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Distillation: Could scale be the reason for an increase in steam consumption?
Q: The steam consumption has increased in our distillation plant. Is it possible that the re-boilers have gotten scale? What are the possible causes of an increase in the steam consumption rate of a multi-pressure distillation plant producing ethanol?
A: I don’t think the increased steam consumption is being cause by scale formation in the re-boilers. If scale is forming, I think it would require the steam pressure to be increased to raise the temperature on the steam side of the re-boilers to offset the decrease in the heat transfer coefficient but this would not increase heat input required.
The only thing I can think of is that something is causing the distillation towers to requiring more reflux to make the separation thereby increasing heat demand. The causes I can think of for this to happen are:
1. A feed concentration change requiring more reflux to remove additional impurities or makeup for a lower ethanol concentration.
2. A fouling of the Trays or Packing & Liquid Distributor(s) lowering their performance (Reduced Theoretical Stages) that would cause a need for more reflux resulting in an increased steam demand.
3. A tray panel has been blown out of place or a down-comer panel has come loose. This would lower the tray efficiency leading to the need for more reflux/steam. And it would also decrease the overall tower pressure drop, which is something that can be measured and compared with the data history of the tower(s).
4. A packed tower liquid distributor has moved out of place lowering separation efficiency leading to the need for more reflux/steam.
5. The tower is operating below the operating range of its internals, which would require and increase in the reflux to feed ratio.
6. The tower is operating above the operating range of the internals leading to liquid entrainment (back mixing) or flooding. In this case, increasing the boil-up rate (increasing steam rate) would make things worse and the pressure drop across the trays or packing should have increased significantly if the tower is flooding. So this is most likely not what is happening, since I am assuming that the separation is being achieved with the higher steam rate.
As I think about this answer, there could be a situation where only one area to the tower is flooding and the remaining sections can handle the increased vapor and liquid rates caused by the need for greater reflux. Has the pressure drop increased in one section of the tower or in one of the towers?
7. A tower operating at a wrong pressure maybe caused by a malfunctioning absolute pressure controller would change the vapor-liquid equilibrium and thereby causing a need to change the reflux ratio leading to a change in steam demand.
8. If the liquid level in the bottom of the tower increases to the level of the vapor inlet from the re-boiler liquid entrainment is created. This would lower the performance of the trays or packing above by back-mixing and lead to a need to increase the reflux ratio/re-boiler steam rate.
I have included an article, “Successfully Troubleshoot Distillation Towers,” by the late Dr. John G. Kunesh of Fractionation Research, Inc. (FRI). Three additional articles by Rukovena and Rukovena & Cai can be found on FRI’s Web site, http://www.fri.org under publications concerning packed tower good practice for proper operation. Also, a book by H. Z. Kister “Distillation Troubleshooting,” (2006, Wiley-Interscience) would be a good reference.
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