J. M. Ortiz de Zárate

Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.

Concentration of Bovine Serum Albumin Aqueous Solutions by Membrane Distillation

ABSTRACT The use of membrane distillation in the concentration of protein solutions is proposed. Semidilute BSA solutions (concentrations ranging between 40 and 100%), at pH 7 were employed. Transmembrane volume fluxes and evolution of the BSA concentration were measured at different conditions. Temperatures ranged between 20 and 38°C. The filtration cell was operated at laminar regime with tangential velocities of 579 and 89cm-s°. Special attention was devoted to analysis of the temperature polarization and fouling effects. The results obtained show that fouling effects are practically absent in membrane distillation operations with BSA solutions, at least for concentrations up to 10% w/w and in the range of crossflow velocities employed. This may be due to the fact that the transmembrane fluxes are below the value of the critical flux for fouling.

Investigation of Fickian diffusion in the ternary mixture of 1,2,3,4-tetrahydronaphthalene, isobutylbenzene, and dodecane

We present a comprehensive analysis of experimental results obtained for Fickian diffusion in the benchmark ternary liquid mixture of 1,2,3,4-tetrahydronaphthalene, isobutylbenzene, and dodecane (nC12) with equal mass fractions. Isothermal diffusion coefficients have been measured by two independent experimental methods: by Taylor dispersion technique, and by a counter flow cell fitted with an optical interferometry device. The experimental diffusion matrices have been critically analyzed regarding the Onsager reciprocal relations, for which we introduce a matrix asymmetry index s(2) that is independent of the frame of reference and the component order.

Finite-size effects on fluctuations in a fluid out of thermal equilibrium

In this paper, we consider a horizontal liquid layer in the presence of a stationary temperature gradient. Specifically, we calculate the structure factor neglecting gravity, but taking into account the finite height of the liquid layer. For this purpose, we consider the linearized Boussinesq equations, in the limit of negligible Rayleigh number, supplemented with Langevin noise terms and assuming free-slip boundary conditions. The nonequilibrium temperature fluctuations are obtained by expanding the solution in a complete set of orthogonal functions satisfying the boundary conditions. It is shown that the finite height of the system restricts the spatial range of the temperature fluctuations not only in the direction of the temperature gradient, but also in the horizontal direction away from the boundaries. It is demonstrated that the q−4 dependence of the structure factor in the absence of finite-size effects now crosses over to a q2 dependence for very small values of the wave number q. Estimates of the wave numbers where light-scattering experiments will be affected by these finite-size effects are presented.

Fickian Diffusion in Ternary Mixtures Composed by 1,2,3,4-Tetrahydronaphthalene, Isobutylbenzene, and n-Dodecane.

We report the Fickian diffusion coefficients in 20 ternary mixtures formed by 1,2,3,4-tetrahydronaphthalene (THN), isobutylbenzene (IBB) and n-dodecane (nC12) measured by the Taylor dispersion technique at 298.1 K and atmospheric pressure. Four diffusion coefficients of the ternary mixtures were measured along six concentration paths starting on one binary subsystem and moving toward the other one. We found expressions for the diffusion matrix of a ternary mixture approaching to the binary limits. The measured diffusion coefficients were thoroughly verified by comparison with the theoretical asymptotic behavior. The main diffusion coefficients vary smoothly over the entire concentration space and D11 is always larger than D22. One of the two cross-diffusion coefficients is of the same order of magnitude as the main ones and, hence, not negligible, whereas the other one is close to zero. The investigated mixtures also comprise compositions that were examined in microgravity experiments in the ESA DCMIX1 project.

Nonequilibrium Casimir-like Forces in Liquid Mixtures

In this Letter, we consider a liquid mixture confined between two thermally conducting walls subjected to a stationary temperature gradient. While in a one-component liquid nonequilibrium fluctuation forces appear inside the liquid layer, nonequilibrium fluctuations in a mixture induce a Casimir-like force on the walls. The physical reason is that the temperature gradient induces large concentration fluctuations through the Soret effect. Unlike temperature fluctuations, nonequilibrium concentration fluctuations are also present near a perfectly thermally conducting wall. The magnitude of the fluctuation-induced Casimir force is proportional to the square of the Soret coefficient and is related to the concentration dependence of the heat and volume of mixing.

Light scattering from nonequilibrium concentration fluctuations in a polymer solution

We have performed light-scattering measurements in dilute and semidilute polymer solutions of polystyrene in toluene when subjected to stationary temperature gradients. Five solutions with concentrations below and one solution with a concentration above the overlap concentration were investigated. The experiments confirm the presence of long-range nonequilibrium concentration fluctuations which are proportional to (del T)(2)/k(4), where del T is the applied temperature gradient and k is the wave number of the fluctuations. In addition, we demonstrate that the strength of the nonequilibrium concentration fluctuations, observed in the dilute and semidilute solution regime, agrees with theoretical values calculated from fluctuating hydrodynamics. Further theoretical and experimental work will be needed to understand nonequilibrium fluctuations in polymer solutions at higher concentrations.

Coupled Phenomena Membrane Distillation and Osmotic Distillation through a Porous Hydrophobic Membrane

Abstract Water transport in the vapor phase through a porous hydrophobic membrane has been studied in different experimental situations. Pure water and/or different aqueous solutions of sodium chloride, ranging from 0.5 to 5 mol/L, were employed on both sides of the membrane. The experiments were carried out under temperature differences varying between 5 and 30 K, and at mean temperatures varying between 20 and 40[ddot]C. The stirring rate was varied between 0 and 350 rpm. The results were interpreted based on the existence of unstirred polarization layers.

Giant Casimir effect in fluids in nonequilibrium steady states

In this Letter, we consider the fluctuation-induced force exerted between two plates separated by a distance L in a fluid with a temperature gradient. We predict that for a range of distances L, this nonequilibrium force is anomalously large compared to other Casimir forces. The physical reason is that correlations in a nonequilibrium fluid are generally of longer range than other correlations, even than those near an equilibrium critical point. This giant Casimir force is related to a divergent Burnett coefficient that characterizes an Onsager cross effect between the pressure and the temperature gradient. The predicted Casimir force should be detectable with currently available experimental techniques.

Non-isothermal solute transport through PTFE membranes

Abstract Experiments of solute transport through porous and hydrophobic membranes have been carried out in non-isothermal conditions and in the absence of net volume flux. Three commercial membranes of PTFE have been used. Aqueous solutions of NaCl and KCl, at various concentrations, have been studied. A model is proposed which permits the determination of the non-isothermal permeability from concentration measurements. The physical nature of the transport mechanism is discussed. The influence of relevant process parameters such as mean temperature, initial concentration and stirring rate has been examined.