Adsorptive Desulfurization of the Gasoline Obtained from Low

May 25, 2017 - In this paper, adsorptive desulfurization of the gasoline produced from low-pressure hydrocracking of the atmospheric residue was studi...
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Adsorptive Desulfurization of the Gasoline Obtained from LowPressure Hydrocracking of the Vacuum Residue Using a Nickel/ Bentonite Catalyst Vagif M. Abbasov,† Hikmat C. Ibrahimov,† Gulbaniz S. Mukhtarova,† Musa I. Rustamov,† and Elshad Abdullayev*,‡ †

Institute of Petrochemical Processes, Azerbaijan National Academy of Sciences, Baku AZ1025, Azerbaijan Independent Researcher, High Point, North Carolina 27260, United States

‡

S Supporting Information *

ABSTRACT: In this paper, adsorptive desulfurization of the gasoline produced from low-pressure hydrocracking of the atmospheric residue was studied. Gasoline obtained from such a process has a high sulfur content and is not suitable for use as a motor fuel without further desulfurization. A nickel-modified natural bentonite catalyst was developed and used in the process. The reaction temperature and flow rate were adjusted to maximize the sulfur removal efficiency while fixing the pressure at 2.5 MPa. Optimal sulfur removal was obtained at 270 °C and 1.0 h−1 feed rate, reducing the gasoline sulfur level from 450 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.

1. INTRODUCTION Deep desulfurization of fuels has become a critical part of refinery processes and fuel production as a result of stringent United States Environmental Protection Agency (U.S. EPA) regulations on the sulfur content put on gasoline and diesel. This has caused the hydrotreatment/desulfurization catalyst market to become the biggest market within the refinery catalyst market, surpassing the fluid catalytic cracking catalyst market as of 2001.1 Another tendency in the petroleum refining industry is directed toward 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 upon the origin, which makes processes such as catalytic cracking and hydrocracking one of the most important processes for increasing the yield of the light fuel production.2,3 Gasoline obtained from heavy fractions pose unique problems as a result of their differing chemical compositions than the atmospheric distillates. The content of aromatic compounds is generally higher in catalytic cracking and lowpressure hydrocracking products, increasing their octane rating. On the other hand, desulfurization of the aromatic compounds generally requires 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 as a result of 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 © 2017 American Chemical Society

has special importance as a result of low hydrogen consumption, stoppage of hydrogen sulfide release preventing recombination reactions with olefins, and significant operating cost savings. ConocoPhillips 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 The Research Triangle Institute has developed the TReND process, which is based on the metal oxide sorbent for H2S removal from liquid fuels.8 Yang et al. reported a 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, the gasoline fraction obtained from lowpressure hydrocracking of the vacuum residue was studied to demonstrate a low-temperature and low-pressure adsorptive desulfurization process on a 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 advantages of the current process over a 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 process safety. This is a contiuation of our previous work,10 in which a suspended halloysite catalyst was demonstrated for low-pressure hydrocracking of the vacuum residue to obtain high-octane gasoline. The current publication along with the previous publication contain important guideReceived: January 8, 2017 Revised: May 17, 2017 Published: May 25, 2017 5840

DOI: 10.1021/acs.energyfuels.7b00081 Energy Fuels 2017, 31, 5840−5843

Article

Energy & Fuels

place. Liquid products are collected from the bottom cock of the receiver. 2.4. Catalyst Regeneration. The catalyst was regenerated under air flow at 500 °C for 3 h, followed by nitrogen purge to remove oxygen from the system. Then, hydrogen gas was fed into the system at 400 °C. High-sulfur gasoline was fed into the system after the regeneration to test the performance of the catalyst. 2.5. Characterization. Elemental composition of the Ni/ bentonite catalyst was determined by an X-ray analytical microscope (XGT-7000, Horiba, Japan). Compositions of the products were analyzed using gas chromatography (AutoSystem XL, PerkinElmer). Chromatographic separations were achieved using helium gas in a Zebron ZB-1 capillary column coated with a dimethyl polysiloxane polymer as the stationary phase. Octane numbers were calculated on the basis of compositions of the gasoline fractions obtained from gas chromatography (AutoSystem XL, PerkinElmer). Distillation fractions were determined with a crude oil distillation system (B/R Instruments Company, Easton, MD, U.S.A.) by ASTM D2892 and ASTM D86 standards. Compositions of gasoline fractions were analyzed using gas chromatography (AutoSystem XL-230, PerkinElmer), Fourier transform infrared spectroscopy (FTIR, Infralum FT-02), and nuclear magnetic resonance (NMR, Tesla BS-487C). The sulfur content in gasoline was determined with a SLFA-20 X-ray fluorescence sulfur-inoil analyzer (Horiba Scientific) by the ASTM D4294 method. Densities of the gasoline were measured with density meters (DMA 4500 M, Anton Paar) by the ASTM D5002 method. The iodine number was determined by reacting the sample with iodine in chloroform solution and titrating residual iodine with sodium thiosulfate as described in ref 11.

lines for obtaining low-sulfur, high-octane gasoline from the vacuum residue at a 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 the Dash-Salakhli deposit in Azerbaijan. Chemical composition of the clay is provided by the manufacturer. Nickel(II) chloride and NH4Cl were obtained from the Ural Plant of Chemicals (former Ural Plant of Chemical Reagents, Sverdlovsk oOblast, Russian Federation, with a purity of 98.5%). Highsulfur gasoline was obtained from catalytic hydrocracking of the vacuum residue obtained from the Haydar Aliyev Petrochemical Refinery (Baku, Azerbaijan) at 440 °C and 4 MPa pressure using a Ni/ Co-modified halloysite catalyst as described in ref 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 g) was dispersed in about 300 mL of 10% NH4Cl solution at 80 °C for 1 day. Then, bentonite was separated from the solution by filtering. The procedure was repeated 5 times. Then, bentonite was dried and ground to a 2 mm particle size. A total of 200 g of bentonite was mixed with 220 mL of 0.3% aqueous nickel(II) chloride solution, heated in a muffle furnace by gradual heating to 150 °C, and kept for 3−4 h for complete drying of the moisture. 2.3. Adsorptive Desulfurization of Gasoline. Adsorptive desulfurization of gasoline was conducted in a 200 mL volume flowtype microreactor (Figure 1). The catalyst (200 g) was pelletized,

3. RESULTS AND DISCUSSION 3.1. Bentonite Clay and Catalyst. High-sulfur gasoline was processed by a nickel/bentonite catalyst to reduce the sulfur level. The use of natural bentonite clay as a catalyst support provides a 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. Its elemental composition includes 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 μm size. X-ray analysis revealed about 1.5% nickel presenting at the surface of bentonite. 3.2. Desulfurization of Gasoline. Gasoline used in the current process was obtained from catalytic hydrocracking of the vacuum residue of the crude oil obtained from the Caspean Sea near the shores of Baku, Azerbaijan. It has a relatively high sulfur content of 450 ppm, which was reduced from 8000 ppm in the original feed during the hydrocracking process. Gasoline is abundant with paraffins and isoparaffins, although the 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. A volumetric flow rate was initially adjusted to 280 mL/h, which corresponds to the weight hourly space velocity of 1.0 h−1 relative to the catalyst weight (200 g in this case). The temperature was varied at the 240−300 °C range to determine the optimal conditions for selective desulfurization. Table 1 summarizes the results obtained for the 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 process, while the sulfur level was reduced to the 30−40 ppm range. Minor changes include a small reduction of naphthenes and olefins and a slight increase of isoparafins at around 300 °C, indicating some isomerization and hydro-

Figure 1. Scheme of the 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, and (PS) pressure gauge. placed into the reactor (1), and activated under air flow at 500 °C for 3 h, followed by nitrogen purge to remove oxygen from the system and reduction under a hydrogen gas flow at 400 °C for a 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 h−1 (corresponding to 140−415 mL/h), where it is mixed with H2 from a cylinder (4). The pressure of the hydrogen gas was set at 2.5 MPa. The reactor is made of stainless steel and set in a block of aluminum bronze, which provides a uniform temperature distribution across its volume during the overall process. The temperature in the reactor was regulated by an electronic potentiometer and set the range of 240−300 °C. The reaction products and leftover hydrogen gas proceed to the shell-andcoil condenser (5) from the bottom of the reactor and then further to the ice-cooled separator−receiver (6), in which the separation of liquid hydrocarbons from gaseous products of reaction and hydrogen takes 5841

DOI: 10.1021/acs.energyfuels.7b00081 Energy Fuels 2017, 31, 5840−5843

Article

Energy & Fuels

increased to 47 ppm during the last run from 32 ppm of the initial run, indicating partial loss of the catalytic activity as a result of the conversion of Ni into NiS. The catalytic activity was completely regained after the regeneration. The hydrocarbon content of the gasoline remained fairly consistent throughout the experiment and after the catalyst regeneration, indicating high selectivity of the catalyst toward desulfurization. 3.3. Reaction Mechanism. The proposed mechanism of desulfurization from aromatic hydrocarbons by a nickel catalyst involves σ-bond formation between metal 4s orbitals and π electrons of the aromatics and back-donating electron density from nickel 3d orbitals to the antibonding π* orbitals (Figure 2). This causes destruction of the π orbital of the thiofene ring

Table 1. Composition and Properties of the Gasoline Feed and Product in the Desulfurization Process Using a Ni/ Bentonite Catalyst sample

fed gasoline

produced gasoline

Reaction Parameters (Pressure, 2.5 MPa; H2 Feed Rate, 150 mL/h) reaction temperature NA 240 270 300 270 270 (°C) flow rate (h−1) NA 1 1 1 0.5 1.5 Fractional Distillation of Feed/Product by ASTM D86 (Temperature in °C) IBPa 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 FBPa 201 198 200 201 199 198 PONA Analysis (Composition in wt %) paraffins 36.6 36.6 36.6 35.5 35.5 35.8 isoparaffins 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 Physicochemical Properties density at 20 °C 724 722 720 718 720 719 (kg/m3) research octane 71 71 71 71 71 71 number sulfur content (ppm) 450 36 32 34 40 36 iodine number 11 8 7 6 9 5 (g of I2/100 g) gasoline yield (%) 99.0 98.5 98.0 98.0 99.0 gas yield (%) 0.6 1.0 1.5 1.6 0.6 a

Figure 2. Schematic representation of the desulfurization reaction mechanism by a nickel/bentonite catalyst.

and formation of nickel sulfide. Elemental analysis of the catalyst was analyzed after the process and yielded about 0.2% 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 benzene,13 allowing for selective desulfurization of thiofene derivatives, without affecting other aromatic species. Hydrocarbon composition of the processed gasoline by a nickel catalyst provides experimental evidence that this also applies for nickel.

IBP, initial boiling point; FBP, final boiling point.

genation reactions. The yield of the gasoline was above 98% in all of the experimental trials, while minor amounts of gases were formed. The composition of the gases is given in Table S1 of the Supporting Information. The octane number of the gasoline remained unchanged. The most efficient desulfurization was obtained at 270 °C and 1.0 h−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 olefin and aromatic contents of the gasoline were higher at a low flow rate (0.5 h−1). It seems that some dehydrogenation processes were taking place at lower flow rates. The performance of the catalyst on sulfur reduction compares reasonably well to the CoMo/Al2O3 catalyst reported by Hancksok et al.12 under similar reaction conditions. Their catalyst yielded above 20 ppm of sulfur at 260 °C temperature, 3.0 MPa pressure, and 1.0 h−1 liquid hourly space velocity on fluid catalytic cracking (FCC) gasoline with 196 ppm of initial sulfur. However, the catalyst also caused 5 units of octane loss as a result of the saturation of olefins. It should be noted that 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 Table S2 of the Supporting Information, properties of the gasoline produced by the catalyst at various stages of the desulfurization process (270 °C temperature, 2.5 MPa pressure, and 1.0 h−1 flow rate) are demonstrated. A total of 22 L of high sulfur gasoline was passed through the reactor loaded with 200 g of the catalyst. Levels of sulfur in produced gasoline have

4. CONCLUSION Nickel-modified bentonite catalysts have been tested for selective desulfurization of the gasoline fraction obtained from hydrocracking of the heavy vacuum residue. The reaction temperature and flow rate were adjusted to maximize the sulfur removal efficiency while fixing the pressure at 2.5 MPa. Optimal sulfur removal was obtained at 270 °C and 1.0 h−1 feed rate, which reduced the sulfur level of the gasoline from 450 to 32 ppm without affecting the other hydrocarbon contents. Sulfur was adsorbed by the catalyst by means of formation of a σ bond. The gasoline fraction used in this process was obtained by low-pressure hydrocracking of the vacuum residue;10 hence, the research allows us to obtain low-sulfur gasoline from the vacuum residue 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. The used catalyst can be regenerated and reused by the atmospheric oxidation, followed by reduction under hydrogen stream. 5842

DOI: 10.1021/acs.energyfuels.7b00081 Energy Fuels 2017, 31, 5840−5843

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00081. Chemical composition of the gases formed during desulfurization of the gasoline at 270 °C temperature, 1.0 h−1 flow rate, and 2.5 MPa pressure (Table S1) and composition and properties of the gasoline obtained by passing high-sulfur gasoline from the 200 g catalyst at 270 °C temperature, 1.0 h−1 flow rate, and 2.5 MPa pressure (Table S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 336-862-4567. E-mail: [email protected]. ORCID

Elshad Abdullayev: 0000-0002-1418-351X Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b00081 Energy Fuels 2017, 31, 5840−5843