Kai Sundmacher

The impact of mass transport and methanol crossover on the direct methanol fuel cell

The performance of a liquid feed direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane is reported. The cell utilises a porous Pt–Ru-carbon supported catalyst anode. The effect of cell temperature, air cathode pressure, methanol fuel flow rate and methanol concentration on the power performance of a small-scale (9 cm2 area) cell is described. Data reported is analysed in terms of semi-empirical models for the effect of methanol crossover by diffusion on cathode potential and thus cell voltage. Mass transfer characteristics of the anode reaction are interpreted in terms of the influence of carbon dioxide gas evolution and methanol diffusion in the carbon cloth diffusion layer. Preliminary evaluation of reaction orders and anode polarisation agree with a previous suggested mechanism for methanol oxidation involving a rate limiting step of surface reaction between adsorbed CO and OH species.

Current status of and recent developments in the direct methanol fuel cell

The direct methanol fuel cell (DMFC) has been discussed recently as an interesting option for a fuel-cell-based mobile power supply system in the power range from a few watts to several hundred kilowatts. In contrast to the favoured hydrogen-fed fuel cell systems (e.g. the polymer electrolyte membrane fuel cell, PEMFC), the DMFC has some significant advantages. It uses a fuel which is, compared to hydrogen, easy to handle and to distribute. It also comprises a fairly simple system design compared to systems utilising liquid fuels (like methanol) to produce hydrogen from them by steam reforming or partial oxidation to finally feed a standard PEMFC. Nevertheless, many severe problems still exist for the DMFC, hindering its competitiveness as an option to hydrogen-fed fuel cells. This work reviews the major research activities concerned with the DMFC by highlighting the problems (slow kinetics of the anodic methanol oxidation, methanol permeation through the membrane, carbon dioxide evolution at the anode) and their possible solutions. Special attention is devoted to the steady state and dynamic simulation of these fuel cell systems.

Dynamics of the direct methanol fuel cell (DMFC) : experiments and model-based analysis

Abstract A laboratory-scale liquid-feed direct methanol fuel cell (DMFC) was operated with different methanol feeding strategies. A proton exchange membrane (PEM) was used as the elecytrolyte. The cell voltage response to dynamic feeding of methanol revealed that a significant voltage increase can be obtained from dynamic changes in methanol feed concentration. The observed fuel cell behaviour was analysed with a mathematical model which consists of anode mass balances, charge balances of both electrodes and electrode kinetic expressions. Anode kinetics were derived from a four-step reaction mechanism with several intermediates bound to the catalyst surface. The model also accounts for the undesired cross-over of methanol, through the PEM, towards the cathode catalyst layer. First, the model is applied to predict steady-state current–voltage characteristics. Then, the cell voltage response to dynamic changes of methanol feed concentration is simulated. The simulated results are in full agreement to experimental observations. It turns out that methanol cross-over can be reduced by periodically pulsed methanol feeding.

A model for the liquid feed direct methanol fuel cell

Mass transport is a factor which limits the performance of solid polymer electrolyte fuel cells operating at relatively high current densities. The direct methanol solid polymer electrolyte fuel cell, unlike the hydrogen cell, suffers from mass transport limitations predominantly at the anode. In the liquid feed cell the mass transport limitations arise from diffusion of methanol in the carbon cloth covering the active electrocatalyst layer and from hydrodynamic limitations in the anode flow channel. A model of the methanol mass transport processes is presented which is used to predict the effective methanol concentration at the catalyst surface and thereby the anode polarisation. This model, together with an empirical model of the open circuit voltage and the cathode overpotential model, is used to predict the overall cell voltage, current density response of the fuel cell.

Steady-state multiplicities in reactive distillation columns for the production of fuel ethers MTBE and TAME: theoretical analysis and experimental verification

In this contribution, the nonlinear dynamic behaviour of reactive distillation columns for the production of MTBE and TAME is studied. The focus is on steady-state multiplicity and a rigorous bifurcation analysis of pilot plant reactive distillation columns for both processes is presented. The different sources and physical causes for the existence of multiple steady-states in MTBE and TAME synthesis are discussed. Further, a rigorous experimental verification of steady-state multiplicity in a pilot plant reactive distillation column for the production of TAME is presented. Finally, some remarks on the implications of multiple steady-states on column operation are made.

Limiting current behaviour of the direct methanol fuel cell

Limiting current density data for a liquid feed direct methanol fuel cell based on a Nafion ® solid polymer electrolyte membrane are reported. The cell uses a membrane electrode assembly composed of porous PtRucarbon supported catalyst anode, Ptcarbon supported cathode covered by carbon cloth backing layers. The effect of cell temperature, air cathode pressure, methanol fuel flow rate, temperature and methanol concentration on the limiting current behaviour are described. Limiting current data are interpreted in terms of hydrodynamics and diffusion limiting effects in the porous catalyst structure and carbon cloth backing layers. In practical fuel cells the temperature of the feed and the cell are different due to heat transfer requirements and this factor is shown to influence performance. It is also demonstrated that cell operation is possible with a methanol solution vaporised into an inert gas stream.

Direct methanol polymer electrolyte fuel cell: Analysis of charge and mass transfer in the vapour–liquid–solid system

Liquid-feed direct methanol fuel cell systems (DMFC) have a number of advantages over hydrogen fuel cells. For a DMFC no additional fuel processing is necessary, therefore, it has a lower volume and a lower weight, the existing infrastructure for fuel supply and distribution can be utilized and the fuel costs are low. However, a number of technical problems have still to be solved. The most important are the cross-over of methanol through the polymer electrolyte membrane, the removal of carbon dioxide from the anode catalyst layer, and the poor anode kinetics. These aspects are analysed by means of a steady state, isothermal cell model which accounts for the essential mass and charge transport processes in the different fuel cell layers. The model is applied to evaluate experimental current–voltage data which were obtained from a small scale cell fuelled with liquid methanol/water solutions.

Investigations on the Synthesis of Methyl Acetate in a Heterogeneous Reactive Distillation Process

The production of methyl acetate in a reactive distillation process – prior art for 15 years – is often used as an example to study the basic phenomena of reactive distillation. The present paper deals with a theoretical and experimental analysis of methyl acetate synthesis in a reactive distillation column. A design method based on the interpretation of reactive distillation line diagrams is used to identify the main process parameters and to provide a foundation for experimental investigation. The significant influence of the reflux ratio on the conversion in the column is shown by mini plant experiments using supported ion exchanger in the form of Raschig rings as a heterogeneous catalyst. These experiments demonstrate the catalytic quality of this packing material. To simulate the reactive distillation column with a simple stage-to-stage method, the separation efficiency of the catalytic rings is investigated. Comparison of experimental and simulation results reveals that a simple model based on the assumption of simultaneous chemical and phase equilibrium describes the experimental data quite well over a wide range of reflux ratios. Furthermore, simulation results show that the conversion depends less on the number of reactive stages than on the use of two feed stages.

Development of a new catalytic distillation process for fuel ethers via a detailed nonequilibrium model

Fuel ethers such as MTBE (methyl tertiary butyl ether), TAME (tertiary amyl methyl ether) and ETBE (ethyl tertiary butyl ether) can be produced very efficiently from alcohols (methanol/ethanol) and olefin cuts (C4/C5) in reactive distillation columns which are packed with acid ion exchange rings. In the present contribution first a detailed three phase nonequilibrium model for such a packed catalytic distillation column will be presented. Then it will be demonstrated how this model can be used for the prediction of suitable operating conditions and optimal arrangement of feed streams and packings. Finally the simulated results will be validated by comparison with experimental data from a laboratory scale MTBE-column.

Residue curve maps for heterogeneously catalysed reactive distillation of fuel ethers MTBE and TAME

The liquid-phase synthesis of octane enhancing ethers like methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME) is increasingly realized as a reactive distillation process catalysed by ion exchange resins. To study the occurring physical and chemical phenomena of those systems, the present paper deals with a reactive batch distillation process in a heated still. The model is based on phase equilibrium between the liquid and the vapour phase and an adequate kinetic formulation of the heterogeneously catalysed etherification by application of the generalized Langmuir-Hinshelwood rate expressions. To describe the non-ideal mixture behaviour, liquid-phase activities are introduced. Simulation results show that the shape of residue curves depends on two essential parameters, the Damkohler number Da and the operating pressure p. The values of these parameters influence the existence and the position of stable nodes, saddles and separatrices in the residue curve map. The evaluated trajectories deviate quantitatively from those recently reported for homogeneous catalysis. The most interesting result lies in the fact that the lightest boiling component can be enriched in the still.