Page 51 - Chemical engineering design

CHAPTER 2

Fundamentals of Material Balances

2.1. INTRODUCTION
Material balances are the basis of process design. A material balance taken over the
complete process will determine the quantities of raw materials required and products
produced. Balances over individual process units set the process stream flows and
compositions.
A good understanding of material balance calculations is essential in process design.
In this chapter the fundamentals of the subject are covered, using simple examples to
illustrate each topic. Practice is needed to develop expertise in handling what can often
become very involved calculations. More examples and a more detailed discussion of the
subject can be found in the numerous specialist books written on material and energy
balance computations. Several suitable texts are listed under the heading of “Further
Reading” at the end of this chapter.
The application of material balances to more complex problems is discussed in “Flow-
sheeting”, Chapter 4.
Material balances are also useful tools for the study of plant operation and trouble
shooting. They can be used to check performance against design; to extend the often
limited data available from the plant instrumentation; to check instrument calibrations;
and to locate sources of material loss.

2.2. THE EQUIVALENCE OF MASS AND ENERGY

Einstein showed that mass and energy are equivalent. Energy can be converted into mass,
and mass into energy. They are related by Einstein’s equation:
E D mc 2 2.1
where E D energy, J,
m D mass, kg,
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c D the speed of light in vacuo,3 ð 10 m/s.
The loss of mass associated with the production of energy is significant only in nuclear
reactions. Energy and matter are always considered to be separately conserved in chemical
reactions.

2.3. CONSERVATION OF MASS
The general conservation equation for any process system can be written as:
Material out D Material in C Generation Consumption Accumulation

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