How to reduce the residue content in the tower bottom liquid
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Since the double-effect distillation tower in my workshop started operating on February 4th, the residue level at the bottom of the tower has been relatively high, around 2%, whereas the normal value last year was 0.8%. The only change that took place was the chemical cleaning of the distillation tower as part of the maintenance work, including the trays in the stripping section. The operations carried out by the staff were similar to those before the New Year. I don’t understand why the methanol content in the residue has increased; please help analyze this issue. PS: The number of trays in the stripping section remains unchanged
Have there been any changes in the various operational conditions of the tower, such as the feed location, feed temperature, feed composition, and the heat load at the bottom of the tower?
If the composition of the material doesn’t change much, try adjusting the bottom temperature of the tower and the reflux rate.
If all other parameters remain unchanged, based on your description it seems that during the tower cleaning process, the residues that were originally in the bottom of the reactor or in the reboiler were flushed into the packing inside the tower, meaning the cleaning was not thorough enough. It is recommended to continue cleaning; avoid the reboiler during cleaning, and the reboiler can be cleaned separately.
Increase the reflux and try changing the operating pressure.
If the operation is fine, it should be checked whether there are any changes in the material components
This is not a simple issue. If the methanol residue levels are within acceptable limits before maintenance, it indicates that the separation efficiency of the tower is still good; it is necessary to check whether there have been any changes in the control parameters. Since methanol is a light component, it should be extracted from the top, and in fact, that may indeed be the case for this tower. It is recommended to check whether there are any errors in the monitoring instruments; although they may show normal readings, after maintenance, various factors can cause incorrect readings, leading to changes in the distribution of the material components within the tower. Another factor is whether there are significant changes in the composition of the feed; within a certain range, this can be adjusted by changing the reflux ratio or the temperature at the bottom of the tower. If the load exceeds the design capacity, it will also result in an excessive level of methanol remaining at the bottom of the tower. Within a certain range, it is possible to appropriately increase the bottom temperature of the tower and reduce the reflux ratio while ensuring that the conditions at the top of the tower remain satisfactory! If the reflux ratio is too high, it will also cause the light components to move downward, resulting in an excessive methanol content at the bottom of the tower. The above is in view of methanol as a light component! This post was last edited by yfq4308 on 2009-2-20 08:15.]
The high methanol content in the residue at the bottom of the tower indicates that the stripping section of the tower is not functioning properly; the possible reasons are insufficient efficiency of the stripping section and an inadequate number of tray levels； The temperature of the feed to the tower is low, below the temperature at equilibrium within the tower under normal conditions, which causes the material to be pushed downward toward the stripping section ； Insufficient heating in the reboiler at the bottom of the tower resulted in the temperature there not reaching the desired level ； An excessively high reflux ratio causes the light components in the column to be pushed downward; although this preserves the components at the top of the column, it affects the components at the bottom.