Page 80 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

63
FUNDAMENTALS OF ENERGY BALANCES
for heat leaving the system. W is the work done by the system; positive for work going
from the system to the surroundings, and negative for work entering the system from the
surroundings.
Equation 3.6 is a general equation for steady-state systems with flow.
In chemical processes, the kinetic and potential energy terms are usually small compared
with the heat and work terms, and can normally be neglected.
It is convenient, and useful, to take the terms U and Pv together; defining the term
enthalpy, usual symbol H,as:
H D U C Pv
Enthalpy is a function of temperature and pressure. Values for the more common
substances have been determined experimentally and are given in the various handbooks
(see Chapter 8).
Enthalpy can be calculated from specific and latent heat data; see Section 3.5.
If the kinetic and potential energy terms are neglected equation 3.6 simplifies to:
H 2 H 1 D Q W 3.7

This simplified equation is usually sufficient for estimating the heating and cooling require-
ments of the various unit operations involved in chemical processes.
As the flow-dependent terms have been dropped, the simplified equation is applicable
to both static (non-flow) systems and flow systems. It can be used to estimate the energy
requirement for batch processes.
For many processes the work term will be zero, or negligibly small, and equation 3.7
reduces to the simple heat balance equation:
3.8
Q D H 2 H 1
Where heat is generated in the system; for example, in a chemical reactor:
3.9
Q D Q p C Q s
Q s D heat generated in the system. If heat is evolved (exothermic processes) Q s is taken
as positive, and if heat is absorbed (endothermic processes) it is taken as negative.
Q p D process heat added to the system to maintain required system temperature.
Hence:
3.10
Q p D H 2 H 1 Q s
H 1 D enthalpy of the inlet stream,
H 2 D enthalpy of the outlet stream.

Example 3.1
Balance with no chemical reaction. Estimate the steam and the cooling water required for
the distillation column shown in the figure.
2
Steam is available at 25 psig (274 kN/m abs), dry saturated.
Ž
The rise in cooling water temperature is limited to 30 C.
Column operates at 1 bar.