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Energy & Fuels 2000, 14, 70-75
Extraction of Petroleum Pitch with Supercritical Toluene: Experiment and Prediction Mark S. Zhuang and Mark C. Thies* Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909 Received June 29, 1999. Revised Manuscript Received October 19, 1999
A heat-soaked isotropic petroleum pitch has been extracted with supercritical toluene in a region of liquid-liquid equilibrium at temperatures from 600 to 640 K, pressures from 60 to 155 bar, and solvent-to-pitch (S/P) ratios of 2.0 and 3.0. The solvent compositions of both the solventrich top phase and the pitch-rich bottom phase were measured, along with the weight fraction of pitch extracted into the top phase (i.e., the extraction yield). A thermodynamic model that incorporates the molecular weight distribution of the feed pitch, mathematically generated pseudocomponents, and the SAFT equation of state was used to predict phase compositions and extraction yields over a range of temperatures, pressures, and S/P ratios; good agreement is obtained with subsequently measured experimental data. The model contains a total of three adjustable parameters, which are determined by fitting a limited set of experimental data (e.g., phase compositions at one temperature and S/P ratio). Because these parameters are independent of temperature and composition, they can be used to predict phase compositions and extraction yields for supercritical operating conditions that have yet to be experimentally investigated.
Introduction Clean petroleum pitches are of considerable interest for a number of applications, particularly as raw materials for carbon artifacts.1 These pitches are made by heat-soaking decant oil, a high molecular weight (MW) aromatic oil that is a byproduct of the catalytic cracking of petroleum distillates. As shown in Figure 1 and Table 1, these pitches are similar to asphaltenes in many ways, but are more aromatic in nature, are only partially soluble in toluene, and contain fewer organic and inorganic impurities. Depending on their molecular weights, percent mesophase content, and softening points, isolated fractions of petroleum pitch can be used for a wide range of applications. For example, the heaviest 10% of the pitch (MW ∼ 2000) forms up to 100% mesophase when isolated and serves as an excellent precursor for highperformance carbon fibers. Other fractions can be used to produce isotropic fibers or for the matrix phase of a carbon-carbon composite. In this study, we report on the fractionation of isotropic petroleum pitch with supercritical toluene and evaluate the ability of a thermodynamic model to predict the effects of the extraction process. Previous workers have fractionated both petroleum and coal-tar pitches with supercritical fluids,5-7 but to date no predictive thermodynamic model has been proposed. * Author to whom correspondence should be addressed. Fax: (864) 656-0784. E-mail:
[email protected]. (1) Edie, D. D. Carbon 1998, 36, 345-362. (2) Dickinson, E. M. Fuel 1985, 64, 704-706. (3) Dauche, F. M. High-Performance Carbon Fibers from Mesophases Produced by Supercritical Fluid Extraction. Ph.D. Dissertation, Clemson University, Clemson, SC, 1997.
Figure 1. Compounds representative of those present in petroleum pitch.2
Experimental Section Experimental Apparatus. A continuous-flow apparatus is available at Clemson for measuring fluid-phase equilibria at temperatures to 400 °C and pressures to 300 bar for mixtures containing heavy fossil fuels (e.g., pitches and heavy oils) and dense supercritical fluids (e.g., toluene, xylene, and (4) Wiehe, I. A. Preprints of AIChE International Conference on Petroleum Phase Behavior and Fouling, Third International Symposium on the Thermodynamics of Asphaltenes and Heavy Oils III, 1999. (5) Poot, M. Extraction of Coal-Tar Pitches with Supercritical Toluene. Ph.D. Dissertation, Potchefstroom University, Republic of South Africa, 1998. (6) Bolan˜os, G.; Thies, M. C. Fluid Phase Equilib. 1996, 117, 273280. (7) Dauche, F. M.; Bolan˜os, G.; Blasig, A.; Thies, M. C. Carbon 1998, 36, 953-961.
10.1021/ef990141q CCC: $19.00 © 2000 American Chemical Society Published on Web 12/09/1999
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Energy & Fuels, Vol. 14, No. 1, 2000 71
Figure 2. Schematic of the continuous-flow apparatus. Table 1. Physical and Chemical Properties of Petroleum Pitches and Asphaltenes petroleum products pitch3 asphaltenes4
metal C H C/H S content MW by (wt %) (wt %) atomic (wt %) ppm VPO 92.3 81.9
5.5 7.9
1.45 0.87
1.0 7.5
∼200 ∼1000
510 2980
water). This apparatus can also be used to fractionate pitches with supercritical fluids by means of a single-stage flash. The resulting fractions are sufficiently large both for subsequent chemical analysis and for their evaluation as potential end products. Up to a kilogram of a pitch or heavy oil can be processed in a 12-hour period with no flow interruption. A final advantage of the flow apparatus is that residence times at the elevated temperatures of operation are only a few minutes. Several hours at temperatures approaching 400 °C would be required for any measurable thermal polymerization reactions to occur with the heat-soaked feed pitch. A simplified schematic of the apparatus is shown in Figure 2. For the experimental work presented herein, a 50/50 by weight homogeneous solution of isotropic petroleum pitch and toluene was pumped indirectly using one of two identical highpressure cylinders. Each cylinder contains a floating piston driven by the working fluid toluene, so the setup operates such as a syringe pump. The two cylinders are mounted in parallel and are used alternately during an experimental run to ensure truly continuous flow through the apparatus with no disturbance to operating temperatures, pressures, or flow rates. The pure toluene solvent was pumped independently at a predetermined flow rate to obtain a specified solvent-to-pitch (S/P) ratio. The solvent and working fluid (and thus the pitchtoluene solution) were pumped at total flow rates ranging from 400 to 500 mL/h. The two streams were preheated and combined in an impingement mixing “tee” before reaching a high-pressure equilibrium cell, which functions as a phase separator. The preheating, mixing, and separation steps are performed at elevated temperatures in an isothermal nitrogen bath. The heavy and light phases were collected independently through lines exiting the bottom and the top of the cell, respectively. A high-temperature micrometering valve on the top-phase line was used to control the system pressure, and a high-temperature regulating valve on the bottom-phase line was used for interface level control in the cell. All sample lines exiting the nitrogen bath were wrapped with electrical heating tapes and were insulated. After expansion to atmospheric pressure, both samples were collected. Because the two phases present in the equilibrium cell are black, visual detection of the phase interface could not be used to assist interface level control. Fortunately, we have discovered that a large difference between the electrical conductivi-
Figure 3. Interface detection device located inside the equilibrium cell. ties of the two phases present in the cell exists and have used this phenomenon to develop an interface detection device. Briefly, an insulated steel plate inserted into the body of the cell serves as the positive electrode, and the cell body serves as the negative electrode, see Figure 3. As the interface level rises and falls, the resistance between the plate and the cell wall also changes. Both AC impedance and DC resistance have been found to reliably correlate with the phase interface level in the cell.8,9 For example, a cell full of only top phase has a DC resistance on the order of 500 kΩ, and a cell full of bottom phase on the order of 50 kΩ. Incidentally, we believe that this technique could also be used to detect phase interfaces in multiphase opaque systems containing asphaltenes and solvents. Additional details of the apparatus and its automated features, and of the interface detection device, are presented elsewhere.3,9 Experimental Measurements. At a specified temperature, pressure, and S/P ratio, the system was brought to steady state, and triplicate samples of the top and bottom phases were collected for about 20 min each in sealed 200-mL jars placed in water-ice baths and connected to cold traps. No detectable losses of toluene occurred during sampling. Top-phase samples consisted primarily of toluene (about 80 wt %) and the lowermolecular-weight portion of the pitch. In contrast, the bottomphase samples consisted of about 20 wt % toluene and the mesophase-containing, higher-molecular-weight pitch fraction. The wt % toluene present in each phase is found by a gravimetric method. The top-phase samples are dried at atmospheric pressure under nitrogen purge for 3 h at 125 °C and then for an additional 15 h at approximately 140 °C. Similarly, the bottom-phase samples are dried at 150 °C under 1.5 Torr for 1 h. An ambient-temperature cold trap in series with an acetone-dry ice cold trap is used to minimize toluene losses to the atmosphere. The resulting solvent-free phases are solids at ambient temperature; typically, the top phase is isotropic, but the bottom phase can consist of up to 100% anisotropic mesophase. Molecular weight distributions (MWDs) of the dried feed pitch were determined by gel permeation chromatography (GPC) using a Waters 150-C ALC/GPC chromatograph equipped with two Polymer Laboratory Gel columns (100 and 500 Å) and a refractive index detector. 1,2,4-Trichlorobenzene (TCB) (8) Hochgeschurtz, T.; Hutchenson, K. W.; Roebers, J. R.; Liu, G.Z.; Mullins, J. C.; Thies, M. C. In Supercritical Fluid Engineering Science; Kiran, E., Brennecke, J. F., Eds.; ACS Symposium Series 514; American Chemical Society: Washington, DC, 1993; Chapter 28. (9) Wince, J. E. Control of a Supercritical Extraction Apparatus using Virtual Instrumentation. M.S. Thesis, Clemson University, Clemson, SC, 1999.
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Table 2. Measured and Predicted Solvent-Phase Compositions in Liquid-Liquid Equilibrium for the Petroleum-Pitch-Toluene System at 600, 620, and 640 K
Figure 4. MWD of the feed pitch by GPC. Log10 MW ) [6.13394-0.19659* (ret. time)]. The 21 pseudocomponents are shown as open circles. extracted into the solvent-rich top phase, was reproducible and accurate to within (0.02 and (0.03, respectively.
Thermodynamic Modeling
a
These data were correlated, not predicted.
was used as the mobile phase. A calibration curve, generated from polynuclear aromatic standards, was used to convert the retention times to molecular weights. Because the molecular weight of the heaviest standard (i.e., rubrene) is only 535, the linear relationship found between the log of molecular weight and retention time for the calibration curve had to be extrapolated for those pitch components eluting at shorter retention times. Materials. A heat-soaked isotropic petroleum pitch was obtained from Conoco Inc. The pitch was received as a solid and dissolved in toluene (at approximately 50/50 by weight) under agitation and solvent reflux at ∼383 K and atmospheric pressure. The mixture was then filtered through a 0.2 µm membrane to separate the toluene-insoluble solids, which consisted of about 5 wt % of the initial pitch. If necessary, extra toluene was added to the remaining liquid to produce the desired 50/50 by weight feed pitch solution. Selected physical and chemical properties of this feed pitch were already presented in Figure 1 and Table 1. ACS-grade toluene (CAS No. 108-88-3) and HPLC-grade 1,2,4-trichlorobenzene (TCB) (CAS No. 120-82-1) were obtained from VWR Scientific Products. TCB was pre-filtered through a 0.2 µm membrane to remove any possible particulate matter. Experimental Results. Using the apparatus described above, liquid-liquid equilibrium compositions were measured at 600, 620, and 640 K over a range of pressures with S/P ratios of 2.0 and 3.0, respectively. Table 2 shows the experimentally determined phase compositions for the different extraction conditions investigated in this work. For an experimental run, the reported temperatures and pressures are believed to be accurate to within (1 °C and (2 bar. Top-phase and bottom-phase compositions were typically reproducible to better than (1 wt % toluene and are believed to be accurate to within (1.5 wt %, respectively. The extraction yield, which is defined as the weight fraction of the feed pitch that is
A thermodynamic model that incorporates the SAFT equation of state,10,11 characterization data for the feed pitch, and the concepts of continuous thermodynamics was used to correlate the experimental results. SAFT was selected for this work because conventional cubic equations of state are generally inaccurate for liquidliquid equilibria; our preliminary calculations confirmed this was also the case for our system. In addition, previous work has shown that SAFT is a useful equation of state for less-well-defined systems containing highmolecular-weight materials, such as polymers and bitumen.12 Finally, methods were already available from the literature for estimating all of the necessary pure component parameters in SAFT.10,12 To calculate the phase equilibria for a complex mixture such as petroleum pitch, the mixture is frequently represented as a set of pseudocomponents. Unfortunately, the physical fractionation of our feed pitch to generate such pseudocomponents would be difficult and expensive. An alternative presented here is the generation of mathematical pseudocomponents that are directly derived from the MWD of the solventfree feed pitch. The first step in this procedure was to normalize the MWD of the feed pitch such that the area below the curve is 1.0. Next, it was determined that the MWD could be represented as the sum of three normal distribution functions (see Figure 4). A Gauss-Chebyshev quadrature method13 was then used to generate 7 quadrature points for each of the three normal distribution curves. Each quadrature point defines a pseudocomponent, whose mass fraction is calculated by multiplying the value of the normal distribution function at each quadrature point by a weighting factor. The molecular (10) Huang, S. H.; Radosz, M. Ind. Eng. Chem. Res. 1990, 29, 22842294. (11) Huang, S. H.; Radosz, M. Ind. Eng. Chem. Res. 1991, 30, 19942005. (12) Huang, S. H.; Radosz, M. Fluid Phase Equilib. 1991, 70, 3354. (13) Bolan˜os, G. Production of Mesophase Pitch by Supercritical Fluid Extraction: A Study of the Region of Liquid-Liquid Equilibrium. Ph.D. Dissertation, Clemson University, Clemson, SC, 1995.
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Energy & Fuels, Vol. 14, No. 1, 2000 73
Table 3. Mathematically Generated Pseudocomponents for Modeling MWD of Feed Pitch molecular weight
mass fraction
1 2 3 4 5 6 7
Normal Function 1 (µ ) 15.45, σ ) 0.942, ω ) 0.4049)a 12.20 5428 13.04 3716 14.17 2230 15.45 1248 16.73 698 17.86 419 18.70 286
0.0002 0.0070 0.0676 0.1720 0.0674 0.0063 0.0006
8 9 10 11 12 13 14
Normal Function 2 (µ ) 16.72, σ ) 0.448, ω ) 0.1986) 15.17 1414 15.57 1181 16.11 926 16.72 702 17.33 533 17.86 418 18.26 349
0.0002 0.0073 0.0697 0.1173 0.0695 0.0064 0.0006
15 16 17 18 19 20 21
Normal Function 3 (µ ) 18.14, σ ) 0.4455, ω ) 0.3965) 16.60 739 17.00 618 17.53 485 18.14 369 18.75 280 19.28 220 19.67 184
0.0003 0.0146 0.1400 0.3561 0.1396 0.0130 0.0012
pseudocomponent
retention time (min)
a µ ) normal distribution mean value; σ ) normal distribution standard deviation; ω ) normal distribution mass fraction.
weight of each pseudocomponent was determined from its retention time using the calibration curve given with Figure 4. Thus, the entire MWD, fitted by three normal distributions, can be adequately represented by 3 × 7 quadrature points. A more detailed description of the pseudocomponent calculations is given elsewhere.13 The retention times, molecular weights, and mass fractions of the 21 pseudocomponents are given in Table 3. Also shown in Table 3 are the parameters used to generate the three normal distributions that represent the solventfree feed pitch. Next, the pure-component parameters were calculated for each of the pseudocomponents. These parameters were estimated from correlations developed by Huang and Radosz for pure polynuclear aromatics and alkanes;10 additional correlations were proposed by these authors for compounds that contain both aromatic and alkyl groups.12 For each component or pseudocomponent, the molecular weight and C/H ratio are required as input. Because no other information was available, we assumed that all pseudocomponents had the same C/H ratio as the feed pitch. Unfortunately, the correlations developed by Huang and Radosz for compounds that contain both aromatic and alkyl groups (such as pitch) yielded less than satisfactory results. These correlations were developed for bitumens, (which have lower molecular weights and are less aromatic than pitch molecules), so we decided to let the C/H ratio vary (while still using one value for all pseudocomponents) in an attempt to achieve a better fit to the equilibrium data. The binary interaction parameters were obtained by fitting the model to the experimentally obtained solventphase compositions. As was done previously by Bolanos and Thies,6 the interaction parameters between the
Figure 5. k1,j as a function of MW at an S/P ratio of 3.0.
solvent and the pseudocomponents were assumed to be linear with respect to the molecular weight of the petroleum pitch molecules:
k1,j ) AMj + B
(1)
where k1,j ) interaction parameter between the solvent and the jth pseudocomponent, Mj ) molecular weight of the jth pseudocomponent, and A and B ) adjustable parameters. The interaction between pitch components was computed using the following expression:
ki,j ) C|Mi - Mj|
(2)
where ki,j ) interaction parameter between pseudocomponents i and j, Mi ) molecular weight of the ith pseudocomponent, and C ) adjustable parameter. For each temperature and S/P ratio, an independent set of parameters A, B, and C was obtained by using the downhill simplex method14 to minimize the following objective function:
f)
1
nd
∑
ndi)1
[( ) ( ) ] xic - xie xie
2
+
yic - yie yie
2
(3)
where xi ) bottom-phase composition (wt % toluene), yi ) top-phase composition (wt % toluene), e ) superscript to designate an experimental value, c ) superscript to designate a value calculated with the model, nd ) number of equilibrium tie lines for a given isotherm and S/P ratio. During the minimization procedure, the C/H ratio was held constant; the process was then repeated for other C/H ratios. For all temperature and S/P ratios, the above objective function was minimized at a C/H ratio of 1.10. Figure 5 shows the binary interaction parameters between the solvent and the pitch pseudocomponents (k1,j) as a function of pseudocomponent molecular weight for the three isotherms measured at an S/P ratio of 3.0. For clarity, the k1,j’s for the three isotherms at an S/P ratio of 2.0 are not shown because they all fall within the lines shown in the figure. The binary interaction (14) Press: W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1986; pp 289-293.
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Figure 6. Pressure-composition isotherms at an S/P ratio of 2.0.
parameters between the pitch components (i.e., the ki,j’s with i * 1) were all found to be very small (i.e., ∼10-5); thus, they were set to zero with no loss in accuracy of the fit. An important goal of our work is to be able to predict the phase behavior for pitch-toluene systems from limited experimental data. The results in Figure 5 appeared to be promising in this regard, as only small differences in the k1,j’s were observed for the various temperatures and S/P ratios; furthermore, the ki,j’s were all found to be essentially zero. Thus, the predictive nature of the binary interaction parameters was tested: k1,j’s and ki,j’s optimized from one set of experimental conditions were used to predict the phase behavior for all other experimental conditions that had been measured. In particular, the interaction parameters optimized from the experimental data at 620 K and an S/P of 3.0 were arbitrarily selected for this test. The equations for calculating these “global” k1,j’s and ki,j’s for a given pseudocomponent molecular weight are given below:
Figure 7. Pressure-compositions isotherms at an S/P ratio of 3.0.
Figure 8. Extraction yield vs pressure for three isotherms at an S/P ratio of 2.0. Table 4. Distribution of Pitch and Toluene between Top and Bottom Phases at 600 K and an S/P Ratio of 3.0
kl,j ) 6.64 × 10-5 Mj + 2.99 × 10-2
(4)
ki,j ) 0.000
(5)
top phase bottom phase
18.7 6.3
The other adjustable interaction parameter, the C/H ratio, was held constant at a value of 1.10 for all calculations.
top phase bottom phase
21.7 3.3
Discussion The phase compositions that were predicted from these “global” interaction parameters are shown in Table 2; pressure-composition isotherms that compare experimental and predicted phase compositions for a given S/P ratio are given in Figures 6 and 7. (Of course the values given for 620 K and an S/P ratio of 3.0 were correlated, not predicted.) Note that in all cases the wt % toluene in both the light and heavy phases decreases as the pressure increases. Such behavior can be explained as follows: As the pressure increases, the solvent-rich top phase becomes denser and is able to extract more pitch, decreasing the concentration of toluene in this phase. As a result of this “extraction”, the remaining pitch molecules in the bottom phase have
pitch (g)
toluene (g)
total (g)
% toluene
P ) 74 bar 73.4 1.6
92.1 7.9
80.0 20.3
P ) 114 bar 74.3 0.7
96.0 4.0
77.5 17.5
a higher average molecular weight. Thus, the solubility of toluene in this increasingly asymmetric phase also decreases. As shown by the representative set of experimental data in Table 4, the mass balance is not violated because the amount of the bottom phase is small. Thus, the fact that SAFT is correctly predicting this trend indicates that it is giving reasonable predictions not only for the phase compositions but also for the relative amounts of each phase. The explanation for the effect of temperature on the phase compositions presented in Figures 6 and 7 is analogous to that given above for pressure, except decreasing temperature increases the density of the solvent-rich top phase. Thus, the effect of decreasing temperature is the same as increasing pressure. This behavior is also correctly predicted by SAFT.
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Energy & Fuels, Vol. 14, No. 1, 2000 75
notice that at a temperature of 640 K and an S/P ratio of 3.0, the predicted extraction yield declines dramatically as the pressure further decreases to around 60 bar (see Figure 9). Additional experiments would be required to determine whether this effect, which is due to a large decrease in the calculated density of the top phase, is being accurately predicted by SAFT. The effect of the S/P ratio on the extraction yield is shown in Figure 10. For the S/P ratios investigated thus far, increases in the S/P ratio decrease the solubility of pitch in the toluene. Once again, the SAFT ki,j’s obtained by correlating data at 620 K and an S/P of 3.0 give reasonable estimates of the system behavior at other conditions. Conclusions Figure 9. Extraction yield vs pressure for three isotherms at an S/P ratio of 3.0.
Figure 10. Effect of S/P ratio on the extraction yield at a temperature of 600 K.
An important variable for the economical operation of a supercritical fluid extraction process for fractionating pitches (and other heavy fossil fuels) is the extraction yield, defined as the weight fraction of the feed pitch extracted into the toluene-rich top phase. As expected, the experimental data in Figures 8 and 9 show that the extraction yields increase with increasing pressure and with decreasing temperature (because of increasing solvent density). SAFT correctly predicts both of these trends, and the accuracy of the predictions is good when one considers that both phase compositions and phase splits are required to calculate the extraction yield. Also
A thermodynamic model developed for predicting equilibrium phase compositions and extraction yields for mixtures of petroleum pitch with dense supercritical fluids has yielded promising initial results. That is, binary interaction parameters (k1,j’s and ki,j’s) obtained by fitting measured phase compositions at one temperature and S/P ratio were used to predict reasonable estimates of phase compositions and extraction yields for other temperatures and S/P ratios. For the ranges of temperature investigated thus far, the ki,j’s are only a function of molecular weight and not of temperature or composition, and the k1,j’s are essentially zero. Thus, the only input required to the model are the molecular weights of the pseudocomponents, and these can be mathematically generated if the MWD of the feed pitch is known and is a continuous smooth curve. The model contains three adjustable parameters (A, B, and the C/H ratio) that are determined by fitting to a limited set of experimental mixture data. Finally, the ultimate potential of the model needs to be determined by (1) comparing experimental measurements to predictions for a wider range of supercritical extraction operating conditions, (2) implementing more comprehensive characterization data into the model, and (3) evaluating other heavy fossil fuels and supercritical solvents. Acknowledgment. This material is based upon work supported by the U.S. Army Research Office under Grant No. DAAG55-98-1-0023. We also thank Conoco Inc. for providing the pitch used in this work. EF990141Q
