Page 102 - 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 79

FIGURE 4.42 Appropriate placement T*
of distillation columns against the
GCC.

ΔH

When the operating conditions of a column result in placing it
across the Pinch, there are several degrees of freedom that can be
utilized to facilitate appropriate placement. It may be possible to
change the operating pressure, which could shift the column with
respect to the temperature scale until it fits above or below the Pinch.
Varying the reflux ratio results in simultaneous changes of the column
temperature span and the duties of the reboiler and condenser.
Increasing the reflux ratio (the ratio between the reflux flowrate
returned to the column and the distillate product flowrate) yields a
smaller temperature span and larger duties, whereas reducing the
ratio has the opposite effect. It is also possible to split the column into
two parts, introducing a “double effect” distillation arrangement. In
this approach, one of the effects is placed below and the other above
the Pinch; this prevents internal thermal integration of the column
effects.
Additional options are available, such as interreboiling and
intercondensing. When using available degrees of freedom, it is
important to keep in mind that the energy–capital trade-offs of the
column and the main process are combined and thus become more
complicated than the individual trade-offs. Another important issue
is controllability of the integrated designs: unnecessary complications
should be avoided, and disturbance propagation paths should be
discontinued. It is usually enough to integrate the reboiler or the
condenser. If inappropriate column placement cannot be avoided,
then condenser vapor recompression with a heat pump can be used
to heat up the reboiler.
Evaporators constitute another class of thermal separators.
Because they, too, feature a reboiler and a condenser, their operation
is similar to that of distillation columns and so the same integration
principles can be applied to them. Absorber-stripper loops and