Suspension Melt Crystallization Emerges as Effective Purification Alternative

Melt crystallization is a process in which chemical compounds can be separated from one another by using the temperature points at which physical states transition from liquid to solid. At the correct temperatures, pure solid crystals can be separated, leaving impurities (and other chemical components) behind. As part of the continuing process, other chemicals can be separated at different temperatures. The separation and purification of chemical compounds is an important separation process step in the chemical industry. Impurities typically represent by-products and can cause unwanted changes in final product quality. Some impurities can cause damage to catalysts, negatively affecting the progress of the continuing process. Distillation is the standard process used in many chemical separations in industry. It has become an established basic operation, frequently used in situations where gradual contact between multi-component liquids and vapors is facilitated. Melt crystallization, however, is an economical and efficient alternative. It is typically used in purification applications where distillation becomes difficult: • Isomers with boiling points close to one another, • Azeotropic systems, • Substances sensitive to temperature, • Components with a tendency to polymerize, • Explosive substances. A typical eutectic system can produce pure crystals of a product. This high selectivity is not possible in other separation methods. The crystallization process is suitable not only for newly established facilities, but also for increasing the purity and capacity of concentration processes such as distillation or adsorption. With minor modifications to equipment in use, product purity can be increased and efficiency improved in an existing process. The hybrid process completes final purification using a proprietary crystallization process. GEA is ready to be your partner in projects developed to overcome the following obstacles: increasing product purity alongside production and recovery, • Transition from batch to continuous operation, • Facility review and feasibility analysis, • Solid-liquid separation in existing crystallization units.

Principles of Melt Crystallization

Melt crystallization systems typically create a driving force for crystal formation and growth by removing heat and cooling the liquid melt. Phase diagrams are used to define the relationship between the composition and temperature of a mixture under equilibrium conditions. Although industrial streams consist of more than one component almost entirely, most organic mixtures can be described as simple binary systems. These binary systems can be divided into two important categories: Eutectic systems – one of the components crystallizes as a pure solid. These systems make up approximately 85% of all chemical mixtures and are extremely important for purification through crystallization. Solid-solution forming systems – the crystallizing solid consists of a mixture of a series of components. These systems make up the remaining 15% and require multiple steps, similar to the vapor-liquid separation used in distillation. A typical p-DCB and o-DCB eutectic mixture is shown in the phase diagram. Suppose the initial liquid composition of the mixture is 85% p-DCB and 15% o-DCB by weight. As the mixture is cooled, pure p-DCB crystals form and the remaining liquid becomes richer in o-DCB. The process continues as heat is removed until the eutectic temperature and composition are reached.  

Phase Diagram for p/oDCB

  Pure crystals can only be obtained under conditions close to equilibrium and with a very slow process. Higher growth rates typically result in the inclusion of concentrated mother liquor in the crystal mass. The eutectic point (xe) represents the theoretical concentration limit for any melt crystallization process. Higher purity at higher concentrations typically slows growth and can affect crystal purity. However, lower growth rates enable pure crystal growth even at this limit. Final recovery depends on the amount of product in the original feed solution. High selectivity results from the fact that impurity molecules typically do not incorporate into the regular crystal lattice structure. Impurity molecules usually have a different shape. Therefore, under slow, near-ideal conditions, these impurities do not incorporate into the crystal product. Analysis of more than five thousand organic mixtures revealed that 85% of these systems exhibit eutectic behavior, demonstrating that melt crystallization is primarily applicable to organic mixtures. • Phosphoric Acid • Benzoic Acid • Ethyl Lactate • Monochloroacetic Acid • o-Phenylphenol • Substituted Aromatics (pDCB) • Paraxylene • Water • MDI, TDI • Lactide • Acetonitrile • Acetic Acid The crystallization process is a highly efficient approach for recovery and purification of chemical components from non-pure solutions. Single-step crystallization and continuous operation provide the lowest auxiliary material consumption among commercially used crystallization processes. Products can be obtained at purities exceeding 99.9% by weight. GEA design adheres to the following principles: • Suspension crystallization – Pure crystal formation, • Product/crystal separation with wash columns – Efficient separation of ultra-pure product.

Suspension Crystallization

In suspension-based crystallization process, a simple vessel-type crystallizer is used that includes stripped surface area growth volume (figure shown below). Thanks to numerous crystals, a very large growth surface is obtained in a relatively small volume. This large surface area results in an extremely low overall growth rate, as it absorbs excess cooling of the solution. Due to this slow, near-ideal growth, pure crystals can be obtained in a single crystallization step. Pure crystals must be completely separated from the impurities remaining in the mother liquor. Separation is accomplished in the GEA proprietary wash column.

Crystallization

 

High-Efficiency Product Crystal Separation

Suspension crystallization provides the large surface needed to create near-ideal growth conditions. Although this results in the formation of pure crystals, an effective wash process is also required to remove residual impurities from the large crystal surface that forms. The GEA wash column separates pure product crystals from the impurity-concentrated mother liquor in an near-perfect manner. High purity is achieved in two ways: 1) the crystal is already pure, and 2) counter-current washing is completed with completely dissolved product in a crystal bed exceeding 50 cm in height. This high-efficiency contact between crystal and wash liquid provides sufficient time for recrystallization of the wash liquid, preventing product losses from excess wash liquid. Crystals entering the wash column are in equilibrium with the mother liquor exiting the crystallizer and are much colder than the melting point of the pure product. Most of the mother liquor is discharged through a filter. This impurity-concentrated solution can now be discarded or sent to a second recovery stage. Crystals are compacted in the wash column, forming a firm crystal bed with crystal density between 60–80%. The compaction device can be a slurry pump or a reciprocating piston.

Wash Column

The mass of unwashed crystal in the piston column accounts for approximately 70–80% of the crystal product, with the remainder being impurity-containing liquid mother liquor. When pure wash liquid (dissolved product) is advanced into the porous crystal bed, it effectively washes the impurities in the unwashed section of the bed and, upon contact with relatively cold crystals, recrystallizes to form new crystal product. The sharp demarcation between washed and unwashed sections of the crystal bed is demonstrated using a wash column with a transparent cylinder. Heat released after the crystallization process warms the crystal mass around it. This is a self-controlling process in which the recrystallizing wash liquid releases enough liquid to warm the crystal temperature to the equilibrium temperature of the pure product. This recrystallization zone, typically called the wash zone, is a relatively narrow section of the column. In this wash zone there are sharp transitions in temperature, concentration, and porosity. No wash liquid mixes with the filtrate, since the new crystal products formed after the wash liquid completes its function are carried back to the fresh wash zone along with the now warmer crystals. In suspension-based crystallization processes, crystals are traditionally separated from the mother liquor using filters or centrifuges. These increase final product purity by allowing cross-flow washing of the relatively thin crystal cake (filter-cake thickness approximately 1 to 5 cm). These methods require high quantities of wash liquid even to achieve average product purities. Excess wash liquid rapidly passes through the cake and creates an additional dirty wash liquid stream. Typically, approximately 10–20% of the final product is used as wash liquid. The crystallization department must be sufficiently large to process this additional wash liquid quantity, resulting in inefficiency and resource waste.

Wash Column Technologies

GEA has introduced different wash columns for commercial use that differ in terms of crystal transport mechanism. Each wash column has certain application-specific characteristics, and GEA provides the optimum separation equipment for each application. Through modular components, it has a wide range of product capacity. The GEA crystallization process can be applied to most eutectic solutions if the chemical composition is stable at the melting point, has liquid viscosity lower than ± 50 mPa·s, and produces crystals with reasonable filtration properties. The initial separation is completed by comparing product properties with our extensive database containing past test results. Tests conducted on a bench scale allow rapid determination of crystal nature and filtration properties with only a very small amount of product. These properties provide a good indicator of whether the GEA crystallization process is applicable.

W6 Pilot Facility

Our W6 pilot facility determines crystallization and separation characteristics through equipment in commercial use. The scale-up process is straightforward and uses standard modular components.

GEA Crystallization Technologies

In addition to melt crystallization, GEA also designs and manufactures specialized and innovative process solutions for organic and inorganic chemistry, environmentally friendly industries, food technology, and mining and metal industries. İlker Damar Sales Manager Industrial Applications GEA Türkiye     Ray Ruemekorf Senior Sales Manager GEA Niro PT B.V.       Jan van Esch Product Manager GEA Niro PT B.V.