US2777889A

US2777889A - Crystallization of polycyclic hydrocarbons

Info

Publication number: US2777889A
Application number: US257144A
Authority: US
Inventors: Naumann Carl
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-11-19
Filing date: 1951-11-19
Publication date: 1957-01-15
Anticipated expiration: 1974-01-15

Description

c. 7 NAUMANN 2,777,889 CRYSTALLIZATION OF POLYCYCLIC HYDROCARBONS Jan. 15, 1957 Filed NOV. 19, 1951 United States Patent C CRYSTALLIZATION OF POLYCYCLIC HYDRCARBON S Carl Naumann, Philadelphia, Pa. Application November 19, 1951, Serial No. 257,144 4 Claims. (Cl. 260-674) The present invention relates to crystallization of poly cyclic hydrocarbons such as naphthalene.

This application has been divided and the apparatus subject matter is contained in copending application Serial No. 324,707, tiled December 8, 1952, for Crystallizer for Polycyclic Hydrocarbons.

A purpose of the invention is to continuously crystallize and remove polycyclic hydrocarbons from hydrocarbon oils by progressing the hydrocarbon oils initially at a temperature of 90 to 170 F. through a plurality of generally horizontal closed cylindrical chambers of a first stage and there cooling the hydrocarbon oil and slowly agitating the oil as by a scraper-stirrer for a time in excess of 8 hours and suitably between 8 and 18 hours, forming crystals, removing the crystals from the first stage by centrifuging, progressing the oil initially at a temperature below 100 F. through a plurality of generally horizontal closed cylindrical chambers of a second stage and there further cooling the oil and slowly agitating it as by a scraper-agitator for a further time in excess of 8 hours and suitably between 8 and 18 hours, forming further crystals, and removing the crystals from the second stage by centrifuging.

A further purpose is to remove a maximum of crystals at a high temperature and then remove a maximum of crystals at a lower temperature.

A further purpose is to employ continuousfeed and discharge in the process.

A further purpose is to dilute the oil optionally before each of the stages and between flights of individual stages by crystal-free eluent diluent.

A further purpose is to introduce alkali solution, suitably sodium hydroxide solution of a concentration of below 8 to 10% and ally prior to either stage and betwen the ights in either stage. The alkali is desirably introduced in a solution in one of the alcohols named to give the combined advantage of both treatments.

A further purpose is to dilute hydrocarbon oil by alcohol such as a straight or branched carbon chain alcohol having carbon atoms in the carbon chain or less, Vsuch as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, isopropyl alcohol, isobutyl alcohol introduced optionally at the beginning of each stage or between the ilights of each stage.

A further purpose is to render the naphthalene vor other polycyclic compound less soluble in the hydrocarbon oil by adding an alcohol having a carbon chain length not in excess of 5 carbon atoms or an alkali dissolved in such alcohol. The alkali may suitably be sodium hydroxide or l,potassium hydroxide.

A further purpose is to apply a coolant in the second stage cooled to a temperature below 20 F.

A further purpose is to obtain maximum Icrystal growth ofv separate crystals through adequate dilution and slowly agitating the crystals as they form.

A further purpose is to discharge the c rysta-ls to the centrifuge by gravity so that 'they are not broken up.

preferably of the order of 5%, option- A further purpose is to obtain hard crystals which can be washed with hot water or the like.

A further purpose is to reduce the adherence of oil to naphthalene or other polycyclic hydrocarbon crystals by washing in the centrifuge which removes hydrocarbon crystals with an alcohol having a carbon chain length not in excess of 5 carbon atoms oran alkali such as sodium or potassium hydroxide dissolved in such alcohol. The alcohol with or without alkali can be used alone or diluted by water, preferably hot.

Further purposesv appear in the specication andv in the claims.

In the drawings I have chosen to illustrate a -few only of the numerous embodiments in which my invention may appear, selectingthe forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

Figure l is a How diagram of the process of the invention.

Figure 2 is a central axial vertical section of one of the crystallizer nights.

Several processes exist for crystallizing naphtha'lene and the like. In one process known as the Vogt process, 'the crystals form on a cooling wall which iselectively scraped by a scraper plate urged against the wall under spring pressure. The crystals never grow very large and it is dicult to separate them from the oil because they are so small that they are dicult to separate in a centrifuge. i

Another commercial process i-s the Swenson-Walker process which uses U-shaped open troughs in which the hydrocarbon oil is agitated by a worm. There is a tendency for great build-up of naphthalene to occur at the tops of the troughs, adding to the power requirement and labor consumption.

The present invention is concerned with crystallizing of polycyclic hydrocarbons such as naphthalene, anthracene, phenanthrene,vand the like. Similar compounds such as polycyclic imides like carbazole may also be treated in the present process. The raw material ordinarily operated upon is a hydrocarbon oil from tar distillation containing naphthalene'crystals although in 'some cases petroleum oils containing naphthalene may be used.

In accordance with the invention, the hydrocarbon oil including the crystals is passed successively through a first stage crystallizer and then a second stage crystallizer. Each crystallizer comprises a plurality of flights, suitably arranged in cascade,'one below another, and discharging by gravity from flight to Hight and from the last flight to a centrifuge which removes the crystals present in that stage. `Each flight is a suitably horizontal cylinder of metal closed from the atmosphere and cooled by heat transfer suitably to cooling water around the wall. A scraper moving very slowly, desirably at a sped not in excess of 5 R. P. M., turns on the inside of each ight, moving and scraping and agitating plate close to butnot in contact with the surrounding wall. Theclearance allowed from the wall is at least Ms, suitably as much as 1%2 and preferably 1A". This allows crystals to grow to a substantial size before they are removed or scraped from the wall.

The system by operation in two stages ismore effective in eliminating crystals since one group of crystals is taken out in the first stage' by cooling to a higher temperature and another group of crystals is taken out in the second stage by cooling to a lower temperature. The cooling in the second stage is suitably to a temperature below 20 F., as the second stage coolant is preferablyat a temperature below 20 F., and the cooling medium temperature may bedown to a temperature as low as minus 10 F.

The hydrocarbon `oil entering the rs'tstage is suitably at a temperature jof 4between n90 and 170 F.' and prefer- Patented Jan. l5, 1957 ably between`90 and 140 F. The hydrocarbon oil entering the second stage is suitably at aternperature below 100 F., and conveniently between 70 and 100 F.

The two stage continuous system of the invention is very effective, and can operate on any naphthalene oil containing frorn 20 to 85% of naphthalene by weight.4

The holding time for crystal growth in the second stage is ample to provide for maximum crystal growth to aid in effective separation in the centrifuge. The time of treatment in each stage is preferably about l2 hours and suitably may vary from S to 18 hours. rhus the total time of treatment is of the order of 16 to 36 hours, preferably about 24 hours. During this treatment, slow agitation goes on, tending to keep the crystals-separated. To further separate the crystals, the hydrocarbon oil is desirably diluted by introducing a suitable liquid which will tend to keep the crystals further apart and will allow for ample room for growth without forming interlocking masses which will trap hydrocarbon oil and contain impurities. The diluent may merely be inert or it may perform a function in the crystallization and purification. In some cases the diluent is crystal-free effluent, that is, hydrocarbon oil from which crystals have been removed from the process, depending upon the tendency of crystals to interlock at different stages, the diluting will be introduced at the beginning of the first stage, at the beginning of the second stage, or between any flights in the first or second stage.

In some cases, it is desirable to incorporate a liquid having special advantages in aiding purification. One of the straight or branched carbon chain alcohols having not in excess of five carbon atoms in the carbon chain will desirably be introducedwith the crystal-free effluent or bon oil. This can conveniently be accomplished by introducing alkali such as sodium hydroxide in a concentration of less than 8 to 10% and suitably of the order of 5%, with the crystal-free ellluent and the alcohol, either or both, or alone, at the beginning of either stage or between v flights in either stage.

By the above procedures, and particularly by the slow crystal growth with slow agitation, l obtain hard crystals which can safely be washed withhot water in the centrifuge as desired. The water wash does not interfere with subsequent operations.

lnaccordance with the invention, the crystals discharge `to the centrifuge by gravity and they are not likely to break up from this cause.

The invention is applicable to treatment of coal tar crude light oil still residue with contaminating petroleum wash oil.

` The process lends itself well to automatic operation, so that one part-time operator can control the entire process.

The raw feed enters at 20 and may consist of napth- Vthalene oil with a concentration of 2.0 to 84 percent of naphthalene by weight. The lower melting point naphthalene material is limpid at roorn temperature and the higher melting point material is limpid at 160 f". The remainder of the material fed is likely `to be methyl, naphthalene, tar acids, tar bases, other coal tar materials or petroleum oils boiling up to 570 F. The feed passes through flow meter 2l having normally

open valves

22 and 23 in series and provided with a by-pass 24 controlled by valve 25 for use ation. From the inlet line the feed enters the first stage feed tank 26 through automatic control 27.

when the flow meter is not in operp y The feed at 27 is responsive to the level at feed tank 26, and includes a liquid level control 30 which closes valve 27 in case the level in first stage feed tank 26 should rise abnormally. Valve 27 has normally open series valves 31 and 32 and by-pass 33 controlled by normally closed valve 34.

Also introduced into the firststage feed tank is crystalfree eilluent coming through pipe 3S and valves 36 and 37 from eluent pump 38.

The effluent will have any one of the following compositions, depending upon the desired limpidity of the hydrocarbon oil:

l. It will in some cases be entirely free of naphthalene, and be a tar distillate oil containing tar acids and/or tar bases and methyl naphthalene.

2. It will in some cases be a tar oil containing the above with naphthalene partially removed by crystallizing.

3. lt will sometimes be a mixture of water and tar oil.

4. lt will sometimes be an alkali solution such as a sodium hydroxide solution in Water or admixed with tar distillate or petroleum oil.

5. lt will in some cases include alcohol.

In some cases it will be necessary to add as much as 30 to 40 percent of crystal-free `effluent in the first stage feed tank, so as to provide adequate spacing between crystals to permit them to grow adequately. ln other instances it Will not be necessary to add crystal-free eflluent at that stage if the hydrocarbon oil feed is sufficiently limpid to allow the crystals to grow. The first stage feed tank 26 has a heating coil 40 adapted to receive a suitable heating medium such as steam, hot water or the like, and adapted to maintain a temperature in the first stage feed tank above the melting point of naphthalene and suitably from to 120 F. and permissibly up to 170 F.

From the first stage feed tank 26, the naphthalene oil, with or without accompanying crystal-free diluting effluent, is discharged through valve 41, which is suitably continuously open to a controlled amount, into the initial flight 42 of first stage crystallizer 43. The first stage crystallizer suitably has second and third flights 44 and .45 desirably arranged one below another for gravity flow from oneto another. Each one of the

flights

42, 44 and 4S is generally of the construction shown in Figure 2, which illustrates any one of the crystallizer flights. A crystallizer cylinder 46, suitably horizontal, has at its opposite ends journals 47 and 4S which rotatably mount a drum S0 extending longitudinally through the crystallizer casing in spaced relation thereto, and turned as by a chain, turning sprocket S1 or a worm drive at this point may optionally be used. The hydrocarbon oil enters at 52 at one end, and is progressed longitudinally by suitably spiral vanes 53 on the outside of the drum, engaging the inside of the cylindrical casing with slight clearance.

AThehydrocarbon oil and separated crystals are discharged by gravity at 54 at the opposite end, and in all of the subsequent flights but the last flight are then progressed oppositely through the next flight. The casing is cooled suitably by jacketing at 55, inlet 56 being provided at one end for a cooling medium such as cooling water or brine, while an outlet 57 is provided at the opposite end.

From the first flight 42 the mixture of hydrocarbon oil and crystals passes through connection 58 to the inlet of the next crystallizer flight 44 and from crystallizer flight 44 it passes through connection 60 to the last crystallizer flight 45 of the first stage.

It will be understood that additional crystallizer flights as required can be added in the first stage and that the length of an individual flight can be increased or reduced as required. v

Cooling water is introduced to the crystallizers through pipe 6l and regulating valve 62 in series with cutoff valves 63 and 64 and by-passed by by-pass connection 65 and 62 is suitably automatically controlled by controller 67 which responds to the temperature of the mixture of the hydrocarbon oil and crystals discharging through pipe 68 from the final flight of the crystallizer. The control of the temperature is indicated by dotted line 70. After leaving the jacket of crystallizer flight 45, the cooling water passes by pipe 61' to the cooling jacket of crystallizer flight 44, then by pipe 612 to the cooling jacket of crystallizer flight 42, and finally leaves by pipe 613.

It has been found desirable to move the agitating drums in the crystallizer flights at approximately l to 2 R. P. M. While the dimensions may vary widely, it has been found convenient to construct the crystallizer flights with a diameter of about five feet and a length of about 12 feet, although other dimensions may be used.

From the pipe 68 the mixture of hydrocarbon oil and crystals passes through first stage centrifuge 71 which is suitably of the continuous type for removing crystals from the oil by centrifugal action. The crystals discharge from the centrifuge through chute 7 2 into crystal receiving tank 73 heated by heating coil 74 supplied with a heating medium such as steam or hot water, producing an adequate temperature to melt the naphthalene. In the centrifuge the crystals before discharging are automatically washed by a washing liquid introduced at 75 and suitably consisting of hot water, caustic soda solution of a concentration of less than 8 to 10%, usually about 5% by weight, or other suitable washing liquid to remove the oil adhering to the outside of the crystals.

The hydrocarbon oil extracted by the centrifuge passes through valve 76 (closed during the char-ging cycle and open during the discharging cycle) into oil tank '77 heated by coil 78 receiving steam or hot water. A float control 80 is provided to control valve 76, while valve 76 lis also manually operable according to the cycles ,of the centrifuge operation. The discharge of the lnaphthalene from receiver 73 is accomplished through valveSl, pump 82 and valve 83 to the naphthalene discharge line `84.

The suitably continuous discharge .of Vthe hydrocarbon oil from the iirst stage tank 77 is accomplished through valve 85, pump S6 and pipe 87 to the top of second stage feed tank 88.

It will be evident that in many cases it is desirable to introduce crystal-free diluent eflluent to the hydrocarbon oil between individual flights of the first stage crystallizer. This is readily accomplished by withdrawing the effluent from pump 38 through pipe 35 and float controlled valve 90 to eluent tank 91 and from thence through Valve 92 and pipe 93 to the ,connection 58 between the first and second flights of the first stage ,crystallizer. Similarly eflluent will in some cases be introduced between the second and third crystallizer flights through valve 94, valve 95' and pipe 96 to the connection .60 between

flights

44 and 45.

Inlet to the second stage feed tank S8 is controlled by valve 97 responsive to float device 97' in feed tank 88, in series with normally open valves 98 and 98', and shunted by by-pass 99 controlled by valve 99.

The second stage feed tank 88 is heated by steam, hot water or the like through .coil 101. Any excess voil from tank 88 is discharged through pipe ,88' controlled `by valve 882 to tank 77. Crystal-free effluent enters the second stage feed tank from pipe 35 through valve 102 and p ipe 103 and the quantity of efuent is controlled to provide the desired limpidity in the hydrocarbonoil. From the second stage feed tank SS the hydrocarbon oil passes through valve 104 to first flight 105 of second stage crystallizer 106. The second stage crystallizer flights will desirably be identical with those used in the first stage.

From the bottom of lirst flight 105 the mixture of hydrocarbon oil and crystals passes through connection 107 to second flight S. From second flight 108 the mixture of crystals and hydrocarbon oil passes through connection 110 to third flight 111. The provision for introducing crystal-free diluent effluent between the first and second flights is made by valve 112 and pipe 113 entering connection 107. Similarly the provision for introducing crystal-free diluent effluent between the second and third flights is accomplished through valve 114 and pipe 115.

In the second stage it has been found to be important to cool by brine to a temperature of approximately 20 F. or below. This is conveniently accomplished by making up brine in a tank 116 having a stirrer 117, withdrawing the brine through pump 118 and regulating valve 120 provided with by-pass 12,1 controlled by valve 122, and feeding through refrigeration unit 123 which is a selfcontained automatic refrigeration unit whose condenser is cooled as by water entering from pipe 124. The valve 120 is suitably regulated from temperature control 125 according to the temperature in pipe 126 issuing from the last flight of the second stage crystallizer to second stage centrifuge 127. The control from `the temperature controller to pipe 126 is indicated by dotted line 128, while the control to the regulator valve 120 and to the refrigeration system which increases its operation when the regulator valve is open is suggested by dotted line 130.

The brine from the refrigeration system 123 passes through pipe 131 and valve 132 to the jacket of the last flight 111 of the second stage crystallizer, and then passes counter-current to the flow of liquor and crystals through pipe 133 to the jacket of the second flight 108 of the second stage crystallzcr, and then through pipe 134 to the jacket of the first flight 105. From the jacket of the first flight the refrigerant passes through valve 135 and pipe 136 to make-up tank 116.

From the centrifuge 127 of the second stage, crystals l discharge through chute 137 to receiver 138 heated by steam or hot water heating coil 139 to melt the naphthalene. From the receiver the molten lnaphthalene passes by valve 140 and pump 141 through pipe 142 and valve 143 to join naphthalene discharge pipe 84.

The naphthalene-free effluent from the second stage passes by valve 144 to receiver 145 heated by steam or hot water in coil 146. A float valve 147 in the receiver controls feed valve 148 from the crystallizer to the centrifuge.

Discharge of crystal-free eluent takes place through valve 150 to pump 38, which discharges through pipe 35 to return 4crystal-free effluent to the system via valve 37 as desired. Crystal-free ellluent which is not required for dilution of the incoming liquor passes through valve 151 and flow meter 152 having series valves v153 and 154, y

by-passed by pipe 155 and valve 156, to the storage.

For the purpose of introducing alkali solution or alcohol I provide connection 157 controlled by valve 158 which introduces the alkali solution or alcohol or both from a metering pump, not shown.

It will be evident that adequate time is allowed in each stage (suitably `about 12 hours) to allow the crystals to grow to `their maximum size in each stage so that they can be readily separated by the centrifuge without the likelihood that microcrystalline material will remain dispersed in the hydrocarbon oil. The introduction of diluent material is very important as it provides an adequate free space between the crystals in which they can grow.

It will be evident that by the constant agitation as the crystals are growing, the crystals are kept separated and prevented from forming interlocking structures or from 4adhering firmly to each other `or adjoining structure, thus avoiding Athe likelihood that hydrocarbon oil will be trapped in the intersticies of the .crystals to increase the quantity of impurities carried away thus affecting the melting point of the iinal product.

The slow agitation, while keeping the crystals separated, does not break them up as would high speed agitation and the cascading of the hydrocarbon oil from one flight systems, particularly as regards tire hazard and discharge of toxic fumes and loss of volume by evaporation.

In accordance with the present invention, sodium hydroxide solution less than 8 to l0 percent will optionally be included. This has the etect or combining with phenols and cresols to form phenolates or cresolates, reducing the strength of the phenols or cresols in the oil, which itself has a tendency to dissolve naphthalene and inhibit growth of naphthalene crystals. In some cases it is desirable to employ the alkali in solution in one of the alcohols named.

It will be evident that in the lirst stage I reduce the temperature from a range of approximately 90 to 170 F. to a range of 70 to 100 F. depending upon the concentration of naphthalene. In the second stage I reduce the temperature from 70 to 100" F. to 20 F. or beiow.

It will be understood of course that a higher finishing temperature can be used if it is not necessary to cool to F. to get the maximum yield of naphthalene.

It will be evident that the invention can be applied on other aromatic polycyclic hydrocarbons such as anthracene or phenanthrene or on analogous polycyclic compounds such as imides like carbazole.

It will be evident that in many cases it is desirable to use alcohol in the hpyrocarbon oil as a diiuent. Any one of the lower straight or branched carbon chain alcohols such as methyl alcohol and ethyl alcohol to and including five carbon atoms in the chain may be used, since the alcohol does not dissolve naphthalene in appreciable quantities but is mutually soluble with the oil. Thus the alcohol serves to wash the naphthalene crystals.

It will be evident that in accordance with the present invention the cycle is completed in about 24 hours from the time of the first feed (l2 hours in each stage), or in a shorter time, depending upon the character of the oil being treated.

It will be evident that the use of two stages removes more total naphthalene than would be accomplished in a single stage because there is less naphthalene present in the feed to the second stage. In the first stage you cannot go below 70 F. with normal coal tar distillates, because the mixture is not pumpable below that temperature, whereas in the second stage you can go down to 20 F. because the mixture minus the crystals from the first stage is more limpid.

It will further be evident that according to the present invention it is not necessary to pump crystals. The materials pumped are entirely oil and molten naphthalene.

It will be evident that the continuous scraping close to the walls of the crystallizers improves the cooling action of the jackets.

It will be evident that the slight clearance between the blades and the housing permits of the retention of only a thin layer of naphthalene on the inside of the housing while avoiding abrasion on the blades. The long period of exposure in the crystallizer makes up for any loss in heat transfer, through the thin layer of naphthalene.

It will further be evident that by employing an alcohol of the character discussed, added to the hydrocarbon oil, I reduce the tendency of the napthalene or other polycyclic compound to remain dissolved in the oil, increase the tendency of crystals to form and reduce the tendency of the oil to adhere to the naphthalene crystals. The advantage of adding alcohol is further increased by incorporating an alkali such as sodium or potassium hydroxide in the alcohol.

I further find that by washing in the centrifuge with an alcohol of the character discussed, with or without alkali and with or without water, the naphthalene crystals are freer from adhering oil and the purity is increased.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the structure shown, and I therefore claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. The process of removing crystallizable polycyclic hydrocarbons from hydrocarbon oil containing such compounds, which comprises continuously progressing hydrocarbon oil, initially at a temperature from to 170 F. through a plurality of generally horizontal closed chambers of a rst stage and there cooling the oil and agitating the oil and crystals which form by rotors at a speed not in excess of 5 R. P. M. with continuous scraping adjacent to but out of contact with the chamber walls for a time in excess of 8 hours, continuously removing the crystals that form from the oil by centrifuging, then continuously progressing the oil remaining after the removal of the crystals initially at a temperature of below F. through a plurality of generally horizontal closedV chambers of a second stage and there cooling the oil and agitating the oil and crystals that form by rotors at a speed not exceeding 5 R. P. M. with continuous scraping adjacent to but out of contact with the chamber walls for a time in excess of 8 hours, and continuously removing the second stage crystals from the oil by centrifuging, the process throughout the first and second stages being carried out in a closed system.

2. Thcprocess of claim 1, in which the time of crystalliz'ing in each of the stages is in excess of l2 hours.

3. The process of claim 1, in which the oil in the second stage is cooled by heat transfer with a coolant having a temperature below 20 F.

4. The process of claim 1, in which crystal-free oil obtained from the end of the process is introduced into the oil at the beginning of the second stage.

References Cited in the le of this patent UNITED STATES PATENTS 1,450,992 Stevens Apr. 10, 1923 1,548,920 Weil Aug. ll, 1925 1,879,686 Jaeger et al. Sept. 27, 1932 1,951,923 Cartonx Mar. 20, 1934 1,988,763 Walter Jan. 22, 1935 2,078,963 Miller May 4, 1937 2,163,581 Boyd June 27, 1939 2,321,117 Wilcock June 8, 1943 2,400,883 Keeling et al. May 28, 1946 2,540,977 Arnold Feb. 6, 1951 2,552,525 Wenzelberger May 15, 1951 2,593,300 Hachmuth Apr. 15, 1952 2,598,449 Scott et al. May 27, 1952 2,617,273 Findlay Nov. 11, 1952 FOREIGN PATENTS 255,429 Great Britain June 23, 1927 960,364 France Apr. 17, 1950

Claims

1. THE PROCESS OF REMOVING CRYSTALLIZABLE POLYCYCLIC HYDROCARBONS FROM HYDROCARBON OIL CONTAINING SUCH COMPOUNDS, WHICH COMPRISES CONTINUOUSLY PROGRESSING HYDROCARBON OIL, INITIALLY AT A TEMPERATURE FROM 90 TO 170* F. THROUGH A PLURALITY OF GENERALLY HORIZONTAL CLOSED CHAMBERS OF A FIRST STAGE AND THERE COOLING THE OIL AND AGITATING THE OIL AND CRYSTALS WHICH FORM BY ROTORS AT A SPEED NOT IN EXCES OF 5 R. P. M. WITH CONTINOUS SCRAPING ADJACENT TO BUT OUT OF CONTACT WITH THE CHAMBER WALLS FOR A TIME IN EXCES OF 8 HOURS, CONTINUOUSLY REMOVING THE CRYSTALS THAT FORM FROM THE OIL BY CENTRIFUGING, THEN CONTINUOUSLY PROGRESSING THE OIL REMAINING AFTER THE REMOVAL OF THE CRYSTALS INITIALLY AT A TEMPERATURE OF BELOW 100* F. THROUGH A PLURALITY OF GENERALLY HORIZONTAL CLOSED CHAMBERS OF A SECOND STAGE AND THERE COOLING THE OIL AND AGITATING THE OIL AND CRYSTALS THAT FORM BY ROTORS AT A SPEED NOT EXCEEDING 5 R. P. M. WITH CONTINUOUS SCRAPING ADJACENT TO BUT OUT OF CONTACT WITH THE CHAMBER WALLS FOR A TIME IN EXCESS OF 8 HOURS, AND CONTINUOUSLY REMOVING THE SECOND STAGE CRYSTALS FROM THE OIL BY CENTRIFUGING, THE PROCESS THROUGHOUT THE FIRST AND SECOND STAGES BEING CARRIED OUT IN A CLOSED SYSTEM.