(676c) Use and Abuse of Crystallization for Process Separations

               Often chemical processes require an intermediate refining step to separate reaction products and the physical properties of the components exclude typical separation unit operations such as distillation or extraction.  Every effort is made to exclude crystallization because of uncertainties in the performance and/or reliability of the methodology.  This presentation will address the challenges (and solutions) of integrating crystallization techniques into processes where component yield and purity are the driving forces and crystal size and shape are generally secondary concerns. 

PRELIMINARY EVALUATION:

               Careful evaluation of the physical and chemical properties of the proposed feed stream from which a desired component is to be recovered is essential. Reliable solubility data for two components or a phase diagram (for more than two components) is necessary for preliminary process evaluation and is best determined experimentally. MATRIC has a variety of analytical equipment and experience in obtaining and evaluating solid/liquid phase data.  For a two component system MATRIC computes a chart which combines product yield and total solids in suspension, as a function of temperature and initial feed concentration.  The chart basically organizes the overall material balance for the system and serves as a basis for preliminary evaluation of product yield as a function of process variables.  Development and use of this chart narrows the choices available for zones of probable operation and also provides guidance regarding necessary staging of the process.  A typical recovery chart will be presented and discussed.

LABORATORY TESTING:

               Bench scale testing is appropriate at this stage in the process development to evaluate the crystallization behavior of the desired product (solute).  A variety of techniques is available to determine the so called meta-stable or labile portion of the phase diagram.  Some systems sustain limited super saturation and others may require seeding to initiate nucleation.  MATRIC has equipment and experienced personnel to investigate the need for a seeding policy.  A chart describing a situation where high supercooling and careful seeding are required is included.  At this stage of investigation it is advisable to conduct small scale batch crystallization tests to obtain information regarding particle shape (habit).  As the preliminary testing proceeds it is advisable to document the crystal habit with photomicrographs to detect any dramatic change as the process development (and impurities) change.  Preliminary information regarding particle size distribution is also desirable at this stage.  Generally product purity studies are a little premature at this stage; however, product recovery via filtration techniques followed by washing will give a reasonable idea of the ease or difficulty in achieving desired purity.

PILOTING:

               Piloting of the proposed process serves several purposes.  If the proposed process operation is batch the techniques for crystallizing, separating and washing the product can be verified on a small scale and in similar equipment to a full scale operation.  Sufficient material can be produced to conduct separation and washing studies and downstream drying investigations if the product must be dried.    Solid-liquid separation schemes should be considered and can be evaluated at MATRIC facilities.  At this stage of process development continuous operation may be of interest and process equipment can be designed to test this concept.  Often product testing by the client for acceptability is also a goal during this phase.

               If particle size distribution is specified in some way studies can be conducted in a continuous mode to develop the kinetics of the system.  MATRIC has piloted several systems and conducted kinetic studies.  

NOTE:  MATRIC also has laboratory equipment to investigate the feasibility of using so-called “Melt Crystallization” to separate and purify components.  The product from this type of separation operation is a “melt” which is ideal if the product is not required to be in a particulate form.

Authors:

Charles Moyers is a senior engineering scientist at Mid-Atlantic Technology, Research and Innovative Center (P.O. Box 8396, So. Charleston, WV 25303). He is a Fellow of AIChE, a PE (WV), and has co-taught several AIChE education courses including Industrial Crystallization, Liquid-Solid Separation, and Drying.  He holds a BS from Virginia Tech, an MS from West Virginia U. and a PhD from U. of Delaware, all in chemical engineering. (E-mail: charles.moyers@matricinnovates.com)

Brooke Albin (presenter) is a chemical engineer at Mid-Atlantic Technology, Research and Innovative Center (P.O. Box 8396, So. Charleston, WV 25303).  She earned a BS in biology and BA in chemistry from Alderson-Broaddus College and an MSE in chemical engineering from West Virginia University.  (Email: brooke.albin@matricinnovates.com)