NL8100515A - METHOD FOR RECOVERING SOLVENT IN A HYDROCARBON EXTRACTION SYSTEM. - Google Patents

Description

Short designation: Process for solvent recovery in a hydrocarbon extraction system.

The invention relates to an improved solvent extraction process of a petroleum oil fraction containing aromatic and non-aromatic components. The invention particularly relates to an improved method for recovering solvents from the hydrocarbon extract in an extraction system using a solvent. Substantial savings in the energy needs of a solvent extraction process compared to processes using the conventional solvent recovery operations are obtained by the process of the invention. The process of the invention accomplishes the recovery of solvents from the extract phase in several separation stages comprising at least three pressure stages.

It is well known that aromatic and unsaturated.

the components of a hydrocarbon oil stock batch from the polyunsaturated hydrocarbon components can be separated by various methods, including solvent extraction of the aromatic and unsaturated hydrocarbons.

Processes used in practice primarily include extractions with furfural, N-methyl-2-pyrrolidone and phenol. The removal of aromatics and other undesirable components from lubricating oil base stocks improves the vicosity index, color, oxidation stability, thermal stability, and base oil inhibition and response. the final lubricating oil products.

A number of solvents are known which have an affinity for at least one component of a mixed 8100515 - 2 - oil charge stock and which are partially immiscible with an oil charge stock under the temperature and pressure conditions used in the solvent refining in the charge stock involving two liquid phases in the extraction 5 zone are formed. The two liquid phases generally consist essentially of an extract phase which is the largest | f. 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 10 together with smaller amounts of solvent. Among the solvents known to be useful for solvent extraction processes. of petroleum base lube stocks are furfural, N-methyl-2-pyrrolidone, phenols and other various known organic and inorganic solvents.

Recently, N-methyl-2-pyrrolidone has replaced interest in furfural and phenol as preferred solvents for the extraction of aromatic hydrocarbons from mixtures of aromatic and non-aromatic hydrocarbons.

The benefits of N-methyl-2-pyrrolidone as a lubricating oil extraction solvent for the removal of undesirable aromatic and polar constituents from lubricating oil base stocks are now well recognized by refiners, some of which are other solvents, such as phenol or furfural, 25 For refining, use or previously used lube oil base stocks. N-methyl-2-pyrrolidone is generally the most preferred solvent. due to its chemical stability, low toxicity, and its ability to produce refined oils of improved quality.

Typical known solvent extraction methods illustrating conventional solvent recovery operations are described in U.S. Pat. Nos. 3,329,606, 3,461,066, 3,470,089, and 4,013,54.9.

The process according to the invention is particularly applicable to existing plants for refining with phenol, furfural and N-methyl-2-pyrrolidone using a solvent recovery system in one or more stages and stripping of the 8100515-3 The invention is particularly suitable for the conversion of furfural and phenol process plants to N-methyl-2-pyrrolidone solvent systems with significant savings in the energy needs of the solvent refiner. manner.

When recovering a hydrocarbon extraction solvent, for example N-methyl-2-pyrrolidone, from the oil-solvent mixtures, i.e., the extract phase and the raf-10finate phase, wherein this solvent is separated from the oil-solvent mixtures by a combination of distillation and stripping, stripping with an inert gas rather than steam simplifies solvent purification and reduces the energy requirement of the process, compared to conventional steam stripping. Steam stripping is common in solvent refining processes. Inert gas strips are disclosed, for example, in U.S. Pat. Nos. 2,923,680, 4,013,549, and 4,057,491.

In conventional lubricating oil refining processes, the solvent extraction step is conducted under conditions effective to recover about 30 to 90 volume percent of the lubricating oil charge as a raffinate and refined oil and about 10 to 70 volume percent of the extract charge as an aromatic extract. The lubricating oil stock is contacted with a solvent such as furfural or N-methyl-2-pyrrolidone at a temperature of at least 5 ° C, preferably at least 50 ° C, below the temperature of full miscibility of the lubricating oil stock in that 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 dosages than oil than most other known solvents.

In the extraction step, the operating conditions are chosen to prepare a primary raffinate having a dewaxed viscosity index of about 70 to 100, preferably about 85 to 96. When furfural 81 00 5 1 5 4 is used as the solvent extraction temperatures in the range of from about 46 to 110 ° C, and preferably from about 60 to 95 ° C, with solvent dosages in the range of from about 100 to 600% are used to provide the desired 5-VI product. When N-methyl-2-pyrrolidone is used as a solvent, solvent extraction temperatures are in the range of from 43 to 100 ° C, preferably in the range of from 54 to 95 ° C, with solvent dosages in the range of from 50 to 500%, and at preferably within the range of 100 to 300%, suitable. Water or wet solvent can be injected into the bottom of the extractor or mixed with the recycled solvent to control dissolving power and selectivity.

To prepare a finished lubricating oil base stock, the primary raffinate is dewaxed to the desired pour point. If desired, the refined or dewaxed oil can be subjected to a final treatment to improve color and stability, for example, a gentle hydrogenation.

The invention provides improvements in the method of stripping solvent from the extract and raffinate products, removing oil contamination in the solvent, and controlling the water content of the solvent in the solvent refining system. The process of the invention simplifies solvent recovery and purification operations compared to conventional processes and involves significant savings in the energy needs of a solvent refining process.

Details of the invention will be apparent from the accompanying drawings and "the following" detailed description. of the method according to the invention.

The drawing is a schematic manufacturing scheme showing a solvent refining process "using a modified solvent recovery operation" according to the "process of the invention.

Lube oil feedstock, which may contain bag water, enters the system through line 5 and is heated in heater 6 to a temperature within the range of 65 to 120 ° C. The preheated feed stock is passed through line 7 into the upper part of an absorber stripper column 8, suitably 8 100 515-5 - maintained at a pressure within the range of 100 to 515 kPa, in which the feed stock of water is stripped, through an inert stripping gas which enters the lower part of the stripping column 8 through conduit 9. Column 8 is provided with suitable means / eg perforated bubble cap or cascade trays to ensure intimate countercurrent contact between the lubricating oil feedstock and the stripping gas. Inert gas containing water vapor is withdrawn from the top of column 8 through line 10. The resulting dehydrated feedstock 10 is withdrawn from the bottom portion of column 8 and passed through pump 11 through heater 12 and line 13 to the bottom portion of extraction tower 14 where it is intimately contacted with solvent which is in the top portion of extraction tower 14 enters through line 17. Wet 15 solvent from solvent purifiers 30 enters the bottom of the extraction tower 14 through line 99.

The raffinate mixture, which typically contains 85% hydrocarbon oil mixed with solvent, is withdrawn from the extraction tower 14 through line 19 and treated to recover raffinate from the solvent. The raffinate, after solvent separation, is the solvent-refined lubricating oil base stock, that is, the desired product of the process.

Most of the solvent appears in the extract mixture which is drained from the bottom of extraction tower 14. In this example, an extract mixture containing about 85% solvent is withdrawn from tower 14 through line 18. The extract mixture is first treated for solvent recovery from the extract and then extract recovery as a salable product of the process. The major portion of the extract mixture, which typically contains about 85% of the solvent, is passed through heat exchangers 20 and 21 which serve to preheat the extract mixture and placed in a low pressure flash tower 22. Flash tower 22 typically operates at a pressure of 170 to 205 kPa. Extract mixture from tower 14 is fed back into the upper part of tower 22 through lines 31 and 32. Solvent separated from the extract in flash tower 22 is vented through line 24 to heat exchanger 20 and, after condensation of solvent vapors. and further cooling in cooler 26, the solvent is transferred through line 27 to solvent purification and storage 30 for reuse in the process.

Most of the extract mixture, part of which has been removed from the solvent, is discharged from the lower part of column 22 by pump 36 and transferred by heater 37 and line 38 to the high pressure flash tower 39.

The high pressure flash tower 39 is conveniently operated at a pressure within the range of 375 to 415 kPa. A small part of the extract mixture from the bottom of extraction tower 14 is passed through lines 31 and 33 to the upper part of high-pressure separator 39 through lines 31 and 33 as a return for high-pressure separator 39. Otherwise, solvent mixture from low-15 pressure flash tower 22 is supplied to tower 39 and tower 22 as a return through lines 40, 31, 32 and 33.

The solvent vapors exiting the top of the high pressure flash tower 39 through line 41 are passed through heat exchanger 21 through indirect heat exchange with the extract-20 mixture from the bottom of extraction tower 14, condensing the solvent vapors and preheating the extract mixture prior to its introduction into low pressure flash tower 22. Reclaimed solvent is passed through line 42 to a solvent accumulator purification and storage 30 to be reused in the process.

The hydrocarbon oil extract withdrawn from the bottom of the high pressure separator 39 through line 44 still contains some solvent, preferably 5 to 15 volume percent solvent and 95 to 85 volume percent hydrocarbons. conduit 44 to vacuum flash tower 46 for further recovery of solvent from the extract. Vacuum flash tower 46 typically includes countercurrent darnp liquid contact trays, suitably of the cascade or bubble-type construction. Part of the extract mixture from ex-35 traction tower 14 or low pressure flash tower 22 is added to the top of vacuum flash tower 46 as backflow through lines 36 and 47. The vacuum flash tower may be at a pressure within the range of 10 to 100 kPa working.

Additional separation of 8100515 solvent extract takes place in the vacuum flash tower 46. Solvent vapors are vented from the top of the flash tower 46 through line 48 to a condenser 49 and solvent accumulator 50. Uncondensed gases that are withdrawn from accumulator 50 through line 51 to a suitable unsigned vacuum source may be siphoned or recycled through line 86 turn into.

An extract-rich fraction is withdrawn from the lower part of flash tower 46 through line 54 and introduced into the upper part of stripper 55. Stripper 55 is typically a countercurrent vapor-liquid contacting column equipped with bubbling trays in which the liquid extract flowing down the column is contacted with inert stripping gas introduced into the lower portion of stripper 55 through conduit 56. Part of the extract mixture of the. bottom of extraction tower 14 is fed back to the upper part of stripper 55 through conduits 36 and 57. Otherwise, part of the extract mixture from the lower part of low pressure flash tower 22 may be fed back to towers 22, 39, 46 and 55 through line 40.

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

Inert stripping gas and stripped solvent vapors are vented from the upper portion 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 vented into conduit 65 for recirculation in the process 35 as described below.

Raffinate mixture withdrawn from the top of extraction tower 14 through line 19 is heated in heat exchanger 59 by indirect heat exchange with stripped extract from extract stripper 55 and then passed through heat exchanger 67 81 00 5 1 5 - 8 - and heater 68 prior to the introduction into vacuum flash tower 70 in which solvent is separated from the raffinate mixture. A small portion of the raffinate mixture from line 19 passes through circulation through heat exchangers 59 and 67 and heater 68 and is recycled in the upper portion of vacuum flash tower 70 through line 71. A further portion of the raffinate mixture from line 19 passes through circulation through heat exchangers 59 and 62 and heater 63 and is introduced into the top portion of stripper 75 through line 72 as return 10.

Solvent vapors separated in flash tower 70 from the raffinate mixture are vented from the top of the tower to conduit 48 and, along with solvent vapors from flash tower 46, are discharged to condenser 49 in which the solvent vapors are condensed. The condensate solvent is collected in condensate accumulator 50 and uncondensed gases are vented through line 51, as explained above.

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

Condensates from accumulator drums 50 and 64 are fed by pumps 79 and 80, respectively, to solvent purification and storage system 30. Various process steps can be used for the purification of solvent for re-use in the process, including, for example, distillation, and azeotropic separation, absorption, gas stripping, etc., primarily for removal of excess water if present, and for removal of polymers, oils , etc. Excess water from an external source can be removed from the solvent 8100515 9 purification and storage system 30 through line 81. Solvent is recycled in the process from pump 82 through line 83 to line 17 as required.

Inert gas from strippers 55 and 75 may still contain solvent vapors after separation of condensate solvent in condensate separator 74. The inert gas from Scheldt 64 is repressurized by compressor 85 and transferred through line 86 to absorber stripper 8 which serves as an absorber for solvent vapors remaining in the inert gas stream. The flow of inert gas leaving compressor 85, suitably with a pressure in the range of 170 to 410 kPa or higher, depending on the pressure in tower 8, is at an elevated temperature due to the compression heat in the compressor. The gas stream can be heated or cooled as desired to maintain the desired temperature in the absorber stripper tower 8, which is suitable at a temperature within the range of 65 to 150 ° C.

In the absorber stripper 8, solvent vapors of the inert gas are absorbed, and water entering the system 20 with the oil feed stream is evaporated in 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 containing 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 contain a substantially inert gas including, but not limited to, nitrogen, methane, carbon dioxide, etc. Nitrogen is preferred as the inert gas for use in the process.

Contacting fresh lubricating oil stock with inert stripping gas previously used in the process for stripping solvent from the extract and raffinate products provides two important advantages. Solvent is recovered from the stripping gas and 8100515-10 water is simultaneously removed from the nutrient stock.

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When refining lubricating oil stocks with solvent, there is a tendency for light oils to build up in the solvent. The accumulation of light oils in the solvent in conventional solvent refining processes requires additional distillation in the solvent purification process steps to remove the accumulated oils. These light oils become common: Transferred from the extract of raffinate flash towers and strippers with the solvent and stripping medium. In the process of the invention, part of the extract mixture from the extraction tower 14 or from the flash tower 22 is used as a backflow for the flash towers 22 and 39 in the solvent recovery section of the process. In the illustrated preferred embodiment, extract-mixture from the extraction tower 14 or from the low-pressure flash tower column 22 is also used as a return for the extract recovery columns, i.e. flash tower 46 and stripper 55. In this embodiment, the return to vacuum exists flash tower 70 and stripper 75 in the raffinate. recovery section of the process from a portion of the raffinate mixture taken from the top of the solvent extraction tower 14.

The use of tower 14 primary extract mixture, or tower 22 extract mixture from which a substantial portion of the solvent has separated, as backflow for flash towers 22 and 39 virtually eliminates the transfer of light oil from the towers with the separated solvent. Also, in the other towers, the relatively low volatility of the extract fraction and of the raffinate fraction significantly reduces or eliminates the transfer of light hydrocarbons into the solvent vapors. At the same time, the streams, raffinate, and extract backflow are good absorbents for light hydrocarbon oil vapors that might otherwise tend to be transferred from the various solvent separation towers. The combination of pretreatment of fresh lubricating oil feedstock with inert gas recycle and the recycle of the various towers with extract and raffinate substantially eliminates or largely reduces the amount of solvent purification required for the process.

8100515 11

In a specific example of the method of the invention, 20,400 kg / hr wax distillate (WD-20) containing 45 ppm of water is fed to an absorber stripper according to the method of the invention in which it is stripped at 115 ° C and 138 kPa with 250 kg / hr nitrogen recycle gas containing 12.7 kg / hr N-methyl-2-pyrrolidone vapors transferred from the raffinate and extract strippers of a solvent refining unit as described above.

In the absorber stripper, the solvent present in the circulating inert gas stream is recovered from the inert gas. At the same time, water contained in the lubricating oil charge stock is almost completely evaporated in the nitrogen stripping gas.

The nitrogen and water vapor mixture leaving the absorber stripper is cooled to 50 ° C to condense the water vapor. Condensate water is separated from the recycled C gas stream and the resulting dry nitrogen is heated to 290 ° C and recirculated to the strippers; 52.2 kg / hr of heated dry nitrogen is fed to the extract stripper 20 and 197.7 kg / hr to the raffinate stripper. The raffinate and extract strippers are operated at a bottom pressure of 35 kPa and a top product pressure of 20.7 kPa. Both strippers are refluxed with N-methyl-2-pyrrolidone at a temperature of 20 ° C. The extract feed stream enters the extract stripper at 290 ° C and the product extract leaves the stripper at 260 ° C. The extract stripper feed contains 2,255 kg / hr extract and 435 kg / hr N-methyl-2-pyrrolidone. 154.2 kg / hr N-methyl-2-pyrrolidone is fed back to the extract stripper. Nitrogen * -30 stripping gas supplied to the stripper at 290 ° C and 35 kPa at 52.2 kg / hr is vented from the top of the stripper at 160 ° C and 20.7 kPa along with 589.2 kg / hr N-methyl -2-pyrrolidone. The overhead of the extract stripper is combined with the overhead of the raffinate stripper 35 which contains 197.8 kg / hr nitrogen and 836 kg / hr N-methyl-2-pyrrolidone at 20.7 kPa and 155 ° C. Nitrogen and N-methyl-2-pyrrolidone vapors from the strippers are transferred to a condenser where they are cooled to 75 ° C at 20.7 kPa condensing 1,425.2 kg / hr solvent. After removal of the condensate solvent, the nitrogen, in an amount of 250 kg / hr and containing 12.7 kg / hr of solvent vapors of 75 ° C, is transferred to a compressor in which the pressure of the mixture is increased to 175 kPa with an increase in temperature to 120 ° C. The compressed mixture is recycled to the absorber stripper for recovery of solvent vapors before being used again in the process.

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Claims (10)

A method of refining a lubricating oil ** feed stock using a solvent comprising contacting this feed stock with a selective solvent for aromatic ingredients of this feed stock in an extraction zone to form a raffinate phase containing a small amount of this solvent and an extract phase containing a large amount of this solvent, which phases are separated separately from that zone, removing part of the solvent from the extract phase in an extract solvent separation zone and then stripping a further part of the extract phase solvent with an inert stripping gas to form a mixture of solvent vapor and stripping gas, characterized by the steps of intimately contacting the feedstock with the mixture of inert stripping gas and solvent vapor in a first contact zone prior to the solvent extraction operation, where by the solvent vapors from this stripping gas are absorbed by the feedstock and at the same time external water contained in this feedstock is at least partially evaporated to form a mixture of inert gas and water vapor which is separated from the feedstock. 2. Process according to claim 1, characterized in that the solvent is removed from each of these phases by flash evaporation, distillation, rectification or a combination thereof, and the residual solvent is stripped from this extract and from this raffinate with an inert stripping gas which is a mixture of solvent vapor and stripping gas which is transferred to be contacted with fresh countercurrent lube oil charge stock in the first contact zone and which mixture of inert gas and water vapor obtained in the first contact zone is transferred to a first condensation zone in which at least a portion of this water vapor is condensed, the inert gas is separated from condensed water, and inert gas from which water and solvent are removed is contacted with said extract and said raffinate is introduced as said stripping gas for the removal of solvent therefrom. 8100515 3. A method according to claim 1, characterized in that the feed stock in said first contact zone is intimately contacted with an inert gas stripping agent at a temperature in the range of about 65 to 150 ° C and at a pressure in the range of about 100 to 515 kPa, and that a further part of that solvent is removed from that extract by contacting this extract in an extract stripping zone at a temperature of 204 to 371 ° C and a pressure of 0 to 415 kPa with the said inert strip-10 gas from the first contact zone / thereby forming said strip gas and solvent vapor mixture / which is transferred for contact with fresh lubricating oil feedstock in said first contact zone. A method according to claim 3, characterized in that said mixture of stripping gas and solvent vapors is cooled to a temperature in the range of about 38 to 150 ° C at a pressure in the range of from about 205 to 515 kPa causing condensation of at least a portion of said solvent vapors are made into a liquid, and condensed liquid solvent is separated from said inert gas before contacting the fresh feed stock. 5. A method according to claim 3 or 4, wherein the feed stock contains water and the inert stripping gas / separated in the first contact zone from the feed stock contains water vapor, characterized by the additional step of feeding this water vapor containing inert gas to a condensing zone in which at least a portion of this water vapor condenses and is separated from this inert gas before this inert gas 30 is contacted with said extract. 6. A method according to any one of claims 3-5, characterized by the additional steps of bringing most of said extract phase from this extraction zone to said extract solvent separation zone and bringing a small part of this extract phase from the extraction zone to the upper part of this extract solvent separation zone as a return for this separation zone. A method according to claim 6, characterized in that an additional small portion of this extract from the extraction zone 8100515 * # {* - IS - is introduced into the upper part of said extract stripping zone as a backflow therefor. A method according to any one of claims 3-7, characterized by the additional steps of removing a part of the solvent from said raffinate phase in a raffinate solvent separation zone, after which a further part of said solvent is removed from the raffinate by contacting the raffinate in a raffinate stripping zone, a portion of said inert gas from said first contact zone to form a mixture of inert gas and solvent vapors, and inert gas-containing solvent vapors from said raffinate stripping zone to said first contact zone . 9. A method according to claim 8, characterized in that a small part of said raffinate phase of the extraction zone is fed to the upper part of said raffinate solvent separation zone as a return therefor. 10. A method according to claim 9, characterized in that an additional small part of said raffinate phase of said extraction zone is supplied to the upper part of said raffinate stripping zone as a return therefor. 81 00 5 15 81504 - Texaco Devl. Corp 8100515