Low-temperature rectification device and method for high-purity carbon monoxide

Abstract

The invention particularly relates to a low-temperature rectification device and method for high-purity carbon monoxide. The device comprises a light component removal tower and a heavy component removal tower, a tower kettle of the light component removal tower is provided with a first reboiler, a tower kettle of the heavy component removal tower is provided with a second reboiler, and the top of the heavy component removal tower is provided with a condenser; one end of the second reboiler is communicated with the top of the light component removal tower, and the other end is communicated with the side wall of the light component removal tower; and the lower part of the light component removal tower is communicated with the side wall of the heavy component removal tower. On one hand, gradient utilization of energy is achieved through double-tower thermal coupling design, and a gas-phase component on the top of the light component removal tower directly serves as a heat source of a reboiler of the heavy component removal tower, so that a low-temperature-difference heat transfer system is formed. And on the other hand, a tower kettle reboiler series structure is adopted to realize cold circulation reuse, and the second reboiler simultaneously has dual functions of light component removal tower top gas phase condensation and heavy component removal tower kettle reboiling, so that compared with the traditional process, the energy consumption is saved by more than or equal to 40%, and the liquid nitrogen is saved by more than or equal to 40%. The method adopts continuous operation, is suitable for continuous industrial production, and can be used for preparing a high-purity carbon monoxide product.

Description

Technical Field

[0001] The present application belongs to the technical field of high-purity gas purification, and specifically relates to a high-purity carbon monoxide low-temperature distillation device and method. Background Art

[0002] Carbon monoxide (molecular formula CO) is an important organic chemical raw material that can be used to prepare ammonia, phosgene, alcohols, acids, anhydrides, esters, aldehydes, etheramines, alkanes and alkenes, various homogeneous reaction catalysts, and the extraction of high-purity nickel. High-purity carbon monoxide is mainly used in the etching of chips larger than 8 inches in the semiconductor field, in the production of polycrystalline diamond films, to provide a carbon source for the chemical vapor deposition (CVD) process, and in the fields of pharmaceutical intermediates, standard gas preparation, carbon monoxide lasers, environmental monitoring and scientific research. The key technical difficulty in purifying carbon monoxide is that the boiling point difference between CO and nitrogen and oxygen is 5.5 and 7.0°C respectively. They are the key light components and heavy components that need to be removed, which requires a lot of energy. At the same time, low-temperature distillation consumes a large amount of liquid nitrogen, which greatly increases the production cost of CO.

[0003] Chinese patent publication number CN115010132B discloses a method for preparing and purifying high-purity carbon monoxide electronic gas. This technical solution uses low-cost nano-alumina or silicon oxide catalysts to solve the problem of high-purity carbon monoxide preparation in the existing technology, achieving the preparation and purification of high-purity carbon monoxide. The mixture after the reaction is separated into crude gas and dilute acetaldehyde solution through a gas-liquid separator, and the dilute acetaldehyde solution is returned to the raw material tank; the crude gas is dehydrated in a drying tower and then enters a cryogenic distillation tower. Methane is removed at the bottom of the cryogenic distillation tower, and high-purity carbon monoxide electronic gas is obtained from the cryogenic distillation tower. It only uses one-step distillation, and the components in the product are relatively complex. Using only one-step cryogenic distillation not only requires high refrigeration performance of the equipment, but also makes it difficult to remove all impurity gases through one-step distillation to achieve a high-purity CO product with a purity higher than 5N.

[0004] Chinese patent publication number CN106288651B discloses an apparatus and method for producing ultra-high-purity carbon monoxide by nitrogen heat pump distillation. The apparatus comprises a raw gas storage tank, a heat pump, an exhaust gas furnace, and a product tank. The raw gas storage tank is connected to the raw gas inlet of a second heat exchanger via a second regulating valve. The raw gas outlet of the second heat exchanger is connected to the raw gas inlet of a first distillation tower via a second throttle valve. The gas phase outlet of the first distillation tower is connected to a first gas-liquid separator via a first raw gas inlet of the first heat exchanger and the first raw gas outlet of the first heat exchanger. The liquid phase outlet of the first gas-liquid separator is connected to the raw liquid inlet of the first distillation tower and the first raw liquid inlet of the second distillation tower, respectively. The liquid phase outlet of the second distillation tower is connected to the product tank via a fourth regulating valve. The apparatus can increase the purity of the product to an ultra-high-purity level of 99.999%. However, the multi-stage heat exchange and circulation system increases the complexity, and the nitrogen heat pump relies on additional energy consumption. Overall, the apparatus has the disadvantages of complex equipment, high energy consumption, and difficulty in operation.

[0005] The Chinese patent with announcement number CN105692552B proposes an efficient and energy-saving distillation process for high-purity carbon monoxide and hydrogen, including the following steps: feeding the raw mixed gas into a hydrogen membrane separator, obtaining high-purity hydrogen on the top side of the tower and outputting it as a product, obtaining the tail gas mixture on the bottom side of the tower, cooling it through heat exchange in the main heat exchanger, and then entering the dehydrogenation tower for further distillation and separation, obtaining a hydrogen-rich mixed gas at the top of the dehydrogenation tower, and outputting it as a hydrogen-rich product after being reheated in the main heat exchanger, transporting the material from the bottom of the dehydrogenation tower into the denitrification tower for further distillation, discharging the material from the top of the denitrification tower and reheating it through the main heat exchanger before entering an external pipeline, transporting the material from the bottom of the denitrification tower into a carbon monoxide extraction tower for further distillation, discharging the material from the top of the extraction tower and reheating it through the main heat exchanger as a high-purity carbon monoxide output product, and discharging the material from the bottom of the extraction tower and reheating it through the main heat exchanger as a fuel gas output product. Compared with the existing technology, the present application has the advantages of high product purity and extraction rate, full energy utilization, and easy operation. However, it adopts a four-stage separation process of hydrogen membrane separator + dehydrogenation tower + denitrogenation tower + CO extraction tower, involving multiple distillation towers, heat exchangers and nitrogen circulation systems, with a large number of equipment and complex control nodes.

[0006] In summary, the existing technology has problems such as high energy consumption, complex equipment and inefficient purification of carbon monoxide. Therefore, there is an urgent need for a high-purity carbon monoxide low-temperature distillation device and method that can not only obtain a CO product with a purity of >5N but also efficiently save energy. Summary of the Invention

[0007] In response to the problems of high energy consumption, complex equipment, and inefficient purification of carbon monoxide in existing technologies for carbon monoxide distillation, this application proposes a high-purity carbon monoxide low-temperature distillation device and method, which can not only obtain a CO product with a purity greater than 5N, but also achieve high efficiency and energy saving. The technical solution of this application is as follows:

[0008] On the one hand, the present application provides a high-purity carbon monoxide cryogenic distillation device, comprising a light removal tower and a heavy removal tower,

[0009] The tower kettle of the light removal tower is provided with a first reboiler, the tower kettle of the heavy removal tower is provided with a second reboiler, and the tower top of the heavy removal tower is provided with a condenser;

[0010] One end of the second reboiler is connected to the top of the light removal tower, and the other end is connected to the side wall of the light removal tower;

[0011] The lower portion of the light removal tower is communicated with the side wall of the heavy removal tower.

[0012] Preferably, the side wall of the light removal tower is provided with a raw material inlet and a light component condensate inlet, the top of the light removal tower is provided with a light component outlet, and the bottom of the light removal tower is provided with a heavy component outlet; the side wall of the heavy removal tower is provided with a heavy component inlet, the top of the heavy removal tower is provided with a carbon monoxide gas extraction outlet, and the bottom of the tower is provided with a heavy component impurity outlet; the first reboiler and the second reboiler are provided with a heat medium inlet and a heat medium outlet.

[0013] Preferably, the light component outlet at the top of the light component removal tower is communicated with the heat medium inlet of the second reboiler, and the heat medium outlet of the second reboiler is connected to a gas-liquid separation tank for separating gas and liquid.

[0014] Preferably, the gas-liquid separation tank is provided with a gas outlet, a liquid outlet and a separation liquid inlet, the gas outlet is used to extract light component impurities, the liquid outlet is connected to the light component condensate inlet of the light removal tower through a reflux pump, and the separation liquid inlet is connected to the heat medium outlet of the second reboiler.

[0015] Preferably, the heavy component outlet of the light removal tower kettle is connected to the heavy component inlet of the heavy removal tower.

[0016] In another aspect, the present application provides a method for cryogenic distillation of high-purity carbon monoxide, comprising the following steps:

[0017] Step S1. The feed gas first enters the light fraction removal tower. The gaseous component is extracted from the top of the light fraction removal tower and used as a heat source to enter the second reboiler of the heavy fraction removal tower kettle. After heat exchange with the material in the heavy fraction removal tower kettle, the gaseous component serving as the heat source is partially condensed and liquefied. The uncondensed gaseous component is extracted as the light component through a gas-liquid separator, and the liquid component is fed to the top of the light fraction removal tower via a reflux pump.

[0018] Step S2. Part of the liquid in the bottom of the light removal tower enters the first reboiler of the bottom of the light removal tower for recycling; part of it is extracted and enters the heavy removal tower, in which high-purity carbon monoxide is extracted from the top of the tower, and the heavy component part enters the second reboiler of the heavy removal tower for recycling, and the remaining heavy component is extracted from the bottom of the tower.

[0019] Preferably, the top pressure of the lightness removal tower is 1.5 to 2.0 MPa, and the operating temperature is -158 to -151°C.

[0020] Preferably, the top pressure of the deweighting tower is 0.5 to 1.0 MPa, and the operating temperature is -173 to -163°C.

[0021] Preferably, the operating temperature of the top of the light removal tower is higher than the operating temperature of the bottom of the heavy removal tower by more than 10°C.

[0022] Preferably, the operating temperature of the top of the light removal tower is higher than the operating temperature of the bottom of the heavy removal tower by more than 20°C.

[0023] Beneficial effects of this application:

[0024] On the one hand, the present application realizes the energy cascade utilization through the double-tower thermal coupling design. The gas phase components at the top of the light-removal tower are directly used as the heat source of the reboiler of the heavy-removal tower, forming a low-temperature difference heat transfer system. On the other hand, the series structure of the tower kettle reboiler is adopted to realize the recycling of cold capacity. The second reboiler simultaneously undertakes the dual functions of condensing the gas phase at the top of the light-removal tower and reboiling the tower kettle of the heavy-removal tower. Compared with the traditional process, it saves energy consumption ≥40% and liquid nitrogen ≥40%. The present application adopts continuous operation, which is suitable for continuous industrial production. The process operation is convenient and easy to implement; it can prepare high-purity CO products with a purity ≥5N. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a diagram of the purification device for high-purity carbon monoxide electronic gas in this application.

[0026] Figure identification: 1. Light removal tower; 2. Heavy removal tower; 3. First reboiler; 4. Second reboiler; 5. Reflux pump; 6. Gas-liquid separation tank; 7. Condenser. DETAILED DESCRIPTION

[0027] In order to further illustrate the technical means and effects adopted by this application to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of this application are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0028] Device Example 1

[0029] This embodiment provides a high-purity carbon monoxide cryogenic distillation system. It utilizes dual-tower coupled distillation and waste heat cascade utilization technology to achieve CO purity exceeding 99.999% through a collaborative separation mechanism of light and heavy components. It includes a light removal tower 1 and a heavy removal tower 2, with the following specific structural features:

[0030] The bottom of the light-removing tower 1 is provided with a first reboiler 3 for material vaporization. The first reboiler 3 adopts an independent external heat source. The first reboiler 3 is provided with a heat medium inlet and a heat medium outlet. The side wall of the light-removing tower 1 is provided with a raw material inlet and a light component condensate inlet. The top of the light-removing tower 1 is provided with a light component outlet, and the bottom of the light-removing tower 1 is provided with a heavy component outlet.

[0031] The bottom of the de-weighting tower 2 is provided with a second reboiler 4, the side wall of the de-weighting tower 2 is provided with a heavy component inlet, the top of the de-weighting tower 2 is provided with a carbon monoxide gas extraction outlet, and the bottom of the tower is provided with a heavy component impurity outlet;

[0032] The heavy component outlet of the kettle of light removal tower 1 is connected to the heavy component inlet of heavy removal tower 2.

[0033] The second reboiler 4 is provided with a heat medium inlet and a heat medium outlet, and the top of the deweighting tower 2 is also provided with a condenser 7 for controlling the complete condensation reflux of CO gas;

[0034] The light component outlet at the top of the light component removal tower 1 is connected to the heat medium inlet of the second reboiler 4 to form a thermal coupling, which fully utilizes the heat. The heat medium outlet of the second reboiler 4 is connected to a gas-liquid separation tank 6 for separating gas and liquid.

[0035] The gas-liquid separation tank 6 is provided with a gas outlet, a liquid outlet and a separation liquid inlet. The gas outlet is used to extract light component impurities. The liquid outlet is connected to the light component condensate inlet of the light removal tower 1 through a reflux pump 5. The separation liquid inlet is connected to the heat medium outlet of the second reboiler 4.

[0036] Principle: After the feed gas enters the light fraction removal tower 1 through the feed inlet, it is heated and vaporized in the first reboiler 3. The light fraction rises to the top of the tower and enters the second reboiler 4 through the light fraction outlet as a heat medium. The gaseous light fraction releases latent heat in the second reboiler 4 and partially condenses. The resulting gas-liquid mixture enters the gas-liquid separator 6 through the separation liquid inlet. Uncondensed gas is discharged from the system through the gas outlet. The condensed liquid enters the reflux pump 5 through the liquid outlet of the gas-liquid separator 6 and returns to the light fraction removal tower 1 for reflux, forming a light fraction self-circulating heating system.

[0037] The heavy component-enriched liquid phase from the bottom of Light Removal Column 1 is transferred through the heavy component outlet to the middle of Heavy Removal Column 2, where it undergoes secondary distillation under heat provided by the second reboiler 4. High-boiling-point impurities in the heavy component sink to the bottom of the column and are discharged through the heavy component impurity outlet. High-purity carbon monoxide gas is fully condensed in the overhead condenser 7 and then discharged through the gas extraction outlet. The two towers are thermally coupled, utilizing waste heat from the light component to power Heavy Removal Column 2, significantly reducing system energy consumption. The synergistic effect of the two-stage distillation ensures CO purity reaches electronic-grade standards exceeding 99.999%.

[0038] Example 1

[0039] Step S1. Feed the raw gas into the de-lightening tower at a feed rate of 100 kg / h. The composition of the raw gas is shown in Table 1. The gas phase at the top of the de-lightening tower is extracted as a heat source and enters the reboiler of the de-heavy tower to exchange heat with the bottom material. After the heat exchange, the gas phase serving as the heat source is partially condensed and liquefied, and the liquid phase is refluxed as the reflux liquid at the top of the de-lightening tower, and the gas phase is extracted as a light component impurity; part of the bottom liquid of the de-lightening tower enters the reboiler of the de-lightening tower, and part of it is extracted and enters the de-heavy tower.

[0040] Step S2: The liquid in the kettle of the de-weighting tower is reboiled in a reboiler, and the vapor phase is extracted from the top of the de-weighting tower and enters the overhead condenser of the de-weighting tower, which serves as a partial condenser. After condensation in the condenser, the liquid phase is refluxed, and the vapor phase is extracted as the electronic CO product. Part of the de-weighting tower kettle material enters the de-weighting tower kettle reboiler, and part is extracted as heavy component impurities. The top pressure of the de-weighting tower is controlled at 1.5 MPa and the operating temperature is -158°C. The top pressure of the de-weighting tower is controlled at 0.5 MPa and the operating temperature is -173°C. The purity of the produced carbon monoxide is 99.999%. The temperature of the vapor phase at the top of the de-weighting tower is 15°C higher than the discharge temperature of the de-weighting tower kettle.

[0041] Table 2 shows the energy consumption comparison between the conventional distillation process and the process of this application. As can be seen from Table 2, compared with the energy consumption of the conventional distillation process, from the comparative data it can be seen that, at the same processing volume and the same product purity and product yield, the heat consumption is only 51.8% of that of the conventional distillation process, and the liquid nitrogen consumption is 57.2% of that of the conventional process. In this way, the energy consumption and liquid nitrogen consumption are greatly reduced.

[0042] Table 1 Composition of raw gas:

[0043] name <![CDATA[H2]]> <![CDATA[N2]]> CO <![CDATA[O2]]> <![CDATA[CH4]]> <![CDATA[CO2]]> Molar composition% 0.01 0.1 99.85 0.02 0.01 0.01

[0044] Table 2 Comparison of energy consumption between ordinary distillation process and this application process

[0045]

[0046] Example 2

[0047] Step S1. The raw gas is fed into the de-lightening tower at a feed rate of 100 kg / h. The gas phase at the top of the de-lightening tower is extracted as a heat source and enters the reboiler of the de-heavy tower kettle to exchange heat with the kettle material. After the heat exchange, the gas phase serving as the heat source is partially condensed and liquefied, the liquid phase is refluxed as the reflux liquid at the top of the de-lightening tower, and the gas phase is extracted as a light component impurity; part of the liquid in the kettle of the de-lightening tower enters the reboiler of the kettle of the de-lightening tower, and part of it is extracted and enters the de-heavy tower.

[0048] Step S2: The gas phase is extracted from the top of the de-weighting tower and enters the de-weighting tower overhead condenser, which is a partial condenser. After condensation in the condenser, the liquid phase is refluxed, and the gas phase is extracted as the electronic CO product. Part of the de-weighting tower bottom material enters the de-weighting tower bottom reboiler, and part is extracted as heavy component impurities. The top pressure of the de-weighting tower is controlled at 1.8 MPa and the operating temperature is -155°C. The top pressure of the de-weighting tower is controlled at 0.8 MPa and the operating temperature is -167°C. The purity of the produced carbon monoxide is 99.999%. The temperature of the gas phase at the top of the de-weighting tower is 12°C higher than the discharge temperature of the de-weighting tower bottom material.

[0049] Table 3 shows the energy consumption comparison between the conventional distillation process and the process of this application. As can be seen from Table 3, compared with the energy consumption of the conventional distillation process, from the comparative data it can be seen that, at the same processing volume and the same product purity and product yield, the heat consumption is only 55.8% of that of the conventional distillation process, and the liquid nitrogen consumption is 52.9% of that of the conventional process. In this way, the energy consumption and liquid nitrogen consumption are greatly reduced.

[0050] Table 3 Comparison of energy consumption between ordinary distillation process and this application process

[0051]

[0052] Example 3

[0053] Step S1. The raw gas is fed into the de-lightening tower at a feed rate of 100 kg / h. The gas phase at the top of the de-lightening tower is extracted as a heat source and enters the reboiler of the de-heavy tower kettle to exchange heat with the kettle material. After the heat exchange, the gas phase serving as the heat source is partially condensed and liquefied, the liquid phase is refluxed as the reflux liquid at the top of the de-lightening tower, and the gas phase is extracted as a light component impurity; part of the liquid in the kettle of the de-lightening tower enters the reboiler of the kettle of the de-lightening tower, and part of it is extracted and enters the de-heavy tower.

[0054] Step S2: The gas phase is extracted from the top of the de-weighting tower and enters the de-weighting tower overhead condenser, which is a partial condenser. After condensation in the condenser, the liquid phase is refluxed, and the gas phase is extracted as the electronic CO product. Part of the de-weighting tower bottom material enters the de-weighting tower bottom reboiler, and part is extracted as heavy component impurities. The top pressure of the de-weighting tower is controlled at 2.0 MPa and the operating temperature is -151°C. The top pressure of the de-weighting tower is controlled at 1.0 MPa and the operating temperature is -163°C. The purity of the produced carbon monoxide is 99.999%. The temperature of the gas phase at the top of the de-weighting tower is 12°C higher than the discharge temperature of the de-weighting tower bottom material.

[0055] Table 4 shows the energy consumption comparison between the conventional distillation process and the process of this application. As can be seen from Table 4, compared with the energy consumption of the conventional distillation process, from the comparative data it can be seen that, at the same processing volume and the same product purity and product yield, the heat consumption is only 61.4% of the conventional distillation process, and the liquid nitrogen consumption is 60.1% of the conventional process. In this way, the energy consumption and liquid nitrogen consumption are greatly reduced.

[0056] Table 4 Comparison of energy consumption between ordinary distillation process and this application process

[0057]

[0058] The above description is merely a preferred embodiment of the present application and does not constitute any form of limitation to the present application. Although the present application has been disclosed as above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to equivalent embodiments using the technical contents disclosed above without departing from the scope of the technical solution of the present application. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present application without departing from the content of the technical solution of the present application are still within the scope of the technical solution of the present application.

Claims

1. A high-purity carbon monoxide cryogenic distillation device, characterized in that: It includes a light removal tower (1) and a heavy removal tower (2), The tower kettle of the light removal tower (1) is provided with a first reboiler (3), the tower kettle of the heavy removal tower (2) is provided with a second reboiler (4), and the tower top of the heavy removal tower (2) is provided with a condenser (7); One end of the second reboiler (4) is in communication with the top of the light removal tower (1), and the other end is in communication with the side wall of the light removal tower (1); The lower part of the light removal tower (1) is communicated with the side wall of the heavy removal tower (2).

2. A high-purity carbon monoxide cryogenic distillation device according to claim 1, characterized in that: The side wall of the light removal tower (1) is provided with a raw material inlet and a light component condensate inlet, the top of the light removal tower (1) is provided with a light component outlet, and the bottom of the light removal tower (1) is provided with a heavy component outlet; the side wall of the heavy removal tower (2) is provided with a heavy component inlet, the top of the heavy removal tower (2) is provided with a carbon monoxide gas extraction outlet, and the bottom of the tower is provided with a heavy component impurity outlet; the first reboiler (3) and the second reboiler (4) are both provided with a heat medium inlet and a heat medium outlet.

3. A high-purity carbon monoxide cryogenic distillation device according to claim 2, characterized in that: The light component outlet at the top of the light component removal tower (1) is connected to the heat medium inlet of the second reboiler (4), and the heat medium outlet of the second reboiler (4) is connected to a gas-liquid separation tank (6) for separating gas and liquid.

4. A high-purity carbon monoxide cryogenic distillation device according to claim 3, characterized in that: The gas-liquid separation tank (6) is provided with a gas outlet, a liquid outlet and a separation liquid inlet. The gas outlet is used to extract light component impurities. The liquid outlet is connected to the light component condensate inlet of the light removal tower (1) through a reflux pump (5). The separation liquid inlet is connected to the heat medium outlet of the second reboiler (4).

5. A high-purity carbon monoxide cryogenic distillation device according to claim 2, characterized in that: The heavy component outlet of the bottom of the light-removing tower (1) is communicated with the heavy component inlet of the heavy-removing tower (2).

6. A method for low-temperature distillation of high-purity carbon monoxide, based on the high-purity carbon monoxide low-temperature distillation device according to claims 1 to 5, characterized in that: The steps include: Step S1. The raw gas first enters the light-removal tower (1), and the gaseous component is extracted from the top of the light-removal tower (1) as a heat source and enters the second reboiler (4) of the bottom of the heavy-removal tower (2), and exchanges heat with the material in the bottom of the heavy-removal tower (2). After the heat exchange, the gaseous phase serving as the heat source is partially condensed and liquefied, and the uncondensed gaseous phase is extracted as the light component, and the liquid phase enters the top of the light-removal tower (1); Step S2. Part of the bottom liquid of the light removal tower (1) enters the first reboiler (3) of the bottom of the light removal tower (1) for recycling; part of it is extracted and enters the heavy removal tower (2). In the heavy removal tower (2), high-purity carbon monoxide is extracted from the top of the tower, and the heavy component part enters the second reboiler (4) of the heavy removal tower (2) for recycling, and the remaining heavy component is extracted from the bottom of the tower.

7. A method for low-temperature distillation of high-purity carbon monoxide according to claim 6, characterized in that: The top pressure of the light removal tower (1) is 1.5 to 2.0 MPa, and the operating temperature is -158 to -151°C.

8. The method for low-temperature distillation of high-purity carbon monoxide according to claim 6, characterized in that: The top pressure of the deweighting tower (2) is 0.5-1.0 MPa, and the operating temperature is -173--163°C.

9. The method for low-temperature distillation of high-purity carbon monoxide according to claim 6, characterized in that: The operating temperature of the top of the light removal tower (1) is higher than the operating temperature of the bottom of the heavy removal tower (2) by more than 10°C.

10. A method for low-temperature distillation of high-purity carbon monoxide according to claim 9, characterized in that: The operating temperature of the top of the light removal tower (1) is higher than the operating temperature of the bottom of the heavy removal tower (2) by more than 20°C.

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