Article pubs.acs.org/EF
Deep Conversion of Venezuela Heavy Oil via Integrated Cracking and Coke Gasification−Combustion Process Yuming Zhang,*,†,‡ Lei Huang,† Xiaoying Xi,‡ Wangliang Li,‡ Guogang Sun,† Shiqiu Gao,*,‡ and Shu Zhang§ †
State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Process Fluid Filtration and Separation, China University of Petroleum-Beijing, Beijing 102249, China ‡ State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China § Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia ABSTRACT: The integrated residue cracking and coke gasification−combustion (RCGC) process was proposed to make hierarchical and value-added utilization of Venezuela vacuum residue. Heavy oil cracking was conducted in a fluidized bed reactor with spent fluid catalytic cracking (FCC) catalysts, finding that both high liquid yield (>76 wt %) and conversion ratio (>90%) could be realized over the hydrothermal treated FCC (A-FCC) catalyst at 524 °C. As characterized by temperature-programmed ammonia desorption analysis, the A-FCC catalyst with moderate cracking ability was essential for optimum product distribution in vacuum residue conversion, where coke formation could be greatly suppressed via efficient oil vaporization and minimized secondary reaction. Coke removal (i.e., catalyst regeneration) of the FCC catalysts was conducted in two ways, that is, coke gasification (G-FCC) and gasification−combustion (GC-FCC). During coke gasification, the sum of H2 and CO took up more than 80 vol % in the syngas, which could be potentially used as a hydrogen source for hydrotreating the cracked oil. Compared with that of the G-FCC catalyst, the regeneration time of the GC-FCC catalyst not only was shortened by 40% but also had higher carbon conversion ratio and a superior recovery of pore structures. As a result, the GC-FCC catalyst showed cracking performance for vacuum residue that was better than that of the G-FCC catalyst because of its higher recovered acidity for heavy oil conversion. The FCC catalyst exhibited good hydrothermal stability during the cycle tests and thus could be potentially used as a candidate for Venezuela heavy oil upgrading via the RCGC process. processing12,13 processes are the representative catalytic conversion technologies in the modern refineries for maximizing the liquid yield with high quality. However, when treating heavy oil with high carbon residue and heteroatoms, the catalysts of these processes would face the problems of catalyst deactivation14 by severe coking formation and poisoning by heteroatoms, like sulfur, nitrogen, and heavy metals. New refining technologies15 are developed to cope with the increasing deterioration of feedstocks by modifying the existing units or combining different upgrading processes together. Taking the FCC process as an example, the millisecond catalytic cracking,16 maximizing iso-paraffins,17 two-stage riser,18 multifunctional two-stage riser,19 and dual-reactions mutual control20 processes were put into a demonstration or industrial utilization either for treating residue oil or optimizing product distribution. Heavy oil with deteriorating quality was hydrogenated with novel catalysts and reactors, such as an ebullated bed or slurry bed hydrocracking.21,22 Various upgrading processes are combined together to take utmost advantage of each other, including solvent deasphalting, delayed coking, residue FCC, and hydroprocessing.23 Actually, heavy oil, as a kind of carbonaceous fuels, could also be converted
1. INTRODUCTION The development of new petroleum refining processes has been stimulated by (1) the deteriorating quality of feedstocks, such as heavy and extra-heavy oil and oil sand bitumen; (2) the increasing demand for transportation fuels, i.e., gasoline, diesel, and jet fuels; (3) the more stringent environmental regulations on extra-low sulfur permission in gasoline and diesel.1 The unconventional oil resources (heavy oil, extra-heavy oil, and bitumen), make up about 70% of the world’s total oil reserves.2 Venezuela heavy crudes take up 17.7% of total proved reserves because of the abundant heavy oil and extra-heavy oil reservoirs in Orinoco Belt, ranking the first in the world.3 Modern refineries incorporate different units for heavy oil processing based on thermal or catalytic conversion technologies.4,5 Thermal cracking processes, such as visbreaking, and coking, without requiring expensive catalysts, are still attractive methods for heavy oil upgrading from an economic viewpoint. As a mild thermal cracking process, visbreaking6,7 could substantially improve the viscosity characteristics of residue for pipeline transportation or using as feedstock for the downstream units. The coking process, particularly delayed coking,8,9 is widely used for treating inferior feedstocks because of its extensive feed adaptability and low investment and operation costs. The main drawbacks of delayed coking are the mass production of low-value petroleum coke with high contents of sulfur and mineral matter when treating inferior feedstocks. Fluid catalytic cracking (FCC)10,11 and hydro© XXXX American Chemical Society
Received: June 5, 2017 Revised: July 23, 2017
A
DOI: 10.1021/acs.energyfuels.7b01606 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
the RCGC process in a fluidized bed reactor. The cracking behaviors of heavy oil were first investigated over commercial FCC catalysts to obtain suitable catalytic activity and operating conditions for both high conversion and liquid yield. Then the catalyst regeneration was carried out in two ways, that is, coke gasification and coke gasification−combustion. The reaction characteristics, such as carbon conversion ratio and catalyst regeneration time, were investigated and compared with each other. Finally, heavy oil cracking was further conducted over the regenerated catalysts via these two methods to justify the technical advantages and feasibility of the RCGC process for heavy oil upgrading.
hierarchically via dual fluidized bed technology.24,25 On the basis of this concept, FCC could be considered as a pneumatic riser for oil cracking and a dense fluidized bed combustor for catalyst regeneration. However, as discussed before, deteriorating feedstocks could not be directly treated with the FCC process because of the quick catalyst deactivation by coking and poisoning of heteroatoms. Also, catalyst regeneration merely by coke burning would generate excessive heat for the FCC process and meanwhile lead to huge waste of carbon source and the heavy burden of flue gas desulfurization. The integrated residue cracking and gasification (RCG) process26,27 was thus proposed for value-added utilization of heavy oil and their residues (atmospheric residue, vacuum residue, oil slurry, etc.). Heavy oil was first converted via contacting with catalysts of moderate activity to maximize liquid oil yield, while some heavy components (i.e., resins and asphaltenes) deposited on the catalyst as coke, which was transported to the gasifier for syngas production and simultaneous catalyst regeneration. The regenerated catalyst circulated back to the cracking reactor to provide the endothermic heat and catalytic activity essential for oil conversion. Fundamental characteristics of the RCG process have been studied using a fluidized bed reactor in terms of operation parameters (temperature, catalyst/oil ratio, etc.), feedstocks (different vacuum residues) and catalysts (silica sand, kaolin, FCC, and self-made bifunctional catalysts) in our past studies.28,29 The results showed that both high liquid yield and the high conversion ratio of heavy oil could be realized by fluid cracking, but catalyst regeneration merely via coke gasification usually needed a long time for complete carbon removal. As a result, the integrated coke gasification and combustion procedure was used to accelerate the catalyst regeneration compared with the RCG process. Figure 1 shows
2. EXPERIMENTAL SECTION 2.1. Materials. The feedstocks used in the present study was the vacuum residue of Venezuela heavy oil with the properties shown in Table 1. The vacuum residue had quite inferior quality with low H/C ratio of 1.41 and high Conradson carbon residue (CCR = 21.15 wt %). The resins and asphaltenes in the group analysis together accounted for about 37 wt %, suggesting its great tendency of coke formation. This kind of Venezuela vacuum residue was in a solidlike phase at room temperature, and its viscosity was up to 19 100 mPa·s at 80 °C. Also, the high contents of heteroatoms (nitrogen, sulfur, and heavy metals of Ni and V) made Venezuela vacuum residue difficult to treat by catalytic conversion processes, such as catalytic cracking and hydrocracking, because of the quick deactivation of the catalyst. A kind of spent FCC catalyst from PetroChina was used for vacuum residue conversion in this study. The spent FCC catalyst was commercially available and cheap, whereas it still had enough catalytic activity for heavy oil upgrading at low operation cost. The FCC catalyst mainly consisted of Al2O3 (45 wt %) and SiO2 (46 wt %), with a small portion of ultrastable Y-zeolite (USY) and some other oxides. The original FCC catalyst was treated at 800 °C for 17 h in a steam atmosphere to adjust its acidity and thus improve the cracking acidity and hydrothermal stability of aged-FCC (A-FCC) catalysts during the cracking−gasification operation. 2.2. Experimental Setup. Venezuela vacuum residue was stepwise converted by oil cracking and coke gasification−combustion in a fluidized bed reactor. The details of the reactor can be found in our previous studies.26,28,29 In brief, the catalyst was first placed on the porous stainless steel sintered plate and fluidized by steam. When the reactor was heated and stabilized at the desired temperature, preheated vacuum residue at about 150 °C together with steam (steam/oil ratio of 0.2 by mass) at 300 °C was injected into the fluidized catalyst particles through a nozzle and converted into light volatiles by contacting with hot catalysts. Light volatiles were quickly entrained out from the hot reaction area by upward steam to suppress excessive oil cracking. After being purified by an inside-mounted filter, the volatiles were condensed and successively collected as heavy oil, light oil, and uncondensable cracking gas, while extra-heavy compounds condensed on the surface of the catalyst as coke deposit. Nitrogen was the purging gas during the interval of coke gasification and/or combustion experiments. The initial temperature of coke gasification was about 750 °C determined by carbon−steam reaction in the pretests, at which nitrogen was switched back to steam for coke gasification. The final temperature of coke gasification was fixed at 800 °C, and during this period of time the produced gas was monitored and collected. When coke was partially removed by gasification (i.e., coke conversion at about 50%), the reaction temperature decreased to 700 °C and changed steam into the air for complete carbon removal by combustion. The generated gas was detected by a flue gas analyzer. The completion time of coke gasification (catalyst regeneration) was recorded until no more carbon-containing gases (i.e., CO and CO2) were detected. The regenerated catalyst could be collected for analysis, or alternatively, to lower the reaction temperature for oil cracking over the regenerated catalyst. Thus, the oil cracking combining coke gasification−combustion could be realized in the single fluidized bed via repeated batch operation simulating the RCGC process.
Figure 1. Conceptual diagram of residue cracking and coke gasification−combustion process.
the conceptual diagram of the integrated residue cracking and gasification−combustion (RCGC) process to regenerate the coked catalyst, where first coke was partially gasified for syngas production and then combusted for complete catalyst regeneration. The RCGC process is similar to the dual flexicoking process in terms of syngas generation from which the produced hydrogen via coke gasification is twice the amount needed for upgrading its extracted liquid.30 In this study, vacuum residue extracted from Venezuela heavy oil, which has more deteriorating quality than the feedstocks previously used,26,28,29 was stepwise converted by B
DOI: 10.1021/acs.energyfuels.7b01606 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Properties of Venezuela Vacuum Residue group analysis (wt %) densitya (g·cm−3)
H/C ratio
CCRb (wt %)
1.025
1.41
21.15
C
H
84.74
9.96
80 19100 a
90 6450
N
saturates
19.14 Elemental Analysis (wt %) O
aromatics
resins
asphaltenes
43.75
24.70
12.41
S
0.75 0.91 3.64 Viscosity (mPa·s) Variation with Temperature (°C) 100 3980
110 1600
120 635
130 500
Ni
V
99 × 10−6
423 × 10−6
140 320
150 204
160 120
170 82
Density: at 20 °C. bCCR: Conradson carbon residue.
2.3. Analysis and Characterization. Heavy oil was converted into three kinds of final products, that is, the cracking gas, liquid oil, and coke, which condensed on the surface of the catalyst. The cracking gas was characterized with a multichannel gas chromatograph (GC, BEIFEN 3420) for components H2, CO, CO2, and hydrocarbons C1− C5. Mass yield of gas species could be calculated according to the total gas volume and gas composition. Coke was analyzed with a coke analyzer (CS-344, LECO) for the carbon content of the catalyst. Simulated GC (Agilent 7890A) was used to measure liquid products following the boiling point range, that is, the distillation fractions of gasoline (initial boiling point to 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 the unconverted oil; thus, we could obtain the cracking conversion ratio (Rc, %) as
Table 2. Cracking Behavior of Vacuum Residue over FCC and A-FCC Catalystsa catalyst
FCC
A-FCC
temperature (°C) 487 485 506 gas yield (wt %) 6.7 4.4 5.3 liquid yield (wt %) 61.0 78.1 77.0 coke yield (wt %) 32.3 17.5 17.7 conversion (%) 96.1 82.8 86.9 Fractional Distribution of the Produced Liquid gasoline fractions 22.5 11.3 14.4 diesel fractions 46.0 20.4 24.3 VGO fractions 25.2 46.3 44.3 heavy oil fractions 6.3 22.0 17.0 a
Rc = 100% − liquid yield × heavy oil fraction An automatic BET analyzer (Autosorb-1, Quantachrome) was used to measure the specific surface area and pore structure of the catalyst via N2 adsorption at −196 °C and then outgassed in vacuum at 300 °C for 24 h. The acidity of catalyst samples was analyzed with temperature-programmed ammonia desorption (NH3-TPD, Quantachrome). 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 was changed to ammonia gas to perform the NH3-adsorption for about 120 min. After that, helium was switched again to purge the reactor until the baseline of mass spectrometry (MS) was stable. 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 online 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°.
524 5.7 76.4 17.9 90.8 (wt %) 16.8 26.4 44.8 12.0
547 7.2 75.5 17.3 93.2 17.0 25.5 48.5 9.0
FCC, fluid catalytic cracking; A-FCC, aged FCC catalyst.
results gave clear evidence of the much inferior properties of Venezuela vacuum residue, in terms of its poor crackability and great coke formation tendency due to the high contents of residue carbon and asphaltenes. Consequently, vacuum residue cracking was conducted using the hydrothermal treated FCC catalysts (A-FCC) with lower activity. Comparing with the FCC catalyst, the liquid yield over the A-FCC catalyst increased by 17 wt % at a similar temperature of 485 °C, with the corresponding conversion ratio of 83%. It was clearly proved that the A-FCC catalyst possessed cracking ability that was much lower than that of original FCC catalysts, as shown by the acidity characterization with the NH3-TPD analysis in Figure 2. Essentially, oil cracking over catalysts occurs in both thermal and catalytic reaction routes, while the activation energy of thermal reaction (210− 290 kJ/mol) is higher than that of catalytic cracking (42−125 kJ/mol).31 Thus, the high temperature is more prone to
3. RESULTS AND DISCUSSION 3.1. Vacuum Residue Cracking. Table 2 shows the product distribution of Venezuela vacuum residue cracking over the spent and aged FCC (A-FCC) catalysts at different temperatures with the catalyst/oil ratio of 6 by mass. When directly using original FCC catalysts for vacuum residue conversion, the conversion ratio was over 96% at a low temperature of 487 °C, whereas the liquid yield was only about 60%, and coke accounted for over 30 wt %. It meant that the original FCC catalyst still had sufficient activity for vacuum residue conversion, and its strong acidity caused excessive cracking of heavy oil. Nevertheless, the fractional distribution of liquid oil indicated that the heavy oil fractions took up 6.3 wt % at such severe cracking conditions. Considering the fact that the gasoline and diesel fractions (i.e., boiling point
