Causes of fluctuations in the distillation column

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Our plant has a column that extracts a portion of the olefins from the FCC feedstock; this column operates intermittently, with the product being obtained from a side stream. The operation of this tower has been very stable in the past. This month, after starting the machine, both the tower temperature and the pressure drop deviated from normal values; all temperature and pressure parameters showed oscillatory fluctuations. The pressure was set at 400 KPA, with fluctuations of around 10 KPA. The overall pressure drop in the tower was 40 KPA higher than normal, as it should be below 40 KPA under normal conditions. The difference is that the temperature fluctuations are relatively large; the maximum fluctuation is 5 degrees, while the minimum is 2 degrees. The liquid level in the tower bottom (manually adjusted) and the steam supply volume (manually adjusted) also fluctuate accordingly. The quality of the side-stream product was substandard, introducing a large amount of components from the bottom of the tower. It is suspected that severe fouling of the tower plates caused flooding; upon restarting the operation with the discharged liquid, a large amount of accumulated liquid was indeed expelled. We checked the raw materials and found no abnormalities; all instruments and control valves are also in good condition. The conditions of the steam and circulating water are also normal. To avoid flooding, we reduced the reflux ratio and also slowed down the rate at which steam was introduced. In the end, the quality of the side-product was satisfactory; the yield was low, and the pressure drop was somewhat high, at around 60 KPA. The most critical issue is that the fluctuations in this tower have not ceased; all the key parameters remain fluctuating. Could you explain how fluctuations occur? It would be best if you could describe the dynamic equilibrium inside the tower as well.