Karina Araus and Feral Temelli * Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

Transcription

1 Separation of Oleic Acid from a Model Mixture of Oleic Acid/α-Tocopherol/β-Sitosterol using Supercritical CO 2 Coupled with Cross-Flow Reverse Osmosis Membrane Filtration Karina Araus and Feral Temelli * Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5 * feral.temelli@ualberta.ca ABSTRACT A reverse osmosis polyamide membrane (SG) was used to test the feasibility of concentrating α-tocopherol and β-sitosterol from a model mixture (Oleic Acid (OA)/αtocopherol/β-sitosterol) solubilized in SC-CO 2 using cross-flow filtration regime. SG membrane was used up to 26 h at pressures of 120 and 280 bar and temperature of 40 C and its performance was measured in terms of CO 2 flux and separation factor at the transmembrane pressure of 10 bar. There was a reduction in the CO 2 flux at the beginning (30 min) of filtration, attributed to fouling, but the permeability did not change over 4.5 h of filtration. CO 2 flux was reestablished after passing pure SC-CO 2 after 4.5 h. Increasing the pressure to 280 bar resulted in a slight reduction in CO 2 flux in comparison to that at 120 bar. Feed pressure of 120 bar showed the best separation factors, where the OA separation factor was higher than 1 and those for α-tocopherol and β-sitosterol were less than 1. The preferential permeation of OA through the membrane in comparison to α-tocopherol and β- sitosterol is due to its lower molecular weight, higher diffusion coefficient, and higher solubility in SC-CO 2. The SG membrane used in this study showed good stability under SC- CO 2 processing conditions over 26 h. INTRODUCTION Separation processes are crucial for food and bioactive products, as most natural targets of interest are present in complex matrices and need to be purified. Extraction processes using supercritical carbon dioxide (SC-CO 2 ) have reached commercial level, due to the main advantages of SC-CO 2 over organic solvents, such as higher diffusivity, lower viscosity, lower extraction operating temperature, solvent power that can be adjusted by pressure and temperature variations, generation of solvent-free extracts, and low cost of CO 2 [1]. A major application of SC-CO 2 technology is the extraction of lipids from different plant matrices. Interest in the recovery of bioactive lipid components has been growing due to their health effects. For example, vegetable oil deodorizer distillate, which is a by-product of the refining process, is the main commercial source of tocopherols and sterols. It is a very complex mixture consisting of tocopherols, sterols, aldehydes, ketones, free fatty acids, glycerides, and other volatiles [2-4]. Phytosterols and tocopherols have been used in the food

2 and pharmaceutical industries, due to their cholesterol lowering and antioxidant activities, respectively [5,6]. Specifically, according to the U.S. Food and Drug Administration [7], the phytosterols may reduce the risk of heart disease. Furthermore, vegetable oil deodorizer distillate can be used as a source of fatty acids due to its high content in comparison to tocopherols and sterols. However, further fractionation of the complex mixture is not always efficient due to the similar solubilities of these target components in SC-CO 2. The coupled use of SC-CO 2 extraction and membrane processing may increase the separation efficiency and reduce the operating cost. Membrane separation processes have many applications at industrial level, including food, dairy, pharmaceutical, chemical and water treatment. Scale up of the process is easy and economically viable due to low energy consumption compared to other methods of separation [8,9]. However, the process has drawbacks associated with the low selectivity when treating highly viscous fluids and when dead-end filtration regime is employed [10]. The reduction in flux and accumulation of solutes on the membrane surface could be mitigated by coupling SC-CO 2 and cross-flow filtration regime. SC-CO 2 has been demonstrated to increase permeate flux by reducing viscosity during transport through the membrane [11] and crossflow filtration would reduce the extent of fouling on the membrane surface, due to hydrodynamic shear forces present at the membrane surface. Considering that there has been an increasing demand for purification of minor bioactive lipid compounds with health benefits and drawbacks associated with using SC-CO 2 (target bioactive compounds present in lipid mixtures, having similar solubility in SC-CO 2 ) and membrane separation (low selectivity) technologies individually, studies about fouling, selectivity, and integrity of membranes using SC-CO 2 coupled with polymeric membranes employing cross-flow filtration are necessary. The aim of this study was to study the separation of oleic acid (OA) from a model mixture of OA/α-tocopherol/β-sitosterol solubilized in SC-CO 2 employing the cross-flow regime at two pressures (120 and 280 bar) and constant temperature (40 C). Two flow rates (1 and 2 L min -1 ) of the retentate stream were tested to evaluate the reduction of fouling. A flat sheet polyamide reverse osmosis membrane (SG) was used to test its performance in terms of CO 2 flux and separation factor (α) at transmembrane pressure ( P) of 10 bar. A model lipid mixture of OA, α-tocopherol, and β-sitosterol at concentrations of 70, 10 and 20% (w/w), respectively, was used as the feed material to reflect the levels present in canola oil deodorizer distillate [12], to assess the feasibility of concentrating phytosterols and tocopherols. MATERIALS AND METHODS Membrane and materials SG membrane was kindly provided by GE Osmonics Inc. (Minnetonka, MN, USA) with NaCl rejection of 98.2%, as reported by the manufacturer. The physicochemical characterization of the membrane was reported by Akin and Temelli [13]. Liquid CO 2 (purity of 99.9%, moisture content < 3 ppm) was obtained from Praxair Canada Inc. (Mississauga, ON, Canada). OA (purity of 100%) was purchased from VWR International (Mississauga, ON, Canada), while α-tocopherol (purity of 95%) and β-sitosterol (purity of 75%) were purchased from Fisher Scientific Ltd. (Ottawa, ON, Canada). Sylon BFT (N,Obis(trimethylsilyl)trifluoroacetamide (BSTFA)/trimethylchlorosilane (TMCS), 99:1) and pyridine were purchased from Supelco Inc. (Bellefonte, PA, USA). Methyl-heptadecanoic acid (purity 99%) used as internal standard was purchased from Sigma-Aldrich (Oakwille, ON, Canada). Chloroform (analytical grade) was obtained from Fisher Scientific Ltd. (Ottawa, ON, Canada).

3 Processing with pure SC-CO 2 and SC-CO 2 + lipid mixtures The supercritical system used in this study (cross-flow filtration) (Fig. 1) is a modified version of the experimental system (dead-end filtration) described by Akin et al. [14], equipped with a new membrane module (Karnin Machine Co Ldt., Edmonton, AB, Canada). The tests of hysteresis and pure CO 2 flux through the membrane (filtration area of 9.6 cm 2 ) were performed at 120 bar and 40 C. P was increased from 10 to 40 bar in 10 bar increments with 1 h of processing at each step, where CO 2 flux was monitored, and then the same procedure was repeated by decreasing the P back to 10 bar. The P between the upstream and downstream was measured using two pressure gauges (GE Druck, Leicester, UK). The system was pressurized using a syringe pump (Teledyne ISCO Inc., Lincoln, NE, USA) by first closing the valves V3, V4, V8, V9, V10, V11, and V12 while slowly opening valves V1, V2 and then opening V5, V6, and V7. When the desired operational conditions (P and T) were reached, valve V7 was closed to build up the P by adjusting V12 on the permeate side, while the CO 2 flow was measured at ambient conditions. The CO 2 flux for each P was measured 30 min after CO 2 started to permeate through the membrane. For the selectivity tests (SC-CO 2 + lipid mixture), 75 g of OA/α-tocopherol/βsitosterol mixture (70, 10, and 20 wt.%) was loaded in the vessel (200 ml) and solubilized in SC-CO 2 at pressures of 120 and 280 bar and temperature of 40 C. Membrane separation was performed at P of 10 bar and 1 or 2 L min -1 flow rate of the retentate stream using the SG membrane. The valves V2, V3, and V4 were opened to pressurize the vessel. When the equilibrium was reached (after 12 h with stirring), valves V5, V6, and V7 were opened to pressurize the system with pure CO 2, while keeping valves V3 and V4 closed. The pressure in the vessel was measured with a gauge (McDaniel Controls, Inc., Luling, LA, USA). The same procedure described above was used to transport CO 2 into the membrane module. When the operational conditions (P and T) were reached, valve V7 was closed to build up the P by adjusting valve V12 on the permeate side, while on the retentate side cross-flow was applied (1 or 2 L min -1 ) by opening valve V11. The CO 2 flow rate (V11) and ΔP (V12) were controlled by micro-metering needle valves (Parker Autoclave Engineers, Erie, PE, USA), which were heated to keep the CO 2 flow rate constant. The vessel and membrane module were heated with electric heating jackets and separately monitored by means of J-type thermocouples connected to temperature controllers (Chromalox, Pittsburgh, PA, USA). The CO 2 flow rate in both streams was measured at ambient conditions using two flow meters (Alicat Scientific, Tucson, AZ, USA). The CO 2 retentate and permeate flow rates were recorded during 30 min after which the SC-CO 2 + lipid mixture was fed to the membrane module by closing valve V5 and opening valves V3 and V4. The mixture was fed during 30 min (conditioning stage), after which the SC-CO 2 + lipid permeate and retentate flow rates were recorded during 4.5 h at time intervals of 30 min. Three samples (at each 1.5 h) were collected in the retentate and permeate separators. At the end of the 4.5 h filtration stage, pure CO 2 was passed through the membrane for 1 h to clean the membrane surface (keeping P and CO 2 flow in the retentate stream constant). The system was kept pressurized overnight. The same piece of membrane was used over 4 days in total (using 1 and 2 L min -1 at 120 or 280 bar). The procedure described above was repeated each day. At the end of the experiment, depressurization of the system was achieved by closing valves V2, V3, V4, V9, V10, V11, and V12 and opening V5, V6, V7, and V8. The flux (kg m -2 h -1 ) was calculated by multiplying the permeate flow rate (L min -1 ) by CO 2 density [15] and dividing by the active filtration area of the membrane.

4 V5 P Filters V1 V2 V6 V9 V11 Syringe pump V3 P V4 T V7 T Retentate Module CO 2 Vessel P V8 V10 V12 Flowmeters Permeate Figure 1: Schematic of the SC-CO 2 extraction-membrane separation coupled system, working in crossflow filtration mode (v, valve). Determination of OA, α-tocopherol, and β-sitosterol separation factors Composition analyses of OA/α-tocopherol/β-sitosterol mixture in the permeate and retentate streams were carried out using a gas chromatograph (GC, Bruker Scion 456, East Milton, ON, Canada) equipped with a flame ionization detector and a capillary DB-5 HT (30 m 0.25 mm 0.1 µm) column (Scientific Agilent Technologies, Palo Alto, CA, USA). The parameters of the GC method were: the injector was operated in splitless mode and the temperature program started at 50 C, immediately ramped to 300 C at a rate of 200 C min -1 and was held for 10 min. The GC oven temperature program was: initial temperature of 50 C held for 2 min, then ramped up to 300 C at a rate of 5 C min -1 and the temperature was held for 10 min. A modified version of the method reported by Verleyen et al. [16] was used to silylate the components before injection. Samples (20 mg) were weighed in a test tube, and dissolved in pyridine (0.5 ml) and BSTFA (0.5 ml) containing 1% of TMCS solution. Then, the tubes were placed in an oven at 70 C for 20 min for completion of the reaction. Samples were diluted with the addition of 9 ml chloroform, and then 0.5 ml of this dilution was transferred into a GC vial together with 0.5 ml methyl-heptadecanoic acid (2 mg/ml chloroform) used as internal standard. Individual standards (10 mg of each) were silylated using the same procedure. Standard solutions (0.05 ml) were transferred into GC vials, where they were diluted by the addition of 0.5 ml chloroform and 0.5 ml internal standard. The quantitative analysis of the samples (weight of components in the sample and relative response factors) was carried out according to Güçlü-Üstündağ and Temelli [12]. The performance of the membrane was evaluated in terms of a separation factor (α), as defined by Semenova et al. [17], according to Eq. (1): α =!!" (1)!!" where C!" and C!" represent the (%) mass of α-tocopherol or β-sitosterol collected in the permeate and feed streams, respectively. Higher retention of α-tocopherol or β-sitosterol compared to OA by SG membrane would lead to α < 1. The amount of each component solubilized in SC-CO 2 at the same operational conditions was assessed by collecting the extract without the use of a membrane in order to determine the composition of the feed stream. RESULTS AND DISCUSSION Pure CO 2 flux measurements Fig. 2 shows the comparison of CO 2 flux versus ΔP results obtained in this study and those reported in the literature. Transmembrane CO 2 flux showed a linear increase with ΔP,

5 which was in agreement with other reports [18-20]. The differences in the magnitude of CO 2 flux shown in Fig. 2 can be attributed to incurred changes in the physicochemical and morphological properties of the membrane used, ΔP employed, and the specific methodologies (e.g., dead-end or cross flow regimes) used. No hysteresis was observed for the membrane tested. The standard deviations based on triplicate runs ranged from 4.4 to 10.8%. Figure 2: Comparison of CO 2 flux through the membrane reported in the literature with that measured in this study, as a function of ΔP at 120 bar and 40 C for SG membrane. Concentration of OA, α-tocopherol and β-sitosterol in the feed stream The composition and the amount solubilized in SC-CO 2 for each component (OA, α- tocopherol, and β-sitosterol) entering the membrane module were measured without the use of membrane, at 120 and 280 bar, using CO 2 flow rate ranging from 1 to 3 L min -1 and fixed temperature of 40 C. Table 1: The composition of the feed stream of OA, α-tocopherol, and β-sitosterol to the membrane module obtained at 40 C and two different pressures. Pressure (bar) OA (% w/w) α-tocopherol (% w/w) ± 1 14 ± 1 11 ± ± 3 13 ± 1 7 ± 2 β-sitosterol (% w/w) As expected, OA concentration in the feed stream was higher than α-tocopherol and β- sitosterol at both pressures (Table 1). Furthermore, the solubilities in terms of mole fraction in the quaternary system showed that increasing the pressure increased the solubility of OA, α- tocopherol, and β-sitosterol from 527 ± 41 to 2209 ± !!, 63 ±3 to 227 ±14 10!!, and 54 ±10 to 145 ±20 10!!, respectively. The increase in solubilities with increasing SC-CO 2 pressure from 120 to 280 bar was due to higher density of CO 2, which increases from 718 to 899 kg m!, respectively. Furthermore, OA solubility was higher than that for α-tocopherol and β-sitosterol, due to its smaller molecular weight (MW OA = g mol) compared with the other solutes (MWα-tocopherol = and MWβ-sitosterol = g mol). α-tocopherol (-OH and -O-) and β-sitosterol (-OH) were less soluble than OA in SC-CO 2 due to the presence of hydrophilic side groups in their structures; furthermore, OA (-COOH) was more soluble due to its long hydrocarbon chain or non-polar nature. As expected, solubility behavior in SC-CO 2 is dependent on density, MW and polarity [21]. The magnitude of solubilities in the quaternary system were in agreement with those reported in the literature for binary systems [22,23], but it is necessary to evaluate the multicomponent

6 systems to study the effect of the presence of other solutes on the solubility behavior of the target components. The differences in the magnitude of solubility could be attributed to the purity of solutes, methodology used, physical state of the solute, and molecular interactions in the multicomponent system [21]. The amount of model mixture loaded in the vessel was enough to saturate the SC-CO 2 and keep the concentration of each component constant at 120 or 280 bar during the experiment and independent of the CO 2 flow used. Permeate flux and separation factors for the model lipid mixture The permeate flux and α were measured at 120 and 280 bar using 1 or 2 L min -1 in the retentate stream, and fixed temperature of 40 C and ΔP of 10 bar. Permeate flux of the mixture showed a slight drop with time, which was attributed to fouling (Fig. 3). At 280 bar, the decrease in flux was higher compared to that at 120 bar, probably due to the higher feed concentration of the lipid mixture at the higher density of CO 2. As expected, the higher pressure of the retentate stream increased the SC-CO 2 solvent power and therefore, the solubility of the lipid mixture [24]. Furthermore, the flux j = β ρ μ ΔP is a function of density and viscosity ρ μ under supercritical conditions [25], where β is a constant and represents membrane structural parameter. Therefore, the flux was lower at higher CO 2 pressures, since the increase in viscosity is greater than that for density [15], when the pressure is increased from 120 (μ = !! Pa s) to 280 (μ = !! Pa s) bar at fixed ΔP. The addition of pure SC-CO 2 to the system for 1 h periods after 5 h of processing with the model mixture at 120 and 280 bar, allowed cleaning and reestablished the initial permeate flux at the original value. Akin et al. [14] reported a pronounced drop in the flux at high pressure (120 versus 280 bar) using dead-end filtration regime due to the accumulation of solutes (OA/triglycerides) on the membrane surface. Even after the addition of pure CO 2 the permeate flux was not reestablished at the original level. Also, Carlson et al. [19] reported a drop in the permeate flux during cross-flow filtration at fixed flow rate (1 L min -1 ) of the retentate stream, targeting the separation of D-limonene. This effect was attributed to membrane clogging, which was reestablished with the addition of pure CO 2 ; however, the flux achieved was too low (1.9 kg/h m 2 ). In this study, the decrease in the CO 2 flux (Fig. 3) was independent of the retentate flow rate used (1 and 2 L min -1 ); however, in comparison to the results of Akin et al. [14] and Carlson et al. [19], the magnitude of the flow rate used mitigates the fouling and the drop in permeate flux. The standard deviations based on duplicate runs ranged from 3.9 to 11.3%. Figure 3: Effect of pressure, flow rate (1 and 2 L min -1 ) in the retentate stream, and periodic pure SC- CO 2 addition as a function of time, on permeate flux for SG membrane at 40 C with ΔP of 10 bar.

7 Fig. 4 shows the α for OA, α-tocopherol, and β-sitosterol at 40 C and ΔP of 10 bar. Separation factors were less than 1 for α-tocopherol and β-sitosterol while it was higher than 1 for OA at 120 and 280 bar. The results exhibited an excellent retention of molecules with larger size (α-tocopherol and β-sitosterol) and a preferential permeation of molecules with smaller size (OA), without sacrificing the permeate flux through the membrane. The preferential retention of α-tocopherol and β-sitosterol in comparison to OA may be justified by their lower diffusion coefficients and solubilities in SC-CO 2 and higher molecular weights. Similar tendencies were reported by Subramanian et al. [26], Nagesha et al. [27] and Akin et al. [14] during the separation of lipid mixtures (OA/triglycerides [14, 26] and tocopherols/fatty acid [27]). Therefore, considering that the diffusion coefficient is higher for solutes with lower MW [27], the greater permeation of OA through the membrane over α- tocopherol and β-sitosterol could be expected. The increase in pressure showed a negative effect on the retention of α-tocopherol and β-sitosterol, which was attributed to a decrease in the selectivity, due to increased solubilization of lipids, similar to previous reports [24]. The retentate stream flow rate did not have an impact on α. The standard deviations based on duplicate runs ranged from 0.4 to 10.1%. Figure 4: Separation factors for OA, α-tocopherol, and β-sitosterol at 40 C and ΔP of 10 bar, (A) 120 and (B) 280 bar. CONCLUSIONS Pure CO 2 flux showed a linear increase with ΔP in agreement with the previous reports in the literature for SG membrane at 120 bar, 40 C, and ΔP ranging from 10 to 40 bar. Separation factors obtained using the coupled process at 120 or 280 bar were higher than 1 for OA while those for α-tocopherol and β-sitosterol were less than 1. The higher diffusion coefficient and solubility in SC-CO 2, and lower molecular weight of OA compared with α- tocopherol and β-sitosterol would be the reasons for the preferential permeation of OA through the membrane. Increasing the pressure from 120 to 280 bar increased the amount of lipid solubilized in SC-CO 2 due to the increased density of CO 2, resulting in a decrease in the selectivity of the process and a slight drop in CO 2 flux due to fouling on the membrane surface, which was reestablished with the addition of pure SC-CO 2. The results demonstrate an excellent opportunity for using SC-CO 2 coupled with reverse osmosis membrane employing cross-flow filtration regime to selectively separate α-tocopherol and β-sitosterol from OA, bioactive components present in vegetable oil deodorizer distillate, which is a byproduct of the oil refining process.

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