The aim of this work was to study the removal of free fatty acids (FFAs) from soybean oil, combining solvent extraction (liquid-liquid) for the separation of FFAs from the oil and membrane technology to recover the solvent through nanofiltration (NF). Degummed soybean oil containing 1.05 ± 0.10% w/w FFAs was deacidified by extraction with ethanol. Results obtained in the experiences of FFAs extraction from oil show that the optimal operating conditions are the following: 1.8 : 1 w : w ethanol/oil ratio, 30 minutes extraction time and high speed of agitation and 30 minutes repose time after extraction at ambient temperature. As a result of these operations two phases are obtained: deacidified oil phase and ethanol phase (containing the FFAs). The oil from the first extraction is subjected to a second extraction under the same conditions, reducing the FFA concentration in oil to 0.09%. Solvent recovery from the ethanol phase is performed using nanofiltration technology with a commercially available polymeric NF membrane (NF-99-HF, Alfa Laval). From the analysis of the results we can conclude that the optimal operating conditions are pressure of 20 bar and temperature of 35°C, allowing better separation performance: permeate flux of 28.3 L/m2·h and FFA retention of 70%.
Oilseeds are one of the most common sources of edible vegetable oils and fats. The process of obtaining oil from them consists in preparing the seed and then extracting the oil (by means of solvent extraction or crushing). After that, the crude oil obtained is refined; these stages are degumming, deacidification, bleaching, and finally deodorizing. These industrial processes are carried out with conventional technology and consume a great amount of energy, such as electricity, natural gas, or liquid fuel; this results in considerable oil loss and high content of effluents [
In the oils and fats processing industry, deacidification (the removal of free fatty acids or FFAs) is important, not only for consumer acceptance, but also for its high economic impact over production. Conventional methods for the removal of FFAs from oils consist in chemical and physical deacidification. The chemical process has disadvantages such as high energy consumption, thermal damage to the oil, high discharge of effluents, and oil loss [
Separation technology using membranes has evolved quickly in the last two decades. This has allowed scientists to isolate, purify, and separate very complex mixtures. Due to its high energetic efficiency, this separation technology is being utilized by industries for the effluents treatment, fruit and vegetable juice processing, products processing, and protein recovery and purification, among other uses [
The main reason for the use of membrane technology in the vegetable oil industry is its advantages compared to conventional processes, since it allows the selective separation of molecules according to the needs in each stage; it minimizes thermal damage and reduces the consumes energy and effluent production. Because of these advantages, its application in different stages of the production of edible oils is under constant research and expansion [
Crude vegetable oils contain FFAs together with triglycerides (oil). The deacidification process involves the removal of FFAs from crude oil to provide more stability and to make it more acceptable for consumers. This stage is the most delicate and difficult one in the refining process, since it determines final product quality; besides, it is the stage with the highest economic impact on the refining process. Conventional chemical and physical methods with which deacidification is industrially performed have already been widely studied [
The aim of the present work was to study the removal of FFAs from soybean oil by liquid-liquid extraction and the solvent recovery by nanofiltration.
The raw material used was degummed soybean oil from regional industry (OLCA S.A.I.C, General Cabrera, Argentina). The FFA extraction experiments were performed using analytical grade ethanol, isopropanol, and acetone. A commercially available polymeric composite nanofiltration membrane called NF-99-HF was used (Alfa Laval, Buenos Aires, Argentina). This is designed to reject organics with molecular weight above 200 Da. The membrane elements are based on a polyamide type thin film composite on polyester membrane.
Ethanol was selected instead of methanol, because it has low toxicity, is easily recoverable, and has good selectivity and distribution coefficient for FFAs, along with minimum losses of nutraceutical component.
The FFAs liquid-liquid extraction from oil was performed in two stages: the first one using the degummed soybean oil and the second one using the portion of FFAs-poor oil from the first stage. There was a varied ratio of solvent/oil (1 : 1 and 1.8 : 1 w : w) and rest times (24 hour and 30 minutes) remained constant: speed stirring (700 rpm), time stirring (30 minutes), and temperature (20 ± 2°C). The FFAs-rich solvent extracts from two extraction stages were combined and used in the nanofiltration experiments.
The nanofiltration setup (Figure
Schematic diagram of dead-end cell.
Permeate flux was evaluated from
The permeate flux at steady state
The FFAs retention factor (
The membrane cleaning procedure consisted in washing the membrane with an enzymatic detergent aqueous solution at ambient temperature for 15 minutes, rinsing it with distilled water, then permeating water during 30 minutes, and finally permeating the pure solvent. If necessary, the cleaning procedure was repeated until the hydrodynamic permeability of cleaned membrane was similar to that of original membrane (95–100%).
The FFA concentration was determined according to the AOCS Ca 5a-40 method, with an automatic titrator Titrino plus 848 (Metrohm, Switzerland) with Solvotrode electrode for titration in nonaqueous media Solvotrode (Metrohm, Switzerland). The FFA concentration was expressed as percent oleic acid (% w/w oleic acid).
Degummed soybean oil deacidification experiences, with an FFA concentration of 1.05 ± 0.10% w/w oleic acid, were performed with the following solvents: ethanol, isopropanol, and acetone. It was observed that isopropanol and acetone dissolved high contents of oil in addition to FFAs, which did not allow an adequate separation of the phases. As a result, ethanol was selected as the solvent used for extraction.
Different tests were performed with the aim of obtaining the best operating conditions during the phase of extraction of FFAs from oil. In relation to phase separation time, trials were conducted with 30 minutes and 24 hours of rest time. With respect to the ethanol/oil ratio, the relations 1 : 1 w : w and 1.8 : 1 w : w were used in both extraction stages. Table
Experimental results of extraction of FFAs from soybean oil.
Ethanol/oil ratio |
% FFAs feed | % FFAs ethanol phase | % FFAs oil phase |
---|---|---|---|
Experience 1 | |||
1 : 1 (First stage) | 1.10 | 0.58 | 0.51 |
1 : 1 (Second stage) | 0.51 | 0.28 | 0.24 |
| |||
Experience 2 | |||
1 : 1 (First stage) | 1.17 | 0.62 | 0.54 |
1 : 1 (Second stage) | 0.54 | 0.24 | 0.36 |
| |||
Experience 3 | |||
1.8 : 1 (First stage) | 0.98 | 0.33 | 0.23 |
1.8 : 1 (Second stage) | 0.23 | 0.10 | 0.07 |
| |||
Experience 4 | |||
1.8 : 1 (First stage) | 0.94 | 0.38 | 0.35 |
1.8 : 1 (Second stage) | 0.35 | 0.11 | 0.11 |
Ethanol permeability was determined from the slope of the curve that results when representing permeate flux values at steady state (
Effect of transmembrane on ethanol permeate flux (
Figure
Permeate flux (
Figure
Effect of transmembrane pressure on permeate flux (
Table
Flux and FFAs retention (
|
|
|
|
|
|
---|---|---|---|---|---|
24 | 20 | 0.17 | 0.05 | 72 | 15.5 ± 1.1 |
10 | 0.20 | 0.05 | 76 | 9.4 ± 0.7 | |
5 | 0.19 | 0.03 | 87 | 5.2 ± 0.4 | |
| |||||
35 | 20 | 0.21 | 0.06 | 70 | 28.3 ± 2.0 |
10 | 0.20 | 0.05 | 76 | 12.6 ± 0.9 | |
5 | 0.25 | 0.06 | 75 | 8.2 ± 0.6 |
The results of this study show that membrane technology combined with solvent extraction is effective in the soybean oil deacidification. From the analysis of the results in the extraction of the oil FFAs, we can conclude that the best condition corresponds to the separation time of 30 minutes and ethanol/oil ratio of 1.8 : 1 w : w for the two stages of extraction. A separation greater than 20% FFAs from oil when varying the ethanol content from 1 to 1.8 was achieved, not observing variations in the rest time. From the analysis of the results of the FFAs/ethanol mixture nanofiltration experiences it can be concluded that permeate flux increases with increasing transmembrane pressure. This behaviour is characteristic of the filtration systems that have pressure difference as a driving force. An analysis of the effect of temperature reveals that permeate flow increases when temperature rises. This can be attributed to the fact that the viscosity of the fluid decreases with increasing temperature, which causes an increase in the permeate flux for a given pressure difference. The best operating conditions are 20 bar and 35°C, obtaining the best permeoselectivity:
When the permeate flux varied as a function of time at the beginning of the experiences, a steady decrease of permeate flux (5–20%) can be observed. For periods over
The authors acknowledge the National Research Council of Argentina (CONICET) and National Agency for Scientific Promotion (ANPCyT) for their financial support.