Refrigeration – Storage of solidified or liquified gas – With vapor discharged from storage receptacle
Reexamination Certificate
2000-03-10
2001-05-29
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
With vapor discharged from storage receptacle
C062S050200, C062S137000, C062S003610, C137S002000, C141S004000, C222S003000, C222S061000
Reexamination Certificate
active
06237347
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the handling of pressurized liquefied natural gas and, more particularly, to a method for loading pressurized liquefied natural gas into containers that are filled with methane-rich vapor.
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 temperature. 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 4,480 kPa (650 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 −162° C. (−260° F.).
In co-pending United States patent application Ser. No. 09/464987 by J. R. Rigby, a process is disclosed for unloading PLNG from ship containers by pressuring out the PLNG with gas, leaving the tanks PLNG-empty but full of pressurized, methane-rich gas. At the end of the PLNG unloading method, all but the last container or group of containers are at low pressure, preferably between about 690 kPa (100 psia) and 1,380 kPa (200 psia), while the last container is at slightly above the original PLNG's bubble point pressure. Having the lower pressure vapor in the containers for the return trip or voyage substantially reduces the mass of methane left in the containers compared to having high-pressure gas contained therein. Depending upon the pressure, temperature, and composition of the PLNG, leaving high pressure vapor in all the containers could constitute from about 10 to 20 percent of the mass of the cargo in the containers before PLNG removal.
During PLNG loading, the methane-rich vapor in the containers is displaced by the entering liquid. It is desirable to liquefy at least part of the methane-rich vapor displaced from containers during PLNG loading. The vapor liquefaction is preferably integrated with the liquefaction process used to manufacture the PLNG being loaded into the containers. During the loading of a multiplicity of gas-filled containers, the flow rate of vapor leaving the containers can vary substantially between the beginning and end of the loading method. To maintain operational stability of the liquefaction plant, it is desirable that the vapor return flow rate be a relatively constant percentage of the plant feed rate. A need exists for a PLNG loading method that provides this type of vapor return flow rate.
SUMMARY
A method is disclosed for loading PLNG into a plurality of containers filled with pressurized vapor. The containers or groups of containers are loaded in succession and the PLNG introduced in a container discharges vapor therefrom. A fraction of the discharged vapor from at least one of the containers is passed into auxiliary storage tanks comprising a first tank and a second tank. Vapor is withdrawn from at least one of the tanks and passed to a vapor utilization means, preferably a liquefaction plant to liquefy the vapor or an engine or turbine that uses the vapor as fuel. The flow of PLNG and vapor to and from the first and second tanks are regulated to assure that the total flow rate of vapor to the vapor utilization means remains at a generally constant flow rate.
In a preferred embodiment of this invention, two auxiliary storage tanks, a first tank and a second tank, are used to buffer the flow rate of pressurized vapor discharged from a plurality of containers that are filled with PLNG in succession. PLNG is introduced into a first container or group of containers and vapor is discharged therefrom. A first fraction of the discharged vapor is passed to a suitable gas utilization means, such as a vapor liquefaction plant or an engine or turbine that uses the vapor as fuel, and a second fraction is passed to another container to be filled with PLNG. Vapor discharged from the last container being filled with PLNG is passed to one of the auxiliary storage tanks and the vapor from the auxiliary storage tanks is then passed to any suitable vapor utilization means. Fluid flow (vapor and PLNG) to and from the storage tanks is regulated to buffer the flow rate of vapor to the vapor utilization means. At the beginning of the PLNG-loading method, the first tank is full of relatively high-pressure vapor and the second tank contains pressurized liquefied gas. During filling of the first container or first group of containers, PLNG is withdrawn from the second tank and passed to the first container or first group of containers and simultaneously vapor is withdrawn from the first tank, pressurized, and a first fraction of the pressurized vapor is passed to the second tank and a second fraction of the pressurized vapor is heated and returned to the first tank. When the second tank is emptied of PLNG, the second tank contains relatively high-pressure vapor and the first tank contains relatively low-pressure vapor. Vapor from the second tank is then withdrawn, pressurized, and a fraction of the pressurized vapor is passed to a vapor utilization means and a second fraction of the pressurized vapor is heated and returned to the second tank. At the end of the foregoing step, the first and second tanks both contain vapor at a relatively low pressure. During PLNG filling of the last container or last group of containers, vapor discharged from the last container or group is passed to a compressor for pressurization, and a first fraction of the pressurized vapor is passed to the vapor utilization means and a second fraction of the pressurized vapor is heated and passed to the second tank. When the last container is filled with PLNG, the first tank contains vapor at a relatively low pressure and the second tank contains vapor at a relatively high pressure. The second tank is then ready to be replenished with PLNG. PLNG is then introduced into the second tank and vapor discharged therefrom. The discharged vapor is split into a first fraction and a second fraction. The first vapor fraction is heated and passed to the first tank and the second fraction is passed to the gas utilization means. At the end of this step, the first tank contains vapor at a relatively high-pressure and the second tank contains PLNG. The storage tanks are now ready for PLNG-loading another set of containers.
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Rigby James R.
Stone Brandon T.
Capossela Ronald
ExxonMobil Upstream Research Company
Lawson Gary D.
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