For extraction of dioxane from water, benzene is used as an extraction solvent.

Question

For extraction of dioxane from water, benzene is used as an extraction solvent. At 298K (25 degree C), the equilibrium distribution of dioxane between water and benzene is as given below: At these concentrations, water and benzene are substantially insoluble. 1000 kg of solution containing 25% dioxane is to be extracted with benzene to remove 95% of the dioxane. The dioxane free benzene is used. Calculate the solvent requirement for single batch operation. If the extraction were done in 4 stages crosscurrent unit using 750 kg of pure benzene each time, determine the final raffinate concentration and the percentage removal of dioxane from feed solution.

Explanation / Answer

a) With a distribution coefficient = 1.2

Kd = dioxane in benzene/dioxane in water

1.2 = (237.5/volume of benzene)/(12.5/750)

Thus,

Volume of benzene required = 11875 l to remove 95% dioxane from water

b) First extraction,

let x amount is extracted

1.2 = (x/750)/[(250 - x)/750)

0.4 - 0.0016x = 0.0013x

x = 137.93 kg

Amount in water = 250 - 137.93 = 112.07 kg

Second extraction,

1.2 = (x/750)/[(112.07 - x)/750)

0.18 - 0.0016x = 0.0013x

x = 62.07 kg

Amount in water = 137.93 - 62.07 = 75.86 kg

Third extraction,

1.2 = (x/750)/[(75.86 - x)/750)

0.12 - 0.0016x = 0.0013x

x = 41.85 kg

Amount in water = 75.86 - 41.85 = 34.01 kg

fourth extraction,

1.2 = (x/750)/[(34.01 - x)/750)

0.054 - 0.0016x = 0.0013x

x = 18.76 kg

Amount in water = 34.01 - 18.76 = 15.25 kg

Total amount of dioxane extracted = 250 - 15.25 = 234.75 kg

% removal of dioxane from feed solution = (234.75/1000) x 100 = 23.475%

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