Mixing

Mixing

INTRODUCTION

Mixing has been defined as an operation “in which two or more ingredients in separate or roughly mixed condition are treated so that each particle of any one ingredient is as nearly as possible adjacent to a particle of each of the other ingredients.” (Perry and Chilton, 1999) The term “blending” is synonymous, and “segregation” or “demixing” is the opposite.

Mixing is a basic step in most process sequences, and it is normally carried out

1. to secure uniformity of composition, so that small samples withdrawn from a bulk material represent the overall composition of the mixture and

2. to promote physical or chemical reactions, such as dissolution, in which natural diffusion is supplemented by agitation.

Danckwerts (Dankwerts, 1953) classified mixing as follows:

Positive mixing, which applies to systems that, given time, would spontaneously and completely mix. Examples are provided by two gases or two miscible liquids, and mixing apparatus is used on such systems to accelerate mixing.

Negative mixing, which is demonstrated by suspensions of solids in liquids. Any two-phase system, in which the phases differ in density, will separate unless continuously agitated.

Neutral mixing, which occurs when neither mixing nor demixing takes place unless the system is acted upon by a system of forces. Examples are found in the mixing of solids and of solids with liquids when the concentration of the former is high.

Mixing must embrace all combinations of the three states of matter. The theory of mixing should be able, when the system to be mixed has been defined, to dictate the type and design of the mixer, such as volume, shape and type of impeller, and the process conditions, such as degree of agitation, and the time and power required. Theoretical knowledge is, however, insufficient to predict the performance of mixers. More commonly, choice is based on broad empirical principles, which are then supported by practical tests.