What is a material balance and why is it fundamental to process design?

A material balance accounts for all material entering, leaving, and accumulating in a system: Input + Generation = Output + Consumption + Accumulation. For steady-state without reaction: Input = Output. Material balances are fundamental because they: determine equipment sizes (flow rates, volumes), establish stream compositions, identify recycle requirements, validate process feasibility, and form the basis for energy balances. They apply to overall process and individual units. Proper material balance closure (mass in equals mass out) is essential for reliable design; closure errors indicate calculation mistakes or missing streams.