Clathrate-based CO2 Capture from CO2-rich Natural Gas and Biogas

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Clathrate-based CO Capture from CO-rich Natural Gas and Biogas Jiyeon Lim, Wonjung Choi, Junghoon Mok, and Yongwon Seo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00712 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Sustainable Chemistry & Engineering

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Clathrate-based CO2 Capture from CO2-rich Natural

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Gas and Biogas Jiyeon Lim†, Wonjung Choi†, Junghoon Mok†, and Yongwon Seo†*

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School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea

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*

Corresponding author. Tel: +82-52-217-2821. Fax: +82-52-217-2859 E-mail address: [email protected] (Y. Seo)

ACS Paragon Plus Environment

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ABSTRACT. In this study, clathrate-based CO2 capture was investigated in the presence of

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thermodynamic promoters such as tetrahydrofuran (THF) and tetra-n-butyl ammonium chloride (TBAC)

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for upgrading CO2-rich natural gas and biogas. The phase equilibria, gas uptakes, gas

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composition measurements, and spectroscopic analyses of CH4 (50%) + CO2 (50%) + promoter

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clathrates were examined with a primary focus on the effects of thermodynamic promoters on

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clathrate stability and cage filling behavior. The addition of THF and TBAC significantly enhanced

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the thermodynamic stability of CH4 (50%) + CO2 (50%) clathrates.

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spectroscopy clearly revealed that CO2 and CH4 are enclathrated in the clathrate cages. THF

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solutions demonstrated a faster growth rate of clathrates but CO2 was less selective than CH4 in

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the THF clathrate phase due to the lower thermodynamic stability of the CO2 + THF clathrate

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compared to the CH4 + THF clathrate. TBAC solutions produced higher CO2 selectivity in the

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semi-clathrate phase due to the presence of distorted small cages which have a strong preference

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for CO2 molecules. The experimental results demonstrated that CO2 selectivity in the clathrate

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phase can be influenced by thermodynamic stability, cage shape and dimension, and cage filling

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behavior in the presence of thermodynamic promoters and thus, a suitable promoter and their

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optimum concentration should be carefully determined in designing and operating clathrate-

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based CO2 capture from natural gas or biogas.

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C NMR and Raman

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Keywords: Clathrate hydrate, CO2 capture, Gas upgrading, Promoters, Biogas


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Introduction

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Methane (CH4) is a main component of natural gas, and it has recently been found in unconventional

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forms, such as shale gas and natural gas hydrates. CH4 is also generated from biogas or landfill gas by the

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anaerobic decomposition of organic substrates1. The importance of natural gas as an energy source has

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been increasing because it has higher energy efficiency and less of an effect on global warming than coal

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and oil2. However, natural gas and biogas contain some impurities, including carbon dioxide (CO2) and

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hydrogen sulfide (H2S), which are called sour gases and can reduce the calorific value of natural gas and

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biogas3. A significant number of natural gas fields remain untapped due to the high concentration of sour

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gas, and some natural gas fields contain even more than 50% CO2 4. Also, biogas typically consists of 50–

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75% CH4 and 25–50% CO2 5. To increase the energy content of the gas, the upgrading of natural gas and

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biogas should be performed by separating CO2 from the gas mixture and, thereby, concentrating CH4.

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Conventional CO2 separation or capture methods include chemical absorption, physical adsorption,

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cryogenic distillation, and membrane separation6. Amine-based chemical absorption is one of the

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conventional and representative methods for CO2 capture from flue gas, but it is applicable only for feed

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gas with lower CO2 concentrations, and it has also solvent degradation and corrosion problems. Other

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methods have their own drawbacks, such as low capture efficiency, intensive energy usage, and low

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reusability of separation media. Recently, clathrate-based gas separation has been suggested as a

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promising option for capturing greenhouse gases with higher concentrations7–16.

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Clathrate hydrates are non-stoichiometric crystalline compounds which incorporate gas molecules (guest)

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into hydrogen-bonded water cages (host) under high pressure and low temperature conditions, and they

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can be generally classified into true clathrates and semi-clathrates17. Gas hydrates which belong to true

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clathrates exist in three different structures: structure I (sI), structure II (sII), and structure H (sH)17, 18. Each


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structure consists of differently sized and shaped cages. Semiclathrates share many physical and chemical

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features with true clathrates, but the primary difference is that in true clathrates guest molecules are not

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physically bonded to host water cages whereas in semiclathrates some guest molecules can participate in

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forming clathrate cage structures. Quaternary ammonium salts (QASs) are known as semi-clathrate

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formers and their cations can be incorporated into large cages whereas their anions can take part in

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building up cage structures by replacing some host water molecules19-22.

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CO2 can be captured by formation of the clathrate hydrate and CO2 capture from CO2-rich natural gas and

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biogas using clathrate formation is affected by the equilibrium pressure difference between CH4 clathrate

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and CO2 clathrate. However, there are main drawbacks of clathrate-based CO2 capture; it requires high

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pressure and low temperature for clathrate formation and similar molecular sizes of CH4 and CO2

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could result in the lower separation efficiency. Natural gas extracted from the reservoir with high

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pressure does not need additional pressurization for clathrate formation23, but biogas is produced at

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atmospheric pressure. Therefore, chemical additives, called thermodynamic promoters, are used to

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enhance the thermodynamic stability of clathrates, and they can reduce the equilibrium pressure at any

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given temperature or raise the equilibrium temperature at any given pressure24. Also, the extent of

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improved thermodynamic stability of gas hydrates in the presence of thermodynamic promoters varies

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depending on guest gases and thus, thermodynamic promoters can exert an influence on gas separation

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efficiency. Thermodynamic promoters that have been commonly used are cyclic ethers and QAS

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materials. Tetrahydrofuran (THF), one of the cyclic ethers, is a water-soluble sII clathrate former which

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can function as both a thermodynamic and a kinetic promoter25, 26, and it can facilitate gas hydrate

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formation at milder conditions 27. Tetra-n-butylammonium chloride (TBAC) is a semi-clathrate

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former which has a large storage capacity and high thermodynamic stability among various

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QASs19. TBAC can form semi-clathrates under atmospheric pressure and the semi-clathrate


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possess vacant small cages that can be used for capturing small-sized gas molecules, so they can

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be effectively applied for gas separation and storage28-35.

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Even though various works on the phase equilibria and formation kinetics of CH4 + CO2

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clathrates have been reported in the literatures16,25,36, the effect of thermodynamic promoters on

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cage filling behavior of guest molecules in different clathrate structures has not yet been clearly

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examined for clathrate-based CO2 capture from natural gas and biogas. In this study, the gas

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mixture of CH4 (50%) + CO2 (50%) was used as a target gas simulating CO2-rich natural gas and

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biogas, and THF and TBAC were selected as thermodynamic promoters for clathrate formation.

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The precise and unique patterns of CH4 (50%) + CO2 (50%) + promoter (THF and TBAC)

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clathrates were investigated with a primary focus on the effects of thermodynamic promoters on

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clathrate stability and cage filling behavior. The thermodynamic stability of CH4 (50%) + CO2

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(50%) + promoter (THF and TBAC) clathrates were experimentally measured at two different

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promoter concentrations (1.0 mol% and stoichiometric concentrations of each clathrate

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structure). The enclathration of guest molecules in the clathrate cages of different structures were

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observed using

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clathrates were examined by measuring the amount of gas consumption during clathrate

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formation. CO2 concentration changes in the vapor phase during clathrate formation and CO2

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concentrations in the vapor and clathrate phases after completion of clathrate formation were

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measured using gas chromatography to examine CO2 selectivity, depending on the types and

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concentrations of thermodynamic promoters.

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C NMR and Raman spectroscopy. The gas uptakes and growth rates of the


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Experimental Section

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Materials and Methods

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CH4 gas (99.95%), CO2 gas (99.99%), and a gas mixture of CH4 (50%) + CO2 (50%) were

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supplied by MS Gas, Ltd. (Republic of Korea). Tetrahydrofuran (THF, 99%) and tetra-n-butyl

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ammonium chloride (TBAC, 97%) were purchased from Sigma-Aldrich Chemical Co. (USA).

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Double distilled, deionized water was used in this study. The experimental apparatus was

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specially designed in order to measure the accurate phase equilibrium by tracking temperature

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and pressure during clathrate formation and dissociation. A high-pressure equilibrium cell with

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an internal volume of 250 cm3 was made of 316 stainless steel, and the inner content of the cell

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was vigorously mixed with an impeller-type agitator (350 rpm). The cell was immersed in a

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water bath, and the temperature of the water bath was controlled by an external circulator (RW-

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2025G; JEIO Tech, Republic of Korea). The temperature was measured using a thermocouple

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which was calibrated with an ASTM 63C liquid-in-glass thermometer (H-B Instrument

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Company, USA) with an accuracy of ± 0.1 K. The pressure was measured using a pressure

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transducer (Model S-10; Wika, Germany) which was calibrated by a Heise Bourdon tube

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pressure gauge (CMM-137219; Ashcroft Inc., USA) within the experimental range. The

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uncertainties associated with the temperature and pressure measurement are 0.1 K and 0.02 MPa,

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respectively.

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Phase equilibrium measurement

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The high-pressure equilibrium cell was initially filled with 70 cm3 of the solution with

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thermodynamic promoters (TBAC and THF). After assembling the apparatus, ventilation was

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conducted several times to remove residual air from the cell. Then, the gas mixture (CO2 + CH4)


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was injected into the cell until the desired experimental pressure was obtained. A stepwise

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heating and cooling method was applied under an isochoric condition in order to accurately

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measure clathrate phase equilibria. The cell was slowly cooled from 298 K until the pressure in

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the cell abruptly dropped due to the clathrate formation. After sufficient time was given for

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complete clathrate formation, the temperature was increased in steps of 0.1 K/90 min to

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dissociate the clathrates. As the clathrates dissociated, the pressure in the cell was increased due

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to the gas released from the clathrates. The point where the clathrate dissociation line and the

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thermal expansion line intersect was determined as the three-phase (clathrate (H) – liquid water

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(LW) – vapor (V)) equilibrium point.

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Gas uptake and composition measurements

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The experiment for measuring gas uptake, which is the amount of gas consumed during clathrate

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formation, was conducted under isobaric and isothermal conditions. The pressure of the cell was

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maintained at 3.0 MPa, and the driving force (∆T), which is defined as the temperature

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difference between experimental and equilibrium temperatures, was set to 5.0 K. A micro-flow

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syringe pump (ISCO 500D; Teledyne Isco, Inc., USA) was used to maintain constant pressure

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and to measure the gas volume consumed during clathrate formation. The volume of gas

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supplemented into the cell at a constant pressure mode was recorded every 10 min for 2 h, and it

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was then converted to the moles of gas consumed per moles of water charged for comparison of

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gas uptakes for each promoter solution. Gas compositions during clathrate formation were

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measured every 10 min using a gas chromatograph (GC, 7890A; Agilent, USA) with a thermal

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conductivity detector (TCD) and a 80/100 Porapak Q Column (Supelco, USA). The vapor phase

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was circulated between the cell and a sampling valve (Rheodyne, USA) with a loop of 20 µL

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using a high-pressure metering pump (Eldex Laboratories, USA) to make a uniform composition


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throughout the system. For the clathrate composition measurement, the cell was immersed in the

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liquid nitrogen vessel for a short time period, and the vapor in the cell was then evacuated by a

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vacuum pump (Rocker 300; GSS Scientific Co., USA) Clathrate compositions were measured

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from the gas retrieved from clathrates at 298 K.

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Spectroscopic analyses

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A solid-state 400 MHz NMR spectrometer (Bruker, USA) that belongs to the Korea Basic

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Science Institute (KBSI) was used to examine the structure and guest distributions of the

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clathrates that were formed. The finely powdered clathrate samples were loaded into a 4 mm o.d.

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Zr-rotor which was placed in a variable-temperature probe at 243 K. The 13C NMR spectra were

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collected at a Larmor frequency of 100.6 MHz with magic angle spinning (MAS) from 2 to 4

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kHz. The downfield carbon resonance peak of adamantane (38.3 ppm at 300 K) was used as an

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external chemical shift reference.

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An in situ fiber-coupled Raman spectrometer (SP550; Horiba, France) equipped with a

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multichannel air cooled CCD detector and a 1,800 grooves/mm grating was used to confirm the

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enclathration of guest molecules. The excitation source was a Nd:Yag laser emitting a 532 nm

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line and providing 100 mW. Raman spectra were collected using a fiber-optic Raman probe that

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was attached to the water-jacked cell. The clathrates were formed under isobaric (3.0 MPa) and

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isothermal conditions. The driving force of ∆T = 5.0 K was given for each experiment. A more

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detailed description of the experimental apparatus and methods were provided in our previous

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papers13, 14, 19, 33.


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Results and Discussion

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Thermodynamic stability and structure of clathrates

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The three-phase (H-LW-V) equilibria of the CH4 (50%) + CO2 (50%) + promoter + water

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systems at two different concentrations (1.0 mol% and stoichiometric concentrations of each

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promoter clathrate) were measured in the pressure range of 0–7.0 MPa. The three-phase

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equilibrium data were plotted in Figure 1 and listed in Table S1. THF and TBAC were used as

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thermodynamic promoters to stabilize the CH4 (50%) + CO2 (50%) clathrate. The THF 5.6 mol%

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corresponds to the stoichiometric concentration of THF·17.0H2O (sII clathrate), whereas the

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TBAC 3.3 mol% corresponds to the stoichiometric concentration of TBAC·29.7H2O (semi-

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clathrate)17, 37. The significant shift of the equilibrium curve to a higher temperature region at a

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given pressure or to a lower pressure region at a given temperature indicates that the

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thermodynamic stability of the CH4 + CO2 clathrate was significantly enhanced in the presence

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of THF and TBAC. The extent of thermodynamic promotion was dependent on the concentration

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of each promoter. Despite the significant thermodynamic promotion of both THF and TBAC for

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the CH4 + CO2 clathrate, THF was a more effective thermodynamic promoter at a higher

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pressure range, whereas TBAC showed higher thermodynamic stability at a lower pressure range

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for both the 1.0 mol% and the stoichiometric concentration of each promoter. The clathrate phase

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equilibria of CH4 + CO2 + promoter + water systems imply that promoter molecules (THF and

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TBAC) are incorporated into the clathrate cages, and, thus, structural transformation of the

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original CH4 + CO2 clathrate can occur due to the inclusion of the promoters.

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in the clathrate cages.38. In particular, cage-dependent

C NMR spectroscopy is a powerful method to detect and quantify the carbon species captured 13

C NMR spectra of enclathrated CH4


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molecules are effective in the identification of clathrate structure. The stacked plot of 13C MAS

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NMR spectra of pure CH4, CH4 (50%) + CO2 (50%), CH4 (50%) + CO2 (50%) + THF (5.6

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mol%), and CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) clathrates in the chemical shift range

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of 0 to -10 ppm are depicted in Figure 2. The pure CH4 clathrate exhibited two resonance peaks

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at -4.3 ppm and -6.6 ppm, which are assigned to CH4 molecules captured in the small 512 and

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large 51262 cages of sI clathrate, respectively. The positions of NMR resonance peaks from the

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CH4 (50%) + CO2 (50%) clathrate were identical to those from the pure CH4 clathrate, indicating

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that the CH4 (50%) + CO2 (50%) clathrate is also sI. The area of each NMR resonance peak is

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proportional to the number of CH4 molecules trapped in each clathrate cage. The theoretical peak

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area ratio of CH4 molecules enclathrated in the large to small cages (AL/AS) for sI hydrate is 3.0

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because the unit cell of sI hydrate consists of two small 512 and six large 51262 cages. The AL/AS

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for the pure CH4 clathrate was found to be 3.3, which is very close to the theoretical ratio (3.0) of

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sI. However, the AL/AS for the CH4 (50%) + CO2 (50%) clathrate was found to be 1.45, which is

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relatively lower than that for the pure CH4 clathrate. This indicates that CO2 molecules were

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preferentially captured in the large 51262 cages of the sI clathrate. On the other hand, the CH4

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(50%) + CO2 (50%) + THF (5.6 mol%) clathrate had only one NMR resonance peak at -4.5 ppm

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which can be assigned to the CH4 molecules enclathrated in the small 512 cages of sII. THF

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molecules can occupy only large 51264 cages of sII, and at the stoichiometric concentration of 5.6

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mol%, the large 51264 cages of sII are completely filled with THF molecules39. Therefore, CH4

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molecules captured only in the small 512 cages of sII were detected at 4.5 ppm. The CH4 (50%) +

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CO2 (50%) + TBAC (3.3 mol%) semi-clathrate had a resonance peak at -4.0 ppm which is also

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assigned to CH4 molecules trapped in the small 512 cage of the semi-clathrate. In the pure TBAC

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semi-clathrate, the large broken cages are occupied by TBA cations, whereas the small 512 cages


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are left vacant, which are then available for capturing CH4 molecules. Even though the small 512

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cages are common for sI and sII clathrates and semi-clathrates, the slight shift in the resonance

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peak position from CH4 molecules in the small 512 cages was observed because there is a slight

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difference in the size and environment of the 512 cages in each structure40-42.

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The enclathration of both CO2 and CH4 in the clathrate cages was identified by Raman

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spectroscopy which is also useful in confirming the clathrate structure with various guests.

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Figure 3 depicts Raman spectra of pure CH4, CH4 (50%) + CO2 (50%), CH4 (50%) + CO2 (50%)

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+ THF (5.6 mol%), and CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) clathrates. The CH4

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(50%) + CO2 (50%) clathrate exhibited two Raman peaks at 2,905 cm-1 and 2,915 cm-1 which

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correspond to CH4 molecules captured in the large 51262 and small 512 cages of sI, and also two

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Raman peaks at 1,276 cm-1 and 1,380 cm-1 which are attributed to CO2 molecules in the clathrate

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cages43, 44. The Raman peak positions of each guest in the CH4 (50%) + CO2 (50%) clathrate

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were identical with those in the pure CH4 and pure CO2 clathrates, confirming that the CH4

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(50%) + CO2 (50%) clathrate is sI. However, the Raman peak of enclathrated CH4 molecules in

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the CH4 (50%) + CO2 (50%) + THF (5.6 mol%) clathrate was detected only at 2,914 cm-1 which

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can be assigned to the CH4 molecules in the small 512 cages of sII. The Raman peaks of

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enclathrated CO2 molecules were observed at 1,274 cm-1 and 1,380 cm-1. In the CH4 (50%) +

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CO2 (50%) + TBAC (3.3 mol%) semi-clathrate, one Raman peak for CH4 molecules captured in

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the 512 cages appeared at 2,912 cm-1, and two Raman peaks for enclathrated CO2 molecules were

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observed at 1,273 cm-1 and 1,380 cm-1, even though there were many broad Raman peaks that

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emanated from the TBA cations captured in the semi-clathrates45. Raman spectra clearly

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demonstrated that both CO2 and CH4 molecules are trapped in the clathrate cages and that the


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corresponding Raman peak positions of each guest are slightly different depending on clathrate

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structures.

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Gas uptake and gas composition change during clathrate formation

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The gas uptakes during clathrate formation were measured at 10 min intervals for the CH4 (50%)

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+ CO2 (50%) gas mixture with pure water, THF, and TBAC solutions and shown in Figure 4.

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The accumulated amount of gas consumption was expressed as moles of gas consumed per

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moles of water charged. The gas uptake of the CH4 (50%) + CO2 (50%) + water system was

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gradually increased during clathrate formation. The THF (5.6 mol%) solution showed a

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relatively higher rate of clathrate growth than the pure water system in the early stage of hydrate

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growth, but the final gas uptake of the THF (5.6 mol%) solution was smaller than that of the pure

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water system. The TBAC solutions demonstrated lower gas consumption than other systems. The

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amount of gas consumption during clathrate formation is closely related to the number of vacant

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cages in the unit cell of each clathrate structure, as well as to the conversion of water into

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clathrate. The unit cell of sI (pure water) and sII (THF solution) clathrates consists of 2(512)

237

·6(51262)·46H2O and 16(512) ·8(51264)·136H2O, respectively, and the TBAC solution forms a

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semi-clathrate that has a tetragonal structure I (TS-I) with a unit cell of 4(51263) ·16(51262)

239

·10(512) ·172H2O17, 37, 46. At the stoichiometric concentration of each structure, the pure water

240

system has a higher ratio of the number of available cages to the number of water molecules in

241

the unit cell than the THF (5.6 mol%) solution system. For the THF (5.6 mol%) solution, only

242

small 512 cages of sII are available for capturing gas molecules because all of the large 51264

243

cages are occupied by THF molecules, whereas for the pure water system, both small and large

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cages of sI can be used for enclathrating gas molecules. Therefore, the gas uptake of the pure

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water system was higher than that of the THF (5.6 mol%) solution. The TBAC solution showed


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the lowest gas uptake due to the lowest ratio of the number of available cages to the number of

247

water molecules in the unit cell. Furthermore, the amount of gas consumption was also

248

dependent on the concentration of the promoters. The gas uptake for the THF (1.0 mol%) and

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TBAC (1.0 mol%) solutions was significantly lower when compared to the solutions with the

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stoichiometric concentration.

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The growth rate of clathrate [mol/mol/min] was defined as moles of gas consumed for every 10

252

min per mole of charged water during the whole clathrate formation process (120 min) to

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observe the growth patterns of the clathrates in each solution, and it is demonstrated in Figure

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S1(a). The growth rate of clathrates formed from pure water was initially increased, then kept

255

almost constant until 60 min, and then, started to decrease. This indicates that the clathrates

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without any promoters grow gradually. On the other hand, the growth rate of the clathrate

257

formed from the THF (5.6 mol%) solution reached the highest peak value in the first 20 min after

258

clathrate nucleation, and then started to decrease abruptly after 20 min. The THF (5.6 mol%)

259

solution demonstrated the relatively faster growth in the early stage of clathrate formation

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compared to the pure water and THF (1.0 mol%) solution. The TBAC solutions generally

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showed the lowest growth rate among all the solutions.

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Furthermore, the normalized formation rate of clathrates for 30 min (NR30)7 is also depicted in

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Figure S1(b). As can be expected from the gas uptakes (Figure 4) and the growth rate, NR30 of

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the THF (5.6 mol%) solution was the highest and that of the TBAC (1.0 mol%) solution was the

265

lowest.

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As gas molecules in the vapor phase are consumed by clathrate growth, the gas compositions in

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the vapor phase change from the feed composition due to the different selectivity of guest gases


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in the clathrate cages. CO2 composition changes in the vapor phase during clathrate formation

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were measured every 10 min, and the results are displayed in Figure 5. The slight decline of the

270

CO2 concentration in the vapor phase before clathrate growth was observed due to high solubility

271

of CO2 in water. The CO2 composition changes in the vapor phase during clathrate formation can

272

be affected by the conversion of water into clathrate, CO2 selectivity in the clathrate phase, and

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the ratio of head space volume to water volume in the reactor. The CO2 concentrations in the

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vapor phase generally were decreased as the clathrate formation proceeded, except for the THF

275

(5.6 mol%) solution. For the pure water system, the CO2 concentration in the vapor phase was

276

gradually but most significantly decreased until 60 min because the CO2 is preferentially

277

captured in the clathrate cages. The final CO2 concentration in the vapor phase was found to be

278

∼35%. The TBAC (3.3 mol% and 1.0 mol%) solutions showed a very slight decrease of CO2

279

composition in the vapor phase and the final CO2 concentrations were ∼45% and ∼44%,

280

respectively. However, the THF (5.6 mol%) solution demonstrated the unusual behavior of an

281

increase in CO2 concentration in the vapor phase during clathrate formation, whereas the THF

282

(1.0 mol%) solution showed a decrease in CO2 concentration.

283

The CO2 compositions in the vapor and clathrate phases after completion of clathrate formation

284

are depicted in Figure 6. The pure water system demonstrated ∼56% CO2 in the clathrate phase,

285

whereas TBAC semi-clathrates produced significantly higher CO2 concentrations in the clathrate

286

phase (∼75% for 3.3 mol% and ∼72 % for 1.0 mol%). However, for the THF solutions, CO2

287

compositions in the clathrate phase were heavily dependent on the THF concentrations. The CO2

288

concentrations in the clathrate phase were found to be ∼45% and ∼66% for 5.6 mol% and 1.0

289

mol%, respectively. It should be noted from Figure 6 that for the THF (5.6 mol%) solution, CO2

290

is less selective than CH4 in the clathrate phase. That is why the CO2 concentration in the vapor


14

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


291

phase continued to increase during clathrate formation, as shown in Figure 5. Even though the

292

gas uptakes of TBAC semi-clathrates were smaller than those of other systems, the CO2

293

selectivity in the clathrate phase was the highest and was not significantly dependent on the

294

TBAC concentrations.

295

The factors affecting CO2 selectivity in clathrates

296

For the CH4 + CO2 clathrates, CO2 selectivity in the clathrate phase can be affected primarily by

297

the thermodynamic stability of each clathrate formed by a single guest. As seen in Figure S2, the

298

pure CO2 clathrate is thermodynamically more stable than the pure CH4 clathrate, and it is so for

299

TBAC semi-clathrates as well33. Therefore, for pure water and TBAC solution systems, CO2 was

300

more preferentially captured in the clathrate phase than CH4 (Figure 5 and 6). However, the

301

thermodynamic stability of the CH4 + THF clathrates is generally higher than that of CO2 + THF

302

clathrates for both THF (1.0 and 5.6 mol%) solutions27,47,48, as seen in Figure S2, which indicates

303

that CH4 can be more selectively captured in the clathrate cages than CO2. For the THF (5.6

304

mol%) solution, it was actually observed that the CO2 concentration in the vapor phase was

305

increased during the clathrate formation and that the CO2 concentration in the clathrate phase

306

after completion of clathrate formation was lower than that in the vapor phase, which clearly

307

demonstrates preferential occupation of CH4 over CO2 in the clathrate phase.

308

For TBAC solutions, cage symmetry, as well as thermodynamic stability, can also affect CO2

309

selectivity in the semi-clathrate cages42, 49. QAS semi-clathrates possess two types of small

310

dodecahedral (D) cages (distorted D cages and regular D cages) which are available for trapping

311

CO2 and CH4 molecules. Due to the presence of distorted D cages, which have a strong


15


Page 16 of 38

312

preference for CO2 molecules over CH4 molecules, CO2 selectivity in the TBAC semi-clathrates

313

was found to be higher compared to other clathrates.

314

For THF solutions, the cage filling behavior that depends on THF concentrations also exerts a

315

significant influence on CO2 selectivity in the clathrate phase. Guest distributions of CH4, CO2,

316

and THF molecules in the clathrates can be determined from NMR spectra shown in Figure 7.

317

The cage occupancy of each guest in the small and large cages of sI and sII clathrates can be

318

calculated through the following statistical thermodynamic equations by considering the area

319

ratios of each guest and the number of carbons in each guest molecule:

320

° ∆ߤ௪ = − ସ଺ [6 ln(1 − ߠ௅,஼ுସ −ߠ௅,஼ைଶ ) + 2 ln(1 − ߠௌ,஼ுସ − ߠௌ,஼ைଶ)]

321

ோ்

° ∆ߤ௪ =−

ோ் ଵଷ଺

[8 ln(1 − ߠ௅,்ுி −ߠ௅,஼ுସ -ߠ௅,஼ைଶ ) + 16 ln(1 − ߠௌ,஼ுସ − ߠௌ,஼ைଶ)]

(1)

(2)

322

° ° where ∆ߤ௪ is the chemical potential of the empty lattice relative to clathrate cages (∆ߤ௪ =

323

ு ° ߤௐ − ߤௐ ), and ߠ௃,௜ is the fractional cage occupancy of guest J in type i cage. The ∆ߤ௪ values

324

of 1,297 J/mol for Eq. (1) and 883.8 J/mol for Eq. (2) were used for sI and sII clathrates,

325

respectively 48,50,51. The occupancy ratios of ߠ௅,஼ுర /ߠௌ,஼ுర and ߠ௅,்ுி /ߠ௦,஼ுర were obtained from

326

the area ratios shown in Figure 7, and the composition ratio of CH4 to CO2 in the clathrate phase

327

was obtained by GC measurement and the assumption of large cage occupancy50. The calculated

328

cage occupancy of each guest for the pure water and THF (5.6 mol%) solution systems is listed

329

in Table 1 along with predicted values determined by CSMGem software17. For the pure water

330

system, the estimated cage occupancy of CO2 in the large 51262 cages (0.669) was larger than that

331

of CH4 (0.311), and the cage occupancy of CO2 in the small 512 cages (0.100) was much lower

332

than that of CH4 (0.655). It indicates that CO2 preferred to occupy the large 51262 cages of sI,

ఉ


16

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


333

whereas CH4 predominantly occupied the small 512 cages of sI. The relative cage occupancy

334

ratio of THF in the large 51264 cages to CH4 in the small 512 cages (ߠ௅,்ுி /ߠ௦,஼ுర ) in the 5.6

335

mol% THF clathrate was found to be 2.88, whereas that in the 1.0 mol% THF clathrate was 1.29.

336

At the stoichiometric concentration (5.6 mol%), THF molecules fully occupy the large 51264

337

cages of sII, whereas, at THF concentrations lower than 5.6 mol%, gas molecules can also

338

occupy the large 51264 cages, which is called “tuning behavior”

339

ߠ௅,்ுி /ߠ௦,஼ுర at 1.0 mol% THF indicates that CO2 molecules preferentially occupied the large

340

51264 cages, thereby resulting in a higher CO2 concentration in the clathrate phase at 1.0 mol%

341

THF.

342

In this study, clathrate-based CO2 capture was investigated for upgrading CO2-rich natural gas

343

and biogas. The phase equilibria, gas uptakes, gas composition measurements, and spectroscopic

344

analyses of CH4 (50%) + CO2 (50%) + promoter clathrates were examined to elucidate the effects

345

of thermodynamic promoters (THF and TBAC) on clathrate stability and cage filling behavior.

346

The addition of both THF and TBAC resulted in a significant thermodynamic promotion, which

347

was dependent on the promoter concentrations. The pure water and THF solutions demonstrated

348

higher gas uptakes and faster growth rates than TBAC solutions, which is attributed to the higher

349

and faster conversion of water into clathrate, as well as the larger number of available cages in

350

the unit cell of the clathrates formed. However, TBAC solutions gave higher CO2 selectivity in

351

the clathrate phase due to the presence of distorted D cages in the semi-clathrate structure which

352

have a strong preference for CO2 molecules over CH4 molecules. For the THF (5.6 mol%)

353

solution, the CO2 concentration in the vapor phase continued to increase during clathrate

354

formation, and the CO2 concentration in the clathrate phase after completion of clathrate

355

formation was found to be lower than that of feed gas because of the lower thermodynamic

39, 52

. The lower ratio of


17


Page 18 of 38

356

stability of CO2+THF clathrates compared to CH4 +THF clathrates and the full occupancy of

357

THF molecules in the large 51264 cages of sII. The CO2 selectivity of the THF (1.0 mol%)

358

solution was higher than that of the THF (5.6 mol%) solution because CO2 can also occupy the

359

large 51264 cages of sII. However, the CO2 selectivity of TBAC solutions was not significantly

360

dependent on TBAC concentrations. The overall experimental results demonstrated that

361

thermodynamic stability, cage shape and dimension, and cage filling behavior can affect CO2

362

selectivity in each clathrate phase. THF solutions produced higher gas uptakes and faster growth

363

rates for clathrates, but lower CO2 selectivity in the clathrate phase, whereas TBAC solutions

364

demonstrated lower gas uptakes, but higher CO2 selectivity. It should also be noted that THF is

365

volatile and toxic, whereas TBAC is non-volatile and reusable. Therefore, TBAC would be one

366

of the efficient thermodynamic promoters to separate CO2, and careful consideration should be

367

given to the selection of a suitable promoter and its optimum concentration in designing and

368

operating clathrate-based CO2 capture from sour natural gas or biogas.

369

Corresponding Author

370

* Tel: +82-52-217-2821. Fax: +82-52-217-2859. E-mail: [email protected].

371

Notes

372

The authors declare no competing financial interest.

373

ACKNOWLEDGMENT

374

This research was supported by the National Research Foundation of Korea (NRF) grant funded

375

by the Ministry of Education (2016R1D1A1A02937037) and also by the Korea Institute of

376

Energy Technology Evaluation and Planning (KETEP) through "Human Resources Program in


18

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


377

Energy Technology" (No. 20164030201010) funded by the Ministry of Trade, Industry and

378

Energy, Republic of Korea.


19


379

Page 20 of 38

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523

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27


525

Page 28 of 38

FIGURE CAPTION

526

Figure 1. Clathrate phase equilibria of the CH4 (50%) + CO2 (50%) + promoter + water

527

systems.

528

Figure 2. 13C NMR spectra of pure CH4, CH4 (50%) + CO2 (50%), CH4 (50%) + CO2 (50%)

529

+ THF (5.6 mol%), and CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) clathrates

530

Figure 3. Raman spectra of pure CH4, CH4 (50%) + CO2 (50%), CH4 (50%) + CO2 (50%) +

531

THF (5.6 mol%), and CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) clathrates

532

Figure 4. Gas uptakes of the CH4 (50%) + CO2 (50%) + promoter + water systems during

533

clathrate formation at 3.0 MPa and ∆T = 5.0 K.

534

Figure 5. CO2 concentration changes in the vapor phase during the clathrate formation.

535

Figure 6. CO2 compositions in the vapor and clathrate phases after completion of clathrate

536

formation.

537

Figure 7.

538

(5.6 mol%), and CH4 (50%) + CO2 (50%) + THF (1.0 mol%) clathrates.

13

C NMR Spectra of CH4 (50%) + CO2 (50%), CH4 (50%) + CO2 (50%) + THF


28

Page 29 of 38

8 pure w ater

7

THF 5.6 mol% THF 1.0 mol% TBAC 3.3 mol% TBAC 1.0 mol%

6

Pressure (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

4

3

2

1

0 270

280

285

290

295

300

Temperature (K)

539 540

275

Figure 1.


29


Page 30 of 38

-6.6

-4.3

pure CH4 clathrate (sI) -6.6 -4.3

CH4 (50%) + CO2 (50%) clathrate (sI) -4.5

CH4 (50%) + CO2 (50%) + THF (5.6 mol%) clathrate (sII) -4.0

CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) semi-clathrate

0

-4

-6

-8

-10

Chemical Shift (ppm)

541 542

-2

Figure 2.


30

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2905

2915

pure CH4 clathrate (sI) 2905

2915

1380 1276

CH4 (50%) + CO2 (50%) clathrate (sI) 2914

1380

1274

CH4 (50%) + CO2 (50%) + THF (5.6 mol%) clathrate (sII) 2912

1380

1273

CH4 (50%) + CO2 (50%) + TBAC (3.3 mol%) semi-clathrate

1200

1400

1500 2850

2900

2950

3000

-1

Wavenumber (cm )

543 544

1300

Figure 3.


31


0.12 pure water

Consumed mol of gas per mol of water (mol/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

THF 5.6 mol% THF 1.0 mol% TBAC 3.3 mol% TBAC 1.0 mol%

0.10

0.08

0.06

0.04

0.02

0.00 0

20

40

60

80

100

120

Time (min)

545 546

Figure 4.


32

Page 33 of 38

55

CO2 concentration in the vapor phase (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

45

40

pure water THF 5.6 mol% THF 1.0 mol% TBAC 3.3 mol% TBAC 1.0 mol%

35

30 0

40

60

80

100

120

Time (min)

547 548

20

Figure 5.


33


100 90

vapor phase clathrate phase

80

CO2 composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

70 60 50 40 30 20 10 0

pure water

THF 5.6 mol%

THF 1.0 mol%

TBAC 3.3 mol%

TBAC 1.0 mol%

549 550

Figure 6.


34

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH4 in sI-L - 6.6 CH4 in sI-s - 4.3

CH4 (50%) + CO2 (50%) clathrate (sI)

THF in sII-L 25.9

THF in sII-L 67.9

CH4 in sII-s - 4.5

CH4 (50%) + CO2 (50%) + THF (5.6 mol%) clathrate (sII) THF in sII-L 25.9

THF in sII-L 67.9

CH4 in sII-s - 4.5

CH4 (50%) + CO2 (50%) + THF (1.0 mol%) clathrate (sII)

80

40

20

0

-20

Chemical Shift (ppm)

551 552

60

Figure 7.


35


553

Page 36 of 38

Table 1. Cage occupancy of each guest molecule in the clathrate cages.

Large cage

Small cage system

ߠ௦, ஼ுସ

ߠ௦, ஼ைଶ

Pure water

0.655

0.100

Pure water (CSMGem)

0.598

THF (5.6 mol%)

0.344

ߠ௦,

ࣂࡿ,࢚࢕࢚ࢇ࢒

ߠ௅, ஼ுସ

ߠ௅, ஼ைଶ

ߠ௅, ்ுி

ࣂࡸ,࢚࢕࢚ࢇ࢒

-

0.753

0.311

0.669

-

0.980

0.187

-

0.785

0.326

0.637

-

0.963

0.270

-

0.614

-

-

0.990

0.990

்ுி

554


36

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


555

Supporting Information. Figures of growth rate of the CH4 (50%) + CO2 (50%) + promoter +

556

water systems, normalized growth rate for 30 min (NR30) of the CH4 (50%) + CO2 (50%) +

557

promoter + water systems, and phase equilibria of CO2 and CH4 clathrates in THF 5.6 mol% and

558

1.0 mol% systems and Table of clathrate phase equilibrium data of the CH4 (50%) + CO2 (50%)

559

+ promoter + water systems.


37


Page 38 of 38

560

Synopsis

561

Clathrate-based CO2 capture was investigated in the presence of thermodynamic promoters for

562

upgrading CO2-rich natural gas and biogas.

563

TOC/Abstract Art

564


38

Clathrate-based CO2 Capture from CO2-rich Natural Gas and Biogas

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