JPS6152149B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recovering unreacted urea and isocyanic acid from waste gas produced during the production of isocyanuric acid.

An example of a method for obtaining isocyanuric acid by heating urea is to heat urea at a temperature range of 240 to 360°C to convert it into isocyanuric acid and its aminated derivatives (ammeride, ammeline, etc.), and then to convert it into isocyanuric acid and its aminated derivatives (ammeride, ammeline, etc.), and then to convert it into isocyanuric acid and its aminated derivatives (ammeride, ammeline, etc.) There is a method of hydrolyzing the aminated derivative to isocyanuric acid under normal conditions.

However, when urea is heated in a temperature range of 240 to 360°C, unreacted urea vapor and isocyanic acid produced by thermal decomposition of urea are present in the waste gas mainly composed of ammonia and carbon dioxide. Since it contains a large amount of urea, discharging this waste gas as it is will lead to a significant deterioration of the basic unit of raw material urea in the production of isocyanuric acid.
Some sort of collection process will be necessary. As a method for recovering urea vapor and isocyanic acid from this waste gas, for example, as described in Japanese Patent Publication No. 42-22375, urea vapor is recovered by cooling the waste gas passage surface to 170°C. and a method of condensing and recovering isocyanic acid as molten urea, or
115-130℃ as described in No. 37-9244
There is a method of collecting the water by passing it through a condenser whose temperature is adjusted to . However, these methods
In both cases, in principle, urea vapor and isocyanic acid in waste gas are recovered as urea using a condenser. Due to isocyanuric acid, adhesion of solids on the condensation surface cannot be avoided, and in order to operate the equipment industrially and stably, two lines of equipment are required, and in addition, it takes a lot of effort to remove the adhering solids. Therefore, there is a demand for the development of further improved methods.

On the other hand, a method for recovering urea from waste gas during melamine production is described in Japanese Patent Publication No. 41-21343. As is clear from the examples, after washing the waste gas with urea containing 20% or more of biuret, cooling the waste gas and separating urea and melamine, the melamine synthesis reaction gas This method is used as a cooling gas for the production of isocyanuric acid, and the method can be applied as is to waste gas containing 20 to 60% urea vapor and isocyanic acid, such as waste gas during the production of isocyanuric acid. It was not possible to achieve the desired effect. In addition, as in Japanese Patent Publication No. 41-21343, in order to maintain the temperature of hot melt urea below 130°C, which is lower than the melting point of urea, it is essential to contain about 20% biuret. If biuret is present in the isocyanuric acid production waste gas which has a high acid content, it is not preferable because the isocyanic acid will be converted to triuret or the like.

The present inventors have conducted intensive research to develop a new method for recovering urea and isocyanic acid from isocyanuric acid production waste gas that has a high content of urea and isocyanic acid, which has the above-mentioned problems.
It has been discovered that the object can be achieved by bringing a melt of raw material urea into contact with isocyanuric acid production waste gas at a temperature of 132 to 145° C. at least 50 times the weight of the waste gas, and has accomplished the present invention. I've reached it.

That is, the present invention is a method for producing isocyanuric acid from urea by a heating method, in which the generated waste gas and the raw material urea melt are heated at 132 to 145°C.
, preferably in a temperature range of 132 to 140°C, and
The present invention provides a method for recovering urea and isocyanic acid from the waste gas of isocyanuric acid production, which is characterized by contacting the waste gas with a molten material of raw material urea at a ratio of 50 times or more by weight.

The waste gases generated during the production of isocyanuric acid by the heating method in the method of the present invention are mostly ammonia, carbon dioxide gas, isocyanic acid, and unreacted urea vapor generated by the following basic reaction. Usually consists of 20~60% isocyanic acid, 40~
Contains 80% ammonia and 1-10% carbon dioxide.

Note that when isocyanic acid becomes liquid or solid in an ammonia atmosphere, it is converted to urea, so it is very difficult to distinguish unreacted urea vapor from isocyanic acid.

In the method of the present invention, it is necessary that the contact temperature between the urea melt and the isocyanuric acid production waste gas be 132 to 145°C. If this temperature is lower than 132℃, urea will aggregate and the inside of the equipment and piping will be blocked.
Continuous operation becomes impossible. The waste gas from the production of isocyanuric acid according to the invention is significantly different from the waste gas from the production of melamine, with a content of isocyanic acid of up to about 50%, whereas in the latter it is only a trace. In the latter case, it always contains melamine.
It is necessary to keep the temperature of the waste gas as low as possible because it becomes an isocyanuric acid complex and easily deposits on the vessel wall. However, since the method of the present invention does not have this fear, the melting temperature of the circulating urea can be relatively high, and can be suitably carried out at 132° C. or higher. on the other hand,
In the method of the present invention, when the temperature exceeds 145°C, the concentration of isocyanuric acid in the urea melt rapidly increases due to decomposition and polymerization of urea, and the isocyanuric acid causes contact equipment, gas-liquid separators, piping, etc. Not only does continuous operation become impossible due to blockage of the contactor, but the amount of urea and isocyanic acid contained in the off-gas discharged from the contactor increases, leading to loss of raw material urea.

Further, the weight ratio of the waste gas and the urea melt to be brought into contact requires 50 times or more of the urea melt per part of the waste gas. If the urea melt is less than 50 times as large, the contact between the waste gas and the urea melt will be uneven, resulting in a large proportion of isocyanuric acid being produced, and the contact equipment,
The gas-liquid separator and piping become clogged, making continuous operation impossible. The method of the present invention does not use a condenser equipped with a cooling device as in the conventional method, but instead performs cooling by gas-liquid contact in a contact tower. If this is not used, isocyanuric acid will be produced in areas where the temperature inside the column is high due to the presence of isocyanic acid in a partially high concentration in the column, causing a disadvantage. However, it is not economically advantageous to use more than the required amount of molten urea.

The method of the present invention can be carried out using a spray tower, a packed tower, a tray tower, etc. as a gas-liquid contact device, but a spray tower is most preferred among them, and can be operated continuously for a long time.

Next, an example of a flow sheet suitable for carrying out the method of the present invention will be explained with reference to the drawings.

Raw material urea is introduced into the gas-liquid contact tower 1 in a molten state or solid form from a pipe 4, and is supplied to the reactor 2 in a molten state through pipes 5 and 6. Reactor 2 has piping 9
The jacket is heated by a heating medium passed through the reactor, and the reaction temperature is maintained at, for example, 280°C. Reactor 2
The urea supplied to the reactor undergoes a thermal decomposition reaction, producing solid products such as isocyanuric acid, ammelide, and ammeline, and waste gases such as unreacted urea vapor, isocyanic acid, ammonia, and carbon dioxide gas. The solid product is sent through pipe 8 to the next isocyanuric acid purification step. The waste gas generated in the reactor 2 is supplied to the gas-liquid contact tower 1 through a pipe 10. On the other hand, the flow rate of molten urea is adjusted through the pipe 11, and the pump 13 sends it to the gas-liquid contact tower 1.
co-current contact with the waste gas while circulating at a preferred temperature range of .degree. Urea vapor and isocyanic acid contained in the waste gas are condensed as urea and absorbed into molten urea. The gas and liquid extracted from the gas-liquid contact tower 1 are introduced into the gas-liquid separator 3 through a pipe 5, where they are separated into gas and liquid, and the molten urea is supplied to the reactor 2 through a pipe 6. The off-gas containing ammonia as a main component separated by the gas-liquid separator 3 is transferred to a urea manufacturing plant, an ammonium sulfate manufacturing plant, etc. through a pipe 7, and is effectively used. Note that 12 is a cooler installed in the pipe 11, which cools the circulating molten urea to an appropriate temperature range and removes heat.

According to the method of the present invention, large amounts of urea and isocyanic acid can be effectively recovered from waste gas during the production of isocyanuric acid. Furthermore, by applying the method of the present invention, adhesion of solid matter can be avoided and continuous stable operation of the isocyanuric acid production process can be achieved.

Next, the present invention will be explained in more detail based on examples.

Example The following operations were performed according to the illustrated flow sheet. 2.3 kg of molten urea per hour was fed into an externally heated reactor from a gas-liquid contact tower to which 1.4 kg of raw material urea was supplied per hour at a reaction temperature of 250°C for a reaction time of 1 hour, and 1.34 kg of waste gas was generated per hour. Occurred. The composition of this waste gas was 48.4% by weight of ammonia, 5.7% by weight of carbon dioxide, and 45.9% by weight of isocyanic acid.

This waste gas is circulated at a flow rate of 100 kg/hour using a spray tower with a diameter of 100 mm and a height of 1000 mm.
When the off-gas was brought into contact with a urea melt at 132°C, the temperature of the off-gas decreased to 139°C, and the amount of isocyanic acid contained in the off-gas was less than 0.1% by weight. This means that almost all of the isocyanic acid in the waste gas was recovered. Furthermore, continuous operation was performed for 45 hours under these operating conditions.
The operation continued for several days, and when the operation was stopped and inspected, almost no solid matter was observed.

Comparative Example 1 Waste gas obtained under the same conditions as Example 1 was
When contacted with a urea melt at 130°C at a circulating flow rate of 50 kg/hour, the temperature of the waste gas reached 147°C. At this time, the isocyanic acid contained in the off-gas is
It had increased to 0.8% by weight. This corresponds to approximately 0.3% of the raw urea used. Furthermore, when continuous operation was continued under these operating conditions, on the 14th day, a large amount of solid matter adhered to the collision plate-type gas-liquid separator that separates urea and off-gas, making it impossible to continue operation.

Comparative Example 2 Waste gas obtained under the same conditions as in Example was
When it was brought into contact with a urea melt at 127℃ at a circulating flow rate of 100 kg/hour, the urea melt stuck.
It became impossible to pass fluid through.

[Brief explanation of the drawing]

The drawing is an example of a flow sheet for carrying out the method of the present invention. Explanation of symbols, 1... Gas-liquid contact device, 2... Isocyanuric acid production reactor, 3... Gas-liquid separator, 1
2...Cooler, 13...Circulation pump.

Claims (1)

[Claims] 1. In a method for producing isocyanuric acid from urea by heating, the generated waste gas and the urea melt are heated at 132 to 145°C, and the urea melt is at least 50 times the weight of the waste gas. A method for recovering urea and isocyanic acid from waste gas from isocyanuric acid production, the method comprising contacting them at a ratio of .