Reasons for unstable heat removal from the external heat exchanger in catalytic cracking
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This unit is of the coaxial catalytic cracking type. Recently, the heat extraction load of the external heat exchanger in this unit has been unstable; with an external heat extraction fluidization air flow of around 3000, the steam generation rate due to heat extraction varies between 50 t/h and 41 t/h. When the external heat extraction fluidization air flow is adjusted to 5000, the heat extraction load changes very little. Due to unstable heat extraction, the liquid level in the outer heat exchanger’s drum fluctuates greatly, resulting in unstable water supply. The medium-pressure to low-pressure control valve adjusts too frequently, with large changes in its opening degree (40-20 or 20-40). May I ask experienced colleagues how to adjust the external heat extraction fluidization air (which consists of upper and lower streams) as well as the lift air? Thank you
At the same time, the temperature change at the pre-cooler is significant (the normal density is around 800; when the temperature drops by 15°C, the density falls to over 600). Could this be caused by a clogged slide valve? The differential pressure of the slide valve is around 50 kPa, while the differential pressure between the two devices is around 45 kPa
To adjust the heat extraction amount from the external heat exchanger, there is only one word: slow. Adjust the fluidization air flow slightly; there may be no immediate reaction after making the adjustment, so there’s no need to rush – just observe for a while~
Heat extraction by the external heat exchanger relies on the flow of fluidized hot catalyst, but this fluidization requires a certain catalyst density to ensure effective heat extraction! Personally, I think the flow rate of the fluidizing air is too high, which causes a short circuit in the catalyst; as a result, the flue gas flows back to the outlet point. You can reduce the flow rate of the fluidizing air and see what happens!
Is there no special valve control? When the external heat absorption for steam generation reaches a certain level, regulation may not be very effective then What is the maximum load?
I agree with you; there is a lag in regulation.
The flow rate of the fluidizing air is controlled by a flow restrictor plate, and there isn’t much room for adjustment. Are your external heat exchangers operating at low load? Under normal high-load operation, the fluidization proceeds smoothly.
It might be due to poor fluidization; based on your situation, it seems that the flow rate of the fluidization air is too high. Adjust both streams of fluidization air to values close to the design levels, and instead of adjusting the heat intake, increase the flow rate of the air used for fluidization
Check whether the loosening of the external heat extraction is normal
Check for any loose points in the inclined tubes at the external extraction inlet; the fluidization air flow in the middle should not be too strong, and it is best to use the air supply from the bottom to adjust the gas production volume.