\"Seeking help\": Expert advice needed – abnormal fluctuations in the liquid level of the flash tank in the solvent regeneration unit
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In our solvent regeneration system, the rich liquid flows into the regeneration tower after being heated to 65 degrees Celsius in a primary heat exchanger between the rich and lean liquids. It then enters the flash tank, from where it is pumped out of the tank using a pump located at the bottom of the tank. There is a control valve before the liquid enters the tower, used to regulate the liquid level in the flash tank. However, when the valve is operated manually, its opening degree remains around 40% constant, yet the liquid level in the flash tank still changes, either decreasing or increasing. When the liquid level drops, the temperature at the top of the regeneration tower decreases, the pressure increases, and the amount of acidic gas generated increases. But when the valve is manually reduced to an opening degree of 35%, the liquid level returns to normal. When the liquid level rises, the temperature at the top of the tower increases, the pressure decreases, and the amount of acidic gas generated decreases. After increasing the valve opening degree to 45%, the liquid level again returns to normal. Each time there is a change in the tower due to operations in the coking unit, the liquid level in the flash tank changes significantly. We have checked the pump and the control valve, and they seem to be fine. I would like to ask the experts here: what could be the reason for this? Logically, if the valve opening degree remains constant, then the amount of rich liquid entering the tower should also remain constant, right? What’s the reason? This issue has persisted for half a year now... and the cause hasn’t been found yet?
When cutting the coking tower, did the pressure in your flash tank change? Is it possible that the gas phase from the amine recovery unit of your dry gas desulfurization tower entered the liquid phase?
When the liquid level in the flash tank drops, the temperature at the top of the regeneration tower decreases, which indicates that the processing capacity of the regeneration tower has increased. As a result, the concentration of acidic gases at the top of the tower rises, causing the temperature there to drop
With the pressure in the flash tank and the valve position remaining unchanged, check whether there are any fluctuations in the liquid levels of the desulfurization tower tank and the tower itself. Also examine the underground solvent tank to see if amine rejection might occur
All the possible situations you mentioned have been checked; the manual valves upstream and downstream of the liquid control valve in the amine recovery unit are normally kept closed and are used only when pressing amine. There are no changes in the underground solvent tank either, and the liquid level in the dry tower remains very stable
Our units have also encountered similar situations; the likely cause is unstable operation of the upstream units, such as instability in the operations of the sections that transfer material to the coking unit or in the high-pressure to low-pressure sections of the hydrocracking unit, which results in gas phase entering the amine liquid system. Treatment: The liquid control valve of the rich liquid flash tank is automatically adjusted, with the valve position being restricted ； The PID control feedback value of the hydraulic control valve is highly sensitive ； The operator should enhance monitoring and adjust the steam flow at the bottom of the regeneration tower as well as the pressure control at the top of the tower in a timely manner.
If it’s not due to reasons related to exiting the tank, then we need to look for reasons related to entering the tank – could it be that the amount of liquid flowing into the tank is unstable, or the temperature or pressure is unstable?
Control the pressure in the stabilizing flash tank; any excess hydrocarbons or hydrogen sulfide from the feedstock are vented to the flare