MXPA98010856A - Method to produce vin acetate - Google Patents

Abstract

A method to produce vinyl acetate using ethylene, acetic acid and argon-containing oxygen that optimizes selectivity and minimizes ethylene losses to purge

Description

METHOD TO PRODUCE VINYL ACETATE FIELD OF THE INVENTION This invention relates to a method for producing vinyl acetate, and more particularly to a method for producing vinyl acetate that maximizes selectivity and minimizes loss of ethylene to purge. BACKGROUND OF THE INVENTION Vinyl acetate is produced commercially by the catalyzed partial oxidation of ethylene in the presence of acetic acid and oxygen.

The source of oxygen may be commercially available oxygen or air. Generally, in an oxygen-based process, ethylene, acetic acid and oxygen are mixed with a recycled gas and fed into the reactor. The reactor comprises a number of tubes which are placed inside a container arranged in a manner similar to a shell and tube heat exchanger. The reactor tubes are filled with preferably a metal catalyst on a porous support containing small amounts of promoters. A refrigerant circulates in the jacket around the reactor tubes to maintain temperature control. In an oxygen-based process, a typical composition of the gas stream that is fed to the reactor tubes includes 40 to 60% in mol of ethylene, 5 to 10% in mol of oxygen, 4 to 10% in mol of argon, 10 to 15% in mol of acetic acid, 5 to 15% in mol of carbon dioxide, with ethane, nitrogen and water constituting the rest of the composition. Ethylene and acetic acid react with oxygen to form vinyl acetate and also in a side reaction to form carbon dioxide and water. Both reactions are exothermic. The reactor effluent is treated in two separate steps by removing vinyl acetate product and remaining reactive acetic acid, and removing carbon dioxide by-product. The remaining gas is recycled after a portion of it is purged. The purge stream is required in order to keep the impurities in the reactor at acceptable levels. Impurities, such as argon, are introduced into the oxygen stream, and like ethane and propane, into the ethylene feed stream. A significant amount of ethylene is lost in the purge stream as a loss of selectivity. Typically, a purge gas composition is 65.0 mol% ethylene, 7.0 mol% oxygen, 5.0 mol% argon, 17.8 mol% carbon dioxide, 4.0 mol% nitrogen, and the rest being Ethane and methane. In general, the argon impurities introduced with the oxygen stream determine the size of the purge stream when the oxygen concentration of the oxygen supplied is between 98 mol% and 99.6 mol%. If the amount of argon introduced into the reactor decreases, then the size of the purge stream can be decreased and ethylene losses can be reduced. An oxygen feed with a higher oxygen concentration (99.6 mol%) will result in a reduction of the purge volumetric flow rate if the argon is the impurity that controls the purge. However, other impurities are also introduced into the process. For example, due to the corrosive nature of acetic acid in the process, the instruments used to control the process require an inverse drag of nitrogen purge. Nitrogen is the inert gas of selection and ends in recycling. The nitrogen must therefore be removed with the purge to prevent nitrogen from accumulating in the recycling. If nitrogen enters the reactor in amounts similar to those of argon, then these steps must be done to reduce the concentration of nitrogen before reducing the concentration of argon. Several methods have been proposed to treat the purge of oxygen-based reactions to recover ethylene. For example, U.S. Patent No. 4,904,807 describes the use of an argon selective membrane that is used to treat the purge and separate it into two streams 1) a stream rich in argon that is vented and 2) a stream rich in ethylene that can be recycled to the ethylene oxide reactor, and the US Patent. No. 4,769,047 discloses the use of pressure swing adsorption to remove ethylene from the purge and recycle it to the reactor. A major disadvantage in these methods is the high capital cost of the associated equipment. It was believed that there has not been a commercially practical solution to reduce the impurities associated with the production of vinyl acetate. Therefore, there is a need to provide a new method for producing vinyl acetate that maximizes selectivity and minimizes ethylene losses to purge, thereby improving the yield of vinyl acetate production.

OBJECTIVES OF THE INVENTION It is therefore an object of the invention to provide a method for improving the performance of the oxygen-based process for the production of vinyl acetate. It is another objective to provide a method to produce vinyl acetate which maximizes selectivity and minimizes losses to purge. It is yet another objective to provide a method to increase the selective production of vinyl acetate by adjusting the concentration of at least one of ethylene, acetic acid, oxygen, nitrogen and carbon dioxide.

BRIEF DESCRIPTION OF THE INVENTION This invention is directed to a method for improving the production yield of vinyl acetate comprising combining ethylene, acetic acid and an oxygen stream containing argon with a recycled gas to form a reaction gas mixture; feeding a reaction mixture stream to a reactor filled with catalyst such that a reaction effluent stream emerges therefrom, passing a portion of the ethylene-rich effluent stream to purge as a purge stream and at least a portion of the stream ethylene-rich effluent to a carbon dioxide removal unit such that an effluent stream of carbon dioxide and a stream rich in ethylene and free of carbon dioxide emerge therefrom; passing a portion of the effluent stream rich in carbon dioxide-free ethylene with the effluent stream rich in ethylene to form the recycle gas; passing the effluent stream of vinyl acetate-acetic acid mixture to a purification unit such that a recycle stream of acetic acid and a stream of vinyl acetate emerge from it; and pass the recycle stream of acetic acid with acetic acid. For purposes of this invention, the gaseous constituents preferably comprise 40-60% mol ethylene, 5-10% mol oxygen, 4-10 mol% argon, 10-15 mol% acetic acid, and -15% mole of carbon dioxide. The reactor filled with catalyst comprises a reactor tube filled with palladium and gold on a porous support. BRIEF DESCRIPTION OF THE DRAWINGS Other objectives, aspects and advantages will occur to those skilled in the art from the following description of preferred embodiments and the attached drawings, in which: Fig. 1 is a schematic representation of a process to produce vinyl acetate by selective oxidation of ethylene and acetic acid with oxygen; and Fig. 2 is a graphical representation of the effects of ethylene savings and of reducing the volume of the purge stream so when the oxygen concentration increases from 99.6% oxygen to 99.95% oxygen, and when the concentration of argon in the recycle stream is maintained at 5% constant. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 provides a schematic representation 100 of a process for producing vinyl acetate or for the selective oxidation of ethylene and acetic acid with oxygen. The acetic acid 102 is combined with the recycle stream 104 of acetic acid forming the 105 stream of acetic acid. Ethylene 101, oxygen 103 containing argon and stream 105 of acetic acid are added to stream 130 of ethylene recycle forming stream 106 of ethylene-acetic acid-oxygen, which is fed to reactor 150. Leaving the reactor 150 is the stream 108 which is passed to the purification unit 152. Departing from the purification unit 152 are two streams: 1) stream 1 10 rich in ethylene, and 2) stream 1 12 containing vinyl acetate and acetic acid. The ethylene-rich stream 1 10 is separated in streams 122 and 123. Stream 123 is further divided into purge stream 124, and stream 126 to pass through carbon dioxide removal unit 156. The purge stream 124 is eliminated. The stream 129 of purified carbon dioxide and the carbon dioxide-free ethylene effluent stream 128 emerges from the carbon dioxide removal unit 156. The stream 129 of purified carbon dioxide is removed. The effluent stream 128 rich in carbon dioxide-free ethylene is added to stream 122 to form recycle stream 130. Stream 1 12 containing vinyl acetate monomer and acetic acid is passed to purification unit 154. Leaving unit 154 are stream 104 of acetic acid recycle and stream 120 of vinyl acetate. The vinyl acetate stream is removed as a product.

In an oxygen-based process, the argon impurities introduced with the oxygen stream determine the size of the purge stream for an oxygen purity between 98 mol% to 99.6 mol%. The amount of argon that is removed in the purge is equal to the product of the argon concentration by the purge volume and this product must be equal to the amount of argon that is added to the reactor by the fresh oxygen feed according to the equation (1) that follows: (concentration x (flow rate = (volume of argon added by volumetric argon) volumetric purge) fresh oxygen feed) (1) If the amount of argon introduced to the process by the fresh oxygen feed decreases and the concentration of argon remains constant, then, according to equation (1), the purge stream size can be decreased and the loss of ethylene to the purge can be reduced. If the amount of argon introduced to the process decreases and the concentration of argon is reduced, then the size of the purge stream will be increased in relation to the case of constant argon. In the vinyl acetate manufacturing process, if the argon is the impurity that controls the volume of the purge then if the concentration of oxygen in the oxygen feed can be increased then the volumetric purge regime can be reduced resulting in substantial savings of ethylene. For example, a plant that produces 363.2 MM k / year of vinyl acetate is used to demonstrate this invention. The gas stream that is fed to the reactor tubes is filled with palladium and gold on a porous support. Fig. 2 represents the reduction in purge stream volume and ethylene savings from an oxygen purity of 99.6% mol to an oxygen purity of 99.95 mol%, and maintaining the concentration of argon in the stream of recycle at 5% in mol. A stream of oxygen containing argon containing more than 99.6 mol% oxygen is defined for the purpose of this invention as high purity oxygen. In Fig. 2, the composition of the purge gas is 65 mol% ethylene, 7 mol% oxygen, 5 mol% argon, 20.5 mol% carbon dioxide, with the remainder being ethane and methane. At an oxygen purity of 99-95 mol% the ethylene saved represents approximately 2.5 mol% of the ethylene that is fed into the process, which is a significant improvement for an industrial process. By reducing the purge flow, while maintaining the concentration of argon in the constant recycle gas, the ethylene losses in the purge are reduced. Instead of maintaining the constant argon concentration and reducing the purge flow rate, this invention also provides a method for reducing the argon concentration using high purity oxygen and regulating the purge flow. This type of operation can be used even if other impurities that control the purge are present. Reducing the concentration of argon allows the increase of the concentration of ethylene, acetic acid, oxygen or carbon dioxide, or a combination of these four gases to obtain a composition of gas feed to the reactor with better heat transport properties. If the argon concentration decreases and is replaced by ethylene or carbon dioxide, then the oxygen concentration can also be increased because argon has an adverse effect on flammability. See U.S. Patent No. 3,855,280. Increasing the concentration of oxygen improves the performance of the reactor. The reaction ratios for the formation of the desired vinyl acetate depend on the concentration of all the reactants and products. Regulating the concentrations of reagents and products in the feed can result in an increased yield of vinyl acetate. The purge flow rate can be even lower than that used when high purity oxygen is not used, thus providing additional ethylene savings. There are two reasons for the improvement of selectivity if the argon concentration decreases: 1) better heat transport properties of the gas feed to the reactor which will reduce the heat point effect and improve the selectivity, and 2) better kinetics through of the adjustment of remaining gas concentrations (reactants and products). Improving the selectivity to vinyl acetate means more ethylene, acetic acid and oxygen that are fed to the reactor and converted to vinyl acetate and fewer by-products. The improvement depends on a variety of conditions including the type and age of the catalyst, and various operating conditions such as temperature, pressure and residence time inside the reactor tubes, and the temperature, pressure and flow rate of the cooling fluid flowing in. the cover around the reactor tubes. The selectivity improvement should be determined on a case-by-case basis due to the differences in the catalyst and the operating conditions used by each commercial plant. For the reasons discussed above, when high purity oxygen is used, selectivity improvements of 0.05 mol% to 1 mol% can be expected for each 1 mol% reduction of the argon concentration if at the same time a combination of concentrations of other gases to replace that of argon. However, reducing the argon concentration will increase the purge flow rate and may also increase the ethylene concentration in the recycle stream, and consequently increase etilene losses in the purge. Thus, when high purity oxygen is used, there is an optimum reduced argon concentration in the flow regime of the recycle stream and the purge stream that will optimize the ethylene yield. If other impurities are introduced into the process in significant quantities, then additional steps must be taken to remove the impurities before decreasing the flow rate of the purge stream. For example, the nitrogen used for instrumentation protection can be replaced by carbon dioxide (another inert gas). The carbon dioxide that replaces nitrogen increases the concentration of carbon dioxide in the recycle stream. The additional carbon dioxide in the recycle stream raises the concentration of carbon dioxide, which will improve the performance of the carbon dioxide removal section (see Fig. 1). The carbon dioxide removal unit may not remove all of the additional carbon dioxide and, as a result, the concentration of carbon dioxide in the recycle stream that is fed to the reactor may increase. Thus, carbon dioxide will replace nitrogen in the stream that is fed to the reactor. Carbon dioxide has better heat transport properties than nitrogen, consequently, a lower concentration of nitrogen and a higher concentration of carbon dioxide are beneficial for the performance of the reactor. The carbon dioxide can be supplied from the product of the carbon dioxide removal section or from an independent source. The elimination of nitrogen reduces the purge requirements. Reducing the concentration of nitrogen means that we can increase the concentration of ethylene, acetic acid or oxygen or a combination of the four to obtain a feed gas composition to the reactor with better heat transport properties and better reaction kinetics. He . hollow left by the decrease in the concentration of nitrogen can be replaced by ethylene, carbon dioxide and oxygen. Increasing concentrations of ethylene, carbon dioxide and oxygen improves reactor performance. This invention provides a number of clear advantages over the art by reducing the amount of argon in the system and / or being introduced into the system. One mode uses high purity oxygen and regulates the volume of the purge stream to effectively reduce the argon concentration. Another mode uses carbon dioxide to replace nitrogen in the system, and reduce the concentration of nitrogen. In another embodiment, one can replace nitrogen with carbon dioxide, use high purity oxygen and reduce the volume of the purge stream. Improving reactor selectivity by reducing the concentration of at least one of argon or nitrogen could potentially allow the increase in vinyl acetate production. If the ethylene feed rate were kept constant and the selectivity improved, additional vinyl acetate would necessarily be produced. That is, one embodiment of this invention is to maintain the constant flow rate, while adjusting the concentration of at least one of ethylene, acetic acid and oxygen containing argon. If the downstream separation equipment could process the additional load; this would be a method of increasing production with zero capital. Production increases in the order of approximately 0.5-5% of the total yield can be expected. Reducing the concentration of at least one of argon or nitrogen also reduces the effects of hot spot formation in the reactor and thus extends the life of the catalyst. This effect is associated with improved selectivity (which reduces the amount of heat generated within the reactor) and improves the thermal properties of the reaction gas mixture (which improves the removal of heat from the reactor). The extension of the life of the catalyst will reduce the consumption of the catalyst. It is expected that the life of the catalyst can be extended to approximately one year. In the reaction, the gas stream is fed to the reactor tubes filled with palladium and gold on a porous support. The catalyst employed in the process of this invention can be a metal-containing catalyst known in the art to catalyze the controlled oxidation of ethylene with acetic acid and molecular oxygen to produce vinyl acetate. The catalyst can be a metal, preferably palladium and gold on a suitable support, preferably porous support. The support may consist of siliceous and aluminous materials. Particularly suitable catalysts are those made of gold metal and promoters essentially in low surface area supports containing alpha alumina together with minor proportions of silica, silicon carbide and other refractory materials. In general, the operating temperature of this invention suitably occurs in the range from about 150 ° C to about 350 ° C, preferably in the range from about 120 ° C to about 200 ° C. The operating pressure for the practice of this invention is suitably in the range from about 2.81 k / cm 2 to about 21. 1 1 k / cm 2, and preferably from about 5.63 to about 14.07 k / cm 2. The space velocity is chosen according to the desired amount of production, and preferably in the range from about 3000 to about 5000 / hr. These ranges of parameters are typically used in the common production of commercial vinyl acetate. The use of high purity oxygen in this invention can also be practiced with a conventional ethylene recovery apparatus for the purge stream, such as separation membrane adsorption or pressure oscillation, or cold box, to treat the purge, recover the remaining ethylene and return it to the reactor. The use of high purity oxygen greatly reduces the capital investment required for such systems. Specific aspects of the invention are shown in the drawings for convenience only, since each aspect can be combined with other aspects according to the invention. Alternate modalities will be recognized by those skilled in the art and are intended to be included within the scope of the claims.

Claims (6)

1 . - A method to improve the production of vinyl acetate by reducing the concentration of argon during the production of vinyl acetate, said method comprising: a) combining ethylene, acetic acid and an oxygen stream containing argon with a recycled gas to form a gaseous reaction mixture; b) feeding a stream of said reaction mixture to a reactor filled with catalyst such that a reaction effluent stream emerges therefrom; c) passing said effluent reaction stream to a purification unit such that a rich effluent stream of ethylene emerges therefrom; d) passing a portion of said ethylene-rich effluent stream to be charged as a purge stream and a portion of said ethylene-rich effluent stream to a carbon dioxide removal unit such as an effluent stream of carbon dioxide and a effluent stream rich in carbon dioxide-free ethylene emerge from it; e) passing a portion of said effluent current stream rich in carbon dioxide-free ethylene with said ethylene-rich stream to form said recycle gas; f) passing said effluent stream of vinyl acetate-acetic acid mixture to a purification unit such that a recycle stream of acetic acid and a stream of vinyl acetate emerge from it; g) combining said recycle stream of acetic acid with said stream of acetic acid; h) increasing the oxygen content of said oxygen stream containing argon; ei) regulating the flow of the purge stream to effectively reduce the concentration of argon

2. - The method of claim 1 wherein step (i) further comprises regulating the flow rate of feed of at least one of said streams of ethylene, acetic acid and oxygen containing argon to increase the selective production of said vinyl acetate.

3. The method of claim 1 wherein step (i) comprises maintaining the flow rate constant and regulating the concentration of at least or not of ethylene, acetic or acetic acid and oxygen containing argon.

4. - The method of claim 1 wherein step (i) further comprises producing a gas with increased heat transport properties to reduce the effects of the heat points formed in the reactor.

5. A method for producing vinyl acetate comprising a) combining ethylene, acetic acid and argon containing oxygen streams with a recycle gas to form a reaction gas mixture; b) feeding a stream of said reaction mixture in a reactor filled with catalyst such that a reaction effluent stream emerges therefrom; c) passing said effluent reaction stream to a purification unit such that an effluent stream of vinyl acetate-acetic acid mixture and an ethylene-rich effluent stream emerge therefrom; d) passing a portion of said ethylene-rich effluent stream to purge as a purge stream and a portion of said ethylene-rich effluent stream to a carbon dioxide removal unit such as an effluent stream of carbon dioxide and a carbon dioxide-free ethylene-rich effluent stream does not emerge from the same; e) passing a portion of said effluent stream rich in carbon dioxide-free ethylene with said ethylene-rich stream to form said recycle gas; f) passing said effluent stream of vinyl acetate-acetic acid mixture to a purification unit such that a recycle stream of acetic acid and a stream of vinyl acetate emerge from it; h) combining said recycle stream of acetic acid with said stream of acetic acid; i) passing a portion of carbon dioxide to replace nitrogen for purging / reverse injection of instruments; and j) increasing the oxygen content of said oxygen stream containing argon.

6. - The method of claim 5 comprising reducing the flow rate of the purge stream. 1. - The method of claim 5 wherein the feed gas introduced into the reaction zone comprises 40-80% mol ethylene, 5-15% in mol of oxygen, 0-10% in mol of argon, 10-15% in mol of acetic acid, and 5-15% in mol of carbon dioxide. 8. A method for producing vinyl acetate comprising a) combining ethylene, acetic acid and oxygen containing argon with a recycle gas, to form a gas mixture of reaction; b) feeding a stream of said reaction mixture to a reactor filled with catalyst such that an effluent stream of the reaction emerges therefrom; c) passing said effluent reaction stream to a purification unit such that an effluent stream of vinyl acetate-acetic acid mixture an ethylene-rich effluent stream emerges therefrom; d) passing a portion of said ethylene-rich effluent stream to purge as a purge stream and a portion of said ethylene-rich effluent stream at a carbon dioxide removal rate such as an effluent stream of dioxide or carbon and a rich effluent stream in the carbon dioxide free ileum emerges therefrom; e) passing a portion of said effluent current stream rich in carbon dioxide-free ethylene with said effluent ethylene-rich stream to form said recycle gas; f) passing said effluent stream of vinyl acetate-acetic acid mixture to a purification unit such that a recycle stream of acetic acid and a stream of vinyl acetate emerge from it; h) passing said recycling stream of acetic acid with acetic acid co-curing acid d; and i) passing a current of carbon dioxide to replace nitrogen for purge / reverse injection of instruments to effectively reduce the concentration of nitrogen in the system. 9. - The method of claim 8 wherein step (i) further comprises regulating the flow rate of feed of at least one of said streams of ethylene, acetic acid and oxygen containing argon, thereby increasing the selective production of said vinyl acetate. 10. - The method of claim 8 wherein step (i) further comprises producing a gas with improved heat transport properties to reduce the effects of hot spots formed in the reactor.