The influence of thermodynamic activity on the solute rejection in multicomponent systems

Abstract

In this study it has been shown that major changes in solute rejection can occur when the solute is part of a multicomponent system instead of a binary system. The effect of changes in the thermodynamic activity of the solute on its rejection has been studied.As a model system the combined ultrafiltration of poly(ethylene glycol) (PEG) and dextran has been chosen. The thermodynamic activities of PEG and dextran have been modelled with the UNIQUAC model.Rejection measurements showed a decrease in the observed PEG rejection when dextran was added to the solutions under conditions where the dextran was mainly retained by the membrane. At high dextran concentrations and low fluxes the PEG rejection even became strongly negative. The PEG concentration in the permeate was at maximum a factor 2 higher than in the permeate (on a dextran-free basis). For a more open membrane the rejection of PEG was found to increase in the presence of dextran.The elevated transport of PEG through the membrane in the presence of dextran was ascribed to the additional driving force due to the increased thermodynamic activity of PEG at the retentate side of the membrane.The results in this paper show that thermodynamic interactions can have a strong impact on the observed rejection and are certainly worth being considered in multicomponent systems.

Fine ultrafiltration of concentrated oligosaccharide solutions – Hydration and pore size distribution effects

“…Under diluted conditions, the effect of including the pressure gradient is negligible; however, under concentrated conditions it becomes quite important, increasing the transport of solutes towards the permeate. This is in line with the observations by Van Oers et al, who considered the reduction in the observed rejection of PEG3400 in the presence of dextran more related to the TMP than to the permeate flux[9].…”

“…At feed concentration of 20% w/w and higher, negative values were observed for DP1 sugars. Figure 3 The observation of negative rejections has been ascribed to different effects [9,35].…”

“…Only few filtration studies can be found in literature in which the effect of different solutes on each other is assessed. Van Oers et al (1997) described the decrease in the observed PEG rejection when combined with dextran in comparison with the observed rejection of PEG as single solute [9]. During these experiments, these authors even obtained negative observed rejection values for PEG under some specific conditions.…”

Modeling of a membrane reactor system for crude palm oil transesterification. Part II: Transport phenomena

“…Looking at the com-plexity of multiphase of the recycle loop operation as shown in Figure 1 of part I of this two-paper series, mechanistic modeling approach to describe the transport phenomena of multicomponent biodiesel process in the membrane reactor with the consideration of heterogeneity behavior and CPE is important for the membrane reactor design. 1,2 There are several mathematical models describing the transport phenomena of binary or multicomponent separation in the membrane system which include the models based on particle-liquid, [3][4][5] liquid-liquid, 6,7 and gaseous separation [8][9][10][11][12] processes. There is also an article describes mass transport owing to molecular diffusion and viscous flow, as well as an instantaneous reversible reaction inside a membrane reactor for catalytic gas-solid reactions.…”