Cold box of argon recovery apparstus for full recovery of argon and hydrogen, and use method therefor

Abstract

A cold box of an argon recovery apparatus for full recovery of argon and hydrogen, and a use method therefor. The cold box (C501) is internally provided with an argon purification column (C21), a waste gas distillation column (C23), a nitrogen production rectification column (C22), main heat exchangers, expanders, and a nitrogen production condensing evaporator (K23), which are connected to each other by means of pipes. Two main heat exchangers are provided, which are a first main heat exchanger (E21) and a second main heat exchanger (E22) respectively. Also, two expanders are provided, which are a first expander (ET501) and a second expander (ET502) respectively, and when in use, one is being used and the other is on standby, with the two being switched for use. The waste gas distillation column (C23) is used for cold capacity balancing of the cold box (C501), and transfers excess cold capacity of the nitrogen production system to an argon separation system. The use of the cold box (C501) allows the argon recovery rate to be maintained close to 100%, achieving a small liquid argon re-injection amount, while also recovering and utilizing hydrogen in the waste gas.

Description

A cold box for argon recovery device with complete argon-hydrogen recovery and its usage method Technical Field

[0001] This invention relates to gas separation technology, specifically, to the design and improvement of the cold box of an argon recovery device, the addition of a nitrogen extraction port to the waste gas distillation tower, and the use of a cryogenic distillation method to design an argon recovery device cold box for complete argon and hydrogen recovery and its application method, belonging to the field of cryogenic technology. Background Technology

[0002] Argon recovery units, by recovering the sealed argon gas from single crystal furnaces and performing decarbonization and deoxygenation pretreatment and cryogenic distillation separation, allow for the recycling of argon gas, significantly reducing the production cost of single crystal rods and enabling the industrial production of single crystal rods. Hydrogen-free argon recovery units, however, do not recover oxygen-containing argon gas collected by auxiliary pumps, resulting in high argon consumption and frequent liquid argon replenishment, leading to poor economic efficiency. Therefore, later-built argon recovery units for single crystal furnaces all adopt hydrogen-based processes. Hydrogen-based argon recovery units involve front-end processes such as dust removal and pressurization of the raw gas, compressor compression, decarbonization and drying, hydrogenation for deoxygenation and moisture adsorption, followed by cryogenic separation in a cold box.

[0003] In recent years, with the development of the social economy and the advancement of industrial technology, the market demand for high-purity argon has been increasing, and the requirements have become more stringent. The development of the photovoltaic and semiconductor industries has also led to a simultaneous increase in the number and scale of argon recovery devices in operation. With the application of two-stage separation of argon gas as raw material in the cold box of argon recovery devices, the argon recovery rate in the cold box is close to 100%. To further reduce equipment investment and operating power consumption, and to recover and utilize hydrogen from the waste gas, a cold box for argon recovery device with complete argon-hydrogen recovery and its usage method are designed. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a cold box for an argon recovery device with complete argon-hydrogen recovery and its usage method. This invention improves the argon recovery rate of the cold box while reducing equipment investment and operating power consumption through a new process. The application of this cold box not only maintains a near 100% argon recovery rate and minimizes liquid argon backfilling, but also recovers and reuses hydrogen from waste gas, thereby reducing equipment investment and operating power consumption. It has significant practical value and economic benefits.

[0005] The application is implemented by the following technical scheme: an argon recovery device cold box for argon and hydrogen full recovery, which comprises a cold box, wherein an argon purification tower, a waste gas distillation tower, a nitrogen production rectification tower, main heat exchangers, expanders, a nitrogen production condensation evaporator are installed in the cold box and connected with each other through pipelines, wherein the main heat exchangers are provided with two main heat exchangers, namely a first main heat exchanger and a second main heat exchanger, and the expanders are provided with two expanders, namely a first expander and a second expander, one of which is used and the other is standby, and the two are switched for use, and the waste gas distillation tower is used for cold balance of the cold box and transfers the excess cold of the nitrogen production system to the argon separation system.

[0006] As preferred: the argon purification tower is composed of a purification tower condenser at the top, a purification tower evaporator at the bottom and a middle tower body section, and the waste gas distillation tower is composed of a waste gas distillation tower condenser at the top, a waste gas distillation tower evaporator at the bottom and a middle tower body section.

[0007] As preferred: the nitrogen production rectification tower can be arranged together with the nitrogen production condensation evaporator or separately.

[0008] As preferred: the first main heat exchanger is provided with a first raw argon gas inlet pipe, a nitrogen-hydrogen-argon circulating gas pipe, a non-condensable waste gas outlet pipe, a low-pressure nitrogen gas outlet pipe and a waste nitrogen gas outlet pipe of an auxiliary tower, and the second main heat exchanger is provided with a second raw argon gas inlet pipe, an oxygen-rich gas outlet pipe, a purified air inlet pipe, a nitrogen gas outlet pipe and a product argon gas outlet pipe.

[0009] As preferred: the cold box is further connected with a return liquid argon pipeline which is connected with the argon purification tower and the waste gas distillation tower, and the nitrogen production condensation evaporator is further provided with a product nitrogen gas pipeline which is connected with the outside of the cold box.

[0010] A use method of the argon recovery device cold box for argon and hydrogen full recovery, the method comprising the following steps:

[0011] Step 1: crude argon gas is divided into two paths and enters the first main heat exchanger and the second main heat exchanger for precooling and temperature reduction, and after temperature reduction, the two paths are combined and enter the argon purification tower evaporator, part of non-condensable gas is reheated out of the cold box through the first main heat exchanger, and the remaining non-condensable gas is condensed into crude liquid argon;

[0012] Step 2: the non-condensable gas out of the cold box is mixed with supplemental hydrogen gas by a circulating compressor and then enters a hydrogenation deoxidization system for recycling;

[0013] Step 3: crude liquid argon is throttled and enters the argon purification tower for rectification, high-purity liquid argon is obtained at the bottom of the argon purification tower, and hydrogen-containing argon waste nitrogen is obtained at the top of the argon purification tower;

[0014] Step 4: The hydrogen-containing argon waste nitrogen is condensed in the condenser of the purification tower, and the condensed liquid is used as the reflux liquid of the argon purification tower; part of the non-condensable gas is sent to the waste gas stripping tower for further distillation and separation.

[0015] Step 5: Non-condensable gas is fed into the lower part of the waste gas stripping tower. Through distillation separation, the bottom rich liquid argon is heated and evaporated by nitrogen from the nitrogen production distillation tower, and used as the rising gas of the distillation tower; the top gas is condensed into liquid by liquid nitrogen and used as the reflux liquid of the waste gas stripping tower. Sludge nitrogen is extracted from the upper part of the tower and used as waste gas. It is reheated in the first main heat exchanger and discharged from the cold box or reused. The hydrogen-containing non-condensable gas at the top of the tower is reheated in the first main heat exchanger and discharged from the cold box to enter the argon recovery unit for recycling.

[0016] Step 6: The liquid nitrogen condensed in the evaporator of the waste gas stripping tower is throttled into the condenser of the waste gas stripping tower and evaporated as a cold source. A portion of crude liquid argon is drawn from the bottom of the waste gas stripping tower as reflux liquid at the top of the argon purification tower. At the same time, liquid nitrogen is drawn from the nitrogen generator evaporator and throttled into the condenser of the waste gas stripping tower to supplement the cooling capacity and increase the reflux ratio of the waste gas stripping tower.

[0017] Step 7: The evaporated low-pressure nitrogen gas is reheated by the first main heat exchanger and exits the cold box as regeneration gas for the deoxygenation purifier.

[0018] Step 8: The high-purity liquid argon obtained from the argon purification tower is throttled and enters the purification tower condenser. The liquid argon is evaporated, and then reheated through the second main heat exchanger before exiting the cold box and being sent to the product argon compressor.

[0019] Step 9: After precooling in the second main heat exchanger, the air enters the nitrogen distillation column. Through distillation, oxygen-enriched liquid air is obtained at the bottom and high-purity nitrogen is obtained at the top. A portion of the nitrogen is extracted from the top of the nitrogen distillation column, reheated in the second main heat exchanger, and then discharged from the cold box for user use and as regeneration gas for the deoxygenation purifier.

[0020] Step 10: After throttling, the oxygen-enriched liquid air enters the nitrogen generator condenser evaporator, evaporates into oxygen-enriched air, and then is reheated to a certain temperature through the second main heat exchanger before entering the first and second expanders for expansion and refrigeration to provide cooling capacity for the cold box. After expansion, the oxygen-enriched air is reheated through the second main heat exchanger and exits the cold box as regeneration gas for the air purifier and decarbonization purifier.

[0021] Step 11: In the nitrogen-generating condenser-evaporator, nitrogen gas is condensed into liquid nitrogen. Most of it is used as reflux liquid in the nitrogen-generating distillation column, part of it is used as a cold source for the condenser of the waste gas stripping column, and part of it is sent to the liquid nitrogen storage tank as product liquid nitrogen.

[0022] Step 12: In order to maintain the balance of argon usage and cold box cooling capacity, a portion of liquid argon is reinjected into the argon purification tower.

[0023] The beneficial effects of the present application are as follows: the present application utilizes the low-temperature rectification separation principle, and performs secondary rectification on the non-condensable waste gas discharged from the first rectification of the argon tower, so that the argon recovery rate of the cold box of the argon recovery device reaches 99.95% or above (when the raw gas contains 1.2% of oxygen), and the hydrogen-containing non-condensable gas at the top is reheated and then enters the gas tank for recycling. The use of the present application maintains the ultra-high argon and hydrogen recovery rate of the cold box of the argon recovery device and the low liquid argon injection amount, while reducing the investment cost of the cold box and the complexity of operation. The increase of the argon recovery rate and the reduction of the liquid argon injection amount reduce the cost of purchasing liquid argon and the dependence on the liquid argon market. At the same time, the recycling of hydrogen reduces the amount of water electrolysis hydrogen and the power consumption. For regions where liquid argon is in short supply, the economic efficiency is more obvious and considerable, so the present application has good practical value and economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0024] Fig. 1 is a process flow diagram of the present application.

[0025] Wherein N2 is a first raw argon gas inlet pipe; N3 is a nitrogen-hydrogen-argon circulating gas inlet pipe; N4 is a product argon gas outlet pipe; N5 is a non-condensable waste gas outlet pipe; N6 is a liquid argon injection pipeline; N7 is a purified air inlet pipe; N8 is a nitrogen gas outlet pipe; N9 is an oxygen-rich gas outlet pipe; N10 is a product nitrogen gas pipeline; N11 is a low-pressure nitrogen gas outlet pipe; and N12 is an auxiliary tower waste nitrogen gas outlet pipe. DETAILED DESCRIPTION

[0026] In order for those skilled in the art to more clearly understand the purpose, technical solution and advantages of the present application, the present application will be further described below in conjunction with the drawings and examples.

[0027] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "up", "down", "left", "right", "inner", "outer", "horizontal", "vertical" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application, and does not indicate or imply that the device or element referred to must have a particular orientation, therefore it cannot be understood as a limitation on the present application.

[0028] A cold box of an argon recovery device for full recovery of argon and hydrogen will be described in detail below with reference to the drawings: As shown in FIG. 1, the cold box of the argon recovery device for full recovery of argon and hydrogen comprises a cold box C501, wherein an argon purification tower, a waste gas distillation tower, a nitrogen production rectification tower, main heat exchangers, expanders, and a nitrogen production condenser evaporator are installed in the cold box C501 and connected with each other through pipelines. The main heat exchangers are provided with two first main heat exchangers E21 and two second main heat exchangers E22, respectively. The expanders are provided with two first expanders ET501 and two second expanders ET502, respectively. One of the expanders is used and the other is standby, and the two are switched for use. The waste gas distillation tower is used for cold balance of the cold box and transfers excess cold of the nitrogen production system to the argon separation system.

[0029] The argon purification tower C21 is composed of a purification tower condenser at the top, a purification tower evaporator at the bottom, and a middle tower body section. The waste gas distillation tower is composed of a waste gas distillation tower condenser at the top, a waste gas distillation tower evaporator at the bottom, and a middle tower body section. The nitrogen production rectification tower can be arranged together with the nitrogen production condenser evaporator or separately. The first main heat exchanger E21 is provided with a first raw argon gas inlet pipe N2, a nitrogen-hydrogen-argon circulating gas pipe N3, a non-condensable waste gas outlet pipe N5, a low-pressure nitrogen gas outlet pipe N11, and an auxiliary tower dirty nitrogen gas outlet pipe N12. The second main heat exchanger E22 is provided with a second raw argon gas inlet pipe N1, an oxygen-rich gas outlet pipe N9, a purified air inlet pipe N7, a nitrogen gas outlet pipe N8, and a product argon gas outlet pipe N4.

[0030] The cold box C501 is further connected with a return liquid argon pipeline N6 connected with the argon purification tower C21 and the waste gas distillation tower C23. The nitrogen production condenser evaporator K23 is further provided with a product nitrogen pipeline N10 connected with the outside of the cold box.

[0031] A method for using the cold box of the argon recovery device for full recovery of argon and hydrogen, the method comprising the following steps:

[0032] Step 1: crude argon gas is divided into two paths and enters the first main heat exchanger E21 and the second main heat exchanger E22 for precooling and temperature reduction. After temperature reduction, the crude argon gas is combined and enters the argon purification tower evaporator K21. Part of the non-condensable gas is reheated by the first main heat exchanger E21 and discharged from the cold box. The remaining non-condensable gas is condensed into crude liquid argon.

[0033] Step 2: The non-condensable gas discharged from the cold box is mixed with supplemental hydrogen by a circulating compressor and then enters a hydrogenation deoxidization system for recycling.

[0034] Step 3: The crude liquid argon is throttled and enters the argon purification tower C21 for rectification. High-purity liquid argon is obtained at the bottom of the argon purification tower C21. Hydrogen-containing argon dirty nitrogen is obtained at the top of the argon purification tower C21.

[0035] Step 4: The hydrogen-containing argon waste gas is condensed in the purification column condenser K22, and the condensed liquid is used as reflux liquid of the argon purification column C21; part of the non-condensable gas is sent to the waste gas rectification column C23 for further rectification and separation;

[0036] Step 5: The non-condensable gas is sent to the lower part of the waste gas rectification column C23, and is separated by rectification; the bottom liquid argon is heated and evaporated by the nitrogen gas from the nitrogen rectification column C22 to serve as the rectification column rising gas; the top gas is condensed into liquid by liquid nitrogen to serve as the waste gas rectification column C23 reflux liquid; the waste gas is extracted from the upper part of the column and is reheated in the first main heat exchanger E21 to be discharged from the cold box or utilized; the top hydrogen-containing non-condensable gas in the column is reheated in the first main heat exchanger E21 to be discharged from the cold box and enters the argon recovery device for recycling;

[0037] Step 6: The liquid nitrogen condensed in the waste gas rectification column evaporator K24 is throttled into the waste gas rectification column condenser K25 to serve as the evaporation source; part of the crude liquid argon is extracted from the bottom of the waste gas rectification column C23 to serve as the reflux liquid at the top of the argon purification column C21; liquid nitrogen is extracted from the nitrogen condensation evaporator K23 and throttled into the waste gas rectification column condenser K25 to supplement the cold quantity and increase the reflux ratio of the waste gas rectification column C23;

[0038] Step 7: The evaporated low-pressure nitrogen gas is reheated in the first main heat exchanger E21 to be discharged from the cold box C501 to serve as the deoxygenation purifier regeneration gas;

[0039] Step 8: The high-purity liquid argon obtained from the argon purification column C21 is throttled into the purification column condenser K22, and the liquid argon is evaporated and reheated in the second main heat exchanger E22 to be discharged from the cold box C501 to be sent to the product argon gas compressor;

[0040] Step 9: The air is pre-cooled in the second main heat exchanger E22 and then enters the nitrogen rectification column C22; the bottom of the column is obtained by rectification and separation to obtain oxygen-rich liquid air, and the top is obtained by rectification and separation to obtain high-purity nitrogen gas; part of the nitrogen gas is extracted from the top of the nitrogen rectification column C22, reheated in the second main heat exchanger E22 to be discharged from the cold box, and used by the user and as the regeneration gas of the deoxygenation purifier;

[0041] Step 10: The oxygen-rich liquid air is throttled into the nitrogen condensation evaporator K23, evaporated into oxygen-rich air, and reheated to a certain temperature in the second main heat exchanger E22 to be expanded in the first expander ET501 and the second expander ET502 to provide cold quantity for the cold box; the expanded oxygen-rich air is reheated in the second main heat exchanger E22 to be discharged from the cold box to serve as the regeneration gas of the air purifier and the decarburization purifier;

[0042] Step 11: In the nitrogen condensation evaporator K23, the nitrogen gas is condensed into liquid nitrogen, most of which is used as reflux liquid of the nitrogen rectification column C22, part of which is used as a cold source of the waste gas rectification column condenser K25, and part of which is used as product liquid nitrogen and sent to the liquid nitrogen storage tank;

[0043] Step 12: In order to maintain the balance of argon gas and the cold box cold balance, the liquid argon is injected into the argon gas purification tower C21. The condenser and the main heat exchanger in the application are both plate-fin heat exchangers.

[0044] In order to improve the argon and hydrogen recovery rate of the cold box of the current argon recovery device, the first thing to consider is to reduce the emission of argon and hydrogen and improve the argon and hydrogen recovery rate of the cold box. The application studies the following methods to improve the argon and hydrogen recovery rate of the cold box of the argon recovery device: 1. An incondensable waste gas rectification tower is added to the original argon recovery device cold box. The lower part of the rectification tower is provided with a reboiler, and the nitrogen gas in the nitrogen tower is used as a heat source; the top of the rectification tower is provided with a condenser, and liquid nitrogen is used as a cold source. In order to obtain usable crude hydrogen at the top, the contaminated nitrogen gas is extracted from the upper part of the tower. 2. Due to the increase of the argon recovery rate, the liquid argon injection amount needs to be reduced, and in order to maintain the cold balance of the device, a reflux gas expander needs to be added to the cold box.

[0045] The design features of the application are as follows:

[0046] 1) The secondary separation of waste gas reduces the content of hydrogen and argon in waste gas and improves the hydrogen and argon recovery amount;

[0047] 2) The setting of the waste gas rectification tower condenser and evaporator improves the reflux ratio and effectively improves the argon recovery amount.

[0048] 3) The selection of the waste gas rectification tower condenser cold fluid adopts liquid nitrogen to ensure a certain temperature difference with the waste gas at the top.

[0049] 4) The waste gas rectification tower C23 is provided with a contaminated nitrogen extraction, which realizes a high hydrogen concentration of incondensable gas at the top and can be recycled.

[0050] The specific embodiments described in the present application are only illustrative of the principles and effects of the application, and are not used to limit the application. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the application should be covered by the claims of the application.

Claims

1. An argon recovery device cold box for argon-hydrogen full recovery, comprising a cold box, characterized in that: The cold box is internally provided with an argon purification column, a waste gas distillation column, a nitrogen production rectifying column, main heat exchangers, expanders, a nitrogen production condensation evaporator, and is connected with each other through pipelines, wherein the main heat exchangers are provided with two, which are a first main heat exchanger and a second main heat exchanger, and the expanders are also provided with two, which are a first expander and a second expander, one is used and the other is standby, and they are switched with each other for use, the waste gas distillation column is used for cold balance of the cold box, and the excess cold of the nitrogen production system is transferred to the argon separation system.

2. The cold box of the argon recovery device for argon and hydrogen full recovery according to claim 1, characterized in that: The argon purification column is composed of a purification column condenser at the top, a purification column evaporator at the bottom and a middle column body section, and the waste gas distillation column is composed of a waste gas distillation column condenser at the top, a waste gas distillation column evaporator at the bottom and a middle column body section.

3. The cold box of the argon recovery device for argon and hydrogen full recovery according to claim 1, characterized in that: The nitrogen production rectifying column can be arranged together with the nitrogen production condensation evaporator or separately.

4. The cold box of the argon recovery device for argon and hydrogen full recovery according to claim 1, characterized in that: The first main heat exchanger is provided with a first raw material argon gas inlet pipe, a nitrogen-hydrogen-argon circulating gas pipe, a non-condensable waste gas outlet pipe, a low-pressure nitrogen gas outlet pipe and a secondary column contaminated nitrogen gas outlet pipe, and the second main heat exchanger is provided with a second raw material argon gas inlet pipe, an oxygen-rich gas outlet pipe, a purified air inlet pipe, a nitrogen gas outlet pipe and a product argon gas outlet pipe.

5. The cold box of the argon recovery device for argon and hydrogen full recovery according to claim 1, characterized in that: The cold box is further connected with a return liquid argon pipeline which is connected with the argon purification column and the waste gas distillation column, and the nitrogen production condensation evaporator is further provided with a product nitrogen gas pipeline which is connected with the outside of the cold box.

6. A method of using a cold box of an argon recovery device for full recovery of argon and hydrogen according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: Step 1: crude argon gas is divided into two paths and enters the first main heat exchanger and the second main heat exchanger for precooling and temperature reduction, is combined after temperature reduction and enters the argon purification column evaporator, part of non-condensable gas is reheated out of the cold box through the first main heat exchanger, and the remaining condenses into crude liquid argon; Step 2: the non-condensable gas out of the cold box is mixed with supplemental hydrogen gas through a circulating compressor and then enters a hydrogen addition and oxygen removal system and is recycled; Step 3: crude liquid argon is throttled and enters the argon purification column for rectification, high-purity liquid argon is obtained at the bottom of the argon purification column, and hydrogen-containing argon contaminated nitrogen is obtained at the top of the argon purification column; Step 4: the hydrogen-containing argon contaminated nitrogen is condensed in the purification column condenser, the condensed liquid is used as reflux liquid of the argon purification column, and part of non-condensable gas is sent into the waste gas distillation column for rectification and separation again; Step 5: the non-condensable gas is sent into the lower part of the waste gas distillation column, is separated and rectified, the bottom liquid argon is heated and evaporated by nitrogen gas from the nitrogen production rectifying column and is used as ascending gas of the rectifying column, the top gas is condensed into liquid by liquid nitrogen and is used as reflux liquid of the waste gas distillation column, contaminated nitrogen gas is extracted at the upper part of the column and is reheated in the first main heat exchanger to be vented out of the cold box or utilized, and the hydrogen-containing non-condensable gas at the top of the column is reheated in the first main heat exchanger, enters an argon recovery device and is recycled; Step 6: liquid nitrogen condensed by the waste gas distillation column evaporator is throttled and enters the waste gas distillation column condenser and is evaporated as a cold source, part of crude liquid argon is extracted from the bottom of the waste gas distillation column and is used as reflux liquid at the top of the argon purification column, liquid nitrogen is extracted from the nitrogen production condensation evaporator, is throttled and enters the waste gas distillation column condenser, cold is supplemented, and reflux ratio of the waste gas distillation column is increased; Step 7: the evaporated low-pressure nitrogen gas is reheated out of the cold box through the first main heat exchanger and is used as regenerative gas of the oxygen removal purifier. Step 8: The high-purity liquid argon obtained from the argon purification column is throttled into the condenser of the purification column, and the liquid argon is evaporated, then reheated by the second main heat exchanger to be sent to the product argon compressor; Step 9: The air is pre-cooled by the second main heat exchanger, then enters the nitrogen production rectification column, and is separated by rectification to obtain oxygen-rich liquid air at the bottom and high-purity nitrogen gas at the top. Part of the nitrogen gas is extracted from the top of the nitrogen production rectification column, reheated by the second main heat exchanger to be sent to the user and used as the regeneration gas of the deoxidization purifier; Step 10: The oxygen-rich liquid air is throttled into the nitrogen production condenser-evaporator, evaporated into oxygen-rich air, then reheated by the second main heat exchanger to a certain temperature, enters the first and second expanders to be expanded to provide cold energy for the cold box. The oxygen-rich air after expansion is reheated by the second main heat exchanger to be sent to the cold box as the regeneration gas of the air purifier and the decarburization purifier; Step 11: In the nitrogen production condenser-evaporator, the nitrogen gas is condensed into liquid nitrogen, most of which is used as the reflux liquid of the nitrogen production rectification column, part of which is used as the cold source of the waste gas stripping column condenser, and part of which is used as the product liquid nitrogen to be sent to the liquid nitrogen storage tank; Step 12: In order to maintain the balance of argon use and the balance of cold energy of the cold box, part of the liquid argon is returned to the argon purification column.

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