Page 140 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 125
The maximum conversion of disordered energy to ordered energy (work) is only
achieved in fully reversible processes.

Exergy is defined as maximum work which can be obtained from a given form of energy
using the environmental parameters as the reference state. (Kotas, 1995).

Exergy is formulated thermodynamically as:
E x =(H±H 0 )±T 0 (S ± S 0 ),
where H is enthalpy, S is entropy, and a zero subscript indicates the reference condition.

Exergy for thermal systems can be written as: E x =Q” T h T 0
T h
Now energy Q can be divided in two terms:
Q = anergy + exergy,
where anergy is that part of the energy Q that cannot be converted into work.
In addition to energy balances being made around thermal processes (which are
based on the conservation law of energy), exergy balances can also be made.
The exergy balance is seen as the law of degradation of energy, this being equivalent
to the loss of the irretrievable loss of exergy due to all real processes being irreversi-
ble (Kotas, 1995).
Exergy loss over a unit is defined as the difference between exergy input and
exergy output or,
E in ±E out =E loss
where any work added or removed to the unit is included in E in and E out .

Exergy efficiency is defined as
E
g = out
E
in
The exergy losses may be of several different types:
. Thermal exergy losses E TH , as in a heat exchanger, where a hot stream heats
a cold stream up to a certain temperature: the smaller the temperature differ-
ence, the less the exergy losses.
. Friction exergy losses due to pressure drop, E DP .
. Heat losses to the environment, which always include an exergy term when a
heat exchanger is considered E HL .
It is these exergy losses which require attention for improvements.
The exergy losses over a counter-flow heat exchanger are (Figure 4.34):

E A1 ±E A2 = DE A
E B4 ±E B3 = DE B
DE A ± DE B =E loss
E loss =E TH +E DP +E HL