Comparison of Diesel Fuel Oxygenate Additives to the Composition

ARTICLE pubs.acs.org/EF

Comparison of Diesel Fuel Oxygenate Additives to the Composition-Explicit Distillation Curve Method. Part 1: Linear Compounds with One to Three Oxygens Thomas J. Bruno,* Tara M. Lovestead, Jennifer R. Riggs, Erica L. Jorgenson, and Marcia L. Huber Thermophysical Properties Division, National Institute of Standards and Technology (NIST), Boulder, Colorado 80305, United States

bS Supporting Information ABSTRACT: There is a great deal of interest in formulating oxygenated diesel fuels that produce low particulate emissions. The most common oxygenate additives for diesel fuels include the glycol ethers, glycol esters, alcohols, ethers, and ketones. It is important to characterize the mixture properties of diesel fuel with oxygenate additives, to assess the degree of departure of the oxygenated fuels from the base fuel. One of the most important properties to use for this purpose is the volatility, as expressed by the distillation curve. We have recently introduced several important improvements in the measurement of distillation curves of complex fluids. The modifications to the classical measurement provide (1) a composition-explicit data channel for each distillate fraction (for both qualitative, quantitative, and trace analysis), (2) temperature measurements that are true thermodynamic state points that can be modeled with an equation of state, (3) temperature, volume, and pressure measurements of low uncertainty suitable for equation of state development, (4) consistency with a century of historical data, (5) an assessment of the energy content of each distillate fraction, and (6) a corrosivity assessment of each distillate fraction. In this paper, we present measurements for dimethoxymethane, butyl methyl ether, 1,2-dimethoxyethane, 2-methoxyethyl acetate, 2-ethoxyethyl acetate, and diethylene glycol dimethyl ether. We find that the more volatile additives cause significant early departures from the distillation curves of diesel fuel, while the less volatile additives act more to displace the entire curve. We also note that the additive affects the curve shape and temperature profile even after being totally depleted, an observation made in earlier studies of oxygenate additive mixtures.

’ INTRODUCTION Diesel engine designers have increasingly come under pressure to improve environmental performance.16 Although very efficient, diesel engines have had difficulties achieving desirable emission targets, especially for particulates (that is, soot) and NOx formation. Over the years, improvements have been made in the design of fuel systems, combustion chambers, and engine control. Indeed, catalytic after treatment of diesel exhaust has become common on large power plants. More recently, reformulation of diesel fuel has been given renewed attention. This has focused on two aspects of the fuel: (1) reformulation to lower the sulfur specification and (2) reformulation to incorporate oxygenates into the fuel.710 The first aspect (lowering the sulfur specification) is more related to issues of acid precipitation rather than soot and NOx formation. Indeed, this goal has substantially been met as of 2007, with the introduction of ultra-low-sulfur diesel (ULSD) fuels that meet the target 15 ppm (mass/mass) specification. The second goal, on the other hand, is still the subject of research.1126 Reformulation of diesel fuel to include oxygenates has proven to be an effective way to reduce catalyst poisoning at the aftertreatment devices, soot, SO2 and acid rain, and NOx emissions, to enhance the ability to recirculate exhaust gases [exhaust gas recirculation (EGR)] and to reduce in-cylinder radiative heat transfer (and thereby improve efficiency). Moreover, reformulation may enable the application of more sophisticated after-treatment technologies. Because the reformulation is usually performed by mixing oxygenates directly into the diesel fuel, such reformulations This article not subject to U.S. Copyright. Published 2011 by the American Chemical Society

are applicable to (and with) biodiesel fuels as well. There are indications that it might be possible to produce completely smoke-free operation of a diesel engine by the incorporation of 38% (mass/mass) oxygen into the fuel.27 Other authors have argued that the addition of more than 5% of any oxygenate additive would be cost-prohibitive or that additive supplies would prevent fuel formulation on such a concentration level for the entire fleet.28,29 Several major classes of chemical additives have been considered for diesel oxygenates: alcohols, ethers, ketones, glycol ethers, glycol esters, lactones, and carbonates. The best studied of these fluids are the lower organic carbonates, organic ethers, acetates, glycol ethers, and glycol esters.12,14,3033 Some bioderived glycol esters have also been proposed and studied.34,35 In all cases, the potential of designing new fuel mixtures that incorporate oxygenates critically depends upon knowledge of the thermophysical properties of the fluids. The mixture volatility is an important part of that knowledge base. In this paper, we present volatility measurements on mixtures of diesel fuel with six compounds that have been proposed or tested as oxygenates. The compounds presented herein are linear molecules and contain between one and three oxygen atoms. Linear Oxygenate Fluids. Dimethoxymethane (DMM, methylal, CAS number 109-87-5) is a relatively high-volatility additive for Received: March 4, 2011 Revised: April 28, 2011 Published: May 02, 2011 2493

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Energy & Fuels diesel fuel that has been studied in research engines with some success.33,3639 DMM has a high oxygen fraction (0.42, mass/ mass), relatively high cetane number, and a low cost. The uses of this fluid as a chemical intermediate, solvent, and anesthetic component contribute to its relatively low cost. In mixtures with diesel fuel, DMM has been reported to produce a lower particulate emission, lower CO emission, longer ignition delay, and higher pressure rise rate than those of diesel fuel alone. A potential disadvantage is that, along with the decreased output of particulate matter, the particulates that are formed have smaller diameters and may be more easily inspired during normal breathing. The high volatility of this fluid can also be problematic because of vapor blocking in the engine fuel system. This fluid is also used as an entrainer in supercritical fluid extraction and azeotropic distillation, which points out another potential problem. The entrainment ability can sometimes cause frothing when added to diesel fuel.40 Butyl methyl ether (BME, CAS number 628-28-4) has a modest oxygen fraction (0.18, mass/mass) and has not received the extent of study that has been applied to DMM. It is of interest because it is expected to perturb the thermophysical properties of neat diesel fuel to a lesser extent than DMM. 1,2-Dimethoxyethane (DME, glyme, CAS number 110-71-4) has been used as an oxygenate for diesel fuel primarily as a constituent in the mixture known as Cetaner [80% diethylene glycol methyl ether (DGME) and 20% DME].7,14 DME has been suggested for testing as an individual oxygenate additive to reduce processing cost and complexity. 2-Methoxyethyl acetate (ethylene glycol methyl ether acetate, CAS number 110-49-6) has a volatility that approaches that of diesel fuel and, therefore, causes less of a perturbation to the fuel properties (that is, it will not cause vapor blocking).41 It has a higher energy density than oxygenates listed above; the oxygen fraction is high (0.40, mass/ mass); and it is very soluble in diesel fuel.42 There are multiple synthetic routes for this fluid, and the cost is relatively low. A similar fluid is 2-ethoxyethyl acetate (ethylene glycol monoethyl ether acetate, CAS number 111-15-9). This fluid has a slightly lower oxygen fraction (0.36, mass/mass) when compared to the methoxy compound discussed above; the ethyl group provides a slightly better solubility in the base diesel fuel. Finally, diethylene glycol diethyl ether (DGDE, CAS number 112-36-7), an oxygenate additive with a 0.3 (mass/mass) oxygen fraction provided by three oxygen atoms, has a higher boiling point than those of the other additives listed above. It has been used in research engine tests of soot morphology, where it was used as a pure (single component) fuel to serve as a reference.43 Some basic information on these six oxygenate additives is provided in Table S1 of the Supporting Information. As mentioned above, when evaluating the suitability of a particular oxygenate additive, one must consider the effect of the additive on the thermophysical and operational properties of the resulting mixture. One of the most important and informative parameters that is measured for fuels (or indeed for any complex fluid mixture) is the distillation (or boiling) curve. It has been possible in recent years to relate the distillation curve to many operational parameters of complex liquid fuels. In diesel engines, it is possible to use the distillation curve (in the same way as for gasoline combustion in spark-ignition engines) to design fuels for operability.44,45 Especially important with diesel fuels are the later regions of the distillation curve, which describe the high relative molecular mass components of the fuel.46,47 It is also possible to correlate the distillation curve of fuels with exhaust emissions, a fact that is especially important with diesel engines.48

ARTICLE

Advanced Distillation Curve (ADC) Metrology. In earlier work, we described a method and apparatus for an ADC measurement that is especially applicable to the characterization of fuels.4958 This method is a significant improvement over current single-stage approaches, featuring (1) a composition-explicit data channel for each distillate fraction (for both qualitative and quantitative analysis), (2) temperature measurements that are true thermodynamic state points that can be modeled with an equation of state, (3) temperature, volume, and pressure measurements of low uncertainty suitable for equation of state development, (4) consistency with a century of historical data, (5) an assessment of the energy content of each distillate fraction, (6) trace chemical analysis of each distillate fraction, and (7) a corrosivity assessment of each distillate fraction. This approach also provides important advantages over other methods, such as the simulated distillation method embodied in procedures such as American Society for Testing Materials (ASTM) D2887.59 In that method, for example, one uses the gas chromatographic behavior of a suite of compounds as a frame of comparison to a fuel. The very significant advantage offered by the approach discussed in this paper is the ability to model the distillation curve resulting from our metrology with an equation of state.6064 Such thermodynamic model development is simply impossible with the classical approach to distillation curve measurement or with any of the other techniques that are used to assess fuel volatility or vapor liquid equilibrium. We have applied this metrology to azeotropes,65 gasolines,56,6668 aviation fuels,50,57,6980 diesel fuels,20,35,8186 crude oils,87,88 and rocket propellants.50,76,89,90 We note that, although the method has multiple features, it is not always necessary or desirable to apply all of them in every application. For highly finished fuels, such as the low-sulfur diesel fuels used today, for example, it is usually unnecessary to assess corrosivity as a function of the distillate fraction.

’ EXPERIMENTAL SECTION The diesel fuel used in this work was obtained from a commercial bulk supplier and was stored at 7 °C to preserve any volatile components. No phase separation was observed as a result of this storage procedure. The fuel was a winter-grade, low-wax, ULSD fuel that incorporated a red dye (specifying off-road use) and was refined locally from petroleum of the DenverJulesburg field. This fluid was used without any purification or modification. The composition was analyzed by gas chromatography (GC) with a 30 m capillary column of 5% phenyl95% dimethyl polysiloxane, having a thickness of 1 μm with a temperature program from 90 to 275 °C, 9 °C/min, using flame ionization detection (FID) and mass spectrometric (MS) detection.91,92 This analysis was unremarkable in that the typical pattern of commercial petroleum-derived diesel fuel was observed. The n-hexane used as a solvent in this work was obtained from a commercial supplier and was analyzed by GC with the same column used above with a temperature program from 50 to 170 °C, 5 °C/min, by use of FID and MS detection. These analyses revealed the purity to be approximately 99.9% (mass/mass), and the fluid was used without further purification. The oxygenates used in this work were obtained as pure fluids from a commercial source. All were reagent-grade fluids with reported purities of 9899% (mass/mass). These fluids were analyzed by GC with the same column used above with temperature programs appropriate to each fluid. These analyses revealed that the purity of the fluids were in fact higher than specified. Mixtures of each oxygenate were prepared as stock solutions of 5 or 10, 20, and 30% (vol/vol). This ensured that there 2494

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Energy & Fuels were no variations in composition among measurements performed with each mixture. Some discussion is warranted regarding our choice of mixture ratios. Mixtures that contain 30% (vol/vol) of a given oxygenate additive might not be practical fuels for all circumstances, and the performance of an engine using such a mixture could potentially be poor. In some instances, however, high oxygenate concentrations in a diesel fuel are usable. Our study of a particular mixture ratio does not imply that mixtures with very high oxygenate concentrations are practical formulations for fuel nor that we advocate such mixtures for use as fuels. Rather, our mixtures were chosen to cover as large a range of composition as practical for the purpose of modeling the results with equations of state. Indeed, our ultimate purpose is to predict the behavior of such diverse mixtures as diesel fuel with glycol ether and ester oxygenates. Likewise, our reason for the use of reagent-grade oxygenate fluids rather than the technicalgrade fluids that might be used industrially speaks to the same issue. The method and apparatus for the distillation curve measurement has been reviewed in a number of sources; therefore, an additional general description will not be provided here (see the references provided earlier). The required volume of fluid for the distillation curve measurement (in each case, 200 mL) was placed into the boiling flask with a 200 mL volumetric pipet. The thermocouples were then inserted into the proper locations to monitor Tk, the temperature in the fluid, and Th, the temperature at the bottom of the takeoff position in the distillation head. Enclosure heating was then commenced with a four-step program based on a previously measured distillation curve. Volume measurements were made in a level-stabilized receiver, and sample aliquots were collected at the receiver adapter hammock. In the course of this work, we performed six complete distillation curve measurements for each of the mixtures. Because the measurements of the distillation curves were performed at ambient atmospheric pressure (measured with an electronic barometer), temperature readings were adjusted for what should be obtained at standard atmospheric pressure (1 atm = 101.325 kPa). This adjustment was made with the modified Sydney Young equation, in which the constant term was assigned a value of 0.000 109.86,9395 This value corresponds to a carbon chain of 12. In the chemical analysis of the diesel fuel sample (see above), as well as in previous work on diesel fuel, it was found that n-dodecane can indeed represent the fluid as a very rough surrogate, not accounting for the presence of the oxygenate additive. The magnitude of the adjustment is of course dependent upon the extent of departure from standard atmospheric pressure. The location of the laboratory in which the measurements reported herein were performed is approximately 1650 m above sea level, resulting in a typical temperature correction of 8 °C. The actual measured temperatures are easily recovered from the Sydney Young equation at each measured atmospheric pressure. We note that we have not considered the effect of the oxygenate fluid, even at high concentrations, on the temperature adjustment; unfortunately, no data are available to allow this. Indeed, thermodynamic models that can be derived from ADC measurements provide one avenue to the explicit consideration of the effect of the oxygenate fluids. Thus, for consistency, we use the constant mentioned above for all of the adjustments.

’ RESULTS AND DISCUSSION Diesel fuel is a commodity fluid with seasonal and regional variations, although the degree of compositional and thermophysical property variability is not as great as that encountered with gasoline. We have found that the diesel fuel that we have used here (winter-grade, low-wax, ULSD, red dye for off-road use) is representative of many diesel fuels that we have examined. In most cases, we measured oxygenate mixture concentrations of 10, 20, and 30% (vol/vol). In the case of BME, we encountered

ARTICLE

Table 1. Summary of the Initial Boiling Behavior of the Diesel Fuel Mixtures with the Oxygenate Additivesa observed temperature (°C)

diesel fuel (83.91 kPa)

vapor rise

216.8

observed

diesel fuel þ

diesel fuel þ

diesel fuel þ

temperature 10% (vol/vol) 20% (vol/vol) 30% (vol/vol) (°C) DMM (82.70 kPa) DMM (83.55 kPa) DMM (83.15 kPa) onset

40.4

38.3

46.7

vapor rise

82.4

61.8

54.4

observed

diesel fuel þ 5%

temperature

(vol/vol) BME

(vol/vol) BME

(vol/vol)

(°C)

(83.28 kPa)

(83.0 kPa)

BME (84.17 kPa)

onset vapor rise

120.8 158.8

93.7 133.8

71.6 105.7

diesel fuel þ 10% diesel fuel þ 20%

observed

diesel fuel þ 10% (vol/vol)

diesel fuel þ

diesel fuel þ

temperature

DME

20% (vol/vol)

30% (vol/vol)

(°C)

(83.89 kPa)

DME (83.65 kPa)

DME (83.18 kPa)

onset vapor rise

87.8

85.9

82.9

136.2

114.2

104.0

diesel fuel þ

diesel fuel þ

diesel fuel þ

10% (vol/vol)

20% (vol/vol)

30% (vol/vol)

observed temperature

2-methoxyethyl acetate



(°C)

(82.70 kPa)

(83.23 kPa)

(83.11 kPa)

onset

107.4

71.7

74.2

vapor rise

178.4

164.2

154.7

diesel fuel þ

diesel fuel þ

diesel fuel þ

10% (vol/vol)

20% (vol/vol)

30% (vol/vol)

observed

2-ethoxyethyl

2-ethoxyethyl

2-ethoxyethyl

temperature (°C)

acetate (83.09 kPa)

acetate (82.72 kPa)

acetate (83.59 kPa)

onset

185.8

174.4

166.5

vapor rise

190.6

178.4

172.4

diesel fuel þ 10%

diesel fuel þ 20%

diesel fuel þ 30%

temperature (vol/vol) DGDE

(vol/vol) DGDE

(vol/vol) DGDE

observed (°C)

(82.85 kPa)

(83.24 kPa)

(83.16 kPa)

onset vapor rise

199.2 211.2

192.7 205.8

187.5 201.4

a

The vapor rise temperature is that at which vapor is observed to rise into the distillation head, considered to be the initial boiling point of the fluid. These temperatures have been adjusted to 1 atm with the Sydney Young equation; the experimental atmospheric pressures are provided to allow for recovery of the actual measured temperatures. The uncertainties are discussed in the text.

immiscibility at the 30% (vol/vol) mixture at ambient temperature (although at slightly elevated temperature, complete miscibility was obtained). For this fluid, we instead measured 5, 10, and 20% (vol/vol). Initial Boiling Temperatures (IBTs). During the initial heating of each sample in the distillation flask, the behavior of the 2495


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Table 2. (a) Representative Distillation Curve Data for Mixtures of Diesel Fuel with (a) DMM, (b) BME, (c) DME, (d) 2-Methoxyethyl Acetate, (e) 2-Ethoxyethyl Acetate, and (f) DGDEa a distillate volume fraction (%)


diesel fuel þ 20%


DMM (83.12 kPa)

(vol/vol) DMM (82.70 kPa)

DMM (83.58 kPa)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5

126.8

57.2

69.9

47.0

60.6

43.2

10 15

221.0 228.7

163.1 192.6

85.9 125.5

56.0 62.4

64.9 72.8

49.7 54.6

20

234.7

204.2

222.9

181.8

85.3

57.3

25

240.2

211.0

231.7

202.5

124.5

58.1

30

245.3

217.9

237.0

211.5

224.6

179.7

35

251.1

225.8

243.3

220.7

232.5

202.0

40

256.8

232.0

250.2

228.2

240.0

213.7

45

263.9

238.8

256.9

235.3

248.2

224.3

50 55

269.6 276.6

245.1 249.9

263.6 271.1

242.5 250.9

254.6 261.6

231.7 240.2

60

284.0

257.5

278.6

259.1

270.5

249.0

65

291.7

263.3

286.6

266.1

278.9

257.5

70

301.2

271.4

295.0

276.6

289.2

267.2

75

310.3

280.8

301.5

284.6

298.4

278.9

80

319.8

290.8

312.9

296.8

310.1

287.8

85

331.0

288.4

325.7

307.4

322.8

298.3

342.5

308.9

337.0

302.9

90 diesel fuel þ 5% (vol/vol)



BME (83.28 kPa)

BME (83.70 kPa)

BME (84.17 kPa)

b distillate volume fraction (%)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5

214.9

104.1

168.4

84.6

120.6

82.1

10

227.4

178.5

203.5

121.8

136.5

88.7

15

233.1

194.9

231.6

173.0

160.9

96.4

20

238.3

203.0

240.0

190.2

212.5

98.0

25

243.6

211.5

247.1

203.0

228.8

197.4

30 35

249.4 254.8

220.4 225.1

252.4 258.0

211.9 217.1

235.6 241.6

208.3 213.7

40

260.2

230.8

263.5

223.7

248.0

221.4

45

266.1

236.9

269.7

230.2

254.5

230.4

50

272.6

243.3

275.9

239.0

261.5

238.5

55

279.4

251.0

281.0

245.5

268.7

247.9

60

286.5

257.8

288.2

254.9

274.7

253.6

65

293.5

262.8

293.3

261.1

284.3

261.4

70 75

301.5 309.9

270.2 278.4

300.0 307.1

269.0 277.2

293.3 302.7

270.9 281.2

80

318.8

285.3

313.8

287.1

312.2

291.2

85

329.5

293.9

323.3

295.4

323.6

302.8

90

340.2

305.2

331.6

302.7




DME (83.89 kPa)

DME (83.65 kPa)

DME (83.18 kPa)

c distillate volume fraction (%)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5

157.9

89.0

123.7

86.4

106.6

92.4

10 15

192.7 232.4

98.3 166.0

136.6 157.4

89.0 95.0

111.1 117.5

94.8 97.3

20

241.7

190.6

192.9

109.3

128.1

100.5

25

247.6

207.6

238.0

196.4

145.4

96.9

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Table 2. Continued diesel fuel þ 10% (vol/vol)



DME (83.89 kPa)

DME (83.65 kPa)

DME (83.18 kPa)

c

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

30

255.3

223.9

247.5

211.1

182.3

102.5

35

261.0

229.7

254.3

219.6

233.5

134.0

40

266.9

236.6

261.6

228.2

247.7

206.0

45

273.4

244.6

268.9

233.0

255.2

213.8

50

280.3

244.8

276.8

244.8

262.7

224.0

55

288.5

245.0

285.4

253.6

271.7

234.7

60

293.7

247.4

292.4

258.5

280.8

244.7

65 70

301.1 308.5

259.0 264.7

301.1 308.0

270.1 283.5

289.9 299.9

253.4 266.5

75

316.1

279.1

317.3

294.8

309.8

275.1

80

323.4

276.4

325.6

305.0

320.2

285.6

85

332.5

286.5

335.3

315.3

332.7

295.9

90

342.3

301.5

347.1

328.6

347.3

305.0

distillate volume fraction (%)

d

diesel fuel þ 10% (vol/vol) 2-



methoxyethyl acetate (82.70 kPa)



Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5 10

193.4 203.2

168.1 169.9

171.7 175.7

149.4 156.4

159.3 161.6

144.3 145.5

15

217.9

188.3

184.7

161.7

164.9

147.9

20

233.5

211.6

197.3

171.7

170.1

153.3

25

245.7

227.7

213.0

186.2

176.9

159.9

30

254.7

236.4

230.1

205.4

187.9

166.9

35

260.7

242.8

245.6

221.5

203.8

176.1

40

268.3

251.8

257.1

235.5

234.9

187.0

45 50

274.8 280.6

258.3 264.1

265.2 272.8

248.5 255.4

255.6 264.8

223.9 239.1

55

287.6

271.9

280.4

262.2

272.9

251.0

60

294.2

277.8

287.9

268.2

281.1

258.6

65

301.2

285.2

295.3

272.4

289.8

269.2

70

308.4

292.6

303.2

279.3

297.8

278.7

75

316.1

300.6

311.4

287.0

307.6

289.9

80

324.1

308.9

320.5

299.1

316.2

301.4

85 90

335.0 347.0

320.0 328.1

332.1 344.4

309.1 323.3

328.7 339.3

315.5 326.5




e

2-ethoxyethyl acetate (83.09 kPa)





Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5

199.0

165.6

183.1

165.3

175.2

162.6

10

207.3

181.5

187.9

171.0

177.9

166.2

15

216.6

191.6

193.5

175.7

180.6

170.2

20

226.9

201.3

200.5

179.9

184.2

173.6

25 30

237.8 247.7

210.0 222.5

210.5 223.0

190.7 200.5

189.4 196.3

178.0 181.6

35

255.5

235.2

236.9

211.1

206.3

188.3

40

262.4

244.5

250.1

225.1

220.6

200.3

45

268.3

252.4

260.8

240.4

239.0

211.9

50

273.7

257.5

268.1

251.1

254.9

229.8

55

279.5

263.6

274.6

259.2

266.2

247.1

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Table 2. Continued e







Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

60

285.2

266.3

280.6

265.8

274.3

258.3

65

291.2

271.6

286.9

272.7

281.7

267.5

70

297.2

275.9

293.6

277.3

288.8

274.7

75

304.4

276.5

301.0

284.3

296.6

281.8

80

311.7

279.2

309.1

288.6

304.8

289.2

85

320.6

278.8

318.5

298.3

314.4

296.5

90

332.2

282.4

329.7

305.5

329.0

304.1


f distillate volume fraction (%)




DGDE (82.85 kPa)

DGDE (83.24 kPa)

DGDE (83.16 kPa)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5

218.0

192.8

211.0

192.5

205.7

191.7

10

222.8

199.9

214.8

199.3

208.3

196.4

15

228.0

209.5

218.4

203.7

211.0

199.9

20

232.9

215.1

221.9

207.5

213.6

202.9

25

238.5

221.3

226.8

212.3

216.8

206.1

30

244.7

227.5

231.7

216.8

220.5

209.4

35 40

251.0 258.1

234.2 241.0

237.6 245.0

220.8 227.6

224.8 230.3

212.9 216.9

45

265.0

248.4

253.3

236.5

237.0

221.3

50

271.7

254.7

261.7

245.0

244.9

229.2

55

278.2

262.2

270.7

253.3

254.9

237.8

60

284.5

269.3

278.8

262.2

265.4

246.8

65

291.0

275.2

286.7

270.4

276.6

257.2

70

297.1

282.1

293.8

277.4

286.3

267.7

75 80

303.8 311.3

287.5 293.4

301.4 309.5

284.7 291.4

295.2 301.6

276.7 284.3

85

320.5

298.5

318.7

297.5

313.8

292.3

90

332.0

307.2

330.9

303.8

326.5

299.6

a

The uncertainties are discussed in the text. These temperatures have been adjusted to 1 atm with the Sydney Young equation; the experimental atmospheric pressures are provided to allow for recovery of the actual measured temperatures.

fluid was carefully observed. Direct observation through the flask window or through the bore scope ports of the apparatus allowed for the measurement of the onset of boiling for each of the mixtures (measured with Tk). Typically, to ascertain the initial boiling behavior, we measure the onset of bubbling, the temperature at which bubbling is sustained, and the temperature at which the vapor rises into the distillation head. We have shown that this last temperature is actually the initial boiling temperature (that is, an approximation of the bubble point temperature at ambient pressure) of the fluid mixture. This measurement is significant for a mixture because it can be modeled with an equation of state. Measurement of these temperatures with mixtures of commercial diesel fuel is complicated by the presence of the dye mentioned above. As a result, we only report the onset and vapor rise temperatures here. Vapor rise is accompanied by a sharp increase in Th and is therefore far less subjective to ascertain and, thus, is less uncertain than the onset of bubbling. Experience with previous mixtures, including n-alkane standard mixtures that were prepared gravimetrically, indicates that the uncertainty in the onset of the bubbling temperature is approximately 1 °C. The uncertainty in the vapor rise temperature is 0.3 °C.

In Table 1, we present the initial temperature observations for mixtures of diesel fuel and diesel fuel with (5 or 10), 20, and 30% (vol/vol) each of the oxygenates. These values have been adjusted to atmospheric pressure with the modified Sydney Young equation, as mentioned earlier. The initial boiling temperature of the base diesel fuel itself, also adjusted, was 216.8 °C. We note that the addition of even 10% (vol/vol) of these oxygenates significantly decreased the IBT of the mixtures relative to base diesel fuel. The magnitude of the departure is directly correlated with the boiling temperature of the oxygenate and the concentration of the oxygenate in diesel fuel. The most striking reduction is seen with the addition of DMM; a 10% (vol/vol) addition to diesel fuel reduced the IBT by 134 °C. At 30% (vol/vol), this reduction was 162 °C. The effect on the IBT because of the addition of the higher boiling oxygenate additives is of lower magnitude but nevertheless pronounced. Distillation Curves. During the measurement of the distillation curves, both the kettle and head temperatures were recorded (Tk and Th, respectively). The ambient atmospheric pressure was also recorded and used to adjust the temperatures to what would be obtained at sea level atmospheric pressure by use of the modified 2498


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Figure 1. Distillation curves of diesel fuel with mixtures of DMM. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text.

Figure 2. Distillation curves of diesel fuel with mixtures of BME. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text.

Sydney Young equation, as mentioned earlier. Each curve was measured 6 times, and the uncertainty in temperature was 0.3 °C. The repeatability of the pressure measurement (assessed by logging a pressure measurement every 15 s for the duration of a typical distillation) was 0.001 kPa. The uncertainty in the volume measurement that is used to obtain the distillate volume fraction was 0.05 mL in each case. Head and kettle temperatures, as well as the measured atmospheric pressure, are presented as a function of the distillate cut for a representative measurement for each mixture in parts af of Table 2. These data are also represented graphically in Figures 16. On each of the figures, the IBT is indicated as a hatch mark on the temperature axis. The behavior of the distillation curves is consistent with that of the IBT data. The magnitude of the departure from neat diesel fuel that is caused by the oxygenate is correlated to its normal boiling temperature; the largest displacements in the distillation curves are observed in the mixtures with oxygenates having the lowest normal boiling temperatures. We also note that the oxygenate additives with the

ARTICLE

Figure 3. Distillation curves of diesel fuel with mixtures of DME. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text.

Figure 4. Distillation curves of diesel fuel with mixtures of 2-methoxyethyl acetate. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text.

Figure 5. Distillation curves of diesel fuel with mixtures of 2-ethoxyethyl acetate. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text. 2499


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Figure 6. Distillation curves of diesel fuel with mixtures of DGDE. Here, we present Tk, the temperature measured directly in the fluid. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties are discussed in the text.

lowest normal boiling temperatures only affect the curve shape early in the distillations, and then the curves return to the familiar sigmoidal shape (similar to that of the base diesel fuel) as the distillation proceeds. This has been noted in much of our work with additives. We note that, with one remarkable exception (the measurements for DMM), the distillation curves for the oxygenate mixtures never completely merge with that of the base diesel fuel, even at the latest stages of the distillation. The permanent offset caused by a fuel additive has been observed in many previous studies and will be discussed in more detail later. We also observe the pronounced dependence of the distillation curves upon the concentration of the oxygenate additive. The addition of 10% (vol/vol) of each additive causes the largest displacement in the temperature seen in each curve, while the jump from 20 to 30% shows a more modest temperature change. Because all of the oxygenate additives that we have measured in this work have normal boiling temperatures below the IBT of the base diesel fuel, the distillate becomes enriched with the oxygenate early in the distillation. Indeed, as we will discuss below, the additive concentration in early distillate cuts exceeds the concentration of the additive in the initial (or stock) starting mixture. As the additive concentration in the starting mixtures increases from 5 or 10% to 30% (vol/vol), we notice the temperature asymptotically approaching the boiling temperature of the pure additive. We note that, for each additive, the addition of the lowest concentration of oxygenate fluid (either 5 or 10%, vol/vol) to diesel fuel produces a distillation curve that is still sigmoidal over the entire curve, despite the initial displacement to lower temperatures. For the 20 and 30% (vol/vol) mixtures, a somewhat different shape is apparent, with an initial flattening of the curve in response to the high concentration of a single component. Subsequent to this flattening, the curve once again becomes sigmoidal. The mixtures of diesel fuel with DGDE and 2-ethyoxyethyl acetate (Figure 5 and 6) show the effect of an additive with a relatively high boiling temperature. We note that the displacement occurs over most of the distillation curve, and the inflections in the curves that are apparent with the more volatile additives are more subtle. This fluid is much closer to the behavior of the base diesel fuel; indeed, it has been used in direct-injection research engines as a single-component diesel fuel.43

ARTICLE

Finally, we note that an examination of the behavior of the temperatures Tk and Th revealed no convergence; thus, we do not find indications of azeotropy among the oxygenate additives and the components of the base diesel fuel. Composition Channel Information. While the gross examination of the distillation curves is instructive and valuable for many design purposes, the composition channel of the ADC approach can provide an even greater understanding and information content. One can sample and examine the individual fractions as they emerge from the condenser, as discussed in the Introduction. Following the analytical procedure described in the Experimental Section, ∼7 μL samples were collected in autosampler vials containing a known mass of n-hexane solvent. Chemical analyses of each fraction were performed by GC with MS detection. Calibration was performed by the external standard method, in which four solutions of known concentration (prepared gravimetrically) of oxygenate were prepared, also in n-hexane or acetone. The chromatographic conditions were optimized for each oxygenate, as summarized in Table 3. In most cases, baseline resolution was easily obtainable for each oxygenate fluid; however, to improve sensitivity and selectivity for determining the mass compositions, selected ion monitoring was primarily used. The measured concentrations of the oxygenate additives are presented in Figures 712 as histograms, representing the mass percent of the additive as a function of the distillate cut. The uncertainty bars represent the propagated uncertainties of the sample and standard measurements and incorporate a coverage factor k = 2 (i.e., having a level of confidence of 95%). While not explicitly measured, we essentially recovered all of the oxygenate in the distillate or residue. Some general comments regarding the analysis are warranted. Occasionally, relatively high uncertainties are encountered in the analysis of polar fluids, such as those analyzed here. This is the result of very different mass spectral behavior (such as the abundance of an abundant parent ion at a relatively high m/z). Moreover, the analysis of polar fluids with a nonpolar column can be challenging, yet this approach was necessary to provide analytical results for the base fluid. Despite these difficulties, the quantitative results are nonetheless very good and are more than adequate. We note that the early eluting fractions are always enriched in the oxygenate additive relative to the starting stock solution. This concentration is observed to increase as the starting concentration of the oxygenate additive increases from 10 to 30% (vol/vol). Because of this, we note that each additive is depleted well before the end of the distillation. For example, for all mixtures of DMM, we note that, after the 0.3 distillate volume fraction, no DMM can be detected. As the normal boiling temperatures of the oxygenate additives increase, we observe two changes in this behavior. First, we note that the enrichment mentioned above (that is, the increase in the concentration seen in the early distillate fractions) generally decreases in magnitude. For one of the high boiling temperature fluids, 2-ethoxyethyl acetate, we observe that this is almost half that of the lowest boiling fluid, DMM. Second, we note that the persistence of the oxygenate fluid into ever later fractions of the distillation curve also increases. Both of these characteristics are similar to our past experience with oxygenate additives in diesel fuel. As reflected in the appearance of the distillation curves, the compositional behavior of the highest boiling point fluids, 2-ethoxyethyl acetate and DGDE, differs from the other fluids. We observe that for each of the mixtures (10, 20, and 30%, vol/vol), a maximum occurs at the 0.1 distillate volume fraction. The lighter 2500


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Table 3. Summary of the Analytical Conditions Used for the Samples Withdrawn as a Function of the Distillate Cut oxygenate DMM

chromatographic method

analysis time (min)

45 °C for 2 min 60 °C/min to 275 °C, held for 3 min

8.83

scan mode, RMM from 15 to 550 BME

60 °C for 1 min 20 °C/min to 100 °C 60 °C/min to 275 °C, held for 3 min

8.92

SIM: m/z 45.1, 56.1, and 88.0 DME


8.60

FID 2-methoxyethyl acetate


8.83

SIM: m/z 59.0, 73.1, and 88.1 2-ethoxyethyl acetate


DGDE

8.83

SIM: m/z 59.0, 73.1, and 88.1 110 °C for 2 min 40 °C/min to 245 °C

5.38

SIM: m/z 59, 72, 73, 103

Figure 7. Histogram plot showing the results of the analysis for DMM as a function of the distillate volume fraction for the three diesel fuel starting mixtures (10, 20, and 30%, vol/vol).

Figure 8. Histogram plot showing the results of the analysis for BME as a function of the distillate volume fraction for the three diesel fuel starting mixtures (5, 10, and 20%, vol/vol).

components of the base diesel fuel are simply more volatile than this additive and vaporize ahead of the additive. This is reflected in both the distillation curve and the composition-explicit channel. Comparison to Predictive Models. As we have demonstrated, the oxygenate concentration information provided by the composition-explicit data channel complements the temperature data grid of the distillation curves. In general, the conclusions are consistent, with the largest displacements in temperature occurring when there is a large concentration of oxygenate additive in the distillate. An interesting feature alluded to earlier is that, even after all of the oxygenate additive had been distilled out, the vaporization temperatures never merge with those of the base diesel fuel. We have observed this in all of our previous studies, in which an additive is present at a relatively high concentration, such as the solutions measured here. As we have argued, this is a consequence of the vapor liquid equilibrium that is established among the components when one of the components (the

additive) is significantly more volatile than the others. Early in the distillation, the applied energy is being used to preferentially vaporize the additive. Hydrocarbon constituents that are less volatile and would otherwise begin to vaporize early in the distillation remain in the liquid phase. Their evaporation is delayed; thus, the distillation curves of the 10, 20, and 30% starting mixtures are seen to lie below that of diesel fuel, although the oxygenate additives are no longer present in the kettle liquid. One can model the volatility (or rather the vapor liquid equilibrium) and, in particular, the behavior of the oxygenate additives with an equation of state. For calculations of the thermodynamic properties of mixtures, we use a mixture model explicit in Helmholtz energy that can use any equation of state, provided that it can be expressed in terms of the Helmholtz energy.96 This form of model has been used successfully for refrigerant and natural gas mixtures.97 We have adapted this approach to calculate the 2501


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Figure 9. Histogram plot showing the results of the analysis for DME as a function of the distillate volume fraction for the three diesel fuel starting mixtures (10, 20, and 30%, vol/vol).

Figure 11. Histogram plot showing the results of the analysis for 2-ethoxyethyl acetate as a function of the distillate volume fraction for the three diesel fuel starting mixtures (10, 20, and 30%, vol/vol).

Figure 10. Histogram plot showing the results of the analysis for 2-methoxyethyl acetate as a function of the distillate volume fraction for the three diesel fuel starting mixtures (10, 20, and 30%, vol/vol).

Figure 12. Histogram plot showing the results of the analysis for DGDE as a function of the distillate volume fraction for the three diesel fuel starting mixtures (10, 20, and 30%, vol/vol).

distillation curve of complex mixtures. As we have done for aviation and rocket kerosenes, we can devise a suite of components (that is, a surrogate mixture) to be representative of the constituents of the fluid. We typically start with a comprehensive chemical analysis of the fluid to develop a list of potential constituents (for example, between 6 and 12) to include in the mixture model. Provided sufficient data are available on those constituents, pure-fluid equations of state can be developed for each. For the hydrocarbon constituents, we used equations of state from the REFPROP database.98 For the oxygenate fluids, we employ the TDE program to generate SpanWagner equations of state.99,100 Then, using the mixture model mentioned above, the distillation curve of a mixture of those constituents can be calculated and refined with other thermophysical property information that includes density, speed of sound, viscosity, thermal conductivity, etc. Here, our goal is not a comprehensive equation of state for mixtures of diesel fuel with the oxygenate additives, but rather we desire to further explain the distillation curve trends and how the composition-explicit channel complements our explanation of those trends. Specifically, we seek to demonstrate theoretically that, although the additives have been boiled out of the mixture early in the distillation curves, the effect of their presence persists

much later, as observed experimentally. To do so, we construct and model (for calculation only) very simple, rough surrogate mixtures for diesel fuel with the oxygenate additives at 10 and 30% (vol/vol).82 The components of the surrogate mixtures and their mole fractions are provided in Table 4. Note that we have taken no pains to make this surrogate physically authentic. There is a complete absence of aromatic and cyclic compounds in this surrogate mixture, and we have not fitted any thermophysical property data (such as density or the transport properties) or operational properties (such as cetane number) to arrive at the compositions. We merely seek to examine the phase behavior trends predicted over the distillation curve. The distillation process is modeled with REFPROP as a simple distillation with no reflux; as vapor is produced, it is removed at a constant flow rate. The most informative presentation for the phase equilibrium information is to present the predicted oxygenate additive concentration (in mole fraction) in the vapor and liquid phases as a function of the distillate volume fraction. These are shown in panels af of Figure 13, in which we also provide the concentration of a light and heavy component (n-nonane and n-hexadecane, respectively) of the base diesel fuel. By convention, xi represents 2502


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Table 4. Composition of the Surrogate Mixture Models Used To Calculate the Predicted Compositions of 10 and 30% Oxygenate Additive Mixtures as a Function of the Distillate Volume Fraction

component DMM

mole fraction, for 10% (vol/vol)

mole fraction, for 30% (vol/vol)

additive

additive

0.2256 0.0201

0.0122

n-decane

0.0403

0.0245

n-undecane

0.1015

0.0617

n-dodecane n-tridecane

0.4105 0.1015

0.2496 0.0617

n-tetradecane

0.0403

0.0245

n-pentadecane

0.0403

0.0245

n-hexadecane

0.0201

0.0122

0.178

0.4551

n-nonane

0.0214

0.0142

n-decane

0.0427

0.0283

n-undecane n-dodecane

0.1077 0.4357

0.0714 0.2888

n-tridecane

0.1077

0.0714

n-tetradecane

0.0427

0.0283

n-pentadecane

0.0427

0.0283

n-hexadecane

0.0214

0.0142

0.2021

0.4942

n-nonane

0.0207

0.0132

n-decane

0.0415

0.0263

n-undecane n-dodecane

0.1045 0.4229

0.0663 0.2681

n-tridecane

0.1045

0.0663

n-tetradecane

0.0415

0.0263

n-pentadecane

0.0415

0.0263

n-hexadecane

0.0207

0.0132

0.1792

0.4571

n-nonane

0.0213

0.0141

n-decane n-undecane

0.0427 0.1075

0.0282 0.0711

n-dodecane

0.435

0.2877

n-tridecane

0.1075

0.0711

n-tetradecane

0.0427

0.0282

n-pentadecane

0.0427

0.0282

n-hexadecane

0.0213

0.0141

DME


2-ethoxyethyl acetate n-nonane n-decane

0.1555

0.4152

0.022 0.0439

0.0152 0.0304

n-undecane

0.1106

0.0766

n-dodecane

0.4476

0.3099

n-tridecane

0.1106

0.0766

n-tetradecane

0.0439

0.0304

n-pentadecane

0.0439

0.0304

n-hexadecane

0.022

0.0152

DGDE

0.1254

0.3560

n-nonane

0.0227

0.0167

n-decane n-undecane

0.0455 0.1146

0.0335 0.0844

mole fraction,

mole fraction,

for 10% (vol/vol)

for 30% (vol/vol)

additive

additive

n-dodecane

0.4636

0.3413

n-tridecane

0.1146

0.0844

n-tetradecane

0.0455

0.0335

n-pentadecane

0.0455

0.0335

n-hexadecane

0.0227

0.0167

component

0.5291

n-nonane

BME

Table 4. Continued

the mole fraction in the liquid phase, while yi is the mole fraction in the vapor phase. Starting with Figure 13a, for the DMM additive, we observe that the trends in behavior for the 10 and 30% (vol/vol) mixtures are consistent with the measured additive concentrations in the vapor (Figure 7). The depletion of the DMM additive in the 10% mixture is nearly complete just before the 0.3 distillate volume fraction, while that of the 30% mixture persists until after this fraction. We also note the very different behavior of the n-nonane in the base diesel fuel in the 10 and 30% mixtures. In both mixtures, the vapor-phase concentration of this light component reaches a maximum early in the distillation but the vaporization of the n-nonane in the 30% mixture is delayed. This is because the energy input is being used to vaporize the more volatile component: DMM. For both mixtures, the concentration of n-hexadecane increases slowly, reaching its maximum later in the distillation, consistent with the high boiling temperature of this component of the base diesel fuel. Turning to the liquid phase plot, we observe the depletion of the additive in the liquid during vaporization. This is consistent with the vapor-phase plot, because the concentrations in the liquid phase must precede what is observed in the vapor phase. We also note the gradual depletion of n-nonane and the enrichment of n-hexadecane. n-Nonane in the liquid is depleted early because it emerges early in the vapor plot; n-hexadecane in the liquid is observed to enrich, because it emerges later in the vapor plot. The predictions for BME and DME, shown in panels b and c of Figure 13, respectively, are similar in structure to what was observed for DMM, although we see the persistence of the oxygenate additives to higher mole fractions. This is consistent with the distillation curves (Figures 2 and 3) and the composition measurements (Figures 8 and 9) for these mixtures. As the boiling temperatures of the oxygenate additives increase further (approaching that of the light and moderate molecular mass constituents of the base diesel fuel), we note a change in the appearance of the phase composition predictions. This can be seen in panels d and e of Figure 13, for 2-methoxyethyl acetate and 2-ethoxyethyl acetate, respectively. First, we note the persistence of the additive to even later distillate volume fractions. This is observed on both the vapor and liquid plots and is consistent with the measured distillation curves (Figures 4 and 5) and the measured distillate compositions (Figures 10 and 11). The concentrations predicted for the liquid phase are seen to lead to those in the vapor phase, as we noted above in more detail for DMM. Moreover, because these oxygenate additives with higher boiling temperatures vaporize in the middle range of the base diesel fuel, we do not observe a peak for the n-nonane concentration in the vapor phase. Instead, we observe a more gradual decline in the concentration, although for the 30% mixture, a slightly longer persistence into the distillation is evident. This behavior is carried 2503


Energy & Fuels

ARTICLE

Figure 13. (af) Predicted vapor and liquid compositions of mixtures of diesel fuel with oxygenate additives at 10 and 30% (vol/vol): (a) DMM, (b) BME, (c) DME, (d) 2-methoxyethyl acetate, (e) 2-ethoxyethyl acetate, and (f) DGDE.

through as the boiling temperature of the oxygenate additive increases to nearly that of the base diesel fuel, as seen in Figure 13f for DGDE.

’ CONCLUSION In this paper, we have used the ADC approach to characterize the vapor liquid equilibrium of mixtures of linear oxygenate additives with a base diesel fuel. All of the additives studied here affect the distillation curve significantly, always resulting in an increase in volatility in the early part of the distillation curve. This effect is seen to persist later into the distillation curve as the normal boiling temperature of the oxygenate additive increases. The data presented here can be used for the development of equations of state for the mixtures; indeed, comparisons to a coarse surrogate mixture model have been included. This comparison has been used to explain how the effect of the oxygenate fluid can persist even after that fluid has been completely vaporized. Evaluation of oxygenate additive performance is a complex undertaking that will require far more work than an examination of a single thermophysical property. Despite this, one can make observations on the basis of the departure of the fuel from our normal experience base, as is often performed for turbine engines. An examination of the volatility behavior of the diesel fuels with the oxygenate mixtures shows a significant volatility departure, at all concentrations and all distillation temperatures, for DGDE and 2-ethoxyethyl acetate. These additives could not be applied at concentrations above a few percentage without bringing the resulting fuel outside the experience base. The presence of three oxygen atoms on each of these molecules would mitigate, because a high concentration would be unnecessary. The other additives, which have fewer

oxygen atoms, affect the volatility behavior mainly at lower distillation temperatures and do not result in as significant of a departure from the experience base.

’ ASSOCIATED CONTENT

bS

Supporting Information. Data on the oxygenate additive fluids studied in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ DISCLOSURE Disclaimer: Certain commercial equipment, materials, or supplies are identified in this paper to adequately specify the experimental procedure or description. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology nor does it imply that the equipment, materials, or supplies are the best available for the purpose. ’ ACKNOWLEDGMENT Tara M. Lovestead gratefully acknowledges a National Academy of Sciences (NAS)/National Research Council (NRC) postdoctoral associateship at NIST. Jennifer R. Riggs gratefully acknowledges a Professional Research Experiences Program (PREP) undergraduate fellowship at NIST. Erica L. Jorgenson 2504


Energy & Fuels gratefully acknowledges a Summer Undergraduate Research Fellowship (SURF) at NIST.

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Comparison of Diesel Fuel Oxygenate Additives to the Composition

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