In the triple-column distillation process, what effect does the reflux temperature in the atmospheric column have on the pressurized column and the atmospheric column?
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During the initial start-up of my three-column distillation process, at first the temperature in the atmospheric column was maintained at around 40 degrees Celsius. It was very difficult to raise the pressure inside the pressurized column; moreover, the pressure difference across the columns was quite large—around 40 kPa. Later, by chance, the reflux temperature in the atmospheric column was increased to 55 degrees Celsius. This led to a rapid start-up process, and no excessive pressure difference occurred in the pressurized column either. I would appreciate it if experts could explain whether this aligns with my understanding.
This post was last edited by guojing9113 on January 13, 2011, at 15:09. Initially, the temperature in the atmospheric distillation column was maintained at around 40 degrees; the rising methanol vapor was quickly condensed entirely, resulting in a relatively low pressure at the top of the column. At this time, the amount of reflux is relatively large; the liquid layer on the trays is thick, and the column pressure difference is relatively high. Due to the large pressure difference, methanol vapor rises rapidly, resulting in poor mass and heat transfer effects. When the reflux temperature in the atmospheric tower is raised to 55 degrees, some gases cannot be condensed, resulting in an increase in the top-side pressure of the tower. The reflux rate and the thickness of the liquid layer on the trays are smaller at around 40 degrees; the pressure difference is also small, resulting in better mass and heat transfer effects.
Although I work in oil refining, my understanding of atmospheric and vacuum distillation can still serve as a useful reference. During the initial stages of startup, when maintaining a low temperature at the top of the tower is necessary, the reflux rate must be quite high. However, at this time there isn’t enough heat provided by the rising vapor stream, making it difficult for gas and liquid phases to reach equilibrium within the tower. Based on our experience in atmospheric and vacuum distillation, during startup we generally don’t strictly control the top-of-tower temperature; it’s acceptable to keep it slightly higher. This poses no risks. Only once there is sufficient vapor load will we proceed to regulate the top-of-tower temperature. Temperature and pressure complement each other; only when they reach equilibrium can the operating conditions within the tower be properly controlled! !
My question is: Does the reflux temperature in the atmospheric column affect the pressure difference in the pressurized column? Could you please explain this?
Reply to 4# Shaojun: Theoretically, there will be a little bit. Because when the reflux temperature in the three columns is high, the pressure in the pressure column will definitely be a bit higher than before. Because the temperature in the third tower’s reboiler affects the reflux temperature in the second tower. This causes the pressure in the pressure tower to rise. After the pressure rises, with the load remaining constant, the gas flow rate inside the tower decreases. Ultimately, this results in a smaller pressure difference.
Well said! It seems like a truly professional individual. I give my support!
In theory, it would, but in reality the impact is minimal. When the reflux temperature is high, the pressure in the column may also be higher than before. Because the temperature in the reboiler also affects the reflux temperature in the second column. This causes the pressure in the pressure tower to rise. After the pressure rises, with the load remaining constant, the rising velocity of the gas in the tower decreases. Ultimately, this results in a smaller pressure difference. Additionally: When the temperature in the atmospheric distillation column is maintained at around 40 degrees, the rising methanol vapor is quickly and completely condensed, resulting in a low pressure at the top of the column. At this time, the amount of reflux is relatively large; the liquid layer on the trays is thick, and the column pressure difference is relatively high. Due to the large pressure difference, methanol vapor rises rapidly, resulting in poor mass and heat transfer effects. When the reflux temperature in the atmospheric tower increases, some gases cannot be condensed in time, resulting in an increase in the top-pressure of the tower. Conversely
So, what is the typical control temperature for the reflux in an atmospheric distillation column? ? 50-60, right?