Crystallization Flashcards
is the formation of solid particles within a homogeneous phase.
Crystallization
Crystallization may be carried out from
vapor, melt, or solution
Most of the industrial applications of crystallization involve crystallization from
Solutions
formation of solid particles in a vapor
Snow
solidification from a liquid melt
as freezing of water to ice
formation of solid crystals from a solution
as the formation of Glauber’s salt from Na2SO4 solution
is the process whereby a solution is supersaturated so as to cause formation of crystals
Crystallization
Its wide use has two - fold basis:
- A crystal formed from an impure solution is itself pure (unless mixed crystals occur)
- Crystallization affords a practical way of obtaining pure chemical substance in a satisfactory condition for packaging and storing.
The rate of crystallization involves two distinct actions:
- The rate of formation of new crystals (nucleation)
- The rate of precipitation on crystals already present (crystal growth to macroscopic size)
The two rates of crystallization’s driving force
supersaturation
The two rates of crystallization can’t occur in
a saturated or undersaturated solution
Methods of supersaturating the solutions:
- By cooling
- By evaporation of solvent
- By adiabatic evaporation with cooling
- By addition of a third component where the solubility of the solute is sharply reduced
- By addition of a third component which will react with original solute to form an insoluble product.
The extent of the supersaturation depends upon
- The number and shape of crystals upon which precipitation occurs
- The temperature levels
- The solution concentration
- The violence of the agitation present
The heat effects in a crystallization process can be computed by two methods:
- A heat balance
- An enthalpy balance
can be made in which individual heat effects such as sensible heats, latent heats, and the heat of crystallization can be combined into an equation for total heat effects.
heat balance
can be made in which the total enthalpy of all leaving streams minus the total enthalpy of all entering streams is equal to the heat absorbed from external sources by the process.
enthalpy balance
The advantage of the enthalpy - concentration diagram method is
both heat and mass effects are taken into account simultaneously.
In using heat balance method, it is necessary to make ________, since the heat effects are related to the quantities of solids produced through the heat of crystallization
a corresponding mass balance
This method has only limited use, however, because of the difficulty in obtaining enthalpy - concentration data.
heat balance
With compounds whose solubility increases with increasing temperature there is an absorption of heat when the compound dissolves which is called the
heat of solution
When there is no change in solubility with temperature,
there is no heat effect.
are generally reported as the change in enthalpy associated with the dissolution of a large quantity of solute in excess of pure solvent.
Heat of solution
is equal and opposite in sign to the heat of solution.
heat of crystallization
Using the heat of solution at infinite dilution as equal but opposite in sign to the heat of crystallization is equivalent to neglecting
heat of dilution
A solution that contains as much of a dissolved solute as it can hold is said to be ________ with that solute.
saturated
If a saturated solution is cooled, the solubility of the solute ___
Decreases
Equilibrium relations for crystallization are expressed in the form
solubility data
Solubility data are ordinarily given as
parts by weight of anhydrous crystals per 100 parts by weight of total solvent
Solubility data can be presented efficiently and clearly in ________, which show equilibrium compositions in a system as functions of one or more system variables.
phase diagrams
The usual phase diagram for a solid-liquid system is
a plot of temperature versus mass fraction of the solid.
