(778d) Effects of the Different Stages of Superstructure Development On the Efficiencies and Designs of Heat-Integrated Process-Water Networks

In a recent paper by Ahmetović and Kravanja (2012), a novel superstructure and mixed-integer non-linear programming (MINLP) model was proposed for the simultaneous synthesis of process-water and heat exchanger networks. This superstructure combines the water network (WN) given by Ahmetović and Grossmann (2011) and the heat exchanger network (HEN) introduced by Yee et al. (1990). It includes both direct and indirect heat exchanges (isothermal and non-isothermal heat transfers) between hot and cold streams, as well as some additional opportunities for the splitting and mixing of freshwater and wastewater streams. This model’s objective is to minimize the total annual costs of the overall network.

This contribution describes and discusses the usage of the simultaneous optimization model proposed by Ahmetović and Kravanja (2012) for studying its effects, during different stages of superstructure development, on the efficiencies and designs of heat-integrated process-water networks. We first present a base case-study of a water network synthesis problem and reports its freshwater and utilities' consumption. Then, different designs of heat-integrated process-water networks are synthesized and compared from the viewpoint of freshwater and utility consumption, total amount of indirect heat exchanged, and the total eat exchanger area, as well as the total annual cost. In addition, the complexities of the resulting different network designs are also discussed. Using the above-mentioned synthesis methodology, some novel water-network designs have been reproduced for certain literature case-studies with   minimum total annual cost significantly better than that reported in the literature. We used different GAMS (2012) MINLP solvers (i.e. BARON, SBB, DICOPT) for model-solving during various stages of the superstructure's development. It is worth pointing out that only small-size MINLP problems regarding  heat-integrated water-networks could be solved with  global optimality. In other cases we used local solvers SBB and DICOPT to solve  problems over reasonable computational times, and compared the obtained results. The results clearly indicated that, after allowing for additional degrees of freedom for heat transfer, and water-splitting and mixing within the network, better network designs with reduced total annual cost resulted, in comparison with those reported results in the literature.

Keywords:water and heat-exchanger networks, MINLP model, simultaneous superstructure optimization.

Acknowledgement:The authors are grateful to the Scholarship scheme for academic exchange between the EU and Western Balkan countries for providing a JoinEU-SEE postdoctoral fellowship. Financial support is also gratefully acknowledged from the Slovenian Research Agency (Program No. P2-0032), and Federal Ministry of Education and Science Bosnia and Herzegovina.

References:

Ahmetović, E. & Grossmann, I. E. (2011). Global superstructure optimization for the design of integrated process water networks. AIChE Journal 57(2): 434-457.

Ahmetović, E. & Kravanja, Z. (2012). Simultaneous synthesis of process water and heat exchanger networks. Manuscript in the preparation.

GAMS (2012). GAMS solvers. http://www.gams.com/solvers/index.htm.

Yee, T. F., Grossmann, I. E. & Kravanja, Z. (1990). Simultaneous optimization models for heat integration—III. Process and heat exchanger network optimization. Computers and Chemical Engineering 14(11): 1185-1200.