Ferric sulphate catalysed esterification of free fatty acids in waste cooking oil

Bioresource Technology 101 (2010) 7338–7343

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Ferric sulphate catalysed esterification of free fatty acids in waste cooking oil Suyin Gan a,*, Hoon Kiat Ng b, Chun Weng Ooi a, Nafisa Osman Motala a, Mohd Anas Farhan Ismail a a b

Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia Department of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia

a r t i c l e

i n f o

Article history: Received 21 January 2010 Received in revised form 8 April 2010 Accepted 10 April 2010

Keywords: Esterification Free fatty acids Waste cooking oil Ferric sulphate Biodiesel

a b s t r a c t In this work, the esterification of free fatty acids (FFA) in waste cooking oil catalysed by ferric sulphate was studied as a pre-treatment step for biodiesel production. The effects of reaction time, methanol to oil ratio, catalyst concentration and temperature on the conversion of FFA were investigated on a laboratory scale. The results showed that the conversion of FFA reached equilibrium after an hour, and was positively dependent on the methanol to oil molar ratio and temperature. An optimum catalyst concentration of 2 wt.% gave maximum FFA conversion of 59.2%. For catalyst loadings of 2 wt.% and below, this catalysed esterification was proposed to follow a pseudo-homogeneous pathway akin to mineral acidcatalysed esterification, driven by the H+ ions produced through the hydrolysis of metal complex [Fe(H2O)6]3+ (aq). Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Ever since its discovery, biodiesel has been the focus of both governments and industries as an alternative renewable fuel to petroleum diesel. Of late, biodiesel has seen increased interest worldwide due to the fluctuating crude oil prices, the environmental benefits associated with biodiesel combustion, and the potential for greater regional development in developing countries (Ali and Hanna, 1994; Van Gerpen, 2005; Shahid and Jamal, 2008; Demirbas, 2009; Ng et al., 2009). The most common approach to producing biodiesel is by transesterification which refers to an alkali-catalysed reaction between vegetable oils or other fats and an alcohol to form fatty acids methyl esters (FAME) or biodiesel and glycerol as a by-product. Waste cooking oil (WCO) has recently been proposed as a cheaper, environmentally friendly feedstock for biodiesel production (Felizardo et al., 2006; Canakci, 2007; Issariyakul et al., 2007; Marchetti and Errazu, 2008; Phan and Phan, 2008). Nevertheless, the biggest challenge in using WCO for biodiesel production is its high level of free fatty acids (FFA) (Canakci, 2007; Jacobson et al., 2008). In alkali-catalysed transesterification, the oil used should contain no more than 1% FFA to avoid the formation of soap which hinders the final separation of FAME from glycerol and reduces the yield of FAME (Freedman et al., 1984; Liu, 1994).

* Corresponding author. Tel.: +60 3 8924 8162; fax: +60 3 8924 8017. E-mail address: [email protected] (S. Gan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.04.028

The two-step catalysis has been proposed as a method of converting WCO with high FFA content to biodiesel. Its first step consists of acid-catalysed esterification which lowers the FFA level in the oil, after which the conventional alkali-catalysed transesterification follows. Although sulphuric acid is often used in the first step because of its high conversion and low cost, its usage is associated with effluent disposal problems, loss of catalyst and high equipment cost due to the corrosive nature of acids (Zhang et al., 2003; Cao et al., 2008). These drawbacks have spurred research for heterogeneous catalysts for the initial esterification process. Ion-exchange resins such as Smopex-101, Amberlyst 15, Amberlyst 16, Amberlyst 35 and Dowex HCR-W2 have been demonstrated to be effective heterogeneous catalysts for esterification reactions (Peters et al., 2006; Özbay et al., 2008). Zeolites, superacids such as sulphated zirconia and niobium acids as well as heteropolyacids have also been studied as possible catalysts for esterification reactions (Peters et al., 2006; Caetano et al., 2008). Recently, ferric sulphate was proposed by Wang et al. (2006, 2007) as a heterogeneous substitute for sulphuric acid for the esterification of FFA in WCO. Ferric sulphate was shown to have much higher catalytic activity compared to sulphuric acid, with maximum FFA conversion of 97.22% when 2 wt.% was added to a reaction mixture with a methanol to oil ratio of 10:1 and reacted at 95 °C for 4 h. In this work, the esterification of FFA in WCO catalysed by ferric sulphate was conducted under varying experimental parameters to elucidate the effects of reaction time, methanol to oil molar ratio, catalyst concentration and temperature on the conversion of FFA to esters. The single factor experimental design was applied for this purpose. The kinetics of the esterification of FFA catalysed by

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2 wt.% ferric sulphate was also studied at a methanol to oil molar ratio of 15:1 and temperature of 60 °C. More importantly, a feasible reaction mechanism for the ferric sulphate catalysed esterification of FFA is proposed for catalyst loadings of 2 wt.% and below. To the best knowledge of the authors, this has not been attempted in previous studies.

This complex named as hexa-aquoiron (III) ion, [Fe(H2O)6]3+, is a stable complex but consequently undergoes hydrolysis as outlined in Eq. (7). This tendency to hydrolyse is continual due to the very high charge density on the central ferric ion (Hill and Holman, 2000). Following further hydrolysis as can be seen in Eq. (8), a pool of hydroxonium ions is yielded hence allowing the continuation of reaction under the Fischer esterification.

2. Theoretical background

½Fe2 ðSO4 Þ3 xH2 OðsÞ þ aq ! 2½FeðH2 OÞ6 3þ ðaqÞ

2.1. Current state of knowledge of reaction mechanism

þ 3½SO4 2 ðaqÞ þ ðx  12ÞH2 O ðaqÞ

ð6Þ

½FeðH2 OÞ6 3þ ðaqÞ $ ½FeðH2 OÞ5 ðOHÞ2þ ðaqÞ þ Hþ ðaqÞ

ð7Þ

½FeðH2 OÞ5 ðOHÞ2þ ðaqÞ ! ½FeðH2 OÞ5 ðOHÞ2 2þ ðaqÞ þ Hþ ðaqÞ

ð8Þ

Although ferric sulphate has previously been proven to be effective for the esterification of FFA in WCO (Wang et al., 2006, 2007), the catalysis mechanism remains debatable. To date, knowledge about the chemistry of ferric sulphate applied to catalysis is scarce and the only postulations that can be gathered from the literature are esterification mechanisms catalysed by mineral acids and by metal ions. The typical mechanism for acid-catalysed esterification in a homogeneous phase is the Fischer esterification, which consists of the following three steps (Ingold, 1969; Barg et al., 1994; Kirbaslar et al., 2001):

On the other hand, should the catalyst be dehydrated, the formation of the indispensable complex will be slowed down because ferric sulphate only dissociates into its respective ions in aqueous form (Eq. (9)). Moreover, once the ferric ions (Fe3+) are moving freely in solution they require six molecules of water each to form the necessary complex as shown in Eq. (10).

Fe2 ðSO4 Þ3ðsÞ þ aq ! Fe3þ ðaqÞ þ SO2 4 ðaqÞ 3þ

Fe

i. Protonation of carbonyl group by acid catalyst

RCOOH þ Hþ $ RCOOHþ2

ð1Þ

ii. Alcohol attack on the protonated carbonyl group

RCOOHþ2 þ R0 OH $ RCOOR0 Hþ þ H2 O

ð2Þ

iii. Formation of ester with the breakaway of proton

RCOOR0 Hþ $ RCOOR0 H þ Hþ

ð3Þ

For reactions catalysed by mineral acids such as sulphuric acid, hydrochloric acid and phosphoric acid, the hydroxonium ion (H+) in the initiating step is generated via the protolysis of the aqueous acids, as shown for the case of sulphuric acid (Ronnback et al., 1997):

H2 SO4 ðaqÞ $ 2Hþ ðaqÞ þ SO2 4 ðaqÞ

ð4Þ

As for reactions catalysed by transition metal salts, the carbonyl group protonation is initiated by the metal ion as can be seen in Eq. (5) (van Santen et al., 2000):

RCOOH þ Mnþ $ RCOOHMnþ

ð5Þ

3þ

ðaqÞ þ 6H2 O ðaqÞ ! ½FeðH2 OÞ6 

ðaqÞ

ð9Þ ð10Þ

2.3. Kinetic model Although several complex models describing the kinetics of heterogeneous catalysis of esterification reactions exist (Lee et al., 2002), this work adopts a simplified pseudo-homogeneous model as used by Peters et al. (2006) and Berrios et al. (2007). The esterification of FFA in WCO is a reversible reaction which occurs in the presence of an acid catalyst:

RCOOH þ R0 OH $ RCOOR0 þ H2 O

ð11Þ

Assuming that the reaction is pseudo-homogeneous and first order with respect to each component, the following kinetic equation holds:

d½A=dt ¼ k1 ½A½B  k2 ½C½D

ð12Þ

where [A], [B], [C] and [D] are the molar concentrations of FFA, methanol, FAME and water, respectively, whereas k1 and k2 are the kinetic constants for the forward and reverse reactions. For compositions corresponding to chemical equilibrium, the concentration-based equilibrium constant K can be related to k1 and k2:

2.2. Proposed reaction mechanism

K ¼ k1 =k2

In this study of esterification of FFA in WCO, the catalyst used was hydrated and existed in the form of solid Fe2(SO4)3xH2O. Ferric sulphate has been classified as a heterogeneous acid catalyst by Wang et al. (2007) but it is arguable whether the catalytic reaction follows a heterogeneous pathway for all catalyst concentrations since experimental observations indicated that although ferric sulphate is insoluble in oil, it is sparingly soluble in methanol (Wang et al., 2006). Additionally, ferric sulphate can dissolve in the water formed during esterification (Moody, 1965). Since ferric sulphate is a metal salt, it is postulated that knowledge about the formation of complexes in transition metal ions can be coupled with the esterification mechanisms discussed previously to propose a feasible reaction mechanism for the esterification of FFA in the presence of metal sulphates. This work suggests that the catalysed reaction follows a pseudohomogeneous route below a certain ferric sulphate concentration whereby ferric sulphate dissolves in the reaction mixture. Following an initial esterification which is catalysed by FFA in WCO, it is suggested that the hydrated salt solubilises in the water produced to eventually form an octahedral complex, as shown in Eq. (6) below.

If [C] and [D] are assumed to be negligible at the start of the reaction (t = 0) and the initial concentrations of A and B are denoted by [A]0 and [B]0, the overall stoichiometry gives the following relations:

ð13Þ

½B ¼ ½A  ð½A0  ½B0 Þ ½C ¼ ½D ¼ ½A0  ½A

ð14Þ ð15Þ

a ¼ ½A0  ½B0

ð16Þ

Combining (13)–(16) into 12, the kinetic equation can be rewritten as:

d½A=dt ¼ k1 f½A2  ½Aa  ð½A0  ½AÞ2 =Kg

ð17Þ

3. Methods 3.1. Materials WCO samples were collected from households and university cafeteria, all of which were then homogeneously mixed. Food

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residues were filtered out and the oil was heated to remove its moisture. It was stored in an air tight bottle at room temperature (25 ± 2 °C), and its FFA content was measured daily for 10 consecutive days, until equilibrium was achieved. After the aforementioned period, the percentage of FFA in the oil was found to be constant at 10.92%, and the same oil was then employed in all experiments. The chemicals used in the esterification reaction were methanol (99.8% purity, Sigma Aldrich) and hydrated ferric sulphate (97% purity, Sigma Aldrich). The reagents employed in the free fatty acids determination included sodium hydroxide (NaOH) solution prepared to 0.1 M using NaOH pellets (99.99% purity, Sigma Aldrich), iso-propanol (99% purity, Merck), phenolphthalein indicator and distilled water. All the chemicals and reagents were of analytical grade. 3.2. Esterification of FFA A laboratory scale reactor in the form of a three-neck glass flask equipped with a reflux condenser was used for the esterification reactions. The reactor was placed in a heating mantle equipped with a stirrer and temperature controller. The stirrer speed was set at 700 rpm. For all tests, 100 g of WCO was mixed with the required amount of ferric sulphate and methanol and heated at the desired temperature in the reactor. Samples were withdrawn either periodically or at the end of a set period for FFA content analysis. Table 1 shows the nominal parametric settings for all the esterification reactions. Each of the listed parameters were subsequently varied in turn to study its effect on the conversion of FFA as can be seen in Table 2, with all other reaction conditions remaining constant at their nominal values. In view of substantiating the proposed ferric sulphate catalysis mechanism, an additional experiment was conducted with dehydrated catalyst. Prior to esterification, the hydrated ferric sulphate was oven-heated to a constant mass at 105 °C for 2 h. The eventual decrease in mass was 5.9%. Following that methanolysis was carried out with the dehydrated ferric sulphate till equilibrium was reached under the nominal reaction conditions. 3.3. Titration analysis Samples of the reaction mixture were analysed for FFA content via titration according to MPOB Test Method p2.5:2004 (MPOB, 2004). Determination of the concentration of FFA (A) and the percentage conversion of FFA in the samples at a certain time were calculated using the following equations:

A ðwt:%Þ ¼ ð25:6  M  VÞ=m

ð18Þ

% FFA conversion ¼ 100ðinitial A  final AÞ=initial A

ð19Þ

where A is the concentration of FFA (wt.%), M is the NaOH solution molarity (mol/dm3), V is the volume of the NaOH solution required for neutralisation (mL) and m is the mass of test sample (g). 4. Results and discussion 4.1. Variation of FFA content during storage Fig. 1 illustrates the variation of FFA content in the collected WCO kept under ambient conditions. The relatively large increase in FFA content over the first 5 days can be attributed to the hydrolysis of remaining triglycerides in the oil in the presence of entrained moisture which was not removed during the heating process. Hydrolysis of triglycerides normally only occur at high temperature and pressure (Chow, 1999; Akoh and Min, 2002), but in the presence of an acid catalyst the reaction is also possible at ambient conditions (Chow, 1999). Both the FFA in the WCO and the infinitesimal amount of burnt food particles have been suggested to be able to catalyse the hydrolysis of triglycerides in oil (Rossell, 2001). After the 5th day, the gradual tailoring off in the increasing trend of FFA level is due to the moisture being depleted. 4.2. Effects of reaction parameters 4.2.1. Reaction time Fig. 2 shows the effect of reaction time on the conversion of FFA. The three-phase division shown in Fig. 2 can be explained from a theoretical viewpoint by the production of water during esterification. In the first half an hour of this study, when the water content was minimal, the esterification rate was fast. As the reaction proceeded, water in the reaction mixture progressively increased resulting in two hindering effects. Firstly, the catalyst was deactivated and secondly, the hydrolysis of methyl ester back to FFA occurred following Le Chatelier’s principle. With increased water concentrations, the protons released by ferric sulphate get encircled by water clusters. These shield the hydroxonium ions from the hydrophobic FFA molecules, hence preventing the progress of the forward reaction (Rived et al., 2001). Moreover, the reaction gets preferentially driven towards the reactants side due to increasing water content in the mixture. The gradual decrease in esterification rate from 30 to 60 min can therefore be attributed to the combination of the two negative effects above. The reaction slowly reached its equilibrium after 60 min, when the rate of ester formation and the rate of hydrolysis were equal.

Table 1 Nominal parametric settings for the esterification of FFA in WCO. Parameter

Set value

Reaction time (min) Methanol to oil molar ratio () Catalyst concentration (wt.%) Temperature (°C)

60 15:1 2 60

Table 2 Variation of parameters for the esterification of FFA in WCO. Parameter

Studied values

Reaction time (min) Methanol to oil molar ratio () Catalyst concentration (wt.%) Temperature (°C)

15, 30, 45, 60, 75, 90, 105, 120 6:1, 9:1, 12:1, 15:1 0, 1, 2, 3, 4 30, 40, 50, 60

FFA content (wt.%)

11.5 11.0

10.5 10.0

9.5 9.0 0

2

4

6

8

10

12

Day Fig. 1. Variation of FFA content in WCO under ambient temperature (25 ± 2 °C) during 10 days storage period.

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The reaction rate constant was evaluated by fitting Eq. (17) to the curve in Fig. 2 using a differential method combined with a non-linear least-squares regression technique (Peters et al., 2006). A value of 4  108 m3 mol1 s1 was determined for k1. The activity of ferric sulphate catalyst expressed as k1 normalised to the catalyst weight, was calculated as 2  108 m3 mol1 g1 s1, which is similar to the activity of ion-exchange resins used for esterification reactions (Peters et al., 2006).

4.2.3. Catalyst concentration Based on Fig. 4, in the absence of catalyst, a minimal FFA conversion of 23.8% was observed. This could have arisen as a result of autocatalysis, whereby esterification gets catalysed by the FFA in the WCO. During autocatalysis, the un-dissociated FFA undergoes protolysis, producing activated carboxylic acids which induce esterification (Liu et al., 2006). Addition of catalyst substantially increased the conversion of FFA by at least twofold, with an optimum catalyst concentration of 2 wt.% producing the maximum FFA conversion of 59.2%. At low catalyst concentrations (1–2 wt.%), ferric sulphate was seen to fully dissolve in the reaction mixture due to its affinity to methanol and water. Here, the catalysis is postulated to follow a pseudo-homogeneous route. Nonetheless, as the concentration of ferric sulphate was raised to 3 and 4 wt.%, a suspension of solid in oil was observed in the reactor. This could be due to several reasons including insufficient agitation intensity to keep all catalyst in the pseudo-homogeneous phase and dead zones within the reactor. This failure to solubilise created mixing difficulties of reactants and products, which eventually resulted in lower conversion of FFA. Above 2 wt.% too, pseudo-homogeneous catalysis is superseded by heterogeneous catalysis due to the formation of a twophase reaction mixture caused by the excess ferric sulphate suspension in the mixture. In heterogeneous catalysis, the rate of esterification could be limited by external/internal diffusion, adsorption/desorption and surface reactions leading to lower FFA conversions compared to that at 2 wt.%, for the same reaction time.

4.2.2. Methanol to oil molar ratio From a stoichiometrical viewpoint, esterification per se only requires one mole of methanol per mole of free fatty acid. Previous studies have however reported that a high methanol to oil molar ratio is necessary to drive the reaction towards completion and produce more methyl esters (Ramadhas et al., 2005; Zheng et al., 2006; Cao et al., 2008). As can be seen in Fig. 3, the FFA conversion increased linearly with the increase in methanol to oil molar ratio. It can be observed that the FFA conversion increased approximately 10% for every increment of 3 units of methanol to oil molar ratio. The increase in conversion trend can again be explained using Le Chatelier’s principle. When methanol concentration increases, the tendency of equilibrium to lie towards the side of products also increases, which yields higher final conversion of FFA to methyl esters. Additionally, from a mass transfer perspective, by increasing the methanol to oil molar ratio, the viscosity of the reacting mixture decreases. This promotes better mixing between reactants and catalyst which enhances the rate of mass transfer and eventually resulting in a higher conversion within a fixed reaction time.

4.2.4. Temperature It can be seen clearly from Fig. 5 that increasing the temperature increased the FFA conversion. At 30 °C, the conversion of FFA was 31.2% and this figure rose steadily to an appreciable 59.2% at 60 °C. Methanol is only sparingly soluble in WCO. In all experiments, agitation has been used to break the methanol drops to create a pseudo-homogeneous reaction mixture. Nevertheless, at low temperatures, due to the low solubility of WCO in methanol, the resistance to mass transfer is significant such that mass transfer controls the overall rate of reaction. With increasing temperatures, the solubility of WCO in methanol increases leading to lesser resistance to mass transfer. The viscosity of the WCO also decreases which promotes better mixing between reactants and catalyst. The overall rate of reaction at higher temperatures is thus chemically controlled. Nonetheless, there is a limit to the extent which the temperature could be raised at atmospheric pressure. Temperatures much greater than 65 °C, the boiling point of methanol, would cause an excessive loss of alcohol. At higher operating pressures, temperatures can be raised accordingly to increase FFA conversion (Zhang et al., 2003; Glišic´ et al., 2009; Lukic´ et al., 2009).

70

% FFA conversion

60 50 40 30 20 10 1st phase

2nd phase

3rd phase

0

0

15

30

45

60

75

90

105

120

135

Reaction time (min)

70

70

60

60

% FFA conversion

% FFA conversion

Fig. 2. FFA conversion versus reaction time. Reaction conditions: methanol to oil molar ratio = 15:1, catalyst concentration = 2 wt.%, temperature = 60 °C.

50 40 30 20 10

50 40 30 20 10

0 6

7

8

9

10

11

12

13

14

15

16

Methanol to oil molar ratio (-) Fig. 3. FFA conversion versus methanol to oil molar ratio. Reaction conditions: reaction time = 1 h, catalyst concentration = 2 wt.%, temperature = 60 °C.

0

0

1

2 3 Catalyst concentration (wt.%)

4

5

Fig. 4. FFA conversion versus catalyst concentration. Reaction conditions: reaction time = 1 h, methanol to oil molar ratio = 15:1, temperature = 60 °C.

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the esterification progress had reached the required intensity to promote formation of the [Fe(H2O)6]3+ complex, as a result of which more hydroxonium ions were released. Catalyst deactivation was insignificant as all water molecules were engaged in complex formation. However, after the 60th minute, it can be inferred from Fig. 6 that both reactions reached equilibrium whereby the FFA levels remained constant with equal rates of esterification and hydrolysis.

70

% FFA conversion

60 50 40 30 20

5. Conclusions

10 0

40

35

30

45

55

50

60

65

Temperature (oC) Fig. 5. FFA conversion versus temperature. Reaction conditions: reaction time = 1 h, methanol to oil molar ratio = 15:1, catalyst concentration = 2 wt.%.

4.2.5. Validation of proposed reaction mechanism Fig. 6 compares the change in FFA content for both hydrated and dehydrated ferric sulphate over a reaction time of 120 min. For the first 15 min, the FFA conversions achieved for both salts were similar. This was because autocatalysis induced esterification, and therefore the reaction progress was independent of ferric sulphate’s activity. Besides, at this stage it can be deduced that the water produced from esterification is insufficient to promote the reaction shown in Eq. (6). From the 15th to the 45th minute, substantial decrease in FFA content can be seen with the use of the hydrated catalyst, but not with the dehydrated one. In line with the proposed reaction mechanism, after 15 min, for hydrated ferric sulphate which had x moles of water of crystallisation, the formation of the [Fe(H2O)6]3+ complex was immediate. Reactions as outlined in Eqs. (6)–(8) were consequently highly favoured, thus increasing the rate of FFA conversion. On the other hand, with the use of the dehydrated catalyst, the formation of the indispensable complex was slow because ferric sulphate only dissociates into its respective ions in aqueous form (Eq. (9)). Moreover, once the ferric ion (Fe3+) was moving freely in solution it requires six molecules of water to form the necessary complex, and the water produced from the esterification at this stage might have been insufficient to promote the reaction shown in Eq. (10). As a result, less hydroxonium ions were formed and the conversion was lower. From the 45th to the 60th minute, it can be observed that FFA conversion rate decreased with the use of the hydrated ferric sulphate whereas for the dehydrated one, the conversion rate increased. In the case of the former, excessive amounts of water hindered further esterification (Rived et al., 2001). For the latter,

12

FFA content (wt.%)

10 8 6 4 2

Hydrated catalyst Dehydrated catalyst

0

0

15

30

45

60

75

90

105

120

135

Reaction time (min) Fig. 6. Comparison of FFA conversion between hydrated and dehydrated catalyst. Reaction conditions: reaction time = 1 h, methanol to oil molar ratio = 15:1, catalyst concentration = 2 wt.%, temperature = 60 °C.

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