Issues related to running the air compressor in reverse during the startup of a catalytic cracking unit – please join the discussion!
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Dear colleagues: During the startup of a catalytic cracking unit, following the conventional approach, the processes are carried out in sequence – that is, fuel injection into the reactor and the establishment of a proper reflux in the distillation column take place first, before the compressor is started. The drawback of this method is that the reaction mixture continues to be vented through the flare before the compressor is activated, which results in significant waste and environmental pollution. In the past, refinery workers would judge whether the catalytic process had been completed by observing the condition of the flares; even cleaners were aware of this common knowledge. As the level of catalytic cracking operations improves and there is a greater focus on economic efficiency, reducing startup losses has always been a goal for refineries. Some catalytic units have managed to start the compressors first during the catalytic startup process (without venting gas), and after the reaction oil is introduced, the oil and gas are fed directly into the compressors without any venting. We would appreciate it if you could share the good practices of your organizations so that others can learn from them! Please participate actively in the discussion!
When we start up, the absorption and stabilization system is pressurized to 0.4–0.6 MPa, and the compressor is prepared for operation. Half an hour before the reaction unit is ready to inject fuel, notify the air compressor unit to start increasing its speed gradually, so that the backflow at the outlet returns to the inlet of the fractionated oil and gas line. As the amount of raw material increases gradually, the compressor speeds up progressively; once the pressure at the compressor outlet exceeds that of the absorption stabilization system, the pressure at the compressor outlet takes precedence over that of the stabilization system. During the entire startup process, coordination among various positions is essential, and the operations must be carried out smoothly. However, since there are many factors that affect the pressure in the settler, it is sometimes necessary to vent some of the gas at the inlet of the air compressor (care must be taken to prevent surging in the air compressor). Fortunately, our plant has gas tanks that can handle this situation. In recent years, since construction has started, our torches have rarely caught fire.
At startup, the pressure in the absorption tower should be kept slightly lower (0.2–0.3 MPa); this way, starting the compressor first will prevent surging (in the centrifuge)
Since there is basically no pressure in the stabilization system, the output pressure can directly enter this system; the main question is whether the flow rate resulting from backflow is sufficient for the medium entering the system Additionally, it is worth exploring whether rapid transcritical operation can be achieved without using a flare during acceleration. After changing to the internal circulation in the tower, we perform warm-up at low speed; when the feed rate reaches 30-40%, the system enters the transcritical state and stabilizes
To reduce the startup time, the absorption and stabilization system is pressurized in advance, gasoline is introduced, and dehydration through circulation takes place. If the pressure in this system is too low, cavitation occurs in the pumps, preventing dehydration through circulation; therefore, during startup, we usually keep the pressure in the absorption and stabilization system at 0.4–0.6 MPa. Of course, it is also possible to reduce the pressure of the absorption stabilization system again after cyclic dehydration.
In our system, a flare is installed at the outlet of the compressor first, before it is connected to the absorption unit for stabilization. It has long been suggested that thorough planning and careful preparation are necessary to ensure that work can begin without any hazards, but safety must come first, with no accidents involving equipment or personnel allowed. The current accountability system is quite strict, yet it has not been put into practice to date.
The oil-gas separator at the top of the fractionation tower is pressurized with gas or liquefied gas for absorption and stabilization; the compressor is started 10 to 15 minutes before oil injection.
None of the above answers are complete; it seems there’s a hint of technical blockade at work!
When the catalytic cracking unit is started, the air compressor is operated in reverse to reduce waste from flaring and environmental pollution. Pay attention to stable operation; gasoline can easily cause tank flooding accidents in areas with light hydrocarbons.
It is necessary to properly address the heat extraction issues related to the absorption tower and the stabilization tower; by ensuring that the stabilization system operates smoothly and quickly during startup, it becomes possible to start the compressor first without using a flare. If the heat source is the middle stage of distillation and there are no other heat sources, the introduction of rich gas during stable operation can lead to difficulties in maintaining stable operations. It can also cause light hydrocarbons in the stable gasoline stream to impact the gasoline tanks in the storage area, resulting in damage to the vent valves of those tanks.
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