(662c) Isolating Petroleum Pitch Oligomers Via Multistage Supercritical Extraction

Abstract

In this investigation, an oligomeric petroleum pitch (M-50) was fractionated using two continuous, counter-current, multistage, packed columns. Pure monomer and dimer were isolated using supercritical toluene in the first column (Col-1), while supercritical pentane was used in the second column (Col-2). A trimer-rich pitch was isolated using supercritical toluene in both columns. As demonstrated, solvent-to-pitch (S/P) ratios >10 and positive temperature gradients (+ΔT) improve the selectivity of the process. In addition, the feed location was compared, i.e., a stripper vs. a rectifier/stripper. Overall, monomer- and dimer-rich pitches with purities greater than 96 mol% and trimer-rich pitches of 60 mol% where obtained on a continuous basis. Based on these results, the composition of M-50 was estimated to be 50 wt% monomer, 27 wt% dimer, and 23 wt% of trimer and heavier species. To our knowledge, this is the first time that the oligomers from an oligomeric pitch are quantified.

Introduction

Petroleum pitches are carbonaceous materials that serve as precursors for advanced carbon products. Because of their highly graphitic nature, such pitches are of particular interest for producing high thermal conductivity carbon fibers for thermal management applications, such as microelectronics packaging for hybrid and plug-in vehicles.

Even though the properties of the final carbon products are impacted by the molecular composition of the starting pitch precursor, a fundamental understanding of this relationship has yet to be established. To this end, dense-gas extraction (DGE), a fractionation technique that allows for the production of pitches of controlled composition for both analytical and rheological characterization, was investigated.

As shown in Figure 1,  M-50 petroleum pitch from Marathon Oil Corporation, is oligomeric in nature with, a molecular weight (mol wt) that extends from approximately 200 to over 1000 [1,2]. The species themselves consist of alkylated polycyclic aromatic hydrocarbons (PAHs). The predominant molecular species present in M-50 pitch have recently been identified by Burgess and Thies [3,4].

Figure 1. Fractionation of M-50 via two-column DGE to produce dimer-rich pitches. Col-1 and Col-2 are operated under toluene and pentane supercritical extraction conditions, respectively.  Spectra shown are actual MALDI spectra of feed pitch M-50 and extraction products obtained in this study. Mo, Di, Dimer+ refers to monomer, dimer, and dimer plus heavier oligomers, respectively.

Experimental

As shown in Figure 1, fractionation of M-50 pitch was carried out using two packed columns. The feed pitch is fed to Col-1 via a single-screw extruder and metering pump. In order to isolate dimer-rich pitches, an evaporator is used to concentrate the top product from Col-1 (i.e., close V₁ and V₃, open V₂). To produce trimer-rich pitches, the top product of Col-1 if fed directly into Col-2 (i.e., close V₁ and V₂, open V₃). In Col-2, supercritical toluene and toluene-pentane mixtures are used as extractive solvents. The maximum operating temperatures and pressures of both columns is 400 °C and 200 bar. Both columns consist of a 2.0 m high x 1.8 cm i.d., with an actual packing height of 1.5 m. Details of the design and construction of the apparatus are given elsewhere [5]. Typical operating variables include column temperature gradient and pressure, solvent-to-pitch (S/P) ratio, solvent composition, and feed location (i.e., stripper vs. stripper/rectifier).

Pitch fractions produced by DGE were analyzed for absolute molecular weight and mol fractions using a Bruker Daltonics Autoflex matrix-assisted, laser desorption/ionization, time-of-flight (MALDI-TOF) mass spectrometer equipped with a 337 nm nitrogen laser.  Instrument settings and sample preparation details are given elsewhere [6].  Considering the oligomeric nature of the feed pitch and the mechanisms of pitch formation [3,4], we have defined monomer species as having a mol wt from 210 to 388, dimer species from 388 to 645, trimer species from 645 to 890, tetramer from 890 to 1120, and pentamer and heavier from 1120 to 1500 Da.  Reported softening points were obtained with a Mettler FP83HT Dropping Point Cell, equipped with softening-point cups.

Results and Discussion

As shown in Figure 1, in order to produce dimer-rich pitches, the operating conditions in Col-1 must be such that only monomer and dimer species are extracted. On the other hand, if trimer-rich pitches are to be produced, the extract from Col-1 should contain monomer, dimer, and trimer species only. Thus, to produce dimer-rich pitches, Col-1 was operated as a stripper/rectifier at a +ΔT (400 top, 380 middle, 350 °C bottom), S/P ratio of 12.1/1, 69.9 bar (1000 psig), and toluene as the supercritical extractive solvent. An evaporator was used to increase pitch concentration in the solvent from 6 to 64 wt%. The concentrated pitch solution, comprised of monomer and dimer species only, was then sent to Col-2, where both pentane and pentane-toluene mixtures were used to extract all monomer. The pitch-rich phase (i.e., dimer-rich), flows down the column and is collected as a bottom product. To separate monomer from dimer in Col-2, different S/P ratios, pressures, temperature gradients, and solvent compositions were investigated. For instance, good selectivity was obtained when Col-2 was operated as a stripper/rectifier at +ΔT (270-260-250-240 °C), S/P ratio of 22/1, 69.9 bar, and pentane as the supercritical extractive solvent. As seen in Table 1, a sharp separation between monomer and dimer was achieved, with impurities in each product (Monomer A and Dimer A) being less than 5 mol%.

In order to extract all monomer from the feed pitch to Col-2, and obtain a dimer-rich product of higher purity, other extraction pressures, temperatures profiles, S/P ratios, and solvent compositions were investigated. Complete extraction of monomer from the feed pitch to Col-2 was obtained by operating Col-2 as a striper/rectifier at +ΔT (270-260-250-240 °C), S/P ratio of 24, 69.9 bar, and 25/75 vol% toluene-pentane mixture as solvent. In this case, a 99 mol% dimer-rich pitch (Dimer B) was produced at 9.3 g/h (overall yield of 14%).

To produce trimer-rich pitches, Col-1 was operated as a stripper/rectifier at +ΔT (380, 350, 330 °C), S/P ratio of 8.0/1, and 69.9 bar. Supercritical toluene was used as the extractive solvent. Under these conditions, a top product (or extract) comprised of 19 mol% monomer, 65 mol% dimer, 15 mol% trimer, and minimal tetramer (1 mol% or lower) is obtained. This top product from Col-1, containing 10 wt% pitch, was then sent to Col-2, where the feed location and S/P ratio where kept constant (stripper/rectifier, 17/1) and both pressure and temperature profiles were varied in order to control the solvent power and selectivity of the extraction.

As shown in Table 1, a 60 mol% trimer-rich pitch (Trimer A) was produced at an overall yield of 7%. Compared to a trimer-rich pitch (Trimer B) produced via one-column DGE (i.e., the bottom product from Col-1), trimer-rich pitches of lower tetramer and pentamer content were obtained with the two-column DGE process.

Table 1. Composition, overall pitch yield and softening point of selected pitches produced in this work.

Conclusions

Pitches of narrow molecular weight distribution were isolated from the oligomeric feed pitch (M-50) on a continuous basis and characterized by MALDI mass spectrometry. In particular, monomer- and dimer-rich pitches with purities greater than 96 mol%, and trimer-rich pitches of 60 mol% purity, were obtained at overall product yields of 50% for the monomer, 14% for the dimer, and 7% for the trimer. The softening point of pure monomer was found to be 41 °C, of pure dimer 217 °C, and of a trimer-rich cut 286 °C. By combining DGE with our analytical characterization techniques, we have been able to obtain the first reliable estimate of the molecular composition of M-50 pitch: 50 wt% monomer, 27 wt% dimer, and 23 wt% trimer and heavier species.

References

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[2] Edwards WF, Jin L, Thies MC. MALDI-TOF mass spectrometry: obtaining reliable mass spectra for insoluble carbonaceous pitches. Carbon 2003; 41:2761-2768.

[3] Burgess WA, Thies MC. Spectral identification of the monomeric fraction of aromatic hydrocarbons present in a petroleum pitch. Submitted for publication in Energy & Fuels, 2010.

[4] Burgess WA, Thies MC. Structural characterization of the oligomeric constituents of petroleum pitches: structures of molecules of higher molecular weight (> ~ 600 g/mol).  To be submitted for publication in Carbon, 2010.

[5] Cervo EG, Thies MC. Control of molecular weight distribution of petroleum pitches via multistage supercritical extraction. Journal of Supercritical Fluids 2010; 51:345-352.

[6] Cristadoro A, Kulkarni SU, Burgess WA, Cervo EG, Räder HJ, Müllen K, Bruce DA, Thies MC. Structural characterization of the oligomeric constituents of petroleum pitches. Carbon 2009; 47:2358-2370.