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.)−202 Synthesiology - English edition Vol.1 No.3 (2009) Ex(N2) = RT0 ln(101.3/76.57) …… (7)Si+(2/3)N2→(1/3)Si3N4 …… (8)Ex(Si) = (-△G0) + Ex(SiO2)-Ex(O2) …… (9)Ex(Si3N4) = 3(△G0)+3Ex(Si)+2Ex(N2) …… (10)Ex(Xi) shows the exergy for material Xi. The section in parenthesis in equation (8) is ratio of total pressure of air and partial pressure of nitrogen. From the above equations, exergy of silicon nitride was calculated to be 1877 kJ/mol. Using similar method, exergies values for major fuels were calculated (Table).3.3 Consumption and efficiency by operation Figure 7 shows the values of exergies that come in and go out throughout manufacturing operation of silicon nitride member and during complete manufacturing. The silicon nitride material is input to this system as artifact made by melting and reducing silicon oxide at a different plant and then reacted with nitrogen. The exergy of material was calculated as 291 MJ for one product. The product begins as starting material in powder form, and is created by mixing, granulation, forming, dewaxing, and sintering. Looking at each operation, large exergy is required for granulation and sintering of 2547 MJ and 776 MJ, respectively, and this is about 80 % of all exergy input. It was found that these were almost entirely emitted outside the system as waste heat, while powder raw material was collected and there was almost no loss between the operations. Exergy fixed in final product was 229 MJ, and this was only 5.5 % of exergy input (4175 MJ). This was an extremely inefficient process where 94.5 % or 3946 MJ was disposed.Figure 8 shows the input-output of exergy by operation in manufacturing using steel member. In case of steel, starting material is iron oxide (Fe2O3) whose exergy is 0 by definition. It was manufactured by effectively utilizing the reaction of material such as reduction, and was found that exergy input and output at all operations was small and equalized. Exergy fixed in product was 126 MJ, which was about 20 % of exergy input (621 MJ), and the amount of exergy input was extremely low, about 1/7 compared to ceramics. Therefore, looking at the system of manufacturing one product, it was confirmed that ceramics consumed significantly higher amount of exergy compared to steel, and efficiency was low.3.4 Exergy analysis at each process 3.4.1 Use①Wear and material disposalWhen steel heater tube is used in molten aluminum, it is corroded by aluminum and worn down by passage of time t. It was assumed that wastage progressed according to the following equation. D=D0･(2-exp(kt)) …… (11)(48)−Intermediate material : 312 MJRaw material : 291 MJElectric power : 300 MJLPG : 2247 MJWaste heat : 267 MJ Input : 4175 MJProduct : 229 MJOutput : 3946 MJSinter-ingDewax-ingFormingGranula-tionMixingIntermediate material : 291 MJIntermediate material : 291 MJIntermediate material : 229 MJElectric power : 98 MJElectric power : 175 MJElectric power : 776 MJElectric power : 267 MJProduct : 229MJWaste heat : 175 MJWaste heat : 776 MJWaste heat : 2464 MJWaste heat : 98 MJWaste : 104 MJWaste : 62 MJIntermediate material : 174 MJRollingDecar-buriza-tionTranspor-tationReduc-tionSinteringRaw material : 0 MJWaste heat : 49 MJ Input : 621 MJProduct : 126 MJOutput : 495 MJWaste heat : 3 MJWaste heat : 94 MJWaste : 28 MJWaste heat : 4 MJWaste heat : 194 MJWaste : 30 MJWaste heat : 50 MJWaste : 43 MJProduct : 126 MJFuel : 49 MJFuel : 211 MJRaw material etc. : 177 MJFuel : 58 MJRaw material etc. : 238 MJPig iron : 164 MJIntermediate material : 170 MJFuel etc.: 49 MJTranspor-tationRaw material and fuelExergyY2O3AI2O3Si3N4N2Fe2O3SiFeAICO2O2PVALPG47 kJ/mol0 kJ/mol1877 kJ/mol7×10-1 kJ/mol0 kJ/mol851 kJ/mol368 kJ/mol788 kJ/mol20 kJ/mol4 kJ/mol49 MJ/kg48 MJ/kgFig. 7 Operations for ceramics parts and exergy balance during manufacture (for one product with weight 19 kg).Fig. 8 Operations for steel parts and exergy balance during manufacture (for one product with weight 19 kg).Table. Calculation of exergy for related main fuel.

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