Detailed Explanation of the Operating Techniques for Catalytic Cracking Fractionation Systems

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Operation techniques for distillation systems 1. Factors affecting the operation of distillation systems. To achieve high processing capacity, high yields, high quality, and low consumption in a production facility, apart from choosing an appropriate process flow and advanced equipment, it depends largely on the quality of operation. This includes knowing how to maintain stable operation when production conditions and tasks remain unchanged, as well as how to establish new stable operating conditions when those conditions change. Stable operation refers to fully utilizing the potential of equipment in production, so as to achieve high yields and high-quality products while reducing consumption levels, and at the same time ensuring material balance and heat balance across all equipment and the entire plant. They are manifested in the relative stability of various process conditions during operation, including flow rate, temperature, pressure, and liquid level ； To this end, it is necessary to first discuss the main process factors affecting distillation operations, in order to identify the key factors that frequently influence the operation. The main indicator of the separation efficiency of a distillation column is the separation accuracy. The accuracy of distillation depends not only on the structure of the distillation column (such as the type of trays, tray spacing, number of trays, etc.), but the main operational factors affecting it include temperature, pressure, reflux rate, steam velocity inside the column, amount of steam introduced, and the liquid level at the bottom of the column. (1) Temperature: The temperature of the oil and gas entering the tower, especially the temperatures at the top of the tower and along the side streams, must be strictly controlled. To maintain stable operation of the distillation column, it is most important to keep the reaction temperature constant. When the processing volume remains constant, the temperature of the oil and gas inlet directly affects the amount of heat entering the tower; accordingly, the temperatures at the top of the tower and along the side streams change, as does the quality of the product. When the temperature of the oil and gas remains constant, changes in the reflux volume, reflux temperature, and the amounts of various distillates can also disrupt the thermal equilibrium within the tower, resulting in temperature variations throughout it. Among these, the temperature at the top of the tower is the most sensitive indicator of changes in thermal equilibrium. (2) Pressure: The temperature required for oil distillation is related to the partial pressure of its vapors. The lower the partial pressure of the vapors, the lower the temperature needed to distill the same oil. The oil vapor partial pressure is the product of the operating pressure inside the equipment and the molecular fraction of the oil ； When the amount of water vapor and the amount of inert gas (introduced as part of the reaction) in the tower remain constant, the partial pressure of oil and gas decreases as the operating pressure in the tower drops. Therefore, provided the load in the tower permits, reducing the operating pressure within the tower or appropriately increasing the amount of steam fed into it can both lower the vapor pressure of oil and gas. (3) Reflux rate and reflux temperature to the column: Reflux creates the conditions for contact between the gas and liquid phases. The reflux rate and the temperature of the reflux returning to the column directly affect the thermal balance of the entire column, thereby influencing the quality of the distillation process. For catalytic fractionators, the amount of reflux and the temperature of the reflux returning to the tower are determined by the overall thermal balance of the tower. As the temperature conditions within the tower change, appropriately adjusting the reflux volume and reflux temperature at the tower top is a means to maintain temperature balance there, thereby achieving the goal of regulating product quality. During normal adjustment, the focus is on adjusting the temperature of the reflux returning to the tower. (4) Bottom liquid level The change in the bottom liquid level reflects changes in material balance, which in turn depends on the stability of temperature, flow rate, and pressure. The reaction depth has a significant impact on the liquid level at the bottom of the tower. 2. Operation control of the distillation system (1) Control of the liquid level at the bottom of the distillation tower: Under normal conditions, the liquid level at the bottom of the distillation tower is controlled manually using the slurry discharge control valve. Main influencing factors and adjustment methods Influencing factor (1) Changes in the amount of slurry reprocessed. (2) Changes in the amount of material processed in the reaction; (3) Increase in the weight of the raw materials and deterioration in the properties of the catalyst. (4) Changes in the temperature of the slurry returning to the tower (5) The re-refining tank becoming full (6) Changes in the degree of reaction (7) Changes in the pressure of the distillation tower (8) Pumps and compressors running dry. (9) Failure of the three-way valve or instrument control results in a low liquid level. Adjustment method (1): Control the amount of slurry reprocessing through reaction control. Control the slurry circulation rate and the temperature returning to the tower. (3) Adjust the operation according to changes in the properties of the raw materials and the catalyst. (4) Adjust the temperature using a three-way valve. (5) Increase the amount of oil recycled in the reprocessing stage, and reduce the amount sent back to the tower in the second stage. (6) Control the reaction depth. (7) Properly control the pressure in the settler, as well as the amount of reaction recycle and the reaction temperature. (8) Adjust the operating conditions of the pump, or switch to a standby pump. (9) If the control valve and instruments fail, consult the instrumentation team for handling. (2) Temperature control at the bottom of the fractionator: The normal method for controlling the temperature at the bottom of the fractionator is by adjusting the circulation rate and the temperature of the slurry returning to the tower. An auxiliary method involves regulating the level of liquid at the tower bottom by controlling the amount of slurry discharged externally. In special circumstances, the valve for directing slurry back into the tower can also be used as a control measure; however, it is essential to ensure that there is sufficient oil flowing back into the tower to prevent flooding. In case of an accident, when the bottom temperature of the tower rises sharply, cold wax can be used to repair the bottom valve in order to regulate the bottom temperature. The bottom temperature of the fractionation tower is controlled such that the liquid phase temperature remains ≤ 360°C, while the gas phase temperature at the tower bottom is kept ≤ 380°C (the factors affecting the bottom temperature are the same as mentioned earlier). (3) Control of slurry solid content: The slurry solid content is related to the amount of slurry discharged, the amount of slurry recycled, the amount of slurry sent upward and downward in the tower, the processing volume, and the properties of the catalyst. When the solid content in the slurry is high, it can cause wear and damage to equipment, particularly severe wear on the slurry pumps. It can also lead to coking and sedimentation at the bottom of the tower, blocking heat exchange equipment and pipelines. Therefore, during normal operation, the allowable value should be ≤6 g/L, while during startup, it should be ≤10 g/L. Under normal conditions, it is manually controlled by the slurry discharge volume FV-212. (4) Control of the distillation column top temperature: The temperature at the top of the distillation column is the most important parameter for controlling the dry point of crude gasoline. A higher temperature results in a higher dry point. Under different column top pressures and with varying properties of the feedstock, the dry point of gasoline will change; therefore, it is necessary to adjust the temperature according to the specific pressure and feedstock conditions in order to ensure that the product meets the required standards. Under normal conditions, it is manually controlled by the top circulation control valve FIC-202. For more interesting content, follow the WeChat official account: Catalytic People. Key influencing factors and adjustment methods. Influencing factor (1): Changes in the properties of raw materials. (2) Change in reaction depth. (3) Change in processing capacity. (4) Changes in top cycle rate and cold reflux. (5) Water carried in the cold reflux zone. (6) Change in tower top pressure. (7) Pump failure. Adjustment method (1): Adjust the temperature in a timely manner according to the properties of the raw materials. (2) Adjust the top temperature as the reaction depth changes. (3) Adjust the top temperature according to the processing volume. (4) Adjust the flow rates and return temperatures of the top cycle reflux and cold reflux to the tower. (5) Control the interface in the reflux drum to prevent water carryover. (6) Contact the reaction control team to stabilize the operating pressure. (7) Switch to the backup pump or increase the cold return flow rate. For more great content, follow the WeChat official account: Catalytic People