CN113654302B - Low-temperature air separation device and method - Google Patents

Low-temperature air separation device and method Download PDF

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Publication number
CN113654302B
CN113654302B CN202110926268.8A CN202110926268A CN113654302B CN 113654302 B CN113654302 B CN 113654302B CN 202110926268 A CN202110926268 A CN 202110926268A CN 113654302 B CN113654302 B CN 113654302B
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liquid
gas
tower
air
nitrogen
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CN113654302A (en
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平芳
阿兰·布里格利亚
曹建伟
赵伯伟
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04006—Providing pressurised feed air or process streams within or from the air fractionation unit
    • F25J3/04048—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of cold gaseous streams, e.g. intermediate or oxygen enriched (waste) streams
    • F25J3/04054—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of cold gaseous streams, e.g. intermediate or oxygen enriched (waste) streams of air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04406—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system
    • F25J3/04412—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system in a classical double column flowsheet, i.e. with thermal coupling by a main reboiler-condenser in the bottom of low pressure respectively top of high pressure column
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04006—Providing pressurised feed air or process streams within or from the air fractionation unit
    • F25J3/04012—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of warm gaseous streams; details of intake or interstage cooling
    • F25J3/04018—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of warm gaseous streams; details of intake or interstage cooling of main feed air
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    • F25J3/04024—Providing pressurised feed air or process streams within or from the air fractionation unit by compression of warm gaseous streams; details of intake or interstage cooling of purified feed air, so-called boosted air
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    • F25J3/04078—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression
    • F25J3/04084—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression of nitrogen
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    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
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    • F25J3/04078—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression
    • F25J3/0409—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression of oxygen
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    • F25J3/04109—Arrangements of compressors and /or their drivers
    • F25J3/04145—Mechanically coupling of different compressors of the air fractionation process to the same driver(s)
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04248—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion
    • F25J3/04284—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams
    • F25J3/0429—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams of feed air, e.g. used as waste or product air or expanded into an auxiliary column
    • F25J3/04296—Claude expansion, i.e. expanded into the main or high pressure column
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
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    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04406—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system
    • F25J3/04418—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system with thermally overlapping high and low pressure columns
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    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/50—Oxygen or special cases, e.g. isotope-mixtures or low purity O2
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    • F25J2270/00—Refrigeration techniques used
    • F25J2270/08—Internal refrigeration by flash gas recovery loop
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    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/34—Details about subcooling of liquids

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Health & Medical Sciences (AREA)
  • Emergency Medicine (AREA)
  • Separation By Low-Temperature Treatments (AREA)

Abstract

In a two-tower air separation system of cryogenic rectification, when a liquid air stream with higher pressure or streams such as oxygen-enriched liquid air, pure liquid nitrogen and the like extracted from a tower with higher pressure are sent into a two-tower and/or a storage tank with lower pressure, throttling valve decompression is needed. The liquid stream after passing through the throttling valve forms a gas-liquid mixed stream due to a flash evaporation phenomenon. Different from the prior art that two phases of the gas-liquid mixed stream are all introduced into a downstream device, the gas-liquid mixed stream is divided into a gas phase and a liquid phase by using gas-liquid separation equipment after throttling, the liquid phase is all sent into a second tower and/or a storage tank, and at least part of the gas phase is converged into polluted nitrogen through a bypass pipeline according to the requirement. Therefore, the diameter of the second tower can be reduced, the transportation requirement can be met, the cold energy contained in the flow can be recovered, and the operation energy consumption is reduced.

Description

Low-temperature air separation device and method
Technical Field
The invention relates to the field of low-temperature air separation, in particular to a device and a method for low-temperature air separation.
Background
The low-temperature rectification is adopted to separate the air into products such as oxygen, nitrogen, liquid oxygen, liquid nitrogen and the like in an air separation tower, and the technology is mature. Pure oxygen and pure nitrogen products are generally produced simultaneously using a two-column system. A two-column system refers to two air separation columns which are connected by heat exchange, for example, a condenser evaporator, and which can be stacked one above the other or placed side by side. The two air separation columns are operated at different pressures, one column being operated at a first, higher pressure, for example 5.0 to 6.5Bar A, and the other column being operated at a second, lower pressure, for example 1.1 to 1.5Bar A. The compressed, precooled, purified, cooled feed air is fed to one and/or both of the columns in a substantially liquid form. In the first tower with higher pressure, lighter nitrogen rises to the top of the tower, heavier oxygen is enriched to the bottom of the tower, and pure liquid nitrogen, lean liquid nitrogen, oxygen-enriched liquid air and other streams are respectively extracted from various positions from the top to the bottom of the first tower and are sent to the second tower to serve as reflux liquid. The pressure of the two towers is obviously lower than that of the first tower, the stream needs to be decompressed in a throttling or expansion mode before entering the two towers, and the throttled stream can be changed into a gas-liquid mixed state from a liquid state due to a flash evaporation phenomenon. Since both the gas phase and the liquid phase contain the components to be separated, it is customary to feed both phases to a two-column rectification in order to increase the oxygen extraction as much as possible. Liquid oxygen is generated at the bottom of the second tower after rectification, and the waste nitrogen is generated at the top of the second tower. The waste nitrogen contains a large amount of cold energy, raw material air is cooled through heat exchange, and the waste nitrogen after reheating can be used as a regeneration purification system or directly emptied.
With the increase of the air separation scale, the size of the air separation column is increased, and the diameter of the two columns is the largest in the double-column system, which causes limitation on transportation. In the prior art, in order to avoid transporting an over-limit air separation column, a plurality of smaller-size air separation column rectification systems are sometimes selected to replace a large-size air separation column rectification system, but the investment and the occupied area of equipment are increased; or adopt the field welding to assemble the air separation tower, but this kind of method consumes time and manpower, also is difficult to guarantee the welding quality of air separation tower.
Chinese utility model patent CN203927396U discloses a liquid oxygen storage tank flash distillation vapour recovery unit, recycles the liquid oxygen flash distillation vaporization oxygen who discharges among the air separation plant, has improved the oxygen content of air separation plant feed gas air, has effectively reduced oxygen manufacturing cost.
Disclosure of Invention
The technical problem to be solved by the invention is that 1) under the condition of ensuring the extraction rate of oxygen as much as possible, the size of the air separation tower is reduced to meet the transportation condition; 2) The energy consumption of the operation of the air separation tower is reduced.
In one aspect, the present invention discloses a cryogenic air separation plant comprising: at least one column operating at a first pressure and at least one second column operating at a second, relatively lower pressure, the first and second columns being in heat exchange communication; a main compressor for pressurizing, pre-cooling, purifying feed air to produce a liquid air stream, an air pre-cooling and purification system, a main heat exchanger, and an expander; and a piping system comprising a pipe for delivering the liquid air stream to the first and/or second column, a pipe for delivering the oxygen-enriched liquid air and the lean liquid nitrogen generated by the first column to the second column, and/or a pipe for delivering the pure liquid nitrogen generated by the first column to a liquid nitrogen storage tank, and a pipe for reheating the dirty nitrogen generated by the second column through the main heat exchanger; the system also comprises a first throttling valve and a first gas-liquid separation device which are sequentially arranged before the liquid air stream enters the second tower, a pipeline for introducing a liquid phase generated by the first gas-liquid separation device into the second tower and a pipeline for merging at least part of a gas phase generated by the first gas-liquid separation device into the polluted nitrogen; and/or a second throttle valve and a second gas-liquid separation device which are sequentially arranged before the oxygen-enriched liquid air enters the second tower, a pipeline for introducing a liquid phase generated by the second gas-liquid separation device into the second tower and a pipeline for converging at least part of a gas phase generated by the second gas-liquid separation device into the polluted nitrogen gas; and/or a third throttle valve and a third gas-liquid separation device which are sequentially arranged before pure liquid nitrogen generated by a tower enters a liquid nitrogen storage tank, a pipeline for introducing a liquid phase generated by the third gas-liquid separation device into the storage tank and a pipeline for converging at least part of gas phase generated by the third gas-liquid separation device into the polluted nitrogen.
Further, the device also comprises a subcooler which further subcools the liquid air and/or the oxygen-enriched liquid air and/or the pure liquid nitrogen.
Furthermore, at least part of gas phase of each gas-liquid separation device is merged into a pipeline of the waste nitrogen before the waste nitrogen enters the subcooler for heat exchange.
Further, each gas-liquid separation device in the above apparatus comprises a gas-liquid separation tank and/or a pipe.
Alternatively, in the above apparatus, a control valve or an on-off valve may be provided in a pipe for introducing at least part of the gas phase of each gas-liquid separation device into the nitrogen purge gas.
In another aspect, the present invention also discloses a method for cryogenic air separation using an apparatus as described above, comprising:
a) Providing at least one column operating at a first pressure and two columns operating at a second, relatively lower pressure, the one and two columns being in heat exchange communication,
b) Providing a main compressor, an air pre-cooling and purification system, a main heat exchanger and an expander for pressurizing, pre-cooling, purifying feed air to produce a liquid air stream,
c) Providing a piping system comprising piping for transporting the liquid air stream to one and/or both of the columns, piping for transporting oxygen-enriched liquid air and lean liquid nitrogen produced in one of the columns to the other column, and/or piping for transporting purified liquid nitrogen produced in one of the columns to the liquid nitrogen storage tank, and piping for reheating the dirty nitrogen produced in the other column via the main heat exchanger,
throttling before the liquid air stream and/or the oxygen-enriched liquid air enters the second tower, separating the throttled stream into a gas phase and a liquid phase, sending the liquid phase into the second tower, and gathering at least part of the gas phase into a pipeline for polluted nitrogen;
and/or throttling the pure liquid nitrogen generated by one tower before the pure liquid nitrogen enters a liquid nitrogen storage tank, separating the throttled stream into a gas phase and a liquid phase, feeding the liquid phase into the storage tank, and merging at least part of the gas phase into the polluted nitrogen before reheating.
If the general practice in the cryogenic air separation field is followed and the entire composition of the liquid air stream and/or the oxygen-rich liquid air stream is fed to the lower pressure two columns, although there is no loss in oxygen extraction, the gas phase flow to the two columns is greater, resulting in the two columns needing to have a larger column diameter, possibly causing the diameter of the two columns to exceed the transport limits. To address transport difficulties, different measures may be taken: one is to replace a large-scale air separation with a plurality of small-scale air separations, which has the disadvantages of increasing the cost and the occupied area of the air separation and reducing the operation efficiency of the air separation; the other type is an air separation tower assembled by welding on site, and has the disadvantages that the field quality control is not easy, and the labor and the working hours are consumed.
By adopting the device and the method, part of gas-phase flow generated by throttling the liquid air flow and/or the oxygen-enriched liquid air flow can be merged into the waste nitrogen gas led out from the top of the two towers through the bypass according to the requirement. The flow of the gas phase stream entering the second tower is reduced, so that the size of the second tower can be effectively reduced, and the requirement of integral transportation is met; meanwhile, as the content of oxygen in the gas phase is far lower than that of nitrogen, discharging part of the gas phase flow into the polluted nitrogen does not cause too large negative influence on the oxygen extraction rate of the whole air separation system.
When pure liquid nitrogen generated by one tower is sent into a storage tank for storage, the pure liquid nitrogen also needs to be throttled and depressurized due to the existence of pressure difference, and a gas-liquid mixture is generated after throttling. It is common practice in the art to feed the liquid phase and the gaseous phase, when they are the same, to a storage tank, and the gaseous phase is vented through a valve above the storage tank. By adopting the device and the method, at least part of gas-phase fluid generated after throttling is merged into the waste nitrogen gas led out from the top of the second tower through the bypass, and the cold energy contained in the waste nitrogen gas is recovered, thereby reducing the energy consumption of the operation of the air separation device.
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 schematic illustration of a cryogenic rectification air separation plant as a comparative example of the present invention.
FIG. 2 is a schematic diagram of a cryogenic rectification air separation plant in accordance with one embodiment of the present invention.
In fig. 1 and 2, like reference numerals designate the same component or stream. The system comprises a main compressor, a pre-cooling system, a purification system, a 4-supercharger, a 5-supercharging end of an expander, an expansion end of an expander 6, a main heat exchanger 7, a subcooler 8, a liquid nitrogen pump 9, a liquid oxygen pump 10, a first tower 11, a second tower 12, a main condensing evaporator 13, a liquid nitrogen storage tank 14, a liquid oxygen storage tank 15, a first throttling valve 21, a first gas-liquid separation device 22, a second throttling valve 23, a second gas-liquid separation device 24, a third throttling valve 25, a third gas-liquid separation device 26, a fourth throttling valve 27, a fourth gas-liquid separation device 28, a first regulating valve 29, a second regulating valve 30 and a third regulating valve 31;
corresponding to each stream, 101-feed air, 102-first partial pressurized air stream, 103-first partial expanded air stream, 104-second partial pressurized air stream, 105-second partial throttled air stream, 106-liquid air stream, 106 a-first partial vapor air stream, 106 b-liquid air stream, 106 c-second partial vapor air stream, 107-oxygen-rich liquid air, 107 a-first partial vapor oxygen-rich liquid air, 107 b-liquid oxygen-rich liquid air, 107 c-second partial vapor oxygen-rich liquid air, 108-first partial pure liquid nitrogen, 108 a-vapor pure nitrogen, 108 b-liquid pure liquid nitrogen, 109-second partial pure liquid nitrogen, 110-first partial pure liquid oxygen, 111-second partial pure liquid oxygen, 112-contaminated nitrogen, 113-lean liquid nitrogen.
Detailed Description
In the present disclosure, the term "feed air" means a mixture comprising primarily oxygen and nitrogen, the composition of which is similar to ambient air; the term "liquid air stream" refers to the feed air after precooling, purification treatment, further cooling, expansion or throttling to form a liquid state, which stream can be fed directly to one and/or both columns.
The term "nitrogen product" covers gaseous fluids having a nitrogen content not less than 99 mole percent, preferably not less than 99.5 mole percent; the term "nitrogen gas" covers gaseous fluids having a nitrogen content of not less than 80 mole percent, and the nitrogen content of "nitrogen gas" is less than the "nitrogen product"; the term "neat liquid nitrogen" refers to a liquid fluid having a mole percent of nitrogen greater than 99, the term "lean liquid nitrogen" refers to a liquid fluid having a mole percent of nitrogen greater than 80, and the "lean liquid nitrogen" has a nitrogen content less than that of "neat liquid nitrogen".
The term "oxygen-rich liquid air" refers to a liquid fluid having a mole percent of oxygen greater than 30, the term "pure liquid oxygen" covers liquid fluids having a mole percent of oxygen greater than 90, and the content of oxygen in "pure liquid oxygen" is higher than that of "oxygen-rich liquid air", the term "oxygen product" covers gaseous fluids having an oxygen content not less than 90 mole percent, preferably not less than 95 mole percent.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically stated otherwise.
In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; 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.
Unless clearly indicated to the contrary, each aspect or embodiment defined herein may be combined with any other aspect or embodiments or embodiment or embodiments. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.
In the invention, bar is a unit of pressure, 1Bar =0.1Mpa; a-absolute means absolute pressure; flow rate unit Nm 3 The term,/h denotes the standard volume flow of the gas at a temperature of 0 ℃ and a standard atmospheric pressure.
Cryogenic rectification of the present disclosure is a rectification process that is conducted at least in part at a temperature of 150K or less than 150K. By "column" herein is meant a distillation or fractionation column or zone wherein liquid and vapor phases are countercurrently contacted to effectively separate a fluid mixture. The operating pressure of the first tower in the disclosure is generally 5-6.5 Bar A, which is higher than the general operating pressure of the second tower by 1.1-1.5 Bar A. The two towers can be vertically mounted on top of one tower or two towers are mounted side by side. The first and second columns are connected in a heat exchange manner by a condensing evaporator located at the bottom of the second column.
The device and the method are suitable for various process flows of low-temperature rectification. As a comparative example, FIG. 1 shows a typical two-column air separation scheme. Feed air 101 is boosted to about 5.5Bar a in primary compressor 1, then cooled, purified in pre-cooling system 2 and purification system 3, and further boosted by booster 4. Thereafter, the stream is divided into a first portion of pressurized air stream 102 and a second portion of pressurized air stream 104. The former 102, after being partially cooled in the main heat exchanger 7, is expanded and cooled by the expansion end 6 of the expander to obtain a first part of an expanded air stream 103, which is fed in gaseous and/or liquid form to the bottom of a column 11. The latter 104 is compressed at the charging end 5 of the expander, cooled in the main heat exchanger 7, throttled by the fourth throttle valve 27 to obtain a second partially throttled air stream 105, which is in the form of a mixture of gas and liquid, and the gas phase and the liquid phase are separated by the fourth gas-liquid separation device 28, the gas phase is completely fed to the lower part of the first column, a part of the liquid phase is fed to the lower part of the first column, and the other part of the liquid air stream 106 is subcooled by the cooler 8 and fed to the upper middle part of the second column 12.
Air streams 103 and 105 entering a column are separated by cryogenic rectification in a column 11, with light nitrogen enriched towards the top of the column and heavy oxygen enriched towards the bottom of the column. Extracting the oxygen-enriched liquid air 107 from the bottom of the first tower, supercooling the oxygen-enriched liquid air by a cooler 8, sending the oxygen-enriched liquid air to the middle of the second tower to continuously participate in rectification separation, extracting the nitrogen-poor liquid 113 from the middle of the first tower, supercooling the nitrogen-poor liquid by the cooler 8, and sending the nitrogen-poor liquid to the upper part of the second tower to serve as reflux liquid. The nitrogen gas from the top of the first tower 11 is collected and sent to the main condensing evaporator 13 at the bottom of the second tower 12 to exchange heat with the liquid oxygen stored therein, the liquid oxygen is partially evaporated, and the nitrogen gas is condensed to obtain pure liquid nitrogen. One part of the pure liquid nitrogen is taken as reflux liquid of a tower, and the other part of the pure liquid nitrogen is led out of the tower to be taken as a product. According to actual needs, the first part 108 of the pure liquid nitrogen product is subcooled by the subcooler 8 and then decompressed by the third throttle valve 25 to generate a gas-liquid mixed stream, and the gas-liquid mixed stream is separated into gas-phase pure nitrogen 108a and liquid-phase pure liquid nitrogen 108b and 108b in the third gas-liquid separation equipment 26 and is input into the liquid nitrogen storage tank 14, and the 108a is discharged. The second portion 109 of the purified liquid nitrogen product is raised to a suitable pressure by liquid nitrogen pump 9 and reheated in main heat exchanger 7 to produce a nitrogen product.
After entering the second column 12, a liquid air stream 106, an oxygen-rich liquid air 107 and a lean liquid nitrogen 113 are separated into pure liquid oxygen at the bottom of the second column and a top nitrogen gas 112 by rectification. The nitrogen content of the dirty nitrogen 112 is generally over 90 mole percent, and this stream contains a large amount of cold energy, which provides necessary cold energy for the subcooler and the main heat exchanger, and can be directly evacuated after reheating, and also can be used for the cooling of a precooling system and/or the regeneration of a purification system. The pure liquid oxygen generated by the second tower can also be divided into a plurality of parts according to the requirements, for example, the first part of the pure liquid oxygen 110 is lifted to the proper pressure in the liquid oxygen pump 10, and an oxygen product is obtained after reheating in the main heat exchanger 7; the second part of pure liquid oxygen 111 enters the liquid oxygen storage tank 15 for storage after being supercooled by the supercooler 8.
Since the operating pressure of one column is much higher than that of the second column, the stream withdrawn from one column must be depressurized in a suitable manner before entering the second column, a throttling valve being a common depressurization device. Under the temperature and pressure conditions of cryogenic rectification, the liquid stream passing through the throttling valve forms two phases of gas and liquid due to flash evaporation. Both the liquid air stream 106 and the oxygen-enriched liquid air 107 serve to feed the air component to be separated into the two columns, and sufficient material for the rectification is ensured only if as much of these streams as possible are fed into the two columns. In view of this, in the prior art, a throttled gas-liquid mixed stream is divided into a gas phase and a liquid phase by using gas-liquid separation equipment, and then the gas phase and the liquid phase are respectively and completely introduced into the two towers at appropriate positions according to different component compositions of the gas phase and the liquid phase. The gas-liquid separation equipment can be a gas-liquid separation tank or a pipeline. Specifically, since liquid air is higher in nitrogen content than oxygen-enriched liquid air, liquid air is introduced into the second column at a higher level than oxygen-enriched liquid air. The liquid air stream 106 is changed into a gas-liquid mixed stream after passing through the throttling valve 21, a first gas-phase air stream 106a and a liquid-phase air stream 106b are obtained through separation in the gas-liquid separation tank 22, and the gas-phase stream 106a and the liquid-phase stream 106b respectively enter the two towers at the same height through a gas-phase feeding pipe and a liquid-phase feeding pipe. Similarly, the oxygen-rich liquid air 107 becomes a gas-liquid mixed stream after passing through the throttling valve 23, a first part of gas-phase air stream 107a and a first part of liquid-phase air stream 107b are obtained by separation in the gas-liquid separation tank 24, and the gas-phase stream 107a and the liquid-phase stream 107b enter the two towers at the same height through a gas-phase feeding pipe and a liquid-phase feeding pipe respectively.
The liquid storage tank is also typically at a much lower pressure than the operating pressure of a column 11, so that the pure liquid nitrogen 108 drawn from a column is also reduced in pressure by the throttle valve 25 before entering the liquid nitrogen storage tank 14. The throttled gas-liquid mixed stream is separated into gas and liquid phases in the gas-liquid separation tank 26, liquid phase pure liquid nitrogen 108b enters the liquid nitrogen storage tank 14, and gas phase pure nitrogen 108a tends to enter the storage tank and then is discharged.
The invention is not limited to the common practice and thinking in the field, but develops a new way to research the influence on the rectification effect and equipment if the gas phase flow generated after throttling is not completely introduced into two towers, and measures and compares the relative importance degree of the influence and the reduced oxygen extraction amount, and selects a proper method according to different situations from the global angle. Meanwhile, the method of recovering the waste nitrogen is adopted to avoid the cold loss of the cryogenic rectification and improve the operation efficiency of the whole process.
Fig. 2 shows three embodiments of the present invention, which need not be employed simultaneously, but may be optionally combined one or more according to actual needs. Which will be described separately below.
The transport diameter of the rectification column depends on the section with the largest column diameter, often the diameter of two columns. The diameter of the two columns is determined by the maximum gas flow introduced at any position, so that the column diameter can be reduced by reducing the maximum gas flow entering the rectifying column at a certain position, thereby reducing the size of the whole cold box and being beneficial to reducing the manufacturing cost, the transportation cost and the occupied area.
The oxygen yield is 90,000Nm 3 The total flow of liquid air stream 106 to both columns is 60,000Nm if the process scheme shown in FIG. 1 is used as an example of a cryogenic rectification double column air separation system 3 H a flash rate of 12% at the first throttle 21, i.e. the total flow rate of the gas-phase air stream separated by the first gas-liquid separation device 22 after throttling is 7200Nm 3 H is used as the reference value. If the entire gaseous air stream (106a + 106c) is fed to the second column, the diameter of the second column is 4850mm, exceeding the 4800mm transportation limit. At this point, the reduction of the vapor air stream entering the second column would be about 4800Nm 3 A second partial vapor phase air stream 106c of/h is introduced by-pass into the conduit for the dirty nitrogen 112The first part of the vapor phase air stream 106a entering the second column is 2400Nm 3 H, about 33% of the total amount, and the diameter of the second column can be reduced to 4800mm, which satisfies the transportation restriction.
The second part of the gaseous air stream 106c entering the nitrogen waste 112 contains a part of the oxygen which naturally reduces the oxygen extraction rate of the whole system, but it is calculated that this part has a very small loss, only accounting for 0.1% of the total production, because throttling causes flashing, the oxygen content of the liquid stream increases after flashing, and the oxygen content of the gaseous stream decreases. Table 1 compares the gas composition in each stream before and after throttling of the liquid air stream and in the nitrogen contaminated.
TABLE 1 gaseous composition of streams after throttling of Nitrogen contaminated and liquid air streams
Stream 106 Stream 106a/106c Stream 106b Stream 112
N 2 ％ 78.0％ 92.4％ 76.0％ 94.1％
O 2 ％ 21.0％ 7.2％ 23.0％ 5.1％
Ar％ 0.9％ 0.5％ 1.0％ 0.8％
Wherein stream 106 represents the liquid air stream prior to throttling; stream 106a/106c represents a throttled vapor phase air stream, the gas compositions of which are consistent; stream 106b represents a throttled liquid air stream; stream 112 represents the nitrogen gas effluent. The data in the table shows that the oxygen content in vapor stream 106a/106c is 7.2% much less than the oxygen content in air itself and the liquid stream (21.0% and 23.0%, respectively). Thus, the loss in oxygen production resulting from the introduction of the partial vapor phase stream 106c into the nitrogen gas effluent is:
4800Nm 3 /h x(7.2％-5.1％)＝101Nm 3 h, about 90,000Nm of total yield 3 0.1% of/h.
In some cryogenic rectification air separation processes, a liquid air stream is fed to only one column and not to both columns, where the most significant stream to both columns is oxygen-rich liquid air. Similar to the case of the aforementioned liquid air stream, the diameter of the two columns is determined by the flow rate of the gas phase oxygen-rich liquid air stream (107a + 107c) obtained after throttling, when the diameter of the two columns is too large, a second portion of gas phase oxygen-rich liquid air stream 107c can be introduced into the dirty nitrogen gas 112, so that the flow rate of the first portion of gas phase oxygen-rich liquid air stream 107a is reduced. Table 2 compares the gas composition of each stream before and after throttling of the oxygen-rich liquid air stream and the contaminated nitrogen.
TABLE 2 gas composition of each stream after throttling of dirty nitrogen and oxygen-enriched liquid air streams
Stream 107 Stream 107a/107c Stream 107b Stream 112
N 2 ％ 61.0％ 84.4％ 58.1％ 94.1％
O 2 ％ 37.5％ 14.8％ 40.3％ 5.1％
Ar％ 1.5％ 0.9％ 1.6％ 0.8％
Wherein stream 107 represents an oxygen-rich liquid air stream prior to throttling; the stream 107a/107c represents a throttled gas-phase oxygen-enriched liquid air stream, and the gas compositions of the two streams are consistent; stream 107b represents a throttled liquid phase oxygen-rich liquid air stream; stream 112 represents the nitrogen gas effluent. The data show that 14.8% oxygen in the post-throttling vapor stream is much less than the oxygen content in the oxygen-rich liquid air itself and the liquid stream before throttling (37.5% and 40.3%, respectively), so the loss of oxygen production from introducing a portion of vapor stream 107c into the contaminated nitrogen gas is not too high.
In addition to directly changing the diameter of the second column, the flow of the gaseous oxygen-enriched liquid air entering the second column can further influence the operation of the crude argon column connected to the second column. On the one hand, when partial gas-phase oxygen-enriched liquid air is directly discharged into the polluted nitrogen, more nitrogen is taken away, so that the nitrogen content in the feed gas of the crude argon removing tower is reduced, and the risk of a nitrogen plug of the crude argon removing tower is reduced. In yet another aspect, the oxygen-rich liquid air used in the crude argon condenser vaporizer can be bypassed into the contaminated nitrogen gas in the event of a shutdown of the crude argon column, thereby avoiding flooding the secondary column gas feed, and helping to maintain stable operation of the main air separation system.
For the liquid air stream and the oxygen-rich liquid air stream, a first regulating valve 29 and a second regulating valve 30, which can be selected to be either an on-off valve or a control valve, can be respectively arranged on a bypass pipeline for introducing the throttled part of the gas phase stream into the polluted nitrogen gas. Compared with direct emptying, the introduction of the waste nitrogen can reduce the cold loss of the device, thereby reducing the operation energy consumption.
A subcooler is not necessary in the cryogenic rectification air separation scheme, but its use can reduce the flash rate of the post-throttling stream. As shown in FIG. 2, taking saturated liquid nitrogen drawn from a column as an example, entering the liquid nitrogen storage tank 14 at atmospheric pressure (1.1 Bar A) from a column at a pressure of 5.5Bar A, a pressure reduction device, such as a throttle valve, must be used. If a subcooler is not used, the temperature of the saturated pure liquid nitrogen 108 before entering the third throttle valve 25 is-178 ℃, the temperature of the mixed stream after passing through the throttle valve 25 is-195 ℃, and the flash evaporation rate is 18 percent, namely the mol percent of the gas-phase pure nitrogen 108a in the total amount is 18 percent. If the pure liquid nitrogen 108 is subcooled, the temperature of the pure liquid nitrogen is reduced to-190 ℃ after passing through the subcooler, the temperature of the mixed stream is-195 ℃ after passing through the throttling valve 25, and the flash evaporation rate is 5%. After the gas-phase pure nitrogen 108a is sent into the liquid nitrogen storage tank, the gas-phase pure nitrogen cannot be recycled and can only be discharged to the air, so that the loss can be greatly reduced by using the subcooler; further, the gaseous phase pure nitrogen 108a is merged into the dirty nitrogen 112 before the dirty nitrogen enters the subcooler, so that the cold energy contained in the dirty nitrogen can be recovered, the cold energy loss of the device is reduced, and the operation energy consumption is reduced.
The above are several embodiments for implementing the present invention, but the present invention is not limited to the embodiments, and various equivalent modifications or substitutions by those skilled in the art according to the present disclosure are included in the scope defined by the claims of the present application.

Claims (6)

1. A cryogenic air separation plant comprising:
a) At least one column operating at a first pressure and two columns operating at a second, relatively lower pressure, the one and two columns being in heat exchange communication,
b) A main compressor for pressurizing, pre-cooling, purifying feed air to produce a liquid air stream, an air pre-cooling and purification system, a main heat exchanger, and an expander,
c) A pipeline system, wherein the pipeline system comprises a pipeline for conveying the liquid air stream to the first tower and/or the second tower, a pipeline for conveying oxygen-enriched liquid air and lean liquid nitrogen generated by the first tower into the second tower, and/or a pipeline for conveying pure liquid nitrogen generated by the first tower into a liquid nitrogen storage tank, and a pipeline for reheating waste nitrogen generated by the second tower through a main heat exchanger;
the system also comprises a first throttling valve and a first gas-liquid separation device which are sequentially arranged before the liquid air stream enters the second tower, a pipeline for introducing a liquid phase generated by the first gas-liquid separation device into the second tower and a pipeline for merging at least part of a gas phase generated by the first gas-liquid separation device into the polluted nitrogen;
and/or a second throttle valve and a second gas-liquid separation device which are sequentially arranged before the oxygen-enriched liquid air enters the second tower, a pipeline for introducing a liquid phase generated by the second gas-liquid separation device into the second tower and a pipeline for converging at least part of a gas phase generated by the second gas-liquid separation device into the polluted nitrogen.
2. The cryogenic air separation plant of claim 1 further comprising a subcooler further subcooling the liquid air and/or oxygen-enriched liquid air and/or pure liquid nitrogen.
3. The cryogenic air separation plant of claim 2 wherein at least a portion of the vapor phase of each vapor-liquid separation device is diverted into the conduit for the dirty nitrogen prior to entering the subcooler for heat exchange.
4. A cryogenic air separation plant according to claim 1 wherein each gas-liquid separation device comprises a gas-liquid separation tank and/or a pipeline.
5. A cryogenic air separation plant according to claim 1 wherein a switch valve and/or a control valve is provided in the conduit for at least part of the gaseous phase of each gas-liquid separation plant into the dirty nitrogen.
6. A method of cryogenic air separation using the apparatus of claim 1, comprising:
a) Providing at least one column operating at a first pressure and two columns operating at a second, relatively lower pressure, the one and two columns being in heat exchange communication,
b) Providing a main compressor, an air pre-cooling and purification system, a main heat exchanger and an expander for pressurizing, pre-cooling, purifying feed air to produce a liquid air stream,
c) Providing a pipeline system which comprises a pipeline for conveying the liquid air stream to one tower and/or two towers, a pipeline for conveying oxygen-enriched liquid air and lean liquid nitrogen generated by one tower into the two towers, and/or a pipeline for conveying pure liquid nitrogen generated by one tower into a liquid nitrogen storage tank, and a pipeline for reheating the waste nitrogen generated by the two towers through a main heat exchanger,
throttling the liquid air stream and/or the oxygen-enriched liquid air before entering the second tower, separating the throttled stream into a gas phase and a liquid phase, sending the liquid phase into the second tower, and gathering at least part of the gas phase into a pipeline for polluted nitrogen.
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