The temperature of the reflux stream from the top of the distillation tower is low
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Some time ago, severe salt deposition occurred at the top of the distillation tower in our unit. After washing with water, the pressure drop was reduced and the salt deposition was eliminated. However, once the operating conditions were stabilized, the temperature of the liquid returning from the top of the distillation tower was only 35 degrees, whereas it was around 65 degrees under normal conditions. I would like to ask everyone what could be the reason for this. Currently, the temperature at the top is 105 degrees and the pressure is 0.125 MPa. During tower washing, the temperature of the liquid taken out at the outlet was 122 degrees, with a set temperature of 85 degrees; the temperature of the liquid taken out from the top circulation system was 93 degrees, and the pressure was 0.1 MPa. The temperature of the gas phase in the layer taken out during washing was 200 degrees, and all the water from the tower has been removed
When severe salt deposition occurs at the top of the distillation tower, the light components circulate multiple times within the tower; since the heat is not removed, this results in a high temperature of the recycle stream at the tower top； After the salt buildup at the top of the distillation tower is removed, the number of cycles that the light components undergo within the tower decreases, and heat is carried away, resulting in a lower temperature of the reflux stream at the top of the distillation tower. If you don’t believe it, you can remove the salt buildup at the top of the distillation column at present and increase the reflux rate; you will then see an increase in the temperature of the liquid returning from the top of the distillation column.
After the salt accumulation at the top of the distillation tower is removed, the flow rate increases and more heat is released, which inevitably leads to a decrease in the return temperature. The traffic volume can be appropriately reduced.
It’s because your heat loss due to backflow is too high; reduce the backflow volume
After washing the top circulation heat exchanger, its heat exchange efficiency has improved compared to before; the temperature after cooling is definitely lower. Considering raising the top circulation temperature, we might give it a try
Previously, the salt buildup on the top circulating water cooler reduced its heat exchange efficiency, resulting in less heat being removed; after cleaning it, more heat is now removed, so it’s advisable to reduce the flow rate of water in the cooler
It is highly likely that salt and scale have formed at the top of the tower; this leads to a decrease in the vapor-liquid load on the tower. Factors such as excessive extraction of light diesel or an increased amount of material returning to the tower contribute to a drop in the temperature at the bottom of the tower. Bonus: The cold reflux should be normal, right?
It is necessary to consider the aspects of heat extraction and heat exchange primarily. During salt formation, the amount of liquid circulating at the top is insufficient, resulting in less heat being extracted and an increase in the temperature at the top of the tower. During the washing process, some water-soluble deposits in the heat exchanger are removed, which improves the efficiency of the heat exchanger; this leads to a decrease in the temperature of the liquid returning to the tower. Reducing the volume of liquid circulating at the top resolves this issue.
It is recommended that you upload the boiling ranges of crude gasoline and diesel; if the density of diesel is low, the 95% point will be low as well, and the dry point of crude gasoline will also be low. And what is the pressure drop in the fractionation tower now? I guess it’s caused by salt and scale formation at the top of the distillation tower. Secondly, the return flow rate from the top cooler is too high (this is less likely), or there is a leak in the top cooler’s water cooling system, resulting in a lower return temperature. Currently, the distillation temperature at the top of my unit is 140 degrees, while the return temperature is around 105 degrees; almost all of the flow goes through the heat path, with the circulation rate being used to control the top temperature. If salt deposition is occurring, it is recommended to use deionized water or deoxygenated water for cleaning the tower; in this situation, a water temperature of around 90 degrees is advised.
1. Remove the scale. 2. See if it’s possible to reduce the cooling water flow rate in order to increase the temperature after cooling
The top pressure is 0.1 MPa, and the temperature for top circulation extraction is 93 degrees. Does your top circulation not involve water? Suggestions: 1. Try to avoid using cold reflux as much as possible ； 2. Increase the circulation volume at the top of the system and make use of more thermal bypasses; as long as the product meets the quality standards, the parameters related to the top temperature can be relaxed. It’s not a big problem; it should be solvable.
The top circulation temperature is only 93 degrees – that’s way too low. Appropriately reduce the top circulation return flow or decrease the top circulation heat exchange to increase the return temperature; furthermore, raise the diesel return temperature.