US2180070A - Manufacture of low pour test lubricating distillates - Google Patents

Description

CRUDE ATMOSPHERIC A F/ R E GAS 011. mo 70 REDUCTION INTERMEDIATE F a UT'I-ET 676 "E DISTILLATE BOTTo/ws I37, 35 LIGHT OVERHEAD cvu - VACUUM FIRE AND STEAM MA in: uP PETROLATUM [IA/MOE) STQCK PH-ousj- WAX N4 FHTHA INTERMEDIATE I T/ ATE CLAY CONTACTOR AY mvo REEUCEIZ 300 F. NnF/ ITHA fli Luaelcarlms 60-70% DISTILLATE Nov. .14, 1939. H, G. SMITH 9 MANUFACTURE OF LOW POUR TEST LUBRICATING DISTILLATES Filed May 9, 1952 r ig T ,(zissgzz REDUCING STILL 0 2o 60 80 POUR Pomrs OF A 600 via NEUTRAL 011..

Herschel Qfifivitly,

0 VER'HEAD RIGHT sToc/c Patented Nov. 14, 1939 UNITED STATES 2,180,070 PATENT OFFICE MANUFACTURE OF LOW POUR TEST LUBRI- CATING DISTILLATES Herschel G. Smith, Swarthmore,.Pa., assignor, by mesne assignments, to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application May 9, 1932, Serial No. 610,200

2 Claims.

wax is substantially completely removedfrom said mixture by centrifuging at low temperatures in a liquid separator, thereby recovering a low pour test neutral oil; all as more fully hereinafter set forth and as claimed.

At the present time there is a considerable demand for low pour test lubricating oils of all kinds; oils sufiiciently free of high melting constituents to withstand low winter temperatures without separation of solid or excessively viscous substances. To produce these oils from crudes containing both wax and asphaltic matter in substantial proportions presents a special problem to the oil industry. Since it is not generally feasible to employ residuum from such crudes for the manufacture of lubricants due to the difiiculty of eliminating asphaltic impurities, it is usually advantageous to recover their lubricating values in the form of distillates. The best grades of these distillates can be roughly divided into neutral oils and bright stocks, the latter class having the higher viscosity. The dewaxing of both classes of these distillates presents difficulties, due to their content of so-called semi-crystalline Wax.

Waxes occur in a wide range of forms in petroleum oils. They may be classified starting with bodies liquid at ordinary temperatures but freezing readily (liquid waxes) in a continuous series having increasing melting points, boiling points and molecular weights. In general, the higher the molecular weight, the smaller is the crystal size of those forms which will crystallize.

At one extreme are the crystalline waxes of low melting point, not over about 132 F., shading into the liquid waxes. Waxy oils in which the wax content is largely of this crystalline type 55 the, chilling and subjecting to centrifugal force,

a wax gel is produced in the chilled mixture of Waxy oil and naphtha. This happens even when the crystalline wax content of the oil is small. This gel formation makes centrifugal separation impracticable since satisfactory centrifugal removal depends upon throwing out of individual solid particles of wax. While it is possible theoretically to modify oils containing large quantitles of crystalline wax to permit centrifugal separation, by adding amorphous wax to modify the crystalline wax, the amount of amorphous wax required is so huge as to rule out the procedure for most practical purposes. It has been found that low pour test oils cannot be produced commercially by such procedures.

Crude crystalline waxes after removal from the oil can be refined and purified by the Well known sweating process to secure familiar translucent white Wax products of low melting point and crystalline structure. Accordingly such waxes are often referred to as sweatable waxes.

Waxes of melting point between 135 and 140 F. form an intermediate class. They have higher molecular weights and the crystal structure is so small that they are classed as semi-amorphous. Oils containing large quantities of them do not readily yield to pressing, and the waxes, when isolated by special means, are not susceptible to refining by sweating. Accordingly, such waxy oils are termed unpressable and the waxes themselves are termed unpressable or unsweatable.

At the other extreme are the high molecular weight waxes of melting point above about 140 F. While these waxes are often regarded as truly crystalline in character, the crystal size is so minute that the waxes are termed amorphous. Petrolatum is largely a blend of wax of this type with sufficient oil to'form a soft gel at room temperatures. The wax content is often not more than per cent of the total weight.

Waxes of this micro-crystalline or amorphous type cannot be practically removed from oil containing them, by pressing. The wax is not of a form which filters well. It clogs the filtering medium. However, it can be removed from oil containing it, by centrifugal separation. By simply chilling, diluting and centrifuging, it separates cleanly. These amorphous waxes like the semi-amorphous types are not sweatable.

Actual oils usually contain mixtures of these waxes in various proportions. One kind of wax may modify another. Presence of semi-amorphous waxes may inhibit crystallization of waxes which, alone, crystallize well.

Certain mixtures of these waxes; and the semiamorphous range of Waxes; are not directly susceptible of removal by either filter pressing or by centrifugal procedures. They may often be modified in appropriate ways to facilitate removal by one or the other procedure.

In Patent No. 1,999,486, issued April 30, 1935,,

residual oil derived from a high grade crude to a heavy unpressable distillate serves to convert contained semi-crystalline wax into a form more amenable to centrifugal separation. By the process of the patent, low grade crudes can be employed in the production of low pour test bright stocks. The amount of residual oil derived from a high grade crude, which is added to the distillate, is determined by the viscosities of the residuum and of the distillate. In the process of this patent, a neutral oil of low pour test can be obtained by reduction of the lubricating stock, after the dewaxing. The bright stock recovered as another final product contains a certain amount of residuum derived from the residual oil added to facilitate wax removal.

It is sometimes desirable to obtain lubricating oils free from all residuals. The process of my present invention is suitable for recovering substantially the entire lubricating values of a low grade crude in the form of overheads which are entirely or nearly free from residuals. In order to accomplish this it has been necessary to de- Velop novel methods of removing the wax content from the various overheads obtained during the distillation of low-grade crudes.

, By low grade crudes is meant crudes containing relatively large proportions of dark-colored and heavy asphaltic materials; enough in many cases to make it uneconomical to refine residuals derived therefrom. Distillation from such crudes of light fractions in the gasoline, kerosene and gas oil boiling range leaves a residual of dark brown or black appearance and having a high fixed carbon test indicative of high asphaltic content. Such residuals are not adaptable for working up into useful cylinder stock or lubricating oils, without the application of expensive and complicated refining operations. The proportion of asphaltic materials usually runs to several per cent. Examples of low grade crudes are West Texas crudes (asphaltic content 9 per cent) Seminole crude of Oklahoma (asphaltic content 6 per cent), East Texas crudes and Louisiana crudes.

I have found that low pour test lubricating oils can be economically manufactured from such crudes, by recovering a so-called pressable distillate from the crude, filter-pressing the distillate at moderately low temperatures to take out as much of the wax content as can be readily and economically removed by filtering (the wax removed hereconstituting a large fraction of the total wax content of the oil), adding sufiicient amorphous wax, in the form of petrolatum stock, for example, to modify the remaining wax content, that is, to render the wax centrifugeable, and finally removing wax by centrifugal force at low temperatures in a liquid separator of the Sharples type, for example. The unpressable distillates recovered from such a crude can likewise be dewaxed with the addition of petrolatum :stock, if required, to recover an overhead bright stock.

For convenience of description I have illustrated the present invention by the accompanying drawing. In this showing:

several process steps being indicated by appropriate legends;

ig. 2 is a chart showing the optimum per cent the oil-wax mixture.

of amorphous wax to be added to a neutral oil for modification purposes, as a function of the pour point of the said neutral oil.

Referring to the flow chart of Fig. 1 a Seminole type Oklahoma crude (Crude A shown at the upper left-hand side of the figure) is fed into a continuous tube still equipped with fractionating tower. This charge is distilled at atmospheric pressure; gasoline, kerosene, gas oil and a pressable distillate being recovered. The pressable distillate represents about 9 per cent of the charge. The residuum remaining, which represents 35 per cent of the charge, is fed into a vacuum tube still and fractionator wherein it is subjected to a vacuum, fire and steam reduction. From this still there is recovered a heavy pressable distillate (about 13 per cent) and a light overhead cylinder stock (about 13 per cent). The latter may be employed to produce an overhead bright stock, as described below.

The heavy pressable distillate from the vacuum still is united with the pressable distillate from the atmospheric still and the mixture is pressed at a temperature of about 25 F. toremove about 1 per cent of total charge (or 5 per cent of the mixture) as slack wax. The pressed oil is then reduced to a residuum representing about 7 per cent of the charge (or about onethird of the pressed oil), the viscosity of this residuum being about 160 seconds S. U. V. at 100 F. Gas oil and intermediate distillates are recovered in this operation. The pressed oil bottoms may then be mixed, as shown, with a lubricating distillate (3 per cent of charge) derived from a reduction (after dewaxing) of the light overhead cylinder stock mentioned above. To this mixture there is added 1 per cent of a composite wax-bearing charge composed of reduced wax discharge and make up petrolatum stock. The reduced wax discharge may be derived, as shown, from the subsequent centrifugal dewaxing and reduction of the same oils to which the charge is now added. The make-up petrolatum stock is usually derived from residual bright stock operation; that is, it is usually a petrolatum stock which is separated out in the manufacture of bright stock from a heavy residue of a high grade or relatively asphalt-free crude. This petrolatum contains about 50 per cent of oil-free amorphous wax, having a melting point of from 125 to 160 F. This composite wax-bearing charge is added to the mixture of pressed oil bottoms and lubricating distillates in the proportion of about 6 per cent by volume. This mixture is then acid treated.

In the acid treating step about 25 pounds of 98 per cent sulphuric acid is added per barrel of The initial temperature is about 100 F. The settled sour oil from the acid treating step, representing about 10 per cent of the original charge may be de-acidified, as shown, in a combined clay contacting and reducing step. The clay used may amount to about 0.3 pound per gallonof oil. The temperature used in this step is about 300 F. A small (0.1 per cent) gas oil distillate is recovered. The oil-clay mixture is freed from clay by filtering and is then ready for wax removal. I

The clay treated oil is first diluted, the diluted mixture containing approximately 60-70 per cent, by volume, of cold settling naphtha and having .a gravity of about 46.9 A. P. I. The dewaxing stock thus produced is usually chilled to a minimum temperature depending upon the pour test 17 desired in the finished neutral oil.

In the example illustrated this temperature is about -35 F. The dewaxing stock is then run through a centrifuge. On account of the addition of amorphous wax in the step preceding acid treatment, the wax in the oil-naphtha mixture separates cleanly in the centrifuge. This result would have been impossible without this addition.

The wax recovered from the centrifuge may be reduced with fire and steam, as shown, or by any other suitable means of reduction without cracking. A naphtha distillate as well as a wax distillate are recovered. The latter contains the bulk of the total wax content. A heavy reduced wax residuum is obtained. If the primary object of the operation of the process is to produce the maximum quantity of neutral oil, thiswax residuum can be advantageously recycled in the process, as shown in the flow sheet. However, the process described can be advantageously employed as a method of recovering valuable high viscosity oil from petrolatum stock containing such oil. Since the final wax residuum recovered in my process contains less oil and oil of lower viscosity than petrolatum stock from bright stock manufacture, for example, it is evident that any high viscosity oil contained in such a petrclatum stock used for wax modification purposes in my process is largely recovered in the lubricating oil product. When my process is used in this manner the heavy reduced wax residuum, ob tained as described above, is not recycled but is withdrawn from the process for any suitable disposition.

The dewaxed oils from the centrifuge are re duced to recover the added naphtha and a small intermediate distillate. The bottoms are clay filtered and are then recovered as a finished neutral oil forming about 8 per cent of the total charge and having the following typical tests:

Gravity 26.1 Via/100 F 305 Vie/210 F Flash, 0. C., "F 420 Fire, 0. 0., F 490 Pour, F +5 Carbon residue, per cent 0.12

The light overhead cylinder stock (1.3 per cent) recovered from the vacuum still, as described above and as shown in the flow chart, may be first subjected to a dewaxing step, after being diluted with naphtha to give a gravity of about 44.2" A. P. I. The diluted mixture may be chilled to a minimum temperature of about 30 F. and then centrifuged. In this particular case, the stock, being an unrefined overhead, can frequently be centrifuged without the addition of amorphous wax. In case centrifuging is difficult, however, it is only necessary to add a small amount of such Wax after the dilution with naphtha. If amorphous wax is added to assist in the centrifuging, the acid treatment of the oil can be accomplished before dewaxing instead of afterwards as shown on the flow chart. The dewaxed oil, as

shown in the flow sheet, may be subjected to a reduction inwhich step there is recovered the lubricating distillate mentioned previously. The reduced oil is then subjected to dilution with kerosene, acid treatment, clay contacting, kerosene removal, filtering, etc., these steps not requiring a special description. The final product is an overhead bright stock, representing about 5 per cent of the original charge and having the following typical tests:

Products having lower pour tests than those illustrated above can be readily obtained by the simple expedient of employing lower temperatures in the chillers before centrifuging. Such temperatures may necessitate a somewhat greater dilution with naphtha. A lowering of about 15 to 2 F. is required to produce a reduction of 1 F. in the pour test.

In carrying out a process such as that described above several factors must be taken into consideration. For example, in dewaxing pressable distillates by the filter pressing and subsequent centrifuging steps of my invention, the pressable distillates which are most suitable have viscosities (before pressing and reduction) ranging from about 50 to 120 seconds S. U. V. at 100 F. The temperature employed during filter pressing should be sufficiently low as to give pour tests (after reduction of the pressed stock) not much over '70 F. Usually pressing temperatures of around 25 to 30 F. are found to be satisfactory.

I have found, as a result of a large number of experiments, that the amount of amorphous wax advantageously used in modifying the semi-crystalline wax content of neutral oils including the above described pressable distillates can be close 1y estimated from the amount of oil-free, semicrystalline wax contained therein. By oil-free Wax in this connection, I mean paraffine wax having a melting point from about 125 to 160 F. I have found that the amount of amorphous wax required for optimum results approximately equals the quantity of semi-crystalline wax contained in the neutral oil. Since the latter quantity bears a rather definite relation to the pour point of the neutral oil, as shown later, the amount of amorphous wax required can also be determined roughly from the pour point of the neutral oil.

The amount of oil-free, semi-crystalline wax contained in a 300 viscosity neutral oil can be estimated from its pour point by referring to the table which follows. This table gives the oil-free wax content (in this case wax having a melting point of 140 F.) of a neutral oil having a viscosity of 300 seconds S. U. V. at 100 F. as a function of its pouring point.

Table I C0?lii%l (if wax with M. P.

o in percent 1.0 l. 5 2. 5 4.. 5 Pouringpoint in F. of 300 o 16' 5 viscosity 011 +8 +13 +52 +77 +93 +107 When amorphous wax is addedto a neutral oil which has not been partially dewaxed by pressing or otherwise, the dewaxing stock formed by adding naphtha to the mixture tends to form a gel or to assume a thick soupy condition upon chilling. Such a dewaxing stock will not respond satisfactorily to centrifugal force due to this physical condition of the mixture. The wax particles will not migrate through the thick chilled mixture under any obtainable centrifugal force at a sufficiently rapid rate to efiect removal.

A typical analysis for the wax discharge from a centrifuge for residual bright stock operation would be approximately 50 per cent naphtha, 25 per cent heavy oil and 25 per cent oil-free wax. But for normal operation the wax discharge amounts to only from 12 up to about 16 per cent of the total material discharged from the centrifuge. It is obvious therefore that the total wax in the oil must be kept within rather well defined limits for practical dewaxing operations.

Since the quantity of wax required to be added to an oil for modification purposes increases in direct proportion to the semi-crystalline wax content of the oil, as mentioned above, it is evident that a practical operating limit of total wax content would soon be reached even if the formation of wax gels did not further complicate the procedure. As a matter of experiment I have found that such a limit is roughly approached, with neutral oils from the reduction of pressed distillates, when the pour points of such reduced, pressed neutral oils reaches about 70 F. before the addition of any amorphous wax. Such an addition of course increases the pour point of the 011.

Of course the above operating limit depends to a large extent upon the pour test desired for the finished oil. When very low pour tests are required, such as 0 F., the best results are usually secured when the wax content of the pressable distillate is first reduced to the minimum consistent with satisfactory pressing operations. For securing pour tests of 0 F. or below, I usually first filter-press the oil under conditions such that a pour test of not over +45 F. is obtained for the reduced pressed oil.

It is also true that my present process cannot in general be employed for the dewaxing of cracked lubricating distillates. The wax contained in such distillates requires a considerably larger proportion of amorphous wax to be added for modification purposes, and such a proportion is likely to prevent satisfactory dewaxing by centrifugal force.

A numerical relation between the optimum requirements of amorphous wax to be added for modifying purposes and the pour point of the oil to be modified can be readily deduced from Table I. The amorphous wax required approximately equals the semi-crystalline wax content indicated for the neutral oil. The data given in the table are shown plotted in Fig. 2. The curved line follows the data given in the table. It will be noted that, for low values of the pour point, that is, for values of the pour point up to about 70 F., these data can be accurately represented by a straight line. Such a line is shown in the figure. This line has the equation W=0.05P+0.5 in which W is the per cent by weight of oil-free wax (semi-crystalline) in the oil and P is the pouring point of the oil in degrees F. In order to calculate the optimum amount of amorphous wax usually required for modification purposes the same equation can therefore be employed but in this case W would represent the per cent by weight of amorphous wax required.

The above equation is intended to indicate the optimum amount of amorphous wax required for modification purposes and therefore includes a slight margin above the minimum which will often produce the desired results. For calculation of approximately the minimum requirements a still simpler method of calculation is often found to be satisfactory. Thus, by taking onetwentieth of the pouring point of an oil, expressed in degrees F., a numerical value is obtained which represents, in per cent by weight, the amount of amorphous wax required. If it is desired to add this amorphous wax in the form of petrolatum stock (containing usually 50 per cent oil-free amorphous wax having a melting point of from to F.) the corresponding factor to employ is of course one-tenth the pouring point in degrees F.

An equation which fits the curved line of Fig. 2 with considerable exactness is there is any considerable variation from the simpler relation given above.

In employing the above relationships for determining the optimum amount of amorphous wax required for dewaxing a given neutral oil, the viscosity of the oil can in general be neglected. At least this is nearly true for viscosities ranging from about 100 to 1000 seconds S.U.V. at 100 F. It is also true that the quantity of wax reported in an oil depends upon the melting point of the wax upon which the estimated wax content is based. The lower the melting point selected the greater becomes the estimated wax content. Some idea of the effect of the viscosity of the neutral oil and of the melting point of the wax selected as a basis can be gained from the following table, which may be compared with Table I.

Table II Content of wax with melting point of 132 F. in percent l 3 6 ll 16 21 Pouring points in F. of

Table II gives the pouring points and corresponding approximate wax contents (semi-crystalline) of neutral oils having three different viscosities, namely, 200, 300 and 450 seconds S. U. V. at 100 F. It is seen that the pouring points of a neutral oil with a given wax content increase slightly with increase of viscosity but that this increase is practically within the experimental error of this determination. The amounts of amorphous wax required to modify the above oils, as calculated from the relations previously deduced, agree with the values for the oil-free (semi-crystalline) wax contents, listed in Table I, well within the limits required to give satisfactory centrifuging operation.

The methods outlined above for estimating the amount of amorphous Wax to be added to a neutral oil relate either to the optimum or to the minimum requirements for producing a dewaxing stock which can then be dewaxed under the most favorable operating conditions. Less amorphous wax than the minimum specified will often cause difficulty in the centrifuging step either through the formation of a Wax gel in the chilled, naphtha-diluted acid-treated oil, or through the formation in the rotor of the centrifuge of a hard wax layer incapable of flowing under available degrees of centrifugal force. The latter effeet is encountered particularly in the dewaxing of dark overhead stocks that have not been treated with sulfuric acid, or refined by other means.

The upper limit for the percentage of amorphous wax in the dewaxing stock is dictated purely by practical considerations with reference to the particular object of the operation. If the primary object is to secure a maximum production of low pour test distillate, it is advantageous to employ as little amorphous wax as possible, and not to exceed, for example, in per cent added, one-fifth of the pour test expressed indegrees Fahrenheit of the pressed distillate. However, if the primary object is to extract valuable residual oil, for example from petrolatum stock used, it is practicable to employ considerably higher proportions of petrolatum stock in the mixture than would correspond to the stated amount of amorphous wax. For example, petrolatum stock can be added to a neutral oil until the pour test of the mixture approaches F., the mixture still remaining capable of. being centrifuged. Theper cent which can be added may amount to as high as four-tenths of the numerical value of the pour test of the neutral oil expressed in degrees F. particularly when the pour test of the neutral oil is rather low, for example +30 F.

For the purposes of this application petrolatum stock may be defined as amorphous Wax associated with approximately an equal weight of oil, and will include tank settlings, the amorphous rod wax obtained from oil wells and the reduced Wax stock recovered from the wax discharge obtained in the centrifugal dewaxing of heavy residua. The latter type, which is the one I consider most advantageous, contains about 50 per cent of heavy oil by Weight and 50 per cent of oil-free high melting point amorphous wax, the actual amorphous wax being the. component of primary importance for this process. By the terms oil-free or high-melting-point amorphous wax employed in the claims I mean a Wax of melting point ranging from about to '160" F.

The usual pressed distillate contains a considerable proportion of oil of lower viscosity than is desired in finished neutral oils. In such cases I reduce the pressed distillate before the prepara- I sulfuric acid before centrifugal dewaxing. It is also often advantageous to mix the amorphous wax with the pressed oil, or with the reduce pressed oil, before acid treating.

The procedure outlined above is capable of considerable variation without falling outside the scope of the present invention. Various changes may be made in the specificsteps of the process. For example, the neutralization with fullers earth can be replaced by a neutralization of the sour oil with a dilute caustic solution or other chemical, followed by washing with hot Water. The pressable distillate may or may not be reduced after pressing or at some other stage of the process. Larger quantities of amorphous wax may be added before the acid treating step, as described previously. The proportions and gravities of the various distillates andbottoms can be varied. The temperatures and pressures of the distilling and reducing steps, the proportions of naphtha employed in dewaxing, the proportion of fullers earth employed etc. may all be varied to a considerable extent. Even the sequence of several of the steps of my process may be altered in some cases. For example, the acid treatment can, if desired, be applied to a crude oil stock directly after a reduction to remove gasoline and part of the kerosene. case the remaining steps of the process can be altered to suit. Other variations from the above procedure which fall within the scope of the following claims will be evident to those skilled in the art.

What I claim is:

1. In the manufacture in high yield of low cold test lubricating distillates from low grade crudes, the process which comprises reducing such a crude and recovering an uncracked pressable distillate and a residuum, distilling off from said residuum a heavy pressable distillate and a light overhead cylinder stock, mixing said pressable distillates and filter pressing the same to remove crystalline, readily pressable wax; diluting said overhead cylinder stock, centrifuging the same for removal of wax, reducing the same to recover a lubricating distillate, uniting said lubricating distillate with said filter pressed distillates, adding sufficient amorphous wax to render the wax content of the mixture centrifugeable, removing the Wax content by centrifuging in a liquid separator at low temperatures and recovering a low cold test neutral oil.

2. In the far-going removal of solidifiable constituents from oil in manufacturing low pour test lubricating oils, the process which comprises recovering a waxy pressable distillate from a low grade crude, filter pressing the distillate to remove part of the wax content as pressable wax, adding to the pressed oil a hydrocarbon residue from the distillation of a waxy oil mixture containing a hydrocarbon wax-separation aid, diluting the mixture with an oil-miscible liquid of relatively low viscosity, chilling the mixture and mechanically separating a mixture of wax, entrained oil and hydrocarbon residue from the diluted oil mixture in a centrifugal liquid separator, and fractionating the wax-oil-residue to remove lower melting point waxes and the oil from the hydrocarbon residue to produce a higher melting point residue which has the property when dissolved in wax-bearing oil of functioning as a wax separation aid.

HERSCHEL G. SMITH.

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