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Catalytic steam gasification of Athabasca visbroken residue by NiO-kaolin-based catalysts in a fixed-bed reactor Azfar Hassan, Lante Carbognani-Arambarri, Nashaat N. Nassar, Gerardo Vitale, Monica Bartolini, and Pedro Rafael Pereira-Almao Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017
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Catalytic steam gasification of Athabasca visbroken residue by NiO-kaolin-based catalysts in a fixed-bed reactor Azfar Hassan*, Lante Carbognani-Arambarri, Nashaat N. Nassar, Gerardo Vitale, Monica Bartolini, Pedro Pereira-Almao Department of Chemical & Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, T2N 1N4, Canada *Corresponding author. ph: (403)2204239; fax: (403)2103973; e-mail:
[email protected] ABSTRACT: In this study, H2 production via low temperature catalytic steam gasification of Athabasca visbroken vacuum residue adsorbed onto NiO-kaolin-based catalysts in a fixed-bed reactor was investigated. The catalytic steam gasification was carried out at 600, 650 and 700°C. Two NiO-kaolin based catalysts were used, containing either K2O or Cs2O along with BaO. XRD data shows the presence of crystalline Ni3S2 phase in the spent catalyst only when K was used instead of Cs in the catalyst preparation. Catalytic steam gasification experiments (CSG) conducted for 100 h and then regenerated, confirmed that both catalysts are good candidates for H2 production via catalytic steam gasification of adsorbed heavy hydrocarbon feed in a fixed-bed reactor unit. However, H2/CO2 ratio obtained during CSG is closer to 2 at all three temperatures studied for 3NiO6K6Ba catalyst. Further, the activation energy values, determined for 3NiO6K6Ba and 3NiO6Cs6Ba catalysts for catalytic steam gasification of visbroken residue, were 83 and 95 kJ/mol, respectively.
Keywords: Athabasca vacuum residue, Kaolin, NiO nanoparticles, Catalytic steam gasification, H2 production
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1. INTRODUCTION The recent fall and fluctuation in the global crude oil price have drastically influenced the projection of the current and future energy supply and demand. Nonetheless, conventional crude oil still remains the main energy source that is needed to meet the global energy demand because of its high quality as well as low production cost. However, conventional oil reserves are now generally agreed to be depleting, as these resources are finite. This generated more focus on exploitation of unconventional oil reserves in recent past.1 One of the main unconventional oil reserves is Alberta’s oil sands, which is comparable to that of Saudi Arabia in the size of oil reserves, and has been gaining importance as a strategic North American energy supply. However, the energy industry continually encounters challenges related to oil upgrading and recovery due to the fluctuation in the global oil price with the increase of environmental burdens and regulations of greenhouse gas emissions. Hence, innovation is required to improve or even replace current upgrading and recovery, which are becoming economically challenging and environmentally unfriendly. In view of this, the main objective here is to develop advanced and innovative techniques to cost-effectively increase the efficiency of oil upgrading, with reducing environmental footprints. Significant research has been carried out on the development of hydrogen generation techniques from heavy hydrocarbons. 2 The most developed technologies for this purpose are reforming and gasification. Gasification usually refers to a thermal process where a carbonaceous feed is transformed to gases, like CO, CO2, CH4 and H2, which subsequently could be used to produce liquid hydrocarbon through Fischer-Tropsch process.3 The gasification reactions depend strongly on temperature, pressure and C to O atomic ratio.
3
In
the recent past, the economics of Alberta bitumen coke gasification has been studied in detail 2 ACS Paragon Plus Environment
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and reported in literature.
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The study showed that bitumen-coke integrated gasification
combined cycle (IGCC) has the potential to produce electricity and H2 at prices competitive with natural gas and coal fed plants. Catalytic steam gasification, on the other hand, refers to a similar process occurring in presence of a catalyst. Sorption enhanced catalytic steam gasification of biomass has been reported
5
where CO2 sorption over dolomite increased H2 production. Also, CaO when used as an additive in atmospheric gasification of biomass acts as a CO2 sorbent when gasification occurs at low temperatures (less than 800 °C) and as a catalyst when gasification is carried out beyond 800 °C.6 Very recently, works have been published on the use of Ni/Al2O3 catalyst in molten eutectic salts for biomass gasification.7 In catalytic steam gasification of coal 8, 9 or coke, 10, 11 it has been well known that the use of alkaline metal augments the reactivity, and that the effect is higher with higher alkaline metal/C ratio.12 It has also been reported that the order of catalytic activity of alkaline metals, for coal gasification follows from bottom to top in the periodic table of elements as follows: Li > Cs > K > Na > Ca for single metal salts.13 In general, the activity was in the order ternary, binary, and single salts.13 Also, the catalytic activity of alkali metals may change depending on the nature of the carbon that is gasified.12 Pereira et al have used K-Ca mixtures for catalytic steam gasification of graphite and chars.14 They demonstrated that these catalysts have superior resistance to sulphur poisoning as compared to K-Ni catalysts. It may be noted that, in the majority of the work published in the literature, the alkali-alkaline earth metals are dispersed in the carbonaceous matrix and subsequently subjected to gasification, mostly in a fluidized bed reactor type unit.8, 15 Recently attempts have been made for CO2 capture during biomass/ coal steam co-gasification to make the process more environment friendly.16 3 ACS Paragon Plus Environment
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Bitumen typically comprises more than 50 wt.% of distillation residua (500 °C+). In search of an economically viable upgrading process for bitumen or heavy oil, we have envisaged an alternative processing scheme that may prove viable. This scheme requires the development of a catalyst capable of preferentially adsorb heavy polar hydrocarbons, like asphaltenes, and subsequently its ability to perform low temperature catalytic steam gasification of the adsorbed species. The overall objective is to separate the unstable heavy polar hydrocarbons (in particular the ones produced during thermal cracking of the residual oil) by adsorbing them onto a solid adsorbent-catalyst, as illustrated schematically in Scheme S1 (Supplementary Information). These selectively adsorbed (therefore separated) hydrocarbon compounds would then be used to produce H2 via catalytic steam gasification. This mode of gasification is different than the conventional gasification and would operate at significantly lower temperatures. In this case, H2 would be produced from the most difficult-to-process asphaltenes rather than natural gas, a cleaner fuel that could better be used for heat and power generation in order to displace coal in the future. Our recent work on asphaltene adsorption using nanoparticles17, 18 aluminas 19, 20 and modified kaolin
21
showed that incorporation of transition metal oxide nanoparticles on/into kaolin
would be advantageous; both in terms of adsorption as well as catalytic activity for steam gasification reactions. This hypothesis was confirmed in a previous study.
22
Very recently we
have prepared catalysts that contained alkali (K or Cs) and alkaline earth (Ca or Ba) metal and NiO nanoparticles incorporated into mesoporous-macroporous meta-kaolin for catalytic steam gasification of heavy hydrocarbons in a fixed-bed reactor unit. 23 The feed used was a mixture of n-C5 asphaltenes mixed with light cyclic oil (LCO) to mimic a real heavy feed. We found that replacement of Ca by Ba increased H2 production. Also, incorporation of NiO 4 ACS Paragon Plus Environment
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nanoparticles further promoted H2 production during the steam gasification of the adsorbed nC5 asphaltenes, leading to CO2 and H2 as major products with less side reactions.
23
Both
3Ni6K6Ba and 3Ni6Cs6Ba formulations were found to be promising catalysts. Based on the results obtained in our previous studies
22, 23
and, in order to get a complete
picture on the usefulness of the aforementioned catalysts for possible industrial application, herein we investigate the catalytic steam gasification of a much heavier feed like Athabasca vacuum residue in a fixed-bed reactor with the two catalysts found most promising in our previous work.
23
The focus of this work as highlighted by the dotted rectangle in Figure S1
(Supplementary Information) is to study in detail the low temperature catalytic steam gasification of adsorbed heavy molecules over these catalysts. Both aforementioned catalysts were tested for long hours and the spent catalysts were regenerated and tested again for steam gasification. This study is significant as it demonstrates for the first time that very heavy feed can also be successfully gasified at low temperatures in presence of steam, for H2 production in a fixed-bed reactor type unit using catalysts having NiO nanoparticles in conjunction with alkali metal eutectic salts dispersed in a mesoporous-macroporous material.
2. EXPERIMENTAL 2.1. Materials. Toluene (HPLC grade, Sigma-Aldrich), NiO nanoparticles (Sigma-Aldrich), Kaolin (Merck, Germany). K(CH3COO) (99%, Sigma-Aldrich) and Cs(CH3COO) (99%, Sigma-Aldrich), Ba(CH3COO)2 ( 99%, Merck (Germany)) were used in catalyst preparation. Air, Ar, H2, He, and N2 gases of UHP grade were purchased from Praxair, Canada. The heavy feed, Athabasca vacuum residue (VR) was provided by Nexen-CNOOC Ltd that was thermally
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cracked to produce the desired material used for adsorption studies. All chemicals were used as received without further purifications.
2.2. Elemental Analysis. S and N analysis were performed using ANTEK 9000 NS Analyzer. A known amount of VR was dissolved in toluene and tested for N and S quantification. A calibration curve using six standards was made containing 100-900 ppm of N and 1000-10000 ppm of S and used for this purpose. Table 1 list the elemental analysis on dry weight basis of the VR sample used in the experiment. The ash analysis of this vacuum residue has been determined to be around 0.1 wt%.
Table 1. Elemental composition analysis of the Athabasca vacuum residue sample C
H
S
N
O
wt%
wt%
wt%
wt%
wt% (by difference)
82.30
9.72
5.31
0.62
2.05
2.3. Adsorbent/Catalyst Preparation. Mesoporous-macroporous adsorbent/catalyst supports were prepared by using kaolin as the solid support. The formula of the kaolin used is H2Al2Si2O6.×H2O showing a Si/Al atomic ratio of 1. Traces of dehydroxylated muscovite (KAl3Si3O11) were found as impurity, which is very stable to thermal and hydrothermal treatments.
23
A known amount of kaolin was mixed with an aqueous solution containing
known amounts of K(CH3COO) or Cs(CH3COO) and Ba(CH3COO)2. These metal acetates were used as binding agents to render kaolin more basic in nature. Also in general, alkali-salts of weak acids make better gasification catalysts. 13 Sucrose was then added to kaolin in known 6 ACS Paragon Plus Environment
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amounts to act as a porogen. NiO nanopowder was then added to the mixture. Extrudates were drawn from the resulting dough, using an in-house built machine, and dried overnight at room temperature. Dried extrudates of kaolin were then calcined at 650 °C under air for 8 h in a muffle furnace to form a mesoporous-macroporous material. The extruded catalysts were cut into small pieces. The extrudates were 1.50 mm in diameter and 15-20 mm in length. Kaolin transformed into meta-kaolin because of calcination in air beyond 600 °C. 22 Table 2 shows the composition of the extruded catalysts used in this study in weight percentage. CO2 TPD study showed that the surface is slightly basic in nature. Detailed catalyst characterization can be found in our previous works. 22, 23
Table 2. List of catalyst composition considered in this study in weight percentage Catalyst
KOAc*
CsOAc*
Ba(OAc)2
NiO
Sugar
Kaolin
3Ni6K6Ba
6
-
6
3
20
65
6
6
3
20
65
3Ni6Cs6Ba *OAc= (CH3COO)
2.4. Fixed-bed Reactor Setup. The construction and operational procedures of the setup capable of performing both adsorption and catalytic steam gasification in a continuous operation mode is described in Section S2 and the setup is shown in Figure S2 in the Supplementary Information. More details details on the setup can be found in our previous study.23
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2.7. XPS Analysis. X-ray photoelectron spectroscopy (XPS) was carried out with a PHI VersaProbe 5000 spectrometer operating under ultrahigh vacuum (
