1. Introduction
Worldwide increase in industrialization has led to an increase in consumption of petroleum oil and coal [
1]. It has raised serious environmental concerns due to the rise in sulphur oxide (SOx) levels in air. Therefore, environmental protection agencies (EPA) such as US EPA limits the sulphur levels to 15 ppm in diesel fuel, which is expected to be further lowered to 10 ppm. On the other hand, the gas oil extracted from unconventional sources such as, oil sands and shale oil contain high amount of sulphur and nitrogen impurities. For instance the oil sands bitumen derived heavy gas oil (HGO) contains ~40,000 ppm sulphur and ~4000 ppm nitrogen [
2]. The bitumen derived gas oils (HGO, light gas oil (LGO) and naphtha) are upgraded onsite via hydrotreating to lower the sulphur and nitrogen content before sending them to further processing in existing refineries. Hydrotreating is a catalytic process operating in the presence of hydrogen at high pressure (8–12 MPa) and temperatures (350–400 °C) to remove sulphur and nitrogen via processes known as hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), respectively. Typically, for HGO the hydrotreating can lower the sulphur and nitrogen content to ~2200 ppm and 1700 ppm, respectively. To further lower the sulphur content from ~2200 ppm during upgrading requires severe hydrotreating operating conditions such as higher pressures, temperatures and hydrogen flowrates to remove sulphur from refractory molecules such as alkyl substituted dibenzothiophenes (DBT). Increasing the temperature leads to cracking of oil and higher pressures lead to an increase in saturation of aromatics and higher hydrogen consumption, thus degrading the oil quality, in addition to decline in catalyst lifespan. Moreover, huge capital investment is required for high-pressure processes.
The alternative processes such as adsorption and oxidative desulfurization and denitrogenation has been of interest to achieve ultra-low sulphur level in oil because of very mild operating conditions and no usage of hydrogen. Various adsorbents including activated carbon, ionic resins, metal organic frameworks, metal oxides and zeolites have been used to adsorb sulphur containing compounds from diesel oil [
3,
4,
5,
6,
7,
8,
9,
10]. Ganiyu et al. [
6] utilized activated carbon doped with 1.0 wt.% boron to selectively adsorb 4,6-DMDBT from model fuel. Srivastav and Srivastava [
7] carried out the adsorption of DBT dissolved in hexanes on commercial grade activated alumina and Li et al. [
4] presented a study on the challenges associated with removal of aromatic sulphur compounds using metal-organic frameworks. McKinley and Angelici [
9] used silver salts on SBA-15 for adsorptive removal of DBT from simulated hydrotreated petroleum feedstocks. They were able to lower the sulphur level from 411 ppm to 8 ppm.
Oxidative desulfurization (ODS) is one of the alternate routes for deep desulfurization. In ODS process (as shown in
Figure 1), the sulphur present in compounds such as DBT is first oxidized to sulfoxides or sulfones in the presence of oxidizing agents such as hydrogen peroxide, organic peroxides, nitrogen oxide or air and catalysed by organic acids, heteropolyic acids or solid catalysts. The oxidation of sulphur leads to increase in polarity of sulphur containing compounds. Therefore, the sulfoxides and sulfones were easily extracted from the oil by using polar solvents or adsorbents, thus achieving deep desulfurization. Most commonly tested solvents include dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, methanol and acetone [
11,
12]. There are several well-known disadvantages with solvent exactions such as toxicity, reusability, disposal, explosiveness and cost. Therefore, selection of solvent is a challenge. DMSO poses challenges during recovery due to similar boiling point, whereas acetonitrile is highly polar and extracts lots of aromatics [
11]. Methanol is a good solvent for extracting sulfones however, it has similar density as diesel and thus separation is difficult. The ease of oxidation of various sulphur containing compounds depends on the electron densities on the sulphur atom. Sulphur with higher electron densities are easier to oxidize, hence, follows the order 4,6-DMDBT > DBT > BT > Thiophene [
13]. Bunthid et al. [
14] have utilized formic acid and H
2O
2 as oxidizing agent to oxidize DBT. The corresponding sulfone from the solution was then extracted by adsorption on pyrolysis char. The small amount of water remaining was extracted by drying over anhydrous sodium sulphate. They reported 72% sulphur removal. In another study, Ahmad et al. [
15], used acetic acid as a catalyst for sulphur oxidation with H
2O
2. They used Fuller’s earth for the adsorption of sulfones and achieved 50% sulphur removal.
Palaic et al. [
16] studied the oxidative desulfurization of diesel fuels for the removal of refractory sulphur compounds which are difficult to remove during conventional hydrotreating. They utilized hydrogen peroxide as oxidant and acetic acid as catalyst and performed reactions in a batch reactor. The effects of process conditions of ultrasound-assisted ODS with
N,
N dimethylformamide and methanol as extraction solvents were also reported. They successfully removed 98% sulphur from 4000 ppm DBT spiked diesel fuel. The usage of acid catalyst such as acetic acid or formic acid for oxidation of sulphur compounds requires recovery of these organic acids after treatment. It requires additional set up for recovery of such corrosive and toxic organic acids. Therefore, in view of this, solid catalysts were utilized. Fattahi et al. [
17] synthesized CoMo/γ-Al
2O
3 catalyst with different Co/Mo ratio and utilized it for the oxidative desulfurization of DBT and benzothiophene (BT) using H
2O
2 as oxidizing agent. They reported 90% removal of DBT and 30% removal of BT. Chica et al. [
18] studied the effect of catalyst on ODS in a continuous fixed bed reactor for model feed containing different types of sulphur compounds including thiophenes and alkyl substituted DBTs. Tert-butyl hydroperoxide was utilized as oxidizing agent. They reported that MoOx/Al
2O
3 was very active but had faster deactivation rate. However, the Ti-MCM-41 was stable and active for longer time. Gatan et al. [
19] from UOP also proposed the oxidation with organic perpoxide in the presence of heterogeneous catalyst followed by separation via adsorption and/or extraction. They proposed ODS as complementary to hydrodesulfurization for the removal of refractory sulphur compounds to attain ultra-low sulphur diesel. Leng et al. [
20] synthesized titanium doped hierarchical mordenite to catalyse ODS of DBT in octane. They used acetonitrile for the extraction of sulfones and were successful in lowering the sulphur content from 1000 ppm to 14 ppm. Lorencon et al. [
21] utilized titanate nanotubes and H
2O
2 for the oxidation of DBT in model feed (500 ppm sulphur) and evidenced ~98% sulphur removal. Tian et al. [
22] performed ODS using H
2O
2 and phosphomolybic acid supported on silica for the removal of DBT and BT from model oil (~400 ppm sulphur) and were successful in removing ~95% sulphur. García-Gutiérrez et al. [
23] utilized heterogeneous tungsten catalyst for oxidation of sulphur compounds in diesel (~320 ppm sulphur) using H
2O
2 as oxidizing agent and achieved ~70% removal. Therefore, it has been seen that various catalyst systems have been tested for the oxidation of sulphur present in model and simulated diesel fuels. Moreover, the extraction of sulfones and sulfoxides from oil was carried out using both solvent extraction and adsorption.
The oxidation using peroxide in the presence of catalyst is not only selective to oxidize sulphur in heterocyclic aromatic compounds such as DBTs and alkyl substitute DMDBTs but it also oxidizes nitrogen containing aromatic compounds present in real feed. The oxidation of nitrogen containing aromatic compounds such as quinoline, indole and carbazole is a complex reaction. According to the literature [
24,
25,
26], it was found that the peroxide group oxidizes that carbon in aromatic ring, which is having least electron density to form -oxy or -oxyl compounds. Further oxidation leads to ring opening and formation of various oxygenated products of ketone and carboxylic acid category. A study by Ogunlaja et al. [
25] also reported the oxidation of nitrogen in quinoline to form of quinoline N-oxide. The oxidation follows the order indole > quinoline > acridine > carbazole. However, the oxidation of nitrogen compounds increases their polarity, which makes it easy to remove organonitrogen compounds from oil by adsorption or extraction, thus resulting in oxidative denitrogenation (ODN). The removal of nitrogen prevents the downstream catalysts from poisoning and hence ODN increases the quality of the treated HGO and LGO.
Therefore, to keep bitumen-based fuels competitive in the current market, the production of low sulphur and low nitrogen HGO and LGO is required, which still remains a challenge to industry. Thus, the potential of ODS and ODN to further lower the sulphur and nitrogen level in hydrotreated LGO and HGO using heterogeneous catalyst needs to be explored. In this work, NiMo/γ-Al2O3 catalyst was synthesized and utilized to hydrotreat the HGO and LGO at typical industrial conditions of 370–390 °C, 9 MPa, 1–1.5 h−1 LHSV and 1:600 oil to H2 ratio, in fixed bed flow reactor, to generate hydrotreated gas oil for ODS and ODN process. Further, alumina and alumina-titania supported Mo, W, Mn and P catalysts were synthesized and tested for the oxidation of sulphur and nitrogen compounds present in real gas oil using tert-butylhydroperoxide (TBHP) as oxidizing agent. The extraction of oxidized sulphur and nitrogen compounds was carried out by adsorption on activated carbon and compared with liquid extraction using methanol. This study on integration of conventional hydrotreating technology with ODS and ODN for upgrading bitumen derived gas oil has resulted in lowering the sulphur levels to less than 500 ppm in both HGO and LGO, which can be further lowered down in existing refineries to meet the EPA regulations for diesel fuel. Additionally, the nitrogen levels for HGO were brought down to ~200 ppm, which enhances the quality of crude oil and eliminates the nitrogen removal process prior to refining of treated HGO. The catalysts were thoroughly characterized using BET, XRD, FTIR and XPS and their physico-chemical properties were related to their catalytic activities.
3. Experimental
3.1. Materials
Titanium isopropoxide, ammonium dihydrogen phosphate, Ammonium heptamolybdate tertrahydrate, Manganese nitrate hexahydrate, tert-butylhydroperoxide (TBHP) (70 v/v%), Ammonium metatungstate hydrate, were purchased from Sigma Aldrich, Edmonton, Canada. Activated carbon was provided by Norit, Canada. Amberlyst 15 ion exchange resin and Amberlite IRA-400 ion exchange resin were purchased from Alfa Aesar, USA. γ-Al2O3 and methanol was purchased from Fischer-Scientific, Toronto, Canada.
3.2. Catalyst Synthesis
All metals were impregnated using incipient wetness method. Al2O3-TiO2 material was synthesized by impregnating the solution of titanium isopropoxide and ethanol on γ-Al2O3 to obtain 10 wt.% TiO2 in Al2O3-TiO2. 15 wt.% W and 15 wt.% Mo were impregnated on γ-Al2O3 and Al2O3-TiO2 materials using ammonium metatungstate hydrate and ammonium heptamolybdate tertrahydrate as precursors for W and Mo, respectively. The materials were dried overnight at 100 °C and then calcined at 550 °C for 6 h. The resulting materials were named as Mo/γ-Al2O3, Mo/γ-Al2O3-TiO2, W/γ-Al2O3 and W/γ-Al2O3-TiO2. Further, 2 wt.% phosphorus was impregnated to Mo/γ-Al2O3-TiO2 using ammonium dihydrogen phosphate as precursor. This was followed by drying and calcination at 400 °C for 4 h to obtain PMo/γ-Al2O3-TiO2 catalyst. Manganese nitrate hexahydrate was used as a precursor for Mn and it was wet impregnated on PMo/γ-Al2O3-TiO2 to obtain 2 wt.% Mn in MnPMo/γ-Al2O3-TiO2 catalyst. The catalyst containing 13 wt.% Mo and 3 wt.% Ni supported on γ-Al2O3 was also synthesized using sequential wetness impregnation method and named as NiMo/γ-Al2O3.
3.3. Material Characterization
All materials were characterized in oxide state. Micrometrics ASAP2020 instrument was used to determine the textural properties. The Brunauer-Emmett-Teller (BET) method was used to determine the surface area and Barrett-Joyner-Halenda (BJH) method was used to calculate the pore diameter and pore volume. The wide angle (20° to 80°) X-ray diffractograms for powder samples were obtained using Bruker Advance D8 series II equipment having Cu Kα radiation. The FTIR analysis for the materials was performed using PerkinElmer Spectrum GX instrument. The details on the sample preparation and method of analysis for XRD, FTIR and BET are mentioned in our previous work [
52]. The X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the oxidation state of active metals. The XPS spectra for catalysts were collected using Kratos Axis SUPRA XPS instrument equipped with monochromatic Al Kα X-ray source. This instrument is located at Saskatchewan Structural Sciences Centre, University of Saskatchewan, Canada. The XPS data were fitted using CasaXPS software.
3.4. Hydrotreating Experiment
The fixed bed flow reactor system was used to perform hydrotreating reactions for HGO and LGO. The physical properties of HGO and LGO are mentioned in
Table 6. The reaction setup consists of a liquid feed pump, H
2 and He gas inlet lines via mass flow controllers, tube type fixed bed reactor heated by furnace, back pressure regulator, NH
3 scrubber and gas-liquid separator to collect the hydrotreated product followed by nitrogen stripping. The schematic and details of the setup are mentioned in our previous work [
52]. 5g of NiMo/
γ-Al
2O
3 catalyst was diluted with SiC (90 mesh size) and loaded in the reactor. The catalyst was sulphided by pumping 5 mL/h of 2.9 vol% butanethiol solution (in transformer oil) through the catalyst bed for 48 hrs. The first 24 h sulphidation was performed at 190 °C and then the temperature was raised to 340 °C and maintained for next 24 h. The reactor was pressurized to 9 MPa with helium prior to start of sulphidation. The entire sulphidation process was carried out at 50 mL/min of hydrogen flow.
The liquid feed was switched to gas oil (HGO or LGO) after sulphidation and the reaction temperature was increased to 370 °C for LGO and 390 °C for HGO. The gas oil feed rate was maintained at 5 mL/h and 7.5 mL/h for HGO and LGO, respectively. The H
2 feed rate was maintained at 50 mL/h for HGO and 75 mL/h for LGO. The hydrotreated oil was collected every 24 h and analysed for sulphur and nitrogen content. The constant conversion was observed from day 5 to day 12 and then the reactor was shut down. Therefore, the hydrotreated oil product from day 6–12 was mixed to obtain a big batch of hydrotreated gas oil, hereafter called as HDT-HGO and HDT-LGO. Hydrotreating of LGO and HGO was done with fresh catalyst in the same experimental set-up. The Antek 9000 N/S analyser was used to determine the nitrogen and sulphur content in liquid samples. ASTM D4629 method using combustion/chemiluminescence technique was adopted to determine the total nitrogen content of the liquid product and the ASTM D5463 method using combustion/fluorescence technique was deployed for measuring sulphur content. The details on catalyst loading procedure and sulphidation procedure are mentioned in our previous works [
52].
3.5. Experimental Procedure for Oxidative Desulfurization and Denitrogenation
20 ml of HDT-HGO was taken in a round bottom flask and 1 g catalyst was added to it. To this mixture 20 ml of TBHP (70% v/v) was added as an oxidizer and stirred at 400 rpm at 90 °C under reflux for 15 h. The 1:1 volume ratio of TBHP and HDT-HGO was used for screening experiments considering the complex nature of bitumen derived heavy gas oil. This makes molar ratio of TBHP to sulphur equal to 110 and TBHP to nitrogen molar ratio equal to 58, which is quite high. The Metal (Mo) to sulphur molar ratio is 1.2. Further, handling equal amount of TBHP and gas oil in existing refineries will be a concern. However, this lab scale experiment was performed for the proof of concept and further optimization studies on the amount of oxidant, catalyst, oil, stirring speed and temperature can help to determine the techno-economical and logistical viability of this process integration with existing refineries. The reaction was cooled down to room temperature and then filtered to recover the solid catalyst. The catalysts tested for this reaction are: Mo/γ-Al2O3, Mo/γ-Al2O3-TiO2, W/γ-Al2O3 and W/γ-Al2O3-TiO2. PMo/γ-Al2O3-TiO2 and MnPMo/γ-Al2O3-TiO2. The reaction products were phase separated due to the difference in the density of oil phase and water phase. The oil phase was collected and water washed to remove dissolved butanol, which is the by-product of oxidation of sulphur and nitrogen compounds by TBHP.
The oxidized sulphur compounds known as sulfones and sulfoxides and oxides of aromatic nitrogen compounds were than extracted from the oil phase using adsorption or solvent extraction. Activated carbon was used as an adsorbent. In a typical experiment, 1 g of activated carbon was mixed with 20 mL of oil for 12 h at room temperature. The mixture was then filtered to separate the solids and the resultant liquid was analysed to determine total S and N content. The type and amount of oxidized sulphur and nitrogen compounds were not determined in this work due to the complex composition of heavy gas oil. Therefore, the total S and N content in product was used as a basis to define the catalyst activity. The liquid was again treated with TBHP in presence of fresh catalyst followed by adsorption with activated carbon to perform double stage adsorptive extraction, when required. Similar procedure was followed to perform multi-stage ODS and ODN.
The solvent extraction process was also tested to extract the sulfones, sulfoxides and oxides of organonitrogen compounds from the oil phase collected after oxidation with TBHP. 10 mL methanol was mixed with 20 mL oil and allowed to settle for 4 h at room temperature. The mixture was phase separated to obtain desulfurized and denitrogenated oil, which was then tested to determine the sulphur and nitrogen content. Similar procedure was followed to perform oxidative desulfurization and denitrogenation on HDT-LGO.
4. Conclusions
Highly efficient upgrading of oil-sands bitumen derived heavy gas oil (~ 41,000 ppm sulphur, 3900 ppm nitrogen) and light gas oil (~24,000 ppm sulphur, 1400 ppm nitrogen) is required to generate synthetic crude, which can compete with conventional crude oil. Therefore, in this work the combination of hydrotreating, oxidative desulfurization (ODS) and oxidative denitrogenation (ODN) was performed to achieve less than 500 ppm sulphur in LGO and HGO. The synthetic crude produced using treated HGO and LGO will be highly competitive and easily processed in existing refineries to produce various petroleum fractions including diesel fuel with less than 15 ppm sulphur. The hydrotreating of gas oils was carried out in a fixed bed flow reactor operating at typical industrial conditions of 370–390 °C, 9 MPa, 1–1.5 h−1 space velocity and 600:1 H2 to oil ratio. NiMo/γ-Al2O3 was used as a catalyst. Hydrotreating resulted in lowering the sulphur and nitrogen content to 2100 ppm and 1750 ppm for HGO and 950 ppm and 175 ppm for LGO, respectively.
Various catalysts including Mo/γ-Al2O3, Mo/γ-Al2O3-TiO2, W/γ-Al2O3, W/γ-Al2O3-TiO2, PMo/γ-Al2O3-TiO2 and MnPMo/γ-Al2O3-TiO2 were synthesized and characterized using X-ray diffractions, N2 adsorption-desorption, FTIR and XPS. All catalysts were tested for the oxidation of sulphur and nitrogen compounds present in gas oil using tert-butyl hydroperoxide (TBHP). The removal of oxides of sulphur and nitrogen compounds was carried out using adsorption and extraction. Methanol was used as solvent for liquid-liquid extraction, however, the minimal difference in densities of hydrotreated LGO and methanol possess challenge in separation and longer settling time was required. Among activated carbon and ion-exchange resins, the higher adsorption capacity for polar oxidized sulphur and nitrogen aromatic compounds was shown by activated carbon. The sulphur and nitrogen removal were higher with activated carbon in comparison to methanol. The catalytic activity measured in terms of percent sulphur and nitrogen removal was related to the catalyst characterization. XPS analysis has confirmed the oxidation state of metals such as Mo, Ti, Mn, P and W, which helped in identifying the catalyst structure and relate it to the metal oxide structures as indicated by XRD and FTIR analysis. Mo supported catalyst outperformed the W supported catalyst due to stronger interaction between 2p O (TBHP) and 4d Mo orbitals in contrast to bonding between 5d W and 2p O orbitals. The Ti acts as an additional site for TBHP activation, which caused oxidation of sulphur and nitrogen containing compounds. P and Mn in MnPMo/γ-Al2O3-TiO2 catalyst makes molybdenum more electrophilic, thereby promoting the nucleophilic attack by TBHP, thus facilitating oxidation. Hence, the catalyst MnPMo/γ-Al2O3-TiO2 performed best among the series tested and removed 44.4 wt.% sulphur and 46.4 wt.% nitrogen from hydrotreated HGO and 54.9 wt.% sulphur and 77.1 wt.% nitrogen from hydrotreated LGO. Further three stage ODS and ODN process was performed using MnPMo/γ-Al2O3-TiO2 catalyst. Thus, the integration of hydrotreating, oxidative desulfurization and oxidative denitrogenation lowered the sulphur and nitrogen content to 478 ppm (~98.8% removal) and 206 ppm (~94.7% removal), respectively, in HGO and 354 ppm (~98.5% removal) and 30 ppm (~97.8% removal), respectively in LGO. The decrease in nitrogen content is beneficial for the downstream processing of gas oils because nitrogen acts as a poison to various catalysts. Therefore, the combination of oxidative desulfurization and denitrogenation with hydrotreating is a promising methodology to improve the quality of synthetic crude derived from oil sands bitumen and keeping them competitive with respect to conventional petroleum crude oil.