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Basis of Material Balances

In this post, I want to share the basis of material balances. Material balances are the basis of design. The amount of raw materials needed and the amount of product produced will be determined by a material balance applied to the entire process. In each process unit, there will be specific stream flows and compositions.

Process design requires a solid understanding of material balance calculation.

As you may experience as a process engineer, material balances are always key to the process. In almost every project, material balance calculation is always required. It is because equipment design and sizing need material balances. Utility consumption calculation is also generated from material balance information.

Why is it important to have good understanding of material balances?

You will find that material balances are useful tools to:

  • study plant operation and troubleshooting
  • check performance against design
  • check instrument calibration
  • locate sources of material losses
Importance to understand basis of material balances
Importance to understand basis of material balances

Conservation of Mass

The basis of material balances is expressed in conservation of mass equation. The equation can be written as:

Material in + Generated – Consumption – Accumulation = Material Out

For a steady-state process the accumulation will be zero. If a chemical reaction takes place, a particular species may be formed or consumed in the process. If there is no chemical reaction and the process is steady-state, then the material balances can be simplified to:

Material in = Material Out


This is a simple example of conservation of mass.

1000 kg of a 5%-weight slurry of calcium hydroxide in water is to be prepared by diluting a 20% slurry. Calculate the quantities required.


Let the unknown quantities of the 20% slurry and water be A and B, respectively.

We will calculate material balance of calcium hydroxide, water, and total.

Material balance of calcium hydroxide:

Calcium hydroxide in = Calcium hydroxide out

A (20%) = 1000 (5%)

A = 250 kg

Material balance of water

Water in = Water out

A (100%-20%) + B = 1000 (100% – 5%)

250 (80%) + B = 950

B = 950 – 200 = 750 kg

Material balance of total quantity

A + B = 250 + 750 = 1000 kg, the answer is correct.

I hope you find this simple post useful.