CA1248486A - Filter-centrifuge series combination for improved oil- wax separation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved method for oil-wax separation is disclosed which comprises the use of a filtration zone (10) followed by a centrifugation zone (20). The filtration zone produces a wax cake product which may be reslurried prior to being centrifuged, and a relatively wax-free filtrate product. The centrifugation zone (20) produces a relatively wax-free liquid which is recycled to the filtration zone (10) and a dry wax cake product having a relatively low solvent and oil content.
A preferred centrifugation zone (20) comprises a scroll decanter centrifuge. The disclosed process produces a relatively dry wax cake product. This improves the dewaxed oil yield by having a reduced amount of oil entrained in the wax cake product. The process also reduces the energy consumption associated with wax recovery by decreasing the amount of solvent entrained in the wax cake.

Description

1 dACKGROUND OF THE lNVENTION

2 The subject lnventlon is directed at an

3 lmproved method for llquld-solid separatlon. More

4 speclfically, the sub~ect Inventlon Is dlrected at a method for separatlng a solvent-oll-wax slurry lnto ~
6 substantlally wax-free oll-solvent mlxture and a waxy 7 solld havlng a reduced solvent content.

8 It ls well-known ln the art to dewax wax g contalnlng hydrocarbon olls, partlcularly the lube oll fr~ctlons of petroleum oll, ln order to remove at least 11 a portlon o the wax therefrom to obtaln a dewaxed oil 12 of reduced cloud and pour polnts. It also ls well-known 13 to deoll wax contalning hydrocarbon olls, partlcularly 14 slack wax from dewaxing operatlons, ln order to remove at least a portlon of the oil therefrom to obtaln a 16 crystalllne wax product.

17 In current refinery practice, wax ls separated 18 from olls by various solvent dewaxlng technlques. A
19 summary of processes for oll wax separatlon ls complled in ~Hydrocarbon Processing~, September, 1978, pages 21 177 210.
22 The wax containing hydrocarbon oll 23 typlcally ls mixed with solvent and cooled to crystal-24 lize the wax. The type, amount and distrlbutlon of solvent, as well as the specific hardware varles depend-26 ing on the process. The resulting slurry of wax crys-27 tals ln an oll-solvent mlxture is separated by one of 28 the methods to be described below.

29 The overall efficiency of the dewaxlng oper-ation depends to a large degree on the efflciency of the 31 method of separating the wax crystals from the slurry.

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1 Several techniques have been developed for this separa-2 tion:

3 1. Decantation 4 2. Filter pressing 3. Rotary filtration 6 4. Filtering centrifugation 7 Decantation and filter pressing, which are the 8 oldest techniques, are not used commonly now due to 9 their batch characteristics, which limit through-put.

One or more rotary filters may be used to 11 separate the slurry into a substantially wax-free 12 filtrate and a filter cake containing entrapped lube oil 13 and about 3 to about 8 volumes of solvent per volume of 14 wax. This method has several undesirable features. The filter cake produced may have an undesirably high 16 entrapped oil and solvent content. This results in 17 unacceptably high lube oil yield losses and necessitates 18 the use of large wax recovery distillation trains in 19 which the wax molecules are separated from the solvent.
When the feed to the wax recovery train contains rela-21 tively high concentrations of solvent, the wax recovery 22 unit must be operated at relatively low throughputs and 23 a relatively high rate of energy input per unit of lube 24 oil product.

Similarly, one or more rotary filters in 26 series have been used to separate a slurry comprising a 27 slack wax feed into a substantially wax-free filtrate 28 and a refined wax filter cake containing entrapped lube 29 oil and about 3 to about 6 volumes of solvent per volume of wax. This method is not preferred for producing a 31 refined wax product, since it may be necessary to use an ~2~ 36 1 excessive amount of solvent to produce a wax having a 2 sufficiently high melting point. Although it may be 3 possible ultimately to recover most of the solvent 4 added, the solvent recovery units must be operated at relatively low throughputs and relatively high rates 6 of energy input per unit of refined wax product.
7 In addition, since significant improvements previously 8 have been made in increasing the throughput from rotary g drum filters, the wax recovery operation frequently may be the rate limiting operation for the entire lube oil 11 production unit and/or the solvent recovery operation 12 may be the rate limiting operation for the entire 13 refined wax production unit. Furthermore, rotary drum 14 filters must be shutdown for washing with hot solvent at 4-10 hour intervals. Considerable energy is required to 16 re-cool the filters to steady-state process conditions.

17 Various designs of centrifuges have been 18 proposed for oil-wax separation. Apart from the 19 batch centrifuges, which currently are not used for 20 wax recovery, the most common design is the filtering 21 centrifuge. Filtering centrifuges are known to be used 22 only in systems where the density of the solvent is 23 higher than that of the wax processed, such as where 24 chlorinated solvent systems are used. In general, the 25 operation of filtering centrifuges also is intermittent.

26 U.S. Patent No. 2,772,210 describes a solvent 27 dewaxing process in which the large wax crystals are 28 separated from the smaller, finer wax crystals present 29 in a hydrocarbon oil by settling when the gravity 30 differential between the wax and solvent oil phases is 31 at least 0.05. When the gravity differential is less, 32 the smaller crystals may be separated from the larger 33 crystàls by several methods including centrifugation.
34 The solution including larger crystals, is then passed ~Z ~

1 through the filter. The patent also discloses at column 2 2 that conventionally wax was separated from oil and 3 solvent by filtration or centrifugation.

4 U.S. Patent Nos. 1,963,498; 1,999,468;
2,180,070; and 2,279,937, disclose the combination of 6 filtration and centrifugation either to separate pro-7 cessing additives and wax from lube oil or amorphous wax 8 from crystalline wax in a lube oil stream.

9 U. S. Patent No. 3,006,839 discloses that a centrifuge may be used for wax deoiling, while U. S.
11 Patent No. 1,989,028 discloses the use of a centrifuge 12 followed by a filter for oil dewaxing.

13 U. S. Patent No. 2,723,941 discloses the use 14 of two rotary or other filtration zones in series for lS wax deoiling.

16 U. S. Patent No. 1,939,946 describes the use 17 of a filtration zone ror separating oil from wax. The 18 filtrate from the filtration zone is passed to a centri-19 fugation zone for further separation.

While the use of centrifuges either alone or 21 in combination with filters may be known, the most 22 common means for wax separation from solvent dewaxing 23 slurries uses one or more filters.

24 Accordingly, to improve the dewaxing process efficiency, it is desirable to provide a process in 26 which the wax separated from a lube oil-solvent-wax 27 slurry contains reduced amounts of entrapped lube oil 28 and solvent while not adversely affecting the cloud and 29 pour points of the lube oil by wax entrainment into the lube oil product.

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Similarly, to improve the slack wax process efficiency, it is desirable to provide a process in which the wax geparated from a wax-lube oil-solvent slurry contains reduced amounts of entrapped lube oil and solvent.
It also is desirable to provide a process in which the solvent consumption is reduced to produce a refined wax product of predetermined purity.
It is also desirable to provide a process which is reliable and may be installed in existin~ facilities without extensive modifications.
It is also desirable to provide a process in which a centrifugation zone can be utilized for processing non-chlorinated solvents.
It has been found that the unique combination of a filtration zone followed by a centrifugation zone for processing a wax slurry produces a higher dewaxed oil yield and lower liquids content in the wax cakes than heretofore possible.
It has been found that the unique combination of a filtration zone followed by a centrifugation zone for processing a slack wax slurry may produce the combination of a higher deoiled wax yield and a wax cake having a lower liquids content than heretofore possible.

SUMMARY OF THE INVENTION

The subject invention is directed at an improved process for separatin~ wax crystals from a hydrocarbon oil-solvent-wax slurry which comprises:
(a) admixin~ the hydrocarbon oil feed with slurry forming solvent in a slurry forming zone to form a hydrocarbon oil-solvent-wax slurry;
(b) passin~ the slurry from the slurry forming zone throu~h a filtration zone to separate the slurry into a substantially wax free filtrate and a filter cake hav;ng entrapped hydrocarbon oil and solvent therein; and - 6- ~ Ç3 (c) passing filter cake from the filtration zone to a centrifugation zone wherein entrapped hydrocarbon oil and solvent is separated from the filter cake and recycled directly to the slurry forming zone for use as slurry forming solvent.
In a preferred method the solvent utilized is less dense than the filter cake. Where a non-autorefrigerative solvent is used, the hydrocarbon feed-solvent mixture to the filter is chilled to a temperature between about -10C
to about -20C to crystallize the wax and form a slurry. A solvent, such as for example MEK/MIBK, MEK/Toluene, or similar solvents, preferably is added in an amount ranging between about 3 and about 8 volumes per volume of feed to improve the feed viscosity and to assist in the separation of the hydrocarbon oil from the wax. When an autorefrigerative solvent such as propane, butane, or propylene alone or in admixture with one or more ketones, such as acetone, methyl ethyl ketone or methyl isobutyl ketone, is added, preferably in an amount ranging between about 1 volume per volume of feed and 3 volumes per volume of feed, the feed-solvent mixture is chilled to a temperature of about - 20 to about -40C to crystallize the wax and form a slurry. Irrespective of the solvent used, the filtration zone preferably comprises one or more rotary drum filters, while the centrifugation zone preferably compr~ses a sedimenting type centrifuge, such as a scroll decanter centrifuge. At least a portion of the liquid separated in the centrifugation zone preferably is recycled to the filtration zone.
The present invention also is directed at a method for producing a refined wax product from a wax-hydrocarbon oil~solvent first slurry which comprises:

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1 A. passing the first slurry through a filtra-2 ~ion zone to separate the slurry into a substantially 3 wax-free filtrate and a filter cake having entrapped 4 hydrocarbon oil and solvent therein;

B. adding additional solvent to the filter 6 cake from the filtration zone to produce a second 7 slurry; and 8 C. passing the second slurry to a centrifu-g gation zone wherein hydrocarbon oil and solvent are separated from the wax.

11 In a preferred process, the feed comprises a 12 slack wax from a dewaxing process, the slack wax having 13 between about 3 and about 20 wt.% oil. The filtration 14 zone preferably comprises a rotary drum filter and the centrifugation zone preferably comprises a sedimentation 16 type centrifuge. Among the preferred sedimentation type 17 centrifuges are the scroll-decanter, vertical decanter, 18 and tubular centrifuges, with the most preferred being 19 the scroll decanter centrifuge.

In a preferred method of operation, the first 21 slurry enters the filtration zone at a temperature 22 ranging between about -20C to about +30C, preferably 23 between about 0C and about +30C, and the second slurry 24 enters the centrifugation zone at a temperature ranging between about -20C and about +30C, preferably between 26 about 0C and about +30C.

27 The solvent added to the first slurry and the 28 second slurry preferably is the same and preferably is 2g selected from the group of solvents consisting of methyl ethyl ketone, methyl isobutyl ketone, aromatic hydro-31 carbons, aliphatic hydrocarbons and mixtures thereof.

1 The liquid, or centrate, separated by the centrifugation 2 zone preferably is recycled to the filtration zone.

4 Figure 1 is a simplified flow drawing of one method for practicing the present invention.

6 Figure 2 is a simplified partial sectional 7 view of a scroll decanter centrifuge useful for practic-8 ing the subject invention.

g Figure 3 shows the four possible combinations of a filtration zone and a centrifugation zone for 11 separating wax from a wax slurry and/or for deoiling a 12 slack wax.

14 Referring to Figure 1, a simplified process flow diagram is shown. In this diagram, valves, piping, 16 instrumentation and equipment not necessary for an 17 understanding of the subject invention have been omitted 18 for clarity. This feed is shown entering filtration 19 zone 10 through line 12. When the feed is to be dewaxed, feed frequently comprises distillate from a vacuum pipe-21 still or deasphalted oil which may have been extracted 22 and hydrotreated. The feed typically is chilled to a 23 temperature ranging between about 0C and about -40C, 24 preferably about -10 to about -20C, by various means well~known in the art, such as indirect chilling utiliz-26 ing a scraped surface heat exchanger, or by direct 27 chilling. A solvent preferably is added to the feed 28 to facilitate the hydrocarbon oil-wax separation. In 29 direct chilling, a chilled non-volatile solvent, such as MEK/MIBK or MEX/Toluene or similar solvents is added to g 1 cool by intermixing, or a liquefied solvent, such as 2 liquid propane, is injected and cools by vaporization.
3 The solvent selected and the amount used are a function 4 of several factors including the feed characteristics, and the desired product cloud and pour points. In 6 addition to cooling the feed, solvents typically are 7 added to improve the viscosity of the slurry for separa-8 tion.

9 Where a refined wax is to be produced, the feed frequently comprises a slack wax from a dewaxing 11 operation which is heated to a temperature between about 12 -20C and about +30C, preferably between about 0C and 13 about +30C, depending on the desired melting point of 14 the refined wax product to facilitate the removal of low melting point wax from the refined wax product.
16 Since the slack wax feed typically is reslurried by the 17 addition of solvent to facilitate the separation, 18 frequently the slack wax feed temperature is adjusted 19 by regulation of the quantity and temperature of the solvent added. The temperature of the feed also may 21 be adjusted by other means well known in the art, such 22 as by indirect heat transfer. The solvent selected and 23 the amount added are a function of several factors, 24 including the slack wax feed characteristics and the desired crystalline melting point of the wax product.
26 Preferred solvents are methyl ethyl ketone (MEK); methyl 27 isobutyl ketone (MIBK); aromatic hydrocarbons, such 28 as toluene or benzene; aliphatic, liquefied normally 29 gaseous hydrocarbons, such as propane, butane, and butylene; and mixtures of these solvents.

31 Filtration zone 10 may comprise any type 32 filter which is effective for liquid-solid separation.
33 In the separation of a slurry of liquid hydrocarbon oil 34 from solid wax crystals, a rotary drum filter previously has been found to be particularly effective, although q`~

1 other type filters, such as plate and frame filter 2 presses, also may be used.

3 In oil-dewaxing, the filtrate from the filtra-4 tion zone, which is essentially wax-free and contains about 55 to about 85 weight % solvent, is sent from 6 filtration zone 10 to dewaxed oil recovery zone 30 7 through line 14. Dewaxed oil recovery zone 30 typically 8 comprises a tower and associated equipment for the 9 separation and recovery of solvent from the hydrocarbon oil. Recovered solvent is removed from recovery zone 30 11 through line 34 for recycle to the process, such as 12 through lines 36, 38, 39 and 40, or to storage, while 13 the dewaxed oil is removed from recovery zone 30 through 14 line 32. The wax, having entrained hydrocarbon oil and lS solvent therein, forms a filter cake in zone 10 which is 16 transported through line 22 to centrifugation zone 20 by 17 conventional means, such as by a centrifugal pump (not 18 shown). In the centrifugation zone 20 the filter cake, 19 preferably at a temperature of about 0C to about -20C, is centrifuged, preferably with additional solvent 21 repuddle, to remove additional quantities of solvent and 22 oil from the wax. Typically, the filter cake entering 23 centrifuge 20 contains about 3 to about 20 volumes of 24 solvent per volume of wax, preferably about 8 to about 14 volumes of solvent per volume of wax, and about 15 to 26 about 50 wt. % oil in the wax. The liquid removed from 27 centrifugation zone 20 generally contains trace amounts 28 of entrained wax. Typically, the liquid exiting centri-29 fugation zone 20 comprises about 3 to about 7 wt. %
hydrocarbon oil, about 97 to about 93 wt. % solvent, 31 and less than 0.5 wt. % wax. Although the wax concen-32 tration in the liquid from centrifugation zone 20 may be 33 unacceptably high for hydrocarbon lube oil product, this 34 liquid may be recycled through lines 24, 39 and 40 where it is refiltered in filtration zone 10. The wax cake L'~36 1 from centrifugation zone 20, preferably having a liquid-2 solid ratio of between about 0.8 and about 2.5, is 3 transported from zone 20 to wax recovery zone 50 by 4 conventional means, such as by melting and pumping. In wax recovery zone 50 the solvent is recovered from the 6 wax by conventional distillation.

7 In refined wax production, the filtrate from 8 the filtration zone, which commonly is referred to as 9 foots oil solution, is essentially wax-free and contains about 50 to about 90 wt.% solvent. The foots oil 11 solution is sent from filtration zone 10 to a foots 12 oil separation zone 30 through line 14. Foots oil 13 separation zone 30 typically comprises a distillation 14 tower and associated equipment for the separation and recovery of solvent from the foots oil. Recovered 16 solvent is removed from separation zone 30 through line 17 34 for recycle to the process, such as through lines 36, 18 38, 39 and 40, or to storage, while the foots oil is 19 removed from separation zone 30 through line 32.

The wax, having entrained hydrocarbon oil and 21 solvent therein, forms a filter cake in zone 10 which is 22 transported from filtration zone 10 through line 22 to 23 centrifugation zone 20 by conventional means, such as 24 by a centrifugal pump (not shown). Prior to entering centrifugation zone 20, the filter cake is reslurried 26 or n repuddled~ by the addition of more solvent. In 27 centrifugation zone 20 the slurry, maintained at a 28 temperature of about -20C to about +30C, preferably 29 maintained at a temperature of about 0C to about +30C, is centrifuged to remove additional quantities of 31 solvent and oil from the wax. Typically, the slurry 32 entering centrifuge 20 also contains about 3 to about 20 33 volumes of solvent per volume of wax, preferably about 8 34 to about 14 volumes of solvent per volume of wax, and 1 about 0.5 to about 2.0 wt.% oil in the wax. Typically, 2 the liquid, or centrate, exiting centrifugation zone 20 3 comprises about 2 to about 8 wt.% hydrocarbon oil, about 4 98 to about 92 wt.% solvent, and less than 0.5 wt.% wax.
The centrate, which may contain minor quantities of wax, 6 may be recycled through lines 24, 39 and 40 where it 7 is refiltered in filtration zone 10. The wax cake from 8 centrifugation zone 20, comprising refined hard wax 9 and solvent, preferably having a liquid-to-solid ratio of between about 0.8 and about 2.5, is transported from 11 zone 20 to solvent separation zone 50 by conventional 12 means, such as by melting and pumping. In solvent 13 separation zone 50 the solvent is recovered from the 14 refined crystalline wax. In the process shown, the solvent passes through line 36 for use in reslurrying or 16 repuddling the filter cake from filtration zone 10. The 17 solvent also may pass through lines 38, 39 and 40 for 18 slurrying and washing the wax cake in filtration zone 19 10. The crystalline wax product is removed from sepa-ration zone 50 through line 52.

21 Where a chlorinated solvent is used for 22 oil-dewaxing or refined wax production, the wax crystals 23 typically are less dense than the solvent, while, 24 when a non-chlorinated solvent is used, the wax crystals typically are more dense than the solvent. There are 26 two general types of centrifuges which might be used 27 in solvent-oil-wax systems, filtering centrifuges and 28 sedimenting centrifuges. Centrifugal separations by 29 filtering centrifuges, in which the liquid is filtered under centrifugal force, may be useful where a chlori-31 nated solvent is used.

32 Where a non-chlorinated solvent is used, 33 sedimenting centrifuges may be useful. The wax crystals, 34 which are denser than the non-chlorinated solvent-oil 1 mixture, such as ketone-oil or propane-oil, are sub-2 jected to a centrifugal field which separates the wax 3 crystals by forcing the crystals radially outward. The 4 crystals, which accumulate at the outer walls of the centrifuge, are removed by various mechanisms.

6 The type of centrifuge utilized may be impor-7 tant. Since the wax crystals may blind or plug filtra-8 tion screens, sedimenting centrifuges, which do not 9 utilize filters for separation, are preferred. There-fore, to avoid the problems associated with filtering 11 type centrifuges, the present invention preferably is 12 practiced utilizing a non-chlorinated solvent and a 13 sedimenting type centrifuge, with the most preferred 14 centrifuge being the scroll-decanter centrifuge.

Referring to Figure 2, a simplified schematic 16 drawing is shown of the centrifuge utilized in testing 17 the subject invention, a Sharples Model P660 scroll 18 decanter centrifuge, 20 often also referred to as a 19 solid-bowl centrifuge, 150 mm in diameter and 350 mm in length. A horizontal cylindrical rotor bowl 110, driven 21 by a motor and gear means, (not shown), contains helical 22 screw conveyor 120, rotating in the same or opposite 23 direction but at a different speed, which is affixed to 24 hollow shaft 130. Feed is introduced through shaft 130 and discharged into bowl 110 through opening 122, 26 typically located near the end of the horizontal section 27 of bowl 110. The slurry feed discharged is forced to 28 travel around helical screw conveyor 120 by centrifugal 29 force, causing the wax and liquid to separate. The wax deposits on the interior wall of bowl 110, while the 31 liquid forms an inner ring, with the thickness of the 32 ring determined by the height of overflow weir 140. As 33 the liquid travels around helical screw conveyor 120, 34 the liquid becomes clearer as it approaches overflow 1 weir 140. Liquid, substantially free of entrained wax, 2 passes over weir 140 for recycle to filtration means 10 3 as previously described. The wax layer is forced to 4 travel in a direction opposite to that of the liquid by the difference in rotary speed between rotating bowl 110 6 and screw conveyor 120. The speed of wax discharged is 7 directly proportional to the relative velocity of bowl 8 110 and screw conveyor 120. When bowl 110 and screw 9 conveyor 120 are rotating in the same direction, bowl 110 typically rotates at a higher speed than screw 11 conveyor 120. Thus, faster rotation of screw conveyor 12 120 in the same direction as the rotation of bowl 110 13 usually reduces the relative velocity between the bowl 14 and the screw conveyor, thereby decreasing the rate of wax movement through centrifuge 20. The wax travels 16 along conical beach section 112 for further drying prior 17 to discharge through ports 150.

18 The ultimate hydrocarbon oil content in the 19 wax discharged from centrifuge 20 is a function of several slurry characteristics and several variables 21 in the operation of centrifuge 20. The slurry charac-22 teristics which affect the oil content of the wax 23 discharged include slurry composition, t~mperature, 24 viscosity and relative liquid-solid density. Variables in the operation of centrifuge 20 which affect the 26 ultimate composition of the wax discharged include the 27 feed rate, the speed of bowl 110 and screw conveyor 120, 28 the height of overflow weir 140, as well as the length, 29 diameter, feed point and beach angle, oC , of the centrifuge.

31 During tests utilizing scroll decanter centri-32 fuge 20 for lube oil dewaxing, the length, diameter, 33 feed point and beach angle of centrifuge 20 were held 34 constant. The specifications for the feeds used in the ~'fV.~ 6 1 tests described hereinafter is presented in Table I.
2 ïn these tests it was found that the scroll decanter 3 centrifuge was able to operate continuously for a longer 4 time than a conventional rotary drum filter. The scroll decanter centrifuge was operable even during feedstock 6 changes.

9 Grade 100N 150N 600N 1200N

10 API 33.1 31.2 27.2 25.0 11 Density at 15C, 0.8592 0.8692 0.8916 0.8979 12 g/cc 13 RI at 20C 1.4756 1.4817 1.4920 1.4970 14 Viscosity at 37.8C, 18.81 27.54 135.43 326.70 15 cSt 16 Viscosity at 98.9C, 3.83 4.74 12.34 23.15 17 cSt 18 Pour Point, C -19 -18 -11 -1 19 Cloud Point, C -17 - -21 EXAMPLE I

21 Table II illustrates the superiority of the 22 use of a filtration zone followed by a centrifugation 23 zone for separating wax and hydrocarbon oil at high 24 through-put rates for 150N distillate feedstock as 25 compared with other filter-centrifuge combinations for 26 dewaxing oil. In this table all possible combinations 27 of centrifugation and filtration zones shown in Figure 28 3 were utilized. It may be seen that the use of a 29 filtration zone and a centrifugation zone produced a wax having a lower oil content than either two filtration 31 zones in series or two centrifugation zones in series.
32 Reducing the oil content in the wax increases the l dewaxed oil yield. Since wax recovery typically is 2 accomplished by distillation, an energy intensive 3 process, the subject invention results, in addition 4 to yield gain, in energy savings over the present two stage filtration zone systems. Moreover, the present 6 invention permits an increase in lube oil production 7 where wax recovery is the production limiting operation.
8 It also should be noted that the second stage centri-9 fugation zones exhibited a much higher capacity than the first stage centrifugation zones. The first stage ll centrifugation zones had to be operated at a reduced 12 through-put to produce a relatively wax-free filtrate.
13 However, even if the first stage centrifugation zones 14 were operated at reduced through-put, the entrained wax still may exceed product specifications. By comparison 16 the second stage centrifugation zones could be operated 17 at much higher through-puts, since the separated liquid 18 was recycled to the first stage thereby eliminating the l9 requirement that the separated liquid from the second stage centrifugation zones be wax-free.

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2 This Example illustrates in Table III, for 3 each feedstock used, that a second stage centrifugation 4 zone produced a wax product having a reduced oil content and also a reduced total liquid (solvent plus oil) 6 content as compared to a second stage filtration zone 7 for all feedstocks tested. Use of the second stage 8 centrifugation zone produced a wax having at least 50%
9 less liquid and a significantly lower oil content in the wax as compared to the wax produced using a second stage 11 filtration æone.

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2 As indicated below, the liquid contained in 3 the wax can be reduced by increasing the differential 4 scroll speed, since this increases the residence time of the wax in the centrifugation zone. A 150 N wax slurry 6 from a first stage filtration zone had an oil content of 7 approximately 40 wt% at -12C. This wax passed to a 8 centrifugation zone maintained at constant operating g conditions except for differential scroll velocity. The data is presented in Table IV below.

13VELOCITY (RPM) IN WAX CAKE (WT%) 14 47 1.8 38 2.2 16 17 2.8 17 Increasing the height of overflow weir 140 18 increases the centrifugation zone capacity for a given 19 liquid content in the wax cake as indicated in Table V
below for a 150 N feedstock where the desired liquid 21 content in the wax cake was 2.2 wt.%.

23 WEIR HEIGHT (cm)MAXIMUM CAPACITY (m3/hr) 24 1.1 0.24 25 1.3 0.32 26 1.5 0.44 27 In the comparative tests and example described 28 below a 600 Neutral slack wax feed was utilized for 29 refined wax production. The entering slack wax feed contained 20 wt.% residual dewaxed oil. Several key 31 properties of the feed from which the slack wax was 32 produced were as follows:

~z~ 36 1 Density API 28.3 2 Density at 15C, g/cc 0~885 3 RI at 20C 1.4630 4 Viscosity at 37.8C, cSt 110 Viscosity at 98.9C, cSt 12 6 Dry wax at -12C, wt.% 17 7 Cloud Point, C 48 8 In the example and comparative tests described herein-9 after, all possible combinations of filtration and/or centrifugation zones were utilized for refined wax 11 manufacture. Simplified schematic flow diagrams of 12 these processes are shown in Figure 3.

14 The slack wax feed from a dewaxing operation was warmed up to a temperature of 25C following dewax-16 ing. Aproximately 3 to 3.5 volumes of 40/60 v/v methyl 17 ethyl ketone/methyl isobutyl ketone solvent was added to 18 the slack wax feed to dissolve low melting wax and form 19 a slurry. The slurry was passed through a filtration zone, comprising a rotary drum filter maintained at a 21 temperature of 25C. The wax cake exiting from the 22 rotary drum filter was reslurried with about 4 volumes 23 of 40/60 v/v MEK/MIBK solvent. This second slurry was 24 passed through a centrifugation zone comprising a Sharples model P850 vertical scroll decanter centrifuge 26 at a feed rate of about 2 liters/min. Several key 27 properties of the wax cake, the filtrate, the product 28 wax and the solvent addition rate are presented in Table 29 IV.

Comparative tests also were conducted in which 31 other combinations of a filtration zone and centrifuga-32 tion zone were utilized at substantially the same 1 through-put rates as in Example IV. The results also 2 are presented in Table IV.

3 From a review of Table IV, it can be seen that 4 the combination of a filtration zone followed by a centrifugation zone for refined wax production had the 6 following advantages over other filtration zone and/or 7 centrifugation zone combinations:

8 A. The filtration zone-centrifugation zone g combination required less solvent than a multi-stage filtration system. This reduced the amount of solvent 11 which subsequently had to be recovered;

12 B. The filtration zone-centrifugation zone 13 combination had a lower loss of refined wax to the foots 14 oil stream than the multi-stage centrifugation system at comparable solvent addition rates; and, 16 C. The filtration zone-centrifugation zone 17 combination had a lower refined wax loss than the 18 centrifugation zone-filtration zone combination.

19 While the subject invention has been shown to be effective for lube oil dewaxing and for refined wax 21 production utilizing a scroll decanter, or solid-bowl 22 centrifuge, other types of sedimenting centrifuges, such 23 as vertical decanting and tubular centrifuges, also may 24 prove effective depending upon the range of oil-wax slurries to be treated, the feed rates and the desired 26 final product characteristics.

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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for removing wax from a hydrocarbon oil feed characterized by:
(a) admixing the hydrocarbon oil feed with slurry forming solvent in a slurry forming zone to form a hydrocarbon oil-solvent-wax slurry;
(b) passing the slurry from the slurry forming zone through a filtration zone to separate the slurry into a substantially wax free filtrate and a filter cake having entrapped hydrocarbon oil and solvent therein; and (c) passing filter cake from the filtration zone to a centrifugation zone wherein entrapped hydrocarbon oil and solvent is separated from the filter cake and recycled directly to the slurry forming zone for use as slurry forming solvent.

2. The process of claim 1 wherein at least a portion of the hydrocarbon oil and solvent separated from the filter cake in the centrifugation zone is returned to the filtration zone.

3. The process of claim 2 wherein additional solvent is added to the filter cake prior to the filter cake entering the centrifugation zone.

4. The process of claim 3 further characterized by the temperature of the slurry in the filtration zone being maintained between about -40°C and about +30°C.

5. The process of claim 4 further characterized by the temperature of the slurry and/or filter cake entering the centrifugation zone being maintained between about -20°C and about +30°C.

6. The process of claim 5 further characterized by the slurry and/or filter cake passed to the centrifugation zone containing from about 3 to about 20 volumes of solvent per volume of wax.

7. The process of claim 6 further characterized by the solvent being less dense than the filter cake.

8. The process of claim 7 further characterized by the centrifugation zone comprising a sedimentation-type centrifuge.

9. The process of claim 8 wherein the sedimentation-type centrifuge comprises a scroll-decanter centrifuge.