Definition of Mass Balance
Mass balance is a fundamental principle in science and engineering stating that for any defined system, the total mass entering must equal the total mass exiting, plus any accumulation or depletion within the system boundaries. It is a practical application of the Law of Conservation of Mass, which asserts that mass cannot be created or destroyed in ordinary chemical and physical changes, only transformed or relocated.
Key Principles and Components
To perform a mass balance, one must define a system boundary and identify all inputs (inlets), outputs (outlets), and internal processes. Within the system, mass can accumulate (increase in storage), deplete (decrease in storage), or be generated (e.g., by a chemical reaction) or consumed (e.g., by a reaction or decay). The general equation is: Mass In + Mass Generated = Mass Out + Mass Consumed + Mass Accumulated.
A Practical Example
Consider a simple mixing tank where two streams of liquid, each containing a certain concentration of salt, are continuously fed in, and a single mixed stream exits. A mass balance on the salt would track how much salt enters from both inlet streams and how much salt leaves in the outlet stream. If the tank volume and salt concentrations remain constant over time (steady state), then the rate of salt entering must exactly equal the rate of salt leaving, meaning no accumulation.
Importance and Applications
Mass balance is a cornerstone for designing, analyzing, and optimizing chemical processes, environmental systems, and biological pathways. Engineers use it to size equipment, predict pollutant dispersion, or monitor raw material usage. In environmental science, it helps understand nutrient cycling in ecosystems. Its broad applicability makes it an essential tool for quantitative analysis across various STEM disciplines.