A gas chromatography system for detecting trace formaldehyde in a gas

Abstract

This invention discloses a gas chromatography system for detecting trace amounts of formaldehyde in gases, comprising a six-way switching valve I, a quantitative tube, a chromatographic column I, an FID detector I, a four-way switching valve I, a methane conversion furnace, a six-way switching valve II, an FID detector II, a chromatographic column II, a four-way switching valve II, a damping needle valve, and an oxidation furnace. This invention uses a platinum catalyst to oxidize formaldehyde in the gas to carbon dioxide under aerobic conditions, and then, after deoxygenation, converts it to methane in the methane conversion furnace, resulting in a signal peak on the FID detector. This system allows for direct injection to detect trace amounts of formaldehyde in gases, with a low detection limit, and eliminates the need for absorption or derivatization pretreatment, making it convenient and rapid.

Description

Technical Field

[0001] This utility model belongs to the field of gas chromatography detection, specifically relating to a gas chromatography system for detecting trace amounts of formaldehyde in gases. Background Technology

[0002] When using gas chromatography to detect formaldehyde content in a gas, the FID detector cannot be used for formaldehyde detection because the formaldehyde signal peak is very weak or even non-existent. If a TCD detector is used, formaldehyde can produce a signal peak, but due to sensitivity limitations, the signal peak may still be very small or non-existent at low concentrations.

[0003] Currently, the common method for detecting trace amounts of formaldehyde in gases using chromatography is to first absorb the gas sample with water, then convert the formaldehyde into a derivative compound under certain conditions using 2,4-dinitrophenylhydrazine or acetylacetone, before analyzing it using gas chromatography or liquid chromatography. When using gas chromatography, the high boiling point of the derivative compound leads to a long retention time, thus prolonging the analysis time. While liquid chromatography can achieve better component separation and faster signal peak elicitation, the high cost of the mobile phase makes the analysis too expensive. Both of these methods require gas absorption before analysis, which is a relatively cumbersome process. Utility Model Content

[0004] Based on the problems existing in the prior art, the purpose of this utility model is to provide a convenient, fast, and low detection limit gas chromatography system for detecting trace amounts of formaldehyde in gases, and to avoid pretreatment such as gas absorption and derivatization, so as to achieve efficient detection by direct sample injection.

[0005] The purpose of this utility model is achieved through the following technical solution: a gas chromatography system for detecting trace amounts of formaldehyde in a gas, comprising a six-way switching valve I, a quantitative tube, a chromatographic column I, an FID detector I, a four-way switching valve I, a methane conversion furnace, a six-way switching valve II, an FID detector II, a chromatographic column II, a four-way switching valve II, a damping needle valve, and an oxidation furnace;

[0006] The six-way switching valve I and six-way switching valve II each have six pipeline connection points and two rotary valve positions, namely: the off position is connected to position 1 and position 2, position 3 and position 4, and position 5 and position 6; the on position is connected to position 2 and position 3, position 4 and position 5, and position 6 and position 1.

[0007] The four-way switching valve I and four-way switching valve II each have four pipeline connection points and two rotary valve positions, namely: the off position is connected to position ① and position ②, and position ③ and position ④; the on position is connected to position ② and position ③, and position ④ and position ①.

[0008] Position ① of the six-way switching valve I is connected to the sample outlet pipeline, positions ② and ⑤ are connected to the two ends of the quantitative tube respectively, position ③ is connected to the first nitrogen inlet pipeline, position ④ is connected to the inlet of chromatographic column I through a pipeline, and position ⑥ is connected to the injection port pipeline.

[0009] Position ① of the four-way switching valve I is connected to the outlet of chromatographic column I via a pipeline, position ② is connected to FID detector I via a pipeline, position ③ is connected to the second nitrogen inlet pipeline, and position ④ is connected to the inlet of the oxidation furnace via a pipeline.

[0010] Position ① of the four-way switching valve II is connected to the inlet of the damping needle valve via a pipeline, position ② is connected to the outlet of the oxidation furnace via a pipeline, position ③ is connected to the inlet of the chromatographic column II via a pipeline, and position ④ is connected to the first hydrogen inlet pipeline.

[0011] The six-way switching valve II has the following connections: position ① is connected to the outlet of chromatographic column II via a pipeline; position ② is connected to the inlet of the methane converter via a pipeline; position ③ is connected to the second hydrogen inlet pipeline; position ④ is connected to a gas venting pipeline; position ⑤ is connected to the outlet of the methane converter via a pipeline; and position ⑥ is connected to the FID detector II via a pipeline.

[0012] Furthermore, the outlet of the damping needle valve is connected to a gas venting pipeline.

[0013] Furthermore, both chromatographic column I and chromatographic column II are packed columns; chromatographic column I is filled with a medium polar stationary phase, and chromatographic column II is filled with a strongly polar stationary phase; both chromatographic column I and chromatographic column II are placed in the column oven of a gas chromatograph, where a constant or gradient temperature can be provided.

[0014] Furthermore, a gas damping tube with a length of 300mm to 500mm is installed at the inlet end of both FID detector I and FID detector II.

[0015] Furthermore, one end of the gas damping tube is connected to the inlet end of FID detector I and FID detector II respectively, and the air inlet lines of FID detector I and FID detector II are connected to the other end of the gas damping tube respectively.

[0016] Furthermore, the methane conversion furnace has a filling tube filled with nickel catalyst, and is equipped with a constant temperature heating device and an insulation jacket. The constant temperature heating device can provide a constant heating temperature to the filling tube.

[0017] Furthermore, the oxidation furnace consists of a T-shaped quartz tube, a heater, a thermocouple, insulating ceramic cotton, and a temperature control circuit module, and has an independent temperature control function, which can provide a constant heating temperature to the quartz tube.

[0018] Furthermore, the T-shaped quartz tube consists of a main pipe and a branch pipe; the main pipe has two openings, an upper one and a lower one; the branch pipe is located on the side near the upper opening of the main pipe and is vertically connected to the main pipe; the upper and lower openings of the main pipe and the air inlet of the branch pipe are all provided with connectors and nuts.

[0019] Furthermore, the oxidation furnace heater is located between the T-shaped quartz tube main pipe and the insulating ceramic cotton, with a gap of 3mm to 5mm between the heater and the T-shaped quartz tube main pipe, and the thermocouple is tightly attached to the outer wall of the T-shaped quartz tube main pipe.

[0020] Furthermore, the main section of the T-shaped quartz tube is filled with a platinum catalyst with a filling length of 25mm to 30mm. A small amount of quartz wool is plugged at both ends of the catalyst filling section, and the upper end of the catalyst filling section is located below the air inlet of the branch pipe of the T-shaped quartz tube.

[0021] Furthermore, the T-shaped quartz tube has a main pipe upper port serving as the inlet for carrier gas to be introduced, a branch pipe inlet for oxygen to be introduced, and a main pipe lower port serving as the outlet for reaction gas to flow out.

[0022] Furthermore, the fuel gas for FID detector I and FID detector II is hydrogen, the combustion gas for oxidizing is air, and the exhaust gas for tail gas is nitrogen.

[0023] Compared with the prior art, the advantages of this utility model are: (1) By setting up an oxidation furnace and a methane conversion furnace, this utility model can convert trace formaldehyde into methane, and the detection limit can reach 0.5 μmol / mol or even lower. It does not require absorption or derivatization pretreatment, thus avoiding excessive manual operation; (2) The gas chromatography system described in this utility model is not only suitable for detecting trace formaldehyde, but also suitable for detecting trace carbon monoxide, carbon dioxide, formic acid, acetic acid, acetaldehyde and other compounds that do not have a signal peak or have a weak signal peak on the FID detector if a suitable chromatographic column is selected. By converting such compounds into methane, the output signal on the FID detector is enhanced, thereby realizing the detection of multiple compounds, especially the detection of low content; (3) The gas chromatography system described in this utility model is equipped with a dual FID detector, which can detect other organic components at the same time as detecting trace formaldehyde, thus realizing one machine for multiple uses. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the device structure of this utility model.

[0025] Figure 2 This is a schematic diagram of the oxidation furnace in the device of this utility model.

[0026] Figure 3 The image shows the chromatogram of the sample to be tested under the instrument conditions described in Example 1, obtained using the FID detector I.

[0027] Figure 4 The image shows the chromatogram of the sample to be tested under the instrument conditions described in Example 1, obtained using the FID detector II.

[0028] Figure 5 The image shows the chromatogram of the sample under the instrument conditions described in Example 2, obtained using the FID detector II.

[0029] Figure 1 In the diagram, 1 is a six-way switching valve I, 2 is a quantitative tube, 3 is a chromatographic column I, 4 is an FID detector I, 5 is a four-way switching valve I, 6 is a methane conversion furnace, 7 is a six-way switching valve II, 8 is an FID detector II, 9 is a chromatographic column II, 10 is a four-way switching valve II, 11 is a damping needle valve, 12 is an oxidation furnace, and 13 and 14 are gas damping tubes.

[0030] Figure 2 In the diagram, 12.1 is a T-shaped quartz tube branch pipe, 12.2 is a temperature control circuit module, 12.3 is a T-shaped quartz tube main pipe, 12.4 is a catalyst filling section, 12.5 is ceramic insulation cotton, 12.6 is a heater, and 12.7 is a thermocouple.

[0031] Figure 3-5 In the spectrum, peak 13 is dimethyl ether, peak 14 is methanol, peak 15 is carbon monoxide, peak 16 is carbon dioxide, and peaks 17 and 18 are formaldehyde. Detailed Implementation

[0032] The technical solutions in the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, and not all embodiments.

[0033] It should be noted that all experimental data and operating conditions described in the following embodiments are for reference and understanding by those skilled in the art only, and are not intended to limit the present invention. The experimental data obtained by those skilled in the art during the operation of the following embodiments may not be exactly the same as the data listed in the embodiments, and the actual measured data shall prevail.

[0034] Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this utility model. Example 1

[0035] See attached document Figure 1This embodiment provides a gas chromatography system for detecting trace amounts of formaldehyde in gases, including a six-way switching valve I1, a quantitative tube 2, a chromatographic column I3, an FID detector I4, a four-way switching valve I5, a methane conversion furnace 6, a six-way switching valve II7, an FID detector II8, a chromatographic column II9, a four-way switching valve II10, a damping needle valve 11, and an oxidation furnace 12. The six-way switching valves I1 and II7 are planar six-way valves, driven electrically or pneumatically, with polytetrafluoroethylene (PTFE) valve cores. They each have six pipeline connection points and two rotary valve positions: the off position connects positions ① and ②, ③ and ④, and ⑤ and ⑥; the on position connects positions ② and ③, ④ and ⑤, and ⑥ and ①. The four-way switching valve I5 and four-way switching valve II10 are planar four-way valves, driven by electric or pneumatic means, with the valve core material being polytetrafluoroethylene. They each have four pipeline connection points and two rotary valve positions: the off position connects position ① with position ② and position ③ with position ④; the on position connects position ② with position ③ and position ④ with position ①.

[0036] The six-way switching valve I1 has the following connections: Position ① is connected to a gas venting line; Positions ② and ⑤ are connected to the two ends of the quantitative tube 2, which is made of stainless steel and can be processed into a coil shape with an internal volume of 0.5 mL; Position ③ is connected to the first nitrogen inlet line, which uses high-purity gas from a cylinder with a purity ≥99.999% and a constant flow rate of 15 mL / min; Position ④ is connected to the inlet of chromatographic column I3 via a pipeline; and Position ⑥ is connected to the sample injection line. The four-way switching valve I5 has the following connections: Position ① is connected to the outlet of chromatographic column I3 via a pipeline; Position ② is connected to the FID detector I4 via a pipeline; Position ③ is connected to the second nitrogen inlet line, which uses high-purity gas from a cylinder with a purity ≥99.999% and a constant flow rate of 15 mL / min; and Position ④ is connected to the inlet of oxidizer 12 via a pipeline. Position ① of the four-way switching valve II7 is connected to the inlet of the damping needle valve 11 via a pipeline. The outlet of the damping needle valve 11 is connected to a gas venting pipeline. The damping value of the damping needle valve 11 is set to be equal to the damping value of the chromatographic column II9 to reduce the instability of the chromatographic baseline caused by gas flow fluctuations during valve switching. Position ② is connected to the outlet of the oxidizer 11 via a pipeline. Position ③ is connected to the inlet of the chromatographic column II9 via a pipeline. Position ④ is connected to the first hydrogen inlet pipeline. The hydrogen is supplied by high-purity steel cylinder gas with a purity ≥99.999% and a constant flow rate of 25mL / min. Position ① of the six-way switching valve II7 is connected to the outlet of chromatographic column II9 via a pipeline; position ② is connected to the inlet of methane converter 6 via a pipeline; position ③ is connected to the second hydrogen inlet pipeline, with high-purity gas from a cylinder (purity ≥99.999%) and a constant flow rate of 25 mL / min; position ④ is connected to a gas venting pipeline; position ⑤ is connected to the outlet of methane converter 6 via a pipeline; and position ⑥ is connected to FID detector II8 via a pipeline. All pipelines are made of stainless steel, with an inner diameter of 1.0 mm and an outer diameter of 1.6 mm.

[0037] In this embodiment, column I3 is a stainless steel packed column with an inner diameter of 2 mm, an outer diameter of 3 mm, and a length of 2 m. The internal packing surface is coated with porous microspheres of a moderately polar stationary phase, styrene-divinylbenzene copolymer, with a particle size of 60-80 mesh. Column II9 is ​​a stainless steel packed column with an inner diameter of 2 mm, an outer diameter of 3 mm, and a length of 4 m. The internal packing surface is coated with porous microspheres of a strongly polar stationary phase, N-vinyl-2-pyrrolidone, with a particle size of 60-80 mesh. Both columns I and II are placed in the column oven of a gas chromatograph. The column oven temperature parameters are set as follows: initial temperature 50°C held for 5 min, first-stage temperature ramp rate 20°C / min, first-stage final temperature 150°C held for 3 min.

[0038] In this embodiment, gas damping tubes 13 and 14, with an outer diameter of 1.60 mm and an inner diameter of 0.25 mm, are respectively installed at the inlet ends of FID detector I4 and FID detector II8. The length of gas damping tube 13 installed in FID detector I4 is 500 mm, and the length of gas damping tube 14 installed in FID detector II8 is 300 mm. One end of each gas damping tube 13 and 14 is connected to the inlet end of FID detector I4 and FID detector II8, respectively, and the air inlet lines of FID detector I4 and FID detector II8 are connected to the other end of each gas damping tube 13 and 14. Gas damping tubes 13 and 14 can be used to buffer the inlet flow rate, preventing the hydrogen flame from extinguishing due to drastic changes in gas flow rate during valve switching. The fuel gas for FID detector I4 and FID detector II8 is hydrogen, with a constant flow rate of 30 mL / min; the combustion gas is air, with a constant flow rate of 300 mL / min; and the tail gas is nitrogen, with a constant flow rate of 10 mL / min. Both hydrogen and nitrogen are supplied from high-purity steel cylinders with a purity ≥99.999%; the air is supplied from an oil-free air compressor. The temperature of both FID detector I4 and FID detector II8 is set to 250℃.

[0039] In this embodiment, the methane conversion furnace 6 has a filling tube made of stainless steel, with an inner diameter of 2 mm, an outer diameter of 3 mm, and a length of 60 mm. The filling tube is filled with a nickel catalyst, which uses aluminum oxide as a support and is coated with a layer of highly active nickel with a particle size of 60-80 mesh. The filling tube is equipped with a constant temperature heating device and a heat insulation jacket. The heat insulation jacket is composed of a heat insulation lining material and an outer shell. The heat insulation lining material is made of ceramic wool, and the outer shell material is made of stainless steel. The constant temperature heater is set to a temperature of 350°C.

[0040] In this embodiment, refer to the appendix Figure 2The oxidation furnace 12 consists of a T-shaped quartz tube, a heater 12.6, a thermocouple 12.7, insulating ceramic cotton 12.5, and a temperature control circuit module 12.2. It has independent heating and temperature control functions, with the quartz tube set to a heating temperature of 450℃. The T-shaped quartz tube consists of a main pipe 12.3 and a branch pipe 12.1. The main pipe 12.3 is 50mm long and 3mm in diameter, with upper and lower inlets. The upper inlet serves as the inlet for carrier gas, and the lower inlet serves as the outlet for reaction gas. The branch pipe 12.1 is located 10mm from the upper inlet of the main pipe 12.3, is 10mm long, and 3mm in diameter, and is vertically connected to the main pipe 12.3. Oxygen is supplied through the inlet of the branch pipe 12.1 using high-purity steel cylinder gas with a purity ≥99.999% and a constant flow rate of 10m³. L / min; the upper and lower inlets of the main pipe 12.3 and the air inlet of the branch pipe 12.1 are all equipped with connector nuts; the heater 12.6 is located between the T-shaped quartz tube main pipe 12.3 and the insulating ceramic cotton 12.5, and there is a 3mm gap between the heater 12.6 and the T-shaped quartz tube main pipe 12.3; the thermocouple 12.7 is tightly attached to the outer wall of the T-shaped quartz tube main pipe 12.3; the middle of the T-shaped quartz tube main pipe 12.3 is filled with a section of platinum catalyst, which is made by impregnating 40-60 mesh alumina microspheres with chloroplatinic acid solution for several hours, then drying and high-temperature calcining. The length of the platinum catalyst filling is 25mm. The catalyst filling section 12.4 is plugged with a small amount of quartz cotton at both ends, and the upper end of the catalyst filling section 12.4 is 5mm away from the air inlet of the T-shaped quartz tube branch pipe 12.1.

[0041] As a further improvement, the internal volume of the metering tube 2 can also be 1.0 mL.

[0042] As a further improvement, the interior of the chromatographic column I3 can also be filled with porous microspheres coated with a neutral polar stationary phase, ethylstyrene-divinylbenzene copolymer, with a particle size of 60-80 mesh.

[0043] As a further improvement, the interior of the chromatographic column II9 can also be filled with porous microspheres coated with a highly polar stationary phase, polyethylene glycol dimethacrylate, with a particle size of 60-80 mesh.

[0044] As a further improvement, the column oven temperature parameters can also be set as follows: initial temperature 50℃ held for 5 minutes, first-stage heating rate 30℃ / min, first-stage final temperature 160℃ held for 2 minutes.

[0045] As a further improvement, the gas damping tubes 13 and 14 installed on the FID detector I4 and FID detector II8 can both be 400mm in length.

[0046] As a further improvement, the platinum catalyst filling length in the oxidation furnace 12 can also be 28 mm, and the gap between the heater 12.6 and the main tube 12.3 of the T-shaped quartz tube can also be 4 mm.

[0047] As a further improvement, the hydrogen supply method can also employ a hydrogen generator.

[0048] The design principles and workflow are as follows.

[0049] 1. Sample injection and separation: After the gas sample is injected through the injection line into position ⑥ of the six-way switching valve I1 to fully purge the quantitative tube 2, the six-way switching valve I1, four-way switching valve I5, four-way switching valve II10, and six-way switching valve II7 are opened simultaneously. Nitrogen gas carries the gas sample from the quantitative tube 2 through the six-way switching valve I1 into the chromatographic column I3. Under the flow of the carrier nitrogen gas, the various components in the gas sample move forward at non-uniform speeds on the stationary phase of the chromatographic column I3, thus achieving separation of the components. After 0.5 minutes, the gas sample in the quantitative tube 2 has been completely purged, at which point the six-way switching valve I1 is closed.

[0050] 2. Oxidation: Carbon monoxide, carbon dioxide, and formaldehyde in the gas sample elute sequentially from chromatographic column I3 and enter oxidation furnace 12 through four-way switching valve I5. Oxygen is introduced through the T-shaped quartz tube branch pipe 12.1 of oxidation furnace 12. In the presence of oxygen, with platinum as a catalyst, and under heating conditions, the carbon monoxide and formaldehyde are oxidized to carbon dioxide. The reaction formula is as follows:

[0051] ;

[0052] The carbon dioxide originally contained in the gas sample passes through directly without reaction, flowing out of the oxidizer 12 and entering the chromatographic column II9 through the four-way switching valve II10. The carbon dioxide is temporarily adsorbed by the stationary phase in the chromatographic column II9 and forms three carbon dioxide residence zones, which move forward at a constant speed on the stationary phase. Nitrogen and oxygen are hardly adsorbed by the stationary phase in the chromatographic column II9 and flow out directly into the FID detector II8.

[0053] 3. When column II is in forward orientation, and all three carbon dioxide segments have just entered column II 9, simultaneously close the four-way switching valve I 5 and the four-way switching valve II 10. The outlet of oxidizer 12 is switched to venting. Hydrogen enters column II 9 through the four-way switching valve II 10 for forward purging. The three carbon dioxide residence zones in column II 9 continue to move forward at a constant speed under the purging of hydrogen. The residual nitrogen and oxygen are quickly purged and removed and leave column II 9.

[0054] 4. Methanation: After closing the four-way switching valve I5 and four-way switching valve II10 for a certain period of time, the residual nitrogen and oxygen in column II9 are completely removed. At this time, the six-way switching valve II7 is closed, and the three segments of carbon dioxide remaining in column II9 flow out sequentially under hydrogen purging. They then enter the methane conversion furnace 6 through the six-way switching valve II7. The carbon dioxide is converted into methane in a hydrogen atmosphere, with nickel as a catalyst, and under heating conditions. The reaction formula is as follows:

[0055] ;

[0056] The three methane segments formed by the conversion enter the FID detector II8 through the six-way switching valve II7. The FID detector II8 generates corresponding signals and displays chromatographic peaks. The order of the signal peaks is carbon monoxide 15, carbon dioxide 16, and formaldehyde 17.

[0057] 5. After heating and cleaning columns I3 and II9, open the six-way switching valve II7, raise the column oven temperature and maintain it for a certain period of time. Other compounds in column I3 will rapidly elute after the column temperature rises and enter FID detector I4 sequentially through the four-way switching valve I5. FID detector I4 will generate a response signal and display the chromatographic peaks of other compounds. Water in column II9 will rapidly elute after the column temperature rises and enter FID detector II8 sequentially through the six-way switching valve II7. This prevents water accumulation in column II9 from affecting the column separation effect and also prevents excessive water from entering the methane conversion furnace 6 and affecting the catalytic activity of the nickel catalyst. After the heating and cleaning is completed, close the six-way switching valve II7 and lower the column temperature to the initial state. The system will then reset.

[0058] In the above workflow, it is important to carefully control the timing of switching the six-way switching valve II7 when column II9 is ​​placed in the correct orientation. This ensures that nitrogen and oxygen are adequately removed from column II9 while preventing carbon dioxide from escaping during purging. Switching the valve too early results in insufficient removal of nitrogen and oxygen from column II9, which will form an oxide film on the nickel catalyst surface upon entering the methane converter 6, affecting its conversion efficiency. Switching the valve too late allows carbon dioxide to escape from column II9 before it enters the methane converter 6. The specific switching timing for each valve can be determined through repeated adjustments using the system provided in this invention. Alternatively, another gas chromatograph can be used, with columns I3 and II9 installed separately under the same chromatographic conditions and a TCD detector, to obtain the retention time and half-peak width of each peak, which can then be used as the basis for valve switching timing.

[0059] The retention times and full width at half maximum (FWHM) of each component, measured under specific instrument conditions, are shown in Table 1.

[0060] Table 1. Retention time and half-peak width of each component under certain instrument conditions

[0061]

[0062] It should be understood that the time when each component begins to elute from the column is the retention time of that component in the column minus the half-peak width, and the time when each component completely elutes from the column is the retention time of that component in the column plus the half-peak width.

[0063] Based on the data in Table 1, calculate the time when the last analyte, formaldehyde, completely elutes during the separation process at column I3. t 1' is:

[0064]

[0065] Therefore, it is inferred that the closing time of the four-way switching valve I5 and the four-way switching valve II10 should be at least 3.06 minutes after the gas sample is injected, and setting it to 3.5 minutes is more appropriate. Similarly, the time for nitrogen and oxygen to completely flow out when the chromatographic column II9 is ​​placed in the correct orientation is calculated. t 2' is:

[0066]

[0067] The time it takes for carbon dioxide to begin eluting column II9 t 3' is:

[0068]

[0069] The time it takes for carbon monoxide to first elute from column I3 t 4' is:

[0070]

[0071] Therefore, it is inferred that the closing time of the six-way switching valve II7 should be 0.92 minutes after the closing of the four-way switching valve I5 and the four-way switching valve II10 and before 6.00 minutes after the gas sample is injected, that is, between 3.98 minutes and 6.00 minutes after the gas sample is injected, and setting it to 5 minutes is more appropriate.

[0072] In this embodiment, the sample to be tested is the tail gas from the formaldehyde absorption tower and the outlet gas from the ECS reactor, which contains approximately 50 μmol / mol of carbon monoxide, 300 μmol / mol of carbon dioxide, 80 μmol / mol of formaldehyde, 200 μmol / mol of dimethyl ether, and 100 μmol / mol of methanol. The operating steps are as follows:

[0073] Step 1: After taking a sample with a gas bag, inject the gas sample into position ⑥ of the six-way switching valve I1 through the injection line and replace the quantitative tube 2 for 15s to 30s. At the same time, open the six-way switching valve I1, the four-way switching valve I5, the four-way switching valve II10 and the six-way switching valve II7 to inject the sample. After 0.5min, close the six-way switching valve I1.

[0074] Step 2: When the carbon dioxide generated by oxidation 3.5 min after the gas sample is injected has just entered column II, simultaneously close the four-way switching valve I5 and the four-way switching valve II10.

[0075] Step 3: 5 minutes after the gas sample is injected, when the residual nitrogen and oxygen in column II are completely removed, close the six-way switching valve II7. The FID detector II8 will generate the corresponding signal and display the chromatographic peaks. The order of the signal peaks is carbon monoxide 15, carbon dioxide 16, and formaldehyde 17.

[0076] Step 4: After FID detector II8 displays the formaldehyde signal peak 17, open the six-way switching valve II7 and increase the column oven temperature to a final temperature of 150℃ at a first-order heating rate of 20℃ / min, and hold for 3 minutes. Dimethyl ether and methanol in column I3 will rapidly elute after the column temperature increases, and FID detector I4 will generate a response signal and display chromatographic peaks. The order of the elution peaks is dimethyl ether 13, followed by methanol 14. The water in column II9 will rapidly elute after the column temperature increases, thus purifying the column. After the purging is complete, close the six-way switching valve II7 and lower the column temperature to 50℃, then reset the system.

[0077] Step 5: Calculate the content of each component using the response factor in the calibration table of the chromatography workstation.

[0078] The chromatogram obtained by FID detector I4 is attached. Figure 3 As shown.

[0079] The chromatogram obtained by the FID detector II8 is shown in the attached figure. Figure 4 As shown. Example 2

[0080] As another preferred embodiment, the difference from Embodiment 1 is that, after the gas sample is injected into position ⑥ of the six-way switching valve I1 through the injection line to fully replace the quantitative tube 2 in the first step of the workflow described in Embodiment 1, only the six-way switching valve I1 is opened. After carbon monoxide and carbon dioxide have just flowed out of the chromatographic column I3 and entered the FID detector I4, the four-way switching valve I5, the four-way switching valve II10 and the six-way switching valve II7 are opened simultaneously to allow only formaldehyde to enter the oxidation furnace 11.

[0081] Based on the data given in Table 1 of Example 1, the time at which carbon dioxide completely eluted during injection separation at column I3 was calculated. t5' is:

[0082]

[0083] Based on the data given in Table 1 of Example 1, the time at which formaldehyde first began to elute from chromatographic column I3 was calculated. t 6' is:

[0084]

[0085] Therefore, it is inferred that the opening time of the four-way switching valve I5, the four-way switching valve II10, and the six-way switching valve II7 in this embodiment should be between 1.52 min and 2.46 min, and setting it to 2 min is more appropriate.

[0086] After the operation in this embodiment, the FID detector II 8 finally only responds to a single formaldehyde signal peak 18.

[0087] As another preferred embodiment, the difference from Embodiment 1 is that the sample to be tested in this embodiment is the air inside a formaldehyde production plant, which contains approximately 5 μmol / mol of formaldehyde. The operating steps are as follows:

[0088] Step 1: After taking a sample with a gas bag, inject the gas sample into position 6 of the six-way switching valve I1 through the injection line. Replace the quantitative tube 2 for about 15 to 30 seconds, open the six-way switching valve I1, and close the six-way switching valve I1 after 0.5 minutes.

[0089] Step 2: 2 minutes after the gas sample is injected, when all the carbon monoxide and carbon dioxide in column I have just flowed out, simultaneously open the four-way switching valve I5, the four-way switching valve II10, and the six-way switching valve II7.

[0090] Step 3: When the carbon dioxide generated by oxidation 3.5 min after the gas sample is injected has just entered column II, simultaneously close the four-way switching valve I5 and the four-way switching valve II10.

[0091] Step 4: 5 minutes after the gas sample is injected, when the residual nitrogen and oxygen in column II are completely removed, close the six-way switching valve II7. The FID detector II will generate a response signal and display the formaldehyde chromatographic peak 18.

[0092] Step 5: After the FID detector II8 displays the formaldehyde signal peak 18, open the six-way switching valve II7 and raise the column oven temperature to the final temperature of 150℃ at a first-order heating rate of 20℃ / min, and hold for 3 minutes. Moisture and other possible impurities in chromatographic columns I3 and II9 will quickly flow out after the column temperature rises, thus cleaning them. After the heating and cleaning are completed, close the six-way switching valve II7 and lower the column temperature to 50℃, then reset the system.

[0093] Step 6: Calculate the formaldehyde content using the response factor in the calibration table of the chromatography workstation.

[0094] The chromatogram obtained by the FID detector II8 is shown in the attached figure. Figure 5 As shown.

Claims

1. A gas chromatography system for detecting trace amounts of formaldehyde in a gas, characterized in that: Includes a six-way switching valve I, a quantitative tube, a chromatographic column I, an FID detector I, a four-way switching valve I, a methane conversion furnace, a six-way switching valve II, an FID detector II, a chromatographic column II, a four-way switching valve II, a damping needle valve, and an oxidation furnace; The six-way switching valve I and six-way switching valve II each have six pipeline connection points and two rotary valve positions, namely: the off position is connected to position 1 and position 2, position 3 and position 4, and position 5 and position 6; the on position is connected to position 2 and position 3, position 4 and position 5, and position 6 and position 1. The four-way switching valve I and four-way switching valve II each have four pipeline connection points and two rotary valve positions, namely: the off position is connected to position ① and position ②, and position ③ and position ④; the on position is connected to position ② and position ③, and position ④ and position ①. Position ① of the six-way switching valve I is connected to the sample outlet pipeline, positions ② and ⑤ are connected to the two ends of the quantitative tube respectively, position ③ is connected to the first nitrogen inlet pipeline, position ④ is connected to the inlet of chromatographic column I through a pipeline, and position ⑥ is connected to the injection port pipeline. Position ① of the four-way switching valve I is connected to the outlet of chromatographic column I via a pipeline, position ② is connected to FID detector I via a pipeline, position ③ is connected to the second nitrogen inlet pipeline, and position ④ is connected to the inlet of the oxidation furnace via a pipeline. Position ① of the four-way switching valve II is connected to the inlet of the damping needle valve via a pipeline, position ② is connected to the outlet of the oxidation furnace via a pipeline, position ③ is connected to the inlet of the chromatographic column II via a pipeline, and position ④ is connected to the first hydrogen inlet pipeline. The six-way switching valve II has the following connections: position ① is connected to the outlet of chromatographic column II via a pipeline; position ② is connected to the inlet of the methane converter via a pipeline; position ③ is connected to the second hydrogen inlet pipeline; position ④ is connected to a gas venting pipeline; position ⑤ is connected to the outlet of the methane converter via a pipeline; and position ⑥ is connected to the FID detector II via a pipeline.

2. The gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 1, characterized in that: The outlet of the damping needle valve is connected to a gas venting pipeline.

3. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 1, characterized in that: Both Column I and Column II are packed columns.

4. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 1, characterized in that: A gas damping tube, 300mm to 500mm in length, is installed at the inlet end of both FID detector I and FID detector II.

5. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 1, characterized in that: The oxidation furnace consists of a T-shaped quartz tube, a heater, a thermocouple, insulating ceramic cotton, and a temperature control circuit module. It has independent heating and temperature control functions and can provide a constant temperature of 400℃ to 600℃ to the quartz tube.

6. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 5, characterized in that: The T-shaped quartz tube consists of a main pipe and a branch pipe; the main pipe has two openings, an upper one and a lower one; the branch pipe is located on the side near the upper opening of the main pipe and is vertically connected to the main pipe; the upper and lower openings of the main pipe and the opening of the branch pipe are all equipped with joints and nuts.

7. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 5, characterized in that: The heater is located between the T-shaped quartz tube main pipe and the insulating ceramic cotton. There is a 3mm to 5mm gap between the heater and the T-shaped quartz tube main pipe. The thermocouple is tightly attached to the outer wall of the T-shaped quartz tube main pipe.

8. A gas chromatography system for detecting trace amounts of formaldehyde in a gas according to claim 5, characterized in that: The main section of the T-shaped quartz tube is filled with a platinum catalyst, with a filling length of 25mm to 30mm. The catalyst filling section is plugged with a small amount of quartz wool at both ends. The upper end of the catalyst filling section is located below the air inlet of the branch pipe of the T-shaped quartz tube.

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