ABSTRACT
The physical state of a body is absolutely defined when the following characteristics are established:
• Quantity of matter and composition • Total energy (internal, electric, magnetic, potential,
kinetic, etc.) • Components of the velocity at which the body is cir-
culating
The changes that take place during a unit operation in raw matter or an intermediate product are
• Changes in mass or composition (phase separation, blending, transformation by biochemical reactions, etc.)
• Changes in level or quality of the energy in the product (cooling, vaporization, heating, pressure increasing, etc.)
• Changes in motion conditions (increasing velocity or changing direction, etc.)
In addition, changes taking place in a system must be in accordance with the following conservation laws (Welty et al., 1997):
• Law of conservation of mass • Law of conservation of energy • Law of conservation of momentum
Generally, the changes that occur in a system, body, or product during a unit operation can be carried out by means of the mass, energy, and momentum transfer phenomena. In fact, when a system is not in a state of equilibrium, it is inclined to achieve it; as the system approaches equilibrium, such transfer phenomena properties will take place. Mass, energy, and momentum are designated as properties of transfer phenomena since they are considered properties of the system.
