As the reflux ratio increases, the number of theoretical plates required decreases. It is possible to achieve a separation at any reflux ratio above the minimum reflux ratio. The Minimum Reflux Ratio (R min) is the lowest value of reflux at which separation can be achieved with an infinite number of plates. As the reflux ratio is reduced (by taking off product), the number of plates required increases. At total reflux, the number of theoretical plates required is a minimum. The slopes of the Operating Lines are then L/V = 1.0, that is by mass balance, between each plate y = x. Total Reflux is the operating condition where vapor and liquid are passing each other in the column but no product is removed (i.e., D = 0 and R = L/D = ∞). Ln+1/Vn is called as internal reflux ratio (IR).ĭ/Vn is called as external reflux ratio (ER). Ln+1 is liquid going down from n+1 plate, Vn is vapor going up from n plate, Xn+1 is the mole fraction of liquid from n+1 plate, D is an overhead product, and XD is the mole fraction of product in the outgoing stream Images for McCabe Thiele diagram: All lines explained Consequently, the composition of the vapor leaving the top tray y1 is the same as that of the liquid distillate product and reflux, xD. In a total condenser, all of the vapor leaving the top of the column is condensed. The use of a partial condenser can avoid the use of costly refrigeration in the condenser.įor a total condenser, however, the operating reflux is often a subcooled liquid at column pressure, particularly if the distillate bubble-point temperature is significantly higher than the inlet cooling-water temperature. This approach is commonly employed when there are very light components in the feed to the column that would require a high column pressure or a low condenser temperature to completely condense these very volatile components. Partial condensers are used in distillation columns when the distillate product is removed as a vapor stream. The vapor is drawn off the top of the tower into the condenser.
Meanwhile, the top of the tower is still hot enough to keep the lighter fraction in vapor form. Refluxing increases the purity of the overhead products because condensation of the heavier fraction keeps them out of the stream of vapor that leaves the top of the tower.
In simple words, as the external reflux cools the top of the tower, vapors made of heavier fraction condense and liquid made of heavier faction flows down the tower and it’s referred to as internal reflux. A balance of heating with a reboiler at the bottom of a column and cooling by condensed reflux at the top of the column maintains a temperature gradient (or gradual temperature difference) along the height of the column to provide good conditions for fractionating the feed mixture. The more reflux that is provided, the better is the tower's separation of the lower boiling from the higher boiling components of the feed. Inside the tower, the downflowing reflux liquid provides cooling and partial condensation of the up-flowing vapors, thereby increasing the efficacy of the distillation tower. Reflux refers to the portion of the condensed overhead liquid product from a distillation tower that is returned to the upper part of the tower.
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