Article pubs.acs.org/EF
High-Pressure Viscosity of Soybean-Oil-Based Biodiesel Blends with Ultra-Low-Sulfur Diesel Fuel Andrew M. Duncan,† Noorbahiyah Pavlicek,† Christopher D. Depcik,‡,§ Aaron M. Scurto,†,‡,∥ and Susan M. Stagg-Williams*,†,‡,∥ †
Department of Chemical and Petroleum Engineering, ‡DOT-KS Transportation Research Institute, §Department of Mechanical Engineering, and ∥Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States ABSTRACT: Current and future injector designs for diesel engines approach pressures of greater than 100 MPa. However, the high-pressure physical properties, such as viscosity, of biodiesel blends with diesel fuel are nearly absent in the literature. This study focuses on the viscosity of biodiesel, diesel, and several biodiesel blends from 283.15 to 373.15 K and pressures up to 131 MPa. Soybean biodiesel (B100) and ultra-low-sulfur diesel (ULSD, B0) were combined to form biodiesel/diesel blends: B5, B10, B20, B40, B60, and B80. The viscosity of the samples increases linearly with the pressure until approximately 35 MPa, followed by a higher order response to pressure. The biodiesel blends with low biodiesel content (B5, B10, and B20) had very similar viscosities to ULSD. The normalized viscosity increased with a decreasing temperature and a decreasing biodiesel blend fraction. The viscosity of the various blends increased above the ambient pressure viscosity ranging from 164% at 373.15 K to 547% at 283.15 K at the highest pressure of 131 MPa. The biodiesel and some biodiesel blend samples at 283.15 K experienced pressureinduced cloud points (solid−liquid equilibrium) from 70 to 131 MPa. The Tait−Litovitz equation was found to correlate the biodiesel and diesel data with an absolute average relative deviation (AARD) of less than 1% over the large range of both temperature and pressure, and Kay’s mixing rule predicted the viscosity of the biodiesel blends with an AARD of 1.75%.
1. INTRODUCTION Biodiesel is a renewable alternative to petroleum diesel that is typically produced via transesterification of seed oils. The most common biodiesel production processes use methanol, which results in a product comprised of a mixture of fatty acid methyl esters (FAMEs). These FAMEs can be used as pure biodiesel (B100) or blended with petroleum diesel (BX), where X represents the volume fraction of biodiesel in the mixture. Biodiesel is produced in many countries, has low aromatic and sulfur contents, and has environmental benefits compared to petroleum diesel.1−3 Researchers have shown that compression ignition (CI) engines that burn diesel fuel can be operated successfully using biodiesel.4,5 Biodiesel fuels show promise as replacement fuels because they do not require any engine hardware modifications for combustion while retaining similar energy content to diesel.5,6 Furthermore, biodiesel is the only fuel available in commercial quantities in the U.S. that meets the definition of biomass-based diesel under Renewable Fuels Standard (RFS2) provided in the Energy Independence and Security Act of 2007.7 While there are many benefits to biodiesel, its use as a fuel does have some limitations. Although biodiesel can be substituted for petroleum diesel in engines, biodiesel typically clouds at temperatures 10−15 degrees higher than petroleum diesel8,9 and crystals that develop because of freezing of the fuel may cause problems with a vehicle’s fuel filters and fuel lines.10 A significant problem for biodiesel is that it is more prone to degradation when exposed to oxygen than petroleum diesel, and as a result, problems can arise with both injection systems and fuel filters as the fuel breaks down.11,12 Biodiesel also has greater ambient viscosity than petroleum diesel, a physical property important to injector systems, as mentioned later. © 2012 American Chemical Society
Finally, the production of biodiesel is not sufficient to replace the entire petroleum-based diesel consumed in the U.S. Blending petroleum diesel and biodiesel can be advantageous in many respects, including increasing the storage stability,13 lowering the cloud point14 of biodiesel, and increasing the lubricity of the fuel. In particular, the literature indicates that biodiesel and blends often have better lubricity characteristics than ultra-low-sulfur diesel (ULSD).15,16 Biodiesel’s higher lubricity reduces frictional losses and can reduce wear and injector nozzle erosion or corrosion.17 For these reasons, and the inability to replace completely the petroleum-based diesel production capacity, blending biodiesel with petroleum-based diesel will continue to be the more common method of biodiesel usage. For older engines that use mechanical fuel injection systems, viscosity is important to analyze because less-viscous fuels can leak through the clearances of the mechanical fuel pump and cause the fuel pressure to rise more slowly inside the pump. This causes the mechanical governor to compensate by increasing the fuel flow, reducing fuel economy.16 The increased viscosity of biodiesel can result in less leakage and a subsequent improvement in fuel economy for these older systems.18 For more modern, high-pressure common-rail injection systems, fuel leakage is not an issue.19,20 These current systems use high-pressure fuel pumps that can reach pressures up to 2200 bar for passenger car and commercial vehicles.21 Next-generation systems are targeting 2500 bar while also expanding the possible number of injections per Received: July 19, 2012 Revised: September 13, 2012 Published: September 17, 2012 7023
dx.doi.org/10.1021/ef3012068 | Energy Fuels 2012, 26, 7023−7036
Energy & Fuels
Article
cycle from five to nine.22 Under these pressures, the liquid viscosity of the fuel can increase hundreds of percent over atmospheric levels.23 Upon injection of the fuel in the cylinder, the large depressurization of the fuel results in a significant gradient of the viscous properties of the fluid. With respect to the increased viscosity of biodiesel over ULSD, this can reduce the atomization and breakup of the fuel spray within the engine, subsequently forming larger droplets, leading to a longer combustion burn.24,25 In brief, the longer the combustion time, the less constant-volume-like the combustion process and the lower the thermal efficiency of the engine. A proper understanding of viscosity is required for modeling the discharge coefficient and flow rate through the injector, which is key to ensuring the proper rate shaping and timing of the fuel injection spray to optimize performance while minimizing emissions.26 With respect to biodiesel combustion, a current paper by the authors18 indicates that the biodiesel molar unsaturation percentage, molar monounsaturation percentage, density, hydrogen/carbon molar ratio, energy content, oxygen content, and viscosity all play significant roles in how an engine performs and its resulting emissions. For instance, it was determined that, as biodiesel saturation increases, the fuel consumption increases, while the nitrogen oxide (NOx) emissions decrease. Similar to what others16,19,23,27−35 have observed, biodiesel use led to higher brake-specific fuel consumption but also resulted in lower emissions of carbon monoxide (CO), hydrocarbons (HCs), and particulate matter (PM). It is important to note that several of the fuels tested produced lower NOx emissions than ULSD because of improved combustion, lower energy content, and advantageous chemical properties. As a result, under the range of projected future injection pressure and temperature conditions (up to 2500 bar and 273.15−373.15 K36,37), the study of viscosity for neat and blended biodiesel plays a large role on engine outcomes. There are few studies in the literature of the high-pressure viscosity of biodiesel or related compounds. Bhide et al.38 used a high-pressure capillary viscometer to measure the viscosity of blends of dimethyl ether with diesel and soybean biodiesel to 24 MPa. Schaschke and co-workers39 have demonstrated a new high-pressure viscosity apparatus that uses a falling sinker to measure the viscosity of oils and free fatty acids to 150 MPa. This method requires very accurate knowledge of the highpressure density of the liquid sample to compute the viscosity, which is often unknown and must be estimated. They report on the viscosity of biodiesel from one sample of sunflower oil and one sample of waste cooking oil, as well as a B20 blend of mineral diesel with sunflower biodiesel at temperatures below ≤293.15 K and pressures to
