US20220196325A1

US20220196325A1 - Method and apparatus for improving start-up for an air separation apparatus

Info

Publication number: US20220196325A1
Application number: US17/125,823
Authority: US
Inventors: Paul Kong; May Yee Wendy YIP
Original assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2020-12-17
Filing date: 2020-12-17
Publication date: 2022-06-23

Abstract

A method for operating an air separation unit having both a startup phase and a steady-state operating phase is provided. The method can include the steps of: introducing a plurality of air streams into a cold box, wherein each stream is cooled in a heat exchanger and then sent to a system of distillation columns for separation therein by cryogenic distillation; producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger; determining whether each stream of the plurality of air streams is a net cold producing stream or net cold consuming streams; and adjusting a flow rate of each stream of the plurality of air streams during the startup phase such that each stream that is a net cold producing stream has a higher flow rate during the startup phase as compared to the steady-state operating phase, and wherein each stream that is a net cold consuming stream has a lower flow rate during the startup phase as compared to the steady-state operating phase.

Description

TECHNICAL FIELD

The present invention generally relates to a method for shortening the cool down process for an air separation unit (ASU).

BACKGROUND OF THE INVENTION

A typical ASU may include use of 1) Main Air Compression (MAC) to medium pressure ˜6 bar, 2) purification unit, 3) Booster Air Compression (BAC) to high pressure ˜50-60 bar, 4) main heat exchanger, and 5) medium pressure, low pressure and argon distillation columns for the production of oxygen, nitrogen, and argon.
During normal operation of the ASU, it is common to feed the cold box multiple air streams at various flow rates, pressures and temperatures. These process conditions are optimized based on the overall design and in an effort to produce particular product fluid mixes at specific pressures, temperatures, and concentrations.
Prior to running at normal operation, the ASU must first undergo a startup operation in which the equipment within the cold box is cooled down to process temperatures that are typically well below zero Celsius. In methods known heretofore, it is common to run the ASU as normally would be done, and sometimes with just increased overall air flows, until the process temperatures are reached, and once that happens, the system can be then switched to production mode. Other methods of improving start-up have also included using an external source of a cryogenic liquid, typically liquid nitrogen, in an effort to more quickly lower the temperature within the system. However, use of liquid nitrogen, which is at very low temperatures can cause thermal stresses on the equipment that is originally at ambient temperatures.
Therefore, it would be beneficial to provide a process and apparatus that could provide for improved start-up times in an economical fashion and without causing undue thermal stresses to the system components.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a device and a method that satisfies at least one of these needs. An objective of the current invention is to be able to reduce the time needed for bringing an ASU from ambient conditions to cryogenic operating temperatures.
In ASU startup process, a cool down step is necessary to cool cold box equipment (e.g., main exchangers, distillation columns, etc.) from ambient temperature to the cryogenic temperature. This process requires an additional amount of refrigeration (cold) relative to the amount required for normal steady-state operation. The cool down can be achieved by passing process air into cold box, in which refrigeration (cold) is generated by turbo expansion or free expansion (Joule expansion).
As noted before, methods known heretofore typically fed as much air as possible to the cold box in the relatively same proportion among various streams as in the normal steady state operation. In certain embodiments of the present invention, the flow rates of each air stream is adjusted to take advantage of each air stream in an effort to optimize cold production as opposed to flow rates based on steady-state design operations.
In one embodiment, a method for operating an air separation unit having both a startup phase and a steady-state operating phase is provided. The method can include the steps of: splitting a purified and compressed air stream downstream of the main air compressor into a first air stream and a second air stream; cooling the first air stream in a heat exchanger and then introducing the first air stream into the system of distillation columns for separation therein by cryogenic distillation; boosting the pressure of the second air stream in the booster air compressor to form a boosted air stream; expanding at least a first portion of the boosted air stream to provide cooling energy, and then introducing the resulting expanded air stream to the system of distillation columns for separation therein by cryogenic distillation; and producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger.
In certain embodiments, during the start-up phase, the first air stream has a start-up flow rate (Q_FAS1) and the boosted air stream has a start-up flow rate (Q_BAS1). Furthermore, during the steady-state operating phase, the first air stream can have a steady state flow rate (Q_FAS2) and the boosted air stream can have a steady state flow rate (Q_BAS2). Moreover, the flow rate of the boosted air stream can be set higher during the start-up phase as compared to during the steady-state operating phase, such that Q_BAS1>Q_BAS2, and wherein the flow rate of the first air stream is set lower during the start-up phase as compared to during the steady-state operating phase, such that Q_FAS1<Q_FAS2.
In another embodiment, the method can further include the steps of splitting the boosted air stream into the first portion and a second portion, wherein the first portion is expanded in a cold turbine after partial cooling in the heat exchanger before being sent to the system of distillation columns for separation therein by cryogenic distillation.
In another embodiment, the method can further include the steps of further compressing the second portion of the boosted air stream in a warm compressor, cooling the second portion of the boosted air stream in the heat exchanger and then expanding the second portion of the boosted air stream in a Joule-Thompson valve before introduction into the system of distillation column for separation therein by cryogenic distillation.
In another embodiment, a method for operating an air separation unit having both a startup phase and a steady-state operating phase is provided. In this embodiment, the air separation unit can include a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns. In this embodiment, the method can include the steps of: introducing a plurality of air streams into the cold box, wherein each stream is cooled in the heat exchanger and then sent to the system of distillation columns for separation therein by cryogenic distillation; producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger; and determining whether each stream of the plurality of air streams is a net cold producing stream or net cold consuming streams, wherein a flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase.
In an optional embodiment, the flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase by increasing a flow rate of each stream of the plurality of air streams that is a net cold producing stream during the startup phase as compared to the steady-state operating phase and decreasing a flow rate of each stream of the plurality of air streams that is a net cold consuming stream during the startup phase as compared to the steady-state operating phase.
In another embodiment, an apparatus for operating an air separation unit having both a startup phase and a steady-state operating phase is provided. In this embodiment, the air separation unit can include a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns. In a preferred embodiment, the cold box is configured to receive a plurality of air streams and cool each stream in the heat exchanger, the system of distillation columns is in fluid communication with the heat exchanger, the system of distillation columns is configured to produce a plurality of air gas streams. Furthermore, the apparatus can also include means for determining whether each stream of the plurality of air streams is a net cold producing stream or net cold consuming streams; and means for increasing a flow ratio between the net cold producing streams and the net cold consuming streams during the startup phase as compared to the steady-state operating phase.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figure(s) is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a process flow diagram of an embodiment of the present invention.

FIG. 2 is a process flow diagram of a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 provides a schematic representation of an embodiment of the present invention outlining how an ASU can operate in both its steady state operational mode, as well as its cooling down (start-up) mode.
An air stream 2 is first compressed in a main air compressor 10 and purified of carbon dioxide and water in a front-end purification unit 30. The purified and compressed air 32 is then split into two streams, with one stream 36 being further compressed in a second air compressor 60, while the second stream 34, which is at 6 bara in this example, is sent to the cold box and cooled in a heat exchanger 40, before being sent to the system of distillation columns 50 as stream 42.
The further compressed air stream 36 can then be further split into a first fraction 62 and a second fraction 64. The first fraction 62, which is at 45 bara in this example, is sent to the cold box and partially cooled in a higher pressure heat exchanger, before being expanded in a turbine 80 and then ultimately introduced to the system of distillation columns 50 as stream 82. The second fraction 64 is further boosted in a booster compressor 70, which is preferably powered by the turbine 80, to a pressure of about 67 bara in this example, before being introduced to the cold box and fully cooled in the heat exchanger 40.
The resulting cooled boosted stream 44 is withdrawn from heat exchanger 40 and expanded across a Joule-Thompson valve to produce additional refrigeration for the system before being introduced to the distillation column system 50 for separation therein. In a preferred embodiment, expansion of the first fraction in the turbine provides the compression energy used by the booster compressor. The embodiment shown preferably includes cooler 71 in order to remove heat of compression from boosted second fraction 72 prior to introduction to main heat exchanger 40.
In the embodiment shown, distillation column system 50 is configured to provide a waste nitrogen stream 51, a medium pressure nitrogen stream 53, a low-pressure nitrogen stream 55 and a high-pressure gaseous oxygen stream 57. In the embodiment shown, liquid oxygen 52 is withdrawn from the sump of the lower-pressure column (not shown) and pressurized in pump 100 before being heated in main heat exchanger 40 to form gaseous oxygen stream 57. Argon product 59 can also be withdrawn from the distillation column system.
In the example shown above, during normal operation, the approximate flows for the various air feed streams, as a percentage of total air flow, is as follows: Second air stream 34 at 6 bara is 50% of air stream 2; first fraction 62 at 45 bara is 25% of air stream 2; and second fraction 64 at 67 bara is 25% of air stream 2.
As noted above, during an ASU startup process, a cool down step is necessary to cool cold box equipment (e.g., main exchangers, distillation columns, etc.) from ambient temperature to the cryogenic temperature. This process requires an additional amount of refrigeration (cold) relative to the amount required for normal steady state operation. The cool down is achieved by passing process air into cold box, in which refrigeration (cold) is generated by turbo expansion or free expansion (Joule-Thompson expansion). The conventional cool down method is to feed as much air as possible to the cold box in the relatively same proportion among various streams as in the normal steady state operation.
However, embodiments of the present invention alter the proportional flow rates of the various air streams in order to significantly speed up this cool down step. Specifically, the proportional flow rates of air are increased for both the first and second air fraction (62, 64), while the second portion 34 proportional flow rate is decreased.
The inventors of the current invention discovered that as air circulates through the cold box, it produces cold by expansion but also carries some so called main exchanger warm end cold loss (WE ΔT cold loss) and not all of the feed air steams produce the same amount of cold. More importantly, some feed air steams produce a net positive amount of cold while others produces a net negative amount of cold. In certain embodiments, it is common for the turbo expansion and high-pressure (HP) streams to produce more cold than WE ΔT loss, and they are therefore considered to be “net cold producing streams.” On the other hand, low-pressure (LP) streams, such as stream 34 produces lower amounts of refrigeration compared to the WE ΔT loss, and therefore, would be considered as “net cold consuming streams.”
Furthermore, the inventors discovered that the breakeven point between cold gain by expansion and cold loss due to WE ΔT is when air pressure of the feed air is at approximately 11 bara for a typical main exchanger WE ΔT of 2° C. Therefore, it is advantageous to disproportionally increase turbo expansion air (e.g., stream 62) and HP air streams (e.g., stream 72) with pressure greater than 11 bara and to decrease air streams with pressure less than 11 bara (e.g., stream 34) during cool down process relative to normal steady state operation. As a non-limiting example, for the embodiment shown in FIG. 1, the second air stream, which is at only 6 bara, can be reduced in proportional flow rate (20%), while the remaining air fractions can be increased (first fraction increased to 45% and second fraction increased to 35%).
In short, embodiments of the invention intentionally increase the flow ratios between the net cold producing streams and net cold consuming streams during the cool down process compared to normal steady state operation in order to shorten the cool down process start-up.
Those of ordinary skill in the art will recognize that the distillation column system 50 can be any column system that is configured to separate air into at least a nitrogen-enriched stream and an oxygen-enriched stream. This can include a single nitrogen column or a double column having a higher and lower pressure column, as is known in the art. In another embodiment, the distillation column system can also include other columns such as argon, xenon, and krypton columns. As all of these columns and systems are well known in the art, Applicant is not including detailed figures pertaining to their exact setup, as they are not relevant to the inventive aspect of the present invention.
FIG. 2 provides an additional embodiment having a different turbo-booster configuration. In the embodiment show in FIG. 2, all of the air coming out of compressor 60 is sent to booster compressor 70, with a portion of this boosted second fraction 72 being expanded in turbine 80, while the remaining portion is fully cooled in the heat exchanger and withdrawn as cooled boosted stream 44.
Additionally, non-boosted air stream can be divided into a first fraction 37 and a second fraction 39, with first fraction being compressed and then expanded in turbo booster (75, 85) before being introduced into system of distillation columns 50 for separation therein. Similarly, second fraction 39 can be expanded across a JT valve and then introduced into system of distillation columns 50 for separation therein.
Moreover, while the embodiments shown in the figures show specific turbo-booster configurations, the invention is not intended to be so limited. Rather, those of ordinary skill in the art will recognize that any known front-end compression system can be used so long as the there are multiple air streams at varying pressures sent to the cold box. Additionally, while the embodiments shown in FIG. 1 and FIG. 2 include a single heat exchanger, those of ordinary skill in the art will recognize that a higher-pressure heat exchanger and a lower pressure heat exchanger can be used. In this embodiment, lower pressure air 34 would be cooled in lower pressure heat exchanger and the two higher- pressure streams 62, 72 would be cooled in the higher-pressure heat exchanger.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step or reversed in order.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

Claims

What is claimed is:

1. A method for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns, wherein the method, for both the startup phase and the steady-state operating phase, comprises the steps of:

splitting a purified and compressed air stream downstream of the main air compressor into a first air stream and a second air stream;

cooling the first air stream in a heat exchanger and then introducing the first air stream into the system of distillation columns for separation therein by cryogenic distillation;

boosting the pressure of the second air stream in the booster air compressor to form a boosted air stream;

expanding at least a first portion of the boosted air stream to provide cooling energy, and then introducing the resulting expanded air stream to the system of distillation columns for separation therein by cryogenic distillation; and

producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger;

wherein during the start-up phase, the first air stream has a start-up flow rate (Q_FAS1) and the boosted air stream has a start-up flow rate (Q_BAS1),

wherein during the steady-state operating phase, the first air stream has a steady state flow rate (Q_FAS2) and the boosted air stream has a steady state flow rate (Q_BAS2),

wherein the flow rate of the boosted air stream is set higher during the start-up phase as compared to during the steady-state operating phase, such that Q_BAS1>Q_BAS2, and

wherein the flow rate of the first air stream is set lower during the start-up phase as compared to during the steady-state operating phase, such that Q_FAS1<Q_FAS2.

2. The method of claim 1, further comprising the steps of splitting the boosted air stream into the first portion and a second portion, wherein the first portion is expanded in a cold turbine after partial cooling in the heat exchanger before being sent to the system of distillation columns for separation therein by cryogenic distillation.

3. The method of claim 2, further comprising the steps of further compressing the second portion of the boosted air stream in a warm compressor, cooling the second portion of the boosted air stream in the heat exchanger and then expanding the second portion of the boosted air stream in a Joule-Thompson valve before introduction into the system of distillation column for separation therein by cryogenic distillation.

4. A method for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns, wherein the method comprises the steps of:

introducing a plurality of air streams into the cold box, wherein each stream is cooled in the heat exchanger and then sent to the system of distillation columns for separation therein by cryogenic distillation;

producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger; and

determining whether each stream of the plurality of air streams is a net cold producing stream or a net cold consuming stream,

wherein a flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase.

5. The method of claim 4, wherein the flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase by increasing a flow rate of each stream of the plurality of air streams that is a net cold producing stream during the startup phase as compared to the steady-state operating phase and decreasing a flow rate of each stream of the plurality of air streams that is a net cold consuming stream during the startup phase as compared to the steady-state operating phase.

6. An apparatus for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns,

wherein the cold box is configured to receive a plurality of air streams and cool each stream in the heat exchanger,

wherein the system of distillation columns is in fluid communication with the heat exchanger, wherein the system of distillation columns is configured to produce a plurality of air gas streams,

wherein the apparatus further comprises:

means for determining whether each stream of the plurality of air streams is a net cold producing stream or a net cold consuming stream; and

means for increasing a flow ratio between the net cold producing streams and the net cold consuming streams during the startup phase as compared to the steady-state operating phase.