Applying Bubbling Fluidized-Bed Reactors for Strongly Exothermic Reactions: Focus on Methanation

Abstract

Strongly exothermic reactions inherently pose the risk of thermal reactor runaway, which may lead to very high increase in temperature, hot spots and potential catalyst deactivation. For such reaction systems, reactors with excellent heat removal performance are needed. In the case of methanation of CO/CO2-rich gases, full conversion is not possible in a single adiabatic reactor due to the equilibrium limitation, and in large-scale plants, e.g., coal-to-synthetic natural gas (SNG) plants, series of four and more reactors with intercooling have been realized. To allow for complete conversion within one reactor, the potential of bubbling fluidized-bed (BFB) reactors with immersed heat exchangers was investigated in the US and Germany from the 1960s to the 1980s. A Swiss consortium started to expand the concept to small- and medium-scale plants to allow the production of renewable methane from decentral renewable sources such as wood gasification and biogas. During their tests, it could be shown that the catalyst particle movement does not only allow for optimal heat removal—close to isothermal operation and thus little catalyst sintering—but that the catalyst particle movement over the height of the reactor with different concentration zones favors the chemical catalyst stability. This contribution will review the fluid-dynamic studies for BFB reactors with immersed heat exchangers in the last decades comprising studies with pressure fluctuation probes, optical probes, X-ray tomography studies, and particle attrition studies.

Keywords

• bubbling fluidized-bed (bfb) reactors
• methanation
• immersed heat exchanger
• chemical catalyst stability
• heat transfer