A method for preparing gaseous nitrous acid and a generating system

Abstract

The present invention discloses a preparation method and a generating system for gaseous nitrous acid. The preparation method includes: placing nitrogen dioxide and water vapor under high-temperature conditions to generate gaseous nitrous acid. Among them, under high-temperature conditions, part of the nitrogen dioxide thermally decomposes to generate nitric oxide, and gaseous nitrous acid is generated by the chemical equilibrium reaction of nitrogen dioxide, nitric oxide and water vapor under the high-temperature conditions. The generating system mainly includes a zero air source, a nitrogen dioxide source, a flow control device, a humidity generating device, a constant-temperature gas mixing chamber, and a heating device. It has a simple structure, easily available raw materials, convenient operation, and can stably and repeatedly generate gaseous nitrous acid with adjustable concentration and humidity.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental quality monitoring, and particularly to a preparation method and a generation system of gaseous nitrous acid. Background Art

[0002] Gaseous nitrous acid (HONO) exists in the atmospheric environment and is an important trace nitrogen-containing gaseous pollutant. In recent years, due to the progress of atmospheric detection technologies, the detection of trace gaseous nitrous acid in the atmosphere has become possible, such as long-path absorption photometers, differential optical absorption spectroscopy, mass spectrometry detection technologies, etc. Gaseous nitrous acid, as an important source of hydroxyl radicals (OH·) in the atmospheric environment, is an important trace gas in the atmospheric chemical cycle. In addition, nitrous acid and its reaction products have carcinogenic properties. Therefore, researchers have carried out a large amount of research work. Whether it is the research on the reaction mechanism of gaseous nitrous acid in the atmosphere or the calibration of gaseous nitrous acid measuring instruments, a simple and stable gaseous nitrous acid generation system is required as a standard source for various studies.

[0003] Existing preparation methods of gaseous nitrous acid: Initially, researchers simply used the equilibrium of nitric oxide, nitrogen dioxide and water vapor at room temperature to generate nitrous acid gas or used the thermal decomposition of ammonium nitrite to generate nitrous acid gas. However, these two methods cannot achieve stability and have low purity. There are also methods using different acid replacements, such as the reaction of oxalic acid, sulfuric acid, hydrochloric acid with nitrites to generate nitrous acid gas. These methods of generating nitrous acid gas by acid replacement have received attention and development in recent years.

[0004] The problems existing in the above-mentioned existing preparation methods of gaseous nitrous acid are as follows: The operation and structure are relatively complex, the raw materials required for the preparation of gaseous nitrous acid need to be prepared or replaced, and most of the raw materials involve chemical dangerous goods such as strong acids (hydrochloric acid, sulfuric acid, etc.), which will generate hazardous waste and require more human intervention. Most of the internal components contain glassware, and some glassware needs to be customized, which is fragile and not convenient for transportation. Summary of the Invention

[0005] Aiming at the problems existing in the prior art, the present invention provides a preparation method and a generation system of gaseous nitrous acid, the raw materials required for the preparation of gaseous nitrous acid are simple and easy to obtain, and can continuously and stably generate gaseous nitrous acid without excessive human intervention.

[0006] The technical solution of the present invention is as follows:

[0007] In the first aspect of the present invention, a method for preparing gaseous nitrous acid is provided, which is characterized by including: placing nitrogen dioxide and water vapor under high-temperature conditions to generate gaseous nitrous acid, wherein under high-temperature conditions, part of the nitrogen dioxide pyrolyzes to generate nitric oxide, and gaseous nitrous acid is generated by the chemical equilibrium reaction of nitrogen dioxide, nitric oxide, and water vapor under the high-temperature conditions.

[0008] In some embodiments of the present invention, the high-temperature conditions are a high-temperature environment with a temperature ≥ 150 °C.

[0009] In some embodiments of the present invention, the concentration of nitric oxide or nitrogen dioxide in the gas after pyrolysis under the high-temperature conditions accounts for 40 - 60% of the sum of the two concentrations.

[0010] In some embodiments of the present invention, after introducing water vapor, the humidity condition for the reaction of nitrogen dioxide and nitric oxide is > 0% RH, and further, the humidity condition is 70 - 90% RH.

[0011] In some embodiments of the present invention, the nitrogen dioxide is derived from any one of a nitrogen dioxide storage gas cylinder, nitrogen dioxide generated by the discharge reaction of a zero-air source, or nitrogen dioxide generated by the reaction of nitric oxide and ozone. In the second aspect of the present invention, a gaseous nitrous acid generation system is provided, including a zero-air source, the zero-air source is connected to a humidity generation device through a pipeline, the nitrogen dioxide source and the water vapor generated by the humidity generation device are mixed and then enter a heating device to be heated to generate gaseous nitrous acid, and zero air is used to dilute and adjust the concentration and humidity of the generated gaseous nitrous acid.

[0012] In some embodiments of the present invention, the nitrogen dioxide and water vapor are mixed through a constant-temperature gas mixing chamber, the constant-temperature gas mixing chamber adopts a metal reaction cavity, and the temperature stability in the cavity is ±1 °C.

[0013] In some embodiments of the present invention, the heating device adopts a tube furnace, a metal tube or a quartz tube for introducing gas is arranged in the tube furnace, the tube furnace heats the metal tube or the quartz tube, and the temperature stability of the gas in the metal tube or the quartz tube is ±1 °C.

[0014] In some embodiments of the present invention, the zero-air source is also connected to a discharge device, and through the discharge of the discharge device, nitrogen and oxygen in the zero air react to generate nitric oxide and ozone, and ozone and oxygen rapidly convert nitric oxide into nitrogen dioxide, serving as the nitrogen dioxide source.

[0015] In some embodiments of the present invention, the zero-air source is also connected to an ultraviolet light ozone generator, and the ozone generated by the ultraviolet light ozone generator is mixed with nitric oxide to generate nitrogen dioxide, serving as the nitrogen dioxide source. One or more technical solutions of the present invention have the following beneficial effects:

[0016] (1) The preparation method of gaseous nitrous acid provided by the present invention has simple raw materials and only generates waste gas due to redundant production of gaseous nitrous acid. The high-temperature environment can pyrolyze impurities such as nitric acid and ozone, and contains fewer other impurities except NOx, improving the purity of the prepared gaseous nitrous acid.

[0017] (2) The gaseous nitrous acid generation system provided by the present invention has a simple structure and is easy to operate. Only the raw material gas needs to be connected, and after starting the power supply of the equipment and waiting for it to stabilize, trace gaseous nitrous acid with a volume concentration in the order of ppb to ppm can be continuously and stably generated without excessive human intervention; this generation system can be used as a standard generation source for calibrating gaseous nitrous acid detection devices and can also be used for related research on the reaction mechanism of gaseous nitrous acid in the atmosphere.

[0018] (3) The gaseous nitrous acid generation system provided by the present invention outputs dilution gases with different flow rates through the set dilution pipeline, so that gaseous nitrous acid with different concentrations and humidities can be obtained according to needs, improving the application range of this generation system.

[0019] (4) The concentration of gaseous nitrous acid generated by the present invention is related to the heating temperature in the tubular furnace, the temperature and humidity in the constant-temperature gas mixing chamber, and the flow rate set by the mass flowmeter, etc. After adopting more precise temperature, humidity and flow control, a more stable HONO output concentration can be obtained, so as to improve the stability of this gaseous nitrous acid generation system. Description of the Drawings

[0020] Figure 1 Shows the influence of heating temperature on the generation concentration of HONO in the preparation method of Example 1 of the present invention;

[0021] Figure 2 Shows the influence of humidity on the generation concentration of HONO in the preparation method of Example 1 of the present invention;

[0022] Figure 3 Shows the influence of the average concentration of NOx on the generation concentration of HONO in the preparation method of Example 1 of the present invention;

[0023] Figure 4 Shows the structural schematic diagram of the discharge-type gaseous nitrous acid generation system of Example 2 of the present invention;

[0024] Figure 5 Is the data image obtained by continuously monitoring the nitrous acid gas generated by Figure 4 the described discharge-type gaseous nitrous acid generation system using the long optical path absorption photometry (LOPAP). The time resolution of the data is 5 s; in the figure, the x-axis is time and the y-axis represents the volume concentration of gaseous nitrous acid, with the unit of ppb;

[0025] Figure 6 For Figure 5 the histogram of data after 4:00

[0026] Figure 7 is the NO of Embodiment 2 of the present invention 2 structural schematic diagram of the gas cylinder type gaseous nitrous acid generation system

[0027] Figure 8 The present invention adopts the long optical path absorption photometry (LOPAP) for Figure 7 the NO 2 data image obtained by continuously monitoring the nitrous acid gas generated by the gas cylinder type gaseous nitrous acid generation system, the time resolution of the data is 5 s; in the figure, the x-axis is time, and the y-axis represents the volume concentration of gaseous nitrous acid, with the unit of ppb

[0028] Figure 9 is the structural schematic diagram of the NO gas cylinder type gaseous nitrous acid generation system of Embodiment 2 of the present invention

[0029] In the figure: 1, zero air source; 2, four-way valve; 3, first mass flowmeter; 4, discharge device; 5, second mass flowmeter; 6, humidity generating device; 7, constant temperature gas mixing chamber; 8, third mass flowmeter; 9, heating device; 10, first three-way joint; 11, NO 2 source; 12, NO source; 13, fourth mass flowmeter; 14, ultraviolet light ozone generator; 15, second three-way joint Specific embodiments

[0030] The present invention will be further described below in conjunction with the drawings and embodiments

[0031] Embodiment 1

[0032] In a typical embodiment of the present invention, a method for preparing gaseous nitrous acid is proposed, including: placing nitrogen dioxide and water vapor under high temperature conditions to generate gaseous nitrous acid, wherein, under high temperature conditions, part of the nitrogen dioxide is pyrolyzed to generate nitric oxide, and gaseous nitrous acid is generated by the chemical equilibrium reaction of nitrogen dioxide, nitric oxide and water vapor under the high temperature conditions

[0033] Further, the high temperature conditions are a high temperature environment with a temperature ≥ 150 °C. Preferably, the temperature under the high temperature conditions is the temperature when the concentration of nitric oxide generated after partial pyrolysis of nitrogen dioxide is approximately equal to the concentration of nitrogen dioxide. At this temperature, the yield of the generated gaseous nitrous acid is the highest, which is the most suitable temperature for generating gaseous nitrous acid, and the most suitable temperature corresponding to different concentrations of nitrogen dioxide is different

[0034] Further, after introducing water vapor, the humidity condition for the reaction between nitrogen dioxide and nitric oxide is >0% RH. The greater the humidity, the higher the yield of HONO, and the smaller the impact of its change on the concentration of HONO generated by the HONO generation device. Considering that excessive humidity easily leads to water vapor condensation, the optimal humidity condition for use is 70% RH - 90% RH. Preferably, the humidity condition is 80% RH.

[0035] In this embodiment, the nitrogen dioxide is sourced from any one of a nitrogen dioxide storage gas cylinder, nitrogen dioxide generated by the discharge reaction of a zero air source, or nitrogen dioxide generated by the reaction of nitric oxide with ozone.

[0036] According to the above preparation method, the following experiments were conducted:

[0037] NO 2 standard gas was used as the NO 2 source. A tube furnace was used as the heating device, and the dry zero air (0.5% RH) generated by a zero air generator as the zero air source was mixed with the NO 2 gas in the standard gas cylinder of NO 2 through a humidity generation device and then introduced into the tube furnace for heating to generate HONO gas, which was then diluted with zero air according to the required concentration and humidity.

[0038] In this experiment, the factors affecting the final concentration of HONO generated were the NO 2 concentration, humidity, and dilution flow rate of the mixed gas before entering the tube furnace, the diameter, wall thickness, length of the heating tube in the tube furnace, the flow rate of the gas inside the tube, and the effective heating length of the tube furnace, etc.

[0039] The fixed experimental conditions in this experiment were:

[0040] 1. The heating tube in the tube furnace was a stainless steel tube with a diameter of 20 mm, a wall thickness of 1.5 mm, and a length of 500 mm, and the effective heating length in the tube furnace was 300 mm;

[0041] 2. The flow rate of the zero air dilution gas was 2000 sccm;

[0042] 3. The temperature of the constant temperature gas mixing chamber (MIX) was 40 °C.

[0043] By changing the temperature of the tube furnace, the flow rate (concentration) of NO 2 , and the humidity of the gas generated by the humidity generation device, the following experimental results were obtained:

[0044] (1) Under the following experimental conditions: the intake air volume of the humidity generation device was 500 sccm of zero air, and 10 μL / min of pure water was added to it to keep the humidity in the constant temperature gas mixing chamber stable at about 80%, NO 2The intake air volume is 10 sccm (NO 2 cylinder concentration is 3.36 ppm) as an example (the optimal temperature corresponding to different concentrations is different).

[0045] The influence of different heating temperatures on the generation of HONO is as Figure 1 shown. It can be known from Figure 1 that when the NO concentration ≈ NO 2 concentration, the generation concentration of HONO is the largest. At this time, the temperature is about 515 °C. This temperature is the optimal temperature for the generation of HONO when the HONO generating device passes through this NOx concentration. The optimal temperature for the generation of HONO under different NOx concentrations can be judged through similar experiments.

[0046] (2) Under the following experimental conditions: NO 2 gas volume is 10 sccm NO 2 (NO 2 cylinder concentration is 7.62 ppm) is mixed with 1000 sccm of zero air with different humidities, heated by a stable temperature tubular furnace, diluted by 2000 sccm of zero air, and the data detected by LOPAP. During this process, the readings of each item of the NOx detector are stable.

[0047] The zero air with different humidities is generated by adding a small amount of pure water to the humidity generating device, and the data is obtained by monitoring the constant temperature gas mixing chamber with a dew point meter. The concentration change of HONO under different humidity conditions is as Figure 2 shown. The data is fitted with a logarithmic function to obtain the fitting formula: y = 1.2284ln(x) + 3.2928, which has a good correlation. The greater the humidity, the smaller the influence of its change on the HONO concentration generated by the HONO generating device. Considering that too high humidity is likely to cause water vapor condensation, the humidity condition used ~80% RH is the best.

[0048] (3) Under the following experimental conditions: the intake air volume of the humidity generating device is 500 sccm of zero air, and 10 μL / min of pure water is added to it to keep the humidity in the constant temperature gas mixing chamber stable at about 80%. Change the NO 2 intake air volume (NO 2 cylinder concentration is 3.36 ppm) to 0 sccm, 5 sccm, 10 sccm, 15 sccm, 20 sccm respectively, and at the same time adjust the optimal heating temperature at this NO 2 concentration. Further, 5 sccm, 10 sccm, 15 sccm, 20 sccm NO 2After being detected by a NOx detector after passing through the gaseous nitrous acid generation system of the present invention, the average NOx concentrations are 6.17 ppb, 13.38 ppb, 20.74 ppb, and 28.00 ppb respectively. The corresponding optimum temperatures measured through the above temperature influence experiment are 475 °C, 515 °C, 545 °C, and 580 °C respectively. The scatter plot obtained from the average NOx concentration and the corresponding average HONO concentration is as shown in Figure 3 shown. Linear fitting is performed on the data to obtain the fitting formula: y = 0.2004x + 0.2022, where y is the HONO concentration and x is the NOx concentration, and the correlation is good. This can be used as a quantitative curve. That is to say, in the absence of LOPAP, the HONO concentration can also be indirectly quantified by the NOx concentration of the produced gas detected by a NOx detector.

[0049] This quantification method takes the NO 2 standard gas as an example of the NO 2 source for elaboration and is equally applicable to the HONO preparation methods of other NO 2 sources.

[0050] Example 2

[0051] In a typical implementation manner of the present invention, a gaseous nitrous acid generation system is proposed for implementing the gaseous nitrous acid preparation method described in Example 1, including a zero air source. The zero air source is connected to a humidity generation device through a pipe. After nitrogen dioxide and the water vapor generated by the humidity generation device are mixed, they enter a heating device for heating to generate gaseous nitrous acid. Zero air can be used to dilute and adjust the concentration and humidity of the generated gaseous nitrous acid.

[0052] In this embodiment, the nitrogen dioxide can be sourced from any one of a nitrogen dioxide storage gas cylinder, nitrogen dioxide generated by the discharge reaction of a zero air source, or nitrogen dioxide generated by the reaction of nitric oxide and ozone.

[0053] When nitrogen dioxide is generated by the discharge of a zero air source, the structure of the generation system is as shown in Figure 4 shown. The present invention refers to the system with this structure as a discharge-type gaseous nitrous acid generation system, including a zero air source 1. The zero air source 1 is respectively connected to three pipelines. Among them, the zero air source 1 is connected to a discharge device 4 through pipeline I, and the zero air source is connected to a humidity generation device 6 through pipeline II; the gas generated by pipeline I and the gas generated by pipeline II enter a constant temperature gas mixing chamber 7 for mixing, and the mixed gas enters a heating device 9 for heating. The heated gas is diluted by the zero air from pipeline III, and gaseous nitrous acid with different concentrations is obtained by changing the added amount of zero air.

[0054] Specifically, the zero air source 1 is connected to three pipelines respectively through a four-way valve 2. The four-way valve is an electromagnetic four-way valve, which automatically opens when powered on. After opening, the first pipeline, the second pipeline and the third pipeline are ventilated, and the connected pipelines are automatically closed when powered off.

[0055] Further, a first mass flowmeter 3 is provided on the pipeline I, a second mass flowmeter 5 is provided on the pipeline II, and a third mass flowmeter 8 is provided on the pipeline III. The zero air flow on each pipeline can be controlled by the provided flowmeters.

[0056] Further, the discharge device 4 is a device that can generate nitrogen dioxide with a stable concentration by discharging.

[0057] Figure 4 The working principle of the generation system in [reference] is as follows: Zero air is used as the gaseous precursor substance for generating gaseous nitrous acid. The zero air is divided into three paths by the four-way valve 2 and then enters the discharge device 4 through the pipeline I after being controlled by the mass flowmeter. The zero air enters the humidity generation device 6 through the pipeline II. The gases generated in the pipeline I and the second pipeline II enter the constant-temperature gas mixing chamber 7. The stable humidity zero gas generated by the humidity generation device 6 is mixed with the nitrogen dioxide gas generated by the discharge device 4 and then enters the heating device 9 for heating. The gas discharged later is the nitrous acid gas. The zero air in the third pipeline is used as the dilution gas after being controlled by the third mass flowmeter 8 and is mixed with the gas output by the heating device 9 in the tee joint 10. By controlling the third mass flowmeter 8 to change the flow rate under the condition that the above structural condition parameters remain unchanged, the output concentration of the nitrous acid gas can be changed. The discharge of the discharge device causes the nitrogen and oxygen in the zero air to react to produce nitric oxide and ozone. Ozone and oxygen cause nitric oxide to rapidly turn into nitrogen dioxide. The nitrogen dioxide is mixed with water vapor and then enters the heating device. The high-temperature environment in the heating device causes part of the nitrogen dioxide to thermally decompose to produce nitric oxide, and nitrous acid is generated by using the balance among nitrogen dioxide, nitric oxide and water vapor. The advantage of the above generation system is that only one zero air source is required as the raw material for producing HONO.

[0058] The following uses a specific embodiment to illustrate the discharge-type gaseous nitrous acid generation system:

[0059] In this embodiment, the system adopted is the present invention Figure 4In the described discharge-type gaseous nitrous acid generation system, zero air is output from a 40L compressed air standard cylinder with a purity of 99.999%. The flow rate through the discharge device is 50 sccm. The average concentration of NOx in the produced gas detected by the NOx monitor is 45.9 ppb (NOx concentration after system dilution); the zero air flow rate in the pipeline where the humidity generation device is located is 50 sccm (the humidity generation device in this embodiment is not started), and the humidity of the gas generated by the humidity generation device is the humidity of the zero air itself, which is 0.8% RH (relative humidity); the temperature of the constant-temperature gas mixing chamber is set to 30 °C, and at the same time, the mass flowmeter output of the dilution pipeline is controlled so that the dilution gas flow rate is 3000 sccm. The gas generated by the above-mentioned gaseous nitrous acid generation system is detected by LOPAP (long optical path absorption photometry), as Figure 5 shown. In addition, the data after 4:00 in Figure 5 is made into a histogram as Figure 6 shown. The stability analysis of the data after 4:00 in Figure 6 is carried out, including mean, standard deviation, range, and mean SE, as shown in Table 1 below:

[0060] Total N Mean Standard Deviation Mean SE Range 3753 3.40952 0.06023 9.83E-04 0.3988

[0061] It can be seen from Figure 5 and Figure 6 and Table 1 that the discharge-type gaseous nitrous acid generation system described in the present invention can generate relatively stable nitrous acid gas.

[0062] The HONO concentration produced by the gaseous nitrous acid generation system provided in this embodiment is related to the heating temperature in the above-mentioned tubular furnace, the temperature and humidity in the constant-temperature gas mixing chamber, and the flow rate set by the mass flowmeter. The concentration fluctuations shown in the above figure are mainly caused by temperature fluctuations and flow rate fluctuations. After adopting more precise temperature and flow rate control, a more stable HONO output concentration can be obtained, so as to improve the stability of the gaseous nitrous acid generation system.

[0063] When a nitrogen dioxide storage cylinder is used as the nitrogen dioxide source, the structure of the generation system is as Figure 7 shown. The present invention refers to the system with this structure as NO 2 cylinder-type gaseous nitrous acid generation system, including an air source 1 and a NO 2 source 11, NO 2The source 11 is connected to the constant-temperature gas mixing chamber 7 through pipeline Ⅰ, and a first mass flowmeter 3 is provided on pipeline Ⅰ. The zero-air source 1 is connected to the humidity generating device 6 through pipeline Ⅱ. Pipeline Ⅰ and pipeline Ⅱ are connected to the inlet of the constant-temperature gas mixing chamber 7. The outlet of the constant-temperature gas mixing chamber 7 is connected to the heating device 9. The zero-air source 1 is also connected to pipeline Ⅲ, and pipeline Ⅲ is connected to the first three-way joint 10. A second mass flowmeter 5 and a third mass flowmeter 8 are respectively provided on pipeline Ⅱ and pipeline Ⅲ. The zero-air source is connected to pipeline Ⅱ and pipeline Ⅲ respectively through the second three-way joint 15.

[0064] Figure 7 The working principle of the generation system in

[0065] NO 2 After the flow rate is controlled by the mass flowmeter, it enters the constant-temperature gas mixing chamber 7 through pipeline Ⅰ. Zero air enters the humidity generating device 6 through pipeline Ⅱ. The water vapor in pipeline Ⅱ enters the constant-temperature gas mixing chamber 7. The stable humidity zero air generated by the humidity generating device 6 is mixed with the nitrogen dioxide gas in the nitrogen dioxide storage cylinder and enters the heating device 9 for heating. The gas discharged later is the nitrous acid gas. The zero air in the third pipeline is controlled by the third mass flowmeter 8 and used as the dilution gas to be mixed with the gas output by the heating device 9 in the three-way joint 10. Under the condition that the above structural condition parameters remain unchanged, controlling the third mass flowmeter 8 to change the flow rate can change the output concentration of the nitrous acid gas. After nitrogen dioxide is mixed with water vapor, it enters the heating device. The high-temperature environment in the heating device causes part of the nitrogen dioxide to pyrolyze to generate nitric oxide, and nitrous acid is generated by using the balance among nitrogen dioxide, nitric oxide, and water vapor.

[0066] The advantage of the above generation system is that the NO 2 in the NO 2 gas cylinder has a stable concentration, so that the generated HONO is more stable.

[0067] The following takes a specific embodiment to illustrate the NO 2 gas cylinder type gaseous nitrous acid generation system:

[0068] The humidity condition is stable at about 80% RH, the temperature of the high-temperature condition is 475 °C, and the NO 2 intake volume (the NO 2 gas cylinder concentration is 3.36 ppm) is 5 sccm.

[0069] Figure 8 After the humidity and temperature conditions are stable, 5 sccm of NO 2 gas is introduced. Through the data image obtained by LOPAP, it can be seen from the following figure that the stable time of the HONO generation device is about 30 min. The data stability analysis after 30 min is as

[0070] As shown in Table 2, it can be reflected that this preparation method can ensure the stability of HONO generation.

[0071] Mean Standard Deviation Mean SE Minimum Median Maximum Range 1.43046 0.01087 5.14E-04 1.4036 1.4316 1.4553 0.0517

[0072] When nitrogen monoxide reacts with ozone to produce nitrogen dioxide, the structure of the generating device is as Figure 9 shown. The present invention refers to the system with this structure as a NO gas cylinder type gaseous nitrous acid generating system, which includes a NO source 12 and a zero air source 1. The NO source 12 is connected to a constant temperature gas mixing chamber 7 through pipeline Ⅰ, and a first mass flowmeter 3 is arranged on pipeline Ⅰ. The zero air source 1 is connected to an ultraviolet light ozone generator 14 through pipeline Ⅱ, and the zero air source 1 is connected to a humidity generating device 6 through pipeline Ⅲ. Pipeline Ⅰ, pipeline Ⅱ, and pipeline Ⅲ are all connected to the inlet of the constant temperature gas mixing chamber 7. The outlet of the constant temperature gas mixing chamber 7 is connected to a heating device 9. The zero air source 1 is also connected to pipeline Ⅳ, and a second mass flowmeter 5, a third mass flowmeter 8, and a fourth mass flowmeter 13 are respectively arranged on pipeline Ⅱ, pipeline Ⅲ, and pipeline Ⅳ. The zero air source is connected to pipeline Ⅱ, pipeline Ⅲ, and pipeline Ⅳ through a four-way valve 2 respectively.

[0073] Figure 9 The working principle of the generating system in

[0074] is as follows: 2 NO enters the constant temperature gas mixing chamber 7 through pipeline Ⅰ after the flow rate is controlled by the mass flowmeter. Zero air enters the ultraviolet light ozone generator 14 through pipeline Ⅱ to generate ozone, and ozone and oxygen rapidly convert nitrogen monoxide into NO 2 , and NO 2 enters the constant temperature gas mixing chamber; zero air enters the humidity generating device 6 through pipeline Ⅲ, and the NO 2 generated by pipeline Ⅰ and pipeline Ⅱ and the water vapor in pipeline Ⅲ enter the constant temperature gas mixing chamber 7 to be mixed, and then enter the heating device 9 for heating. The gas discharged later is nitrous acid gas. The zero air in pipeline Ⅳ is used as a dilution gas after being controlled by the fourth mass flowmeter 13 and is mixed with the gas output by the heating device 9 in a tee joint 10. By controlling the fourth mass flowmeter 13 to change the flow rate under the condition that the above structure condition parameters remain unchanged, the output concentration of nitrous acid gas can be changed. Ozone and oxygen rapidly convert nitrogen monoxide into nitrogen dioxide, and nitrogen dioxide is mixed with water vapor and then enters the heating device. The high-temperature environment in the heating device causes part of the nitrogen dioxide to thermally decompose to produce nitrogen monoxide, and nitrous acid is generated by using the balance among nitrogen dioxide, nitrogen monoxide, and water vapor. The advantage of the above generating system is that NO gas can use a filter tube with an alkaline filter material to remove other acidic impurities in the NO gas cylinder. Since the principle of this generating system is similar to that of the above-mentioned discharge type gaseous nitrous acid generating system, no specific embodiments are provided for elaboration.

[0075] In this embodiment, the humidity generating device 6 is a device capable of continuously generating a stable humidity gas.

[0076] In this embodiment, the nitrogen dioxide and water vapor are mixed in a constant temperature gas mixing chamber. The constant temperature gas mixing chamber 7 is a metal reaction chamber, and a heating device, a temperature control device and a temperature and humidity sensor are arranged in the metal reaction chamber. The temperature in the chamber is maintained above 30 °C, and the stability is ±1 °C. The function of the constant temperature gas mixing chamber is to make the gas mixing more uniform, and the condensation of water vapor can be reduced by heating to maintain a constant temperature. In this embodiment, the heating device 9 is a tube furnace. The function of the tube furnace is to create a high temperature environment for the pyrolysis of impurities such as ozone and nitric acid. A metal tube or a quartz tube for introducing gas is arranged in the tube furnace, and the tube furnace heats the metal tube or the quartz tube. The temperature of the tube furnace is controlled above 150 °C, and the temperature stability of the gas in the metal tube or the quartz tube is ±1 °C. Further, the metal tube or the quartz tube is wrapped with a heat insulating material to insulate the gas flowing through the tube.

[0077] In the gaseous nitrous acid generation system provided in this embodiment, the discharge device, the humidity generating device, the constant temperature gas mixing chamber and the tube furnace used are all existing devices. The connecting pipelines between the components are connected in sequence as shown by the arrows in Figure 4 , 7 , 9. The connecting pipe and the joint are made of metal or Teflon.

[0078] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation to the protection scope of the present invention. Those skilled in the art should understand that on the basis of the technical solution of the present invention, various modifications or deformations that can be made by those skilled in the art without creative labor are still within the protection scope of the present invention.

Claims

1. A method for preparing gaseous nitrous acid, characterized in that: include: Placing nitrogen dioxide and water vapor under high temperature conditions to produce gaseous nitrous acid, wherein under the high temperature conditions, part of the nitrogen dioxide is thermally decomposed to produce nitric oxide, and the gaseous nitrous acid is produced by a chemical equilibrium reaction among nitrogen dioxide, nitric oxide and water vapor under the high temperature conditions; The high temperature condition is a high temperature environment with a temperature ≥ 150°C; The concentration of nitrogen monoxide or nitrogen dioxide in the gas after pyrolysis under the high temperature conditions accounts for 40-60% of the sum of the concentrations of the two; After the water vapor is introduced, the humidity condition for the reaction of nitrogen dioxide and nitric oxide is 70-90% RH; The gaseous nitrous acid generation system is used to realize the gaseous nitrous acid preparation method, comprising a zero air source, wherein the zero air source is connected to three pipelines respectively, wherein the zero air source is connected to the discharge device through pipeline I, and the zero air source is connected to the humidity generating device through pipeline II; the gas generated by pipeline I and the gas generated by pipeline II enter the constant temperature gas mixing chamber for mixing, and after mixing, enter the heating device for heating, and the heated gas is diluted by the zero air from pipeline III, and gaseous nitrous acid of different concentrations is obtained by changing the amount of zero air added; It can continuously and stably produce trace gaseous nitrous acid with a volume concentration of ppb to ppm; Discharge through the discharge device causes the nitrogen and oxygen in the zero air to react to produce nitric oxide and ozone. Ozone and oxygen rapidly convert nitric oxide into nitrogen dioxide, which serves as a source of nitrogen dioxide.

2. The method for preparing gaseous nitrous acid as claimed in claim 1, characterized in that: The nitrogen dioxide source zero air source generates nitrogen dioxide through a discharge reaction.

3. The method for preparing gaseous nitrous acid as claimed in claim 1, characterized in that: The constant temperature gas mixing chamber adopts a metal reaction chamber, and the temperature stability in the chamber is ±1°C.

4. The method for preparing gaseous nitrous acid as claimed in claim 1, characterized in that: The heating device adopts a tubular furnace, in which a metal tube or a quartz tube for passing gas is arranged. The tubular furnace heats the metal tube or the quartz tube, and the temperature stability of the gas in the metal tube or the quartz tube is ±1°C.

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