Appropriate Placement of Compressors and Expanders

The term “Appropriate Placement” was introduced in Pinch Analysis during the 1980s to indicate how special unit operations should be correctly integrated with the background process to ensure maximum energy savings. Pinch Analysis has provided guidelines for the design of heat exchanger networks as well as the Appropriate Placement of distillation columns, evaporators, heat engines and heat pumps. These guidelines are based on insight about the process Pinch as a bottleneck for heat recovery, and a fundamental decomposition effect by temperature level into a heat deficit region above Pinch and a heat surplus region below Pinch.

Glaviè and co-workers while considering the Appropriate Placement of chemical reactors also made statements about operation of compressors, however, without completing the discussion and without developing a methodology based on insight from thermodynamics. This paper will conclude on this issue and provide very accurate guidelines for the correct integration of compressors and expanders, such as optimal inlet temperatures.

The discussion in the paper assumes that streams subject to heat integration also may undergo pressure changes. In subambient processes, even streams with the same supply and target pressure may be utilized as utility streams by a sequence of compression and expansion to provide refrigeration for the process.

Established design guidelines for rotating machinery suggest to operate compressors at low temperature (possibly with inter-stage cooling) to reduce work consumption and to operate expanders at high temperature (possibly with inter-stage re-heating) to increase work production. Based on insight developed in this paper that in turn relates to principles from Pinch Analysis, design guidelines are presented that are the opposite of the established ones. Compressors provide additional heating to the heat recovery system and should be placed above the Heat Recovery Pinch. Likewise, expanders provide additional cooling and should be placed below Pinch.

The extended problem definition (vs. the classical heat recovery problem) for systems of process streams that may undergo changes in both temperature and pressure is the following:

Given a set of process streams with a supply state (temperature, pressure, and the resulting phase) and a target state, as well as utilities for power, heating and cooling; design a system of heat exchangers, expanders, pumps and compressors in such a way that an objective function is minimized.

The objective function could be based on economics (such as minimum Total Annual Cost) or thermodynamics (such as minimum irreversibilities or exergy losses). The operating cost is related to consumed and produced shaftwork and external heating and cooling (both at different temperature levels). An evaluation of or comparison between design alternatives will have to deal with both mechanical and thermal utility systems, and the obvious solution is to use a parameter that measures the quality of different energy forms. Thus there is a motivation for using exergy, and in this paper a newly developed Exergetic Transfer Effectiveness (ETE) is used to measure exergy efficiency of the design alternatives.

As indicated above, compression and expansion are used deliberately in below ambient processes to provide refrigeration as cold utility. For these types of processes, and in developing the ETE, the true exergy sources and sinks of a process were identified through a proper classification of exergy types and the decomposition of these exergy forms into exergy components.

The paper will use a case study to conclude its discussion by identifying the optimal inlet temperature for compressors and expanders that are part of a heat recovery system. It should be no surprise that these optimal inlet conditions are the hot or cold Pinch temperature depending on whether the actual process stream that is subject to pressure change is a hot (to be cooled and/or condensed) or a cold (to be heated and/or evaporated) stream. In addition, the paper will discuss the importance of the Heat Recovery Pinch concept when process streams not only change temperature but also pressure. With changes in both these state variables, the path from supply to target state is not fixed and will be determined as part of the search for a system that satisfies the objective of the design exercise.