US2591525A - Process for the catalytic desulfurization of hydrocarbon oils - Google Patents

Description

used to alimited extent in practice.

Patented Apr. 1, 1952 PRQCESS FOR THE CATALYTIC DESUL- FURIZATION OF HYDROCARBON OILS Willem Frederik Engel, Amsterdam, and Peter van t Spijker, The Hague, Netherlands, assign? ors to Shell Development Company, San Francisco, Calif., a corporation of Delaware No Drawing. Application December 21, 1948, Se-

rial No. 66,578. In the Netherlands December 6 Claims. 1

This invention relates to an improved method for the treatment of sulfur-bearing hydrocarbon oils under conditions of elevated temperature and high pressure to remove sulfur without substantial decomposition of the oil treated. More particularly the invention relates to an improved method for the removal of organically combined sulfur from heavy oils such as gas oils, lubricating oils and the like materials boiling above the gasoline boiling range.

The partial removal of sulfur from hydrocarbon oils takes place, at least to a certain degree, in numerous oil-treating processes. Thus, for example, such oil-treating processes as thermal cracking, catalytic cracking, thermal reforming, catalytic reforming, hydrcforming, alkylation, polymerization and isomerization usually result in some removal of sulfur from the oil treated.

In fact, an appreciable amount of sulfur is often problem. While many solutions have been proposed and some of them have been tried, it is nevertheless a fact that at the present state of knowledge the desulfurization of hydrocarbons in anything approaching an eflicient manner is a very costly operation which is rarely justifiecfin the economy presently pervailing.

While certain other types of processes may have some limited application, the desulfurization process generally considered the best at prescut is one in which the oil is treated in the vapor phase in the presence of a large excess of hydrogen and a sulfactive hydrogenation catalyst under moderate conditions of temperature and pressure. This process is described in numerous patents and articles in the trade journals and has been This proeess, of which ther are numerous minor variations, is fairly efiicient, but the cost of the plant and the cost of operation are so high that the process is rarely economical. This vapor phase desulfurization process is theoretically applicable to the desulfurization of any hydrocarbon oil, but from a practical standpoint it is limited to the desulfurization of light hydrocarbon oils such as gasoline, kerosene, light gas oils and the like, oils .which are easily vaporizable without decomposition.

The desulfurization of heavy hydrocarbon oils (Cl. 19t 28) hydrogenation conditions.

presents a still more diflicult problem. Such oils can be and have been desulfurized fairly efiiciently by treating them under otherwise similar conditions while they are completely or partly in the liquid phase. The plant costs and the operating costs are also prohibitively high in this case. Also, the reaction rate is slower than in the vapor phase process and the periodic regeneration of the catalyst is mademuch more difficult by the presence of liquid oil in the reactor.

The largest single factors causing the high capital costs and operating costs in the mentioned processes are the cost of the considerable amounts of hydrogen required and the cost of the facilities for handling (storage, separation, compression, recycling, preheating, etc.) the large amounts of high pressure hydrogen. In an attempt to overcome this disadvantage it was tried to supply the required hydrogen by the simultaneous, dehydrogenation of added naphthenic hydrocarbons. While the presence of added naphthenes afforded a small improvement in desulfurization over that obtained without their presence, not more than half, and usually only about one quarter of the sulfur could be removed by this method, even when treating a light oil in the vapor phase. Also, under these conditions the life of the catalyst was too short to bring the process into practical consideration. When operating the process with large amounts of recycled hydrogen to improve the lif of the catalyst the presence of added naphthenes decreased the consumption of hydrogen somewhat when operating at the heighest permissible temperatures. However, there was still a prohibitive consumption of hydrogen and the main items of cost were not materially reduced. The only metho found for eliminating the net hydrogen consumption was to treat the napthenic hydrocarbon separately under dehydrogenation conditions and then use the product gas for the treatment of the oil to be desulfurized under This increases, rather than decreases, the plant costs and operating costs and is in effect only the substitution of a separate dehydrogenation process for one of the other usual methods for the production of hydrogen. It is, therefore, only a solution in the few isolated cases where the dehydrogenation of large amounts of naphthenes is a profitable operation of itself. This was frequently the situation during the warwhen large amounts of toluene were produced by the dehydrogenation of methyl cyclohexane fractions from petroleum, but it is rarely the situation now.

It has now been found that hydrocarbon oils boiling above the gasoline boiling range may be desulfurize in a practical and eflicient manner without the use of extraneous hydrogen if the desulfurization is carried out in the specific manner now to be described. In the method used by us the oil to be desulfurized is first mixed with a lower boiling hydrocarbon oil (preferably having a lower sulfur content and being not too paraffinic in nature) to lower the critical temperature to well below the operating temperature, and the mixture is then treated in the absence of added hydrogen under specific conditions of temperature and pressure where transfer of hydrogen is favored and at the same time the deposition of polymers, asphaltic materials, and similar materials on the catalyst is largely prevente by the solvent action of the mixture.

The process of the invention is applicable for the desulfurization of various hydrocarbon oils containing organically combined sulfur as impurity, such, for example, as diesel fuels, socalled jet fuels, stove oils, gas oils, spray oils, spindle oils, transformer oils, lubricating oil distillates and the like materials boiling above the gasoline boiling range. The oils to be treated may have been obtained from petroleum, coal, oil shale, or the like carbonaceous materials by widely divergent physical or chemical treatments such as distillation, extraction, retorting, destructive hydrogenation, and thev like. The process is particularly adapted for the treatment of heavy hydrocarbon oils, i. e., hydrocarbon oils having mid-boiling-points (A. S. T. M.) of at least 200 C. and preferably above 250 C. The process is best adapted for the treatment of hydrocarbon oils having sulfur contents of at least 0.1% and preferably at least 0.5%.

The process of the invention can be carried out with any of the sulfactive hydrogenation catalysts such as conventionally used heretofore for hydrodesulfurization, destructive hydrogenation, hydrofining and related processes. The preferred catalysts have as their main active ingredient one or more hydrogenating oxides or sulfides of the transition metals (see J. Chem. Ed. 21, 532 (1944)) particularly those of groups I, II, VI, and VIII of the periodic system of the elements. These materials may be used in various combinations with or without such stabilizers and promoters as the oxides or carbonates of K, Ag, Be. Mg, Ca, Sr, Ba, Ce, Bi, Cr, Th, Si, Al and Zr. These various catalysts may be applied per se or in combination with various conventional supporting or carrying materials which in certain cases may impart very important characteristics to the catalyst. Examples of a few typical and preferred materials are activated aluminas, activated magnesias, activated silicas, activated zirconias, activated clays, activated bauxites and activated bleaching earths. (The term activated is here used as denoting a material having a microporous structure affordinga large inner surface.)

While any of the mentioned classes of conventional catalysts may be used, it is found that in this particular process some catalysts of the general type are much better suited than others. .It was found that a molybdenum oxide catalyst promoted by a minor amount of cobalt oxide and supported upon an activated alumina is quite superior to the several other catalysts of the same general type. This preferred catalyst is somewhat similar to that described in U. S. Patent No. 2,393,288, but differs therefrom in that the oxides of molybdenum and cobalt are incorporated by impregnation in a previously activated alumina and the mole ratio of molybdenum to cobalt is about 5 to 1.. The pro-activated alumina is also preferably pretreated with a dilute solution of hydrochloric acid or more preferably a 10% solution of hydrofluoric acid prior to the impregnation. This is the subject matter of copending application Serial No. 82,285, filed March 18, 1949. The catalyst may be further improved for the present purpose by the inclusion of a small amount (for example, 2 to 6%) of silica and/or a small amount (for example, 1 to 4%) of zinc oxide, as described in U. S. Patent 2,508,014 of D. D. Davidson. The zinc oxide, it is found, should be impregnated into the activated alumina prior to the impregnation with the molybdenum compound and cobalt compound. While these particular, more or less special catalysts are superior in the present process to all of the other catalysts tried, the process of the invention is not restricted to the use of these catalysts since in some cases it will be more economical and practicable to use a somewhat less efficient but cheaper catalyst which can be discarded or easily regenerated after a short period of use. The particular choice of catalyst will in practice depend to a large extent upon the particular material to be treated and its intended use. In desulfurizing costly materials for final use (for example, transformer oils and lubricating oils) where it is particularly important to avoid side reactions such as cracking, isomerization, aromatization and the like, the catalytic agent is preferably combined with a special carrier material having relatively large micropores, such as the new so-called beta alumina carriers or ordinary activated alumina which has been subjected to a relatively drastic pretreatment with steam (for example, at a temperature in the order of 4.00 to 600 0.). In other cases, for example in the partial desulfurization of reduced crude petroleums, shale oils or the like materials preparatory to converting them into more useful products by catalytic cracking or related processes, a cheap catalyst such as an activated bauxite impregnated with small amounts of molybdenum oxide and cobalt oxide may be advantageously used.

The sulfur removed from the hydrocarbon oil in the present process is removed in the form of hydrogen sulfide. This hydrogen sulfide is not formed by hydrogenation of the sulfur compounds with free hydrogen added or produced in situ, but by hydrogen transfer within the oil itself. A carefully controlled temperature is necessary. Hydrogenation and dehydrogenation are opposite directions of one and the same fundamental reaction. The direction of the .reaction, (1. e., Whether hydrogenation or dehydrogenation takes place) depends upon the temperature in accordance with the known laws of thermodynamics. At low temperatures in the order of 300 C. hydrogenation is favored; at high temperatures in the order of 475 C. and above dehydrogenation is favored. In the small intermediate range from about 325 C. to about 450 C. the system is near equilibrium and any hydrogenation or dehydrogenation is of small extent. It is within this narrow range and most preferably between 375 C. and. 425 C. that the desulfurization according to the present process is carried out. In this range of temperatures the hydrogen bonds are relatively labile in the presence of the catalyst allowing hydrogen transtier reactions to take place with a minimum amount of dehydrogenation. As will be explained later, it is not only important that temperatures in this range be applied in order to take advantage of the optimum activity of the catalyst and the optimum conditions for hydrogen transfer, but also for other reasons.

The desulfurization is carried out in the presout process under high pressures in the absence of a liquid phase. In order that a liquid phase may be positively excluded it is necessary to operate at a temperature which is above the critical temperature of the material treated, and preferably above the cricondentherm (the secand critical point). The critical temperature of any ,given oil to be treated can be calculated with sufiicient accuracy for the present purpose as described in Industrial Engineering Chemis try, vol. 20, pages 1169-1172 (1928). A somewhat more refined method is described in Industrial Engineering Chemistry, vol. 24, page 819 (1932). The oils to be desulfurized generally have critical temperatures above 405 C. and often above 450 C. The cricondentherm is generally only a few degrees (e. g., 20 C.) above the critical temperature. It is, therefore, preferred to operate at a temperature at least 20 C. above the critical temperature of the material treated.

The above-specified operating temperatures of 325-450 C. are either below the critical temperature of the oil to be desulfurized or sufficiently close thereto that separation of a liquid phase is not precluded. According to the present method .a sufficient quantity of a lower boiling hydrocarbon oil is added to the oil to be desolfurized to bring the critical temperature of the mixture down to below the operating temperature and preferably at least 20 C. below the op erating temperature. The amount to be added in any given case is easily calculated as indicated above.

The oil added, as indicated above, is a lower boiling hydrocarbon oil. However, for reasons which will be apparent the'added oil should not be too volatile. The preferred material is a naphtha boiling substantially within the range of about 100 C. to about 205 C. As will be further explained, the added oil is not materially altered in the treatment; it is frequently desired to recycle it in the process after separating it from the desulfurized oil. When it is desired to do this the boiling range of the added oil should also be chosen to allow easy separation from the desulfurized oil by distillation.

The character of the added oil is also important. It is found that if the added oil is too paramnic in nature it tends 'to cause precipitation of tarry materials from the reaction mixture. It is, therefore, desirable to choose an oil containing less than 50% parafiins. Naphthenic naphthas are quite suited. The added oil may contain some sulfur impurities, but it preferably contains less sulfur than the oil to be clesulfurlzed and preferably less than about 0.15% sulfur.

Naphtha fractions of the type described are readily available, inexpensive and quite satisfactory. They may be improved somewhat for the present purpose, however, by the addition of a small amount (for instance, 0.5 to 10%) of a phenolic compound such as phenol, cresol, o=cyclohexylphenol or 1,2,3, i-tetrahydro--hydroxynaphthalene.

In order that the desulfurization may be effected under the described conditions it is necessary that the reaction mixture be maintained at a high density. This condition is obtained firstly by working in the absence of added hydrogen or other gases, secondly by choosing a lower boiling hydrocarbon oil which is not too volatile, thirdly, by choosing conditions of temperature and space velocity affording a minimum amount of cracking and dehydrogenation, and fourthly, by imposing a very high pressure. Thus, it is necessary that the pressure be sufiicient to maintain the hydrocarbon mixture in the reaction zone at a density of at least 0.25 g./cc. and preferably at least 0.30 g./oc. The material under a pressure above the critical pressure and at a temperature above the critical temperature is sometimes referred to as being in a supercritical dense phase. The specified density is necessary in order to prevent precipitation of tarry deposits on the catalyst. Theoretically, there is no upper limit to the pressures that may be applied; however, a practical upper limit in the order of 1000 atmospheres'is set by the available apparatus.

The process of the invention will be further illustrated by the following specific example.

Example The process of the invention was applied in the desulfurization of a gas oil having the fol lowing inspection data:

Gas Oil Naphtha Density, 20/4 Bromine Numbc Aniline Point, C Critical Temperature, calc Sulfur, per cent by wt ASliM Distillation:

A quantity (1.15 volumes) of a lower boiling naphtha fraction having the inspection data indicated the-above table was added to produce a blend having a critical temperature of about 368 C. The naphtha contained by weight 5.2% aromatics, 53.4% naphthenes and 41.4% parafflns. The mixture was passed at a space velocity of 1 (based on the gas oil) in the absence of hydrogen or other gas through a bed of catalyst while maintaining a temperature of 400 C. and a pressure kg./cm. sufhcient to compress the reaction mixture to a density of 0.35 g./cc. The naphtha fraction was recovered substantially unchanged from the desulfurized gas oil. As seen in the above data the untreated gas oil contained 0.65% sulfur. The treated gas oil contained 0.08% sulfur during the first hours of opera-- tion, after which the sulfur content increased to 0.19% at the end of 300 hours.

The catalyst in this particular case was prepared by treating an activated alumina with hydrochloric acid, impregnating it with cobalt nitrate and ammonium molybdate, and calcining in a current of nitrogen at 370 C. It contained 7 parts of cobalt plus molybdenum, in an atomic ratio of 1 to 5, per 93 parts of alumina.

In other experiments with the same'gas oil and catalyst and under substantially the same conditions, but without the added naphtha, and hence in the liquid phase, the desulfurization activity. of they catalyst declined at a rapid rate. At the end of 100 hours. of operation the treated gas oil already contained about 0.3% sulfur.

In still another case where a parafiinic oil, e. g. octane, was. substituted for the naphtha. the sulfur content of the treated gas oil rose rapidly to over 0.5% sulfur in less than 100 hours.

Also. when operating in the vapor phase the activity of the catalyst. is quickly poisoned unless alarge amount of hydrogen is added. Even when operating in the vapor phase in the presence of large amount of hydrogen, it is not possible to supply the required hydrogen by the simultaneous dehydrogenation of added naphthenic hydrocarbons. For example, in the vapor phase desulfurization of No. 1 range fuel (Ex West Texas. petroleum) boiling between about 182 C. and, 252 C. in the. presence of recycled hydrogen a methylcyclohexane concentrate was added in amounts up to 40 7G. The process was dependent in all cases upon an external source of hydrogen. All attempts to supply the hydrogen by the dehydrogenation of the added methylcyclohexane were unsuccessful due to the inception of substantial cracking (which consumes large amounts of hydrogen) as soon as the temperature was increased sufiiciently to establish dehydrogena ing conditions.

As pointed out, the desulfurization when operating according to the present invention is not efiected through the use of added hydrogen and added gas is detrimental. The small amount of hydrogen required for the desulfurization reaction is supplied by hydrogen exchange within the oil itself. No appreciable amount of dehydrogenation of hydrocarbon constituents in the oil to give free hydrogen takes place at the temperatures used. The process, therefore, is not one which is carried out in the conventional way with hydrogen while attempting to supply the required hydrogen by the simultaneously dehydrogenating added naphthenic hydrocarbons. When a naphthenic naphtha, such as that illustrated, is used to depress the critical temperature, the naphtha, therefore, suffers little change. It is, therefore. desirable in some cases to recycle the naphtha to the reaction zone after separating it from the desulfurized oil. Before reusing the naphtha it is desirable to free it substantially of dissolved hydrogen sulfide.

We claim as our invention:

1. The method of desulfurizing ahydrocarbon oil having a mid-boiling point (ASTM) "of at least 250 C. and having at least 0.5% of organically combined sulfur which comprises adding to said oil to be desulfurized a recycled naphtha of lower sulfur content boiling substantially within the range of 106 C. and 205 C. and containing not more than 50% paraffms in an amount sufiicient to, reduce the critical temperature or the mixture to at least 20 C. below the desired working to: perature, then contracting the resulting mixture in the absence of added gas and in the absence of a liquid phase as. a homogeneous supercritical dense phase with a molybdenum oxide-cobalt oxide-aluminum oxide catalyst containing molybdenurn and cobalt in an atomic ratio of about 1:5 at a temperature above the critical tempera-- ture of the mixture and between 375 C. and. 425 C. while compressed to a density of at least 0.30 g./cc., separating the said naphtha from the desulfurizcd oil, and recycling the said separated naphtha for the treatment of further quantities of said oil to bc desulfurized.

2. The method for desulfurizlng a hydrocar- 8 bon oil having a mid-boiling point ASTM). of at least 200 C. and having organically bound S111!- fur impurities which comprises adding to the said oil to be desulfurized a lower boiling naphthenic hydrocarbon distillate in an amount sufficient to reduce the critical temperature of the resulting mixture to at least 20 C. below the desired operating temperature, then contacting-the resulting mixture as a homogeneous supercritical dense phase in the absence of added gas with a sulfactive hydrogenation catalyst at a temperature at least 20 C. above the critical temperature of the mixture and between 325 C. and 450 C. while compressed to a density of at least 0.25 g./cc., separating the said lower boiling hydrocarbon distillate from the desulfurized hydrocarbon oil, and adding said separated lower boiling hydrocarbon distillate to a fresh quantity of. the said oil to be desulfurized as above specified.

3. The method for desulfurizing a hydr0car bon oil having a mid-b0lling point (AS'I'M) of at least 250 C. and having organically bound sulfur impurities which comprises adding to the oil to be desulfurized a lower boiling non-paraffinic hydrocarbon oil in an amount. sulficientto reduce the critical temperature of the resulting mixture to at least 20 0. below the operating temperature, then contracting the resulting mixture in the absence of added hydrogen and in the absence of a liquid phase with a molybdenum oxide-cobalt oxide-aluminium oxide catalyst at. a. temperature at least 20 C. above the. critical temperature of the mixture and between 325 C. and 450 C. while compressed to a density of at least 0.25 g./cc., and then separating the said lower boiling hydrocarbon oil from the desulfurized hydrocarbon oil.

The method for desulfurizing a hydrocarbon oil boiling above the gasoline boiling range or. having organically bound sulfur impurities which comprises adding to'the oil to be desulfurized a lower boiling straight run fraction of naphthenic petroleum in an amount suiiicient to reduce the critical temperature of the resulting mixture to at least 20 C. below the operating per-ature, then contacting the resulting mixture in the absence of added gas with a sulfactive hydrogenation catalyst at a temperature at least 20 C. above the critical emperature of the mixture and, between 3'75 C. and 425 C. while comto a density oi at least.0.25 g./cc., and then separating the said lower boiling fraction from the desuliurized hydrocarbon oil.

5. The method for desulfurizing a hydrocarbon oil having a mid-boiling point (ASTM) of at least 200 C. and having at least 0.5% of organically bound sulfur which comprises adding tothe. hydrocarbon oil to be desulfurizeol a lower boiling naphtha containing not more than 50% paraffins in an amount sufficient to reduce the critical temperature of the mixture to at least 23 C. below the desired operating temperature. then contacting the resulting mixture in the absence of added gas with a sulfactive hydrogenation catalyst at a temperature at least 20 0. above the critical temperature of the mixture and between 325 C. and 450C. while compressed to a density of at least 0.3 g./cc., and then separating the said naphtha from the desulfurized hydrocarbon oil.

5. The method for desuliurizing a sulfur-bearing hydrocarbon oil boiling above the gasoline boiling range which comprises adding to the hydrocarbon oil to be desulfurized a lower boiling naptha. of lesser sulfur content and. containing not more than 50% paraflins in an amount sufflcient to reduce the critical temperature of the mixture to at least 20 0. below the desired operating temperature, then contacting the resulting mixture in the absence of added hydrogen with a sulfactive hydrogenation catalyst at a temperature at least 20 0. above the critical temperature of the mixture and between 325 C. and 450 C. while compressed to a density of at least 0.25 g./cc., and then recovering the said naphtha from the desulfurized hydrocarbon oil.

WILLEM FREDERIK ENGEL. PETER VAN r SPIJKER.

REFERENCES CITED The following references are of record in the file of this patent:

Number 10 UNITED STATES PATENTS Name Date Rosen Aug. 19, 1941 Schulze Feb. 17, 1942 Bent et al Feb. 8, 1944 Hays June 12, 1945 Huffman Mar. 9, 1948 Byrns July 20, 1948

Claims (1)

1. 4. THE METHOD FOR DESUFLURIZING A HYDROCARBON OIL BOILING ABOVE THE GASOLINE BOILING RANGE AND HAVING ORGANICALLY BOUND SULFUR IMPURITIES WHICH COMPRISES ADDING TO THE OIL TO BE DESULFURIZED A LOWER BOILING STRAIGHT RUN FRACTION OF A NAPHTHENIC PETROLEUM IN AN AMOUNT SUFFICIENT TO REDUCE THE CRITICAL TEMPERATURE OF THE RESULTING MIXTURE TO AT LEAST 20* C. BELOW THE OPERATING TEMPERATURE, THEN CONTACTING THE RESULTING MIXTURE IN THE ABSENCE OF ADDED GAS WITH A SULFACTIVE HYDROGENATION CATALYST AT A TEMPERATURE AT LEAST 20* C. ABOVE THE CRITICAL TEMPERATURE OF THE MIXTURE AND BETWEEN 375* C. AND 425* C. WHILE COMPRESSED TO A DENSITY OF AT LEAST 0.25 G./CC., AND THEN SEPARATING THE SAID LOWER BOILING FRACTION FROM THE DESULFURIZED HYDROCARBON OIL.