US2041277A - Method for treating oils - Google Patents

Description

y '19, 1936 u. B. BRAY METHOD FOR TREATING OILS Original Filed May 23, 1935 II'VVENTOR. Ulric B. Bray ATTORNEY Patented May 19, 1936 PATENT OFFICE METHOD FOR TREATING oILs Ulric B. Bray, Palos Verdes Estates, Califl, as-

signor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application May 23, 1933, Serial No. 672,433 Renewed March 26, 1935 14 Claims.

The present invention relates to a method for separating asphalt and wax from oil. More specifically, it relates to a method for separating asphalt from asphalt and wax containing oils in a plurality of stages. This invention is a continuation in part of my co-pending applications Serial No; 466,189, filed July 7, 1930; Serial No. 610,130, filed May 9, 1932; and Serial No. 659,854, filed March 6, 1933.

In my co-pending application Serial No. 466,189, I disclosed a two-stage process for the separation .of asphalt and wax from crude'oils or topped crudes containing the same to produce lubricating oil. In the first stage of the process, the asphalt and wax containing oil was commingled with a liquefied normally gaseous hydrocarbon solvent, such as liquid propane, under pressure sufficient to maintain the propane in a liquid state. This permitted theasphalt to precipitate from the solution of oil and propane and settle to the bottom of the decanter. The oil solvent solution was separated from the precipitated asphalt and was then chilled'by evaporating a portion of the propane under reduced pressure. This caused the wax to precipitate from the solution. The wax was separated and then the propane was removed from the remaining oil solvent solution to produce a lubricating oil which was substantially free from asphalt and wax.

In my co-pending application Serial No. 610,130, Idisclosed a process for the co-precipitation of asphalt and wax by means of a liquefied normally gaseous hydrocarbon solvent, in particular, liquid propane, wherein the asphalt and wax containing oil was commingled with the solvent under pressure sufficient to maintain the solvent in a liquid state. The oil dissolved in the liquid propane and'the asphalt was thrown out of solution. A portion of the liquid propanev was allowed to vaporize under reduced pressure to chill the mixtureof oil, solvent, wax and asphalt to a sufliciently low temperature to cause precipitation of the wax. The ebullitionor boiling which accompanies the va-porization of the liquid propane caused a violent agitation of the mass and the precipitated asphalt acted as nuclear .orgathering bodies for the a glomerationof thewax as it was precipitated at the lower temperatures attained The agglomerated particles ofwax and asphalt v were then separatedfromthe oil solvent solution and the solvent was-;remov,ed from-the oil. This 'oil was a lubricati "g o'ilsubStantially free from asphaltf andfwax.

In the fqregoing processfl.have 'observed that the lubricating oil produced is also free froin the maintained-in a liquid; state at a pressure of 125 55 heaviest viscous oil fractions and has a lower Conradson carbon value. It is my belief that the presence of these heavy viscous oil fractions in lubricating oils is largely responsible for the high Conradson carbon values of lubricating oil. Such fractions were thrown out of solution by chilling the oil containing both asphalt and wax and settled to the bottom of the decanter. In other words, I have discovered that chilling an oil to precipitate wax will also eiTect precipitation of the heavy viscous oil fractions, which, when separated from the oil, will produce a lubricating oil of lower Conradson carbon value than when such fractions are retained in the lubricating oil.

In my co-pending application Serial No. 659,854, I disclosed a process for the co-precip-' itation of asphalt and heavy viscous oil fractions from asphalt containing residues substantially free from wax. Such precipitation was effected by mixing a liquefied normally gaseous hydrocarbon solvent, in particular, liquid propane, with the asphalt containing oil under pressure sufficient to maintain the solvent liquid and then reducing the pressure on the mixture to permit evaporation of a portion of the propane which chilled the oil solvent solution and precipitated the asphalt and heavy viscous oil fractions from solution. The precipitated mixture was removed from the oil solvent solution and was then heated to vaporize residual propane. The asphalt was recovered as a mixture of hard bitumen and heavy viscous oil fractions. The oil solvent solution sub- 'stantially free from asphalt and heavy viscous oil fractions was de-propanized and a lubricating oil. having a low Conradson carbon valuewas thus produced. 1

- In the present discussion of my invention by the term propane I intend to include such'hy- .drocarbons as are normally vaporous at ordinary 4 temperature andpressureysuch hydrocarbons inelude methane, ethane, propane,butane, isobua tane and mixturesthereof. These normally gase--' ous hydrocarbon solvents may be obtained-by rectification of casinghead gasoline by the soealled .stabilizing method now conventional in.

v the natural gasoline industry. v Theyare theoverhead obtained." They are liquefiedbycompression and cooled in the conventional manner and are drawn ofi, intopressure chambers where they.

are maintained in a liquid state until used. A'

typical analysis for.. such ,a -fraction is 6.72% propane, 19.91%gisobutane and.

ethane, 72.2%

normal butane. Such a fraction may be lbs. per square inch at a temperature of about 75 F. The predominating hydrocarbon member in this mixture is propane and hereafter when I use the word propane or liquid propane I am referring to such a fraction.

The present invention is related to a treatment of asphaltic oils containing wax. As stated in my aforementioned co-pending application, Serial No. 659,854 substantially all petroleum oils and particularly asphalt containing oils contain such heavy viscous oil fractions irrespective of whether they contain wax. It will be observed that all asphaltic oils contain more or less wax; some contain large quantities of wax, while others are substantially free but nevertheless contain a small quantity. Thus, such California crude oils as McKittrick, Poso Creek, Coalinga, Summerland and Sunset are typical asphaltic crudes containing very little wax, while other" crudes as Montebello, Kettleman Hills, Santa Fe Springs, Elwood, Torrance and Santa Barbara are classed as mixed base crudes, that is, they contain large quantities of wax as well as asphalt, Nevertheless, substantially all of these oils; contain high molecular weight bodies which are responsible for the high Conradson carbon value of lubricating oils. In treatingoils substantially free of wax, it is customary to maintain the temperature for separation of asphalt at approximately atmospheric, i. e. about 70 F. to 80 F. At such temperatures, the heavy viscous oil fractions are soluble-in the oil solventsolution and will not precipitate from solution. My present invention is directed towards removal of such fractions from oils. It is particularly directed to treatment of asphaltic oils containing wax.

By thepres ent invention, I propose to separate the asphaltic material from oils containing as phalt and wax in a plurality of stages, in particular, in two stages. Ihave now. discovered that I may precipitate and recover two entirely differenttypes of. asphalt, that is, a hard asphalt and a soft asphalt, if the precipitation by solvents and separation is effected in two stages. In one stage of the process, the oil containing asphalt ismixed with a liquefied normally gaseous hydrocarbon solvent under pressure suflicient to maintain the solvent in the liquid state to precipitate hard asphaltic material and at a temperature of substantially atmospheric, i. e. about 70 F. to 80 F. The precipitated asphaltic materlal is separated from the oil solvent solution and then the oil solvent solution which has been freed from the hard asphalt is gradually chilled, preferably at a rate not in excess of substantially 4 F. per minute in a second stage to a sufficiently low temperature; 1. e. about -40' F. to 0 F. to precipitate further quantities of asphaltic material. The character of the asphalt recovered in the first stage comprises a substantially hard asphalt or substantially pure bitumen,

while the asphaltic material recovered in the second stage comprises the heavy viscous oil fractions aforementioned which are soluble in the oil solvent solution at atmospheric temperatures. Chilling of the oil solvent solution from which the hardasphalt has been removedwill congeal such heavy viscous oil fractions and cause them to precipitate out of solution.

It will be observed that by the term "asphalt I intend to include such asphaltic. and non-waxy fractionswhich will separate or precipitate from asphalt containing oils when these oils are commingled with liquid propane or other solvents. Thus, the heavy viscous oil fractions precipitated from solution by means of solvents at low temperatures will be considered an asphalt or asphaltic material as well as the hard asphalt or substantially pure bitumen separated from the oil in the first stage at atmospheric temperature. Such asphaltic material recovered in the second stage may be considered a secondary asphalt or a pseudo asphalt. It is of light brown color and of about road oil consistency.

I have discovered that if the oil and propane subsequent to the separation of the hard asphalt or substantially pure bitumen I at atmospheric temperature is gradually chilled to the low temperatures suflicient to precipitate wax, that the heavy viscous oil fractions orpseudo asphaltic material soluble in the oil and propane at atmospheric temperatures will precipitate from solution and settle to the bottom of the chilling column before any substantial amount of precipitated wax will settle to the bottom of the column. I have observed that as the temperature is gradually lowered from atmospheric to the low dewaxing temperatures, asphaltic material will also gradually precipitate from solution even before'the temperature has been lowered sufiiciently to precipitate the wax from solution; such astially free from the light fipcculent wax also precipitated from solution. The light iiocculent wax will remain in suspension in the solution of oil and solvent during the period required for settling the heavier asphaltic material. The supernatant solution of oil and solvent containing the suspended precipitated wax may then be decanted and transferred to a wax settling chamber where a longer settling period may be provided to permit the wax to settle; If desired, the decanted oil solvent and precipitated wax may be passed to a centrifuge or to a filter press in order to separate the wax from the remaining oil and solvent. To obtain the chilling effect in the second stage of precipitation, I prefer to gradually reduce the pressure in the chilling column and thus vaporize propane which will chill the oil and effect precipitation. The ebullition accompanying the vaporization of the propane will agitate the solution of oil, solvent and precipitated wax and thus will prevent substantial amounts of the light wax from settling and mixing with the asphaltic material settling to the bottom of the column, although some of the heavy or hard wax will contaminate the asphalt. The agitation will also cause the precipitated asphaltic material to contact the sides of the chilling column which will adhere thereto and then run down to the bottom of the chamber.

Instead of employing a uniform slow cooling rate of not in excess of 4 F. per minute as stated above over the entire range of '70 to F. to 40 F., I have found that the propane solution may be chilled at a much more rapid rate, i. e. 10 F. perminute, until a temperature of approximately 35 to 45 F. is reached, provided the gins to precipitate from the propane solution. With the above discussion in mind, it will be perceived that it is an object of my invention to sep-- arate asphalt and wax from oil by means of a solvent in two stages, the first stage being operated.

rate asphaltand wax from oils by means of solvents in.-. two;stages wherein a substantially hard asphaltlis Precipitated and separated in the first stage and aspft asphalt and wax are precipitated and separated in the second stage, the precipitation and separation in the second stage being carriedoutata lower temperature and pressure than that iinlthe first stage.

A furtherobject of my invention resides in commingling an asphalt and wax-containing oil with :a liquefied normally gaseous hydrocarbon solvent under pressure suiiicient to maintain the solvent in'a': liquid state and at atmospheric temperatures of about 70 F. to 80 F. to precipitate a substantially hard asphalt,-separating the precipitated asphalt from the oil solvent solution and then gradually reducing the temperature of the oil solvent-solution to approximately 40 F.

' to F. by lowering the pressure to eifect vaporization of a portion of the solvent and thus precipitate heavy viscous oil fractions or malthas or secondary soft asphalt and also to precipitate wax, retaining the bulk of the precipitated wax in suspension of oil and solvent, separating the precipitated asphaltic material from the oil solvent andv suspended precipitated wax and then separating the-precipitated wax from the oil and solvent and then separating the solvent from the oil.

Other objects and advantages of my invention will be apparent from the following description refers to a schematic arrangement of of my invention taken from the drawing which apparatus for carrying out my invention:

Referring to the drawing, an asphalt and wax containing oil is taken from tank I and is drawn into line 2 by pump 3 where it meets a liquefied normally gaseous hydrocarbon solvent such as,

liquid propane taken from storage tank 4 via line 5 and pump 6 which forces the liquid propane through valve 1 into line 2. The amount of propane introduced into line 2 will depend upon the character of the oil and the temperatures desired in the second decgiter or chilling column 38. The mixture of liquid prdpane and oil containing asphalt and wax is passed through turbulence or mixing coil 8 into the first decanter or asphalt precipitator 9 where the asphalt in the oil is precipitated and settles rapidly as a slurry of substantially pure bitumen or hard asphalt containing some oil and propane. A pressure of approximately 125 lbs..per square inch and a tem- .perature of about 70 F. to 80 F. is maintained in the decanter 9. The amount of propane introduced into the oil will also depend upon whether it is desired to remove the asphalt at the bottom of the decanter 9 as a slurry of asphalt and propane,.i'. e. as a mixture. of substantially pure bitumen or hard asphalt and propane or as a liquid fraction of asphalt containing propane and some oil. If it is desired to separate the as phalt in decanter 9 as a liquid fraction, the volumetric ratio of propane to oil in decanter 9 will be within a range of 0.6 to 6 to 1, depending upon the character of the oil and the degree to which it was topped. Higher volumetric ratios will precipitate the asphalt as a slurry of substantially 'pure bitumen and propane.

sure reduction valve IS on line I5 into evaporator l'l which operates at a lower pressure. Superheated steam is introduced into the evaporator through perforated pipe iii to supply additional heat and to reduce the asphalt to the proper specification, generally only as regards flash and fire points. The overhead from evaporator ll passing through mist extractor I 9 is sent through line to cooler 2| and then to separator 22. The uncondensed propane from separator 22 is sent through line 23 controlled by valve 24 to lines 25 and 21 to the suction of compressor 28, where its pressure is raised to that in the high pressure system and is then sent via line 29 to cooler 38 where it liquefies and runs down into propane storage tank 4. Any condensed light oil in separator 22 is withdrawn through line 3i and condensed water through line 32. The asphalt is taken from the bottom of the evaporator l1 and sent through line 32 controlled by valve 34 to storage tank 35.

The overflow from decanter 9 consisting of propane and oil free from the asphaltic material the remaining material to a sufficiently low temperature which causes the aforementioned heavy viscous oil fractions and the like to precipitate from solution. The chilling of the oil and solvent also precipitates the wax from solution. However, as the temperature is gradually lowered to a dewaxing temperature in the column 38, the heavy viscous oil fractions and the like precipitate and settle to the bottom of the'column before substan- 5 tial amounts of precipitated wax will settle and be commingled with the precipitated asphaltic material. The agitation accompanying the ebullition causes the precipitated-asphaltic material to contact the sides of the chilling column, ad-

here thereto and then run down to the bottom of the column as asphaltic material substantially free from precipitated wax. The temperature is gradually lowered to the desired temperature, i. e. at a rate not in excess of 4 F. per minute, by controlling the pressure in the column 38 by proper operation of valve 48 on line 39 and compressor 28 which is connected to the column by lines 21 and 39.- The pressure will gradually be lowered in column 38 to about 0 to 25 lbs. gauge which corresponds to .a temperature of approximately 40 F. to 0 F. If desired the temperature in column 38 may be rapidly lowered as for example, at a' rate of approximately 10 F. per minute until a temperature of approximately to 45 F. is attained after which the temperature may be gradually lowered to approximately F. to 0 F. at a rate of approximately 3 F. per minute. The propane evaporating in'the column 38 will pass out of the top through line 39 con- Inchiller 38, sufficient propane vaporizes to reduce the temperature of trolled by valve 49 and then into line 21 to suction of compressor 28 where the vapors are cornpressed, passed through line 29, liquefied in cooler 38 and run down to propane storage tank 4. An equilibrium line 4| controlled by valve 42 connects decanter 9 with the. high pressure storage tank 4.

Due to the fact that suflicient time must be given to gradually chill the solution of oil and solvent passing into the column 38 and to make the process continuous, a plurality of chilling columns 38 may be provided and operated alternately. However, when operating a batch process, merely one of such chilling columns will be sufficient. Thus, the entire material from decanter 9 may be transferred to the column 38 and the chilling may be accomplished by controlling the operation of valve 48 on line 39..

The asphaltic material precipitated and settled in the bottom of column 38 is removed by line 43 controlled by valve 44 and pump 45 as a mixture of propane and heavy viscous oil fractions or asphaltic material which was not separated in the first decanter 9. Pump forces this mixture under pressure through heating coil 46 where it is heated to a sufficiently high temperature to render the precipitated matter fluid and to vaporize residual propane. The heated mixture is then passed into line'4'i and flashed through pressure reduction valve 48 on line 41 into evaporator 59 which operates. at a lower pressure. superheated steam is introduced into evaporator 59 through perforated pipe 5! for supplying additional heat and to reduce the asphaltic and heavy viscous oil fractions to a proper specification, such as flash and fire points. The overhead from the evaporator 58 passing through mist extractor 52 is sent through line 53 to cooler 54 and then to separator 55. The uncondensed propane from separator 55 is sent through line 5% controlled by valve 51 to lines 25 and 21 to the suction of compressor 28, where its pressure is raised to that in the high pressure system and is sent via line 29 to cooler-38 where it liquefies and runs down into propane storage tank 4. Any condensed pseudo asphaltic material or heavy viscous oil fractions are taken from the bottom of evaporator 58 and sent through line 60 controlled by valve 6| to storage tank 62.

Chilling column 38' is provided with a plurality of side draw-off lines 65 located at any desired position above the bottom outlet employed for withdrawing the secondary asphaltic material. Each of these lines is provided with valves 66. By proper operation of such valves the supernatant solution of oil, solvent and precipitated wax will be withdrawn from the column 38 as a solution substantially free from precipitated asphaltic materials. The propane, oil and precipitated wax suspended in the solution are withdrawn from column 38 via said draw-off line 55 and pumped by pump 51 into the vapor tight wax separator or settler In. In batch operation, only one of such settlers will sufiice but to make the process continuous a plurality of settlers may be provided, operated alternately in order to provide for sufficient settling periods. In order to prevent ebullition in the wax separator during the wax settling operation, pressure is imposed upon the solution of oil. This is accomplished by maintaining pressure within the separator by pump 61. As the chilled mass in the wax separator remains in a non-ebullient state, the Wax settles out and is col-' lected by vanes 'Il operated by pulley 12 con nected to a suitable source of power not shown.

The wax free oil dissolved in propane is withdrawn I from the separator 10 and pumped by pump 13 through lines 14 controlled by valves 15 into heater 16 provided with mist extractors 11 where the propane present is vaporized by the aid of steam circulated through the closed steam coil '18. The vaporized propane passes out of the heater through line 19 controlled by valve 88, cooled in cooler8l and then passes through lines 82 and 21 to compressor 28, cooler 30 into -storage tank 4. The dewaxed propane free oil passes from heater 16 by means of line 83 controlled by valve 84 to pump 85 which forces the heated oil solution through heat exchanger 86 and then through line 8'! into tank 88.

The precipitated wax slurry settling at the bottom of wax separator I9 is removed from the separator through line 89 and is passed through heat exchanger 85 where it extracts heat from the dewaxed oil from heater 16. The wax slurry is then pumped by pump 9| into line 92 through heater 93 and line 94 into separator 95. vaporized propane is passed to the storage tank 4 via line 96 controlled by valve 91, cooler 98, lines 99, and 21, compressor 28 and cooler 30. The propane free wax is withdrawn from the separator 95 through line I08 and pumped by pump 19| into line I02 controlled by valve I03 into storage tank I84.

It is to be understood that the above is merely illustrative of preferred embodiments of my invention of which many variations may be made by those skilled in the art without departing from the spirit thereof.

I claim:

1. A process for separating asphalt and wax from oil containing the same which comprises mixing said oil with liquefied normally gaseous hydrocarbon solvent, chilling the oil and solvent to precipitate asphalt at low temperature and to precipitate wax in the supernatant solution of oil and solvent, withdrawing the precipitated as-. phalt from the supernatant solution of oil, solvent and precipitated wax and separating the precipitated wax from the oil and solvent.

2. A process as in claim 1 in which the solvent comprises liquid propane.

3. A process as in claim 1 in which the precipitated wax in the supernatant solution of oil and solvent after withdrawal of the precipitated asphalt is separated from the solution of oil and solvent by filtration.

4. A process for separating asphalt and wax from oils containing the same which comprises mixing said oil with an asphalt precipitating solvent to precipitate asphalt, separating said precipitated asphalt from the oil solvent solution containing wax, chilling said solution to precipitate further quantities of asphalt and to precipitate wax, causing the chilled mixture to stratify into a lower layer of said'further quantities of precipitated asphalt and an upper layer of oil, solprecipitated wax to settle from the solution of oil and solvent.

7. A process as in claim 4 in which the precipitated wax in the supernatant solution of oil and solvent after withdrawal of the second precipitated asphalt is separated from the solution of oil and solvent by filtration.

8 A process for separating asphalt and wax from oils containing the same which comprises mixing said oil with a liquefied normally gaseous hydrocarbon solvent under superatmospheric pressure to precipitate asphalt, separating said precipitated asphalt from the oil solvent solution containing wax, reducing the pressure on said oil solvent solution to vaporize a portion of the solvent thereby chilling the oil and precipitating further quantities of asphaltic material and precipitating wax, causing the chilled mixture to stratify into a lower layer of said further quantitles of precipitated asphalt and an upper layer of oil, solvent and precipitated wax, withdrawing said further quantities of precipitated asphalt from the supernatant solution of oil, solvent and precipitated wax and separating the precipitated wax from the oil solvent solution.

" 9. A process for separating asphalt and wax from oils containing the same which comprises mixing said oil with liquid propane under pressure sufiicient to.maintain said propane liquid to precipitate asphalt, separating said precipitated asphalt from the oil and propane solution containing wax, releasing the pressure on the oil and propane solution containing wax to vaporize a portion of the propane and-rthus chill the oil to precipitate further quantities of asphalt and to precipitate wax, said chilling being under controlled conditions to cause said asphalt to settle asa lower fraction and to retain precipitated wax in suspension in the supernatant solution of oil and propane, separating said precipitated asphalt fromthe solution of oil and propane and precipitated wax, separating the precipitated wax from the oil and propane and separating the propane from the oil.

10. A process for separating asphalt and wax from oil containing the same which comprises mixing said oil with an asphalt precipitating solvent to precipitate asphalt, separating said precipitated asphaltfrom the oil solventvsolution containing wax, chilling said oil solventfsolution to a'temperature' su'fiiciently low to precipitate further quantities of asphaltic material and to precipitate wax, causing the chilled mixture to stratify into a lower layer of said further quantities of precipitated asphalt and an upper layer of oil, solvent arid precipitated wax, separating said second precipitated asphaltic material from the bulk of the precipitated wax and supernatant oil solvent solution, then separating said precipitated wax from the oil solvent solution and separating the solvent from the oil.

l1. A process for separating asphalt and wax from oils containing the same which comprises mixing said oil with propane under pressure sufficient to maintain the propane liquid to precipbull:

7 itate a substantially hard asphalt and at substantially atmospheric temperature, separating said precipitated hard asphalt from the oil propane solution containing wax, reducing the pressure on the oil propane solution to vaporize a portion of the propane and thus chill the oil to precipitate a soft asphalt and to precipitate wax causing said chilled mixture to stratify into a. lower layer of said precipitated soft asphalt and an upper layer of oil, remaining propane and precipitated wax, separating the said precipitated soft asphalt from the oil, remaining propane and precipitated wax, separatingthe precipitated wax and separating the remaining propane from the oil.

12. A process -for separating asphalt and wax from oils containing the same which comprises mixing said oil with an asphalt precipitating solvent, precipitating the asphalt from the oil and solvent in a plurality of stages, precipitating the wax from the oil and solvent in one of said asphalt precipitating stages, said precipitation of asphalt and wax insaid stage being under controlled conditions to cause said asphalt to settle as a lower fraction and to retain wax in suspension in the supernatant solution of oil and solvent, withdrawing said last mentioned precipitated asphalt from the bulk ofsupernatant precipitated wax oil and solvent and separating the precipitated wax from the oil and solvent,

13. A process for separating asphalt and wax from oils containing the same which comprises mixing said oil with a liquefied normal y gaseous hydrocarbon solvent under pressure and precipitating the asphaltfrom the oil solvent solution in two stages, the second of said stages being operated'at a lower temperature than the first stage and at a sufiiciently low temperature to precipitate wax, causing the chilled mixture to stratify into a lower layer of said further quantities of precipitated asphalt andan upper layer of oil, solvent and precipitated wax, separating the asphalt precipitated in said stages from the of supernatant solution of oil and solvent and precipitated wax and'separatlng the precipitated wax from the oil solvent solution.

14. A process for separating asphalt and wax from oils containing the same which comprises mixing said oil witlrliquid propane under pressure, precipitating asphalt from said mixture in two stages, the first of said stages being separated at a higher temperature and pressure than'the second of said stages whereby a substantially hard asphalt isprecipitated in said first stage and a substantially soft asphalt is precipitated in said second stage, precipitating wax insaid second stage and causing the same to remain in the supernatant layer of oil and propane, separating the hard asphalt precipitated in said first stage and separating'the substantially soft asphalt precipitated in said second stage from the bulk of precipitated wax and separating the pre-- cipitated wax from the oil solvent solution and then separating the solvent from the oil.

' ULRIC B. BRAY.

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