ARTICLE pubs.acs.org/EF
Renewable Oxygenate Blending Effects on Gasoline Properties Earl Christensen,† Janet Yanowitz,‡ Matthew Ratcliff,† and Robert L. McCormick*,† † ‡
National Renewable Energy Laboratory, Golden, Colorado 80401, United States Ecoengineering, Inc., Boulder, Colorado 80304, United States
bS Supporting Information ABSTRACT: The oxygenates ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol (isobutanol), 1-pentanol, 3-methyl-1-butanol (isopentanol), methyl levulinate, ethyl levulinate, butyl levulinate, 2-methyltetrahydrofuran (MTHF), 2-methylfuran (MF), and 2,5-dimethylfuran (DMF) were blended in three gasoline blendstocks for oxygenate blending (BOBs) at levels up to 3.7 wt % oxygen. Chemical and physical properties of the blends were compared to the requirements of ASTM specification D4814 for spark-ignited engine fuels to determine their utility as gasoline extenders. Vapor pressure, vapor lock protection, distillation, density, octane rating, viscosity, and potential for extraction into water were measured. Blending of ethanol at 3.7% oxygen increased vapor pressure by 57 kPa as expected. 2-Propanol slightly increased vapor pressure in the lowest-volatility BOB, while all other oxygenates caused a reduction in vapor pressure of up to 10 kPa. Coefficients for the Wilson equation were fitted to the measured vapor pressure data and were shown to adequately predict the vapor pressure of oxygenategasoline blends for five individual alcohols and MTHF in different gasolines. Higher alcohols and other oxygenates generally improved vapor lock protection. Butyl levulinate blended at 2.7% oxygen caused the distillation end point to exceed 225 °C, thus failing the specification. Distillation parameters were within specification limits for the other oxygenates tested. Other than ethanol, MF, and DMF, the oxygenates examined will not produce blends with satisfactory octane ratings at these blend levels when blended into lower-octane blendstocks designed for ethanol blending. However, all oxygenates tested except 1-pentanol and MTHF produced an increase in octane rating. For ethanol, the propanol isomers, and methyl levulinate, 20 wt % or more of the oxygenate could be extracted into water in a room-temperature water tolerance experiment. For the butanol isomers and ethyl levulinate, the percent extracted ranged from about 4% to 8%. Extraction for other oxygenates was 2% or lower. Methyl levulinate separates from gasoline as a separate liquid phase at temperatures below 0 °C.
’ INTRODUCTION Oxygenates produced from renewable resources, including higher alcohols (those containing more than two carbon atoms) and several other oxygenated compounds, have been proposed as blend components in gasoline for reduction in petroleum consumption and greenhouse gas emissions. The higher alcohols, including propanols, butanols (with the exception of tertbutanol) and pentanols, can be produced from sugars via fermentation or by gasification to form synthesis gas, followed by catalytic conversion to a single or mixture of alcohols.13 Fivecarbon alcohols (pentanols) can also be produced from sugars or lignocellulosic biomass by chemical means, as can levulinic acid esters (methyl, ethyl, and butyl), methyl pentanoate, 2-methylfuran (MF), 2,5-dimethylfuran (DMF), and 2-methyltetrahydrofuran (MTHF).47 The chemical structures of the nonalcohol oxygenates included in this study are illustrated in Figure 1. The physical and chemical properties of these compounds as well as those of several other oxygenates were compiled and evaluated in a recent report.8 It was concluded that some of these oxygenates may have acceptable properties as gasoline blend components. Relative to ethanol, blends of gasoline with higher alcohols or cellulose-derived oxygenates may have higher energy density, lower vapor pressure, and lower affinity for water. The information available on these properties is summarized in Table 1 for the compounds examined here. Note that many of these properties do not blend linearly into gasoline. r 2011 American Chemical Society
ASTM D4814, Standard Specification for Automotive SparkIgnition Engine Fuel, sets minimum performance requirements for gasoline in the United States whether or not the gasoline contains oxygenates. Generally, the standard was developed with functional vehicle engine performance requirements, rather than chemical-specific requirements, and therefore it provides a basis for determining the properties that will be required of any acceptable gasolineoxygenate blend regardless of its chemical makeup. Many of the requirements are listed in Table 2, including vapor pressure, distillation temperatures, and driveability index. In this work, we determined the properties of blends of these oxygenates with three different gasoline blendstocks and compared them to the ASTM D4814-10b requirements. Octane rating, density, viscosity, and potential for extraction into water were also measured. In addition to the technical issues addressed in ASTM D4814, the legal impediments to bringing a new oxygenate to market are daunting. Under the Clean Air Act, gasoline used in the United States must be “substantially similar” to gasoline used in certification of cars for emission compliance. To meet the substantially similar requirement of the U.S. Environmental Protection Agency (EPA), this is generally interpreted as meaning that fuels Received: July 11, 2011 Revised: August 12, 2011 Published: August 16, 2011 4723
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’ EXPERIMENTAL SECTION
Figure 1. Chemical structures of nonalcohol oxygenates examined in this study.
must be substantially similar to gasoline as defined by U.S. EPA. Aliphatic ethers, alcohols (which include MTHF and all of the higher alcohols but not the other oxygenates considered here), and their mixtures were deemed substantially similar for concentrations up to 2.7 wt % oxygen by an EPA rule.30 Prior to EPA’s recent ruling allowing 15 vol % ethanol in gasoline for some cars,31 ethanol blends up to 10 vol % in gasoline were allowed. This volumetric level corresponds to about 3.7 wt % oxygen in gasoline. Waivers for the addition of higher alcohols at concentrations up to 3.7 wt % oxygen have also been granted,32,33 but these typically have additional requirements such as inclusion of specific corrosion inhibitors. Methyl levulinate, ethyl levulinate, butyl levulinate, methyl pentanoate, MF, and DMF do not meet the definition of “substantially similar” and may require a large body of test data to show that they do not “cause or contribute” to the failure or deterioration of any emission control device in order to obtain a waiver. The Clean Air Act also requires that new fuels be registered with the U.S. EPA, which requires, in part, submission of basic data on the composition and health impacts of the fuel. None of the higher alcohols or other oxygenates discussed in this report have fully completed fuel registration requirements, although the process is underway for 2-methyl-1-propanol (isobutanol),34 suggesting it will likely be the first non-ethanol renewable oxygenate registered. There are many other state and federal requirements that must be met, including Underwriters Laboratories’ listing of storage and dispensing equipment, development of appropriate fuel standards, and the revision of a number of requirements in ASTM D4814 that were tailored specifically to ethanol. These and other federal and state requirements are detailed elsewhere.35 New fuels must also be fully compatible with the materials of construction commonly used in vehicle fuel systems and retail distribution equipment.
Three gasoline blendstocks for oxygenate blending (BOBs) were procured from BP and stored in a temperature-controlled fuel storage facility maintained at 24 °C. Measured properties of the BOBs are presented in Table 3. BOB 1 and BOB 2 are intended for blending with 10 vol % ethanol as E10 in the autumn and summer, respectively, both as regular gasolines. BOB 3 is intended for blending as E10 in winter and is sold as a premium gasoline. BOB 3 may also have been sold as a clear gasoline (E0) in some markets. Oxygenategasoline blends were prepared volumetrically to the desired nominal weight percent oxygen. Aliquots of the BOBs and oxygenates were stored in a freezer at 20 °C for 24 h prior to blending. Blends were prepared cold, and care was taken to minimize the amount of loss from evaporation. Blends with fuel-grade ethanol were made at 10 vol % ethanol (nominally 3.7 wt % oxygen) for comparison of oxygenated blend properties. Most of the alcohols used in this study were blended volumetrically to achieve nominal concentrations of 1.0, 2.7, and 3.7 wt % oxygen. 3-Methyl-1-butanol (isopentanol) was blended at 2.7 wt % oxygen only. Methyl levulinate, ethyl levulinate, butyl levulinate, MTHF, MF, DMF, and methyl pentanoate were mixed at a level of 2.7 wt % oxygen. Methyl pentanoate was also blended at 7.2 wt % oxygen and methyl levulinate at 9.7 wt % oxygen. Methyl pentanoate, methyl levulinate, MF, and DMF were tested only in BOB 2. A complete list of the blends prepared in each BOB is provided in Tables S1S3 of the Supporting Information. Properties of the BOBs and oxygenate blends were determined by ASTM standard test methods with the exception of water solubility and low-temperature phase separation experiments, which are described below. To examine the tendency for these oxygenates to be extracted from blends with gasoline into water, 25 mL of the highest concentration blend of each oxygenate, up to 3.7 wt % oxygen, was saturated with water by exposure to 2.5 mL of water. Graduated centrifuge tubes designed for ASTM method D1796 were used to allow determination of the volume increase of the aqueous layer from oxygenate extracted out of the hydrocarbon phase. Each blend was shaken approximately 1 min by hand and left to separate overnight at room temperature. The BOBs were also saturated in the same manner for comparison of the amount of water absorbed by the pure hydrocarbon phase. The volume of the aqueous layer present after phase separation was measured by use of the graduation marks on the centrifuge tube. An increase in volume of the aqueous phase was notable for ethanol, 1-propanol, 2-propanol, and methyl levulinate blends, but phase separation of the remaining oxygenate blends did not result in a visually measurable change in volume of the aqueous layer. Loss of oxygenate to the aqueous layer of each sample was measured by gas chromatography of the aqueous layer, and the amount of oxygenate remaining in the gasoline was calculated by difference. Approximately 1 g of the aqueous layer was removed and diluted by weight at a ratio of approximately 1:10 with deionized water. Methyl levulinate was added to each aqueous sample gravimetrically as an internal standard, with the exception of the methyl levulinate, MF, and DMF blends, for which isobutanol was used as an internal standard. A minimum of three calibration standards were prepared for each of the oxygenates. The internal standard was added gravimetrically to each calibration standard in the same concentration as with the samples. Analysis of the diluted aqueous layers was performed on an Agilent 7890 gas chromatograph equipped with a flame ionization detector. The column used for compound separation was an Agilent DB-Wax with dimensions of 30 m 0.25 mm and 0.25 μm internal coating (catalog no. 122-7032). For each compound, a linear area response curve was generated. R2 exceeded 0.995 for each compound. The hydrocarbon layer of each blend was analyzed by Karl Fischer titration following ASTM method D6304 to determine the amount of water absorbed by the oxygenated gasoline. 4724
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Table 1. Chemical and Physical Properties of Gasoline and Alcohols lower heating compd
valuea (MJ/L)
heat of
water solubility
boiling pta neat vaporizationa (°C) (kJ/kg from 25oC) RON ∼351
solubility in
neat in oxygenate at water MON 20 °C,b wt % at 20 °C,c wt %
kinematic
specific
RVPd (kPa)
viscosity at 20 °Ce (cSt)
gravity @ 20 °C
gasolinef
3033
27225
8898 8088
negligible
negligible
54103
0.370.44
ethanolg
21.4
78
919.6
109
90
miscible
miscible
16
1.5
0.794
1-propanol
24.7
97.2
792.1
104h
89h
miscible
miscible
6.2
2.7
0.804
2-propanoli
24.1
82.3
756.6
106
99
miscible
miscible
12.4
3.1
0.789
1-butanol
26.9
117.7
707.9
98h
85h
20.1
7.7
2.2
3.6
0.81
2-butanol 2-methyl-1-propanol
26.7 26.6
99.6 107.9
671.1 686.4
105h 105h
93h 90h
60 20
12.5 8.7
5.3 3.3
4.7 8.3
0.808 0.802
0.720.78
(i-butanol) 1-pentanol
28.5
137.8
647.1
10.6
2.2
0.83
5
0.816
3-methyl-1-butanol
28.3
132
617.1
9.8
2.5
1.0
5.5
0.8
0.57
0.888
1.52
0.855
(i-pentanol) 2-methylfuranj
27.6
64.7
389.0
103
2,5-dimethylfurank
30.1
94
389.1
119
2-methyltetrahydrofuranl
28.2
78
375.3
86h
methyl levulinatem
86 73h
0.3
18.5
0.26
13.4
5.1
12.1
196
332.5
miscible
miscible
ethyl levulinaten
24.8
206
306.7
8.5
15.2
butyl levulinateo
27.1
237.5
277.5
2.6
1.3
methyl pentanoatep
25.9
126
371.5
