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Energy & Fuels 2005, 19, 426-432
Effect of Alcohol and Copper Content on the Stability of Automotive Gasoline Rita C. C. Pereira and Vaˆnya M. D. Pasa* Laborato´ rio de Combustı´veis, Departamento de Quı´mica, ICEx, Universidade Federal de Minas Gerais Av. Antoˆ nio Carlos 6627, Belo Horizonte, Minas Gerais, Brasil CEP: 31.270-901 Received June 29, 2004. Revised Manuscript Received November 18, 2004
Gasoline is a volatile liquid that is used in internal combustion engines. It is formed by a mixture of paraffins, naphthenes, olefins, and aromatic hydrocarbons (C4-C12). During storage, the hydrocarbons present in gasoline react with atmospheric oxygen, and with each other, promoting changes in its physical and chemical properties, because of the formation of gums. These materials present macromolecular character and normally cause undesirable effects, such as deposits in filters and distribution lines. The present work evaluated the formation of gums in Brazilian gasoline added with different anhydrous ethyl alcohol and copper contents in the fuel. A discussion of the oxidation reaction mechanisms is also presented. The results show that the addition of anhydrous ethyl alcohol to gasoline does not contribute to the formation of gum in gasoline. It can be stated that the addition of anhydrous ethyl alcohol to gasoline provides benefits such as a reduction of gum deposits in an engine per liter of fuel consumed. Now, copper interferes significantly in the gum content of gasoline, because it is a catalyst of radicalar oxidation reactions and it accelerates peroxidation. Therefore, this metal must be avoided in any metal alloy that comes into contact with gasoline in the feeding system of engines.
1. Introduction Gasoline is a fuel that is extensively used for internal combustion engines. It is a volatile, flammable liquid that is constituted mainly by a complex mixture of paraffins, naphthenes, olefins, and aromatic hydrocarbons (C4-C12). In addition to hydrocarbons, gasoline has small quantities of sulfur, oxygen, and traces of nitrogen. This petrol fraction distills in the temperature range of 30-220 °C.1 Over a period of storage, the hydrocarbons present in gasoline react with atmospheric oxygen and with each other, with resulting changes in its physical and chemical properties. The products of the primary oxidation reaction continue to react and form molecules with high molar mass.2 These nonvolatile materials, commonly called gums, form deposits along the vehicle fuel system, from the tank to the combustion chamber.1,3 These deposits build up in large quantities in the carburetor and the admission valves, which makes it difficult from the parts to work properly and results in an inappropriate mixture and deficient fuel burn. After some time of vehicle use, the deposit volume causes starting and low-gear problems, and it is necessary to remove the carburetor for thorough cleaning. A similar * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Oliveira, F. O.; Teixeira, L. S. G.; Arau´jo, M. C. U.; Korn, M. Screening Analysis to Detect Adulterations in Brazilian Gasoline Samples Using Distillation Curves. Fuel 2004, 83, 917-923. (2) Nagpal, J. M.; Joshi, G. C.; Singh, J.; Kumar, K. Studies on the Nature of Gum Formed in Cracked Naphthas. Oxid. Commun. 1998, 21 (4), 468-477. (3) Kinoshita, M.; Saito, A.; Matsushita, S.; Shibata, H.; Niwa, Y. Study of Deposit Formation Mechanism on Gasoline Injection Nozzle. JSAE Rev. 1998, 19, 355-357.
process occurs inside the injection nozzles and plugs in electronic injection cars. Gum deposits in carburetors, injectors, admission valves, and combustion chambers affect3 (i) drivability (engine choking, hesitation), (ii) engine performance (power loss, reduced acceleration, increased fuel consumption, detonation), and (iii) exhaust gas emissions (CO, NO, etc.) The formation of deposits in fuel stored for a long time in normal temperature conditions is mainly dependent on gasoline composition, petrol origin, type of refinement, and storage room conditions. In Brazil, ∼25% anhydrous ethyl alcohol is added to gasoline as an oxygenated additive, which contributes to an increase in its motor octane number (MON), research octane number (RON), and the reduction of carbon monoxide (CO) emissions.1 Nevertheless, the possibility of damage to engines caused by oxidation reactions that could occur in the gasoline/alcohol mixture, due to the use of alcohol, has been debated. Another factor that can favor oxidation is traces of transition metals, which strongly accelerate gasoline oxidation, catalyzing the decomposition of hydroperoxides into radicalar species. The most effective metallic ions are those which suffer the transference of one electron, such as the copper, iron, cobalt, and manganese ions.4-6 In past decades, copper was one of the (4) Waynick, J. A. The Development and Use of Metal Deactivators in the Petroleum Industry: A Review. Energy Fuels 2001, 15 (6), 13251340. (5) Morris, R. E.; Hasan, M. T.; Su, T. C. K.; Wechter, M. A.; Turner, N. H.; Schreifets, J. A. Significance of Copper Complex Thermal Stability in the Use of Metal Deactivators at Elevated Temperature. Energy Fuels 1998, 12, 371-378.
10.1021/ef049849h CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
Effect of Alcohol and Copper on Gasoline Stability Table 1. Physicochemical Properties of the Brazilian Gasoline Studied characteristic
method
ethanol content distillation temperature 10% evaporated, maximum 50% evaporated, maximum 90% evaporated, maximum final boiling point, maximum maximum distillation residue MON,a minimum (MON + RON)/2, minimum RON,b minimum gum concentration, maximum induction period at 100 °C aromatics content, maximum olefins content, maximum saturates content, maximum density a
Energy & Fuels, Vol. 19, No. 2, 2005 427 Table 3. Percentage of Copper Present in Ethyl Alcohol Added to Gasoline
value
NBR 13992
0%, v/v
ASTM D 86 ASTM D 86 ASTM D 86 ASTM D 86 ASTM D 86 ASTM D 2700 ASTM D 2699 ASTM D 2699 ASTM D 381 ASTM D 525 CG CG CG ASTM D 4052
58.1 °C 106.0 °C 179.6 °C 214.7 °C 1.0%, v/v 80.3 88.8 97.4 2 mg/100 mL 1247 min 30.7 vol % 29.1 vol % 40.2 vol % 753.4 kg/m3
Motor octane number. b Research octane number.
Table 2. Physicochemical Properties of the Anhydrous Ethyl Alcohol Fuel Studied characteristic
method
value
color electrical conductivity specific mass at 20 °C alcohol content hydrocarbon content copper content
visual ASTM D 1125 ASTM D 4052 ASTM D 512 ABNT/NBR 13993 ABNT/NBR 10893
colorless 400 µS/m 792.4 99.5 vol % 1.0 vol % 0.006 mg/kg
main contamination sources that contributed to the formation of gum in Brazilian gasoline, because the alcohol added to gasoline was produced in copper fermenting tanks and/or distillers.7,8 At present, anhydrous ethyl alcohol fuel is produced in a stainless-steel system. However, one of the possible sources of contamination of gasoline by copper may be the car feeding system, because some of the metallic parts in the system have copper in their alloy composition.9 The present work is intended to study the formation of gum in gasoline that has, as an additive, anhydrous ethyl fuel and anhydrous ethyl fuel spiked with different proportions of copper. It is also our objective to discuss the oxidation reaction mechanisms in gasoline. 2. Experimental Section Alcohol-free gasoline samples and anhydrous ethyl alcohol fuel samples from the main Brazilian oil refining company, Petrobra´s, were used. The main physicochemical properties of gasoline and anhydrous ethyl alcohol are presented in Tables 1 and 2, respectively. 2.1. Samples. 2.1.1. Influence of Anhydrous Ethyl Fuel. Ethyl and gasoline mixtures were prepared with varying ethyl percentages, from 0% to 30%, with intervals of 5%. The copper content in alcohol was dosed via atomic absorption spectrometry, using graphite furnace equipment (PerkinElmer model 5000 with a longitudinal Zeeman module). The calibration curve was prepared with 93% v/v alcohol. The sample was directly introduced into the graphite tube without any previous treatment. Palladium was used as a permanent (6) Lodwick, J. R. Chemical Additives in Petroleum Fuels: Some Use and Action Mechanisms. J. Inst. Pet. 1964, 50 (491), 297-308. (7) Solomons, T. W. G.; Sryhle, C. B. Quı´mica Orgaˆ nica, 7th Edition; LTC: Rio de Janeiro, Brazil, 2001; Vol. 1. (8) March, T. Advanced Organic Chemistry: Reactions, Mechanisms and Structure; McGraw-Hill: New York, 1968. (9) http://www.metalleve.com.
sample
copper content (mg/kg, per 25.00 mL of alcohol)
01 02 03 04 05 06 07 08
0.03 0.05 0.07 0.09 0.10 0.50 1.00 2.00
modifier. The copper content was 0.006 mg/kg for the alcohol that was added to gasoline. 2.1.2. Influence of Copper Content. Samples of gasoline and ethyl alcohol were prepared by adding 25.00 mL of ethyl alcohol that was spiked with copper. Later, the volume was composed of gasoline with a copper concentration of up to 100.0 mL in ethyl alcohol was varied in each sample, as shown in Table 3. The ethyl alcohol solution that was spiked with copper was prepared by leaving pieces of copper in a container of ethyl alcohol for a month. Copper content was then determined by flame atomic absorption spectrometry (Perkin-Elmer model 5000, according to standard NBR-10893). The ethyl alcohol stock solution had a copper content of 3.19 mg/kg. Three points close to the maximum content copper content allowed (0.07 mg/kg) in anhydrous ethyl fuel, as determined by the Brazilian government agency ANP,10 and four values farther from the limit were chosen for measurement. 2.2. Density. Sample densities were measured using a digital densimeter (Density Meter DMA 4500, Anton Paar) at room temperature (20 °C), in accordance to ASTM Standard D4052. 2.3. Induction Period and Potential Insoluble Gum Content. An induction period test was conducted with induction period equipment from Petrotest, according to ASTM Standard D525 with modifications11 at 110 °C. A potential insoluble gum test was performed with induction period equipment from Petrotest and Gumtest from Herzog according to ASTM Standard D 873/94 with modifications.12 Aging was performed at 110 °C for 4 h. Potential insoluble gum content measurements were performed in quadruplicate. To accelerate oxidation, a work temperature of 110 °C was chosen.13 2.4. Characterization of Potential Insoluble Gum. 2.4.1. Elemental Analysis. Potential insoluble gum was submitted to elemental analysis using a Perkin-Elmer analyzer (model 2400). 2.4.2. Absorption Spectrometry Analysis in Infrared Region. Gum samples were prepared in KBr (1%) pellets and absorption spectra in the infrared region were performed by ABB Bomen (model M102). 2.4.3. Thermal Analysis. Thermogravimetric curves of gums were obtained using Shimadzu model TGA 50 equipment in N2 and a dynamic air (100 mL/min) atmosphere at a heating (10) Ageˆncia Nacional Do Petro´leo (ANP); Portaria n. 309 de 27 dez, 2001; Dia´rio Oficial da Unia˜o, Brası´lia, 28 dez, 2001. (11) Test Method for Oxidation Stability of Gasoline (Induction Period Method), ASTM D525; 1995 ASTM Annual Book of Standards; American Society for Testing and Materials: West Conshohoken, PA, 1995; 6 pp. (12) Standard Test Method for Oxidation Stability of Aviation Fuels (Potential Residue Method), ASTM D873; 1994 ASTM Annual Book of Standards; American Society for Testing and Materials: West Conshohoken, PA, 1994; 3 pp. (13) Pereira, R. C. C. Estudo dos Paraˆmetros que Influenciam a Formac¸ a˜o de Gomas em Gasolinas, Dissertac¸ a˜o de Mestrado, Departamento de Quı´mica, Universidade Federal de Minas Gerais, 2003.
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Figure 1. Variation of the density of gasoline spiked with alcohol (GAL), as a function of alcohol content (before and after aging at 110 °C for 4 h). rate of 10 °C/min from room temperature up to 750 °C. The weight used was ∼3 mg for each analysis.
Figure 2. Gum content variation, as a function of alcohol content in gasoline; their respective standard deviations are also given.
3. Results 3.1. Study of the Effect of Alcohol Content on the Formation of Gum in Gasoline. 3.1.1. Density. It was observed that the sample density increased as the alcohol percent in the sample increased. This is due to the higher density of alcohol (0.7924 g/mL), in relation to that of gasoline (0.7534 g/mL). The comparison of the curves of oxidized and non-oxidized samples reveals a nonsignificant increase in density due to oxidation (Figure 1). This increase can be attributed to the formation of substances with larger molar masses (i.e., gum). The percentage variations in the non-oxidized and oxidized sample densities were ∼0.50%. The curve for the theoretical density of the samples was constructed, taking into consideration an ideal solution behavior, as described in eq 1:
Ftheoretical ) χgasolineFgasoline + χalcoholFalcohol
(1)
where
χgasoline ) χalcohol )
gasoline mass gasoline mass + alcohol mass
alcohol mass gasoline mass + alcohol mass
and F represents density (in units of g/mL). It is observed that the theoretical density curve practically coincides with the experimental curve for non-oxidized samples. 3.1.2. Gum Content and Induction Period. Figure 2 shows that the gum content decreased as the alcohol content increased. This value of gum content was due only to the gasoline present in the gasoline/alcohol mixture, because there was a dilution effect of the gum derived from gasoline. The curve for the theoretical gum content was obtained taking into consideration the measured gum content (40.1 mg/100 mL) in a sample with 100% gasoline (G). Next, the theoretical gum content for the samples with 95%, 90%, 85%, 80%, and 70% gasoline
Figure 3. Oxidation of ethyl alcohol to carboxylic acid.
were determined by taking into consideration the effect of alcohol dilution. From the analysis of the curves in Figure 2, it was observed that they are very close, with a maximum gum content variation of 1.5 mg/100 mL. This indicates that alcohol did not cause either an increase or a reduction in gasoline gum content during analysis and it did not behave as a catalyst or inhibitor of oxidation reaction of the olefins present. Alcohol did not suffer oxidation reactions that affected gum content. One of the possible explanations for the absence of influence of anhydrous ethyl alcohol fuel in the formation of gum is the fact that the oxidation mechanism of ethyl alcohol is not radicalar, in contrast to the oxidation process of olefins. When ethyl alcohol suffers oxidation, it changes to aldehyde and, later, to carboxylic acid7,8 without forming gum (Figure 3). Figure 4 shows the result of the induction period of the gasoline/alcohol samples. It was observed that they present a close induction period with a maximum variation of 35 min, which corresponds to 7%-8%. This difference may be associated to an error in the method itself. From the analysis of the curve in Figure 4, it was observed that the stability of the gasoline sample with 25% ethyl alcohol at the moment of the study10 presented a value close to that of the Brazilian gasoline sample with 0% ethyl alcohol. 3.2. Study of the Effect of Copper Content Present in Anhydrous Ethyl Alcohol on the Stability of Gasoline. 3.2.1. Density. Because the alcohol content was kept at 25%, only the percentage of copper present in each alcohol sample varied. It was observed that there was no variation in density (0.40%) for the whole of both sets of non-oxidized and oxidized samples. Nevertheless, a nonsignificant variation in density was
Effect of Alcohol and Copper on Gasoline Stability
Figure 4. Variation of induction period as a function of alcohol content present in the GAL mixture.
Figure 5. Density variation as a function of copper content present in 25.00 mL of ethyl alcohol (low-content region).
Figure 6. Density variation as a function of copper content present in 25.00 mL of ethyl alcohol (high-content region).
observed between oxidized and non-oxidized samples (Figures 5 and 6). Figure 5 shows the density values for low copper contents (0.09 mg/kg, large gum contents were observed. The sample with a copper content of 0.03 mg/kg, with 25% ethyl alcohol, presented a gum content (30.4 mg/ 100 mL) close to the measured gum content (30.7 mg/ 100 mL) for the sample with 25% alcohol and a copper content of 0.006 mg/kg, in Section 3.1.2. Figure 7 shows gum content values for low copper contents, whereas Figure 8 gives density values for high copper contents. For a copper concentration of 0.09 mg/ kg, the gum content (38.6 mg/100 mL) was similar to the value obtained for the alcohol-free sample (40.1 mg/ 100 mL). This indicates that, at this concentration (0.09 mg/kg), copper content does not influence the formation of gum yet.
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Figure 9. Variation of induction period as a function of copper content present in anhydrous ethyl alcohol.
One of the main contamination sources of copper in gasoline is the feeding line of cars through some metallic parts that have copper in their alloy composition.9 The stability of the gasoline samples studied here with different copper contents was determined and is presented in Figure 9. The induction period decreased significantly as the copper content in alcohol increased, indicating that the copper present in the fuel diminishes its stability. Alcohol samples with a copper content of 0.03-0.09 mg/kg presented a superior induction period: >360 min, a value which is recommended by ANP.10 Therefore, they presented stability to oxidation. The copper ions that are added to gasoline accelerate its peroxidation, catalyzing the decomposition of the hydroperoxides. The equations usually quoted in the literature are4,5,7,8
ROOH+ Cu+ f RO• + Cu2+ + OHROOH + Cu2+ f ROO• + Cu+ + H+ where the global equation is
2ROOH f ROO• + RO• + H2O In these oxidation-reduction reactions, the formation of a metal-hydroperoxide complex probably occurred, followed by an electron transfer to the free radical. In this case, copper can behave both as an oxidizing agent and a reducing agent, provoking a considerable increase in the rate of formation of free radicals and accelerating the beginning of peroxidation. Another type of oxidation process that can occur is the formation of a complex between copper and oxygen present in gasoline.4,5,7,8
Cun+ + O2 f [Cun+.........O• - O•] + RH f Cun+ + HOO + R• It is immediately followed by the peroxidation process and the consequent attack of olefins, which leads to an increase of gum content with the decrease in induction period. 3.3. Gum Characterization. The results obtained by absorption spectroscopy in the infrared region show that some bands were observed in all samples, as, for
Figure 10. (a) Absorption spectrum in the infrared region of gums formed in GAL, gasoline + alcohol spiked with copper (GCu), and gasoline (KBr pellet). (b) Enlargement of the absorption spectrum in the IR region (1900-600 cm-1) of gums formed in GAL and GCu mixtures.
example, that which indicates the O-H stretching of alcohols with a hydrogen bond (∼3400 cm-1).2,14,15 This band is large and indicates polymeric associations, as shown in Figure 10a. Other important bands are those which indicate the C-H stretching (∼2962, ∼2930, and ∼2870 cm-1; see Figure 10a). Now, the bands at 1655 and 1500 cm-1 indicate an axial deformation of C-C bonds within the ring and the bands at 1457 and 1378 cm-1 are symmetrical angular deformations β-CH2 and β-CH3, respectively,2,14,15 as shown in Figure 10b. Finally, there is a characteristic stretching band of carbonyl at 1708 ( 10 cm-1 and bands 1167 and 1000 cm-1, which indicates asymmetrical axial deformation of C-O-C,2,14,15 as shown in Figure 10b. Elemental analysis of the potential insoluble gum samples (Table 4) indicated high oxygen and carbon contents, which can be attributed to the macromolecular structure2 with the presence of carbonyl and hydroxyl groups, as identified in infrared analysis. Thermogravimetry (TG) and differential thermogravimetry (DTG) curves of potential insoluble gums of all samples were obtained in nitrogen and in an air atmosphere. GAL (gasoline + alcohol) and GCU (gasoline + copper-spiked alcohol) presented several overlapping events in which there were mass losses of ∼60%(14) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Identificac¸ a˜ o Espectrome´ trica de Compostos Orgaˆ nicos, 5th Edition; Guanabara Koogan: Rio de Janeiro, 1994. (15) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd Edition; Holden-Day: San Francisco, CA, 1977.
Effect of Alcohol and Copper on Gasoline Stability
Energy & Fuels, Vol. 19, No. 2, 2005 431
Table 4. Percent of Carbon, Hydrogen, Nitrogen, and Oxygen of the Different Gum Samples Elemental Content (%)
a
gum sample
C
H
N
Oa
O/C
gasoline gasoline + alcohol gasoline + alcohol spiked with copper
74.20 ( 0.10 73.10 ( 0.10 75.10 ( 0.10
7.40 ( 0.06 7.10 ( 0.10 7.60 ( 0.04
1.30 ( 0.04 4.40 ( 0.80 1.60 ( 0.10
17.10 15.40 15.70
0.23 0.21 0.21
Obtained by difference.
Figure 11. Thermogravimetry (TG and DTG) curves in a dynamic N2 atmosphere, using a heating rate of 10 °C/min for samples of potential insoluble gum from GAL and GCu mixtures and gasoline (G).
Figure 12. TG and DTG curves in a dynamic air atmosphere, using a heating rate of 10 °C/min, for samples of potential insoluble gum from GAL and GCu mixtures and gasoline (G).
81% (see Figure 11 and Table 5) up to 600 °C. The GAL gum sample presented thermal stability superior to that of the GCu sample in a N2 atmosphere. The analysis of the TG and DTG curves obtained in a dynamic air atmosphere revealed that mass loss occurred throughout the entire analysis (Figure 12), but two rather significant events were detected (at 220 and 600 °C; see Table 6).
Table 5. Thermogravimetric Results Obtained for Samples of Potential Insoluble Gum in a Dynamic N2 Atmosphere
sample
event
onset temp. (°C)
G GAL GCu
1° 1° 1°
208 250 214
maximum temp. (°C)
mass loss (%)
residual mass (%)
266 290 282
81.5 66.0 70.0
18.5 34.0 30.0
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Table 6. Thermogravimetric Results Obtained for Samples of Potential Insoluble Gum in a Dynamic Air Atmosphere sample
event
onset temp. (°C)
maximum temp. (°C)
mass loss (%)
residual mass (%)
G G GAL GCu GCu
1° 2° 2° 1° 2°
207 512 528 206 532
278 570 570 260 573
44 42 56 41 39
1 2 2
Gum samples from the study of the effect of ethyl alcohol/copper on the oxidation of gasoline presented thermal distinct stability with air (see Figure 12). The GAL gum sample presented thermal stability superior to that of the GCu sample. The GCu sample presented two thermal events with an onset temperature close to that of the G sample. The results of the TG and DTG curves in air showed that, at temperatures of >600 °C, gum degradation is practically complete. An automotive engine does not reach this temperature; therefore, clogging problems of the combustion chamber and admission valves will occur. Thus, the control of gum content in gasolines is extremely important.
(1) It was concluded that alcohol content does not affect gum formation. The reduction in potential insoluble gum content occurred because of dilution of the gasoline-derived gum present in the gasoline/alcohol mixture. Therefore, alcohol neither catalyzes nor inhibits gum-forming reactions, because it does not have a radicalar oxidation mechanism. It is even possible to state that the addition of alcohol to gasoline is beneficial, because it reduces the gum deposit in the engine, per liter of fuel consumed. (2) It can be concluded that copper interferes significantly in the gum formation/content in Brazilian gasoline that has been subjected to oxidation, because it is a catalyst of radicalar oxidation reactions, which accelerate peroxidation. This behavior is intensified when copper concentrations are >0.09 mg/kg. Therefore, this metal must be avoided in any metallic alloy that will be in contact with gasoline in the engine feeding circuit. (3) The potential gums obtained in this work are macromolecules with high carbon and oxygen contents. The oxygen present in the structure can be associated to ketone carbonyl groups and O-H groups of alcohols. Gums derived from gasoline added with alcohol present larger thermal stability both in air and nitrogen.
4. Conclusions The studies performed have allowed us to reach the following conclusions on the stability of gasoline, in relation to oxidation reactions, taking into account the contents of ethyl alcohol and copper:
Acknowledgment. The authors thanks to Ageˆncia Nacional do Petro´leo (ANP), FINEP/CETPetro, and Capes for financial assistance. EF049849H
