Pre-Cooled Liquefaction Process

Abstract

A system and method for liquefying a natural gas stream, the method including the steps of providing a dehydrated natural gas stream for liquefaction, pre-cooling the dehydrated natural gas stream in a pre-cooling apparatus, where the pre-cooling is performed by using a pre-coolant that consists essentially of a hydroflorocarbon (HFC) refrigerant, further cooling the pre-cooled dehydrated natural gas stream in a main heat exchanger through indirect heat exchange against a vaporized hydrocarbon mixed refrigerant coolant to produce a liquefied natural gas product stream, where the mixed refrigerant coolant comprises ethane, methane, nitrogen, and less than or equal to 3 mol % of propane.

Description

BACKGROUND[0001]Previously pre-cooled natural gas liquefaction processes were disclosed for use on Floating Production Storage and Offloading platforms (FPSO's) that used CO2 to pre-cool a natural gas feed stream while a mixed refrigerant hydrocarbon mixture (HMR) was used to further cool the pre-cooled stream to provide a liquefied natural gas (LNG) product. In these processes, the natural gas feed stream is pre-cooled against boiling CO2 at one or more pressure levels. The CO2 vaporizes while pre-cooling the natural gas feed stream to a temperature of approximately −35° C. The CO2 vapors are then compressed, cooled, and condensed to form the liquid CO2 refrigerant to be re-circulated back into the process.[0002]Using CO2 as a precoolant for liquefaction of LNG on a FPSO, however, has several disadvantages. First, CO2 has a freezing point of −56.6° C. at which temperature dry ice formation begins to occur. To prevent operational issues associated with the formation of dry ice in a ...

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Benefits of technology

[0013]Embodiments of the present invention satisfy this need in the art by providing an improved liquefaction system and method that utili

Problems solved by technology

Use of CO2, therefore, may preclude the removal of required amounts of such impurities in the pre-cooling section.

Second, the critical temperature (i.e., the temperature above which a fluid cannot be condensed irrespective of pressure) of CO2 is approximately 31.1° C., which means that CO2 cannot be condensed above 31.1° C. A pre-cooling cycle, however, requires a condensed refrigerant which supplies refrigeration by boiling against a load stream.

Thus, if the sea water is warm, for example, 27° C., which is the typical surface water temperature in the tropical regions, and assuming a typical 10° C. approach in the condenser, the CO2 cannot be condensed using the mild sea water, thereby degrading the efficiency and limiting the applicability of CO2 based cycles for latitudes with sea water below approximately 20° C.

The use of special high pressure barrel type casings thereby raises the capital cost.

Thus, the whole system must be designed with high pressure piping, pressure relief devices, etc. that increase capital cost and increase gas pressure safety concerns, especially on an offshore platform where distances between equipment and personnel is much less than compared with land-based plants.

While propane has been widely used in land-based plants as a pre-cooling refrigerant, use of propane also has disadvantages for use on LNG FPSOs.

Propane may leak from gas compressor seals, propane evaporators, and other points in the system that can lead to hazardous explosive conditions at and near the surface of the FPSO.

Because the propane vapors settle to the floor or surface of the FPSO platform and because propane is extremely flammable, the presence of a propane “cloud” on the FPSO platform can be extremely hazardous.

If a propane leak were to occur on an FPSO, the flammable propane vapors would likely propagate to all major areas of the FPSO in a short span of time, exposing the plant to explosion and / or fire risk.

On Jul. 6, 1988, a back-up propane condensate pump which was allegedly under repair was accidentally started which led to the discharge of propane into the air just above the surface of the platform.

The resultant leaked propane was accidentally ignited and resulted in two very large explosions that immediately engulfed the control room and most of the ship.

The accident resulted in 167 fatalities and the platform melted.

Use of propane as a refrigerant can, therefore, be extremely hazardous, especially since it will require propane inventories not just in the pre-cooling section but also in storage and in a de-propanizer column to extract the propane.

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