The product gases have the same volume in each reaction:
Hence, the additional work of compression (Wc) is essentially due to the PDV orDn RT where Dn ¼2 moles. The remaining question is what is the value of T? We can select 400C as an estimated value of the temperature. This value can be calculated from an adiabatic compression of the gas (mostly air) from T1¼350 K and assuming that CR¼10.
It is worth considering an extension to this argument by considering the increase fuel economy if we remove the nitrogen from the combustion process. Firstly, a higher temperature is obtained, and secondly, less work is done in compressing the gas. If we assume that gasoline is represented by octane, C8H18, then in air, the reaction is
C8H18þ12:5O2þ50N2! 8 CO2þ9H2O þ50N2 (5.9) 63.5 moles !67 moles R3¼1.055
C8H18þ12:5O2! 8 CO2þ9H2O (5.10)
13.5 moles !17 moles R4¼1.259, R4/R3¼1.19
The difference in moles of compression of reaction 5–9 and 5–10Dn is 63.5–13.5 orDn¼50.
Hence,Dn RT¼508.314673¼280 kJ. The heat of combustion of octane is 5,470 kJ/mol.
This extra work of compression is only about 5% of the combustion energy. However, the calculated adiabatic combustion flame temperature for octane in air is about 1,400 K, whereas in oxygen, the flame temperature is over 9,000 K. This represents a substantial increase in energy of combustion due to the removal of nitrogen.
Thus, besides obtaining more heat from the reaction (since the heat capacity of nitrogen absorbs some of the energy to reach the high temperature), the removal of nitrogen can give rise to about 20%
higher fuel economy. This is now being done for stationary furnaces where only the thermal improvement is obtained. It would be interesting to run an automobile on enriched oxygen which can be obtained by using suitable membranes. More will be said about this later.
One aspect which is important to note is that methanol is toxic and its TLV (threshold limit value) is 200 ppm requiring its dispensing in well-ventilated areas.
The ultimately efficient fuel system would be an electric vehicle running on a methanol fuel cell.
Such a system could meet the most stringent environmental emission requirements as well as high energy efficiency. This will be discussed more fully in Chap.9.
The ethylene is formed by the thermal cracking of ethane by the reaction
C2H6ðgÞ!C2H4ðgÞþH2ðgÞ (5.12) Ethanol from ethane is a fossil-based fuel and must be distinguished from grain alcohol. Ethanol from syngas has been extensively studied, but catalysts have not been found, as yet, to be efficient enough to make the process cost-effective. Similarly, the conversion of syngas to ethanol by alternate routes has been considered:
CH3OHðgÞþ COðgÞþ2H2ðgÞ! CH3CH2OHðgÞþH2OðgÞ
2 COðgÞþ4H2ðgÞ!C2H5OHðgÞþH2OðgÞ (5.13) or
2 CH3OðgÞH ! CH3OCH3ðgÞþH2OðgÞ (5.14)
CH3OCH3ðgÞ!C2H5OHðgÞ (5.15)
These catalytic processes are attractive because the relative value of ethanol/methanol is about 4, whereas the C2H6/CH4value is about 2, though in the last year ending in June 1995, the price of methanol doubled. Hence, the direct conversion of ethane to ethanol by the reaction analogous to reaction 5.4
3C2H6þ31=2O2!2C2H5OH þ 2CO þ3H2O (5.16) is a convenient approach to the utilization of surplus ethane.
According to Vaclav Smil’s analysis of ethanol production in the USA, commercial production of fuel ethanol began in 1980, and it took nearly 15 years to surpass 5 billion liters, accelerated Fig. 5.6 US fuel ethanol production, million US liquid gallons (2000–2010)
5.4 Ethanol 79
expansion began only in 2002; more than 15 billion liters were shipped in 2005; 25 billion liters in 2007; and 35 billion liters (9 billion gallons) in 2008 (see Fig. 5.6). Much higher targets were established in June 2007 when the American Senate passed an energy bill that mandated no less than 36 billion gallons of ethanol by the year 2022. The USA produced about 45 GL of ethanol (2010) or 52% of the word’s production from over 110 plants in comparison to 4 GL in 1992 from over 15 plants. The estimated annual US market is about 100 GL today. The total annual world ethanol production was 85.9 GL in 2010 and over 31 GL in 1998, respectively. The production of ethanol in the European Union was slightly reduced from 22 GL (1998) to 4.3 GL (2009). Russia increased ethanol production in three times from 2.5 GL (1998) to 7.6 GL (2010).
The present use of ethanol as a fuel additive is made possible by the subsidy which varies from one location to another but is approximately 60¢/US gal. of ethanol or approximately 16¢/L. This subsidy applies only to ethanol formed from grain, that is, nonfossil fuel sources. The present processes of production include fermentation using yeasts as the enzyme catalyst which converts sucrose to CO2
and ethanol
C6H12O6ð Þ!aq catalyst 2C2H5OH aqð Þ þ2 CO2ð Þg (5.17) This is then followed by distillation of the alcohol which normally reaches about 12% in the fermentation process. The distillation produces 95% alcohol. The residual 5% water is removed either by the addition of a third component to form a two-phase azeotrope or by passing the wet alcohol through a 3A molecular sieve which removes the water leaving 99.9 + % alcohol.
There is, however, enzymes which can still function up to almost 20% ethanol. The energy balance in ethanol production which takes into account the energy used to grow the corn, the energy to ferment and distillate the alcohol, and the energy in the remaining mash results in a ratio for the input energy/output energy of 1.3 which varies somewhat depending on the efficiency of the various steps.
It would appear that it makes little sense to spend 1.3 kJ of fossil fuels to obtain 1 kJ of grain alcohol fuel and to consume grain which is a badly needed food supply for an expanding world population.
Also Vaclav Smil noticed that because of America’s extraordinary high gasoline consumption and the inherently low power density of ethanol production (only about 0.25 W/m2of cultivated land), corn- derived ethanol can never supply more than a relatively small part of the overall demand for fuel in the USA. If America’s entire corn harvest—just over 280 million tons in 2005—was converted to ethanol at the best conversion ratio of 0.4 kg/kg of grain, the country would produce fuel equivalent to 13% of the total gasoline consumption.
Improved efficiency may be achieved by using pervaporation to separate out the alcohol instead of distillation. This consists of a membrane through which water and ethanol permeate with greatly different rates. When fermentation is continuously conducted in a membrane reactor, it is possible to continuously remove the alcohol as it is formed enriching it by a factor of six- to tenfold. This can reduce the costs of distillation which is as much as half the production cost of the ethanol.
The separation of ethanol from water can also be effected by freezing. The effect of ethanol concentration on the freezing point is given in Table5.4. Thus, a liter of fermented brew with 12.5%
ethanol by volume was completely frozen and then allowed to thaw. The first 500 mL of solution was 17% ethanol. When this 500 mL solution was frozen and allowed to thaw, again the first 250 mL was 23% ethanol. Various freeze-thaw cycles can thus concentrate ethanol. Another process which has been studied extensively for more than 70 years is the conversion of cellulose from wood and straw to glucose by enzyme or by acid hydrolysis.
The reaction is
C6H10O5
ð Þx acellulose
þxH2O !x Cð 6H12O6Þ
glucose
(5.18)
Cellulose is the most abundant naturally occurring biomass material on earth, and its utilization as a fuel has not as yet been fully exploited. Paper, though presently recycled to some degree, can be included with straw and wood for conversion to ethanol fuel. According to Vaclav Smil’s informa- tion, the US Department of Energy invested to build six cellulosic plants with combined capacity to 0.1% of transportation fuel used in the USA (2005). The construction of these plants (one based solely on corn stover, one on waste wood, and the rest on a mixture of agricultural wastes and waste wood) should be completed in Silicon Valley by 2011.
Ethanol has been used as a gasoline additive in Brazil since the 1920s. The conversion to straight alcohol during World War II has continued to the present. Ethanol is used in its pure form in the Otto cycle engine for cars and light trucks and in ethanol-gasoline blends (22% ethanol) where it replaces lead as antiknock additive and octane enhancer. Ethanol is also used in diesel engines. This is accomplished by the addition of 4.5% of a diesel ignition improver such as isoamyl nitrate, hexyl nitrate, and the ethylene glycol dinitrate to the fuel which consumes 65% more fuel volume than regular diesel fuel and 2% less than an Otto cycle engine running on pure ethanol. In some cases, 1%
castor oil is added to the ethanol to hold lubricate engine parts.
The ethanol fuel program in Brazil is made possible by the large sugarcane crop as well as other sugar crops such as cassava and sorghum. According to Vaclav Smil’s information, the total area planted to sugarcane in tropical and subtropical countries was about 19 million hectares in 2005, and if all that were devoted to ethanol production, the annual yield would be equivalent to 6% of the world’s 2005 gasoline consumption. To cover the entire demand, sugarcane would have to be planted on some 320 million hectares—that is, on 20% of the world’s arable land.
Though liquid fuels are used for automotive fuels throughout the world, its precise choice is determined by local conditions, which vary considerably.
Exercises
1. The standard heat of formation isDHfo
(NO) ¼90.25 kJ/molDGfo¼86.57 kJ/mol.
(a) Calculate the value of the equilibrium constant Kp for the reaction N2+ O2⇄2NO
at 800, 1,000, and 1,200 K and determine the % NO in air at these temperatures (see Exercise 2.18).
Table 5.4 Some properties of ethanol-water (E/W) solutions
[E] (Wt. %) Density (g/mL) [E] (Molarity) F.P. (C)
2.5 0.9953 0.539 1.02
5.0 0.9911 1.974 2.09
10.0 0.9838 2.131 4.47
15.0 0.9769 3.175 7.36
20.0 0.9704 4.205 10.92
25.0 0.9634 5.21 15.4
30 0.9556 6.211 20.5
35 0.9466 7.145 25.1
40 0.9369 8.120 29.3
44 0.9286 8.853 32.7
50 0.9155 9.919 37.7
54 0.9065 10.607 40.6
60 0.8927 11.606 44.9
64 0.8786 12.565 48.64
68 0.8739 12.876 49.52
5.4 Ethanol 81
(b) Determine the thermodynamic values of the constants a and b in the equation of Exercise 2.18.
2. Using the data in Table5.5, calculate the equilibrium constant for the reaction CO + 2H2!CH3OH(g)
at 298, 400, and 600K.
3. Using the data in Table5.5, calculate the equilibrium at 298, 400, and 600 K constant for reaction (5.3)
CH4+ H2O(g)!CO + 3H2
4. The air fuel ratio (AFR)¼weight of air available for combustion/weight of fuel available for combustion
Show that for complete combustion (stoichiometric), (a) the AFR¼15 for gasoline or diesel fuel; (b) calculate the AFR for pure methanol fuel.
5. What are the reasons for pursuing the development of alternate fuels for the automobile?
6. How can NOx formation be minimized when using air (with the N2present) in a combustion process?
7. Explain why propane is a “clear” fuel.
8. Explain why the heat of combustion by weight of propane is greater than that of gasoline (see Table4.9).
9. Why would butane, if available in large quantities, be a better fuel than propane?
10. Why is it ill-advised to use methanol as a fuel in a vehicle designed to run on gasoline?
11. Can you suggest a nonfossil fuel source for methanol?
12. Calculate the temperature reached in the adiabatic compression (CR¼10) of an air/fuel mixture.
Note: The heat capacity of air Cu(air)¼21.5 J/K mol.
13. Would it be an advantage to convert methanol to dimethyl ether for an SI-ICE?
14. Calculate the energy required (if any) to convert natural gas (CH4) to ethanol by the reaction 2CH4(g)+ O2(g)!C2H5OH(l)+ H2O(l)
15. Discuss the energetics and feasibility of reactions (5.14) and (5.15) to produce ethanol from methanol.
Further Reading
1. Ramadhas AS (ed) (2011) Alternate fuels for transportation, CRC Press, Taylor & Francis Group
2. Smil V (2010) Energy myths and realities. Bringing science to the energy policy debate. The AEI Press, Washington, DC
Table 5.5 Selected thermodynamic values
Chemical formula DHfo(kJ/mol) DGfo(kJ/mol) So(JK1mol1) DHfo(kJ/mol) DGfo SoJK1mol1
CO 110.5 137.2 197.6
H2 0 0 130.6
O2 0 0 205.0
CH3OH(g) 201.6 162.4 239.7
CH3OH(l) 239.1 166.4 126.8
C2H5OH(g) 234.4 167.9 282.6
C2H5OH(l) 277.1 174.9 160.7
H2O(g) 241.8 228.6 188.7
H2O(l) 285.8 237.2 70.0
CH4(g) 74.7 50.8 186.2
CH4(l) 184.4 114.2 266.5
3. Kitasei S, Mastny L (2010) Powering the low-carbon economy: the once and future roles of renewable energy and natural gas. Worldwatch report 184. Worldwatch Institute, Washington, DC
4. (http://en.wikipedia.org/wiki/2008_Toronto_explosions).
5. (1983) Propane carburetion, EMR Canada 6. (1981) Switching to propane. MTC Energy Ontario 7. (1991) Focus on propane. Superior Propane Inc.
8. Ecklund EE, Mills GA (1989) Alternate fuels: progress and prospects. Part 1 and 2. Chemtech Sep p. 549 Oct. p. 626 9. Semanaitis D (1989) Alternate fuels. Road Track Nov. p 72
10. Chang TY et al (1991) Alternative transportation fuels and air quality. Environ Sci Technol 25(7):1190 11. Morton L et al (1990) Methanol: a fuel for today and tomorrow. Chem Ind 16:457
12. Gray CL, Alson JA (1989) The case for methanol. Sci Am p 108–114
13. Lincoln JW (1976) Methanol and other ways around the gas pump. Garden Way Publ, Charlotte Vermont 14. Marsden SS Jr (1983) Methanol as a viable energy source in today’s world. Ann Rev Energy 8:333
15. Kohl WL (ed) (1990) Methanol as an alternative fuel choice: an assessment. John Hopkins University, Washington 16. Goodger EM (1981) Alternate fuels for transport, vol 1, Alternate fuels technology series. Cranfield Press, UK 17. Willkie HF, Kalachov PJ (1942) Food for thought—a treatise on the utilization of farm products for producing farm
motor fuel as a means of solving the agricultural problem. Indiana Farm Bureau Inc., USA
18. Van Koevering TE et al (1987) The energy relationships of corn production and alcohol fermentation. J Chem Educ 64:11
19. Biomass Energy Institute (1893) Canadian alternate fuels. Biomass Energy Institute, Winnipeg
20. World Bank (1980) Alcohol production from biomass in the developing countries. World Bank, Washington, DC 21. Anderson EV (1992) Ethanol from corn. Chem Engin News 2:7
Further Reading 83