Scientific Approach for a Cleaner Environment Using Ionic Liquids

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Scientific Approach for a Cleaner Environment Using Ionic Liquids Erin M. Broderick, Manuela Serban, Beckay Mezza, and Alak Bhattacharyya ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02953 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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ACS Sustainable Chemistry & Engineering

Scientific Approach for a Cleaner Environment Using Ionic Liquids Erin M. Broderick, Manuela Serban, Beckay Mezza, and Alak Bhattacharyya* *[email protected] Honeywell UOP, 50 E. Algonquin Rd., Des Plaines, IL 60016, United States. Ionic Liquids, denitrogenation, desulfurization, diesel, VGO, caprolactam

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 catalysis. Volatile organic compounds (VOCs) may cause air pollution and health issues. As a result industrial processes

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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 FCC and hydrocracking units. The design, synthesis, separation, and catalytic application of novel, green ionic liquids for making low sulfur, low nitrogen fuels, such as ultra-low sulfur diesel are discussed.

1. 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 2,000 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 bio-refineries.1

Diesel is a hydrocarbon fuel used throughout the world. However, diesel fuel contains sulfur as well as nitrogen-containing molecules, which upon combustion yield the well-known 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, where as Bharat Stage III (Euro III) is applicable nationwide. Incorporating a pretreatment process can have significant impact in cutting back on the catalyst and hydrogen requirements and enable easier add on revamps for existing units to produce ultra-low sulfur diesel in the future.

It is, however, becoming more expensive to hydrodesulfurize diesel fuels to even lower levels of sulfur now required. Ionic 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


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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 to 90 percent of these contaminants by ionic liquid extraction.2,3

2. 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.

Hydrodesulfurization of Feed. The pilot plant runs were done at 800 psig and at a linear hourly space velocity (LHSV) of 1 hr-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.

3. Significant Results and Discussion:


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For the past several years, Honeywell UOP has been working with various nitrogen and phosphorus-based ionic liquids to invent new -- or to improve current -- processes or products for the refining and petrochemical industries. separation, pretreatment, and catalysis.

These applications deal with absorption,

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-3-methylimidazolium hydrogen sulfate or 1-butyl-3-methylimidazolium methane sulfonate 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 are longer chain carbazoles (Fig. 1).4

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.


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Hydrodesulfurization of the untreated feed (Fig. 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 ppm S to 10 ppm.

Increasing the temperature to 360 °C improved the

hydrodesulfurization process with 10 ppm S being obtained by 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 25% of the catalyst bed (Fig. 1 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.

Figure 2. Hydrodesulfurization Runs of Various Feeds: Product Sulfur vs. Catalyst Bed Utilization

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 % nitrogen removal decreased with each cycle (Fig. 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 percent of its nitrogen extraction capacity even after fourth recycle. The HDS


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improvement coupled with the regeneration capability of the ionic liquid with stream stripping indicated the IL pretreatment process of HDS may be a promising advancement for hydroprocessing. Steam Stripping at 150oC Stripping conditions: 1L/min steam at 150oC for 1 h + 1L/min N2 at 150oC for 1 h

100

Nitrogen Removed (%)

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Steam

90

No Regeneration

80

N2 Stripping at 150oC

70

Steam Stripping I at 150oC

60

Steam Stripping II at 150oC

50

2nd, 3rd, 4th Cycles 2nd, 3rd Cycles

IL Level

3rd Cycle

2nd Cycle 3rd Cycle

40 30

1st Cycle

Condensate + Stripped Species

4th Cycle

20

Feed = LCO:LCGO:SR = 3:3:4

10

652 ppm N; 220 ppm basic N; 1.35% S; 104 ppm H2O

0 0

10

20

30

40

50

60

70

80

90

100

Basic Nitrogen Removed (%)

Figure 3. Efficient Ionic Liquid Regeneration by Steam Stripping

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.46

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


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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.

Ionic Liquid

Weight % N Removal

Trihexyl(tetradecyl)phosphonium bromide

78.5a

Triisobutyl(methyl)phosphonium tosylate

55.4a

Tributyl(ethyl)phosphonium diethylphosphate

55.7a

1-Butyl-3-methylimidazolium chloride

30.2b

1-Butyl-3-methylimidazolium octylsulfate

45.2b

Trihexyl(tetradecyl)phosphonium chloride

84.7b

Table 1. b

Nitrogen removal of VGO by ionic liquids. aVGO with 1330 ppm nitrogen.

Hydrotreated VGO with 451 ppm nitrogen.

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 7-membered cyclic 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

Feed

% Nitrogen Removal

Hydrotreated VGOa

58


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Diesel Blendb

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Table 2. Nitrogen removal of caprolactam based ionic liquid. aVGO feed had 430 ppm N. b

Diesel blend feed had 155 ppm 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

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 mole 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 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.

Absorption Solvent

CO2 Capacity (mol/L)

N-Methylpyrrolidone (NMP)

0.022

Methanol (MeOH)

0.007

Dimethylether of Polyethyleneglycol (DEPG)

0.025

PmixOAc + NMP (14 wt%)

0.094


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PmixOAc + MeOH (14 wt%)

0.280

PmixOAc + DEPG (13 wt%)

0.142

Table 3. Capacity of an Phosphonium Based Ionic Liquid and Physical Absorption Solvents

4. 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. One major advantage of this pretreatment followed by an HDS process is the economic production of ultra-low 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 may offer a viable alternative for large scale applications. The recent progress in ionic liquids in both applications and cost effective synthesis may lead to exciting breakthroughs in future environmentally friendly industrial processes.

ACKNOWLEDGMENT The authors acknowledge Solvay for a generous supply of custom ionic liquids, and Honeywell UOP for permission to publish.

References: [1]

Ionic Liquids Market Global Forecast to 2021. May 2016. marketsandmarkets.


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[2]

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Serban, M.; Kocal, J. A. Methods of denitrogenating diesel fuel. US 7749377 B2, July 06, 2010.

[3]

Serban, M.; Bhattacharyya, A.; Vanden Bussche, 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. 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.

[12]

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. 2005, 109, 19542-19546.

[13]

Serban, M.; Levy, A.; Tang, L.; Bhattacharyya, A. Process for removing nitrogen from fuel streams with caprolactamium ionic liquids. US8709236 B2, April 29, 2014.


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For Table of Contents Use Only

Scientific Approach for a Cleaner Environment Using Ionic Liquids

Erin M. Broderick, Manuela Serban, Beckay Mezza, and Alak Bhattacharyya* Ionic liquids give off essentially no volatile organic compounds, which make them green alternatives for various applications, such as an extraction solvent for nitrogen removal.


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Scientific Approach for a Cleaner Environment Using Ionic Liquids

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