Christof Hamel

Theoretical and Experimental Investigation of Concentration and Contact Time Effects in Membrane Reactors

A potential field of applying membrane reactors is the enhancement of selectivity and yields in complex reactions like networks of parallel and series reactions. To evaluate, design and optimise dosing concepts exploiting membranes it is necessary to perform systematic studies. In this paper the possibility of enhancing selectivity and yields by application of optimised dosing concepts is investigated theoretically and experimentally. In the first, the theoretical part of the study presented, dosing of one reactant at discrete reactor positions (cascade of fixed-bed reactors) was compared with continuous dosing through a porous reactor wall (packed-bed membrane reactor). The effects of manipulating the local reactant compositions (and thus the local reaction rates) and the component residence time distributions via the different dosing strategies is elucidated. In the second part of this study, it is illustrated by experimental data from oxidative dehydrogenation of ethane, that membrane reactors possess indeed the potential to improve selectivity and yields of desired intermediates. By application of membrane reactors it is possible to optimise the selectivity for different products in a given reaction network.

Compatibility of Transport and Reaction in Membrane Reactors Used for the Oxidative Dehydrogenation of Short-Chain Hydrocarbons

The possibility of process intensification by enhancing selectivity and yield in networks of parallel and series reactions was investigated applying asymmetric multilayer ceramic and sintered metal membranes in a dead-end configuration for a controlled distributed reactant feeding. The oxidative dehydrogenation of ethane to ethylene was selected as a model reaction applying three different doped and/or active VOx/?-Al2O3 catalysts. Experimental investigations were performed in a pilot scale in order to evaluate the potential of a distributed dosing via membranes with respect to operation conditions and compatibility of reaction and membrane properties. It was demonstrated that the rates of reaction and trans-membrane mass transfer have to be compatible for an optimal membrane reactor operation avoiding back diffusion of reactants out of the catalytic zone as well as achieving safety aspects. Therefore, a detailed modeling of the trans-membrane mass transfer under reaction conditions was carried out. As a main result, it was found metal membranes possess a favorable mechanical stability, relatively low costs for production and the possibility to control mass transfer if the rate of reaction and mass transfer in the membrane is compatible which can adjusted by the trans-membrane pressure and the catalyst activity, respectively.

Impact of minor amounts of hydroperoxides on rhodium-catalyzed hydroformylation of long-chain olefins

The effect of minor amounts of impurities on the course of chemical reactions is often overlooked. Analyzing commercial 1-dodecene feeds, hydroperoxides were identified as critical impurities. The influence of varying hydroperoxide concentrations in olefin feeds was systematically investigated, experimentally studying rhodium-catalyzed hydroformylation using a diphosphite ligand. A significant loss of n-aldehyde selectivity and linear-to-branched ratio (l/b) was observed for increasing hydroperoxide concentrations. Feeding of an additional ligand and/or purification of 1-dodecene restored the catalyst performance.

Model-Based Optimal Design of Experiments for Determining Reaction Network Structures

A new approach for optimal experimental design has been developed to support the work of chemists and process engineers in determining reaction kinetics of complex reaction networks. The methodology is applied on sub-networks of the hydroformylation process of 1-dodecene with a Biphephos-modified rhodium catalyst in a DMF-decane thermomorphic solvent system (TMS). The isomerization and hydrogenation sub-networks are systematically analyzed with respect to parameter estimability. They are determined in a sequential approach using model-based optimal experimental design via perturbations with respect to temperature and synthesis gas pressure, and subsequently used to build up the reaction network. The focus of this contribution is the parameter estimation procedure at the very early investigation stage where model uncertainties are high. Sensitivities of sensitive parameters are increased while others are suppressed, which are carried over from the estimated sub-networks or structurally more difficult to determine. This subsequently leads to more reliable parameter estimations.

Experimental and Model Based Study of the Hydrogenation of Acrolein to Allyl Alcohol

The hydrogenation of acrolein was investigated experimentally in a fixed-bed reactor (FBR) using several classical and a newly developed hydrogenation catalyst. The aim was to evaluate selectivity and yield with respect to the desired product allyl alcohol. The kinetics of the two main parallel reactions of acrolein hydrogenation were quantified for a supported silver catalyst which offered the highest performance. In a second part the reaction kinetics identified were used in a theoretical study applying a simplified isothermal 1D reactor model in order to analyse the hydrogenation of acrolein performed in single- and multi-stage packed bed membrane reactors (PBMR). The goal of the simulations was to evaluate the potential of dosing one reactant in a distributed manner using one or several membrane reactor stages. The results achieved indicate that the membrane reactor concept possesses the potential to provide improved yields of allyl alcohol compared to conventional co-feed fixed-bed operation.