GB2023641A - Improved Filtration of Wax in a Solvent Dewaxing Process - Google Patents

Abstract

In the separation of wax crystals from dewaxed oil and solvent using a rotary drum vacuum filter having a filter cloth around its periphery embedded wax and/or resin is removed from the filter cloth on the drum by backwashing the filter cloth with cold solvent at not more than 30 DEG F above the filtration temperature introduced at 12-100 psig from within the drum over an area defined by an arc of the filter drum of 12 DEG - 72 DEG . Cake on the primary filter 3 is treated with an inward flow of inert gas over segment 14, washed with a spray 16 of cold solvent, treated with a second inward flow of inert gas over segment 20 and then backwashed with cold solvent or repulp filtrate 22 over segment 23. A backwashing process can also be applied to the repulp filter 26. <IMAGE>

Description

SPECIFICATION Improved Filtration of Wax in a Solvent Dewaxing Process The present invention relates to processes for solvent dewaxing distillate and/or residual petroleum oil stocks. More particularly, the invention relates to solvent dewaxing processes wherein a solution of waxy petroleum oil stock and dewaxing solvent is cooled at a uniform rate of about 1 to 80F/min (0.56 to 4.40C/min) to a selected temperature in the range of about +25 to -400F (-3.9 to L400C) for forming a mixture comprising wax crystals in a dewaxed oil/solvent solution, and wherein said mixture is separated in a solid-liquid separation zone into a wax cake and dewaxed oil/solvent solution.More particularly, the present invention relates to improvements in separating crystalline wax from said dewaxed oil/solvent solution employing a rotary drum vacuum filter.

It is known in the prior art to dewax waxy petroleum oil stocks by cooling oil-solvent solutions at uniformly slow rates, of e.g. 1 to 80F/minute (0.56 to 4.40C/min) under controlled conditions for crystallization of wax from said solutions. Commercially, such oil-solvent solutions are cooled by several methods such as indirect heat exchange in scraped surface exchangers; dilution chilling wherein waxy oil stock is contacted in a multi-stage chiller with chilled solvent under conditions of high levels of agitation (U.S. Patent No. 3,773,650); and direct chilling, wherein a low boiling solvent, e.g.

propane, mixed with waxy oil stock is vaporized under conditions of reduced pressure.

In such commercial processes, the waxy oil charge, or solutions of waxy oil charge and solvent, are heated to a temperature at which all the wax present is dissolved. The heated charge is then passed into a cooling zone wherein cooling is undertaken at a uniform slow rate in the range of about 1 to 8 F/minute (0.56 to 4.40C/min) until a temperature is reached at which a substantial portion of the wax is crystallized, and at which dewaxed oil product has a selected pour point temperature. Upon achieving the desired dewaxing temperature, the mixture of wax crystals, oil and solvent is subjected to solidliquid separation for recovery of a dewaxed oilsolvent solution, and a solid wax containing a minor proportion of oil (slack-wax).The separated oil-solvent solution is subjected to fractional distillation for recovery of a solvent fraction and a product dewaxed oil fraction. The slack wax may be recovered as is, or may be subjected to additional processing, such as repulp filtration for removal of additional oil therefrom.

Waxy petroleum distillate oil stocks and waxy residual petroleum oil stocks may be dewaxed according to the dewaxing process contemplated herein. Waxy distillate stocks boil in the range of about 600 to 6500F (31 6-3450C) initial boiling point to about 1050 to 11 000F (565 to 5930C) end point. Residual oil stocks have a viscosity 65 7C 75 80 85 90 95 100 105 110 115 120 125 greater than about 1200 SUS at 1 000F and have an initial boiling point of about 700+ OF (3700C).

Such waxy petroleum oil stocks may be derived from raw lube oil stocks the major portion of which boil above 6000F (3160C). Such raw lube oil stocks can be vacuum distilled, yielding overhead and side draw distillate streams and a bottom residual oil stream. Considerable overlap in boiling ranges of distillate streams and the residual stream may exist, depending upon distillation efficiency. Some heavier distillate streams may have almost the same distribution of molecular species as the residual stream.

Preferably, paraffinic crude oils are used as sources of petroleum fractions to be charged to solvent dewaxing units.

Petroleum fractions contain aromatic and polar compounds which are undesirable in lubricating oils. Such compounds may be removed, by means such as solvent extraction, hydrogenation, and similar means well known in the art, either before or after solvent dewaxing. Treatment of waxy oil stocks for aromatic and polar compounds removal before solvent dewaxing reduces the volume of oil to be dewaxed, which concomitantly reduces the amount of solvent employed, heat load, etc., in the solvent dewaxing process.

Residual fractions, and perhaps heavy distillate fractions, contain asphaltic materials in addition to aromatic and polar compounds discussed above. Such stocks are generally deasphalted, by methods well known in the art such as propane deasphalting, etc., prior to charge to a solvent dewaxing process.

Wax content of a waxy petroleum fraction is defined by the amount of wax removed to produce a dewaxed oil with a selected pour point, generally in the range of about +25 to -400F (-3.9 to -400C). Wax content of waxy petroleum fractions will vary in the range of about 5 to 35 weight percent. The wax removed in solvent dewaxing is a complex mixture of straight chain and branched chain paraffinic and naphthenic hydrocarbons. Wax in light distillate oil stocks generally predominantly comprises normal paraffinic hydrocarbons which have relatively high crystal growth rates, whereas wax in heavier petroleum fractions comprise mixtures of normal and isoparaffin waxes having relatively slower crystal growth rates. In solvent dewaxing processes, wax is separated as solid crystals.

Dewaxing solvents which may be used in solvent dewaxing processes include known dewaxing solvents. Commonly used solvents include aliphatic ketones of 3-6 carbon atoms, C2-C4 range hydrocarbons, C6~C7 aromatic hydrocarbons, halogenated C1-C4 hydrocarbons and mixtures of such solvents. Solvent dilution of waxy oil stocks maintains fluidity of the oil for facilitating easy handling, obtaining optimum wax-oil separation and obtaining optimum dewaxed oil yields. The extent of solvent dilution depends upon the particular oil stocks and solvents used, the approach to filtration temperature in the cooling zone, and the desired final ratio of solvent to oil in the separation zone.

For processes employing indirect cooling in scraped surface exchangers, cooling and wax crystallization is accomplished under conditions of very little agitation at a rate in the range of about 1 to 80 F/minute (0.56 to 4.40C/min).

Under such conditions, without wall scrapers, wax tends to accumulate on the cold exchanger walls, interfering with heat transfer, and causing increased pressure drop. Thus, scrapers are employed to remove the accumulated wax.

Dewaxing solvents are employed to maintain fluidity of the oil in the coolers and chillers, and may be added before the oil is cooled or in increments during cooling. Often the oil is given a final dilution with solvent at the separation temperature for reducing solution viscosity such that wax separation is more efficient. Commonly, solvent added to the oil in such processes is at the same temperature, or somewhat higher temperature than the oil. Cold solvent, added at substantially lower temperatures than the oil, shock chills the oil, resulting in formation of many small wax crystals which are difficult to separate.

Under controlled conditions, elongated wax crystals of good size are formed which are easy to separate and which contain little occluded oil.

Dilution chilling processes employ incremental addition of cold solvent, e.g. +20 to -250F (-6.7 to -320C), to the oil under conditions of high degrees of agitation, such that oil and solvent are completely mixed in less than one second. Under such conditions, wax precipitates in small, hard balls rather than elongated crystals. Such wax precipitates are easy to separate and retain very little oil.

Direct chilling processes employ a low boiling hydrocarbon, e.g. propane, as dewaxing solvent and refrigerant. Waxy oil stock is diluted with sufficient low boiling hydrocarbon to provide the necessary cooling and provide the desired final dilution to facilitate separation of solid wax from the oil-solvent solution. The low boiling hydrocarbon is vaporized from the oil-low boiling hydrocarbon solution under conditions of reduced pressure, at a rate sufficient to cool the solution about 1 to 80F per minute (.56 to 4.40C/min).

Such cooling is continued until the desired separation temperature and degree of wax crystallization are obtained. At the separation temperature, sufficient low boiling hydrcarbon remains in solution with the oil to provide the desired fluidity for good separation of wax.

Agitation of the mixture being cooled is commonly provided for reduction of temperature and concentration gradients. Solid-liquid separation techniques which may be employed for separation of wax crystals from the dewaxed oil-solvent solutions include known solid-liquid separation techniques, such as gravity settling, centrifugation and filtration.

In commercial solvent dewaxing processes, separation of crystalline wax from dewaxed oilsolvent solutions, at a filtration temperature selected to provide dewaxed oil of desired pour point, is commonly accomplished in rotary drum vacuum filters. Such rotary drum vacuum filters are common articles of commerce, and are well understood by those skilled in the art.

Consequently, a detailed description of the mechanics of their operation is not required herein for a full understanding of the filtration process.

According to the prior art, a mixture of wax crystals, dewaxed oil and solvent flows into the vat of a primary rotary vacuum filter. A rotary drum having a filter cloth thereon, rotates through the body of chilled mixture contained in the filter vat. A wax cake forms on the filter cloth as dewaxed oil and solvent flow through the filter cloth under the influence of an imposed vacuum.

As the rotary drum leaves the chilled mixture, excess dewaxed oil and solvent are displaced from the wax cake by the flow of inert gas through the cake, also under the influence of the imposed vacuum. Next the wax cake passes under cold solvent sprays where additional dewaxed oil is displaced from the wax cake by cold solvent flowing therethrough. As the wax cake leaves the solvent spray area, inert gas is flowed therethrough once again under the influence of imposed vacuum for displacing additional solvent and drying the wax cake. After this second treatment with inert gas, the wax cake passes into a blow back zone wherein the wax cake is loosened from the filter cloth by a flow of inert gas under pressure from inside the filter drum. The loosened cake is deflected and falls into a wax boot, from which the wax is removed for further treatment.The rotary drum filter then passes into a dead area, where neither vacuum nor pressure is applied to the filter cloth, before re-entering the body of chilled mixture for another cycle.

In commercial dewaxing processes, wax separated from dewaxed oil-solvent mixture in the primary filter is referred to as slack wax, and the filtration step is referred to as primary filtration.

This slack wax, in the primary filter boot contains a quantity of dewaxed oil entrained therein. In order to improve recovery of dewaxed oil and improve quality of recovered wax, the slack wax in the primary filter boot may be slurried with additional cold dewaxing solvent for dissolving dewaxed oil, and the slurry is separated into a wax cake and a solvent-dewaxed oil solution in a repulp filter. The process steps in the repulp filter are the same as described above for the primary filter.

As both the primary and repulp rotary drum vacuum filters continue through their cycles, the filter cloths gradually become more and more plugged or "blinded", presumably by tiny wax particles and/or resinous material from the charge oil. As a result, the filtration rate of the filters gradually decreases. At a sufficiently low filtration rate, it becomes necessary to restore filtration rate by cleaning the filter cloths of plugging materials. According to generally accepted commercial practice, such cleaning of filter cloths is presently accomplished by "hot washing" the filter cloths with dewaxing solvent.Such "hot washing" comprises: removing the filter to be cleaned from service and draining chilled mixture from the filter vat; spraying the exterior of the filter cloth on the rotating filter drum with "hot" dewaxing solvent, at a temperature in the range of about 120 to 1 800F, for a time to dissolve wax and wash it from the filter cloth; draining the washings from the filter vat; rechilling the filter cloth with cold solvent, and returning the filter to service.

Different dewaxing stocks, or even various lots of the same dewaxing stock, may differ in the rapidity with which they will blind filter cloths.

When processing an average filtering stock, the decline in filter rate is gradual and a time schedule is used whereby a particular filter is washed about six hours. When a stock is processed which blinds the filter more quickly, the hot washing frequency may be increased to maintain a higher average filtration rate.

Backwashing rotary drum vacuum filters with solvent or other fluid from the underside of the filter cloth to the outside, while maintaining the filter in service, is a well and widely practiced technique in processes other than solvent dewaxing of petroleum oils. However, it has been considered by those familar with solvent dewaxing processes that hot solvent is required for dissolving wax from between the threads of the filter cloth. Hot solvent would heat the filter drum, causing additional wax to melt thus increasing the rate of blinding of the filter cloth and increasing the pour point of dewaxed oil as a result of resolution of some of the wax crystals.

Consequently, backwashing of rotary drum vacuum filters with solvent in solvent dewaxing service has never been seriously considered.

Now, according to the method of the present invention, we have discovered an improved method for filtering wax crystals from a cold dewaxed oil, solvent, wax crystal mixture employing a rotary drum vacuum filter. The improved method of the present invention is equally applicable to primary filtration and repulp filtration.

According to the present invention, the filter cloth of a rotary drum vacuum filter is backwashed with cold dewaxing solvent under conditions such that the filter cloth is cleansed of wax and/or resin accumulations.

Advantages of the present invention include elimination of necessity for taking a filter off-line for cleansing the filter cloth; improved filtration rates for the filter cleaned according to the process of the present invention; and improved dewaxing of oils filtered through rotary drum vacuum filters cleaned according to the process of the present invention. These advantages, and others are discussed in the detailed description of the invention which follows.

Fig. 1 is a schematic representation of a primary filter-repulp filter combination system for separating crystalline wax from a dewaxed oilsolvent-wax mixture, employing an embodiment of the present invention.

Fig. 2 of the Drawing is a reproduction of a strip chart record of a primary dewaxing filter charge rate versus time, when following the process of the prior art and when following the process of the present invention.

For an understanding of the process of the present invention, reference is now made to Fig. 1 of the drawing. Fig. 1 is a schematic representation of a primary rotary drum vacuum filter in association with a repulp rotary drum vacuum filter, as commonly found in commercial solvent dewaxing processes, incorporating improvements of the present invention. Only those elements of the process and mechanical details of the rotary drum vacuum filters necessary for an understanding of the present invention are included in Fig. 1.Mechanical features and process equipment unnecessary for an understanding of the present invention have been omitted for the sake of clarity. Fig. 1, and the description which follows, are exemplary of one eombodiment of the present invention, and are not to be construed as limiting the scope of the present invention which is set-out in the claims appended to this application.

In Fig.1, a chilled mixture of wax crystals, dewaxed oil and solvent, from a dewaxing chilling step not shown, flows in line 1. The chilled mixture generally comprises solvent and oil in a volume ratio of about 1:1 to about 10:1 respectively and about 5-35 wt.% wax crystals based upon the total weight of wax and oil present. The temperature of the chilled mixture is generally in the range of about +200F to about -400F (~6,5 to -400C). In line 1, the chilled mixture is mixed with repulp filtrate from line 2.

Said repulp filtrate comprises solvent and dewaxed oil recovered from a repulp filter as will be described herein below.

In Fig. 1, the chilled mixture and repulp filtrate flow into the lower vat portion of the primary rotary drum vacuum filter 3 forming a pool of liquid containing wax crystals, having an upper surface 4. Primary filter 3 comprises a vapor tight case 5, the lower portion of which forms the vat area, in open communication with a primary filter boot 6. Inert gas, such as flue gas or nitrogen is supplied to the interior of filter case 5 via line 10.

Within filter case 5 a rotating cylindrical filter drum 7, covered by a filter cloth, is concentrically mounted upon a stationary trunion 8. Stationary trunion 8 forms a plurality of open areas beneath canvas of filter drum 7, separated by bridge blocks 9.

In Fig.1, as rotating filter drum 7 rotates into the pool of cold mixture, a vacuum applied via opening 11 to a first trunion open area 12 causes dewaxed oil and solvent to flow through the cloth of filter drum 7 into trunion open area 12, and a cake of wax forms on the exterior of filter drum 7.

As filter drum 7 rotates out of the cold mixture, inert gas flows under influence of vacuum applied via opening 13 from the interior of filter case 5 through the wax cake, displacing entrained oil and solvent therefrom, into second trunion open area 14.

In Fig. 1, as filter drum 7 rotates further, the wax cake is sprayed with cold solvent, from line 15 and sprinkler header 16. The cold solvent, at about the filtration temperature of the cold mixture, is a dewaxing solvent and is generally the same as solvent employed for admixture with oil in the dewaxing process. The sprayed cold solvent flows through the wax cake, and displacing entrained oil therefrom, into third trunion open space 17 under influence of vacuum applied via opening,18. As filter drum 7 rotates further, inert gas from the interior of filter case 5 flows through the wax cake, under influence of vacuum applied via opening 19, into fourth trunion open area 20.

This flowing inert gas displaces solvent and entrained oil from the wax cake.

In Fig. 1, as filter drum 7 rotates further, wax cake is dislodged from the filter cloth, and is deflected by deflector blade 21 into wax boot 6.

In the present embodiment of the present invention, the wax cake is dislodged by the flow of cold solvent via line 22 and a fifth trunion open area 3 through filter drum 7 as a backwash. In addition to dislodging wax cake from filter drum 7, the backwash, when applied according to the method of the present invention, additionally frees the filter cloth of wax and/or resins embedded therein which tend to "blind" the filter cloth and decrease capacity of filter 3. In the embodiment of the present invention depicted in filter 3, a portion of the solvent flowing as backwash, dislodges wax cake above deflector blade 21 and flows with dislodged wax cake into wax boot 6. The remainder of the backwash solvent flows through rotary drum 7 below deflector blade 21 and flows into the pool of cold mixture contained in the vat area of filter case 5.It is necessary that the pressure applied to the filter cloth by backwash solvent be sufficient to accomplish the purpose of cleaning wax and/or resins from the filter cloth. This pressure will vary somewhat, depending upon the characteristics of the filter cloth. However, pressures in the range of about 12-100 psig are appropriate. At pressures lower than about 12 psig the solvent will not clean wax and resin from the cloth, and at pressures substantially exceeding about 100 psig the filter cloth may be damaged or displaced from its position on filter drum 7. Additionally, to ensure maintaining the filter cloth free of embedded wax and resin, we have discovered that backwash solvent flow through each portion of the filter cloth must be sustained for a period sufficient to dislodge embedded wax and resins.

Experience has shown, at filter drum rotations of about 1.25 RPM, solvent backwash must be sustained over an area of the filter cloth defined by an arc of the rotating drum of at least 120 (equivalent to filter cloth exposure to backwash for about 1.5 seconds). Solvent backwash sustained over a lesser area does not remove embedded wax and resin from the filter cloth.

Preferably, cold solvent backwash is sustained over an area of the filter cloth equivalent to about 120 to 720 arc of rotating drum 7. The minimum area of sustained backwash is necessary whether backwash is continuously applied or intermittently applied to filter drum 7.

In Fig. 1, cold solvent employed as backwash may be cold fresh solvent or repulp filtrate, as will hereinafter be described. Preferably, when cold fresh solvent is employed for backwash one bridge block 9 is placed in trunion 8 such that fifth trunion open area 23 is limited to a position above deflector blade 21, such that all the backwash flows into wax boot 6. When repulp filtrate, which comprises a mixture of solvent and dewaxed oil, is employed as backwash, it may flow into wax boot 6, into the vat area of filter case 5, or in to both.Cold fresh solvent, if allowed to flow in large quantity into the cold mixture contained in the vat area of filter case 5, may upset the solvent-oil balance of said cold mixture and adversely affect filter rate or product quality of dewaxed oil. Repulp filtrate, being a mixture of solvent and dewaxed oil, is not likely to upset the solvent-oil balance of the cold mixture.

Temperature of solvent used as backwash solvent is preferably near the filtration temperature of the cold mixture. Should the temperature of cold backwash solvent exceed the filtration temperature by more than about 1 OOF, filter drum 7 will be heated sufficiently to melt a portion of the wax cake. The melted wax may blind the filter cloth thus decreasing filter rate, and may flow through the filter cloth with oil and solvent thus degrading quality of dewaxed oil.

In Fig. 1, filtrate, comprising dewaxed oil and solvent, is accumulated from the interior of trunion 8 and flows via line 50 to further processing, not shown, wherein dewaxed oil and solvent are separated. In filter boot 6 wax cake and any backwash solvent are mixed with cold fresh solvent from line 24 forming a slurry of wax crystals in solvent. Oil present in the wax dissolves into the solvent. This wax-solvent slurry flows via line 25 to a repulp filter 26. This slurrying of wax cake, known as "slack wax", with additional solvent followed by repulp filtration serves to recover additional dewaxed oil which may be entrained in the slack wax, thus increasing dewaxed oil recovery and improving quality of repulp wax product.

In Fig. 1, repulp filter 26 is similar to primary filter 3 and comprises a repulp filter vapor tight case 27 directly communicating with a repulp filter wax boot 28. Inert gas, such as flue gas or nitrogen is supplied to filter case 27 via line 29.

The lower portion of filter case 27 forms a vat for holding a pool of cold slurry having an upper surface 30. Within filter case 27, repulp filter cylindrical rotary drum 31 having a filter cloth thereon, rotates about repulp filter stationary trunion 32. Stationary trunion 32 is designed to provide a plurality of open spaces communicating with the underside of rotary drum 31, wherein the open spaces are separated by a plurality of bridge blocks 33. Spray header 34 and line 35 are arrayed within filter case 27 for spraying cold wash solvent onto an exterior portion of rotary filter drum 31.

In Fig. 1, cold repulp slurry flows via line 25 into the lower, vat portion of filter case 27 forming a slurry pool having an upper surface 30.

Rotary filter drum 31 rotates clockwise through the slurry pool. In the slurry pool, vacuum is applied to first trunion open space 36 via opening 37 such that solvent, containing some dewaxed oil, flows through the filter cloth and a wax cake forms on the outer surface of rotary filter drum 31. As rotary filter drum 31 rotates out of the slurry pool, vacuum applied to second trunion open space 38 via opening 39 causes inert gas to flow through the wax cake, displacing entrained solvent and oil therefrom. As rotary filter drum 31 rotates further, cold solvent from spray header 34 is sprayed on the wax cake outer surface and flows therethrough into third trunion open space 40 under the influence of vacuum applied via opening 41. Cold solvent displaces additional solvent containing dewaxed oil from the wax cake.Rotary filter drum 31 continues to rotate, and inert gas is drawn through the wax cake into fourth trunion open space 42 as a result of vacuum applied via opening 43, thus displacing and evaporating additional solvent from the cake.

In Fig. rotary filter drum 31 continues rotation, and inert gas, under pressure, from line 44, enters fifth trunion open space 45 and blows through rotary filter drum 31, breaking the wax cake loose from the filter cloth. The loose wax cake, deflected by deflector blade 46 falls into wax boot 28, from which it is recovered as wax product via line 47.

In Fig. 1 , filtrate, comprising solvent containing dewaxed oil, is accumulated from trunion open spaces 36, 38, 40 and 42 and flows, as repulp filtrate, from repulp filter 26 via line 2. A portion of the repulp filtrate flows o line 1 for admixture with additional cold mixture, as hereinabove described. A second portion of repulp filtrate may flow via line 48 for use as backwash solvent on either primary filter 3 or repulp filter 26.

In Fig. 1, rotary filter drum 31, as it rotates past deflector blade 46 passes over sixth trunion open space 49 before re-entering said slurry pool for picking additional wax cake. The filter cloth of rotary drum filter 31, as it passes deflector blade 46 has wax and/or resins embedded between the threads. For removing such wax and/or resins, backwash solvent from line 51, at about the temperature of the repulp filtration step, flows into sixth trunion open space at a pressure in the range of about 1 2-100 psig. From sixth trunion open space 49, cold backwash solvent flows through rotary drum filter 31 filter cloth, washing wax and resins therefrom, thus increasing capacity of repulp filter 26.As in the case of primary filter 3, backwash of each portion of the filter coth of repulp filter 26 with cold solvent must be sustained for a period sufficient to dislodge the embedded wax and resins.

Experience has shown that such backwash must be sustained, at a pressure in the range of about 12-100 psig, over an area equivalent to at least about 12 degrees of arc of rotary filter drum 31, and preferably over about 12 to 72 degrees of arc.

The temperature of backwash solvent, whether repulp filtrate from line 48, or cold fresh solvent, should be about the repulp filtration temperature.

Backwash solvent having a temperature about 1 00F or more greater than the repulp filtration temperature warms rotating filter drum 31 sufficiently such that a portion of the wax cake may melt. The melted wax may blind the filter cloth, thus severely reducing filter capacity, or may pass through the filter cloth with repulp filtrate, thus contaminating dewaxed oil contained therein. Pressure upon backwash solvent in sixth trunion open space 49 is maintained at least about 40 psig to ensure sufficient flow through the filter cloth for dislodging wax and resins.

Backwash solvent pressure is maintained below about 100 psig such that the filter cloth is not damaged or separated from rotary filter drum 31.

For repulp filter 26, backwash solvent is employed to backwash the filter cloth at a location below deflector blade 46, such that solvent does not become admixed with wax product in wax boot 28. Consequently, backwash solvent flows into the vat portion of filter case 27, mixing with repulp slurry contained therein.

Example In order to demonstrate the advantages of the improved wax filtering method of the present invention, the following comparative example is provided. For comparison, a primary rotary vacuum filter similar to primary filter 3 in Fig. 1 was operated according to the prior art method, without solvent backwash, wherein the filter cloth was washed with hot solvent to dissolve wax and resins and then placed in service. The filter was maintained in service until filter capacity was reduced to a low value as a result of wax and resins embedded in the filter cloth. At this low level of filter capacity, the rotary vacuum filter was maintained in service, and the filter cloth before wax cake discharge was continuously backwashed with cold fresh solvent over an area of about 72 degrees of arc on the filter drum for a period of two revolutions.

For the example, a raw wax distillate having a gravity of 34.2 OAPI, an IBP of about 6000F, an end point of about 10000 F, a viscosity of about 107.6 SUS at 1000F and 40.5 SUS at2100F, a pour point of 850F, and containing about 15 wt.% wax was mixed with about 2.1 volumes dewaxing solvent comprising 35% toluene and 65% methyl ethyl ketone. The wax distillate-solvent solution was chilled, in scraped surface chillers to a temperature of about -1 00F, for crystallizing wax therefrom. The cold mixture was flowed into a primary filter similar to primary filter 3 of Fig. 1. In the primary filter, the filter drum rotated through the cold mixture, wherein a wax cake accumulated upon the rotary drum filter.Upon rotating out of the cold mixture, solvent and oil were displaced from the wax cake by a first inert gas flow, followed by a cold solvent wash. The cold solvent wash was followed by a second inert gas flow for purging solvent from the wax cake.

For the period of this experiment the rate of cold mixture charge to the primary filter was recorded upon a strip-chart recorder. Fig. 2 of the drawings is a reproduction of the strip chart showing the primary filter charge rate as O to 100 percent vs.

time in hours.

Referring to Fig. 2, prior to hour 1 the filter charge rate was reduced to about 60 percent of capacity as a result of wax and resin accumulation in the filter cloth. At hour one (1) the primary filter was taken out of service and the filter cloth washed with hot solvent to remove accumulated wax and resin. Upon completion of the hot wash, the filter was returned to service, having a filter capacity of about 99%. Filter capacity declined rapidly, and at about hour two (2) was reduced to about 65% of capacity, where it remained until hour eight (8). At about hour eight (8) of the experimental run, the filter was maintained in service and the filter cloth was subjected to a backwash with cold fresh solvent in that portion of the filter just prior to discharge of the wax cake.

The cold solvent backwash was applied at a pressure of 18 psig over a filter cloth area described by 72 degrees of arc upon the rotary filter drum, for a period equivalent to two (2) revolutions (90 seconds). Filter capacity increased to 100% capacity. The filter was maintained in service, without backwash, for a period of six (6) hours, until hour fifteen (15), during which time the filter capacity decreased slowly to about 72% of capacity. At hour fifteen (1 5) filter cloth was again backwashed for two revolutions with cold solvent, whereupon filter charge rate increased to 96% of capacity.

Thus, from Figure 2, it is seen that cold solvent backwash of a rotary drum vacuum filter employed in filtering wax crystals from a dewaxed oil-solvent mixture increases average capacity of the filter, and allows cleaning the filter cloth of accumulated wax and/or resins without taking the filter from service.

From the above description and examples, modifications and variations will be obvious to those skilled in the art, which modifications and variations are within the spirit and scope of the present invention. Therefore all such modifications and variations within the spirit and scope of the present invention are considered to be included herein and the only limitations to the present invention are those included within the

Claims (9)

appended claims. Claims

1. A solvent dewaxing process in which a cold mixture comprising wax crystals, dewaxed oil and solvent at a selected temperature is charged to a rotary drum vacuum filter case to form a pool of said cold mixture therein, in which a filter drum having a filter cloth mounted thereon rotates through said pool of cold mixture, in which dewaxed oil and solvent flow through said filter cloth under the influence of a vacuum applied within the drum and a wax cake accumulates upon the face of the filter cloth, in which the filter drum having the wax cake thereon rotates out of the pool of cold mixture, and in which the wax cake is disengaged from the filter drum at a position remote from where the drum emerged from the pool of cold mixture and wherein the filter drum, after the disengagement of the wax cake, rotates into the pool of cold mixture for accumulation of additional wax cake upon the filter cloth, wherein the filter cloth is backwashed with a liquid comprising solvent at a temperature not greater than about 300F (1 70C) above said selected temperature, at a pressure in the range of about 12 to 100 psig (1.9 to 7.9 bars absolute), over an area of the filter cloth defined by an arc of the filter drum in the range of about 120 to 720 at a position preceding re-entry of the filter cloth into the pool of cold mixture, for removing embedded wax and/or resin from the filter cloth and increasing the capacity of the filter.

2. A process according to claim 1, wherein the wax cake on the filter cloth which emerges from said pool of cold mixture is treated with a first inert gas flow, a cold solvent wash, and a second inert gas flow for displacing dewaxed oil and solvent therefrom before the wax cake is disengaged from the filter cloth, said backwashing taking place after said second inert gas flow through the filter cloth.

3. A process according to claim 1 or claim 2 wherein said backwashing is applied for a period equivalent to at least one revolution of said rotating drum.

4. A process according to claim 3, wherein wax cake discharge is accomplished by action of said backwashing liquid.

5. A process according to claim 3, wherein said backwashing follows said wax discharge and precedes re-entry of said backwashed filter cloth into said cold mixture.

6. A process according to claims 2 to 5, in which said filter is a primary rotary drum vacuum filter and the discharged slack wax cake therefrom is slurried with additional cold solvent for dissolving entrained dewaxed oil therefrom, and in which the slurry is filtered on a repulp rotary drum vacuum filter having a fabric repulp filter cloth bound to a rotating repulp filter drum which rotates through a pool of said slurry to form a second wax cake thereon, and, after emerging from the pool of slurry, is treated with a first inert gas flow, a cold solvent wash, and a second inert gas flow, the second wax cake thereafter being disengaged from the repulp filter drum before it rotates again into the pool of slurry, wherein said repulp filter cloth, following discharge of the second wax cake therefrom, is backwashed with a liquid stream comprising solvent at a temperature not greater than about 1 00F (5.60C) above said slurry temperature, at a pressure in the range 12 to 100 psig (1.9 to 7.9 bars absolute) over an area of said filter cloth defined by about 120 to 720 of arc upon said rotating repulp filter drum, for dislodging embedded accumulated wax resins from the fabric of said repulp filter cloth.

7. A process according to claim 6, wherein repulp filtrate from said repulp rotary drum vacuum filter is employed as backwashing liquid for said repulp rotary vacuum filter.

8. A process according to claim 6 or claim 7, wherein repulp filtrate from said repulp rotary drum vacuum filter is employed as backwashing liquid for said primary filter cloth and wherein said backwashing of said primary filter cloth is applied following said slack wax cake discharge.

9. A solvent dewaxing process substantially as hereinbefore described with reference to the accompanying drawing.