(430g) Work and Heat Exchange Networks - from Thermodynamic Insight to Optimization Based Design Procedures

Design of Work and Heat Exchange Networks (WHENs) represents an emerging area of research, and it involves three distinct activities that briefly can be described as: (1) Heat Integration, (2) Work Integration, and (3) Pressure Integration. The first is a classical Process Synthesis problem trying to recover as much heat as possible from hot process streams to heat up cold process streams while selecting an optimal set of hot and cold utilities to provide additional heating and cooling. The second involves the use of work (power) from expansion processes to supply work (power) to compression processes. This work integration can be done on the same shaft (e.g. the use of so-called companders for the extraction of heavier hydrocarbons in natural gas liquefaction plants) or separate shafts where electricity is produced and consumed. The third involves the utilization of additional heating and cooling resulting from changing the pressure of process streams, and this has been the focus of research in our group for the last 10 years [1].

The sparse and relatively recent literature on WHEN design can be grouped into publications on Work Exchange Networks (WENs, e.g. [2]), Work and Heat Exchange Networks (WHENs, e.g. [3]) and Work, Heat and Mass Exchange Networks (e.g. [4]). The extended problem definition for WHEN synthesis that has been used in our research group is:

Given a set of process streams with supply and target states (temperature and pressure), as well as utilities for power, heating and cooling; design a network of heat exchangers, expanders and compressors in such a way that exergy consumption is minimized or exergy production is maximized.

At this stage of methodology development, exergy is used in the objective function since energies of different qualities (thermal and mechanical) are involved. Later, when moving from a thermodynamic approach to an optimization-based approach, a more appropriate objective function involving total annual cost will be used.

The motivation for including compression and expansion processes in the traditional heat recovery problem is that considerable additional energy savings can be obtained. The main issue is to sacrifice modest amounts of mechanical energy to make significant savings in thermal energy, and/or increase work production by efficient utilization of low temperature thermal energy. As such, the research on WHENs has strong relations to heat pumping [5] and refrigeration cycles. It is also closely related to the research on the so-called self-heat recuperation technology [6].

In developing a design methodology for WHENs, the Appropriate Placement concept from Pinch Analysis applied to pressure changing equipment (primarily compressors and expanders) has been a key issue. New challenges compared to traditional Heat Exchanger Network (HEN) design are: (1) The thermodynamic path from supply to target state is unknown (changing both temperature and pressure), (2) Pinch points and utility demand may (often will) change, so that no explicit targeting is possible, and (3) stream identity (hot/cold) may (often will) temporarily change.

A set of 4 theorems have been proposed and proven using thermodynamics and mathematics for expanders above ambient temperature [7], with minor modifications for compressors above ambient as well as expanders and compressors integrated below ambient. Thus, 4 scenarios with a total of 16 theorems are handled with considerable symmetry between the 4 cases. Simultaneous compression and expansion has also been handled with the introduction of a new theorem that deals with the integration sequence for compression and expansion [8]. In line with earlier findings [9], Pinch Expansion and Compression (i.e. expander and compressor inlet temperature should be equal to the Pinch temperature) will often result in significant exergy savings.

An early version of a design procedure for WHENs, based on the Appropriate Placement concept with detailed guidelines from the above mentioned theorems, uses the Grand Composite Curve (GCC) to quantify the maximum amount of heating/cooling that can be accommodated from compression/expansion. The procedure is time consuming even for very small cases and a number of simplifying assumptions. In addition, using the GCC with its modified temperatures ignored the different features for hot and cold streams as well as the relative position of supply and target temperatures to the pinch temperature. As a result, minor (in most cases) errors in estimation of utility consumption were introduced.

To overcome problems related to prohibitive time consumption and inaccuracies in utility estimation due to ignoring the identity of streams (hot/cold), work has been initiated to develop new superstructures with corresponding mathematical models of varying complexity (from LP to MINLP) that can be used in an optimization framework [10]. In fact, early applications of such optimization models have revealed new insight on the WHEN synthesis problem. One example is that Pinch Compression and Expansion should start at the Pinch temperature (hot or cold) that corresponds to the identity (hot or cold) of the stream segment prior to compression or expansion, and not to the original identity of the stream. Two classes of optimization models were developed, i.e. with and without utilizing the insight provided by the theorems. The latter class was used as a final validation of the theorems, while the former class offered advantages in model complexity and thus reduced time to solve the optimization problem.

In conclusion, based on several case applications using the original thermodynamic approach as well as the optimization framework under development, the original hypothesis that Pinch Compression and Expansion represent optimal placement of pressure changing equipment is valid fully or partly in the majority of cases. However, there are cases where expansion/compression at hot/cold utility temperature or ambient temperature provides better solutions. No other temperatures result in WHENs with lower exergy consumption (or higher exergy production).

Preliminary studies on industrial applications such as CO₂ capture processes [11] show that considerable energy savings can be achieved following the new design procedures. An interesting feature of this research is that Exergy Analysis has been moved from post-design evaluation to conceptual stage design.

References

[1] A. Aspelund, D.O. Berstad and T. Gundersen, An Extended Pinch Analysis and Design Procedure utilizing Pressure based Exergy for Subambient Cooling, Applied Thermal Engineering, 27, 2633-2649, 2007.

[2] M.S. Razib, M.M.F. Hasan and I.A. Karimi, Preliminary Synthesis of Work Exchange Networks, Comput. & Chem. Engng., 36, 262-277, 2012.

[3] V.C. Onishi, M.A.S.S. Ravagnani and J.A. Caballero, Simultaneous Synthesis of Heat Exchanger Networks with Pressure Recovery, AIChE Journal, 60, 893-908, 2014.

[4] R. Dong, Y. Yu and Z. Zhang, Simultaneous Optimization of integrated Heat, Mass and Pressure Exchange Network using Exergoeconomic Method, Applied Energy, 136, 1098-1109, 2014.

[5] C. Fu and T. Gundersen, A Novel Sensible Heat Pump Scheme for Industrial Heat Recovery, Ind. Eng. Chem. Res., 55, 967-977, 2016.

[6] Y. Kansha, N. Tsuru, K. Sato and A. Tsutsumi, A Self-Heat Recuperation Technology for Energy Saving in Chemical Processes, Ind. Eng. Chem. Res., 48, 7682-7686, 2009.

[7] C. Fu and T. Gundersen, Integrating Expanders into Heat Exchanger Networks above Ambient Temperature, AIChE Journal, 61, 3404-3422, 2015.

[8] C. Fu and T. Gundersen, Correct Integration of Compressors and Expanders in above Ambient Heat Exchanger Networks, Energy, in review, 2016.

[9] T. Gundersen, D.O. Berstad and A. Aspelund, Extending Pinch Analysis and Process Integration into Pressure and Fluid Phase Considerations, Chem. Engng. Trans., 18, 33-38, 2009.

[10] C. Fu, P. Maurstad Uv, B. Nygreen and T. Gundersen, Compression and Expansion at the right Pinch Temperature, Chem. Engng. Trans., 51, 2016.

[11] C. Fu and T. Gundersen, Heat and Work Integration: Fundamental Insights and Applications to Carbon Dioxide Capture Processes, Energy Conversion and Management, 2016, DOI: 10.1016/j.enconman.2016.04.108.