(en)A dewaxing process is described wherein a residual waxy petroleum oil stock characterized by having a viscosity greater than about 75 SUS at 210*F. and containing less than about 10 percent of material boiling below about 950*F., is mixed with at least about 0.3 volumes of a dewaxing solvent per volume of residual waxy oil stock, thereby depressing the cloud point of same. In one embodiment of the invention, the resultant mixture is introduced into a cooling zone, at a temperature above the depressed cloud point of the oil. Precooled dewaxing solvent is incrementally added to the cooling zone which is divided into a plurality of stages with agitation means present in each of the stages. The resultant solvent-oil mixture is cooled and agitated as it passes through the cooling zone, thereby reducing the temperature of the oil to below its depressed cloud point and precipitating at least a portion of the wax therefrom. A residual oil stock of diminished wax content is thereafter recovered. In a second embodiment of the invention, the waxy oil stock is introduced into the cooling zone in the absence of solvent at a temperature above the cloud point of the oil. Precooled dewaxing solvent is introduced incrementally into the initial stages of the cooling zone, coming into contact with the waxy oil and depressing its cloud point. The oil is gradually cooled to a temperature no less than the depressed cloud point of the oil whereupon additional precooled dewaxing solvent is added to the oil in the remaining stages of the cooling zone, thereby gradually cooling the oil to a temperature below the depressed cloud point and precipitating at least a portion of the wax therefrom.

1.ApplicationNumber: US-28464772-A
1.PublishNumber: US-3850740-A
2.Date Publish: 19741126
3.Inventor: GUDELIS D,CA
SHAW D,CA

4.Inventor Harmonized: GUDELIS D()
SHAW D()

5.Country: US
6.Claims:
(en)A dewaxing process is described wherein a residual waxy petroleum oil stock characterized by having a viscosity greater than about 75 SUS at 210*F. and containing less than about 10 percent of material boiling below about 950*F., is mixed with at least about 0.3 volumes of a dewaxing solvent per volume of residual waxy oil stock, thereby depressing the cloud point of same. In one embodiment of the invention, the resultant mixture is introduced into a cooling zone, at a temperature above the depressed cloud point of the oil. Precooled dewaxing solvent is incrementally added to the cooling zone which is divided into a plurality of stages with agitation means present in each of the stages. The resultant solvent-oil mixture is cooled and agitated as it passes through the cooling zone, thereby reducing the temperature of the oil to below its depressed cloud point and precipitating at least a portion of the wax therefrom. A residual oil stock of diminished wax content is thereafter recovered. In a second embodiment of the invention, the waxy oil stock is introduced into the cooling zone in the absence of solvent at a temperature above the cloud point of the oil. Precooled dewaxing solvent is introduced incrementally into the initial stages of the cooling zone, coming into contact with the waxy oil and depressing its cloud point. The oil is gradually cooled to a temperature no less than the depressed cloud point of the oil whereupon additional precooled dewaxing solvent is added to the oil in the remaining stages of the cooling zone, thereby gradually cooling the oil to a temperature below the depressed cloud point and precipitating at least a portion of the wax therefrom.

7.Description:
(en)[451 Nov, 26, 1974 PARTIAL PREDILUTION DILUTION CHILLING t [75] Inventors: David A. Gudelis; David H. Shaw,
both of Sarnia, Ontario, Canada [73] Assignee: Exxon Research and Engineering Company, Linden, NJ.
[22] Filed: Aug. 29, 1972 [21] Appl. No.: 284,647
Primary Examiner-Herbert Levine [57] ABSTRACT A dewaxing process is described wherein a residual waxy petroleum oil stock characterized by having a viscosity greater than about 75 SUS at 210F. and containing less than about 10 percent of material boiling below about 950F., is mixed with at least about 0.3 volumes of a dewaxing solvent per volume of residual waxy oil stock, thereby depressing the cloud point of same.
In one embodiment oi the invention, the resultant mixture is introduced into a cooling zone. at a temperature above the depressed cloud point of the oil. Precooled dewaxing solvent is incrementally added to the cooling zone which is divided into a plurality of stages with agitation means present in each of the stages. The resultant solvent-oil mixture is cooled and agitated as it passes through the cooling Zone, thereby reducing the temperature of the oil to below its depressed cloud point and precipitating at least a portion of the wax therefrom. A residual oil stock of diminished wax content is thereafter recovered.
in a second embodiment of the invention, the waxy oil stock is introduced into the cooling zone in the absence of solvent at a temperature above the cloud point of the oil. Precooled dewaxing solvent is introduced incrementally into the initial stages of the cooling zone, coming into contact with the waxy oil and depressing its cloud point. The oil is gradually cooled to a temperature no less than the depressed cloud point of the oil whereupon additional precooled dewaxing solvent is added to the oil in the remaining stages of the cooling zone, thereby gradually cooling the oil to a temperature below "the depressed cloud point and precipitating at least a portion of the wax therefrom.
l .29 4(u) I 8 7 SEPARATOR\ SEPARATOR SEPARATOR I9 Pmmm vz 131850.140
SHEEI 1 BF 2 FIGURE COOLING TOWER PATENTEDNBIZBISH ,740
SHEET 2 BF 2 Figure 2 MEK/TOLUENE (55/45) DILUTION CHILLING ARAMCO 2500 BRIGHT STOCK FEED FILTER RATE VERSUS PREDILUTION 4.0 l I I 3.0 T I v u; 20?- c: O E
3 LU Ll.
o I I l PREDILUTION SOLVENT/ FEED (V/V) 1 PARTIAL PREDILUTION DILUTION CHILLING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention'relates to a process for the dewaxing of a waxy residual petroleum oil stock. More particularly, this invention relates to a solvent predilution dewaxing process wherein a residual waxy petroleum oil stock is admixed with a solvent prior to the cooling of the oil to a temperature below its depressed cloud point.
2. Description of the Prior Art It is known in the prior art to dewax petroleum oil stocks by cooling an oil-solvent solution in scraped surface exchangers. In this process the oil and selective solvent are admixed at a temperature sufficient to effect complete solution of the oil and its contained wax in the solvent. The extent of dilution is dependent upon the particular oil and solvent employed and is adjusted to facilitate easy handling and optimum filtration rates and oil yields. The solution is cooled at a uniformly slow rate, e.g., 1 to 5F. per minute, under controlled conditions so as to avoid any substantial agitation of the solution during precipitation of the wax. Notwithstanding the carefullycontrolled conditions used in this pro cess, there are several deficiencies which hamper successful commercial operation. Most significant among these deficiencies is the loss of good heat transfer due to wax deposition on the exchange surfaces. Such failing has been repeatedly noted after short periods of operation, e.g., 24 to 48 hours. Associated directly with the loss of good heat transfer is the loss of careful control of the cooling rate and the corresponding loss of uniform crystal growth. This non-uniform crystal growth results in lower filtration rates. The highpressure drop through the chilling section also reduces the maximum feed rate obtainable. Physical mashing of the wax crystals by the action of the scrapers mayalso contribute to poor filtration.
It is also known in the prior art to dewax petroleum oil stocks by cooling same in scraped surface exchangers using an incremental'solvent addition technique. In this process, the solvent is added at several points along the chilling apparatus. The waxy oil is chilled without solvent until some wax crystallization occurs and the mixture thickens considerably. The first increment of solvent is introduced at this point and cooling continues. Each incremental portion of solvent is added, if necessary, to maintain fluidity until the desired separation temperature is reached, at which point the remainder of the solvent desired for filtration is added.
Using this common industrial technique, it is well known, and has been repeatedly demonstrated, that the temperature of the solvent should be the same as that of the main stream at the point of addition. Having the solvent at a lower temperature causes shock chilling of the slurry at that point, with resulting formation of crystal fines, and impairment of filter rate; having the solvent warmer throws an unnecessary additional load on the scraped surface chillers. The bulk of the chilling of the slurry in this well-known process is accomplished through the walls of the scraped surface chillers rather than by means of cold solvents.
It is also known in the art, as described in US. Pat. No. 2,361,503 to Schutte et al, to subject lubricating oil fractions to agitation in a multi-staged tower with water or brine, which serves as the cooling medium. This process suffers from the disadvantage that the cooling medium is completely immiscible with the wax and oil, and rapid separation occurs between the feed and water unless the mixture is maintained in an agitated state or is emulsified. The use of water as a cooling medium also practically limits the process to an upfiow operation. In US. Pat. No. 2,410,483 to Dons et al a twostage dewaxing process is described wherein a waxcontaining oil stock, heated above its cloud point, is introduced into an elongated cooling vessel divided into a plurality of stages. Cold dewaxing solvent is injected into each of the stages thereby cooling the oil. The resultant oil solvent/mixture is withdrawn from the cooling vessel at the temperature of the cloud point of the oil or no more than about lOF., above the cloud point. This mixture is then introduced into a pipe wherein additional cold dewaxing solvent is added thereto in the substantial absence of any agitation, thereby cooling the oil to below its cloud point and precipitating wax therefrom. The process contemplates no substantial agitation in the crystallization phase of the dewaxing process. As a consequence, the filtration rates are inferior to other dewaxing processes.
In US Ser. No. 129,973, filed Mar. 31, 1971 now US. Pat. No. 3,773,650, which application is a C.l.P. of US. Ser. No. 17,869, filed Mar. 9, 1970 (now abandoned) which is, in turn, a C.I.lP. of US. Ser. No. 666,268, filed Sept. 8, 1967 (now abandoned), the disclosures of which are incorporated herein by reference, there is taught a method for dewaxing oils, i.e., dilution chilling, wherein a waxy oil stock is introduced into a cooling zone divided into a plurality of stages. Dewaxing solvent is introduced into the cooling zone at a plurality of spaced points situated along the cooling zone, coming into contact with the oil. High levels of agitation are provided in at least a portion of the solvent-containing stages, thereby providing substantially instantaneous mixing of the solvent and oil, e.g., within a second or less. As the oil passes through the cooling zone, it is cooled to a temperature sufficient to precipitate at least a portion of the wax therefrom resulting in the formation of a wax slurry wherein the wax particles have a unique crystal structure, thereby providing superior filtering characteristics such as high filter rates and high dewaxed oil yields. While the process of Ser. No. 129,973 overcomes many of the disadvantages of the prior art, it has not been found to be very effective with waxy residual lubricating oil stocks, such as residual bright stocks. The filter rates with these heavy oils have been found to be quite low and there is considerable incentive to improving the filterability characteristics'of these oils.
SUMMARY OF THE INVENTION In accordance with the invention, it has now been discovered that waxy residual lubricating oil feedstocks can be dewaxed and that substantial improvements in filtration rates can be obtained by use of the solvent predilution process of the subject invention.
The process is particularly suitable for dilution chilling dewaxing and comprises, in one embodiment of the invention, prediluting a waxy residual oil with at least about 0.3 volumes ofa predilution solvent per volume of residual oil stock, resulting in the depression of the cloud point of the oil stock. The process feedstock comprises a residual waxy petroleum oil stock characterized by having a viscosity greater than about SUS at 210F. and containing less than about percent of material boiling below about 950F., (all temperatures are reported at atmospheric pressure, unless otherwise stated).
The cloud point of the oil is defined as the temperature at which a cloud or haze of wax crystals first appears when an oil is cooled under prescribed conditions (modified ASTM D2500-66 procedure). Predilution," as the term is used herein, refers to the mixing of solvent and oil prior to cooling of the oil to a temperature below its depressed cloud point.
The resultant solvent-oil mixture is introduced into a cooling zone divided into a plurality of stages, at a temperature above the depressed cloud point of the oil. Additional dewaxing solvent, which may be the same or different than the predilution solvent used to form the initial solvent-oil mixture, is introduced into at least a portion of the stages and high levels of agitation are maintained in at least a portion of the solventcontaining stages thereby providing efficient mixing of solvent and oil. The high levels of agitation referred to above are only necessary during the initial phases of wax crystal nucleation and growth. Once good crystal growth is effected, lower agitation levels may be used, e.g., in the later stages of the cooling zone.
The solvent-oil mixture is cooled as it passes through the cooling zone to a temperature below the depressed cloud point of the waxy oil stock, thereby precipitating at least a portion of the wax therefrom, and a residual oil stock of diminished wax content is recovered.
In another embodiment of the invention, the predilution of the oil is conducted in situ, i.e., within the cooling zone itself. To this end, the feedstock is introduced into the cooling zone at a temperature above its cloud point and in the substantial absence of solvent. At least about 0.3 volumes of solvent per volume of oil is added to the initial stages of the cooling zone, coming into contact with the oil stock and forming an oil-solvent mixture. The mixture is gradually cooled, as it passes through the initial cooling stages, to a temperature no less than the depressed cloud point of the oil stock. Thereafter, additional solvent is introduced into at least a portion of the remaining stages of the cooling zone, and the oil is further cooled to a temperature below its depressed cloud point thereby precipitating at least a portion of the wax.
Although it is preferred that a substantial portion of the cooling of the oil be provided by the contacting of same with prechilled solvent, it is contemplated that other cooling means, such as autorefrigeration, wherein cooling is effected in part by vaporization of solvent, may also be employed.
The feedstock that is used in the process of the invention comprises a residual waxy oil stock having an initial boiling point above about 800F., with less than about 10 percent (by weight) of material boiling below about 950F. and less than about 50 percent (by weight) of material boiling below about 1,050F. The oil is further characterized by having a viscosity greater than about 75 SUS at 2 lOF. and ranging between about 75 and 300 SUS, preferably between about 100 and 200 SUS and most preferably between about 125 and 175 SUS at 210F.
The residual oil contains the most difficulty vaporizable components of petroleum hydrocarbons including asphaltenes and pitch, which are undesirable not only in the finished lubricating oil product, but also in the LII intermediate refining operations, as discussed in more detail infra. It is thus preferred, prior to the dewaxing operation of the subject invention, to remove as much of these components from the residual oil as possible, such as by a deasphalting operation, e.g., propane deasphalting. Further, the residual oil may contain aromatic and polar molecules which would impart undesirable properties to the finished lube oil product. These molecules may be removed by using such process techniques as solvent extraction, comparatively severe hydrogen treatment and the like either before or after the dewaxing step.
Preferably, the residual oil is derived from a raw lube oil stock, the major portion of which boils above about 650F. This oil stock can be vacuum distilled with the resultant overhead and sidestreams being termed distillates and the bottoms being termed residua or residual oil stocks. Considerable overlap may be encountered in the boiling ranges of the distillates and residua dependent, in part, on the efficiency of the distillation, with certain of the higher boiling distillates containing almost the same distribution of molecular species as the residua and therefore showing similar responses to the dewaxing operating variables of the subject invention. These distillates are therefore included in the term residual as used herein. The crude sources from which the instant feedstocks may be obtained are exemplified by the paraffinic crudes such as Aramco, Kuwait, the Panhandle, North Louisiana, Tia Juana and the like.
In general, the wax content of the feedstock as defined by the amount of material to be removed to produce an oil with a pour point in the range of +25 to 0F. will vary between about 5 and 35 wt. percent based on total feed, preferably between about 10 and 30 wt. percent. The initial pour and cloud points of the oil will range, respectively, between about and 175F. and about and 180F.
The predilution solvent is selected from any of the dewaxing solvents known in the prior art such as the aliphatic ketones having from three to six carbon atoms, e.g., acetone, methylethyl ketone (MEK), methylisobutyl ketone (MlBK) and the like, the lower molecular weight hydrocarbons such as ethane, propane, butane and propylene, as well as mixtures of the foregoing ketones and mixtures of the ketones with hydrocarbon compounds such as propylene, and aromatics such as benzene and toluene. In addition, halogenated low molecular weight hydrocarbons such as the C -C chlorinated hydrocarbons, e.g., dichloromethane, dichloroethane and mixtures thereof, may be used. Specific examples of effective predilution solvents include toluene, MlBK, MEK/toluene, MEK/MIBK and the like.
The depressed cloud point of the oil is dependent, in part, upon the degree of predilution of the oil with solvent and will preferably range between about 50 and l75F., most preferably between about 50 and F. In general, the amount of predilution solvent added to the oil will be dependent, in part, on the nature of the feedstock, the cooling zone, the extent of cooling within the cooling zone, i.e., approach to the filtration temperature, and the desired final ratio of solvent to oil in the wax/oil/solvent slurry withdrawn from the cooling zone. Preferred amounts of predilution dewaxing solvent range between about 0.3 and 2.0 volumes per volume of residual oil stock, most preferred between about 0.5 to 1.5 volumes of solvent per volume of oil stock.
The dewaxing solvent that is used during the phase of the dewaxing operation conducted at a temperature below the depressed cloud point of the oil may be the same as or different than the predilution solvent and is selected from the same group of solvents mentioned in connection with the predilution solvents. Specific examples of suitable dewaxing solvent mixtures include methylethyl ketone/methylisobutyl ketone, methylethyl ketone/toluene and propylene/acetone. The preferred solvents are the C -C ketones with methylethyl ketone being particularly preferred. It is noted that when the dewaxing solvent is MEK, a particularly preferred predilution solvent comprises toluene or MIBK.
While all the cooling of the oil stock to the subsequent filtration temperature may take place in the dilution chilling zone, this is not necessarily required for the successful operation of the subject process. In fact only a portion of the cooling need be done therein. Further cooling of the wax/oil/solvent slurry withdrawn from the cooling zone to the filtration temperature may take place in conventional cooling apparatus such as scraped-surface equipment, an autorefrigeration vessel and the like. A description of this aspect of the process is found in U.S. Ser. No. 257,435, filed May 26, 1972, the disclosures of which are incorporated herein by reference.
Quite surprisingly, the predilution process has been found to be specific to waxy residual oils as described hereinabove, and is, in fact, detrimental to the dewaxing of light distillates. The term light distillates" as used herein refers to a feedstock having a 90 percent or end boiling point as high as about 1,050F. and containing at least 50 percent (by weight) of material boiling below about 1,050F. In addition, these light distillates usually contain more than about percent (by weight) of material boiling below about 950F. but contain less than about 5 percent (by weight) of material boiling below about 650F. The distillate is further 1 characterized by having a viscosity at 210F below about 75 SUS.
While the exact mechanism of the predilution process is not known, it is speculated that trace amounts of asphaltene and pitch components present in the residual feedstocks, possibly as a result of contamination in the vacuum pipestill or inadequate deasphalting, interfere with the formation of wax crystals of the desired structure. Specifically, it is thought that at some temperature above the depressed cloud point of the feedstock, these asphaltene and pitch components precipitate from the oil as very small crystals which interfere with the uniform nucleation and growth of the wax crystals. It is contemplated that solvent predilution facilitates solution of these very small crystals, and delays their precipitation until after the cloud point temperature of the oil is reached, at which time, they cocrystallize with the wax components of the oil, thereby substantially reducing wax crystal growth interference.
It is also thought that predilution techniques in dilution chilling dewaxing reduce the overall viscosity of the oil stock in the critical early stages of crystal nucleation and growth thereby removing diffusion limitations to crystal growth and facilitating the development of larger particles.
This may be particularly important with the residual fecdstocks and the like, since the wax crystallizing from such high boiling, high molecular weight feedstocks comprises highly branched paraffins and naphthenes, which have very low crystal growth rates. In contrast, the wax crystallizing from lower boiling distillate feedstocks, generally contains predominantly normal paraffins, which have relatively high crystal growth rates and would therefore not be as sensitive to diffusion limitations.
Further improvements in filter rate may be obtained when dewaxing aids are used in conjunction with solvent predilution for dewaxing residual feedstocks. A preferred dewaxing aid comprises a Ziegler type mixed normal alpha olefin copolymer described in more detail in U.S. Ser. No. 164,892, filed July 21, 1971, having a number average molecular weight between about 2,000 and 60,000 or higher, and having pendant side chains of C and higher. A particularly preferred dewaxing aid composition comprises 38 wt. percent nhexene-l, 26percent n-hexadecene-l, 21 percent noctadecene-l and 15 percent n-eicosene-l. Other dewaxing aids may also be used such as polymeric higher alkyl methacrylates, long-chain alkyl 1,2-oxiranes, po-
lymerized higher fatty acid esters of vinyl alcohol, a mixture of at least two homopolymers of a C C alpha olefin, a Friedel-Crafts condensation product of a halogenated hydrocarbon such as chlorinated paraffin wax with an aromatic hydrocarbon such as naphthalene, mixtures thereof and the like.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified flow scheme of a preferred embodiment of the dewaxing process of the subject invention.
FIG. 2 is a graph relating filter :rate to the amount of predilution in MEK/toluene dilution chilling dewaxing of an Aramco 2,500 bright stock.
DETAILED DESCRIPTION Referring to FIG. 1, a waxy lubricating oil stock is taken from tankage and introduced into predilution mixing zone 1 via line 28 while dewaxing solvent is introduced therein via 1ine-29QAfter a sufficient contact time, the resultant solvent-oil mixture is introduced via line 2 into cooling zone 3, at a temperature above the depressed cloud point of the feedstock. Although not shown, heating means may be provided in mixing zone 1 to ensure that the feed temperature is above the depr'essed cloud point of the oil prior to introduction into the cooling zone. The cooling zone is depicted herein as a vertical cooling tower; however, it is noted that the design is not limited to this configuration. The solventoil mixture enters the cooling tower and into the first stage of the cooler, i.e., 4(a). Dewaxing solvent is passed from storage tank 5 through line 6, and heat exchangers 7 and 8, where the solvent temperature is reduced to that sufficient to cool the oil to the desired temperature. Coolant enters the heat exchangers 7 and 8 through lines 24 and 25, respectively and leaves through lines 26 and 27. It will be apparent to those skilled in the art that the exact solvent temperature employed will depend upon the amount of oil to be cooled and the amount of solvent to be added to the oil, i.e., the degree of oil dilution which is sought during the filtration step.
The solvent leaves the heat exchanger 8, through line 9, and enters manifold 10. The manifold comprises a series of spaced solvent inlet points 11 to the several stages of the cooling tower 3. The rate of solvent flow through each inlet is regulated by flow control means (not shown) and is adjusted, so as to maintain a desired temperature gradient along the height of the cooling tower. Preferably, the incremental solvent addition is such that the chilling rate of the oil is below about lF./minute and most preferably between about 1 and 5F./minute. In general, the amount of solvent added thereto will be sufficient to provide a liquid/solid weight ratio between about 5/1 and 100/1 at the dewaxing temperature and a solvent/oil volume ratio between about 1.0/1 and 7/1.
The-first portion or increment of solvent enters the first stage, 4a, of the cooling tower 3, where it is substantially instantaneously mixed with oil due to the action of the agitator 12a. The agitator is driven by a variable speed motor 13 and the degree of agitation, as defined in more detail below, is controlled by variation of the motor speed, with due allowance for the flow rate through the cooling tower. It is noted that while a rotating blade is shown as the agitation source, any other mixing means that is able to produce the high levels of agitation required can be used herein. The oil-solvent mixture may pass upwardly or downwardly through the cooling tower 3 (downwardly flow only has been shown). At various heights along the cooling tower, additional prechilled solvent is introduced to each of the several stages 4, through inlets 1 1 so as to maintain substantially the same temperature drop from one mixing stage to the next and at the same time to provide the desired degree of dilution. It should be noted that any number of stages up to 50 may be employed; however,
at least six is preferred.
The cooling of the oil stock continues to a temperature substantially below the depressed cloud point of the oil stock, thereby precipitating at least a portion of the wax therefrom and forming a wax-oil-solvent mixture.
The oil-solvent solution with precipitated wax passes from the final stage of the cooling tower through line 14 to wax separation means 15. If desired the wax-oil solvent mixture may be further cooled by conventional means not shown. Any suitable separation means such as filtration or centrifugation may be employed. The wax-solvent mixture is removed through line 16 and the solvent recovered therefrom in a suitable separating system 19, which preferably comprises stripping with an inert gas such as nitrogen, steam or air, or straight distillation. The solvent leaves the separator 19 through line 17 and the wax exits through line 18.
The oil-solvent mixture leaves separator through line 20 and passes to oil separation means 21. Any suitable means to effect this separation may be used, such as distillation, selective adsorption, or stripping with an inert gas such as nitrogen, air or steam. The solventfree oil is removed from the separator and recovered through line 22. The solvent is removed through line 23 and may be recycled directly to the dilution chilling tower or first scrubbed to remove impurities before reuse.
As indicated previously, the degree of agitation, during the initial stages of crystal nucleation and growth, must be sufficient to provide substantially instantaneous mixing of solvent and oil, i.e., preferably within a second or less, The degree of agitation required in the process can be achieved by increasing the agitator rpm, when all other mixing variables, e.g., flow rates through the mixer, vessel and agitator design, viscosity of the ingredients and the like, are maintained constant, so that the modified Reynolds Number (Perry, Chemical Engineers Handbook, 3rd, pp. 1224, McGraw-Hill, New York, 1959), Me, which is defined by the equatron:
where L agitator diameter, ft.
1 liquid density, pound/feet n agitator speed, revolution/second u liquid viscosity, pound/feet second ranges between about 200 and about 150,000. The dimensionless ratio of cooling tower diameter to agitator diameter is between about 15/1 and about 10/1 and the ratio of the impeller blade length to impeller blade width ranges from about 0.75 to 2 and preferably from about 1 to 1.5. The ratio of the mixing stage height to the diameter of the stage will generally range from about 0.2/1 to about l/l. A turbine type agitator is preferred, however, other types of agitators such as propellers may be used.
The cooling tower may or may not be baffled, but a baffled tower is preferred. Each stage will generally contain from about two to eight baffles and preferably from two to four baffles, located about the outer periphery of each stage, The width of the baffles may range from about 5 to 15 percent of the diameter of the tower. In general, the dimentional ratio of the crosssection of the restricted flow opening to the crosssection of the tower will be between about H20 and about l/200.
The cooling tower of the present invention is preferably operated at a pressure sufficient to prevent flashing of the solvent. Atmospheric pressure is sufficient when the ketones are employed as solvent; however, superatmospheric pressures are required when low molecular weight hydrocarbons such as propyleneacetone and related autorefrigerative solvents are used. As noted above, however, in situations where propylene-acetone and related autorefrigerative type solvents are used, low pressures will be required. A process combining both vaporization of the solvent to provide in situ refrigeration and direct cooling from cold dewaxing solvent is disclosed in US. Pat. No. 3,658,688 patented Apr. 25, 1972, the disclosures of which are incorporated herein by reference.
The recovered lube oil products may, if so desired, be subjected to various finishing operations such as clay contacting, hydrofinishing, acid treatment and the like.
PREFERRED EMBODIMENT The invention will be more apparent from the working examples set forth hereinbelow.
EXAMPLE 1 A laboratory experiment was performed in a 6 inches diameter single stage batch unit provided with a 2 inches diameter flat-bladed turbine impeller, a means for solvent introduction and an overflow device to maintain a constant volume of slurry. This batch unit does not completely duplicate continuous multi-staged operations but has been found to give approximately EXA P equivalent results. M LE 2 The feedstock used in this example was a deasphalted Thefexpehmhhts dlsclosed 1h E pl 1 Supra; were phenol-extracted residual distillation fraction from an rerun m a wmmuous 16 Stage pll'ot compnsed of Arabian light crude. having less than 10 percent of maa 6 inches dhhheter tower q pp with 2 inches terial boiling below 975F. and less than 50 percent of "lmeteh 6 bladeflat la disc turbine 'fiE l l H material boiling below 1,150F. The feedstock had an Experiments were also conducted in which the P initial pour point f 145%? an i i i Cloud point f lution solvent was toluene and the composition of the 150F., a viscosity at 210F. of 140 sus, and required MEK/wluene dewaxing Solvent was adjusted to give removal f 5 percent (wt) dry wax to give a bright the desired end solvent composition of MEK/toluene,
stock lubricating oil product with a +F. pour point. 55/45 LV%i at the desired final Solvent/feed ratio This feed is hereinafter referred to as an Aramco 2500 of I H bright stock. The data displayed below in Table 11 relate degree of Methylethyl ketone/toluene, 55/45 LV%, was used as predilution, cloud point reduction in the oil stock and both the predilution solvent and as the dewaxing soll5 process performance as measured by filtration rate. vent during the chilling operation. The solvent compo- The data are displayed for the lab single stage equipsition in the dilution chilling dewaxing operation was ment in addition to the pilot plant 16 stage dilution adjusted to obtain approximately a 4:1 final solvent/oil chilling tower.
TABLE 11 Feed Filter Rate USG/fP-hr v Cloud Point F. Pilot Unit Lab Single Stage Predilution MEK/Tol MEKfI'oI MEK/Tol V/V Tolu- (55/45) Toluene* (55/45) Toluene* (SS/45) ene Composition of predilution solvent. Solvent composition to tower adjusted to give outlet MEX/T01 (55/45).
dilution ratio. 6th; variables such as average chilling The data indicate that predilution is an effective rate, agitation levels and the like are displayed below means for increasing the overall filtration rate in the in Table 1. Excess slurry comprising precipitated wax, dewaxing of waxy residual feedstocks. Additionally, the oil and solvent was allowed to overflow the apparatus. 5 data indicate that predilution solvent systems such as wh the slurry h d a ifi d t pe t th mixtures of methylethyl ketone and toluene perform as contents were drawn off and chilled further by conven- Well, if hot better than P Solvent Systems Such 33101- tional means in order to reach a common filtration tem- Ilene in rrying Out h process Objectives. The advanperature. I tage of using toluene rather than MEK/toluene (55/45 p A LV%) as the predilution solvent relates to the greater The data Show that with pre ilution n t range of cloud point depression obtained with toluene for a 0.5 to 1.5 volumes of solvent/volumes of oil, DWO filgiven ratio of predilution solvent/feed. Since the prediter rates were increased by nearly 100 percent. The lution solvent, in addition to the feed, has to be chilled data, which have been graphed and are displayed in from a few degrees above the dpress e d Eloud poirTtto FIG. 2, show the tremendous enhancement in filtration the filter temperature, there are obvious savings in rerate when predilution is used. It is further noted that frigeration from operating with the lowest possible dethe major improvement is observed in the filtration rate pressed l d im as pp 10 the dewaxed 0h Yields which remain The use of a single solvent composition for predilufairly constant throughout the Various ruhstion and dilution chilling has the obvious advantage 5 TABLE I MEK/TOLUENE. (/45 LV%) DILUTION CHILLING DEWAXING QF ARAMCO 2500 BRIGHT STOCK Laboratory Simulation of a 16 Stage DiIutiOn Chilling Tower Stage volume I500 ccs Impeller centrally mounted 2" diameter 6 bladed, flat bladed. disc turbine Agitation 770 rpm Solvent for predilution, stage injection and filter wash: MEK/toluene 55/45 LV% Solvent injected into stage at 20F.
Filter temperature. +5F. Wash k Filter Time.
Dilution Chilling Performance Cloud Dilution Wash DWO Filter DWO Feed DWO Run Predilntion Point Start End End Feed Rate Yield Filter Rate Pour Point No. V/V F. F. F. V/V V/V USGIft -hr. wt.% on feed uso/re-m.
the Aramco 2,500 bright stock waxy raffinate, described in Example 1. was introduced into the chilling zone in the absence of solvent. Dilution chilling was performed with F. MEK/toluene (55/45 LV'/() and the effect of varying the feed temperature on effective predilution and performance is shown in Table IV below.
TABLE IV EFFECT OF IN SITU PREDILUTION ON MEK/TOLUENE DILUTION CHILLING DEWAXING ARAMCO 2500 BRIGHT STOCK Initiation of Wax Crystallization In stage Solvent/Feed* Stage Feed Filter Rate Feed Temperature, F. number V/V in Stage Temperature, F. USGIft -hr.
Effective predilution EXAMPLE 3 TABLE III The data confirm that in situ predilution is an alternative means ofincreasing the overall filtration rate in the dewaxing of residual feedstocks, althrough it suffers from the disadvantage that less effective use is being made of the first few stages prior to wax crystallization.
EXAMPLE 5 This example illustrates the detrimental effect of predilution on a phenol extracted light distillate feedstock from a Western Canadian Crude. The feedstock KETONE DILUTION CHILLING DEWAXING ARAMCO 2500 BRIGHT OCK EFFECT OF PREDILUTION ON DWO FILTER RATE Lab single stage simulation of 16 stage dilution chilling. Same solvent composition used for predilution and subcloud point cooling. Dilchill solvent temperature 20F. Agitation 770 rpm (2" impeller) chilling rate 2F/minute Solvent/feed to filter 4/1.
'DWO Filter Rate, wash filter time, relative to MEK/toluene case with no predilution The data indicate that a similar beneficial effect of contained less than 5 percent of material boiling below predilution on the filter rate was obtained using all 660F. or above 890F., and its viscosity at 210F. was
three solvents.
EXAMPLE 4 This example demonstrates the performance advantage obtained by in situ predilution. The experiments were carried out in the laboratory single stage unit, and
41 SUS. The initial feed pour and cloud points were F. and F. respectively, and it required the removal of 22 percent dry wax to yield a lubricating oil with a 0F. pour point. The process conditions are disclosed in Table V. The data typify the effect of predilution on light distillates.
TABLE v MEK/MIBK CHlLLlNG ON A LOW BOILING DlSTlLLATE FROM A WESTERN CANADIAN CRUDE Effect of Predilution Ahead of Tower Lab single stage simulation of 16 stage dilution chilling.
l 130 rpm. Chilling rate 2F./min. Filtered at F.
Solvent/Feed (wt/wt) Performance In Feed DWO Filter DWO Yield Run to tower to as Wash Rate Wt.% on No. (predilution) filter to filter USG/ft hr. feed EXAMPLE 6 b. introducing said first mixture at a temperature ln this example, the feedstock was a phenol-extracted distillate from a Western Canadian crude, with 45 percent of material boiling below 950F., 85 percent of material boiling below l,050F., and also characterized by having a viscosity of 63.1 SUS at 210F. and requiring removal of 19 percent dry wax to yield a lubricating oil of +20F. pour point. The initial feed pour point was 125F. and the initial feed cloud point was 130F. In one instance, the feed was introduced into the 16 stage dilution chilling pilot unit, described in Example 2, at l35F., while in another instance the feed was introduced into the 16 stage pilot unit at 155F. under in situ predilution conditions. Other conditions, and the deleterious effect on performance of in situ predilution obtained by elevating the feed temperature is illustrated in Table VI below.
TABLE V] DlLUTlON CHlLLlNG DEWAXING A WESTERN CANADIAN MEDIUM BOlLlNG DlSTlLLATE EFFECT OF IN SITU' PREDlLUTlON 16 Stage pilot unit. 2" impellers. Solvent MEK/MIBK 45/55 Lvvr. Dilchill solvent at -20F. Agitation 1 I rpm. Chilling rate 2F./min. Filter at +20F.
Feed temperature. F. initial wax cloud point reached at: stage number I 4 stage temperature. F. I28 I26 sol\'ent/feed (effective predilution) .06 .27
Solvent/Feed to filter 2.8 2.7
-to wash* |.l 0.8 DWO Filter Rate. UsG/ft -hrfl 4.8 4.2
DWO Yield, Wt.% on feed 77.2 67.8
' Wash time filter time above the depressed cloud point of said residual oil stock into a cooling zone divided into a plurality of stages and passing said mixture from stage to stage of said cooling zone;
c. introducing dewaxing solvent into at least a portion of said stages of said cooling zone at a plurality of spaced points therealong;
d. mixing said dewaxing solvent: with at least a portion of said first mixture as it passes from stage to stage of said cooling zone under conditions of high agitation, thereby forming a second mixture comprising said dewaxing solvent, said predilution solvent and said residual oil stock; and,
e. cooling said residual oil stock contained in said second mixture as it passes from stage to stage of said cooling zone, thereby reducing the temperature of said residual oil stock to below its depressed cloud point and precipitating atleast a portion of said wax therefrom under said conditions of high agitation.
2. The process of claim 1 wherein said waxy residual petroleum oil stock is characterized by containing less than about 10 percent (weight) of material boiling below about 950F., at atmospheric pressure, and less than about 50 percent (weight) of material boiling below about l,050F., at atmospheric pressure.
3. The process of claim 1 wherein the solvent in step (a) is a dewaxing solvent and is selected from the group consisting of aliphatic ketones containing from 3 to 6 carbon atoms per molecule, the lower molecular weight hydrocarbons, aromatic compounds, halogenated lower molecular weight hydrocarbons and mixtures thereof.
4. The process of claim 3 wherein said solvent is selected from the group consisting of methylethyl ketone, methylisobutyl ketone, toluene and mixtures thereof.
5. The process of claim 1 wherein the solvent used in step (a) is a dewaxing solvent and is the same as or different than the dewaxing solvent used in step (c).
6. The process of claim 1 wherein the degree of agitation is sufficient to provide substantially instantaneous mixing of solvent and oil.
7. The process of claim 1 wherein the dewaxing solvent used in step (c) is prechilled prior to introduction into said cooling zone.