Selective Oxidation of a Hydrotreated Light Catalytic Gas Oil To

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Energy & Fuels 2009, 23, 799–804

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Selective Oxidation of a Hydrotreated Light Catalytic Gas Oil To Produce Low-Emission Diesel Fuel Roberto Galiasso Tailleur*,†,‡ and Pedro Casanova Caris† Department of Thermodynamics, Simon BoliVar UniVersity, Sartenejas, Baruta, Miranda 8900, Venezuela, and Department of Chemical Engineering, Texas A&M UniVersity, College Station, Texas 78413 ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 13, 2008

LCO (light catalytic gas oil) hydrotreating was performed on a laboratory-scale trickle bed reactor to obtain a 15 ppm sulfur fuel. The fuel was then selectively oxidized using a CuCr/IP(4-PVP) catalyst in air at different operating conditions on a laboratory-scale continuously stirred tank reactor. The oxygenated and polyoxygenated compounds formed were measured by a previously developed HPLC GC-MS technique, and acid content and fuel stability were measured using standard ASTM analytical procedures. Additionally, a simplified reaction model was developed to predict oxygen incorporation into the LCO, and the oxygenated fuel was tested in a diesel engine. The results show a decrease in emissions by low-sulfur diesel oxidation, as well as the benefits of having a high selectivity toward ketone formation when using a CuCr/IP(4-PVP) catalyst. The need to carefully control the depths of oxygen incorporation to preserve fuel stability was verified. A simplified kinetic model is proposed to predict oxygen incorporation in the fuel.

Introduction Recent studies have demonstrated a positive effect on the reduction of diesel emissions by adding oxygenated compounds.1 Advanced diesel engines have shown much lower NOx and PM emissions than traditional diesel engines using low-sulfur fuels. Additionally, emission reduction might be achieved by increasing the in-place oxygen by adding oxygenated compounds to the fuel that improve burning in the hot diffusion flame. The formation of a fuel-lean, premixed region inside of the fuel jet in the diesel combustion chamber may also be influenced by several fuel properties, including oxygen content, viscosity, and boiling point range. These properties can all be affected by blending diesel with other oxygenated compounds. Combustion initiation within the lean region of the diffusion flame may also be ameliorated by tuning other fuel properties such as rate of vaporization and cetane number. Oxygenate addition requires that the blend of compounds fulfill diesel specifications, such as diesel storage stability, lubricity, viscosity, density, and a boiling point compatibility with the diesel flash point. Hence, there may be opportunities to tailor the fuel properties in ways that expand the diesel engine operating regime. The effects of adding oxygenated additives are covered in more detail in the literature.2,3 The conventional blending method for producing oxygenated diesel fuel significantly increases production cost by adding the costs of oxygenate manufacturing, distribution, * To whom correspondence should be addressed. Present address: Department of Chem. Biol. and Mat. Eng., Oklahoma University, Norman, Oklahoma 73019. Telephone: (405) 795-0432. E-mail: [email protected]. † Simon Bolivar University. ‡ Texas A&M University. (1) Gonzalez, M. A.; Liney, E.; Piel, W.; Natarajan, M.; Asmas, T.; Naegeli, D. W.; Yost, D.; Frame, E. A.; Clark, W.; Wallace, J. P.; Garback, J. SAE Paper. 2001, No. 01-01-3632. (2) Hilden, D. L.; Eckstrom, J. C.; Wolf, L. R. The Emissions Performance of Oxygenated Diesel Fuels in a Prototype Di Diesel Engine. SAE World Congress, Detroit, MI, 2001. Session: Alternative Fuels for CI Engines (Part C). (3) Zannis, Th. C.; Hountalas, D. T. Energy Fuels 2004, 18 (3), 659– 666.

and blending. Another disadvantage associated with conventional oxygenated fuels is lower volumetric heating value compared with nonoxygenated fuel. However, modification of the diesel by chemical reaction can moderate the negative effects of polyaromatic and napththoaromatic compound formation. Oxygenated diesel can be obtained by direct oxidation of diesel, either by nonselective autothermal4 oxidation or via selective catalytic oxidation.5 I-Ching et al.,6 for example, patented a distillate fuel containing hydroxyl and/or carbonyl groups chemically bound to paraffinic carbon atoms within the distillate fuel molecules. Additionally, they have presented a process for reducing particulate emissions in diesel engines by using distillate fuel containing less than 3% olefins and less than 10% aromatics. In particular, We5 developed a process that partially hydrogenates diesel to produce naphthoaromatic compounds, and then selectively oxidizes these compounds into ketones, raising the cetane number leading to reduced emissions. Wender et al.7 have oxidized a Fischer-Tropsch diesel fraction that contains pure paraffins and obtained an improved diesel fuel. Oxidation of naphtoaromatics can be performed by reaction with hydrogen peroxide and catalyst8,9 or with air.5,10 For example, Ti-silicalite-type catalysts have been used together with H2O2 oxidant in the presence of a polar solvent, such as a ketone or a ketol.9 However, the use of nonselective catalysts and, in some cases, the employment of free radical activators (TBP) can modify the types of compounds formed and might result in undesired reactions that can negatively affect diesel properties. (4) Naegeli, D. W.; Childress, K. H.; Moulton, D. S. Lacey, P. I. Method for producing oxygenated fuels. United States Patent 6488727, 2002. (5) Galiasso Tailleur, R.; Casannova Caris, P. IJCRE 2007, 5, A105. (6) I-Ching Yeh, L.; Schlosberg, R. H.; Miller, R.; Cae, R. F. Diesel fuel composition. United States Patent 6458176, 2002. (7) Wender, I.; Tierney, J. W. Oxygenated Fischer-Tropsch Derived Diesel Fuel. DOE Reports; 1990, No. 40540. (8) Gaoyou, T.; Xia, D.; Zhan, F. Energy Fuels 2004, 18 (1), 49–53. (9) Beaver, B.; Fedak, M.; Wei, Y.; Clitford, C. B.; Sobkowiak, M. Chemistry of tetralin in an oxygenated model fuel system. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 2004, 49, 461–462. (10) Galiasso Tailleur, R.; Gomez, C. J. Catal. 2007, 250, 1–110.

10.1021/ef8007135 CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

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Linares et al.11 oxidized tetralin, indane, and fluorene using a homogeneous catalyst containing Cu and demonstrated the importance of the ligand on the oxidation selectivity. We have demonstrated that ketones can be formed by selective oxidation of tetralin or diesel using nonimprinted Cu/PVP or CuCr/PVP catalysts.5,10 Chromium deposited on a polymer, such as poly(2vinylpyridine), was also used in tetralin oxidation by Goe et al.12 1-Tetralone, acetophenone, and 2-methyl-5-acetylpyridine were found to be the majority products when tetralin, ethylbenzene, and the 2-methyl-5-ethylpyridine were oxidized with these catalysts. This oxidation process can be carried out in batches, as well as continuously. Goe et al. used temperatures between 393 and 423 K and an oxygen partial pressure of 2.6 MPa in a batch operating process to achieve 10% conversion of tetralin with a ketone/ketol (one/ol) ratio near 2. This paper deals with the application of an imprinted CuCr/ IP(4-PVP) catalyst to upgrade a low-value light cycle oil (LCO) stream. First hydrogenation is used to adjust the sulfur concentration to required specifications (∼15 ppm) and to hydrogenate most of the alkylpolyaromatics and alkylnaphthoaromatics into alkylnaphthoaromatics and alkylnaphthenes for further oxidation. We studied different operating conditions to develop a kinetic model for additional process simulation and to evaluate the effect of oxygen incorporation in diesel emissions. Experimental Section Fuel Characterization. A hydrotreated LCO to ultralow sulfur (LSLCO) diesel was oxidized using air and a CuCr/IP(4-PVP) catalyst to selectively incorporate oxygen into the naphthenic part of the alkylnaphthoaromatic molecules (Al-[N]Ar) and in the alkyl part of the akyl compounds (Al-[C]). The hydrotreated product was characterized using HPLC with three different solvents to separate the naphthoaromatics from the other aromatics and naphthenes. The naphthoaromatic fraction was then separated into families using a HPLC with PDVB column coupled with a GC-MS and by HNMR analysis.5 The analysis of the naphthoaromatic fraction was targeted to measure the potentially oxidizable compounds before and after reaction. The oxygenated concentrations obtained by GC-MS analysis were lumped into three families of compounds: ketones (Al-[NO]Ar), ketols (Al-[NOH]Ar),andpolyoxygenates(pol,forexampleAl-[HONOH]Ar and Al-[ONOH]Ar). The acids (AlOOH-[C]) were measured by titration (ASTM D664); in this way the oxidized alkyl compounds were measured. (Al-[C]: alkylnaphthene, alkylaromatic and -polyaromatic, and alkylnapththoaromatic species). The analytical techniques were calibrated adding tetralol, tetralone, 1,4 dihydroxytetralin, and naphtoic acid diluted in LSLCO, with the analysis of oxygenates performed in triplicate for the aromatic and naphthoaromatic fractions. The cetane number was obtained using our own correlation, based on HNMR, the composition measured by GC, and density analysis (ASTM). This correlation was developed using 100 different blends of LSLCO and oxygenate model molecules and was verified by engine testing (ASTM D643). All oxidized lowsulfur LCO (OLSLCO) samples were prepared at 10% in CDCl3 v/v. Spectra were acquired in a Bruker 200 at ambient temperature using a frequency of 200.057 MHz (sw, 4001.6; d1, 0s; nt, 128; pw, 45°; fn, 65 536; lb, 0.3). TMS signal was used as reference. (11) Linares, F.; Castro, K.; Karma, A.; Navarro, M. React. Kinet. Catal. Lett. 2005, 84, 303–310. (12) Goe, G.; Marston, Ch.; Scriven, E.; Showers, E. Application of Pyridine Containing Polymers in Organic Chemistry, Chapter 17; Prentice Hall: New York, 1990; pp 275-285.

Galiasso Tailleur and CasanoVa Caris Table 1. Properties of ULSD, LSLCO, and Their Cuts cuts (°C) 180-220 220-300 300-360 180-360 ULSD mass (wt %) di-ring aromatics mono-ring aromatics tri-ring aromatics naphthoaromatics dicycloparaffins monocycloparaffins paraffins density (kg/m3) viscosity 40 °C (cSt) cetane number

43.12 2.5 3.0 0.32 6.5 7.15 8.74 14.91 802.3 0.16 44

38.11 2.2 2.93 0.91 5.6 7.33 7.6 11.54 832.7 1.5 42

18.84 1.12 1.59 0.85 2.0 5.73 2 4.55 847.9 3.2 40

100.0 5.82 7.52 2.08 16.1 20.21 18.34 301 823.0 1.24 42.5

100.0 3.88 6.44 0.2 12.3 16.88 22.44 37.2 812.4 0.99 47.0

Diesel Oxidation. Oxidation of diesel was carried out at three temperatures and six space velocities in a continuously stirred tank reactor (CSTR5), operating at 0.8 MPa of total pressure. An imprinted CuCr/IP(4-PVP) catalyst was used for a total of 55 experiments. The oxygenated products were characterized by the same technique to identify the product composition, specifically the extent of oxygen incorporation. Total oxygen, acid, and cetane numbers were also measured using an elemental analyzer (AE) and ASTM methods. In particular, an in-house-developed storage stability test was used to control fuel quality. This test involves storing 1 L of sample in a steel tank for 45 days at 50 °C under 0.1 Pa of air partial pressure. Samples were taken every 3 days. Color, solids formation, oxygen uptake, and acid number analyses were performed on each sample to assess the degree of fuel deterioration. This method was compared to the storage of a ULSD (a control fuel that meets the requirements of U.S. Federal Specification VV-F-800a; see properties in Table 1). Diesel Engine Tests. Diesel engine tests were performed using a direct injection 1-cylinder AVL engine positioned on a test stand. The following test conditions were chosen: rotation speeds of 1900 and 1000 rpm and torques of 43.1 and 22.4 N · m. The torque was measured using a dynamometer and the rotation speed by an impulse meter (JP 1414). NOx and CO concentrations in the exhaust gas were measured using a chemiluminescence detector and an infrared detector, respectively. The Bosch index was measured using a Bosch smoke meter, and the PAH content was determined using liquid chromatography (HPLC, Varian) on toluene-extracted samples collected on a membrane filter. The emissions tests were compared with that of the ULSD reference fuel. Results and Discussion Hydrotreated Product. A large sample of LCO was hydrogenated in the pilot plant operating at 350 °C, 2 h residence time, 6 MPa of total pressure, and H2/HC ratio of 6 to obtain an on-spec sulfur fuel (∼15 ppm, referred to here as LSLCO), using a self-prepared WNiPd/TiO2Al2O3 catalyst. The LSLCO was fractionated in three cuts whose compositions are shown in Table 1. It can be seen in the analyses of the fractions that the cetane number decreases with molecular weight. This phenomenon is attributed to (1) the negative contribution of aromatics, which generally predominates, and (2) the positive effect of the naphthene and paraffin content as a function of boiling point range (or hydrocarbon molecular weight). The mono/di-ring aromatics ratio also increases, while that of mono/ di-ring naphthenes decreases with the boiling point of the cut. For comparison, the full-range LSLCO and reference diesel

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Table 2. Alkylnaphthoaromatic (Al[N]Ar) Families in LSLCO cuts (°C) wt/wt feed tetralin indane fluorene TH pentalene DH and TH anthracene others (difference)

180-220

220-300

300-360

180-360

2.03 1.73 1.06 0.40 0.25 1.03

1.83 1.47 0.76 0.43 0.47 0.67

0.51 0.42 0.28 0.23 0.13 0.45

4.37 3.62 2.1 1.06 0.85 2.15

Table 3. Oxygenated Families Content and Boiling Range (OLSLCO 1) (353 K, 0.6 h, O2/LSLCO: 3) wt % on Feed cuts (°C) ketones ketols pol acid total O2 (GC-MS + tit.) total oxygen (EA)

180-220

220-300

300-360

180-360

0.264 0.048 0.010 0.010 0.332 0.310

0.176 0.032 0.020 0.010 0.238 0.240

0.050 0.009 0.030 0.005 0.094 0.100

0.490 0.089 0.060 0.025 0.664 0.670

composition are shown in the two rightmost columns in Table 1. The LSLCO presents larger concentrations of naphthoaromatics and polyaromatics, lower cetane number, and higher viscosity, heat power, and density than the ULSD sample. It is important to determine the amount of naphthoaromatics in the LSLCO that represents the potentially oxidizable compounds.5 The HPLC/GC-MS analyses allowed for the identification of the quantitative distribution of some families of compounds, depicted in Table 2. The polyalkyltetralin, polyalkylindane, and polyalkylfluorene families were the most abundant. The prevalence of the first family increases with boiling point, while the other two follow an opposite trend. The others represent more than 20 different types of compounds in minor proportion. Calibration of the HPLC/GC-MS techniques, by blending model molecules, indicated that 95% of the oxygen added to the LSLCO was detected by this methodology. Acid and pol compounds present in most of the oxidized diesel contribute less than 5% of the total oxygen content, determined by elemental analysis. LSLCO Thermal Oxidation. Thermal oxidation of the LSLCO was performed at 353, 363, and 373 K at five space velocities. The product after reaction (designated here as OLSLCO) was filtered using the ASTM 2274 method, and the liquids and solids were analyzed. Example results are shown in Figure 1. The left axis represents the wt % of oxygen uptake, measured by elemental analysis of OLSLCO. The wt % of hydroperoxide was determined by iodimetric titration,10,13 and the acid formed was determined by titration with a standard base and reported in wt %. The right ordinate depicts the thermal stability, in milligrams of solid formed per liter. It can be seen that the rate of oxygen uptake (measured in the liquid phase) was initially very high, but eventually slowed. The initial high rate of oxygen uptake is likely associated with peroxide formation. When the residence time increases, the amount of peroxide is gradually reduced, while that of acid and insoluble material starts to increase very rapidly. Soon after this shift in oxygen uptake, the quality of the fuel was totally deteriorated. The cetane number could not be measured in the diesel engine, and the families of oxygenates could not be separated by HPLC, due to the high fuel instability. The previous thermal studies performed with tetralin show very poor selectivity toward ketone formation (tetralone/tetralol < 2 mol).13 (13) Casanova, O.; Galiasso Tailleur, R.; Corma, A. Effect of tetralol and tetralone in the tetralin oxidation with copper and chromium. Preprint of ECCE, Copenhagen, Denmark, 2007; pp 287-295.

Figure 1. Effect of residence time on thermal oxidation (363 K; O2/ LSLCO, 3 mol): pink, oxygen; orange, peroxide; blue, acid; gray, pol; black, thermal stability.

Figure 2. Effect of residence time on catalytic oxidation (353 K; O2/ LSLCO, 3 mol): pink, oxygen; blue, acid; gray, pol; red, cetane number.

Catalytic Oxidation of LSLCO. The effect of residence time on the activity and selectivity of the LSLCO oxidation in the presence of CuCr/IP(4-PVP) catalyst is reported in Figure 2 for 353 K and in Figure 3 for 363 K. The left axis represents the wt % of oxygen uptake for the OLSLCO samples, measured by elemental analysis. The amount of hydroperoxide was determined by iodimetric titration by wt %, but for almost all operating conditions, the concentration was very low. Thus, these values are not reported in Figures 2 and 3; however, the wt % of acid, measured by titration, is reported. The left axis in Figures 2 and 3 depicts the cetane number for the OLSLCO products calculated by the correlation based on 1HNMR, GC, and density analysis. The cetane number was observed to increase with the oxygen uptake. At both temperatures, hydroperoxide content was negligible. We know that hydroperoxides are predominantly produced by autothermal reactions since they were also formed in a higher proportion in the absence of catalysts at the same operating conditions. However, in the presence of surface nitrogen (support (IP/4PVP), peroxide formation is suppressed. The initial formation of the hydroperoxide activates the catalyst, which starts to form its own “on-surface” peroxide, the intermediaries of reaction, which limit the further need of thermal ones and contribute to improve the product stability.10 The shape of the oxygen uptake curve is the result of a fast initial formation of ketone and a

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Galiasso Tailleur and CasanoVa Caris Table 4. Storage Stability (Acid and Solid Formation) in LSLCO, LSLCO 1 and 2, ULSD, ULSD + 2% Tetralin fuel LSLCO days 3 45 oxygen 0 0 (wt %) acida 0.17 0.24 13 16 solidb a

Figure 3. Effect of residence time on catalytic oxidation (363 K; O2/ LSLCO, 3 mol): pink, oxygen; blue, acid; gray, pol; red, cetane number.

Figure 4. Distribution of oxygenated families in the product OLSLCO 1 (353 K, 0.6 h) and OLSLCO 2 (363 K, 0.6 h): red, tetralone and tetralol; orange, indanone and indanol; blue, florenone and fluorenol; green, other; gray line, total oxygen.

slow production of ketol; there appeared some contribution of secondary’s products due to very slow formation of pol and acid by catalytic reactions. The cetane number initially increases proportionally with the oxygen content, but then the rise is proportionally lower than the oxygen uptake. Within the naphthoaromatic families, oxidation is favored by the electronically induced effect of paraffinic branches in alpha and or beta positions in the naphthenic ring. As the oxygen uptake progresses, the reaction rates decrease because the concentration of reactant decrease and the already oxidized compounds and alkylaromatic molecules are less reactive compounds than the alkylnaphthoaromatics. These secondary slow oxidation reactions lead to polyoxygenates and carboxylic acid. We will see later that during storage the polymerization of unconverted peroxide and polyoxygenated compounds produces gums and insoluble carboides. It can be see in Figures 2 and 3 that the higher the residence time, the higher the amount of acid formed and the higher the product instability. Oxidation can proceed further until the product of reaction intermediatesswashed with caustic solutionsreaches the required specifications for acid number and/or product stability. All products were washed with caustic solution to remove most of the acid- and alkali-soluble compounds. It was required that the remaining acid content be below 0.02 mg KOH/g (ASTM D974-95). We observed that the optimum oxygen uptake was different for the two oxidation

OLSLCO 1

OLSLCO 2

ULSD

ULSD + 3% tetralol

3 45 3 45 0.45 0.45 0.48 0.48

45 0

45 0.2

0.08 6

0.14 14

0.28 18

0.11 0.13 0.14 9 9 13

Units, mg KOH/g. b Units, mg/100 cm3 (similar to ASTM 2274).

temperatures due to different amounts of KOH-acid-insoluble compounds formed (the maximum acid content is indicated in Figure 2 by a horizontal dashed line). After this point, the reaction became unfeasible because no commercially viable fuel could be derived. The individual analysis of the oxygenate families in the cut, by HPLC/GC-MS and oxygen content, confirmed a different oxygen distribution for different temperatures, given a similar space velocity. This analysis was calibrated using model molecules incorporated in the LSLCO samples; it showed that, on average, 90% of the blended oxygenates were detected in the naphthoaromatics-eluted fraction (methanol), and only 10% were detected in the aromaticseluted fraction (toluene). Figure 3 shows that an increase in the oxidation temperature resulted in a proportionally larger oxygenated content in the OLSLCO light portion (180-250 °C) compared with the heavy portion (300-360 °C). Nevertheless, the oxygen distribution changes are due in part to incorporation of multiple oxygen molecules in the naphthoaromatic (pol and acid) compounds. Even when the temperature favored incorporation of oxygen and an increase in cetane number, the trend of increasing acid and polyoxygenated compound content at increasing temperature was maintained for the OLSCO. It seems that the polyoxygenates and acid compounds accumulated in the middle and heavy parts of the diesel, probably due to the presence there of more reactive to oxidation alkylnaphthoaromatic precursors. Diesel Stability. The effect of temperature on diesel stability is now discussed. These tests were performed for samples with the around the same total level of oxygen content after washing with caustic solution to extract acids and peroxides. Table 4 shows the results of the solid formed in ULSD, LSLCO, OLSLCO 1, and OLSLCO 2 products after 3 and 45 days in storage. The results shown in Table 4 indicate that, at the beginning of the storage time, the LSLCO and the ULSD (0% of oxygen) were the most unstable of the five fuel samples studied. This is due to the loss of some oxidation inhibitors, mostly nitrogen compounds, eliminated during the deep hydrotreating. After three days of storage, the oxidized samples (OLSLCO 1 and OLSLCO 2) showed a reduction in the amount of acids and solids formed with respect to the LSCO for the same period of storage. This observation can be attributed to the presence, in the former, of more stable compounds for further oxidation (ketones formed during the previous catalytic oxidation). Now, comparing the two oxidized fuels after 45 days in storage, in can be seen that OLSLCO 1 (353 K, 0.45% oxygen) produces less solid and acid compounds than OLSLCO 2 (363 K, 0.48% oxygen). Additionally, both are more stable than the hydrogenate fuels (LSLCO and ULSD). Therefore, the storage results indicate that controlled oxidation of naphthoaromatics eliminates very reactive compounds and generates stable compounds for storage, where only mild additional oxidation occurred. It was demonstrated by a comparative study that these storage conditions correspond to 2 years of standard storage in

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commercial operation. By storing 100% tetralone or 100% tetralol for 45 days at the same conditions mentioned above, the tetralone was not significantly oxidized, while tetralol was converted into polyoxygenate compounds and insoluble gum. By subsequently adding 0.2% oxygen to ULSD as tetralol, its stability was dramatically deteriorated (fourth column in Table 4). The oxidation of LSLCO at low temperature using a selective catalyst predominantly produces ketones which are more stable than ketols. Additionally, by increasing the temperature, other unwanted compounds, such as phenol, polyoxygenates, and acid, begin to form. These ultimately contribute to deteriorated storage stability and other properties, at similar oxygen content. The caustic treatment is able to eliminate 80-85% of the acid formed (titration analyses) with more than 99% fuel yields. This improves stability at the expense of higher operating costs. The amount of insoluble compounds formed during the 45 day storage increased with the depth of the oxidation, due to the presence of an increasing amount of polyoxygenates and peroxides, which were not extracted during the caustic treatment. During the storage, all samples showed a small increase in viscosity (363 K), the role of thermal reaction becomes important and changes the product distribution. But at 353 K, the thermal reaction plays a minor role in the presence of CuCr/IP(4-PVP) catalyst, as was demonstrated in previous studies using tetralin and indane in diesel,5 using a similar catalyst: CuCr/P(4-PVP) (none imprinted). The simplified path of reactions can be written as follows: k1cat

Al-[N]Ar + O2 98 Al-[NO]Ar k2cat

(3)

k3cat

Al-[N]Ar + O2 98 Al-[NOH]Ar 98 pol

(4)

k4cat

Al-[C] + O2 98 0.75AlOOH-[C] + 0.25AlO-[C] (5) This results in the following primary kinetic equations:

reaction

koj (m3/kmol)0.5 h

Ej (cal/mol)

Al-[N]Ar f Al-[NO]Ar Al-[N]Ar f A[NOH]Ar Al-[NOH]Ar f pol Al-[C] f AlOOH[C]

7.00E + 07 1.10E + 07 2.30E + 08 1.80E + 09

11 500 12 000 14 500 17 000

m1 rHOOAl[C] ) k5catKAl[C]KO2CAl[C]CO2

m1 k1catKAl[N]ArKO2CAl[N]ArCO2 (6)

(7)

rAl[NOH]Ar ) k3catKAl[N]ArKO2CAl[N]Ar - k4catKAl[NOH]ArCAl[NOH]Ar (9)

(10)

where rj is the reaction rate for species j (in mol/L h), k1 to k5 are the reaction rate constants for catalytic (cat) conversion of the species, KAl[N]Ar, KAl[NO]Ar, KAl[NOH]Ar, KAl[C], and KO2 are the adsorption constants for the alkylnaphthoaromatics, the oxygenate naphthoaromatics, the oxygenate alkyl compounds, and the oxygen on surface, respectively, and Cj is the concentration of species j in the liquid phase. Based on the results for pure TO and TOH oxidation,13 it was assumed that Al[NO]Ar does not decompose into other products and that Al[NOH]Ar is converted into Al[ONOH]Ar and Al[HONOH]Ar (pol) via catalytic reaction. Numerical Method Used To Obtain the Rate Constant. A computational program in Visual Basic was developed to simulate the semibatch isothermal reactor without mass transfer control.ThematrixofequationswassolvedusingaRunge-Kutta-Felberg numerical method that adjusts space steps according to local truncation errors. For all components, the ideal semibatch mass balance equations, such as that depicted below for the Al[N]Ar disappearance (eq 11), were solved simultaneously with boundary conditions according to eq 12. A similar equation is used for product formation. dCAl[N]Ar ) (k’1cat + k’2cat)CAl[N]ArsCO2l (11) dt t ) 0 f CAl[N]Arl ) CA[N]Arlo ; CO2l ) CO2 . f T ) To ; CO2l ) CO2 ; CAl[N]Arl ) CAl[N]Arl f for all t > 0 (12) -

where t is the reaction contact time. The program internally converts mass into moles and vice versa, and calculates the oxygen concentration using an empirical equation. The program integrates the differential equations assuming a reaction order of 1 for hydrocarbons and 0.5 for oxygen, for a catalytic reaction path based on previous results.5 The program is seeded with the pre-exponential factors and activation energies for the catalytic reaction and then proceeds to calculate the amount of oxygen present as Al[NO]Ar, Al[NOH]Ar, Al[NOOH]Ar, and Al[ONO] + Al[OHNO]Ar, at the outlet of the reactor for particular operating conditions. The results were then compared with the experimental data, and the difference was used for the genetic algorithm (GA) optimization tool to adjust (by mutation) the values of the seeded constants. The GA uses a random search and evolution technique to determine the new constant values. The objective function used by this algorithm is the sum of the relative errors between the predictions (pred) for the ketone families (main product) formed minus the experimental value (exp), as shown in eq 13. errorAl[NO]Ar )

rAl[N]Ar ) k1catKAl[NAr]KO2CAl[N]ArCOm12 +

1 m1 rAl[NO]Ar ) k2catKAl[N]ArKO2CAl[N]Ars CO2

Table 5. Kinetic Rate Constants for the Lumped Catalytic Reactions

∑W

Al[NO]Ar - WAl[NO]Ar pred exp

(13)

The GA converged ((2%) after 25 iterations and obtained apparent kinetic constants for the simplified catalytic reaction path. The obtained values for the apparent activation energies and pre-exponential factors of the reaction rates are reported in Table 5. The difference in the activation energies should be noted, as they indicate the negative effect of temperature increase

804 Energy & Fuels, Vol. 23, 2009

Figure 5. Effect of oxygen on diesel emission at constant cetane number (OLSLCO 353 K, 9• red, 0.2; 9• black outline, 0.4; 9• gray, 0.9 h, O2/LSLCO: 3). 1 red, PM; 2 red, NOx ULSD. ) red, PM. ) blue, NOx ULSD. (1900 rpm, 40% load, without EGR.)

on selectivity discussed above. Additionally, it is important to note that thermal reaction increases more than the catalytic ones, producing a progressive deterioration of fuel quality due to lack of selectivity (lower ketone/alcohol ratio + acid), adding another reason to work at low temperatures. Diesel Engine Testing. To demonstrate the effect of oxygen content, three oxidized diesel fuels, obtained by oxidation of LSLCO at constant temperature (353 K) and different residence times, were engine tested. Additionally, LSLCO and ULSD were also tested. Recent literature14 indicates that CO, HC, and aldehyde emissions depend on cetane number, NOx emissions are controlled by fuel density, and PAHs and smoke are directly related to aromatic content. Prior research has demonstrated that soot emissions correspond to the percentage of aromatics in fuel,15 and that aromatic content negatively affects NOx and CO emissions. In this study, four fuels have been studied with equal aromatics contents (28 wt %), similar densities (∼0.678 kg/L), and similar boiling point contributions, but with different oxygen contents. We added a cetane enhancer (ethyl) to raise the cetane number of the four fuels to 47, the same as ULSD which was used as reference. In this way, all the fuel was evaluated with a constant cetane number effect. The diesel engine tests show that higher oxygen content resulted in lower emissions (Figure 5), with respect to the ULSD (reference). When the fuel is injected into the chamber after the engine is cranked, the liquid is atomized and ignition begins. In this phase, referred to as the “uncontrolled burning” or “flame propagation” period, most of the PM precursors are formed by thermal cracking in which aromatics play an important role. The second phase starts with full combustion, resulting in peaks of pressure and temperature. From this point on, most of the NOx formed is associated to the maximum temperature attained in the chamber. The pressure-volume curves for the three oxygenated products are flatter than those of ULSD and LSLCO. This result is equivalent to an earlier (and better) detonation and a lower average maximum temperature for the oxygenated fuels. The ultimate effect is slightly lower NOx and PM production, as shown in Figure 5. It has been described in the literature16,17 (14) Martin, B.; Aakko, P.; Beckman, D.; Del Giacomo, N.; Giavazzi, F. SAE Paper. 1997, No. 97-03-2966. (15) Kouremenos, D. A.; Hountalas, D. T.; Pariotis, E. G.; Papagiannakis, R. G. SAE Paper. 2000, No. 00-01-1172. (16) Beatrice, C.; Bertoli, C.; Giacomo, N. D. Combust. Sci. Technol. 1998, 137, 31–50.

Galiasso Tailleur and CasanoVa Caris

that adding oxygenated compounds (methanol, diglyme, biodiesel, and others) contributes to increased NOx emissions. It was also shown that an opposite trend occurred with paraffinic fuels, which led to a retarding of injection timing. The present results, at constant cetane number, indicate a clearly favorable effect of oxygen on NOx emission at 1900 rpm and 40% load and a negligible effect at 1000 rpm and 30% load. Smoke index analysis showed that, on increasing the fuel oxygen content from 0.0% to 0.9%, the smoke index decreased considerably (by 2.2 times) at 1900 rpm. When the rotation speed was reduced to 1000 rpm, the values of the smoke index became low and the effect of oxygen content was also reduced (0.34 times). Special attention was given to the analysis of PAH in exhaust gases. The amount of PAH decreased by half when oxygen content was increased to 0.6%. Increasing the oxygen content in the OLSLSO from 0.0 to 0.6% (wt) caused the hydrocarbon content in the exhaust to increase by 20% at 1900 rpm and by 10% at 1000 rpm. A detailed analysis of PAH is now required. The reduction of soot with oxygenate addition is attributed to the increased oxygen atom concentration in the fuel-rich regions of the jet.18 Nevertheless, in the present case, the effect of oxygen might not be due to the formation of hydroxyl free radicals as was suspected from the literature for methanol or diglime addition to the fuel. Alternatively, the effect in the present case is likely due to the direct combustion of the associated aromatics. The presence of oxygen within the fuel favors high-temperature fuel oxidation, especially in fuel-rich areas of the flame region, resulting in a decrease of total unburned hydrocarbons. In this case, the values measured for HC emissions are lower for the three oxygenated sample than any other fuel in this study with equivalent aromatic content. CONCLUSIONS Oxidation of hydrotreated LCO demonstrated that there is a set of optimal operating conditions (temperature and residence time) for catalytic oxidation that allows for selective incorporation of oxygen to maintain the stability of the diesel product within the diesel specification range. A simplified reaction path was proposed and an apparent kinetic rate constant was calculated from experimental data. This information confirms that selectivity for ketone production depends on the reaction temperature. For the CuCr/IP(4-PVP) catalyst, the amounts of acid and polyaromatics produced were very low. The diesel emissions study indicated a positive effect of fuel oxygen content on NOx, CO, HC, and PM emissions at high speed and medium load. The effect was less important at lower speed and loading. The emissions were comparable to that of a ULSD reference, despite differences in density, viscosity, and aromatic content. The effect of oxygen content in diaromatics in-cylinder combustion largely compensates for the presence of aromatics in the hydrogenated LCO. Nevertheless, the amount of oxygen incorporated must be controlled to avoid the formation of unwanted oxygenate that can deteriorate other fuel properties. Acknowledgment. The authors would like to acknowledge the experimental effort of Y. Quinteros and Professor K. Ramisof for engine testing. EF8007135 (17) Nabi, M. N.; Minami, M.; Ogawa, H.; Miyamoto, N. SAE Paper. 2000, No. 00-01-0231. (18) Song, J.; Cheenkachorn, K.; Wang, J.; Perez, J.; Boehman, A. L.; Young, P.-J.; Waller, F. J. Energy Fuels 2002, 16, 294–301.