US2100662A - Dewaxing petroleum oils - Google Patents

Description

Nov. 30, 1937. A, L. LYMAN ET AL DEWAXING PETROLEUM OILS Filed March 12, 1935 I Ammo: mmn mm mmEE E2315 :0 Qmx m m F4 L m 2% $42 8 S ES "/0 BYVWEIGHT ADDED CONDITIONER In Vent ops ARTHUR L. L YMA/V MELV/A/ M IHOLM Patented Nov. 30, 1937 UNITED STATES PATENT OFFICE Calif, assignors to Standard Oil Company of California, San Francisco, Calif., a corporation of Delaware Application March 12, 1935, Serial No. 10,594

2 Claims.

This invention relates to the separation of wax from mineral oil. In particular it relates to a process of the type in which the oil to be dewaxed is first dissolved in a low viscosity, low specific 5 gravity hydrocarbon diluent, the solution chilled to a temperature low enough to congeal or precipitate the desired amount of Wax, and the congealed wax separated from the fluid solution.

In general, three methods of separating wax from diluted oil are practiced, namely: (a) by centrifuge; (b) by the addition of diatomaceous earth or equivalent filter aid to the solution and subsequent filtration; and (c) by filtration (or cold settling) without filter aid. Each of these three methods has certain disadvantages and limitations.

In the first named, centrifugal process, it is necessary to avoid the formation of wax of certain non-flowing crystalline types which cannot be separated continuously by centrifuge. Processes of this type use, as diluent, a petroleum naphtha, ordinarily of about 60 A. P. I. The solution of waxy oil and naphtha is cooled at a rate of a few degrees per hour to the desired temperature, 25 which may vary from F. to -60 F. Slow cooling is necessary in order to insure the formation of the type of wax crystals which can be separated effectively by centrifuge. This cooling normally requires many hours, often in excess of 48.

30 The chilled solution is then fed to centrifuges for separation of the Wax and oil. This process is not applicable to most distillates, since the character of wax formed, even with very slow chilling rates, will not permit centrifugal separation.

The filter aid process also makes use of naphtha as a diluent. The filter aid is added to the solution before chilling, whereupon chilling rates of 50 to 100 F. per hour may be employed. This process is at a great disadvantage where the wax 40 content is high, because of the excessive quantities of solid filter aid requirement, and the consequent high recovery cost and low dewaxing yield.

Comparatively recently it has been found that dilution with normally gaseous hydrocarbons, such as propane, makes possible either cold settling or filtering without filter aid, as a means of removal of solidified wax. The use of normally gaseous diluents also makes it possible to obtain the desired refrigeration by evaporation of a part '0 of the diluent from the waxy solution. Relatively rapid cooling rates may be used with this method.

The outstanding disadvantages are the necessity for high pressure storage and equipment which will function safely under wide extremes of tem- 5 perature and pressure, and the fact that on many types of waxy oils a low filtering rate and a low dewaxing yield result.

The primary object of the present invention is to provide a dewaxing process using hydrocarbon diluent, in which these various disadvantages are eliminated.

Specifically, it is the object of our invention to provide a process by which a wide variety of oils of high and low wax content can be substantially completely dewaxed with the direct production of a high melting point wax containing so little entrained oil as to require, in most cases, no sweating or other processing for separation of oil from the wax cake.

A further object is to provide a process which is capable of precipitating wax in such form that it can be separated from the oil by simple filtration, continuously and at a high rate, without filter aid.

Another object is to provide a process which will permit the use of high chilling rates, but which employs moderate pressures and can be carried out in conventional chilling, filtering and diluent recovery equipment.

Another object is to provide a process which can be operated continuously and at low unit cost. 7

Other objects and advantages may be realized by the practice of our invention as will be apparent from the following description.

We have discovered that results superior to those attainable by known dewaxing methods can be had provided the diluent used is a hydrocarbon selected from a. particular range of boiling points. Added improvement may be realized by proper control of two factors, namely: (1) the presence in the oil to be dewaxed of a wax crystal conditioner; and (2) the cooling of the oil-diluent solution in such a way as to minimize local temperature mixing of comparatively small increments, as well as to avoid shock chilling of major portions. These terms will be more fully described below.

As diluent we use a hydrocarbon, or mixture of hydrocarbons, the major portion of the constitucuts of which is liquid at normal temperature and pressure (1. e., vapor pressure not over 14.7 lb. per sq. in. at 60 F.). The presence of normally gaseous hydrocarbons tends to affect the yield of dewaxed oil adversely. On the other hand, the presence of moderately high-boiling hydrocarbons renders the precipitated wax more diflicult to separate by settlement or filtration. Consequently we avoid the use of diluent containing such proportions of high boiling constituents as would raise the 50% distillation point over 250 F. By 50% point we mean the temperature indicated in A. S. T. M. Distillation Method D8630 when 50% of the charge has been collected in the receiver. While hydrocarbon solvents conforming to the above specifications are satisfactory for use in the process, we prefer, for best results, to use a liquid solvent whose 50% point is not over 180 F. Hydrocarbons boiling in the range represented by the pentanes and hexanes make particularly good solvents, or diluents, for use in our method. The chemical classification of the hydrocarbons used is of less importance than the proper choice of boiling points. For example, we may use a petroleum fraction comprising paraflines, naphthenes and aromatics, or a cracked petroleum fraction containing in addition a high percentage of olefines. Likewise, we may use a synthetic material such as may be obtained by polymerization of normally gaseous olefines, or a substantially pure hydrocarbon such as hexane. A very satisfactory material, on the basis of cost and availability as well as results, is the petroleum ether fraction from crude petroleum oil. Such fraction boils between about 90 F. and 180 F.

The effect of higher boiling constituents on filter rate is illustrated in Figure 1. Each of the curves A, B and C represents the results obtained when using a different diluent, all other conditions being constant. In the case of curve D, the diluent used was the same as that of curve C, but a different kind of conditioner was used. The diluents were all fractions of the same crude oil,

but compared as follows:

Boiling points F. (A. S. T. M.)

Diluent used in Initial 50% End Curve A 255 301 396 201 228 298 98 130 220 Curve D 98 130 220 These curves also show the efiect of added conditioner as will be more fully described later, but the effect of change in diluent alone may be seen by noting the filter rates obtained with a fixed amount of conditioner. Thus, when using .25% of conditioner, a relative filter rate of about 0.12 was obtained with diluent A; 0.60 with B; and 3.3 with C. The yields of dewaxed oil varied moderately, the lower yields occurring on the treatments which gave low filter rates, indicating greater entrainment of oil in the cake.

The volume ratio of diluent to waxy oil may be varied to fit particular conditions. A satisfactory ratio for the typical oil of 10-15% wax content is 3 to 4'volumes of solvent to one volume of oil. Oils having low wax content (e. g., 1-3%) may often be dewaxed satisfactorily with a smaller diluent-to-oil ratio.

By wax-crystal conditioner we mean substances which, When present in an oil in comparatively small amounts. influence the form in which Wax solidifies out of solution. We are unable to specify the exact chemical and physical properties which distinguish suitable conditioning agents. Examples of suitable materials are (a) pour point depressors, such as the synthetic material prepared by the reaction of chlorinated paraifins with aromatic hydrocarbons in the presence of aluminum chloride; (1)) certain high molecular weight residues derived from petroleum or coal tar; (c) certain substances well known as productive of green bloom when dissolved in petroleum lubricating oils, e. g., residues obtained by cracking petroleum, or the synthetic bloom agent resulting from the treatment of naphthalene with aluminum chloride; (d) the pitch bottoms obtained by reducing mildly acid treated tar from a petroleum cracking operation. The material d) is a particularly good conditioner for use in our process.

Certain waxy oils, for example, residual oils and long range distillates, inherently contain sufficient conditioner. Other oils, such as pale oils, wax distillates, lubricating distillates of medium viscosity and relatively narrow boiling range, residual oils and long range distillates which have been exposed to acid or solvent treatment, may be deficient in Wax crystal conditioner and therefore it may be possible to obtain improved results by adding conditioner.

The proportion of conditioner to be added depends upon the amount of conditioner naturally contained in the oil, upon the wax content of the oil and upon the nature of the diluent used, but ordinarily ranges from a trace to 1% by weight of the oil to be dewaxed. A convenient way of adding conditioner, when such addition is necessary, is to dissolve it in the Waxy oil prior tomixing with diluent.

An indication of the need for conditioner and the desirable amount may be easily obtained by laboratory test as follows: A sample of the oil to be dewaxed is completely dissolved in three volumes of petroleum ether. One portion (e. g. 100 cc.) of this solution is measured into a glass graduate (e. g. in. diameter). In another equal portion there is dissolved 25% by weight of conditioner, and this solution is placed in a similar graduate. A third portion containing an increased amount of conditioner (e. g., 50% by weight) may be prepared and placed in a third graduate. All of the graduates are placed in an insulated bath of suitable non-freezing liquid (e. g., petroleum ether) so that the graduates are submerged at least up to the level of contained oil. The bath is then cooled at a uniform rate (e. g., 2 F. per minute). The graduates are left undisturbed until the desired low temperature (e. g., i0 F.) has been reached. The graduates are then lifted out (without shaking) and observed. Those samples in which the wax has settled to the bottom in a distinct, firm layer have adequate conditioner. Those which contain no well defined oil layer require more conditioner. More quantitative information may be obtained, if desired, by shaking the graduate to make a uniform slurry, then filtering the slurry by suction through cloth. note being taken of filter time, yield, oiliness of cake, etc.

The presence of conditioner greatly decreases the sensitivity of wax to agitation and temperature mixing (discussed below), aids materially in the attainment of high filtering rates, and is particularly desirable if the oil is to be dewaxed to substantially the theoretical yield.

boiling a diluent by the addition of more conditioner, however, because it is found that, beyond a certain point (which may vary for different oils) the use of more conditioner produces no added benefit. Other things being equal, it is desirable to use as little conditioner as possible, not only on account of the added cost, but because some conditioners, if used in relatively large amounts, impair the color and other qualities of the oil.

An illustration of the efiect of varying the proportionate amount of conditioner is seen in the figure. The oil being dewaxed was a lubricating distillate from Kettleman Hills, California, crude. The conditioner used in the case of curves A, B and C was the pitch resulting from a reducing distillation of acid-treated petroleum cracking still residue. The conditioner used in the case of curve D was a synthetic pour point depressor of the type previously referred to. The volume ratio of diluent to waxy oil was in every case 3 to 1. The characteristics of the diluent have been given above. Chilling operation was the same in every case, the final temperature being -40 F. The filter rate scale is an arbitrary one, but accurately represents the relative value of the rates obtainable in practice under the described conditions.

It will be noted that, in the cases illustrated, very little additional improvement results from the use of more than .75% of conditioner. It will also be noted that, when using diluent A, the maximum rate obtainable is only about oneninth that obtainable with diluent B. The fact that the use of conditioners of different types sometimes affects the results (in degree) is illustrated by a comparison of curves C and D, where the same diluent was used.

It is well known that, in many of the dewaxing processes now in use, shock chilling is to be avoided. By this is meant the avoidance of a condition in which portions of the oil solution are subjected to sudden great temperature drop. Examples of this kind of shock chilling are the case where a quantity of relatively warm oil is poured into a quantity of oil at a considerably lower temperature, e. g., 20 F. or more lower, and the case where a quantity of the waxy oil solution is brought into contact with a very much colder solid surface.

Likewise, we have found that turbulence (induced, for example, by liquid flow and/or revolution of the scrapers used for removing wax from the walls of conventional pipe chillers) affects the formation of wax crystals. Turbulence of itself disturbs the formation of wax crystals and, in addition, may promote a mild form of shock chilling i. e., the mixture of small portions of relatively cold oil with relatively warm oil, thereby subjecting portions of the oil to repeated small but precipitate temperature drops. In order to obtain best results with our process, we therefore minimize shock chilling and turbulence. For example, a conventional 6 in. double-pipe chiller such as the one described on page 7 of the 1933 Proceedings of the American Petroleum Institute, Division of Refining (third mid-year meetin, vol. 14 M III) is equipped with a motor driven wall scraper. We prefer to operate such scraping mechanism at rates considerably below those normally used in equipment of this kind. We have found that, in the practice of our process, in apparatus of the type above referred to, a wall-scraper speed of 0.2 to 0.8 of a revolution per minute is adequate for maintenance of high heat transfer rates through the chiller wall, and yet will not seriously impair filter rates.

It is realized that in a continuous chiller provided with a wall-scraper (agitation and temperature mixing, as described above as resulting from turbulence, are closely interrelated and would usually occur simultaneously. The elimination of one effect and not the other is conceivable, however. For example, the use of a very long chiller might eliminate the objectionable features of lengthwise mixing and permit more violent agitation than could be used in a relatively short chiller.

By our process the rate of or low (e. g., 2 F., per minute or 10 F., per hour). The maximum overall practical chilling rate is obviously influenced by the particular apparatus, i. e., by such factors as diameter of chiller, temperature differential through chiller wall, etc.

Our invention will be more clearly understood from the following description of an example of an operation in accordance therewith, it being understood that the particular temperatures, oil, etc., used therein are illustrative only.

The oil to be dewaxed was an untreated lubricating distillate derived from Kettleman Hills, California, crude oil. This oil had a viscosity of 65 seconds Saybolt at 210 F., and a wax content by the Holde method of 10.4% by weight. The cold test was 100 F. In this oil there was dissolved .25% by weight or conditioner consisting of pitch obtained by reducing acid treated pressure still (cracking) tar. The oil containing the conditioner was dissolved in three volumes of closely fractionated straight run gasoline from Midway, California, crude oil. This fraction boiled between 95 and 176 F. by the A. S. T. M. distillation method. It is, of course, essential that the solution of oil and diluent be complete and homogeneous. In order to insure this condition the waxy oil and diluent were mixed at about 120 F. The solution was then charged continuously at 10 gallons perhour to a vertical double-pipe chiller 0.5 feet x 20.0 feet equipped with a wall-scraper operated at 0.2 revolution per minute. Oil was used as a cooling medium in this case, flowing upwardly through the shell of the chiller countercurrent to the flow of Waxy oil. The cooling oil temperature at the point of entrance was -40 F. and at the point of exit +40 F. The waxy oil entered the top of the chiller at +80 F. and left the bottom at F. The chilled oil was passed from the bottom of the chiller to a filter where it was filtered through #6 canvas at approximately 10 lbs. per square inch pressure difierential. As soon as a 4 inch wax cake had built up on the filter it was removed from the slurry and washed with fresh diluent at 30 F. The wax cake was then blown off by back pressure of gas and the filter was reinserted in the slurry. This sequence Was repeated continuously. The filtrate, i. e., the dewaxed oil, was heated to distill off the diluent which was condensed and returned for reuse.

The results of this treatment were as follows: the yield of dewaxed oil of +10 F., cold test was 89.0% by weight; the filter rate after 15 hours continuous filtering time was 12 gallons of dewaxed oil per square foot of filter area per hour; the filter rate after 40 hours continuous filtering time was substantially unchanged; the oil con tent of the wax was 1.56% by weight (by the ether-alcohol method). A substantial proportion of this oil in the wax was conditioner.

chilling may be high The effect of temperature mixing as induced by agitation in the chiller is shown by a treatment identical with the foregoing, except that the scraper speed was .9 revolution per minute. The filter rate in this case after 15 hours was 6 gallons per square foot per hour.

Another identical treatment in which the scraper speed was 7 revolutions per minute gave a filter rate of 1.8 gallons per square foot per hour after 15 hours.

Inasmuch as our process permits high chilling rates and produces a rapidly-filtering slurry, it is particularly Well adapted to continuous operation. We prefer, therefore, to take advantage of this, and, in doing so, use a standard continuous filter of the rotating drum type, although this type of filter is obviously not essential. Wax which filters rapidly also settles rapidly. Hence it is possible to separate the wax produced by our process by settlement, although we prefer to filter in order to obtain more complete separation of oil and wax.

While we have described examples of the operation of our process in which we chilled to 30 F., it should be understood that higher or lower dewaxing temperatures may be used. Likewise, the volume ratio of solvent. (or diluent) to waxy oil may be varied to suit the particular situation.

As in the case of conventional solvent dewaxing processes, the dewaxed oil is distilled to remove the diluent (which may then be reused). Like- Wise, we prefer to follow the usual practice of washing the wax cake to recover dewaxed oil with which it is wet. Ordinarily, however, in our process, the cake contains so little oil that we find that it is sufficient to wash it with cold diluent and then use the wash liquor directly as diluent without distillation. In most cases we find that the cake after such washing is substantially oilfree, so that (after heating to drive off the diluent with which it is moistened) it is available as high melting-point wax without sweating.

We claim:

1. A method for separating wax from oil by continuous filtration at a rapid rate with a high yield of dewaxed oil and production of a wax filter cake having low oil content, comprising the steps of dissolving together the oil, a wax crystal conditioner, and a hydrocarbon solvent of which the major portion is liquid at normal temperature and pressure and the A. S. T. M. point is below 250 F., and chilling the solution to a point at which wax solidifies by directing the solution through cooling means in non-turbulent flow and in such manner as to prevent shock chilling and temperature mixing of any minor portions of the solution to thereby insure the obtaining of a high filter rate during prolonged continuous filtration.

2. A method for separating wax from oil by continuous filtration at a rapid rate with a high yield of dewaxed oil and production of a wax filter cake having low oil content, comprising the steps of dissolving together the oil, a wax crystal conditioner in the form of not more than 1.0% by weight of the pitch obtained by distillation of acid treated tar from a petroleum cracking operation, and a hydrocarbon solvent of which the major portion is liquid at normal temperature and pressure and the A. S. T. M. 50% point is below 259 F., and chilling the solution to a point at which wax solidifies by directing the solution through cooling means in non-turbulent flow and in such manner as to prevent shock chilling and temperature mixing of any minor portions of the solution to thereby insure the obtaining of a high filter rate during prolonged continuous filtration.

ARTHUR L. LYMAN. MELVIN M. HOLlVL

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