Fractional Crystallization

Problem - I Problem: Sodium sulphate sample contained ammonium sulphate as contaminant Objective: To develop a method to remove ammonia from this mixture Strategy employed: Fractional Crystallization

 A process by which a chemical compound is separated into components by crystallization.  In fractional crystallization the compound is mixed with a solvent, heated, and then gradually cooled so that, as each of its constituent components crystallizes, it can be removed in its pure form from the solution. What is Fractional Crystallization?

 Crystallization is one of the most powerful purification methods available in process engineering.  Fractional crystallization is a stagewise separation technique that relies upon liquid-solid phase transition and enables multicomponent mixtures to be split into narrow fractions, ultimately leading to top purities of selected components, by virtue of selectivity found in solid liquid equilibria.  Fractional crystallization is most frequently encountered in the separation of organic materials ranging from isomer separations to tar chemical mixtures and from organic acids to monomers. Fractional Crystallization – Why is this method important to Chemical Industry?

Fractional Crystallization – Different Ways - From Melt Vs Solvent Crystallization can be performed both from melts and from solutions. The addition of solvents generally reduces the operating temperature and increases the selectivity of purification. The presence of solvents leads to reduced viscosity and, in some cases, modifies crystal habit. On the other hand, the absence of a solvent involves smaller equipment volumes and Absence of solvent does not require additional equipment to recover the solvent

 Industrial crystallization features generation of crystals by nucleation, crystal growth and the recovery of crystals from residual mother liquor.  Both generation and recovery of crystals can be performed either from suspension or through a crystal layer Suspension crystallization. Individual crystals are freely suspended in liquid. Crystallization proceeds by cooling the liquid. Crystallization in Suspension With this technique a melt or a solution is cooled below saturation temperature. Crystals grow under adiabatic conditions with supersaturation as driving force being present throughout the liquid phase. A very special knowhow is required to secure crystal shape, crystal size and crystal size distribution. Since the product from suspension crystallization is a slurry, the separation of residual mother liquor from large specific surface area is usually carried out in dedicated equipment.

Layer Crystallization. Crystal mass is frozen onto the cooling surface. Crystallization proceeds by cooling the crystal mass. Layer Crystallization Crystals are allowed to grow onto a cooled surface Crystals grow in a non adiabatic environment in such a way that subcooling is supplied through the crystal layer rather than through the liquid phase. The driving force results from the net effect of temperature and concentration gradients across both solid and liquid phase. Under these conditions crystal growth rate is 10 to 100 times faster than in suspension crystallization.

i. Taken known amount (15 g) of the salt (sodium sulphate with ammonium sulphate as contamination) ii. The salt is dissolved in known amount of water (50 ml) iii. Both sodium sulphate and ammonium sulphate dissolved in water completely iv. The contents are heated to 80 °C with stirring on a magnetic stirrer for 1 h V. The contents are cooled slowly vi. Crystallization started after nearly 4 h of aging the saturated solution vii. The supernatent liquid and the crystals were separated by decantation vii. The supernatent liquid is tested for ammonia using Nesslers reagent. A brown precipitate is formed indicating that the liquid contained ammonia Viii. The crystals were presumed to be of Na 2 SO 4. But addition of few drops of Nesslers reagent to a little of the crystals gave reddish brown precipitate. This indicate that The crystals are not exclusively of Na 2 SO 3. But ammonium sulphate too crystallized with Na 2 SO 4. ix. Since the crystallization is carried out at room temperature no fractions (Na2SO4 and (NH 4 ) 2 SO 4 ) could be separated. Procedure I (Batches I and II)

Strategy employed: Fractional Crystallization i. Taken known amount (10 g) of the salt (sodium sulphate with ammonium sulphate being present as contamination) and dissolved in known amount (30 ml) of distilled water (30 ml) ii.Both sodium sulphate and ammonium sulphate dissolved in water completely. iii. The contents were heated to 80 °C with stirring on a magnetic stirrer for 2 h v. The reaction temperature is lowered to 60 °C and kept at that temp. for 6 h stand still in water bath but with out stirring allowing fractional crystallization to take place vi. After 6 h of aging at 60 °C, the crystals formed at the bottom of the beaker were separated from the supernatent liquid. The crystals were dried in water bath at 80 C and the yield was was g. The percentage yield was 57% vii. The crystals were subjected to Nesslers reagent test to look for the presence of ammonia. Reddish brown precipitate is formed even with a speck of the cystal clearly indicating that the compound that is crystallized at 60 C is ammonium sulphate viii. To check the purity of the supernatant liquid containing sodium sulphate, little of the supernatent liquid is taken and subjected to Nesslers reagent test. No reddish precipitate formed. Procedure II (Batch I – )

iX. To confirm that whether ammonium sulphate is completely absent in the supernatent liquid, excess supernatent liquid is taken in a test tube and Nesslers reagent was added. With excess amount of the supernatent liquid reddish brown precipitate is obtained. X. This indicate that even though major amount of (NH 4 ) 2 SO 4 could be crystallized at 60 °C and removed, still a little of (NH 4 ) 2 SO 4 is present in the supernatent liquid along with Na 2 SO 4 which is responsible for the redish brown precipitate with Nessler’s reagent Xi. Thus the method of fractional crystallization is successful in separating (NH 4 ) 2 SO 4 from Na 2 SO 4 Xii. To remove the (NH 4 ) 2 SO 4 to the maximum extent possible the reaction is repeated by increasing the factional crystallization or aging time at 60 °C so that most of the (NH 4 )SO 4 will be crystallized from the solution and the supernatent will be almost free from (NH 4 ) 2 SO 4. Procedure II (Batch I – )

Procedure II (Batch II – )  The procedure adopted is exactly the same as the previous one except that The fracational crystalliation or the aging time at 60 °C is 12 h rather than 6 h so that The crystallization of (NH 4 ) 2 SO 4 is almost complete.  The crystal obtained are saparated from the supernatent liquid. The crystals are being dried. The yield (of ammonium sulphate) will be calculated.  The supernatent liquid is test for ammonia, using Nesslers reagent. Even with excess of supernatent liquid no reddish brown precipitate is formed with Nessler’s Reagent. This indicates absence of (NH 4 ) 2 SO 4 in the supernatent liquid.  The supernatent liquid is being aged at room temperature for crystallizing Na 2 SO 4 from solution. The yield of Na2SO4 is g.

Purity of Supernatent liquid containing Na 2 SO 4 – Nessler’s Test Parent compound From Industry (Na 2 SO 4 with (NH 4 ) 2 SO 4 as impurity) Crystals obtained at 60 °C (NH 4 ) 2 SO 4 alone Supernatent liquid containing Na 2 SO 4 separated at 60 °C from the crystal of (NH 4 ) 2 SO 4 Absence of ammonia Nessler’s test with excess supernatent liquid obtained at 60 °C allowed to crystallize at RT Commercial Na 2 SO 4 from Rankem

Crystallization of Na 2 SO 4 supernatent liquid at room temperature