crystallization
TRANSCRIPT
CRYSTALLIZATION
Introduction:
What is crystallization?
CRYSTALLIZATION
It is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.
It is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.
It is the process by which a chemical is separated from solution as a high-purity, definitively shaped solid.
CRYSTALLIZATION
The crystallization process consists of two major events:
Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters
The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size
CRYSTALLIZATION
Methods of crystallization includes;Solution Cooling (if solubility is dependent on temperature
Solvent Evaporation (solubility is nearly independent on temperature)
Adiabatic (combination of cooling and evaporation under vacuum)Chemical Reaction
CRYSTALLIZATION
Others includes:Anti-solvent/drown-out (addition of second solvent to reduce solubility)Changing the pH
CRYSTALLIZATION
Two major groups of applications:
Crystal production (macroscopic crystal production and tiny crystal production)
Purification ( improving and/or verifying purity of substance)
CRYSTALLIZATION
The nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control.
T(Sliquid − Ssolid) > Hliquid − Hsolid
Gliquid < Gsolid
It is easy to dissolve a perfect crystal than to grow again a new one from the resulting solution
Evaporating Crystallizers
EVAPORATING CRYSTALLIZATION
a process that is conducted under vacuum. chosen when solubility of the solute is nearly independent of temperature.
EVAPORATIVE CRYSTALLIZER
There are four common types of evaporative crystallizers namely:Steam-heated EvaporatorsForced-circulation Evaporative CrystallizersOslo-crystal Evaporating CrystallizersDraft-tube Baffle Crystallizers
STEAM-HEATED EVAPORATORS
EVAPORATIVE CRYSTALLIZERS
STEAM-HEATED EVAPORATORS
a typical evaporator body used in evaporative crystallization is the short tube vertical typethe type of most evaporator unitsa steam chest or calandria with a large central downcomer allows the magma to circulate through the tubes
STEAM-HEATED EVAPORATORS
downcomerVertical tubes
FORCED CIRCULATION EVAPORATORS
EVAPORATIVE CRYSTALLIZERS
FORCED CIRCULATION EVAPORATORS
a circulating magma unit operated under reduced pressurefor feeds where high rates of evaporation are required, where there are scaling compounds, where crystallization is achieved in inverted solubility solutions, or where the solution is of relatively high viscosity
FORCED CIRCULATION EVAPORATORS
most frequent use is in the continuous processing of such materials:
• NaCl (food or technical grade)• KNO3
• Na2 SO4, K2 SO4
• NH4Cl• Na2CO3H2O• Citric acid
FORCED CIRCULATION EVAPORATORS
can be either single or multiple effects and the vapor recompression concept (either thermal or mechanical) is often applied. Usually, they operate from low vacuum to atmosphere pressure.used for high evaporation rates and when crystal size is not of the utmost importance or if crystal grows at a fair rate.
FORCED CIRCULATION EVAPORATORS
Features:Active volumeAgitationSpecial design of the liquid vapor separation areaInternal bafflesElutriation legInternal scrubbing section
FORCED CIRCULATION EVAPORATORS
OSLO-KRISTAL EVAPORATING CRYSTALLIZER
EVAPORATIVE CRYSTALLIZERS
OSLO-KRISTAL EVAPORATING CRYSTALLIZER
also called classified-suspension crystallizer is the oldest design developed for the production of large, coarse crystals
The classifying crystallization chamber is the lower part of the unit.
The upper part is the liquor-vapor separation area
OSLO-KRISTAL EVAPORATING CRYSTALLIZER
The basic design criteria are twofold:desupersaturation of the mother liquor by contact with the largest crystals present in the crystallization chamberkeeping most of the crystals in suspension without contact by a stirring device, thus enabling the production of large crystals of narrow size distribution
OSLO-KRISTAL EVAPORATING CRYSTALLIZER
Closed type Open type
DRAFT -TUBE BAFFLE EVAPORATING CRYSTALLIZER
EVAPORATIVE CRYSTALLIZERS
DRAFT -TUBE BAFFLE EVAPORATING CRYSTALLIZER
DTB crystallizer by incorporating a steam-heated exchanger in the clear liquor recycle line.operate with a rather low supersaturation, which is sometimes a limitation to crystal growth, so that very large crystals can be produced only by providing extensive and costly dissolving of fines.
DRAFT -TUBE BAFFLE EVAPORATING CRYSTALLIZER
includes a baffled area (settling zone), peripherical to the active volume
for processes exhibiting a moderate evaporation rate
Contains elutriation leg
Advantages :
A. Capable of producing large singular crystals. B. Longer operating cycles. C. Lower operating costs. D. Minimum space requirements, single support elevation.
Advantages :
E. Adaptable to most corrosion resistant materials of construction. F. Can be easily instrument-controlled. G. Simplicity of operation, start-up and shutdown. H. Produces a narrow crystal size distribution for easier drying and less caking. I. The product size varies only slightly with large changes in production rate.
DRAFT- TUBE BAFFLE EVAPORATING CRYSTALLIZER
elutriation leg
Reactive/Reaction Crystallizers
REACTIVE/REACTION CRYSTALLIZERS
A crystallizer where a solid phase crystalline material results from the reaction of two components, can often be performed more profitably than in a separate reactor. Can operate conventional evaporative type crystallizer Reactive crystallization can also be performed in a forced circulation type crystallizer where the reactants are mixed in the circulation piping.
REACTIVE/REACTION CRYSTALLIZERS
Vacuum Crystallizer
-Crystallizer in which a warm saturated solution is fed to a lagged, closed vessel maintained under vacuum. The solution evaporates and cools adiabatically, resulting in crystallization.
Draft Tube Agitation-is widely used in chemical industry for suspending solids in liquids.- Flow pattern may be upward or downward.
Swenson draft-tube baffled (DTB) crystallizer
-A vacuum unit capable of producing large crystals of narrow size distribution .-Has a low speed propeller agitator-The baffle forms an annular space in which agitation effect is absent.
Swenson Draft tube Baffled Crystallizer
Messo turbulence crystallizer
-is another draft tube agitated unit.- Circulation is effected by the variable speed agitator.-Two liquor flow circuits are created by concentric pipe.
- Outer injection tube- Guide Tube
Fluidized Bed Agitation-it operates in which mass of crystals is suspended in an upward flowing stream of liquor.
Oslo Crystallizer Unit
-is a fluidized bed agitated crystallizer in which the gentle action minimizes secondary nucleation allows large crystals to grow.
Multistage Vacuum Crystallizer
Standard Messo Multistage Vacuum Crystallizer
-provides a number of cooling stages in one vessel. - Hot feedstock is sucked into the first compartment which is operated at the highest pressure and temperature.
COOLING CRYSTALLIZER
Tank Crystallizer-includes cooling of hot saturated solution in open tanks. After a long period of time the mother liquor is drained and the crystals removed. -High labor costs-Tank is cooled by coils or a jacket and a agitator used to improve heat transfer rate.
Non Agitated Vessel
-the simplest type of cooling crystallizer, a form of tank crystallizer-because of slow cooling, large interlocked crystals are usually obtained
Agitated Vessel-The installation of an agitator in an open-tank crystallizer generally results in smaller, more uniform crystals and reduced batch time.
Large Agitated Cooling Crystallizer
-has an upper conical section which slows down the upward velocity of liquor and prevents the crystalline product from being swept out with the spent liquor.
Trough Crystallizer
The Wulff-Bock crystallizer
-Frequently referred to as the crystallizing cradle or rocking crystallizer.
Swenson Walker Crystallizer
-is a trough crystallizer with internal agitation and a cooling system.
-It has a feature where a concentrated solution, which is continuously cycled through the crystallizer, is supersaturated in one part of the apparatus, and the saturation solution is conveyed to another part, where it is gently released into a mass of growing crystals.
Oslo-Crystal Cooling Surface
Direct Contact Cooling Surface -is effected by blowing air into a hot crystallizing solution.
Cerny Direct Coolant Crystallizer
-A continuous crystallizer which has been used for the large scale production of calcium nitrate tetrahydrate, wherein aqueous feedstock enters at the top of crystallizer at 25˚C and flows counter currently to the immiscible coolant droplets.