Use of a turboexpander cycle in liquefied natural gas process

A process for producing liquefied natural gas from a pressurized natural gas feed stream, the process comprising the steps of:

• introducing the feed stream into heat exchange contact with a mechanical refrigeration cycle to cool the feed stream to a first cooling temperature;
• and
• passing at least a portion of the feed stream through a turboexpander loop to provide auxiliary refrigeration for the mechanical refrigeration cycle to thereby cool the feed stream to a second, relatively lower cooling temperature and condense the feed stream to produce a liquefied natural gas stream;
• reducing the pressure of the liquefied natural gas stream in a flash vessel to produce a liquefied natural gas product stream and an overhead gaseous stream;
• compressing the overhead gaseous stream;
• and
• recycling the compressed overhead gaseous stream to be combined with the feed stream entering the mechanical refrigeration cycle.

A process for producing liquefied natural gas from a pressurized lean natural gas feed stream which is predominantly methane and has an initial pressure above about 800 psig, the process comprising the steps of:

• introducing the feed stream into heat exchange contact with a mechanical refrigeration cycle to cool the feed stream to a first cooling temperature;
• passing at least a portion of the feed stream through a turboexpander step to provide auxiliary refrigeration for the mechanical refrigeration cycle to thereby cool the feed stream to a second, relatively lower cooling temperature and condense the feed stream to produce a liquefied natural gas stream;
• reducing the pressure of the liquefied natural gas stream in a flash vessel to produce a liquefied natural gas product stream and an overhead gaseous stream;
• compressing the overhead gaseous stream;
• recycling the compressed overhead gaseous stream to be combined with the feed stream entering the mechanical refrigeration cycle;
• wherein the turboexpander step includes a turboexpander for reducing the pressure of the feed gas stream and for extracting useful work therefrom during the pressure reduction, the turboexpander also producing an effluent stream;
• passing the turboexpander effluent to a separator or column which separates the effluent into a heavy liquid stream which subsequently is expanded to provide further refrigeration to the process and a gas stream which is also used for further refrigeration effect, both the expanded heavy liquid stream and the gas stream from the separator or column being passed in indirect heat exchange contact with the entering feed gas stream.