I n d . Eng. Chem. Res. 1988,27, 1222-1228
1222
K, and 45 min. Dithionate ion which becomes COD source in the wastewater was not detected in the extraction solutions by precipitation-spectrometry. Registry No. V, 7440-62-2; H2S03,7782-99-2.
Literature Cited Baird, S. S. Talanta 1961, 7, 237. Burriesci, N. J. Chem. SOC.,Faraday Trans. 1 1984,80, 1777. Gmelin Handbook; Verlag Chemie: Weinheim, 1968, Vanadium 48, A(2), p580. Okuwaki, A.; Chida, T. Nippon Kagaku Kaishi 1980, 1230. Ono, T.; Hoshizawa, K.; Suzuki, J.; Ishihara, Y. Kuryoku Genshiryoku Hatsuden 1973,24, 1349. Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon: New York, 1966; p 234.
Rokukawa, N.; Kato, S. Abstracts of Papers, 1987 Spring Meeting of the Mineral and Mining Institution of Japan, Tokyo, p 127. Sarma, P. L. Anal. Chem. 1964, 36, 1037. Sato, T.; Aita, K. Sulfuric Acid Znd. 1976, 29, 69. Sato, T.; Okabe, T. Nippon Kagaku Kaishi 1977, 1124. Sato, T.; Okabe, T. Bull. Chem. SOC.Jpn. 1983,56, 3511. Sato, T.; Aita, K.; Komatsubara, T.; Okabe, T. Nippon Kagaku Kaishi 1978, 1221. Tsukagoshi, K. Abstracts of Papers, 1986 Fall Meeting of the Mineral and Mining Institution of Japan, Sapporo, Q-5, p 20. Whigham, W. Chem. Eng. 1965, 72, 64. Yatabe, T. Denryoku Kenkyusho Gijutsu Daiichi Kenkyusho Hokoku 1972, 71558. Received for review June 9, 1987 Revised manuscript received January 5, 1988 Accepted March 8, 1988
Waste Lubricating Oil Rerefining by Extraction-Flocculation. 1. A Scientific Basis To Design Efficient Solvents M. Alves dos Reis* and M. Silva Jer6nimo Faculdade de Engenharia, Departamento de Engenharia &mica, Portugal
Rua dos Bragas, 4099 Porto Codex,
Waste motor oils may be rerefined by treatment with a solvent that dissolves the base oil and flocculates some of the additives and impurities. This paper compares the performance of ketones and alcohols that are miscible with base oils a t room temperature. The results indicate that the flocculating action of polar solvents in waste oils is, basically, an antisolvent effect exerted on some nonpolar macromolecules. In this context, it has been shown that the difference IS, - S,l between the solubility parameters of the solvent and of a typical polyolefin (polyisobutylene) may be correlated with sludge removal from waste oils and thus be used as a preliminary criterion to select the components of composite solvents. In some cases the polar solvent induces the formation of an electrically stabilized dispersion. The addition of potassium hydroxide in alcoholic solution easily destabilizes the dispersion and increases sludge removal from waste oils. 1. Introduction
Two fundamental reasons explain the interest in the recycling of waste lubricating oils: (a) the elimination of a pollution source, and (b) the need to conserve crude oil reserves. Waste oil is, in fact, an important cause of water pollution if it is discharged to the environment and may be a cause of air pollution when it is used as a fuel. Many technological processes have been proposed to recover base oil from waste lubricating oils, but only a few have been tested industrially. The acid-clay process is still the most commonly used. In this process, the oil is flash distilled at about 150 " C and 1atm to remove water and light hydrocarbons and is then treated with 5-10% by volume concentrated sulfuric acid. After 24-72 h of reaction time, a sludge containing the sulfuric acid and the majority of the additives and other impurities is removed from the bottom of the reactor, and the oil is decanted and finished by adsorption with activated clays. In some plants, this oil is fractionated into several base oil stocks by vacuum distillation. This technology is very attractive economically, but the acid sludge is a source of pollution that causes more environmental problems than the waste oil itself. No environmentally acceptable application has been found for this sludge, which is very difficult to incinerate and impossible to mix with other fuels. An additional problem is the
* To whom
~
correspondence should be addressed.
0888-5885/88/2627-1222$01.50/0
growing difficulty of separating this sludge from waste oils by the sulfuric acid treatment, as a result of the current trend to increase the consumption of highly additive oils. In fact, this treatment is ineffective to segregate the additives and other impurities from these oils. Among the alternative processes proposed in recent years, we have given special attention to those in which the acid treatment step is replaced by a solvent extraction operation. In this process, an organic sludge containing the additives and impurities is flocculated and separated by sedimentation or centrifugation. Our interest was based on the fact that this technology seems to overcome the main problem of the acid treatment, since the organic sludge produced, after solvent recovery, may be used as an asphalt component or may be mixed with liquid fuels and burned. The best use for this sludge is, according to a patent of the authors' (1982a,b), to be an offset ink component. Some polar solvents have been proposed for this operation, namely l-butanol by Brownawell and Renard (1972); butanone by Jordan and Mc Donald (1973);a solution of l-butanol, 2-propanol, and butanone by Whisman et al. (1978a,b); and a solution of n-hexane and 2-propanol containing 3 g/L of potassium hydroxide by Reis (1982). Apart from the extensive work done by Whisman and co-workers (1974) at the Bartlesville Energy Research Center, work from which one US. patent (Whisman et al., 197813) resulted, no systematic study has been published on the action of organic solvents on waste oil. This paper 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1223 intends to be a contribution to this study. It explores the scientific basis of an explanation of the action of polar solvents in segregating some waste oil components. The principles of the developed theory make it possible to formulate new efficient and cheap solvents. 2. Definition of Extraction-Flocculation Solvent
Waste motor oils are composed of (a) unchanged base oil molecules that one intends to recover; (b) oxidized base oil molecules; and (c) polymers, such as polyolefins and polymethacrylates, generally introduced in motor oils as viscosity index improvers, pour point depressants, and dispersants; (d) other additives such as dispersants, detergents, antifoaming and extreme pressure agents, and others, including a great variety of chemical products such as, for example, succinimides, sulfonates, phosphonates, dialkyldithiophosphates of calcium, zinc, and barium; phosphites; amines; phenols; silicones; carboxylic acid salts; and many others; (e) water, originating from fuel combustion in the engine and accidental contamination by rain; (f) light hydrocarbons from incompletely burned gasoline and diesel oil; (g) carbonaceous particles formed by partial coking of fuels, graphitic particles from graphitic oils, and metallic particles produced by motor wear. The bulk of these particles are kept in stable dispersion by the dispersant additives. The water and light hydrocarbons are easily removed by flash distillation and separated by continuous decantation in a florentine tank. After this operation, one addresses the removal of the additives and dispersed particles. If a solvent is to be designed to recover the base oil, separating it from the additives and particulate matter, the solvent must have the following properties: (1)it must be miscible with the base oil contained in the waste oil being processed; and (2) when mixed with the waste oil, it must reject from the solution the additives and the dispersed particles (or part of them), allowing their aggregation to particle sizes big enough to separate from the liquid by sedimentation. Of course, for industrial use, other properties are important: stability, ease of recovery, and low price. A liquid meeting requirements 1and 2 will be called in this paper an extraction-flocculation solvent. Butanone, 1-butanol, and their solutions are extraction-flocculation solvents. As shown by Reis (1982),a large number of other pure organic liquids and their multicomponent solutions exhibit those properties and have potential utility for separating the base oil from the additives and other undesirable impurities in waste oils. In this study, it was also shown why and how mixtures of hydrocarbons with alcohols or ketones having less than four carbon atoms can be formulated to be very efficient extraction-flocculation solvents. In order to determine what kinds of organic liquids are the most suitable to include in the composition of a good extraction-flocculation solvent, it is necessary to understand the action on waste oils of typical groups of pure organic liquids. 3. One-Component Solvents Hydrocarbons. When mixed with waste oils, the liquid hydrocarbons at room temperature keep stable the solution of macromolecular and other additives as well as the dispersion of carbonaceous and other particles. No destabilization is observed after many days of gravity settling or equivalent centrifugation time, no particles are seen by direct observation, and no sludge settles at the bottom of the test tubes. It must be remarked that the macromol-
ecules of polymeric additives and the black dispersed particles are submicrometer in size, smaller than 0.1 pm. Flocculation is necessary if such components are to be separated by gravity settling, and this does not occur when any waste oil is mixed with most of hydrocarbons at room temperature, as, for example, n-pentane, n-hexane, isoctane, cyclopentane, cyclohexane, benzene, and toluene. The same is observed for complex hydrocarbon mixtures such as gasolines and kerosene. In this study, many tests were performed on a medium waste oil that we shall refer as M.W.0, obtained by mixing 20 2-L samples collected from 20 different 200-L drums at 20 different garages or service stations. Each drum was full, and its contents were homogenized before collecting the sample. This oil is, probably, representative of the global waste oil that would be collected by an industrial rerefining plant in Portugal. In what follows, all compositions are based on weight. When one of the above-mentioned pure or complex solvents is mixed with the M.W.0 in proportions ranging from 0.4 M.W.O.:l hydrocarbon to 1 M.W.O.:20 hydrocarbon, one does not observe any sludge separation after centrifugation equivalent to 30 days of gravity settling and no particles are detected by visual observation of the glass centrifuge tubes. If the same experiments are repeated with replacement of the M.W.O. by a virgin oil rich in polymeric additives such as a typical multigrade oil (GALP Super 20W50, manufactured by PETROGAL) or by a polymethacrylate solution in base oil (15% Rohm and Haas Plexol954 or Plexol 702 in SAE 20 base oil) or by a polyolefin solution in base oil (15% TEXACO TLA-374A in SAE 20 base oil), using n-hexane as hydrocarbon, one always obtains very clear and stable solutions, and no flocculation of macromolecules or other additives occurs. Ketones and Alcohols. A t room temperature, the ketones and alcohols having more than three carbon atoms satisfy property 1 of the extraction-flocculation solvents, e.g., are miscible with base oils at room temperature. The only ketone with less than four carbon atoms, propanone, is almost immiscible with base oils at room temperature. Methanol, ethanol, and the propanols are also almost immiscible with base oils at room temperature, the mutual solubility oil/alcohol increasing as the number of carbon atoms in the alcohol increases. This shows that ketones and alcohols having less than four carbon atoms are not extraction-flocculation solvents at room temperature. The ketone and the alcohols having four carbon atoms, butanone and butanols (1-butanol, 2-butanol, sec-butyl alcohol, and tert-butyl alcohol), are extraction-flocculation solvents. In fact, when mixed with the waste lubricating oils, they dissolve the desired base oil and are segregated from the solution part of the undesirable materials, which form large flakes that settle by gravity. The same occurs for alcohols and ketones having more than four carbon atoms, but in each of these groups the quantity of separated impurities, for similar experimental conditions, decreases as the molecular weight of the solvent increases. Percent Sludge Removal. To compare the capability of the different solvents to remove the undesirable components, we have determined curves for sludge removal, where the weight percentage of dry solids separated is plotted as a function of the solvent to waste oil weight ratio. The following experimental procedure was used. Six previously weighed glass centrifugal tubes such as those used in ASTM Method D-96 are filled with about 20 g of waste oil and solvent, varying the proportions of the two components. After strong agitation to miscibilize oil and
1224 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
melhyl-n -propylkelone
5 10 Butanone : Waste oil weight ratio
Figure 1. Sludge removal curves for the system butanone/M.W.O. at three temperatures.
solvent, the tubes are introduced in the six tube supports of a temperature-controlled centrifuge (MSE Model 58316/B). We shall define percent sludge removal as the mass of dry sludge, expressed in grams, separated from 100 g of waste oil when this mass of oil is treated by a certain amount of solvent and the dispersion settles during 24 h under gravity action or during the equivalent time in a centrifuge. For example, in the centrifuge, the equivalent time at 750 rpm is 1min. The sludge removal curve will be the curve obtained when the percent sludge is plotted as a function of the solvenkoil ratio. In the practical procedure, after centrifugation, the liquid is rejected and the sludge redispersed by 7.0 cm3of n-hexane. Then 28.0 cm3 of 2-propanol is added to separate the sludge again. This addition of 2-propanol immediately produces large flakes. The purpose of this treatment is to wash the sludge to reduce the interstitial oil content as much as possible. This oil would not be evaporated by the following drying operation and, as preliminary determinations have shown, the results would be less reproducible. It has been verified that the solubility of the oil-free sludge in the washing solvent at room temperature is less than 5%, and it was considered preferable to tolerate this error (introduced in the same way in all the sludge removal measurements that we wish to compare) than to introduce an error of unknown magnitude. Finally, the washing liquid is discarded, and the tubes are introduced into an oven in which the sludge is dried for 30 min at 100 OC. After cooling in a desiccicator, the tubes are weighed and the percent sludge removal is calculated. Figure 1 shows the curves obtained for the M.W.O./ butanone system at three temperatures. Figure 2 compares the butanone removal performance with a five-carbon and a six-carbon ketone at room temperature. Figure 3 compares the sludge removal capability of alcohols having four to six carbon atoms with the best sludge removing ketone (butanone). 2-Butanol was excluded from this study because it yields two liquid phases at room temperature. tert-Butyl alcohol was also excluded because it is solid below 25.5 "C. Sedimentation Curves. The sedimentation operation is of immediate practical importance to the application of a process based on extraction-fldcculation to waste oil
0 Solvent
Waste oil weight ratio
Figure 2. Sludge removal curves obtained for M.W.O. treated by several ketones a t 20 O C .
Solvent: Waste oil weight ratio
Figure 3. Sludge removal curves obtained for M.W.O. treated by several alcohols at 20 OC,compared to butanone curve.
rerefining. In this paper, however, our intention is mainly to present more experimental data for a future discussion explaining the action of polar solvents on waste oils. In order to make possible the observation of the settling front, determination of sedimentation curves must be done in tubes of small diameter, not exceeding 2 cm. This introduces a small wall effect error in all curves. For these these determinations, 25-cm3tubes have been used. After the tubes are fded with 20.0 g of solvent and waste oil in the desired proportions, the mixture is strongly agitated and then the tubes are gently inverted 20 times to promote flocculation by velocity gradients. After this, the settling curve is obtained in the usual manner, by plotting the height of the settling front as a function of time. Figure 4 shows the sedimentation curves obtained for the M.W.O./butanone system, considering three different solvenkoil weight ratios. It is impossible to obtain settling
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1225
I
i Butanone M.W 0 ratio 1-Pentanol' MWO.ratto
3 1
5 1 8 10 1 0
0
3 1
5 1 0 10 1
D
r
50
0
100 Time, m i n
Time, m i n
Figure 4. Sedimentation curves for butanone/M.W.O. system, considering three solventoil weight ratios, a t 20 OC.
1 -Butanol :
i
0
0 0
\
M.W.O. ratio
1.5:1 3 .l
Figure 6. Sedimentation curves for 1-pentanol/M.W.O. system, considering three so1vent:oil weight ratios, at 20 "C.
Table I. Critical Clarifying Ratio a n d Minimum Removal Ratio for Butan0ne:M.W.O. System at Three Temperatures temD, "C C.C.R. M.R.R. 20 2.2 0.90 35 2.6 1.15 50 3.0 1.40
5.1
IO
'1
Table 11. Critical Clarifying Ratio a n d Minimum Removal Ratio for M.W.O. Treated by Several Ketones a n d Alcohols at 20 OC solvent C.C.R. M.R.R. butanone 2.2 0.90 methyl n-propyl ketone 1.82 4.7 methyl isobutyl ketone 7.5 2.52 1-butanol 1.2 0.60 sec-butyl alcohol 1.4 0.70 1-pentanol 1.8 0.78 1-hexanol 3.3 1.40 1-octanol >15
'0
0 0
IO0
50
50
100 Time, m i n
Figure 5. Sedimentation curves for 1-butanol/M.W.O. system, considering four solventoil weight ratios, at 20 "C.
curves for ketones having more than four carbon atoms because the extent of destabilization is so small that no settling front is observed. The sedimentation curves are plotted for the systems M.W.O./l-butanol and M.W.O./l-pentanol (Figures 5 and 6). The settling front is very difficult to observe for 1hexanol. Critical Clarifying Ratio (C.C.R.). We shall define the critical clarifying ratio (C.C.R.) as the minimum solvent to oil weight ratio necessary to destabilize the dispersion contained in a column of 10-cm height, producing aggregates that settle by gravity action in 24 h (or under equivalent centrifugal conditions) and leaving a clear supernatant solution. This experimental parameter is most readily obtained by a centrifugal technique similar to the one used for the percent sludge removal determinations. First, a set of preliminary observations in 25-cm3test tubes enables one to obtain an approximate value of the C.C.R.. The tubes are fiied with solvent and waste oil in several proportions, strongly agitated and observed 24 h later. If, for example, for a 2:l so1vent:oil ratio the liquid is still black and for
a 3:l ratio a clear solution is observed, the C.C.R. lies between 2.0 and 3.0. Now a set of six centrifugal glass tubes such as those used in ASTM Method D-96 is filled with solvent/oil mixtures in proportions varying from 2:1 to 3:l and centrifuged under conditions equivalent to 24 h of gravity settling. The critical clarifying ratio is now obtained more accurately, but a new set of compositions near this second value may be necessary to obtain a value correct to two significant figures. In this test, to determine if the solution is clear, an arbitrary visual definition of clearness was accepted. The tube is observed at 10-cm distance from a 25-W cold light source. If a blue horizontal trace is seen through the thicker part of the tube, the solution is considered clear. There is another experimental parameter that experimental evidence has shown to be approximately one-half as large as the C.C.R. The sludge removal curves are initially linear. This suggests the definition of a minimum removal ratio (M.R.R.) as the minimum so1vent:oil ratio necessary to begin the sludge segregation in the conditions of the percent sludge removal test. This M.R.R. is found at the intersection of the sludge removal curve with the horizontal axis. Table I shows the C.C.R. and M.R.R. for butanone: M.W.O. at several temperatures. Table I1 compares the same parameters for M.W.O. treated by the ketones and alcohols studied in this work, at 20 "C.
1226 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table 111. Mass of Dry Polymer (g) Removed from 100 g of 15% Solution of TEXACO TLA-347A Polyolefins in SAE 20 Base Oil (Dispersion C) by Several Solvents in Proportions of So1vent:DisDersion c 3:1, at 20 "c solvent % polymer removal 3.95 butanone methyl n-propyl ketone 3.11 methyl isobutyl ketone 1.50 1-butanol 4.83 sec-butyl alcohol 4.52 4.10 I-pentanol 3.24 1-hexanol
T h e Stability of Waste Oil Carbonaceous Dispersions. To study the stability of (and to know how to destabilize) carbonaceous dispersions in solutions of dispersing oil additives, a base oil was burnt and a porcelain plate was placed over the flame to collect carbon black. This carbon black was dispersed in 50.0 cm3 of a 10% solution of dispersing polymethacrylates in SAE 10 base oil (Rohm and Haas Plexol954). A very fine suspension was obtained by adding 100 mg of these carbon particles to 50.0 cm3of the polymer solution. To remove some large particles, the dispersion was diluted in 100 cm3of n-hexane and filtered on Whatman 42 paper. After removing the n-hexane by vacuum distillation at 40 "C, a stable dispersion of carbonaceous particles in base oil containing polymethacrylates was obtained. This dispersion, which we shall call dispersion A, resembles waste motor oils in its dark color. If one adds 2.0 g of a multipurpose additive for multigrade oils (Lz-4456, supplied by LUBRIZOL) to 20.0 cm3 of the dispersion A, the result is still a very stable dispersion that we shall refer to as dispersion B. This dispersion is similar to a multigrade oil in its initial additive composition and in having the carbon particles artificially introduced. A solution of polyolefins in base oil was also prepared. Dispersion C was a 15% solution of TEXACO polyolefins TLA-347A in SAE 20 base oil. The following experiments were done on those dispersions. Upon treatment of dispersion A with any of the ketones or alcohols having more than three carbon atoms in proportions solvenkdispersion A 3:l to 5:1, no flocculation is observed, and the resulting dispersions remain stable for months. If one replaces dispersion A by dispersion B and performs the same tests, the following is observed: (a) the ketones produce no flocculation, and the dispersions obtained remain stable for months; (b) all the alcohols from 1-butanol and its isomers to 1-hexanol, in the proportions used, promote the formation of large black flakes that settle down very quickly; nevertheless, the dispersion remains black and opaque, showing that not all the carbonaceous matter and, eventually, not all the segregated polymers have flocculated to sizes big enough to settle by gravity or by centrifugal action under conditions equivalent to 30 days of gravity settling. On repeating these experiments on dispersion C , one observes fast flocculation for all the ketones and alcohols tested. The polymeric flakes so formed settle rapidly and aggregate at the bottom of the settling column in a mass like chewing gum, and clear solutions are obtained for all these solvents in all proportions tested. Table I11 shows the percent polymer removal (obtained by the sludge percent removal technique) obtained by mixing 1part of dispersion C with 3 parts of solvent. It was also considered important for an understanding of the action of polar solvents on waste oils to study their
Table IV. Percent Sludge Removal from GALP Super 20W50 Oil, New and after Use for 10000 km in a Gasoline Motor, by Treatment with Polar Solvents in Proportions of So1vent:Oil 3:1, at 20 OC '70 sludge removal solvent new oil used oil remarks butanone 0 =O a clear and stable methyl n-propyl ketone 0 =O soln and a stable methyl isobutyl ketone 0 .=O black dispersion are obtained for the new and used oils, respectively 1-butanol 4.74 4.85 for the new oil, the sec-butyl alcohol 4.39 4.41 liquid remains 1-pentanol 3.44 3.57 white clouded; for 1-hexanol 2.90 3.30 the used oil, the liquid remains black, opaque; in both cases, KOH destabilizes the dispersion and more sludge settles
action on some specific oils before and after use in a motor. Results obtained on the typical multigrade oil GALP Super 20W50 provide material for the following discussion. When this oil is treated by butanone and the higher carbon chain ketones in proportions ranging from 3:l to 5:l solvenkoil, no destabilization is observed and a clear and stable solution is obtained. Contrary to the ketones, in these proportions the alcohols show very sharp destabilizing action, immediately producing large flakes of very obvious polymeric nature. Nevertheless, after sedimentation of the flocculated matter, the solutions remain opaque, (cloudy white), showing that some particles have been segregated from the liquid phase but have formed a stable dispersion. This is similar to what was observed for dispersion B. Upon treatment of 25.0 cm3 of the stable dispersions obtained by treating the GALP Super 20W50 oil and dispersion B by alcohols, with a few drops of a saturated solution of potassium hydroxide in 1-butanol, those dispersions are immediately destabilized, large flakes being formed and settled, and clear solutions are obtained. The GALP Super 20W50 oil was introduced in a previously washed gasoline motor, and the vehicle was then driven 10000 km. The used oil was previously vacuum distilled at 10 mmHg absolute pressure and 150 "C to remove water and light hydrocarbons and, after cooling, was submitted to the same solvents as the new oil. It was observed that the ketones are completely ineffective in removing the additives and particles from this oil. Alcohols exhibited the same performance observed for the new oil, the percent solids removal being very similar, as Table IV shows. Without the addition of KOH in 1-butanol, it is not possible to obtain a clear solution, exactly as was observed for the new oil. 4. Discussion
Segregation of Polymers by Antisolvency. The following was observed: (a) Ketones and alcohols exert a flocculating action on waste oils but no flocculating action on polymethacrylate solutions with which they are completely miscible, exerting, conversely, a strong flocculating action on polyolefins, which are known not to be soluble in those solvents. The hydrocarbons, which are miscible with the polymethacrylates and polyolefins, are not able to flocculate them from their solutions in base oils nor to flocculate any sludge from waste oils, originating new dispersions ap-
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1227 1 2 3 4 6
1 -pentand
I
1 -hexanol
I
7
1
butanone
L
methyl-n-propylketone
3
methylisobutylketone
5 s e c - butyl alcohol 6 1 -penlano1 7
16,-6,1
/
methylisobutylketone 1 -bulanol
5 s e c - b u t y l alcohol
2
‘9;
butanone methyl. n. propylkelone
./
1 -bulanol
I-hexanol
I Jicm3)4
Figure 7. Polymer removal results from Table I11 versus difference between the solubility parameters of solvent and polyisobutylene, at 20 o c .
parently as stable as the waste oils to which they are added. (b) The sludge removal decreases as the temperature increases, as shown in Figure 1. The critical clarifying ratio and minimum removal ratio shown in Table I also reveal this increased difficulty in separating impurities from waste oils as the temperature is increased. (c) Figure 2 and Table I1 show that butanone exhibits a more active flocculating action than methyl n-propyl ketone, which in turn is more effective than methyl isobutyl ketone. The flocculating action decreases from 1-butanol to 1-hexanol (Figure 3). These observations suggest that the segregation of sludge from waste oils by polar solvents is, basically, an antisolvent effect exerted on some macromolecules. Considering the polar nature of ketones and alcohols, this antisolvent action will be greater for nonpolar or slightly polar molecules, like polyolefins. More generally, this segregation due to antisolvency will take place for all substances present in waste oils that are only slightly soluble in the polar solvent of interest. It is important for the development of new composite solvents for extraction-flocculation to relate the single solvents flocculating action with measurable physical properties of solvent and polymers used as oil additives, e.g., the properties associated to miscibility. The properties that are best correlated with the miscibility are the solubility parameters, 6. The solubility of polymers in organic solvents has been discussed in relevant books by Hildebrand and Scott (ISM), Hildebrand et al. (1970), Krevelen (1976),and Elias (1977). It is known that the interaction energy between solvent and polymer molecules, A€, is near zero when miscibility is good. On assuming some simplifying hypotheses, it can be expressed as (Elias, 1977) A€ = -K(6, - 6,)2
(1)
where K is a positive constant and 61 and a2 are the solubility parameters of the solvent and the polymer. If the sludge removal from waste oil by polar solvents is, basically, the segregation by antisolvency of polymers of nonpolar or slightly polar nature, the results of polymer removal from Table I11 should correlate with the 16, - Szl
I
2
3
L
5
6
7
[ 6 1 - 6 ~ ( 1J / c m 3 i i
Figure 8. Sludge removal for M.W.O. treated by several solvents at 3:l solvent to oil ratio versus difference between solubility parameters of solvent and polyisobutylene, at 20 “ C .
values, where 62 refers to a typical polyolefin used as motor oil additive, polyisobutylene. Figure 7 shows this correlation. A similar correlation was obtained for waste oil (Figure 8). In both plots, it was not possible to obtain a satisfactory correlation taking the ketones and alcohols together, a fact indicating that the solubility is not an exclusive function of the solubility parameter difference but also depends on the group to which the solvents belong. Coflocculation of Polymers and Black Particles. The tests performed on dispersions A and B and on the GALP Super 20W50 used oil showed that, when there is no segregation of polymeric molecules from the solution, the black particles remain in stable dispersion. When flocculation occurs, macromolecules and carbonaceous particles coflocculate and settle together. Comparison of the values obtained for the new and used oils in Table IV shows that the amount of impurities above and beyond the additives is very small. Colloidal Dispersions Formed by Electrostatic Repulsion. It has been observed that in some cases, namely in the treatment of dispersion B and the GALP Super 20W50 new and used oils, even the best of the solvents tested is incapable of promoting the flocculation of all the mater it segregates from the liquid phase. I t is thought that the polar nature of the alcohol is the origin of the stabilization by electric repulsion of some of the particulate matter. If so, addition of a solution containing ions that neutralize those charges should break this stability. In fact, addition of KOH promotes a fast flocculation process. The sedimentation curves (Figures 4-6)also support this conclusion. In Figure 5 the best observed so1vent:M.W.O. ratio is 3:l. The existence of an optimum (somewherenear 3:1), above which the settling velocity decreases, is interpreted as follows: (i) The increase of the so1vent:M.W.O. ratio increases the difference between the medium solubility parameter of the liquid where the nonpolar or slightly polar macromolecules are dissolved (solution of base oil with some additives and 1-butanol) and the solubility parameter of those macromolecules. Is also decreases the viscosity and specific gravity of the liquid. All these factors tend to increase the veiocity of the settling front.
1228 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
Another factor increasing the velocity of the settling front when the so1vent:M.W.O. increases is the smaller hindrance effect in falling particles, which according to Richardson and Jerdnimo (1979) may be expressed by the equation where u,, is the velocity of the particle under interest in the solids free liquid and u h is the hindered velocity of the particle, e.g., the velocity in a dispersion where the particle volume fraction is C. (ii) However, on increasing the so1vent:M.W.O. ratio, the settlable matter concentration decreases, resulting in a slower flocculation and a decrease in the velocity of the settling front. The growth of the electric repulsion between particles as the polarity of the medium increases as a consequence of increasing the so1vent:M.W.O. ratio also hinders the growth of particles to large sizes and, thus, reduces the dispersion settling velocity. Factors i are more important than factors ii below the observed optimum. Above the optimum, the effect of factors ii prevails. The settling curves for the 1-pentanol/M.W.O. system (Figure 6) also show the existence of this optimum solvent:M.W.O. ratio, somewhere near 5:l. This value is greater than the value observed for the 1-butanol/M.W.O. system. This is to be expected because 1-pentanol is less polar than 1-butanol, which makes it necessary to add more solvent to the oil to make factors ii prevail above factors i. The electrical repulsion effects are, possibly, associated with links between the alcohol OH groups and ions from the additives. For the ketones (Figure 4),these effects probably do not occur, because of the presumed incapability of the carbonyl group to link with those ions. In fact, the results obtained by adding a ketone to the GALP Super 20W50 oil, new and used, did not show the segregation of any settlable matter, nor the formation of a stable colloidal dispersion, as observed for the alcohols. From the previous discussion, it is obvious that the settling of flocculating dispersions depends on many parameters and should be quantitatively discussed. A mathematical theory to solve this problem has been published (Reis, 1982; Reis et al., 1987). 5. Conclusions The main conclusions from this work are as follows: (I) Sludge removal from waste oils by polar solvents is related to the segregation by antisolvency of nonpolar or slightly polar polymers. Research to find new single or composite solvents may be guided by the difference IS, - S21 where S1 and S2 are the solubility parameters of the solvent and polyisobutylene, respectively. When ISl - S,l is high, one should expect to
obtain high sludge removal values. (11) The carbonaceous particles and macromolecules coflocculate and settle together. Since the amount of carbonaceous particles in the sludge removed is very small as compared to the polymeric matter, the development of new extraction-flocculation solvents must be directed to find polar compounds showing good removal of the additives from the new oils present in the market. (111)To avoid the formation of stable colloidal dispersions due to electric repulsions in polar medium, one must add ionizable inorganic substances to the solvents. For example, the addition of KOH to alcohols increases their capability to remove sludge from waste oils.
Literature Cited Brownawell, D.; Renard, R. “Rerefining of Used Lubricating Oils”. U S . Patent 3 639 229, February 1972. Elias, H. G. Macromolecules, 1st ed.; Plenum: New York, 1977; Vol. 1, pp 205-210. Hildebrand. J. H.: Scott. R. L. The Solubilitv of Nonelectrolvtes. 3rd ed.; Dover: New York, 1964; Chapter XX: Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions, 1st ed.; Van Norstrand Reinhold: New York, 1970; pp. 188-196. Krevelen, D. W. Properties of Polymers. Their Estimation and Correlation with Chemical Structure, 2nd ed.; Elsevier Scientific: Amsterdam, Holland, 1976; pp. 130-159. Jordan, T.; Mc Donald, W. “Method of Reducing the Lead Content of a Used Hydrocarbon Lubricating Oil by Adding Methylethylketone to Separate the Resulting Mixture into a Coagulated Insoluble Phase”. U S . Patent 3363 036, October 1973. Reis, M. A. “RegeneraGHo de Oleos Lubrificantes Usados por ExtracGHo-Flocula@o”. Ph.D. Dissertation, Oporto University, Faculty of Engineering, Oporto, Portugal, 1982. Reis, M. A,; Jerbnimo, M. S. “Regenera@o de Oleos Usados por ExtracgHo-FloculaqBo Rtipidas”. Addition to Portuguese Patent 69 392, October 1982a. Reis, M. A.; Jerhimo, M. S. “Fabrico de-Tinta de Base para Fabrico de Tinta de Impressa0 a Partir de Oleos Usados”. Portuguese Patent 75 702, October 1982b. Reis, M. A.; Jerbnimo, M. S.; Wilson, D. J. “Mathematical Simulation of Quiescent Settling of Flocculating Dispersions. Comparison of Different Breakage Models. Part I. Theoretical Analysis”. Sep. Sci. Technol. 1987,22(10),2143-2163. Richardson, J. F.; Jerbnimo, M. S. “Velocity-Voidage Relations for Sedimentation and Fluidisation”. Chem. Eng. Sci. 1979, 34, 1419-1422. Whisman, M. L.; Goetzinger, J. W.; Cotton, F. 0. “Some Innovative Aproaches to Reclaiming Used Crankcase Oil”. Report RI-7925, 1974; Bartlesville Energy Research Center, Bartlesville, OK. Whisman, M. L.; Reynolds, J. W.; Goetzinger, J. W.; Cotton, F. 0.; Brinkman, D. W. ”Re-refining Makes Quality Oils”. Hydrocarbon Proc. 1978a, Oct, 141-145. Whisman, M. L.; Reynolds, J. W.; Goetzinger, J. W.; Cotton, F. 0. “Process for Preparing Lubricating Oil from Used Waste Lubricating Oil”. U S . Patent 4073 720, February 197813. Received f o r review July 20, 1987 Revised manuscript received February 16, 1988 Accepted March 1, 1988
