In Theory and Practice

GEA MessoProcess EngineeringDivision

CrystallizationCrystallizationCrystallization

GEA Messo GmbH • Friedrich-Ebert-Straße 134 · 47229 Duisburg · GermanyTel. +49-(0)20 65/903-0 · Fax +49-(0)20 65/903-199 · info@messo.net · www.messo.com

GEA Messo GmbH Process EngineeringDivision

A company of GEA Group & member of GEA Evaporation & Crystallization

BN

A/G

B 1

1/05

Range of Products

Individual plants for the

chemical, pharmaceutical

and food processing

industries

(Unit operation, evaporation,crystallization, thermocom-pression)■ Crystallization plants ■ Evaporation plants

(all concentration under significant scaling conditions)

Crystallization and

evaporation technologies

for the chemical, pharma-

ceutical and food processing

industries, e.g.:

(Entire technology concepts,based on precipitation, evaporation, crystallization)■ Common salt production

plants■ Reaction crystallization

plants for several base/acidreactions

Plants for environmental

protection

(Based on precipitation, evaporation, crystallization)■ Pickling bath liquor

recycling plants■ Landfill leachates

concentration plants■ Industrial waste water

ZLD plants■ Treatment plants for slags

from the secondary aluminum industries

Inhalt

Messo in the field of crystallization 4

Crystallization in history and presence 5

Crystallization in theory and practice 6–8

Types of crystallizersForced circulation crystallizer

Turbulence (DTB) crystallizer

OSLO crystallizer

Peripheral components 10–12

Classical applicationsSurface cooling crystallization

Vacuum cooling crystallization

Evaporative crystallization 13–15

Modern applicationsFlue gas desulfurization (FGD): scrubber effluent

Recovery of caffeine

Salt from secondary aluminum slag

Ammonium sulfate from the

caprolactam process 16–17

Research and development servicesChemical laboratory and pilot plant facilities 18

Our product experience 19

Crystals of citric acid in polarized light

4 5

Crystallization in history and presence

In antiquity, settlements de-veloped around, and exploit-ed sites where salt waseasily available, whether asrock, brine, or derived fromsolar evaporation. For exam-ple, salt was produced inthe Pharaonic Egypt at theNile Delta; similarly, theRomans recovered salt atOstia seacoast (near Rome)and the same happened allover the world (e.g. in China).These and many other pro-duction sites prove thatcrystallization from solutionis one of the oldest unitoperations practiced byhumankind.

While crystallization in solar ponds is still in regionswith plentiful sunshine, itslow production rate andmediocre product purityprevents this technology tobe used generally. As theworld developed throughindustrial age, the demandfor crystalline chemicalsincreased in variety, quanti-ty, and quality. This led to

the birth of crystallizationtechnology, that aimed atimproving the methods andequipment used in crystal-lization operations. Moderncrystallizers can boast spe-cific production rates thatare several orders of mag-nitude higher than solarponds, have low manpowerrequirements, and low pro-duction costs.

The specific requirementsof a crystallizer can varywidely, depending on thenature of the product, andits intended use: pharma-ceutical and food productsrequire higher purity, forexample, while fertilizersneed larger crystal size; thecrystal size and final mois-ture are not as important incrystallization systemswhich produce an interme-diate compound. There arecases where the real prod-uct of the crystallizer is thesolvent: crystallization isused to separate from thesolvent the compounds thatmake it impure. Further,there are cases where crys-tallization is used to concen-trate a solution, by crystalliz-ing and removing the sol-vent (freeze concentration).

One quality that is presentin all crystallization systems,regardless of the final useof the solvent or the crystal,is the ability to separate thecrystals from the motherliquor. This ability is a func-tion of the crystal size, and,by extension, a function ofthe separation equipmentthat can be used. Centri-fugation is by far the mostefficient separation method,if the average crystal size islarge enough. It is therefore

logical to expect that of thecharacteristics of a crystal-lizer, the crystal size it pro-duces is of great impor-tance.

The possible crystal size of a given compound is dependent on its chemicaland physical properties, and those of the solution in which it is dissolved. In parallel, the crystal size is

dependent on the equip-ment used to crystallize it,and the method by whichthe equipment is operated.

The crystallizer used cancontribute to improving thecrystal size, within physicaland energy boundaries, bycontrolling the nucleation,

the attrition, and the growth rate of crystals, and by destroying a fractionof the smaller crystals present in the crystallizeritself. Inattention of theseparameters, on the otherhand, can contribute to adegradation of the crystalsize.

Messo, synonym for crystallization

MESSO’s contributions tothe field of crystallizationbegan about fifty years ago,originally under the nameStandard-MESSO Duisburg;since that time, MESSO hasbeen a crucial participant incrystallization and relatedpurification technologies,throughout the world.MESSO’s broad and verysuccessful involvement inthe chemical, food, steel,and environmental indus-tries is proof of the accept-ance it has gained withinthe international chemicalengineering community. Atthe turn of the century,MESSO boasts of overthousand installations andreferences of its abilities inthe design, construction,start-up, de-bottlenecking,and turnkey delivery ofevaporation and crystalliza-tion systems.

MESSO’s expertise encom-passes all basic types ofcrystallizers for the crystal-lization from solutions, suchas the forced circulation ordraft-tube (MSMPR) crys-tallizer, the turbulence (DTB)crystallizer, and the fluidized-bed (OSLO) crystallizer.MESSO is thus in a uniqueposition to address the spe-cial needs of each of itsclients, depending on therequired product crystalquality and size.

In addition to the center-piece of a crystallizationsystem, the crystallizer,MESSO offers its clientsthe supply of optimizedperipheral equipment, inseveral levels of involve-ment. MESSO routinelysupplies upstream anddownstream components,such as the preconcentra-tion (in multiple effect,mechanical vapour recom-pression, flash, and otherevaporator configurations),the dewatering (thickening,filtration or centrifugation),drying, solids handling andpackaging. MESSO alsosupplies piping and

instrumentation and processcontrol systems for itsplants, installations in pre-fabricated and modularizedsections, and turnkey instal-lations, as required by theclient.

MESSO is a leader in itsfield through in-depth re-views of its operating instal-lations and research and de-velopment. The Researchand Development Depart-ment of MESSO is housedin a two-hundred-square-meter facility, equippedwith test units that simu-late batch and continuousoperation of all basic typesand configurations of crys-tallizers. It has in-house ana-lytical capabilities for directdetermination of concentra-tion, supersaturation, andother physical properties ofthe subject process liquors.Not only the design of crys-tallizers but the develop-ment of optimized separa-tion processes for ourclients’ needs is in thefocus of MESSO chemistsand engineers.

In order that its know-howis continually enriched withcutting-edge developmentsin the field of crystallization,MESSO has established theclose cooperation of severalleading European Univer-sities’ Research Centers forinformation exchange.

MESSO engineers sharetheir wealth of practicalexperience with the interna-tional chemical engineeringcommunity through presen-tations during scientificsymposia, publications ofpertinent articles and thearrangement of national andinternational crystallizationseminars. MESSO contribu-tions have been included inseveral technical hand-books and the well-knownUllmanns’ Technical Ency-clopedia.

This state of experience,broad technical background,updated skills and in-depthresearch are brought tobear on each and everyproject MESSO handles.The results are tailor-madesolutions that combine optimal investment withplant functionality, reliability,modern technology, safety,and respect for the environ-ment. MESSO engineersalways look for the mostfeasible plant configurationwhich – at the end – is alsothe cheapest investment.

A large part of MESSO’sbusiness comes fromrepeat clients; this is thebest testimony for the quali-ty of our work, the commit-ment of our engineers, andthe performance of ourequipment.

A 16th century salt works

7

most common sources ofsecondary nucleation.Secondary nucleation istherefore affected by themixing energy input to thecrystallizer.

Combining these character-istics of crystallization, itcan be said that large singu-lar crystals are formed atlow nucleation rates. Fig. 3is a simplification (exagger-ated for purposes of illustra-tion) of this premise, andconcerns two crystallizersthat have the same amountof supersaturation, 10 g,from which crystals willgrow. This is to demon-strate the strong influence of the nucleation rate onthe mean crystal size. Due to the two differentnucleation rates (5 nucleiand 40 nuclei) the result aretwo different crystal sizes;either 5 crystals of 2 g eachor 40 crystals of 250 mgeach.

Most crystallizers need toproduce large singular crys-tals, because this improvescrystal purity and handlingcharacteristics, and veryoften the crystalline prod-uct’s marketability. Toachieve a larger crystal size,it is therefore important to:

■ Control the supersatura-tion in the crystallizer sothat it does not exceedthe metastable region;

■ Choose an operating pointof such supersaturationthat growth rate is maxi-mized;

■ Optimize the mixing ener-gy input so that super-saturation is controlled,while secondary nuclea-tion is minimized.

As can be seen from theabove, the method andintensity of mixing in a crys-tallizer is very critical, as it is what most influences thesupersaturation and second-ary nucleation of the sys-tem. Mixing, therefore, is a basic design feature in acrystallizer unit. The instan-

taneous operation cycle of atypical vacuum cooling FCcrystallizer, with respect tothe solubility of a system, is illustrated in Fig. 4. Thefresh feed at temperatureand concentration repre-sented by point (1) entersthe crystallizer and is mixedwith the crystallizer con-tents that are at concentra-tion and temperature (3).The resultant mixture is atpoint (2), passes throughthe pump, and reaches theboiling surface of the slurryin the crystallizer. Upon boil-ing, the solution reachespoint (4), which is well intothe metastable zone. Thesupersaturation generatedin this way is consumed bycrystal growth of crystalspresent in the crystallizervessel, as the supersaturat-ed liquid is cooled adiabati-cally to point (3), and thecycle is completed.

Crystallization in theory and practice

A key parameter for crystal-lization is the supersatura-tion. Supersaturation is thetemporary increase of con-centration of the solute inthe solvent above its equi-librium, and is produced byevaporation, cooling, chemi-cal reaction, salting out, etc.The area over the normalsolubility, in which a systemcan be supersaturated, isalso called the ”metastable”region. Supersaturation isthe driving force of crystal-lization. Proper control ofsupersaturation is of criticalimportance in achievingacceptable results.

The most common crystal-lization operations today arethose of evaporative crystal-lization and of indirect anddirect (vacuum) cooling: Inthe former, crystallizationoccurs after some amountof solvent is removed, andthis is due to the relatively”flat” solubility of the sys-tem at hand (Fig. 1a). In thelatter case, the solubility israther steep (Fig. 1b), andsupersaturation can beachieved by cooling easily.

The crystal growth rate, aparameter that measureshow fast a crystal grows, is,for most systems, exponen-tially dependent on thesupersaturation (Fig. 2).However, the end result ofthe crystal size obtained ina crystallizer is not a matteronly of the growth rate, butalso of the nucleation rate(how many crystals takepart in crystal growth), andthe attrition rate (how easilycrystals break, and howsmall are the broken frag-ments). The nucleation rateis also a function of the

supersaturation, and isaffected by supersaturationto a far greater degree thanthe growth rate (Fig. 2). As a result of these verycomplex relationships, thesupersaturation at which a crystallizer will operatemust be chosen with greatcare.

There are two commontypes of nucleation mechan-isms. Primary (homoge-neous) nucleation occurs atthe onset of crystallization,when the concentration ofthe solvent exceeds themetastable region, and sec-ondary nucleation, which is caused by contactsbetween a crystal andanother surface, and occurswithin the metastableregion (Fig. 2). Crystal-to-crystal and crystal-to-impeller contacts are the

1. Crystallization processes, shown in equilibrium (solubility) diagrams

2. Kinetics of crystallization

e. g.NaCl

Na2SO4(NH4)2SO4

CaCl2conc

entr

atio

n C

temperature T

∆Cevaporation

e. g.KCl

NiSO4.6H2OCuSO4.5H2O

AgNO3melamineco

ncen

trat

ion

C

temperature T

∆C

cooling

3. Influence of nucleation on crystal size

∆ C = 10g

5 nuclei

40 nuclei

super-saturation

super-saturation

5 final crystals

40 final crystals

B0,

G

∆Cmet ∆ C

primary nucleationB0=kN*∆ C>18

secondary nucleationB0=kn*εr*M*∆ C1-10

crystal growthG=kg*∆ C1-2

4. Control of tip supersaturation(vacuum cooling crystallization)

conc

entr

atio

n C

unstable supersaturated field

metastable zone

solubility curve

temperature θ

∆C

4

3

2

∆θ

1

undersaturatedfield

1 2

3

4

8 9

MESSO-type crystallizers: FC, DTB, OSLO

Since it is important tomaintain the peak supersat-uration (point 4) within themetastable zone, the loca-tion of point (2), and moreimportantly, that of point (4),can be adjusted, by design-ing the recirculation rate inthe crystallizer.

If the supersaturation gen-erated in one cycle is notcompletely consumed bythe end of the cycle, thestarting point for the nextcycle will be somewhat fur-ther from the saturationcurve. After some time, thewhole cycle will migrate sofar into or even above themetastable zone, that it willadversely affect crystalgrowth and nucleation. It istherefore important to pro-vide sufficient opportunity(efficient mixing) and suit-able sites (sufficient crystalsurface) for the supersat-uration to be consumed.Otherwise, the crystal sizewill suffer, and the crystal-lizer will be subject to in-crustations.

The above ideas are embod-ied in the two kinetic equa-tions below. The mass dep-osition rate (dm/dt) resp. theconsumed supersaturationper cycle time is dependenton the surface of suspend-ed crystals (A) and on thelevel of supersaturation(∆C). Secondary nucleationB0 depends on dissipatedmixing energy (ε), suspen-sion density (m) and level ofsupersaturation (∆C):

Crystal size is influenced by the time that the crystalstays in the crystallizer (re-tention time), where, underproper operating conditions,it may grow. There is, how-ever, a competing quality in this arrangement, thataffects the crystal size ad-versely. Mechanical attrition(Ga) is the rate of removal of material from a crystal(as opposed to Gk, the lin-ear, kinetic crystal growthrate), and it is dependent onthe crystal retention time,

the magma density, themixing energy and thehydrodynamic design of the system. It is thereforeto be expected that undercertain conditions, crystalsize will peak at a certainretention time, and willthereafter become smaller,as Ga overpowers Gk andthe effective crystal growthrate is minimized.

Types of crystallizers

10 11

Turbulence (DTB) crystallizer

The MESSO turbulencewith draft tube and baffle(DTB) crystallizer (Fig. 2) isthe typical modern type ofcrystallizer in the industry.This crystallizer has beennamed so because itprovides for two dischargestreams, one of slurry thatcontains the product crys-tals, and another, that ismother liquor (saturated sol-vent) with a small amountof fines. The configurationof the crystallizer is such thatit promotes crystal growth,and can generate crystals ofa larger average size thancould be achieved in an FC.Most conventional turbu-lence crystallizers operateunder vacuum, or at slightsuperatmospheric pressure.

The turbulence (DTB) crys-tallizer has been studiedwidely in crystallization the-ory, and can be modeledwith accuracy. Its distinctzones of growth and clari-fied mother liquor make it

possible to define in termsof kinetic parameters, andthus growth and nucleationrates can be determined.These features make theturbulence crystallizer verysuitable to mathematicaldescription, and thus sub-ject to good operating control.

OSLO crystallizer

This crystallizer type (Fig. 3)originally represented thefirst major step in moderncrystallization technologyand equipment design. Itwas invented by F. Jeremi-assen of Krystal A/S, Oslo,Finland, in 1924, and it tookthe name of the city inwhich the design originated.It is also referred to as“growth-“, “fluid-bed-”, and“Krystal-”type.

As the successor of DavyPowergas’ and A. W. Bam-forth’s crystallization tech-nology, MESSO owns alldocumentation of OSLOinstallations built by these

Types of crystallizers

All this is considered inmodern types of continuouscrystallizers. Crystallizerswith longer retention timesare operated with less spe-cific energy input ε, result-ing in lower nucleation rates.The impacts between crys-tals and the impeller pumpblades are the most effec-tive source for the nucleiproduction. These impactsare at least 100fold moreeffective than crystal/walland crystal/crystal impacts.Therefore, types of crystal-lizers differ mainly in designand the position of theimpeller pump.

Forced circulation crystallizer

The forced circulation (“FC”) crystallizer (Fig. 1) isthe most common type ofcrystallizer in the industry.The average FC crystallizerevaporates solvent, thusincreasing the supersatura-tion in the process liquor,and causing crystallizationto occur. Most conventionalFC units operate under vacuum, or at slight super-atmospheric pressure.

The FC consists of fourbasic components: the crys-tallizer vessel, which pro-vides most of the volumedictated by the residencetime requirements, the cir-culating pump, which pro-vides the mixing energy, theheat exchanger, which sup-plies energy to the crystal-

lizer (in a typical evaporativecrystallization operation),and the vacuum equipment,which handles the vapoursgenerated in the crystallizer.

Slurry from the crystallizervessel is circulated, in plug-flow fashion, through theheat exchanger, and return-ed to the crystallizer vesselagain, where its supersatu-ration is relieved by deposi-tion of material on the crys-tals present in the slurry.The supersaturation is con-trolled so as to avoid spon-taneous nucleation, by suf-ficient circulation capacity.The evaporated solvent isconducted to the vacuumsystem, where it is con-densed and removed.

The FC crystallizer is usedfor general, simple crystal-lization operations, wherelarge crystal size is not arequirement. The FC designaims to protect the crystalsize from reduction fromthe crystallizer environment,but has no features toaggressively increase thecrystal size.

2. Turbulence (DTB) crystallizer 3. OSLO crystallizer1. Forced circulation (FC) crystallizer

Turbulence (DTB)d’ = 0.5 – 1.5 mm

FCd´ = 0.2 – 0.6 mm

OSLOd’ > 1.5 mm

4Simplified flow sheet

5Spin bath regeneration plant

6Planning model of an evaporative crystallization plant (Abu Dhabi)

5

4

6

The slurry is removed fromthe crystallizer’s fluidizedbed and sent to typical cen-trifugation sections. Clearliquor may also be purgedfrom the crystallizer’s clarifi-cation zone, if necessary.

From each of these basictypes of crystallizers a num-ber of different applicationsare designed from MESSOengineers to fulfill the spe-cial needs of the customers.

two companies. This back-ground, added to MESSO’sown extensive experiencemakes MESSO the premierdesigner of OSLO crystal-lizers in the world.

The primary advantage ofthe OSLO crystallizer untiltoday is the ability to growcrystals in a fluidized bed,which is not subject tomechanical circulationmethods. A crystal in anOSLO unit will grow unhin-dered, to the size that itsresidence time in the fluidbed will allow. The result isthat an OSLO crystallizerwill grow the largest crys-tals, as compared to othercrystallizer types.

steam

cooling water

crystallizer

prethickener

centrifuge

drier

feed product

13

Application examples

The selection of equipmentand the design of a crystal-lization operation is depend-ent on, and influenced byseveral process-specific fac-tors. The following exam-ples illustrate how thesefactors influence the choiceof crystallizer type:

Surface cooling crystallization

Surface cooling crystalliza-tion will be selected if thesolubility of the substanceto be crystallized is stronglydependent on temperature,and if vacuum cooling crys-tallization cannot be chosen,e.g. the vapour pressurerequired to achieve the end-point temperature is toolow for the plant utilities, ortoo expensive.

Melt crystallization of

bisphenol A adduct

Beside a number of applica-tions for inorganic products(e.g. Glauber’s salt, carnal-lite) modern surface coolingsuspension crystallizers areused also for melt crystal-lization, e.g. to crystallizebisphenol-A (BPA) phenol-adduct, the prepurificationstep for the melt from syn-thesis. The eutectic pointof the melt is about 39°C,and the crystallizer is oper-ated at 45–50°C. The BPA,which is used in the pro-duction of polycarbonate, iscrystallized in a surface-cooled crystallizer from themelt, which consists ofBPA, phenol and someimpurities. The pure crys-tals are separated in cen-trifuges and washed withphenol. The design shouldrecognize the tendency ofthe product to form incrus-tations on the heat ex-change surfaces, therebeing the place of the high-est supersaturation in theentire system. Polishedsurfaces and small temper-ature differences are someof the techniques used inmodern designs to controlthe effects of this problem.

The surface-cooled crystal-lizer is a simple FC unit, in which the process slurryis cooled in the tube-and-shell heat exchanger in the circulating loop (loopcrystallizer). Growth-type crystallizers are notcommonly chosen, due to the low settling rate of the crystals, which makesfines separation at the crystallizer very difficult.

12

Details from a pickling bath liquor regeneration unit

Peripheral components

The crystallizer is the heartof a crystallization system,but there are several com-ponents, in the periphery,that need to be consideredbefore the final product canbe collected. The suspen-sion from the crystallizerhas to be separated, thecrystals have to be driedand packed. The vapourshave to be condensed andthe noncondensables to be extracted by vacuumpumps.

Fig. 4 shows a simplifiedflow sheet of a completecrystallization plant operatedon the principle of evapora-tive crystallization undervacuum. Depending onprocess considerations(crystal size, evaporativeduty, etc.) one of severaltypes of crystallizer can beinstalled instead of the FCcrystallizer shown, includingmultiple-effect units.

Instead of using steam forheating (as shown), onecould utilize mechanical orthermal vapour recompres-sion. In the illustration, thevapours from the (last) crys-tallizer are condensed in asurface condenser; how-ever, a mixing condensercould be chosen, instead.The suspension in the crys-tallizer can be withdrawn byoverflow, as shown, orpumped out, using pumpswith special specifications.Because suspension densi-ties are usually between 15to 25%wt. in the crystalliz-er, while a centrifuge oper-ates best at 50 to 60%wt.suspension, the suspensionis preconcentrated in thick-eners or hydrocyclones.

The underflow of the thick-ener or hydrocyclone is sentto the centrifuge for separa-tion. Depending on theproduct CSD (and to a less-er degree on the physicalproperties of the suspen-sion) there is a choice be-tween types of centrifuges:generally, the decanter andpeeler are used for smallerparticles, and the screenbowl and the pusher for

larger particles. In somecases of very small particlesizes or very fragile crystals,filters are used, instead ofcentrifuges. Filters, how-ever, are usually not as effi-cient as centrifuges in sepa-rating the solvent from thecrystals.

The small amount of resid-ual solvent left on the crys-tals after the separationstep, is removed in a dryer.There are several types ofdryers that are used,depending on crystal size,crystal chemistry (reactivenature, tendency to decom-pose, oxidize, etc.), crystalfragility, and initial solventcontent. The most commontypes of dryers used arefluid bed (stationary orvibrating), and the flashdryer.

Spin bath regeneration plant

Vacuum cooling crystallization

Vacuum cooling crystalliza-tion is usually chosen if thesolubility of the substanceto be crystallized is stronglydependent on temperature,and if the vapour pressure ofthe solvent is high enoughfor this application to allowthe use of conventional vac-uum equipment. Vacuumcooling crystallization is thepreferred cooling crystalliza-tion method for continuousoperation conditions, due tothe fact that the supersatu-ration is generated by adia-batic cooling of the solventat the liquor level. Thismeans that the energy is

removed from the crystal-lizer at a location that is farless prone to encrustations,and with a method thatrequires far less mixingenergy input to the crystal-lizer slurry.

Loop crystallizer forbisphenol A adduct

15

Evaporative crystallization

Evaporative crystallization isusually a process that isconducted under vacuum,just like the vacuum coolingcrystallization. This processis chosen when solubility ofthe solute is nearly inde-pendent of temperature. Asin vacuum cooling crystal-lization, special scaling prob-lems are not a serious prob-lem as long as boiling onthe heater surface is avoid-ed, and the special case ofinverse solubility (solubilitydecreases with tempera-ture) is recognized andtaken into consideration.

Crystallization of

sodium chloride

This example shows a crys-tallization plant for tablesalt, operated with concen-trated brine from a solarpond. In addition to threeFC crystallizers, there is anOSLO crystallizer, used bythe plant to produce a frac-tion of its output as coarsecrystals. The plant is operat-ed as a quadruple-effectunit. The coarse crystalsfrom the OSLO are separat-ed on a pusher centrifuge,whereas the salt producedin the FC crystallizers isseparated on screen bowlcentrifuges, after beingcounter-currently washedwith fresh feed liquor in awashing thickener. Theproduct crystals are driedand packed. In order to

maintain the level of impuri-ties in the system to anacceptable level, somemother liquor is removed ashydrocyclone overflow, andpurged.

Some plants of this typehave been supplied for theproduction of up to 2.5 t/hcoarse (mean size greaterthan 2 mm) salt and addi-tionally 10 t/h of normalsalt.

14

Salt recovery from solar brine withOSLO crystallization

Vacuum cooling

crystallization of

potassium chloride

This example shows therecrystallization ofpotassium chloride in ind.grade from fertilizer quality.The crude KCl is dissolvedat elevated temperatures ina recycled stream of motherliquor. The resulting solu-tion, now saturated withpotassium chloride, is fed toa multiple-stage, vacuumcooling crystallizer train. In order to fulfill the require-ment of coarser crystals,the type selected is theDTB crystallizer. Fines dissolving is possible, byadding water to each crys-tallizer’s clear liquor over-flow. The number of stagesis optimized on the basis ofmaximum heat recovery(the recycled mother liquoris reheated in condensersusing the vapours leavingthe hotter crystallizers).

Barometric (direct-contact)condensers are usuallyemployed, so that the watercontent of the mother liquoris increased, and thus itsdissolving capacity is im-proved. Steam is used (inseparate heat exchangers)to heat the recycled, anddiluted, mother liquor to thetemperature required by thedissolver step, and the loopis closed by returning themother liquor to the dis-solver.

The crystals are separatedin pusher centrifuges,washed and dried. The typi-cal crystal sizes averagesare 0.8 to 1.0 mm.

Recovery of pickling

bath effluent liquors

Vacuum cooling crystalliza-tion can also be used topurify solutions, by crystal-lization of the solute. Thepickling of mild steel sheetswith sulfuric acid producesan aqueous waste streamcontaining ferrous sulfateand sulfuric acid. Cooling ofthat solution forces ferroussulfate to crystallize asFeSO4.7H2O. From theviewpoint of the motherliquor composition, this is away to purify the solution.

At the same time, the sevenmolecules of water that isremoved with the crystal-lized ferrous sulfate causesthe reconcentration of thesulfuric acid. The solutionthus treated can be recy-cled to the pickling bath.The vacuum cooling isachieved in a single-effectdraft-tube crystallizer whichis operated together with ahigh-vacuum generator (asteam ejector or chilled-water surface condenser).This modern process maybe operated for a couple ofmonths without interrup-tions for washouts.

Vacuum evaporators for brine concentration

Hot dissolving station

Potassium chloride refinery for ind. grade quality, triple-effect vacuum cooling crystallization

Filtration Crystallization and recoverySeparation

and drying stationSilo

16

under controlled processconditions and to dissolvethe salts. The degassing ismade in the absence ofatmospheric air, thus ensur-ing that the whole systemoperates above the gasmixture’s upper explosionlimit. The gases evolvedare purified and fed to anincinerator that allows theplant to recover the com-bustion energy. Afterdegassing, the remainingresidue is separated fromthe salt solution by filtrationon a belt filter. The filtercake containing aluminumoxide is usually sent to alandfill. The solution is fedto a single-effect mechani-cal vapour recompressionevaporative FC crystallizer,operated at about atmos-pheric pressure. The result-ing crystallized mixed saltproduct is separated on apusher centrifuge, driedand compacted into pellets

which can be reused in thegeneration of the moltensalt layer (in the beginningof this process).

This crystallization methodcan be used for the recov-ery of a single salt, as wellas salt mixtures.

Ammonium sulfatefrom the caprolactamprocess

Ammonium sulfate is a by-product of the synthesisof caprolactam. Multiple-effect evaporative crystal-lization is the well-estab-lished process to recovercrystalline ammonium sulfate and market it as fertilizer. In the last fewyears, the fertilizer market-place has seen an increaseddemand for larger crystals,and for a narrower size distribution.

The example shows a triple-effect evaporative crystal-lization plant using DTBcrystallizers for the produc-tion of ammonium sulfateof an average crystal size ofabout 2 mm. The solution isfed counter-currently (withrespect to the steam flowsequence) in order toimprove crystal growth con-ditions by combining thehighest process tempera-ture and highest impurityconcentration. The MESSOturbulence (DTB) crystal-lizers use bottom-entry,custom-designed axial flowinternal recirculation pumps,which provide superior mix-ing characteristics at alower power requirementthan common agitators. The product crystal size isenhanced by fines destruc-tion systems. Each DTBcrystallizer discharges slurryto a common slurry collec-tion tank. The slurry is then

fed to pusher centrifuges,where the crystals arewashed and separated fromthe mother liquor. The cen-trifuged crystals are driedand screened, and theundersize fraction is recy-cled for recrystallization.

The centrate is taken overby an aftercrystallizer (FC)to improve yield. These fineand impure crystals are concentrated by means ofan hydrocyclone and theconcentrated suspension isredissolved in the feedliquor. A part of the hydro-cyclone overflow is taken asthe liquid process purge.

Modern applications inenvironmental protection

Flue gas desulfurization (FGD):scrubber effluent

Concentration of scrubbereffluent from FGD systemsin thermal power plants hasbeen practiced for abouttwo decades. Plants forconcentration of thesewaste waters to drynessare in fact evaporative crys-tallization units, and shouldnot be designed as simpleevaporation units. Usually,FGD concentration units arecombined with pretreat-ment facilities, such asheavy metals precipitation,in order that it may be pos-sible to recover a brine or aproduct salt pure enough tobe recycled in the process.

This example shows aninstallation for the concen-tration-to-dryness of the liq-uid effluent from a flue gas cleaning system in anindustrial waste incinerationfacility. This plant consistsof a heavy metals precipita-tion and a double-effectevaporative crystallizationunit, with two FC crystalliz-

ers supplied to recover cal-cium chloride dihydrate salt.The first stage is a gypsum-seeded preconcentrator,and the second stage is thecalcium chloride crystallizer.The crystal product is sepa-rated on a screen bowl cen-trifuge, dried, packed andreused in another industrialapplication.

Recovery of caffeine

When caffeine is extractedfrom coffee by the super-critical carbon dioxidemethod, a caffeine-contain-ing waste water is pro-duced. Evaporation, com-bined with a surface coolingcrystallization separates thiswaste water into a caffeineof food grade quality, andpure distillate which can bereused for the decaffeina-tion process. Short resi-dence times at the highertemperatures is importantin the evaporative step ofthis process, in order tominimize caffeine decom-position.

The process encompassesactive carbon treatmentused to remove impuritiesthat influence the productcolour, followed by a two-stage falling film evaporatordriven by process vapourscompressed to a higherpressure by a singlemechanical compressor. Inorder to minimize residencetime in the evaporators, thefinal concentrate is pro-duced in a separate, smallerunit. This concentrate isfinally cooled to ambienttemperature in a surface-cooled loop crystallizer, to

crystallize caffeine mono-hydrate. The crystals arecalcined to remove thewater of hydration, andpacked. This caffeine prod-uct is used in the manufac-ture of soft drinks.

Salt from secondaryaluminum slag

When aluminum scrap ismolten down, the liquefiedaluminum must be protect-ed from exposure to theatmosphere to avoid its oxi-dation and to absorb theimpurities of the scrap. Thisprotection is provided by alayer of molten salts, prefer-ably a mixture of potassiumand sodium chloride thatfloats over the aluminum.This salt remains after therecovery of the aluminum,and is cast into ingots.Because of the impurities inthis solid mass, left overfrom the scrap aluminumresidue, exposure of theingots to humidity causesevolution of various gases.The gases evolved are poi-sonous and explosive, andthe leachate generatedfrom dissolution of the ingot

is saturated with salts.Consequently, landfill-dis-posal of this waste wasseverely restricted inEurope during the eighties,and processes had to bedeveloped to solve thisproblem in an environ-mentally sound way.

The slag is treated mechani-cally to recover most of themetallic aluminum for directrecycling, and is then fed toa cascade leaching processat elevated temperatures to achieve fast degassing

Vacuum filter for citrate extraction

Salt crystallizer; modified FC-type Vacuum cooling crystallizer for copperas

Triple-effect four stage evaporationcrystallization of ammonium sulphatein DTB (caprolactam process) withaftercrystallization (FC)

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Research and development services

Chemical laboratoryand pilot plant facilities

The MESSO chemicallaboratory and pilot plantfacility is available to devel-op the basic informationnecessary for the design ofcrystallization plants as wellas the most appropriateoverall processes for ourclients. The chemical labor-atory is able to define physi-cal properties to the crystal-lizer designer, such as asolution’s metastable zone(supersaturation), desuper-saturation rates, viscosity,density of a range of com-positions, the system solu-bility, formation of mixedcrystals, as well as thechemical compositions ofsolutions and minerals.

Our research anddevelopment facility hasequipment that accuratelyrepresents most types ofcrystallizers, and this is

used as necessary to simulate the specific designenvisioned for our clients.These process designs canbe tested in small pilotplants brought togetheraccording to the specificprocess requirements, andsamples can be producedfor further (market) tests.

In case of products that aretoo sensitive to be shippedto our facility, or that requirespecial handling (due tosafety or health concerns)our team may perform thenecessary tests or investi-gations in our clients’ facili-ties.

We are proud to have de-veloped and optimized pro-duction processes for thechemical, pharmaceuticaland food industry jointlywith our customers and tailor-made for the individualproject.

We continue to improve –for the benefit of our customers.

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Turnkey in Kuwait; salt recovery plantfor a chloralkali electrolysis

Our product experience

Acetylsalicylic acid & saltsAdipic acidAmmonium bromideAmmonium dimolybdateAmmonium hydrogenfluorideAmmonium sulfate (also by reaction)Ammonium thiosulfateAscorbic acid & salts

Benzoic acid & saltsBisphenol A

CaffeineCalcium chlorideCalcium formateCalcium tartrateCarnalliteCitric acid & saltsCooling Tower BlowdownCopper chlorideCopper sulfate

Dextrose (Glucose)DichlorobenzeneDicyandiamideDipentarythritol

Epichloro hydrine processeffluent ZLD

Ferrous sulfate from process effluentsFerrous sulfate from TiO2

Fumaric acid & salts

Glutamic acid & saltsGuanidine nitrate

Hexachlorocyclohexane

Isomaltulose

Ketogulonic acid & salts

Lactic saltsLactoseLandfill Leachate Concentration & ZLDLauryllactam

Magnesium ammonium sulfateMagnesium chlorideMagnesium hexafluoro-silicateMagnesium sulfateMalic acid & saltsMethionine

Nickel acetateNickel nitrate

PentaerythritolPotash from various sourcesPotassium bromidePotassium carbonatePotassium chloratePotassium chloridePotassium dichromatePotassium hydrogen-carbonatePotassium permanganatePotassium phosphate (Industrial)Potassium sulfatePotassium sulfate fromNa2SO4 & KCl (conversion)

Salicylic acid & saltsSalt (based on sea salt respectively brines)Silver nitrateSodium acetateSodium ascorbateSodium carbonateSodium chlorateSodium chloride from sea salt

Sodium chromate (& Na2SO4)Sodium cyanideSodium dichromateSodium disulfiteSodium dithioniteSodium fluoride saltsSodium formateSodium glutamateSodium ketogulonateSodium nitrite (waste)Sodium perborateSodium perchlorateSodium phosphates (industrial)Sodium salicylateSodium sulfate Sodium tartrateSodium thiocyanateSorbic acid & saltsSulfanilic acid & salts

Tartaric acid & saltsTMPTrimellitic acid

Urea

Zinc sulfateZn salt slag recovery