WO2020233030A1 - Device and method for synergistic recover of sulfur and hydrogen resources from hydrogen sulfide acid gas - Google Patents

Abstract

A device and method for synergistic recover of sulfur and hydrogen resources from hydrogen sulfide acid gas. Said device comprises a catalytic unit, a sulfur-hydrogen separation unit, an amine solution regeneration unit, pipelines connecting each unit, delivery pumps and valves provided on the connecting pipelines, and an instrument device for automatic control. A method for synergistic recovery of sulfur and hydrogen resources from hydrogen sulfide acid gas by using said device, comprising subjecting hydrogen sulfide acid gas to a catalytic reaction by means of a thermal catalytic reactor (2), recovering sulfur produced by cooling sulfur vapor, and purifying and concentrating hydrogen after hydrogen has been absorbed and purified by an amine solution. Hydrocarbons and water vapor in the acid gas react at elevated temperature to produce CO and H₂, and NH₃ is thermally decomposed into N₂ and H₂ under the action of elevated temperature and a catalyst.

Description

Device and method for synergistic recovery of hydrogen sulfide acid gas sulfur and hydrogen resources Technical field

The invention belongs to the technical field of hydrogen sulfide acid gas decomposition and recovery, and relates to a hydrogen sulfide acid gas sulfur and hydrogen resource cooperative recovery device and method.

Background technique

In the chemical industry (petrochemical industry, coal chemical industry and natural gas chemical industry), a large amount of H ₂ S acid gas is generated during the generation process. H ₂ S is a highly toxic, odorless, colorless gas, which not only harms human health, but also causes corrosion of metal materials. At present, the H ₂ S acid gas treatment in the chemical industry mainly adopts the traditional Claus process method, which converts hydrogen sulfide into elemental sulfur and water:

1)H ₂ S+3/2O ₂ →SO ₂ +H ₂ O

2) 2H ₂ S+SO ₂ →3/xS _x +2H ₂ O

Chinese patent CN201610052763.X discloses a SWSR-7 sulfur recovery process, which utilizes a sulfur recovery and tail gas treatment process formed by an optimized combination of Claus process, hydrogenation or oxidation process and hydrogen peroxide desulfurization technology. Although the Claus process can realize the harmless treatment of hydrogen sulfide, it converts hydrogen resources with higher added value into water, wasting precious resources. Hydrogen energy is the most promising fuel to replace fossil energy in the future. At present, industrial hydrogen is produced by reforming or electrolyzing water from light hydrocarbons, coal, natural gas, methanol, etc., which is costly and expensive, making it difficult to be widely used as a fuel.

Therefore, if hydrogen sulfide can be decomposed, not only can hydrogen sulfide be made harmless, but also high value-added hydrogen and elemental sulfur can be obtained. The current methods for the direct decomposition of H ₂ S to produce hydrogen and sulfur mainly include: thermal decomposition, thermal catalytic decomposition, electrochemical decomposition, photocatalytic decomposition, and plasma methods. Thermal catalytic decomposition is one of the most promising H ₂ S decomposition technologies for industrial applications. Chinese patent CN201510730163.X uses iron powder and H ₂ S pyrolysis reaction to generate hydrogen and elemental sulfur. The process requires a magnetic field to separate the iron powder from solid gas. The process is complicated and the sulfur iron ion content produced is high, which does not meet the requirements of industrial sulfur qualified products. Fe≤0.02%(w) is required, and the FeS generated spontaneously ignites in the air, and there is a risk of generation safety.

Therefore, if a safe and reliable process technology with high hydrogen production rate can be developed, it will have important research significance and practical value for realizing the synergistic recovery of sulfur and hydrogen resources in the chemical industry.

Summary of the invention

The purpose of the present invention is to overcome the above-mentioned shortcomings of the existing hydrogen sulfide acid gas decomposition and recovery process, provide a hydrogen sulfide acid gas sulfur and hydrogen resource synergistic recovery device and method, and maximize the conversion rate of sulfide decomposition into hydrogen and sulfur, and the development is reasonable The technological process of this technology can recover hydrogen and elemental sulfur and increase the total conversion rate of hydrogen sulfide.

The present invention is realized by adopting the following technical solutions:

The present invention provides a hydrogen sulfide acid gas sulfur and hydrogen resource synergistic recovery device, which includes a catalytic unit, a sulfur and hydrogen separation unit, an amine liquid regeneration unit, and pipelines connected to the above-mentioned systems, and delivery pumps, valves and valves arranged on the connecting pipelines. Instrumentation devices for automatic control;

The catalytic unit includes a raw material heater, a thermal catalytic reactor, and a sulfur condenser that are connected in sequence, and the sulfur and hydrogen separation unit includes a process gas heater, a hydrogenation reactor, a steam generator, a quench tower, and Absorption tower, the inlet of the hydrogenation reactor is connected with the outlet of the sulfur condenser; the top of the absorption tower is provided with a hydrogen-rich tail gas outlet, and the bottom is provided with a rich liquid outlet, which is connected to the amine liquid regeneration unit through a rich liquid pump The amine liquid regeneration device is connected.

Preferably, the hydrogen sulfide gas outlet of the amine liquid regeneration device is in communication with the raw material heater inlet, and the lean liquid outlet is in communication with the upper part of the absorption tower.

Preferably, a process gas raw material gas heat exchanger is arranged between the thermal catalytic reactor and the sulfur condenser, and the pipes entering and exiting the thermal catalytic reactor are heat exchanged in the process gas raw material heat exchanger.

Wherein, a liquid sulfur trapping device is arranged below the sulfur condenser; the process gas feed gas heat exchanger and the steam generator are provided with a deoxygenated water inlet and a steam outlet.

Wherein, the thermal catalytic reactor is a catalytic pyrolysis device with heat insulation and wear-resistant lining arranged inside, and the thermal catalytic reactor is a fixed bed reactor with a pyrolysis catalyst inside, and one side of the hydrogen sulfide acid gas autothermal catalytic reactor Enter, the reacted process gas enters the next device from the other side.

The present invention also provides a method for synergistically recovering hydrogen sulfide acid gas sulfur and hydrogen resources using the above-mentioned device. The hydrogen sulfide acid gas is catalytically reacted by a thermal catalytic reactor to recover sulfur produced by sulfur vapor cooling, and hydrogen is purified after being absorbed and purified by amine liquid. And enrichment to realize the synergistic recovery of sulfur and hydrogen resources; a small amount of hydrocarbons and steam in the acid gas react at high temperature to generate CO and H ₂ , and a small amount of NH ₃ thermally decomposes into N ₂ and H ₂ under the action of high temperature and catalyst.

Specifically include the following steps:

(1) Thermal catalytic decomposition of hydrogen sulfide acid gas:

The high-temperature acid gas formed by the heating of hydrogen sulfide acid gas enters the thermal catalytic reactor. Under the action of the catalyst, hydrogen sulfide is decomposed into elemental sulfur and hydrogen. Hydrocarbon and water vapor react to form CO and H ₂ , and NH _{3 is} thermally decomposed into N ₂ And H ₂ ;

(2) Liquid sulfur capture:

The process gas containing hydrogen sulfide, sulfur vapor and hydrogen is cooled down, then enters the sulfur condenser for further cooling, and collects and recovers the liquid sulfur produced by the cooling of the sulfur vapor;

(3) Hydrogenation reaction:

The cooled process gas is heated to enter the hydrogenation reactor, and the non-hydrogen sulfide sulfide and elemental sulfur in the process gas are reduced and hydrolyzed by hydrogenation;

(4) Separation of sulfur and hydrogen:

After the hydrogenated tail gas is cooled, it enters the quench tower to be further cooled to 40°C and enters the absorption tower for absorption. The lean liquid absorbs hydrogen sulfide in the hydrogenation tail gas, and the hydrogen-rich tail gas is enriched and purified to obtain hydrogen;

(5) Regeneration of amine solution:

The rich liquid that has absorbed hydrogen sulfide enters the amine liquid regeneration unit for regeneration, the separated hydrogen sulfide is returned to the thermal catalytic reactor for cyclic reaction, and the lean liquid is returned to the absorption tower for recycling to absorb hydrogen sulfide.

Wherein, the temperature of the high-temperature acid gas entering the thermal catalytic reactor in step (1) is 700°C to 1000°C. In step (2), the temperature of the process gas is reduced to 250°C to 300°C, and in step (3), the process gas is heated to 280 to 300°C.

In the present invention, the control of the operating temperature can be realized by the process gas sour gas heat exchanger or the electric heating automatic control system configured with the equipment and related connecting pipes of the present invention.

In addition to inert gas components, acid gas components also contain NH ₃ , H ₂ O and hydrocarbons. Under the action of a pyrolysis catalyst, NH ₃ can be pyrolyzed into N ₂ and H ₂ ; H ₂ S pyrolysis reaction temperature At 700-1000°C, at this temperature hydrocarbons react with water vapor to generate CO and H ₂ .

Compared with the prior art, the beneficial effects of the present invention are:

(1) The recovery of hydrogen requires the absorption of unreacted hydrogen sulfide. During the reaction, sulfides such as CS ₂ and COS will inevitably be generated. At the same time, some of the elemental sulfur generated cannot be captured. In the process gas, non-hydrogen sulfide must be sulfided Hydrogen and elemental sulfur are reduced and hydrolyzed to H ₂ S to separate hydrogen. The invention combines hydrogen sulfide thermal catalytic decomposition method, hydrogenation process and amine liquid regeneration process for the first time to decompose hydrogen sulfide acid gas into elemental sulfur and hydrogen, not only fully recovering sulfur resources, but also recovering hydrogen resources with higher added value. The sulfur recovery efficiency of the invention is high, which is in line with the first-class sulfur products in GB/T2449. The recovery rate of sulfur and hydrogen in the device can reach more than 99%. The greater the amount of amine liquid absorbed, the more sulfur is carried away from the rich hydrogen. The less hydrogen, the higher the total recovery rate; the conversion rate of thermal catalytic reaction hydrogen sulfide is 30%-50%, and the regenerated hydrogen sulfide is returned to the catalytic reactor for cyclic conversion.

(2) While the present invention improves the hydrogen sulfide conversion rate, the amine liquid regeneration selects mature, reliable, and low-energy thermal regeneration technology. The invention simplifies the process flow, reasonably utilizes the reaction heat, generates 0.3MPa (g) saturated steam, and is used for jacket heating and amine liquid regeneration reboiler heating, thereby reducing device energy consumption.

(3) The present invention has simple process flow, simple startup, shutdown and normal operation, high economic performance, small floor space, low investment cost, and at the same time effectively reduces labor intensity and saves costs.

Description of the drawings

The accompanying drawings are used to provide a further understanding of the present invention, and constitute a part of the specification, together with the embodiments of the present invention, are used to explain the present invention, and do not constitute a limitation to the present invention.

In the attached picture:

Figure 1 is a process flow diagram of the device and method for synergistic recovery of hydrogen sulfide acid gas sulfur and hydrogen resources of the present invention;

The labels in the figure are as follows: 1 raw material heater, 2 thermal catalytic reactor, 3 process gas feed gas heat exchanger, 4 sulfur condenser, 5 hydrogenation reactor, 6 steam generator, 7 quench tower, 8 absorption tower, 9 rich liquid pump, 10 quench water pump, 11 quench water cooler, 12 amine liquid regeneration device, 13 process gas heater, A raw material acid gas inlet direction, B deoxygenated water inlet, C steam outlet, D liquid sulfur outlet , E hydrogen-rich tail gas outlet.

Detailed ways

In order to make the objectives and technical solutions of the present invention clearer, the present invention will be further described in detail with reference to the accompanying drawings. The experimental methods described in the following examples, unless otherwise specified, are conventional methods; if specific techniques or conditions are not indicated in the examples, they shall be carried out in accordance with the techniques or conditions described in the literature in the field or in accordance with the product specifications; The reagents and materials, unless otherwise specified, can be obtained from commercial sources.

The thermal catalytic reactor is a conventional fixed bed reactor, and the catalyst used is the hexaaluminate composite oxide material disclosed in Chinese Patent CN201810780422.4.

A solution such as amine solution for absorbent (MDEA) or low temperature methanol is used in the absorption tower. The amine solution regeneration process is a well-known single-tower stripping thermal regeneration process, and the amine solution regeneration device 12 is a regeneration tower.

As shown in Figure 1, the hydrogen sulfide acid gas sulfur and hydrogen resource synergistic recovery device used in the following embodiments includes a catalytic unit, a sulfur and hydrogen separation unit, an amine liquid regeneration unit, and pipelines connected to the above-mentioned systems and installed on the connecting pipelines. Delivery pumps, valves and instrumentation devices for automatic control;

Among them, the catalytic unit includes a raw material heater 1, a thermal catalytic reactor 2 and a sulfur condenser 4 connected in sequence, and a process gas raw material gas heat exchanger 3 is arranged between the thermal catalytic reactor and the sulfur condenser to enter and exit the thermal catalytic reaction. The pipes of the reactor exchange heat in the process gas feed gas heat exchanger. A liquid sulfur trapping device is installed below the sulfur condenser to trap the cooled liquid sulfur.

The thermal catalytic reactor is a fixed bed reactor with a lining structure. The catalyst is packed in the catalytic reactor. The acid gas enters from one end of the thermal catalytic reactor, and the outlet end is on the other side, and is connected to the process gas feed gas heat exchanger.

The sulfur and hydrogen separation unit includes a process gas heater 13, a hydrogenation reactor 5, a steam generator 6, a quench tower 7 and an absorption tower 8, which are connected in sequence. The gas inlet of the hydrogenation reactor is connected with the gas outlet of the sulfur condenser. . Both the process gas raw material heat exchanger and the steam generator are provided with a deoxygenated water inlet B and a steam outlet C. The bottom of the quenching tower is connected to the quenching water cooler 11 through the quenching water pump 10, and the cooled water is returned to the quenching tower from the upper part of the quenching tower.

The top of the absorption tower is provided with a hydrogen-rich tail gas outlet E, and the bottom is provided with a rich liquid outlet. The rich liquid outlet is communicated with the amine liquid regeneration device 12 in the amine liquid regeneration unit through the rich liquid pump 9. The hydrogen sulfide gas outlet of the amine liquid regeneration device is connected with the inlet of the raw material heater, and the lean liquid outlet is connected with the upper part of the absorption tower.

In order to avoid repetition, the raw materials and preparation condition parameters involved in this specific embodiment are now uniformly described as follows, and the details are not repeated in the specific examples:

(1) Thermal catalytic decomposition of hydrogen sulfide acid gas:

At the beginning of construction, the raw material gas is heated to the required temperature of 700～1000℃ by the raw material gas heater, and the acid gas containing hydrogen sulfide is heated into the thermal catalytic reactor. The reactor is a fixed bed lined reactor, and the acid gas is from the reactor. One side enters the catalyst bed. Under the action of the catalyst, hydrogen sulfide is decomposed into elemental sulfur and hydrogen. Hydrocarbon and water vapor react to form CO and H ₂ , and NH _{3 is} thermally decomposed into N ₂ and H ₂ . One side of the thermal catalytic reactor is directly connected to the process gas heat exchanger of the raw material device, and the temperature of the process gas after the thermal catalytic reaction is used to heat the raw gas. The insufficient heat is provided by the raw gas heater, and the high-temperature heat is effectively used. The temperature drops to 250～300℃;

(2) Liquid sulfur capture:

The process gas containing hydrogen sulfide, sulfur vapor and hydrogen is cooled down, and then enters the sulfur condenser for further cooling, and collects and recovers the liquid sulfur produced by the cooling of the sulfur vapor. At the same time, the sulfur condenser uses low-temperature heat to generate low-pressure steam 0.3MPa(g) to reduce Equipment energy consumption, sulfur vapor is cooled into liquid sulfur and captured as industrial sulfur as a product;

(3) Hydrogenation reaction:

The temperature of the cooled process gas is about 160℃. Because the process gas contains a small amount of sulfur vapor, as well as impurities COS, CS ₂ and a small amount of SO ₂ produced by the reaction, it must be hydrogenated or hydrolyzed into hydrogen sulfide to effectively remove hydrogen sulfide. , To obtain relatively pure hydrogen-rich gas, the cooled process gas is heated to 280 ~ 300 ℃ into the hydrogenation reactor, hydrogenation reduction and hydrolysis process gas non-hydrogen sulfide sulfide and elemental sulfur, easy to absorb hydrogen sulfide and separate high Pure hydrogen;

(4) Separation of sulfur and hydrogen:

The hydrogenated tail gas is cooled to 170°C in the steam generator, and then enters the quench tower to be further cooled to 40°C and enters the absorption tower for absorption. The lean liquid is used to absorb the hydrogen sulfide in the hydrogenation tail gas, and the hydrogen-rich tail gas is concentrated and combined. Purify to obtain hydrogen;

(5) Regeneration of amine solution:

The rich liquid that has absorbed hydrogen sulfide enters the regeneration tower for regeneration, and the separated hydrogen sulfide is returned to the thermal catalytic reactor for cyclic reaction. The outlet pressure at the top of the regeneration tower is 0.06～0.08MPa(g), and the temperature at the top of the regeneration tower is 122℃; the lean amine liquid is returned The absorption tower is recycled to absorb hydrogen sulfide. The rich liquid regeneration uses mature and reliable thermal regeneration technology with low energy consumption, and the steam uses self-produced 0.3MPa(G) steam.

Example 1

In this embodiment, in the catalytic reactor, the conversion rate of H ₂ S is about 30% to 50%. Hydrocarbon and water vapor react at high temperature to produce CO and H ₂ , and a small amount of NH ₃ is thermally decomposed under the action of high temperature and catalyst. For N ₂ and H ₂ , the H ₂ S absorbed by the amine solution is regenerated and desorbed and then returned to the catalytic reactor. The total recovery rate of H ₂ S can reach more than 99%. The recovered sulfur meets the requirements of GB/T2449-2014 Industrial Sulfur Superior Product Standards.

Amine Amine absorption tower circulating volume, the collection of H ₂ S down to the more, the less the hydrogen rich gas carrying away of H ₂ S, H ₂ S trap down after regeneration Amine return heat catalytic reactor Reaction, the higher the total recovery rate of H ₂ S.

The pressure of acid gas entering the catalytic reactor is generally 0.06MPa(g), the pressure drop of each equipment is calculated as 5kPa, the pressure of acid gas regenerated by amine solution is 0.05～0.08MPa(g), the outlet pressure of hydrogen-rich gas is 0.03～0.04MPa( g).

Example 2

In this embodiment, the source, composition and quantity of raw materials for a 10,000-ton/year sulfur recovery unit of a chemical plant are shown in Table 1 below.

Table 1. Inlet acid gas composition and flow table of thermal catalytic reactor

After the above-mentioned mixed acid gas that enters the hydrogen sulfide acid gas sulfur and hydrogen resource cooperative recovery device of the present invention, after three processes of hydrogen sulfide thermal catalytic decomposition, hydrogenation and amine regeneration, the gas composition changes significantly. The detailed data is shown in the table 2 shown.

Table 2. The composition and flow table of the process gas, purified hydrogen rich and regenerated acid gas at the outlet of the thermal catalytic reactor

The conversion rate of the thermal catalytic reactor in Table 2 is calculated as 40%, and the hydrogen sulfide content in the purified hydrogen is 1% (v).

From the gas phase composition changes in Table 1 and Table 2, it can be seen that the hydrogen sulfide acid gas sulfur and hydrogen resource synergistic recovery device and method of the present invention can efficiently treat hydrogen sulfide in the hydrogen sulfide acid gas to obtain liquid sulfur resources, while recovering more added value. High hydrogen resources, and simultaneous treatment of hydrocarbons and ammonia in acid gas, improving the acid gas treatment efficiency of the entire device.

Of course, the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still compare the foregoing embodiments. The recorded technical solutions are modified, or some of the technical features are equivalently replaced. Any modification, equivalent replacement, improvement, etc., made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (9)

1. A synergistic recovery device for hydrogen sulfide acid gas and sulfur and hydrogen resources, which is characterized by comprising a catalytic unit, a sulfur and hydrogen separation unit, an amine liquid regeneration unit, and pipelines connected to the above-mentioned systems, and delivery pumps, valves and valves arranged on the connecting pipelines. Instrumentation devices for automatic control;

   The catalytic unit includes a raw material heater (1), a thermal catalytic reactor (2) and a sulfur condenser (4) connected in sequence, and the sulfur and hydrogen separation unit includes a process gas heater (13) connected in sequence, The hydrogenation reactor (5), the steam generator (6), the quench tower (7) and the absorption tower (8), the gas inlet of the hydrogenation reactor (5) is connected with the gas outlet of the sulfur condenser (4); The top of the absorption tower (8) is provided with a hydrogen-rich tail gas outlet (E), and the bottom is provided with a rich liquid outlet. The rich liquid outlet is connected with the amine liquid regeneration device (12) in the amine liquid regeneration unit through the rich liquid pump (9).
2. The hydrogen sulfide acid gas hydrogen sulfide resource synergistic recovery device according to claim 1, wherein the hydrogen sulfide gas outlet of the amine liquid regeneration device (12) is connected with the inlet of the raw material heater (1), and the lean liquid The outlet is connected to the upper part of the absorption tower (8).
3. The hydrogen sulfide acid gas sulfur hydrogen resource synergistic recovery device according to claim 1, characterized in that a process gas feed gas heat exchanger (4) is provided between the thermal catalytic reactor (2) and the sulfur condenser (4). 3) The pipes entering and exiting the thermal catalytic reactor (2) exchange heat in the process gas feeder heat exchanger (3).
4. The hydrogen sulfide acid gas sulfur hydrogen resource synergistic recovery device according to claim 3, characterized in that, a liquid sulfur trap device is provided below the sulfur condenser (4); a process gas feed gas heat exchanger (3) and The steam generator (6) is provided with a deaeration water inlet (B) and a steam outlet (C).
5. The hydrogen sulfide acid gas sulfur hydrogen resource synergistic recovery device according to claim 1, characterized in that the thermal catalytic reactor (2) is a catalytic pyrolysis device with a heat-insulating and wear-resistant lining inside, which is a fixed bed reaction Device.
6. A method for synergistically recovering hydrogen sulfide acid gas sulfur and hydrogen resources based on the device of any one of claims 1 to 5, characterized in that the hydrogen sulfide acid gas is catalytically reacted by a thermal catalytic reactor to recover sulfur generated by sulfur vapor cooling, Hydrogen is purified and concentrated by amine liquid absorption and purification to realize the synergistic recovery of sulfur and hydrogen resources; a small amount of hydrocarbons and water vapor in acid gas react at high temperature to form CO and H ₂ , and a small amount of NH ₃ is thermally decomposed under the action of high temperature and catalyst It is N ₂ and H ₂ .
7. The method for recovering hydrogen sulfide acid gas sulfur and hydrogen resources according to claim 6, characterized in that it specifically comprises the following steps:

   (1) Thermal catalytic decomposition of hydrogen sulfide acid gas:

   The high-temperature acid gas formed by the heating of hydrogen sulfide acid gas enters the thermal catalytic reactor. Under the action of the catalyst, hydrogen sulfide is decomposed into elemental sulfur and hydrogen. Hydrocarbon and water vapor react to form CO and H ₂ , and NH _{3 is} thermally decomposed into N ₂ And H ₂ ;

   (2) Liquid sulfur capture:

   The process gas containing hydrogen sulfide, sulfur vapor and hydrogen is cooled down, then enters the sulfur condenser for further cooling, and collects and recovers the liquid sulfur produced by the cooling of the sulfur vapor;

   (3) Hydrogenation reaction:

   The cooled process gas is heated to enter the hydrogenation reactor, and the non-hydrogen sulfide sulfide and elemental sulfur in the process gas are reduced and hydrolyzed by hydrogenation;

   (4) Separation of sulfur and hydrogen:

   After the hydrogenated tail gas is cooled, it enters the quench tower to be further cooled to 40°C and enters the absorption tower for absorption. The lean liquid absorbs hydrogen sulfide in the hydrogenation tail gas, and the hydrogen-rich tail gas is enriched and purified to obtain hydrogen;

   (5) Regeneration of amine solution:

   The rich liquid that has absorbed hydrogen sulfide enters the amine liquid regeneration unit for regeneration, the separated hydrogen sulfide is returned to the thermal catalytic reactor for cyclic reaction, and the lean liquid is returned to the absorption tower for recycling to absorb hydrogen sulfide.
8. The method for recovering hydrogen sulfide acid gas sulfur hydrogen resources according to claim 7, characterized in that the temperature of the high temperature acid gas entering the thermal catalytic reactor in the step (1) is 700°C to 1000°C.
9. The method for recovering hydrogen sulfide acid gas sulfur and hydrogen resources according to claim 7, characterized in that, in the step (2), the temperature of the process gas is reduced to 250°C to 300°C, and the process gas is heated to 280°C in step (3). ～300℃.

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