Page 78 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
Energy can exist in many forms and this, to some extent, makes an energy balance
more complex than a material balance. 61
3.3. FORMS OF ENERGY (PER UNIT MASS OF MATERIAL)

3.3.1. Potential energy
Energy due to position:
Potential energy D gz 3.1
where z D height above some arbitrary datum, m,
2
g D gravitational acceleration (9.81 m/s ).

3.3.2. Kinetic energy
Energy due to motion:
u 2
Kinetic energy D 3.2
2
where u D velocity, m/s.

3.3.3. Internal energy

The energy associated with molecular motion. The temperature T of a material is a
measure of its internal energy U:
U D f T 3.3

3.3.4. Work
Work is done when a force acts through a distance:
1

W D F dx 3.4
0
where F D force, N,
x and l D distance, m.
Work done on a system by its surroundings is conventionally taken as negative; work
done by the system on the surroundings as positive.
Where the work arises from a change in pressure or volume:
2
W D P dv 3.5
1
2
where P D pressure, Pa (N/m ),
3
v D volume per unit mass, m /kg.
To integrate this function the relationship between pressure and volume must be known.
In process design an estimate of the work done in compressing or expanding a gas is