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Water Science & Technology Vol 65 No 11 pp 2034–2041 © IWA Publishing 2012 doi:10.2166/wst.2012.109
Enhanced removal of sodium salts supported by in-situ catalyst synthesis in a supercritical water oxidation process
F. Takahashi, Z. R. Sun, K. Fukushi, Y. Oshima and K. Yamamoto
Faculty of Urban and Environmental Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan E-mail: f-takahashi@doc.kyushu-u.ac.jp
School of Environmental and Energy Engineering, Beijing University of Technology, Pingleyuan 100, Chaoyang District, Beijing 100022, P. R. China
Transdisciplinary Initiative for Global Sustainability, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8563, Japan
Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
ABSTRACT
For practical applications of supercritical water oxidation to wastewater treatment, the deposition of inorganic salts in supercritical phase must be controlled to prevent a reactor from clogging. This study investigated enhanced removal of sodium salts with titanium particles, serving as a salt trapper and a catalyst precursor, and sodium recovery by sub-critical water. When Na2CO3 was tested as a model salt, sodium removal efficiency was higher than theoretically maximum efficiency defined by Na2CO3 solubility. The enhanced sodium removal resulted from in-situ synthesis of sodium titanate, which could catalyse acetic acid oxidation. The kinetics of sodium removal was described well by a diffusion mass-transfer model combined with a power law-type rate model of sodium titanate synthesis. Titanium particles showed positive effect on sodium removal in the case of NaOH, Na2SO4 and Na3PO4. However, they had negligible effect for NaCl and negative effect for Na2CrO4, respectively. More than 99% of trapped sodium was recovered by sub-critical water except for Na2CrO4. In contrast, sodium recovery efficiency remained less than 50% in the case of Na2CrO4. Reused titanium particles showed the same performance for enhanced sodium removal. Enhanced salt removal supported by in-situ catalyst synthesis has great potential to enable both salt removal control and catalytic oxidation.
Keywords: kinetics; salt deposition control; sodium removal; sodium titanate synthesis; supercritical water oxidation
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