Distillation column pressure issues
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For the pressure at the top of a distillation column, in the case of simple two-phase separation, is the pressure at the top constant? Does the temperature determine the composition of the gas phase at the top of the column? Conversely, if the temperature is constant, can we ensure that the gas phase composition remains the same at any pressure, or is it the composition that determines the pressure? Could an expert give some advice? What causes a sudden drop in the tower top pressure? No leaks, and no significant temperature changes
When the pressure at the top of the tower remains constant, changes in temperature will inevitably affect the gas composition at the top of the tower! At high temperatures, the composition of light components at the top of the tower increases; conversely, it decreases! When the temperature is constant, it cannot be guaranteed that the gas phase composition will remain the same at any pressure; this also depends on whether there are changes in the reflux flow rate! Regarding the sudden drop in pressure at the top of the tower you mentioned, I think it’s very likely that the backflow has increased!
Personally, I don’t think it’s the temperature that determines the components at the top of the tower; rather, under constant pressure, the temperature changes as the components at the tower top change. The factors that affect these components include feed rate, reflux rate, pressure, and so on. I believe there are generally four possible reasons for a sudden drop in pressure at the top of the tower: first, an increase in reflux rate; second, issues with the condenser, such as an increase in the amount of material that can be condensed (higher flow rate of condensate water, lower temperature of the condensate water, or an increase in the condensation area due to certain reasons); third, a sudden increase in the output from the bottom of the tower; fourth, it’s possible that there is some leakage somewhere that hasn’t been detected (such as in areas like safety valves, which are difficult to locate)
I also worked in the large-scale chemical industry for three years. We can exchange ideas; my QQ number is 851642758. Please mention it when adding me
Distillation operations are carried out at constant pressure, as this is the easiest to control. Moreover, in equipment design, pressure is a crucial parameter regarding safety and economics; therefore, no large margin is allowed. Furthermore, at a constant pressure, temperature is a function of composition; composition is the essential factor, while temperature is merely a macroscopic manifestation of it. There may be some forum users who disagree with my view. For example, in a distillation column, there is a section where the solvent concentration remains constant; the temperature in this section is relatively high, and it is even higher than the temperature of the plates below it. Although this goes against the rule that temperature increases from the top to the bottom of the column, it reflects the fact that composition determines temperature. Therefore, when the pressure remains constant, an increase in reflux leads to a higher content of light components at the top of the tower, which in turn results in a lower temperature at the tower top; similarly, the temperature at the bottom of the tower increases.
The decrease in tower top pressure may be due to three reasons: first, the reflux flow increases, causing an excess of vapor phase to turn into liquid phase. Secondly, the composition of the feed has become heavier, which prevents the reboiler from vaporizing enough of the broth at a given load. Third, the raw materials contain more components with low potential.