JPS60209575A - Preparation of melamine - Google Patents

Abstract

PURPOSE:In preparing melamine from urea or its thermal decomposition product, to reuse a recovered catalyst as a reaction catalyst, by removing a bulky material from the recovered catalyst dropped and accumulated at the bottom of the porous plate of a reactor, using the prepared granular or powdery solid acid catalyst directly as the reaction catalyst. CONSTITUTION:In preparing melamine from urea and/or its thermal decomposition product by a fluidized bed reaction in an ammonia atmosphere in the presence of a granular or powdery solid acid catalyst, the porous plate 2 is set at the bottom 6 of the reactor, a bulky material is removed from the recovered catalyst accumulated at the bottom, and the prepared granular or powdery solid acid catalyst is directly reused without regenerating it. EFFECT:Preparation process can be simplified and a unit of catalyst is improved.

Description

[Detailed description of the invention] The present invention relates to a method for producing melamine.

A method of synthesizing melamine from urea and/or its thermal decomposition products by a fluidized bed reaction in the presence of a granular or powdered solid acid catalyst in an ammonia atmosphere is known. Conventionally, the one-body acid catalyst used in synthesis has the disadvantage of reducing the conversion rate of urea to melamine and the quality of the product melamine, and this deteriorated catalyst is unable to absorb water vapor. It was used by bringing it into contact with oxygen containing oxygen at high temperature, burning off adhering organic matter, and regenerating it (Japanese Patent Application Laid-Open No. 54-2238
No. 5). By the way, in a fluidized bed reactor, a perforated plate is installed at the bottom of the reactor in order to improve the dispersion of the gas introduced from the bottom of the reactor into the reactor and to prevent the catalyst from depositing at the bottom of the reactor. .

However, in reality, as the reaction progresses, the catalyst falls and accumulates under the porous plate. Part of the recovered catalyst that has fallen and accumulated has become agglomerated, and it is impossible to reuse it as it is.

The inventors of the present invention conducted extensive research on the reuse of the recovered catalyst accumulated at the bottom of the perforated plate, and found that if the lumps were removed from the recovered catalyst accumulated at the bottom of the perforated plate,
It was discovered that the obtained granular or powdery solid acid catalyst can be used as a reaction catalyst as it is without any regeneration treatment from 1 dJ, and the present invention was created based on this finding.

That is, the present invention synthesizes melamine from urea and/or its thermal decomposition products in the presence of a granular or powdery solid acid catalyst using a moving bed reactor equipped with a perforated plate at the bottom of the reactor. The method is characterized in that all or part of the granular or powdery solid acid catalyst from which lumps have been removed is supplied to the reactor from the recovered catalyst accumulated at the bottom of the perforated plate of the reactor. .

Hereinafter, the present invention will be explained with reference to the drawings.

FIG. 1 is a diagram schematically showing a reactor of the present invention.

A solid acid catalyst 5 is charged into the reactor 1 . The solid acid catalysts used in the present invention include activated alumina, silica gel, silica alumina, silica maguey shea, magnesia, chromia, chromia alumina, silica zirconia, alumina boria, titania boria, zeolite, acid clay, montmorillonite, kaolinite, Bentonite, diatomaceous earth phosphoric acid, silica phosphoric acid, etc. are used.The shape of the catalyst may be granular or powdery. This solid catalyst is fluidized by a mixed gas consisting of ammonia and carbon dioxide introduced from the lower nozzle 8 of the reactor. The perforated plate 2 installed at the top of the gas inlet at the bottom of the reactor is to prevent the catalyst from falling to the bottom of the reactor, but there are many fine holes for gas circulation. The internal temperature of the reactor during melamine synthesis is 300℃.
~400°C is preferred. Urea and/or its pyrolysis products are introduced into the reactor via line 7. Urea and/or its thermal decomposition products are usually used in the form of molten urea in a fluidized state. In the reactor, urea is converted to melamine, which is separated from the catalyst by a cyclone and taken out from the top of the reactor.

Most of the catalyst flowing in the reactor is present in the upper part of the perforated plate 2, but as the reaction progresses, some of the catalyst passes through the holes in the perforated plate and accumulates in the lower part 6 of the reactor and is recovered. . The accumulated recovered catalyst contains partially solidified agglomerates.

In the method of the present invention, the accumulated recovered catalyst is taken out, the lumps are removed, and all or part of the granular or powdery solid acid obtained is recycled and used as a reaction catalyst. As a means for removing lumps from the recovered catalyst, ordinary sieving may be used. Further, the lumps to be removed are removed by sieving coarse particles having a particle size of 3 to 4 mm or more that do not form a fluidized state under the reaction conditions.

The reuse of the granular or powdered solid acid catalyst from which lumps have been removed is limited to approximately 5 to 40 parts by weight, preferably 10 to 25 parts by weight of the catalyst added to the reactor as a reaction catalyst for the reaction to be carried out. .

In addition, the catalyst recovered at the bottom of the perforated plate is taken out continuously or intermittently, lumps are removed from the recovered catalyst, and the reaction is carried out while continuously replenishing the insufficient amount of catalyst in the reaction system by combining it with new catalyst. You can also.

According to the method of the present invention, the catalyst that has conventionally been collected and accumulated under the lower porous plate of the reactor, by simply physically removing the lumps, can be regenerated into new catalyst or catalyst without any regeneration treatment. Since it exhibits exactly the same performance as a regenerated catalyst, there is no need to regenerate the catalyst layer that has deteriorated over time, which simplifies the melamine production process and improves the catalyst consumption rate.

Next, the method of the present invention will be explained in more detail with reference to Examples.

Example 1 A fluidized bed reactor 1 with an inner diameter of 8rrL and a height of 17ya has a diameter of 0. Fluidize the mixed gas through 32,000 tf per hour 39
13 tons of molten urea was supplied to the reactor per hour at a reaction temperature of 0° C., and continuous operation was carried out for one month. When the operation was stopped, 50% of catalyst had accumulated in the lower part of the reactor under the reactor perforated plate. Part of the catalyst was solid.

The catalyst accumulated at the bottom was taken out and sieved through a 6-mesh sieve to remove lumps. The amount of catalyst from which lumps were removed was 26 tons.
Met. This catalyst was supplied to the catalyst bed of the Lot reactor. When the melamine synthesis reaction was restarted under the same conditions, it was confirmed that the melamine synthesis reaction had exactly the same performance as when using the new catalyst. Table 1 shows changes over time in the conversion rate of urea to melamine and the rate of resinization of the product melamine. Reference Example 1 shows the case where the new catalyst was used entirely.

Table 1

[Brief explanation of drawings]

FIG. 1 shows a cross-sectional view of an example of a reactor for producing melamine according to the present invention. In the figure, each symbol indicates the following. 1, Reactor 2 Perforated plate 3 Cyclone 4 Sparger 5 Catalyst fluidized bed 6, Reactor lower part 7, Urea supply line 8, Reactor lower nozzle 9, Heating coil 10, Reactor outlet line Patent applicant Mitsui Toatsu Chemical Co., Ltd. company

Claims (1)

[Claims] 1) In a method for synthesizing melamine from urea and/or its thermal decomposition products in the presence of a granular or powdery solid acid catalyst using a fluidized bed reactor with a perforated plate at the bottom of the reactor, the reaction A method for producing melamine characterized by removing lumps from a solid acid catalyst collected at the bottom of a perforated plate of a container and reusing it.