The methanol recovery tower keeps fluctuating
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As shown in the figure, it is a simple distillation tower for methanol recovery; methanol is at the top of the tower while wastewater is at the bottom, and it operated smoothly previously. During the recent fluctuations, the temperature and pressure at the bottom of the tower decreased, while the pressure at the top of the tower became slightly negative (normally 1 KPA). At the same time, the feed rate to the methanol tank increased, whereas the feed rate to the wastewater tank decreased. I’d appreciate it if an expert could analyze what the cause might be – is there a change in the composition of the feed material, or are there blockages or leaks in the pipeline equipment?
It should be that the feed composition has changed: the methanol content has increased
Explanation for changes in methanol and water feed: the methanol content in the feed has increased； When the temperature and pressure at the bottom of the tower decrease, the pressure at the top of the tower also drops; this is an indication of a reduced evaporation rate, which may be caused by a lower feed temperature, insufficient heat supply, or heavier feed components
There is a high possibility that the light components will increase; check whether the methanol content has gone up!
Check the feed components first, then check the equipment…
It is necessary to judge by the changes in heating amount; how does the heating amount change? Are there any testing parameters for the tower feed, the methanol at the top of the tower, and the wastewater at the bottom of the tower? By observing the changes in these parameters, it is possible to determine the cause.
After reading it, it seems that the light components in the feed mixture have increased, that is, there are more light components that are more volatile than methanol. Previously, I was involved in methanol separation; at that time, the methanol contained C4 components. Whenever these C4 components from an upper-stage system entered the methanol recovery system, it caused significant pressure fluctuations as well as a sharp drop in the temperature at the bottom of the tower. Also, after looking at this tower top condensation system, it seems like it could function as a single heat exchanger, with only the piping needing to be modified. It’s purely my personal opinion; we are separating methanol from water, and I’m not sure what this system is used to separate.
This post was last edited by wgqmqez on 2020-2-20 09:48. For a simple distillation tower, it seems that the flow rate this time isn’t being pumped back; can the flow be controlled? Regarding your question, here is my analysis: 1) An increase in the methanol content among the components entering the tower, that is, a higher proportion of light components, will lead to a decrease in the temperature at the bottom of the tower. 2) A decrease in column pressure will cause the temperature at the bottom of the column to drop. 3) An increase in backflow will cause the tower temperature to drop and the pressure to decrease. 4) A decrease in the reflux temperature will lead to a drop in the tower temperature and pressure. Based on what you said about the decrease in wastewater and the increase in methanol, it must be caused by changes in the composition. If the product quality is satisfactory, then congratulations – it’s time to give out a bonus. At present, it is necessary to ensure that the tower is not under negative pressure and that the pressure remains stable; this can be achieved by adjusting the tower top water cooler in order to maintain a stable reflux temperature. If nothing else works, introduce some nitrogen without affecting the operating conditions to maintain tower pressure
In simple terms, the distillation power is not enough; it can no longer climb uphill
I work in the production site, and I have experienced periodic fluctuations in distillation columns. At that time, the pressure curve at the column top on the computer showed very regular peaks and troughs, with fluctuations occurring once or twice per minute. I won’t go into too much theoretical detail; based on my experience, I consider two factors: 1. Unstable heat input. Since there is a reboiler in your PID control system, you need to check whether the drain valve on the reboiler’s exhaust gas is damaged； 2. The cooling capacity of the components at the top of the tower is unstable; you need to consider whether scaling in the cooling water channels of the two horizontal heat exchangers is affecting the heat exchange efficiency ； 3. I don’t see the vent valve for the product tank at the top of your tower; please check whether there is any problem with this valve. This is just my personal opinion; I hope other forum members can offer their guidance. Thank you!