WO2007129152A1 - Cryogenic air separation process - Google Patents

Cryogenic air separation process Download PDF

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Publication number
WO2007129152A1
WO2007129152A1 PCT/IB2007/000841 IB2007000841W WO2007129152A1 WO 2007129152 A1 WO2007129152 A1 WO 2007129152A1 IB 2007000841 W IB2007000841 W IB 2007000841W WO 2007129152 A1 WO2007129152 A1 WO 2007129152A1
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WO
WIPO (PCT)
Prior art keywords
air
oxygen
separation unit
liquid
air separation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/IB2007/000841
Other languages
French (fr)
Inventor
Jean-Renaud Brugerolle
Bao Ha
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Air Liquide SA
LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Original Assignee
Air Liquide SA
LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Air Liquide SA, LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude filed Critical Air Liquide SA
Priority to AU2007246784A priority Critical patent/AU2007246784A1/en
Publication of WO2007129152A1 publication Critical patent/WO2007129152A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04151—Purification and (pre-)cooling of the feed air; recuperative heat-exchange with product streams
    • F25J3/04187—Cooling of the purified feed air by recuperative heat-exchange; Heat-exchange with product streams
    • F25J3/04193—Division of the main heat exchange line in consecutive sections having different functions
    • F25J3/04206—Division of the main heat exchange line in consecutive sections having different functions including a so-called "auxiliary vaporiser" for vaporising and producing a gaseous product
    • F25J3/04212—Division of the main heat exchange line in consecutive sections having different functions including a so-called "auxiliary vaporiser" for vaporising and producing a gaseous product and simultaneously condensing vapor from a column serving as reflux within the or another column
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04006—Providing pressurised feed air or process streams within or from the air fractionation unit
    • F25J3/04048—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of cold gaseous streams, e.g. intermediate or oxygen enriched (waste) streams
    • F25J3/0406—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of cold gaseous streams, e.g. intermediate or oxygen enriched (waste) streams of nitrogen
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    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04006—Providing pressurised feed air or process streams within or from the air fractionation unit
    • F25J3/04078—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression
    • F25J3/0409—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression of oxygen
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    • F25J3/04309—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams of nitrogen
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    • F25J3/04248—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion
    • F25J3/04333—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using quasi-closed loop internal vapor compression refrigeration cycles, e.g. of intermediate or oxygen enriched (waste-)streams
    • F25J3/04351—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using quasi-closed loop internal vapor compression refrigeration cycles, e.g. of intermediate or oxygen enriched (waste-)streams of nitrogen
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J3/04406—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system
    • F25J3/04418—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system with thermally overlapping high and low pressure columns
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04472—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using the cold from cryogenic liquids produced within the air fractionation unit and stored in internal or intermediate storages
    • F25J3/04496—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using the cold from cryogenic liquids produced within the air fractionation unit and stored in internal or intermediate storages for compensating variable air feed or variable product demand by alternating between periods of liquid storage and liquid assist
    • F25J3/04503—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using the cold from cryogenic liquids produced within the air fractionation unit and stored in internal or intermediate storages for compensating variable air feed or variable product demand by alternating between periods of liquid storage and liquid assist by exchanging "cold" between at least two different cryogenic liquids, e.g. independently from the main heat exchange line of the air fractionation and/or by using external alternating storage systems
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    • F25J3/04503—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using the cold from cryogenic liquids produced within the air fractionation unit and stored in internal or intermediate storages for compensating variable air feed or variable product demand by alternating between periods of liquid storage and liquid assist by exchanging "cold" between at least two different cryogenic liquids, e.g. independently from the main heat exchange line of the air fractionation and/or by using external alternating storage systems
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    • F25J3/04533—Integration with an oxygen consuming unit, e.g. glass facility, waste incineration or oxygen based processes in general for the direct combustion of fuels in a power plant, so-called "oxyfuel combustion"
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/40—Processes or apparatus involving steps for increasing the pressure of gaseous process streams the fluid being air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2250/00—Details related to the use of reboiler-condensers
    • F25J2250/30—External or auxiliary boiler-condenser in general, e.g. without a specified fluid or one fluid is not a primary air component or an intermediate fluid
    • F25J2250/50—One fluid being oxygen

Definitions

  • the present invention relates to a cryogenic air separation process. All pressures listed in the document are absolute pressures. Because of the global warming effect caused by the increased release of CO 2 (carbon dioxide) generated by combustion processes, efforts have been made by utility companies and governments worldwide to reduce and minimize the CO2 emission.
  • One major source of CO 2 emission is the power generation plant's combustion process. There are mainly two types of power plants based on combustion processes: coal combustion and natural gas combustion. Both of these processes produce CO 2 when generating power.
  • the most efficient approach to reduce or minimize the CO 2 emission is to capture most of the CO2 emitted by the power plants. For this effort to be efficient, it must also target the existing coal combustion plants that represent a large portion of the power generation plants worldwide.
  • the oxy- combustion technique is quite advantageous since it can be adapted to existing facilities as well.
  • a traditional power plant uses air as the source of oxidant (oxygen) to combust the coal.
  • Steam is generated by heating boiler feed water with the hot combustion products (steam, CO 2 , N 2 , excess air).
  • the steam is then expanded in steam turbines to produce power.
  • the combustion produces CO 2 as a by-product, which, along with other gases such as residual nitrogen of combustion air, moisture etc., constitutes the flue gas of the combustion. Due to the high content of nitrogen in air (78 mol%), the CO 2 is very much diluted in the flue gas. To insure full combustion, the power plants must also run with an excess air ratio, which further dilutes the CO 2 .
  • the concentration of CO 2 in the flue gas of an air combustion plant is only about 10-20 mol%.
  • the diluted composition of CO 2 increases the size and the power consumption of the CO 2 recovery unit. Because of this dilution, it becomes very costly and difficult to recover and capture the CO2 especially with the low pressure of the flue gas. Therefore, it is desirable to produce more concentrated CO 2 in flue gas, about 95 mol% of CO2 purity is preferred, to minimize the abatement cost.
  • An alternative technology for CO2 recovery from flue gas utilizes an amine contact tower to scrub out the CO 2 . However, significant amount of heat is needed to regenerate the amine and to extract the CO 2 such that the amine process is not cost effective.
  • the flue gas rich in CO 2 exiting the boiler is cooled and treated to recover the CO 2 for subsequent disposal.
  • the potential main users of oxycombustion technology are existing pulverized coal power plants since an oxygen plant, a CO 2 recycle blower and a CO2 recovery from flue gas can be added to the existing plant to retrofit it so that the converted plant can comply with new CO 2 emission standard. New grass root plants are likely to base on cleaner IGCC.
  • For retrofitted oxycombustion coal plants it is clear that the effort to capture CO 2 is hindered by the cost of the oxygen plant.
  • the power consumption of the oxygen plant which can be about 10% of the power plant output, also introduces additional cost issues: part of the power generated by the power plant must be diverted to supply the oxygen plant.
  • the Integrated Gasification Combined Cycle is a new highly efficient power plant wherein, instead of performing the direct combustion of coal to generate hot flue gas for steam generation, the coal is subjected to a partial oxidation process in which it is gasified to yield a mixture containing mostly of H 2 and CO called fuel gas.
  • This fuel gas after being treated to remove various pollutants or corrosive chemicals, is sent to a gas turbine where it is combusted to heat the compressed feed air prior to the expansion. Pure oxygen of about 95% supplied by an air separation unit is used in the partial oxidation reaction of the coal gasifier.
  • the resulting CO 2 is also mixed with the nitrogen of the gas turbine feed air such that the CO 2 recovery is also a costly and difficult task.
  • the fuel gas is subjected to a shift conversion wherein the CO reacts with steam to produce H 2 and CO 2 .
  • the CO 2 can then be recovered economically by scrubbing with a solvent like in the Rectisol process.
  • the fuel gas free of CO and CO 2 and containing mostly H 2 and steam is then burned in the gas turbine to yield an almost CO 2 -free exhaust gas.
  • the oxygen plant is usually needed for the partial oxidation portion, regardless of the need of the CO2 capture.
  • This invention addresses the potential savings in power and cost of an oxygen plant integrated with an oxycombustion power plant for CO 2 capture or an IGCC plant.
  • Power plants supply electricity to the grid. It is well known that power demand varies during the day, there are “peak” periods of high demand, hence high power cost, and there are “off-peak” periods of low demand, low power cost. Peaks usually occur in the daytime of the weekdays, for example, from 9 AM to 5 PM. Off-peaks usually take place at nighttime for example from 9 PM to 5 AM and week ends. There are also some intermediate demands and costs. The duration of peaks and off-peaks also depends on the seasons, the variations of local temperature, the weather changes etc.
  • Off-peak demand can be as low as 15-20% of the rated capacity. Power cost is high during peaks and sometimes the utility companies must purchase additional power from other suppliers to satisfy demand. The situation is reversed for off-peaks: power supply is abundant but demand drops sharply such that the power generating equipment must be turned down to minimum and sometimes shut down. Utility companies encourage users to consume more power during off-peaks to avoid costly equipment shutdown by lowering the power cost sharply for off-peaks.
  • the power cost for peak or high demand periods can be 3 to 5 times higher than the power cost of the off-peak or low demand periods.
  • an oxygen plant to the power generation plant worsens the power cost structure especially for peak periods. Indeed, the oxygen plant must generate maximum oxygen flow to match with the maximum power output; its power consumption is therefore at the highest during peaks. This additional power consumption is quite costly because it deprives the utility companies from having the available kW to sell on the grid at the premium value. As an indication, an oxygen plant consumes as much as 10% of the power output of a power plant. During off-peaks, the power output is at the minimum level; the consumption of the oxygen plant is also at its lowest level and cannot take advantage of the lower power cost.
  • an oxygen production process capable of tracking economically the demand curve of a power generation plant such that: the oxygen plant can minimize its power consumption during peaks while maintaining its supply of oxygen to the power plants at the rated level. This reduction in power consumption will free up more kW for the grid.
  • the oxygen plant can maximize its power consumption during off-peaks to take advantage of the lower power cost while remaining capable of supplying oxygen at reduced level. This high power consumption creates a power demand and keeps the power generating equipment running above it minimum rate, thus potentially avoid costly equipment shutdown.
  • a cryogenic process of supplying oxygen to a power generation plant comprising at least an air separation unit, a liquid oxygen tank, and an air derived component liquid tank, said process comprising: a. During a first period: i) feeding a first air stream to the air separation unit at a first flowrate; ii) feeding liquid oxygen from the liquid oxygen tank to the air separation unit; iii) recovering a gaseous oxygen stream with a higher flow than the liquid oxygen stream from the air separation unit; and iv) sending at least one air derived component liquid to at least one air derived component liquid tank.
  • the air separation unit produces substantially the same flowrate of gaseous oxygen during the first and second periods.
  • the air separation unit produces a higher flowrate of gaseous oxygen during the first period than in the second period.
  • the power costs are average during the second period and below average during a third period, wherein during the third period, the process includes: i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit; ii) extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank; iii) recovering a gaseous oxygen stream from the air separation unit; iv) increasing the flowrate of the first air stream feeding the air separation unit to a value greater than its flowrate in the first period; and v) wherein the flowrates of the first air stream in the second period and the third period are substantially equal.
  • the power costs of the first period are higher than average.
  • the power costs of the second period are average or lower than average.
  • the power costs of the first period are higher than average and the power costs of the second period are average or lower than average.
  • the power demand of the first period is higher than average.
  • the power demand of the second period is average or lower than average.
  • the power demand of the first period is higher than average and the power demand of the second period is average or lower than average.
  • the power generation plant is an oxycombustion plant.
  • the power generation plant is an IGCC plant.
  • At least one air derived component liquid is liquid nitrogen and wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
  • At least one air derived component liquid contains 80 mol% nitrogen or greater.
  • At least one air derived component liquid is liquid air. At least one air derived component liquid contains 35 mol% oxygen or greater wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
  • Figure 1 represents the Prior art approach in which the air separation unit simply supplies the oxygen to the power plant.
  • Figures 2 and 3 represent an air separation unit operating according to the invention at different periods
  • Figures 4 and 5 represent different phases of operation of plant according to the prior art
  • Figures 6, 7, 8 and 9 represent air separation units capable of operating according to the invention.
  • ASU Air Separation Unit
  • the performance of an Air Separation Unit can be approximated by a simple oxygen recovery ratio of about 20%: for 1000 Nm 3 /h feed air to the ASU, the corresponding recovery rate of oxygen is 200 Nm 3 /h.
  • the power consumption of the oxygen plant is mainly the power consumption of the air feed compressor, hence the air feed flow.
  • the prior art process consists of a basic Air Separation Unit, its oxygen production output is adjusted simply by adjusting the air feed flow to the unit. As an approximation, the power consumption is assumed to be proportional to the feed air flow.
  • the plant size for the off- peaks in this example must be increased by 20%.
  • the saving achieved is still significant at 10% and the ASU power can be cut back by 40% during the periods of high demand.
  • the air flow can be kept constant and the oxygen demand can vary during the peaks and off-peaks. This strategy results in 25% reduction in plant size while preserving a good 10% reduction in power cost.
  • liquid nitrogen as a means to transfer and store the refrigeration during periods of peaks and off-peaks.
  • the process can be applicable to a liquid of another composition derived from air such as liquid air, a liquid rich in oxygen (greater than 35% O 2 ) or a liquid rich in nitrogen (greater than 80% N 2 ).
  • Two or more liquid streams can also be used if needed, for example, during peaks, liquid oxygen is fed and vaporized in the ASU, a stream of liquid N 2 and a stream of liquid air can be extracted from the ASU to compensate for the refrigeration.
  • basiccule is used to describe the cryogenic air separation process in which, in one phase, a first liquid stream is used to liquefy an oxygen stream.
  • liquid oxygen produced is then fed to the process to allow extraction and restoration of the first liquid stream. Since the process simply exchanges refrigeration between liquid oxygen and the first liquid stream, it does not require power intensive equipment to liquefy a gaseous stream like in traditional liquefaction equipment.
  • the ASU can increase the air feed to restore the liquid oxygen inventory by re-feeding the liquid nitrogen produced in the high demand periods back into the system.
  • the higher feed air coupled with low power cost can provide an added advantage: some small amount of liquid can be extracted from the cryogenic cold box of the oxygen plant with almost no power or cost penalty, for example by simply increasing the flow of a cold box's expander. This additional liquid can be fed back to the cold box during peaks, reducing the need to operate the cold box's expander(s) during peaks thus increasing the efficiency and ability of the system to better track the demand.
  • the cryogenic oxygen plant may be equipped with cold compression equipment, which consumes refrigeration.
  • Oxygen plants equipped with "bascule" features have been utilized in the industry for some time. However, this usage has been limited to the tracking of the usage demand of the clients of the oxygen plant, independently of the power demand of the utility companies like the object of this new invention.
  • the bascule has been applied previously to the client side of electricity business, however this new invention addresses the integration of the bascule oxygen plant to the supply side, and in particular to the power generation aspect created by the need of oxycombustion or the partial oxidation requirement of IGCC plants.
  • the economics of an air separation plant can also be improved by liquefying a first liquid stream during the off-peak periods when power cost is low.
  • the liquid is then vaporized in the air separation unit (ASU) allowing reducing the air flow to minimize the power.
  • ASU air separation unit
  • the basic difference between this technique and the present invention is that the excess of refrigeration produce by vaporizing the liquid is mainly used to compress a cold gas stream of the ASU at cryogenic temperature to higher pressure, and not to recover an equivalent liquid flow for subsequent use.
  • Power intensive liquefaction equipment such as high pressure compressors and additional gas expanders must be provided to run the liquefaction unit during off hours.
  • the liquefaction equipment can be integrated with the oxygen plant. This prior art is illustrated schematically in figures 4 and 5.
  • Figure 4 shows the high power demand phase wherein nitrogen compressed in a cold compressor 16 is sent to the power plant's gas turbine, a portion of this cold compressed nitrogen can be optionally recycled to the air separation unit 13 to improve the distillation or to vaporize liquid oxygen, and liquid air from storage tank 18 is sent to the air separation unit.
  • Figure 5 shows the low power demand phase wherein air is sent via a warm compressor to the power generation plant 10. Liquid air formed in liquefier 14 is sent to liquid air storage tank 18.
  • the concept of this invention can use a combination of both techniques: a bascule feature and some ability to generate additional liquid during low demand and low power cost.
  • the added liquid can, for example, be fed back to the system during peaks to enable an economical cold compression of gaseous nitrogen to higher pressure for the IGCCs gas turbine injection to lower the compression power requirement in the peak demand period.
  • Figure 6 shows an air separation unit capable of operating according to the invention.
  • the plant uses a double column with a medium pressure column 9 operating at around 3.6 bar to 4.0 bar and a low pressure column in a dual reboiler configuration operating at around 1.32 bar.
  • Oxygen enriched liquid 19 is sent from the medium pressure column 9 to column 11.
  • Medium pressure nitrogen is used to reboil condenser 21 at an intermediate location between oxygen enriched feed and the bottom of column 11.
  • Part of the medium pressure nitrogen 25 is compressed by motor- driven compressor 27 and used to reboil condenser 29 at the bottom of column 11.
  • the liquid formed is expanded in valve 22 and sent back to the top of the column 9.
  • the condensed medium pressure nitrogen is used as reflux 35 for column 9, reflux 51 for column 11 and feed 49 for nitrogen tank 17.
  • a stream of nitrogen 37 at the pressure of column 9 is sent to the exchanger 7 where it warms and is then sent to turbine expander 39 where it is expanded and then fully warmed in the exchanger 7 to form waste stream 43.
  • Product oxygen 45 is withdrawn as a gas from a section between the two reboilers 21 , 29.
  • Low pressure nitrogen is warmed in exchanger 7 and exits as stream 47.
  • the expander 39 does not function or sees its flow sharply reduced.
  • Liquid nitrogen is sent to the tank 17 as stream 49 and liquid oxygen 53 is sent from tank 15 to the bottom of column 11 wherein it vaporizes.
  • the air flow is reduced by reducing the flow of compressor 1. Because of the wide flow fluctuations of the nitrogen expander in various modes, it is not practical to use the power generated by the nitrogen expander to drive the cold compressor 27. Indeed, in peak mode, the duty required by the cold compressor is very high to vaporize maximum flow of oxygen, meanwhile the flow of the expander is sharply reduced or even zero such that there is not sufficient power of the expander to drive any equipment. Therefore an electric motor is a proper choice to drive the cold compressor. During low power demand, the expander 39 functions at or near its peak. Liquid nitrogen is sent from the tank 17 to section 11 as stream 49 and liquid oxygen 53 is sent to tank 15 from section 31.
  • the process according to the invention could of course be operated using other types of apparatus, for example that of Figure 7 wherein the oxygen from section 31 is optionally pumped by pump 32 and then vaporized in an external exchanger 50.
  • Part of the cold compressed medium pressure nitrogen 55 condenses in exchanger 50 to provide the necessary heat for vaporization of oxygen.
  • the cold compressor 27 provides the pressurized nitrogen needed for condensation in both exchangers 50 and 29.
  • FIG. 8 Another embodiment is shown in Figure 8: air to vaporize the oxygen is produced by booster 8, which compresses about a quarter of the feed air flow. This air condenses in exchanger 7 against the vaporizing liquid oxygen of stream 45 extracted from the bottom of the column.
  • Liquid oxygen from storage 15 can be fed to the column 11 or directly an external vaporizer without passing to the column. It can also be vaporized in the exchanger 50, 7 or another exchanger and the resulting gaseous oxygen is mixed with gaseous oxygen produced by the column.
  • Figure 9 shows a variant of Figure 7 in which all the air is compressed to a single pressure in compressor 1 , purified in purification unit 5 and sent to the column 9 as gaseous stream 6. The rest of the figure is as in Figure 7 except that there is no longer a liquid air reflux stream sent from column 9 to column 11.
  • the apparatus uses a single turbine, that turbine being a high pressure nitrogen turbine.

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Abstract

A cryogenic process of supplying oxygen to a power generation plant including at least an air separation unit(9,ll), a liquid oxygen tank(15) and an air derived component liquid tank(17), comprises: During a first period: feeding a first air stream to the air separation unit at a first flowrate, feeding liquid oxygen (53) from the liquid oxygen tank (15) to the air separation unit (11), recovering a gaseous oxygen stream(45) with a higher flow than the liquid oxygen stream (53) from the air separation unit, sending at least one air derived component liquid (49) to at least one air derived component liquid tank (17). During a second period: feeding the at least one air derived component liquid stream(49) from the at least one air component liquid tank(17) to the air separation unit(ll), extracting a liquid oxygen stream(53) from the air separation unit (11) to the liquid oxygen tank, recovering a gaseous oxygen stream (45) from the air separation unit and increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.

Description

CRYOGENIC AIR SEPARATION PROCESS
Background
The present invention relates to a cryogenic air separation process. All pressures listed in the document are absolute pressures. Because of the global warming effect caused by the increased release of CO2 (carbon dioxide) generated by combustion processes, efforts have been made by utility companies and governments worldwide to reduce and minimize the CO2 emission. One major source of CO2 emission is the power generation plant's combustion process. There are mainly two types of power plants based on combustion processes: coal combustion and natural gas combustion. Both of these processes produce CO2 when generating power. The most efficient approach to reduce or minimize the CO2 emission is to capture most of the CO2 emitted by the power plants. For this effort to be efficient, it must also target the existing coal combustion plants that represent a large portion of the power generation plants worldwide. The oxy- combustion technique is quite advantageous since it can be adapted to existing facilities as well.
A traditional power plant uses air as the source of oxidant (oxygen) to combust the coal. Steam is generated by heating boiler feed water with the hot combustion products (steam, CO2, N2, excess air). The steam is then expanded in steam turbines to produce power. The combustion produces CO2 as a by-product, which, along with other gases such as residual nitrogen of combustion air, moisture etc., constitutes the flue gas of the combustion. Due to the high content of nitrogen in air (78 mol%), the CO2 is very much diluted in the flue gas. To insure full combustion, the power plants must also run with an excess air ratio, which further dilutes the CO2. The concentration of CO2 in the flue gas of an air combustion plant is only about 10-20 mol%. The diluted composition of CO2 increases the size and the power consumption of the CO2 recovery unit. Because of this dilution, it becomes very costly and difficult to recover and capture the CO2 especially with the low pressure of the flue gas. Therefore, it is desirable to produce more concentrated CO2 in flue gas, about 95 mol% of CO2 purity is preferred, to minimize the abatement cost. An alternative technology for CO2 recovery from flue gas utilizes an amine contact tower to scrub out the CO2. However, significant amount of heat is needed to regenerate the amine and to extract the CO2 such that the amine process is not cost effective.
In order to avoid the dilution of CO2 in N2, the power generation industry can switch to an oxy-combustion process: instead of utilizing air as oxidant, pure oxygen of 95% purity or better is used in the combustion process. The combustion heat is dissipated in a recycled flue gas concentrated in CO2. By doing so, since there is very small amount or almost no nitrogen in the system, it becomes possible to achieve a flue gas containing about 75 - 95 mol% CO2, which is a significant improvement over the previous 20 mol% of air combustion. The purity of CO2 in oxy- combustion's flue gas depends on the amount of air leakage into the system and the purity of oxygen being utilized. An air separation unit normally supplies the pure oxygen for combustion. The flue gas rich in CO2 exiting the boiler is cooled and treated to recover the CO2 for subsequent disposal. The potential main users of oxycombustion technology are existing pulverized coal power plants since an oxygen plant, a CO2 recycle blower and a CO2 recovery from flue gas can be added to the existing plant to retrofit it so that the converted plant can comply with new CO2 emission standard. New grass root plants are likely to base on cleaner IGCC. For retrofitted oxycombustion coal plants, it is clear that the effort to capture CO2 is hindered by the cost of the oxygen plant. Furthermore, the power consumption of the oxygen plant, which can be about 10% of the power plant output, also introduces additional cost issues: part of the power generated by the power plant must be diverted to supply the oxygen plant. Therefore, less power will be available to supply the grid, especially during peak demand when power is scarce and power costs are premium, resulting in reduction of power plant's revenue. In this situation, the economics of CO2 capture, and disposal by oxycombustion technique, depend strongly on the cost and power consumption of the oxygen plant. Without an efficient setup for the oxygen generation, the cost penalty would be such that it would become uneconomical to operate the clean and Cθ2-free oxycombustion power plants.
In order to optimize the oxygen supply for oxycombustion scheme, several studies conducted by the power industry with the co-operation of various oxygen suppliers have concluded that a low purity oxygen (about 95 mol%) is sufficient for oxycombustion and provides a low cost, low power consumption oxygen plant. This takes into account the impact of the purity of CO2 in flue gas caused by the oxygen purity on the subsequent CO2 concentrating and purification equipment. However further cost reduction of the cost of oxygen is needed to improve the economics of the CO2 capture. The Integrated Gasification Combined Cycle (IGCC) is a new highly efficient power plant wherein, instead of performing the direct combustion of coal to generate hot flue gas for steam generation, the coal is subjected to a partial oxidation process in which it is gasified to yield a mixture containing mostly of H2 and CO called fuel gas. This fuel gas, after being treated to remove various pollutants or corrosive chemicals, is sent to a gas turbine where it is combusted to heat the compressed feed air prior to the expansion. Pure oxygen of about 95% supplied by an air separation unit is used in the partial oxidation reaction of the coal gasifier. Since the CO of the fuel gas is combusted to yield CO2 in the burner of the gas turbine, the resulting CO2 is also mixed with the nitrogen of the gas turbine feed air such that the CO2 recovery is also a costly and difficult task. To avoid difficult CO2 recovery due to a high flow, low pressure and much diluted CO2 of the gas turbine, the fuel gas is subjected to a shift conversion wherein the CO reacts with steam to produce H2 and CO2. The CO2 can then be recovered economically by scrubbing with a solvent like in the Rectisol process. The fuel gas free of CO and CO2 and containing mostly H2 and steam is then burned in the gas turbine to yield an almost CO2-free exhaust gas. In IGCC facilities, the oxygen plant is usually needed for the partial oxidation portion, regardless of the need of the CO2 capture.
It can be seen from the above simple process description that pure oxygen gas, supplied by an air separation plant, is used in either the direct combustion of an oxycombustion or the partial oxidation of an IGCC process. The production of oxygen requires additional capital investment and consumes significant power to drive the compression equipment. It is obvious that the power consumption of the oxygen plant and the oxygen plant cost must be optimized to reduce the impact of CO2 capture on the final cost of electricity. In addition to the oxygen plant, the CO2 recovery from flue gas of an oxycombustion or from the fuel gas of an IGCC also consumes power and requires significant investment since the CO2 must be further concentrated to about 95 mol% and then compressed to about 100 bar or higher for disposal.
This invention addresses the potential savings in power and cost of an oxygen plant integrated with an oxycombustion power plant for CO2 capture or an IGCC plant.
Power plants supply electricity to the grid. It is well known that power demand varies during the day, there are "peak" periods of high demand, hence high power cost, and there are "off-peak" periods of low demand, low power cost. Peaks usually occur in the daytime of the weekdays, for example, from 9 AM to 5 PM. Off-peaks usually take place at nighttime for example from 9 PM to 5 AM and week ends. There are also some intermediate demands and costs. The duration of peaks and off-peaks also depends on the seasons, the variations of local temperature, the weather changes etc.
Because of the variable demand, power plants usually run at or near its design capacity during peaks, but must idle at very low output during off- peaks. Off-peak demand can be as low as 15-20% of the rated capacity. Power cost is high during peaks and sometimes the utility companies must purchase additional power from other suppliers to satisfy demand. The situation is reversed for off-peaks: power supply is abundant but demand drops sharply such that the power generating equipment must be turned down to minimum and sometimes shut down. Utility companies encourage users to consume more power during off-peaks to avoid costly equipment shutdown by lowering the power cost sharply for off-peaks. The power cost for peak or high demand periods can be 3 to 5 times higher than the power cost of the off-peak or low demand periods.
The addition of an oxygen plant to the power generation plant worsens the power cost structure especially for peak periods. Indeed, the oxygen plant must generate maximum oxygen flow to match with the maximum power output; its power consumption is therefore at the highest during peaks. This additional power consumption is quite costly because it deprives the utility companies from having the available kW to sell on the grid at the premium value. As an indication, an oxygen plant consumes as much as 10% of the power output of a power plant. During off-peaks, the power output is at the minimum level; the consumption of the oxygen plant is also at its lowest level and cannot take advantage of the lower power cost.
Therefore, there exists a need for an oxygen production process capable of tracking economically the demand curve of a power generation plant such that: the oxygen plant can minimize its power consumption during peaks while maintaining its supply of oxygen to the power plants at the rated level. This reduction in power consumption will free up more kW for the grid. The oxygen plant can maximize its power consumption during off-peaks to take advantage of the lower power cost while remaining capable of supplying oxygen at reduced level. This high power consumption creates a power demand and keeps the power generating equipment running above it minimum rate, thus potentially avoid costly equipment shutdown.
Summary of the Invention
According to this invention, there is provided a cryogenic process of supplying oxygen to a power generation plant comprising at least an air separation unit, a liquid oxygen tank, and an air derived component liquid tank, said process comprising: a. During a first period: i) feeding a first air stream to the air separation unit at a first flowrate; ii) feeding liquid oxygen from the liquid oxygen tank to the air separation unit; iii) recovering a gaseous oxygen stream with a higher flow than the liquid oxygen stream from the air separation unit; and iv) sending at least one air derived component liquid to at least one air derived component liquid tank. b. During a second period: i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit; ii) extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank; iii) recovering a gaseous oxygen stream from the air separation unit; and iv) increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.
According to further optional features:
- The air separation unit produces substantially the same flowrate of gaseous oxygen during the first and second periods.
- The air separation unit produces a higher flowrate of gaseous oxygen during the first period than in the second period.
- The power costs are average during the second period and below average during a third period, wherein during the third period, the process includes: i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit; ii) extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank; iii) recovering a gaseous oxygen stream from the air separation unit; iv) increasing the flowrate of the first air stream feeding the air separation unit to a value greater than its flowrate in the first period; and v) wherein the flowrates of the first air stream in the second period and the third period are substantially equal.
The power costs of the first period are higher than average. - The power costs of the second period are average or lower than average.
The power costs of the first period are higher than average and the power costs of the second period are average or lower than average.
The power demand of the first period is higher than average. - The power demand of the second period is average or lower than average.
The power demand of the first period is higher than average and the power demand of the second period is average or lower than average.
The power generation plant is an oxycombustion plant. - The power generation plant is an IGCC plant.
At least one air derived component liquid is liquid nitrogen and wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
At least one air derived component liquid contains 80 mol% nitrogen or greater.
At least one air derived component liquid is liquid air. At least one air derived component liquid contains 35 mol% oxygen or greater wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
Brief Description of the Invention
Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The invention will be described in greater detail with reference to the figures, wherein Figure 1 represents the Prior art approach in which the air separation unit simply supplies the oxygen to the power plant. Figures 2 and 3 represent an air separation unit operating according to the invention at different periods, Figures 4 and 5 represent different phases of operation of plant according to the prior art and Figures 6, 7, 8 and 9 represent air separation units capable of operating according to the invention.
As shown on Figure 1 for the Prior art, during peaks in power demand, 1000 Nm3/h of feed air 6 is treated to yield 200 Nm3/h of oxygen 12 required for peak demand by power generation plant 10. If the demand is reduced, less air is sent to the oxygen plant 13 to yield less oxygen. The air flow is essentially proportional to the oxygen demand.
As shown on Figure 2 for the bascule approach, during peaks in power demand, 1 10 Nm3/h of liquid oxygen 53 from a liquid oxygen tank 15 is fed to the oxygen plant 13, vaporized and combined with the oxygen produced from the feed air to yield 200 Nm3/h of a high oxygen stream 12 required for peak demand by the power generation plant 10. The recovered refrigeration from the vaporization of liquid oxygen is used to liquefy liquid nitrogen 49 and to store it in a liquid nitrogen tank 17. Since a portion of the oxygen is provided by vaporizing the liquid oxygen, the air flow 6 to the oxygen plant can be reduced by about the same proportion to 450 Nm3/h resulting in significant power reduction while maintaining the total rated flow of oxygen to satisfy the peak demand.
As shown in Figure 3, during average periods, 900 Nm3/h of feed air 6 are sent from compressor 1 to the oxygen plant 13 to produce 150 Nm3/h of oxygen 45. 30 Nm3/h of liquid oxygen are sent from the oxygen plant to the oxygen tank 15 whilst liquid nitrogen 49 is sent from the nitrogen tank 17 to the oxygen plant.
As shown in Figure 3, during off-peaks, instead of being reduced, the air flow 6 is at 900 Nm3/h to produce more oxygen than the demand. 80 Nm3/h of the excess gaseous oxygen is liquefied by feeding and vaporizing the liquid nitrogen 49 produced during peaks. The produced liquid oxygen 53 is then stored in the liquid oxygen tank 15 to restore the oxygen inventory which will be needed in the subsequent peak periods. In order to illustrate this concept, a simple model of power demand (or generation rate) can be used: Power generation:
Figure imgf000012_0001
The performance of an Air Separation Unit (ASU) can be approximated by a simple oxygen recovery ratio of about 20%: for 1000 Nm3/h feed air to the ASU, the corresponding recovery rate of oxygen is 200 Nm3/h. For low purity, low pressure oxygen for oxycombustion application, the power consumption of the oxygen plant is mainly the power consumption of the air feed compressor, hence the air feed flow.
The prior art process consists of a basic Air Separation Unit, its oxygen production output is adjusted simply by adjusting the air feed flow to the unit. As an approximation, the power consumption is assumed to be proportional to the feed air flow.
Let us compare the air feed rate of this new process with the prior art process:
Figure imgf000013_0001
Note: Assuming the air compressor can be turned down to 50% (for example by using 2 compressors in parallel) It can be seen from above table that: a) The maximum air flow of the new ASU is 90% of the maximum air flow of the prior art. This represents smaller equipment and a reduction of plant cost. b) The cost of power to operate the new process is reduced by about 19% based on the above model. This is a significant cost reduction.
The economics of oxycombustion and CO2 capture in particular can therefore be improved. c) During peak periods, the power consumption of the oxygen plant is sharply reduced by 55%; this represents an important availability of power to supply the demand of the grid. If demand cannot be satisfied, utility companies usually have to purchase additional power from another network at a very high cost. This reduction of consumption of the oxygen during peaks can alleviate the situation and will result in major savings for utility companies. The concept can also be applicable to situations where the demand remains constant throughout the high demand periods (highest power cost) or low demand periods (lowest power cost). In this situation, during high demand, the air flow to the ASU is reduced to the limit of machinery's turndown to minimize its power consumption. During low demand, the air flow is increased not only to satisfy the demand but also to produce liquid oxygen to be vaporized during the periods when power cost is high.
Figure imgf000014_0001
Since the oxygen plant must supply a constant oxygen flow and the additional oxygen liquid for the high demand periods, the plant size for the off- peaks in this example must be increased by 20%. However, the saving achieved is still significant at 10% and the ASU power can be cut back by 40% during the periods of high demand.
This new process can be used to minimize the plant cost and still provides significant power cost saving. Indeed, in the above example of Case 1 we can arrange to have the following configuration:
Figure imgf000015_0001
As can be seen, the air flow can be kept constant and the oxygen demand can vary during the peaks and off-peaks. This strategy results in 25% reduction in plant size while preserving a good 10% reduction in power cost.
A similar approach can be used to estimate the savings in some other models and the concept appears to be advantageous in most situations.
It is useful to note that by liquefying liquid nitrogen when liquid oxygen is vaporized, or vice versa by liquefying liquid oxygen when liquid nitrogen is vaporized, we can recover and store the refrigeration under the liquid form such that there is no major power expenditure to liquefy these important amounts of liquid involved in the transfers.
The above example use liquid nitrogen as a means to transfer and store the refrigeration during periods of peaks and off-peaks. The process can be applicable to a liquid of another composition derived from air such as liquid air, a liquid rich in oxygen (greater than 35% O2) or a liquid rich in nitrogen (greater than 80% N2). Two or more liquid streams can also be used if needed, for example, during peaks, liquid oxygen is fed and vaporized in the ASU, a stream of liquid N2 and a stream of liquid air can be extracted from the ASU to compensate for the refrigeration. The term "bascule" is used to describe the cryogenic air separation process in which, in one phase, a first liquid stream is used to liquefy an oxygen stream. In a next phase, liquid oxygen produced is then fed to the process to allow extraction and restoration of the first liquid stream. Since the process simply exchanges refrigeration between liquid oxygen and the first liquid stream, it does not require power intensive equipment to liquefy a gaseous stream like in traditional liquefaction equipment.
In the new invention, during low demand periods, the ASU can increase the air feed to restore the liquid oxygen inventory by re-feeding the liquid nitrogen produced in the high demand periods back into the system. The higher feed air coupled with low power cost can provide an added advantage: some small amount of liquid can be extracted from the cryogenic cold box of the oxygen plant with almost no power or cost penalty, for example by simply increasing the flow of a cold box's expander. This additional liquid can be fed back to the cold box during peaks, reducing the need to operate the cold box's expander(s) during peaks thus increasing the efficiency and ability of the system to better track the demand. The cryogenic oxygen plant may be equipped with cold compression equipment, which consumes refrigeration. Such small amount of additional liquid generated inexpensively during off-peaks, coupled with the liquid resulting from refrigeration exchange of the bascule, can improve the cold requirement of the system during peaks. Therefore, the cold box's expander(s) can be throttled or even shut down to further cut back the air flow and still be able to maintain good efficiency of the distillation columns and satisfy the refrigeration need of cold compression equipment, thus increasing the saving of the bascule.
Oxygen plants equipped with "bascule" features have been utilized in the industry for some time. However, this usage has been limited to the tracking of the usage demand of the clients of the oxygen plant, independently of the power demand of the utility companies like the object of this new invention. In another word, the bascule has been applied previously to the client side of electricity business, however this new invention addresses the integration of the bascule oxygen plant to the supply side, and in particular to the power generation aspect created by the need of oxycombustion or the partial oxidation requirement of IGCC plants.
In prior art, the economics of an air separation plant can also be improved by liquefying a first liquid stream during the off-peak periods when power cost is low. When power cost is high, the liquid is then vaporized in the air separation unit (ASU) allowing reducing the air flow to minimize the power. The basic difference between this technique and the present invention is that the excess of refrigeration produce by vaporizing the liquid is mainly used to compress a cold gas stream of the ASU at cryogenic temperature to higher pressure, and not to recover an equivalent liquid flow for subsequent use. Power intensive liquefaction equipment such as high pressure compressors and additional gas expanders must be provided to run the liquefaction unit during off hours. The liquefaction equipment can be integrated with the oxygen plant. This prior art is illustrated schematically in figures 4 and 5.
Figure 4 shows the high power demand phase wherein nitrogen compressed in a cold compressor 16 is sent to the power plant's gas turbine, a portion of this cold compressed nitrogen can be optionally recycled to the air separation unit 13 to improve the distillation or to vaporize liquid oxygen, and liquid air from storage tank 18 is sent to the air separation unit.
Figure 5 shows the low power demand phase wherein air is sent via a warm compressor to the power generation plant 10. Liquid air formed in liquefier 14 is sent to liquid air storage tank 18.
It is clear the concept of this invention can use a combination of both techniques: a bascule feature and some ability to generate additional liquid during low demand and low power cost. The added liquid can, for example, be fed back to the system during peaks to enable an economical cold compression of gaseous nitrogen to higher pressure for the IGCCs gas turbine injection to lower the compression power requirement in the peak demand period.
In summary, all power plants are subjected to daily usage variations and this variable characteristic can be utilized advantageously by the bascule approach of the new invention such that the cost of oxygen supply for oxycombustion power plants can be minimized. The concept is directly applicable to IGCC plants.
Figure 6 shows an air separation unit capable of operating according to the invention. The plant uses a double column with a medium pressure column 9 operating at around 3.6 bar to 4.0 bar and a low pressure column in a dual reboiler configuration operating at around 1.32 bar.
For a normal run, air is compressed in compressor 1 and purified in purification unit 5. The air is then cooled in exchanger 7 as stream 6 and sent in essentially gaseous form to column 9.
Oxygen enriched liquid 19 is sent from the medium pressure column 9 to column 11. Medium pressure nitrogen is used to reboil condenser 21 at an intermediate location between oxygen enriched feed and the bottom of column 11. Part of the medium pressure nitrogen 25 is compressed by motor- driven compressor 27 and used to reboil condenser 29 at the bottom of column 11. The liquid formed is expanded in valve 22 and sent back to the top of the column 9. The condensed medium pressure nitrogen is used as reflux 35 for column 9, reflux 51 for column 11 and feed 49 for nitrogen tank 17. A stream of nitrogen 37 at the pressure of column 9 is sent to the exchanger 7 where it warms and is then sent to turbine expander 39 where it is expanded and then fully warmed in the exchanger 7 to form waste stream 43. Product oxygen 45 is withdrawn as a gas from a section between the two reboilers 21 , 29. Low pressure nitrogen is warmed in exchanger 7 and exits as stream 47. During high power demand, the expander 39 does not function or sees its flow sharply reduced. Liquid nitrogen is sent to the tank 17 as stream 49 and liquid oxygen 53 is sent from tank 15 to the bottom of column 11 wherein it vaporizes.
The air flow is reduced by reducing the flow of compressor 1. Because of the wide flow fluctuations of the nitrogen expander in various modes, it is not practical to use the power generated by the nitrogen expander to drive the cold compressor 27. Indeed, in peak mode, the duty required by the cold compressor is very high to vaporize maximum flow of oxygen, meanwhile the flow of the expander is sharply reduced or even zero such that there is not sufficient power of the expander to drive any equipment. Therefore an electric motor is a proper choice to drive the cold compressor. During low power demand, the expander 39 functions at or near its peak. Liquid nitrogen is sent from the tank 17 to section 11 as stream 49 and liquid oxygen 53 is sent to tank 15 from section 31.
The process according to the invention could of course be operated using other types of apparatus, for example that of Figure 7 wherein the oxygen from section 31 is optionally pumped by pump 32 and then vaporized in an external exchanger 50. Part of the cold compressed medium pressure nitrogen 55 condenses in exchanger 50 to provide the necessary heat for vaporization of oxygen. In this arrangement, the cold compressor 27 provides the pressurized nitrogen needed for condensation in both exchangers 50 and 29. One can also opt to further compress stream 55 by another cold compressor (not shown) should the required pressure of oxygen stream 45 be higher.
Another embodiment is shown in Figure 8: air to vaporize the oxygen is produced by booster 8, which compresses about a quarter of the feed air flow. This air condenses in exchanger 7 against the vaporizing liquid oxygen of stream 45 extracted from the bottom of the column.
Liquid oxygen from storage 15 can be fed to the column 11 or directly an external vaporizer without passing to the column. It can also be vaporized in the exchanger 50, 7 or another exchanger and the resulting gaseous oxygen is mixed with gaseous oxygen produced by the column.
Figure 9 shows a variant of Figure 7 in which all the air is compressed to a single pressure in compressor 1 , purified in purification unit 5 and sent to the column 9 as gaseous stream 6. The rest of the figure is as in Figure 7 except that there is no longer a liquid air reflux stream sent from column 9 to column 11.
For all figures, the apparatus uses a single turbine, that turbine being a high pressure nitrogen turbine.

Claims

CLAIMS:
1. A cryogenic process of supplying oxygen to a power generation plant comprising at least an air separation unit, a liquid oxygen tank and an air derived component liquid tank, said process comprising: a. During a first period: i) feeding a first air stream to the air separation unit at a first flowrate; ii) feeding liquid oxygen from the liquid oxygen tank to the air separation unit; iii) recovering a gaseous oxygen stream with a higher flow than the liquid oxygen stream from the air separation unit; and iv) sending at least one air derived component liquid to at least one air derived component liquid tank. b) During a second period: i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit; ii) extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank; iii) recovering a gaseous oxygen stream from the air separation unit; and iv) increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.
2. Process according to Claim 1 wherein the air separation unit produces substantially the same flowrate of gaseous oxygen during the first and second periods.
3. Process according to Claim 1 wherein the air separation unit produces a higher flowrate of gaseous oxygen during the first period than in the second period.
4. Process according to Claim 1 wherein the power costs are average during the second period and below average during a third period, wherein during the third period, the process includes: i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit; ii) extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank; iii) recovering a gaseous oxygen stream from the air separation unit; iv) increasing the flowrate of the first air stream feeding the air separation unit to a value greater than its flowrate in the first period; and v) wherein the flowrates of the first air stream in the second period and the third period are substantially equal.
5. Process according to claim 1 wherein the power costs of the first period are higher than average.
6. Process according to claim 1 wherein the power costs of the second period are average or lower than average.
7. Process according to claim 1 wherein the power costs of the first period are higher than average and the power costs of the second period are average or lower than average.
8. Process according to claim 1 wherein the power demand of the first period is higher than average.
9. Process according to claim 1 wherein the power demand of the second period is average or lower than average.
10. Process according to claim 1 wherein the power demand of the first period is higher than average and the power demand of the second period is average or lower than average.
11. A process according to Claim 1 , in which the power generation plant is an oxycombustion plant.
12. A process according to Claim 1 , in which the power generation plant is an IGCC plant.
13. A process according to Claim 1 , in which at least one air derived component liquid is liquid nitrogen and wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
14. A process according to Claim 1 , in which at least one air derived component liquid contains 80 mol% nitrogen or greater.
15. A process according to Claim 1 , in which at least one air derived component liquid is liquid air.
16. A process according to Claim 1 , in which at least one air derived component liquid contains 35 mol% oxygen or greater wherein step iv) of period a) of Claim 1 comprises removing liquid nitrogen from a column of the air separation unit.
17. A process according to Claim 1 wherein in at least one of step a) ii) of Claim 1 the liquid oxygen is fed to a column of the air separation unit.
18. A process according to Claim 1 wherein in step a) ii) of Claim 1 the liquid oxygen is fed to an exchanger of the air separation unit without passing via a column of the air separation unit.
19. A process according to Claim 4 wherein in at least one of step i) of Claim 4 the liquid oxygen is fed to a column of the air separation unit.
20. A process according to Claim 4 wherein in step i) of Claim 4 the liquid oxygen is fed to an exchanger of the air separation unit without passing via a column of the air separation unit.
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