Treatment of cooling tower blowdown water containing silica, calcium and magnesium by electrocoagulation.

This research investigated the effectiveness of electrocoagulation using iron and aluminium electrodes for treating cooling tower blowdown (CTB) waters containing dissolved silica (Si(OH)(4)), Ca(2 + ) and Mg(2 + ). The removal of each target species was measured as a function of the coagulant dose in simulated CTB waters with initial pH values of 5, 7, and 9. Experiments were also performed to investigate the effect of antiscaling compounds and coagulation aids on hardness ion removal. Both iron and aluminum electrodes were effective at removing dissolved silica. For coagulant doses < or =3 mM, silica removal was a linear function of the coagulant dose, with 0.4 to 0.5 moles of silica removed per mole of iron or aluminium. Iron electrodes were only 30% as effective at removing Ca(2 + ) and Mg(2 + ) as compared to silica. There was no measurable removal of hardness ions by aluminium electrodes in the absence of organic additives. Phosphonate based antiscaling compounds were uniformly effective at increasing the removal of Ca(2 + ) and Mg(2 + ) by both iron and aluminium electrodes. Cationic and amphoteric polymers used as coagulation aids were also effective at increasing hardness ion removal.

Simulation of electrophoretic separations by the flux-corrected transport method.

Electrophoretic separations at typical experimental electric field strengths have been simulated by applying the flux-corrected transport (FCT) finite difference method to the transient, one-dimensional electrophoresis model. The performance of FCT on simulations of zone electrophoresis (ZE), isotachophoresis (ITP), and isoelectric focusing (IEF) has been evaluated. An FCT algorithm, with a three-point, central spatial discretization, yields numerical solutions without numerical oscillations or spurious peaks, which have plagued previously-published second-order solutions to benchmark ZE and ITP problems. Moreover, the FCT technique captures sharp zone boundaries and IEF peaks more accurately than previously-published, first-order upwind schemes.

Simulation of electrophoretic separations: effect of numerical and molecular diffusion on pH calculations in poorly buffered systems.

A poorly buffered cationic isotachophoresis separation, first simulated by Reijenga and Kasica, has been revisited to demonstrate that an inconsistent description of solute and charge transport can lead to significant errors in the pH calculation. The separation is first simulated using a second-order finite difference scheme to show that omission of molecular diffusion from the charge balance results in a pH profile with spurious dips in the steady-state zone boundaries. The separation is also simulated using two first-order methods that employ numerical diffusion to stabilize solutions against spatiotemporal oscillations. Similar pH dips are generated by these first-order schemes, even when molecular diffusion is included in the charge balance, if numerical diffusion is not considered amongst the charge transport mechanisms. When numerical diffusion, inherent in the discretization of the component balances, is introduced to the charge balance, the spurious pH dips are eliminated. The results indicate that (i) pH dips originally reported by Reijenga and Kasicka are merely artifacts of their numerical model, and (ii) nonoscillatory numerical techniques, such as upwinding and flux limiters, should incorporate artificial transport mechanisms in the charge as well as the solute balances.

Electrohydrodynamic effects in continuous flow electrophoresis.

We demonstrate experimentally and theoretically the importance of electrohydrodynamic (EHD) flows in continuous-flow electrophoresis (CFE) separations. These flows are associated with variations in the conductivity or dielectric constant, and are quadratic in the field strength. They appear to be the main cause of extraneous and undesired flows in CFE which have degraded separation performance and have until now not been explained. We discuss the importance of EHD flows relative to other effects. We also describe possible techniques for reducing the associated degradation of CFE separations.