Environ. Sci. Technol. 2007, 41, 4454-4457
Process for Recycling Waste Aluminum with Generation of High-Pressure Hydrogen TAKEHITO HIRAKI,† SATORU YAMAUCHI,‡ MASAYASU IIDA,‡ HIROSHI UESUGI,§ AND TOMOHIRO AKIYAMA† Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan, ITEC Co., Ltd., Kannanbechou 4 132, Sakai 590-0984, Japan, and Waseda University, Wasedatsurumaki-cho 513, Shinjuku-ku, Tokyo 162-0041, Japan
conforms to the Arrhenius equation with an activation energy of 69 kJ/mol (5). Here, the beaker experiments were carefully performed under controlled isothermal conditions. Further, the coproduction system was quantitatively evaluated on the basis of Life Cycle Assessment (LCA) of carbon dioxide emission and the energy requirement under the assumptions that hydrogen and aluminum hydroxide were produced only at normal pressure. The LCA results clarified that the proposed system had only 4% carbon dioxide emissions and 2% energy requirement when compared to the conventional system (5). In contrast, a thermodynamic consideration reveals that the following major reaction in the system can proceed extremely rapidly to form products as on the righthand side due to a large equilibrium constant (6-9).
Al + OH- + 3H2O f 1.5H2 + Al(OH)4-
(1)
∆H ) - 415.6 kJ, ∆G ) - 437.1 kJ, and Ka ) 3.78 ×
An innovative environmentlly friendly hydrolysis process for recycling waste aluminum with the generation of highpressure hydrogen has been proposed and experimentally validated. The effect of the concentration of sodium hydroxide solution on hydrogen generation rate was the main focus of the study. In the experiments, distilled water and aluminum powder were placed in the pressure-resistance reactor made of Hastelloy, and was compressed to a desired constant water pressure using a liquid pump. The sodium hydroxide solution was supplied by liquid pump with different concentrations (from 1.0 to 5.0 mol/dm3) at a constant flow rate into the reactor by replacing the distilled water, and the rate of hydrogen generated was measured simultaneously. The liquid temperature in the reactor increased due to the exothermic reaction given by Al + OH- + 3H2O ) 1.5H2 + Al(OH)4- + 415.6 kJ. Therefore, a highpressure hydrogen was generated at room temperature by mixing waste aluminum and sodium hydroxide solution. As the hydrogen compressor used in this process consumes less energy than the conventional one, the generation of hydrogen having a pressure of almost 30 MPa was experimentally validated together with Al(OH)3, a useful byproduct.
Introduction According to the database of aluminum supply and demand in Japan, the amount of missed and landfilled aluminum is as high as 0.77 Mt (1-3). The metallic aluminum present in waste aluminum, such as cutting chips, has high purity and shows a high potential with a large chemical exergy of 788.61 kJ/g (4). However, the recycling of aluminum chips is still insufficient due to the technical difficulties involved in handling very fine chips. The coproduction of hydrogen and aluminum hydroxide from waste aluminum is fairly attractive from the viewpoint of reproduction of latent material energy. In a previous paper, we demonstrated that aluminum powder can react with water at normal pressure to generate hydrogen, and the temperature dependence of the generation rate * Corresponding author phone: +81-11-706-6842; fax: +81-11706-6849; e-mail:
[email protected]. † Hokkaido University. ‡ ITEC Co., Ltd.. § Waseda University. 4454
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1076 (at 298 K) This suggests the possibility that the aforementioned reaction is strongly exothermic, thereby directly generating highpressure hydrogen. Ka value was calculated by commercial software code of HSC chemistry 5.11. It is well-known that a hydrogen compressor consumes a large amount of energy due to large differences in the gas density. To compress hydrogen up to 40 MPa, we have to charge a power of 1.30 MJ/Nm3 of hydrogen through several steps of adiabatic compression; 1.26 MJ of the power charged is then wasted as heat. Therefore, hydrogen production according to eq 1 using waste aluminum shows sufficient potential for eliminating the disadvantages of conventional energy-consuming hydrogen compression process. However, the direct production of high-pressure hydrogen from waste aluminum has not been studied so far in spite of its feasibility from the engineering perspective. The purpose of this study is, therefore, to produce high-pressure hydrogen by the experimental hydrolysis of aluminum, in which the effect of alkali concentration on the rate of high-pressure hydrogen generation was chiefly examined by using the pressureresistance reactor (autoclave).
Experimental Section Figure 1 shows the schematic diagram of the experimental apparatus used. It consists of four main sections: a liquid pump, pressure-resistance reactor (autoclave), gas-liquid separator, and liquid storage tank. The liquid pump supplies compressed and distilled water or a sodium hydroxide solution into the autoclave. The pipe connected between the liquid pump and autoclave can be preheated by using an electric heater. Distilled water is compressed in the reactor, which is made of Hastelloy C-22, to a maximum pressure of 35 MPa, and it is heated to a maximum temperature of 573 K. Prior to the experiments, aluminum powder, having a grain size of 180-425 µm and 99.9% purity, was weighed to 0.5 mol and charged into a metallic filter cage in the cylindrical reactor (5). Distilled water was then filled up to the controlled regulator level of pressure 10, 20, and 30 MPa, followed by the heating up of the reactor and connecting pipe to the desired temperature using the electric heater. The experiments were initiated by replacing the compressed water with sodium hydroxide solution at concentrations of 1.0 and 5.0 mol/dm3 (M). A cooler was placed between the reactor and the gas-liquid separator to condense the steam generated in the reactor. In the separator, high-pressure gas containing Al3+, Na+, H+, and OH- ions pushed the liquid to the lower 10.1021/es062883l CCC: $37.00
2007 American Chemical Society Published on Web 05/17/2007
FIGURE 1. Schematic diagram of the experimental apparatus used for producing high-pressure hydrogen.
FIGURE 2. Changes in pressure under different experimental conditions of water pressure and sodium hydroxide concentration. region. That is, the inflowing liquid was pumped out from the bottom of separator into the storage tank, whereas the gas remained in the upper region of the separator. We confirmed in the preliminary tests that the water pressure in the equipment can be easily controlled using the liquid pump, which consumes less energy during water compression when compared to a gas compressor due to a smaller volume change. The gas volume present in the upper region of separator was recovered after the experiments by opening the valve attached to the top of the separator. The weight of the liquid recovered in the storage tank was monitored using an online balance to evaluate the change in gas generation with time. Further, a gas flowmeter was used to double check the amount of hydrogen generated. The gas recovered was later introduced into a gas chromatograph for confirming the hydrogen purity. During the experiments, the local pressure and temperatures were measured using a pressure gauge and thermocouples, as shown in Figure 1. The reactor product was also analyzed by X-ray diffractometer.
Results and Discussions Figure 2 shows the changes in pressure with time for five runs at different pressures and concentrations of sodium hydroxide solutions. The replacement time was approximately 30 s, which was easily calculated from the piston flow of sodium hydroxide solution (see the dash-dotted line). The entire experimental data showed a constant value within a scattering of 5%. It should be noted that the hydrogen generated in out-flowing liquid remained in the upper region
FIGURE 3. XRD spectra of the product after crystallization process.
FIGURE 4. Diagram of the proposed process system producing 1 kg of hydrogen at 30 MPa and 26 kg of aluminum hydroxide from waste aluminum containing 15 mass % metallic aluminum. of the separator due to its low density. It was recovered by opening the valve and measured by using a dry flowmeter. The liquid in the recovery bath yielded aluminum hydroxide by means of the crystallization process. Figure 3 shows the XRD pattern of the product obtained from the crystallization process and the production of very pure aluminum hydroxide was confirmed. Based on the results of XRD patterns of the product, the process system diagram was drawn as shown in Figure 4. In this work, with 60 kg of waste Al and 18 kg of water as starting materials, 1 kg of H2 of pressure 30 MPa and Al(OH)3 were obtained as products. VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Phase diagram of water (10, 11), in which plotted data were based on experimental one in Figure 5. Note that closed square, 9, data of rapid reaction reaches the subcritical zone.
FIGURE 5. Changes in temperature (a) and reaction degree (b) under different experimental conditions of water pressure and sodium hydroxide solution concentration. Calculation was done under the assumptions of no heat loss and ideal piston flow. SCW; Subcritical water. Figure 5 (a) shows the changes in temperature with time, which were measured using a thermocouple placed in the upper part of the reactor. The plotted symbols indicate the experimental data, whereas the broken lines indicate the calculated data. The data calculated at 10, 20, and 30 MPa were obtained from the piston flow and adiabatic conditions. The entire experimental data revealed a sudden increase in the temperature due to an exothermic reaction caused by the replacement of water by the alkali solution after the induration period. The replacement time was approximately 30 s, which was easily calculated from the piston flow of sodium hydroxide solution (see the dash-dotted line). The use of a low-concentration sodium hydroxide solution (1.0 M) caused a marginal increase in the temperature where the maximum temperature was only 360 K. Under the assumptions of a piston flow of the sodium hydroxide solution up to the reactor and a uniform temperature in the reactor, the temperature history was roughly estimated by using eq 1. The temperature increased rapidly due to the exothermic heat when 5.0 M sodium hydroxide solution was used in the experiments, reaching a maximum value of 420 K within a couple of minutes; thus, the calculated data gradually decreased. This was probably caused by the aluminum consumption and heat loss from the reactor. The observed data showed a similar trend, exhibiting a peak in temperature. For a more sophisticated simulation, a fluid flow and heat transfer model should be developed in the future together with an accurate evaluation of heat loss from the reactor. Figure 5(b) shows the reaction curves for five runs, which were obtained from the liquid quantity in the storage tank. In the three experiments using 5.0 M sodium hydroxide solutions, all curves exhibited a sharp increase as soon as the sodium hydroxide solution reached the reactor at approximately 30 s. Similar to Figure 5(a), no significant difference was observed among the three curves. In other words, the effect of pressure on the reaction curves in the 4456
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experiments using 5.0 M sodium hydroxide solution is fairly small. On the contrary, for the 1.0 M sodium hydroxide solution case, effect of water pressure was dominant, when 1.0 M sodium hydroxide solution and a water pressure of 10 MPa was used, the reaction was very slow showing an induration period of 100 s. A comparison with solutions of 5.0 and 1.0 M, represented by open and closed circles, respectively, indicates that 5.0 M sodium hydroxide solution was very effective for obtaining a rapid and large reaction degree because there was no duration time and the reaction curve was accorded with the theoretical one even at the relatively lower pressure of 10 MPa. In addition, a remarkable effect of pressure on the reaction curves using 1.0 M sodium hydroxide solution in Figure 5(b) was shown. (See the closed circles and closed squares.) Using 1.0 M sodium hydroxide solution and 30 MPa water pressure, the reaction degree corresponds, approximately, to the condition using 4.0 M sodium hydroxide solution. The slope angle was about four times that of the theoretical value. Compressing the water to a pressure of 30 MPa resulted in a very rapid reaction. The increased reaction degree when using 1.0 M sodium hydroxide solution and 30 MPa water pressure can probably be explained on the basis of the structure of the subcritical water. Figure 6 shows the phase diagram of water in which the subcritical and supercritical zones are defined (10, 11). It is well-known that subcritical water can easily oxidize due to small clusters of water (12-17). Thus, from the results, a possibility of an innovative process of eliminating the use of an alkali to produce hydrogen from low-grade waste aluminum under the certain temperature and pressure condition was ascertained.
Acknowledgments This study was supported by the project “The Model of GreenHydrogen Community in Honjo-Waseda Area” of the Ministry of Environment, Japan, and by a Grant-in-Aid for Scientific Research (grant no. B-17360365) from the Japan Society for the Promotion of Science (JSPS). The technical support and worthwhile discussions provided by the staff of ITEC Co., Ltd. are highly appreciated.
Literature Cited (1) JATIS: Japan Technical Information Services Corporation. Investigation Report of Aluminum Flow; ISIJ: Japan, 2002. (2) Ohnishi, T. Social environment and problems in aluminum recycling. J. Jpn. Inst. Light Met. 1996, 46, 525-532. (3) Murata, F. Research and development of technology to promote recycling of aluminium materials. J. Jpn. Inst. Light Met. 1996, 46, 551-556. (4) Nobusawa, T. Exergy Nyuumon; Ohmu Co., Ltd.: Tokyo, 1980.
(5) Hiraki, T.; Takeuchi, M.; Hisa, M.; Akiyama, T. Hydrogen production from waste aluminum at different temperatures, with LCA. Mater. Trans., JIM 2005, 46, 1052-1057. (6) Knacke, O.; Kubaschewski, O.; Hesselman, K. Thermochemical Properties of Inorganic Substances, 2nd ed.; Springer-Verlag: Berlin, 1991; pp 1-1113. (7) Chase, M. W.; Davies, C. A.; Downey, J. R.; Frurip, D. J.; Mcdonald, R. A.; Syverud, A. N. JANAF thermochemical tables, 3rd edition. 1. Al-Co. J. Phys. Chem. Ref. Data 1985, 14, 1-926. (8) Shock, E. L.; Helgeson, H. C. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures-correlation algorithms for ionic species and equation of state predictions to 5-KB and 1000-degrees-C. Geochim. Cosmochim. Acta. 1988, 52, 2009-2036. (9) Nordstrom, D. K.; Plummer, L. N.; Langmuir, D.; Busenberg, E.; May, H. M.; Jones, B. F.; Parkhurst, D. L. Revised ChemicalEquilibrium Data for Major Water-Mineral Reactions and their limitations; ACS Symposium Series 416; American Chemical Society: Washington, DC, 1990; pp 398-413. (10) Ohtaki, H.; Radnai, T.; Yamauchi, T. Structure of water under subcritical and supercritical conditions by solution X-ray diffraction. Chem. Soc. Rev. 1997, 26, 41-51. (11) Sako, T. Supercritical Fluid; Agune Shofusya Co., Ltd: Japan 2001.
(12) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K. Dissolution and hydrolysis of cellulose in subcritical and supercritical water. Ind. Eng. Chem. Res. 2000, 39, 2883-2890. (13) Lachance, R.; Paschkewitz, J.; DiNaro, J.; Tester, J. W. Thiodiglycol hydrolysis and oxidation in sub- and supercritical water. J. Supercrit. Fluids 1999, 16, 133-147. (14) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. Cellulose hydrolysis in subcritical and supercritical water. J. Supercrit. Fluids 1998, 13, 261-268. (15) Marrone, P. A.; Gschwend, P. M.; Swallow, K. C.; Peters, W. A.; Tester, J. W. Product distribution and reaction pathways for methylene chloride hydrolysis and oxidation under hydrothermal conditions. J. Supercrit. Fluids 1998, 12, 239-254. (16) Sasaki, M.; Furukawa, M.; Minami, K.; Adschiri, T.; Arai, K. Kinetics and mechanism of cellobiose hydrolysis and retroakdol condensation in subcritical and supercritical water. Ind. Eng. Chem. Res. 2002, 41, 6642-6649. (17) Krammer, P.; Vogel, H. Hydrolysis of esters in subcritical and supercritical water. J. Supercrit. Fluids 2000, 16, 189-206.
Received for review December 5, 2006. Revised manuscript received March 19, 2007. Accepted March 30, 2007. ES062883L
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