1. Field of the Invention
The present invention relates to a catalytic vapor-phase oxidation method and a shell-and-tube reactor and, more particularly, to a catalytic vapor-phase oxidation method capable of effectively removing reaction heat, that is generated when a raw material gas passes through reaction tubes and reacts, thereby suppressing the occurrence of hot spot (excessively high temperature region localized in catalyst layer), during catalytic vapor-phase oxidation reaction wherein a reaction accompanied by a very large amount of heat generation is carried out at a high temperature in a fixed bed shell-and-tube reactor, and a shell-and-tube reactor preferably used in the method.
2. Description of the Prior Art
In a shell-and-tube reactor, a space outside (shell side) of reaction tubes is filled with a heating medium to circulate the heating medium therein, and a reaction raw material is supplied into the reaction tubes to react therein, while the heat generated during the reaction is removed by means of the heating medium, thereby maintaining predetermined reacting conditions.
FIG. 1 shows a typical example of the shell-and-tube reactor of the prior art. In FIG. 1, numeral 1 denotes a reactor, 2 denotes a raw material gas inlet, 3 denotes a catalyst, 4 denotes a reaction tube, 5 denotes a product gas discharge port, 6a denotes an upper tube supporting plate, 6b denotes a lower tube supporting plate, 7a, 7b, 7c denote baffle plates, 8 denotes a heating medium inlet nozzle, and 9 denotes a heating medium outlet nozzle. The raw material gas comprising a mixture of a reaction raw material and air is supplied through the raw material gas inlet 2 into the reactor 1, allowed to flow in the reaction tube 4 that is filled with the catalyst 3 and, after being oxidized in the reaction tube to turn into a reaction product, is discharged through the product gas discharge port 5. The catalyst 3 for the catalytic vapor-phase oxidation reaction may consist of either a single kind of material or two or more kinds of material, and is packed in the plurality of reaction tubes 4 that are fixed onto the upper and lower tube supporting plates 6a, 6b. 
The heating medium that fills and circulates in the space of the reactor on the shell side for removing the reaction heat is introduced through the heating medium inlet nozzle 8 formed on the lower part of the shell of the reactor, and is discharged to the outside of the system through the heating medium outlet nozzle 9 after removing the reaction heat while allowing to flow in a direction to counter the flow of the raw reaction material gas in the axial direction. The baffle plates 7a, 7b, 7c are installed in the shell of the reactor so that the direction of the heating medium flow is changed and the temperature difference across a horizontal plane is reduced, thereby making it possible to keep the heating medium flowing uniformly in the reactor shell and keep the temperature in all reaction tubes at similar levels. The raw material gas may be, for example, propylene, isobutylene, benzene, xylene, naphthalene, acrolein and methacrolein, that produce acrolein and methacrolein, maleic anhydride, phthalic anhydride, acrylic acid and methacrylic acid, etc. through catalytic vapor-phase oxidation.
In most of the shell-and-tube reactors of the prior art, the heating medium is circulated to flow in the axial direction of the reaction tube to counter the flow of the reaction raw material that is charged through the top of the reactor and flows through the reaction tubes, as shown in FIG. 1. As a consequence, the heating medium charged from the outside of the reactor to the lower part of the shell of the reactor, gradually heated to high temperatures through heat exchange while moving up in the reactor, and heat exchanging capacity decreases near the raw material gas inlet of the reaction tubes (top of the reaction tube) where the reaction heat is generated at the highest rate.
Particularly the catalytic vapor-phase oxidation is a process carried out at a high temperature with a very large amount of heat generation, and is often accompanied by excessively high temperature regions localized (hot spot). Therefore it has been required to take such measures as keeping the concentration of the reaction material at a low level or reducing the reaction temperature, in the catalytic vapor-phase oxidation using the shell-and-tube reactor of the prior art, thus making it difficult to maintain the reacting conditions steady. There has also been such a problem that the occurrence of the hot spot may decrease the catalytic activity thus resulting in lower productivity. Thus it is very important to suppress the occurrence of hot spot for a shell-and-tube reactor where catalytic vapor-phase oxidation is carried out in an industrial scale.
Japanese Unexamined Patent Publication (Kokai) No. 48-85485 discloses a method of suppressing the occurrence of the hot spot in reactor tubes by reducing the reactor tube diameter and diluting the catalyst with an inert material. However, this method employs such reacting conditions that do not cause hot spot, and requires it to increase the number of reactor tubes and use a much larger reactor in order to ensure a certain level of production volume.
Japanese Unexamined Patent Publication (Kokai) No. 8-92147 discloses a method of carrying out catalytic vapor-phase oxidation by allowing the heating medium to flow upward from the lower part of the shell in the reactor while supplying a reaction gas at a lower portion of the reactor and discharging it through the top of the reactor. This method makes it possible to prevent hot spot from occurring near the raw material gas inlet. However, since the raw material gas inlet is located at a lower portion, when two or more types of catalyst are used and life expectancy is different between the catalyst used in the first stage and that of the second and subsequent stages, all the catalysts must be changed even in such a case as it is desired to change only the catalyst located at a lower position near the raw material gas inlet. Also in such a case as the catalyst is in the process of partial deterioration due to adsorption blocking of carbide or the like near the raw material gas inlet, differential pressure in the reactor increases that makes it difficult to maintain steady reacting conditions unless the part of catalyst deteriorated due to adsorption blocking of carbides or the like near the raw material gas inlet is removed and replaced. Moreover, since the deteriorated catalyst affects the entire catalyst and leads to shorter active life of the catalyst, it is necessary to periodically remove the catalyst located near the raw material gas inlet. In this case, much labor is required to replace the catalyst with the construction of allowing the raw material gas to flow from below upward.
Therefore, it is common to allow the raw material gas to flow from the top downward when carrying out such a reaction that requires it to periodically remove the catalyst, located near the raw material gas inlet, that is subject to adsorption blocking of carbides or the like, and the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-92147 is generally not in use.
Under the circumstances described above, the present invention has been accomplished, and an object thereof is to provide a catalytic vapor-phase oxidation method that makes it easy to replace only the catalyst located near the raw material gas inlet and to suppress the occurrence of hot spot with a construction of allowing the raw material gas to flow from the top of the reactor downward, and a shell-and-tube reactor that can be preferably used in the method.
The catalytic vapor-phase oxidation method according to the present invention that solved the problems described above is a catalytic vapor-phase oxidation method wherein, using a shell-and-tube reactor comprising a plurality of reaction tubes incorporated between an upper tube supporting plate and a lower tube supporting plate of the reactor, a heating medium that absorbs reaction heat is circulated while surrounding the reaction tubes, while the reaction tubes are filled with a catalyst and a raw material gas is supplied, thereby to proceed the reaction, said method comprising: allowing the raw material gas to flow downward from the top of the reaction tubes and also allowing the heating medium to flow downward from the upper part of the shell in the reactor, as well as discharging gas, that is introduced along with the heating medium and stores below the upper tube supporting plate of the reactor, to the outside of the reactor. In the method described above, it is desirable to take out the heating medium taken out of the reactor at the bottom thereof, force it to move upward and charge it into the reactor through the upper part of the shell in the reactor, while replacing a predetermined portion of the heating medium taken out of the reactor with a cold heating medium newly supplied from the outside.
The shell-and-tube reactor according to the present invention that solved the problems described above is a shell-and-tube reactor having a plurality of reaction tubes incorporated therein, a circulation path for the heating medium formed outside the reaction tubes, a heating medium introducing section provided on the upper part of the shell in the reactor and a heating medium discharge section provided on the lower part of the shell in the reactor, said reactor further comprising a back pressure applying means for the heating medium in the heating medium discharge section. Further in the shell-and-tube reactor having a plurality of reaction tubes incorporated between an upper tube supporting plate and a lower tube supporting plate of the reactor thereof having a circulation path for the heating medium formed outside the reaction tubes, a pipe may also be installed right below the upper tube supporting plate for discharging a gas, that is introduced together with the heating medium and stores therein, to the outside of the reactor.