Liquid level control issue in the hydrocracking fractionation tower!
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Since the start of operation of my hydrocracking unit, the liquid level in the distillation tower has been fluctuating greatly! In theory, all the tail oil is recycled to the feedstock buffer tank, and the liquid level in the distillation tower is controlled by adjusting the amount of tail oil recycled. However, in practice, frequent adjustments to the amount of tail oil recycled have a significant impact on the reaction, causing fluctuations in the reaction system; this in turn leads to even greater fluctuations in the separation system. At present, the only option is to maintain a fixed amount of tail oil recycling and control the liquid level in the distillation tower by adjusting the amount of heavy diesel extracted! ! ! Have colleagues encountered similar phenomena! How was it solved! Thank you! !
I have seen the full cycle process; the distillation tower discharges the material to heavy sludge oil. At the beginning of operation, the liquid level in the distillation column fluctuates greatly, and when the level rises too high, the material is discharged to heavy sludge oil. If diesel is used to extract liquid and control the level of the distillation tower, can the flash point and freezing point of the diesel meet the specified requirements? Secondly, the circulating oil affects the liquid level in the crude oil buffer tank. I have seen situations where the crude oil feed consists of cold and hot vacuum distillate waxes as well as circulating tail oil; in such cases, the liquid level in the buffer tank is still relatively easy to control, although the composition of the feed varies significantly. However, the recycled tail oil has undergone hydrodesulfurization and denitration, so it releases little heat and remains quite stable. I wonder what your cold and hot feed rates as well as the circulation volume are?
Using diesel flow rate to control the bottom liquid level of the tower is not a very wise choice; although it stabilizes the properties of the feed to some extent, the quality of the diesel cannot be guaranteed. A small portion of it can be sent to the feed tank, while most of the bottom residue oil is discharged outside. Afterwards, it is pressurized by a feed pump and fed into the feed tank at a steady flow rate. This might be more stable.
Reply to 1# haijiang417: In my opinion, the liquid level in a fractionation tower should be stable when it’s normal. If it’s as you described, possible reasons could include frequent adjustments to the amount of circulating oil fed into the tower; however, the adjustment of the reaction temperature may not keep up with these changes, resulting in incomplete cracking reactions and thus a vicious cycle； If the feed rate of the circulating oil has stabilized, it may be due to significant changes in the properties of the feed; alternatively, it could be that the adjustments to the distillation tower and furnace have not yet stabilized. Personally, I think the most likely reasons are significant changes in the properties of the raw materials and incomplete separation of the light components in the recycled oil.
With a fixed amount of circulating oil, the ratio of the mixed feed oils remains constant, which in turn ensures stability in the reaction temperature and a constant level of reaction depth. As for the liquid level in the distillation tower, it cannot follow a straight line; fluctuations within a certain range are normal, typically between 15% and 90%, with a wider range being possible. The quality of the diesel product can only be adjusted by controlling the reaction depth – once this depth is achieved, the product will be of satisfactory quality. It is not recommended to dispose of large amounts of residual oil; 1%-2% is sufficient (this is done to remove polycyclic aromatic hydrocarbons and prevent an increase in pressure in the reactor). Otherwise, the material balance calculations will show significant losses