Membrane technology has been gaining momentum in industrial processes, especially in food technology. It is believed to simplify processes, reduce energy consumption, and eliminate pollutants. The objective was to study the performance of polyvinylidene fluoride (PVDF) and polyethersulfone (PES) polymeric membranes in the degumming of the miscella of crude rice bran oil by using a bench-scale tangential filtration module. In addition, oil miscella filtration techniques using hexane and anhydrous ethyl alcohol solvents were compared. All membranes showed the retention of phospholipids and high flow rates. However, the best performance was observed using the 50-kDa PVDF membrane in miscella hexane solvent, with a 95.5% retention of the phosphorus concentration (by a factor of 1.4), resulting in a permeate with 29 mg·kg−1 of phosphorus and an average flow rate of 48.1 L·m−2·h−1. This technology can be used as a low-pollution, economical alternative for the de-gumming of crude rice bran oil, being effective in the removal of hydratable and non-hydratable phospholipids, resulting in oils with a low phosphorus content.
Degumming is a refining step, wherein the phospholipids are removed. The presence of large quantities of phospholipids in oil leads to a dark coloration; in addition, phospholipids act as emulsifiers, leading to a loss in neutral oils and resulting in a low quality product (Erickson,
The conventional oil refinement process is characterized by high energy, water, and chemical product requirements, loss in neutral oil and nutrients, and a high rate of effluent production (Subramanian
The similar molecular weights of triacylglycerols (900 Da) and phospholipids (700 Da) could complicate the membrane-based separation of these components. However, phospholipids have some specific features that could assist in their separation; they are natural surfactants and express hydrophilic and hydrophobic groups that are capable of forming micelle under non-aqueous conditions (Ochoa
Pardun (
Hexane solvents have a dielectric constant of 1.88 D (Rydberg
The interaction of the solvent with the membranes may result in expansion (swelling), lamination, or dissolution of the membrane; this would subsequently cause structural changes to the membrane, leading to changes in the separation properties and reduced mechanical resistance to pressure (Tsui and Cheryan,
The solvent flow through the polymeric membrane mainly depends on solvent polarity; therefore, the solvent flow is decreased in hydrophobic membranes and increased in hydrophilic membranes, as the polarity in organic solvents is closely related to surface tension. Polyvinylidene fluoride (PVDF) membranes were observed to possess greater stability towards hexane than polyethersulfone (PES) membranes in the studies conducted by Ochoa
The main aim of this project was to study the performance of PVDF and PES polymer membranes in the degumming of crude rice bran oil using a tangential filtration module bench, and to compare our results to those of previous studies.
The ultrafiltration unit (
Schematic diagram of the ultrafiltration cell used to test the polymeric membranes.
The following commercially available flat polymeric membranes were used in this study: PVDF with a 0.05 µm pore size filter DBD (Mauá, Brazil) denoted as 50-kDa PVDF, PVDF with a 0.075 µm pore size Nadir® (Wiesbaden, Germany) denoted as 75-kDa PVDF, and PES with a pore size of 0.005 µm Nadir® (Wiesbaden, Germany) denoted as 5-kDa PES.
The membranes were cut into 7 cm×11 cm rectangles, covering a permeation area of 0.0077 m2 and conditioned in the solvents used in the ultrafiltration process (hexane for PVDF and anhydrous ethyl alcohol for PES). The membranes were kept immersed in the solvent for 24 h at room temperature prior to use (Kesting,
The crude rice bran oil was characterized based on the peroxide index, calculated iodine index, and the calculated saponification index, according to the protocols Cd 8b-90, Cd 1c-85, and Cd 3a-94, detailed by the AOCS (
A stainless steel 2 L Invict (Campinas, Brazil) tangential-flow filtration module bench-scale was coupled to support the flat rectangular (7 cm×11 cm; permeation area 0.0077 m2) polymer membrane. The process temperature and tangential velocity were set to 40 °C and 0.20 ms−1, respectively, for the miscella in hexane, and 40 °C and 0.24 m·s−1, respectively, for the miscella in anhydrous ethyl alcohol because of equipment limitations. The tank was fed with 1 kg of miscella; the temperature was stabilized to the following working conditions: the pressure was set at 5 bar by closing the needle type valve of a Nuovafima (Belenzinho, Brazil) manometer with a scale of 0 to 7 kgf·cm−2, the tangential velocity was set using the WEG (Jaraguá do Sul, Brazil) frequency inverter pump, and a Hydra-Cell (Minneapolis, United States of America) electrically driven diaphragm pump was started. The permeate was collected in a graduated cylinder, where the cumulative volume was recorded as a function of time in order to obtain a concentration factor (CF) of 1.4 by weight. For all experiments new membranes were used.
The tangential velocity (TV) was calculated using
The membrane performance was expressed in terms of the permeate flow, phosphorus retention coefficient, and concentration factor. These were calculated using the following equations:
The permeate flow (J) was calculated using
The retention coefficient (% R) of phosphorus was calculated in percentage using
The Concentrantion Factor was calculated using
The central portion of the unused membranes and the membranes used for ultrafiltration was cut into a 1 cm×1 cm square. The surfaces of the unused and post-use membranes were analyzed by high vacuum scanning electron microscopy. For this purpose, a TM 3000 microscope Hitachi (Tokyo, Japan) was used, with a magnification of 15×to 30000×, and an accelerating voltage of 15 kV (Analysis mode), coupled to a back scattered electron detector (Swift ED 3000; Hitachi) composed of a high sensitivity semiconductor (with a resolution of 148 eV or more in MnK), which analyzes the elements from boron to uranium.
The fatty acid composition of crude rice bran oil (
Characterization of crude rice bran oil
Composition of FattyAcids | % |
---|---|
C14:0 | 0.30 |
C16:0 | 19.20 |
C16:1 | 0.20 |
C18:0 | 2.20 |
C18:1 | 37.20 |
C18:2 | 36.50 |
C18:3 | 2.00 |
C20:0 | 1.00 |
C20:1 | 0.50 |
C22:0 | 0.30 |
C24:0 | 0.60 |
Calculated iodine index (cg I2·100 g−1) | 101 |
Calculated saponification index (mg KOH·g−1) | 193 |
Peroxide value (meq·kg−1) | 9.8 |
Phosphorus content (mg·kg−1) | 640.5 |
All values were determined in duplicate.
The iodine index measurement reflected the presence of double bonds of fatty acids (Lawson,
Saravanan
Approximately 95.5% of the phospholipid content of the crude rice bran oil was retained in the form of miscella using the 50-kDa PVDF membrane (hexane solvent) (
Ultrafiltration of crude rice bran oil miscella as a function of the material and porosity of the polymeric membrane, to a concentration factor of 1.4 (initial oil contained 640 mg·kg−1 phosphorus)
Membrane | Porosity | Pressure (bar) | Solvent | J (L·m−2·h−1) | % R | Residual P in the permeate (mg·kg−1) |
---|---|---|---|---|---|---|
50-kDa PVDF | 0.05 µm | 5 | Hexane | 48.1 | 95.5 | 29±5 |
75-kDa PVDF | 0.075 µm | 5 | Hexane | 55.8 | 82.3 | 113±11 |
5-kDa PES | 0.005 µm | 5 | AnhydrousEthyl Alcohol | 49.5 | 62.0 | 243±15 |
All values were determined in triplicate; J=accumulated average flow of permeate; % R=Retention of phospholipids; P=Phosphorus
Manjula and Subramanian (
According to Yang
The 50-kDa PVDF, 75-kDa PVDF, and 5-kDa PES polymeric membranes showed the same flow behavior of up to 1.4 CF at 5 bar pressure (
Ultrafiltration of crude rice bran oil miscella using PVDF (hexane solvent) and PES (anhydrous ethyl alcohol solvent) polymeric membranes with differing porosities, to a CF of 1.4.
According to Kaimal
The initial flow through the 75-kDa PVDF and 5-kDa PES membranes (at 5 bar pressure) was higher than that observed through the 50-kDa PVDF membrane. However, a sharp decline in the flow was observed because of the increase in concentration polarization throughout the process, which in turn accelerated the formation of the polarized layer.
The 75-kDa PVDF membrane displayed a high average flow of 55.8 L·m−2·h−1 (
Photomicrographs of intact 50-kDa PVDF (A), 75-kDa PVDF (B), and 5-kDa PES (C) membranes, and 50-kDa PVDF (D), 75-kDa PVDF (E), and 5-kDa PES (F) membranes after ultrafiltration, with a 2000×increase in magnification.
The EDS analysis of the intact 50-kDa and 75-kDa PVDF polymeric membranes revealed the presence of carbon, fluorine, and oxygen (
Results of the energy dispersive system (EDS) analysis of intact polymeric membranes, and membranes used in the ultrafiltration process
Element | Intact membrane | Membrane after ultrafiltration | ||||
---|---|---|---|---|---|---|
50-kDa PVDF (%) | 75-kDa PVDF (%) | 5-kDa PES (%) | 50-kDa PVDF (%) | 75-kDa PVDF (%) | 5–kDa PES (%) | |
Carbon | 44.97 | 53.71 | 68.87 | 67.72 | 71.23 | 84.90 |
Oxygen | 13.60 | 18.43 | 21.44 | 7.62 | 13.59 | 12.11 |
Fluorine | 41.43 | 27.86 | – | 24.38 | 14.71 | – |
Sodium | – | – | 0.83 | – | – | – |
Silicon | – | – | 0.08 | – | – | 0.25 |
Sulfur | – | – | 7.90 | – | – | 2.62 |
Chlorine | – | – | 0.60 | – | – | – |
Potassium | – | – | 0.28 | – | – | – |
Phosphorus | – | – | – | 0.16 | 0.36 | 0.12 |
Calcium | – | – | – | 0.12 | – | – |
Magnesium | – | – | – | – | 0.11 | – |
Following permeation, the 50-kDa PVDF and 75-kDa PVDF membranes showed small quantities of phosphorus, 0.12% calcium and 0.11% magnesium; these compounds were possibly released from non-hydratable phospholipids present in the crude rice bran oil, which remained adhered to the membrane surface.
The 5-kDa PES membrane (after permeation) did not show the presence of chlorine or potassium, which are originally present in small quantities. However, a small quantity of phosphorus (0.12%) was detected, which might have been released from the phospholipids during the de-gumming of crude rice bran oil.
The commercial flat polymeric membrane prepared using PVDF (50-kDa molecular weight cut-off) showed the best retention of phospholipid miscella present in the crude rice bran oil (obtained using a hexane solvent), with a retention value of 95.5%. The obtained permeate had a phosphorus content of 29 mg·kg−1, and was associated with a flow rate of 48.1 L·m−2·h−1. The 5-kDa PES polymer membrane showed interesting results, with respect to the retention of phospholipids (62%) and the permeate flow (49.5 L·m−2·h−1), for crude rice bran oil miscella (obtained using anhydrous ethyl alcohol). The degumming of crude oils using the membrane separation process shows better viability, with regard to the rate of phospholipid retention. SEM and EDS analyses are indispensable tools for the evaluation of the microstructure of the polymeric membrane surface before and after the permeation process.
The authors would like to thank Capes, CNPq and FAPESP.