C RYS YST TALLIZATION C rys ysta tall lliz iza a tio tio n is the p ro c e ss o f so so lid lid p a rtic tic le s for fo rming ming within within a homo ho mog g e nous no us p has ha se . It It is is the slow slow p re c ipitation ipitatio n of o f so so lid c rys ysta tals ls fro fro m the so so lution. lution. It is refer fe rre d a s liq liq uid so so lid lid se p a ra tio tio n and a nd p urif urific ica a tio tio n technique. In che c hemi mic c a l enginee eng ineerring c rys ysta tall lliz iza a tio tio n oc o c c urs urs in cr c rys ysta tall lliz ize e rs. It is a n a sp ec t of prec prec ipita pita tion tion whi w hic c h is is o btai bta ined thro thro ugh a var va ria tion tion of o f the the so lubi ub ility lity co c o ndit nd itions ions o f the so lute lute in the the so lve lve nt.
C RYSTALLIZERS C rys ysta talllizer is is the type o f equi eq uip p ment that tha t ar a re used used to pr p ro d uce uc e c rys ysta tals ls.. They he y ma ma ybe lab sc a le or o r industr industrial ial sc sc a le in size ize. Co C o mmercial mmerc ial c rys ysta talllize rs may ma y op o p e ra te e ither co c o ntinuo ntinuous uslly or ba tch tc h wi w ise . Ex Exc e p t for sp sp e c ia l a p p lic a tio tio ns ns,, co c o ntinuo ntinuous us o p erati era tio o n iis s p re ferre ferre d . Ther he re a re foll fo llo o wing type o f C rys ysta tall lliz ize e rs. • • •
Va c uum Cr C rys ysta tall lliz ize e rs Dra Dra ft Tub Tube e -B -Ba a ffle ffle C rys ysta tall lliz ize er Fo rc ed C irc irc ula ula tio tio n C rys ysta talllize r
Vac Vac uum uum C ryst ystallize llizers Vacuum crystallizers are those in which adiabatic evaporative c o o ling is us use e d to c re a te sup supe e r sa tura tura tio tio n. In its its o riginal igina l and and simples imple st for form, suc suc h a c rys ysta tall lliz ize e r is a c lose lose d ves ve sse l in which a vacuum is maintained by a condenser, usually with the help of a steam-jet vacuum pump, or booster, placed between the crystallizer and the condenser. A warm saturated solution at a temperature well above the boiling point at the pressure in the c rys ysta tall lliz ize e r is fed fe d to the ves ve sse l.
A magma volume is maintained by controlling the level of the liquid and crystallizing solid in the vessel, and the space 'above the magma is used for release of vapor and elimination of entrainment. The feed solution c ools spontaneously to the equilibrium temperature; since both the enthalpy of cooling and the enthalpy of crystallization appear. as enthalpy of vaporization, a portion of the solvent evaporates. The super saturation generated by both cooling and evaporation causes nucleation and growth. Product magma is drawn from the bottom of the crystallizer. The theoretical yield of crystals is proportional to the difference between the concentration of the feed and the solubility of the solute at equilibrium temperature.
ADVANTAGES & DISADVANTAGES
The phenomena of Evaporation a nd c oo ling occ ur only in the liquid layer near the magma surface thus Gradients of temperature and c oncentrations are formed nea r the surfac e. • The newly formed C rystals tend to settle at the bottom of the c rystallizer and at the bottom small saturation oc c urs. • It provides no good method for nuclea tion control, for c lassific ation, or for removal of excess nuc lei and very small c rystals.
Draft Tube-Baffle Crystallizer
Draft tube baffle crystallizers are more versatile and effective equipment. The crystallizer body is equipped with a draft tube, which also ac ts as a ba ffle to c ontrol the c irc ulation of the magma, and a downward-directed propeller agitator to provide a controllable c irc ulation within the c rystallizer. An additiona l c irc ulation system, outside the crystallizer body and driven by a circulating pump, contains the heater and feed inlet. Product slurry is removed through an outlet near the bottom of the conical lower section of the crystallizer body. For a given feed rate both the internal and external circulations are independently variable and provide controllable variables for obtaining the required C SD. Draft tube-ba ffle crystallizers can be equipped with an elutriation leg below the body to classify the crystals by size and may also be equipped with a baffled settling zone for fines removal.
ADVANTAGES & DISADVANTAGES •
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They c an be equipped with a baffled settling zone for fines removal They c an also be used with elutriation leg below the body to c lassify c rystal size of the newly formed crystals The efficienc y of this c rystallizer is low for high density slurries
To minimize the deposits and acc umulations, frequently flushing is required.
Forced Circulation Crystallizer
The forc ed c irculation c rystallizer (FC ) is the prac tical applic ation of the MSMPR (mixed suspension, mixed produc t removal) crystallizer. It is basically composed by a crystallization body and an external circulation loop whose flow rate is assured by a propeller pump, completed with a shell and tube heat exchanger. Internal agitation is provided by an external c irculator that must also satisfy the requirements of thermal exchange giving rise to the operating super saturation. Due to its simplic ity and easiness of design and operation, it is the work horse of the c hemical industry. The feed slurry is first heated in a typical heat exchanger followed by the pumping to the main body of the crystallizer. Vaporization basically occurs at the top surface of the slurry whereas the nucleation occurs approximately near the bottom
of the crystallizer body. The crystals are removed and the vaporized solvent is c ondensed and returned to the c rystallizer body.
ADVANTAGES & DISADVANTAGES • •
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It has high range of crystallizers varying in sizes It can provide high rate of circulation which reduces particle deposition It is less expensive than other crystallizers C rystals are difficult to control
