- Email: [email protected]
Pretreatment of yellow grease for efficient production of fatty acid methyl esters
biomass and bioenergy 33 (2009) 558–563
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Pretreatment of yellow grease for efficient production of fatty acid methyl esters Walterio Diaz-Felixa, Mark R. Rileya,*, Werner Zimmta, Michael Kazzb a
Department of Agricultural and Biosystems Engineering, Shantz Building, Room 403, The University of Arizona, Tucson, AZ 85721, USA Zelen Environmental, Tucson, AZ, USA
b
article info
abstract
Article history:
Biodiesel is a renewable fuel comprised of fatty acid methyl esters (FAME) derived from
Received 3 September 2007
vegetable oils or animal fats. Comparisons between biodiesel and petroleum-based diesel
Received in revised form
have shown biodiesel to be effective in reducing exhaust emissions of carbon monoxide,
25 July 2008
hydrocarbons, particulate matter, and sulfur dioxide. While there are advantages of
Accepted 15 September 2008
biodiesel over the traditional petroleum based diesel, biodiesel commercialization is
Published online 18 November 2008
limited by production cost that is dominated by the price of the feedstock (soybean oil). Yellow grease has the potential to be an effective feedstock with lower cost, but the
Keywords:
chemical composition of these oils is variable depending on the source of collection and
Yellow grease
differs from that of virgin oil due to the presence of free fatty acids (FFA). Esterification has
Fatty acid methyl esters
been previously demonstrated to reduce the FFA levels of YG; however, large quantities of
Free fatty acid
methanol were required to drive the reaction to high yield. Methanol usage for processing
Biodiesel pretreatment
and FFA content are the main factors affecting the economics of FAME production from YG. In this study, the relationship between composition and process variables was systematically studied. The effect of FFA ranging from 2% to 32% (w/w) was studied at three different molar ratios of methanol to FFA (4.5:1, 9:1, 18:1) and was found to have a non-linear relationship. Data obtained from this full factorial screening was used to develop a predictive statistical model to forecast the conversion based on initial FFA level and proportion of alcohol applied for esterification. ª 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Since traditional fossil energy sources are limited and greenhouse emissions are becoming a greater concern, research on alternative, renewable fuels has increased in recent years [1]. Fatty acid methyl esters (FAME) are the primary component of biodiesel and provide an alternative fuel with a number of merits. FAME are biodegradable, non-toxic and can be derived from renewable sources, such as vegetable oils or animal fats. Comparisons between FAME and fossil-based diesel have shown FAME to be effective in reducing exhaust emissions of
carbon monoxide, hydrocarbons, particulate matter, and sulfur dioxide [2]. In addition, the carbon dioxide formed by combustion of FAME can be recycled by photosynthesis, which minimizes the impact of FAME combustion on greenhouse emissions [3]. While there are advantages of FAME over the traditional fossil diesel fuel, FAME commercialization is limited by cost, which is dominated by the price of the feedstock which can represent 70% of total costs [3]. This has motivated the search for less expensive, but still renewable feedstocks [4–8]. Yellow grease (defined as vegetable oils or animal fats with a FFA less
* Corresponding author. Tel.: þ1 520 626 9120; fax: þ1 520 621 3963. E-mail address: [email protected] (M.R. Riley). 0961-9534/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.09.009
biomass and bioenergy 33 (2009) 558–563
than 15%) has the potential to be an effective feedstock with lower cost [9]. According to the National Renderer’s Association, approximately 7.2 billion pounds of inedible tallows and greases are produced in commercial US restaurants annually [10]. Used frying oils differ from refined and crude oils (such as soybean and canola) primarily by the presence of free fatty acids (FFA) which are generated through the hydrolysis of triglycerides resulting from the high temperatures of typical cooking processes in the presence of water released from the foods [11]. Oxidation products also can be found in yellow grease (YG). The amounts of FFA are dependent on the cooking process and so vary with the source of collection and storage conditions; they also create a process challenge that greatly impacts conversion efficiency of the triglycerides to FAME. The production of FAME on an industrial scale most frequently utilizes an alkali-catalyzed transesterification of oils to yield methyl esters [3]. Conversion to FAME by this process is challenging if the oil contains large amounts of free fatty acids (>1% w/w) which form soaps with the alkaline catalyst [12,13] and hence requires additional downstream operations. A number of researchers working with feedstocks with elevated FFA levels employed an excess of alkaline catalyst to neutralize the FFA, which could then be removed from the process stream as a waste product [14]. This approach increased the costs associated with higher catalyst usage and recovery and also was limited to YG with no more than 3% (w/w) FFA [15]. An alternative process utilizes acid catalysts to esterify the FFAs before the transesterification. Acid catalysts are quite effective at converting FFAs to esters fast enough to be of practical application [7]. Thus, an acid-catalyzed process should provide an effective and efficient method to convert high FFA feedstocks into a suitable feedstock for FAME production. In a previous study, Canakci and Gerpen [14] developed a two-step pretreatment process to reduce the FFA levels of yellow (<15% FFA) and brown grease (>15% FFA) to less than 1%. The first reaction employed an approximate 1 h reaction time, 10% sulfuric acid (based on FFA) and 20:1 molar ratio of methanol/FFA. The second step involved the removal of water produced from the first step. Then, using the product from the first step, and a reaction time of 1 h, the molar ratio (methanol:FFA) for the second step was 40:1. After this two-step pretreatment process, the oils were successfully converted into FAME by transesterification; however, large molar ratios of methanol were required, leading to high material and energy for recovery costs. Ghadge and Raheman [12] used a single acidpretreatment process in which the FFA content was decreased to less 1% using a 0.35 v/v (methanol/oil) ratio for the treatment (a molar ratio of between 70 and 260, depending on the FFA content). Research conducted by Freedman and coworkers [13] concluded that in order for the transesterification of vegetable oils to be successful, the vegetable oils should contain no more than 1% FFA. Higher levels of FFA would reduce the completeness of the reaction and affect the FAME quality. However, Dorado et al. [15] showed that YG can be transesterified with up to 3% FFA with the addition of catalyst to neutralize the FFAs. We have performed a series of transesterification reactions to determine the threshold for FFA in
559
the transesterification of YG and determined that the target FFA should be 2% (shown below). One major disadvantage of acid pretreatments (esterifications) of high FFA oils is that they require large excesses of methanol in order for the reaction to reach a high yield. In addition, the performance of the esterification of FFA is remarkably different when using low FFA oils than high FFA oils. The main variables affecting the economics of FAME production from YG are the FFA content and methanol usage for processing. The FFA content directly dictates the price of the feedstock (low FFA being higher cost), and the methanol usage for processing further impacts the economics of the process due to increasing costs for methanol recovery. Since YG from different collection sources vary widely in FFA levels, one of our objectives was to systematically characterize the relationship between FFA level and the required molar ratio of methanol. The second objective of this research was to develop a predictive model to better understand the process and optimize methanol usage (molar ratio methanol/FFA). The third objective was to perform a process scale up from laboratory (0.10 L) to small pilot plant (190 L) and assess performance.
2.
Materials and methods
2.1.
Materials
The yellow grease, originally a mixture of canola and soybean oil, was supplied by Grecycle Arizona LLC (Tucson, AZ). The free fatty acid (FFA) content varied across samples ranging from approximately 1 to 32% (analysis method described below). Anhydrous methanol, sodium hydroxide (used as a standard in titrations) and sulfuric acid (for esterification) were from Sigma–Aldrich (St. Louis, MO). Sodium methoxide (for transesterification) was obtained from BASF (Ludwigshafen, Germany).
2.2.
Acid catalyzed pretreatment
The esterification reaction was conducted in a 0.250 L round bottom flask, with thermostat, magnetic stirring, and a watercooled condenser that returned vaporized methanol to the reaction mixture. The reaction was maintained at 333 1 K by using an oil bath. The general procedure was as follows. The reactor was washed, rinsed with acetone, preheated to approximately 348 K to eliminate moisture, and then 60 g of YG were introduced for each experiment. When the oil reached 336 K, the catalyst and alcohol were added, in the amount established for each experiment (described in detail below). This addition led to a drop in oil temperature to 333 1 K, which was considered to be time zero of the reaction. Each experiment was run for 60 min, a time at which the conversion to methyl esters is practically complete (data not shown, but similar results were presented by Canakci and Gerpen [14]). Mixing, temperature and reaction time were kept constant for all tests. Since the time for the methanol/catalyst and oil phases to separate spontaneously depended on the molar ratio of methanol used, the phase separation was standardized using centrifugation with an Eppendorf 5325
560
biomass and bioenergy 33 (2009) 558–563
centrifuge operated at 7500 rpm for 20 min. The conversion of FFA into methyl esters was determined by titration using a modified version of the American Oil Chemists’ Society’s official method Cd-3a-63 for acid value [16]. The method was adjusted and validated to reduce sample mass for titrations. The percentage of conversion was calculated as follows: % Conversion ¼ ½ðinitial FFA final FFAÞ=initial FFA 100%
Table 1 – Reaction parameters for model validation (MR, the molar ratio of methanol to fatty acids (mole:mole)). Experiment
MR
% FFA
1 2 3 4 5
4.5 7 6 15 11
10.3 5.33 26 16.3 7
All experiments were performed in triplicate with statistical tests using the statistical software Statgraphics version 5.0 with a ¼ 0.05 as the significance level.
The same equipment and configuration employed for esterification was also used for transesterifications, with only minor modifications in the general procedure. Since there is a large difference in the density of the methyl esters and glycerol phases, they were allowed to separate by gravity in a graduated cylinder after the reaction was complete. The volume in each phase was recorded. The conditions for transesterification were 6:1 molar ratio (methanol to triglyceride), 333 1 K, 60 min reaction time, and 0.25% sodium methoxide, based on the weight of the oil. The amount of catalyst was adjusted for any FFAs present in the oil. For control (theoretically fully reacted YG), 60 g of pure FAME were mixed with 0.00825 L of methanol and 6 g of pure glycerol (to simulate the reaction of 60 g of vegetable oil reacted with a 6:1 molar ratio of methanol). For the transesterifications, 60 g of YG were reacted at 6:1 molar ratio with methanol. Additional catalyst was used to neutralize the FFAs present in the YG.
2.4.
3.
Results and discussion
3.1.
Transesterification of YG
Transesterification
Model development and validation
100
18
90
16
80
14
70
12
60
10
50 8
40
6
30
2.5.
Pilot plant equipment
The YG was stored in a cone-bottom tank in which the solids were separated and removed. The reaction temperature was maintained using an immersion heater and stirred with a 186 W mixer (fixed speed) provided with an axial flow propeller. This configuration resulted in a Reynolds number of 16,500, a power number equal to 1.
% Glycerol phase
A statistical model to explain the sensitivity of the esterification process to the molar ratio (MR) and the initial FFA (iFFA) content was developed using forward selection and backward elimination of variables. A subset of samples was used for calibration (70% of measurements) with the remaining ones utilized as a validation set. Independent variables were MR and iFFA with percent conversion as the dependent variable. Selection and evaluation of the model was based on minimizing R2 (adjusted for degrees of freedom) for model comparison, mean squared error (MSE), and residual plots to assess prediction bias. For model validation, five experiments were selected to minimize the correlation between input conditions and reduce bias in model construction. The conditions for these experiments are presented in Table 1.
Since prior works employed a variety of targets for the maximum allowable initial FFA (iFFA), our studies evaluated the sensitivity of the transesterification reaction over a range of levels. First, a control to determine the volumetric distribution of phases (glycerol phase and FAME phase) of a successful transesterification of 60 g of YG was formulated. This blend was mixed vigorously in a graduated cylinder and allowed to separate into the FAME and glycerol phases (note that methanol partitions itself between the FAME and the glycerol phases) until the mixture reached equilibrium. The relative volume of these two phases was then considered as the control to compare with the transesterifications of the experimental 60 g batches. After establishing this control, transesterifications of YG containing different levels of FFA were performed. These results are shown in Fig. 1. The volumetric footprint of the oils as shown in Fig. 1 was found to change only slightly between 0% FFA (control), 1% FFA YG and 2% FFA YG. However, the volumetric footprint of the yield from the 3% FFA YG increased significantly. This may be due to the higher FFAs present, which leads to lower conversions [13]. With lower conversions, the more polar phase (glycerol phase) tends to attract any mono- and
% FAME phase
2.3.
4
20
% FAME phase
10
2
% Glycerol phase
0
0
Control
<1% FFA
2% FFA
3% FFA
Fig. 1 – Volumetric yield of transesterification of YG (60 g using a 6:1 molar ratio, 333 K, 60 min reaction time).
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biomass and bioenergy 33 (2009) 558–563
3.2.
Acid esterification: conversion vs. time
In order to study the progress of the esterification rate over time, two different oils with low (4.6% w/w) and high (19.2% w/w) FFA were treated under analogous conditions. A molar ratio of 9:1 (methanol:FFA), 10% w/w sulfuric acid (based upon the FFA weight) were used. Nearly the entire conversion occurred during the first 30 min of the reaction (Fig. 2). The lack of conversion at later times may be due to accumulation of water released by the esterification of FFAs that would reduce the reaction rate over time. This behavior is in accordance with that obtained by Ghadge and Raheman [12] and Canakci and Gerpen [14]. A 1 h reaction time appears suitable for pretreatment of YG under these conditions since longer reaction times would not increase conversion.
3.3.
Acid esterification: effect of process variables
In order to investigate the relationship between the level of FFA in the waste cooking oil and the molar ratio for esterification, a two-variable screening was conducted. The first variable, FFA amount, was studied at 5 levels (2%, 4.5%, 7%, 19% and 32%). The second variable, molar ratio (methanol/ FFA) was studied at 3 levels (4.5:1, 9:1, and 18:1). Fig. 3 shows the output of all the combinations of FFA vs. molar ratio that were conducted in laboratory experiments. The percentage of conversion (Fig. 3) represents the degree to which the FFAs in the YG oil were converted to its respective FAME. Higher molar ratios and higher iFFA led to greater percent conversion. The percentage of conversion (FFA to FAME) showed an asymptotic response to the increase in molar ratio for any given oil. This is most clearly seen with 20
% FFA
4.6% Initial FFA 16
19.2% Initial FFA
12
Pilot Plant (10.5% FFA)
8 4 0
0
30
60
90
120
Time (mins) Fig. 2 – Free fatty acid esterification progress of yellow grease.
150
32% FFA
4.5% FFA
19% FFA
2% FFA
7% FFA 100 90
% Conversion of FFA to Methyl Esters
di-glycerides that emulsify with the FFAs and thus increase the volume of the glycerol phase. Based on data here, the target FFA level for an acid-pretreatment ideally should be 1% or less since this amount requires no additional catalyst to neutralize the FFAs. Given the energy and methanol required to reduce the FFA level to this point, it may not be feasible to accomplish this task, economically in all cases. Transesterification of YG with FFA levels of 2% can be performed without significantly affecting the yield by adjusting the amount of catalyst to the FFA content (Fig. 1).
80 70 60 50 40 30 20 10 0
0
4.5
9
13.5
18
Molar ratio (alcohol / FFA) Fig. 3 – Results of experiments on esterification of YG as pretreatment (each point represents the average of 3 experiments, summing up for a total of 45 experiments).
the 2% iFFA which displays an increase in conversion from 25% to 40% when the alcohol content is doubled. On the other hand, high iFFA oils could readily be processed reaching conversions of 95%. In general, the amount of FFA converted by the acid pretreatment was greater for higher initial FFA YG than for lower FFA YG. The lower the iFFA of the YG, the lower percent conversion is needed to pretreat the YG but also, the higher molar ratios that are needed to accomplish a given conversion. It should also be noted that even at the highest molar ratio for the low iFFA content the quantity of methanol used is quite small. One reasonable explanation for this behavior is that the solubility of the reactants are also dependent on the iFFA %. The composition of YG is complex but can be divided into polar and non-polar compounds. During the reaction, the non-polar phase is comprised mainly of triglycerides. The polar phase is comprised of a variety of compounds resulting from the frying process and includes low molecular weight aldehydes and ketones in addition to the methanol used for the esterification of FFAs. The FFAs, being amphiphylic in nature, could be distributed in either phase. In fact, an increase in the FFA content of YG leads to a nearly linear increase in the solubility of methanol (Fig. 4). As the FFA level in YG is increased, the system can readily solubilize more methanol which increases the methanol concentration in the reacting phase. On the other hand, it had previously been reported that short chain alcohols, especially methanol, have low solubility in oils and so a new liquid polar phase appears in the system at moderate concentrations [4]. With this consideration in mind, it is also possible that methanol may play a role as a solvent in the system in addition to being the esterifying agent. Under this scenario, the FFA may migrate into the polar methanol phase (which is very rich in catalyst), is esterified, and then returns to the non-polar phase in the
562
biomass and bioenergy 33 (2009) 558–563
100
250 80
200 150
Experimental
Meethanol in Oil (mI/L)
300
100 50 0
0%
5%
10%
15%
20%
25%
60
40
30% 20
% FFA Fig. 4 – Solubility of methanol in YG as a function of FFA level.
0 0
20
40
60
80
100
Predicted form of methyl ester. This would also increase the rate of the reaction. If this is so, there would be almost no methanol phase when iFFA is low since the standard selection of the amount methanol used is dependent on the FFA level. Another possibility is that the water could also be extracted by this polar phase. By doing this, the backward reaction is reduced at the reaction site (oil plus methanol/acid).
3.4.
Model selection
To provide a means to predict the amount of methanol and catalyst needed to reduce the iFFA to <1%, a model was constructed using the data shown in Fig. 3. A variety of model constructions utilizing linear and non-linear combinations of the input variables: methanol content, FFA, and interactive terms were evaluated. The equation that gave the best fit to the data is as follows: % Conversion ¼ 4:12 17:94ðFFAÞ þ 47:27ðMeOHÞ 38:89ðFFAÞðMeOHÞ þ 135:15ðFFAÞ2 5:04ðMeOHÞ2 þ7:14ðFFAÞ2 ðMeOHÞ 26:58ðFFAÞðMeOHÞ2 where FFA is moles of FFA and MeOH is moles of methanol (mol/L). The selected model explains 95% of the variation in the process; a prediction plot is shown in Fig. 5. A good one-to-one correspondence is obtained with no bias at either end of the valid range. Prediction errors are evenly distributed. Note that the requirement of including interaction terms between FFA and MeOH supports the hypothesis of MeOH impacting the solubility of FFA. Removal of such interactive terms reduces the predictive capability of the model to an R2 of 72.5%.
3.5.
Pilot plant implementation
After model development and validation, the next challenge was to increase the process scale from laboratory size (0.08L reaction) into a larger scale pilot plant of approximately 189 liters per batch. Experiments were performed using the amounts of methanol that were predicted by the model for
Fig. 5 – Predicted percent conversion by the model vs. experimentally obtained data.
96% conversion of FFA to FAME. Trials consisted of 151.2 liters of YG reacted with 39.31 liters of methanol plus 0.83 liter of sulfuric acid. After addressing some reactor configuration problems during experimentation (such as vortex formation), the results of the pilot plant system were similar to the laboratory scale process. The molar ratio on these pilot experiments was 18:1. The FFA level of the yellow grease was reduced from 10.5% down to 0.4% (Fig. 2), which represents 95.8% conversion (versus 96% predicted by the model), which proves that the model is reliable. This also shows that there is room for optimization as only 85% conversion is really needed for a successful transesterification. The model predicts that for 85% conversion a molar ratio of 13:1 for approximately 85% conversion. Eighty-five percent conversion translates to a FFA less than 2%, which can be still successfully transesterified with an alkaline catalyst.
4.
Conclusions
1. Prior studies report conflicting data as to what the FFA level of YG for a successful transesterification should be. While it is ideal to reduce the FFA level to the lowest feasible value, the results of this research indicate that the presence of 2% FFA (w/w) in YG does not affect the transesterification yield significantly. 2. The acid-catalyzed esterification successfully reduces the FFA level of YG. One of the key factors on the esterification of YG is methanol usage, which is closely related to the FFA of the YG. The results obtained in this research suggest that methanol plays multiple roles in the reaction. It is the esterifying agent that displaces the hydroxyl group from the FFA, creating the methyl ester, but some other important solvent properties for methanol are suggested. As
biomass and bioenergy 33 (2009) 558–563
shown above, the solubility of methanol increases with FFA content of the YG due likely to hydrogen bonding interactions. The more FFAs in YG, the more methanol is soluble in the oil phase readily available to react with the FFA. 3. The data obtained from the screening was used to develop a model to better understand the process and aid in scale up. The model was found to be reliable on validation experiments (which were selected to be the least correlated to the data used for model construction) and in pilot plant experiments. The coefficient of correlation obtained was 95%, under 0.05 as significance level. 4. The major advantage of the proposed model is that it provides a guide for expected conversion efficiency of acid esterification of YG, based solely on the initial FFA of the YG and a given molar ratio for the reaction. This is especially useful in increasing the scale of the process.
Acknowledgements We thank the University of Arizona’s Agricultural and Biosystems Engineering machine shop for support on pilot plant construction. We also thank Pedro Romero for his help in developing the statistical model. This research was funded by Zelen Environmental.
references
[1] Lopez DE, Goodwin JG, Bruce DA, Lotero E. Transesterification of triacetin with methanol on solid acid and base catalysts. Applied Catalysts 2005;295:97–105. [2] Nelson R, Schrock M. Energetic and economic feasibility associated with the production, processing, and conversion of beef tallow to a substitute diesel fuel. Biomass and Energy 2005;30:584–91.
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[3] Zhang Y, Dube MA, Mclean CC, Kates M. Biodiesel production from waste cooking oil: 2. economical assessment and sensitivity analysis. Bioresource Technology 2003;90:229–40. [4] Royon D, Daz M, Ellenrieder G, Locatelli S. Enzymatic production of biodiesel from cotton seed oil using t-butanol as a solvent. Bioresource Technology 2006;98:648–53. [5] Tomasevic A, Siler-Marinkovic S. Methanolysis of used frying oil. Fuel Processing Technology 2003;81:1–6. [6] Canakci M. The potential of restaurant waste lipids as biodiesel feedstocks. Bioresource Technology 2005;98:183–90. [7] Zheng S, Kates M, Dube M, McLean D. Acid-catalyzed production of biodiesel from waste frying oil. Biomass and Bioenergy 2005;30:267–72. [8] Hass M. improving economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock. Fuel Processing Technology 2005;86:1087–96. [9] Lai CC, Zullaikah S, Vali SR, Ju YH. Lipase-catalyzed production of biodiesel from rice bran oil. Journal of Chemical Technology and Biotechnology 2005;80:331–7. [10] National Renderers Association. Market report. Render Magazine 2002;59:10–6. [11] Encinar J, Gonzalez J, Rodrı´guez-Reinares A. Biodiesel from used frying oil. variables affecting the yields and characteristics of the biodiesel. Industrial & Engineering Chemistry Research 2005;44:5491–9. [12] Ghade S, Raheman H. Biodiesel production from mahua (Madhuca indica) oil having high free fatty acids. Biomass and Bioenergy 2005;28:601–5. [13] Freedman B, Pryde H. Variables affecting the yields of fatty esters from transesterified vegetable oils. Journal of American Oil Chemists Society 1984;61:1638–43. [14] Canakci M, Gerpen V. Biodiesel production from oils and fats with high free fatty acids. Transactions of ASAE 2001;44(6): 1429–36. [15] Dorado MP, Ballesteros E, Almeida JA, Schellert C, Lohrlein HP, Krause R. An alkali-catalyzed transesterification process for high free fatty acid waste oils. Transactions of ASAE 2002;45(3):525–9. [16] AOCS. Official test method Ca-40 for free fatty acids. Official and Tentative Methods of the American Oil Chemists’ Society 1970;1:1–2.
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Pretreatment of yellow grease for efficient production of fatty acid methyl esters Walterio Diaz-Felixa, Mark R. Rileya,*, Werner Zimmta, Michael Kazzb a
Department of Agricultural and Biosystems Engineering, Shantz Building, Room 403, The University of Arizona, Tucson, AZ 85721, USA Zelen Environmental, Tucson, AZ, USA
b
article info
abstract
Article history:
Biodiesel is a renewable fuel comprised of fatty acid methyl esters (FAME) derived from
Received 3 September 2007
vegetable oils or animal fats. Comparisons between biodiesel and petroleum-based diesel
Received in revised form
have shown biodiesel to be effective in reducing exhaust emissions of carbon monoxide,
25 July 2008
hydrocarbons, particulate matter, and sulfur dioxide. While there are advantages of
Accepted 15 September 2008
biodiesel over the traditional petroleum based diesel, biodiesel commercialization is
Published online 18 November 2008
limited by production cost that is dominated by the price of the feedstock (soybean oil). Yellow grease has the potential to be an effective feedstock with lower cost, but the
Keywords:
chemical composition of these oils is variable depending on the source of collection and
Yellow grease
differs from that of virgin oil due to the presence of free fatty acids (FFA). Esterification has
Fatty acid methyl esters
been previously demonstrated to reduce the FFA levels of YG; however, large quantities of
Free fatty acid
methanol were required to drive the reaction to high yield. Methanol usage for processing
Biodiesel pretreatment
and FFA content are the main factors affecting the economics of FAME production from YG. In this study, the relationship between composition and process variables was systematically studied. The effect of FFA ranging from 2% to 32% (w/w) was studied at three different molar ratios of methanol to FFA (4.5:1, 9:1, 18:1) and was found to have a non-linear relationship. Data obtained from this full factorial screening was used to develop a predictive statistical model to forecast the conversion based on initial FFA level and proportion of alcohol applied for esterification. ª 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Since traditional fossil energy sources are limited and greenhouse emissions are becoming a greater concern, research on alternative, renewable fuels has increased in recent years [1]. Fatty acid methyl esters (FAME) are the primary component of biodiesel and provide an alternative fuel with a number of merits. FAME are biodegradable, non-toxic and can be derived from renewable sources, such as vegetable oils or animal fats. Comparisons between FAME and fossil-based diesel have shown FAME to be effective in reducing exhaust emissions of
carbon monoxide, hydrocarbons, particulate matter, and sulfur dioxide [2]. In addition, the carbon dioxide formed by combustion of FAME can be recycled by photosynthesis, which minimizes the impact of FAME combustion on greenhouse emissions [3]. While there are advantages of FAME over the traditional fossil diesel fuel, FAME commercialization is limited by cost, which is dominated by the price of the feedstock which can represent 70% of total costs [3]. This has motivated the search for less expensive, but still renewable feedstocks [4–8]. Yellow grease (defined as vegetable oils or animal fats with a FFA less
* Corresponding author. Tel.: þ1 520 626 9120; fax: þ1 520 621 3963. E-mail address: [email protected] (M.R. Riley). 0961-9534/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.09.009
biomass and bioenergy 33 (2009) 558–563
than 15%) has the potential to be an effective feedstock with lower cost [9]. According to the National Renderer’s Association, approximately 7.2 billion pounds of inedible tallows and greases are produced in commercial US restaurants annually [10]. Used frying oils differ from refined and crude oils (such as soybean and canola) primarily by the presence of free fatty acids (FFA) which are generated through the hydrolysis of triglycerides resulting from the high temperatures of typical cooking processes in the presence of water released from the foods [11]. Oxidation products also can be found in yellow grease (YG). The amounts of FFA are dependent on the cooking process and so vary with the source of collection and storage conditions; they also create a process challenge that greatly impacts conversion efficiency of the triglycerides to FAME. The production of FAME on an industrial scale most frequently utilizes an alkali-catalyzed transesterification of oils to yield methyl esters [3]. Conversion to FAME by this process is challenging if the oil contains large amounts of free fatty acids (>1% w/w) which form soaps with the alkaline catalyst [12,13] and hence requires additional downstream operations. A number of researchers working with feedstocks with elevated FFA levels employed an excess of alkaline catalyst to neutralize the FFA, which could then be removed from the process stream as a waste product [14]. This approach increased the costs associated with higher catalyst usage and recovery and also was limited to YG with no more than 3% (w/w) FFA [15]. An alternative process utilizes acid catalysts to esterify the FFAs before the transesterification. Acid catalysts are quite effective at converting FFAs to esters fast enough to be of practical application [7]. Thus, an acid-catalyzed process should provide an effective and efficient method to convert high FFA feedstocks into a suitable feedstock for FAME production. In a previous study, Canakci and Gerpen [14] developed a two-step pretreatment process to reduce the FFA levels of yellow (<15% FFA) and brown grease (>15% FFA) to less than 1%. The first reaction employed an approximate 1 h reaction time, 10% sulfuric acid (based on FFA) and 20:1 molar ratio of methanol/FFA. The second step involved the removal of water produced from the first step. Then, using the product from the first step, and a reaction time of 1 h, the molar ratio (methanol:FFA) for the second step was 40:1. After this two-step pretreatment process, the oils were successfully converted into FAME by transesterification; however, large molar ratios of methanol were required, leading to high material and energy for recovery costs. Ghadge and Raheman [12] used a single acidpretreatment process in which the FFA content was decreased to less 1% using a 0.35 v/v (methanol/oil) ratio for the treatment (a molar ratio of between 70 and 260, depending on the FFA content). Research conducted by Freedman and coworkers [13] concluded that in order for the transesterification of vegetable oils to be successful, the vegetable oils should contain no more than 1% FFA. Higher levels of FFA would reduce the completeness of the reaction and affect the FAME quality. However, Dorado et al. [15] showed that YG can be transesterified with up to 3% FFA with the addition of catalyst to neutralize the FFAs. We have performed a series of transesterification reactions to determine the threshold for FFA in
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the transesterification of YG and determined that the target FFA should be 2% (shown below). One major disadvantage of acid pretreatments (esterifications) of high FFA oils is that they require large excesses of methanol in order for the reaction to reach a high yield. In addition, the performance of the esterification of FFA is remarkably different when using low FFA oils than high FFA oils. The main variables affecting the economics of FAME production from YG are the FFA content and methanol usage for processing. The FFA content directly dictates the price of the feedstock (low FFA being higher cost), and the methanol usage for processing further impacts the economics of the process due to increasing costs for methanol recovery. Since YG from different collection sources vary widely in FFA levels, one of our objectives was to systematically characterize the relationship between FFA level and the required molar ratio of methanol. The second objective of this research was to develop a predictive model to better understand the process and optimize methanol usage (molar ratio methanol/FFA). The third objective was to perform a process scale up from laboratory (0.10 L) to small pilot plant (190 L) and assess performance.
2.
Materials and methods
2.1.
Materials
The yellow grease, originally a mixture of canola and soybean oil, was supplied by Grecycle Arizona LLC (Tucson, AZ). The free fatty acid (FFA) content varied across samples ranging from approximately 1 to 32% (analysis method described below). Anhydrous methanol, sodium hydroxide (used as a standard in titrations) and sulfuric acid (for esterification) were from Sigma–Aldrich (St. Louis, MO). Sodium methoxide (for transesterification) was obtained from BASF (Ludwigshafen, Germany).
2.2.
Acid catalyzed pretreatment
The esterification reaction was conducted in a 0.250 L round bottom flask, with thermostat, magnetic stirring, and a watercooled condenser that returned vaporized methanol to the reaction mixture. The reaction was maintained at 333 1 K by using an oil bath. The general procedure was as follows. The reactor was washed, rinsed with acetone, preheated to approximately 348 K to eliminate moisture, and then 60 g of YG were introduced for each experiment. When the oil reached 336 K, the catalyst and alcohol were added, in the amount established for each experiment (described in detail below). This addition led to a drop in oil temperature to 333 1 K, which was considered to be time zero of the reaction. Each experiment was run for 60 min, a time at which the conversion to methyl esters is practically complete (data not shown, but similar results were presented by Canakci and Gerpen [14]). Mixing, temperature and reaction time were kept constant for all tests. Since the time for the methanol/catalyst and oil phases to separate spontaneously depended on the molar ratio of methanol used, the phase separation was standardized using centrifugation with an Eppendorf 5325
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centrifuge operated at 7500 rpm for 20 min. The conversion of FFA into methyl esters was determined by titration using a modified version of the American Oil Chemists’ Society’s official method Cd-3a-63 for acid value [16]. The method was adjusted and validated to reduce sample mass for titrations. The percentage of conversion was calculated as follows: % Conversion ¼ ½ðinitial FFA final FFAÞ=initial FFA 100%
Table 1 – Reaction parameters for model validation (MR, the molar ratio of methanol to fatty acids (mole:mole)). Experiment
MR
% FFA
1 2 3 4 5
4.5 7 6 15 11
10.3 5.33 26 16.3 7
All experiments were performed in triplicate with statistical tests using the statistical software Statgraphics version 5.0 with a ¼ 0.05 as the significance level.
The same equipment and configuration employed for esterification was also used for transesterifications, with only minor modifications in the general procedure. Since there is a large difference in the density of the methyl esters and glycerol phases, they were allowed to separate by gravity in a graduated cylinder after the reaction was complete. The volume in each phase was recorded. The conditions for transesterification were 6:1 molar ratio (methanol to triglyceride), 333 1 K, 60 min reaction time, and 0.25% sodium methoxide, based on the weight of the oil. The amount of catalyst was adjusted for any FFAs present in the oil. For control (theoretically fully reacted YG), 60 g of pure FAME were mixed with 0.00825 L of methanol and 6 g of pure glycerol (to simulate the reaction of 60 g of vegetable oil reacted with a 6:1 molar ratio of methanol). For the transesterifications, 60 g of YG were reacted at 6:1 molar ratio with methanol. Additional catalyst was used to neutralize the FFAs present in the YG.
2.4.
3.
Results and discussion
3.1.
Transesterification of YG
Transesterification
Model development and validation
100
18
90
16
80
14
70
12
60
10
50 8
40
6
30
2.5.
Pilot plant equipment
The YG was stored in a cone-bottom tank in which the solids were separated and removed. The reaction temperature was maintained using an immersion heater and stirred with a 186 W mixer (fixed speed) provided with an axial flow propeller. This configuration resulted in a Reynolds number of 16,500, a power number equal to 1.
% Glycerol phase
A statistical model to explain the sensitivity of the esterification process to the molar ratio (MR) and the initial FFA (iFFA) content was developed using forward selection and backward elimination of variables. A subset of samples was used for calibration (70% of measurements) with the remaining ones utilized as a validation set. Independent variables were MR and iFFA with percent conversion as the dependent variable. Selection and evaluation of the model was based on minimizing R2 (adjusted for degrees of freedom) for model comparison, mean squared error (MSE), and residual plots to assess prediction bias. For model validation, five experiments were selected to minimize the correlation between input conditions and reduce bias in model construction. The conditions for these experiments are presented in Table 1.
Since prior works employed a variety of targets for the maximum allowable initial FFA (iFFA), our studies evaluated the sensitivity of the transesterification reaction over a range of levels. First, a control to determine the volumetric distribution of phases (glycerol phase and FAME phase) of a successful transesterification of 60 g of YG was formulated. This blend was mixed vigorously in a graduated cylinder and allowed to separate into the FAME and glycerol phases (note that methanol partitions itself between the FAME and the glycerol phases) until the mixture reached equilibrium. The relative volume of these two phases was then considered as the control to compare with the transesterifications of the experimental 60 g batches. After establishing this control, transesterifications of YG containing different levels of FFA were performed. These results are shown in Fig. 1. The volumetric footprint of the oils as shown in Fig. 1 was found to change only slightly between 0% FFA (control), 1% FFA YG and 2% FFA YG. However, the volumetric footprint of the yield from the 3% FFA YG increased significantly. This may be due to the higher FFAs present, which leads to lower conversions [13]. With lower conversions, the more polar phase (glycerol phase) tends to attract any mono- and
% FAME phase
2.3.
4
20
% FAME phase
10
2
% Glycerol phase
0
0
Control
<1% FFA
2% FFA
3% FFA
Fig. 1 – Volumetric yield of transesterification of YG (60 g using a 6:1 molar ratio, 333 K, 60 min reaction time).
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3.2.
Acid esterification: conversion vs. time
In order to study the progress of the esterification rate over time, two different oils with low (4.6% w/w) and high (19.2% w/w) FFA were treated under analogous conditions. A molar ratio of 9:1 (methanol:FFA), 10% w/w sulfuric acid (based upon the FFA weight) were used. Nearly the entire conversion occurred during the first 30 min of the reaction (Fig. 2). The lack of conversion at later times may be due to accumulation of water released by the esterification of FFAs that would reduce the reaction rate over time. This behavior is in accordance with that obtained by Ghadge and Raheman [12] and Canakci and Gerpen [14]. A 1 h reaction time appears suitable for pretreatment of YG under these conditions since longer reaction times would not increase conversion.
3.3.
Acid esterification: effect of process variables
In order to investigate the relationship between the level of FFA in the waste cooking oil and the molar ratio for esterification, a two-variable screening was conducted. The first variable, FFA amount, was studied at 5 levels (2%, 4.5%, 7%, 19% and 32%). The second variable, molar ratio (methanol/ FFA) was studied at 3 levels (4.5:1, 9:1, and 18:1). Fig. 3 shows the output of all the combinations of FFA vs. molar ratio that were conducted in laboratory experiments. The percentage of conversion (Fig. 3) represents the degree to which the FFAs in the YG oil were converted to its respective FAME. Higher molar ratios and higher iFFA led to greater percent conversion. The percentage of conversion (FFA to FAME) showed an asymptotic response to the increase in molar ratio for any given oil. This is most clearly seen with 20
% FFA
4.6% Initial FFA 16
19.2% Initial FFA
12
Pilot Plant (10.5% FFA)
8 4 0
0
30
60
90
120
Time (mins) Fig. 2 – Free fatty acid esterification progress of yellow grease.
150
32% FFA
4.5% FFA
19% FFA
2% FFA
7% FFA 100 90
% Conversion of FFA to Methyl Esters
di-glycerides that emulsify with the FFAs and thus increase the volume of the glycerol phase. Based on data here, the target FFA level for an acid-pretreatment ideally should be 1% or less since this amount requires no additional catalyst to neutralize the FFAs. Given the energy and methanol required to reduce the FFA level to this point, it may not be feasible to accomplish this task, economically in all cases. Transesterification of YG with FFA levels of 2% can be performed without significantly affecting the yield by adjusting the amount of catalyst to the FFA content (Fig. 1).
80 70 60 50 40 30 20 10 0
0
4.5
9
13.5
18
Molar ratio (alcohol / FFA) Fig. 3 – Results of experiments on esterification of YG as pretreatment (each point represents the average of 3 experiments, summing up for a total of 45 experiments).
the 2% iFFA which displays an increase in conversion from 25% to 40% when the alcohol content is doubled. On the other hand, high iFFA oils could readily be processed reaching conversions of 95%. In general, the amount of FFA converted by the acid pretreatment was greater for higher initial FFA YG than for lower FFA YG. The lower the iFFA of the YG, the lower percent conversion is needed to pretreat the YG but also, the higher molar ratios that are needed to accomplish a given conversion. It should also be noted that even at the highest molar ratio for the low iFFA content the quantity of methanol used is quite small. One reasonable explanation for this behavior is that the solubility of the reactants are also dependent on the iFFA %. The composition of YG is complex but can be divided into polar and non-polar compounds. During the reaction, the non-polar phase is comprised mainly of triglycerides. The polar phase is comprised of a variety of compounds resulting from the frying process and includes low molecular weight aldehydes and ketones in addition to the methanol used for the esterification of FFAs. The FFAs, being amphiphylic in nature, could be distributed in either phase. In fact, an increase in the FFA content of YG leads to a nearly linear increase in the solubility of methanol (Fig. 4). As the FFA level in YG is increased, the system can readily solubilize more methanol which increases the methanol concentration in the reacting phase. On the other hand, it had previously been reported that short chain alcohols, especially methanol, have low solubility in oils and so a new liquid polar phase appears in the system at moderate concentrations [4]. With this consideration in mind, it is also possible that methanol may play a role as a solvent in the system in addition to being the esterifying agent. Under this scenario, the FFA may migrate into the polar methanol phase (which is very rich in catalyst), is esterified, and then returns to the non-polar phase in the
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100
250 80
200 150
Experimental
Meethanol in Oil (mI/L)
300
100 50 0
0%
5%
10%
15%
20%
25%
60
40
30% 20
% FFA Fig. 4 – Solubility of methanol in YG as a function of FFA level.
0 0
20
40
60
80
100
Predicted form of methyl ester. This would also increase the rate of the reaction. If this is so, there would be almost no methanol phase when iFFA is low since the standard selection of the amount methanol used is dependent on the FFA level. Another possibility is that the water could also be extracted by this polar phase. By doing this, the backward reaction is reduced at the reaction site (oil plus methanol/acid).
3.4.
Model selection
To provide a means to predict the amount of methanol and catalyst needed to reduce the iFFA to <1%, a model was constructed using the data shown in Fig. 3. A variety of model constructions utilizing linear and non-linear combinations of the input variables: methanol content, FFA, and interactive terms were evaluated. The equation that gave the best fit to the data is as follows: % Conversion ¼ 4:12 17:94ðFFAÞ þ 47:27ðMeOHÞ 38:89ðFFAÞðMeOHÞ þ 135:15ðFFAÞ2 5:04ðMeOHÞ2 þ7:14ðFFAÞ2 ðMeOHÞ 26:58ðFFAÞðMeOHÞ2 where FFA is moles of FFA and MeOH is moles of methanol (mol/L). The selected model explains 95% of the variation in the process; a prediction plot is shown in Fig. 5. A good one-to-one correspondence is obtained with no bias at either end of the valid range. Prediction errors are evenly distributed. Note that the requirement of including interaction terms between FFA and MeOH supports the hypothesis of MeOH impacting the solubility of FFA. Removal of such interactive terms reduces the predictive capability of the model to an R2 of 72.5%.
3.5.
Pilot plant implementation
After model development and validation, the next challenge was to increase the process scale from laboratory size (0.08L reaction) into a larger scale pilot plant of approximately 189 liters per batch. Experiments were performed using the amounts of methanol that were predicted by the model for
Fig. 5 – Predicted percent conversion by the model vs. experimentally obtained data.
96% conversion of FFA to FAME. Trials consisted of 151.2 liters of YG reacted with 39.31 liters of methanol plus 0.83 liter of sulfuric acid. After addressing some reactor configuration problems during experimentation (such as vortex formation), the results of the pilot plant system were similar to the laboratory scale process. The molar ratio on these pilot experiments was 18:1. The FFA level of the yellow grease was reduced from 10.5% down to 0.4% (Fig. 2), which represents 95.8% conversion (versus 96% predicted by the model), which proves that the model is reliable. This also shows that there is room for optimization as only 85% conversion is really needed for a successful transesterification. The model predicts that for 85% conversion a molar ratio of 13:1 for approximately 85% conversion. Eighty-five percent conversion translates to a FFA less than 2%, which can be still successfully transesterified with an alkaline catalyst.
4.
Conclusions
1. Prior studies report conflicting data as to what the FFA level of YG for a successful transesterification should be. While it is ideal to reduce the FFA level to the lowest feasible value, the results of this research indicate that the presence of 2% FFA (w/w) in YG does not affect the transesterification yield significantly. 2. The acid-catalyzed esterification successfully reduces the FFA level of YG. One of the key factors on the esterification of YG is methanol usage, which is closely related to the FFA of the YG. The results obtained in this research suggest that methanol plays multiple roles in the reaction. It is the esterifying agent that displaces the hydroxyl group from the FFA, creating the methyl ester, but some other important solvent properties for methanol are suggested. As
biomass and bioenergy 33 (2009) 558–563
shown above, the solubility of methanol increases with FFA content of the YG due likely to hydrogen bonding interactions. The more FFAs in YG, the more methanol is soluble in the oil phase readily available to react with the FFA. 3. The data obtained from the screening was used to develop a model to better understand the process and aid in scale up. The model was found to be reliable on validation experiments (which were selected to be the least correlated to the data used for model construction) and in pilot plant experiments. The coefficient of correlation obtained was 95%, under 0.05 as significance level. 4. The major advantage of the proposed model is that it provides a guide for expected conversion efficiency of acid esterification of YG, based solely on the initial FFA of the YG and a given molar ratio for the reaction. This is especially useful in increasing the scale of the process.
Acknowledgements We thank the University of Arizona’s Agricultural and Biosystems Engineering machine shop for support on pilot plant construction. We also thank Pedro Romero for his help in developing the statistical model. This research was funded by Zelen Environmental.
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