Which three equilibria are controlled by the distillation column?

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The operation of a distillation column requires an understanding of material balance, gas-liquid phase equilibrium, and heat balance. Material balance means that the amount of material entering the tower per unit time should be equal to the sum of the amounts of material leaving the tower. Material balance reflects the production capacity of the tower, which is primarily regulated by the feed rate and the outlet rates from the top, sides, and bottom of the tower. During operation, changes in material balance are specifically reflected in the liquid level at the bottom of the tower. When the operation of the tower does not conform to the overall material balance, this can be reflected in changes in the tower pressure difference. For example, if more fluid enters and less exits, the pressure difference across the tower increases. For a fixed distillation column, the pressure difference across the column should remain within a certain range. If the pressure difference across the tower is too high, the velocity of the vapor rising inside the tower becomes too great, resulting in severe entrainment of foam; in extreme cases, flooding may occur and normal operation will be disrupted. If the pressure difference across the tower is too low, the velocity of the vapor rising inside the tower is too low, which reduces the efficiency of mass transfer between the vapor and liquid on the tray surfaces; in extreme cases, liquid leakage may occur, thereby reducing the efficiency of the trays. Poor control of material balance can lead to chaos in the operation of the entire tower; mastering material balance is a key aspect of tower operation. If the normal material balance is disrupted, it will affect the other two balances: the vapor-liquid phase balance will not achieve the desired results, and the heat balance will also be disrupted, requiring readjustment. Vapor-liquid equilibrium mainly reflects the quality of the product and the amount of loss. It is maintained by adjusting the operating conditions of the tower (temperature, pressure) and the vapor-liquid contact on the tray. Only when temperature and pressure are fixed is there a definite vapor-liquid equilibrium composition. When temperature and pressure change, the composition determined by the vapor-liquid equilibrium changes, and as a result, the quality of the product as well as any losses associated with it also change. Vapor-liquid equilibrium is closely related to material balance. By ensuring an appropriate vapor rise velocity within the tower and good vapor-liquid contact, heat and mass transfer efficiency can be improved, as well as the efficiency of the tower plates. Of course, temperature and pressure also change as the material balance changes. Heat balance refers to the equilibrium between the heat entering the tower and the heat leaving it, which is reflected in the temperature at the top of the tower. Heat balance is the foundation upon which material balance and vapor-liquid phase balance are achieved, and in turn it depends on them. Without a hot vapor phase and a cold reflux, the entire distillation process cannot take place ； And as the operating pressure and temperature of the tower change (that is, the composition of the vapor-liquid equilibrium changes), the heat released due to vapor condensation on each tray, as well as the heat absorbed during liquid vaporization, also change accordingly. Mastering material balance, gas-liquid phase balance, and heat balance is key to distillation operations; these three balances influence and restrict each other. In operation, control of material balance is usually the main focus, with heat balance adjusted accordingly, in order to ultimately achieve gas-liquid phase equilibrium. To maintain a stable liquid level at the bottom of the tower, it is necessary to stabilize: ① the feed rate and feed temperature; ② the withdrawal rates from the top, side lines, and bottom of the tower; ③ the pressure at the top of the tower. Maintain a stable top temperature of the tower. Stability is essential: ① feed rate and feed temperature; ② top reflux, recycle reflux, reflux rates and temperatures at each intermediate stage; ③ tower top pressure; ④ steam stripping volume; ⑤ the feed and reflux must be free of water. By closely monitoring the top temperature of the tower and the liquid level at the bottom, analyzing the causes of fluctuations, and making timely adjustments, it is possible to maintain the three balances of the tower and ensure its proper operation.