Phase Transitions and Thermal Behavior of Fuel−Diluent Mixtures

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Phase Transitions and Thermal Behavior of Fuel-Diluent Mixtures George Anitescu,*,† Lawrence L. Tavlarides,† and Dan Geana‡ Department of Biomedical and Chemical Engineering, Syracuse UniVersity, Syracuse, New York 13244, and Department of Applied Physical Chemistry and Electrochemistry, UniVersity Polytechnica, Bucharest, Romania ReceiVed February 18, 2009. ReVised Manuscript ReceiVed April 23, 2009

Phase transitions and thermal behavior of fuel-diluent mixtures were investigated at temperatures and pressures up to 750 K and 600 bar, respectively, as part of a breakthrough approach directed to mitigate the harmful emissions of diesel fuel combustion and provide simultaneous benefits on the engine efficiency and environmental thermal impact. (Anitescu, G. PhD Thesis, Syracuse University, Syracuse, 2008. Tavlarides, L. L.; Anitescu, G. US Patent No. 7,488,357, 2009.) The mole fraction of fuels (e.g., n-hexadecane or cetane and diesel fuel No. 2) was in the range of 0.100-0.786 in mixtures with chemically inert diluents such as CO2 and mixtures of CO2, H2O, and N2 as substitutes for exhaust gases of diesel engines. Most of the experiments were conducted in laboratory flow and batch reactors equipped with view cells to visualize the phase transitions associated with the heating processes of the fuel-diluent mixtures. The Soave-Redlich-Kwong equation of state with one-parameter conventional mixing rule was used to construct P-T phase diagrams to identify the conditions for the experiments carried out with no view cell included in the laboratory setups. The extent of thermal decomposition of both cetane and diesel fuel No. 2 was relatively low in the presence of the selected diluents, proving the role of these additives as anticoking agents in the newly proposed diesel engine applications of the heated fuels. The major reaction products of cetane thermolysis, identified by GC-MS analytical technique, were all pairs of C8-C14 n-alkanes and homologue R-olefins. Only under highest P-T-X conditions were some polycyclic aromatic hydrocarbons and higher molecular alkanes produced in low amounts.

Introduction The thermal stability of diesel fuels is an important issue for smooth and efficient engine operation during the fuel widespread use in compression-ignition engines for transportation, power generation, and other industrial applications. Diesel fuel reactivity is higher compared to that of gasoline, which allows selfignition of the former fuel at the end of the compression step of the thermodynamic engine cycle. This higher reactivity is mainly due to the presence in fuel compositions of n-alkanes with long molecular chains (from ∼C10H22 to C20H42). The buildup of solid deposits on fuel injectors is one of the problems of concern attributed to fuel thermal instability. In addition, there are other important problems with the conventional combustion of diesel fuels such as high emissions and relatively low efficiency. A potential breakthrough approach directed to nearly eliminate the harmful NOx and particulate matter (PM) emissions of diesel fuel combustion and concomitantly increasing the engine efficiency (up to 55%) and mitigating environmental thermal impact was recently proposed.1,2 The new method consists of injecting near- or supercritical (SC) fuel-diluent mixtures in the combustion chambers of diesel engines. It is expected that these fluids, heated by one third to one half of the exhaust gas heat content, will near instantaneously mix with SC air upon * Corresponding author. E-mail: [email protected]; phone: (315)-4431917; fax: 315-443-9175. † Syracuse University. ‡ University Polytechnica. (1) Anitescu, G. Supercritical Fluid Technology Applied to the Production and Combustion of Diesel and Biodiesel Fuels. Ph.D. Thesis, Syracuse University: Syracuse, 2008. (2) Tavlarides, L. L.; Anitescu, G. US Patent No. 7,488,357, 2009.

injection and then the fuel will autoignite and combust in a homogeneous fuel-air charge. The temperature spikes and fuelrich regions observed in conventional combustion of fuel droplets can thus be essentially eliminated and consequently leading to PM and NOx emission reduction, higher efficiency, and a net cutback in carbon dioxide (CO2) generation. Also, the aftertreatment filtering and catalytic devices will become obsolete. By minimizing the need of excess air, the engine power density is increased and a significant amount of environmental air is spared. The individual steps of the proposed method, which are based on proven processes, are arranged in a new sequence that is designed specifically for in situ preparation, injection, and combustion of SC fuels. If only neat diesel fuel will be used, it will coke before reaching SC states.1,3-5 The use of a diluent with diesel fuels will permit heating these mixtures to SC states prior to injection in a combustion chamber.1,2 Among others, particularly attractive diluents are the ubiquitous, relatively inert at moderate temperatures, and environmentally compatible water and CO2. Since these compounds are components of engine exhaust gas recycled (EGR), EGR appears to be the best choice as diluent in the process of bringing diesel fuels to SC states. The properties of SC diesel fuels are difficult to obtain due to structural complexity and thermal decomposition before reaching SC conditions.6-12 However, the fuel components can (3) Rahmani, S.; McCaffrey, W. C.; Dettman, H. D.; Gray, M. R. Energy Fuels 2003, 17 (4), 1048–1056. (4) Miwa, K.; Mohammadi, A.; Kidoguchi, Y. Intl. J. Engine Research, 2001, 2 (3), 189–198. (5) Scharnweber, D. H.; Hoppie, L. O. SAE Paper No. 850089; 1985. (6) Nikitin, E. D.; Pavlov, P. A.; Skripov, P. V. J. Chem. Thermod. 1993, 25 (7), 869–80.

10.1021/ef900141j CCC: $40.75  2009 American Chemical Society Published on Web 05/21/2009

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be grouped into structural classes and a selection of surrogate model compounds is often used to deal with this complexity.13 Straight-chain n-alkanes are the main components of diesel fuels and are more easily ignited under engine conditions than branched-chain or aromatic compounds. The reference compound for diesel fuels is n-hexadecane (cetane), the fuel that defines 100 cetane number (easy ignition).13-15 The choice of cetane in this study was based on several factors: (i) critical temperature, molecular weight, and other properties close to those of diesel fuel No. 2; (ii) major component of diesel fuel No. 2, positioned in the middle of n-alkanes class; (iii) a conservative approach in respect to coking of diesel fuel components during moderate heating (the heavier alkanes are more prone to coking); and (iv) representative model fuel compound for the combustion of diesel fuels.13 Cetane was proved an acceptable substitute to study important properties of diesel fuels such as phase equilibria, transport properties, and chemical reactivity.16-18 However, conducting experiments with this fuel at high temperatures is challenging due to coking processes. By adding a diluent such as CO2 or EGR, these processes can be eliminated, at least at moderately high temperatures (e.g., up to ∼800 K).1,3,19 P-T phase diagrams for different compositions of the fuel-diluent mixtures are needed to define the phase boundaries of the mixtures of interest. Many data sets and theoretical models are available for cetane-diluent systems in different temperature and pressure ranges, although some of them seem inconsistent with the rest.20-30 We used the information reported in these references for cetane-CO2 mixtures covering the temperature

rangefrom298to673KtoparametrizetheSoave-Redlich-Kwong (SRK) equation of state (EOS) to construct our phase diagrams. In turn, we used these diagrams to guide the experiments with much more complex diesel fuel-EGR mixtures to establish the corresponding homogeneous/heterogeneous phase regions. By using SRK EOS, P-T diagrams have been constructed for binary, ternary, and quaternary cetane-diluent mixtures. Also, experimental P-T data have been acquired and placed on the theoretical P-T frames to confirm the accuracy of theoretical approaches. The vaporization of fuel droplets under known SC conditions in the combustion chambers of diesel engines was extensively studied.31,32 For n-pentane through n-dodecane, a vaporizing droplet can reach the critical state for ambient temperatures and pressures greater than approximately twice the critical values of the fuel.31 These results show the possibility of a rapid fuel transition from liquid to SC states in a time-scale of a few milliseconds. However, if the fuel has to be vaporized in a preinjection step, then the time will be longer at lower temperatures accommodated by the fuel system, thus a diluent will be required to avoid coking. Guided by the P-T diagrams of the fuel-diluent mixtures, both batch and flow experiments were carried out to study the P-T-X conditions under which phase regions and phase transitions of interest occurred. The fluids subjected to these experiments were then chromatographically characterized to determine the extent of the fuel thermal decompositions at these conditions.

(7) Scheidgen, A. L.; Schneider, G. M. Fluid Phase Equilib. 2002, 194197, 1009–1028. (8) Charoensombut-Amon, T.; Martin, R. J.; Kobayashi, R. Fluid Phase Equilib. 1986, 31, 89–104. (9) Tanaka, H.; Yamaki, Y.; Kato, M. J. Chem. Eng. Data 1993, 38, 386–388. (10) NIST. Thermophysical Properties of Fluid Systems; URL: http:// webbook.nist.gov/chemistry/fluid/. (11) Anselme, M. J.; Gude, M.; Teja, A. S. Fluid Phase Equilib. 1990, 57 (3), 317–26. (12) Fenghour, A.; Trusler, J. P. M.; Wakeham, W. A. Fluid Phase Equilib. 2001, 185 (1-2), 349–358. (13) Westbrook, C. K.; Pitz, W. J.; Herbinet, O.; Curran, H. J.; Silke, E. J. Combust. Flame 2009, 156 (1), 181–199. (14) Batts, B. D.; Fathoni, A. Z. Energy Fuels 1991, 5 (1), 2–21. (15) Farrell, J. T.; Cernansky, N. P.; Dryer, F. L.; Law, C. K.; Friend, D. G.; Hergart, C. A.; McDavid, R. M.; Patel, A. K.; Mueller, C. J.; Pitsch, H. Society of AutomotiVe Engineers, SP-2100 (Homogeneous Charge Compression Ignition Engines) 2007, pp 249–277. (16) Watanabe, M.; Tsukagoshi, M.; Hideyuki Hirakoso, H.; Tadafumi Adschiri, T.; Arai, K. AIChE J. 2000, 46 (4), 843–856. (17) Khorasheh, F.; Gray, M. R. Ind. Eng. Chem. Res. 1993, 32 (9), 1853–63. (18) Depeyre, D.; Flicoteaux, C. Ind. Eng. Chem. Res. 1991, 30 (6), 1116–30. (19) Karakas, G.; Dogu, T.; Tarik, G. S. Ind. Eng. Chem. Res. 1997, 36 (11), 4445–4451. (20) Scheidgen, A. Fluid Phase Equilibria of Binary and Ternary Carbon Dioxide Mixtures with Low-volatile Organic Substances up to 100 Mpa — Cosolvency Effect, Miscibility Windows and Holes in the Critical Surface. Ph.D. Thesis, Ruhr-University: Bochum, 1997. (21) Spee, M.; Schneider, G. M. Fluid Phase Equilib. 1991, 65, 263– 274. (22) Garcia, J.; Lugo, L.; Fernandez, J. Ind. Eng. Chem. Res. 2004, 43, 8345–8353. (23) Sebastian, H. M.; Simnick, J. J.; Lin, H.-M.; Chao, K.-C. J. Chem. Eng. Data 1980, 25, 138–140. (24) Virnau, P.; Muller, M.; MacDowell, L. G.; Binder, K. Comput. Phys. Commun. 2002, 147 (1-2), 378–381. (25) Virnau, P.; Muller, M.; MacDowell, L. G.; Binder, K. J. Chem. Phys. 2004, 121 (5), 2169–2179. (26) Polishuk, I.; Wisniak, J.; Segura, H. J. Phys. Chem. B 2003, 107 (8), 1864–1874. (27) Polishuk, I.; Wisniak, J.; Segura, H. Chem. Eng. Sci. 2003, 58 (12), 2529–2550.

Experimental Section Phase transitions from ambient P-T conditions to SC regions of fuel mixtures are associated with high mechanical and thermal stress for the vessels housing these fluids and, consequently, are challenging to monitor and characterize by common procedures. The most-employed method in this regard is to use view cells coupled with adequate photographic systems. However, it is well established that at high temperatures the materials of different expansion coefficients used to contain reactive streams (e.g., 316SS and sapphire/quartz) expand significantly different, leading to leaks or mechanical damage. To overcome these problems, special view cells for our experimental SC conditions (up to 600 bar and 750 K) were designed, constructed, tested, and used for these studies at Syracuse University.1 View Cell Design. The most useful tool for examining phase transitions is a view cell, whose contents can be viewed with a video camera through transparent sapphire windows. Basically, our view cells have been constructed from available components in the market: 316SS crosses and fittings (High Pressure Equipment Company), sapphire windows (Meller Optics, Inc.), Bellville washers (Key Bellevilles, Inc.), and gold gaskets (Scientific Instrument Services). A K-type thermocouple (Omega) was inserted by silver-welding in the view cell block to be in contact with the fluids. Work to retrofit the cross block to house the sapphire windows was done at the Syracuse University mechanical shop. A view cell is shown in Figure 1 and can be used in any position although a vertical flow is preferred to avoid accumulation of denser fluids at the bottom of the optical chamber. (28) Galindo, A.; Blas, F. J. J. Phys. Chem. B 2002, 106 (17), 4503– 4515. (29) Horstmann, S.; Fischer, K.; Gmehling, J.; Kolar, P. J. Chem. Thermod. 2000, 32 (4), 451–464. (30) Brunner, G.; Teich, J.; Dohrn, R. Fluid Phase Equilib. 1994, 100, 253–68. (31) Givler, S. D.; Abraham, J. Prog. Energy Combust. Sci. 1996, 22, l-28. (32) Ra, Y.; Reitz, R. D. Int. J. Multiphase Flow 2009, 35 (2), 101– 117.

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Figure 1. Schematic of the shadowgraph optical arrangement (left) with a photograph of the view cell (right). Heating cartridges and a thermocouple are shown on the left and on the right sides of the view cell, respectively.

Optical System. To acquire reliable information on fluids of interest in either batch or flow modes for adequate characterization of phenomena associated with fluid phase transitions to SC domains, two optical arrangements have been set up and tested: shadowgraph and schlieren. The former arrangement (Figure 1) proved to be more convenient for our studies since the latter was affected by the temperature gradients around the heated view cell. A high-speed video camera system was used to record up to 2000 frames/s (fps) under highest resolution (512 × 512) through the visual field (6.75 mm in diameter) of the view cell (FASTCAM 512 PCI; Photron Limited). The imaging data were processed with the Photron FASTCAM viewer software. Higher fps values were also available but with lower resolutions (max. 32 000 fps at 512 × 32 pixels). The view cell was heated by four cartridges housed within the cell block. In the shadowgraph images, 2D spatial variations of the refractive index n (as second derivatives) of the fluid in the visual field are caused by density (F) gradients:33

(

)

n0 - 1 ∂2 F ∂2n ) F0 ∂y2 ∂y2

(1)

where n0 and F0 refer to a standard fluid state. For a gas phase at constant pressure with the Gladstone-Dale constant C ) (n0 - 1)/ F0, if the density changes are induced by temperature gradients, then the second spatial derivative of n is:33

[

( )]

F ∂2T 2F ∂T ∂2n )C + 2 2 T ∂y2 ∂y T ∂y

2

(2)

Since n is a function of the thermodynamic state, the visual effects induced by its space variations are particularly associated with fluid phase transitions. Therefore, when the striations disappear in the visual field, a state of thermodynamic equilibrium is reached without temperature or density gradients. Batch Experiments. For easy monitoring of phase transitions from liquid or liquid-vapor to SC states and for miscibility studies, batch-type experiments were preferred. For this purpose, solutions with known synthetic compositions were charged in the view cell and subjected to heat sources under the appropriate heat flux. Change of position of the meniscus separating two phases or meniscus disappearance when a transition from two-phase system to a SC state occurred was conveniently monitored, and the associated P-T conditions were recorded. Flow Experiments. To closely mimic the real-world processes taking place in a fuel delivery system of a diesel engine, flow experiments were carried out with the view cell connected at the outlet end of the mixing pipe as shown in Figure 2. As temperature gradients can play a significant role on refractive index according to eq 2, the view cell was also positioned inside the electrical (33) Goldstein, R. J. Hemisphere 1983, 377–397.

Figure 2. A schematic of the laboratory apparatus for mixing studies and phase transitions at high temperatures and pressures. Thermocouple location is shown by the T1-T5 positions.

furnace with the upstream coil, and the temperature along the mixing pipe and in the view cell was carefully monitored. Figure 2 shows five thermocouples inserted in different positions of the laboratory setup. The desired pressure in the system was manually controlled by a micrometric valve (MMV) positioned on the exit stream line. The fuel was collected in L-G separators and samples were prepared for chromatographic analysis. The residence time τ of the fluid mixture flowing through the mixing pipe was calculated by dividing the volume V of the pipe by the total volumetric flow rate V of the fuel-diluent stream at the P-T conditions:

τ ) V/ν ) V/(ν0 × F0 /F)

(3)

where V0 is the total volumetric flow rate at the pump (V0,cetane + V0,CO2), and F0 and F are the experimental average densities of the mixture at pump conditions and in the reactor, respectively.

Results and Discussion P-T Phase Diagrams from Equations of State (EOS). Initialization of the SRK EOS with one-parameter classical mixing rule (1-PCMR) has been performed based on the reported data for the binary system cetane-CO2.20-30 This EOSbased model (see Appendix for more details) was used to predict the complex phase behavior (critical curve, bubble- and dewpoint curves). Emphasis was on the high temperature portion of the isoplets, with the details of the phase regions near the ambient temperatures being somewhat neglected. The calculations were made using the software package Phase Equilibria

Phase Transitions of Fuel-Diluent Mixtures

Figure 3. A P-T phase envelope for the binary system cetane (C16)-CO2 with two values of the binary interaction parameter k12.

(PHEQ),34 developed in the Department of Applied Physical Chemistry (University Polytechnica, Bucharest). The critical curves were calculated using the method proposed by Heidemann and Khalil,35 with analytical derivatives for fugacity.36 For all calculations, the theory and the computational aspects are presented in the ref 37. The topography of the phase envelope (bubble- and dew-point curves, or isopleth) is dependent on the binary interaction parameter k12 as outlined in Figure 3. In other phase diagrams, k12 was adjusted to force the points of the diagram to pass through available known values such as critical points. The value of k12 ) 0.1 for the SRK with 1PCMR model was obtained by comparing the calculated critical curve to that available from reported experimental data.20 To predict the behavior of multicomponent mixtures, the interaction parameters kij can be estimated from data of each binary subsystem. However, such data are not available for our multicomponent systems, and we choose kij ) 0 as a first approximation. Interestingly, the homologous series of CO2 + n-alkane binary mixtures fall into four of the six types of phase behavior26,27 in Scott-van Konynenburg classification.38 Mixtures of CO2 with short n-alkanes, up to n-hexane, exhibit type I phase behavior. Binary systems of CO2 with n-alkanes from n-heptane up to n-dodecane show phase behavior of type II. Mixtures of CO2 with n-tetradecane and with longer n-alkanes, including nhexadecane, exhibit type III phase behavior, whereas CO2 + n-tridecane mixtures correspond to type IV. These different types of P-T diagrams mainly refer to the shapes of the critical lines for binary systems. The differences are more pronounced for the low temperature regions, which are not of interest for this study. (34) Geana˜, D.; Rus, L. Proceedings of the 14thRomanian International Conference on Chemistry and Chemical Engineering, Bucharest, Romania, 2005; pp 170-178. (35) Heidemann, R. A.; Khalil, A. M. AIChE J. 1980, 26, 769–779. (36) Michelsen, M. L.; Heidemann, R. A. AIChE J. 1981, 27, 521–523. (37) Geana˜, D.; Feroiu, V. Equations of State: Applications to Fluid Phase Equilibria; Tehnica: Bucharest, 2000. (38) van Konynenburg, P. H.; Scott, R. L. Phil. Trans. R. Soc. London 1980, 298, 495–540.

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Figure 4. A P-T phase diagram with four phase envelopes (isopleths) for binary system cetane-CO2 calculated with SRK EOS by using 1-PCMR and interaction parameter k12 ) 0.1. The critical points 1 and 2 are for CO2 (1) and for cetane (2) and the marked phases (L, L-V, V, and SC) are for X2 ) 0.2 only.

Figure 4 shows a P-T diagram of type III according to Scott-van Konynenburg classification for the binary system cetane-CO2 with four phase envelopes (isopleths). Each isopleth exhibits an upper and a lower branch, the bubble-point and the dew-point curves, respectively, which define the L-V binary phase envelope. In this figure, conventional L, V, L-V, and SC regions are marked only for the isopleth of XHD ) 0.2. This diagram was calculated with a SRK EOS with 1PCMR and the value of the binary interaction parameter k12 ) 0.1. In Figure 4 is also shown the critical curve of the system which is the loci of critical P-T values for all of the mixture compositions. Both pressure and temperature exhibit maxima for each isopleth (maxcondenbar and maxcondentherm, respectively). For pressures beyond the critical value, above the bubble-point curves there is only a liquid phase up to the critical temperature or a conventional SC region at T > Tc. Between the bubble- and the dew-point curves the system is a mixture of liquid and vapor phases with different compositions, and below the dew-point curves the system is monophasic, in a vapor phase. Somewhat misleading is the use of terms liquid and vapor (or gas) beyond the critical curve (L ) V), more appropriate being the terms liquid- and vapor- (gas) like density regions. Figure 5 shows two P-T phase envelopes (isopleths) for two compositions of the ternary and quaternary mixtures cetane(0.6)-CO2(0.1)-N2(0.3) and cetane(0.48)-CO2(0.07)-N2(0.38)-H2O(0.07), respectively. Due to higher volatility of the mixtures induced by N2, the pressure on the bubble-point curves is higher than that for the similar isopleth of the cetane-CO2 diagram. However, these systems can be still easily brought to a liquid phase (before transitions to SC phases) for temperatures up to Tc if the system pressure is above ∼215 and 300 bar, respectively. For the quaternary mixture, the diluent molar composition was closer to that of EGR (74% N2, 13.5% CO2, and 12.5% H2O) resulting from the reaction: CH1.85 + 6.96(0.21O2 + 0.79N2) f CO2 + 0.925H2O + 5.50N2 (4)

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Figure 5. P-T phase envelopes (isopleths) calculated with SRK EOS (1PCMR; kij ) 0): (1) ternary system cetane (0.6)-CO2(0.1)-N2(0.3) and (2) quaternary mixture cetane(0.48)-CO2(0.07)-N2(0.38)H2O(0.07).

written with a simplified, theoretical, diesel fuel surrogate (CH1.85). Disregarding the possible cetane-water immiscible phases at low temperatures, which are not of interest for SC combustion applications, the diagram shows the phase boundary and L, L-V, V, and SC regions but with a similar topography of bubble- and dew-point curves as that of cetane-CO2 mixtures. It is important to observe that beyond ∼300 bar the above mixtures are in a liquid phase for all of subcritical temperatures. The thermodynamic states of a fluid can be characterized by 3D P-V-T representations. Due to the difficulties both in construction and interpretation of these diagrams for multicomponent systems, the 2D sections such as P-T or P-V are preferred for given compositions. P-T Phase Diagrams from Experimental Data. Three sets of different compositions, each of 11-16 experimental P-T curves of different densities, have been generated from the data acquired with a high-temperature, high-pressure cell. Along each of these curves of given composition and density can be located the points of phase change from L-V to L states. Since these points can be positioned on P-T curves of different compositions, a locus of an isopleth of the bubble-point curve can be generated as shown in the above P-T diagrams. An example of comparison of these diagrams is shown in Figure 6 for three experimental P-T lines overlapped with a calculated P-T phase diagram. The experimental lines exhibit two segments of different slopes for L-V and for L phases (lower and the upper end, respectively). The global composition is the same for all of these lines (Xcetane ) 0.40) but the density is different (0.57, 0.51, and 0.37 g/cm3 from left to right). The points A, B, and C of the phase change are placed well on the calculated bubble-point isopleth of the same composition. Because the transition from the L to the SC region through the point C′ is not associated with a discontinuity, this behavior would permit accurate injections of these fuel mixtures into a diesel engine either before or after C′ (liquid and SC, respectively).

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Figure 6. P-T data for the binary system cetane(2)-CO2(1): experimental (the three thick lines of X2 ) 0.4 and different densities) and the isopleths calculated with SRK EOS (0.2-0.8 cetane mole fraction). The (L-V) to (L) transition points A, B, and C are positioned on the calculated isopleth of X2 ) 0.4.

Due to prohibitively long experimental time needed to acquire a high number of points required to build full isopleths of a P-T phase diagram, it is more advantageous to construct P-T phase diagrams with a selected EOS and adjust their topography with a few experimental points. Nevertheless, these phase diagrams, calculated based on selected available data17-27 and confirmed by our experiments, have been extensively used throughout this study to guide our attention on different potential points of interest. The focus on this aspect was the sequence of diesel fuel-diluent phase changes from initial L-V to L or V and, finally, to SC regions of interest. Phase Transitions Involving SCFs. To ensure that desirable transitions occur from the initial L-V to a one phase region (L or SC) of fuel-diluent mixtures in a synchronized tandem within limited space and time of diesel engine cycles, a comprehensive, yet detailed, characterization is required of the phenomena associated with these transitions. A fluid in a liquid state can be brought to a SC state by different but simple ways such as: (1) isothermal compression above Pc followed by an isobaric heating; (2) isobaric heating above Tc, followed by an isothermal compression; and (3) heating to T > Tc associated with continuous pressure increase to P > Pc. Comparably, phase transitions in multicomponent systems are quite intricate. This aspect can be exemplified, schematically, by an isochoric heating of a binary system (e.g., cetane-CO2) which exhibits a P-T diagram of type III as shown in Figure 6. If a starting point C0 is assumed positioned in a liquid-vapor region of a given composition of 0.4 mol fraction of cetane, the bubble curve is crossed in point C with increasing temperature, where vapors condense and the system transitions to the liquid region. At some point further on the liquid P-T curve of higher slope compared to that of L-V, the system can cross the temperature of the mixture at point C′ into the SC region. Such P-T diagrams can be constructed for fuel-diluent mixtures of various compositions and densities, as shown in Figure 7. With the information from these composition-density maps at different pressures and temperatures, one can design a fuel system to function under desired thermodynamic conditions in a retrofitted diesel engine by avoiding the coking region (at

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Figure 9. Selected images of cetane-CO2 (0.5 mL/min, each; Xcetane ) 0.2) mixing experiments carried out at 300 bar and heating the flowstream from 313 to 700 K. The first image stands for CO2(SC). The critical temperature of the mixture from Figure 6 is ∼645 K. (The visual field is of 6.75 mm in diameter.)

Figure 7. An experimental P-T diagram mapping the liquid, liquid-vapor, and SC regions for diesel fuel-CO2 mixtures with different compositions and densities (numbers in red and black, respectively). The fuel may coke for temperatures higher than ∼750 K (at the right of the red line).

Figure 10. Phase transitions in diesel fuel-EGR mixtures (Vfuel ) 1.5 mL/min; VEGR ) 1.9 mL/min; Xfuel ) 0.28) from 628 K and 101 bar (SC EGR only) to liquid-vapor fluids (next four photographs, from left to right), and then to a homogeneous, single phase of SC diesel fuel-EGR mixture at 664 K and 232 bar. In a central position is a 0.2 mm thick wire for image focusing.

Figure 8. Selected images of cetane (C16)-CO2 mixtures flowing in a view cell with a 6.75 mm visual field at 250 bar and 623 K.

the right of the red line in Figure 7). For example, a composition of 0.52 mol fraction of SC diesel fuel-CO2 mixture with a density of 0.49 g/cm3 can be injected at 600 bar and 700-750 K. Transitions from the liquid to a SC region can be monitored with a view cell as shown in the next figures. Figures 8-12 show selected images of various compositions of cetane/diesel fuel-CO2/EGR mixtures heated under different P-T conditions in both batch and flow modes. Figure 8 is a set of six images selected from experiments performed in a flow mode at 250 bar and 623 K with the binary system cetane-CO2. The calculated Reynolds numbers and residence times for the given flow streams, are provided in the figure captions. When a steady flow of SC CO2 was achieved, cetane was mixed with the CO2 stream at the inlet of the coiled pipe. The time when the bicomponent stream entered the view cell (90 s, image 1) was a near perfect match with the time calculated by eq 3. Further, the flow rates of the mixture components were

reduced and the flowstream characteristics clearly showed a laminar flow pattern (images 4-6). Experiments similar to those described above were conducted at both higher pressure and temperature. Figure 9 shows selected images of these experiments carried out at 300 bar and from 313 to 700 K for Xcetane ) 0.2. The first image is of CO2(SC). Although the system has to be liquid at 300 bar for all of the subcritical temperatures (Figure 4), images 2-4 still show striations of the heterogeneous, not fully mixed flow. However, the last two images show a homogeneous, SC flow of the cetane-CO2 binary system as expected from Figure 4 beyond 645 K, the critical temperature of this mixture. Figure 10 shows selected representative images of fluid streams in the view cell, taken during flow experiments in which diesel fuel-EGR mixtures were continuously heated from 628 K and 101 bar to 664 K and 232 bar. In the center of the view cell, coaxial with the flow, a 0.2 mm wire was inserted for the purpose of image focusing. The mixing and heating time of the diesel fuel-EGR mixture was assessed at ∼2 min based on the fluid flow rates, pipe dimensions, and the properties of the diesel fuel-EGR fluids. Similarly with the phase topography shown early in Figure 5 for a cetane-EGR composition (Xcetane ) 0.48), this figure shows a pathway of phase transitions from

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Figure 11. Selected images from a set of batch experiments with a fuel-diluent mixture showing phase transitions. Image 1 is for L-V CO2 (57 bar, 297 K) while images 2-6 are for cetane-CO2 (image 2: L-L, 257 bar, 297 K; image 3: L-V, 60 bar, 323 K; image 4: L-V, 200 bar; 473 K; image 5: L-V, 210 bar, 573 K; and image 6: SC, 210 bar; 673 K). In a central position is a 0.2 mm thick wire for image focusing.

Figure 12. Images taken during the heating of a diesel fuel-CO2 mixture in a batch mode: (1) pure diesel fuel(L); (1-2) diesel fuel(L) + CO2(L); (3-5) three phases, L1-L2-V; (6-7) L-V; (8) transition to a SC phase; (9) one SC phase. (The visual field is of 10.15 mm in diameter).

subcritical, heterogeneous diesel fuel-EGR (L-V; Xfuel ≈ 0.28; images 2-5) to homogeneous diesel fuel-EGR (L/SC; image 6) for this much more complex mixture. Figures 11 and 12 provide selected images of cetane-CO2 and diesel fuel-CO2 systems, respectively, which show phase transitions from L-V to SC states by heating these mixtures in a batch mode. In Figure 11 are six selected images from a set of batch experiments with pure CO2 and cetane-CO2 mixtures showing phase transitions from CO2(L-V) (image 1; 57 bar; 297 K) to cetane-CO2 (image 2: L-L, 57 bar, 297 K; image 3: L-V, 60 bar, 323 K; image 4: L-V, 200 bar, 473 K; image 5: L-V; 210 bar; 573 K; image 6: SC, 210 bar, 673 K). The

Anitescu et al.

disappearance of the meniscus between the liquid and vapor phases when the system transitioned from the biphasic states shown in the image 4 to a single, homogeneous SC phase (image 6) is clearly illustrated. These images also show a wire of 0.2 mm diameter which was inserted in a central vertical position for image focusing purpose. In Figure 12 are shown selected images from a set of experiments with the diesel fuel-CO2 system being heated in a batch mode from ambient temperature and 70 bar to SC conditions. In the first image is only diesel fuel (L), and in the second image there are two liquid phases rich in diesel fuel (bottom) and in CO2 (top), respectively. In the third image a vapor phase is formed at the top of the visual field that expands (images 4-5). The images 6-7 show a liquid phase at bottom and a vapor phase on top. Of a particular interest are images 7-9, which show the L-V meniscus disappearance when the system transitions from a subcritical to a SC state. Different phase transitions are clearly illustrated in these images. Fuel Thermal Decomposition. Due to widespread use of hydrocarbons as fuels, thermal decomposition of these chemicals has been extensively studied both experimentally and using different modeling approaches.1,6-19,39 In our study, flow experiments were conducted using the reactor system described in Figure 2 for a variety of conditions to determine the extent of decomposition of cetane. Aliquots of each of the effluent streams under the P-T-X conditions shown in Table 1 were analyzed by GC-MSD. Two representative chromatograms in Figure 13 for the experiments at 250 bar, 723 K and Xcetane of 0.2 (top) and 0.8 (bottom) confirm that the cetane main decomposition products are pairs of n-alkanes and n-alkenes from C8 to C15. The yield and distribution of these products depend upon P-T conditions and cetane-CO2 compositions. Higher CO2 concentrations decrease the level of cetane conversion and inhibit the formation of heavier hydrocarbon compounds. It was reported that a diesel engine functioned with drastically reduced PM and NOx emissions for about half an hour with SC fuels.5 At that time the application failed due to fuel coking with no diluent added. To explore the coking phenomena in diesel fuel-diluent mixtures, GC-MSD analysis was performed on the neat diesel fuel No. 2 and samples taken from the effluent streams after being heated. The top chromatogram in Figure 14 is of the diesel fuel-EGR heated for ∼2 min through a 4 m long coiled pipe at 700 K and 200 bar and is compared with the chromatogram for unheated diesel fuel (bottom). Diesel fuel coking phenomena can be monitored by the amount of the PAHs (e.g., naphthalene, anthracene/ phenanthrene, pyrene, etc.) formed during the heating process. The arrows point to the peaks of naphthalene (left) and anthracene/ phenanthrene (right). Since the peak pattern is not significantly different in the two chromatograms, it is concluded that no significant diesel fuel coking occurred under these experimental conditions. Regarding the possibility of flowing diesel fuel-EGR mixtures for longer times in order to achieve complete mixing before fluid transitioning to desirable SC states in diesel engines, additional experiments were carried out at much longer residence times. In each set, for example, the first fuel aliquot charged to a batch cell was heated up to 700 K and then cooled to room temperature for 11-14 times for about 3 h every time. Despite this harsh thermal treatment, diesel fuel was not significantly decomposed. The proposed method appears to be well suited to leverage (39) Savage, P. E. J. Anal. Appl. Pyrolysis 2000, 54 (1-2), 109–126.

Phase Transitions of Fuel-Diluent Mixtures

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Table 1. Thermal Stability of Cetane-CO2 Mixtures under Experimental Conditions P (bar)

T (K)

XHD

VHD (mL/min)

VCD (mL/min)

τ (min)

observations and estimated conversion (%)

100 100 100 100 100 100 100 100 100 100 100 250 250 250 250 250 250 250 250 250 250 250 250 250 250 103

623 723 723 723 623 623 723 723 723 623 623 498 623 723 623 623 623 623 623 723 723 723 723 723 723 297

0.100 0.100 0.207 0.100 0.379 0.379 0.379 0.647 0.786 0.786 0.647 0.100 0.100 0.100 0.207 0.200 0.400 0.645 0.786 0.200 0.400 0.647 0.786 1.000 0.238 0.200

0.10 0.10 0.10 0.10 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.10 0.10 0.10 0.10 0.143 0.200 0.230 0.240 0.143 0.200 0.230 0.240 0.250 0.100 0.143

0.15 0.15 0.07 0.15 0.06 0.06 0.06 0.02 0.01 0.01 0.02 0.15 0.15 0.15 0.07 0.100 0.060 0.230 0.012 0.100 0.060 0.023 0.012 0.000 0.060 0.100

14 8 13 8 18 18 9 11 12 25 24 18 14 8 13 17 18 8 21 9 9 10 10 10 14 33