JPH0138110B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing cyanuric acid by thermally decomposing urea, and its purpose is to process urea, biuret, cyanuric acid, ammelide, ammeline, and The objective is to recover ammonium cyanate, etc. without hindering continuous operation and improve the production efficiency of cyanuric acid.

When urea is heated to a temperature of 200 to 330℃ in a reactor, ammonia gas is generated while ammelide,
It is widely known that ammeline and some unreacted urea are converted to cyanuric acid, but
According to this method, about 10-40% of the urea normally supplied
The substances vaporize and dissipate together with the reaction product gas, and are accompanied by the sublimation of reaction products such as biuret, cyanuric acid, ammeline, ammonium cyanate, etc.
The yield of cyanuric acid based on the raw material urea is extremely low.

To solve this problem, US Patent No.
According to the specification of No. 2943088, the reaction product gas containing urea etc. scattered from a rotary kiln maintained at specific circumferential speed and temperature conditions is
A method of condensing and recovering the urea by introducing it into a condenser maintained at 115-130℃ and a scrubber filled with a Raschig ring with an approximately 72% urea aqueous solution at 70-100℃.
A method is described in which urea and the like are recovered by circulating the gas at a temperature of 0.degree. C. and introducing a reaction gas into the scrubber.

However, in the former method, most of the urea, etc. in the reaction gas passes through the condenser without being condensed, so sufficient recovery is not possible, and in order to improve the recovery effect, the condensation area must be increased. , the flow of the condensate flowing down along the condensing surface becomes uneven, and solidifying dust with poor solubility such as cyanuric acid and ammelide present in the reaction product gas adheres to the condensing surface, causing problems in the condenser. However, although the latter method has a better recovery efficiency for urea, etc. than the former, it does not allow cyanuric acid, ammelide, etc. to be recovered from an approximately 72% urea aqueous solution. The solubility of the sublimable mixture is extremely low at 0.3 to 0.5%, and it is recovered in a so-called slurry state, which adheres to the Raschig ring in the scrubber and makes continuous operation difficult.Also, when using a urea solution containing water, Decomposition of ammonia from urea into carbon dioxide gas occurs significantly, and the yield of cyanuric acid is extremely reduced.

In view of these circumstances, the inventor of the present invention repeatedly conducted intensive research and research, and found that in a method for producing cyanuric acid by heat-treating urea, the reaction product gas is washed by contacting with substantially anhydrous molten urea, and this reaction The intended purpose was achieved by thermally decomposing the washed urea that absorbed the urea present in the generated gas and its decomposition products.

In carrying out the present invention, it is desirable to use substantially anhydrous molten urea to wash the reaction product gas discharged from the reactor and to supply this washed urea to the reactor in liquid form. For example, a predetermined amount of cyanuric acid is charged in advance into a reactor equipped with an externally heated stirrer, and while the cyanuric acid is heated to a predetermined temperature and stirred, the washed urea and additional raw material urea are added to the reactor as necessary. Cyanuric acid may be continuously supplied and a part of the generated cyanuric acid may be recycled to the reactor, or heat treatment may be performed without recycling.

In carrying out the present invention, in order to bring the molten urea into contact with the reaction product gas discharged from the reactor, a conventional tray type or spray tower type (or rotating disk type) liquid-gas contact can be carried out efficiently. A structure that allows for this can be used. In addition, the piping for supplying the washed urea to the reactor is designed to prevent urea from sticking to the inner wall.
The wall temperature should be maintained at 120-170°C by heating from the outside.

The present invention will be explained in detail below using examples.

Example: A reactor with an externally heated stirrer with a heat transfer area of approximately 4 m ² and an effective volume of approximately 150 mm was equipped with a plate tower type structure, piping that served as an exhaust gas line and a washing urea supply line, and a pipe for heating the reactor. A heating medium supply inlet and an outlet were provided, and the exhaust gas line was connected to a packed tower type scrubber, and the gas discharged from the scrubber was passed through a normal type mist catcher attached to the scrubber outlet. Afterwards, using a reaction device that blows air into the exhaust gas treatment equipment and concentrates the valuables recovered by the scrubber and feeds it back to the molten urea supply tank, crude cyanuric acid (cyanuric acid 78.0% by weight, ammelide and Total amount of ammeline 21.5
0.4 weight percent of urea, 0.1 weight percent of biuret) was charged, and the crude cyanuric acid was heated to a temperature of about 250 ° C.
While maintaining the same temperature, substantially anhydrous molten urea heated to a temperature of about 150 DEG C. was continuously fed at a rate of 40 kg per hour through a urea feed port attached to the exhaust gas line of the reactor.

The supplied molten urea was effectively contacted with the reaction product gas, and a portion of it continuously flowed down along the inner wall and was directly introduced into the reactor.

The operation continued for 48 hours under the above conditions, but no trouble was observed during that time, and 1265 kg of crude cyanuric acid was added to the 1885 kg of raw urea (including 35 kg of valuables recovered by the scrubber) fed into the molten urea supply tank.
I was able to recover it.

In addition, approximately 90% of the crude cyanuric acid obtained here is granular with a diameter of about 3 mm or less, and the rest is granular and powdery with a diameter of 3 mm to 5 mm and has extremely high fluidity. Its composition is 77.0% cyanuric acid, the total amount of ammelide and ammeline 22.5%, urea 0.4%, biuret 0.1%, which is purified with a purity of 99.5% or more and recovered by crushing it and boiling it in an aqueous sulfuric acid solution. The amount of cyanuric acid was 1258 kg, and the yield of cyanuric acid based on the raw material urea was 93.1% of the theoretical value.

Furthermore, among the active ingredients, cyanuric acid, ammelide, ammeline, and biuret were not detected in the exhaust gas from the outlet of the mist catcher, and only trace amounts of urea and ammonium cyanate were detected.

Claims (1)

[Claims] 1. A method for producing cyanuric acid by heat treating urea, which includes a step of contacting a reaction product gas with substantially anhydrous molten urea for washing, and absorbing urea and its decomposition products present in the reaction product gas. A method for producing cyanuric acid, comprising a step of thermally decomposing washed urea.