Rheology of bio-edible oils according to several rheological models and its potential as hydraulic fluid

Industrial Crops and Products 22 (2005) 249–255

Rheology of bio-edible oils according to several rheological models and its potential as hydraulic fluid W.B. Wan Nik a, ∗ , F.N. Ani b , H.H. Masjuki c , S.G. Eng Giap a a

b

Faculty of Science and Technology, Kolej Universiti Sains Dan Teknologi Malaysia, Mengabang Telipot, 21030 Kuala Terengganu, Malaysia Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia c Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 13 May 2004; accepted 19 January 2005

Abstract Today’s concern of protecting the environment has encouraged the research and the use of environmental friendly products. Bio-edible oils are potential energy transport media in hydraulic and lubricating systems. The use of bio-edible or vegetable oils as hydraulic fluid would help to minimize hazardous pollution caused by accidental spillage, lower disposal costs of the used fluid, and help the user industry to comply with environmental safety regulations. In order to successfully use these oils, an understanding of the oil properties is necessary in order to overcome the possible failures or obstacles that might occur in real operating conditions. Rheological property is one of the most important parameters and for this reason this parameter was investigated. The present work evaluates the temperature and shear rate effects of food grade oils that include palm, coconut, canola, corn and sunflower oils. A couette-type viscometer was used to determine the flow behavior of the oils at different temperatures and discrete shear rates that are ranged 40–100 ◦ C and 3–100 rpm, respectively. Various empirical models such as Ostwald de-Waele, Cross, Carreau, Herschel–Bulkley and Arrhenius-type relationship were used to evaluate the experimental data. The influence of shear rate and temperature on the variation of viscosity was clearly observed but temperature has more significant influence. Interpretations of rheological models indicate that these food grade oils belong to pseudo-plastic category. The palm and sunflower oils are highly stable in terms of shear rate and temperature, respectively. The overall results suggest the potential substitution of food grade oils as an energy transport media. © 2005 Elsevier B.V. All rights reserved. Keywords: Rheology; Temperature; Viscosity; Vegetable oil; Lubricants; Hydraulic fluid

1. Introduction

∗

Corresponding author. Tel.: +60 9 6683342; fax: +60 9 6694660. E-mail address: [email protected] (W.B. Wan Nik).

0926-6690/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2005.01.005

Rheology is a field which tends to combine continuum mechanics with ideas obtained by considering the microstructure of the fluid under study. Rheological properties play a major role in describing heat transfer

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250

Nomenclature Ea K, KH m, N n, nH R T

activation energy (N m mol−1 ) consistency index (Pa sn ) constant flow behavior index universal gas constant (N m K−1 mol−1 ) temperature (K)

Greek letters τ shear stress (kg m−1 s−2 ) τg gel strength (kg m−1 s−2 ) γ shear rate (s−1 ) η, η∞,T , η0,γ , η∞,γ viscosity, viscosity at infinitetemperature, zero-shear, infinite-shear rate (Pa s) λc , αc characteristic relaxation time (s)

or in the design, evaluation and modeling of continuous treatment (Marcotte et al., 2001) and its measurements are useful to determine behavioral and predictive information for various products, as well as knowledge of the effect of processing, formulation changes and aging phenomena (Brookfield, 2000). Therefore, it is necessary to have theoretical knowledge as related to rheological aspects. It has been recognized that rheological properties of oil depends on many factors that include temperature, shear rate, concentration, time, pressure, chemical properties, additive and catalyst (Rewolinski and Shaffer, 1985; Toro-Vazquez and Infante-Guerrero, 1993; Diaz et al., 1996; Guo et al., 1997; Rosana et al., 2002; Barreto et al., 2003; Chauvelon et al., 2003; Chen et al., 2003; Georgopoulos et al., 2004). Most of the researches focusing on the effects of temperature, shear rate, concentration and pressure. However, it is normally found that the effect of temperature is much more apparent on fluid viscosity. Vegetable oils were once had widespread use as lubricant before the discovery of petroleum oil in the late 1800s. Due to the advance of petrochemical industry development, the readily available of petroleum oils replaced vegetable oils for reasons of lubricity, stability and economics. Recently, environmentally related issues that include biodegradability, toxicity, occupational health and safety, and emissions have created

important issues to be revealed and reconsidered especially the use of mineral oils in environmental sensitive areas. This has attracted a number of researchers involving in the research of vegetable oil in non-food application. Mostly, vegetable oils are applied in situation where accidental spillage and leakage would cause serious impact on environment. Such application includes marine, construction and agriculture activities. Allawzi et al. (1998) reported that jojoba oil is suitable to be used as a component in lubricating oil formulations for two-cycle gasoline engines. Masjuki et al. (2001) reported that coconut oil and its blends can be used as alternative biofuel in a diesel engine. In addition, vegetable oils have been reported suitable as raw materials for a wide range of products such as lubricants, fuels, printing ink, skin care products and alkyd resins (Cermak and Isbell, 2002; Eromosele and Paschal, 2002; Erhan and Bagby, 1995). Currently, the use of palm based oil as energy transport media or lubricating fluid in hydraulic system is being jointly investigated by researchers at Kolej Universiti Sains dan Teknologi Malaysia, Universiti Teknologi Malaysia and Universiti Malaya. The hydraulic fluid is normally used to actuate linear or rotary devices used in industrial automation systems, construction equipment, automotive braking systems and many others. The objective of the present study is to evaluate the effects of shear rate and temperature on the rheological properties of five selected food grade oils through graphical observation of experimental data plots. The effects of temperature and shear rate will be further elucidated through comparison of parameters obtained from four well known empirical rheological models. This present work would help to promote and diversify the application of natural resources as well as to safe guard the environment. For the betterment of mankind, environmental friendly and sustainable source of energy is to be sought for future use.

2. Materials and methods The coconut, sunflower, canola and corn oils used in this study were purchased from a local supermarket. Palm oil was obtained direct from a refinery. It has gone through a double fractionation process. All oils used are of food grade. Since several grades of palm oil were used in our joint research, the double frac-

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251

η0,γ − η∞,γ 1 + (αc γ)m

(4)

tionated palm oil will be referred as superolein in this paper. A Brookfield (Viscometer model DV-I+) rotationaltype viscometer was used to measure the viscosity of oil samples. Before use, the viscometer (accuracy, ±1% full-scale range; repeatability, 0.2% full-scale range) was calibrated with 4.7 cP Brookfield silicone viscosity standard. The viscosity of the oils was measured in triplicate at ten different shear rates. SP-18 spindle was operated at different speeds between 3 and 100 rpm. A temperature controller (temperature accuracy of ±1%) was used to increase the temperature of the oil samples from 40 up to 100 ◦ C with an increment of 10 ◦ C. For each increment of 10 ◦ C, the oil samples were left 15 min until steady-state heat transfer was achieved. The viscosity and percentage of torque were manually recorded when the viscosity reading reached apparent equilibrium (appears relatively constant for reasonable time). The viscosities were calculated at ten different shear rates in mPa s for each combination of plant oils and temperature. The shear stress and shear rate were calculated using formulas suggested for non-Newtonian fluid by Mohsenin (1986). Shear stress (τ) was given by: τ=

M 2πR2b h

η = η∞,γ + η = η∞,γ +

η0,γ − η∞,γ

N

[1 + (λc γ)2 ]

(5)

η = KH γ nH −1 + η∞,γ

(6)

η = η∞,T eEa /RT

(7)

3. Results and discussion It is observed that viscosity values decrease with increasing shear rates. The reduction in viscosity is much more apparent at low shear rate. At high shear rate, viscosity leveling off is observed as in Fig. 1. Similar observation was reported by Al-Zahrani and AlFariss (1998) for the viscosity of waxy oils. This shear-thinning behavior is commonly known as pseudo-plastic behavior with n and nH < 1 (Table 1). This behavior was explained by Al-Zahrani (1997) that the shear applied in the fluid breaks down the internal structure within the fluid very rapidly, reversible and

(1)

where M = torque (N m), Rb = radius of the SP18 spindle (m) and h = height of the spindle (m). The shear rate was calculated as: γ = 1.318 × N

(2)

where N = speed of spindle (rpm). The shear rate dependence of the oils was investigated using Ostwald de-Waele or also known as Power Law, Cross, Carreau and Herschel–Bulkley (Eqs. (3)–(6), respectively) rheological models. The temperature effect on the flow behavior index (n or m) was investigated. The temperature dependence of the oil was investigated using the Arrhenius type relationship (Eq. (7)) and the effect of shear rate on activation energy (Ea ) was investigated. The experimental data were fitted to the five models (shear rate dependence at 50 and 90 ◦ C and temperature dependence at 20, 60 and 100 rpm) by using common mathematical software. Following are the models used: η = Kγ n−1

(3)

Fig. 1. The effect of shear rate on viscosity measured at 40 and 100 ◦ C.

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252

Table 1 Predicted parameters for bio-edible oils at selected temperatures (50 and 90 ◦ C) Model

Parameter

Condition (◦ C)

Superolein

Corn

Canola

Sunflower

Ostwald de-Waele

K

50 90

2.83E−02 9.23E−03

7.22E−02 6.03E−02

4.41E−02 2.71E−02

4.66E−02 3.01E−02

9.80E−02 8.93E−02

n

50 90

9.81E−01 9.94E−01

7.24E−01 5.47E−01

8.26E−01 7.25E−01

8.24E−01 7.12E−01

6.16E−01 4.56E−01

KH

50 90

1.37E−02 6.31E−03

1.56E−01 7.14E−02

5.82E−02 2.78E−02

8.78E−02 3.28E−02

2.11E−01 1.11E−01

nH

50 90

2.44E−02 7.99E−01

−1.35E−01 3.21E−01

1.30E−01 4.88E−01

−1.28E−01 4.21E−01

−1.35E−01 2.22E−01

η∞,γ

50 90

2.54E−02 6.34E−03

2.32E−02 5.47E−03

2.01E−02 5.49E−03

2.25E−02 6.35E−03

2.05E−02 5.86E−03

η0,γ

50 90

7.66E+01 1.80E−02

1.71E−01 5.03E−02

4.12E−02 2.91E−02

6.98E−02 3.18E−02

8.42E−02 4.83E−01

η∞,γ

50 90

2.54E−02 7.45E−03

2.36E−02 8.03E−03

2.16E−02 7.15E−03

2.29E−02 7.93E−03

2.25E−02 6.21E−03

αc

50 90

6.92E+03 1.29E+00

6.54E−01 1.91E−01

1.37E−01 2.10E−01

3.44E−01 2.14E−01

1.79E−01 4.85E+00

m

50 90

9.76E−01 3.86E−01

1.34E+00 1.30E+00

2.17E+00 9.77E−01

1.54E+00 1.09E+00

2.17E+00 8.32E−01

η0,γ

50 90

2.32E−01 1.69E−02

1.18E−01 6.03E−02

4.19E−02 2.11E−02

6.04E−02 3.61E−01

8.74E−02 1.97E−01

η∞,γ

50 90

2.54E−02 6.34E−03

2.35E−02 5.89E−03

2.16E−02 7.52E−03

2.29E−02 8.68E−03

2.24E−02 5.92E−03

λc

50 90

1.61E+01 1.27E+01

5.32E−01 5.90E−01

1.32E−01 1.68E−01

3.32E−01 9.59E+00

1.78E−01 1.96E+00

N

50 90

4.88E−01 1.01E−01

6.34E−01 3.67E−01

1.11E+00 5.17E−01

7.28E−01 4.57E−01

1.11E+00 3.92E−01

Herschel–Bulkley

Cross

Carreau

no time dependence. From Fig. 1, it is also found that the viscosity of superolein is less affected by shear rate and followed by corn, canola, coconut and sunflower oils. The average percentage variation of viscosity (at 40 and 100 ◦ C) between low and high shear rate are 69.5, 58.9, 47.1, 44.3 and 5.2% of low shear rate (4.0 s−1 ) for sunflower, coconut, canola, corn and superolein, respectively. By comparing the values of consistency index obtained from Ostwald de-Waele (K) and Herschel–Bulkley (KH ) models for various oils, it is found all the values follow this sequence: (K, KH )sunflower > (K, KH )coconut > (K, KH )canola > (K, KH )corn > (K, KH )superolein (Table 1). This sequence indicates sunflower and superolein are having the highest and lowest values of viscosity, respectively. As the tem-

Coconut

perature increases, the K and KH values decrease. Similar observation was reported by Kokini (1992), Rao et al. (1993) and Rosana et al. (2002) for tomato pastes, edible oils and formate-based fluids. Comparing the values of flow behavior index obtained from Ostwald de-Waele (n) and Herschel– Bulkley (nH ) models, it is found most of the values follow this sequence: (n, nH )superolein > (n, nH )corn > (n, nH )canola > (n, nH )coconut > (n, nH )sunflower (Table 1). There are only nH values for coconut and sunflower at 50 ◦ C deviate from this sequence. The temperatures 50 and 90 ◦ C were arbitrarily chosen to show the variation of consistency index and flow behavior index of different oils. The analyzed results for other temperatures show similar pattern. The sequence indicates that superolein and sunflower exhibit the most Newtonian

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and non-Newtonian alike behavior, respectively. If the non-Newtonian behavior oil is used in a hydraulic system, the system will require high torque during starting due to the increased viscosity at low shear rate. Thus, the results suggest the suitability of superolein to be used as blending oil to reduce variation of viscosity with respect to shear rate of other vegetable oils. Fig. 2 shows the variation of viscosity with shear rate for sunflower oil at 90 ◦ C. Experimental plots for different oils and at different temperatures yield similar superimposed plot pattern. The superimposed experimental data and model curves plotted can indicate the suitability of the models used. Among the four models fitted, Cross model seems very well fitted to the experimental data. It is followed by Carreau, Herschel–Bulkley and Ostwald de-Waele models. The average correlation coefficients for the five bio-edible oils at 90 ◦ C according to Cross, Carreau, Herschel–Bulkley and Ostwald de-Waele models are 0.99384, 0.99127, 0.99040 and 0.86455, respectively. The experimental data are well fitted by Cross and Carreau models because each model consists of four parameters whereas Ostwald de-Waele and Herschel–Bulkley only consist of two and three parameters to be determined, respectively. Normally, typical behavior of fluid need for more than one or two parameters in order to provide a good fit into experimental data. The extremity viscosities (η∞,γ and η0,γ ) were determined through Herschel–Bulkley, Cross and Carreau models. Most of the infinite-shear rate viscosity

253

(η∞,γ ) estimated by Cross is greater than Carreau and value estimated by Carreau was greater than Herschel–Bulkley as shown in Table 1. The different values of infinite-shear rate viscosity predicted by Herschel–Bulkley, Cross and Carreau were compared. The differences in the highest and lowest values of infinite-shear rate viscosity for five bio-edible oils at 50 and 90 ◦ C are 2.54 × 10−2 and 5.47 × 10−3 Pa s, respectively. The differences of the η∞,γ estimated by different models are not very significant and therefore, the results are considered acceptable. The η∞,γ is found to decrease as the temperature increases, which suggests that less internal friction is encountered as the temperature increases. However, the zero-shear rate viscosity (η0,γ ) estimated by Cross and Carreau models is not acceptable. This is because large variation of zero-shear rate viscosity was significantly observed when different models were used. The flow behaviors of the oils only consist of one initial-Power Law and final-Newtonian regions. The existence of characteristic time λc and αc in Carreau and Cross models, respectively, which reflect the onset of Power Law region has incorrectly estimated the η0,γ value. Wrong prediction of η0,γ value can be avoided by using an appropriate method. Alternatively, gel strength (τ g ) was used to describe the low shear rate viscosity as an approximation to η0,γ value. Gel strength is equal to shear stress at which fluid movement begins. This measurement is commonly conducted at the lowest speed to note the shear stress reading at which the gel structure is broken (usually at 3 rpm). It is found gel strength of the oils followed this sequence: τ g(sunflower) > τ g(coconut) > τ g(canola) > τ g(corn) > τ g(superolein) (Table 2). This indicates superolein requires less energy, but sunflower requires more energy before fluid movement begins. Moreover, the gel strength is found to decrease as the temperature increases. Hence, temperature increase of the oils would reduce an excessive drag and crankTable 2 Gel strength of five bio-edible oils at four different temperatures

90 ◦ C,

Fig. 2. Experimental plotted data of sunflower oil at with shear rate ranged 26.3–131.6 s−1 fitted by four shear rate dependent models.

Gel strength, τ g (kg m−1 s−2 )

Temperature (◦ C)

Sunflower Coconut Canola Corn Superolein

40 60 80 100

28.4 22.9 18.6 15.8

23.3 19.4 15.0 12.2

19.4 13.4 9.1 7.1

16.6 11.8 8.3 6.7

16.2 8.3 5.1 3.2

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Table 3 Predicted parameters for bio-edible oils at selected spindle speeds Parameter

Speeds (rpm)

Superolein

Coconut

Corn

Canola

Sunflower

η∞,T

100 60 20

2.10E−06 1.71E−06 2.58E−06

2.91E−06 2.98E−06 2.40E−05

3.47E−06 4.30E−06 1.46E−05

3.94E−06 3.56E−06 1.75E−05

5.83E−06 7.52E−06 1.47E−04

Ea

100 60 20

2.52E+04 2.59E+04 2.48E+04

2.42E+04 2.42E+04 1.88E+04

2.35E+04 2.29E+04 1.98E+04

2.32E+04 2.36E+04 1.95E+04

2.21E+04 2.15E+04 1.39E+04

ing difficulty during the start-up operation especially in a fluid power system, which commonly operated with temperature range 35–55 ◦ C. Therefore, in terms of gel strength consideration, it would be an advantage for superolein oil in a hydraulic fluid power system where the unwanted energy consumption, drag and cranking difficulty during start-up would be reduced. Temperature also affects the viscosity. The influence of temperature on viscosity is even more significant than shear rate. From Fig. 3, it is found that the viscosity of all bio-edible oils converged and approached low viscosity oil as the temperature increases. Similar behavior was also observed for peanut oil–diesel fuel blends (Goodrum and Law, 1982), biodiesel blends with diesel (Mustafa and Gerpen, 1999) and Slovenian wines (Kosmerl et al., 2000). The increase in temperature tends to increase molecular interchange (motion) and reduce attractive forces between molecules. However, in liquid, the reduction in attractive forces is much more significant than increases in molecular interchange and therefore viscosity decreases with increasing temperature. With reference to Fig. 3, it is graphically observed that sunflower exhibits best viscosity stability as tem-

perature varies from 40 to 100 ◦ C. For percentage variation of viscosity between 40 and 100 ◦ C, sunflower has 73.9% (at 60 rpm) of viscosity at 40 ◦ C. While corn, canola, coconut and superolein exhibit 75.7, 76.6, 77.8 and 79.8%, respectively (at 60 rpm), over the same temperature range. Among all oils, sunflower and superolein have the best and worst temperature stability behavior as temperature varies from 40 to 100 ◦ C, respectively. The Arrhenius-type relationship was used to elucidate the experimental data to further support the statement mentioned previously. The estimated activation energy (Ea ) reflects the viscosity-temperature stability of the oil. Hence, oil with the smallest and the highest values of Ea indicate the best and worst viscosity-temperature stability oil, respectively. The activation energies for sunflower, corn, canola, coconut and superolein are 2.15 × 104 , 2.29 × 104 , 2.36 × 104 , 2.42 × 104 and 2.59 × 104 N m mol−1 , respectively at 60 rpm (Table 3). This is inline with the graphical observation of Fig. 3. Table 3 also shows activation energy values for spindle speeds at 100 and 20 rpm. It shows that sunflower and superolein have the lowest and highest Ea values, respectively irrespective of spindle speeds. Again, this indicates that in terms of temperature effect, sunflower has outstanding viscosity stability with respect to temperature. This suggests that sunflower is suitable to be used as a blending oil to reduce variation of viscosity with respect to temperature of blended vegetable oils. A good hydraulic fluid should poses high viscosity index. 4. Conclusions

Fig. 3. The effect of temperature on viscosity of bio-edible oils.

In this study, five bio-edible oils were subjected to rheological evaluations. It was found that the viscosity of all oils investigated had shown shear rate and

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temperature dependence where the viscosity of the oils reduces as the temperature and shear rate increase. The effect of temperature on the changes of viscosity was found more significant than the effect of shear rate. Interpretation of rheological properties by Ostwald de-Waele, Herschel–Bulkley, Cross and Carreau models indicate that all oils under study exhibit pseudoplastic behavior. This rheology behavior is important where excessive reduction in viscosity might fail its function especially for a lubricating fluid or as energy transport media in hydraulic system. Moreover, this shear-thinning behavior was found less significant as the temperature increased for all the oils investigated. Therefore, heating the oil would eventually shift the oils towards Newtonian behavior. With the lowest gel strength and the outstanding Newtonian behavior, superolein with shear rate ranged 3.9–131.6 s−1 showed a promising characteristic for industrial application, especially in conditions where the fluid power system frequently performed start-up and shut-down operations. In addition, sunflower oil was found to have the highest viscosity stability with respect to temperature. Acknowledgements We greatly acknowledge the financial support from KUSTEM through vot 55002 and MOSTE vot 74033. Assistance from Mr. Azhar and Mr. Zaki during the experiment is also acknowledged. References Allawzi, M., Abu-Arabi, M., Al-Zoubi, K., Tamimi, A., 1998. Physicochemical characteristics and thermal stability of jordanian jojoba oil. JAOCS 75, 57–62. Al-Zahrani, S.M., 1997. A generalized rheological model for shear thinning fluids. J. Pet. Sci. Eng. 17, 211–215. Al-Zahrani, S.M., Al-Fariss, T.F., 1998. A general model for the viscosity of waxy oils. Chem. Eng. Process. 37, 422–437. Barreto, P.L.M., Roeder, J., Crespo, J.S., Maciel, G.R., Terenzi, H., Pires, A.T.N., Soldi, V., 2003. Effects of concentration, temperature and plasticizer content on rheological properties of sodium caseinate and sodium caseinate/sorbitol solutions and glass transition of their films. Food Chem. 82, 425–431. Brookfield Engineering Laboratories, Inc., 2000. More Solution to Sticky Problems: A Guide to Getting More From Your Brookfield Viscometer, Middleboro, USA, pp. 1–50.

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