At present, the purification methods of ammonia gas mainly include chemical pickling method, cooling water washing method and high temperature decomposition method.
Chemical absorption method is a process method that is widely used in exhaust gas control, with mature technology, stable operation and good treatment effect. The method can carry out multi-stage continuous treatment of various malodorous gas components, such as two-stage, three-stage and four-stage absorption systems, and it has wide adaptability. This process is suitable for the treatment of atmospheric volume and high concentration of malodorous gas. Its characteristic is to maximize the gas-liquid contact, increase the gas-liquid mass transfer rate, and achieve the purpose of high-efficiency treatment. In the chemical scrubbing absorption process, the concentration and dosage of the chemical can be changed according to the change of gas concentration and composition, so as to reduce the operating cost and improve the treatment efficiency.
Low-temperature water washing mainly adopts low-temperature cooling spray, and after multi-stage circulating water washing, NH3 gas is recovered into ammonia water for reuse. The method is simple in operation, moderate in operating cost and good in effect.
The pyrolysis method is to introduce the gas into the calciner and increase the temperature to above 700 degrees Celsius to decompose the ammonia gas into nitrogen monoxide, carbon monoxide and water vapor, so as to achieve a satisfactory comprehensive degradation effect on the production waste gas. However, it has the disadvantages of large one-time investment, high operating cost, complex structure and inconvenient maintenance.
Process principle
The NH3 gas cooled to room temperature enters the impurity-removing washing buffer tank, and the lye solution absorbs carbon dioxide and other chemical components in the exhaust gas, and uses the coil to exchange heat, so that the temperature of the exhaust gas does not rise, and the ammonia gas is insoluble in the lye solution at room temperature. middle. The water ejector is used as the pumping power, and the NH3 gas is sent to the high-efficiency low-temperature water washing tower. The ammonia absorption tower mainly uses the relevant principles of fluid mechanics. The gas and the absorption liquid are mixed in the pipeline. When the absorption liquid is mixed with the gas, it passes through the pipeline. At this time, the contact surface between the gas and liquid phases and the pipe wall is large, and the liquid on the contact surface is continuously cooled. After the gas and liquid collide violently to make the gas and liquid fully mixed, it falls into the circulating water tank with the gravity of the liquid to form NH3 water. Because the inlet pipe of the absorption tower is large and the outlet pipe is small, a slight positive pressure is generated in the tower, so as to achieve the purpose of efficient absorption. In order to prevent the volatilization of NH3 water, a pickling tower is set up at the end to absorb the volatilized NH3 gas, so as to meet the environmental protection requirements.
The principle of ammonia absorption
1. Physical properties
The relative molecular weight is 17.031, and the density of ammonia gas under standard conditions is 0.7081g/L. NH3 gas is easily soluble in water, with a solubility of 1:700, with a pungent odor and colorless.
2. Chemical properties
A. to react with water
When ammonia dissolves in water, NH3 molecules and water molecules combine to form ammonia monohydrate (NH3+H2O) through * hydrogen bonds. NH3 monohydrate can be partially ionized into ammonium ions and hydroxide ions, so ammonia water is weakly alkaline and can make The phenolphthalein solution turns red. The reaction of ammonia in water can be expressed as: Ammonia monohydrate is unstable and decomposed by heat to generate ammonia and water. There are three molecules, three ions and three equilibriums in ammonia water. Molecules: NH3, NH3?H2O, H2O; ions: NH4+, OH-, H+; three balances: NH3+H2O NH3?H2O NH4++OH-H2O H++OH-, three applications in ammonia chemistry experiments:
- Use a glass rod dipped in concentrated ammonia water to test the existence of HCl and other gases;
- The laboratory uses it to react with aluminum salt solution to produce aluminum hydroxide;
- Prepare silver ammonia solution to test the existence of aldehyde groups in organic molecules.
B. to react with acid
2NH3+H2SO4===(NH4)2SO4
3NH3+H3PO4===(NH4)3PO4
NH3+CO2+H2O===NH4HCO3
(The essence of the reaction is that the lone pair electrons of the nitrogen atom in the ammonia molecule combine with the hydrogen ions with empty orbitals in the solution to form ionic crystals through coordination bonds. If the reaction is carried out in an aqueous solution, the ionic equation is:
8NH3+3Cl2===N2+6NH4Cl (yellow-green fades, producing white smoke)
The essence of the reaction: 2NH3+3Cl2===N2+6HCl, NH3+HCl===NH4Cl
Overall reaction formula: 8NH3+3Cl2===N2+6NH4Cl
According to the improvement of our company's long-term ammonia scrubber equipment, three elements of ammonia absorption are obtained: 1. Absorption time - the time for the absorption liquid to repeatedly absorb the gas. 2. Absorption liquid temperature - ammonia gas will generate a large amount of temperature during the process of being absorbed by water when it encounters water. If it is not cooled, the temperature of the gas itself plus the temperature generated during absorption will cause the temperature in the tower to rise sharply. The process of ammonia dissolving in water is prevented, and the absorption liquid is quickly saturated. If the absorption liquid is cooled with cooling water, the absorption process will resume again. The lower the temperature of the absorption liquid, the better the absorption effect (the temperature above 0). ). 3. Gas pressure - the higher the gas pressure, the better the absorption effect, which can shorten the absorption time and increase the concentration of ammonia water, thereby reducing the operating cost of the equipment.
Absorption effect
In the treatment of NH3 gas containing non-viscous gas, the exhaust gas containing ammonia gas is usually collected into the absorption device by means of induced draft. During the absorption process, ammonia gas and water will generate a lot of heat again, preventing the absorption of ammonia gas. The concentration is very low, and it is volatilized while being absorbed by water, so the concentration of ammonia water rises very slowly, and the absorption efficiency is very low. Only by cooling and pressurizing the absorption liquid can NH3 gas be quickly absorbed by water. Since the gas is continuously cooled, the volatilization speed is slow and the absorption speed is fast, so that the effect of efficient absorption can be achieved. Usually the temperature of the absorption liquid is controlled at 0-5 degrees Celsius, the pressure in the tower is controlled at more than 1 kg, and the concentration of the absorbed NH3 water can reach more than 20%. Ammonia storage tank, and then put clean water in the tower to continue to absorb.