Page 77 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition
P. 77
CHAPTER 3
Fundamentals of Energy Balances
(and Energy Utilisation)
3.1. INTRODUCTION
As with mass, energy can be considered to be separately conserved in all but nuclear
processes.
The conservation of energy, however, differs from that of mass in that energy can be
generated (or consumed) in a chemical process. Material can change form, new molecular
species can be formed by chemical reaction, but the total mass flow into a process unit
must be equal to the flow out at the steady state. The same is not true of energy. The
total enthalpy of the outlet streams will not equal that of the inlet streams if energy is
generated or consumed in the processes; such as that due to heat of reaction.
Energy can exist in several forms: heat, mechanical energy, electrical energy, and it is
the total energy that is conserved.
In process design, energy balances are made to determine the energy requirements of
the process: the heating, cooling and power required. In plant operation, an energy balance
(energy audit) on the plant will show the pattern of energy usage, and suggest areas for
conservation and savings.
In this chapter the fundamentals of energy balances are reviewed briefly, and examples
given to illustrate the use of energy balances in process design. The methods used for
energy recovery and conservation are also discussed.
More detailed accounts of the principles and applications of energy balances are given
in the texts covering material and energy balance calculations which are cited at the end
of Chapter 2.
3.2. CONSERVATION OF ENERGY
As for material (Section 2.3), a general equation can be written for the conservation of
energy:
Energy out D Energy in C generation consumption accumulation
This is a statement of the first law of thermodynamics.
An energy balance can be written for any process step.
Chemical reaction will evolve energy (exothermic) or consume energy (endothermic).
For steady-state processes the accumulation of both mass and energy will be zero.
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