Why can full reflux reduce the start-up time of distillation?

This post was last edited by qy576100527 on 2017-8-5 at 23:16. Full reflux reduces the operation time: (1) it reduces the time required for warming the pipes; (2) it reduces the time needed to establish the liquid level; (3) the equipment remains in operating condition, so there is no need to restart it. The reason why the minimum number of theoretical plates is required in full reflux mode is that, in this mode, no feed is added to nor product is removed from the distillation column. The two operating lines merge into one and coincide with the diagonal line; the equation for the operating line is therefore yn+1 = xn. This is an important characteristic of full reflux: at any cross-section between two plates, the composition of the rising vapor, yn, is equal to the composition of the descending liquid, xn. Obviously, at full reflux the distance between the operating line and the equilibrium line is greatest, so the minimum number of theoretical plates required to achieve a specified degree of separation is attained.

Could the second question be explained in simpler terms? I can’t understand the equations for operation lines; I didn’t study chemistry well enough

It is not extracted; there are no product requirements, and the number of mass and heat transfer processes is reduced. The tray is a platform that provides gas-liquid mass transfer and heat transfer.

Is my understanding correct? Because during full reflux, there is neither feed nor drawdown, the frequency of gas-liquid mass and heat transfer within the tower increases. Moreover, the liquid-phase reflux and gas-phase reflux are at their maximum levels; as a result, the concentration of the light components in the bottom of the tower gradually decreases, while the concentration of these light components at the top of the tower gradually increases. The concentration differences and temperature differences between the different tray levels increase, which makes it easier for the heavier components in the gas phase to condense, and for the lighter components in the liquid phase to vaporize. Consequently, the number of theoretical trays required to achieve the same separation requirements is reduced, as is the time needed for stabilization. In other words, by increasing the liquid-phase and gas-phase reflux, the time required to establish the concentration profile within the tower is decreased.

Full reflux means it is in equilibrium, with neither inflow nor outflow

In simple terms, when driving the process, the heating and circulating water, as well as the composition and temperature of the distillates at each tray, are not very stable. The product obtained through distillation will certainly not meet the required standards. After full reflux, the composition and temperature of the materials on each tray will stabilize fairly quickly, and the reboiler and circulating water systems will also stabilize. Only then can the distillation process proceed slowly, with changes in these four aspects occurring gradually and being accurately controlled.