طراحی سیستمهای انرژی استفاده از آب موثر برای امکان تقسیم عملیات

This article deals with the design of energy efficient water utilization systems allowing operation split. Practical features such as operating flexibility and capital cost have made the number of sub operations an important parameter of the problem. By treating the direct and indirect heat transfers separately, target freshwater and energy consumption as well as the operation split conditions are first obtained. Subsequently, a mixed integer non-linear programming (MINLP) model is established for the design of water network and the heat exchanger network (HEN). The proposed systematic approach is limited to a single contaminant. Example from literature is used to illustrate the applicability of the approach.

Energy and water are important resources to process industries. Since the modem society faces energy shortage and water scarcity problems, considerable attention has been paid to reduce energy and water consumption. For water using operations such as absorption, extraction, and washing, it is required to operate at particular temperature levels. The water streams need to be heated up or cooled down to satisfy the temperature requirements of the operations. Under such circumstances, energy consumption should be considered in the design of water utilization systems. During the past decade, considerable design techniques have been developed to minimize the energy and water consumption simultaneously. Conceptual design tools such as two dimensional grid diagram [ 1-31, separate system [ 1-31, source-demand energy composite curve [4], and graphical thermodynamic rule [5] have been introduced to design the water network and the corresponding heat exchanger network (HEN). Mathematical programming techniques have also been used for the problem. Bagajewicz et al. [6] proposed two sequential linear programming (LP) problems to determine the minimum water usage and energy consumption targets. Once these targets are identified, a mixed integer linear programming (MILP) model is generated to obtain the detailed network. Zheng 171 considered the multi-contaminant situation and obtained different water networks under different economic targets. Other researchers [8-11] treated the water network and the HEN sequentially. Since the water network obtained in the first step was not unique, the resulting HEN may not be optimal. Du et al. 1121 introduced genetic algorithm and simulated annealing algorithm to solve the problem simultaneously. Leewongtanawit et al. [13, 141 developed a mixed integer non-linear programming (MINLP) model that relies on the combining of water superstructure and HEN superstructure to provide an overall network design. Recently, Liao et af. [15] introduced a modified transshipment model that treats direct and indirect heat transfers separately. Kuo and Smith [16] first addressed that for particular processes such as extracting and washing, the operations are allowed to split, as seen in Fig. 1. They applied these split options to further reduce the freshwater usage. Later, Xu et al. [I71 developed a mathematical programming model, which allows operation split for the water network with regeneration reuse. When the energy aspect is taken into account, the split of operations may further reduce the energy consumption. Consider now a two operation example of Table 1 to see how the operating cost can be decreased by operation split. Given that the water and energy cost are specified at 1.5 RMB Yuan.t-' and 38 YuamGJ-', the approach of Liao et al. [15] is applied to ths problem. The resulting minimum utility cost is 1114.6 RMB Yuan-h - ', with the corresponding network shown in Fig. 2. If operation 1 is allowed to split at the concentration of 83.67 pg.g-', then a network with 999.6 RMB Yuan-h-' utility cost can be obtained as shown in Fig. 3. On the other hand, the split of operations may cause additional equipment cost. Therefore, the tradeoffs between capital cost increasing and operating cost reducing should be evaluated. +Q operation split +-q-+... -++z+ Figure 1 Split of original operation The designer now faces the question of whether a better network may be possible using a different design strategy. What is really the correct utility target?How many number of splits are required to achieve this? It should be noted that when operation split is allowed in the water network design, the design variables will increase rapidly. For example, if the maximum number of N is permitted to split in one operation, the total number of streams increases from N& to (NMW2w, here, NM is the number of operations. In this article, an effective two step procedure is introduced to deal with such a problem. In the first step, a rigorous determination of the targets including utility consumption and the number of the operation splits is obtained. Once these targets are identified, the detailed network design is carried out in the second step by an MINLP formulation.water-stream interconnections among the processes and to design a network of heat exchangers between these streams. The objective is the simultaneous minimization of the freshwater usage and the energy consumption of the whole system. Several assumptions are specified: 0 all operations operate isothermally; 0 the contaminant load is fixed and is independent of the flow rate; 0 the operations are allowed to split by concentration; 0 the water flow rate does not change through an operation; 0 single contaminant is permitted.water-stream interconnections among the processes and to design a network of heat exchangers between these streams. The objective is the simultaneous minimization of the freshwater usage and the energy consumption of the whole system. Several assumptions are specified: 0 all operations operate isothermally; 0 the contaminant load is fixed and is independent of the flow rate; 0 the operations are allowed to split by concentration; 0 the water flow rate does not change through an operation; 0 single contaminant is permitted.

In the design of energy efficient water utilization systems, the split of water using operations should be considered to reduce the total utility cost. The synthesis of networks allowing split is now carried out in two stages. The utility and split targets are first obtained. Then, the detailed network design is carried out based on the MINLP model. Although this strategy may lead to local optimum solution, the design procedure of the example shows fairly low complexity, which is favorable in the industry.