Fractional Crystallization

21.2.09

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.

http://www.thefreedictionary.com/Fractional+crystallization

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.

http://www.sulzerchemtech.com/en/PortalData/11/Resources/Brochures/MTC/Fractional_Crystallization-e.pdf

Fractional Crystallization – Why is this method important to Chemical Industry?

http://www.sulzerchemtech.com/en/PortalData/11/Resources/Brochures/MTC/Fractional_Crystallization-e.pdf

Fractional Crystallization – Different Ways - From Melt Vs Solvent

Crystallization can be performed bothfrom melts and from solutions.

The addition of solvents generally reducesthe operating temperature and increasesthe selectivity of purification.

The presence of solvents leads to reducedviscosity and, in some cases, modifiescrystal habit.

On the other hand, the absence of a solvent involves smaller equipment volumes and

Absence of solvent does not requireadditional equipment to recoverthe solvent

http://www.sulzerchemtech.com/en/PortalData/11/Resources/Brochures/MTC/Fractional_Crystallization-e.pdf

Industrial crystallization features generation of crystals by nucleation, crystalgrowth 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 SuspensionWith this technique a melt or a solutionis cooled below saturation temperature.

Crystals grow under adiabaticconditions with supersaturation as drivingforce being present throughout theliquid phase.

A very special knowhow is required to secure crystal shape, crystal size and crystal size distribution.

Since the product from suspensioncrystallization is a slurry, the separationof residual mother liquor from largespecific surface area is usually carriedout 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 acooled surface

Crystals grow in a non adiabatic environment in such a way that subcooling is supplied through the crystal layer ratherthan through the liquid phase.

The driving force results from the net effectof temperature and concentrationgradients across both solid and liquidphase.

Under these conditions crystalgrowth rate is 10 to 100 times fasterthan 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 Na2SO4. But addition of few drops of Nesslersreagent to a little of the crystals gave reddish brown precipitate. This indicate thatThe crystals are not exclusively of Na2SO3. But ammonium sulphate too crystallized with Na2SO4.

ix. Since the crystallization is carried out at room temperature no fractions (Na2SO4 and(NH4)2SO4) 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 5.707 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 – 16.2.09)

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 (NH4)2SO4 could be crystallized at 60 °C and removed, still a little of (NH4)2SO4 is present in the supernatent liquid along with Na2SO4 which is responsible for the redish brown precipitate with Nessler’s reagent

Xi. Thus the method of fractional crystallization is successful in separating (NH4)2SO4 from Na2SO4

Xii. To remove the (NH4)2SO4 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 (NH4)SO4 will be crystallized from the solution and the supernatent will be almost free from (NH4)2SO4.

Procedure II (Batch I – 16.2.09)

Procedure II (Batch II – 16.2.09)

The procedure adopted is exactly the same as the previous one except thatThe fracational crystalliation or the aging time at 60 °C is 12 h rather than 6 h so that The crystallization of (NH4)2SO4 is almost complete.

The crystal obtained are saparated from the supernatent liquid. The crystals arebeing 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 (NH4)2SO4 in the supernatent liquid.

The supernatent liquid is being aged at room temperature for crystallizing Na2SO4

from solution. The yield of Na2SO4 is 0.981 g.

Purity of Supernatent liquid containing Na2SO4 – Nessler’s Test

Parent compoundFrom Industry(Na2SO4 with(NH4)2SO4 as impurity)

Crystals obtained at 60 °C(NH4)2SO4 alone

Supernatent liquidcontaining Na2SO4

separated at 60 °Cfrom the crystal of (NH4)2SO4

Absence of ammonia

Nessler’s testwith excess supernatent liquid obtained at 60 °Callowed to crystallize at RT

CommercialNa2SO4 from Rankem

Crystallization of Na2SO4 supernatent liquid at room temperature