ARTICLE pubs.acs.org/IECR
Fluid Behavior of Woody Biomass Slurry during Hydrothermal Treatment Nobusuke Kobayashi,* Nobuhiko Okada, Yasuhiro Tanabe, and Yoshinori Itaya Department of Chemical Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Hydrothermal experiment of woody biomass was carried out in an autoclave, and a property change of the woody slurry associated with hydrothermal reaction was analyzed. The reaction conditions, such as reaction temperature, reaction period, and slurry concentration, were varied in this experiment, and the effect of the hydrothermal conditions on the slurry characteristics was evaluated. To evaluate the property change of the woody slurry, apparent viscosity, particle size, pH, and ζ potential measurements of the hydrothermally influenced slurry were conducted. The fluid resistance of the slurry during hydrothermal experiment was also measured with an autoclave reactor. With application of hydrothermal treatment on a woody slurry, significant change of the slurry property was observed. The particle size decreased during hydrothermal experiment with increasing reaction temperature, and this particle size change influenced the slurry behavior. Due to the particle size reduction, the apparent viscosity of the treated slurry became smaller, and the treated slurry showed strong pseudoplastic fluid behavior. With a longer reaction period, the particle size was getting smaller by the hydrolysis reaction; however, particle size enlargement caused by a swelling and/or agglomeration occurred when the reactor’s temperature started to increase. This particle size enlargement behavior at the beginning of the hydrothermal experiment increased the fluid resistance of the slurry significantly. pH value and the ζ potential value were also influenced by the hydrothermal treatment. pH and absolute ζ potential values dropped with increasing reaction temperature; however, the absolute ζ potential value of the slurry reversely increased when the reaction temperature was over 523 K. The increment of carboxyl radical on the surface of the reacted particle changed particle-particle and particle-solvent interactions.
1. INTRODUCTION Hydrothermal treatment has been employed in various fields, such as synthesis and/or decomposition of organic materials.1 Application of the field of hydrothermal treatment is expanding to biomass gasification and liquefaction, and now hydrothermal technique is available in cellulose saccharification as one of the efficient handling methods. Research work on a biomass hydrothermal process is actively implemented, because hydrolysis reaction of cellulose and hemicellulose components are capable of yielding oligosaccharide, monosaccharine, and other valuable chemicals, such as glucose, xylose, and hydroxyl methyl furfural (HMF). Sakaki et al.2,3 and other researchers4,5 conducted hydrothermal experiments with lignocellulose materials in a batch reactor. After this research work, many researchers became involved in biomass hydrothermal experiment and then developed enzyme saccharification pretreatment technique and chemicals production technology. Yamada et al.,6 Hayashi et al.,7 Kumagai et al.,8-10 and Kobayashi et al.11-14 conducted hydrothermal experiment with various biomass species. This research work clarified decomposing material of biomass, illustrated the hydrolysis mechanism of lignocellulose associated with the hydrothermal reaction, and explained the optimum reaction conditions of different biomasses, in a batch type reactor. With expanding hydrothermal applications on lignocellulose biomass, demand for a new application device and systems has been growing in order to increase the energy efficiency and productivity of the desired materials. A conventional hydrothermal treatment system divides into three categories, which are batch type reactor, semibatch reactor, and continuous reactor. In r 2011 American Chemical Society
a batch reactor, water and reactant are sealed in the same reactor. The reactor is heated from the outside or inside with microwaves. Due to the easy handling and operation of a batch reactor, many results and analysis data in various operation conditions have been reported. But productivity in a batch system does not meet commercial demands. In a semibatch reactor, reactant is filled in a reactor and hot compressed water is introduced to the reactant separately. Temperature control of the slurry and flow rate control of the hot water are simple, and moreover product is able to yield continuously. However, reactants have to be refilled in the reactor for a continuous production. Sakaki et al. developed a semibatch system,15 but productivity of the semibatch system was still very low. There are two methods in a continuous system; one is a separate type, and the other is a slurry type. Feeding of solid feed stock into a high-pressure reactor is the biggest challenge on the operation of the separate process. On the other hand, a commercial high-pressure slurry pump is available for continuous feeding of high-concentration slurry. Moreover enormous knowledge of slurry handling and operation in a coal-liquefaction process is also available; those technologies16,17 would help in the development of a continuous biomass hydrothermal treatment process. Kobayashi et al.14 proposed a continuous hydrothermal system of slurry flow, but energy efficiency was still inadequate, because the concentration of the slurry was not sufficiently high (
