Crystallisation

Question 1

Define crystals

Answer 1

A solid where atoms, ions or molecules are organised in a regular and repeating 3D

Question 2

define amorphous solid

Answer 2

a solid where there is only short range positional order

Question 3

define liquid crystals

Answer 3

a substances which properties fall in between a liquid and a solid. It can flow like a liquid but has the organised atomic arrangement of a crystal

Question 4

define allotropy

Answer 4

an element that can exist in two or more chemical forms

Question 5

define polymorphy

Answer 5

a substance that can exist in two or more crystalline forms

Question 6

how does fractional crystallisation work?

Answer 6

A and B are in solution together
Evaporation or cooling forms crystals of A and leaves B in mother liquor
Excessive evaporation (or cooling) causes crystallisation of A and B and impure crystals

Question 7

draw a graph showing fractional crystallisation (solubility vs temp)

Question 8

what do we need for crystallisation from a solution to occur

Answer 8

supersaturation of that solution and nuclei generation.

Question 9

how do we supersaturate a solution?

Answer 9

evaporation, cooling, adding a second solvent (anti solvent), change in PH

Question 10

define labile, metastable and stable regions on a supersaturated solution graph

Answer 10

labile- supersaturated, nucleation more likely spontaneously
metastable-supersaturated, nucleation non spontaneous
stable-not supersaturated, no nucleation

Question 11

draw a diagram showing the distribution of solute conc in a supersaturated solution and indicate on the diagram the driving force for diffusion

Question 12

How does forced circulation crystalliser work

Answer 12

feed mixes with slurry
2. stream is pumped up to heat exchanger (heated by condensing steam)
4. stream enters crystallizer and this is where supersaturation occurs by flashing and evaporating

Question 13

disadvantages of FC crystalliser

Answer 13

high circulation- difficult to scale up
Pump can lead breaking of crystals/secondary nucleation.

Question 14

derive critical nucleus equation in nucleation

Answer 14

1. Write out surface term and volume term
2. Expand out delta G in the volume term and put back into the first equation
3. differentiate full expression to get dG/dt
4. At Gmax, rearrange the expression to find r.

Question 15

define primary nucleation

Answer 15

where no other crystals are present

Question 16

define secondary nucleation

Answer 16

where other crystals already present

Question 17

define homogenous nucleation

Answer 17

spontaneous

Question 18

define heterogenous nucleation

Answer 18

induced by foreign particles

Question 19

DT baffle crystallisation (fine crystals)

Answer 19

impeller pumps stream to boiling surface in draft tube where supersaturation occurs
cooled slurry is recirculated or discharged
annual baffle is a settling zone where fines and mother liquor are separated

Question 20

oslo crystalliser

Answer 20

stream pumped to vaporaise where supersaturation occurs
suspension chamber which fluidises growing crystals
also acts a settling zone which classifies crystals in bed based on settling velocities
large crystals are seperated.