Title of Invention

METHOD FOR RECOVERY OF ETHYLENE IN A RECIRCULATING GAS PROCESS FOR THE PRODUCTION OF VINYL ACETATE

Abstract

The invention relates to a method for the recovery of ethylene in a recirculating gas process for the production of vinyl acetate by means of a) heterogeneously-catalysed reaction of ethylene, acetic acid and oxygen at a pressure of 1 to 50 bar and a temperature of 50 DEG C to 200 DEG C, b) separation of the product gas stream containing essentially ethylene, vinyl acetate, acetic acid, water, carbon dioxide and further inert gases and c) recycling of ethylene into the recirculating gas process, characterized in that d) the product gas stream is fed to a recirculating gas scrubber coated with acetic acid at system pressure and vinyl acetate removed from the recirculating gas, e) the recirculating gas containing no vinyl acetate is then fed to a C02 absorber for removal of carbon dioxide and then f) a part of the recirculating gas stream comprising ethylene is recycled to the reaction system and the remainder of the gas stream containing ethylene is exhausted and reused in methods for recycling or reaction of ethylene.

Full Text

Method for recovery of ethylene in a recirculating gas process for the production of vinyl acetate The invention relates to a process for ethylene recovery in a cycle gas process for preparing vinyl acetate, in which a sub-stream of the ethylenic product stream is discharged and sent to a process for recovering or converting ethylene. Vinyl acetate is prepared in continuous processes with recy-cling of the purified product stream. In a heterogeneously catalysed gas phase process, ethylene reacts with acetic acid and oxygen over fixed bed catalysts which generally comprise palladium and alkali metal salts on a support material, and may additionally also be doped with gold, rhodium or cadmium. The ethylene, oxygen and acetic acid reactants are reacted in an exothermic reaction generally at a pressure of from 8 to 12 bar and a temperature of from 130°C to 200°C in a fixed bed tubular reactor to give vinyl acetate: C2H4 + CH3COOH + 0..5 02 => CH3COOCH=CH2 + H20 The ethylene conversion is about 10%, the acetic acid conver-sion from about 20 to 30% and the oxygen conversion up to 90%. A problem in this reaction is that inert substances are intro-duced via the ethylene and oxygen reactants, and are difficult to remove, but increasingly reduce the selectivity of the re-action when they are recycled into the reactor and accumulate in the cycle gas. The inerts nitrogen and argon are introduced via the oxygen, and ethane and to a slight extent methane via ethylene. These inerts have to be removed continuously from the system, otherwise their accumulation would inhibit the re-action. Since ethylene constitutes the majority of the cycle gas mixture at from 60 to 70% by volume, the removal of inerts is generally accompanied by a distinct loss of ethylene of from about 1 to 4% by volume of the amount fed. In addition, side reactions form carbon dioxide and further by-products such as methyl acetate and ethyl acetate. Owing to the incomplete conversion of the reactants, the gase-ous product stream is worked up in a continuous process: in a cycle gas scrubber operated with acetic acid, the vinyl ace-tate target product is washed out of the cycle gas and worked up in subsequent distillation processes. The vinyl acetate-free cycle gas is sent through a cycle gas compressor to the acetic acid saturator and subsequently to the reactor. In or-der to reduce the C02 by-product, a portion of the vinyl ace-tate-free cycle gas is discharged on the pressure side of the cycle gas compressor and sent to a water scrubber. Subse-quently, a small fraction is sent to incineration for inerts discharge, and the remainder is passed into a C02 absorption column and then sent back to the cycle gas in C02 free form. The inerts discharge by means of removal of ethylene from the cycle gas prevents accumulation of ethane, methane, argon and nitrogen in the cycle gas stream. The amount of the inert stream discharged is controlled depending on the ethylene con-centration in the cycle gas. When the amounts discharged are too small, the inerts become concentrated in the cycle gas and the ethylene concentration in the cycle gas falls. However, the ethylene selectivity increases with the ethylene content of the cycle gas. The higher the ethylene content in the cycle gas, i.e. the more ethylene comprising inerts from the cycle gas is removed and “fresh” ethylene is supplied, the better the ethylene selectivity is. However, from a certain propor-tion, a more extensive discharge of ethylene comprising inerts is uneconomic, since each additional ton of vinyl acetate monomer has to be paid for with a disproportionately high fraction of discharged ethylene comprising inerts. Since eth-ylene is expensive, the recovery of ethylene has the highest priority as a cost-lowering measure. WO-A 01/00559 describes two common alternatives for ethylene recovery in vinyl acetate preparation by means of gas phase reaction of ethylene, acetic acid and oxygen. Carbon dioxide is removed from the gas stream leaving the re-actor which then comprises primarily ethylene, methane, oxy- gen, nitrogen and argon. At system pressure, the gas stream is passed into an absorption column and washed with vinyl ace-tate, and a mixture of methane, nitrogen, oxygen and argon is drawn off at the top of the column and sent to incineration. At the bottom of the column, vinyl acetate and ethylene are withdrawn, the gas mixture is decompressed and ethylene is re-moved from vinyl acetate. The ethylene is subsequently com-pressed and passed back into the reactor. Disadvantages in this context are the energy-intensive decom-pression-compression step, and the fact that the inerts cannot be removed fully, and thus become increasingly enriched and distinctly lower the selectivity of the reaction. In a further variant, to which WO-A 01/00559 is directed, the majority of the gaseous product stream is contacted at system pressure with acetic acid in an absorption vessel. At the top of the column, methane, nitrogen, oxygen and argon are re-moved, and a mixture of vinyl acetate, acetic acid and ethyl-ene is drawn off at the bottom of the column. This mixture is contacted in a gas scrubber with the remaining fraction of the gaseous product stream. The ethylene is drawn off at the top and recycled into the reactor; the vinyl acetate is obtained at the bottom of the column and sent to further workup. The decompression/compression step becomes unnecessary, but here too the inert gases accumulate increasingly in the cycle gas. A similar process is the subject matter of US-A 3,714,237, in which the gaseous stream is likewise worked up by scrubbing with acetic acid, vinyl acetate is removed, and the residual gas is recycled into the reactor after the carbon dioxide has been washed out. Here too, the inert gases accumulate increas-ingly in the cycle gas. It is therefore an object of the present invention to config-ure the cycle gas process in the preparation of vinyl acetate in such a way that the accumulation of the inert gases men-tioned is very substantially prevented. The invention provides a process for ethylene recovery in a cycle gas process for preparing vinyl acetate by means of a) heterogeneously catalysed reaction of ethylene, acetic acid and oxygen at a pressure of from 1 to 5 0 bar and a temperature of from 50°C to 200°C/ b) separation of the product gas stream comprising substan-tially ethylene, vinyl acetate, acetic acid, water, carbon di-oxide and further inert gases, and c) recycling of ethylene into the cycle gas process, characterized in that d) the product gas stream is fed at system pressure to a cycle gas scrubber charged with acetic acid, and vinyl acetate is removed from the cycle gas, and e) the vinyl acetate-free cycle gas is subsequently fed to a C02 absorption to remove carbon dioxide, and then f) a portion of the ethylenic cycle gas stream is recycled into the reaction system, and the remainder of the ethylenic gas stream is discharged and reused in processes for recover-ing or converting ethylene. In the continuous preparation of vinyl acetate, operation is effected in tubular reactors which are charged with a fixed bed catalyst. These catalysts are generally supported cata-lysts doped with noble metals (or noble metal salts) and pro-moters, for example bentonite spheres doped with palladium and with gold (cadmium) and potassium salts. The reactor is char-ged with ethylene, oxygen and acetic acid, and the reaction is carried out preferably at a pressure of from 8 to 12 bar and a temperature of from 130°C to 200°C. The product gas stream leaving the reactor comprises substantially vinyl acetate, ethylene, acetic acid, water, oxygen, C02 and the inerts ni-trogen, argon, methane and ethane. The product gas stream is subsequently separated in a cycle gas scrubber operated with acetic acid, in which vinyl ace-tate, acetic acid, water and further condensable fractions are removed, and the vinyl acetate monomer is obtained by means of distillative workup. After the removal of the condensable fractions (vinyl acetate, acetic acid, water), the cycle gas typically has the following composition: from 60 to 65% by volume of ethylene, from 12 to 18% by volume of C02, from 5 to 8% by volume of ethane, from 4 to 9% by volume of oxygen, from 4 to 6% by volume of nitrogen, from 1 to 2% by volume of argon, from 0.5 to 1% by volume of methane. This composition makes clear that, to effectively remove meth-ane, ethane, argon and nitrogen, a relatively high proportion has to be sent to incineration, with a correspondingly high ethylene loss. In the inventive procedure, the cycle gas is now conducted in-to a C02 absorption/desorption, customarily operated with aqueous potassium carbonate solution. After the C02 scrubbing, the cycle gas generally has the following composition: from 80 to 83 % by volume of ethylene, from 1 to 4 % by volume of C02/ from 2 to 4 % by volume of ethane, from 3 to 5 % by volume of oxygen, from 3 to 4 % by volume of nitrogen, from 0.5 to 1 % by volume of argon, from 0.2 to 0.4% by volume of methane. After the C02 scrubbing, the product stream is divided. A ma-jority of the ethylenic cycle gas stream is recycled into the reactor via a cycle gas compressor and acetic acid saturator. The remainder of the ethylenic gas stream is discharged and reused in processes for converting ethylene. Preference is given to discharging from 1 to 2 5% by volume, more preferably from 5 to 2 0% by volume, of the ethylenic gas stream. The car-bon dioxide laden with traces of hydrocarbons is passed to thermal disposal. The recycling is effected preferably into processes for con-verting ethylene. Examples thereof are oxidation processes for preparing acetaldehyde and for preparing acetic acid, the oxy-chlorination of ethylene for preparing dichloroethane and the direct chlorination of ethylene to dichloroethane. Further ex-amples are the preparation of ethylene oxide and ethylene gly-col, alkylation of benzene to ethylbenzene and optionally de-hydrogenation to styrene, the carbonylation to acrylic acid, the polymerization to polyethylene, the hydroformylation to propionaldehyde, the Reppe carbonylation to propionic acid, and the Alfol process for preparing long-chain, primary alco-hols. The ethylene may also be used for recycling in processes for refining hydrocarbons. With the process according to the invention, the inerts dis-charge is no longer inevitably associated with the incinera-tion of the valuable ethylene raw material. Virtually 100% utilization of the ethylene is possible; typically, 2% of the ethylene used is lost as a result of the inerts discharge. The generation of environmentally polluting carbon dioxide owing to ethylene incineration is prevented. This corresponds to a drop of 50 kg of C02 per metric ton of vinyl acetate monomer. The examples which follow serve to further illustrate the in-vention: Figure 1 shows a simplified diagram of the process: Example 1: A 25 m3 tubular reactor 1 which was equipped with a Pd/Au sup-ported catalyst was charged at a pressure of 8.5 bar and a temperature of 160°C via line 2 with a gas mixture having a gas hourly space velocity (GHSV) of 3500 h”1. The gas mixture (cycle gas) had the following composition: 61.7% by volume of ethylene, 10.8% by volume of C02, 12.7% by volume of acetic acid, 3.4% by volume of ethane, 8.0% by volume of oxygen, 0.8% by volume of nitrogen, 0.8% by volume of argon, 1.0% by volume of methane 0.8% by volume of water. The cycle gas leaving the reactor 1 was fed via line 3 to a vinyl acetate scrubber 4 operated with acetic acid, and subse-quently via line 6 to a carbon dioxide scrubber 7 operated with potash. A vinyl acetate/acetic acid/water mixture was withdrawn via line 5 from the vinyl acetate scrubber 4 and sent to further processing. After the carbon dioxide scrub-bing, 200 kg/h, corresponding to approx. 7% by volume, of the cycle gas going to the C02 removal were passed via line 8 into the acetic acid preparation for ethylene recovery, and the re-mainder was recycled via line 9 and the cycle gas compressor 10 into the reactor 1. The C02 discharge for thermal disposal was effected via path 11. Under these conditions, the catalyst exhibited a space-time yield of 650 g/l’h at an ethylene selectivity of 91.5%. Example 2 : The procedure was analogous to Example 1, with the difference that, after the C02 scrubbing, 300 kg/h, i.e. approx. 10% by volume, of the cycle gas going to the C02 removal, have been discharged. This increased the ethylene content in the cycle gas to 64% by volume; the other components were adjusted cor-respondingly. The reaction conditions (GHSV, cycle gas pres-sure , etc.) remained the same. The increase in the amount discharged resulted in an increase in the space-time yield to 660 g/l’h with an improvement in the ethylene selectivity to 92.5%, which corresponds to an at least 0.5% increase in the vinyl acetate production. Example 3: The procedure was analogous to Example 1, with the difference that, after the C02 scrubbing, 450 kg/h, i.e. approx. 15% by volume, of the cycle gas going to the C02 removal have been passed into the acetic acid preparation for ethylene recovery. This resulted in an increase in the ethylene content in the cycle gas to 66% by volume; the other components were adjusted correspondingly. The reaction conditions (GHSV, cycle gas pressure, etc.) remained the same. The increase in the amount discharged resulted in an increase in the ethylene selectivity to 93.0%, which corresponds to a 1% increase in the vinyl acetate production. New version of the Claims: 1. Process for ethylene recovery in a cycle gas process for preparing vinyl acetate by means of a) heterogeneously catalysed reaction of ethylene, acetic acid and oxygen at a pressure of from 1 to 5 0 bar and a temperature of from 50°C to 200°C, b) separation of the product gas stream comprising sub-stantially ethylene, vinyl acetate, acetic acid, water, carbon dioxide and further inert gases, and c) recycling of ethylene into the cycle gas process, characterized in that: d) the product gas stream is fed at system pressure to a cycle gas scrubber charged with acetic acid, and vinyl acetate is removed from the cycle gas, and e) the vinyl acetate-free cycle gas is subsequently fed to a C02 absorption to remove carbon dioxide, and then f) a portion of the ethylenic cycle gas stream is recycled into the reaction system, and from 1 to 25% by volume of the ethylenic gas stream is discharged and reused in proc-esses for converting ethylene or in processes for refining hydrocarbons. 2. Process according to Claim 1, characterized in that the discharged proportion of the ethylenic gas stream is re-used in oxidation processes for preparing ethylene oxide and ethylene glycol, acetaldehyde and for preparing acetic acid, in the oxychlorination of ethylene for the prepara-tion of dichloroethane, or in the direct chlorination of ethylene to dichloroethane. 3. Process according to Claim 1, characterized in that the discharged proportion of the ethylenic gas stream is re-used in processes for alkylating benzene to ethylbenzene, for carbonylation to acrylic acid, for polymerization, in the hydroformylation to propionaldehyde, in the Reppe car-bonylation to propionic acid, or in the Alfol process for preparinglong-chain primary alcohols.

Documents:

3794-CHENP-2006 EXAMINATION REPORT REPLY RECEIVED 25-09-2012.pdf

3794-CHENP-2006 FER REPLY AMENDED PAGES OF SPECIFICATION 25-09-2012.pdf

3794-CHENP-2006 FER REPLY AMENDED CLAIMS 25-09-2012.pdf

3794-CHENP-2006 FER REPLY FORM-3 25-09-2012.pdf

3794-CHENP-2006 FER REPLY POWER OF ATTORNEY 25-09-2012.pdf

3794-CHENP-2006 OTHER PATENT DOCUMENT 25-09-2012.pdf

3794-CHENP-2006 CORRESPONDENCE OTHERS 21-03-2012.pdf

3794-chenp-2006-abstract.pdf

3794-chenp-2006-claims.pdf

3794-chenp-2006-correspondnece-others.pdf

3794-chenp-2006-description(complete).pdf

3794-chenp-2006-drawings.pdf

3794-chenp-2006-form 1.pdf

3794-chenp-2006-form 3.pdf

3794-chenp-2006-form 5.pdf

3794-chenp-2006-pct.pdf