روش واحد برای بهبود کارایی مصرف انرژی حرارتی: کاربردی برای فرایند کرافت Unified methodology for thermal energy efficiency improvement: Application to Kraft process

۱٫ Introduction The enhancement of the energy efficiency of processes is of paramount importance for the energy intensive chemical industries that still rely on expensive fossil fuels. The globalization of markets has led the industries to reduce all costs, including that of energy, without reducing the quality of the end product. The implementation of energy reduction programs is essential to enable manufacturing industries in industrially mature countries to remain competitive. Increased awareness of the importance of energy efficiency in the context of environmental sustainability has led governments to implement programs and incentives in support of energy enhancement in the manufacturing sector and other areas. The industry has responded by developing methodologies and technologies aimed at improving the chemical processes. These methodologies tend to focus on specific enhancing techniques and on specific process sections without regard to the interactions between the utility systems, water and steam, and unit operations. The application of individual techniques will indeed lead to the decrease of energy consumption of a plant but they will not accomplish the full reduction potential. A preliminary step to the process analysis, often overlooked, is the definition of the base-case. It consists of developing the process and utility systems diagrams and, the overall steam and water balances. The construction of a computer simulation, representative of the complete process, is fundamental for any energy study (Paris, 2000). The simulation is the principal source of data and also an instrument to assess the impacts of possible process modifications. The evaluation of the process sections or units is achieved by benchmarking (Towers and Turner, 1998; Francis et al., 2006). Benchmarking typically consists of comparing a given process with the industrial practices. However, the results of this analysis do not provide information on the means to improve the process. Paris (2000) proposed a systematic approach for implementing water reutilization measures that can also be extended to the improvement of the energy efficiency. This approach consists of three sequential steps: good housekeeping, good engineering practice and advanced energy optimization. Measures to improve pipes isolation, stop leaks and take other simple measures should be implemented first. The optimization of the operating conditions and the control systems should be performed before applying more complex energy enhancing techniques. The energy enhancing techniques most frequently utilized are internal heat recovery, water reutilization and energy conversion and upgrading. To this end, milestone methodologies such as Pinch analysiss, Water Pinch and Exergy Analysis have been developed. The objective of Pinch analysiss (Linnhoff et al., 1982) is to increase process to process heat exchanges by the design of an optimum retrofit heat exchanger network (HEN). It was first applied with success to complex industrial sites such as petroleum refineries. It can incorporate complementary techniques such as energy conversion and upgrading. Water Pinch (El-Halwagi and Manousiouthakis, 1989; Wang and Smith, 1994; Dhole, 1998) is an extension of Pinch Analysiss but in a water reutilization perspective. Its objective is to maximize the reutilization of water streams within the process or to determine appropriate water regeneration measures. Exergy is a measure of both the quality and quantity of the energy involved in transformations within a system and the transfers across its boundary. The internal destruction of exergy is associated with the irreversible transformations that occur in the system; examples relevant to this work are the destruction of exergy caused by heat transfer in the heat exchangers and by the adiabatic expansion of a steam in a valve. Therefore, the exergy is an indicator of the inefficiencies of a process. Exergy analysis is an approach based on the principle of exergy destruction and exergetic efficiency used to analyze the performance of certain operations or identify the bottlenecks of a process (Kotas, 1985; Szargut and Morris, 1988). The objective is to propose enhancement measures that reduce the exergy destroyed by the way the process operations are performed. Exergetic efficiency is a term that can have several interpretations (Brodyansky et al., 1994) and that may lead to different results. Moreover, the concept of exergy might seem unrelated to the engineering practice when compared to other more familiar quantities such as enthalpy and concentration. In fact thermal pinch, water pinch and exergy analysis complement each other and should all be part of a unified methodology for the improvement of the energy efficiency. They have been combined to analyze specific sections of a process. Staine and Favrat (1996) proposed the utilization of exergy composite curves for the energy optimization of a process in the context of a life cycle analysis. In this representation, the exergy destroyed in a heat exchange is graphically displayed as a function of the temperature approach; it can thus be used to determine a realistic efficiency compatible with heat exchange conditions. This method is also used to quantify the exergy destroyed in the production and utilization of steam as shown by Brown et al. (2005) and Mateos-Espejel et al. (2007). Sorin and Paris (1999) integrated exergy and pinch analysis for improving the operating conditions of a process and the retrofit of the HEN. They introduced the concept of transit exergy (Sorin et al., 1998) in the computation of the exergetic efficiency. Marechal and Favrat (2005) demonstrated the combined used of exergy analysis and process integration techniques to analyze the implementation of utility systems in industrial processes. Linnhoff and Alanis (1991) applied Pinch Analysiss and Gaggioli et al. (1991) exergy analysis separately to the same case study. Their results only highlight the domains of action for each methodology. Exergy analysis identifies the operations with poor performances and Pinch Analysiss the heat transfer inefficiencies or lack of internal heat recovery. The interactions between water and energy have also been studied. Savulescu et al. (2005) proposed a method to reduce water and steam consumption in water networks by the utilization of a two-dimensional grid diagram. This diagram incorporates the temperature and contaminants concentration in order to develop complementary measures for internal heat recovery and water reutilization. Leewongtanawit and Kim (2008) extended the method proposed by Savulescu et al. (2005) to consider mathematical optimization and multiple contaminants. Schaareman et al. (2000) applied Pinch Analysiss and Water Pinch in sequence but without analyzing the impacts of water reutilization strategies on the thermal side of the process. Savulescu and Alva-Argaez (2008) proposed a methodology for the appropriate utilization of non-isothermal mixing used for direct heat recovery, to reduce the steam demand in water and steam systems. In regard to exergy and water studies, Asselman et al. (1996) employed the exergetic efficiency, to choose the appropriate equipment for water regeneration. Energy upgrading is performed by external devices such as absorption heat pumps. Absorption heat pumps is an energy technology that can upgrade low temperature heat by exploiting the effect of pressure on an absorption–desorption cycle (Ziegler and Riesch, 1993). They can reduce the heating and cooling requirements if their positioning into the process respects the pinch rules (Bakhtiari et al., 2007). Absorption heat pumps have been used in combination with pinch analysis (Marinova et al., 2007; Bakhtiari et al., 2009) to increase the energy savings achieved by an energy reduction program. A unified methodology for improving the energy efficiency of a process in a global process perspective has been developed and is presented below. The definition and characterization, and the benchmarking analysis of the process base-case are done in a steam and water perspective. Several energy enhancing techniques are combined and their effects on the process systems identified and taken into account. Energy, water and exergy indicators are used to asses cumulative enhancements. An implementation strategy considering technical and economic constraints is finally proposed. The methodology is illustrated by a case study based on an operating Kraft pulping process situated in Eastern Canada, which is characterized by strong interactions between the steam and water systems.