JPH09227493A - Recovery of urea dust and ammonia from exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To clean exhaustion gas by simultaneously recovering urea and ammonia from exhaust gas containing urea dust and ammonia. SOLUTION: When urea and ammonia are recovered by bringing a gas containing urea dust and ammonia into contact with an aqueous urea solution, an aqueous urea solution with a pH of 3.5-5.5 is used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for recovering urea and ammonia from a gas containing urea dust and ammonia and cleaning the gas.

[0002]

2. Description of the Related Art Gases containing urea and ammonia include a concentrator of a urea plant, a prill urea granulating tower, a urea granulating tower using a fluidized bed or an exhaust gas of a chemical fertilizer manufacturing plant. For example, in a granulation tower used for prill urea production, urea droplets falling from the top of the tower are cooled with air and solidified. The air used for cooling is discharged from the top of the granulation tower, and the discharged air contains urea dust, ammonia contained in molten urea, which is cooled as it solidifies and is released into the air. The concentration varies depending on the size of the granulation tower and the equipment for dispersing molten urea, but generally urea dust 70-200
It is in the range of mg / Nm ³ and ammonia 50 to 150 mg / Nm ³ .
Also in the large-sized urea production method in which a urea solution is sprayed and granulated in a spouted bed or a fluidized bed, the heat of solidification of urea is removed by air in the fluidized bed and the spouted bed in the granulator. Since the temperature of urea particles from these granulators is as high as 90 to 110 ° C, it is further cooled to a temperature which is easy to handle, for example 70 to 80 ° C, by a fluidized bed cooler separately provided for cooling the particles by air. The air discharged from these granulators and fluidized bed coolers also contains urea dust and ammonia. In this method, the air velocity in the granulator is faster than in the granulation tower method, so the dust concentration of urea is as high as 5000 to 15000 mg / Nm ³ , and the ammonia concentration is also smaller than in the granulation tower method, so Its concentration is high, 100-500 mg / Nm ³ . drum,
Even in the granulation by the plate-type granulation method, cooling air is necessary, and the cooled air also contains urea dust and ammonia. The air discharged from these facilities cannot be discharged as it is because it contains a considerable amount of urea dust and ammonia as described above, and must be discharged after being treated by the recovery facility. Regarding the recovery of urea dust, there is a technology of contacting with a dilute aqueous urea solution circulating in a packed column to recover urea, and urea can be reduced to a concentration of 20 to 50 mg / Nm ³ .

[0003]

However, the solubility of ammonia in the urea aqueous solution is very small and hardly absorbed, and ammonia is released at the above high concentration, which is one of environmental pollution. The present invention provides a means for simultaneously recovering urea dust and ammonia and cleaning exhaust gas.

[0004]

The present invention is a method for recovering urea by bringing a gas containing urea dust and ammonia into contact with an aqueous urea solution to obtain a urea solution having a pH of 3.5 to 5.
The method for recovering urea dust and ammonia is characterized in that Furthermore, the present invention provides the pH of the urea aqueous solution.
Nitric acid, hydrochloric acid, sulfuric acid,
It includes a method of using any one or more of phosphoric acid in combination.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. FIG. 1 is a conceptual diagram showing a urea dust and ammonia recovery method of the present invention including a urea granulator as an example of a source of an exhaust gas containing urea dust and ammonia.
Urea from the concentration step of the urea synthesis is sent to the urea granulator 1 via the line 11 in the molten state and is fed as urea particles into the line.
Sent out from 13. The air used for the purpose of removing the heat of solidification of the molten urea is introduced into the granulator 1 via the line 12. The air used for cooling is discharged from the granulator through line 14, and the discharged air contains urea dust and ammonia of 5000 to 15 depending on the granulation type and granulation conditions.
000 mg / Nm ³ and 100-500 mg / Nm ³ are included. The exhausted air is sent to the exhaust cleaning equipment 2 via a line 14 to avoid air pollution. The exhaust gas cleaning facility 2 circulates the urea aqueous solution in the packed bed, and recovers the urea dust contained during the contact between the urea aqueous solution and the exhaust air in the packed bed. Ammonia cannot be removed with urea solution, so acid is added
Keep the pH at 3.5-5.5 and also collect the ammonia contained in the exhaust gas. The type of acid used to maintain pH 3.5 to 5.5 is not particularly limited, but the salts produced by the reaction of nitric acid, hydrochloric acid, phosphoric acid and sulfuric acid with ammonia are ammonium nitrate, ammonium chloride, ammonium phosphate and ammonium sulfate, respectively. Is more preferable because it also becomes a nitrogen fertilizer. For example, when nitric acid is used as the acid, a part of the circulating liquid is extracted via line 17 so that the concentrations of urea and ammonium nitrate in the urea aqueous solution circulating liquid do not rise. The circulating water extracted is the so-called UAN (urea-
Aqueous ammonium nitrate solution), which is concentrated and used as liquid fertilizer. Water for supplementing the circulating water and acid for keeping the pH within a predetermined range are supplied to the exhaust cleaning equipment 2 via lines 15 and 16, respectively. The pH of the circulating urea solution is
If it exceeds 5.5, there is no acid in the circulating water of the exhaust gas cleaning facility 2 and a liquid with excess ammonia is formed, and the reaction between the acid and ammonia is insufficient, and the ability to absorb ammonia is reduced, and no effect can be obtained.

On the other hand, when the pH is less than 3.5, a large amount of acid remains in the circulating water, the amount of ammonia for fixing the acid in the extracted liquid increases, and the operating cost increases. In this way, the urea dust and ammonia contained in the exhaust gas are absorbed and removed to 50 mg / Nm ₃ or less in the exhaust gas cleaning facility, and are released into the atmosphere through the line 18. The exhaust gas containing urea dust and ammonia and the aqueous urea solution adjusted to pH 3.5 to 5.5 can be contacted by the venturi method regardless of the packed column method. INDUSTRIAL APPLICABILITY The present invention can be applied to all types in which gas and liquid are brought into contact with each other.

[0007]

[Function] It is possible to absorb not only urea but also ammonia for using the aqueous urea solution at pH 3.5 to 5.5.

[0008]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto. Example 1 Cooling air was supplied to a granulation tower with a urea particle production capacity of 50 t / h at a temperature of 18
The present invention was applied when 0000 Nm ³ / h was used. When this cooling air is discharged from the granulator, urea dust 2160kg
/ H (12000 mg / Nm ³ ), ammonia 90 kg / h (500 mg /
Nm ³ ) was included. The amount of this urea air circulating fluid is 1
It was introduced into the exhaust cleaning equipment, which is a packed tower rotating at 80 t / h. To the exhaust gas cleaning equipment, 310 kg / h of nitric acid was supplied so that the pH of the urea aqueous solution would be 4.5. Water was supplied to the exhaust cleaning facility at 13360 kg / h together with the replenishment of water discharged as vapor contained in the discharged air so that the concentration of urea and ammonium nitrate solution recovered from the exhaust cleaning facility became 40%. The aqueous solution recovered at this time was 8920 kg / h. The air discharged from the exhaust cleaning tower contained urea dust and ammonia of 7 kg / h (39 mg / Nm ³ ), respectively. Comparative Example 1 When the pH was adjusted to 6.5 under the conditions of Example 1, nitric acid was 133 kg / h.
Was the amount. At this time, urea dust in the air discharged from the exhaust gas cleaning equipment showed a low value of 7 kg / h (39 mg / Nm ³ ), but ammonia was 54 kg / h (300 mg / Nm ³ ). Comparative Example 2 When the pH was adjusted to 3.0 under the conditions of Example 1, nitric acid was 537 kg / h.
Was the amount. At this time, urea dust and ammonia in the air discharged from the exhaust cleaning equipment were 7 kg / h each.
(39 mg / Nm ³ ), 5 kg / h (28 mg / Nm ³ ), which was a low value, but the recovery solution contained 33 kg / h nitric acid, and 9 kg / h ammonia was used to fix it as ammonium nitrate. Was required separately.

[0009]

According to the present invention, since the urea aqueous solution has a pH of 3.5 to 5.5 in order to absorb the urea dust and ammonia contained in the exhaust gas into the urea aqueous solution, not only urea dust but also ammonia can be absorbed and recovered at the same time. Further, when an acid selected from nitric acid, hydrochloric acid, sulfuric acid and phosphoric acid is used as the acid, there is an advantage that the recovered urea aqueous solution can be concentrated and used as a liquid fertilizer.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a method for recovering urea dust and ammonia contained in exhaust gas according to the present invention.

[Explanation of symbols]

 1 Granulator 2 Exhaust cleaning equipment

Claims (2)

[Claims] 1. A method for recovering urea by bringing a gas containing urea dust and ammonia into contact with an aqueous urea solution, wherein the pH of the aqueous urea solution is 3.5 to 5.5. 2. The urea dust according to claim 1, wherein any one or more of nitric acid, hydrochloric acid, sulfuric acid and phosphoric acid is used as an acid in order to keep the pH of the aqueous urea solution at 3.5 to 5.5. Ammonia recovery method.