A process for the purification of a diolefin hydrocarbon stream

Abstract: A process for the selective hydrogenation of trace quantities of acetylene compounds contained in a stream of diolefins to achieve extended on-stream performance by contacting the selective catalyst with a diolefin feed, hydrogen and a polymer solvent.

Description: The field of art to which this invention pertains is the purification of a diolefin hydrocarbon stream containing trace quantities of acetylene compounds. The production of diolefins is well known and widely practiced to produce a wide variety of products and precursor products utilizing a variety of diolefin production processes including naphtha cracking processes and by-products from fluid catalytic cracking processes. Most of these diolefin production processes produce undesirable trace quantities of acetylene. One technique which is used to purify diolefin streams selectively hydrogenates the acetylene while minimizing the destruction or hydrogenation of the diolefin compounds. The selective hydrogenation of the acetylene compounds is generally conducted in the presence of a selective hydrogenation catalyst and hydrogen and conducted at an elevated pressure and temperature. Such selective hydrogenation catalysts are well known in the art and include, for example, a catalyst containing copper metal associated with one or more activator metals impregnated on an alumina support. During the acetylene hydrogenation polymers are formed and deposited on the catalyst thereby reducing the activity of the catalyst. One known method of regenerating spent or partially spent catalyst is to perform a controlled carbon bum and subsequent metal reduction to remove catalyst contaminants which are formed as an undesirable by-product of the acetylene hydrogenation. The carbon bum regeneration techniques necessarily require that the reaction zone containing the spent catalyst be taken off-line and that ancillary regeneration equipment be provided. INFORMATION DISCLOSURE US-A-3,634,536 discloses a process for selectively hydrogenating acetylenic impurities in an isopropene- or butadiene-containing stream whereby carbon monoxide is utilized during hydrogenation over a copper-based catalyst. US-A-4,440,956 discloses a catalyst for the removal of acetylenes from liquid hydrocarbon streams with a minimum loss of diolefinic unsaturation present in the liquid composition. Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new selective hydrotreating processes which provide lower costs, higher selectivity and longer on-stream operation. The present invention continuously maintains the high activity of the selective hydrogenation catalyst during an extended run length without shutdown for catalyst regeneration. Higher average product quality when integrated over time on-stream improves the process economics and demonstrates the unexpected advantages. SUMMARY The present invention is a selective acetylene hydrogenation process which is able to produce a high quality diolefin having extremely low levels of acetylene over an extended period of time compared with the prior art. The process of the present invention provides a selective hydrogenation reaction zone wherein the catalyst activity is maintained at a high level while the process unit remains on stream by contacting the selective hydrogenation catalyst with a polymer solvent, diolefin feed and hydrogen. The contacting may take place by introducing the solvent in admixture with the olefin feed or by cyclicly alternating contacting of the catalyst between the feed and the solvent in two or more beds. In accordance with one embodiment, the present invention relates to a process for the purification of a diolefin hydrocarbon stream containing trace quantities of acetylene compounds which process comprises introducing the diolefin hydrocarbon stream containing trace quantities of acetylene compounds and elemental hydrogen into a selective hydrogenation zone to selectively hydrogenate at least a portion of the acetylene compounds. A polymer solvent contacts the selective hydrogenation catalyst in said selective hydrogenation zone. One or more resulting effluents from the selective hydrogenation zone pass to at least one fractionation zone to produce a diolefin hydrocarbon stream having a reduced concentration of acetylene compounds and a stream containing polymer solvent and polymer compounds. Recycling at least a portion of the stream containing polymer solvent and polymer compounds provides at least a portion of the polymer solvent to the selective hydrogenation zone. At least another portion of the stream containing polymer solvent and polymer compounds and the diolefin hydrocarbon stream having a reduced concentration of acetylene compounds produced are recovered. BRIEF DESCRIPTION OF THE DRAWING The drawings are simplified process flow diagrams of preferred embodiments of the present invention. The drawings are intended to be schematically illustrative of the present invention and not be a limitation thereof. DETAILED DESCRIPTION OF THE INVENTION It has been discovered that a selective hydrogenation zone for the hydrogenation of trace quantities of acetylene contained in a stream of diolefins may achieve continued start-of-run activity, yields and product quality by contacting the selective catalyst with a polymer solvent, diolefin feed and hydrogen. These advantages enable superior performance and economic results. The process of the present invention is particularly useful for the production of high quality diolefin streams in a process having an extended on-stream capability. The diolefin feed stream may be any convenient hydrocarbon stream containing diolefin compounds and having undesirable trace quantities of acetylene compounds. It is contemplated that the diolefin feedstream contains diolefins containing from 3 to 5 carbon atoms. A preferred diolefin feedstream contains butadiene. In accordance with the present invention, the selected diolefin feedstock is introduced along with a polymer solvent and hydrogen into a selective hydrogenation reaction zone operating at selective hydrogenation conditions and containing a selective hydrogenation catalyst to produce an improved diolefin stream having a reduced concentration of acetylene compounds. The polymer solvent may be selected from any compound or mixtures of compounds and which polymer solvent is capable of acting as a solvent for polymers which are produced during the selective hydrogenation reaction. Suitable solvents may be selected from alkane compounds having from 4 to 8 or more carbon atoms. In the case where the fresh feedstock is a stream of butadiene, a particularly preferred polymer solvent is hexane. For the case of feed and solvent in admixture the polymer solvent may be present in an amount of 5 to 100 weight percent based on the weight of diolefin. It is preferred that the polymer solvent has a boiling point greater than the diolefin feedstream. The selective hydrogenation conditions will depend upon the selected diolefinic feed and may be selected from a pressure from 1379 kPa to 3447 kPa and a temperature from 32°C to 83°C. The process may produce one or more effluents depending on whether the process mixes the solvent with the feed or contacts the catalyst separately with the feed and solvent in a cyclic operation. Each resulting effluent from the selective hydrogenation reaction zone is passed to a fractionation zone to produce a diolefin hydrocarbon stream having a reduced concentration of acetylene compounds and/or a stream containing the polymer solvent and polymer compounds. A small drag stream of polymer solvent containing dissolved polymer compounds is removed from the process to prevent an accumulation of polymer compounds in the polymer solvent. Fresh make-up polymer solvent is added in order to maintain a suitable inventory of solvent. At least a portion of the polymer solvent recovered from the fractionation zone is recycled to the inlet of the selective hydrogenation zone. In a cyclic mode of operation the selected diolefin feedstock is introduced along with hydrogen into an on-line selective hydrogenation reaction zone operating at selective hydrogenation conditions and containing a selective hydrogenation catalyst to produce an improved diolefin stream having a reduced concentration of acetylene compounds. In an alternating fashion, an off-line reaction zone containing selective hydrogenation catalyst, either spent or partially spent, is preferably contacted with a polymer solvent and hydrogen at catalyst regeneration conditions including a pressure from 1034 kPa to 3447 kPa, a temperature from 32ºC to 260ºC and a solvent liquid hourly space velocity from 0.5 to 10 hr -1 . The resulting effluent containing polymer solvent, dissolved polymer and hydrogen from the off-line reaction zone undergoing regeneration is introduced into a fractionation zone to remove gaseous hydrogen and to recover the polymer solvent which is preferably recycled together with fresh, make-up polymer solvent. At least a portion of the polymer solvent recovered from the fractionation zone is preferably recycled to the inlet of the off-line selective hydrogenation zone. The selective hydrogenation catalyst may be any suitable known catalyst and may contain one or more beds of the same or different selective hydrogenation catalyst. Suitable catalysts for the selective hydrogenation of acetylene contain copper metal, activated with one or more of the metals from the group of silver, platinum, palladium, manganese, cobalt, nickel, chromium and molybdenum on an alumina support. The hydrogenation catalysts contemplated for use in the process of the present include any support types, sizes and shapes, for example, spheres, cylinders, tri-lobes, quadralobes and rings. The process of the present invention is not limited by the type of hydrogenation catalyst and any suitable selective hydrogenation catalyst is contemplated for use therein. DETAILED DESCRIPTION OF THE DRAWINGS In the drawings, the process of the present invention is illustrated by means of a simplified schematic flow diagram. With reference now to Figure 1, a feedstream comprising butadiene, trace quantities of acetylene and steam condensate is introduced into the process via line 1 and is passed into feed surge drum 2. A condensed steam stream is removed from feed surge drum 2 via line 3 and recovered. A stream containing butadiene and trace quantities of acetylene is removed from feed surge drum 2 via line 4 and is admixed with a recycle stream containing a polymer solvent transported via line 14 and the resulting admixture is transported via line 5 and is admixed with a hydrogen-rich gaseous stream provided via line 15 and the resulting admixture is transported via line 16 and introduced into selective hydrogenation zone 6. An effluent stream containing butadiene and having a reduced concentration of acetylene compounds is removed from selective hydrogenation zone 6 via line 7 and introduced into fractionation zone 8. A stream containing butadiene and having a reduced concentration of acetylene compounds is removed from fractionation zone 8 via line 9 and is recovered for further purification and subsequent use. A stream containing polymer solvent and polymer compounds is removed from fractionation zone 8 via line 10 and at least a portion is transported via line 11 and introduced into polymer solvent storage drum 13. Another portion of the stream removed from fractionation zone 8 via line 10 is removed via line 12 as a drag stream in order to prevent an undue accumulation of polymer compounds in the process. A stream containing polymer solvent and dissolved polymer compounds is removed from polymer solvent storage vessel 13 via line 14 and is admixed with a fresh make-up stream of polymer solvent which is introduced via line 17 and the resulting mixture is carried via line 14 and contacts the butadiene stream carried via line 4 as hereinabove described. With reference now to Figure 2, a feedstream comprising butadiene, trace quantities of acetylene and steam condensate is introduced in a similar manner to that described in Figure 1. A stream containing butadiene and trace quantities of acetylene is removed from feed surge drum 2 via line 4' and is admixed with a hydrogen-rich gaseous stream provided via line 5' and the resulting admixture is introduced into on-line selective hydrogenation zone 7' via line 6'. An effluent stream containing butadiene and having a reduced concentration of acetylene compounds is removed from on-line selective hydrogenation zone 7' via lines 8' and 9' and introduced into fractionation zone 10'. A stream containing butadiene and having a reduced concentration of acetylene compounds is removed from fractionation zone 10' via line 11' and is recovered for further purification and subsequent use. A stream containing polymer solvent and polymer compounds is removed from surge drum 15' via line 16' and is admixed with a hydrogen-rich gaseous stream introduced via line 17' and the resulting admixture is transported via line 18' and introduced into off-line selective hydrogenation zone 19'. An effluent stream containing polymer solvent and polymer compounds is removed from off-line selective hydrogenation zone 19' via line 20' and line 9' and is introduced into fractionation zone 10'. A stream containing polymer solvent and polymer compounds is removed from fractionation zone 10' via line 12' and at least a portion is transported via line 14' and introduced into polymer solvent storage drum 15'. Another portion of the stream removed from fractionation zone 10' via line 12' is removed via line 13' as a drag stream in order to prevent an undue accumulation of polymer compounds in the process. Fresh make-up polymer solvent is introduced via line 21' into polymer solvent storage drum 15'. The process of the present invention is further demonstrated by the following illustrative embodiments. The following are considered prospective and reasonably illustrative of the expected performance of the invention based upon sound engineering calculations. FIRST ILLUSTRATIVE EMBODIMENT A raw butadiene stream in an amount of 100 mass units and having the characteristics presented in Table 1 is introduced into a fresh feed drum and entrained or condensed water is decanted therefrom. The raw butadiene stream is then admixed with 90 mass units of hexane solvent and the resulting mixture is introduced along with 1 mass units of hydrogen into a fixed bed of selective hydrogenation catalyst. The catalyst contains copper metal. The resulting effluent from the selective hydrogenation is introduced into a fractionation zone to produce a butadiene stream containing less than 3 wppm acetylene compounds (a 99.9% reduction). A bottoms stream containing polymer solvent and dissolved polymer was removed and introduced into a polymer solvent storage drum. A stream containing polymer solvent and polymers and in an amount of 0.35 mass units is removed from the process as a drag stream and recovered. Another stream containing polymer solvent and dissolved polymers is admixed with a fresh make-up stream of 0.3 mass units and is introduced into the selective hydrogenation zone as described hereinabove. The selective hydrogenation zone is operated at conditions which are selected to selectively hydrogenate the acetylene compounds while minimizing any hydrogenation of the butadiene compounds including a temperature of 35°C and a pressure of 2758 kPa. RAW BUTADIENE STREAM ANALYSIS Butadiene 50 weight % Acetylene 0.8 weight % SECOND ILLUSTRATIVE EMBODIMENT A raw butadiene stream in an amount of 100 mass units and having the characteristics presented in Table 1 is introduced into a fresh feed drum and entrained or condensed water is decanted therefrom. The raw butadiene stream is then admixed with 1 mass units of hydrogen and the resulting admixture is introduced into a fixed bed of selective hydrogenation catalyst contained in an on-line selective hydrogenation zone. The catalyst contains copper metal. The resulting effluent from the on-line selective hydrogenation zone is introduced into a fractionation zone to produce a butadiene stream containing less than 3 wppm acetylene compounds (a 99.9% reduction). An off-line selective hydrogenation zone containing a selective hydrogenation catalyst having a copper metal component is contacted with a stream containing hexane and hydrogen at regeneration conditions including a pressure of 1930 kPa, a temperature of 149ºC and a liquid hourly space velocity (LHSV) of 1.3 hr -1 . The resulting effluent containing hexane, hydrogen and polymer compounds from the off-line selective hydrogenation zone is also introduced into the previously mentioned fractionation zone to produce a stream containing hexane and dissolved polymer compounds. At least a portion of the recovered hexane in an amount of 0.35 mass units is removed from the process as a drag stream to prevent undue polymer compound accumulation and recovered. At least another portion of the recovered hexane is recycled along with fresh make-up hexane in an amount of 0.3 mass units to the off-line selective hydrogenation zone in order to continue the regeneration thereof. RAW BUTADIENE STREAM ANALYSIS Butadiene 50 weight % Acetylene 0.8 weight %

Claims: A process for the purification of a diolefin hydrocarbon stream containing trace quantities of acetylene compounds which process comprises: (a) introducing said diolefin hydrocarbon stream containing trace quantities of acetylene compounds and elemental hydrogen into a selective hydrogenation zone to selectively hydrogenate at least a portion of said acetylene compounds; (b) contacting the selective hydrogenation catalyst in said selective hydrogenation zone with a polymer solvent; (c) passing one or more resulting effluents from said selective hydrogenation zone to at least one fractionation zone to produce a diolefin hydrocarbon stream having a reduced concentration of acetylene compounds and a stream containing polymer solvent and polymer compounds; (d) recycling at least a portion of said stream containing polymer solvent and polymer compounds to provide at least a portion of said polymer solvent in step (b); (e) recovering at least another portion of said stream containing polymer solvent and polymer compounds; and (f) recovering said diolefin hydrocarbon stream having a reduced concentration of acetylene compounds produced in step (c). The process of Claim 1 wherein the diolefin hydrocarbon stream and the polymer solvent are introduced together in admixture into the selective hydrogenation zone. The process of Claim 1 wherein the stream containing polymer solvent cyclicly regenerates at least partially spent selective hydrogenation catalyst by introducing said diolefin hydrocarbon stream and elemental hydrogen into a selective hydrogenation zone to selectively hydrogenate at least a portion of said acetylene compounds and to produce an at least partially spent selective hydrogenation catalyst; contacting the at least partially spent selective hydrogenation catalyst in said selective hydrogenation zone with the polymer solvent and hydrogen to reduce the polymer content of the selective hydrogenation catalyst to thereby increase hydrogenation activity; and introducing said diolefin hydrocarbon stream and elemental hydrogen into said selective hydrogenation zone after contact with said polymer solvent and hydrogen. The process of Claims 1, 2 or 3 wherein the diolefin hydrocarbon stream comprises butadiene and said polymer solvent is an alkane having from 4 to 8 carbon atoms. The process of Claims 1, 2 or 3 wherein said selective hydrogenation zones contain a catalyst comprising copper metal. The process of Claims 1, 2 or 3 wherein said selective hydrogenation zone is operated at conditions including a pressure from 1379 to 3447 kPa and a temperature from 32°C to 83°C. The process of Claim 3 wherein the contacting of the at least partially spent selective hydrogenation catalyst is conducted at conditions including a pressure from 1034 kPa to 3447 kPa, a temperature from 32°C to 260°C, a solvent liquid hourly space velocity from 0.5 to 10 hr -1 . The process of Claims 1, 2 or 3 wherein said diolefin hydrocarbon stream comprises a compound selected from the group of diolefins containing from 3 to 5 carbon atoms. The process of Claim 1 comprising: contacting a butadiene hydrocarbon stream as said diolefin hydrocarbon stream containing trace quantities of acetylene compounds with a polymer solvent comprising hexane and introducing the resulting admixture together with elemental hydrogen into the selective hydrogenation zone containing a copper catalyst to selectively hydrogenate at least a portion of said acetylene compounds; and passing the resulting effluent from said selective hydrogenation zone in step (a) to the fractionation zone to produce the butadiene hydrocarbon stream having a reduced concentration of acetylene compounds and a stream containing the polymer solvent comprising hexane and polymer compounds; The process of Claim 1 comprising: introducing a butadiene hydrocarbon stream as said diolefin hydrocarbon stream containing trace quantities of acetylene compounds and elemental hydrogen into said selective hydrogenation zone to selectively hydrogenate at least a portion of said acetylene compounds and to produce an at least partially spent selective hydrogenation catalyst; passing the resulting effluent from said first selective hydrogenation zone into the fractionation zone to produce said butadiene hydrocarbon stream having a reduced concentration of acetylene compounds; contacting the at least partially spent selective hydrogenation catalyst in said selective hydrogenation zone with a polymer solvent comprising hexane and hydrogen to reduce the polymer content of the selective hydrogenation catalyst to thereby increase hydrogenation activity; passing the resulting effluent from said selective hydrogenation zone to said fractionation zone to produce a stream comprising hexane solvent and polymer compounds; recycling at least a portion of said stream comprising hexane solvent and polymer compounds to provide at least a portion of said polymer solvent comprising hexane; and introducing said butadiene hydrocarbon stream and elemental hydrogen into said selective hydrogenation zone after contact with said hexane solvent and hydrogen.

Inventors: COTTRELL PAUL R, COTTRELL, PAUL R.
Applicants: UOP LLC
Application year: 2000
Main classification: C07C7
IPC codes: B01J23, C07C11, C07C7