Abstract:
The refrigeration system for an ethylene plant comprises a closed loop tertiary refrigerant system containing methane, ethylene and propylene. The tertiary refrigerant from a compressor final discharge is separated into a methane-rich vapor fraction and two levels of propylene-rich liquids so as to provide various temperatures and levels of refrigeration in various heat exchange stages while maintaining a nearly constant refrigerant composition flowing back to the compressor and with the bulk of the total return refrigerant flow going to the first stage compressor section. This tertiary system can also be applied to an ethylene plant with a high pressure demethanizer.

Description:
This application is a continuation-in-part of application Ser. No. 10/121,151, filed Apr. 11, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention pertains to a refrigeration system to provide the cooling requirements of an olefin plant. More particularly, the invention is directed to the use of a tertiary or trinary refrigerant comprising a mixture of methane, ethylene and propylene for cooling in an ethylene plant. 
     Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, a propylene and an ethylene refrigerant are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus three separate refrigeration systems are required, cascading from lowest temperature to highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc. are required. An additional methane refrigeration compressor, either reciprocating or centrifugal, can partially offset the capital cost savings resulting from the use of low pressure demethanizers. 
     Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple refrigerants are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants. The application of a binary mixed refrigeration system to ethylene plant design is disclosed in U.S. Pat. No. 5,979,177 in which the refrigerant is a mixture of methane and either ethylene or ethane. However, such a binary refrigeration system cascades against a separate propylene refrigeration system which provides the refrigeration in the temperature range of −40° C. and warmer. Therefore, two separate refrigeration systems are required. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, therefore, to provide a simplified, single refrigeration system for an olefin plant, particularly an ethylene plant having a low pressure demethanizer, utilizing a mixture of methane, ethylene and propylene as a tertiary refrigerant. This tertiary system replaces the separate propylene, ethylene and methane refrigeration systems associated with a recovery process using a low pressure demethanizer. The invention involves the separation of the tertiary refrigerant from the discharge of the final stage of a compressor into a methane-rich vapor fraction and two levels of propylene-rich liquids so as to provide various temperatures and levels of refrigeration in various heat exchange stages while maintaining a nearly constant refrigerant composition, as measured by molecular weight, in the compressor and with the bulk of the total return refrigerant flow going to the first stage compressor suction. This enables the tertiary refrigerant system to compete favorably on a thermodynamic basis with the use of separate compressors for separate refrigerants. This tertiary system can also be applied to an ethylene plant with a high pressure demethanizer in which case the tertiary system only supplies propylene and ethylene refrigeration temperature levels. The objects, arrangement and advantages of the refrigeration system of the present invention will be apparent from the description which follows. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic flow diagram of a portion of an ethylene plant illustrating one embodiment of the refrigeration system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to an olefin plant wherein a pyrolysis gas is first processed to remove methane and hydrogen and then processed in a known manner to produce and separate ethylene as well as propylene and some other by-products. The process will be described in connection with a plant which is primarily for the production of ethylene. The separation of the gases in an ethylene plant through condensation and fractionation at cryogenic temperatures requires refrigeration over a wide temperature range. The capital cost involved in the refrigeration system of an ethylene plant can be a significant part of the overall plant cost. Therefore, capital savings for the refrigeration system will significantly affect the overall plant cost. 
     Ethylene plants with high pressure demethanizers operate at pressures higher than 2.76 MPa (400 psi) with an overhead temperature typically in the range of −85° C. to −100° C. Ethylene refrigeration at approximately −100° C. to −102° C. is typically used to chill and produce overhead reflux. An ethylene plant designed with a low pressure demethanizer which operates below about 2.41 MPa (350 psi) and generally in the range of 0.345 to 1.034 MPa (50 to 150 psi) and with overhead temperatures in the range of −110° C. to −140° C. requires methane temperature levels of refrigeration to generate reflux. The advantage of the low pressure demethanizer is the lower total plant power requirement and the lower total plant capital cost while the disadvantage is the lower refrigeration temperature required and, therefore, the need for a methane refrigeration system in addition to the ethylene and propylene refrigeration systems. 
     The tertiary refrigerant of the present invention comprises a mixture of methane, ethylene and propylene. The percentage of these components can vary depending on the ethylene plant cracking feedstock, the cracking severity and the chilling train pressure among other considerations, but will generally be in the range of 7 to 20 mol percent methane, 7 to 20 mol percent ethylene and 50 to 90 mol percent propylene as measured at the compressor discharge. A typical composition for an ethylene plant with a low pressure demethanizer would be 10% methane, 10% ethylene and 80% propylene. The use of the tertiary refrigerant provides all the refrigeration loads and temperatures required for an ethylene plant while obviating the need for two or three separate refrigerant systems. 
     The purpose of the present invention is to provide the necessary refrigeration to separate the hydrogen and methane from the charge gas and provide the feed for the demethanizer as well as provide for the other refrigeration requirements of the entire plant. Referring to the specific embodiment of the invention shown in the drawing which is for a low pressure demethanizer, the tertiary refrigeration system is arranged to provide all of the required levels of refrigeration for an ethylene plant in the series of heat exchangers  10 ,  12 ,  14 ,  16 ,  18  and  20 . These heat exchangers can be combined as fewer units or expanded into a greater number of units depending on the particular needs for any particular ethylene process and in particular on the specific charge gas composition. They are typically plate fin type heat exchangers and are preferably packed inside of a heavily insulated structure referred to as a cold box to prevent heat gain and to localize the low temperature operation. Before describing the tertiary refrigeration system, the flow of the charge gas through the system will be described with examples of specific temperatures for purposes of illustration only. 
     The charge gas feed  22 , which is the pyrolysis gas conditioned as required and cooled, is typically at a temperature of about 15° C. to 20° C. and a pressure of about 3.45 MPa (500 psi), and is typically a vapor stream. The charge gas contains hydrogen, methane, and C 2  and heavier components including ethylene and propylene. The charge gas  22  is progressively cooled by the refrigeration system of the present invention in the heat exchangers  10 ,  12 ,  14 ,  16 ,  18  and  20  with appropriate separations being made to produce demethanizer feeds. The charge gas  22  is first cooled in the heat exchangers  10  and  12  down to about −35° C. at  23 . In heat exchanger  14 , the charge gas is cooled from −35° C. to −60° C. at  24 . In heat exchanger  16 , it is cooled from −60° C. to −72° C. with the condensate  25  in the effluent  26  being separated at  28 . The condensate  25  is a lower feed to the demethanizer (not shown). The remaining vapor  30  is then cooled from −72° C. to −98° C. in heat exchanger  18  with the condensate  32  in the effluent  34  being separated at  36 . This condensate  32  is a middle feed to the demethanizer. The vapor  38  is then further cooled in heat exchanger  20  from −98° C. to −130° C. with the condensate  40  in the effluent  42  being separated at  44 . The condensate  40  is a top feed to the demethanizer. The remaining vapor  46  is then separated (not shown) to produce the hydrogen stream  48  and the low pressure methane stream  50 . The cooling loop  52  in heat exchanger  20  is for cooling and partially condensing the low pressure demethanizer overhead to generate reflux. The remaining overhead vapor from the demethanizer forms the high pressure methane stream  54 . The hydrogen stream  48  and the low and high pressure methane streams  50  and  54  provide additional cooling in the heat exchangers. To complete the description of the charge gas flow, it is the demethanizer bottoms which contains the C 2  and heavier components which is sent for the recovery of the ethylene and propylene and other components. 
     In addition to the charge gas stream and the tertiary refrigerant streams, the streams  55 ,  56 ,  57  and  58  are various ethylene plant streams at various temperatures which also pass through the heat exchangers for recuperation of cold. Merely as examples, stream  55  is for the recuperation of the cold from the low pressure demethanizer side reboiler. Stream  56  recuperates the cold from the demethanizer feed and the low pressure demethanizer bottom reboiler. Stream  57  is for recuperation of the demethanizer feed, the ethane recycle, the ethylene fractionator side reboiler and bottom reboiler and the ethylene product. The last stream  58  covers the recuperation of cold from the lower deethanizer feed, the ethylene product and the ethane recycle. 
     The maximum efficiency of heat transfer between a warm fluid and a cold fluid is achieved when the temperature difference is low. A mixed refrigerant, such as proposed in this invention, has an increasing temperature with increasing vaporization, at a fixed pressure. This is as distinguished from a pure component refrigerant which vaporizes at a constant temperature at a fixed pressure. Pure component refrigeration systems therefore tend to be more efficient when the process condensing temperatures are unchanged, or relatively unchanged, when being cooled, and relatively less efficient when process temperatures decrease when being cooled. For mixed refrigeration systems, such as proposed in this invention, the relative advantages are reversed. 
     In an ethylene plant, some of the cooling services requiring refrigeration are at relatively constant temperatures and some are at decreasing temperatures. In the pending U.S. patent application Ser. No. 09/862,253, entitled, Tertiary Refrigeration System for Ethylene Plants, and filed May 22, 2001, a mixed refrigerant system for ethylene plants is described which emphasizes a constant composition throughout the system. Thus, a somewhat lower efficiency in the constant temperature heat transfer services has been understood. The present invention proposes to improve the efficiency of the mixed refrigeration system by varying the composition of the mixed refrigerant used for these constant temperature heat transfer services. This invention is especially directed to the refrigeration system utilized in the separation of ethylene from ethane which has a very large refrigeration requirement. The concept can also be utilized for other constant temperature heat transfer services with lower heat transfer duty such as the deethanizer. 
     For the purposes of the present invention, the total duty of the ethylene fractionator condenser  59 , the total duty of the deethanizer condenser  69  and the total duty of the low pressure depropanizer condenser  79  are handled outside the coldbox with special consideration. As known from the thermodynamics, the condensation of the process stream with constant temperature, such as the ethylene fractionator overhead and the deethanizer overhead, as well as the depropanizer overhead if a single low pressure tower is employed, will be less efficient if a mixed refrigeration system is used where the vaporization curve is sloped with temperature. The wide cold-end temperature approach indicates inefficiency and results in higher power consumption for the tertiary refrigeration system. To make the tertiary system competitive in power consumption to a system designed with separate compressors, a concept to generate a heavy refrigerant stream approaching the conventional propylene refrigeration is called for in the tertiary system of the present invention. In the present invention, the composition of stream  80 , which supplies the refrigeration normally supplied by a separate propylene compressor, is typically greater than 80 mol percent propylene. 
     Turning now to the refrigeration system per se, the tertiary refrigerant as identified earlier is a mixture of methane, ethylene and propylene and is compressed by the multistage refrigeration compressor  60 . In the illustrated embodiment, there are three compressor stages  61 ,  64 , and  66  with one interstage coolers. The interstage cooler  70  is at the second stage discharge. The final discharge  76  is partially condensed in discharge cooler  74  by cooling water and then separated in the drum  78  to provide the heavy liquid refrigerant  80 . The remaining vapor  82  from drum  78  is cooled in exchanger  10  by heavy refrigerant from drum  78  and partially condensed and then separated in drum  88  to generate a medium liquid refrigerant  90  and a light vapor refrigerant  92  by phase separation. The light vapor refrigerant generated from drum  88  is cooled in exchanger  12 ,  14  and  16  by medium liquid refrigerant and then condensed in exchanger  18  by self-refrigeration. The typical operating conditions and the range of operating conditions for the compressor are as follows: 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Range of Suction Pressure 
                 Typical Suction Conditions 
               
             
          
           
               
                   
                 MPa 
                 MPa 
                 Degree C. 
               
               
                   
                   
               
             
          
           
               
                 1 st  Stage 
                 0.011-0.016 
                 0.014 
                 −40 
               
               
                 2 nd  Stage 
                 0.40-0.55 
                 0.50 
                 −10 
               
               
                 3 rd  Stage 
                 0.90-1.40 
                 1.20 
                  30 
               
               
                   
               
             
          
         
       
     
     The light refrigerant  92  from the drum  88  passes through the heat exchangers  12  to  18  and is condensed and sent to light refrigerant drum  89 . It is then subcooled to about −130° C. at the exit  94  from heat exchanger  20  and then flashed through valve  96  to provide the lowest refrigeration temperature of −140° C. to −145° C. This level of refrigeration provides the cooling of the charge gas stream at  42  down to −130° C. or lower and to provide sufficient cooling in the loop  52  to generate reflux from the demethanizer overhead. 
     The charge gas temperature in stream  34  is typically at −98° C. by controlling the flow of the light refrigerant in stream  100 . Typically, the refrigeration supplied by the stream  102  will meet the refrigeration demand in heat exchangers  20 , and  18 . The light refrigerant is finally superheated to about −45° C. in heat exchanger  14 . This provides the desired superheat temperature of 5 to 15° C. when it is mixed with portions of the heavy and medium refrigerate streams for return to the first stage suction drum  104 . 
     The liquid  90  from the drum  88  is the medium refrigerant which is subcooled as it passes through heat exchangers  12 ,  14  and  16 . This medium refrigerant controls the temperature of the charge gas at  24  and  26  by flashing the subcooled refrigerant through valves  98  and  108 . From valve  98  and  108 , the medium refrigerant flows back through heat exchangers  16 ,  14  and  12  and then to the suction drum  104  for the first stage  61  of the compressor. From valve  106 , the medium refrigerant flows back through heat exchangers  12  and  10  and then to the suction drum  112  for the second stage  64  of the compressor. The heavy refrigerant  80  from the drum  78  is about 88% propylene. This liquid supplies four major duties, i.e., the cooling for the ethylene fractionator condenser  59 , the cooling for the deethanizer condenser  69 , the cooling for the low pressure depropanizer condenser  79  and the major refrigeration demand in heat exchanger  10  to support the self-refrigeration of the tertiary refrigeration system. The degrees of subcooling of the heavy refrigerant exiting the heat exchanger  12  are flexible between −10° C. and −35° C. The following table is a summary of the suction streams to the compressor and the compressor flows. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Wt % of 
                 Ave. 
               
               
                 Stages 
                 Type of Refrigerant 
                 total flow 
                 MW 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 st  Stage Suction 
                 100% Light Refrigerant 
                 10.0 
                   
               
               
                   
                 Medium Refrigerant 
                 5.0 
               
               
                   
                 Heavy Refrigerant 
                 60.0 
               
               
                 1 st  Stage Flow 
                   
                 75.0 
                 38.1 
               
               
                 2 nd  Stage Side Inlet 
                 Medium &amp; Heavy Refrigerant 
                 10.0 
               
               
                 2 nd  Stage Flow 
                   
                 85.0 
                 38.2 
               
               
                 3 rd  Stage Side Inlet 
                 Heavy Refrigerant 
                 15.0 
               
               
                 3 rd  Stage Flow 
                   
                 100 
                 38.6 
               
               
                   
               
             
          
         
       
     
     As shown by the above table, the split of the refrigerant for the purpose of energy saving and then the recombination of the refrigerants, particularly the recombination in the first compressor stage of the light and most of the heavy refrigerants along with some medium refrigerant to provide almost 75% of the total flow in the first stage stabilizes the compressor wheels. With 75% of the total flow in the first stage and a relatively uniform molecular weight throughout preferably varying less than 5% and most preferably varying less than 2%, a normal speed control of the turbine by the first stage suction drum pressure becomes equally applicable to the tertiary refrigerant compressor system as to a single refrigerant compressor system. With respect to the control of the process chilling duties, the variables which can be used include the control of the critical temperature, the adjustment of the overall refrigerant composition, the adjustment of the temperatures in the separation drums  78  and  88  and the adjustment of the compressor operating conditions. 
     The closed loop tertiary refrigeration system with three or more inter-stages of the present invention provides a versatile system in which various refrigerant compositions can be formed and various refrigeration levels can be provided. This provides precise temperature control in an efficient and economical manner. Therefore, a single closed loop tertiary refrigeration system can adequately provide all the necessary refrigeration to the entire ethylene plant with either a low pressure or high pressure demethanizer at a competitive power consumption and a lower overall plant cost.