Thread Content
The second heavy oil catalytic cracking unit at the Maoming branch was put into operation in 1989. In recent years, with an increase in imported crude oil and a rising proportion of residue used in blending, the feedstock for this unit has become of lower quality and heavier in nature. The oil slurry stays for longer periods at the bottom of the distillation tower, has a low linear velocity in the heat exchange equipment, and contains high levels of solids and aromatics; as a result, coking problems in the oil slurry system have become increasingly severe. Therefore, the causes affecting coking in the device’s fractionation system and corresponding countermeasures are discussed. (1) Bottom temperature of the distillation column: The properties of the slurry change depending on the proportion of heavy oil blended in and the bottom temperature of the distillation column. If the bottom temperature is too high, coking at the bottom of the column can occur even with a short residence time; therefore, it is very important to select an appropriate bottom temperature. When putting the cleaned heat exchanger into use, maintain the circulation rate of the oil slurry, reduce the temperature at the bottom of the distillation tower from 355°C–360°C to 345°C–350°C, and continuously inject scale inhibitors to prevent insoluble substances in the oil slurry from sticking to the tube walls. While using the bottom stream oil slurry as quench oil to lower the bottom temperature of the tower, reprocessed oil can be directly injected at the bottom of the distillation tower if necessary. There are two purposes for injecting quench oil at the bottom of the distillation tower: one is to lower the temperature at the bottom of the tower, thereby reducing high-temperature polymerization and coking of the oil slurry ； Secondly, it can stir the oil slurry at the bottom of the tower, preventing the formation of flow dead zones and stopping catalyst particles from settling and clogging the filter for the oil slurry at the tower bottom. (2) Residence time of the slurry at the bottom of the distillation tower: The length of time the slurry stays at the bottom of the tower depends on the level of the liquid at that location and the volume of slurry circulating in the tower. The processing capacity of the second catalytic unit at the Maoming branch is 1 Mt/a, with 50.29% residue being blended in; the circulation rate of the slurry ranges from 50 t/h to 360 t/h. Based on this, it is possible to calculate the residence time of the slurry at different liquid levels at the bottom of the distillation tower. When the slurry circulation rate is normal, it is appropriate to keep the residence time of the slurry at the bottom of the fractionation tower between 5 minutes and 6 minutes. Therefore, by adjusting the reaction depth and bottom temperature, the liquid level at the bottom of the distillation column should be kept around 50%, with 60% being an appropriate upper limit. By maintaining an appropriate liquid level, it is possible to avoid situations where the low liquid level causes the slurry pump to run dry, resulting in an excess of heat in the distillation tower and thereby disrupting the thermal balance of the entire distillation tower ； It can also prevent the liquid level from being too high, which would cause the slurry to coking due to prolonged exposure to high temperatures. Furthermore, the three-way valve at the slurry evaporator should direct as much flow as possible to the cold side, in order to maintain a high flow rate of the slurry within the heat exchanger tubes. Meanwhile, while keeping the flow rate of the slurry pump at its maximum, scale inhibitors should be continuously added to the slurry system to prevent the slurry from sticking to the inner walls of the heat exchanger tubes, thus avoiding blockages in the slurry evaporator and ensuring a stable, high circulation rate as well as a short residence time for the slurry in the system. (3) Relative density and solid content of the bottom oil slurry: Controlling the relative density of the oil slurry is also very important for reducing coking in the slurry system, as the density of the slurry indirectly reflects its composition as well as properties such as residue. Controlling the relative density of the slurry is mainly achieved by adjusting the amount of slurry discharged ； Next is adjusting the reaction depth; if necessary, the properties of the raw materials can be adjusted. The solid content in the slurry consists of catalyst particles and coke particles. It is also very necessary to analyze the solid content of the slurry in order to reduce coking in the slurry system. High solid concentrations cause severe wear on equipment, especially on high-speed components such as slurry pumps; extremely high concentrations can also lead to serious coking. The main approach adopted is to increase the amount of slurry discharged outside, thereby removing the large quantity of catalyst suspended at the bottom of the tower through the slurry pump ； To reduce the time it takes for the slurry pump to return to normal operation, a reprocessing oil line is connected to the inlet of the slurry pump; a small amount of reprocessed oil is pumped into the inlet of the slurry pump using the reprocessing oil pump as a supplement, or the inlet pipeline of the slurry pump is opened so that it can return to normal operation in as short a time as possible ； An slurry pump with a high net positive suction head is used to ensure that it does not experience cavitation within the temperature range of 50°C to 390°C.