Organic solvent nanofiltration of binary vegetable oil/terpene mixtures: Experiments and modelling
Introduction
Vegetable oil extraction and purification is an important industry, with an average annual production of more than 380 million metric tonnes [1]. In the primary stages, the crude oil is extracted from oilseeds by means of an organic solvent; typically n-hexane. N-hexane has been used for many years due to its high solubility for oil, availability and low price. Importantly, its low boiling point (69 °C) and heat of vaporization (30 kJ mol−1) make the recovery by conventional evaporation feasible [2]. However, n-hexane has recently been classified as a CMR (carcinogenic, mutagenic and reprotoxic) solvent and may be banned for industrial use in the future [3], [4], [5]. Moreover, the high volatility and low flash point of n-hexane increase the risk of fire and explosion [6]. Therefore, finding environmentally friendly alternative solvents has become imperative.
Bio-derived solvents, particularly terpenes represent promising substitutes for n-hexane [7]. These solvents are extracted from sustainable agricultural materials and have been proved to be safer to humans and the environment [8]. These solvents are used widely in perfumes, pharmaceuticals and in the food industry as flavouring agents [9], [10], [11]. Several studies have reported the utilization of terpenes for the extraction of vegetable oil and microalgae lipids [12], [13], [14], [15], [16], [17], [18]. These studies concluded that terpenes can replace n-hexane as extractive solvents due to their affinity for oil and lipid constituents. However, their high boiling points and heats of vaporization restrict their utilization at an industrial scale [19]. Furthermore, exposure to high temperatures during the recovery of these solvents using conventional evaporation results in the thermal decomposition of heat sensitive components such as natural antioxidants and the formation of harmful peroxides and unsaturated aldehydes [19], [20]. Therefore, finding an alternative solvent recovery process with minimal energy consumption has become inevitable.
Membrane separation technologies are increasingly used for aqueous separations in the food industry. This success has triggered interest in such membrane technology for organic solutions, an approach known as solvent resistant nanofiltration (SRNF), or organic solvent nanofiltration (OSN). SRNF represents a feasible alternative for energy-intensive conventional separation processes such as distillation, evaporation and liquid-liquid extraction. SRNF has advantages over these conventional processes of low energy consumption, mild operating temperatures, ease of installation and operation and low operating cost [21]. It is also considered as an environmentally friendly technology that minimizes harmful emissions as well as carbon dioxide to the atmosphere [1], [22], [23]. In the vegetable oil industry, SRNF has the potential to replace different stages of crude oil processing such as solvent recovery, degumming, de-acidification, deodorization and decolourisation [22].
The utilization of membrane technology in the recovery of solvents from vegetable oil mixtures has been reported in some studies [20], [24], [25], [26], [27], [28], [29], [30], [31]. Kuk et al. [32] investigated the recovery of ethanol from ethanol-cotton oil mixtures using a commercial reverse osmosis membrane. Tres et al. [33] used a polymeric ultrafiltration membrane for the separation of refined soybean-hexane mixtures and observed a permeate flux up to 65 kg m−2 h−1 and oil rejections up to 30%. In another study, Tres et al. [34] observed higher rejections and fluxes for soybean oil/n-butane mixtures relative to oil/n-hexane mixtures using ceramic membranes (MWCO 5 and 10 kDa). Rama et al. [35] investigated the separation of 20 wt% soybean oil in hexane using a polymeric nanofiltration membrane in a multistage process which resulted in a net oil recovery of 99%. In our own prior work, we investigated the permeation of pure terpenes through a polydimethylsiloxane (PDMS) membrane and observed a non-linear increase in permeate flux that was readily explained using the solution-diffusion model. However, this work considered the permeation of terpenes alone and not oil/solvent mixtures [36].
While the solution-diffusion model is effective for single component systems, it ignores coupling effects between penetrants that occur in binary mixtures [37]. For this reason, it fails to predict the negative solute rejection which is observed in some studies [38], [39]. The Maxwell–Stefan (M–S) approach overcomes these limitations [40]. These equations were originally developed to describe multicomponent diffusion through low-density gases but have since been extended successfully to dense gases, liquids and polymers [40], [41]. The approach has been widely used for describing transport in reverse osmosis, pervaporation, gas permeation and organic solvent nanofiltration systems [42], [43], [44], [45], [46].
The M–S equations originate from applying an inter-species force balance and can be written as follows [47].where , and are the mole fraction, chemical potential (J mol−1) and velocity (m s−1) of component , respectively. (m2 s−1) are the M–S diffusivities which represent the inverse of the drag coefficient between species and . In dealing with a system where a polymer is one component, it is more convenient to express the M–S equations in terms of volume fractions to be consistent with the thermodynamic models that describe species interactions with polymers [48]. Heintz and co-workers [49] adapted the M–S equations to be in terms of species volume fraction () as follows:
The formula developed by Heintz has been widely used in modelling multicomponent transport in pervaporation [50], [51], [52], [53]. However, the formula does not satisfy the Gibbs–Duhem relationship as pointed out by Fornasiero et al. [54]. To overcome the drawback of Eq. (2), Fornasiero et al. [54] reformulated the original M–S equation (Eq. (1)) in terms of the molar segment concentration. With the assumption of no volume change of the multicomponent system upon mixing, Fornasiero obtained the following expression:where is the molar concentration of the segment (mol cm−3), is the segment molar volume (cm3 mol−1) and is the MS diffusivity of the segments comprising species and . Several studies have reported the application of Eq. (3) to modelling the transport of water through polymeric materials [55], [56], [57], diffusion of asphaltene [58], [59], [60], and the formation of polymeric nanoparticles [61]. The only limitation of Eq. (3) is that a reference molar volume consistent with the thermodynamic model must be chosen. Ribeiro et al. [42] adapted the original M–S equations in terms of volume fraction to overcome the limitations of Heintz [49] and Fornasiero [54] and the following relation was obtained:where and are the partial molar volume (cm3 mol−1) and the molar flux (mol cm−2 s−1) of component , respectively. is the modified M–S diffusivity, cm2 s−1. This formulation assumes that the friction factor between species are not symmetric (i.e. ) [42]. While this is generally not an issue for steady-state transport through a membrane (Nm = 0), it would make difficult a treatment of transient phenomena with Nm ≠ 0. Ribeiro et al. [42] used Eq. (4) combined with the Flory–Huggins model to develop a set of ordinary differential equations to describe the steady-state transport of CO2/C2H6 mixtures in a cross-linked poly(ethylene oxide) membrane. These differential equations were solved by numerical methods. In an attempt to avoid the complexity associated with solving such differential equations, Krishna [44] used a linearized M–S formulation for the transmembrane fluxes. This expression combines the original M–S equations and the Flory–Huggins model and it provides comparable accuracy.
In this work, we aim to investigate the feasibility of SRNF for the recovery of terpenes from their binary mixture with canola (rapeseed) oil under different experimental conditions. Free-standing PDMS films are prepared and used to determine the sorption isotherm of terpenes and their binary mixtures. The performance of PDMS/PAN composite membranes for the recovery of terpenes from canola oil solutions is then investigated. The linearized Maxwell–Stefan formulation combined with the Flory–Huggins model is used to model these results.
Section snippets
Model development
According to the Flory–Huggins model for a ternary system (polymer (m), solvent (s) and oil(o)), the activity of the solvent () and the oil () are related to volume fractions assuming concentration independent interaction parameters as follows [48], [62]:andwhere is the mutual interaction parameter between the oil and the solvent.
For
Materials
Polyacrylonitrile (PAN) membranes (MWCO = 20 kDa) were purchased from AMI, USA. A PDMS kit consisting of a pre-polymer (KE106) and a crosslinker (CAT-RG) was supplied by Shin-Etsu Chemicals (Japan). Food grade , and were obtained from Sigma-Aldrich. A refined canola oil (Pure Vita – Australia) was used for the preparation of solvent-oil mixtures. The oil consists of a mixture of triglycerides (6 wt% unsaturated C16 and C18, 67 wt% monounsaturated C18 and 26 wt%
Sorption and swelling
The average uptakes of pure pinene, limonene, cymene and canola oil in PDMS films at both 25 °C and 40 °C were determined in our previous paper as 3.13, 2.77, 2.58 g/g dry membrane [36]. In the present case, we determined that the uptake of pure canola oil was three orders of magnitude lower at 3.90 10−3 g/g dry membrane. Stafie et al. [31] reported similar behaviour and attributed this to the large molecular size of the oil compared with the solvent.
The partial uptakes of the solvents/canola
Conclusions
In this work composite PDMS/PAN membranes were prepared and used for the recovering of pinene, limonene and cymene from their binary mixtures with canola oil as a solute. The highest solvent fluxes were observed with oil/limonene mixtures whereas the oil rejections were comparable in the different oil/solvent mixtures and up to 90% was achieved. The concentration polarisation effects on the system were studied and the mass transfer correlation of the system was developed. The sorption levels of
Acknowledgements
M. H. Abdellah acknowledges the University of Melbourne for the Melbourne Research Scholarship and Alexandria University for the financial support. L. Liu and S.E. Kentish acknowledge funding support from the Australian Research Council (ARC) Discovery Program (DP150100977). B.D. Freeman gratefully acknowledges support from the Australian-American Fulbright Commission for the award of a U.S. Fulbright Distinguished Chair in Science, Technology, and Innovation sponsored by the Commonwealth
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