Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-12-22
2001-09-18
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S614000
Reexamination Certificate
active
06289692
ABSTRACT:
This invention concerns a method and an apparatus for improving the efficiency of an open-cycle refrigeration process for LNG production by the employment of a liquid expander to recover energy associated with the flashing of a pressurized liquid stream and employing said recovered energy to compress at least one flashed vapor stream in the open cycle.
BACKGROUND
It is common practice to cryogenically treat natural gas to liquefy the same for transport and storage. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. Often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas which in turn requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where it possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas or the like in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane and mixtures thereof. In the art, the refrigerants are frequently arranged in a cascaded manner and each refrigerant is employed in a closed refrigeration cycle. When the condensed liquid is at an elevated pressure, further cooling is possible by flashing the liquefied natural gas to atmospheric pressure in one or more expansion stages. The flashing is generally accomplished via the use of expansion valves. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a storage or transport temperature suitable for liquefied gas storage at near-atmospheric pressure. In this expansion to near-atmospheric pressure, significant volumes of flash vapors are produced. The flash vapors from the expansion stages are generally collected and recycled for liquefaction or utilized as fuel gas for power generation.
In an open cycle cascaded refrigeration process, the cycle comprises the steps of flashing a pressurized LNG-bearing stream in discrete steps, warming the resulting flash vapor streams by employing such streams as refrigeration streams, recompressing a substantial portion of the resulting warmed flash vapor streams, cooling said compressed gas stream and returning the compressed cooled gas stream to the liquefaction process for liquefaction. As previously noted, the flashing of a pressurized LNG-bearing stream to near-atmospheric pressure is generally performed with expansion valves. From a thermodynamic perspective, such flashing is a highly irreversible process.
SUMMARY OF THE INVENTION
It is an object of this invention to increase the efficiency of an open-cycle cascaded refrigeration process for LNG production.
It is a further object of this invention to increase the efficiency of an open-cycle cascaded refrigeration process for LNG production by recovering energy associated with the flashing of a pressurized LNG-bearing stream to near atmospheric pressure.
It is a still further object of the present invention to increase the efficiency of an open-cycle cascaded refrigeration process for LNG production by recovering energy associated with the flashing of a pressurized LNG-bearing stream and employing said energy in the liquefaction process.
It is still yet a further object of this invention to increase the efficiency in of an open-cycle cascaded refrigeration process for LNG production by recovering energy associated with the flashing of a pressurized LNG-bearing stream and employing said energy in the open-cycle of said liquefaction process.
It is yet a further object of the invention to increase the efficiency of an open-cycle cascaded refrigeration process for LNG production by recovering mechanical energy from the flashing of a pressurized LNG-bearing stream and directly employing said mechanical energy to compress flash vapors or gases in the open-cycle of said liquefaction process.
In one embodiment of this invention wherein an open-cycle cascaded refrigeration process for LNG production produces a LNG-bearing stream at an elevated pressure and said stream is flashed in an open methane refrigeration cycle via multiple stages of pressure reduction to a near-atmospheric pressure, an improvement has been discovered comprising in at least one of said pressure reduction stages:
(a) flashing a pressurized LNG-bearing stream in an expander thereby generating a two-phase stream and energy;
(b) separating said two-phase stream into a gas stream and lower pressure predominantly LNG-bearing stream;
(c) compressing said gas stream in a compressor thereby producing a pressurized gas stream and wherein said compressor is powered at least in part by the energy of step (a); and
(d) returning said pressurized gas stream to the multi-stage compressor employed in the open methane refrigeration cycle.
In another embodiment of this invention, an apparatus for conducting the preceding process has been discovered.
REFERENCES:
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patent: 5537827 (1996-07-01), Low et al.
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patent: 5722255 (1998-03-01), Brasz
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Swearingen, “Compare Methane Liquefying Processes”, Hydrocarbo Processing, vol. 45, No. 8. Aug. 1966.
Baudat Ned
Houser Clarence G.
Lee Rong-Jwyn
Doerrler William
Haag Gary L.
Phillips Petroleum Company
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