Crystallization
Crystallization is defined as a process by which a
chemical is converted from a liquid solution into a
solid crystalline state.
Why is Crystallization Important?
Crystallization touches every aspect of our lives from the
foods we eat and the medicines we take, to the fuels we
use to power our communities.
The majority of agrochemical and pharmaceutical
products go through many crystallization steps during
their development and manufacture.
Key food ingredients, such as lactose and lysine, are
manufactured using crystallization and the unwanted
crystallization of gas hydrates in deep sea pipelines is a
major safety concern for the petrochemical industry.
• Types of Crystallization:
• Crystallization occurs when the solubility of a solute in solution is reduced by some
means. Common methods to reduce solubility include:
a.Cooling crystallization
In a cooling crystallization process the feed is cooled in a heat exchanger, which can
be situated inside the crystallizer. Crystallization can take place when the liquid is
cooled to a temperature below the equilibrium solubility.
b. Antisolvent Addition
c. Evaporation crystallization
In evaporative crystallization, the crystallization is resulting from the evaporation of
the solvent. So, this process creates a vapor and a suspension of crystals in mother
liquor.
d. Reaction (Precipitation)
In precipitation the super saturation is created by the mixing of two streams. The
most common forms of precipitation are: (1) reactive crystallization, (2) pH shift
crystallization and (3) anti-solvent (or extractive) crystallization. In reactive
crystallization, the solid phase is formed due to a reaction between components that
were originally present in the (two) feed streams
• The choice of crystallization method depends on the
⁕Equipment available for crystallization,
⁕The objectives of the crystallization process ,
⁕The solubility and
⁕ Stability of the solute in the chosen solvent.
Methods of crystallization:-
• Crystallization is the oldest “unit operation” in a chemical engineering sense. For
example, Sodium Chloride has been manufactured this way since the dawn of
civilization.
• Various traditional methods for crystallization exist, with each technique having unique
benefits and drawbacks. The method chosen must be selected based on the properties
of the material being crystallized.
• Solvent Evaporation – Easy to set-up, requires air stable samples, requires a minimum
solvent volume to work effectively. Large amount of material required.
• Slow Cooling – Requires solvents with boiling points less than 100 °C and moderate
solute solubility. Large amount of material required.
• Solvent / Vapour Diffusion – Works well with small amounts of material, however finding
two suitable solvents can be challenging. Can “oil out”.
• Sublimation – Not the method of choice for diffraction quality crystals. Typically
performed at high temperatures, causing crystals to grow too quickly.
• How does the crystallization process occur?
• The crystallization process consists of two major events:
Nucleation:-
– Molecules gather together in clusters in a defined manner. Clusters
need to be stable under current experimental conditions to reach the
“critical cluster size” or they will redissolve. It is this point in the
crystallization process that defines the crystal structure.
Crystal Growth:-
– Nuclei that have successfully achieved the “critical cluster size”
begin to increase in size. Crystal growth is a dynamic process, with
atoms precipitating from solution and becoming redissolved. Super
saturation and supercooling are two of the most common driving
forces behind crystal formation.
The crystallization process - crystal growth rate vs. nucleation rate
• Crystallization Steps
1. Choose an appropriate solvent. Common considerations included how
much solute can be dissolved (solubility) and how practical the solvent is to
handle (safety)
2. Dissolve the product in the solvent by increasing the temperature until the
last product molecule disappears. At this insoluble impurities may be
filtered from the hot solution
3. Reduce solubility via cooling, anti-solvent addition, evaporation or
reaction. The solution will become supersaturated.
4. Crystallize the product. As solubility is reduced a point is reached where
crystals will nucleate and then grow. Highly pure product crystals should
form and impurities should remain in solution.
5. Allow the system to reach equilibrium after cooling (or another
crystallization method stops).
6. Filter and dry the purified product.
TYPES OF CRYSTALLIZER
1- DRAFT TUBE BAFFLE (DTB) 2-VACUUM CRYSTALLIZER
CRYSTALLIZERS ARE USED IN INDUSTRY TO ACHIEVE LIQUID- SOLID
SEPARATION.
THEY ARE CAPABLE OF GENERATING HIGH PURITY PRODUCTS WITH
A RELATIVELY LOW ENERGY INPUT.
-
1 DRAFT TUBE BAFFLE (DTB)
DRAFT TUBE BAFFLE CRYSTALLIZERS ARE USED TO CONTROL CRYSTAL
SIZE AND CHARACTERISTICS.
DRAFT TUBE BAFFLE (DTB) CRYSTALLIZERS ARE USED IN APPLICATIONS IN
WHICH EXCESSIVE NUCLEATION OCCURS.
• DTB crystallizers have two distinct volumes.
• In the baffled region gravitational settling separates larger crystals
from fines.
• The larger crystals settle between the baffle and draft tube, as
shown above, and are removed in the product slurry, while the
fines are recirculated after being redissolved in a heat exchanger.
• The evaporated solvent is then condensed, cooled, and returned
to the region of active crystallization.
• DTB crystallizers typically have circulation rates of 1 to 4
times/min and a total residence time of 4 to 6 hours.
USAGES OF DRAFT TUBE BAFFLE (DTB)
• TO PRODUCE AMMONIUM SULFATE.
• ARE USED FOR THE PURIFICATION OF ORGANIC COMPOUNDS.
• ARE USED FOR THE PURIFICATION OF PRODUCTION OF SALTS.
• Examples of crystals that are formed using a DTB crystallizer :
ADVANTAGES AND DISADVANTAGES OF DRAFT TUBE BAFFLE (DTB)
2-VACUUM CRYSTALLIZER
VACUUM CRYSTALLIZERS MAY BE CONTINUOUS OR BATCH.
THE BATCH VACUUM CRYSTALLIZER IS PARTICULARLY USEFUL
WHEN PROCESSING MATERIALS THAT TEND TO GROW ON THE
WALLS OF CONTINUOUS CRYSTALLIZATION EQUIPMENT.
Vacuum crystallizers use a condenser with a booster to
maintain a vacuum inside the crystallizer body.
This vacuum makes it possible to generate a
supersaturated solution when very low operating
temperatures are needed
The crystallizer body can be seen on the left, with the
condenser and booster slightly above and to the right
of it.
USAGES OF VACUUM CRYSTALLIZER
• TO PRODUCE GLAUBER'S SALT AT A MINERAL PLANT.
