Preparation Of Mineral Free Asphaltenes

Abstract

Disclosed herein is an invention directed to a method of deasphalting in which a desired amount of asphalt is removed by adjusting the solvent power of the solvent-feed system to obtain a desired cotangent theta for the system. Also disclosed is the application of the cotangent theta principle to two-stage deasphalting of Athabasca bitumen.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to a method of removing mineral contaminants from petroleum fractions. More particularly, the present invention is concerned with the deasphalting of crude petroleum fractions and the removal of mineral contaminants during a deasphalting operation. The invention is particularly useful in treating bitumen recovered from bituminous sands.

Large deposits of bituminous sand are found in various localities throughout the world. The term, bituminous sand, is used herein to include those materials commonly referred to as oil sand, tar sand and the like. One of the most extensive deposits of bituminous sand occurs, for example, in the Athabasca District of the Province of Alberta, Canada.

Typically, the composition of these sands by weight is: from about 5 percent to about 20 percent oil; from about 1 percent to about 10 percent water; and from about 70 percent to about 90 inorganic solids. The specific gravity of the bitumen varies from about 1 to about 1.05 at 60.degree. F. The major portion of the inorganic solids, by weight, is a fine grain porous sand, having a particle size greater than about 45 microns and less than 2,000 microns. The remaining inorganic solid matter has a particle size of less than 44 microns and is referred to as fines. The fines content typically varies from about 5 percent to about 30 percent by weight of the solid inorganic content of bituminous sand. However, it is not uncommon for the composition of bituminous sand to vary from the above-mentioned ranges. Also, in mining the bituminous sand, clay, which is found in layers of varying thickness in such sand areas, may be mixed with the bitumen, thus increasing the inorganic solids content, particularly the fines content of the material to be processed. The fines portion contains minor amounts of various metals including, for instance, titanium and zirconium. Such metals are sometimes present in economically recoverable concentrations. In any event, the presence of such metals in recovered bitumen is frequently considered undesirable, especially in connection with upgrading of the bitumen to saleable products.

In refining crude petroleum, including bitumen recovered from bituminous sands, a variety of processes are available for converting the lower boiling distillate portion of the crudes into more valuable products. Materials boiling below about 750.degree. F. are usually recovered by atmospheric distillation and materials boiling up to 950.degree. to 1,150.degree. F., or higher, by vacuum distillation. The remaining residuum generally contains high concentrations of the high molecular weight organic compounds with sulfur, nitrogen, oxygen, metals and other non-hydrogen species, as well as high molecular weight hydrocarbons, including condensed ring aromatics. The non-hydrocarbon compounds are often poisonous to catalysts used in upgrading processes with metal compounds or mineral contaminants being particularly deleterous to cracking catalysts.

The residual portions of crude petroleum are sometimes described in terms of relative solubility as comprising: firstly, a pentane soluble heavy oil fraction resembling distillate except for its high molecular weight and boiling point; secondly, a less soluble resin or maltene fraction; and thirdly, a pentane insoluble asphaltene fraction. The term "asphaltenes" as used herein refers to material which is insoluble in pentane under temperature and pressure conditions used for the extraction. When so extracted, the asphaltenes separate as solid particles or granules. "Resins" are less clearly defined terms in the art and the term is used herein to describe that portion of the bitumen which adheres to the insoluble asphaltene particles, but which may be solubilized by a further extraction with pentane. The term "deasphalted heavy oil" is used herein to describe that portion of the crude petroleum or bitumen that is solubilized by the first extraction with a solvent.

The deasphalting of petroleum crude is well known in the art. Descriptions of deasphalting operations may be found in U. S. Pat. No. 3,278,415, by Doberenz, et al; U. S. Pat. No. 2,188,013, by Pilat, et al; and U. S. Pat. No. 2,853,426, by Peet. In the above processes, the metal contaminants are normally remove together with the resins and asphaltenes and, depending upon their origin, generally about 60 percent of the crude, by addition to the crude of an excess (from 2 to 10 volumes) of a low boiling liquid hydrocarbon. Propane, propylene, normal or isobutanes, butylenes, normal or isopentanes, hexanes, or their mixture, light straight run naphthas, or other light aromatic free fractions of mineral oil have been claimed to be satisfactory solvents.

However, the above art teaches that the crude or asphaltic materials yield low recoveries of deasphalted heavy oil, generally below 40 percent of the residuum, and it has been stated that if high ratios of solvent to crude are utilized, to obtain oil yields above about 60 percent, excessive amounts of metal compounds and some asphaltenes appear in the extract phase.

It is an object of the present invention to provide a process for the removal of metal contaminants from an Athabasca bitumen using normally liquid or liquefied hydrocarbon solvents.

It is a further object of the present invention to provide a process whereby hydrocarbon solvents, for example the isomeric and paraffinic hydrocarbons having from three to eight carbon atoms, saturated substituted or unsubstituted cycloparaffins having five or more carbon atoms are utilized to deasphalt petroleum crude.

It is still a further object of the present invention to provide a process for deasphalting the Athabasca bitumen whereby the deasphalting medium is adjusted by the addition thereto of any of the aforementioned solvents employed either individually or as blends of two or more individual solvents.

It is still a further object of the present invention to provide a process whereby a low mineral content asphaltene fraction is produced which may be employed in the manufacture of high grade metallurgical coke, electrocarbon, or other processes wherein a low mineral content is a necessary prerequisite.

With these and other objects in mind, the present invention may be more fully understood by specific referral to the following discussion and drawings.

SUMMARY OF THE INVENTION

The objects of the present invention are accomplished by a process for treating an asphaltene containing petroleum crude which may be contaminated by both metals and water. The petroleum crude may contain a considerable water concentration, being represented by a froth. The process comprises the introduction, singularly or in multiple steps, of a normally liquid or liquefied hydrocarbon solvent into the asphaltene containing petroleum crude so as to adjust the total asphaltene solvent power of the system. By asphaltenes is meant those hydrocarbons which are pentane insolubles. By solvent power of the system is meant the ability of the total hydrocarbon system, including solvent, to dissolve and contain the asphaltenes present.

In general, the process uses hydrocarbon solvents which may be selected from the group consisting of one or more paraffinic or isomeric hydrocarbons having from three to eight carbon atoms and saturated substituted cycloparaffins and saturated unsubstituted cycloparaffins having five or more carbon atoms. The solvent power of the system including solvent and feed is adjusted by latering the proportions of the solvent or the solvent type according to the required cotangent .theta., as described hereafter, so as to yield a desired solvent power for the total system of solvent and petroleum crude. In general, in treating petroleum crude, for example an Athabasca bitumen, it is preferred that from about 5.0 to about 40.0 barrels of solvent are introduced in each step of the process per barrel of liquid to be treated.

In a two-step process, a solvent having a high solvent power is introduced to form two fractions. The first fraction preferably contains about 10.0 to about 25 weight percent of the asphaltenes and resins contained within the petroleum crude, at least 85.0 weight percent of the water and at least 98.0 weight percent of the mineral contaminants. A liquid fraction or heavy oil is formed which contains the remaining mineral contaminants and asphaltenes originally present in the petroleum crude. Subsequently, the two fractions are separated and a second normally liquid or liquefied hydrocarbon solvent forming a low solvent power system is introduced into the liquid fraction, thereby forming two additional liquid fractions. A first additional liquid fraction is formed containing the remainder of the water, mineral contaminants and asphaltene content of the heavy oil and a second additional liquid fraction is formed consisting essentially of a mineral, water and asphaltene free heavy oil. These two additional liquid fractions are then separated to form the desired products of the present invention, that is, a heavy oil significantly free of water or mineral contaminants in a solvent phase and an asphaltene fraction of the bitumen substantially free of mineral and water contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more fully understood by referral to the following drawings in which:

FIG. 1 represents the relationship between as phaltene percent recovery and the Cot .theta. of the total system for determining solvent type and quantity of solvent introduced for use in the process of the present invention; and

FIG. 2 represents one embodiment of a two-step process utilizing the solvent power relationship of the present invention for treating a petroleum crude to remove the asphaltenes therefrom and concentrate and remove the metal contaminants and water.

DETAILED DESCRIPTION OF THE INVENTION

Solvent power is generally measured as the solubility of an oil in a particular solvent system. As the asphaltenes are the least soluble components of the oil, it is necessary to define solvent power as the measurement of the critical solubility of the asphaltenes in given oil systems containing solvents so that deasphalting and deimineralizing of asphaltene containing oils may be regulated. Conventional deasphalting processes for the deasphalting of heavy oils by pentane or other paraffinic solvents generally utilize the solvents singularly, wherein the asphalts are all concentrated in one deasphalting tower, but in most instances the asphaltenes recovered having a high mineral content.

It has been found that particularly useful solvent combinations may be formed by the mixing of one or more high power solvents with heavy oils in definite relationships, so that exact precipitation of asphaltenes in deasphalting tower occurs. Specifically, the yield of asphaltene precipitate obtained from the heavy oil is related to solubility of the normally precipitating asphaltenes in the solvent system. The major influence on precipitation is governed by the solvent power of the single solvent or solvent blend, in conjunction with the heavy oil system, with the contribution of the solvent powers of the soluble oils and resins in the heavy oil being considered. Additional information concerning solubility characteristics may be found in "Regular Solution" by Hildebrand and Scott, published by Prentice Hall in 1963 (Library of Congress Catalogue No. 62-11984).

The relationship of asphaltene solvent power of the heavy oil-solvent precipitating medium may be expressed by the solubility parameter Contangent .theta. (Cot .theta.). Cot .theta. is derived from experimental data in which the percent asphaltenes precipitated from a system for each solvent or solvent blend mixture with the heavy oil is plotted against the individual solvent concentrations or blend concentrations in the system, thereby deriving a definite linear relationship. Cot .theta. being the contangent of the angle .theta. formed between the X-axis or the axis containing the solvent concentration and the linear line formed by the experimental data. The Cot .theta. relationship and means for determining .theta. for deasphalting system is fully presented by J. A. Bichard, Oil Solubility, 19th Canadian Chemical Engineering Conference and 3rd Symposium on Catalysis, Oct. 19-22, 1969, Edmonton, Alberta. As originally conceived, the Cot .theta. relationship was intended to provide a means for insuring that undesirable precipitation would not occur. Using this concept, it was found that precipitation would be precluded by maintaining the Cot .theta. of the system at a positive value. It has now been found that by use of the Cot .theta. relationship, solvent power relationships may be placed on a single functional graph as illustrated by FIG. 1. One may choose the desired percent of asphaltenes to be precipitated from FIG. 1 and thereby determine what Cot .theta. relationship is required for the system, thus adjusting the solvent content of the system to the exact specifications required to obtain the necessary Cot .theta. relationship in the system and thereby control the degree of precipitation of asphaltenes.

The significance of selecting Cot .theta. for deasphalting becomes apparent when one realizes that a solvent system exhibiting a negative value yields an excellent asphaltene precipitation. If the solvent system exhibits a very low solvent power, its respective Cot .theta. is liable to lie close to the X-axis and exhibit a value of minus infinity. If the system exhibits a good solvent power, it will lie closer to the Y-axis. A good solvent has a positive value by definition, the cotangent .theta. having a value greater than zero. Therefore, a solvent having a cotangent .theta. less than zero will be a relatively poor solvent. It is preferred that the cotangent .theta. of the solvent and oil system utilized in the deasphalting of heavy oil be less than about minus 0.3 if a significant degree of asphaltene precipitation is desired.

To fully understand the meaning of solvent power for the Athabasca oil systems, it is necessary first to consider the components, composition and structure of the oil. The existence of saturates, aromatics, resins and asphaltenes in the oil has been measured showing the oil to be a mixture of isogels or heterogels, a structure composed of an ordered state consisting of asphaltenes surrounded by still higher molecular weight aromatic type hydrocarbons with layers of resin molecules about them. These molecules form micelles in an oil saturated environment. Weak inner molecular attraction and repulsion forces between the nonpolar hydrocarbons are essentially responsible for the structure. The process of solution proceeds by the addition of solvent toward the nucleus. These components held by the weakest association forces dissolve first. The energy supplied to overcome the association forces of the micelles is supplied by the solution energy of the solvent system. The solution energy is different for different solvent systems and is a function of chemical structure. Paraffin containing solvent systems have the least amount of solution energy, with solution energy decreasing with increasing molecular size, this is due apparently to parallel self-association.

Polarizability of aromatics, such as benzene, result in greater dispersion forces and hence, high solution energy. This is even more prevalent with condensed aromatics with side chain substitution. Partial moments through substitution are responsible for the good solvent powers of chlorinated solvents. When the solution energy of the solvent molecules is insufficient to overcome the strong cohesive forces acting on the nucleus of the micelle, floculation or precipitation of these nuclei occur. The experimental end point of asphaltene solubility is a measure of the critical cohesive energy of the system at which the solvation of the nuclei can no longer be maintained and precipitation of the asphaltene occurs. Therefore, in deasphalting it is desirable to obtain the effective rate of floculation required such that the exact Cot .theta. or solvent blend in the oil system may be chosen whereby an exacting percent asphaltene will be precipitated in the deasphalting tower. In multiple stage deasphalting for the treatment of materials such as metal contaminated Athabasca bitumens, it is preferred that the first deasphalting tower exhibit a relatively low asphaltene precipitation with a high precipitation of the minerals contained therein. Between about -0.3 and -0.6 Cot .theta. would be preferred in the first deasphalting tower, while the second and subsequent deasphalting towers would utilize a lower Cot .theta. such as between about -0.3 and -2.0, preferably between about -1.4 and -2.0 so that a significant precipitation of asphaltenes, comparatively mineral-free, will be obtained with a low asphaltene content in the oil-solvent mixture effluent produced from the top of the deasphalting tower.

Through use of FIG. 1, in which is illustrated the relationship of asphaltene yield and solvent power of the precipitating medium as expressed as a solubility parameter Cot .theta., it has been shown that the more concentrated the solution of the bitumen in the solvent, the more limited the solvent power of the system becomes so that the solvent cannot retain as large a quantity of the asphaltenes. As bitumen often contains considerable aromatic constituents, which act in a similar manner as the added solvents, these properties must also be considered in obtaining the Cot .theta. of the solvent system shown in FIG. 1. The Cot .theta. of the system is a function of the type and amount of solvent utilized, as well as the type and amount of oil being treated.

In the processing of Athabasca tar sands for the formation of useful refinery products, one finds a residual product of asphaltenes and ash content being made up of mineral contaminants in a water froth mixture. This mixture is extremely difficult to decompose into the desirable product of a heavy oil for coking or catalytic cracking and asphaltenes free from minerals. The minerals are recovered for their economic value and the asphaltenes separated as a suitable source for asphalt and derivative materials for use in commercial processes. Required is a solvent dilution technique whereby the heavy oils and asphaltenes may be separated and the asphaltenes then separated from the minerals contained therein for the reduction of the bitumen froth to commercially desirable products.

In accordance with a preferred embodiment of the invention, a heavy crude oil such as Athabasca bitumen which is contaminated by substantial amounts frequently exceeding 3 weight percent of mineral contaminants is treated in a two-stage process to produce substantially deasphalted heavy oil and an asphaltene product sufficiently free of minerals to be useful in the manufacture of metallurgical grade coke. In the first stage, the feedstock is contacted with hydrocarbon solvent of a type and quantity chosen to adjust the Cot .theta. of the system to between about -0.3 and about -0.6. This results in two first stage product fractions, one of which is an asphaltene fraction comprising between about 10 and about 25 weight percent of the asphaltenes present in the feedstock (in the case of Athabasca bitumen frequently between about 0.5 and about 10 wt. percent of the total bitumen) and which contains more than about 90 wt. percent, preferably more than about 98 wt. percent, of the total minerals content of the feedstock. The second first-stage product fraction is a heavy oil fraction which contains the remainder of the asphaltenes and solvent. Where the feedstock is Athabasca bitumen, this fraction usually contains between about 90 and about 98 wt. percent of the total bitumen feedstock and less than 0.1 percent of the minerals originally present in the bitumen. This heavy oil fraction is then subjected to a second-stage deasphalting treatment with hydrocarbon solvent in which the solvent power of the system is adjusted to a Cot .theta. less than about -0.3 and preferably less than about -0.8 to produce a second-stage asphaltene precipitate containing less than about 0.5 wt. percent minerals and which is therefore of sufficiently low mineral content for use in the manufacture of metallurgical coke. In order to obtain maximum yields of such substantially mineral-free asphaltenes, it is preferred that the Cot .theta. of the second-stage deasphalting unit be maintained less than about -1.4, preferably between about -1.4 and -2.0 to insure precipitation of at least 90 and preferably 98-100 percent of the asphaltenes present in the second-stage feed. Maintaining the Cot .theta. of the system less than about -1.6, may result in precipitation of some materials such as resins which are soluble in pentane in addition to precipitation of pentane-insoluble asphaltenes. Where Athabasca bitumen is the original feed, the second-stage asphaltene product frequently will be equivalent to between about 5 and about 10 wt. percent of the original bitumen. The other second stage product is, of course, a deasphalted heavy oil fraction which is substantially free of minerals and preferably also substantially free of asphaltenes and therefore does not present problems during further refining operations.

In another preferred embodiment of the invention, material such as Athabasca bitumen having a high water content, as well as a high mineral content, is treated by a two-stage process similar to that described above. For instance, in typical processes for recovery of bitumen from Athabasca tar sands, the bitumen is recovered in the form of a froth containing between about 5 and about 30 wt. percent by water. In treating such a froth by the two-stage process described immediately above, the majority and usually between 85 and 100 percent of the water present will be found in the first-stage asphaltene product. It can thus be seen that the treatment of such bituminous froth as described above has the added advantage of removing water, as well as minerals, from the useful products, i.e., the second stage deasphalted heavy oil product and second-stage asphaltene product, thus eliminating the need for separate processing steps for removal of water.

In practicing the invention, from about 0.5 to about 40 barrels per barrel of oil treated of either isomeric paraffin hydrocarbons having five or more carbon atoms or saturated or unsaturated substituted cycloparaffin hydrocarbons are frequently used to adjust the solvent power of the deasphalting system. The invention relates to the use of any one or more of the aforementioned hydrocarbon types to adjust the system's solvent power in the enhancement of the deasphalting process. The deasphalting process is preferably conducted at from about 50.degree. F. to about 140.degree. F. and at atmospheric pressure.

To more fully understand the process of the present invention, FIG. 2 is presented in which a two-step deasphalting and demineralizing process for treating bitumen recovered from tar sands is depicted. In the process, bitumen 101 is introduced simultaneously with a hydrocarbon solvent 102 into a deasphalting tower 103. The solvent power of this system is preferably adjusted to a Cot .theta. of between about -0.3 and about -0.6. A solvent-heavy oil mixture 105, containing the majority of the asphaltenes, is produced from the top of the deasphalting tower 103 and an asphaltic residue 104, preferably containing greater than 90 percent of the minerals, is produced from the lower portion of the deasphalting tower 103. The solvent-heavy oil stream 105 is then introduced into a second deasphalting tower 107 simultaneously with additional hydrocarbon solvent 106. For maximum recovery of mineral-free asphalt, the solvent power of the second-stage deasphalting system is preferably adjusted to a Cot .theta. between about -1.4 and -2.0. A solvent-oil mixture 109 is produced from the top of deasphalting tower 107 and asphalt 108, being substantially mineral-free, is produced from the lower portion thereof. The solvent-oil mixture 109 is then fed to a fractionating tower 110 from which a mineral free-cracking stock oil 111 is produced from the lower portion thereof and solvent vapor 112 is produced from the upper portion thereof. The solvent vapor is then introduced into a condensor 113 from which a liquid solvent stream 114 is produced. The liquid solvent stream may be used for solvent 102 and solvent 106 makeup in the first and second deasphalting towers 103 and 107, respectively.

To exhibit the use of solvent power adjustment of the system to control the yield of precipitated asphaltenes, the following Examples are presented:

EXAMPLE 1

The solvent powers in terms of Cot .theta. of various systems of solvents and oils were determined by mixing one-to-one volumes of normally liquid or liquefied solvents with an Athabasca bitumen containing 17 wt. percent asphaltenes. A measured amount of 100 grams of each solvent was poured into 130 grams of a water-bitumen froth, equivalent to 100 grams of dry bitumen. The mixture was shaken vigorously for 5-10 minutes at 0.5 hr. intervals and allowed to settle; this procedure was conducted for approximately 8 hours. At the end of the 8 hour period the fractions were easily separated by decantation followed by filtration, with light suction of the solvent/heavy oil solution.

After removal of any residual solvent by blowing, the water content of the asphaltene fraction was determined by the standard ASTM D-9558 method for the determination of water in petroleum products. For the heavy oil fraction formed, the solvent was removed by blowing followed by evaporation and the water separated mechanically.

Results of this analytical procedure are set forth in Table I as follows:

TABLE I

Results of Deasphalting Bitumen

Solvent Power Weight Percent of System Asphaltenes Solvent (Cot .theta.) Precipitated Propane -(2.5 to 100 3.5) est. Butane -2 100 N-Pentane -1.5 100 Hexane Isomers -1.2 75 Heptane Isomers -1.1 67 Pentane/Hexane -1.3 83 Pentane/Heptane -1.2 75 Pentane/Cyclopentane (1:1) -0.7 35 Pentane/Cyclohexane (1:1) -0.7 35 Cyclopentane -0.1 1 Cyclohexane 0 0 Heptane/Cyclopentane (1:1) -0.5 22 Heptene/Cyclohexane (1:1) -0.5 23

EXAMPLE 2

A solution of bi-constituent solvent consisting of liquid propane and varsol was utilized in a sample of Athabasca tar bitumen in order to determine the percent asphaltenes precipitated. As liquid propane itself precipitates asphaltenes plus resins and possibly other fraction of bitumen, initial concentrations with 100 percent propane solvent were determined. Yields of precipitated materials ranged from between 40 to 48 percent with liquid propane used in large excess. The solution of the propane with varsol produced a solvent blend of increased solvent power. Consequently, the yield of precipitated material decreased. For example, a solvent bitumen weight ratio of 3.5 to 1 and a propane-varsol concentration of 2.5 to 1 resulted in a 30.7 percent yield of precipitate at a Cot .theta. of -2.6. In a similar manner, when bitumen was present in a higher concentration, the contribution of the soluble aromatic portion was added to the solvent power, the precipitating media in the yield of precipitating material decreased as the Cot .theta. value of the system increased. In the second case, an approximate propane-varsol weight ratio of between 1.5 and 3.5 to 1 was blended as a solvent. The bitumen ratio was between 1.0 and 1.6 to 1 as compared to four to one in the first instance and the Cot .theta. was -2.5. The yield of precipitating material was 18.4 percent in this case, showing considerable decrease in precipitate as compared to the 30.7 percent as illustrated in the first case.

Therefore, it can be seen through this experimentation and FIG. 1 that the amount of asphaltene precipitate can be varied over a wide range, from say, approximately 0.5 percent to 100 percent of the total asphaltene content of the system, dependent upon the adjusted solvent power of the precipitating medium or solvent system. The inherent advantage of this control is that subsequent crops of asphaltenes can be precipitated as essentially mineral-free material in a second deasphalting tower with the majority of the minerals precipitated with a first crop of material in a first deasphalting tower.

The process of the present invention discloses a technique whereby suitable solvents such as paraffinic and isomeric hydrocarbons having from three to eight carbon atoms and saturated substituted or unsubstituted cycloparaffins having five or more carbon atoms may be blended in exacting compositions of solvents for the use in the deasphalting of bitumen or petroleum crudes containing mineral and water contaminants and high asphaltene concentrations.

The present invention has been described herein with respect to particular embodiments and aspects thereof and it will be appreciated by those skilled in the art that various changes and modifications can be made, however, without departing from the scope of the present invention.

Claims

Therefore, we claim:

1. A process for deasphalting a petroleum crude oil feedstock containing asphaltenes and mineral contaminants which comprises:

a. contacting said petroleum crude with hydrocarbon solvent in a first deasphalting zone maintained at a cot .theta. solubility parameter between about -0.3 and about -0.6, whereby between about 10 and about 25 wt. percent of the asphaltenes are precipitated together with about 90 to 100 wt. percent of the mineral contaminants; and

b. recovering said precipitate from said first deasphalting zone;

c. recovering from said first deasphalting zone a heavy oil fraction containing between about 75 and about 90 wt. percent of the asphaltenes originally present in the feedstock and less than 10 wt. percent of the mineral contaminants present in such feedstock;

d. contacting said heavy oil fraction from the first deasphalting zone with hydrocarbon solvent in a second deasphalting zone wherein the cot .theta. solubility parameter is maintained less than about -0.8;

e. recovering from said second deasphalting zone an asphaltene fraction containing between about 70 and about 90 wt. percent of the asphaltenes originally contained in the feedstock together and less than about 0.5 wt. percent of the mineral contaminants present in such feedstock; and

f. recovering a deasphalted heavy oil fraction from the second deasphalting zone.

2. The process of claim 1 in which the hydrocarbon solvents are formulated as a single solvent or solvent blend selected from the group consisting of paraffinic or isomeric hydrocarbons having from three to eight carbon atoms, saturated substituted cycloparaffins having five or more carbon atoms and saturated unsubstituted cycloparaffins having five or more carbon atoms with the concentration of the solvent or solvent blend introduced to form the system adjusted by altering the proportions of the solvent used according to the requisite Cot .theta. so as to yield the desired Cot .theta. for the system.

3. The process of claim 1 in which the feedstock is bitumen recovered from bituminous sand, which bitumen contains between about 0.5 and about 10 wt. percent asphaltenes and at least about 3 wt. percent minerals.

4. The process of claim 3 in which the feedstock also contains between about 5 and about 30 wt. percent water and in which at least about 85 percent of such water is recovered in the first asphaltene fraction.

5. A process for deasphalting a petroleum crude oil feedstock containing asphaltenes and mineral contaminants which comprises:

a. contacting said petroleum crude with hydrocarbon solvent in a deasphalting zone maintained at a cot .theta. solubility parameter between about -0.3 and about -0.6, whereby between about 10 and about 25 wt. percent of the asphaltenes are precipitated together with about 90 to 100 wt. percent of the mineral contaminants; and

b. recovering said precipitate from said deasphalting zone.

6. The process of claim 5 in which the hydrocarbon solvents are formulated as a single solvent or solvent blend selected from the group consisting of paraffinic or isomeric hydrocarbons having from three to eight carbon atoms, saturated substituted cycloparaffins having five of more carbon atoms and saturated unsubstituted cycloparaffins having five or more carbon atoms with the concentration of the solvent or solvent blend introduced to form the system adjusted by altering the proportions of the solvent used according to the requisite cot .theta. so as to yield the desired cot .theta. for the system.

7. The process of claim 5 in which the feedstock is bitumen recovered from bituminous sand, which bitumen contains between about 0.5 and about 10 wt. percent asphaltenes and at least about 3 wt. percent minerals.