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Stability and microwave demulsification of water in castor oil biodiesel emulsions Bruno Bôscaro França, Bruno Nogueira, Cristine Carretoni, Fernando L P Pessoa, Márcio Figueredo Portilho, José Carlos Pinto, and Márcio Nele Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef401234w • Publication Date (Web): 13 Jan 2014 Downloaded from http://pubs.acs.org on January 24, 2014
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Stability and microwave demulsification of water in castor oil biodiesel emulsions
Bruno Bôscaro França1; Bruno Nogueira2; Cristine Carretoni2; Fernando Luiz Pellegrini Pessoa2; Márcio F. Portilho3; José Carlos Pinto1, Marcio Nele2* 1 – Programa de Engenharia Química, Instituto de Tecnologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, CP 68502, RJ, Brazil. 2 – Departamento de Engenharia Química, Escola de Química, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, CP 68502, RJ, Brazil.
[email protected] 3- Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello (Cenpes), Av. Horácio Macedo, 950, Cidade Universitária, Rio de Janeiro - RJ. CEP: 21.941-915 Keywords: Castor oil biodiesel; microwave; emulsion.
Abstract: Biodiesel may be produced by vegetable oil transesterification followed by purification steps of product sedimentation and water washing. The occurrence of stable emulsions during the purification steps in the biodiesel production process from castor oil is a severe problem that precludes its use as an industrial raw material. The stability behavior of emulsions formed by castor oil biodiesel-water with 10, 20 and 30% (w/w) of water content was studied using laser light profiling. Emulsion stability showed to decrease for emulsions with 10 to 20% of water, but emulsions with 30% of water showed a very high stability. Demulsification of water in biodiesel emulsions using microwave irradiation was also investigated in function of water content, separation temperature and stirring speed. Analysis of the separation efficiency results showed that the all variables were statistically significant but water content was the most important factor. Comparison of microwave irradiation with gravitational sedimentation showed that higher separation efficiency is obtained for emulsions submitted to the microwave irradiation.
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1. Introduction It is undisputable that the pathway to a more sustainable world is the diversification of the energetic matrix. In this context, the production of biodiesel from different sources is a subject of great relevance. Biodiesel can be produced by the transesterification of diverse oils and fats, followed by purification steps. The purity of final product from transesterification has a significant effect on its combustible properties1. Alcohol, generally in excess on the reaction to favor high conversions, catalyst and glycerol need to be removed from the biodiesel. Consecutive water washing steps of undiluted1-4 or diluted (ether1, hexane5) organic phase are suggested for base catalyzed transesterification. Furthermore, hexane extraction followed by water washing is also suggested6. Given its simplicity and good efficiency, water washing of the undiluted organic phase is the preferred method. During the base catalyzed
transesterification,
esters
saponification
can
occur,
or
even
the
7
triacylglycerides hydrolysis. According to Rinaldi et al., saponification consumes the basic catalyst and the presence of soap produced may stabilize emulsions formed during the washing process. Although soap formation is a serious process problem that leads to stable emulsions, the amphiphilic character of the ricinoleic acid methyl esters present biodiesel from castor oil promotes the stabilization of emulsions even under absence soap formation, preventing its use as an industrial feedstock. In an investigation about the influence of temperature, type of catalyst, agitation and water quantity on reaction and separation steps of biodiesel production, Korus et al.8 emphasize emulsion formation during the product separation. The emulsion formation lead to an increase on time needed for phase separation. Those results showed the need for investigations about emulsion stability from biodiesel-water and biodieselglycerol systems and mechanisms of demulsification. A number of demulsification techniques are used, for example, for water removal of crude oil emulsions could such as chemical demulsification, gravity or centrifugal settling, pH adjustment, filtration, heat treatment, membrane separation, and electrostatic demulsification9,10. Comparative studies11,12 show that the use of microwave promotes a better separation efficiency, in relation to classical heating treatments, that the use of microwave irradiation is associated to relatively smaller piece of equipments and that is a very fast method, which selectively heats the aqueous dispersed emulsion phase.
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Several variables may affect demulsification processes by microwave irradiation, such as initial emulsion properties, separation temperature, stirring speed, heating rate and time of irradiation12. The use of statistical experimental designs can lead to a quantitative variable effect evaluation in microwave demulsification. The main objective of this work is to investigate the stability of water-inbiodiesel (w/b) emulsions simulating washing steps in biodiesel synthesis. Tests of emulsion stability as a function of time were performed through a light scattering profiling analysis. A statistical experimental design was carried out to evaluate the effect of initial water content, temperature and agitation on the separation efficiency by microwave demulsification of w/b systems as function of final water content. A comparison between microwave and gravitational sedimentation was conducted in order to evaluate the efficiency of both processes with time.
2. Experimental Section 2.1 – Materials The castor oil used to produce biodiesel was supplied by Petrobras. Transesterification was conducted with methanol (Vetec; 99,8%) and sodium methoxide catalyst (Merck; Solution in methanol). For emulsion preparation deionized water was used. 2.2 – Biodiesel and Emulsion Preparation The biodiesel preparation involved a two-step transesterification using sodium methoxide as catalyst at 298 K and atmospheric pressure, followed by a purification process. In the first step (R1), methanol:oil molar ratio of 6:1 (14 wt%) and a catalyst concentration of 3.3 wt% were used for a reaction time of 1h. The reaction product was decanted for 24h and the methyl esters phase was used for the second reaction. In the second reaction (R2), in which 5 wt% of methanol was used in relation to the mass of the methyl esters phase. In this step, the catalyst concentration was similar to R1 and the reaction time was 1h. The methyl esters phase derived from R2 was washed initially with a 5% HCl aqueous solution (10:1 methyl esters to HCl solution ratio), decanted for 24h, followed by two washing and 24h decanting steps with distilled water (10:1 methyl esters to water ratio).
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In order to characterize the final methyl esters, some relevant properties were obtained. Table 1 summarizes the main characteristics of this product. Density was measured in a digital Anton Paar DMA 4500 densimeter. Interfacial tension analyses between methyl esters and water was determined the Du Nouy ring method using a Kruss tensiometer. The dynamic viscosity of the biodiesel was determined using a rheometer (AR-G2, TA Instruments) equipped with a temperature control system (298.15 ± 1 K). Water content (WC) of the methyl esters was determined by the Karl Fischer (KF) titration method, in accordance with ASTM D1744 procedure. The solvent used during the analysis was a mixture of dry methanol and chloroform (20 % v/v). A Metrohm KF titrator (model 836 Titrando) equipped with a double platinum electrode was employed during the WC determination tests. The methyl esters-water emulsions prepared in this work had water contents of 10, 20 and 30%. All of those were prepared using an Ultra Turrax T25 mixer during 4 min, at a stirring speed of 13500 rpm.
2.3 – Emulsion Characterization The emulsions stabilities were determined with the optical analyzer Turbiscan Lab. This equipment has a detection head composed of a pulsed near infrared laser light source and two synchronous detectors. A transmission detector records the light, which goes through the sample, while the backscattering detector records the light scattered backward by the sample. Turbiscan makes scans at various programmed times and overlays the profiles on one graph in order to show the destabilization. The measurement result is the light transmission and backscattering percentage from the sample in function of the height of the tube (mm), from the bottom to the top of the tube. From the obtained profiles, the backscattering percentage average values (%BS) were calculated for w/b emulsion in the test tube in the 15 to 35 mm height zone (%BS15-30), corresponding to the middle part of the tube. A method proposed by Marquez et al.13 was used as an index of emulsion stability. The sedimentation percentage (%S), related to emulsion destabilization rate, was defined as shown in Eq. (01).
%
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where BSin 15-35 is the initial average sample backscattering in the test tube height from 15 mm to 35 mm, and BSf 15-35 is the average final sample backscattering in the test tube height from 15 mm to 35 mm value at the required time. The backscattering signal (BS) is a complex function of the water content and the water droplet size, therefore the %S is not linearly related to the amount of water that sedimented, it can be used only as an indicator. The Droplet Size Distribution (DSD) of emulsion samples was measured with a Mastersizer 2000 (Malvern) particle size analyzer based on laser light diffraction. Droplet size distributions were measured from samples highly diluted in a transparent mineral oil (white spindle) in three replicates. In the current work, the optical constants (refractive index, n, and absorption index, k) employed as internal parameters for the particle-size analyzer were 1.33 and 0.01 for dispersed and 1.467 for dispersing medium. 2.4 – Microwave and Conventional Demulsification Tests A commercial laboratory-scale microwave reactor system (Anton Paar Synthos 3000) was used for demulsification tests. The processes involved 4 quartz tubes, each one with about 50 g of emulsion. A magnetic stirrer was used for sample stirring to guarantee the contact between water drops. Several microwave irradiation demulsification runs were carried out at different temperatures (333, 343, 353, 373 and 393 K), using methyl esters-water emulsions containing different amounts of water (10, 20 and 30% (w/w)), different magnetic stirring levels (1, 2 and 3), two heating ramp times (30 s and 150 s) and two irradiation times (5 and 10 min). For the tests at temperatures higher than 373 K, the test tubes were pressurized with nitrogen to avoid losses of water by boiling/evaporation. Demulsification tests under conventional heating (sedimentation method) were employed for comparison with microwave irradiation results. Conventional heating systems were carried out using a thermostatic water bath and a quartz tube identical to that applied in the microwave tests. The use of same vessel for both methods allowed performing tests with the same geometry and volumes. Finally, the efficiency on separation for both microwave and sedimentation methods was verified by measuring the final water content on samples taken from the top of the emulsions.
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3 – Results and Discussions 3.1 – Emulsion Stability The DSD of water droplets in emulsions with 10, 20 and 30% (w/w) of dispersed phase are shown in Figure 1. The water droplet volume distributions are multimodal with a large range of droplet sizes. It is observed an increase of droplet size with water content. Droplets size for emulsion with 10% of WC are concentrated from 10 to 70 µm, while for emulsions with 20 and 30% of WC this range varying from approximately 10 to 100 µm. The values of medium diameter D(0.5) are 17, 24, 24 µm and mean volume weight D(4.3) are 18, 28 and 31 µm for emulsions with 10, 20 and 30 % of WC, respectively. The stability results investigated by the sample light backscattering profiles showed a slow sedimentation processes for emulsions with 10 and 20% WC (Figure 2). Measurements for emulsion with 10% WC indicate faster sedimentation detected by a decrease of the light backscattering at the top of the sample. In the first 4 hours, a clarification occurs in the top of the tube, indicating dispersed water sedimentation, and after 24 hours it can be observed a peak on the backscattering profile approximately at 5mm of tube length due of water droplet accumulation at in methyl esters water interface. After 24 hours, a clear water phase can be observed at the bottom of the sample. All the backscattering profiles for w/b emulsions are showed in Figure 2. Emulsions with 20% WC showed a similar behavior, but more unstable compared to those with 10% WC, droplet sedimentation was faster and the interfacial peak of water and methyl esters formed in only two hours. A very high stability was observed for the emulsion with 30% WC. There are no changes on the backscattering profile in the first 3 hours. After 24 hours, a slight decrease on the backscattering occurs and it can be observed a small amount of water at the bottom of the sample at 2.5mm height, demonstrating a slow sedimentation process. In dense emulsions, every droplet is closely surrounded by other droplets and this configuration leads to the formation of structures14-16 that hinders particle sedimentation. This is clearly observed in 30% WC emulsion. The sedimentation percentages, described by Eq. (01), for w/b emulsion with different WC are shown in Table 2. During the first hour of measurements the emulsion with 20% water present a higher %S, while the more concentrated emulsion have an insignificantly %S. After 24 hours, the sedimentation percentage reaches 80% o for
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emulsions with 10 and 20% WC. At the final of measurements (48 h) the emulsion with 30% WC has a %S of 1.92, showing a high stability even after two days. This behavior shows that purification of castor oil methyl esters by water washing may be very difficult for high water content treatments.
3.2 – Biodiesel Demulsification Tests
Microwave irradiation as a nonconventional heating source was used to accelerate the demulsification process of w/b emulsions. Three variables were used to develop a full factorial experimental design and evaluate separation efficiency (EF) of water from emulsions: initial water content, separation temperature and stirring. Three replicates were added to the central point to estimate the experimental error. Table 3 summarizes the initial emulsions properties, microwave operational conditions and separation efficiency results of the experimental design (runs 1-11) for w/b demulsification. The analysis for separation efficiency response was conducted employed following a methodology described in detail by Nele et al.17. A linear model (02) relates the separation efficiency (EF) to the experimental variables (Xi) and their linear interactions (Xi Xj). 3
3
EF = aa + ∑ ai X i + ∑ aij X i X j i
(02)
i< j
The coefficients ai are calculated by linear regression and a variable (interaction) is considered statistically significant if its coefficient has a confidence level above 95%. The linear model was not able to describe the experimental data adequately, as shown in Fig. (3-a). A better description of the separation efficiency was obtained using a model containing a quadratic term that takes into account a non linear behavior. The nonlinear model is shown on Eq. (03) and results for this model are presented in Fig. (3-b). 82.25 2.61 18.47 1.59 ∙ "# 5.78 1.55 ∙ $ 2.92 1.60 ∙ %& 2.98 1.60 ∙ "# ∙ $ 14.68 3.06 ∙ ' (
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where EF is the separation efficiency, WC the water content, T means temperature, Ag is the microwave stirring and Y represents the quadratic effect. It may be concluded that the separation efficiency is strongly dependent on water content as appointed its coefficient in Eq. (03). Results showed that only interaction between WC and T had statistical significance in this case. Another strong effect comes from the quadratic effect (Y). The experimental design used in this study does not allow to positively identify the effect source, however the water content showed the strongest effect and the measurements indicated that stability was high for the 10% water emulsion, decreased for the 20% water emulsion and had large increase when water content went to 30%, therefore one may speculated that it may be the responsible by the quadratic effect. The linear model (Eq.(3)) also shows that separation temperature has a positive influence on the efficiency explained by the well known viscosity reduction effect. More interestingly, was the positive effect of stirring. An increase of the stirring speed, within the range used in this study, increases the separation efficiency by promoting droplet coalescence. Preliminary experiments, in absence of stirring, showed negligible water separation. In order to evaluate demulsification process by microwave irradiation at temperatures equal and above water boiling point (runs 12 and 13) and the effect of others operational conditions like heating ramp (time to reach the treatment temperature) and irradiation time (runs 14 and 15) complementary data was obtained. The experimental results showed a slight increase on separation efficiency at temperatures up to water boiling point and no improvement at higher temperatures. Figure 4 shows the behavior of the final WC of emulsions with 30% of initial water content. A comparison between microwave and gravitational sedimentation was performed with regard to the water content of the final product (Figure 5) using the same reaction vessel used for microwave demulsifications and the same treatment temperature. The experimental points labeled 14 and 15 in Table 3 were reproduced for this purpose. During the first 5 minutes, the microwave process showed a very good performance in comparison to gravitational sedimentation, yielding a product with a final water content four times smaller. This difference in efficiency decreased after 10 minutes but it remained statistically significant. The final water content of the emulsion treated by gravitational sedimentation was about twice as much as that of the emulsion
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treated by microwave and this was far superior than the estimated experimental error of these experiments, which was about 10%.
4 – Conclusions For low water volume fraction, the stability of water-in-biodiesel emulsions decreases with increase of water content. However, when emulsion concentration increase approximately to 30% of water the stability increases significantly. This may be related to the creation of organized structures typical of dense emulsions or of a layer of water droplets that prevent the droplet to interface coalescence. Microwave demulsification can be used as an alternative to the traditional techniques, as it causes the emulsion break up with a high separation speed and results in a low value of residual water in castor oil methyl esters. In relation to the demulsification conditions, it is possible to suggest that higher stirring speed, higher temperature and higher the amount of water present on the emulsion (up to 20%), leads to a more efficient the separation with a lower content of residual water.
Acknowledgements: The authors wish to thank CAPES (Conselho de Aperfeiçoamento de Pessoal de Nível Superior) for providing a scholarship (BBF), CNPq (Conselho Nacional de Pesquisa e Desenvolvimento) and FAPERJ (Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro) for their financial support. The authors are also very grateful to the reviewers for their careful and meticulous reading of the manuscript.
References
(1) Karaosmanoğlu, F.; Cığızoğlu, K. B.; Tüter, M.; Ertekin, S. Energy & Fuels 1996, 10, 890. (2) Al-Widyan, M. I.; Al-Shyoukh, A. O. Bioresource Technol. 2002, 85, 253.
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(3) Encinar, J. M.; Gonzalez, J. F.; Rodriguez-Reinares, A. Ind. Eng. Chem. Res. 2005, 44, 5491. (4) Felizardo, P.; Correia, M. J. N.; Raposo, I.; Mendes, J. F.; Berkemeier, R.; Bordado, J. M. Waste Management 2006, 26, 487. (5) Nye, M. J.; Williamson, T. W.; Deshpand, S.; Schrader, J. H.; Snively, W. H.; Yurkewich, T. P.; French, C. L. J. Am. Oil Chem. Soc. 2006, 83, 457. (6) He, H. Y.; Guo, X.; Zhu, S. L. J. Am. Oil Chem. Soc. 2006, 83, 457. (7) Rinaldi, R.; Garcia, C.; Marciniuk, L. L.; Rossi, A. V.; Schuchardt, U. Química Nova 2007, 30, 1374. (8) Korus, R. A.; Hoffman, D. S.; Bam, N.; Peterson, C. L.; Drown, D. C. 1996, Acessed
june
(2009):
http://journeytoforever.org/biofuel_library/EthylEsterofRapeOil.pdf. (9) Nour, A. H.; Abu Hassan, M. A.; Yunus, R. M. J. App. Sci. 2007, 7, 1437. (10) Ríos, G.; Pazos, C.; Coca, J. Colloid and Surfaces A 1997, 138, 383. (11) Xia, L.; Lu, S.; Cao, G. J. Colloid Interface Sci. 2004, 271, 504. (12) Fortuny, M.; Oliveira, C. B. Z.; Melo, R. L. F. V.; Nele, M.; Coutinho, R. C. C.; Santos, A. F. Energy & Fuels 2007, 21, 1358. (13) Marquez, A. L.; Palazolo, G. G.; Wagner, J. R.; Colloid Polym. Sci. 2007, 285, 1119. (14) Mishchuk, N. A.; Sanfeld, A.; Steinchen, A. Adv. Colloid Interface Sci. 2004, 112, 129. (15) Chu, X.; Wasan, D. T.; J. Colloid Interface Sci. 1996, 184, 268. (16) Ruckenstein, E. Adv. Colloid Interface Sci. 1998, 75, 169. (17) Nele, M.; Vidal, A.; Bhering, D. L.; Pinto, J. C.; Salim, V. M. M. Applied Catalysis A: General 1999, 178, 177.
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Table 1. Castor oil methyl esters properties. Property Water content Density Viscosity (298 K) Interfacial tension with water (298 K)
Unit
Value
%
0.18
g.cm-3
0.91
Pa.s
0.0264
mN.m-1
23.4
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Table 2. Effect of WC on sedimentation percentage for w/b emulsions. Water Content (%)
Time = 0 %BS15-35
Time = 1h %BS15-35
%S
Time = 24h %BS15-35
%S
Time = 48h %BS15-35
%S
10
15.68
14.83
5.42
2.99
80.93
1.46
90.69
20
17.21
15.02
12.73
2.23
87.04
1.46
91.52
30
73.06
72.81
0.34
71.82
1.70
71.66
1.92
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Table 3. Operational conditions and final WC for methyl esters-water microwave demulsifications. Run
Water Initial WC1
Microwave operational conditions Temperature Agitation
(%)
(K)
1
30
353
2
10
3
Ramp Irradiation time2 time
Separation Final WC
EF3
(s)
(min)
(%)
(%)
3
30
5
2.54
91.54
353
3
30
5
3.70
62.98
30
353
1
30
5
4.16
86.15
4
10
353
1
30
5
4.73
52.75
5
30
333
3
30
5
4.46
85.14
6
10
333
3
30
5
5.77
42.30
7
30
333
1
30
5
5.60
81.33
8
10
333
1
30
5
6.16
38.40
9
20
343
2
30
5
3.35
83.25
10
20
343
2
30
5
3.72
81.40
11
20
343
2
30
5
3.58
82.10
12
30
373
3
30
5
2.24
92.53
13
30
393
3
30
5
2.24
92.53
14
30
353
3
30
10
2.25
92.50
15
30
353
3
150
5
2.30
92.33
1
WC: water cut
2
Ramp time: Time to reach the treatment temperature
3
EF: Separation efficiency defined as a fraction of dispersed water removed from continuous
methyl esters phase. )
*+, -.
+, -. /∙ *+, -.
100
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8
water content 6
volume - ln(%)
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10%
20%
30%
4
2
0 0.01
0.1
1
10
100
size - µm
Figure 1. Effect of water content on a water droplet size distribution in w/b emulsions.
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80 00:00:00
backscattering (%)
10 % of water
01:00:00 02:00:00
60
03:00:00 24:00:00 48:00:00
40
20
0 0
5
10
15
20
25
30
35
40
45
tube length (mm) 80 00:00:00
backscattering (%)
20 % of water
01:00:00 02:00:00
60
03:00:00 24:00:00 48:00:00
40
20
0 0
5
10
15
20
25
30
35
40
45
tube length (mm)
80
Backscattering (%)
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60 00:00:00
30 % of water
01:00:00 02:00:00
40
03:00:00 24:00:00 48:00:00
20
0 0
5
10
15
20
25
30
35
40
45
tube length (mm)
Figure 2. Results of laser light scattering profiling as function of time and tube length for w/b emulsions.
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(a)
(b)
linear model
quadratic model
100
100
80
80
predicted value
predicted values
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60
40
20
60
40
20
0
0 0
20
40
60
80
100
0
20
observed values
40
60
80
100
observed value
Figure 3. Predicted vs. observed values of separation efficiency (experiments 1 to 11). (a) linear model, (b) quadratic model.
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5.0 4.0 WC(%)
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3.0 2.0 1.0 0.0 330
340
350
360
370
380
390
400
T (K)
Figure 4. Biodiesel final water content as function of separation temperature (microwave stirring level 3, heating ramp of 30s and irradiation time 5min).
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microwave heating thermal heating
9 8
Water final content (%)
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Time (min)
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Figure 5. Comparison of final water content for microwave and thermal sedimentation processes at 80°C and 30% of water cut.
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