Serena Bandini

Nanofiltration modeling: the role of dielectric exclusion in membrane characterization

Abstract A general model is presented, called Donnan steric pore model & dielectric exclusion (DSPMD the hindered nature of diffusion and convection of the species inside the membrane is considered. Ionic partitioning at the interfaces between the membrane and the external phases takes into account of three separation mechanisms: steric hindrance, Donnan equilibrium and dielectric exclusion. The role of the difference existing between the dielectric constant of the aqueous solution in the pores and the dielectric constant of the membrane material is assumed dominant in determining the rejection mechanism related to the dielectric effects. The membrane is characterized through the use of adjustable parameters such as the average pore radius, the effective membrane thickness and the volumetric charge density. A general assessment is presented for membrane characterization. A new procedure is introduced for the membrane parameters calculation, based on a simple analytical relationship developed to describe rejection of single symmetric electrolytes. Two versions of the DSPM&DE model are presented, in which the “integral” model is a weak simplification of the general “differential” model. The validity of the models presented is fully demonstrated in the case of negatively charged membranes, through the comparison with experimental results performed in a wide range of operative conditions. The separation effect related to the dielectric exclusion is relevant with respect to Donnan equilibrium in determining bivalent counter-ions rejection, such as CaCl 2 as well as MgSO 4 , whereas dielectric effects are not so remarkable in the case of mixtures containing various co-ions, such as NaCl+Na 2 SO 4 .

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.

Donnan equilibrium and dielectric exclusion for characterization of nanofiltration membranes

Abstract Experimental and modelling analysis is presented for characterization of polymeric nanofiltration membranes; the application considered is the separation of aqueous mixtures containing strong electrolytes and/or uncharged solutes. Mass transfer through the membrane is based on the extended Nernst—Planck equation; ion partitioning is described at the interfaces between the membrane and the external solution taking into account of steric hindrance, Donnan equilibrium and dielectric exclusion. The membrane is characterized through the pore radius, the charge density and the membrane thickness. Weak simplifications are introduced into the general model to obtain analytical relationships for the salt rejection, as well as for the ion concentration profile, as a function of membrane parameters and operative conditions. Model predictions are successfully compared with experimental data for all the mixtures investigated. Dielectric exclusion phenomenon is relevant with respect to Donnan equilibrium in determining the rejection of bivalent ions.

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.

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.

Gas membrane extraction of ethanol by glycols: Experiments and modelling

Abstract The removal of ethanol from fermentation broth by gas membrane extraction was analysed. Aqueous solutions of glycols were used as extractants. The extractant and the broth were separated by a microporous hydrophobic membrane which was not wetted by the broth or the extractant. Ethanol and water vapors passed across the membrane by diffusion through the stagnant air entrapped within the pores. Since the glycols alter the liquid-vapor equilibrium of the ethanol-water mixtures by reducing the ethanol content in the vapor phase with respect to the binary system, ethanol vapors preferentially diffused through the stagnant gas layer. 1,3-Butanediol, ethylene, diethylene, triethylene and tetraethylene glycols were tested as extractants. A detailed model including all the transport phenomena involved in the process was developed and compared with the experimental results.

Ethanol removal from fermentation broth by gas membrane extraction

Abstract A new type of membrane extraction for in situ removal of ethanol from fermentation broth is presented. Aqueous solutions of propylene glycol are used as extractants. The extractatant and the broth are separated by a microporous hydrophobic membrane which is not penetrated by the broth or by the extractant. As a consequence a thin gas layer, essentially air, is immobilised within the membrane pores and separates the two liquid phases (i.e. a gas membrane). Vapour-liquid equilibria are established at both membrane sides; because glycols reduce the ethanol content of the equilibrium vapour phase with respect to the binary system, ethanol vapours preferentially diffuse through the stagnant gas layer.

Transport Phenomena in Nanofiltration Membranes

In this chapter, a general assessment of nanofiltration modeling is presented. Transport phenomena across the membrane as well as partitioning mechanisms between the membrane and the external phases are described in detail. First, the more interesting methods, which had been applied to approach the solution of the membrane characterization problem, are reviewed. Second, the Donnan steric pore and dielectric exclusion model is introduced. It is the evolution of various versions developed in different steps by some authors and it can be recognized as the simpler and complete model developed at present to simulate the behavior of nanofiltration membranes. The extended Nernst–Planck equation, appropriately modified on account of hindered diffusion and convection inside narrow pores, is used to describe mass transfer across the membrane. Steric hindrance, solution nonideality, Donnan equilibrium, as well as dielectric exclusion are considered as the main partitioning mechanisms existing at the interfaces between the membrane and the external phases. The strengths and critical features of the model are discussed in detail; a general assessment of the membrane characterization procedures is also reported.

Ethanol Production by Extractive Fermentation: A Novel Membrane Extraction Technique

A new type of membrane extraction for in situ removal of ethanol from fermentation broth is presented. Aqueous solutions of Ethylene or Propylene Glycol are used as extractants. The extractant and the broth are separated by a microporous hydrophobic membrane which is not penetrated by the broth or by the extractant. As a consequence a thin gas layer, essentially air, is inmobilized within the membrane pores and separates the two liquid phases (gas membrane). Glycols lower the relative volatility of ethanol with respect to water and as a consequence ethanol vapours preferentially diffuse through the stagnant gas layer.