CN110411973B - Method for detecting concentration of non-methane total hydrocarbons in gas - Google Patents
Method for detecting concentration of non-methane total hydrocarbons in gas Download PDFInfo
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- CN110411973B CN110411973B CN201910812908.5A CN201910812908A CN110411973B CN 110411973 B CN110411973 B CN 110411973B CN 201910812908 A CN201910812908 A CN 201910812908A CN 110411973 B CN110411973 B CN 110411973B
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
Abstract
The invention discloses a method for detecting the concentration of non-methane total hydrocarbons in gas. The method comprises the following steps: the method comprises the following steps of enabling a gas to be detected to pass through a medium-temperature selective catalytic oxidation column, carrying out catalytic oxidation at 260-380 ℃, detecting the concentration of carbon dioxide in the gas after catalytic oxidation by using an NDIR instrument, and obtaining the concentration of NMHC in the gas to be detected through conversion; the composition of the catalyst loaded in the selective catalytic oxidation column was as follows: comprises a carrier and an active metal loaded on the carrier; the carrier is a molecular sieve; the active metal is palladium; the mass content of active metal in the catalyst is 0.1-10%. The invention detects non-methane total hydrocarbons based on the catalytic oxidation-NDIR technology, and the selective catalytic oxidation of methane-ethane mixed gas is used as a model reaction (ethane is the most difficult to be oxidized in the non-methane total hydrocarbons), so that the adopted catalyst has the very high-efficiency capability of selectively converting NMHC (such as ethane) without converting methane, and is completely suitable for application in the detection of the non-methane total hydrocarbons.
Description
Technical Field
The invention relates to a method for detecting the concentration of non-methane total hydrocarbons in gas, belonging to the field of gas monitoring.
Background
In the field of environmental monitoring, Total Volatile Organic Compounds (TVOCs) are defined by the world health organization (WHO, 1989) as: the total name of the volatile organic compounds with the melting point lower than room temperature and the boiling point between 50 ℃ and 260 ℃. Non-Methane hydrocarbons (NMHC) generally refer to a generic term for hydrocarbons other than Methane (mainly C2-C8), and include Hydrocarbon derivatives such as alcohols, aldehydes, acids, esters, and ketones, and C, in addition to hydrocarbons8The above volatile organic substances. Total Volatile Organic Compounds (TVOCs) and non-methane total hydrocarbons (NMHC) are two different concepts, defined as total volatile organic compounds covering a range greater than non-methane total hydrocarbons, i.e. with or without a difference in methane concentration. At present, NMHC is a main pollution degree index of environmental air and industrial factory source exhaust gas, the approximate condition of the concentration of the polluted gas can be obtained through the concentration value of the NMHC, and the rapid, simple and convenient measurement index is provided for environmental monitoring and environmental law enforcement. As the emission of organic pollutants in the atmosphere is gradually and widely concerned by governments, society and the public, total volatile organic compounds and non-volatile organic compoundsThe sensitivity, convenience, safety and other properties of monitoring instruments for methane total hydrocarbons are always the focus of attention of environmental protection departments and manufacturers of monitoring instruments. At present, among the detection and analysis methods for non-methane total hydrocarbons, the analysis methods commonly used include gas chromatography, fluorescence photometry, capillary electrophoresis, and the like. Among them, the method of separating methane and NMHC by using a chromatographic column and adding a FID detector (GC + FID) is a non-methane total hydrocarbon monitoring means widely used in China because of the high sensitivity and linear width of a hydrogen Flame Ion Detector (FID) to hydrocarbon organic matters. However, the GC + FID method has many defects such as high-purity hydrogen safety problem, environmental gas interference, detection concentration error caused by low response value of organic matters containing unsaturated bonds or heteroatoms caused by 'equal carbon effect', incapability of continuous sampling analysis and the like, and the application of the method in NMHC monitoring in different pollution emission sources is severely restricted.
The prior art discloses a method for detecting non-methane total hydrocarbons by using a catalytic method, but the method uses FID to detect methane, converts TVOCs into methane by using a catalytic technology, and then uses FID to detect the generated methane to obtain the concentration of NMHC.
The monitoring technology of non-methane total hydrocarbons (NMHC) based on the catalytic oxidation-NDIR technology is to utilize a high-selectivity and high-stability catalyst to catalytically oxidize the NMHC into carbon dioxide and water within a certain temperature interval, and then measure the concentration of the generated carbon dioxide through the currently mature non-linear infrared (NDIR) so as to calculate the concentration of the NMHC. The use of NDIR detector technology has several advantages: the volume is small, and the carrying is convenient; has high safety and does not contain H in FID2Potential safety hazards brought by assistance; the economy is high, and compared with an FID detector, the NDIR technology can greatly reduce the detection cost; the detection process is rapid, compared with the defect that the separation time of a chromatographic column in a GC + FID method is as long as several minutes, the catalytic oxidation NDIR technology can be usedMonitoring in real time; the measurement sensitivity is high, and the current mature NDIR sensor detects CO2The sensitivity of the concentration can reach 1ppm (0.536 mg/m)3In terms of C) the following; the target organic matters in the NMHC can be completely converted into CO by the catalytic oxidation technology2The method effectively avoids the detection concentration error caused by the response difference of the FID/PID to different types of organic matters, and has excellent full-spectrum detection capability.
The key core technology of the catalytic oxidation-NDIR technology lies in the selection determination and the synthetic preparation of catalytic materials. Whether the catalytic material can meet corresponding requirements generally needs to meet the following conditions: (1) high catalytic selectivity and complete oxidation of NMHC to CO2While retaining methane and having a wide temperature operating window; (2) the stability is good, and the catalyst can still maintain high catalytic activity and selectivity after long-term use; (3) the anti-environmental interference capability is strong, and the gas with different concentrations and different components can be maintained to convert NMHC into CO in corresponding temperature intervals2Catalytic oxidation ability of (1).
For the catalytic oxidation-NDIR technique used for NMHC testing, the choice of catalytic oxidation materials in NMHC testing has certain challenges, such as selectively achieving complete catalytic oxidation of NMHC. Although methane is the most stable hydrocarbon, NMHC is completely oxidized to CO in a certain temperature range when the organic matter contains ethane, propane and the like with similar chemical properties2While retaining methane so that it is not converted is a key and challenge of the technology. Therefore, if a class of catalytic materials were developed that could possess the ability to fully oxidize NMHC while retaining methane that was not oxidized, then it could be applied to selective oxidation and detection analysis of NMHC as a catalytic material.
Disclosure of Invention
The method realizes the detection of the non-methane total hydrocarbon based on a catalytic oxidation-NDIR technology, can completely convert the non-methane total hydrocarbon into carbon dioxide within a certain temperature interval, selectively and completely does not convert the methane, and avoids the interference of the methane on the NMHC detection; the carbon dioxide concentration is then measured using currently established non-dispersive infrared (NDIR) techniques to convert to mass concentrations of non-methane total hydrocarbons.
Specifically, the method for detecting the concentration of non-methane total hydrocarbons in the gas provided by the invention comprises the following steps:
the method comprises the following steps of enabling a gas to be detected to pass through a medium-temperature selective catalytic oxidation column, carrying out catalytic oxidation at 260-380 ℃, detecting the concentration of carbon dioxide in the gas after catalytic oxidation by using an NDIR sensor, and obtaining the concentration of NMHC in the gas to be detected through conversion;
the catalyst loaded in the medium-temperature selective catalytic oxidation column comprises the following components:
comprises a carrier and an active metal loaded on the carrier;
the carrier is a molecular sieve;
the active metal is palladium;
in the catalyst, the mass content of the active metal is 0.1-10%, preferably 0.2-1% or 0.5%.
In the above method, the molecular sieve may be a ZSM series molecular sieve, a Beta molecular sieve, an MOR molecular sieve, a Y type molecular sieve, an a type molecular sieve or an X type molecular sieve, and preferably a ZSM-5 molecular sieve in the ZSM series molecular sieve.
In the above process, the catalyst further comprises an auxiliary metal component supported on the molecular sieve;
the auxiliary metal is at least one of potassium, sodium, manganese, iron, cobalt, nickel, cerium, lanthanum, chromium, vanadium and tin.
In the method, the silica-alumina ratio of the molecular sieve can be 20-80, preferably 25-50, 25 or 50, the size is 100 nm-10 μm, such as micron-sized molecular sieve with the size of about 1-10 μm and nanometer-sized molecular sieve with the size of about 100-1000 nm.
In the method, when molecular sieves with different silicon-aluminum ratios are used as carriers, and the active metal is palladium, the mass content of the palladium is 0.2-1% (preferably 0.5%), the catalytic oxidation temperatures applicable to the catalyst are different, and specifically the following are adopted:
1) when the molecular sieve is a nano-scale ZSM-5 molecular sieve with the silicon-aluminum ratio of 25, the temperature of catalytic oxidation is 260-360 ℃;
2) when the molecular sieve is a micron-sized ZSM-5 molecular sieve with the silica-alumina ratio of 25, the temperature of catalytic oxidation is 350-370 ℃;
3) when the molecular sieve is a micron-sized ZSM-5 molecular sieve with the silica-alumina ratio of 50, the temperature of catalytic oxidation is 300-340 ℃;
4) when the molecular sieve is a micron MOR molecular sieve with the silicon-aluminum ratio of 25, the temperature of catalytic oxidation is 340-360 ℃.
In the method, the mass concentration of NMHC in the gas to be detected is 1-1500 mg/m3。
In the above method, the carbon dioxide (i.e. background concentration) in the gas to be measured needs to be subtracted, and then conversion is performed; can be determined by the following method: and directly detecting the concentration of the carbon dioxide in the gas to be detected by adopting NDIR.
The catalyst used in the process of the invention can be prepared by conventional methods, such as: an isometric impregnation method, a vacuum impregnation method, an ion exchange method, a liquid phase reduction loading method and the like.
The method of the present invention can be used to detect the concentration of carbon dioxide using existing NDIR instruments.
The invention detects non-methane total hydrocarbons based on the catalytic oxidation-NDIR technology, and the selective catalytic oxidation of methane-ethane mixed gas is used as a model reaction (ethane is the most difficult to be oxidized in the non-methane total hydrocarbons), so that the adopted catalyst has the very high-efficiency capability of selectively converting NMHC (such as ethane) without converting methane, and is completely suitable for application in the detection of the non-methane total hydrocarbons. In order to further verify the selective catalytic oxidation performance of the catalytic conversion material, other typical non-methane total hydrocarbons such as propane, benzene, toluene and ethyl acetate are added to represent pollutants, and the catalytic conversion material also has very excellent catalytic oxidation activity.
Drawings
FIG. 1 is a graph showing the activity of the micron Pd/ZSM-5(50) catalyst material in the catalytic oxidation of methane and ethane in example 1 of the present invention.
FIG. 2 is a graph showing the activity of the nano-sized Pd/ZSM-5(25) catalyst material in the catalytic oxidation of methane and ethane in example 2 of the present invention.
FIG. 3 is a graph showing the activity of the micron Pd/ZSM-5(25) catalyst material in the catalytic oxidation of methane and ethane in example 3 of the present invention.
FIG. 4 is a graph showing the activity of the micron-sized Pd/MOR (25) catalytic material in example 4 of the present invention for catalytic oxidation of methane and ethane.
FIG. 5 is a graph showing the activity of the micron Pd/ZSM-5(200) catalytic material of comparative example 1 of the present invention in catalytic oxidation of methane and ethane.
Fig. 6 is a flow chart of a method of measuring NMHC concentration based on catalytic oxidation-NDIR technique according to the present invention.
Detailed Description
The experimental procedures used in the following examples are all conventional procedures unless otherwise specified.
Materials, reagents and the like used in the following examples are commercially available unless otherwise specified.
The catalysts used in the following examples were prepared as follows:
under the condition of room temperature, 0.1g of palladium acetylacetonate is dissolved in 1mL of trioctylphosphine, the solution gradually changes from yellow to orange under the atmosphere of nitrogen protection, then the solution is dripped into 9mL of trioctylphosphine and stirred, the temperature is gradually increased to 300 ℃, and the solution is kept for 30min, so that a black colloidal solution is obtained. After cooling to room temperature, ethanol was added for washing, centrifugal separation was performed, washing was repeated several times, and then the mixture was dispersed in 50mL of ethanol solution. Adding a certain amount of molecular sieve carrier, stirring and soaking overnight, evaporating the solvent to dryness, and calcining in a muffle furnace at 500 ℃ for 3 h. Obtaining the Pd/molecular sieve catalytic material with corresponding load.
Example 1: Pd/ZSM-5(50) molecular sieve catalytic material is used for catalytic oxidation reaction of methane-ethane mixed gas at different temperatures
A small-sized fixed bed continuous flow reaction evaluation device is adopted, the silica-alumina ratio of the micron-sized molecular sieve carrier ZSM-5 is 50, and the Pd loading amount is 0.5 wt%. The inner diameter of the reaction tube is 4mm, the reaction gas is a mixed gas of 508ppm methane, 515ppm ethane and 21% oxygen, and the space velocity is 18000 g.ml.h-1. At different temperaturesThe results of the conversion rates of methane and ethane under normal pressure are shown in fig. 1, and it can be seen that ethane is completely oxidized into carbon dioxide while methane remains unchanged at a temperature range of 300 to 340 ℃.
Example 2, Pd/ZSM-5-3(25) molecular sieve catalytic material is used for catalytic oxidation reaction of methane-ethane mixed gas at different temperatures
A small-sized fixed bed continuous flow reaction evaluation device is adopted, the silica-alumina ratio of the nano-scale molecular sieve carrier ZSM-5-3 is 25, and the Pd loading amount is 0.5 wt%. The inner diameter of the reaction tube is 4mm, the reaction gas is a mixed gas of 508ppm methane, 515ppm ethane and 21% oxygen, and the space velocity is 18000 g.ml.h-1. The results of the conversion rates of methane and ethane at different temperatures and under normal pressure are shown in fig. 2, and it can be seen that ethane is completely oxidized into carbon dioxide while methane remains unchanged at a temperature range of 260-360 ℃.
Example 3, Pd/ZSM-5(25) molecular sieve catalytic material used for catalytic oxidation reaction of methane-ethane mixed gas at different temperatures
A small-sized fixed bed continuous flow reaction evaluation device is adopted, the silica-alumina ratio of a micron-sized molecular sieve carrier ZSM-5 is 25, the inner diameter of a reaction tube is 4mm, reaction gas is mixed gas of 508ppm of methane, 515ppm of ethane and 21% of oxygen, and the space velocity is 18000 g.ml.h-1. The results of the conversion rates of methane and ethane at different temperatures and under normal pressure are shown in fig. 3, and it can be seen that ethane is completely oxidized into carbon dioxide while methane remains unchanged at the temperature range of 340-350 ℃.
Example 4 Pd/MOR (25) molecular sieve catalytic material used for catalytic oxidation reaction of methane and ethane mixed gas at different temperatures
A small-sized fixed bed continuous flow reaction evaluation device is adopted, the silicon-aluminum ratio of a micron-sized molecular sieve carrier MOR is 25, the inner diameter of a reaction tube is 4mm, reaction gas is mixed gas of 508ppm methane, 515ppm ethane and 21% oxygen, and the space velocity is 18000 g/ml/h-1. The results of the conversion rates of methane and ethane at different temperatures and under normal pressure are shown in fig. 4, and it can be seen that ethane is completely oxidized into carbon dioxide within the temperature range of 340-360 DEG CWhile methane remains unchanged.
Comparative example 1, Pd/ZSM-5(200) molecular sieve catalytic material is used for catalytic oxidation reaction of methane-ethane mixed gas at different temperatures
A small-sized fixed bed continuous flow reaction evaluation device is adopted, the silica-alumina ratio of a micron-sized molecular sieve carrier ZSM-5 is 200, the inner diameter of a reaction tube is 4mm, reaction gas is mixed gas of 508ppm of methane, 515ppm of ethane and 21% of oxygen, and the space velocity is 18000 g/ml/h-1. The results of the conversion of methane and ethane at various temperatures and atmospheric conditions are shown in fig. 5, and it can be seen that the catalytic material with the less acidic ZSM-5(200) has no selective oxidation capability for non-methane total hydrocarbons.
Comparing the catalytic oxidation results of examples 1-4 and comparative example 1, it can be seen that the synergistic effect of excellent surface and suitable supported active metal is a key factor for the functioning of the catalyst, since there is a great difference in surface properties, particularly in surface acidity properties, of the molecular sieves of different sizes and different silica to alumina ratios, which can play a very large role in the catalytic oxidation of alkanes.
Comparative example 2, ZSM-5(50) comparison of activity performance of catalytic material with different noble metals on molecular sieve for selective catalytic oxidation reaction of methane and ethane
A small-sized fixed bed continuous flow reaction evaluation device is adopted, a molecular sieve carrier is micron-sized ZSM-5, the silica-alumina ratio is 50, and the noble metal loading is 0.5 wt%. The inner diameter of the reaction tube is 4mm, the reaction gas is a mixed gas of 508ppm methane, 515ppm ethane and 21% oxygen, and the space velocity is 18000 g.ml.h-1. The results of the catalytic oxidation of methane and ethane under atmospheric conditions are shown in table 1 below. Wherein, the selectivity of the carbon dioxide is over 98 percent.
From the data in table 1, it can be seen that only Pd of the noble metals has the ability to selectively catalyze the oxidation of non-methane total hydrocarbons.
TABLE 1 catalytic Oxidation results of different catalytic materials
Example 5 moisture and CO2The influence of the environmental gas on the activity of the Pd/ZSM-5(50) catalytic material is evaluated by adopting a small-sized fixed bed continuous flow reaction evaluation device, wherein a molecular sieve carrier is micron-sized ZSM-5, the silica-alumina ratio is 50, the Pd loading amount is 0.5 wt%, the inner diameter of a reaction tube is 4mm, the concentrations of single-component reaction gas are methane 508ppm and ethane 515ppm, and added H2O concentration is 50% RH, CO2The concentration is 2 percent, and the airspeed is 18000 g-ml.h-1. The results of the catalytic oxidation of methane and ethane under atmospheric conditions are shown in table 2. Wherein, the selectivity of the carbon dioxide is over 98 percent.
From the data in Table 2, it can be seen that CO is common in the ambient atmosphere2And H2O does not have a major effect on the catalyst activity.
TABLE 2 addition of moisture and CO2Result of catalytic oxidation of
Example 6 Effect of typical NMHC materials such as propane, toluene, acetylene, Ethyl acetate, dichloroethane on the Activity of Pd/ZSM-5(50) catalytic Material
A small-sized fixed bed continuous flow reaction evaluation device is adopted, a molecular sieve carrier is ZSM-5, the silica-alumina ratio is 50, and the Pd loading amount is 0.5 wt%. The inner diameter of the reaction tube is 4mm, typical NMHC substances of 200ppm propane, 200ppm toluene, 100ppm acetylene, 200ppm ethyl acetate and 200ppm dichloroethane are respectively added into the methane-ethane combined gas, and the space velocity is 26000h-1. The results of catalytic oxidation of each non-methane total hydrocarbon single component gas under atmospheric conditions are shown in table 3. Wherein, the selectivity of the carbon dioxide is over 98 percent.
From the data in table 3, it can be seen that the catalyst has very excellent selective catalytic activity for several other typical non-methane total hydrocarbons.
TABLE 3 catalytic oxidation results with different NMHC additions
Example 7,
A flow chart of a method for detecting NMHC concentration using non-linear infrared (NDIR) technology is shown in fig. 6, and the specific steps are:
(1) the flow direction of the gas to be detected is controlled by a three-way valve, when the gas passes through the first hollow column, organic matters in the gas to be detected are not changed, then the gas enters an NDIR detector, and the measured CO is2Background CO in concentration of gas2Concentration, subtracted as a blank value;
(2) gas is tangential to a third path of medium-temperature selective catalytic oxidation column through a three-way valve, a catalyst in the catalytic oxidation column is a Pd/ZSM-5(50) catalytic material, all NMHC can be completely oxidized into carbon dioxide within the temperature range of 300-340 ℃, methane can be kept unchanged, and CO measured by an NDIR sensor2The NMHC concentration (calculated as C) can be converted by subtracting the background value from the concentration.
The concentration of NMHC in a mixed gas of 100ppm of methane, 100ppm of ethane and 100ppm of ethyl acetate was measured according to the above-mentioned flow. The catalyst uses micron Pd/ZSM-5(50), and the temperature of the NMHC catalytic column is 340 ℃. The results are shown in table 4:
TABLE 4 detection results of NMHC in the mixed gas
From the above results, it can be seen that the detection of NMHC by catalytic oxidation-nonlinear infrared method has very high accuracy, which is more than 99%.
Claims (6)
1. A method of detecting the concentration of non-methane total hydrocarbons in a gas comprising the steps of:
the method comprises the following steps of enabling a gas to be detected to pass through a medium-temperature selective catalytic oxidation column, carrying out catalytic oxidation at 260-380 ℃, detecting the concentration of carbon dioxide in the gas after catalytic oxidation by using an NDIR instrument, and obtaining the concentration of NMHC in the gas to be detected through conversion;
the catalyst loaded in the medium-temperature selective catalytic oxidation column comprises the following components:
comprises a carrier and an active metal loaded on the carrier;
the carrier is a molecular sieve, and the molecular sieve is a ZSM series molecular sieve, a Beta molecular sieve, an MOR molecular sieve, a Y type molecular sieve, an A type molecular sieve or an X type molecular sieve;
the silicon-aluminum ratio of the molecular sieve is 20-80, and the size of the molecular sieve is 100 nm-10 mu m;
the active metal is palladium;
in the catalyst, the mass content of the active metal is 0.1-10%;
the mass concentration of NMHC in the gas to be detected is 1-1500 mg/m3。
2. The method of claim 1, wherein: the catalyst also comprises an auxiliary metal component loaded on the molecular sieve;
the auxiliary metal is at least one of potassium, sodium, manganese, iron, cobalt, nickel, cerium, lanthanum, chromium, vanadium and tin.
3. The method according to claim 1 or 2, characterized in that: when the molecular sieve is a nano-scale ZSM-5 molecular sieve with the silica-alumina ratio of 25, the temperature of catalytic oxidation is 260-360 ℃.
4. The method according to claim 1 or 2, characterized in that: when the molecular sieve is a micron-sized ZSM-5 molecular sieve with the silica-alumina ratio of 25, the temperature of catalytic oxidation is 350-370 ℃.
5. The method according to claim 1 or 2, characterized in that: when the molecular sieve is a micron-sized ZSM-5 molecular sieve with the silica-alumina ratio of 50, the temperature of catalytic oxidation is 300-340 ℃.
6. The method according to claim 1 or 2, characterized in that: when the molecular sieve is a micron MOR molecular sieve with the silicon-aluminum ratio of 25, the catalytic oxidation temperature is 340-360 ℃.
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