Process and installation for cryogenic air separation integrated with an associated process

Abstract: In a process for separation of air by cryogenic distillation integrated with an associated process, air is separated in a separation unit (1), fluid is sent from the separation unit to an associated process, steam (3) is derived from the associated process, at least part of the steam is used in the separation unit and at least one fluid stream (11) is sent from the air separation unit to the atmosphere, at least when the steam is used in the air separation unit.

Description: The present invention relates to a process and installation for separation of air by cryogenic distillation, integrated with an associated process comprising the features of the preamble of claim 1 respectively of claim 14. Such a process and installation are known from US-A-3 731 495. Air separation units are frequently integrated with associated processes producing large amounts of water vapour, such as gas-to-liquid (GTL) and gas-to-olefins (GTO) processes. On remote sites where the vapour cannot be used to generate energy and the vapour cannot be exported, the excess vapour, representing between 5 and 30% of the steam production, is generally sent to a condenser where it is converted into water, as described in EP-A-0748763. The air separation units generally supply oxygen enriched gas at a pressure exceeding 5 bar abs. to the associated process. One object of the present invention is to reduce the size of the steam condenser or even eliminate it completely, thereby reducing the capital costs of the plant. It is known from The Future of Air Separation', a conference given by Dr.T.Rathbone at LTEC90, held in 1990, to couple a steam turbine using steam from a partial oxidation system with the compressor of an air separation unit. EP-A-0562893 describes an air separation unit in which the air compressor and nitrogen compressor are powered by a steam turbine. According to the present invention, there is provided a process according to claim 1. In this way, the air separation unit functions in a way which is deliberately chosen to be less than optimal, in order to use the steam in the air separation unit and avoid using a steam condenser or reduce the size of the condenser, so as to reduce the overall costs for the whole of the site including the air separation unit. Certainly energy is wasted by operating the air separation unit in this way but the overall cost of the wastage is reduced. Preferably the first fluid stream sent to the atmosphere is previously used to regenerate the purification unit used to purify the air and a third fluid stream or streams sent to the atmosphere is/are enriched in oxygen, nitrogen and/or argon and is preferably at a pressure of at least 5 bar abs. The third fluid stream or stream is/are preferably warmed to ambient temperature in a heat exchanger and then sent directly to the atmosphere, possibly after an expansion step. Preferably, at least two air separation units supply fluid to the associated process, each air separation unit being dimensioned to produce N/N-1 multiplied by at least 80%. preferably 90% or even 100%, of the nominal flow, N being the number of air separation units supplying the associated process. Preferably the process comprises expanding at least part of the vapour in at least one turbine coupled to at least one compressor of the air separation unit Preferably, at least one steam turbine is used to produce work and the work is used to supply at least part of the energy needs of at least one main compressor compressing air treated in the air separation unit and/or an air booster compressing air which has already been compressed to a superatmospheric pressure and/or a compressor for gas enriched in oxygen or nitrogen. For example, the at least one turbine may be coupled to a main compressor compressing air treated in the air separation unit and/or to an air booster compressing air which has already been compressed to a superatmospheric pressure and/or to a compressor for gas enriched in oxygen or nitrogen. Alternatively the steam turbine may be used to generate electricity and that electricity may be used to power at least one of the compressors of the air separation unit. According to another embodiment of the process, the process comprises sending energy to the atmosphere by sending refrigeration from the air separation unit to the atmosphere. For example, vapour from the associated process may be sent to at least one heat exchanger forming part of the air separation unit, at least one cryogenic liquid produced in the air separation unit is sent to the at least one heat exchanger, at least one cryogenic liquid vaporises at least partially in the heat exchanger and is sent to the atmosphere and/or to an associated process in gaseous form . Alternatively, vapour from the associated process is sent to at least one heat exchanger of the air separation unit, at least one cryogenic fluid produced in the air separation unit is sent to the at least one heat exchanger wherein it is warmed and the warmed cryogenic fluid is then expanded in a turbine before being sent to the atmosphere. At least one fluid stream, other than that used for regeneration, is sent to the atmosphere from the air separation unit constantly or when the amount of steam derived from the associated process exceeds a given value. In a particular embodiment, the fluid sent to the associated process is an oxygen rich gas and the associated process is a partial oxidation process associated with a catalytic conversion process producing excess steam. Preferably the at least one fluid stream is not used or is only partly used to regenerate a unit used to remove humidity and carbon dioxide from the feed air for the air separation unit or an air separation unit and is not used or is only partly used in a water chilling unit. Preferably, steam is sent constantly or substantially constantly to the air separation unit. According to another embodiment of the invention, there is provided an installation according to claim 14. Thus the air separation unit is voluntarily operated so as to waste energy, in the form of a compressed air stream by sending it to the atmosphere. This in fact proves to be more economical for the overall cost of the plant than the present techniques for disposing of the excess steam, which are costly in terms of equipment and maintenance. An oxygen enriched stream contains at least 30 mol.% oxygen, preferably at least 60 mol.% oxygen and still more preferably at least 80 mol.% oxygen. An argon enriched stream contains at least 30 mol.% argon, preferably at least 60 mol.% argon and still more preferably at least 80 mol.% argon. A nitrogen enriched stream contains at least 85 mol.% nitrogen, preferably at least 90 mol.% nitrogen and still more preferably at least 95 mol.% nitrogen. The air stream released to the atmosphere is at a pressure of at least 5 bar abs. preferably at least 10 bar abs. or at least 20 bar abs or at least 30 bar abs. The oxygen enriched stream and/or nitrogen enriched stream released to the atmosphere is/are at a pressure of at least 10 bar abs. or preferably at least 20 bar abs or at least 30 bar abs. It will be understood that the term air separation unit may include the main air compressors(s), booster compressor(s), product compressor(s), product storage tanks or buffer tanks, heat exchangers, distillation columns, pump(s) and turbine(s). The term thus may cover elements within and without the cold box. An air separation unit may include a single column, a double column (for example as described in FR-A-2477276, EP-A-0504029, FR-A-2688052 or EP-A-0583189) or a triple column( for example as described In EP-A-0538118) and possibly additionally at least one argon enrichment column and/or a mixing column (for example as described in EP-A-0531182). The associated process may be any process consuming a fluid produced by the air separation unit, such as an oxygen enriched stream and/or an argon enriched stream and/or a nitrogen enriched stream and/or compressed air and which produces steam either directly from the stage of the process consuming the enriched stream or another stage of the process upstream or downstream that stage. The term treated in the air separation unit covers separation by cryogenic distillation within the unit but also covers the case where a stream is simply compressed by the main air compressor of the unit or by another process upstream of the columns. The nominal flow of the air separation unit is the maximum real product flow to the customer for which it is designed. It will of course be understood that the gaseous stream may be sent to the atmosphere either by sending them into the air, for example using a device such as claimed in FR-A-2 815 549, or by sending them into a tank of water or a bed of solid material. The invention will now be described in further detail with reference to the figures: Figure 1 is a schematic drawing of an air separation unit and a GTL process integrated to function according to the process of the invention, with at least one compressor of the air separation unit being coupled to a steam turbine. Figure 2 is a schematic drawing of an air separation unit and a GTL process integrated to function according to an optional feature of the invention, with a heat exchanger in which steam is used to vaporise a cryogenic liquid of the air separation unit. Figure 3 is a schematic drawing of an air separation unit and a GTL process integrated to function according to an optional feature of the invention, with a heat exchanger in which steam is used to warm a cryogenic fluid of the air separation unit, before the fluid is expanded in a turbine. In Figure 1, natural gas is sent to a partial oxidation process using oxygen from an air separation unit 1 to produce a synthesis gas containing carbon monoxide and hydrogen. The synthesis gas is reacted catalytically to produce higher molecular weight hydrocarbon products and excess steam 3. The air separation unit may be of any known type and may comprise a classical double column or a triple column. The air to be treated is first compressed in at least one main air compressor 5, which is coupled to a steam turbine 7 in which the excess steam 3 is expanded. The main air compressor or compressors preferably compress the feed air to between 5 and 35 bar abs. Part of the air may then be compressed in a booster compressor 9, which is also coupled to the or a steam turbine. The Figure shows the compressor 9 as a cold booster but it may of course have an inlet temperature equal to or higher than the ambient temperature. The air is sent to the air separation unit wherein it is separated to form at least a waste nitrogen stream 37 containing at least 90 mol. % nitrogen, a nitrogen enriched gaseous product stream 27 containing between 90 and 99.99 mol. % nitrogen (optional), a product argon stream 31 containing between 90 and 99,99 mol. % argon (optional), an oxygen enriched liquid stream 43 (optional), a nitrogen enriched liquid stream 45 (optional) and an oxygen enriched gaseous stream 23 containing between 70 and 99,8 mol. % oxygen with a yield of less than 95%, preferably less than 90%. Preferably the nitrogen and argon streams each contain less than 1ppm oxygen. The waste nitrogen stream 37 only is used to regenerate the purification unit 35 of the air separation process. The heat exchanger 21 used to cool the air to a cryogenic temperature against product streams 23,27,31 is operated to have a temperature difference of at least 5 K, preferably 10K between the temperature of the entering air and at least one of the product streams coming from the warm end. The product nitrogen and oxygen streams in gaseous form may be removed from the column system in gaseous form or may be removed in liquid form from the column system and optionally pressurised in a pump (not shown). It will be appreciated that, given the demands of the partial oxidation process, there are commonly several air separation units used to provide the oxygen requirements and connected in parallel, for example four air separation units, each having their own main air compressor or compressors. There may be a common air network for the compressed air linking the compressors of several air separation units. Similarly, there may be an oxygen network linking the oxygen outputs of several air separation units. If the amount of air compressed in the main air compressor or compressors is such that the amount of oxygen produced would be surplus to the requirements of the partial oxidation process, various solutions are possible according to the invention. Firstly, the excess compressed air can be sent to the atmosphere in a stream 11 upstream of the purification unit 35 and/or a stream 11A downstream the purification unit and/or a stream 11B removed following further compression in booster 9. In all cases the pressure of the air 11,11A,11B exceeds 5 bar abs. and may exceed 15 bar abs. In this case the columns of the air separation unit are dimensioned to produce the maximum amount of oxygen required by the partial oxidation process and no streams are sent to the atmosphere except the air stream or streams 11,11A, 11B and the stream 37 used for the regeneration. Alternatively or additionally, the columns of the air separation unit can be dimensioned to receive the excess compressed air and a stream enriched in oxygen 25, nitrogen 29 or argon 33 can be released to the atmosphere, since the amount of products produced exceeds the requirements of the partial oxidation process. It will of course readily be seen that the excess air can be released to the atmosphere following distillation in the form of different streams having different compositions. Air is additionally sent to the atmosphere in the form of streams 11, 11A, 11B. In the case of the figure, the streams form part of the normal product streams but it will readily be seen that the streams sent to the atmosphere may have a purity greater than or less than the product stream purity. For example, in the case where excess steam is available, a stream of oxygen enriched gas less pure than stream 23 may be sent to the atmosphere. Should the partial oxidation process require additional oxygen, the oxygen can be supplied by no longer rejecting the oxygen stream 25 to the atmosphere or by reducing the oxygen enriched stream 25. During start-up, the steam turbine 7 is driven by steam produced by a boiler fuelled by natural gas. A or the steam turbine may additionally or alternatively be coupled to a compressor 13 for the oxygen enriched gas 23 or a compressor 15 for the nitrogen enriched gas 27, as shown in dashed lines. In Figure 2, natural gas is sent to a partial oxidation process using oxygen from an air separation unit 101 to produce a synthesis gas containing carbon monoxide and hydrogen. The synthesis gas is reacted catalytically to produce higher molecular weight hydrocarbon products and excess steam 103. The air separation unit may be of any known type and may comprise a classical double column or a triple column as described in the patents mentioned above. The air to be treated is first compressed in a main air compressor, which may or may not be coupled to a steam turbine in which part of the excess steam is expanded, as in Figure 1. Alternatively in the case of Figure 2, there need be no steam expansion step. The main air compressor preferably compresses the feed air to between 5 and 35 bar abs. Part of the air may then be compressed in a booster compressor between 10 and 70 bar abs., which could also be coupled to the steam turbine. The air separation unit produces at least a gaseous oxygen enriched stream 123 and a liquid oxygen enriched stream 143.

Claims: Process for separation of air by cryogenic distillation integrated with an associated process comprising the steps of a) cooling compressed and purified air to a cryogenic temperature in a heat exchanger (21) by heat exchange with fluids separated in an air separation unit c) separating compressed, purified and cooled air in an air separation unit (1.101) to produce at least one fluid enriched in oxygen (23.43,123) and/or at least one fluid enriched in nitrogen (27,45) and possibly at least one fluid enriched in argon (31), c) sending at least part of one said fluid (23,43,123) to an associated process, d) deriving at least one stream of steam (3,103) from the associated process, e) using at least part of the steam in the air separation unit by using at least one steam turbine (7) to produce work and the work being used to supply at least part of the energy needs of at least one main compressor (5) compressing air treated in the air separation unit and/or an air booster (9) compressing air which has already been compressed to a superatmospheric pressure and/or a compressor (13,15) for gas enriched in oxygen or nitrogen. f) operating the air separation unit using the process feature of sending at least first and second fluid streams (11,11A,11B,37) from the air separation unit to the atmosphere characterized in that the second fluid stream or streams (11,11A,11B) is compressed air, removed before or after purification, preferably at a pressure of at least 5 bar abs. Process according to Claim 1 comprising sending third fluid streams (25,29,33) from the air separation unit to the atmosphere wherein the first fluid stream (37)sent to the atmosphere is previously used to regenerate the purification unit used to purify the air and the third fluid stream or streams (25,29,33) sent to the atmosphere is/are enriched in oxygen, nitrogen and/or argon and is preferably at a pressure of at least 5 bar abs. Process according to any preceding claim wherein at least two air separation units (1,101) supply fluid to the associated process, each air separation unit being dimensioned to produce N/N-1 multiplied by at least 80%, preferably 90% or even 100%, of the nominal flow, N being the number of air separation units supplying the associated process. Process according to any preceding claim comprising expanding at least part of the steam (3,103) in at least one turbine (7) coupled to at least one compressor (5,9,13,15) of the air separation unit. Process according to any preceding claim wherein at least one steam turbine (7) is coupled to the air booster (9) compressing air which has already been compressed to a superatmospheric pressure and/or to a compressor (13,15) for gas enriched in oxygen or nitrogen. Process according to any preceding claim comprising warming a fluid stream separated in the air separation unit against a stream of steam wherein steam (103) from the associated process is sent to at least one heat exchanger (17) forming part of the air separation unit, at least one cryogenic liquid (143) produced in the air separation unit is sent to the at least one heat exchanger, the at least one cryogenic liquid vaporises at least partially in the heat exchanger and is sent to the atmosphere and/or to an associated process in gaseous form (125). Process according to any preceding claim comprising warming a fluid stream separated in the air separation unit against a stream of steam wherein steam (203) from the associated process is sent to at least one heat exchanger (117) of the air separation unit (201), at least one cryogenic fluid produced in the air separation unit is sent to the at least one heat exchanger wherein it is warmed and the warmed cryogenic fluid is then expanded in a turbine (227) before being sent to the atmosphere. Process according to any of claims 1 to 8 wherein at least one fluid stream (11,11A,11B,28,29,33,125), preferably an oxygen enriched gaseous stream, is sent to the atmosphere from the air separation unit constantly. Process according to any of claims 1 to 7 wherein at least one fluid stream (11,11A,11B,25,29,33,125), preferably an oxygen enriched gaseous stream, is sent to the atmosphere from the air separation unit when the amount of steam derived from the associated process exceeds a given value. Process according to any preceding claim wherein the fluid sent to the associated process is an oxygen rich gas (23, 123) and the associated process is a partial oxidation process associated with a catalytic conversion process producing excess steam. Process according to any preceding claim wherein the at least one fluid stream (11,11A,11B,25,29,33,125) is not used or is only partly used to regenerate a unit (35) used to remove humidity and carbon dioxide from the feed air for the air separation unit or another air separation unit and is not used or is only partly used in a water chilling unit. Process according to any preceding claim wherein a fluid sent from the air separation unit to the associated process and a fluid sent from the air separation unit to the atmosphere have the same principal component, the fluid sent to the associated process being less pure or purer than the fluid sent to the atmosphere. Process according to any preceding claim wherein steam is sent constantly or substantially constantly to the air separation unit (1,101,201). Process according to any preceding claim wherein the heat exchanger is operated to have a temperature difference between a warm stream entering the heat exchanger and a stream leaving the heat exchanger, having been warmed, of at least 5 K, preferably at least 10 K, at its warm end. Process according to any preceding claim wherein the fluid enriched in oxygen is produced with a yield of less than 95%, preferably less than 90%. Installation for separation of air by cryogenic distillation integrated with an associated process including : i) at least one air compressor (5) for compressing air to be treated in an air separation unit ii) an air separation unit comprising a purification unit (35), at least one heat exchanger (21), and at least one cryogenic distillation column (41) iii) means for supplying compressed air from the main air compressor to the air separation unit iv) means for removing a fluid enriched in a component of air from the air separation unit and sending it to an associated process v) means for transferring steam (3) from the associated process to the air separation unit and vi) means for sending at least one fluid stream (11,11A,11B) from the air separation unit to the atmosphere, without previously sending the fluid stream to regenerate the air purification unit, vii) at least one steam turbine (7) producing work and means to use the work for the energy needs of the main air compressor (5) and/or an air booster (9) of the air separation unit and/or a gaseous product compressor (13,15) of the air separation unit and means for feeding at least part of the steam (3) from the associated process to the steam turbine(s). characterised in that the means for sending the at least one fluid stream (11,11A,11B) from the air separation unit to the atmosphere is connected to the main air compressor (5) least one fluid stream (25,29,33,125) from the air separation unit to the atmosphere is connected to a column of the air separation unit.

Inventors: GUILLARD ALAIN, GUILLARD, ALAIN
Applicants: AIR LIQUIDE, L'AIR LIQUIDE, SOCIETE ANONYME A DIRECTOIRE ET CONSEIL DE SURVEILLANCE POUR L'ETUDE ET L'EXPLOITATION DES
Application year: 2000
Main classification: F25J3
IPC codes: F25J3