Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
2003-03-26
2004-01-06
Bennett, Henry (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S048200, C062S614000
Reexamination Certificate
active
06672104
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an improved process for reliquefaction of boil-off from methane-rich liquefied gas such as boil-off from liquefied natural gas (“LNG”) or boil-off from pressurized liquefied natural gas (“PLNG”).
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 (“LNG”) for transport to market at or near ambient pressure and at a temperature of about −162° C. (−260° F.).
The source gas for making LNG is typically obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). Associated gas occurs either as free gas or as gas in solution in crude oil. Although the composition of natural gas varies widely from field to field, the typical gas contains the hydrocarbon methane (C
1
) as a major component. The natural gas stream may also contain the hydrocarbon ethane (C
2
), higher hydrocarbons (C
2+
), and minor amounts of contaminants such as carbon dioxide (CO
2
), hydrogen sulfide (H
2
S), nitrogen (N
2
), iron sulfide, wax, and crude oil. The solubilities of the contaminants vary with temperature, pressure, and composition. At cryogenic temperatures, CO
2
, water, other contaminants, and certain heavy molecular weight hydrocarbons can form solids, which can potentially plug flow passages in liquefaction process equipment. These potential difficulties can be avoided by removing such contaminants and heavy hydrocarbons from the natural gas stream prior to liquefaction.
It has also been 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. This pressurized liquid natural gas is referred to in this specification as “PLNG” to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about −162° C. (−260° F.).
Because PLNG typically contains a mixture of low molecular weight hydrocarbons and other substances, the exact bubble point temperature of PLNG is a function of its composition. For most natural gas compositions, the bubble point pressure of the natural gas at temperatures above −112° C. (−170° F.) will be above 1,380 kPa (200 psia). One of the advantages of producing and shipping PLNG at a warmer temperature than LNG is that PLNG can contain considerably more C
2+
components than can be tolerated in most LNG applications.
Depending upon market prices for ethane, propane, butanes, and heavier hydrocarbons (collectively referred to herein as “NGL products”), it may be economically desirable to transport the NGL products with the PLNG and to sell them as separate products. International Publication No. WO 90/00589 (Brundige) discloses a process of transporting pressurized liquid heavy gas containing butane and heavier components, including condensable components that are deliberately and intentionally left in the liquefied natural gas. In the Brundige process, basically the entire natural gas composition, regardless of its origin or original composition, is liquefied without removal of various gas components. This is accomplished by adding to the natural gas an organic conditioner, preferably C
2
to C
5
hydrocarbons to change the composition of the natural gas and thereby form an altered gas that is in a liquid state at a selected storage temperature and pressure. Brundige allows the liquefied product to be transported in a single vessel under pressurized conditions at a higher temperature than conventional LNG.
In the storage, transportation, and handling of PLNG, there can be a considerable amount of boil-off, which boil-off is primarily in the gaseous or vapor phase. In many applications in which boil-off is produced, it is desirable to reliquefy the boil-off and combine it with the liquid that produced the boil-off. PLNG boil-off can typically be reliquefied using the same process used to produce PLNG. However, since PLNG often contains an appreciable quantity of nitrogen, this nitrogen will, as a result of its lower boiling point compared with other constituents of natural gas, evaporate preferentially and form a significant portion of the boil-off. For example, for PLNG at 450 psia containing 0.1% nitrogen, boil-off may contain as much as 3% nitrogen. At a given pressure, reliquefaction of the boil-off will therefore require cooling of the boil-off to a lower temperature than required to liquefy the liquid from which the boil-off was produced. Various reliquefaction processes have been proposed for handling nitrogen-rich boil-off.
U.S. Pat. No. 3,857,245 (Jones) discloses a process of condensing a nitrogen-containing boil-off in which LNG is injected into the nitrogen-containing boil-off vapor and the combined mixture is then condensed. The injection of the LNG into the nitrogen-containing boil-off increases the volume of vapor that must be reliquefied.
U.S. Pat. No. 6,192,705 (Kimble) discloses a process of passing boil-off through a heat exchanger followed by compressing and cooling stages, and then recycling the boil-off back through the heat exchanger. The compressed, cooled, and then heated boil-off is subsequently expanded and passed to a gas-liquid separator for removal of liquefied boil-off. The liquefied boil-off is then combined with a second liquefied gas stream to produce a desired product stream.
One problem encountered with reliquefaction processes proposed in the past is that the reliquefied boil-off may have a lower (colder) bubble point temperature than that of the bulk cargo liquid that produced the boil-off. This lower temperature can be undesirable if it exceeds the lower allowable limit of the operating temperature of the transport containers. A need therefore exists for an improved process for re-liquefying PLNG boil-off to overcome the temperature disparity between the bulk bubble point temperature of the liquefied cargo and the bubble point temperature of the liquefied boil-off.
SUMMARY OF THE INVENTION
This invention relates to a method of converting a boil-off stream comprising methane to a liquid having a preselected bubble point temperature, comprising the steps of: (a) pressurizing the boil-off stream; (b) cooling the pressurized boil-off stream of step (a); (c) expanding the cooled, pressurized boil-off stream of step (b), thereby producing pressurized liquid; and (d) obtaining the preselected bubble point temperature of the pressurized liquid by performing at least one of the following steps:
i. before, during, or after one or more of steps (a) to (c), removing from the boil-off stream a first predetermined amount of one or more components having a vapor pressure greater than the vapor pressure of methane, and
ii. before, during, or after one or more of steps (a) to (c), adding to the boil-off stream a second predetermined amount of one or more additives having a molecular weight heavier than the molecular weight of methane and having a vapor pressure less than the vapor pressure of methane,
wherein the first predetermined amount of the one or more components removed and the second predetermined amount of the one or more additives added are controlled to obtain the preselected bubble point temperature of the pressurized liquid. If desired, the multi-component boil-off stream can be warmed prior to the first pressurization. In one embodiment of the method of this invention, the one or more components removed from the boil-off stream comprise nitrogen. In one embodiment of this invention, the one or more additives added to the boil-off stream comprise one or more C
2+
hydrocarbons. One embodiment of this invention further comprises combining the pressurized l
Bowen Ronald R.
Kimble E. Lawrence
Rigby James R.
Bennett Henry
Drake Malik N.
ExxonMobil Upstream Research Company
Hoefling Marcy
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