Synergistic Coconversion of Refinery Fuel Oil and Methanol over H

Article Cite This: Energy Fuels 2019, 33, 5761−5765

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Synergistic Coconversion of Refinery Fuel Oil and Methanol over H‑ZSM‑5 Catalyst for Enhanced Production of Light Olefins Mohammad Ghashghaee,*,† Samira Shirvani,†,‡ Mehdi Ghambarian,§ and Søren Kegnæs∥ Faculty of Petrochemicals and §Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, P.O. Box 14975-112, Tehran 1497713115, Iran ‡ Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada ∥ Department of Chemistry, Technical University of Denmark, Kongens Lyngby 2800, Denmark Downloaded via UNIV OF SOUTHERN INDIANA on July 28, 2019 at 06:26:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Application of methanol as a coreactant in the atmospheric catalytic cracking of refinery fuel oil (abbreviated as the MFOCC process) over the H-ZSM-5 catalyst has been investigated for the first time with the aim to improve the olefin productivity of the heavy hydrocarbons and mitigate coke formation and catalyst deactivation. The results clearly proved the synergistic influence of cocracking in the MFOCC scenario. The integrated MFOCC process increased both light olefin (C=2 − C=4 ) yield and gasoline (C5−C11) share of the liquid products to more than 36.7 and 78.7 wt %, respectively. Influence of contact time and temperature was also discussed.

1. INTRODUCTION Catalytic cracking of viscous heavy hydrocarbons1−5 is one of the leading technologies in petrorefineries and petrochemical industries to produce light olefins (including ethylene, propylene, and butenes) and middle distillates to meet the growing worldwide demands.6−10 Lower-energy consumption, CO2 emission, and coke formation, as well as improved selectivity toward the desired products, form the pillars of preference of fixed-bed or fluid catalytic cracking (FCC) approaches over thermal cracking of these heavy feedstocks.11−13 Despite the progress in the FCC and residue FCC (RFCC), it is still challenged by the market demands in the opposite direction of the regular changes in the quality of the feedstock, which entail rapid deactivation of the catalyst because of fouling and poisoning.14,15 Among the most widely applied catalysts in these types of reactions are zeolites, which have astonishing features, such as shape selectivity, high thermal and mechanical stability, tunable acidity, and hydrophilicity.14,16−18 As an example of various types of zeolites, HZSM-5 (MFI), probably the most well-known solid acid catalyst, has attracted considerable attention in many reaction systems including the catalytic cracking.19,20 By far, intensive research has been devoted to modify the zeolite characteristics14 or improve the quality of the applied feedstock prior to the cracking reactions5,21−23 to tackle the abovementioned obstacles. Considering all these issues, in the present research, methanol was evaluated as a cofeedstock in the upgrading of refinery fuel oil over the H-ZSM-5 catalyst to circumvent or alleviate the problems mentioned above. Application of methanol as a hydrogen donor in the upgrading of vacuum residue has been investigated by Ouchi et al.24 over ZnO− Cr2O3 and CuO−ZnO−Al2O3 under high-pressure nitrogen in a liquid-phase batch reactor. The obtained results have shown that methanol was cracked and produced hydrogen, which © 2019 American Chemical Society

increased the overall gas production during the reactions. In another study, the coupled conversion of four-carbon hydrocarbons and methanol to light olefins over MFI zeolite at atmospheric pressure was investigated by Martin et al.25 The authors found that the thermal equilibrium between the endothermic reactions of hydrocarbons and the exothermic conversion of methanol was formed at the methanol-tohydrocarbon ratio of 3:1. The beneficial influence of methanol cofeeding is advocated economically taking into account the enormous low-priced availability of methanol from the socalled mega-plants in the US and Middle East regions.26,27 The application of the H-ZSM-5 catalyst to the conversion of methanol to olefins (referred to as the MTO process) has been widely explored.28 However, the applicability of this costeffective and abundant alcohol as a coreactant in the vaporphase catalytic cracking of refinery fuel oil under atmospheric pressure still stands in need of further determination, especially with a microporous catalyst as H-ZSM-5. Notably, the possible synergistic effect of cocracking of fuel oil in the presence of methanol (MFOCC), which can be viewed industrially as a combined MTO-FCC process, has not been addressed so far. These issues are investigated here for the first time.

2. EXPERIMENTAL SECTION 2.1. Materials. Refinery fuel oil with the specifications listed in Table S1 was applied as a heavy feedstock. Methanol with a purity of 99.99% was purchased from Merck and applied as a cofeedstock. The acidic (H-ZSM-5) catalyst has been synthesized according to the established procedures,29 More details of the preparation method can be found in the Supporting Information. Received: February 1, 2019 Revised: May 4, 2019 Published: May 16, 2019 5761

DOI: 10.1021/acs.energyfuels.9b00347 Energy Fuels 2019, 33, 5761−5765

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Figure 1. Microactivity test apparatus for the catalytic cracking of fuel oil. 2.2. Characterization Techniques. A wide range of characterization methods was employed to analyze the feed and product samples and to assess the physicochemical properties of the catalyst. The H-ZSM-5 zeolite was analyzed using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, ammonia temperature programmed desorption, midrange Fourier transform infrared spectroscopy (mid-FTIR), nitrogen physisorption and the subsequent Brunauer−Emmett−Teller analysis, and X-ray fluorescence spectrometry. Feed and product samples were also analyzed in terms of density, pour point, ash content, flash point, viscosity, sulfur content, Conradson carbon residue, hydrocarbon type (PONA) analysis through 13C and 1H nuclear magnetic resonance spectroscopy, and componential analysis using gas chromatography. More details can be found in the Supporting Information. 2.3. Catalytic Microactivity Tests. Upgrading of the refinery fuel oil over H-ZSM-5 was conducted in a fixed-bed reactor placed in an electrical furnace according to the standard microactivity test units (Figure 1), which is also regarded as an accepted representative for the real FCC conditions. More specifications of the experimental procedure4,5,30 appear in the Supporting Information. In order to precisely assess the impact of methanol as a coreactant on fuel oil catalytic cracking, two blank tests with fuel oil and methanol as sole feedstocks were also performed under the same experimental conditions.

3. RESULTS AND DISCUSSION The crystal structure of the H-ZSM-5 catalyst was investigated by XRD analysis, as presented in Figure 2 (upper panel). Intense features were observed at 2θ = 7.9°, 8.9°, 23.1°, 23.8°, and 24.4°, which match well with the standard patterns of the H-ZSM-5 catalyst.31 On the basis of the obtained XRD data, the crystallite size was calculated by the modified Scherrer equation to be 16.2 nm. The FTIR spectrum (Figure 2, the lower panel) exhibited bands at 3370 and 1600 cm−1, which were assigned, respectively, to the surface-physisorbed siloxane and/or hydroxyl groups and bending vibrations of lattice water molecules. The observed bands at 1160, 1070, 780, 540, and 445.5 cm−1 were attributed to the external asymmetric stretching of AlO4 and SiO4, internal asymmetric stretching of siloxane structures, external symmetric stretching of siloxane groups, structure-sensitive five-membered rings of the pentasil structure, and bending vibrations of the Si−O or Ti−O bonds in the H-ZSM-5 crystal, respectively.32 Further details of the catalyst properties are presented in the Supporting Information. Figure 3 presents the catalytic performance of the H-ZSM-5 catalyst in the catalytic cocracking of fuel oil and methanol, the MFOCC process, and also in the cracking of fuel oil and methanol separately (all experiments were performed at the

Figure 2. XRD pattern and FTIR spectrum of the H-ZSM-5 catalyst.

same experimental conditions). As can be seen in this figure, the amount of gases produced from a combined fuel oil/ methanol feedstock was more than those obtained from a separate cracking of fuel oil or methanol (76.0 wt % vs 59.9 wt % and 36.44 wt %, respectively) at the same operating conditions. Moreover, the obtained light olefins (ethylene, propylene, and butenes) through the integrated MFOCC scenario (the combined feedstock) was higher (36.6 wt %) than that with the refinery fuel oil alone in the catalytic cracking over H-ZSM-5 (25.8 wt %). In both cases, however, nearly half of the obtained light olefins were occupied by propylene. On the basis of these observations, one may contemplate that the effect of methanol cofeeding is not of a matter of simple additive contributions of the two coreactants. In contrast, a clear synergistic effect was observed when methanol was applied as a cofeedstock in the upgrading of refinery fuel oil. In other words, apart from the additive participation of methanol in olefin production, there has been a synergistically increased productivity from the heavy feedstock upon cofeeding of methanol. This enhancement could have been the result of hydrogen donation capability of methanol 5762

DOI: 10.1021/acs.energyfuels.9b00347 Energy Fuels 2019, 33, 5761−5765

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Figure 3. Catalytic efficiency of the MFOCC cocracking process over the H-ZSM-5 catalyst (reaction conditions: WHSV = 11.7 h−1, temperature = 823 K, and atmospheric pressure).

Figure 4. Effect of reaction temperature and space velocity on the catalytic performance in the integrated MFOCC process.

proceeding to secondary addition reactions that produce long-chain liquid-range hydrocarbon products.33 More importantly, the total productivity of light olefins per mass of the catalyst increased remarkably (by a factor of 6), which proved the high potential of the integrated process with the combined feedstock for light olefins production in the ultrahighthroughput reaction−regeneration cycles. The coke yield of 0.11% in the combined process has been much smaller than the corresponding value of 0.27% without methanol feeding at the same conditions. According to the thermogravimetric analysis of the spent samples, the coke deposits could be classified into soft and hard coke types appearing at 663 and 923 K, respectively. Further discussion of the data can be found in the Supporting Information, under Table S3. To further probe the advantage of the integrated process, we made first-principles calculations using the cluster modeling approach34−42 within a density functional theory framework (see the Supporting Information for more details). The crystallographic model was taken from the literature.43 Methanol and benzene were adsorbed on two representative sites of the H-ZSM-5 catalyst, in which benzene was taken as a model aromatic precursor of coke.44,45 The obtained data predict that methanol can bind much more strongly to the acidic sites than benzene as indicated from the average enthalpy changes of −21.46 and −7.98 kcal/mol at the M06/ Def2-TZVP level of theory for methanol and benzene, respectively. The analysis indicated that the preferred adsorption of methanol beside the aromatic rings formed via cyclization and dehydrogenation reactions could at least lower the probability of the subsequent condensation reactions, thus mitigating catalyst deactivation due to coke formation. Finally, one would be interested in comparing the obtained results with those from the few similar studies. A previous report on the interactions of ethylene with methanol at 823 K in a fixed-bed reactor46 has indicated the advantage of their

principally with a 2-fold benefit: (1) a saturation of coke precursors that accelerate catalyst deactivation and cover the active sites and (2) a recovering effect of methanol with respect to the Brønsted acidic sites. As such, methanol molecules have acted as the regenerative agents of the zeolite catalytic sites in the course of reactions. Meanwhile, methanol could act as a diluent that is expected to reduce the concentration of heavy molecules thus helping fuel oil catalytic cracking shift to lower products. Indeed, methanol cofeeding provides thermal equilibrium, which prohibits the formation of hot spots that exacerbate coke formation in the catalyst particles. Discussion of the liquid products including gasoline (C5−C11) and diesel fuel (C12−C22) is given in the Supporting Information. To elucidate the effects of temperature and residence time as the two most influencing factors on the activity, complementary tests were implemented with the results shown in Figure 4. As was expected, by decreasing the reaction temperature to 723 K at a constant weight-hourly space velocity (WHSV) of 11.7 h−1, the obtained yields for the gases and light olefins were dramatically decreased to 48.3 and 16.0 wt %, respectively, while the corresponding liquid yields increased by more than 2-fold (to 23.9 and 51.7 wt %, respectively). Interestingly, the catalyst was not only more prone to gas production but also more selective toward light olefins at higher temperatures in terms of the total gases. Figure 4 clearly demonstrates the effect of contact time and correspondingly the influence of coke mitigation in the MFOCC process. Although the amount of gaseous products decreased upon shortening of the contact time, the olefins yield and their selectivity on a total gas basis increased substantially from 36.7 and 48.2 wt % to 44.0 and 69.8 wt %, respectively (the selectivity changes are not shown here) for a change in WHSV from 11.7 to 58.5 h−1, which could be attributed to a faster quenching of light olefins before 5763

DOI: 10.1021/acs.energyfuels.9b00347 Energy Fuels 2019, 33, 5761−5765

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cofeeding with respect to the propylene selectivity because of the ethylene methylation. Martin et al.25 investigated the coupled conversion of four-carbon hydrocarbons and methanol to light olefins over H-ZSM-5 and obtained the olefins yield of 44.1 wt % at 953 K, WHSV of 3 h−1, atmospheric pressure, and the methanol-to-hydrocarbon ratio of 3:1. In a relevant study, Liu et al.47 presented the cocracking of methanol and naphtha in different reactors and found olefin yields in the range of 44− 58 wt %. In their study of methanol and n-hexadecane cocracking, Chen and Degnan48 found that the exothermic nature of methanol cracking could significantly mitigate the coke formation on acid sites. Such an improvement makes the process more sustainable to operate continuously without applying serious catalyst regeneration steps.48 No previous report was found on the coconversion of a similar heavy feedstock from refinery with methanol. Taking into account that the present study has employed much heavier feedstock at a lower methanol-to-hydrocarbon ratio and higher space velocity, and more importantly comparing the obtained results with the expected yields from an RFCC base case, the improvements reported here clearly point to the promising potential of the proposed integrated process for the production of light olefins from low-ranked heavy hydrocarbons.

Mohammad Ghashghaee: 0000-0003-1677-8434 Mehdi Ghambarian: 0000-0001-6151-9007 Søren Kegnæs: 0000-0002-6933-6931 Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00347. Further data for the performance of the reactions, materials, catalyst characterization, and computational data (PDF)

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ACKNOWLEDGMENTS

Assistance from Mahboobeh Balar is gratefully acknowledged as is the partial support from Iran National Science Foundation (INSF) under grant 94016123.

4. CONCLUSIONS For the first time, it was demonstrated that the cofeeding of methanol with the refinery fuel oil (the MFOCC process) could provide several prospects in the catalytic cracking process with the H-ZSM-5 catalyst under atmospheric pressure and 723−823 K conditions. The likely impacts included the remarkable enhancement in the catalyst activity and the stability, light olefins productivity (to more than 36 wt % of the virgin feedstock at T = 823 K and WHSV = 11.7 h−1), and an increase in the gasoline selectivity (78.7 wt %). Besides the exothermicity of methanol conversion at these conditions, which could appropriately counterbalance the endothermic nature of the catalytic cracking process, the resulting dilution of the reaction mixture, hydrogen donation, and in situ regeneration of the catalyst during the reactions are the astonishing features of an application of methanol cofeeding in olefin production from heavy hydrocarbons. By reducing the contact time to one-fifth of the initial value (WHSV of 58.5 h−1), the yield of light olefins increased to more than 44 wt % of the heavy feedstock as a consequence of fast quenching of alkenes prior to their secondary reactions. It was also observed that increasing the reaction temperature from 723 to 823 K doubled the production of light olefins in the integrated MFOCC process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +98 21 48662481. Fax: +98 21 44787032. 5764

DOI: 10.1021/acs.energyfuels.9b00347 Energy Fuels 2019, 33, 5761−5765

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DOI: 10.1021/acs.energyfuels.9b00347 Energy Fuels 2019, 33, 5761−5765