Distillation column pressure control method
Thread Content
May I ask everyone, what is the principle behind using circulating water flow to control the pressure in a distillation tower?
Control the amount of vapor in the product stream after cooling at the top of the tower; a smaller amount of vapor results in lower pressure, and to increase the pressure it is necessary to reduce the amount of water circulating.
The amount of condensable gas at the top of the tower is controlled by the circulating water volume, thereby achieving control over the pressure at the top of the tower.
High flow rate means more heat is carried away. Fast flow rate, improving heat exchange efficiency
With the tower’s processing capacity remaining unchanged, increasing the circulation water volume boosts the heat exchange rate of the heat exchanger and reduces the temperature of the material after cooling. When the reflux volume stays constant, a decrease in the post-cooling temperature is equivalent to an increase in the reflux volume. With the bottom temperature of the tower unchanged, this leads to a decrease in the pressure at the top of the tower, resulting in lighter components at the top.
The pressure in a distillation tower is determined by the vapor flow rate at the outlet of the condenser; the more that is condensed, the lower the pressure in the tower. When everything is condensed and the effect of non-condensable gases is ignored, then there is no pressure in the tower. Conversely, the higher the pressure, the greater the amount of water needed for circulation. With constant evaporation rates and heat exchange area, a lower pressure is achieved in the tower.
Increase the flow rate of the cooling water in the condenser to cause more vapor phase at the top of the tower to condense, thereby reducing the pressure at that location; provided that the vapor phase at the top of the tower does not contain excessive amounts of non-condensable gases (either none or only trace amounts).
Unless changes in the circulating water temperature require adjustments to the amount of circulating water, steam is generally used for controlling the distillation tower (which corresponds to control based on changes in the reflux flow). The control of emissions from polysilicon distillation off-gases is important for product quality, and such control is also necessary.
I agree with what was said above; currently, polysilicon manufacturers basically operate based on this principle as well