Jörg Frauhammer

Autothermal fixed-bed reactor concepts

The principles, properties and applications of autothermal fixed-bed reactor concepts are presented. First we focus on different reactor types for weakly exothermic reactions and discuss their basic behavior, their stability and nonlinear dynamic features. The second part is devoted to the autothermal coupling of endothermic and exothermic reactions. A systematic classification is proposed for the process alternatives developed so far and a simplified model is developed from which basic features of an optimal design can be deduced.

A new reactor concept for endothermic high-temperature reactions

A new reactor concept for the autothermal operation of endothermic high-temperature reactions is presented. An endothermic synthesis reaction is coupled with a combustion reaction in such a way that both reactions take place in adjacent channels of a countercurrent fixed-bed reactor. Due to the countercurrent heat exchange the feed and the exit of both reactions have low temperatures while a high-temperature zone in the middle of the reactor allows for high conversion with a subsequent rapid quench. The applicability of the concept is shown through detailed simulation studies and first experiments. As an example reaction methane steam reforming is considered. A ceramic honeycomb monolith has been used as countercurrent heat exchanger where the catalyst was deposited on the monolith walls. First experimental results show the general applicability of the concept. Due to the limited thermal stress resistance of the ceramic monolith used the maximum operation temperature has so far been limited to 800°C.

Efficient reactor concepts for coupling of endothermic and exothermic reactions

Multifunctional autothermal reactors are a novel concept in process integration and intensification. They can be implemented as a countercurrent or reverse-flow reactor. A promising field of application is the coupling of endothermic and exothermic reactions. Methane steam reforming coupled with methane combustion is considered as a particular example. Several novel reactor configurations with co- and countercurrent flow in the reaction zone will be discussed by numerical simulation and an example for experimental verification will be presented.

Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor

A one-dimensional two-phase reactor model for the oxidation of methane to synthesis gas over platinum in a monolith reactor is presented. The model incorporates a detailed elementary step reaction mechanism for methane oxidation, which is verified against experimental data. A good quantitative agreement with steady-state experiments and qualitative agreement with ignition experiments is achieved. The importance of individual reaction steps, homogeneous side reactions, and main reactor parameters are investigated. Essential steps in the catalytic reaction mechanism as well as crucial reactor parameters are identified. It is shown that the reaction system is strongly dominated by competition for oxygen on the catalyst surface. Conclusions about optimal reactor configurations are discussed.

A simplified procedure for the optimal design of autothermal reactors for endothermic high-temperature reactions

Abstract The occurrence of excessive maximum temperatures is presently the main obstacle in the design of countercurrent reactors for the autothermal coupling of endothermic and exothermic reactions. The reasons for the appearance of these high-temperature maxima are elucidated and discussed using a simplified reactor model. Based on it, measures are derived of how the maximum temperature can be reduced and the thermal efficiency of the integrated reactor can be improved. Simple formula and an efficient graphical procedure for a short-cut reactor design are provided. The results of the simplified design procedure have been verified through simulation with a more detailed reactor model.

Catalytic ignition during methane oxidation on platinum: Experiments and modeling

Abstract The ignition behaviour of methane-air mixtures on a platinum-foil catalyst was studied at atmospheric pressure over the entire range of fuel to air ratios. The measured surface ignition temperatures showed a continously decreasing trend with increasing fuel:air ratio up to very fuel rich mixtures, which could be reproduced using a very simple analytical model indicating strong site competition between methane and oxygen on the catalyst surface. In parallel, steady state experiments during methane oxidation in a Pt coated monolith were modeled with a detailed monolith reactor model, comprising heat-, mass- and impulse-balance and a detailed elementary step surface reaction mechanism. A close fit between experimental and modelling results was obtained. The two approaches were then combined by comparing the results of dynamic simulations of the ignition behaviour of the detailed monolith model with the experimental results on the Pt foil catalyst.

Solving moving boundary problems with an adaptive moving grid method : Rotary heat exchangers with condensation and evaporation

Abstract A numerical method for the treatment of moving discontinuities in model equations of chemical engineering systems is presented. The derived model describing the effect of condensation and evaporation in a regenerative air-to-air heat exchanger yields an illustrative example for the so called moving boundary problems. The presented adaptive moving grid method is based on the algorithm Pdex for parabolic partial differential equations. It is shown that the method is well suited for problems where the discontinuities cause low rates of convergence if the equations are solved with a static grid.

Flow distribution concepts for new type monolithic co- or countercurrent reactors

For co- and countercurrent processes two different concepts of gas distribution for monoliths have been studied. The concept of separate reactor heads has been compared with a monolith preparation concept. The advantage of the new heads is that the distribution of the separate gas streams into the different monolith channels is more flexible and that a more uniform flow profile can be achieved over the monolith cross section.

Solving Chemical Engineering Problems with Front Propagation Using an Adaptive Moving Grid Method

A simple and efficient moving grid method for the simulation of one dimensional chemical engineering problems with steep propagating fronts is presented. A combination with a fully adaptive static regridding technique as implemented in PdexPack was used. The moving grid method was tested for the simulation of an adiabatic fixed bed adsorption process. Compared to conventional adaptive methods on a static grid a significant reduction of the CPU time and a higher stability of the simulation could be achieved.