CN2103 — Material & Energy Balances
Systematic mass and energy accounting for chemical process systems.
Part 1 — Material Balances
Weeks 1–6 · A/Prof Lanry Yung
Classify the system before writing any equation
Before writing a single balance equation, you must classify the process. The type of process (batch vs. continuous, steady vs. unsteady) determines which terms in the general balance equation are non-zero. The general material balance is: Input − Output + Generation − Consumption = Accumulation. For steady-state: accumulation = 0. For non-reactive systems: generation = consumption = 0.
Every chemical process falls into one of two operation modes. This determines how you set up the time dimension of your balance.
Feed is charged to a vessel at the start. No mass crosses the system boundary during processing. The vessel is then emptied at the end. Example: a pressure reactor loaded with reactants at t=0, allowed to react, then drained. Key variable is TIME.
Inputs and outputs flow throughout the duration of the process. Industrial processes are almost always continuous at steady state. Key variables are FLOW RATES (kg/h, mol/min, etc.).
One or more streams flow while the system contents are not fully discharged. Example: a reactor with continuous feed but batch product removal. Treated as a transient (unsteady) balance.
Q1.The degree of freedom (DOF) for a process system equals: