master of crystallography and crystallization october 2012...

Master of Crystallography and Crystallization October 2012-Sevilla

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Crystallization reactors

Béatrice Biscans Université de Toulouse. CNRS -Laboratoire de Génie Chimique UMR 5503 –FRANCE


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Crystallizer selectionMethod of creation of supersaturation

Can be imposed:precipitation, extractive crystallization, salting out

Must be determinedFrom the temperature-solubility relationship

high variation: coolingweak variation: evaporation (or vacuum cooling)

Type of crystallizer Batch crystallizer : low production, incrustation problemsContinuous crystallizer: high production, controlled crystallization

Type of contactor Stirred vessel: high energy cost and maintenanceRecycling slurry: efficient but risk of incrustation Recycling liquorFluidized bed (classification): narrower product size distributioneasier solid-liquid separation


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Continuous crystallizers

Advantages:Economical in operating and labour costsReduced spaceAmount of mother liquor needed reworking is smallElimination of impurities by flushing outHigher reproducibilty of crystal size distributionHigh production rate

Drawbacks:Encrustation of heat exchangers and pipesProblem of continuous withdrawalNeed denses slurry for limitation of the volumes of filtrationPeriodical stops problem of steaty state recoveryDifficulty for controlling secondary nucleationSeeding: more difficult


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Batch crystallizersAdvantagesInteresting for 1 to 100t/week productionsSimplicity of equipment without complicated mechanical systemsEasy scale-up Minimization of encrustation on heat exchangersCan be cleaned thoroughly at the end of each batchLarge crystals easy to obtain because nucleation can be controlled at the beginning of the batch cycleSeeding: easyEasy cleaning

Drawbacks:The product is not regular, lack of reproducibilityHigh labour costs , high working costsHigh sizeSmall productionCycles rather long


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Cooling crystallizers

The simplest type of cooling crystallizer is the unstirred tank:A hot feedstock solution is charged to the open vessel where it is allowed to cool, often for several days, predominantly by natural convection. Metallic rods may be suspended in the solution so that largeCrystals can grow on them and reduce the amount of product that sinks to the bottom of the crystallizer. The product is removed by hand

Non agitated vessels


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The design of tank crystallizers variesfrom shallow pans to large cylindricaltanks.The use of external circulation allowsmixing inside the crystallizer and highrates of heat transfer between the liquorand coolant. An internal agitator maybe installed in the crystallization tank ifneeded.The liquor velocity in the tubes is high,therefore, small temperature differencesare usually adequate for coolingpurposes and encrustation on heattransfer surfaces can be reduced

Agitated vessels

Batch or continuous

External circulation through an heat exchanger


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The large agitated crystallizershown here has an upper conicalsection which slows down theupward velocity of liquor andprevents the crystalline productfrom being swept out with thespent liquor. An agitator located inthe lower region of a draft tubecirculates the crystal slurry throughthe growth zone of the crystallizer.If required cooling surfaces may beprovided inside the crystallizer.

Agitated vessels

Batch or continuous

Internal circulation with a draft tube


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It consists of two interconnectedsimple crystallizers, each sectionoperating at a differenttemperature. Hot feed liquor entersand mixes with the circulatingcontents of the crystallizer, whichpass downwards through thewater-cooled draft tube A, underthe influence of an agitator. Part ofthe cold magma passes under theadjustable gate B, into the secondcompartment of the crystallizer,where it mixes with the circulatingmagma in the second draft tube C,operated at a lower temperature(crystallizers in series)

Agitated vessels

Batch or continuous

Twin or double crystallizer:


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The Wulff-Bock crystallizer

It consists of a long shallow through, about 1.2m wide, rocked on supporting rollers. The solution to be crystallized is fed inat one end and the crystals are discharged at the other end continously.The slope of the trough towards the discharge end, is varied according to the required residence time of the liquor in the crystallizer. Heat is lost by natural convection in the atmosphere.

Cooling crystallizersTrough crystallizers


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The Swenson-Walker crystallizer

The Swenson Walker crystallizer is a trough crystallizer with internal agitation and a cooling system. An helical agitator-conveyor rotates at a slow speed inside the trough to aid the growth of the crystals by lifting them and then allowing them to fall back through the solution.

Cooling crystallizersTrough crystallizers


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Oslo-Krystal cooling crystallizer


continuous

A small quantity of warm concentratedfeed solution enters the crystallizer vesselat point A, located directly above the inlet tothe circulation pipe B. Saturated solution fromthe upper regions of the vessel together withThe small amount of feed liquor, is circulatedby pump C through the tubes of heatexchanger D, which is cooled rapidly by aforced circulation of water or brine. On coolingthe solution becomes supersaturated, but notsufficiently for spontaneous nucleation tooccur (metastable). The supersaturatedsolution flows down pipe E and emerges fromthe outlet F, directly into a mass of crystalsgrowing in the vessel. The rate of the liquor issuch that the crystals are maintained in afluidized state


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Direct contact cooling crystallizer


The Cerny direct coolant crystallizer (continuous)

Avoid the use of conventionalheat exchanger andencrustation. The coolant isinsoluble in the solution andits latent heat of phase change(sublimation or vaporization)is the main cause of heatremoval.The low densitycoolant collects in the upperlayers and passes to a cycloneto separate aqueous solutiondroplets before being recycled


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Evaporating crystallizers

Steam heated evaporators

Most evaporation units are steamheated and a typical evaporator bodyused in evaporative crystallization isthe short tube vertical type inwhich steam condenses on theoutside of the tubes. A steam chestor calandria with a large centraldowncomer allows the magma tocirculate through the tubes; duringoperation the tops of the tubes are justcovered with liquor. To increase therate of heat transfer, especially indealing with viscous liquors, a forcedcirculation of liquor may be effectedby installing an impeller in thedowncomer.

Typical evaporating crystallizer with a calandria with a large central downcomer


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Forced circulation evaporators

The typical forced circulationevaporator-crystallizer is acirculating magma unit operatedunder reduced pressure. Magma iscirculated from the conical base ofthe evaporator body through thevertical tubular heat exchanger andreintroduced tangentially into theevaporator below the liquor level tocreate a swirling action and preventflashing.

Forced circulation evaporative crystallizer


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Oslo-Kristal evaporating crystallizer

The principle of the Oslo-Krystal process, already shown in coolingcrystallizer, can also be applied to evaporative crystallization. Three forms of the Oslo-Krystal evaporating crystallizer are shown hereThe vaporizer is directly connected with the crystallizer body to form a sealed unit.


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Vacuum crystallizers

Draft tube agitation

Swenson draft-tube-baffled (DTB) crystallizer

A vacuum unit capable of producinglarge crystals of narrow size distribution is the Swenson draft-tube-baffled (DTB) crystallizer.A slow-speed propeller agitator islocated in a draft tube .The internalbaffle in the crystallizer forms anannular space in which agitationeffects are absent. This provide asettling zone that provide regulationof the magma density and control ofthe removal of excess nuclei. Anintegral elutriating leg may be installedunderneath the crystallization zone toeffect some degree of productclassification.


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Draft tube agitation

Standard Messo turbulence crystallizer

The standard Messo turbulencecrystallizer is another draft tubeagitated unit. Two liquor flow circuits arecreated by concentricpipes: an outer injector tube with acircumferential slot and an innerguide tube. Circulation is effected by avariable speed agitator in theguide tube. The principle of the Oslocrystallizer is used in the growthzone; partial classification occurs in thelower regions, and fine crystals segregate in the upper regions. Feedstockis introduced into the guide tube in thelower region of the vessel and passes intothe vaporizersection where flash evaporation takesplace. Nucleation thereforeoccurs in this region and nuclei are sweptinto the primary circuit


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Vacuum crystallizersFluidized bed agitation

Oslo-Krystal vacuum crystallizer showing two different methods of operation: a) classified suspension b) Mixed suspension

The Oslo-Krystal unit is a fluidized bed agitated crystallizer in which the gentle action minimizes secondary nucleation allows large crystals to grow. Two different methods of operation exist: classified suspension (circulating liquor) or mixed suspension (circulating magma)


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Multistage vacuum crystallizer

The standard Messo multistage vacuumcrystallizer provides a number of coolingstages in one vessel. The horizontalcylinder is divided into severalcompartments by vertical baffles that permitunderflow of magma from one section toanother but isolate the vapor spaces. Eachvapor space is kept at its operating pressureby a thermocompressor, which dischargesto a barometric condenser. Hot feedstock issucked into the first compartment which isoperated at the highest pressure andtemperature. Flash evaporation and coolingoccur and the resulting crystal slurry passesinto the successive compartments

Standard Messo multistage vacuum crystallizer


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reactive crystallizers

Swenson reaction-type DTB crystallizer

Swenson atmospheric up-pumping reaction-type DTB crystallizer

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