GB2069525A - Recovery of Solvent in Lubricating Oil Extraction System - Google Patents

Abstract

In a process for solvent refining a lubricating oil feedstock in which a portion of the solvent is recovered from the extract phase (e.g. by distillation or flash separation) and a further portion of the solvent is recovered by stripping the extract with an inert gas to form an inert gas/solvent vapour mixture, the inert gas/solvent vapour mixture is contacted with the lubricating oil feedstock to absorb the solvent from the gas and to remove extraneous water from the feedstock.

Description

SPECIFICATION Recovery of Solvent in Hydrocarbon Extraction System The invention relates to an improved process for the solvent extraction of a petroleum oil fraction containing aromatic and non-aromatic components. In one of its more specific aspects, the process relates to an improved method of recovering solvent from the hydrocarbon extract in a solvent extraction system. A considerable savings in the energy requirements of a solvent extraction process, as compared with processes employing conventional solvent recovery operations, is realized by the process of this invention. The process of the invention effects the recovery of solvent from the extract phase in a plurality of separation steps comprising at least three pressure stages.

It is well known that aromatic and unsaturated components of a hydrocarbon oil charge stock may be separated from the more saturated hydrocarbon components by various processes involving solvent extraction of the aromatic and unsaturated hydrocarbons. Foremost among the processes which have received commercial acceptance are extraction with furfural, N-methyl2-pyrrolidone and phenol. The removal of aromatics and other undesirable constituents from lubricating oil base stocks improves the viscosity index, color, oxidative stability, thermal stability, and inhibition response of the base oils and the ultimate lubricating oil products.

A number of solvents are known which have an affinity for at least one component of a mixed oil charge stock and which are partially immiscible with an oil charge stock under the temperature and pressure conditions employed in solvent refining the charge stock forming two liquid phases in the extraction zone. The two liquid phases generally consist essentially of an extract phase containing the major amount of the solvent together with dissolved aromatic components of the charge stock and a raffinate phase containing non-aromatic components of the charge stock together with minor amounts of solvent. Among the solvents which are known to be useful for solvent extraction processing of petroleum base lubricating oil stocks are furfural, N-methyl-2pyrrolidone, phenols and other various well known organic and inorganic solvents.

Most recently N-methyl-2-pyrrolidone has displaced furfural and phenol in importance as preferred solvents for extracting aromatic hydrocarbons from mixtures of aromatic and nonaromatic hydrocarbons. The advantages of Nmethyl-2-pyrrolidone as a lubricating oil extraction solvent for the removal of undesirable aromatic and polar constituents from lubricating oil base stocks is now well recognized by refiners, some of whom currently use or previously used other solvents, such as phenol or furfural for the refining of lubricating oil base stocks. N-methyl-2pyrrolidone is generally the most preferred solvent because of its chemical stability, low toxicity, and its ability to produce refined oils of improved quality.

Typical of prior art solvent extraction processes illustrating conventional solvent recovery operations are those disclosed in U.S. 3,329,606; 3,461,066; 3,470,089; and 4,013,549.

The process of this invention is particularly adaptable to existing phenol, furfural and Nmethyl-2-pyrrolidone refining installations employing a single or multiple stage solvent recovery system and steam or inert gas stripping of the solvent from the products. The invention is particularly suited to the conversion of furfural and phenol process installations to N-methyl-2pyrrolidone solvent systems with substantial savings in the energy requirements of the solvent refining process.

In recovering a hydrocarbon extraction solvent, e.g., N-methyl-2-pyrrolidone, from the oil-solvent mixtures, i.e., the extract phase and the raffinate phase, wherein said solvent is separated from said oil-solvent mixtures by a combination of distillation and stripping, stripping with an inert gas rather than with steam simplifies solvent purification and reduces the energy requirements of the process, as compared with conventional steam stripping. Steam stripping is common in solvent refining princesses. Inert gas stripping has been disclosed, for example, in U.S. 2,923,680; 4,013,549 and 4,057,491.

In conventional lubricating oil refining processes, the solvent extraction step is carried out under conditions effective to recover about 30 to 90 volume percent of the lubricating oil charge as raffinate or refined oil and to extract about 10 to 70 volume percent of the charge as an aromatic extract. The lubricating oil stock is contacted with a solvent, such as furfural or Nmethyl-2-pyrrolidone, at a temperature at least 5 C, preferably at least 500C, below the temperature of complete miscibility of said lubricating oil stock in said solvent.

Particularly preferred solvents are furfural and N-methyl-2-pyrrolidone, both of which are effective for the solvent extraction of aromatic components from lubricating oil charge stocks at relatively lower temperatures and lower solvent to oil dosages than most other known solvents.

In the extraction step, operating conditions are selected to produce a primary raffinate having a dewaxed viscosity index of about 75 to 100, and preferably about 85 to 96. When employing furfural as solvent, extraction temperatures within the range of about 46 to 11000 (115 to 2300F), and preferably about 60 to 950C (140 to 2050F), with solvent dosages within the range of about 100 to 600 percent are employed in order to provide the desired VI product. When N-methyl-2pyrrolidone is employed as solvent, solvent extraction temperatures within the range of 43 to 1000C (110 to 21 20F), preferably within the range of 54 to 950C (130 to2050F), with solvent dosages within the range of 50 to 500 percent, and preferably within the range of 100 to 300 percent, are suitable.Water or wet solvent may be injected into the bottom of the extractor or admixed with the recycled solvent to control solvent power and selectivity.

To produce a finished lubricating oil base stock, the primary raffinate is dewaxed to the desired pour point. If desired, the refined or dewaxed oil may be subjected to a finishing treatment for color and stability improvement, for example, mild hydrogenation.

The present invention provides improvements in the methods of stripping solvent from the extract and raffinate products, eliminating oil contamination in the solvent, and controlling the water content of the solvent in the solvent refining system. The process of this invention simplifies solvent recovery and purification operations as compared with conventional processes and effects substantial savings in the energy requirements of a solvent refining process.

Details of the invention will be evident from the accompanying drawings and the following detailed description of the process of this invention.

The accompanying drawing is a schematic flow diagram illustrating a solvent refining process employing a modified solvent recovery operation in accordance with the process of this invention.

Lubricating oil feedstock, which may contain extraneous water, enters the system through line 5 and is heated in heater 6 to a temperature within the range of 65 to 1200C (about 150 to 2500F). The preheated feedstock is introduced through line 7 into the upper part of an absorberstripper column 8, suitably maintained at a pressure within the range of 100 to 515 kPa (0 to 60 psig), wherein the feedstock is stripped of water by an inert stripping gas entering the lower part of stripping column 8 through line 9. Column 8 is provided with suitable means, for example, perforated, bubble cap or cascade trays, for insuring intimate countercurrent contact between the lubricating oil feedstock and the stripping gas.

Inert gas containing water vapor is discharged from the top of column 8 through line 10. The resulting dehydrated feedstock is withdrawn from the lower portion of column 8 and passed by pump 11 through heater 12 and line 13 to the lower portion of extraction tower 14 where it is intimately countercurrently contacted with solvent entering the upper portion of extraction tower 14 through line 17. Wet solvent from solvent purification means 30 enters the bottom of extraction tower 14 via line 99.

The raffinate mixture, comprising typically 85 percent hydrocarbon oil admixed with solvent, is discharged from the extraction tower 14 through line 1 9 and processed for the recovery of raffinate from the solvent. The raffinate, after the separation of solvent, is the solvent refined lubricating oil base stock, i.e., the desired product of the process.

The major portion of the solvent appears in the extract mixture withdrawn from the bottom of extraction tower 14. In this example, an extract mixture comprising about 85 percent solvent is withdrawn from tower 14 through line 1 8. The extract mixture is processed first for the recovery of solvent from the extract and then from recovery of the extract as a marketable product of the process. The major portion of the extract mixture, typically containing about 85 percent of the solvent, is passed through heat exchangers 20 and 21 which serve to preheat the extract mixture, and introduced into a low pressure flash tower 22. Flash tower 22 typically operates at a pressure 170 to 205 kPa (10 to 1 5 psig). Extract mixture from tower 14 is introduced into the upper part of tower 22 as reflux through lines 31 and 32.Solvent separated from the extract in flash tower 22 is discharged through line 24 to heat exchanger 20 and, after condensation of solvent vapors and further cooling in cooler 26, the solvent is passed through line 27 to solvent purification and"storage 30 for reuse in the process.

The major portion of the extract mixture, from which part of the solvent has been removed, is withdrawn from the lower portion of column 22 by pump 36 and passed through heater 37 and line 38 to high pressure flash tower 39. The high pressure flash tower 39 suitably is operated at a pressure within the range of 375 to 41 5 kPa (40 to 45 psig). A minor portion of the extract mixture from the bottom of extraction tower 14 is passed through lines 31 and 33 to the upper portion of high pressure separator 39 through lines 31 and 33 as reflux for high pressure separator 39.

Alternatively, solvent mixture from low pressure flash tower 22 may be supplied to tower 39 and tower 22 as reflux through lines 40, 31, 32 and 33.

The solvent vapors leaving the top of high pressure flash tower 39 through line 41 are passed through heat exchanger 21 in indirect heat exchange with the extract mixture from the bottom of extraction tower 14, condensing the solvent vapors and preheating the extract mixture prior to its introduction to low pressure flash tower 22. Recovered solvent is passed through line 42 to a solvent accumulator purification and storage 30 for reuse in the process.

The hydrocarbon oil extract withdrawn from the bottom of high pressure separator 39 through line 44 still contains some solvent, for example, 5 to 1 5 volume percent solvent and 95 to 85 volume percent hydrocarbons. This extract mixture is passed through line 44 to vacuum flash tower 46 for the further recovery of solvent from the extract. Vacuum flash tower 46 typically comprises countercurrent vapor-liquid contact trays, suitably of the cascade or bubble-tray type construction. A portion of the extract mixture from extraction tower 14 or low pressure flash tower 22 is supplied to the top of vacuum flash tower 46 as reflux through lines 36 and 47. The vacuum flash tower may operate at a pressure within the range of 10 to 100 kPa.

In the vacuum flash tower 46, additional separation of extract from solvent takes place.

Solvent vapors are withdrawn from the top of flash tower 46 through line 48 to a condenser 49 and solvent accumulator 50. Uncondensed gases withdrawn from accumulator 50 through line 51 to a suitable vacuum source, not illustrated, may be discarded or recirculated through line 86.

An extract rich fraction is withdrawn from the bottom of flash tower 46 through line 54 and introduced into the upper portion of stripper 55.

Stripper 55 is typically a countercurrent vaporliquid contact column provided with bubble trays in which the liquid extract flowing downwardly through the column is contacted with inert stripping gas introduced into the lower portion of stripper 55 through line 56. A part of the extract mixture from the bottom of extraction tower 14 is supplied as reflux to the upper portion of stripper 55 through lines 36 and 57. Alternatively, a part of the extract mixture from the bottom of low pressure flash tower 22 may be supplied as reflux to towers 22, 39, 46 and 55 via line 40.

Extract oil containing less than about 50 parts per million solvent, and typically comprising 80 percent unsaturated hydrocarbons and about 20 percent saturated hydrocarbons, is withdrawn from the lower end of stripper 55 by pump 58 and passed through heat exchanger 59 where it is cooled by indirect heat exchange with the raffinate mixture taken overhead from extractor 14 and discharged from the system through line 60 as a product of the process.

Inert stripping gas and stripped solvent vapors are discharged from the upper part of stripper 55 through line 62 to condenser 63 where solvent vapors are condensed. Solvent condensate is collected in condensate accumulator 64. Inert gas separated from the condensate is discharged into line 65 for recirculation to the process as described hereinafter.

Raffinate mixture taken overhead from extraction tower 14 via line 1 9 is heated in heat exchanger 59 by indirect heat exchange with stripped extract from extract stripper 55 and then passed through heat exchanger 67 and heater 68 prior to introduction into vacuum flash tower 70 wherein solvent is separated from the raffinate mixture. A minor portion of the raffinate mixture from line 1 9 by-passes heat exchangers 59 and 67 and heater 68 and is introduced into the upper portion of vacuum flash tower 70 through line 71 as reflux. A further portion of the raffinate mixture from line 19 by-passes heat exchangers 59 and 62 and heater 63 and is introduced into the upper portion of stripper 75 through line 72 as reflux.

Solvent vapors separated from the raffinate mixture in flash tower 70 are withdrawn from the top of the tower to line 48 and passed, together with solvent vapors from flash tower 46, to condenser 49 wherein the solvent vapors are condensed. The condensate solvent is collected in condensate accumulator 50 and uncondensed gases are withdrawn through line 51, as explained hereinabove.

Raffinate, still containing some solvent, is withdrawn from the lower part of vacuum flash tower 70 through line 74 to the upper part of stripping column 75 wherein the residual solvent is removed from the raffinate by stripping with inert gas entering the lower part of stripper 75 through line 76. Raffinate, substantially free from solvent, is withdrawn as a product of the process from the lower portion of stripper 75 by pump 77, passed in indirect heat exchange with raffinate mixture from line 19 in heat exchanger 67, and discharged through line 78 as the refined lubricating stock, the principal product of the process.

Condensates from accumulator drums 50 and 64 are passed by pumps 79 and 80, respectively, to solvent purification and storage system 30.

Various process steps may be utilized in the purification of solvent for reuse in the process, including, for example, distillation, and azeotropic separation, absorption, gas stripping, and the like, primarily for removal of excess water, if present, and for removal of polymers, oils, and the like.

Excess water from any extraneous source may be removed from the solvent purification and storage system 30 through line 81. Solvent is recycled to the process by pump 82 through line 83 to line 17, as required.

Inert gas from strippers 55 and 75, after separation of condensate solvent in condensate separator 64, may still contain solvent vapors.

The inert gas from separator 64 is repressured by compressor 85 and passed through line 86 to absorber-stripper 8 which serves as an absorber for solvent vapors remaining in the inert gas stream. The inert gas stream leaving compressor 85, suitably at a pressure in the range of 170 to 410 kPa (10 to 45 psig) or higher, depending upon the pressure in tower 8, is at an elevated temperature due to the heat of compression in the compressor. This gas stream may be heated or cooled as required to maintain the desired temperature in absorber-stripper tower 8 which is suitably at a temperature within the range of 65 to 150 C (about 1 50 to about 3000F).

In the absorber-stripper 8, solvent vapors are absorbed from the inert gas, and water entering the system with the lube oil feedstream is vaporized into the inert gas. Inert gas containing water vapor leaves the absorber-stripper column 8 through line 10 and is cooled in condenser 88 to a temperature sufficient to condense water vapor from the inert gas stream. Condensate separated from the inert gas is collected in condensate separator 89 from which condensate comprising water is discharged through line 90.

Inert gas from which water and solvent vapors have been removed is passed through line 91 and through heater 92 to lines 56 and 76 for introduction into strippers 55 and 75, respectively.

The stripping gas may comprise a substantially inert gas including, but not limited to, nitrogen, methane, carbon dioxide, and the like. Nitrogen is a preferred inert gas for use in the process.

By contacting fresh lubricating oil feedstock with inert stripping gas previously used in the process for stripping solvent from the extract and raffinate products, two important advantages are obtained. Solvent is recovered from the stripping gas and water is simultaneously removed from the feedstock.

In solvent refining of lubricating oil stocks, there is a tendency for light oils to accumulate in the solvent. The accumulation of light oils in the solvent in the conventional solvent refining process requires additional distillation in the solvent purification process steps to remove the accumulated oils. These light oils usually are carried over from the extract and raffinate flash towers and strippers with the solvent and stripping medium. In the process of this invention, a portion of the extract mixture from the extraction tower 14 or from flash tower 22 is employed as reflux to the flash towers 22 and 39 in the solvent recovery section of the process.In the preferred embodiment illustrated, extract mixture from the extraction tower 14 or from the low pressure flash tower column 22 is also utiiized as reflux for the extract recovery columns, i.e., flash tower 46 and stripper 55. In this embodiment, the reflux to vacuum flash tower 70 and stripper 75 in the raffinate recovery section of the process consists of a part of the raffinate mixture taken overhead from solvent extraction tower 14.

The use of primary extract mixture from tower 14, or extract mixture from tower 22 from which a substantial portion of the solvent has been separated, as reflux to flash towers 22 and 39 substantially eliminates the carryover of light oil from the towers with the separated solvent.

Similarly, in the other towers, the relatively low volatilities of the extract fraction and of the raffinate fraction substantially reduces or eliminates the carryover of light hydrocarbons in the solvent vapors. At the same time, the raffinate and extract reflux streams are good absorbents for any light hydrocarbon oil vapors which might otherwise tend to be carried over from the various solvent separation towers. The combination of pretreatment of fresh lubricating oil feedstock with the inert gas recycle and the reflux of the various towers with extract and raffinate effectively eliminates or substantially reduces the amount of solvent purification required for the process.

In a specific example of the process of the present invention, 20,400 kg/hr (45,000 pounds per hour) of wax distillate (WD-20) containing 45 ppm water is fed to an absorber-stripper in accordance with the process of this invention where it is stripped at 11 50C and 138 kPa (20 psia) with 250 kg/hr (551 pounds per hour) of nitrogen recycle gas containing 12.7 kg/hr (28 pounds per hour) of N-methyl-2-pyrrolidone vapors carried over from the raffinate and extract strippers of a solvent refining unit as described hereinabove. In the absorber-stripper, the solvent contained in the recycled inert gas stream is recovered from the inert gas. At the same time, water contained in the lubricating oil charge stock is substantially completely vaporized into the nitrogen stripping gas.

The mixture of nitrogen and water vapor leaving the absorber-stripper is cooled at 500C condensing the water vapor Condensate water is separated from the recycle gas stream and the resulting dry nitrogen heated to 290cm and recirculated to the strippers; 52.2 kg/hr (115 pounds per hour) of the heated dry nitrogen is supplied to the extract stripper and 197.7 kg/hr (436 pounds per hour) to the raffinate stripper.

The raffinate and extract strippers are operated at a bottom pressure of 35 kPa (5 psia) and an overhead pressure of 20.7 kPa (3 psia). Both strippers are refluxed with N-methyl-2pyrrolidone at a temperature of 200C. The extract feed stream enters the extract stripper at 2900C and the product extract leaves the stripper at 2600 C. The feed to the extract stripper contains 2,255 kg/hr (4,971 pounds per hour) of extract and 435 kg/hr (959 pounds per hour) N-methyl2-pyrrolidone. 154.2 kg/hr (340 pounds per hour) N-methyl-2-pyrrolidone is supplied to the extract stripper as reflux.Nitrogen stripping gas supplied to the stripper at 2900C and 35 kPa (5 psia) at the rate of 52.2 kg/hr (115 pounds per hour) is discharged from the top of the stripper at 1 600C and 20.7 kPa (3 psia) together with 589.2 kg/hr (1,299 pound per hour) of N-methyl-2pyrrolidone. The overhead from the extract stripper is combined with overhead from the raffinate stripper which comprises 197.8 kg/hr (436 pounds per hour) nitrogen and 836 kg/hr (1,843 pounds per hour) N-methyl-2-pyrrolidone at 20.7 kPa (3 psia) and 1 55 OC. Nitrogen and Nmethyl-2-pyrrolidone vapors from the strippers are passed to a condenser where they are cooled to 750C at 20.7 kPa (3 psia) condensing 1,425.2 kg/hr (3,142 pounds per hour) of solvent. After the removal of the condensate solvent, the nitrogen, amounting to 250 kg/hr (551 pounds per hour) and containing 12.7 kg/hr (28 pounds per hour) of solvent vapors at 750C, is passed to a compressor where the pressure of the mixture is raised to 175 kPa (about 25 psia) with an increase in temperature to 1200 C. The compressed mixture is recycled to the absorberstripper for recovery of the solvent vapors prior to reuse in the process.

Claims (14)

Claims

1. A process for solvent refining a lubricating oil feedstock comprising contacting said feedstock with a selective solvent for aromatic constituents of said feedstock in an extraction zone to form a raffinate phase comprising a minor amount of said solvent and an extract phase comprising a major amount of said solvent, which phases are separately withdrawn from said zone, removing a portion of the solvent from said extract phase in an extract solvent separation zone and thereafter stripping a further portion of said solvent from the extract phase with an inert stripping gas forming a mixture of solvent vapour and stripping gas, and including the steps of intimately contacting said feedstock with said mixture of inert stripping gas and solvent vapour in a first contacting zone prior to said solvent extraction operation whereby solvent vapours are absorbed from said stripping gas by said feedstock and simultaneously extraneous water contained in said feedstock is at least partially vaporized to form a mixture of inert gas and water vapour which is separated from said feedstock.

2. A process according to claim 1 wherein said solvent is removed from each of said phases by flash vaporization, distillation, rectification or a combination thereof, and residual solvent is stripped from said extract and from said raffinate with an inert stripping gas forming a mixture of solvent vapour and stripping gas which is passed to contact fresh lubricating oil feedstock in countercurrent fashion in said first contacting zone and said mixture of inert gas and water vapour obtained from said first contacting zone is passed to a first condensing zone wherein at least a portion of said water vapour is condensed, separating inert gas from condensed water, and inert gas from which water and solvent have been removed is passed into contact with said extract and said raffinate as said stripping gas for the removal of solvent therefrom.

3. A process according to claim 2 wherein said solvent is N-methyl-2-pyrrolidone.

4. A process according to claim 2 wherein said selective solvent is furfural.

5. A process according to claim 2 wherein said selective solvent is phenol.

6. A process according to claim 1 wherein said feedstock is intimately contacted in said first contacting zone with an inert gas stripping agent at a temperature in the range of about 65 to 1 500C (150 to 3000F) and at a pressure in the range of about 100 to 51 5 kPa (0 to 60 psig), in th-at said further portion of said solv.ent is removed from said extract by contacting said extract in an extract stripping zone at a temperature of 204 to 371 OC (400 to 7000F) and a pressure of 0 to 41 5 kPa (0 to 60 psia) with said inert stripping gas from said first contacting zone thereby forming said mixture of stripping gas and solvent vapour which is passed for contact with fresh lubricating oil feedstock in said first contacting zone.

7. A process according to claim 6 wherein said mixture of stripping gas and solvent vapours is cooled to a temperature in the range of from about 38 to 1 500C (100 to 3000 F) at a pressure in the range of from about 205 to ski 5 kPa (15 to 60 psig) effecting condensation of at least a part of said solvent vapours to a liquid, and condensed liquid solvent is separated from said inert gas prior to contact with said fresh feedstock.

8. A process according to claim 6 or claim 7 wherein said feedstock contains water and said inert stripping gas separated from said feedstock in said first contacting zone contains water vapour, characterised by the additional step of passing said inert gas containing water vapour to a condensing zone wherein at least a part of said water vapour is condensed and separated from said inert gas prior to contacting said inert gas with said extract.

9. A process according to any of claims 6 to 8 including the additional steps of passing the major portion of said extract phase from said extraction zone to said extract solvent separation zone and passing a minor portion of said extract phase from the extraction zone to the upper part of said extract solvent separation zone as reflux for said separation zone.

10. A process according to claim 9 wherein a further minor portion of said extract from said extraction zone is introduced Into the upper part of said extract stripping zone as a reflux therefor.

11. A process according to any of claims 6 to 10 also comprising the additional steps of removing a portion of the solvent from said raffinate phase in a raffinate solvent separation zone, thereafter removing a further portion of said solvent from said raffinate by contacting said raffinate in a raffinate stripping zone with a portion of said inert gas from said first contacting zone forming a mixture of inert gas and solvent vapours and passing inert gas containing solvent vapours from said raffinate stripping zone to said first contacting zone.

12. A process according to claim 11 wherein a minor portion of said raffinate phase from said extraction zone is supplied to the upper part of said raffinate solvent separation zone as reflux therefor.

1 3. A process according to claim 12 wherein a further minor portion of said raffinate phase from said extraction zone is supplied to the upper part of said raffinate stripping zone as reflux therefor.

14. A process for solvent refining a lubricating oil feedstock substantially as hereinbefore described with reference to the accompanying drawings.