High Pressure Behavior of Oil Extracted from Green Alga Botryococcus braunii

Transcription

1 40th Leeds-Lyon Symposium on Tribology & Tribochemistry Forum 013 September 4-6, 013, Lyon, France High Pressure Behavior of Oil Extracted from Green Alga Botryococcus braunii B. Zhang 1*, T. Mawatari 1, N. Ohno 1, K. Tajima 1 M. Ozeki, H. Kuno, S. Bunne, H. Fukuda, M. M. Watanabe 3 1) Graduate school of science and engineering, Saga University, 1 Honjo-machi, Saga-shi, Saga , Japan. ) DENSO ORPORATION, 1-1 Showa-cho, Kariya-shi, Aichi , Japan. 3) University of Tsukuba, Tennodai, Tsukuba, Ibaraki 30-87, Japan * orresponding author: zhang@me.saga-u.ac.jp Abstract Green alga Botryococcus braunii is regarded as a potential source of bio-diesel fuel because of its ability to produce large amounts of hydrocarbons and its widespread in the world. One of the most potential oil extracted from green alga B. braunii was investigated for its rheological behavior and the state transition from liquid to solid under high pressure at different temperatures. The results were compared with that of other natural oils such as squalene and squalane, and synthetic oil of PAO3. It is found that the B. braunii oil has the highest pressure viscosity coefficient among the studied four oils. A novel method of measuring the fractional free volume which is related to various high pressure properties of liquids, such as the bulk modulus and pressure viscosity coefficient, is proposed. The proposed method has advantages over the existing one in convenience of experiments. The fractional free volume can be estimated from lower pressure test, instead of higher pressure as the existing method. 1. Introduction Green alga Botryococcus braunii is regarded as a potential source of bio-diesel fuel because of its ability to produce large amounts of hydrocarbons and its widespread in the world. Depending on the strain and growth conditions, up to 86% of algal dry mass can be hydrocarbons [1]. Being a photosynthetic organism it has been reported to reduce O emissions by 18 ton/yr/ ha []. There are three different races of B. braunii, based on the characteristic hydrocarbon they produce. The A race produces odd-numbered to 31, n-alkadienes, and trienes. These linear olefins can constitute up to 61% of the dry cell mass [3]. The B race produces polymethylated unsaturated triterpenes known as botrycoccenes ( n H n-10, n=30-37) which may constitute from 7 to 86% of the dry cell mass [1]. The L race produces lycopadiene, a 40 tetraterpene which may constitute from to 8% of the dry cell mass [4]. The B. braunii oil used in this study is triterpene 34 H 8 extracted from botrycoccenes produced by the B race. The fuel in diesel engine is generally supplied from a common rail in which the pressure may be as high as 00 MPa, which is obtained by using a high pressure pump. The pressure in the common rail may be increased up to 300 MPa for the next generation diesel engine to improve the fuel efficiency and to reduce the exhaust emissions. The high pressure pump of fuel supply is also lubricated with diesel fuel, in which highly loaded contacts occur. It is reported that the lubricity of fuel determines the performance of the high pressure pump []. A primary experimental study has shown that the B. braunii oil of triterpene is also with high potentiality as a lubricant [6]. In addition to the use of bio-diesel fuel, the B. braunii oil is expected to provide a renewable lubricant. Like vegetable oils, the B. braunii oil is easy in biodegradation, and, hence, environmentally friendly. In this paper, the high pressure properties of the B. brauii oil were investigated, which are necessary for the uses of both bio-diesel fuel and renewable lubricant. omparison was conducted with other natural oils and synthetic oil.. Experimentation.1. Bulk modulus at high pressure Figure 1 shows the schematic of the high pressure densitometer used in this study. ml of test fluid is filled into a cylindrical pressure space of 1 mm in diameter, which is ended with the upper and lower plunges. The lower plunge is fixed to a load cell and the upper plunge is driven by a hydraulic power unit. The volume change of test fluid during pressing is monitored by measuring the displacement of the upper plunge with a linear gauge. To eliminate the effect of friction force between plunge and cylinder both the loads of the upper and the lower plunges are measured and the pressure within the pressure space is calculated by the following expression (1) where A is the plunge section area, L up and L lw are the loads of the upper and lower plunges, respectively. The test temperature is measured with thermocouple thermometer which is inserted into the lower plunge, and is kept in constant by using a thermostatic bath during test. The isothermal bulk modulus K is calculated by

2 IT Hiroshima densities of the four test oils are almost the same and, except PAO3 which has much higher viscosity, the viscosity of the other three oils are in the same order of magnitude. Figure 3 shows the structures of the B. braunii oil, Squalane, and Squalene. All the three oils have a linear chain structure. Squalane has only single bonds while the B. braunii oil has 6 double bonds but only one in the middle of backbone chain and Squalene has 6 double bonds all in the backbone chain. Hence the flexibility of backbone chain increases in an order of Squalene, the B. braunii oil, and Squalane. PAO3 has only single bonds like Squalane, but has much longer branches. The molecular weights of the B. braunii oil, squalane, and squalene are 466, 43, and 411 g/mol, respectively. 1 high pressure vessel, upper plunge, 3 high pressure space, 4 lower plunge, thermocouple, 6 load cell, 7 thermostatic bath Fig. 1 Schematic of high pressure densitometer ().. Viscosity at high pressure Viscosity at high pressure was measured by using a high pressure falling ball viscometer (Fig.). The structure of the viscometer is very similar to that of the densitometer. The high pressure space of the viscometer is much longer than that of the densitometer to give the falling ball a sufficiently long moving distance. The ball is captured by an electromagnet which is attached to the end surface of the upper plunge. The falling velocity of the ball is estimated by measuring the passage time of ball through a sapphire window which is located in the lower part of the high pressure space. When the ball passes through the windows, the light across the windows will be blocked and the breakout time is a measurement of the ball velocity. This method is only applicable to the transparent test liquid. The test temperature is measured with a thermocouple thermometer inserted in the end of the lower plunge and is kept in constant by using a thermostatic bath. 3. Experimental Results and Discussions 3.1. Bulk modulus Figure 4 shows the dependence of the density of the B. braunii oil on pressure at four different temperatures. The density increases with pressure monotonically, but in a concave function, that is, the increase rate reduces with pressure. In the studied temperature range of 6.6 K to 93 K, the effect of temperature on density is minor. The bulk modulus of the B. braunii oil, which is calculated by equation () from the results of Fig. 4 is shown in Fig.. It is clear, from Fig., that the bulk modulus increases with pressure in two different states: low pressure state and high pressure state. In both the states the bulk modulus almost increases linearly with pressure, but the increase rate is much lower in low pressure state. The discontinuity point of the bulk Test fluids Besides the B. braunii oil, squalane, squalene and PAO3 were also investigated for comparison. The density and dynamic viscosity at atmosphere pressure of the four test oils were summarized in Table 1. The Table 1 Viscosity at atmosphere pressure of typical natural oils Dens. (g/cm 3 ) Dyn. Visc., mm /s 88 K 313 K 373 K B. braunii Squalane Squalene PAO high pressure vessel, upper plunge, 3 high pressure space, 4 lower plunge, thermocouple, 6 load cell, 7 thermostatic bath, 8 electromagnet, 9 falling ball, 10sapphire windows Fig. Schematic of high pressure viscometer 6

3 Bulk modulus K, GPa Template for Extended Abstract H H H H H 3 H H H (a) the B. braunii oil ( 34 H 8, Mw=466) H 3 H H H H H H H (b) Squalane ( 30 H 6, Mw=43) H H H H H (c) Squalene ( 30 H 0, Mw=411) Fig. 3 hemical structure of the B. braunii oil triterpene, Squalane, and Squalene modulus is the liquid-to-solid transitional point of the B. braunii oil. The volume of liquids can be divided into two different parts: the free volume and the occupied volume. In liquid state the decrease in the volume with pressure is mainly due to the decrease in the free volume, while, in solid state, the compressibility is attributed to the occupied volume which is much stiffer than the free volume. Therefore the sudden increase in the bulk modulus with pressure is an indicator of the transition of liquid to solid. The bulk modulus K can be decomposed as follows (3) where and are the bulk moduli of the free volume and the occupied volume, respectively. The bulk Density, g/cm B. braunii oil 6.6 K 69.3 K 83 K 93 K 10 B. braunii oil 6.6 K 69.3 K 83 K 93 K Fig. 4 Dependence of density of the B. braunii oil on pressure at different temperature Fig. Dependence of bulk modulus of the B. braunii oil on pressure at different temperatures 3

4 IT Hiroshima 011 modulus of the free volume defined above is the nominal value and its real value should be given by (4) () where f is the fractional free volume. If the real bulk modulus of the free volume is independent of pressure, the nominal bulk modulus will be inversely proportional to the fraction of the free volume which decreases as pressure increases. It is expected that the bulk modulus will increase with pressure smoothly since it is reasonable to assume that the free volume decreases with pressure continually. The sudden increase in the bulk modulus suggested that the free volume is blocked at the same time when the solidification occurs, that is, the free volume loses its fluidity in a solid. The solidification is the blocking of the free volume rather than the vanishing of the free volume. Therefore after the liquid to solid transition the bulk modulus is given by (6) From Fig. it is known that for both liquid and solid the bulk modulus may be related to pressure by the Murnaghan equation [7] (7) where are the constants, depending on both temperature and the state of material. are summarized in Tables and 3 for liquid and solid, respectively. As published in the literature [8], tables and 3 show that are almost independent of temperature for both liquid and solid. This means that are not structure sensitive parameters. This is a natural conclusion if we remind that the bulk modulus is not structure sensitive. Table onstants of equation (7) for liquid Temperature (K) Bulk modulus K p0 (GPa) Pressure coef. kp Table 3 onstants of equation (7) for solid Temperature (K) Bulk modulus K p0 (GPa) Pressure coef. kp Now let us explore the physical meaning of kp in equation (7). For liquid it is reasonable to assume that the occupied volume has a much high bulk modulus than the free volume and then we have (8) Furthermore the real bulk modulus is expected to be independent of pressure. Substituting this into equation (8) gives Since equation (10) can be approximated by (9) (10) (11) (1) omparing equation (1) with equation (7), we know that (13) (14) The value of kp in Table gives an estimation of the fractional free volume for the B. braunii oil about 18~%, and the real bulk modulus of the free volume about 193~ MPa. The fractional free volume at pressure p may be obtained from logarithmic strain for large deformation Then we have (1) (16) Another way to estimate the fractional free volume is to assume that the bulk modulus of the occupied volume (solid) is independent of the pressure and any deviation 4

5 Temperature T, K Viscosity, Pa s from the constant bulk modulus is attributed to the free volume [9]. The bulk modulus of the occupied volume is obtained from the volume change under such a high pressure that the liquid has been solidified. Template for Extended Abstract (17) in Fig. 6. The B. braunii oil, PAO3 and squalene have a similar dependence of the transition temperature on pressure, but the transition temperature decreases in an order of the B. braunii oil, squalene and PAO3. Squalane has a strongest dependence of the transition temperature on pressure among the four materials. where K o is constant. Therefore the occupied volume at any pressure is estimated by (18) where om is the occupied volume density at high pressure p m. Then the fractional free volume can be approximated by (19) where is the measured density. This method has much more experimental difficulties than the method proposed in this paper since the bulk modulus of the occupied volume has to be measured at pressure as high as, say, 1 GPa. 3.. Phase diagram The discontinuity of the bulk modulus in Fig. is the liquid to solid transition above which configurational rearrangements of molecular chain backbones are extremely slow. Figure 6 shows the phase diagram of the B. braunii oil, together with squalane, squalene and PAO3 for comparison. The curves follow the empirical equation as follows. (0) where T 0 is the solid transition temperature at atmosphere pressure, and and D are constants. It is known, from Fig. 6, that, for the four oils, the solid transition temperature is almost linear to pressure. This is because D in equation (0) is less than 1 for all the oils 3.3. Viscosity The viscosity of the B. braunii oil increases with the pressure at all the experimental temperatures, and the dependence of viscosity on pressure may be expressed approximately by the Barus equation. (1) The pressure viscosity coefficient at different temperatures by fitting the measured viscosity into equation (1) are summarized in Table 4, and the calculated viscosity from equation (1) by using the estimated pressure viscosity coefficient is also shown in Fig. 7 for comparison. It is known, from Fig. 7, that the estimation from equation (1) is in very good agreement with the experimental results within the range of pressure applied in this study. The pressure viscosity coefficient decreases as temperature increases. In Table 4 the pressure viscosity coefficients of squalene, squalane, and PAO3 are also listed for comparison. It is known that the B. braunii oil has the highest pressure viscosity coefficient among the four oils. The high pressure viscosity of the B. braunii oil may be related to its chemical structure of multi double bonded branches as shown in Fig. 3. The double bonded branch will be interlocked under high pressure, and therefore the viscosity increases. Table 4 Pressure viscosity coefficient at different temperatures Pressure Viscosity oefficient, GPa K 93 K 313 K 333 K B. braunii Squalene Squalane (11.3) - PAO :B. braunii oil exp. :B. braunii oil fitted :Squalane fitted :Squalene fitted :PAO3 Liquid B. braunii oil T=73 K 93 K 313 K 333 K Solid Fig.6 Phase diagram of B. braunii oil together with squalane, squalene and PAO Fig.7 Dependence of viscosity on pressure at different temperatures for the B. braunii oil

6 4. onclusions High pressure experiments on the B. braunii oil were carried out for its uses of diesel fuel and lubricant, and comparison with other three different oils were conduced. The conclusions are as follows. (1) The fractional free volume of liquid can be estimated by measuring the dependence of the liquid bulk modulus on pressure. () The dependence of the fractional free volume on pressure can be estimated by equation (16). (3) The B. braunii oil has the highest pressure viscosity coefficient among the four oils: the B. braunii oil, Squalene, Squalane, PAO3. IT Hiroshima 011. References [1] Brown, A. G. and Knights, B. A., Hydrocarbon content and its relationship to physiological state in the green alga Botryococcus braunii, Phytochemistry, 8(1969), [] Sawayama, S., Minowa, T., and Yokayama, S., Possibility of renewable energy production and carbon dioxide mitigation by thermochemical liquefaction of microalgae, Biomass Bioenergy, 17 (1999), [3] Gelpi, E., Schneider, H., Mann, J., and Oro, J., Hydrocarbons of geochemical significance in microscopic algae, Phytochemistry, 9 (1970), 603- [4] Metzger, P. and asadevall, E., Lycopadiene, a tetraterpendoid hydrocarbon from new strains of the green alga Botryococcus braunii, Tetrahedron Letters, 4 (1987), 30-. [] Matzke, M., Litzow, U., Jess, A., aprotti, R., and Balfour, G., Diesel lubricity requirements of future fuel injection equipment, SAE International, [6] Ikejima, S., Nagakubo, M., and Watanabe, M., Lubricant oil extracted from microalgae, Petrotech, 3 () (01), [7] Murnaghan, F. D., Finite deformation of an elastic solid, Wiley, New York, 191. [8] utler, W. G., McMickle, R. H., Webb, W., and Schiessler, R. W., Study of the compressions of several high molecular weight hydrocarbons, J. hem. Phys., 9(4) (198), [9] Ohno, N., Sunahara, K., Kumamoto, T., and Hirano, F., Prediction of liquid lubricant viscosity at high pressure from the density measurements, Journal of JAST, Tribologist, 44(7) (1999),