CRYSTALLISATION Introduction
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STUDENTS SHOULD BE ABLE TO:
1. Understa Understand nd the conc concept ept of crystallization crystallization 2. Perform Perform materia materiall & heat heat balan balance ce 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 formation of solid s olid particles particles from liquid melts, melts , formation formation of s now particles from vapour or formation formation of s olid crystals from a liquid s olution
(MOST IMPORTANT & COMMERCIALIZED)
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The The solu solution ion is concentrated and and usually cooled until until the solute s olute concentr concentration ation becomes greater greater than its sol solubil ubility ity at that temperature. temperature. Then Then,, the solu solute com comes out out of the solu solution ion, form orming ing crystals of approximately pure solute.
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Good yie yield High purity urity Size, shapes Uniform niformity
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Why Unif niformity im important??? a) Minim nimize caking in the package b) For ease of pouring ring c) For ease of washing hing and fil filtering ring d) Unifor niform m behavior vior 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 fa ces & sharp corners.
The ANGLE between between the correspon corresponding ding faces faces of all all crystals of THE SAME MATERIAL are EQUAL…CHARACTERISTIC CHARACTERIST IC OF THE PARTICULAR MATERIAL. 8
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Development of different types of faces of crystal may differ depending on the solute crystallizing. Eg. NaCl crystallizes crystallizes from aqueous aq ueous solution with cubic faces NaCl crystallizes crystallizes from aqueous aqueo us solution, impurity present, will will have octahedral octahed ral faces, BUT BOTH are in the CUBIC CUBIC system.
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When When water fr eezes eezes into ice, the water molecules stack together together to form a regular crystalline lattice, and the ice lattice has s ix-fold s ymmetry ymmetry
A hexagonal hexagonal pr ism inclu des two hexagonal hexagonal " basal" faces faces and six rectangular rectangular " prism" faces. faces. Hexagonal Hexagonal prism can be plate-like or columnar , depending on which facet surfaces grow most quickly.
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When snow crys tals tals are very s mall, mall, th ey are are mostly in the form of simp le hexagonal hexagonal prisms. But as as they grow, branche branches s sprout from the cor ners to make mor e complex shapes .
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The crystalliz crystalli zation in shapes shapes of of plates or needles usually depends upon the process conditions under which the crystals are grown. grown.
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ATTAIN EQUILIBRIUM WHEN THE SOLUTION/MOTHER LIQUOR IS SATURATED.
Represented by a SOLUBILITY CURVE.
Solubility is dependent is dependent mainly m ainly on TEMPERATURE . 14
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Generally, the solubilities of most salts increase with increasing temperature. That is why normally we cooled coo led down the solution solution to get ge t the crystal so that the concentration exceed the solubility solubility at that temperature 15
DURING CRYSTALLIZATION PROCESS, THE SOLUTION (MOTHER LIQUOR) & THE SOLID CRYSTALS CRYSTALS ARE IN CONTACT CONTACT FOR ENOUGH EN OUGH TIME TO REACH EQUILIBRIUM.
At the end of o f the process, pro cess, AT AT FINAL TEMPERATU TEMPERATURE RE T, the solution is is saturated, satu rated, 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 Na 2CO3/100 kg of total water? Assume that no water is evaporated.
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W kg H2O
10,000 KG SOLUTION
30% Na2CO3
COOLER & CRYSTALLIZER
S kg soln
21.5 kg Na2CO3/100 kg H2O C kg crystals, Na2CO3 • 10H2O 23
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1. Perform material material balance for water and Na2CO3 Feed = Solution stream + Crystals stream + Other stream Water
0.7(10000)
Na2CO3
0.3(10000)
Molecular Weight:
100 100
21.5
21.5 100
21.5
( S)
(S )
180.2 286.2 106 286.2
(C )
0
(C )
0
10H2O = 180.2 Na2CO3 = 106 Na2CO3 • 10H2O = 286.2
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2. Solving the two equation simultaneously, C = 6370 kg of Na 2CO3 • 10H2O crystals S = 3630 kg solution
Assume that 3% of the total weight of the solution is LOST by evaporation of water in cooling….RECALCU cooling….RECA LCULA LATE TE C & S VALUES.
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W hen a compound compound (whose solubility solubility increases as tem temperat perature ure increases) increases ) heat; dissolves , there is an absorption of heat; called the hea heatt of solu tion . W hen a compound compound (whose solubility solubility decreases as tem temperat perature ure increases) increases ) heat. dissolves , there is an evolution of heat. For compounds dissolving whose solubility does not ch ange with tempe temperature rature, there is no heat of evolution or heat of dissolution. dissolution. 26
In crystallization, the opposite of dissolution occurs. At equ eq u il i b ri u m , th e heat o f c ryst ry st all izat iz atio io n is equal to t he negative negative of t he heat heat of solu tion at the same same concentration concentration in solutio n.
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q = (H 2 +H V ) – H 1 H 1 = enthalpy of the entering solution at the initial temperature H 2 = enthalpy of the final mixture of crystals and mother liquor, liquor, at the final temperature temp erature H V = enthalpy of water vapor vapo r (if evaporation occurs) q = Total Total heat absorbed a bsorbed (kJ) = +ve: heat must be added to the system = -ve: heat is evolved evo lved or given off
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Books 1. Gean Geank kopl oplis C. J., J., Transp anspor ortt Proces Processe sess and and Proce ss Principl Principles, es, 4th Edition, Edition, Prentice Hall, Hall, 2003. 200 3.
Separ Separat atiion
2. McCabe W. M., Smith J. C. and Harriott Harriott P., Unit Operations of Chemical Chemica l Engineeri Enginee ring, ng, 7th Ed., McGraw Hill, Hill, 2005. 20 05.
Websites:
http://www.its.caltech.edu/~ato http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm mic/snowcrystals/primer/primer.htm
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THANK YOU.
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