Ethylene plant refrigeration system

Abstract

The refrigeration system for an ethylene plant comprises a tertiary refrigerant containing methane, ethylene and propylene. In the closed loop system, a portion of the constant composition refrigerant from the compressor is separated into a methane-rich vapor portion and a propylene-rich liquid portion. The various refrigerant streams are then used to cool the charge gas to separate the C 2 and heavier hydrocarbons from the hydrogen and methane. The separated refrigerant streams are then recombined to form the constant composition before recycle to the compressor.

1 . In a process for the production of ethylene from a charge gas containing hydrogen, methane, ethylene and other C 2  and heavier hydrocarbons wherein said process includes a low pressure demethanizer operating at a pressure below 2.41 MPa (350 psi) and wherein said charge gas is cooled by a refrigeration system, a method for cooling said charge gas by the use of a tertiary refrigerant in said refrigeration system comprising the steps of: 
 (a) compressing a tertiary refrigerant comprising a mixture of methane, ethylene and propylene having a selected composition;    (b) cooling and partially condensing said tertiary refrigerant and forming a vapor refrigerant stream having an increased percentage of methane and a liquid refrigerant stream having an increased percentage of propylene;    (c) progressively cooling said vapor refrigerant stream and said liquid refrigerant stream in a series of heat exchangers;    (d) bringing said charge gas into heat exchange contact in said series of heat exchangers with said progressively cooled vapor refrigerant stream and said progressively cooled liquid refrigerant stream thereby cooling said charge gas and producing a remaining gas stream containing said hydrogen and a portion of said methane and producing liquid demethanizer feed streams containing another portion of said methane and concentrated in said ethylene and other C 2  and heavier hydrocarbons;    (e) feeding said liquid demethanizer feed streams to said low pressure demethanizer and producing a demethanizer overhead stream consisting essentially of methane and producing a demethanizer bottoms product stream;    (f) contacting said demethanizer overhead stream with said progressively cooled refrigerant streams; and    (g) combining said vapor refrigerant stream and said liquid refrigerant stream to form a combined refrigerant stream having said selected composition and returning said combined refrigerant stream to said step of compressing.    
     
     
         2 . In a process as recited in  claim 1  and further including the steps of separating a portion of said liquid refrigerant stream, superheating said separated portion of said liquid refrigerant stream and combining said superheated portion of said liquid refrigerant stream with said combined refrigerant stream for return to said step of compressing.  
     
     
         3 . In a process as recited in  claim 1  wherein said demethanizer overhead stream is partially condensed in said heat exchanger and said condensed part is returned as reflux to said demethanizer.  
     
     
         4 . In a process for the production of ethylene from a charge gas containing hydrogen, methane, ethylene and other C 2  and heavier hydrocarbons wherein said process includes a low pressure demethanizer and wherein said charge gas is cooled by a refrigeration system, a method for cooling said charge gas by the use of a tertiary refrigerant in said refrigeration system comprising the steps of: 
 (a) compressing a tertiary refrigerant comprising a mixture of methane, ethylene and propylene having a selected composition;    (b) cooling and partially condensing at least a portion of said tertiary refrigerant;    (c) separating said partially condensed tertiary refrigerant into a vapor refrigerant stream having an increased percentage of methane and a liquid refrigerant stream having an increased percentage of propylene;    (d) cooling said charge gas by heat contact with said vapor refrigerant stream and said liquid refrigerant stream thereby cooling said charge gas and producing a remaining gas stream containing hydrogen and a portion of said methane and producing a liquid demethanizer feed stream containing another portion of said methane and concentrated in said ethylene and other C 2  and heavier hydrocarbons;    (e) combining said vapor refrigerant stream and said liquid refrigerant stream to form a combined refrigerant stream having said selected composition and returning said combined refrigerant stream to said step of compressing.    
     
     
         5 . In a process as recited in  claim 4  and further including the steps of separating a portion of said liquid refrigerant stream, superheating said separated portion of said liquid refrigerant stream and combining said superheated portion of said liquid refrigerant stream with said combined refrigerant stream for return to said step of compressing.