Multifunctional Diesel Fuel Additives from Triglycerides - Energy

Energy & Fuels 2001, 15, 151-157

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Multifunctional Diesel Fuel Additives from Triglycerides G. J. Suppes,* M. Goff, M. L. Burkhart, and K. Bockwinkel The University of Kansas, Department of Chemical and Petroleum Engineering, Lawrence, Kansas 66045-2223

M. H. Mason, J. B. Botts, and J. A. Heppert The University of Kansas, Department of Chemistry, Lawrence, Kansas 66045 Received June 8, 2000. Revised Manuscript Received October 12, 2000

Nitrate derivatives of soybean oil were synthesized and evaluated as diesel fuel additives. The goal of this phase I commercialization effort was to develop a renewable cetane improver, alternative to 2-ethylhexyl nitrate (EHN) which currently dominates the cetane improver market. The products exhibited NOx-reducing capabilities similar to that of EHN when used in a diesel fuel. They also provided significant lubricity enhancement to the fuels at the same concentrations used to provide the cetane enhancement. Depending on the product, these additives exhibit increased stability and lower volatility than EHN. Commercially competitive enhancements of both ignition-related properties and lubricity where achieved in a single product.

Introduction Ignition quality and its direct and indirect impacts on fuel performance are some of the most important properties of a diesel fuel. When a diesel fuel has deficient ignition quality, cetane improvers (CI) are commonly added to improve the ignition. Improved ignition is detected as a decrease in the ignition delay time, the ignition delay time being measured as the time between start of fuel injection and detectable ignition. The cetane number scale is defined by reference fuels specified in the ASTM D-613 standard. This test method is based on comparative analysis of the engine compression ratio necessary to ignite a fuel at TDC in 2.4 ms when operating at 900 rpm. Shorter ignition delay times have been directly correlated with faster startup in cold weather, reduced NOx emissions, and smoother engine operation. A diesel fuel’s cetane number is one of five properties that can be used to justify a “premium” rating for that fuel1sthe analogous octane rating of gasoline is used to determine quality and price of gasoline. Due to the fast growth of premium diesel utilization, the National Conference of Weights & Measures has acted to define “premium” diesel. Energy content, cetane number, fuel injector cleanliness, low-temperature operability, and thermal stability were the five properties most commonly associated with premium diesel.1 Most companies guarantee a cetane number >45 for premium diesel. Additives that boost the performance of diesel in one or more of these five premium categories are of great value and interest. This paper describes the performance and general syntheses of nitrates of fatty acid derivatives (NFAD). * Author to whom correspondence should be addressed. Phone: (785) 864-3864. Fax: (785) 864-4967. E-mail: [email protected]. (1) Peckham, J. Growing your business with premium diesel. Special Report from Diesel Fuel News; Hart Publications Inc.: January, 1998.

Table 1. Example CI Compounds

The performance of NFAD both as cetane improvers and as lubricity improvers is presented. Being derived from biomass, these products also are renewable. Since biobased products also tend to have improved biodegradability as compared to petroleum counterparts, these products are also likely to have advantages associated with biodegradability. The fats and oils used to make the NAFD also sell at less than half the price of 2-ethylhexanol (see Chemical Marketing Reporter for latest prices). Background Cetane Improvers. The predominant commercial CI is 2-ethylhexyl nitrate (EHN). Table 1 illustrates the structure of EHN as well as two alternative CI. Di-tertbutyl peroxide (DTBP) is often presented as an alternative to EHN when the nitrogen content of EHN is

10.1021/ef000122c CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000

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undesirable in the fuel; however, DTBP is both more expensive than EHN and must be transported in a diluted form (e.g., 32% DTBE in diesel).2 MODN is the dinitrate of the oleic acid methyl ester and is one of several possible products resulting from the nitration of fatty acid derivatives (FAD). For comparison purposes, methyl oleate (MO) is presented as a reaction precurser to MODN. Being a methyl ester of a common fatty acid, MO is a reasonable model of biodiesel. The commercial market considers several factors when selecting and using CI, these include: (1) efficacy toward improving ignition properties; (2) hazards associated with storage and transport; (3) additional costs associated with diluting cetane improves to allow safe transport (e.g., diluting DTBE to 32% in diesel); (4) nitrogen content (to meet CARB regulations). As legislation mandates lower emissions from diesel engines, fuel additives will be increasingly relied upon to reduce emissions and to compensate for changes in physical and chemical properties associated with meeting reduced sulfur specifications. Fuel distributors will rely more on multifunctional treatment packages. In this competitive market, a fifth consideration comes forth: (5) multifunctional treatment capability from a single additive. On the basis of the first three criteria, EHN is typically favored over DTBP. However, when all five criteria are considered, there are opportunities to develop additives that provide more overall value. Of greatest importance toward multifunctional treatment (criteria 5) are additional advantages associated with boosting a diesel fuel’s performance in fuel injector cleanliness, low-temperature operability, thermal stability, or lubricity. To a large extent, the molecular properties of additives that contribute to good performance are trade secrets. For the additive described in this paper, efficacies for reducing ignition delay times and increasing lubricity were evaluated. Evaluation of Ability to Improve Ignition. When fundamentally interpreted, the cetane number correlates with the time between initiation of fuel injection and the time when the fuel starts to ignite.3 This is commonly referred to as the ignition delay time. Ignition delay times in engines are between 0.7 and 2 ms for cetane numbers between 40 and 50; the shorter the ignition delay time, the higher the cetane number of a fuel. Improved ignition (shorter ignition delay time) has been directly correlated with faster startup in cold weather, reduced emissions of nitrogen oxides (NOx), and smoother engine operation.4 Cetane numbers are routinely evaluated by cetane engine tests as well as direct measurements of ignition delay times.3,5 Higher cetane fuels are promoted for use in diesel engines due to advantages associated with (1) quicker (2) Peckham, J. Peroxide CI could help some diesel refiners. Growing Your Business with Premium Diesel, special report from Diesel Fuel News; Hart Publications Inc.: 1998b. (3) (a) Diesel Engine Reference Book, 2nd ed.; Challen, B., Barabescu, R., Eds.; Butterworth Hieneman: Oxford, 1999. (b) Diesel Fuel Injection; Ulrich, A., Ed.; Robert Bosch GmbH: Germany, 1994. (4) Bacha, J., et. al. Diesel Fuels Technical Review (FTR-2). Cheveron Ornite Division; Chevron Products Co: Houston, TX, 1998. (5) (a) Allard, L. N.; Webster, G. D.; Hole, N. J.; Ryan, T. W.; Ott, D.; Fairbridge, C. W. Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT). SAE Paper 961182. (b) Allard, L. N., et al. Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)-Part II. SAE Paper 971636.

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startup in cold weather, and (2) simultaneous reduction of all emissions.2-4 The ability to simultaneously reduce all major emissions is a significant advantage and must be taken with a grain of salt because varying engine models and operating conditions cause substantial variance in performance. However, when focusing on the predominant trouble emissions from diesel, namely NOx and particulate matter (PM), evidence of the correlation between higher cetane and reduced [NOx + yPM] emissions is rather overwhelming. Since injector timing can be used to decrease NOx at the expense of increasing PM, cetane improvers will not always reduce both NOx and PM,but the weighted sum of NOx and PM (y being weighting factor) will be consistently reduced. Trends of reducing NOx emissions with reducing ignition delay times (increasing cetane numbers) has been frequently observed and can be fundamentally explained. For a diesel fuel injection that lasts about 4 ms, a typical ignition delay time is about 1.2 ms. Higher cetane numbers lead to shorter ignition delay times while lower cetane numbers lead to longer ignition delay times. With everything else held constant, longer ignition delay times result in more injection/dispersion of fuel prior to ignition, and more injection/dispersion of fuel translates to a stronger detonation, higher temperatures, and greater NOx generation as a direct result of higher temperatures. Hence, higher cetane number generally result in lower NOx emissions. Conversely, higher temperatures can lead to more or less PM, depending upon the fuel density and spray dynamics. While PM emissions tested in the controlled environment of a laboratory may decline with increasing cetane number, actual results vary considerably and the fundamental basis for increasing or lowering PM emissions is not obvious. Schwab et al.6 specifically evaluated the impact of cetane improver application rates on NOx, PM, HC, and carbon monoxide (CO) emissions. HC emissions were unaffected by the cetane number; however, each of the other emissions decreased. At an application rate of 1000 ppm EHN raised the cetane value of the test fuel by 5 cetane numbers and resulted in a 3% reduction in NOx, a 4% reduction in PM, and a 5% reduction in CO emissions. Nitrates have historically been the chemical functional groups that lead to good cetane improver performance. For our research7-10 as well as the work of Poirier et al.,11 agricultural feedstocks were pursued on (6) Schwab, S. D.; Guinther, G. H.; Henly, T. J.; Miller, K. T. The effects of 2-ethylhexyl nitrate and di-tertiary-butyl peroxide on the exhaust emissions from a heavy-duty diesel engine. SAE Paper 199901-1478. (7) Suppes, G. J.; Mason, M.; Tshung, Y. T.; Aggarwal, R.; Heppert, J. A. Performance Advantages of CI Produced from Soybean Oil, Bioenergy ’98, Improver Analysis and Impact of Activation Energy on Relative Performance of 2-Ethylhexyl Nitrate and Tetraethylene Glycol Dinitrate. IEC Res. 1997, 36 (10), 4397-4404. (8) Chen, Z. Chemical Synthesis and Evaluation of Cetane Improvers. M.S. Thesis, Department of Chemical and Petroleum Engineering, The University of Kansas, 1997. (9) Mason, M. H. Ph.D. Dissertation, Department of Chemistry, The University of Kansas, 1999. (10) Suppes, G. J.; Tshung, T. T.; Mason, M. H.; Heppert, J. A. Performance Advantages of Cetane Improvers Produced From Soybean Oil. Proceedings of the Fourth Biomass Conference of the Americas, DOE Publication, Oakland, CA, 1999. (11) Poirier, M. A.; Steere, D. E.; Krogh, J. A. Cetane improver compositions comprising nitrated fatty acid derivatives. U.S. Patent 5,454, 828, 1995.

Diesel Fuel Additives from Triglycerides

the basis of the following criteria: (1) relatively low cost ($0.50 per pound), and (2) chemical functional groups that allow nitration at a carbon:nitrogen ratio >6. Triglycerides meet these criteria. In addition, triglycerides have carbon-carbon π-bonds and ester bonds, both of which can be chemically modified leading to high nitration yields. Hydrocarbon emissions (HC) generally do not correlate well with cetane number since, to a first approximation, hydrocarbon emissions are dominated by fuel volatility. In addition, typical variations in engine operation will have a greater impact than a cetane improver additive. Lubricity Additives. Lubricity additives enhance the lubricity of diesel fuels. Lubricity is extremely important to minimize wear between close tolerance metal-on-metal moving parts in diesel fuel injectors. The lubricity mechanism of a fuel consists of both boundary and hydrodynamic lubricitysadditives typically only address the boundary lubricity component. Molecular architectures having both polar groups and >C14 hydrocarbon chains are attracted to the metal surfaces and form a thin protective film thereby enhancing boundary lubricity. Munson and Hertz12 reported the performance of twelve different additives, none of the additives were described in detail. Three were described as fuel conditioners, one as a sulfur substitute, three as lubricity additives, one as a commercial biodiesel additive, two as biodiesel additive A, and two as biodiesel additive B. The industrially accepted definition of biodiesel is a methyl or ethyl ester of a fatty acid (such as MO of Table 1). This type of nondetailed description of fuel additives is fairly common in the literature. The fat-derived nitrate products reported in this paper were evaluated as lubricity enhancers. Since these products are not commercially available, no data was previously available on their performance as lubricity enhancers. No data has been published on how the addition of nitrate groups impacts a molecule’s ability to enhance lubricity in a diesel fuel. ASTM standards D 6078 and D 6079 describe scuffing load ball-on-cylinder lubricity evaluation (SLBOCLE) and high-frequency reciprocating rig (HFRR) testing procedures. The SLBOCLE was used to evaluate the additives of our program. SLBOCLE studies have indicated that test values 2800 g usually will perform satisfactorily. For purposes of discussion, an apparent lubricity number (ALN) is defined by the following equation:

ALN ) L/2000 where L is the SLBOCLE load in grams. NFAD Additives. For the past few decades, the cetane improver market has been dominated by EHN, with some competition from DTBP, dinitrates of polyethyleglycol, and several alkyl nitrates containing up to 10 carbons. Progress on cetane improvers synthesized from fatty acids has been dominated by work supported (12) Munson, J. W.; Hertz, P. B. Seasonal Diesel Fuel and Fuel Additive Lubricity Survey Using the “Munson ROCLE” Bench Test. SAE Paper 1999-01-3588.

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by Exxon11 and recent work in the laboratories of Suppes and Heppert.8,-10 Poirier11 reports at least three different methods for producing nitrates from triglyceride feedstocks. The first of these includes steps of (1) hydrolysis of triglycerides to fatty acids, (2) esterification of the fatty acids with a diol such as ethylene glycol, and (3) nitration of the primary alcohol group of the glycol ester of the fatty acid. Nitration is performed using nitric acid in combination with another strong acid to form primary alchohols. Although not described in the patent, the long chain nitrates formed by this reaction had performance limitations related to the location of the ester bond between the nitrate groups and the major portion of the hydrocarbon chain. The ester tends to reduce the effectiveness of the free-radical decomposition that produces desired cetane improver performance. The second method described by Poirier includes (1) hydrolysis of triglycerides to fatty acids, (2) hydroxylation of double bonds in the fatty acid chain with hydrogen peroxide catalyzed by performic acid, and (3) nitration of the secondary alcohol groups. The resulting long chain nitrates had performance limitations due to limited solubility in diesel. The third method described by Poirier includes (1) hydrolysis of triglycerides to fatty acids, (2) hydrogenation of the double bonds in the fatty acid chain, (3) esterification of the acids with methanol, (4) reduction of the ester bond to a primary alcohol, and finally (5) nitration of the alcohols. The process involves more steps than would be desired for a competitive production process. The performance was >60% that of EHN. This third product was the basis of U.S. Patent 4,454,842 with claims of good performance and high solubility. More recent studies by Suppes et al.7-10 include specific improvements toward the nitration of FAD such as (1) direct nitration of double bonds to form nitrates, (2) improved solubilities (>0.5%), and (3) improved performance by mixing nitrates produced from triglycerices with nitrates produced from polyglycols. Suppes et al. have achieved and reported FAD nitrate cetane improver performances approximately equal to that of EHN; however, until the work reported in this paper, only the nitration of relatively pure fatty acids was successful enough to be considered competitive with EHN. The difficulties of synthesis based on derivatives of naturally occurring mixtures of fatty acids exceed the difficulties of producing good products from model fatty acids such as oleic acid. Experimental Procedure Chemical synthesis procedures are presented in terms of synthesis with oleic acid to provide general details of the chemical synthesis while preserving proprietary synthesis procedures until patent protection is received. Details of preferred synthesis means should be available in patent literature in due time. To produce the MODN product of Table 1, the MO reactant is produced by esterification of oleic acid with methanol. Epoxidation of the carbon-carbon π bond followed by hydrolysis resulted in a diol susceptible to nitration. Nitration was performed with a mixture of acetic anhydride and nitric acid. Epoxidation of Methyl Oleate (MO). A 250 mL, threeneck round-bottom flask was equipped with a thermometer, then charged with methyl oleate (20 g, 0.0675 mol) and formic

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acid (2.54 mL, 0.0673 mol; from Lancaster.) This mixture was warmed in a water bath to +30 °C. A 10 mL syringe was charged with 50% H2O2 (4.59 g, 0.675 mol.) Over a period of 1 h, this oxidant was added to the reaction vessel. The rate of addition was adjusted to keep the reaction temperature below +45 °C. After the addition was complete, the temperature was kept at +40 °C for 6 h. The reaction was quenched with ice water, and the epoxide was washed several times with dilute NaHCO3, extracted into ether, dried over MgSO4 and isolated in vacuo. 1H NMR data: δ 3.63 (s), 2.9 (m), 2.3 (t), 1.61 (m), 1.47 (m), 1.31 (m), 0.87 (m). IR Data: small peak at 3470 cm-1 (likely from traces of ring-opened dihydroxy product) major peaks at 2925, 2864, 1745, 1466, 1369, 1177, 1011, 845 cm-1. Hydrolysis of MO Epoxide. In a 250 mL round-bottom flask, 100 mL of 9,10-epoxy methyl stearate was reacted with 500 mL of water and 25 mL 18 M H2SO4. This heterogeneous mixture was heated to 90 °C for 4 h. The crude product was neutralized with NaHCO3(aq), extracted into diethyl ether, washed with water, dried over NaSO4, and isolated in vacuo. Complete conversion to 9,10-dihydroxy methyl stearate is possible. 1H NMR data: δ 5.3 (m), 3.85 (m), 3.64, 3.6 (m), 2.3 (m), 1.8 (m), 1.5 (m), 1.25 (m), 0.85 (m). FT-IR data: 3476, 2925, 2855, 1745, 1466, 1378, and 1169 cm-1. Nitration of MO Diol. In a 250 mL round-bottom flask, 8.0 g (0.024 mol) of 9,10-dihydroxy methyl stearate was mixed with an equivalent volume of CH2Cl2.. Next, 3.73 g (0.06 mol) of HNO3 was reacted with 7.49 g of 99% acetic anhydride in a graduated cylinder below 5 °C. (Caution! Explosion Hazard! Mix these reagents slowly, with stirring, and efficient cooling!) The acid mixture was dropped into the oil solution slowly to maintain the temperature at about +5 °C. The reaction temperature was maintained at 5 °C in the first hour, then raised to 10 °C in the second hour. The reaction was quenched with ice water and the product was washed with dilute NaHCO3, extracted into ether, and dried over NaSO4. 1 H NMR data: multiplets δ 3.95 (m), 3.64, 2.28 (t), 1.7 (m), 1.25 (m), 0.87 (m). FT-IR data: 2934, 2855, 1745, 1640.7, 1562, 1465, 1274, and 854 cm-1. Alternative to the MODN described above, the nitrate product evaluated in this study is a soybean oil derivative referred to as BK1P41. This product was produced in a 1.5liter reactor capable of producing 0.7 to 1.2 Ls of product. Testing. Product BK1P41 was evaluated as a cetane improver in a constant volume combustor that directly measured the ignition delay times.13 Application rates of the cetane improvers at 0.1%, 0.2%, and 0.4% in hexanes were evaluated. These applications rates provide a good balance between typical application rates (about 0.05%) and a good signal-tonoise ratio for the method. HPLC grade hexanes were used rather than diesel since they have a good testing cetane number of about 40 and are available at a consistent quality. The ignition delay times were plotted as a function of temperature to observe trends. About 10 ignition delay times were measured in an automated procedure at 750, 800, and 850 K. The actual ignition delay times are visually identified and read from pressure traces. The higher temperature is considered the most relevant since it is closer to the actual engine conditions. ASTM D 6078 standard lubricity tests were performed at Southwest Research Institute on sample BK1P49, which is a second 1.5 L batch of the BK1P41 product produced under identical conditions and exhibiting similar physical properties. Kerosene was used as the reference fuel since its lubricity is low and sensitive to improvement with lubricity enhancers. The additized, final lubricity of kerosene may not meet diesel specifications, but the test provides insight into the lubricity enhancing abilities of the additive. (13) Suppes, G. J.; Terry, M.; Burkhart; Cupps, M. P. Compressionignition fuel properties of FTL. IEC Res. May, 1998, 37 (5), 20292038.

Suppes et al. Table 2. Cetane Engine Results Used to Identify Producta

sample

test date

base fuel/ cetane #

∆ application cetane blending rate (ppm) number cetane #

EHN

6/99 diesel CN ) 48.7 ALTERNATIVE 6/99 diesel CN ) 48.7 EHN 7/99 diesel CN ) 42.5 ALTERNATIVE 7/99 diesel CN ) 42.5 BK1 7/99 diesel (small batch) CN ) 42.5 EHN 8/99 diesel CN ) 44.1 BK1 8/99 diesel (small batch) CN ) 44.1 ALTERNATIVE 8/99 diesel CN ) 44.1

1000

+ 3.8

3849

1000

+ 2.0

2048

1000

+2.8

2843

1000

+2.9

2942

1000

+3.1

3142

1000

+3.9

3944

1000 250 1000 250

+3.0 +2.2 +2.5 +1.8

3044 8844 2511 7244

a The BK2 and BK4 products were recorded in USDA NRI project research books.

Figure 1. Ignition delay times versus concentration for EHN and BK1P41, both at 1000 ppm. The HPLC grade hexanes (from Fisher) were used as the reference fuel. EHN (from Aldrich) was used as the reference additive. The NFAD products of this paper are referred to as the BK1 series products. Two 700 mL batches were prepared from the same recipe and with essentially identical properties and spectra, a BK1P41 product and a BK1P49 product.

Results Cetane Improver Characteristics. Based on a three-year compilation of potential cetane improvers synthesized from soybean oil in 2-20 g batches, two of the best samples were selected from archived (refrigerated) samples and tested in a cetane engine. The cetane engine test data are summarized in Table 2. On the basis of these data, the BK1 series product was chosen for scale-up. The BK1P41 was the first batch of product successfully produced in the 1.5-L pilot reactor. The ignition delay times of the BK1P41 product were compared to those of EHN tested at the same concentrations. The testing results are summarized in Figures 1-3. In addition to direct measurement of ignition delay times, MODN, BK1P41, and EHN were evaluated in a cetane engine. Table 3 summarizes the results of these tests.

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Figure 2. Ignition delay times versus concentration for EHN and BK1P41, both at 2000 ppm.

Figure 4. Impact of additives on NOx emissions from VW 1.9 L TDI engine before and after the exhaust catalytic converter. The fuels are U.S. #1 diesel fuel with a cetane of 37.3, this base diesel with 10% ethanol, the ethanol blend with 1% EHN and the ethanol blend with 1% BK1P41.

Figure 5. Impact of additives on emissions’ opacity (left bar) and particulate matter emissions (right bar) from VW 1.9 L TDI engine. Figure 3. Ignition delay times versus concentration for EHN and BK1P41, both at 4000 ppm.

Table 4. Lubricity Performance of BK1P49 Product Compared to Biodiesel Additivesa

Table 3. Summary of Cetane Engine Tests fuel

cetane number

test diesel test diesel + 250 ppm BK1P41 test diesel + 250 ppm EHN test diesel test diesel + 1000 ppm MODN test diesel + 1000 ppm EHN

37.3 38.2 40.3 42.5 44.8 45.3

Because of discrepancies between the ignition delay time and cetane engine tests, engine tests were performed in a VW 1.9 L TDI engine on fuels with and without the additives. A US1D test fuel (same as Table 3) was used with 10% ethanol added to lower the cetane number to 2800 g due to the extremely low initial lubricity of kerosene(1650 g). In addition, the Jet-A fuel had a viscosity of 1.07 cSt at 40 °C as compared to a 1.3 cSt minimum for US 1D and 1.9 cSt minimum for US 2Ds after additive addition the low lubricity is primarily due to a lack of hydrodynamic lubricity and not a lack of additive enhancement. Storage and Transport Issues. A complete set of explosivity and reactive chemicals testing was outside the scope of the present investigation. However, dif-

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Figure 6. DSC of EHN illustrating evaporation.

Figure 7. DSC of BK1P41 superimposed over reference compound.

ferential scanning calorimetry (DSC) analyses were performed in accordance with ASTM standard method E537-98 (for accessing thermal stability) on EHN, BK1P41 product, and a reference compound similar to BK1P41 but without the nitrate functional groups. Figure 6 shows the DSC for EHN. Figure 7 superimposes the DSC for BK1P41 and its reference compound. Discussion Cetane Improver Characteristics. At each of the three compositions evaluated in the constant volume combustor, the performance of BK1P41 was the same as EHN within the 10% standard deviation of the data. Their performances were indistinguishable. Mixed results were observed in ASTM-613 cetane engine tests. The small batch samples of Table 2 performed similar to EHN while the pilot scale BK1P41 sample (producing an increase of 0.9 cetane numbers at 250 ppm in a diesel fuel having a base cetane number of 37.3) was substantially inferior to EHN used in the same fuel. The cetane engine performance of the pilotscale batch of BK1P41 contradicted both the performance of the smaller batch samples of Table 2 and the combustor results of Figures 1-3. Engine tests that evaluated the impact of both additives on emissions indicated that within testing error both additives had the same impact on NOx, opacity, and particulate matter emissions. These emission tests are the most definitive because reduced NOx and PM emissions are the primary reason for using cetane improvers. Possible advantages associated with improved cold-start were not evaluated.

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In the 10% mixture of ethanol in US1D, the additives decrease NOx by about 10%, but increased opacity and PM by about 15%. It is well-known that NOx can be decreased at the expense of higher PM emissionssthe most important aspect of the emissions data is that both additives created the same changes in emissions. On the basis of these data, the question arises as to whether an engine test at 900 rpm that yields a cetane number is a thorough, or best, indicator of the benefits that can be realized due to improved ignition quality of a fuel. The gasoline industry has accepted the fact that one octane test is insufficient to characterize the octane of a gasoline. Consequently, the octane of gasoline is typically characterized by the average of the RON and MON octane ratings. A similar approach may be appropriate for diesel due to the lack of precision in characterizing a cetane rating based on the measurement of an engine operating at only one condition. Especially for diesel, the cetane test is performed at 900 rpm while engine startup can be substantially less than 900 rpm and operating speeds >1500 rpm are very common. Under cetane test conditions, ignition delay times are 2.4 ms while actual engine operation may have ignition delay times less than 1.2 ms. All-in-all, the cetane number is expected to and has been demonstrated to correlate with quicker startup and lower NOx emissions; however, this correlation is not without error. For the present BK1 series additive, the measures of efficacy as determined by ignition delay time analysis and cetane engine analysis disagree substantially. Part of the discrepancies were likely due to the difference in test fuels, and part of the discrepancies are likely due to the inaccuracy of the cetane engine test. The NOx emission results agreed with the ignition delay time measurement resultssEHN and the BK1 series product have similar efficacies on ignition-related performance. In addition to cetane improving capabilities, the BK1 series product has the following potential performance advantages: • reduced transport and storage hazard, • value-added applications associated with a multiple functional additive, • reduced hazards associated with chemical synthesis (see following discussion on storage and transport), • potentially lower feedstock costs, • use of renewable feedstocks, • improved biodegradability, • reduced nitrogen content as compared to EHN, • substantially improved performance-to-nitrogen content ratios. Lubricity Characteristics. The BK1P49 batch used for lubricity tests is essentially identical to the BK1P41 batch. SLBOCLE lubricity tests showed this product to be a good lubricity improver. The performance-to-cost ratio of the BK1P49 product is on par with best products evaluated by Munson and Hertz.12 Storage and Transport Issues. A nitrate-related decomposition was not detected with EHN due to the evaporative endotherm at ∼125 °C. Nitroglycerin undergoes a similar endotherm (no exotherm) at 150 °C. Ayres and Bens14 report this endotherm accompanied by nitrate decomposition. Such decomposition is under(14) Ayres, W. M.; Bens, E. M. Differential thermal studies with simultaneous gas evolution profiles. Anal. Chem. April, 1961, 33, 4.

Diesel Fuel Additives from Triglycerides

standable since the energy state of the vapor is considerably greater than that of the liquid at the same temperature. The BK1P41 product experienced the onset of an exotherm at ∼190 °C with a peak at 220 °C. The reference compound only displayed two minor endotherms that could be due to a number of factors including trace amounts of lower molecular weight compounds in the product. In practice, decomposition exotherms for EHN and BK1P41 would both be lower than the DSC exotherms since heat removal capability and free radical-wall effects will impact decomposition temperatures. Since the EHN endotherm is accompanied by a significant generation of gases, this endotherm would likely present very dangerous and explosive conditions as described by EHN’s flash point of 76 °C. The DSC data of Figure 6 indicates the high vapor pressure of EHN near the temperatures at which the nitrate begins to decompose. The vapor pressure of fatty acid derivatives are considerably lower. Upon exothermic decomposition, EHN will have tendencies to both build up considerable pressure and to explosively ignite (77 and 75 °C flash point for 2-ethyl-1-hexanol and EHN, respectively). These aspects of safety apply for storage, transport, and synthesis of the products. The BK1P41 exotherm is predominantly a liquidphase phenomenon with some generation of gases. Most importantly, the majority of the energy of the compounds is retained in a tar-like substance and tendencies for explosive detonation are reduced if not eliminated. The flash point of BK1P41 is greater than 95 °C. This observation is based on the boiling point data of the DSC runs where the boiling point of BK1P41 is in excess of 200 °C. The formation of the tar-like substance is overwhelmingly preferred as compared to the type of explosion that can happen when a substance simultaneously exhibits high vapor pressures, flash points, and tendencies toward exothermic decomposition. The flash point should, obviously, be confirmed with ASTM standard tests before the compound is employed outside the controlled laboratory setting. However, these tests provide insight into the ignition properties in the absence of more conclusive tests.

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tions. Lubricity tests indicated that the product also has good lubricity-enhancing capabilities. Preliminary tests were performed on safety related issues of the BK1 product. Safety advantages likely include lower volatilities, lower flash point temperatures, and a substantial reduction in the release of vapor-phase hydrocarbons during the decomposition exotherm. These safety issues suggest that safetyrelated benefits can be realized for BK1P41 during synthesis, transport, and storage. Acknowledgment. This work was supported by the Kansas Soybean Commission, the USDA SBIR program, the NRI Competitive Grants Program/USDA award number 97-35504-4244, and the MidContinental Chemical Company. Funding from The University of Kansas was also provided through the New Faculty Award, Department of Chemical and Petroleum Engineering, and the Energy Research Center. Paul Lacey of Southwest Research Institute provided assistance in measuring and interpreting lubricity data. Alix Botts assisted with the DSC analyses. Appendix Table A1. Engine Emission Test Data Used To Generate Figures 4 and 5a engine load idle

50%

75%

50%

Conclusions A nitrate product (BK1 series) of a fatty acid derivative was produced having a naturally occurring distribution of fatty acids. It exhibited cetane improving abilities similar to EHN as evaluated by direct measurement of ignition delay times. The performance of the BK1 product was less than EHN in cetane engine tests; however, the ability of the BK1P41 product to reduce NOx emissions in a test fuel comprised of 10% ethanol in US1D was identical to that of EHN. At 4.87% nitrogen, the BK1 product has about 25% less nitrogen content than EHN and will exhibit better performance-to-nitrogen-content in some applica-

75%

engine fuel

before after before after NOx NOx HC HC opacity

1430 rpm #1 diesel 137 #1 diesel + 10% EtOH 153 #1 diesel + 10% EtOH + 140 1% EHN #1 diesel + 10% EtOH + 142 1% BK1941 #1 diesel 368 #1 diesel + 10% EtOH 426 #1 diesel + 10% EtOH + 369 1% EHN #1 diesel + 10% EtOH + 386 1% BK1941 #1 diesel 578 #1 diesel + 10% EtOH 635 #1 diesel + 10% EtOH + 628 1% EHN #1 diesel + 10% EtOH + 602 1% BK1941 1800 rpm #1 diesel 342 #1 diesel + 10% EtOH 395 #1 diesel + 10% EtOH + 355 1% EHN #1 diesel + 10% EtOH + 343 1% BK1941 #1 diesel 540 #1 diesel + 10% EtOH 583 #1 diesel + 10% EtOH + 541 1% EHN #1 diesel + 10% EtOH + 554 1% BK1941

138 151 137

45 64 71

33 47 54

0.27 0.14 0.23

139

76

60

0.18

341 387 351

43 66 73

25 40 48

1.25 0.68 0.88

351

72

50

0.74

494 541 530

41 64 69

25 41 48

1.55 1.07 1.04

506

76

53

1.16

287 344 306

50 81 77

26 48 49

0.75 0.38 0.57

299

88

56

0.63

468 510 475

47 77 76

27 46 51

0.85 0.57 0.67

486

83

56

0.66

aNO are in ppm HC and opacity are in %. Emissions are before x and after a catalytic converter.

EF000122C