Page 80 - Sustainability in the Process Industry Integration and Optimization

P r o c e s s I n t e g r a t i o n f o r I m p r ov i n g E n e r g y E f f i c i e n c y 57

MPR-oriented authors employ the equivalent “transshipment” model
(Cerdá et al., 1990). The steps are as follows:

1. Shift the process stream temperatures.
2. Set up temperature intervals.
3. Calculate interval heat balances.
4. Assuming zero hot utility, cascade the balances as heat
flows.
5. Ensure positive heat flows by increasing the hot utility as
needed.
The algorithm will be illustrated using the sample data in Table 4.2.
Step 1
Because the PTA uses temperature intervals, it is necessary to set up
a unified temperature scale for the calculations. If the real stream
temperatures are used, then some of the heat content would be left
out of the recovery. The problem is avoided by obtaining shifted
stream temperatures (T*) for PTA calculations. The hot streams are
shifted to be colder by ΔT /2 and the cold streams are shifted to be
min
hotter by ΔT /2. If the shifted temperatures (T*) of a cold and a hot
min
stream (or their parts) are the same, then their real temperatures are
still actually ΔT apart, which allows for feasible heat transfer. This
min
operation is equivalent to shifting the Composite Curves toward
each other vertically, as illustrated in Figure 4.13. The last two
columns in Table 4.2 show the shifted process stream temperatures.
Step 2
Temperature intervals are formed by listing all shifted process
stream temperatures in descending order (any duplicate values are

Shifter Composite Curves
T*
T*
T
Temperature − 2 min Hot Cold + ΔT 2 min
T
ΔT

ΔH

FIGURE 4.13 Temperature shifting to ensure feasible heat transfer.