Lng process using feedstock as primary refrigerant

Abstract

Described is a process for production of Liquefied Natural Gas (LNG) that optimises an offset turbo expansion chilling curve attributed to the refrigerant properties inherent to the natural gas feedstock for the majority of chilling, followed by a smaller externally sourced heat exchange to make the final phase transition to LNG. This process reduces the complexity and high capital costs associated with traditional LNG systems.

1 . A process for producing liquefied natural gas from a natural gas feedstock of a pipeline comprising:
 a) compressing and/or heating the natural gas feedstock upstream of an inlet of a turbo expander, to form a conditioned feedstock with a pressure and temperature that will enable chilling of the flow stream through J-T expansion along an offset chilling curve profile that terminates at the lower pressure levels of the gaseous region of its phase envelope;   b) delivering the conditioned feedstock to the inlet of the turbo expansion device at the elevated temperature and elevated pressure; and   c) expanding the conditioned feedstock in the turbo expansion device; and   d) discharging an expanded gas, from an outlet of the turbo expansion device at a temperature of between about −175° F. and about −262° F. for pure methane or light NGL gas mixtures and between about −145° F. and about −175° F. for rich NGL mixes and at a pressure of between about 5 and about 15 psig,   wherein the conditioned feedstock gas is not further compressed after delivery to the inlet of the turbo expansion device and before discharge from the outlet of the turbo expansion device.   
     
     
         2 . The process of  claim 1 , wherein the turbo expansion device is a stepped series of turbo expansion devices. 
     
     
         3 . The process of  claim 1  wherein, when the final temperature of the expanded gas leaving the turbo expansion device is above about −262° F., the expanded gas is further subjected to extraction of sensible heat to render the liquid state at a temperature of about −262° F. 
     
     
         4 . The process of  claim 1 , wherein the conditioned feedstock at the turbo expansion device inlet is 100% methane, or it is a mixture of methane or methane and NGLs that has been preconditioned to remove undesirable quantities of water vapor, acid gas, excess NGLs and heavier hydrocarbon liquids, CO 2 , N 2 , and mercury. 
     
     
         5 . The process of  claim 1 , wherein the feedstock comprises up to:
 a) 100 mol % methane;   b) about 25 mol % ethane;   c) about 12.5 mol % propane; and   d) about 8.5 mol % i-butane and/or n-butane   
     
     
         6 . The process of  claim 1 , wherein the molecular weight of the feedstock does not exceed about 23.2, HHV of the feedstock does not exceed about 1395BTU/ft3, and modified Wobbe Index of the feedstock as calculated for 60° F. does not exceed about 62.20. 
     
     
         7 . The process of  claim 1 , wherein the elevated pressure of the conditioned feedstock is between about 3400 psig and about 600 psig, the elevated temperature of the conditioned feedstock is between about −20° F. and about 210° F., and the elevated temperature and the elevated pressure of the conditioned feedstock intersect on the offset turbo expansion chilling curve profile to provide an expanded gas having a temperature of between about −145° F. and about −262° F. and a terminal pressure of between about 5 and about 15 psig. 
     
     
         8 . The process of  claim 1 , wherein the offset turbo expansion chilling curve profile is the 170° F. curve or 190° F. curve of  FIG.  5   . 
     
     
         9 . The process of  claim 1 , wherein the turbo expansion device is coupled to a shaft, to recover energy released by the expansion. 
     
     
         10 . The process of  claim 9 , wherein the shaft is either a single shaft or a multi shaft configuration wherein the shafts operate at different or the same speeds. 
     
     
         11 . The process of  claim 1 , further comprising the step of interrupting the expansion of the conditioned feedstock step to remove excess liquid fractions formed during the process. 
     
     
         12 . The process of  claim 11 , wherein the step of interrupting the expansion includes the introduction of guide vanes within liquid bleed off chambers. 
     
     
         13 . The process of  claim 1 , wherein the expanded gas is further cooled using externally refrigerated heat exchange equipment situated downstream of the outlet of the turbo expansion device to extract sensible heat for final liquefaction of the expanded gas. 
     
     
         14 . The process of  claim 13 , wherein a condensation loop of refrigerant in the heat exchange equipment is integrated with the turbo expansion device or with the expanded gas emerging flowstream. 
     
     
         15 . The process of  claim 13 , further comprising chilling a stream of NGLs with the heat exchange equipment, to enhance the HHV heat content of the produced LNG by intermixing of the streams.