L13- Crystallisation

Question 1

Crystallisation

Answer 1

Taking supersaturated material and forming solids/crystals

Question 2

Saturated solution

Answer 2

Solution in thermodynamic equilibrium with solid phase at given temp

Question 3

Supersaturated

Answer 3

Solution containing more dissolved solute than at equilibrium

Question 4

Degree of supersaturation, Delta c

Answer 4

Delta c = c-c*

c = actual supersaturated concentration
c* = equilibrium concentration

Question 5

Supersaturation ratio, S

Answer 5

S = c/c*

Question 6

Relative supersaturation, phi

Answer 6

phi = delta c/c* = S-1

Question 7

Crystallisation driving force

Answer 7

Supersaturation is the driving force for crystallisation;
fundamentally is the difference in chemical potential

Question 8

Solubility-supersolubility diagram: width of metastable zone

Answer 8

delta c = (dc/dT)delta T

Question 9

Solubility-supersolubility diagram: zones & factors affecting

Answer 9

Stable: no crystals readily form

Metastable: crystals do not form spontaneously but existing crystals would grown

Labile: crystals form since conc. above max show in supersolubility line

Depends on agitation, impurities and how got supersaturated

Question 10

Solubility-supersolubility diagram: Moving from stable to labile (crystal growth)

Answer 10

Constant temp: saturate at metastable boundary, crystals form spontaneously at labile boundary

Cooling: saturate at metastable boundary, crystals form spontaneously at labile boundary

Question 11

Nucleation

Answer 11

Creation of crystals within supersaturated liquor

Question 12

Nucleation types

Answer 12

Primary: nucleation in absence of existing crystals
Homogeneous if only solute involved; heterogeneous when other substances involved or present

Secondary: nucleation in presence of existing crystals- nucleates more readily, i.e. lower supersaturation

Question 13

Crystal growth kinetic stages

Answer 13

Diffusional stage: solute transported from bulk through solution boundary layer adjacent to crystal surface

Deposition stage: adsorbed solute deposited and integrated into crystal lattice

Question 14

Overall mass deposition rate, RG

Answer 14

RG = KG(delta c^s) = [(6ap)/B](dr/dt) = [(6ap*u’)/B]

where particle mass, m = ap(d^3)
surface area, A = B*(d^2)
KG - overall crystal growth coefficient
s = constant between 1-2
u’ = mean linear velocity (dr/dt, m/s)

For spheres 6a/B = 1; octahedra 6a/B = 0.816

Question 15

Crystal yield basis

Answer 15

estimated from conc. of initial solution and solubility at final temp, allowing for evaporation

Question 16

Solvent (Water) balance

Answer 16

For solvent (usually water), the initial solvent present is equal to the sum of the final water in mother liquor + water of crystallisation within crystals and water evaporated

w1 = w2 + y[(R-1)/R] + w1E

w1, w2 = initial and final masses of solvent in liquor
y = yield of crystals
R = molecular mass of hydrate/anhydrous salt
E = mass of solvent evaporated/solvent in initial solution

Question 17

Solute balance

Answer 17

w1c1 = w2c2 + y/R

w1, w2 = initial and final masses of solvent in liquor
y = yield of crystals
R = molecular mass of hydrate/anhydrous salt

Question 18

Proof for crystal yield

Answer 18

Sub solvent balance (w2 =…) in solute balance:
w1c1 = c2{w1(1-E) - y[(R-1)/R]} + y/R

y = Rw1{[c1-c2(1-E)]/[1-c2(R-1)]}

Question 19

Crystallisation processes & equipment

Answer 19

Reaction -> Evaporator -> Crystalliser -> Filter/centrifuge -> dryer

Cooling crystalliser: allow vessel to cool over time, surfaces provided for crystal growth
Economic for small batches, but uncontrolled crystal sizes,i.e. agitated/agitated w/ circulation through external HEx

Evaporating crystalliser: used when temp not decreased enough
Increases supersaturation, i.e. steam heated