SMILEY PATRICK HAROLD
MENON KRISHNA S
WELLS DANNY JOSEPH
US5360530A | 1994-11-01 | |||
US5358625A | 1994-10-25 |
1. | A process for solvent dewaxing a waxy petroleum oil feed to obtain petroleum oil lubricating stock under dynamic membrane selectivation conditions, which comprises: (i) chilling a waxy oil feed to crystallize and precipitate the wax in the oil feed and form a multiphase oil/solvent/wax mixture containing wax particles having a size range of 0. |
2. | to 5 microns and larger filterable wax particles; (ii) prefiltering the mixture to remove filterable wax particles having a size greater than 20 microns from the cold oil/solvent/wax mixture to recover a cold wax cake and a cold oil/solvent filtrate stream; (iii) feeding the cold oil/solvent filtrate stream containing wax particles having a size range of 0.2 to 20 microns under pressure to a selective permeable membrane for selectively separating the cold filtrate into a cold solvent permeate stream and a sold oilrich retentate stream which contains the dewaxed oil and the remaining solvent. |
3. | 2 A process according to claim 1 in which the cold oil/solvent filtrate stream contains wax particles having a size range up to 20 microns including 0.2 to 1 microns. |
4. | A process according to claim 2 in which the cold oil/solvent filtrate stream contains wax particles having a size range of 0.2 to 1 micron. |
5. | A process according to any of claims 1 to 3 in which the cold oil/solvent filtrate stream contains wax particles having a size predominantly in the range of 1 to 5 microns which wax particles comprise 20 to 500 parts per million by weight (ppm/wt) of the filtrate stream. |
6. | A process according to any of claims 1 to 4 in which membrane permeation temperature is maintained substantially isothermal to prevent dissolution of small wax particles. |
7. | A process according to any of claims 1 to 5 in which membrane permeation temperature is maintained at a temperature not greater than 5°C higher than the temperature of the filtration. |
8. | A process according to any of claims 1 to 6 in which the cold oil/solvent filtrate stream containing wax particles having a size range of 0.2 to 5.0 microns is fed under pressure of at least 2750 kPa at a temperature of 23 to 12°C to the a selective permeable membrane. |
9. | A process according to any of claims 1 to 9 in which the membrane consists essentially of 5 (6) amino1 (4 ' aminophenyl) 1, 3trimethylindane polymer. |
10. | A process according to any of claims 1 to 8 in which the retentate stream, after heat exchange with the warm waxy oil feed, is passed to an oil/solvent separation operation in which residual solvent is removed from the dewaxed oil and recycled to the dewaxing process and the wax free lubricating oil stock product is recovered. |
11. | A process according to any of claims 1 to 9 in which the cold solvent permeate stream at the filtration temperature is split into a first split stream for injection into the filter feed stream and a second split stream to cool the warm waxy oil feed by indirect heat exchange. |
This invention relates to processes for separation of oil/solvent mixtures containing waxy particles. In particular, it relates to recovery of solvent from a solvent dewaxing process using selectively permeable membranes.
Typical solvent dewaxing processes mix waxy oil feed with solvent from a solvent recovery system. The waxy oil feed/solvent mixture is cooled by heat exchange and filtered to remove solid wax particles. A filtrate comprising a mixture of oil and solvent is recovered from the filtration step. At present, dewaxing of waxy feed is performed by mixing the feed with a solvent to completely dissolve the waxy feed at a suitable elevated temperature. The mixture is gradually cooled to an appropriate temperature required for the precipitation of the wax and the wax is separated on a rotary filter drum. The dewaxed oil is obtained by evaporation of the solvent and is useful as a lubricating oil of low pour point.
This type of dewaxing apparatus is expensive and complicated. In many instances the filtration proceeds slowly and represents a bottleneck in the process because of low filtration rates caused by the high viscosity of the oil/solvent/wax slurry feed to the filter. The high viscosity of the feed to the filter is due to a low supply of available solvent to be injected into the feed stream to the filter. In some cases, lack of sufficient solvent can result in poor wax crystallization and ultimately lower lube oil recovery.
The use of solvents to facilitate wax removal from lubricants is energy intensive due to the requirement for separating from the dewaxed oil and recovery of the expensive solvents for recycle in the dewaxing process.
The solvent is conventionally separated from the dewaxed oil by the addition of heat, followed by a combination of multistage flash and distillation operations. The separated solvent vapors must then be cooled and condensed and further cooled to the dewaxing temperature prior to recycle to the process.
Membrane separation of solvent from the filtrate is a promising process, if suitably selective membranes can be found and operated a low temperature to achieve thermodynamic efficiencies. Such membranes are found m US Patents No. 5,264,166 (White et al) and 5,360,530 (Gould et al) . The present invention also relates to improved operation of selectively permeable membranes. These membranes are found to have a high permeabilility for solvent at low temperature, while rejecting oil, and are suitable for use in solvent recovery from the oil/solvent filtrate mixture.
It has been discovered that the membrane separation can be improved under dynamic operating conditions by retaining small particles of wax m the filtrate feed stream. By retaining particles mainly m the 0.2 to 5 micron size range in the filtrate, separation selectivity can be improved under normal operating conditions. It is believed that the small wax particles seal off micro- defects in the membrane structure without blinding the membrane surface. This improved selectivity can be secured without a loss in the baseline permeability of the membrane as may be the case with uncontrolled dynamic layer foramtion. The present invention therefore provides a process for solvent dewaxing a waxy petroleum oil feed to obtain petroleum oil lubricating stock under dynamic membrane selectivation conditions. The process is carried out by:
(1) chilling a waxy oil feed to crystallize and precipitate the wax in the oil feed and form a multiphase oil/solvent/wax mixture containing wax particles having a size range of 0.2 to 5 microns (meter X10 " °) and larger filterable wax particles;
(ii) pre-filtenng the mixture to remove filterable wax particles having a size greater than 20 microns from the cold oil/solvent/wax mixture to recover a cold wax cake and a cold oil/solvent filtrate stream; (m) feeding the cold oil/solvent filtrate stream containing wax particles having a size range of 0.2 to 20 microns under pressure, normally at filtration temperature, to a selective permeable membrane for selectively separating the cold filtrate into a cold solvent permeate stream and a sold oil-rich retentate stream which contains the dewaxed oil and the remaining solvent.
The retentate stream, after heat exchange with the warm waxy oil feed, is normally passed to an oil/solvent separation operation m which residual solvent is removed from the dewaxed oil and recycled to the dewaxing process and the wax free lubricating o l stock product is recovered.
By operating in this manner, the dynamic membrane selectivity is controlled by circulating the cold oil/solvent filtrate stream containing wax particles m the 0.2 to 20 micron size range in contact with the membrane surface during pressurized separation. The wax particles m this size range act to improve the selectivity of the membrane under what are normal operating conditions. It has been found that the preferred size range for the wax particles which are brought into contact with the membrane is 0.2 to 1 micron since this results in an increase in solvent purity with solvent permeation rates being
essentially the same as those through defect-free membranes under controlled laboratory conditions. Increase of the particle size of the wax particles from 1 to 5 microns has no incremental effect although improvement in selectivity is maintained. The drawings
Fig. 1 is a schematic process flow sheet; and
Fig. 2 is a graphic plot of membrane throughput and permeate composition vs. operating time. Detailed description
The following description of the process of the present invention is given with reference drawing. Metric units and parts by weight are employed unless otherwise indicated. ^ In Fig. 1, a waxy oil feed, after removal of aromatic compounds by convention phenol or furfural extraction, is introduced through line 1 at a temperature of 55 to 95°C [about 130 to 200°F] and is mixed with MEK/toluene solvent fed through line 2 at a temperature of 35-60°C [95 to 140°F] from the solvent recovery section, not shown. The solvent is added at a volume ratio of 0.5 to 3.0 solvent per part of waxy oil feed. The waxy/oil solvent mixture is fed to heat exchanger 3 and heated by indirect heat exchange to a temperature above the cloud point of the mixture of about 60-100°C [140 to 212°F] to ensure that all wax crystals are dissolved and in true solution. The warm oil/solvent mixture is then fed through line 4 to heat exchanger 5 in which it is cooled to a temperature of 35- 85°C [about 95 to 180°F] . The waxy oil feed in line 101 is then mixed directly with solvent at a temperature of 5-60°C [40 to 140°F] fed through line 102 to cool the feed to a temperature of 5-
60°C [40 to 140°F] , depending on the viscosity, grade and wax content of the waxy oil feed through line 102 in an amount of 0.5 to 2.0 parts by volume per part of waxy oil in the feed. The temperature and solvent content of the cooled waxy oil feed stream in line 101 is controlled at a few degrees above the cloud point of the oil feed/solvent mixture to preclude premature wax precipitation. A typical target temperature for the feed in line 101 would be 5-60°C [40-140°F] . The cooled waxy oil feed and solvent are fed through line 101 to scraped-surface double pipe heat exchanger 9.
The cooled waxy oil feed is further cooled by indirect heat exchange m heat exchanger 9 against cold filtrate fed to the heat exchanger 9 through line 110. It is in heat exchanger 9 that wax precipitation typically first occurs. The cooled waxy oil feed is withdrawn from exchanger 9 by line 103 and is injected directly with additional cold solvent feed through line 104. The cold solvent is injected through line 104 into line 103 m an amount of 0 to 1.5, e.g. 0.1 to 1.5, parts by volume based on one part of waxy oil feed. The waxy oil feed is then fed through line 103 to direct heat exchanger 10 and is further cooled against vaporizing propane in scraped-surface, double pipe heat exchanger 10 m which additional wax is crystallized from solution. The cooled waxy oil feed is then fed through line 105 and mixed with additional cold solvent injected directly through line 106. The cold solvent is fed through line 106 in an amount of 0.1 to 3.0, e.g. 0.5 to 1.5, parts by volume per part of waxy oil feed. The final injection of cold solvent at or near the filter feed temperature through line 106 serves to adjust the solids content of the oil/solvent/wax mixture feed to the main filter 11 at a rate of 3-10 volume percent, in order to facilitate filtration and removal of the wax from the waxy
oil/solvent/wax mixture feed to the main filter 11. The mixture is then fed through line 107 to the main filter 11 and the bulk of the large particle wax is removed. The temperature at which the oil/solvent/wax mixture is fed to the filter is the dewaxing temperature and can be -10 to +20°F] -23 to -7 C C and determines the pour point of the dewaxed oil product.
If desired, a slipstream 19 from line 104 can be combined with the solvent in line 106 to adjust the solvent temperature prior to injecting the solvent in line 106 into line 107. The remaining solvent in line 104 is injected into line 13 to adjust the solvent dilution and viscosity of the oil/solvent/wax mixture feed prior to feeding the mixture through line 103 to the exchanger 10. The oil/solvent/wax mixture in line 107 is then fed to rotary vacuum drum filter 11 in which the wax is separated from the oil and solvent.
One or more main filters 11 can be used and they can be arranged in parallel or in a parallel/series combination. A separated wax is removed from the filter through line 112 and is fed to indirect heat exchanger 13 to cool solvent recycled from the solvent recovery operation. The cold filtrate is removed from filter 11 through line 108 and at this point contains a solvent to oil ratio of 15:1 to 2:1 parts by volume and is at a typical temperature of -23 to +10°C [-10 to +50°F] .
The cold filtrate in line 108 is increased in pressure by pump IIA and fed to a pre-filter 109, according to the present invention, prior to contacting selective permeable membrane module Ml at the filtration temperature. Pre- filter 109 is selected to pass fine wax particles while retaining particles greater than about 20 microns in size to provide dynamic selectivity in the membrane separation.
Particles smaller than about 20 microns including the higly effective particles in the 0.2 to 5 micron range, are permitted to pass to provide the selectivation of the membrane separation. The membrane module Ml contains a low pressure solvent permeate side 6 and a high pressure oil/solvent filtrate side 8 with the selective permeable membrane 7 in between. Membrane permeation temperature is maintained substantially isothermal to prevent dissolution of small wax particles, preferably maintained not greater than 5°C higher than filtration temperature, and the process conditions are maintained at substantially steady state. In the preferred embodiments the cold oil/solvent filtrate stream contains wax particles having a size predominantly in the range of 0.2-20 microns (preferably 1 to 5 microns), with these wax particles comprising 20 to 500 parts per million by weight (ppm/wt) of the filtrate stream.
The cold oil/solvent filtrate at the filtration temperature is fed through line 108 to the membrane module Ml. The membrane 7 allows the cold MEK/tol solvent from the oil/solvent filtrate side 8 to selectively permeate through the membrane 7 into the low pressure permeate side
6 of the membrane module. The cold solvent permeate is recycled directly to the filter feed line 107 at the filter feed temperature.
The solvent selectively permeates through the membrane
7 in an amount of 0.1 to 3.0 parts by volume per part of waxy oil in the feed.
About 10 to 100%, typically 20 to 75% and more typically 25 to 50% by volume of the MEK/tol. solvent in the cold filtrate permeates through the membrane and is recycled to the filter feed line 107. The removal of cold solvent from the filtrate and the recycle of the removed solvent to the filter feed reduces the amount of solvent
needed to be recovered from the oil/solvent filtrate and reduces the amount of heat required to subsequently heat and distill the solvent from the filtrate in the solvent recovery operation, respectively. Higher oil filtration rates and lower oil-m-wax contents are obtained as a result.
The filtrate side of the membrane is maintained at a positive pressure of 1500-7400 kPa [about 2000-1000 psig] and preferably 2750-5500 kPa (400-800 psig) greater than the pressure of the solvent permeate side of the membrane to facilitate the transport of solvent from the oil/solvent filtrate side of the membrane to the solvent permeate side of the membrane. The solvent permeate side of the membrane is typically at 100-4000 kPa (0-600 psig, preferably 5-50 psig, for example at about 25 psig) .
The membrane 7 has a large surface area which allows very efficient selective solvent transfer through the membrane. The cold filtrate removed from the membrane module Ml is fed through line 110 to indirect heat exchanger 9, in which it is used to indirectly cool warm waxy oil feed fed through line 101 to the heat exchanger 9. The amount of solvent to be removed by the membrane module Ml is determined, to some extent, by the feed pre-cooling requirements. The cold filtrate is then fed through line 111 to line 115 and sent to an oil/solvent separation operation in which the remaining solvent is removed from the dewaxed oil.
The cold solvent permeate stream at the filtration temperature may be split into a first split stream for recycle and injection by way of lines 105, 106 into the filter feed stream 105 to further chill the feed to the filter with a second split stream for use in parallel with the cold filtrate stream passing through line 109 to cool
the warm waxy oil feed by indirect heat exchange m heat exchanger 9.
The solvent is separated from the oil/solvent filtrate in the oil/solvent recovery operation, not shown, by heating and removing the solvent by distillation. The separated solvent is recovered warm and returned through line 2 to the dewaxing process. The wax and solvent free oil product is recovered and used as lubricating oil stock. A portion of the solvent from the solvent recovery operation is fed through line 2 at a temperature of about 35-60°C (95 to 140°F) to be mixed with waxy oil feed through line 1. Another portion of the recovered solvent is fed through line 2 to line 16 into heat exchangers 17 and 13 m which the solvent is cooled to about the dewaxing temperature by indirect heat exchange against cooling water and wax/solvent mixture, respectively. Another portion of the recovered solvent is fed through lines 2, 16 and 14 to heat exchanger 15 in which it is cooled by indirect heat exchange with cold refrigerant, e.g. vaporizing propane, to about the fluid temperature in line 103 and fed through line 104 and injected into the oil/solvent/wax mixture in line 103.
The filtrate stream in line 111 can be fed through valve 15a and line 114 to membrane module M2. The filtrate is fed to module M2 at a temperature of 15 to 50°C and solvent is selectively transferred through the membrane 7a and is fed through line 116 and recycled to the dewaxing process. The membrane module M2 is operated in the same manner as membrane module Ml, except for the temperature of separation, and can contain the same membrane as module Ml. The use of the membrane module M2 embodiment allows reducing cooling capacity requirement and reducing utility consumption in the solvent/oil recovery section. However, since the recovered solvent permeate is at a higher
temperature than the solvent recovered from module Ml the solvent from the membrane module M2 must be cooled prior to being used in the dewaxing process, as for example in heat exchangers 15 or 17 and 13. The higher temperature, however, allows more solvent to be recovered because of the higher permeate rate of the higher temperature as compared to Ml. Membranes
In the present invention, a membrane module comprised of either hollow fibers or spiral wound or flat sheets may used to selectively remove cold solvent from the filtrate for recycle to the filter feed. For the solvent-oil separation of the present invention, the membrane materials that can be used include, but are not limited to isotropic or antisotropic materials constructed from polyethylene, polypropylene, cellulose acetate, polystyrene, silicone rubber, polytetrafluoroethylene, polyimides, or polysilanes. Asymmetric membranes may be prepared by casting a polymer film solution onto a porous polymer backing, followed by solvent evaporation to provide a permaselective skin and coagulation/washing.
In the following examples a polyimide membrane is case from 5(6) -amino-1- (4 ' -ammophenyl) -1, 3-trimethylindane polymer (commercially available as "Matnmid 5218") . The membrane is configured as spiral wound module, which is preferred due to its balance between high surface area and resistance to fouling.
The membranes are usually cast as a very thin layer on a substrate support. Very small imperfections in the layer may permit the non-selective passage of oil into the low pressure side. Micro-defects result in decreased selectivity and high oil contents in the permeate stream. Such defects may be inherent in the polymeric material or
manufacturing process and may be difficult to avoid using known membrane technology. It is believed that micro- defects can be sealed under operating pressure by correspondingly small wax particles having size range of 0 to 20 microns, especially in the range of about 0.2 to 5.0 microns. It is recognized that the optimal size range is dictated by the size of the surface defects. Examples As an example of the invention, a light lubricating oil feed is treated in the manner described in US Patent No. 5,360,530 (Gould et al), incorporated by reference, to remove aromatic compounds and is pre-diluted with solvent, heated to melt wax crystals and cooled. A MEK/tol. solvent is used at a ratio of MEK/tol. Of 25:75 to 100:0,, preferably 60:40 to 90:10, and more preferably 70:30 to
80:20. The total solvent to oil dilution ratio is 6:1 to 1:1, preferably 5:1 to 2:1, and more preferably 4:1 to 2:1.
The dewaxing temperature, i.e., the temperature at which the oil/solvent/wax mixture is fed to the filter, is preferably -23 to -12°C. The oil/solvent filtrate from the filter contains a ratio of solvent to oil of 6:1 to 1:1, preferably 5:1 to 3:1. The oil/solvent filtrate is fed to the membrane module Ml at the dewaxing temperature.
The operating temperature of the selective membrane is preferably -23 to -12°C, but in any case not more than about 5°C above the filtration temperature. There is transferred through the membrane module Ml 10 to 100 vol . % of the solvent in the oil/solvent filtrate stream, preferably 20 to 75 vol.%, and more preferably 25 to 50 vol.%.
A sufficient amount of solvent is transferred through the membrane to add 0.1 to 2.0 parts and preferably 0.5 to 1.5 parts of solvent per part of oil feed to the filter feed to provide a dewaxed oil product.
To demonstrate the process a large scale dewaxing unit is operated at steady state conditions of pressure and temperature using a light neutral base stock designed in Fig. 2 as Stock A. The unit is operated for 7 days with full pre-filter 109 in service removing wax particles greater than 0.2 micron from filtrate stream 108. The prefilter consists of two parallel banks of filters. Each filter bank is identical and handles half of the total process flow, each filter bank contains three filter housings in series. Each filter housing contains standard cartridge-type filter element. For this Example, the first housing in each bank contains filters which removal all particles greater than 75 microns in diameter. The second housing in each bank contains filters which remove all particles greater than 25 microns in diameter. The third housing in each bank contains filters which remove all particles greater than 0.2 micron in diameter. The measured permeate rate through the unit is about 2.8 metric tons/hr (T/H) . Oil content of the permeate ranges from 11- 15 wt.%. At an arbitrary point in time (at the 18 hr point indicated in Fig. 2) , each bank is removed from service one at a time and the 0.2 microns filters replaced with 5 micron filters. Unit operating conditions are held constant. Within 6 hrs, the permeate rate decreases by about 40% to 1.6 metric tons/hr. Sample analyses show that the oil content of the permeate is reduced to the range of 2-3 wt.%.
Satisfactory performance is maintained when unit operation is switched to a medium neutral base stock designated in Fig. 2 as Stock B. Oil content of the permeates for both stocks reach equilibrium at 0.5-1.5 wt.% after about 2 weeks on continuous operation.
The membrane selectivation is reversible. Upon re- installation of the 0.2 micron pre-filters, both the
permeate rates and the oil contents increase to their original values within a four hour period. This occurs because the solvent in the feed is sub-saturated with wax and therefore continually dissolves the wax particles from the surface. Therefore, a continuous supply of wax particles is needed to maintain their superior selectivity. Improved performance is re-established upon replacing the 0.2 micron prefilters with 5 micron filters.
Operation with no filter medium in the third housing (25 micron prefiltration only) results in a gradual increase of flow resistance and pressure drop across the membrane due to fouling of the membrane module feed channels.
Though the process and economic advantages of the present invention have been described as they apply to solvent lube dewaxing using MEK/toluene solvent, the invention can also be utilized in a similar manner in other solvent dewaxing systems, such as in propane dewaxing.