Reflux Ratio, Importance And It's Effect On Distillation Operation

Reflux Ratio: 
                      It is the ratio of liquid flow rate (L) from the reflux drum to the flow rate of distillate (D) (also called top product).

 R = L/D

Importance Of Reflux Ratio:
                                               Any change in reflux ratio will modify the slope of operation line, as can be seen in figure, this will alter the number of plates required for a given separation to be achieved.

     If R is known the top line is most easily drawn by joining point A (mole fraction of distillate) to B. If no product is withdrawn from the still, that is D=0 (R is infinite), then the column is said to operate under conditions of total reflux and, as seen from the figure above, and coincides with the line x=y. 

     

     If reflux ratio is reduced (or if distillate rate D is increased compared to L) the slope of the operating line is reduced and more stages are required to pass from xf to xd or to achieve desired concentration (i.e, from point K to A). Furthur, reduction in R will eventually bring the operating line to AE, where an infinite number of stages is needed to pass from xd to xf. This arises from the fact that under these conditions the steps become very close together at liquid compositions near to sf, and no enrichment occurs from the feed plate to the plate above. These conditions are known as minimum reflux. Any small increase in R beyond this reflux will give a workable system, although a large number of plates will be required. 

     Two important deductions may be made.

1) The minimum number of plates is required for a given separation at conditions of total reflux.

2) There is a minimum reflux ratio, at and below which it is impossible to achieve desired enrichment, no matter many plates are used.

Effect of Reflux Ratio On Distillation Operation:

                                                                       Distillation process is done to achieve a specific level of enrichment. To achieve this enrichment level, one must specify a reflux ratio. Decreasing that reflux ratio (i.e, increasing D), has an advantage that the duty of condenser and reboiler decreases, since the load in distillation column decreases (more mass goes out as D increases) . In other sense, the operating cost of distillation column decreases. However, it would be at expense of less enrichment achieved than required.

     Increasing reflux ratio would act in reverse i.e, it would have a disadvantage of increase in duty of condenser and reboiler as load increases. This increases the operation cost of distillation column, however, more enrichment is achieved.