CrystallizationEquipment

NGO. TIONGCO.

Outline of Report

I. Crystallization: Definition and Industrial ImportanceII. Crystallization Equipment

A. Tank and Batch-Type Crystallizers1. Agitated Batch Crystallizer2. Swenson-Walker Crystallizer3. Wulff-Bock Crystallizer

B. Evaporative CrystallizersC. Vacuum Crystallizers

Crystallization: Definition and Industrial Importance

• It is a process whereby a solution is supersaturated so as to cause the formation of crystals.

• It is the removal of a solute such as a salt from a solution by precipitating the solute from the solution.

• Crystallization is an important operation in the chemical industry as a method of purification and as a method of providing crystalline materials in the desired size range.

Crystallization Equipment• Crystallizing equipment can be classified according to the

methods used to bring about supersaturation as follows:

1. Tank and Batch-Type Crystallizers – supersaturation is produced by cooling the solution with negligible evaporation.

2. Crystallizing Evaporators – supersaturation is produced by evaporation of the solvent with little or no cooling.

3. Vacuum Crystallizers – supersaturation is produced by combined cooling and evaporation in an adiabatic evaporator.

Tank and Batch-Type Crystallizers• These are crystallizers which produce supersaturation by cooling.• The material must have a solubility that varies greatly with

temperature.• 3 types:

Tank and Batch-Type Crystallizers

Agitated Batch

Crystallizer

Swenson-Walker

Crystallizer

Wulff-Bock Crystallizer

Agitated Batch Crystallizer• Water is circulated

through the cooling coils and the solution is agitated by the propellers mounted on the central shaft.

• See Figure 18-79, Perry’s HB, 8th ed.

Agitated Batch Crystallizer• FUNCTIONS OF THE AGITATOR1. It increases the rate of heat transfer and keeps the

temperature of the solution more uniform.2. It also keeps the fine crystals in suspension, thus it gives

them an opportunity to grow uniformly instead of forming large crystals or aggregates.

Agitated Batch Crystallizer• ADVANTAGES1. Production of more uniform crystals compared to older

tanks.2. The crystals formed are very much finer than that from the

older tanks.

• DISADVANTAGES1. It is a batch or discontinuous crystallizer.2. The solubility is the least at the surface of the cooling coils.

Therefore, crystal growth is most rapid at this point and the coils rapidly build up with a mass of crystals that decreases the rate of heat transfer.

Swenson-Walker Crystallizer• It consists of an open

trough with a semicircular bottom having a cooling water jacket welded outside.

• It is about 2 ft wide and 10 ft long.

• The hot concentrated solution to be crystallized is fed at one end of the trough and cooling water usually flows through the jacket in counter current to the solution.

Swenson-Walker Crystallizer• A slow-speed spiral agitator, set as close as possible to the

bottom of the trough, rotates and suspends the growing crystals on turning.

• Blades pass close to the wall and break off any deposits of crystals on the cooled wall.

• In order to control crystal size, it is sometimes desirable to introduce an extra amount of water into certain sections in the jacket.

• A number of units may be joined together to give increased capacity.

Swenson-Walker Crystallizer• ADVANTAGES:1. Large saving in floor space.2. Large saving in material in process.3. Saving in labor.4. Free from inclusions and aggregations.

• DISADVANTAGES:1. The product generally have a somewhat wide crystal-size distribution.

Wulff-Bock Crystallizer• It has similar characteristics

as that of the Swenson-Walker crystallizer, however, it depends on air cooling.

• It consists of a shallow trough set inclined and mounted on rollers so that it can be rocked from side to side.

• The slow rate of cooling in this crystallizer results in low capacity but it gives uniform crystals.

Wulff-Bock Crystallizer• ADVANTAGE:It gives more uniform crystals as compared to Swenson-Walker Crystallizer.

• DISADVANTAGE:The slow rate of cooling in this crystallizer results in low capacity.

Evaporative Crystallizers• These are crystallizers which produce supersaturation by

evaporation of solvent.• The material must have a solubility that changes little with (or

is independent of) temperature.

Vacuum Crystallizers• These are crystallizers which produce supersaturation by

adiabatic evaporation with cooling.• The method of producing supersaturation in these crystallizers

is the most important one for large-scale production.• Hot solution is introduced into a vacuum where the solvent

evaporates and the solution is cooled adiabatically.

Forced Circulation Crystallizer• an "active volume“ to get both required residence time for

crystal growth and mother liquor desupersaturation• agitation rate • Control the extent of supersaturation arising from the evaporation, • Control the temperature difference in the heat exchanger

• a special design of the liquid-vapor separation area to avoid the formation of an excessive amount of fines, which is highly detrimental to crystal growth.

Forced Circulation Crystallizer• These systems can be either single or multiple effects. • Usually operate from low vacuum to atmosphere pressure.• Used for high evaporation rates and when crystal size is not of

the utmost importance.• Almost any material of construction can be considered for the

fabrication of these crystallizers.• Use vacuum cooling or evaporation method

Forced Circulation Crystallizer

Typical products are:• NaCl (food or technical grade)• KNO3Na2 • SO4• K2 SO4• NH4Cl• Na2CO3H2O• Citric acid

Draft Tube Baffle (DTB) Crystallizer• AKA Messo-turbulence• Employ magma recirculation to control supersaturation

generation• Use cooling, vacuum cooling and evaporation• The concept is such that if no (or little) heat make-up is

required, it results in a rather compact arrangement; therefore the initial investment is minimized

• operate with a rather low supersaturation so that very large coarse and uniform crystals can be produced only by providing extensive and costly dissolving of fines.

Draft Tube Baffle (DTB) Crystallizer• When destruction of fines not

needed or wanted, baffles are omitted and the internal circulation rate is set to have the minimum nucleating influence on the suspension

Draft Tube Baffle (DTB) CrystallizerTypical products are:• boric acid• Na2SO4. 10H20 (Glauber salt)• melamine• citric acid• NaCIO3

Induced Circulation Crystallizer• provide additional agitation of the active volume of forced

circulation crystallizers with the use of only one pump.• operates similarly to a Draft Tube Baffle crystallizer but without

the internal agitation device• main applications are for evaporative crystallization cases• produce a narrow crystal size distribution• can be fabricated in almost any material of construction• limited to non-viscous solutions

Induced Circulation CrystallizerTypical products are:• NaCl• NH4ClO4

• NH4Cl

Surface-cooled (SC) Crystallizer • Same with surface-cooled-baffle (SCB) crystallizers• Use only surface cooling to generate supersaturation• Employ magma recirculation to control supersaturation

generation• Do not provide mechanism for classified product removal• provide a mechanism for fines dissolution when baffle is

present (SCB ONLY)

• The heat exchanger surface is the coldest part of the process and is prone to solids build-up so it is operated such that the tube-side and shell-side does not exceed 5-10°C

Oslo Type Crystallizer• AKA classified-suspension crystallizer• Oldest design for large, coarse crystals

Design Criteria:• Employ liquor recirculation to control supersaturation generation• Use surface cooling, evaporation or adiabatic evaporative cooling

to generate supersaturation• Provide a built-in mechanism for fines dissolution and classified

product removal.• keeping most of the crystals in suspension without contact by a

stirring device, thus enabling the production of large crystals of narrow size distribution

Oslo Type Crystallizer• classifying crystallization

chamber is the lower part of the unit

• upper part is the liquor-vapor separation area

• Used for reaction-crystallization and separation-crystallization when several chemical species are involved.

Oslo Type Crystallizer

• Usually “close type”• the "open" type is to be considered when very large settling areas are required

or when the vessel must be fabricated out of high cost alloys or metals.

Oslo Type Crystallizer

Typical products are:• (NH4)2 SO4

• Na2SO4

• AgNO3

• hydrated mono sodium glutamate• mono ammonium phosphate (MAP)

Crystallizer Configurations

Crystallizer ComparisonCrystallizer Equipment Rough Cost

Estimate (1=highest)

Forced Circulation Crystallizer 4

Induced Circulation Crystallizer 2

Draft Tube Baffle (DTB) Crystallizer

1

Surface-cooled (SC) Crystallizer 5

Oslo Type Crystallizer 3

Crystallizer Equipment Advantages Disadvantage

Forced Circulation Crystallizer

• Least expensive type of crystallizer

• Large range of sizes available• High rate of circulation reduces

particle deposits on vessel walls

• Crystal size difficult to control

Draft Tube Baffle (DTB) Crystallizer

• Crystal size easy to control• Economic due to recyclability

of fines• More energy efficient than

forced-circulation crystallizers

• Frequent flushings required to minimize deposits on the crystallizer wall

• Cannot effectively handle a high density slurry

• Not easily reproduced in small scale

Surface-cooled (SC) Crystallizer

• Can handle high boiling point solution

• Can handle solution that has such low temperature boiling point that evaporation by vacuum is not possible

Crystallizer Equipment Advantages Disadvantage

Oslo Type Crystallizer • operating costs of the Oslo type crystallizer unit are much lower than with any other type when both large and coarse crystals are required

• Since crystals are not in contact with any agitation device, the amount of fines to be destroyed is lower and so is the corresponding energy requirement.

• allows long cycles of production between washing periods.

• Not easily reproduced in small scale

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Crystallization of Monosodium Glutamate

Video• Forced Circulation Crystallizer

C:\Users\Chong\Documents\PARTECH\Crystallization\gea-wiegand-animation-forced-video.mp4

References• Samant, K.D. & O’Young, L. (2006). Understanding

Crystallization and Crystallizer. Clearwaterbay Technology, Inc.• http://www.niroinc.com/evaporators_crystallizers/

crystallization.asp• http://www.alaquainc.com/Crystalizers.aspx• http://video.geap.com/video/852192/gea-wiegand-

animation-forced• http://video.geap.com/channel/4103319/crystallization• http://www.slideshare.net/saravanamoorthy/crystallization