US3234122A - Process for treating crude petroleum oils - Google Patents

Description

United States Patent 3,234,122 PROCESS FOR TREATING CRUDE 1 PETROLEUM OILS Bernard Charles Allibone, St. Mary Cray, Kent, and

Reuben Joseph Russell, Sunbury-on-Tharnes, Middlesex, England, assignors to The Burmah Oil Company, 'Limited and The British Petroleum Company Limited, both of London, England, both British joint-stock corporations v No Drawing. Filed Oct. 24, 1961, Ser. No. 147,199 Claims priority, application Great Britain, Nov. 1, 1960,

10 Claims. (Cl. 208-370) This invention relates to a process for treating crude petroleum oils, particularly crude oils having high wax contents and low asphaltene contents, whereby they may be pumped more easily below their normal pour points.

'W-h'en crude oils are cooled, wax separates and eventually -forms a structure in the oil which prevents it flowing under the force of gravity.

i Waxy crude oils usually have a relatively high pour point and they may have to be handled at ambient temperatures below, sometimes considerably below, their pour points and pumping such crude oils along pipe lines under such conditions is usually an expensive and difficult operation if it is possible at all. For example, in certain parts of India, waxy crude oils are produced which have pour points in the region of 90 F. and which will require to be conveyed by long distance pipe line to a refinery or ocean terminal. The ground temperature during the winter months in the regions in question is often as low as 65 F. so'that very high pumping pressures would be necessary to pump the oil along the pipe line.

- The object of the present invention is to provide a process for treating a crude petroleum oil which enables the oil to be pumped more easily below its normal pour point than if it had not been so treated and which enables pumping to be restarted more easily after shut down.

Accordingto the invention, there is provided a process for treating a .wax'containin'g crude petroleum oil, in which the oil is first heated at least to a temperature at which all the. wax is in solution in the oil and the heated oil is then cooled to a temperature below about 75 F. to reprecipitate the wax, the cooling over at least a major part of the temperature range Within which the Wax is precipitating being at a rate within the range 0.1-5 F., especially 0.5 2L5 F., per minute, and under conditions such that nosubstantial mixing movement is induced in the oil. The oil should preferably be cooled to a temperature close to the minimum temperature at which it is subsequently to be pumped. In most cases this will be between 40 F. and 75 F.

The temperature at which solution of the wax is complete' in a wax-containing crude oil depends on the source of the, oil but is usually within the range 120 to 200 F. For best results, the oil should preferably be heated to a temperature 20-30 F. above the temperature at which all the wax goes into solution. It will clearly not be normally desirable for economic reasons to raise the oil to any higher temperature than this, and, in any case, the temperature of the oil should preferably not be raised above 250 F. Again, from economic considerations, it is desirable to restrict as far as possible the temperature range over which the cooling conditions of the oil are controlled. In many cases it will not be necessary to conice trol the cooling conditions in the manner stated over the whole of the period during which the wax is precipitating. In the case of some crude oils it is possible to allow the oil to cool in conventional manner to 30 to 40 F. below the temperature at which solution of the wax is complete before cooling it in the controlled manner specified above. It will be clear that the optimum temperature range over which controlled cooling should be maintained may easily be determined by experiment for any particular crude oil, bearing in mind the demands of economy on the one hand and satisfactory results of the treatment on the other.

Cooling the oil in the manner specified results in the precipitation of wax in the form of large discrete crystals which remain in suspension in the oil and are non-sensitive to shearing; They thus have a very much reduced tendency to thicken the oil on cooling than the smaller crystals which separate from untreated oil. The treated oil may, therefore, be pumped more easily below its normal pour point than if it had not been treated. In some cases economical pumping over long distances is possible at temperatures as low as 30 F. below the normal pour point of the oil.

The process of the invention is particularly suitable for treating very waxy crude oils of low asphaltene content, i.e. those having a wax content (as determined by dewaxing the deasphalted oil with methylene chloride at 25 F.) of 10% or more by weight and an asphaltene content (as determined by the British Institute of Petroleums standard method No. 143) of 0.3% or less by weight. The normal pour point of such oils is usually at least about C. Crude oils of this type are found in many parts of the world, e.g. Iran, India, Burma, the East Indies and the USA.

In any given body of liquid there is, of course, a certain amount of invisible molecular movement, caused by intermolecular forces, continually takingplace. It is important, however, that during that part of the cooling step, during which the cooling rate is controlled as defined above, the amount of additional movement induced in the oil, e.g. by convection currents or by mechanical means, should be kept to a minimum. Thus, the cooling should be carried out so that no substantial temperature gradient occurs across the body of the oil in a horizontal direction and, if any temperature gradient occurs in a vertical direction, the colder oil should be at the bottom and the hotter oil at the top.

A particularly satisfactory form of apparatus to use for the cooling step is a vertical tube heatexchanger, the oil to be treated being preferably contained in the shell and the cooling medium, e.g. Water, being passed through the tubes. The latter should be arranged in parallel and not in series with respect to the flow of cooling medium. Both the tubes and the shell should be positioned vertically or approaching the vertical and any internal stifiening devices should be arranged so as to minimize non-vertical surfaces on which treated oil or waxy sludge might be held up.

In order to ensure that no substantial temperature gradient occurs across the cross-section of the oil in a horizontal direction, the ratio of heat transfer surface to oil volumetric capacity should preferably not be less than 10 sq. ft. per cubic foot. With this arrangement it is usually possible to ensure that the temperature difference between the cooling medium and that part of the oil farthest away from it in the same horizontal plane enriched waxy material deposited on the tube surfaces may interfere with heat transfer.

It is most easy to ensure static conditions when required during the precipitation of the wax if that part of the process may be carried out in a batchwise manner but the rest of the process may be carried out continuously. Slightly enriched waxy material tends to adhere to the surfaces of the tubes carrying the cooling medium and should preferablybe removed by melting out with hot water before a fresh charge is introduced.

It has been found that the very best results are obtained if a rate of cooling is chosen, within the limits specified above, in accordance with the size of the vessel in which the oil is being cooled. In practice, vessels of a capacity within the range '00'10,000 gallons are most convenient for the purpose in question. In general, for

most satisfactory results, the rate of cooling should be.

slower when using vessels with large spacings and a lesser heat exchange surface relative to volumetric capacity, and vice versa.

Example 1 By way of example, an Assam crude .oil having, a pour point of 90 F. and a wax' content of 15.4% by weight was treated according tothe process of the inventionin a pilot'plant consisting of a cylindrical chamber feet long by 30inches in diameter arranged with its axis vertical- The chamber. contained;l27 vertical one inch di-' ameter tubes arranged in a triangular spacing with a distance of 2%" between the tube'centres and held rigid by;

two intermediate bracing structures designed so as to keep the non-vertical surfaces to a minimum. The chamber was filled with the crudeoil which was then raised to a temperature of 200 P. which was F. above the temperatureat which all the wax had gone intosolution: The oil was then cooled by passing Water through the tubes at a temperature diminishing from 200 F. to 65 F. at a rate of 1 F. per minute, and thereafter held at 65 F. The water was supplied to each tube simultaneously so as to ensure that the temperature of the water at any given level would be the same in all the tubes.

The rate at which the oil cooled varied from 1 F. per

minute at the higher temperature to 0.1 F. per minute when its temperature approached 65 F. The flow of' water was continued until the temperature of the oil was in the region of 65 F. when the charge was run out.

in a highly fluid condition, showing an apparent viscosity at a rate of shear of 16 reciprocal seconds of about 1 poise, which value was not materially affected by prolonged shearing either in a test line or in a viscometer.

The oil in question is required to be pumped at ternperature of the order of 65 F. during the wintermonths in Assam. Without the treatment according to the invention the oil had a yield value at 65 F. of 100-700 dynes/ sq. cm. depending on its shearing history, whereas after.

the treatment its yield Value was only 10 dynes/ sq. cm. It will thus be readily appreciated that the oilwas much more easily pumpable at 65 F. after the treatment according to the invention.

Such a treated oil whilst being sheared at 65 F. at a rate of 13 reciprocal seconds showed an apparent viscosity of l poise or less whereas untreated crude under similar conditions did not fall below 6 poises.

then caused the viscosity to fall once more. In contrast,

untreated crude under similar conditions showed an apparent viscosity of poises falling eventually to about .8.pois-es.

Continued shearing Furthensuch treated oil-conditioned to 65 F. was,

Another characteristic of such treated oil'is that the a relatively low'one, albeitv still tangibly higher than.

that of treated oiL.

Example 2 The same Assam crude oil was passedthrough two shell and tube heat exchangers in series, in the. first of which it .was steam heated to 200 F. and'in the second water cooled to 15 F., after which. it waspassed into the chamber described in Example 1, also held at 150. F. by circulating Water. On the chamber becoming filled with crude, the chamber water temperature wasdecreased at 1 F./minute until the crude temperature thadireached 65 F., the. water temperature. then being about 45 F.

The water temperature was then. quickly raised to t 65 F: and the-crude oil run out. The highly fluid con-- dition described in Examplegl was again evidenced.

We claim:

LA processfor treating a wax-containing crude pe-.

I roleum oil to enablethe: oil to be pumped more easily below itsnormalpour point, which comprises-heating the oil to a temperature at least at :which all the wax in the oil is placed in; solution therein; cooling the. heatedxoilto a temperature below about F; to reprecipitate thewax in the form of'large discrete-crystals and-maintaining over at least a major part of the temperature range within'which the wax is precipitating, a cooling rate within, the .range 0.15 F. per'minute, and maintaining the voiliduring the cooling thereof in a substantially quiescent state.

2. :A process for. treating a .waxwcont'ainingxcrudeper "troleum oil havinga wax content of at least 10% by weight and an 'asphaltene content of up to 0.3%. by weight, to.

enable the oil to be pumped more. easily below its Inorrn-al pour point, which "comprises heating the oil to atemperature at least at which all the wax present in thepoil is in solution therein, cooling the heatedioil toa temperature within the range 40-75 F. to reprccipitate the wax in the form of large discrete crystals and maintainingcver at least a major part of the temperature rangewithin which the.

wax is precipitating a cooling rate: within the: range 0.1- 5 F. per minute, andmaintaining the. oil during the:

cooling thereof in a substantially quiescent'state, that part.

of the cooling process carriedouton a controlled rate;v

of cooling being in a batchwise manner.

3. A process in accordance with?claim-1 in which the cooling of the "heated oil is achieved by placing the heated. oil in a vertically disposed heat exchanger zone.

4. A process in:accordance with claimd inwhich the.

heated oil is maintained in a vertically disposed heat ex-n changer zone and a cooling medium isgpassed. through the heat exchanger zone in parallel relationship to. the.

heated oil to effect a cooling thereof- 5. A process in accordance withclaim 3 .in which the.

ratio of heat transfer surface-area.t0 oil volumetric. capacity in the heat exchanger zoneis at least 10 square feet per cubic foot.

6. A process in accordance: with? claim 1 :in'jwhich att least a part of the cooling ofthe heated oil is carried out;

in a batchwise manner.

ture at which-all the wax goes into solution.

9. A process according to claim 1, in which the oil is cooled to a temperature within the range 4075- F.

10. A process according to claim 1, in which lthe cruder petroleum oil is one which has a wax content of at least 10% by weight and an .asphaltene content of up to 0.3% by weight.

(References on. following page),

3,234,122 5 6 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS Sachanen: The Chemical Constituents of Petroleum, Reinhold Pub. C0rp., New York, 1945, pages 302 to 306.

1,974,398 9/1934 Ellsbeig 208-37 2,100,662 11/1937 Lyman 208-37 5 DELBERT E. GANTZ, Primary Examiner.

FOREIGN PATENTS ALPHONSO D. SULLIVAN, JOSEPH R. LIBERMAN,

Examiners.

851,135 10/1960 Great Britain.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,234,122 February 8, 1966 Bernard Charles Allibone et a1.

ears in the above numbered pat- It is hereby certified that error app d Letters Patent should read as ent requiring correction and that the sai corrected below.

line 8, for "may be" read is column 4,

Column 3, 13, for "15 F." read 150 F.

Signed and sealed this 24th day of January 1967 (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents

Claims (1)

1. A PROCESS FOR TREATING A WAX-CONTAINING CRUDE PETROLEUM OIL TO ENABLE THE OIL TO BE PUMPED MORE EASILY BELOW ITS NORMAL POUR POINT, WHICH COMPRISES HEATING THE OIL TO A TEMPERATURE AT LEAST AT WHICH ALL THE WAX IN THE OIL IS PLACED IN SOLUTION THEREIN, COOLING THE HEATED OIL TO A TEMPERATURE BELOW ABOUT 75*F. TO REPRECIPITATE THE WAX IN THE FORM OF LARGE DISCRETE CRYSTALS AND MAINTAINING OVER AT LEAST A MAJOR PART OF THE TEMPERATURE RANGE WITHIN WHICH THE WAX IS PRECIPITATING A COOLING RATE WITHIN THE RANGE 0.1-5*F. PER MINUTE, AND MAINTAINING THE OIL DURING THE COOLING THEREOF IN A SUBSTANTIALLY QUIESCENT STATE.