How to determine the minimum reflux ratio for the distillation of ethanol-water, in a sieve tray tower

There is a range of operation for the distillation tower, and production tests should be conducted on-site to determine the maximum and minimum loads; these values should be specified in the user manual. As for how to control the equipment in a sieve tray tower, it is necessary to analyze it from the perspective of its principles. The liquid layer formed on the sieve trays by the downward-flowing liquid is caused by the pressure of the rising gas and the weir installed on the sieve trays to retain the liquid on those trays; this weir is called an overflow weir. The reflux ratio refers to the ratio of the rising gas to the flowing downward liquid. The two factors involved are rising gas and flowing liquid. In a normal distillation process, this operating condition is based on the resistance created by the rising vapor and the reflux liquid. On this basis, changes in purity occur. During operation, if the load drops to a level that is not sufficient to maintain the minimum purity required for production, this load represents the minimum reflux ratio for the distillation column. Since the reflux ratio is a conceptual term, its actual value cannot be determined in practical production; only the purity existing under that condition represents the most concrete numerical indicator of it. (Pirate language: lol, for reference only)

The reflux ratio is the ratio of the amount of reflux to the amount of distillate. It has two extreme values: infinity (full reflux) and the minimum reflux ratio. The minimum reflux ratio occurs when the operating line of the distillation section and the operating line of the stripping section gradually approach each other as the reflux ratio decreases; when the intersection point of these operating lines falls on the equilibrium line, that value represents the minimum reflux ratio. This value can be calculated by determining the slope of the operating line in the distillation section. In practical work, the PROII or ASPEN simplified tower model can be used for calculations

It is sufficient to use the PROII or ASPEN simplified column model for calculations; manual calculation is also an option and it’s not difficult. Based on the composition and separation requirements, one needs to derive the equations for the operating line, equilibrium line, and q-line in the distillation section, then find the intersection point of the q-line and the equilibrium line, and the values can be calculated using relevant formulas

This is closely related to your system, separation requirements, and the heat conditions of the feed material. In a pure ethanol and water system, due to the presence of an azeotrope, it is essentially impossible to achieve a purity of over 95 wt% without a minimum reflux ratio； A purity of 95wt% enables a very low minimum reflux ratio to be achieved ； If only a slight concentration increase is used to achieve product purity, a lower minimum reflux ratio results. You can determine it very intuitively by using graphical methods.