JPH0827126A - Production of high-purity melamine - Google Patents

Abstract

PURPOSE:To suppress the formation of impurities in a stage of synthesizing melamine and produce the high-purity melamine by heating and reacting urea and/or its thermal decomposition product using a solid acid catalyst having an amount of an acid within a specific range. CONSTITUTION:This method for producing high-purity melamine is to heat urea and/or its thermal decomposition product at 300-500 deg.C (preferably 350-450 deg.C) reactional temperature in a fluidized bed by using a granular or a powdery solid acid catalyst having 130-400mumol/g amount of an acid measured according to a heat-up eliminating method of NH3 and synthesize the melamine. The average grain diameter of the solid acid catalyst used herein is preferably 10-300mum. Gases in a mixed state produced by the reactional formula or further cyanamide and dicyandiamide coexisting in the reactional system, etc., are cited as the thermal decomposition product of the urea used herein.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing high-purity melamine, and more particularly to a method for producing high-purity melamine using a solid acid catalyst having an acid amount in a specific range.

[0002]

2. Description of the Related Art Melamine is generally used as a raw material for thermosetting resins, paints, adhesives, decorative boards and the like. The thermosetting resin, the adhesive, and the raw materials for the decorative board are prepared by reacting melamine and formaldehyde to form methylolated melamine, and further prepared according to each application and made into a resin. At this time, it becomes difficult to manufacture a stable product because the reaction time until resinification varies.
The reaction time of resinification is affected by impurities in melamine,
The more impurities, the greater the variation. Further, in the case of paints, if the concentration of acidic impurities, ammeline, ammelide, cyanuric acid, etc. in melamine is high, turbidity occurs, which is not preferable as a raw material melamine. Therefore, melamine quality with less impurities and less variation in reaction time is required.

As a process for producing high-purity melamine containing few impurities, for example, a low-pressure gas phase catalytic reaction adopting a wet crystallization method in which synthetic gas is absorbed in water or an alkaline aqueous solution and concentrated to crystallize melamine. DSM method, C
The hemie Linz method and the like are known. However, in the BASF method which employs a dry crystallization method in which melamine is crystallized by a cooling gas, the purification step for removing impurities in melamine is only a gas filter. This only removes catalyst fine particles and high-boiling-point impurities such as melam and melem flying from the reactor, and it is difficult to produce a high-purity product. Therefore, not to mention the process adopting the wet crystallization method in the gas phase catalytic reaction process,
Particularly in a process employing a dry crystallization method, it is desired to develop a catalyst that suppresses the production of impurities in the reaction stage and synthesizes melamine.

[0004]

The object of the present invention is to synthesize melamine from urea and / or its pyrolysis products in the presence of a granular or powdery solid acid catalyst in a fluidized bed at 300 ° C to 500 ° C. In doing so, it is an object of the present invention to achieve the development of a catalyst that suppresses the generation of impurities at the melamine synthesis stage, and to provide a catalyst for producing high-purity melamine.

[0005]

Means for Solving the Problems The inventors of the present invention have noticed that the melamine synthesis reaction in the low pressure gas phase catalytic reaction method proceeds on an acid catalyst and the acid property thereof is closely related to the melamine synthesis reaction. However, as a result of intensive studies aimed at achieving the above object, it was found that a solid acid catalyst having appropriate acid properties is advantageous as a catalyst for synthesizing melamine. That is, the present invention provides an acid amount of 130 measured from urea and / or a thermal decomposition product thereof by the temperature programmed desorption method of NH _3.
Melamine is synthesized at a reaction temperature of 300 ° C. to 500 ° C. in a fluidized bed in the presence of a granular or powdery solid acid catalyst having a concentration of ˜400 μmol / g.

The type of solid acid catalyst used in the present invention is as follows:
It is usually available on the market and is not particularly limited, for example, activated alumina, silica, silica-alumina, zeolite,
Examples thereof include metal oxides such as TiO ₂ and SnO ₂ , and one or more metal composite oxides thereof.

This solid acid catalyst is a solid acid catalyst having an acid amount of 130 to 400 μmol / g measured by NH ₃ temperature programmed desorption method, preferably 140 to 300 μm.
It is mol / g. That is, it has an acid point with a strength suitable for the melamine synthesis reaction, and the larger the amount, the more advantageous it is to produce melamine. Although the mechanism is not clear, impurities are likely to be generated and the melamine purity is lowered when the acid strength is too small or too large as compared with an appropriate value. For example, it was found that when an acid having an acid strength of about 100 μmol / g was used, the number of compounds having an —OH group such as cyanuric acid increased as impurities.

The average particle diameter of the granular or powdery solid acid catalyst in the present invention is 10 to 300 μm, preferably 20 to 250 μm, and more preferably 30 to 2 μm.
It is 00 μm. If the average particle size is smaller than 10 μm, the catalyst particles will fly out from the reactor together with the reaction gas, and it will be impossible to operate at a practical linear velocity of the reaction gas. On the other hand, if it is larger than 300 μm, the apparent bulk specific gravity of the catalyst becomes large, and normal flow cannot be obtained regardless of the shape of the catalyst particles, which is not preferable.

The particle size distribution of the granular or powdery solid acid catalyst is not particularly specified, but it is preferable that the maximum particle size does not exceed 500 μm. Further, even if a small amount of fine powder of 10 μm or less is mixed with the reaction gas, it flows out of the reactor, so that the reaction itself is not adversely affected, but an amount that does not cause clogging of the filter for removing fine powder in the subsequent process. If

This solid acid catalyst can be used in place of or mixed with a conventionally used γ-alumina catalyst having an acid strength of less than 130 μmol / g. The ratio in the case of mixing depends on the properties of the particles to be mixed and therefore cannot be specified in particular, and it is desirable to determine the optimum amount by actually conducting a reaction test in a mixed state.

In the present invention, urea is used as a raw material for synthesizing melamine, and urea is a solid at room temperature, and is generally in a liquid state in a reactor while being kept at its melting point (132 ° C.) or higher. Is supplied to. Since the reactor is usually maintained at a temperature considerably higher than the decomposition temperature of urea (about 160 ° C.), the supplied urea immediately undergoes thermal decomposition to produce cyanic acid and ammonia according to the following formula (Formula 1).

[0012]

[Chemical Formula 1] (NH ₂ ) ₂ CO → NCOH + NH ₃

The thermal decomposition product of urea in the present invention is
It is defined by including the mixed gas produced by the above formula (Formula 1) or cyanamide or dicyandiamide which coexists in the reaction system.

In general, in addition to the above-mentioned raw material urea, a gas obtained by removing only melamine from the outlet gas flow of the reactor as a gas for flowing the catalyst is circulated and supplied to the reactor for melamine synthesis simultaneously with urea.

The reaction temperature for synthesizing melamine in the fluidized bed of the present invention is in the range of 300 to 500 ° C, preferably 350 to 450 ° C. If the reaction temperature is less than 300 ° C., the reaction rate will decrease and the conversion rate to melamine will decrease. Further, when the temperature exceeds 500 ° C, side reactions other than the main reaction in which melamine is produced from isocyanic acid frequently occur, and not only the by-products such as ureidomelamine and cyanomelamine increase, but also the melamine once produced becomes a polymer. By doing so, impurities such as melam and melem increase, which is not preferable.

[0016]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 A fluidized bed reactor having an inner diameter of 0.3 m and a height of 6 m was charged with an acid amount of 167 μmol / g measured by a NH ₃ thermal desorption measuring device (ATD-700: manufactured by Okura Riken). 150 kg of a solid acid catalyst of γ-alumina having an average particle size of 55 μm and a particle size distribution of 10 to 300 μm (CATAPAL B alumina manufactured by VISTA) was filled. Ammonia 75% by volume, carbon dioxide 25
11 kg / hour of molten urea was supplied to the reactor while fluidizing a mixed gas consisting of volume% through 40 Nm ^{3 /} hour, and the reaction temperature was 400 ° C. and the pressure was 0.5 to 1.0 kg / cm ² -G. It was made to react with. At this time, the conversion rate of urea to melamine and the resinification rate of the product melamine were measured by the following methods. As a result, a high-purity melamine having a conversion rate to melamine of 77% and a resinification rate of 94 minutes was obtained. In addition, the result of liquid chromatography analysis of cyanuric acid in melamine at this time showed that the detection limit was 1
It was 0 ppm or less.

The conversion rate of urea to melamine is expressed by (amount of melamine produced) × 2.86 ÷ (amount of urea fed).
The resinification rate is an index showing the quality of melamine and is measured as follows. Product Melamine 40g
88m of 37wt% formalin adjusted to pH 8.1
1 was added and the resinification reaction was carried out at a constant temperature of 85 ° C.
ml is added to 10 ml of distilled water at 15 ° C., and the presence or absence of white turbidity is checked, and the shortest reaction time when white turbidity is observed is the resinification rate. Therefore, this resinification rate depends on the amount of acidic impurities such as cyanuric acid and ammeline contained in melamine, and if the amount of these impurities increases, resinification is promoted. It is considered that the slower the resinification rate is, the better the quality of melamine with less impurities is, and it is used as an index of quality.

Example 2 60 kg of a solid acid catalyst of γ-alumina having an average particle size of 55 μm and a particle size distribution of 10 to 300 μm (CATA PAL B alumina manufactured by VISTA) having an acid amount of 167 μmol / g and an acid amount of 114 μmol / g. Γ-alumina synthetic catalyst having an average particle size of 150 μm and a particle size distribution of 45 to 500 μm (COND
EA, CONDEA NW; conventional catalyst) 90kg,
A total of 150 kg of catalyst was charged, and melamine was synthesized under the same conditions as in Example 1. The result is a conversion to melamine of 78
%, And the resinification rate was 85 minutes. In addition, as a result of analyzing liquid cyanuric acid in melamine at this time by liquid chromatography, the detection limit was 10 ppm or less.

Comparative Example 1 Average particle size 150 μm having an acid amount of 114 μmol / g,
Γ-alumina catalyst with a particle size distribution of 45 to 500 μm (CONDEA
150 kg of CONDEA NW (conventional catalyst manufactured by K.K.) was charged into the reactor, and melamine was synthesized under the same conditions as in Example 1. As a result, the conversion rate to melamine was 76%, and the resinification rate was 7%.
Only melamine containing a large amount of impurities was obtained at 0 minutes. Further, the result of liquid chromatography analysis of cyanuric acid in melamine at this time was 38 ppm.

[0021]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a method capable of producing melamine of extremely high purity as compared with conventional catalysts by defining the acid amount of the solid acid catalyst, which is industrially advantageous.

Front page continuation (72) Inventor Tsutomu Kajiri 1-6 Takasago, Takaishi-shi, Osaka Mitsui Toatsu Chemical Co., Ltd.

Claims (2)

[Claims] 1. An acid amount of 130 measured from urea and / or its thermal decomposition product by thermal desorption of NH _3.
A method for producing high-purity melamine, which comprises synthesizing melamine in a fluidized bed at a reaction temperature of 300 to 500 ° C. in the presence of a granular or powdery solid acid catalyst having ˜400 μmol / g. 2. The average particle size of the solid acid catalyst is 10 to 300 μm.
The method of claim 1, wherein m is m.