Abstract:
A process and apparatus are presented for the removal of carbon monoxide from ethylene streams. The removal of carbon monoxide before selective hydrogenation protects the catalyst in the selective hydrogenation reactor. Carbon monoxide levels are controlled with the water gas shift process to convert the carbon monoxide to carbon dioxide, with the carbon dioxide removed in an acid gas removal process.

Description:
FIELD OF THE INVENTION 
     The field of the invention is the production of ethylene. In particular, the invention pertains to the removal of carbon monoxide from an ethylene stream. 
     BACKGROUND OF THE INVENTION 
     Light olefins are important feed materials for the production of many chemicals, and products, such as polyethylene. The production of light olefins, and in particular ethylene, is 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, ethane. Also contaminants are generated in the process, such as CO, CO 2 , and H 2 S. To produce a high quality ethylene product, the contaminants and byproducts are removed to achieve a purity level of greater than 99.9% by volume of ethylene. In order to achieve this, the acid gases must be removed as well as the other by products. 
     In the process of purification, a portion of the ethylene is lost to the waste streams. Methods of reducing loss and increasing yields can have significant economic benefits. 
     SUMMARY OF THE INVENTION 
     Carbon monoxide is a problem contaminant for the selective hydrogenation process. A system for the selective hydrogenation of acetylene in an ethylene rich gas is presented. The system includes a first acid gas removal unit having an inlet for the ethylene rich gas, and an outlet for the egress of the ethylene gas with a depleted acid gas. The system includes a water gas shift reactor having an inlet in fluid communication with the first acid gas removal unit outlet, and a water gas shift reactor outlet for an effluent stream of ethylene having a reduced CO concentration. The system includes a water supply to the water gas shift reactor. The system includes a second acid gas removal unit having an inlet in fluid communication with the water gas shift reactor effluent stream. The second acid gas removal unit has an outlet for the de-acidified effluent stream. The system further includes a selective hydrogenation unit having an inlet in fluid communication with the second acid gas removal unit outlet, and an outlet for an ethylene stream with reduced acetylene content. 
     In one embodiment, the system includes a CO detector positioned near or at the outlet of the water gas shift reactor. The CO detector controls the water supply inlet to the water gas shift reactor through feedback to control the water based upon CO concentrations found in the water gas shift effluent. 
     Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a process flow diagram of the invention; 
         FIG. 2  is a diagram of one embodiment of the acid gas treating; 
         FIG. 3  is a diagram of a second embodiment of the acid gas treating; and 
         FIG. 4  is a diagram of a third embodiment of the acid gas treating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The production of ethylene includes catalytic and thermal cracking of hydrocarbon feedstocks. The cracking process generates a vapor stream comprising light olefins and other hydrocarbons. The vapor stream is passed to a compression system. The compressed stream is separated into a light vapor stream comprising ethylene and lighter components, and a heavier liquid stream comprising C 3  and heavier hydrocarbons. The light vapor stream is passed through a primary absorber to collect C 3  and higher HCs from the vapor phase. The primary absorber flow overhead (or vapor stream) is passed to a sponge absorber to remove residual heavy hydrocarbons, mostly C 5 s. The sponge absorber uses a lean oil to absorb the residual heavier HCs, and the lean oil is either unstrapped light cycle oil or unstrapped heavy naphtha from the main fractionator in a refinery. The lean oil is cooled by exchange with the sponge absorber bottoms and then with either air or water coolers. The sponge oil is regenerated by returning it to the main fractionator to strip out the absorbed gases. 
     After the compression separation of C 3 + components form the cracking process stream an effluent rich in ethylene is left, but includes byproducts such as acetylene, ethane, hydrogen and methane, and contaminants such as carbon monoxide (CO), carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S). Acetylene can be converted to ethylene through selective hydrogenation, to reduce the acetylene concentration to a level sufficiently low for a high purity ethylene product. The typical level of acetylene in polymer grade ethylene is less than 5 ppm by volume. The selective hydrogenation is adversely affected by the presence of carbon monoxide. Polymer grade ethylene is typically 99.9 vol. % or greater ethylene, with less than about 10 ppm acid gases and less than about 0.1 vol % methane and ethane. 
     The selective hydrogenation catalyst is adversely affected by too much CO, and the removal of CO obviates the need for special reactor designs to overcome relatively high CO concentrations. In some cases the CO concentration is too low, and CO is purposefully added to attenuate the selective hydrogenation catalyst activity. The control with a water gas shift reactor can allow maintenance of CO concentrations in acceptable ranges. 
     The present invention is a process for removing the carbon monoxide in the ethylene process stream to increase the performance of the selective hydrogenation of acetylene and therefore increase the yield of ethylene. The present invention also produces hydrogen for use in the selective hydrogenation of acetylene. 
     The process of the present invention is as shown in  FIG. 1 . A light olefin product stream  10  is passed to an acid gas removal unit  20 , to create a de-acidified light olefin product stream  22 . The de-acidified olefin stream  22  is passed to a water gas shift reactor  30 , where water is reacted with carbon monoxide to produce carbon dioxide and hydrogen, creating a treated stream  32  having a reduced CO concentration and an increased H 2  concentration. The treated stream  32  is passed to a selective hydrogenation unit  40  for the conversion of acetylene to ethylene, creating an enriched ethylene product stream  42 . The enriched product stream  42  is passed to an ethylene recovery unit  50  to recover a purified ethylene product. 
     The process can further include passing water  24  to the water gas shift reactor  30  to ensure sufficient water for the equilibrium to consume the CO. The water gas shift reactor is operated at a temperature of at least 210° C., with a preferable temperature of the reactor between 230° C. and 260° C. In one embodiment, the process further includes passing the water gas shift reactor effluent stream  32  to a second acid gas removal unit  60 , thereby creating a treated stream  62  with reduced CO 2  content. The second acid gas removal unit  60  removes CO 2  created by the water gas shift reactor  30 . The flow in the water stream  24  to the water gas shift can be controlled to minimize the CO content in the effluent gas stream  32 . The CO concentration in the effluent stream  32  can be monitored with a CO detector, with a feedback to control the amount of water admitted to the reactor in response to the CO detector. 
     The water gas shift reactor  30  is a catalytic reactor that contacts a gas having CO and water to form CO 2  and H 2 . The catalyst in the reactor can be a metal oxide, a metal oxide on a support, or a mixture of metal and metal oxides. The preferred choices of metal oxides include iron oxide, chromic oxide, or mixtures of copper, zinc oxide, and alumina. 
     The enriched product stream  42  entering the ethylene recovery unit  50  is passed through a demethanizer, to remove methane and lighter gases from the product stream  42 . The effluent from the demethanizer is passed to an ethylene-ethane splitter to separate the demethanized stream into an ethylene product stream and a bottoms stream comprising mostly ethane, but also including heavier components that have passed through the process. 
     The acid gas removal units comprise an amine treatment system for reducing the CO 2  and H 2 S concentrations to ppm levels and sub-ppm levels respectively. The acid gas removal units can also comprise other chemical or physical treatment systems for the removal of CO 2  and H 2 S. 
     One embodiment of the invention is a system for reducing the CO content in the ethylene stream being passed to the selective hydrogenation reactor. The system comprises a first acid gas removal unit  20  having an inlet for admitting an ethylene rich gas stream and an outlet for passing the de-acidified ethylene stream to a water gas shift reactor  30 . The ethylene rich gas stream is produced from a catalytic or steam cracking unit and heavier hydrocarbons having 3 or more carbons are separated in a compression separation system. The water gas shift reactor  30  has an inlet in fluid communication with the first acid gas removal unit  20  and an outlet for the water gas effluent stream having a reduced CO content and increased H 2  content. The water gas shift reactor  30  includes a CO sensor positioned in the effluent stream from the reactor  30 , and provides control to a water supply to the reactor  30 . A second acid gas removal unit  60  has an inlet in fluid communication with the reactor  30  effluent, and an outlet for the treated gas stream. A selective hydrogenation unit  40  has an inlet in fluid communication with the second acid gas removal unit  60  outlet, and an outlet for passing the selective hydrogenation effluent stream. An ethylene recovery unit  50  has an inlet in fluid communication with the selective hydrogenation unit  40  effluent. 
     The acid gas removal units  20 ,  60  include regenerators, where each acid gas removal unit comprises an absorber for contacting the ethylene rich stream with a solvent for absorbing the acid gases. The primary acid gases absorbed are CO 2  and H 2 S. The absorbers include an inlet for admitting the ethylene gas and an outlet for passing the de-acidified ethylene gas, and an inlet for admitting a lean solvent, and an outlet for removing the solvent enriched with CO 2  and H 2 S. The enriched solvent is passed to a regenerator for removing the acid gases, and regenerating the solvent. The regenerated, or lean, solvent is returned to the absorbers. 
     The acid gas removal units  20 ,  60  can have several configurations in the present invention. A first configuration is shown in  FIG. 2  where the first acid gas removal unit  20  includes an absorber and a regenerator  25 . The ethylene rich stream  10  containing CO 2  and H 2 S is contacted with a solvent for removing the acidic components of the gas. The rich solvent, containing the acidic components, is passed to the regenerator  25  and the acidic components are stripped from the solvent. The lean solvent is passed back to the absorber. The second acid gas removal unit  60  includes an absorber and a first regenerator  65 . The effluent stream from the water gas shift reactor  32  is passed to the second acid gas unit  60  where the acidic components are removed from the water gas shift effluent. The primary component is CO 2  that is generated as a result of the water gas shift reaction, and is absorbed in a solvent passed to the absorber. The solvent is enriched with the acidic components and passed to a second regenerator  65  where the acidic components are stripped from the solvent, and the lean solvent is passed back to the absorber. 
     A second configuration of the acid gas removal systems is shown in  FIG. 3 . The second configuration utilizes a single regenerator for both absorbers in the acid gas removal units  20 ,  60 . The ethylene rich stream  10  containing CO 2  and H 2 S are stripped of the acid gases in the first absorber through contact with a solvent. The rich solvent is passed to the regenerator  27 . The treated ethylene gas stream  22  after passing through the water gas shift reactor  30  generates an ethylene stream  32  with increased CO 2  content. The ethylene stream  32  is passed to the second absorber in the second acid gas removal unit  60  to generate a de-acidified stream  62 , through contact with a solvent. The rich solvent is passed to the regenerator  27 , and stripped of the acid gases, generating a lean solvent. The lean solvent stream is split and passed back to the first and second absorbers. 
     A third configuration for the removal of acid gas is shown in  FIG. 4 . The third configuration utilizes a single regenerator  27  for both absorbers in the acid gas removal units  20 ,  60 . The third configuration also takes advantage of the design where most of the acid gas is removed in the first acid gas removal unit  20 . The ethylene rich stream  10  containing CO 2  and H 2 S are stripped of the acid gases in the first absorber through contact with a solvent. The rich solvent is passed to the regenerator  27  where the acid gases are stripped from the solvent. The lean solvent is passed to the second absorber where the solvent strips acid gas from the effluent stream  32  from the water gas shift reactor, creating a partially enriched solvent. The partially enriched solvent is passed to the first absorber in the first acid gas removal unit  20 , where the partially enriched solvent strips more acid gas from the ethylene rich stream  10 . 
     The process was simulated using a typical process stream from a fluidized catalytic cracking unit with the following results. 
     EXAMPLE 1 
     The process of the present invention is shown with out water addition to the water gas shift reactor. The results are shown in Table 1. The water gas shift reaction conditions were a temperature of 232° C. (450° F.) and 1700 kPa (246.7 psia). 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Stream “A” 
                 Stream “B” 
               
             
          
           
               
                   
                 Mole 
                   
                   
                 Mole 
                   
               
               
                   
                 Fraction 
                 Mole % 
                   
                 Fraction 
                 Mole % 
               
               
                   
                   
               
             
          
           
               
                 H 2 O 
                 0.003893 
                 0.3893 
                 H 2 O 
                 0.002525 
                 0.2525 
               
               
                 Nitrogen 
                 0.063767 
                 6.3767 
                 Nitrogen 
                 0.063767 
                 6.3767 
               
               
                 Hydrogen 
                 0.107293 
                 10.7293 
                 Hydrogen 
                 0.108660 
                 10.8660 
               
               
                 CO 
                 0.002632 
                 0.2632 
                 CO 
                 0.001264 
                 0.1264 
               
               
                 CO 2   
                 0.000005 
                 0.0005 
                 CO 2   
                 0.001373 
                 0.1373 
               
               
                 Methane 
                 0.250013 
                 25.0013 
                 Methane 
                 0.250013 
                 25.0013 
               
               
                 Acetylene 
                 0.000506 
                 0.0506 
                 Acetylene 
                 0.000506 
                 0.0506 
               
               
                 Ethylene 
                 0.488891 
                 48.8891 
                 Ethylene 
                 0.488891 
                 48.8891 
               
               
                 Ethane 
                 0.076927 
                 7.6927 
                 Ethane 
                 0.076927 
                 7.6927 
               
               
                 Propylene 
                 0.006073 
                 0.6073 
                 Propylene 
                 0.006073 
                 0.6073 
               
               
                   
               
             
          
         
       
     
     Stream “A” is the feed stream  22  entering the water gas shift reactor, and stream “B” is the effluent stream  32  exiting the water gas shift reactor. 
     EXAMPLE 2 
     The process was also run with water added to the water gas shift reactor. The reaction conditions of temperature and pressure were the same as in Example 1. 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Stream “A” 
                 Stream “B” 
               
             
          
           
               
                   
                 Mole 
                   
                   
                 Mole 
                   
               
               
                   
                 Fraction 
                 Mole % 
                   
                 Fraction 
                 Mole % 
               
               
                   
                   
               
             
          
           
               
                 H2O 
                 0.009843 
                 0.9843 
                 H2O 
                 0.007828 
                 0.7828 
               
               
                 Nitrogen 
                 0.063386 
                 6.3386 
                 Nitrogen 
                 0.063386 
                 6.3386 
               
               
                 Hydrogen 
                 0.106652 
                 10.6652 
                 Hydrogen 
                 0.108667 
                 10.8667 
               
               
                 CO 
                 0.002616 
                 0.2616 
                 CO 
                 0.000600 
                 0.0600 
               
               
                 CO2 
                 0.000005 
                 0.0005 
                 CO2 
                 0.002021 
                 0.2021 
               
               
                 Methane 
                 0.248519 
                 24.8519 
                 Methane 
                 0.248519 
                 24.8519 
               
               
                 Acetylene 
                 0.000503 
                 0.0503 
                 Acetylene 
                 0.000503 
                 0.0503 
               
               
                 Ethylene 
                 0.485971 
                 48.5971 
                 Ethylene 
                 0.485971 
                 48.5971 
               
               
                 Ethane 
                 0.076467 
                 7.6467 
                 Ethane 
                 0.076467 
                 7.6467 
               
               
                 Propylene 
                 0.006037 
                 0.6037 
                 Propylene 
                 0.006037 
                 0.6037 
               
               
                   
               
             
          
         
       
     
     Although the water gas shift reaction reduced the CO content by more than 50%, there was still CO that could be removed. The use of additional water passed to the water gas shift reactor reduced the CO concentration by more than 75% of the CO in the gas stream  22  entering the water gas shift reactor. 
     While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.