The liquid which can be obtained by condensing the overhead vapor and returning some of the liquid to the top of the column is known as reflux.
Reflux Ratio of Distillation
The reflux ratio of the reflux and overhead product is known as the reflux ratio it is denoted by Rd. The total reflux operation is necessary only to know the minimum number of plates required and is not a practical method of operation as total reflux the product rate is zero.
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A column operation under total reflux is in Fig. In this case, enough material is charged to the distillation assembly, and the column is operated under total reflux. During the operation of the column, the vapor issuing from the top is condensed and all the condensed steam is feedback to the column as reflux.
Reflux Ratio Formula
The reflux ratio (Rd) is mathematically expressed as the ratio of the liquid returned (L) to the vapor taken off (D)
Rd = D÷L
Where:
Rd is the reflux ratio.
L is the liquid returned to the column.
D is the vapor taken off the column (distillate).
Reflux Ratio Advantage and Disadvantage
(i) When the reflux ratio is maximum meaning to say Rd ~=1.50 total reflux the number of plates minimum and so the overhead is zero for many still of finite size for minimum reflux overhead product is less.
(ii) When the reflux ratio is maximum the fixed cost is low because of less number of plates.
(iii) For maximum reflux ratio the operating cost is high.
(iv) For reflux ratio minimum to the number of plates is infinities.
(iv) For the minimum reflux ratio the operating costs are low.
(v) For minimum reflux ratio fixed cast is high due to the infinite no of plates.
Effect of Reflux Ratio on Distillation
Effect on Purity
A higher reflux ratio leads to a higher purity distillate, as it provides more opportunities for the vapor and liquid phases to contact each other and exchange components. This is because the fluid returned to the column is enriched with more volatile components, which are then vaporized and carried to the column.
Effect on the number of stages
A higher reflux ratio also leads to less number of stages required to achieve the desired separation. This is because higher reflux rates provide more phases with higher degrees of separation.
Other influences
In addition to the purity and number of stages, the reflux ratio is also affected by other factors such as energy consumption and the diameter of the distillation column. Higher reflux ratios result in more energy consumption, as more heat is required to vaporize the liquid reflux. This also leads to a larger diameter column, as more stages are required.
Minimum Reflux Ratio of Distillation
This represents a condition of minimum reflux. Thus the minimum reflux ratio is the reflux ratio at which an infinite number of plates are required for the desired separation. At the minimum reflux ratio, the required heat supply for the reboiler and coolant supply for the condenser is minimal.
Reflux Ratio Formula
These are many effects of reflux ratio and distillation.
Rd = Reflux / Overhead product
Rd = L/D = V-D/ D
In other words, reflux is the ratio of the reflux to the vapor
Rw = L/V = L /L + D
Optimum Reflux Ratio of Distillation Column
Any reflux ratio between an infinite reflux ratio requiring a minimum number of plates and a minimum reflux ratio requiring an infinite number of plates is a workable system that requires finite stages for the desired degree of separation. At a minimum reflux ratio, as an infinite number of plates are required, the fixed cost is also infinite while the cost of heat supply to the reboiler and condenser coolant is minimum.
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As the reflux ratio is increased, the number of plates decreases and the fixed cost decreases at first passes through a minimum and then increases as with a higher reflux ratio the diameter of the column and sizes of the reboiler and condenser increase.
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