Reactive Desorption of Fatty Acid Adsorbed on γ-Alumina Using

Sep 6, 2016 - acid and the corresponding fatty acid methyl ester (FAME) for γ-alumina. Palmitic acid was selected as a model fatty acid. Batch-type r...
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Reactive Desorption of Fatty Acid Adsorbed on γ‑Alumina Using Supercritical Methanol Hee Suk Woo, Seungmok Shin, Tae Joon Youn, and Youn-Woo Lee* School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea ABSTRACT: A novel method for regeneration of fatty acidadsorbed γ-alumina using supercritical methanol was developed. This method is based on a difference in the affinity of a fatty acid and the corresponding fatty acid methyl ester (FAME) for γ-alumina. Palmitic acid was selected as a model fatty acid. Batch-type reactors were used to investigate the effect of the operating parameters (temperature, methanol to γ-alumina weight ratio, and reaction time) on supercritical methanol regeneration. Almost all of the adsorbed palmitic acid was desorbed at temperatures above 300 °C, or when the weight ratio of methanol to γ-alumina was higher than 75:1, and/or if the reaction time was longer than 15 min. The crystal structure and BET surface area of γ-alumina were unchanged after supercritical methanol regeneration. The developed technique requires relatively lower operating temperature, is ecofriendly, and generates fuel, although the process is more complex than the thermal regeneration method.

1. INTRODUCTION Vegetable oil is a major source of nutrition in the human diet and is an important fuel source for production of biodiesel. Most vegetable oil is composed of triglycerides (TG), diglycerides (DG), monoglycerides (MG), free fatty acids (FFA), and other impurities. For use as a food or fuel source, crude oil must be purified to remove impurities including FFA. FFA in vegetable oil inactivates the base catalyst used in biodiesel production by forming soaps. Deacidification, which refers to removal of FFA in vegetable oil, is an important refining process for reducing the high level of FFA that may promote rancidity of oils. The deacidification process is also important for preventing equipment corrosion in other fields. The most common deacidification method is neutralization with alkali, generally sodium hydroxide solution. However, neutralization via the alkaline method is not suitable for vegetable oils that contain a high concentration of FFA. Saponification of vegetable oil due to alkaline treatment results in high loss of oils during the washing step. It is reported that the refining loss reaches three times the FFA content during the deacidification of soybean and cottonseed oil.1 Other methods of deacidifying vegetable oil include treatment with an adsorbent,2 organic solvent extraction,3−6 supercritical fluid extraction,7−9 distillation, membrane deacidification,10 and chemical and biological re-esterification.11−13 The deacidification process using distillation consumes a large amount of energy, and a secondary reaction might occur because of the high operating temperature. The organic solvent extraction process also expends a large amount of energy owing to solvent recovery. Adsorption of FFA using activated alumina or ion-exchange resin is a potential alternative for neutralization by the alkaline © XXXX American Chemical Society

method. A chromatographic process using activated alumina at room temperature without physical contact between the oils and alkaline reagent leading to decreased loss of oils has been proposed.14 Furthermore, vegetable oils may be bleached to some extent due to adsorption of some color pigments on most adsorbents. Activated alumina, which is manufactured from aluminum hydroxide by dehydroxylation, has been widely applied as an adsorbent or a catalyst support due to its thermal stability, mechanical properties, low cost, and high surface area. Activated porous alumina is commonly used as a desiccant. When activated alumina is exposed to moisture, the surface functional groups of alumina are converted to hydroxyl groups due to moisture adsorption. Chemisorption of carboxylic acids on alumina can occur via surface esterification with hydrated alumina surfaces.15 The higher reactivity of the hydrated alumina surface toward carboxylic acids relative to other molecules is thought to arise from the enhanced acidity of the hydroxyl group as compared to the hydrated silica surface (for instance), which does not show such reactivity toward the carboxylate group.16 Therefore, adsorption of FFA using activated alumina is a potential alternative for neutralization by the alkaline method. A chromatographic process using activated alumina at room temperature without physical contact between the oils and alkaline reagent leading to decreased loss of oils has been proposed.14 Furthermore, vegetable oils may be bleached to some extent due to adsorption of some color Received: July 17, 2016 Revised: September 5, 2016 Accepted: September 6, 2016

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DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

experimental conditions. Since supercritical methanol is a homogeneous one phase, the density of supercritical methanol in a batch reactor is easily calculated if the exact weight of methanol is loaded. Densities of methanol are presented at various temperatures and pressures in the NIST Reference Fluid Thermodynamic and Transport Properties Database Version 8.0. The pressure of a batch reactor can be controlled such that the exact amount of methanol could be introduced into a known internal volume of the batch reactor (23 mL) which lead to the desired pressure after the temperature reaches the desired point. A proportional-integral-derivative (PID) controller with a K-type thermocouple was used for temperature control of the molten salt comprising KNO3/NaNO3/ CaNO3 in a ratio of 46:24:30. After supercritical methanol treatment, the γ-alumina was separated from the methanol solution by filtration and washed with dichloromethane. The washed γ-alumina was dried under vacuum (600 mmHg) at 40 °C for 5 h. Dried γ-alumina was analyzed via TGA, BET, and XRD. The filtrate was evaporated under vacuum (600 mmHg) at 40 °C for 5 h to remove methanol. The dried filtrate was analyzed using GC/MS. To measure adsorption capacity of γalumina, it was washed and dried repeatedly after a supercritical methanol treatment. 2.3. Characterization. 2.3.1. Gas Chromatography. An Agilent 7890A instrument with a 5975C mass spectrometer was used to analyze the liquid product obtained by supercritical methanol treatment. The liquid product was dried to evaporate the methanol. Hexane was used as a solvent for dissolution of the dried product. To conduct a quantitative analysis, methyl heptadecanoate was used as an internal standard. An HP-5 ms (length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 μm) column and a triple-axis HED-EM detector were loaded into the instrument. The detection conditions are described below. High purity helium (99.9999%) was used as a carrier gas at a flow rate of 1 mL/min. The inlet (split mode; ratio 5:1) and the detector temperature were maintained at 260 °C. The oven temperature was programmed to increase from 150 to 250 °C at 5 °C/min and maintained for 10 min. The temperature of the ionization source and quadrupole were respectively maintained at 230 and 150 °C. 2.3.2. Titration with Base Solution. After the adsorption process, filtrated palmitic acid solution was titrated with 0.005 M sodium hydroxide solution to measure an amount of adsorbed palmitic acid on γ-alumina. The solution was titrated using the ASTM D664-11a method by an automatic titrator (TitroLine 6000 SI Analytics). 2.3.3. Thermal Gravimetric Analysis (TGA). The thermogram of γ-alumina was obtained using a Q-5000 IR (TA Instruments, USA) instrument with a heating ramp of 10 °C/ min up to 700 °C under nitrogen atmosphere. The minimum sample consumption for a single analysis was 2 mg, and an identical weight of sample was used for each analysis. 2.3.4. X-ray Diffraction (XRD). A D-MAX-2500 PC (Rigaku, Japan) diffractometer was used to obtain the X-ray diffraction pattern of γ-alumina after supercritical methanol treatment. The powder was scanned from 10° to 90° at a scan speed of 10°/ min. The obtained XRD pattern was compared to the JCPDS reference. 2.3.5. Fourier-Transform Infrared Spectrophotometer (FTIR). A Nicolet 6700 (Thermo Scientific, USA) was used to examine surface functional group change of γ-alumina after supercritical methanol treatment. The scanned wavenumber range was from 4000 to 650 cm−1.

pigments on most adsorbents. To reduce the material and operation cost, the spent alumina must be efficiently regenerated. However, regeneration of spent alumina is energy intensive due to the high dissociation energy of the covalent bond created by chemisorption. The calcination process, which requires high temperature, is conventionally used for regeneration of spent alumina. Supercritical fluids have recently been applied to various fields. Specifically, supercritical methanol has been used in the production of biodiesel, synthesis of metal oxides, and recycling of plastics. Biodiesel production via transesterification of vegetable oil with supercritical methanol was first studied by Saka and Kusdiana in Japan,17 and many other studies have ensued owing to the merits of the process. The supercritical methanol process offers the advantage of a high transesterification reaction rate relative to other processes. Thus, high conversion can be achieved in a short time, and the reaction products can be easily separated after reaction due to the lack of side reactions such as saponification. In this study, a new regeneration method is proposed for regeneration of the fatty acid-adsorbed γ-alumina through transesterification, in which the chemisorbed fatty acid is converted to fatty acid methyl ester (FAME) by supercritical methanol. This regeneration method is based on the difference in the strength of adsorption of fatty acid and FAME on γalumina. The product of the proposed process realized with supercritical methanol was analyzed by GC/MS to identify the fatty acid methyl esters. The experiments were carried out with variation of the temperature, weight ratio of methanol to γalumina, and reaction time to determine the optimum conditions for regeneration of fatty acid-adsorbed γ-alumina. After supercritical methanol treatment, the γ-alumina was analyzed by TGA to quantify the amount of adsorbed fatty acid. In addition, XRD and BET analyses were used to monitor transition of the crystal structure and changes in the surface area of γ-alumina.

2. EXPERIMENTAL SECTION 2.1. Preparation of Fatty Acid-Adsorbed γ-Alumina. The experiments were carried out using fatty acid-adsorbed γalumina. Palmitic acid (C16 fatty acid) purchased from SigmaAldrich (Korea) was used as a model fatty acid and utilized as an adsorbate. The adsorbent comprised γ-alumina (AL-3992 1/ 8, Engelhard) ground into a micron powder size of between 45 and 150 μm. The BET surface area of powdered γ-alumina was measured in triplicate to yield an average surface area of 205 m2/g. One gram of palmitic acid was dissolved in 400 mL of dichloromethane. Two grams of γ-alumina was poured into a glass bottle containing palmitic acid solution and stirred for 24 h with a magnetic stirrer to reach equilibrium at room temperature. After the adsorption process, γ-alumina was separated by filtration and washed twice with neat dichloromethane. Filtrated solution was titrated by base solution to measure an amount of adsorbed palmitic acid on γ-alumina. Separated γ-alumina was dried for 2 h in a vacuum oven at 40 °C under a pressure of 600 mmHg to evaporate the dichloromethane. 2.2. Regeneration of Palmitic Acid-Adsorbed γAlumina Using Supercritical Methanol. The regeneration of γ-alumina was carried out using a 23 mL batch type reactor made of SUS316 in a molten salt bath with an electric shaker. The palmitic acid-adsorbed γ-alumina and methanol (J.T. Baker) were loaded into the reactor under the selected B

DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

first derivative of the thermogravimetric curve that specifies the corresponding rate of weight loss. The weight of raw γ-alumina decreased drastically up to 100 °C due to desorption of physically adsorbed moisture, which constituted about 3 wt % as shown in Figure 1(a). When the temperature exceeded 150 °C, removal of chemisorbed moisture caused about 3% weight loss of γ-alumina with a low weight loss rate. When the raw γ-alumina was treated with supercritical methanol, the weight loss rate was higher in the higher temperature range of >150 °C based on comparison of Figure 1(c) and (d). On the surface of γ-alumina, the Al−O−Al bonds dissociate upon treatment with methanol to form Al− OH and Al-OCH3 by introduction of the methoxy group, resulting in a higher weight loss rate. There was a significant difference between the thermogram of raw γ-alumina and that of palmitic acid-adsorbed γ-alumina. The maximum rate of desorption of chemisorbed palmitic acid (0.1276 wt %/°C) was observed around 452 °C and accounted for a large proportion of the weight loss, as shown in Figure 2. It was measured that an average of 165.50 mg of palmitic acid was adsorbed on 1000 mg γ-alumina in consequence of titration which resulted in major weight loss of palmitic acid-adsorbed γalumina. The weight loss rate corresponding to moisture desorption decreased because some adsorption sites were occupied by palmitic acid instead of moisture. This result demonstrated that temperatures exceeding 550 °C were necessary to thermally regenerate the palmitic acid-adsorbed γ-alumina. 3.2. Mechanism of Supercritical Methanol Regeneration. The fatty acid is chemisorbed onto γ-alumina via the esterification with the hydrated γ-alumina surfaces.15 A reactive desorption mechanism that converts adsorbed fatty acid to fatty acid methyl ester using supercritical methanol was suggested for regeneration of the fatty acid-adsorbed γ-alumina as shown in Figure 3. Methylation of the surface functional groups occurred by dissociation of the Al−O−Al groups to form Al−OCH3 and Al−OH in the presence of methanol.18 The regeneration process could be accomplished in a short time owing to the properties of supercritical fluids. It is expected that the FAME produced via the transesterification reaction was not readsorbed on γ-alumina in the reactor for two reasons. First, FAME does not have the carboxylic group to confer affinity to the hydrated surface of γalumina. Second, phase separation occurs during quenching due to the very low solubility of FAME in methanol. Thus, FAME is not readily adsorbed due to high mass transfer resistance, given that most FAME species are solid or viscous liquids at room temperature. 3.3. Effect of Temperature on Regeneration of Palmitic Acid-Adsorbed γ-Alumina. Temperature is expected to be an important variable for regeneration of γalumina owing to its importance in the transesterification reaction. Figure 4 shows the result of regeneration of palmitic acid-adsorbed γ-alumina using supercritical methanol with

2.3.6. Brunauer−Emmett−Teller (BET) surface area. To determine the surface area of γ-alumina, N2 adsorption/ desorption isotherms were acquired at liquid nitrogen temperature using a gas sorption analyzer (Micromeritics, ASAP 2020). The samples were degassed at 473 K under a vacuum of below 10−3 Torr for 6 h prior to the measurement. The specific surface area was calculated from the BET equation.

3. RESULTS AND DISCUSSION 3.1. Characterization of Raw γ-Alumina, Supercritical Methanol-Treated Raw γ-Alumina, and Palmitic AcidAdsorbed γ-Alumina. Figures 1 and 2 represent TG/DTG

Figure 1. TG/DTG analysis of raw γ-alumina (a and c) and supercritical methanol treated raw γ-alumina (b and d), (a): weight of raw γ-alumina, (b): weight of supercritical methanol treated raw γalumina, (c): weight derivative of raw γ-alumina, (d): weight derivative of supercritical methanol treated raw γ-alumina.

Figure 2. TG/DTG analysis of palmitic acid-adsorbed γ-alumina.

analysis of raw γ-alumina, supercritical methanol-treated raw γalumina, and palmitic acid-adsorbed γ-alumina. The left y-axis indicates the weight percent, and the right y-axis indicates the

Figure 3. Regeneration mechanism of fatty acid-adsorbed γ-alumina using supercritical methanol. C

DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 4. Weight derivative curves of raw γ-alumina, supercritical methanol treated raw γ-alumina, palmitic acid-adsorbed γ-alumina, and regenerated γ-alumina with supercritical methanol at temperatures of 300, 350, and 400 °C.

Figure 5. Weight derivative curves of supercritical methanol treated raw γ-alumina and regenerated γ-alumina in supercritical methanol with the methanol to γ-alumina weight ratios of 53:1, 75:1, 105:1, and 211:1.

ratio of methanol to γ-alumina of 75:1 is sufficient for regeneration of palmitic acid-adsorbed γ-alumina in supercritical methanol. 3.5. Effect of Reaction Time on Regeneration of Palmitic Acid-Adsorbed γ-Alumina. Figure 6 shows the

varying temperature (300, 350, and 400 °C). To evaluate the effect of temperature, the methanol to γ-alumina weight ratio, pressure, and reaction time were kept constant at 53:1, 300 bar, and 30 min. A derivative of the weight was used to compare the degree of regeneration of γ-alumina. The rate of weight loss of regenerated γ-alumina decreased to a level similar to that of supercritical methanol-treated γ-alumina. This result demonstrated that almost all of the palmitic acid was desorbed from γalumina. The rate of weight loss of regenerated γ-alumina (300 °C) was slightly higher than that of the other samples in the temperature range of 400−500 °C. The maximum rate of weight loss of regenerated γ-alumina (300 °C) was 0.0224 (wt %/°C) at 452 °C. The weight loss rates of the other samples were respectively 0.0125 (wt %/°C, 350 °C) and 0.0167 (wt %/°C, 400 °C) at the same temperature. The difference in the rate of weight loss from the thermograms at each regeneration temperature was trivial even at the maximum. Thus, a regeneration temperature of 300 °C was high enough for regeneration of palmitic acid-adsorbed γ-alumina using supercritical methanol. 3.4. Effect of the Weight Ratio of Methanol to γAlumina on Regeneration of Palmitic Acid-Adsorbed γAlumina. From the thermodynamic perspective of the mechanism of regeneration of γ-alumina, as the ratio of methanol to γ-alumina increases, more fatty acid-adsorbed γalumina is regenerated. As shown in Figure 5, the palmitic acidadsorbed γ-alumina was treated with supercritical methanol by varying the weight ratio of methanol to γ-alumina as 53:1, 75:1, 105:1, and 211:1 to evaluate the effect of the weight ratio of methanol to γ-alumina. The other experimental conditions were kept constant at 300 °C, 300 bar, and a reaction time of 30 min. The rate of weight loss of γ-alumina decreased in the temperature range of 400−530 °C when the weight ratio of methanol to γ-alumina was increased from 53:1 to over 75:1 owing to a shift in the equilibrium. The maximum weight loss rate (0.0224 (wt %/°C)) of γ-alumina (53:1) was attained at 452 °C. When the weight ratio of methanol to γ-alumina was higher than 75:1, the respective maximum weight loss rates were all less than 0.0166 (wt %/°C). Almost identical weight loss rates were obtained, except at a methanol to γ-alumina weight ratio of 53:1. This result demonstrated that the weight

Figure 6. Weight derivative curves of supercritical methanol treated raw γ-alumina, palmitic acid-adsorbed γ-alumina, and regenerated γalumina in supercritical methanol with reaction times of 5, 15, 30, 60, and 90 min.

TGA curve of γ-alumina regenerated by supercritical methanol treatment for various reaction times (5, 15, 30, 60, and 90 min). The other experimental conditions were kept constant as 300 °C, 300 bar, and the weight ratio of methanol to γ-alumina of 75:1. The maximum rate of weight loss (0.1276 wt %/°C) of palmitic acid-adsorbed γ-alumina decreased to 0.0199 wt %/°C after 5 min of supercritical methanol treatment. This result demonstrated that most of the adsorbed palmitic acid was removed from γ-alumina in 5 min. The rate of weight loss of γalumina in the temperature range of 400−530 °C further decreased as the reaction time increased from 5 to 15 min, indicating that almost all of the adsorbed palmitic acid was desorbed from γ-alumina. The weight loss rates of the γalumina samples treated with supercritical methanol for longer D

DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

and 400 °C using a pressure of 300 bar, a methanol to γalumina weight ratio of 53:1, and a reaction time of 30 min. This result confirmed that the crystal structure of γ-alumina was not altered after supercritical methanol treatment regardless of the operating temperature. 3.8. Alteration of the Surface Functional Group of γAlumina during Supercritical Methanol Regeneration. FT-IR measurements were conducted to figure out the surface functional group of γ-alumina during supercritical methanol regeneration. In Figure 9(b), the methylene peak (2850−2950 cm−1, 1470 cm−1) and carbonyl peak around 1500 cm−1 were observed due to chemisorption of palmitic acid on γ-alumina. The intensity of both methylene peak and carbonyl peak were decreased, and the C−O peak (1050−1100 cm−1) was observed after supercritical methanol treatment at 300 °C, a pressure of 300 bar, a methanol to γ-alumina weight ratio of 75:1, and a reaction time of 30 min. Almost the same FT-IR patterns of the other regenerated γ-alumina were obtained. These results were consistent with suggested regeneration mechanism in Figure 3. After second adsorption of palmitic acid on regenerated γ-alumina, intensity of methylene peak, hydroxyl peak, carbonyl peak was decreased due to adsorption capacity shrink of γ-alumina. On the other hands, C−O peak which was not detected at (a) (b) was observed by alteration of surface functional group from Al−O−Al to Al−O−CH3 and Al−O during supercritical methanol regeneration. 3.9. Analysis of Reaction Product of Supercritical Methanol Regeneration. All reaction products were analyzed by gas chromatography after supercritical methanol regeneration and palmitic acid methyl ester was the only detected reaction product. Table 1 shows a yield of palmitic acid methyl ester formed during the regeneration of the palmitic acidadsorbed γ-alumina by supercritical methanol treatment. These results demonstrate that adsorbed palmitic acid was converted to palmitic acid methyl ester by transesterification in supercritical methanol. At 300 °C, the yield of palmitic acid methyl ester was increased with an increase in time. After 15 min of reaction time, most of the adsorbed palmitic acid was converted to palmitic acid methyl ester. This result was consistent with TGA/DTG curves of regenerated γ-alumina which were presented in Figure 6. At 400 °C, the yield of palmitic acid methyl ester was much lower than yields of temperature 350 and 300 °C. According to previous research, palmitic acid methyl ester undergoes slight decomposition at 350 °C and 43 MPa but undergoes negligible decomposition below 350 °C in supercritical methanol.19 Even at 400 °C, methyl palmitate remained almost stable and that may account for thermal decomposition of adsorbed palmitic acid occurring.20

than 15 min were identical, indicating that a treatment time of 15 min was sufficient to attain the reaction equilibrium. 3.6. Constancy of Adsorption Capacity of γ-Alumina. Several cycles of adsorption and desorption of palmitic acid on γ-alumina were performed to investigate the repeatability of adsorption by γ-alumina regenerated with supercritical methanol. The adsorption conditions were identical to those described in the Experimental Section. Supercritical methanol regeneration was carried out at 300 °C, 300 bar, using a methanol to γ-alumina weight ratio of 75:1 and a reaction time of 30 min. The results are presented in Figure 7. The largest

Figure 7. Weight derivative curves of raw γ-alumina, supercritical methanol treated raw γ-alumina, palmitic acid-adsorbed γ-alumina (1st, 2nd, 3rd, 4th), and regenerated γ-alumina with supercritical methanol (1st, 2nd, 3rd).

adsorption capacity was obtained with the use of virgin γalumina on which palmitic acid was adsorbed. The adsorption capacity was reduced during the series of adsorption processes. This is attributed to hindrance of the adsorption by the surface methoxy groups of γ-alumina that were formed during supercritical methanol regeneration. The rate of weight loss of γ-alumina regenerated by supercritical methanol was similar after all cycles, indicating that reactive desorption of palmitic acid occurred successfully regardless of the number of adsorption cycles. 3.7. Alteration of Crystal Structure of γ-Alumina in Supercritical Methanol. The γ-alumina was analyzed with XRD to identify a change in the crystal structure after supercritical methanol regeneration. Figure 8 presents XRD patterns of γ-alumina which was regenerated at 300 °C, 350 °C,

Figure 8. XRD patterns of γ-alumina samples regenerated at 300 °C, 350 °C, and 400 °C of supercritical methanol. E

DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. FT-IR patterns of (a) raw γ-alumina; (b) palmitic acid-adsorbed γ-alumina; (c) regenerated γ-alumina with supercritical methanol; (d) palmitic acid-adsorbed γ-alumina (2nd adsorption); (e) regenerated γ-alumina with supercritical methanol (2nd regeneration).

regeneration methods have several common features. The first is that the crystal structure of regenerated γ-alumina is identical to that of raw γ-alumina. Second, the BET surface area of regenerated γ-alumina was not changed during either process (thermal regeneration: 193 m2/g, supercritical methanol regeneration: 200 m2/g). The supercritical methanol regeneration process is a prospective alternative to the thermal regeneration process owing to its advantages.

Table 1. Palmitic Acid Methyl Ester Yield According to Experimental Conditions with a Pressure of 300 bar no.

temp (°C)

reaction time (min)

wt ratio of methanol to γ-alumina

palmitic acid methyl ester yield (%)

1 2 3 4 5 6 7 8

400 350 300 300 300 300 300 300

30 30 30 5 15 30 60 90

53:1 53:1 53:1 75:1 75:1 75:1 75:1 75:1

56.27 95.18 85.38 55.36 87.64 94.35 97.32 88.56

4. CONCLUSION A novel method for regeneration of fatty acid-adsorbed γalumina based on the difference in the affinity of fatty acid and FAME on γ-alumina was introduced. Palmitic acid adsorbed on γ-alumina was converted to palmitic acid methyl ester via transesterification using supercritical methanol without a catalyst. A series of experiments were carried out in a batch reactor to investigate the optimum conditions for regeneration of palmitic acid-adsorbed γ-alumina. The optimum conditions were 300 °C, 300 bar, a methanol to γ-alumina weight ratio of 75:1, and a reaction time of 15 min. Supercritical methanol regeneration, which is an ecofriendly process, has a lower operating temperature than thermal regeneration and produces biodiesel.

3.10. Comparison of Methods for Regeneration of Fatty Acid Adsorbed γ-Alumina. Thermal regeneration is a conventional method for regeneration of fatty acid adsorbed γalumina. A comparison of thermal regeneration and supercritical methanol regeneration is presented in Table 2. Thermal Table 2. Comparison of Methods for Regeneration of Fatty Acid-Adsorbed γ-Alumina

temperature (°C) process adsorbed fatty acid surface area (m2·g−1) crystal structure

thermal regeneration

supercritical methanol regeneration

550 simple decomposition identical identical

300 complex FAME (biodiesel) identical identical

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+82) 2 880-1883. Fax: (+82) 2 883-9124. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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regeneration is an energy-intensive process requiring an operation temperature of at least 550 °C based on the TGA thermogram in Figure 2. In contrast, the operating temperature of supercritical methanol regeneration is much lower than thermal regeneration. Thermal regeneration may lead to environmental pollution due to the decomposition product of adsorbed fatty acid. In contrast to thermal regeneration, the fatty acid methyl ester synthesized by transesterification of the adsorbed fatty acid during supercritical methanol is an ecofriendly product that can be utilized as an automobile fuel. However, additional processes such as filtration and drying are required in supercritical methanol regeneration. Both

ABBREVIATIONS BET Brunauer−Emmett−Teller DG diglycerides FAME fatty acid methyl ester FFA free fatty acids MG monoglycerides TG triglycerides TGA thermal gravimetric analysis XRD X-ray diffraction FT-IR Fourier transform infrared spectroscopy F

DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.6b02663 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX