US20070101762A1

US20070101762A1 - Method for designing a cryogenic air separation plant

Info

Publication number: US20070101762A1
Application number: US11/269,662
Authority: US
Inventors: Herbert Schaub; Todd Skare; John Fassbaugh; Douglas May; Scott Spriggs
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2005-11-09
Filing date: 2005-11-09
Publication date: 2007-05-10
Also published as: WO2007055965A3; CN101351681A; WO2007055965A2

Abstract

A method for designing a cryogenic air separation plant wherein a particular plant which fits into a certain classification is designed by first employing at least one predesigned subsystem for that classification to form a base system and then completing the design by adding to the base system at least one auxiliary subsystem designed specifically for that particular plant.

Description

TECHNICAL FIELD

This invention relates generally to cryogenic air separation and, more particularly,. to designing a cryogenic air separation plant.

BACKGROUND ART

A cryogenic air separation plant is a very complicated process plant which includes many different units and subsystems such as distillation columns, condensers, reboilers, prepurification systems, air compression systems, liquid pumps, heat exchangers, storage tanks, process control systems, buildings and other infrastructure. Accordingly, the designing of a cryogenic air separation plant is a complicated and thus costly endeavor which adds significantly to the overall costs of the plant beyond the equipment and construction costs. This is because in most cases each particular cryogenic air separation plant must be specifically designed. Rarely will an existing design for a previously constructed cryogenic air separation plant be exactly suited for use in the construction of a new cryogenic air separation plant. Any method which can reduce the complexity, time and cost for designing a new cryogenic air separation plant would be very useful.

SUMMARY OF THE INVENTION

A method for designing a cryogenic air separation plant comprising:
(A) electing a plant classification from a set of plant classifications;
(B) defining a group of predesigned subsystems for the elected plant classification;
(C) initiating the design of a specific cryogenic air separation plant within the elected plant classification by choosing at least one predesigned subsystem to form the base system of the cryogenic air separation plant; and
(D) completing the design of the cryogenic air separation plant by adding to the base system an auxiliary system comprising at least one subsystem designed specifically for the cryogenic air separation plant.
As used herein the term “predesigned subsystem” means an arrangement comprising a plurality of engineered components and wherein each engineered component is connected to at least one other engineered component of the subsystem.
As used herein the term “engineered component” means a fully designed unit that performs at least one process step that is part of an overall process comprising heat exchange, distillation, compression and/or purification. Examples of engineered components include feed pretreatment units (for example prepurifiers), distillation columns, reboiler/condensers, heat exchangers, direct contact coolers, chillers, liquid pumps, gas compressors, cooling water towers, fluid expanders, process control units, liquid storage vessels, motors and electrical switch gears.
As used herein the term “fully designed” means design work, such as materials of construction specification, process equipment sizing and selection, major valves sizing and selection, equipment arrangement and location of all permanent support and loads, process piping sizing and routing, process instrumentation and controls specification, process analyzers selection and specification, vent and pressure relief devices sizing and specification, drain valves and headers sizing and specification, casing sizing and specifications, static and operating weights estimations, and shipping outline, has been completed.
As used herein the term “connected” means related by way of material transfer, energy transfer, and/or data transfer.

BRIEF DEXRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of one embodiment of a cryogenic air separation plant which may benefit from the application of the method of this invention.
FIG. 2 is a schematic representation of a predesigned subsystem which may be used with the plant illustrated in FIG. 1.
FIG. 3 is a schematic representation of another predesigned subsystem which may be used with the plant illustrated in FIG. 1.
The numerals in the Drawings are the same for the common elements.

DETAILED DESCRIPTION

In the method of this invention cryogenic air separation plant classifications based on specific requirements such as plant size, i.e. capacity, product slate (oxygen, nitrogen, argon and/or clean dry air, etc.), product type (gas and/or liquid), product specification (purity and/or pressure) and location (back-up needs, ambient conditions, local factors etc.), are defined. The desired cryogenic air separation plant fits into one of the classifications and is designed by fitting together at least one predesigned subsystem for that classification with one or more subsystems specifically designed for that particular cryogenic air separation plant.
In the practice of this invention a base system is defined to comprise at least one, preferably two or more, predesigned subsystems that are common to meet different requirements regarding, for example, product type and purity. An auxiliary system is defined to comprise one or more subsystems that are designed specifically to provide the complete plant. For example the base system may provide for air compression, prepurification, heat exchange, refrigeration supply, cryogenic distillation, condensation/reboil, liquid pumping, liquefaction, process control; and the auxiliary system may provide for product compression, liquids storage, switchgear and transformers, cooling water, motor control, buildings and other infrastructure. Any base system can operate over a range of pre-determined conditions, and the specific application requirements will fall within the allowable limits. Generally the auxiliary system will address the specific application requirements associated with product specifications or location factors. These could include factors such as product purity, pressure, backup needs, or cooling water needs. Once the engineering work for the base system is completed, it can be reused for all of the specific applications that have similar requirements.
The invention will be more particularly described and exemplified with reference to the Drawings. Referring now to FIGS. 1-3, ambient air 61 from air suction filter 101 is compressed in main air compressor 102 which is driven.by motor 103. Resulting air stream 63 is cooled in cooler 104, and cooled stream 64 is subjected to free water removal in moisture removal system 105. Resulting elevated pressure air stream 5 is then fed to prepurification system 107, which is a continuously operating two bed pressure swing adsorption (PSA) process. One bed purifies the air of water, carbon dioxide, and most of the hydrocarbons in stream 5 while the other bed is being regenerated by waste nitrogen stream 47. The exiting contents of the regenerating bed leaves prepurification system 107 as waste stream 50. Prepurified air stream 6 then enters a dust filter (not shown) for the removal of any remaining solid particles. Dust free prepurified air stream is split into streams 8 and 11 and further compressed in compressors 109 and 113 respectively. Aftercoolers 110 and 114 remove the heat of compression in the resulting air streams. The compressors 109 and 113, turbine 117, and motor 116 can be configured as a single component or as combinations of one or more of them. Motor 116 can supply additional power if the work generated by turbine 117 is not sufficient to drive compressors 109 and 113. Likewise, if turbine 117 generates more work than is required by compressors 109 and 113, motor 116 removes the excess power from the system.
In primary heat exchanger 115, stream 15 is condensed against boiling oxygen product and warming nitrogen gas, whereupon it exits the cold end of primary heat exchanger 115 as subcooled liquid air stream 17. Stream 17 is split into streams 19 and 20. Stream 19 is fed to medium pressure column 118 several stages from the bottom and stream 20 is fed to the middle of low pressure column 121. Stream 10 is cooled in primary heat exchanger b 115 and removed from primary heat exchanger 115 at an intermediate point. Cooled air stream 16 is then fed to expansion turbine 117, which supplies the refrigeration needs of the plant. Turbine discharge air stream 18 is then fed to the bottom of medium pressure column 118. In column 118 the air is separated by cryogenic rectification into oxygen-enriched and nitrogen-enriched portions. Oxygen-enriched liquid 21 is removed from the bottom of the column and passed into heat exchanger 120 where it is cooled against warming nitrogen gas and from which it exits as a sub-cooled liquid 26. Subcooled oxygen-enriched liquid stream 26 is split into streams 27 and 33. Stream 27 is fed directly to low pressure column 121 l below the feed point for stream 20 but above the bottom of the column. Stream 33 is fed to the boiling side of condenser/reboiler 122 where it is partially vaporized. Oxygen-enriched vapor and liquid streams 29 and 30 exit condenser/reboiler 122 and are fed to an intermediate point of low pressure column 121, below that point where stream 27 enters the column.
Nitrogen-enriched vapor 22 exits the top of the medium pressure column 118 and enters the condensing side of condenser/reboiler 119. Stream 22 is liquefied against vaporizing bottoms liquid in column 121. Liquid nitrogen 23 leaving condenser/reboiler 119 is split into two streams; stream 24 is returned to column 118 as reflux and stream 25 is sent to heat exchanger 120. Stream 25 is subcooled against warming nitrogen vapor. Subcooled liquid nitrogen stream 31 is split into two streams; stream 32 enters low pressure column 121 at or near the top and stream 28 is sent liquid nitrogen storage vessel 127.
Low pressure distillation column 121 further separates its feed streams into oxygen-rich and nitrogen-rich portions. An oxygen-rich liquid stream 34 is removed from the bottom of column 121, where it is split into two streams; stream 35 is fed to liquid oxygen storage vessel 125 and stream 36 is fed to cryogenic oxygen pump 124 and raised to the pressure at which it will boil in primary heat exchanger 115. High pressure liquid stream 37 is fed to the cold end of primary heat exchanger 115 where it is warmed and boiled against the condensing high pressure air stream 15. Warmed, high pressure oxygen vapor product 48 exits the warm end of primary heat exchanger 115.
Vapor stream 38 is removed from an intermediate point of low pressure column 121 and fed to the bottom of argon column 123. Liquid stream 39 exits the bottom of argon column 123 and is returned to low pressure column 121 at the same point at which stream 38 was withdrawn. Argon-enriched liquid stream 40 is removed from the top of argon column 123 and fed to liquid argon storage vessel 126. Also, argon-enriched vapor stream 41 exits the top of argon column 123 and is fed to the condensing side of condenser/reboiler 122. Argon-enriched liquid stream 42 exits condenser/reboiler 122 and is returned to the top of argon column 123 as reflux.
Two nitrogen-rich streams are withdrawn from the top portion of low pressure column 121. Product nitrogen-rich vapor 44 exits the top of the low pressure column 121, is fed to heat exchanger 120, is warmed against cooling streams, and leaves as superheated nitrogen vapor product stream 46. Waste nitrogen-enriched vapor 43 is removed from low pressure column 121 a few stages from the top, is fed to heat exchanger 120, is warmed against cooling streams, and leaves as superheated nitrogen vapor waste stream 45. Both superheated nitrogen streams 45 and 46 are fed to the cold end of primary heat exchanger 115 where they are warmed against cooling air streams and exit primary heat exchanger 115 to form waste and product streams 47 and 49, respectively.
As mentioned earlier, warm nitrogen-enriched waste stream 47 is fed to prepurifier vessel 107 in order to regenerate one of the PSA beds. If nitrogen product is desired at elevated pressure, it is compressed in compressor 134 (driven by motor 135) to form nitrogen product 53. If the oxygen is boiled below its final delivery pressure, it is compressed in oxygen compressor 129 (driven by motor 130) to form oxygen product 51.
In one embodiment of this invention the base system comprises two sets of pre-engineered and fully designed components (subsystems) 107 and 108. The first subsystem 107 (FIG. 2) comprises all of the pieces of equipment needed to treat stream 5 and produce stream 6. The design of this portion of the plant is common to every particular application that has similar air throughput requirements regardless of any product slate variations or further air compression requirements. This subsystem includes process equipment, associated valves, piping, analyzers, electrical connections, and other infrastructure needed to remove contaminants of air.
The second subsystem 108 (FIG. 3), a cryogenic processing unit, comprises contents of the air separation plant cold section such as in the cold box. Again, the design of this portion of the plant is common to applications that have similar product requirements. The equipment in this pre-engineered and fully designed subsystem includes distillation columns 118, 121, and 123, condenser/ reboilers 119 and 122, heat exchangers 115 and 120, all of the associated valves and piping that are used to connect these pieces together, analyzers, and all the necessary electrical connections.
In any plant classification those subsystems required to complete the plant and are not included in the base system are part of an auxiliary system. Because of the differences in product slate, purities, delivery pressures and location issues, each auxiliary system is custom designed rather than operating the plant in an inefficient manner. For example, one plant may require more liquid back-up on account of its remote location, hence custom designing storage vessels 125, 126, and 127 is preferred to over-designing the vessels to fit all liquid makes and including it in a base system. Likewise, main expansion turbine 117 will be custom designed for each application to account for the differences in liquid making requirements of each particular application. As another example, if one application requires the delivery of oxygen product at twice the pressure of another application, then compressors 113 and 130 and pump 124 will be custom designed for each plant. In another variation, if liquid air stream 17 is at a sufficiently high pressure for a particular application, then it would be advantageous to use a liquid turbine (not shown) to generate additional refrigeration from that liquid prior to feeding it to the two columns. Hence, the liquid turbine would be part of an auxiliary system of this different application.

Table 1 examples of four different plants belonging to the air separation process plant classification illustrated in FIG. 1. This table lists only some of the subsystems required to form a complete plant by the practice of this invention. The numerals in parentheses listed in first column correspond to the labels in FIG. 1. This table is not exhaustive with respect to plant subsystems and/or process variations, as there are many ways in which the invention could be applied. The plant subsystems are categorized as belonging to the base system (B) or belonging to an auxiliary system (A). By definition, any subsystem that is part of the base system (B) is necessarily the same for every plant belonging to a given classification. Additionally, by definition, every plant designed according to a given classification has to include each and every subsystem that is part of the base system (B). Therefore, Table 1 illustrates that the prepurification subsystem (107) and cryogenic processing subsystem (108) in all four plants are part of the base system. An auxiliary system in any plant will be different from that in any other plant belonging to the same classification. However, any subsystem that is part of an auxiliary system (A) by definition can have either the same design or a different design than that used in one or more of the other plants belonging to a given classification. Furthermore, any subsystem that is part of an auxiliary system (A) by definition does not have to be included in the design of every plant belonging to a given classification. For example, considering plant 1 as a base or a reference plant in a given classification, if the delivery pressure of product nitrogen from plant 2 is same or lower than that of product nitrogen leaving cryogenic processing subsystem (108), then the auxiliary system of plant 2 need not contain product nitrogen compression subsystem, hence this is left blank. Similarly, in plant 3 the product oxygen compression subsystem is not required, hence this is left blank. In plant 4 more liquid nitrogen production is required, hence the auxiliary system contains the nitrogen liquefier subsystem which was not required in plant 1. Thus, in accordance with this invention, plant subsystems are categorized as belonging to a base system or an auxiliary system, and engineering costs are reduced by custom designing only the auxiliary system.

	TABLE 1


	Plant Number

	1	2	3	4

Product Nitrogen (49) Delivery Pressure	Base	Lower	Lower	Base
Product Oxygen (48) Delivery Pressure	Base	Base	Lower	Base
Liquid Nitrogen Make	Base	Base	Base	Higher

Plant Subsystem/

Infrastructure Categorization

Ambient Air Filtration (101)	A	A	A	A
Feed Air Compression (102, 103, 104, 105)	A	A	A	A
Prepurification (107)	B	B	B	B
Air Compression for Oxygen Boiling (113, 114)	A	A	A	A
Air Compression for Refrigeration Supply (109, 110)	A	A	A	A
Air Expansion for Refrigeration Supply (117)	A	A	A	A
Cryogenic Processing (17, 120, 118, 119, 121, 122, 123)	B	B	B	B
Liquid Oxygen Pumping (124)	A	A	A	A
Oxygen Compression (129, 130)	A	A		A
Nitrogen Compression (134, 135)	A			A
Liquid Oxygen Storage (125)	A	A	A	A
Liquid Nitrogen Storage (127)	A	A	A	A
Liquid Argon Storage (126)	A	A	A	A
Cooling Tower (not shown)	A	A	A	A
Cooling Water Treatment/Supply (not shown)	A	A	A	A
Switchgear/Transformer (not shown)	A	A	A	A
Motor Control Center (not shown)	A	A	A	A
Compressor Building (not shown)	A	A	A	A
Controls Infrastructure (not shown)	A	A	A	A
Nitrogen Liquefier (not shown)				A

Additionally, this invention can be applied to cryogenic air separation plants employing processes or classifications that are substantially different than the one illustrated in FIGS. 1-3.

Claims

1. A method for designing a cryogenic air separation plant comprising:

(A) electing a plant classification from a set of plant classifications;

(B) defining a group of predesigned subsystems for the elected plant classification;

(C) initiating the design of a specific cryogenic air separation plant within the elected plant classification by choosing at least one predesigned subsystem to form the base system of the cryogenic air separation plant; and

(D) completing the design of the cryogenic air separation plant by adding to the base system an auxiliary system comprising at least one subsystem designed specifically for the cryogenic air separation plant.

2. The method of claim 1 wherein the base system comprises an air prepurification system.

3. The method of claim 1 wherein the base system comprises a primary heat exchanger.

4. The method of claim 1 wherein the base system comprises a cryogenic distillation column.