15 Microemulsions as Diesel Fuels
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G. GILLBERG and S. FRIBERG Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65401 and The Swedish Institute for Surface Chemistry, Stockholm, Sweden
Microemulsions with greater than 15 wt % water and cetane numbers larger than 35 show simultaneous large decreases in emitted NO and smoke (soot) when used as diesel fuels. Important factors for microemulsion stability are described since these are essential to obtain thermodynamically stable microemulsions. The regions for these are found easily with a systematic knowledge of phase equilibria in surfactant systems. Most surfactants and the microemulsified water had a negative effect on the diesel fuel ignition performance. Conventional cetane number improvers yielded only minor improvements. However, certain emulsifiers function by themselves as cetane number improvers. Relations between emitted NO, smoke, and fuel consumption are given at various injection timings for microemulsions with 10, 20, and 30% water, and the effect in other types of emissions is discussed. x
Tl/Ticroemulsions are transparent thermodynamically stable colloidal dispersions containing high amounts of both water and hydrocarbons. The colloidal state is stabilized by a proper balance between a hydrophobic and a hydrophilic surfactant. Initially microemulsions were considered to be different from colloidal solutions (I, 2, 3, 4, 5); an opinion that is still held by some (6) although it is accepted generally that microemulsions belong to micellar systems (7, 8, 9, 10). This chapter describes the liquid phases in some four-component systems, a treatment that demonstrates the identity between colloidal solutions and microemulsions and gives information about the important factors for their stability. 0-8412-0383-0/78/33-166-221$05.00/0 © American Chemical Society Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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Water/Oil (W/O) Uicroemulsions The basis for the W/O microemulsions is the association between a hydrophobic surfactant (commonly a medium-chain (C -Ce) alcohol) and a hydrophilic surfactant (usually a soap in the model systems) such as potassium oleate or an alkyl sulfate. A combination of two such amphiphiles gives ranges of high water solubility capacity according to Figure la. In order to show that the three components are equally important, a triangular diagram is often used; the results of Figure la then are presented as in Figure lb. The promotion of solubility of the ionic surfactant from 8 to 45% (Figure lb) by water is not obtained by inverse micelles. The aggregates in this part of the system contain ion pairs; the notation "micellar systems" which is still used (6) is not correct, simply because no micelles exist in this area. 5
A
CD
0
0.2
0M
0.6
Ionic surfactant Ionic surfactant • Alcohol
Figure 1. Solubility of an ionic surfactant in a medium chain length alcohol (C , C ) is promoted by water; ion pairs are formed. The solubility of water is promoted by the presence of the surfactant through the formation of inverse micelles. 6
6
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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Microemulsions as Diesel Fuels 223
W a t e ^ surfactant a n d alcohol (co-surfactant) form
the basis
inverse micellar
for the area
_B_ MICROEMULSIONS also contain a fourth component; the hydrocarbon
I.S. _C_ The W / Q MICROEMULSIONS are a part of the inverse micellar solution
Figure 2. Starting with the conditions of the three basic components—water, surfactant, and co-surfactant—the microemulsions are easily shown to be a part of the inverse micellar area The enhanced water solubility, on the other hand, is caused by the existence of inverse micelles. The maximum size of these to allow maximum water solubilization depends critically on the alcohol/soap ratio; in Figure lb maximum water solubility is obtained at a weight ratio of 5:2. Identical conditions exist if the corresponding solubility region is determined at constant hydrocarbon content in a microemulsion. Such compositions mean that a fourth component is introduced, and a tetrahedral representation is necessary such as in Figures 2a, b, and c. From this and other diagrams (8,9,10) an important conclusion concerning microemulsion conditions may be drawn—the alcohol/soap ratio necessary to obtain maximum water solubilization remains identical at different hydrocarbon contents. This result is important since it saves much experimental labor in order to find optimum compositions for microemulsions. These systems
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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do not differ from normal colloidal solutions, and the term microemulsion is unnecessary. It has, however, been so well established that it will be difficult to change. Oil/Water (O/W) Microemulsions These systems have not been investigated as thoroughly as the W/O microemulsions have been. One determination (11) has been reported, and Prince has suggested (6) that the O/W microemulsions exist within a limited oil/emulsifier ratio in a sectorial solubility region emanating from the aqueous corner. This is true only for nonionic systems (12); the combination ionic substance and alcohol gives a more complicated pattern.
AN PPTli^AJL^tQtiaJiiAJJj: OF THE TWO COMPOUNDS MAY GIVE RISE TO A
H0 2
i.S.
Figure 3. An optimum combination of surfactant and co-surfactant gives microemulsions The most simple representation, found in Figure 3, demonstrates the solubilization dependence on the amount of ionic surfactant dissolved in water. The amount of ionic surfactant in water is the critical condition to obtain O/W microemulsions.
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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Microemulsions as Diesel Fuels 225
Factors of Importance for the Stability of Microemulsions
The earlier concepts of microemulsion stability stressed a negative interfacial tension and the ratio of interfacial tensions towards the water and oil part of the system, but these are insufficient to explain stability (13). The interfacial free energy, the repulsive energy from the compression of the diffuse electric double layer, and the rise of entropy in the dispersion process give contributions comparable with the free energy, and hence, a positive interfacial free energy is permitted. One important problem connected with the stability of microemulsions is the long-term stability against separation of a liquid crystalline phase. Such a phase is always present in the base diagram, and the liquid crystalline phase which forms a separate phase at equilibrium may be distributed in the form of small spherical particles which are not observed on optical inspection. Prolonged storage, however, may lead to the separation of the liquid crystal, causing turbidity, clogging, and other problems. Phase diagrams such as those described above are a useful tool for clarifying and avoiding problems of this kind since they demonstrate the kind of instability that may be expected at different compositions. Diesel Fuels
Although several patents concerning the use of microemulsions based on gasoline (I, 14, 17) exist, and articles have been published claiming extremely good engine performance with such fuels (18,19, 20), we limit ourselves here to diesel fuels. We believe that microemulsified fuels have their greatest potential when used in the diesel oil engine since the consumption of gasoline is too large for the world production to supply the necessary amounts of emulsifiers for the microemulsified fuels. Furthermore, in contrast to the gasoline engine, no changes in construction of the diesel oil engine will cause its emissions to fulfill the EPA standards. Many experiments have shown that combustion of petroleum fuels is cleaner and more efficient if small water droplets are dispersed in the oil by ultrasonic, hydrodynamic, or chemical techniques (21). The presence of the water leads to a lower generation of nitrogen oxides and to less soot, probably because of lower combustion temperatures and better fuel atomization. The most tested system is a mechanical one for furnaces and ovens produced by the French oil company, E L F (22). The diesel engine requires a fuel with ignition and combustion characteristics opposite from those for the ordinary spark ignition gasoline engine. In the diesel engine an enhanced ignition speed is desired since the fuel is injected, in general, directly into the combustion space.
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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For its maximum efficiency, the delay period between injection and ignition must be short. The investigations by Valdmanns and Wulfhorst (23) concerning the effect of emulsified water on diesel combustion showed that the increase in ignition delay was large enough to increase fuel consumption, and consequently there were no net reductions in the amount of exhaust pollutants. The emulsion fuels showed no net changes in the amount of NO emitted and only a slight decrease in smoke compared with a standard fuel. Since the economy and benefits of the diesel emulsions were low, the project was abandoned. Microemulsion Fuels Ignition delays similar to those observed for the emulsion fuels may be experienced with microemulsion fuels as illustrated by Table I, which shows cetane numbers of microemulsion fuels determined on a CFR engine. Table I.
Cetane Numbers of Microemulsion Fuels Composition of Fuel (%) (w/w)
Series
Diesel Oil
Emulsifiers
Water
Cetane No.
A
100 80 76 68
0 20 19 17
0 0 5 15
43 37 34 31
B
70 49
30 21
0 30
33 18
The test shows that the emulsifier has a negative effect on the ignition performance and that this effect is further enhanced by adding water. However, an important difference was noted. With microemulsion fuels, the NO* content of the exhaust gases was reduced substantially while with the emulsion fuels, the detected amount of NO* increased considerably. A fuel with a cetane number higher than 40 is generally required for high-speed diesel engines. The cetane number, and thus the ignition of a microemulsion fuel, may be improved using two different approaches. The first one is the conventional addition of cetane-number improvers such as amyl nitrate and kerobrisol MAR. However, at least 10% (w/w) of the improver was needed to restore the cetane number of the microemulsion containing 30% water (Table I) to that of the pure diesel oil. The second and most promising approach is based on the fact that certain emulsifiers function by themselves as cetane number improvers. Observed cetane numbers of microemulsion fuels produced by this type
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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Microemulsions as Diesel Fuels 227
I
I
I
I
0
10
20
30
I 40 %(WW)
WATER C O N T E N T IN MICROEMULSION FUELS
Figure 4. Cetane numbers of microemulsion fuels in which the emulsifiers act as cetane number improvers. (A) Diesel fuel pure, (%) diesel fuel plus emulsifier and water. of emulsifier are given in Figure 4. The addition of emulsifier increased the cetane number from 43 to 49, a value which was not reduced by water additions less than 10%. Tests According to a Limited California Cycle
The engine used was a one-cylinder direct-injection test engine at Saab-Scania, Sodertalje, Sweden. During the tests, maximum pressure, fuel consumption, exhaust temperature, CO, C0 , NO, N0 , 0 , HC, and smoke were registered at varied injection timings, loads, and speeds. The performance of three microemulsion fuels was compared with a reference fuel of pure diesel oil with the cetane number of 43. The data of the microemulsion fuels are given in Table II. 2
2
2
Table II. Tested Microemulsion Fuels Existence TemperaSample Composition (%) (w/w) No. Diesel Emulsifiers Water 1 2 3
63 56 49
27 24 21
10 20 30
Range (°C)
Viscosity Cetane (cP) No.
0-80 15-73 7-65
15.7 14.3 19.8
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
44-45 40-42 35
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Figure 5. Relation between the relative NO emission and fuel consumption various injection timings; • 28, • 23, * 18, and © 13 °BTDC. The reference fuel ( ••••• )isa diesel oil with cetane number 43. Fuel 1 (mm*), fuel 2 (ickit) and fuel 3 ( ) are microemulsion fuels with 10, 20, and 30% water respectively, (a) Speed 1380 rpm and 100% load (P = 8.4 kp/cm ). (b) Speed 2300 rpm and 75% load (P = 5.4 kp/cm ). 2
e
e
2
Zung; Evaporation—Combustion of Fuels Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
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GILLBERG AND
Microemulsions as Diesel Fuels 229
FRIBERG
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