(197a) Practical Considerations of Melt Crystallization

Crystallization involves purification of a chemical from a liquid mixture by solidification of the desired component.  Crystallization falls into two main categories: solution crystallization and crystallization from the melt (also referred to as “fractional crystallization”).  Solution crystallization means a material is crystallized from a mixture containing a solvent.  Crystals are formed by either concentrating the desired component by evaporation or cooling or both, and the solid phase is typically formed at a temperature well below its pure component freezing point.  In melt crystallization, no solvent is added.  The crystals are generated by cooling of the “melt” and are formed at a temperature near the pure-component freezing point.  The product is collected in its melt form, and solids handling can be largely avoided for some process designs particularly if the product is desired in a liquid form.   

APPLICATIONS:

Melt crystallization technology is somewhat uncommon, but can be very effective in particular applications.  It is a separation technique that offers certain advantages over traditional separation methods including distillation or solution crystallization.  It is capable of producing an extremely high product purity of >99.99%.  Producing a material which is a liquid melt can be a great advantage for processes in which a solid product is not required.  Typical challenges associated with handling a solid can be altogether avoided (except within the crystallizer itself).  In addition, a cost savings may be realized since solid-liquid separation and drying equipment are unnecessary.  Relative to distillation, energy usage can be low since the heat of fusion is generally smaller that the heat of evaporation for organic compounds.  It may be desirable for separation of materials which are thermally unstable and for separating components which have low relative volatilities (such as isomers) or that form azeotropes.  Finally, crystallizing from the melt rather than from solution means that the use of solvents and subsequent concentration or solvent removal steps are unnecessary.  Without the addition of solvents, volumes of process streams are smaller and associated equipment is smaller as well.    

This presentation will provide an overview of this process technology and describe the variations in available equipment for employing this technology.  Criteria indicating whether melt crystallization is appropriate for processing a given stream will be presented.  Current uses of this technology in industry will also be discussed. 

FEASIBILITY AND SCALE-UP STUDIES:

MATRIC has constructed laboratory scale falling film melt crystallizers and has experience in conducting feasibility studies to evaluate the use of melt crystallization for a given purification process.  Small scale testing can provide valuable information at an earlier stage in a project when large quantities of material are not available.  The work process employed by MATRIC consists first of conducting a simple “bottle test” (in which the process stream is subjected to a static crystallization) as a preliminary indicator of performance.  A partition coefficient (K) is calculated based on the impurity concentrations in the solids and mother liquor.  A K value of <1 indicates that the impurities preferentially stay in the liquid phase, and systems with K ≤0.25 are considered ideal candidates for melt crystallization technology.  At this stage, phase diagrams are also obtained and assessed to determine what recoveries are possible and whether the operating temperatures are practical. 

Next, crystallization trials are carried out in batch mode in the falling film crystallizer, where crystal formation can be easily observed through the glass crystallizer walls.  Crystallization, sweating and melting steps are carried out, and the resulting fractions are collected and analyzed.  Individual and overall interfacial distribution coefficients are calculated.  These data can be used to construct a material balance and estimate the number of stages which would be required to reach a specified purity and recovery.  The technique for subcooling the solution can be determined through this exercise.  Observations can be made regarding how well crystals adhere to the tube wall and whether seeding is required.  If seeding is required, special start-up techniques may be necessary.  Recommendations for sweating can also be made.