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Ind. Eng. Chem. Res. 2002, 41, 78-84
SEPARATIONS Stagewise Fractionation of Petroleum Pitches with Supercritical Toluene Mark S. Zhuang, Kai Gast, and Mark C. Thies* Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909
A stagewise fractionation technique employing sequential single-stage extractions with supercritical toluene was used to fractionate the heaviest portion of an isotropic petroleum pitch. Each stage was operated in a region of liquid-liquid equilibrium (LLE) at a temperature of 614 K and a solvent-to-pitch ratio of 2.0, and the pressure was sequentially reduced so as to precipitate out ∼5 wt % of the feed pitch in each stage. Five single-stage flashes could be carried out in the LLE region at pressures ranging from 138 to 52 bar, and five pitch fractions comprising the heaviest 28.8 wt % of the feed pitch were obtained. No flashes were performed below the onset of vapor-liquid equilibrium at 49 bar, because the vapor phase is a relatively poor solvent for pitches. Softening points, C/H ratios, and DRIFTS were all used to characterize the pitch fractions that were isolated. The heaviest three fractions had to be hydrogenated to increase their solubility in the mobile phase before molecular weight distributions could be determined by gel permeation chromatography. Results from the characterization techniques all indicate that stagewise fractionation is separating the pitch according to molecular weight. Introduction Petroleum pitches are produced by polymerizing decant oil, a byproduct of the fluid catalytic cracking (FCC) of petroleum distillates, by heat-soaking and serve as important raw materials for the production of carbon products.1-3 Generally speaking, these pitches consist of polynuclear aromatic (PNA) hydrocarbons with methyl and ethyl side chains (see Figure 1) and have a broad molecular weight distribution (MWD) ranging from less than 300 to above 3000. When carbon products are processed from a pitch-based precursor, the molecular composition of the pitch (i.e., the MWD and the molecular structures present) acquires special importance, because composition directly affects the physical properties of the pitch and ultimately affects the properties of the carbon products themselves. For example, certain heavy pitch molecules (also called mesogens; normally with molecular weight (MW) > 800) will spontaneously form discotic liquid crystals (or mesophase) when they are isolated from the lighter pitch molecules;2,5 previous workers have found that the heaviest 10-20% of petroleum pitch forms up to 100% mesophase when isolated and serves as an excellent precursor for high-performance carbon products, such as fibers.6,7 On the other hand, the lighter cuts of pitch are of interest for the production of isotropic pitch fibers, which can be treated to produce activated carbon fibers.8 Because raw, untreated pitches have such a broad MWD, they do not always have the desired bulk properties (e.g., 100% mesophase, softening points above * To whom correspondence should be addressed. Phone: (864) 656-3055. Fax: (864) 656-0784. E-mail: mark.thies@ ces.clemson.edu.
Figure 1. Compounds representative of those present in petroleum pitch.4
∼250 °C, and easily oxidizable) for making pitch-based carbon products. Thus, techniques for altering both the MWD and composition of pitches have been developed. To date, researchers have carried out extractions, thermal treatments,9,10 and reactions11 for producing isotropic and mesophase pitches. Here, our interest is with those methods that employ extraction. Both conventional solvent extraction and supercritical extraction have been used to isolate pitch fractions in both coal tar and petroleum pitches. In 1980, Diefendorf and Riggs5 discovered that when isotropic petroleum pitches were extracted with toluene/heptane mixtures at ambient conditions, the unextracted solid precipitate formed mesophase pitch when heated above the phasetransition temperature. The same basic extraction process was used by both Exxon and DuPont to produce
10.1021/ie010528s CCC: $22.00 © 2002 American Chemical Society Published on Web 12/05/2001
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mesophase pitch for subsequent melt-spinning into carbon fibers for about 10 years. To our knowledge, Beneke and Peter12 were the first to investigate the extraction of pitches with supercritical fluids. They carried out a continuous countercurrent multistage extraction of coal tar pitch in a pilot-scale, rotating disk column at 473 K and 200 bar; the mixed supercritical solvent consisted of equal parts of toluene and propane. About 75% of the feed pitch was extracted, but only 30-40% of the heaviest portion of the pitch (i.e., the quinoline insolubles) was recovered. The extract phase was found to be free of ash and suitable for additional processing. Over the past decade, a number of other groups have investigated the supercritical extraction of coal tar pitches in the laboratory as a semibatch operation.13-15 These researchers were generally successful in using supercritical toluene to extract a lower MW ash-free fraction from the parent pitch for subsequent processing. In some cases, researchers were hoping to extract a fraction that was high enough in MW to form mesophase pitch, but they were not always successful in this regard. Dauche´ et al.6 extracted an isotropic petroleum pitch with supercritical toluene at pressures above the threephase liquid-liquid-vapor region and, thus, at pressures corresponding to true liquid-liquid equilibrium (LLE). Single-stage liquid-liquid extraction was carried out in a continuous manner, and the pitch that was recovered in the heavier liquid phase contained from 40% to 100% mesophase. Several of the 100% mesophase fractions were subsequently melt-spun into high thermal conductivity carbon fibers. Ideally, a technique for fractionating pitches should be versatile enough that the entire pitch (and not just the lighter fractions) can be separated into cuts on the basis of MW, because applications exist for both low and high MW fractions. Furthermore, such a separation method could be attractive for producing fractions for the analytical characterization of pitches. Supercritical extraction would seem to be ideally suited to accomplish this task, but the results of previous work are inconclusive, because the emphasis of previous work has generally been on the low MW portions of the pitch. In this paper, we investigate a technique for fractionating the heaviest portion of petroleum pitch by carrying out stagewise supercritical extraction in a region of liquid-liquid equilibrium. Typically, supercritical extraction is operated in a region of vapor-solid or vaporliquid equilibrium where the density of the solvent phase is between that of a gas and a liquid. Thus, the process favors the extraction of the lower MW components in a continuously distributed mixture. Here, we consider the operation of a supercritical fractionation process at conditions such that there are two liquid phases so that even the high MW mesogens may remain soluble in one or both of the liquid phases. After the extraction, the chemical and physical properties of the resultant fractions were examined to evaluate the effectiveness of this process. Stagewise Fractionation of Petroleum Pitches. In this work, we proposed to fractionate the pitch with a series of single-stage flashes of mixtures of pitch and supercritical toluene. A schematic diagram of our proposed stagewise fractionation process is shown in Figure 2. Here, a basis of 100 g of feed pitch is chosen for illustrative purposes. For each step of pressure reduction, the goal is to fractionate out ∼5 g “cuts” of the feed
Figure 2. Proposed stagewise fractionation process.
pitch. For all of the stages, the fractionation is carried out in the LLE region, using toluene as the solvent, at a constant temperature of 614 K and a solvent-to-pitch (S/P) ratio of 2.0. For the first stage, the pressure is selected such that 95 g of the feed pitch is extracted into the top toluene-rich phase, with 5 g remaining in the bottom pitch-rich phase. The precipitated 5 g of pitch is the product of the first stage, while the extracted 95 g of pitch in the top phase becomes the feed for the second stage. To obtain the correct S/P ratio, toluene must be removed from the first-stage extract before it can serve as the second-stage feed. The second stage of fractionation is operated at the same temperature and S/P ratio but at a lower pressure such that another 5 g of pitch precipitates out in the bottom liquid phase. Once again (after removal of toluene to obtain the desired S/P ratio), the extracted 90 g of pitch is used as the feed for the third stage. This liquid-liquid extraction process continues, decreasing the pressure so as to obtain another 5 g of pitch precipitate, until the region of vapor-liquid equilibrium is reached. Because the vapor phase is a relatively poor solvent for pitch, the process is terminated at this point. Our results in applying this fractionation method to an isotropic petroleum pitch are described in the following sections. Experimental Section Preparation of Pitch Feedstock Solution. An isotropic petroleum pitch was supplied as a solid by Conoco Inc. (Ponca City, OK). The pitch was made by heat-soaking a cut of aromatic decant oil obtained from an FCC unit for 6 h at ∼400 °C. To ensure that this pitch was free of particulate matter such as catalyst fines and coke particles and to facilitate its handling, it was converted into a liquid solution by the following procedure. The raw pitch was dissolved in boiling liquid toluene (i.e., at ∼110 °C) in a 50/50 weight ratio and stirred under reflux for about an hour. The resulting solution was then filtered through a 0.2 µm membrane filter with the use of a filter aid such as diatomaceous earth. Typically 5-7 wt % of the feed pitch is filtered out as insolubles by this procedure; some desirable high MW mesogens are lost as part of the precipitate. The final homogeneous 50/50 by weight solution of pitch and toluene (ACS-grade toluene from VWR Scientific, West Chester, PA) served as the feedstock to our fractionation process. Experimental Apparatus and Procedure. A continuous-flow apparatus with a single-stage flash was
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Figure 3. Schematic of the continuous-flow apparatus with a single-stage flash.
used to carry out the stagewise fractionation process (see Figure 3). This apparatus, which is rated for 673 K and 300 bar, was previously used to measure LLE for mixtures containing petroleum pitches and dense supercritical fluids16 and was also used to produce a mesophase pitch that was subsequently converted into high-performance carbon fibers.6,7 A brief description of the apparatus and procedure, along with specific modifications that were required for these experiments, is given below; more detailed information can be found elsewhere.17 For the first stage of fractionation, the 50/50 by weight homogeneous solution prepared previously is pumped indirectly using one of two identical, 1 L high-pressure cylinders. Each cylinder contains a floating piston driven by the working fluid, which in our case is toluene. The two cylinders are used alternately (i.e., one can be refilled while the other is in use) to produce continuousflow conditions. The total flow rate of the solvent plus the pitch-toluene solution is maintained at ∼540 g/h; the individual flow rates of the solvent and feedstock solutions are adjusted such that the desired S/P ratio of 2.0 is obtained. The two streams are preheated and combined in an impingement mixing “tee” before reaching the high-pressure equilibrium cell, which is maintained at 614 K by the nitrogen bath. This equilibrium cell serves as a liquid-liquid phase separator, in which the light (top) and heavy (bottom) liquid phases separate by gravity. Each liquid phase contains significant amounts of toluene, with the top phase averaging about 70 wt % toluene and the bottom phase about 20 wt % toluene. The temperatures of the phases are monitored with sheathed 0.159 cm o.d. thermocouples inserted through ports in the sides of the cell; this differential thermocouple setup was referenced to a secondary standard platinum resistance temperature detector (RTD) located in a constant-temperature aluminum block inside the nitrogen bath. The light and heavy liquid phases are collected independently through lines exiting the top and bottom of the cell into 500 mL fournecked glass reaction kettles. Typical flow rates of the top and bottom phases are 530 ( 10 and 10 ( 1 g/h, respectively. Top-phase kettles were changed hourly, while bottom-phase kettles were changed every 3 h. As referred to in Figure 2, for a basis of 100 g of feed pitch dissolved in the pitch-toluene solution, 95 g distributes into the lighter liquid phase and 5 g into the heavier liquid phase. The 5 g cut should contain the highest MW portion of the feed pitch. The desired system pressure
is controlled by the top-phase micrometering valve and a computer-controlled servo motor, and a second servo motor mounted on the bottom-phase regulating valve is used for liquid-level control in the cell. Because the two phases present in the equilibrium cell are black, visual detection of the phase interface cannot be used to assist interface level control. Instead, we use an insulated cell insert in combination with the cell body to measure the dc resistance of the two-phase mixture in the cell, which we have found can be correlated with the liquid-level interface in the cell.16 Because the top liquid phase consists of about 70 wt % toluene, a solid pitch phase precipitates out of solution when the mixture is cooled to ambient conditions. To redissolve the pitch, toluene is boiled off under stirring until a 50/50 by weight homogeneous toluene-pitch solution is reconstituted. This solution then serves as the feed for the second stage of fractionation. The operation of the flow apparatus is then essentially identical to that described previously except that a new operating pressure is used. Of the 95 g of feed pitch contained in the second-stage feed solution, 5 g precipitates out in the bottom liquid phase, and the remaining 90 g of feed pitch remain in the top liquid phase (see Figures 2 and 3). The feed for the third stage is then produced as before (i.e., by distilling off toluene from the light phase to recreate a 50/50 homogeneous solution), and the fractionation process is repeated. The procedure is discontinued when the operating pressure has been reduced to the point that the coexisting phases are in vapor-liquid equilibrium (VLE). As discussed previously and indicated in Figure 2, our goal was to obtain a cut containing 5 wt % of the original feed pitch for each fractionation stage. Because the temperature and S/P ratio were being held constant throughout the fractionation process, a specific operating pressure was required to obtain a nominal 5 g cut of pitch for a given stage; how this was achieved is described next. For the feed to the first stage, the total flow rate (i.e., toluene plus pitch) was ∼540 g/h; for an S/P ratio of 2.0, this corresponded to a neat pitch flow rate of ∼180 g/h. Thus, to obtain a 5% cut of the original feed pitch in the heavy liquid phase, the pitch flow rate in the bottom phase should have been 9 g/h. With the weight fraction of toluene in the bottom phase being approximately 20%, this corresponded to a desired total bottom-phase flow rate of 11.3 g/h. Next, the bottom-phase flow rate was correlated to the rate of change of the interface level inside the equilibrium cell. To carry out this procedure, the bottomphase valve was fully closed, whereas the top-phase valve was regulated to maintain the system pressure; as a result, the top phase was withdrawn continuously from the cell, and the heavier bottom phase accumulated in the cell. Figure 4 displays the change in dc resistance (and, hence, the interface level) as a function of time as the bottom phase builds up in the cell. Initially, the cell is full of the top phase, which has a resistance of 7000 Ω. Then, the bottom phase starts to accumulate in the cell at point A, with the dc resistance continuously decreasing down to 6000 Ω, at which point the cell is full of the bottom phase. For a given rate of accumulation of the bottom phase in the cell, a constant slope is obtained. For example, a bottom-phase accumulation rate of 10 g/h corresponds to a slope of -17 Ω/min. Thus, the operator can monitor the change in resistance for 10-15 min, obtain a slope, and increase or decrease the
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Figure 5. Birch reduction for the hydrogenation of aromatic species.
Figure 4. Change in dc resistance due to the accumulation of the bottom phase in the equilibrium cell.
operating pressure as necessary until the bottom-phase flow rate corresponding to a 5% cut is obtained. Once the appropriate bottom-phase flow rate is obtained, the interface level is maintained in the middle of the cell by controlling the bottom-phase valve such that the dc resistance is constant, and the top and bottom phases are collected at the newly determined operating pressure. Finally, it should be noted that we attempted to use the SAFT equation as modified by our group for supercritical solvent-pitch mixtures16 to predict the required operating pressure for each stage of fractionation. Although SAFT gave reasonable predictions for the solvent composition in each phase, estimates of the pressures required to obtain a nominal 5 g cut greatly exceeded those obtained in practice. Thus, the trial-anderror procedure described previously had to be used. Analytical Characterization. Solvent compositions of the top and bottom phases from each fractionation stage were determined by drying to remove all of the toluene, followed by weighing. Bottom-phase samples were dried at 425 K under 1.5 Torr for 1 h; essentially all of the toluene and none of the pitch components were removed at these conditions. An ambient-temperature cold trap in series with an acetone-dry ice cold trap was used to minimize toluene losses to the atmosphere. For the top phase, toluene was removed from the collection kettles by batch distillation until the remaining solutions were homogeneous and contained about 50 wt % toluene. Then, triplicate samples containing about 30 mL of the solutions were dried in a vacuum oven at atmospheric pressure under a nitrogen purge for 3 h at 400 K, followed by an additional 15 h at approximately 410 K. Softening points of both top- and bottom-phase samples were estimated using a Fisher-Johns melting-point apparatus (model 12-144). The softening points measured by this technique give numbers significantly lower than those obtained with a Mettler melting-point apparatus, which is the ASTM standard (D 3461-85) for pitch softening-point measurements. Unfortunately, the temperature limitations of the Mettler apparatus prevent its use for the determination of softening points for the higher-melting fractions derived from petroleum pitch. The carbon and hydrogen contents of the various pitch fractions were determined by elemental analysis (Galbraith Laboratories, Inc., Knoxville, TN). Bottom-phase cuts were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Samples were prepared by using a mortar
and pestle to mix 10 mg of a pitch cut with 600 mg of potassium bromide (KBr). A Nicolet Avatar 360 FTIR ESP with a diffuse reflectance attachment was used to collect the IR spectra. Sixty-four scans were performed with a 4.0 cm-1 resolution for each sample and for the background. The sample chamber was purged with air, which was filtered by a Whatman FTIR purge gas generator to remove water and CO2. The KubelkaMunk spectrum was then used to calculate indices that characterized the chemical structure of the analyte. Molecular Weight Distributions by Gel Permeation Chromatography (GPC). The MWDs of both top- and bottom-phase cuts were determined by GPC. Prior to carrying out the GPC analyses, the solubilities of the top- and bottom-phase cuts (see Figure 2) in the GPC mobile phase (1,2,4-trichlorobenzene, TCB; HPLC reagent grade from VWR Scientific) were determined. For each test, triplicate samples of 50 mL solutions with a pitch concentration of 0.5 wt % were prepared. Each sample was independently vacuum-filtered at 50 °C through a preweighed 0.45 µm membrane filter. After the samples were filtered, the filter papers were dried for 10 h at 50 °C and 200 Torr in a vacuum oven and were then reweighed. The percentage of pitch solubles in TCB was then calculated. Cuts that were greater than 90 wt % soluble in TCB were analyzed by GPC without further treatment, as described in the following paragraphs. Cuts exhibiting lower solubilities in TCB were hydrogenated in an attempt to render the entire sample solvent-soluble. In this work, a Birch reduction18,19 was employed to hydrogenate the heaviest pitch cuts, with liquid ammonia being used to dissolve the lithium and with isopropyl alcohol as the proton donor (see Figure 5). Reproducible results were obtained only by using the following procedure. First, a pitch sample was finely ground and screened with a 230 mesh (63 µm) microsieve. A total of 500 mg of the screened pitch was combined with 12 mL of anhydrous THF to form a homogeneous suspension. Approximately 250 mL of liquid ammonia was then slowly added with stirring under an argon purge to a three-necked reaction flask that had been precooled with a dry ice-acetone mixture to 195 K. A total of 0.25 g of lithium (which was stored in mineral oil up to the point of addition) was then introduced into the flask in approximately 0.02 g portions. Twenty minutes after the ammonia-lithium mixture turned dark blue, the pitch-THF suspension was added to the reaction flask. The reaction mixture was stirred for 1 h at 195 K in the dry ice-acetone bath. A total of 15 mL of isopropyl alcohol, which served as the proton source, was then added to the flask. After the reaction mixture was stirred for 2 h more at 195 K to ensure completion of the reaction, the addition of dry ice to the acetone bath was discontinued so that the ammonia would evaporate overnight. The product that remained was then rinsed with 50 mL of deionized water to remove lithium salts and other impurities. The rinse water was removed by pipetting after obtaining solid-liquid separation by centrifugation. This rinsing
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Table 1. Operating Pressures, Bottom-Phase Yields, and Solvent-Phase Compositions for a Stagewise Fractionation Process at 613.7 K and an S/P Ratio of 2.0 stage pressure bottom-phase wt % toluene wt % toluene no. (bar) yielda (g) in top phase in bottom phase 1 2 3 4 5 a
138 110 84 71 52
6.97 3.31 5.84 7.61 5.93
68.0 67.7 70.5 71.4 70.7
14.7 18.9 21.0 23.3 29.4
Yields are shown on a basis of 100 g of feed pitch.
process was repeated six times. Final drying of the solid, hydrogenated pitch precipitate was carried out by vacuum filtration at 323 K using a 0.20 µm membrane filter followed by drying in a vacuum oven at 333 K and 200 Torr. Duplicate reactions were performed for each fraction. DRIFTS was used to examine the extent of hydrogenation that was obtained from the reduction of a given pitch fraction. The solubilities of the hydrogenated samples were measured following the procedures described previously. The MWDs of both unhydrogenated and hydrogenated pitch samples were determined with a Waters 150C ALC/GPC chromatograph (Milford, MA) equipped with a 100 Å 10-µm-particle-size column (Polymer Laboratories, PLGel, Amherst, MA), a differential refractometer, and a differential viscometer (Viscotek, Houston, TX). The mobile phase (i.e., TCB) was pumped at 1.0 mL/min, and pump, injector, and column temperatures were all maintained at 358 K. A calibration curve, generated from PNA standards, was used to convert the retention times to MWs. Because the MW of the heaviest standard was only 535 (i.e., rubrene), the calibration curve had to be extrapolated for those pitch components eluting at shorter retention times. Results and Discussion Stagewise Fractionation Results. Table 1 shows the results of our stagewise fractionation process. A total of five single-stage flashes could be carried out in the LLE region, with decreasing pressure being used to precipitate out each succeeding cut. It is of interest to note that above a pressure of ∼170 bar, a homogeneous solution existed; that is, no bottom-phase precipitation was detected. At a pressure of about 49 bar, a phase transition from LLE to VLE occurred (with the top phase becoming a vapor phase with relatively poor solvent power), so no additional flashes were performed. The yields that are shown are the grams of pitch recovered in the bottom phase on the basis of 100 g of feed pitch and are believed to be accurate to (0.4 g. Actual yields differed somewhat from our goal of 5 g of pitch/100 g of feed pitch, because only a limited portion of the feed solution to each stage could be used in locating the desired operating pressure (or else we would have run out of feed solution prematurely). Despite this limitation, the fractionation process was relatively successful, because the heaviest 28.8% of the pitch was separated into five different cuts. The solvent compositions of the top and bottom phases for each stage are believed to be accurate to (1.0 and (0.3 wt %, respectively. The observed increase in toluene weight percent in each phase with decreasing operating pressure is a result of the decreasing MW of the pitch fed to each stage and has been previously observed.16 Although the operating temperature setpoint was held constant at 613.7 K, temperature variations are
Figure 6. DRIFTS spectrum of the bottom-phase cut from stage 1.
always part of a flow apparatus. Considering both of these variations and the accuracy of the thermocouple/ RTD setup that was used to measure the view cell temperature, we believe that the reported fractionation temperature is accurate to (0.5 K. The operating pressures for each flash were controlled to better than (2 bar with a PI controller. A Heise HPO precision pressure transducer (0-5000 psi, 4-20 mA) that had been calibrated to an accuracy of (0.2 bar was used to measure the system pressure and to provide a signal output to the PI controller. Thus, the accuracy of the reported pressures is believed to be (2 bar. Analytical Characterization of Pitch Fractions. Selected properties of the bottom-phase cuts obtained from each stage of fractionation (see Figure 2) are given in Table 2. The reported softening points are believed to be accurate to (2 K. Previous researchers have found that the softening points of pitches are roughly proportional to the MW of the pitch,17 so our results are consistent with their work. The softening points of the bottom phases from stages 1 and 2 are high enough that they would be expected to contain mesophase. Topphase cuts do not change much as the fractionation process proceeds; thus, their softening points decrease only from 373 (stage 1) to 354 K (stage 5) over the entire fractionation process. The atomic C/H ratios of the bottom-phase cuts shown in Table 2 are consistent with a decrease in aromaticity (and, as a result, MW) as the fractionation proceeds. On the basis of the reproducibility of a duplicate sample that was analyzed for cut 1, the results are believed to be accurate to (0.03. DRIFTS was used to analyze the differences in chemical structures between the bottom-phase cuts obtained for each stage. A typical IR spectrum for petroleum pitch and its fractions is given in Figure 6. Information about the chemical structure of a pitch fraction can be obtained from seven characteristic peaks. Four peaks lay in the so-called fingerprint region at wavenumbers of 750, 814, 840, and 880 cm-1. These peaks represent aromatic C-H out-of-plane bending. The peaks at 2920, 2960, and 3050 cm-1 are the C-H stretching bands for methylene (-CH2-), methyl (-CH3), and aromatic groups, respectively.20,21 We have defined two indices that can be related to molecular structure. The first index, referred to as the ortho-substitution index (IOS),17 is calculated from the fingerprint region and corresponds to the fraction of aromatic rings with ortho substitutions (i.e., four neighboring hydrogen atoms) as compared to all aromatic rings with at least one hydrogen atom; thus, IOS gives the relative size of the aromatic molecules in a given sample, with smaller
Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002 83 Table 2. Selected Properties of the Bottom-Phase Cuts Obtained from Each Fractionation Stage DRIFTS indices
solubility in TCB (wt %)
stage no.
softening point (K)
C/H ratio
IOS
ICHS
original
hydrogenated
1 2 3 4 5 feed pitch
570 520 465 439 426 375
1.815 1.747 1.702 1.670 1.593 1.463
0.269 0.283 0.294 0.294 0.309 0.317
0.475 0.492 0.504 0.516 0.526 0.572
5 45 78 92 97 98
94 92 95
molecules having a higher IOS. The second index, the C-H substitution index (ICHS), represents the fraction of aromatic carbons substituted with aliphatic (-CH3) or methyl (-CH2-) groups versus aromatic hydrogen (-H) groups, so smaller molecules would also have a higher ICHS. Dauche´ et al.17 present a detailed description of these DRIFTS indices. Table 2 shows how both indices increase for the bottom-phase cuts as the fractionation progresses, indicating a decrease in the MW of the bottom-phase cuts. The reported values for IOS and ICHS are estimated to be accurate to (0.003 and (0.006, respectively. Hydrogenation of Pitch Fractions. Before they could be analyzed by GPC, the bottom-phase cuts from stages 1-3 had to be hydrogenated to increase their solubility in the mobile phase (i.e., in TCB). The Birch reductions performed in this work were fairly reproducible, with the product yields ranging from 94% to 105%. The degree of hydrogenation was qualitatively monitored by DRIFTS. Figure 7 compares the spectrum of an unhydrogenated fraction, with its strong aromatic C-H stretching peak at 3050 cm-1 and strong peaks in the aromatic fingerprint region, to the same fraction after hydrogenation. Dramatic reductions in these aromatic peaks were observed for the hydrogenated sample, indicating their conversion to naphthenic character. Table 2 shows how the solubilities in TCB of bottomphase cuts from stages 1-3 increased to more than 90 wt % after the hydrogenation treatment, indicating that hydrogenation is an effective technique to increase the solubility of the heavy pitch fractions. The increase in solubility is due to the conversion of aromatic C-H into naphthenic C-H2 bonds in the pitch molecules. Even though the carbon backbone structures are preserved, the pitch molecules have become more flexible after the hydrogenation treatment, lowering their melting points and, thus, increasing their solubilities in solvents. To quantify the effect of hydrogenation on the MWD of our pitch fractions, samples of the original feed pitch and of top-phase cuts (both of which were originally
Figure 7. DRIFTS spectra of the bottom-phase cut from stage 1: (a) the original cut and (b) the same cut after hydrogenation.
soluble in TCB) were hydrogenated by Birch reduction, and the elution behavior of hydrogenated versus unhydrogenated samples was compared. A representative chromatogram is shown for the top-phase cut from stage 1 in Figure 8. No significant change in the MWD is observed, so we conclude that the original carbon backbone structure in the pitch molecules was preserved. However, the decrease in the retention time for the hydrogenated sample indicated that its hydrodynamic volume had increased. Thus, a calibration curve developed specifically for hydrogenated pitch fractions was generated as follows. The MWDs of the feed pitch and the top-phase cuts from stages 1-3 were obtained by GPC for both the original and hydrogenated samples. The average shift in retention time between these hydrogenated and unhydrogenated samples was then applied as a correction factor to the previously generated calibration curve. MWDs of Pitch Fractions. GPC runs with PNA standards indicated that peak response factors for the viscosity detector were well-behaved with respect to MW, whereas for the refractive index, no discernible relationship was found. Thus, all of chromatograms presented here were performed with the viscosity detector, and the peak heights shown have been adjusted so that they are proportional to the mass fraction of that species in the pitch cut. The MWDs of the bottom-phase cuts from stages 1 and 3, and for the feed pitch, as determined by GPC are shown in Figure 9; for readability, the other cuts are not shown. Cuts 1-3 were hydrogenated to increase their solubility in TCB; for cuts 4 and 5 and the feed pitch, such treatment was unnecessary. For comparison purposes, each chromatogram was normalized to a unit area. Clearly, the bottom-phase cuts contain significantly more of the higher MW species than the feed pitch. However, recall that the calibration curve must be extrapolated above MWs of 535, so the presence of
Figure 8. MWD of the top-phase cut from stage 1 as measured by GPC with a viscosity detector: (a) the original cut and (b) the same cut after hydrogenation.
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Literature Cited
Figure 9. MWD of selected bottom-phase cuts by GPC: (a) hydrogenated bottom-phase cut 1, (b) hydrogenated bottom-phase cut 3, and (c) original feed pitch.
pitch species in the two bottom-phase cuts shown at MWs greater than ∼1500 should be interpreted with caution. Conclusions When carried out as a series of single-stage flashes in the liquid-liquid region, supercritical extraction can be used to fractionate the heaviest portion of petroleum pitches. This technique would also be applicable to other high MW carbonaceous materials. A key requirement is the selection of an initial S/P ratio and pressure (for a given temperature) for which the entire pitch is completely miscible with the supercritical solvent. This technique is in marked contrast to the more typical applications of supercritical extraction to carbonaceous materials, in which only the lighter portions of the material are recovered. Additional work is needed to determine whether, by the proper manipulation of operating variables, the entire pitch can be fractionated into cuts, with each containing ∼5 wt % of the feed pitch, or whether the change in phase behavior from VLE to LLE with increasing pressure would make such a process impractical. Results from all of the characterization methods were consistent with one another, because they all indicate that our stagewise fractionation technique is separating the pitch in terms of MW. Nevertheless, additional analytical techniques to confirm and extend the GPC results need to be developed to obtain more definitive MW information on the fractions that were produced. Acknowledgment This material is based upon work supported by the U.S. Army Research Office under Grant No. DAAG5598-1-0023. This work also made use of ERC Shared Facilities supported by the National Science Foundation under Award No. EEC-9731680. The authors thank Conoco Inc. for providing the isotropic petroleum pitch used in this work. We are indebted to Mr. Scott G. Bowers for performing the GPC measurements and also thank Profs. David Bruce and Ya-Ping Sun for their helpful discussions on Birch reduction.
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Received for review June 14, 2001 Revised manuscript received October 17, 2001 Accepted October 18, 2001 IE010528S
