Conventional hydrothermal treatment processes are divided into three categories: batch — type reactor, semibatch reactor, and continuous reactor.

In a batch reactor, water and reactant are sealed in the same reactor. The reactor is heated from outside or inside. 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 demand. Steel batch autoclaves are used in most cases. Steel autoclaves have the disadvantage of heating slowly, and thus some time is re­quired to reach reaction temperature (Manarungson et al., 1990). Other reactor types include capillaries and tubular steel reactors. Quartz capillaries have also been used as batch microreactors.

In a semibatch reactor, a reactor is filled with reactant 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 obtained continuously. However, reactants have to be refilled in the reactor for continuous production. Sakaki et al. developed a semibatch system (Sakaki et al., 1998), but productivity 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 feedstock into a high-pressure reactor is the biggest challenge to 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 (Kobayashi et al., 2011). For continuous operation, tubular steel reactors are often used. Other types of reactors, such as the stirred tank reactor, can be used in principle, but to date this configuration has not yet been applied (Navarro et al., 2009).