US2153116A - Extraction process - Google Patents

Description

April 4, 1939. E. ElcHwALD EXTRACTION PROCESS Filed July 26, 1937 Soll/enf Figa 4 Inventor: Egon Ei hwa/d Patented Apr. 4, 1939 UNITED STATES EXTRACTION PROCESS Egon Eichwald, Amsterdam, Netherlands, assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Applicants July 26, niet, serial No. 155,635 In Great Britain August 14, 1936 14 Claims.

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carbons, is generally undesirable, in view of the' fact that such hydrocarbons increase the change in the viscosity of the oil upon variations in the temperature, and alsor in view of the relatively low resistance against oxidation. and the high Conradson carbon residue of oils containing these hydrocarbons. Therefore, the paraflinicity of lubricating oils is often associated with their quality.

The paraflinicity of an oil is. frequently represented by the viscosity-gravity constant, which is dependent upon the specific gravity and the viscosity of the oil, and may be calculated by the formula set forth by Hill and Coates in the Journal of Industrial and Engineering Chemistry, vol. 20, page 641, of 1928. -A high value represents a high content of hydrocarbons which are poor in hydrogen, such as aromatics and naphthenes, while a low value` indicates relatively greater parafnicity. Lubricating oils from natural crudes range from about 0.903 viscosity-gravity constant for an extreme Gulf Coast type to about 0.807 or lower for an extreme Pennsylvania type. The Conradson carbon residue, which is taken as an indication of the behaviour of oil in` internal combustion engines and similar applications, is the residue left on evaporating an oil under specied conditions from a glazed porcelain or silica Crucible. The test is described as test method D 189-36 of the American Society for Testing Materials.

Various methods have already been proposed for improving the properties of mineral oil fractions, both distillates and residues, of lubricating oil viscosity range such as, inter alia the process in which a removal of aromatic substances, and often of naphthenic substances, is eifected by extraction with the aid of selective solvents to form an extract phase consisting of a solution of these substances in the selective solvent. As selective solvents there have been used, inter alia: liquid SO2, furfural, nitrobenzeney benzonitrile, aniline, Chlor-aniline, BB dichloroethyl ether (known under the Trade-Mark Chlorex,) croton aldehyde, cresol, etc., or mixtures thereof, if desired in the presence of diluents, such as benzene, CS2, ethers, and carbon tetrachloride. These diluents increase the solubility of the hydrocarbons to be removed in the solvents mentioned.

In order to remove a larger quantity of aromatic constituents several successive extractions were often eected, if desired, at different tern peratures.

It is known that in the more intensive extractions, besides the aromatics and naphthenes, also parann hydrocarbons-especially the lower molecular praiiin hydrocarbons-are dissolved in the solvent and separated from the insoluble raffinate oil together with the soluble or extract oil.

It has.. already been proposed to separate lower molecular hydrocarbons with a relatively low viscosity-gravity constant from intermediate extra-cts by subjecting the latter to a fractional distillation and subsequently returning the light distillate to the ultimate raffinate. (See the U. S. patent tov Merrill,` 1,988,803.) It was found impossible by operating in this manner to obtain rai'linates of the highest quality. Among other properties, the Conradson` carbon residue and the tendency to cause so-called ring-sticking and the like was not satisfactory.

It is: an object of the present invention to refine an oil so as to remove therefrom a substantially greater proportion of the heavy aromatic hydrocarbons than has been the case with the processes now used, and to accomplish this without an undue lowering in the quantitative yield of refined oil. It is a further object of the present invention to provide a process which will produce a reined lubricating oil having a low Conradson carbon residue and which is particularly valuable because of a reduced tendency to cause ringsticking.

In accordance with the present invention it was found that a surprising improvement in the Conradson carbon residue and in the ring-sticking tendency is effected by subjecting oils of lubrieating viscosity range to an intensive solvent extraction, as defined below. It was, moreover, found that the yield of raflinate oil can be greatly improved without substantially aifecting these properties by recovering a light fraction from the extract phase and incorporating it into the raiiinate oil.

It has been found that the higher molecular aromatics are the substances unfavourably affecting the value of the Conradson carbon residue and causing the tendency to ring-sticking. In solvent extraction processes, these higher molecular aromatics, however, are not quantitatively dissolved in the extract phase when an extraction treatment of. normal intensity is applied.

By the expression intensive extraction, as used in the present specification and claims, is meant one or a series of solvent extractions in which over 50% by volume of the initial oil is removed in the extract phase or in the aggregate of extract phases, when several extractions are employed, and, further, the kinematic Viscosity at 50n C. (as determined, for example, by the Engler viscosimeter) of the final rainate is greater than the kinematic viscosity at 50 C. of the final extract. By iinal rainate and extract are meant the oil portions of the respective isotonic phases i. e., phases in osmotic equilibrium obtained at the end of intensive extractions. The intensive extraction may be applied either to a fraction which has been preliminarily extracted by a normal extraction, or to a fraction which contains substantially al1 of the normally liquid components naturally contained in the petroleum fraction, and which may or may not contain asphalt and/or wax. When the intensive extraction is carried out in several stages, or is applied to an oilwhich has been subjected to a preliminary extraction of lesser intensity, it is preferred to carry out the intensive extraction to the point at which the kinematic viscosity at 50 C. of the nal raffinate is greater than the Viscosity of the total extract obtained in the several extractions, starting with the fraction containing substantially all of the normally liquid components naturally contained in the fraction, (see Examples 5 and 7, which satisfy this limitation both as to the extracts from the preliminary and intensive extractions, and differ in this respect from Example 6), it being understood that this viscosity relationship merely denes the intensive extraction without implying that it must begin with a fraction actually containing substantially all normally liquid components naturally present in the fraction.

Such an intensive extraction should be distinguished from the less intensive extractions normally employed in that the latter are carried out only to the point at which a relatively satisfactory viscosity-gravity constant or viscosity index is reached, the ratio of the kinematic viscosities of the raffinate and extract being always less than 1.0. When, however, the oil is more intensively extracted, this ratio exceeds 1.0. Such more intensive extraction is not intended to improve the viscosity index or the viscositygravity constant, but exclusively to remove the substances which cause the formation of large amounts of Conradson carbon residue, and are responsible for the ring-sticking tendency. For example, Pennsylvania oils which are highly paranic are not normally subjected to solvent extraction, but may advantageously be intensively extracted in accordance with the present process to effect an improvement in the Conradson carbo-n residue.

'Ihe extract obtained in the intensive extraction contains, besides the lower molecular parain hydrocarbons, also the more valuable parains of medium molecular Weight, those of lower molecular Weight having already been partially removed during the less intensive treatment of the oil. Y

According to the process of this invention paraiin hydrocarbons (i. e., those having low viscosity-gravity constants) are subsequently separated from the extract or extracts obtained in the intensive extraction and then incorporated into the raffinate obtained in the intensive extraction. This separation of a light paraiiinic fraction may be effected in a simple manner by any fractionating process, such as, for example, by means of a distillation treatment in the presence or absence of a preferential solvent for aromatics, the higher molecular aromatics being left behind as distillation residue, or by means of a treatment with a low-molecular treating agent under para-critical conditions, hereinafter referred to as gas fractionation.

In most cases the light fraction or fractions recovered from the extract are suitable as such to be added wholly or partly to the raffinate obtained in the intensive extraction treatment, or to the feed to the intensive extraction, preferably in one of the last stages in the case of a multistage process, thus yielding a lubricating oil of superior quality, but they may also first be subjected to a rening treatment, e. g., to a separate solvent extraction, or a treatment with sulphuric acid and clay or neutralization. The light recovered fraction may, if desired, also be refined after it has been added to the raffinate following the intensive extraction.

This refining treatment will in general be effected when lower molecular aromatics or other impurities, e. g., sludge-formers or sulphur compounds, are still present in the distillates.

The refining treatment may be wholly or partly avoided when the intensive extraction is preceded by a normal refining, e. g., a sulphuric acid treatment, or an extraction of normal intensity, e. g., with 3 100% by vol. liquid SO2 at 10 'C., so as to remove a large portion of the aromatics, viz., the aromatics of lower and medium molecular weight. The extracts obtained in such preliminary extraction, indeed, also contain paraiiin hydrocarbons, but only those of the lowest molecular weight, which are not valuable for lubricating oils, and moreover in a small percentage, so that there is usually no purpose in recovering them. In general the extracts are only of value as fuel.

Even if the intensive extraction is applied to an initial oil already stripped of a large part of its aromatics by a normal extraction or naturally containing little or no aromatics, according to the process of the present invention more than 70% of the hydrocarbons are often obtained as extract, from which often up to 60% (based on the initial oil) of valuable parailin hydrocarbons are recovered by fractionation.

Both the intensive and the preceding normal extraction (if employed) may take place several times in succession.

The process according to the invention is preferably applied to heavy hydrocarbon oils having a relatively long range of molecular weights (or a relatively wide boiling range), i. e., both to distillate lubricating oils and to residual lubricating oils.

'I'he presence of asphalt in the oils to be treated does not impair the practicability of the process according to the invention, but may even have the special advantage that, in proportion to the intensity of the extraction, 60 to 90% of the asphaltic constituents present pass into the extract phase and on fractionation by distillation or by gas fractionation remain behind in the residue. Any asphaltic substances still present in the raiiinate can be removed therefrom in a known manner, as by adding naphtha or a low boiling fraction low in aromatics to precipitate the asphaltic substances, if desired after addition of the distillate obtained from the extract.

The presence of paraflin wax as a rule does not constitute an impediment in carrying out the process. The dewaxing may be effected without difficulty at any suitable time, either prior to, as an intermediate step of, or following the execution of the process.

The invention may be further understood by referring to the drawing in which Figure 1 is a simplified flow diagram of an illustrative process embodying the invention, Figure 2 is a fragmental view illustrating a modification of the fractionation steps thereof, and Figure 3 is a fragmental view illustrating a modification of the intensive extraction step.

Referring to Figure l, an initial residual or distillate oil of lubricating viscosity range, which may or may not have been deasphalted and/or dewaxed is introduced into the rst extraction stage I through the conduit 2, and treated with a selective solvent for aromatic hydrocarbons fed through the conduit 3. The extraction may be either batch or continuous or semi-continuous, and may comprise a single stage, or a plurality of stages, arranged to effect a countercurrent extraction. A rst extract phase is withdrawn at 4 and a first rainate phase at 5. The latter is intensively extracted in the intensive extraction stage 6 with a selective solvent for aromatics introduced at 1. This'extraction is carried out under conditions causing a large quantity of oil (at least 50% of the feed to the stage 6) to be dissolved in the solvent. This is effected by employing a larger quantity of selective solvent, using for this purpose a plurality of stages arranged for multiple extractions, and/or by operating under conditions which increase the solvent power of the solvent, as by employing a higher temperature than in the rst stage, and/or utilizing a diluent, such as benzol or kerosene extract with the selective solvent. The extract phase (or a blend of several extract phases, in case multiple extractions are employed as shown in Figure 3) is withdrawn at 8 and treated in the solvent removal stage 9, wherein the selective solvent is separated from the extracted oil, herein designated as the intermediate extract. The latter is fractionated in the distilling apparatus I0, from which the residue is withdrawn at II and the distillate at I2, which may be of any desired size and may, for example, contain between 40% and of the extract fed into the apparatus I0. rIhe latter, or a portion thereof, if desired after further refining, is fed through the conduit I3 and blended with the raiiinate withdrawn from the extraction stage at I4 and freed from the selective solvent in the solvent removal stage I5. Distillate not blended with the raiinate may be withdrawn at I6. It should be understood that if several extracts are produced in the stage 6, all or only some of them may be separately treated.

'I'he method of producing several extracts and blending them is shown in Figure 3, wherein 6a and 6b are extraction stages which may be used in place of the extraction stage 6 according to Figure 1. The rst raffinate phase from the extractor I is introduced at 5a, and extracted with solvent introduced at la, producing a raffinate, which is fed via line 5b to the extractor 8b and therein extracted with solvent introduced at 1b. The final raffinate phase is withdrawn at Ida. The extract phases, withdrawn at Bc and 8d may be blended and treated, introduced into the apparatus I0, either directly, or through the solvent removal stage, as explained above.

If desired, the separation effected in the column I0 may be improved by carrying out the distillation in the presence of a separating agent of the type of preferential solvent for aromatics. The designation separating agent of the type of preferential solvent for aromatics is used to include (l) true preferential solvents, i. e., solvents which possess such solvent characteristics that they are capable of forming two liquid layers when mixed with an oil containing paraninic and aromatic hydrocarbons, and in which the distribution ratio between the solvent and the oil is greater for the aromatics than for the paraflins, and (2) solvents which do not form two layers when commingled with an oil, but which have a preferential affinity for aromatics. Examples of true preferential solvents are furfural, cresylic acid, aniline, chlorophenol; examples of the separating agents which do not form two liquid layers are quinaldine, picolines, and refinery nitrogen bases. It is usually preferable to employ a separating agent having a relatively high boiling point, boiling near to or within the boiling temperature range of the oil being distilled, but low enough to be distilled together with the oil.

If the separating agent to be used is different from the selective solvent employed in the stage 6, the extract phase may be freed from the solvent in the stage 9, as before, and the separating agent may be introduced into the column l0, either separately or together with the extract as through an inlet Illa. If the same substance is employed as the selective solvent in the extracting stage 6 and as the separating agent in the column Ill, the solvent removal stage 9 may be by-passed through a line 8a and valve 8b.

The column is preferably operated to distill the separating agent overhead together with the light fraction. It may be removed from the light fraction in the solvent removal stage I2a, which may be operated in any desired manner, such as by washing the distillate with an extracting agent, and/or chilling it to cause phase separation.

The process may be modified by returning all or a portion of the distillate from the apparatus II! to the feed of either the first or intensive extraction stages. Thus, the valve I'I may be partially or completely closed, and the distillate introduced into the extraction stage 6 through the valve I8 and conduit I9.

The rst extraction may be omitted entirely, and the initial fraction introduced directly into the stage 6 through an inlet 20. This operation is preferred when working with a highly parafnic oil, such as one which has been previously extracted, or which is naturally highly parainic, such as a Pennsylvania oil. When an oil of low paraiiinicity is fed directly into the stage 6, it will generally be desirable to further rene either the intermediate extract prior to distillation, or the distillate withdrawn at I2 prior to blending, or the blended raffinate.

According to a modification of this process, the extract phase, instead of or in addition to being distilled, may be subjected to a gas fractionation, by which is meant any fractionation in which the separation of an oil into two liquid phases is effected by means of a low molecular treating agent having a relatively low critical temperature (generally below 200 C.), the treating agent being in its para-critical state under conditions of the treatment. In such fractionations, a substance is said to be in its para-critical state when its temperature is above, at, or not more than about 40 C. below its critical temperature. The pressure is such that the low molecular treating agent is dissolved in the material being fractionated to cause the formation of two liquid phases o'f different specific gravities. Several of such gas fractionation processes are known, and it is understood that any one of them may be employed.

For purposes of illustration, one form thereof is illustrated in Figure 2, according to which the extract from the intensive extraction stage 6 of Figure 1 or the stages 6d, Bb, of Figure 3 (preferably after the removal of the solvent, but permissibly together with it) is introduced into a phase separator 2| by means of a pump 22. A low molecular treating agent, such as a hydrocarbon having less than seven carbon atoms, preferably methane, ethane, or propane, or a mixture thereof, or a similar non-hydrocarbon low molecular substance having a low critical temperature, preferably below about 200 C., such as carbon dioxide, is introduced from the tank 23 through the valve 24. 'Ihe phase separator 2| is provided with a temperature regulating device 25 in the form of a coil through which heating or cooling fluid may be flowed; it is employed to maintain the temperature of the mixture of the extract and treating agent at a temperature at which the latter is in its para-critical state, whereby two liquid phases are formed, the high molecular aromatic hydrocarbons being precipitated as a heavy liquid or semi-liquid phase, and the light and intermediate parailinic hydrocarbons being in the lighter liquid phase, together with the greater portion of the treating agent. The pressure is suiciently high to dissolve enough treating agent in the oil to induce the formation of two phases. Solubility may be enhanced b y adding a mutual solvent, as a liquid having a critical temperature higher than that of the low molecular treating agent. For example, when methane or ethane is employed as the low molecular treating agent, propane or butane may be employed as the mutual solvent, and may be introduced into the system together with the methane, or separately. In this case the temperature may, for example, be ordinary room temperature, and the pressure between about 15 and 150 atmospheres.

The heavy phase may be withdrawn continuously or intermittently through the valve 26, and the entrained treating agent (together with the mutual solvent, when employed) separated from higher molecular substances in a pressure-reducing flash apparatus 21: the high molecular aromatics, together with asphaltic material, when present, are withdrawn at 28; the treating agent is returned to the tank 23 Via the conduit 29, pump 30, and cooler 3l. The lighter phase containing the desired light oil fraction and treating agent, is withdrawn from the separator 2l through the valve 32, and treated in the apparatus 33 to isolate the light oil fraction, which is Withdrawn at 34. This fraction may, for example, contain from 40% to 95% of the extract fractionated. All or a part of this light oil fraction is incorporated into the refined raffinate oil from the intensive extraction according to either method shown in Figure l, either directly, or after a further distillation in the distilling stage I9.

The following examples illustrate the improved results obtainable by the process of the invention. The following abbreviations are employed: E/50 for viscosity Engler viscosity at 50 C., in degrees; C. C. R. for Conradson carbon residue; aV. G. C. for viscosity-gravity constant; and V. I. for viscosity index. All percentages are by weight and on the basis of the initial oil prior to extraction.

Example 1.-A deasphalted La Concepcion residue (E/50=7.9; V. I.=63.4; V. G. C.=0.8363; C. C. R.=1.21) was extracted successively at 90 C., 120 C., and 130 C., each time with an equal Weight of furfural, a total of 67% of extract (El/50:60) and 33% ramnate (E/50=15) being obtained. The ratio of the kinematic viscosities Was, therefore, 15/6=2.5. The extract Was distilled to produce 8% of aromatic residue and 59% of distillate (V. 1:83). This distillate was mixed with the raffinate and the mixture (92%) was dewaxed and treated with bleaching earth. The nal product had the following properties: E/50=7.65; V. I.=97; C. C. R.=0.44.

Example 2.-Deasphalted and dewaxed La Concepcion residue (E/50=10.5; V. I.=82;

V. G. 0:08456; C. C. R.=1.54)

was extracted successively at 18 C., 25 C., and 35 C., each time with an equal weight of nitrobenzene, '70% of exaract (El/50:96) and 30% of raffinate (E/50=13.0) being obtained; the ratio of viscosities was, therefore, 1.35. The extract was distilled, yielding 17% of aromatic residue and 53% of distillate. This distillate was mixed with the raffinate and the mixture (83%) had the following properties; E/50=7.6; V. I.=93; C. R. R.=0.30.

Example 3.--Non-deasphalted and non-dewaxed La Concepcion residue was extracted once with 200% by vol. furfural at 75 C. This extraction yielded:

% by weight of rafnate I 15% by weight of extract I Extract I was distilled in vacuum, 10% distillate I being thus obtained (E/50=5.45; V. I.=5: V. G. C.=0.9411; C. C. R.=0.35). Rafnate I was extracted twice with two volumes of furfural (at C. and 140 C.). This extraction yielded 24% raffinate II and 61% extract II.

Rafiinate II was more viscous than extract II, and also more viscous than the mixture of extract I and extract II blended in the ratio of 15 to 61. Extract II Was distilled in vacuum, 45.5% of a distillate II being thus obtained (E/50=4.65; V. I.=98; C. C. R.=0.23). This distillate II was mixed with raiiinate II and the mixture thus obtained (69.5%) was dewaxed, subsequently dissolved in one volume of petroleum ether and acidied with one tenth weight of H2SO4 of 96% concentration at room temperature. After sedimentation of the acid sludge the mixture was treated with lye and bleaching earth in the usual manner. The product obtained from the distillate II and raffinate II (about 53%) had the following properties: E/50=6.2; V. I.=103.8; V. G. C.=0.8266; C. C. R.=0.41.

Example 4.-A Pennsylvania residual lubricating oil (E/50=8.4) was deasphalted by dissolving it in three volumes aromatic-free gasoline and a treatment with one tenth weight of 96% sulfuric acid at room temperature, and then -dewaxed by dissolving the deasphalted oil, freed from gasoline, in ve Volumes of a dewaxing solvent consisting of benzene and acetone in the ratio of 7:3. The deasphalted and dewaxed residual oil had the following properties:

:El/50:23.80; V. I.=109;

V. G. C.=0.8139; C. C. R.=0.87

This oil was successively extracted, twice at 20 C., and once each at the following temperatures in the order stated: 30 C., 35 C., 50 C., and 55 C., each time With a volume of nitrobenzene equal to the oil being extracted. The extract phases were combined and freed from solvent, yielding in all 68.5% of extract (E/50=7.97). The 31.5% of raffinate oil had the following properties: E/50=10.35; V. I.=116; V. G. C.=0.'78'79; C. C. R.=0.41. In this extraction the ratio of the kinematic viscosities was 1.3. Of the extract, 59.5% Was distilled olf in vacuo and added to the raiiinate, yielding 91% of blended refined oil with the following properties:

E/50=6.l5; V. I.=110.4;

V. G. C.=0.8089; C. C. R.=0.21

It should be noted that the process had only a small effect upon the viscosity index, but that the Conradson carbon residue was very materially reduced. Such a Pennsylvania oil would not ordinarily be subjected to solvent extraction, and its properties would notl be substantially altered by the usual extractions of normal intensity.

Example 5.-In Iraq lubricating oil residue, deasphalted andy dewaxed as described in Example 4, had the following properties: E\/50=6.27; V. G. C.=0.8593; C. C. R.=l.90. This oil was' subjected to a normal extraction in four successive stages, twice at 20 C., and once each at C. and at 100 C., respectively, each time with an equal volume of furfural, yielding 33% of combined extract (E/50=5.68) and 67% of rafiinate (E1/50:51; V. G. C.=0.828; C.C.R.=1.96). The ratio of kinematic viscosities was 0,90. It isseen that such a normal extraction, in which the ratio of viscosities of the raflinate and extract does not exceed 1.0 is insufficient to cause a satisl factory improvement in the Conradson carbon residue. The above rafiinate was then further subjected to an intensive extraction consisting of one extraction at 120 C. followed by one at 130 C., each time with an equal volume of furiural, yielding an additional combined extract of 45% with a viscosity E/50=4.24, leaving 22% of a rafnnate (E/50=8.10; V. G. C.=0.8125). In this intensive extraction, therefore, the ratio of kinematic viscosities was 8.10/4.24=1.91. (Based on the viscosity of the total extract from all extractions, this ratio was about 8.10/5.1=1.59.) Of the 45% extract from the intensive extraction, 36% was distilled off in vacuo, and the distillate was added to the final raflinate, resulting in 58% of a refined oil having the following properties:

n E/50=3.35; V. G. C.=0.8164; C. C. R.=0.51

Example 6.-A deasphalted and dewaxed Kettlemen Hills lubricating oil residue had the following properties:

V. G. C.=0.8683; C. C. R.=".56

The oil was subjected to a normal extraction, successively at the following temperatures: 30 C., 50 C., 70 C., 90D C., and 110 C., each time with an equal Volume of furfural, yielding 58.7% of a combined extract (El/50:50.66) and 41.3% rafiinate (lil/50:15.53; V. I.=92.4; V. G. C.=0.8164; C. C. R.=1.46). The ratio of kinematic Viscosiities was, therefore, 0.31. While this raffinate oil was of good quality when judged by its viscosity index and viscosity-gravity constant and would, therefore, not ordinarily be extracted beyond this point, its Conradson carbon residue was too high, indicating that an extraction in which the ratio of kinematic viscosities is 0.31 does not improve this residue sunciently. This rafiin-ate was then subjected to an intensive extraction consisting of one extraction at 125 C. followed by another at 130 C., each time with an equal volume of furfural, resulting in an additional 24.5% or combined extract (El/50:14.95) and 16.8% of final rainate (ri/50:16.72; v. r.=95.7; v. G. C.=0.81o9) In this intensive extraction the ratio of kinematic viscosities Was 16.72/14.95=1.12, although the corresponding ratio, based upon the viscosity of a proportional blend of the total extracts obtained both from the normal and intensive extractions was below 1.0. This combined extract from the intensive extraction was distilled to produce 20.6% of a distillate, which was added to the iinal rainate, yielding 37.4% of a refined oil having the following properties: Eli/50:10.25;

4V. I.=94.4; V. G. C.=0.8204; C. C. R.:0.50`.

Example 7.-A deasphalted and dewaxed La Concepcion lubricating oil residue had the following properties: E/50=5.47; V. G. C.=0.8399; C. C. R.=1.23. Upon a normal extraction at 70 C. with an equal volume of furfural, there were obtained 15% of raiiinate (E/50=5.43; V. G. C.=0.8286; C. C. R.=l.10), indicating a ratio of kinematic viscosities of 0h99 and an unsatisfactory Conradson carbon residue. When this raffinate was further intensively extracted, once at C., once at C., and then twice at 130 C., each time with an equal volume of furfural, an additional combined 56% of extract was obtained leaving 29% of a final rainnate (E`/50=10.73; V. G. C.=0.8055). The ratio of kinematic Viscosities for the intensive extraction was 2.78, and, computed on the basis of the total extract from the normal and intensive extractions, about 2.5. Of the extract from the intensive extraction, 46% was distilled oi in vacuo, and added to the final rainate, resulting in 75% of a rened oil having the following properties: E/50=4.29; V. G. C.=0`.8228; C. C. R.=0.35.

` The process of the invention is especially suitable for the treatment of lubricating stocks with a viscosity gravity constant of 0.850 or less, and more particularly residual lubricating stocks. For the present purpose, a lubricating oil stock may be defined as amineral oil having a viscosity at 50 C. of at least 1.6 degrees Engler (50 seconds Saybolt Universal) and an A. S. T. M. initial boiling poirit of at least 230 C., and which contains a substantial portion normally free owing components.

I claim as my invention:

1. A process for producing a lubricating oil having a low Conradson carbon residue, comprising the steps of extracting a mineral lubricating oil stock in an intensive extraction stage with -a selective solvent for aromatic hydrocarbons to produce an extract soluble in said solvent and a raiiinate insoluble therein, said extract containing at least 50% by weight of the oil introduced into said extraction stage and the kinematic viscosity at 50 C. of the rainate being greater than the kinematic viscosity at 50 C. of the extract in the extract phase in equilibrium with said raffinate in said intensive extraction stage, separating the rainate and extract, fractionating at least a portion of said extract into a light fraction containing oil having a low Conradson carbon residue and a heavier residue, and incorporating at least a portion of said light fraction into the raffinate to form a lubricating oil of low Conradson carbon residue.

2. The process according to claim 1 in which the light fraction is incorporated into the raffinate after the latter has been removed from the intensive extraction stage.

i 3. The process according to claim 1 in which the light fraction is incorporated into the raffinate by introducing it into the intensiveextractionstage. Y, Y

extract (E/50==5.53) and` 4. The process according Yto claim 1 in which the fractionation is carried out by distillation.

5. The process according to claim 1 in which the fractionation is carried out by distillation in the presence of a separating agent of the type the lubricating oil stock is extracted to the point at which the kinematic viscosity at 50 C. of the raffinate is greater than the kinematic viscosity at 50 C. of the total extract obtainable by extracting the fraction containing all of the normally liquid components naturally present in said fraction to produce a like raffinate.

8. The process according to claim 1 in which the lubricating oil stock subjected to the intensive extraction has a viscosity-gravity constant of less than 0.850.

9. A process for producing a lubricating oil having a low Conradson carbon residue, comprising the steps of extracting a mineral lubricating oil stock in an intensive extraction stage with a selective solvent for aromatic hydrocarbons to produce an extract soluble in said solvent and a rafnate insoluble therein, said extract containing at least 50% by Weight of the oil introduced into said extraction stage and the kinematic viscosity at 50 C. of the rainate being greater than the kinematic viscosity at 50 C. of the total extract produced in said intensive extraction stage, separating the raffinate and extract, fractionating at least a portion of said extract into a fraction containing oil having a low Conradson carbon residue and a heavier residue, and adding at least a portion of said light fraction to the raffinate.

10. A process for producing a lubricating oil having a low Conradson carbon residue, comprising the steps of extracting an initial mineral lubricating oil fraction in a first extraction stage with a selective solvent for aromatic hydrocarbons to produce a first extract soluble in said solvent and a first raffinate insoluble therein, separating said first extract and first rainate, extracting said first raffinate in an intensive extraction stage with a selective solvent for aromatic hydrocarbons to produce a second extract soluble in said solvent and a second raffinate insoluble therein, said combined extracts containing at least 50% by Weight of the initial lubricating oil fraction and the kinematic viscosity at 50 C. of the second raffinate being greater than the kinematic viscosity at 50 C. of the extract in the extract phase in equilibrium with said second raffinate, separating the second raffinate and second extract, fractionating at least a portion of said second extract into a light fraction containing oil having a low Conradson carbon residue and a heavier residue, and adding at least a portion of said light fraction to the second raffinate.

11. The process according to claim 10 in which the intensive extraction stage contains a plurality of successive extraction steps producing a plurality of extracts, and the extracts are combined and fractionated together.

12. A process for producing a lubricating `oil having a low Conradson carbon residue, comprising the steps of extracting an initial mineral lubricating oil fraction in a rst extraction stage with a selective solvent for aromatic hydrocarbons to produce a rst extract soluble in said solvent and a first raffinate insoluble therein and having a lower kinematic viscosity at 50 C. than that of the said first extract, separating said first extract and first raffinate, extracting said first l rafiinate in an intensive extraction stage with a selective solvent for aromatic hydrocarbons to produce a second extract soluble in said solvent and a second raffinate insoluble therein, said second extract containing at least 50% by weight of the first raffinate and the kinematic viscosity at 50 C. of the second raffinate being greater than the kinematic viscosity at 50 C. of the second extract, separating the second rafiinate and second extract, fractionating at least a portion of said second extract into a light fraction containing oil having a low Conradson carbon residue and a heavier residue, and adding at least a portion of said light fraction to the second raffinate.

13. A process for producing a lubricating oil having a 10W Conradson lcarbon residue, comprising the steps of extracting an initial mineral lubricating oil fraction in a rst extraction stage with a selective solvent for aromatic hydrocarbons to produce a first extract soluble in said solvent and a first raffinate insoluble therein and having a lower kinematic viscosity at 50 C. than that of the said first extract, separating said first extract and first rainate, extracting said first raffinate in an intensive extraction stage with a selective solvent for aromatic hydrocarbons to produce a second extract soluble in said solvent and a second rafnate insoluble therein, said second extract containing at least 50% by weight of the first raffinate and the kinematic viscosity at 50 C. of the second rainate being greater than the kinematic viscosity at 50 C. of a blend of all extracts obtained from the several extractions, separating the second raffinate and second extract, fractionating at least a portion of said Second extract into a light fraction containing oil having a low Conradson carbonresidue and a heavier residue, and adding at least a portion of said light fraction to the second raffinate.

14. A process for producing a lubricating oil having a low Conradson carbon residue, comprising the steps of extracting an initial mineral lubricating oil fraction in a first extraction stage With a selective solvent for aromatics to produce a first extract soluble in said solvent and a first raffinate insoluble therein, said first raffinate having a viscosity-gravity constant below- 0.850, separating said first extract and first rafiinate, extracting said first raffinate in an intensive extraction stage with a selective solvent for aromatic hydrocarbons to produce a second extract soluble in said solvent and a second raffinate insoluble therein, said combined extracts containing at least 50% by weight of the initial lubricating oil fraction and the kinematic viscosity at 50 C. of the second raiiinate being greater than the kinematic viscosity at 50 C. of the extract in the extract phase in equilibrium with said second rafiinate, separating the second rafiinate and second extract, fractionating at least a portion of said second extract into a light fraction containing oil having a low Conradson carbon residue and a heavier residue, and adding at least a portion of said light fraction to the second rafnate.

EGON EICHWALD.

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