Gas-liquid equilibrium issues in distillation tower operation

In the case of gas-liquid equilibrium, the temperature in each tower section remains relatively stable, as does the pressure.

In one case, the material exists in a two-phase state of vapor and liquid during distillation. Under certain conditions, if there are two or more phases in the material system, and the relative amounts of the material in each phase as well as the temperatures of the various components in each phase do not change over time, we say that the system is in equilibrium. At equilibrium, the substances are still in constant motion, but the number of phases and the concentrations of components in each phase do not change over time. When conditions change, a new equilibrium is established; thus, phase equilibrium is a kind of dynamic equilibrium. However, equilibrium is relative and temporary, rather than static and absolute.

Distillation involves multiple countercurrent contacts between the gas and liquid phases, with reflux provided, to facilitate mass and heat transfer through interphase diffusion. The concentration of light components in the gas phase increases gradually as one moves upward along the tower, while the concentration of heavy components in the liquid phase increases gradually as one moves downward. This process allows for the separation or purification of certain components present in the original material. Thus, the difference between distillation and distillation is determined by the presence or absence of reflux. The essence of the distillation process is that as long as a liquid with a high content of light components is supplied to the topmost tray, and a vapor with a low content of light components is supplied to the bottommost tray, and the contact between the different phases is organized effectively, then the vapor rising from each tray will have an increasing content of light components as it moves upward, while the liquid flowing downward from each tray will have an increasing content of heavy components as it moves downward. In principle, if a liquid containing pure light components is supplied from above and a vapor containing pure heavy components is supplied from below, and there are enough trays, then by introducing the solution that needs to be separated into an appropriate tray within the tower, we can obtain nearly pure light components at the top of the tower and nearly pure heavy components at the bottom. Moreover, the temperature decreases as one moves upward, and it increases as one moves downward, thus creating a temperature gradient that increases upward and a concentration gradient of light components that decreases upward. This allows for the continuous separation of different components.

Gas-liquid phase equilibrium refers to a state in which, at a certain operating pressure, the composition of the liquid on each tray remains constant, and the gas phase composition is in dynamic equilibrium with the liquid phase composition. If the temperatures at all the temperature measurement points in the distillation tower remain unchanged, then are the temperatures measured at these points those of the gas phase or the liquid phase?

If the thermometer is inserted into the liquid phase, it measures the temperature of the liquid phase; if it is inserted into the vapor phase, it measures the temperature of the vapor phase.

A distillation column can be divided into several sections; each section is different – there is more vapor higher up, and more liquid lower down

That’s not necessarily the case; it depends on which component has a higher vaporization latent heat.