When the reflux ratio (actual reflux ratio, divided into liquid gas ratio in distillation section and gas-liquid ratio in stripping section) is defined, the problem of minimum reflux ratio and maximum reflux ratio can be discussed. Taking the liquid gasification in the distillation section as an example, the so-called minimum liquid gas ratio is the ratio of the number of reflux liquid (liquid nitrogen in the air separation column) and the number of rising gas moles required to obtain pure low boiling point products at the top of the distillation section under the condition of infinite number of trays. In the data diagram of gas-liquid equilibrium, it is shown as a parallel line parallel to the horizontal axis with the distillation feed gas phase group as the starting point. This of course can not draw the operating line, is a limit of the situation. So what is the maximum liquid-gas ratio of distillation section? It is the reflux ratio without low boiling point product removal (total reflux), which is of course a limit state. Of course, the actual liquid gas ratio can only be greater than the minimum liquid gas ratio and less than the maximum liquid gas ratio. In the data diagram of gas-liquid equilibrium, it is a line with the distillation feed gas phase group as the starting point upward. The angle between the line and the horizontal axis is determined by the ratio of the international liquid gas ratio and the minimum liquid gas ratio. According to this, we can draw the operation line.
The operation line of the stripping section is a line with the starting point downward from the equilibrium liquid phase group of the distillation raw material, and the angle between the line and the horizontal axis is determined by the ratio of the actual gas-liquid ratio and the minimum gas-liquid ratio.
With the concept of maximum reflux ratio, we will understand that under the condition of limited plate number, any distillation process has the limit of product purity.
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