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

Influence of temperature gradients on mono- and divalent ion transport in electrodialysis at limiting currents

Temperature gradients in electrodialysis (ED) stacks can potentially enhance the efficiency of charge separation and the selective transport of ions. We have previously investigated temperature gradients in the Ohmic regime but not in the limiting current regime, where diffusion of ions towards the membrane determines the transport rate and temperature gradients potentially have the largest influence. In this research, commercial ion exchange membranes (FAS and FKS, FUMATECH, Germany) are used for the investigation of temperature gradients in the limiting current regime. In contrast to the Ohmic regime, we find that heating the diluted stream increases the current obtained (at a constant applied potential) when compared to heating the concentrate stream in systems containing monovalent KCl and NaCl solutions. For mixtures of mono- and divalent ions, the temperature gradient has a larger influence on the selectivity of the separation. If the desalinated stream is heated, divalent Mg2+ ions show a higher transport than the monovalent K+ and Na+ ions. This is due to the enhanced competitive transport of the mono- and divalent ions under the application of a temperature gradient. These results show the potential application and relevance of temperature gradients to enhance the selective separation of mono- and divalent ions

Influence of temperature gradients on charge transport in asymmetric nanochannels

Charge selective asymmetric nanochannels are used for a variety of applications, such as nanofluidic sensing devices and energy conversion applications. In this paper, we numerically investigate the influence of an applied temperature difference over tapered nanochannels on the resulting charge transport and flow behavior. Using a temperature-dependent formulation of the coupled Poisson-Nernst-Planck and Navier-Stokes equations, various nanochannel geometries are investigated. Temperature has a large influence on the total ion transport, as the diffusivity of ions and viscosity of the solution are strongly affected by temperature. We find that the selectivity of the nanochannels is enhanced with increasing asymmetry ratios, while the total current is reduced at higher asymmetry cases. Most interestingly, we find that applying a temperature gradient along the electric field and along the asymmetry direction of the nanochannel enhances the selectivity of the tapered channels even further, while a temperature gradient countering the electric field reduces the selectivity of the nanochannel. Current rectification is enhanced in asymmetric nanochannels if a temperature gradient is applied, independent of the direction of the temperature difference. However, the degree of rectification is dependent on the direction of the temperature gradient with respect to the channel geometry and the electric field direction. The enhanced selectivity of nanochannels due to applied temperature gradients could result in more efficient operation in energy harvesting or desalination applications, motivating experimental investigations

Enhanced ion transport using geometrically structured charge selective interfaces

A microfluidic platform containing charged hydrogels is used to investigate the effect of geometry on charge transport in electrodialysis applications. The influence of heterogeneity on ion transport is determined by electrical characterization and fluorescence microscopy of three different hydrogel geometries. We found that electroosmotic transport of ions towards the hydrogel is enhanced in heterogeneous geometries, as a result of the inhomogeneous electric field in these systems. This yields higher ionic currents for equal applied potentials when compared to homogeneous geometries. The contribution of electroosmotic transport is present in all current regimes, including the Ohmic regime. We also found that the onset of the overlimiting current occurs at lower potentials due to the increased heterogeneity in hydrogel shape, owing to the non-uniform electric field distribution in these systems. Pinning of ion depletion and enrichment zones is observed in the heterogeneous hydrogel systems, due to electroosmotic flows and electrokinetic instabilities. Our platform is highly versatile for the rapid investigation of the effects of membrane topology on general electrodialysis characteristics, including the formation of ion depletion zones on the micro-scale and the onset of the overlimiting current

Effect of temperature gradients in (reverse) electrodialysis in the Ohmic regime

Electrodialysis (ED) and reverse electrodialysis (RED) are processes for the production of desalinated water (ED) and power (RED). Temperature of the feed streams can strongly influence the performance of both processes. In this research, commercial membranes are used for the investigation of temperature and temperature gradients on ED and RED processes. We find that the energy required for ED processes can be reduced by 9% if the temperature of one of the feed streams is increased by 20 °C, while maintaining the charge-selectivity of the membranes. The direction of the temperature gradient did not have a significant influence on the efficiency and selectivity of ED in the Ohmic regime. In RED, we find an increase in obtained gross power density over 25% for the process when one feed stream is heated to 40 °C instead of 20 °C. This work experimentally demonstrates that utilization of low-grade waste heat from industrial processes can yield significant reduction of energy costs in ED processes, or result in higher power densities for RED systems where the increase in temperature of a single feed stream already yields significant efficiency improvements

Desalination by electrodialysis using a stack of charged hydrogels in a microfluidic platform

We show a new approach for desalinating water, using a stack of periodic hydrogel structures in a microfluidic platform. This technique utilizes alternating anion- (AEH) and cation-exchange hydrogels (CEH) locally fabricated in confined compartments by capillary line pinning. Parallel streams of concentrated and ion-depleted water are formed in continuous flow when applying a potential difference across the microchip. Different electric fields (10-100 V/cm) and fluid flow rates (0-5 μl/min) are investigated

Design and intensification of industrial DADPM process

Process intensification is an essential method for the improvement of energy and material efficiency, waste reduction and simplification of industrial processes. In this research a Process Intensification methodology developed by Lutze, Gani and Woodley at the Computer Aided Process Engineering Center (CAPEC) at DTU in Denmark is used for the intensification of the 4,4′-methylenedianiline (DADPM) process at Huntsman B.V. in the Netherlands. The goal of this research was the extension of the DTU methodology for applicability on running, industrial processes and improvement of the Huntsman process, focus is on reduction of operation costs. We have shown in the DADPM case that an analysis of the performance per section or unit operation and the mutual interactions provide essential additional information that is not being detected by the DTU method. We demonstrated how good engineering practice and heuristics can also reduce the number of process options that have to be modelled in detail. Selection of the optimal process is done based on a quantitative analysis of several intensified process options which all obey all required constraints. Equipment models were built in Excel and integrated in an Aspen Plus process flowsheet containing 27 different process options. A sensitivity analysis is done using Aspen, yielding the optimized and intensified process for DADPM production. Energy costs for the DADPM process are reduced by 24% using a combination of both heuristic and methodology-based intensification. We conclude that the method developed by Lutze et al. is a valuable tool for PI and process analysis and synthesis. The extension developed using heuristics, provides additional insight, traces the process weak points, facilitates implementation of new technology and reduces calculations

Influence of temperature gradients on charge transport in asymmetric nanochannels

Charge selective asymmetric nanochannels are used for a variety of applications, such as nanofluidic sensing devices and energy conversion applications. In this paper, we numerically investigate the influence of an applied temperature difference over tapered nanochannels on the resulting charge transport and flow behavior. Using a temperature-dependent formulation of the coupled Poisson-Nernst-Planck and Navier-Stokes equations, various nanochannel geometries are investigated. Temperature has a large influence on the total ion transport, as the diffusivity of ions and viscosity of the solution are strongly affected by temperature. We find that the selectivity of the nanochannels is enhanced with increasing asymmetry ratios, while the total current is reduced at higher asymmetry cases. Most interestingly, we find that applying a temperature gradient along the electric field and along the asymmetry direction of the nanochannel enhances the selectivity of the tapered channels even further, while a temperature gradient countering the electric field reduces the selectivity of the nanochannel. Current rectification is enhanced in asymmetric nanochannels if a temperature gradient is applied, independent of the direction of the temperature difference. However, the degree of rectification is dependent on the direction of the temperature gradient with respect to the channel geometry and the electric field direction. The enhanced selectivity of nanochannels due to applied temperature gradients could result in more efficient operation in energy harvesting or desalination applications, motivating experimental investigations