CN1034988C

CN1034988C - Method for removing and recovering nitrogen oxides from tail gas of nitric acid production process

Info

Publication number: CN1034988C
Application number: CN92101018A
Authority: CN
Inventors: 殷斌友
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-02-14
Filing date: 1992-02-14
Publication date: 1997-05-28
Anticipated expiration: 2007-02-14
Also published as: CN1068753A

Abstract

The invention relates to a method for solving the problems of NOx pollution and waste in tail gas in nitric acid production, which is characterized by comprising the following steps: after tail gas is oxidized, cooled and acid mist is removed, ammonium nitrate solution containing ammonia is used as an absorption medium, all processes including absorbing NOx, converting generated ammonium nitrite into ammonium nitrate, washing ammonium salt particles in a scrubbing liquid phase and the like are completed in a tricyclic absorption tower made of granite or acid-proof bricks, the absorption rate reaches over 90 percent, the NOx is recovered in the form of ammonium nitrate, the full cost of compensation operation is enough, and the equipment cost is only 1/5-1/10 of common stainless steel equipment.

Description

Method for removing and recovering nitrogen oxides from tail gas of nitric acid production process

The invention relates to a method for removing nitrogen oxides in tail gas, in particular to a method for removing and recovering nitrogen oxides from the tail gas in a nitric acid production process.

In the production of nitric acid, a large amount of discharged tail gas contains about 2000PPM of nitrogen oxides. Not only causes the waste of resources and energy, but also causes serious pollution to the atmosphere. Various methods have been reported at home and abroad for eliminating or controlling the nitrogen oxides, but all have some disadvantages, such as: the selective ammonia reduction method has high ammonia consumption, energy consumption, high running cost, NO product and energy recovery, and the discharged tail gas contains NO_x400 PPM; the non-selective reduction method is mainly 'decoloration', and NO in tail gas_xIf the total amount is not reduced, further reducing NO to N₂The process will be complicated and the catalyst is susceptible to poisoning; tail gas treated by alkali absorption method contains NO_x1000PPM, and evaporation crystallization equipment is also needed for recovering nitrite; mercerized boiling waterStone method (solid adsorption method), the treated tail gas containing NO_x400PPM, low adsorption capacity and high investment and operation cost; nitric acid absorption method with absorption rate of 70%, low gas-liquid ratio, dissolved NO_xBleaching is needed, the cold and heat consumption is large, and the operation cost is high; high pressure enhanced absorption of NO in the tail gas without treatment_xCan reach below 200PPM, but the investment is huge once, is not suitable for the existing enterprises, and can be considered only when new factories are built. A good approach would be: 1. can be used forMake NO_xReducing the content of the active carbon to below 200 PPM; 2. adding NO_xRecycling the product to become usable; 3. the system is kept stable and easy to operate; 4. not consume too much energy; 5. the investment is saved at one time, and the equipment is easy to manufacture and install; 6. no secondary pollution is generated. According to the actual of ammonium nitrate factory, it is ideal to develop a continuous oxidation absorption method using dilute nitric acid and air as oxidant and ammonium nitrate solution of ammonia water as absorption medium, and can recover waste ammonia water and NO in nitric acid tail gas_xBecome usable ammonium nitrate product by conversion and eliminate NO_xThe toxic and harmful pollution to the environment can achieve the effect of treating wastes with processes of wastes against one another and achieving multiple purposes.

The ammonification ammonium nitrate absorption method for treating nitric acid tail gas has been reported industrially abroad and is still in a test stage at home. The main principle is to utilize ammonia in the absorption medium and NO in the tail gas_xThe reaction is carried out. The company GOOdpastUre firstly published the industrialized report in 1976, and the tail gas treatment device for treating 100T/day nitric acid plant was established in Dimmit Taxes in 3 months in 74 years, and then the tail gas treatment device for 250T/day nitric acid plant was approved to be established in RichmOnt California. U.S. Pat. No. 3453071(1969) describes a 9-stage absorption column. West German patent 2513619(1976) describes the process as a 5-stage absorption column, in which a further stage uses a urea solution as absorbent. The ammonified ammonium nitrate absorption method in China has been tried in two stages of packed towers, and the result is positive.

The method is characterized in that NO special catalyst and cold heat are needed, the raw material is ammonia water, ammonium nitrate is used as an intermediate medium, and NO in the treated tail gas_xCan be reduced to below 200PPM and absorbed NO_xRecovered as ammonium nitrate. From the view of the many methods which have been industrialized internationally, the ammoniated ammonium nitrate absorption method for treating nitric acid tail gas is the most economical industrial method.

The invention comprises an industrial process route for treating nitric acid tail gas by an ammoniated ammonium nitrate absorption method and two main devices: one is a dual venturi oxidation cooling tower; the second is a three-ring absorption tower. Its advantages are low resistance, low cost, high strength, anticorrosion nature, long service life, compact structure, less connection of large pipeline, simple operation, easy manufacture and installation, low investment, low cost, high absorption rate and no secondary pollution. The cost is very low, only one fifth to one tenth of the cost of the stainless steel absorption tower, and the method is particularly suitable for the national conditions of developing countries.

The main process and reaction principle of the process are as follows:

1. oxidation of tail gas: since less than 30% nitric acid is non-oxidizing, the process uses a mixture of 40-50% nitric acid and air (or oxygen-enriched air) to oxidize NO in the tail gas. The oxidation degree is improved to more than 50 percent. The reaction formula is as follows:

2. absorbing NO with ammonium nitrate solution of ammonia_x(ii) a Gas-phase reaction:

(in the case of high-temperature drying

Side reactions can be avoided) liquid phase reaction:

3. conversion of ammonium nitrite: adding proper amount of nitric acid and air, and making them flow concurrently with the absorption liquid and tail gas. Converting nitrosamine to ammonium nitrate:

4. washing and absorbing nitrate particles and residual NO by using ammonium nitrate solution (or water)_x: because a large amount of ammonium nitrate existing in gas phase is generated in the ammoniation absorption sectionThe ammonium nitrite particles (aerosol) may not be completely eliminated after absorption and conversion in the co-current conversion absorption section. Residual NO_xFurther absorption is required to obtain NO in the exhaust gas_xAnd ammonia loss is minimized. The absorption of the section does not use ammoniation absorption liquid, and the ammonia in the absorption liquid is prevented from being carried to a gas phase by gas stripping and is prevented from being mixed with residual NO_xNitrate particles are generated again, so that ammonia loss is increased and secondary pollution is generated.

II, equipment structure and process flow:

thestructure and the process flow of the double-venturi cyclone oxidation cooling tower are as follows:

the double-venturi cyclone oxidation cooling tower is a cylindrical integral hollow tower (as shown in figure 1), the upper part of the double-venturi cyclone oxidation cooling tower is provided with a tail gas filtering device (8) and a perforated pipe sprayer (7), the middle lower part of the double-venturi cyclone oxidation cooling tower is provided with a gas distribution plate (5), the upper part of the double-venturi cyclone oxidation cooling tower is provided with a cooling area (6), and the lower part of the double-venturi cyclone oxidation cooling tower. The inlet of the tail gas inlet pipe (11) is provided with a double-layer Venturi structure and an oxidation zoneThe tangential position is communicated, a double-layer nozzle (10) arranged in the inlet pipe sprays 40-50% of mixture of nitric acid and air into the tail gas, the mixture enters an oxidation area in the tower in the tangential direction after being mixed with the tail gas entering the tower, and the oxidation reaction is carried out with NO in the tail gas in the rotating process, so that the oxidation degree of the tail gas is improved to more than 50%. The tail gas is separated from the unreacted nitric acid under the action of centrifugal force in the cyclone process, enters a cooling area (6) at the upper part of the tower through a distribution plate (5), is in countercurrent contact with cooling water (desalted water) sprayed from the upper part, reduces the temperature of the tail gas to below 50 ℃ (preferably, the tail gas can be cooled to enter a three-ring suction deviceThe reaction with ammoniation absorption liquid after tower collection is carried out at the temperature of 20-30 ℃), most of acid mist in the tail gas can be washed away in the cooling process, and NO in the tail gas can be preliminarily absorbed_xAnd very dilute nitric acid is generated, the cooled tail gas passes through a filtering device (8) at the upper part of the cooling area, acid mist is removed, then the tail gas enters a three-ring absorption tower from an outlet pipe (9), and the cooling water preliminarily absorbs NO_xThen falls down and passes through a guide pipe (4) below the distribution plate to be mixed with the rest dilute nitric acid flowing down from the tower wall after the oxidation reaction in the oxidation zone, flows out from the tower bottom, is sent to an acid absorption production position for acid absorption (part of NO is recycled) by a pump (1) through a storage tank (2)_xTo generate nitric acid into the production system).

(II) a tricyclic absorption tower structure and a process flow:

description of the reference numerals

FIG. 1-double Venturi Oxidation Cooling Tower and schematic flow sheet

1-pump 7-sprayer

2-storage tank 8-filter

3-oxidation zone 9-tail gas outlet pipe

4-cooling water guide pipe 10-Venturi type double-nozzle

5-gas distribution plate 11-tail gas inlet pipe

6-Cooling zone

FIG. 2-schematic diagram of a three-ring absorber and process:

1-liquid sump at bottom of tower 9-demister

2-inlet pipe for tail gas 10-outlet pipe for tail gas

3-venthole 11-storage tank

4-outer ring tower 12-storage tank

5-medium ring tower 13-storage tank

6-inner round tower 14-pump

7-sprayer 15-pump

8-swirl plate 16-pump

The three-ring absorption tower is formed by sleeving three absorption tower bodies with different diameters together in a concentric circle mode (as shown in figure 2), the cross sections of an outer-layer tower and a middle-layer tower are all annular, an inner tower is circular, the outer-ring tower (4) is not provided with filler, the inner-ring tower (6) and the middle-ring tower (5) are all packed towers, the inner-ring tower and the middle-ring tower are communicated with a vent hole (3) reserved on the tower wall at the lower part of the filler layer, liquid storage tank (1) spaces are reserved below the vent hole, the lower parts of the three liquid storage tanks are respectively communicated with liquid storage tanks (11), (12) and (13) outside the towers by using a guide pipe, the upper part of the outer-ring tower is separated and communicated with the middle-ring tower by a rotational flow plate liquid blocking device (8), a foam device (9) is arranged at the upper part of the inner-ring tower, annular porous pipe sprayers (7) are arranged at the three towers below the positions of the, air and nitric acid inlet pipes are additionally arranged on the middle ring tower sprayer, and clear water inlet pipes are additionally arranged on the upper part of the inner round tower demister and the middle ring tower sprayer. The ammonium nitrate solution led out from the liquid storage tank at the bottom of the inner tower is stored outside the towerAmmonia water is added into a tank (13) for ammoniation, ammoniated ammonium nitrate solution is sent to each sprayer at the upper part of an outer ring tower by a pump (14) and is annularly sprayed from top to bottom along the hollow part of the outer ring tower and the walls of two towers in multiple layers, a large-area liquid film is formed on the walls of the two towers, foggy raindrops fall down in an annular space, tail gas after oxidation cooling enters the outer ring tower from an inlet pipe (2) along the tangential direction, spirally rises along the outer ring tower (4) in a tornado mode and contacts ammoniated liquid sprayed in the multiple layers at the upper part of the outer ring tower at a high speed to form foggy air of the raindropsThe liquid mixed state is fully contacted, and absorption reaction is carried out. The solution after the absorption reaction flows to a storage tank at the bottom of an outer ring tower along the tower wall under the action of centrifugal force, is sent to a sprayer at the upper part of a middle ring tower (5) by a pump (16) through a guide pipe and the storage tank (11) to be sprayed with tail gas and nitric acid and air added from the outside of the tower (such as the air or oxygen-enriched air added when the tail gas is oxidized has enough amount, the air can not be added any more), and flows through a packing layer of the middle ring tower from top to bottom to convert the ammonium nitrite in the solution. Tail gas absorbed by ammoniation is recovered from the upper part of the outer ring tower through a cyclone plate (8) and separated from the entrained ammoniation absorption liquid, then enters a middle ring tower (5), flows together with air, nitric acid and absorption liquid fed by a pump (16) from the upper part of the middle ring tower in parallel through a middle ring tower filler layer from top to bottom, converts ammonium nitrite in the absorption liquid, and continues to absorb NO in the tail gas_x. The converted absorption liquid flows to a middle ring tower bottom storage tank, flows to an outer tower storage tank (12) through a guide pipe, is added with ammonia, is adjusted to pH about 7, and then one part of the absorption liquid is sent to the upper part of an inner circular tower by a pump (15) to be sprayed and washed with washing tail gas (or washed by water), and the other part of the absorptionliquid can be sent toEvaporating and recovering ammonium nitrate in the production post. The tail gas after secondary absorption in the middle ring tower flows into the inner circular tower from a vent hole (3) at the lower part of the middle ring tower, passes through the packing layer from bottom to top, and is in countercurrent contact with water or ammonium nitrate washing absorption liquid sprayed from top to bottom to absorb NO in the tail gas again_xAnd ammonium salt particles (aerosol) in the tail gas are washed and removed, and the tail gas absorbed by the washing section passes through a demister (9) at the upper part of the inner circular tower and then enters an exhaust funnel through an outlet pipe (10) for emptying. The ammonium nitrate washing absorption liquid flowing down from the inner circular tower is led out of the tower through a tower bottom storage tank, ammonia water is added into the storage tank (13) for ammoniation, and the ammoniated ammonium nitrate washing absorption liquid is sent to the upper part of the outer ring tower by a pump (14) for circulating spraying. In order to prevent the ammonium nitrate solution from cooling and crystallizing (the ammonium nitrate solution cannot crystallize in the temperature and concentration range of the process) and blocking equipment, a filler layer of the three-ring absorption tower is flushed by clean water when the machine is stopped, and a small amount of steam can be added for heat preservation.

Thirdly, explaining several problems in the process:

1. oxidation and cooling of tail gas:

due to NO in the tail gas_xMost of the NO is NO, and NO and NH₄Does not react chemically, so must beThe oxidation degree of the tail gas is improved to more than 50 percent. Generally, the higher the acid concentration, the better the oxidation effect, and the acid consumption is about 6g/m when the tail gas is oxidized by using 40-50% nitric acid³And (4) tail gas. The process adopts a double-venturi cyclone oxidation cooling tower, uses 40-50% nitric acid and part of air (or oxygen-enriched air), and in the double-layer venturi structure the tail gas, acid and air can be well mixed and contacted, and its oxidation property can be improved, and the dosage of nitric acid can be reducedIn (1). This is an important feature of the process. The desalted water is used for cooling so as to recycle the cooled dilute acid water solution, and the tail gas is cooled so as to supply NO in the tail gas of the following working procedure_xThe absorption of the ammonia and the water creates better conditions, most of the acid mist after oxidation in the tail gas can be washed away, and the generation of 'white mist' in the ammoniation and absorption section can be reduced. (if the heat is not balanced, a cooling coil can be added at the tower bottom to cool the cooling water for recycling)

2. Reaction mechanism of ammoniation absorption section, ammonia concentration in ammoniation absorption liquid to NO_xInfluence of absorption:

according to the accepted absorption double-membrane theory, the larger the gas-liquid flow rate, the thinner the gas-membrane liquid membrane, the smaller the resistance to absorption and diffusion. The ammoniation absorption section adopts an absorption process of multi-spray and cyclone contact in the outer ring tower, so that the gas-liquid contact speed is increased, high-speed convection diffusion is formed, namely, the absorption process is accelerated, and the diffusion and absorption reaction speed is accelerated because ammonia participates in the reaction in the gas phase, so that the outer ring tower absorbs NO relatively_xIs relatively high, which is a main feature of the present process.

The ammonia content of the ammoniation absorption liquid is too high, so that the ammonia loss is increased, and the ammonia content is too low, so that the NO content in tail gas is reduced_xThe absorption rate of the ammonia absorption process is that the ammonia content of the ammoniated solution is 0.05-0.5% (weight) concentration (the operation and analysis are more convenient). NO in the tail gas_xCan be reduced to below 200PPM, and the discharged tail gas has no white fog. In operation according to the NO of the exhaust gas_xContent (wt.)The ammonia content in the absorption liquid is adjusted in time, so that the ammonia leakage amount and NO can be effectively controlled_xThe content is below 200 PPM. The process adopts three-stage absorption to properly increase the absorption of the ammoniation stageAmmonia content of the liquor, ammoniation stage to NO_xThe absorption rate can be further improved, the ammonia loss can not be increased after the absorption of the cocurrent flow conversion section and the absorption and the trapping of the countercurrent absorption washing section, and NO in the tail gas_xThe content will be lower.

3. Spraying amount to NO in tail gas_xInfluence of absorption rate and selection of column diameter:

according to the diffusion principle of the absorption process, the larger the spraying amount is, the NO in the tail gas is_xThe better the absorption, but an excessive amount of spraying increases power consumption. The domestic laboratory proves that the NO is guaranteed_xThe absorption rate of the spray is more than 90 percent, and the spray density is up to 8m³/m²hr (two-stage absorption, gas-liquid ratio is about 500: 1), power consumption is greatly increased, the absorption effect is not obvious, a three-ring absorption tower is adopted at present, the absorption process of a cooling section is increased, theabsorption efficiency is improved undoubtedly, the circulation quantity is properly reduced, and the gas-liquid ratio is possibly increased. But the spraying density can not be too small, and the theory proves that the minimum spraying density which is required to completely soak the surface of the filler in the packed tower is 4-5m³/m²hr, therefore, when selecting the tower diameter, the selection of a larger tower diameter, i.e., a smaller spray density, can be considered.

4. Presence and conversion of nitrosamine:

NH can be generated quantitatively in the reaction process of the ammoniation absorption section₄NO₂Theory of qiIt is proved that the nitrosamine is easily soluble in water, the nitrosamine is stable in aqueous solution, and the nitrosamine is stable in ammonia-containing condition and is stable to NO_xThe absorption reaction of (A) has no obvious influence. The tail gas of the middle ring tower directly comes from the outer ring tower, and the gas phase contains NH₄NO₂And water and ammonia, the ammonium nitrite is stable in the gas phase at the upper part of the outer ring tower and the middle ring tower, and is also stable in the absorption solution, so that the ammonium nitrite is not decomposed and exploded.

To make NH not present₄NO₂Into NH₄NO₃In the product, the process can effectively mix the pH value of the product within the range of 0.1-5.0, particularly about 0.5-2.0 with a proper amount of airNH₄NO₂Conversion to NH₄NO₃. In order to increase the NH content of the tail gas₄NO₂The process adopts a mode of injecting nitric acid and air at the top of the conversion tower to add NH₄NO₂And NO_xIs advantageous, the absorption liquid NH converted by the mediumring tower₄NO₂The content can be controlled below 0.1%, the ammonium nitrate is discharged out of the tower, and ammonia is added into a storage tank (12) to adjust the pH value to 7, so that the ammonium nitrate can be evaporated and recovered.

5. Concentration and crystallization problems of ammonium nitrate in the absorption liquid:

the ammoniated absorption liquid has too high concentration of ammonium nitrate to produce crystallization and block the equipment, and if the concentration is too low, the viscosity of the absorption liquid is too low, the partial pressure of water vapor and ammonia in the gas phase are increased, so that the produced NH₄NO₂The white fog is more, the process adopts 40-50% ammonium nitrate solution, the ratio of water vapor to ammonia vapor in gas phase is low on the absorption liquid surface, the generated ammonium nitrite is less,this white mist is particularly easily absorbed rapidly and quantitatively by the aqueous ammonium nitrate solution. The solubility of ammonium nitrate in water solution is 65.18% at normal temperature (20 deg.C), and 54.49% even at 0 deg.C, and the reaction process is not lower than 20 deg.C. Therefore, under the condition of normal driving, as long as the packing in the tower is reasonably stacked, the gas does not generate bias flow, and the equipment cannot be blocked due to crystallization.

6. Resistance problems with exhaust gas treatment devices:

the oxidation cooling tower and the outer ring tower are hollow towers which are not provided with fillers and tower plates, so that the resistance is very small. The resistance of the whole device mainly occurs in a middle ring tower and an inner circular tower with fillers, and the fillers with smaller resistance can be selected and measures such as larger tower diameter and the like are selected during construction design, so that the resistance is the minimum. The theory indicates that: at a practically operating gas flow (assuming a permissible velocity of 85-95%), the pressure drop for several conventional packings (ceramic rings, saddles) is approximately in the range of 40-70 (mm water/m. height of packing), and if the total height of the two columns of packing is 40 m, the resistance drop is only 0.16-0.28 atm. According to the compression curve of the rotary compressor, the rise of the system pressure is always close to (does not exceed) the increase of the resistance drop. Therefore, increasing such resistance is fully tolerable for the production system (see description 7). In special cases, this resistance is not allowed to increase if the process and the equipment of the production system do not allow it. It is contemplated to add a blower after the production system to overcome the drag of the tail gas treatment device, which allows the turbine compressor to recover more energy for the production system and increase the pumping capacity to increase the productivity of the system.

7. Turbo compressor electric power increase and change of production system:

as described in the explanation 6, since the increase of the tail gas treatment device after the production system inevitably increases the system resistance, the energy recovered by the expansion stage of the turbo compressor is quantitatively reduced, and the power consumption of the electric turbine is quantitatively increased, which is an inevitable phenomenon. However, after the new equilibrium is reached, the result will be an increase in system pressure, an improvement in acid absorption conditions, an increase in efficiency, an increase in product quality, (an increase in acid concentration), and a corresponding increase in exhaust gas pressure, thereby increasing the driving force for exhaust gas treatment absorption. Taking a certain total medium pressure 270T/day nitric acid plant as an example: the design pressure is 4.5kg/cm²Rated current 168/159A of turbine, and actual operation pressure is only 3.8kg/cm²The current is only 100/98A. The design level and the rated index are not reached, and the operation practice proves that the outlet of the turbine is improved by 0.3kg/cm²The pressure and the current are only increased by 5A, so the potential is completely bearable for overcoming the resistance of the tail gas treatment device, although the turbine air-inflating quantity is reduced, the production efficiency can be improved only by consuming more electric energy without expanding and replacing equipment, the cost is absolute, the production efficiency is improved, and necessary conditions are created for eliminating the poison of the tail gas and polluting the atmosphere. A certain domesticIn order to install a tail gas treatment device in a large chemical fertilizer plant, the resistance of the device is overcome by a method of increasing the turbine back pressure of tail gas, actual measurement is carried out on a plurality of units, the turbine back pressure is increased from 170mm water column to 2500mm water column, no abnormity is found, the turbine power is increased, equipment leakage is caused,The vibration and voice conditions are good. Therefore, it is fully feasible to add an exhaust gas treatment device after the production system.

The above analysis of the system pressure is based on a full medium pressure nitric acid production plant. If the tail gas treatment for producing nitric acid by the normal pressure method and the full low pressure method is considered, a blower (or an induced draft fan) can be added before (or after) the tail gas treatment device to overcome the resistance of the tail gas treatment device. If the comprehensive method is used for producing the nitric acid, the tail gas treatment device can be installed better in the same way as the full medium-pressure method, and if the tail gas treatment device is not installed, the blower is additionally installed in the same way as the normal-pressure method and the full low-pressure method.

8. The three-ring absorption tower and the Venturi oxidation cooling tower can be used for treating nitric acid tail gas, and can also be used for processes of nitric acid absorption production, sulfuric acid absorption, tail gas treatment, furnace gas purification and drying before contact oxidation and the like, and processes of alkali absorption of acid gas and the like. Because the tower body is made of the granite or the acid-proof brick, when the airtight performance and the overall compressive strength of the three-ring absorption tower and the Venturi oxidation cooling tower are not enough during construction design, the joint can be filled with the acid-proof mortar during tower construction, and the exterior of the tower is integrally plastered with the acid-proof mortar. And the 100 x 100-300 x 300 phi 6-phi 12 reinforcing mesh is assembled to pour the 100-200-thickness acid-resistant concrete to increase the air tightness and the overall compressive strength.

In order to overcome the stress of expansion with heat and contraction with cold, the vehicle should be slowly preheated during driving (or stopping) and gradually heated (or cooled). For the security of the device.

9. And (3) economic benefit analysis:

take 270 tons/day full medium pressure nitric acid plant as an example. Its tail gas quantity is 38000m³Hr, containing NO_x0.2% by weight of total carbon monoxide₂156.4 kg/hr. 140kg/hr in terms of 90% recovery. 3360kg daily, 4.6 tons per day in 100% nitric acid, 1380T in 300 days all year, 1770 tons of ammonium nitrate and 1274400 yuan (720 yuan/ton) in value can be recovered each year because the ammonia used is recovered waste ammonia water and the final product recovered is ammonium nitrate.

The operation cost is calculated according to the nitric acid absorption method of 300T/day nitric acid plant for treating tail gas (U.S. dollar/year)

Managing	Operation of	Maintenance	Cooling water	Steam generating device	Electric power	Aggregate meter
Managing	Operation of	Maintenance	Cooling water	Steam generating device	Electric power	Aggregate meter	4400	2200	2200	14980	17500	43250	84530

The RMB is 45 ten thousand yuan (the actual cost is low). The annual profit is 127-45 to 82 ten thousand yuan, and the ammonia reduction method adopted by many families in China. The tail gas treatment cost of each ton of nitric acid produced is 11.17 yuan (80 years without changing price) and no product is recovered, for example, the annual production of 80000 tons of nitric acid plants requires about 89.36 ten thousand yuan (80 years without changing price) of tail gas treatment cost. In contrast, the superiority of the method is shown.

Fourth, the embodiment:

example 1A composition containing NO_x2420PPM tail gas with oxidation degree of 20-30%, flow rate of 12-15M/SeC, gas amount of 3350Nm³At 70 deg.C/hr, adding 0.65% ammonia water (without ammonium nitrate) at a concentration of 2-3kg/cm³Pressure of 8m³Spraying amount per hr with 10 nozzles, performing parallel flow absorption treatment, and discharging with NO_xThe absorption rate of 710PPM is 70.8 percent, the discharged tail gas is about 30 ℃, the ammonia content is 0.49mg/L, the ammonium nitrate content is 1.9mg/L, and the ammonium nitrite content is 3.6 mg/L.

Example 1 illustrates the absorption of NO at higher non-oxidation temperatures using aqueous ammonia alone without the addition of ammonium nitrate as the medium_xThe tail gas has large ammonia loss, low absorption rate and white fog.

Example 2A Tail gas, Normal temperature and pressure, Linear speed 0.9-1.08M/SeC containing NO_x1940PPM gas amount 1.5m³A/hr oxidation rate of 90% by about 40% nitric acid and NO_xThe content is increased to 2820PPM, the tail gas is filtered to remove acid mist, and then passes through two series absorption towers, and is absorbed and treated by ammoniated ammonium nitrate solution with concentration of 40-50%, the ammonia content of absorption liquid is imported 0.123%, exported 0.062%, absorbedThe spraying density is 8m³/m²hr, ammoniation absorption, and tail gas NO_x280PPM in content and 86% in absorption rate, and NO in 43PPM after ammonia leakage of tail gas is washed with water_xThe content is 200PPM, the absorption rate is increased to 90 percent, the ammonia leakage of the tail gas is 0PPM, and the total ammonium yield is 88.7 percent.

Example 3A gas, Normal temperature and pressure, Linear speed 0.9-1.08M/SeC containing NO_x2060PPM, gas amount 1.5m³The oxidation degree of the nitric acid after oxidation with about 40 percent of nitric acid is 92.4 percent of NO_xThe content is increased to 2640PPM, and the acid mist is filtered and then passes through two absorption towers which are connected in series for use40-50% of ammoniated ammonium nitrate solution is absorbed, the ammonia content of the absorption solution is 0.126% at the inlet and 0.064 at the outlet, and the absorption spray density is 8m³/m²And (5) hr. Ammoniated NO absorption_xThe content is 200PPM, the absorptivity is 90.5%, the tail gas contains 66PPM of ammonia, and NO is obtained after the tail gas is washed by water containing 0.01% of ammonia_xThe content is 20PPM, the absorption rate is increased to 98 percent, the tail gas ammonia leakage is 37PPM, and the total ammonium yield is 65 percent.

Example 4 Another gas, Normal temperature, Normal pressure, Linear velocity 0.9-1.08M/SeC containing NO_x2270PPM at 1.5m³A/hr oxidation rate of 90% by about 40% nitric acid and NO_xThe content is increased to 2520PPM, after filtering and removing acid mist, the filtrateis passed through a two-stage packed tower and absorbed by ammoniated liquid of 40-50% ammonium nitrate, the content of absorbed liquid ammonia is imported 0.198%, the output is 0.051% and the absorption spray density is 8m³/m²hr, ammoniation of NO absorbed_xContent oPPM, absorption rate 100%. Tail gas containing ammonia 73PPM, NO washed by acid water_xIn an amount of20PPM, the absorption rate is reduced to 90.2 percent, the tail gas runs out of 86PPM, and the total ammonium yield is 75.8 percent.

Example 2 example 3 example 4 all show NO in tail gas after absorption in ammoniation absorption section_xThe content can reach about 200PPM, the ammonia leakage of the tail gas is below 200PPM, if the tail gas is washed by water or ammonia-containing water or acidic water, the absorption rate is above 90 percent, and the ammonia leakage and NO of the tail gas_xThe content is below 200 PPM. The viscosity of the ammonium nitrate solution is larger than that of water at the same temperature. It is therefore also possible to carry out the ammoniation absorption followed by the reabsorption in the acidic reforming section and finally to wash the cleaned off-gas with an aqueous ammonium nitrate solution having a PH of about 7.

Claims

1. A method for removing nitrogen oxides from a tail gas discharged from a nitric acid production process, the method comprising the steps of:

(1) contacting said off-gas with a mixture of nitric acid and air at a concentration of 40-50% by weight to oxidize said off-gas;

(2) contacting the oxidized tail gas with an ammonium nitrate solution of ammonia water so as to enable nitrogen oxides in the tail gas to be absorbed by the solution;

(3)contacting the mixture of the tail gas generated in the step (2) and the absorption solution with nitric acid and air or oxygen, wherein the concentration and the addition amount of the nitric acid are such that the pH value of the mixed solution after the nitric acid is mixed with the absorption solution is 0.1-5.0;

(4) contacting the tail gas generated in the step (3) with ammonium nitrate solution or water to wash and absorb solid particles and residual nitrogen oxides in the tail gas.

2. The method according to claim 1, wherein the concentration of ammonium nitrate in the ammonium nitrate solution of the aqueous ammonia in the step (2) is 40 to 50% by weight, and the concentration of aqueous ammonia is such that the ammonia content in the absorption solution produced after the solution is brought into contact with the off-gas is 0.05 to 0.5% by weight.