CN102078752A - Method for preparing sodium nitrite from nitric oxide waste gas through multistage oxidation absorption - Google Patents

Abstract

A method for preparing sodium nitrite from low-oxidation nitrogen oxide waste gas in an industrial process, which is to mix the nitrogen oxide waste gas with oxygen-containing air in a certain proportion after dehydration, and enter the multi-stage absorption tower from the bottom of the first-stage absorption tower. Alkali absorption towers are connected in series, the temperature of the absorption liquid is below 50°C, and the pressure inside the tower is 0.03-0.8 MPa. The purified gas is discharged from the top of the tail tower up to the standard, and the sodium nitrite solution is extracted from the bottom of the first-stage absorption tower. During the absorption process, the NOx gas discharged from the previous stage tower is dewatered, and according to the _NO2 and NO concentration measured by the online nitrogen oxide analyzer, it is controlled by PRC and input with oxygen distribution air. During the absorption process, the circulating absorption liquid of each absorption tower is first sent to the cooling device to reduce the temperature, so as to control the temperature in the absorption tower. The invention adopts common low-cost tower equipment to obtain sodium nitrite solution with sodium nitrate/sodium nitrite<0.02, reduces NOx discharge, and achieves the goals of energy saving, emission reduction and resource utilization.

Description

Method for preparing sodium nitrite by absorbing nitrogen oxide waste gas through multistage oxidation

technical field

       The invention relates to the treatment and resource utilization of nitrogen oxide waste gas, in particular to a method for preparing sodium nitrite through multistage oxidation and absorption of nitrogen oxide waste gas.

Background technique

Nitrogen oxides (mainly referring to NO and NO ₂ , commonly known as NOx) exhaust gas will not only cause acid rain and acid fog, but also destroy the ozone layer, causing serious harm to the natural environment and human production and life. The nitrogen oxides in the atmosphere mainly come from the production waste gas of the nitric acid industry, the fuel combustion waste gas of oil-fired boilers and the exhaust gas of motor vehicles. Therefore, reducing nitrogen oxide emissions and strengthening the control of nitrogen oxide pollution have become a very urgent task in environmental protection.

The mainstream technology of nitrogen oxide removal process can be roughly divided into four categories: catalytic reduction method, solvent absorption method, solid adsorption method and biological treatment technology. Catalytic reduction methods include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Urea and ammonium sulfite are the most commonly used reducing agents in the catalytic reduction method. These reducing agents can reduce NOx to harmless nitrogen and other nitrogen-free components under the action of a certain temperature and catalyst. Solvent absorption technology is divided into direct absorption method, oxidation absorption method, redox absorption method, liquid phase absorption reduction method and complex absorption method according to the different combination methods of acid gas and solvent. Among them, the direct absorption method can be divided into water absorption, nitric acid absorption, alkaline solution (sodium hydroxide, sodium carbonate, ammonia and other alkaline liquids) absorption and concentrated sulfuric acid absorption according to the different main solvents used. The traditional solvent absorption denitrification technology has a simple process and low investment, so it is widely used in enterprises. The solid adsorption method uses an adsorbent to absorb nitrogen oxides to prevent its pollution. Currently, commonly used adsorbents include molecular sieves, activated carbon, and silica gel. Biochemical treatment of nitrogen oxide waste gas technology has only been gradually developed in the past 10 years. The essence of biological treatment is to use the life activities of microorganisms to convert NOx into harmless inorganic substances and microbial cytoplasm (Jiang Ran, Huang Shaobin, Fan Lirong, Removal of NOx by biological filtration system under aerobic conditions. Journal of Environmental Science, 2007(09): 1469-1475.). In addition, foreign countries have also developed plasma flue gas denitrification technology, such as electron beam irradiation method, pulse current corona discharge method, etc. These methods are relatively advanced, but there are problems of complex technology and high cost (Wei Enzong, Lin He, Gao Xiang, Coal Boiler Flue gas nitrogen oxide pollution plasma treatment technology. Environmental Pollution Treatment Technology and Equipment, 2003. 4(1): 58-62.). Absorbing nitrogen oxides with water or dilute nitric acid is a commonly used nitrogen oxide treatment method in China. This method can simultaneously obtain nitric acid and other chemical raw materials. This method generally requires higher concentration and oxidation degree of nitrogen oxide waste gas. The patent (ZL100396360C) proposes a method for the treatment and recycling of nitrogen oxide waste gas in industrial processes, which not only ensures the environmental protection of NOx emissions, but also effectively recovers NOx from emphysema, and the nitric acid produced can be used for industrial recycling. Alkaline absorption treatment technology is also a commonly used environmental protection process in nitrogen oxide treatment. This method has the advantages of fast absorption speed and high exhaust gas purification rate. It can not only achieve the standard discharge of NOx exhaust gas, but also remove most of the Part of the NOx is converted into nitrate and nitrite for recycling by the enterprise.

气相中NO₂/NO的比例；

严格控制气相中的水分。 The mechanism of lye absorbing nitrogen oxides is the same as that of water absorbing nitrogen oxides. The reaction is extremely complicated. The absorption process includes multiple reversible and irreversible chemical reactions and intermediate products of nitrogen oxides that are controversial in the academic circle. Due to the mutual influence of each reaction It is difficult to figure out each step link, so it is usually simplified into two steps in the research process: the first step is the gas-phase oxidation reaction: add air to the exhaust gas containing nitrogen oxides, so that NO can be oxidized NO ₂ absorbed by water: 2NO+O ₂ =2NO ₂ (1); the second step is the liquid phase absorption reaction, NO ₂ and NO react with alkali in the liquid phase to generate sodium nitrate and sodium nitrite: 3NO ₂ +2NaOH =2NaNO ₃ +NO+H ₂ O (2); NO ₂ +NO+2NaOH=2NaNO ₂ +H ₂ O (3), the generated NO reacts with O ₂ again to generate NO ₂ , thus cyclic reaction absorbs and prepares nitrate and nitrites. The reaction between sodium carbonate and nitrogen oxide gas is similar, except that carbon dioxide is released during the reaction. In general, _NO2-- can undergo a rapid irreversible reaction with water or lye (2), so simple lye absorbs nitrogen oxides and finally obtains a mixture of nitrate and nitrite. The traditional method to increase the ratio of nitrite is to use a lower NOx oxidation degree and a larger spray volume (Jiang Yan, Talking about the way to increase the ratio of NaNO ₂ to NaNO ₃ in the neutralization solution obtained by absorbing NOx in alkaline solution, Nitrogen Fertilizer Information, 1994. 2:11-13). NO alone does not react with water or lye, but when NO ₂ /NO is 1, reaction (3) can occur very quickly. Since the price of sodium nitrite is much higher than that of sodium nitrate at present, it is best to obtain a higher NaNO ₂ /NaNO ₃ in the absorption liquid for lye to absorb nitrogen oxides, and then concentrate it to utilize the low solubility of NaNO ₂ . The crystals were filtered to remove sodium nitrate. It is reported in China that the nitric acid tail gas is used to produce sodium nitrite, and a higher absorption temperature is used, so the absorption efficiency of NOx is not high. Because the concentrated crystallization process requires more equipment investment, enterprises generally tend to obtain sodium nitrite solutions with lower sodium nitrate content. During the NOx absorption process, NO- ₂ reacts with alkali in the liquid phase to release NO and emit a large amount of heat. During the absorption process, some water enters the gas phase, and this part of water easily reacts with _NO2 to form nitric acid. The nitrate content is increased, so in order to control the nitrate content, two aspects must be ensured:

The ratio of NO ₂ /NO in the gas phase;

Strictly control the moisture in the gas phase.

Contents of the invention

Technical scheme of the present invention is as follows:

A method for preparing sodium nitrite from waste gas with low oxidation degree nitrogen oxides (NOx) in an industrial process, as shown in Fig. It includes the following steps:

Step 1. After the low-oxidation nitrogen oxide waste gas is dewatered by the water removal device, the content of water and nitric acid in the nitrogen oxide waste gas is kept below 1%, and then according to the _NO2 and The concentration of NO is controlled by PRC, and oxygen-mixed air is input to control the oxidation degree of nitrogen oxides, so as to ensure that the ratio of NO ₂ /NO is between 0.4 and 1, and the preferred ratio of NO ₂ /NO is less than 0.8;

Step 2. Pass the low-oxidation nitrogen oxide exhaust gas after oxygenation into the first-stage absorption tower from the bottom of the first-stage absorption tower, and spray the absorption lye from the top of the first-stage absorption tower. The absorption lye can be NaOH or Na ₂ CO ₃ solution, the temperature of the absorption liquid in the absorption tower is kept below 50°C to control the amount of moisture in the gas phase, the temperature of the absorption liquid can be cooled by circulating the absorption liquid outside the tower or installing a cooling heat exchange device inside the tower, Take out the heat released by the absorption reaction in the tower to control the temperature in the absorption tower. The pressure in the tower is between 0.03-0.8MPa, and the best absorption pressure is 0.2-0.8MPa;

Step 3. After the first-stage absorption, the low-oxidation nitrogen oxide waste gas discharged from the top of the first-stage absorption tower is dewatered by the water removal device again, and the concentration of NO ₂ and NO measured by the online nitrogen oxide analyzer is passed through the PRC. Control, input oxygen-mixed air with an oxygen content greater than 40% to control the oxidation degree of nitrogen oxides, so as to ensure that the ratio of NO ₂ /NO is between 0.2 and 1;

Step 4. Pass the mixed gas of step 3 input oxygen distribution air into the second-stage absorption tower from the bottom of the second-stage absorption tower, spray and absorb lye from the top of the second-stage absorption tower, and absorb lye can be NaOH or Na ₂ CO ₃ aqueous solution. The temperature of the absorption liquid in the absorption tower is kept below 50°C to control the amount of moisture in the gas phase. The temperature of the absorption liquid can be cooled by circulating the absorption liquid outside the tower or installing a cooling heat exchange device inside the tower to bring out The heat released by the reaction is absorbed in the tower to control the temperature in the absorption tower. The pressure in the tower is between 0.03 and 0.8 MPa, and the best absorption pressure is 0.2 to 0.8 MPa;

Repeat steps 3 and 4 in this way, and continue to absorb in the next-level absorption tower until the concentration of nitrogen oxides in the low-oxidation nitrogen oxide exhaust gas discharged from the top of the nth-level absorption tower reaches the emission standard.

In the above method, the nitrogen oxide exhaust gas absorption tower described in step 2 can be a packed tower or a bubble cap tower, preferably a packed tower.

In the above method, the alkali absorption solution described in step 2 may be NaOH or Na ₂ CO ₃ solution with a concentration of 5-30% by mass.

In the above method, the spraying density of the absorbing lye sprayed on the top of the absorption tower described in step 2 is 2-20.

In the above-mentioned method, the absorption lye discharged from the bottom of the next-level absorption tower can be used as the absorption lye of the upper-level absorption tower, so that the output at the bottom of the first-level absorption tower contains 10-50% of the quality of sodium nitrite solution.

In the above-mentioned method, the theoretical plate number of the absorption tower is 5-30, and the first stage tower theoretical plate number is below 15. As the number of stages increases, the oxidation degree and the plate number of nitrogen oxides are all corresponding increase.

The above-mentioned method is suitable for treating the nitrogen oxide exhaust gas whose nitrogen oxide concentration in the original exhaust gas is above 200 mg/m ³ and the molar ratio of NO ₂ /NO is less than 0.4. If NO ₂ /NO>0.4, first use water Absorb and remove part of NO ₂ .

The present invention adopts the multi-stage oxidation of lye to absorb nitrogen oxide gas with low oxidation degree, which can greatly improve the absorption efficiency of NOx, and adopts common low-cost tower equipment to obtain sodium nitrite solution with sodium nitrate/sodium nitrite<0.02, It can realize the benefits of exhaust gas treatment, reduce NOx emissions, and achieve the purpose of energy saving and emission reduction.

The advantages of the patent of the present invention are:

1. Before the nitrogen oxide lye is absorbed, in view of the problem that NO ₂ is easily irreversible with water, the method of drying and removing water and liquid is adopted to reduce the content of nitrate in the absorption liquid;

2. Use the method of cooling the absorption liquid or setting a heat exchanger in the tower to control the temperature of the absorption liquid within 50°C to reduce the volatilization of water.

3. Before the nitrogen oxide exhaust gas enters each absorption tower, use an online nitrogen oxide analyzer to measure the concentration of NO ₂ and NO, and then pass in air with oxygen to adjust the nitrogen oxides according to the molar ratio of NO ₂ /NO The degree of oxidation is between 0.2 and 1.

4. The NOx in the exhaust gas is recovered and converted into a certain concentration of sodium nitrate, which can be sold for profit or recycled.

5. No secondary waste acid and waste water are produced during the treatment process, which is green and environmentally friendly.

6. Exhaust exhaust gas is detected by nitrogen oxide analysis equipment to ensure compliance with emission standards.

7. The invention has a high output/investment ratio and adopts the common tower absorption process, which is especially suitable for NOx waste gas with low oxidation degree and high NO concentration, and conforms to the principles of environmental protection and resource utilization.

Description of drawings

Fig. 1 is the flow chart of embodiment 1 of the present invention, wherein: 1, 2 and 3 are gas-phase dewatering/liquid removal devices; 4, 5, 6 are oxidation tanks; 7, 8, 9 are absorption towers; 10 is NOx on-line Analytical equipment; 11, 12, and 13 are liquid storage tanks of towers 7, 8, and 9; 14, 15, and 16 are condensers of towers 7, 8, and 9, respectively.

Detailed ways

Three towers are used in series to absorb, the absorbent is NaOH solution, the concentration is 20%, the oxygen content in the oxygen distribution air is 40%, the intake air is NOx mixed gas, the gas volume is 600Nm ³ /h, the pressure is 0.3MPa, NO ₂ /NO is 0.2. Sodium nitrite solution is obtained through three-stage tower absorption operation, and all three towers use 350Y structured packing. The original gas of nitrogen oxides first enters the molecular sieve fixed bed for dehumidification and drying. After drying, the moisture content is 0.8ppm. In the oxidation box 4, the input of oxygen distribution air is 125Nm ³ /h, and the gas pressure at the outlet of the oxidation box is 0.5MPa. The packing height of tower 7 is 4m, the operating pressure is 0.6MPa, the absorption temperature is 40°C, the spray density of NaOH liquid is 8.6m ³ /m ² .h, and the condensation area is 20m ² ; the gas at the top of absorption tower 7 enters the molecular sieve fixed bed Carry out dehumidification and drying. After drying, the moisture content is 0.8ppm, and the NO ₂ /NO is 0.03. In the oxidation box 5, the input volume of oxygen distribution air is 250Nm ³ /h, and the gas pressure at the outlet of the oxidation box is 0.6MPa. The second absorption tower The packing height of 8 is 6m, the operating pressure is 0.7MPa, the absorption temperature is 45℃, the spray density of NaOH liquid is 7.2m ³ /m ² .h, and the condensation area is 20m ² . The gas at the top of the absorption tower 8 enters the molecular sieve fixed bed for dehumidification and drying. After drying, the moisture content is 0.8ppm, and the NO ₂ /NO is 0.01. In the oxidation box 6, the input of oxygen distribution air is 125Nm ³ /h. The pressure is 0.6MPa, the packing height of the third absorption tower 9 is 7m, the operating pressure is 0.7MPa, the absorption temperature is 45°C, the spray density of NaOH liquid is 7.2 m ³ /m ² .h, and the condensation area is 20m ² . The NO _X emission concentration in the exhaust gas is 68ppm, and the rate is 0.085kg/h. A nitrous acid solution with a mass concentration of 35% flows out from the bottom of the first-stage absorption tower, wherein the ratio of sodium nitrate/sodium nitrite is 0.015.

Claims (9)

1. an industrial process suboxides degree nitrous oxides exhaust gas prepares the method for natrium nitrosum, it is characterized in that it is that the oxidation of a kind of plural serial stage alkali lye absorption tower combination absorbs the method for preparing natrium nitrosum, and it comprises the steps:

Step 1. dewaters suboxides degree nitrous oxides exhaust gas through de-watering apparatus after, processing method can be the combination of one or more drying means in low temperature process, drier drying, the gas compression, with the content that keeps water in the nitrous oxides exhaust gas and nitric acid below 1%, the NO that measures according to online nitrogen-oxide analyzer, then ₂With NO concentration, by PRC control, the input oxygen content greater than 40% join the oxidizability of oxygen air with the control nitrogen oxide, to guarantee NO ₂The ratio of/NO is between 0.2～1;

The suboxides degree nitrous oxides exhaust gas that step 2. will be joined behind the oxygen feeds first order absorption tower from the bottom on first order absorption tower, absorb alkali lye from the top spray on first order absorption tower, and absorbing alkali lye is NaOH or Na ₂CO ₃Solution, the absorption liquid temperature remains on below 50 ℃ in the absorption tower, amount with moisture in the control gas phase, the absorption liquid temperature is cooled off or installation cooling heat transferring device in tower in the tower outer circulation by absorption liquid, take absorption reaction liberated heat in the tower out of, with the temperature in the control absorption tower, pressure is between 0.03～0.8MPa in the tower;

After the suboxides degree nitrous oxides exhaust gas that step 3. is discharged from top, first order absorption tower through first order absorption dewaters through de-watering apparatus once more, according to the NO of online nitrogen-oxide analyzer, mensuration ₂With NO concentration, by PRC control, the input oxygen content greater than 40% join the oxidizability of oxygen air with the control nitrogen oxide, to guarantee NO ₂The ratio of/NO is between 0.2～1;

Step 4. is joined the bottom feeding second level absorption tower of oxygen Air mixing gas from absorption tower, the second level with step 3 input, and from the top spray absorption alkali lye on absorption tower, the second level, absorbing alkali lye is NaOH or Na ₂CO ₃The aqueous solution, the absorption liquid temperature remains on below 50 ℃ in the absorption tower, amount with moisture in the control gas phase, the absorption liquid temperature can be cooled off or installation cooling heat transferring device in tower in the tower outer circulation by absorption liquid, take absorption reaction liberated heat in the tower out of, with the temperature in the control absorption tower, pressure is between 0.03～0.8MPa in the tower;

Repeating step 3 like this and step 4, down first grade absorption tower is proceeded to absorb, and the concentration of the nitrogen oxide until the suboxides degree nitrous oxides exhaust gas of discharging from top, n level absorption tower is to qualified discharge.

2. method according to claim 1 is characterized in that: the described input oxygen content of step 1 greater than 40% join the oxidizability of oxygen air with the control nitrogen oxide, to guarantee NO ₂The ratio of/NO is less than 0.4.

3. method according to claim 1 is characterized in that: pressure is 0.2～0.8Mpa in the described tower of step 2 and step 4.

4. method according to claim 1 is characterized in that: the described nitrous oxides exhaust gas of step 2 absorption tower is packed tower or bubble column.

5. method according to claim 1 is characterized in that: the described absorption alkali lye of step 2 is that mass percentage concentration is NaOH or the Na of 5-30% ₂CO ₃Solution.

6. method according to claim 1 is characterized in that: the sprinkle density that the described absorption tower of step 2 top spray absorbs alkali lye is 2-20.

7. method according to claim 1 is characterized in that: the absorption alkali lye of discharging at the bottom of the described next stage absorption Tata is as the absorption alkali lye on upper level absorption tower, and so, output is the sodium nitrite solution that contains quality 10-50% at the bottom of the first order absorption Tata.

8. method according to claim 1 is characterized in that: the theoretical cam curve on described absorption tower is 5-30, and first order tower theoretical cam curve is below 15, along with the increase of progression, and all corresponding increase of the oxidizability of nitrogen oxide and the number of plates.

9. method according to claim 1 is characterized in that: this method is fit to handle described original nitrogen oxides of exhaust gas concentration at 200mg/m ³More than, NO ₂The mol ratio of/NO is less than 0.4 nitrous oxides exhaust gas, if NO ₂The mol ratio of/NO〉0.4, then adopt water to absorb earlier and remove part NO ₂