Removal of Organic Sulfur Compounds From FCC Gasoline Using Regenerable Adsorbents

Diverse types of petroleum feedstocks contain sulfur compounds whose removal is an indispensable prerequisite for commercial utilization of the feedstock, for subsequent processing of the feedstock, or both. Consequently, it is no surprise that substantial efforts have been expended to eliminate sulfur-containing materials from petroleum products. For example, the Claus process is commercially employed in removing hydrogen sulfide from feedstocks, at least for large streams containing large amounts (greater than about 1000 ppm) hydrogen sulfide. The Stretford process is a vanadium-based oxidative conversion of hydrogen sulfide to sulfur. A non-oxidative method of hydrogen sulfide removal is exemplified by the work of Bricker and Imai, U.S. Pat. No. 5,034,118.

Various oxidative processes also are known for removal of mercaptans by converting them to disulfides; many of these are available as the Merox.TM. process (see Handbook of Petroleum Refining Processes, R.A. Meyers, editor-in-chief, chapter 9.1, McGraw-Hill Book Company (1986)). It is also known to remove mercaptans and disulfides from petroleum feedstocks by adsorption with clays; see U.S. Pat. No. 5,360,536. In fact, adsorptive processes for sulfur removal may have elements of generality not shared by some oxidative processes.

U.S. Pat. No. 3,051,646 uses molecular sieves to selectively remove sulfur and sulfur-containing compounds such as mercaptans and disulfides. By using molecular sieve adsorbents with an average pore diameter of 8-20 angstroms the patentee avoided significant removal of hydrocarbon components. Hydrogen subsequently was used to desorb the adsorbed sulfur compounds and thus regenerate the molecular sieve. U.S. Pat. No. 3,211,644 teaches the use of crystalline zeolitic molecular sieve materials with an approximate pore size of at least 3.8 A, including zeolite X, to adsorb sulfur-containing compounds from liquid hydrocarbon feedstocks with subsequent desorbtion of the sulfur compounds from the molecular sieves using a non-adsorbable purge gas, e.g., methane, hydrogen, nitrogen, and carbon dioxide. The patentee of U.S. Pat. No. 3,620,969 teaches that zeolitic molecular sieves dehydrated to a stated residual water loading may be used as an adsorbent for liquid hydrocarbon feeds with thermal swing desorption of the adsorbed sulfur compounds using a conventional purge gas with a high water content. U.S. Pat. No. 5,114,689 recognized the disadvantages and problems associated with the regeneration of molecular sieve adsorbents used in the desulfurization of hydrocarbon streams and suggested solutions thereto.

Although the prior art relating to the use of molecular sieves as adsorbents for removal of sulfur compounds is relatively well developed, there are unique problems in attempting to utilize molecular sieves for the analogous purification of FCC feedstocks. In particular, the different nature of FCC streams insures a substantially different nature in the sulfur-containing organic material. In fact, many sulfur-containing organic materials in FCC streams are highly aromatic, in contrast to the sulfur streams of, for example, distillate gasoline, which has two important consequences. One consequence is that the nature of some major sulfur-containing organic materials is quite analogous to the major components of the FCC hydrocarbon matrix, making it more difficult to find molecular sieves which will selectively adsorb the offending sulfur-containing materials. A second consequence is that the adsorbed organic heterocyclic sulfur-containing materials are sufficiently strongly adsorbed that regeneration of the molecular sieve by conventional means is ineffective. Since once-through use of adsorbents simply is impractical and quite uneconomical, regenerability of molecular sieves is a sine qua non for any commercially viable process.

We have devised an effective process to remove sulfur-containing compounds from FCC feedstocks based on certain molecular sieves loaded with active hydrogenation metals. In particular, where the adsorbent is a potassium-exchanged zeolite X with palladium or platinum dispersed thereon, we have found it is not only possible to selectively adsorb heterocyclic sulfur-containing compounds so prevalent in the FCC feedstocks without a concomitant significant loss of aromatic hydrocarbons, but it is also possible to effectively regenerate the sulfur-laden adsorbents. Regeneration is performed in a hydrogen atmosphere at elevated temperatures in what is in effect a reductive desulfurization stage.

The purpose of our invention is to provide a process for removal of organic sulfur compounds from petroleum feedstocks using a regenerable sorbent. Although our invention is quite general in scope, it is particularly applicable to FCC feedstocks where our invention provides a process for selective removal of organic sulfur compounds, especially heterocyclic sulfur compounds, using a regenerable sorbent. An embodiment comprises contacting a petroleum feedstock with a zeolite X exchanged with an alkali or alkaline earth metal cation and loaded with a group VIII metal. In a more specific embodiment the zeolite X is exchanged with an alkali metal. In a still more specific embodiment the alkali metal is potassium. In another embodiment the group VIII metal is platinum. In yet another specific embodiment the zeolite X is potassium exchanged and loaded with zerovalent platinum.

We have developed a process for removal of organic sulfur compounds, particularly heterocyclic sulfur compounds, from petroleum feedstocks, and especially FCC feedstocks, which overcomes the prior art limitations of poor selectivity and non-regenerability of the sorbent. We have observed that zeolite X exchanged with an alkali or alkaline earth cation metal selectively adsorbs the organic sulfur compounds from petroleum feedstocks generally, and FCC feedstocks in particular, with little attendant adsorption of aromatic hydrocarbons from the feedstock. We also have observed that if the alkali metal or alkaline earth metal cation exchanged zeolite X also is loaded with a group VII metal, then regeneration of the sorbent is achieved by heating the sulfur-laden adsorbent in a hydrogen atmosphere at temperatures in the range from 25.degree. C. up to about 300.degree. C. These observations afforded our invention, which is a process of selective adsorption of organic sulfur compounds, and especially heterocyclic sulfur-containing compounds, from petroleum feedstocks, particularly FCC feedstocks, with subsequent regeneration of exhausted sorbent.

It needs to be stressed that our invention is applicable to petroleum feedstocks generally. Exemplary of petroleum feedstocks which may be used in the practice of this invention include kerosine, middle distillates, light gas oil, coker naphtha, and so forth. However, the petroleum feedstocks to which our invention is particularly applicable are FCC feedstocks. The FCC feedstocks referred to herein are typically withdrawn as a particular boiling point range from the upper portion of the so-called FCC Main Column. FCC gasoline is characterized as having a boiling point in the range of C5 paraffins up to about 450.degree. F. Such material is composed of many kinds of discrete hydrocarbons, including olefins, paraffins, and aromatics. Such material also has sulfur-containing materials such as benzothiophene and thiophene, which are representative of heterocyclic sulfur compounds, and various types of mercaptans (thiols) with the total concentration amounting to as much as several thousand ppm. The subsequent description shall refer almost exclusively to FCC feedstocks, but it is to be clearly understood that this is done not only to reflect the relative importance of this particular feedstock in the practice of our invention but also to represent illustratively the feedstocks for which our invention may be practiced.

One characteristic of FCC feedstocks is that the nature of the sulfur impurities generally is significantly different from the nature of sulfur-containing materials, in, for example, distillate fuels. In particular, FCC feedstocks contain aromatic heterocyclic sulfur compounds in addition to mercaptans, whose adsorptive properties are quite similar to the aromatic compounds of the hydrocarbon matrix in FCC feedstocks. As previously mentioned, this makes it significantly more difficult to selectively remove sulfur-containing materials from FCC feedstocks than for other feedstocks. Among the aromatic heterocyclic compounds of particular interest in this application are thiophene, 2-methylthiophene, 3-methylthiophene, 2-ethylthiophene, benzothiophene, and dimethylbenzothiophene. Mercaptans which will be removed by the process of this invention often contain from 3-10 carbon atoms, and are illustrated by materials such as 1-mercaptopropane, 2-mercaptopropane, 1-mercaptobutane, 2-mercaptobutane, 2-methyl-2-mercaptopropane, mercaptopentanes, mercaptohexanes, mercaptoheptanes, mercaptooctanes, mercaptononanes, and mercaptodecanes. The total sulfur content in FCC feedstocks usually is in the range from about 150 to as much as several thousand ppm. After treatment according to our invention the sulfur content is desirably no more than about 100 ppm, and most desirably under about 50 ppm. The process which is our invention is particularly suitable for feedstocks with relatively low aromatic content, or for fractions high in benzothiophene or alkylated benzothiophene.

We have found that zeolite X is suitable for adsorption of sulfur compounds from FCC feedstocks without significant loss of FCC hydrocarbons. In particular, we have found that zeolite X, which has been exchanged with an alkali or alkaline earth metal cation, shows good adsorption capacity for aromatic heterocyclic sulfur compounds. Among the cations which may be used are included lithium, sodium, potassium, rubidium and cesium, exemplifying the alkali metal cations, and beryllium, magnesium, calcium, strontium, and barium as exemplifying the alkaline earth metal cations. Although any of the alkali and alkaline earth metal cation exchanged zeolite X materials may be used in the practice of our invention, the alkali metal exchanged materials are preferred, and among these the sodium and potassium exchanged materials are most desirable. In the absence of exchange, oligomerization of olefins present in the FCC feedstock often occurs to the extent of affording significant gum formation and attendant lower octane of the FCC material. The alkali metals in particular reduce the acidity of the zeolite X affording lower oligomerization with minimal effect on octane. In the practice of our invention we most prefer the potassium exchanged material. Typically, there is at least 50% of the exchangeable sites occupied by an alkali or alkaline earth metal cation, although our preference is to have essentially all of the available sites exchanged with the alkali or alkaline earth metal cation. Adsorption of organic sulfur compounds by the adsorbents of our invention is conveniently effected by contacting at temperatures from about 25.degree. to about 200.degree. C. for a time sufficient to adsorb the organic sulfur compounds present and reduce sulfur content to less than 100 ppm, and preferably less than 50 ppm.

Even though the alkali metal or alkaline earth metal cation exchanged zeolite X is an effective selective adsorbent for the aromatic heterocyclic sulfur materials present in FCC feedstocks, nonetheless its lack of regenerability precludes successful use in a commercial process. Thus, treatment with hydrogen in accord with prior art regeneration procedures fails to regenerate the sorbent from a sulfur-laden sorbent bed. However, if the sorbent is loaded with a group VIII metal, we have observed that sorbent can be regenerated upon contact with hydrogen, especially at somewhat elevated temperatures. By "loaded" we mean that the group VII metal may be placed on the sorbent by cation exchange, by simple impregnation, or by vapor phase deposition; the manner in which the group VIII metal is placed on the sorbent is not critical to the success of our invention. Among the group VII metals which may be employed in the practice of our invention are nickel, ruthenium, rhodium, palladium, and platinum, with palladium and platinum apparently the most effective materials. The zeolite X adsorbents of our invention typically have between 0.05 and about 1.0 wt. % of palladium or platinum (as a zerovalent metal) loaded thereon. Sulfur-laden zeolite X having at least one of the group VIII metals loaded thereon may be readily regenerated by treatment with hydrogen at temperatures from about 25.degree. C. up to about 350.degree. C. but typically at temperatures between about 80 and about 300.degree. C.

The following examples are illustrative of our invention and are not intended to limit it in any way. Specifically, we emphasize again that our invention is applicable to removal of a broad range of organic sulfur compounds from petroleum feedstocks generally; our description is couched in terms of FCC feed solely for expository convenience. These examples show the preparation of adsorbents, and clearly demonstrate the lack of regenerability of the zeolite X adsorbents in the absence of a group VIII hydrogenation metal.

Preparation of Adsorbents. Typically zeolite X is prepared in the sodium form according to procedures well known in the art. The NaX then is bound with a clay which is present generally to the extent of 10-20 wt. % and meshed to a 20-40 mesh size. The bound NaX is then exchanged with an alkali or alkaline earth salt to completely exchange the sodium form, with, for example, the potassium form. This material was used as an adsorbent with an FCC model feedstock whose composition is given in Table 1.

The adsorbent cyclic capacity and selectivity for adsorbents was determined by column breakthrough and desorption. A column was equipped with low dead volume fittings and loaded with a measured weight of dried adsorbent material. The column was heated to 650.degree. C., and normal heptane was pumped through the column at a measured liquid hourly space velocity (LHSV). At time zero, the heptane flow was stopped and model solution flow was commenced at the same space velocity. The effluent of the column was collected into fractions and analyzed by gas chromatography. By measuring the breakthrough volumes of the various components, the adsorbent capacities can be obtained.

A column of X type zeolite, bound with clay and formed into beads, was used in the breakthrough experiment as described above. The beginning of the benzothiophene breakthrough under the aforedescribed standard conditions was observed after feeding the column about 36 cc of the simulated FCC gasoline.

A column of zeolite X, saturated with sulfur compounds in the manner of the prior paragraph, was desorbed by heating the bed in accordance with the teachings of U.S. Pat. No. 4,404,118. The regeneration was conducted according to these prior art teachings by first contacting the adsorbent with hydrogen at 21.degree. C. for 20 minutes. Then, the temperature was raised and the column was contacted with hydrogen at 61.degree.-63.degree. C. for 15 minutes. Next the temperature was raised, and the column held at 300.degree.-303.degree. C. for 13 minutes. During all these steps, the pressure was 20-21 psig hydrogen and the hydrogen flow rate was in the range of 630-680 GHSV. After the desorption step was completed, another breakthrough was conducted as before to determine the adsorption behavior of the sulfur components. The bed exhibited a dramatic decrease in capacity for benzothiophene; breakthrough was observed after less than 5cc feedstock. This shows that the regeneration step was ineffective.

The potassium-exchanged zeolite X as described above was used in the determination of cyclic capacity, except that it was first partially ion exchanged with platinum using the following procedure. 15 gms KX zeolite, bound with clay and formed into beads, 200 mg KCl, 160 mg Pt(NH.sub.3).sub.4 Cl.sub.2, and 400 cc water were mixed at 70.degree. C. for 6 hours. The zeolite was filtered from the solution and washed with 200 cc deionized water, than dried at 90.degree. C. It was then dried in a muffle furnace at 250.degree. C. for 2 hours and then treated with 2 liters/minute hydrogen at 250.degree. C. for 2 hours. The cyclic capacity was determined by repeating the breakthrough and desorption as described above. After three regenerations breakthrough was observed at 25 cc feedstock. The capacities estimated from the cyclic capacity experiments are given in Table 2. Although there is a decline in benzothiophene capacity compared to the fresh material, the Pt-KX zeolite has significant cyclic capacity of about 1 wt. % or more for benzothiophene, compared to KX zeolite without platinum ion exchange which has less than 0.1 wt. % capacity.