Air separation method and air separation plant

Abstract: In an air separation plant comprising heat exchangers (11, 20) utilizing liquefied natural gas as a cold source and a cryogenic rectifying unit (10) for separating air, a product gas obtained in said cryogenic rectifying unit (10) is supplied and a closed cycle is provided, in which a heat medium which has been cooled down and liquefied in the first heat exchanger (20) by said liquefied natural gas is led to the second heat exchanger (11) by a pump (21) so as to be evaporated, the evaporated heat medium is then introduced into said first heat exchanger (20) again, said product gas is led to said second heat exchanger (11) so as to be cooled down by said heat medium and then compressed by a compressor (12) and the cooled and compressed product gas is then warmed by a heater (13).

Description: The present invention relates to an air separation method for supplying to the outside a product gas obtained by carrying out air separation, while utilizing liquefied natural gas (LNG) as a cold source and an air separation plant therefor, which are useful for supplying high pressure oxygen and nitrogen especially to an integrated gasification combined cycle power generation plant or the like. Various techniques have existed which are intended to utilize effectively cold generated on gasification of liquefied natural gas (LNG) because it is finally used as a gas. One of these techniques has been known which utilizes LNG as a cold source for an air separation plant for separating air through rectification to produce nitrogen and oxygen For instance, JP-A-59045054 discloses a method of directly utilizing the cold of LNG for the purpose of cooling feed air, and JP-A-52041224 gives a method of utilizing the cold of LNG for the purpose of cooling and liquefying nitrogen which is compressed and recycled, with its temperature returned to normal temperature. Furthermore, JP-A-46016081 gives a method of directly utilizing the cold of LNG for the purpose of cooling both recycled nitrogen and feed air. FR-A-1196821 discloses an air separation unit in which cold from evaporating LNG is transferred, using a closed cycle, to the feed air for the unit US-A-3339370 discloses a process and apparatus according to the preambles of the independent claims. Refrigeration is produced by the compression-liquefaction-expansion of nitrogen when a nitrogen cycle is adopted for the purpose of supplying cold necessary for air separation. However, it is known that electricity requirements can be reduced in the case of compressing low temperature gas. In JP-A-46016081, it is attempted to save the power expense by adopting the so-called cryogenic compression which comprises compressing low temperature nitrogen which has been cooled down by LNG or low temperature gas separated in an air separation plant. This invention is intended to produce product nitrogen in liquid form by the compression and liquefaction of low temperature nitrogen. In an integrated gasification combined cycle power technique (IGCC) which has been recently observed, on the other hand, large quantities of nitrogen and oxygen are consumed in high pressure state. Since nitrogen and oxygen supplied directly from a usual air separation plant are usually at too low a pressure, these gases have been generally supplied to IGCC plant after they are compressed by a normal temperature compressor. In such a usual air separation technique as has been hitherto used, however, the power expense based by a normal temperature compressor becomes larger because the consumption of nitrogen and that of oxygen are larger in IGCC plant. Accordingly, an improvement for reducing said power expense has been sought. Since the cold of LNG is utilized for the purpose of supplementing cold necessary for air separation or for the purpose of producing products in liquid in such an air separation plant as mentioned above, on the other hand, there has hitherto not existed an ideal of adopting said cryogenic compression for the purpose of enhancing the pressure of products. It is therefore an object of the present invention to provide an air separation method capable of supplying high pressure nitrogen gas and oxygen gas for use in, for example, an IGCC plant, at a lower power expense by utilization of the cold of LNG and an air separation plant therefor. The aforementioned purpose can be achieved by the present invention as defined in claim 1, respectively in claim 8. According to the present invention, there is a provided an air separation method, according to claim 1. According to optional features of the invention: at least part of said feed air is led to said second heat exchanger so as to be cooled down by said heat medium and then compressed and the cooled and compressed feed air is led to said cryogenic rectifying unit, the medium circulates in a closed circuit, only part of the liquefied natural gas is evaporated in the first heat exchanger, the medium is nitrogen or argon, the compression of the product and/ or of the air takes place at a sub-ambient temperature, the compression of the product and/ or of the air takes place at below -100°C. The term 'high pressure gas' referred here is intended to denote a gas at a pressure higher than that of a product gas obtained in a conventional air separation method and is indicative of, for example, a pressure of 10 bara or more. According to a further aspect of the invention, there is provided an air separation plant according to claim 8. There may also be provided means for sending feed air to said second heat exchanger, at least one further compressor, means for sending air from the second heat exchanger to the at least one further compressor and means for sending air from the at least one further compressor to the rectifying unit. Preferably the plant comprises means for warming compressed gas connected to the product compressor and/or the air compressor(s). Since a product gas is cooled down by a heat medium which has been cooled down by liquefied natural gas and compressed and the cooled and compressed product gas is then destined to be a high pressure gas, according to the air separation method of the present invention, power expenses for compression can be reduced owing to the compression carried out at low temperatures. By the way, the compressed gas is optionally warmed and destined to be a high pressure gas for supply, where this warming does not require any special heat energy (for example, water or the like is usable), and hence the supply of a high pressure gas can be carried out at a lower power expense by utilizing the cold of LNG. Furthermore, since the latent heat of the heat medium is utilized as it is recycled, thereby transferring its cold to the product gas so that it is cooled down, the product gas can be effectively cooled down by the cold of LNG. Since a heat medium independent of the product gas or feed gas is permitted to be used as this heat medium at that time, the safety can be secured even when LNG is mixed, if an inert heat medium is selected. As a result, high pressure nitrogen gas and/or oxygen gas for use in, for example, an IGCC plant can be supplied at a lower power expense by utilizing the cold of LNG. In a case where said feed air is led to said second heat exchanger so as to be cooled down by said heat medium and compressed and the cooled and compressed feed air is then led to said cryogenic rectifying unit, the power expense can be saved also on supply of the feed air and high pressure product gas can be supplied at a lower power expense in total. According to the air separation plant of the present invention, on the other hand, high pressure nitrogen gas and/or oxygen gas for use in, for example, an IGCC plant can be supplied at a lower power expense by utilizing the cold of LNG, owing to the same operational effects as mentioned above. In a case where a route is provided, through which at least part of said feed air is led to said second heat exchanger so as to be cooled down by said heat medium and compressed by a compressor and the cooled and compressed feed air is then led to said cryogenic rectifying unit, high pressure product gas can be supplied at a lower power expense in total, similarly to the aforementioned case. Now referring to the accompanying drawings, embodiments of the present invention will be described in detail. Fig. 1 is a schematic flow diagram of an air separation plant according to the present invention, and Fig. 2 is a schematic block diagram showing the air separation plant of another embodiment. The air separation plant according to the present invention comprises a cryogenic rectifying separation unit (ASU) 10, where pre-purified feed air is subjected to air separation. In the pre-purification of feed air, an air purifying operation is carried out in order to remove impurities, for example, components difficult to remove in a rectification column or solid components such as dust. Concretely, feed air taken in from a filter 1 and freed of dust is compressed by a feed air compressor 2, and then cooled down by brine (sea water or the like) in a cooler 3 and freed of water-soluble components in a water separator 4. After the feed air is then led to an adsorber 5 filled with molecular sieve so as to be freed of moisture and carbon dioxide, a major part thereof (between 60 and 80%) is introduced, for instance, at 4 barg into the cryogenic rectifying separation unit 10 by way of a line L1, and the remaining part thereof will be described below. This cryogenic rectifying separation unit 10 is generally composed of a single or plural rectification columns, heat exchangers and equipment accompanied therewith (not shown). In the present invention, any of such known units as mentioned above can be adopted. As for the cryogenic rectifying separation unit 10, detailed explanation of its construction will be omitted. For the purpose of supplying product gases at high pressure, there can be preferably used a cryogenic rectifying separation unit 10, in which the liquid oxygen pumping system is adopted. In this cryogenic rectifying separation unit 10, feed air (for example, 30 bara) for evaporating product oxygen is required. After said feed air is led to a second heat exchanger 11 from a line L2 so as to be cooled down (for example, cooled down to -147°C), accordingly, it is elevated in pressure by a cryogenic air booster 6 and thereafter warmed by brine (sea water or the like) in a warmer 7, and the warmed feed air is then fed to the cryogenic rectifying separation unit 10 through a line L3 and can be used to vaporise pumped cryogenic liquids such as nitrogen or oxygen. In addition, this cryogenic compression also contributes to the saving of the power expense in total. A major part of product nitrogen gas led out of the cryogenic rectifying separation unit 10 through a line L4 is led into the second heat exchanger 11 through a line L5 so as to be cooled down (for example, cooled down to -147°C) and compressed by a cryogenic nitrogen compressor 12, and then warmed by brine (sea water or the like) in a warmer 13, and thereafter supplied, for example at 30 bara to the outside by way of a line L6. The remaining part thereof is led to the adsorber 5 through a line L7 so as to be used as a regeneration gas therefor, and then compressed by a nitrogen compressor 14, joined in the line L6 by way of a line L8, and thereafter supplied to the outside. In order to utilize liquefied natural gas as a cold source in the present invention, on the other hand, a recycle route is provided. This recycle route serves to ensure that a heat medium (e.g. -150°C) which has been cooled down and liquefied by liquefied natural gas in a first heat exchanger 20 is led to the second heat exchanger 11 by a pump 21 so as to be evaporated and the evaporated heat medium is then introduced into the first heat exchanger 20 again. As the heat medium used here, nitrogen or a rare gas such as argon is preferably used so that safety can be secured even if liquefied natural gas is mixed therein. In addition, the temperature rise of said heat medium caused by the pump 21 is slight. Concretely, as shown in Fig. 1, liquefied natural gas is introduced under a high pressure (e.g. 40 bara) and at a low temperature (e.g. -155°C) into the first heat exchanger 20 through a line L10 so as to be evaporated through heat exchange with the heat medium introduced therein from lines L16, L18 so that the same heat medium is cooled down. Evaporated natural gas is led out at different temperatures through a line L11 or L12 and fed to a warm water evaporator (ORV) 22 and a cooler 23 using cooling water or brine for refrigeration so that its cold is recovered, and then supplied to the outside. Into the cooler 23, in addition, cooling water (CW) is fed through a line L21 and brine for refrigeration (BR) is fed through a line L20. The heat medium which has been cooled down and liquefied in the first heat exchanger 20 is led out under a high pressure (e.g. 45 bara) and at a low temperature (e.g. -150°C) through a line L15 and led into the second heat exchanger 11 by the pump 21. In the second heat exchanger 11, the heat medium is evaporated through heat exchange with the feed air and product gas introduced therein through the line L2, L5 so that they are cooled down. Thereafter, the evaporated heat medium is introduced into the first heat exchanger 20 again through the line L16 so as to be cooled down, and led (for example, at -130°C) into the second heat exchanger 11 through a line L17, and further introduced for recycle use into the first heat exchanger 20 through the line L18. The reason why one cycle is made up by two trips of the heat medium between the first heat exchanger 20 and the second heat exchanger 11, as mentioned above, is to effectively utilize the cold of liquefied natural gas. On the other hand, product oxygen gas is led out of the cryogenic rectifying separation unit 10 through a line L9, compressed by an oxygen compressor 15, and then fed under a high pressure (e.g. 80 bara) to the outside. In addition, cooling water and brine for refrigeration which have been cooled down in the cooler 23 will be used for cooling in the air separation plant of the present invention or other plants. Calculated examples of the reducing effect of electric power obtained in a case where air separation is carried out under a conditions shown in Table 1 by use of such an air separation plant of the present invention as mentioned above, will be given (provided that the same effect of the prior art is 100). In addition, values obtained in other embodiments which will be described below are given. NT Compression Of Prior Art This Embodiment Other Embodiment Feed Air (L1) 127,000 Nm3/h 13 barA Feed Air (L3) 56,100 Nm3/h 30 barA Product O2 Gas (L9) 40,000 Nm3/h 14 barA Product N2 Gas (L4) 142,800 Nm3/h 4 barA HP Product O2 Gas 40,000 Nm3/h 80 barA HP Product N2 Gas 142,800 Nm3/h 30 barA Feed Air Compressor 54.5 54.5 38.2 Cryogenic Air Compressor --- --- 9.7 Air Booster 5.5 --- --- Cryogenic Air Booster --- 2.9 2.9 02 Compressor 8.6 8.6 8.6 N2 Compressor 31.4 6.3 7.3 Cryogenic N2 Compressor --- 16.0 15.3 Total Value 100 88.3 82.0 HP: High Pressure, NT: Normal Temperature Figures of pressure and temperature given above exhibit one example of the operating condition, and the technical scope of the present invention is not limited thereby. Although the air separation method and air separation plant of the present invention are useful for supplying high pressure oxygen and nitrogen, especially to an integrated gasification combined cycle power generation plant or the like, as mentioned above, both of them are applicable to other plants which require the supply of high pressure oxygen and nitrogen such as an iron manufacturing furnace. Other embodiments of the present invention will be described below. (1) Although an example of subjecting the product nitrogen gas and the feed air for evaporation of product oxygen to cryogenic compression has been given in the aforementioned embodiment, it is preferred in the present invention that feed air for rectification use is also subjected to cryogenic compression. Referring to Fig. 2, this embodiment will be described below, where only the different points thereof from the case of Fig. 1 will be explained. All of the feed air coming from the adsorber 5 filled with molecular sieve is led into the second heat exchanger 11 through a line L30 so as to be cooled down to an intermediate temperature of the second heat exchanger, for example to -120°C by said heat medium. Afterwards the feed air is then compressed by a cryogenic air compressor 30, it is warmed by brine (sea water or the like) in a warmer 31 and supplied in part to the cryogenic rectifying separation unit 10 through the line L1 without further cooling or heating and in part to the second heat exchanger 11 and subsequently to compressor 6, heater 7 and then to the ASU 10.In this way it is the cryogenic compressor 30 which brings the feed air to the pressure at which it is to be separated so the compressor 2 may be smaller.As for a recycle route, on the other hand, one cycle is made up by five trips of the heat medium between the first heat exchanger 20 and the second heat exchanger 11, whereby the utilizing efficiency of the cold of liquefied natural gas is enhanced. Through a line L17, in addition, the heat medium is introduced, for example at -130°C to the second heat exchanger 11. (2) In the present invention, product oxygen and/or argon gas may be subjected to cryogenic compression, similarly to the aforementioned case . In this case, product oxygen or argon gas will be led to the second heat exchanger so as to be cooled down by the heat medium and compressed, and thereafter warmed, similarly to the product nitrogen gas. (3) Although an example having a recycle route constructed such that one cycle is made up by plural trips of the heat medium between the first heat exchanger 20 and the second heat exchanger 11 has been given in the aforementioned embodiment, it is a matter of course that a recycle route may be constructed such that one cycle is made of one trip of the heat medium. In addition, a recycle route may be also constructed by changing the trip frequency of the heat medium.

Claims: An air separation method, in which pre-purified feed air is led to a cryogenic rectifying unit (10) so as to be subjected to air separation, while utilizing liquefied natural gas as a cold source and a resulting product gas from the cryogenic rectifying unit is supplied to the outside, and a medium which has been cooled down and liquefied in a first heat exchanger (20) by said liquefied natural gas is led to a second heat exchanger (11) so as to be evaporated and the evaporated medium is then introduced into said first heat exchanger again, the product gas is compressed and is then destined to be a high pressure gas for supply, characterized in that before being compressed, said product gas is led to said second heat exchanger so as to be cooled down by said heat medium. A method, according to claim 1, in which at least part of said feed air is led to said second heat exchanger (11) so as to be cooled down by said heat medium and then compressed and the cooled and compressed feed air is led to said cryogenic rectifying unit (10). A method according to claim 1 or 2 in which the medium circulates in a closed circuit(L15,L16,L17,L18). A method according to any preceding claim wherein only part of the liquefied natural gas is evaporated in the first heat exchanger (20). A method according to any preceding claims wherein the medium is nitrogen or argon. A method according to any preceding claims wherein the compression of the product and/ or of the air takes place at a sub-ambient temperature. A method according to claim 6 wherein the compression of the product and/ or of the air takes place at below -100°C. An air separation plant comprising heat exchangers (11,20) for utilizing liquefied natural gas as a cold source and a cryogenic rectifying unit (10) for subjecting pre-purified feed air led therein to air separation, where a product gas obtained in said cryogenic rectifying unit is supplied to the outside, a cycle (L15,L16,L17,L18), through which a medium which has been cooled down and liquefied in the first heat exchanger (20) by said liquefied natural gas is led to the second heat exchanger (11) so as to be evaporated and the evaporated heat medium is then introduced into said first heat exchanger again, and means (L4) for sending said product gas to said second heat exchanger and a compressor (12), characterised in that it comprises means for sending the product gas from the second heat exchanger to the compressor (12). A plant, according to claim 8, comprising means (L2) for sending feed air to said second heat exchanger (11), at least one further compressor (6,30) and means for sending air from the second heat exchanger to the at least one further compressor and means (L3)for sending air from the at least one further compressor to the rectifying unit. A plant according to claim 8 or 9 comprising means (7,13) for warming compressed gas connected to the product compressor(12) and/or the air compressor or compressors(6,30).

Inventors: DEN RYO, TOMITA SHINJI, DEN, RYO, TOMITA, SHINJI, Den, Ryo, Harima Technical Center, Tomita, Shinji, Harima Technical Center
Applicants: AIR LIQUIDE, L'AIR LIQUIDE, S.A. A DIRECTOIRE ET CONSEIL DE SURVEILLANCE POUR L'ETUDE ET L'EXPLOITATION DES PROCEDES GEORGES CLAUDE
Application year: 2000
Main classification: F25J3
IPC codes: F25J5, F25J3, F25J1