US3136711A

US3136711A - Process for reducing the pour points of crude oils

Info

Publication number: US3136711A
Application number: US98668A
Authority: US
Inventors: Marvin B Glaser; Lichtenstein Ira
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1961-03-27
Filing date: 1961-03-27
Publication date: 1964-06-09
Anticipated expiration: 1981-06-09
Also published as: GB979772A; DE1216468B; NL6401913A; BE644537A; FR1392602A

Description

June 9, 1964 M. a. GLASER ETAL 3,135,711

PROCESS FOR REDUCING THE FOUR POINTS OF CRUDE OILS Filed March 27, 1961 2 Sheets-Sheet 1 FlG.-l

IA A I I I I Q 60- I2 I; vIs. TO I-IIaII T0 GENTRIFUGE 2 8 3 a i I; 0 40-- 4 H z 2 III 50 l l l l 0 I0 20 so 40 so so 10 AVE. CENTRIFUGATION TEMR, F.

FIG.- 2

SYNERGISTIC ASPHALTENE-RESIN INTERACTION I l I I Izo u. HO- o I e fis E 80- 2 g LEGEND 8 A vmou: RESID A OIL WAX Q on. WAX ASPH. 0 on. WAX nesms 20 I l l I so I00 I40 me 220 260 REHEAT TEMP. F.

Marvin B. Glaser lro Lichtenstein PATENT ATTORNEY INVENTORS June 9, 1964 M. B. GLASER ETAL 3,136,711

PROCESS FOR REDUCING THE FOUR POINTS OF CRUDE OILS Filed March 27, 1961 2 Sheets-Sheet 2 GAS ...9 FIG-3 P T PIPE snu. H

RES") MAIN on. PRoouc'T Lu lc semen-use MIDDLE PREHEATEI: DISTILLATE s I I I A A A A A A '\I\I\VA \I\I\I\I CRUDE v v v v v 6 2 coouzn ,wax mcu FRACTION Morv| n B. Glo er NVENTORS Iro Llchtensiem PATENT ATTORNEY United States Patent 3,136,711 PROCESS FOR REDUCING THE POUR POINTS OF CRUDE OILS Marvin B. Glaser, Scotch Plains, and Ira Lichtenstein,

Fords, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Mar. 27, 1961, Ser. No. 98,668 8 Claims. (Cl. 208-37) This invention relates to a process for improving the pour point of waxy crude oils by selective removal of the waxes causing high pour points. Specifically, this process relates to a method of improving the pour point of a residual fuel oil by heating the entire crude, followed by cooling the crude to agglomerate and precipitate wax, centrifuging to remove precipitated wax, and fractionating the crude to obtain a residual fuel oil of low pour point. Thermal pretreatment of the entire waxy crude to an elevated temperature, followed by controlled cooling below its pour point and subsequent centrifugation selectively removes from the crude those waxes which cause high pour point in the residual fuel oil component of the crude.

It is known to improve the pour point of the residual fuel component by thermal cracking of the waxy bottoms or gas oil fraction of the crude; this process reduces the pour point of the residual fuel oil by about 10 to F. This process, however, results in loss in the residual fuel component of approximately 40% by volume due to poor selectivity in cracking of the wax that causes the high pour point. Other means of improving fuel oil pour point, such as undercutting residuum, blending with low pour stocks or fiuxing with middle distillates are unattractive because of loss of valuable middle distillate yield and decreased refinery flexibility. The conventional methods of removing wax from a residual fraction involved introduction and recovery of large amounts of valuable solvent material, the necessity of having essentially all of the wax separated in a pure form from the oil fraction, and very low temperatures at which the separation of the wax from the oil was normally carried out; e.g., to F.

It is an object of this invention to reduce the pour point of waxy crudes so that they may be handled with greater ease and flexibility in climates where the ambient temperatures are near their upper pour points. It is a further object of this invention to reduce the pour point of a residual oil fraction obtained from a waxy crude oil while at the same time minimizing the loss of the residual fuel oil component to other fractions. A still further object of this invention is to obtain a lower pour point residual fuel by use of inexpensive apparatus and use of a process wherein the separation of the wax from the oil can be accomplished at relatively high temperatures.

In accordance with our invention, a crude oil containing a large concentration of wax is preheated to a temperature sufliciently high to melt the highest melting wax and to disperse the asphaltenes and resins present. heated crude is subsequently cooled at a controlled rate to a temperature near its pour point in order to precipitate the wax, which causes the high pour point, in the form of agglomerates which can be easily separated by centrifuging. A wax-rich fraction is removed from the The I 3,136,7ll Patented June 9, 1964 crude oil. The separated oil is fractionally distilled to obtain the various desired fractions. A residual fuel cut boiling from 650 to 750+ is withdrawn. A waxy crude having an original pour point of about 35 to 55 F. can in this manner have its pour point reduced 15 to 25 F. The residual fuel component cut from the preheated crude has its pour point reduced l5-25 F.

Many advantages are realized in utilizing our process of thermal treating a waxy crude to improve its pour point, as well as the pour point of the residual fuel fraction. This process is selective in removing the waxes which cause the high pour point in the residual fuel component. The pour point of the residual fuel may be reduced from about 105 F. to to F. or less with a minimum loss of this component. In order to obtain the same reduction in pour by thermal cracking, there is a loss of about 40% of the residual component. In accordance with this process, all of the wax present does not have to be removed, but only the waxes causing the high pour in the resid. fraction. Relatively simple apparatus is required. No additional solvents need to be added, and the crude may be centrifuged at relatively high temperatures of 35 to 55F. The centrifuge temperature is generally within about 10 F. below the upper pour point. A further disadvantage of thermal cracking is that the hot filtration sediment problem which usually limits the amount of pour reduction with thermal cracking is nonexistent when centrifuging for pour reduction.

Referring to the drawings:

FIGURE 1 is a graph showing the effect of centrifugation temperature on the resulting pour point of a waxy crude, as well as the percentage of dense phase which is separated. The crude used was whole Zelten crude which had a pour of 55 F. and which was heated to 200 F. and cooled to the indicated centrifugation temperature.

FIGURE 2 is a graphic presentation showing the synergistic effect on pour point of the asphaltenes and resins.

FIGURE 3 is a schematic diagram of the process used in accordance with applicants invention to obtain a resid. fuel fraction of low pour point.

Our process is advantageously used in reducing the high pour point of waxy crudes which contain from 3 to 12 wt. percent Wax, based on crude, and which contain in the residual fuel fraction from 7 to 25 wt. percent wax, based on the resid. fraction. The high waxy crudes which are normally treated are the Middle East and North African crudes; for example, Zelten, Kuwait, and Aramco.

By carefully controlling the thermal history of the crude oil to be treated, the wax present in the crude can be put in optimum condition to be precipitated and centrifuged. In practicing this invention, the crude is heated to a temperature between F. to 220 F. and then cooled to the centrifuge temperature. The temperature to which the crude is heated should be sufiiciently high to melt all the wax present and to disperse the asphaltenes and resins. Preferably, the crude should be heated to a temperature of 180 F. to 210 F. Specifically, temperatures of to 200 F. are used. The exact temperature of pretreatment will depend upon the wax and asphaltene content of any particular crude. The temperature of centrifugation is critical and is close to the standard ASTM upper pour point of the oil to be centrifuged. Generally, this will be below 100 F. The centrifuge temperature should be such that the maximum amount of waxes causing the high pour point will precipitate, but high enough that the viscosity of the oil centrifuged is not so lowered that the material is too viscous to separate the precipitated wax from the oil treated. Generally, the waxy crudes are centrifuged at temperatures between 35 and 55 F. Specifically, temperatures between 40 and 50 F. are preferred. However, the optimum centrifugation temperature will vary from crude to crude, depending on the relative amount of wax and asphaltenes present. The pressure is not critical and the process is normally carried out at atmospheric pressure. Obviously, when heating to 200 or more, some light ends in the crude will boil out. These can be refluxed, condensed and returned to the crude, or cooled and returned to a specific fraction. If it is desired to prevent escape of these light ends, the heating to 200 can be carried out under pressure.

The rate at which the heated crude is cooled is critical. The crude may be shock cooled to just above its cloud point; however, cooling from this point to the temperature of centrifugation must be carried out at a controlled rate of about 0.5 to 2 per minute. If the crude is cooled too rapidly, the wax precipitates in fine crystals and does not have the opportunity to form agglomerates with the dispersed asphaltenes. The centrifugal force applied to the wax agglomerates that form is critical to the extent that if too low a force is used, the wax agglomerates do not settle out and no change in pour point results. Generally, a centrifuge operating between 6,000 and 13,000 gs is used. The present commercial centrifuges that are available operate in the range of 7,000 to 10,000 gs and these are the units that we prefer to use. Utilizing equip- I ment of this type, high throughout rates of crude may be attained. Residence times as low as a few seconds up to 5 minutes or more have been used. Preferably, residence times of about 2 seconds to about 1 minute are used. The particle size of the asphaltene-wax agglomerates that are separated by the centrifuge generally range in the size of 30 to about 500 microns. A centrifuge operating at about 7600 gs obtains a 5 to 6% separation of waxy material, based on entire weight of crude, at a flow rate of 2000 b./d. This relates to a residence time of about 10 seconds.

Conventional apparatus may be used to provide continuous heating of the waxy crude to be treated to a temperature up to 220 F. and also provide controlled cooling of the oil to the centrifuge temperature. In order to conserve heat in the process, the residual oil fraction separated after centrifugation may be recycled through appropriate heat exchange equipment to the pretreatment heating zone to assist in raising the temperature of the waxy crude in the pretreatment step.

The mechanism causing the variation of residual fuel pour point with thermal history has been postulated which involves the action of natural pour inhibitors, asphaltenes and resins, on wax. When properly dispersed, these asphaltenes and resins cause the agglomeration of wax crystals into large clusters, which when centrifuged produce a fuel that has a low pour point. When the asphaltenes are not properly dispersed, the wax forms a gellike structure of fine crystals which cannot be centrifuged, resulting in a high pour state. The presence of large crystal agglomerates at the lower pour point and the existence of a gel-like network of small crystals at the high pour point were confirmed by photomicrographic studies. Residual fuels, therefore, have pour points that are sensitive to temperature history because the extent of dispersion of the natural inhibitors is affected by prior thermal history.

In order to determine the residuum components that are responsible for the inhibition of pour points, a sample of visbroken Zelten 680 F.+resid., having a sensitive pour point, was separated into several key component groups, containing asphaltenes, aromatic resins, saturated oil and wax. (See FIGURE 3 of the drawings.) Pour point data on blends of these components showed that, when present together, asphaltenes and resins interact synergistically by means of a peptization mechanism to form a potent pour point depressant. When properly dispersed, this depressant can produce low pour point fuels. Taken separately, asphaltenes and resins are only moderately effective as pour inhibitors and the pours of the resulting fuels are independent of temperature history. Of the two, asphaltenes are slightly more potent inhibitors and probably reduce pour by means of a powerful surfactant effect. Aromatic resins, on the other hand, act either as very weak surfactants or as diluents. In order to be effectively dispersed, the asphaltenic inhibitors must be liberated from previously formed wax-asphaltene agglomerates, and this occurs when the fuel is heated slightly above the temperature sufficient to dissolve the highest melting waxes present.

FIGURE 1 shows the criticality of the centrifugation temperature on the pour point of preheated whole Zelten crude. Zelten crude having a 55 F. upper pour was preheated to 200 F. and centrifuged at the indicated temperatures. Thhe centrifuge was operated at 13,000 gs and the residence time was 15-20 seconds. It can readily be seen that for Zelten crude, an unexpected reduction in pour point can be obtained by centrifuging, after preheating, at a temperature of 4050 F. while, at the same time, minimizing the amount of resid. fraction loss to the dense phase.

FIGURE 2 of the accompanying drawings shows the effect of the natural pour inhibitors; i.e., asphaltenes and resins, on whole visbroken resid. The curved line shows the effect of reheat temperature on the pour point of whole visbroken resid. The top line shows that reheat temperature has no effect on pour point of the resid. in the absence of the natural pour inhibitors. The middle line shows that the asphaltenes have a slightly better effect on pour point than the resins. It is noted, however, with only the resins or asphaltenes present that the reheat temperature has little or no effect on pour point. The data was presented to show the synergistic effect of the asphaltenes and resins on the pour point of a whole crude which was preheated up to 220 F. The effect of preheating on pour points is characteristic of the waxy crudes which may be treated in accordance with our invention.

Table I, below, shows the effectiveness of reducing the pour point of a waxy crude and the resulting improvement in reduction of pour point of a resid. fraction obtained .by using the process of this invention.

1 Whole Zelten crude having a pour of 55 F. was heated to either 200 F.

or 115 F. and cooled to the optimum centriiugation temperature in each case. 2 Attempts to centrifuge virgin Zelten 680 resid. gave no separation independent of thermal pretreatment of entire crude. The high viseositv of the resid. fraction (i.e., 500 SSU at F. vs. 70 SSU for crude at 50 1 2 at temperatures close to the upper pour make it practically impossible to centrifuge the resid. fraction.

The above data shows that the process of the invention will reduce the pour point of the entire crude and of the residual fraction by about 20 F.

Table II is a presentation of data showing the effects of centrifugal force, temperature of centrifugation, residence time, and thermal history of the crude treated on the percentage of material separated and pour point.

charged to pipestill 8 which separates the dewaxed crude into the desired fractions. A C fraction is taken overhead through line 9. A naphtha and middle distillate fraction are taken off at 10a and 10b. A 650 to 750 TABLE II Continuous Pilot Unit Relative centrifugal force, qs

13000 r sd tm 2 Whole Zelteu Centrif. e 1 ence 1 8 (Sec) E g og?) crude pretreattemp.

ment (F) Pour 1 Wt. Pour Wt. Pour Wt. Pour Wt. Pour Wt. Pour Wt.

percent 2 percent percent percent percent percent Heat to 115 F 43 55 0 70 55 0.2 None 70 55 0.3 Heat to 200 F 20 45 12. 0

1 AS TM upper pour point in F. of light phase from centrifugation. 2 WI}. percent dense wax-rich phase sedimented during centrifugation.

. These data show that the centrifuging operation is selective for removal of the material in the crude causing the high upper pour. It can be concluded that the thermal pretreatment of the feed to the centrifuge and the temperature of centrifugation both must be carefully controlled. Pretreatment of the feed by heating through a cycle of ambient temperature to up to 220 F. and cooling to the centrifuge temperature is best because this places the feed in a high fluidity state with the higher melting point waxes in large agglomerates for easy removal. On the other hand, heating from ambient temperature to 115 F., for example, and cooling to the centrifuge temperature produces small individual wax crystals which do not separate easily by centrifugation. centrifugation temperature is important in order to opti; mize conditions of crude viscosity and wax agglomerate size to yield maximum selectivity for separating the high pour wax. This occurs in the range of 40 to 50 F. If the centrifuge temperature is too high, the wax agglomerates will be too small to be selectively separated. In fact, at 'sufliciently high temperatures, the heavy asphaltic materials which act as pour inhibitors may be preferentially separated and the pour of the crude can actually be increased. At temperatures below about 40 F., the selectivity of wax separation progressively decreases because of separation of viscous nonwaxy material with the Wax-rich fraction. Moreover, if the temperature is sufficiently low, the viscosity of the oil will become so high that solid particles can no longer move through the medium and no separation will be possible. This is brought out by the data presented in FIGURE 2 of the drawings.

An understanding of the various aspects of the invention may be had by referring to FIGURE 3 of the accompanying drawings and the discussion thereof. This drawing is a schematic flow diagram showing one arrangement of apparatus which may be used in the practice of one embodiment of the invention.

Referring now to the drawing, a Middle East or North African crude containing 6 to 10 wt. percent of wax, based on the crude, is charged to preheater 2 through line 1 where it is continuously heated to a temperature between 190 and 210 F. The heated crude is withdrawn through line 3 and introduced to cooler 4 where it is cooled to a temperature between 40 and 60 F. The cooled crude is then charged to centrifuge 6 through line 5 wherein it is continuously centrifuged under a force of 7,000 to 13,000 gs for a residence time of 5 to seconds. The main oil is taken off through line 7 and F. resid. fraction is withdrawn through line 11a and may be cycled through a heat exchange to the preheater to partially heat the incoming crude. The crude charged to preheater 2 has a pour point between 45 and 55 F.; whereas, the main oil withdrawn through line 7 has a pour point of 25 to 35 F. The wax-rich fraction withdrawn through line 12 from centrifuge 6 may be sent to a conventional pipestill 13 where it is separated into various desired fractions. For example, an overhead fraction containing 680 F. is withdrawn through line 14 and would constitute 40% by volume of the wax-rich fraction. A 1100 F. l-l-I fraction is withdrawn through line 16 which would constitute 10% by volume of the wax-rich fraction. A very good feed for a steam cracker, 680/ 1100 R, which constitutes about 50% by volume of the Wax-rich fraction and which is almost all wax, is withdrawn through line 15. The pour point of the resid. fraction is about 75 to F., as compared to a pour point of F. of the resid. fraction obtained from the untreated crude.

This process of selectively removing the waxes which cause high pour point in crudes and resid. fractions may be utilized to efficiently and economically reduce the pour point of crudes for handling at low temperatures, as well as improving the pour point of the resid. fractions so that they themselves may be more easily handled as residual fuels. The residual fuels thus improved in pour point can, of course, be blended with other oils of lower pour point to make a more marketable product. Further, applicants process of utilizing the natural pour inhibitors present in the crude may be used to remove wax from other high wax concentration fractions by combining the crude with said high wax content fraction and carrying out applicants process wherein waxes which cause high pour point are selectively removed.

The invention is illustrated by the following example:

A Zelten crude containing 7 wt. percent Wax, based on volume of crude, was processed in accordance with this invention. The entire crude was preheated at atmospheric pressure to a temperature of 200 F. The lighter ends that distilled off were refluxed and returned to the preheater. The crude had an initial pour point of 55 F. The preheated crude was cooled rapidly to a temperature just above its pour point and then cooled at a rate of 1 to 2 to the centrifugation temperature of 43 F. The cooled crude was continuously centrifuged in a conventional centrifuge operating at 13,000 gs with a residence time of about 6 seconds for the crude in the centrifuge. A wax-rich phase was separated and amounted to about 7.5% by weight, based on the crude. The main oil separated had a pour point of 35 F, and a 680 F. H-I residual oil fraction distilled from the crude had a pour point of about 80 F. This pour point represents a reduction in pour in the residual fraction of about 25 F.

The next most economical method of improving the pour point of this residual oil fraction that has been tried is thermal cracking of the crude. This process results in obtaining an equivalent reduction of pour point in the resid. fraction, as obtained with this inventive process. However, it is nonselective to the high pour point waxes and results in cracking up to 40% of the residual fraction which is a desired product. Therefore, though an equivalent pour point is o tained, almost 40% of the desired product is cracked to lighter products and, there fore, not available as residual oil.

In accordance with our invention, the waxes responsible for high pour point in the residual fraction may be selectively removed from the crude with a minimum loss of resid. This process is carried out in relatively inexpensive equipment and in an efiicient and economical manner. By carefully controlling the thermal history of a high waxy crude and centrifuging at a critical temperature, the natural pour inhibitors in crude oil are utilized to selectively separate the undesirable high pour waxes.

The invention is not intended to be restricted by the described theory or operation, or the examples given.

What is claimed is:

1. An improved process for the refining of a crude oil of high pour point, containing wax and asphalt fractions, to reduce its pour point, which comprises heating an entire crude to a temperature of from 160 to 220 F., cooling the oil to a point just above the cloud point and thereafter slowly at a controlled rate until a temperature between 35 and 55 F. has been reached thereby precipitating a portion of the said wax fraction from the said crude oil, centrifuging the precipitated wax agglomerates at a temperature between 35 and 55 F. to separate a dense wax phase from the crude, and recovering a crude of substantially lower pour point.

2. The process of claim 1 wherein the controlled cooling is at a rate of from one-half to two degrees Fahrenheit per minute.

3. The process of selectively removing Wax which causes high pour points in crudes containing 3 to 12 wt. percent wax based on crude corresponding to residual fractions containing 7 to 25% wt. Wax, based on residual fraction comprising heating the entire crude to a temperature of 160 to 220 F., cooling the heated crude to a point just above the cloud point and thereafter at a controlled rate to a temperature of -55 F., and centrifuging the cooled crude at a temperature of 35 to 55 F. to separate a dense wax phase and a dewaxed crude oil substantially reduced in pour point.

4. The process of claim 3 wherein a 650 to 750 F.+ residual fraction is separated from the dewaxed oil.

5. The process of claim 3 wherein the crude is reduced in pour point by about 15-25 F.

6. The process of claim 3 where the 650 to 750 F. residual fraction is reduced in pour point by 15-25" F.

7. A process for improving the pour point of a Zelten crude containing about 6-10 wt. percent of wax, based on crude, and of the 680 F.+ resid. fraction containing about 19-22 wt. percent of wax, based on resid., comprising heating the entire Zelten crude to a temperature of about 180-210 F., shock cooling the heated crude to just above its cloud point, then cooling at a controlled rate of 0.5 to 2 per minute to a temperature of about -50" F. and continuously centrifuging the cooled crude at a temperature of 40 to F. to separate a dense Wax phase and a main oil substantially reduced in pour point.

8. The process of claim 3 wherein the dense wax phase consists of 68 wt. percent, based on crude, of waxy material.

References (Zited in the file of this patent UNITED STATES PATENTS 369,902 Aab et al Sept. 13, 1887 1,416,890 Sharples May 23, 1922 1,732,143 Travis Oct. 15, 1929 1,974,398 Ellsberg Sept. 18, 1934 2,100,662 Lyman et al Nov. 30, 1937 2,770,577 Stossel Nov. 13, 1956 3,038,854 Kiersted June 12, 1962 FOREIGN PATENTS 851,135 Great Britain Oct. 12, 1960

Claims

1. AN IMPROVED PROCESS FOR THE REFINING OF A CRUDE OIL OF HIGH POUR POINT, CONTAINING WAX AND ASPHALT FRACTIONS, TO REDUCE ITS POUR POINT, WHICH COMPRISES HEATING AN ENTIRE CRUDE TO A TEMPERATURE OF FROM 160 TO 220*F., COOLING THE OIL TO A POINT JUST ABOVE THE CLOUD POINT AND THEREAFTER SLOWLY AT A CONTROLLED RATE UNTIL A TEMPERATURE BETWEEN 35 AND 55*F. HAS BEEN REACHED THEREBY PRECIPITATING A PORTION OF THE SAID WAX FRACTION FROM THE SAID CRUDE OIL, CENTRIFUGING THE PRECIPITATED WAX AGGLOMERATES AT A TEMPERATURE BETWEEN 35 AND 55*F. TO SEPARATE A DENSE WAX PHASE FROM THE CRUDE, ADN RECOVERING A CRUDE OF SUBSTANTIALLY LOWER POUR POINT.