The Law of Mass Conservation states that matter can neither be created nor destroyed. The Law of Energy Conservation states that energy can neither be created nor destroyed. However, both mass and energy may change forms, and even change into one another in the case of nuclear phenomena.
Conversion of mass into energy, or of energy into mass, is quantitatively described by Albert Einstein’s famous equation:
\[E = mc^2\]
\(E\) = Energy (joules)
\(m\) = Mass (kilograms)
\(c\) = Speed of light (approximately \(3 \times 10^8\) meters per second)
Conservation laws find practical context in many areas of science and life, but in the realm of process control we have the principles of mass balance and energy balance which are direct expressions of these Laws. “Mass balance” refers to the fact that the sum total of mass entering a process must equal the sum total of mass exiting the process, provided the process is in a steady-state condition (all variables remaining constant over time). To give a simple example of this, the mass flow rate of fluid entering a pipe must be equal to the mass flow rate of fluid exiting the pipe, provided the pipe is neither accumulating nor releasing mass within its internal volume. “Energy balance” is a parallel concept, stating that the sum total of energy entering a process must equal the sum total of energy exiting a process, provided a steady-state condition (no energy being stored or released from storage within the process).