Experimental Investigation of the Application of Ionic Liquids to

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 11811−11820

Experimental Investigation of the Application of Ionic Liquids to Methanol Synthesis in Membrane Reactors Fatemeh Sadat Zebarjad, Sheng Hu, Zhongtang Li,§ and Theodore T. Tsotsis* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, University Park, Los Angeles, California 90089-1211, United States

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S Supporting Information *

ABSTRACT: In this study, a high-pressure membrane reactor (MR) was employed to carry out the methanol synthesis (MeS) reaction. Syngas was fed into the MR shell side where a commercial MeS catalyst was used, while the tube side was swept with a high boiling point liquid with good solubility toward methanol. A mesoporous alumina ceramic membrane was utilized after its surface had been modified to be rendered more hydrophobic. The efficiency of the MR was investigated under a variety of experimental conditions (different pressures, temperatures, sweep liquid flow rates, and types of sweep liquids). The results reveal improved per single-pass carbon conversions when compared to the conventional packed-bed reactor. An ionic liquid (IL), 1ethyl-3-methylimidazolium tetrafluoroborate, was utilized in the MR as the sweep liquid. The experimental results are compared to those previously reported by our group (Li, Z.; Tsotsis, T. T. J. Membrane Sci. 2019, 570, 103) while using a conventional petroleum-derived solvent as sweep liquid, tetraethylene glycol dimethyl ether (TGDE). Enhanced carbon conversion (over the petroleum-derived solvent) was obtained using the IL.

1. INTRODUCTION Since the Industrial Revolution, human activities like fossil-fuel combustion have produced CO2, thought to contribute to global warming, thus leading to an increase in its atmospheric concentration of ∼40%.1 During the same period, the world’s rising demand for energy increased the use of fossil fuels, such as oil, gas, and coal. Combined with the limited availability of these resources, this has intensified the need for finding new technologies to meet the world’s energy needs. CO2 capture and utilization (CCU) to produce fuels and chemicals and the use of renewable energy resources (e.g., biomass) in the place of fossil fuels are two approaches intensively studied today for reducing the CO2 environmental impact and for diminishing the world’s reliance on fossil fuels. Recently, CO2 conversion into methanol (MeOH), as a CCU process, and MeOH production from biomass have attracted increased attention. MeOH is one of the most commonly used industrial chemicals; its widespread applications includes utilizing it as a feedstock for producing chemicals (e.g., C2−C4 olefins and aromatics2−4), fuels and fuel additives (e.g., DME, MTBE, and DMC5−7), and as a H2 carrier in energy storage. The most common method to produce MeOH is from a mixture of CO, CO2, and H2 known as syngas; it is generated from the catalytic reforming of natural gas or the gasification of coal and biomass. The following three global reactions are thought to take place during MeOH synthesis (MeS). (Though there are differing opinions among © 2019 American Chemical Society

researchers on the carbon source for MeOH during MeS, and all three reactions are noted in the literature, only two out of three are linearly independent. It is noted that only the first and third reactions are considered in the kinetics part of this study, to be discussed later.) CO2 + 3H 2 → CH3OH + H 2O CO + 2H 2 → CH3OH

ΔH ° = − 49.5 (kJ/mol)

ΔH ° = − 90.6 (kJ/mol)

CO + H 2O → CO2 + H 2

ΔH ° = −41.2 (kJ/mol)

(R1) (R2) (R3)

8,9

Today’s commercial MeS processes utilize the so-called low-pressure (pressures range from 35 to 80 bar) Cu−ZnO− Al2O3 MeS catalyst (also employed in this study), which is highly selective (selectivity, typically, >99%), with DME being the main byproduct.10 They have different technical features, but are all designed to overcome two key challenges: first, low per-pass conversions dictating recycle of unreacted syngas,8 and second, the need to remove the reaction heat efficiently. Newer MeS processes have also been developed,11−15 but none has, as yet, reached commercial maturity. The first, and foremost, challenge (low per-pass conversion) is particularly problematic for MeOH production from small-scale, disReceived: Revised: Accepted: Published: 11811

March 5, 2019 May 17, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b01178 Ind. Eng. Chem. Res. 2019, 58, 11811−11820

Article

Industrial & Engineering Chemistry Research

Our group previously investigated42 a different MR methanol synthesis process; a high-temperature inorganic membrane was utilized in MeS as a porous selective barrier in between the reaction side and a liquid solvent flowing in its tube side (permeate side). Tetraethylene glycol dimethyl ether (TGDE) was used as the sweep liquid, in which MeOH has good solubility, while other components of syngas (like H2 and CO) do not. Removing the MeOH in situ from the reaction side allows us to achieve conversion for MeOH beyond its equilibrium value. An advantage of this over other MR processes for alcohol synthesis is that the selective separation is done by the sweep liquid; thus, commercial, off-the-selfinorganic membranes can be used (as in this study), which require no further development effort beyond needing to modify the hydrophobicity of their surface. Thus, one is no longer constrained by the limited commercial availability (or lack of robustness) of appropriate membranes, and can, instead, rely on the greater range of available solvents to attain desired selective properties. Though employing a petroleum-derived solvent like TGDE in a MR system (but even in a conventional trickle-bed reactor) shows good benefits, there are still disadvantages relating to its application to the MeS reaction. Its relatively low boiling point (275 °C) dictates that the reaction temperature is kept rather low. In addition, though its vapor pressure is quite low (85%), preliminary process scale-up simulations employing a data-validated model indicate the potential of the proposed concept to meet such a target. We presently continue to investigate the MeS−MR technology by focusing on optimization and on technical and economic analysis for process scale-up. There are several important optimization conditions one must consider, such as syngas composition, gas and liquid flow rates, temperature, pressure, catalyst weight and activity, membrane area, the concentration of MeOH in the solvent, and the fraction of the MeOH product recovered in the product stream. In our studies, we employ a MR model validated by our MR experimental data that allows us to investigate process performance under operating conditions that are not readily accessible by the lab-scale experimental system. Results of these studies will be presented in an upcoming publication.

Figure 11. Effect of pressure on PBR, MR and equilibrium conversion. T = 220 °C, liquid sweep rate = 1 cc/min, W/F = 42.7 g·h/mol.

and PBR conversion as well as the thermodynamic equilibrium conversion while keeping constant the reactor temperature (220 °C), the sweep liquid flow rate (1 cc/min), and W/F (47.2 g*h/mol). Predictably, both the thermodynamic equilibrium and PBR conversions (which at this relatively high temperature closely track the equilibrium values) increase with increasing pressure, due to the fact that the total number of moles reduces as a result of MeS reaction which is thermodynamically favored at higher pressures. The MR conversion also increases with increasing pressure, the added reason for that being that a greater pressure means that a higher amount of methanol will also transport through the membrane to be removed by the sweep liquid, and thus, the equilibrium condition is further shifted toward the methanol generation. Comparing the MR performance when employing the two different sweep liquids, higher conversions are obtained for the IL over the whole range of pressures studied, thus manifesting the advantage in terms of MeS reactor efficiency of using the IL when compared to TGDE. As noted previously, using the IL portends additional advantages related to diminished solvent loss, lower potential for damage to the catalyst, and simplified downstream separation requirements.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSIONS This paper presents experimental data in support of a novel concept of carrying out MeS in a MR. In this reactor, the membrane divides its volume into two sections, the membrane

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01178. Experimental kinetic data (PDF) 11818

DOI: 10.1021/acs.iecr.9b01178 Ind. Eng. Chem. Res. 2019, 58, 11811−11820

Article

Industrial & Engineering Chemistry Research

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fatemeh Sadat Zebarjad: 0000-0002-2330-1987 Present Address

§ Palcan Energy Corporation, No. 111 Xiangke Road, Pudong New District, Shanghai, 201203

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the National Science Foundation (Award No. CBET-1705180) is gratefully acknowledged. ABBREVIATIONS MR, membrane reactor; MeS, methanol synthesis; IL, ionic liquid; PBR, packed-bed reactor; FAS, fluoroalkylsilane; DI, deionized; W/F, catalyst weight/total molar flow rate; TEA, technical and economic analysis

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DOI: 10.1021/acs.iecr.9b01178 Ind. Eng. Chem. Res. 2019, 58, 11811−11820