Identification of a Polycyclic Aromatic Hydrocarbon Indicator for the

Mar 18, 2009 - the Onset of Coke Formation during Visbreaking of a Vacuum ... To identify hydrocarbons that might be sensitive indicators of coke form...
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Energy & Fuels 2009, 23, 2157–2163

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Identification of a Polycyclic Aromatic Hydrocarbon Indicator for the Onset of Coke Formation during Visbreaking of a Vacuum Residue Kingsley U. Ogbuneke,† Colin E. Snape,*,† John M. Andre´sen,† Simon Crozier,‡ Christopher Russell,‡ and Ron Sharpe‡ Department of Chemical and EnVironmental Engineering, Faculty of Engineering, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom, and Refinery and Fuel Management Research Group, Nalco Limited (Energy SerVices DiVision), Block 102, Cadland Road, Hardley, Hythe, Southampton SO45 3NP, United Kingdom ReceiVed December 2, 2008. ReVised Manuscript ReceiVed January 27, 2009

To identify hydrocarbons that might be sensitive indicators of coke formation in visbreaking, experiments have been conducted on a vacuum residue at 410 °C with residence times of up to 60 min in a microreactor. An induction period of 40 min prior to coke formation was observed, consistent with previous laboratory studies, with the changes in bulk aromaticity being small. As an example of trace aliphatic components, the distribution of hopanes proved to be highly sensitive to cracking severity but not to the onset of coke formation. In contrast, the concentrations and distributions of polycyclic aromatic hydrocarbons (PAHs) are sensitive to coke formation. Small ring size PAHs and their substituted equivalents were present in the feed, and their concentrations increased with coke formation. However, for the 6-ring PAH, benzo[ghi]perylene, the concentration was constant at ca. 0.1 ppm of the total aromatics during the coke induction period and then increased sharply thereafter to ca. 0.3 ppm. Therefore, it is proposed that tracing the concentrations of large ring PAHs, such as benzo[ghi]perylene, offers a novel and sensitive approach to assess the onset of coke formation in visbreaking.

Introduction Viscosity breaking or “visbreaking” is a mild thermal cracking process used in the petroleum refinery to upgrade heavy crude oils or residual or asphaltic feedstocks with the aim of reducing viscosity and pour-point characteristics.1,2 Similar to any other thermal process, visbreaking suffers from the disadvantage of producing coke, which causes fouling of reactor heater tubes and reduces the efficiency of operation in the refinery.3 In general, conversion to distillates improves with increasing severity up to a certain level beyond which coke will be formed. Both conversion and coke formation are chiefly dependent upon the feedstock characteristics, temperature, and residence time of operation.4 In terms of previous work addressing some of the factors that can affect coke formation, Del Bianco et al.5 studied the effect of the presence of hydrogen donor diluents on visbreaking and reported from their findings that visbreaking performances can be substantially improved by adding hydrogen-donor * To whom correspondence should be addressed. E-mail: colin.snape@ nottingham.ac.uk. † University of Nottingham. ‡ Nalco Limited. (1) Dominici, V. E.; Sieli, G. M. Handbook of Petroleum Refining Processes; McGraw-Hill: New York, 1997. (2) Ballard, W. P.; Cottington, G. I.; Cooper, T. A. Petroleum Processing Handbook; Marcel Dekker, Inc.: New York, 1992. (3) Schabron, J. F.; Pauli, A. T.; Rovani, J. F., Jr.; Miknis, F. P. Fuel 2001, 80, 1435–1446. (4) Omole, O.; Olieh, M. N.; Osinowo, T. Fuel 1999, 78, 1489–1496. (5) Del Bianco, A.; Garuti, G.; Pirovano, C.; Ruso, R. Fuel 1995, 74 (5), 756–760.

solvents. Di Carlo and Janis6 studied the effect of composition of feedstocks on the thermal cracking behavior, where they also derived kinetic parameters for the feed that they studied. Krishna et al.7 carried out studies on the effects of temperature and residence time on the yields and properties of visbroken distillates and Aghajari long residues. They also reported data showing the kinetics of cracking to be that of a first-order reaction. Benito et al.8 studied the kinetics of visbreaking and coke formation for an asphaltene-rich residue and determined the viscosity, coke content, boiling point distribution, elemental analysis, and aromaticity of the reaction products. Kataria et al.9 also reported detailed kinetic studies on the visbreaking of vacuum residues. From their findings, they concluded that the product yields and activations energies were a function of the severity and feed characteristics. Changes in vacuum residue feedstock characteristics have been investigated by Fainberg et al.,10 who used a combination of nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and column chromatography. Wang et al.11 carried out analysis of products from thermal cracking of Shengli vacuum residue, where they showed a decrease in aromatics and resins (6) Di Carlo, S.; Janis, B. Chem. Eng. Sci. 1992, 47, 2695–2700. (7) Krishna, R.; Kuchhal, Y. K.; Sarna, G. S.; Singh, I. D. Fuel 1988, 67, 379–383. (8) Benito, A. M.; Maria, T. M.; Fernandez, I.; Miranda, J. L. Fuel 1995, 74 (6), 922–927. (9) Kataria, K. L.; Kulkarni, R. P.; Pandit, A. B. Ind. Eng. Chem. Res. 2004, 43, 1373–1387. (10) Fainberg, V.; Podorozhansky, M.; Hetsroni, G.; Brauch, R.; Kalchouck, H. Fuel Sci. Technol. Int. 1996, 14 (6), 839–866. (11) Wang, Z.; Guohe, Q.; Liang, W.; Qian, J. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem., March 29-April 3, 1998.

10.1021/ef801047f CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

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Figure 1. Summary of experimental procedures for recovery and analysis of the visbroken tars. Table 1. Changes in the Liquid Product and Coke Yields as a Function of the Time from Visbreaking of the Vacuum Residue at 410 °C run time (min)

aliphaticsa (wt %)

aromaticsa (wt %)

resinsa (wt %)

asphaltenesa (wt %)

cokea (wt %)

gas + lossesb (wt %)

0 30 35 40 45 60

20.0 26.6 29.5 29.0 28.3 29.8

30.6 19.9 24.1 18.4 18.6 15.2

36.8 24.5 29.9 25.1 27.8 24.2

6.5 14.0 13.4 14.4 15.7 25.6

0.1 0.1 0.1 0.2 0.9 2.4

6.0 14.9 3.0 12.9 8.7 2.8

a

Results based on actual recoveries of products from the experimental runs. b Light ends lost during rotary evaporation.

with a corresponding increase in asphaltenes and toluene insolubles, with the later attributed to condensation reactions. Toluene insolubles or coke from thermal cracking were reported to be comprised of polycyclic aromatic hydrocarbons (PAHs) with large aromatic ring sizes,12 and it is also believed that olefins and PAHs have the strongest tendencies to form coke when thermally stressed.13 However, a detailed understanding of the mechanisms associated with coke formation during visbreaking is yet to be established. This paper presents findings that indicate that PAH distributions are sensitive to coke formation, and in particular, a 6-ring PAH, benzo[ghi]perylene, is identified, whose concentration increases markedly at the point of coke induction. Further, it is demonstrated that, as an example of aliphatic biomarkers present in vaccum residue, hopanes are extremely sensitive to cracking but not to the onset of coke formation. Experimental Section

washed out extensively in toluene. The vistars recovered were then refluxed in 300 mL of toluene. This mixture was filtered through a 0.5 µm glass fiber filter paper, and the toluene-insoluble coke deposited was further washed with more toluene until this became colorless after passing through the filter paper. The coke was then dried and weighed. Toluene was then removed from the filtered solutions by gentle rotary evaporation. Precipitation of asphaltenes was conducted in n-heptane by adding a 40-fold excess to the toluene solubles, after which the mixture was transferred to centrifuge tubes and spun for 5 min at 2500 rpm. The maltenes (n-heptane solubles) were transferred to a separate beaker, leaving the asphaltenes for collection. Excess solvent from the maltenes was removed by gentle rotary evaporation. Separation of the maltenes was carried out on a silica/alumina column. Aliphatic fractions were eluted using n-hexane. Aromatic fractions were eluted using a mixture of 40% (v/v) dichloromethane in n-hexane. The resins were then eluted using a mixture of 50% (v/v) dichloromethane in methanol. The fractions obtained were then concentrated by gentle rotary evaporation of the solvents, with care being taken to minimize the loss of

Sample and Pyrolysis Procedure. The vacuum residue sample used was from Urals crude. Pyrolysis of the sample was performed batchwise in a stainless-steel mini-autoclave reactor. Approximately 2 g of the feed was used for each experiment. The reactor was first evacuated and then refilled with nitrogen to an initial pressure of 2 bar before being placed in a preheated fluidized sandbath. An independent thermocouple inserted into the reactor was used to monitor the feed temperature. Experiments were conducted at 410 °C for different residence times. At the end of each experiment, the reactor was quickly quenched with dry ice to cool the sample and prevent further reactions. Separation of the Products. Figure 1 schematically presents the experimental and analytical procedures followed for recovery and separation of the visbroken tars (vistars). The reactor was first (12) Tanabe, K.; Gray, M. R. Energy Fuels 1997, 11, 1040–1043. (13) Guisnet, M.; Magnoux, P. Appl. Catal., A 2001, 212 (1-2), 83– 96.

Figure 2. Coke yields obtained after visbreaking of the vacuum residue at 410 °C.

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Figure 3. TICs of aliphatics with n-alkanes indicated for the feed and the products from visbreaking at 410 °C.

light ends, after which they were transferred to gas chromatography (GC) vials for analysis. Analysis of the Liquid Products. Gas chromatography/mass spectrometry (GC/MS) analyses of the aliphatics and aromatics were carried out using a Varian 1200 Triple Quadrupole GC/MS/MS instrument, with a sensitivity of ca. 10-15 g. The GC column used was a Varian factor four VF-1MS and was heated from 50 to 300 °C at a rate of 6 °C min-1. Solution state 1H NMR was carried out on the toluene solubles using a DRX500 NMR equipment. For this, approximately 5 mg of the toluene solubles were dissolved in 0.3 mL of CDCl3 and then charged into 5 mm tubes.

Results and Discussion Yields and Evaluation of the Liquid Products. Table 1 lists the concentrations of aliphatics, aromatics, resins, asphaltenes, and coke (toluene insolubles) in the initial feed and the products from the visbreaking experiments, and Figure 2 plots the coke yields. It should be noted here that Figure 2 shows a linear rise in coke amount with time for 40, 45, and 60 min and that a linear climb is diagnostic of zero-order kinetics, which others have observed also for other oils. The initial coke and asphaltene contents were 0.1 and 6.5 wt %, respectively. Upon increasing cracking severity, the

coke levels remained unchanged at first before experiencing an increase after an induction period of roughly 40 min. Such induction periods prior to coke formation have also been previously reported by other investigators14,15 and usually would vary from feed to feed depending upon the relative paraffinic or asphaltenic contents. Cracking and combination reactions result in the decreased levels of aromatics from 30.6 to 18.4 wt % and resins from 36.8 to 25.1 wt % after 40 min. Thus, the production of lighter distillates up to this point appears to have occurred mainly from the aromatics and resins, thus contributing to the increase in the aliphatics from 20.0 to 29.0 wt % (Table 1). The thermal conversion pathway proposed by Wiehe16 further supports this observed phenomenon. The asphaltene content increases from 6.5 to 14.0 wt % within 30 min of initial heating residence time, after which it remains relatively constant until significant amounts of coke begin to appear at 45-60 min (Table 1). Aliphatic Hydrocarbon Distributions. The n-alkane distributions are a particularly useful indicator for the extent of (14) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32, 2447–2454. (15) Sanaie, N.; Watkinson, A. P.; Bowen, B. D.; Smith, K. J. Fuel 2001, 80, 1111–1119. (16) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530–536.

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Figure 4. Tri- and pentacyclic terpane distributions for the feed and the products from visbreaking at 410 °C (m/z 191 SIC).

Figure 5. Proportion of aromatic hydrogen of the total hydrogen for the feed and the products from visbreaking at 410 °C.

cracking that has occurred. Figure 3 indicates that the relative abundances of the n-alkane in the range of C13-C30 increased significantly with residence time regimes when compared to the initial feed, where most of the aliphatics are of high molecular mass and only partially eluting from the GC column. As the

residence time is increased, the relative abundances of n-alkanes of lower molecular mass increase markedly. The results confirm that the higher molecular mass n-alkanes crack under increasing severity, with the same phenomenon being reported by Song et al.17

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Figure 6. Relative SIC distributions measured for m/z 178 (3 rings, including phenanthrene plus anthracene), m/z 202 (4 rings, including fluoranthene plus pyrene), m/z 252 (5 rings, including benzopyrenes), and m/z 278 (6 rings, benzo[ghi]perylene) to represent the changes in aromatic ring sizes and alkyl substitution with increasing reaction time.

the C35-Rβ S + R isomers. Second, the relative increase in tricyclics is considered to be due to their preferential release from the asphaltenes and resins. Whereas the aliphatic hydrocarbons can provide quite sensitive indicators of the extent of cracking, they do not provide any indications of the onset of coke formation, with all of the parameters sensitive to cracking increasing continuously.

Figure 7. Normalized SIC distributions measured for m/z 178 (3 rings, including phenanthrene plus anthracene), m/z 202 (4 rings, including fluoranthene plus pyrene), m/z 252 (5 rings, including benzopyrenes), and m/z 278 (6 rings, benzo[ghi]perylene) to represent the changes in aromatic ring sizes and alkyl substitution with increasing reaction time.

As well as the overall n-alkane distributions, biomarkers present in the aliphatics also provide a sensitive measure of the extent of cracking in visbreaking. From Figure 4, a significant relationship with cracking between the tri- and pentacyclic terpanoids can be observed. Virtually no tricyclic terpanes are present in the initial feed, with the m/z 191 single ion chromatogram being dominated by the hopanes (pentacyclics) present. As the residence time is increased going from 30 to 60 min, the relative abundances of the hopanes decrease with a notable increase in the tricyclics, with the highest proportion appearing at the highest cracking severity. As one of the many potential indicators that can be chosen, the ratio of tri- to pentacyclics can thus be seen to gradually increase from 0 in the initial feed to almost 0.25 after pyrolysis at 40 and 60 min. Two mechanisms are responsible for this overall pattern. First, the increasing thermal stress causes cracking of the extended side-chain hopanes, thereby explaining the marked decrease in

Bulk Aromaticity Measurements and PAH Distributions. The plot in Figure 5 shows that change in the bulk hydrogen aromaticities as determined by 1H NMR was relatively small, with only a total increase of 2.6 mol % hydrogen with measurement errors of (0.5%, and thus, this provides a relatively insensitive measure of the extent of cracking and associated coke formation. The GC/MS analyses of the aromatic fractions for the different reaction residence times in general indicated that a wide range of substituted 2-4-ring PAH accounted for the bulk of the samples. The absolute and normalized self-interaction correction (SIC) distributions measured for m/z 178 (3 rings, including phenanthrene plus anthracene), m/z 202 (4 rings, including fluoranthene plus pyrene), m/z 252 (5 rings, including benzopyrenes), and m/z 278 (6 rings, benzo[ghi]perylene) are presented to visualize the changes in aromatic ring sizes and alkyl substitution that occur with increasing reaction time. Figure 6 indicates that the absolute SICs all increase because of dealkylation and ring-growth reactions occurring with increasing reaction time. The normalized distributions (Figure 7) indicate that, while the 3-5-ring PAH concentrations all increase before the point of coke induction (i.e., up to 35 min), this does appear to be the case for 6-ring PAHs. The individual mass spectra shown in Figure 8 reveal that the m/z 276 intensity was close to the noise level during the coke induction period and thereafter increased systematically with the residence time. The dominant PAH of mass 276 present was identified as benzo[ghi]perylene, and quantification was carried out using indeno[1,2,3-c,d]pyrene as an internal standard, because this was not present in significant amounts in the feed and products. The concentration of benzo[ghi]perylene was not much more than ca. 0.1 ppm of the total aromatics in the feed and remained at this level up to the point of coke induction.

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Figure 8. SIC m/z 276 for the feed and the products from visbreaking at 410 °C, showing the peaks for benzo[ghi]perylene and the internal standard, indeno[1,2,3-c,d]pyrene.

However, thereafter, the concentration increased sharply to ca. 0.3 ppm as the coke yield increased. Among all of the other PAHs of e6 rings investigated, only the formation of benzo[ghi]perylene was found to correlate well with the onset of coke formation and subsequent rise in concentration. Thus, the smaller PAH rings present in the feed all continue to be formed during the coke induction period and thus do not appear to be

direct contributors to coke formation but form larger ring systems first. Coke Formation Pathways during Visbreaking of the Vacuum Residue. The results obtained clearly indicate that the larger ring PAHs are sensitive to coke formation, with 6-ring and also possibly larger PAHs being present in negligible concentrations until the onset of coke formation. In terms of

PAH Indicator for the Onset of Coke Formation Scheme 1

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reactions put forward as a proposed process of formation of 6-ring PAHs from smaller aromatic substrates. The thermal cracking of aromatics during the coke induction period leads to the formation of free-radical sites on 1-2-ring units, with hydrogen being abstracted or side chains being cracked. The free-radical species will eventually combine to lead to a more condensed aromatic unit being formed as a termination step. This and other possible mechanisms involving the formation of 6-ring PAHs in visbreaking may be used to develop a more fundamental understanding toward coke formation that can potentially be probed in the future via 13C isotope labeling studies.

Scheme 2

Conclusions (1) In visbreaking, the distributions of aliphatic biomarkers are very sensitive to cracking severities but not to coke formation. (2) Although changes in bulk aromaticity observed are relatively small, GC/MS has clearly revealed that PAH distributions are sensitive to coke formation. (3) In particular, the concentration of benzo[ghi]perylene is sensitive to the onset of coke formation. (4) Smaller ring size PAHs are evident before the end of the coke induction period and do not provide a welldefined indicator of the onset of coke formation. (5) The monitoring the concentrations of specific large-ring PAHs is thus a sensitive new approach to assess the extent of coke formation in visbreaking. Acknowledgment. The authors are grateful to Nalco Ltd. for their financial support. EF801047F

reaction pathways to account for these new observations, Scheme 1 shows arylation and cyclization reactions of PAHs via radical mechanisms described previously by other authors.18,19 Scheme 2 illustrates a combination of both mechanisms of

(17) Song, C.; Lai, W.-C.; Schobert, H. H. Ind. Eng. Chem. Res. 1994, 33, 548–557. (18) Dyker, G.; Borowski, S.; Heiermann, J.; Korning, J.; Opwis, K.; Henkel, G.; Kockerling, M. J. Organomet. Chem. 2000, 606, 108–111. (19) Speybroeck, V.; Hemelsoet, K.; Waroquier, M.; Marin, G. Int. J. Quantum Chem. 2003, 96, 568–576.