Background. One of the most important organizing principles in chemical engineering conservation of mass (recall Table 1.) in quantitative terms, the principle is expressed as a mass balance that accounts for all sources and sinks of a material that pass in and out of a volume (Fig. 12.1). Over a finite period of time, this can be expressed as (12.1) Accumulation = inputs - outputs . The mass balance represents a bookkeeping exercise for the particular substance being modeled. For the period of the computation, if the inputs are greater than the outputs, the mass of the substance within the volume increases. If the outputs are greater than the inputs, the mass decreases. If inputs are equal to the outputs, accumulation is zero and mass remains constant. For this stable condition, or steady state, Eq. (12.1) can be expressed as (12.2) Inputs= outputs