Abstract:
A system and process for acetylene selective hydrogenation of an ethylene rich gas stream. An ethylene rich gas supply comprising at least H 2 S, CO 2 , CO, and acetylene is directed to a first treatment unit for removing H 2 S and optionally CO 2  from the gas stream. A CO oxidation reactor is used to convert CO to CO 2  and form a CO-depleted gas stream. A second treatment unit removes the CO 2  from the CO-depleted gas stream and an acetylene selective hydrogenation treats the CO-depleted gas stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Division of prior copending U.S. application Ser. No. 12/857,709 which was filed on Aug. 17, 2010, the contents of which are incorporated herein by reference thereto. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a system for reducing CO concentration in an ethylene rich stream. 
       DESCRIPTION OF RELATED ART 
       [0003]    Industrial processes for producing ethylene include catalytic and thermal cracking of hydrocarbon feedstocks. In at least some cases, the cracking process effluent contains carbon monoxide. For example, certain product separation and recovery systems produce a vapor stream rich in ethylene and containing hydrogen, methane, acetylene, ethane and other contaminants such as CO, CO 2 , and H 2 S that must be removed in order to produce a high purity ethylene product. Acetylene in polymer grade ethylene is typically limited to a maximum of 5 vol ppm. A typical polymer grade ethylene specification is shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Typical Polymer Grade Ethylene Specifications 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ethylene 
                 99.90 
                 vol % min 
               
               
                   
                 Methane plus ethane 
                 1000 
                 vol ppm max 
               
               
                   
                 Ethane 
                 500 
                 vol ppm max 
               
               
                   
                 Acetylene 
                 5 
                 vol ppm max 
               
               
                   
                 C3 and heavier 
                 10 
                 vol ppm max 
               
               
                   
                 CO 
                 2 
                 vol ppm max 
               
               
                   
                 CO 2   
                 5 
                 vol ppm max 
               
               
                   
                 Sulfur 
                 2 
                 wt ppm max 
               
               
                   
                   
               
             
          
         
       
     
         [0004]    Acetylene removal is typically effected by acetylene conversion to ethylene via selective hydrogenation. Carbon monoxide (CO) attenuates the activity of the commonly used acetylene selective hydrogenation catalysts and thus excessive CO concentration can be problematic. 
         [0005]    Hence it would be beneficial to be able to control the amount of CO that enters the acetylene conversion unit. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to controlling CO concentration in a stream prior to subjecting the stream to an acetylene selective hydrogen catalyst. 
         [0007]    One embodiment of the invention is directed to a system for acetylene selective hydrogenation of an ethylene rich gas stream comprising: (a) an ethylene rich gas supply comprising at least H 2 S, CO 2 , CO, and acetylene; (b) a first treatment unit for removing H 2 S and, optionally, CO 2  from the gas stream; (c) a CO oxidation reactor to convert CO to CO 2  and forming a CO-depleted gas stream; (d) a second treatment unit for removing the CO 2  from the CO-depleted gas stream; and (e) an acetylene selective hydrogenation downstream of the CO oxidation reactor. 
         [0008]    Another embodiment of the invention is directed to a process for acetylene selective hydrogenation of an ethylene rich gas stream comprising: (a) supplying an ethylene rich gas comprising at least H 2 S, CO 2 , CO, and acetylene to a first treatment unit and removing H 2 S and, optionally, CO 2  from the gas stream; (b) supplying the H 2 S and CO 2  free gas stream to an CO oxidation reactor and converting CO to CO 2  to form a CO-depleted gas stream; (c) supplying the CO-depleted gas stream to a second treatment unit to remove the CO 2  from the CO-depleted gas stream; and (d) treating the CO-depleted or CO-depleted gas stream to an acetylene selective hydrogenation unit to convert the acetylene to ethylene. 
         [0009]    These and other embodiments relating to the present invention are apparent from the following Detailed Description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic of part of a PetroFCC™ product treating and recovery system. 
           [0011]      FIG. 2  is a schematic of an ethylene rich lean gas preferential CO oxidation reactor system in accordance with one embodiment of the invention. 
           [0012]      FIG. 3  is a schematic of ethylene rich lean gas preferential CO oxidation reactor system in accordance with another embodiment of the invention. 
       
    
    
       [0013]    The same reference numbers are used to illustrate the same or similar features throughout the drawings. The drawings are to be understood to present an illustration of the invention and/or principles involved. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a schematic of a separation scheme for recovering ethylene and propylene. A vapor stream ( 2 ) comprised of ethylene and propylene is compressed ( 4 ) to produce a propylene rich liquid stream ( 6 ) and an ethylene rich vapor stream ( 7 ). The ethylene vapor stream ( 7 ) is treated and concentrated in a primary absorber ( 8 ) and a sponge absorber ( 10 ) to form an ethylene rich lean sponge gas ( 12 ). The lean sponge gas ( 12 ) includes other light hydrocarbons, primarily hydrogen, methane, acetylene, ethane and other contaminants such as CO, CO 2 , and H 2 S that must be removed in order to produce a high purity ethylene product. 
         [0015]    The ethylene and propylene stream may be obtained from any industrial process for producing ethylene including catalytic and thermal cracking of hydrocarbon feedstock product streams. For example, US20080078692 discloses a hydrocarbon cracking process and subsequent treatment of the effluent streams. US 20080078692 discusses various conventional terms and process steps used in processes for recovering ethylene and propylene after a hydrocarbon cracking process, see especially paragraphs 0012-0018, 0034-0041, 0045-0055, and is hereby incorporated by reference in its entirety, 
         [0016]    The ethylene purification scheme shown in  FIG. 1  includes an amine treatment unit ( 14 ) to remove H 2 S and CO 2  from the ethylene rich lean sponge gas ( 12 ) forming stream ( 22 ). Treatment in the amine treatment unit reduces the H 2 S to less than about 0.1 ppm and CO 2  to less than about 50 ppm. The stream ( 22 ) is then fed to an acetylene selective hydrogenation unit ( 16 ) to hydrogenate the acetylene into ethylene. 
         [0017]    In the scheme shown in  FIG. 1 , acetylene is hydrogenated upstream of the demethanizer ( 18 ) and ethane-ethylene splitter fractionators ( 20 ). For this example, stream ( 22 ) includes sufficient hydrogen for hydrogenating the acetylene in the gas to ethylene. Hence, no additional hydrogen is required to be added to the feed stream into the acetylene selective hydrogenation unit ( 16 ). Additionally, the acetylene selective hydrogenation unit ( 16 ) normally operates above ambient temperature while the demethanizer ( 18 ) and ethane-ethylene splitter ( 20 ) typically operate sub-ambient. Positioning the CO oxidation reactor and acetylene conversion reactor upstream of the demethanizer lessens feed heating and effluent cooling duty compared to an arrangement that includes CO oxidation and acetylene conversion in the sub-ambient section of the process. 
         [0018]    The concentration of CO in stream ( 22 ) is variable, generally in a range of 0 to 6 vol %. It is desirable to maintain the CO concentration of the stream ( 22 ) (the acetylene selective hydrogenation reactor feed stream) within a certain operating range, typically about 1 to 0.2 vol %. In general, as the CO concentration of the acetylene selective hydrogenation reactor feed stream increases, the operating window of the acetylene selective hydrogenation reactor system and the time between catalyst regenerations decreases. 
         [0019]    The operating window is the set of operating conditions that enables selective and stable performance. Specifically, allowing complete hydrogenation of acetylene while minimizing hydrogenation of ethylene to ethane. The operating window is affected by process conditions including reactor inlet temperature, feed acetylene, hydrogen, and CO concentrations, space velocity, and catalyst type. 
         [0020]    Thus, as discussed above, the feed stream ( 22 ) entering the acetylene selective hydrogenation unit ( 16 ) often contains unacceptably high carbon monoxide (CO) concentrations. The present invention is directed to a process of controlling or reducing the amount of CO in feed stream ( 22 ) entering an acetylene selective hydrogenation unit ( 16 ). 
         [0021]    The feed stream ( 12 ) from the sponge absorber contains unacceptably high levels of CO. An oxidation reactor will oxidize CO in the feed stream using elemental oxygen as an oxidant: “CO+0.5 O 2 →CO 2 ”. The CO to CO 2  conversion selectivity depends on the catalyst choice and composition of the feed stream. However, the feed stream ( 12 ) from the sponge absorber contains H 2 S which is a catalyst poison for oxidation and must be removed from the feed stream prior to entering the oxidation reactor. 
         [0022]    It was discovered that placing a CO oxidation reactor downstream of the amine treatment unit ( 14 ) enables control of the CO concentration in the feed to the acetylene selective hydrogenation unit ( 16 ). As shown in  FIG. 2 , the ethylene rich stream ( 12 ) from the sponge absorber (not shown) flows to a first amine treatment unit ( 14 ). For this illustration, it is assumed that the first amine treatment unit ( 14 ) removes both H 2 S and CO 2  even though CO 2  removal upstream of the oxidation reactor is not required. Thus, the process does not require CO 2  removal at this stage. 
         [0023]    The ethylene rich stream from the amine treatment unit ( 14 ), essentially H 2 S and CO 2  free, is combined with a stream ( 32 ) that provides a source of elemental oxygen, for example, air or oxygen enriched air. The combined gases A (H 2 S and CO 2 -depleted stream) flow to the CO oxidation reactor ( 30 ). After CO conversion to CO 2 , the effluent stream B (CO-depleted stream) continues to a second amine treatment unit ( 34 ) downstream of CO oxidation reactor ( 30 ). This second amine treatment unit ( 34 ) removes CO 2  from effluent stream B. The CO 2 -depleted effluent then continues to the acetylene selective hydrogenation unit ( 16 ). 
         [0024]    As also shown in  FIG. 2 , the amine treating arrangement uses a common amine regenerator ( 36 ) to regenerate rich amine from both the first and second amine treatment units ( 14 ) and ( 34 ). In doing so, amine treating equipment is minimized. The combination of the preferential CO oxidation reactor ( 30 ) and amine treatment unit ( 14 ) to remove H 2 S enables control of the CO concentration within a suitable range for subsequent acetylene conversion via conventional selective hydrogenation technology. 
         [0025]    A sensor ( 38 ) may be placed in the effluent B stream after the CO oxidation reactor ( 30 ) to detect the amount of CO in the stream. The sensor may be placed at any position subsequent to the CO oxidation reactor where CO is present in detectable levels. The sensor may signal whether the amount of oxygen or air supplied by line ( 32 ) should be modified. The effluent stream B ideally comprises less than about 50 ppm-vol CO. 
         [0026]    The oxidation temperature in the CO oxidation reactor ( 30 ) is typically between about 70° C. and about 160° C. 
         [0027]    Suitable catalysts for selectively oxidizing CO using air or oxygen enriched air include, but are not limited to ruthenium metal disposed on an alumina carrier, such as those described in U.S. Pat. No. 6,299,995, hereby incorporated by reference in its entirety. The ruthenium metal comprises well dispersed ruthenium crystals having an average crystal size less than or equal to about 40 angstroms. Other suitable catalysts utilize platinum and copper. 
         [0028]    Other treatments may be used instead of amine treatment units. Alternative treatment units include absorbers with amine or solvent flow arranged in a cascading relationship. As shown in  FIG. 3 , an ethylene rich feed gas ( 12 ) flows into a first absorber ( 40 ) wherein H 2 S and CO 2  are removed by absorption. The ethylene rich stream from the first absorber ( 40 ), essentially H 2 S and CO 2  free, is combined with a stream ( 47 ) that provides a source of elemental oxygen. The combined gases A (H 2 S and CO 2 -depleted stream) flow to the CO oxidation reactor ( 42 ). After CO conversion to CO 2 , the effluent stream B (CO-depleted stream) continues to a second absorber ( 44 ) downstream of CO oxidation reactor ( 42 ). This second absorber ( 44 ) removes CO 2  from effluent stream B. The CO 2 -depleted effluent then continues to an acetylene selective hydrogenation unit (not shown). The CO 2  rich amine from the second absorber ( 44 ) flows to first absorber ( 40 ). The CO 2  and H 2 S rich amine from the first absorber ( 40 ) flows to an amine regenerator ( 46 ). The lean amine from the amine regenerator ( 46 ) then flows into the second absorber ( 44 ). A CO sensor (not shown) may be placed downstream of CO oxidation reactor ( 42 ) similar to the system shown in  FIG. 2  in order to control the amount of air or oxygen added to combined gases A. 
         [0029]    In the amine treatment unit ( 14 ) shown in  FIGS. 1 , ( 14 ) and ( 34 ) shown in  FIGS. 2 , ( 40 ) and ( 44 ) shown in  FIG. 3  selective removal of H 2 S and CO 2  can be achieved using amine-containing chemical solvents. For example, UOP AMINE GUARD™ FS may be used to remove the H 2 S and CO 2 . Such solvents provide selective removal of H 2 S via amine selection. Other treatment units may use other chemical solvents. Chemical solvents are used to remove the acid gases by a reversible chemical reaction of the acid gases with an aqueous solution of various alkanolamines or alkaline salts in water. 
         [0030]    Other treatment units may utilize physical solvents. With a physical solvent, the acid gas loading in the solvent is proportional to the acid gas partial pressure. For example, the UOP SELEXOL™ process may be used which uses a physical solvent made of dimethyl ether of polyethylene glycol. Chemical solvents are generally more suitable than physical or hybrid solvents for applications at lower operating pressures. 
         [0031]    As discussed above, in accordance with the present invention, a CO oxidation reactor is placed upstream of the acetylene selective hydrogenation unit to enable control of the CO concentration within a suitable range for the acetylene selective hydrogenation reaction occurring in the acetylene selective hydrogenation unit. Further aspects of the invention are therefore directed to a method for controlling the CO concentration in an acetylene selective hydrogenation unit feed stream by preferential CO combustion (i.e. oxidation) with air or oxygen enriched air providing the oxygen. 
       EXAMPLES 
       [0032]    The following examples and tables summarize the expected performance of the preferential CO oxidation reactor processing a typical ethylene-rich lean gas as shown in  FIG. 2 . Stream “A” is oxidation reactor feed and “B” is oxidation reactor effluent. The examples assume selectivity based on a ruthenium on alumina catalyst. 
       Example 1 
       [0033]    The lean gas (i.e. H 2 S and CO 2  removed) from the amine treatment unit is mixed with air. The oxygen available for oxidizing CO is controlled to limit the CO conversion to ˜50%. As shown in Table 2, the CO concentration is reduced from ˜2600 ppm to ˜1300 ppm. 
         [0034]    Specifically, stream A is introduced into a CO oxidation reactor and stream B exits the reactor under the following conditions: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Inlet 
                 Outlet 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Reactor Temperature (° F.) 
                 194 
                 221 
               
               
                   
                 Reactor Pressure (psia) 
                 246.7 
               
               
                   
                 Air to Reactor (lbmol/hr) 
                 66 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Stream “A” 
                 Stream “B” 
               
             
          
           
               
                   
                 Mole 
                   
                   
                 Mole 
                   
               
               
                   
                 Fraction 
                 Mole % 
                   
                 Fraction 
                 Mole % 
               
               
                   
                   
               
             
          
           
               
                 H 2 O 
                 0.003869 
                 0.387 
                 H2O 
                 0.005189 
                 0.519 
               
               
                 Oxygen 
                 0.001314 
                 0.131 
                 Oxygen 
                 0.000000 
                 0.000 
               
               
                 Nitrogen 
                 0.068311 
                 6.831 
                 Nitrogen 
                 0.068401 
                 6.840 
               
               
                 Hydrogen 
                 0.106621 
                 10.662 
                 Hydrogen 
                 0.105446 
                 10.545 
               
               
                 CO 
                 0.002615 
                 0.262 
                 CO 
                 0.001303 
                 0.130 
               
               
                 CO 2   
                 0.000005 
                 0.001 
                 CO 2   
                 0.001321 
                 0.132 
               
               
                 Methane 
                 0.248449 
                 24.845 
                 Methane 
                 0.248775 
                 24.878 
               
               
                 Acetylene 
                 0.000503 
                 0.050 
                 Acetylene 
                 0.000504 
                 0.050 
               
               
                 Ethylene 
                 0.485833 
                 48.583 
                 Ethylene 
                 0.486472 
                 48.647 
               
               
                 Ethane 
                 0.076446 
                 7.645 
                 Ethane 
                 0.076546 
                 7.655 
               
               
                 Propylene 
                 0.006035 
                 0.604 
                 Propylene 
                 0.006043 
                 0.604 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0035]    The lean gas (i.e. H 2 S and CO 2  removed) from the amine treatment unit is mixed with air. The oxygen available for oxidizing CO is controlled to limit the CO conversion to ˜75%. The CO concentration is reduced from ˜2600 ppm to ˜600 ppm, see Table 3. Undesirable side reactions include “H 2 +0.5 O 2 →H 2 O”, as well potential oxidation of light hydrocarbons including olefin products. Assuming sufficient reactant, the CO oxidation reactor essentially completely removes CO. 
         [0036]    Stream A is introduced into a CO oxidation reactor and stream B exits the reactor under the following conditions: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Inlet 
                 Outlet 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Reactor Temperature (° F.) 
                 194 
                 234 
               
               
                   
                 Reactor Pressure (psia) 
                 246.7 
               
               
                   
                 Air to PreFOX Reactor (lbmol/hr) 
                 99 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Stream “A” 
                 Stream “B” 
               
             
          
           
               
                   
                 Mole 
                   
                   
                 Mole 
                   
               
               
                   
                 Fraction 
                 Mole % 
                   
                 Fraction 
                 Mole % 
               
               
                   
                   
               
             
          
           
               
                 H 2 O 
                 0.003857 
                 0.386 
                 H 2 O 
                 0.005833 
                 0.583 
               
               
                 Oxygen 
                 0.001965 
                 0.196 
                 Oxygen 
                 0.000000 
                 0.000 
               
               
                 Nitrogen 
                 0.070561 
                 7.056 
                 Nitrogen 
                 0.070700 
                 7.070 
               
               
                 Hydrogen 
                 0.106289 
                 10.629 
                 Hydrogen 
                 0.104530 
                 10.453 
               
               
                 CO 
                 0.002607 
                 0.261 
                 CO 
                 0.000644 
                 0.064 
               
               
                 CO 2   
                 0.000005 
                 0.001 
                 CO 2   
                 0.001973 
                 0.197 
               
               
                 Methane 
                 0.247674 
                 24.767 
                 Methane 
                 0.248161 
                 24.816 
               
               
                 Acetylene 
                 0.000501 
                 0.050 
                 Acetylene 
                 0.000502 
                 0.050 
               
               
                 Ethylene 
                 0.484318 
                 48.432 
                 Ethylene 
                 0.485271 
                 48.527 
               
               
                 Ethane 
                 0.076207 
                 7.621 
                 Ethane 
                 0.076357 
                 7.636 
               
               
                 Propylene 
                 0.006016 
                 0.602 
                 Propylene 
                 0.006028 
                 0.603 
               
               
                   
               
             
          
         
       
     
         [0037]    In view of the present disclosure, it will be appreciated that other advantageous results may be obtained. Those having skill in the art, with the knowledge gained from the present disclosure, will recognize that various changes can be made in the above apparatuses and methods without departing from the scope of the present disclosure. Mechanisms used to explain theoretical or observed phenomena or results, shall be interpreted as illustrative only and not limiting in any way the scope of the appended claims.