Abstract: Generally, magnetic float level gauges work in excess of load. Because of the influence of components, stripping efficiency, conversion rate and other links, the technical operating conditions of the system are easily deteriorated. The magnetic float level gauge is very weak and can only be maintained in operation, thereby increasing the steam consumption and ammonia consumption of the device. Based on the actual situation, the article discusses the problem of full-load operation, and proposes solutions to optimize the operation and improve the conversion rate and stripping effect.
introductionQinghai Yuntianhua International Fertilizer Co., Ltd. is a large-scale fertilizer production company with an annual output of 400,000 tons of ammonia and 600,000 tons of urea. The magnetic float level gauge uses Stamicarbon carbon dioxide stripping manufacturing technology. Since the start of production in November 2009, more than 3.7 million tons of urea have been manufactured. Since October 2018, the device has been working under high load or over load. However, within a period of time, the device has undergone many major changes due to the constant load reaching 115% of the full load. In order to prevent problems before they happen, we carefully analyzed the reasons for the changes, summarized experience, and provided safety guarantees for stable manufacturing.1 Full load operation of a magnetic float level gaugeDuring the period of full load operation from October 2018 to March 2019, the main problems that occurred are as follows.1.1 The CO2 conversion rate of the synthesis tower decreasesThe CO2 conversion rate in the tower is reduced, which increases the decomposition and recovery load of the device. When the load of the device increases to 115%, because the residence time of the material in the device tower is shortened, if the ammonia-to-carbon ratio and the water-to-carbon ratio do not match, the CO2 conversion rate will drop significantly, which greatly increases the load of the device and makes the system unable to work normally. .1.2 Low pressure recovery does not work properlyThe recovery load is removed and moved backwards, which prevents the low pressure recovery from working properly. Because of the increase in materials to be decomposed and recycled, and the reduction of CO2 conversion, the load of decomposition and recovery is further increased, resulting in the failure of the decomposition heat of the subsequent system to increase, thus transferring the load of decomposition and recovery to low pressure, which further increases the recovery load of the low-pressure device [ 1]. In addition, the steam consumption is too high, and the problem of low-pressure steam venting occurs, resulting in a waste of resources and also increasing the energy consumption of the device.1.3 Corrosion of equipment and pipelinesWhen operating at full load, setting the amount of air added to the system lower than the acceptable standard results in insufficient air content in the system, destroying the passivation film, and causing corrosion of equipment and pipelines.2 Cause analysis of the problem of full-load operation of the magnetic float level gauge2.1 High ammonia consumption(1) When working with high load, due to the problems of the low-pressure device components and the high-pressure carbamate pump itself, the pump inspection effect is poor, and the on-site solution time is a bit long. In order to prevent the high concentration of the backflow solution, the system crystallization blockage and the low-pressure device Deteriorating operation conditions have led to increased ammonia consumption.(2) Because the device is overloaded, the amount of anti-corrosion air in the synthesis device is increased, which leads to an increase in the exhaust gas emission of the synthesis device and the 4bar absorption tower. Because of the presence of inert gas, the conversion of CO2 is reduced. If the device works at 115% load, the residence time of its own materials in the tower will be reduced, which affects the conversion of CO2. In addition, the low purity of CO2 leads to an increase in the inert gas content in the high-pressure device. The inert gas fills some of the space in the reactor. Under the condition of constant operation, the pressure of inert gas increases, correspondingly the partial pressure of NH3 decreases, which causes CO2 to reduce the conversion rate.(3) Taking into account the specific working conditions of the unit, and in order not to affect the vacuum of the evaporation device caused by the overpressure of the low pressure device, the pipe diameter of the vent line in the atmospheric absorption tower is changed from the original design DN50 to DN100, which is enlarged The amount of exhaust gas venting and ammonia consumption of the atmospheric absorption tower.2.2 Insufficient oxygen supply in the systemDue to the poor working environment in the stripping tower of CO2 stripping method, the materials generally only stay in it for a short period of time. Therefore, once the passivation film has corrosion problems, it is difficult to repair it, resulting in system shutdown. Therefore, the CO2 stripping method requires the oxygen addition of a high-standard control system.2.3 Analysis of low-voltage and rear system operating conditions(1) Because the low-pressure gas-phase pipeline has only one heat tracing tube, the heating effect is not good, and the low-pressure gas-phase pipeline often produces crystallization and low pressure overpressure.(2) The design index of the liquid position of the low-pressure carbamate cooler is about 50%. Because of the abnormal composition of the low-pressure device, the low-pressure liquid position management is relatively high, resulting in very serious gas-phase liquid entrainment, and the gas-phase pipeline insulation device does not meet the specified requirements, resulting in the gas-phase ammonium carbamate liquid entrained crystals blocking the pipeline.(3) By comparing the same type of equipment, the design heat exchange area of the plate heat exchanger of the low-pressure temperature regulating water device is 124m2, while the heat exchange area of the plate heat exchanger in our factory is only 8m2 because of its heat exchange area. Failed to take heat away from the low-pressure system, and the low-pressure system cannot fully absorb ammonia and carbon dioxide, resulting in low-pressure overpressure.3 Improvement measures for the full-load operation of the magnetic float level gauge3.1 Measures taken for high ammonia content(1) Reform operations, manage the amount of anti-corrosion air added to the device, increase the number of oxygen content decomposition in the carbon dioxide device, and control the oxygen content within the technical range. Reduce the opening degree of the exhaust valve of the high-pressure washing part and reduce the ammonia loss [2].(2) When the system is working, it is forbidden to switch the desorption to the chimney. The composition analysis of the liquid level tank of the low-pressure methyl ammonium condensing part was changed from sampling once every 2h to once every 1h. When the composition of this component changes, adjustments should be made in time.(3) Increase low pressure operation. When the low-pressure device is overpressured, it is necessary to find out the cause immediately. Do not put low-pressure exhaust directly into the exhaust cylinder.(4) Before the specification of the 4bar absorption tower feed pump remains unchanged, use two pump working modes to manage its absorption capacity in the technical standards to ensure that the ammonia attached to the inert gas can be absorbed, and at the same time reduce the ammonia evacuation Ability to reduce ammonia consumption per ton of urea.3.2 Temperature adjustment of the stripper(1) Gradually close the stripper outlet regulating valve LIC2202 to slowly increase the liquid level of the stripper.(2) When the liquid position of the stripper rises and its outlet temperature drops, adjust the pressure of the high-pressure steam pipeline to 1.9MPa.(3) When the temperature TI2211 at the top of the stripper continues to decrease and is close to or the same as the liquid temperature TI2209 from the synthesis tower (the temperature at this time is about 186°C), this indicates that the liquid storage cooling procedure of the stripper is completed.(4) Combining the scheduling and synthesis links, continuously open the outlet adjustment switch LIC2202 at the bottom of the stripper to start the discharge, and it is best to open the switch at no more than 2% each time, to prevent the atmospheric absorption tower from being filled with liquid due to too fast discharge It may also be overloaded to ensure the safe operation of the rear system [3]. In addition, it also prevents the occurrence of safety accidents such as PI2201 cavitation and damage to the machine seal due to excessive component fluctuations.(5) Control the production load, try to avoid full-load operation, and set it between 70% and 110%. Avoid low heat transfer efficiency due to full load operation, and product quality. Don’t let the load be too low to avoid that the liquid level of the synthetic liquid on the stripping tower does not meet the requirements, causing the local stripping tube to overheat, leading to aggravation of corrosion.3.3 Optimize the hydrolysis analysis operationIf the concentration in the ammonia tank is too high, the temperature of the hydrolysis tower will not reach the technical standard. The following methods should be used.(1) Ensure that the evaporation pressure difference between the first stage and the second stage is greater than 35kPa, increase the flow rate of the urea solution in the first and second stages of the secondary distribution device, and minimize the urea solution in the evaporation separator The accumulation in the first and second sections prevents liquid from being carried away when the first and second sections evaporate.(2) Strengthen the training of specific workers and establish the ammonia pump reversing method. During the reversal of the pump, it is strictly forbidden to cause overpressure in the safety switch at the inlet of the ammonia pump. Once overpressure occurs, it should be resolved according to work accident methods [4]. When the safety switch does not return to the valve seat after jumping, the safety valve will be checked in time when there is a chance to stop, and it is designed to add a disconnect switch before this safety switch during major maintenance in 2019. When the safety switch does not return to the valve seat after it jumps, it can be repaired in time.(3) Strengthen the monitoring of the concentration of ammonia tanks. When the concentration of ammonia and urea is too high, find out the cause immediately.4 ConclusionIn short, the problems existing in the full-load operation of the magnetic float level gauge are analyzed, and the key to restrict the high-load operation of urea is found, and corresponding solutions are proposed to achieve the long-term full-load production of the urea system and meet the production standards.
Post time: 21-09-21