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.  Crystallization may be excluded because of uncertainties in the performance and/or reliability of the methodology.  Let’s address the challenges (and solutions) of integrating crystallization techniques into processes where component yield and purity are the driving forces!

PRELIMINARY EVALUATION:

Crystallization behavior is highly empirical. Crystallization is often an afterthought in process design, yet proper evaluation is required for optimum performance. 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 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 organizes the material balance 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.

LABORATORY TESTING:

After preliminary evaluation of the system, bench scale testing is appropriate to evaluate the crystallization behavior of the desired product (solute). A variety of techniques is available to determine the so called meta-stable region 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. It is advisable to conduct small scale batch crystallization tests to obtain information regarding particle shape (habit).  Photomicrographs (see Fig. 1) can be used to track the crystal habit and detect any dramatic change as the process variables and impurity profile change. Information regarding particle size distribution is also desirable at this stage.  Generally, product purity studies are 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. If continuous operation is of interest, 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.

MATRIC also has laboratory equipment to investigate the feasibility of using “Melt Crystallization” to separate and purify components. The product from this type of separation operation is a liquid “melt”.  This is ideal if the product is not required to be in a particulate form, since solids handling can be largely avoided.  This technique is capable of producing extremely high product purity and can be scaled up easily.  Melt crystallization has been used commercially for purification of several materials, such as acrylic acid.  The solubility curve for such a system is presented in Figure 2.

photomicrographs of crystals

 

 

 

 

 

 

 

Figure 1.  Photomicrographs of crystals.

Solubility cure for a binary mixture 

 

 

 

 

 

 

Figure 2.  Solubility curve for a binary mixture.

– For more information on MATRIC’s crystallization capabilities, contact Brooke Albin.