Biodiesel production using oil from fish canning

Energy Conversion and Management 74 (2013) 17–23

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Biodiesel production using oil from fish canning industry wastes J.F. Costa a,b, M.F. Almeida a, M.C.M. Alvim-Ferraz b, J.M. Dias a,⇑ a b

LEPAE, Departamento de Engenharia Metalúrgica e de Materiais, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 8 February 2013 Accepted 23 April 2013

Keywords: Fish oil Biodiesel Acid esterification Experimental planning

a b s t r a c t The present study evaluated biodiesel production using oil extracted from fish canning industry wastes, focusing on pre-treatment and reaction conditions. Experimental planning was conducted to evaluate the influence of acid catalyst concentration (1–3 wt.% H2SO4) in the esterification pre-treatment and the amount of methanolic solution (60–90 vol.%) used at the beginning of the further two-step alkali transesterification reaction. The use of a raw-material mixture, including waste oil obtained from olive oil bagasse, was also studied. The results from experimental planning showed that catalyst concentration mostly influenced product yield and quality, the best conditions being 1 wt.% catalyst and 60 vol.% of methanolic solution, to obtain a product yield of 73.9 wt.% and a product purity of 75.5 wt.%. Results from a one-step reaction under the selected conditions showed no advantage of performing a two-step alkali process. Although under the best conditions several of the biodiesel quality parameters were in agreement with standard specifications, a great variation was found in the biodiesel acid value, and oxidation stability and methyl ester content did not comply with biodiesel quality standards. Aiming to improve fuel quality, a mixture containing 80% waste olive oil and 20% of waste fish oil was evaluated. Using such mixture, biodiesel purity increased around 15%, being close to the standard requirements (96.5 wt.%), and the oxidation stability was in agreement with the biodiesel quality standard values (P6 h), which are promising results clearly showing the potential of using such wastes, of very low value, for biodiesel production. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction A biofuel is, in simple terms, a combustible material obtained from renewable biomass resources, commonly used as an alternative, cleaner fuel. When properly produced, biodiesel can be used pure or in mixture in most current diesel vehicles, with minor adaptations. The quality requirements for this automotive diesel substitute can be found in standards. At present, the standard used in Europe for 100% fatty acid methyl esters (FAME) is the EN 14214:2008+A1:2009. In case of the produced biodiesel fail the standard requirements, due to raw material characteristics, it might be used in boilers for heat generation (depending on its characteristics); alternatively, raw materials might be mixed or the fuel blended with diesel to improve its quality [1]. Biodiesel production is generally made at a larger scale by a transesterification reaction, using selected feedstock, normally virgin vegetable oils [2]. However, their high prices and use as food

⇑ Corresponding author at: LEPAE, Departamento de Engenharia Metalúrgica e de Materiais, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal. Tel.: +351 22 5081422; fax: +351 22 5081447. E-mail address: [email protected] (J.M. Dias). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.04.032

resource (if edible) are limiting factors. Therefore, efforts have been made to find alternative raw materials. The use of waste materials brings a valuable contribution, not only in the reduction of biodiesel production costs but also in the better waste management results, avoiding subsequent environmental impacts. Waste frying oils are deeply studied as alternative raw materials [3–7]. The impact of this responsible application, even though relevant, is limited, considering their availability. Industrial waste oils and fats are however more abundant. When compared with biodiesel from vegetable origin, biodiesel from fats has the advantages of a higher calorific value and cetane number, being however less stable to oxidation and presenting a higher cold filter plugging point [8]. For countries with a considerable expression of the fish canning industry there is the possibility to use oily fish discarded parts as feedstock for the biodiesel production. The use of such alternative raw material, that represents a challenging waste management problem for the industries, is, however, still understudied. The oil extraction is performed mainly in marine oily fishes such as mackerel, salmon, tuna and codfish, which present a considerable oil content [9–13]. The usable parts, that can represent around 25 wt.% of the fish, are, in general, the viscera, head, fins, and tails, amongst others [12]. On the other hand, considering

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J.F. Costa et al. / Energy Conversion and Management 74 (2013) 17–23

the 2006 Best Available Techniques in the Food, Drink and Milk Industries Reference Document (BREF) [14], the expected range for solid wastes generated during fish processing is 20–60% of the catch, comprising heads, viscera, skins and other parts. Regarding Northern Portugal’s sea canning industry, the oily fishes transformed are mainly sardine, mackerel and tuna. Considering the information gathered from contact with the existing plants (information given on a confidential basis), it can be estimated that more than 150 t of fish oil are produced annually and, also, that more than 40 t are, in addition, washed away by the wastewaters. Currently, the reported fish oil that is extracted is being, essentially, commercialized for incorporation in animal feed or processed for use as a soil supplement. The energetic potential of such waste is however not being used by the industries. Table 1 presents the characteristics of the raw oil extracted from the discarded parts of various marine fishes. The elementary composition shows around 77 wt.% of carbon and 12 wt.% of hydrogen [15]. The most common fatty acids found in these fish oils, ordered by carbon chain increase, are: myristic (C14:0, up to 7 wt.%), palmitic (C16:0, up to 20 wt.%), palmitoleic (C16:1, up to 28 wt.%), stearic (C18:0, up to 7 wt.%), oleic (C18:1, up to 42 wt.%) and also significant amounts of polyunsaturated fatty acids (PUFA) such as arachidonic (C20:4, up to 3 wt.%), eicosapentaenoic (C20:5, up to 11 %), docosapentaenoic acid (C22:5, up to 15 %), and docosapentaenoic (C22:6, up to 39 wt.%) [9,10,12,16,17]. It should be noted that the PUFA are considered beneficial for feed purposes [18]; regarding biodiesel production, fluidity of the fuel is improved compared to other raw materials, which might be an advantage. On the other hand, the presence of PUFA might increase fuel instability since degradation tends to be accelerated in more unsaturated esters. Biodiesel current production process includes several steps: raw material pre-treatment (if necessary) to remove impurities and undesirable characteristics, transesterification reaction, phase separation and product purification (usually washing and drying) [2]. Common feedstock characterization to evaluate pre-treatment needs includes water content and acid value determination. In particular, the reduction of the acid value is an important pre-treatment as the removal of the excessive amount of free fatty acids is vital, especially when using alkaline catalyzed transesterification. The idea is to prevent the formation of soaps and consequently avoid a poorer catalytic activity and emulsifying soap effects that reduce biodiesel yield and quality [1,19]. Biodiesel transesterification reaction occurs between the triglyceride source and a short-chain alcohol (mostly methanol) to produce a mixture of FAME and glycerol. The reaction might be catalyzed by enzymes, acid or alkaline homogenous catalysts [20] and heterogeneous catalysts [21]. The most commonly used are the homogeneous alkaline catalysts [2,20] due to low cost and high

Table 1 Characteristics of fish oil extracted from marine wastes [10,15,16,20,25]. Parameter

Result

General characteristics

Dark brown, viscous liquid with a distinctive smell 0.05–0.26 0.1–28.4 88a 875.3–978.9 39.71–40.21 156.0–178.5 3.883–4.360

Water content (wt.%) Acid value (mg KOH g 1) Iodine value (g I2/100 g) Density (kg m 3) Calorific value (MJ kg 1) Flash point (°C) Kinematic viscosity at 40 °C (mm2 s 1) a

Wiggers et al., 2009.

efficiency. NaOH, KOH and their methoxydes are the commonly used catalysts. Since it is a reversible reaction, an excess of alcohol (usually 6:1 methanol to oil molar ratio) is usually used to force the reaction towards the products. When the fish oil presents high acidity, an acid esterification pre-treatment might be performed. In a study by El-Mashad et al. [10], an acid esterification using 1 wt.% H2SO4 and a reaction temperature around 52 °C, during 1 h, was performed (molar ratio methanol:oil of 6:1, 600 rpm stirring) to reduce the acid value of a salmon oil (3.5 or 12 mg KOH g 1) to values acceptable for alkaline transesterification (considering a maximum of 2 mg KOH g 1). The fish oil biodiesel synthesis, that follows pre-treatment, is usually, but not exclusively, reported as single stage alkaline transesterification process (methanolic route). Few studies have been performed in biodiesel production from fish oil; however, according studies found [9–13], common conditions include: (i) NaOH (1 wt.%) or KOH (0.5 wt.%) as catalysts; (ii) 6:1 to 9:1 methanol to oil molar ratio; (iii) reaction temperature between 50 and 60 °C; (iv) reaction time between 30 and 60 min; and, (v) stirring from 600 to 6000 rpm. These are conditions conventionally used in most alkaline transesterification reactions [2]. After phase separation, methyl esters/biodiesel purification is usually performed. First, the methanol in excess is recovered by distillation, water washing is used to remove the homogeneous catalyst and drying is performed to remove residual water [10– 13,16], as in general alkali transesterification processes. Regarding the characterization of the obtained biodiesel, Table 2 reports data collected from the revised literature and compares it with the general applicable requirements according to the European Biodiesel Standard EN 14214. The studies show that biodiesel quality varies considerably and that it might be difficult to fulfill some of the required parameters. Therefore, further studies are required to evaluate and if possible improve biodiesel quality. The biodiesel composition varies according to the fish oil characteristics and fatty acids found agree with the ones reported previously for oils. It should be, however, mentioned, that a greater range of values were found for the composition of FAME due to the great variability of fish oils and mixtures used as raw materials. In order to improve product quality, pre-treatment conditions should be studied and optimized. In a study by Dias et al. [1], the catalyst concentration during the acid esterification pre-treatment of a waste fat was found to be a key parameter.

Table 2 Quality parameters of biodiesel obtained from waste fish oil [9–13,16,17,20,27,28] and European Biodiesel Standard (EN 14214) requirements. Parameter

Result

EN 14214

Aspect

Transparent yellow, but varies depending on the feedstock nature/condition and the processing 619a

NA

0.26–1.19

60.50

860–889

860– 900 P51.0 P101 °C 3.50– 5.00

Water content (mg kg 1) Acid value (mg KOH g 1) Density (kg m 3) Cetane number Flash point (°C) Kinematic viscosity at 40 °C (mm2 s 1) Methyl ester content (wt.%) NA – Not applicable. a Fan et al., 2010. b Lin and Li, 2009.

b

50.9 103–220 4.0–7.2

95.74–100.00

6500

P96.5


The purity of the product is a very important parameter and it is generally difficult to achieve purities imposed by EN 14214 (>96.5 wt.%) using waste raw materials [1], even though according to some of the revised studies, high purities could be obtained. For the same raw material, higher purities are associated with lower viscosities, because triglycerides have higher viscosities than FAME. Using a two-step alkali transesterification, removing the produced glycerol in the first step, a biodiesel yield increase from 91% to 97% could be obtained by Ye et al. [22], through the reduction of reaction equilibrium constraints. Such process was however not previously studied for the transesterification of waste fish oil. Another way to improve fuel quality is the use of raw-material mixtures, preferably by incorporation of other wastes [8]. Olive is one of the most important agricultural products of Mediterranean countries [23] and Portugal is widely known for its production; however, the waste management associated is still a challenge. Olive bagasse is the solid residue obtained from the mechanical oil extraction of olives. Such waste still presents relevant oil content; however, it usually has a high acidity, impairing its application for food purposes. The olive oil presents high content of linoleic acid and lower content of PUFA [24] which makes it a suitable raw material for mixture to balance quality issues resulting from the use of fish oil. According to what was previously stated, the goal of the present work was to study the pre-treatment and reaction conditions to produce biodiesel using oil extracted from Portuguese fish canning industry wastes. With that purpose, the variation of the acid catalyst concentration in the esterification pre-treatment was studied and a two-step alkali transesterification was after performed aiming to maximize FAME production and deal with the quality issues previously referred. Finally, the use of a mixture, including waste oil obtained from olive oil bagasse was studied aiming biodiesel quality improvement. 2. Materials and methods 2.1. Experimental design The present study was conducted using Design of Experiments (DOE). This is a useful way to test factors and their interaction effects in the process response, within a limited range and a minimum of experiments and time. JMP 5.0.1 software was used, considering the following approaches: Response Surface Design (two factors, three levels) – RSD 32, with Central Composite Design (two central points, one replica). Therefore, 20 experiments were due to be made in advance, in a random way, to minimize undesirable disturbances or uncontrollable external factors effects. The chosen factors, or process variables, were: catalyst concentration during esterification, ranging from 1 to 3 wt.%, and, the amount of methanolic solution used in the first transesterification stage, varying from 60 to 90 vol.%. Apart from the biodiesel quality parameters selected as response variables, product yield (weight of product/weight of oil 100) was also used. The determination of product yield is important, because one can have very high purities but low yields (less product, with high purity), namely due to the formation of soaps when high catalyst concentrations are used [2]. For economical evaluations, the product yield is very important. 2.2. Materials Fish oil was donated by Savinor S. A. (Trofa, Portugal) and was kept in the refrigerator at 4 °C during the experimental period. Waste oil, extracted from olive oil bagasse, was supplied by a local company and kept under the same conditions as the fish oil. The

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waste olive oil presented an acid value of 21.85 mg KOH g 1 and an iodine value of 71 cg I2 g 1. The methanolic acid solution for the esterification reaction was prepared with methanol min. 99.8% assay, AnalaR NORMAPUR from VWR International S.A.S. and sulfuric acid 95–97%, for analysis, EMSURE ISO from Merck. The methanolic solution prepared for the transesterification reaction used the same methanol but sodium hydroxide 97%, reagent grade, from Sigma–Aldrich. The biodiesel washings were performed using distilled water and 0.2 wt.% hydrochloric acid (prepared from hydrochloric acid 37%, A. C. S. reagent, from Aldrich). For GC analysis, n-heptane (for analysis, Merck) was used as solvent and, as internal standard, methyl heptadecanoate, analytical standard, Fluka Analytical, was used. A SupelcoÒ 37 Component FAME Mix (10 mg/mL in methylene chloride) was also used as analytical standard. 2.3. Methods 2.3.1. Oil and biodiesel characterization Oil properties measured were: (i) acid value, by volumetric titration as reported in NP EN ISO 660:2002; (ii) water content, by coulometric Karl Fischer titration, according to ISO 8534:996; (iii) iodine value by volumetric titration using Wijs reagent, according to the standard ISO 3961:1996; and (iv) oxidation stability at 110 °C, according to EN 14112. Oil composition was obtained from the methyl ester profile evaluated by GC analysis according to NP EN 5508:1996 and EN 14103:2003. Biodiesel quality was accessed by measuring: (i) density, by a hydrometer method according to ISO 3675:1998; (ii) kinematic viscosity, according to ISO 3104:1994; (iii) flash point, using a rapid equilibrium closed cup tester according to ISO 2160:1998; (iv) methyl ester content, using GC analysis according to EN 14103:2003; and (v) acid value, as reported in EN 14104:2003. Based on preliminary experiments, where all mentioned biodiesel quality properties were determined, and on a literature review and previous work [1], only some biodiesel properties were selected for the experimental planning. All parameters were measured in replicates and taking into account the error requirements imposed by each standard. Results are presented as mean values. 2.3.2. Biodiesel production The production process included dehydration, acid esterification and alkaline transesterification, divided in two steps. Dehydration: Fish oil was dehydrated by heating at around 100 °C until constant weight. Esterification: Acid esterification was performed in a 250 mL round-bottom glass reaction vessel, with three necks, immersed in a temperature controlling bath and equipped with a watercooled condenser fed by a circulating cooling bath at 4 °C. A thermometer was used to certify the reaction temperature. The vigorous stirring was performed using a magnetic stirrer, regulated to 900 rpm. For the reaction, the dehydrated fish oil was fed to the reactor followed by the homogeneous methanolic solution of methanol and catalyst (sulfuric acid). The esterification conditions were: 90 g of oil, 1, 2 or 3 wt.% of catalyst relative to oil weight and amount of methanol corresponding to a 6:1 methanol to oil molar ratio. The reaction was conducted at 65.0 °C, at ambient pressure, during 1 h. After reaction, excess methanol was removed from biodiesel in a rotary evaporator and the product was washed four times using distilled water (in equal volume). The final product was dehydrated by heating at around 100 °C, until constant weight was achieved.

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Reaction conditions were selected according to the literature review. Preliminary experiments were performed to evaluate the progress of the esterification reaction with time and support the selection of the reaction period. For that, two experiments were conducted using 120 g of oil, 2 wt.% catalyst and 65 °C during 5 h of esterification. Around 2 mL were sampled at 0.5, 1, 2, 3, 4 and at the end of the reaction. Samples were treated and acid value was determined at each sampling, according to Dias et al. [1]. The product obtained after 5 h was subjected to alkaline transesterification, described below, to evaluate the FAME quality in advance and select key quality parameters. Alkaline transesterification: The same apparatus as for the esterification reaction was used. The following reaction conditions were selected: 1 wt.% NaOH, methanol amount corresponding to a 6:1 methanol to oil molar ratio, temperature of 65 °C and 1.5 h of reaction. Two steps of 45 min each were performed. The amount of methanolic solution used in the first step varied from 60 to 90 vol.% (corresponding to the addition of 40% to 10% of solution in the second step, respectively) according to the experimental planning previously described (Section 2.1). After the first reaction, the glycerol was separated and the rest of the methanolic solution was added to perform the second stage of reaction for another 45 min. The final product was decanted overnight. The biodiesel purification consisted on distillation under vacuum to remove excess methanol followed by washing once with a hydrochloric acid solution (0.2 wt.% (m/m)) and after three times with distilled water (in equal volume). The remaining water was finally removed by heating at around 100 °C, until constant weight.

Table 3 Waste fish oil properties. Property

Result 1

a

Acid value (mg KOH g ) Water content (wt.%) Iodine value (cg I2 g 1) Oxidation stability @ 110 °C (h)

10.04 0.28 164 0.1

Methyl ester profile, wt.% Myristate (C14:0) Palmitate (C16:0) Palmitoleate (C16:1) Stearate (C18:0) Oleate (C18:1) Linoleate (C18:2) Linolenate (C18:3) Eicosenoate (C20:1) Eicosapentanoate (C20:5) Docosenoate (C22:1) Docosadienoate (C22:2) Docosapentanoate (C22:6) Mean molecular weight (g mol

6.6 21.6 8.0 4.1 17.3 1.7 2.9 4.2 13.3 3.8 1.7 14.8 882.6

1 a

)

Obtained from the methyl ester composition.

3. Results and discussion 3.1. Raw material properties The resume of the fish oil properties is presented in Table 3. Regarding the acid value, the values agree with the range reported in previous studies (between 0.1 [20] and 28.4 mg KOH g 1 [25]). Taking into account the high value found, it is clear the need of a pre-treatment to allow alkali transesterification (usually less than 2 mg KOH g 1 is demanded). The iodine value of many fish oils is in fact higher than 150 cg I2 g 1 [26], as it was found for the present oil, although in the study by Wiggers et al. [25], the acid value of the waste oil was determined as being 88 cg I2 g 1. The differences relate mostly to the type of fish but also to the source of the oil and the conditions at which the oil was exposed. The water content was not very high, as found in revised studies [25]. The composition of fish oil agrees with the values generally reported, described in the introduction chapter. As previously stated, fish oil differs considerable from other oils, namely due to its content of PUFA [18]. However, due to its high content, these oils are very prone to oxidation. In fact, results from the present study indicate that this oil presents very low oxidation stability. From the point of view of nutrition, this is a very important subject. Considering the use for biodiesel production, raw material oxidation is also detrimental, as it increases the acid value and reduces the quality of the oil. The results show that a great care is required during the storage of such oil to prevent oxidation.

Fig. 1. Evolution of waste oil acid value during the esterification reaction (mean values of two experiments, differences below 5%; reaction conditions: 65 °C, 2 wt.% H2SO4, 6:1 M ratio of methanol to oil).

3.2.1. Progress of the esterification reaction Fig. 1 shows the evolution of the acid value with time during the acid esterification of the oil. According to the results, the time required for the esterification was selected as 1 h. The acid values are high due to the presence of H2SO4 in the reaction media. The results agree with what has been reported for the esterification of acid waste lard [1]. 3.2.2. FAME quality The product from the esterification reaction was further transesterified (according to Section 2.3.2) and the quality of the FAME obtained was analyzed. Results are presented in Table 4. Table 4 Biodiesel quality in preliminary experiments, using waste fish oil, and European biodiesel quality requirements (results outside the standard presented in bold). Property

3.2. Preliminary experiments Preliminary experiments were conducted to evaluate the progress of the esterification reaction as well as the quality of the produced FAME to support the selection of the conditions for the design of experiments.

1

Acid value (mg KOH g ) Density (kg m 3) Kinematic viscosity (mm2 s 1) Flash point (°C) Methyl ester content (wt.%) a

Resulta

EN 14214

0.90 891.8 4.41 162 78.2

60.50 860–900 3.50–5.00 P101 °C P96.5

Mean values, differences always less than 5%.

J.F. Costa et al. / Energy Conversion and Management 74 (2013) 17–23 Table 5 Resume of results obtained regarding yield, biodiesel quality parameters and comparison with European biodiesel quality requirements (results outside the standard presented in bold). Parameter

Result

EN 14214

Yield Kinematic viscosity at 40 °C (mm2 s Acid value (mg KOH g 1) Methyl ester content (wt.%)

66.4–73.9 4.32–4.59 0.32–0.85 73.0–75.5

NA 3.50–5.00 60.50 P96.5

1

)

NA – Not applicable.

Fig. 2. Product yield obtained varying the concentration of the H2SO4 catalyst during esterification and the volume of the methanolic solution used in the first transesterification stage (mean of relative percentage differences around 6%).

The preliminary quality results showed that the acid value of FAME was still higher than the one imposed by the European quality standard. The flash point was considerably higher than the minimum imposed and the density and viscosity of biodiesel were within the standard requirements. The methyl ester content was below the minimum required value. Taking into account the preliminary results, the experimental plan considered the evaluation of the acid value, methyl ester content and kinematic viscosity. The kinematic viscosity was still evaluated as it is closely related with the methyl ester content and could help to evaluate specific trends. 3.3. Experimental planning According to the planning described in Section 2.1, 20 experiments were performed aiming to evaluate the impact on product

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quality of the acid concentration during esterification and the two step transesterification using different ratios of the methanolic solution at each step. Table 5 presents the resume of the results obtained for the analyzed variables in all conditions studied. It can be seen that the parameters concerning the yield and the acid value presented some degree of variation within the experimental range whereas the viscosity and methyl ester content presented small variation. In addition, some of the parameters were not in agreement with the biodiesel standard. Considering the preliminary experiments, the results differ on the acid value; the maximum purity obtained was in agreement with the values found previously (differences less than 3%); however, under different reaction conditions. In order to understand the influence of the studied variables in each parameter, an individual discussion of the results will be performed in more detail from now on. 3.3.1. Product yield The variation of the product yield with the studied variables is presented in Fig. 2. In general, the higher yields were obtained for the lowest catalyst amount (1 wt.%) and the increase in catalyst concentration led to a decrease in product yield. This is probably related with the fact that, as it was observed experimentally, at higher catalyst concentrations the purification of the lipid phase during washing leads to the formation of emulsions, responsible for high product losses. The highest yield was obtained at the lowest catalyst concentration and 60 vol.% of the methanolic solution in the first stage (73.9 wt.%). The lower yield was obtained for 3 wt.% catalyst and 90 vol.% of the methanolic solution (66.4 wt.%). The results also showed that biodiesel yield generally decreased when the amount of methanolic solution used in the first step increased. The results relate to the amounts in the first step of 60, 75 and 90 vol.% of solution at the different catalyst concentrations, which results in the addition of 40, 25 and 10 vol.% in the second step, respectively. Therefore, it seemed to be more relevant, considering this parameter, to use more methanolic solution in the second step (in this case the maximum was 40 vol%.) to allow greater product conversion (note that the difference between the highest and lowest yield was 7.5 wt.%). 3.3.2. Kinematic viscosity and biodiesel purity The kinematic viscosity of biodiesel presented a small variation among experiments, ranging between 4.32 and 4.60 mm2 s 1. The lower viscosities were generally obtained at the lowest catalyst concentration (Fig. 3A), except when 75 vol.% solution was used in the first step, where the lowest viscosity was obtained at

Fig. 3. (A) Kinematic viscosity @ 40 °C, and (B) purity obtained varying the concentration of the H2SO4 catalyst during esterification and the volume of the methanolic solution used in the first transesterification stage (mean of relative percentage differences less than 4%).

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2 wt.%. Under all the reaction conditions studied, none of the products reached the minimum purity imposed by the EN 14214, of 96.5 wt.% (Fig. 3B). The results seem to indicate that no advantage regarding the viscosity occurs using higher catalyst concentrations, since higher viscosities can many times be attributed to lower purity of the product. The trend verified for the viscosity was confirmed and validated by the purity results, which clearly present an inverse trend. In fact, the highest purity was achieved generally at the lowest catalyst concentration, except when 75 vol.% solution was used in the first step, where the highest purity was obtained at 2 wt.%. Although trends were identified, the differences between the results for purity, as for the viscosity, were not relevant (the differences between the maximum and minimum values were overall less than 3%).

3.3.3. Acid value The general applicable requirements according to the European Biodiesel Standard EN 14214 state that the fuel should present a maximum acid value of 0.50 mg KOH g 1. The experimental results showed a great variation of this parameter (between 0.32 and 0.85 mg KOH g 1) and also, a difficulty in obtaining low relative percentage differences between replicates. In fact, using the different amounts of methanolic solution and also different acid catalyst amounts in esterification, it was possible to obtain a product that either complied or was very far from the standard limit. These results are even so relevant, as they seem to indicate that differences occurred not by changing the studied variables but by variations during the purification procedures, when using such waste oil. As this is a raw material with high degree of PUFA, the results indicate that the acid value of the product depends upon the purification procedures and storage, and that minor variations, for instances during washing procedures and storage period prior to analysis might result in changes in this parameter. This study shows, therefore, that this is a parameter that requires additional care when designing the biodiesel production process using waste fish oil. According to the results obtained from the experimental planning, and taking into consideration the preliminary results, it seems that using such waste fish oil as raw material, the use of a catalyst concentration of 1 wt.% H2SO4 during the esterification leads to better results regarding the product yield and quality. However, the use of a two-step transesterification, using different amounts of the methanolic solution in each step does not seem to bring advantages in terms of the results found. To confirm this hypothesis, an experiment was conducted by performing an acid esterification using the selected catalyst concentration (1 wt.% catalyst) followed by an alkali one step transesterification under the conditions previously described, during 1.5 h. The results are presented in Table 6, together with the results retained from experimental planning and preliminary results, for

comparison. The oxidation stability of the product was also measured, being very low (0.1 h). From the results, it is clear that for such raw-material, there is no advantage of performing a two-step alkali transesterification, since results obtained using a one-step reaction showed that the same product yield and quality is achieved. Therefore, the best conditions were selected as 1 wt.% H2SO4 for the esterification reaction followed by a one step alkali transesterification, considering all the other conditions previously described. However, it should not be ignored that using such raw material, the methyl ester content is significantly below the minimum required to be used as automotive fuel in 100% and also that oxidation stability is very low. The methyl ester content was in fact a very difficult parameter to fulfill and such challenge has been reported for acid raw materials and other wastes [1,8]. The FAME content obtained was less than expected according to the revision performed (95.74–100.00 wt.%). The analysis of such studies confirms that different fish oils were used. Behçet [9] used a anchovy fish oil with low acidity; El-Mashad et al. [10] used salmon oil, with lower acid value (3.47 mg KOH g 1); Fan et al. [16] used a fish oil with a much lower acidity, 0.28 mg KOH g 1; Lin and Li [13] used a refined mixture of marine fish oil obtained from discarded parts of various marine fishes (after the crude fish oil production, a refinement process followed, removing water, fish residue, saline compounds and others); Parente et al. [17] used commercial fish oil. These last two studies used only alkali transesterification due to low acidity. Apparently, the waste fish oil used in the present study lacked the optimum qualities required to achieve the high methyl ester contents reported. Therefore, it seems that this type of fish oil cannot be used pure as raw material for biodiesel production. Ideally, a mixture of such fish oil with other lipid waste of high acid value (presenting low commercial value and management difficulties) should be used, aiming to attenuate the quality issues found. 3.4. Mixture of waste fish oil and waste olive oil for biodiesel production For the present work, and as a preliminary study to access potential, a mixture containing 80 wt.% waste olive oil and 20 wt.% of waste fish oil was evaluated. The selection of the mixture composition was performed according to the amount of available raw material and expected impact on product quality; namely the need to increase fuel stability and methyl ester content. Regarding the fatty acid composition, the results showed that the waste olive oil presented a high content of oleic acid (C18:1, 72.9 wt.%) and the other most relevant acids found were the palmitic (C16:0, 13.9 wt.%) and the linoleic acid (C18:1, 8.6 wt.%). Small contents of stearic, palmitoleic and linolenic acid were also found (2.2, 1.4 and 0.9 wt.% respectively).

Table 6 Results comparing the yield and quality parameters, obtained using one-step transesterification (OST) and two-step transesterification (TST) after acid esterification of fish oil under selected catalyst concentration. Parameter Yield (wt.%) Kinematic viscosity @ 40 °C (mm2 s Acid value (mg KOH g 1) Methyl ester content (wt.%)

1

)

1 wt.% catalyst, OST

1 wt.% Catalyst, TSTa

2 wt.% Catalyst, OSTb

73.5 4.27 0.55 75.0

73.9 4.32 0.62 75.5

NA 4.41 0.90 78.2

NA – Not applicable. a Results from experimental planning, where 60 vol.% of methanolic solution was used in the first stage. b Results from preliminary experiments.


The conditions under which biodiesel production was performed were the ones selected according to the previous results (1 wt.% H2SO4 for esterification followed by one step alkali transesterification). The results of the quality of the fuel obtained showed a methyl ester content of 90.2 wt.% (compared to 75 wt.% using the fish oil alone) and an oxidation stability of 6.2 h (compared to 0.1 h obtained using the fish oil alone). These results clearly show the potential of using such wastes, of very low value, in mixture for biodiesel production. Biodiesel purity increased 15.2 wt.% and the oxidation stability is in agreement with the biodiesel quality standard values (P6 h). Results are promising; however, it is a fact that biodiesel purity, although much higher, is still slightly lower than the one required for use 100% as automotive fuel. The application of such fuel might be done for heating purposes (boilers) but the impact of its use in diesel motors will depend upon the blending percentages with diesel and also on long hour engine operation behavior. Accordingly, detailed studies are being conducted regarding the impact of using such fuel in a diesel motor, pure, and in mixture with fossil diesel fuel.

4. Conclusions The present study showed that it is possible to produce biodiesel using acid oil extracted from the fish canning industry wastes. The process established comprised an acid esterification pre-treatment followed by a basic transesterification. The results from experimental planning showed that catalyst concentration during esterification mostly influenced product yield and quality, the best conditions being 1 wt.% catalyst and 60 vol.% of methanolic solution. Under such conditions, a product yield of 73.9 wt.% and a product purity of 75.5 wt.% was obtained. With the selected catalyst concentration, results from a one-step reaction showed that no significant improvement occurs by using two steps (a product yield of 73.5 wt.% and product purity of 75.0 wt.% was obtained). Although several quality parameters were in agreement with standard specifications, a great variation was found in the biodiesel acid value, which showed a high sensitivity of the product to purification and storage, due to the low oxidation stability; in addition, methyl ester content was always below the minimum required for use as 100% diesel fuel (96.5 wt.%). Due to the fact that the waste fish oil used in the present study lacked the optimum qualities required to achieve the high methyl ester content and oxidation stability, a mixture containing 80 wt.% waste olive oil and 20 wt.% of waste fish oil was evaluated using the reaction conditions previously selected. Using such mixture, biodiesel purity increased to 90.2 wt.% and the oxidation stability was in agreement with the biodiesel quality standard values (P6 h), which are promising results.

Acknowledgments The authors thank the Project ‘‘ValorPeixe’’ (QREN 13634) and J.M. Dias thanks the FCT for the fellowship SFRH/BPD/73809/2010.

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