Continuous tower type chlorination method for nitrotoluene

Abstract

The invention discloses a nitrotoluene continuous tower-type chlorination method, and relates to the technical field of nitrotoluene production, and the method comprises the following steps: S1, putting p-nitrotoluene into a pre-preparation kettle, adding ferric trichloride and refined iodine, and stirring until the materials are completely dissolved to form a uniform mixed solution; s2, feeding the mixed solution into a first tower at a set flow rate for reaction to obtain a first tower product; s3, the first-tower product is fed into a secondary tower for a reaction, a secondary-tower product is obtained, and chlorine needs to be introduced in four stages during the first-tower reaction and the secondary-tower reaction; s4, monitoring the feeding quantity of the prefabricated kettle, the first tower and the secondary tower in real time through a flow control system, and finely adjusting parameters by combining online detection data of a product, so as to realize continuous and automatic operation in the whole process; a multi-stage continuous reaction mode of the first tower and the secondary tower is adopted, the material remaining amount in single equipment is greatly reduced, meanwhile, whole-process automatic operation is achieved through the flow control system and the temperature control device, the manual intervention frequency is reduced, and the safety risk is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of nitrotoluene production technology, specifically relating to a continuous tower chlorination method for nitrotoluene. Background Technology

[0002] The existing nitrobenzene chlorination process is based on batch batch reactor production. It uses a reactor as the reaction vessel, p-nitrotoluene as raw material, chlorine as chlorinating agent, and a ferric chloride + iodine composite catalytic system. By fixing parameters such as reaction temperature and the ratio of raw material to chlorine, the chlorination reaction is completed in the reactor. The target product is chlorinated products such as 2-chloro-4-nitrotoluene.

[0003] The reaction of p-nitrotoluene with chlorine gas produces o-chloro-p-nitrotoluene and hydrogen chloride, and the reaction proceeds according to the following mechanism:

[0004] ;

[0005] The main side reactions of the chlorination process are as follows:

[0006] ;

[0007] However, due to the large size of the reactor equipment and the large amount of material stored, the intermittent operation requires frequent manual intervention in the material addition, reaction, and discharge processes, which does not meet the requirements for automatic control of hazardous chemical production and poses significant safety hazards. Furthermore, the temperature and material mixing uniformity of each batch of reaction are difficult to be completely consistent, resulting in large fluctuations in the content of 2-chloro-4-nitrotoluene and the content of dichloronitro compounds in the byproducts of each batch of products, and insufficient product quality stability. Summary of the Invention

[0008] The purpose of this invention is to provide a continuous tower chlorination method for nitrotoluene, which adopts a two-tower multi-stage continuous reaction mode with a first tower and a second tower, significantly reducing the amount of material remaining in a single unit and improving the stability of product quality, thereby solving the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A continuous tower chlorination method for nitrotoluene includes the following steps:

[0011] S1. Add p-nitrotoluene to the pre-mixing kettle, add ferric chloride and refined iodine, and stir until completely dissolved to form a homogeneous mixture;

[0012] S2. The mixture is fed into the first column at a set flow rate to react and obtain the product of the first column.

[0013] S3. The product from the first tower is sent to the second tower for reaction to obtain the product from the second tower. Chlorine gas needs to be introduced in four stages during the reactions in the first and second towers.

[0014] S4. The feed rate of the pre-processing vessel, the first tower, and the second tower is monitored in real time by the flow control system. The parameters are fine-tuned by combining the online detection data of the product to achieve continuous and automated operation throughout the process.

[0015] Preferably, in step S1, 3000L of p-nitrotoluene is added to the pre-preparation reactor, ferric chloride is added to make the concentration of p-nitrotoluene reach 5%, and then refined iodine is added to make the concentration of p-nitrotoluene reach 0.25%.

[0016] Preferably, in step S2, the mixture is fed into the first tower at a flow rate of 630 kg / h, and chlorine gas is introduced in four stages at rates of 90 kg / h, 65 kg / h, 47 kg / h, and 34 kg / h respectively. The mass ratio of p-nitrotoluene to chlorine gas in each stage is 6-7:1. The reaction temperature of the first tower is 75±10℃, and the pressure is 5-10 kPa.

[0017] Preferably, both the first tower and the second tower are packed towers, with four layers of packing material inside. Each layer of packing material is provided with a gas pipeline below it, and a gas flow valve is provided on the gas pipeline. All the gas pipelines are connected to the vaporization pipeline of liquid chlorine through a main gas valve.

[0018] Preferably, the bottom of the packed tower is connected to a feed pipe, one end of which is connected to a p-nitrotoluene feed pump for connecting to a catalyst mixing tank. A discharge pipe is connected to the connection between the feed pipe and the packed tower. The liquid at the bottom of the packed tower is transferred to the top of each packing layer through a circulation pipeline, and a recovery pipeline for recycling the circulating water is provided on the circulation pipeline. The upper end of the first tower is connected to the feed pipe of the second tower through a transmission pipe.

[0019] Preferably, the flow control system includes online flow meters installed on the feed pipe, the vent pipe, and the circulation pipe; an online temperature meter installed on the tower wall below each packing layer; an online material monitoring device installed on the inner wall at the upper end of the packed tower; and an online pressure meter installed at the upper end of the packed tower.

[0020] Preferably, the feed flow rate of the secondary tower is controlled to 630 kg / h, and chlorine gas is introduced in four stages at rates of 21 kg / h, 16 kg / h, 12 kg / h and 10 kg / h respectively, with a mass ratio of p-nitrotoluene to chlorine gas of 7-8:1 per stage; the reaction temperature of the secondary tower is 75±10℃ and the pressure is 5-10 kPa.

[0021] Preferably, after the secondary tower reaction is completed, the product needs to be detected by online chromatography. When the content of 2-chloro-4-nitrotoluene is detected to be <90%, the flow control system automatically adjusts the chlorine feed rate of the secondary tower, and the chlorine flow rate of each stage is finely adjusted within the range of ±5% of the set value.

[0022] Preferably, the tail gas generated during the reaction process of the first and second towers is transported to the HCl absorber for treatment via pipeline, and the tail gas transport pipeline is equipped with a shut-off valve.

[0023] Preferably, the packing layer in the first and second towers is made of polypropylene or ceramic material, and the specific surface area of ​​the packing is ≥150m². 2 / m 3 .

[0024] The continuous tower chlorination method for nitrotoluene proposed in this invention has the following advantages compared with the prior art:

[0025] 1. This invention adopts a two-tower, multi-stage continuous reaction mode with a first tower and a second tower, which greatly reduces the amount of material remaining in a single unit. At the same time, it achieves fully automated operation through a flow control system and a temperature control device, reducing the frequency of manual intervention. It fully meets the automated control requirements of hazardous chemical production and significantly reduces safety risks.

[0026] 2. This invention uses a flow control system to precisely control the reaction temperature, pressure, and raw material ratio of the two towers, thereby achieving stable and controllable parameters and improving the stability of product quality.

[0027] 3. This invention improves the conversion rate of p-nitrotoluene by dynamically adjusting the chlorine feed rate through a stepwise strategy of suppressing side effects in the first tower and optimizing the second tower.

[0028] 4. This invention enables continuous production throughout the entire process, eliminates intermittent downtime, increases product output per unit time, and is more suitable for the needs of large-scale, high-capacity industrial production. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the connection system between the first and second towers of the present invention;

[0030] Figure 2 This is a flowchart of the present invention;

[0031] In the diagram: 1. First tower; 2. Second tower; 3. Ventilation pipeline; 4. Main gas valve; 5. Feed pipe; 6. p-Nitrotoluene feed pump; 7. Circulation pipeline; 8. Discharge pipe; 9. Transfer pipe; 10. Recovery pipeline; 11. Packing layer. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The specific embodiments described herein are merely used to explain the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] This invention provides, for example Figure 1-2 The continuous tower chlorination method for nitrotoluene shown includes the following steps:

[0034] S1. Add p-nitrotoluene to the pre-mixing vessel, along with ferric chloride and refined iodine, and stir until completely dissolved to form a homogeneous mixture. Add 3000L of p-nitrotoluene to the pre-mixing vessel, add ferric chloride to achieve a concentration of 5% in the p-nitrotoluene, and then add refined iodine to achieve a concentration of 0.25% in the p-nitrotoluene. This method allows for precise control of the catalyst concentration, ensuring that the ferric chloride and refined iodine composite catalytic system achieves optimal catalytic effect, avoiding excessively slow reaction rates or increased side reactions due to improper catalyst concentration, ensuring the homogeneity of the mixture, and improving the stability and efficiency of subsequent reactions.

[0035] S2. The mixture is fed into the first tower 1 at a set flow rate to obtain the first tower product. The mixture is fed into the first tower 1 at a flow rate of 630 kg / h, and chlorine gas is introduced in four stages at rates of 90 kg / h, 65 kg / h, 47 kg / h, and 34 kg / h respectively. The mass ratio of p-nitrotoluene to chlorine gas in each stage is 6-7:1. The reaction temperature in the first tower 1 is 75±10℃, and the pressure is 5-10 kPa. The first tower 1 uses a high feed ratio to suppress side reactions. At the same time, the appropriate temperature and pressure are controlled to provide a good environment for the reaction and initially promote the chlorination reaction. The 2-chloro-4-nitrotoluene content in the product of the first tower 1 reaches 71%-72%, the dichloronitro compound content is <0.6%, the p-nitrotoluene conversion rate is >71%, and the chlorine utilization rate is >95%, achieving initial chlorination and effectively suppressing the formation of by-products.

[0036] S3. The product from the first tower is sent to the second tower 2 for reaction to obtain the product from the second tower 2. Chlorine gas needs to be introduced in four stages during the reaction of the first tower 1 and the second tower 2.

[0037] S4. The feed rates of the pre-processing vessel, first tower 1, and second tower 2 are monitored in real time by the flow control system, and the parameters are fine-tuned in combination with the online detection data of the product to achieve continuous and automated operation throughout the process.

[0038] Both the first tower 1 and the second tower 2 are packed towers, each containing four layers of packing material 11. Each layer of packing material 11 has a venting pipe 3 below it, and each venting pipe 3 is equipped with a gas flow valve. All venting pipes 3 are connected to the vaporization pipeline of liquid chlorine via a main gas valve 4. The packing material in the packed tower enhances gas-liquid mixing. The four layers of packing material 11, combined with the staged venting pipes 3, ensure sufficient contact between chlorine and the mixed liquid. The flow valves precisely control the amount of chlorine introduced at each stage, increasing the gas-liquid contact area and uniformity, preventing incomplete local reactions or excessive chlorine, reducing byproducts, and improving reaction efficiency and product purity.

[0039] The packing layer 11 in the first tower 1 and the second tower 2 is made of polypropylene or ceramic material, and the specific surface area of ​​the packing is ≥150m². 2 / m 3 High specific surface area corrosion-resistant fillers are selected to enhance the gas-liquid two-phase contact effect, while resisting the corrosiveness of the reaction system, extending the service life of the equipment, further improving the uniformity of gas-liquid mixing, ensuring the full reaction, reducing filler loss, reducing equipment maintenance costs, and extending the continuous operation time of the process.

[0040] The bottom of the packed tower is connected to a feed pipe 5, one end of which is connected to a p-nitrotoluene feed pump 6 for connecting to a catalyst mixing tank. A discharge pipe 8 is connected to the connection between the feed pipe 5 and the packed tower. The liquid at the bottom of the packed tower is transferred to the top of each packing layer 11 through a circulation pipe 7, and a recovery pipe 10 for recycling the circulating water is provided on the circulation pipe 7. The upper end of the first tower 1 is connected to the feed pipe 5 of the second tower 2 through a transmission pipe 9. The feed pump ensures stable delivery of raw materials, the circulation pipe 7 allows unreacted liquid to flow back for further reaction, the transmission pipe 9 enables continuous material transfer between the two towers, and the recovery pipe 10 reduces resource waste, ensures a continuous and stable supply of materials, improves raw material utilization, avoids material accumulation or interruption of delivery, and ensures smooth continuous production.

[0041] like Figure 1 As shown, the circulation pipeline 7 includes a liquid storage chamber located on the side wall of the packed tower. A branch pipe is connected to one side of the liquid storage chamber above the packing layer 11 and the lower end of the packed tower. An online flow meter and a control valve are installed on the branch pipe. Multiple branch pipes are connected to the circulating water supply source through connecting pipes. The recovery pipeline 10 includes a branch water pipe connected to one side of the liquid storage chamber below the packing layer 11. A solenoid valve is installed on the branch water pipe. Multiple branch water pipes are connected to the circulating water cooling equipment through connecting pipes. The circulating water supply source is transmitted to each branch pipe through the connecting pipes. The liquid in each branch pipe enters the liquid storage chamber from different directions and is then transmitted to the circulating water cooling equipment through the branch water pipes, so that the liquid inside the liquid storage chamber is in a flowing state, which facilitates precise temperature control of the packed tower.

[0042] The flow control system includes online flow meters installed on the feed pipe 5, the vent pipe 3, and the circulation pipe 7; online temperature meters installed on the tower wall below each packing layer 11; online material monitoring equipment installed on the inner wall at the upper end of the packed tower; and online pressure monitoring equipment installed at the upper end of the packed tower. By collecting flow rate, temperature, pressure, and material status data in real time through various online monitoring devices, the system provides a basis for parameter adjustment, achieves closed-loop control, facilitates real-time monitoring of key parameters in the reaction process, timely detection and adjustment of abnormalities, ensures reaction stability, improves product quality consistency, and reduces manual monitoring costs.

[0043] The feed flow rate of the secondary tower 2 is controlled to 630 kg / h. Chlorine gas is introduced in four stages at rates of 21 kg / h, 16 kg / h, 12 kg / h, and 10 kg / h, with a mass ratio of p-nitrotoluene to chlorine of 7-8:1 per stage. The reaction temperature of the secondary tower 2 is 75±10℃, and the pressure is 5-10 kPa. Based on the content of unreacted raw materials in the product of the primary tower 1, the secondary tower 2 adopts the optimal ratio and further increases the chlorination depth under the same temperature and pressure conditions as the primary tower 1. The product of the secondary tower 2 contains >90% 2-chloro-4-nitrotoluene, <1.0% dichloronitro compounds, >90% p-nitrotoluene conversion, and >95% chlorine utilization, significantly improving product purity and raw material conversion rate.

[0044] After the reaction in secondary tower 2 is completed, the product needs to be detected by online chromatography. When the content of 2-chloro-4-nitrotoluene is detected to be <90%, the flow control system automatically adjusts the chlorine feed rate of secondary tower 2. The chlorine flow rate of each stage is finely adjusted within ±5% of the set value. Through online detection feedback, the chlorine feed rate is dynamically adjusted to achieve on-demand control ratio, ensure that the product quality meets the standards, avoid product purity failure due to fluctuations in raw material properties or slight changes in reaction conditions, improve product quality stability, and reduce the rate of defective products.

[0045] The tail gas generated during the reaction process of the first tower 1 and the second tower 2 is transported to the HCl absorber for treatment through pipelines. The tail gas transport pipeline is equipped with a shut-off valve. The tail gas contains harmful gases such as hydrogen chloride. After being treated by the HCl absorber, it meets the emission standards. The shut-off valve can be quickly closed in case of abnormality to prevent tail gas leakage, reduce the emission of harmful gases, meet environmental protection requirements, reduce the safety risks caused by tail gas leakage, and ensure the safety of the production environment.

[0046] By adopting a two-tower, multi-stage continuous reaction mode (first tower 1 and second tower 2) to replace the traditional batch reactor process, the reaction is gradually advanced through precise temperature, pressure, and raw material ratio control in stages. At the same time, the parameters are adjusted in real time by an automated system to ensure stable and continuous reaction. This solves the problems of high safety risks, poor stability, and low resource utilization in traditional processes, achieving continuous and automated production, and improving production efficiency, product quality stability, and raw material utilization.

[0047] Based on the nitrobenzene continuous tower chlorination process provided above, the following specific examples are provided to determine the conversion rate of p-nitrotoluene and the chlorine utilization rate in the chlorination process:

[0048] Example 1

[0049] Raw materials and reagents: p-nitrotoluene (purity ≥99.8%, moisture ≤0.1%, Jiangsu Huaihua), chlorine (purity 99.8%, moisture ≤0.01%, Anhui Hongsifang), ferric chloride (industrial grade, Anhui), refined iodine (purity 99.8%, Chile).

[0050] Equipment used: 2 x 3000L catalyst pre-mixing kettles, 2 x feed pumps (magnetically driven centrifugal pumps, F4 steel-lined), 2 x tandem packed towers (with four layers of packing material, polypropylene or ceramic, specific surface area ≥150m² / m³), temperature control device (jacketed heat exchange structure, with DCS control system), flow control system (including FICO, FCO modules and online flow meter), online detection equipment (temperature meter, pressure meter, material monitoring equipment, chromatograph), HCl absorber, tail gas delivery pipeline and shut-off valve (VT0101-65 model).

[0051] The specific process is as follows:

[0052] 1. Add 3000L of p-nitrotoluene to a 3000L pre-mixing reactor, add ferric chloride to achieve a concentration of 5‰ in the p-nitrotoluene, and then add refined iodine to achieve a concentration of 0.25‰. Stir at a rate of 80-120 rpm for 30-45 minutes until the catalyst is completely dissolved and a homogeneous mixture is formed. Two pre-mixing reactors should be operated alternately to ensure a continuous supply of the mixture.

[0053] 2. The mixed liquid is fed into the bottom of the first tower at a flow rate of 630 kg / h via a feed pump. Chlorine gas is introduced into the first tower in four stages via a vaporization pipeline, a main gas valve, and a staged gas supply pipeline, with flow rates of 90 kg / h, 65 kg / h, 47 kg / h, and 34 kg / h for each stage, respectively. The mass ratio of p-nitrotoluene to chlorine gas in each stage is controlled at 6-7:1. The reaction temperature in the first tower is maintained at 75±10℃, and the pressure is controlled at 5-10 kPa. The liquid in the tower is returned to the top of each packing layer through a circulation pipeline to ensure sufficient gas-liquid contact. During the reaction, online monitoring equipment collects flow rate, temperature, and pressure data in real time, and the flow control system dynamically fine-tunes the parameters. The product from the first tower contains 71%-72% 2-chloro-4-nitrotoluene, <0.6% dichloronitro compounds, >71% p-nitrotoluene conversion, and >95% chlorine utilization.

[0054] 3. The product from the first tower is fed into the second tower via a transfer pipe at a flow rate of 630 kg / h. Chlorine gas is introduced in four stages at flow rates of 21 kg / h, 16 kg / h, 12 kg / h, and 10 kg / h respectively, with a p-nitrotoluene to chlorine mass ratio of 7-8:1 in each stage, maintaining the same temperature and pressure conditions as the first tower. The second tower product is analyzed by online chromatography. If the 2-chloro-4-nitrotoluene content is <90%, the flow control system automatically adjusts the chlorine flow rate for each stage (within ±5% of the set value). The final second tower product should have a 2-chloro-4-nitrotoluene content >90%, a dichloronitro compound content <1.0%, a p-nitrotoluene conversion rate >90%, and a chlorine utilization rate >95%.

[0055] 4. The hydrogen chloride-containing tail gas produced by the two-tower reaction is transported to the HCl absorber for treatment via pipeline. The shut-off valve on the tail gas pipeline automatically closes in case of equipment malfunction. The entire process is automated through a DCS control system that integrates various monitoring data, achieving full automation of feeding, reaction, detection, and adjustment without frequent manual intervention, ensuring continuous and stable operation of the process.

[0056] Based on Example 1 above, it can be concluded that after the secondary tower reaction, the content of 2-chloro-4-nitrotoluene is >90%, the content of dichloronitro compounds is <1.0%, the conversion rate of p-nitrotoluene is >90%, and the chlorine utilization rate is >95%, which effectively improves the quality of chlorinated products such as 2-chloro-4-nitrotoluene, and also improves the conversion rate of p-nitrotoluene and the chlorine utilization rate.

[0057] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A continuous column process for the chlorination of nitrotoluene, characterized in that: It comprises the following steps: S1, put p-nitrotoluene into the pre-preparation kettle, add ferric chloride and iodine, stir until completely dissolved, form a uniform mixture; S2, the mixture is sent to the first tower at a set flow rate for reaction to obtain a tower product; S3, the first tower product is sent to the second tower for reaction to obtain a second tower product, and the first tower reaction and the second tower reaction need to be fed with chlorine in four stages; S4, the feeding amount of the pre-preparation kettle, the first tower and the second tower is monitored in real time by a flow control system, and the parameters are adjusted in combination with online detection data of the product to realize continuous and automatic operation.

2. A continuous nitro-toluene tower chlorination process according to claim 1, characterized in that: In step S1, 3000L of p-nitrotoluene is put into the pre-preparation kettle, ferric chloride is added to make the concentration of p-nitrotoluene in the kettle reach 5%, and iodine is added to make the concentration of p-nitrotoluene in the kettle reach 0.25%.

3. A continuous nitro-toluene tower chlorination process according to claim 1, characterized in that: In step S2, the mixture is sent to the first tower at a flow rate of 630Kg / h, and chlorine is fed in four stages at flow rates of 90Kg / h, 65Kg / h, 47Kg / h and 34Kg / h, respectively, the mass ratio of p-nitrotoluene to chlorine in each stage is 6-7:1, the reaction temperature of the first tower is 75±10℃, and the pressure is 5-10KPa.

4. A continuous nitro-toluene tower chlorination process according to claim 1, characterized by: The first tower and the second tower are both packed towers, and four layers of packing are arranged in the packed tower. An air pipe is arranged below each layer of packing, and a gas flow valve is arranged on the air pipe. The multiple air pipes are connected with the gasification pipe of liquid chlorine through a total gas valve.

5. A continuous nitro-toluene column chlorination process according to claim 4, characterized by: The bottom of the packed tower is connected with a feeding pipe, one end of the feeding pipe is connected with a p-nitrotoluene feeding pump for communication with a catalyst batching tank, a discharge pipe is connected with the connection between the feeding pipe and the packed tower, and the bottom liquid of the packed tower is transmitted to the upper part of each layer of packing through a circulating pipe, and a recovery pipe for recycling circulating water is arranged on the circulating pipe. The upper end of the first tower is connected with the feeding pipe of the second tower through a transmission pipe.

6. A continuous nitro-toluene column chlorination process according to claim 5, characterized by: The flow control system comprises online flow monitoring meters arranged on the feeding pipe, the air pipe and the circulating pipe, online temperature monitoring meters arranged on the tower wall below each layer of packing, online material monitoring equipment arranged on the inner wall of the upper end of the packed tower, and online pressure monitoring meters arranged on the upper end of the packed tower.

7. A continuous nitro-toluene column chlorination process according to claim 6, characterized by: The feeding flow rate of the second tower is controlled to 630Kg / h, chlorine is fed in four stages at flow rates of 21Kg / h, 16Kg / h, 12Kg / h and 10Kg / h, respectively, the mass ratio of p-nitrotoluene to chlorine in each stage is 7-8:1, the reaction temperature of the second tower is 75±10℃, and the pressure is 5-10KPa.

8. A continuous nitro-toluene tower chlorination process according to claim 7, characterized by: After the reaction of the second tower is completed, the product needs to be detected by an online chromatography, and when the content of 2-chloro-4-nitrotoluene is detected to be less than 90%, the flow control system automatically adjusts the feeding amount of chlorine in the second tower, and the flow rate of chlorine in each stage is adjusted within the range of ±5% of the set value.

9. A continuous nitro-toluene column chlorination process according to claim 8, characterized by: The tail gas generated in the reaction process of the first tower and the second tower is transported to an HCl absorber through a pipeline for treatment, and a shut-off valve is arranged on the tail gas conveying pipeline.

10. A continuous nitro-toluene column chlorination process according to claim 9, characterized by: The packing layer in the primary column and the secondary column is made of polypropylene or ceramic material, and the specific surface area of the packing is greater than or equal to 150 m 2 / m 3 .

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