Gun Trägårdh

Membrane emulsification — a literature review

Abstract Membrane emulsification is a simple method that has received increasing attention over the last 10 years, with potential applications in many fields. Experimental studies which have focused mainly on investigations of process parameters such as, membrane type, average pore size and porosity, crossflow velocity, transmembrane pressure and emulsifier, are reviewed. By careful choice of these parameters, emulsions with narrow emulsion droplet size distributions have been produced, with average droplet sizes ranging between 2 and 10 times the nominal membrane pore diameter. The effects of individual parameters are reasonably well understood, particularly at a qualitative level. Results can be explained by a direct influence of the membrane pore size, diameter and distribution. Interfacial tension and the action of wall shear stress are also important. In comparison with conventional turbulence based methods, such as homogenization and rotor-stator systems, less energy is needed to produce droplets of a given size using membrane emulsification. However, one of the main limiting factors with regard to industrial scale-up can be the often low level of dispersed phase flux through the membrane, especially for small submicron droplets.

The effect of protein fouling in microfiltration and ultrafiltration on permeate flux, protein retention and selectivity: A literature review

Abstract The application of ultrafiltration and microfiltration to protein solutions is hindered by membrane fouling that results in a decrease in the permeate flux and protein transmission with time. This review describes the evidence for fouling both on the membrane surface and within the membrane pores. The effect of the feed properties, membrane properties and the operating conditions on membrane behaviour and fouling are discussed. In ultrafiltration, fouling occurs predominantly on the membrane surface where the dynamic membrane controls membrane behaviour. In microfiltration, severe pore plugging by protein occurs, in spite of the pores being an order of magnitude larger than the protein.

Pervaporation of dilute organic-waters mixtures. A literature review on modelling studies and applications to aroma compound recovery

Pervaporation as a tool for the removal of organic substances from dilute aqueous solutions has been reviewed. During pervaporation the membrane governs the performance of the process. The removal of organics from water is accomplished with membranes made of elastomeric or hydrophobic polymer materials. The performance of these membranes, measured as fluxes and enrichment factors, differs considerably. Total fluxes vary between 24 and 2728 g/m2-hr while the enrichment factor varies between 0.6 and 372. Modelling of the process involves four successive steps, a mass transfer from the bulk of the feed to the feed-membrane interface, partition of the penetrants between the feed and the membrane, diffusion in the membrane and desorption at the membrane-permeate interface. All these steps are crucial for the overall performance of the process. Several models with different approaches and validities exist for each step. The possible of pervaporation for aroma compound recovery has been verified in a number of studies. These studies indicate that aroma compounds, especially volatile aroma compounds, can be enriched several hundreds of times with pervaporation and that the process could be profitable.

Food emulsions using membrane emulsification : conditions for producing small droplets

Abstract Ceramic membranes were used to produce oil-in-water (O/W) emulsions consisting of vegetable oil as the dispersed phase and skim milk as the dispersion medium. The purpose of the work was to find operating conditions suitable for producing small emulsion droplets, a small size being important for emulsion stability. The main parameters investigated were the effect of wall shear stress, emulsifier concentration and membrane pore size. Formation of small droplets was favoured at higher emulsifier concentrations and for a high wall shear stress using a membrane with a small nominal pore size. Submicron particles were produced at an 8% emulsifier concentration for a wall shear stress of 135 Pa using a 0.1 μm pore size membrane. Under these conditions the flux was >100 kg m−2 h−1. A high flux is important for industrial-scale production of food emulsions using membrane emulsification.

The effect of the hydrophobicity of microchannels and components in water and oil phases on droplet formation in microchannel water-in-oil emulsification

Abstract The effects of the hydrophobicity of silicon microchannels and components in the oil and water phases in the production of monodispersed water-in-oil (W/O) emulsions by microchannel emulsification was investigated. Several types of silane coupler reagents were applied to make the microchannel hydrophobic. The hydrophobicity of the surface was evaluated by contact angle measurements. Hexane, decane and oleic acid were used as the oil phase, and pure water and sucrose solution were used as the water phase. Tetraglycerol polyricinoleate, sodium bis(2-ethylhexyl) sulfosuccinate and sorbitan fatty acid esters were used as surfactants. The contact angle of a water droplet on the modified silicon microchannel plate in an oil phase was found to be dependent on the composition of the oil phase, the surfactant type and the silane coupler reagent used for surface modification. When the contact angle was greater than 120°, monodispersed water droplets were formed and the geometric standard deviation was below 1.06. The average droplet size varied from 17 to 23 μm, and was affected by the characteristics of the surfactant and the viscosities of the water and oil phases.

Crossflow Membrane Filtration Enhanced by an External DC Electric Field: A Review

Techniques for fouling prevention in membrane filtration are needed. This paper reviews the process design, theory and some applications of crossflow membrane filtration enhanced by a DC electric field. An electric field can be applied across the membrane or the membrane itself can be the electrode. The performance of the filtration process is, in both cases, primarily improved due to electrophoresis. Electro-osmosis was found to be significant in some cases when an electric field was applied across the membrane. The theory of electrically enhanced crossflow membrane filtration is quite complex and there is no generally accepted model describing this process. Significant enhancement of the flux compared with the flux with no electric field has been found in the filtration of particles or colloids with high electrophoretic mobility using a pressure at which the cake was formed without an electric field and an electric field strength at which the net particle migration was away from the membrane. Apart from enhancement of the flux, the quality of the permeate has been found to improve by applying an electric field.

A new integrated membrane process for producing clarified apple juice and apple juice aroma concentrate

An integrated membrane process for producing apple juice and apple juice aroma concentrates is proposed. The process involves the following operations: an integrated membrane reactor to clarify the raw juice; reverse osmosis (RO) to preconcentrate the juice up to 25°Brix; pervaporation (PV) to recover and concentrate aroma compounds, and a final evaporation step to concentrate apple juice up to 72°Brix. These operations were tested in laboratory and pilot plant units. Promising results were obtained with the membrane operations involved. In order to have an economic process assessment, the pilot plant units were assembled into an integrated unit and operated with raw apple juice. The products were more clear and brilliant than apple juice produced by conventional methods. The integrated membrane process also seemed to be more advantageous on the basis of economics than the conventional one.

Pervaporation of a model apple juice aroma solution: comparison of membrane performance

Abstract The production of concentrated apple juice is associated with major physical and chemical losses of aroma compounds, due mainly to evaporation, which results in a product with an inferior sensory quality. One method of improving the sensory quality of the juice is to recover the lost aromas by pervaporation and to feed them back to the final product. The purpose of this study was to investigate the performance of six different pervaporation membranes for the recovery of apple juice aromas by pervaporation. As pervaporation of natural apple juice would cause analytical problems, a model apple juice aroma solution was developed, by identification of aroma compounds in apple juice by GC and GC-MS, and used in the experiments. Two polyoctylmethyl siloxane membranes with different porous support layers and a polydimethyl siloxane membrane proved to have very good performance, resulting in 100-fold to 1000-fold enrichment of aromas from the model apple juice aroma solution with mass transfer coefficients up to 300 kg/m2 h. Further investigations on the long-term effects, the influence of the temperature and the effect of interaction between the membrane and the apple juice, must be performed. Economical aspects must also be considered.

MODELLING OF PERVAPORATION: MODELS TO ANALYZE AND PREDICT THE MASS TRANSPORT IN PERVAPORATION

The modelling of the ma ss transfer in pervaporation is one of the fundamental aspects to understand and therefore improve the process performance. This paper reviews the different models to analyse and predict the mass transport through the selective layer in pervaporation. It therefore provides an overview combined with some guidance about the different models proposed in the literature and the applicability range of these models. The different models reviewed cover the two key mass transport steps in pervaporation: (1) sorption into the membrane, and (2) diffusion through the membrane. For the two different steps individual models will be proposed as well as models covering the mass transport across the membrane as a whole. The different models will be grouped with respect to the nature of the models: theoretical, semi-empirical or empirical. Further the applicability range of the different models regarding the different polymer classes such as glassy, semi-crystalline, or rubbery will be shown. Finally, it will be commented on the applicability of the models with regard to two main research fields in pervaporation: development of membranes and design of processes and modules. Inorganic and composite-material membranes involve additional models to analyse and predict the mass transport and have been excluded from this review.

Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect

Abstract The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The zeta-potential of Membralox® ceramic microfiltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zeta-potential is zero. The iso-electric points of new α-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the α-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point.