Flag number ( coefficient of irrigation ) is a parameter of the distillation column , which determines the efficiency of the separation process in the concentration section of the column ; along with the number of theoretical plates, it is an important parameter determining the quality of separation.
The reflux ratio is the ratio of the amount of liquid flowing from any plate in the concentration section of the column (from the point of introduction of the feed to the selection of the top product — the distillate), to the amount of distillate taken.
The equation of material balance for an arbitrarily selected contour of the concentration section of the column:
{\ displaystyle G_ {j} = g_ {j-1} + D}
Where - flow rate of steam leaving the j- th plate, Is the flow rate of liquid entering the j- th plate from the overlying plate, D is the flow rate of the distillate.
Then the reflux number is written as the ratio .
The reflux ratio is a characteristic of only the strengthening (concentration) section of the column; below the supply zone, a steam number acts as a similar characteristic, which is the ratio of the amount of steam rising from any plate in the distant section of the column (from the place of input of raw materials to the selection of the lower product — residue) to the amount of residue taken. In the case of a complete distillation column (having both concentration and distillation sections), only reflux number is often mentioned, since its change affects the value of the steam number. However, it is worth remembering that in the case of an incomplete distant column, the quality of the product (residue) taken will depend only on the steam number.
The reflux number can take any value in the range from F min (in the minimum irrigation mode ) to infinity (in the full irrigation mode ) for a given quality of distillate, as well as other parameters that determine the separation process in the column (pressure, proportion of steam in the feed to column feed zone). If this condition is not fulfilled in practice, the specified purity of the distillate will not be achieved.
An increase in the reflux number on a running column (with a known number of theoretical plates), i.e., a decrease in distillate selection with a simultaneous increase in the amount of irrigation sent back to the column, will lead to an increase in separation efficiency and an increase in the content of low boiling components in the distillate.
Literature
- Skoblo A.I., Tregubova I.A., Molokanov Yu.K. Processes and apparatuses of the oil refining and petrochemical industries. - 2nd ed., Revised. and add. - M.: Chemistry, 1982, p. 112, 134-136.
- Bagaturov S. A. Fundamentals of the theory and calculation of distillation and rectification. - 3rd ed., Revised. - M .: Chemistry, 1974, p. 125, 180-181. * Handbook of a chemist. Volume 5. - Leningrad: Chemistry, 1968, p. 669-670.