US2303721A - Art of dewaxing oils - Google Patents

Description

DCC l 1942- D. G. BRANDT ART OF DEWAXING' OIL Filed May 9, 1935 Patented Dec. 1, 1942 scam ART F DEWAXING OILS David G. Brandt, Westfield, N. J., assignor to- Cities Service Oil Company, New York, N. Y., a corporation of Pennsylvania Application May 9, 1933, Serial No. 670,066

4 Claims.

This invention relates to process and apparatus for removing wax or petrolatum from pertroleum oils, particularly such petroleum oil stocks as are to be used for motor lubricants.

Various types of procedure have been developed for removing wax from petroleum oils, the most common of which is to dilute the oil stocks, such as a bright stock, with petroleum naphtha and chill the mixture to a temperature sulciently low to precipitate the Wax in the oil mixture. The wax is then removed either by a long time cold settling or by ltration or centrifuging. Such processes are expensive and in many respects unsatisfactory because poor wax separations are obtained, extensive refrigerating plants are necessary for the indirect chilling of the naphtha-oil mixture, and comparatively high temperatures must be used for removing the naphtha from the dewaxed oil.

The primary object of the present invention is to overcome certain of the diiiiculties and disadvantages referred to above, and at the same time provide a process for dewaxing petroleum oils which will be highly efficient and economical.

Another object of the present invention is to provide a process for dewaxing petroleum oils such as petroleum residues and wax distillates in which a low boiling point, low molecular weight hydrocarbon or hydrocarbon mixture isused as a diluent and internal refrigerant for the oil, and which when mixed with the oil produces a blend having a relatively low specic gravity and viscosity.

A further object of the invention is to provide an 'apparatus suitably adapted to carry out the steps and features of the improved process.

In general the process of the present invention includes the steps of forming a mixture or blend of the oil stock to be dewaxed with a low boiling point hydrocarbon, such as liquid propane, chilling the resulting mixture to a low temperature under controlled pressure conditions, which chilling includes the vaporization of regulated proportions of the low boiling point hydrocarbon from the mixture, controlling the proportions of the constituents in the mixture during the chilling operation, and separating from the chilled mixture the wax or petrolatum precipitated therein.

Other objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description taken in connection with the accompanying drawing in which:

Fig. l is a diagrammatic view partly in secstock to be dewaxed is introduced into the system through a pipe 2 and forced by means of a pump Il and pipes and 8 into the upper part of a cooling chamber l0. In the pipe 8 the oil stock is mixed with a regulated amount of a hydrocarbon such as propane, andy these constituents are more thoroughly mixed in the upper part oi cooling chamber l0 by means of mixing blades l2 which are operated by a motor i4. The oil and propane are introduced into the chamber i0 at a pressure of approximately 300 pounds per square inch and at a temperature of about 110 F., so that a thorough solution of the oil in the propane is secured.

The chamber i0 is preferably constructed so as to hold a substantial volume of the oil-propane mixture, and is cooled by indirect heat exchange with water conducted through the cooling coil I6. The oil passing through chamber il) may be cooled to the normal temperature of water or vapproximately F.

The oil at the pressure maintained in chamber l0 is conducted therefrom through a pipe i8 and passed into the upper part of a heat exchanger 20 which is provided with a plurality of spiral coils 22. These coils are provided with solid supporting shafts 23 and spiders 25, and are connected by suitable packing glands into lower and upper header boxes 24, so that the coils may be rotated to prevent as far as possible the accumulationA of wax thereon.V lThe necessary mechanism 2S for rotating the coils includes a motor and suitable interconnecting gearing by which the spiral coils are slowly rotated` Dur-` ing the rotation of the coils, each coil contacts and rubs on two other coils, so that a substantial surface is kept free of any wax which may accumulate. The chilling medium used for cool'- ing the oil in tower 20 is the cold dewaxed oilpropane mixture resulting from the process (or other suitable cooling medium), which is introduced into the lower header 24 and discharged through the upper header.

The oil chilled in the tower 20 is discharged therefrom at a temperature of from 25 to 40 F. through a line 28 provided with 'an automatic pressure controlled' discharge valve 30V and discharged into the upper part of a two-chamber cooling tower 32. The oil discharged into the upper chamber of tower 32 is further chilled by reducing the pressure on the oil mixture and permitting the vaporization of a certain proportion of the propane. The vaporized propane is withdrawn from the upper part of tower 32 through an automatic pressure valve controlled line 34 and conducted through a line 86 to a l compressor 38.

The oil mixture collected in the upper part of tower 32 is withdrawn therefrom through a float valve controlled discharge line 48 and passed into the lower part of tower 32 where a further reduction in pressure is made so that additional quantities of propane are vaporized and further cooling of the oil accomplished. The vaporized propane is withdrawn through an automatic pressure valve controlled line 42 into the vapor line 36. The valve in line 42 controls the pressure in the lower chamber of tower 32.

The oil mixture collected in the bottom of tower 32 is withdrawn through a iioat valve controlled line 44 into the upper part of a second cooling tower 46 which is similar to tower 32. A lower pressure is maintained in the upper chamber of tower 46 than in the lower part of tower 32 so that further vaporization of propane may be effected in order to further chill the oil mixture. The vapors thus separated are withdrawn through an automatic pressure valve controlled line 50 into the Vapor line 36. The further cooling of the mixture may be accomplished by passing the same from the upper chamber of tower 46 through a oat Valve controlled line 52 into the lower part of tower 46 where further cooling is effected by vaporization of propane from the mixture. The vaporized propane is discharged from the lower chamber of tower 46 into vapor line 36 through an automatic pressure valve controlled line 54.

The oil mixture in the lower part of tower 46 now chilled to a temperature of approximately 40 F., or other desired temperature, and reduced to nearly atmospheric pressure, is conducted through a oat valve controlled line 56 by means of a centrifugal pump 51, and passed into a wax separating device 58 through a hollow rotary shaft 68 mounted therein.

The separating chamber 58 is a double walled or jacketed chamber which may be cooled by the evaporation of liquid propane in the manner referred to hereinafter. The separator 58 is constructed somewhat similar to a thickener and is provided with a plurality of sloping trays 62 supported at Various elevations in the separator and mounted on brackets 64 so as to leave a free space between the outer edge of each tray and the inside wall of the separator. The trays slope toward the center of the separator and are spaced from the central shaft 68 which extends to the lower part of the separator. This shaft as stated above, is hollow and provided with a plurality of oil discharge conduits 66, so that oil is discharged beneath each tray, except the lowermost. 'I'he chamber 58 is operated completely lled with oil which rises around the outer edges of the tra-ys 62, and is discharged from the upper part of the separator through a discharge line 68. The wax content of the oil precipitates slowly on the trays and is scraped therefrom by means of sweeps 18 which are attached to the branch conduits 66. As the shaft 60 is slowly rotated, the sweeps 18 force the wax toward the center of the chamber 58 so that it falls through the openings in the trays around the shaft 68. The wax accumulates in the lower part of the separator 58 and is Withdrawn by means of a gear or other suitable pump 12 and passed through a line 14 to any suitable washing device, or directly into a still 16 where any of the low boiling hydrocarbon (propane) is removed.

The still 16 may be heated by indirect steam or any other suitable heating medium as shown, and the vapors produced therein conducted through a baiiled still-head or short tower 18, then through surface condensers 88 where the propane is condensed. The still 16 may be operated at a pressure sufficient to effect the condensation of the propane in condensers 88. A part of the propane condensate produced in the first condenser may be returned to the tower 18 through a valved line 82, to control the outlet temperature therein, while the other part, together with the condensate from the second condenser 88 is conducted through lines 84 to a propane storage tank 86.

The dewaxed oil is conducted from the separator 58 through the line 68 and at a temperature in the neighborhood 40 F. into a tank 81 and forced therefrom by means of a pump 88, the operation of which is controlled by a oat in chamber 81, into the lower header 24 of the heat exchange tower 28, through a pipe 89. The cold oil from lower header 24 passes through the coils 22 in indirect heat exchange with the oil mixture to be dewaxed. The dewaxed oil after this heat exchange is conducted from the upper header 24 at a temperature of approximately 60 F., through a pipe 88 and heat exchanger 96 into a pipe still 98 in which the oil is heated under a high pressure to a temperature sufficient to vaporize the propane content of the oil. The heated mixture is conducted through a transfer line |88 into a baiiled tower |82 wherein the dewaxed oil stock is separated from the propane vapors and withdrawn through a float valve controlled line |84 into a baffled stripper |86 where the oil is stripped at low pressure with steam to remove any traces of propane which may be present. The propane vapors separated in tower |82 are conducted through condensers 88 where the. propane is condensed under high pressure, conducted to propane storage 86 and otherwise handled in the same manner as that described in connection with the tower 18.

The propane vapors and steam removed in stripper |86 are conducted through a surface condenser |88 and the resulting condensed water and uncondensed propane discharged therefrom into a receiver ||8 from which the water and propane are separately withdrawn and the propane conducted through a line ||2 (part not shown) to compressor 38. The wax residue discharged from still 16 may be stripped in the same manner as the oil withdrawn from tower |82.

The propane vapors or gas picked up by the l compressor 38 are passed at a high pressure to condensers ||4 in which the propane is condensed, and the condensate conducted through a line I6 into the liquid propane storage tank 86. Liquid propane to be used in the process may be withdrawn from storage tank 86 through a line ||8, in which is mounted a pump |28, and conducted through a valved line |22 into the line 8, where the liquid propane is mixed with the stock to be dewaxed.

In carrying out the chilling of the oil mixture passed through the chambers of chilling towers 32 and 46, the vaporization of portions of the propane in the oil mixture is relied upon for cooling the remaining mixture. In certain instances, it may be desirable to add additional quantities of cold liquid propane to the oil mixture passed through these towers to maintain the desired gravity and viscosity. This may be accomplished by withdrawing liquid propane from the line |22 through a line |24 in which is mounted a cooler |26, and then passing the cooled liquid propane into the line so that it is mixed with the oil mixture being discharged into the lower chamber of the tower 32. Similarly, liquid propane may be passed from the line ||8 through a line |28, a branch line |38 in which is mounted a cooler |32, and through valved lines |34 and |36 into the lines 44 and 52 respectively, which discharge into the upper and lower chambers of cooling tower 4E.

It is important in carrying out the process that the oil mixture be of such a viscosity that it is readily handled, and that its gravity is such that a separation of the wax or petrolatum may be made. Different types of oil stocks which are to be dewaxed obviously differ in viscosity and gravity as do the hydrocarbon diluents, so that the exact proportions of light hydrocarbon diluent must be adjusted accordingly. The oil wax mixture however should not be so dilute that the wax crystals when formed are too widely dispersed in the mixture. Preferably the dilution of the oil is maintained as low as possible until the wax precipitation is substantially complete; thereafter a greater dilution will aid in the separation of the wax from the oil without materially affecting the solubility of the precipitated wax.

Where the mixture is too dilute with light hydrocarbon, the wax crystals have a tendency to be very small and so widely separated in the mixture that large crystals or nuclei cannot be built up. In general, it has been found that with various types of petroleum oils, the mixture may comprise from about to 85% of the light hydrocarbon, it being understood of course that where vaporization of propane is relied upon for eiecting the chilling of the mixture, the higher percentages may be used.

In carrying out the process, the oil and low molecular weight hydrocarbon mixture may be cooled very slowly until it reaches a temperature within the range of about to 40 F., after which it may be chilled as rapidly as possible to the desired low temperature which may be from to 60 F. The rate of chilling however may depend on the type of oil. The mixture is then discharged into the separator 58 in which the temperature is maintained at the desired low point by introducing liquid propane into the jacket through a valved extension of line |28 which connects with the storage tank 86. In the jacket of separator 58 a part of the propane may be vaporized to maintain the desired low temperature and the vapors discharged therefrom into the gathering line 3B through a valved connecting pipe |38.

As a specific example of the process, a filtered cylinder stock having a viscosity at 210 F. of 151 (Saybolt) and a pour point of 70 F. was mixed with liquid propane at 110 F. The mixture was made up so that it contained 70% propane by volume. It was cooled slowly until it reached a temperature of about 34 F., and thereafter rapidly chilled to -58 F. The precipitated petrolatum was separated from the resulting chilled mixture by gravity. The oil decanted from the settled petrolatum after removal of propane had a pour point of 5 F.

The process of the present invention has been described in detail in connection with the use of propane as an example of the low boiling point hydrocarbon used as a precipitant and diluent. It is to be understood that the invention is not limited to the use of propane but that low molecular Weight hydrocarbons such as methane, ethane, propane, the butanes, the corresponding olens and mixtures thereof, may be used. In the production of natural gas gasoline, large quantities of substantially pure propane and butane are now separated from natural gas, and these products may be used in the process of the present invention. Where a low molecular weight hydrocarbon is specically prepared for use in the process no great care need be' taken to separate such constituents as methane, ethane, propane and the butanes, since any mixture of these constituents or with olens, may be used. The use of a hydrocarbon diluent having a narrow boiling range is preferred however, since more accurate regulation of temperatures and pressures is possible in passing the oil-diluent blend through the various stages of the process. It is apparent that where hydrocarbons such as methane, ethane or propane are used, much higher pressures must be maintained on the system than when the butanes are used in the process.

The wax residue discharged through the line 14 may be washed with cold diluent to remove occluded oil prior to its discharge into, the still 16.

The process as carried out in the apparatus shown in the drawing, may be varied in many respects without departing from the scope of the present invention. Likewise the apparatus may be varied. In the use of methane, ethane and ethylene or mixtures thereof, the oil blend may contain as low as from 20 to l0 per cent by volume of the diluent, and the blend may be chilled entirely by indirect heat exchange with a refrigerant. These hydrocarbons appear to affect the character of the blend to such a substantial extent that with some oils a good wax precipitation can be eiected at temperatures slightly below the pour point desired for the finished oil.

Mixed asphaltic and paraffin base oils may be treated by the process of the present invention because the asphalt is readily precipitated. In fact, oils containing any substantial percentage of asphalt may be first treated for the removal of the major part of the asphalt at one temperature and dilution, and then for the removal of wax at a lower temperature or greater dilution and lower temperature.

The procedure and apparatus elements used for separating the precipitated wax from the oil may be varied to suit particular types of oils or wax, and the wax may be separated in whole or in part by filtration or centrifuging. Crystalline wax may be settled from the major part ofni the oil and the remaining oil in the wax residue ltered.

Having thus described the invention in its preferred form, what is claimed as new is:

l. The process of separating wax or petrolaturn from mineral oils containing the same, which comprises forming a mixture comprising said oil and a hydrocarbon having not more than four carbon atoms to the molecule, agitating the resulting mixture at a temperature of approximately F. to insure thorough mixing of the constituents, slowly cooling the mixture to a temperature of from 25 to 40 F., said cooling step including the passage of the mixture in indirect heat exchange with cold dewaxed oil from the process, thereafter rapidly chilling the Wax-oil mixture to a temperature of approximately 40 F., said rapid chilling including the steps of passing the Wax-containing mixture through a series of large zones of gradually decreasing pressure in which portions of said low boiling point hydrocarbon are vaporized for the purpose of effecting said chilling, maintaining a predetermined proportion of low boiling point hydrocarbon in said mixture during said chilling operations by adding additional quantities of cold liquid hydrocarbon thereto, and separating the resulting precipitated wax from the chilled mixture.

2. The process of separating wax or petrolatum from mineral oil lubricants, which comprises mixing the mineral oil to be dewaxed with a low boiling point hydrocarbon at; a temperature of approximately 100 F., slowly cooling the resulting mixture to a temperature of from 25 to 40 F., and then rapidly chilling the mixture to a temperature of about 40 F. by vaporizing a portion of the low boiling point hydrocarbon contained in the mixture, maintaining a predetermined proportion of W boiling point hydrocarbon in the mixture during said chilling by adding necessary quantities of cold liquid 10W boiling hydrocarbon thereto, and separating the precipitated wax from the chilledv mixture.

3. An apparatus for dewaxing petroleum oil, comprising means for mixing the oil to be dewaxed with a diluent, means for cooling the liquid mixture by passing the same in heat exchange with a cooler liquid, said cooling means comprising a heat exchanger provided with movable, intermeshing helical cooling coils, a series of chambers for cooling the mixture by Vaporizing portions of the diluent, means for passing the cooled mixture through said chambers in series, automatic means for maintaining said chambers at progressively lower pressures, and means for separating precipitated Wax or petrolaturn from the chilled oil mixture.

4. In an apparatus for dewaxing petroleum oils in which means is provided for passing the cold dewaxed oil in heat exchange with relatively cool wax containing oil to be dewaxed, the improvement which comprises a heat exchanger for effecting said heat exchange, which includes a plurality of parallel helical coils intermeshing with each other, means for introducing the cold dewaxed oil into said coils, means for passing the oil to be dewaxed over said coils, and means for rotating said coils whereby a substantial proportion of the surface of said coils is kept free from precipitated wax.

DAVID G. BRANDT.

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