In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz
A Classroom Demonstration of Water-Induced Phase Separation of Alcohol–Gasoline Biofuel Blends
Kutztown University Kutztown, PA 19530
submitted by: Sherry A. Mueller,* James E. Anderson, and Timothy J. Wallington
Research and Advanced Engineering, Ford Motor Company, Dearborn, MI 48121; *
[email protected] checked by:
Robert M. Hammond Department of Chemistry, East Carolina University, Greenville, NC 27858
Fuel ethanol use is increasing in the United States because of concerns about domestic energy security, climate change, and air quality. In 2007, the United States produced nearly 6.5 billion gallons of fuel ethanol, an increase of nearly 35% over 2006 production, and production is expected to increase as more ethanol biorefineries are built (1). Ethanol can be readily produced locally from biomass feedstocks such as corn, sugar cane, or other starch or sugar-containing plant material, thereby reducing dependence on foreign petroleum supplies. In the future, it is anticipated that ethanol will also be made from lignocellulosic biomass, such as wood and grass. In fact, the Energy Independence and Security Act of 2007 mandates the use of 36 billion gallons of renewable fuels by 2022, including 21 billion gallons of advanced biofuels such as cellulosic ethanol. Climate change, largely driven by increasing atmospheric CO2 concentration from the burning of fossil-based fuels, and deforestation to a lesser degree, can be partially addressed by biofuels such as ethanol, despite the use of fossil fuels during production. Owing to the recycling of atmospheric CO2 in the biomass feedstock, biofuels can offer significant reductions in greenhouse gas (mostly CO2) emissions, as evaluated on a full life-cycle or “well-to-wheels” basis (2). The Clean Air Act requires the use of oxygenated gasoline additives in some areas of the United States to reduce unwanted vehicle emissions. Since the late 1970s when the phase-out of leaded gasoline began, methyl tert-butyl ether (MTBE) and ethanol have been added to gasoline to improve the octane rating and to reduce emissions. However, ethanol has recently become the most attractive oxygenate owing to environmental and health concerns with MTBE. Butanol, another alcohol that can be produced biologically, is also being proposed as a biofuel (3). Ethanol is an oxygenated hydrocarbon with a high octane rating that is often blended with unleaded gasoline in various proportions. Typical blends in the United States are E10 (10% ethanol and 90% gasoline by volume) and E85 (85% ethanol and 15% gasoline by volume). Blends up to E10 are approved for use in any gasoline-powered vehicle currently sold in the United States. Flexible-fuel vehicles (FFVs) are designed to run on gasoline or gasoline blended with up to 85% ethanol (4). Fuel distribution and storage systems typically contain some moisture, but because of the low solubility of water in gasoline (up to 150 mg/L at 21 °C depending on aromatic content), some water ingress does not pose a problem. Contaminating water in excess of the solubility limit forms a layer on the bottom of storage tanks, can be separated relatively easily from the gasoline, and does not adversely affect gasoline quality or performance
(e.g., drivability). However, ethanol–gasoline blends are sensitive to moisture and require special handling during distribution and storage. In practice, ethanol and ethanol–gasoline blends are not transported via current distribution systems. Instead, ethanol is proportionally blended with gasoline directly in the tanker trucks that deliver the blends to retail outlets (5). A major issue of some alcohol–gasoline blends is their tendency to “phase separate” (form two layers) upon addition of relatively small volumes of water. Ethanol is miscible with gasoline in all proportions (6). If exposed to water, an ethanol– gasoline blend will initially absorb the water because of hydrogen bonding between the water and alcohol molecules. When the quantity of added water exceeds its solubility in that blend, a separate aqueous layer will form (also observed with pure gasoline). However, when an ethanol–gasoline blend undergoes phase separation, a large fraction of the ethanol preferentially partitions into the aqueous layer, producing an ethanol-deficient gasoline layer and an ethanol-rich aqueous layer. As a fuel, the ethanol-depleted gasoline may cause engine knock due to the reduced octane rating. The ethanol-rich aqueous layer may not provide satisfactory vehicle performance, for example, difficulty in starting due to lack of volatility (usually provided by the gasoline) and engine stalling due to lack of energy content, as the aqueous layer may contain mostly water. Thus, the issue is not with the formation of a separate aqueous layer, but with the resulting change in fuel composition for alcohol–gasoline blends (i.e., loss of alcohols to the aqueous phase) and the decreased performance as a fuel. The solubility of water and the potential for phase separation in alcohol–gasoline blends depend on several factors: (i) the proportions of gasoline and alcohol; (ii) the temperature (i.e., cooling a nearly water-saturated alcohol–gasoline blend may result in phase separation); (iii) the polarity of the alcohol, largely determined by its hydrocarbon chain length (e.g., the potential for phase separation is lower in butanol–gasoline blends than ethanol–gasoline blends); and (iv) the composition of the gasoline, particularly aromatic content (7–10). A ternary phase diagram for an alcohol–gasoline–water mixture provides information about the potential for phase separation of the alcohol– gasoline blend upon the addition of water (12, 13). The phase diagram for ethanol–gasoline–water mixtures at 25 °C is shown in Figure 1. The composition of a mixture indicated by a point on the phase diagram is related to the proximity of the point from the three corners. Each side of the phase diagram represents from 0 to 100 mass fraction of a component. To determine the percentage of a component in the mixture, a line is drawn from
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 9 September 2009 • Journal of Chemical Education
1045
In the Classroom
0
r (% nW a te
70
40
60
50
40
A
water
30
90
)
ethanol
80
l (%
70
no
60
tha
rac
50
nE
tio
80
E
30
tio
sF
90
rac sF
Ma s
D
s Ma
)
10 20
single phase two phases
100
20
gasoline
C B
0
10 0
10
30
20
50
40
70
60
0
10
90
80
10
0
Mass Fraction Gasoline (%) Figure 1. Ternary phase diagram for ethanol–gasoline–water at 25 oC; data reproduced from ref 11.
0
A
an
(% ter Wa cti
Eth
50
60
ol
40
A
70
30
)
80
(%
Ma
60
tie lines
50
on
on
70
B
cti
Fra
Fra
80
ss
ss
90
20 30
40
single phase two phases
100
Ma
)
10
20
90
10
C
0
10 0
30
20
10
40
50
70
60
90
80
0
10
0
Mass Fraction Gasoline (%) 0
B
(% Wa ter on cti Fra
60
50
C
60
50
no uta nB
tio 40
A
30
)
80
l (%
ss
70
40
rac
Ma
80
30
F ss
0
90
20
70
90 10 0
100
Ma
)
10
20
tie lines
10
B 10
20
30
40
50
60
70
80
90
0
10
0
Mass Fraction Gasoline (%) Figure 2. Ternary phase diagrams and selected tie lines for (A) ethanol–gasoline–water and (B) butanol–gasoline–water systems at 25 oC; data reproduced from ref 11.
1046
the corner indicating 100% of the component to the mid-point on the opposite side. For reference, a dotted line has been drawn from the 100% ethanol corner to the 50% mark on the opposite (gasoline) side of the diagram. The line is intersected by gridlines that run parallel to the side opposite the corner to independently define the percent of that component in the mixture; gridlines in 10% increments are shown in Figure 1. For example, mixture A is composed of 30% ethanol as shown by the arrow that lies on the gridline and points to the 30% value on the mass fraction ethanol side. The values for water and gasoline, 20% and 50%, respectively, are found in the same manner. In general, as the quantity of ethanol increases and the quantity of water absorbed decreases, a mixture has greater tolerance for water and is more resistant to phase separation (as shown by the white area above the binodal curve in Figure 1). An ethanol–gasoline mixture whose composition falls within the white area of the diagram will exist as one homogenous layer (e.g., mixture B, dry E10 or mixture D, dry E85). Any mixture represented by a point within the gray-shaded area (below the binodal curve) will separate into two layers, an ethanol-depleted gasoline layer and an ethanol-containing aqueous layer. In general, much more water can be added to higher ethanol–gasoline blends than blends containing a lower percentage of ethanol before phase separation occurs. For example, E85 (mixture D) will not phase separate until the water content exceeds approximately 17% (mixture E) whereas E10 (mixture B) will phase separate upon addition of approximately 0.5% water (difficult to read on diagram). Addition of water to mixtures B or D, which exist independently as a single phase, will move the composition of the new mixture (either C or E) to the left and down into the gray shaded area toward the corner representing 100% water as shown by the arrows. Tie lines connect two points on portions of the binodal curve and indicate the composition of the two layers in equilibrium after phase separation has occurred. Several example tie lines are shown in Figure 2 for discussion purposes. Mixture A contains 43% alcohol (i.e., ethanol in Figure 2A or butanol in Figure 2B), 39% gasoline, and 18% water. Because this mixture falls within the gray-shaded area in both diagrams, phase separation will result in the formation of two different layers in equilibrium (mixture B, an aqueous layer and mixture C, a gasoline-containing layer). The relative quantities of alcohol that partition into each of these layers depends on the characteristics of the individual alcohol. For ethanol, the aqueous layer will contain nearly all of the water, much of the ethanol, and only a small quantity of the gasoline. With butanol, the aqueous layer will contain nearly all water with only trace quantities of butanol and gasoline. The slope of the tie lines is dependent on the solubility properties of the alcohol, specifically the distribution coefficients of the alcohol between gasoline and water. Smaller alcohols, such as ethanol, are more hydrophilic than alcohols with longer hydrocarbon tails (e.g., butanol) and will partition more into the aqueous phase owing to the relatively greater hydrogen bonding effects. Butanol is being considered for gasoline blending and, similar to ethanol, can be produced from bio-based feedstocks via fermentation. Butanol–gasoline blends are less susceptible to phase separation in the presence of water than ethanol–gasoline blends because of the decreased solubility of butanol in water, as indicated by the slope of the tie lines in Figure 2B. When water
Journal of Chemical Education • Vol. 86 No. 9 September 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
is added to butanol–gasoline blends, two layers will form just as with pure gasoline and water, but very little of the butanol will partition into the aqueous layer. Therefore, butanol–gasoline blends could be transported using the existing distribution infrastructure. We report a simple demonstration that compares the addition of water to gasoline, an ethanol–gasoline blend, and a butanol–gasoline blend using dyed solutions to improve visualization of the separate layers that form. Materials Anhydrous Ethanol (or Butanol) with Dye Add 0.2 mg eosin B (red dye; 4´,5´-dibromo-2´,7´dinitrofluorescein disodium salt; Aldrich) per mL anhydrous ethanol (or anhydrous butanol). Other dyes [e.g., Janus green (blue dye) or brilliant blue] may be substituted as long as they are soluble in alcohol and water, but not in gasoline.
gasoline
E10
Bu10
Figure 3. Identical volumes of water added to gasoline, gasoline with 10% ethanol (E10), and gasoline with 10% butanol (Bu10).
Unleaded Gasoline with Dye Add 0.025 mg oil blue N [1,4-bis(pentylamino)anthraquinone; Aldrich] per mL gasoline. (The gasoline sample used must not contain ethanol.) As ethanol use expands, it may be more difficult to find non-ethanol-containing gasoline and some states do not require ethanol content to be labeled on gasoline retail pumps. The presence of ethanol in the gasoline can be checked by mixing a known volume of water (e.g., 10 mL) with a known volume of the gasoline (e.g., 100 mL) and letting the mixture settle into two phases. If the volume of the lower aqueous layer is greater than the volume of water initially mixed with the gasoline (10 mL in this case), the gasoline contains some ethanol and cannot be used for this demonstration. Alternatively, the presence of ethanol in the gasoline sample may be determined by water extraction followed by gas chromatography (14). Hexane or iso-octane (2,2,4-trimethylpentane) may be used as a substitute for non-ethanol-containing gasoline if non-ethanol-containing gasoline is unavailable. Other dyes (e.g., Sudan III or Sudan IV, both red dyes) may be substituted as long as they are soluble in hydrocarbons but not in water. Different dye colors should be used for the alcohol and gasoline for better visualization of the phase separation. Combinations of dyes other than the recommended eosin B and oil blue N dyes may result in colored layers different from those described in this demonstration. Test Tubes Glass test tubes (13 mm × 100 mm) or 10 mL conical, glass centrifuge tubes with screw caps are preferred. If other sizes of tubes are substituted, volumes of the individual chemicals should be adjusted such that the ratios of volumes given below are maintained. Containers with a larger length-to-diameter ratio allow better visualization of the phase separation. Plastic tubes may not be compatible with the chemicals used in this demonstration and their use is discouraged. Distilled or Deionized Water The water added to alcohol–gasoline blends should be colorless, but that added to the gasoline-only tube may be dyed (eosin B or alternative dyes) for better visualization.
Pipets and Pipettors Glass pipets or automatic pipettors with disposable plastic tips, capable of measuring from 0.5 to 5 mL of liquid are required to provide proper proportions of solutions. Glass Pasteur pipets may be used when adding a small drop of water to the prepared solutions to initiate the phase separation. Hazards Ethanol, butanol, gasoline, hexane, and iso-octane are flammable. Ethanol, butanol, hexane, and iso-octane inhalation may cause central nervous system depression. Gasoline inhalation may cause anesthetic effects (dizziness, nausea, headache, intoxication). Demonstration Gasoline and Water Pipet 5 mL of dyed gasoline into a test tube and add 60 μL (approximately one drop; to achieve 1.2 wt %) of water (or dyed water) to the tube, seal with screw cap, and shake well. The water will form a small separate layer at the bottom of the tube because of its low solubility in gasoline (Figure 3). Ethanol–Gasoline and Water Pipet 4.5 mL of dyed gasoline and 0.5 mL of dyed anhydrous ethanol into a test tube, seal with screw cap, and shake to mix. The contents of the tube (10% ethanol in gasoline, E10) should turn purple upon mixing (not shown in Figure 3). Remove the cap, add 60 μL (approximately one drop; to achieve 1.2 wt %) of water to the tube, replace the cap, and shake well (Figure 3). Observe phase separation, resulting in an upper layer (blue) containing primarily gasoline with some ethanol and a lower layer (red) containing the water and most of the ethanol. Note that the volume of the red layer is greater than the volume of the aqueous layer added to the tube containing gasoline only owing to the preferential partitioning of ethanol into the aqueous layer.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 9 September 2009 • Journal of Chemical Education
1047
In the Classroom
Butanol–Gasoline and Water
Literature Cited
Pipet 4.5 mL of dyed gasoline and 0.5 mL of dyed anhydrous butanol into a test tube, seal with screw cap, and shake to mix. The contents of the tube (10% butanol in gasoline, Bu10) should turn purple upon mixing. Remove the cap, add 60 μL (approximately one drop; to achieve 1.2 wt %) of water to the tube, replace the cap, and shake well (Figure 3). Because butanol has a longer, hydrophobic hydrocarbon chain than ethanol, butanol–gasoline blends have a higher tolerance for water and there is far less alcohol extraction from the gasoline layer upon water addition (3). The water will extract the red dye and some butanol from the dyed gasoline and form a primarily aqueous layer on the bottom of the tube. The aqueous layer may climb up the side walls of the tube making the exact volume difficult to compare to the lower layers in the other test tubes. The aqueous layers formed with the addition of water to gasoline and to the Bu10 blend are about the same volume, indicating that the addition of water to the butanol–gasoline blend results in very little butanol extraction into water after the phase separation. However, the addition of the same volume of water to the E10 sample results in the extraction of a significant percentage of the ethanol from the gasoline layer into the lower aqueous layer.
1. Renewable Fuels Association (RFA) Page. http://www.ethanolrfa. org/ (accessed Jul 2009).
Extension To reinforce basic concepts of experimental design, the demonstrator may also wish to include E85 in the demonstration to illustrate the greater solubility of water in this blend owing to the higher ethanol content. One drop of water (e.g., 60 μL) could be added resulting in no phase separation and then additional water could be added to initiate the phase separation. The demonstrator may also wish to add a smaller volume of water (e.g., 15 μL, 0.3 wt %, at room temperature) to all of the above blends to demonstrate the greater solubility of water in the ethanol–gasoline blend. The volume of water used in the demonstration (60 μL; 1.2 wt %) was selected to ensure that phase separation occurs. Classroom Topics The simple demonstration can be used to discuss several important alternative fuels topics. Among these discussion topics are (i) chemical properties, such as hydrophobicity and hydrophilicity; (ii) ethanol–water phase separation; (iii) properties of mixtures, such as solubility; and (iv) butanol–gasoline blends.
1048
2. Brinkman, N.; Wang, M.; Weber, T.; Darlington, T. Well-toWheels Analysis of Advanced Fuel/Vehicle Systems—A North American Study of Energy Use, Greenhouse Gas Emissions, and Criteria Pollutant Emissions. http://www.transportation.anl.gov/ pdfs/TA/339.pdf (accessed Jul 2009). 3. Brekke, K. Ethanol Today 2007, 36–39. 4. International Energy Agency (IEA). Biofuels for Transport: An International Perspective; Chirat: France, 2004. http://www.iea.org/ textbase/nppdf/free/2004/biofuels2004.pdf (accessed Jul 2009). 5. U.S. Department of Energy. Alternative Fuels and Advanced Vehicles Data Center Page. http://www.afdc.energy.gov/afdc/ethanol/ distribution.html (accessed Jul 2009). 6. Powers, S. E.; Hunt, C. S.; Heermann, S. E.; Corseuil, H. X.; Rice, D.; Alvarez, P. J. J. CRC Crit. Rev. Env. Sci. Techn. 2001, 31, 79–123. 7. Gramajo de Doz, M. B.; Bonatti, C. M.; Sólimo, H. N. Energy and Fuels 2004, 18, 334–337. 8. French, R.; Malone, P. Fluid Phase Equilibria 2005, 228–229, 27–40. 9. Stephenson, R. M. J. Chem. Eng. Data 1992, 37, 80–95. 10. Peng, C.; Lewis, K. C.; Stein, F. P. Fluid Phase Equilibria 1996, 116, 437–444. 11. Letcher, T. M.; Heyward, C.; Wootton, S.; Shuttleworth, B. Fuel 1986, 65, 891–894. 12. Campbell, A. N.; Smith, N. O. The Phase Rule and Its Applications, 9th ed.; Dover Publications: Mineola, NY, 1951; pp 277–295. 13. Atkins, P. W. Physical Chemistry, 3rd ed.; W. H. Freeman and Company: New York, 1986; pp 203–208. 14. Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056–1058.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Sep/abs1045.html Abstract and keywords Full text (PDF)
Links to cited URLs and JCE article
Figure 3 in color
Journal of Chemical Education • Vol. 86 No. 9 September 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
