Anne M. Benneker

Temperature effects on the electrohydrodynamic and electrokinetic behaviour of ion-selective nanochannels.

A non-isothermal formulation of the Poisson-Nernst-Planck with Navier-Stokes equations is used to study the influence of heating effects in the form of Joule heating and viscous dissipation and imposed temperature gradients on a microchannel/nanochannel system. The system is solved numerically under various cases in order to determine the influence of temperature-related effects on ion-selectivity, flux and fluid flow profiles, as well as coupling between these phenomena. It is demonstrated that for a larger reservoir system, the effects of Joule heating and viscous dissipation only become relevant for higher salt concentrations and electric field strengths than are compatible with ion-selectivity due to Debye layer overlap. More interestingly, it is shown that using different temperature reservoirs can have a strong influence on ion-selectivity, as well as the induced electrohydrodynamic flows.

Hydrogenation of Carbon Dioxide for Methanol Production

A process for the hydrogenation of CO2 to methanol with a capacity of 10 kt/y methanol is designed in a systematic way. The challenge will be to obtain a process with a high net CO2 conversion. From initially four conceptual designs the most feasible is selected and designed in more detail. The feeds are purified, heated to 250 °C and fed to a fluidized bed membrane reactor equipped with a Cu/ZnO/Al2O3 catalyst. Zeolite membranes mainly remove the methanol and shift the equilibrium reaction towards methanol. A yield of 25 % per pass is obtained. The permeate and the water-methanol mixture from the phase separator is finally separated in a distillation column. In the final design 15.4 kt/y of carbon dioxide is needed in order to produce 10 kt/y methanol. The net CO2 reduction is about 2/3, which is significant. The process is technical but currently not economically feasible.

Observation and experimental investigation of confinement effects on ion transport and electrokinetic flows at the microscale

Electrokinetic effects adjacent to charge-selective interfaces (CSI) have been experimentally investigated in microfluidic platforms in order to gain understanding on underlying phenomena of ion transport at elevated applied voltages. We experimentally investigate the influence of geometry and multiple array densities of the CSI on concentration and flow profiles in a microfluidic set-up using nanochannels as the CSI. Particle tracking obtained under chronoamperometric measurements show the development of vortices in the microchannel adjacent to the nanochannels. We found that the direction of the electric field and the potential drop inside the microchannel has a large influence on the ion transport through the interface, for example by inducing immediate wall electroosmotic flow. In microfluidic devices, the electric field may not be directed normal to the interface, which can result in an inefficient use of the CSI. Multiple vortices are observed adjacent to the CSI, growing in size and velocity as a function of time and dependent on their location in the microfluidic device. Local velocities inside the vortices are measured to be more than 1.5 mm/s. Vortex speed, as well as flow speed in the channel, are dependent on the geometry of the CSI and the distance from the electrode.

From small to big : ion transport at interfaces

Ion transport is crucial in many applications such as desalination and fuel cells. This thesis describes fundamental investigations on the effect of temperature and permittivity gradients on ion transport at charge selective interfaces for electrodialysis systems. Numerical and experimental investigations on different length scales were combined to investigate charge transport in the characteristic Ohmic, limiting and overlimiting current regimes of electrodialysis systems. Temperature gradients can enhance the selective transport of ions through changing ionic diffusivity. In systems with permittivity gradients at the charge selective interface, an immediate electro-osmotic flow is induced even in the Ohmic regime. This can greatly enhance ion transport towards the interface, without having to operate in overlimiting current conditions. The results of this work have implications in numerous processes dependent on ion transport phenomena such as fuel cells and desalination and yields fundamental insights into the nature of charge transport.