Patent Grant
12061045
This application is a § 371 of International PCT Application PCT/CN2018/122047, filed Dec. 19, 2018, which is herein incorporated by reference in its entirety.
The present invention relates to a method for starting up a cryogenic air separation unit and to an associated air separation unit.
In particular, it may apply to an air separation unit (ASU) having a booster which compresses air which has been compressed and then cooled in a main heat exchanger and is subsequently cooled after compression in the booster and sent to the column system or to a turbine, coupled to the booster.
In an air separation unit, the feed air is compressed cooled, distilled in a column system and the gaseous and/or liquid products of the column system are warmed. The warming of the products and the cooling of the feed air generally take place in a main heat exchanger, having an entry temperature at the warm end above 0° C.
In some plants, which may have little or no liquid production, air can be compressed in a main air compressor and part of the air is then further compressed in a compressor called a booster. The air from the booster is generally cooled in a separate heat exchanger before being sent to the main heat exchanger in which the products of the air separation unit are warmed.
Eliminating this separate heat exchanger, called a turbine booster aftercooler, saves the cost of the heat exchanger and also allows compression power to be reduced, since the pressure drop in the separate heat exchanger is eliminated.
However it poses some problems when the air separation unit is started up from a warm condition, for example, when the unit has been closed down for maintenance or for an initial start-up.
These include:
In certain embodiments, t invention may include a method which reduces at least one of these risks when starting the ASU from a warm state using a venting conduit to release heat from the system to produce cold.
According to one object of the invention, there is provided a process for starting up an air separation unit which is at a temperature of above 0° C., the air separation unit comprising a main air compressor for compressing the feed air, a main heat exchanger, a conduit for sending compressed air from the main air compressor to the main heat exchanger to be cooled, a booster, a conduit for sending at least part of the compressed air cooled in the main heat exchanger to the booster, means for sending air to the main heat exchanger from the booster, there being no means for cooling the air downstream of the booster and upstream of the main heat exchanger, a column system, at least one turbine connected to receive compressed air from the main air compressor and possibly from the booster, the at least one turbine being connected to the column system to provide air to be distilled in the column system, a conduit from removing an oxygen enriched product from the column system and sending it to be warmed in the main heat exchanger, a conduit from removing an nitrogen enriched product from the column system and sending it to be warmed in the main heat exchanger wherein in normal operation, air is sent from the main air compressor to the heat exchanger, cooled in the heat exchanger, compressed in the booster, cooled in the heat exchanger and separated in the column system, air is sent form the heat exchanger to be expanded in the turbine and is separated in the column system and a nitrogen enriched product and an oxygen enriched product are warmed in the heat exchanger wherein the air separation unit comprises a venting conduit connected downstream of the booster and upstream of the main heat exchanger and in that in order to start up the air separation unit,
According to further optional features:
According to a further object of the invention, there is provided an air separation unit, comprising a main air compressor for compressing the feed air, a main heat exchanger, a conduit for sending compressed air from the main air compressor to the main heat exchanger to be cooled, a booster, a conduit for sending at least part of the compressed air cooled in the main heat exchanger to the booster, means for sending air to the main heat exchanger from the booster, there being no means for cooling the air downstream of the booster and upstream of the main heat exchanger, a column system, at least one turbine connected to receive compressed air from the main air compressor and possibly from the booster, the at least one turbine being connected to the column system to provide air to be distilled in the column system, a conduit from removing an oxygen enriched product from the column system and sending it to be warmed in the main heat exchanger, a conduit from removing an nitrogen enriched product from the column system and sending it to be warmed in the main heat exchanger characterised in that the air separation unit comprises a venting conduit connected downstream of the booster and upstream of the main heat exchanger.
The unit preferably includes a conduit for sending air from the booster to the conduit for sending compressed air from the main air compressor to the main heat exchanger to be cooled.
The unit may include a device for detecting the outlet temperature of the booster and controlling the opening of a valve to send air compressed in the booster to be mixed with an airstream compressed in the main air compressor as a function of the outlet temperature of the booster.
Preferably the booster is driven by the least one turbine.
The unit may comprise means for detecting the speed of the at least one turbine and for opening the venting conduit once the turbine reaches a given speed.
The unit may comprise means for detecting the outlet temperature of the booster and for opening a valve to allow air to flow directly from the booster to the main heat exchanger without being mixed with another air stream once the outlet temperature is below a given value.
Further features, advantages and possible applications of the invention are apparent from the following description of working and numerical examples and from the drawings. All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims the way in which said claims refer back to one another.
The invention will now be described in detail with reference to
In
In normal operation, air 7 is sent to a main air compressor 3 in which air is compressed to a pressure above the pressure of the first column 28. Part of the air 18 is cooled in the main heat exchanger 6 and divided in two. One part 8 of the air is removed at a temperature just below that of the warm end of the heat exchanger 6 and is compressed in a cool booster 4a. The term “cool booster” is used since the air has simply been slightly cooled in the heat exchanger 6. All the boosted air is then sent back to the warm end, without having been cooled, is then cooled in the main heat exchanger 6 to an intermediate temperature and sent as stream 9 to a second booster 4b which is designated as a cold booster, since the air arriving in the booster is significantly colder than that arriving in booster 4a. The air from cold booster 4b is sent back to the main heat exchanger 6, cooled to the cold end temperature, removed as stream 10, expanded and send as liquefied stream 25 to the first column 28 and as liquefied stream 26 to the second column 30.
The rest 20 of stream 18 is cooled to a temperature lower than the inlet temperature of the cold booster 4b and expanded in turbines 5a and 5b as parallel streams 11, 22. The cool booster 4a is driven by turbine 5a and the cold booster 4b is driven by turbine 5b. The expanded air streams from the turbines 5a, 5b are mixed to form stream 12 and sent as gaseous feed to the first column.
Oxygen enriched liquid and nitrogen enriched liquid are sent from the first column to the second column in the usual way.
A nitrogen enriched gaseous stream 15 is removed from a minaret at the top of the second column 30 and warmed in exchanger 6. Liquid oxygen 13 is removed close to the bottom reboiler 34 of the second column 30 and vaporised in the exchanger 6.
A nitrogen enriched gaseous stream 14 is removed from the top of the second column 30 and warmed in exchanger 6.
It will be appreciated that in normal operation, none of the boosted air from either booster 4a or 4b is sent to a turbine.
In order to start up the air separation unit, in a basic version of the start-up process, valve 80 is used to isolate the high pressure air from cool booster 4a and allow the bypass air from main air compressor 3 to go to the inlet of cold booster 4b for initial start up.
When the unit required to start up whilst being at a temperature above 0° C., air is compressed in main air compressor 3 and is sent to the main heat exchanger, but in warm condition there is no cold stream 13,14,15 from the coldbox to cool down this air. So the air is sent to the inlet of cool booster 4a in warm condition. At the same time, air is sent to turbines 5a and 5b. Air 9 from main air compressor 3 is sent to cold booster 4b via bypass conduit 70.
Once the turbine 5a is functioning at a speed higher than its no-dwell zones, corresponding to a range of speeds around a critical speed or speeds, the venting valve 50 is opened to release boosted air from booster 4a to the atmosphere via venting conduit 24. Part of the air from booster 4a is released to the atmosphere via conduit 24, however the rest of the air is sent via conduit 60 to join air stream 18. Valve 80 is closed.
Air is sent from the turbine 5a to the column 28 and begins to be separated in the column.
If this venting conduit 24 is opened by opening the venting valve 50 before flow is introduced to the cool booster turbine 5a, the cool booster turbine could begin to rotate, which could damage the machine. This valve should therefore remain closed before cool booster turbine is started. Additionally, this venting valve should be fail closed to ensure that the machine does not rotate while the plant is shutdown.
Because this turbine booster 4a has no aftercooler, the hot air downstream the turbine booster cannot be introduced into the suction of the booster 4a or the inlet of the turbine 5a, which will make the surge worse. So here an anti-surge line 60 is installed from booster discharge to the air conduit upstream the main heat exchanger 6. This allows the air from the booster 4a to be mixed with the main feed air 18 from compressor 3. Because the boosted air is reduced in quantity and is mixed with another cooler stream, stream 18, the risk of damaging the heat exchanger is reduced.
In order to prevent the trip of the main heat exchanger, we need to open the venting line 24 as soon as possible, immediately after turbine has passed the no-dwell zone (if a no dwell zone exists).
For this basic variant, independent start-up of the turbines is not possible due to an incompatibility of flow between the cold booster and its turbine.
In a basic version of the invention, during start-up, the air from cool booster 4a is sent either to the atmosphere or to the turbine 5a. None of the air from the cool booster 4a is sent to the cold booster 4b during at least part of the start-up process. During at least part of the start up process, any air which is boosted in booster 4a and which is not sent to the atmosphere, is sent to turbine 5a and/or turbine 5b.
Ideally it is desired to startup the cool booster 4a simultaneously with the cold booster 4b. In this case a bypass line 70 is used to have the incoming air for cold booster 4b, together with an isolation valve 80 after cool booster 4a. Additionally, bypass line 70 must be connected in such a way as to avoid a short circuit sending the flow from anti surge line 60 directly to the main heat exchanger 6.
Warm Startup steps:
Before the temperature in main heat exchanger 6 is sufficiently low, the air from the cool booster 4a is either sent entirely to the atmosphere or else sent in part to the atmosphere and in part to turbines. None of the air is sent to the cold booster 4b as it is sent in normal operation.
The temperature in the main heat exchanger 6 is detected and when it is cool enough, the venting line 24 and bypass line 70 are closed so that all the boosted air from booster 4a goes to the heat exchanger 6 and from there to cold booster 4b.
The opening of the venting line 24 is adjusted as a function of the refrigeration needs. It is maintained at least partially open until other equipment that produces cold is started. It may also be maintained partially open to help establish normal liquid inventories in the cold box. Once isolation valve 80 is open, some of the flow from the cool booster 4a will be sent to the heat exchanger and cold booster There is no need to bypass air through valve 70 once the discharge temperature of the cool booster 4a is cold enough to send the air directly the heat exchanger.
The anti-surge line 60′ sends air from the cold booster 4b to the inlet of the turbine 5b without passing through the heat exchanger if the booster outlet temperature is too high.
The apparatus may also include:
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. 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.
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.
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 a range is expressed, it is to be understood that another embodiment is from the one 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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2018/122047 | 12/19/2018 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2020/124427 | 6/25/2020 | WO | A |
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| Number | Date | Country | |
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| 20220074657 A1 | Mar 2022 | US |
