Analysis of Excessive CO Content in Nitrogen After Liquid Nitrogen Washing

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The last edit to this post was made by 1587328380 on 2016-11-9 at 13:20. Summary of the analysis on the reason for excessive CO levels in N2 during liquid nitrogen washing: The low-temperature liquid nitrogen washing system uses liquid nitrogen to absorb CO gas, which is harmful to ammonia synthesis catalysts. Keywords: liquid nitrogen washing, air separation distillation tower, CO content. Our company is a large-scale urea manufacturer that uses coal as raw material; for gas purification, we employ low-temperature methanol washing and low-temperature liquid nitrogen washing technologies. The liquid nitrogen washing unit receives the purified gas from the upstream low-temperature methanol washing process, and then uses liquid nitrogen at low temperatures to remove substances such as CO, CH4, and Ar from this gas, thereby producing hydrogen-nitrogen synthesis gas with a H2/N2 ratio of 3:1. The hydrogen-nitrogen syngas is first sent to the H2-N2 gas compressor in the ammonia synthesis process for compression, and then liquid ammonia, the product, is produced in this process. I. Description of the liquid nitrogen washing process: Liquid nitrogen washing is carried out in a washing tower at a temperature of -188°C to -193°C and a gauge pressure of 5.27 Mpa. Liquid nitrogen enters from the top of the tower, while the feed gas enters from the bottom; the two come into contact in counterflow and heat transfer occurs on the tray surfaces. Impurities such as CO, CH4, and Ar are washed away by the liquid nitrogen, resulting in a purified gas. Purified hydrogen containing 8.9% nitrogen exits from the top of the tower; by passing through inert gases, a pure gas with a CO content of <5×10-6 is obtained. When the oxide content in the ammonia synthesis tower exceeds 20×10-6, it can cause the ammonia synthesis process to stop; therefore, it is very important to control the CO content in the syngas. II. Course of Events On August 29, 2014, at 19:00, the online analysis value of CO at the liquid nitrogen washing outlet, AI17002, fluctuated around 0.7 ppm and was within normal limits; by 19:30 it rose to a maximum of 1.5 ppm before dropping back to 0. At 20:00, AI17002 once again rose to 1.5 ppm, prompting the control room staff to monitor it more closely. At 20:10, AI17002 increased to 2.0 ppm. At this point, the dispatch team informed the personnel in charge of purification to reduce the flow of N2 and N3. However, the control room staff told the dispatch team that since AI17002 was showing an upward trend, more N2 was needed, and they requested that an manual analysis of AI17002 be conducted. Subsequently, adjustments were made to the valves used for washing with nitrogen, FV17010, the valve used for regulating the temperature of the process gas, TV17039, and the valve used for supplying crude nitrogen, FV17009. 21:02 AI17002 rose to 6.4ppm and continued to rise. At this point, the control room staff informed the dispatch team to reduce the amount, and also asked the analysis team to conduct a second analysis of component A17002; the result of that analysis was 11 ppm. The inlet flow rate of liquid nitrogen has been reduced from 94,384 Nm3/h to 83,872 Nm3/h. At 21:21, AI17002 rose to a maximum value of 9.5 ppm, while the manual analysis showed 15 ppm; at this point, the nitrogen consumption for washing reached its maximum level of 13,023 kg/h. 21:50 AI17002 dropped to 5 ppm, the target level. At 22:04 it dropped to 2 PPM, while manual analysis indicated 3 ppm. Since August 29, the CO level in the syngas has remained volatile, showing an upward trend in its concentration; in such cases, the liquid nitrogen washing system is activated to reduce the volume of gas. The syngas production was halted twice due to excessive CO levels, and the system was used for reduction measures 29 times. Subsequently, a specialized analysis was conducted on liquid nitrogen washing ； Causes of CO content in syngas and treatment methods ； (1) The amount of nitrogen used for liquid nitrogen washing is insufficient ； (2) The temperature for liquid nitrogen washing is too high ； (3) The operating pressure is too low ； (4) Fault in the methanol washing startup line ； (5) Tray reasons ； Solution ； (1) Increase the amount of nitrogen used for washing; (2) Lower the operating temperature TI17039 for liquid nitrogen washing to below -185°C ； (3) Increase the system pressure to above the normal operating pressure of 5.0 MPa ； (4) Check the actual operating status of the valve assemblies HV16003A/B/C/D on the methanol washing startup line to confirm that they are in their proper positions ； (5) Parking for maintenance. After confirming that there were no faults with the heat exchangers, trays, or valves, we focused once again on analyzing the purity of nitrogen. With the cooperation of the Production Department and the Quality Control Center (manual analysis of N2 composition was carried out whenever there were fluctuations in CO levels in the syngas), three days of sampling and comparative analysis were conducted, and finally, on September 13, it was determined that the high CO level in the nitrogen was the cause. III. Consequences of exceeding the limits 1. It caused the ammonia synthesis process to stop twice, affecting the ammonia synthesis system. 2. Multiple reductions in the amount of liquid nitrogen used for washing led to a decrease in the production of ammonia and urea. IV. Cause analysis (1) Comparison of gas compositions ； Under standard conditions, pure carbon monoxide (CO) is a colorless, odorless, and non-irritating gas. The relative molecular mass is 28.01, the density is 1.25 g/l, the freezing point is -205.1°C, and the boiling point is -191.5°C. Its solubility in water is very low; it is extremely difficult to dissolve in water. Nitrogen is a colorless, odorless, and tasteless gas under normal conditions, and it is generally non-toxic. Nitrogen accounts for 78.12% of the total atmosphere by volume. Its density under standard conditions is 1.25 g/L. Nitrogen is poorly soluble in water; at normal temperature and pressure, only about 0.02 volumes of nitrogen dissolve in 1 volume of water. Melting point: -210°C, boiling point: -195.8°C, relative molecular weight: 28. In terms of gas composition, CO and N2 have similar boiling points and molecular masses of 28. Moreover, our company is separated from the flue gas vents of the steel plant and the inlet filters of the air separation compressors by only one wall. Due to the low height of the steel plant’s flue gas vents, when the wind direction changes and the flames do not ignite, CO can enter the compressors along with the wind. Air separation distillation: It utilizes the difference in boiling points of substances to carry out multiple cycles of partial condensation of the mixed vapor and partial evaporation of the mixed liquid, in order to achieve separation. Important components of air Name Chemical symbol Volume percentage Boiling point °C (101.325 KPa) Nitrogen N2 78.09 -195.8 Carbon monoxide CO -191.5 Argon Ar 0.932 -185.7 Oxygen O2 20.95 -183 As can be seen from the table above, the boiling points of CO and N2 are very close to each other; carbon monoxide emitted by steel mills enters the air separation distillation tower along with the air. (2) The process flow of the air separation distillation column is described below: In the lower tower, air is initially separated into nitrogen and oxygen-enriched liquid air. The gaseous nitrogen at the top is liquefied in the main condensation evaporator, while the liquid oxygen on the low-pressure side of the main condensation evaporator is vaporized. The vast majority of the liquid nitrogen returns to the lower column as reflux liquid, while the remaining liquid nitrogen is subcooled by a cooler using pure nitrogen and contaminated nitrogen, and then throttled before being sent to the top of the upper column. The contaminated liquid nitrogen drawn from the upper part of the lower column is subcooled in a cooler before being sent to the upper part of the upper column via throttling. The oxygen-enriched liquid air drawn from the bottom of the lower column is subcooled in a subcooler and then sent to the middle section of the upper column via throttling as reflux liquid. Dirty nitrogen is drawn out from the upper part of the upper tower. After being reheated in the subcooler as well as the high-pressure and low-pressure main heat exchangers, it is sent outside the distillation tower; part of it serves as regeneration gas for the molecular sieve purifier, while the rest goes into the water-cooled tower. Pure nitrogen is drawn from the top of the upper tower; after being reheated in the subcooler and the low-pressure main heat exchanger, it exits the cold box and is sent to the water-cooled tower to serve as a cooling source for cooling external water. CO is drawn out from the top of the lower column along with nitrogen; after being reheated in the low-pressure main heat exchanger, it exits the cold box. It is then compressed to 6.0 MPa by the nitrogen compression unit and sent to the liquid nitrogen washing unit for nitrogen blending before being fed into the synthesis process. Cause analysis ； (1) The design of the steel plant’s flare system was unreasonable; the emission of CO into the air separation system was the main cause of this accident. (2) There was insufficient awareness regarding CO, as it entered the distillation tower along with the air compressor and then made its way into the N2 main pipeline. (3) The on-site operators detected a CO alarm but failed to report it in a timely manner, which delayed action to address the excess levels of CO. IV. Preventive measures: (1) Inform the steel plant that flares must not go out. (2) Instruct on-site inspectors to report any CO alarms immediately to the control room staff. (3) Work in conjunction with the air separation team to regularly take samples of CO in N2 for analysis. References: [1] Lu Xiaofeng, Operation Manual for Nitrogen Washing Process, Global Design; [2] Li Yongkang, Shen Weileng, \"Deep Cryogenic Technology\", a core journal of Chinese science and technology, 1961