CRYSTALLIZATION

In crystallization, the solute is separated out of the feed solution by saturating it through heating or cooling. Crystallization is preferred operation in some cases because heat of crystallization is much lower than energy of vaporization thereby requiring lower energy requirement. Heat of crystallization is the energy released or absorbed when crystallizing a mole of the solute from a saturated solution.

Solubility is the measure of the maximum amount of the solute that can be dissolved in a solvent at a given temperature. For list of solubility of solute in water, refer to 8th Perry’s Handbook, p: 216, T: 2 – 122.

 

Example 1:
You hope to recover salt crystals of sodium chloride by crystallization operation. 500 kg of salt solution whose salt content is 20% was cooled from 80^oC to 10^oC. The resulting saturated solution has a solubility of 35.72 kg salt/100g water and crystals are deposited. Calculate the amount of the crystal produced.

Solution:

(anhydrous)

OMB:
500 = S + C

KCl Solute Balance:



 

Example 2:
Sodium sulfate was recovered by crystallizing it out of the saturated solution from 25 ^oC to 10 ^oC. Calculate the percent recovery if the sulfate was crystallized as Na₂SO₄.10H₂O.

Solution:

Perry’s Handbook:

Temperature (T), oC	S(Solubility) = weight of anhydrous solute /100g water
20	19.4
30	40.8

In the calculator, choose STATISTICS option, then linear equation ( Y = A + BX). Input data and evaluate.
For T = 25 ^oC ,

Therefore,

x_c for hydrated salt: (MM_Na2SO4 = 142.06 g/mol, MM_Na2SO4.H2O = 322.06 g/mol):

OMB:
F = S + C

Na₂SO₄ Solute Balance:

0.1488F = 0.3585C
F = 2.409C

Percent Yield: