Vol.1 No.3 2009

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Research paper : A rationalization guideline for the utilization of energy and resources considering total manufacturing processes (H. Kita et al.)−204 Synthesiology - English edition Vol.1 No.3 (2009) (50)−・Steel: 126 (MJ/ tube) × 14 (tubes) = 1764 MJ・Silicon nitride: 229 (MJ/ tube) × 1 (tubes) = 229 MJLooking over the entire process, the exergy inputs for steel and silicon nitride were 130999 GJ and 130637 GJ respectively, while exergy outputs were 5417 GJ and 5055 GJ. Using silicon nitride reduced 362 GJ of input and output exergy compared to steel.From the above results, it was shown that although one silicon nitride tube required 7 times more exergy in manufacturing process, frequency of replacement decreased due to its high conservative property, which allowed furnace with highly efficient structure that reduced electricity consumption, and therefore, exergy consumption level was smaller compared to steel in total throughout the lifecycle of manufacture, use, and disposal.3.5 Rationalization consideration Assuming current system, we proposed guideline for rationalization for using ceramics or steel. Then we summarized the current state and direction of rationalization of casting system.3.5.1 Steel memberHighly economical steel member is mainstream of heater tube. When use of steel member is assumed, development of material or coating technology to prevent corrosion by molten aluminum is necessary to increase lifespan. Also, steel has excellent recyclability, and it is important to increase recycling efficiency.3.5.2 Ceramics memberTo promote rationalization of ceramics manufacture, as mentioned above, increased efficiency of granulation and sintering operations that have particularly high consumption among all operations is mandatory. This improvement is highly significant to reduce environmental impact and to counter steel members that are highly economical.①GranulationWhile metal process such as in steel involves melting raw material at high temperature and mixing and reaction by dispersal ability of liquid medium, ceramics use solid powder without dispersal ability in gravitational field. Therefore, it is a process with inherent inefficiency, where water and binder that do not remain in the final product are added between solid particles for mixing, and energy is required to remove them.When water is introduced, distance between particles can be reduced and mixing becomes easy, but large amount of latent heat must be consumed to evaporate the water in the slurry in the post-operation dry granulation. The input energy is transferred to water as heat, evaporates, and released outside the system along with entropy as steam. Therefore, to reduce exergy consumption during granulation, although reduction of water including selection of deflocculant and adjustment of granularity are necessary, the time required for mixing in preliminary operation will increase. To reduce exergy consumption in the granulation operation, optimization of water content while considering effect on preliminary and following operations is necessary.LPG is used as fuel in granulation. When LPG is used, input exergy will produce water and carbon dioxide unavoidably during combustion, other than in drying of slurry and granulation, and exergy is consumed for releasing them from the system. If electricity is used instead of LPG, input exergy may seem to be reduced. In this case, exergy consumption in plant may be reduced, but exergy is actually consumed outside (at a power plant). This time, LPG was used in the granulation operation since cost was prioritized.②SinteringFigure 11 is a conceptual diagram that summarizes the relationships of material, product, and input exergy for rationalization. It is assumed that reference material (exergy = 0) and material have chemical exergy and exergy derived from surface energy, and material and product (sintered body) have difference in exergies derived from surface and interface, as well as arrangement. Stable material with highly covalent silicon nitride requires large amount of exergy in running the furnace and heating the refractory materials, in addition to exergy equivalent to the barrier of activation energy. These unavoidably become waste exergy, and waste heat recovery must be considered.To reduce exergy consumption, input and output can be lessened by using low exergy material and by utilizing the energy of the material. Chemical exergy of silicon nitride is high at 1877 kJ/mol. Moreover silicon nitride particles undergo separate operations for nitrification of silicon and sintering of silicon nitride obtained, and each operation produces waste heat.On the other hand, chemical exergy of silicon is calculated to be 851 kJ/mol, or about half of silicon nitride. To reduce exergy consumption, it is effective to shift from silicon nitride powder to silicon, powder and to conduct nitriding and sintering in one operation. Although this process is known as post sintering involving reaction-bonding, it is not widely done because control of heating in nitrification process is difficult and mixing using water medium is difficult since silicon itself is active. In the future, to make the process practical, it is necessary to develop a catalyst that allows mixing with water medium in short time and allows nitrification at low temperature using rough silicon particles. Also, to increase efficiency, one way is to increase sintering temperature by increasing size of drying and sintering

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