JPS5946888B2 - Urea hydrolysis method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for hydrolyzing urea dissolved in a small amount in an aqueous solution to ammonium carbonate.

Separating and recovering a small amount of urea dissolved in an aqueous solution from water is extremely important in urea synthesis and processes that use urea, from the standpoint of effective resource utilization and wastewater purification. This is important.

However, urea is highly soluble in water, so
If a small amount of urea is dissolved, it is very difficult to separate it directly, so the conventional method is to hydrolyze urea to form ammonium carbonate, and then distill it to separate and recover it from water as ammonia. It has become a means.

As a method for hydrolyzing urea in a dilute aqueous urea solution, a method using heating is known, and usually 180 to 210
At temperatures of °C urea is hydrolyzed.

However, even at such high temperatures, the rate of hydrolysis of urea is slow, and it takes a considerable amount of time to hydrolyze substantially all of the urea in water.

This tendency becomes more pronounced as the urea concentration decreases, and for this reason, in today's world where more complete wastewater purification is required, it is necessary to heat at high temperatures for long periods of time for this hydrolysis. A large hydrolysis vessel is required due to the heat source and long residence time of the reaction.

Accordingly, it is an object of the present invention to provide a method for substantially completely hydrolyzing urea in an aqueous urea solution.

Another object of the present invention is to provide a method for hydrolyzing urea in an aqueous urea solution in a short time.

Still another object of the present invention is to provide a method for hydrolyzing urea in an aqueous urea solution at a low temperature.

Another object of the present invention is to provide a method for removing urea from urea-containing wastewater.

In order to achieve the above object, the present inventors conducted intensive research on the hydrolysis rate of urea and found that the hydrolysis rate of urea becomes significantly faster in the presence of carbonate or bicarbonate. , we have completed the present invention.

That is, in the method of the present invention, the weight ratio o
, oooi to 0.1 carbonate and/or bicarbonate and heating the aqueous solution to 80 to 250°C.

The heating temperature of the urea aqueous solution in the present invention is 80 to 250
Preferably, the temperature is between ℃.

This is because the hydrolysis rate is very slow at temperatures below 80°C, so even if the amount of carbonate or bicarbonate added is large, the reaction takes a long time, and heating above 250°C This is because a high-temperature heating source is required for this purpose, and the object of the present invention cannot be achieved.

The carbonate or bicarbonate added in the present invention includes:
Carbonates or bicarbonates of alkali metals, alkaline earth metals, other metals, ammonia or amines are used.

For example, as alkali metal salts, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, etc. are used, and as alkaline earth metal salts, calcium carbonate,
Magnesium carbonate or the like is used.

As salts of other metals, for example, iron carbonate, nickel carbonate, cobalt carbonate, etc. can also be used.

As the salt of ammonia or amines, ammonium carbonate, ammonium bicarbonate, methylamine carbonate, ethanolamine carbonate, guanidine carbonate, etc. are used, and in addition, carbamines such as ammonium carbamate produced by dehydration of ammonium carbonate are used. Acids also have essentially the same effect as carbonates.

It is a matter of course that the present invention is not limited to only these examples.

These carbonates or bicarbonates can be added to the aqueous urea solution in solid, solution or slurry form.

When ammonia or a small amount of ammonium carbonate is previously contained in the aqueous urea solution, it is preferable to add carbon dioxide gas to the aqueous solution to generate ammonium carbonate or ammonium bicarbonate.

The higher the amount of carbonate or bicarbonate added or the amount of ammonium carbonate or ammonium bicarbonate produced by adding carbon dioxide, the faster the hydrolysis rate will be. For,
The weight ratio is preferably in the range of 0.0001 to 0.1, and more preferably in the range of o, ooi to 0.05.

According to the method of the present invention, when urea in water is hydrolyzed at the same temperature as conventional methods, more complete hydrolysis can be achieved in a shorter reaction time; More complete hydrolysis can be achieved at higher temperatures.

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

Examples 1 to 8 and Comparative Example 1 Carbonate or bicarbonate was added to a urea aqueous solution containing 2000 ppII of carbon, and hydrolysis was performed at 175°C.

As a comparative example, urea was hydrolyzed under the same conditions without adding carbonate or bicarbonate.

The reaction time and the residual concentration of urea in the aqueous solution (ppm) are:
It was as shown in Table 1.

Example 9 and Comparative Example 2 Carbon dioxide gas was added to an aqueous solution containing 5000 ppm of ammonia and 5000 ppm of urea at 40°C and 2 Ky/cr.
tt.

The mixture was blown for about 5 minutes under a pressure of 100 ml.

As a result, about 0.8% by weight of ammonium carbonate and about 1% by weight of ammonium bicarbonate were produced in the aqueous solution.

This aqueous solution was heated to 175°C to hydrolyze urea,
Table 2 shows the residual urea concentration after each hydrolysis time in comparison with a comparative example in which hydrolysis was carried out under the same conditions without blowing in carbon dioxide gas.

Examples 10 to 12 and Comparative Examples 3 to 5 An aqueous solution containing about 1000 ppIII of urea was prepared in advance at 100 ppIII.
Heat to ~200'C and add ammonium bicarbonate to it for 20? /l, and hydrolysis was carried out for a certain period of time.

The concentration of urea remaining in water is shown in Table 3 in comparison with a comparative example in which hydrolysis was carried out under substantially the same conditions without adding ammonium bicarbonate.

Claims (1)

[Claims] 1. Weight ratio of 0.0001 to 0.1 relative to the urea-containing aqueous solution
of carbonate and/or bicarbonate to bring the aqueous solution to 80~
A method for hydrolyzing urea, which comprises heating to 250°C. 2. The method according to claim 1, wherein the amount of carbonate or bicarbonate added is o, ooi to 0.05 in weight ratio to the urea aqueous solution. 3. The method according to claim 1 or 2, wherein the carbonate is ammonium carbonate. 4. The method according to claim 1 or 2, wherein the bicarbonate is ammonium bicarbonate. 5. The method according to claim 1 or 2, wherein the carbonate is sodium carbonate. 6. The method according to claim 1 or 2, wherein the bicarbonate is sodium bicarbonate. 7 Claim 1 in which the carbonate is potassium carbonate
or the method described in paragraph 2. 8. The method according to claim 1 or 2, wherein the bicarbonate is potassium bicarbonate. 9. Claim 1 or 2, wherein the method for adding carbonate or bicarbonate is to add carbon dioxide gas to an aqueous solution containing urea to form carbonate or bicarbonate in the aqueous solution. The method described in Section 2.