US2673831A - Cracking of dewaxed wax slops - Google Patents

Description

Patented Mar. 30, 1954 Everett CQHughes, Shaker Heights, Ohio, assignor to The'Standa'rd Oil Company, Cleveland, Ohio,

a corporation of Ohio No Drawing. Application June 13, 1950, Serial No. 167,910

1 Claim.

This invention relates to a process of removing wax from a petroleum fraction known as wax slops.

As is well known, crude petroleum is reduced by fractional distillation into a large number of cuts, depending upon their boiling points. The cuts taken vary withthe refinery, but in general a fairly clean separation of the different types of hydrocarbon mixtures present in petroleum is thereby obtained. In one type of system in standard use, a reduced crude is frac'tioned in a vacuum tower. A series of fractions are obtained. The lightest is a gas oil having a viscosity of less than '15 SUS at 100 F. The next fraction is known as parafiin distillate and contains lubrieating oil base stocks having a viscosity of from 75 to 300 SUS which contain varying amounts of parafiin wax; this wax must be removed in most instances before the cut is useful commercially as lubricating oil. the parafiin distillate crystallizes readily and may be separated therefrom by cooling the fraction and then removin the wax by filtration. The dewaxed oil can be redistilled into fractions of various viscosities. The next fraction contains a micro-crystalline wax and is known as wax slops." The micro-crystalline wax in the wax slops inhibits the formation of filterable wax crystals and as a result the wax slops cannot be dewaxed by chilling in this way as can the paraffin distillate. Since the wax cannot readily be removed, and its high wax content makes it useless for most purposes, it is often used as cracking feed stock, and the wax cracked'with the other hydrocarbons in this fraction. This is particularly true ina plant which does nothave a commercial solvent dewaxing process. Such a dewaxing process comprises diluting the petroleum fraction with from 1 to 10 parts of a liquefied normally gaseous hydrocarbon, usually methyl ethyl ketone, heating the mixture to dissolve the wax and form a homogeneous solution and then vaporizing a portion of the liquefied hydrocarbon to chill the mixture to a temperature ranging from +10 to 40 F, in order to precipitate the wax. The mixture is then centrifuged, filtered or subjected to other treatment for separation of the precipitated wax, while the hydrocarbon is recovered from the oil fraction.

Such a solvent dewaxing process is not required to dewax parafiin distillate, and the cost of such a process is not always justifiable to process the wax slops cut referredto above. However, since the wax in wax slops commands a fair price on the market, it is worth-recovering by an inex- The paraffin wax present in pensive process, if one could be devised, even though the dewaxed fraction is then cracked in the usual way.

Accordingly, it is an object of the present invention rapidly, easily and inexpensively to recover, at least in part, wax contained in the petroleum fractions characterized as wax slops.

This is accomplished by blending from about 0.1 to about 0.5 part liquid butane with each part of wax slops, such that the butane comprisesfrom about 10 to about 33% by weight of themix'ture, subjecting the mixture to conditions of pressure, temperature and time suilicient to former homogeneous solution, cooling the mixture to a temperature from about 50 to about F. to effect precipitation of wax, and separating precipitated wax from the mixture. Butane may then be recovered from the dewaxed petroleum fraction either at once or after cracking and then recycled. 'The process is applicable to'the wax slops fraction from any crude oil. By wax slops is'meant a fraction boiling higher than distillate containing solely paraffin wax and having up to 12% wax at 70 F. atleast a part of which is micro-crystalline so that the wax cannot be separated by chilling and filtering. In general it has a boiling range of 600 to 1100 F. and a viscosity of 50 to150 SUS at F. 7

It is possible to employ liquefied n-bu'tane, isobutane ormixtures thereof in carryin out this process. However, it is preferred to employ lique i'ied refinery butanes, which are mixtures of normal and iso-butanes together with small quantities of butylenes, because they are less expensive. The-butanes employed in'accordance with the invention have the following physical prop erties:

n-outane iso-butane Molecular Weight .l 58.12 58.12 Boiling Point l l Fl 33 14 Critical Temperature F 308 274 Critical Pressure Hatml 36 '37 v Specific Gravity at 0 0. (grams per col) 0.600 0. 603

This process is tobe'distinguished from conventional solvent dewaxing processes referred to above, which hav several disadvantages. Most important, from acommercial standpoint, is the expense of the equipment and the process, which renders it uneconomic to treat wax slops. The equipment required to reduce the temperature to the low levels at which the commercial processes are carried out is especially expensive both to install and to operate.

The process of the invention, on the other hand, possesses several advantages over the customary commercial solvent dewaxing procedures. It employs small quantities of butane, thereby greatly decreasing the bulk of the material to be handled, reducing time consumed in processing and lowering solvent costs. The precipitation of wax is carried out at ordinary room temperatures, thus eliminating special apparatus and procedures for cooling the mixture. The butane employed as solvent is plentiful and, therefore. inexpensive. It is easy to separate from the dewaxed fractions either before or after cracking them and is readily condensed and recycled. Because an evaporative chilling step is avoided, no bumping, flashing or violent boiling of the mixture with resultant loss of material may occur, nor is there a change in proportions of solvent to oil during cooling. In consequence, the ratio of solvents to oil in the final mixture is always known, and is constant, permitting reproducible results. Moreover, as applied to wax slops the butane appears to overcome the effect of any crystallization inhibitor present and good wax crystals are obtained. In the process of the invention a normally gaseous liquefied butane or mixture of butanes, preferably liquefied refinery butane, is drawn from a storage tank and blended with the above-described wax slops in a mixing tank. The blend ordinarily formed therein comprises 20% butane by weight. That is, about one part butane is blended with about four parts wax slops by weight. This ratio gives the highest yield of wax commensurate with the cost of carrying out the process. It is preferable to employ the minimum quantity of butane necessary to eifect precipitation of the ma,or proportion of wax present. With this object in view, it is possible to employ as little as one part butane to nine parts wax slops, on the one hand, or as high as one part butane to two parts wax slops, on the other hand, so as to obtain a blend containing from to 33% butane by weight.

The mixture, while under sufficient pressure to maintain the butane in a liquid state, is passed through heating equipment wherein the temperature is raised to a point at which a homogeneous solution is obtained and all the wax contained in the wax slops dissolved after heating for a sufficient length of time. It has been determined that temperatures ranging from about 90 F. to the critical temperature of the butane, i. e., 274 F. in the case of isobutane or 295 F. for refinery butanes, or 308 F. for n-butane, and pressures from about 2 to 3'7 atmospheres are satisfactory. Temperatures from 100 to 150 F. are usually high enough, however. Under these conditions it requires from about minutes to about 1 hour for the solution to become homogeneous.

If desired, the blending and heating steps may be combined, as for example by blending hot butane and wax slops, or hot wax slops with butane.

It is essential in order to obtain a wax precipitate of satisfactory crystal structure that the mixture or blend be made homogeneous, that is, that it be heated to temperatures within this range for the necessary period of time. Otherwise, it may be impossible to filter the mixture, or, if filterable, the filtrate may be cloudy and the filter cake may contain a large amount of oil.

The solution is then conducted to a precipitation or settling tank in which it is allowed to cool to a temperature in the range from about 50 to about F. At the butane concentrations employed, cooling to temperatures below about 50 F. does not noticeably increase the yield of wax. On the other hand, yields are usuallyv only a fraction of those obtainable if the mixture is not cooled below about 80 F. Since these temperatures are ordinary room temperatures, it is not necessary to cool the mixture by artificial means, and for reasons of economy none ordinarily are employed. Under certain conditions, however, it may be feasible in order to save time to provide the tank with a cooling jacket or pipes in which water or other heat-transfer liquid may be circulated. Cooling may also be eifected by vaporizing a part of the butane, but conditions must be carefully controlled since the minimum quantity 'of butane required to dissolve the wax and form a homogenous solution is preferably employed, so that loss of appreciable amounts of butane may cut down the yield of wax obtained.

The cooled mixture containing precipitated wax is then filtered or centrifuged in order to separate the wax therefrom. The presence of the butane considerably increases the rate of filtration of the mixture compared to petroleum fractions not containing butane. It is also possible to separate the wax by cold settling, apparently because of the low specific gravity and viscosity of liquid butane. Filter aids may be added to expedite filtration, but usually their presence does not materially increase the filtration rate.

The slack wax thus recovered has an oil content of from 40 to 60%, which may be removed therefrom by treating the Wax with a solvent for the oil in which the wax itself is insoluble. Thereafter, the solvent may be separated from the oil 'by distillation, and recycled. Solvents such as butane, methyl ethyl ketone and acetone have been found to be satisfactory.

The partiaLy dewaxed wax sops containing butane may be passed directly to a catalytic or thermal cracker and the wax slops cracked in the usual way. The butane is unaffected in the cracking operation and after cracking the butane is recovered for example, by means of the gas plant used in connection with the cracker, and recycled to the dewaxing operation. This procedure is preferred in the commercial application of the process, because it avoids both the capital and operating expense of a separate butane recovery system.

Alternatively the partially dewaxed crude Wax slops may first be freed of butane by distil ation at atmospheric temperatures and pressures and then cracked. The butane gas thus recovered is condensed and recycled.

The fraction obtained by this procedure before cracking, has a wax content of about 4.4% at 70 F.

The following examples illustrate various modes of practicing the invention.

Example I Liquefied refinery butane and wax slops having a boiling range of 365 to 1037 F. and con-, taining about 10% wax, some of which was suspended therein, were blended in a closed system under a pressure of 3 atm. in the ratio of one part butane to four parts wax slops and then heated at F. until all the wax had dissolved. The mixture, which contained 20% butane, was then allowed to cool to 60 F. and filtered at a filter pressure differential of 20 lbs. per square 33 inch. The mixture passed through the filter at a rate of 0.791 gallon per square foot per hour. A yield of 15.6% wax containing 64% oil was obtained.

By way of comparison, the same wax slops not containing liquid butane filtered at a rate of 0.022 gallon per square foot per hour. Thus, addition of butane to the material increases the filtration rate by a factor of about 40.

The slack wax obtained was examined microscopically and found to have a structure intermediate between amorphous paraffin distillate slack wax and true microcrystalline wax.

The partially dewaxed wax slops obtained in the filtrate after removal of butane therefrom had a wax content of about 4.4% at 70 F. It was cracked in the usual Way.

Example 2 heated to 150 F. for V hour at system pressure L at the end of which period the mixture had become homogeneous. After it had been allowed to stand overnight (18 hrs), cooling slowly during that time to a mimimum of 80 F., the mixture was filtered to separate the wax. Filtration was clean and rapid, and a good yield of wax was obtained. The filtrate, still containing about 6% wax at 70 F. was stripped of butane by allowing the latter to evaporate at room temperature and pressure, and then passed to the crackers.

Example 3 with a system of pipes through which cold water was circulated, and allowed to stand at that temperature for 18 hours. During this period the wax precipitated and settled at the bottom and sides of the tank. The clear supernatant liquid was removed by decantation, and butane stripped therefrom at room temperature and pressure and condensed for recycling. The partially dewaxed wax slops containing about 6% wax at 70 F. was then cracked in standard towers in the usual way. The wax recovered was de-oiled by washing with methyl ethyl ketone, and then packaged for sale as refined wax.

Thus it will be seen that the process of the invention permits rapid and easy recovery of wax from wax slops at nominal cost. The procedure is simple and requires only inexpensive, conventional equipment and relatively small quantities of solvent. Because only small quantities of solvent are used, it is unnecessary to cool the mixture to very low temperature to precipitate the Wax, saving time and making it unnecessary to evaporate solvent therefrom, avoiding bumping, flashing and violent boiling, and special equipment. Also, the bulk of the mixture to be handled is reduced.

It will be understood that various changes may be made in details within the scope of the claim without departing from the spirit of the invention.

In the specification and claim all parts and percentages are by weight.

I claim:

A process of treating a wax slops cracker feed stock having a boiling range of 600 to 1100 F. and a viscosity of to 150 SUS at 100 F. and having up to 12% wax at F., at least a portion of which is microcrystalline, which comprises blending liquid butane therewith to an amount between about 10 and about 33% by weight of the resulting mixture, subjecting the mixture to conditions of pressure, temperature and time suificient to form a homogeneous solution, cooling said solution to a temperature from about 50 to about F. to effect precipitation of wax, separating wax therefrom, recovering butane therefrom, recycling butane to treat an additional portion of feed stock and cracking the dewaxed wax slops.

EVERETT C. HUGHES.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,939,946 Baylis Dec. 19, 1933 1,943,236 Giles Jan. 9, 1934 2,103,898 Fellows Dec. 28, 1937 2,166,893 Holsten July 18, 1939 2,229,658 Jenkins Jan. 28, 1941 2,484,728 Patillo Oct. 11, 1949