Physical chemistry and engineering of membranes for fat - fatty acid separations

J. Keurentjes

    Research output: Thesisinternal PhD, WU

    Abstract

    <p>Fatty acids have to be removed from non-mineral oil for several purposes. In the refining of edible oils and fats they have to be removed as a contaminant. In the enzymatic hydrolysis of oils, a high content in fatty acids results in a reduced conversion rate. In order to maintain a sufficiently high reaction rate they will have to be removed, preferably in-line. The conventional separation method is the caustic refining process, in which alkali is added to the fatty acid containing oil, resulting in the formation of the sodium salts of the fatty acids (soapstock). Then the soapsock is separated from the oil by centrifugation. However, this soapstock contains considerable amounts of triglycerides (about 50%), which have to be considered as a loss. For this reason, caustic refining of oils containing high concentrations of fatty acids is not an economical process.<p>In this thesis two alternative processes for the separation of fatty acids from oil are presented, with the common feature that membranes are used in a two-phase environment. It is the aim of this thesis to study the engineering and physico-chemical phenomena that are relevant for the operation of these processes.<p>In the first process, alkali is added to the oil in order to form the sodium salts of the fatty acids. Additionally, 2-propanol is added to solubilize the soapstock thus formed, resulting in a twophase system. The water phase contains the fatty acid salts dissolved in a water/2-propanol mixture and the oil phase merely contains triglycerides and a trace amount of 2-propanol. This two-phase system can be separated into its two phases by a hydrophilic and a hydrophobic membrane in series.<p>In chapter 2 the nature of the two-phase system and the permeation behaviour of the water phase through the hydrophilic membrane have been investigated. It appears, that both the oil and the water phase are present as a continuous phase between 20 and 65% water phase in the dispersion. Above 20% water phase, the flux through the membrane is merely determined by the hydrodynamic membrane resistance, provided that the membrane is entirely wetted by the water phase. Below 20% water phase, the water phase is present as dispersed droplets in oil. At the transition from a continuous into a discrete water phase the permeation flux drops almost stepwise to a value close to zero.<p>When in the system described in chapter 2, a concentrated NaCl solution is circulated at the permeate side of the cellulose membrane, initially a flux reversed to the normal permeation flux is observed. After some time the flux changes direction and becomes 2 to 10 times larger than it would be based on the pressure difference over the membrane. These effects cannot be accounted for using the classical Fickian diffusion theory. In chapter 3 it is shown that the flux changes can be explained qualitatively using the Maxwell-Stefan diffusion theory.<p>It appeared, that commercially available hydrophobic membranes were incapable of separating the oil phase from this dispersion. As the preliminary requirement to solve this problem, chapter 4 describes a method that has been developed to measure hydrophobicities of membranes. When a piece of membrane is submerged in a liquid, air bubbles will adhere to the membrane when the surface tension of the liquid is high. Decreasing the surface tension of the liquid yields a transition from adhesion to non-adhesion. The surface tension of the liquid at which this transition occurs can be compared to the critical surface tension as is commonly used to characterize polymeric surfaces.<p>In chapter 5 the adsorption is measured of a surfactant in water/2-propanol mixtures onto surfaces that vary in hydrophobicity. Adsorption appears to occur in three regions: "tail down" adsorption on hydrophobic surfaces, "head down" adsorption on hydrophilic surfaces, and a very small region in between, where adsorption is absent. A membrane that possesses a hydrophobicity belonging to this region appears to be capable of separating the oil phase from the dispersion selectively.<p>The second system for the removal of fatty acids from oil consists of a membrane extraction step, using 1,2-butanediol as a selective extractant. When water is added to the fatty acid containing 1,2-butanediol, the system demixes in a fatty acid phase dispersed in a 1,2-butanediol/water mixture. After phase separation, water has to be removed from the 1,2-butanediol, which can be reused as extractant. In chapter 6 extraction of fatty acids from oil has been investigated. In order to obtain a stable system, it is necessary to use rather dense membranes. This results in relatively high mass transfer resistances and hence in large surface areas for extraction. Due to the fact that the mass transfer coefficients vary significantly with fatty acid chain length, it appears to be possible to fractionate a fatty acid mixture.<p>In chapter 7 membrane cascades for the separation of binary mixtures have been investigated. Calculations based on a McCabe-Thiele diagram show that, for permeabilities and selectivities commonly found for reverse osmosis membranes, permeability is the parameter on which improvements have to be made when a minimum total membrane surface area is required.<p>Finally, in chapter 8 some implications of the work described in this thesis have been discussed. The two systems descibed for the separation of fatty acids from oil are capable to perform this separation selectively and at mild conditions. Two future implications that require further investigations are discussed. The first one is the separation of emulsions using membranes. Secondly, tailoring membranes for special applications and in order to reduce fouling are possible applications that require further investigations.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    Supervisors/Advisors
    • van 't Riet, K., Promotor
    • Cohen Stuart, Martien, Promotor
    Award date6 Mar 1991
    Place of PublicationS.l.
    Publisher
    Publication statusPublished - 1991

    Keywords

    • membranes
    • reverse osmosis
    • ultrafiltration
    • oils
    • fats
    • waxes
    • animal products
    • plant products
    • fatty acids
    • carboxylic acids
    • cum laude

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