Principles of material (mass) balance ACCUMULATION = INPUT - OUTPUT + GENERATION - CONSUMPTION Input – enters through the boundary of the system Output – leaves through the boundary of the system Generation – produced within the system Consumption – consumed within the system Accumulation – build-up within the system
General approach to material balance The general approach for carrying out the material balance on a chemical process can be summarised into the following steps: Step 1
Draw a flow diagram and fill in all the given variables
Step 2
Label unknown stream variables with symbols
Step 3
Decide on a “basis” of calculation (do you need one?)
Step 4
If there are mixed mass and mole units, convert to the same units
Step 5
Write material balance equations using all available information. Write the balance with the fewest unknowns first
Step 6
Check if the equations are solvable or not. You can Degree of Freedon analysis (DOF) to check if the equations are solvable or not. DOF is a powerful tool and used by many chemical engineers.
Step 7
Solve for the unknowns progressively Substitute values on the flow diagram and into other equations as they are found
Step 8
Scale the final values as necessary. Make sure the units are consistent.
Step 9
Check your final answer using dependent equations
Note: Based on the nature of the problem, some of these steps can be avoided.