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Journal of Industrial and Engineering Chemistry 21 (2015) 1039–1043

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Selected physical properties of binary mixtures of crude glycerol and methanol at various temperatures Reza Afshar Ghotli, Abdul Raman Abdul Aziz *, I.M. Atadashi, D.B. Hasan, Pei San Kong, M.K. Aroua Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

A R T I C L E I N F O

Article history: Received 17 January 2014 Received in revised form 9 May 2014 Accepted 12 May 2014 Available online 20 May 2014 Keywords: Glycerol Dynamic viscosities Densities Excess molar volumes Refractive indexes

A B S T R A C T

Crude glycerol, the main byproduct of biodiesel production, consists of excess methanol and other impurities and can be converted into other useful products through purification or conversion processes. In this work, dynamic viscosities, densities, excess molar volumes and refractive indexes for several mixtures of biodiesel crude glycerol and methanol with were determined at different temperatures. The physiochemical characteristics of crude glycerol in mixture form with methanol could be useful for the design of industrial equipment and purification or conversion process. The results demonstrated temperature dependent behaviors of all binary mixtures. The empirical correlations were obtained based on the experimental results. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Biodiesel (or methyl esters) is a clean and biodegradable alternative to petroleum-based diesel fuel. The most common method to produce biodiesel is catalytic transesterification of vegetable oils and animal fats using a homogeneous acid or base catalyst in stirred reactors [1]. In the transesterification reaction, triglyceride reacts with alcohol, producing a mixture of fatty acids; alkyl esters and glycerol (see Fig. 1). Different homogeneous catalysts such as potassium hydroxide [2], sodium hydroxide [3], sodium ethoxide [4], sodium methoxide [4], sulfuric acid [5] and hydrochloric acid [6] are used. Despite many benefits from the production and use of biodiesel, there are also challenges involved such as the use of byproducts generated in the transesterification process. Glycerol is the main transesterification byproduct. There is about 1 kg of a crude glycerol byproduct is formed for every 9 kg of biodiesel produced [7]. Glycerol, or 1,2,3-propanetriol, is a trihydric alcohol. It is a colorless, odorless, sweet-tasting, syrupy liquid that melts at 17.8 8C, boils at 290 8C, and is miscible with water and ethanol [8]. Since biodiesel production is increasing rapidly, crude glycerol as the by-product of the production process is also generated in

* Corresponding author. Tel.: +60 379675300; fax: +60 379675319. E-mail addresses: [email protected], [email protected] (A.R. Abdul Aziz).

large quantity. Thus, conversion of crude glycerol to value added products can reduce the price of biodiesel. The crude glycerol byproduct can be converted to many useful products including 1,3propanediol [9], 1,2-propanediol [7], dihydroxyacetones [10], hydrogen [11], polyglycerols [12], succinic acid [13], and polyesters [14]. The quality of crude glycerol generated from biodiesel production is low, and therefore, it should not be used directly. Hence, further refining and purification of the crude glycerol is necessary [15]. Generally, crude glycerol treatment consists of filtration, chemical additions, fractional vacuum distillation, bleaching, deodorization, and ion exchange. The impurities in crude glycerol include methanol and soaps. As a result of the application of excess methanol to enhance biodiesel production, methanol is present in the glycerol byproduct. Due to the presence of free fatty acids in the oil feedstock, soap can exists in the glycerol layer. In addition to methanol and soap, crude glycerol may also contain a variety of elements such as calcium, magnesium, phosphorous, or sulfur [16]. It has been reported that glycerol makes up 65% to 85% (w/w) of the crude glycerol streams [17,18]. The remaining weight in the crude glycerol streams is mainly methanol and soaps [16]. Physiochemical properties of the crude glycerol mixtures are fundamental variables that need to be considered for the design of industrial equipment for purification or conversion of crude glycerol to other products. The properties such as density and viscosity are needed to determine behavioral and predictive information to design and optimize the unit operations. Density,

http://dx.doi.org/10.1016/j.jiec.2014.05.013 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

[(Fig._1)TD$IG]

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R. Afshar Ghotli et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1039–1043

Fig. 1. Transesterification of triglycerides to produce biodiesel and glycerol.

viscosity and refractive index data of binary liquid mixtures are very important from the theoretical point of view, to understand the liquid theory [19,20]. The present work is motivated by the fact that there has not been enough data published on the properties of crude glycerol from biodiesel production process (e.g., viscosity, density, and refractive index). The specific objective of this work is to prepare the binary mixtures of glycerol and methanol and measure the dynamic viscosities, densities and refractive indexes of these mixtures as a function of temperature and concentration. 2. Methodology 2.1. Materials Mostly, vegetable oils from rape seeds, sunflowers, and soybeans are used for the production of biodiesel [15]. However, the oil used in this study was palm oil as Malaysia is one of the largest producers of palm oil in the world [21]. Refined, bleached and deodorized palm oil with an acid value of 0.5 was obtained from the local market. The oil had an iodine value and water content of 53.2 and 400 ppm, respectively. Methanol (99.8%) supplied by Sigma-Aldrich, Malaysia was used as the alcohol reactant. Pure potassium hydroxide (98.9%) in pellet form which was also purchased from the same company was used to catalyze the reaction. 2.2. Crude glycerol preparation Crude glycerol was produced through transesterification of palm oil. The reaction was carried out in a 2.5 l jacketed batch reactor. The reactor was equipped with an overhead stirrer (Kika1 Werke) fitted with a stainless steel propeller, a thermometer and a water cooled reflux condenser. The reaction temperature was established and controlled using a hot water circulation bath (RC6 LAUDA). The transesterification reaction was started with a 6:1 molar ratio of methanol to oil and 1 wt% potassium hydroxide catalyst at 60 8C [22,23]. After 60 min of reaction, the glycerol byproduct was separated from biodiesel by sedimentation and the excess methanol was removed by atmospheric evaporation. The purity of the crude glycerol was 79.2%, determined using high performance liquid chromatography. The major impurities in the crude glycerol are soap, methanol, water and methyl esters. 2.3. Mixture preparation Sufficient amount of crude glycerol and pure methanol were blended at the volume fractions of 90%, 80%, 70%, and 50% at 25 8C to prepare the binary mixtures. Mechanical mixing was not needed for blending because glycerol is fully miscible in methanol.

circulated through the jacket of the viscometer and temperatures were checked with two digital thermometers in the water bath and the viscometer. The viscometer was calibrated using published viscosity values for ethanol and water [24]. Measurement started after 5 min when the temperature reached the value over the range of 30 to 60 8C with 10 8C intervals. Each measurement was replicated three times. The uncertainties of the viscosity values were within the range of 0.01 mPa s. 2.5. Density measurement Density of the binary mixtures of glycerol and methanol was measured using a DMA 4500 vibrating tube density/specific gravity meter (Anton Paar, Austria). Density of water (degassed bi-distillated) was measured at 25 8C to check the density meter adjustment. Comparison with the corresponding value in the density tables [25] showed a difference of 0.00003 g cm3 which confirmed the accuracy of the machine. Density measurements were carried out at temperatures from 20 to 70 8C with three replicates for each reading. The uncertainty in density measurements was 0.00001 g cm3. The excess molar volumes of the binary mixtures were calculated using the formula below: E ¼ Vm

x1 M 1 þ x2 M2

r

x1 M 1

r1

x2 M 2

(1)

r2

where x, M, and r are mole fraction, molecular mass and density, respectively. 2.6. Refractive index measurement Refractive indexes of the binary mixtures of glycerol and methanol were determined using an RE50 refractometer (MetllerToledo, US). The refractometer was calibrated with distilled water before each run. The accuracy of the refractive index measurement was 0.00005. Pure ethanol was used to clean the surface of the refractometer prism. The measurements were performed at atmospheric pressure and temperatures from 15 to 55 8C. The mean data was derived from the repetitions (three times) of each measurements with a repeatability of 0.02%. 3. Results and discussion 3.1. Viscosity measurement Dynamic viscosities of the binary mixtures of crude glycerol and methanol were determined from 30 to 60 8C at a 10 8C intervals. The viscosity data reported here was derived by means of triplicate determinations. The measured viscosities are presented in Table 1. Fig. 2 shows the viscosities of the binary mixtures of crude glycerol and methanol together with the composition and temperature. The results demonstrated temperature-dependent behavior of the binary mixtures. It was found that their viscosities decreased Table 1 Dynamic viscosity of mixture of crude glycerol and methanol. Temp. (8C)

Viscosity (mPa s) Glycerol:methanol volume ratio

2.4. Viscosity measurement Viscosities of binary mixtures were measured using a VT550 rotary viscometer (HAAKE, Germany) with a NV sensor. A circulating water bath (RCS and RC6 LAUDA) was used to conduct the measurements at different temperatures. Hot water was

30 40 50 60

0:100

50:50

60:40

70:30

80:20

90:10

100:0

0.52 0.47 0.39 0.34

27.41 17.32 10.60 8.50

58.01 42.20 35.10 32.65

95.62 68.60 55.91 50.21

132.33 93.57 66.61 55.52

187.65 111.73 79.56 62.30

521.03 401.42 301.00 242.10

Viscosity h of mixture of crude glycerol and methanol.

[(Fig._2)TD$IG]

[(Fig._3)TD$IG]


Fig. 2. Viscosity h of mixture of crude glycerol and methanol at different volume fractions as a function of temperature. ~, 90% glycerol; &, 80% glycerol; &, 70% glycerol; D, 60% glycerol; *, 50% glycerol.

non-linearly with temperature, especially at low methanol volume fractions. Decrease in the methanol volume fractions resulted in considerable reduction in viscosity values of the mixtures. The viscosity decreased from 27.41 mPa s at 30 8C to 0.34 mPa s at 60 8C at the volume ratio of 50:50. It also reduced significantly from 187.65 to 62.30 mPa s at the volume ratio of 90:10. When the liquid mixtures are heated, the cohesive forces between the molecules reduce. This eventually leads to reduction in viscosities of the liquid due to reduced attraction forces between the molecules. At a fixed temperature, the viscosities of the crude glycerol and methanol mixture decreased with increasing volume fraction of methanol in the mixture. The viscosity values changed considerably at higher volume fraction (50%) compared to lower volume fractions (10%) of methanol. This is attributed to the fact that the viscosity of crude glycerol is higher than the viscosity of methanol. An empirical correlation was proposed to determine the viscosity of the binary mixtures of crude glycerol and methanol in

1041

Fig. 3. Density r of crude glycerol + methanol mixtures at different volume fractions as a function of temperature. &, 90% glycerol; *, 80% glycerol; D, 70% glycerol; &, 60% glycerol; ~, 50% glycerol.

an easy way. The empirical correlation for viscosity is presented in Table 5. 3.2. Density measurement Density is defined as the ratio of the mass per unit volume. Densities of the binary mixtures were measured from 20 to 70 8C. No bubbles were observed and no significant variation was noted at any temperatures during the density measurements. The measured densities are listed in Table 2. Fig. 3 presents the densities of the binary mixtures of crude glycerol and methanol which show the effects of temperature and composition on density of crude glycerol and methanol mixtures. Base on the figure, it is apparent that densities presents almost decrease with increasing temperature. Therefore, the binary mixtures demonstrate temperature-dependent behavior. The liquid density of the mixtures decreased linearly with increase

Table 2 Density r of mixture of crude glycerol and methanol. Density (g cm3)

Temp. (8C)

Glycerol:methanol volume ratio

21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69

0:100

50:50

60:40

70:30

80:20

90:10

100:0

0.79118 0.78876 0.78630 0.78406 0.78163 0.78181 0.77939 0.77707 0.77461 0.77222 0.77224 0.76984 0.76748 0.76510 0.76264 0.76260 0.76018 0.75769 0.75534 0.75256 0.75274

1.00512 1.00365 1.00105 1.00018 0.99895 0.99738 0.99585 0.99411 0.99253 0.99113 0.99026 0.98864 0.98715 0.98584 0.98424 0.98252 0.98121 0.98003 0.97802 0.97688 0.97538 0.97416 0.97302 0.97166 0.97030

1.03827 1.03680 1.03420 1.03333 1.03210 1.03053 1.02900 1.02726 1.02568 1.02428 1.02341 1.02179 1.02030 1.01899 1.01739 1.01567 1.01436 1.01318 1.01117 1.01003 1.00853 1.00731 1.00617 1.00481 1.00345

1.08340 1.08193 1.07933 1.07846 1.07723 1.07566 1.07413 1.07239 1.07081 1.06941 1.06854 1.06692 1.06543 1.06412 1.06252 1.06080 1.05949 1.05831 1.05630 1.05516 1.05366 1.05244 1.05130 1.04994 1.04858

1.12842 1.12695 1.12435 1.12348 1.12225 1.12068 1.11915 1.11741 1.11583 1.11443 1.11356 1.11194 1.11045 1.10914 1.10754 1.10582 1.10451 1.10333 1.10132 1.10018 1.09868 1.09746 1.09632 1.09496 1.09360

1.17473 1.17326 1.17066 1.16979 1.16856 1.16699 1.16546 1.16372 1.16214 1.16074 1.15987 1.15825 1.15676 1.15545 1.15385 1.15213 1.15082 1.14964 1.14763 1.14649 1.14499 1.14377 1.14263 1.14127 1.13991

1.22121 1.21943 1.21618 1.21780 1.21650 1.21514 1.21371 1.21332 1.21090 1.20957 1.20810 1.20671 1.20530 1.20391 1.20252 1.20115 1.20060 1.19831 1.19492 1.19553 1.19412 1.19276 1.19134 1.18999 1.18852


1042

Table 3 Excess molar volume VE of mixture of crude glycerol and methanol. Excess molar volume (cm3 mol1)

Temp. (8C)


21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69

50:50

60:40

70:30

80:20

90:10

1.035730 1.022502 1.101701 1.023878 0.997401 0.988955 0.978222 1.029693 0.970421 0.952216 0.904915 0.898268 0.951066 0.860396 0.852192 0.850496 0.825937 0.872083 0.807943 0.773234 0.758454 0.727716 0.692189 0.668816 0.645206

1.813556 1.806685 1.889755 1.822007 1.801502 1.799783 1.795735 1.846659 1.801976 1.790307 1.748467 1.748906 1.798466 1.724405 1.723374 1.729149 1.711239 1.751978 1.707886 1.679603 1.672056 1.648006 1.619044 1.602749 1.586265

1.966274 1.964770 2.048989 1.991539 1.976244 1.980159 1.981737 2.029846 1.999639 1.993571 1.956789 1.963156 2.007143 1.950153 1.955154 1.967131 1.955023 1.988107 1.964017 1.941489 1.940125 1.922008 1.898941 1.888822 1.878561

2.145272 2.150200 2.236302 2.190771 2.181685 2.192368 2.200702 2.246005 2.232647 2.233285 2.202468 2.215969 2.253966 2.216730 2.228985 2.248447 2.243268 2.267929 2.267119 2.251431 2.257491 2.246442 2.230383 2.227649 2.224831

2.272447 2.284860 2.373297 2.341519 2.339651 2.358218 2.374421 2.416624 2.422740 2.431185 2.407286 2.429106 2.460335 2.445897 2.466615 2.494812 2.497702 2.512784 2.538890 2.531158 2.545877 2.543055 2.535149 2.541023 2.546879

in temperature. It is obvious that because of the greater molecular motion at high temperatures, contributes to expansion in volume is expanded and reduction in density. At constant temperature, densities of the binary mixtures decreased with increasing volume fraction of methanol in the mixture. Crude glycerol and methanol have different densities with maximum values of 1.22121 and 0.79118 g cm3, respectively. The calculated excess molar volume, VE, for the five samples of crude glycerol and methanol at temperatures from 20 to 70 8C are listed in Table 3. Positive VE values were obtained for excess molar over the whole range of mixtures. It can be explained by the physical effects of dispersion forces and nonspecific interactions in the mixture [26]. Moreover, it can be the effects of strong hydrogen bonding interaction between the molecules [23], The VE values increased with decrease in volume fraction of methanol at the same temperature. The same trend was repeated throughout the whole range of temperatures. The maximum VE values were obtained for the binary mixtures with 10% methanol. The increase in excess molar volume can be due to the weak dipole–dipole interactions in higher hydrogen bonding and dispersion forces (London interactions) [23,26–28]. Although, reduction trend was seen in the VE values between the volume ratio of 50:50 to 70:30 with increase in temperature, it rose at the volume ratio of 80:20 and 90:10. This behavior can be explained by Table 4 Refractive index nD of mixture of crude glycerol and methanol. Temp. (8C)

Refractive index

volume expansion in which the interactions between glycerol and methanol decrease with increase in temperature [28]. The density and excess molar volume of the binary mixtures of crude glycerol and methanol are correlated and the empirical correlations are presented in Table 5. 3.3. Refractive index measurement The Refractive index, nD, of binary mixtures of crude glycerol and methanol at atmospheric pressure and temperatures from 15 to 55 8C are listed in Table 4. It can be seen that refractive indexes of glycerol and methanol binary mixtures decreased with increase in temperatures. The highest refractive index in a range of 1.4348 to 1.4218 at the temperatures of 15 to 55 8C were obtained for the mixture with 10% (volume) methanol. The result also indicated that, at constant temperature, the refractive index of mixtures decreased with increasing volume fraction of methanol in the mixture. Moreover, it was also observed that the methanol volume ratio exerted more effects on refractive index compared to temperature. The empirical correlation of the refractive index for the binary mixtures of crude glycerol and methanol are presented in Table 5.

Table 5 Empirical correlations for viscosity, density, excess molar volume and refractive index of mixture of crude glycerol and methanol. Property

Empirical correlation

R

Viscosity

h (mPa s) = 524.03/[(1 +

0.92


15 25 35 45 55

0:100

50:50

60:40

70:30

80:20

90:10

100:0

1.3319 1.3280 1.3238 1.3197 1.3157

1.3902 1.3860 1.3826 1.3795 1.3767

1.4034 1.3997 1.3971 1.3942 1.3910

1.4157 1.4121 1.4088 1.4059 1.4021

1.4282 1.4249 1.4213 1.4185 1.4160

1.4348 1.4313 1.4275 1.4246 1.4218

1.4409 1.4374 1.4336 1.4309 1.4278

Density Excess molar volume

Refractive index

((T (8C) + 30)/30)2) (1 + ((F1 + 100)/5)2] r (g cm3) = 0.7996 0.0007T (8C) + 0.0043F1 E (cm3 mol1) = 4.8796 + Vm 0.0059T (8C) + 0.1592F1 8.3 105 T2 (8C) 0.0009F12 R.I. = 1.338 0.0003T (8C) + 0.0011F1

0.99 0.98

0.99


1043

Table 6 Two-factor variance analysis for viscosity, density, excess molar volume and refractive index. Source of variation

SS

df

Glycerol:Methanol Temperature Error Total

27044.78 370461.5 32392.37 429898.6

3 6 18 27


380.9229 29332.61 2699.09 32412.62

20 7 140 167


0.153396 36.08454 0.613929 36.85186

24 4 96 124


0.000784 0.043007 8.33E 06 0.0438

4 6 24 34

MS

F

P-value

Fcrit

Viscosity 9014.927 61743.58 1799.576

5.009473 34.31007

0.010656 6.65E 09

3.159908 2.661305

Density 19.04614 4190.373 19.27921

0.987911 217.3519

0.480442 6.58E 72

1.646027 2.075589

Excess molar volume 0.006392 9.021135 0.006395

0.999441 1410.633

0.474868 2.57E 84

1.63128 2.466476

Refractive index 0.000196 0.007168 3.47E 07

564.7655 20651.13

2.34E 23 2.53E 43

2.776289 2.508189

3.4. Analysis of variance (ANOVA) ANOVA was used as a statistical mean to define the significance and effect of each parameter involved in the determination of the mixture properties. The effects of temperature and different ratios of crude glycerol and methanol were statistically investigated through a ‘‘two-factor without replication’’ method in ANOVA. In this analysis, the default value of the significance level (Alpha) was 5%. Table 6 illustrates the variance analysis for viscosity, density, excess molar volume and refractive index of the mixtures of crude glycerol and methanol. Based on Table 6, F values were greater than the corresponding Fcrit values for viscosity, density, excess molar volume and refractive index at different temperatures. Furthermore, the P-values were smaller than ‘‘alpha’’ (P < 0.05) at different temperatures. Small P-values together with large F-values indicate that the results are statistically significant [29]. This reveals that the effects of temperature on the mentioned properties were significant. The results also showed that the glycerol:methanol volume ratio had significant influences on viscosity and refractive index. 4. Conclusion The physical properties (viscosity, density, excess molar volume and refractive index) of crude glycerol and methanol were determined at various temperatures and atmospheric pressure. The binary mixtures demonstrated temperature-dependent behaviors. The data obtained in this work will serve as an important information for designing and optimizing reactors for producing biodiesel and byproducts. Acknowledgments The authors are very appreciative for the financial support provided by the High Impact Research Grant (UM.C/HIR/MOHE/ ENG/38) and the Department of Chemical Engineering, University

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