WO2001033150A1

WO2001033150A1 - Compressed air energy storage system with an air separation unit

Info

Publication number: WO2001033150A1
Application number: PCT/US2000/041602
Authority: WO
Inventors: Lindsay Leveen
Original assignee: Lectrix Llc
Priority date: 1999-11-03
Filing date: 2000-10-26
Publication date: 2001-05-10
Also published as: AU2618901A

Abstract

A system which is used to produce oxygen, nitrogen and/or argon in gas or liquefied form that uses the air separation unit (13) with a compressed air energy storage unit (3). This system allows the manufacture of such air separation gases to be performed with electric power purchased during off-peak or semi-peak periods when the unit cost of electric power is much less expensive. The compressed air is stored in above surface vessels, subterranean caverns or aquifers. In this system, the air compression unit (2) for the air separation unit (13) is shared with the compressed air energy storage unit (3). During periods of off-peak or semi-peak power costs, air is compressed and stored. A side stream of air from the air compressor (3) is also directly and continuously passed through the air separation unit (13).

Description

COMPRESSED AIR ENERGY STORAGE SYSTEM WITH AN AIR SEPARATION UNIT

Field of the Invention: The present invention relates in general to a method and apparatus to improve and optimize the product of air separation gases (oxygen, nitrogen, and/or argon) by a compressed air energy storage system and an air separation process.

Background of the Invention:

The manufacture of air separation gases via cryogenic, pressure or vacuum swing adsorption, or membrane separation is a well established enterprise. The technology that is mostly employed is that of cryogenic separation as this is the most economical method for large scale (tonnage) oxygen, nitrogen and argon manufacture. Examples of air separation units can be found in the following technical brochures and specifications of L'Air Liquide S.A. Engineering Division, Air Liquide Engineering: (1 ) Large Air Separation Units, 1995; (2) Compact VSA: Economical, Reliable and Versatile On-Site Oxygen Supply, Tech Spec: #PTS-OX2R1 , 3/96; (3) APSA Advanced Product Supply Approach, 1996; (4) On-site Nitrogen Membrane System, 1997; and (5) Liquid Air engineering Corporation. Innovation and Efficiency in Air and Gas Separation Plants; all of which are incorporated herein by reference.

All of above methods require compressed air as the source of the manufactured gas. In the case of cryogenic separation, the compressed air is passed through a cold box heat exchanger to recover refrigeration from product gases and then is introduced into a two stage distillation process (high pressure and low pressure) to cause the components of air to separate due to their varying volatility. In the case where oxygen gas is required as moderate to high pressures (400 to 600 psi) the manufacturing cycles that are used to perform the air separation employ incoming air at a pressure or 1.2 to 1.5 times the product pressure. Likewise there are cryogenic cycles to produce nitrogen at pressure that may also use elevated air pressure. In all events the lowest pressure for make up air that is employed is above 100 psi.

Membrane, vacuum swing adsorption, or pressure swing adsorption air separation processes require compressed air above a pressure of 50 psi to be passed through the air separation unit. These units do not require heat exchange as the separation is not performed at reduced temperature as is the case with cryogenic separation. The cost of producing oxygen, nitrogen and argon is split between the cost of power and the amortization of the capital needed for the plant. Labor, maintenance, cooling water, lubricants and consumables, insurance, property taxes make up the remaining costs which are small by comparison. A typical oxygen plant producing gas oxygen in tonnage quantities at 600 psi might require 450 KWh per ton to produce 99.5% pure oxygen. Nitrogen plants will require 225 KWh per ton to produce 99.9% pure nitrogen at 200 psi.

By way of example only, consider a 1000 ton per day oxygen plant producing oxygen at 600 psi. This plant will require approximately 18 MW of power. The capital cost of such a plant would be in the $20 million range. A plant producing this amount of gas oxygen is normally dedicated to supply, via pipeline, the oxygen requirements of a steel mill or petrochemical plant. The oxygen manufacturer, in order to assure constant supply, has to purchase firm electrical power in order to run the air separation plant continuously. Incremental production of liquefied products for the merchant market will be supplied with interruptible power. Such merchant plants are sized between 400 and 1000 tons per day of total liquid nitrogen, oxygen and argon production. To produce 1000 tons per day of liquid product after the air separation an additional a16 MW of power is typically needed.

The cost of electric power is by far the largest cost in the manufacture of air separation gases in either gaseous or liquid form, therefore manufacturers are always eager to obtain the lowest unit electrical cost and site their facilities accordingly in areas where competitive power can be obtained.

Summary of the Invention:

The present invention relates in general to a method and apparatus to optimize the production of air separation gases (oxygen, nitrogen and/or argon) with a compressed air energy storage system having an air separation system. The manufacture of oxygen, nitrogen and argon via air separation through cryogenic, pressure swing adsorption, or membrane separation is a well established enterprise. The technology that is mostly employed is that of cryogenic separation as this is the most economical method for large scale (tonnage) oxygen, nitrogen and argon manufacture. All of above methods require compressed air as the source of the manufactured gas. The typical cryogenic air separation process has to have a continuous stream of compressed air flowing into the separation unit as the unit operates at reduced temperature and cannot be run in an on/off mode. Membrane and pressure swing adsorption units also require continuous air flow to produce air separation gases on a continuous basis. Therefore, manufacturers of air separation gases run these units on a continuous basis. The compression of the air stream requires significant amounts of energy as air is compressed from ambient condition to pressures that range from 80 psi to 1000 psi. Electric power is the most common form of purchased energy to power the air compression in an air separation plant. Standard air separation plants, therefore, purchase electricity during all periods of the day and night and the power purchased is charged at rates that cover peak, semi-peak and off peak periods.

An embodiment of this invention allows the manufacture of such air separation gases to be performed with electric power purchased during off- peak or semi-peak periods when it the unit cost of electric power is much less expensive. The facility is a combination of a compressed air energy storage system and the air separation unit. Compressed air energy storage units compress air during periods of off-peak or semi-peak electric power costs. This compressed air is stored in above surface vessels, subterranean caverns or aquifers. In an embodiment of this invention, the air compression system for the air separation unit is shared with the compressed air energy storage unit and economies of scale are, therefore, gained. During periods of off-peak or semi-peak power costs, air is compressed by these air compressors and is introduced into the storage system. A side stream of air from the air compressors is also directly and continuously passed through the air separation unit. During on peak electric power periods the air compressors are turned off and air is extracted from the storage system and is flowed through the air separation system and/or a turbine expander that produces electricity for resale at a higher price. The air separation unit therefore has a uninterrupted flow of air, and an uninterrupted supply of low cost energy, and can produce air separation gases continuously. The reduction in the unit price paid for electric energy is significant and as the predominant operating cost to produce air separation gases is electric power, the gases are produced at a significantly lower cost than in the standard air separation process.

Accordingly, an object of the invention is an apparatus and method for producing air separation gases of oxygen, nitrogen and/or argon with a compressed air energy system having an air separation plant.

Another object of the invention is an apparatus and method for manufacturing air separation gases as by compressing denser air, thereby requiring less energy.

Another object of the invention is an apparatus and method for manufacturing air separation gases as air is compressed during off-peak and semi-peak periods, thereby requiring less energy at a reduced cost rate.

A further object of the invention is an apparatus and method for manufacturing air separation gases at a wider range of production rates as the method and apparatus are not limited by the "turn down" characteristics of the air compression system in a typical air separation plant.

Brief Description of the Figures:

FIG. 1 is a schematic flow chart of the process and apparatus for a compressed air energy storage with an air separation unit for making component gases of oxygen, nitrogen, and or argon.

Detailed Description of the Preferred Embodiment:

The present invention is based on a compressed air energy storage system with an air separation process, such as for example a cryogenic air separation unit, resulting in the optimized manufacture of the air separation gases of oxygen, nitrogen and or argon. Examples of compressed air energy storage systems can be found in the following: (1 ) Importance of Adequate Geotechnical Evaluation for CAES Siting in Aquifers, by Cole R. McClure and John H. Ross, May 1985, Bechtel Group, Inc.; (2) An Answer to Energy Storage, Dresser-Rand, Steam Turbine, Motor & Generator Division, 9303-CAES; (3) Why is our engineering generating so much interest?, Dresser-Rand, Steam Turbine, Motor & Generator Division; (4) Gas Turbine World, November-December 1991 , Vol.21 , No.6, AEC commissions the nation's first air energy storage plant, by Marc De Piolenc; (5) Mclntosh CAES Plant, Co-op manages system load with compressed- air storage, Power, April 1992, McGraw-Hill, Inc.; all of which are incorporated herein by reference. An embodiment of the invention is generally denoted with reference numbers 1 , 3 and 13 as illustrated in FIG. 1.

In FIG. 1 during periods of off-peak or semi peak electric power costs, electric power 7 is fed into the motor of the compressed air energy storage plant 2 to suck ambient air 1 into the compressor section of the compressed air energy storage plant 2. This air 1 is compressed such that compressed air 16 leaves the compressed air energy storage plant 2 at a pressure between 50 and 1500 psia. This compressed air 16 then is split into two streams. Stream 11 is fed into the air separation plant, and stream 17 is sent to the above or below ground air storage system 3. The quantity of air flowing in stream 11 is controlled by the flow control valve 8. During such periods of off-peak or semi-peak power cost, the compressed air in stream 17 is constantly added to the quantity of air stored under pressure in the storage system 3. The air in stream 11 is fed to the air separation unit 13 to produce air separation gases of oxygen, nitrogen and or argon in gas or liquid form 14 . During periods of peak power cost, the compressed air energy storage plant 2 no longer draws electric power 7 to its motor. Instead compressed air 4 is extracted for the air storage system 3 and is split into two streams of compressed air 5 and 12. Stream 5 is routed to the turbine expander section of the compressed air energy storage plant where it is expanded in the presence of natural gas or a liquid fuel 15 to produce power 6 which is dispatched and sold to the electric power grid. This power can be used to run the air separation unit. The quantity of compressed air in stream 12 is controlled by flow control valve 9. This stream of compressed air 12 replaces the air in stream 11 to feed the air separation plant 13 to produce air separation gases 14. As periods of off-peak and semi-peak electric power costs are often at night, the ambient temperature during these periods is lower, therefore the density of the air in stream 1 will be higher than the density of ambient air that would have been fed to the air separation plant 13 during daytime periods. Therefore, the amount of electricity needed to compress a given mass of air is lower at night than during the day yielding a savings in the amount of energy that is required to make a given quantity of air separation gases 14. Control valve 9 can be set to regulate the flow of air 12 over a wide range from 5% to 100% of the rated capacity of the air separation plant 13. This range is larger than the range of operation of a standard air separation plant given the limited performance characteristics of a compressor used in the standard air separation plant.

Unit 20 includes a set of automatic controls that can be run by a computer to perform the method of the invention. Such control 20 can be programmed to turn on and of the various values and regulate flow of compressed air and air separation products. Further controls can be programmed to take into consider any or all of the following as well as other factors: air temperature, time of day, humidity, gas output requirement of the air separator units, and power rate costs. Economics of an Embodiment of the Invention:

An embodiment of the invention includes a compressed air energy storage system and an air separation system which allows manufacturers of air separation gases to essentially purchase their entire electric power requirements at the cost of off-peak or semi-peak period and avoid the cost of purchasing power during peak periods when power is the most costly. There is also an added advantage of compressing during the night (off peak period) when the ambient temperature is lower and the specific energy required to compress a given mass of air is less than during the day (peak period) when temperatures are warmer. In the summer the savings for compressing during the cooler nights versus the warmer days could equal as much as 10% of the total quantity of kilo-watt-hours (KWh) needed. Because of design limitations inherent in air compressors, air separation plants can turn down to approximately 70% of their nominal capacity without much sacrifice in their unit energy efficiency (KWh/Ton). Beyond this level of turndown, no further savings of energy can be affected in the typical air separation system. Withdrawing air from a compressed air energy storage system does not have this limitation, therefore, the quantity of air extracted from the compressed air energy storage can be regulated to meet the optimum flow required in the air separation process and further energy savings will be gained.

Typical Economics To Produce Oxygen Gas Sold By

Pipeline To A Steel Mill or A Petrochemical Plant:

Assuming that off-peak power can be purchased for $0.02 per KWh and peak power can be purchased for $0.05 per KWh, a savings of $0.03 per KWh is achievable. As the air separation plant would require an air compressor, no additional capital is required in the compressed air energy storage (CAES) plant. In fact as the CAES plant will have to only provide a fraction of incremental compression capacity this will cost less capital than the corresponding air compressor in a stand-alone air separation plant due to economies of scale. The additional capital cost for storage of the air used in the air separation process over and above that already needed for a compressed air energy storage plant, particularly where the air is stored in an aquifer will be very low as all of this infrastructure is needed for the compressed air energy storage. As an example, if the power savings (peak minus off-peak) are $0.03 per KWh and we have a plant producing oxygen at a specific power rate of 400 KWh per ton, this results in a savings of $12 per ton of oxygen. The value of the oxygen under normal circumstances with power costing $0.05 per KWh would be as follows:

Power $20 per ton Capital Amortization $12 per ton (five year payback)

Other $3 per ton Total $35 per ton

Therefore, the savings afforded by an air separation system producing 1000 Ton per day of oxygen with a compressed air energy storage plant is as much as 35% of the total cost of producing this quantity of oxygen in a stand alone air separation unit. The additional savings by compressing air during colder night periods, than during the day, will add a further 5% to these savings. Additionally producing oxygen at the optimum rate to match its usage and not having the turn-down rate limitation of the air compression system on a stand-alone air separation unit, again adds to these savings. These savings are significant and this invention have widespread utility. Industrial Applicability:

Accordingly, the present invention has significant utility in the production of air separation gases from a compressed air storage system with an air separation process.

Other features, aspects and objects of the invention can be obtained from a review of the figures and the claims.

It is to be understood that other embodiments of the invention can be developed and fall within the spirit and scope of the invention and claims.

Claims

Claim:

1. A method for producing air separation gases including the steps of: compressing air storing compressed air; using the compressed air in order to at least one of (1 ) generate power and (2) as a source of air for an air separation unit; and feeding the compressed air into an air separation unit in order to produce air separation gases.

2. The method of claim 1 including the step of: using the compressed air to generate power; and using the generated power to operate the air separation unit.

3. The method of claim 1 wherein: said step of storing compressed air is performed during periods of either one of off-peak and semi-peak power cost.

4. The method of claim 1 wherein: said step of storing compressed air is performed during periods when the air to be compressed is denser in comparison the air at other periods.

5. The method of claim 3 wherein: said step of storing compressed is additionally performed during periods of either one of off-peak and semi-peak power cost.

6. The method of claim 1 wherein said storing step is performed at night.

7. The method of claim 1 wherein said feeding step is performed continuously.

8. The method of claim 6 wherein said feeding step is performed continuously.

9. The method of claim 1 wherein: said step of feeding the compressed air into an air separation unit in order to produce air separation gases is not limited by turndown characteristics.

10. The method of claim 1 wherein: said storing step includes storing compressed air in one of above surface vessels, subterranean caverns, and aquifers.

1 1. The method of claim 2 wherein: said step of using compressed air to generate power occurs at periods of peak power cost.

12. The method of claim 3 wherein: said step of using compressed air to generate power occurs at periods of peak power cost.

13. The method of claim 4 wherein: said step of using compressed air to generate power occurs at periods of peak power cost.

14. The method of claim 1 wherein: the step of feeding the compressed air into an air separation unit in order to produce air separation gases is used to produce gases selected from the group consisting of oxygen, nitrogen, and argon.

1 5. The method of claim 1 wherein: said feeding step includes the step of feeding stored compressed air into an air separation unit during periods of peak power costs.

1 6. The method of claim 1 wherein the feeding step includes: selectively regulating the flow of the compressed air into an air separation unit over a wide range of the rated capacity of the air separation unit to produce air separation gases.

1 7. The method of claim 1 6 wherein said step of selectively regulating includes: selectively regulating the flow of stored compressed air to an air separation unit.

1 8. The method of claim 1 7 wherein said step of selectively regulating includes: selectively regulating the flow of stored compressed air to an air separation unit during periods of one of off-peak and semi-peak power cost.

19. A method for producing air separation gases including the steps of: compressing air during periods of one of off-peak and semi-peak power costs; using the compressed air during periods of one of off-peak and semi- peak power costs as a source of air for an air separation unit and feeding the compressed air into an air separation unit in order to produce air separation gases during the periods of one of off-peak and semi-peak power costs; storing the compressed air during periods of one of off-peak and semi-peak power costs; and using the stored compressed air (1 ) to generate power during periods of peak power costs and (2) as a source of air for an air separation unit during periods of peak power costs and feeding the compressed air into an air separation unit in order to produce air separation gases at periods of peak power costs.

20. The method of claim 19 including the step of: using the generated power to operate the air separation unit.

21 . An apparatus that can produce air separation gases comprising: a compressor unit that can compress air; a storage unit that can store compressed air; an air separation unit that can separate air into desired gases; said compressor unit communicating with said storage unit and said air separator unit in order to provide compressed air to both of said storage unit and said air separator unit; and said storage unit communicating with said air separator unit in order to provide compressed air to said air separator unit.

22. The apparatus of claim 21 wherein: said storage unit includes one of an above ground storage system and a below ground storage system.

23. The apparatus of claim 21 wherein: said storage unit includes a below ground storage system including one of a subterranean cavern and an aquifer.

24. The apparatus of claim 21 wherein: a generator unit; said storage unit communicating with said generator unit in order to generate power from compressed air provided from said storage unit.

25. The apparatus of claim 21 including: a regulator unit that regulates the flow of compressed air from said storage unit to said air separator unit so said regulator unit can match the output of compressed gas therefrom to the air separation unit to the usage requirements of the air separator unit.

26. The apparatus of claim 21 including: a controller unit; said controller unit communicated with said compressor unit, said storage unit and said air separation unit; said controller unit causing said compressor unit to compress air during at least one of off-peak and semi-peak power cost periods; said controller unit causing said storage unit to provide compressed air to said air separation unit during peak power cost periods; and said controller unit for causing said storage unit to provide compressed air to said compressor unit during peak power cost periods in order that said compressor unit can generate power.