CN111981760B

CN111981760B - Heating and unfreezing method of low-temperature air separation device

Info

Publication number: CN111981760B
Application number: CN202010896752.6A
Authority: CN
Inventors: 肖鸾; 曾庆华; 高飞
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: 2020-08-31
Filing date: 2020-08-31
Publication date: 2021-09-28
Anticipated expiration: 2040-08-31
Also published as: CN111981760A

Abstract

The invention discloses a heating and unfreezing method of a low-temperature air separation device. Starting an air compressor, a precooling system and a purifying system, and heating and purging heating gas at an outlet of the purifying system until most of an air separation device reaches 0 ℃; starting an air supercharger, compressing at least part of the heated gas by the air supercharger, decompressing by a decompression valve, inputting the compressed gas into a part of the air separation device with the temperature not reaching 0 ℃, such as a main condensation evaporator, and further heating and unfreezing the compressed gas. The return water flow of cooling water in a cooler behind the air supercharger is reduced, so that the temperature of the heating gas at the outlet of the air supercharger is raised to 50-60 ℃, the temperature of the heating gas at the outlet of the purification system is obviously higher, and the heating and thawing process is accelerated. When all the equipment and pipelines of the low-temperature air separation device are heated to about 0 ℃, the heating and the unfreezing are finished. The invention adopts a simple, energy-saving and convenient method to provide heating gas with higher temperature, thereby achieving the effect of reducing the heating and thawing time.

Description

Heating and unfreezing method of low-temperature air separation device

Technical Field

The invention relates to a heating purging method of a low-temperature air separation device, in particular to a method for providing high-temperature heating gas.

Background

Over long periods of operation of cryogenic air separation plants, the cryogenic equipment and fluid lines of the rectification system may experience increased resistance due to ice, dry ice or mechanical powder deposits inside (all of which are plugging components). Thus, after the air separation unit has been in operation for a period of time, the rectification system should be purged with heat to remove these plugging components. In addition, when the air separation plant has a failure such as the water inflow of the molecular sieve adsorber or the water inflow of the main heat exchanger, the plant must be stopped and warmed. When the temperature is increased, the temperature of each part of the device is slowly and uniformly increased so as to prevent the equipment or the pipeline from being damaged due to stress caused by excessive temperature difference. All measurement, analysis and instrumentation signal lines must also be thoroughly warmed and purged.

Chinese utility model with publication number CN204128276U provides a quick heating apparatus for oxygen generator, through addding the control system that heats, utilizes the pressurized air in the air pipe network to heat for the oxygen generator, has solved the problem of diffusing as the air of the gas that heats by the compressed air that the air compressor machine produced, but is not suitable for near the situation that does not have the air pipe network.

The chinese patent application publication No. CN109163506A discloses that the purified air extracted from an air supercharger is cooled by a post-cooler and then divided into two streams, one stream is directly sent to a main heat exchanger of a cold box, and the other stream is used as instrument air of the device itself, a heating thawing air source and a self-cleaning filter back-blowing air source.

Disclosure of Invention

The invention aims to solve the technical problem of how to utilize the existing equipment of a low-temperature air separation device to provide an efficient, energy-saving and convenient method for heating and unfreezing the low-temperature air separation device.

The invention discloses a heating and unfreezing method of a low-temperature air separation device, which comprises an air compressor for pressurizing, precooling and purifying raw material air, a precooling system, a molecular sieve purification system, a downstream main heat exchanger and an air supercharger, at least one rectification air separation tower containing a condensation evaporator and a corresponding pipeline system, wherein equipment needing to operate at low temperature is arranged in a cold box; the method comprises the following steps: after the low-temperature air separation device stops and discharges liquid, a pipeline for heating and purging in a pipeline system is opened, an air compressor, a precooling system and a purification system are operated, and dry air F1 with the temperature of T1 and taken from an outlet of the purification system is used for heating and purging a main heat exchanger and a rectification air separation tower until the low-temperature air separation device reaches a first unfreezing state; next, operating the air booster by inputting at least part of F1 into the air booster and further purging the condensing evaporator with dry air F2 at a temperature of T2 and taken from the outlet of the air booster until the low-temperature air separation device reaches a second defrosting state; the third step of heating is finished, the pipeline for heating and purging is closed, and the air compressor and the air supercharger are optionally shut down; the air supercharger comprises an after cooler cooled by cooling water, the return flow of the cooling water is controlled to adjust T2, and T2> T1.

The first defrosting state in the method means that the temperature of the main heat exchanger and/or the tower body of the rectification air separation tower reaches the preset temperature t1 after warming and purging. The second defrosting state means that the temperature of the condensing evaporator reaches the preset temperature t1 after the condensing evaporator is heated and blown. t1 is equal to about 0 ℃.

Alternatively, T1 is approximately equal to ambient temperature; t2 is about 50-60 ℃.

Optionally, the pressure of the F1 and/or F2 after decompression by a decompression valve is less than the design pressure of the distillation air separation column. The two are used as warming gas to be sent into a main heat exchanger and/or a rectification air separation column.

Optionally, the pressure of F1 and/or F2 after depressurization by a depressurization valve is between 3barg and 5 barg.

Alternatively, the return valve of the air booster is kept open, so that the output pressure of the air booster after reaching the steady state is minimized.

Alternatively, the cooling water return flow rate is adjusted by a return valve provided on the cooling water circuit, and the temperature T2 increases as the opening degree of the return valve decreases.

The heating and thawing process of the low-temperature air separation device is divided into two stages, in the first stage, the low-temperature dry air taken from the outlet of the purification system is used for heating and blowing the low-temperature air separation device, so that the heating process is ensured to be slow and uniform, and the possible damage to equipment caused by rapid heating is avoided.

When most of the cryogenic air separation plant, including the main heat exchanger, the pipeline and equipment in the rectification air separation column except the condensing evaporator, are heated to a predetermined temperature, the second stage of the heating and thawing process is started, and the equipment with lower thawing degree, such as the condensing evaporator or the optional crude argon column, in the cryogenic air separation plant is heated and purged by using the dry air with higher temperature taken from the outlet of the air supercharger, so that the heating and purging efficiency is improved, and the required time is reduced.

In the present invention, the temperature of the drying air at the outlet of the air supercharger is adjusted by controlling the amount of the cooling water returned in the aftercooler of the supercharger. Compared with the prior art in which an additional heater is used for heating the drying air, the operation is simple and convenient, and simultaneously, the investment and the energy consumption are saved.

Drawings

The drawings in the present disclosure are only for illustration of the invention for understanding and explaining the spirit of the invention, but not for limiting the invention in any way.

FIG. 1 is a flow chart of an apparatus for warming thawing process.

Detailed Description

In the present invention, it should be understood that the terms "upper", "lower", "front", "rear", "vertical", "parallel", "top", "bottom", "inner", "outer", etc. indicate orientations and positional relationships based on those shown in the drawings, and are only for convenience of description of the present invention, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention.

The appearances of the phrases "a" or "an" in various places herein are not necessarily all referring to the same quantity, but rather to the same general description. Similarly, modifiers similar to "about" or "approximately" that occur before a numerical term herein typically include the same number, and their specific meaning should be understood in conjunction with the context. Similarly, unless a specific number of a claim recitation is intended to cover both the singular and the plural, and also that claim may include both the singular and the plural.

In the present invention, unless otherwise specifically stated or limited, the terms "mounted," "connected," and "fixed" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral part; can be mechanically or electrically connected; either directly or indirectly through intervening media, either internally or in any other suitable relationship. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

The low-temperature air separation device is a device which takes air as a raw material, changes the air into liquid by a compression cycle deep freezing method, and separates inert gases such as oxygen, nitrogen, argon and the like from the liquid air by rectification. The low-temperature air separation device mainly comprises a raw material air compressor, a precooling system, a molecular sieve purifying system, an air supercharger, an expansion machine, a main heat exchanger, a rectification air separation tower and the like. By "column" is meant herein a distillation or fractionation column or zone in which liquid and vapor phases are countercurrently contacted to effectively separate a fluid mixture. The distillation air separation column may comprise only one column, more typically two columns. The operation pressure of the first tower in the disclosure is generally 5-6.5 Bar A, which is 1.1-1.5 Bar A higher than the general operation pressure of the second tower. The two towers may be mounted vertically on top of one tower or two towers side by side. A condensing evaporator at the top of a column refers to a heat exchange device that produces vapor from liquid in the column, typically comprising a bath condenser and a falling film condenser, which, due to their compact plate and channel structure, warm up more slowly during defrosting than the other components of the rectification column. Optionally, the cryogenic air separation plant further comprises a crude argon column, a fine argon column, a subcooler, and the like. The air separation plant can be operated at low temperatures of-150 ℃ to-180 ℃, so that the cryogenic equipment is arranged in an insulated cold box.

The pipeline system in the low-temperature air separation device comprises a process fluid pipeline and a warming and purging pipeline. In the case of two distillation air separation columns, the process fluid line typically includes at least one waste nitrogen line connected to the second column of the distillation column and a feed air line connected to the first column of the distillation column. "dirty nitrogen" refers to a gaseous fluid having a nitrogen content of not less than 95 mole percent. In consideration of saving cost and simplifying design and installation, the heating channel line and the purging line can be respectively connected in a way of adding branches on different process fluid pipelines instead of adding pipelines specially used for heating and purging on the rectifying tower. Thus, when the air separation device normally operates, the valve on the branch is closed, so that the heating channel line and the purging line are in a standby state; when heating purging is needed, a valve on the process fluid pipeline is closed, the valve on the branch is opened, the heating channel line and the purging pipeline are in working states, heating gas is introduced into the corresponding heating channel line, purging is respectively performed on the main heat exchanger, the first rectifying tower and the second rectifying tower, and then the purging impurities are discharged into the atmosphere through the purging pipeline.

Warming herein purges the cryogenic air separation plant using clean dry air as the warming gas to remove any accumulated moisture, carbon dioxide, and mechanical fines and other impurities. The raw material air is pressurized to 3-6 barg by an air compressor, and then is cooled to 14-15 ℃ by a precooling system, generally a water cooling tower, and then is input into a molecular sieve purification system. The air at the outlet of the molecular sieve purification system is clean dry air with the temperature of 22-25 ℃. In normal operation, the clean dry air is cooled by a downstream main heat exchanger, pressurized by an air booster, and subjected to expansion refrigeration by an expansion machine and then input into a rectification air separation column. During the heating and thawing process, the temperature at the outlet of the molecular sieve purification system is T1, and clean dry air with the flow rate of F1 directly passes through a heating channel line and optionally enters a rectification air separation column through a main heat exchanger.

In normal operation, the air supercharger continuously boosts the clean dry air of 3-6 barg to 15-60 barg. This compression process is typically accomplished by multiple stages of compression, followed by intercoolers and aftercoolers to cool the temperature of the air stream and improve the energy efficiency of the compressor. A conventional cooler performs a cooling function using cooling water flowing in a cooling water circuit, and the cooling effect depends on the flow rate and temperature of the cooling water. The temperature of the cooling water is generally 20-30 ℃, the flow is regulated through a water return valve arranged on a cooling water loop, when the opening degree of the water return valve is reduced, the flow of the cooling water is reduced, the cooling effect is reduced, and the temperature of the correspondingly output air flow is increased.

The warming and thawing must be performed after the air separation plant is shut down for large discharge. In the first stage, the valves for the respective process fluid lines in the piping system are closed and the valves for the respective warming purge lines into the main heat exchanger and the rectification air separation column are opened. And restarting the air compressor, the precooling system and the purification system, obtaining clean dry air with the temperature of T1 and the flow rate of F1 at the outlet of the purification system, selectively inputting the clean dry air into each fluid channel of the main heat exchanger from the hot end, then respectively introducing the clean dry air into the first tower and the second tower of the rectification air separation tower, wherein the first tower and the second tower comprise main condensation evaporators positioned at the bottoms of the second tower, and finally discharging the clean dry air to the atmosphere from a purging pipeline. T1 is generally in the range of 22-25 deg.C, and is also influenced by the ambient temperature. The temperature of the purge gas is measured at the outlet of the purge line and when this temperature is around 0 c, it is indicated that the first thawing condition is reached, when the main heat exchanger and most of the equipment of the rectification air separation column are at temperatures around 0 c. The temperature of the main condensation evaporator is lower due to the structure, and the temperature of the main condensation evaporator can still be about 50 ℃ below zero. It generally takes 10 hours to reach the first thawing state.

In the second stage of warm purge, the air booster is further started. The clean dry air flowing out of the purification system with the flow rate of F1 is totally or partially input into an air supercharger, and the clean dry air with the temperature of T2 and the flow rate of F2 is obtained at the outlet of the air supercharger. T2 is controlled by adjusting the amount of return cooling water flow through the booster cooler. Specifically, a temperature probe TIC2 may be provided on the fluid line at the outlet of the supercharger, and the opening degree of the water return valve may be adjusted electrically or manually according to a predetermined temperature control value. When the device is normally operated, the temperature T2 is 30-40 ℃, and when the device is heated and thawed, the temperature T2 can be preferably increased to 50-60 ℃ by reducing the return water flow of cooling water. Further warming of the plant having a temperature not reaching 0 ℃ is carried out using stream F2, such plant generally comprising a main condensing evaporator, or alternatively a crude argon column, a fine argon column, etc. When all parts of the whole low-temperature air separation device are heated to be higher than 0 ℃ and reach a second unfreezing state, heating is completed, a pipeline for heating and purging is closed, and the air compressor and the air supercharger are optionally shut down.

During the warming and purging process, since the expander is not started to expand and cool clean dry air, a pressure reducing valve is used to ensure that the pressure of F1 and F2 does not exceed the design pressure of the rectification air separation column when necessary. The pressure of the two air streams after depressurization is preferably 3 to 5 barg. Based on this, if the pressure of the pressurized F2 is too high, unnecessary energy consumption may be caused. Therefore, when the supercharger is started, the return valve is opened simultaneously, so that a part of air at the outlet returns to the inlet of the air supercharger, and after the operation reaches a stable state, the output pressure is the lowest and can be about 15 barg.

An embodiment of the invention is described below with reference to fig. 1. The present invention is applicable to a variety of different cryogenic air separation plants and different piping systems, and thus the embodiment of fig. 1 is exemplary and not intended to limit in any way the scope and spirit of the present invention.

A part of the warming purge line in fig. 1 is separately provided, and a separate inlet is arranged on the distillation air separation column; the other part is partially overlapped with the process fluid pipeline. The heating channel line is provided with a manual valve, a switching device and an optional filter, the filter is used for further filtering the dry air, and the switching device preferably adopts a splayed blind plate to ensure that the dry air does not leak to a connected process fluid pipeline to influence the purity of the gas in the process fluid pipeline when the rectifying air separation tower normally operates. Preferably, a temperature probe and a blow-off valve are arranged on a purge line connected with the cryogenic equipment, the temperature probe is used for measuring the temperature of the purge gas, and the blow-off valve is used for controlling the on-off of the purge line. When the low-temperature air separation device normally operates, the manual valve on the heating channel line and the blow-off valve on the blow-off pipeline are closed, and the switching device is adjusted to be a closed circuit, so that the heating channel line and the blow-off pipeline are in a standby state; when heating and purging are needed, the manual valve on the heating channel line and the purging valve on the purging pipeline are opened, and the switching device is adjusted to be a passage, so that the heating channel line and the purging pipeline are in a working state.

The cryogenic air separation plant shown in FIG. 1 comprises two distillation air separation columns, wherein the second column 9 is placed in the upper part of the first column 8 and is in heat exchange communication with the first column through the main condensing evaporator 7. When the stopping and the liquid discharging are finished, the heating and unfreezing process is started. Raw material air 20 is compressed by an air compressor 1, precooled to 14-15 ℃ in a water-cooling tower 2 and then conveyed into a purification system 3 to remove impurities, water and CO in the raw material air₂And the like. The common purification system can be a molecular sieve purification system, a membrane purification system and the like. When the purification system is a molecular sieve containing two adsorbers, R01 and R02, regeneration of the adsorbers utilizes the air separation product, namely, the waste nitrogen 35, which is reheated by the main heat exchanger during normal operation; during heating and thawing, regeneration gas can be input from an external pipeline (not shown in FIG. 1). The temperature of the clean dry air at the molecular sieve outlet is about22 to 25 ℃. The warming gas is fed to each warming conduit branch line through a main line 23. Immediately after the warming-up, the air supercharger 4 is in a stop or low load operation state, the valves V2 and V3 are closed, and there is no warming-up gas flow to the branch line 24. The warmed gas enters the main heat exchanger 6 at the warm end via warmed

conduit branches

28, 29 and 30, the 3 branches representing schematically all the paths within the main heat exchanger 6 which need to be warmed and thawed, some of which are not identified in figure 1 for simplicity. After passing through the main heat exchanger 6, the warmed gas in the

branches

28, 29 is discharged to the atmosphere via

purge lines

34, 33, respectively. The branch line 30 is partially overlapped with a product pipeline of the polluted nitrogen or the low-pressure pure nitrogen, the warming gas in the branch line 30 enters the top of the second tower 9 of the rectification air separation tower at the interface 36, and is discharged into the atmosphere through the purging line 31 after the whole second tower is warmed and purged from top to bottom. The warming main line 23 is also connected to a separately provided warming conduit branch 27 which is connected directly to an inlet 37 at the top of a column 8. The warmed gas in this branch line warms and defrosts the condenser evaporator 7, then warms and purges the entire column from top to bottom, and is discharged to the atmosphere at a purge line 32 at the bottom of the column.

The temperature probes arranged on the purging lines 31-34 display the temperature of the heating air at the opening in real time. The starting temperature of the air separation plant to be warmed and thawed lies in the range-130 to-150 c, the temperature of the warmed gas measured at the outlet of each purge line after about 10 hours of purging is close to 0 c, with the only exception of the outlet temperature of the warmed gas stream 37 measured at the purge line 32, which may be as low as-50 c. This is due to the fact that stream 37 flows through the main condenser-evaporator 7, whereas the temperature rise of the main condenser-evaporator, the crude argon column, the fine argon column, etc. with a complicated compact structure is slower than the other components of the rectification column. At this time, the low-temperature air separation device is considered to reach a first thawing state, and the first stage of heating and thawing is completed.

In order to accelerate the thawing of the main condensing evaporator with still lower temperature, heating gas with higher temperature and larger flow is introduced into equipment with lower temperature in the second stage of heating and thawing. Referring to fig. 1, starting or operating the air supercharger 4 at high load opens valves V1, V2, V4, respectively, keeping V3 closed. V4 is the return valve of the air supercharger, when it is open, part of the air at the supercharger outlet is returned to the supercharger inlet, and after reaching a steady state, the air pressure at the supercharger outlet is at or slightly above the factory set minimum, for example about 15 barg. V3 is closed and V2 is opened so that clean dry air does not flow into the process fluid line during normal operation, but rather flows into warming line 25, joins warming line 27, is reduced in pressure by a pressure reducing valve and flows into the main condensing evaporator from a port 37 at the top of a tower 8. Alternatively, the valves in the warming lines 28, 29, 30 may be closed, and the total clean dry air obtained at the outlet of the molecular sieve purification device is treated by the air booster and then introduced into the main condensing evaporator 7 for more efficient warming and defrosting.

Since the compressed air is an exothermic process, in order to prevent the compressed air having an excessively high temperature from damaging the compressor itself, the air supercharger 4 is provided with a cooler after each stage of compression, and the cooler installed after the last stage of compression is called an after-cooler 5. The cooler is generally cooled by flowing cooling water. In normal operation, the temperature and the flow of cooling water are controlled to enable the temperature of an outlet stream of the air supercharger to be 30-40 ℃, in the invention, a higher outlet temperature T2 can be preset, the temperature of the outlet stream of the air supercharger is monitored by adopting a temperature probe TIC2, if the temperature is lower than T2, the opening degree of a water return valve V1 on a cooling water loop is reduced in an automatic or manual mode, a smaller amount of cooling water flows through a cooler, the cooling effect is reduced, and the temperature of the outlet stream of the air supercharger is increased. This adjustment was continued until the stream temperature was equal to T2. Compared with the heating gas with the temperature of about ambient temperature, the heating gas with the temperature T2 of about 50-60 ℃ can shorten the time of heating the condensation evaporator of the oxygen production air separation with 60,000Nm3/h from-50 ℃ to 0 ℃ for about 4-8 hours.

The temperature of the condensing evaporator reaches 0 ℃ as a mark, and the heating and thawing reach a second thawing state. The lines for warm purge can be shut down and the corresponding equipment, such as the air compressor, purification system, air booster, etc., shut down.

The above is an embodiment of the present invention, but the present invention is not limited to the above embodiment, and those skilled in the art should be able to make various equivalent modifications or substitutions according to the present invention, which is included in the scope of the present invention defined by the claims.

Claims

1. A heating and thawing method of a cryogenic air separation plant is characterized by comprising the following steps:

a) providing a cryogenic air separation plant, wherein the plant comprises an air compressor for pressurizing, precooling and purifying raw air, a precooling system, a molecular sieve purification system, a downstream main heat exchanger and an air supercharger, at least one rectification air separation tower containing a condensation evaporator and a corresponding pipeline system, and equipment needing to operate at low temperature is arranged in a cold box;

b) after the low-temperature air separation device stops and discharges liquid, a pipeline for heating and purging in a pipeline system is opened, an air compressor, a precooling system and a purification system are operated, and dry air F1 with the temperature of T1 and taken from an outlet of the purification system is used for heating and purging a main heat exchanger and a rectification air separation tower until the low-temperature air separation device reaches a first unfreezing state;

c) operating the air booster by feeding at least a portion of the F1 into the air booster and further purging the condenser evaporator with dry air F2 at a temperature T2 from the outlet of the air booster until the cryogenic air separation plant reaches a second defrost state;

d) after the heating is finished, closing the pipeline for heating and purging;

e) the air supercharger comprises an after cooler cooled by cooling water, the return flow of the cooling water is controlled to adjust T2, and T2> T1.

2. The warming defrosting method for a cryogenic air separation plant according to claim 1, characterized in that the first defrosting state means that the temperature of the main heat exchanger and/or the body of the rectifying air separation column reaches the preset temperature t1 after warming and purging.

3. The warming thawing method for a cryogenic air separation plant according to claim 1, wherein the second thawing state is a state in which the temperature of the condensing evaporator reaches a preset temperature t1 after warming and purging.

4. A method for warming and defrosting a cryogenic air separation plant as claimed in claim 2 or 3, wherein t1 is equal to 0 ℃.

5. A method for warming and defrosting a cryogenic air separation plant as claimed in claim 1 wherein T1 is equal to ambient temperature.

6. A heating and thawing method for a cryogenic air separation plant as in claim 1, wherein T2 is equal to 50-60 ℃.

7. The warming defrosting method of a cryogenic air separation plant according to claim 1, characterized in that F1 and/or F2 is decompressed by a decompression valve and then sent to a main heat exchanger and/or a rectification air separation column as a warming gas.

8. The warming defrosting method of a cryogenic air separation plant according to claim 7, characterized in that the pressure of the F1 and/or F2 after decompression by a decompression valve is less than the design pressure of the rectification air separation column.

9. A process according to claim 8, wherein F1 and/or F2 are depressurized by a pressure reducing valve to a pressure in the range of 3barg to 5 barg.

10. A warming and thawing method for a cryogenic air separation plant according to claim 1, characterized in that in step c), the return valve of the air supercharger is kept open so that the output pressure of the air supercharger is minimized after reaching a steady state.

11. The warming-up thawing method for a cryogenic air separation plant according to claim 1, wherein the flow rate of the return cooling water is adjusted by a return valve provided in the cooling water circuit, and the temperature T2 is raised when the opening degree of the return valve is decreased.