Method to produce plng and ccng at straddle plants

Abstract

There is provided a method to produce PLNG and CCNG at Straddle Plants. In contrast to present practice at Straddle Plants, there is added a slipstream of high pressure, pre-treated, pre-cooled natural gas stream to feed a new PLNG and or CCNG production section. This slipstream is further cooled in a heat exchanger by a counter-current vapour fraction of the expanded gas before entering an expander apparatus. The cooled gas is expanded into a separator. The cold vapour fraction from the separator is routed and expanded to the Straddle Plant fractionator. The produced PLNG is routed to storage.

What is claimed is: 
     
         1 . A method to produce pressurized liquid natural gas (PLNG) and cold compressed natural gas (CCNG), comprising the steps of:
 passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through one or more heat exchangers to pre-cool the natural gas stream;   passing the pre-cooled natural gas stream through a gas column where natural gas liquid fractions and natural gas fractions are separated;   passing the pre-cooled natural gas fractions at pressures of between 450 psig and 1200 psig through one or more heat exchangers to further cool the natural gas fractions;   passing the further cooled natural gas fractions at pressures of between 450 psig and 1200 psig through a gas expansion apparatus where pressure of the natural gas fractions is lowered to a pressure of less than 300 psig; and   passing the natural gas fractions at a pressure of less than 300 psig through a separator where they are separated into a PLNG stream and a CCNG stream at a pressure of less than 300 psig.   
     
     
         2 . The method of  claim 1 , further comprising the step of modifying an existing straddle plant to perform the steps. 
     
     
         3 . A method to produce pressurized liquid natural gas (PLNG) where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus, comprising the steps of:
 providing heat exchangers on a high pressure, pre-treated, pre-cooled natural gas PLNG feed line to a gas expansion apparatus;   providing a gas liquid separator downstream of the expansion apparatus; and   providing a pressure reducing device for a separated gaseous stream into a straddle plant fractionator.   
     
     
         4 . The method of  claim 3 , wherein the gas expansion apparatus is a Joule-Thompson (JT) valve or a gas expander turbine. 
     
     
         5 . The method of  claim 3 , wherein cooling is provided by a vapour fraction of the expanded gas. 
     
     
         6 . The method of  claim 3 , wherein the gaseous stream from the gas liquid separator is returned to the straddle plant fractionator. 
     
     
         7 . The method of  claim 3 , wherein the gas to be liquefied is PLNG. 
     
     
         8 . A method to produce cold compressed natural gas (CCNG) where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus. 
     
     
         9 . A cold compressed natural gas (CCNG) section of a straddle plant, the CCNG section comprising:
 a gas expansion apparatus;   a feed line feeding gas to the gas expansion apparatus;   a CCNG receiver to separate vapour and liquid fractions; and   a liquids return line to a straddle plant fractionator.   
     
     
         10 . The CCNG section of  claim 9 , further comprising a pressurized liquid natural gas line to storage. 
     
     
         11 . The CCNG section of  claim 9 , wherein the CCNG liquid fraction is routed to the straddle plant fractionator for NGL's recovery. 
     
     
         12 . A method to produce Pressurized Liquid Natural Gas (PLNG), the method comprising the steps of:
 passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through a first phase separator to obtain a natural gas liquid fraction and a natural gas fraction, the separated natural gas fractions being at a pressure of between 450 psig and 1200 psig;   passing at least a portion of the natural gas fraction through a gas expansion apparatus to reduce the pressure of the natural gas fraction to less than 300 psig; and   passing the pressure-reduced natural gas fraction through a second phase separator to obtain at least one of a PLNG stream and a CCNG stream at a pressure of less than 300 psig.   
     
     
         13 . The method of  claim 12 , wherein the dewatered natural gas stream is a natural gas stream in a straddle plant. 
     
     
         14 . The method of  claim 12 , further comprising the step of passing a natural gas stream through a dewaterer and a heat exchanger to cool the natural gas stream to obtain the dewatered natural gas stream. 
     
     
         15 . The method of  claim 12 , further comprising the step of cooling at least a portion of the separated natural gas fraction in one or more heat exchangers. 
     
     
         16 . The method of  claim 12 , further comprising the step of collecting each of the at least one of the PLNG stream and the CCNG stream separately in a storage vessel. 
     
     
         17 . The method of  claim 12 , wherein the gas expansion apparatus comprises a Joule-Thompson valve or a gas expander turbine. 
     
     
         18 . The method of  claim 12 , wherein at least a portion of an output of the first or the second phase separator is used to cool the natural gas stream. 
     
     
         19 . The method of  claim 12 , wherein at least a portion of the output of the second phase separator is used to cool a fractionator of the saddle plant.