Agitation and Mixing: Principles, Purposes, and Chemical Impact

Agitation and Mixing: Core Definitions

Agitation is the operation of creating violent, irregular motions within a fluid material. Its primary purpose is to ensure particles or molecules of one or more phases are distributed efficiently, achieving desired results in the shortest time and with minimum energy.

If two or more substances, whether miscible or not, receive this movement, the process is called mixing. Mixing aims to achieve a random distribution of these substances or phases.

Overall Aims of Agitation

Overall, agitation aims to:

• Produce and maintain an even distribution of materials undergoing treatment, or increase the speed at which this occurs.
• Produce and maintain an even heat distribution.
• Increase the active specific surface area of the different phases within the agitated mixture.

Specific Purposes of Liquid Agitation

Specific purposes of liquid agitation include:

• Suspending solid particles.
• Mixing immiscible fluids.
• Dispersing a gas in a liquid as small bubbles.
• Dispersing a second immiscible liquid to form an emulsion or suspension of droplets.
• Promoting heat transfer between the fluid and a coil or casing.

Chemical Significance of Agitation

Impact on Product Properties and Yield

The mechanical and physical properties of a product are significantly influenced by the degree of homogeneity achieved through mixing. This directly impacts the product's composition and subsequent analysis.

Adequate agitation is essential to prevent unequal concentrations of reactants or temperature gradients. Such inconsistencies can lead to unwanted byproducts (e.g., isomers, waste) in the final product, making separation difficult and reducing reaction yield.

For example, Morton demonstrated that when two substances, A and B, are reacted in a vigorously agitated reactor (high RPM), the finished product results from the condensation of two molecules of A. However, if the agitation is less intense, the condensation of reactants leads to a product of type A₂B.

Reaction Types in Heterogeneous Systems

In heterogeneous systems, reactions can be categorized into three types based on their sensitivity to agitation:

• Type I: Increasing the degree of agitation (RPM) leads to a more or less linear increase in the reaction rate. Temperature influence can be considered negligible.
• Type II: Agitation has a positive, progressive influence up to a certain degree. Beyond this point, the reaction becomes insensitive to further agitation and begins to be sensitive to temperature.
• Type III: The reaction rate is not affected by agitation but is sensitive to temperature.