Method and apparatus for producing a cooled hydrocarbon stream

Abstract

Method and apparatus for producing a cooled hydrocarbon stream ( 60 ). The method employs cooling, at at least two consecutive pressure levels, a first stream and a first mixed refrigerant stream, using portions of the first mixed refrigerant from the first mixed refrigerant stream in first and second heat exchangers ( 125, 145 ); first and second expansion devices ( 135, 165 ); and a first compressor ( 105 ) to provide the first mixed refrigerant stream. The cooling process is controlled using an advanced process controller based on model predictive control to determine simultaneously control actions for a set of manipulated variables in order to optimise at least one of a set of parameters whilst controlling at least one of a set of controlled variables. The set of manipulated variables comprises: the composition of the mixed first refrigerant, the setting of the first expansion device ( 135 ), and the setting of the second expansion device ( 165 ).

1 . Method of producing a cooled hydrocarbon stream comprising cooling of a hydrocarbon stream, the method comprising at least the steps of:
 (a) cooling a first stream and a compressed first refrigerant stream in a first refrigerant circuit comprising a first compressor and a first mixed refrigerant, against a first expanded first refrigerant stream in a first heat exchanger at a first pressure level, to provide a cooled first stream, a first cooled first refrigerant stream and a first warmed first refrigerant stream;   (b) expanding a first portion of the first cooled first refrigerant stream in a first expansion device to provide the first expanded first refrigerant stream;   (c) cooling one or more second streams, and a second portion of the first cooled first refrigerant stream against a second expanded first refrigerant stream in a second heat exchanger to provide one or more cooled second streams, a further cooled first refrigerant stream, and a second warmed first refrigerant stream, wherein the one or more second streams comprise at least the cooled first stream or a dependent stream derived therefrom, whereby the hydrocarbon stream is comprised in the one or more second streams in order to provide at least the cooled hydrocarbon stream;   (d) expanding at least a first portion of the further cooled first refrigerant stream in a second expansion device to provide the second expanded first refrigerant stream, said second expanded first refrigerant stream being at a lower pressure than said first expanded first refrigerant stream, and   (e) compressing gaseous fractions of the first warmed first refrigerant stream and the second warmed first refrigerant stream in the first compressor and subsequently cooling the compressed stream from the first compressor, to provide the compressed first refrigerant stream;   the method further comprising controlling the steps (a) to (e) using an advanced process controller based on model predictive control to determine simultaneous control actions for a set of manipulated variables in order to optimise at least one of a set of parameters to be optimized, whilst controlling at least one of a set of controlled variables, wherein the set of manipulated variables comprises:
 the composition of the first mixed refrigerant inventory in the first refrigerant circuit, 
 the setting of the first expansion device, and 
 the setting of the second expansion device, 
   wherein the set of controlled variables comprises:
 the temperature of at least one of the one or more cooled second streams, 
 the temperature difference between the first warmed first refrigerant stream and at least one of (i) the compressed first refrigerant stream and (ii) the first stream, 
 the temperature difference between the second warmed first refrigerant stream and at least one of (i) the second portion of the first cooled first refrigerant stream and (ii) one of the one or more second streams in the form of the cooled first stream and/or the dependent stream derived from the cooled first stream, 
 at least one of (i) the temperature difference between the first cooled first refrigerant stream and the first expanded first refrigerant stream and (ii) the temperature difference between the further cooled first refrigerant stream and the second expanded first refrigerant stream, and 
 the power being consumed by the first compressor; 
   and wherein the set of parameters to be optimised comprises the production rate of the cooled hydrocarbon stream and/or the cooling efficiency of the first refrigerant circuit, said cooling efficiency reflecting the power consumed in the first compressor in relation to the mass of cooled hydrocarbon stream produced over time.   
     
     
         2 . The method according to  claim 1 , wherein the first mixed refrigerant comprises at least methane, ethane and propane and wherein manipulating the composition of the first mixed refrigerant comprises manipulating one or more of the methane, ethane and propane inventory of the first mixed refrigerant in the first refrigerant circuit. 
     
     
         3 . The method according to  claim 1 , wherein the first stream comprises a hydrocarbon feed stream, whereby the cooled first stream comprises a cooled hydrocarbon feed stream, and wherein the one or more second streams comprises the cooled hydrocarbon feed stream or the dependent stream derived from the cooled first stream. 
     
     
         4 . The method according to  claim 3 , further comprising, prior to step (c), deriving the hydrocarbon from the cooled first stream by fractionating the cooled hydrocarbon feed stream in a natural gas liquids recovery column to provide the hydrocarbon stream and a liquid bottoms stream. 
     
     
         5 . The method according to  claim 1 , wherein the first stream comprises a second refrigerant stream of a second mixed refrigerant in a second refrigerant circuit, whereby the cooled first stream comprises a cooled second refrigerant stream, and wherein the one or more second streams comprises the cooled second refrigerant stream as well as the hydrocarbon stream, and the one or more cooled second streams comprises a further cooled second refrigerant stream and the cooled hydrocarbon stream. 
     
     
         6 . The method according to  claim 1 , wherein cooling the first stream comprises cooling two or more first streams in the same first heat exchanger or each in a separate high pressure first heat exchanger. 
     
     
         7 . The method according to  claim 1 , wherein cooling the one or more second streams comprises cooling two or more second streams in the same second heat exchanger or each in a separate second heat exchanger. 
     
     
         8 . The method according to  claim 1 , further comprising the step of:
 (h) at least partially liquefying at least a fraction of the cooled hydrocarbon stream in a main heat exchanger.   
     
     
         9 . The method according to  claim 8 , further comprising the step of:
 (f) separating the cooled hydrocarbon stream in a first separator to provide the fraction in the form of a methane-rich stream overhead and a methane-depleted bottoms stream.   
     
     
         10 . The method according to  claim 8 , wherein the at least partially liquefying the fraction of the cooled hydrocarbon stream comprises heat exchanging the methane-rich stream against the second refrigerant to provide an at least partially liquefied hydrocarbon stream. 
     
     
         11 . The method according to  claim 8 , wherein the at least partially liquefying the fraction of the cooled hydrocarbon stream comprises heat exchanging the fraction against a second portion of the further cooled first refrigerant stream, to provide an at least partially liquefied hydrocarbon stream. 
     
     
         12 . The method according to  claim 8 , further comprising the step of:
 (i) reducing the pressure of the at least partially liquefied stream to provide a liquefied hydrocarbon product stream and an end flash gas stream.   
     
     
         13 . The method according to  claim 1 , wherein the hydrocarbon stream is a natural gas stream. 
     
     
         14 . The method according to  claim 1 , wherein the composition of the first portion being expanded in step (b) is substantially the same as the composition of the at least the portion of the further cooled first refrigerant stream being expanded in step (d). 
     
     
         15 . Apparatus for producing a cooled hydrocarbon stream from a hydrocarbon stream, comprising:
 a first refrigerant circuit comprising an inventory of a first mixed refrigerant; a first expansion device; a second expansion device; a first compressor and one or more first coolers to cool a compressed stream from the first compressor thereby to provide a compressed first refrigerant stream;   a first heat exchanger arranged to cool a first stream and the compressed first refrigerant stream, against a first expanded first refrigerant stream at a first pressure level, to provide a cooled first stream, a first cooled first refrigerant stream and a first warmed first refrigerant stream, the first expansion device being arranged to receive a first portion of the first cooled first refrigerant stream and to provide the first expanded first refrigerant stream;   a second heat exchanger arranged to cool one or more second streams and a second portion of the first cooled first refrigerant stream, against a second expanded first refrigerant stream to provide one or more cooled second streams, a further cooled first refrigerant stream and a second warmed first refrigerant stream, the second expansion device being arranged to receive at least a first portion of the further cooled first refrigerant stream to provide the second expanded first refrigerant stream, said second expanded first refrigerant stream being at a lower pressure than said first expanded first refrigerant stream, said one or more second streams comprising at least the cooled first stream or a dependent stream derived therefrom, whereby the hydrocarbon stream is comprised in the one or more second streams in order to provide at least the cooled hydrocarbon stream;   the first compressor being arranged to compress gaseous fractions of the first warmed first refrigerant stream and the second warmed first refrigerant stream;   an advanced process controller comprising computer-executable coded based on model predictive control to determine simultaneous control actions for a set of manipulated variables in order to optimise at least one of a set of parameters to be optimized, whilst controlling at least one of a set of controlled variables, wherein the set of manipulated variables comprises:
 the composition of the first mixed refrigerant inventory in the first refrigerant circuit, 
 the setting of the first expansion device, and 
 the setting of the second expansion device, 
   wherein the set of controlled variables comprises:
 the temperature of at least one of the one or more cooled second streams, 
 the temperature difference between the first warmed first refrigerant stream and at least one of (i) the compressed first refrigerant stream and (ii) the first stream, 
 the temperature difference between the second warmed first refrigerant stream and at least one of (i) the second portion of the first cooled first refrigerant stream and (ii) one of the one or more second streams in the form of the cooled first stream and/or the dependent stream derived from the cooled first stream, 
 at least one of (i) the temperature difference between the first cooled first refrigerant stream and the first expanded first refrigerant stream and (ii) the temperature difference between the further cooled first refrigerant stream and the second expanded first refrigerant stream, and 
 the power being consumed by the first compressor; 
   and wherein the set of parameters to be optimised comprises the production rate of the cooled hydrocarbon stream and/or the cooling efficiency of the first refrigerant circuit said cooling efficiency reflecting the power consumed in the first compressor in relation to the mass of cooled hydrocarbon stream produced over time.   
     
     
         16 . The method according to  claim 9 , further comprising:
 (g) pressurising the methane-depleted bottoms stream to provide a methane-depleted bottoms stream and passing said methane-depleted bottoms stream to the natural gas liquids recovery column as reflux.   
     
     
         17 . The method according to  claim 16 , wherein the methane-depleted bottom stream is in pressurized condition. 
     
     
         18 . The method according to  claim 1 , further comprising the step of:
 (h) fully liquefying at least a fraction of the cooled hydrocarbon stream in a main heat exchanger.   
     
     
         19 . The method according to  claim 18 , further comprising the step of:
 (i) reducing the pressure of the fully liquefied stream to provide a liquefied hydrocarbon product stream and an end flash gas stream.