PROCESS FOR PURIFYING USED LUBRICATING OIL

Abstract: A process for re-refining used lubricating oil (1) by treating a lubricating oil distillate (6) with a minor amount of caustic (8) in the presence of a controlled quantity of water (9). A salt phase (10) is formed which is easily separated from the mixture, producing a purified lubricating oil (19) which has no objectionable odor, has a low total acid number and is highly stable to oxidation. A portion of the salt phase (14) can be recycled for enhanced efficiency.

Description: TITLE OF THE INVENTION Process for Purifying Used Lubricating Oil BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to the field of treating waste lubricating oil and the like and, more particularly, to the purifying of used lubricating oil for reuse by treating a lubricating oil distillate with sufficient caustic and water to form a salt phase which contains the undesirable impurities of the oil and which can be easily separated, resulting in a lubricating oil having a low total acid number, no objectionable odor, and improved oxidation stability. This salt phase can be recycled to enhance the efficiency of the process. DESCRIPTION OF THE PRIOR ART Lubricating oil can be used for only a limited amount of time in the crankcase of an engine before it must be discarded because of the build up of liquid and solid impurities. Because of advancements in engine design resulting in more severe operating conditions, there has been a widespread use of additives in motor oils. These additives, which are designed to hold dirt, water, wear metals, etc. in suspension to prevent the contaminants from corroding the engine, have made the job of re-refining, or purifying, used lubricating oil much more difficult than it was before such extensive additive use became common. However, it is desirable to reclaim used lubricating oils for reuse to conserve world resources. In addition, used lubricating oils, which may contain heavy metals and unidentified organic by-products of oxidation and reactions with additives, are now recognized as a hazardous waste product which cannot be simply disposed of without environmental considerations. Thus, there is a need for an economically feasible, environmentally acceptable process for purifying waste lubricating oil and the like. Numerous methods for reclaiming or re-refining used lubricating oil have been tried. Some of the earlier methods used and considered are summarized in a report by G.J. Mascetti and H.M. White to the U.S. Department of Energy, Utilization of Used Oil, (August 1978) , and in Used Oil Re-Refining, (1980) , by Richard Bigda and Ted Cowan. In general, these methods have been unsuccessful due to economic factors, the creation of large, hazardous by-products and the difficulty and expense of complying with environmental regulations. For example, the acid-clay method, which relied on severe treatment of the oil with acid, was expensive and created a hazardous acid sludge which was corrosive, flammable and leached toxic heavy metals into the environment; it also produced an oily clay residue which was also difficult to dispose of. Caustic pretreatment to reduce the amount of concentrated sulfuric acid which was used did not solve the problems. Heat treatment of the oil with clay was expensive, not selective, and resulted in low yields and an oily clay disposal problem. Several newer approaches to re-refining utilize distillation to separate water, fuel and lube oils from sludges. However, the lube oil distillate resulting from such distillation still contains organic acids, inorganic acids, sulfonic acids, hydrogen sulfide, mercaptans and several classes of oxidation products which, if they are not removed, cause the characteristic objectionable odor of re-refined oil and substantially reduce the oxidation stability of the re-refined lube oil. The two most widely used treatment steps to the lube oil distillate are clay treating and hydrotreating of the lube oil distillate. Clay treating uses clay to adsorb the impurities from the lube oil distillate, resulting in contaminated clay which must be disposed of or reactivated by incineration and, in either case, the adsorbed oil is lost. In addition, the yield loss with clay treating is significant. Other re-refiners use hydrotreating of the lube oil distillate, which involves reaction of the lube oil impurities with hydrogen at elevated temperatures and pressures in the presence of a catalyst and driving off hydrogen sulfide, water and light hydrocarbons from the distillate. Hydrotreating is expensive and relatively dangerous due to the use of high pressure hydrogen and the creation of hydrogen sulfide, which is a poisonous gas. Since it also requires a hydrogen source and the disposal of hydrogen sulfide, this treatment fails to provide an economic, safe and environmentally acceptable process for treating used lubricating oil. Thus, there has been a need in the field of re-refining lubricating oil for a process which is economical and does not have high energy requirements nor require expensive chemicals or reactants, does not create substantial amounts of hazardous or environmentally objectionable by-products and yet provides a good yield of a high quality lubricating oil which can be comparable to virgin lube oil in terms of acid content and oxidation stability. The present invention provides a process which solves these needs. In the present invention, the waste lubricating oil is first subjected to conventional steps of pretreatment, such as dehydration and vacuum distillation, to remove most of the water, light hydrocarbons, oxidation gum, lube oil additives, and heavy metals from the lube oil distillate. That distillate is then mixed with a solution of caustic and water, under moderate conditions, which generates a liquid salt phase that precipitates in a short time, thereby removing the contaminants from the lubricating oil distillate and resulting in a lubricating oil which has an acid number at least as low as typical virgin lube oils and has improved oxidation stability. A remarkable advantage of this process is that dissolving the caustic and water in some portion of the recycled salt phase prior to mixing with the lube oil distillate causes the process to be faster, less expensive, more efficient and more tolerant of variances in the feed stock. Any salt phase unrecycled to the feed can be recycled with the waste oil to further purify it or it can be used as a fuel. Since the heavy metals and other bottoms of the vacuum distillation can be used in making asphalt and the light hydrocarbons driven off by the distillation can be captured and used, there are no environmentally harmful by-products, except a small amount of water which can be cleaned up before it is discharged. This process is safe, and it requires only a few steps, no expensive reactants, only moderate energy use, and reasonable capital expenditures. It also provides a very high yield of a high quality product which has been well-received in the market. SUMMARY OF THE INVENTION The present invention is an improved process for purifying used lubricating oil. A lubricating oil distillate is prepared by conventional means and is then mixed with a minor amount of a caustic in the presence of a controlled quantity of water. The resulting mixture is allowed to settle until a salt phase has separated from a lubricating oil fraction, the lubricating oil fraction being a highly stable purified lubricating oil. OBJECTS OF THE INVENTION It is, therefore, one object of the invention to provide an improved process for re-refining • used lubricating oil by treatment of a lubricating oil distillate with a minor amount of caustic and water to generate an easily separable salt phase from a lubricating oil fraction. It is another object of this invention to produce from used lubricating oil a re-refined lubricating oil which has no objectionable odor, a low total acid number, and is highly stable to oxidation. It is a further object of this invention to remove the contaminants from used lubricating oil in such a way that they can be beneficially used at a substantial value. It is yet another object of this invention to provide a safe, inexpensive and efficient process which produces a high yield of lubricating oil comparable in quality to virgin lubricating oil. It is yet a further object of this invention to provide a process which can be continuous and which can be made more efficient by recycling its by-products. Other objects and advantages of the invention will become apparent when it is considered in conjunction with the accompanying drawing described hereafter. DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow sheet illustrating the treatment of used lubricating oil in accordance with the process of this invention, including the conventional pretreatment steps, optional subsequent treatment and optional variations in the process. DESCRIPTION OF THE PREFERRED EMBODIMENT In this process waste, or used, lubricating oil is purified for reuse. Since the waste oil is collected from various sources, the composition of each batch will vary depending on the type and brand of oil, the additives used, the degree to which the oil and additives were used, and other factors. Typically, the feedstock for this process consists of lubricating oils from automobiles and diesel engines and some industrial oils. Some of the difficulties in re-refining used lubricating oil is that any re-refining process must account for these variations in feedstock. Oxidation stability is critical to the performance of a lubricating oil since oxidation limits the useful life of the lubricant. Once oxidation begins, chain reactions involving the oxidation products increase the oxidation reaction rates until the lubricating oil has been spent. Some of the oxidation products formed are: organic acids, such as naphthenic acids, carboxylic acids, and phenols, hydroperoxides, anhydrides, esters, aldehydes and ketones. In order to extend the useful life of lubricating oil, antioxidant and oxidation retardant additives are added to commercial lubricating oils. These additives are intended to slow the oxidation rate and to react with the oxidation products to form stable products which will be suspended in the oil until the lubricating oil is drained, thus preventing the contaminants from corroding or causing excessive wear to the engine. Used lubricating oil contains contaminants such as heavy metals, used additives, oxidation products, water, solvents, motor fuels, etc. To practice the process of this invention, a lubricating oil distillate is first prepared by conventional means from the used lube oil. Referring to FIG. 1, typically, the used lubricating oil (1) is subjected to a conventional dehydration step (2) , in which water, solvents, motor fuels, and light oils and oxidation products are first boiled out or separated (3) from the used oil. These overheads can be captured and separated by conventional means. Used lubricating oil which has been conventionally dehydrated will have a reduced water content, although it is not necessarily completely free of water. This dehydrated oil (4) is next preferably subjected to a vacuum distillation step (5) to separate a lubricating oil distillate (6) from high boiling point and non-boilable components (7) of the oil. Various types of distillation equipment can be suitably used to perform this step of the process, such as a thin film evaporator or a high vacuum flash distillator, or other conventional distillation equipment used in refining and re-refining oil. Typically, the vacuum created will be between 1/2 and 100 Torr and the temperatures applied may be in the range of 600-750°F (315.5°-398.9°C) . Different techniques and equipment designs, known to those skilled in the art, can be used to minimize fouling and other problems associated in the early prior art with the distillation of waste oil. Vacuum distillation (5) separates a lubricating oil distillate (6) from the dehydrated oil. The vacuum residue (7) , or the bottoms from the distillation treatment, consists of high boiling point and non-boilable components, such as heavy metals, additives, oxidation gum, and other contaminants of the used lubricating oil. This residue or flux from the vacuum distillation may be sold as an asphalt softener for paving asphalts or air blown industrial asphalts. This use has been accepted as one of the safest uses of the heavy metals and high boiling components of used oil. Although dehydration and vacuum distillation are the preferred pretreatment steps for the process of this invention, it is also possible to remove some of the water and the nonboilable components of used lubricating oil by other means, such as steam stripping, and it is intended that the lubricating oil distillate referred to herein includes lubricating oil products prepared by other means which reduce the water content and remove the non-boilable components of the used oil. The lubricating oil distillate (6) which has been separated by vacuum distillation or other means from the waste oil (1) is free of most contaminants and additives but it typically contains oxidation products which are in the same boiling range as the lubricating oil, and ercaptans and other decomposition products from the additives contained in the used lubricating oil. Thus, this lube oil distillate is not comparable to virgin lube oil: it has a characteristic foul odor and excessive acidity. If the lubricating oil distillate is not processed further, it would have a dramatically shortened useful life unless excessive dosages of anti-oxidants and oxidation retardants are used to prolong its useful life. However, this invention provides further treatment which refines the lubricating oil distillate until it at least meets the acid standard of virgin lube oil. In the process of this invention, the lube oil distillate (6) is mixed with a minor amount of caustic (8) in the presence of a controlled quantity of water (9) . The caustic will preferably be the hydroxide of an alkali metal such as sodium, lithium or potassium or ammonium hydroxide. The preferred caustic is sodium hydroxide because of its low cost and ease of handling. Generally, the only added water to this mixture will be that in which the caustic is dissolved. The caustic and water are mixed with the lubricating oil distillate in sufficient quantities to generate an extractable salt phase from the mixture; in sufficient quantities, the caustic solution will hydrolyze oxidation products and neutralize the acids contained in the lubricating oil distillate and will form a distinct and relatively easily extractable salt phase (10) . Good results are obtained when the caustic is present in one to ten times, and preferably two to three times, the stochiometric quantities relative to the total acid number (TAN) of the lubricating oil distillate. Total acid number will obviously vary as the feedstock varies and will also vary with the vacuum distillation conditions: the TAN of the lubricating oil distillate may range from 0.3 to 5.0, although it is typically close to 1.0. A titration measurement, such as ASTM D-974, can determine the initial TAN quite easily and then the amount of caustic can be calculated. Usually, the stochio¬ metric value, referring only to the measured total acid number, must be exceeded to complete the reactions. It has been found, for example, that a lubricating oil distillate having a viscosity of 100-150 SUS at 100°F (37.8°C) and having a TAN of approximately 1.0 can be successfully treated with a solution of 1.5 to 2.0 mg. sodium hydroxide (on a dry basis) per gram of lubricating oil distillate. The use of insufficient caustic may generate a salt phase but the resulting oil fraction will not be stable to oxidation. Although the caustic (8) need not be dissolved in water (9) when mixed with the lubricating oil distillate (6) , it is important that the proper amount of water be present in the mixture for the salt phase to form. Ideally, the salt phase (10) should be 15 to 50% by weight water. It has been found that mixing the lubricating oil distillate with a solution of 20 to 50% by weight of the proper amount of sodium hydroxide to water generates the salt phase and a solution of 30 to 35% by weight provides the best results. If insufficient water is present, there will be poor separation of the salt phase and a high salt content in the oil phase of the mixture. If excess water is present an undesirable water in oil emulsion will be formed. The lube oil distillate (6) and caustic (8) and water (9) can be mixed by any conventional means, such as a propeller mixer, non-dispersive mixing, or mixing with a system of valves, but it has been found that a static mixer (11) works most effectively in this process. After the components have been mixed in mixer (11) they are pumped to a vessel (12) in which they are allowed to settle until the salt phase (10) completely forms. This salt phase typically has a higher specific gravity than the oil phase and easily separates therefrom. Generally, it is desirable to have the mixture at a relatively moderate temperature and a pressure sufficient to suppress vaporization in the settling vessel. A temperature of approximately 100°-400°F (37.8°-204°C) , and preferably 200°-250°F (93.3°-121.1°C) , with pressure of approximately 50 pounds per square inch works effectively in this step. While higher temperatures tend to promote lower oil phase viscosity and greater density differences between the salt phase and the oil phase, and thus enhance the separability of the salt phase, higher temperatures require higher energy inputs and result in an increase in the solubility of water (and salt) in the oil and a lubricating oil fraction with a higher salt content. The preferred temperature is thus a compromise of competing factors. In the preferred embodiment the temperature of the lubricating oil distillate (6) will be controlled by controlling the temperature of the lubricating oil distillate as it is removed from the distillation equipment (5) and then treating the distillate promptly with the caustic solution. If the discharge temperature of the lube oil distillate needs to be raised or lowered, it can be put through a heat exchanger (6a) to adjust it to the preferred mixing temperature. The use of an insulated settling vessel (12) reduces the need for the application of additional heat; however, additional heat can be easily provided by conventional means if desired. When sufficient quantities of caustic solution (8, 9) are mixed with the lubricating oil distillate (6) a salt phase (10) forms, which can be separated from a lube oil fraction (13) . Although some salt phase forms immediately, the mixture is allowed to settle for a sufficient time to precipitate the remainder. Generally, the salt phase can be removed from the settler in less than an hour after the caustic solution has been mixed with the lubricating oil distillate. Preferably the caustic added was 50-95% consumed as salt and the salt phase is 15 to 50% water. It is seen that the salt phase may be considered a solution of water in salt, rather than the reverse. It is noted that caustic has been used for various purposes in other processes in the prior art. In the treatment of virgin crude oil, where the oils have a low molecular weight, large amounts of dilute caustic in water have been successful in removing contaminants from the oil. However, large amounts of dilute caustic have not worked satisfactorily in the case of high-molecular weight fractions like lubricating oil. It has been the experience in the prior art that the use of excess caustic and water creates very stable and almost irreversible emulsions between the water and oil, with the salts formed acting as powerful surfactants. Also, dilute caustic does not hydrolyze the oxidation products and the lubricating oil fraction formed does not have good oxidation stability. It is not fully understood why the processes of this invention accomplish such striking results. However, although it is not intended that this invention be bound by the explanation it is believed that the use of only so much caustic as is sufficient to hydrolyze substantially all the oxidation products in the lubricating oil distillate and to neutralize substantially all the acids present forms salts which are drawn to the limited amount of water available instead of acting as surfactants. The salt phase that forms thus contains the sodium salts of most of the contaminants and it has such a different specific gravity from the lube oil fraction in the settler that it can be easily separated from the oil and removed from the settler. Upon removal, the salt phase (10) , which contains salts, unused caustic, water and some minor amount of oil, can be collected and used as a fuel. However, it has been found that there are substantial advantages to recycling a portion of the salt phase in the process, as illustrated by phantom line 31 in FIG. 1. Thus, a portion of the salt phase (14) is preferably recirculated and mixed with the caustic (8) and water (9) prior to mixing with the lubricating oil distillate (6) . Although the process can be performed by mixing the salt phase recycle directly with the lube oil distillate, it has been found that the process works most efficiently if the caustic and water are first mixed with the salt phase recycle before mixing with the lubricating oil distillate. The salt phase contains some caustic, which can be determined by measuring the Strong Base Number, as by ASTM D-974. The amount of caustic to be mixed with the lube oil distillate can thus be adjusted to maintain the desired range of caustic. It has been found that use of the recycled salt phase (14) has many unexpected benefits. For example, the subsequent salt phase separates much faster and the average droplet size of the new salt phase formed is larger when a portion of the previous cycle's salt phase is included, thus increasing the settling speed of salt phase droplets from the lubricating oil fraction (13) . It is believed that the caustic dissolves in the salt phase and that when this pre-formed salt phase is mixed with the lubricating oil distillate, it enhances the separation process. There are additional benefits of recycling a portion of the salt phase: the caustic is used more efficiently and the amount of caustic can be reduced and, for some reason, there is a damping effect on the entire process, that is, there is an increased tolerance for changes in the composition of the feedstock of used lubricating oil and for changes in the amounts of reactants added. The outer limits on the amount of recycled salt phase which can be used has not yet been determined. The remaining portion of the salt phase (10) which is removed from the settler vessel (12) but which is not used to pre-mix with the caustic (8) and water (9) is referred to as the "net salt phase" (15) . This net salt phase can be burned as a fuel or subjected to further refinement to extract any further salts, water and oil from it, and in that case is referred to as "net salt phase product" (16) . Preferably, the net salt phase (15) will also be recycled (17) back into the pretreatment of the used lubricating oil so that it mixes with the new feedstock of used lubricating oil (1) before it is dehydrated (2) . This optional step is illustrated by phantom line 33 in FIG. 1. Any caustic remaining in the recycled net salt phase (17) will neutralize some of the stronger acids in the feedstock. The water contained in the recycled net salt phase (17) can be driven off in the dehydration step (2) ; any entrained or dissolved oil will be separated in the distillation step (5) and recovered as part of the lube oil distillate (6) ; and the organic and inorganic salts which are non-boilable will become part of the vacuum residue (7) , where their presence does not affect the asphalt for which these bottoms are used. The lubricating oil fraction (13) which is present in the settler (12) when the salt phase (10) separates from the mixture is removed and cooled by conventional means (18) . It can then be collected and stored for use as a low acid lubricating oil (indicated by 19 on FIG. 1) , which is highly stable to oxidation. Typical virgin lubricating oils may have 0.0-0.2 TAN. The TAN of the lubricating oil produced by this process is at least as low as virgin lube oil and typically is in the range of 0.00-0.03 TAN. It should be noted that while 0.10 TAN is acceptable, any TAN value lower than 0.05 is considered exceptional for re-refined lubricating oil. The oxidation stability and color stability of the re-refined lubricating oil of this process are significantly improved, the characteristically objectionable odor of re-refined lubricating oil is removed and the viscosity index of the re-refined lubricating oil is increased. There may be some salts present in the lubricating oil fraction (13) which were dissolved at the higher temperature of the settler (12) . For many uses, especially where detergents are to be added to the lube oil after it is removed from this process, such minor amounts of salt can be tolerated in the final product (19) since the salts will act as detergents. Where it is desired to have a salt-free re-refined lube oil with no detergency, the lubricating oil fraction (13) from the settler (12) can be cooled off (18) and subjected to the treatment of a coalescer (20) , which separates the final residue of salt (21) present in the lube oil fraction (13) . This optional variation in the process is illustrated by phantom line 35 in FIG. 1. Any water present may be removed by vacuum flash. Any salt (21) removed by the coalescer can be added to the net salt phase (15) and treated with that product as described above. It has been found that the yield of treated lube oil from this process is generally greater than 98% of the lube oil distillate, and 99% of the distillate when the entire salt phase is recycled in either the mixing step or the dehydrating step. It should be noted that 100% of the hydrocarbons of the lubricating oil ends up in usable products and none is disposed of. While the final salt-free lubricating oil (22) prepared according to this invention will be as suitable for most purposes as virgin lube oil, it may sometimes be advantageous for certain specialized needs to subject the salt-free lubricating oil (22) to further treatment, such as steam-stripping or mild clay (or activated carbon) treating. The severity of any additional treatment would be substantially reduced by first using the process of this invention. If a steam-stripped lube oil is desired, the salt free lube oil may be sent through heat exchanger (23) and conventional steam-stripping means (24) to drive off any water and light hydrocarbons (25) still present, resulting in a steam-stripped lube oil (26) . Such a steam-stripped lube oil (26) will have an increased flash point, increased viscosity and reduced volatility, which is an advantage since high volatility can increase oil consumption in some cases. This optional process step is illustrated by phantom line 37 in FIG. 1. The following nonlimiting examples are presented to further illustrate the results obtained by treating lube oil distillate (6) according to the process of this invention. Example 1 Twelve gallons per minute (GPM) (45.4 liters per minute) of a lubricating oil distillate was commercially treated in a continuous process as described above. The distillate had the following properties: Total Acid Number 0.80 Gravity, °API 29.7 Viscosity, SUS @ 100°F 98.5 Viscosity Index 85 Flash Point, °F 315 Characteristic Odor Strong One-half GPM (1.893 liters per minute) of salt phase was drawn from the bottom of the settler and mixed with 0.10 Ib/min (45.4 grams per minute) of sodium hydroxide dissolved in 0.20 lb/min (90.8 grams per minute) of water. This mixture was then combined with the above 12 GPM of lube oil distillate, which had been preheated to 225°F (107.2°C), and the entire mixture was passed through an eight element static mixer into the settling vessel. The purified lube oil flowing from the top of the settler exhibited the following properties which illustrates the improvements to the quality of the oil: Total Acid Number 0.03 Gravity, "API 30.0 Viscosity, SUS 100F 97.8 Viscosity Index 94 Flash Point, °F 315 (157.2°C) Characteristic Odor None The salt phase which settled and was withdrawn had the following properties: Strong Base Number 9.5 Total Base Number 150 Water, wt% 21 Density, lb/gal @ 204°F 8.8 Caustic Utilization, % 94 Strong Base Number (SBN) was determined by ASTM D-974 and Total Base Number (TBN) by ASTM D-2896. Caustic Utilization was calculated by the formula: Caustic Utilization = (TBN-SBN) /TBN x 100% It is to be noted that in a commercial context, there is no significant difference in TAN values between 0.00 and 0.03 TAN, since any value less than 0.05 TAN is considered exceptional for re-refined oil. Example 2 The purified lubricating oil from Example 1 was commercially vacuum steam stripped to remove light hydrocarbons resulting in lubricating oil with the following improved properties: Gravity, "API 29.8 Viscosity, SUS @ 100°F 134.8 Flash Point, °F 410 This stripped oil was blended with additives in the same manner as virgin lube oil to make a high quality SAE 10W-40 API SF motor oil. Example 3 A lubricating oil distillate from a different base oil was treated in a similar manner to Example 1, except that more caustic was used. One-half GPM of salt phase was mixed with 0.17 lb/min (77.2 grams per minute) of sodium hydroxide dissolved in 0.34 lb/min (154.4 grams per minute) of water. This mixture was then combined with 12 GPM (45.4 liters per minute) of lube oil distillate having the following properties: Total Acid Number 0.95 Viscosity, cSt g 100°C 2.97 Viscosity Index 88 Characteristic Odor Strong The purified lube oil was improved substantially, as can be seen by its properties: Total Acid Number 0.01 Viscosity, cSt @ 100°C 3.01 Viscosity Index 94 Characteristic Odor None The salt phase which separated exhibited the following characteristics: Strong Base Number 62.3 Total Base Number 205 Water, wt% 36 Viscosity, cSt @ 100°C 15.75 Caustic Utilization, % 70 The lower caustic utilization indicated that excessive caustic was being mixed with the salt phase and that it could have been reduced to 0.13 lb/min (59 grams per minute) dissolved in 0.26 lb/min (118 grams per minute) water with little or no effect on purified lube oil quality. However, at $0.10/lb for sodium hydroxide (dry basis) there would be nominal savings from this reduction. This example also illustrates that a purified lubricating oil having substantially consistent qualities can be produced with variations in the quantities and characteristics of the reactants. The net salt phase remaining (after a portion of salt phase was recycled in the distillate-caustic mixture) was recycled to the feedstock and mixed with the used lube oil being fed to the dehydration step. As a result, the organic and inorganic salts ended up in the vacuum residue which was sold as asphalt air blower feedstock which ultimately became roofing tar, roofing felt saturant, and shingle coating. Example 4 The purified lube oil from Example 3 was vacuum steam stripped as in Example 1 to produce a stripped oil having the following properties: Total Acid Number 0.01 Viscosity, cSt @100°C 3.94 Flash Point, °F 370 (187.8°C) This stripped oil was blended with additives in the same manner as virgin lube oil to make a high-quality automatic transmission fluid. Example 5 A lubricating oil distillate was continuously treated in accordance with the subject invention in a pilot facility which did not provide for recycling any portion of the salt phase. Three gallons per hour of lube oil distillate (A) , preheated to 250°F (121.1°C), was mixed directly with 0.025 lb/hr (11.35 grams per hour) of sodium hydroxide dissolved in 0.05 lb/hr (22.7 grams per hour) of water and the mixture was fed to the settler. The resulting purified lube oil (B) was cooled to approximately 100°F (37.8°C) and fed to a coalescer which contained a fiberglass coalescing pad. The resulting salt-free purified oil (C) was collected. The following is a comparison of lube oil distillate, purified oil, and salt-free purified oil inspections, illustrating the improvements to the quality of the oil: Lube Oil Purified Salt-Free Distillate Lube Oil Lube Oil (A) (B) (C) Total Acid Number 0.40 0.00 0.01 Viscosity, SUS @ 100°F 191.5 189.3 Viscosity Index 97 99 Total Base Number 0.25 0.45 0.30 Flash Point, °F 430 430 (221°C) (221°C) Characteristic Odor Moderate None None A minor quantity of salt phase was obtained from the coalescer and mixed with the salt phase from the settler. This mixture had the following properties: Strong Base Number 27 Total Base Number 126 Water, wt% 24 Caustic Utilization, % 79 It is to be understood that variations and adjustments in the amount of caustic and water and temperature, pressure and time conditions have been made successfully within the scope of this invention to accommodate differences in the characteristics of the feedstock, temperature of the lube oil distillate, and desired characteristics of the final re-refined lubricating oil. By measuring the total base and total acid numbers of the various fractions produced during the process, compensating adjustments to the concentration and amount of caustic solution or other conditions can be easily made during the continuous process so that the TAN of the lube oil fraction remains within desirable limits yet no undesirable emulsions are formed. And, remarkably, after the salt phase has first been generated and a portion of it has been recycled with the caustic solution, the process can tolerate even greater variations in process conditions. Thus, the process of this invention solves the problems of the prior art. This process can be used in batches, although it is used to better advantage as a continuous process, to produce high yields of a high quality, low acid lubricating oil. This process can accommodate wide variations in feedstock and, when the process includes recycling the salt phase with the caustic solution, it can tolerate generous changes in conditions and concentrations. The process is also more selective than alternative commercial treatments. The process of this invention is also advantageous because its by-products are not only environmentally unobjectionable but actually have a substantial value and the process can accommodate recycling of any of the by-products which are not immediately usable to further refine them for use. Thus, this invention provides a safe, flexible and economic process with few steps, which produces treatable or usable by-products, and requires only moderate energy use and reasonable capital expenditures. It will be seen that the above-described process for purifying used lubricating oil will achieve all the advantages and objects attributed to it and, while it has been described in detail, it is not to be limited to such details or examples except as may be necessitated by the appended claims. Others may practice the invention in any of a number of ways which will be suggested to one skilled in the art by the present disclosure, but such practices are considered to be within the scope of the invention.

Claims: WE CLAIM: 1. In a process for purifying used lubricating oil wherein said used lubricating oil has been pretreated to remove excess water and lower boiling components of said oil and to separate a lubricating oil distillate from high-boiling and non-boilable components of said used oil, the improvement comprising mixing said lubricating oil distillate with a minor amount of caustic in the presence of a controlled quantity of water; settling and separating said mixture by gravity into a lubricating oil fraction and a salt phase; and removing said separated salt phase from said lubricating oil fraction to provide a purified lubricating oil having a reduced odor and being highly stable to oxidation. 2. In the process of claim 1 wherein a portion of said salt phase is recirculated and mixed with said caustic and water and said lubricating oil distillate. 3. In the process of claim 1 or 2 wherein the caustic is an alkali metal hydroxide or ammonium hydroxide and said caustic and water are mixed with said lubricating oil distillate in sufficient quantities to generate an extractable salt phase from said mixture. 4. In the process of claim 1 wherein the caustic is an alkali metal hydroxide or ammonium hydroxide and the quantities of said caustic and water are sufficient to hydrolyze oxidation products contained in said lubricating oil distillate and to neutralize acids contained therein and to form a distinct and relatively easily extractable salt phase. 5. In the process of claim 1 wherein said caustic is present in one to ten times the stochiometric quantities relative to the total acid number of the lubricating oil distillate. 6. In the process of claim 1 wherein a caustic solution of approximately 20-50% by weight sodium hydroxide to water is mixed with said lubricating oil distillate. 7. In the process of claim 1 or 2 wherein the mixture of said lubricating oil distillate, caustic and water is allowed to settle at an elevated temperature and at a pressure sufficient to suppress vaporization. 8. In the process of claim 1 or 2 wherein the purified lubricating oil fraction is cooled and further salt is separated by coalescence from said oil fraction to obtain a substantially salt-free re-refined lubricating oil. 9. In the process of claim 1 wherein at least a portion of said separated salt phase is recirculated and mixed in minor amounts with the used lubricating oil before said oil is dehydrated. 10. A process for treating a lubricating oil distillate said distillate having been prepared from used lubricating oil by first removing excess water, lower boiling components, higher boiling components and non-boilable components from the used oil comprising mixing said lubricating oil distillate with a minor amount of a relatively concentrated solution of caustic in water wherein said caustic is present in one to ten times the stochiometric quantities relative to the total acid number of the lubricating oil distillate; settling and separating said mixture by gravity into a lubricating oil fraction and a salt phase, wherein said mixture is at least initially at a temperature of approximately 100°-400°F (37.8°-204°C) and sufficient pressure is applied to suppress vaporization; removing said separated salt phase from said lubricating oil fraction and cooling said lubricating oil fraction to provide a purified lubricating oil being highly stable to oxidationand free of the characteristic odor of re-refined lubricating oil. 11. In the process of claim 10, wherein a portion of said salt phase is recirculated and pre-mixed with said caustic solution before said caustic solution is mixed with said lubricating oil distillate. 12. In the process of claim 10 or 11, wherein the caustic solution mixed with the lubricating oil distillate is approximately 20 to 50% by weight sodium hydroxide to water. 13. In the process of claim 10 or 11 wherein the purified lubricating oil fraction is subjected to a coalescing treatment to obtain a substantially salt-free purified lubricating oil. 14. In the process of claim 10 or 11 wherein said caustic is present in two to three times the stochiometric quantities relative to the total acid number of the lubricating oil distillate. 15. In the process of claim 10, 11, or 13 wherein the purified lubricating oil is steam stripped to remove any low boiling point components of said lubricating oil fraction to provide a lubricating oil having a relatively high flash point, increased viscosity, and low volatility. 16. In a process for re-refining used lubricating oil wherein excess water and lower boiling components are first removed from said used oil, and said used oil is then distilled to separate a lubricating oil distillate from high boiling and non-boilable components of said used oil, the improvement comprising mixing said lubricating oil distillate with a minor amount of a solution of caustic and water wherein said caustic is present in two to three times the stochiometric quantities relative to the total acid number of the lubricating oil distillate; settling and separating said mixture by gravity into a lubricating oil fraction and a salt phase, wherein said mixture is at least initially at a temperature of approximately 200-250°F (93.3°-121.1°C)and at a pressure sufficient to suppress vaporization; removing said separated salt phase from said lubricating oil fraction and recycling a portion of said salt phase by premixing said recycled salt phase with said caustic solution before said caustic solution is mixed with said lubricating oil distillate removing said lubricating oil fraction from said salt phase and cooling said lubricating oil fraction to provide a purified lubricating oil being highly stable to oxidation. 17. In the process of claim 16, wherein the caustic solution is approximately 30 to 35% by weight sodium hydroxide to water. 18. In the process of claim 16, wherein a further portion of said separated salt phase is recirculated and mixed in minor amounts with the used lubricating oil before said oil is dehydrated. 19. In the process of claim 16 wherein the purified lubricating oil fraction is subjected to a coalescing treatment to obtain a substantially salt-free purified lubricating oil. 20. In the process of claim 16 or 19, wherein the purified lubricating oil is steam stripped to remove any low boiling point components of said lubricating oil fraction to provide a lubricating oil having a relatively high flash point, increased viscosity, and low volatility.

Inventors: FORESTER ALLEN R, STRAHORN DAVID A, FORESTER, ALLEN, R., STRAHORN, DAVID, A.
Applicants: FORESTER ALLEN R, STRAHORN DAVID A, FORESTER, ALLEN, R., STRAHORN, DAVID, A.
Application year: 1987
Main classification: C10M175
IPC codes: C10M175