Article
Upgrading of Canadian Oil Sand Bitumen via Cracking and Coke Gasification: Effect of Catalyst and Operating Parameters Yuming Zhang, Lei Huang, Xiaochen Zhang, Guogang Sun, Shiqiu Gao, and Shu Zhang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017
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Upgrading of Canadian Oil Sand Bitumen via Cracking and Coke Gasification: Effect of Catalyst and Operating Parameters Yuming Zhanga*, Lei Huanga, Xiaochen Zhanga,b, Guogang Suna, Shiqiu Gaob*, Shu Zhangc a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, China
b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing 100190, China c
Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia
ABSTRACT Canadian oil sand bitumen was stepwise converted via integrated cracking and coke gasification (ICCG) process. The cracking behaviors of oil sand bitumen were investigated in a fluidized bed reactor with FCC and bifunctional (BFC) catalysts, respectively. The BFC catalyst specialized for ICCG process was designed with both cracking and gasification activities. The results showed that the liquid yield of 78 wt.% and conversion ratio of 87% of oil cracking could be simultaneously realized over hydrothermally-treated catalysts (A-FCC and A-BFC) at 510 °C. The cracking performance of catalysts was closely related with their acidity, as indicated by NH3-TPD analysis. Coke deposit on the catalyst could lead to severe pore blockage. Catalyst regeneration via coke gasification could recover its pore structures and meanwhile produce high-quality syngas with the sum of H2 and CO up to 80 vol.%. Comparing with FCC catalysts, the regeneration (coke gasification) time over BFC catalysts was shortened by about 30% due to its well-developed pores and high alkaline oxides content, and these effects could be further enhanced by impregnating potassium oxides as catalytic additives. Finally, the alternating cracking-gasification operation was conducted four cycles to justify the stability of BFC catalyst. The newly-designed amorphous BFC catalyst exhibited high hydrothermal stability and could be potentially used for oil sand bitumen upgrading via ICCG process in a large scale. KEYWORDS: Oil sand bitumen; Bifunctional catalyst; Cracking; Coke gasification.
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1. INTRODUCTION With the depletion of conventional crude oil, feedstocks of deteriorating quality, such as heavy oil, extraheavy oil and oil sand bitumen, have been widely utilized and will be increasingly utilized in refineries to meet the oil demand. 1 Oil sand, also known as tar sand or bituminous sand, is a mixture of sand, water, clay and bitumen, particularly found in extremely large quantities in Alberta, Canada. 2 About 10% of the world's oil reserves are located in the Alberta oil sands. Oil sand bitumen is the organics which could be extracted from oil sand via open-pit mining or in-situ recovery methods depending on the depth of the deposit. 3,4 As an unconventional oil resource, oil sand bitumen5 is characterized by high density, high viscosity, high contents of carbon residue and heteroatoms (sulfur, nitrogen and metals), and usually could not be directly used as the feedstock for the conventional refinery processes. Thus, different upgrading processes are required to convert these resources into synthetic crude oil for pipeline transportation or acceptable feedstocks for the downstream refining units. The upgrading technologies of oil sand bitumen are typically divided into thermal and catalytic cracking conversion processes considering if there is the presence of catalysts or not. Visbreaking and coking,6-9 in terms of reaction severity, are known as the most widely operated thermal cracking processes. Visbreaking6, 7 is a relatively mild thermal cracking process to improve the viscosity characteristics of the residue without coke formation. Coking8, 9 could achieve deep conversion of heavy oil with long reaction time, especially for delayed coking with hours of residence time. However, the formation of large quantities of petroleum coke with highsulphur content hinders its popularization when treating heavy oil with deteriorating quality. Fluid catalytic cracking (FCC) process10, 11 could markedly enhance liquid yield (i.e., gasoline and diesel) comparing with thermal cracking process, but the high molecular weight and heteroatom contents in heavy feedstocks greatly limited the direct conversion by FCC process due to catalyst deactivation caused by coke and metal deposition. Hydrogenation12 involves feedstock in presence of catalyst and hydrogen under desired temperature, highpressure and residence time, thus to increase H/C ratio and remove sulfur, nitrogen, heavy metals of the oil products. Nevertheless, when hydrotreating or hydrocracking high-heteroatom heavy oil, catalysts should be optimized and together with new-type reactors, such as, moving bed, ebullating bed and slurry bed. 13 Besides, there is usually a great hydrogen deficiency when processing these heavy feedstocks with low H/C ratio and limited hydrogen source in the refinery. 14 The above upgrading processes are generally classified into two technical ways, that is, carbon rejection and hydrogen addition, respectively. Carbon rejection essentially is to redistribute the hydrogen and carbon among the cracked products. Hydrogen addition could compensate hydrogen deficiency for the heavy 2
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feedstock, thus to obtain higher yield of liquid with lower impurities. From the economic viewpoint, hydrogen addition is the most expensive process in terms of high-pressure equipment, catalyst and hydrogen consumption. Also, the high-content resins and asphaltenes in heavy oil have high tendency of coke formation to shorten the catalyst life and make the hydrogenation reaction hard to proceed.15 As a result, it comes the question that whether it is cost-effective to initially hydrogenate heavy/extra-heavy oil with high contents of carbon residue and heteroatoms, or alternatively to first convert the heaviest components via cracking and then to hydrotreat the cracked oil. As a result, the integrated cracking and coke gasification (ICCG) process16, 17 was proposed in our research group to make hierarchical utilization of heavy oil, i.e., oil sand bitumen. Figure 1 shows the reaction flow of heavy oil during ICCG process. Heavy oil was first cracked into volatiles (gas and oil) and coke by contacting with hot catalyst (also serving as heat carriers). Volatiles could be distillated into cracking gas and oil with different fractions. Coke on the catalyst surface was gasified for syngas production and simultaneously for spent catalyst regeneration. Sulfur in coke was converted into H2S afterwards to be recovered as sulfur product. Syngas transforming into pure hydrogen further could be used as the hydrogen resource for hydrotreating the cracked oil. The regenerated catalyst circulating back to the cracking reactor provided the heat and catalytic activity for heavy oil conversion. In fact, ICCG process combined two kinds of chemical reactions essentially for converting heavy oil into the light-medium boiling point distillates, that is, chemical bond-breaking to convert large molecules into smaller ones and hydrogenation to remove the impurities from the product oil. The heaviest compounds, such as, resins and asphaltenes, were first converted into coke to avoid their adverse effects on catalyst under the conditions of hydrotreating heavy oil directly. Coke was further gasified as syngas for hydrogen addition to the light cracked liquid for clean oil. Coke gasification together with subsequent syngas reforming by steam could convert some hydrogen from water into final oil products, thus to make efficient utilization of carbon and hydrogen elements of heavy feedstock. In fact, ICCG process makes more sense when treating heavy oil with high contents of resins and asphaltenes, because during cracking these deteriorating feedstocks could yield more coke, which is necessary for syngas production by coke gasification. This study is devoted to investigating the fundamental characteristics of ICCG process using oil sand bitumen to acquire the suitable operating conditions and catalysts for oil cracking and coke gasification. For ICCG process, the catalyst is of vital role considering that it should not only possess proper activity for oil cracking to maximize oil yield and remain stable in the regeneration via coke gasification, but also have some catalytic effects on coke gasification for fast catalyst regeneration. Junaid et al. 18, 19 studied the effects of catalyst structure and acidity on oil sand bitumen cracking with natural and activated zeolite catalysts, respectively. 3
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used calcium-aluminate catalyst for vacuum residue conversion. The kaolin catalyst and spent
FCC catalyst have been applied for vacuum residue treating in our previous studies.16, 17 A kind of bifunctional (BFC) catalyst 21 was also prepared with both catalytic activities of oil cracking and coke gasification. In this study, oil sand bitumen conversion was carried out with BFC catalysts to compare its cracking and regeneration performance with commercial FCC catalysts. After that, the regeneration characteristics of BFC and FCC catalysts were studied via gasifying the cracking-generated coke and producing syngas. Finally, the regenerated catalyst after coke gasification was recycled several times to determine the catalyst life and operating characteristics of ICCG process. 2. EXPERIMENTAL SECTION 2.1. Oil Sand Bitumen and Catalyst The properties of Canadian oil sand bitumen are shown in Table 1. This feedstock is quite heavy in terms of its high density and low H/C molar ratio (1.47). The high contents of carbon residue and asphaltenes in oil sand bitumen indicate the high tendency of coke formation during thermal and catalytic cracking. Moreover, the high heteroatoms of sulphur (4.9 wt.%) and heavy metals (Ni and V) could potentially cause catalyst deactivation during fluid catalytic cracking (FCC) or hydrocracking processes. Oil sand bitumen at room temperature is in solid-like phase and the viscosity decreases at higher temperature. In this study, the feedstock is preheated to 120°C for easy pumping and atomizing. The reaction behaviors of oil sand bitumen were investigated with a commercial FCC catalyst and a selfdesigned BFC catalyst, respectively. The properties of FCC and BFC catalysts are shown in Table 2. The FCC catalyst was mainly composed of matrix, binder and Y-zeolite (ultra-stable Y-zeolite). The BFC catalyst with moderate cracking activity and catalytic effects of coke gasification was specially prepared for ICCG process. The preparation procedures of BFC catalysts were detailed in our past publication.21 Briefly, the cracking acidity of BFC catalysts was adjusted by changing the Si/Al ratio of matrix and the amount of zeolite, while its coke gasification performance could be improved by enhancing the pore structures and adding some alkaline metal (Na and K) oxides. In order to optimize its cracking acidity and hydrothermal stability, FCC and BFC catalysts were treated at 800 °C steam for 14h and denoted as aged FCC (A-FCC) and BFC (ABFC) catalysts, respectively. The acidity of BFC catalysts was also adjusted by doping potassium carbonate of 0.1 mol/L. The impregnated BFC catalysts (named as K-BFC) were filter-washed, dried and calcined as potassium oxides for the catalytic gasification. 2.2. Apparatus and Operation Oil sand bitumen was stepwise converted by oil cracking and coke gasification in a fluidized bed reactor, 4
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and the details of the reactor and operation procedures can be found in our past studies.16, 17 In brief, certain amounts of weighed catalysts (catalyst/oil ratio of 4-6 by mass) were previously placed on the porous frit and fluidized by steam. When the reactor was heated-up and stabilized at the desired temperature, preheated oil sand bitumen of about 25g at 120 °C together with hot steam (steam/oil mass ratio of 0.2) at 300 °C was continuously injected into the fluidized catalyst particles through a nozzle. The tiny oil droplets vaporized and converted into light volatiles by contacting with hot catalysts. Light volatile fractions were quickly entrained out from the hot reaction area by upwards steam to minimize over-cracking effects. After purified by an inside-mounted filter, volatiles were condensed and collected respectively as heavy, light oil fractions and cracking gas. Heavy compounds condensed on the surface of catalyst as coke deposit, which was successively gasified in-situ for syngas production. Steam from the bottom was switched into nitrogen to purge the whole system and raised the temperature of reactor for gasification. When the temperature was reached and stabilized, nitrogen was switched to steam for coke gasification. The produced gas was monitored until no more gas was generated. The time for coke gasification (i.e., catalyst regeneration) was recorded and catalyst after regeneration could be collected for analysis. Alternatively, we could further lower temperature of the reactor for oil cracking over the regenerated catalyst and evaluate the catalyst stability by cycle tests. The oil cracking combined coke gasification could be conducted by repeating this procedure in a single fluidized bed to simulate ICCG process. The mass balance was over 95% for the repeated experiments and the average value of the tests was used for the calculation. 2.3. Analysis and Characterization The gas from oil cracking was characterized with multi-channel gas chromatograph (GC, BEIFEN 3420) for components of dry gas (H2, CO, CO2, and hydrocarbons C1-C2) and liquid petroleum gas (LPG, hydrocarbons C3-C5). Mass yield of gas species could be calculated from total gas volume and gas composition according to the ideal gas law. Simulated GC (Agilent 7890A) was used to measure liquid products following with the boiling point range, that is, the distillation fractions of gasoline (initial boiling point-180 °C), diesel (180-350 °C), vacuum gas oil (VGO, 350-500 °C) and heavy oil (>500 °C). The heavy oil fraction in liquid after cracking was considered as unconverted oil, thus we could obtain the cracking conversion ratio (Rc, %) as, Rc=100% - liquid yield × heavy oil fraction. Coke on the spent catalyst was measured with a coke analyzer (CS-344, LECO) by burning the coke with pure oxygen. The generated gas was analyzed with a GC for the CO2 content, which was used to calculate the carbon content. The composition of FCC and BFC catalysts was determined using the X-ray fluorescence (XRF) spectrometry (AXIOS). An automatic BET analyzer (Autosorb-1, Quantachrome) was used to measure the 5
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specific surface area and pore structure of the catalyst via N2 adsorption at 77 K and then out-gassed in vacuum at 300 °C for 24 h. The acidity of catalyst samples was analyzed with the temperature-programmed ammonia desorption (NH3-TPD, Quantachrome). The catalyst sample of 50 mg was loaded into the U-tube quartz reactor and heated to 200 °C in helium to remove its impurities. Then, the temperature was lowered to 150 °C and the helium stream was changed to NH3 stream to perform the NH3-adsorption for about 120 min. After that, helium was switched back again to purge the reactor until the baseline of mass spectrometry (MS) was stable. Then the NH3 desorption was tested by heating the catalyst sample to 900 °C at a rate of 10 °C/min. The desorbed ammonia with temperature was monitored by on-line MS (PROLINE, AMETEK). The X-ray diffraction analyzer (XRD, X’Pert MPD Pro, Panalytical) was equipped with a 1.54 Å (λ) Cu-Kα radiator of 40 kV and 20 mA, and the adopted scanning rate was 4°/min in the range of 5-90°. 3. RESULTS AND DISCUSSION 3.1. Cracking Behaviors over Catalysts Based on our previous studies on cracking experiments, the reaction temperature was chosen at 510 °C for all catalysts with catalyst/oil ratio of 4-6 by mass. Table 3 shows the product distribution of oil sand bitumen cracking. Fresh FCC and BFC catalysts exhibited excessive cracking activity on oil sand bitumen, and the liquid yield over fresh catalysts was only about 52 and 57 wt.%, respectively. The liquid yield of BFC catalyst was higher than that of FCC catalyst, correspondingly with lower gas and coke yield. This indicated that the activity of BFC catalysts was slightly weaker for oil cracking, which could be also demonstrated by the acidity characterization of FCC and BFC catalysts with NH3-TPD analysis (Figure 2). The FCC catalyst had two NH3desorpotion peaks both at low and high temperature, while the NH3-intensity of BFC catalysts was mainly in the low-to-medium temperature range. Oil conversion via thermal cracking could tend to produce more dry gas components, while LPG species usually was much favored in catalytic cracking.22 So the ratio of LPG/(dry gas) could be used as an indicator of oil conversion over catalytic and thermal cracking. The LPG/(dry gas) ratio was both above 2.0 using fresh FCC and BFC catalysts, also justifying the high catalytic activity for two catalysts. It needs to point out that the conversion ratio was around 96% over the two fresh catalysts and heavy oil fractions took up about 6 wt.% in the cracked liquid oil. Our previous studies16,17,21 conducted several vacuum residues upgrading over fresh FCC and BFC catalysts, finding that the conversion ratio reached 100% and more than 95 wt.% of cracked oils were gasoline and diesel fractions (i.e., boiling point < 350 °C). The reason for the results was probably caused by the deteriorating grade of oil sand bitumen, which had high contents of carbon residue and heteroatoms comparing with former vacuum residue feedstocks. The sum contents of resins and asphaltenes were over 50 wt.% in oil sand bitumen, making it hard to be converted. 6
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Oil sand bitumen upgrading over fresh catalysts was mostly converted into gas and coke. Thus, the hydrothermally-treated catalysts were applied for cracking oil sand bitumen, and finding that the liquid yield greatly increased from about 57% to 78 wt.% over A-BFC catalysts with more suitable oil cracking activity. The conversion ratio over A-BFC catalysts (87.2%) was slightly higher than that of A-FCC catalysts (86.6%), and taking into account the fact that fresh BFC catalysts had lower activity than that of FCC catalysts (from Table 3 and NH3-TPD in Figure 2). Thus, it demonstrated that BFC catalysts could reserve more stable cracking activity during high-temperature steam atmosphere comparing with FCC catalysts. As shown in Figure 2, oil conversion behaviors were closely related to the acidity of catalysts, meaning that catalytic activity had crucial effects on cracking performance. The potassium impregnated BFC (K-BFC) catalyst showed higher cracking activity than that of the A-BFC catalyst in terms of both higher conversion ratio and LPG/(dry gas) ratio because of the higher cracking acidity of K-BFC as shown in Figure 2. Nevertheless, the LPG/(dry oil) ratio in the produced gas from A-FCC, A-BFC and K-BFC was quite smaller (
