GB2162534A - Silicalite reforming process - Google Patents

Abstract

A process for reforming a low octane feedstock composed of predominantly normally liquid hydrocarbons within the gasoline range to a product predominantly within the gasoline range and having a higher octane rating.The feedstock, containing branched and straight aliphatic hydrocarbons, is passed into a reaction zone and into contact with a catalyst system comprising a discrete physical mixture of a metal or metal oxide dehydrogenation catalyst and a shape-selective silicate catalyst. The silicalite catalyst is preferentially selective to a straight chain aliphatic hydrocarbons while being restrictive to branch chain isomers thereof. Conversion conditions are established within the reaction zone to produce a reformed product having an increased aromatic content and a decreased normal paraffin content.

Description

SPECIFICATION Silicalite reforming process This invention relates to the reforming of hydrocarbon feedstocks and more particularly to reforming processes employing a catalyst system comprising a discrete physical mixture of zinc oxide and a silicalite conversion catalyst.

In the chemical and refining industry,thereare various processes which involve the conversion of hydrocarbon feedstocks into corresponding products of higher economic value. Reforming operations are carried out on petroleum fractions in the gasoline boiling range in order to increasetheiroctane ratings.

Octane numbers may be increased from values of from 30to 60 octane to values within the range of from 70to 90 octane. Typical operating conditionsfor reforming operations include temperatures ranging from 400 to 600"C and pressures ranging from 5 to 35 bars. Normally, commercial reforming operations are carried out employing noble metal catalysts such as platinum, often in combination with another metal such as rhenium. Reforming reactions which produce a product of increased octane rating include isomerization of linear hydrocarbons, conversion or linear hydrocarbons to cyclic hydrocarbons, aromatization ofcyclicnapthenes by dehydrogenation and hydrocracking of heavier paraffins to lighter products.

A disadvantage of the conventional platinum-type catalysts is their low tolerance to catalyst "poisons" such as sulfur and nitrogen compounds. Such catalysts are particularly susceptible to sulfur poisoning and require feedstocks of very low sulfur content normally less than a few parts per million (ppm).

Reforming operations normally do not effect any substantial change in boiling points. The boiling point range of the product is about the same, or perhaps slightly lessthan, the boiling point range ofthe feedstock.

Reforming and other hydrocarbon conversion processes are also carried out in the presence of molecular sieve type catalysts. Such molecular sieves may be employed as a composite with a metal catalyst. For example, U.S. Patent4,276,151 (Planket al.) discloses a catalytic reforming process in which the catalyst is a composite of a zeolite, i.e., a crystalline aluminosilicate, and a conventional metallic reforming catalyst such as a platinum/rhenium catalyst.

U.S. Patent No.4,197,214 (Chen etal.) discloses the use of metal-zeolite combinations in various conversion reactions including the formation of aromatics from normal hexane. Composite catalysts disclosed in that patent include zeolite with zinc incorporated by wet impregnation, ion exchange, and an extrudate formed from zinc oxide and ammonium exchanged ZSM-5 zeolite. The impregnated, ion exchanged and extruded forms showed substantially increased aromatization activity relative to the ammonium exchanged zeolite alone. The ion exchanged composites showed an improved steam stability, the zinc oxide added by physical mixing did not.

U.S. Patent Nos. 4,288,645 (Wag staff) and 4,291,182 (Dautzenberg metal.) disclose aromatization processes based upon feedstreams predominantly of propane and butane respectively. Those processes are carried out at conversion conditions of 400-700 C and 5-10 bars in the presence of a crystalline silicate containing zinc as a promotor. Zinc is present in amounts within the range of 0.05-20 weight percent, preferably 0.1 -5 weight percent and may be incorporated into the silicate by ion exchange or by impregnation.

U.S. Patent No.4,403,044 (Post et al.) discloses a large number of conversion procedures employing silicalite based catalyst systems. The processes disclosed in Postetal. include isomerization, hydrocracking, improving the octane number of gasoline, and aromatization processes. Feedstreams for the aroma- tization processes include acyclic organic compounds and aliphatic and/or cycloaliphatic hydrocarbons and mixtures thereof. The silicalite may be impregnated with metals or metal combinations of nickel, copper, zinc, cadmium, platinum, palladium, nickel-tungsten, cobalt-molybdenum, nickel-molybdenum, zinc-palladium, zinc-copper, and zinc-rhenium, iron-chromium oxide, and zinc oxide-chromium oxide.

The present invention provides a method of reforming a low octane feedstock composed of predominantly normally liquid hydrocarbons within the gasoline rangeto a product predominantlywithinthe gasoline boiling range but having a higher octane rating, the method comprising the steps of passing the feedstock containing branched and straight-chain aliphatic hydrocarbons into a reaction zone and into contact with a catalyst system within said reaction zone, the catalyst system comprising a discrete physical mixture of a metal or metal oxide dehydrogenation catalyst and a shape selective crystalline silica polymorph silicalite catalystwhich is preferentially selective to straightchain aliphatic hydrocarbons while restrictive to branch chain isomers thereof, and establishing conversion conditionswithinthe reaction zoneto produce a reformed product having an increased aromatic content and a decreased normal paraffin content in the product relative to the feedstock.

In accordance with the present invention there is provided a new and improved process for reforming a relatively low octane feedstock to produce a product having a substantially higher octane rating. The gasoline range feedstock containing branched and straight-chain aliphatic hydrocarbons is passed into a reaction zone into contact with a catalyst system comprising a discrete physical mixture of a shape selective crystalline silica polymorph silicalite catalyst and a metal or metal oxide dehydrogenation catalyst.

The silicalite catalyst is preferentially selectiveto straight-chain aliphatic hydrocarbons while restrictive to the corresponding branch chained isomers. Conversion conditions are established within the reaction zone to produce the reformed product having an increased aromatic content and a decreased paraffin content relative to the aromatic and paraffin content of the feedstock.

In one embodiment of the invention, the catalyst system is in a particulate form comprising pellets formed of crystallites ofthe silicalite and the dehydrogenation catalyst oxide mixed with a binder. In another embodiment, the catalyst system is in a particulate form comprising granules ofthe silicalite and granules ofthe dehydrogenation catalyst which are mixed togetherto provide the discrete physical mixture. In a preferred embodimentofthe invention the dehydrogenation catalyst is zinc oxide. In yet a further preferred aspect of the invention, the feedstock contains sulfur in an amount greaterthan 2 parts per million.

Embodiments ofthe present invention will now be described in greater detail by way of example only.

In carrying outthe reforming process of the present invention,the hydrocarbon feedstock is passed into a reaction zone containing a catalyst system which comprises a discrete physical mixture of dehydrogenation catalyst, preferablyzincoxide, and a crystalline silica polymorph silicalite catalyst. In this zone, the feedstock contacts the catalyst system under the appropriate conditions of temperature, pressure, and residence time to effectthe desired reforming reactions to arrive at the higher-octane product As will be described in greater detail hereinafter, the reaction zone temperatures normally will be maintained within the range of groom 400 to 600"C and the pressure within a range of from 30 to 41 bars.For operating temperatures and pressures within these rangesthefeedstock normallywill be passed through the reaction zone at a ata to provide a weight hour space velocity (WHSV) within the range of from 1 to 50.

As will be described in greater detail hereinafter, feedstocks suitable for use in the present invention may vary widely in their compositions. However they may be identified generally as being composed predominantly of normally liquid hydrocarbons within the gasolinefraction boiling range. Whilethe gasoline range may vary somewhat depending upon the refining operation and the crude oil charge, it can be characterized as varying from the boiling point of pentane atthe low end to the middle kerosene range (from 220 to 230"C) and in any case less than the initial boiling point of the gas-oil fraction. The feedstock may contain normal and branced chain aliphatic hydrocarbons, cycloaliphatic hydrocarbons, and aromatic hydrocarbons.The aromatic hydrocarbons may be substituted or unsubstituted and thus include benzene and alkyl aromatics such astoluene, ethylbenzene and xylenes.

The shape selective catalyst is a crystalline silica material as contrasted with a zeolitic material which by definition is a silicate of aluminium and either sodium or calcium, or both, which demonstrates ion exchange capacity. These crystalline silica materials are silica polymorphs whose structures have been designated in the art as "silicalite". These materials, in contrastto aluminosilicatezeolites, demonstrate no appreciable ion exchange properties since Al04tetrahedra do not comprise a portion ofthe crystalline silica framework. Aluminium may be present in these silicalite catalyst materials. However, its presence is a result of impurities in the silica source used to prepare the material and silicalite containing such alumina or other metal oxide impurities can in no sense be considered to be a metallo-silicate.Further description and methods of preparing silicalitetype catalysts are setforth in U.S. Patent No.4,061,724 (Grose).

Dehydrogenation catalysts which may be employed in the present invention include metal or metal oxide compounds which include platinum, rhenium, zinc, iron, copper, gallium, antimony,tin, lead, bismuth, indium, thallium, cadmium and chromium. Zinc oxide is the preferred dehydrogenation catalyst as noted previously. Other suitable catalysts which preferen tiallyacceleratethedehydrogenation reactions may be employed in carrying out the invention provided thatthey are compatible with the silicalite catalyst.

The dehydrogenation catalyst and the silicalite catalyst may be mixed together in any suitable manner so long as the two catalysts retain their discrete physical characteristics in the final product Thus, the catalyst system may be in a particulate form comprising granules of the silicalite and granules of the dehydrogenation catalyst mixed togetherto provide the discrete physical mixture. The granular particles which normally will include the particular catalyst material in a binder matrix should be mixed thoroughlyto provide a relativelyhomegeneous mixture. Alternatively, the catalyst system may be in a particulate form comprising pellets formed of mixtures ofcrystallites of the dehydrogenation catalysts and crystallites ofthe silicalite.Such pellets may be formed by any suitable technique, buttypicallywill take the form of extrudates. The extrudates may be formed by mixing crystallites of the dehydrogenation catalyst and the silicalite catalysttogetherwith a binder material to form a plastic material which is then extruded through a suitable die structure to form pellets of the desired size. Where pelletized mixed products formed by extrusion techniques are employed,the granular material typicallywill range in sizefrom 0.5to 4 millimeters.Where mixtures of granularsilicalite and granulardehydrogenation catalyst are used, the individual catalyst particles normally will range from 0.2 to 2 millimeters.Crystallites ofthe dehydrogenation catalyst may be mixed with crystallitesofsilicalitewithoutbinders in which case the size ofthe catalyst particles will be extremely small ofthe order of 10 mirons or less.

The use of a catalyst system comprising a physical admixture fo discrete catalysts offers several important advantages over catalyst systems in which a metallic catalyst is impregnated into a silicalite base, as disclosed,forexample, in the aforementioned United States Patent in the name of Post et al. The physical mixture permits the use of relatively large concentrations of zinc oxide or other dehydrogenation catalyst without plugging of the pore structure ofthe silicalite as would resultfrom the use oftoo much metal in the impregnated catalystform. Also, use of the particulate catalyst system will not be attended by wide swings in catalytic activity which may occur in the case of the metal impregnated catalyst as metal is lostfrom the pore structure. The advantages derived from the use ofthe particular mixture over an impregnated catalyst system usually can be achieved withoutanyincrease in cost In fact, in addition to providing greater constancy of performance, the particulate mixture will normally be simpler in formulation and lower in cost than the impregnated catalyst system.

Regardless ofthe form ofthe discrete physical mixture, the weight ratio of zinc oxide to silicalite normally will be with in the range of from 0.1 to 1 although as indicated by the experiments set forth hereafter greater concentrations of zinc oxide can be used. For most feedstocks, the preferred zinc oxide/ silicalite ratio in the multicomponent catalyst is within the range of from 0.3 to 0.8.

Feedstocks employed in carrying outthe present invention may be derived from any suitable source.

Feedstocks with respect to which the invention is considered to be especially applicable include raffinate streams from aromatic purification units, primary reforming charges, and reformates. With respect two the first alternative, it is a conventional practice to su bjectthe product stream from an aromatization process to a solvent refining extraction technique. As will be understood by those skilled in the art, aromatic hydrocarbons can be readily separated from aliphatic hydrocarbons an naphthenes by extraction with a preferential aromatic solvent such as the ethylene or propylene glycol and water mixtures employed in the Udex process. The raffinate phase derived from solvent aromatics extraction normally is relatively rich in straight chain paraffins and, of course, contains only minor amounts of aromatic hydrocarbons.

Another application ofthe present invention is in a post reforming operation. In this case, the feedstock will taketheform ofthe product effluent (reformate) from a primary reforming unit and the feedstock will have a relatively high aromatic content. This application of the present invention is particularly advantageous since it is effective in the dealkylation of heavyaromatics (Cg + aromatics)aswellasinthe conversion of straight-chain aliphatic hydrocarbons.

Another use ofthe present invention is in a primary reforming unit. In this case, as indicated by the experimental data described hereinafter,the use of the composite catalyst system in according with the present invention results in higher benzene and toluene yields and lower yields of Cg + aromatics than commercially available platinum-rhenium catalysts.

Standards for reforming operations employing noble metal reforming catalysts require extremely low sulfur concentrations, about 1 ppm or less. In many cases, the charge stock to the primary reforming unit contains substantially higher amounts of sulfur, e.g., upto lotto 20 ppm sulfur and sometimes even greater sulfur concentrations. Thus a particularly useful application of the present invention is with regard to thosefeedstockswhich contain sulfurinamounts greater than can be tolerated by commercially avail- able noble metal catalysts i.e. sulfur concentrations, of 2 ppm and above. It is believed that materially higher sulfur concentrations can be tolerated over long periods of time without catalytic poisoning ofthe catalyst system occurring.Thus another application of the invention is with respect to feedstreams containing even greater amounts ofsulfur, ofthe order of 10 ppm and above.

The reforming process can be carried out employing any appropriate processing equipment including a reactor vessel which defines the reaction zone within which the catalyst system is contained.

The composite zinc oxide-silicalite catalyst may be arranged in either single bed or multiple beds within the reaction zone. The reaction vessel may be ofthe fixed bed or moving bed type as will be understood by those skilled in the art. As a practical matter, it usually will be desirable to provide a plurality of reaction vessels to allowfor regeneration of the catalyst system without interruption of the reforming process.

Normally, the feedstockwill be heated prior to introduction into the reaction zone. Afterthe feedstock is maintained in contact with the catalystforthe desired residence time, the reformed product is withdrawn and subjected to suitable purification and separation procudures.

Experimental work was carried out in which a primary reformer charge was subjected to a reforming process employing a zinc oxide-silicalite catalyst mixture in accordance with the present invention. The catalystsystem was a homogeneous mixture of particulate zinc oxide and silicalite in an amount providing a weight ratio of zinc oxide to silicalite of 1.5.

Each ofthe zinc oxide and silicalite were ground to a particle size offrom 60to 80 mesh. The zinc oxide was a commercial catalyst identified as UCI G72D. The silicalite catalyst had a crystalline size of less than 8 microns and a ratio of silica to alumina in the tetrahedra molecular sieve network of at least about 200.

The test was carried out over number of hours during which the temperature was increased from about 490into about 550"C and then to about 590"C.

The reaction zone temperature was maintained constant at 20 bars and thefeed rate was maintained constant to provide a space velocity of about 14.5 LHSV. The effluent from the reaction vessel was collected and three samples corresponding to the reaction temperatures of 490,550 and 590"C were analyzed to determinetheirindividual liquid hydrocarbon components. The experimental work is setforth in summaryform in Table I which also sets forth the analyses of a plantreformateproducedfromthe reformer charge. The plant reformer was a three reactor reformer with the first, second and third reaction stages operated at inlettemperatures of 458, 463 and 493"C. The reformerwasoperated at an average pressure of about 17 bars. The catalyst employed was a commercial platinum-rhenium reforming catalyst and reformer feed was supplied art a LHSV of about 1.2. In Table I, the first column designates the hydrocarbon components. The second column presents analysis data forthe reformer charge and the third, fourth, and fifth columns give the liquid analyses of the liquid effluent samples attempera tures of 490O,550 and 590 C respectively. The last column gives the liquid analyses of the plant reformate.

Table I Liquid Comp* 1 2 3 4 5 6 B2 1.300 2.235 3.605 5.482 3.572 4.000 5.216 8.733 13.973 10.627 EB 6.400 3.167 3.285 3.744 5.001 P-XYL 0.600 1.025 2.003 2.942 1.697 M-XYL 1.200 2.142 2.724 3.543 4.912 O- YL 0.900 1.211 1.122 1.538 2.411 C9+ 38.400 35.209 28.002 18.319 32.684 C3 - 0.838 0.641 0.267 C4 - 1.863 1.683 0.869 - IC5 - ' 0.652 0.801 0.401 NC5 - 0.373 0.721 0.535 - 2,2-DMB - - - 0.067 0.089 CP 0.100 0.093 0.G80 0.067 - 2,4-DMB 0.100 0.093 0.080 0.067 0.268 2-MP 0.800 0.745 0.561 0.267 1.429 3-MP 0.800 0.838 0.641 0.334 1.250 N-C6 1.900 1.770 1.522 0.936 2.233 MCP 1.700 1.304 0.001 0.468 1.875 2,2-DMP 0.100 0.093 0.080 0.067 0.268 2,4-DMP 0.100 0.093 0.080 0.067 0.268 CYCL-HEX 2.400 1.583 1.202 0.735 0.179 1.100 0.931 0.561 0.267 1.875 1,1-DMCP - - - - 2,3-DMP 0.900 0.931 0.561 0.267 0.982 3-MH 1.600 1.304 0.721 0.334 2.333 1C-3-DMCP - - - - Table I Continued IT-3-DMCP 0.800 0.559 0.240 0.134 0.268 IT-2-DMCP 0.700 0.559 0.240 0.134 0.268 3-EP 1.400 1.397 1.122 0.735 0.536 NC7 3.200 2.422 1.763 1.003 2.411 1C-2-DMCP 0.200 0.186 0.160 0.067 0.089 M-CH 4.400 3.539 1.683 0.802 0.447 2,5-DMH 0.200 0.186 0.160 0.067 0.268 ECP 0.500 0.373 0.160 0.201 0.179 2,4-DMH 0.200 0.186 0.160 0.210 0.447 2,2,3-TMP 0.500 0.466 0.441 0.267 0.089 IT2C4TMCP - - - - 111TMCP 0.500 0.838 0.961 - 2,3-DMH 0.500 0.466 0.481 0.267 0.357 2-MH 1.500 1.118 0.561 0.267 1.161 4-MH 0.400 0.279 0.160 0.067 0.536 3-EH 1.000 0.931 0.481 0.267 1.697 3M8 0.400 ; 0.373 0.320 0.134 0.268 1,1-DMCH 1.700 1.583 1.202 0.669 0.089 1C-3-DMCH 0.300 0.186 0.080 - 0.089 2,2,4-TMH 0.800 0.559 0.481 0.267 CYC-HEP - 0.093 0.481 - IT-2-DMCH 0.600 0.279 0.160 0.134 - NCB 3.300 2.701 1.442 1.003 1.518 2,4,4-TMH 0.100 0.373 0.080 0.134 0.179 2,3,4-TNi 0.100 0.745 0.080 0.067 0.179 2,2-DMH 0.100 0.093 0.080 0.134 0.179 2,4-DMH 0.100 0.186 0.080 0.067 0.179 1C-2-DMCH 1.100 0.279 0.641 0.401 0.179 N-PRO-CP 1.900 1.211 0.801 0.401 0.536 ET-CYCHEX 1.100 1.211 1.602 0.735 1,1,3-TMCH 0.200 0.279 0.160 0.134 0.089 lC3C5TM53 0.300 0.279 0.160 0.134 0.089 3,4-1 C.300 0.931 0.561 0.134 0.714 Table I Continued 4-M-OCT 0.700 0.466 0.240 0.267 0.625 3-EH 0.400 0.186 0.080 0.602 0.179 2-M-OCT 0.600 0.652 0.240 0.134 0.714 1T2C3TMCH 1.600 0.745 0.561 0.401 1,1.2ThC8 0.500 0.466 0.320 0.267 1MC3ECH 0.700 0.466 0.320 0.134 NC9 2.700 1.583 0.961 0.468 0.893 WT%NA LIQ < C1041.200 42.940 30.646 17.316 28.397 WT% C68 AR 14.400 14.996 21.472 31.223 28.219 wRt C8+AR+SAT 38.400 35.209 28.002 18.319 32.684 LIQUID YIELD wr 8 100.00 93.14 80.12 66.86 89.30 DMB-dimethylbenzene, CP-cyclopentane, MP methylpentane, MCP-methylcyclopentane, DMP dimethylpentane, DMCP-dimethylcyclopentane, CP ethylpentane, MH-methylhexane, M-CH- methylcyclohexane, EP-ethylpentane, ECP ethylcyclopentane, TMCP-trimethylcyclopentane, DMH dimethylhexane or dimethylheptane, TNP- trimethylpentane, PRO-CP-propylcyclopentane, TMH trimethylhexane or trimethylhapten, EH-ethylhexane, TMCH-trimethylcyclohexane, MCEH methylethylcyclohexane Based upon a review of column 4 it can be seen that the liquid product profile obtained atthe intermediate temperature of 550 C is similar to that resulting from the current reformer operation. At this temperature, however,the composite silicalite zinc oxide catalyst yielded less Cg + product than the conventional reformercatalyst, indicating that some alkylation of the heavieraromaticcompounds occurred. In addi tion,the normal hexane and normal heptane concen trations forthe composite catalysts are somewhat less than those forthe commercial platinum-rhenium catalysts. At the highertemperature of 590 C (column 5) the normal hexane and heptane concentrations were reduced further and the benzene and toluene concentrations were significantly higher than for the reformer operation employing the commercial catalyst. In addition, the highertemperature resulted in a substantial decrease in the concentration ofthe C9 + compounds.

In the experimental work, equal volume amounts of the particulate zinc oxide and silicate catalysts were used to provide the previously mentioned weight ratio of 1.5. In actual operations is will usually be desirable to employ a substantially lower relative amount of zinc oxide to provide zinc oxide-silicalite ratios within the ranges described previously.

Claims (12)

1. A method of reforming a low octane feedstock composed of predominantly normally liquid hydrocarbons within the gasoline range to a product predominantly within the gasoline boiling range but having a higher octane rating, the method comprising the steps of passing thefeedstock containing branched and straight-chain aliphatic hydrocarbons into a reaction zone and into contact with a catalyst system within said reaction zone, the catalyst system comprising a discrete physical mixture of a metal or metal oxide dehydrogenation catalyst and a shape selective crystalline silica polymorph silicalite catalyst which is preferentially selective to straightchain aliphatic hydrocarbons while restrictive to branch chain isomers thereof, and establishing conversion conditions within the reaction zone to produce a reformed product having an increased aromatic content and a decreased normal paraffin content in the product relative to the feedstock.

2. A method according to Claim 1, wherein the catalyst system is in a particulate form comprising pellets formed of crystallites ofthe silicalite and the dehydrogenation catalyst to provide the discrete physical mixture.

3. A method according to Claim 1 wherein the catalyst system is in a particulate form comprising granules ofthe silicalite and granules of the dehydrogenation catayst mixed together to provide the discrete physical mixture.

4. A method according to any one of Claims 1 to 3, wherein th efeedstock is a raffinate mixture resulting from the solvent refining ofthe product stream from an aromatization process.

5. A method according to any one of claims 1 to 3, wherein the feedstock is the reformate product from a reforming unit in which the aromatic hydrocarbons present in the feedstock are predominantly in the form of Cg + hydrocarbons.

6. A method according to anyforegoing Claim, wherein thefeedstockcontains sulfur in an amount greaterthan 2 parts per million.

7. A method according to Claim 6, wherein said feedstock contains sulfur in an amountgreaterthan 10 parts per million.

8. A method according to any foregoing Claim, wherein the hehydration catalyst is zinc oxide.

9. A method according to Claim 8, wherein the weight ratio of zinc oxide to silicalite in the discrete physical mixture is within the range offrom 0.1 to 1.

10. A method of reforming a low octane feedstock as claimed in Claim 1 substantially as herebefore described.

11. A method of reforming a lowoctanefeedstock as claimed in Claim 1 substantially as hereinbefore described with reference to Table I.

12. Hydrocarbonswheneverproduced bythe method of any foregoing claim.