Total Reflux (minimum no. of plates)

The total reflux condition represents operation with no product removal. All the overhead product is condensed and returned as reflux. Consequently D = 0 and reflux ratio becomes infinite. (RD = L/D = ∞). This in turn, makes the operating line, 45° line where V = L and slope = 1. The operating line then coincides with diagonal. This condition is called total reflux.

With operating line on diagonal, they are as far as they can get from equilibrium curve, so if number of plates are stepped off using diagonal and equilibrium curve, number of theoretical stages will be minimum. Often columns are operated at total reflux during their initial startup and product is not withdrawn until separation close to that desired is achieved. Therefore, rate of feed and rate of both overhead and bottom product are zero. Since there is no feed in the column operating under total reflux, there is no discontinuity between upper and lower sections. Fenske equation is another method of determining minimum number of trays for given separation.