Rapid detection method for judging internal leakage of organic matters in heat exchanger of coal chemical device

Abstract

The invention discloses a rapid detection method for determining internal leakage of organic matters in a heat exchanger of a coal chemical device, which comprises the following steps: sampling at a water inlet and a water outlet of circulating water of the heat exchanger respectively to obtain a water inlet sample and a water outlet sample; detecting the total organic matter content in the samples by using chromatography to obtain the total organic matter content in the water inlet sample and the water outlet sample; if the total organic matter content in the sample at the water inlet exceeds 10% of the total organic matter content in the sample at the water outlet, the internal leakage of the heat exchanger occurs. The total organic matter content in the circulating water of the heat exchanger is detected through chromatography, whether the heat exchanger has internal leakage or not is judged according to the change of the total organic matter content in the samples at the water inlet and the water outlet, whether the heat exchanger has internal leakage or not in use can be rapidly and accurately checked, precious time is saved for internal leakage elimination, device safety is protected, VOCs emission is reduced, cost is low, and popularization is easy. Good popularization and application prospects are realized.

Description

Technical Field

[0001] This invention belongs to the field of detection technology, specifically relating to a rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants. Background Technology

[0002] Heat exchangers in coal chemical plants transfer heat from high-temperature fluids (such as gases produced during coal gasification / coking) to low-temperature media (water) through heat conduction and convection, enabling energy transfer between different process stages. They play a crucial role in ensuring dynamic heat balance and stable reaction conditions throughout the production process. Coal chemical heat exchangers are key equipment for improving process energy efficiency, achieving environmental compliance, and ensuring safe production; their function spans multiple dimensions, including energy conversion, process optimization, and sustainable development.

[0003] In actual production, internal leaks can occur in heat exchangers of coal chemical plants. Existing methods for detecting internal leaks in heat exchangers include pressure testing, tracer gas detection, temperature monitoring, ultrasonic testing, infrared thermography, and acoustic emission technology. Pressure testing is the most common method, involving pressurizing one side and observing the pressure change on the other. However, it has potential problems: if the leak is small, the pressure change may not be noticeable, or it may require shutdown testing, affecting production. Furthermore, if the system is complex, other factors may cause pressure changes, leading to misjudgments. For tracer gas detection, such as using helium as a tracer and mass spectrometry, this method has high sensitivity but can be very expensive, requiring costly equipment and potentially requiring specialized personnel, limiting its application in the field. Additionally, if the heat exchanger structure is complex, the gas may not easily reach the leak point, making detection difficult. Other methods also suffer from insufficient sensitivity, the need for shutdown, susceptibility to environmental interference, high cost, operational complexity, lack of real-time monitoring, or inaccurate location.

[0004] The "Sinopec 2020 Volatile Organic Compounds (VOCs) Control Action Plan" (Group Works and Energy

[2020] No. 39) requires that the volatile organic compounds in the circulating water and heat exchanger inlet and outlet water be analyzed using the TOC method every six months. For outlet concentrations exceeding inlet concentrations by 10%, the leak point must be traced and repaired promptly. During heat exchanger leak checks, the sample volume is large, the analysis time is long, and many current TOC analyzers are not suitable for high-salt samples, resulting in rapid consumption of consumables and high maintenance costs. TOC analysis of a single sample requires six injections to analyze total carbon (TC) and inorganic carbon (TIC) separately, with the total organic carbon (TOC) obtained by subtraction. Therefore, the method's precision is also poor, and many indicators in the circulating water exceed the analytical range of this method. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a rapid detection method for determining organic leakage in heat exchangers of coal chemical plants. This invention uses chromatography to detect the total organic matter content in the circulating water of the heat exchanger, which can quickly and accurately identify whether the heat exchanger in use has internal leakage, saving valuable time for internal leakage elimination, protecting the safety of the equipment, and protecting the environment.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants includes the following steps:

[0008] Sampling should be carried out on the inlet and outlet of the heat exchangers and circulating water systems that have been put into operation and involve organic materials. When testing the total organic matter in the circulating water, at least three sets of samples should be collected at the inlet and outlet, and the average value of each sample should be calculated. The water sample should be collected in a brown bottle and should be filled completely without leaving a headspace. The water sample should be measured within 24 hours. Otherwise, sulfuric acid should be added to acidify the water sample to pH ≤ 2. The sample can be stored for 7 days at 4°C.

[0009] The inlet and outlet samples were analyzed using chromatography, as follows:

[0010] Select the chromatographic column based on the type of organic matter leaking from the heat exchanger; set the column temperature, split ratio, column flow rate, and needle washing times;

[0011] Select the type of organic matter leaking from the heat exchanger, prepare standard samples of organic matter, and plot the standard curve of organic matter.

[0012] Place the sample to be tested into the autosampler, start the chromatograph according to the set conditions, and after the chromatograph finishes running, open the spectrum and calculate the organic content in the sample based on the organic standard curve.

[0013] If the organic matter content in the inlet sample exceeds the organic matter content in the outlet sample by 10%, it indicates that the heat exchanger has an internal leak; otherwise, it indicates that there is no internal leak.

[0014] Further, the selection of chromatographic column type and conditions is as follows: Based on the types of organic matter leaking from the coal chemical heat exchanger and their solubility in water, the possible organic matter is generally determined to be low-carbon hydrocarbons, high-carbon hydrocarbons, oxygen-containing compounds, etc., with the principle of rapid elution and minimal interference from water. Therefore, a deactivated fused silica capillary column (0.32mm*1.5m) and a DB-FFAP (30m*0.32mm*0.5um) polar column are selected. The advantage of the deactivated fused silica capillary column is that all organic matter is concentrated in a single elution within one minute, resulting in fast analysis speed; even water samples containing nearly 30% (m / m) of oxygen-containing compounds can elute within one minute. However, its disadvantage is that water interference cannot be avoided, leading to insufficient accuracy. The advantage of the DB-FFAP polar column is that it effectively avoids water interference, allowing for better separation of water and organic matter; the disadvantage is a longer analysis time, requiring at least 2.5 minutes for heavy olefin samples containing 100% organic matter to elute. Taking into account both analysis time and separation effect, this invention selects the second type of DB-FFAP polar chromatographic column.

[0015] The exploration of chromatographic operating conditions is detailed below:

[0016] Regarding column temperature, when the column temperature is set to 40℃, the analysis time is long; the peak of the 300 mg / kg methanol standard did not fully emerge after 5 minutes, and the peak shape exhibited severe tailing. When the column temperature is set to 110℃, the peak of the 300 mg / kg methanol standard emerged completely in 2.5 minutes, and the peak shape was symmetrical. Through comparison, a column temperature of 110℃ is the appropriate condition.

[0017] Regarding the split ratio, when a split ratio of 10:1 is selected, the peak area of ​​a 10 mg / kg methanol standard can reach more than 6 PA·S, and the spectrum is good.

[0018] When the column flow rate is set to 2.5 mL / min, isopentane and n-heptane can be completely separated.

[0019] The injection needle needs to be cleaned at least 6 times to remove any residue from the previous sample.

[0020] In summary, the experimental results show that the optimal chromatographic conditions are: column temperature 110℃, split ratio 10:1, needle washing 6 times, and flow rate 2.5 mL / min.

[0021] Select appropriate organic compounds to prepare standard samples: Determine the type of organic compounds leaking from coal chemical heat exchangers and their solubility in water. Possible organic compounds are generally low-carbon hydrocarbons, high-carbon hydrocarbons, oxygen-containing compounds, etc. Based on this, select isopentane, n-heptane, and methanol as standard samples.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention uses chromatography to detect the total organic matter (TOC) content in the circulating water of a heat exchanger. By observing changes in the TOC content in samples from the inlet and outlet, it determines whether the heat exchanger has an internal leak. This allows for rapid and accurate troubleshooting of internal leaks in heat exchangers in use, saving valuable time, protecting the equipment, and reducing VOC emissions. Compared with methods for determining TOC, the precision (RSD) of chromatography is improved from 6.02% to 0.76%; the analysis time for a single sample is reduced from 30 minutes to within 3 minutes; and the results are even more significant for multiple sample analyses. It is easy to operate, does not require phosphoric acid reagent, has simple instrument maintenance, and is low-cost, making it suitable for rapid leak detection in circulating water and heat exchanger inlet / outlet leak checks in basic laboratories. Detailed Implementation

[0024] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0025] In addition, unless otherwise specified, the preparation processes in the following examples are all conventional methods in the prior art, and therefore will not be described in detail. The reagents used in the following examples are all commercially available products.

[0026] Analysis principle:

[0027] Gas chromatography (GC) is based on the fact that different substances have different partition coefficients in two relatively moving phases. When a sample enters the vaporization chamber, it is converted into a gaseous state and carried into the chromatographic column by a carrier gas. Due to differences in boiling point, polarity, or adsorption properties, the components will form different partition or adsorption equilibria between the mobile and stationary phases. The flow of the carrier gas causes these components to repeatedly undergo partition or adsorption / desorption processes in motion. Ultimately, the components with higher partition concentrations will elute from the column first, while the components with lower partition concentrations will elute later. After these components elute, they immediately enter the detector, which converts the presence of these components into electrical signals, the magnitude of which is proportional to the amount or concentration of the component. These signals are amplified and recorded to form a chromatogram.

[0028] The TOC method involves introducing the sample along with purified gas into a high-temperature combustion tube and a low-temperature reaction tube, respectively. In the high-temperature combustion tube, the sample undergoes high-temperature catalytic oxidation, converting both organic and inorganic carbon into carbon dioxide. In the low-temperature reaction tube, the sample is acidified, causing the inorganic carbon to decompose into carbon dioxide. The carbon dioxide generated in both reaction tubes is then introduced into a non-dispersive infrared detector. At a specific wavelength, within a certain mass concentration range, the infrared absorption intensity of carbon dioxide is directly proportional to its mass concentration. This allows for the quantitative determination of total carbon and inorganic carbon in the sample. The difference between total carbon and inorganic carbon is the total organic carbon.

[0029] instrument:

[0030] The 7890B gas chromatograph (Agilent) is equipped with a DB-FFAP (30m*0.32mm*0.5um) polar column, a fully automated liquid sampler, and an OpenLab chemistry workstation.

[0031] Reagents and materials:

[0032] Isopentane, n-heptane, and methanol solutions as standard substances; pure water; quartz beakers, pipettes, volumetric flasks; microsyringes.

[0033] Instrument parameter conditions

[0034] The typical operating conditions for the instrument were selected through experiments and are shown in Table 1.

[0035] Table 1 Chromatographic columns and chromatographic operating conditions

[0036] chromatographic column DB-FFAP (30m*0.32mm*0.5um) Column temperature 110℃ constant temperature carrier gas Nitrogen Column flow rate 2.5 mL / min Flow split ratio 10：1 Number of times to clean the injection needle 6 times Analysis time 2.5min

[0037] Example

[0038] A rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants includes the following steps:

[0039] Sampling: Samples should be taken from the inlet and outlet of the VOCs-containing heat exchangers and circulating water systems that have been put into operation. When testing the total organic matter in the circulating water, three sets of samples should be collected at the inlet and outlet, and the average value of each sample should be calculated. The water sample should be collected in a brown bottle and should be filled completely without leaving a headspace. The water sample should be measured within 24 hours.

[0040] Establish a standard curve for total organic matter in water by chromatography, and plot the standard curve for organic matter.

[0041] Establish the chromatographic method according to the above chromatographic conditions, start the chromatograph, and adjust the method until the instrument is stable. Transfer approximately 1 ml of water sample into a sample vial, place it in the corresponding position of the autosampler, start the instrument, and the run will end after 2.5 minutes. After the run, open the chromatogram and calculate the organic matter content according to the standard curve. The result is expressed as methanol, in mg / kg, and retained to one decimal place. If it is less than the detection limit, it is reported as undetectable. The result is used as the basis for determining whether there is an internal leak in the heat exchanger.

[0042] Comparative Example

[0043] Using the same sampling method as in Example 1, the total carbon (TC) and inorganic carbon (TIC) values ​​of the inlet sample were determined by the TOC method. The difference between the total carbon and inorganic carbon is the total organic carbon (TOC) value.

[0044] Test sample analysis and comparison analysis

[0045] For the same heat exchanger test sample, compare the advantages and disadvantages of the two methods in the examples and comparative examples to examine the expected effects.

[0046] Analysis results of the TOC method in the comparative example:

[0047] Sample at the inlet: TC: 135.6 mg / L, TIC: 123.0 mg / L; TOC = TC - TIC = 135.6 - 123.0 = 12.6 mg / L

[0048] Sample at the outlet: TC: 818.6 mg / L, TIC: 7639.0 mg / L; TOC = TC - TIC = 818.6 - 7639.0 < 0 mg / L. The TOC method analyzes the TOC values of the samples at the inlet and outlet of the circulating water of the heat exchanger, compares their values, and determines that there is an internal leak in the heat exchanger if the TOC value at the outlet is greater than 10% of the inlet.

[0049] However, according to the above test results, it can be seen that the TOC value of the sample at the outlet is actually lower than that of the sample at the inlet, which is unreasonable. Considering the reason, it is because the TOC method analyzes TIC first and then TC. Since the injection direction is vertically upward, the density of the organic liquid leaked from the heat exchanger is smaller than that of water and floats on the water surface, so the abnormal data of TC < TIC appears. For hydrocarbon substances with serious internal leaks and small solubility of the leaked organic matter in water, this method has large errors, resulting in inaccurate test results and the failure of this test method.

[0050] Analysis results of the chromatography method in the example:

[0051] The chromatography analysis results are as follows:

[0052] Sample at the inlet of the heat exchanger (total organic matter calculated as methanol): Not detected;

[0053] Sample at the outlet of the heat exchanger (total organic matter calculated as methanol): 608 mg / kg.

[0054] Whether it is the difference between the data at the inlet and outlet of the heat exchanger or the data of the outlet alone, the chromatography method can determine that there is a very serious internal leak in the heat exchanger. It can be seen that the chromatography method can accurately determine whether there is an internal leak in the heat exchanger, and the test results are intuitive and clear.

[0055] Comparison of the precision of the two methods in the example and the comparative example

[0056] Take the above two samples, the sample at the inlet of the heat exchanger (sample 1 in the following table) and the sample at the outlet of the heat exchanger (sample 2 in the following table), and test them 10 times respectively with the two methods. The statistical data is shown in Table 2 below:

[0057] Table 2 Precision test results of samples (n = 10)

[0058]

[0059] Regarding the data in Table 2, it should be noted that due to the different mechanisms of the two analytical methods, the TOC method includes not only VOCs (volatile organic compounds) but also stable organic carbon components (such as polymers and humic acids) that require high-temperature oxidation (≥680℃) to decompose. In contrast, the organic matter in the chromatographic method consists of volatile organic compounds. Therefore, the numerical values ​​differ significantly between the two methods. The results in Table 2 show that the precision (RSD) of the chromatographic calculations for both heat exchanger outlet and inlet samples is significantly better than that of the TOC method.

[0060] Leak detection sensitivity comparison test:

[0061] Take a non-compliant sample and dilute it with the inlet sample at times of 20, 40, 80, 160, 320, and 640 times. Perform a comparison test of the detection limit using two different methods. The results are shown in Table 3.

[0062] Table 3 Comparison of Detection Limits for TOC Method and Chromatographic Method (Unit: mg / kg)

[0063]

[0064] As shown in Table 3, the TOC at the inlet using the TOC method was 9.73 mg / kg. When diluted 80 times, the value was 10.18 mg / kg, which no longer met the internal leakage assessment criterion of exceeding the inlet TOC value by 10%, thus indicating no internal leakage. In contrast, the chromatographic method, even after a 640-fold dilution, still detected a result of 1.55 mg / kg, indicating the presence of internal leakage. This demonstrates that the chromatographic method has a lower detection limit and better sensitivity.

[0065] Prepare standard samples for leak detection sensitivity testing

[0066] Standards of isopentane, n-heptane, and methanol were prepared using pure water at a concentration of 10 mg / kg. The data were compared using chromatographic and TOC methods, respectively, as follows:

[0067] Table 4 Sample sensitivity test results

[0068]

[0069] The data above indicates that chromatography is more sensitive than the TOC method.

[0070] Comparison of analysis time for individual samples

[0071] The total analysis time using the TOC method is 30 minutes; while the time using chromatography is reduced to less than 3 minutes.

[0072] Comparison of operating steps

[0073] The TOC method requires 6 injections per analysis, while the chromatography method requires only 1 injection.

[0074] Whether to perform two analyses using sampling difference subtraction

[0075] The TOC method requires two analyses using the difference method to determine the total organic matter content in a water sample; the chromatographic method only requires one analysis to obtain the results.

[0076] Injection volume comparison

[0077] The injection volume for the TOC method is 1.6 mL; the injection volume for the chromatographic method is 1 μL.

[0078] Comparison of instrument consumables and maintenance costs

[0079] The TOC method requires a lot of consumables every year, and it also requires phosphoric acid. The maintenance time is about ten days per year, which is relatively high in both cost and time.

[0080] Chromatography has low maintenance costs, requiring only half a day of maintenance per year. The chromatographic column is not easily damaged, resulting in low costs.

[0081] result

[0082] The above experiments revealed that chromatography has the following advantages over the TOC method: 1. Accuracy: RSD improved from 6.02% to 0.76%. 2. Sensitivity: Screening sensitivity is more than 16 times higher than the original TOC method. 3. Speed: Time reduced from 30 minutes to 3 minutes. 4. Efficiency: Injection volume reduced from 1.6 ml to 1 μL. 5. Convenience: Number of injections reduced from 6 to 1 per analysis. 6. Cost-effectiveness: Significant reduction in annual consumable and maintenance costs. 7. Environmental friendliness: The original method required phosphoric acid, while the new method requires no additional reagents, thus protecting the environment.

[0083] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants, characterized in that: Includes the following steps: Samples were taken from the inlet and outlet of the circulating water in the heat exchanger to obtain inlet and outlet samples respectively. Chromatography was used to detect the total organic matter content in the inlet and outlet samples. If the organic matter content in the inlet sample exceeds 10% of the total organic matter content in the outlet sample, it indicates that the heat exchanger has internal leakage.

2. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 1, characterized in that: At least three samples should be taken from the inlet and outlet of the circulating water in the heat exchanger.

3. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 1, characterized in that: The method for detecting inlet and outlet samples using chromatography is as follows: Select the chromatographic column based on the type of organic matter leaking from the heat exchanger; set the column temperature, split ratio, column flow rate, and needle washing times; Select the type of organic matter leaking from the heat exchanger, prepare standard samples of organic matter, and plot the standard curve of organic matter. Place the sample to be tested into the autosampler, start the chromatograph according to the set conditions, and after the chromatograph finishes running, open the spectrum and calculate the organic content in the sample based on the organic standard curve.

4. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 3, characterized in that: The chromatographic column is a DB-FFAP polar chromatographic column.

5. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 3, characterized in that: The column temperature is 110°C, and the split ratio is 10.

1. The flow rate of the chromatographic column is 2.5 mL / min.

6. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 3, characterized in that: The needle washing process shall be repeated at least 6 times.

7. The rapid detection method for determining organic internal leakage in heat exchangers of coal chemical plants according to claim 3, characterized in that: The organic compounds used to prepare the standard samples include isopentane, n-heptane, and methanol.

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