Melt Crystallization
The Efficient Purification Alternative
GEA Crystallization
Preface
GEA Processing Engineering operates worldwide through
local companies specializing in a wide variety of process
technologies.
Recently the German based GEA Messo GmbH and the
Netherlands based GEA Niro PT B.V consolidated into one
operational entity: GEA Messo PT. The newly formed company
will combine the two technology centers for solution
crystallization and melt crystallization into one operational
entity using all cross-fertilizing synergies between melt and
solution crystallization. GEA Messo focusses on the recovery
and purification of chemicals via crystallization.
The companies operate through a worldwide network of sales
and service offices and authorized representatives, which
ensures quick and professional customer support anywhere in
the world.
Apart from chemical applications GEA Messo PT also
provides technology for the liquid food processing industry.
Together the GEA Process Engineering companies form the
largest crystallization technology supplier in the world. By
combining our capabilities we can provide our clients the
most complete and advanced range of concentration,
purification and recovery systems available.
Principles of Melt Crystallization
Melt crystallization systems generally remove heat and cool
the liquid melt to create a driving force for the formation and
growth of crystals. Phase diagrams are used to describe the
relationship between composition and temperature of a
mixture at equilibrium conditions. Although industrial
streams almost exclusively consist of multiple components,
most organic mixtures can be described as simple binary
systems. These binary systems can be subdivided into two
important categories:
Eutectic systems, one component crystallizes as a pure solid.
These systems are extremely important for purification via
crystallization.
Solid-solution forming systems, in which the crystallizing
solid consists of a mixture of components. These systems
require multiple stages and are quite similar to the
vapor-liquid separation used in distillation.
A typical eutectic mixture of p-DCB and o-DCB is illustrated
in the phase diagram. Assume that the mixture has an initial
melt composition of 85wt% p-DCB and 15wt% o-DCB. Upon
cooling the mixture pure p-DCB crystals will be formed and
the remaining liquid becomes richer in o-DCB. Additional
heat removal will continue the process until the eutectic
temperature and composition are reached.
Crystal Purity
Pure crystals will only be obtained if they are grown very
slowly at near equilibrium conditions. Higher growth rates
generally result in concentrated mother liquor being included
into the crystal mass.
Product Recovery
The eutectic point (xe) represents the theoretical concentration
limit for any melt crystallization process. Higher
concentrations of impurities generally inhibit growth and can
affect the crystal purity. However, slow growth rates allow
pure crystal growth even near this limit. The final recovery
depends on the amount of product in the original feed solution.
The Power of Melt Crystallization is its High Selectivity
This is caused by the fact that impurity molecules will generally not be incorporated
into the highly ordered crystal lattice, provided that solid solutions do not form and
that crystal growth is slow in near ideal conditions.
Analysis of more than 5000 relevant organic mixtures revealed that more than 85%
of these systems exhibit eutectic behavior indicating that melt crystallization should
be a feasible process option for most organic mixtures.
Water
Acrylic Acid
Benzoic Acid
o-Phenyl Phenol
Ethyl Lactate
Paraxylene
Mono Chloro Acetic Acid
Melt Crystallization as a Chemical Process Unit Operation
The chemical industry is very much concerned with
the separation and purification of chemical
compounds. Impurities generally represent wasted
product and cause undesirable variations to the final
product quality. Specific impurities can damage
catalysts and lead to failure of downstream
processes.
Distillation is the industry standard for most
chemical separations. It has matured into a reliable
unit operation that is widely used when conditions
allow the stage-wise contacting of multi-component
liquid and vapor.
Melt crystallization is an economic and efficient
alternative. It is typically used in purification
applications where distillation becomes difficult:
♣♣Isomers with close boiling points
♣♣Azeotropic systems
♣♣Temperature sensitive substances
♣♣Components that tend to polymerize
♣♣Explosive substances
The typical eutectic system can form pure crystals of
a product. This specific selectivity is not possible
with any other separation technique. The
crystallization process is not only applicable for new
grass root plants, but ideally suited to upgrade
capacity and purity of existing concentration
processes such as distillation or adsorption. Small
changes to existing units can significantly increase
throughput by relaxing the product purity
requirement of an existing process. The hybrid
process completes the final purification using the
GEA Messo PT crystallization process. For such
de-bottlenecking projects GEA Messo PT is your
partner for:
♣♣Increased production, recovery and product
purity
♣♣Conversion from batch to continuous operation
♣♣Plant review and feasibility analysis
♣♣Solid-Liquid separation on existing
crystallization units
The GEA Messo PT Crystallization ProcessThe innovative
alternative for attainment of pure chemicals
The GEA Messo PT Crystallization Process is a highly
efficient approach for the recovery and purification of
chemical components from impure solutions. Single-step
crystallization and continuous operation help to provide the
lowest consumption of utilities of any other commercially
available crystallization process. Product purities of greater
than 99.9 wt% are typically achieved. The GEA Messo PT
design adheres to the following principles:
♣♣Suspension crystallization
Pure crystal formation
♣♣Product/crystal separation using wash columns
Efficient separation for ultra-pure product
Suspension Crystallization
The suspension-based crystallization process operates with
mainly vessel type crystallizers. The large number of crystals
provide a massive growth surface in a relatively small volume.
Since this large surface absorbs the under-cooling of the
solution, the resulting overall growth rate is extremely low.
This slow, near ideal, growth allows the formation of pure
crystals in a single crystallization step.
A typical suspension crystallizer may contain more than 60 billion
crystals per ton of final product. This represents nearly 20,000 m2 of
surface area.
Product Crystal Separation
The pure crystals must be completely separated from the
impurities remaining in the mother liquor. The separation is
accomplished within the unique GEA Messo PT Purifier.
Internals of a vessel crystallizer
Skid mounted melt crystallization plant for purification of enantiomers
Melt crystallization plant for
production of electronic grade
phosphoric acid Melt crystallization plant for purification of paraxylene
Highly Efficient Product Crystal Separation
Suspension crystallization provides the
massive surface needed to create near
ideal growth conditions. While this
results in pure crystals it also requires an
efficient washing process to remove the
remaining impurities from this massive
crystal surface.
In the GEA Messo PT process the
product crystal separation is effected
with proprietary wash column
technology. The unique GEA Messo PT
Purifier provides a near perfect
separation between the pure product
crystals and the impurity rich mother
liquor. High purity is possible because 1)
the crystal is already pure and 2) the
counter-current wash is completed with
pure melted product in a crystal bed of
more than 50 cm height. This highly
efficient contact between crystal and
wash liquid allows sufficient time to
recrystallize the wash liquid within the
packed bed thereby essentially
eliminating product losses from excess
wash liquid.
The crystals entering the wash column
are at equilibrium with the mother liquor
composition from the crystallizer and are
significantly colder than the melting
temperature of the pure product. Most
of the mother liquor is discharged
through a filter. This concentrated
stream of impurities can now be
discharged as reject or passed to a second
recovery stage.
The unwashed crystal mass is roughly
70-80% crystalline product with the
remainder being the impure liquid
mother liquor. As the pure wash
liquid (product melt) is forced through
the porous crystal bed it will effectively
wash away any impurities in the
unwashed part of the bed and
recrystallize as new crystal product
upon contacting the relatively cold
crystals.
The heat released by this crystallization warms the surrounding crystal mass.
This is a self controlling process where the recrystallizing wash liquid will
release just enough heat so that the crystals will reach the equilibrium
temperature of the pure product.
This recrystallization zone, generally
called the washfront, is a relatively
narrow portion of the column. This
washfront marks steep gradients in
temperature, concentration and porosity.
No wash liquid is lost to the filtrate
since after completing its task as wash
liquid, the new crystal product is
transported together with the now
warmer crystals back towards the pure
wash circuit.
The sharp separation between the
washed and unwashed portion of the
crystal bed is illustrated in a wash
column with transparent cylinder.
A reciprocating piston/filter draws a
charge of crystal slurry into The GEA
Messo PT Purifier and compresses this
charge into a compact bed of crystals
while allowing the mother liquor to
leave through the filter. The scraper
starts and the piston/filter continues to
force the existing crystal bed through
the column as the scraper disintegrates
the bed at the opposite end of the
column. The pure melted product is
forced counter-current to the crystal
bed flow. The porous bed provides a
unique environment where the pure
melt contacts the significantly colder
crystals mass and results in complete
recrystallization of the wash liquid.
This counter-current wash flow
effectively removes the impurities
remaining around the crystals and
returns the wash liquid as pure product
crystals.
The washed crystal bed is
disintegrated by a rotating scraper. The
crystals are then reslurried with
circulating pure melt and melted in a
heat exchanger. The final product is
removed through a control valve.
Restricting this discharge will result in
an increase in the pressure of the
circulation loop.
Wash Column Technologies
This unique environment, not found with shorter crystal beds,
allows a rather simple control strategy for maintaining product
purity and eliminating loss of wash liquid.
The position of the wash front can be measured and used to
control the washing pressure that determines the position of
this washfront; higher pressure forces the washfront further
away from the pure melt circuit.
Maintaining the product purity is easy: Since the washfront
does not need to be precisely located, small changes in the
operating parameters, which move the wash front, have little
effect on the performance of the GEA Messo PT Purifier.
Traditionally suspension-based crystallization processes use
filters or centrifuges for the separation of crystals from the
mother liquor. They utilize cross-flow washing of relatively
thin crystal cakes (filter-cake thickness of about 1 to 5 cm) to
increase the final product purity. These methods require 10 -
20% of the final product as wash liquid to achieve even
moderate product purities. The excess wash liquid quickly
passes through the cake and produces an extra stream of con-
taminated wash liquid. The crystallization section has to be
sufficiently sized to treat this extra quantity of product and
represents wasted resources for this inherent inefficiency.
GEA Messo PT offers for license various wash columns that
differ in the type of crystal transport mechanism. Each wash
column possesses special characteristics specific to an applica-
tion and GEA Messo PT can therefore assure a client of the
optimal separation device for any specific application. Modu-
lar components allow a wide range of capacities.
Pilot Plant Testing
The GEA Messo PT Crystallization Process can be applied to
most eutectic solutions when the chemical compound:
Is stable at its melting temperature,
Has a liquid viscosities lower than ± 50 mPa•s
Forms crystals with reasonable filtration properties.
Initial screening is completed by comparing the product
properties to our extensive database of previous test results.
Bench scale testing will provide a quick method to determine
crystal habit and filtration properties using only a small volume
of product. These properties give a good indication if the
GEA Messo PT Crystallization Process is feasible.
The W6 pilot plant determines crystallization and separation
properties on commercially relevant equipment. Scale-up is
straightforward and based on standardized modular
components.
◊ 5-15 kg/h product
◊ 30 liter crystallization unit
◊ Piston wash column
◊ Temperature range -60 °C to +130 °C
◊ Designed according to ATEX II3G IIB T3 (or T2 when heat
tracing is required)
◊ Transportable for on-site installation in most industrial locations
Our product experience
◊ Acetic acid
◊ Acetonitrile
◊ Acrylic acid
◊ Caprolactam
◊ DMT
◊ Durene
◊ Ethylene carbonate
◊ Hydrogen peroxide
◊ Maleic anhydride
◊ 4,4’ MDI
◊ m-Xylene
◊ Methacrylic acid
◊ Naphthalene
◊ o-Phenylphenol
◊ p-Diisopropylbenzene
◊ p-Dichlorobenzene
◊ p-Chlorotoluene
◊ Phenol
◊ Phosphoric acid
◊ p-Nitrochlorobenzene
◊ p-Xylene
◊ 2,4’ TDI
◊ Benzoic acid
◊ Chloroacetic acid
Scope of Supply Technology Highlights
GEA Messo PT designs and builds freeze concentration and
melt crystallization plants for virtually every liquid food and
organic product, with capacities ranging from 2,000 to
500,000 tons per year.
GEA Messo PT can provide you with the following range of
services:
♣♣Feasibility Studies (Technical and Economic)
♣♣Pilot Testing
♣♣Basic Engineering
♣♣Technology License
♣♣Detailed Engineering
♣♣Supply of proprietary equipment
♣♣Supply of skid mounted preassembled installations
♣♣Project Management
♣♣Erection Supervision
♣♣Commissioning and Start-up supervision
♣♣Training of operating personnel
♣♣After Sales Services
♣♣Ultra high purities Product
purities >99.999 wt-% are possible
♣♣Low energy consumption The utility cost is lower
than with any competing crystallization process based on
equivalent recovery and product purity. The higher
efficiency of the GEA Messo PT process results from the
single crystallization step.
♣♣Low capital cost The inherent efficiency and the
continuous nature of the GEA Messo PT process lead to
lower capital cost than for any of the competing
technologies. Modular design and standard component
sizes provide both easy scale-up and economy of scale
♣♣Ease of operation The GEA Messo PT process offers
reliable and simple control strategies. Low
maintenance and high service factor components
provide years of continuous operation. Apart from
pumps only low rotational speed components are applied.
Bulk system volume absorbs normal operating fluctuations to
stabilize production and reduce sensitivity to fluctuations
in feed composition.
♣♣Small footprint Modular designs allow custom fit into
existing facilities. More efficient use of equipment requires
less plot space. No intermediate vessels are needed for the
continuous GEA Messo PT process.
♣♣High recovery High purity crystal growth means lower
reject concentrations are possible in single-stage operation.
A second cold stage can boost this even further
without affecting the energy efficiency since the majority of
the product crystals are grown under favorable growth
conditions in the larger warm-stage crystallizer.
♣♣Slurry handling With over 30 years experience in slurry
handling the GEA Messo PT suspension
crystallization units typically have high on-stream times.
Industrial screw type GEA Messo PT Purifier. The crystal
bed fills the cylinder between the main cylinder flanges near
the center of the photo.
Contact us at:
www.gea-crystallization.com
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Process Engineering / GEA Crystallization
Kestner Division
GEA Process Engineering France
4, Rue J.P. Timbaud, BP 80
Montigny le Bretonneux
78185 ST Quentin en Yvelines (France)
Tel. + 33 1 301 46 110 Fax + 33 1 300 71 819
geakestner@ geagroup.com
GEA Messo PT
Germany:
Friedrich-Ebert-Strasse 134
47229 Duisburg
Tel. + 49 2065-903 0, Fax + 49 2065-903 199
info.geamesso.de @ geagroup.com
The Netherlands:
De Beverspijken 7b
5221 EE ‘s-Hertogenbosch
Tel. + 31 73 6390 390, Fax + 31 73 6312 349
sales.niropt.nl @ geagroup.com
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