C. Gostoli

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.

Thermal effects in osmotic distillation

Abstract Osmotic distillation (OD) is a concentration technique for aqueous mixtures based on porous hydrophobic membranes in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. The driving force for the water vapour diffusion through the gas phase immobilised within the membrane pores is sustained by an activity difference by using a hypertonic solution, typically concentrated brines, downstream the membrane. The mass transfer causes a cooling down of the feed and a warm up of the brine, as a consequence a temperature difference is created which reduces the effective driving force for mass transfer. This ‘thermal effect’ is investigated both theoretically and experimentally, it is shown that the effect on the flux is substantial.

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.

Role of heat and mass transfer in membrane distillation process

Abstract Membrane Distillation is a separation process based on the evaporation through porous hydrophobic membranes. The membrane plays the role of physical support for the vapor-liquid interface. The process is characterized by simultaneous heat and mass transfer. In this work a simple criterion is presented to predict whether the overall permeation rate is mass or heat transfer controlled, simply based on the knowledge of the physical properties and of the transport coefficients of each intervening phase. The influence of the membrane properties and of the operating conditions on mass transfer rate and energy efficiency is discussed in some details.

The heat and mass transfer phenomena in osmotic membrane distillation

Abstract This paper presents an experimental procedure to analyse the heat and mass transfer phenomena involved in osmotic membrane distillation and to measure the related parameters. Use is made of a module containing a few membrane sheets in co-current flow configuration. Whatever the inlet feed and extractant temperatures may be (equal to each other or not), the temperature difference between the two streams reaches an asymptotic value as the residence time increases. After the asymptotic conditions are established the module can be analysed as a pseudoisothermal case.

Mass transfer in a hollow fiber dialyzer

Abstract A mathematical model is given for a hollow fiber dialyzer in which the dialysate side resistance to mass transfer is taken into account. The mass balance equations are integrated by a numerical procedure for cocurrent and countercurrent flow. Results for both cases are given in terms of the local and average Sherwood Number and of the dialyzer efficiency. Dialysate-side Sherwood Numbers are also evaluated for the boundary condition of uniform wall mass flux, and asymptotic solutions are also obtained for this boundary condition. p]A comparison of the Sherwood Numbers and of the efficieneies obtained for the various boundary conditions points out that the use of the mass transfer coefficients for uniform wall mass flux can assure reliable modelling in all the actual working conditions of a hollow fiber hemodialyzer.