Solvent Extraction with liquid Carbon Dioxide ALFRED W. FRANCIS Socony-Vocuurn Oil Co., Inc., Poulsboro, N. J.
The unusual miscibility relations and low cost of liquid carbon dioxide merit consideration of means to apply it to solvent extraction of hydrocarbon mixtures Since cosolvents are necessary to make its properties effective, an extensive investigation was made of ternary liquid systems of carbon dioxide, many of which are novel in type. The cosolvents serve to dilute the oil and increase its solubility in carbon dioxide; to provide a consecutive double extraction of oil, first with carbon dioxide, then with the cosolvent; or to provide simultaneous double extraction of oil using three liquid layers. Alternatively, carbon dioxide serves to increase solubility of oil in a conventional solvent; or to recover a nonvolatile solvent from the oil b y re-extraction instead of by distillation. The 132 cosolvents found pertinent to one or more of the processes mentioned are listed.
IQUID carbon dioxide at ordinary temperatures has miscibility relations with hydiocarbons and other liquids which are unusual and in many cases novel in type of diagram (7, 8). Carbon dioxide is incompletely miscible with dicyclic hydrocarbons but mixes with aliphatic and monocyclic hydrocarbons in the same boiling range as the dicyclics. This relation is the reverse of that with almost all other solvents in which the more highly cyclic hydrocarbons are more soluble, Carbon dioxide exhibits a precipitating action similar to but more intense than that of propane in deasphalting operations (1, 8, 16, 17, 18, 89). On the other hand, a t moderate concentrations, up to about 40% by weight, carbon dioxide has a strong homogenizing action, rendering miscible most pairs of partially miscible liquids (8). Because of this unique combination of properties liquid carboii dioxide offers promise as a selective solvent for separating mixtures of hydrocarbons in spite of the high pressure required, nearly 1000 pounds per square inch a t room temperature. It has the advantages of being cheap, noncorrosive, and nontoxic Moreover, it is easy to recover from the hydrocarbons. Buchner (3) stated that no hydrocarbon was known which fails to mix with liquid carbon dioxide. However, he did not test any hydrocarbon above CS. Auerbach ( 2 ) proposed liquid carbon dioxide as a single solvent for refining light lubricating oils. The extract is highly paraffinic and almost colorless. He noted a solubility of about 3% for the oil which he tested. However, Quinn ( 1 9 ) indicated a solubility of only 0.72% lubricating oil in carbon dioxide, and this is more consistent with present results. This solubility is too low for practical operation and cannot be increased by use of higher temperature. In fact over the range 10' to 31" C. the solubility of oil in liquid caibon dioxide decreases slightly bith rising temperature (19).
Apparatus, Materials, and Procedure The observations on liquid carbon dioxide miscibilities and extractions were made in a visual autoclave (4). This is a Jerguson gage of 116-ml. capacity with narrow borosilicate glass windom about 17-mm. thick, front and back. It has been tested to 400 atmospheres. Incandescent lamps are mounted behind the vertical position. Agitation results from rotation end over end within a heat insulated case. The reagents used were mostly from Eastman Kodak Co., first grade, but not further purified except to dehydrate those 230
suspected of containing water. The lubricating oil used in most of the tests had the properties API gravity Density Refraotive index, na$ CST with aniline Pour point, O F. Flash (open cup). F. l7;*a
0
P
KsCbsiiy; cs. at 1000 F. Viscosity, cs. at 210O F. Viscosity index Viscosity gravity constant Color, Lovibond
2 3 . SO 0.910 1.5076 72' C. 20 395 455 28.66 4.51 60 0.871
1s
Binary and ternary miscibilities were observed by charging the liquid reagent or pair of reagents through a small glass funnel, and then carbon dioxide from a steel lecture bottle through a valve and cone joints. The amounts added were estimated by the liquid voIunie changes in combination vith the apparent density of dissolved carbon dioxide. The latter was calculated in typical cMes from observations on densities of mixtures (8). The mutual binary solubilities of carbon dioxide with 261 other substances and the triangular phase diagrams for 464 ternary systems are reported in another paper (8). A few miscibility relations were given also by Buchner (3). Some actual extractions were made. Systems with two or three liquid layers were made up in the visual autoclave including carbon dioxide and the lubricating oil as components. After agitation and settling with the valve down, the layers were withdrawn separately into beakers. The liquids emerged as foams which broke as the carbon dioxide was released. The cosolvent was removed by evaporation or washing, and the lubricating oil fractions were evaluated by refractive index and aniline point. The volumes of oil obtained were usually insufficient for viscosity index observations.
High Pressure One niethod of increasing the solubility of oils in liquid carbon dioxide is by the use of pressure higher than the vapor pressure of carbon dioxide. This is suggmted by the very considerable contraction on mixing of carbon dioxide with other liquids, often amounting to 10 or 15% (8). To test the idea, the visual autoclave wai charged with 25 ml. of n-octadecane and cooled to 5' C., crystallizing the hydrocar-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 2
HYDROCARBON SEPARATIONS
NITROBENZEK
OIL
Figure 1. Effect of nitrobenzene on solubility of lubricating oil in carbon dioxide
Figure 2. Effect of butane or benzaldehyde on solu-
Cosolvents that have similar graphs Benzoyl chloride Carbon disulflde o-Chlorophenol p-Dichlorobenzene a,a-Dichlorotoluene p-Dimethoxybenzene Dimethylaniline Hexyl alcohol Kerosine Methyl salicylate Nitrobenzene 0-Nitrotoluene
Cosolvents that have similar graphs Acetophenone Benzaldehyde Benzene n-Butane 2-Butanone (MEK) n-Butyl ether . Caproic acid Caprylic acid Carbon tetrachloride p-Dioxane Ethyl acetate Ethyl carbonate Methylal Phenyl isocyanide Phosphorus trichloride Propane
bility of lubricating oil in carbon dioxide
bon (m.p. 28’ C.). This hydrocarbon was selected as a typical pure paraffin of moderately high molecular weight. It has a solubility in carbon dioxide which is definite and large enough, 3970, so that the change with pressure could be measured more accurately than that of the oil. Its high freezing point facilitated the development of the requisite high pressure. The autoclave was then filled completely at 5” C. with liquid carbon dioxide a t about 1000 pounds, and allowed to warm to 24’ C. with continued agitation, melting the hydrocarbon. The pressure as measured by a strain gage, increased to 3800 pounds per square inch (260 atmospheres); and the solubility as indicated by the position of the interface, was increased from 3% a t 1000 pounds per square inch to a t least 11%. The change was the more spectacular since the density of the solution was increased enough (because of compression due to expansion of the oil layer) to make the undissolved oil float on the solution. “Cracking” of the valve, releasing a little oil, inverted the layers again and increased the volume of undissolved oil. The method of controlling solubility in carbon dioxide (“low molecular treating agent”) by means of high pressure was patented by Van Dijck ($1). The phenomenon is related to the supposed complete miscibility of carbon dioxide with water a t pressures over 500 atmospheres postulated by Wiebe and Gaddy (dS,Z4). They made several observations on mutual solubility of the two liquids but none on the densities of the solutions. I n this investigation water saturated with liquid carbon dioxide a t 26’ C. (6% carbon dioxide) was found to have a density of 1.016 (8).
Cosolvents More general methods of employing liquid carbon dioxide in solvent extraction of lubricating oil involve cosolvents. These methods take advantage of the homogenizing‘effectsin ternary systems of liquid carbon dioxide (8). Not only does carbon dioxide augment the mutual solubilities of two other liquids, but almost any liquid augments the mutual solubilities of another dissimilar liquid with carbon dioxide. Thus solubility of oil in carbon dioxide can be increased by
Figure 3. Effect of hydrogen sulfide on solubility of lubricating oil in carbon dioxide
Figure 4. Effect of benzonitrile on solubility of lubricating oil in carbon dioxide
Cosolvents that have similar graphs
Cosolvents that have similar graphs
Acetal Acetyl chloride Acrolein Anisole fert-Butyl alcohol n-Butyl oxalate n-Butyraldehyde Chlorobenzene Chloroform Crotonaldehyde Ethyl benzoate Ethyl chloroformate Ethylene bromide Ethylene glycol monobutyl ether Ethyl ether Ethyl formate Ethyl salicylate 1-Heptaldehyde Hydrogen sulfide Isopropyl ether Limonene Mesityl oxide Methyl acetate Methyl benzoate Paraldehyde Propionaldehyde Valeraldehyde
Benzonitrile Camphor Cyclohexanone &P’-Dichloroisopropyl ether Ethyl phenylacetate 2-Octanone p-Oxathiane [Thioxane) 2-Picoline Pyridine Thiophene Tolunitriles [mixed)
Similar results are obtained if the tie lines are nearly parallel to the side line (Figure 3), especially if the binodal curve is shallow (Figure 4). However, such processes depending only on the selectivity of carbon dioxide give only a moderate improvement in the oil. A double extraction with carbon dioxide and another solvent, taking advantage of their opposite selectivities, is more effective. The two extractions may be consecutive, employing a system such as shown in Figure 5 (IS). A composition S separates into a raffinate layer R and an extract layer E. The former contdins
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1955
awmm - -*
many different cosolvents. The latter may be miscible with either carbon dioxide or the oil or both or neither. I n these systems if the tie lines are orientated so that the upper or carbon dioxide layer is close to the top corner in composition (Figure l), the increase in solubility and yield of higher quality oil are slight. If the tie lines are orientated in the opposite direction (Figure 2), the solubility and yield are increased; but the upper carbon dioxide layer also has a higher concentration of cosolvent than the lower layer. Such a mixture of carbon dioxide and benzaldehyde, for example, is unfavorable for extracting lubricating oil, because the opposite selectivities of the two solvents might make the mixture nearly neutral in this quality. Certainly the two would not be cooperative. On the other hand, butane and propane, which are practically neutral in selectivity, do not detract from that of carbon dioxide and so give good results. Lantz (16) has patented this combination. A similar arrangement (6) employing 20% sulfur dioxide and SO% carbon dioxide waa applied to fuel oil.
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ENGlNEERING, DESIGN, A N D PROCESS DEVELOPMENT
OIL
ACETONE
Figure 5. Effect of Chlorex on solubility of lubricating oil in carbon dioxide
Figure 6. Effect of acetone on solubility of lubricating oil in carbon dioxide
Cosolvents that have similar graphs P,P'-DichIoroethyl ether (Chlarex) Diethylene glycol monoethyl ether (Carbitol) Hhoxyethanol (Cellosolve) Ethyl acetoacetate" Ethyl chloroacetatea Ethyl lactate Ethyl oxalate Ethyl phthalate Ethyl succinate 4-Hydroxy-4-methyCZ-pentanone isopropyl alcohol a-Nitrophenol 1-Nitropropane Salicylaldehyde
Cosolvents that have similar graphs Acetic acida Acetic anhydride' Acetone Acetonitrile Chloroacetonea P-Chloroethyl acetatea Dimethyl formamidea Ethyl alcohol Ethylene diformate' Ethyl maleatea Ethyl sulfatea Furfurala 2,5-Hexanedione' Methanol 0-MethoxyethanoP Methyl formate Methyl sulfatea Nitroethanea Nitromethonea Sulfur dioxidea Triacetina
Graphs of these systems have twin density lines, connecting compositions of liquids in equilibrium with equal dentities (6).
low quality oil. The extract layer on release of the carbon dioxide (to F ) gives two new layers. These are raffinate layer D containing high quality oil, which has been refined by both solvents, and extract layer C, which is recycled. The position of D between C and R' on the base line of Figure 5 indicates an intermediate concentration of Chlorex in the phase but does not indicate intermediate quality of oil. Typical results with @@'dichloroethyl ether (Chlorex) are as follows (one stage):
Oil charge Ra5nate (from RO Extract-extract (from C) Extract-raffinate (from D)
Refractive Index, n zg
Bniline Point,
1.5076 1.5228 1.5028 1.4790
72 35 73 89
0
co,
T. E.G.
OIL
Figure 7. Effect of aniline on solubility of lubricating oil in carbon dioxide
Figure 8. Effect of triethylene glycol on solubility of lubricating oil in carbon dioxide
Cosolvents that have similar graphs Aniline 0 - Anisidine Benzoic anhydride Benzyl alcohol Castor oil m-Chloroaniline p-Chlorophenol Cinnamaldehyde Hydrocinnamaldehyde Methyl phthalate o-Nitrobiphenyl o-Nitrochlorobenzene Phenylacetonitrile Phenylethanol Phthalyl chloride Pinacol Tefrahydrafurfuryl alcohol a-Toluidine
Cosolvents that have similar graphs Aldol Chloroacetic acid P-Chloroethanol a-Chloropropionlc acid Cinnamyl alcohol 2,4-Dinitrochlarobenzene Dipropylene glycol Furfuryl alcohol P-Hydroxyethyl acetate Maleic anhydride o-Nitroanisole Phenylethanolamine Triethylene glycol
c.
The last product has a viscosity index of about 99, an improvement of 39 units over that of the charge. It is not necessary that the two binodal curves be separate. Systenis with merged curves (Figure 6) give similar results ( 1 1 ) . I n a system illustrated in Figure 7 each layer, E and R, in the first extraction (with carbon dioxide) separates into two layers on release of carbon dioxide. Its extract (from E) is more paraffinic giving the higher of the two binodal curves, while the raffinate (from R ) gives the lower curve. The final results are in order of decreasing quality (viscosity index) an extract-raffinate Er, a raffinate-raffinate Rr, an extract-extract Ee, and a raffinateextract Re. The fourth fraction is discarded, and the second and third are mixed and recycled. The double extraction can also be simultaneous. For this arrangement three liquid layers are required. This combination is more readily adapted to continuous operation (9). Graphs for such a system resemble Figure 8. The top carbon dioxide layer is removed with its content of high quality oil. The bottom triethylene glycol layer is removed with low quality oil; while the middle or oil layer remains and is replenished with feed stock.
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The carbon dioxide is introduced a t the bottom and the triethylene glycol a t the top, so that they flow countercurrent to each other. For this arrangement it is obviouely necessary that the cosolvent have a low miscibility with liquid carbon dioxide. Each solvent dissolves a larger fraction of the oil than it would if used alone because of the partial elimination from the oil of the constituents least soluble in that solvent (but most soluble in the other solvent). Results of such a simultaneous double extraction are illustrated in Table I by extraction of a synthetic mixture of aromatic hydrocarbons of different types in comparison with extractions with the solvents separately (single stage in each case).
Another process employing three liquid phases for simultaneous double extraction has been patented recently (16). Liquid carbon dioxide also has applications in solvent extraction not involving its own selectivity for hydrocarbon type. The solvent power of a conventional solvent like furfural may be
Table 1. Extractions of Aromatic Hydrocarbon Mixture with Carbon Dioxide and with Triethylene Glycol (66,7% a-methylnaphthalene, 33.3 % di-sec-butylbenxene by volume) Extraction 1 2 3 4 Solvent Cog TEG TEG Simultaneous COz and TEG 2.13 2.0 2.0 2.75 1.25 Vol./vol. hydrocarbon Temperature, ' C. 10 20-5 20-5 19 Pressure, atm. 45 1 1 I31 Yo CIIHIOin COY extract" 59 .. .. 47 % CuHm in TEG extract" .. 73 76 67,89 b % CIIHIOin raffinatea 75 54 57 G3
...
..
a Analysis of hydrocarbon product by refractive index. b Glycol extract separated in two layers on release of, carbon dioxide. 89% a-methylnaphthalene was the hydrocarbon composition still dissolved in glycol.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 2
HYDROCARBON SEPARATIONS
NITROBENZENE
PARAFFINIC O I L
Figure 9. Effect of furfural on solubility of lubricating oil in carbon dioxide
Figure 10. Effect of nitrobenzene on solubility of paraffinic oil in carbon dioxide
Cosolvents that have graphs Acetic acid' Acetic anhydrideQ Acetone Acetonitrile Chloroacetone' 6-Chloroethyl acetalea Dimethyl formamide' Ethyl alcohal Ethylene diformate" Ethyl maleatea Ethyl sulfate" Furfural" 2,5-HexanedioneQ Methanol @-Mathoxyethanola Methyl formate Methyl sulfatea Nitroethanea Nitromethanea Sulfur dioxidea TriacetinD
Cosolvenfs that have similar graphs &P'-Dichloroethyl ether (Chlorex) Diethylene glycol monoethyl ether (Carbitol) Ethoxyethanol (Cellosolve) Ethyl acetoacetatea Ethyl chloroacetatea Ethyl lactate Ethyl oxalate Ethyl phthalate Ethyl succinate 4-Hydroxy-4-methyl-2-pentanone (diacetone alcohol) Isopropyl alcohol o-Nitrophenol 1 -Nitropropone Salicylaldehyde
similar
(8).
The cosolvents studied in this investigation that are pertinent to the processes described are listed under the appropriate figures. Chemicals in each group do not have identical graphs, which are presented in greater detail in another paper (8). Moreover, the classification in groups depends to some extent upon the oil. The difference between Figures 1 and 10 is apparent; and benzonitrile, for example, is listed in Group 4, because it is miscible with the oil described. With a heavier oil like bright stock or a more paraffinic one, with which it is not miscible, benzonitrile shows two separate binodal curves like Figure 5. Liferature Cited
a Graphs of these systems have twin density lines, connecting compositions of liquids in equilibrium with equal densities (6).
undesirably low for a lubricating oil stock a t ordinary temperatures-e.g., 3%. Although the mutual solubilities of these two liquids are too low to permit homogenization by liquid carbon dioxide, the latter does increase the solubility of oil in the solvent to about 15% (8, 12) (Figure 9, point E). Saturation with carbon dioxide is nearly equivalent to an increase in temperature of about 80" C . On release of the carbon dioxide from the extract layer E most of the extracted oil is thrown out of solution ( E ' F ) and can be discarded. The remaining dilute furfural solution F can be used again without distillation, thus saving on an expensive step in that process. The dashed line TD is a twin density line indicating that T and D have the same densities (6).
Another use for liquid carbon dioxide is in the separation of the primary solvent by re-extraction instead of by distillation. In one patent (14) propylene is re-extracted by carbon dioxide from silver nitrate solution, which has been used to separate it from paraffin mixtures, This obviates the necessity of heating the concentrated silver nitrate solution. The sensitivity of the latter to impurities or metals is greatly increased by higher temperatures.
February 1955
In lubricating oil extraction carbon dioxide permits the use of a nonvolatile or high boiling solvent, provided it is soluble in liquid carbon dioxide (10). The operation is particularly applicable to the raffinate and avoids long time heating of it. For the extract layer it may be advantageous to distill a portion of the solvent before treating with carbon dioxide. Nitrobenzene is well adapted to recovery by this system (Figure lo), applied to a paraffinic oil with which the solvent is not miscible. Liquid carbon dioxide has relatively little application to lighter hydrocarbon products like gasoline and kerosine, which mix with it. Sullivan (20) employed a mixture of carbon dioxide and sulfur dioxide a t about - 4 0 " C. to get a high antiknock gasoline. I n the present investigation several ternary systems were observed with carbon dioxide, gasoline hydrocarbons, and cosolvents; but the variations with different types of pure hydrocarbons in this boiling range were too slight to offer much promise
(1) Andrews, C. E., and Fenske, M. R., U. S. Patent 2,346,639 (April 18, 1944). (2) Auerbach, E. B., Brit. Patents 277,946 (Sept. 25, 1926); 285,064 (Feb. 12, 1927); Can. Patent 285,782 (Dec. 25, 1928); U.S. Patent 1,805,751 (May 19, 1931). (3) Buchner, E. H., Z.physik. Chem., 54, 665 (1906). (4) Caldwell, W. F., IND.ENG.CHEM.,38, 572 (1946). (5) Edeleanu, G.m.b.H., Ger. Patent 546,123 (Jan. 26, 1928). (6) Francis, A. W.,
[email protected].,45, 2789 (1953). (7) Francis, A. W., J. Am. Chem. SOC.,76, 393 (1954). (8) Francis, A. W., J . Phys. Chem., 58, 1099 (1954). (9) Francis, A. W., U.S. Patent 2,463,482 (March 1, 1949). (10) Zbid., 2,631,966 (March 17, 1963). (1 1) Ibid., 2,632,030. (12) Ibid., 2,646,387 (July 21, 1953). (13) Ibid., 2,698,276 (Dec. 28, 1954). (14) Ibid., 2,698,277-8. (15) Francis, A. W., and Johnson, G. C., Zbid., 2,663,670 (Dec. 22, 1953). (16) Lantz, V., Zbid., 2,188,051 (Jan. 23, 1940). (17) Milmore, O., Ibid., 2,130,147 (Sept. 13, 1938); 2,166,503 (July 18, 1939). (18) Pilat, S., and Godlewicz, M., Ibid., 2,188,013 (Jan. 23, 1940); 2,315,131 (March 30, 1943). (19) Quinn, E. L., IND.ENG.CHEM.,20, 735 (1928); Quinn, E. L., and Jones, C. L., "Carbon Dioxide," pp. 109-10, Reinhold, New York, 1936. (20) Sullivan, F. W., Jr., U. S. Patent 2,034,495 (March 17, 1936). (21) Van Dijck, W. J. D., Ibid., 2,281,865 (May 5, 1942). (22) Webb, W. A,, Ibid., 2,246,227 (June 17, 1941). (23) Wiebe, R., Chem. Rev., 29, 475 (1941). (24) Wiebe, R., and Gaddy, V. L., J . Am. Chem. SOC.,62, 815 (1940). RECEIVED for review June 1, 1954.
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ACCEPTED September 15, 1954.
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