Giulio C. Sarti

Separation efficiency in vacuum membrane distillation

Abstract Vacuum membrane distillation has been analyzed as a separation process for aqueous mixtures. The total permeate flux obtained is affected by two simultaneous resistances, those due to the heat and mass transfer processes which take place through the liquid phase and through the membrane respectively. On the other hand, the separation factor is highly sensitive to the mass transfer resistance existing within the liquid phase. Different applications have been considered such as extraction of organic components, degassing of water and evaporation of pure water. In all cases appropriate design equations for shell and tube equipment have been obtained and solved. In parallel, appropriate criteria for a priori recognition of the principal resistances have been formulated.

Extraction of organic components from aqueous streams by vacuum membrane distillation

Abstract The removal of volatile organic compounds from aqueous streams by vacuum membrane distillation (VMD) has been analyzed. VMD is an evaporation process which takes place through microporous hydrophobic membranes; at low pressure the mass transfer through the membrane is generally dominated by the Knudsen mechanism, while the process selectivity is essentially determined by the liquid-vapor equilibrium conditions existing at the interface. Dilute aqueous mixtures containing ethanol or methylterbutyl ether have been experimentally investigated, in a wide range of operating conditions. The role of concentration-polarization phenomena on the separation factor was also investigated. A detailed model of the transport phenomena involved in the process is developed and compared with the experimental data. A VMD system is finally designed for the purification of waste waters and the related treatment costs are evaluated.

Low energy cost desalination processes using hydrophobic membranes

Abstract A thermally driven mass transport process across hydrophobic membranes has been investigated. A hydrophobic membrane separates two aqueous liquid phases and a temperature difference is maintained as the driving force for desalination. A quantitative theory for the process has been developed based on the evaporation-condensation steps occurring at the membrane interfaces. The role of the relevant process parameters has been investigated both theoretically and experimentally.

Separation of liquid mixtures by membrane distillation

Abstract An experimental and theoretical analysis of the membrane distillation process is presented for aqueous solutions containing ethanol. Different operational conditions are investigated for the air gap membrane distillation case. A computer simulation of the process is presented and compared with the experimental results; the effects of the relevant process parameters on the separation factor are then analyzed. The relevant role of the temperature difference between the evaporation and the condensation surfaces is pointed out, and the unexpected changes in selectivity with the feed composition are discussed.

Predictions of the solubility of gases in glassy polymers based on the NELF model

Abstract A reliable, predictive model for the solubility of gases and vapours in glassy polymers has been recently presented: it is based on the free energy expression for the polymer penetrant mixture as obtained from the lattice fluid theory and on the idea that the partial polymer density is an internal state variable for the system. When the polymer density for the sorption condition is known, the model can be used in a pure predictive way as the solubility is estimated on the basis of pure penetrant and pure polymer PVT properties. In this work the basic assumptions of the model and its development are discussed in detail. The pseudo-equilibrium condition and the calculation of the penetrant solubility under pseudo-equilibrium conditions are clearly stated. In closure, several examples comparing model predictions and experimental results for the solubility in glassy polymers are presented. In all the cases considered, the comparison confirms the model capability to satisfactorily predict solubility data in nonequilibrium glassy polymers and supports the validity of the underlying physical model.

Solvent osmotic stresses and the prediction of Case II transport kinetics

Abstract The rate controlling step in Case II transport kinetics is the swelling which occurs at the internal moving boundary. A physical model describing the swelling kinetics of glassy polymers in liquids is presented here. Using a thermodynamic argument, the stress induced by the penetrant on the glassy matrix is evaluated in terms of the penetrant concentration. The velocity of the swelling front is expressed in terms of the solvent stress, using the same functional relationship which gives the mechanical craze propagation rate, in terms of the mechanical stress. The resulting model permits the prediction of the kinetics of the swelling front from an independent characterization of mechanical properties.

Hydrocarbon and fluorocarbon solubility and dilation in poly(dimethylsiloxane): Comparison of experimental data with predictions of the Sanchez–Lacombe equation of state

Sorption and dilation isotherms are reported for a series of gases (N2, O2, CO2), hydrocarbon vapors (CH4, C2H6, C3H8), and their fluorocarbon analogs (CF4, C2F6, C3F8) in poly(dimethylsiloxane) (PDMS) at 35°C and pressures up to 27 atmospheres. The hydrocarbons are significantly more soluble in hydrocarbon-based PDMS than their fluorocarbon analogs. Infinite dilution partial molar volumes of both hydrocarbons and fluorocarbons in PDMS were similar to their partial molar volumes in other hydrocarbon polymers and in organic liquids. Except for C2H6 and C3H8, partial molar volume was independent of penetrant concentration. For these penetrants, partial molar volume increased with increasing concentration. The Sanchez–Lacombe equation of state is used to predict gas solubility and polymer dilation. If the Sanchez–Lacombe model is used with no adjustable parameters, solubility is always overpredicted. The extent of overprediction is more substantial for fluorocarbon penetrants than for hydrocarbons. Very good fits of the model to the experimental sorption and dilation data are obtained when the mixture interaction parameter is treated as an adjustable parameter. For the hydrocarbons, the interaction parameter is approximately 0.96, and for the fluorocarbons, it is approximately 0.87. These values suggest less favorable interactions between the hydrocarbon-based PDMS matrix and the fluorocarbon penetrants than between PDMS and hydrocarbons.

Concentration of must through vacuum membrane distillation

Vacuum Membrane Distillation is studied for the concentration of fruit juices up to 50°Brix. The fluxes of water and of the relevant aroma compounds are experimentally examined for a typical must, as a function of temperature, sugar content and downstream pressure. The process leads to juice concentrates still retaining interesting amounts of the aroma compounds. A process analysis is also shown for must concentration up to 50°Brix.

A desalination process through sweeping gas membrane distillation

Abstract A continuous desalinaton process is analyzed, based on a membrane distillation procedure. Both flat and tubular porous hydrophobic membranes are considered. The aqueous solution flows in direct contact with one side of the membrane, while on the other side a sweeping gas flows and strips out water vapour. The process is throughly studied experimentally by inspecting the influence on the evaporation efficiency of the relevant process parameters such as inlet temperatures and flow rates. A comprehensive mathematical model is then presented and compared with the experimental results.

Low temperature distillation through hydrophobic membranes

Abstract The use of hydrophobia porous membranes makes it possible to maintain liquid-vapour interfaces localized at a membrane surface. Based on that, thermally driven separation processes were obtained through the membrane and thoroughly analyzed both experimentally and theoretically, Two experimental conditions were used: i) the porous membrane is in direct contact with two liquid aqueous phases on both sides and the vapour phase is trapped inside the pores (capillary distillation); ii) on one side of the porous membrane there is a warm aqueous solution, while an additional gaseous gap is maintained on the opposite side of the porous membrane; the vaporizing component diffuses through the entire gas phase and condenses at a cold surface confining the gaseous gap (cold wall distillation). The mathematical model, describing both the separation rate and the energy flux is presented and compared with the experimental results. The influence of the gas membrane thickness is also discussed.