Basic information about material and energy balances in mass transfer processes.

1. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
Material and
energy balances

2. Balances
• In this topic, we will briefly discuss
mass balances (also called material
balances) and energy balances.
• In this course, the mass balances
have bigger role. Many calculations
are based on material balances.
• Block diagram or flowsheet helps to
demonstrate the problem.
• Material balances can also be used
on troubleshooting, e.g. to locate
leaks.
A simple flowsheet for continuous distillation.
(Parameter Estimation Strategies in Thermodynamics -
Scientific Figure on ResearchGate. Available from:
https://www.researchgate.net/figure/Flowsheet-for-the-
distillation-column_fig3_333547266)

3. Steady state and unsteady state systems
• To do calculations, it is essential to
understand the terms steady state and
unsteady state.
• In steady state system, variables do
not change with time. These are the
most common systems in continuous
processes.
• In unsteady state systems, variables
(e.g. flow rate) can change with time.
Batch processes are examples of
unsteady state operations. Diagram of batch distillation. In the beginning the
pot is filled. The rate of flows changes with time.
Picture: Mbeychok CC BY-SA 3.0

4. Mass balance (material balance)
• The simple concept of material balance is: "What comes in will come out."
• The common version of material balance can be written:
Material in + Generation - Consumption - Accumulation = Material out
• If there is no chemical reaction and the system is steady state, the equation
reduces to: Material in = Material out
• Mass balances can be written for total mass and for
each independent component. When composites are
presented as a percentage, it is crucial to state the basis:
mass, volume or molar.
• The system boundary defines the part of the process
being considered. In complex systems it is often
necessary to draw several system boundaries to solve the problem.

5. General procedure for material balance problems
1. Draw a block diagram of the process.
2. List all the available data.
3. List all the information requireded from the balance.
4. Decide the system boundaries.
5. Write out the chemical reactions involved in the process.
6. Note any other constraints.
7. Note any stream compositions and flows that can be approximated.
8. Determine the dergees of freedom. (Number of independent equations –
unknows. This should be zero to solve the problem.)
9. Decide the basis for calculations (e.g. mass, volume, moles, flow rate).
10. Do the calculations and check your answer.
Order of the steps may vary and not all steps are always needed.

6. Energy balance
• The general equation for energy balances can be written:
Energy in + Generation - Consumption – Accumulation = Energy out
• This can be written for any step of the process.
• If the reaction is exothermic, there will be generation of energy.
• If the reaction is endothermic, there will be consumption of energy.
• For steady state processes the accumulation will be zero.
• Energy balances are often more complex than material balances,
because energy can exist in many forms.
• Energy can also be lost for surroundings.
• Energy balances will be studied more detailed
in further modules, e.g in crystallization.

7. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
References
Hipple, J. 2017. Chemical Engineering for Non‐Chemical Engineers. John Wiley & Sons,
Inc, pp. 35-45.
Sinnot, R. K. 2005. Chemical Engineering, Volume 6, Chemical Engineering Design,
Fourth edition. Oxford: Elsevier Butterworth-Heinemann, pp. 34-57.
Videos:
• An example about material balances: https://youtu.be/Hsm6kwINKLc
• Steady vs unsteady flow: https://youtu.be/-a7EtooUf5U
• Solving material balances on multiple units: https://youtu.be/my1ZTIDSMbs