7/23/2019 Discussion Mass Crystalization

1/3

Discussion.

In crystallization, the supersaturation is a measure of the driving force for the

crystallization process. Quantifying the supersaturation is important as it concerns the

theoretical yield of the process. The theoretical yield of any crystallization can be determined

by the differences between the actual composition of the feed solution at the starting point of

the process and the equilibrium composition at the end-point of the process. This formula

applies for any temperature gradients employed in the process.

In this eperiment, !g of solid containing "#$ of benzoic acid and %#$ of &ulphur

were mied with water to form a solution. 's the solution is heated, energy was supply to

benzoic acid and used to brea( the intermolecular force that bond the benzoic acid molecule.

)ence, benzoic acid dissolve in water. &ulphur on the other hand is insoluble in water thus it

floats forming precipitate in the solution. Then the solution was filtered to remove the

&ulphur. Then, the solution was cooled down. *nly then crystallization occurred.

+rystallization occurs when the molecules in the liquid, which have no set structure, begin to

arrange themselves in a set pattern that followed the shape of the molecules themselves when

undergo cooling process.

rom the result as calculated refer appendi, the crystal yield obtain from this

eperiment is //$. The actual yield of the process may differ from the theoretical yield. This

is because of the result of percentage yield obtain from calculation in material balance is

"#$, where are higher compare to eperimental result. 'ccording to 0.1lrich and 2.0.0ones,

as the supersaturation decreases, the crystal growth rates also decrease and the theoretical

yield will only be reached in the limit of infinite time. This will result in a lower yield than

possible. )owever, if growth is fast and if the crystals obtained contain a significant amountof liquid and3or impurity inclusions, the yields may be higher than theoretically possible.

There are also some other factors influenced the yield of crystal. The first factor is

temperature. 's temperature rises, more energy is supplied to bea( the molecular bond of

benzoic acid thus easier for the benzoic acid to dissolve in water. 'llan &. 2yerso nd states

that the temperature difference between the cooling surface and the solution is at maimum at

the beginning of process4 which brings the solution to maimum critical supersaturation

level due to the huge and quic( subcooling.

7/23/2019 Discussion Mass Crystalization

2/3

5et, concentration of the feed solution to be crystallized are also factor that affect its

yield and economy. In cooling crystallization, concentrated solution at high temperature is

delivered into a 6ac(eted stirrer which consists of a cooling coil to increase the cooling

surface area. +old water or cooling agent is pumped through the 6ac(et as the solution is

stirred. The process continues until the temperature of the solution in the crystallizer is

approaching the cooling medium temperature. 7astly, theviscosityof a solution increases as

temperature decreases. )ydraulic problems andlaminar flow may arise, due to too high

viscosity and thus will affect the crystallization dynamics. This method is not suitable to

compounds having reverse inverse solubility, which means that solubility increases with

temperature decrease eg8 sodium sulphate 5a/&*" where solubility is reversed above 9/.:

;+.

There are some possible errors that might occur during the crystallization eperiment.

The benzoic acid and &ulphur might not mied evenly thus #."g of benzoic acid might not

obtained from !g of the solid. 5et, not all the benzoic acid is filtered during the filtration

process. *ther than that, the crystal yield might be affected by the miture of the feed

solution with some other fluid due to improper flushing by the previous eperimenter.

http://en.wikipedia.org/wiki/Viscosityhttp://en.wikipedia.org/wiki/Viscosityhttp://en.wikipedia.org/wiki/Laminar_flowhttp://en.wikipedia.org/wiki/Laminar_flowhttp://en.wikipedia.org/wiki/Laminar_flowhttp://en.wikipedia.org/wiki/Viscosity

7/23/2019 Discussion Mass Crystalization

3/3

!. )andboo( of industrial crystallization. 'llan &. 2yerso.