Energy & Fuels 1989, 3, 76-79
76
VI1 presents the total hydrogen and aliphatic hydrogen contents of the five coals as determined from elemental analysis and CRAMPS. Examination of the short time (5 min) conversion data in Table IV shows that the effect of hydrogen in both the 1-MN and DHP systems is a strong function of coal, with the following general effects noted: coal Stockton, Pittsburgh No. 8 Illinois No. 6, Upper Freeport Pocahontas
effect of hydrogen negligible (0-6%) small to moderate (11-25%) large (50-7170)
Correlation of this observation with coal properties can be made with the data in Table VI1 on aliphatic hydrogen. As can be seen, the coals with the greatest amount of aliphatic hydrogen (Stockton and Pittsburgh No. 8) have the least sensitivity to gas-phase molecular hydrogen. Conversely, the coal with the lowest quantity of aliphatic hydrogen (Pocahontas) has the greatest sensitivity to gas-phase molecular hydrogen. The other two coals with intermediate levels of aliphatic hydrogen are correspondingly intermediate in their sensitivity to the effect of hydrogen on conversion. This correlation serves to reiterate the importance of the aliphatic hydrogen content of coal in coal liquefaction and coal reactivity. As the aliphatic hydrogen content of coal increases, more hydroaromatic and aliphatic protons are available as radical scavengers. Hence, self-donation of hydrogen from coal to primary radicals generated during the initial stages of thermolysis is an important feature in the sensitivity of hydrogen utilization for liquefaction reactivity. The rationalization
shown in Figure 1for the results of the model compound study also explains the lack of sensitivity to the presence of molecular hydrogen seen in the coal liquefaction results. It is noteworthy that the coal conversion, particularly at 40-min reaction time, is always substantially higher in DHP than in 1-MN. Regardless of the nature of the solvent used (DHP, 1-MN, or tetralin), the fraction of H-transfer capability that is present as ArH' increases dramatically when solvent is present. These observations are thus consistent with recent suggestions that hydrogenolysis engendered by H transfer from ArH' species constitutes a significant part of the bond cleavage that occurs during 1iq~efaction.l~ Acknowledgment. We acknowledge the financial support of the US.Department of Energy under Grant No. DE-FG22-85PC80907. The cooperation of the Korean Ministry of Energy and Resources is also gratefully acknowledged. We are indebted to Dr. D. F. McMillen of SRI, International, for his careful review of this work and many helpful suggestions. Registry No. Dibenzyl, 103-29-7; 1-methylnaphthalene, 9012-0; 9,10-dihydrophenanthrene,776-35-2; tetralin, 119-64-2; 1,2-diphenylethane, 103-29-7;toluene, 108-88-3;benzene, 71-43-2; ethylbenzene, 100-41-4; diphenylmethane, 101-81-5; 1,l-diphenylethane, 612-00-0; 1,2-diphenylethylene, 588-59-0; n-butylbenzene, 104-51-8; 1-methylindan, 767-58-8; decahydronaphthalene, 91-17-8; 1,2,3,4-tetrahydronaphthalene, 119-64-2; naphthalene, 91-20-3; chrysene, 218-01-9; biphenyl, 92-52-4. (14)McMillen, D. F. Private communication, October, 1988.
Effects of Heptanoic and Oleic Acids on the Phase Separation Temperatures of Methanol/Hydrocarbon/Water Mixtures Rathin S. Pate1 and H. Michael Cheung* Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325 Received April 14, 1988. Revised Manuscript Received September 19, 1988
The efficacy of heptanoic and oleic acids as phase separation temperature depressants for methanol/hydrocarbon/water blends was examined. A simulated fuel consisting of 50% 2,2,4-trimethylpentane (isooctane) and 50% toluene (by volume) was used as the hydrocarbon. The amount of water present was varied from 0% to 0.3% (w/w). The amount of carboxylate present was varied from 0% to 4% (w/w). The amount of methanol present was varied from 0% to 25% (w/w). The mixed samples were cooled in an ethylene glycol bath in a Missimers refrigerator and the phase separation temperatures recorded. The heptanoic acid proved to be more effective in depressing the phase separation temperature. The results for heptanoic and oleic acids were compared with those for 2-propanol, which is commonly used as a methanol cosolvent.
Introduction Shortages of petroleum have frequently motivated the examination of alternate synthetic fuels, especially those containing or consisting of alcohols. High octane ratings, lean flammability, and resultant lower regulated exhaust
* Author to whom correspondence should be addressed.
emissions of alcohols make them attractive candidate fuels for spark-ignition internal combustion engines and gas turbines. Gasohol, a mixture of 90 vol % unleaded gasoline and 10 vol 70 ethanol, has been in commercial use in the U.S. since 1977. The possibility of blending methanol with gasoline has also received considerable attention. This is primarily because of the potential of extending the diminishing gasoline supplies in a form that is compatible
0887-0624/89/2503-0076$01.50/00 1989 American Chemical Society
Fuel Phase Separation Temperatures
Energy & Fuels, Vol. 3, No. 1, 1989 77
with existing vehicles, with only minor alterations. The technology for producing ethanol and methanol is available. While ethanol is currently available, its higher cost and limited resource base mitigate against it meeting a significant percentage of fuel needs. However, methanol can be manufactured from a number of renewable as well as conventional resources. It is presently made primarily from natural gas, but it can be made from coal, agricultural residue, wood, or municipal solid waste.l When coal and flared natural gas are included, the potential resource base for producing methanol is very large and could sustain a major fuel industry. At present the cost of producing methanol is much higher, on a gasoline equivalent basis, than the cost of producing gasoline. For the foreseeable future, barring significant technological improvements in methanol production processes, methanol (and ethanol to an even greater degree) would require substantial tax subsidies to compete with gasoline. In spite of this, considerable attention has been focused on neat methanol as a long-term solution for transportation fuel needs. Methanol blends could well serve as transition fuels to such a situation. The octane quality of a fuel is indicative of a fuel’s ability to resist detonation or knock.2 Methanol has a higher octane number than gasoline. Test results3p4have shown that adding methanol to a low-octane-base gasoline, although not to a high-octane-base gasoline, increases the road octane number significantly more than the proportion added. High octane ratings allow considerably higher compression ratios, thus permitting greater efficiency and power in automobile engines. Exhaust emissions such as oxides of nitrogen (NO,) and CO are less for straight methanol, but aldehyde emissions are generally higher than those for gasoline. Blends of methanol and gasoline result in changes in emissions proportional to their content^.^ On the whole, for gasoline-methanol blends, CO and hydrocarbon emissions are reduced while aldehyde and NO, emissions are increased somewhat. Due to the strict desulfurization required in the manufacture of methanol, sulfur dioxide and sulfuric acid mist emissions would be less than those for blended gasoline. Unfortunately, along with these advantages, methanol-gasoline blends also have their own drawbacks from both physical and chemical standpoints. Alcohol blends inevitably experience some materials incompatibility, and methanol blends are incompatible with copper, magnesium, brass, aluminum, and some plastics and rubbers. Methanol-gasoliw mixtures also exhibit a large deviation from Raoult’s law. That is the volatility of the base gasoline is increased. This results in an increase in the frequency of engine vapor lock during warm weather. Also, if the volatility is not adjusted, an increase in evaporative emissions will occur. Some other problems with these blends are poor drivability, cost, and toxicity. However, the principal problem with methanol blends is phase separation. Methanol-gasoline blends have been studied for a number of years, but the problem of phase separation at low temperatures in the presence of water is still a major barrier to their widespread commercialization. This is a physical process in which two phases are formed a heavier
alcohol-rich aqueous phase and a lighter gasoline-rich phase. Much research has been undertaken in the last two decades to identify additives that would stabilize these blends and at the same time not debilitate the other characteristics of the fuel. The separation temperature of methanol-gasolineblends is dependent on the composition of the base gasoline and the percentage of methanol and water present. It is well-known that the addition of higher alcohols increases the water tolerance of methanol-blended fuels and reduces the risk of phase separation.6 Various blending agents have been proposed for this purpose. These can broadly be classified as surfactants, alcohols, and alcohol mixes. COX’studied a number of higher alcohols, ethers, esters, and soaps and found that the best surfactant tested (a poly(oxyethy1enealkyl ether)) was only half as effective as 1-hexanol. Terzoni et al.s also studied the solubilizing effects of alcohols such as 1- and 2-propanol, n-, sec-, isoand tert-butyl alcohols, and n-amyl, n-hexyl, and n-octyl alcohols. Other functional compounds such as methyl tert-butyl ether (MTBE) and monoethylene glycol tertbutyl ether (MEGTBE) were also evaluated by them. Their conclusion was that linear alcohols have a higher efficiency than branched ones. Also, the hydroxylic function had a higher efficiency than the ethereal or estereal ones. Osten and Sell9 compared tert-butyl alcohol with 2-propanol as blending agents. Their experiments showed that the 2-propanol generally resulted in a larger depression of the separation temperature, particularly at a blending agent concentration of 5% or less and a water concentration of 0.2% or less. This particular study examined the effectiveness of carboxylic acids as separation temperature depressants for fuel mixtures containing gasoline, methanol, and water. The carboxylates used were oleic acid and heptanoic acid. Oleic acid is a fairly surface-active carboxylate, while heptanoic is relatively less surface active and can be expected to behave more as a cosolvent. The efficacy of these carboxylates was compared with that of 2-propanol in preventing phase separation. Carboxylate dosage in the fuel mixture ranged from 1to 4 wt % ,while methanol and water compositions ranged from 0 to 25 and 0 to 0.3 wt % , respectively.
(1)Reed, T.B.;Lerner, R. M. Science 1973,182,1299-1304. (2)Benson, J. D. CHEMTECH 1976,5,16-22. (3)Hagen, D.L. SAE Paper No. 770792. (4)CRC Fuel Rating Program: Road Octane Performance of Oxygenates in 1982 Model Cars. Coordinating Research Council Report No. 541;Coordinating Research Council: Atlanta, GA, 1985. (5) Sauter, N. A. SAE Paper No. 820261.
(6)Ecklund, E. E. Presented at the Fourth International Symposium on Alcohol Fuels Technology, Guaruja, Brazil, 1980. (7)Cox, F. W.Presented at the Third International Symposium on Alcohol Fuels Technology, Asilomar, CA, 1979. (8)Terzoni, G.;Pea, R.; Ancillotti, F. Presented at the Third International Symposium on Alcohol Fuels Technology, Asilomar, CA, 1979. (9)Osten, D.W.;Sell, N. J. Fuel 1983,62,268-270.
Experimental Section A simulated hydrocarbon fuel was used in this work. This was a mixture of pure 2,2,4-trimethylpentane (isooctane) and toluene, which is fairly representative of the two major hydrocarbon classes in commercial gasoline, namely alkanes and aromatics. The methanol, isooctane, and toluene used were all chemically pure grade. The oleic acid, heptanoic acid, and 2-propanol were all reagent grade, and the water used for mixing was deionized and distilled. The samples were prepared by mixing the hydrocarbon, methanol, carboxylate, and water in that order. The methanol content examined ranged from 0 to 25 wt % and was examined in 5% increments. The carboxylate content was varied from 0 to 4 wt % and was examined in 1% increments. The water content studied was from 0 to 0.3 w t % and was examined in 0.1% increments. All calculations were based on weight and were determined prior to mixing. The samples were prepared in Pyrex screw cap culture tubes and sealed by using shrink wrap tape. The phase separation behavior was examined by gradually cooling the samples. A 4-6-h equilibration period was used a t
Pate1 and Cheung
78 Energy & Fuels, Vol. 3, No. 1, 1989
- 501
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0.1% water (Figure 2) where a slight increase in the phase separation temperature was noted for 1% olecic acid below approximately 12% methanol content. Figures 5-7 show
the results obtained for the heptanoic acid. Heptanoic acid was more effective in depressing the phase separation temperature than oleic acid. With moderate water content (0.2%) the heptanoic acid lowered the phase separation temperature from 0 "C to approximately -30 "C for a solution containing 15% methanol and 2 % heptanoic acid. 2-Propanol was also tested with the same samples since considerable research has already been carried out on it. The results obtained are shown in Figures 8-10. When the results are compared, it seems that 2-propanol is in
Energy & Fuels, Vol. 3, No. 1, 1989 79
Fuel Phase Separation Temperatures CT
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greater than 15%. And in the case of low water content (0.1%) and methanol content greater than 15%, 2% heptanoic acid lowered the phase separation temperature slightly more than 2% 2-propanol. These experimental results seem to indicate that the carboxylates examined (heptanoic and oleic acid) were not superior to 2-propanol, and in fact the oleic acid was significantly less effective in lowering the phase separation temperature than 2-propanol. In many instances, particularly for relatively low water content, heptanoic acid and 2-propanol had comparable effects on the phase separation temperature. This is promising, and other carboxylates or combinations of surface-active carboxylates and other alcohols or carboxylates (cosurfactants)may prove useful as stabilizers for methanol-containinggasolines. Obviously, considerably more detailed studies than simple phase separation temperature determination would be required prior to commercialization of any fuel additive. The affects of the additives on corrosivity, deposit formation, performance, and other fuel properties would have to be examined carefully before commercialization. This research is continuing and will examine other carboxylates as well as other C, H, and 0 compounds as possible stabilizers for gasohol type fuels. Acknowledgment. We acknowledge the support of the
State of Ohio and the Ohio Board of Reagents through the Research Challenge program. Registry No. Heptanoic acid, 111-14-8;oleic acid, 112-80-1; methanol, 67-56-1; water, 7732-18-5; 2,2,4-trimethylpentane, 540-84-1; toluene, 108-88-3.
