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CRYSTALLISATION

Introduction

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STUDENTS SHOULD BE ABLE TO:

1. Understand the concept of crystallization

2. Perform material & heat balance for crystallization process

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Process where solid particles are formed from homogeneous phase.

Occur in the: freezing of water to form ice,

formation of solid particles from liquid melts,

formation of snow particles from vapour or

formation of solid crystals from a liquid solution (MOST IMPORTANT & COMMERCIALIZED)

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The solution is concentrated and usually cooled until the solute concentration becomes greater than its solubility at that temperature.

Then, the solute comes out of the solution, forming crystals of approximately pure solute.

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Good yield

High purity

Size, shapes

Uniformity

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Why Uniformity important???

a) Minimize caking in the package

b) For ease of pouring

c) For ease of washing and filtering

d) Uniform behavior when used

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Crystallographic systems

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A Solid composed of atoms, ions or molecules which are arranged ORDERLY and REPETITIVE MANNER.

Appear as polyhedrons, having flat faces & sharp corners.

The ANGLE between the corresponding faces of all crystals of THE SAME MATERIAL are EQUAL…CHARACTERISTIC OF THE PARTICULAR MATERIAL.

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Development of different types of faces of crystal may differ depending on the solute crystallizing.

Eg. NaCl crystallizes from aqueous solution with cubic faces

NaCl crystallizes from aqueous solution, impurity present, will have octahedral faces,

BUT BOTH are in the CUBIC system.

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A hexagonal prism includes two hexagonal "basal" faces and six rectangular "prism" faces. Hexagonal prism can be plate-like or columnar, depending on which facet surfaces grow most quickly.

When water freezes into ice, the water molecules stack together to form a regular crystalline lattice, and the ice lattice has six-fold symmetry

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When snow crystals are very small, they are mostly in the form of simple hexagonal prisms. But as they grow, branches sprout from the corners to make more complex shapes.

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The crystallization in shapes of plates or needles usually depends upon the process conditions under which the crystals are grown.

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ATTAIN EQUILIBRIUM WHEN THE SOLUTION/MOTHER LIQUOR IS SATURATED.

Represented by a SOLUBILITY CURVE.

Solubility is dependent mainly on TEMPERATURE.

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Generally, the solubilities of most salts increase with increasing temperature.That is why normally we cooled down the solution to get the crystal so that the concentration exceed the solubility at that temperature

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DURING CRYSTALLIZATION PROCESS, THE SOLUTION (MOTHER LIQUOR) & THE SOLID CRYSTALS ARE IN CONTACT FOR ENOUGH TIME TO REACH EQUILIBRIUM.

At the end of the process, AT FINAL TEMPERATURE T, the solution is saturated, hence the final concentration of the solution can be obtained from SOLUBILITY CURVE.

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THE YIELD OF CRYSTALS CAN BE CALCULATED FROM:

INITIAL CONCENTRATION OF SOLUTE

FINAL TEMPERATURE &

ITS SOLUBILITY AT THE TEMPERATURE

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Material balance is straightforward , when SOLUTES are anhydrous.

When the crystals are hydrated, some of the water in the solution is removed with the crystals are hydrate.

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A salt solution weighing 10 000 kg with 30% Na2CO3 is cooled to 293 K (200C). The salt crystallizes as the decahydrate. What will be the yield of Na2CO3 • 10H2O crystals if the solubility is 21.5 kg anhydrous Na2CO3/100 kg of total water? Assume that no water is evaporated.

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COOLER & CRYSTALLIZER

10,000 KG SOLUTION

30% Na2CO3

W kg H2O

S kg soln

21.5 kg Na2CO3/100 kg H2O

C kg crystals, Na2CO3 • 10H2O

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1. Perform material balance for water and Na2CO3

Feed = Solution stream + Crystals stream + Other stream

Water

0)(2.286

106)(5.21100

5.21)10000(3.0

0)(2.2862.180)(

5.21100100)10000(7.0

CS

CS

Na2CO3

Molecular Weight: 10H2O = 180.2

Na2CO3 = 106

Na2CO3 • 10H2O = 286.2

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2. Solving the two equation simultaneously,

C = 6370 kg of Na2CO3 • 10H2O crystals

S = 3630 kg solution

Assume that 3% of the total weight of the solution is LOST by evaporation of water in cooling….RECALCULATE C & S VALUES.

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When a compound (whose solubility increases as temperature increases) dissolves, there is an absorption of heat; called the heat of solution.

When a compound (whose solubility decreases as temperature increases) dissolves, there is an evolution of heat.

For compounds dissolving whose solubility does not change with temperature, there is no heat of evolution or heat of dissolution.

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In crystallization, the opposite of dissolution occurs.

At equilibrium, the heat of crystallization is equal to the negative of the heat of solution at the same concentration in solution.

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q = (H2 +HV) – H1H1 = enthalpy of the entering solution at the initial temperature

H2 = enthalpy of the final mixture of crystals and mother liquor, at the final temperature

HV = enthalpy of water vapor (if evaporation occurs)

q = Total heat absorbed (kJ)

= +ve: heat must be added to the system

= -ve: heat is evolved or given off

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Websites:

http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm

1. Geankoplis C. J., Transport Processes and Separation Process Principles, 4th Edition, Prentice Hall, 2003.

2. McCabe W. M., Smith J. C. and Harriott P., Unit Operations of Chemical Engineering, 7th Ed., McGraw Hill, 2005.

Books

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THANK YOU.