WASTE LUBRICATING OIL PRETREATMENT PROCESS

Abstract: A method of reducing the acidity and fouling tendencies of distillates, when waste lubricating oil is distilled, which comprises treating the oil in a state when the oil has some water content, with one or more alkali metal salts of an acid that is weaker than sulphonic, or with a hydroxide of an alkali metal salt.

Description: WASTE LUBRICATING OIL PRETREATMENT PROCESS This invention relates to re-refining of waste lubricating oil. Waste lubricating oil includes such material as used crankcase lubricating oil from internal combustion engines of motor vehicles. Such waste oil is often collected and is subjected to re-refining processes to remove contaminants and spent additives from the oil in order to produce a base oil from which new lubricants are manufactured. Prior methods of re-refining typically involved the following steps: 1) water removal by atmospheric distillation; 2) precipitation of additives and impurities as a sludge using a reagent such as sulphuric acid; 3) removal of sludge; 4) treatment of remaining waste oil with activated earth with simultaneous steam stripping; and 5) filtration of the oil. The aforementioned method has been in use for decades. However, increasing amounts of additives used in lubricating oil formulations have made the aforementioned processes increasingly difficult and expensive to operate. New methods have evolved that are aimed at avoiding the production of acid sludge and also the avoidance of the use of activated earth. For example, hydro-treatment is used to avoid treatment with activated earth. In order to prepare oil for earth treatment or hydro-treatment, it is necessary to remove the original additives present in the oil. The current method of choice, in place of treatment with a reagent such as sulphuric acid, is to distil the oil so as to leave the additives in the distillation residue. However, distillation results in thermal decomposition of some oil additives producing acids and tars or polymers, particularly in the vapour spaces of the distillation plant. Wiped-film evaporators using indirect heating, high turhulence, low residence time and high vacuum have been used to reduce thermal decomposition of additives. However it has not been possible to eliminate thermal decomposition and the consequent production of corrosive acid compounds and tars when using distillation procedures on a plant scale. In most countries where waste lubricating oil is collected for re-refining, the oils are mixed and stored in large quantities. When such oil is subjected to high temperature, for example during distillation of the oil, the pH of the distillate will be low and the acidic distillate produced will soon cause corrosion problems in the plant. In addition tars and polymeric products in the distillate will cause severe plant fouling. At laboratory scale, the pH of the distillates can be measured and the production of tars and polymers may be monitored by observing a blackening of the laboratory glassware. It has been discovered that addition of a base to waste lubricating oil that is undergoing heat treatment (such as during distillation) will not necessarily result in a decrease in the production of acidity in the distillate. However, pre-treatment of such oil while in a state when the oil has some water content, with an alkali metal salt of a weak acid, will result in a reduction in the acidity of the distillate when the oil is subsequently subjected to the high temperature. Accordingly, this invention provides a method of reducing the acidity of the distillates, and of reducing the formation of polymers and tars which cause fouling of plant equipment, when used oil is distilled, which method comprises treating said oil while in a state when the oil has some water content with one or more basic compounds of an alkali metal. Suitable basic compounds for use in the method of this invention includes basic compounds such as sodium hydroxide. However, if such a basic compound is injected into the plant apparatus at the point of distillation, the added compound will not prevent the production of acidity in the distillate or reduce plant fouling. However, if the basic compound is added to the waste lubricating oil before distillation and while the oil still has some water content, it is possible to prevent the production of acidity in the vapour space and distillates during subsequent distillation. As will be described in more detail below, the alkalinity of the reagent used to treat the waste oil is not the important factor in the process but rather, the reagent used must be able to provide an alkali metal to substitute for di- basic metals such as calcium that are present in certain lubricating oil additives. Waste lubricating oil that has not been subjected to a heat treatment or a drying process will contain some water. Typically the water content will be at least five percent by weight. It is preferable that the method of this invention be carried out that while the waste lubricating oil subjected to the treatment contains at least one percent by weight of water. The alkali metal component of the reagent to be used in the method of this invention may include sodium, potassium, and lithium. Generally, the hydroxide, or a salt of any acid weaker than sulphonic (such as carbonate, borate, and sulphide, etc.), may be used. It has been found that the pre-treatment process of this invention is most advantageously carried out if the alkali metal reagent is added to the waste lubricating oil in sufficient quantities such that the alkali metal is present in an amount between 70-200% of the stoichiometric equivalent of the total amount of calcium, magnesium, and zinc present in the waste lubricating oil. The latter metals are the most common di-basic metals found in waste lubricating oil but there may be others such as barium. Preferably r the stoichiometric range employed will be 80-150% of the total calcium, magnesium, and zinc. Most preferably, the aforementioned stoichiometric relationship is maintained within the range 95-105%. It is possible to carry out the method of this invention wherein the aforementioned stoichiometric relationship is less than 70% but an increase in acidity will be experienced and equipment fouling may occur. It is also possible to employ the reagent at greater than 200% but generally, a gelling of the distillation residue will be experienced which may cause other operating problems. The amount of time it takes to dry waste lubricating oil that is subjected to heat varies depending upon the conditions employed. For example, when such oil is heated at approximately 100°C in laboratory conditions, it will usually take at least ten minutes to dry the oil. Under plant conditions, oil may undergo "flash drying" when plant feed is injected into dried oil maintained at 150°C. It is important that the pre-treatment process of this invention be carried out before the oil is dried in order that the reaction in the pre-treatment may proceed. It has also been found that the chemical reaction that takes place in the pre-treatment process of this invention is time and temperature dependant. Therefore, the pre-treatment process of this invention may be more quickly carried out if the temperature is elevated but, it is necessary that the oil not be dried before the reaction is complete. The reaction may also be carried out at room temperature and it has been found that typically, such reaction will be complete at approximately three days. At about 60°C, approximately one hour is required for the reaction to be complete. At about 99°C, the reaction will be complete in approximately ten minutes. This invention is most preferably carried out on a plant scale by carrying out the pre-treatment process in a continuous fashion using a large reactor vessel. Such a vessel may be a tank having a capacity of about 9900 litres. In such a tank a feed rate of approximately 4000 litres per hour will provide a residence time of about 2.5 hours. The level of the contents of the reactor vessel may be controlled using methods and devices known in the art for controlling the feed rate to the vessel and the pumping rate of the contents of the vessel to the next stage of process. The reactor vessel will preferably have some agitation means such as a motorized paddle. Preferably, the temperature of the aforementioned reactor vessel will be controlled so that it is maintained between 60°C and the boiling point of water (for example, at 82°C) . Methods and devices for maintaining the temperature of the reactor vessel are well known. A circulating pump may be used to circulate the reactor vessel contents through a heat exchanger and the pump may be controlled according to the temperature of the reactor vessel. A recirculation loop through a heat exchanger will also serve to keep the contents of the vessel agitated. The reaction vessel may be fully vented and is operated at atmospheric pressure. Preferably, the rate of addition to the oil of the alkali metal reagent is continuously adjusted to maintain an ideal stoichiometric relationship. For example, the oil treated in the reactor vessel may be analyzed after leaving the reactor vessel by ICP emission spectroscopy for calcium, magnesium, zinc, and sodium content and the amount of reagent added is adjusted so as to maintain the desired stoichiometric relationship. Preferably, the following formulae is used and the amount of sodium is adjusted to maintain the relationship within the range 95-105 (all percentages are weight) : The above relationship is for sodium; 45.98 is twice the atomic weight of sodium. The proportion of each other element is divided by that element's atomic weight. It may also be advantageous to carry out the method of this invention using several reagents. Sodium hydroxide is often preferred for its availability and low cost. However, a mixture of sodium hydroxide and sodium sulphide may also be used with the sodium sulphide being provided in the re-refining plant as a byproduct of a hydro-treating step. The oil that been subjected to the pre-treatment process of this invention may be distilled and the distillates therefrom may be subjected to other processes such as hydro-treatment. The following examples are illustrative of the method of this invention. Example 1 500 grams of waste lubricating oil taken from a large waste oil storage tank was heated, with stirring, in an open beaker to 150°C to remove the water. 250 grams of the aforementioned oil was placed in a 1 litre flask and a thermometer was used to measure the temperature of the liquid under distillation while the oil was heated to a temperature of 385°C using a gas ring. The pH of the distillate was measured to within one pH unit using pH paper and the flask vapour space and condenser were examined for tar and fouling deposits. The distillate was analyzed to determine the identity of any acid present by water extraction followed by identification of the acid by the barium chloride reaction. In this example, it was found that the pH was <1 and that the glassware was blackened in the neck of the flask and in the condenser inlet. It was shown that sulphuric acid was present in the distillate. Example 2 The method of experiment one was repeated but 2% (by weight) of a 50% sodium hydroxide solution was added to the oil before dehydration at 150°C. In this example, the glassware remained clean and the pH of the distillate was 6-7. Example 3 The method of example 2 was repeated except that 1% of calcium hydroxide was added to the oil rather than sodium hydroxide. In this case, the pH of the distillate was <1 and the glassware was fouled as in example 1. Example 4 The method of example 2 was repeated except that a solution of sodium carbonate was added to the used oil (rather than sodium hydroxide) in order to give a 1.33% (by weight) treatment. In this case, the pH of the distillate was 6-7 and the glassware remained clear. This result demonstrates that the functional reagent does not service to neutralize acidic compounds since calcium hydroxide in a water suspension is more alkaline than sodium carbonate. Example 5 The method of example 1 was attempted on a plant scale with sodium hydroxide being added to plant feed oil. The plant feed oil was flash dried at 150°C before entering a wiped film evaporator. However low distillate pH and plant fouling occurred. When oil from the flash dehydration unit was taken from the plant and distilled according to the method in example 1, the resulting pH was 1-2 and the glassware showed blackening. The difference in conditions between this example and example 1 was that in this case the oil that was treated with sodium hydroxide was subjected to flash drying (almost instantaneous drying) in the plant whilst the oil in example 1 had been boiled dry over a period exceeding 10 minutes. The process of this invention is time and temperature dependant; is ineffective when a basic reagent such as calcium hydroxide is used; and, requires the presence of water to proceed. This indicates that the mechanism of this process is not the neutralisation of sulphuric acid produced in distillation but rather the prevention of a reaction in which the sulphuric acid is formed. Detergent additives used in the formulation of motor oils typically contain di-basic metals (e.g. calcium, magnesium, nd barium salts of aromatic sulphonic acids of the general formula (using calcium as an example) : When such sulphonate compounds decompose at high temperature in the presence of available water, the following reaction occurs: When sodium sulphonates decompose under similar circumstances, sulphuric acid is not produced. + Na HSO, Thus, the process of this invention appears to involve an ion exchange reaction in which di-basic metal sulphonates are converted to alkali metal sulphonates. However, the ideal stoichiometric relationship in this invention is determined according to the predominant di¬ basic metals present in waste oil but not all of such metals are necessarily present in the form of sulphonates. Various changes and modifications may be made in practising this invention without departing from the spirit and scope thereof.

Claims: WE CLAIM: 1. A method for reducing acidity of distillates and for reducing the formation of polymers and tars which cause fouling of plant equipment, when waste lubricating oil is distilled, the method comprising treating said oil while in a state when the oil has some water content with one or more basic compounds of an alkali metal. 2. The method of claim 1 that comprises the additional step of heating the oil to remove the water in the oil or to distil the oil, after treatment with the basic compound of an alkali metal. 3. The method of claims 1 - 2, wherein the waste lubricating oil contains at least 1% by weight water. 4. The method of claims 1 - 3, wherein the basic compound is a hydroxide. 5. The method of claim 4, wherein the hydroxide is sodium hydroxide. 6. The method of claim 4, wherein the basic compound is lithium hydroxide or potassium hydroxide. 7. The method of claims 1 - 3, wherein the basic compound is an alkali metal salt of an acid weaker than sulphonic acid. 8. The method of claim 7 in which the salt is sodium sulphide. 9. The method of claim 7 in which the salt is a carbonate, borate or acetate of sodium, potassium or lithium. 10. The method of claims 1 - 9, wherein the amount of the basic compound of an alkali metal employed is in the range 70 - 200% of the total zinc, calcium and magnesium present in the waste lubricating oil. 11. The method of claim 10, wherein the amount of basic compound is in the range 85 - 150%.

Inventors: WILSON TERRY A, WILSON, TERRY, A.
Applicants: MOHAWK OIL CO LTD, MOHAWK OIL CO. LTD.
Application year: 1993
Main classification: C10M175
IPC codes: C10M175, C10N40