This is part one of a two-part feature that examines the fundamentals and discuss equipment selection and process design. Part 2, which appeared in December 2003 Chemical Processing, addresses scaleup, simulation and new technologies

Crystallization combines particle formation and purification in a single operation, and is used to make a wide variety of products, from commodity chemicals to chiral pharmaceuticals. Crystallization is often more energy efficient than distillation, since the heat of crystallization is usually 1/5th ," 1/10th of the heat of evaporation. In addition, the growing crystal surface can be extremely selective, allowing crystallization to be used in challenging separations such as ultrapure powders or heat sensitive materials.

Part 1 will focus on the basics, providing guidance for process design and equipment selection.

Purification and crystal growth

The regular order of the crystalline lattice allows purification to take place during crystal growth (photos). Most purification occurs at the crystal surface, which provides a boundary between the ordered lattice of the crystalline solid state and the disordered liquid phase. A solute atom, ion or molecule must find a site where it can incorporate into the growing lattice. If the "guest" atom, ion or molecule differs from the "host" material, it will not fit into the growth site as well as the host solute species if it fits at all.

To optimize purification, the desired solute must be incorporated into the crystal surface as the rate-limiting step for growth. The driving force for crystallization is the level of supersaturation during crystallization. Supersaturation must be maintained at a low level, while providing enough mass transfer from the bulk solution to the growing surface. If supersaturation is too high, impurities can be incorporated and remain within the lattice, leading to problems that include caking and product instability.

Nucleation

In a supersaturated solution where solute concentration exceeds equilibrium solubility, two competing processes occur during crystallization: nucleation and crystal growth.

Nucleation, or formation of a new crystal, can be either primary or secondary. Primary nucleation occurs in the absence of product crystals and usually at high levels of supersaturation. Solute entities cluster together in solution to achieve an orderly arrangement with minimum free energy. If the clusters exceed a minimum size, they survive and grow. If not, they dissolve. This mechanism often is the major nuclei source for precipitations created by reaction, salting-out or pH adjustment since they often have high levels of supersaturation and a low percent solids slurry density. Typically, a thick mass of small crystals will suddenly appear as the system "crashes out."

In contrast, secondary nucleation requires that product crystals exist in the slurry. Usually the dominant nucleation force in a mixed-suspension crystallizer operating at low supersaturations, it occurs through contact, fluid shear on semiordered surface layers or attrition or breakage. The mechanism depends on the level of supersaturation, percent solids, circulation rate, type of agitator, tip speed and power input/unit slurry volume. Contact nucleation is the predominant type and results from collisions between crystals, or between crystals and walls or agitators.

The basic engineering equation for secondary nucleation is:

B Degrees = kNSbMTJA1 (1)

Where:

B Degrees = the nucleation rate; number of nuclei/unit volume/unit time.

kN = the nucleation rate constant, which depends on temperature.

S = the supersaturation

MT = the slurry density such as grams solids per liter

A = a measure of the degree of agitation such as rpm and power per unit slurry volume.

b, j, l are exponents that can be experimentally determined, with j often close to 1.0.

Crystal growth involves the diffusion of solute to the crystal surface, followed by a surface reaction. One of these mechanisms will control the rate. As with nucleation, growth is driven by supersaturation and can be described by a simple empirical equation:

G = kg Sg = dL/dt (2)

Where:

G = the growth rate in, for example, mm/min.

kg = the growth rate constant dependent on temperature, agitation and solution composition.

g = the exponential relating supersaturation to growth

L = a characteristic dimension such as an average screen opening

t = time

Since both Equations 1 and 2 contain the supersaturation term, a value difficult to measure, one can combine them to obtain a new expression for nucleation:

B Degrees =k'NGiMTjAl (3)

Where i = b/g, and i is normally greater than 1.0, indicating that nucleation has a higher order of dependence on supersaturation than on growth.

Supersaturation

Supersaturation can be expressed as ratios, or as differences in chemical potential, concentration, mole fraction and temperature vs. equilibrium values.

Supersaturation results from cooling and evaporation. It can also stem from chemical reaction, salting-out and pH adjustment, techniques that are common for precipitations that can yield high levels of supersaturation. In these cases, mixing can have a great influence on product characteristics. Equilibrium relationships for crystallization systems are expressed as solubility data, which are plotted as phase diagrams or solubility curves. Solubility data are usually stated as parts by weight of anhydrous material per hundred parts of solvent or solution, or weight percent anhydrous material. If hydrates are present, they are indicated as a separate phase.

The concentration is normally plotted as a function of temperature. If there are two components in solution, concentrations can be plotted on the x and y axes, and solubilities represented by isotherms. With three or more compounds, two- and three-dimensional models can be used [1]. Generally, the solubility curve for a given crystallization will determine which method to use for crystallization. Figure 1 shows a typical curve for an unseeded batch cooling crystallization.