- Email: [email protected]
Electrochemical impedance: A new alternative to assess the soap removal from biodiesel in the washing process
Fuel 265 (2020) 116880
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Electrochemical impedance: A new alternative to assess the soap removal from biodiesel in the washing process
T
⁎
L. Díaz-Ballotea, , L. Maldonadoa, J. Genescab, E.R. Hoil-Canula, T. Vega-Lizamaa a b
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Km 6, antg. Carr a Progreso, Mérida, Yuc. 97310, Mexico Departamento Ingeniería Metalúrgica. Facultad Química. Universidad Nacional Autónoma de México, UNAM. Ciudad Universitaria. 04510 ciudad de México, Mexico
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Soap Biodiesel Impedance Sensor Soybean Waste cooking oil
An impedance- based sensor was built with stainless steel circular plates (7.0 cm2) to evaluate the soap limit for both biodiesels produced from commercial soybean oil and from waste cooking oil. Conversion of soybean oil to biodiesel was achieved with KOH and waste cooking oil was converted to biodiesel using NaOH. Various biodiesel batches were dry washed using a mixed ionic resin to remove charged carriers such as soaps. Based on the Nyquist and Bode plots as well as the physical shape of the sensor, an RbCb parallel circuit was used for modelling the experimental data. It was found that washing had a significant effect on Rb in comparison with the effect on Cb. From the impedance data acquired using the present sensor, an Rb value of 5 GΩ·cm2 was chosen as the minimum biodiesel resistance to assure a soap limit of below 41 ppm to comply with the currently accepted soap level in biodiesel.
1. Introduction The washing process is one of the most important steps in biodiesel production, it has a tremendous impact on biodiesel quality and, consequently, on public acceptance. High quality biodiesel is achieved by meeting the ASTM D6751 standard, and public acceptance is needed for
⁎
assuring its commercial success. Wet washing is a common method of biodiesel purification, but the use of dry washing by ion exchange resins has been growing during the last decade among biodiesel producers, as well as novel methods such as the use of membrane technology separation [1,2] or ionic liquids. Regardless of the washing method chosen, it is useful to know the effectiveness of the washing process in
Corresponding author. E-mail address: [email protected] (L. Díaz-Ballote).
https://doi.org/10.1016/j.fuel.2019.116880 Received 9 July 2019; Received in revised form 11 December 2019; Accepted 13 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
electrodes, determined the content of biodiesel in a diesel/biodiesel mixture. Their method was based on the linear relationship between the Rct (charge transfer resistance) and the biodiesel level in the diesel/ biodiesel blends. Kung [22] also described an impedimetric sensor made of carbon paste used to determine the biodiesel content in diesel/ biodiesel blends. Based on measuring the dielectric constant, De Souza et al. [23] have developed another sensor for measuring the biodiesel content in diesel/biodiesel blends. The electrochemical impedance has also been demonstrated to be useful in determining the water content of biodiesel [19]. According to the procedure described by Delfino et al. [19] the biodiesel sample was diluted with concentrated acetonitrile, poured into an electrochemical cell (500 mL) with two parallel stainless steel plates to determine the charge transfer resistance which was used as an indicator of the water content. The key impurities (soap and catalysts) combined with the successful use of impedance-based sensors in biofuels applications encouraged the development of a novel impedance-based sensor to detect the soap removal. Therefore, the aim of the present study was to evaluate the potential use of an impedancebased sensor in a two-electrode arrangement as an alternative reagentless method to estimate the satisfactory threshold limit of soap content in a biodiesel sample. The sensor is designed to be as simple as possible, robust and with few maintenance requirements. For this reason, the sensor was constructed based on the two-plate capacitor design with biodiesel between plates.
removing impurities, which should be evaluated by some form of assessment. However, there is currently no standard indicator or criteria that is widely accepted to evaluate the effectiveness of the washing process. Subsequent to the washing process, different properties related to the impurities are often measured and the requirement for further reduction of the impurities is used as an assessment of the effectiveness of the washing process [3]. For example, acidity index, alkalinity, glycerin, and turbidity of biodiesel were used as indicators of the efficiency of a purification process in the study of starch and cellulose as absorbents [4]. The impurities commonly found in the biodiesel can be grouped in nonelectrolytes (biodiesel, methanol, glycerol and water) and electrolytes (soaps and catalyst) [5]. One of the main undesired contaminants is the soap, which is formed through a saponification reaction (Free fatty acids + KOH → Water + Potassium soap) that occurs in the same reactor as the transesterification [6]. Basically, soaps are metallic salts of aliphatic carboxylic acids with a long nonpolar hydrophobic tail and a charged hydrophilic head [7], and its content in biodiesel has been considered one of the main criteria to evaluate the wash process efficiency [8]. Various methods have been used to determine soaps: Titration (AOCS method Cc 17–79) [9], atomic absorption [10], flame photometry [11] and neutron activation [10]. Although none of the international standards (ASTM D6751 nor UN 14214) provide a maximum threshold limit for soap level as contaminants in biodiesel, the ASTM D6751 specify a maximum for the total combined amount of alkali metals (5 ppm) that can be expressed as soap. The accepted soap content corresponds to 66 ppm or 41 ppm depending on the alkali metal used in the biodiesel preparation, potassium (K) or sodium (Na) respectively [12]. As previously mentioned, soaps and catalysts are considered electrolytes, meaning that they can transport charge [13,14], and therefore their presence can confer a degree of conductivity in biodiesel [15]. Therefore, the removal of soap is expected to increase the impedance of biodiesel. In the last decade, impedance-based sensors have been demonstrated to be an environmentally friendly [16] alternative because commonly it is reusable; the measurement is free of solvent/chemicals; it is not destructive; easy to use and can be operated relatively quickly. Impedance describes the resistance of an electrical circuit to the charge flow in the presence of an alternating current (ac). It is commonly represented with a phasor which is mathematically described by a complex number, ZT(w) = ZRe(w) + j·ZIm(w), with magnitude and phase given by Eqs. (1) and (2) [13,17].
|ZT (w )| =
θ = tan−1
ZRe (w )2 + ZIm (w )2
ZIm (w ) ZRe (w )
2. Materials and methods 2.1. Materials The main chemicals used in this study were methanol, KOH, NaOH, bromophenol blue, hydrochloric acid and isopropyl alcohol, all reagent grade, purchased from Sigma-Aldrich. The raw materials for the transesterification process were purified soybean oil acquired from local convenience stores and waste cooking oil (WCO) from different local restaurants. Resin from a mixed bed deionization cartridge (intelifil, series-SM, IF-SM-DIO10C), designed to remove all electrically charged contaminants (anions and cations), usually in water, was used as the absorbent for the biodiesel purification by the dry washing process.
2.2. Biodiesel preparation
(1)
Biodiesel from soybean oil (hereafter referred to as BioSoy) and biodiesel from WCO (hereafter referred to as BioWCO) was prepared following the procedure described by Díaz-Ballote et al. [24]. However, the catalyst used in each case was different, KOH was used to obtain biodiesel from soybean oil and NaOH was used to obtain biodiesel from WCO. Basically, 150 g of soybean oil or WCO was used with a 6:1 methanol/oil molar ratio, and 1% (mass/mass oil) of the catalyst at 600 rpm and 40 °C was used for soybean oil or 60 °C for WCO. The WCO was filtered before the transesterification process (250-mm sieve) to remove residual food.
(2)
where ZT(w) is the total impedance of the circuit, ZRe(w) is the real component, ZIm(w) is the imaginary component, j is the imaginary number, w is the angular frequency and [is the phase of the phasor. ZRe(w) provides information about the pure resistive component, meanwhile ZIm(w) provides information about the reactive component (capacitance or inductance). The electrical parameters of the circuit are obtained by fitting the response of an electrical circuit model to the experimental curves generated by plotting ZIm(w) against ZRe(w) (Nyquist plot) or ZT(w) against frequency (Bode plot). It is worth mentioning that one of the simplest electrical circuit models is a twoparallel plate capacitor in which the capacitance depends on its geometry and the conductive nature of the material between the plates. The electrical properties of the material have been key to the development of some impedance-based sensors. Although, biodiesel is nonconductive and the application of electrochemical methods in this media using traditional three-electrode cell (working, auxiliary and reference electrode) to obtain reliable data is impractical [18], the electrochemical impedance using the two-electrode setting has demonstrated to be a useful tool to address the problem of high biodiesel resistance [18–21]. Pereira et al. [20] using two identical stainless-steel
2.3. Dry washing process Biodiesel derived from refined soybean and waste cooking oils was dry washed with an ion exchange resin three times (steps). The number of washes carried out on a biodiesel sample are coded as 0TF for biodiesel unwashed, 1TF, 2TF and 3TF for one, two and three times dry washed. In each washing step, 10% (mass/mass biodiesel) of fresh-resin and general-purpose filter paper (5–13 mm particle retention) were placed inside a glass funnel (diameter 10 cm) to filter the biodiesel sample by gravity. At the end of the washing step, the soap content and impedance were determined from a sample of biodiesel. 2
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
2.4. Electrochemical impedance
3. Experimental results and discussions
The impedance measurements were carried out with a two-electrode mode Gamry potentiostat (model reference 600). The frequency range and amplitude of the AC signal were from 100 kHz to 5 mHz and 50 mV, respectively. Impedance was determined from samples of unwashed biodiesel taken immediately following the transesterification process and from samples of biodiesel dry washed with the ion exchange resin. The experiments were conducted in the following order; firstly, at least three measurements were taken to determine the viability of the sensor and the impedance measurements. Then, the impedance spectrum was recorded at intervals of 30 min for 8 h. Finally, three impedance curves were generated omitting waiting time between each measurement.
In this study, the effect of dry washing (filtration with ion exchange resin) was assessed by measuring the impedance of biodiesel samples using the sensor. Fig. 2a, shows the Nyquist plot of a typical unwashed (inside Fig.) or zero times filtered (0TF) Biosoy as well as a Nyquist plot of Biosoy three times filtered (3TF). The Nyquist plot of the 0TF biodiesel sample shows at low frequency, a dispersion which is attributed to uneven distribution of time constants [21]. This dispersion approaches zero the less contaminated the biodiesel. In Fig. 2a it can be observed that the radii of the semicircle increases dramatically with the washing process. This result agrees with the expected result because, in practice, the charge carrier is removed from the biodiesel sample in each washing step and because of the reduction in the charge carrier, the biodiesel impedance increases. In particular, biodiesel, derived from waste cooking oil, commonly contains a high level of soap due to the high level of free fatty acids, which are inherited from the parent oil [26]. It is important to highlight that the impedance not only increases as a function of the washing step, but there is also two magnitudes of order between the impedance of biodiesel 0TF and biodiesel 3TF which indicates that the impedance is highly sensitive to the presence of charged species. Fig. 2b, shows the Bode plots of an unwashed biodiesel (0TF) and a biodiesel three times filtered (3TF). The impedance modulus behavior confirms the large difference between both unwashed and washed biodiesel.
2.5. Sensor Two-parallel plates (1 mm thick) composed of 304 stainless steel with a circular shape and 3 cm in diameter (approx. area, 7 cm2) were used to build the sensor. The sensor itself is a capacitor filled with biodiesel as dielectric material. Separation between plates was achieved using a polyvinyl chloride (PVC) disc of 4 mm in diameter with a hole centered about ⅛” (approx. 3.1 mm) in diameter. The disc thickness was 0.8 mm. Both plates were fixed with a PVC screw of ⅛” (approx. 3.1 mm). Two stainless steel rods were used as the contact leads for the plates. Fig. 1 shows an image of the sensor measuring the impedance of a biodiesel sample after the washing process. The stainless-steel sensor is also shown alone in the inset Figure.
3.1. Kramers-Kroning test The results are encouraging, and it was decided to continue validating the impedance curves. The reliability of the impedance measurement is a necessary condition to propose a new potential method to determine the efficiency of the washing process. Since impedance results must obey the principles of linearity, causality and stability in order for the data to be reliable, some typical impedance measurements were checked by kramers–kronig transformation [20,27]. Fig. 3 displays a good fit between the impedance data of biodiesel samples derived from different feedstocks (soybean and WCO), and even between different catalysts (KOH and NaOH). The residual error for the real and imaginary components of the impedance were also below 2% which indicates that the samples obey the principles of linearity, causality and stability demonstrating the reliability of the impedance data [28]. Randomly chosen samples gave similar results.
2.6. Soap test Bromophenol blue soap tests were carried out to determine the soap content after each washing step. Briefly, 10 g of biodiesel were dissolved in 100 mL of isopropyl alcohol and 20 drops of bromophenol blue were added. Then the sample was titrated with 0.01 M HCl. Each sample was conducted in triplicate [25].
3.2. Equivalent circuit Physically, the sensor is a two-plate capacitor and, on the other hand, Nyquist and Bode curves clearly represent the typical behavior of a capacitor [14]. Therefore, a resistance and capacitor in parallel is the more appropriate equivalent circuit for modelling the impedance data (see Fig. 4). The real and imaginary components of the impedance of the equivalent circuit in terms of Rb and Cb are expressed by [23] Eqs. (3) and (4),
ZRe (w ) =
Rb 1 + w 2Rb2Cb2
jZIm (w ) = j
wRb2Cb 1 + w 2Rb2Cb2
(3)
(4)
Rb represents the electrical resistance of biodiesel due to the presence of ionic species and Cb represents the capacitance of the two plate capacitor used as a sensor with washed or unwashed biodiesel filling the gap between plates. Both parameters Rb and Cb were calculated from the real and imaginary components of the impedance obtained by fitting the equivalent circuit to the experimental impedance data using the software Echem Analyst from Gamry Instrument. It is noted in Fig. 5a
Fig. 1. The impedance-based sensor measuring biodiesel impedance within a Faraday cage. The isolated device is shown in the inset Figure. 3
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
Fig. 2. Typical (a) Nyquist and (b) Bode plots for unwashed (0TF) and three times filtered (3TF) biodiesel sample.
these results, Rb was chosen as an indicator of the soap level in biodiesel.
that Rb clearly increases as a function of the number of washing steps but this value rapidly increases for biodiesel derived from commercial soybean oil as would be expected because it is derived from a purified and clean oil. On the other hand, biodiesel derived from waste cooking oil usually contains more free fatty acids and undesirable contaminants. Therefore, it is harder to remove ionic contaminants by the dry wash process. In contrast, it is observed (Fig. 5b) that the washing step does not have a significant influence on the sensor capacitance. Based on
3.3. Sensor application in biodiesel washing process. Five biodiesel samples obtained from two different feedstocks and two catalysts (NaOH and KOH) were dry washed three times, using a fresh and clean ionic resin each time. Impedance and soap content were
Fig. 3. Experimental data of typical Nyquist curves of biodiesel from a) soybean and, c) waste cooking oils three times filtered, and the Kramers-Kronig fit (solid line). The corresponding residual variations are also displayed in Figs. b and d. 4
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
Fig. 4. The equivalent electrical circuit used for modelling the two-plate capacitor as a sensor. Rb represents the resistance of the biodiesel between the plates and Cb represents the capacitance of the capacitor used as a sensor.
determined in unwashed biodiesel as well as in biodiesel after each washing step. Fig. 6 shows the behavior between soap content and Rb derived from the impedance curve. It can be observed that each sample of biodiesel has a different initial soap content, and that soap removal is not the same amount in each wash step. A potential explanation is the influence of various factors that can change between each batch and in turn, the initial impurity content. Some examples of such factors are stirring, presence of ionic species and catalyst, biodiesel contamination with water, and free fatty acids. In addition, the path that biodiesel takes when it transitions through the adsorbent particles and the activity of each particle that contacts the biodiesel can be different in any new wash. As can be seen in Fig. 6, samples of BioWCO, which was derived from the most contaminated feedstock, has the greatest initial soap content. For this reason, it possesses a low resistance (high conductivity). In contrast, biodiesel derived from refined soybean oil (BioSoy) exhibits an initial low soap content and decreases rapidly with increasing number of washes. In general, the real component of impedance increases when the soap content decreases. The recommended amount of soap in biodiesel is 41 ppm (0.0041%) in the case of NaOH and 66 ppm (0.0066%) for KOH [12,25]. The lowest soap limit of 41 ppm is indicated in Fig. 6. It is evident that the ion exchange resin was more effective for cleaning biodiesel derived from refined soybean oil than for cleaning biodiesel derived from WCO. Note that below the threshold soap limit (41 ppm for NaOH) there is a sudden increase of Rb indicating a low content of charge carrier impurities and the effective removal of soap. Below 41 ppm of soap, impurities are well diluted and under this condition the ion-ion interaction becomes complex making it hard to obtain a linear correlation between soap and impedance. This especially true when soap is used as an indicator but instead represents several charge carrying species. Without a linear correlation an analysis of accuracy (recovery test) [23], precision and sensitivity would not be suitable and reliable in
Fig. 6. Behavior of the electrical resistance (Rb) as a function of the soap content in biodiesel from different batches.
the present study. However, a threshold of impedance as an indicator of the purification level would be a very useful alternative. A Rb value equal to or greater than 5 GΩ·cm2 fulfills the maximum soap content that is widely accepted in biodiesel. It seems to be that the resistance value is neither dependent on the type of biodiesel nor the catalyst used in the present study, during the conversion process. Therefore, the results suggest that 5 GΩ⋅cm2 could be used as a minimum resistance value for biodiesel to assure that the soap content is low enough to comply with the maximum amount of soap allowed in biodiesel. Therefore, this study proposes an alternative to the soap test which is typically carried out by titration of biodiesel dissolved in a solvent such as acetone [9,29] or isopropyl alcohol (100 mL) [25,30] using bromophenol blue to determine the end point when titrated with 0.01 N HCl. The amount of HCl consumed is recorded and used to calculate the soap content in the biodiesel sample as described previously. Other methods found in the literature for soap determination are: Flame atomic absorption spectrometry (FAAS), this method requires a sample preparation of a biodiesel microemulsion using n-propanol, water and nitric acid before the analysis [31]. Inductively coupled plasma optical emission spectrometer (ICP-OES) this method determines alkali metals that could be converted into soap later. The sample requires microwave assisted acid digestion before the metal determination [32]. Neutron activation analysis is also used for soap determination, in which up to forty elements can be determined using this highly sensitivite, and
Fig. 5. average values of a) Rb and b) Cb calculated from the impedance curves of biodiesel samples from different batches; showing changes made to each parameter as a function of the washing step. The error (one sigma deviation) cannot be seen in each plot because it is small in comparison with the corresponding bar. 5
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
accurate method. However, the instrument is not readily available to most laboratories [33]. Due to the pretreatment some of these methods are time consuming, costly or use uncommon instruments [10]. Nevertheless, mathematics for the data analysis, [34] reproducibility [35] and interpretation of results [36] are highlighted as disadvantages for the impedance measurements. The method has been widely used and has advantages in sensor applications such as its potential miniaturization for in-situ determinations, relative simplicity, use of cheaper electrodes with simple electronics, [37] non-destructiviness, [38] and its unique capability to provide information about the properties of the solution and different processes at the electrode surface [27]. For example, charge transport, mass diffusion or the resistance of the solution [39]. Moreover, the impedance measurement does not require laborious sample pretreatments that usually include use of reagents and solvents. A sensor like this one can stimulate the development of novel washing processes and consequently, can contribute to the improvement of the quality of commercial biodiesel efficiently.
[3]
[4]
[5]
[6] [7] [8]
[9]
[10]
[11]
4. Conclusions [12]
In the present study, a simple capacitor was built with a pair of parallel circular plates made of stainless steel AISI type 304. Impedance measurements in two-electrode arrangements combined with the capacitor, which functions like a sensor, have been demonstrated to be a simple, yet novel method to determine whether biodiesel fulfills the accepted soap limit. The physical shape of the sensor and the impedance curves (one constant time) suggest the use of a resistor (Rb) and a capacitor (Cb) in parallel as the equivalent electrical circuit for modelling the impedance spectra. From the results, a value Rb equal or greater than 5 GΩ·cm2 was proposed as an indicator to determine whether the soap content in biodiesel is low enough to comply with the maximum amount of soap allowed in biodiesel. The resistance value of 5 GΩ⋅cm2 is independent of the biodiesel type or of the catalysts (KOH or NaOH) used in the oil to biodiesel conversion. The use of this impedance-based sensor can reduce the use of solvents during soap measurements, among other reagents used to determine soap in biodiesel, and it could be a potential alternative to the use of complex analytical tools.
[13] [14]
[15]
[16] [17] [18]
[19]
[20]
CRediT authorship contribution statement
[21]
L. Díaz-Ballote: Conceptualization, Visualization, Writing - original draft. L. Maldonado: Writing - review & editing, Validation. J. Genesca: Writing - review & editing, Validation. E.R. Hoil-Canul: Investigation. T. Vega-Lizama: Investigation.
[22]
[23]
Declaration of Competing Interest [24]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[25]
Acknowledgements
[26]
Special thanks to Ernesto A. Barrera-Chavelas (research student) for conducting the impedance measurements of several samples. The measurements were performed on the electrochemical systems acquired through CONACYT grant 205050/2013.
[27]
[28]
References [29] [1] Atadashi IM. Purification of crude biodiesel using dry washing and membrane technologies. Alex Eng J 2015;54(4):1265–72. https://doi.org/10.1016/j.aej.2015. 08.005. [2] Wang Y, Wang X, Liu Y, Ou S, Tan Y, Tang S. Refining of biodiesel by ceramic
[30]
6
membrane separation. Fuel Process Technol 2009;90(3):422–7. https://doi.org/10. 1016/j.fuproc.2008.11.004. Hingu SM, Gogate PR, Rathod VK. Synthesis of biodiesel from waste cooking oil using sonochemical reactors. Ultrason Sonochem 2010;17(5):827–32. https://doi. org/10.1016/j.ultsonch.2010.02.010. Gomes MG, Santos DQ, de Morais LC, Pasquini D. Purification of biodiesel by dry washing, employing starch and cellulose as natural adsorbents. Fuel 2015;155:1–6. https://doi.org/10.1016/j.fuel.2015.04.012. Di Felice R, De Faveri D, De Andreis P, Ottonello P. Component distribution between light and heavy phases in biodiesel processes. Ind Eng Chem Res 2008;47(20):7862–7. https://doi.org/10.1021/ie800510w. Gerpen JV. Biodiesel processing and production. Fuel Process Technol 2005;86(10):1097–107. https://doi.org/10.1016/j.fuproc.2004.11.005. Meyers RA. Encyclopedia of physical science and technology. 3rd ed. California, USA: Academic Press; 2001. Ott LS, Riddell MM, O'Neill EL, Carini GS. From orchids to biodiesel: Coco coir as an effective drywash material for biodiesel fuel. Fuel Process Technol 2018;176:1–6. https://doi.org/10.1016/j.fuproc.2018.02.023. American Oil Chemical Society (AOCS) Method Cc 17-79, Soap in oil, titrimetric method. Official Methods and Recommended Practices of the AOCS 7th Ed, Urban IL 1996. Gegiou D. Determination of soap in refined vegetable oils by atomic-absorption spectrophotometry. Analyst 1974;99(1184):745–8. https://doi.org/10.1039/ AN9749900745. Black LT. Direct determination of sodium in soybean oil by flame photometry. J Am Oil Chem Soc 1970;47(9):313–5. https://doi.org/10.1007/BF02638992. White JM, Shah P, Sanford S, Valverde M, Meier G. Feasibility Study to Determine Soap Concentration in Biodiesel from Alkali Metal Content. Ames IA: Renewable Energy Group, Inc.; 2010. Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and applications. 2nd ed. New York, USA: John Wiley and Sons, Inc.; 2001. Rocha JW, Vicente MA, Melo BN, Maria de Lourdes S, Guimarães RC, Sad CM, et al. Investigation of electrical properties with medium and heavy Brazilian crude oils by electrochemical impedance spectroscopy. Fuel 2019;241:42–52. Sekar N, Ramasamy RP. Electrochemical impedance spectroscopy for microbial fuel cell characterization. Microbial Biochem Technol 2013;6(2):1–14. https://doi.org/ 10.4172/1948-5948.S6-004. Amais RS, Teixeira LS, Rocha FR. Greener procedures for biodiesel quality control. Anal Methods 2015;7(11):4396–418. https://doi.org/10.1039/c5ay00530b. Barsoukov E, Macdonald JR. Impedance spectroscopy: theory, experiment, and applications. New York, USA: John Wiley & Sons, Inc.; 2018. Park I-J, Nam T-H, Kim J-H, Kim J-G. Evaluation of corrosion characteristics of aluminum alloys in the bio-ethanol gasoline blended fuel by 2-electrode electrochemical impedance spectroscopy. Fuel 2014;126:26–31. https://doi.org/10.1016/ j.fuel.2014.02.030. Delfino JR, Pereira TC, Viegas HDC, Marques EP, Ferreira AAP, Zhang L, et al. A simple and fast method to determine water content in biodiesel by electrochemical impedance spectroscopy. Talanta 2018;179:753–9. https://doi.org/10.1016/j. talanta.2017.11.053. Pereira TC, Conceição CA, Khan A, Fernandes RM, Ferreira MS, Marques EP, et al. Application of electrochemical impedance spectroscopy: a phase behavior study of babassu biodiesel-based microemulsions. Spectrochim Acta A 2016;168:60–4. https://doi.org/10.1016/j.saa.2016.05.034. Perini N, Prado A, Sad C, Castro E, Freitas M. Electrochemical impedance spectroscopy for in situ petroleum analysis and water-in-oil emulsion characterization. Fuel 2012;91(1):224–8. https://doi.org/10.1016/j.fuel.2011.06.057. Kung Y, Hsieh B-C, Cheng T-J, Huang C-K, Chen RL. Impedimetric sensing of the biodiesel contents in diesel fuels with a carbon paste electrode pair. Fuel 2012;102:724–8. https://doi.org/10.1016/j.fuel.2012.06.023. De Souza J, Scherer M, Cáceres J, Caires A, M’Peko J-C. A close dielectric spectroscopic analysis of diesel/biodiesel blends and potential dielectric approaches for biodiesel content assessment. Fuel 2013;105:705–10. https://doi.org/10.1016/j. fuel.2012.09.032. Díaz-Ballote L, Gómez-Hernández K, Vega-Lizama ET, Ruiz-Gómez MA, Maldonado L, Hernández E. Thermogravimetric Approach for Assessing the oxidation Level of a Biodiesel Sample. Quím Nova 2018;41(5):492–6. https://doi.org/10.21577/01004042.20170199. Gross EM, Williams S, Williams E, Dobberpuhl D, Fujita J. Synthesis and characterization of biodiesel from used cooking oil: a problem-based green chemistry laboratory experiment. American Chemical Society; 2016. Leung Y, Chen G. Biodiesel production using waste cooking oil from restaurant. In: Cheng P, ed. Symposium on Energy Engineering for the 21st Century (SEE2000). 21. KowLoon, Hong Kong: Begell House, Inc.; 2000:1553-9. Pereira TC, Delfino JR, Ferreira AA, Barros FJS, Marques EP, Zhang J, et al. Stainless steel electrodes to determine biodiesel content in petroleum diesel fuel by electrochemical impedance spectroscopy. Electroanalysis 2017;29(3):814–20. https://doi.org/10.1002/elan.201600504. De Faria RAD, Lins VdFC, Nappi GU, Matencio T, Heneine LGD. Development of an Impedimetric Immunosensor for Specific Detection of Snake Venom. BioNanoScience 2018;8(4):988–96. https://doi.org/10.1007/s12668-018-0559-7. Wall J, Van Gerpen J, Thompson J. Soap and Glycerin Removal from Biodiesel Using Waterless Processes. T ASABE 2011;54(2):535–41. https://doi.org/10. 13031/2013.36456. Atadashi I, Aroua M, Aziz AA, Sulaiman N. Crude biodiesel refining using membrane ultra-filtration process: An environmentally benign process. Egypt J Pet 2015;24(4):383–96. https://doi.org/10.1016/j.ejpe.2015.10.001.
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
https://doi.org/10.1016/0300-9440(95)00581-1. [36] Bandarenka AS. Exploring the interfaces between metal electrodes and aqueous electrolytes with electrochemical impedance spectroscopy. Analyst 2013;138(19):5540–54. https://doi.org/10.1039/C3AN00791J. [37] Hammond JL, Formisano N, Estrela P, Carrara S, Tkac J. Electrochemical biosensors and nanobiosensors. Essays Biochem 2016;60(1):69–80. https://doi.org/10.1042/ EBC20150008. [38] Rocha MdS, Simões-Moreira J. A simple impedance method for determining ethanol and regular gasoline mixtures mass contents. Fuel 2005;84(4):447–52. https://doi. org/10.1016/j.fuel.2004.09.011. [39] Kashyap D, Dwivedi PK, Pandey JK, Kim YH, Kim GM, Sharma A, et al. Application of electrochemical impedance spectroscopy in bio-fuel cell characterization: A review. Int J Hydrogen Energy 2014;39(35):20159–70. https://doi.org/10.1016/j. ijhydene.2014.10.003.
[31] Lyra FH, Carneiro MTWD, Brandão GP, Pessoa HM, de Castro EV. Determination of Na, K, Ca and Mg in biodiesel samples by flame atomic absorption spectrometry (F AAS) using microemulsion as sample preparation. Microchem J 2010;96(1):180–5. https://doi.org/10.1016/j.microc.2010.03.005. [32] Iqbal J, Carney WA, LaCaze S, Theegala CS. Metals determination in biodiesel (B100) by ICP-OES with microwave assisted acid digestion. Open Anal Chem J 2010;4:18–26. [33] Umar I. Determination of trace elements in some Nigerian vegetable based oils by neutron activation analysis. J Radioanal Nucl Chem 2001;249(3):669–71. https:// doi.org/10.1023/A:1013231222908. [34] Macdonald DD. Review of mechanistic analysis by electrochemical impedance spectroscopy. Electrochim Acta 1990;35(10):1509–25. https://doi.org/10.1016/ 0013-4686(90)80005-9. [35] Amirudin A, Thieny D. Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals. Prog Org Coat 1995;26(1):1–28.
7
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Electrochemical impedance: A new alternative to assess the soap removal from biodiesel in the washing process
T
⁎
L. Díaz-Ballotea, , L. Maldonadoa, J. Genescab, E.R. Hoil-Canula, T. Vega-Lizamaa a b
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Km 6, antg. Carr a Progreso, Mérida, Yuc. 97310, Mexico Departamento Ingeniería Metalúrgica. Facultad Química. Universidad Nacional Autónoma de México, UNAM. Ciudad Universitaria. 04510 ciudad de México, Mexico
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Soap Biodiesel Impedance Sensor Soybean Waste cooking oil
An impedance- based sensor was built with stainless steel circular plates (7.0 cm2) to evaluate the soap limit for both biodiesels produced from commercial soybean oil and from waste cooking oil. Conversion of soybean oil to biodiesel was achieved with KOH and waste cooking oil was converted to biodiesel using NaOH. Various biodiesel batches were dry washed using a mixed ionic resin to remove charged carriers such as soaps. Based on the Nyquist and Bode plots as well as the physical shape of the sensor, an RbCb parallel circuit was used for modelling the experimental data. It was found that washing had a significant effect on Rb in comparison with the effect on Cb. From the impedance data acquired using the present sensor, an Rb value of 5 GΩ·cm2 was chosen as the minimum biodiesel resistance to assure a soap limit of below 41 ppm to comply with the currently accepted soap level in biodiesel.
1. Introduction The washing process is one of the most important steps in biodiesel production, it has a tremendous impact on biodiesel quality and, consequently, on public acceptance. High quality biodiesel is achieved by meeting the ASTM D6751 standard, and public acceptance is needed for
⁎
assuring its commercial success. Wet washing is a common method of biodiesel purification, but the use of dry washing by ion exchange resins has been growing during the last decade among biodiesel producers, as well as novel methods such as the use of membrane technology separation [1,2] or ionic liquids. Regardless of the washing method chosen, it is useful to know the effectiveness of the washing process in
Corresponding author. E-mail address: [email protected] (L. Díaz-Ballote).
https://doi.org/10.1016/j.fuel.2019.116880 Received 9 July 2019; Received in revised form 11 December 2019; Accepted 13 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
electrodes, determined the content of biodiesel in a diesel/biodiesel mixture. Their method was based on the linear relationship between the Rct (charge transfer resistance) and the biodiesel level in the diesel/ biodiesel blends. Kung [22] also described an impedimetric sensor made of carbon paste used to determine the biodiesel content in diesel/ biodiesel blends. Based on measuring the dielectric constant, De Souza et al. [23] have developed another sensor for measuring the biodiesel content in diesel/biodiesel blends. The electrochemical impedance has also been demonstrated to be useful in determining the water content of biodiesel [19]. According to the procedure described by Delfino et al. [19] the biodiesel sample was diluted with concentrated acetonitrile, poured into an electrochemical cell (500 mL) with two parallel stainless steel plates to determine the charge transfer resistance which was used as an indicator of the water content. The key impurities (soap and catalysts) combined with the successful use of impedance-based sensors in biofuels applications encouraged the development of a novel impedance-based sensor to detect the soap removal. Therefore, the aim of the present study was to evaluate the potential use of an impedancebased sensor in a two-electrode arrangement as an alternative reagentless method to estimate the satisfactory threshold limit of soap content in a biodiesel sample. The sensor is designed to be as simple as possible, robust and with few maintenance requirements. For this reason, the sensor was constructed based on the two-plate capacitor design with biodiesel between plates.
removing impurities, which should be evaluated by some form of assessment. However, there is currently no standard indicator or criteria that is widely accepted to evaluate the effectiveness of the washing process. Subsequent to the washing process, different properties related to the impurities are often measured and the requirement for further reduction of the impurities is used as an assessment of the effectiveness of the washing process [3]. For example, acidity index, alkalinity, glycerin, and turbidity of biodiesel were used as indicators of the efficiency of a purification process in the study of starch and cellulose as absorbents [4]. The impurities commonly found in the biodiesel can be grouped in nonelectrolytes (biodiesel, methanol, glycerol and water) and electrolytes (soaps and catalyst) [5]. One of the main undesired contaminants is the soap, which is formed through a saponification reaction (Free fatty acids + KOH → Water + Potassium soap) that occurs in the same reactor as the transesterification [6]. Basically, soaps are metallic salts of aliphatic carboxylic acids with a long nonpolar hydrophobic tail and a charged hydrophilic head [7], and its content in biodiesel has been considered one of the main criteria to evaluate the wash process efficiency [8]. Various methods have been used to determine soaps: Titration (AOCS method Cc 17–79) [9], atomic absorption [10], flame photometry [11] and neutron activation [10]. Although none of the international standards (ASTM D6751 nor UN 14214) provide a maximum threshold limit for soap level as contaminants in biodiesel, the ASTM D6751 specify a maximum for the total combined amount of alkali metals (5 ppm) that can be expressed as soap. The accepted soap content corresponds to 66 ppm or 41 ppm depending on the alkali metal used in the biodiesel preparation, potassium (K) or sodium (Na) respectively [12]. As previously mentioned, soaps and catalysts are considered electrolytes, meaning that they can transport charge [13,14], and therefore their presence can confer a degree of conductivity in biodiesel [15]. Therefore, the removal of soap is expected to increase the impedance of biodiesel. In the last decade, impedance-based sensors have been demonstrated to be an environmentally friendly [16] alternative because commonly it is reusable; the measurement is free of solvent/chemicals; it is not destructive; easy to use and can be operated relatively quickly. Impedance describes the resistance of an electrical circuit to the charge flow in the presence of an alternating current (ac). It is commonly represented with a phasor which is mathematically described by a complex number, ZT(w) = ZRe(w) + j·ZIm(w), with magnitude and phase given by Eqs. (1) and (2) [13,17].
|ZT (w )| =
θ = tan−1
ZRe (w )2 + ZIm (w )2
ZIm (w ) ZRe (w )
2. Materials and methods 2.1. Materials The main chemicals used in this study were methanol, KOH, NaOH, bromophenol blue, hydrochloric acid and isopropyl alcohol, all reagent grade, purchased from Sigma-Aldrich. The raw materials for the transesterification process were purified soybean oil acquired from local convenience stores and waste cooking oil (WCO) from different local restaurants. Resin from a mixed bed deionization cartridge (intelifil, series-SM, IF-SM-DIO10C), designed to remove all electrically charged contaminants (anions and cations), usually in water, was used as the absorbent for the biodiesel purification by the dry washing process.
2.2. Biodiesel preparation
(1)
Biodiesel from soybean oil (hereafter referred to as BioSoy) and biodiesel from WCO (hereafter referred to as BioWCO) was prepared following the procedure described by Díaz-Ballote et al. [24]. However, the catalyst used in each case was different, KOH was used to obtain biodiesel from soybean oil and NaOH was used to obtain biodiesel from WCO. Basically, 150 g of soybean oil or WCO was used with a 6:1 methanol/oil molar ratio, and 1% (mass/mass oil) of the catalyst at 600 rpm and 40 °C was used for soybean oil or 60 °C for WCO. The WCO was filtered before the transesterification process (250-mm sieve) to remove residual food.
(2)
where ZT(w) is the total impedance of the circuit, ZRe(w) is the real component, ZIm(w) is the imaginary component, j is the imaginary number, w is the angular frequency and [is the phase of the phasor. ZRe(w) provides information about the pure resistive component, meanwhile ZIm(w) provides information about the reactive component (capacitance or inductance). The electrical parameters of the circuit are obtained by fitting the response of an electrical circuit model to the experimental curves generated by plotting ZIm(w) against ZRe(w) (Nyquist plot) or ZT(w) against frequency (Bode plot). It is worth mentioning that one of the simplest electrical circuit models is a twoparallel plate capacitor in which the capacitance depends on its geometry and the conductive nature of the material between the plates. The electrical properties of the material have been key to the development of some impedance-based sensors. Although, biodiesel is nonconductive and the application of electrochemical methods in this media using traditional three-electrode cell (working, auxiliary and reference electrode) to obtain reliable data is impractical [18], the electrochemical impedance using the two-electrode setting has demonstrated to be a useful tool to address the problem of high biodiesel resistance [18–21]. Pereira et al. [20] using two identical stainless-steel
2.3. Dry washing process Biodiesel derived from refined soybean and waste cooking oils was dry washed with an ion exchange resin three times (steps). The number of washes carried out on a biodiesel sample are coded as 0TF for biodiesel unwashed, 1TF, 2TF and 3TF for one, two and three times dry washed. In each washing step, 10% (mass/mass biodiesel) of fresh-resin and general-purpose filter paper (5–13 mm particle retention) were placed inside a glass funnel (diameter 10 cm) to filter the biodiesel sample by gravity. At the end of the washing step, the soap content and impedance were determined from a sample of biodiesel. 2
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
2.4. Electrochemical impedance
3. Experimental results and discussions
The impedance measurements were carried out with a two-electrode mode Gamry potentiostat (model reference 600). The frequency range and amplitude of the AC signal were from 100 kHz to 5 mHz and 50 mV, respectively. Impedance was determined from samples of unwashed biodiesel taken immediately following the transesterification process and from samples of biodiesel dry washed with the ion exchange resin. The experiments were conducted in the following order; firstly, at least three measurements were taken to determine the viability of the sensor and the impedance measurements. Then, the impedance spectrum was recorded at intervals of 30 min for 8 h. Finally, three impedance curves were generated omitting waiting time between each measurement.
In this study, the effect of dry washing (filtration with ion exchange resin) was assessed by measuring the impedance of biodiesel samples using the sensor. Fig. 2a, shows the Nyquist plot of a typical unwashed (inside Fig.) or zero times filtered (0TF) Biosoy as well as a Nyquist plot of Biosoy three times filtered (3TF). The Nyquist plot of the 0TF biodiesel sample shows at low frequency, a dispersion which is attributed to uneven distribution of time constants [21]. This dispersion approaches zero the less contaminated the biodiesel. In Fig. 2a it can be observed that the radii of the semicircle increases dramatically with the washing process. This result agrees with the expected result because, in practice, the charge carrier is removed from the biodiesel sample in each washing step and because of the reduction in the charge carrier, the biodiesel impedance increases. In particular, biodiesel, derived from waste cooking oil, commonly contains a high level of soap due to the high level of free fatty acids, which are inherited from the parent oil [26]. It is important to highlight that the impedance not only increases as a function of the washing step, but there is also two magnitudes of order between the impedance of biodiesel 0TF and biodiesel 3TF which indicates that the impedance is highly sensitive to the presence of charged species. Fig. 2b, shows the Bode plots of an unwashed biodiesel (0TF) and a biodiesel three times filtered (3TF). The impedance modulus behavior confirms the large difference between both unwashed and washed biodiesel.
2.5. Sensor Two-parallel plates (1 mm thick) composed of 304 stainless steel with a circular shape and 3 cm in diameter (approx. area, 7 cm2) were used to build the sensor. The sensor itself is a capacitor filled with biodiesel as dielectric material. Separation between plates was achieved using a polyvinyl chloride (PVC) disc of 4 mm in diameter with a hole centered about ⅛” (approx. 3.1 mm) in diameter. The disc thickness was 0.8 mm. Both plates were fixed with a PVC screw of ⅛” (approx. 3.1 mm). Two stainless steel rods were used as the contact leads for the plates. Fig. 1 shows an image of the sensor measuring the impedance of a biodiesel sample after the washing process. The stainless-steel sensor is also shown alone in the inset Figure.
3.1. Kramers-Kroning test The results are encouraging, and it was decided to continue validating the impedance curves. The reliability of the impedance measurement is a necessary condition to propose a new potential method to determine the efficiency of the washing process. Since impedance results must obey the principles of linearity, causality and stability in order for the data to be reliable, some typical impedance measurements were checked by kramers–kronig transformation [20,27]. Fig. 3 displays a good fit between the impedance data of biodiesel samples derived from different feedstocks (soybean and WCO), and even between different catalysts (KOH and NaOH). The residual error for the real and imaginary components of the impedance were also below 2% which indicates that the samples obey the principles of linearity, causality and stability demonstrating the reliability of the impedance data [28]. Randomly chosen samples gave similar results.
2.6. Soap test Bromophenol blue soap tests were carried out to determine the soap content after each washing step. Briefly, 10 g of biodiesel were dissolved in 100 mL of isopropyl alcohol and 20 drops of bromophenol blue were added. Then the sample was titrated with 0.01 M HCl. Each sample was conducted in triplicate [25].
3.2. Equivalent circuit Physically, the sensor is a two-plate capacitor and, on the other hand, Nyquist and Bode curves clearly represent the typical behavior of a capacitor [14]. Therefore, a resistance and capacitor in parallel is the more appropriate equivalent circuit for modelling the impedance data (see Fig. 4). The real and imaginary components of the impedance of the equivalent circuit in terms of Rb and Cb are expressed by [23] Eqs. (3) and (4),
ZRe (w ) =
Rb 1 + w 2Rb2Cb2
jZIm (w ) = j
wRb2Cb 1 + w 2Rb2Cb2
(3)
(4)
Rb represents the electrical resistance of biodiesel due to the presence of ionic species and Cb represents the capacitance of the two plate capacitor used as a sensor with washed or unwashed biodiesel filling the gap between plates. Both parameters Rb and Cb were calculated from the real and imaginary components of the impedance obtained by fitting the equivalent circuit to the experimental impedance data using the software Echem Analyst from Gamry Instrument. It is noted in Fig. 5a
Fig. 1. The impedance-based sensor measuring biodiesel impedance within a Faraday cage. The isolated device is shown in the inset Figure. 3
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
Fig. 2. Typical (a) Nyquist and (b) Bode plots for unwashed (0TF) and three times filtered (3TF) biodiesel sample.
these results, Rb was chosen as an indicator of the soap level in biodiesel.
that Rb clearly increases as a function of the number of washing steps but this value rapidly increases for biodiesel derived from commercial soybean oil as would be expected because it is derived from a purified and clean oil. On the other hand, biodiesel derived from waste cooking oil usually contains more free fatty acids and undesirable contaminants. Therefore, it is harder to remove ionic contaminants by the dry wash process. In contrast, it is observed (Fig. 5b) that the washing step does not have a significant influence on the sensor capacitance. Based on
3.3. Sensor application in biodiesel washing process. Five biodiesel samples obtained from two different feedstocks and two catalysts (NaOH and KOH) were dry washed three times, using a fresh and clean ionic resin each time. Impedance and soap content were
Fig. 3. Experimental data of typical Nyquist curves of biodiesel from a) soybean and, c) waste cooking oils three times filtered, and the Kramers-Kronig fit (solid line). The corresponding residual variations are also displayed in Figs. b and d. 4
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
Fig. 4. The equivalent electrical circuit used for modelling the two-plate capacitor as a sensor. Rb represents the resistance of the biodiesel between the plates and Cb represents the capacitance of the capacitor used as a sensor.
determined in unwashed biodiesel as well as in biodiesel after each washing step. Fig. 6 shows the behavior between soap content and Rb derived from the impedance curve. It can be observed that each sample of biodiesel has a different initial soap content, and that soap removal is not the same amount in each wash step. A potential explanation is the influence of various factors that can change between each batch and in turn, the initial impurity content. Some examples of such factors are stirring, presence of ionic species and catalyst, biodiesel contamination with water, and free fatty acids. In addition, the path that biodiesel takes when it transitions through the adsorbent particles and the activity of each particle that contacts the biodiesel can be different in any new wash. As can be seen in Fig. 6, samples of BioWCO, which was derived from the most contaminated feedstock, has the greatest initial soap content. For this reason, it possesses a low resistance (high conductivity). In contrast, biodiesel derived from refined soybean oil (BioSoy) exhibits an initial low soap content and decreases rapidly with increasing number of washes. In general, the real component of impedance increases when the soap content decreases. The recommended amount of soap in biodiesel is 41 ppm (0.0041%) in the case of NaOH and 66 ppm (0.0066%) for KOH [12,25]. The lowest soap limit of 41 ppm is indicated in Fig. 6. It is evident that the ion exchange resin was more effective for cleaning biodiesel derived from refined soybean oil than for cleaning biodiesel derived from WCO. Note that below the threshold soap limit (41 ppm for NaOH) there is a sudden increase of Rb indicating a low content of charge carrier impurities and the effective removal of soap. Below 41 ppm of soap, impurities are well diluted and under this condition the ion-ion interaction becomes complex making it hard to obtain a linear correlation between soap and impedance. This especially true when soap is used as an indicator but instead represents several charge carrying species. Without a linear correlation an analysis of accuracy (recovery test) [23], precision and sensitivity would not be suitable and reliable in
Fig. 6. Behavior of the electrical resistance (Rb) as a function of the soap content in biodiesel from different batches.
the present study. However, a threshold of impedance as an indicator of the purification level would be a very useful alternative. A Rb value equal to or greater than 5 GΩ·cm2 fulfills the maximum soap content that is widely accepted in biodiesel. It seems to be that the resistance value is neither dependent on the type of biodiesel nor the catalyst used in the present study, during the conversion process. Therefore, the results suggest that 5 GΩ⋅cm2 could be used as a minimum resistance value for biodiesel to assure that the soap content is low enough to comply with the maximum amount of soap allowed in biodiesel. Therefore, this study proposes an alternative to the soap test which is typically carried out by titration of biodiesel dissolved in a solvent such as acetone [9,29] or isopropyl alcohol (100 mL) [25,30] using bromophenol blue to determine the end point when titrated with 0.01 N HCl. The amount of HCl consumed is recorded and used to calculate the soap content in the biodiesel sample as described previously. Other methods found in the literature for soap determination are: Flame atomic absorption spectrometry (FAAS), this method requires a sample preparation of a biodiesel microemulsion using n-propanol, water and nitric acid before the analysis [31]. Inductively coupled plasma optical emission spectrometer (ICP-OES) this method determines alkali metals that could be converted into soap later. The sample requires microwave assisted acid digestion before the metal determination [32]. Neutron activation analysis is also used for soap determination, in which up to forty elements can be determined using this highly sensitivite, and
Fig. 5. average values of a) Rb and b) Cb calculated from the impedance curves of biodiesel samples from different batches; showing changes made to each parameter as a function of the washing step. The error (one sigma deviation) cannot be seen in each plot because it is small in comparison with the corresponding bar. 5
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
accurate method. However, the instrument is not readily available to most laboratories [33]. Due to the pretreatment some of these methods are time consuming, costly or use uncommon instruments [10]. Nevertheless, mathematics for the data analysis, [34] reproducibility [35] and interpretation of results [36] are highlighted as disadvantages for the impedance measurements. The method has been widely used and has advantages in sensor applications such as its potential miniaturization for in-situ determinations, relative simplicity, use of cheaper electrodes with simple electronics, [37] non-destructiviness, [38] and its unique capability to provide information about the properties of the solution and different processes at the electrode surface [27]. For example, charge transport, mass diffusion or the resistance of the solution [39]. Moreover, the impedance measurement does not require laborious sample pretreatments that usually include use of reagents and solvents. A sensor like this one can stimulate the development of novel washing processes and consequently, can contribute to the improvement of the quality of commercial biodiesel efficiently.
[3]
[4]
[5]
[6] [7] [8]
[9]
[10]
[11]
4. Conclusions [12]
In the present study, a simple capacitor was built with a pair of parallel circular plates made of stainless steel AISI type 304. Impedance measurements in two-electrode arrangements combined with the capacitor, which functions like a sensor, have been demonstrated to be a simple, yet novel method to determine whether biodiesel fulfills the accepted soap limit. The physical shape of the sensor and the impedance curves (one constant time) suggest the use of a resistor (Rb) and a capacitor (Cb) in parallel as the equivalent electrical circuit for modelling the impedance spectra. From the results, a value Rb equal or greater than 5 GΩ·cm2 was proposed as an indicator to determine whether the soap content in biodiesel is low enough to comply with the maximum amount of soap allowed in biodiesel. The resistance value of 5 GΩ⋅cm2 is independent of the biodiesel type or of the catalysts (KOH or NaOH) used in the oil to biodiesel conversion. The use of this impedance-based sensor can reduce the use of solvents during soap measurements, among other reagents used to determine soap in biodiesel, and it could be a potential alternative to the use of complex analytical tools.
[13] [14]
[15]
[16] [17] [18]
[19]
[20]
CRediT authorship contribution statement
[21]
L. Díaz-Ballote: Conceptualization, Visualization, Writing - original draft. L. Maldonado: Writing - review & editing, Validation. J. Genesca: Writing - review & editing, Validation. E.R. Hoil-Canul: Investigation. T. Vega-Lizama: Investigation.
[22]
[23]
Declaration of Competing Interest [24]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[25]
Acknowledgements
[26]
Special thanks to Ernesto A. Barrera-Chavelas (research student) for conducting the impedance measurements of several samples. The measurements were performed on the electrochemical systems acquired through CONACYT grant 205050/2013.
[27]
[28]
References [29] [1] Atadashi IM. Purification of crude biodiesel using dry washing and membrane technologies. Alex Eng J 2015;54(4):1265–72. https://doi.org/10.1016/j.aej.2015. 08.005. [2] Wang Y, Wang X, Liu Y, Ou S, Tan Y, Tang S. Refining of biodiesel by ceramic
[30]
6
membrane separation. Fuel Process Technol 2009;90(3):422–7. https://doi.org/10. 1016/j.fuproc.2008.11.004. Hingu SM, Gogate PR, Rathod VK. Synthesis of biodiesel from waste cooking oil using sonochemical reactors. Ultrason Sonochem 2010;17(5):827–32. https://doi. org/10.1016/j.ultsonch.2010.02.010. Gomes MG, Santos DQ, de Morais LC, Pasquini D. Purification of biodiesel by dry washing, employing starch and cellulose as natural adsorbents. Fuel 2015;155:1–6. https://doi.org/10.1016/j.fuel.2015.04.012. Di Felice R, De Faveri D, De Andreis P, Ottonello P. Component distribution between light and heavy phases in biodiesel processes. Ind Eng Chem Res 2008;47(20):7862–7. https://doi.org/10.1021/ie800510w. Gerpen JV. Biodiesel processing and production. Fuel Process Technol 2005;86(10):1097–107. https://doi.org/10.1016/j.fuproc.2004.11.005. Meyers RA. Encyclopedia of physical science and technology. 3rd ed. California, USA: Academic Press; 2001. Ott LS, Riddell MM, O'Neill EL, Carini GS. From orchids to biodiesel: Coco coir as an effective drywash material for biodiesel fuel. Fuel Process Technol 2018;176:1–6. https://doi.org/10.1016/j.fuproc.2018.02.023. American Oil Chemical Society (AOCS) Method Cc 17-79, Soap in oil, titrimetric method. Official Methods and Recommended Practices of the AOCS 7th Ed, Urban IL 1996. Gegiou D. Determination of soap in refined vegetable oils by atomic-absorption spectrophotometry. Analyst 1974;99(1184):745–8. https://doi.org/10.1039/ AN9749900745. Black LT. Direct determination of sodium in soybean oil by flame photometry. J Am Oil Chem Soc 1970;47(9):313–5. https://doi.org/10.1007/BF02638992. White JM, Shah P, Sanford S, Valverde M, Meier G. Feasibility Study to Determine Soap Concentration in Biodiesel from Alkali Metal Content. Ames IA: Renewable Energy Group, Inc.; 2010. Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and applications. 2nd ed. New York, USA: John Wiley and Sons, Inc.; 2001. Rocha JW, Vicente MA, Melo BN, Maria de Lourdes S, Guimarães RC, Sad CM, et al. Investigation of electrical properties with medium and heavy Brazilian crude oils by electrochemical impedance spectroscopy. Fuel 2019;241:42–52. Sekar N, Ramasamy RP. Electrochemical impedance spectroscopy for microbial fuel cell characterization. Microbial Biochem Technol 2013;6(2):1–14. https://doi.org/ 10.4172/1948-5948.S6-004. Amais RS, Teixeira LS, Rocha FR. Greener procedures for biodiesel quality control. Anal Methods 2015;7(11):4396–418. https://doi.org/10.1039/c5ay00530b. Barsoukov E, Macdonald JR. Impedance spectroscopy: theory, experiment, and applications. New York, USA: John Wiley & Sons, Inc.; 2018. Park I-J, Nam T-H, Kim J-H, Kim J-G. Evaluation of corrosion characteristics of aluminum alloys in the bio-ethanol gasoline blended fuel by 2-electrode electrochemical impedance spectroscopy. Fuel 2014;126:26–31. https://doi.org/10.1016/ j.fuel.2014.02.030. Delfino JR, Pereira TC, Viegas HDC, Marques EP, Ferreira AAP, Zhang L, et al. A simple and fast method to determine water content in biodiesel by electrochemical impedance spectroscopy. Talanta 2018;179:753–9. https://doi.org/10.1016/j. talanta.2017.11.053. Pereira TC, Conceição CA, Khan A, Fernandes RM, Ferreira MS, Marques EP, et al. Application of electrochemical impedance spectroscopy: a phase behavior study of babassu biodiesel-based microemulsions. Spectrochim Acta A 2016;168:60–4. https://doi.org/10.1016/j.saa.2016.05.034. Perini N, Prado A, Sad C, Castro E, Freitas M. Electrochemical impedance spectroscopy for in situ petroleum analysis and water-in-oil emulsion characterization. Fuel 2012;91(1):224–8. https://doi.org/10.1016/j.fuel.2011.06.057. Kung Y, Hsieh B-C, Cheng T-J, Huang C-K, Chen RL. Impedimetric sensing of the biodiesel contents in diesel fuels with a carbon paste electrode pair. Fuel 2012;102:724–8. https://doi.org/10.1016/j.fuel.2012.06.023. De Souza J, Scherer M, Cáceres J, Caires A, M’Peko J-C. A close dielectric spectroscopic analysis of diesel/biodiesel blends and potential dielectric approaches for biodiesel content assessment. Fuel 2013;105:705–10. https://doi.org/10.1016/j. fuel.2012.09.032. Díaz-Ballote L, Gómez-Hernández K, Vega-Lizama ET, Ruiz-Gómez MA, Maldonado L, Hernández E. Thermogravimetric Approach for Assessing the oxidation Level of a Biodiesel Sample. Quím Nova 2018;41(5):492–6. https://doi.org/10.21577/01004042.20170199. Gross EM, Williams S, Williams E, Dobberpuhl D, Fujita J. Synthesis and characterization of biodiesel from used cooking oil: a problem-based green chemistry laboratory experiment. American Chemical Society; 2016. Leung Y, Chen G. Biodiesel production using waste cooking oil from restaurant. In: Cheng P, ed. Symposium on Energy Engineering for the 21st Century (SEE2000). 21. KowLoon, Hong Kong: Begell House, Inc.; 2000:1553-9. Pereira TC, Delfino JR, Ferreira AA, Barros FJS, Marques EP, Zhang J, et al. Stainless steel electrodes to determine biodiesel content in petroleum diesel fuel by electrochemical impedance spectroscopy. Electroanalysis 2017;29(3):814–20. https://doi.org/10.1002/elan.201600504. De Faria RAD, Lins VdFC, Nappi GU, Matencio T, Heneine LGD. Development of an Impedimetric Immunosensor for Specific Detection of Snake Venom. BioNanoScience 2018;8(4):988–96. https://doi.org/10.1007/s12668-018-0559-7. Wall J, Van Gerpen J, Thompson J. Soap and Glycerin Removal from Biodiesel Using Waterless Processes. T ASABE 2011;54(2):535–41. https://doi.org/10. 13031/2013.36456. Atadashi I, Aroua M, Aziz AA, Sulaiman N. Crude biodiesel refining using membrane ultra-filtration process: An environmentally benign process. Egypt J Pet 2015;24(4):383–96. https://doi.org/10.1016/j.ejpe.2015.10.001.
Fuel 265 (2020) 116880
L. Díaz-Ballote, et al.
https://doi.org/10.1016/0300-9440(95)00581-1. [36] Bandarenka AS. Exploring the interfaces between metal electrodes and aqueous electrolytes with electrochemical impedance spectroscopy. Analyst 2013;138(19):5540–54. https://doi.org/10.1039/C3AN00791J. [37] Hammond JL, Formisano N, Estrela P, Carrara S, Tkac J. Electrochemical biosensors and nanobiosensors. Essays Biochem 2016;60(1):69–80. https://doi.org/10.1042/ EBC20150008. [38] Rocha MdS, Simões-Moreira J. A simple impedance method for determining ethanol and regular gasoline mixtures mass contents. Fuel 2005;84(4):447–52. https://doi. org/10.1016/j.fuel.2004.09.011. [39] Kashyap D, Dwivedi PK, Pandey JK, Kim YH, Kim GM, Sharma A, et al. Application of electrochemical impedance spectroscopy in bio-fuel cell characterization: A review. Int J Hydrogen Energy 2014;39(35):20159–70. https://doi.org/10.1016/j. ijhydene.2014.10.003.
[31] Lyra FH, Carneiro MTWD, Brandão GP, Pessoa HM, de Castro EV. Determination of Na, K, Ca and Mg in biodiesel samples by flame atomic absorption spectrometry (F AAS) using microemulsion as sample preparation. Microchem J 2010;96(1):180–5. https://doi.org/10.1016/j.microc.2010.03.005. [32] Iqbal J, Carney WA, LaCaze S, Theegala CS. Metals determination in biodiesel (B100) by ICP-OES with microwave assisted acid digestion. Open Anal Chem J 2010;4:18–26. [33] Umar I. Determination of trace elements in some Nigerian vegetable based oils by neutron activation analysis. J Radioanal Nucl Chem 2001;249(3):669–71. https:// doi.org/10.1023/A:1013231222908. [34] Macdonald DD. Review of mechanistic analysis by electrochemical impedance spectroscopy. Electrochim Acta 1990;35(10):1509–25. https://doi.org/10.1016/ 0013-4686(90)80005-9. [35] Amirudin A, Thieny D. Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals. Prog Org Coat 1995;26(1):1–28.
7