RESEARCH ACTIVITY

Heat and Mass Transfer
Crystallization
* Mixed-Suspension, Mixed Product Removal (MSMPR) Crystallizer * Crystallization from Melt

Objective of Research
This study is inclined in the discussion of a type of equipment in crystallization operations, the Mixed-Suspension, Mixed-Product-Removal Crystallizer, its operations, as well as the assumptions integrated in its practice.
It also discusses the basic principles of Melt Crystallization and a brief overview of its applications in the modern society.

Introduction A crystal may be defined as a solid composed of atoms or molecules arranged in an orderly, repetitive array. The interatomic distances in a crystal of any definite material are constant and are characteristic of that material. Because the pattern or arrangement of the atoms or molecules is repeated in all directions, there are definite restrictions on the kinds of symmetry that crystals can possess.
There are five main types of crystals, and these types have beenarranged into seven crystallographic systems based on the crystal interfacial angles and the relative length of its axes. The treatment of the description and arrangement of the atomic structure of crystals is the science of crystallography. (Perry's Chemical Engineer Handbook - 8th ed – 2007, page 18-50)
Crystallization, in its essence, is just then the process of crystal production from liquid solutions through the aid of different cooling equipment. It is one of the most used operations in sugar industries, as well as fertilizer producing companies.
Different equipment designing companies also had diverted some technological upgrading in crystallization equipment to suit desired product quality and quantity.

Mixed-Suspension, Mixed-Product-Removal Crystallizers
This type of equipment, sometimes called the circulating-magma crystallizer, is by far the most important in use today. In most commercial equipment of this type, the uniformity of suspension of product solids within the crystallizer body is sufficient for the theory to apply. Although a number of different varieties and features are included within this classification, the equipment operating with the highest capacity is the kind in which the vaporization of a solvent, usually water, occurs.
Although surface-cooled types of MSMPR crystallizers are available, most users prefer crystallizers employing vaporization of solventsor of refrigerants. The primary reason for this preference is that heat transferred through the critical supersaturating step is through a boiling-liquid-gas surface, avoiding the troublesome solid deposits that can form on a metal heat-transfer surface. In this case very low LMTDs are required to stay within the metastable zone to promote growth and reduce scaling. The result is multipass, large-surface-area heat exchangers.
Summary Discussion
MSMPR Crystallizer is an idealized crystallizer model which served well as a basis for identifying the kinetic parameters and showing how knowledge of them can be applied to calculate the performanceof such a crystallizer.
Assumptions
* Steady state operation * At all times the crystallizer contains a mixed-suspension magma, with no product classification * At all times uniform super saturation exists throughout the magma * ΔL law applicable * No size-classified withdrawal system is used * No crystals in the feed * The product magma leaves the crystallizer in equilibrium, so the mother liquor in the product magma is saturated * No crystal breakage into finite particle size occurs

Interpretation * Constant nucleation rate at all points in the magma * Constant growth rate and independent of crystal size and location * All volume elements of mother liquor contain a mixture of particles ranging in size from nuclei to large particles * Particle size distribution is independent of location in the crystallizer and is identicalto the size distribution in the product
Population-Density
* Basic quantity in the theory of the * Crystal Size Distribution * The population density n is defined as the slope of the cumulative distribution curve ( N/V vs. L) at size L

N - no of crystals of size L and smaller in the magma
V - volume of mother liquor in the magma
L - crystal size
At
L=0, N=0
L=LT, N=NT n≡d(NV)dL=1VdNdL

* n ≡ function of L and invariant in both time and location in the magma * Dimensions of n: Number/Volume-Length

Moment Equations * The normalized jthmoment is defined by µj≡0znzjdz0∞nzjdz * The differential distributions are dµ0dz=e-z dµ1dz=ze-z dµ2dz=z2e-z2 dµ3dz=z3e-z6

CRYSTALLIZATION FROM MELTS 1. SOLUBILITIES AND EQUILIBRIA
The variation of the solubilities of most substances with temperature is fairly regular, and usually increases with temperature. When water is the solvent, breaks may occur in solubility curves because of formation of hydrates.
A convenient unit of solubility is the mass of solute per unit mass of solvent, or commonly g solute/100g solvent. Interconversions with molal units and mol fractions are made readily when densities of the solutions are known. Under quiescent conditions a concentration substantially in excess of normal solubility or a temperature lower than the normal saturation temperature can be maintained. The maximum supersaturation appears to be a fairly reproducible quantity, but is reduced or even eliminated by stirring or by the introduction of dust or seed crystals. a. ENTHALPY BALANCES
Although the thermal demands of crystallization processes are small compared with those of possibly competitive separation processes such as distillation or adsorption; nevertheless, they must be known. For some importantsystems, enthalpy-composition diagrams have been prepared.
Calculations also may be performed with the more widely available data of heat capacities and heats of solution. The latter are most often recorded for infinite dilution, so that their utilization will result in a conservative heat balance.

2. CRYSTAL SIZE DISTRIBUTION
Crystal size distribution (CSD) is measured with a series of standard screens. The size of a crystal is taken to be the average of the screen openings of successive sizes that just pass and just retain the crystal. Thecumulative wt 70 either greater or less than a specified screen opening is recorded. The amount of a size less than a particular screen opening and greater than the next smaller size is called the differential amount. Typical size distribution data are plotted in two cumulative modes, greater than or less than, and as differential polygons or histograms. For some purposes the polygon may be smoothed and often is shown that way.
Cumulative data often are represented closely by the Rosin-Rammler-Sperling (RRS) equation
Y = 100 exp[-(d/dm)n], where d is the diameter, d,,, is a mean diameter corresponding to y = 100/e = 36.8% and n is called the uniformity factor. The greater n, the more nearly uniform the distribution. The log-log plot of thisequation should be linear.
Two other single numbers are used to characterize size distributions. The median aperture, MA or d50, is the screen opening through which 50% of the material passes. The coefficient of variation is defined by the equation
CV = 100(d16 – d84)/2d50.

3. THE PROCESS OF CRYSTALLIZATION
The questions of interest are how to precipitate the crystals and how to make them grow to suitable sizes and size distributions. Required sizes and size distributions are established by the need for subsequent recovery in pure form and ease of handling, and by traditional commercial practices or consumer preferences.

b. CONDITIONS OF PRECIPITATION
The most common methods of precipitating a solid from a solution are by evaporation of the solvent or by changing to a temperature at which the solubility is lower. Usually solubility is decreased by lowering the temperature. The limit of removal is determined by the eutectic composition. Complete recovery, however, is accomplished by evaporation.
A precipitatemay be formed as a result of chemical reaction between separately soluble gases or liquids. Commercial examples are productions of sodium sulfate, ammonium sulfate, and ammonium phosphate.
Precipitation also can be induced by additives, a process generally called salting out because salts with ions common to those whose precipitation is desired are often used for this purpose. For instance, ammonium chloride is recovered from spent Solvay liquors by addition of sodium chloride and the solubility of BaCI, can be reduced from 32% to 0.1% by addition of 32% of CaCl.
Other kinds of precipitants also are used, for instance, alcohol to precipitate aluminum sulfate from aqueous solutions. Foreign substances even in minute amounts may have other kinds of effects on crystallization: They may inhibit or accelerate growth rate or change the shape of crystals, say from rounded to needlelike, or otherwise. One of the problems sometimes encountered with translating laboratory experience to full scale operation is that the synthetic liquors used in the laboratory may not contain the actually occurring impurities, and thus give quite different performance. Substances that modify crystal formation are very important industrially and many such materials have been the subject of patents.

c. SUPERSATURATION
A saturated solution is one that is in equilibrium with the solid phase and will remain unchanged indefinitely at a particular temperature and composition of other constituents. Greater than normal concentrations also can be maintained in what is called a supersaturated condition which is metastable. Metastability is sensitive to mechanical disturbances such as agitation, ultrasonics, and friction and the introduction of solid particles. Under those conditions, solids will separate out until normal saturation is obtained. When great care is taken, the metastable state is reproducible. A thermodynamic interpretation of metastability can be made in terms of the Gibbs energy of mixtures.

d. GROWTH RATES
Crystallization can occur only from supersaturated solutions. Growth occurs first by formation of nuclei and then by their gradual growth. At concentrations above supersaturation, nucleation is conceived to be spontaneous and rapid. In the metastable region, nucleation is caused by mechanical shock or friction and secondary nucleation can result from the breakup of already formed crystals. It has been observed that the rate depends on the extent of supersaturation; thus dCnd∅=k(C-Co)m Values of the exponent m have been found to range from 2 to 9, but have not been correlated to be of quantitative value for prediction. Nucleation rates are measured by counting the numbers of crystals formed over periods of time.
The growth rates of crystals depend on their instantaneous surface and the linear velocity of solution past the surface as well as the extent of supersaturation, and are thus represented by theequation dWsd∅=kuA(C-co)m Values of the exponent have been found of the order of 1.5, but again no correlation of direct use to the design of crystallizers has been achieved.
In laboratory and commercial crystallizations, wide size distributions usually are the rule, because nuclei continue to form throughout the process, either spontaneously or by breakage of already formed crystals. Large crystals of more or less uniform size are desirable. This condition is favored by operating at relatively low extents of supersaturation at which the nucleation rate is low but the crystals already started can continue to grow. The optimum extent of supersaturation is strictly a matter for direct experimentation in each case.
Growth rates of crystals also must be measured in the laboratory or pilot plant, although the suitable condition may be expressed simply as a residence time. In most instances the recommended supersaturation measured as the ratio of operating to saturation concentrations is less than 1.1. It may be noted that at a typical rate of increase of diameter of lo-' m/sec, the units used in this table, the time required for an increase of 1 mm is 2.8 hr.
Batch crystallizers often are seeded with small crystals of a known range of sizes. The resulting crystal size distribution for a given overall weight gain can be estimated by an approximaterelation known as the McCabe Delta-L Law, which states that each original crystal grows by the same amount ΔL. The relation between the relative masses of the original and final size distributions is given in terms of the incremental ΔL by
R=Σwi (LOi + ∆L)3ΣwiLOi3
When R is specified, AL is found by trial, and then the size distribution is evaluated.

Typical Updated Applications of Melt Crystallization
(A summary)
Delimitation
This research features only the GEA MESSO PT Technology, a consolidation of two companies, (German based GEA Messo GmbH and the Netherlands based GEA Niro PT B.V) into one operational entity. This is done mainly to visualize real applications of the unit operations studied and not to advertise any of these two GEA companies. Although, acknowledgement to their articles, as sources, is given.
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 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.

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, advantages to be expected shall be: * 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 Process
The 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.

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.
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.
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 contaminated 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.

Skid mounted melt crystallization plant for purification of enantiomers