MIXING PROCESSES FOR AGRICULTURAL AND FOOD MATERIALS .4. ASSESSMENT AND MONITORING OF MIXING SYSTEMS

Mixing of ingredients, or dispersion of one phase into another, is an important process in many agricultural and food production systems. In most batch mixing processes, the object is to make a spatially homoge neous product, using the minimum amount of energy, or sometimes in the minimum amount of time. For continuous processes, the object is to ob tain a given (uniform) concentration of one, or more, of the mixture c omponents in the outflow product, which does not vary with time. This is the fourth part of a series of papers reviewing mixing processes: i n this paper, various methods of describing the completeness of mixing for batch and, to a lesser extent, continuous processes are discussed . Assessment of the degree of mixedness of a system involves proper sa mpling procedures and methods of quantifying the mixing state. A varie ty of mixing indices and their use as measures of mixture quality are reviewed for both fluid and particulate mixtures, and one, designated M7 in this paper, is recommended as it ranges conveniently from 0 to 1 , provides good discrimination over the full range of mixing time and is relatively insensitive to the number of particles in a sample of pa rticulate mixture. Techniques for monitoring the state of a mixing pro cess at both laboratory and full industrial scales are also described.