Page 323 - Biosystems Engineering

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The blades pass close to the wall and break off any deposits of crys-
tals on the cooled wall. The product generally has wide crystal-size
distribution.

Double Pipe–Scraped Surface Crystallizer This type of crystallizer con-
sists of an internal agitator fitted with spring-loaded scrapers that
wipe the wall and provide good heat transfer coefficients. Cooling
water passes in the annular space. This crystallizer is used in crystal-
lizing ice cream and in plasticizing margarine.
Circulating Liquid Evaporator Crystallizer In this type of crystallizer, the
solution is supersaturated by evaporation. The circulating liquid is
drawn down by the screw pump inside the tube side of the condens-
ing stream heater. The heated liquid then flows into the vapor space
where flash evaporation occurs, resulting in supersaturation. The
vapor leaving is condensed. The supersaturated liquid flows down
the downflow tube and then up through the bed of fluidized and
agitated crystals, which are growing in size. The leaving saturated
liquid then goes back as a recycle stream to the heater, where it is
joined by the entering fluid. The larger crystals settle out, and a slurry
of crystals and mother liquid is withdrawn as a product.

Circulating Magma Vacuum Crystallizer In this type of crystallizer, a
steam-jet ejector provides a vacuum. The suspension of crystals is cir-
culated out of the main body through a circulating pipe by a pump.
The suspension flows though a heater, where along with the heated
liquor it then mix with body slurry. This causes supersaturation in the
swirling liquid near the surface, which is deposited in the swirling
suspended crystals until they leave again via the circulating pipe.
Vapors leave through the top.
Continuous Oscillatory Baffled Crystallizer This crystallizer is a tubular
baffled crystallizer that offers plug flow with a superior heat transfer
coefficient and also allows controlled cooling profiles. This crystal-
lizer has much better control over crystal size and consistent crystal
products.

9.3.8 Chromatographic Techniques
Chromatography techniques are based on a combination of chemical
principles of hydrophobic and hydrophilic interactions, partition
behavior, affinity, adsorption, and elution of molecules. In this tech-
nique, there are two phases: the stationary phase, where components
of a mixture are selectively adsorbed, and the mobile phase, which
helps to dislodge the components from the stationery phase at differ-
ent rates. Pure compounds are thus obtained.
Various types of chromatographic methods are developed depend-
ing on the physical states of the different phases. The stationary phase