Process for sweetening a sour hydrocarbon fraction using a mixture of a supported metal chelate and a solid base

Abstract

A catalytic mixture of discrete solid materials and a mercaptan oxidation process for using the catalytic mixture have been developed. The catalytic mixture comprises a metal chelate dispersed on a non-basic solid support and a solid base. The process involves contacting a sour middle distillate hydrocarbon fraction which contains mercaptans with the supported metal chelate and the solid base mixture in the presence of an oxidizing agent and a polar compound. The process is unique in that both the catalyst and the base are discrete solid materials.

Description

BACKGROUND OF THE INVENTION

Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides, a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fractions as well as naphtha, kerosene, jet fuel, fuel oil, and the like.

A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. Doctor sweet means a mercaptan content in the product low enough to test "sweet" (as opposed to "sour") by the well-known doctor test. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent usually air. Alternatively, the sour hydrocarbon fraction may be effectively treated by contacting it with a metal chelate catalyst dispersed on a high surface area adsorptive support-usually a metal phthalocyanine on an activated charcoal at oxidation conditions in the presence of a soluble alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.

The prior art shows that alkaline agents are necessary in order to catalytically oxidize mercaptans to disulfides. Thus, U.S. Pat. Nos. 3,108,081 and 4,156,641 disclose the use of alkali hydroxides, especially sodium hydroxide, for oxidizing mercaptans. Further, U.S. Pat. No. 4,913,802 discloses the use of ammonium hydroxide as the basic agent. U.S. Pat. No. 5,232,887 discloses the use of solid base materials which are used both as the support for the metal catalyst and as the alkaline agent. The activity of the metal chelate systems can be improved by the use of quaternary ammonium compound as disclosed in U.S. Pat. Nos. 4,290,913 and 4,337,147.

We have developed a catalytic mixture of solid materials and a process using the catalytic mixture which is significantly different from all the sweetening processes previously disclosed in the art. The prior art describes numerous types of oxidation catalysts used in combination with an alkaline agent which are usually liquid alkaline agents, and in one case a solid base which is the support for a metal chelate catalyst. In contrast, our invention involves the use of a physical mixture of a solid base and a metal chelate dispersed on a non-basic solid support. Moreover, the demonstrated high conversion of mercaptans to disulfides of our invention was contrary to expectations set by the generally accepted working hypothesis of how the alkaline agent functions and by mercaptan oxidation of kerosine studies using the oxidation catalyst alone and the solid base alone.

SUMMARY OF THE INVENTION

The purpose of this invention is to provide a new catalytic mixture for use in a mercaptan oxidation process to sweeten a sour middle distillate hydrocarbon fraction. An embodiment comprises oxidizing the mercaptans by contacting the middle distillate hydrocarbon fraction in the presence of an oxidizing agent and a polar compound with a mixture of a solid base and a supported metal chelate. In a specific embodiment, the metal chelate is a cobalt phthalocyanine dispersed on charcoal. In another specific embodiment the solid base is a metal oxide solid solution. In a still more specific embodiment the metal oxide solid solution is a magnesium oxide and aluminum oxide solid solution. In yet another specific embodiment the catalyst is a cobalt phthalocyanine dispersed on charcoal, and the solid base is a magnesium oxide and aluminum oxide solid solution. Other objects and embodiments of this invention will become apparent in the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a process for treating a sour middle distillate hydrocarbon fraction that contains mercaptans and to a catalytic mixture of discrete yet synergistic solid materials for use in said process. The process involves, in the presence of an oxidizing agent and a polar compound, contacting the middle distillate hydrocarbon fraction with a mixture of a solid base and a metal chelate dispersed on a non-basic solid support. Said middle distillate hydrocarbon fraction is intended to include those hydrocarbon fractions boiling in the range of about 149.degree. C. to about 371.degree. C., such as kerosine, jet fuel, and fuel oil. Said solid base is an alkaline earth metal oxide, a metal oxide solid solution, a layered double hydroxide, or a mixture thereof.

Thus, one necessary component of the instant invention is a metal chelate. The metal chelate employed in the practice of this invention can be any of the various metal chelates known to the art as effective in catalyzing the oxidation of mercaptans contained in a sour petroleum distillate to disulfides. The metal chelates include the metal compounds of tetrapyridinoporphyrazine as described in U.S. Pat. No. 3,980,582, e.g., cobalt tetrapyridinoporphyrazine; porphyrin and metaloporphyrin catalysts as described in U.S. Pat. No. 2,966,453, e.g., cobalt tetraphenylporphyrin sulfonate; corrinoid catalysts as described in U.S. Pat. No. 3,252,892, e.g., cobalt corrin sulfonate; chelate organometallic catalysts such as described in U.S. Pat. No. 2,918,426, e.g., the condensation product of an aminophenol and a metal of Group VIII; and the metal phthalocyanines as described in U.S. Pat. No. 4,290,913, etc. As stated in U.S. Pat. No. 4,290,913, metal phthalocyanines are a preferred class of metal chelates. The metal phthalocyanines and their derivatives which can be employed to catalyze the oxidation of mercaptans generally include those described in U.S. Pat. No. 4,908,122 with the most preferred being a cobalt phthalocyanine, sulfonated cobalt phthalocyanine, or vanadium phthalocyanine.

The metal chelate is dispersed on any of the various non-basic solid adsorbent support materials generally known and utilized as catalyst supports in the prior art as described in U.S. Pat. No. 4,908,122 which is incorporated by reference. Examples of such non-basic solid adsorbent supports are days, silicates, charcoal, and non-basic inorganic oxides. Charcoal and particularly activated charcoal is preferred because of its capacity for metal chelates and because of its stability under treating conditions. Generally, the metal chelate is present at a concentration from about 0.1 to about 10 weight percent of the catalyst.

Another necessary component of this invention is a solid base. The solid base can be an alkaline earth metal oxide, a metal oxide solid solution, a layered double hydroxide, or a mixture thereof, with the most preferred being the metal oxide solid solution. The alkaline earth metal oxide has the formula MO where M is a divalent metal selected from the group consisting of magnesium, barium, calcium, and strontium. The most preferred alkaline earth metal oxides are magnesium oxide and calcium oxide.

The metal oxide solid solution has the formula M.sub.a (ll)M.sub.b (lll)O.sub.(a+b) (OH).sub.b where M(ll) is a divalent metal and M(lll) is a trivalent metal. The M(ll) metals are selected from the group consisting of magnesium, nickel, zinc, copper, iron, cobalt and mixtures thereof. The most preferred divalent metals are magnesium and nickel, and the most preferred mixture is magnesium and nickel. M(lll) is selected from the group consisting of aluminum, chromium, gallium, scandium, iron, lanthanum, cerium, yttrium, boron, and mixtures thereof. The most preferred trivalent metals are aluminum and gallium. Finally, a and b are chosen such that the ratio of a/b is between 1 and about 15 with about 1.5 to about 10 being the most preferred. Two types of metal oxide solid solutions are the most preferred. The first type are those metal oxide solid solutions where M(ll) is magnesium, M(lll) is aluminum, and a/b is in the range of about 1.5 to about 5. The second type are those metal oxide solid solutions where M(ll) is a combination of magnesium and nickel in all molar ratios, with the magnesium to nickel molar ratio range of about 1:1 to about 1:9 being especially preferred, M(lll) is aluminum, and a/b is in the range of about 1.5 to about 10.

The metal oxide solid solutions are prepared by heating the corresponding layered double hydroxide materials (LDH) (see below) at a temperature of about 300.degree. C. to about 750.degree. C. When preparing the solid solution from the LDH precursor, the precursor must have as its counterion (anion) one which decomposes upon heating, e.g., nitrate or carbonate. Counterions such as chloride or bromide would be left on the solid solution support and may be detrimental to catalyst activity.

Layered double hydroxides (LDH) are basic materials that have the formula M.sub.a (ll)M.sub.b (lll)(OH).sub.(2a+2b) (X.sup.-n).sub.(b/n) .multidot.cH.sub.2 O. The M(ll) and M(lll) metals are the same as those described for the solid solution. The values of a and b are also as set forth above. X.sup.- is an anion selected from the group consisting of carbonate, nitrate, halides and mixtures thereof with carbonate and nitrate preferred, and n is 1 for the halides and 2 for carbonate and nitrate. Finally, cH.sub.2 O is the water of hydration and is not of consequence to the instant invention's function. C usually varies from about 1 to about 100. These materials are referred to as layered double hydroxides because they are composed of octahedral layers, i.e. the metal cations are octahedrally surrounded by hydroxyl groups. These octahedra share edges to form infinite sheets. Interstitial anions such as carbonate are present to balance the positive charge in the octahedral layers. The preparation of layered double hydroxides is well known in the art and can be exemplified by the preparation of a magnesium/aluminum layered double hydroxide which is known as hydrotalcite. The formula of hydrotalcite is Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).multidot.4H.sub.2 O, and it can be prepared by coprecipitation of magnesium and aluminum carbonates at a high pH. Thus magnesium nitrate and aluminum nitrate (in the desired ratios) are added to an aqueous solution containing sodium hydroxide and sodium carbonate. The resultant slurry is heated at about 65.degree. C. to crystallize the hydrotalcite and then the product is isolated and dried. Extensive details for the preparation of various LDH materials may be found in J. Catalysis, 94, 547-557 (1985) which is incorporated by reference.

The catalytic effectiveness of the mixture of the present invention to effect mercaptan oxidation was completely unexpected and is without theoretical or experimental precedent. Use of a metal chelate catalyst dispersed on a non-basic solid support alone led to mercaptan oxidation of a kerosine in only low yield. Use of a solid base material alone led to mercaptan oxidation of a kerosine in a somewhat higher yield which was still low. However, the mixture of a solid base and a metal chelate dispersed on a non-basic support afforded mercaptan oxidation in a yield far greater than that expected from the sum of the yields of the two components demonstrating that our mixture is truly synergistic.

EXAMPLE 1

A reactor bed was loaded with 7.5 cc of sulfonated cobalt phthalocyanine supported on high surface area carbon. A sour kerosine feedstock boiling in the range of 172.degree. C. to 281.degree. C. and containing about 328 ppm mercaptan sulfur was processed through the reactor bed at a liquid hourly space velocity of 6 hours.sup.-1, an inlet temperature of 38.degree. C. and a pressure of 100 psig. The feedstock was charged under sufficient air pressure to provide 2 times the stoichiometric amount of oxygen required to oxidize the mercaptans. Water, 7,000 ppm, and quaternary ammonium hydroxide, 8.75 ppm, were added to the feedstock. The measured percent conversions of mercaptans to disulfides under this system are in Table 1 in the column marked Metal Chelate.

A reactor bed was loaded with 38 cc of metal oxide solid solution where the divalent metals were magnesium and nickel in a 1:3 molar ratio, the trivalent metal was aluminum, and the ratio of all divalent metals to all trivalent metals was 2:1. The same type of feedstock as used above with identical water and quaternary ammonium hydroxide content and operating conditions was passed through the bed at a liquid hourly space velocity of 1.2 hours.sup.-1. It is important to note the space velocity in this experiment is significantly lower than the space velocity of the other two experiments described. For comparison, we have included an estimate of what we reasonably believe the results would be at a space velocity of 6 hours.sup.-1. This data is in Table 1 in the column marked Solid Solution.

A reactor bed was filled with a mixture of 7.5 cc of Sorbplus obtained from Alcoa and 7.5 cc of sulfonated cobalt phthalocyanine and dimethyl benzylalkyl ammonium chloride impregnated on high surface area carbon. Sorbplus is a commercial metal oxide solid solution where the divalent metal is magnesium, the trivalent metal is aluminum, and the ratio of divalent to trivalent metals is 3.8:1. Sorbplus also contains some magnesium oxide as an impurity. A sour kerosine feedstock boiling in the 172.degree. C. to 281.degree. C. range and containing about 381 ppm mercaptan sulfur was processed through the reactor bed at a liquid hourly space velocity of 3 hours.sup.-1, an inlet temperature of 38.degree. C. and a pressure of 100 psi. The feedstock was charged under sufficient air pressure to provide about 2 times the stoichiometric amount of oxygen required to oxidize the mercaptans. Methanol, 8,000 ppm, was added to the feedstock. Note that the liquid hourly space velocity of 3 hours.sup.-1 used in this experiment is equivalent to a mixture of the metal chelate at a liquid hourly space velocity of 6 hours.sup.-1 and the solid solution at a liquid hourly space velocity of 6 hours.sup.-1. This data is in Table 1 in the column marked Mixture.

TABLE 1______________________________________Percent Conversion of Mercaptans to DisulfidesHours Metal Solid Solutionon Chelate LHSV 6 MixtureStream LHSV 6 LHSV 1.2 (estimate) LHSV 3______________________________________4 17 68 (15-35) 928 19 60 (15-35) 9212 17 62 (15-35) 88Average 18 63 (15-35) 91______________________________________

As a comparison of the data demonstrates, the conversion achieved by the invention is substantially greater than the expected sum of the components.

The catalytic effectiveness of the invention was a further surprise since our historic working hypothesis has been that the alkaline agent functions to form a mercaptide which then reacts quickly with the supported metal chelate to form disulfide. Since we have a discrete solid base particle, a physically separate particle from the supported metal chelate, we expected the mercaptide, when formed at the alkaline agent, would be unable to move to the metal chelate particle due to the lack of an available cation. According to this hypothesis, we expected our invention to provide only low conversion of mercaptan to disulfide. Our experimental results to the contrary were wholly unexpected.

Physically separating the alkaline agent and the metal chelate into discrete particles has additional benefits. For example, a solid base which is separate from the metal chelate may have greater basicity than a solid base which also serves as a support for the metal chelate since the metal chelate will cover basic sites on the solid base. Consequently, the separate solid base may have increased activity due to greater basicity and extended life due to its increased capacity for poisons before deactivating.

In order to improve the activity and stability of the catalyst, an onium compound can be added to the hydrocarbon feed or dispersed on the non-basic support along with the metal chelate. Onium compounds are ionic compounds in which the positively charged (cationic) atom is a nonmetallic element, other than carbon, not bonded to hydrogen. For the practice of this invention it is desirable that the onium compounds have the general formula [R'(R).sub.W M].sup.+ X.sup.-. In said formula, R is a hydrocarbon group containing up to about 20 carbon atoms and selected from the group consisting of alkyl, cycloalkyl, aryl, alkaryl and aralkyl. It is preferred that one R group be an alkyl group containing from about 10 to about 18 carbon atoms. The other R group(s) is (are) preferably methyl, ethyl, propyl, butyl, benzyl, phenyl, and naphthyl groups. R' is a straight chain alkyl group containing from about 5 to about 20 carbon atoms and preferably an alkyl radical containing about 10 to 18 carbon atoms. M is phosphorus (phosphonium compound), nitrogen (ammonium compound), arsenic (arsonium compound), antimony (stibonium compound), oxygen (oxonium compound) or sulfur (sulfonium compound). X.sup.- is hydroxide, sulfate, nitrate, nitrite, phosphate, acetate, citrate and tartrate, w is 2 when M is oxygen or sulfur and w is 3 when M is phosphorous, nitrogen, arsenic or antimony. The preferred cationic elements are phosphorus, nitrogen, sulfur, and oxygen. The onium compounds which can be used in this invention are discussed in U.S. Pat. Nos. 4,913,802 and 4,156,641 which are incorporated by reference.

When the optional onium compound is added as a liquid to the middle distillate hydrocarbon fraction, it is desirable that it be present in a concentration from about 0.05 to about 500 wppm and preferably from about 0.5 wppm to about 100 wppm based on hydrocarbon. When the onium compound is dispersed onto the non-basic support as described in U.S. Pat. No. 4,824,818, it is desirable that the onium compound be present in a concentration from about 0.1 to about 10 weight percent of the supported metal chelate. Furthermore, the onium compound may be initially dispersed onto the non-basic support and then desired amounts within the range 0.05 to 500 ppm may be added intermittently to the middle distillate hydrocarbon fraction.

Another necessary component of the process of this invention is a polar compound which is generally present in a concentration from about 10 ppm to about 15,000 ppm based on hydrocarbon. It is believed that the function of this polar compound is to serve as a proton transfer medium. Specifically, the compound is selected from the group consisting of water, alcohols, esters, ketones, diols and mixtures thereof. Specific examples include methanol, ethanol, propanol, isopropyl alcohol, t-butyl alcohol, n-butyl alcohol, benzyl alcohol and s-butyl alcohol. Examples of diols which can be used include ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, 1,3-butylene glycol and 2,3-butylene glycol. Examples of ketones and esters are acetone, methyl formate and ethyl acetate. Of these compounds the preferred are water and alcohols, with methanol being an especially preferred alcohol.

As previously stated, sweetening of the sour middle distillate hydrocarbon fraction is effected by oxidizing the mercaptans to disulfides. Accordingly, the process requires an oxidizing agent, preferably air, although oxygen or other oxygen-containing gases may be employed. The sour middle distillate hydrocarbon fraction may contain sufficient entrained air, but generally added air is admixed with the fraction and charged to the treating zone concurrently therewith. In some cases, it may be advantageous to charge the air separately to the treating zone and countercurrent to the fraction separately charged thereto.

The treating conditions and specific methods used to carry out the present invention are those that have been disclosed in the prior art. The sour middle distillate hydrocarbon fraction may be contacted with a mixture of the catalyst and the solid base which is in the form of a fixed bed. The contacting is thus carried out in a continuous manner and the middle distillate hydrocarbon fraction may be flowed upwardly or downwardly through the mixture of materials. The process is usually effected at ambient temperature conditions, although higher temperatures up to about 105.degree. C. are suitably employed. Pressures of up to about 1,000 psi or more are operable although atmospheric or substantially atmospheric pressures are suitable. Contact times equivalent to a liquid hourly space velocity of from about 0.5 to about 20 hours.sup.-1 or more are effective to achieve a desired reduction in the mercaptan content of a sour middle distillate hydrocarbon fraction, an optimum contact time being dependent on the size of the treating zone, the quantity of catalyst and solid base contained therein, and the character of the fraction being treated.

The following example is presented in illustration of this invention and is not intended as an undue limitation on the generally broad scope of the invention as set out in the appended claims.

EXAMPLE 2

A reactor bed was filled with a mixture of 7.5 cc of Sorbplus obtained from Alcoa and 7.5 cc of sulfonated cobalt phthalocyanine and dimethyl benzylalkyl ammonium chloride impregnated on high surface area carbon. Sorbplus is a commercial metal oxide solid solution where the divalent metal is magnesium, the trivalent metal is aluminum, and the ratio of divalent to trivalent metals is 3.8:1. Sorbplus also contains some magnesium oxide as an impurity. A sour kerosine feedstock boiling in the 172.degree. C. to 281.degree. C. range and containing about 381 ppm mercaptan sulfur was processed through the reactor bed at a liquid hourly space velocity of 3 hours.sup.-1, an inlet temperature of 38.degree. C. and a pressure of 100 psi. The feedstock was charged under sufficient air pressure to provide about 2 times the stoichiometric amount of oxygen required to oxidize the mercaptans. Methanol, 8,000 ppm, was added to the feedstock.

The conversion achieved by the above mixture (System A) compared well with the conversion of mercaptans to disulfides obtained by an equal loading of a cobalt phthalocyanine catalyst dispersed onto a metal oxide solid solution where the divalent metals were magnesium and nickel in a 1:3 molar ratio, the trivalent metal was aluminum, and the ratio of all divalent metals to all trivalent metals was 2:1 (System B), at the same conditions except 7,000 ppm of water instead of 8,000 ppm methanol and 8.75 ppm of quaternary ammonia hydroxide was added to the feedstock. See Table 2.

TABLE 2______________________________________Percent Conversion of Mercaptans to DisulfidesHours on Stream System A System B______________________________________4 92 938 92 9812 88 98______________________________________

As the data shows, the catalytic mixture of a metal chelate dispersed on a non-basic support, and a solid base is effective in sweetening a sour kerosine fraction.

Claims

1. A catalyst for oxidizing mercaptans comprising a mixture of (1) a non-basic solid support on which a metal chelate is dispersed, and (2) a physically discrete and separate solid base selected from the group consisting of a) alkaline earth metal oxides, b) metal oxide solid solutions having the formula M.sub.a (ll)M.sub.b (lll)O.sub.(a+b) (OH).sub.b where M(ll) is a divalent metal selected from the group consisting of magnesium, nickel, zinc, copper, iron, cobalt, calcium, and combinations thereof, M(lll) is a trivalent metal selected from the group consisting of aluminum, chromium, gallium, scandium, iron, lanthanum, cerium, yttrium, boron, and combinations thereof, and a/b is between 1 to about 15, and c) layered double hydroxides represented by the formula M.sub.a (ll)M.sub.b (lll)(OH).sub.(2a+2b) (X.sup.-n).sub.b/n .multidot.cH.sub.2 O where X.sup.- is an anion selected from the group consisting of carbonate, nitrate, halide, and combinations thereof, n is 1 where X.sup.- is a univalent anion and 2 where X.sup.- is a divalent anion, and cH.sub.2 O is water of hydration.

2. The catalyst of claim 1 where the non-basic solid support is selected from the group consisting of charcoal, activated charcoal, clays, silicates, and non-basic inorganic oxides.

3. The catalyst of claim 2 where the non-basic solid support is charcoal.

4. The catalyst of claim 1 where the metal chelate is a metal phthalocyanine.

5. The catalyst of claim 4 where the metal phthalocyanine is cobalt phthalocyanine.

6. The catalyst of claim 1 where the metal chelate is present in a concentration from about 0.1 to about 10 weight percent of the metal chelate dispersed on the non-basic solid support.

7. The catalyst of claim 1 where the solid base is a metal oxide solid solution.

8. The catalyst of claim 1 where M(ll) is magnesium, M(lll) is aluminum, and a/b is in the range of about 1.5 to about 5.

9. The catalyst of claim 1 where M(ll) is a combination of magnesium and nickel in all molar ratios, M(lll) is aluminum, and a/b is in the range of about 1.5 to 10.

10. The catalyst of claim 1, where M(ll) is a combination of magnesium and nickel and where the magnesium to nickel molar ratio is in the range of about 1:1 to about 1:9, M(lll) is aluminum, and a/b is in the range of about 1.5 to about 10.

11. The catalyst of claim 1 where the solid base is an alkaline earth metal oxide.

12. The catalyst of claim 11 where the alkaline earth metal oxide is magnesium oxide.