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.