SCIENCE CHINA Technological Sciences • Article •
February 2015 Vol.58 No.2: 258–265 doi: 10.1007/s11431-014-5691-1
A study on the catalytic performance of carbide slag in transesterification and the calculation of kinetic parameters LIU MengQi, NIU ShengLi, LU ChunMei, LI Hui & HUO MengJia School of Energy and Power Engineering, Shandong University, Jinan 250061, China Received August 5, 2014; accepted October 8, 2014; published online October 28, 2014
The catalytic performance of carbide slag in transesterification is investigated and the reaction kinetic parameters are calculated. After being activated at 650oC, calcium compounds of carbonate and hydroxide in the carbide slag are mainly transformed into calcium oxide. The activated carbide slag utilized as the transesterification catalyst is characterized by X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), nitrogen adsorption-desorption and the Hammett indicator method. Compared with the carbide slag activated at 700 and 800oC, the largest surface area of 22.63 m2 g 1, the smallest particle size of 265.12 nm and the highest catalytic efficiency of the carbide slag activated at 650oC guarantee its capacity in catalyzing transesterification. Then, the influences of activated temperature (Ta), molar ratio of methanol to oil (γ), catalyst added amount (ζ), reaction temperature (Tr) and reaction time (τ) on the catalytic performance are investigated. Under the optimal transesterification condition of Ta=650oC, γ=15, ζ=3%, Tr=60oC and τ=110 min, the catalytic efficiency of 92.98% can be achieved. Finally, the kinetic parameters of transesterification catalyzed by the activated carbide slag are calculated, where activation energy (E) is 68.45 kJ mol 1 and pre-exponential factor (k0) is 1.75×109 min 1. The activated carbide slag shows better reused property than calcium hydroxide. transesterification, catalyst, carbide slag, kinetic parameters Citation:
Liu M Q, Niu S L, Lu C M, et al. A study on the catalytic performance of carbide slag in transesterification and the calculation of kinetic parameters. Sci China Tech Sci, 2015, 58: 258265, doi: 10.1007/s11431-014-5691-1
1 Introduction Biodiesel, which is derived from vegetable oils or animal fats, is one of the most promising alternative sources for the fossil fuels [1,2]. The advantages of biodiesel are renewability, biodegradability, nontoxicity and atmospheric carbon dioxide production balance [3,4]. Dilution, microemulsification, pyrolysis and transesterification are the methods for biodiesel production, where the last one has been demonstrated to be the most effective [5,6]. Transesterification is defined as the reaction between triglycerides (TGs), which are the main composition of vegetable oils or animal fats, *Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2014
and methanol to produce fatty acid alkyl esters (FAMEs), namely, biodiesel, and glycerol under acidic or basic catalytic effect. Broad researches about the transesterification catalysts, which play an important role in the reaction, have been conducted over the past decades. The homogeneous catalysts supply high transesterification efficiency [7], but the process suffers from the accompanied waste water because of the intersolubility between homogeneous catalysts and liquid products. This problem is overcome by the heterogeneous catalysts which can be easily separated from the liquid products [8,9]. A number of the basic heterogeneous catalysts such as alkaline earth metal oxide [10], anion exchange resin [11], hydrotalcite [12], organic heterogeneous alkali [13] and zeolite [14] gain the ability to catalyze a tech.scichina.com link.springer.com
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complete transesterification under the mild condition. Calcium oxide is the representative of the heterogeneous basic catalysts [15,16], but its application in transesterification is restricted to the rigorous condition [17]. The mechanism of the transesterification catalyzed by calcium oxide is shown in Figure 1. To improve the catalytic performance, the supported catalysts based on pure calcium oxide are prepared [20,21], but the cost is negatively boosted. Therefore, searching for the low-cost catalysts from the waste materials becomes quite attractive. Dolomite [22], coral [23], oyster shell [24], pumice [25] and egg shell [26] have been successfully tested as the transesterification catalysts and they make the resources of the heterogeneous calcium-based basic catalysts more broad. Carbide slag (CS) is the by product for ethylene production and millions of tons of CS are generated every year. However, the effective utilization of CS has not been conducted [27,28]. CS is dominantly composed of calciumbased compounds with small quantities of magnesium, aluminum, iron, silicon, etc. Compared with other heterogeneous basic catalysts, CS as the transesterification catalyst has not been comprehensively reported. In a previous study [29], the activated CS was certified as the transesterification catalyst, and an excellent catalytic efficiency was achieved by 90.07% under the reaction condition of molar ratio of methanol to oil of 20, catalyst added amount of 5.5 %, reaction temperature of 54°C, and reaction time of 180 min. Therefore, more fundamental investigations on catalytic performance of CS in transesterification are urgent to be put
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into effect. The kinetic parameters are critical for process simulation and reactor design, but the kinetic calculation on the transesterification catalyzed by the heterogeneous basic catalysts has been seldom studied, especially for the heterogeneous calcium-based waste catalysts. Zanette et al. [30] used a semi-empirical model to capture complex transesterification mechanism. Dossin et al. [31] applied a three-step “EleyRideal” type mechanism to simulate the transesterification of rapeseed oil with methanol catalyzed by magnesium oxide. Vujicic et al. [32] established a pseudo-first order reaction to describe the kinetics of sunflowers oil transesterification over calcium oxide. Kouzu et al. [33] demonstrated that the reaction order of the soybean oil transesterification catalyzed by calcium oxide was varied from the zero order to the first order. The above studies imply that the transesterification kinetics is controversial, and more fundamental researches are obligatory. In this study, the catalytic performance of CS in transesterification is investigated. The chemical ingredient of the original CS is analyzed by X-ray fluorescence (XRF). Then CS is activated and the activated temperature (Ta) range is determined by thermogravimetric (TG) result. The activated CS is further checked for the crystalline phases and the functional groups through X-ray diffraction (XRD) and Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), respectively. At the same time, nitrogen absorption-desorption for the textural parameters measurement and the Hammett indicator method for the basic strength determination of the achieved catalysts are carried out. Based on characterization, the catalytic performance of the activated CS in transesterification is tested from the aspects of Ta, molar ratio of methanol to oil (γ), catalyst added amount (ζ), reaction temperature (Tr) and reaction time (τ). Also, the kinetic parameters of reaction rate constant (k), reaction order (α), pre-exponential factor (k0) and activation energy (E) are calculated and the kinetic model of transesterification catalyzed by the activated CS is subsequently established. Finally, reused property of the activated CS as the transesterification catalyst is studied for stability estimation.
2 Methods 2.1
Figure 1 [18,19].
Mechanism of transesterification catalyzed by calcium oxide
Catalyst preparation and characterization
CS is acquired from an ethylene gas manufacturer in Shandong Province, China and its composition (Table 1) is analyzed by XRF (WDX-200X, Bandwise Technology Development Co., Ltd, China). After the loss of volatile (33.79%), the calcium-based compounds account for primary composition. Further, the crystalline phases existing in the original CS and the activated CS are determined through XRD. To investigate the thermal transition of CS, TG is conducted on TGA/SDTA 851e thermogravimetric analyzer,
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Table 1 CaO 59.20
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Composition of carbide slag (%) SiO2 3.80
Al2O3 2.03
Fe2O3 0.36
SO3 0.37
MgO 0.24
Na2O 0.01
Loss 33.79
Others 0.20
Mettler-Toledo Instruments Co., Ltd, Switzerland, and the initial sample is weighted to 10±0.1 mg in an aluminum oxide ceramic crucible of 5 mm depth and 5 mm diameter. During the experiment, gas flux of nitrogen (99.99% purity) is kept at 50 mL/min and CS is heated from 40 to 900oC at the heating rate of 20°C/min. TG curve, which detects the mass loss signal with a resolution of 0.1 μg, is continuously recorded. Then, the Ta range is determined through the TG result. CS is activated in a muffle furnace under air atmosphere. To check the calcination outcomes at different temperatures, ATR-FTIR (Vertex 70, Bruker Instrument Co., Ltd, German) is employed to provide information about the functional groups of the samples with resolution of 4 cm1 and scanning range from 400 to 4500 cm1. The crystalline phases of the original CS and the activated CS are analyzed by XRD (Advanced D8, Bruker Instrument Co., Ltd, German), which uses Cu radiation source with electricity at 100 mA, voltage at 40 kV, 2θ range from 10° to 90°, step size of 0.1° and scanning speed of 4°/min. Besides, the Maud software is adopted to quantify calcium-based compounds in the catalysts. The textural parameters of the activated CS are measured through nitrogen adsorption-desorption instrument (ASAP2020, Micromeritics Instruments Co., Ltd, America). After the sample is degassed at 300°C under vacuum, nitrogen adsorption isotherms are obtained at 196°C using nitrogen as the adsorption medium. Then, the surface area is calculated by Brunauer-Emmett-Teller (BET) equation and the average particle size and the pore volume distribution are calculated through Barrett-Joyner-Halenda (BJH) model. The basic strength of the activated CS is tested using the Hammett indicator method and the indicators mainly include bromthymol (H=7.2), phenolphthalein (H=9.8), 2,4dinitroaniline (H=15) and 4-nitroaniline (H=18.4). Approximately 0.1 g catalyst is shaken with 2 mL methanol where the Hammett indicator is dissolved. Then, the mixture is left to be equilibrated for color variation. A color change indicates that the catalyst is stronger than the indicator and vice versa. 2.2
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formed on a DB–INNOWAX capillary column where the temperature of the oven is kept at 80°C for 1 min, subsequently programmed to 260°C at 10°C/min and then kept for 15 min; the injection temperature is maintained at 260°C and the volume of the injected sample is 0.1 μL; the temperature of the flame ionization detector is kept at 300°C. The main physicochemical indexes of the oil are also determined, which are water and volatile matter content (0.01%, GB/T9696-2008), density (0.94 g/mL, GB/T25401981), acid value (0.44 mg KOH g1, GB/T5530-2005) and saponification value (190.39 mg KOH g1, GB/T55342008). The mean molar mass is finally calculated to be 886.10 g/mol. The transesterification of peanut oil with methanol to test the catalytic performance of the activated CS is carried out on a batch-type experimental system. The temperature of a three-necked glass reactor is controlled by the thermostatic water bath and a reflux condenser is arranged to avoid the methanol escape. When heated to the preset temperature, the peanut oil, the catalyst and the methanol are sequentially added into the reactor and the transesterification is vigorously affected by the stirrer. As the transesterification is terminated, the catalyst is filtered from the liquid products under vacuum effect. Then the liquid products are transferred into a separatory funnel for delamination under density difference effect. The upper and the bottom part of the liquid products are crude biodiesel and crude glycerol, respectively. The catalytic efficiency of the activated CS in transesterification is determined on a 722-model visible range spectrophotometer (York Instrument Co., Ltd, China) through glycerol yield (GY) [34,35]: GY
(1)
where mcrude glycerol is the total mass of crude glycerol produced Table 2
Fatty acid composition of peanut oil
Category
Saturated fatty acids
Transesterification experiments
Peanut oil is commercially purchased from the market as the transesterification feedstock. The composition of the fatty acid in the oil is determined by gas chromatography (GC, GC-2010, Shimadzu Co., Ltd, Japan) and is listed in Table 2. The operating conditions of GC are as follows: Nitrogen (99.99%) is used as the carrier gas with a flow rate of 1.4 mL/min; the chromatographic separation is per-
mcrude glycerol , mpeanut oil M glycerol M oil
Monounsaturated fatty acids
Pofyunsaturated fatty acids
Formula
Component
C14:0
Myristic acid
w (%) 0.63
C16:0 C17:0 C18:0 C20:0 C22:0
Palmitic acid Heptadecanoic acid Stearic acid Arachidic acid Behenic acid
19.46 0.07 2.10 0.18 0.13
C24:0
Tetracosanoic acid
0.09
C16:1
Palmitoleic acid
0.58
C17:1
Heptadecenoic acid
0.08
C18:1
Octadecenoic acid
16.41
C20:1
Eicosaenoic acid
0.29
C18:2
Linoleic acid
59.91
C18:3
Linolenic acid
0.07
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by transesterification, g; α is the mass fraction of pure glycerol in the crude glycerol, %; mpeanut oil is the mass of the peanut oil, g; Mpeanut oil is the mean molar mass of the peanut oil, g/mol; Mglycerol is molar mass of the glycerol, g/mol.
3 Results and discussion 3.1 Characterization of CS as the transesterification catalyst As shown in Figure 2, the major phases of the original CS mainly include calcium hydroxide and calcium carbonate. It has been confirmed that CS gains strong thermal stability to be used as the transesterification catalyst [29]. The last mass loss segment of CS thermal decomposition process emerges at 600°C as shown in Figure 3 and the activated CS is labeled as CS-600. Figure 4 exhibits the FTIR spectra for the original CS and the activated CS. The functional group peaks of the original CS are higher than the activated ones. The bands at 848–873 and 1407–1419 cm1 can be assigned to symmetric and asymmetric stretching vibrations of O-C-O bond, which is the unidentate carbonate at calcium oxide surface [36]. The
Figure 2
XRD profiles of the original CS and the activated CS.
Figure 4
band at 3643–3647 cm1 is assigned to O-H bond of calcium hydroxide for exposure in the air [37]. The intensity of all bands for the activated CS at 650°C (CS-650) is approximate to 675°C (CS-675) and 700°C (CS-700) and is tremendously different from CS-600 and 625°C (CS-625). If Ta is increased to 650°C, the transformation of hydroxide and carbonate is realized, and calcium-based compounds are mainly in the form of calcium oxide. Further increasing Ta to 700°C, the crystalline phases of calcium oxide presents stronger intensity and it is also observed at 800°C (CS-800). The composition of calcium carbonate, calcium hydroxide and calcium oxide for the original CS, CS-650, CS-700 and CS-800 is shown in Table 3. The calcium carbonate in CS is almost completely decomposed when Ta is higher than 650°C. The small quantities of calcium hydroxide and calcium carbonate in CS-650, CS-700 and CS-800 are ascribed to the contamination of water and carbon dioxide in the air, which can be proved by calcium carbonate content of 1.71% for CS-600, 0% for CS-700 and 0.43% for CS-800. The textural parameters of the activated CS are measured by nitrogen adsorption-desorption. The surface area of 22.63 m2/g is achieved for CS-650 and it is gradually decreased to 21.34 m2/g for CS-700 and 11.82 m2/g for CS-800, which can be ascribed to the sintering of the powder catalyst under higher temperature condition. This phenomenon is also confirmed by the variation of the particle size, which is continuously expended from 265.12 nm for CS-650 to 507.53 nm for CS-800. As shown in Figure 5, the pore volume distribution is measured and all the catalysts investigated gain the micropore and the mesoporous. For Table 3
Figure 3
TG and DTG curves of CS.
FTIR spectra of the original CS and the activated CS.
Composition of calcium-based compounds in catalysts
Content (%) Original CS CS-650 CS-700 CS-800
CaO 0 79.06 91.21 89.51
Ca(OH)2 69.38 16.44 5.15 6.25
CaCO3 29.93 1.71 0 0.43
Others 0.69 2.79 3.64 3.81
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Though the increment of 20.78% can be achieved when γ is increased from 6 to 12, the transesterification is not up to the equilibrium state, as shown in Figure 6(b). At γ=15, CS-650 supplies the catalytic efficiency of 92.98%. Further increasing γ, the increment in catalytic efficiency is reduced.
Figure 5
Pore volume distributions of CS-650, CS-700 and CS-800.
CS-800, the pore diameter is narrowed less than 10 nm, and for CS-650 and CS-700, the largest pore volumes in the mesoporous pore diameter range are respectively centralized at 23 and 30 nm, which is beneficial to the adsorption of reactants onto the catalyst surface to prompt transesterification [38]. In conclusion, CS-650 is preferred for optimal textural parameters. The basic strength plays an important role in catalytic performance for the transesterification catalyst [39]. For the CS activated from 600 to 800°C, they change the color of phenolphthalein (H=9.8) from colorless to pink, but the color of 2,4-dinitroaniline (H=15) remains unchanged. So the basic strength is 9.8
