Fluctuations in the bottom liquid level of the distillation column in the diesel hydrorefining unit
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The distillation column of the diesel hydrorefining unit is heated by a bottom reboiler. Diesel fuel is extracted at the bottom of the tower, while naphtha is produced at the top; there is no extraction from the middle section. The bottom temperature of the tower is 275–283°C, while the top temperature is around 125°C. The processing capacity is 210 t/h; the naphtha product at the tower top is 3–4 t/h, and the reflux volume is 4–6 t/h. The initial boiling point of the diesel at the bottom of the tower is 190–196°C, the dry point of the naphtha is around 160°C, and the flash point of the diesel is kept between 65–78°C. When the flash point is low, the bottom temperature of the fractional distillation tower is reduced; when the temperature drops to around 275°C, the liquid level in the tower begins to fluctuate by 30–70%. Once the temperature rises, the liquid level stabilizes, but fluctuations occur again whenever the temperature is lowered. Attempts were made to further reduce the temperature to 270°C, but the liquid level still fluctuated. We also have a set of liquid-phase diesel equipment that is not in use at the moment, and the same applies to the fractionation tower. I’d like to hear from the forum members their analysis on what causes these fluctuations in liquid level, and how such fluctuations can be avoided when it’s necessary to lower the temperature at the bottom of the distillation tower. :handshake
There can be various reasons for fluctuations in the liquid level at the bottom of the fractionation tower, such as changes in the composition of the feed, unstable heating in the reboiler, damage to the trays inside the tower, and faults in the instrumentation. To avoid fluctuations, it is necessary to ensure a stable feed flow, improve temperature control of the reboiler, check that the tower’s structure is in good condition, and verify that the instruments are functioning properly. At the same time, adjustment of operating parameters should be gradual to avoid sudden changes. If the problem persists, further inspection and repair of the equipment may be required. .
How long has it been driving? There’s a possibility of liquid leakage from the tray
I have been driving for 4 months. The distillation column of another diesel hydrogenation unit from the previous cycle also experienced such level fluctuations.
This unit was modified to a hydrofining operation in the second half of last year； With a processing capacity of 200 t/h, are the current reflux amount and naphtha extraction amount in the distillation tower insufficient? ； Only if the liquid level fluctuates after cooling and returns to normal after heating, issues with the instruments or internal components should be ruled out ； Is it the fluctuation in liquid level caused by a low amount of evaporation of the light components and poor upward movement of these components within the tower?
This way, you can consult the manufacturer of tower internals to get an estimate. I think what should be considered more is your operational pressure
The diesel distillation column is essentially an atmospheric pressure column, with a pressure of 0.06 MPa.
If there is sufficient light component in the reflux tank, and assuming that there are no significant changes in product quality, consider whether the circulation rate of the reboiler is too low, or whether there is any uneven flow in the feed to the reactor or in the pipelines. Determine whether the bottom pump operates at fixed frequency or variable frequency; if a variable frequency pump is being used, and the circulation rate is low, it may be possible to switch to a pump that operates at fixed frequency to increase the circulation rate. Also check the pipelines for reflux at the top of the tower to see if the flow meters are reaching their full scale. Depending on the situation, it might be advisable to activate a bypass line for the reflux control valve as an auxiliary measure.
The processing capacity is 200 t/h, and the circulation rate in the reboiler is around 240 t/h, which is slightly low; however, the flow rates to the various branches feeding into the furnace are balanced； Since it is a modified unit, the pumps are second-hand, so there is no significant improvement for now. The opening degrees of the reflux control valves are relatively low; the naphtha at the tower top is approximately 2 t/h. The initial boiling point of the raw materials used in the unit is around 185°C, while the initial boiling point of the diesel product reaches over 190°C, with very little light fraction present. When the bottom temperature of the tower is above 280°C, the top temperature is controlled at around 130°C only. There always seems to be some material in the middle of the tower, neither rising nor falling, which causes significant fluctuations in the liquid level.
Was this problem resolved later on? Was it due to the operating conditions of the tower, the composition of the materials, or the feed to the tower?