JP2002316816A - Method for separating ammonia by distillation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient method for separating ammonia by distillation in separating ammonia from an aqueous solution containing ammonia and a metal ingredient by preventing a metal hydroxide from precipitating on the upper part of a distillation column and preventing the metal hydroxide from precipitating in the case of preheating the aqueous solution. SOLUTION: In separating ammonia from the aqueous solution containing ammonia and the metal ingredient by distillation, this method for separating ammonia by distillation is characterized by mixing and preheating the aqueous solution with steam using a steam ejector and introducing it into the distillation column as a gas-liquid two phases flow.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for distilling and separating ammonia from a waste water containing ammonia and a metal component without depositing a metal hydroxide.

[0002]

2. Description of the Related Art Various methods for distilling and separating ammonia from an aqueous solution containing ammonia have been conventionally known. For example, according to Japanese Unexamined Patent Publication No. 55-42239, a multi-stage distillation column having a plurality of trays as shown in FIG.
Ammonia water a is injected from the upper part of the tower, and steam b is blown from the lower part of the tower, whereby gas is brought into gas-liquid contact at each shelf in the tower to strip ammonia. That is, in the upper stage of the tower, the ammonia water a and the vapor b are brought into gas-liquid contact to strip volatile ammonia salts (so-called free ammonia) that decompose only by heat such as ammonium carbonate in the ammonia, and then the reaction tank 1-1. After mixing the alkaline agent c for residual decomposition such as sodium hydroxide and ammonia water a in a lower stage, the mixture is brought into gas-liquid contact with steam to form a nonvolatile ammonium salt such as ammonium sulfate and ammonium chloride (so-called fixed ammonia). Thermal decomposition and stripping.

[0003] Further, the present invention can be carried out in a general distillation apparatus. For example, FIG. 2 is a process flow chart for distilling ammonia using a conventional plate-type distillation apparatus. The ammonia-containing liquid supplied from the liquid supply unit 2-4 provided on the upper part of the tray tower-type main body 2 is vaporized by heating and evaporating by the heating unit 2-5 provided on the lower part of the main body 2 while descending. Ammonia is stripped by the countercurrent contact with. The upper steam discharge section 2-
Ammonia taken out from 6 is recovered as ammonia water by the full condenser 2-7.

In any of the methods, it is well known that the q value [q = (the amount of heat required to convert one mole of the raw material into its saturated vapor) / (the latent heat of vaporization of the raw material)] is 1 or more. Since the raw material is fed at a temperature equal to or lower than the boiling point, a portion having a higher ammonia concentration than the raw material is formed in the recovery section. In the case of a water-ammonia system containing no metal component, no particular problem occurs. Is higher than the undiluted solution, so that precipitation of hydroxide is likely to occur. The precipitation of the hydroxide causes a blockage or the like in a tray column or a packed column as shown in FIGS. 1 and 2, and the distillation column cannot be operated. In order to prevent the operation of the distillation column from being stopped due to the blockage or the like, a method such as periodic cleaning or switching of two or more distillation columns is generally employed.

[0005] In order to avoid this, it is conceivable that the raw material introduced into the distillation column is preheated by a heat exchanger to q <1 as shown in FIG. However, depending on the dissolved metal component, the solubility of the hydroxide decreases with increasing temperature,
During the temperature rise to the boiling point, hydroxide precipitates and scales the preheater, resulting in reduced heat transfer efficiency and plugged heating tubes.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to separate ammonia from an aqueous solution containing ammonia and a metal component in the distillation of ammonia by increasing the pH due to the concentration of ammonia in the upper part of the distillation column. In the case of preventing the precipitation of oxides and preheating the aqueous solution, it is to prevent the precipitation of metal hydroxide due to the temperature rise of the aqueous solution and to efficiently distill and separate ammonia.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, a steam ejector has been proposed as a heating method for setting the aqueous solution before introducing it into the distillation column to q <1. The ammonia is quickly evaporated into the gaseous phase by mixing steam with water and instantaneously using the pressure-reducing effect created by the ejector to make a gas-liquid two-phase flow of q <1. It has been found that by reducing the ammonia concentration in the part, scaling and blockage of the preheater and the distillation column due to precipitation of metal hydroxide can be prevented.

That is, according to the present invention, when ammonia is separated from an aqueous solution containing ammonia and a metal component by distillation, the ammonia is mixed with steam using a steam ejector, and the aqueous solution is preheated to obtain a distillation column as a gas-liquid two-phase flow. And a method for separating ammonia by distillation.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, metal components contained in an aqueous solution together with ammonia include, for example, Fe, C
a, such as a, Mg, Ni, Co, and Cd, is a metal whose hydroxide decreases in solubility due to an increase in pH or an increase in temperature.

Other than using a steam ejector for preheating, a simple and general distillation apparatus such as a Raschig ring packed tower with a low number of stages, a tray column type distillation apparatus and the like is used without requiring a complicated column tower. The operating pressure of the column can be either normal pressure or pressurization.

FIG. 4 shows an example of the distillation operation of the present invention. The steam ejector 4-1 is started with the preheated steam 4-4, and the wastewater is injected by the pump 4-2.
The gas-liquid two-phase flow generated by the steam ejector in 4-1 is led to the distillation column, and steam is blown from 4-6 to perform gas-liquid contact in the distillation column. The vapor containing ammonia from the upper part of the distillation column of 4-2 is recovered as ammonia water by the condenser of 4-7. If there is a steam ejector capability, the presence or absence of the pump 4-3 may be either.

By using an ejector as in the present invention, the precipitation of hydroxide can be prevented without using a preheater, and the operation stop of the distillation column due to blockage of the column top can be avoided. . Also, the stock solution insertion pump can be replaced by an ejector.

[0013]

EXAMPLES Hereinafter, the method of the present invention will be described in detail with reference to FIGS. The analysis of the concentration of ammonia was determined by neutralization titration with hydrochloric acid.

The inner diameter of the distillation column of 4-2 is 4 inches and the height is 3
m, 4-2 was packed with a 1/4 inch Raschig ring at a height of 1.5 m. The inner diameter of the pipe before and after the steam ejector of 4-1 is 1/2 inch, the inner diameter of the pipe from the distillation column of 4-2 to the condenser of 4-7 is 2 inches, and the inner diameter of the pipe for extracting the residual liquid of the vessel of 4-5 is 1 inch. did. 17kg / h
37 kg / h containing 1.3% ammonia and magnesium was introduced into 4-1 and heated to form a gas-liquid two-phase flow and introduced into distillation column 4-2. 12 kg / h of steam was blown from the lower part of the distillation column. As a result, a 2.7% aqueous ammonia solution was supplied from the condenser outlet of 4-7 at 19 kg /
h recovered. After several days, the upper portion of the distillation column was confirmed, but no precipitation of magnesium hydroxide occurred and no clogging occurred.

COMPARATIVE EXAMPLE In Example, wastewater containing 1.3% of ammonia and magnesium was introduced into a 37 kg / h distillation column without using a steam ejector, and 29 kg / h steam was blown from a lower portion of the distillation column. In a short time of the distillation, magnesium hydroxide was deposited on the upper portion of the distillation column, and the inside of the distillation column was blocked.

[0016]

According to the method of the present invention, scaling and clogging caused by precipitation of metal hydroxide in the distillation column and in the preheater from the wastewater containing ammonia and metal components can be prevented. The reaction vessel described in JP-A-42239 is not required, and the ammonia can be stably separated by distillation, eliminating the necessity of the washing operation and the switching operation of the distillation tower and the preheater.

[Brief description of the drawings]

FIG. 1 is a schematic view of a known multi-stage distillation column type ammonia stripping apparatus.

FIG. 2 is a process flow diagram for distilling ammonia using a known tray type distillation apparatus.

FIG. 3 is a process flow diagram for distilling ammonia using a tray type distillation apparatus which preheats a liquid and supplies it to a distillation column.

FIG. 4 is an example of a process flow diagram for distilling ammonia of the present invention.

[Explanation of symbols]

 1-1. Reaction tank a ... Ammonia water b ... Steam c ... Alkaline agent 2-1. Distiller 2-2. Main body 2-3. Shelves 2-4. Liquid supply section 2-5.・ Heating section 2-6 ・ Steam discharge section 2-7 ・ Full contractor 2-8 ・ Discharge section 3-1 ・ Distiller 3-2 ・ Main body 3-3 ・ Shelves 3-4 ・ Liquid supply section 3-5・ Heating unit 3-6 ・ Steam discharge unit 3-7 ・ Full contractor 3-8 ・ Discharge unit 3-9 ・ Preheating unit 4-1 ・ Steam ejector 4-2 ・ Distillation column 4-3 ・ Pump 4-4 ・Preheating steam 4-5 ・ Removal of pot residue 4-6 ・ Steaming for stripping 4-7 ・ Condenser

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Atsushi Utsunomiya, 1-6-6 Takasago, Takaishi-shi, Osaka Mitsui Chemicals, Inc. (72) Inventor Akira Yamamoto 1-6-6 Takasago, Takaishi-shi, Osaka Mitsui Chemicals, Inc. F Terms (reference) 4D034 AA27 BA01 CA18 4D076 AA15 AA22 BB21 CC12 CD21 DA14 DA25 HA06 JA02

Claims (1)

[Claims] When separating ammonia from an aqueous solution containing ammonia and a metal component by distillation, the ammonia is mixed with steam using a steam ejector, the aqueous solution is preheated, and introduced into the distillation column as a gas-liquid two-phase flow. A method for separating ammonia by distillation.