CHAPTER ONE
1.0     INTRODUCTION
          Propeller agitator mixing is one of the most common operations carried out in chemical, processing and allied industries.  The terms “mixing” is applied to the processes used to reduce the degree of non uniformity or gradual of a property in a system such as concentration, viscosity, temperature and so on.
          Mixing is achieved by moving material from one region to  another. It may be of interests simply as a means of achieving a desired degree of homogeneity or it may be used two proote heat undergoing a chemical reaction.
          At the outset I t is useful to consider some common examples of problems encountered in industrial mixing operations, since this will nto only reveal the ubiquitous nature of the process but will also provide an appreciation of some of the associated.  Difficulties.  Several attempts have been made to classify mixing problems, and for example “REAVELL” used as a criterion for mixing of powders, the flowability of the final products, HARNEY et al based their classification on the phases present, that is liquid – liquid, solid – liquid and so on.  This is probably the most useful description of mixing as it allows the adoption of a unified approach to the problems encountered in a range of industries.
          The term agitation is applied to a variety of operations differing in the degree of homogeneity of the mixed material.  In chemical engineering, it is conventional to treat agitation and multi purpose operations for which the principles of equipment design and operation can be stated generally, such operations in agitation like crystallization, Ion exchange and colliding are advanced of this study and construction.
          In most homes, industries and chemical operations, two or more substances are required to be mixed for users demand, in mixing as a unit operation, the substance is fed into the vessel or mixing tank containing the agitator in either one form, liquid, solid or gas.  The product obtained should either be homogenous or heterogeneous depending on demand or desired product.
          The selection of a particular agitator for a particular purpose requirement, the flow properties of the process fluid, equipment cost and the construction material required must be considered.  Ideally, the equipment chosen should be that which has the lowest cost but meets all process requirement.  In a given problem, the agitator given must handle the material when it is in it’s worst condition and may not be so effective as other design during other parts of mixing cycle.  As with other equipments, the choice of an agitator for heavy material is always a compromise and arbitrary.