The term unit operation arose from the need to standardize and systematize the teaching of chemical engineering as a discipline, due to the growth of the industry activity generated by the industrial revolution. Unit operations nowadays are diverse, since new types of industries have arisen, some consisting of very specific features. A classification of unit operations is not an easy task and some can be found in literature. The criteria used to categorize unit operations are varied and may include types of phases involved in a process, mass transfer or heat transfer possibility, governing force (e.g., physical or mechanical) in an operation, and so on. Properties of solids in large pieces differ from properties of particulate solids. The most important properties of pieces of solids include density, hardness, fragility, and tenacity. One of the standard methods to evaluate the flowability of a particulate system is to calculate the Hausner ratio after tapping.