Dense-Gas Fractionation and MALDI ... - ACS Publications

Petroleum pitches are formed by the thermal polymerization of decant oil, a high-molecular-weight (MW) aromatic oil that is a byproduct of the fluid c...
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Energy & Fuels 2005, 19, 984-991

Dense-Gas Fractionation and MALDI Characterization of Carbonaceous Pitches William F. Edwards and Mark C. Thies* Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909 Received June 28, 2004. Revised Manuscript Received January 18, 2005

The combination of multistage, dense-gas extraction (DGE) and matrix-assisted, laser desorption/ionization (MALDI) mass spectrometry (MS) was investigated for the fractionation and characterization of an oligomeric model pitch synthesized via the thermal polymerization of anthracene. Our approach was shown to be effective both for elucidating the molecular properties of the anthracene pitch and for generating narrow molecular weight (MW), oligomeric cuts that can serve as MW standards. DGE was used to fractionate 10- to 15-g charges of anthracene pitch into relatively pure, gram- and decigram-sized cuts containing 99+% monomers, 90+% dimers, and 80+% trimers, respectively. The multistage, semi-batch DGE process was carried out in two steps with toluene being used as the dense-gas solvent at both subcritical and supercritical conditions. In the first step, lower-MW species were stripped off as the overhead; for the second step, the remaining residue was extracted, yielding the desired oligomeric cuts as overhead. Computer process simulation was used to guide the experimental work and demonstrated the effect of column temperature gradient on the liquid reflux ratio and, as a result, product purity. MALDI-MS played a critical role in guiding the selection of DGE operating conditions, yielding absolute MW information on the fractions that were produced.

Introduction Petroleum pitches are formed by the thermal polymerization of decant oil, a high-molecular-weight (MW) aromatic oil that is a byproduct of the fluid catalytic cracking of petroleum distillates.1,2 The polymerization reaction, which is called “heat-soaking” and is carried out for ∼6 h at 350-450 °C, produces a material with a broad molecular weight distribution (MWD) ranging from less than 300 to several thousand g/mol.3 These pitches are generally regarded to be alkylated polycyclic aromatic hydrocarbons (PAHs), with the side chains being primarily methyl, along with some ethyl groups. For example, Dickinson4 used simple solvent extraction and a wide range of analytical techniques to propose representative structures for A-240 petroleum pitch; his results are shown as Figure 1. Compared to other heavy fossil fuels such as petroleum residua, pitches are more aromatic, have essentially no metals incorporated into their structures, and contain fewer heteroatoms.5,6 They are also far less soluble than residua, being only partially soluble in solvents such as toluene, pyridine, and quinoline.2,7 * Author to whom correspondence should be addressed. Tel.: +1-864-656-5424. Fax: +1-864-656-0784. E-mail: [email protected]. (1) Edie, D. D. Carbon 1998, 36, 345-362. (2) Greinke, R. A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: Basel, Switzerland, 1994; Vol. 24, pp 1-43. (3) Edwards, W. F.; Jin, L.; Thies, M. C. Carbon 2003, 41, 27612768. (4) Dickinson, E. M. Fuel 1985, 64, 704-706. (5) 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. (6) Dauche, F. M. High-Performance Carbon Fibers from Mesophases Produced by Supercritical Fluid Extraction. Ph.D. Dissertation, Clemson University, Clemson, SC, 1997.

Figure 1. Representative molecular structures in petroleum pitch.4

Pitches are versatile materials that serve as feedstocks for a wide range of carbon products, such as highthermal-conductivity carbon fibers, high-modulus fibers, the matrix phase of carbon-carbon composites, and activated carbon fibers.8 Of interest to us is the fact that the molecular composition of pitches not only affects the physical properties (e.g., viscosity and softening point) of the pitch itself but also can significantly impact the (7) Zhuang, M. S.; Gast, K.; Thies, M. C. Ind. Eng. Chem. Res. 2002, 41, 78-84. (8) Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F. R., Eds; University of Alicante: Alicante, Spain, 1997.

10.1021/ef040060g CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005

Characterization of Carbonaceous Pitches

final properties (e.g., thermal conductivity and adsorption properties) of the resultant carbon products. For example, to form high-thermal-conductivity carbon fibers, a discotic liquid crystalline pitch phase (also called mesophase) must be used as the starting material for melt-spinning the fibers. This mesophase only forms in higher-MW fractions of pitch from which most of the lower-MW, “disordering” molecules that naturally occur in pitch have been removed.1,9 To produce pitches for a given application, then, it would seem logical to both measure and control the MWD of the starting pitch. To this end, we propose a new combination of characterization and separation for application to carbonaceous pitches and to heavy fossil fuels in general. Of particular interest to us are those fractions that are only partially soluble in solvents and thus cannot be analyzed or processed by conventional means such as gel permeation chromatography or simple solvent extraction, respectively. For the fractionation of pitches, we are investigating the use of a multistage separation technique called dense-gas extraction (DGE).10,11 The terms destraction and gas extraction have also been used. With DGE, the solvent is a dense gas in the vicinity of its critical temperature, with the pressure being adjusted to vary the extractive power of the solvent. Depending on the characteristics of the pitch and the depth of the cuts desired, solvents such as pentane and toluene can be used. As practiced by our group, the dense gas and pitch to be extracted are contacted in a packed column that has an efficiency equal to several theoretical stages, so that relatively narrow MW fractions of the pitch can be produced. Liquid reflux down the column, which serves to enhance product purity, is dictated by the phase behavior of the system and is generated by inducing specific temperature gradients along the column. A final advantage of semibatch DGE is that relatively large (i.e., gram- and decigram-sized) quantities of the pitch fractions can be produced for subsequent analysis and investigation. For the characterization of our pitch fractions, we are using matrix-assisted, laser desorption/ionization (MALDI), time-of-flight (TOF) mass spectrometry (MS) (or MALDI for short).12 MALDI is particularly appropriate for our insoluble pitches, as they can be analyzed on a dry basis, so dissolution in a solvent is not required. Recently, Ra¨der and co-workers13 identified a new matrix, 7,7,8,8-tetracyanoquinodimethane (TCNQ), for the analysis of giant, synthetic PAHs. We have found TCNQ to be useful for the analysis of our pitches by MALDI. Recently, we reported on the use of MALDI for determining the MWD of petroleum and anthracene pitches,3 with the oligomeric nature of these pitches being clearly shown. (9) Dauche´, F. M.; Bolan˜os, G.; Blasig, A.; Thies, M. C. Carbon 1998, 36, 953-961. (10) Zosel, K. In Extraction with Supercritical Gases; Schneider, G. M., Stahl, E., Wilke, G., Eds.; Verlag Chemie: Weinheim, 1980; pp 1-23. (11) Brunner, G. Gas Extraction: An Introduction to Fundamentals of Supercritical Fluids and the Application to Separation Processes; Steinkopff: Darmstadt, 1994. (12) Cotter, R. J. Time-of-Flight Mass Spectrometry: Instrumentation and Applications in Biological Research; American Chemical Society: Washington, DC, 1997. (13) Przybilla, L.; Brand, J. D.; Yoshimura, K.; Ra¨der, H. J.; Mu¨llen, K. Anal. Chem. 2000, 72, 4591-4597.

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Figure 2. MALDI mass spectrum of the anthracene pitch.

The term “dense-gas extraction” was first used by Whitehead and Williams,14 who used single-stage extraction (and its inherent selectivity limitations) with supercritical toluene to obtain liquid fuels from coal. Only two fractions, one toluene-soluble and the other insoluble, were produced. No molecular properties were reported. Multistage DGE was first applied to heavy fossil fuels by Warzinski15 (coal-derived residua were fractionated), but only preliminary results were reported and no quantitative characterization was performed. More recently, Shi et al.16 used multistage DGE with the solvent pentane to fractionate petroleum residua into a number of cuts. However, because VPO was used for the analyses, they only determined average MW information (and no MWDs) for their cuts. To our knowledge, no previous investigators have used the combination of (multistage) DGE with an absolute MS method such as MALDI for the fractionation and characterization of heavy fossil fuels. Experimental Section Materials. A model pitch was synthesized by the thermal polymerization of anthracene (CAS 120-12-7). Briefly, 5080 g of anthracene (98%, Aldrich) was loaded into a 300-cm3 section of stainless steel, 3.81 cm o.d. tubing rated for 200 bar, which was then filled with nitrogen to 40 bar. The tube was then placed in a custom-built aluminum block heater that had been preheated to 475 °C, where it was heat-soaked for 2 h. The pressure was maintained at 40 bar throughout the process using a backpressure regulator. The typical yield per gram of anthracene charge was 80-90%. This process is described in detail elsewhere.17,3 A typical MALDI spectrum of anthracene pitch is seen in Figure 2. The oligomeric nature of the pitch is obvious. Of course, the oligomers are not pure, as degradation and polymerization reactions have led to the formation of numerous ringed structures of similar molecular weight. The monomer peaks centered around 178 Da (i.e., anthracene) are not shown because species with molecular weights less than ∼250 volatilize too quickly in the high-vacuum environment of the MALDI instrument to be reliably analyzed. We call the group of peaks centered around 352 Da “dimers” because the molecular weight is centered around a dimer of pure anthracene. Similarly, we refer to the succeeding peaks as “trimers”, “tetramers”, etc. The peaks of maximum height for each oligomer are consistent with what one would expect from (14) Whitehead, J. C.; Williams, D. F. J. I. Fuel 1975, 48, 182-184. (15) Warzinski, R. P. In Supercritical Fluids: Chemical and Engineering Principles and Applications; Squires, T. G., Paulaitis, M. E., Eds.; ACS Symposium Series 329; American Chemical Society: Washington, DC, 1987; pp 229-240. (16) Shi, T.-P.; Hu, Y.-X.; Xu, Z.-M.; Su, T.; Wang, R.-A. Ind. Eng. Chem. Res. 1997, 36, 3988-3992. (17) Scaroni, A. W.; Jenkins, R. G.; Walker, P. L., Jr. Carbon 1991, 72, 4591-4597.

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Figure 3. Flowsheet of the DGE process for fractionating pitches. Table 1. Operating Conditions Used for DGE Experiments column temperature gradient experiment

extraction time (min)

solvent flow (g/min)

column pressure (bar)

stillpot (°C)

bottom packing (°C)

top packing (°C)

reflux finger (°C)

(1) monomer-rich overhead (2) monomer-free residue (3) dimer-rich overhead (4) monomer- and dimer-free residue (5) trimer-rich overhead

80 120 420 300 300

1.5 5.0 4.9 7.0 5.0

14.8 14.8 35.5 42.4 52.7

340 340 330 330 330

325 325 335 335 335

310 310 340 340 340

300 300 360 360 360

our knowledge of the reactions that occur.17 The peaks at ∼200 Da are from the matrix TCNQ. On the basis of this information, and on the success of Przybilla et al.13 in obtaining accurate MW spectra for giant PAHs, we conclude that the MALDI spectrum is representative of the MW species that exist in anthracene pitch. HPLC grade toluene (CAS 108-88-3) with a stated purity of 99.9% and HPLC grade 1,2,4-trichlorobenzene (TCB, CAS 120-82-1) with a stated purity of 99.6% were both obtained from Fisher Scientific. Carbon disulfide (Fisher Scientific, certified, CAS 75-15-0), perylene (Aldrich, 99+%, CAS 198-55-0), and TCNQ (Aldrich, 98%, CAS 1518-16-7) were used for the GC and MALDI analyses. All chemicals were used as is without further purification. Experimental Apparatus and Procedure. A flowsheet of the DGE process used to fractionate the pitch is shown in Figure 3, with the operating conditions used in each of the experiments being shown in Table 1. Our goal in this work was to isolate high-purity fractions of monomers, dimers, and trimers, respectively. To obtain a monomer-rich fraction, the conditions shown in Experiment 1 were used; note that this process required only one step. The operating pressure was relatively low, so the solvent power of the gas was modest and only monomer was extracted. To recover a dimer-rich cut, two steps were required. First, Experiment 2 was used to extract all monomer from the pitch, producing a monomer-free residue; operating conditions were similar to those of Experiment 1, except that higher solvent flow rates and longer extraction times were used. For Experiment 3, the monomer-free residue was used as the feed and column operating conditions were altered so as to extract the dimer species as overhead product. Extraction of a trimer-rich cut was carried out in a similar manner, where Experiment 4 was chosen such that the monomer and dimer species were extracted in the overhead stream, leaving a monomer- and dimer-free residue. Experiment 5 was then subsequently used to produce a trimer-rich overhead cut from this residue. The apparatus that was designed and constructed to carry out the DGE process is shown in Figure 4. Briefly, the equipment consists of a pump and preheater for supplying solvent, a 1.8-cm i.d. × 115-cm tall packed column that contains a stillpot for holding a charge of pitch, and a reflux finger at the top. A regulating valve is used to control the

column pressure by manipulating the flow of overhead product exiting the column. The apparatus, which is operated in the semibatch mode, is rated for 400 °C and 200 bar and can hold a charge of up to 15 g of pitch. The packed section in the column is 70 cm in height. In preparation for an experiment, the stillpot (i.e., the first 23 cm of the column, which is free of packing) is charged with pitch. This is accomplished by loading the charge cartridge with pitch and introducing the cartridge into the bottom of the stillpot. The cartridge consists of a medium-pressure adapter (High-Pressure Equipment Co. (HIP), part no. 10-21AF4LM16) with a sintered, 20-µm disk (Micromeritics, part no. 32232) press-fit into the 1/4-in. end of the adapter. The disk serves both to support the pitch charge and to evenly distribute the flow of solvent upward through the charge during operation. The adapter is fitted at the top with a section of wound spring steel, which serves to hold the pitch charge. This unusual type of container was selected because it was found to be much easier to disassemble after an experimental run, when the pitch charge had melted and solidified. After the cartridge is charged with pitch and inserted into the stillpot, the solvent line is reconnected and the apparatus is purged with nitrogen through the solvent line. For a typical experimental run, the extraction solvent reservoir is filled with the desired solvent (in this work, toluene) and pressurized with nitrogen to ∼3 bar. Nitrogen pressure ensures reliable operation of the reciprocating piston pump (Milton Roy Minipump, Model 92014803), which delivers solvent at a constant mass flow rate (typical flows are 1.5-10 g/min) and at the desired column operating pressure. The compressed solvent then flows through the preheater, where it is heated to the desired dense-gas conditions. The preheater consists of coiled 3.18-mm o.d. × 1.75-mm i.d. tubing wrapped with three 1.8-m sections of high-temperature heating tape (Thermolyne, Model BWH051-060), rated at 480 W each and controlled with a proportional-integral-derivative (PID) controller (Omega, Model CN9122). The dense-gas solvent then flows into the bottom of the charge cartridge. Here, the sintered disk distributes the entering solvent evenly across the pitch sample. The solvent mixes with the pitch charge in the stillpot, selectively extracting a portion of the pitch. The temperature of the stillpot is controlled with a PID controller (Omega, Model CN77344) and

Characterization of Carbonaceous Pitches

Figure 4. Schematic of the dense-gas extraction (DGE) apparatus. a system of band heaters. The dense-gas mixture of solvent and extracted pitch then rises from the stillpot and enters the packed section of the column, which is filled with 4-mm random packing (Cannon Instrument Co., part no. 3947-A20). Upon exiting the packed section, the dense-gas mixture then contacts the reflux finger, which is housed in the top 14 cm of the column. This finger, which serves as a condenser, consists of a cone-shaped aluminum cylinder 13 cm long, with an o.d. of 1.3 cm that tapers to a point at the bottom. The reflux finger is controlled to its setpoint temperature using a 6.4-mm (1/4-in.) o.d. × 20.3-cm (8-in.) heating cartridge (Watlow) mounted in the centerline of the finger. If the temperature of the finger or the packed column is different from the stillpot temperature, a pitch-rich liquid phase can condense. This liquid-phase flows down the column as reflux, further purifying the overhead vapor fraction. The overhead vapor product exits out the side of the column, adjacent to the top of the reflux finger, and is expanded to ambient pressures by means of the regulating valve (Autoclave Engineers, part no. 30VM4084-GY). The product is then collected and condensed in a 500-mL glass kettle cooled in an ice bath. Typically, 5-10 samples are taken for 15-60 min each, with sample sizes ranging from 100 to 300 g each (and from 0.050 to 10 g each on a solventfree basis), depending on the extraction conditions used. The column itself is composed of three custom-made 316SS manifolds (Autoclave Engineers), each rated at 690 bar (10 000 psi), that are connected end-to-end to form one column. The manifolds are externally hexagonal in shape, measure 4.45 cm (1.75 in.) from flat to opposing flat, and have a 1.8-cm (0.6875 in.) inner diameter. The bottom manifold is 30.5 cm (12 in.) long, holds the charge cartridge, and is connected to the middle manifold at its top end. The middle manifold is of the same basic design, but is 46 cm (18 in.) in length and is packed with random packing for the entire available internal length of 42 cm. The top manifold is identical to the middle one, except it has an additional port at the top for the vapor phase to exit. This column contains 28 cm of packing, with a 14-cm high open space at the top to accommodate the reflux finger. The reflux finger is mounted to the top of the third manifold by swaging a compression fitting onto the heating cartridge. Along the length of the column are six 1/16-in. HIP compression fitting ports, which allow thermocouple probes to be inserted directly into the fluid stream. Three of the thermocouples are used only for monitoring, and the other

Energy & Fuels, Vol. 19, No. 3, 2005 987 three are used to control the temperature of the column at various positions (one in the stillpot region, two in the packing), see Figure 4. The PID controllers (Omega, Model CN77344) power a set of ten band heaters. Band heaters (10.16 cm (4 in.) i.d., Watlow, Model Thinband) are clamped around an aluminum jacket. The aluminum jacket is a 10.16-cm (4-in.) o.d. split cylinder that was machined to fit tightly around the hexagonal exterior shape of the column and allows for precise heating control along the entire column length. A calcium silicate split cylinder is used for insulating the column. Column thermocouples were calibrated to an accuracy of (1.0 °C against a secondary standard platinum RTD (Burns Engineering, 200 Series). The temperature of the column at a given location can be controlled to (1.0 °C, giving a total uncertainty in the reported temperatures of (2 °C. Column pressure was controlled by means of the regulating valve (see Figure 4), which was actuated via a National Instruments NuDrive motion control device (Model 4CX-001) along with a DC servomotor and gear reducer (ECM Motor Co., Model 5471). The system pressure was measured with a pressure transducer (Heise, Model HPO) that was monitored by National Instruments Labview software. The transducer was calibrated against a Budenberg deadweight tester (Model 380H) to an accuracy of (0.07 bar. The column pressure was maintained to within (0.07 bar of the desired setpoint at all times, giving an uncertainty in the reported pressures of (0.14 bar. Analysis of Pitch Samples. The compositions of the overhead fractions and residues collected from the DGE apparatus during Experiments 1-5 (see Figure 3) were analyzed using both a simulated-distillation (Sim-Dis) gas chromatograph (GC) and a MALDI mass spectrometer. A Hewlett-Packard 5890 Series II GC equipped with a flame ionization detector and modified by Analytical Controls (Model SIMDIS Analyzer) for Sim-Dis operation was used. Although the GC was not actually operated according to a Sim-Dis method, several of the instrument modifications, including a splitless inlet port designed for temperature programming and for samples that are only partially volatile, were necessary for the analysis of our samples. Separation was achieved with a 0.53-mm i.d. × 5-m long section of a high-temperature, aluminum-coated, fused silica capillary column (Agilent, part no. 19095S-200, Tmax ) 430 °C). For sample analysis, the inlet temperature was ramped from 130 to 380 °C at 60 °C/min and was then held constant for the duration of the run. The column temperature was ramped from 70 to 380 °C at a rate of 15 °C/min, with a final holding time of 4 min. Perylene (MW ) 252) was selected as the internal standard because it eluted between the monomer and dimer species. We have found that our Sim-Dis GC can be used for the analysis of carbonaceous, polyaromatic pitches with MWs up to approximately 550. For MALDI analysis, a Bruker Daltonics Autoflex equipped with a reflectron detector was used. As our method for the MW determination of pitches using MALDI is discussed in detail elsewhere,3 only a brief synopsis is given below. For analysis of the feed pitch and the pitch charge residue, which are not completely soluble in solvents, the sample and the matrix TCNQ were combined at a ratio of 1:20 by mass and ground in a mortar and pestle; the resultant powder was then deposited on the target cell as a film on the surface of a bead of water. Most of the water was withdrawn from under the film with a pipet, and the target was allowed to completely dry before MALDI analysis. In the case of the solvent-soluble overhead fractions, a film of pure TCNQ was cast on the target cell using the method described above. Then a solution of the pitch fraction dissolved in carbon disulfide was applied directly to the film and allowed to dry before analysis. For samples that were completely soluble in carbon disulfide, the above “solution method” was preferred to the previously described “powder method” because it consumed far less sample. For

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Figure 5. GC chromatogram of anthracene pitch, showing oligomers through trimer. Perylene is the internal standard peak at 8 min. each MALDI analysis, the laser power was adjusted as high as possible to maximize response but not so high as to distort the spectra by detector saturation (cropping) or poor resolution. The spectra obtained are the summation of 100 laser shots and acquisitions. Although MALDI can be used to accurately determine the MWs of the species present in pitches, the technique has yet to be established as a quantitative method of analysis. In other words, the relationship between the intensity, or height, of a MALDI peak and the mass or mole fraction of that species in the pitch mixture is unknown. Preliminary results at Clemson indicate that, for the same number of moles, the intensity of MALDI peaks decreases as their MW increases; this is in agreement with trends reported elsewhere.18 In this study, we have chosen to report results in terms of area fraction. To obtain the area fractions by MALDI, the mass spectrum is integrated by dividing the data into MW ranges, with each range representing an oligomer. Dimers were defined as the MW range from 267 to 445 Da, trimers as 445-623, tetramers as 623-801, pentamers as 801-979, and hexamers as 979-1187, with higher oligomers having MWs higher than 1187. Because the MALDI instrument creates a high-vacuum environment (∼10-7 Torr) that volatilizes species with MWs less than ∼250, a monomer fraction was not calculated.

Results and Discussion Experimental. As outlined in schematic form in Figure 3, five experiments were conducted in an attempt to produce relatively pure cuts of selected anthracene pitch oligomers. As discussed later in this paper, both empiricism and process simulations of the DGE process were used to guide the selection of the operating conditions shown in Table 1. For Experiment 1, a highpurity monomer cut was extracted as overhead from the anthracene pitch feedstock at a pressure of 14.8 bar. None of the dimer or trimer species present in the anthracene feed pitch (see Figure 5) were detectable by GC, so we estimate that this fraction contained less than 1 wt% of nonmonomeric species (see Figure 6). By using these monomers as a GC calibration standard, the massto-area response of monomers to internal standard was determined to be 1.28. This response value was used in all subsequent experiments to estimate the weight percent monomers present in a given overhead cut or residue. For Experiment 2, DGE conditions were modified by increasing the solvent flow rate and the extraction duration from Experiment 1, so as to thoroughly strip the monomer species from the anthracene pitch charge. A total of eight runs were made at the Experiment 2 (18) Williams, J. B.; Chapman, T. M.; Hercules, D. M. Anal. Chem. 2003, 75, 3092-3100.

Edwards and Thies

Figure 6. GC chromatogram of a monomer-rich overhead cut obtained from Experiment 1. No dimer and higher oligomers were detected.

Figure 7. GC chromatogram of the monomer-free residue obtained from Experiment 2. Estimated monomer content is 1 wt%.

Figure 8. MALDI mass spectra of a dimer-rich overhead cut (black) and the residue (grey) from Experiment 3. The overhead yielded a 90+% pure dimer fraction, while the residue was concentrated in the higher oligomers.

conditions to accumulate enough of this monomer-free residue to serve as the feed for Experiment 3. The eight combined residues were determined to have a monomer content of 1 wt% by GC (see Figure 7). This feedstock was then used in Experiment 3 to produce overhead fractions with estimated dimer purities of 90+ wt% (see Table 2 and Figure 8). Although in Table 2 the overhead and residue masses should in principle add up to equal the mass of the feed, holdup in the packed column makes complete recovery impractical. In a manner similar to what was done with the pure monomer cut above, we used a high-purity dimer overhead cut as a GC calibration standard, and the response of dimers to internal standard was determined to be 1.30. This response value was then used to estimate the weight percent dimers present in a given overhead cut or residue. The purpose of Experiment 4 was to produce a monomer- and dimer-free residue to serve as the feed for Experiment 5, where a trimer-rich overhead was to

Characterization of Carbonaceous Pitches

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Table 2. Experiment 3: Collection of a Dimer-Rich Overhead mass fraction by GC

area fraction by MALDI

fraction

collection time (min)

dry mass (g)

monomer

dimer

dimer

trimer

tetramer

pentamer +

anthracene pitcha feedb overhead 1 overhead 2 overhead 3 overhead 4 overhead 5 overhead 6 overhead 7 residue

60 60 60 60 60 60 60 -

26 9.59 0.35 0.25 0.20 0.16 0.14 0.13 0.10 7.00

0.350 0.011 0.293 0.040 0.026 0.038 0.053 0.033 0.045 0.001

0.171 -

0.529 0.525 0.876 0.874 0.889 0.853 0.916 0.960 0.951 0.147

0.225 0.251 0.055 0.040 0.049 0.069 0.045 0.032 0.047 0.371

0.118 0.120 0.047 0.066 0.043 0.053 0.033 0.006 0.000 0.245

0.128 0.104 0.022 0.021 0.019 0.025 0.005 0.002 0.002 0.235

a

Feed to Experiment 2. b Monomer-free residue from Experiment 2. Table 3. Experiment 5: Collection of a Trimer-Rich Overhead mass fraction by GC fraction

anthracene feedb overhead 1 overhead 2 overhead 3 overhead 4 overhead 5 residue a

pitcha

area fraction by MALDI

collection time (min)

dry mass (g)

monomer

dimer

dimer

trimer

tetramer

pentamer +

60 60 60 60 60 -

29.112 9.250 0.405 0.146 0.096 0.040 0.051 7.563

0.350 0.001 0.069 0.089 0.076 0.084 0.060 0.001

0.171 0.005 0.053 0.039 0.049 0.051 0.025 0.003

0.529 0.078 0.065 0.013 0.027 0.017 0.007 0.064

0.225 0.285 0.805 0.845 0.765 0.644 0.694 0.220

0.118 0.302 0.113 0.132 0.187 0.266 0.267 0.312

0.128 0.335 0.017 0.010 0.021 0.073 0.033 0.404

Feed to Experiment 4. b Monomer- and dimer-free residue from Experiment 4.

Figure 9. MALDI mass spectra of a trimer-rich overhead cut (black) and the residue (grey) from Experiment 5.

be produced (see Figure 3). As seen for the Feed in Table 3, we were able to select DGE conditions for Experiment 4 such that the monomer and dimer contents were stripped down to relatively low levels. (The discrepancy between the dimer contents for the feed calculated by GC vs MALDI serve to remind us that MALDI results are uncalibrated area percentages; we would expect the GC results to be more reliable.) Four runs of Experiment 4 were carried out in order to produce enough feedstock for Experiment 5. For Experiment 5, overhead cuts containing relatively pure, 80+% trimers (see Table 3 and Figure 9) were produced. Table 3 also shows that some overhead and residue material was held up in the column and was not recovered. It should be noted that, for Experiment 4, both positive and negative temperature gradients were initially examined. Using a positive gradient yielded a residue lower in dimer content, while the residue acquired using the negative gradient had higher trimer content. Ultimately, the positive gradient was chosen for the operating conditions for Experiment 4, as given in Table 1.

Finally, although our experiments were directed toward obtaining the highest-purity fractions possible, changes that could lead to improvements are, in hindsight, apparent. For example, the residue from Experiment 2, and thus the feed to Experiment 3 (see Table 2), contained 1 wt% monomers. Thus, the overhead products in Experiment 3 were contaminated with too many monomers. On the other hand, in the preparation of monomer- and dimer-free residue in Experiment 4, too many trimers were lost in the overhead. Thus, the dominant oligomer in the monomer- and dimer-free feedstock for Experiment 5 was tetramer (see feed in Table 3), limiting the purity of the trimer-rich sample that could be extracted in the overhead products. Process Simulation. In Table 1, the operating conditions for the DGE experiments are summarized. For the case of monomer extraction (i.e., Experiments 1 and 2), we see that a negative temperature gradient (i.e., a decrease from the stillpot to the reflux finger) was employed. On the other hand, for dimer and trimer extractions (Experiments 3-5), positive temperature gradients were found to yield the best results. In addition, column operating pressures increased along with the MW of the fraction to be recovered. To guide our selection of these temperature gradients and pressures, a computer simulation of the DGE process was developed. Specifically, the steady-state simulation package from HYSYS (Hyprotech, Version 2.4.1, Build 3870) was combined with our own, externally written Visual Basic (VB) code to simulate the semi-batch operation of our DGE apparatus. Our computer simulations of the DGE process employed a pseudo-steadystate approximation and made step changes in time. To carry out the simulation, pseudocomponents for anthracene pitch first had to be assigned for input into HYSYS as follows. First, the MW of the highestintensity MALDI peak for a given set of oligomers was

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Figure 10. Monomer (a), dimer (b), and trimer (c) pseudocomponents selected for anthracene pitch. Table 4. Peng-Robinson EOS Parameters for Anthracene Pitch Pseudocomponents19

monomer dimer trimer tetramer pentamer

MW (amu)

aC (bar(cm3/mol)2)

b (cm3/mol)

κ

178 352 526 703 877

7.58 × 107 1.97 × 108 3.20 × 108 4.40 × 108 5.63 × 108

172.51 358.21 544.14 732.64 918.65

1.132 1.526 1.917 2.331 2.720

selected as the representative pseudocomponent. (For example, for dimers, the peak at 352 Da was the most intense, see Figure 2.) Next, the work of Scaroni et al.,17 who proposed a reaction scheme for the creation of oligomers from the thermal polymerization of anthracene, was used to select the most likely molecular structure corresponding to that pseudocomponent. (For example, the most likely structure for dimer is shown as Figure 10b.) This procedure was performed to create pseudocomponents for monomer through pentamer peaks; structures for the first three oligomers are shown in Figure 10. Finally, we note that no additional analytical work (other than MALDI) was used to produce more-refined estimates of the pseudocomponent structures, as our previous modeling work19 indicated that only the molecular weight of the PAHs has a significant impact on the parameters in Hutchenson’s modification of the Peng-Robinson (P-R) equation of state (see discussion below). After the pseudocomponents were defined, an equation of state had to be selected within HYSYS to model the phase behavior in the DGE column. We chose the P-R equation of state as modified by Hutchenson for PAHs.19 Briefly, Hutchenson developed a correlation for calculating the pure component P-R parameters ac, b, and κ directly from the molecular structure of the PAH of interest. The P-R parameters obtained for our five pseudocomponents are given in Table 4. To obtain the necessary binary interaction parameters, we used the values obtained by Hutchenson for his work with petroleum pitch/toluene mixtures in vapor-liquid equilibrium (VLE).19 Thus, the binary interaction parameters (kij’s) between all toluene-pitch pseudocomponents were set to 0.09, and those between the pitch pseudocomponents were 0.00. As shown in Figure 11, the simulated column contained four equilibrium stages, each with a temperature defined by the user. (Although the actual column probably provided somewhat more than these four stages, this number was chosen for convenience so as to represent the four temperature-controlled sections of the column: the stillpot, bottom packed section, top packed section, and reflux finger.) Note that the simulated column contained two pseudo-streams: a pitch (19) 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.

Figure 11. Process simulation results showing the influence of column temperature profile on L/V at 45.8 bar.

feed (which in the actual process was a batch charge) and a bottoms product (which in the actual process was the residue). When the simulation was initialized, the pitch feed stream composition was set to the composition of the batch charge fed to the column and the pitch flow rate was defined as the mass of the batch charge divided by the residence time of the solvent in the stillpot. Note that this pseudo-flow-rate correctly simulates the solventto-pitch ratio seen in the stillpot. A steady-state simulation was then performed in HYSYS, yielding predicted overhead and bottoms flow rates and compositions. The overhead and solvent flow rates were then multiplied by the desired time step to yield the amounts of material removed and added to the column, respectively. By subtracting the overhead amount and adding the solvent amount to the pitch charge, a new value for the contents of the stillpot was obtained. (The bottoms flow rate was discarded.) The new stillpot contents (which now include solvent) were then converted to the new pitch-rich feed stream for input into the steady-state HYSYS simulation, and a new set of overhead and bottoms flow rates and compositions were calculated. The above process of time steps and steady-state simulations was repeated for a time period representative of the longest practical operating time for a given DGE experiment. Simulations were used to find appropriate operating conditions for each of the experiments described in this work. During actual experiments, these conditions were used as starting points, with the simulated temperatures being maintained and the operating pressures being adjusted empirically from the simulated values to improve the fractionation process. The simulations revealed several interesting aspects of DGE operation. For example, a negative temperature gradient was found to yield the highest reflux and thus the highest monomer purity (i.e., for Experiment 1), but for this case, the slope of the column temperature gradient had little effect on results as long as the stillpot and reflux temperatures were not changed. For the purification of dimer, adequate reflux and equivalent purities could be generated by using either a positive or negative temperature gradient. On the other hand, for the purification of trimer, a positive temperature gradient was required. Furthermore, as seen in Figure 11, there was a significant advantage to using a specific temperature profile in this case. Thus, a linear temperature profile (the left column) produced very low reflux ratios (reflux ratios are shown as L/V on a solvent-free mass basis) at the top of the column, reducing overhead product purity, while a profile that was essentially flat but

Characterization of Carbonaceous Pitches

Energy & Fuels, Vol. 19, No. 3, 2005 991

at 350 °C. However, for the trimer mixture, the retrograde region encompasses most of the practical operational range of the apparatus, so a positive gradient is required to produce liquid reflux. In summary, although the flash calculations do not by themselves prove that that higher liquid refluxes were experimentally achieved and were responsible for improved product purities, it is encouraging that the results achieved by experiment and by simulation are consistent. Conclusions Figure 12. Constant-composition phase envelopes predicted by the modified PR-EOS, showing the vapor pressure curve of toluene (a); dew-point curves for (b) 1% monomer, (c) 1% dimer, and (d) 1% trimer in toluene; the critical point of toluene (CP); retrograde regions for the dimer (e) and trimer (f) mixtures.

increased sharply at the reflux finger produced significantly higher reflux ratios and, thus, higher overhead purities. By examining the phase behavior of the dense-gas solvent toluene with the respective pseudocomponents, we can observe the effect of the pseudocomponent being purified on the recommended temperature gradient. To this end, flash calculations were performed in HYSYS using Hutchenson’s modification to P-R for three binary mixtures: toluene with 1 wt% monomer, with 1 wt% dimer, and with 1 wt% trimer. (These weight percents are representative of the vapor-phase concentrations that were obtained in the column during our simulations.) The VLE phase envelopes that were obtained are given in Figure 12, including bubble- and dew-point curves, mixture critical points, and the vapor pressure curve and critical point for pure toluene. In all three cases, the bubble-point curves essentially coincide with the vapor pressure curve for pure toluene. This is not surprising, as the first bubble that vaporizes will be essentially pure toluene. Similarly, the mixture critical points are all too close to the critical point of pure toluene to be differentiated. However, the dewpoint curves are dramatically different, as the first drop of liquid that condenses will contain a significant percentage of the pitch pseudocomponent. Finally, we also note that with DGE one generally prefers to operate at higher pressures within the phase envelope, so that the density (and thus the solvent power) of the densegas solvent is maximized. For the monomer-toluene mixture, the dew-point curve (see line b) exhibits conventional behavior, so that a temperature decrease always causes condensation as one moves toward the bubble-point curve. Thus, liquid reflux in the DGE column is induced by a negative temperature gradient, as in a conventional distillation column. However, for the dimer and trimer mixtures, there are retrograde regions (see regions e and f in Figure 12) where increases in temperature cause liquid condensation. For the dimer mixture, the retrograde region is small, so one can use either a positive or negative temperature gradient to induce reflux, provided the appropriate temperatures are chosen. In fact, reasonable dimer purities were obtained in an experiment20 where a negative temperature gradient was used, with the stillpot at 380 °C and the reflux finger (20) Edwards, W. F. Ph.D. Dissertation, Clemson University, Clemson, SC, 2005.

The goal of this work was to produce narrow-MW fractions of the oligomers present in anthracene pitch by DGE. To a significant extent we succeeded, as several grams of a 99+% pure monomer cut, several hundred milligrams of a 90+% pure dimer cut, and several hundred milligrams of an 80+% pure trimer cut were produced from 10 to 15 g charges of anthracene pitch. In addition to these oligomeric cuts, several residues were collected that are concentrated in the highest MW species present in pitches. Both the DGE process and the unique capabilities of MALDI for determining absolute MW information for insoluble pitches played a key role in this development. A two-step DGE process was used to obtain the dimer and trimer cuts: In the first step, the lower-MW species were stripped off in the overhead; in the second step, the desired oligomer was extracted from the residue remaining from step 1. Such a mode of operation allowed us to optimize each step independently, as products could be analyzed before attempting the next extraction step. Our two-step process can also be applied to the isolation of the higherMW oligomers. To our knowledge, previous workers10,15,16 have reported only on the use of positive temperature gradients with DGE. In this work, we examined the fractionation of pitch oligomers with significantly different relative volatilities and observed how optimum column operation is more complicated in this case. Thus, there is a transition from preferring negative column temperature gradients to preferring positive temperature gradients as the volatility of the pitch component to be isolated decreases. Compared to conventional chromatographic methods, DGE is preferred for the production of significant (e.g., decigram-sized) quantities of narrow-MW pitch fractions, which can serve as calibration standards. For example, in earlier work21 silica gel column chromatography was evaluated for isolating a dimer fraction of petroleum pitch, but only milligram quantities of dimer could be recovered and purities were lower than what can be achieved by DGE. Subsequent efforts to produce trimer-rich cuts by chromatographic methods were even less successful. Acknowledgment. This work was supported by ConocoPhillips Inc., the Engineering Research Centers Program of the National Science Foundation under NSF Award No. EEC-9731680, and by ERC Corp. The authors thank Chris Cutshall, Esther Brown, and Robert Hammett for their assistance with the MALDI and GC analyses. EF040060G (21) Southard, M. Conoco, Ponca City, Oklahoma. Private communication, 2002.