Precipitation, which is the process of coming out of solution as a solid, is an important method in the purification of proteins that usually comes early in the purification process. Precipitation is frequently used in the commercial separation of proteins. The primary advantages of precipitation are that it is relatively inexpensive, can be carried out with simple equipment, can be done continuously, and leads to a form of the protein that is often stable in long-term storage. Since precipitation is quite tolerant of various impurities, including nucleic acids and lipids, it is used early in many bioseparation processes. The goal of precipitation is often concentration to reduce volume, although significant purification can sometimes be achieved. For example, all the protein in a stream might be precipitated and redissolved in a smaller volume, or a fractional precipitation might be carried out to precipitate the protein of interest and leave many of the contaminating proteins in the mother liquor. In this chapter the focus is first upon protein solubility, which is the basis of separations by precipitation. Then we discuss the basic concepts of particle formation and breakage and the distribution of precipitate particle sizes. The specific methods that can be used to precipitate proteins are treated next. The chapter concludes with methodology to use for the design of precipitation systems. After completing this chapter, the reader should be able to do the following: • Explain the various factors that influence protein solubility. • Use the Cohn equation to predict solution equilibria (precipitation recoveries). • Identify the distinct steps in the development of a precipitate. • Calculate mixing times in an agitated precipitator, the kinetics of diffusion-limited growth of particles, and the kinetics of particle-particle aggregation. • Perform particle balances as a function of particle size in a continuous-flow stirred tank reactor (CSTR). • Explain the methods used to cause precipitation. • Outline the advantages and disadvantages of the three basic types of precipitation reactor: the batch reactor, the CSTR, and the tubular reactor. • Implement simple scaling rules for a precipitation reactor.
Diffusion-limited kinetics of the solution-solid phase transition of molecular substances