. per gram; silicic acid (Nallinckrodt Chemical Co., chromatographic grade) 0.6 nil. per gram: silica gel (Davison Chemical Corp., Grade 922) through 200 mesh, 0.4 ml. per gram; silica gel (Davison Chemical Corp., Grade 70) 100 t o 200 mesh, first lot, 0.9 ml. per gram, second lot, 0.7 nil. per gram. EXPERIMESTS WITH ADDED COMPOSENT IN ADSORBED PHASE ONLY
I n this section results are given of experiments performed using an added component in the adsorbed phase only. I n this case the hydrocarbon mixture alone is allowed t o filter through the pretreated adsorbent containing the alcohol-ether compound. The
58
ANALYTICAL CHEMISTRY
flow of the mixture of hydrocarbons is continued until material of the original composition issues as filtrate, so that all of the fractionation occurs in the material preceding this. These experiments were performed a t room temperature in column 1, packed with 155 grams of silica gel (Davison Chemical Corp., Grade 70), 100 to 200 mesh, plus 139 ml. of added component, either diethylene glycol monomethyl ether or ethylene glycol monomethyl ether. The results are given in Figures 3 t o 6. T h e refractive indices plotted in these figures are those of the filtrate after washing with water in order t o remove very small amounts of the added component. Figure 3 shows the results of an experiment with an equivolume mixture of n-heptane and methylcyclohexane using diethylene glycol monomethyl ether as the added component. Three additional experiments, not shown, gave essentially identical results. Figure 4 shows the results of a n experiment with an equivolume mixture of n-heptane and methylcyclohexane using ethylene glycol monomethyl ether as the added component.
k---
Figure 5 shows the results of an experiment with an equivolume mixture of 2,4dimethylpentane and cyclohexane using diethylene glycol monomethyl ether as the added component. Figure
1
,=--ORIGINAL
j
MIXTURE
-
-
,--ORIGINAL
MIXTURE
~
-
I
-1
I
c
J
1 0
Yn-HEPTANE 10
VOLUME
23 IN MlLLlLlTE?S
_ &
Figure 3. Fractionation of mixture of n-heptane and methylcyclohexane 2
Scale of ordinates gives refractive index, and scale of sbacissas gives volume of hydrocarbon part of filtrate
-io
'I"'
1.370
VOLUME
IN MILLILITERS
Figure 6. Fractionation of three mixtures of n-hexane and cyclohexane
? 1.400
See legend to Figure 3. Curves A , B , and C refer t o solutions containing 25, 50, and 76 volume % n-hexane, respectively
1.390
W
501305 2'
VOLUME IN MILLILITERS
2
0
IC0
I
1
ZOO
303
i
4w
V O L U M E IN WILL'LITERS
Figure 4. Fractionation of mixture of n-heptane and methylcyclohexane
Figure 7. Fractionation of mixture of n-hexane and cyclohexane
See legend to Figure 3
See legend t o Figure 8
59
V O L U M E 28, NO. 1 , J A N U A R Y 1956 6 shows the results of three experiments with mixtures of nhexane and cyclohexane, using diethylene glycol monomethyl ether as the added component. The curves A , B , and C refer to solutions containing 25, 50, and 75 volume % n-hexane, respectively. From Figures 3 to 6 it is apparent that significant separations were obtained, with the first portion of the filtrate consisting of material greatly enriched in the paraffinic component. In addition to the preceding experiments, one experiment was performed in column 1, using 167 grams of silicic acid (Mallinckrodt Chemical Co., chromatographic grade) pretreated with 100 ml. of diethylene glycol monomethyl ether. The separation waR not as good as with the silica gel. EXPERIMENTS WITH TWO ADDED COMPONENTS, ONE Ih ADSORBED PHASE 4 N D ONE IN LIQUID PHASE
In this section results are given of experiments in which small quantities (8 to 14 ml.) of binary mixtures of hydrocarbons were added to a column containing the adsorbent (silica gel, Grade 70) "loaded" with the added polar organic compound. These hydrocarbons, which were eluted with a fluorochemical, gave results as shown in Figures 7 t o 14.
r L
- C-A
-a-
was recovered from portions A and B, and substantially pure cyclohexane from portion C. Figure 8 shows the results of two experiments with an equivolume mixture of 2,4dimethylpentane plus cyclohexane. The first experiment, shown in the upper part of the figure, was performed with 8 ml. of the mixture in column 1 at 57" C., using 155 grams of silica gel pretreated with 139 ml. of diethylene glycol monomethyl ether. The hydrocarbon material was eluted with heptacosafluorotributylamine. The second experiment, shown in the lower part of the figure, vias performed in exactly the same manner without repacking the column. In both experiments substantially pure 2,4dimethylpentane was recovered by distillation from portions A and B and substantially pure cyclohexane from portions C and D. The results shorn- that it i p not necessary to repack the column after each experiment. Figures 9 and 10 give the results of an experiment with an equivolume mixture of 2,2,4trimethylpentane and methylcyclohexane. The experiment was performed with 14 ml. of the mixture in column 2 a t 50" C., using 480 grams of silica gel pretreated with 336 ml. of diethylene glycol monomethyl ether. The hydrocarbon material was eluted with the perfluorocyclic ether Portions A , B, and C were extracted separately with ethyl alcohol a t -80" C. t o recover the hydrocarbon material. Figure 10 shows the excellent results obtained. Figures 11 and 12 give the results of an experiment with 14 ml. of an equivolume mixture of n-heptane and methylcyclohexane. The experiment was performed at 50' C. after the ex-
D-
t
...
-A-i-By
u-
rC---tDI
I
3c1 0
200
IC0 VCJME
300
i
1
402
N MlLLI..ITERS
Figure 8. Fractionation of mixture of 2,4dimethylpentane and cyclohexane Scale of ordinates gives refractive index and scale of abscissas gives volume of filtrate. F denotes refractive index of fluorochemical. I gives results of first experiment, I1 gives results of a second experiment performed without repacking column
I-
I
0
5
IO
I5
VOLUME IN MILLILITERS
Figure 10. Fractionation of mixture of 2,2,4-trimethylpentane and methylcyclohexane
1
See legend to Figure 3. Results are from same experiment H E for Figure 9 I2750 L
15d
I
200
G 300
400 U
V'3LUME IN MILLILITERS
Figure 9. Fractionation of mixture of 2,2,4-trimethylpentane and methylcyclohexane
n
I
Legend same as for Figure 8
Figure i shows the results of an experiment with 8 ml. of an equivolume mixture of n-hexane plus cyclohexane. The experiment was performed in column 1 a t 57' C., using 155 grams of silica gel pretreated with 139 ml. of diethylene glycol monomethyl ether. The hydrocarbon material was eluted with heptacosafluorotributylamine. Portions 8,B , and C were distilled separately t o recover the hydrocarbon. Substantially pure n-hexane
2 1.2751----w
v
i
1
200
I
I
300
I
I
400
I
VOLUME IN MILLILITERS
Figure 11. Fractionation of mixture of n-heptane and methylcyclohexane Legend sitme as for Figure 8
500
ANALYTICAL CHEMISTRY
60 periment shown in Figure 9, without repacking the column. The hydrocarbon material was eluted with the perfluorocyclic ether. Portions -4, B, C, D, and E were extracted separately with ethyl alcohol a t -80" C. to recover the hydrocarbon material. Figure 12 shows the excellent results obtained. Figures 13 and 14 give the results of a n experiment \Tith an equivolume mixture of 2,2,5-trimethylhexane and n-propylcyclohexane. The experiment was performed with 14 ml. of the mixture in column 2 at 67' C. using 492 grams of silica gel pretreated with 336 ml. of diethylene glycol monomethyl ether. The hydrocarbon material was eluted with the perfluorocyclic ether. Portions A , B , C, D , E, F , and G were extracted separately with ethyl alcohol at -80" C. t o recover the hydrocarbon material. Figure 14 shows the excellent separation obtained. DISCUSSION OF RESULTS
The amount of separation which can be obtained depends (other things being equal) on the amount of interchange between the phases, and this depends on the volume of the phases. Thus, t o keep the size of the apparatus a t a minimum, it is desirable to select a n adsorbent for which the volume of the adsorbed phase per unit volume of adsorbent is as large a8 possible. I t was for this reason, t h a t silica gel, 100 t o 200 mesh (Davison Chemical Corp., Grade 70), was used for the experiments shown in Figures 3 t o 14, inclusive.
The experiments given in Figures 3 t o 6 with the added component in the adsorbed phase only, indicate that significant separations of paraffin from cycloparaffin hydrocarbons can be obtained using either diethylene glycol monomethyl ether or ethylene glycol monomethyl ether. I n all cases, the paraffin hydrocarbon is concentrated in the first portion of the filtrate. This is the reverse of what frequently happens in regular adsorption; for example, with an equivolume mixture of cyclohexane and n-hexane the initial portion of the filtrate is enriched in cyclohexane ( 5 ) . The experiments displayed in Figures 7 to 14 show that almost complete separations of paraffins from cycloparaffins are obtained by adsorption n-ith two added components, one in the adsorbed phase and one in the liquid phase. K i t h a greater ratio of mass of adsorbent to volume of charge, it is expected that quantitative separation.c can he achieved.
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,
1
