Summary of Gas Analysis for Small Nitrogen Fertilizer Enterprises

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Xing Yutang (Shanxi Heshun Galaxy Chemical Co., Ltd., Heshun, Shanxi 032700), January 24, 2008: The entire production process of nitrogen fertilizers involves gas flow and reactions. Gas analysis is not given enough importance in some small nitrogen fertilizer manufacturers. In recent years, computer control has been increasingly adopted in production management; without accurate composition analysis results, computers cannot carry out production control based on precise inputs. Therefore, it is very important to pay attention to and study gas analysis in low-nitrogen fertilizers. The main methods for gas analysis in small nitrogen fertilizer enterprises are as follows. 1. Austen analyzer method: The Austen gas analyzer is a classic manual analyzer for determining chemical formulas; it is easy to operate and maintain. The precautions for use are discussed in more detail in the \"Production Analysis Procedures for Small Nitrogen Fertilizer Plants\" (first edition, September 1996, Chemical Industry Press). 1.1 Analysis of semi-water gas (shift gas): The composition of semi-water gas is relatively complex, with a high CO content. If the CO absorption method is used, a large amount of reagents is required, and the operation is also complex. If the ternary explosion method is used, the volume fraction of CH4 in semi-water gas cannot be determined. A more feasible approach is to combine the two methods: the ternary explosion method is used for normal analysis, while CO is periodically absorbed to verify the CH4 content in semi-water gas and shifted gas. The CO content in the transformed gas is low, so CO absorption can be used entirely, and the gas composition can be determined by the H2/CH4 binary explosion method. When production is normal, the CH4 content in the gas can be adjusted to determine the CH4 content in semi-water gas. 1.2 Analysis of purified gas (urea feed gas): The purified gas is primarily tested for its degree of purification, while the urea feed gas is analyzed for the purity of CO2 and the amount of O2 present. Therefore, the tests at these two locations only measure the concentrations of CO2 and O2, which can be determined using the absorption method as per the operating procedures. 1.3 Analysis of refined gas (circular gas) The composition of the refined gas is relatively simple; in addition to H2 and N2, it also contains small amounts of CH4 and Ar (inert gas). (1) Due to the high H2 content, special attention should be paid to the influence of the comb tube volume on the analysis results in this case. Calculations show that when measuring a sample gas with an H2 content of 75%, an error of 3.8% can occur for every 1 ml of comb tube volume. In the discussion of the usage precautions for the austenite analyzer, the issue of volume calibration was specifically mentioned. (2) The Austen analyzer is not suitable for analyzing Ar in gases, as the content of Ar affects the calculation of the hydrogen-to-nitrogen ratio in the synthesis cycle gas. Although the content of N2 can be determined using the methods provided in the \"Production Analysis Procedures for Small Nitrogen Fertilizer Plants,\" it is not easy to implement due to the complex preparation of the instruments, cumbersome operations, and high requirements for analysts. The Ar content can be approximated by calculation: in air, N2% : Ar% = 78 : 0.94. Both N2 and Ar in the process gas are added from air, and they do not participate in any reactions before synthesis; therefore, this ratio remains unchanged in the purified gas as well. Components of the refined gas: H2, N2, CH4, Ar, with trace amounts of CO + CO2 (negligible). The ratio of N2% to Ar% is 78:0.94. In the refined gas produced by the methanation process, the CH4 content is approximately 3.0%, while the H2 content is 78.4%; therefore, the N2 content is 100 – CH4 – H2 – Ar. Ignoring the Ar content, we get: N2% = 100 – 3.0 – 78.4 = 18.6. Meanwhile, Ar% = 0.94 × (N2/78) × 18.6 ≈ 0.3. Then, in the refined gas, the ratio of CH4 to Ar is 3.0:0.3 = 10:1. Once inside the synthesis system, neither CH4 nor Ar participates in the reaction, and their ratio remains at 10:1. The N2 content in the recycle gas can be calculated using this value and CH4, thereby determining the hydrogen-to-nitrogen ratio. In actual production, such calculations still contain errors due to the inability to estimate the impact of liquid ammonia on the dissolution of N2 and H2. Corrections still need to be made during the production process. The methane content in the refining gas during the copper washing process is low, making it difficult for an OES analyzer to determine the CH4 level. The sampling ratio of refining gas to air can be changed (for example, to 40:60) in order to obtain more accurate results. 1.4 Safe hot work analysis: To conduct hot work analysis using an oxygen analyzer, an explosion bottle must be attached to the analyzer. Two points should be noted when using it: a) Attention must be paid to the degree of completion of the chemical reaction. During normal production, conventional sample gases yield correct results when analyzed according to the established procedures. In hot work analysis, the gas composition can vary greatly; therefore, sufficient time, temperature, and oxygen are necessary to ensure that the reaction proceeds completely. b) Before each replacement of sample points and their introduction into the combustion flask, a test explosion must first be conducted in the explosion flask. To prevent explosions of the gas during combustion, which could damage the equipment or cause injury to people. 2. Iodometric analysis of H2S in gases: Iodometric analysis of H2S is widely used both domestically and internationally, with absorption solutions such as (CH3COO)2Cd (or CdCl2) and (CH3COO)2Zn being employed. Cadmium salts are toxic and can easily cause water pollution. Except when using H2S in high concentrations, zinc acetate should be used under normal conditions. There are two sources of error in the iodometry method: one is that iodine is volatile and prone to loss ； Secondly, I– is easily oxidized by O2 in the air in acidic solutions, resulting in the release of I2: 4I– + 4H+ + O2 = 2I2 + 2H2O. Therefore, when determining H2S using the iodometric method, it should be carried out in an iodometry flask and exposure to sunlight should be avoided. To reduce contact between I- and air and minimize the volatilization of I2, the solution should not be shaken excessively during titration; starch should be added only near the end of the titration to prevent the starch from adsorbing iodine. 3 Analysis of circulating gaseous ammonia content: The usual method is the Blaus method. In addition to the points discussed in the \"Production Analysis Procedures for Small Nitrogen Fertilizer Plants,\" this method requires attention to be paid to the accurate determination of the endpoint: gas supply should be stopped when half of the solution changes from red to yellow. Shaking the ammonia reaction tube up and down, if the entire solution turns yellow, it indicates that the endpoint has been reached; if it remains red, a small amount of gas still needs to be introduced. At the same time, this method converts volume based on the amount of substance, and the influence of temperature must be taken fully into account. 4 Trace sulfur analysis: The analysis of trace sulfur in gases is very important in production, and specialized analytical instruments are now commonly used for this purpose. Typical examples include the HC series of chromatographs from Hubei Chemical Research Institute, and the LC series of microcoulometers from Jiangsu Jiangyan Analytical Instrument Factory. These two instruments have their own advantages: chromatographic analysis can detect different forms of sulfur, while microcoulometric analysis provides good results even for sample gases with high concentrations. 5 Other gas analysis methods 5.1 Chromatography Gas chromatographs are undoubtedly the most ideal analytical instruments in fertilizer production, offering a wide range of applications, fast analysis speeds, and accurate results. However, the equipment investment is high, and a high level of maintenance for the chromatography columns is required. At the same time, environmental requirements also limit its use. 5.2 Combustible gas detectors: Using an oxygen analyzer for hot work analysis takes a long time, and there is sometimes some error. It can be measured using a combustible gas detector, in combination with an oxygen meter to prevent false alarms. When using a detonation detector, in addition to regular calibration, it is also necessary to frequently calibrate the instrument using standard gases of the substance to be detected, and appropriate sample gas desulfurization agents must be used to ensure its proper functioning. Care should be taken to protect the probe of the explosion detector; prolonged exposure can cause premature aging of the probe, affecting its performance. 5.3 Applications of various online analyzers (1) Trace (CO+CO2) analyzers: The concentration of trace (CO+CO2) in refined gas is very low (20×10‑6), making it impossible to analyze using manual methods. Currently, the DD-10 type automatic analyzer for trace CO and CO2 produced by Nanjing Analytical Instrument Factory is mostly used. The following points should be noted when using this instrument. ① This instrument converts CO in the sample gas through I2O5; if the sample gas contains unsaturated hydrocarbons, these can also react with I2O5 to produce CO2, resulting in higher values. A mixture of Ag2SO4 and HgSO4 should be used to remove the unsaturated hydrocarbons from the sample gas. ② Since the conversion of CO from I2O5 must take place at a specific temperature, it is necessary to strictly control the temperature of I2O5. When its temperature is below 105 °C, the conversion rate does not meet the required levels, which results in lower readings on the instrument. Sometimes, to ensure that the analysis results meet the required standards, operators deliberately lower the conversion temperature to bring the minor analysis values within the specified range. This will lead to chronic poisoning of the ammonia catalyst, thereby reducing the ammonia synthesis rate. It must be strictly prohibited. ③ The free iodine generated by the reaction of CO with I2O5 must be removed using thiourea. (2) Constant infrared gas analyzers: Constant infrared analyzers are commonly used to continuously measure the concentrations of substances such as CO, CO2, NH3, and CH4 in various mixed gases; they represent an important category of online analyzers. When using an infrared analyzer, the sample gas must be dry, clean, and free of corrosive gases; therefore, chemicals are often used to remove harmful impurities. The proper design of the sample gas pretreatment system plays a crucial role in determining the instrument’s service life and the accuracy of the measurement results. To address the common issues encountered in preprocessing, the AR series of preprocessing systems produced by Chengdu Shucheng Analysis Technology Development Company feature excellent corrosion resistance, a fully enclosed design for safety and explosion protection, and a high capacity for preprocessing sample gases – representing an advancement in the field of online analysis systems. The infrared analyzers recommended in the \"Analysis Procedures for Small-scale Nitrogen Fertilizer Production\" include models such as FTH-1, QGS, FQC, HQG, and HW. (3) Thermal conductivity analyzer: Thermal conductivity analyzers are the earliest type of online analyzers to appear, with a large variety of models and wide-ranging applications; they are commonly used to automatically measure the volume fractions of various gases such as H2, Ar, and SO2 in mixtures. Domestic models include: QRD-110Z, QRD-111A, QRD-111Z, RD-004, etc. (4) Oxygen analyzer: Due to the use of electrostatic coking towers, the analysis of oxygen content in semi-water gas has become even more important. Online analysis often employs thermomagnetic oxidation analyzers; common models in China include the QZS series. The former Ministry of Chemical Industry had recommended the use of the KY-ZB type oxygen control instrument, which features high sensitivity and an alarm function; however, its drawback is a short electrode lifespan. (5) Calibration of online gas analyzers Online analyzers are calibrated using standard gases, and it is very important to select the appropriate standard gases in order to ensure the accuracy of the analysis instruments. Standard gases in cylinder form should be purchased from qualified professional gas supply stations. If preparation is to be done manually, the accuracy of the instrument used to determine the content of the standard gas must be higher than that of the analyzer being calibrated. As the quality and performance of analyzers continue to improve, online automatic analyzers combined with portable automatic analyzers will surely replace many of the current cumbersome manual analyses. The development of modern instruments, with improvements in ease of operation and maintenance as well as in their compatibility, will surely free analytical laboratory personnel from manual labor.