Browsing by Author "Kolios, Grigorios"

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Open Access
Fixed bed reactors with periodic flow reversal : experimental results for catalytic combustion
(1994) Nieken, Ulrich; Kolios, Grigorios; Eigenberger, Gerhart
The influence of design and operating parameters on the behavior of a fixed-bed reactor with periodic flow reversal has been studied in a laboratory set-up for the case of catalytic total oxidation. The results are in accordance with detailed model simulations published elsewhere. They show that the periodic operation is completely dominated by the regenerative heat exchange and that steady-state kinetics can be used. Like any other autothermal reactor, a fixed-bed reactor with periodic flow reversal has to be operated in the ignited steady state. It was shown that totally and partially ignited steady states may exist under the same operating conditions if several combustible components with different ignition temperature are present in the feed. Hot gas withdrawal from the middle of the packed bed proved to be a suitable method to utilize almost all of the heat of reaction at the highest temperature in the reactor and to prevent high temperature peaks at the respective exit valves. Together with an appropriate design of the fixed bed, composed of inert front and end sections with low effective axial conductivity and an active portion with large axial conductivity, hot gas withdrawal allows for an efficient control of the reactor under the conditions of both high and low feed concentrations.
Open Access
Regenerative fixed-bed processes : approximative analysis and efficient computation of the cyclic steady state
(2014) Kolios, Grigorios; Eigenberger, Gerhart (Prof. Dr.-Ing. habil.)
The aim of the thesis is to combine heuristic knowledge with theoretical principles to a simple, comprehensive procedure for calculating the state of operation of regenerative fixed-bed processes and their parametric dependence. Regenerative fixed-processes are heat-, mass-transfer or reaction processes with fluids flowing through fixed-beds, where the fixed-bed is periodically exposed to changing fluid inlet conditions. Their principle of operation is relying on the phenomenon of travelling fronts. Travelling fronts occur during the transition state after a sudden change of the fluid inlet conditions, virtually flushing out the original state of the fixed-bed. The properties of travelling fronts can be described by a low number of characteristic variables, based on the wave theory. A second ingredient of the procedure is the equivalence in the behavior of regenerative and continuous countercurrent processes. A theoretically sound explanation of the equivalence relation is provided. Accordingly, the cyclic steady state of regenerative processes results from superposition of the steady state of the equivalent continuous process and the axial displacement caused by travelling fronts. The procedure is evaluated using the reverse-flow reactor as a representative test case. The reverse-flow reactor is a multifunctional reactor concept, originally designed to carry out weakly exothermic reactions in an autothermal mode of operation. The cyclic steady can be closely approximated by an explicit short-cut procedure. Further, the theory is applied to design a reverse-flow reactor for coupling endothermic with exothermic reactions. As attractive this concept seems on a first sight, the major challenge toward a viable implementation is the inherent tendency of endothermic and exothermic reaction zones to repel each other. The problem can be solved by imposing an axially distributed heat supply along the fixed-bed. A rational short-cut procedure is developed for specifying the major design parameters such as flow-rates, cycle period, axial structure of fixed-bed as well as an approximate solution for the cyclic steady state. Finally, the theory is useful in developing dedicated algorithms for detailed simulation and parametric analysis of cyclic processes. The underlying concept is to construct the exact solution by combining the approximate solution of the short-cut procedure with a minor correction term. The performance of these algorithms is demonstrated in the final part of the thesis. The developed methodical framework is directly applicable only to simple process configurations. Nevertheless, the fundamental insight gained from simplified modelling is useful for a rational design of complex, technically relevant configurations.