IMPROVING STORAGE STABILITY OF OILS

Abstract: A method and apparatus for the improvement of storage stability of oils contacts them with an at least partly organic solvent solution comprising an oxidising agent and an acid and then separates said oil and solution. The solvent is wholly or substantially immiscible with the oil and preferably of a different density to it. The oxidising agent and the acid may conveniently comprise the same compound but preferably comprise separate components, particularly an oxidising valency of a transition metal and an organic sulphonic acid respectively.

Description: IMPROVING STORAGE STABILITY OF OILS This invention relates to a method of improving the storage stability of oils, in particular diesel fuels, and to apparatus for putting the method into effect. Oils are widely used industrially as fuels and as lubricants. A problem with some oils, particularly with some heavier oil fuels such as diesel fuels, gas oils and gas turbine fuels. Is the form¬ ation of sediments on storage, whether in tanks etc or when vehicles are left standing for a long period. Such sediments can cause blockages in fuel pipes, filters etc and fouling of engine components. It is believed that one important mechanism by which sediments are formed is reaction between trace compounds present in the oils to form solid or gummy products. Therefore even repeated filtration of the oils may not solve the problem of sediment formation. "Fuel" 67 (August 1988) 1124-1130 and 68 (January 1989) 27 - 31 suggest one of the principal reactions involved in sediment formation in diesel oils is the reaction between phenalanones (I) or Phenalenones (II), which are themselves produced by oxidation of phenalenes (III). and indoles to form sediment precursor compounds of formula (IV) where R is alkyl: where n is an integer 1 to 3 or more. Phenalenes, phenalanones, phenalenones and indoles are believed to be introduced into the oil, in the course of upgrading the oils by incorporation of catalytically cracked oils.. This is increasingly being done with diesel and gas turbine fuels. The 1989 article describes reactions between synthesised phenalanones or phenalenones and indoles under acid conditions to yield indolyl phanalene salts. Applicant's co pending PCT/AU90/00467 describes a method for testing diesel fuel for phenalenes, phenalenones, phenalanones and indoles which involves forming these blue salts by addition of an acid and an oxidising agent to the fuel, extracting the resulting blue salts into a fuel-imiscible solvent and observing the colour of the solvent phase. Having detected such compounds in the oil, or suspecting their presence, it is highly desirable to remove them from the oil in an attempt to improve storage stability, and it is an object of the present invention to provide a method and apparatus for doing this. The term "oil" as used herein includes all types of oils regard¬ less of their origin, which may be natural eg petroleum oils or shale oils, or may be wholly or partly synthetic, eg prepared from natural gas, from coal or by heating shale, or in any other way. The term specifically includes distillate fuels such as petroleum spirit (gasolines), naphthas, paraffins (kerosine), fuel oils such as tractor fuels, diesel fuels, gas oils, gas turbine fuels, lubricating oils, cutting oils, hydraulic oils etc. According to this invention, a method for at least partly remov- ing potentially sediment-forming compounds from an oil comprises contacting the oil with a solution of an oxidising agent and an acid in an at least partly organic solvent, said oxidising agent, acid and solvent being wholly or substantially immiscible with the oil, then separating the oil and solvent phases. Although it is not limited to any specific scientific theory it is believed that the method exploits the oxidation of phenalenes to phenalenones or phenalanones and the subsequent reaction of these latter two with indoles, if these compounds are present in the oil, then with the acid to form an indolyl phenalene salt. These salts are insoluble or only sparingly soluble in oils such as diesel fuel but are soluble in organic solvents, and therefore are extracted into the organic solvent. These salts are also intensely blue in colour. Any oxidising agent which is known to oxidise phenalenes to phenalenones may be used. Inorganic oxidising agents are preferred, particularly transition metals in an oxidising oxidation state such as Ce IV, Mn VII (eg permanganate) or Cr VI (eg dichromate) . These latter two give a rapid reaction. Other suitable oxidising agents include I_, I0„ , BrO , and CIO . H„0 p may also be used but can give a slower reaction. Preferred acids are those which are known to form salts with the indolyl phenalenones or phenalanones condensation product. For example mineral acids such as hydrochloric or sulphuric acid/or percholoric acid may be used and give a rapid reaction. Preferred acids are organic sulphonic acids, such as alkyl or especially aryl sulphonic acids for example alkylphenylsulphonic acids such as p-toluene sulphonic acid. Carboxylic acids, which may be a aliphatic or aromatic, such as acetic acid may be used but may give a slower reaction. Some acids are also oxidising agents, such as perchloric and nitric acids. Such oxidising acids may therefore in some cases be used in place of both the oxidising agent and the acid, or may be used in addition to either or both. Perchloric and nitric acids may for example be used as both an acid and oxidising agent or may be used together with an additional oxidising agent such as dichromate. Preferably the solvent is selected so as to be as immiscible as possible with the oil, and is a water-miscible organic solvent or an organic solvent-water mixture. It is desirable that the solvent has a different density to the oil so that the solvent and oil phases tend to separate spontaneously. The solvent may be more or less dense than the oil. Preferred solvents are therefore polar solvents such as alcohols, particularly lower alcohols (ie up to C_ alcohols), b especially methanol or ethanol, which are less dense than diesel fuel, or mixtures of these with water. If alcohols are used then the use of nitric acid is inadvisable because of the vigorous reaction that may take place between the two. If the solvent is an organic solvent- water mixture, for example or methanol - or ethanol - water mixture, then the proportions of organic solvent and water in the solvent may be important. If too much water is present then the solvent may only separate with difficulty from the oil, or not at all. If too much organic solvent is present then if the preferred transition metal ion is used as an oxidising agent then it may be difficult to dissolve in the solvent. The concentration of oxidising agent and acid in the solution may be varied between wide limits. Typically if a transition metal oxidising agent such as potessium permanganate or dichromate is used, a concentration of 2-3 weight % in the solution is suitable, which is conveniently the solubility limit of these oxidising agents in a methanol - water mixture containing 70-80% by volume of methanol. Typically the concentration of acid should be 10% by weight or less. The method works rapidly at ambient temperatures (20 -30 C) and may be accelerated by gentle warming. The relative volume ratios of oil to solution will vary from oil to oil depending inter alia upon the amount of phenalenones, phenalanones, phenalenes and indoles in the oil, and the way in which the method is put into effect as discussed below. The principal chemical reaction involved in the method of the invention is believed to be: where A is the acid counter anion. It is clearly desirable to use an amount of the oxidising agent and acid in excess of the amount of the sediment - forming compounds believed to be present in the oil, so as to force the reaction rapidly to completion. A typical diesel fuel for example may contain 1-2 ppm of these compounds, and an acceptable upper limit of sediment formation on long term standing, bu a UK Ministry of Defence Standard is 2 mg sediment per lOOg of diesel fuel, ie 20 ppm. These figures and the stoichiometry of the above reaction may be used as a rough guide to the minimum amounts of the oxidising agent and acid necessary. The method of the invention may be put into effect in a variety of ways, distinguished broadly as batch (or static) processes or continuous processes. On an industrial scale it is convenient to provide apparatus for putting the method into effect. Therefore according to a further aspect of the invention there is provided an apparatus for at least partly removing potentially sediment-forming compounds from an oil, in which the oil may be contacted with a solution as defined above, then separating the oil and solvent phases. in one embodiment of a batch process a quantity of the solution is added to a container of the oil, such as a storage tank, and the mixture simply left to stand. Preferably the mixture of oil plus solution is periodically agitated, especially immediately after addition of the solution, so as to encourage contact between the solution and the oil, preferably by breaking the solution up into small droplets thereby increasing the interfacial surface area. After a suitable period of standing the oil and solvent phases are separated. If the solution and the oil have different densities, spontaneous separation into phases will occur, with the solution forming an upper or lower layer which can be removed by simple drainage. Suitable apparatus for putting such a batch process into effect will be apparent to those skilled in the art. It may for example in its simplest form consist merely of a container of convenient volume optionally provided with agitator means, and having inlet and outlet means for introducing and removing the oil and the solution. In an example of a continuous process the oil and the solvent are brought into contact as counter-currents which remain in moving contact for a suitable period of time. Preferably the streams are brought into contact in such a way that turbulence occurs and breaks up the streams into droplets, thereby increasing their interfacial surface area. Turbulence may optionally be increased by agitator means. In such a process the separation of the oil and solution may be achieved by known methods, for example if the oil and solution have different densities, separation can again be on this basis. For example from the region of turbulent contact the oil plus SUBSTITUTE SHEET solution may be passed into a relatively static region where spontaneous separation into phases which can be separately drained off occurs. Suitable apparatus for putting such a continuous process into effect will be apparent to those skilled in the art. It may for example consist of a mixing chamber provided with inlet means for the oil and the solution and constructed so that the oil and solution may be brought into contact, connected to one or more separation chambers wherein the physical mixture of oil and solution may separate as a consequence of a difference in density, the separation chamber(s) being provided with separate outlet means for the oil and solution. Whether the method is applied by a batch or continuous process it is desirable to test the treated oil for the presence of potentially sediment-forming compounds. This may for example be achieved using the test method described in PCT/AU90/00467. Alternatively, as the indoyl phenolene salts formed in the method of this invention have an intensely blue colour, completeness of removal of sediment-forming compounds may be monitored by observing the colour of the solution phase that separates from the oil. This may be performed using a colourimetric instrument measuring absorbance in the solution phase in the 600-850 nm wavelength region, and the electrical output of this instrument may be electronically linked to the control system of the apparatus. After the oil has been treated by the method of the invention it may be necessary to further purify the oil to remove traces of the solvent, acid, oxidising agent etc. Generally filtration will suffice for this purification. The invention will not be described by way of example only with reference to Figure 1 which shows schematically apparatus for carrying out the method of the invention in a continuous process. 1. Laboratory Example A saturated aqueous solution of potassium dichromate and a 10 weight % solution of p-toluene sulphonic acid in methanol were prepared. 50 ml of the acid solution was mixed with 15 ml of the dichromate solution. This mixed solution was added to 500 ml of a straight run gas oil (known to be completely stable) which had been previously mixed with 10 volume % of a catalytically cracked oil (known to be unstable toward sedimentation) in a separating funnel. The combine solution and oil were shaken to form an unstable emulsion and then left to stand for 5-10 minutes. At the end of this time the methanolic solution had separated as a blue coloured upper phase. The lower oil phase was drained off and tested both by the method described in the above PCT and by a present standard test for sedimentation (eg the ASTM method D2274). Both tests showed the absence of sediment forming species to the above mentioned UK MOD Standard. The above experiment was repeated using as alternative oxidising agents potassium permanganate, hydrogen peroxide and percholoric acid, and as alternative acids hydrochloric, sulphuric, acetic and perchloric acids at concentrations similar to those described above. Perchloric acid could be used by itself as an oxidising acid in place of both oxidising agent and acid. The method was less rapid when hydrogen peroxide or perchloric acid were used. Example 2 450 ml portions of a catalytically cracked North Sea gas oil were treated as follows: a) 25 ml of 5% p-toluene sulphonic acid in methanol and 24 ml of 2% aqueous potassium dichromate in methanol were added to the fuel and the mixture was shaken vigorously for 1 minute and then allowed to settle for approximately 30 to 40 minutes. The fuel portion was drawn off and washed with methanol (4 x 100 ml) to remove excess reacted material. The fuel layer was then drawn off, washed with water and allowed to settle. The portion was centrifuged to remove excess water and then stored in the dark for a 6 month period under ambient conditions before filterable and adherent insoluables were determined. b) A smiliar volume of fuel to that used in a) was treated with 25 ml of each of the acid and oxidant used therein. The mixture was shaken as described and then separated fuel portion was washed with 5 x 50 ml portions of methanol to remove reacted material and then reoxidised using the above system except using a 2% acid solution. The fuel portion was finally washed twice SUBSTITUTE SHEET with 50 ml methanol and stored as in a). c) 500 ml of untreated fuel were stored under identical conditions as a control for a) and b). The results of the insolubles determinations are shown in Table I. Control a) Single oxidn. b) Double oxidn. Example 3 A continuous form of the method of the invention was carried out using two fuel blends made up of 5 and 10% light cycle oil in straight run distillate, these being typical concentrations of cracked material found in commercial blends. The blends were treated as follows: a) The fuel was mixed with 1% p-toluene sulphonic acid in methanol and in a ratio of 1:20 acid: fuel and saturated aqueous potassium dishromate was added in a ratio of 1:200 oxidant to fuel. The mixture was stirred vigorously, giving a contact time of approximately 30 minutes and then passed to a separate vessel to allow phase separation. The fuel phase was pumped to a further vessel for extraction with methanol in a ratio of 1-10 methanol to fuel, was separated and then the fuel phase treated with 1% potassium hydroxide in methanol in a ratio of 1:40 alkali to fuel. The treated fuel phase was given a final methanol rinse. b) The 5% blend was treated according to a) above and the 10% was treated using regime lacking the alkali rinse but in all other ways the same. The stability of each blend was determined using ASTM D4626 (storage at 43°C for 12 weeks) and filterable and adherent insolubles were measured. An untreated aliquot of each blend was also subjected to ASTM D4625 as a control. The results of the insolubles determinations is shown in Table II. 5% LCO blend Untreated Treated 10% LCO blend Untreated Treated Each reading given as two independent determinations of the same treatment. As a rule of thumb the ASTM D4625 test is generally accepted as being aquivalent to 12 weeks storage at 43 C to a year at ambient temperature. Thus 1 week at 43 C is equivalent to a month at ambient temperature. Significant improvement in storage performance with regard to insolubles precipitation and colour, being indicative of storage stability, was shown using the method of the present invention. it should be noted from Table I that if oxidation is not carried out sufficiently then the fuel may be rendered more unstable. Thus use of the test method of the co-pending PCT patent application PCT/AU90/ 00467 will be of use in determining the relevant amount of oxidant required prior to actually carrying out the present method. t will be realised that additional methanol washes are desirable for the removal of all the soluable reaction products from the treated fuel but that the alkali wash demonstrated in the examples in un¬ necessary. The methanol used in the method will, of course, be most conveniently recycled to keep costs down and reduce waste problems. Apparatus Referring to Fig 1, an apparatus for performing the method by a continuous process is shown; this comprises a mixing chamber 1 and a setting chamber 2; the mixing chamber 1 is cylindrical, vertically mounted and has an axially orientated vertical shaft 3 passing through and end-wall via an oil tight seal. The shaft 3 has radial blades 4, mounted thereon and on the interior wall of the mixing chamber are SUBSTITUTE SHEET radially inward pointing baffles 5. The mixing chamber 1 is provided with an inlet pipe 6 and an outlet pipe 7 communicating with the settling chamber 2 via control valve 8. The inlet 6 is connected via control valve 9 to oil inlet pipe 10 and to solution feed pipe 11. The control valve 9 is of a type that oil passing along inlet 10 cannot mix with solution passing along feed line 11 until both enter inlet 6. The settling chamber 2 is also cylindrical and vertically mounted and is provided with an oil outlet 12 near the bottom of the chamber 2, communicating with the control valve 13, a solution outlet 14 near the top of the chamber 2 communicates with the control valve 15 which in turn communicates with solution vent 16 and solution recycle pipe 17. The recycle pipe 17 communicates with solution feed 11 and with solution inlet pipe 19, the control valve 18 is of a type that allows solution to pass only from recycle pipe 17 into feed 11 and/or from inlet 19 into feed 11. The settling chamber 2 has a drain valve 20, and drain outlet 21 at it's lower end. In use a flow of diesel oil is passed into inlet 10, and a flow of a solution of an oxidising agent and an acid, for example having a composition as used in the laboratory example above, is passed into inlet 19, through valve 18 into feed 11. The flow of oil and solution and oil are allowed to flow via valve 9 and inlet 6 into mixing chamber 1, the proportions of oil and solution being metred by valve 9, so that chamber 1 is filled with the oil plus solution. The shaft is rotated by a motor (not shown), and in co-operation with the baffles 5 turbulence is set up in the chamber 1, which breaks up large globules 22 of the solution into an unstable emulsion 23 of small droplets in an oil matrix in the chamber 1, with a large inter¬ facial surface area. This emulsion is allowed to leave chamber 1 via outlet 7 and valve 8 to enter and substantially fill settling chamber 2, valves 13 and 30 being closed. As the emulsion 23 fills chamber 2 it separates as a consequence of the lower density of the solution into an upper solution phase 24, and an oil phase, free of solution 25. The purified oil 25 may be collected via outlet 12 and valve 13. The solution 24 may be collected via outlet 14, valve 15 and vent 16. Alternatively, if the ability of SUBSTITUTE SHEET the solution 24 to remove sediment forming compounds has not been exhausted it may be recycled and used for second and subsequent treat¬ ments of fresh oil by allowing the solution 24 to flow via outlet 14, and valve 15 back into feed 11 via recycle pipe 17. The rate of flow of oil and solution through the apparatus the degree of agitation in chamber 1 and the relative sizes of chambers 1 and 2 may be arranged so that the extent of contact between the solution and the oil is sufficient to remove sediment-forming compounds to an acceptable standard, whilst the flow through chamber 2 is slow enough to allow separation and coalescence of an upper solution phase 24 and a lower oil phase 25. The rate of flow may be controlled by electronic control of valves 8, 9, 13, 15 and 18 and of the oil and solution feed pumps (not shown). The apparatus also includes a colourimetric instrument 26 moni- toring the colour of the solution phase 24 so as to enable an assess¬ ment of inter alia whether the solution phase 24 can be recycled. The whole apparatus may be electronically, eg microprocessor, controlled. From time to time it may be necessary to drain the apparatus for cleaning etc and this may be done via valve 20 and outlet 21, and valve 9 and drain outlet 27.

Claims: CLAIMS 1. A method for the removal of potential sediment-forming compounds from an oil comprising: a. contacting the oil with a solution of an oxidising agent and an acid in an at least partly organic solvent, said oxidising agent, acid and solvent being wholly or substantially immiscible with the oil and; b. separating the oil and solvent phases. 2. A method for the removal of potential sediment-forming compounds from an oil comprising: a. contacting the oil with a solution of an oxidising agent and an acid in an at least partly organic solvent, said oxidising agent, acid and solvent being wholly or substantially immiscible with the oil; b. separating the oil and solvent phases; c. washing the oil phase from step b. with a solvent which is wholly or substantially immiscible with the oil and; d. separating the oil and solvent phases produced in step c. 3. A method for the removal of potential sediment-forming compounds from an oil comprising: a. contacting the oil with a solution of an oxidising agent and an acid in an at least partly organic solvent, said oxidising agent, acid and solvent being wholly or substantially immiscible with the oil; b. separating the oil and solvent phases; c. washing the oil phase from step b. with a solvent which is wholly or substantially immiscible with said oil; d. separating the oil and solvent phases produced in step c; e. repeating step a. with the exception that the acid concen¬ tration and/or oxidising agent concentration in said solution is reduced over that used in step a. f. separating the oil and solvent phases produced in step e.; g. washing the oil phase from step f. with a solvent which is wholly or substantially immiscible with said oil and; h. separating the oil and solvent phases produced in step g. 4. A method according to any one of Claims 1 to 3 wherein the oxidising agent used comprises an inorganic agent. 5. A method according to Claim 4 wherein the oxidising agent comprises a transition metal in an oxidising valency state. 6. A method according to Claim 5 wherein the oxidising agent comprises a CeVI, MnVII and/or CrVI containing compound. 7. A method according to Claim 4 wherein the oxidising agent comprises hydrogen peroxide, iodine, iodate ions, bromate ions or chlorate ions. 8. A method according to any one of Claims 1 to 3 wherein the acid comprises an anion capable of forming a salt with indolyl phenalone or it's phenalanone condensation products. 9. A method according to Claim 8 wherein the acid comprises a mineral acid. 10. A method according to Claim 9 wherein said acid comprises nitric, hydrochloric, perchloric and/or sulphuric acid. 11. A method according to Claim 8 wherein said acid comprises an organic acid. 12. A method according to Claim 11 wherein said acid comprises a sulphonic or carboxylic acid. 13. A method according to Claim 12 wherein said acid comprises an alkylphenylsulphonic acid. 14. A method according to Claim 13 wherein said acid comprises a para- tolueneεulphonic acid. 15. A method according to any one of Claim 1 to 3 wherein said oxidising agent component and acid component are the same. 16. A method according to Claim 15 wherein said oxidising agent com¬ ponent and acid component are comprised of nitric acid and/or perchloric acid. 17. A method according to Claim 4 wherein the concentration of oxidising agent comprises it's solubility limit concentration in the solvent being used. 18. A method according to Claim 5 wherein the concentration of oxidising agent comprises from 2 to 3 weight %. 19. A method according to any one of Claims 1 to 3 wherein said solvent comprises a water miscible organic solvent or an organic solvent-water mixture. 20. A method according to Claim 19 wherein said solvent comprises an oil immiscible alcohol. 21. A method according to Claim 20 wherein said solvent comprises an alcohol comprising from 1 to 5 carbon atoms. 22. A method according to any one of Claims 1 to 3 wherein said solvent has a different density to said oil. 23. A method according to Claim 19 wherein said solvent comprises a methanol - water mixture comprising 70 to 80% volume of methanol. 24. A method according to any one of Claims 1 to 3 wherein the concentration of the acid comprises up to 10% by weight. 25. A method according to any one of Claims 1 to 3 wherein the contacting and washing steps are carried out at from 20 to 30 C. 26. A method according to any one of Claims 1 to 3 wherein the removal is carried out by way of batch treatment. 27. A method according to any one of Claims 1 to 3 wherein the removal is carried out continuously. 28. A method according to any one of Claims 1 to 3 wherein the contact and/or washing steps are carried out for a time sufficient for the solution phase to obtain a colour indicative of the desired degree of purity. 29. A method according to any one of Claims 1 to 3 wherein the contact and/or washing steps are repeated a sufficient number of times for the solution phase to obtain a colour indicative of the desired degree of purity. 30. A method according to any one of Claims 1 to 3 wherein the contact and/or washing steps are carried out using a means to agitate the two phases and thus promote mixing. 31. A method according to any one of Claims 1 to 3, substantially as described herein with regard to the description. 32. A method according to any one of Claims 1 to 3, substantially as herein with regard to the Examples 1 to 3 and the description of the apparatus of Figure 1. 33. An apparatus for the removal of potential sediment-forming com¬ pounds from oil comprising a vessel having an inlet for a supply of said oil, an inlet for the supply of an at least partly organic solvent solution comprising an oxidising agent and am acid as solutes, both the oxidising agent and the acid, and the solvent being wholly or substantially immiscible in said oil, an agitating device within or SUBSTITUTE SHEET acting upon the vessel interior, and an outlet from said vessel leading to a settling device. 34. An apparatus according to Claim 33 substantially as described herein with regard to Figure 1. 35. A method or apparatus according to any one of Claims 1 to 34 wherein a filtering step follows the final separation of oil and solvent. SUBSTITUTE SHEET

Inventors: MARSHMAN SHEILA JOYCE, MARSHMAN, SHEILA, JOYCE
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1990
Main classification: C10G53
IPC codes: C10G27, C10G53, C10L1, C10M175, C10N30