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The general balance equation is a fundamental concept of chemical engineering which is based on the principals of conservation of mass and conservation of energy. The general balance equation states that the total or component mass or energy of any system can be modeled by:

In - Out + Generation - Consumption = Accumulation

The equation is used to describe and design almost every process unit within chemical engineering.

Mass Balance

When the general balance equation is applied to the mass of a component or of the entire system, it is called a mass balance. Mass balances are the simplest forms of the general balance equation and one of the first forms a chemical engineer learns. One of the ways chemical engineers ensure their calculations on linked process units have been performed correctly is to perform a mass balance over the entire system.

Using Mass Balances

Below is a general description of how to use mass balances for different situations.

Steady State

If the system is at steady state then there are no terms which very over time. This means that the in, out, generation, and consumption terms are constants, and the accumulation term is 0. Steady state can only be applied to flow systems, or batch systems in which no changes are taking place. The mass balance for a steady state system is:

In - Out + Generation - Consumption = 0

Batch Systems

A batch system is one in which all of the materials of the system are placed into a fixed vessel and left to react for an amount of time.

Overall Mass Balance

An overall mass balance is a mass balance which models all of the mass of the system, as opposed to the mass of one component of the system. Since mass can not be created or destroyed, the consumption and generation terms for an overall mass balance are 0. Thus the overall balance is shown by:

In - Out = Accumulation

If the system is also at steady state, then the the equation reduces to:

In = Out

Note that an overall mass balance can not be performed on a batch system.