Research Article pubs.acs.org/journal/ascecg
Scientific Approach for a Cleaner Environment Using Ionic Liquids Erin M. Broderick, Manuela Serban, Beckay Mezza, and Alak Bhattacharyya* Honeywell UOP, 50 E. Algonquin Road, Des Plaines, Illinois 60016, United States ABSTRACT: Ionic liquids (ILs) are ionic compounds composed of a bulky organic cation and a smaller anion. The size mismatch leads to the low melting points relative to typical ionic compounds such as NaCl. ILs have become a very fruitful area of academic and industrial research primarily because of their beneficial and diverse properties. Ionic liquids characteristically have low vapor pressures, good solvency, high ionic conductivities, and good heat transfer properties. Our work on different combinations of cations and anions has led to a wide span of properties, such as acidity, water solubility, chemical reactivity, viscosity, melting points, etc. Further optimization of a property for a particular application was accomplished through synthesis and compositional variations. Ionic liquid compositions can be tailored to make them much greener than many traditional solvents and catalysts. Volatile organic compounds (VOCs) may cause air pollution and health issues. As a result industrial processes are looking for solvents with decreased VOCs. Since ionic liquids are nonvolatile, they do not produce VOCs. These novel materials are used successfully for the removal of poisonous contaminants such as sulfur, nitrogen, Concarbon, and metals from traditional hydrocarbon feeds for fluid catalytic cracking and hydrocracking units. The design, synthesis, separation, and catalytic application of novel, green ionic liquids for making low sulfur, low nitrogen fuels, such as ultralow sulfur diesel, are discussed. KEYWORDS: Ionic liquids, Denitrogenation, Desulfurization, Diesel, VGO, Caprolactam
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INTRODUCTION Ionic liquids (ILs) are ionic compounds composed of a bulky organic cation and a smaller anion. This class of materials characteristically has low vapor pressures, good solvency, high ionic conductivities, and good heat transfer properties. ILs are attracting significant industrial and academic interest. There are currently more than 10 000 publications and 2000 patents in this area, a majority within the last 15 years. There is a double digit growth rate of IL production in China and India driven by the massive growth in both traditional refineries and biorefineries.1 Diesel is a hydrocarbon fuel used throughout the world. However, diesel fuel contains sulfur as well as nitrogencontaining molecules, which upon combustion yield the wellknown pollutants SOx, and NOx. Therefore, there is an ever increasing need to provide diesel fuels that have ultralow sulfur content. A typical way of removing sulfur from diesel is by catalytic hydrodesulfurization (HDS). Many countries are setting up their own standards and guidelines. For example, one of the heavy users of diesel fuel, India, currently has two separate Diesel Fuel quality specifications. Bharat Stage IV (Euro IV) is applicable to 14 major cities, whereas Bharat Stage III (Euro III) is applicable nationwide. Incorporating a pretreatment process can have a significant impact in cutting back on the catalyst and hydrogen requirements and enable easier add on revamps for existing units to produce ultralow sulfur diesel in the future. It is, however, becoming more expensive to hydrodesulfurize diesel fuels to even lower levels of sulfur now required. Ionic © 2017 American Chemical Society
liquids provide a new means to efficiently hydrodesulfurize diesel fuel. Our approach is to use suitable ionic liquids to extract difficult-to-hydrotreat sulfur and nitrogen compounds first, followed by HDS, increasing the economics and efficiency of the process. We have performed systematic experiments and designed milder conditions to extract 40−90% of these contaminants by ionic liquid extraction.2,3
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MATERIAL AND METHODS
Materials. Experiments were carried out using room-temperature ionic liquids either purchased or procured from IL manufacturers. A variety of cations based on imidazoles, amines, pyridines, lactams, and phosphines were used in combination with anions such as halides, sulfates, phosphates, acetates, carboxylates, etc. Extraction of Feed with Ionic Liquid. All extraction experiments were performed on two high nitrogen containing diesel blends consisting of straight run diesel (SR), light cycle oil (LCO), and light coker gas oil (LCGO) in a 4:3:3 and 1:1:1 weight ratios simulating the components in a typical refinery diesel fuel pool or a vacuum gas oil (VGO) feed. The liquid−liquid extraction experiments were performed at room temperature and atmospheric pressure for the diesel feed and at 60 °C for the VGO feed. Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: December 7, 2016 Revised: February 3, 2017 Published: February 27, 2017 3681
DOI: 10.1021/acssuschemeng.6b02953 ACS Sustainable Chem. Eng. 2017, 5, 3681−3684
Research Article
ACS Sustainable Chemistry & Engineering Hydrodesulfurization of Feed. The pilot plant runs were done at 800 psig and at a linear hourly space velocity (LHSV) of 1 h−1 with a commercial high activity Ni/Mo catalyst. Carbon Dioxide Removal with Ionic Liquid. Tris(propyl/ butyl)methylphosphonium acetate (2.20 g, 0.0087 mol) was added to dimethyl ethers of polyethylene glycol (DEPG) (2.13 g, 0.0085 mol) in a glass insert for a 75 mL autoclave and stirred until well mixed with a magnetic stir bar. The 75 mL autoclave was loaded with the glass insert containing the ionic liquid mixture. The autoclave was flushed with nitrogen and then pressurized with 2068 kPa (300 psi) of a 10 mol % carbon dioxide/methane gas mixture. The mixture was stirred for an hour with a magnetic stir bar at 500 rpm. A sample of the headspace was obtained and submitted for GC analysis.
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SIGNIFICANT RESULTS AND DISCUSSION For the past several years, Honeywell UOP has been working with various nitrogen and phosphorus-based ionic liquids to invent newor to improve currentprocesses or products for the refining and petrochemical industries. These applications deal with absorption, separation, pretreatment, and catalysis. Due to the increasing regulations on the sulfur concentration of various petroleum feedstocks, our main focus has been on the improvement of hydrotreating processes. The goal was to pretreat the feed with an ionic liquid extraction before HDS and have the ionic liquid remove the highly aromatic N and S compounds. Once the diesel feed is substantially pretreated, the pretreated feed may be more easily decontaminated. The feed was stirred with an ionic liquid, such as 1-butyl-3methylimidazolium hydrogen sulfate or 1-butyl-3-methylimidazolium methanesulfonate, at 25 °C. After stirring, the mixture was allowed to settle to form two layers. The organic layer was decanted and used as a feed for the desulfurization process. The ionic liquid extraction removed nitrogen and some sulfur from the feed. A large portion of the high boiling point nitrogen compounds with low polarity were removed by the IL extraction. A majority of the nitrogen left after the extraction is longer chain carbazoles (Figure 1).4
Figure 2. Hydrodesulfurization runs of various feeds: product sulfur vs catalyst bed utilization.
25% of the catalyst bed (Figure 2 in red). Overall, the HDS process became much less severe with an ionic liquid pretreatment. The ionic liquid pretreatment could enable using a smaller amount of the catalyst bed or running at a lower temperature to achieve similar sulfur levels in the product without an IL pretreatment. After using the ionic liquid to remove a portion of the N and S from the feed, the ionic liquid needed to be regenerated. The ionic liquid was treated with steam to strip the organic components present in IL after the extraction. Without regeneration, the percent nitrogen removal decreased with each cycle (Figure 3). Nitrogen stripping resulted in a decreased loss of nitrogen removal but regeneration with steam had the best outcome. Steam-stripped, spent ionic liquid retains about 95% of its nitrogen extraction capacity even after fourth recycle. The HDS improvement coupled with the regeneration capability of the ionic liquid with steam stripping indicated the IL pretreatment process may be a promising advancement. Another major area of application is pretreatment of VGO. ILs can selectively remove contaminants such as nitrogen, sulfur, Concarbon, and metals drastically improving the efficiency and economics of FCC (fluid catalytic cracking) and HDS units while minimizing air pollution.4−6 Inexpensive regeneration and processes to recycle the ionic liquid have been developed as well. Ionic liquids were stirred with VGO in a 2:1 feed to IL ratio at 80 °C at 300 rpm for 30 min. In order to compare a feed with two different levels of nitrogen, hydrotreated VGO containing 451 ppm nitrogen, and untreated VGO containing 1330 ppm nitrogen were used as feed. After allowing the IL and organic phase to separate, the organic phase was analyzed for nitrogen content. A variety of ionic liquids, both phosphonium and imidazolium based, were able to remove nitrogen from a VGO feed (Table 1). The highest N removal was observed with phosphonium halide ionic liquids for both types of VGO tested with >75% of the nitrogen removed. The amount of nitrogen in the feed has a minimal effect on the performance of the IL for nitrogen removal. While ionic liquids may be beneficial for the decontamination of feeds, the cost and biodegradeability of the materials may be prohibitive on an industrial scale. The use of an inexpensive cation that is also biodegreadable may lead to an industrially attractive ionic liquid. Caprolactam is a seven-membered cyclic
Figure 1. Comparison of nitrogen compounds before and after extraction. Carbon number prior to the compound refers the alkylated nitrogen species. IL1 is 1-butyl-3-methylimidazolium hydrogen sulfate.
Hydrodesulfurization of the untreated feed (Figure 2 in green) at 346 °C retained 50 ppm of S in the feed after using 50% of the catalyst bed. Over 90% of the bed had to be used in order to decrease the content of S to 10 ppm. Increasing the temperature to 360 °C improved the hydrodesulfurization process with 10 ppm of S being obtained using 50% of the catalyst bed. In contrast, the feed that had been treated with the ionic liquid achieved a similar S concentration after using only 3682
DOI: 10.1021/acssuschemeng.6b02953 ACS Sustainable Chem. Eng. 2017, 5, 3681−3684
Research Article
ACS Sustainable Chemistry & Engineering
Figure 3. Efficient ionic liquid regeneration by steam stripping.
Ionic liquids are capable of absorbing carbon dioxide through both physical and chemical absorption.10 Acetate based ILs have been shown to absorb up to 1 mol of CO2 per mole of IL. During the chemisorption of carbon dioxide by ionic liquids, the ionic liquid often becomes too viscous making the material difficult to handle. By combining a physical absorption solvent, such as diemethyl ether of polyethylene glycol, with an acetate based IL, the capacity is increased compared to the physical absorption solvent (Table 3).11 The best capacity with a
Table 1. Nitrogen Removal of VGO by Ionic Liquids ionic liquid
weight % N removal
trihexyl(tetradecyl)phosphonium bromide triisobutyl(methyl)phosphonium tosylate tributyl(ethyl)phosphonium diethylphosphate 1-butyl-3-methylimidazolium chloride 1-butyl-3-methylimidazolium octylsulfate trihexyl(tetradecyl)phosphonium chloride
78.5a 55.4a 55.7a 30.2b 45.2b 84.7b
a
VGO with 1330 ppm nitrogen. bHydrotreated VGO with 451 ppm nitrogen.
Table 3. Capacity of Phosphonium Based Ionic Liquid and Physical Absorption Solvents
amide that is often used to make polymers for applications such as plastics. Due to the widespread use of caprolactam, the compound is inexpensive and readily available. Caprolactam has been used as the cation for a variety of ionic liquids.7 A caprolactam hydrogen sulfate based IL was shown to remove nitrogen from various hydrocarbon feeds. Through a liquid− liquid extraction of the caprolactam based IL and the feed, the IL extracted 58% of the nitrogen from a hydrotreated VGO and 76% from a diesel blend (Table 2).8
hydrotreated VGO diesel blendb a
N-methylpyrrolidone (NMP) methanol (MeOH) dimethyl ether of polyethylene glycol (DEPG) PmixOAc + NMP (14 wt %) PmixOAc + MeOH (14 wt %) PmixOAc + DEPG (13 wt %)
0.022 0.007 0.025 0.094 0.280 0.142
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% nitrogen removal a
CO2 capacity (mol/L)
phosphonium acetate based ionic liquid was when a protic physical absorption solvent was added to the ionic liquid, such as methanol. The chemisorption may be analogous to the formation of a carbonate with an aqueous amine.
Table 2. Nitrogen Removal of Caprolactam Based Ionic Liquid feed
absorption solvent
58 76
CONCLUSION IL materials can be designed and used successfully for the removal of contaminants such as nitrogen, which drastically improves the efficiency and economics of FCC and HDS units while minimizing air pollution. A number of types of ionic liquids can be used to pretreat a wide range of feeds, such as naphtha and VGO with a varying amount of nitrogen. Major advantages of this pretreatment followed by an HDS process are the economic production of ultralow sulfur diesel due to the saving of catalyst cost and producing less oxides of sulfur and nitrogen. In addition to removing contaminants from liquids, ionic liquids are capable of removing contaminants, such as carbon dioxide, from natural gas to prevent the greenhouse gas from entering the atmosphere. While ionic liquids have many potential applications, the biodegradability and economics of the material also need to be considered. Caprolactam based ILs
VGO feed had 430 ppm of N. bDiesel blend feed had 155 ppm of N.
In addition to decontamination of liquid feeds, ionic liquids have been used to remove contaminants, such as carbon dioxide, from gas feeds. The removal of carbon dioxide is necessary to improve the fuel quality of the natural gas or to use syngas. The combination of carbon dioxide and water can be corrosive to metal pipes, which makes the removal of CO2 necessary for transportation of natural gas. Carbon dioxide is a greenhouse gas that needs to be captured from flue gases to avoid harming the environment. Ionic liquids by themselves or in combination with solvents have been found to be useful for carbon dioxide capture from natural gas.9 3683
DOI: 10.1021/acssuschemeng.6b02953 ACS Sustainable Chem. Eng. 2017, 5, 3681−3684
Research Article
ACS Sustainable Chemistry & Engineering may offer a viable alternative for large scale applications. The recent progress in ionic liquids in both applications and costeffective synthesis may lead to exciting breakthroughs in future environmentally friendly industrial processes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alak Bhattacharyya: 0000-0001-7406-9746 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge Solvay for a generous supply of custom ionic liquids and Honeywell UOP for permission to publish.
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REFERENCES
(1) Ionic Liquids Market Global Forecast to 2021. May 2016. marketsandmarkets.com. (2) Serban, M.; Kocal, J. A. Methods of denitrogenating diesel fuel. US 7749377 B2, July 06, 2010. (3) Serban, M.; Bhattacharyya, A.; Vanden Bussche, K. M., Mezza, B.; Nicholas, C.; Kocal, J. A.; Bennion, W. Process for removing nitrogen from vacuum gas oil. US8608943 B2, December 17, 2013. (4) Mezza, B. J.; Bhattacharyya, A.; Nicholas, C. P.; Wang, H. Process for removing sulfur compounds from vacuum gas oil. US9068127 B2, June 30, 2015. (5) Mezza, B. J.; Wang, H.; Nicholas, C. P.; Bhattacharyya, A.; Huovie, B. E.; Gattupalli, R. R. Hydrocarbon conversion process to remove metals. US9133403 B2, September 15, 2015. (6) Mezza, B. J.; Wang, H.; Nicholas, C. P.; Bhattacharyya, A.; Huovie, B. E.; Gattupalli, R. R. Hydrocarbon conversion process to remove carbon residue contaminants. US9133400 B2, September 15, 2015. (7) Du, Z.; Li, Z.; Guo, S.; Zhang, J.; Zhu, L.; Deng, Y. Investigation of Physicochemical Properties of Lactam-Based Brønsted Acidic Ionic Liquids. J. Phys. Chem. B 2005, 109, 19542−19546. (8) Serban, M.; Levy, A.; Tang, L.; Bhattacharyya, A. Process for removing nitrogen from fuel streams with caprolactamium ionic liquids. US8709236 B2, April 29, 2014. (9) Brennecke, J. F.; Maginn, E. J. Purification of Gas with Liquid Ionic Compounds. US6579343 B2, June 17, 2003. (10) Chinn, D.; Vu, Q.; Driver, M.; Boudreau, L. C. CO2 removal from gas using ionic liquid absorbents. US7527775 B2, May 5, 2009. (11) Broderick, E. M.; Bhattacharyya, A. Mixtures of physical absorption solvents and ionic liquids for gas separation. US9321004 B2, April 26, 2016.
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