JPH04112838A - Production of benzene - Google Patents

Abstract

PURPOSE:To obtain benzene by bringing toluene into contact, in a vapor phase flow-through system in the absence of H2 Gas, with an exothermic body installed inside a reaction tube with the inner wall surface kept at a temperature lower than the thermal decomposition temperature of toluene and heated at a temperature higher than said thermal decomposition temperature. CONSTITUTION:Toluene is dealkylated by internal heating in a vapor phase flow-through system, thus giving the objective benzene in high selectivity without the need for hydrogen while suppressing the formation of by-products such as bibenzyl. Specifically, an exothermic body with a surface temperature of 900-1100 deg.C is put inside a reaction tube, and toluene is brought into contact with this exothermic body at a flow rate of 0.01-0.3mol/hr. It is important that the inner wall surface of the reaction tube be lower than the thermal decomposition temperature of toluene (pref. <=100 deg.C), and it is preferable that the distance between the surface of said exothermic body and the inner wall surface be 1mm to 5cm. The exothermic body is e.g. in the form of linear or planar nichrome wire or tungsten wire, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing benzene. More specifically, the present invention relates to a method for producing benzene at a high production rate by dealkylating toluene in the gas phase using an internal heating method.

[Prior Art and Problems to be Solved by the Invention] Hydrocracking using hydrogen gas is widely used to produce benzene from toluene.

The reason for using hydrogen is to suppress the production of by-products such as bibenzyl and increase the yield of benzene. However, this production method cannot necessarily be said to be economically satisfactory since it involves handling a large amount of hydrogen.

Also, when toluene is thermally decomposed without adding hydrogen (for example, G, M, Badger and T, M, Spots
wood J, ChemSoc, 1960.442
0), the selectivity of the reactants is as shown below, and the selectivity of benzene is poor, and many by-products such as bibenzyl are produced.

Product Selectivity (%) Benzene
39.0 bibenzyl
15.44.4 Dimethylbiphenyl 1
5.2 An object of the present invention is to provide a method for producing benzene from toluene without using hydrogen, suppressing the production of by-products such as bibenzyl, and obtaining benzene with high production selectivity. It is in.

[Means for Solving the Problems] The above objects of the present invention are achieved by the following manufacturing method.

That is, the present invention provides toluene in a gas phase and a flow system,
A heating element having a surface temperature heated to the thermal decomposition temperature of toluene, which is provided inside the reactor and substantially separated from the inner wall surface of the reactor, the inner surface of which is maintained at a temperature lower than the decomposition temperature. The present invention relates to a method for producing Hensen at a high production rate without using hydrogen, characterized by contacting with hydrogen.

The invention will be explained in more detail below.

The present invention produces benzene with high selectivity by bringing toluene into contact with a heating element installed inside a reactor and having a surface temperature of 500 to 1200°C in a gas phase and flow system without using hydrogen. This is the way to do it.

When toluene is brought into contact with a heating element, gases such as Hensen, methane, and ethylene are produced. At the same time, bibenzyl and hydrogen are produced as one of the side reactions. As in the present invention, the reactor is placed inside the reactor.
When brought into contact with a heating element having a surface temperature of 1200°C, unlike methods in which the reactants are heated externally, the surface of the heating element, which is the high-temperature part where the reaction is actually occurring, and the reaction part, which is the low-temperature part, is heated. Heat diffusion occurs during this time. A large amount of free radicals are generated on the surface of the heating element, and at this time, hydrogen gas quickly diffuses and moves to the heating element, which is the high temperature part, and other free radicals diffuse to the low temperature part due to the thermal diffusion phenomenon. The hydrogen gas transferred to the heating element suppresses the formation of bihendyl, which is the above-mentioned side reaction, and increases the benzene production rate. Also, as a result,
It is even possible to obtain products with a thermodynamic equilibrium composition or higher.

Furthermore, since the heating element is installed inside the reactor and its surface is substantially covered and thermally insulated by the reaction gas, there is theoretically no amount of heat that is not used for the reaction. Thermal efficiency is extremely good.

The reaction method is a flow type. In the batch method, the hydrogen produced in the reaction diffuses from the heating element, which reduces the benzene production rate. A heating element may be installed inside the reaction tube, and the starting materials may be fed into the reaction tube at a predetermined rate. The reaction tube itself can be cooled from the outside by an appropriate method such as water cooling or air cooling.
The reactant may be preheated to a gaseous state before being sent to the reaction system, or it may be vaporized inside the reactor.

The temperature of the inner surface of the inner wall of the reaction tube that comes into contact with the pyrolysis raw material is
It is important that the temperature at all locations is lower than the thermal decomposition temperature of toluene, which is the starting material. Preferably 1
The temperature is below 00℃. If the temperature of the inner wall surface of the reaction tube is high enough to cause thermal decomposition, the yield and selectivity of Hensen will decrease, which is undesirable. For this reason, the reaction tube is usually externally cooled by appropriate means, such as water cooling or air cooling.

The reaction tube may be installed vertically or horizontally. In the case of a vertical reactor, the reactant may be introduced from the top or the bottom of the reactor. That is, the movement of reactants may be either downstream or upstream.

A flow rate that is high enough to cause turbulent flow inside the reaction tube is not preferable because the selectivity will be insufficient. Therefore,
It is sufficient to cause the reaction to occur at a flow rate slower than turbulent flow, that is, so-called laminar flow. The flow rate of toluene can be selected appropriately, usually 0.001- per system.
1.0moJ/hr, preferably 0.01-0.3
moJ/hr.

The surface temperature of the heating element may be a temperature at which the starting material is thermally decomposed. For toluene, the temperature is usually 500 to 1200°C, preferably 900 to 1100°C. If the temperature is lower than this, it will be difficult to thermally decompose toluene and not enough benzene will be obtained. Further, temperatures exceeding this range are disadvantageous because side reactions such as carbonization of the raw material progresses, and some heating elements become easily destroyed or become unusable.

The material of the heating element is not particularly limited as long as it can maintain a predetermined temperature. Typical examples include nichrome wire,
Examples include tungsten wire, platinum wire, and carbon fiber. These are heated directly by the electric current themselves, but also indirectly by other suitable heating sources.
It may also be something that generates heat. As for the shape of the heating element, it is preferable to use a linear heating element, but a heating element of any shape such as a planar heating element can be used. However, if the shape is too complicated, the yield and selectivity are not necessarily good, and rather, relatively simple shapes such as linear or planar shapes are appropriate.

In order to further increase the effect, in the above-mentioned reaction format, it is appropriate to set the temperature gradient due to the heating element to 20 to 20 b.

Furthermore, it is important that at least a large portion of the heat generating portion of the heating element is maintained at a temperature lower than the decomposition temperature. However, it is undesirable if they are too far apart. That is, the selectivity of the target compound is improved when the distance between the heating element surface and the wall surface maintained at a temperature lower than the decomposition temperature is narrower. Normally, this interval is 5 squares or less, but if it is too narrow, the throughput will decrease, which is not preferable. Therefore, the lower limit interval is approximately ts+.

As long as the temperature gradient and the distance between the vessel wall surface and the heating element are set to the above conditions, the dimensions of the decomposition reactor and the heating element, such as length and diameter, are not particularly limited, and any reaction type can be used. Any size can be used.

In the method of the present invention, the desired product can be directly recovered from the reaction solution obtained after the thermal decomposition as described above by an appropriate method such as distillation. Further, after thermal decomposition, decomposed light components, polymerized bones, etc. can be removed, and unreacted raw materials can be recycled to the reaction system again.

[Effects of the invention] In the conventional method for obtaining benzene from toluene, it is necessary to add hydrogen to suppress the production of by-products, and in thermal decomposition without adding hydrogen, many by-products such as bibenzyl are produced, resulting in the production of benzene. The selection rate becomes worse. However, by the method of the present invention, the selectivity of benzene could be improved without adding hydrogen.

[Example] The present invention will be explained in more detail below with reference to Examples.

Example 1 A flow-type reaction system was used, and the heating element had a diameter of 1.0 JIII.
, a length of 420# nichrome wire was used and placed in the center of the tube. In addition, the reaction tube was externally cooled by water cooling.

In this system, the tube diameter was closed at 20#, and the reaction temperature was set at 107mm.
The temperature was 0°C. Toluene was passed in gaseous form from the top of the system at a flow rate of about 0.05 moJ/hr, and the reactant was taken out from the bottom. When this reaction product was analyzed by gas chromatography, the selectivity was as follows, and benzene was produced with a very high selectivity.

Product Selectivity (%) Benzene
72.1 xylene
5.4 Biphenyl 5
．． 0 Methylbiphenyl 5.1 Dimethylbiphenyl 0.2 Diphenylmethane
1.0 Benzyltoluene 0
．． 3 Bibenzyl 1.4 Others
9.5 Example 2 In the above system, when the reaction temperature was 1030'C1 flow rate was about 0.15 IIoJ/hr, the selectivity of the reactants was as follows.

Product Selectivity (%) Benzene
71.1 xylene
6.0 biphenyl 3
．． 3-methylbiphenyl 3.6-diphenylmethane 0.6-benzyltoluene
0.1 bibenzyl 1.
1 Others 14.2 Example 3 In the above system, the reaction temperature was set to 900, and the C1 flow rate was set to about 0.
．． When set to 05 moJ/hr, the selectivity of the reactants was as follows.

Product Selectivity (%) Benzene
5B.

xylene 17.5 biphenyl
1.1 Methyl bilayenyl
3.3 Dimethylbiphenyl 0.
6 diphenylmethane I4 benzyltoluene 1.7 bibenzyl
12・l Others
6.3 Example 4 In the above system, the diameter of the reaction tube was 50 mIn, the reaction temperature was 1080°C, and the flow rate was about 0. ], OmoJ/hr
The selectivity of the reactants was as follows.

Claims (1)

[Claims] 1. Toluene is produced in a gas phase and in a flow-through manner without adding hydrogen gas, and is installed inside a reactor whose inner wall surface is maintained at a temperature lower than the thermal decomposition temperature of toluene. 1. A method for producing benzene, which comprises bringing the benzene into contact with a heating element having a surface temperature higher than the thermal decomposition temperature of toluene, which is provided in isolation.