Abstract:
A selective hydrogenation process is described. The process includes dissolving acetylene and hydrogen in a solvent to form a liquid feedstream. The solvent comprises a mixture of a polar organic solvent and a non-polar organic solvent. The liquid feedstream is contacted with a heterogeneous supported selective hydrogenation catalyst at selective hydrogenation conditions to convert at least a portion of the acetylene to ethylene forming a liquid reaction mixture comprising the ethylene produced.

Description:
BACKGROUND OF THE INVENTION 
     Light olefin materials, including ethylene and propylene, represent a large portion of the worldwide demand in the petrochemical industry. Light olefins are used in the production of numerous chemical products, via polymerization, oligomerization, alkylation and other well-known chemical reactions. These light olefins are essential building blocks for the modern petrochemical and chemical industries for the production of items such as polyethylene. Producing large quantities of light olefin material in an economical manner, therefore, is a focus in the petrochemical industry. 
     The production of light olefins, and in particular ethylene, can be through steam or catalytic cracking processes. The cracking processes take larger hydrocarbons, such as paraffins, and convert the larger hydrocarbons to smaller hydrocarbons products. The primary product is ethylene. However, there are numerous other chemicals produced in the process. Among the many byproducts are hydrogen, methane, acetylene, and ethane. 
     Historically, naphtha cracking has provided the largest source of ethylene, followed by ethane and propane pyrolysis, cracking, or dehydrogenation. Due to the large demand for ethylene and other light olefinic materials, however, the cost of these traditional feeds has steadily increased. 
     Energy consumption is another cost factor impacting the pyrolytic production of chemical products from various feedstocks. Over the past several decades, there have been significant improvements in the efficiency of the pyrolysis process that have reduced the costs of production. 
     More recent attempts to decrease light olefin production costs include utilizing alternative processes and/or feed streams. In one approach, hydrocarbon oxygenates and more specifically methanol or dimethylether (DME) are used as an alternative feedstock for producing light olefin products. Oxygenates can be produced from available materials such as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. Making methanol and other oxygenates from these types of raw materials is well established and typically includes one or more generally known processes such as the manufacture of synthesis gas using a nickel or cobalt catalyst in a steam reforming step followed by a methanol synthesis step at relatively high pressure using a copper-based catalyst. 
     Once oxygenates are formed, the process includes catalytically converting oxygenates, such as methanol, into the desired light olefin products in an oxygenate to olefin (OTO) process. Techniques for converting oxygenates, such as methanol to light olefins (MTO), are described in U.S. Pat. No. 4,387,263, which discloses a process that utilizes a catalytic conversion zone containing a zeolitic type catalyst. This indirect route of production is often associated with energy and cost penalties, often reducing the advantage gained by using a less expensive feed material. 
     Another alternative process used to produce ethylene involves using pyrolysis to convert natural gas to ethylene. U.S. Pat. No. 7,183,451 discloses heating natural gas to a temperature at which a fraction is converted to hydrogen and a hydrocarbon product such as acetylene or ethylene. The product stream is then quenched to stop further reaction and subsequently reacted in the presence of a catalyst to form liquids to be transported. 
     A similar process is disclosed in U.S. Pat. No. 7,208,647 in which natural gas is combusted under suitable conditions to convert the natural gas into primarily ethylene and acetylene. The acetylene in the gaseous product stream is separated from the remaining products and converted to ethylene. 
     More recent efforts have focused on the use of supersonic reactors for the pyrolysis of natural gas into acetylene. For example U.S. Pat. Pub. No. 2014/0058149 discloses a reactor in which a fuel is combusted and accelerated to a supersonic speed. Natural gas is injected into the reactor downstream of the supersonic combustion gas stream, and the natural gas is converted into acetylene as an intermediary product. The reaction is quenched with a liquid to stop the reaction, and the acetylene may be converted to the desired product ethylene in a hydrogenation zone. 
     Whether an undesired byproduct or one of the desired products, acetylene will irreversibly bond with many downstream catalysts, in particular with polymerization catalysts. Therefore, the production streams which include acetylene must be treated to remove or reduce the amount of acetylene. Additionally, in those processes that produce acetylene as an intermediary product, the majority of the acetylene must be converted to ethylene. One method of converting or reducing the amount of acetylene is selective hydrogenation. 
     Selective hydrogenation processes can be utilized to reduce the acetylene concentration to a sufficiently low level and can be done in either a gas phase or a liquid phase. Since selective hydrogenation is a highly exothermic reaction, the liquid phase is sometimes preferred as it can better control the temperature of the reaction. For example, U.S. Pat. No. 8,460,937 discloses a process in which acetylene is absorbed into a solvent and passed into a reactor in which a catalyst and hydrogen are present. Under proper reactive conditions, the acetylene is converted into ethylene. The solvent is described as a non-hydrocarbon polar solvent, such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), acetone, tetrahydrofuran (THF), dimethylsulfoxide (DMSO), and monomethylamine (MMA). 
     A byproduct of selective hydrogenation is C 4+  hydrocarbons (hydrocarbons with four or more carbon atoms). The C 4+  hydrocarbons are undesirable because they can accumulate on catalysts causing coke and fouling the catalyst. Additionally, the creation of the C 4+  hydrocarbons needlessly consumes the acetylene and can make ethylene separation from the rest of products more complicated. 
     Therefore, it would be desirable to have a process which reduces the production of the C 4+  hydrocarbons in a selective hydrogenation of acetylene to ethylene. 
     It would also be desirable for such a process that is not limited by a specific catalyst. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a selective hydrogenation process. In one embodiment, the process includes dissolving acetylene and hydrogen in a solvent to form a liquid feedstream. The solvent comprises a mixture of a polar organic solvent and a non-polar organic solvent. The liquid feedstream is contacted with a heterogeneous supported selective hydrogenation catalyst at selective hydrogenation conditions to convert at least a portion of the acetylene to ethylene forming a liquid reaction mixture comprising the ethylene produced. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating a model of the solubility of hydrogen in N-methyl-2-pyrrolidone and p-diethylbenzene at 150° C. 
         FIG. 2  is a process flow diagram for the liquid phase selective hydrogenation of acetylene to ethylene according to one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that for a liquid phase selective hydrogenation of acetylene to ethylene, mixtures of polar solvents and non-polar solvents allow the process to be operated at lower temperatures than with pure N-methyl-2-pyrrolidone, while maintaining high selectivity for ethylene and conversion of acetylene. 
     The liquid phase acetylene selective hydrogenation process (SHP) utilizes the polar aprotic solvent N-methyl-2-pyrrolidone (NMP), which is a safer alternative to the gas phase acetylene process. However, NMP requires elevated temperature (e.g., greater than about 120° C.) to achieve commercially competitive performance (e.g., 99.5% conversion and about 98% selectivity to ethylene). The ability to operate at lower temperatures while achieving similar performance would result in potentially longer catalyst stability and life, as well as lower operational costs. 
     It was discovered that the use of a non-polar organic solvent, e.g., p-diethylbenzene (p-DEB), results in an activity advantage allowing operation at lower temperatures. However, p-DEB has much lower solubility for acetylene than NMP. Consequently, the acetylene absorber conditions must be adjusted to higher pressure and/or lower temperature to match the performance in NMP. 
     Therefore, an improved solvent system was developed that results in higher activities without a loss in performance while maintaining high acetylene and hydrogen solubility by improving the solvent system. For example, the dual solvent system combines a polar organic solvent, such as NMP, with a non-polar organic solvent, such as p-DEB, to gain higher activity and high acetylene and hydrogen solubility. Hydrogen is less soluble than acetylene, particularly in polar solvents. Although not wishing to be bound by theory, the improved activity in the non-polar solvent may be due to better hydrogen availability in the dissolved state. However, hydrogen is not completely soluble at the conditions used; it may form a gas phase in addition to the liquid phase. 
     The improved mixed solvent system results in equivalent or better performance than using a polar aprotic solvent alone. The mixed solvent takes advantage of the higher activity of the non-polar organic solvent while maintaining the acetylene solubility in the polar organic solvent. As a result, the process can be run at lower temperatures saving utility costs and potentially extending catalyst stability and life. Although not wishing to be bound by theory, the improved results may be related to improved hydrogen solubility in non-polar solvents compared to polar ones, for example, as illustrated by the modeled solubility of hydrogen in N-methyl-2-pyrrolidone and p-diethylbenzene at 150° C. shown in  FIG. 1 . 
       FIG. 2  illustrates an exemplary process  100  for a liquid phase selective hydrogenation of acetylene to ethylene in which an acetylene rich vapor steam  105  may be passed to an absorption zone  110 . The absorption zone  110  may include one or more absorption columns  115 . The acetylene in the stream  105  may be obtained from any industrial process. For example, in some embodiments, the stream  105  may have only a small amount of acetylene which must be treated to remove the acetylene to avoid damaging a downstream polymerization catalyst. In other embodiments, the acetylene rich vapor stream  105  is obtained from a process in which methane is pyrolyzed in a reactor to produce acetylene as an intermediate product, and in some embodiments, the methane is pyrolyzed in a supersonic reactor. In embodiments where acetylene is pyrolyzed to produce ethylene, it is desirable to convert acetylene to ethylene economically and efficiently, and the acetylene conversion must be relatively complete. A second, or downstream conversion can be utilized to polish and remove the remaining trace amounts of acetylene. Thus, in the processes in which the acetylene is the intermediary product and ethylene is the desired product, it is undesirable to convert acetylene to products other than ethylene. 
     In the absorption zone  110 , acetylene in the acetylene rich vapor stream  105  is absorbed into a mixed solvent  120 . The mixed solvent  120  contains a polar organic solvent and a non-polar organic solvent. Suitable polar organic solvents are those with high acetylene solubility, including, but not limited to, N-methyl-2-pyrrolidone, dimethylformamide, acetone, tetrahydrofuran, dimethylsulfoxide, and monomethylamine, acetonitrile, or combinations thereof. Among these solvents, the ones with higher boiling point and low chemical reactivity are preferred. The most preferred polar solvent is N-methyl-2-pyrrolidone. The non-polar organic solvent is chemically inert and has a boiling point of at least about 100° C., or at least about 120° C., or at least about 140° C. Suitable non-polar organic solvents include, but are not limited, to, monoalkyl substituted aromatics, dialkyl substituted aromatics, trialkyl substituted aromatics, and paraffins with 8 or more carbon atoms and chemically inert. By “chemically inert” it is meant that the solvents, both polar and non-polar, do not react under process conditions with each other, nor do they react with either the feed or the products of selective hydrogenation: acetylene, hydrogen, CO, and ethylene. Examples of suitable non-polar organic solvents include, but are not limited to, diethylbenzenes, (e.g., p-diethylbenzene, m-diethylbenzene, o-diethylbenzene), xylenes (p-xylene, m-xylene, o-xylene), cumene, mesitylene, decane, dodecane, or combinations thereof. In one embodiment, the polar organic solvent is N-methyl-2-pyrrolidone, and the non-polar organic solvent is p-diethylbenzene, 
     The polar organic solvent can be present in amounts of about 90 vol % to about 10 vol % and about 10 vol % to about 90 vol % of the non-polar organic solvent, or about 85 vol % to about 15 vol % polar organic solvent and about 15 vol % to about 85 vol % of the non-polar organic solvent, or about 80 vol % to about 20 vol % and about 20 vol % to about 80 vol % of the non-polar organic solvent. The exact ratio of polar to non-polar solvent may depend on process economics and may vary between commercial units; each of them is unique due to variance of the feedstock and product price, utilities and labor costs, etc. If higher activity is preferred, then this ratio is higher, and if the higher selectivity is more important, then this ratio is lower. 
     The concentration of acetylene in the solvent mixture is desirably less than 5 wt %, or between 0.1% to about 5% by weight, between about 0.1% to about 3% by weight, or between about 1% to about 2% by weight, or between about 0.5% to about 5% by weight, or between about 0.5% to about 3% by weight, or between about 0.5% to about 2% by weight, or between about 1% to about 5% by weight, or between about 1% to about 3% by weight. 
     A first stream  125  being a liquid and comprising solvent and dissolved acetylene is removed from the absorption zone  110 . A second stream  130  being an acetylene lean vapor stream and comprising at least hydrogen gas is also removed from the absorption zone  110 . In order to allow downstream reactors to operate at higher pressures, the second stream  130  (or a portion thereof) may be passed to a compression zone  135  to provide a compressed second stream  140 . 
     The compressed second stream  140  and the first stream  125  from the absorption zone  110  may be combined into a combined stream  145  which is passed to a hydrogenation zone  150 . Alternatively, the compressed second stream  140  and the first stream  125  can be introduced into the hydrogenation zone  150  separately. Carbon monoxide may also be passed to the hydrogenation zone  150 . While the second stream  130  from the absorption zone  110  may include carbon monoxide, carbon monoxide can also be recovered from a downstream reaction effluent stream or carbon monoxide may be added to the process from another source. The hydrogen and other gases supplied to hydrogenation zone  150  in stream  130  may be supplemented by any suitable source of for example purified hydrogen or carbon monoxide. The concentration of carbon monoxide in stream  130  may vary depending on the source of the acetylene rich stream  105  entering absorption zone  110 . In an embodiment, the carbon monoxide concentration will be in the range of about 1 to about 50 mol %, or about 5 to about 35 mol %, or about 5 to about 20 mol %. 
     The hydrogenation zone  150  may include at least one hydrogenation reactor  155 . Each hydrogenation reactor  155  includes a hydrogenation catalyst, typically a hydrogenation metal in an amount between 0.01 to 5.0 wt % on a support, wherein the hydrogenation metal is preferably selected from a Group VIII metal. Preferably, the metal is palladium (Pd), platinum (Pt), nickel (Ni), rhodium (Rh), or a mixture thereof. More preferably, a Group VIII metal is modified by one or more metals, selected from Group IB through IVA, such as zinc (Zn), indium (In), tin (Sn), lead (Pb), copper (Cu), silver (Ag), gold (Au) in an amount between 0.01 and 5 wt %. Preferred supports are aluminum oxides (aluminas), pure or doped with other metal oxides, synthetic or natural (i.e. clays). More preferred supports are alpha-aluminas of various shape and size (i.e. spheres, extrudates), with high degree of conversion to alpha phase. 
     In the hydrogenation reactor  155 , in the presence of the catalyst, under hydrogenation conditions, the hydrogen reacts with the acetylene to produce ethylene. The hydrogen may be in the second stream  130  from the absorption zone  110 , or hydrogen may come from a portion of a downstream reaction effluent, or hydrogen may be added to the process. 
     Typical hydrogenation reaction conditions in the hydrogenation reactor  155  include a temperature that may range between about 50° C. and about 250° C., or between about 50° C. and about 200° C., or between about 50° C. and about 160° C., or between about 70° C. and about 250° C., or between about 70° C. and about 200° C., or between about 70° C. and about 160° C. Additionally, the hydrogenation reactor  155  is operated at a high pressure which may range between approximately 0.69 MPa (100 psig) and about 3.4 MPa (500 psig), preferably between approximately 1.0 MPa (150 psig) and about 2.8 MPa (400 psig). The liquid hour space velocity (LHSV) at the reactor inlet of the hydrogenation reaction can range between about 1 and about 100 h −1 , with preferred ranges being between about 5 and about 50 h −1 , between about 5 and about 25 h −1 , and between about 5 and about 15 h −1 . 
     The acetylene reacts with the hydrogen present to form ethylene. The reactor effluent stream  160 , which contains the hydrogenation reaction products, is passed to a separation zone  165  which contains, for example, a separator vessel  170 . 
     In the separator vessel  170  of the separation zone  165 , the reaction effluents are separated into an overhead vapor stream  175  and a bottoms liquid stream  180 . The separation may be obtained using any suitable separation process, including, but not limited to, reducing the pressure in the separator vessel  170 , and/or increasing the temperature of the separator vessel  170 . The majority of the ethylene will separate from the liquid phase even when the conditions are not changed due to the decreased solubility of ethylene in the solvent. 
     The separation zone  165  can be a separation vessel from the hydrogenation zone  150 , or the separation zone  165  and the hydrogenation zone  150  can be the same vessel. 
     The overhead vapor stream  175  is rich in ethylene and may contain other gases. The further processing of the overhead vapor stream  175  and the bottoms liquid stream  180  is not necessary for an understanding and practicing of the present invention. However, since the overhead vapor stream  175  may include carbon monoxide and hydrogen, a portion  185  of this stream  175  may be recycled back to the combined stream  145  entering the hydrogenation zone  150  to provide carbon monoxide and hydrogen for the hydrogenation reactions. Alternatively, portion  185  can be mixed with either the compressed second stream  140  or the first stream  125 , or it the can be introduced into the hydrogenation zone  150  separately. 
     The selectivity for ethylene for the process using the mixed solvent is at least about 95%, or at least about 96%, or at least about 97%, or at least about 98%. The selectivity is calculated by taking the ratio of ethylene weight, to the sum of weights of ethylene and all other products, including ethane, C 4  hydrocarbons, and oxygen-containing hydrocarbons, in %. The latter are believed to be formed upon reaction between hydrocarbons and CO. 
     The conversion of acetylene is at least about 95%, or at least about 96%, or at least about 97%, or at least about 98%, or at least about 99%, or at least about 99.5%. Conversion is calculated by taking the ratio: (weight of acetylene fed less weight of acetylene unconverted)/weight of acetylene fed, in %. 
     EXAMPLES 
     The selective hydrogenation catalyst was 0.35% Pd-0.64% Ag, supported on Alpha-Alumina spheres. It was activated (reduced) by treating with hydrogen at 400° C. for 1 hr. 
     The selective hydrogenation process was run using a pure NMP solvent and a solvent mixture containing 80 vol % NMP and 20 vol % p-DEB. The feedstock contained about 1.8-2.0 wt % acetylene in the solvent, either pure or mixed. The temperature, liquid hourly space velocity, hydrogen to acetylene molar ratio, and hydrogen to carbon monoxide molar ratio for the runs are shown in Table 1. The pressure was 1.8 MPa (250 psig). 
     The data shows that, at the same acetylene/solvent concentration (maintained at 1.8-2.0 wt % acetylene), the acetylene conversion reaches the same level (about 99.2%, and about 99.8%) at a temperature 10° C. lower in mixed p-DEB/NMP than in pure NMP, while high selectivity to ethylene was maintained. In another words, the same temperature conversion in mixed p-DEB/NMP is higher than in pure NMP. Without being bound to any specifics, on the basis of our experience it is believed that a 10° C. difference in temperature is equivalent to at least 10% difference in acetylene conversion. 
     Although not wishing to be bound by theory, the fact that the C 2  selectivity is a little higher and the C 4+  selectivity is a little lower may support the suggestion that the origin of the increased activity in p-DEB is H 2  availability at the surface of the catalyst as a result of improved H 2  solubility in non-polar media. However, the increase in selectivity could also be a result of the slightly higher ratio of hydrogen to acetylene. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 LHSV 
                 Molar ratios 
                   
                 Conv. 
                 Selectivity, wt % 
               
             
          
           
               
                 R# 
                 Solvent 
                 hr −1   
                 H 2 :C 2 H 2   
                 H 2 :CO 
                 T, ° C. 
                 wt % 
                 C 2=   
                 C 2   
                 C 4   
                 C 3  Oxy 
                 C 6+   
                 Σ of C 4+   
               
               
                   
               
             
          
           
               
                 1 
                 100NMP 
                 10 
                 9 
                 10 
                 120.7 
                 99.15 
                 98.06 
                 0.22 
                 1.3 
                 0.3 
                 0.1 
                 1.45 
               
               
                 2 
                   
                 10 
                 9 
                 10 
                 129.4 
                 99.79 
                 98.05 
                 0.33 
                 1.1 
                 0.3 
                 0.2 
                 1.28 
               
               
                 3 
                 80NMP- 
                 10 
                 ~10 
                 10 
                 110.2 
                 99.23 
                 98.01 
                 0.47 
                 0.9 
                 0.5 
                 0.1 
                 0.99 
               
               
                 4 
                 20PDEB 
                 10 
                 ~10 
                 10 
                 119.5 
                 99.86 
                 97.43 
                 0.79 
                 1.0 
                 0.7 
                 0.1 
                 1.12 
               
               
                   
               
             
          
         
       
     
     As used herein, the term about means within 10% of the value, or within 5%, or within 1%. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.