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Winterization of vegetable oil blends for biodiesel fuels and correlation based on initial saturated fatty acid constituents Heng Zhong, Masaru Watanabe, Heiji Enomoto, Fangming Jin, Atsushi Kishita, Taku Michael Aida, and Richard Lee Smith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00310 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 22, 2016
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Winterization of vegetable oil blends for biodiesel fuels and correlation based on initial saturated fatty acid constituents Heng Zhong,† Masaru Watanabe,*,†,‡ Heiji Enomoto,‡ Fangming Jin,‡,§ Atsushi Kishita,‡ Taku Michael Aida,‡ and Richard Lee Smith, Jr.†,‡ †
Research Center of Supercritical Fluid Technology, Tohoku University, Aoba-ku, Sendai
980-8579, Japan ‡
Graduate School of Environmental Studies, Tohoku University, Aoba-ku, Sendai 980-8579,
Japan §
School of Environmental Science and Engineering, Shanghai Jiao Tong University,800
Dongchuan Road, Shanghai 200240,China
*
Corresponding Author: Masaru Watanabe
Tel/Fax: +81-22-795-5864 E-mail:
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ABSTRACT Winterization is a simple method to remove saturated fatty acid contents in biodiesel fuels for improving their cold flow properties. In this work, biodiesel fuels with different initial long-chain (C16 and above) saturated fatty acid constituents (Si) were prepared from blends of palm, canola, and corn oils. The prepared biodiesels were treated at various winterization temperatures (Tw) to investigate the effect of Tw and Si on the final saturated fatty acid constituents (Sw) of the winterized biodiesel fuel. Optical microscopy showed that ball-like crystals formed with fluid regions at moderate cooling rates (−6 oC/h) allowed solid-liquid separation by filtration. A saturated fatty acid reduction ratio, Rs, defined as (Si – Sw)/Si × 100, was used with the experimental results on large samples (ca. 600 mL) to develop a correlation for winterization temperature as: Tw (ºC) = 0.659 Si (wt%) – 0.104 Rs (wt%) – 10.197. The correlation can provide an estimation of the required winterization temperature for reducing a specified ratio of fatty acid in a biodiesel fuel that mainly contains long-chain fatty acids from the initial saturated fatty acid constituents. When used with literature relationships between cold filter plugging point (CFPP) and Sw, estimation of the CFPP of winterized biodiesel fuels is possible without requiring actual winterization treatment.
Keywords: renewable energy; cold flow properties; saturated fatty acid reduction; biodiesel crystallization; physical treatment.
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1. INTRODUCTION Biodiesel derived from bioresources (plant or animal) is regarded as an important renewable energy, since it can be used directly or after blending with fossil fuel diesels without fundamental modifications to diesel engines.1-3 Therefore, biodiesel fuels can be integrated into modern industry easier than other typical renewable energy carriers such as biogas or H2. Palm oil, which is derived from the mesocarp (reddish pulp) of the fruit of the oil palms, is produced on the largest scale among renewable oils.4 However, the use of palm oil to produce biodiesel is limited due to its high saturated fatty acid ester content (ca. 50%).4-6 The high concentration of saturated fatty acid constituents in palm oil biodiesel would lead to poor cold flow properties and cause operability problems, such as plugging the filters and fuel lines, if applied to areas that have cold weather.7 Therefore, improving cold flow properties of highly saturated fatty acid-containing vegetable biodiesels, such as palm oil, is necessary. Methods to improve the cold flow properties of biodiesels with high saturated fatty acid content include blending with fossil diesel or other low saturated fatty acid containing biodiesels, adding additives such as olefin-ester copolymers (OECP) or polymethyl acrylate (PMA),8-10 and/or winterizing the biodiesel with cold temperature treatment to reduce saturated fatty acid content.11,12 Among these methods, winterization, which involves the refrigeration of oils at a specific temperature for a prescribed period followed by decanting of the liquid, is regarded as a simple, economical and effective way to modify the properties of a crude biofuel.11 Winterization has been widely studied for vegetable oil applications.13,14 Winterization of biodiesels and its effect on the cold flow properties of
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various fuels have been also examined.15,16 Those research works show that biodiesels derived from different oil sources usually require different winterization processes to achieve specific saturated fatty acid reductions because the oils have different initial saturated fatty acid constituents. However, it is inefficient to examine all biodiesels to determine the required winterization process to satisfy a specific operating temperature requirement. Therefore, the correlation among the initial saturated fatty acid constituent fraction (Si), winterization temperature (Tw) and reduction ratio of saturated fatty acid constituents after winterization (Rs, defined as the ratio of reduced weight percentage of saturated fatty acid constituents in the biodiesel during the winterization to the weight percentage of saturated fatty acid constituents in the biodiesel before winterization) needs to be developed to characterize conditions for the winterization process. The objective of this work was to develop a correlation for estimating the winterization temperature and reduction ratio of saturated fatty acid constituent content based on a series of prepared biodiesel blends characterized by their initial saturated fatty acid constituent content and properties of the winterized biodiesel blends.
2. Experimental 2.1. Materials. Commercial refined and edible-grade canola, palm and corn oils (Nisshin OilliO Group, Ltd. Japan), which mainly contain long-chain fatty acid constituents, were used as the sources to prepare the biodiesels. Since palm oil has high saturated fatty acid constituents while corn and canola oils mainly contain unsaturated ones, it is possible to prepare oil blends containing different saturated fatty acid constituents by mixing these
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three oils. Most importantly, these three vegetable oils are widely used as cooking oils, thus, the model compounds made by mixing these three oils can be used for further studies on winterizing waste cooking oil methyl esters (WCOME). Fatty acid compositions of these oils were analyzed by GC-FID and are shown in Table 1. Densities and viscosities of these oils were measured and are shown in Table S1 (Supporting Information). Seven different oil blends with palm:canola:corn volumetric ratios of 0:50:50, 10:45:45, 20:40:40, 30:35:35, 40:30:30, 50:25:25, 60:20:20 were used to produce the biodiesels for further experiments and analyses. The biodiesels derived from the above blends were denoted as P0, P10, P20, P30, P40, P50 and P60 based on the palm oil percentage, respectively. Commercially available waste cooking oil methyl esters (WCOME) (saturated FAME composition: 20.2%) provided by Oil Plant Natori Co., Ltd, Japan was used for the biodiesel crystallization study with optical microscope. Potassium hydroxide (KOH, 85%, Kanto Chemical Co., Inc, Japan) and methanol (CH3OH, 99.8%, Kanto Chemical Co., Inc, Japan) were used for the transesterification reactions directly without further purification. Standard reagents of fatty acid methyl esters for gas chromatography (GC), such as methyl myristate (99.5%), methyl palmitate (99.0%), methyl stearate (99.5%), methyl heptadecanoate (99.0%), methyl myristoleate (98.5%), methyl palmitoleate (98.5%), methyl oleate (99.0%), methyl linoleate (98.5%), and methyl linolenate (99.0%), were purchased from Sigma-Aldrich Co. LLC. Tetrahydrofuran (THF, 99.9%, inhibitor-free, Sigma-Aldrich Co. LLC) was used as GC sample diluent. 2.2. Reaction procedures. 2.2.1. Biodiesel production. Biodiesels, which are also referred to as fatty acid methyl esters (FAME), were prepared through transesterification
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reactions of the oil blends with methanolic KOH at 60 ºC for 1 h in a 1.5 liter (Φ 12 cm) glass beaker as schematically illustrated in Figure S1 (Supporting Information). A general procedure for the transesterification was as follows. First, 600 mL oil blend was poured into a 1.5 L glass beaker. Then, methanolic KOH was prepared by dissolving 6.26 g KOH (85%) in 152.3 mL of methanol in another beaker. The oil blend and the methanolic KOH were separately placed into a constant-temperature (60 ºC) water bath equipped with a temperature controller. Agitation was provided to the oil blend right after the oil blend was placed into the water bath and this was continued during the reaction at a constant stirring rate of 500 rpm. The methanolic KOH solution was added into the oil blend to initiate the transesterification reaction after both the oil blend and the methanolic KOH were heated to the desired temperature. After 1 h reaction time, the reactor was transferred quickly into a cold water bath to terminate the reaction. Subsequently, the mixture was held at room temperature for 24 h to separate the upper FAME phase from the lower glycerol phase. The upper FAME phase was collected and washed with deionized water to remove any remained glycerol and unreacted methanol. The water washed FAME was centrifuged at 3000 rpm for 10 min with a Hitachi CF 16RXII centrifuge. The upper FAME layer was collected and then dried in an oven at 120 ºC for 1 h to ensure a complete elimination of the remaining water. The treated FAME was collected and used for further analysis and winterization experiment.
2.2.2. Winterization experiments. Winterization experiments were carried out in a refrigerator equipped with a programmable controller by holding 80 mL FAME derived
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from various oil blends (P0, P10, P20, P30, P40, P50, P60) under a specific temperature program. The refrigerator temperature was first set at an initial temperature that was 6.5 ºC higher than the winterization temperature (Tw), and then the biodiesel sample was placed in the refrigerator. When the temperature of the biodiesel decreased to the initial temperature, the temperature program was started. Details of the temperature program are as follows. The refrigerator temperature was first held at the initial temperature for a 1 h period, and then decreased at a rate of -1 ºC/h for a 6 h period that was followed by a further decrease at a rate of -0.5 ºC/h for a 1 h period. Finally, the temperature was held constant at Tw for 16 h. The total time of the temperature program was 24 h. The actual temperatures of the biodiesel samples were measured continuously by thermocouples and the data were recorded by a multichannel temperature recorder (MCR-4TC, T&D Corporation, Japan). Temperature profiles for the winterization experiments of various biodiesels at different Tw are shown in Figure S2 (Supporting Information). All of the winterization temperatures in this paper are denoted as actual sample temperatures at the final temperature program step unless specified. After the winterization, the liquid phase was separated from the crystallized phase by filtration through a Millipore glass fiber filter paper (pore size: 2.0 µm; diameter: 4.7 cm) at the final winterization temperature at a pressure drop of 20 – 40 kPa. Both liquid and solid samples after the filtration were analyzed. 2.3. Analysis. The FAME samples before and after the winterization experiments were diluted by tetrahydrofuran (THF) to ca. 4 wt% and then analyzed with a Agilent 7890 gas chromatograph equipped with a flame ionization detector (GC-FID) and a DB-FFAP
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capillary column (50 m × 320 µm × 0.5 µm). The temperature program of the column was as follows: 40 ºC for 1 min, then increased to 70 ºC at 5 ºC/min, then increased to 170 ºC at 20 ºC/min, then increased to 200 ºC at 1 ºC/min, then increased to 240 ºC at 20 ºC /min, and held at 240 ºC for 3 min to have a total run time of 47 min. The injector and detector temperatures were 250 ºC. The FAMEs in the samples were identified by comparison of retention times with standard reagents and quantified by both area normalization and calibration curves of corresponding standard reagents. A temperature controllable microscope (Olympus BX51, Japan) as shown in Figure S3 (Supporting Information) was used to study the effect of temperature cooling rate and holding time on the biodiesel crystallization. The cooling stage of the microscope used liquid nitrogen as its cold source.
3. Results and discussion 3.1. Initial saturated FAME fraction of the produced biodiesels. Biodiesels with different initial saturated fatty acid constituent fractions were first prepared from oil blends with different ratios of palm, canola and corn oils (Table 2). Table 2 shows the fatty acid methyl ester compositions of the biodiesels produced from oil blends for palm oil content from 0 to 60 vol% (P0 to P60) and Figure S4 (Supporting Information) shows a plot of the tabular values. The main saturated ingredients in the prepared biodiesels were methyl palmitate (C16:0) and methyl stearate (C18:0) while the major unsaturated components were methyl oleate (C18:1) and methyl linoleate (C18:2). Small amounts (
