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Fractionation of pitches by molecular weight using continuous and semibatch dense-gas extraction
Carbon 44 (2006) 243–252 www.elsevier.com/locate/carbon
Fractionation of pitches by molecular weight using continuous and semibatch dense-gas extraction William F. Edwards, Mark C. Thies
*
Center for Advanced Engineering Fibers and Films, Department of Chemical Engineering, Clemson University, Clemson, SC 29634-0909, USA Received 27 May 2005; accepted 23 July 2005 Available online 21 September 2005
Abstract A countercurrent, multistage, dense-gas extraction technique with reflux was investigated for the fractionation of carbonaceous pitches. Two modes of operation were investigated: continuous-stripping and semibatch operation. For example, continuous stripping with dense-gas toluene in the supercritical state, a positive column temperature gradient from 330 to 380 C, and a pressure of 49 bar was used to strip the monomer and dimer species from an A-240 petroleum pitch feed, yielding a high molecular weight (mol wt) bottoms product rich in trimer and higher oligomers. Afterwards, semibatch operation was used with supercritical, dense-gas toluene, a temperature gradient of 330 to 380 C, and pressures from 84 to 111 bar to fractionate the above bottoms product, yielding a trimer-rich overhead (average mol wt (Mw) = 800) and a tetramer and higher residue with Mw 1000. Considering the two operations as a unit, a combined selectivity factor of 350 was obtained. Not only is this at least an order of magnitude better than what can in principle be accomplished by conventional, single-stage solvent extraction, but such extraction is inapplicable to our system because of the insolubility of the pitch fractions of interest in typical liquid solvents. Matrix-assisted, laser desorption/ionization time-of-flight mass spectrometry (MALDI) was used to verify that separation was indeed occurring by mol wt and to study the relationship between the Mw, softening point, and C/H ratio of the fractions produced. 2005 Elsevier Ltd. All rights reserved. Keywords: Coal-tar pitch; Petroleum pitch; High pressure; Mass spectrometry; Thermodynamic properties
1. Introduction Carbonaceous pitches can serve as precursors for advanced carbon materials, such as high-modulus carbon fibers, high thermal conductivity carbon fibers, and the matrix phase of carbon–carbon composites. Depending on the final product application, pitches with different average mol wts and properties are desired. For example, if high-performance fibers are to be manufactured, the starting pitch must consist primarily of higher
*
Corresponding author. Tel.: +1 864 656 5424; fax: +1 864 656 0784. E-mail address: [email protected] (M.C. Thies). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.07.042
mol wt (>800–1000) molecules that form a liquid crystalline phase, or mesophase [1,2]. On the other hand, if a pitch is to be used as the matrix phase of a composite, a lower mol wt, lower-viscosity pitch that gives a high carbon yield is desired [3]. To produce pitches for a given application, then, it would be logical to control the mol wt distribution (MWD) of the starting pitch, similar to what is done for the manufacture of articles from polymers. Unfortunately, traditional separation technologies for processing pitches do not allow us to control their mol wt in anything but a qualitative sense. For example, petroleum pitches produced by thermal polymerization (also known as heat-soaking) can be fractionated by singlestage solvent [4] or supercritical [5] extraction to eliminate a significant portion of the low mol wt, disordering
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
0.0040 Normalized Intensity
species from the pitch, but allow little control of the remaining, high mol wt mesophase pitch that is used to make the final carbon product. Similarly, mesophase pitches produced by the catalytic polymerization of naphthalene are stripped with nitrogen to remove low mol wt material [6]. In both cases, the limited available information [7,8] indicates that the process-ready pitches still have broad MWDs, ranging from about 300 to 2000 Da. In this paper, we present a multistage separation technique, which we call dense-gas extraction (DGE), for the fractionation of carbonaceous pitches, both on an analytical and a small pilot scale. The extractive solvent is a dense gas in the vicinity of its critical temperature, with the pressures being high enough so that the solvent has significantly more extractive power than an ideal gas (the pressure and temperature do not have to be supercritical, thus the more general term ‘‘dense gas’’ in lieu of the more specific ‘‘supercritical fluid’’). Because the goal is to obtain narrow mol wt fractions of pitch, the dense gas and solute are contacted in a packed column with a separation power equivalent to a number of equilibrium stages, and liquid reflux back down the column is used to further enhance product purity. In essence, then, DGE is a countercurrent, multistage supercritical extraction technique with reflux. Both the phase behavior and mass transfer properties of the solvent-pitch system can have a significant impact on the fractionation of pitch components that can be achieved. The basic concept of DGE was pioneered by Zosel [9,10] and was applied to a wide range of compounds, including a-olefin mixtures and cod liver oil. Warzinski [11] and Shi et al. [12] were the first researchers to apply DGE to heavy fossil fuels, in particular coal-derived and petroleum residua. However, neither of these researchers had access to characterization techniques appropriate for evaluating the efficacy of the DGE process. An initial evaluation of DGE for pitches was recently carried out by Clemson researchers on a prototype apparatus of the semibatch type [13]. Results for single-stage extraction, DGE, and column chromatography are compared in Fig. 1a and b. Here we see that DGE can be used to produce a narrow mol wt dimer cut from an A-240-type pitch, with the cut being of even higher purity (and in amounts 10 times greater) than that produced by preparative-scale chromatography. In this work, the DGE process was operated in two modes. First, the column was operated as a continuous stripper in which the lighter pitch fractions were stripped out as the overhead product and the heaviest pitch fractions were recovered in the bottoms. Next, one of the heavy bottom products was fractionated using the DGE column in the semibatch mode. MALDI was then used to examine the fractions and determine the extent to which the separation was by mol wt.
a 0.0030
Dimer Trimer
0.0020 0.0010 0.0000 250
500
750 1000 1250 1500 1750 Molecular Weight
0.012 Normalized Intensity
244
b 0.008 0.004
0.000 250
500 750 Molecular Weight
1000
Fig. 1. MALDI mass spectra showing the MWD of (a) Ashland A-240 feed pitch (black) and a pitch residue (grey) obtained by extraction with toluene per the method of Diefendorf (100 ml solvent per 1 g pitch feed) [4] and (b) dimer-rich fractions prepared by preparative liquid column chromatography (red) and DGE (black) [8]. In this and all succeeding MALDI spectra, the areas under the curves have been normalized to sum to 1.0. (For interpretation of the reference in colour in this figure legend, the reader is referred to the web version of this article.)
2. Experimental 2.1. Materials An isotropic petroleum pitch (A-240, CAS 68187-586) was obtained from Marathon Ashland Petroleum. This pitch is representative of a starting material that, after fractionation, can be used to manufacture carbon products such as fibers and composites [14]. As shown in Fig. 1, the pitch is oligomeric in nature, with the monomer centered around 280, dimer around 500, trimer around 750, and tetramer around 1000. (The offset of monomer to higher mol wts is probably due to either the sublimation of the low mol wt compounds from the target cell during MALDI or a devolatilization process in the manufacture of the pitch.) HPLC grade toluene (CAS 108-88-3) with a stated purity of 99.9% was obtained from Fisher Scientific. 2.2. Experimental apparatus A multistage extraction unit, capable of continuous or semibatch operation, was constructed to study the dense-gas extraction of pitches. The apparatus, which is shown in Fig. 2, is rated for 400 C and 200 bar. When operated in the continuous mode, the apparatus can be
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252 T
P
Regulating Valve
Reflux Finger
T
Extruder and Metering Pump
Overhead Product
T
Packed Section of Column
T
Level Detector T
T
Solvent Preheater Pump Center Conductor
A
DC Power Supply
Bottom Product
Fig. 2. Dense-gas extraction apparatus for continuous or semibatch operation.
used to separate up to 300 g/h of feed pitch into overhead and bottom products. On the other hand, semibatch operation can be used to fractionate 25-g charges of pitch into multiple overhead cuts and an unextracted residue. Briefly, the equipment consists of a pump and preheater for supplying solvent, a screw extruder and metering pump for supplying molten pitch (used only in continuous operation), a 1.8 cm i.d. · 202 cm tall column that contains a 147 cm high section of packing, and a reflux finger at the top. A regulating valve is used to control the column pressure by regulating the flow of overhead product exiting the column. In continuous operation, a bottom-phase level detector and bottom valve are used to collect the bottom product as it is generated. In semibatch operation, the level detector is removed, the stillpot is charged with the pitch to be fractionated, and the unextracted residue is collected from the stillpot at the conclusion of the experiment. 2.2.1. Dense-gas extraction for continuous operation For continuous operation, pitch is supplied at a constant mass flow rate (typically about 100–200 g/h) by a screw extruder (Alex James and Associates, Model AJA 58) in series with a metering pump (Zenith Pumps, Model HPB, .160 ml/rev). Molten pitch enters the column through a port located just above the packing. The dense-gas extractive solvent (in this work, toluene) is supplied at a constant mass flow rate (typically about 200–1000 g/h) by a two-piston reciprocating pump. The compressed solvent then flows through a preheater, where it is heated to the desired dense-gas conditions. The dense-gas solvent then flows into the bottom section of the column, adjacent to the top of the bottom-phase level detector, and rises through the packed section of the column, which is filled with
245
4-mm random packing (Cannon Instrument Co., part no. 3947-A20). The upward flow of dense-gas solvent is contacted with the downward, countercurrent flow of pitch in the packed section, stripping the lower mol wt species out of the pitch feed. Upon exiting the packed section, the solvent-extracted pitch contacts the reflux finger, which is housed in the top 14 cm of the column. This finger consists of a cone-shaped aluminum piece 13 cm long, with an o.d. of 13 mm that tapers to a point at the bottom and directs liquid condensate onto the packing. 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. The overhead vapor product exits the side of the column, adjacent to the top of the reflux finger, and is expanded to ambient pressures by means of a regulating valve (Autoclave Engineers, part no. 30VM4082-GY). The product is then cooled, condensed, and collected. Typically, sample sizes range from 200 to 600 g each (and from 5 to 100 g each on a solvent-free basis), depending on the extraction conditions used. The unextracted pitch, enriched in high mol wt species, exits the bottom of the packed section as the heavy phase. This heavy phase accumulates in the bottom of the column, where a custom-built, electrical-resistance level detector is used to indicate the bottom-phase level. The bottom product is expanded through a regulating valve (Autoclave Engineers, part no. 30VM4081-GY), cooled, and collected. The column itself consists of four custom-made 316SS manifolds (Autoclave Engineers) connected endto-end to form the column. The manifolds have a 1.8 cm (0.6875 in.) inner diameter and are rated to 690 bar (10,000 psi). The bottom manifold holds the pitch charge during semibatch operation and houses the level detector during continuous operation. The level detector operates by measuring the conductivity of the fluid filling the annular space between a center conductor and the vessel wall (see Fig. 2). The center conductor consists of a hexagonal rod, which is electrically insulated from the manifold wall by ceramic spacers. A 0.99-mm, SS wire is connected to the center conductor by a setscrew; this wire exits the column through a custom electrical feedthrough that uses a crushed soapstone cone as an electrically insulating pressure seal. Using this wire and an electrical contact on the outside of the manifold, a variable-voltage, DC power supply is used to apply an electrical potential (typically 5–10 VDC) across the gap between the center conductor and manifold wall (see Fig. 2). This potential induces a current flow proportional to the conductance of the fluid that fills this gap. Because the heavy phase is more electrically conductive than the light phase [15], the level can be obtained by monitoring the current. The voltage is adjusted so that a full-scale reading on the lA meter corresponds to a completely filled level detector. During
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operation, the liquid level is maintained between the empty lA reading (typically about 1 lA) and the full lA reading (45 lA) by opening the bottom valve and releasing the heavy phase into its collection vessel. The second and third manifolds from the bottom are of the same basic design, but are packed with random packing for the entire available internal length of 57 cm each. The top manifold has ports at the top for the pitch feed to enter and for the vapor phase to exit and contains 33 cm of packing, with a 14-cm-high open space at the top to accommodate the reflux finger. The temperature of the column is controlled by a PID controller and a system of band heaters (Watlow, Model Thinband) clamped around an aluminum jacket. The aluminum jacket is a 10.2 cm (4 in.) o.d. split cylinder machined to fit tightly around the hexagonal exterior shape of the column and allows for precise heating control along the entire column length. Finally, a 14.0 cm o.d. · 11.4 cm i.d. calcium silicate split cylinder is used for insulating the overall column setup. Column thermocouples were calibrated to an accuracy of ±1.0 C against a secondary standard platinum RTD (Burns Engineering, 200 Series) accurate to within ±0.1 C. 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 Fig. 2), 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 ±0.14 bar of the desired setpoint at all times, giving an uncertainty in the reported pressures of ±0.21 bar. 2.2.2. Dense-gas extraction for semibatch operation The dense-gas extraction apparatus modified for semibatch operation is shown in Fig. 3. In preparation for semibatch experiments, the level detector assembly, including the center conductor, electrical feedthrough wire, and related components, are removed. The bottom-phase valve is replaced with a medium-pressure adapter (High Pressure Equipment Co. (HIP), part no. 10-21AF4LM16), with a sintered, 20 lm disc (Micromeritics, part no. 32232) press-fit into the 1/4-in. end of the adapter. The disc serves both to support the pitch charge and to evenly distribute the flow of solvent upwards through the charge during operation. For an experimental run, the bottom manifold is first charged with the pitch to be extracted, typically 10–20 g, is then connected to the rest of the column, and the solvent line is connected to the HIP adapter. The apparatus
T Reflux Finger
Regulating Valve
Overhead Fraction
T
Packed Section
P
T
of Column T Stillpot T
Solvent Pump
Preheater
Pitch Charge T
Sintered Disc
Fig. 3. Dense-gas extraction apparatus for semibatch operation.
is purged with nitrogen through the solvent line overnight to prevent oxidation of the sample charge as the column is heated to operating temperature. Dense-gas solvent is supplied by the solvent pump and preheater and enters the bottom of the column through the HIP adapter. After passing through the sintered disc, the solvent mixes with the pitch charge in the bottom manifold, selectively extracting a portion of the pitch. The densegas mixture of solvent and extracted pitch then rises from the bottom manifold and enters the packed section of the column. Upon exiting the packed section, the dense-gas mixture contacts the reflux finger. If the temperature of the finger or the packed column is different from the bottom manifold 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 through the regulating valve. Typically, 5–10 samples are taken for 15–60 min each, with sample sizes ranging from 200 to 600 g each (and from 0.100 to 10 g each on a solvent-free basis), depending on the extraction conditions used. At the conclusion of the experiment, the column is depressurized, purged with nitrogen, and allowed to cool. The bottom manifold is removed and the unextracted residue is collected. 2.3. Solvent removal Samples collected from DGE experiments were dried to remove toluene before analysis. The samples were ramped to 160 C and held at that temperature for 24 h in an oven purged with nitrogen. Subsequent analysis (MALDI, softening point, and C/H ratio) was performed on the dried pitch samples.
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
2.4. MALDI analysis The molecular weight of the overhead fractions and residues collected from the DGE apparatus were analyzed using a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer. For analysis of the feed pitch and bottom products, which are not completely soluble in solvents, about 10 mg of sample and about 200 mg of the matrix 7,7,8,8-tetracyanoquinodimethane (TCNQ) were combined and mixed in a grinding mill (Thermo Electron Corp., model Wig-L-Bug). The resultant powder was then deposited onto 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 pipette, and the film was allowed to completely dry before MALDI analysis. In the case of the lower mol wt, overhead fractions (which are solvent soluble), a film of pure TCNQ was cast onto 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. The spectra presented herein are the summation of 1000 laser shots and acquisitions. Details of the procedures developed in our group for the analysis of pitches by MALDI are given elsewhere [7]. Although MALDI can be used to accurately determine the mol wts 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. Thus, in this study we have chosen to report results in terms of area percent. To obtain the area fractions by MALDI, the mass spectrum data was integrated by dividing the data into mol wt ranges, with each range representing an oligomer. For A-240 pitch, monomer was defined as the mol wt range from 250 to 375, dimer as 375 to 625, trimer as 625 to 875, tetramer as 875 to 1125, and pentamer as 1125–1375. In addition to determining the relative peak areas, the weight average mol wt (Mw) of a given pitch or pitch fraction (but based on areas and not true concentrations) was also calculated. The major source of variability in the MALDI data presented here is in the manual selection of laser power, which is believed to give a variability of ±6% in the reported Mw values. 2.5. Softening point and C/H analyses The softening point of the feed and bottom fractions was determined using a Fisher–Johns melting-point apparatus (Fisher Scientific, 150 W). For a measurement, the pan was heated to an estimated initial temperature, and the sample was checked for softening by applying about 5 mg of powder to the pan. If the sample did not soften, the sample was brushed away, the pan
247
was heated further, and a fresh sample was added and checked for softening. The measurement was done in air, hence a fresh, unoxidized sample was required for each test. The reproducibility of this measurement is ±2 C. The atomic C/H ratio was determined for the A-240 feed pitch and for selected fractions by elemental analysis (Atlantic Microlab, Inc.), with a stated absolute accuracy of 0.3%.
3. Results and discussion 3.1. DGE as a continuous-stripping operation Initial experiments with the DGE apparatus were performed in the continuous-stripping mode, that is, the lighter pitch fractions were stripped out as overhead products and the heaviest fractions were recovered in the bottoms (see Fig. 2). The effect of column temperature and pressure on product yield and composition was investigated at a constant toluene-to-pitch mass ratio of 5:1 (toluene and pitch flow rates were 598 and 119 g/h, respectively). The operating conditions examined in these experiments (Runs 1–19) are shown in Table 1, along with the bottoms yield (fraction of pitch feed that is collected in the bottom product), toluene content of products, Mw of dried overhead and bottoms products, and softening points (SP) of bottom products (overhead products often had SPs below room temperature). For Runs 1–3, 4–8, and 9–14, the column was operated isothermally, and the effect of increasing pressure for a given temperature was investigated. As seen in Table 1 and Fig. 4, increasing extraction pressure for a given column temperature results in a lower bottoms yield. This trend is expected, as an increase in pressure increases the density, and therefore solvent power, of the dense-gas toluene [15]. The more powerful solvent extracts more pitch, which is collected as the overhead product, while the unextracted pitch is collected as the bottom phase. At each temperature, a pressure was found at which no bottom phase was detected (for example, 49.3 bar at 330 C), where all of the pitch is extracted and is collected in the overhead product. On the other hand, the effect of temperature is more complex. As shown in Fig. 4, an increase in column temperature lowers pitch solubility (and increases bottoms yield) when the temperature is increased from 330 to 350 C. Over this temperature range, the density decrease of the supercritical toluene that occurs with temperature increase dominates. However, a further increase in temperature to 380 C actually increases pitch solubility at the lower pressures (now the volatility increase in pitch components with temperature is more important than the density decrease of the solvent), and only at pressures greater than 50 bar is solvent density the dominant effect on pitch solubility.
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Table 1 Operating conditions and product compositions for continuous-stripping experiments Bottoms yield
Bottom product SP (C)
Mw (by MALDI)
Pressure (bar)
Bottom
Top
Overhead
Bottom
Overhead
Bottom
Feed 1 2 3
– 35.5 42.4 49.3
– 330 330 330
– 330 330 330
– 0.92 0.16 0.00
– 0.99 0.84 –
– 0.25 0.27 –
87 88 180 –
572 323 563 –
572 740 750 –
4 5 6 7 8
35.5 42.4 49.3 56.2 63.1
350 350 350 350 350
350 350 350 350 350
0.95 0.88 0.87 0.24 0.00
0.99 0.98 0.98 0.87 –
0.17 0.17 0.35 0.12 –
91 104 93 278 –
336 319 379 524 –
680 707 702 848 –
9 10 11 12 13 14
35.5 42.4 49.3 63.1 70.0 83.8
380 380 380 380 380 380
380 380 380 380 380 380
0.86 0.85 0.81 0.55 0.29 0.00
0.98 0.97 0.97 0.92 0.88 –
0.13 0.06 0.16 0.23 0.11 –
92 92 91 162 243 –
315 340 339 462 549 –
682 720 699 700 795 –
15 16 17 18 19
49.3 52.7 56.2 70.0 83.8
330 330 330 330 330
380 380 380 380 380
0.06 0.08 0.10 0.24 0.00
0.85 0.85 0.85 0.87 –
0.11 0.16 0.13 0.16 –
378 358 315 288 –
576 602 458 477 –
967 885 947 850 –
Compositions are estimated to be accurate to ±0.01 wt fraction for the overhead and ±0.02 wt fraction for the bottom.
Bottom Phase Yield
1.00
330˚C 350˚C 380˚C 330-380˚C
0.80 0.60 0.40 0.20 0.00 30
40
50
60
70
80
90
Pressure (bar) Fig. 4. Bottoms yield (fraction of pitch fed that is recovered in the bottom product) is a function of column temperature and pressure.
As seen in Table 1, the heaviest bottom product obtained in our experiments using isothermal operation was that of Run 7. In an attempt to find conditions that would generate a heavier product, the column was operated with a temperature gradient, with the top of the column being held at 380 C, the middle of the column at 350 C, and the bottom at 330 C (Runs 15–19). By comparing Runs 3, 11, and 15 through both Table 1 and Fig. 4, we see the impact of using such a gradient. For Run 3, the complete solubility of the pitch in the solvent-rich phase is the result of both the relatively high solvent density at 330 C and 49.3 bar and the strong co-solvent interaction between the low mol wt and high mol wt species. Run 11 was performed at the same pressure, but the reduced solvent power of toluene at 380 C results in a bottoms yield of 81%. Here the
supercritical solvent is too weak to produce a high mol wt bottom fraction, but does remove some of the lightest pitch components that can act as co-solvents. On the other hand, Run 15 shows that by using a positive temperature gradient, one has additional control over the stripping process. With the lower mol wt species being extracted in the top section of the column, these components are no longer present to act as co-solvents. Thus, complete solubility of the pitch no longer occurs at 330 C, and a high mol wt bottoms product is obtained. Fig. 5 provides quantitative evidence of the significant shift in the MWD of bottoms product that can be produced by employing a column with a positive thermal gradient. Thus, in Run 7 only the monomer species were
0.0040 0.0035 Normalized Intensity
a
Column T (C)
Toluene wt fractiona
Run
0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 250
500
750
1000
1250
1500
1750
Molecular Weight Fig. 5. MALDI spectra of the feed pitch (thick black line) and bottoms fractions from Runs 7 (grey) and 15 (thin black line).
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
400 300 o
SP ( C)
completely stripped from the A-240 pitch feed, but in Run 15 almost all of the dimer components were also successfully removed. As shown in Fig. 4, the bottoms yield obtained when using a column temperature gradient does not follow the same trend with changing pressure as do the isothermal extractions. For pressures from 49.3 to 70 bar, yield increases with increasing pressure. This is opposite the trend observed in the isothermal experiments, and can be explained by the competing effects of pressure and species co-solvation as follows: Higher column pressures result in the extraction of more of the co-solvent-pitch species in the top section of the column; thus, there are lower concentrations of co-solvent species in the lower sections of the column to facilitate extraction of the higher mol wt species, and bottom yield increases. Because the yield increases with increasing pressure, the effect of co-solvation is more pronounced than that of column pressure. Finally, at some higher pressure one would expect the bottoms yield for the 330–380 C column to approach that of the 380 C column, as more and more of the pitch species are extracted in the top section of the column and never contact the solvent in the lower-temperature, bottom part of the column. As can be seen in Fig. 4, this occurs at a pressure of about 70 bar, with further increases in pressure to 83.8 bar leading to extraction of the entire pitch in the top section of the column. The softening point of each dried bottoms fraction was measured for comparison with data in other studies and with the mol wt information obtained using MALDI. Previous studies by Dauche [16] and Zhuang [5], both of whom used single-stage extraction with supercritical toluene, yielded bottoms products with softening points ranging from 203 to 319 C. As seen from Runs 1–14, these softening points are comparable to those produced in this work using an isothermal gradient (the softening point of the feed pitch used in the works of Dauche and Zhuang was slightly higher, about 102 C). In contrast, the use of multistage extraction with a temperature gradient allows for the continuous production of fractions with significantly higher softening points than have been previously reported. As shown in Fig. 6, the softening points of the bottoms fractions were found to increase with Mw as a general trend; however, some deviation from this rule was seen in the lower-melting cuts. Finally, we note that the bottom-phase pitches, which contained 10–25 wt% toluene when present at column operating conditions (see Table 1), had dramatically lower softening points before being dried to remove the toluene. For example, the bottoms product from Run 15 was easily collected as a hot, homogeneous liquid from the bottom of the column, which was at a temperature of 330 C. However, after the 11 wt % toluene was removed from this sample, the softening point
249
200 100 0 500
600
700 800 Mw (by MALDI)
900
1000
Fig. 6. The softening points of bottom products obtained by continuous stripping generally increase with Mw.
of this fraction increased to 378 C. These results are consistent with those of Conoco researchers [17], who recognized that this phenomenon could be used to process high-melting pitches at reduced temperatures. 3.2. DGE as a semibatch operation Following the continuous-stripping step, the high mol wt bottom product generated in Run 15 was fractionated using DGE in the semibatch mode (see Fig. 3). The bottom manifold was charged with 10 g of the bottoms product, and the column was heated to establish a temperature gradient, with the bottom of the column at 330 C, the middle of the column at 350 C, and the temperature increasing to 380 C at the top of the column and reflux finger. The dense-gas solvent toluene was supplied at a rate of 598 g/h and was preheated to 330 C before entering the column. Each overhead fraction reported in Table 2 is the result of a 60-min collection, so the entire semibatch operation lasted 6 h. The residue was removed at the conclusion of the experiment and was dried along with the six overhead fractions. As seen in Table 2 and Fig. 7, trimer purity increased during the extraction as the dimer species were exhausted from the pitch charge. By the third hour (Fraction 3), the dimer concentration had decreased such that trimer purities of over 60 area % were obtained. By the sixth hour (Fraction 6), tetramer concentrations had risen (as dimer and trimer were exhausted) and trimer purity again fell below 60%. MALDI spectra for the highest-purity trimer fraction and the heavy unextracted residue are shown in Fig. 8. If we consider Fractions 3 through 5 (Table 2), a cumulative total of 1.65 g of pitch having a trimer purity of over 60 area % was produced, a significantly purer high mol wt fraction than what was obtained in our prototype work with DGE [13]. In that study, the highestpurity trimer was only 40 area % (by MALDI) and the dominant oligomer was dimer, at 44 area %. We attribute the increase in trimer purity obtained in this work to the higher mol wt starting material (generated
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Table 2 Operating conditions and product compositions for semibatch dense-gas extraction Fraction
Pressure (bar)
Mass collected (g)
MALDI area fraction Monomer
Dimer
Trimer
Tetramer
Pentamer +
Feeda 1 2 3 4 5 6 Residue
– 56.2 83.8 83.8 83.8 111.4 111.4 –
10.00 0.44 0.53 0.60 0.40 0.65 0.25 6.25
0.012 0.256 0.068 0.003 0.027 0.005 0.014 0.002
0.048 0.617 0.319 0.161 0.149 0.109 0.105 0.002
0.391 0.103 0.461 0.626 0.601 0.652 0.595 0.325
0.339 0.019 0.132 0.184 0.193 0.205 0.236 0.488
0.211 0.005 0.020 0.025 0.030 0.028 0.049 0.184
a
Mw 967 505 717 785 785 803 824 992
Bottoms product from continuous-stripping Run 15 (see Table 1) served as Feed to the semibatch DGE.
MALDI Area Fraction
0.80
SFi;j ¼
ð1Þ
0.60 0.40 0.20 0.00 0
1
2 3 4 5 Collection Time (hr)
Dimer
Trimer
6
Tetramer
Fig. 7. MALDI area fractions of the three most abundant oligomers found in the overhead fractions produced by semibatch DGE.
0.0040 Normalized Intensity
areaproduct =areaproduct i j areafeed =areafeed i j
0.0030 0.0020
where the ratio of MALDI peak areas of oligomers i and j of the product are compared to those of the feed pitch. Our selectivity factor is similar to the ‘‘separation power’’ used for the evaluation of distillation columns [18], but compares only one product to the feed, rather than two products with each other. SF is reported here because it is applicable to both semibatch processes (which can have multiple products) and to our stripping process (where the overhead product is of little interest). When calculated for the separation of dimer and trimer (i.e., where i = trimer and j = dimer), the SF for traditional solvent extraction is 5.8 (see Fig. 1a) and for DGE Run 15 is 17.4 (see Fig. 5). The SF is for the semibatch process (here, the feed is the Run 15 bottoms product) is 19.9; thus, the combined effect of the continuous and semibatch DGE processes results in an overall SF of 346. Clearly, continuous and semibatch DGE are both more selective in separating the dimer and trimer species than traditional solvent extraction.
0.0010
3.3. C/H analyses of selected samples
0.0000 250
Five samples from the continuous and semibatch experiments were selected for elemental analysis. As shown in Table 3, the A-240 feed pitch has a C/H ratio of 1.41, the three higher mol wt fractions have higher C/H ratios, and the one lighter fraction has a lower C/H ratio. The observed trend is consistent with higher mol wt species having more condensed, polycyclic-aromatic structures and has also been observed in other fractionation work [5,16,19]. As seen in Fig. 9, the relationship between Mw and C/H ratio is essentially linear at Mw below about 800, but above 800 the C/H ratio begins to rise quickly. This may be due to experimental error (e.g., the Mw determination by MALDI) or it could be an indication that the highest mol wt molecules have much more condensed structures than the lower
500
750
1000 1250 1500 1750
Molecular Weight Fig. 8. MALDI spectra of the trimer-rich fraction (cut 5, thin black line) and high mol wt residue (grey) obtained by semibatch DGE. The feed to this extraction was the heavy bottoms product obtained from continuous DGE (Run 15, heavy black line).
by our continuous DGE setup) and the increased packing height of this apparatus (147 cm) as compared to the 70-cm packing height used in the previous work. To compare the separation offered by DGE to that of traditional, single-stage solvent extraction, we define the selectivity factor (SF) as
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252 Table 3 C/H analysis of selected DGE fractions Fraction
Mw
Atomic C/H
Run 9, top A240 Semibatch cut 5 Run 15, bottom Semibatch residue
315 572 803 967 992
1.20 1.41 1.55 1.78 1.86
251
be used to infer Mw. On the other hand, the C/H ratio increased consistently with mol wt and increased more sharply for the highest mol wt fractions. Such behavior is consistent with a pitch that has been polymerized by condensation reactions, yielding oligomers that are more aromatic than the monomers.
Acknowledgements 2.0
This work was supported by ERC Corp., ConocoPhillips Inc., and the Engineering Research Centers Program of the National Science Foundation under NSF Award Number EEC-9731680. The authors thank Robert Hammett and Austin Crooks for their assistance with the MALDI and GC analyses.
Atomic C/H
1.8 1.6 1.4 1.2 1.0 0
400
800
1200
Mw Fig. 9. Atomic C/H ratio increases with average mol wt Mw for fractions produced by DGE.
mol wt species. Interestingly, a similar result has been obtained in the analysis of petroleum vacuum residua [12].
4. Conclusions A multistage, dense-gas extraction apparatus has been constructed that can be operated in both the continuous and semibatch modes. Continuous, multistage fractionation of pitch has been validated in our laboratory on the small pilot scale and has proven itself to be a preferred alternative to the single-stage extractions used by previous researchers [4,16]. Compared to conventional solvent extraction, continuous DGE offers greater economic potential because of the lower solvent requirements and simplified processing technique. Of equal importance is the fact that a column temperature gradient can be used, so that enhanced selectivities are obtained compared to conventional separation processes. The results of this study indicate that we should be able to generate narrow mol wt calibration standards from pitches by the sequential application of continuous and semibatch DGE as follows. First, the lower mol wt pitch species are stripped away in the continuous mode; then the semibatch mode is applied to the isolated, higher mol wt bottoms product, and the desired mol wt cut is extracted as the overhead fraction. Finally, although softening point is clearly related to the molecular structures present in the pitch, it cannot
References [1] Dauche´ FM, Barnes AB, Gallego NC, Edie DD, Thies MC. Ribbon-shaped carbon fibers from supercritically extracted mesophase pitches. Carbon 1998;36:1238–40. [2] Hurt RH, Hu Y. Thermodynamics of carbonaceous mesophase. Carbon 1999;37:281–92. [3] Fathollahi B, Chau PC, White JL. Injection and stabilization of mesophase pitch in the fabrication of carbon–carbon composites. Part I. Injection process. Carbon 2005;43:125–33. [4] Diefendorf RJ, Riggs DM. Forming optically anisotropic pitches. US patent 4208267, 1980. [5] Zhuang MS, Gast K, Thies MC. Stagewise fractionation of petroleum pitches with supercritical toluene. Ind Eng Chem Res 2002;41:78–84. [6] Mochida I, Yoon S-H, Korai Y, Kanno K, Sakai Y, Komatsu M. Carbon fibers from aromatic hydrocarbons. Chemtech 1995;25:29–36. [7] Edwards WF, Jin L, Thies MC. MALDI-TOF mass spectrometry: obtaining reliable mass spectra for insoluble carbonaceous pitches. Carbon 2003;41:2761–8. [8] Edwards WF. Fractionation of carbonaceous pitches by molecular weight using dense-gas extraction, Clemson, SC, USA, Clemson University, PhD dissertation, 2005. [9] Zosel K. Process for the separation of mixtures of substances. US patent 3969196, 1976. [10] Zosel K. Separation with supercritical gases: practical applications. In: Schneider GM, Stahl E, Wilke G, editors. Extraction with supercritical gases. Weinheim: Verlag Chemie; 1980. p. 1–23. [11] Warzinski RP. Fractional destruction of coal-derived residuum. In: Squires TG, Paulaitis ME, editors. Supercritical fluids: chemical and engineering principles and applications. ACS symposium series 329. Washington, DC: American Chemical Society; 1987. p. 229–40. [12] Shi T-P, Hu Y-X, Xu Z-M, Su T, Wang R-A. Characterizing petroleum vacuum residue by supercritical fluid extraction and fractionation. Ind Eng Chem Res 1997;36:3988–92. [13] Edwards WF, Thies MC. Dense-gas fractionation of mixtures of petroleum macromolecules. Fluid Phase Equilibr 2004;224:231–6. [14] Edie DD, Dunham MG. Melt spinning pitch-based carbon fibers. Carbon 1989;27(5):647–55. [15] Zhuang MS, Thies MC. Extraction of petroleum pitch with supercritical toluene: experiment and prediction. Energy Fuels 2000;14:70–5.
252
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
[16] Dauche´ FM et al. Control of mesophase pitch properties by supercritical fluid extraction. Carbon 1998;36(7–8):953–61. [17] Kalback WM, Romine HE, Bourrat XA. Solvated mesophase pitches. US patent 5259947, 1993.
[18] Seader JD, Henley EJ. Separation process principles. New York: John Wiley & Sons; 1998. [19] Dickinson EM. Average structures of petroleum pitch fractions by 1 H/13C NMR spectroscopy. Fuel 1985;64:704–6.
Fractionation of pitches by molecular weight using continuous and semibatch dense-gas extraction William F. Edwards, Mark C. Thies
*
Center for Advanced Engineering Fibers and Films, Department of Chemical Engineering, Clemson University, Clemson, SC 29634-0909, USA Received 27 May 2005; accepted 23 July 2005 Available online 21 September 2005
Abstract A countercurrent, multistage, dense-gas extraction technique with reflux was investigated for the fractionation of carbonaceous pitches. Two modes of operation were investigated: continuous-stripping and semibatch operation. For example, continuous stripping with dense-gas toluene in the supercritical state, a positive column temperature gradient from 330 to 380 C, and a pressure of 49 bar was used to strip the monomer and dimer species from an A-240 petroleum pitch feed, yielding a high molecular weight (mol wt) bottoms product rich in trimer and higher oligomers. Afterwards, semibatch operation was used with supercritical, dense-gas toluene, a temperature gradient of 330 to 380 C, and pressures from 84 to 111 bar to fractionate the above bottoms product, yielding a trimer-rich overhead (average mol wt (Mw) = 800) and a tetramer and higher residue with Mw 1000. Considering the two operations as a unit, a combined selectivity factor of 350 was obtained. Not only is this at least an order of magnitude better than what can in principle be accomplished by conventional, single-stage solvent extraction, but such extraction is inapplicable to our system because of the insolubility of the pitch fractions of interest in typical liquid solvents. Matrix-assisted, laser desorption/ionization time-of-flight mass spectrometry (MALDI) was used to verify that separation was indeed occurring by mol wt and to study the relationship between the Mw, softening point, and C/H ratio of the fractions produced. 2005 Elsevier Ltd. All rights reserved. Keywords: Coal-tar pitch; Petroleum pitch; High pressure; Mass spectrometry; Thermodynamic properties
1. Introduction Carbonaceous pitches can serve as precursors for advanced carbon materials, such as high-modulus carbon fibers, high thermal conductivity carbon fibers, and the matrix phase of carbon–carbon composites. Depending on the final product application, pitches with different average mol wts and properties are desired. For example, if high-performance fibers are to be manufactured, the starting pitch must consist primarily of higher
*
Corresponding author. Tel.: +1 864 656 5424; fax: +1 864 656 0784. E-mail address: [email protected] (M.C. Thies). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.07.042
mol wt (>800–1000) molecules that form a liquid crystalline phase, or mesophase [1,2]. On the other hand, if a pitch is to be used as the matrix phase of a composite, a lower mol wt, lower-viscosity pitch that gives a high carbon yield is desired [3]. To produce pitches for a given application, then, it would be logical to control the mol wt distribution (MWD) of the starting pitch, similar to what is done for the manufacture of articles from polymers. Unfortunately, traditional separation technologies for processing pitches do not allow us to control their mol wt in anything but a qualitative sense. For example, petroleum pitches produced by thermal polymerization (also known as heat-soaking) can be fractionated by singlestage solvent [4] or supercritical [5] extraction to eliminate a significant portion of the low mol wt, disordering
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
0.0040 Normalized Intensity
species from the pitch, but allow little control of the remaining, high mol wt mesophase pitch that is used to make the final carbon product. Similarly, mesophase pitches produced by the catalytic polymerization of naphthalene are stripped with nitrogen to remove low mol wt material [6]. In both cases, the limited available information [7,8] indicates that the process-ready pitches still have broad MWDs, ranging from about 300 to 2000 Da. In this paper, we present a multistage separation technique, which we call dense-gas extraction (DGE), for the fractionation of carbonaceous pitches, both on an analytical and a small pilot scale. The extractive solvent is a dense gas in the vicinity of its critical temperature, with the pressures being high enough so that the solvent has significantly more extractive power than an ideal gas (the pressure and temperature do not have to be supercritical, thus the more general term ‘‘dense gas’’ in lieu of the more specific ‘‘supercritical fluid’’). Because the goal is to obtain narrow mol wt fractions of pitch, the dense gas and solute are contacted in a packed column with a separation power equivalent to a number of equilibrium stages, and liquid reflux back down the column is used to further enhance product purity. In essence, then, DGE is a countercurrent, multistage supercritical extraction technique with reflux. Both the phase behavior and mass transfer properties of the solvent-pitch system can have a significant impact on the fractionation of pitch components that can be achieved. The basic concept of DGE was pioneered by Zosel [9,10] and was applied to a wide range of compounds, including a-olefin mixtures and cod liver oil. Warzinski [11] and Shi et al. [12] were the first researchers to apply DGE to heavy fossil fuels, in particular coal-derived and petroleum residua. However, neither of these researchers had access to characterization techniques appropriate for evaluating the efficacy of the DGE process. An initial evaluation of DGE for pitches was recently carried out by Clemson researchers on a prototype apparatus of the semibatch type [13]. Results for single-stage extraction, DGE, and column chromatography are compared in Fig. 1a and b. Here we see that DGE can be used to produce a narrow mol wt dimer cut from an A-240-type pitch, with the cut being of even higher purity (and in amounts 10 times greater) than that produced by preparative-scale chromatography. In this work, the DGE process was operated in two modes. First, the column was operated as a continuous stripper in which the lighter pitch fractions were stripped out as the overhead product and the heaviest pitch fractions were recovered in the bottoms. Next, one of the heavy bottom products was fractionated using the DGE column in the semibatch mode. MALDI was then used to examine the fractions and determine the extent to which the separation was by mol wt.
a 0.0030
Dimer Trimer
0.0020 0.0010 0.0000 250
500
750 1000 1250 1500 1750 Molecular Weight
0.012 Normalized Intensity
244
b 0.008 0.004
0.000 250
500 750 Molecular Weight
1000
Fig. 1. MALDI mass spectra showing the MWD of (a) Ashland A-240 feed pitch (black) and a pitch residue (grey) obtained by extraction with toluene per the method of Diefendorf (100 ml solvent per 1 g pitch feed) [4] and (b) dimer-rich fractions prepared by preparative liquid column chromatography (red) and DGE (black) [8]. In this and all succeeding MALDI spectra, the areas under the curves have been normalized to sum to 1.0. (For interpretation of the reference in colour in this figure legend, the reader is referred to the web version of this article.)
2. Experimental 2.1. Materials An isotropic petroleum pitch (A-240, CAS 68187-586) was obtained from Marathon Ashland Petroleum. This pitch is representative of a starting material that, after fractionation, can be used to manufacture carbon products such as fibers and composites [14]. As shown in Fig. 1, the pitch is oligomeric in nature, with the monomer centered around 280, dimer around 500, trimer around 750, and tetramer around 1000. (The offset of monomer to higher mol wts is probably due to either the sublimation of the low mol wt compounds from the target cell during MALDI or a devolatilization process in the manufacture of the pitch.) HPLC grade toluene (CAS 108-88-3) with a stated purity of 99.9% was obtained from Fisher Scientific. 2.2. Experimental apparatus A multistage extraction unit, capable of continuous or semibatch operation, was constructed to study the dense-gas extraction of pitches. The apparatus, which is shown in Fig. 2, is rated for 400 C and 200 bar. When operated in the continuous mode, the apparatus can be
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252 T
P
Regulating Valve
Reflux Finger
T
Extruder and Metering Pump
Overhead Product
T
Packed Section of Column
T
Level Detector T
T
Solvent Preheater Pump Center Conductor
A
DC Power Supply
Bottom Product
Fig. 2. Dense-gas extraction apparatus for continuous or semibatch operation.
used to separate up to 300 g/h of feed pitch into overhead and bottom products. On the other hand, semibatch operation can be used to fractionate 25-g charges of pitch into multiple overhead cuts and an unextracted residue. Briefly, the equipment consists of a pump and preheater for supplying solvent, a screw extruder and metering pump for supplying molten pitch (used only in continuous operation), a 1.8 cm i.d. · 202 cm tall column that contains a 147 cm high section of packing, and a reflux finger at the top. A regulating valve is used to control the column pressure by regulating the flow of overhead product exiting the column. In continuous operation, a bottom-phase level detector and bottom valve are used to collect the bottom product as it is generated. In semibatch operation, the level detector is removed, the stillpot is charged with the pitch to be fractionated, and the unextracted residue is collected from the stillpot at the conclusion of the experiment. 2.2.1. Dense-gas extraction for continuous operation For continuous operation, pitch is supplied at a constant mass flow rate (typically about 100–200 g/h) by a screw extruder (Alex James and Associates, Model AJA 58) in series with a metering pump (Zenith Pumps, Model HPB, .160 ml/rev). Molten pitch enters the column through a port located just above the packing. The dense-gas extractive solvent (in this work, toluene) is supplied at a constant mass flow rate (typically about 200–1000 g/h) by a two-piston reciprocating pump. The compressed solvent then flows through a preheater, where it is heated to the desired dense-gas conditions. The dense-gas solvent then flows into the bottom section of the column, adjacent to the top of the bottom-phase level detector, and rises through the packed section of the column, which is filled with
245
4-mm random packing (Cannon Instrument Co., part no. 3947-A20). The upward flow of dense-gas solvent is contacted with the downward, countercurrent flow of pitch in the packed section, stripping the lower mol wt species out of the pitch feed. Upon exiting the packed section, the solvent-extracted pitch contacts the reflux finger, which is housed in the top 14 cm of the column. This finger consists of a cone-shaped aluminum piece 13 cm long, with an o.d. of 13 mm that tapers to a point at the bottom and directs liquid condensate onto the packing. 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. The overhead vapor product exits the side of the column, adjacent to the top of the reflux finger, and is expanded to ambient pressures by means of a regulating valve (Autoclave Engineers, part no. 30VM4082-GY). The product is then cooled, condensed, and collected. Typically, sample sizes range from 200 to 600 g each (and from 5 to 100 g each on a solvent-free basis), depending on the extraction conditions used. The unextracted pitch, enriched in high mol wt species, exits the bottom of the packed section as the heavy phase. This heavy phase accumulates in the bottom of the column, where a custom-built, electrical-resistance level detector is used to indicate the bottom-phase level. The bottom product is expanded through a regulating valve (Autoclave Engineers, part no. 30VM4081-GY), cooled, and collected. The column itself consists of four custom-made 316SS manifolds (Autoclave Engineers) connected endto-end to form the column. The manifolds have a 1.8 cm (0.6875 in.) inner diameter and are rated to 690 bar (10,000 psi). The bottom manifold holds the pitch charge during semibatch operation and houses the level detector during continuous operation. The level detector operates by measuring the conductivity of the fluid filling the annular space between a center conductor and the vessel wall (see Fig. 2). The center conductor consists of a hexagonal rod, which is electrically insulated from the manifold wall by ceramic spacers. A 0.99-mm, SS wire is connected to the center conductor by a setscrew; this wire exits the column through a custom electrical feedthrough that uses a crushed soapstone cone as an electrically insulating pressure seal. Using this wire and an electrical contact on the outside of the manifold, a variable-voltage, DC power supply is used to apply an electrical potential (typically 5–10 VDC) across the gap between the center conductor and manifold wall (see Fig. 2). This potential induces a current flow proportional to the conductance of the fluid that fills this gap. Because the heavy phase is more electrically conductive than the light phase [15], the level can be obtained by monitoring the current. The voltage is adjusted so that a full-scale reading on the lA meter corresponds to a completely filled level detector. During
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operation, the liquid level is maintained between the empty lA reading (typically about 1 lA) and the full lA reading (45 lA) by opening the bottom valve and releasing the heavy phase into its collection vessel. The second and third manifolds from the bottom are of the same basic design, but are packed with random packing for the entire available internal length of 57 cm each. The top manifold has ports at the top for the pitch feed to enter and for the vapor phase to exit and contains 33 cm of packing, with a 14-cm-high open space at the top to accommodate the reflux finger. The temperature of the column is controlled by a PID controller and a system of band heaters (Watlow, Model Thinband) clamped around an aluminum jacket. The aluminum jacket is a 10.2 cm (4 in.) o.d. split cylinder machined to fit tightly around the hexagonal exterior shape of the column and allows for precise heating control along the entire column length. Finally, a 14.0 cm o.d. · 11.4 cm i.d. calcium silicate split cylinder is used for insulating the overall column setup. Column thermocouples were calibrated to an accuracy of ±1.0 C against a secondary standard platinum RTD (Burns Engineering, 200 Series) accurate to within ±0.1 C. 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 Fig. 2), 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 ±0.14 bar of the desired setpoint at all times, giving an uncertainty in the reported pressures of ±0.21 bar. 2.2.2. Dense-gas extraction for semibatch operation The dense-gas extraction apparatus modified for semibatch operation is shown in Fig. 3. In preparation for semibatch experiments, the level detector assembly, including the center conductor, electrical feedthrough wire, and related components, are removed. The bottom-phase valve is replaced with a medium-pressure adapter (High Pressure Equipment Co. (HIP), part no. 10-21AF4LM16), with a sintered, 20 lm disc (Micromeritics, part no. 32232) press-fit into the 1/4-in. end of the adapter. The disc serves both to support the pitch charge and to evenly distribute the flow of solvent upwards through the charge during operation. For an experimental run, the bottom manifold is first charged with the pitch to be extracted, typically 10–20 g, is then connected to the rest of the column, and the solvent line is connected to the HIP adapter. The apparatus
T Reflux Finger
Regulating Valve
Overhead Fraction
T
Packed Section
P
T
of Column T Stillpot T
Solvent Pump
Preheater
Pitch Charge T
Sintered Disc
Fig. 3. Dense-gas extraction apparatus for semibatch operation.
is purged with nitrogen through the solvent line overnight to prevent oxidation of the sample charge as the column is heated to operating temperature. Dense-gas solvent is supplied by the solvent pump and preheater and enters the bottom of the column through the HIP adapter. After passing through the sintered disc, the solvent mixes with the pitch charge in the bottom manifold, selectively extracting a portion of the pitch. The densegas mixture of solvent and extracted pitch then rises from the bottom manifold and enters the packed section of the column. Upon exiting the packed section, the dense-gas mixture contacts the reflux finger. If the temperature of the finger or the packed column is different from the bottom manifold 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 through the regulating valve. Typically, 5–10 samples are taken for 15–60 min each, with sample sizes ranging from 200 to 600 g each (and from 0.100 to 10 g each on a solvent-free basis), depending on the extraction conditions used. At the conclusion of the experiment, the column is depressurized, purged with nitrogen, and allowed to cool. The bottom manifold is removed and the unextracted residue is collected. 2.3. Solvent removal Samples collected from DGE experiments were dried to remove toluene before analysis. The samples were ramped to 160 C and held at that temperature for 24 h in an oven purged with nitrogen. Subsequent analysis (MALDI, softening point, and C/H ratio) was performed on the dried pitch samples.
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2.4. MALDI analysis The molecular weight of the overhead fractions and residues collected from the DGE apparatus were analyzed using a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer. For analysis of the feed pitch and bottom products, which are not completely soluble in solvents, about 10 mg of sample and about 200 mg of the matrix 7,7,8,8-tetracyanoquinodimethane (TCNQ) were combined and mixed in a grinding mill (Thermo Electron Corp., model Wig-L-Bug). The resultant powder was then deposited onto 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 pipette, and the film was allowed to completely dry before MALDI analysis. In the case of the lower mol wt, overhead fractions (which are solvent soluble), a film of pure TCNQ was cast onto 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. The spectra presented herein are the summation of 1000 laser shots and acquisitions. Details of the procedures developed in our group for the analysis of pitches by MALDI are given elsewhere [7]. Although MALDI can be used to accurately determine the mol wts 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. Thus, in this study we have chosen to report results in terms of area percent. To obtain the area fractions by MALDI, the mass spectrum data was integrated by dividing the data into mol wt ranges, with each range representing an oligomer. For A-240 pitch, monomer was defined as the mol wt range from 250 to 375, dimer as 375 to 625, trimer as 625 to 875, tetramer as 875 to 1125, and pentamer as 1125–1375. In addition to determining the relative peak areas, the weight average mol wt (Mw) of a given pitch or pitch fraction (but based on areas and not true concentrations) was also calculated. The major source of variability in the MALDI data presented here is in the manual selection of laser power, which is believed to give a variability of ±6% in the reported Mw values. 2.5. Softening point and C/H analyses The softening point of the feed and bottom fractions was determined using a Fisher–Johns melting-point apparatus (Fisher Scientific, 150 W). For a measurement, the pan was heated to an estimated initial temperature, and the sample was checked for softening by applying about 5 mg of powder to the pan. If the sample did not soften, the sample was brushed away, the pan
247
was heated further, and a fresh sample was added and checked for softening. The measurement was done in air, hence a fresh, unoxidized sample was required for each test. The reproducibility of this measurement is ±2 C. The atomic C/H ratio was determined for the A-240 feed pitch and for selected fractions by elemental analysis (Atlantic Microlab, Inc.), with a stated absolute accuracy of 0.3%.
3. Results and discussion 3.1. DGE as a continuous-stripping operation Initial experiments with the DGE apparatus were performed in the continuous-stripping mode, that is, the lighter pitch fractions were stripped out as overhead products and the heaviest fractions were recovered in the bottoms (see Fig. 2). The effect of column temperature and pressure on product yield and composition was investigated at a constant toluene-to-pitch mass ratio of 5:1 (toluene and pitch flow rates were 598 and 119 g/h, respectively). The operating conditions examined in these experiments (Runs 1–19) are shown in Table 1, along with the bottoms yield (fraction of pitch feed that is collected in the bottom product), toluene content of products, Mw of dried overhead and bottoms products, and softening points (SP) of bottom products (overhead products often had SPs below room temperature). For Runs 1–3, 4–8, and 9–14, the column was operated isothermally, and the effect of increasing pressure for a given temperature was investigated. As seen in Table 1 and Fig. 4, increasing extraction pressure for a given column temperature results in a lower bottoms yield. This trend is expected, as an increase in pressure increases the density, and therefore solvent power, of the dense-gas toluene [15]. The more powerful solvent extracts more pitch, which is collected as the overhead product, while the unextracted pitch is collected as the bottom phase. At each temperature, a pressure was found at which no bottom phase was detected (for example, 49.3 bar at 330 C), where all of the pitch is extracted and is collected in the overhead product. On the other hand, the effect of temperature is more complex. As shown in Fig. 4, an increase in column temperature lowers pitch solubility (and increases bottoms yield) when the temperature is increased from 330 to 350 C. Over this temperature range, the density decrease of the supercritical toluene that occurs with temperature increase dominates. However, a further increase in temperature to 380 C actually increases pitch solubility at the lower pressures (now the volatility increase in pitch components with temperature is more important than the density decrease of the solvent), and only at pressures greater than 50 bar is solvent density the dominant effect on pitch solubility.
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Table 1 Operating conditions and product compositions for continuous-stripping experiments Bottoms yield
Bottom product SP (C)
Mw (by MALDI)
Pressure (bar)
Bottom
Top
Overhead
Bottom
Overhead
Bottom
Feed 1 2 3
– 35.5 42.4 49.3
– 330 330 330
– 330 330 330
– 0.92 0.16 0.00
– 0.99 0.84 –
– 0.25 0.27 –
87 88 180 –
572 323 563 –
572 740 750 –
4 5 6 7 8
35.5 42.4 49.3 56.2 63.1
350 350 350 350 350
350 350 350 350 350
0.95 0.88 0.87 0.24 0.00
0.99 0.98 0.98 0.87 –
0.17 0.17 0.35 0.12 –
91 104 93 278 –
336 319 379 524 –
680 707 702 848 –
9 10 11 12 13 14
35.5 42.4 49.3 63.1 70.0 83.8
380 380 380 380 380 380
380 380 380 380 380 380
0.86 0.85 0.81 0.55 0.29 0.00
0.98 0.97 0.97 0.92 0.88 –
0.13 0.06 0.16 0.23 0.11 –
92 92 91 162 243 –
315 340 339 462 549 –
682 720 699 700 795 –
15 16 17 18 19
49.3 52.7 56.2 70.0 83.8
330 330 330 330 330
380 380 380 380 380
0.06 0.08 0.10 0.24 0.00
0.85 0.85 0.85 0.87 –
0.11 0.16 0.13 0.16 –
378 358 315 288 –
576 602 458 477 –
967 885 947 850 –
Compositions are estimated to be accurate to ±0.01 wt fraction for the overhead and ±0.02 wt fraction for the bottom.
Bottom Phase Yield
1.00
330˚C 350˚C 380˚C 330-380˚C
0.80 0.60 0.40 0.20 0.00 30
40
50
60
70
80
90
Pressure (bar) Fig. 4. Bottoms yield (fraction of pitch fed that is recovered in the bottom product) is a function of column temperature and pressure.
As seen in Table 1, the heaviest bottom product obtained in our experiments using isothermal operation was that of Run 7. In an attempt to find conditions that would generate a heavier product, the column was operated with a temperature gradient, with the top of the column being held at 380 C, the middle of the column at 350 C, and the bottom at 330 C (Runs 15–19). By comparing Runs 3, 11, and 15 through both Table 1 and Fig. 4, we see the impact of using such a gradient. For Run 3, the complete solubility of the pitch in the solvent-rich phase is the result of both the relatively high solvent density at 330 C and 49.3 bar and the strong co-solvent interaction between the low mol wt and high mol wt species. Run 11 was performed at the same pressure, but the reduced solvent power of toluene at 380 C results in a bottoms yield of 81%. Here the
supercritical solvent is too weak to produce a high mol wt bottom fraction, but does remove some of the lightest pitch components that can act as co-solvents. On the other hand, Run 15 shows that by using a positive temperature gradient, one has additional control over the stripping process. With the lower mol wt species being extracted in the top section of the column, these components are no longer present to act as co-solvents. Thus, complete solubility of the pitch no longer occurs at 330 C, and a high mol wt bottoms product is obtained. Fig. 5 provides quantitative evidence of the significant shift in the MWD of bottoms product that can be produced by employing a column with a positive thermal gradient. Thus, in Run 7 only the monomer species were
0.0040 0.0035 Normalized Intensity
a
Column T (C)
Toluene wt fractiona
Run
0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 250
500
750
1000
1250
1500
1750
Molecular Weight Fig. 5. MALDI spectra of the feed pitch (thick black line) and bottoms fractions from Runs 7 (grey) and 15 (thin black line).
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
400 300 o
SP ( C)
completely stripped from the A-240 pitch feed, but in Run 15 almost all of the dimer components were also successfully removed. As shown in Fig. 4, the bottoms yield obtained when using a column temperature gradient does not follow the same trend with changing pressure as do the isothermal extractions. For pressures from 49.3 to 70 bar, yield increases with increasing pressure. This is opposite the trend observed in the isothermal experiments, and can be explained by the competing effects of pressure and species co-solvation as follows: Higher column pressures result in the extraction of more of the co-solvent-pitch species in the top section of the column; thus, there are lower concentrations of co-solvent species in the lower sections of the column to facilitate extraction of the higher mol wt species, and bottom yield increases. Because the yield increases with increasing pressure, the effect of co-solvation is more pronounced than that of column pressure. Finally, at some higher pressure one would expect the bottoms yield for the 330–380 C column to approach that of the 380 C column, as more and more of the pitch species are extracted in the top section of the column and never contact the solvent in the lower-temperature, bottom part of the column. As can be seen in Fig. 4, this occurs at a pressure of about 70 bar, with further increases in pressure to 83.8 bar leading to extraction of the entire pitch in the top section of the column. The softening point of each dried bottoms fraction was measured for comparison with data in other studies and with the mol wt information obtained using MALDI. Previous studies by Dauche [16] and Zhuang [5], both of whom used single-stage extraction with supercritical toluene, yielded bottoms products with softening points ranging from 203 to 319 C. As seen from Runs 1–14, these softening points are comparable to those produced in this work using an isothermal gradient (the softening point of the feed pitch used in the works of Dauche and Zhuang was slightly higher, about 102 C). In contrast, the use of multistage extraction with a temperature gradient allows for the continuous production of fractions with significantly higher softening points than have been previously reported. As shown in Fig. 6, the softening points of the bottoms fractions were found to increase with Mw as a general trend; however, some deviation from this rule was seen in the lower-melting cuts. Finally, we note that the bottom-phase pitches, which contained 10–25 wt% toluene when present at column operating conditions (see Table 1), had dramatically lower softening points before being dried to remove the toluene. For example, the bottoms product from Run 15 was easily collected as a hot, homogeneous liquid from the bottom of the column, which was at a temperature of 330 C. However, after the 11 wt % toluene was removed from this sample, the softening point
249
200 100 0 500
600
700 800 Mw (by MALDI)
900
1000
Fig. 6. The softening points of bottom products obtained by continuous stripping generally increase with Mw.
of this fraction increased to 378 C. These results are consistent with those of Conoco researchers [17], who recognized that this phenomenon could be used to process high-melting pitches at reduced temperatures. 3.2. DGE as a semibatch operation Following the continuous-stripping step, the high mol wt bottom product generated in Run 15 was fractionated using DGE in the semibatch mode (see Fig. 3). The bottom manifold was charged with 10 g of the bottoms product, and the column was heated to establish a temperature gradient, with the bottom of the column at 330 C, the middle of the column at 350 C, and the temperature increasing to 380 C at the top of the column and reflux finger. The dense-gas solvent toluene was supplied at a rate of 598 g/h and was preheated to 330 C before entering the column. Each overhead fraction reported in Table 2 is the result of a 60-min collection, so the entire semibatch operation lasted 6 h. The residue was removed at the conclusion of the experiment and was dried along with the six overhead fractions. As seen in Table 2 and Fig. 7, trimer purity increased during the extraction as the dimer species were exhausted from the pitch charge. By the third hour (Fraction 3), the dimer concentration had decreased such that trimer purities of over 60 area % were obtained. By the sixth hour (Fraction 6), tetramer concentrations had risen (as dimer and trimer were exhausted) and trimer purity again fell below 60%. MALDI spectra for the highest-purity trimer fraction and the heavy unextracted residue are shown in Fig. 8. If we consider Fractions 3 through 5 (Table 2), a cumulative total of 1.65 g of pitch having a trimer purity of over 60 area % was produced, a significantly purer high mol wt fraction than what was obtained in our prototype work with DGE [13]. In that study, the highestpurity trimer was only 40 area % (by MALDI) and the dominant oligomer was dimer, at 44 area %. We attribute the increase in trimer purity obtained in this work to the higher mol wt starting material (generated
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Table 2 Operating conditions and product compositions for semibatch dense-gas extraction Fraction
Pressure (bar)
Mass collected (g)
MALDI area fraction Monomer
Dimer
Trimer
Tetramer
Pentamer +
Feeda 1 2 3 4 5 6 Residue
– 56.2 83.8 83.8 83.8 111.4 111.4 –
10.00 0.44 0.53 0.60 0.40 0.65 0.25 6.25
0.012 0.256 0.068 0.003 0.027 0.005 0.014 0.002
0.048 0.617 0.319 0.161 0.149 0.109 0.105 0.002
0.391 0.103 0.461 0.626 0.601 0.652 0.595 0.325
0.339 0.019 0.132 0.184 0.193 0.205 0.236 0.488
0.211 0.005 0.020 0.025 0.030 0.028 0.049 0.184
a
Mw 967 505 717 785 785 803 824 992
Bottoms product from continuous-stripping Run 15 (see Table 1) served as Feed to the semibatch DGE.
MALDI Area Fraction
0.80
SFi;j ¼
ð1Þ
0.60 0.40 0.20 0.00 0
1
2 3 4 5 Collection Time (hr)
Dimer
Trimer
6
Tetramer
Fig. 7. MALDI area fractions of the three most abundant oligomers found in the overhead fractions produced by semibatch DGE.
0.0040 Normalized Intensity
areaproduct =areaproduct i j areafeed =areafeed i j
0.0030 0.0020
where the ratio of MALDI peak areas of oligomers i and j of the product are compared to those of the feed pitch. Our selectivity factor is similar to the ‘‘separation power’’ used for the evaluation of distillation columns [18], but compares only one product to the feed, rather than two products with each other. SF is reported here because it is applicable to both semibatch processes (which can have multiple products) and to our stripping process (where the overhead product is of little interest). When calculated for the separation of dimer and trimer (i.e., where i = trimer and j = dimer), the SF for traditional solvent extraction is 5.8 (see Fig. 1a) and for DGE Run 15 is 17.4 (see Fig. 5). The SF is for the semibatch process (here, the feed is the Run 15 bottoms product) is 19.9; thus, the combined effect of the continuous and semibatch DGE processes results in an overall SF of 346. Clearly, continuous and semibatch DGE are both more selective in separating the dimer and trimer species than traditional solvent extraction.
0.0010
3.3. C/H analyses of selected samples
0.0000 250
Five samples from the continuous and semibatch experiments were selected for elemental analysis. As shown in Table 3, the A-240 feed pitch has a C/H ratio of 1.41, the three higher mol wt fractions have higher C/H ratios, and the one lighter fraction has a lower C/H ratio. The observed trend is consistent with higher mol wt species having more condensed, polycyclic-aromatic structures and has also been observed in other fractionation work [5,16,19]. As seen in Fig. 9, the relationship between Mw and C/H ratio is essentially linear at Mw below about 800, but above 800 the C/H ratio begins to rise quickly. This may be due to experimental error (e.g., the Mw determination by MALDI) or it could be an indication that the highest mol wt molecules have much more condensed structures than the lower
500
750
1000 1250 1500 1750
Molecular Weight Fig. 8. MALDI spectra of the trimer-rich fraction (cut 5, thin black line) and high mol wt residue (grey) obtained by semibatch DGE. The feed to this extraction was the heavy bottoms product obtained from continuous DGE (Run 15, heavy black line).
by our continuous DGE setup) and the increased packing height of this apparatus (147 cm) as compared to the 70-cm packing height used in the previous work. To compare the separation offered by DGE to that of traditional, single-stage solvent extraction, we define the selectivity factor (SF) as
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252 Table 3 C/H analysis of selected DGE fractions Fraction
Mw
Atomic C/H
Run 9, top A240 Semibatch cut 5 Run 15, bottom Semibatch residue
315 572 803 967 992
1.20 1.41 1.55 1.78 1.86
251
be used to infer Mw. On the other hand, the C/H ratio increased consistently with mol wt and increased more sharply for the highest mol wt fractions. Such behavior is consistent with a pitch that has been polymerized by condensation reactions, yielding oligomers that are more aromatic than the monomers.
Acknowledgements 2.0
This work was supported by ERC Corp., ConocoPhillips Inc., and the Engineering Research Centers Program of the National Science Foundation under NSF Award Number EEC-9731680. The authors thank Robert Hammett and Austin Crooks for their assistance with the MALDI and GC analyses.
Atomic C/H
1.8 1.6 1.4 1.2 1.0 0
400
800
1200
Mw Fig. 9. Atomic C/H ratio increases with average mol wt Mw for fractions produced by DGE.
mol wt species. Interestingly, a similar result has been obtained in the analysis of petroleum vacuum residua [12].
4. Conclusions A multistage, dense-gas extraction apparatus has been constructed that can be operated in both the continuous and semibatch modes. Continuous, multistage fractionation of pitch has been validated in our laboratory on the small pilot scale and has proven itself to be a preferred alternative to the single-stage extractions used by previous researchers [4,16]. Compared to conventional solvent extraction, continuous DGE offers greater economic potential because of the lower solvent requirements and simplified processing technique. Of equal importance is the fact that a column temperature gradient can be used, so that enhanced selectivities are obtained compared to conventional separation processes. The results of this study indicate that we should be able to generate narrow mol wt calibration standards from pitches by the sequential application of continuous and semibatch DGE as follows. First, the lower mol wt pitch species are stripped away in the continuous mode; then the semibatch mode is applied to the isolated, higher mol wt bottoms product, and the desired mol wt cut is extracted as the overhead fraction. Finally, although softening point is clearly related to the molecular structures present in the pitch, it cannot
References [1] Dauche´ FM, Barnes AB, Gallego NC, Edie DD, Thies MC. Ribbon-shaped carbon fibers from supercritically extracted mesophase pitches. Carbon 1998;36:1238–40. [2] Hurt RH, Hu Y. Thermodynamics of carbonaceous mesophase. Carbon 1999;37:281–92. [3] Fathollahi B, Chau PC, White JL. Injection and stabilization of mesophase pitch in the fabrication of carbon–carbon composites. Part I. Injection process. Carbon 2005;43:125–33. [4] Diefendorf RJ, Riggs DM. Forming optically anisotropic pitches. US patent 4208267, 1980. [5] Zhuang MS, Gast K, Thies MC. Stagewise fractionation of petroleum pitches with supercritical toluene. Ind Eng Chem Res 2002;41:78–84. [6] Mochida I, Yoon S-H, Korai Y, Kanno K, Sakai Y, Komatsu M. Carbon fibers from aromatic hydrocarbons. Chemtech 1995;25:29–36. [7] Edwards WF, Jin L, Thies MC. MALDI-TOF mass spectrometry: obtaining reliable mass spectra for insoluble carbonaceous pitches. Carbon 2003;41:2761–8. [8] Edwards WF. Fractionation of carbonaceous pitches by molecular weight using dense-gas extraction, Clemson, SC, USA, Clemson University, PhD dissertation, 2005. [9] Zosel K. Process for the separation of mixtures of substances. US patent 3969196, 1976. [10] Zosel K. Separation with supercritical gases: practical applications. In: Schneider GM, Stahl E, Wilke G, editors. Extraction with supercritical gases. Weinheim: Verlag Chemie; 1980. p. 1–23. [11] Warzinski RP. Fractional destruction of coal-derived residuum. In: Squires TG, Paulaitis ME, editors. Supercritical fluids: chemical and engineering principles and applications. ACS symposium series 329. Washington, DC: American Chemical Society; 1987. p. 229–40. [12] Shi T-P, Hu Y-X, Xu Z-M, Su T, Wang R-A. Characterizing petroleum vacuum residue by supercritical fluid extraction and fractionation. Ind Eng Chem Res 1997;36:3988–92. [13] Edwards WF, Thies MC. Dense-gas fractionation of mixtures of petroleum macromolecules. Fluid Phase Equilibr 2004;224:231–6. [14] Edie DD, Dunham MG. Melt spinning pitch-based carbon fibers. Carbon 1989;27(5):647–55. [15] Zhuang MS, Thies MC. Extraction of petroleum pitch with supercritical toluene: experiment and prediction. Energy Fuels 2000;14:70–5.
252
W.F. Edwards, M.C. Thies / Carbon 44 (2006) 243–252
[16] Dauche´ FM et al. Control of mesophase pitch properties by supercritical fluid extraction. Carbon 1998;36(7–8):953–61. [17] Kalback WM, Romine HE, Bourrat XA. Solvated mesophase pitches. US patent 5259947, 1993.
[18] Seader JD, Henley EJ. Separation process principles. New York: John Wiley & Sons; 1998. [19] Dickinson EM. Average structures of petroleum pitch fractions by 1 H/13C NMR spectroscopy. Fuel 1985;64:704–6.