1-s

R

Ff

GT

a

ARR2A

KBLSFS

C

pt

T

0d

J. of Supercritical Fluids 63 (2012) 133–149

Contents lists available at SciVerse ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

eview

luid properties needed in supercritical transesterification of triglycerideeedstocks to biodiesel fuels for efficient and clean combustion – A review�

eorge Anitescu ∗, Thomas J. Bruno ∗∗

hermophysical Properties Division, National Institute of Standards and Technology, Boulder, CO 80305, United States

r t i c l e i n f o

rticle history:eceived 23 September 2011eceived in revised form2 November 2011ccepted 22 November 2011

eywords:iodiesel fuelipid feedstockupercritical processingluid propertiesupercritical combustion

a b s t r a c t

This review focuses on the potential synergy between fluid properties and supercritical (SC) processing/combustion of biodiesel fuels. These fuels are the extenders/expanders of choice for petroleum-deriveddiesel fuels (PDDF) due to overall performance in the environment, safety, feedstock, and fuel quality. Atypical biodiesel fuel meets commercial specifications of the American Society for Testing and Materials(ASTM D6751) or European Union (EN 14214). Biodiesel fuels, mainly mixtures of fatty acid methyl orethyl esters (FAMEs or FAEEs), are currently produced by base/acid catalytic transesterification (BAC-TE) of triglyceride feedstocks with methanol or ethanol. These methods require refined oil feedstocksand complex product separation/purification that leads to noncompetitive prices compared with PDDFs.Alternatively, a noncatalytic technology based on SC-TE processing of various lipid feedstocks has beenreported to mitigate these drawbacks. One version of this technology, the one-step SC-TE method, poten-tially has major advantages over the BAC-TE, mainly due to shorter reaction times (5–9 min versus 1–6 h)and the reduction of glycerol to acceptable ASTM levels in fuels of superior quality. The latter advantageoriginates from glycerol and polyunsaturated FAME/FAEE thermal conversion to lighter fuel products.Based on technical and economic analyses, the manufacturing cost of biodiesel fuels from a one-stepSC-TE process could be one half of the BAC-TE current cost. To optimize biodiesel fuel production andquality, leading to a more efficient and clean combustion, a close connection between fluid properties
and fuel processing/combustion must be considered. Insights from recent case studies and real-worldexamples of applications of the principles of sustainability in the development and implementation ofbiodiesel fuel projects are given. The review includes sustainability metrics, resource efficiency, and sus-tainable process integration. These themes are woven together into a perspective on how sustainabilityand green-chemistry principles are being implemented for cost-effective biodiesel fuel production and advanced combustion.
Published by Elsevier B.V.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342. Triglyceride feedstock – methanol/ethanol properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

2.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362.2. Triglyceride feedstock composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362.3. Heat capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362.4. Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1372.5. Density and thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
2.6. Miscibility of triglycerides with alkyl alcohols . . . . . . . . . . . . . . . . . . . . .2.7. Thermodynamic parameters of triglyceride feedstocks and mixtu2.8. Phase transitions in triglyceride–alcohol mixtures . . . . . . . . . . . . . . . .
� This article is not subject to U.S. copyright. Certain commercial equipment, materialsrocedure or description. Such identification does not imply recommendation or endorsehat the equipment, materials or supplies are the best available for the purpose.∗ Corresponding author. Present address: Biomedical and Chemical Engineering Departel.: +1 315 443 1917; fax: +1 315 443 9175.∗∗ Corresponding author. Tel.: +1 303 497 5158; fax: +1 303 497 5044.

E-mail addresses: [email protected], [email protected] (G. Anitescu), brun

896-8446/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.supflu.2011.11.020

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137res with alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

or supplies are identified in this paper to adequately specify the experimentalment by the National Institute of Standards and Technology, nor does it imply

ment, Syracuse University, Syracuse, NY 13244, United States.

[email protected] (T.J. Bruno).

dx.doi.org/10.1016/j.supflu.2011.11.020

http://www.sciencedirect.com/science/journal/08968446

http://www.elsevier.com/locate/supflu

mailto:[email protected]



dx.doi.org/10.1016/j.supflu.2011.11.020

134 G. Anitescu, T.J. Bruno / J. of Supercritical Fluids 63 (2012) 133–149

3. Properties involved in supercritical transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393.2. Energies of activation from the kinetics of TE reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393.3. Reversibility/irreversibility of SC-TE reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1403.4. Heat of the overall TE reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1403.5. SC-TE process design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

4. Properties of biodiesel fuels produced by SC-TE processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414.2. Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414.3. Volatility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424.4. Thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.5. Density and speed of sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.6. Fit-for-purpose properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4.6.1. Higher heating value (HHV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.6.2. Cetane number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4.7. Properties from computational techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.8. Blended PDDF-biodiesel fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

5. Combustion of biodiesel fuels produced by SC-TE methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.2. Spray/jet characteristics and fuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.3. Biodiesel fuel properties and combustion quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.4. In-engine biodiesel fuel generation and SC combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

. . . . . .

1

eaa(scAUsRlhoubsvTngtte

rfNtftofbbtTi

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Biodiesel fuels, mainly mixtures of fatty acid methyl or ethylsters (FAMEs or FAEEs), have been the focus of significant mediattention, industrial interest, and scientific research. These fuelsre potential blending additives for petroleum-derived diesel fuelsPDDFs) due to their overall performance on environmental impact,afe handling, feedstock availability, and fuel quality [1,2]. A mer-hantable biodiesel fuel meets commercial specifications of themerican Society for Testing and Materials (ASTM) or Europeannion norms (EN), and is the only fuel produced at a commercial

cale that qualifies as an “advanced biodiesel” under the US EPAenewable Fuels Standard [3–5]. Biodiesel fuels exhibit increased

ubricity compared to ultra-low-sulfur PDDF, have no significantealth risk, are biodegradable, and decrease combustion emissionsf carbon monoxide, unburned hydrocarbons (UHC), and partic-late matter (PM) [1]. To be a viable PDDF extender/expander,iodiesel fuels should be profitable without significant governmentubsidies, should not compete with food supplies, and should pro-ide a net energy gain through the entire production-use cycle.hese fuels could provide ∼93% more usable energy than the energyeeded for their production and could reduce net greenhouseas emission by ∼41% compared with PDDFs [6]. Disadvantageshat must be addressed include oxidative instability, mois-ure absorption, higher viscosity, cold-flow properties, and NOx

missions [1].Triglyceride feedstocks and methanol/ethanol availability and

enewability have favored production of commercial biodieseluels by base/acid catalytic transesterification (BAC-TE) methods.evertheless, various technical and economic aspects require fur-

her improvement of both processing and combustion of theseuels [6–11]. Sustainable production pathways must be establishedo implement cost-effective processing technologies [12–16]. Toptimize TE processes leading to competitive and high-qualityuels with more efficient and clean combustion, the connectionetween fluid properties and fuel processing/combustion muste addressed. Fluid properties represent the integrating factor
o keep the fuel production and application steps synchronized.hese steps include feedstock selection and pretreatment, biorefin-ng, blending, transport, storage, and combustion. Fuel properties
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

contribute to the selection of the optimum upstream processingpathways and a downstream combustion technology. While for fin-ished fuels there are sets of specifications (ASTM D6751 and EN14214) [3,4], the most wide-ranging and demanding needs are forproperties required for chemical process design in biodiesel fuelproduction.

Biodiesel fuels considered in this review are largely limitedto those produced by emerging supercritical (SC) TE methods(triglyceride–alcohol mixtures in SC states). Various aspects ofthese methods are described in recent review articles [17–20]. Rep-resentative references on this topic for laboratory-scale researchare shown in Table 1 [21–51]. Reaction conditions for the feed-stocks studied are included along with reactor type and maximumconversion attained. As potential alternatives to the current BAC-TE commercial technologies, the SC-TE methods can give nearlycomplete conversion in a short processing time (minutes versushours). In addition to the high conversion and reaction rates, aSC-TE process can tolerate feedstocks with a high concentration offree fatty acids (FFAs) and water, up to 36% and 30% (mass/mass),respectively [17–19]. The development of the early SC-TE processeswas, however, hindered by concerns that temperatures higher than350 ◦C would be detrimental to fuel quality, while large alcohol-to-oil molar ratios (∼42:1) would be required to achieve high levels ofconversion at lower temperatures [17–20].

To overcome the main disadvantages of the emerging SC-TEmethods (e.g., large excess of methanol and glycerol separation),an advanced, cost-effective biodiesel fuel technology based on aone-step SC-TE process at ∼400 ◦C was reported [7,21]. This contin-uous processing method claims major advantages over both BAC-TEand early SC-TE methods, due to shorter processing time (<10 min),lower molar ratio of alcohol to triglycerides (<12:1), and elimina-tion of the glycerol to acceptable ASTM standard levels in the biofuel[7,21,50–53]. The latter advantage is due to thermal decomposi-tion of the resulting glycerol to fuel products of lower molecularweight, or the formation of acceptable oxygenated fuel productsdue to etherification. Consequently, the quantity (on a mass basis)of fuel produced by this method is approximately the same as the
mass of the reactants, and its quality appears to be superior to thatof biodiesel fuels produced by lower-temperature methods: lowerviscosity, better cold-flow properties, and improved volatility [52].

G. Anitescu, T.J. Bruno / J. of Supercritical Fluids 63 (2012) 133–149 135

Table 1Reaction conditions reported for noncatalytic supercritical transesterification (SC-TE).

Oil (fat)/cosolvent T (◦C) P (MPa) ROH:TGa (molar) Residence time (min) Reactor Maximum conversion (%) Ref.

Soybean 350–425 10–25 3–6 2–6 Continuous ∼100 [21]Soybean 250–350 12–43 43 15–90 Batch ∼100 [22]Coconut; Palm 270–350 10–19 6–42 1–33 Continuous 95; 96 [23]Canola 420–450 40 11–45 4 Continuous ∼100 [24]Soybean 280 6.2–7.4 15–20 20–120 Batch 96 [25]Chicken fat; Tall oil 275–325 16.5 10–40 20 Batch 91 [26]Soybean 310 35 40 25 Continuous 96 [27]Castor; Linseed 200–350 20 10–70 10–300 Batch 98 [28]Soybean 350 20 40 N/A Continuous 78 [29]Soybean/CO2 280 14.3 24 10 Batch 98 [30]Soybean/C3H8 280 12.8 24 10 Batch 98 [31]Sunflower 200–400 20 40 40 Batch 98 [32]Soybean/C3H8 270–315 8.8–21.1 64 10 Batch 99 [33]Palm 270–350 20–40 40 5–25 Continuous ∼100 [34]Soybean/C6H14/CO2 260–350 N/A 42 10 Batch 98 [35]Soybean/acid 270–350 0.1–15 20–60 8–21 Continuous 91 [36]Palm; Jatropha 200–400 20 50 40 Batch ∼100 [37]Palm kernel/C6H14 290–350 15–22 12–42 10 Batch 87 [38]Palm 200–400 40 3–80 0.5–20 Batch ∼95 [39]Jatropha 239–340 5.7–8.6 10–43 4 Batch ∼100 [40]Soybean 200–375 7–20 10–100 1–17 Continuous ∼85 [41]Rapeseed 200–487 7–105 42 4 Batch 95 [42]Camelina/C6H14 290 11.4 45 40 Batch ∼90 [43]Sesame; Mustard 275–350 19–20 30–80 45–70 Batch ∼100 [44]Jatropha/C6H14/CO2 200–300 24 ∼25 45–80 Batchb ∼100 [45]Radish 295–325 9–14 32–52 15–29 Batch 95 [46]Castor 300 20 20–40 15–50 Continuous 75 [47]Soybean/CO2 250–325 10–20 20–40 15–50 Continuous 80 [48]Camelina/CO2 400 19.6 42 15 Batch N/A [49]Chicken fat 350–400 10–30 3–12 3–10 Continuous ∼100 [50]Triolein 355–400 15 9 2–10 Continuous 98 [51]

Fi

eblpipstodpc[paiobi

otswnoiipc

a ROH is either methanol or ethanol.b Extraction + in situ TE.

urthermore, the manufacturing cost of the one-step SC-TE methods assessed to be about half that of BAC-TE [7,21].

To assess the technological feasibility and obtain material andnergy balances for economic analyses, process simulations muste performed. Despite some differences between a process simu-

ation and a real-world operation, the former is commonly used torovide reliable information on process operation. This capability

s based on component libraries, comprehensive thermophysicalroperty packages, and advanced computational methods. The firstimulation steps consist in selecting the chemical components forhe process along with a thermodynamic model. Additionally, unitperations and operating conditions, plant capacity and input con-itions must all be selected and specified [7–11]. To meet theroperty requirements for biodiesel fuel production and appli-ations, comprehensive databases are being created or extended15,54]. For biodiesel fuel-processing steps, process simulationackages such as ChemCad and Aspen Plus or HYSYS (ASPEN Tech)re available [7–11,55–57]. By using fluid properties containedn the databases of these programs as well as properties fromther available databases, one can design a process for optimizediodiesel fuel production [7–11]. An example of such an application

s available as supporting information (Appendix).Specification of a process chemical component requires input

f a number of properties, such as the normal boiling point, densi-ies, molecular weight, as well as the critical properties of the pureubstance. Complex mixtures, including blends of biodiesel fuelsith PDDFs, are typically modeled as surrogate blends of a limitedumber of components that capture the essential characteristicsf the fluid mixtures. Property measurements for pure compounds
n a surrogate include but are not limited to densities, heat capac-ties, vapor pressures, enthalpies, thermal conductivities, criticalroperties, speeds of sound, and viscosities. Thermodynamic con-epts such as structure-reactivity relationships, group contribution
methods, and extended corresponding-states models are currentlyapplied to generate acceptable surrogate blends [15,58,59].

Various specifications that a biodiesel fuel must meet are con-tained in relevant standards, such as ASTM D6751 and EN 14214[3,4]. Among the fundamental and fit-for-purpose properties inthese standards are cetane number, kinematic viscosity, oxidativestability, and cold-flow properties. Other important properties toconsider that are not contained in these standards are exhaustemissions under specified combustion parameters, lubricity, andheats of combustion. Property ranges of biodiesel fuels obtainedfrom various feedstocks by different TE methods are wide, becauseany property is dependent on fuel composition. Moreover, proper-ties reported without compositional data cannot be used to developproperty models. To assess the completeness of the TE reactionsaccording to the total glycerol level specified (maximum 0.24%(mass/mass)), analytical methods such as gas chromatographic(GC) analysis are required.

In this review we focus on fluid properties relevant to biodieselfuel production by SC-TE methods and to engine performance. Thisapproach will be useful to a broad range of scientists and engineersworking on a scientific foundation for biodiesel fuels and advancedcombustion in diesel engines. Despite recent progress, there ispresently no single comprehensive source of reliable propertydata for biofuels that can be called upon by industry. An exampleis representative for the status of this information: “During thedesign and test phases associated with the development of theportable biodiesel equipment, public domain information on thematerial properties of supercritical methanol with any lipid wasfound to be incomplete. Hence, the portable equipment was con-
servatively designed to account for these uncertainties” [60]. Thenew analyses and insights to the field can stimulate researchers inthis area for new R&D projects. Contributions that dealt explicitlywith the ecological landscape and regulatory issues that will likely


Table 2Relevant properties of SC-TE reactants and reaction products compared to those of n-hexadecane [54,57].

Property (SI unit) Methanol Ethanol Triolein Glycerol Methyl oleate n-Hexadecane

Molar mass (kg/kmol) 32 46 885 92 296 226Density (kg/m3)/25 ◦C 789 785 909 1,261 872 769Normal boiling point (◦C) 65 78 629 288 344 287Melting point (◦C) −98 −114 5 18 20 18Critical temperature (◦C) 239 241 705 577 491 449Critical pressure (MPa) 8.1 6.1 0.33 7.5 1.28 1.43Critical density (kg/m3) 274 276 286 348 279 218Dipole moment (D) 1.70 1.69 3.06 2.68 1.44 0.00Acentric factor, ω 0.566 0.644 2.095 0.513 1.049 0.717–�cH◦ (MJ/kg) 23.875 29.696 37.077 16.054 37.500 47.341

◦ 1301179669

aa

2

2

firiiridptsan

fqvdcttaTnoptGw

2

ee(arsciC

Cp,G (J/(mol K))/25 C 44 65Cp,L (J/(mol K))/25 ◦C 81 112Viscosity (mPa s)/25 ◦C 0.54 1.08

ccompany the continuous growth of the biodiesel fuel industryre also in high demand, but these issues are not discussed herein.

. Triglyceride feedstock – methanol/ethanol properties

.1. Overview

Triglyceride feedstocks are discussed as reagents in TE reactionsor biodiesel fuel production, neglecting their direct use as fuels orn blends with PDDFs. The needed reactant properties for the TEeactions are those of both pure reactants (a brief selection is givenn Table 2) and glyceride–alcohol mixtures. Relevant thermophys-cal properties include thermodynamic (feedstock composition,elative triglyceride–alcohol miscibility, density, heat capacity, crit-cal parameters) and transport (viscosity, thermal conductivity,iffusivity, and Reynolds number) properties. The most importantroperty is the composition, which, in turn, controls other proper-ies. The unrefined oils and fats usually contain FFAs, phospholipids,terols, water, and other impurities [61]. Water and FFA contentsre important quality parameters of the raw material and vary sig-ificantly with feedstock origin.

Reliable information is needed for reactor design to optimizeeedstock use and processing to produce biodiesel fuels of highuality. Accordingly, thermophysical and chemical properties ofarious reactive mixtures, as well as those of TE reaction interme-iates and final products, are required over appropriate ranges ofonditions. Due to the lack of property information for triglycerides,riolein is often selected as a surrogate lipid feedstock in designshat require data for liquid and vapor equilibria. For comparison,pproximate values for some of the triolein properties are given inable 2, along with those of methanol, ethanol, and major compo-ents of biodiesel (methyl oleate), and diesel fuels (n-hexadecaner cetane) [54,57,62]. While the volatility and viscosity, for exam-le, of methyl oleate are dramatically lower than those of triolein,hese properties are still significantly higher than those of cetane.lycerol stands apart by its unusually high density and viscosity,hich make it difficult to handle and process.

.2. Triglyceride feedstock composition

The most important input factors in the production andconomics of biodiesel fuels are the quality and cost of triglyc-ride feedstocks. The oil content varies, reportedly up to 70%mass/mass) for some algal strains [63]. The feedstocks for currentnd potential use for biodiesel fuel production include soybeans,apeseed/canola, cottonseed, sunflower seeds, groundnut, copra,
esame, linseed, castor seed, corn/maize, palm, coconuts, jatropha,uphea, camelina, and pennycress. Soybean is a main feedstockn the United States, whereas rapeseed dominates in Europe andanada and palm/coconut in Southeast Asia [12,17].
115 445 369222 596 500866 5.41 3.06

The major supplies of animal fats and greases for biodieselfuel production are tallow, poultry fat, and lard (white grease).Yellow grease is manufactured from spent cooking oil. The FFAcontent of these feedstocks is usually high (10–30% (mass/mass)),making them rather unattractive for conventional catalytic TE,which requires less than 0.5% (mass/mass) FFA content [61].This composition is, however, advantageous for SC-TE methodsbecause the easiness to esterify FFAs [17–19]. Although insuf-ficient in scale to significantly affect PDDF consumption, thesefeedstocks are low-cost resources for biodiesel fuel production viaSC-TE.

Biodiesel fuel from algae has been seen as a potential substitutefor diesel fuels [63,64]. Microalgae are attractive because of theirhigh triglycerides or hydrocarbon content, and they can be grownby use of water and land resources that do not interfere with foodcrop production [63,64]. Despite the optimistic evaluations, someaspects in life-cycle analyses are difficult to quantify [64,65]. Themain concerns are with water resources and economic feasibility.Production of liquid biofuel from algae is currently at the R&D phase[66,67].

The differences in quality characteristics between various lipidfeedstocks produce variations in the biodiesel fuel properties. Sat-urated triglycerides tend to produce biodiesel fuel with slightlyinferior cold-flow properties but with better storage stability thanthat of unsaturated oils. Some tests have shown animal fats to pro-duce biodiesel fuels with higher cetane number and a slightly betterengine emission profile [1,68].

2.3. Heat capacity

Heat capacity is an important fluid property to be consideredwhen designing heat exchangers and calculating heats of reaction.Heat exchangers play a major role in the energy balance of SC-TEprocesses by improving heat recovery from the reaction productswhich lowers the process cost. For pure fluids, the divergenceof the heat capacity in the vicinity of the critical point is a wellknown feature, whereas for fluid mixtures this property is a con-tinuous function of temperature. Our literature search yielded noheat-capacity data for triglyceride–alcohol mixtures at high tem-peratures, including the SC region. To illustrate the heat-capacitybehavior for complex mixtures, a plot of heat capacity versus tem-perature is given in Fig. 1 for a ternary mixture of a diesel-fuelsurrogate, n-hexadecane, with the main components of exhaustgas recycle (EGR), namely N2, CO2, and H2O [53]. In a TE process,N2 is often used as an inert gas to displace air, CO2 may be used ascosolvent, and water is present as impurity.

Heat capacities at constant pressure (Cp) of the reactants areneeded over wide P–T ranges and can be determined by use of adifferential scanning calorimeter (DSC) capable of acquiring Cp datawith uncertainty of less than 1% up to 35 MPa and 300 ◦C [69]. The


150

200

250

300

350

400

0 100 200 300 400 500T (oC)

Cp (

J/m

ol. K

)

Bubble-point curve

Dew-point curve

Fig. 1. Molar heat capacity of n-hexadecane–CO2–N2–H2O mixture as function oftemperature calculated by Peng–Robinson equation of state (adapted from [53]).Ta

bm

�

wst

C

Ihwo

2

fllawerr[

2

tsuabo

products, often an overlooked issue, is essential for biodiesel fuelproduction design and process operation. Different SC-TE reactionconditions reported for near complete triglyceride conversionoriginate from triglyceride–alcohol properties with miscibility

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Palm

Oil

Prop

ertie

s

Heat capacity (kJ/kg .K)

Density (g/cm3)

Thermal conductivity (W/m .K)

he molar composition of the isopleth is 0.4859 (C16), 0.3804 (N2), and 0.0668 (CO2

nd H2O). The critical value is reached at ∼427 ◦C.

asic equation for heat capacity determination by this calorimetricethod is:

˚sr = ˚s − ˚r = CpsdTs

dt− Cpr

dTr

dt= (Cps − Cpr)ˇ (1)

here ˚ is the heat flow rate, ˇ is the average heat flow rate, andubscripts s and r indicate the sample and the reference, respec-ively. The specific heat capacity of any sample is given by [69]:

s = ˚s − ˚0

˚ref − ˚0· mref

ms· Cref (2)

n this equation, ˚0 is the heat flow rate of the baseline, ˚ref is theeat flow rate of a known amount (mref) of calibration substanceith known specific heat capacity (cref), and ˚s is the heat flow rate

f any known amount (ms) of the sample (reactant or product).

.4. Viscosity

Viscosity is one of the most important transport propertiesor fluids involved in the continuous-flow SC-TE methods. Simi-ar to the triolein behavior illustrated in Fig. 2 [54], most of theipid feedstocks for biodiesel fuel production are very viscous atmbient temperature and pressure. The viscosity decreases rapidlyith heating of the feedstock. The thermal variation of this prop-

rty at ambient pressure can be considered linear within limitedanges of both low and high temperatures (T < 60 ◦C and T > 230 ◦C,espectively). There are also available viscosity data for methanol62,70].

.5. Density and thermal conductivity

Density and thermal conductivity are some of the other proper-ies needed in the process design of SC-TE, including the preheatingtep of the reactants. The reported data for these properties are
sually limited to low temperatures. However, available datat moderately higher temperatures show a quasi-linear thermalehavior along pressure isobars that permits some data extrap-lation. As illustrated in Fig. 3 for palm oil, the density, thermal
Fig. 2. Triolein viscosity at atmospheric pressure as function of temperature. Withdata from [54].

conductivity, and heat capacity exhibit such a behavior up to300 ◦C [71]. This characteristic is important for modeling feed-stock properties in the liquid phase. This linear behavior changesrapidly around and beyond the critical point of the fluids. Reli-able data for the SC region of oil–alcohol solutions are urgentlyneeded.

2.6. Miscibility of triglycerides with alkyl alcohols

Information on mutual solubility of the reactants and reaction

50 100 150 200 250 300 350

Temperature (oC)

Fig. 3. Heat capacity, thermal conductivity, and density of palm oil versus temper-ature at atmospheric pressure [71].


F y heaA

pptta(rmtmbcufpmr

i4vtpcc

ig. 4. Miscibility of a soybean oil–ethanol mixture (1:16 molar ratio) being rapidlCS permission from [21].

laying a major role [21,33,72–80]. At similar P–T conditions, thisroperty depends strongly on the triglyceride composition andhe alcohol-to-triglyceride ratio. The former parameter affectshe solubility by different properties of the FA components suchs polarity and volatility, while the latter acts by lowering thepseudo)critical points of the oil–alcohol mixtures. Stoichiomet-ic amounts of liquid triglycerides and methanol are not veryiscible in the liquid phase, so that reactions occur mainly at

he interface between the two liquid phases [1]. If the reactionixture is a homogeneous phase, higher conversion rates can

e achieved than if the reaction occurs under heterogeneousonditions. Reactants and the TE products are partially solublender various P–T–x conditions. With an increase in the massraction of FAMEs, the alcohol solubility in the triglyceride–FAMEshase increases [77]. Glycerol has a high affinity for alcohol,aking a significant portion of the reactant unavailable for TE

eactions.The miscibility of soybean oil and ethanol (1:16 molar ratio)

s illustrated in Fig. 4 for the case of rapid heating from 26 ◦C to00 ◦C in a high pressure, high temperature view cell at constantolume/density [21]. With two liquid phases at ambient conditions,
he system shows increasing homogeneity with increasing tem-erature and pressure until a single SC phase is reached. At theseonditions, the conversion of triglyceride to FAMEs is very rapidompared to that at subcritical conditions [17,21,52].
ted from room temperature to 400 ◦C (the volume of the view cell is ∼1 mL). With

2.7. Thermodynamic parameters of triglyceride feedstocks andmixtures with alcohols

High-pressure and high-temperature P–V–T data are lacking formany natural lipids that are processed with SC fluids. P–V–T datawere measured with a static-type bellows apparatus for variousfats and oils at 1–150 MPa and 30–80 ◦C and compared to those ofpure components trilaurin, triolein, and tridecane [81]. The P–V–Tbehavior for the fats and oils above their melting points was similarand was explained in terms of molecular weight, iodine value, andfatty acid composition. Tait, Flory, and simplified perturbed hardchain theory (SPHCT) equations provided satisfactory correlation.With the correlated parameters, the P–V–T behavior of the oils wasdescribed to within 4.9% average deviation in pressure.

While the critical properties of triglycerides are difficult to mea-sure due to thermal decomposition, the critical points of variouslipid–alcohol mixtures are typically determined by using a viewcell/quartz tube and a suitable optical system [21,33,53,76,82,83].In addition to Fig. 4, Fig. 5 shows a phase transition from an L-Vtwo-phase mixture of diesel fuel #2 and CO2 to a SC phase [83].In practice, however, the reactivity of the triglycerides and alcohol
could generate esters, which would affect the critical conditions bychanging the system composition. This effect will likely be negligi-ble at low ester concentrations, with high heating flux that will leadto short residence time until the desired temperature is reached.


ixtur

Am[

2

tgtspiasS

bhcrpat

Fam

Fig. 5. Vanishing L-V meniscus in the diesel fuel–CO2 m

lternatively, the critical parameters can be calculated approxi-ately by using a group-contribution technique or other methods

51,52,76,80].

.8. Phase transitions in triglyceride–alcohol mixtures

By observing the P–T–x conditions of fluid phase transitions likehose in Figs. 4 and 5, one can build various binary phase dia-rams [53,83]. With a relatively simple methodology, it is possibleo experimentally observe and construct P–T–x phase diagrams forelected binary systems of triglyceride–alcohol solutions. For theurpose of lipid–alcohol phase transition studies, special attention

s required to minimize the interference of the TE conversion byrapid heating of these solutions. By mapping these phase tran-

itions to SC states, a set of conditions to carry out an optimizedC-TE process can then be selected.

Fig. 6 shows the critical temperatures for two pseudoinary triglyceride–methanol mixtures as functions of the alco-ol:triglyceride molar ratio (data from [23]). For a givenomposition, at T < Tc, the system is usually in a heterogeneous
egion, while at T > Tc the system is in a single homogeneous SChase. A large excess of alcohol is required to bring the system toSC state at relatively low temperatures. For example, at 300 ◦C,
he mixture is in SC states for a methanol:triglyceride molar ratio

Tc(oC)

MeOH:TG (molar)

0

50

100

150

200

250

300

350

400

450

500

550

600

0 5 10 15 20 25 30 35 40 45

Tc/PKO

Tc/CCO

ig. 6. Critical temperatures for pseudo binary mixtures of methanol with palmnd coconut oils (PKO and CCO, respectively) versus methanol to triglycerides (TG)olar ratio. Data from [23].

e transitioning to a SC state at 420 ◦C and 10 MPa [83].

beyond 35–40. As shown in Table 1, most of the reported resultsin the literature are for molar ratios of 40–42, claiming the shift ofthe TE equilibrium to the right under excess alcohol. Instead, theseratios position the oil–alcohol mixtures in a SC state at 300 ◦C, withhigh and rapid conversion. For this reason, at 350 ◦C and 400 ◦C, forexample, SC phases can be reached at ratios of 15 and 8, respec-tively (Fig. 6), significantly cutting the cost of methanol recycling.This critical temperature versus feedstock composition behavior issimilar for all feedstocks rather than just the two pseudo binarymixtures mentioned in Fig. 6.

3. Properties involved in supercritical transesterification

3.1. Overview

This section briefly describes the SC-TE processing of lipid feed-stocks to biodiesel fuels and how these specifications determinefuel quality. A comparison with BAC-TE is also included. While theavailability of widely diversified feedstocks is a significant advan-tage for biodiesel fuel production, the compositional variability ofthese feedstocks renders the selection of the processing conditionsrather difficult. For example, the available feedstocks of triglyc-erides for biodiesel fuel production contain mainly five fatty acids:palmitic, stearic, oleic, linoleic, and linolenic; There are, however,35 triglycerides containing these acids with different structures andcorresponding properties.

Despite significant progress, the science behind biodiesel fuelproduction is not yet well developed. For example, the heat of theTE reactions, TE reversibility, and catalytic activity at high tem-peratures are a few issues that have not yet been convincinglyaddressed. Often, the role of fluid mixing in increasing TE con-version conferred by a solid mesh/bed in a TE reactor could bemistakenly assigned to catalytic activity. Also, as we have alreadydiscussed, the effect of excess alcohol used in SC-TE reactions is, insome cases, erroneously interpreted as a shifting factor of reactionequilibrium.

3.2. Energies of activation from the kinetics of TE reactions

It has been reported that the conversion of various vegetableoils into the corresponding FAMEs was enhanced considerably in SCmethanol (Table 1). The apparent activation energies of the TE reac-tions were found to be different for the subcritical and the SC statesof methanol (e.g., 11.2 and 56.0 kJ/mol, respectively, at 28 MPa anda molar ratio of methanol to oil of 42) [84].

The global TE process between a triglyceride (TG) of three iden-tical fatty acids (e.g., triolein) and methanol can be written as

TG + 3CH3OH → 3RCOOCH3 + C3H5(OH)3 (3)


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 43 5 6 7 8 9 10

FAMEs

TG

DG

MG

Prod

uct y

ield

(wt%

)

Residence �me (min)

Fig. 7. Typical temporal profiles of monoglyceride (MG), diglyceride (DG), triglyc-e[

Tsmp

T

D

M

Ae

C

TTiivi

itmbirSthemcrBlsfe

Fig. 8. GC-FID chromatographs of methyl oleate before (bottom) and after reaction

∂�=

∂T= �i · Cp,i (8)

ride (TG), and FAME in the SC-TE of chicken fat with methanol at 400 ◦C and 20 MPa50].

his overall reaction is a sequence of other competitive or con-ecutive reactions, with TG going to diglycerides (DG), DG toonoglycerides (MG), and MG to glycerol; FAMEs (RCOOCH3) are

roduced in each step of these TE reactions:

G + CH3OH → RCOOCH3 + DG (4)

G + CH3OH → RCOOCH3 + MG (5)

G + CH3OH → RCOOCH3 + C3H5(OH)3 (6)

t SC conditions, glycerol can also react with methanol to generatethers and thermally reacts to glycerol products (GPs):

3H5(OH)3 + 3CH3OH → Ethers + GPs (7)

he chemistry described above forms the basis of the one-step SC-E method for biodiesel fuel production [7,21]. Reaction (7) is verymportant for the SC-TE processes at high temperature by produc-ng smaller molecular fuel components to improve the overall fuelolatility, viscosity, and cold flow properties, as well as by consum-ng the undesired byproduct, glycerol [21,50–52].

The complexity of reactants and reaction products masks thentrinsic kinetics and reaction mechanisms/pathways that con-rol SC-TE conversion processes. This motivates the use of simple

odel compounds, which mimic the important reactive moietiesut also allow reliable developments of reaction pathways, kinet-

cs, and mechanisms. To study the kinetics of this complex set ofeactions, individual triglycerides (e.g., triolein) were subjected toC-TE conditions of 300–400 ◦C, 20 MPa, and up to 9:1 methanol toriglyceride molar ratio for 2–10 min residence time [51]. A compre-ensive chromatographic analysis was performed to determine theffect of the reaction conditions on triglyceride conversion, inter-ediate species temporal profile, and final-product yield. Rendered

hicken fat was also studied at SC-TE conditions [50]. The tempo-al profiles of the main reaction species are shown in Fig. 7 [50].eyond ∼6 min, the polyunsaturated FAMEs start to decompose to

ower molecular-weight products, which mainly include esters and
traight-chain hydrocarbons. To obtain the energy of activation (Ea)or the main reactions involved in the SC-TE process, the kineticxperimental data can be modeled with suitable software.
(top) with glycerol at 400 ◦C and 10 MPa for 10 min. Peak identification: 1 – glycerol,2 – internal standard (butanetriol), 3 – monoolein, 4 –internal standard (tricaprin),and 5 – diolein [51].

3.3. Reversibility/irreversibility of SC-TE reactions

Theoretically, TE is a reversible reaction, particularly for smallester-alcohol molecules. In the production of FAMEs, however, thereverse reaction is negligible, especially at high temperature. Toprove that the SC-TE is not an overall significantly reversible reac-tion, a mixture of glycerol and methyl oleate was heated at thereaction conditions for 10 min. No formation of methanol or trioleinwas observed [51,52]. Only small amounts of monoglycerides andtraces of diglycerides were generated, as shown in Fig. 8. Accord-ingly, the claim of excess methanol as a driving force of the TEequilibrium toward the right side is not persuasive under practicalconditions. The large excess is actually needed to facilitate mix-ing and to bring reactants to one homogeneous phase, as discussedearlier [21,52,72–80].

3.4. Heat of the overall TE reactions

We have not found information regarding the endo- or exo-thermic characteristics of the overall TE reaction (3). A calculationbased on the values of the heat of formation of the TE reactants andproducts [54] shows a thermal effect of ∼13 kJ/mol for the reaction(3). This endothermic effect is in agreement with the increasingtriglyceride conversion with increasing TE temperature. DSC stud-ies of the TE reactions for selected triglyceride–methanol systemscan produce reliable data. The thermal behavior of selected sys-tems at different molar ratios should be examined for individualtriglyceride and oil/fat feedstocks with methanol.

The heat of the overall TE reaction, �rH(T), can be obtained intwo ways. First, as soon as the Cp,i(T) functions of reactants andproducts are determined, �rH(T) can be calculated by the Kirchhoffequation [69]:(

∂Cp,�

) (∂�rH

) ∑
T,p p,� i
In this relationship, Cp,� is the heat capacity of the system at con-stant pressure and extent of reaction (�), �i are the stoichiometric

percrit

ntft(

Rhthhi

〈

3

nmbHds

po[TSittgbctcas

pahdgihhhTafi

spaatccti

G. Anitescu, T.J. Bruno / J. of Su

umbers, and Cp,i are the partial, molar heat capacities of the reac-ants and products. Second, the reaction enthalpy can be obtainedrom the measured heat flow rates in a DSC by the following equa-ion:

dQ

dt

)p

= Cp,�(T) · dT

dt+ 〈�rH〉 · d�

dt(9)

eactions can be conducted quasi isothermally. The reactants areeated as rapidly as possible from room temperature to the reactionemperature. The reaction equilibrium is reached when a constanteat flow rate, ˚end, is achieved. Since there is no contribution fromeat capacity in the isothermal mode, the average reaction enthalpy

s given by

�rH〉 =∫ t

0(˚m − ˚end)dt∫ �

0d�

(10)

.5. SC-TE process design

Alternative methods to produce biodiesel fuels through theoncatalytic TE of triglyceride feedstocks in SC alcohols, mainlyethanol and ethanol, have been described (Table 1). Methods

ased on SC alcohols were reported as simplifying the TE processes.owever, the total manufacturing cost of these methods is still high,ue to the large excess of alcohol used (molar ratio of ∼42:1 versustoichiometric 3:1) and glycerol–FAME separation issues.

A continuous, one-step, SC fluid technology coupled withower cogeneration reportedly produces biodiesel fuels with-ut the highly complex conventional separation/purification steps7,21,50–53]. The core of the integrated system consists of the SC-E of various triglyceride sources with methanol/ethanol (Fig. 9a).ome of the reaction products can be combusted in a diesel enginentegrated in the system, which in turn provides the power neededo pressurize the system and the heat for the SC-TE reactions fromhe exhaust gases. The process has the advantage of near-completelyceride conversion in less than 10 min to fuels with negligi-le glycerol content due to its secondary reactions to viable fuelomponents (e.g., ethers). Further, the molar ratio of methanol toriglyceride content in the feedstock is much closer to the stoi-hiometric amount of 3:1, due to the complete miscibility of lipidsnd methanol at SC conditions. A catalytic TE process flowsheet ishown for comparison in Fig. 9b.

Biodiesel fuel production by the one-step SC-TE method is a sim-le process because of the direct triglyceride and FFA reactions withlkyl alcohols in a SC state to produce alkyl esters [7,21,50–53]. Aigh reaction temperature in the proposed method (350–400 ◦C)ecomposes some of the unsaturated esters and the byproductlycerol further react to valuable fuel components, consequentlymproving the fuel quality (e.g., lower viscosity and cloud point, andigher volatility). Also, a low molar ratio of methanol to triglycerideas the important effect of eliminating the step of excess alco-ol recycling, bringing simplicity and cost savings to the method.he high alkyl ester yield of this SC-TE method is not significantlyffected by the impurity content (e.g., water and FFAs) of the unre-ned lipid feedstocks.

When the process schematic diagram of the SC-TE methodhown in Fig. 9a is compared to a current base catalytic industrialrocess as shown in Fig. 9b, the simplicity of the former method ispparent. The simplicity of the SC-TE process is due to low excesslcohol and glycerol decomposition to valuable fuel componentshat eliminates the need for complex separations. In the SC-TE pro-
ess no catalysts need to be recovered, and continuous flow reactorsan be used instead of batch reactors. All of these processing advan-ages result from the convenient properties of the SC fluids involvedn the SC-TE reactions.
ical Fluids 63 (2012) 133–149 141

Based on the process diagram in Fig. 9a, a SC-TE commercialplant for biodiesel fuel production was designed with a CHEMCADsimulation (Fig. 10) [7]. A list of fluid properties for each stream inthe process is generated by the modeling software. For a reliabledesign, careful attention has to be devoted to the uncertainty ofthese properties as well as to the equations used in the modeling.Of particular importance are the fluid densities in the reactor, whichwill permit the calculation of the residence time, �, of the reactants:

� = V

qv0

�

�0(11)

In Eq. (11), V is the volume of the reactor, qv0 is the total vol-umetric flow rate at the reactor inlet, and � and �0 are thetriglyceride–alcohol mixture densities at steady-state in the reactorand at the reactor inlet, respectively. A list of the CHEMCAD prop-erties of the fluid streams in a pilot plant for one-step biodieselproduction is provided as supporting information (Appendix).

4. Properties of biodiesel fuels produced by SC-TE processes

4.1. Overview

Chemical differences between thermally stressed biodiesel fuelsproduced by a high-temperature SC-TE process and those from aconventional BAC-TE method lead to significant differences in theirphysicochemical properties. These properties ultimately affect fuelquality vis-à-vis the engine performance, efficiency, combustioncharacteristics, and emissions. The fuel quality of FAME mixturesproduced by a SC-TE process is influenced by several factors, includ-ing the quality of the feedstock, the FA profile of the parent lipidfeedstock, the processing conditions, especially temperature, othermaterials used in the process, and post-production parameters[1]. Different processing temperatures mainly affect specific fuelproperties, such as volatility, viscosity, and cold-flow behavior illus-trated by cloud and pour points.

Books and review articles analyzing the research data of themost representative studies on biodiesel fuel properties have beenpublished [1,58,85,86]. While the information on biodiesel fuelsfrom conventional BAC-TE processing is well documented, theavailable properties of the fuels from SC-TE are limited. Theseproperties are affected to different extents by chain length andbranching, the molecular degree of unsaturation, and the positionand geometric configuration (cis or trans) of the double bonds. Mostcommercial biodiesel fuels are composed mainly of C16–C18 chainsof FAMEs, for which the main structural difference is the numberof double bonds. As mentioned earlier, biodiesel fuels produced bya SC-TE process at temperatures higher than ∼350 ◦C exhibit sig-nificant compositional variance, which can significantly affect theirproperties. In particular, the highly unsaturated FAMEs thermallydecompose to smaller C7–C15 FAMEs, both saturated and mono-unsaturated [50,87]. A strong correlation between the degree ofunsaturation and the properties of alkyl esters has been reported[88–90]. With increasing unsaturation, some of the fit-for-purposefuel property values decrease (heating value, melting point, cetanenumber, viscosity, and oxidation stability), while other increase(density, bulk modulus, fuel lubricity, and iodine value) [90].

4.2. Viscosity

Of the transport properties with reported data for biodieselfuels, the viscosity is one of the most sensitive to the compositionaldifferences [91,92]. While most of the biodiesel fuel properties
compare favorably with those of PDDFs, higher viscosity can affectthe fuel injection parameters, particularly at low engine operat-ing temperature. The viscosity of commercially produced biodieselfuels (4.0–4.6 mm2/s at 40 ◦C and 0.1 MPa) is outside the allowed


F criticat

r[ait

4

psat

ig. 9. Process diagram for (a) thermally stressed biodiesel fuel production by a superransesterification process. With Elsevier permission from [8] (bottom).

ange by ASTM standard D975 for PDDFs (2.5–3.2 mm2/s at 40 ◦C)91]. Limited experimental data for biodiesel fuels produced by

SC-TE process at 400–425 ◦C show that this property can bemproved through the thermal decomposition of some of the reac-ion products to smaller molecular fuel components [24,31,52].

.3. Volatility

Volatility is a critical fluid property for fuel replacement
urposes and proper engine operation. It is very sensitive to compo-itional variability of the biodiesel fuel produced by the TE reactionsnd can be determined by the advanced distillation curve (ADC)echnique [93–95]. This technique provides temperature and vol-
l transesterification (SC-TE) method [7,21] (top) and (b) pre-treated alkali-catalyzed

ume measurements of low uncertainty and composition-explicitdata that allows precise analysis of each fraction of the distilla-tion. The fuel composition versus extent of distillation can thereforebe determined for a variety of complex fuels, including biodieselfuels [94–96]. This approach provides VLE measurements of lowuncertainty, and defined thermodynamic state points can be mod-eled with an appropriate equation of state (EOS). These distillationcurves can be easily compared with those of selected fuels thatare considered typical (e.g., diesel fuel #2 (DF2) and commercial
soybean biodiesel fuel) as shown in Fig. 11 [87].
The experimental results (Fig. 11) show that biodiesel fuelsamples obtained from SC-TE processing exhibit higher volatilitycompared to commercial biodiesel fuels produced by a conven-


4

7

8

9

10

3

4

5

6

7

9

1

2

2

5 6

31

10

12

8

Oil

MethanolBiodiesel

11

F operti

ttfBmtoialbhuba

4

nd

Fecs

ig. 10. CHEMCAD simulation process diagram for biodiesel fuel production. The pr

ional catalytic method [87]. This volatility is very close to that ofhe DF2 at the start of vaporization, while commercial biodieseluel starts boiling at a temperature higher by more than 100 ◦C.iodiesel fuel samples processed at 400 ◦C by the one-step SC-TEethod [21] showed no significant thermal decomposition over

he full ADC temperature range. In contrast, the distillation curvef the commercial biodiesel fuel sample exhibits a sharp increasen temperature at volumetric fractions higher than ∼60% (vol/vol)s a result of significant thermal decomposition [87]. The distil-ation profiles of various test fuels show some differences, whichecame more noticeable at the medium-temperature (T50) andigh-temperature (T90). These points were higher for the morensaturated fuels. Those increases in distillation temperatures maye partly due to the slight differences in chain length and densitymong fuels [90].

.4. Thermal conductivity

Although thermal conductivity is involved in heat-transfer phe-omena during SC-TE processing and fuel combustion, availableata are at present limited for biodiesel fuels and their compo-

200

220

240

260

280

300

320

340

360

380

400

420

440

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Liqu

idTe

mpe

ratu

re(o C

)

BD com

CF_BD

Diesel Fuel

Volume Frac�on Dis�lled

ig. 11. Distillation curves for biodiesel fuels obtained under supercritical trans-sterification (SC-TE) conditions from chicken fat (CF-BD) compared to those ofommercial biodiesel fuel (BD com) and diesel fuel #2 [87] (the fuel volume mea-ured at 25 ◦C and 0.83 MPa).

es of each stream are generated by the model for a given set of input conditions [7].

nents [54,57,71]. The thermal conductivity of methyl oleate as amain component of biodiesel fuels was recently measured over awide range of temperature (27–327 ◦C) and pressure (0.1–60 MPa)[97]. The experimental data were used to develop correlations forthermal conductivity that are estimated to have a relative uncer-tainty of 4% at a 95% confidence level. Work is in progress on trioleinas a representative triglyceride for biodiesel fuel processing.

4.5. Density and speed of sound

The densities of the test biodiesel fuels increase with the degreeof unsaturation [90]. Values of this property for neat biodiesel fuelslay within the range limited in the European standard EN 14214(860–900 kg/m3) [4]. Other important fuel properties that are moredifficult to measure can be correlated with density. Speed of soundin a fluid is an important property, because it is related to theisothermal compressibility. This property is used for EOS devel-opment, and fuel level indicators in many modern aircraft functionvia a speed-of-sound measurement. The isentropic bulk moduli ofpure methyl esters and several biodiesel fuels as a function of thedensity and speed of sound were obtained at elevated pressures[98–100]. It was found that the speed of sound and the isentropicbulk modulus of biodiesel fuels tend to increase as the degree ofunsaturation increases.

4.6. Fit-for-purpose properties

Since biodiesel fuels obtained by SC-TE at T < 350 ◦C do not differsignificantly from those from BAC-TE, their fit-for-purpose proper-ties are similar and must meet the ASTM D6751 specifications [3].Several of these properties are discussed below and included inTable 3 for comparison. For biodiesel fuels obtained at T > 350 ◦C,there are very limited data for most of these properties [31].

4.6.1. Higher heating value (HHV)Because the HHV values of the most common saturated and

unsaturated long-chain FAMEs vary within a narrow range, thetest fuels as well as commercial biodiesel fuels have similar val-ues of this property [88]. More detailed insights on the saturatedand unsaturated biodiesel fuels through fuel testing show that HHVunderwent a slight decrease with the degree of unsaturation [90].

4.6.2. Cetane number
Cetane number is very sensitive to the FAME degree of unsatu-
ration. The variation of the estimated cetane number of test fuelswith their unsaturated index is practically linear [90]. The linearitybetween the degree of unsaturation and cetane number shows that


Table 3Comparative fuel properties linked to fuel combustion quality assessment (averaged values).

Property DF2 B20 B100 SC B100a

Chemical formula ∼C14H27 ∼C15H29O0.4 ∼C19H36O2 ∼C18H34O2

Molecular weight 195 215 296 282Carbon (wt.%) 86.9 85.1 77.3 77Hydrogen (wt.%) 13.1 12.6 11.7 12Oxygen (wt.%) 0.0 2.1 11.0 11Specific gravity 0.856 0.862 0.886 0.880Boiling temperatureb (◦C) 216–343 <343 340–440 230–420Cetane no. (min 40 required) 43.3 46.0 47.5Cloud pointc (◦C) −17 −14 10 4Pour pointc (◦C) −21 −15 1 0Flash point (min 52 required) (◦C) 62 90 146Heat of combustion (MJ/kg) 44.0 41.4 37.2Viscosityd (mm2/s)/40 ◦C 2.7 2.9 4.1 3.8

a Biodiesel fuel obtained by SC-TE of chicken fat with methanol [50].

bsofp4

4

cteFbmtbbttdd

cdpvaiaS

4

wrweioltAB

b The last value is for 90% distilled.c Limits are not imposed but the values must be reported.d A range of 1.9–6.0 mm2/s is required for biodiesel fuels.

iodiesel fuels with an unsaturated index higher than 200 are notuitable as neat fuels for modern diesel engines. A compendiumf experimental cetane number data including oils and biodieseluels is available in the public domain [101]. A recent paper alsorovides the cetane numbers for biodiesel fuels obtain by SC-TE at00 ◦C [31].

.7. Properties from computational techniques

In addition to experimental data for biodiesel esters, thermo-hemical property data estimated by use of various computationalechniques have been recently reported [100,102,103]. These prop-rties include density and speed of sound data of the five primaryAMEs found in soybean/rapeseed biodiesel fuels. The data cane used to develop EOS for the individual FAMEs, and then aodel can be developed to estimate the thermodynamic proper-

ies of ester mixtures. This approach was applied to soy-derivediodiesel fuels. The predictions for density, speed of sound, andubble point were found to agree well with values measured fromwo soy biodiesel fuel samples [96]. Studies on physical proper-ies of biodiesel fuels and individual methyl esters [92,104–106]etermined critical properties, vapor pressure, heat of vaporization,ensity, surface tension, and liquid viscosity.

A wide range of detailed property data for several FAMEs isontained in the Design Institute for Physical Properties (DIPPR)atabase [54,107]. This database includes critical properties, vaporressure, liquid density, liquid surface tension, and latent heat ofaporization, as well as liquid and vapor heat capacities, viscosities,nd thermal conductivities. A brief selection of these propertiess shown in Table 3 along with limited properties determined for

biodiesel fuel obtained from chicken fat with methanol via theC-TE method [50].

.8. Blended PDDF-biodiesel fuels

Although biodiesel fuels (B100) can be directly combustedithin slightly modified engines, this is not a realistic option. For

easons of availability, it is likely that biodiesel fuels will be blendedith PDDFs for the foreseeable future. Hence, more attention is

xpected to be devoted toward the implications to refinery upgrad-ng. In comparison, a biodiesel fuel contains ∼94% of the energyf the same mass of a PDDF, but has a higher cetane number and
ubricity. Typically, 20% (vol/vol) biodiesel fuels are added to PDDFso form a B20 blend. B100 and the PDDF must meet their respectiveSTM specifications before blending [3]. Selected properties for a20 blended fuel are included in Table 3 [108]. Blends of over 20%
biodiesel with diesel fuel should be evaluated on a case-by-casebasis until further experience is available [3–5].

5. Combustion of biodiesel fuels produced by SC-TEmethods

5.1. Overview

The combustion of biodiesel fuels is significantly different fromthat of PDDFs, due to property differences: higher viscosity, lowervolatility, higher cetane number, higher cloud points, etc. This dif-ference is due mainly to the oxygen content of the former fuels andtheir unsaturated hydrocarbon content. While exhaust emissionsfrom biodiesel combustion typically have less particulate matter,due to a higher combustion temperature, this same effect leads tohigher NOx content of the exhaust gases. Particular compositions ofbiodiesel fuels (higher on saturated palmitic/stearic FAMEs) renderpoor behavior of these fuels in cold-weather conditions. However,PDDF-FAME blends mitigate this disadvantage and also lead to amore complete combustion, with reduced CO and PM emissions.

In a SC state, any fuel (including biodiesel fuels) will combustwith higher efficiency and cleaner exhaust emissions than a fuelinjected as a liquid at near ambient temperatures. The emissionbenefit of combusting SC fuels is apparent when compared to con-ventional methods. This advantage may lead to the implementationof a new concept of coupling the SC combustion with the one-step SC-TE process in a diesel engine, as shown in the last section.Computational fluid-dynamic simulations assist the studies on flowvisualization and extend the study of phenomena for which exper-imental techniques are either unavailable or limited.

5.2. Spray/jet characteristics and fuel properties

The spray/jet characteristics of fuels injected into a combustionchamber are major factors affecting combustion phenomena andhence the engine efficiency and emission cleanliness [109]. Themain issue with fuel sprays (for liquid fuels) and jets (for gaseousand SC fuels) is their level of mixing with air under tight engineconditions of very short thermodynamic cycles (on a scale of afew milliseconds). Obviously, in the case of sprays, the mixingis much lower compared to that in jets. Fuel sprays require asequence of complex events such as fuel atomization, heating,
vaporization, diffusion, and reaction with the oxygen in the aircharge. In computational fluid-dynamic models for biodieselfuel combustion, the physical properties of the liquid phaseneeded for fuel spray characterization are density, vapor pressure,


F supec

stnc

fiobsfhiw

rtteOsd

O

Hsd

saaef

∂

Emrvt

ig. 12. Spray/jet shadowgraph images of a liquid fuel (left), fuel–CO2 (center) and aonditions [114]. The total length of the visual field is 6.5 cm.

urface tension, viscosity, thermal conductivity, heat of vaporiza-ion, and heat capacity. The properties of the gas-phase specieseeded are diffusivity, thermal conductivity, viscosity, and heatapacity.

Recent research is devoted toward replacing the combustion ofuel sprays with that of the homogeneous fuel–air mixtures such asn homogeneous charge compression ignition (HCCI) and SC meth-ds [110–113]. Superior ignition-combustion characteristics cane achieved by injecting the fuel in the form of a SC fluid. Fig. 12hows shadowgraph images of a biodiesel fuel injected in open airrom 30 MPa and two temperatures (ambient and SC), taken with aigh-speed digital camera (at 1000 frames/s) [114]. When the fuel is

njected under SC conditions, it achieves the highest level of mixingith air upon injection.

The main parameters affecting sprays (the spray penetrationate and spread angle) are the injector nozzle orifice diame-er, injection pressure and fuel temperature, and ambient gasemperature and pressure (density). Both experimental and mod-ling results are available in the public domain [109–111]. Thehnesorge number, Oh, is often used to describe the disper-

ion of liquids in gases (e.g., spray technology) [115]. It isefined as

h =

(�L)1/2= viscous forces

(inertia × surface tension)1/2(12)

ere, is the dynamic viscosity, � is the liquid density, is theurface tension, and L is the characteristic length scale (typicallyrop diameter).

To simulate the injection of a SC fuel into a SC environment, thepecies, temperature, and velocity distributions were obtained byfinite-volume solution of the species, enthalpy, Navier–Stokes,

nd turbulent energy equations [113]. The time-dependent gov-rning equations written in the cylindrical (z, r) coordinate systemor axisymmetric flow are:

∂�

∂t+ ∂�u

∂z+ ∂�vr

∂r+ �vr

r= 0 (13)

�˚/∂t + ∂�u˚/∂z + ∂�vr˚/∂r = ∂/∂z(� ˚∂˚/∂z)

+ ∂/∂r(� ˚∂˚/∂r) − �vr˚/r + (� ˚/r)∂˚/∂r + S˚ (14)

q. (13) is the continuity equation, and Eq. (14) represents the
omentum, species, or energy equation depending on the variable
epresented by ˚. The symbols u and vr are the axial and radialelocities, respectively, � ˚ are transport coefficients, and S˚ arehe source terms of the governing equations. The fuel exiting the

rcritical fuel–CO2 (right) injected at 30 MPa (upstream pressure) into air at ambient

nozzle with an internal diameter I.D. has Reynolds numbers thatare characteristic of turbulent jet flows:

Re = �u(I.D.)

(15)

The rate of turbulent mass transport is several orders of magnitudegreater than that of the concentration-driven (molecular-diffusive)mass transport with the mass diffusivity D. Thus, a constantSchmidt number of unity can be used for simplicity:

Sc =

�uD= 1 (16)

On the basis of the above observations, it would be desirableto implement an experimental method to determine a fuel qual-ity by studying its sprays/jets. Of particular interest will be to mapthe fuel concentration gradient in the air upon injection. This is,however, a difficult task, due to the harsh conditions in combus-tion chambers. An easier task would be to determine the spray/jetcone angle of the fuels injected into a controlled atmosphere. Asshown in Fig. 12, a larger spray/jet cone angle indicates a betterfuel–air mixing process. This method could complement the volatil-ity property of fuel determined by the advanced distillation curvemethod.

5.3. Biodiesel fuel properties and combustion quality

The quality of fuel combustion is obviously closely connectedwith fuel properties. Under the conditions of an engine cycle, thefuel has to follow a complex sequence of events from fuel injectionto the evacuation of the exhaust gases. The fuel properties neededinclude volatility, viscosity, thermal stability, reactivity with air,ignition delay, ignition temperature, heat of combustion, density,critical P–T, thermal conductivity, rate of heat release, and Cp. Otherphysical properties that are needed to develop models for fuel com-bustion are transport properties (viscosity, thermal conductivity,species diffusivity) of the chemical species in the gas phase. Themost important of these gases are the fuel components and O2,along with the intermediate and final products, such as OH•, H•, O•,CO2 and H2O, of the combustion reaction [116]. Recent progress inimproving knowledge of the physical properties of surrogate com-ponents, their mixtures and real fuels is discussed in the literature[58,117].

The influence of biodiesel fuel properties on injection, spray,and engine characteristics with the aim to reduce harm-ful emissions has been extensively discussed in the literature[1,58,90,111,118,119]. Most of the SC-TE reaction products were


F odiese

rfwaar[cte

5

tttairbw

tgSwoesccvce

6

ipitbSva

ig. 13. Conceptual design of a retrofitted diesel engine running on supercritical bi

efined in order to meet the required specifications for biodieseluels. As a result of this post-processing step, the resulting fuelsere close to those obtained by the BAC-TE methods. Standing

part from these laborious post-processed fuels are those obtainedt ∼400 ◦C due to the thermal decomposition of the polyunsatu-ated FAMEs, mainly to lower molecular esters and hydrocarbons21,24,50–53]. Although there are no experimental data yet for aomprehensive characterization of these fuels, the knowledge inhe field points out to higher combustion efficiency and cleanermissions in this case.

.4. In-engine biodiesel fuel generation and SC combustion

Vegetable oil feedstocks exhibit better behavior at low tempera-ures than that of most of biodiesel fuels. The transition from liquido solid states occurs at lower temperatures for the former, while forhe latter solidification occurs at higher temperatures and includestwo-phase heterogeneous transition [1]. This different behavior

s due to the uniform distribution of the saturated and unsatu-ated FAs in triglyceride molecules in the feedstocks. In cooling ofiodiesel fuels, the saturated and unsaturated FAMEs are separated,ith the former solidifying first.

Based on the above observation, a new method was designedo inject into combustion chambers the SC-TE products that wereenerated in a retrofitted engine fuel system (Fig. 13) [53,112]. TheC-TE products could, in this case, include higher concentrations ofater and glycerol than those imposed by current standards. The oil

r the alcohol could be preheated by being used as a coolant for thengine block. The fluid properties needed for alcohol, lipid feed-tock, oil–alcohol mixtures, and SC-TE reaction products includeritical P–T–� properties, density, viscosity, Reynolds number, heatapacity, thermal conductivity, solubility, thermal stability, andolatility. This approach could solve the emission problems asso-iated with the conventional combustion while improving enginefficiency and TE-associated costs.

. Summary

The connection between fluid properties and process-ng/combustion, leading to optimized steps of biodiesel fuelroduction and combustion, is discussed. The review provides

nformation and state-of-the-art knowledge on feedstock proper-ies, SC-TE reaction characteristics, fit-for-purpose properties of
iodiesel fuels, and SC injection/combustion. Superior results withC-TE processing/combustion are outlined and compared to con-entional methods. The support for this statement is presented inChemCad application of the SC-TE process. Emerging knowledge
l fuel produced in situ by supercritical transesterification (SC-TE) reactions [53].

on fluid properties involved in SC-TE reactions and SC combustionis outlined.

Despite the recent progress, much remains unknown about non-catalytic SC-TE processes. The reaction mechanism, pathways, anddetailed kinetics have not yet been resolved. The phase behaviorof this reacting system is extremely important, and recent litera-ture is focused on this topic [21,72–80]. In early SC-TE studies, veryhigh alcohol-to-oil ratios are used to ensure the existence of a sin-gle phase at reaction conditions. With a better understanding ofthe phase behavior, one could reduce the amount of excess alcoholused and thereby reduce the processing cost. Moreover, advancedunderstanding of the phase behavior would allow adequate reac-tion models to be developed.

Several directions for new or additional research that would leadto important advances in this field are suggested. Among them,a better understanding of the reaction kinetics and phase behav-ior (e.g., mapping liquid-liquid-vapor, liquid-vapor, and SC regionsduring the TE reactions) is needed. There are critical needs forfluid properties versus biodiesel fuel processing (properties versusconditions, reaction, and reactor design) and combustion (prop-erties versus spray characteristics, burning efficiency, pollution,etc.). The SC-TE process requires higher temperatures, pressures,and alcohol-to-oil ratios. Consequently, a process well engineeredfor unspent reactant recycle and energy recovery is a prerequisite.There is a need for additional work on the comparative process eco-nomics so that the precise tradeoffs are better understood. Finally,a new conceptual design of in situ generation of biodiesel fuelscoupled with SC fuel injection and combustion is presented. Suchdesigns may be able to mitigate emissions of conventional combus-tion, while improving engine efficiency and TE-associated costs.

Abbreviation list�rH(T) heat of reaction (reaction enthalpy)�cH◦ heat of combustionCp,i constant-pressure molar heat capacity of a fluid iA, B, C fluid constants in Eq. (12)ADC advanced distillation curveASTM American Society for Testing and MaterialsB100 neat biodiesel fuel (100% biodiesel)B20 blend of 20% biodiesel fuel and 80% petroleum diesel fuel

(vol/vol)ci specific heat capacity of a fluid iCN cetane number
CP cloud pointDF2 diesel fuel #2DG diglycerideDIPPR Design Institute for Physical Properties

percrit

DEEFFFFFFGGHHLmMMNNNPPPPQqR

SSTT

TTUVVxX

Gˇ��˚

A

t

R


SC differential scanning calorimetrya energy of activationOS equation of stateA fatty acidAMEs fatty acid methyl estersAEEs fatty acid ethyl estersFA free fatty acidID flame ionization detectorP flash pointC gas chromatographyP glycerol productsC hydrocarbon(s)CCI homogeneous charge compression ignition

liquid phasemass

G monoglycerideSD mass selective detectorIST National Institute of Standards and TechnologyOx nitrogen oxides (noxes)RTL non-random two-liquid (thermodynamic model)M particulate matterP pour pointR Peng–Robinson–V–T pressure–volume–temperature

heatv0 total volumetric flow rate at the reactor inletEFPROP NIST Reference Fluid Thermodynamic and Transport

Properties database (Standard Reference Database 23)C supercriticalC-TE supercritical transesterificationDC top dead center (the upmost position of an engine piston)DE NIST ThermoData Engine (Standard Reference Database

103)E transesterificationG triglycerideHC unburned hydrocarbons

reactor volume, vapor phaseLE vapor–liquid equilibrium

mass fractionmole fraction

reek symbolsaverage heat flow rate;kinematic viscositydynamic viscosity

i stoichiometric numbersextent of the reaction

, �0 fluid densityresidential timeheat flow rate

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.supflu.2011.11.020.

eferences

[1] G. Knothe, J. van Gerpen, J. Krahl, The Biodiesel Handbook, American OilChemists’ Society (AOCS) Press, Champaign, IL, 2005.

[2] L.T.T. Dinh, Y. Guo, M.S. Mannan, Sustainability evaluation of biodieselproduction using multicriteria decision-making, Environmental Progress &
Sustainable Energy 28 (2009) 38–46.
[3] ASTM-International, Standard Specification for Biodiesel Fuel Blendstock(B100) for Middle Distillate Fuels. ASTM Int. Specif. ASTM D6751, 2008.

[4] http://www.worldenergy.net/products/biodiesel/us specs.php (retrieved02.06.11).

ical Fluids 63 (2012) 133–149 147

[5] http://www.epa.gov/otaq/fuels/renewablefuels/index.htm (retrieved02.06.11).

[6] J. Hill, E. Nelson, D. Tilman, S. Polasky, D. Tiffany, Environmental, economic,and energetic costs and benefits of biodiesel and ethanol biodiesels, Proceed-ings of the National Academy of Sciences of the United States of America 103(2006) 11206–11210.

[7] A. Deshpande, G. Anitescu, P.A. Rice, L.L. Tavlarides, Supercritical biodieselproduction and power cogeneration: technical and economic feasibilities,Bioresource Technology 101 (2010) 1834–1843.

[8] A.H. West, D. Posarac, N. Ellis, Assessment of four biodiesel production pro-cesses using HYSYS Plant, Bioresource Technology 99 (2008) 6587–6601.

[9] Y. Zhang, M.A. Dube, D.D. Mclean, M. Kates, Biodiesel production from wastecooking oil: 2. Economic assessment and sensitivity analysis, BioresourceTechnology 90 (2003) 229–240.

[10] M.J. Haas, A.J. McAloon, W.C. Yee, T.A. Foglia, A process model to estimatebiodiesel production costs, Bioresource Technology 97 (2006) 671–678.

[11] J.M.N.V. Kasteren, A.P. Nisworo, A process model to estimate the cost ofindustrial scale biodiesel production from waste cooking oil by supercrit-ical transesterification, Resources Conservation and Recycling 50 (2007)442–458.

[12] A. Elbheri, W. Coyle, E. Dohlman, H. Kmak, J. Ferrell, Z. Haq, J.Houghton, B. Stokes, M. Buford, W. Nieh, M. Patton-Mallory, D. Perla,L. Draucker, R. Dodder, P.O. Kaplan, D. Okamuro, J. Steiner, H. Shapouri,D. O’Toole, The Economics of Biomass Feedstocks in the United States,Biomass Research and Development Board, Technical Report, 2008, October,http://www.usbiomassboard.gov/pdfs/7 Feedstocks Literature Review.pdf.

[13] K. Arai, R.L. Smith Jr., T.M. Aida, Decentralized chemical processes with super-critical fluid technology for sustainable society, Journal of Supercritical Fluids47 (2009) 628–636.

[14] M. Poliakoff, P. Licence, Sustainable technology: green chemistry, Nature 450(2007) 810–812.

[15] M.O. McLinden, T.J. Bruno, M. Frenkel, M.L. Huber, Standard reference data forthe thermophysical properties of biofuels, ASTM International 7 (2010) 1–18.

[16] G. Anitescu, T.J. Bruno, Liquid biofuels: integrating feedstock selection, pro-cessing, refining, blending, storage and combustion with fluid properties – areview, Energy & Fuels, (2011) doi:10.1021/ef201392s.

[17] T. Pinnarat, P.E. Savage, Assessment of noncatalytic biodiesel synthesis usingsupercritical reaction conditions, Industrial Engineering Chemistry Research47 (2008) 6801–6808.

[18] R. Sawangkeaw, K. Bunyakiat, S. Ngamprasertsith, A review of laboratory-scale research on lipid conversion to biodiesel with supercritical methanol(2001–2009) – review article, Journal of Supercritical Fluids 55 (2010) 1–13.

[19] J-S. Lee, S. Saka, Biodiesel production by heterogeneous catalysts and super-critical technologies, Bioresource Technology 101 (2010) 7191–7200.

[20] S. Glisic, D. Skala, The problems in design and detailed analyses of energyconsumption for biodiesel synthesis at supercritical conditions, Journal ofSupercritical Fluids 49 (2009) 293–301.

[21] G. Anitescu, A. Deshpande, L.L. Tavlarides, Integrated technology for supercrit-ical biodiesel production and power cogeneration, Energy & Fuels 22 (2008)1391–1399.

[22] P. Olivares-Carrillo, J. Quesada-Medina, Synthesis of biodiesel from soybeanoil using supercritical methanol in a one-step catalyst-free process in batchreactor, Journal of Supercritical Fluids 58 (2011) 378–384.

[23] K. Bunyakiat, S. Makmee, R. Sawangkeaw, S. Ngamprasertsith, Continuousproduction of biodiesel via transesterification from vegetable oils in super-critical methanol, Energy & Fuels 20 (2006) 812–817.

[24] W. Iijima, Y. Kobayashi, K. Takekura, K. Taniwaki, The non-glycerol process ofbiodiesel fuel treated in supercritical methanol, ASAE/CSAE Annual Interna-tional Meeting, Ottawa, Ontario, Canada, August 1–4, 2004, Paper no. 046073.

[25] N. Aimaretti, D.L. Manuale, V.M. Mazzieri, C.R. Vera, J.C. Yori, Batch studyof glycerol decomposition in one-stage supercritical production of biodiesel,Energy & Fuels 23 (2009) 1076–1080.

[26] W.B. Schulte, Biodiesel production from tall oil and chicken fat via supercrit-ical methanol treatment, Master thesis, University of Arkansas (2007).

[27] H. He, T. Wang, S. Zhu, Continuous production of biodiesel fuel from vegetableoil using supercritical methanol process, Fuel 86 (2007) 442–447.

[28] M.N. Varma, G. Madras, Synthesis of biodiesel from castor oil and linseed oilin supercritical fluids, Industrial Engineering Chemistry Research 46 (2007)1–6.

[29] I. Vieitez, C. da Silva, G.R. Borges, F.C. Corazza, J.V. Oliveira, M.A. Grompone,I. Jachmanian, Continuous production of soybean biodiesel in supercriticalethanol–water mixtures, Energy & Fuels 22 (2008) 2805–2809.

[30] H. Han, W. Cao, J. Zhang, Preparation of biodiesel from soybean oil using super-critical methanol and CO2 as a co-solvent, Process Biochemistry 40 (2005)3148–3151.

[31] R. Sawangkeaw, S. Teeravitud, K. Bunyakiat, S. Ngamprasertsith, Biofuel pro-duction from palm oil with supercritical alcohols: effects of the alcohol to oilmolar ratios on the biofuel chemical composition and properties, BioresourceTechnology 102 (2011) 10704–10710.

[32] G. Madras, C. Kolluru, R. Kumar, Synthesis of biodiesel in supercritical fluids,Fuel 83 (2004) 2029–2033.

[33] P. Hegel, G. Mabe, S. Pereda, E.A. Brignole, Phase transitions in a biodiesel reac-tor using supercritical methanol, Industrial Engineering Chemistry Research46 (2007) 6360–6365.

http://dx.doi.org/10.1016/j.supflu.2011.11.020

http://www.worldenergy.net/products/biodiesel/us_specs.php

http://www.epa.gov/otaq/fuels/renewablefuels/index.htm

http://dx.doi.org/10.1021/ef201392s

1 percrit
48 G. Anitescu, T.J. Bruno / J. of Su
[34] C.-S. Choi, J.-W. Kim, C.-J. Jeong, H. Kim, K.-P. Yoo, Transesterification kineticsof palm olein oil using supercritical methanol, Journal of Supercritical Fluids58 (2011) 365–370.

[35] J.Z. Yin, M. Xiao, J.B. Song, Biodiesel from soybean oil in supercritical methanolwith co-solvent, Energy Conversion and Management 49 (2008) 908–912.

[36] C.W. Wang, J.F. Zhou, W. Chen, W.G. Wang, Y.X. Wu, J.F. Zhang, R.A. Chi, W.Y.Ying, Effect of weak acids as a catalyst on the transesterification of soybeanoil in supercritical methanol, Energy & Fuels 22 (2008) 3479–3483.

[37] V. Rathore, G. Madras, Synthesis of biodiesel from edible and non-edible oilsin supercritical alcohols and enzymatic synthesis in supercritical carbon diox-ide, Fuel 86 (2007) 2650–2659.

[38] R. Sawangkeaw, K. Bunyakiat, S. Ngamprasertsith, Effect of co-solvents onproduction of biodiesel via transesterification in supercritical methanol,Green Chemistry 9 (2007) 679–685.

[39] E-S. Song, J-W. Lim, H-S. Lee, Y-W. Lee, Transesterification of RBD palmoil using supercritical methanol, Journal of Supercritical Fluids 44 (2008)356–363.

[40] S. Hawash, N. Kamal, F. Zaher, O. Kenawi, G. Diwani, Biodiesel fuel from Jat-ropha oil via non-catalytic supercritical methanol transesterification, Fuel 88(2009) 579–582.

[41] C. Silva, T. Weschenfelder, S. Rovani, F. Corazza, M. Corazza, C. Dariva, J.Oliveira, Continuous production of fatty acid ethyl esters from soybean oilin compressed ethanol, Industrial Engineering Chemistry Research 46 (2007)5304–5309.

[42] S. Saka, D. Kusdiana, Biodiesel fuel from rapeseed oil as prepared in super-critical methanol, Fuel 80 (2001) 225–231.

[43] P.D. Patil, V.G. Gude, S. Deng, Transesterification of camelina sativa oil usingsupercritical and subcritical methanol with cosolvents, Energy & Fuels 24(2010) 746–751.

[44] M.N. Varma, P.A. Deshpande, G. Madras, Synthesis of biodiesel in supercriticalalcohols and supercritical carbon dioxide, Fuel 89 (2010) 1641–1646.

[45] S. Lim, S-S. Hoong, L-K. Teong, S. Bhatia, Supercritical fluid reactive extrac-tion of Jatropha curcas L. seeds with methanol: a novel biodiesel productionmethod, Bioresource Technology 101 (2010) 7169–7172.

[46] P. Valle, A. Velez, P. Hegel, G. Mabe, E.A. Brignole, Biodiesel production usingsupercritical alcohols with a non-edible vegetable oil in a batch reactor, Jour-nal of Supercritical Fluids 54 (2010) 61–70.

[47] I. Vieitez, M.J. Pardo, C. da Silva, C. Bertoldi, F. de Castilhos, J.V. Oliveira,M.A. Grompone, I. Jachmanian, Continuous synthesis of castor oil ethyl estersunder supercritical ethanol, Journal of Supercritical Fluids 56 (2011) 271–276.

[48] C.M. Trentin, A.P. Lima, I.P. Alkimim, C. da Silva, F. de Castilhos, M.A. Mazutti,J.V. Oliveira, Continuous catalyst-free production of fatty acid ethyl estersfrom soybean oil in microtube reactor using supercritical carbon dioxide asco-solvent, Journal of Supercritical Fluids 56 (2011) 283–291.

[49] C-Y. Lin, C-L. Fan, Fuel properties of biodiesel produced from Camellia oleiferaAbel oil through supercritical–methanol transesterification, Fuel 90 (2011)2240–2244.

[50] V.F. Marulanda, G. Anitescu, L.L. Tavlarides, Biodiesel fuels through a contin-uous flow process of chicken fat supercritical transesterification, Energy &Fuels 24 (2010) 253–260.

[51] T. Cong, G. Anitescu, L.L. Tavlarides, Supercritical transesterification of tri-olein as model compound for biodiesel synthesis: a kinetic study, (2012) inpreparation.

[52] V. Marulanda, G. Anitescu, L.L. Tavlarides, Investigations on supercriticaltransesterification of chicken fat for biodiesel production from low-cost lipidfeedstocks, Journal of Supercritical Fluids 54 (2010) 53–60.

[53] G. Anitescu, Supercritical fluid technology applied to the production and com-bustion of diesel and biodiesel fuels, Ph.D. thesis, Syracuse University (2008).

[54] R.L. Rowley, W.V. Wilding, J.L. Oscarson, N.A. Zundel, T.L. Marshall, T.E.Daubert, R.P. Danner, DIPPR Data Compilation of Pure Compound Proper-ties, Design Institute for Physical Properties, New York, NY, 2004 (retrieved08.06.11) http://www.aiche.org/dippr/.

[55] http://aspentech.com/products/aspen-plus.aspx (retrieved 19.05.11).[56] CHEMCAD Version 5.6.4, Chemstations Inc., Houston, TX, 2007, http://www.

chemstations.com/content/documents/Technical Articles/BiodieselWhitePaper.pdf.

[57] E.W. Lemmon, M.L. Huber, M.O. McLinden, NIST Standard Reference Database23, Reference Fluid Thermodynamic and Transport Properties – REFPROP,Version 9.0, Standard Reference Data Program, National Institute of Standardsand Technology, Gaithersburg, MD, 2010.

[58] W.J. Pitz, C.J. Mueller, Recent progress in the development of diesel surrogatefuels, Progress in Energy and Combustion Science 37 (2011) 330–350.

[59] M.L. Huber, E. Lemmon, A. Kazakov, L.S. Ott, T.J. Bruno, Model for the thermo-dynamic properties of a biodiesel fuel, Energy & Fuels 37 (2009) 3790–3797.

[60] G. Mowry, A portable, automated and environmentally friendly biodieselprocessing system, International Journal of Sustainable Energy (2011),doi:10.1080/1478646X.2010.542816.

[61] J. Satyarthi, D. Srinivas, P. Ratnasamy, Estimation of free fatty acid content inoils, fats, and biodiesel by 1H NMR spectroscopy, Energy & Fuels 23 (2009)2273–2277.

[62] W. Chen, C. Wang, W. Ying, W. Wang, Y. Wu, J. Zhang, Continuous production
of biodiesel via supercritical methanol transesterification in a tubular reactor.Part 1: Thermophysical and transitive properties of supercritical methanol,Energy & Fuels 23 (2009) 526–532.
[63] P.T. Pienkos, A. Darzins, The promise and challenges of microalgal-derivedbiofuels, Biofuels, Bioproducts & Biorefining 23 (2009) 43–440.

ical Fluids 63 (2012) 133–149

[64] A. Singh, S.I. Olsen, A critical review of biochemical conversion, sustainabil-ity and life cycle assessment of algal biofuels, Applied Energy 88 (2011)3548–3555.

[65] A.F. Clarens, E.P. Resurreccion, M.A. White, L.M. Colosi, Environmental lifecycle comparison of algae to other bioenergy feedstocks, Environmental Sci-ence & Technology 44 (2010) 1813–1819.

[66] R.B. Levine, T. Pinnarat, P.E. Savage, Biodiesel production from wet algalbiomass through in situ lipid hydrolysis and supercritical transesterification,Energy & Fuels 24 (2010) 5235–5243.

[67] R.F. Service, Algae’s second try, Science 333 (2011) 1238–1239.[68] G. Knothe, Designer biodiesel: optimizing fatty ester composition to improve

fuel properties, Energy & Fuels 22 (2008) 1358–1364.[69] G.W.H. Höhne, W.F. Hemminger, H.-J. Flammersheim, Differential Scanning

Calorimetry, 2nd ed., Springer-Verlag, Berlin/Heidelberg, 2003.[70] H.W. Xiang, A. Laesecke, M.L. Huber, A new reference correlation for the vis-

cosity of methanol, Journal of Physical Chemistry Reference Data 35 (2006)1597–1620.

[71] http://www.chempro.in/palmoilproperties.htm.[72] D.G.B. Boocock, S.K. Konar, H. Sidi, Phase diagrams for oil/methanol/ether

mixtures, Journal of the American Oil Chemists Society 73 (1996) 1247–1251.[73] W. Zhou, S.K. Konar, D.G.B. Boocock, Ethyl esters from the single-phase base-

catalyzed ethanolysis of vegetable oils, Journal of the American Oil ChemistsSociety 80 (2003) 367–371.

[74] W. Zhou, D.G.B. Boocock, Phase behavior of the base-catalyzed transesterifi-cation of soybean oil, Journal of the American Oil Chemists Society 83 (2006)1041–1045.

[75] S. Glisic, O. Montoya, A. Orlovic, D. Skala, Vapor–liquid equilibria oftriglycerides–methanol mixtures and their influence on the biodiesel synthe-sis under supercritical conditions of methanol, Journal of Serbian ChemicalSociety 72 (2007) 13–27.

[76] S. Glisic, D. Skala, Phase transition at subcritical and supercritical conditionsof triglycerides methanolysis, Journal of Supercritical Fluids 54 (2010) 71–80.

[77] H. Zhou, H. Lu, B. Liang, Solubility of multicomponent systems in the biodieselproduction by transesterification of Jatropha curcas L. oil with methanol,Journal of Chemical and Engineering Data 51 (2006) 1130–1135.

[78] D. Geana, R. Steiner, Calculation of phase equilibrium in supercritical extrac-tion of C54 triglyceride (rapeseed oil), Journal of Supercritical Fluids 8 (1995)107–118.

[79] T. Tsuji, M. Kubo, N. Shibasaki-Kitakawa, T. Yonemoto, Is excess methanoladdition required to drive transesterification of triglyceride toward completeconversion? Energy & Fuels 23 (2009) 6163–6167.

[80] Z. Tang, Z. Du, E. Min, L. Gao, T. Jiang, B. Han, Phase equilibria ofmethanol–triolein system at elevated temperature and pressure, Fluid PhaseEquilibria 239 (2006) 8–11.

[81] G.M. Acosta, R.L. Smith Jr., K. Arai, High-pressure PVT behavior of natural fatsand oils, trilaurin, triolein, and n-tridecane from 303 K to 353 K from atmo-spheric pressure to 150 MPa, Journal of Chemical and Engineering Data 41(1996) 961–969.

[82] R.J. Goldstein, Fluid Mechanics Measurements, Hemisphere, Washington DC,1983.

[83] G. Anitescu, L.L. Tavlarides, D. Geana, Phase transitions and thermal behaviorof fuel–diluent mixtures, Energy & Fuels 23 (2009) 3068–3077.

[84] H. He, S. Sun, T. Wang, S. Zhu, Transesterification kinetics of soybean oil forproduction of biodiesel in supercritical methanol, Journal of the American OilChemists Society 84 (2007) 399–404.

[85] M.S. Graboski, R.L. McCormick, Combustion of fat and vegetable oil derivedfuels in diesel engines, Progress in Energy and Combustion Science 24 (1998)125–164.

[86] M. Lapuerta, O. Armas, J. Rodriguez-Fernandez, Effect of biodiesel fuels ondiesel engine emissions, Progress in Energy and Combustion Science 34(2008) 198–223.

[87] G. Anitescu, T.J. Bruno, Characterization of the biodiesel fuel obtained bysupercritical fluid processing by the advanced distillation curve method,(2012) in preparation.

[88] G. Knothe, Dependence of biodiesel fuel properties on the structure of fattyacid alkyl esters, Fuel Processing Technology 86 (2005) 1059–1070.

[89] A.A. Refaat, Correlation between the chemical structure of biodiesel and itsphysical properties, International Journal of Environmental Science and Tech-nology 6 (2009) 677–694.

[90] P. Benjumea, J.R. Agudelo, A.F. Agudelo, Effect of the degree of unsaturationof biodiesel fuels on engine performance, combustion characteristics, andemissions, Energy & Fuels 25 (2011) 77–85.

[91] M.E. Tat, J.H. van Gerpen, The kinematic viscosity of biodiesel and its blendswith diesel fuel, Journal of the American Oil Chemists Society 76 (1999)1511–1513.

[92] W. Yuan, A.C. Hansen, Q. Zhang, Predicting the temperature dependent vis-cosity of biodiesel fuels, Fuel 88 (2009) 1120–1126.

[93] T.J. Bruno, Improvements in the measurement of distillation curves – part 1:a composition-explicit approach, Industrial Engineering Chemistry Research45 (2006) 4371–4380.

[94] L.S. Ott, T.J. Bruno, Variability of biodiesel fuel and comparison to petroleum-
derived diesel fuel: application of a composition and enthalpy explicitdistillation curve method, Energy & Fuels 22 (2008) 2861–2868.
[95] B.L. Smith, L.S. Ott, T.J. Bruno, Composition-explicit distillation curves of com-mercial biodiesel fuels: comparison of petroleum derived fuel with B20 andB100, Industrial Engineering Chemistry Research 47 (2008) 5832–5840.

http://aspentech.com/products/aspen-plus.aspx

dx.doi.org/10.1080/1478646X.2010.542816

percrit

[

[

[

[

[

[

[


[96] M.L. Huber, E.W. Lemmon, A. Kazakov, L.S. Ott, T.J. Bruno, Model for thethermodynamic properties of a biodiesel fuel, Energy & Fuels 23 (2009)3790–3797.

[97] R.A. Perkins, M.L. Huber, Measurement and correlation of the thermal con-ductivities of biodiesel constituent fluids: methyl oleate and methyl linoleate,Energy & Fuels 25 (2011) 2383–2388.

[98] M.E. Tat, J.H. van Gerpen, S. Soylu, M. Canakci, A. Monyem, S. Wormley, Thespeed of sound and isentropic bulk modulus of biodiesel at 21 ◦C from atmo-spheric pressure to 35 MPa, Journal of the American Oil Chemists Society 77(2000) 285–289.

[99] M.E. Tat, J.H. van Gerpen, Speed of sound and isentropic bulk modulus of alkylmonoesters at elevated temperatures and pressures, Journal of the AmericanOil Chemists Society 80 (2003) 1249–1256.

100] A. Osmont, L. Catoire, P. Dagaut, Thermodynamic data for the modeling ofthe thermal decomposition of biodiesel – 1. Saturated and monounsaturatedFAMEs, Journal of Physical Chemistry A 114 (2009) 3788–3795.

101] M.J. Murphy, J.D. Taylor, R.L. McCormick, Compendium of ExperimentalCetane Number Data. Technical Report NREL/SR-540-36805, National Renew-able Energy Laboratory, 2004, September.

102] M. Lapuerta, J. Rodríguez-Fernández, F. Oliva, Determination of enthalpy offormation of methyl and ethyl esters of fatty acids, Chemistry and Physics ofLipids 163 (2010) 172–181.

103] L.S. Ott, M.L. Huber, T.J. Bruno, Density and speed of sound measure-ments on five fatty acid methyl esters at 83 kPa and temperatures from(278.15 to 338.15) K, Journal of Chemical and Engineering Data 53 (2008)2412–2416.

104] W. Yuan, A.C. Hansen, Q. Zhang, Predicting the physical properties ofbiodiesel for combustion modeling, Transactions of the ASAE 46 (2003)1487–1493.

105] W.Q. Yuan, A.C. Hansen, Q. Zhang, Z.C. Tan, Temperature-dependent kine-
matic viscosity of selected biodiesel fuels and blends with diesel fuel, Journalof the American Oil Chemists Society 82 (2005) 195–199.
106] W. Yuan, A.C. Hansen, Q. Zhang, Vapor pressure and normal boiling pointpredictions for pure methyl esters and biodiesel fuels, Fuel 84 (2005)943–950.

ical Fluids 63 (2012) 133–149 149

[107] T.E. Daubert, R.P. Danner, H.M. Sibul, C.C. Stebbins, R.L. Rowley, W.V.Wilding, Physical and Thermodynamic Properties of Pure Chemicals, 2011,http://dippr.byu.edu/.

[108] National Biodiesel Board, Specification for Biodiesel Blends B6-B20,ASTM 7467-10, 2011 (retrieved 17.08.11) http://www.biodiesel.org/pdffiles/fuelfactsheets/B20 Specification.pdf.

[109] L.M. Pickett, C.L. Genzale, G. Bruneaux, L.-M. Malbec, L. Hermant, C. Chris-tiansen, J. Schramm, Comparison of diesel spray combustion in differenthigh-temperature, high-pressure facilities, SAE International 2010-01-2106,1–40.

[110] R. Reitz, D. Arcoumanis, T. Kamimoto, B. Johannson, Special Issue on homoge-neous charge compression ignition engines, International Journal of EngineResearch 8 (2007) 1–3.

[111] S. Um, S.W. Park, Numerical study on combustion and emission characteristicsof homogeneous charge compression ignition engines fueled with biodiesel,Energy & Fuels 24 (2010) 916–927.

[112] G. Anitescu, R-H. Lin, L.L. Tavlarides, Preparation, injection and combustionof supercritical fuels; Poster P-2 presented at Directions in Engine-Efficiencyand Emissions Research (DEER) Conference, Dearborn, MI, August 3–6, 2009.

[113] T. Doungthip, J.S. Ervin, T.F. Williams, J. Bento, Studies of injection of jet fuel atsupercritical conditions, Industrial Engineering Chemistry Research 41 (2002)5856–5866.

[114] G. Anitescu, Syracuse University, unpublished data.[115] W. Ohnesorge, Formation of drops by nozzles and the breakup of liquid jets,

Journal of Applied Mathematics and Mechanics 16 (1936) 355–358.[116] A.T. Holley, X.Q. You, E. Dames, H. Wang, F.N. Egolfopoulos, Sensitivity of

propagation and extinction of large hydrocarbon flames to fuel diffusion,Proceedings of the Combustion Institute 32 (2009) 1157–1163.

[117] O. Herbinet, W.J. Pitz, C.K. Westbrook, Detailed chemical kinetic mechanismfor the oxidation of biodiesel fuels blend surrogate, Combustion and Flame
157 (2010) 893–908.
[118] B. Kegl, Effects of biodiesel on emissions of a bus diesel engine, BioresourceTechnology 99 (2008) 863–873.

[119] H.J. Curran, P. Gafuri, W.J. Pitz, C.K. Westbrook, A comprehensive modelingstudy on n-heptane oxidation, Combustion and Flame 114 (1998) 149–177.

1-s

Documents

Transcript of 1-s