Adiabatic simulated moving bed reactor : principle, nonlinear analysis and experimental demonstration
Year: 2012
Type: dissertation
Abstract: This work investigates a heat integrated reactor concept based on unsteady state operation of several adiabatic catalytic fixed-bed reactors. Exothermic reaction fronts are guided through such a cascade in the gas flow direction, while cold reactor segments are switched to the end for efficient thermal regeneration. This inherently periodic operation attempts to trap a self-sustained exothermic front allowing an autothermal operation. As such, the switching policy simulates a countercurrent of the solid compared to the fluid phase and is therefore denoted as a simulated moving bed reactor (SMBR). Potential applications are seen in catalytic total oxidation of volatile organic compounds (VOC) in diluted off-gases or in performing equilibrium limited exothermic reactions. This work contains a comprehensive theoretical analysis and an experimental proof-of-concept of the adiabatic simulated moving bed reactor. Thermal and reaction fronts occurring in a SMBR were characterized regarding their constant pattern behavior, required ignition temperatures, dynamics and spatio-temporal patterns. Stable ignited operation can be maintained, when switching times are adjusted to the velocity of the fronts. Within a specific windows of switching times complete conversion without slip of unconverted reactants can be achieved. In order to avoid expensive dynamic simulations, a true moving bed reactor model was derived, which retains all relevant properties of the periodic reactor needed for reactor analysis and design. This does not only allow a direct approximation of temperature and concentration profiles, but provides an efficient numerical nonlinear analysis essential to understand and control ignition-extinction phenomena. Diverse reactor multiple steady states were encountered and structured with the help of higher order singularities. In this way, reactor control and the identification of feasible reactions are available. Three discrete-time control concepts were investigated to maintain the reactor in the limited range of switchting times. Already a simple temperature control was shown to be appropriate. The model-system experimentally investigated is the total oxidation of propene and ethene on a CuCrOx/Al2O3 catalyst in air. Systematic experiments confirmed the applicable range of switching times as well as the control characteristic. Step experiments altering flow rates and concentrations demonstrated the proper disturbance rejection. As long as rates of the oxidation reactions are similar, mixtures can be well processed. Otherwise, incomplete conversion of the less oxidizable component can occur. Due to sufficiently high start-up temperatures and an excess of this component the total oxidation of such mixtures can be assured.
Source: MPG.PuRe (Max Planck Society)
Author: V. Zahn
Language: English
Subfield: Computational Mechanics
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W840057334