Energy & Fuels 2008, 22, 1073–1079
1073
Predicting the Effect of Cetane Improvers on Diesel Fuels Prasenjeet Ghosh* ExxonMobil Research and Engineering, 1545 Route 22 East, Annandale, New Jersey 08801 ReceiVed February 28, 2007. ReVised Manuscript ReceiVed August 29, 2007
A composition-based model is presented that predicts the effect of cetane improver (specifically 2-ethyl hexyl nitrate) on the cetane number of diesel fuels. A total of 206 different diesel fuels were considered in this work containing varying amounts of improver. The fuels were chosen to span a wide range of compositions, from highly paraffinic to highly naphthenic and, in some cases, highly aromatic fuels. Improver concentrations were varied between 0 to 3500 ppm (v/v), which exceeds the usual commercial application range of 500–1000 ppm. Detailed molecular composition of all the fuels was analyzed using a combination of GC-MS and supercritical fluid chromatographic techniques, while cetane number measurements were made using an Ignition Quality Tester (IQT). Molecular composition was correlated to the cetane number (CN) boost using a simple correlative equation, derived from phenomenological considerations. The model predicts CN boost with a standard error of (0.8 CN, which is within the experimental error of the measurements.
1. Introduction The cetane number (CN) is a measure of the ignition quality of a diesel fuel and is determined by a standard engine test as specified by ASTM (ASTM D613).1 Ignition quality is quantified by measuring the ignition delay, that is, the period between the time of injection and the start of combustion (ignition) of the fuel. Thus, a fuel with a high CN has a short ignition delay and starts to combust shortly after it is injected into an engine. The ignition quality of the diesel fuel depends on its molecular composition. Some of the simpler molecular components like the n-paraffins can ignite in a diesel engine with relative ease, but others like aromatics, that yield more stable ring structures, require higher temperature and pressure to ignite. Thus, depending on the molecular composition of the diesel fuel, a wide range of CNs ranging from 35 to 55 is observed. However, quite frequently, some special molecules, called cetane improvers, are added to the diesel fuel to increase its CN, much like oxygenates (e.g., ethanol and MTBE) are added to gasoline fuel to increase its octane number. The most common cetane improvers are alkyl-nitrates which act by decomposing into chain-initiating radicals, thereby facilitating the subsequent autoignition reactions. Among the various alkyl-nitrates, 2-ethylhexyl nitrate (EHN) is the most widely used improver commercially. The objective of this work is to develop a predictive model for CN based on molecular composition of the diesel fuel that would quantitatively describe the effect of cetane improver on the CN of the fuel. With the increasing use of such improvers in diesel fuels in Europe and North America, such predictive models are timely and would be of considerable value. ASTM defines the CN of a diesel fuel as the percentage by volume of normal cetane (C16H34), in a blend with 2,2,4,4,6,8,8heptamethylnonane (sometimes called HMN or isocetane), which matches the ignition quality of the diesel fuel being rated * E-mail:
[email protected]. (1) American Society for Testing and Materials. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1980; Part 47, D-613 and D-975.
under the specified test conditions. By definition and somewhat arbitrarily, normal cetane has been assigned a CN of 100 whereas HMN has a CN of 15. This implies that CN ) % (ncetane) + 0.15(% HMN). The ASTM D613 method involves running the fuel in a single cylinder compression-ignition engine with a continuously variable compression ratio under a fixed set of conditions. Despite being the traditional test method for CN, it was soon recognized that the test suffered from many disadvantages, some of which include a relatively large fuel sample volume requirement (∼1 L), significant time consumption (approximately a few hours), and a high reproducibility error. ASTM documents a maximum reproducibility error of 3–4 numbers for this test.1 Consequently, there have been many attempts to develop alternate tests to replace the ASTM D613 method. These include devising better engine tests and developing correlative models to predict CN from bulk properties of the fuel that may be measured more quickly and reliably. Among the different engine tests developed, the Ignition Quality Tester (IQT) has been the most promising for reliable measurement of CNs of middle distillates and alternate fuels. It uses much smaller sample volumes (
