The operation of distillation tower should master material balance, gas-liquid phase balance, and heat balance.
Material balance refers to the situation where the amount of material entering the tower per unit time should be equal to the sum of the amounts of materials leaving the tower. Material balance reflects the production capacity of a tower, which is mainly adjusted by the amount of feed and the amount of discharge from the top and bottom of the tower. During operation, changes in material balance are specifically reflected on the liquid level at the bottom of the tower. When the operation of the tower does not conform to the overall material balance, it can be reflected from the change in tower pressure difference. For example, if there is more in and less out, the tower pressure difference will increase. For a fixed distillation tower, the tower pressure difference should be within a certain range. If the tower pressure difference is too large, the speed of rising steam inside the tower will be too high, resulting in severe entrainment of mist and even causing flooding and disrupting normal operation; The tower pressure difference is too small, the velocity of rising steam inside the tower is too low, the mass transfer effect of gas-liquid two-phase on the tower plate is reduced, and even liquid leakage occurs, greatly reducing the efficiency of the tower plate. Poor control of material balance can lead to chaotic 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, that is, the gas-liquid phase balance cannot achieve the expected effect, and the thermal balance is also disrupted and needs to be readjusted.
The gas-liquid equilibrium mainly reflects the quality and loss of the product. It is achieved by adjusting the operating conditions of the tower (temperature, pressure) and the gas-liquid contact on the tray. Only when the temperature and pressure are fixed, can there be a determined gas-liquid equilibrium composition. When the temperature and pressure change, the composition determined by the gas-liquid equilibrium changes, and the quality and loss of the product also change accordingly. The gas-liquid phase equilibrium is closely related to the material equilibrium. Once the material equilibrium is well controlled, the rising vapor velocity in the tower is appropriate, and the gas-liquid contact is good, the heat and mass transfer efficiency is high, and the tray efficiency is also high. Of course, temperature and pressure will also change with changes in material balance.
Heat balance refers to the balance between the heat entering and leaving the tower, which is reflected in the temperature at the top of the tower. Heat balance is the foundation for achieving material balance and gas-liquid equilibrium, which in turn depend on them. Without hot gas phase and cold reflux, the entire distillation process cannot be achieved; As the operating pressure and temperature of the tower change (i.e., the composition of gas-liquid equilibrium changes), the amount of heat released by gas-phase condensation and the amount of heat absorbed by liquid vaporization on each tray will also change accordingly, reflected in the changes in feed heating and tower top heat extraction.
Mastering material balance, gas-liquid phase balance, and heat balance is the key to distillation operations, as these three balances interact and constrain each other. In operation, the main focus is usually on controlling material balance, adjusting heat balance accordingly, and ultimately achieving gas-liquid phase equilibrium.
(1) To maintain a stable equilibrium of the liquid level at the bottom of the tower, it is necessary to stabilize: ① feed rate and feed temperature; ② The amount of extraction from the top, side lines, and bottom of the tower; ③ Tower top pressure.
(2) To maintain a stable tower top temperature, it is necessary to stabilize: ① feed quantity and feed temperature; ② Return flow rate and temperature in each middle section of top reflux and circulating reflux; ③ Tower top pressure; ④ Steam extraction volume; ⑤ The raw materials and reflux do not contain water.
As long as we pay close attention to the temperature at the top of the tower and the liquid level at the bottom of the tower, analyze the reasons for fluctuations, and adjust them in a timely manner, we can grasp the three equilibria of the tower and ensure its normal operation.