Water Network Optimisation with Consideration of Network Complexity

Mass integration was extended by El-Halwagi and Manousiouthakis (1989) from heat integration (Linnhoff et al., 1982), following the analogy of heat and mass transfer. Water minimisation was then developed by Wang and Smith (1994) as a special case of mass integration. The main driving force for the developments of water minimisation is the awareness on environmental sustainability, which calls for the efficient use of water resources among industrial processes (Sueviriyapan et al., 2014). Insight-based pinch analysis and mathematical optimisation are the two major approaches developed rapidly in the past decades (Foo, 2012). Superstructural approach is one of the commonly-used mathematical optimisation technique for water minimisation. In recent years, some works on mathematical optimisation were reported for pre-treatment system (Ahmetović and Grossmann, 2011), as well as flexible network synthesis (Poplewski, 2014). In this paper, a superstructural model that incorporates different process constraints is proposed to synthesise a water network for the ease of process operation. When water minimisation is implemented for process plants with many water-using processes, it may lead to complex piping system. This may lead to controllability issue, due to the decrease in its degree of freedom. A less complex network is always desired as it will reduce operational and controllability issues of the process plant. Different model-size reduction techniques had been developed for different areas of process integration work. Lam et al. (2011) presented few model-size reduction techniques for large-scale biomass production and supply network. Amidpour and Polley (1997) presented decomposition approaches for heat exchanger network synthesis. Ng et al. (2012) on the other hand, decomposed an integrated heat exchanger network by dividing the integrated structure into two or more clusters. In this work, a superstructural model is developed to enable the synthesis of a less complex water network, which considers piping length and number of piping connections.