Fluent material handling – with receiver or receiver coacting mea – Processes
Reexamination Certificate
1999-12-15
2001-03-20
Jacyna, J. Casimer (Department: 3751)
Fluent material handling, with receiver or receiver coacting mea
Processes
C141S009000, C141S004000, C141S231000, C141S005000, C048S190000, C062S050100, C062S053200
Reexamination Certificate
active
06202707
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the handling of pressurized liquefied gas and, more particularly, to a method for unloading containers having pressurized liquefied gas contained therein.
BACKGROUND OF THE INVENTION
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called “LNG”) for transport to market.
It has been recently proposed to transport natural gas at temperatures above −112° C. (−170° F.) and at pressures sufficient for the liquid to be at or below its bubble point. For most natural gas compositions, the pressure of the natural gas at temperatures above −112° C. will be between about 1,380 kPa (200 psia) and about 3,500 kPa (500 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about −160° C.
If PLNG is unloaded from a container by pumping the PLNG out and allowing the container pressure to decrease, the decompression of the PLNG can lower the temperature in the container below the permitted design temperature for the container. If the pressure in the container is maintained as the PLNG is removed to avoid such temperature reduction, the vapor remaining in the container will contain a significant volume of the container's original cargo. Depending upon the pressure and temperature of storage and the composition of the PLNG, the vapors may constitute from about 10 to 20 percent of the mass of PLNG in the container before the liquid was removed. It is desirable to remove as much of this gas as is economically possible while keeping the container at approximately the same temperature as the PLNG before unloading.
SUMMARY
This invention relates to a method for unloading a plurality of containers containing liquefied gas and ullage gas in which the liquefied gas has a temperature above −112° C. and a pressure essentially at its bubble point. In the first step of the method, a pressurized displacement liquid is fed to a first of the plurality of containers to discharge the pressurized liquefied gas and ullage gas therefrom. The displacement liquid has a pressure greater than the pressure of the liquefied gas and is sufficient to displace the liquefied gas from the container. The displacement liquid is then pumped from the first container to a second container of the plurality of containers to discharge liquefied gas and ullage gas therefrom. As the displacement liquid is removed from the first container, the void space caused by the removal of the displacement liquid is filled with a vapor at a lower pressure than the pressure of displacement liquid in the second container. The pressure of the low pressure gas preferably ranges between about 50 and 200 psia and is preferably derived from the liquefied gas. The low pressure gas may for example be produced by revaporization of the liquefied gas or it may be boil-off from liquefied gas. Fluid communication between the first container and the second container is severed and these steps are repeated for all of the containers in succession except that for the last container in the series the displacement liquid is pumped therefrom to an auxiliary container for storage rather than another container.
In the practice of this invention, all the containers are emptied of the pressurized liquefied gas without significant decompression of the liquefied gas and the containers are filled with the lower pressure vapor. The lower pressure vapor in the containers will comprise substantially less mass than if the containers are emptied of liquefied gas and filled with high pressure gas. The gas in the containers is typically reliquefied when the containers are reloaded with liquefied gas. Reducing the amount of gas to be reliquefied at the liquefaction plant can significantly reduce the overall cost of transporting the liquefied gas.
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Minta Moses
Woodall Robert M.
ExxonMobil Upstream Research Company
Jacyna J. Casimer
Lawson Gary D.
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