JPH0680593A - Production of vinyl chloride monomer by pyrolysis of 1,2-dichloroethane - Google Patents

Abstract

PURPOSE:To provide a method capable of thermally decomposing 1,2- dichloroethane in a high decomposition rate into vinyl chloide, while retaining the operation life of a tubular reactor for a long time. CONSTITUTION:The method for producing vinyl chloride monomer by thermally decomposing 1,2-dichloroethane in a tubular reactor comprising a front tubular reactor and a rear tubular reactor having a larger inner diameter and a smaller length than the front tubular reactor and connected to the front reactor tubular comprises feeding 1,2-dichloroethane gas into the front tubular reactor, thermally decomposing the 1,2-dichloroethane in a prescribed decomposition rate, while performing a temperature control so that the temperature of the thermally decomposed gas at the exit of the rear tubular reactor is lower than the temperature of the thermally decomposition gas at the exit of the front tubular reactor, and subsequently thermally decomposing the unreacted 1,2-dichloroethane contained in the thermally decomposed gas transferred from the front tubular reactor in the rear tubular reactor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing vinyl chloride monomer by thermally decomposing 1,2-dichloroethane.

[0002]

2. Description of the Related Art Conventionally, when 1,2-dichloroethane is pyrolyzed by a pyrolyzer to produce vinyl chloride monomer, first, 1,2-dichloroethane is vaporized in an evaporator to obtain 1,2-dichloroethane. -Introducing dichloroethane gas into a heating tube provided in the cracking furnace, heating there to 360 ° C, then introducing it into a reaction tube having a constant inner diameter with respect to the gas flow direction, and further raising the temperature. Thermal decomposition is carried out while adjusting the outlet temperature of the reaction tube to 490 to 495 ° C. so that the decomposition rate of 1,2-dichloroethane is 58 to 60%.

When operated at a high decomposition rate, side reaction products such as methyl chloride and butadiene, which are problems in the polymerization of vinyl chloride monomer, increase, and the coking rate in the reaction tube is promoted to continuously operate the decomposition furnace. The life is shortened.

On the other hand, when operating at a low decomposition rate in order to extend the operating life, energy efficiency such as an increase in fuel consumption in the cracking furnace and an increase in the amount of distillation energy used in the next step of separating and refining decomposition products by distillation are carried out. I feel bad.

Usually, when the pressure loss of the reaction tube reaches 2.5 times the pressure loss at the beginning of the operation due to coking, it becomes difficult to continue the operation, and the reaction tube may be damaged due to overheating. Must be stopped to remove carbon in the reaction tube. The rate of increase in pressure loss depends on the coking rate, and usually begins to increase rapidly over a period of 2 to 3 months, and then reaches 2.5 times that at the beginning of operation in a short period of time.

The continuous operating period from the start of operation until the pressure loss reaches about 2.5 times and the operation is stopped is called the operating life. However, since the economical loss due to the operation stop is avoided,
Efforts to extend operating life have been made to date in parallel with efforts to increase decomposition rates.

A method of increasing the inner diameter of the reaction tube is known as a method of maintaining the predetermined decomposition rate of 1,2-dichloroethane and the quality of vinyl chloride monomer and extending the operating life of the reaction tube (Japanese Patent Publication No. 59-8245). Issue). However, the reaction tube of the publication is composed of only one kind of reaction tube having a predetermined inner diameter, and the operation life can be doubled in the method of the publication, but the operation life is still the same as that of the embodiment. As mentioned in (4), it is 4 months and cannot be said to be sufficiently long. The decomposition rate is relatively low at 50.49%.

Japanese Unexamined Patent Publication No. 63-139140 discloses a high-temperature decomposition outflow gas from a decomposition furnace and introduction into the decomposition furnace.
By heat exchange with 2-dichloroethane, 1,2-
It is described that the decomposition reaction zone of dichloroethane can be expanded and an increase in the decomposition rate of 1,2-dichloroethane can be achieved without increasing by-products. However, there is no description about the inner diameter of the reaction tube in the publication, and the relationship between the temperature of the expanded decomposition reaction zone and the temperature of the existing decomposition reaction zone is not clear, and in the method of the publication, the decomposition rate is 5 to 10 %, But the operating life extension is not clear.

[0009]

In the production of vinyl chloride monomer, it is necessary to greatly increase the decomposition rate without shortening the operating life of the decomposition furnace in order to reduce the operating cost. Therefore, it is conceivable to connect a new reaction tube to the rear of the existing reaction tube, but with this method, caulking in the newly provided reaction tube is remarkable, and the decomposition rate is greatly increased to a desired degree. Is difficult.

The present invention has been made in view of the above problems, and an object thereof is to be capable of thermally decomposing 1,2-dichloroethane at a high decomposition rate, and reducing the degree of coking in the reaction tube to reduce the reaction tube. The object of the present invention is to provide a method for producing a vinyl chloride monomer by thermal decomposition of 1,2-dichloroethane, which can prolong the life of the vinyl chloride monomer.

[0011]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made earnest studies on a method of achieving a high decomposition rate without causing coking, and as a result, have found a controlling factor of the coking rate, of which the contribution rate is particularly large. We succeeded in quantifying the effects of decomposition reaction temperature and decomposition rate and their synergistic effects, and found a method to increase the decomposition rate without increasing the coking rate. Generally, the higher the decomposition rate, the faster the coking rate, but the present invention suppresses the coking rate by lowering the decomposition reaction temperature in the reaction tube, and the simultaneous decomposition of 1,2-dichloroethane by lowering the decomposition temperature. The decrease in speed is compensated by increasing the reaction time, and as a result, a high decomposition rate is realized. Specifically, the reaction tube has a two-stage structure, and the decomposition temperature of the second stage is lower than the decomposition temperature of the first stage for thermal decomposition.

That is, according to the present invention, a vinyl chloride monomer is produced by thermal decomposition of 1,2-dichloroethane by using a reaction tube formed by connecting a front reaction tube and a rear reaction tube having an inner diameter larger than the front reaction tube and a shorter length. 1,2-dichloroethane gas is supplied to the front reaction tube so that the temperature of the pyrolysis gas at the outlet of the rear reaction tube becomes lower than the temperature of the pyrolysis gas at the outlet of the front reaction tube. The present invention relates to a method of thermally decomposing 1,2-dichloroethane to a predetermined decomposition rate while controlling temperature, and thermally decomposing undecomposed 1,2-dichloroethane in a thermally decomposed gas from a front reaction tube in a rear reaction tube.

[0013]

OPERATION AND EXAMPLES In the production method of the present invention, 1,2-dichloroethane is thermally decomposed in the front reaction tube up to a predetermined decomposition rate, and the backward reaction is performed so that a higher decomposition rate can be obtained at the outlet of the rear reaction tube. The tube is used for thermal decomposition, and includes means for suppressing an increase in the coking rate caused by an increase in the decomposition rate and means for mitigating the adverse effects of coking.

According to the manufacturing method of the present invention, first, in the front reaction tube, 1,2-dichloroethane introduced from the heating tube can be thermally decomposed quickly and efficiently at 450 ° C. to 490 ° C.
Up to. In order to suppress coking, the concentration of chloroprene by-produced at such a relatively high temperature must be kept low.

Therefore, by decreasing the inner diameter of the front reaction tube to increase the gas flow rate of 1,2-dichloroethane and ensuring a high heat transfer coefficient, the temperature can be rapidly raised to a predetermined decomposition temperature. Pyrolyze in a short time. That is, in the front reaction tube, although the temperature is relatively high, the thermal decomposition is performed in a short time, so that the reaction is performed to a relatively low predetermined decomposition rate and coking is suppressed.

Next, in the rear reaction tube, in order to increase the decomposition rate at the outlet of the rear reaction tube to the target decomposition rate, the undecomposed 1,2-dichloroethane introduced from the front reaction tube is thermally decomposed. Here, the decomposition rate at the outlet of the rear reaction tube is the decomposition rate in the front reaction tube based on the amount of 1,2-dichloroethane introduced into the front reaction tube, and the 1,2-dichlore introduced into the front reaction tube. It is the sum of the decomposition rate in the rear reaction tube based on the amount of ethane gas. As the decomposition rate increases, the chloroprene concentration also increases, so coking tends to occur. However, the temperature of the wall in the rear reaction tube is lowered and the decomposition temperature is gradually lowered from the outlet temperature of the front reaction tube to 400
Control coking by lowering to ~ 485 ° C.

In order to increase the decomposition rate at a low decomposition temperature, it is necessary to lengthen the reaction time as described above. Therefore, the inner diameter of the rear reaction tube is made large and the gas flow rate is made low. In the rear reaction tube, it does not undergo thermal decomposition while raising the temperature like in the front reaction tube, but because it undergoes thermal decomposition while lowering the temperature,
The heat flux (heat transfer amount per unit area) may be smaller than that of the front reaction tube, and there are no problems related to heat transfer such as relative decrease in heat transfer area and decrease in heat transfer coefficient due to the increase in inner diameter of the reaction tube. Not only does it not occur, but rather there is the advantage that the degree of pressure loss due to coking can be reduced by increasing the inner diameter.

As described above, according to the production method of the present invention, a high decomposition rate of 1,2-dichloroethane can be obtained without causing significant coking. On the other hand, when it is attempted to obtain a high decomposition rate of 60% or higher in a conventional cracking furnace reaction tube composed only of a single-stage reaction tube, the outlet temperature of the reaction tube is usually 4
It must be maintained at a high temperature of 95-520 ° C.
However, the occurrence of coking is remarkable at such a high temperature. If the outlet temperature is lowered to suppress coking, a high decomposition rate cannot be obtained.

In the present invention, the inner diameter of the front reaction tube is generally 10 to 15 cm, and the inner diameter of the rear reaction tube may be larger than this, but is preferably 1.1 of the inner diameter of the front reaction tube. ~ 2 times as much is used.

Because the inner diameter of the rear reaction tube is usually reduced by about 1 cm due to the carbon adhering to the inside of the tube by coking, if it is less than 1.1 times the inner diameter of the front reaction tube, which is about 10 to 15 cm, it will be narrowed by coking. This is because the inner diameter becomes smaller than the inner diameter of the front reaction tube, resulting in a large pressure loss. Further, if the inner diameter is made small, the ratio S / V of the surface area S of the reaction tube to the volume V per unit length becomes large, so that heat can be easily transferred to the gas fluid in the reaction tube, but the flow velocity is high. The reaction time becomes short and it becomes difficult to increase the decomposition rate.

On the other hand, if the inner diameter exceeds twice, it becomes difficult to transfer heat to the gas fluid in the reaction tube, the reaction time becomes longer, the side reaction products increase and coking is promoted.

The ratio of the inner diameter of the rear reaction tube is more preferably 1.2 to 1.5 times, most preferably 1.3 to 1.
4 times.

The length of the rear reaction tube is 1 of the length of the front reaction tube.
The length should be / 20 to 1/2, preferably 1/5 to 1/3, but if the length of the rear reaction tube is too short, the decomposition rate in the rear reaction tube becomes small and the front reaction tube The increase in the decomposition rate at the outlet of the rear reaction tube with respect to the decomposition rate at the outlet tends to be small, and the sufficient effect of the present invention cannot be obtained.
On the other hand, if the length of the rear reaction tube becomes too long, the decomposition rate at the outlet of the rear reaction tube greatly increases with respect to the decomposition rate at the outlet of the front reaction tube, but not only the equipment cost increases, but also caulking and pressure loss occur. Tends to increase and shorten the operating life. The lengths of the front reaction tube and the rear reaction tube are usually 200 to 250 m and 10 to 100 m.

The rear reaction tube is not limited to a straight tube, but is preferably a curved tube and a straight tube.

The outlet temperature of the rear reaction tube may be lower than the outlet temperature of the front reaction tube, but a preferable temperature difference is 5 to 150 ° C. This is because if the outlet temperature of the rear reaction tube is the same as or higher than the outlet temperature of the front reaction tube, coking in the rear reaction tube becomes remarkable and the operating life is shortened. Further, when the outlet temperature of the rear reaction tube is significantly lower than that of the front reaction tube, the thermal decomposition reaction is stopped, and the production amount of by-products such as butadiene and ethyl chloride, which are unfavorable for the vinyl chloride monomer quality, increases. This is not preferable. Therefore, the most preferable outlet temperature of the rear reaction tube is 360 ° C. or higher, which is a low temperature that does not promote coking but does not substantially stop the thermal decomposition reaction, and the outlet temperature of the front reaction tube (470 to 490 ° C.). The temperature is lower by 5 to 150 ° C. This temperature difference is more preferably 10 to 50 ° C, and most preferably 20 to 30 ° C.

In the production method of the present invention, in order to keep the temperature of the pyrolysis gas at the outlet of the rear reaction tube lower than the temperature of the pyrolysis gas at the outlet of the front reaction tube, in principle, the rear reaction tube and the front reaction tube are reacted. Temperature control the tubes separately.

This temperature control may be carried out by installing the rear reaction tube and the front reaction tube in the same decomposition furnace, or by installing them in separate decomposition furnaces.

When installed in the same decomposition furnace, a partition wall is provided between the front reaction tube and the rear reaction tube to adjust the temperature inside the rear reaction tube to a temperature lower than the temperature inside the front reaction tube. By making it possible, the outlet temperature of the rear reaction tube can be kept low. Further, the fuel supply amount to the burner for the rear reaction tube may be limited. In this embodiment, the front reaction tube and the rear reaction tube are directly connected, and the outlet temperature of the front reaction tube is the inlet temperature of the rear reaction tube.

A general apparatus for carrying out the manufacturing method of the present invention in this embodiment is schematically shown in FIG. In FIG. 1, 1 is a front reaction tube, 2 is a rear reaction tube, 3 is a rear communication tube, 4 is a quench tower, 5 is an evaporator, 6 is a decomposition furnace, and 7 is a heating tube. In this apparatus, first, 1,2-dichloroethane is sent to the evaporator 5. Evaporated here 1,2-
The dichloroethane gas is sent to the heating pipe 7 and preheated to a predetermined temperature. The preheated 1,2-dichloroethane gas is sent to the front reaction tube 1 and thermally decomposed to a predetermined decomposition rate. The thermally decomposed gas is sent to the rear reaction tube 2, where 1,2-dichloroethane in the gas is further thermally decomposed, and the decomposition rate at the outlet of the rear reaction tube 2 is increased. here,
In the decomposition furnace 6, a partition wall 8 is provided between the front reaction tube and the rear reaction tube, and heating is performed so that the temperature of the rear reaction tube outlet is lower than the temperature of the front reaction tube outlet. The gas exiting the rear reaction tube outlet is sent to the quenching tower 4 through the rear communication tube and cooled.

When the rear reaction tube and the front reaction tube are installed in separate cracking furnaces, the outlet temperature of the rear reaction tube can be kept low by adjusting the temperature inside each cracking furnace. In this aspect, an intermediate connecting pipe for connecting both the front reaction tube and the rear reaction tube,
That is, there is an intermediate connecting pipe for connecting the two decomposition furnaces. In this case, the outlet temperature of the front reaction tube is the inlet temperature of the intermediate connecting tube.

A general apparatus for carrying out the manufacturing method of the present invention in this aspect is schematically shown in FIG. In FIG.
Reference numeral 9 represents an intermediate connecting pipe, and other numerals are the same as those in FIG. In this apparatus as well, the same thermal decomposition as in the apparatus of FIG. 1 is carried out, but the temperature of the thermal decomposition gas exiting the front reaction tube outlet drops during passage through the intermediate connecting tube 9, so the temperature is reduced to the extent of decrease. The heating of the rear reaction tube 2 is adjusted accordingly.

The temperature of the rear reaction tube may be adjusted by putting it in a container other than the decomposition furnace. If you put it in a container,
Liquid or gas may be passed through the container as a heat medium, and the heat generated by heating or cooling the rear reaction tube may be used for other devices. Also in this embodiment, there is an intermediate connecting pipe for connecting the front reaction pipe and the rear reaction pipe, that is, an intermediate connection pipe for connecting the decomposition furnace and another container.

A general apparatus for carrying out the manufacturing method of the present invention in this aspect is schematically shown in FIG. 1 in FIG.
0 represents a container.

Although the intermediate connecting tube does not exchange heat with the outside, it has a function similar to that of the rear reaction tube because sensible heat of the decomposition gas from the front reaction tube causes thermal decomposition in the connecting tube. Therefore, the inner diameter of the front reaction tube is 1.1 to
It is preferable to double the length and to set the length to 1/20 to 1/2 of the backward reaction tube.

Normally, the number of channels of the rear reaction tube is the same as the number of channels of the front reaction tube, but when the number of channels of the rear reaction tube is smaller than the number of channels of the front reaction tube, Y The channels can be assembled using an intermediate connecting tube in the shape of a mold.

The rear reaction tube is generally directly connected to the quenching tower in the next step by a rear connecting tube. The inner diameter of the rear communication tube is usually the same as the inner diameter of the rear reaction tube.However, when several rear reaction tubes are assembled into one rear communication tube, the sum of the cross-sectional areas of the rear reaction tubes is Make it equal to the cross-sectional area.
The backward reaction tube may be connected to a quenching tower after passing through a quenching heat exchanger using the same heat medium as the backward reaction tube. The tube used for this heat exchanger may be a single tube having the same inner diameter as the rear reaction tube, or may be composed of a plurality of tubes having small inner diameters.

Unlike the above-mentioned intermediate connecting pipe, the rear connecting pipe is not intended for thermal decomposition reaction, and is not related to the rear connecting pipe in terms of inner diameter, length, and temperature inside the pipe.

The lengths, inner diameters, and outlet temperatures of the front reaction tube and the rear reaction tube can be freely selected so as to satisfy the above relationships.

In order to increase the final decomposition rate and extend the operating life, it is preferable that the decomposition rate at the outlet of the rear reaction tube is higher than that at the outlet of the front reaction tube by 2 to 15%. This means that the decomposition rate of 1,2-dichloroethane in only the rear reaction tube is 2 based on the amount of 1,2-dichloroethane introduced in the front reaction tube.
It means ˜15%. The difference in the decomposition rate is more preferably 5 to 10%, most preferably 6 to 8%.
Is.

It is also preferable to operate at a decomposition rate of 50 to 60% at the outlet of the front reaction tube. When the decomposition rate is higher than this, the front reaction tube tends to be clogged earlier than the rear reaction tube, and when the decomposition rate is lower than this, the rear reaction tube is clogged and the operating life tends to be shortened. Absent. The decomposition rate at the outlet of the front reaction tube is more preferably 53 to 58%, most preferably 55 to 56%.

The process of the present invention is also effective in the presence of a catalyst such as carbon tetrachloride which promotes the rate of thermal decomposition of 1,2-dichloroethane or in the presence of a coking promoter such as chloroprene.

The present invention will be described with reference to the following specific examples, but the present invention is not limited to these examples.

Example 1 Gaseous 1,2-dichloroethane at 350 ° C. was added with an inner diameter of 11
The pressure was 15 atm and the supply rate was 6600 kg / hr to the front reaction tube (length 200 m), which is a low decomposition region of cm, and the temperature of the outlet was 485 ° C. to perform thermal decomposition, and then the thermal decomposition gas was partitioned by a partition wall. It is supplied to a rear reaction tube (50 m in length) which is a high decomposition area having an inner diameter of 15 cm provided in the same decomposition furnace, and the outlet temperature of the rear reaction tube is set to 48
Further, thermal decomposition was performed at 0 ° C.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 56%, the decomposition rate at the outlet of the rear reaction tube was 65%, and the pressure loss was 1.5 after continuous operation for 6 months.
There was a two-fold increase from kg / cm ² to 3.0 kg / cm ² .

The decomposition rate X at the outlet of the front reaction tube was determined by sampling the gas at the outlet of the front reaction tube and rapidly cooling it. Then, the number of moles H of hydrogen chloride produced by thermal decomposition and undecomposed 1,2-dichloro The ethane mole number E was measured and calculated by the following formula. The decomposition rate at the outlet of the rear reaction tube was also determined by the same method.

X = H / (H + E) × 100 Example 2 Pyrolysis was carried out under the same conditions using the same front reaction tube as in Example 1, and then the cracked gas was transferred to an intermediate connecting tube having a length of 10 m. Introduced. Next, the cracked gas from the outlet of the intermediate connecting tube was introduced into the same rear reaction tube as in Example 1, which was placed in a container different from the cracking furnace, and the outlet temperature was set to 475 ° C. to perform further thermal decomposition.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 56%, the decomposition rate at the outlet of the rear reaction tube was 66%, and the pressure loss was 1.5 after continuous operation for 6 months.
It increased 2.1 times from kg / cm ² to 3.2 kg / cm ² .

Example 3 Rear reaction tube Pyrolysis was carried out under the same apparatus and conditions as in Example 1 except that the outlet temperature of the reaction tube was 445 ° C.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 56%, the decomposition rate at the outlet of the rear reaction tube was 62%, and the pressure loss was 1.5 after continuous operation for 6 months.
It doubled from kg / cm ² to 3.0 kg / cm ² .

Comparative Example 1 Gaseous 1,2-dichloroethane at 360 ° C. was used with an inner diameter of 11
The pressure is 15 atm and the supply rate is 6600 kg per hour to the front reaction tube (200 m in length), which is a low decomposition region of cm, and is thermally decomposed at an outlet temperature of 497 ° C. Then, this thermally decomposed gas is divided by the same decomposition wall. Rear reaction tube (length 50m), which is a high-resolution area with an inner diameter of 11 cm installed in the furnace
And further pyrolyzed at an outlet temperature of the rear reaction tube of 510 ° C.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 61%, and the decomposition rate at the outlet of the rear reaction tube was 64%. However, after continuous operation for 3 months, the total pressure loss was as early as 1%. It increased from .6kg / cm ² to about 2.4 times to 3.8kg / cm ^2. That is, the decomposition rate was almost the same as in Example 1, but the operating life was about half.

Comparative Example 2 Gaseous 1,2-dichloroethane at 360 ° C. was added to the inner diameter 11
A reaction tube (length: 200 m) having a pressure of 15 atm and a supply rate of 6600 kg / hour was supplied to the reaction tube having a length of 200 cm, and pyrolysis was performed at an outlet temperature of 470 ° C.

Under the above-mentioned conditions, the increase in pressure loss after the continuous operation for 6 months was 1.4 kg / cm ² to 3.5 kg / c.
It could be suppressed to about 2.5 times to m ^2, but the decomposition rate of the reaction tube outlet was only 52%. That is,
The decomposition rate was 13% lower in the same operation period as in Example 1.

Comparative Example 3 Comparative Example 1 except that the outlet temperature of the rear reaction tube was 445 ° C.
Pyrolysis was carried out using the same equipment and conditions.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 61% and the decomposition rate at the outlet of the rear reaction tube was 62%. However, after continuous operation for 3 months, the total pressure loss was as early as 1%. It increased from .6kg / cm ² to 2.1 times to 3.4 kg / cm ^2.

That is, the operating life was about half even though the decomposition rate and the outlet temperature of the rear reaction tube were made smaller than those in Example 1.

Comparative Example 4 Pyrolysis was carried out under the same apparatus and conditions as in Example 1 except that the outlet temperature of the front reaction tube was 490 ° C and the outlet temperature of the rear reaction tube was 495 ° C.

Under the above conditions, the decomposition rate at the outlet of the front reaction tube was 58%, the decomposition rate at the outlet of the rear reaction tube was 72%, and the pressure loss was 1.5 kg / after continuous operation for 2.5 months. from cm ² to 3.8kg / cm ² 2.5
Doubled.

[0059]

According to the method of the present invention, 1,2-dichloroethane can be thermally decomposed at a high decomposition rate while keeping the operating life of the reaction tube long.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus for carrying out the manufacturing method of the present invention.

FIG. 2 is a schematic view of another apparatus for carrying out the manufacturing method of the present invention.

FIG. 3 is a schematic view of another apparatus for carrying out the manufacturing method of the present invention.

[Explanation of symbols]

 1 Front Reaction Tube 2 Back Reaction Tube 3 Back Communication Tube 4 Quenching Tower 5 Evaporator 6 Decomposition Furnace 7 Heating Tube 8 Partition Wall 9 Intermediate Communication Tube 10 Container

Claims (6)

[Claims] 1. A method for producing a vinyl chloride monomer by thermal decomposition of 1,2-dichloroethane using a front reaction tube and a reaction tube having an inner diameter larger and a length shorter than the front reaction tube connected to each other. Then, 1,2-
Dichloroethane gas is supplied to decompose 1,2-dichloroethane to a predetermined temperature while controlling the temperature so that the temperature of the pyrolysis gas at the outlet of the rear reaction tube becomes lower than the temperature of the pyrolysis gas at the outlet of the front reaction tube. Pyrolysis up to rate
A method which comprises thermally decomposing undecomposed 1,2-dichloroethane in the pyrolysis gas from the front reaction tube in the rear reaction tube. 2. The method according to claim 1, wherein the temperature is controlled such that the temperature of the pyrolysis gas at the outlet of the rear reaction tube is lower than the temperature of the pyrolysis gas at the outlet of the front reaction tube by 5 ° C. to 150 ° C. 3. The method according to claim 1, wherein the inner diameter of the rear reaction tube is 1.1 to 2 times the inner diameter of the front reaction tube. 4. The method according to claim 1, 2 or 3, wherein the length of the rear reaction tube is 1/20 to 1/2 of the length of the front reaction tube. 5. An intermediate connecting pipe is present between the front reaction pipe and the rear reaction pipe, and the length of the intermediate connecting pipe is 1 times that of the rear reaction pipe.
The method according to claim 1, 2, 3 or 4, which is / 20 to 1/2. 6. The decomposition rate of 1,2-dichloroethane at the outlet of the front reaction tube is 50 to 60%.
The method according to 2, 3, 4 or 5.