Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-05-24
2001-10-02
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S434000, C062S912000
Reexamination Certificate
active
06295837
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-145962, filed May 26, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for air separation utilizing liquefied natural gas (LNG) as a cold source. Specifically, the invention relates to an air separation apparatus suitable for supplying high-pressure nitrogen gas and oxygen gas to an integrated gasifier combined cycle plant.
Since LNG is consumed in a gas phase in a final stage, various kinds of processes have been proposed wherein cold generated during gasification of LNG is effectively used. For example, there is known a process utilizing LNG as a cold source in an air separation apparatus for producing nitrogen and oxygen by cryogenic separation.
Jpn. Pat. Appln. KOKOKU Publication No. 49-45054, for instance, discloses a process in which cold is utilized for directly cooling feed air. Jpn. Pat. Appln. KOKOKU Publication No. 52-41224 discloses a process wherein cold of LNG is utilized for cooling and liquefying nitrogen, which is restored to room temperature, compressed and recycled. Further, Jpn. Pat. Appln. KOKOKU Publication No. 46-16081 describes a process wherein cold of LNG is utilized directly for cooling both nitrogen, which is to be recycled, and feed air.
In a case where a nitrogen cycle is adopted in order to supply cold needed in an air separation process, cold is produced by compression, liquefaction and expansion of nitrogen. In this case, it is known that less power is consumed when nitrogen gas at a low temperature is compressed, than when nitrogen gas at room temperature is compressed. For this reason, of the above publications, Jpn. Pat. Appln. KOKOKU Publication No. 46-16081 adopts a process wherein nitrogen gas is cooled by LNG (or a low-temperature gas separated in an air separation apparatus) and then compressed, thereby reducing power cost. In the process described in this publication, however, nitrogen is recovered in a liquid phase by compression and liquefaction of separated nitrogen gas.
In an integrated gasifier combined cycle plant to which special attention has recently been paid, a large amount of high-pressure nitrogen gas and oxygen gas is consumed. On the other hand, the pressure of nitrogen gas or oxygen gas produced by a conventional air separation apparatus was insufficient. Thus, in general, such gases were compressed using a compressor at room temperature, and then supplied to the plant. Therefore, power cost was high for the operation of the compressor. Under the circumstances, an improvement was desired for reducing power cost in an integrated gasifier combined cycle plant.
On the other hand, cold of LNG is utilized in conventional air separation apparatuses in order to supply cold needed for air separation or to supply separated products to the outside in a liquid phase. However, there was no idea that cold of LNG is utilized for the purpose of increasing the pressure of a product gas.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide an air separation apparatus for producing, at low power cost, high-pressure nitrogen gas and oxygen gas used in, for example, an integrated gasifier combined cycle plant, by utilizing cold of liquefied natural gas.
In order to achieve the object described above, the present invention provides an apparatus comprises: a cryogenic separation unit for receiving pre-purified feed air and separating the same into nitrogen gas and oxygen gas; a first heat exchanger for receiving liquefied natural gas as a cold source and for cooling and liquefying a refrigerant by indirect heat exchange with the liquefied natural gas; a second heat exchanger; a refrigerant supply line for supplying the refrigerant liquefied in the first heat exchanger to the second heat exchanger; a refrigerant return line for returning the refrigerant vaporised in the second heat exchanger to the first heat exchanger; a first nitrogen gas line for sending at least part of the nitrogen gas separated in the cryogenic separation unit to the second heat exchanger; a second nitrogen gas line for supplying the nitrogen gas cooled within the second heat exchanger to the outside; and a low-temperature nitrogen compressor provided on the second nitrogen line for compressing the nitrogen gas and supplying the compressed nitrogen gas as high-pressure product gas to the outside.
The phrase “high-pressure product gas” in this context means a product gas having a higher pressure than that obtained in a conventional air separation process. For example, a product gas with a pressure of 10 bar (abs.) or more is referred to.
According to the air separation apparatus of the present invention, nitrogen gas separated in the cryogenic separation unit is introduced into the second heat exchanger, cooled by indirect heat exchange with the refrigerant (cooled by LNG), and then compressed by the low-temperature nitrogen compressor. In this manner, since the separated nitrogen gas is compressed at a low temperature, power cost for compressing the nitrogen gas can be reduced.
In addition, the compressed nitrogen gas is heated in order to supply the product nitrogen gas to an external plant for consumption, if necessary. No extra thermal energy is required for heating the compressed nitrogen gas (for example, seawater can be used). Thus the heating thereof does not cause an increase in power cost.
The oxygen gas separated in the cryogenic separation unit can also be compressed at a low temperature, using a process similar to the above. Thereby, power cost for compressing the oxygen gas can be saved.
Preferably, the air separation apparatus of the present invention further comprises: a first feed air line for sending the pre-purified feed air to the second heat exchanger; a second feed air line for sending the feed air cooled within the second heat exchanger to the cryogenic separation unit; and a low-temperature air compressor provided on the second feed air line for compressing the feed air.
As described above, the feed air is cooled in the second heat exchanger and then compressed in the low-temperature air compressor, thereby reducing the power cost for compressing the feed air.
According to the air separation apparatus of the present invention, the refrigerant is circulated between the first heat exchanger and the second heat exchanger, and the nitrogen gas is cooled by latent heat of this refrigerant. Thereby, the nitrogen gas can be cooled with efficiency. In such a case, a thermal medium other than product nitrogen gas or feed gas can be used as a refrigerant. Therefore, safety is ensured when LNG is mixed in the thermal medium by adopting a chemically inert substance as a thermal medium. As a result, high-pressure nitrogen gas and oxygen gas used in, for example, an integrated gasifier combined cycle plant can be produced at low power cost, by utilizing cold of liquefied natural gas.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
REFERENCES:
patent: 2729953 (1956-01-01), Schilling
patent: 3339370 (1967-09-01), Streich et al.
patent: 4054433 (1977-10-01), Buffiere et al.
patent: 4345925 (1982-08-01), Cheung
patent: 5220798 (1993-06-01), Nagamura et al.
patent: 5475980 (1995-12-01), Grenier et al.
patent: 804944 (1958-11-01), None
patent: 46-16081 (1971-05-01), None
patent: 49-45054 (1974-12-01), None
patent: 52-41224 (1977-10-01), None
English abstract of SU 488 967, XP-002147583.
English abstract of BE 803171.
English abstract of JP 46-16081.
English abstract of JP 52 41224.
Den Ryo
Tomita Shinji
Burns Doane Swecker & Mathis L.L.P.
Capossela Ronald
L'Air Liquide, Societe Anonyme pour l'Etude et l'
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