Low-temperature Properties of Biodiesel: Rheological Behavior and Crystallization Morphology

Transcription

1 Oil Product Research Low-temperature Properties of Biodiesel: Rheological Behavior and Crystallization Morphology Chen Boshui; Sun Yuqiu; Fang Jianhua; Wang Jiu; Wu Jiang (Department of Petrochemistry, Logistical Engineering University, Chongqing 43, China) Abstract: A soybean oil derived biodiesel was prepared and blended with a conventional No. petrodiesel. The pour points (PP) and the cold filter plugging points (CFPP) of biodiesel blends were evaluated on a low-temperature flow tester. Dynamic viscosities of the blends at different temperatures and different shear rates were measured on a rotary rheometer. The crystal morphologies of biodiesel blends at low temperatures were analyzed using a polarizing microscope. The results indicated that blended fuels demonstrated slight decrease in PPs and CFPPs as compared with those of neat soybean oil derived biodiesel and pure petrodiesel. Below the temperatures of PPs or CFPPs, the dynamic viscosity of biodiesel blends dramatically increased with a decreasing temperature, but decreased with an increasing shear rate, so that biodiesel blends exhibited non-newtonian behavior. At temperatures higher than PPs or CFPPs, a linear relationship appeared between the dynamic viscosity and shear rate and biodiesel blends became Newtonian fluids. At low temperatures, wax crystals of biodiesel blends grew and agglomerated rapidly. Loss of fluidity for biodiesel blends at low temperatures could therefore be attributed on one hand to the sharp increase of viscosity and on the other hand to the rapid growth and agglomeration of wax crystals. Key words: biodiesel; cold flow property; viscosity; rheology; crystallization Introduction Biodiesel, derived via transesterification of renewable vegetable oil. One of the major problems associated with the use of biodiesel as the diet for diesel engines is its poor low-temperature flow property. It has been found that crystallization or thickening of biodiesel at low temperatures causes fuel flow interruption and operability problems as solidified materials can clog fuel lines and filters, which are mainly caused by large amount of saturated FAME components [7-9] in the fuel blends. In recent years, several approaches to solve the low-temperature problems of biodiesel have been investigated including blending with conventional petrodiesel, winterization, use of additives and branched-chain esters, and incorporating bulky constituents in the alkyl chain [-6]. In practice, biodiesel is often used at different blends with petrodiesel such as B, B5 and B8 rather than using B. The present work deals with the cold flow properties of blends consisting of a soybean-based biodiesel with a conventional No. petrodiesel. The pour points, the cold filter plugging points, the viscosity-temperature relationship, and the viscosity-shear rate relationship of biodiesel blends were tested. The crystal morphologies of the blends at low temperatures were also investigated. Experimental. Preparation of soybean biodiesel Soybean oil based biodiesel (SME) was prepared by reacting 7 g of soybean oil upon 5g of CH 3OH and 7g of NaOH. The reaction was carried out in minutes under reflux at 6 65 with vigorous stirring. After termination of the reaction, the reaction mixture was allowed to stand overnight and the methyl ester layer was separated from the glycerol layer using a separatory Corresponding author: Dr. Chen Boshui, Telephone: ; chenboshui@yahoo.com.cn; Postal address: Dept.of Petrochemistry, Logistical Engineering University, University town, Shapingba District,Chongqing 43, P. R. of China. 9

2 funnel. Residual amount of glycerol in the crude methyl ester was removed by centrifugation. The methyl ester was purified by distilling out the unreacted methanol under atmospheric pressure, followed by washing several times with water, centrifugation and drying with anhydrous Na SO 4.. Blending of biodiesel with petrodiesel Different blends of soybean-based biodiesel and conventional No. petrodiesel, i.e.: B (% biodiesel and % petrodiesel), B (% biodiesel and 8% petrodiesel), B5 (5% biodiesel and 5% petrodiesel), B8 (8% biodiesel and % petrodiesel) and B (% biodiesel and % petrodiesel), were prepared by uniformly blending a proper amount of biodiesel with petrodiesel on a volume basis. No. petrodiesel was obtained from the Yanshan oil refinery of Sinopec in Beijing. Some typical specifications of No. petrodiesel are presented in Table. Table Specifications of No. petrodiesel(diesel fuel standard: GB/T947 3) Properties Value Flash point (closed cup), 55 Boiling point, Kinematic viscosity ( ), mm /s Density ( ), kg/m Pour point, Cold filter plugging point, 4 Cetane number 49 Sulfur content, m%.5 Ash content, m%..3 PP and CFPP measurements Pour point (PP) and cold filter plugging point (CFPP) are important indicators related to low-temperature operability of diesel fuels. The pour point is the temperature at which solid crystals are first visually observed as the fuel cools, while the cold filter plugging point is the temperature at which a fuel will cause a fuel filter to plug due to fuel components which have begun to crystallize or gel. In the present work, PP and CFPP of the biodiesel blends were measured in a low-temperature flow tester in accordance with the SH/T48 procedures. (SH/T48 is a Chinese method for determining PP and CFPP and is equivalent to ASTM D 97 and EN 6 for the determination of PP and CFPP, respectively.).4 Viscosity measurements Diesel fuel viscosity not only affects atomization and density, but also can influence the cold flow property. In the present investigation, dynamic viscosities of the blends at different temperatures and different shear rates were measured on a VIARMES 957 rotary rheometer (made by Sanchez Technologies, France). The temperatures in the present test ranged from 6 down to 5, while the shear rates were set to, 4, 6, 8, s, respectively..5 Crystal morphology observation Crystal morphologies of a biodiesel blend, i.e. B8, at low temperatures were observed using a polarizing microscope modeled DMLP (made by Leica, Germany). The microscope was equipped with a cooling system and, prior to observation, the biodiesel blend was progressively cooled down with liquid nitrogen from room temperature at a rate of per minute. When observable crystals began to appear at certain temperatures, the cooling rate was slowed down and carefully controlled at ca. per minute so as to ensure plenary growth of wax crystals. 3 Results and Discussion 3. PP and CFPP of fuel blends Pour points and cold filter plugging points of biodiesel blends are shown in Table. It can be seen from Table that PPs and CFPPs of the blended fuels, i.e. B, B5 and B8, decreased slightly as compared with those of neat soybean biodiesel (B) and pure petrodiesel (B). This might be due to the specific interactions between FAME molecules of the soybean biodiesel and hydrocarbon molecules of the # grade petrodiesel, although Table PPs and CFPPs of biodiesel blends Blends PP, CFPP, B B B5 B8 3 B 3

3 the magnitude of temperature decreases was not obvious. 3. Viscosity The dynamic viscosities of biodiesel blends as a function of temperature are shown in Figure. It can be seen from Figure that dynamic viscosities of biodiesel blends increased with a decreasing temperature. It is also clear that viscosities of the biodiesel blends dramatically increased with a decreasing temperature below, which was just close to the pour point or cold filter plugging point of the individual fuel as shown in Table. The sharp increase in viscosity at temperatures below PP or CFPP might be an attribute to poor cold flow properties of biodiesel blends, although the viscosity increases identified below PP or CFPP temperatures somewhat differed among blends with different contents of biodiesel. Figure shows the dynamic viscosity of B8 as a function of shear rate at 5,, 5 respectively. Dynamic viscosity, mpa s It can be seen from Figure that the relationship between dynamic viscosity and shear rate was different for B8 at different temperatures. At temperatures higher than PP, or CFPP of B8, e.g. and 5, an approximate linear relationship between shear rate and dynamic viscosity could be detected. This indicates that at temperatures higher than PP or CFPP, blends of soybean biodiesel with # petrodiesel exhibited a Newtonian behavior, although at (only higher than CFPP of B8 as shown in Table.) and at lower shear rates, a slight decrease in dynamic viscosity with an increasing shear rate could be seen. However, at temperatures lower than PP or CFPP, e.g. 5, the decrease in viscosity with an increasing shear rate was obvious, indicating that blends of biodiesel with petrodiesel became non- Newtonian fluids below PP or CFPP. 3.3 Crystal Morphology Shown in Figure 3 are polarizing microscopic images of crystal morphologies of B8 at 5,, 5,, respectively. It is known that in polarizing microscopic analysis, crystal substances can effectively reflect polarized light and can therefore be observed clearly by exhibiting white images, while amorphous substances on the contrary can Temperature, Figure Dynamic viscosity versus temperature of biodiesel blends B; B; B5; B8; B a 5 b Dynamic viscosity, mpa s Shear rate, s Figure Dynamic viscosity of B8 versus shear rate 5 ; ; 5 c 5 d Figure 3 Polariscopic images of crystal morphologies of B8 at low temperatures ( 4) 3

4 only exhibit black images. Therefore it is realized from Figure 3 that the amount and the size of crystals produced by B8 at different temperatures were quite different. At a temperature just below PP and CFPP, e.g. 5, only a small amount of tiny wax crystals were observed (Figure 3 a), while at lower temperatures such as, 5,, respectively, larger amount and bigger size of wax crystals were produced, and the crystals rapidly grew and agglomerated with decreasing temperatures (as shown in Figure 3 b, c and d). It can therefore be inferred from the crystal morphological analysis that rapid growth and agglomeration of wax crystals might be another attribute for biodiesel blends to lose their fluidity at low temperatures. 4 Conclusions The following conclusions can be drawn from the abovementioned facts: () Blends of soybean biodiesel and No. petrodiesel demonstrated a slight decrease in PPs and CFPPs as compared with those of neat soybean biodiesel and pure No. petrodiesel. () Below the temperatures of pour points or cold filter plugging points, dynamic viscosities of biodiesel blends dramatically increased with a decreasing temperature, which might be attributed to poor cold flow properties of biodiesel blends. At temperatures higher than PPs or CFPPs, biodiesel blends exhibited Newtonian behavior, while at temperatures lower than PPs or CFPPs, they became non-newtonian fluids. (3) At temperatures below PPs and CFPPs, wax crystals of biodiesel blends rapidly grew and agglomerated with decreasing temperatures, which might be another attribute for biodiesel blends to lose their fluidity at low temperatures. Acknowledgement: The authors gratefully acknowledge the financial support of the Natural Science Foundation of Chongqing(project No. CSTC6BA63)and the Program for New Century Excellent Talents in Chinese Universities (project No. NCET-4-). Special thanks go to Dr. Lun Zengmin of The Oil Exploration Research Institute, SINOPEC, for his technical support with the viscosity measurements. References [] Nye M J, Southwell P H. Esters from rapeseed oil as diesel fuel. Proceedings of Vegetable Oil as Diesel Fuel- Seminar III, 983, Peoria, IL [] Ali Y, Hanna M A. Alternative diesel fuels from vegetable oils[j]. Bioresource Technology, 994, 5: [3] Agarwal A K, Das L M. Biodiesel development and characterization for use as a fuel in compression ignition engines[j]. Transactions of the ASME, Journal of Engineering for Gas Turbines and Power,, 3: [4] Demirbas A. Progress and recent trends in biofuels[j]. Progress in Energy and Combustion Science, 7, 33: -8 [5] Demirbas A. Diesel fuel from vegetable oil via transesterification and soap pyrolysis[j]. Energy Sources,, 4: [6] Knothe G. Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Processing Technology, 5, 86: 59 7 [7] Rushang M J, Michael J P. Flow properties of biodiesel fuel blends at low temperatures[j]. Fuel, 7, 86: 43-5 [8] Kerschbaum S, Rinke G. Measurement of the temperature dependent viscosity of biodiesel fuels[j]. Fuel, 3, 83: 87-9 [9] Dunn R O, Bagby M O. Low-temperature properties of triglyceride-based diesel fuels: transesterified methyl esters and petroleum middle distillate/ester blends. Journal of the American Oil and Chemical Society, 995, 7: [] Dunn R O, Shockley M W, Bagby M O. Improving the low-temperature properties of alternative diesel fuels: vegetable oil-derived methyl esters[j]. J Am Oil Chem Soc, 996, 73: [] Gomez M, Howard H R, Leahy J J, Rice B. Winterization of waste cooking oil methyl ester to improve cold temperature fuel properties[j]. Fuel,, 8: [] Lee I, Johnson L A, Hammond E G. Use of branchedchain esters to reduce the crystallization temperature of biodiesel [J]. J Am Oil Chem Soc, 995, 7: 55-6 [3] Lee I, Johnson L A, Hammond E G. Reducing the crystallization temperature of biodiesel by winterizing methyl soyate [J]. J Am Oil Chem Soc, 996, 73: 63 [4] Chandler J E, Horneck F G, Brown G I. The effect of cold flow additives on low-temperature operability of diesel fuels[j]. SAE Technical Paper Series, Paper No Warrendale: Society of Automotive Engineers. 99 3

5 [5] Nestor U, Soriano J, Veronica P M, Masatoshi M. Ozonized vegetable oil as pour point depressant for neat biodiesel [J]. Fuel, 6; 85: 5-3 [6] Chuang W C, Leon G S, Galen J S. Impact of cold flow improvers on soybean biodiesel blend[j]. Biomass and Bioenergy, 4, 7: Till December 6, 9 the first in China large unit for manufacture of kt/a of styrene via direct alkylation of ethanol at Yuhuang Chemical Co., Ltd. in Heze city, Shandong province has been smoothly operating for two months with all process indicators meeting or exceeding the design values. This method has provided a new route to styrene production enterprises that do not have ethylene resources and has been highly appraised by a lot of styrene production enterprises located in Eastern China. This unit adopts a new process for manufacture of ethyl benzene via direct alkylation of ethanol followed by production of styrene through dehydrogenation of ethyl benzene. This unit since its startup on October 6, 39 has been working smoothly with the styrene purity reaching 99.9% and the product output equating to % of design target. Compared to traditional ethylene process, the new process features less heavy components in the alkylation products with ethyl benzene yield accounting for 5% % of the alkylation liquid product. The tail gas emission is reduced and the benzene tower overhead almost does not discharge the alkylation tailgas. The material balance of this process shows that the benzene and ethanol consumptions on one ton of ethylbenzene is less than the design value and the ethanol consumption is close to the theoretical value of 463 kg on one ton of ethylbenzene with the benzene consumption being equivalent to that achieved by the ethylene method. Furthermore, since the temperature of catalyst during reactions is lower than that of high-temperature ethylene alkylation process, the energy consumption of this unit is much lower. The Yuhuang Chemical Co. does not possess ethylene resources; it would pay more investment and operating cost if it is necessary to follow a long process route comprising dehydration of ethanol to manufacture ethylene followed by alkylation to manufacture ethylbenzene. Thereby the Yuhuang Chemical Co. has selected the process technology consisting of direct gas-phase alkylation of benzene with ethanol followed by adiabatic dehydrogenation of ethylbenzene under reduced pressure to manufacture styrene. The kt/a unit for production of ethylbenzene via direct alkylation of benzene in the presence of ethanol and the PDP for manufacture of 5 kt/a ethylbenzene developed by the Jiangsu Danyang Chemical and Pharmaceutical Design and Research Institute can guarantee less side reactions during dehydration and alkylation to achieve a higher than 99.5% of ethanol conversion, an ethylbenzene output of 6 8 t/h with the product purity reaching 99.6% 99.9%. This unit adopts the catalyst for ethanol dehydration and alkylation of benzene with ethylene jointly developed by the Dalian University of Technology and Nanjing Xuanda Chemical Co., Ltd. to ensure an over 99.5% of ethanol conversion with less side reactions and high ethylene utilization rate. Since the alkylation reaction is conducted under a lower temperature the tar produced during operation on the said catalyst is apparently less than that achieved by other catalysts. The dehydrogenation reactor is a domestically developed axial-radial reactor with a high utilization rate of reactor volume and catalyst. The dehydration products are subjected to fractionation and purification in a four-column process flow scheme, in which the raw styrene column and pure styrene column are equipped with high-efficiency packing coupled with rectification under reduced pressure. The process condensate and tail gas are thoroughly treated to minimize discharge of waste gas, liquid and solids. 33