Membrane emulsification: process principles

With membrane emulsification in principle monodisperse emulsions can be produced, requiring a relatively low energy density which implies that the shear stress exerted on the ingredients is low. In membrane emulsification the to-be-dispersed phase is pressed through the membrane pores; under certain conditions droplets are formed at the membrane surface. In cross-flow membrane emulsification the droplets are detached by the continuous phase flowing across the membrane surface. A limiting factor for emulsion production on a commercial scale will be a low disperse phase flux. Better knowledge of how membrane parameters affect the disperse phase flux would enable the targeted development of membranes, optimal for the process of cross-flow membrane emulsification for a given application. Therefore, the objective of this research is to gain a fundamental understanding of the mechanism of droplet formation at the membrane surface and of the flow of the disperse phase through the membrane as a function of the membrane characteristics.Droplet formation was studied at a microscopic level with computational fluid dynamics (CFD) simulations and by microscopic experiments of droplet formation at a very thin microsieve with uniform pores. Since these membranes are extremely well defined, they are a good model system for detailed study. Results from both simulations and experiments indicate that to prevent coalescence and steric hindrance of droplets, the membrane porosity should be very low. Steric hindrance resulted in polydisperse emulsions and led to coupling of droplet detachment from neighboring pores. Furthermore, although the pores all had the same diameter, the number of pores at which droplets were formed only increased gradually with increasing transmembrane pressure. This effect was further studied with a scaled-up analogon and could be modeled by taking the resistance of the pores and the resistance of a membrane substructure into account. This model is compared with a model for flow through an isotropic membrane with interconnected uniform pores and extended to describe flow through a membrane with a pore size distribution. This model is used to show that in most cases the estimation of a membrane pore size distribution by using the liquid displacement method is not correct. Just as in membrane emulsification, pores become active at higher transmembrane pressures than expected. Finally, the effects of several membrane parameters on membrane emulsification performance are summarized. As an example, the membrane area required for a typical industrial application is estimated using the models mentioned above, for different types of membranes.<br/

Fiber failure frequency and causes of hollow fiber integrity loss

Ultrafiltration (UF) and microfiltration (MF) have been successfully applied in water treatment plants to remove particles, microorganisms and viruses. Up to 5 logs removal can be obtained; nevertheless, the water quality is influenced by compromised fibers. Several methods have been developed to monitor the integrity of a membrane system, but there is no clear picture of the occurrence and causes of fiber failure. In the present study, for different commercially available membranes, an annual fiber failure rate was found, mostly between I to 10 per million fibers. Data were gathered from the literature and acquired by contacting membrane manufacturers and water treatment plants. An overview of factors playing a role in fiber failure is given: strength of the membrane material (reviewed from the literature), operating conditions and incidents, such as failing pre-treatment, were identified to be important. It is believed that a certain level of membrane leakage is acceptable as long as the integrated multi-barrier treatment process has sufficient disinfection credits. From our data it was concluded that fiber failure rates of UF/MF membranes are acceptable, but a further reduction of this rate can reduce repair costs and installation down-time

Emulsies uit membranen

Membraanemulgeren is een veelbelovende jonge techniek waarbij met weinig energie een betere productkwaliteit wordt verkregen. Op 4 juni werd door onderzoeksschool VLAG in Wageningen een themadag georganiseerd over het betreffende onderwerp. De aanleiding to deze dag was de promotie van ir. A. Gijsbertsen - Abrahamse. Zij promoveerde als eerste in Nederland op deze nieuwe technie

Influence of membrane morphology on pore activation in membrane emulsification

The low throughput of the disperse phase is one of the issues in cross-flow membrane emulsification. This is apparent in the low percentage of pores at which droplets are formed (few active pores). To determine the effect of membrane morphology on pore activation, we developed and experimentally validated a model that describes the flow phenomena in and under a membrane with uniform pores (microsieve). In this model the membrane is divided into two parts: the toplayer and the membrane substructure. The model was validated with a larger-scale physical analogon. It predicts a linear increase of the number of active pores with increasing transmembrane pressure, while the pressure difference over the active pores is independent of the transmembrane pressure as long as not all pores are active. Although the resistance of the microsieve substructure was found to be four times lower than the resistance of a single pore, the resistance of the membrane substructure had a large effect on the activation of pores. Hence, the number of active pores can be increased by increasing the ratio of flow resistance in the pores and the flow resistance in the membrane substructure. Preliminary experiments show that the gradual increase in active pores at a ceramic membrane surface can be explained in the same way. (C) 2003 Elsevier Science B.V. All rights reserved

Why liquid displacement methods are sometimes wrong in estimating the pore-size distribution

The liquid displacement method is a commonly used method to determine the pore size distribution of micro- and ultrafiltration membranes. One of the assumptions for the calculation of the pore sizes is that the pores are parallel and thus are not interconnected. To show that the estimated pore size distribution is affected if this assumption is not satisfied, we developed two models: (1) a model describing the flow through an isotropic porous membrane with uniform pores, and (2) a two-layer model for uniform pores that approximates the first model if the membrane thickness is larger than 10 times the pore radius. In the two-layer model the membrane skin layer is divided into two parts: the unconnected pore layer and a sublayer. This model is extended to describe pore activation as a function of pressure with a pore size distribution in the unconnected pore layer (that is, membrane surface). It is shown that, depending on the membrane thickness or the sublayer resistance, the transmembrane pressure needs to be much larger than the critical pressure of the pores, to activate all the pores. If the sublayer resistance is over 10% of the resistance of the unconnected pore layer, the pore size is underestimated with the liquid displacement method; thus the number of pores is overestimated. Because the sublayer resistance is always larger than the unconnected pore layer resistance in an isotropic membrane with interconnected pores, we conclude that the estimated pore size distribution is always shifted toward smaller pore sizes than they really are. To use the liquid displacement method correctly, we suggest either counting the number of (active) pores or measuring the flux-pressure curve several times, while covering each time a different fraction of the membrane surface

Status of cross-flow membrane emulsification and outlook for industrial application

Cross-flow membrane emulsification has great potential to produce monodisperse emulsions and emulsions with shear sensitive components. However, until now, only low disperse phase fluxes were obtained. A low flux maybe a limiting factor for emulsion production on a commercial scale. Therefore, the effects of membrane parameters on the disperse phase flux are estimated. Besides, the effects of these parameters on the droplet size and droplet size distribution are qualitatively described. Wetting properties, pore size and porosity mainly determine the droplet size (distribution). Membrane morphology largely determines the disperse phase flux. As an example, industrial-scale production of culinary cream was chosen to evaluate the required membrane area of different types of membranes: an SPG membrane, an alpha-Al2O3 membrane and a microsieve. Due to the totally different morphologies of these membranes, the fraction of active pores is I for a microsieve and is very low for the other membranes. The choice of the optimal membrane did not depend on the production strategy: either to produce large quantities or to produce monodisperse emulsions, the best suitable was a microsieve with an area requirement of around I m(2). In general, the total membrane resistance should be low to obtain a large disperse phase flux. In contrast, the membrane resistance should be high to obtain monodisperse emulsions when using membranes with a high porosity. (C) 2003 Elsevier B.V. All rights reserved

Influence of membrane morphology on pore activation in membrane emulsification

The low throughput of the disperse phase is one of the issues in cross-flow membrane emulsification. This is apparent in the low percentage of pores at which droplets are formed (few active pores). To determine the effect of membrane morphology on pore activation, we developed and experimentally validated a model that describes the flow phenomena in and under a membrane with uniform pores (microsieve). In this model the membrane is divided into two parts: the toplayer and the membrane substructure. The model was validated with a larger-scale physical analogon. It predicts a linear increase of the number of active pores with increasing transmembrane pressure, while the pressure difference over the active pores is independent of the transmembrane pressure as long as not all pores are active. Although the resistance of the microsieve substructure was found to be four times lower than the resistance of a single pore, the resistance of the membrane substructure had a large effect on the activation of pores. Hence, the number of active pores can be increased by increasing the ratio of flow resistance in the pores and the flow resistance in the membrane substructure. Preliminary experiments show that the gradual increase in active pores at a ceramic membrane surface can be explained in the same way. (C) 2003 Elsevier Science B.V. All rights reserved