GB2099015A - Combined dewaxing-deoiling process using crystal modifier - Google Patents

Abstract

Waxy oil 1 is mixed with a polyalkyl acrylate crystal modifier 2 and a ketone dewaxing solvent 5, the solution is cooled indirectly at 10 and the precipitated slack wax is separated from the dewaxed oil 12 at 11. The wax cake is repulped with additional solvent 14, separated at 15 to form a repulp wax cake which reslurried in deoiling solvent 18. The slurry formed is warmed at 21 to a temperature sufficient to dissolve only the low melting components and the undissolved high melting wax 26 is separated at 22. <IMAGE>

Description

SPECIFICATION Combined dewaxing-deoiling process using crystal modifier This invention relates to the separation of oil and wax from wax-oil mixtures. More particularly, this invention relates to the use of a crystal modifier in a combined dewaxing-deoiling process.

Dewaxing is one of the more important processes used in the refining of hydrocarbon oils, since removal of the wax results in an oil of markedly improved pour point. The process is usually carried out by chilling the oil to a sufficiently low temperature in order to precipitate the wax, and then filtering the wax from the oil. It is common practice to add to the oil solvents which tend to dissolve the oil and precipitate the wax. After the waxy constituents of the oils have precipitated, there is a marked tendency for the wax crystals to block the filters during the subsequent filtration step. This blockage considerably increases the time of filtration and also the amount of oil trapped in the wax cake.

United Kingdom patent No. 1,145,427, complete specification published 12th March, 1969, and whose disclosure is incorporated herein by reference, discloses that the above-described dewaxing process can be considerably improved by precipitating the wax in the presence of a polyalkyl acrylate of which the average number of carbon atoms in the alkyl side chains is at least 14. The presence of only small amounts of these polyalkyl acrylates is sufficient to improve the filtration rate.

Further work indicates that at least one of the polyalkyl acrylates appears to need a solvent to oil ratio of three or greater at the point of incipient crystallization. Where a single dilution of charge with solvent is employed, this ratio may be easily achieved. Another means for achieving a ratio for solvent to oil of three or greater may be by incremental addition of the oil feed, such as claimed in U.S. patent No. 4,191,631.

In the dewaxing process discussed above, the wax component obtained is known as slack wax.

This slack wax contains oil and low-melting waxes which must be removed to obtain a high quality wax.

In the typical deoiling process, the slack wax is mixed with a solvent, heated to a temperature above the cloud point for complete solution, and then chilled down again in a scraped surface equipment to a temperature which results in the desired wax product. This heating and cooling requires significant expenditure of energy.

A new combined dewaxing-deoiling process has now been found which results in significant improvements over the prior art.

The present invention broadly covers an improved process for dewaxing and deoiling a petroleum feedstock. Specifically, the present invention covers a combined dewaxing-deoiling process comprising: a) contacting a wax-containing petroleum oil feed, a ketone dewaxing solvent and an effective amount of a polyalkyl acrylate crystal modifier in a contacting zone to produce a solvent/oil/crystal modifier mixture having a solvent to oil volume ratio of two or greater; b) cooling said solvent/oil/crystal modifier mixture by indirect cooling, therein precipitating at least a portion of said wax from said mixture; c) withdrawing a wax/solvent/oil/crystal modifier mixture from said chilling zone; d) separating a slack wax containing an oil component along with low-melting and high-melting wax components from said wax/solvent/oil/crystal modifier mixture;; e) mixing said slack wax with additional solvent in a repulping zone at about the same temperature as that employed in step b) therein producing a repulp slack wax component and a repulp dewaxed oil component; f) separating said repulp slack wax from said repulp dewaxed oil in a separating zone; g) mixing said repulp slack wax with additional solvent in a contracting zone and heating the resulting mixture to a temperature sufficient to dissolve only the low-melting wax components in said repulp slack wax; and h) recovering the undissolved high-melting wax from said repulp slack wax/solvent mixture.

As shown in the illustrative Embodiments which follow, the use of the polyalkyl acrylate crystal modifier may reduce the time required for dewaxing and deoiling. In a specific laboratory embodiment, the total filter time was reduced from 351 seconds to 65 seconds by use of a specific polyalkyl acrylate crystal modifier. Further, the combined dewaxing-deoiling process using the specifically claimed crystal modifier significantly increases the yield of dewaxed oils.

It is particularly surprising that a single appiication of the crystal modifier in the feed to the dewaxing section has an effect which persists throughout the entire dewaxing-deoiling sequence.

Further, the warm-up deoiling results in the production of a high quality, high melting point wax at reduced energy costs.

In a specific embodiment of the present invention, repulping of the slack wax is accomplished at dewaxing temperatures (about 0 F) instead of at the higher deoiling temperatures. This modification reduces the loss of low-pour oil to the soft wax component of the slack wax.

The polyalkyl acrylates employed are those described in United Kingdom patent No. 1,145,427, i.e., polyalkyl acrylates in which the average number of carbon atoms in the alkyl side chains is at least 14. Preferred are polyalkyl acrylates wherein the long alkyl side chains contain the group CH3$CH2CH2-, in which n is greater than 12. Polyalkyl acrylates whose average number of carbon atoms in the alkyl side chains is at least 16 and at most 26 are preferred A most preferred polyalkyl acrylate is one in which the average number of carbon atoms in the alkyl side chains is 20.This polyalkyl acrylate, known in the art as Shell SWIM-S additive, is a poly n-C20 average alkyl acrylate (wt. average mol. wt. = 220,000; No. average mol. wt. = 60,000) in which the alkyl is 45% C18, 10% C20 and 45% C22.

The polyalkyl acrylates to be employed in the present process may be prepared in any suitable way for the polymerization of alkyl acrylates. The polymers may be either homopolymers or copolymers. If the polyalkyl acrylates are homopolymers, the starting material is one specific alkyl acrylate with at least 14 carbon atoms in the alkyl group. If the polyalkyl acrylates are copolymers, the starting material is a mixture of alkyl acrylates which in addition to one specific alkyl acrylate with at least 14 carbon atoms in the alkyl group contains one or more other alkyl acrylates which may or may not have at least 14 carbon atoms in the alkyl groups.As examples of alkyl acrylates having at least 14 carbon atoms in the alkyl group and being suitable for the preparation of homo- or copolymers which may be applied according to the invention may be mentioned: n-tetradecyl acrylate, n-hexadecyl acrylate, n-octadecyl acrylate, n-eicosyl acrylate, n-docosyl acrylate, n-tetracosyl acrylate and n-hexacosyl acrylate. As examples of alkyl acrylates having less than 14 carbon atoms in the alkyl groups and being suitable for the preparation of copolymers which may be applied according to the invention may be mentioned: methyl acrylate, ethyl acrylate, butyl acrylate and hexyl acrylate. If the polyalkyl acrylates to be employed according to the invention are copolymers, preference is given to copolymers of two or more alkyl acrylates, each having at least 14 carbon atoms in the alkyl group.The homopolymers of n-hexadecyl acrylates, n-octadecyl acrylate and n-eicosyl acrylate are preferred.

The molecular weight of the polymers may vary between wide limits. For application in practice it is preferable to choose polymers whose average molecular weight (number of average Mn) ranges between 1,000 and 1,000,000, in particular between 4,000 and 100,000. An effective amount of the polyalkyl acrylate, i.e., an amount effective to provide the advantages sought, in conjunction with the apportionment of the feed mentioned, will be employed. This amount may be determined by experimentation, and may vary, depending on the type of hydrocarbon oil being dewaxed. The preferred range is 0.01 to 0.4% by weight of oil. The modifier is preferably added with the incoming feed.

The present dewaxing process may be applied to a great variety of wax-containing high wax content petroleum oils. The invention is especially of importannce for the dewaxing of oils such as short residues which remain as a bottom product from topped crude oils from which all lighter fractions down to and including distillate oil fractions have been removed. Very suitable are waxy raffinates produced from residual or distillate petroleum oils by the extraction of aromatics. Specific feedstocks which are suitable include bright stocks such as Basrah, East Texas/Louisiana, Kirkuk and Qatar Marine bright stocks.

As mentioned before, the precipitation of wax from the hydrocarbon oil is suitably effected by chilling the oil in the presence of a dewaxing solvent. Such solvents tend to dissolve the oil and precipitate the wax. Examples of solvents which can be used for this purpose are ketones such as methyl ethyl ketone and acetone and mixtures of them with an aromatic solvent such as benzene or toluene. Particularly preferred as a dewaxing solvent is a mixture of methyl ethyl ketone and toluene.

The latter mixture may vary in composition, e.g., from 70 percent (by volume) to 40 percent of methyl ethyl ketone. A mixture containing from 60 percent (by volume) to 40 percent methyl ethyl ketone is preferred. in multiple dilution processes, the composition of the solvent, as weil as the amounts added, may vary from stage to stage, as is known in the art. The terms "zone", "zones", or "stages", as used herein in relation to chilling, are not meant merely to imply single pieces of equipment, but are to be considered to include one or more units which have the function of lowering the temperature a desired amount. Thus, for example, included in the first "zone" of a given continuous multiple dilution process may be one or more heat exchangers of differing types.

The oil treated and the solvent employed will normally be heated before chilling. In the case of methyl ethyl ketone and residual petroleum oils, heating of the feed to a temperature of above about 1 700F is desirable.

The invention is particularly applicable to single or multiple dilution dewaxing procedures utilizing the aforementioned solvents. The invention is especially applicable to that continuous process, of the type described, in which the solvent-oil mixture is heated to above 1 70OF, the mixture is then cooled in a first chilling zone or stage to a temperature below the cloud point of the mixture, or below the point of incipient crystallization, the remaining solvent is added in portions in succeeding chilling zones or stages, preferably four to six, each zone or stage being progressively cooler, and the waxy siurry is filtered.A typical multiple dilution operation is to introduce the feed oil and solvent continuously, after heating, into the initial chilling zone at a temperature of about 1 600F to 1 70OF, to operate the second chilling zone or stage at an inlet temperature of about 80OF; to operate the third chilling stage at an inlet temperature of about 60OF; to introduce the mixture to the fourth chilling stage at a temperature of about 450F; to introduce the mixture to the fifth chilling stage at about 1 20F, and to chill the same in the sixth chilling zone or stage to a filtering temperature of about +5OF. The number of the respective chilling zones or stages as well as their arrangement may be varied appreciably and a variety of chilling means may be utilized. For purposes of this illustration it is assumed that the solvent comprises methyl ethyl ketone and toluene. It is also assumed that greater than 2.0, and preferably about 2.5 to 3 volumes of total solvent mixture are utilized per volume of waxy oil being dewaxed. The solvent mixture comprises from 65 to 70 percent by volume of methyl ethyl ketone in the first two chilling zones, and 46 to 64 percent methyl ethyl ketone in the remaining stages or zones. Application of the invention to single dilution processes is preferred.

The deoiling portion of the process is an important part of the invention and is referred to as warm-up deoiling. In this warm-up deoiling process the cold filter cake from the repulping section is reslurried in warm solvent (1050 to 1 400F solvent temperature) to yield a wax slurry having a temperature at or slightiy above that at which the low melting wax components will dissolve in the solvent. Deoiling temperatures of about 400F to about 8O0F are preferred, with temperatures between about 500F and 650F being most preferred.

The invention is further illustrated by reference to the drawing. Referring to the drawing, waxy raffinate from storage (1) is charged to the dewaxing plant by a pump (4), with Shell SWIM-8 crystal modifier (2) injected into the suction of the pump (4). If incremental feed addition is practised to obtain high solvent/oil ratio as taught in U.S. patent No.4,191,631, then a portion of the waxy feed (8) is routed directly to the chillers (10), before dilution with solvent. Dewaxing solvent (9) is mixed with the main stream of feed along line (5), then the solution is heated (6), then cooled (7) before charging to the chilling zone (10).

The dewaxing slurry from the chilling zone (10) is separated into wax cake and primary dewaxed oil filtrate (12) in the first filtration stage (11). The primary wax cake is washed with dewaxing solvent (13), re-slurried with additional solvent (14) and charged to the repulp dewaxing filter (1 5). The repulp dewaxed oil filtrate ( 6) produced is combined with the primary filtrate (1 2) and routed to dewaxed oil recovery (19). The repulp wax cake is washed with dewaxing solvent (17) and reslurried with a deoiling solvent stream (18) from deoiling solvent storage (20). This slurry is warmed up to an appropriate deoiling temperature in an exchanger (21) and charged to the deoiling filter (22). The filtrate (23) is routed to soft wax (foots oil) recovery (24).

The deoiling wax cake is washed with deoiling solvent (25) and routed to product wax recovery (26).

The invention is further illustrated by reference to the following Illustrative Embodiment, which is given for the purpose of iilustration only and is not meant to limit the invention to the particular reactants and conditions employed therein.

ILLUSTRATIVE EMBODIMENT In the Illustrative Embodiment, various laboratory experiments were made in batch, bench-scale dewaxing/deoiling equipment. The crystallizer is a modified ice cream freezer, immersed in a coolant bath. The vessel is 4.9 inches l.D. x 9 inches and is fitted with a counter-rotating scraper. The vessel and scraper each rotate at 28 rpm. The chilling rate in these studies was 30 F/minute, controlled by a Foxboro temperature programmer which circulates cold acetone through a coil in the crystallizer bath.

Multiple dilutions with solvent can be made during the cooling sequence by halting the stirring momentarily and adding the appropriate amount of solvent The filter is a Buechner-type funnel, fitted with cotton filter cloth and immersed in a second thermostatted bath. The surface area is 0.55 ft2. The degree of vacuum used in these studies was 1 5 inches Hg. The filtrates are stripped free of solvent in conventional glass stills. The final stripping conditions are 3500F kettle temperature and 22 inches Hg vacuum, with a small nitrogen purge.

The general procedure used is as follows. The waxy charge is dissolved in the initial dilution solvent at 165-1 700 F, in the crystallizer. The crystallizer is transferred to the chilling bath which is at 165--170"F. The slurry is chilled at 30F/minute to the filtration temperature, with subsequent additions of solvent made at the appropriate temperatures. When the filtration temperature is reached, the slurry is poured onto the filter. Vacuum is applied and the filter time is measured with an electric timer connected to the vacuum soienoid valve. After the primary filtration, the wax cake is washed with additional prechilled solvent.

The wax is usually repulped, as foilows. The cake is returned to the crystallizer and reslurried with repulp solvent at the same temperature used in the primary filtration. The slurry is again filtered and washed as before. The repulped slack wax is then mixed with warm solvent at a temperature slightly above that at which the low melting wax components will dissolve in the solvent (above about 55OF).

The resulting solvent/wax mixture is then filtered and washed to separate the low melting point wax and any remaining oil in order to produce a high melting point, saleable wax. Filtration times are measured for each of the six steps: primary filtration, primary wash, repulp filtration, repulp wash, warm-up deoiling, and deoil washing. The stripped products are weighed to permit material balance and yield calculations.

The solvent was a mixture of methyl ethyl ketone (MEK) and toluene.

The petroleum feedstock employed in this series of experiments was East Texas/Louisiana bright stock which is a residual waxy raffinate obtained from an aromatics extraction process. The crystal modifier was Shell SWIM-5 polymer. The process conditions and results for this series of experiments are summarized in Tabies 1 and 2. All filter rates are based on the volume of solvent added prior to the filtration.

TABLE 1 DEWAXING/WARM-UP DEOILING OF HOUSTON BRIGHT STOCK Run No. LM- 383 385 387 micro micro high crystal- crystal- melting Type of wax produced line line point Charge wt, grams 300 ppm of SWIM-5 0 750 750 Dewax Solvent ratio 2.5 3.0 3.0 % MEK 50 50 50 Filter temp., OF 5 5 5 Rate, ml/sec. 15 40 42 Wash Solvent ratio 1.0 1.0 1.0 % MEK 50 50 50 Rate, ml/sec. 3.7 18 21 Repulp Solvent ratio 0.6 0.6 0.6 % MEK 50 50 50 Filter temp., OF 5 5 5 Rate, ml/sec. 5.2 69 111 Repulp wash 1.0 1.0 1.0 Solvent ratio 50 50 50 Rate, ml/sec. 3.4 31 47 Cake thickness, inches 0.30 0.20 0.23 Warm-up deoil Solvent ratio 1.2 1.2 1.5 % MEK 60 60 60 Solvent temp., OF 100 95 105 Filter temp., OF 57 56 98 Rate,ml/sec. 16 121 21 TABLE 1 (continued) DEWAXING/WARM-UP DEOILING OF HOUSTON BRIGHT STOCK Run No.Lm- 383 385 387 micro micro high crystal- crystal- melting Type of wax produced line line point Deoil wash Solvent ratio 0.6 0.6 0.35 Rate, ml/sec. 5.0 60 5.1 Cake thickness, inches 0.15 0.15 0.05 Total filtration time, sec. 351 65 96 TABLE 2 YIELDS AND RAW WAX PROPERTIES FROM BRIGHT STOCK Run No., LM- 383 385 387 micro- micro- high crystal- crystal- melting Type of wax line line point Yields, %w no loss Dewaxedoil 74 75 74 Soft wax 7 7 18 Raw wax 19 18 8 Recovery, %w 99.5 99.0 100.0 Raw wax properties Congealing point, CF 173 173 182 Specification melting point, OF 177 177 180 min.

%w oil in wax 0.3 nil 0.1 Specification 1.0 1.0 0.3 Dewaxed oil, pour point, OF 25 25 25 The dewaxed oil yields (74 5%w) are significantly better than the typical plant yield (57%w). The additional recovery (at the expense of soft wax) results from repulping during dewaxing, rather than deoiling.

The use of Shell SWIM-5 increases filter rates dramatically and the effect persists throughout the repulp. deoiling, and wash steps. The total filtration time required (six steps) with Shell SWIM-5 is less than 20% of that in the absence of a dewaxing aid. The improved crystallization with this additive is also reflected in the reduced oil content of the microcrystalline wax.

Claims (10)

1. A combined dewaxing-deoiling process comprising: a) contacting a wax-containing petroleum oil feed, a ketone dewaxing solvent and an effective amount of a polyalkyl acrylate crystal modifier in a contacting zone to produce a solvent/oil/crystal modifier mixture having a solvent to oil volume ratio of two or greater; b) cooling said solvent/oil/crystal modifier mixture by indirect cooling, therein precipitating at least a portion of said wax from said mixture; c) withdrawing a wax/solvent/oil/crystal modifier mixture from said chilling zone; d) separating a slack wax containing low-melting and high-melting wax components from said wax/solvent/oil/crystal modifier mixture; e) mixing said slack wax with additional solvent in a repuiping zone at about the same temperature as that employed in step b) therein producing a repulp slack wax component and a repulp dewaxed oil component;; f) separating said repulp slack wax from said repulp dewaxed oil in a separating zone; g) mixing said repulp slack wax with additional solvent in a contacting zone and heating the resulting mixture to a temperature sufficient to dissolve only the low-melting wax components in said repulp slack wax; and h) recovering the undissolved high-melting wax from said repulp slack wax/solvent mixture.

2. The process according to claim 1, wherein the solvent employed is a mixture of methyl ethyl ketone and toluene.

3. The process according to claim 2, wherein the volume ratio of methyl ethyl ketone and toluene is between about 70:30 and about 40:60.

4. The process according to claim 1, wherein the crystal modifier is a poly n-C20 average alkyl acrylate (wt. average mol wt. = 220,000; No. average mol. wt. = 60,000) in which the alkyl is 45% C18, 10% C20 and 45% C22.

5. The process according to claim 1, wherein said solvent/oil/crystal modifier mixture is heated before being cooled in step b).

6. The process according to claim 1, where the temperature of the repulp slack wax/solvent mixture in step g) is between about 400F and about 80OF.

7. The process according to claim 6, wherein the temperature in step g) is between about 500F and about 65OF.

8. The process according to claim 1 or claim 4, wherein the solvent to oil volume ratio is between about 2.5 and 3.0.

9. The process according to claim 1, wherein the temperature in the repulping zone of step e) is about +50F.

10. The process according to claim 1, wherein the petroleum oil feed is a residual waxy raffinate obtained from an aromatics extraction process.