US2893951A - Sweetening petroleum hydrocarbons and method for regenerating the treating solution - Google Patents

Description

y 1 w J. PIETERS ET AL 2,393,951

SWEETENING PETRC ELEUM HYDROCARBONS AND METHOD FOR REGENERATING THE TREATING SOLUTION Filed May 28. 1956 9O 8O 7O 6O 5O 4O 3O 20 IO CRESOL INVENTORS WILLEM J. PIETERS 2 CORNELIS o. TEN HAVE THEIR ATTORNEY SWEETENING PETROLEUM HYDROCARBGNS AND METHOD 1? OR REGENERATHNG THE TREATING SQLUTION Willem ll. Pieters and Cornelis D. Ten Have, Amsterdam, Netherlands, assignors to Shell Development Company, New York, N.Y., a corporation of Delaware Application May 28, 1556, Serial No. 587,633

Claims priority, application Netherlands May 31, 1955 6 (Ilaims. (*Cl. 208-204) This invention relates to a method for the direct sweetening of hydrocarbon oils containing acidic sulfur compounds. More particularly, it relates to a method for converting acidic and malodorous mercaptans in petroleum hydrocarbon fractions into organic disulfides, and especially to such a method for the treatment of petroleum hydrocarbon fractions which contain a relatively small proportion of such undesirable sulfur compounds.

It is well known commercial practice to extract mercaptans from hydrocarbon oils by means of an aqueous solution of an alkali metal hydroxide and a solutizer such as an alkali metal phenolate and/ or an alkali metal organic carboxylate, such as naphthenates, butyrates, and the like. The resulting spent caustic extraction solution is usually regenerated by stripping the mercaptans therefrom, as by steam stripping, or by oxidation with air, usually in the presence of an oxidation catalyst.

It is further known (cf. US. Patents 2,015,038 and 2,550,905) that the mercaptans present in hydrocarbon oils can be oxidized to disulfides if the hydrocarbon oil is brought into contact, in the presence of an oxidizing agent, for example, oxygen, with an alkali metal hydroxide solution containing a comparatively small amount of a special type of phenol which is active as oxidation catalyst, particularly a polyhydric phenol or aminophenol having the hydroxyl groups or the hydroxyl and amino groups in the orthoor para-position with respect to each other. According to US. Patent 2,550,905, the aqueous alkali metal hydroxide solution may also contain cresols and xylenols which promote the solubility of mercaptans in the aqueous alkali metal hydroxide solution.

With methods by which mercaptans present in hydrocarbon oils, particularly in gasoline or kerosene, are oxidized to disulfides by contacting the oil with an aqueous alkali metal hydroxide solution in the presence of oxygen, the oxidation itself takes place in the aqueous alkali metal hydroxide solution. The mercaptans are first extracted from the hydrocarbon oil by the alkali metal hydroxide solution and in this solution, in which they are present as mercaptides, they are oxidized by the oxygen present to disulfides which then pass into the hydrocarbon oil. At the same time, the oxygen, being much more soluble in the-oil than in the aqueous solution, is supplied to the aqueous solution for the oxidation primarily via the oil. However, the transfer of oxygen from the hydrocarbon oil to the aqueous alkali metal hydroxide solution proceeds with relative difiiculty.

A method is described in a copending application, now I Ten Have, U.S. 2,763,594, by which the oxidation of mercaptans to disulfides in processes of the type referred to in the preceding paragraph is elfected much more rapidly. This method involves the use of aqueous alkali metal hydroxide solutions containing phenolates and having a water content not greater than 54% by volume. This method has been widely adopted in commercial practice because of its effectiveness and simplicity, especially in the sweetening of hydrocarbon fractions containing Patented July 7, 1959 an amount of mercaptans which is not so great as to justify the relatively expensive mercaptan extraction processes wherein a large volume of extraction solution must be applied to the hydrocarbon oil and subsequently treated or regenerated in a separate step for the removal of the extracted mercaptans.

In this previous process, the treating solution is gradually diluted with water of reaction formed during the oxidation of rnercaptans to disulfides as well as by Water which is usually present in small quantities in the hydrocarbon oil being treated. Thus, while the solution need not be subjected to the major stripping or regeneration steps necessary in mercaptan extraction processes for the removal of large quantities of mercaptans, nevertheless, the water of dilution must be removed continuously or intermittently to avoid dilution of the solution beyond the limit of 54 percent by volume whereupon the mercaptan oxidation reaction is drastically slowed. Heretofore, it has been necessary to vaporize the water with which the solution has been diluted, an expedient which is relatively expensive because it involves the heating of the entire treating solution to the vaporizing temperature and the use of stripping steam or the like. Also, in that process the oxygen which is generally present during the regeneration, and the high temperatures involved, operate to convert some of the alkyl phenolate present to highly acidic oxidation products which irreversibly combine with some of the alkali metal hydroxide present, thus causing a loss of both alkali metal hydroxide and phenolate.

It is therefore a principal object of the present invention to provide an improved process for the sweetening of hydrocarbon oils. Another important object is to provide an improved process for the conversion of mercaptans to disulfides in an aqueous caustic solution. A further object is to provide an improved process for the conversion of mercaptans to disulfides by the use of an aqueous alkali metal hydroxide solution which can be easily and inexpensively regenerated. Other objects will be apparent in the description of the invention.

It has now been discovered that the foregoing objects are accomplished by contacting a light hydrocarbon oil containing mercaptans, in the presence of oxygen, with an aqueous treating solution of an alkali metal hydroxide and a phenolate, the initial composition of which is the same as that of an aqueous solution of an alkali metal hydroxide and a phenolate which can co-exist as a separate liquid phase in heterogeneous equilibrium with one or more solid compounds of the alkali metal of the alkali metal hydroxide with the phenoxy radical of the phenolate.

Further description of the invention will be made with reference in part to the accompanying drawing, wherein:-

Figure I is the phase diagram of the ternary system water-potassium hydroxide-cresol at 20 C., and is exemplary of the water-alkali metal hydroxide-phenol systerns suitably used in the practice of the invention. The numerals along the axes of the diagram are the percentages by weight of the various components.

Figure II is a flow diagram of a typical application of the process of the invention.

The definition of the treating solutions suitable in the practice of the invention means in effect that the process is carried out with solutions which constitute only an extremely small part of the ternary phase diagram for a water-alkali metal hydroxide-phenol system. For example, in Figure I, the area ABCDE represents solutions which can be used in the practice of the process of the copending application, now Ten Have, US. 2,763,594, referred to hereinbefore. However, in the practice of the present invention, for this particular system, only solutions are used in contacting the hydrocarbon oil which lie on the lines ED and DC because onlysuch solutions can co-exist as a separate liquid phase in heterogeneous equilibrium with a solid compound of the alkali metal and the phenol.

The process of the invention, whereby a mercaptancontaining light hydrocarbon oil is contacted in the presence of oxygen with a treating solution as defined above, is advantageous for several reasons. In the first place, the oxidation reaction by which mercaptans are converted to disulfides takes place in the aqueous treating solution. The speed of the reaction is thus determined by the rate the mercaptans are transferred into the treating solution and also by the rate the solution can absorb the oxygen required for the reaction. The treating solutions used in the process of the invention are superior to more dilute solutions used heretofore because the distribution coefiicient, i.e., the ratio of the equilibrium concentration of the mercaptan sulfur in the aqueous phase to that in the hydrocarbon oil phase, is much higher for the present solutions. Also, the rate of transfer of oxygen into such solutions is much higher. As a result, a smaller quantity of the treating solution is required for treating the same amount of hydrocarbon oil than was heretofore the case, for example, half or even less, so that with existing contacting equipment more hydrocarbon oil can be sweetened.

Still another advantage of the invention is that the regenerating agent which may be used to remove excess water from the treating solution will not absorb hydrocarbons, so that provision for hydrocarbon recovery in any evaporation step is not necessary as it would be if the water of reaction were removed by direct evaporation of a treating solution according to prior processes.

More especially, however, the regeneration of the treating solutions of the invention is considerably simplitied and less expensive. Instead of vaporizing excess water from the used treating solution, with the attendant high cost of supplying heat and the loss of alkali metal hydroxide and phenolate because of the oxidation of phenolates at the high regeneration temperatures, as was heretofore necessary, the used treating solution of the process of the invention is regenerated merely by contacting it with a solid compound derived from the alkali metal hydroxide and the phenol, which can co-exist as a separate phase in heterogeneous equilibrium with the original treating solution. This solid compound is aptly referred to as the regenerating agent because by contacting the used treating solution with such a compound, the treating solution reverts to its original composition, the excess water being removed therefrom by absorption into the regenerating agent to form, in part, another solid compound containing the absorbed water as water of crystallization.

. This will be better understood by referring to Fi ure I. A preferred treating solution in accordance with the invention has a composition on the lines ED and DC, for example, at point G. This solution will be diluted with excess water during the treatment of the mercaptancontaining light hydrocarbon oil, as discussed above. The composition of the treating solution will thus shift from point G to a point in the direction of the water apex of the phase diagram. However,when the used treating solution containing the excess water is contacted with solid potassium cresolate, which has a composition indicated by the point P, the excess water is removed from the used treating solution and its composition again reverts to that of the original solution at point G. During this regeneration step, the composition of the regenerating agent will, of course, change in the direction of the water apex of the phase diagram. After the solid compound has absorbed sufiicient water of crystallization from the used treating solution to render it ineffective for further use in the regenerating step, it is replaced by a further amount of solid compound containing little or no water of crystallization. The exhausted .regenerated agent is then reactivated by removing the absorbed water of crystallization therefrom by any of the conventional techniques, for example, by heating to vaporize the water.

It will be noted that the reason that the composition of potassium cresolate (F) lies outside of the phase diagram of Figure I is that it is calculated by subtracting one mol of water from a mol per mol mixture of potassium hydroxide and cresol, and thus giving a negative water content in respect to the system waterpotassium hydroxide-cresol.

The alkali metal hydroxide used in the process of the invention is preferably either sodium hydroxide or potassium hydroxide. Potassium hydroxide is particularly preferred because of the lower viscosity of solutions thereof at the concentrations required in the process.

The alkali metal phenolates (phenates) in the alkali metal hydroxide treating solution can be derived from unsubstituted phenol or from monohydric alkylphenols, such as phenol, o-, mand p-cresols, the various xylenol isomers, the ethylphenols, the propylphenols, and mixtures of any of these, the alkyl groups of which contain no more than three carbon atoms in total; these phenols are understood to be otherwise unsubstituted. Thus, the suitable phenolates are those monohydroxy phenolates which contain only alkali metal, carbon, hydrogen and oxygen atoms and which contain from 6 to 9 carbon atoms. Such individual phenolates and alkylphenolatcs and mixtures thereof are referred to throughout the description of the invention by the generic designation phenolate. Of the suitable phenolates, the cresolates and xylenolates are preferred, especially the cresolates. It is also preferred that the alkali metal of the phenolate be the same as the alkali metal of the alkali metal hydroxide.

The solid compound with which the treating solution is initially capable of existing in heterogeneous equilibrium is a compound of (A) the alkali metal of the alkali metal hydroxide of the treating solution, with (B) the phenoxy radical of the phenolate. In this connection the term phenoxy as used herein is meant to be analogous in scope to the phenolates defined above. Thus, it includes not only the phenoxy radical, C H O--, but also the oxy radicals of the other suitable phenolates, i.e., it also includes the cresoxy, xyloxy and C -alky1- phenoxy radicals, CH C H O-, (CH C H O and (CH C H O--, respectively.

This compound, as the regenerating agent, preferably contains no Water of crystallization as it is first used in regenerating the treating solution, but some water of crystallization may be present initially as long as the compound contains less than the saturation amount of water of crystallization and is therefore able to absorb additional amounts of water from the used treating solution. At the time it is replaced with fresh regenerating agent, the compound may of course be fully saturated with water of crystallization. It will then still be in equilibrium with regenerated treating solution, but will be incapable of regenerating additional amounts of used treating solution.

The ratio of the volume of the alkali phase to the volume of the hydrocarbon oil in the contacting step may vary within wide limits and usually lies between 0.03 and 3; a ratio of 0.03 to 0.3 is preferably used, particularly 0.05 to 0.1.

The oxidation step of the present process, i.e., the contacting of the mercaptan-containing hydrocarbon with the treating solution in the presence of air, is generally carried out at temperatures of 0 C. to 70 C., prefer ably 10 C. to 45 C.

The oxygen for the oxidation step may be supplied to the two-phase system to be treated either as such or in the form of a mixture of oxygen with another gas which is inert under the reaction conditions, e.g., in the form of air. Oxygen-yielding compounds such as ozone or peroxides may also be introduced into the two-phase system to be treated. Some of these possibilities may, of course, be combined.

TW A

greener If oxygen is used as such, it may either be dissolved in the hydrocarbon oil in advance, or be injected into the oil while the latter is being brought into contact with the aqueous solution of alkali metal hydroxide and phenolate. The oxygen is preferably present in an excess of 50% to 200%, particularly 75% to 125%, over the quantity of oxygen theoretically required for the conversion of the mercaptans in the hydrocarbon oil to disulfides. If peroxide is also used, for instance, in a quantity of to 40% of the stoichiometric quantity with respect to the mercaptans or mercaptides to be converted, in accordance with the process of Pieters, US. 2,744,054, the quantity of oxygen supplied as such can be considerably reduced. In this connection it should be noted that hydrogen peroxide is not readily soluble in hydrocarbon oils and rapidly decomposes in the aqueous alkali metal hydroxide solution. It is therefore preferably injected into the hydrocarbon oil in the form of a concentrated solution in water or mixed with the oil in the form of an alcoholic solution. Organic peroxides may frequently be directly dissolved in the hydrocarbon oil, while ozone mixed with oxygen or air may be supplied, for instance by first passing the oxygen or air through an ozonizer.

If the process is used for removing mercaptans from gasoline or kerosene with a mercaptan sulfur content of not more than 0.04% to 0.5% by weight and if the gasoline or kerosene is in equilibrium with atmospheric air, the quantity of oxygen present in the gasoline or kerosene will generally be suflicient to effect the desired oxidation. However, the mercaptans are usually removed from the hydrocarbon oils shortly after the latter have been obtained from the crude oil and after any other pre-treatments have been carried out, with the result that they are not saturated with air. In this case it is often necessary for air or another oxygen-containing gas to be dissolved in the hydrocarbon.

In general, the process is carried out at atmospheric pressure. If the process is used for removing mercaptans from hydrocarbon oils with a relatively high mercaptan sulfur content, e.g., more than 0.05% by weight, and air is used as the oxygen-containing gas, it may be advisable to operate at elevated pressure in order to dissolve a sutficient quantity of oxygen in the hydrocarbon oil. If peroxides are also used, the increase in pressure may be considerably less than when these compounds are not used.

In order to promote the transfer of the oxygen from the hydrocarbon oil to the aqueous alkali metal hydroxide solution containing the phenolate, care must be taken to effect an intense contact between the two phases. This contact may be brought about, for example, by means of a propeller mixer, a centrifugal mixer such as the so-called tnrbomixer (see John H. Perry, Chemical Engineers Handbook, 1941, pp. 1554-1555), or a colloid mill, for instance the so-called Hurrell mill. The alkali metal hydroxide solution may also be sprayed very finely into the hydrocarbon oil under high pressure, for instance, by means of a spray nozzle, or the hydrocarbon oil may be sprayed in this way into the aqueous alkali metal hydroxide solution.

The various agents customarily employed in extractions for increasing the surface between the phase to be extracted and the extraction agent may also be used for the present purpose. Thus, the process may be carried out in a column provided with packing, projections or rotating discs. In this connection, it should, however, be borne in mind that sudden drops in temperature of the alkali metal hydroxide solution should be avoided, since in this case there is a risk of a second liquid alkali phase forming, or of solid components crystallizing from the solution.

The process can be applied for removing mercaptans from light hydrocarbon oils, i.e., hydrocarbon oils with a boiling point or final boiling point of at most 370 C., particularly gasoline and kerosene, of different origin,

including gasoline and kerosene obtained by straightrun distillation from crude oils, as well as gasoline and kerosene obtained from heavy basic materials by cracking. The so-called reformed gasoline may also be freed from mercaptans according to the present process. When using the process for hydrocarbon oils containing unsaturated components, especially cracked gasoline and reformed gasoline, it is preferred to add to the oil an anti-oxidant such as an aryl amine or an alkyl phenol, the alkyl groups of which contain a total of 4 or more carbon atoms, to prevent the formation of peroxides and gum from the unsaturated components of the oil. In general, a quantity of 0.0001% to 0.01% by weight of such an anti-oxidant is sulficient.

It is often desirable to remove from the hydrocarbon oils any acids present therein, such as hydrogen sulfide, which are stronger than the mercaptans by means of a dilute aqueous alkali metal hydroxide solution, before oxidizing the mercaptans according to the present process. Particularly with products obtained by catalytic cracking a pre-treatment with dilute caustic alkali solution has the further advantage that aromatic mercaptans, which are more diflicult to oxidize than aliphatic mercaptans, are removed at least to a considerable extent. It is preferable to carry out this pre-treatment before the cracked products come into contact with oxygen so as to prevent gulm formation.

Since when applying the process for removing mercaptans from hydrocarbon oils the disulfides formed during oxidation pass again into the hydrocarbon oil, the process is primarily suitable for treating light hydrocarbon oils with a low mercaptan content, viz. lower than 0.05 by weight, and preferably lower than 0.02% by weight, calculated as mercaptan sulfur.

When gasoline or kerosene with a considerable mercaptan sulfur content, e.g., 0.05 by weight or more, is to be freed from mercaptans, the greater portion of the mercaptans, if desired together with other sulfur compounds, may be first removed by any of the hitherto usual methods, and then the remainder of the mercaptans oxidized according to the process of the invention. A more advantageous application of the invention to such high mercaptan-content oils is to use a two-step integrated process, the first step being a conventional mercaptan extraction with a relatively dilute extracting solution of an alkali metal hydroxide and an alkali metal phenolate whereby most of the mercaptans are removed and either stripped from or oxidized in the dilute solution, and the second step being the use of the concentrated solutions defined herein for the oxidation of the remaining mercaptans. In this embodiment of the invention the mercaptan-containing dilute solution from the first step is stripped or oxidized for the removal or conversion of the mercaptans; water is removed by then contacting the solution (and equilibrating it) with a solid compound as defined herein; the thus-reconcentrated solution is used in the second step oxidation of the remaining mercaptans; and finally the solution from the second step, which now contains excess water, is recycled to the first step and there used as the relatively dilute extracting solution.

The present process provides a very simple method by which light hydrocarbon oils can be freed from mercaptans in a short time, which in most cases varies between 2 and 20 minutes. If the hydrocarbon oil contains mercaptans which are diificult to oxide, it may be necessary to keep the oil and the aqueous solution of the alkali metal hydroxide and phenol-ate in contact with each other in the manner described for a somewhat longer period. With a sufficiently intense contact between the hydrocarbon oil to be treated and the aqueous solution of alkali metal hydroxide and phenolate, it is, however, also possible in the latter case to free the hydrocarbon oil from mercaptans to such an extent that the oil has a negative doctor tes within an hour.

The invention is further illustrated by the following ex- 70 cu. m. of gasoline with a mercaptan sulfur content of 0.02% by weight, 4 cu. m. of cresolate-containing potassium hydroxide treating solution with a composition as indicated by point G in Figure I, and 10 standard cu. m. of air are introduced continuously per hour via lines 2, 3, and 4, respectively, into a propeller mixer 1, While stirring vigorously. During the residence time in the propeller mixer, i.e., 6 minutes, the mercaptans are almost completely oxidized to disulfides. The mix ture of gasoline and hydroxide solution is drawn oil continuously via line into settling tank 6, from which the gasoline with a \mercaptan sulfur content of less than 0.0005 by weight is led via line 7 to storage or blending operations (not shown).

The used cresolate-containing potassium hydroxide treating solution is drawn oil from the bottom of the settling tank 6 via line 8 to a regenerator tower 9. This tower contains 8 grids on each of which is a layer of the regenerating agent, which in this case is solid potassium cresolate in crystal form free from water of crystallization with a composition which in Figure I is indicated by point F. The solution leaving the settling tank 6 flows down over these grids, giving up water to the solid cresolate. At the bottom of the tower 9 there is obtained a solution which has a composition which is indicated by point G in Figure I. The thus regenerated treating solution is again led to the propeller mixer via the line 3' for use in treating further amounts of mercaptan-containing gasoline. When the mass free from water of crystallization on the grids in tower 9 has been completely or substantially converted into a mass containing water of crystallization, the water content of the solution returning to the mixer via line 9 will rise because of the decreased absorption of water from the treating solution by the exhausted regenerating agent. At this point line 8 is closed and subsequent amounts of the used treating solution are led via line 10 to tower 11. This tower has the same construction and capacity as tower 9. If desired, tower 13 can also be connected via line 12. This tower is also similar to tower 9. If desired, more than one tower may be used at the same time for regenerating the solution used according to the invention. Exhausted treating agent in the tower is replaced by a fresh mass or reactivated in the tower itself, for example, by heating to drive off the water of crystallization absorbed from the used treating solution. It is then suitable for further use in regenerating used treating solution as before.

We claim as our invention:

1. In a process for removing mercaptans from a light 1 hydrocarbon oil through intimate liquid contacting of the oil in presence of oxygen with an aqueous treating solution of an alkali metal hydroxide and an alkali metal phenolate to effect an oxidation of the mercaptans to disulfides with an accompanying formation of water of reaction which causes an objectionable dilution of the treating solution, the improvement comprising utilizing in the process a treating solution initially saturated in the alkali metal phenolate, recovering the water-diluted treating solution from the light hydrocarbon oil, passing said recovered treating solution into contact with a solid mass of the alkali metal phenolate with the mass initially containing less than the amountof water of crystallization that it is capable of retaining, holding the treating solution in contact with said mass until a heterogeneous equilibrium is reached therebetween and there has geen a transfer of the excess water from the treating solution to the solid mass where it is retained as water of crystallization, said transfer of said excess water from the diluted treating solution being achieved without resort to distillation, separating the regenerated treating solution saturated in the alkali metal phenolate from the solid mass, and recycling said treating solution into contact with additional amounts of the light hydrocarbon oil.

2. A process in accordance with claim 1, wherein the.

alkali metal is potassium.

3. A process in accordance with claim 2, wherein the light hydrocarbon oil has -a boiling range within the boiling range of gasoline and kerosene, and, before it is contacted with said aqueous treating solution, has a mercaptan sulfur content not greater than 0.05% by weight.

4. A process in accordance with claim 3, wherein said light hydrocarbon oil is gasoline and is contacted with said aqueous treating solution at a temperature of from 0 C. to C.

5. A process in accordance with claim 2, wherein the phenolate is a cresolate.

'6. A process in accordance with claim 5, wherein the solid mass consists of solid potassium cresolate which initially contains substantially no water of crystallization.

References Cited in the file of this patent UNITED STATES PATENTS 2,556,438 Parker et al June 12, 1951 2,645,602 Tom et al July 14, 1953 2,663,674 Krause et a1. Dec. 22, 1953

Claims (1)

1. IN A PROCESS FOR REMOVING MERCAPTANS FROM A LIGHT HYDROCARBON OIL THROUGH INTIMATE LIQUID CONTACTING OF THE OIL IN PRESENCE OF OXYGEN WITH AN AQUEOUS TREATING SOLUTION OF AN ALKALI METAL HYDROXIDE AND AN ALKALI METAL PHENOLATE TO EFFECT AN OXIDATION OF THE MERCAPTANS TO DISULFIDES WITH AN ACCOMPANYING FORMATION OF WATER OF REACTION WHICH CAUSES AN OBJECTIONABLE DILUTION OF THE TREATING SOLUTION, THE IMPROVMENT COMPRISING UTILIZING IN THE PROCESS A TREATING SOLUTION INITALLY SATURATED IN THE

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