Optimization of Biodiesel Synthesis from Waste Frying Soybean Oil

Article pubs.acs.org/IECR

Optimization of Biodiesel Synthesis from Waste Frying Soybean Oil Using Fish Scale-Supported Ni Catalyst Rajat Chakraborty* and Sujit K. Das Chemical Engineering Department, Jadavpur University, Kolkata, India ABSTRACT: A novel Ni−Ca−hydroxyapatite solid acid catalyst was prepared through wet impregnation of Ni(NO3)2·6H2O on pretreated waste fish scale (PWFS) support, and the catalyst was characterized by TGA, XRD, FESEM, BET, BJH, and FTIR methods. The efficacy of the developed catalyst possessing a specific surface area of 90 m2/g, 0.1823 cm3/g pore volume, 54.83 nm modal pore diameter, and 7.52 mmol NaOH/g catalyst acidity was evaluated through esterification of free fatty acids (FFAs) of pretreated waste frying soybean oil (PWFSO) in a semibatch reactor . Optimal parametric values for esterification computed using response surface methodology (RSM) corresponding to maximum (i.e., 59.90%) conversion of FFAs to yield FAME (biodiesel) were 0.80 mL/min methanol flow rate, 30 wt % Ni(NO3)2·6H2O precursor dosage, and 300 °C calcination temperature. Subsequent batch transesterification of the unreacted triglycerides present in upstream product catalyzed by calcined fish scale (base catalyst) resulted in an ultimate 98.40% yield of biodiesel.

1. INTRODUCTION Biodiesel is a substitute for or an additive to petrodiesel derived from oils and fats of plants and animals.1 It can be defined as monoalkyl esters of long chain fatty acids2 synthesized by mainly transesterification or esterification with alcohols. Over a few recent years, it has been attracting remarkable attention as an alternative fuel due to its biodegradability, nontoxicity, and better quality of exhaust gas emissions.3 Soybean oil is widely consumed in many parts of the world as a frying oil. Biodiesel can be produced from waste frying soybean oil (WFSO)4 owing to its low cost. In spite of the low cost of WFSO, because of its high free fatty acid (FFA) content,5 an acid catalytic esterification process of WFSO is necessary to reduce the FFA content to below 0.5 wt %.6−8 Base-catalyzed transesterification of WFSO with FFAs greater than 0.5 wt % renders an undesired saponification reaction.9 In our previous study, biodiesel (FAME, fatty acid methyl ester) was synthesized by pretreatment of WFSO through homogeneous acid-catalyzed esterification followed by homogeneous base-catalyzed transesterification.10 However, due to the advantage of catalyst recovery and simplification in downstream product purification, heterogeneous catalysts may gradually substitute their homogeneous counterparts. As a consequence, preparation of heterogeneous catalysts has recently received remarkable significance owing to the strict pollution rules.11 Prospective deployment of low-cost, reusable heterogeneous catalyst for biodiesel synthesis necessitated preparation of several heterogeneous catalysts from various waste animal shells,12−14 waste fish scale,15 and combination of waste eggshell and fly ash.16 Hydroxyapatite (HAp) has several fascinating properties, viz. ion-exchange capability, adsorption capacity, acid−base properties, nontoxicity, and thermal stability,17 making it appealing for catalytic applications. Substitution of Ca2+ with Ni2+ or Cu2+18 makes HAp materials active catalysts for several reactions. In recent years, HAp-supported Pd19 and Ru20 catalysts found application in aerobic oxidation of alcohols. However, all © 2012 American Chemical Society

foregoing illustrations refer to chemically synthesized HAp. Interestingly, fish scale is a potential source of HAp;21 hence, there remains immense scope for its application as a promising catalyst support. Continuous multiphase reactor (130 °C) could perform simultaneous FFA esterification and triglyceride transesterification of simulated grease using a commercial tungstated zirconia (WZ) solid catalyst for biodiesel synthesis. The results indicated a four times faster rate of esterification than transesterification.22 Thus, a semibatch mode of reactor operation, through a regulated continuous methanol dosing (less methanol usage) into a batch of pretreated WFSO (PWFSO), can lead to significant FAME yield via esterification using acid catalyst. Semibatch esterification23 was conducted at a quite high temperature (230−290 °C) and pressure (8.5 bar) for palm fatty acid distillate to synthesize biodiesel. Nevertheless, the availability of literature on semibatch mode of reactor operation for biodiesel production from waste oil is very scanty; thus, it is of utmost importance to investigate and optimize the semibatch reactor operation. The present study reports on the preparation and characterization of a novel Ni−Ca−HAp heterogeneous acid catalyst and its subsequent application in esterification of FFAs (present in PWFSO) with methanol to FAME (biodiesel) employing semibatch reactor. The active salt precursor, i.e., Ni(NO3)2·6H2O was impregnated on pretreated waste fish scale (PWFS) by the wet impregnation method and subsequently calcined at different temperatures to achieve optimal catalytic performance. Characterization of the developed catalyst has been performed through thermogravimetric analysis (TGA), Xray diffraction (XRD), Fourier transform infrared (FTIR), Brunauer−Emmett−Teller (BET), and field emission scanning Received: Revised: Accepted: Published: 8404

January 3, 2012 May 14, 2012 June 9, 2012 June 10, 2012 dx.doi.org/10.1021/ie2030745 | Ind. Eng. Chem. Res. 2012, 51, 8404−8414

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Figure 1. Simplified schematic of semibatch setup for esterification reaction.

(designated as 20Ni and 30Ni, respectively), and the pH of the aqueous solutions was 4.8 and 4.3, respectively. 2.3. Catalyst Characterization. In order to evaluate the effect of calcination temperature on PWFS and the Ni−Ca− HAp framework, TGA analyses of both PWFS and uncalcined sample of 30Ni were performed with a Perkin−Elmer TGA analyzer (Pyris Diamond TG/DTA) in a platinum crucible under nitrogen atmosphere (150 mL/min) from 30 to 900 °C with an increasing temperature rate of 15 °C/min using a 6.65 mg sample and alpha alumina as a reference powder. The specific surface area of the catalyst sample was measured by BET method (Quantachrome make NOVA 4000e). The pore volume and pore size distribution of the catalyst was investigated using the BJH (Barret, Joyner, and Halenda) method. The XRD patterns (Rigaku Miniflex Co., Japan) of the developed catalysts were detected using a Cu Kα source equipped with an Inel CPS 120 hemispherical detector. Analysis was performed at 2θ ranging from 0° to 80° at a scanning speed of 1° min−1. The surface morphology of the prepared catalyst was determined using FESEM at 5.0 kV (JEOL Ltd., Japan, model JSM 6700F). FTIR spectra of the catalyst was detected with a FTIR, SHIMADZU (Alpha), from 350 to 4000 cm−1, while the acidity of the prepared catalyst was determined as per the methods of Singh and Fernando.25 2.4. Esterification of WFSO. Waste frying soybean oil (acid value 1.4 mg KOH/g WFSO) was initially pretreated by filtration using a laboratory-scale filter to separate the suspended solid particles. Subsequently, the filtrate oil underwent washing with hot deionized water several times to remove the dissolved salt impurities. The oil phase was decanted using a high-speed centrifuge (4000 rpm, 10 min) and subsequently dried with anhydrous Na2SO4 and filtered to obtain the pretreated WFSO (PWFSO) for the semibatch esterification process. The semibatch reactor setup (Figure 1) for FAME (biodiesel) synthesis consisted of a three-necked borosil flask. One neck of the flask was fitted with a reflux condenser, whereas the second one was connected to the discharge pipe of a peristaltic pump (Cat. No. 50171001, RIVIERA) for feeding methanol at a regulated rate. A mechanical stirrer (with a digital speed indicator and controller) placed at the central neck provided necessary mixing. A PID temperature controller

electron microscopy (FESEM) analyses. Face-centered central composite design (FCCD) has been employed to correlate FFA conversion (response) and process parameters (factors), viz. precursor dosage, i.e., Ni(NO3)2·6H2O loading (XCL), flow rate of methanol (XFR), and calcination temperature (XCT). Optimal process conditions leading to maximum FFA conversion (i.e., maximum FAME yield) have been determined by response surface methodology (RSM). The upstream product (FAME, unreacted FFAs and glycerides) obtained at optimal esterification conditions was subsequently processed in a batch reactor for the downstream transesterification (methanolysis) using a heterogeneous base catalyst developed from waste fish scale (as reported in our previous study15) for maximization of biodiesel yield.

2. EXPERIMENTAL METHODS 2.1. Materials. Local restaurants and markets provided WFSO and waste fish scale (WFS), respectively. All chemicals used were of analytical reagent (AR) grade. Methanol (>99% purity), Ni(NO3)2·6H2O, anhydrous Na2SO4, acetone, etc. were purchased from Merck. 2.2. Catalyst Preparation. WFS was washed thoroughly with hot deionized water several times to remove the flesh and gelatinous matter. Subsequent drying of WFS in a hot air oven at 105 °C for about 6 h followed by grinding in a laboratory ball mill yielded a fine powder of PWFS (average of 100BSS and 200 BSS). Consequently, 10, 20, and 30 wt % Ni(NO3)2·6H2O were impregnated on PWFS by the wet impregnation method24and followed by calcination at 300, 500 and 700 °C. Typically, in order to prepare 10 wt % Ni impregnated Ca− HAp catalyst (designated as 10Ni), 5 g of Ni(NO3)2·6H2O was added to 100 mL of water to prepare an aqueous solution of nickel nitrate; this solution was subsequently added to 45 g of PWFS and mixed vigorously using a mechanical agitator under total reflux for 4 h at 100 °C and a pH of 5.20. The solution was aged for 24 h for formation of the nickel nitrate precipitate on PWFS carrier. Excess water was driven out using a hot air oven at 105 °C over 20 h. The dried mass was finally calcined in a muffle furnace at a preset temperature to obtain the Ni− Ca−HAp heterogeneous acid catalyst. A similar procedure was used for preparation of 20 and 30 wt % Ni2+-loaded catalysts 8405

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(Honeywell) regulated the reactor temperature (measured by RTD Pt-100). The FFAs present in PWFSO underwent esterification at a regulated 700 rpm stirrer speed and 60 °C temperature.26 In all tests, 100 mL (87.02 g, acid value 1.205 mg if KOH/g PWFSO; mean molecular weight 891.44 g/mol; saponification value 190 mg KOH/g) of PWFSO was mixed with 2.0 wt % (based on PWFSO weight) of the novel catalyst for a few minutes to make an oil−catalyst slurry. Subsequently, methanol was fed to the slurry at a regulated flow rate (varied from 0.40 to 0.80 mL/ min) for 2 h. Afterward, the reaction mixture was filtered under vacuum to separate the solid catalyst followed by methanol recovery through vacuum distillation at 50 °C. A centrifuge (5000 rpm, 10 min) separated the water from the remaining mixture. The upper product layer (i.e., ‘oil layer’), i.e., a mixture of FAME (biodiesel), unreacted FFA, and glycerides, was subsequently subjected to the following step, i.e., downstream transesterification. Analysis of the ‘oil layer’ for the free residual acidity followed the AOAC Official Method Cd 3a-63 acid−base titration procedure.27 The repeatability of the analysis was improved by removing traces of methanol and water through heating at 150 °C under stirring for 15 min.28 Conversion of FFA present in the PWFSO into FAME was calculated from the mean of acid value (AV) of the ‘oil layer’ using the following equation according to Wang et al.29 ⎛ AVOL ⎞ FFA conversion = ⎜1 − ⎟ × 100 AVPWFSO ⎠ ⎝

Table 2. Face-Centered Central Composite Design (FCCD) Layout for Esterification of FFA Present in PWFSO

The functional groups present in the ‘oil layer’ were detected by FTIR, SHIMADZU (Alpha), from 600 to 4000 cm−1. 2.5. Experimental Design. Statistical analysis of semibatch esterification procedure was performed using the Design Expert 8.0 software. The FCCD was used with three factors, viz. precursor dosage, i.e., Ni(NO3)2·6H2O loading (XCL), flow rate of methanol (XFR), and calcination temperature (XCT) (Table 1).

units

−1 level

0 level

+1 level

XCL XFR XCT

Ni(NO3)2·6H2O loading methanol flow rate calcination temperature

wt % mL/min °C

10 0.40 300

20 0.60 500

30 0.80 700

calcination temp. (xCT)

FFA conversion (C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.00 1.00 1.00 0.00 1.00 0.00 −1.00 −1.00 1.00 −1.00 −1.00 −1.00 0.00 0.00 1.00

−1.00 −1.00 1.00 0.00 −1.00 1.00 −1.00 1.00 1.00 −1.00 0.00 1.00 0.00 0.00 0.00

0.00 −1.00 1.00 −1.00 1.00 0.00 −1.00 1.00 −1.00 1.00 0.00 −1.00 1.00 0.00 0.00

32.00 42.00 49.00 45.00 36.00 49.09 30.00 47.00 59.76 31.00 44.00 55.00 42.00 44.00 49.99

⎛W ⎞ YFAME = ⎜ b ⎟PFAME × 100 ⎝ WO ⎠

(3)

The purity of overall (final) biodiesel obtained through downstream transesterification was determined by FTIR, SHIMADZU (Alpha), from 400 to 4000 cm−1. The important fuel properties of prepared biodiesel were tested as per ASTM standard methods.

Table 1. Experimental Ranges and Levels of the Independent Variables/Process Factors Used in RSM name

Ni(NO3)2·6H2O loading (xCL)

2.6. Downstream Base-Catalyzed Transesterification of Semibatch Product (‘Oil Layer’). Base-catalyzed batch transesterification was convenient to perform when the majority of the FFA in the PWFSO could be esterified in the upstream semibatch operation. Transesterification of the ‘oil layer’ (obtained through esterification under optimal conditions) was conducted using the calcined fish scale catalyst following the optimal conditions described in our previous work.15 The overall yield of FAME obtained after transesterification was calculated using eq 3

(1)

factors

run

methanol flow rate (xFR)

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. TGA Analysis. Figure 2 elucidates TGA for 30Ni (uncalcined) sample and PWFS sample depicting weight losses with progressive increase in temperature. Almost equal weight loss was observed for both

On the basis of the three level factorial values, two extreme points (highest and lowest) were used for each factor, viz. 10 and 30 wt % for XCL, 0.40 and 0.80 mL/min for XFR, and 300 and 700 °C for XCT. Experimental runs were performed based on 15 different combinations (Table 2) of the following coded variables X − 20 xCL = CL (2a) 10 x FR =

XFR − 0.6 0.20

(2b)

xCT =

XCT − 500 100

(2c)

Figure 2. TGA analyses of 30Ni () and PWFS (- - -) in the temperature range from 30 to 900 °C. 8406

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Figure 3. Powder XRD patterns of 30Ni catalyst obtained by calcination at various temperatures: (a) 300, (b) 500, and (c) 700 °C. Characteristic peaks due to HAp (red triangles), Ni(NO3)2 (green diamonds), β-Ca3(PO4)2 (orange circles), and NiO (blue squares).

30Ni and PWFS samples up to around 300 °C (the decomposition temperature of Ni(NO 3) 2 ); however, a relatively steeper weight loss was observed for 30Ni sample compared to PWFS above this temperature. This can be attributed principally to decomposition of Ni(NO3)2 into NiO. At the outset, 5% and 8% weight losses occurred, respectively, for 30Ni and PWFS between 30.47 and 130 °C due to elimination of free and hydrated water. Respective 12% and 2% weight losses happened between 130 and 230 °C owing to removal of adsorbed and lattice water. An additional weight loss of 11% and 17% took place over 230−300 °C, which can be ascribed to fragmentation of macromolecules and the other organic matters and partial conversion of Ni(NO3)2 to NiO. Further 30% and 10% respective weight losses occurred up to 500 °C due to elimination of gaseous species which can be ascribed to added formation of β-Ca3(PO4)2 and NiO as confirmed by XRD analysis (Figure 3). An additional amount of β-Ca3(PO4)2 was developed at around 700 °C that represented nearly a 2% weight loss for both samples, which can be confirmed through the presence of intense peaks of βCa3(PO4)2 at 700 °C (Figure 3). The sample obtained up to 900 °C represented 35 wt % of the initial 30Ni sample in comparison with 55 wt % of initial PWFS sample, with no further significant weight loss on heating above 900 °C, indicating high thermal stability of the prepared catalyst. Thermal activation beyond 1000 °C30 could reduce the activity of the HAp-supported catalyst by reducing the specific surface area. 3.1.2. XRD Analysis. Figure 3a−c demonstrates the X-ray diffraction configurations for the 30Ni catalyst samples obtained over a calcination temperature range from 300 to 700 °C. At 300 °C, the presence of both Ni(NO3)2 and NiO

crystalline phases are visible (Figure 3a), corresponding to angles 36.85°, 42.81°, and 62.69° and angles 37.28°, 43.30°, and 62.92° respectively.31 This indicates that Ni(NO3)2 was partially converted into NiO at 300 °C. It is worth noting that the main XRD patterns at lower angles (2θ = 31.67°, 31.82°, and 32.15°) corresponding to HAp at 300 °C were transformed into β-Ca3(PO4)2 (β-TCP) upon thermal activation at 500 °C (Figure 3b).15 Calcination at 500 °C also resulted in transformation of Ni(NO3)2 to NiO at all equivalent angles (2θ = 36.85°, 42.81°, and 62.69°) of Ni(NO3)2 observed at 300 °C. Although the existence of more prominent peaks of NiO at 500 °C compared to those at 300 °C could possibly increase catalyst acidity, the pronounced formation of β-Ca3(PO4)2 at 500 °C counterbalanced the acidic property of the catalyst due to its inherent basicity. Calcination at 700 °C resulted in more intense peaks of β-Ca3(PO4)2 and NiO (Figure 3c) compared to those at lower temperatures, which led to dual acidic and basic properties (amphoteric nature) of the catalyst (6.5 mmol NaOH/g acidity and 8.2 mmol HCl/g basicity).Thus, the acidic nature of the catalyst gradually decreased from 300 to 700 °C owing to the relatively greater extent of β-Ca3(PO4)2 crystalline phase formation in comparison with those of NiO. 3.1.3. Morphological Analysis. The electron micrograph (Figure 4A) of the pretreated waste fish scale (PWFS) sample demonstrated the existence of nonagglomerated, unevenly porous, coarse structured particles. FESEM images of the prepared uncalcined 30Ni sample (Figure 4B) exhibited the presence of nonagglomerated granular particles, whereas the micrograph (Figure 4C) of the representative catalyst (calcined 30Ni) revealed the existence of agglomerated particles with irregular-shaped flat shapes having nonuniform voids distributed over the relatively smoother catalyst surface. 8407

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Figure 5. (a) Pore volume vs relative pressure (P/P0) of the PWFS; (inset) BJH distribution curve. (b) Pore volume vs relative pressure (P/P0) of the 30Ni catalyst at 300 °C calcination temperature; (inset) BJH distribution curve. (c) Penetration curve of the 30Ni/300 °C catalyst.

g as compared to 12.77 m2/g for the PWFS (Figure 5a). The adsorption isotherm profile (Figure 5b) advocates that the catalyst is essentially macroporous solid (>50 nm). The isotherm for this material can be assigned to Type II of the IUPAC classification,32 which signifies unrestricted monolayer−multilayer adsorption. The catalyst pore volume of 0.1823 cc/g (0.0331 cc/g for PWFS) and modal pore size of 54.83 nm (20.14 nm for PWFS) (insets of Figure 5a and 5b show differential pore size distribution) were evaluated using the BJH33 method. The data clearly indicate a higher specific surface area, pore volume, and modal pore diameter for the 30Ni/300 °C catalysts as compared to PWFS, which signify the catalytic efficacy. The segments of the penetration curve (Figure 5c) in the 4.4−49 and 50−76 nm ranges (approx-

Figure 4. FESEM images of the (A) pretreated waste fish scale and prepared 30Ni catalyst samples: (B) before calcination and (C) after calcination at 300 °C.

3.1.4. Brunauer−Emmett−Teller (BET) Analysis. The specific surface area (SBET) of the representative 30Ni catalyst (calcined at 300 °C, hereafter referred to as 30Ni/300 °C), which demonstrated optimal catalytic performance, was 90 m2/ 8408

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Figure 6. (a) FTIR spectra of (a) PWFS and (b) 30Ni/300 °C catalyst. (b) Possible mechanism for esterification of fatty acid with methanol in the presence of 30Ni/300 °C catalyst (R1, alkyl group of fatty acid).

imately) exhibited log arithmetic normal distribution. Figure 5c also elucidates that 96.77% of the total catalyst pore volume corresponds to the mesoporous size range (4.4−49 nm). The remaining 3.23% of the total pore volume represents

macroporous catalyst in the 50−76 nm size range.These size ranges indicate easy transport of the voluminous FFA molecules (e.g., oleic acid molecular length 1.97 nm and width 0.5 nm) into catalyst pores facilitating their conversion 8409

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Table 3. ANOVA result for FFA Conversion (C) Corresponding to eq 4 source

sum of squares

df

model xCL xFR xCT xCLxFR xCLxCT xFRxCT x2CL x2FR residual cor. total

1039.94 789.43 88.51 71.61 1 1 1 1 1 6.08 1046.01

9 1 1 1 13.11 23.67 11.91 25.56 27.96 5 14

mean square 115.55 789.43 88.51 71.61 10.78 19.47 9.80 21.03 23.00 1.22

F value 95.07 649.54 72.82 58.82 0.0219 0.0069 0.026 0.0059 0.0049

p value prob >F F

standard deviation

R2

adjusted R2

predicted R2

press value

linear 2FI quadratic