Oil washing circulation volume

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Methods for Stabilizing the Quality of Wash Oil in the Crude Benzene Processing Process Yan Hongfu (Shanxi Lüliang Energy Development Corporation, Lüliang 033000) 1 Process Overview The crude benzene recovery method used by Zhonglu Coking Co., Ltd. involves absorption using tar wash oil, as well as benzene removal through heating the rich oil in a tubular furnace. The coke oven gas coming from the ammonia washing and ammonia decomposition units enters directly at the bottom of the benzene washing tower, where it comes into countercurrent contact with the circulating washing oil sprayed from the top of the tower. The benzene compounds in the gas are absorbed by the circulating washing oil, and the benzene content in the gas exiting from the top of the tower is required to be less than 2 g/m3. The oil-enriched stream, which has absorbed crude benzene, is pumped to the crude benzene condenser by an oil-enriching pump; it is preheated using crude benzene vapor, then passes through a heat exchanger with hot lean oil, and finally is heated to 180–190°C in a tubular furnace before entering the debenzening tower. Superheated steam heated to 400°C in a tube furnace enters the debenzene tower through an oil-washing regenerator, serving as the heat source for debenzening. The crude benzene vapor evaporated from the benzene removal tower enters the crude benzene condenser, where it is cooled to around 30°C before entering the crude benzene oil-water separator. The separated water enters the control separator, while the separated crude benzene goes into the crude benzene reflux tank; part of this crude benzene is sent to the top of the benzene removal tower as reflux. The hot lean oil at the bottom of the benzene removal tower exchanges heat with the rich oil before entering the lean oil cooler; thereafter, it is sent to the lean oil tank and then to the benzene washing tower for reuse. 2 Existing problems: The recovery rate of crude benzene is unstable, averaging less than 0.8%; increasing the volume of the recycled washing oil does not yield satisfactory results. Upon analyzing the quality of the recycled washing oil, it was found that its quality is deteriorating. To address the issue of declining quality in the recycled washing oil, we conducted follow-up analyses on the benzene content in the oil after the benzene washing tower, as well as in the lean and rich oils. We inspected the operational conditions of each piece of equipment and carried out a thorough analysis of the operations at every stage. The main reasons are as follows. (1) Since there is no gas final cooling tower in the crude benzene processing unit, the gas flows directly from the ammonia washing towers in the ammonia washing and ammonia decomposition processes to the benzene washing tower in that unit. In the ammonia washing and ammonia decomposition processes, ammonia washing is primarily accomplished by using ammonia-evaporation wastewater to absorb ammonia from the gas; the high temperature of this wastewater causes the temperature of the gas inside the benzene washing tower to be higher than that of the oil being washed. As a result, the moisture in the gas condenses and ends up in the oil, thereby reducing the quality of the oil. (2) The superheated steam at the regenerator inlet experiences large fluctuations; the opening degree of the superheated steam inlet valve is low, resulting in a low amount of steam entering the regenerator and thus preventing the temperature from rising. (3) The gas nozzle in the tube furnace gas heating system is unreasonable, resulting in a low oil-rich temperature; the oil-rich temperature is less than 160°C. (4) Improper operation caused the temperature at the top of the benzene removal tower to rise, vaporizing the light components in the washing oil and compromising its quality. 3 Measures to stabilize the quality of oil washing in the circulation process: (1) In the ammonia washing and ammonia decomposition steps, low-temperature soft water spraying was added to Tower No. 2 for ammonia washing, which reduced the temperature of the gas and ensured that the temperature of the gas entering the benzene washing tower could be maintained between 25 and 27°C. (2) Replace the gas nozzle of the tubular furnace, adjust the air intake volume of the tubular furnace to stabilize the amount of superheated steam, raise the temperature of the superheated steam to 380–400°C, and raise the temperature of the rich oil to 180–190°C. (3) Improve operations to stabilize the crude benzene reflux rate in the debenzing tower, adjust the steam supply, and keep the temperature at the top of the debenzing tower below 100°C. (4) Stabilize the amount and temperature of superheated steam at the regenerator inlet. To ensure the quality of the recycled washing oil, 1.5% of the amount of recycled washing oil is introduced into the washing oil regenerator through the pipeline prior to the oil entering the tower; by adjusting the steam volume, the temperature of the oil vapor and water vapor exiting the top of the regenerator is maintained at around 190°C. 4 Effects: By implementing the above improvement measures, the operation of the crude benzene system was stabilized, with a crude benzene recovery rate of over 0.94%. It has prevented the deterioration in the quality of recycled oil, providing a viable solution for stabilizing production. It is recommended that during the production process, testing data should be comprehensive, accurate, and timely to prevent a vicious cycle of oil washing. When adding new washing oil, testing methods should also be employed to ensure strict control over the quality of the washing oil, thereby preventing significant variations in its quality that could affect the absorption efficiency