RECYCLE OF BUTADIENE PRODUCT IN CATALYTIC DEHYDROGENATION PROCESS

Abstract: A process for dehydrogenation of a feedstock contain­ ing isobutane, n-butane, and butadiene using a steam ac­ tive dehydrogenation catalyst which is (a) a calcined sup­ port of at least one aluminate spinel composition, (b) at least one Group VIII metal, (c) with or without at least one Group IA metal and (d) at least one tin group metal selected from among tin, germanium, and lead under dehydrogena­ tion conditions to produce a product stream of isobutane, n-­ butane, butenes, isobutene, and butadiene with a separa­ tion of the product stream to provide a recycle feedstock composition containing isobutane, n-butane and butadiene which is recycled to the reaction zone along with fresh feed­ stock providing a combinped feedstock that has up to 20 mol percent n-butane and up to 5 mol percent butadiene. Op­ tionally, repeating product separation and recycling of feedstock along with fresh feedstock without producing a buildup of by-product butadiene in the cycle.

Description: RECYCLE OF BUTADIENE PRODUCT IN CATALYTIC DEHYDROGENATION PROCESS Background of the Invention This invention relates to catalytic dehydrogenation. In one of its aspects this invention relates to the dehydrogenation of butane. In another of its aspects it relates to the production of butadiene as a by-product in a butane dehydrogenation process. In still another aspect of this invention it relates to a process for recycling butadiene produced in a dehydrogenation process back to the reaction. In a process for dehydrogenation of butane feedstock in which both isobutane and n-butane are present there are produced the desirable butenes and isobutene and there is also produced a by-product of butadiene. The real problem with some of the processes currently used is that there is also a large amount of unconverted butane feedstock that passes through into the product. For a process to be most efficient there needs to be a separation of the product butenes and isobutene from the feedstock butane so that the butane can be recycled to the reaction in consecutive passes over the catalyst. Unfortunately, however, butadiene produced as a by-product is not easily separated from the butane in the product mixture so that any feedstock for recycle is contaminated with the butadiene which is a well known coke producer in dehydrogenation processes. A logical way to overcome the presence of this contaminant by-product in the recycle feedstock separated from the dehydrogenation product is to provide a hydrotreating system which rehydrogenates the butadiene before it is passed into the dehydrogenation reaction. It has now-been discovered, however, that using the preferred catalyst as set out below the dehydrogenation reaction itself acts to maintain only a low level of butadiene in the product stream even though recycle containing butadiene is returned directly to the dehydrogenation reaction. It has been proposed as explanation of these results that the butadiene is rehydrogenated probably with some structural isomerization during the dehydrogenation reaction. It is therefore an object of this invention to provide a method for dehydrogenation using a catalyst that permits direct recycle of butadiene contaminant through the reaction system. Other aspects, objects and the various advantages of this invention will become apparent upon study of this disclosure and the appended claims. Statement of the Invention In accordance with this invention, a method is provided for dehydrogenation of a feedstock of isobutane and n-butane to produce butenes, isobutene and butadiene in which the feedstock is contacted in a reaction zone with a steam active dehydrogenation catalyst which is (a) a calcined support of at least one aluminate spinel composition, (b) at least one Group VIII sepal, (c) with or without at least one Group IA metal and (d) at least one tin group metal selected from among tin, germanium, and lead under dehydrogenation conditions thereby producing a product stream containing isobutane, n-butane, butenes, isobutene, and butadiene; separating the product stream to provide a separated product containing butenes and isobutene and a recycle feedstock composition containing isobutane, n-butane, butadiene and residual butenes; and recycling the recycled feedstock along with fresh feedstock to the reaction zone in a combined feedstock that has up to 20 mol percent n-butane and up to about 5 mol percent butadiene thereby producing à combined product stream having an amount of butadiene substantially equivalent to the amount of butadiene produced without the presence of butadiene in the feedstock. In another embodiment of the invention, the combined product stream produced by recycling the recycled feedstock along with fresh feedstock through the reaction zone is separated to produce a recycle feedstock composition containing isobutane, n-butane and butadiene which is then recycled along with fresh feedstock to the reaction zone to produce a stream containing isobutane, n-butane, butenes, isobutene and butadiene with the repeating of these steps to obtain the desired amount of product. In another embodiment of the invention a process is provided for producing butenes and isobutene from a feedstock that contains isobutane, n-butane and butadiene. In the process the feedstock is contacted with a steam active dehydrogenation catalyst under dehydrogenation conditions to produce a product containing butenes, isobutene, isobutane, n-butane and an amount of butadiene substantially equivalent to that in the feedstock. It has been found that highly active and exceptionally selective catalysts for the dehydrogenation of steam-diluted alkanes, cycloalkanes, and arylalkanes can be prepared by highly calcining a Group II aluminate spinel support. These Group II aluminate spinels are particularly effective and excellent results are obtained with zinc aluminate spinel. A catalyst composition is then formed by combining the highly calcined support with a metal selected from the group consisting of nickel, platinum, palladium, ruthenium, iridium, rhodium, osmium, and mixtures thereof. Other catalyst compositions are then formed by combining with this composition a material selected from the group consisting of a Group IA metal and a tin group metal and mixtures thereof. The groups of metals referred to herein are as classified in the Periodic Table published in the Chemical Rubber Company's "Handbook of Chemistry and Physics", 45th Edition to964), page B-2. Aluminate spinels, as referred to herein, are compounds of the formula N(AlO2)2 or MO A1203 wherein M is a Group IIA or IIB metal with a valence of 2 such as Zn, Mg, Be, Ca, and the like. The highly calcined supports useful in this invention can be prepared by calcining for a sufficient time and at a sufficient temperature Group II aluminate spinels. The spinels can be prepared by any known method and in a preferred embodiment are prepared by the coprecipitation method wherein appropriate amounts of suitable aluminum compounds and suitable Group II metal compounds are combined under spinel-forming conditions. It is preferred that the proportions of these ingredients be approximately stoichiometric or, alternatively, that the Group II metal compound be up to 10 percent or more in excess of stoichiometric. The highly calcined support is prepared, preferably, by calcination for about 1-100 hours at about 15000-25000F. In another embodiment, a highly calcined support is produced by calcining the support, preferably, from about 2-50 hours at about 20000-25000F. In a more preferred embodiment, the support is calcined by heating for about 6 hours at about 16000F. The first catalyst composition of this invention can be prepared by combining, in any manner -known to the art, certain Group VIII metals, or metal compounds capable of reduction to the metal, including nickel, platinum, ruthenium, palladium, iridium, rhodium, osmium, and mixtures thereof, with the highly calcined support of this invention. Platinum, which is very effective, is preferred. The Group VIII metal content of the catalyst can be in the range of about 0.01-5 weight percent of the support and, in one embodiment, is in the range of 0.1-1 weight percent of the support. Throughout this application the term "weight percent of the support" means parts by weight per 100 parts by weight of support. Any platinum group metal compound that produces the desired results can be used. In the discussion of the compounds that can be used the platinum compounds will be used as nonlimiting examples. It is to be understood that similar compounds of the other platinum group metals can be used. Examples of simple or non-coordination compounds that can be used are platinic chloride, chloroplatinic acid, ammonium chloroplatnate, and the like. Nonlimiting examples of coordination platinum compounds that can be used are: platinum aminoacetate, platinum dimethyl dioxime, tetraamineplatinum hydroxide, platinum diamine dinitrate, platinum tetraamine dihydroxide, platinum diamine dihydroxide, platinum hexamine dihydroxide, platinum hexamine tetrahydroxide, platinum diamine tetrahydroxide, platinum diamine dihydroxide dinitrate, platinum diamine tetranitrate, platinum diamine dinitrite, platinum tetraamine dicarbonate, platinum diamine oxalate, and the like. Additionally, many complex or coordination divalent and tetravalent platinum compounds are known and can be used. When added to the support by impregnation from solution, some of the compounds can be added from aqueous solution, but others will require non-aqueous solvents such as alcohols, hydrocarbons, ethers, ketones, and the like. The second catalyst composition of this invention is prepared by combining a Group IA metal or metal compound with the components of the first composition with or without a tin group metal. This can be conveniently done by conventional impregnation. The amount of each alkali metal compound or combination of compounds can exist in the range of about 0.01-5 weight percent of the total catalyst; however, in one embodiment, a range of about 0.1-1 weight percent of said support is used. Any Group IA metal in compound form is suitable for use in this invention. Other convenient compounds are the carbonates, acetates, and hydroxides and the like of sodium, barium, potassium, calcium, and the like. The third catalyst composition of this invention is prepared by combining a tin group metal including lead, tin and germanium with the components of the first catalyst composition with or without a Group IA metal. The tin group metal can be incorporated in the composition in the range of about 0.01-5 weight percent of said support and, in one embodiment, can be incorporated in the composition in the range of about 0.1-1 weight percent of said support. Although any tin group metal in compound form is fully within the scope of this invention, some convenient tin group compounds are the halides, nitrates, oxalates, acetates, carbonates, propionates, tartrates, bromates, chlorates, oxides, hydroxides, and the like of tin, germanium and lead. Tin, itself, is the preferred tin group metal and impregnation of the supports with tin compounds such as the stannous halides is particularly effective and convenient. Generally speaking, the Group IA, VIII and tin group compounds, which are combined with the calcined supports to form the improved catalysts of the present invention, can be sequentially combined with the support, in any order, or for convenience, can be applied simultaneously in a single impregnation operation. After impregnation, the composite solids are dried and calcined. The catalyst systems of this invention are conveniently employed at temperatures between 7500 to 12500F., preferably between 10000 to 11000F, and at total pressures in the range of 0 to 500 psig, preferably 0 to 250 psig. Steam to hydrocarbon mol ratios of 0.5:1 to 30:1, preferably 2.5:1 to 15:1, are employed. Liquid hourly space velocity (LHSV) of hydrocarbon feed is in the range of 2.0 to 6.0 volumes HC per hr per volume of catalyst preferably 3.5 to 4.5 her 1. The process cycle is 0.5 to 24 hrs and the regeneration period is 0.5 to 4.0 hours. The catalyst is regenerated with steam diluted air such that the 02 content of the regeneration gas mixture is about 0.5 to 4.0 mol percent. The regeneration treatment can be carried out at temperatures and pressures within said dehydrogenation operating ranges. The separation of the product stream to provide butenes and isobutene as product and a stream containing butanes and butadiene as recycle for the reaction zone is well known in the art. The usual form of separation is fractionation which does not allow the separation of the butadiene contaminant from the isobutane and n-butane. The recycle stream is then combined with an amount of fresh feedstock to provide a composition of feed stock that is equivalent to the original feedstock with the exception of the addition of butadiene contaminant. The process of reacting the combined feedstock, separating the product stream, and recycling the separated recycle material to the reaction is repeated as desired. The following example describes the basic process of the invention. The example should be taken as illustrative and not as restrictive. Example To illustrate that in the process of this invention the butadiene contaminant will have so small effect as to go unnoticed in the reaction of the recycled separated reaction product a series of tests was conducted in which 95 mol percent isobutane feed was spiked or deliberately increased with one percent butadiene. A slurry consisting of distilled water, finely divided alumina, finely divided reagent grade zinc oxide and finely divided reagent stannic oxide was ball milled for one hour to obtain an intimate mixture. The slurry was dried overnight at 2000-2200F in a forced draft oven. The resulting dry cake was crushed, sieved to remove coarse particles and the powder was compounded with 8 weight percent of a polyethylene lubricant. The mixture was formed into 1/8-inch pellets and calcined in air in a muffle furnace which was programmed as follows: 2 hours at 8000F, 2 hours at 1,1000F and 3 hours at 1,8500F. Thus prepared tin-containing support had a surface area of 12 square meters per gram, a pore volume of 0.48 cc per gram. The support contained 35 weight percent zinc. 26.1 weight percent aluminum, 1.0 weight percent tin and 57.9 weight percent combined oxygen. A portion of the tin-containing catalyst support the above calcined slurry was impregnated with platinum from an aqueous solution of chloroplatinic acid to form a catalyst composition containing 0.6 weight percent platinum based on the weight of the final catalyst. The mixture was dried 3 hours at 2300F and calcined 3 hours at 11000F. Alternating runs with the spiked and regular isobutane feed were made. The conversion of isobutane and the selectivity to isobutane through the cycle are illustrated in Tables I and II below which sets out typical results with and without the butadiene in the feedstock. Table I gives test results obtained early in the runs (1.3 and 1.5 hours) while Table II gives test results obtained later in the runs (5.4 and 5.5 hours) for the not spiked and spiked with butadiene in the feed hydrocarbon. TABLE I PRODUCT DISTRIBUTION All concentrations shown are C4 equivalent mole percent. Not Spiked with Butadiene Spiked with 1 mol X Butadiene Run 2201 Run 2202 Products 1.32 hours into cycle 1.50 hours into cycle C5+ 0.0 0.0 C2/C2= 0.2 0.2 C02 1.8 1.8 C3 1.1 1.1 C3= 0.7 0.6 iC4 44.4 44.2 nC4 2.3 2.6 iC4= 46.4 46.2 C4=1 0.8 0.8 tC4=2 0.9 1.0 cC4=2 0.6 0.7 1,3 C4= 0.0 0.2 C1 0.6 0.6 CO 0.0 0.0 Feed iC4 95.0 94.9 nC4 4.8 4.0 C3 0.2 0.2 1,3 C4=: : 0.0 0.9 Conversion of isobutane 53.2X 53.4Z Selectivity to isobutene 92.2% 91.15 Conversion of butadiene 0; 77.8% Conditions of All Runs of Table I and II Liquid Hourly Space Velocity, Vol/Hr Feed/Vol Catalyst 4.0 Mols Steam/Mol Hydrocarbon 5.0 Average Bed Temperature OF 1050 Average Pressure psig 50 TABLE II PRODUCT DISTRIBUTION Not Spiked Run 2203 Spiked Run 2205 Products 5.4 hours into cycle 5.5 hours into cycle C5+ 0.0 0.0 C2/C2= 0.1 0.1 C02 1.1 1.1 C3 0.7 0.7 C3= 0.5 0.5 iC4 49.8 48.6 nC4 2.4 2.6 iC4= 42.8 43.4 C4=1 0.7 0.8 tC4=2 0.8 0.9 cC4=2 0.6 0.7 1,3 C4= 0.1 0.2 C1 0.4 0.4 CO 0.0 0.0 Feed iC4 95.0 94.9 nC4 4.8 4.0 C3 0.2 0.2 1,3 C4== 0.0 0.9 Conversion of isobutane 47.5% 48.8 Selectivity to isobutene 94.8 mol : 93.8 mol Z Conversion of butadiene 0 mol X 77.8 mol Z In the run, performance through the cycle was virtually unchanged. Had the butadiene been converted directly or extensively to coke in the reactor a rapid decline in conversion would be expected or a dramatic increase in carbon dioxide in the reactor effluent could be seen. Neither of these results were observed. It can be concluded, therefore, that the butadiene was not being converted extensively to coke in the reactor. Since 77.8% of the butadiene fed to the reaction zone was converted there are two possibilities as to what happened to it: either the butadiene was being rehydrogenated or the butadiene was coking in the preheaters before the reactor. Table I shows the product distributions for a regular and a spiked feed run. Inspection of these data show that the normal butane, trans-butane-2 and cis-butane-2 concentrations were all higher in the spiked effluent than in the regular effluent even though the spiked feed contained less normal butane. This indicates that some rehydrogenation was occurring. It cannot be directly determined if all of the converted butadiene became normal butane products because some skeletal isomerization could occur, however,-it is evident that at least the majority of the butadiene was rehydrogenated. While butadiene is generally considered to be a coke precursor, in the environment of the present reaction near the reactor exit there is a 700:1 hydrogen to butadiene molar ratio which shifts the equilibrium away from coke and towards the formation n-butenes. Additionally, there was no indication of increased pressure drop across the coils of the preheater which would indicate coking in the preheaters and the evidence showed little or no coke formation on the catalyst itself that could permanently deactivate it.

Claims: Claims 1. A method for dehydrogenation of the feedstock of isobutane and n-butane to produce butenes, isobutene, and butadiene, said method being c h a r a c t e r i z e d b y (1) contacting the said feedstock in a reaction zone with steam and a steam active dehydrogenation catalyst under dehydrogenation conditions thereby producing a product stream comprising isobutane, n-butane, butenes, iso butene, and butadiene; (2) separating said product stream to provide a separated product comprising butenes and isobutene and a re cycle feedstock composition comprising isobutane, n-butane, and butadiene and residual butenes; and (3) recycling said recycle feedstock along with fresh feed stock to the reaction zone in a combined feedstock that has up to 20 mol percent n-butane and up to 5 mol per cent butadiene thereby producing a combined product stream having an amount of butadiene substantially equivalent to the amount of butadiene produced without the presence of the butadiene of the feedstock. 2. The method of claim 1 characterized by further compri sing: (4) separating said combined product stream to produce a recycle feedstock composition comprising Isobutane, n-butane and butadiene and residual butenes and a product containing butenes and isobutene; and (5) recycling said recycle feedstock composition along with fresh feedstock to the reaction zone to produce a stream comprising Isobutane, n-butane, butenes, isobutene and butadiene and residual butenes. 3. The method of claim 2 characterized by repeating seria tim steps (4) and (5) until the desired amount of product is obtained. 4. A method for producing butenes and isobutenes from a feedstock that contains Isobutane, n-butane and buta diene and residual butenes, said method being characte rized by contacting said feedstock with a steam active dehydrogenation catalyst under dehydrogenation condi tions to produce a product containing butenes, isobutene, isobutane, n-butane and an amount of butadiene substan tially equivalent to that in the feedstock. 5. The method of any of claims 1 to 4 characterized in that said steam active dehydrogenation catalyst is comprised of a calcined support of at least one aluminate spinel composition, at least one Group VIII metal, and at least one tin group metal selected from among tin, germanium, and lead. 6. The method of any of claims 1 to 4 characterized in that said steam active dehydrogenation catalyst is comprised of a calcined support of at least one aluminate spinel composition, at least one Group VIII metal, at least one Group IA metal and at least one tin group metal selected from among tin, germanium, and lead. 7. The method of claim 5 or 6 characterized in that said calcined support is zinc aluminate spinel, said Group VIII metal is platinum, and said tin group metal is tin.

Inventors: BRINKMEYER, FRANCIS MAURICE, OLBRICH, MICHAEL EUGENE
Applicants: PHILLIPS PETROLEUM COMPANY
Application year: 1985
Main classification: C07C11
IPC codes: C07C67, C07C11, C07C5, B01J23, C07C1, C07B61