Mass Balance Equation

\[\boxed{\text{input} + \text{generation} - \text{output} - \text{consumption} = \text{accumulation}}\] where,

  • input is the material entering through the system. This will include feed and makeup streams.

  • generation is the material produced within the system, such as the reaction products in a reactor;

  • output is the material that leaves through the system boundaries. These will typically be the product streams of the process.

  • consumption is the material consumed within the system, such as the reactants in a reactor;

  • accumulation is the amount of material that builds up within the system.

’Energy balance’ also has the same form of equation.

Special Cases:

In a steady-state continuous process, the accumulation is zero, which leads to a more simple mass balance equation: \[\text{input} + \text{generation} = \text{output} + \text{consumption}\] Further, in the case of systems with no reaction, where mass is neither generated nor consumed, the result is even simpler: \[\text{input} = \text{output}\] In a unsteady-state process, the accumulation is nonzero. Here, conditions are changing with time.

Atom Balance:
Since atom can NOT be created or destroyed, for any process, a number of atoms entering the process must be equal to that leaving the process. \[\text{input (of some species of atom)} = \text{output (of the same species of atom)}\] The chemical reactions that take place in a complicated process can be solved by atom balances without knowing the actual chemical reactions taking place in that.