Question

Pulp containing paraffin wax is to be dewaxed in a continuous countercurrent extraction system using kerosene. The pulp containing the wax is kerosene-free when entering the extractor, and contains 20% paraffin wax and 80% pulp. After extraction, it must not contain over 0.2 lb wax / 100 lb wax-free pulp. The kerosene used for the extraction contains 0.05 lb of wax per 100 lb kerosene. Experiments show that the pulp retains 2.0 lb of kerosene per lb of kerosene- and wax-free pulp as it is transferred from cell to cell. The extract from the extraction battery is to contain 5 lb wax per 100 lb of kerosene. How many stages are required?

Answer 1

Basis 100 lb of pulp - wax and kerosene free.

Data provided 20 % wax and 80% pulp

So,

Wax in pulp = 20 lb

Wax in solvent (kerosene ) given = 0.05 lb / 100 lb of kerosene = 0.0005 lb wax / lb of kerosene

Lets denote "s" by weight of solvent (kerosene) used

Tking mass balance over wax

Wax in = wax out

Wax in = wax entering with pulp + wax entering with kerosene

= 20 lb + 0.0005 * s lb

Wax in exiting pulp = 100 b of pulp * 0.2 lb wax / 100 lb of wax free pulp = 0.2 lb wax

Now wax in solution leaving is given by

wax in solution leaving = ( Solvent entering - solvent carried away with pulp ) * ( Weight ratio of wax to solvent in leaving soluton )

  

So, the number stages required = 5