The Study Of Water And Energy System Integration With Multiple Contaminants

Water and energy resources play extremely significant roles in the developments of human society and economic activity. Due to the increasing shortage of water and energy, minimizing water and energy consumptions has become a research focus recently. Being one of the most efficient technologies for saving freshwater and energy, system integration is widely used in process integration and design optimization. This dissertation presents a few new methods to design water networks and heat integrated water networks. The main contents are as follows:In Chapter 1, an introduction is made to the research background, significance and methods for water and energy system integration.In Chapter 2, this dissertation presents a new design method for regeneration recycling water networks with internal water mains. For the regeneration water networks with the model of fixed removal ratios, the initial regenerated concentrations are estimated because they are unknown before detailed design. Then, the water-using processes are divided into three parts based on their limiting flowrates and the concentration potential concepts. The precedence order of the processes is determined by the values of the Concentration Potential of the Demands(CPDs). The formation of the internal water mains is determined by the values of the Concentration Potential of the Sources(CPSs). An initial network with one regeneration unit is developed based on the concentration potential values. The final design can be obtained in a few iterations by adjusting the initial network. The results of the illustrated examples show that the designs obtained in this work are comparable to that obtained in the literature with computer programming methods. It is shown that the proposed method is simple and effective. The significance of the work presented is that all the design steps, including the formation and adjustment of the internal water mains, are orientated with clear engineering meanings.In Chapter 3, this dissertation proposes a new design method for water and heat networks with single contaminant. The temperature of freshwater will be determined first before detailed design of water and heat networks. Then, the water networks will be designed based on the concentration and temperature of processes:(i) determine the performing order of processes based on their limiting inlet and outlet concentrations;(ii) satisfy the demand stream by the source streams in ascending order of their concentrations. For the sources with the same concentration, the temperature will be taken as the main factor to be considered;(iii) perform all the processes in sequence until all the processes are performed. The thermal information of water streams in the water networks obtained will be used to design the corresponding heat exchanger networks. The results of the illustrated examples show that the network structure obtained by using the proposed method is simple.In Chapter 4, this dissertation presents a two-step approach for the water and heat networks with multiple contaminants. The water-using network is designed first by considering both the concentration and temperature specifications of water-using processes. Then the heat exchanger network is designed based on the water network obtained in the first step. Two design strategies are adopted in the design of heat exchanger networks:(i) self circulation heating will be considered for the processes with high temperature. That means the outlet stream of a process will be used as the heat source for its inlet stream. The number of connections can be reduced due to the self heating of processes, and this will simplify the network structure. However, more heat exchangers may be required. To reduce the number of heat exchangers, only high-temperature processes will be considered for self heating in this dissertation;(ii) to reduce the energy degradation caused by non-isothermal mixing, the indirect heat transfer between cold and hot streams will be carried out before mixing. A few matching rules are proposed to improve the energy-usage efficiency and reduce the utility consumptions. The results of the illustrated examples show that the designs obtained in this work are simple.In Chapter 5, the processes in the water and heat networks are highly integrated, and this will make the network difficult to operate and control. To overcome the aforementioned problems, this dissertation presents a new method to increase the flexibility of the network. The key processes and streams of the system are determined first based on the temperature and the freshwater usage of processes. Then, the concept of the impact coefficient of key stream is proposed to identify the normal operation temperature interval of the key stream. The improvement of the normal operation temperature interval is investigated. The results of the illustrated examples show that the designs obtained in this work can effectively reduce the utility consumptions and improve the flexibility of the system.In Chapter 6, conclusion.