(229e) New Insight for the Design of Networks with Heat Exchange, Compression and Expansion

While massive amounts of
research have been performed over the last 40 years to target, design and
optimize Heat Exchanger Networks (HENs), rather limited efforts have been made
to develop systematic tools for the integration of pressure changing equipment
such as compressors, pumps, expanders and valves into HENs. A few recent
publications have studied Work Exchange Networks (WENs), some of these even
with consideration of heat exchange. New insight has been developed in our
research group during the last 10 years with increasing rigor to address the
design of networks with both heat exchange and pressure change.

The integration of compressors
and expanders into heat exchanger networks is guided by the Appropriate
Placement concept from Pinch Analysis in the same way as integration of
reactors, distillation columns, evaporators, heat pumps and heat engines. With
pressure change in some of the process streams, however, the use of this
concept becomes significantly more complicated. This is due to the fact that
the thermodynamic path from supply state (pressure, temperature and the
corresponding phase) to the target state depends on the inlet temperature to
the compressors and expanders. Appropriate Placement thus translates into
finding an optimal inlet temperature to these units.

The initial findings indicated
that compression should be done above the Pinch (reduce heating demand for a
cold stream or increase heat available from a hot stream) and expansion should
be done below the Pinch (reduce cooling demand for a hot stream or increase
cooling available from a cold stream). This is contrary to current industrial
practice where compressors are operated at low (near ambient) temperatures to
reduce work consumption and expanders are operated at high temperatures to
increase work production.

Through detailed studies of
compressors and expanders above and below ambient temperature, it became clear
that additional rules for integration of such equipment were required. The
outlet temperature of pressure changing equipment must also be taken into
account. Emerging insight from a number of case studies has been used to formulate
a set of Theorems for the four cases of compression or expansion above or below
the Pinch. As expected, there is considerable symmetry between these cases. The
new insight supports previous findings that Pinch Expansion or Compression
(inlet temperature equals the Pinch temperature) is optimal in a majority of
cases, however, depending on the outlet temperature from compression and
expansion, the optimum may shift to combined solutions (Pinch
compression/expansion combined with expansion at ambient temperature or
hot/cold utility temperature) and even solutions where Pinch
compression/expansion is not the optimal solution.

The above-mentioned set of
Theorems will be used to present the complete picture for the optimal
integration (i.e. Appropriate Placement) of compressors and expanders into heat
exchanger networks. Since both heat (thermal energy) and power (mechanical
energy) are involved, exergy is used to measure quality and optimality of the
various solutions. With correct integration of compressors and expanders
significant savings are also obtained for energy, not only exergy.

In addition to providing
thermodynamic explanations for the Theorems for Appropriate Placement of
compressors and expanders without going into the mathematical proofs (these are
about to be published as journal papers), illustrative examples will be used to
show the savings in energy and exergy for the correct integration of
compressors and expanders into heat exchanger networks. A design procedure
based on the Theorems will also be presented and illustrated with an industrial
example.