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Adsorptive Desulfurization of the Gasoline obtained from Low Pressure Hydrocracking of the Vacuum Residue using Nickel/Bentonite Catalyst Vagif M. Abbasov, Hikmat C. Ibrahimov, Gulbaniz S. Mukhtarova, Musa I. Rustamov, and Elshad Abdullayev Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017
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Adsorptive Desulfurization of the Gasoline obtained from Low Pressure Hydrocracking of the Vacuum Residue using Nickel/Bentonite Catalyst Vagif M. Abbasov1, Hikmat C. Ibrahimov1, Gulbaniz S. Mukhtarova1, Musa I. Rustamov1, Elshad Abdullayev*2
1
Institute of Petrochemical Processes, Azerbaijan National Academy of Sciences, Baku, Azerbaijan,
AZ1025 2
Independent Researcher, High Point, NC 27260
*Corresponding Author, Phone: (336) 862-4567, e-mail:
[email protected] (E. Abdullayev)
Abstract In this article adsorptive desulfurization of the gasoline produced from low pressure hydrocracking of the atmospheric residue was studied. Gasoline obtained from such process has high sulfur content and is not suitable for using as motor fuel without further desulfurization. Nickel modified natural bentonite catalyst was developed and utilized in the process. Reaction temperature and flow rate was adjusted to maximize the sulfur removal efficiency while fixing the pressure at 2.5 MPa. Optimal sulfur removal was obtained at 270 ⁰C and 1.0 hr-1 feed rate, reducing the gasoline sulfur level from 450 ppm to 32 ppm without affecting the hydrocarbon content and hence preserving the octane number. Sulfur adsorbs on nickel nanoparticles by forming nickel sulfide and the catalyst can be regenerated by subsequent oxidation with air followed by reduction with hydrogen gas.
Key words: adsorptive desulfurization; nickel; bentonite; gasoline; hydrocracking
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1. Introduction Deep desulfurization of fuels has become critical part of the refinery processes and fuel production due to stringent environmental protection agency (EPA) regulations on sulfur content put on gasoline and diesel. This has caused hydrotreatment/desulfurization catalyst market to become the biggest market within refinery catalysts market, surpassing the fluid catalytic cracking catalysts market as of 2001 [1].
Another tendency in the petroleum refining industry
is directed towards increasing the yield of the motor fuels by cracking of the heavy petroleum fractions such as atmospheric and vacuum residues. Heavy petroleum may constitute up to 85% of the crude oil depending on the origin, which makes processes like catalytic cracking and hydrocracking one of the most important ones for increasing the yield of the light fuel production [2, 3]. Gasoline obtained from heavy fractions pose unique problems due to their differing chemical composition than the atmospheric distillates. Content of aromatic compounds are generally higher in catalytic cracking and low pressure hydrocracking products, increasing their octane rating. On the other hand, desulfurization of the aromatic compounds generally require higher energy than the aliphatic hydrocarbons [1]. High pressure hydrodesulfurization (HDS) units, running at about 20-30 MPa pressure, are commonly used for sulfur removal. These units are not perspective due to the trade off between octane rating and sulfur level in addition to the high installation and operation costs and safety concerns. Various processes have been developed to overcome this problem such as selective/adsorptive desulfurization [1], mercaptan extraction by caustic solution followed by atmospheric oxidation (MEROX) [4-6] and post HDS octane recovery by isomerization [1]. Of these processes, adsorptive desulfurization has special importance due to low hydrogen consumption, stoppage of hydrogen sulfide release preventing recombination reactions with olefins and significant operating cost savings. ConocoPhillips
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developed S-Zorb technology for making low-sulfur gasoline, where the sulfur-containing compounds adsorb and react with the sorbent while the hydrocarbon portion is released back into the product [7]. Research Triangle Institute has developed TReND process, which is based on metal oxide sorbent for H2S removal from liquid fuels [8]. Yang et al reported desulfurization catalyst by means of π-complexation of sulfur containing compounds on Cu(I), Ni(II) and Zn(II) exchanged fujasite type zeolites [9]. In this work gasoline fraction obtained from low pressure hydrocracking of the vacuum residue was studied to demonstrate low temperature and low pressure adsorptive desulfurization process on nickel modified natural bentonite catalyst. Sulfur deposits on the surface of the nickel catalyst by converting it into nickel sulfide. The catalyst can be regenerated to recover its activity by subsequent air oxidation followed by reduction with hydrogen gas. The advantage of the current process over traditional HDS process are numerous, including the capability of selective sulfur removal without compromising high octane aromatic components, possibility to run the process at low pressure resulting in reduced operation costs and improved the process safety. This is contiuation of our previous work [10], in which suspended halloysite catalyst was demonstrated for low pressure hydrocracking of vacuum residue to obtain high-octane gasoline. Current publication along with the previous one contain important guidelines for obtaining lowsulfur, high-octane gasoline from vacuum residue at low pressure throughout the entire manufacturing process.
2. Materials and Methods 2.1. Materials Bentonite was obtained from AzRosPromInvest (AZRPRI) Llc, which mines bentonite clay at Dash-Salakhli deposit in Azerbaijan republic. Chemical composition of the clay is
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provided by the manufacturer given in Table 1. Nickel (II) chloride and NH4Cl were obtained from Ural plant of chemicals (former Ural plant of chemical reagents, Sverdlovsk oblast, Russian Federation, purity 98.5%). High sulfur gasoline was obtained from catalytic hydrocracking of vacuum residue obtained from Haydar Aliyev Petrochemical Refinery (Baku, Azerbaijan) at 440 ⁰C and 4 MPa pressure using Ni/Co modified halloysite catalyst as described in [10]. Ultrahigh purity hydrogen gas (99.999% purity) was obtained from Sharjah Oxygen Company (United Arab Emirates). 2.2. Preparation of the Ni/bentonite catalyst Bentonite (300 grams) was dispersed in about 300 mL of 10% NH4Cl solution at 80 ⁰C for 1 day. Then bentonite was separated from the solution by filtering. The procedure was repeated five times. Then bentonite was dried and grinded to 2 mm particle size. 200 grams of the bentonite was mixed with 220 mL of 0.3% aqueous nickel (II) chloride solution and heated in muffle furnace by gradual heating to 150 ⁰C and kept for 3-4 hours for complete drying of the moisture. 2.3. Adsorptive desulfurization of gasoline Adsorptive desulfurization of gasoline was conducted in
200 mL volume flow-type
microreactor (Figure 1). Catalyst (200 grams) was pelletized, placed into the reactor (1) and activated under air flow at 500 ⁰C for 3 hours followed by nitrogen purge to remove oxygen from the system and reduction under hydrogen gas flow at 400 ⁰C for few days, until no moisture is observed at the outlet of the reactor (tested by filter paper). Hydrogen gas is used to sustain the pressure inside the reactor and inhibit coke formation on the catalyst. High-sulfur gasoline is pumped to the reactor from the reservoir (2) with varying liquid hourly space velocities in the range of 0.5 – 1.5 hr-1 (corresponds to 140 – 415 mL/hr), where it is mixed with H2 from cylinder
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(4) . Pressure of the hydrogen gas was set at 2.5 MPa. The reactor is made of stainless steel, and set in a block of aluminium bronze, which provides uniform temperature distribution across its volume during the overall process. Temperature in the reactor was regulated by electronic potentiometer and set the range of 240 – 300 oC. The reaction products and leftover hydrogen gas proceed to the shell-and-coil condenser (5) from the bottom of the reactor and then further to the ice-cooled separator-receiver (6) in which separation of liquid hydrocarbons from gaseous products of reaction and hydrogen takes place. Liquid products are collected from the bottom cock of the receiver.
Figure 1. Scheme of experimental unit for desulfurization of gasoline. 1 – reactor; 2 – feed vessel; 3 – feed pump; 4 – H2 gas cylinder; 5 – condenser; 6 – separator-receiver; 7 – gas flowmeter; T – thermocouple; PR-pressure regulator, PS-pressure gauge.
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2.4.Catalyst regeneration Catalyst was regenerated under air flow at 500 ⁰C for 3 hours followed by nitrogen purge to remove oxygen from the system. Then hydrogen gas was fed into the system at 400 ⁰C. High sulfur gasoline was fed into the system after the regeneration to test the performance of the catalyst. 2.5. Characterization Elemental compositions of the bentonite and Ni/bentonite catalyst were determined by XRay Analytical Microscope (XGT-7000, Horiba, Japan). Compositions of the products were analyzed using gas chromatography (Autosystem XL, Perkin Elmer). Chromatographic separations were achieved using helium gas in Zebron ZB-1 capillary column coated with dimethyl polysiloxane polymer as stationary phase. Octane numbers were calculated based on compositions of the gasoline fractions obtained from gas chromatograph (Autosystem XL, Perkin Elmer). Distillation fractions were determined with crude oil distillation system (BR Instruments Company, USA) by ASTM D2892 and ASTM D86 standards. Composition of gasoline fractions were
analyzed using gas chromatography (PerkinElmer Auto System XL-230), FT-IR (Infralum FT02) and NMR (Tesla BS-487С). Sulfur content in gasoline was determined with SLFA-20 X-ray fluorescence sulfur-in-oil analyzer (Horiba Scientific) by ASTM D-4294 method. Densities of the gasoline were measured with density meters (DMA 4500 M, Anton Paar) by ASTM D5002 method. Iodine number was determined by reacting sample with iodine in chloroform solution and titrating the residual iodine with sodium thiosulfate as described in [11].
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3. Results and Discussion 3.1. Bentonite Clay and Catalyst High-sulfur gasoline was processed by nickel/bentonite catalyst to reduce the sulfur level. The use of natural bentonite clay as a catalyst support provides cheap source for the preparation of the catalyst significantly reducing the cost. Bentonite was predominantly made of montmorrilionite (over 85%) with minor quantities of dickite, illite and kaolinite. It’s elemental composition include 58.5% SiO2, 15.7% Al2O3, 12.4% K2O and 12.9% H2O with minor quantities of iron, calcium, sodium and titanium oxides. Clay is comprised of particles of 5 - 30 micron size. X-ray analysis revealed about 1.5% of nickel presenting at the surface of the bentonite.
3.2. Desulfurization of Gasoline Gasoline used in the current process was obtained from catalytic hydrocracking of vacuum residue of the crude oil obtained from the Caspean Sea near the shores of Baku, Azerbaijan. It has relatively high sulfur content of 450 ppm, which was reduced down from 8000 ppm in the original feed during hydrocracking process. Gasoline is abundant with parafins and iso-parafins, although aromatic content is also significant. After in situ activation of the adsorbent-catalyst with air and hydrogen gas, gasoline was allowed to pass through the reactor. Volumetric flow rate was initially adjusted to the 280 mL/hr, which corresponds to the weight hourly space velocity of 1.0 hr-1 relative to the catalyst weight (200 grams in this case). Temperature was varied at 240 – 300 ⁰C range to determine the optimal conditions for selective desulfurization. Table 1 summarizes the results obtained for total sulfur content of the produced gasoline along with other physiochemical properties. It is clearly evident that the hydrocarbon composition of the fed gasoline did not undergo major changes during the
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process, while sulfur level was reduced to 30 – 40 ppm range. Minor changes include small reduction of naphthenes and olefins and slight increase of iso-parafins at around 300 ⁰C, indicating some isomerization and hydrogentation reactions. Yield of the gasoline was above 98% in all the experimental trials while minor amount of gases were formed. Composition of the gases is given in electronic supporting information (Table S1). Octane number of the gasoline remained unchanged. Most efficient desulfurization was obtained at 270 ⁰C and at 1.0 hr-1 liquid hourly space velocity, yielding 32 ppm of sulfur. Changing the flow rate did not yield better results in terms of sulfur elimination, although olefins and aromatic content of the gasoline was higher at low flow rates (0.5 hr-1). It seems that some dehydrogenation process was taking place at lower flow rates. Performance of the catalyst on sulfur reduction compares reasonably well with the CoMo/Al2O3 catalyst reported by Hancksok et al [12] under similar reaction conditions. Their catalyst yielded above 20 ppm sulfur at 260 ⁰C temperature, 3.0 MPa pressure and 1.0 hr-1 liquid hourly space velocity on FCC gasoline with 196 ppm initial sulfur. However, the catalyst also caused five units of octane loss due to the saturation of olefins. It should be noted that the Nickel in the catalyst gradually turns into NiS causing the loss of the catalytic activity. This necessitates the regeneration of the catalyst during certain process intervals to regain the catalytic activity. In electronic supporting information (Table S2) properties of the gasoline produced by the catalyst at various stages of the desulfurization process (270 ⁰C temperature, 2.5 MPa pressure, 1.0 hr-1 flow rate) is demonstrated. Twenty two liters of high sulfur gasoline was passed through the reactor loaded 200 grams of the catalyst. Levels of sulfur in produced gasoline has increased to 47 ppm during the last run from 32 ppm of the initial run, indicating partial loss of the catalytic activity due to the conversion of Ni into NiS. Catalytic activity was completely regained after the regeneration. Hydrocarbon content of the gasoline
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remained fairly consistent throughout the experiment and after the catalyst regeneration, indicating high selectivity of the catalyst towards desulfurization.
Table 1. Composition and properties of the gasoline feed and product in desulfurization process by using Ni/Bentonite catalyst Sample
Fed Gasoline
Produced Gasoline
Reaction parameters (pressure 2.5 MPa, H2 feed rate 150 mL/hr) Reaction Temperature ( ⁰C)
NA
240
270
300
270
270
Flow rate (hr-1)
NA
1
1
1
0.5
1.5
Fractional distillation of feed/product by ASTM D-86 (Temperature in оC) IBP*
60
58
60
59
61
59
10 %
98
96
98
97
97
97
50 %
125
124
127
121
123
126
90 %
187
185
195
193
194
193
FBP*
201
198
200
201
199
198
PONA analysis (Composition in wt %) Paraffins
36.6
36.6
36.6
35.5
35.5
35.8
Iso-paraffins
35.6
36.6
37.0
39.0
32.9
37.4
Olefins
3.50
1.5
1.2
1.0
4.9
1.7
Naphthenes
13.5
14.5
14.5
13.0
13.9
14.7
Aromatics
10.7
10.7
10.7
10.7
12.8
10.4
Density at 20 °С (kg/m3)
724
722
720
718
720
719
Research octane number
71
71
71
71
71
71
Sulfur content (ppm)
450
36
32
34
40
36
Iodine number (gr J2/100gr)
11
8
7
6
9
5
Gasoline yield (%)
99.0
98.5
98.0
98.0
99.0
Gas yield (%)
0.6
1.0
1.5
1.6
0.6
Physicochemical properties
*
IBP, FBP – Initial and final boiling point
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3.3. Reaction mechanism The proposed mechanism of the desulfurization from aromatic hydrocarbons by nickel catalyst involves σ-bond formation between metal 4s-orbitals and π-electrons of the aromatics and back-donating electron density from nickel 4d-orbitals to the antibonding π*-orbitals (Figure 2). This causes destruction of π orbital of the thiofene ring and formation of nickel sulfide. Elemental analysis of the catalyst was analyzed after the process and yielded about 0.2% of sulfur, indicating sulfur adsorption. It has been proven by means of molecular orbital calculations that the strength of Cu and Ag – thiophene complexation is stronger than the complexation with the benzene [13], allowing selective desulfurization of thiofene derivatives, without affecting other aromatic species. Hydrocarbon composition of the processed gasoline by nickel catalyst provides experimental evidence that this also applies for the nickel.
Figure 2. Schematic representation of the desulfurization reaction mechanism by nickel/bentonite catalyst
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4. Conclusions Nickel modified bentonite catalyst have been tested for selective desulfurization of gasoline fraction obtained from hydrocracking of the heavy vacuum residue. Reaction temperature and flow rate was adjusted to maximize the sulfur removal efficiency while fixing the pressure at 2.5 MPa. Optimal sulfur removal was obtained at 270 ⁰C and 1.0 hr-1 feed rate, which reduced sulfur level of the gasoline from 450 ppm to 32 ppm without affecting the other hydrocarbon content. Sulfur was adsorbed by the catalyst by means of formation of σ-bond. The gasoline fraction used in this process was obtained by low pressure hydrocracking of the vacuum residue [10], hence the research allows obtaining low sulfur gasoline from vacuum residue by using low pressure processes (hydrocracking and adsorptive desulfurization). This can potentially provide significant cost savings in addition to improved process safety over the existing HDS units that operate at high pressures. Used catalyst can be regenerated and reused by the atmospheric oxidation followed by reduction under hydrogen stream.
Supporting Information Available: Adsorptive Desulfurization of the Gasoline obtained from
Low Pressure Hydrocracking of the Vacuum Residue using Nickel/Bentonite Catalyst. Chemical composition of the gases formed during desulfurization of the gasoline at 270 C, 1.0 hr-1 flow rate and 2.5 MPa pressure. Composition and properties of the gasoline obtained by passing high sulfur gasoline from the 200 g catalyst at 270 C, 1.0 hr-1 flow rate and 2.5 MPa pressure. This information is available free of charge via the Internet at http://pubs.acs.org/.
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3. Abbasov, V.; Mammadova, T.; Andrushenko, N.; Hasankhanova, N.; Lvov, Y.; Abdullayev, E. Fuel 2014, 117A, 552-555. 4. Farshi, A.; Rabiei, Z. Petroleum & Coal 2005, 47, 49-56. 5. Farshi, A.; Shiralizadeh, P. Petroleum & Coal 2015, 57, 295-302. 6. Seyedi, M. S.; Bahmaei, M.; Farshi, A. Orient. J. Chem. 2015, 31, 2409-2413. 7. Gislason J. Oil Gas J. 2001, 99, 72-76. 8. Turk, B. S.; Gupta, R. P.; Gangwal, S. K. A novel vapor-phase process for deep desulfurization of naphtha/diesel, Final Report DOE Cooperative Agreement No. DE-FC2601BC15282, http://www.osti.gov/scitech/servlets/purl/823164; 2003 [accessed 10.22.16]. 9. Hernandez-Maldonado, A. J.; Yang, F. H.; Qi, G.; Yang, R. T. Appl. Catal. B Env. 2005, 56, 111–126. 10. Abbasov, V. M.; Ibrahimov, H. C.; Mukhtarova, G. S.; Abdullayev, E. Fuel 2016, 184, 555– 558. 11. Abbasov, V.; Mammadova, T.; Aliyeva, N.; Abbasov, M.; Movsumov, N.; Joshi, A.; Lvov, Y.; Abdullayev, E. Fuel 2016, 181, 55–63. 12. Hancsók, J.; Magyar, S.; Kalló, D. Petroleum & Coal 2004, 46, 1-12. 13. Yang, R. T.; Takahashi, A.; Yang, F. H. Ind. Eng. Chem. Res. 2001, 40, 6236-6239.
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Scheme of experimental unit for desulfurization of gasoline. 1 – reactor; 2 – feed reservoir; 3 – feed pump; 4 – H2 cylinder; 5 – condenser; 6 – separator-receiver; 7 – gas clock; T – thermocouple; PR-pressure regulator, PS-pressure sensor. 257x266mm (96 x 96 DPI)
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Schematic representation of the desulfurization reaction mechanism by Nickel catalyst 763x405mm (96 x 96 DPI)
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