توسعه فرایند مورد پایه برای بهبود کارایی مصرف انرژی، برای استفاده در کارخانه تولید خمیر کرافت. بخش ۲: تجزیه تحلیل معیار Base case process development for energy efficiency improvement, application to a Kraft pulping mill. Part II: Benchmarking analysis

۱٫ Introduction An approach to the definition and characterization of the base case model of an operating process has been presented in Part I of this paper. It has been applied to an operating Kraft pulping mill. The model was specifically designed to support an in depth energy analysis of the mill, it has been implemented as a steady state simulation on the CADSIM PLUS® software. It is focused on the steam and water systems. Both utilities are traced rigorously from production (for steam) or preliminary treatment (for water), through their distribution, utilization and post-utilization fate: recovery, reutilization, and eventual reject to the environment. The simulation generates mass balances (water, fiber and total dissolved solids) as well as heat balances on all the major unit operation and for the global process and its principal sectors. Part II of the paper presents a fundamental analysis which must be performed before the development and evaluation of energy enhancing measures is undertaken. This analysis is the process benchmarking. The object of this task is to asses the current energy performance of the process globally and by sector in order to identify areas of inefficiencies and to establish enhancement targets. Benchmarking can also be used to identify where the most likely energy gains can be obtained and to guide engineering efforts. ۲٫ Literature review The pulp and paper (P&P) industry is among the largest industrial consumers of energy and water. Rising energy costs and more stringent environmental regulations have led the industry to refocus its efforts towards identifying ways to improve energy and water conservation. In a typical Kraft process, the larger the amount of water consumed and effluent produced, the larger will be the energy required for heating, cooling and pumping. The evaluation of a process before implementing enhancement measures is often based on a comparison of its efficiency to that of other similar processes by the utilization of performance indicators (Francis et al., 2004). The utilization of performance indicators as a benchmarking tool is common practice to measure the variability and correct the operation of a process (Klatt and Marquardtb, 2009). Francis et al. (2006) proposed indicators that are normalized to the production rate. These indicators include fuel consumption, boilers efficiency and thermal energy consumption of the overall process and of each individual operation. Lang and Gerry (2005) used indicators to monitor control systems by identifying the periods where control loops are out of normal mode or oscillating. Buckbee (2007) defined indicators such as the ratio between the set points and the actual targets achieved. Van Gorp (2005) proposed a methodology where the ratio of the steam consumption of a unit and the final product tonnage were compared to the goals set for the energy reduction projects. A mathematical relation is used to target the potential energy consumption, which is compared to the actual. Retsina (2006) suggested a similar methodology, with the same type of indicators, adding a real-time analysis to identify gaps between target and actual values so as to take measures to maintain the energy efficiency. Sivill et al. (2009) used indicators to link energy efficiency monitoring with business strategy and process integration options. They monitor the changes to the minimum energy requirements as the operation of the process fluctuates. Sivill and Ahtila (2009) relate the production of the paper machine with the energy efficiency and monetary parameters. Retsina (2005) has also developed a software for monitoring the indicators of different processes. However, there are no indicators that reflect the causes of possible inefficiencies such as the equipments maintenance, internal heat recovery or water reutilization. The current performance indicators that monitor energy efficiency quantify the energy utilization without focusing on the quality of the energy used and produced by the process. Exergy is not often used in engineering analysis despite its usefulness to assess the efficiency of energy transfer and conversion operations. It combines in a single function the quality (temperature) and quantity (enthalpy) of the heat content of material streams. Therefore, while energy is preserved in transformation processes by virtue of the first law of thermodynamics, exergy can be destroyed by virtue of the second law. As the thermodynamic efficiency of a process operation increases, less exergy is destroyed; however, ultimate efficiency is only achieved at equilibrium, i.e. for infinitely slow processes which are not practical engineering options. Much work has been devoted to the use of exergy in process design (Kotas, 1985; Szargut et al., 1988; Brodyansky et al., 1994; Sorin and Paris, 1997; Sorin et al., 1998). It has been applied in the P&P industry (Wall, 1988; Asselman et al., 1996; Brown et al., 2005; Gong, 2005; Mateos-Espejel et al., 2007). The destruction of exergy is associated with the irreversible transformation that occurs in the process. Exergy is destroyed in the heat exchangers because of the temperature difference between hot and cold streams or by the adiabatic expansion of steam in a valve. The exergy which is no longer useful or available for the process is considered lost; it encompasses the streams vented or sewered, the flue gases or losses to the environment. Reduction of the exergy destroyed and lost can be accomplished by internal heat recovery, effluents reutilization, cogeneration and energy upgrading. Therefore, exergy can also be used as an indicator of process inefficiencies, although it rarely is. Energy and water efficiencies are typically analyzed individually by the application of Pinch Analysis® and Water Pinch respectively (Noel, 1995; Noel and Boisvert, 1998; Koufos and Retsina, 1999, 2001; Jacob et al., 2002; Wising, 2003; Axelsson and Berntsson, 2005; Axelsson et al., 2006; Lutz, 2008). Pinch Analysis is used to determine the minimum heating and cooling requirements to be supplied by utilities (Linnhoff et al., 1994; Smith, 1995). The core of Pinch Analysis is the display in a temperature vs. enthalpy diagram of all possible heat transfers within a process. It consists of the hot and cold composite curves, which respectively represent the heat availability and demand in the process. Water Pinch is used to determine the minimum water requirements and minimum effluent production. El-Halwagi and Manousiouthakis (1989) have first proposed a method which is a direct extension of thermal pinch based on the analogy between heat and mass exchanges. Shafiei et al. (2003) applied the method to different types of water using operations. Wang and Smith (1994) have proposed a method for networks of washing operations of organic process streams immiscible with water. Dhole (1998) proposed another approach for single phase processes (as the Kraft process), generally water based, where the main streams content of the desired product are enriched by reducing the level of contamination through a succession of operations, such as dilution, displacement and thickening. The basis is Fig. 4 – Overall thermal consumption and thermal energy production. the representation in the purity vs. mass flow rate diagram of the aggregate of all possible mass transfers between water streams. It consists of two composite curves, one for water sources and the other for water sinks, which respectively represent the effluents produced and the water demand in a process. However, these individual analyses ignore the interactions between the water and steam systems and this may result in counter productive measures and increased energy cost (Mateos-Espejel et al., 2008). The development of improvement scenarios with regards to energy and water issues could lead to more attractive projects, because appropriate water reutilization reduces the surface area needed for increasing internal heat recovery (Savulescu et al., 2005). A benchmarking procedure has been developed to evaluate a process as a prerequisite step to an energy enhancement retrofit project. The procedure highlights issues that should be considered in the water and energy data extraction stage. The conventional comparison with current practice is performed. The procedure evaluates energy, water and exergy characteristics of the process. New performance indicators of the internal heat recovery have been developed. These indicators quantify the excess utilization of steam that is reflected in the energy rejected by the process in hot effluents and flue gases. The more energy is rejected in these heat sources the more hot utility will have to be supplied to the process. Furthermore, the excess water utilization is also reflected in the production of effluents. Exergy indicators have been defined to take also into account the quality of the energy produced, supplied to and used by the process and rejected to the environment. A targeting step involves the utilization of the thermal and water composite curves to determine the maximum heat recovery and water reutilization theoretically possible. The final phase of the procedure consists of a synthesis of all results previously obtained. This is a crucial task as the main water and energy efficiency problems of a process are identified and the targets for the posterior development of energy efficiency measures are fixed.