Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Effect of HFC-134a as a Promoter of CO2 Hydrate: Phase Equilibrium, Dissociation Enthalpy and Kinetics Hyunju Lee,† Chansu Park,‡ Eunkyung Lee,† Ju Dong Lee,§ and Yangdo Kim*,∥ †
Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States Department of Research and Development, AcroLabs, Inc., Seongnam 13516, Korea § Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 46749, Korea ∥ School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea ‡
ABSTRACT: HFC-134a gas was investigated as a potential guest molecule to improve the thermodynamic conditions and formation rate for CO2 hydrate. In the phase equilibrium study, the equilibrium pressure of CO2 + HFC-134a was lower than that of pure CO2 gas, and the equilibrium pressure decreased gradually with increasing HFC-134a concentration. The dissociation enthalpy (ΔHd) was calculated using the Clausius−Clapeyron equation, and the ΔHd value also changed with increasing HFC-134a concentration. In particular, the ΔHd of 8 mol % HFC-134a-added CO2 hydrate was 143.2 kJ/mol, which was similar to that of pure HFC-134a (structure-II). In the kinetic study, the reactor was initially filled with CO2 + HFC-134a gas only and pure CO2 gas was then supplied as a source when the hydrate reaction proceeded. As a result, the formation rate of the HFC-134a mixture in the initial 2 min was faster than that of pure CO2. This was consistent with the gas chromatography results, which showed that HFC-134a occupies the cage at the beginning of hydrate formation. These results suggest that the addition of HFC-134a influences the CO2 hydrate thermodynamic equilibrium and kinetic characteristics.
1. INTRODUCTION Gas hydrates are crystalline compounds formed by a physical reaction between water and guest molecules. Guest molecules, such as CH4 and CO2, occupy the cages of crystal structures composed of hydrogen bonds. The cages can be divided into three types: pentagonaldodecahedron (512, S-cage), tetrakaidecahedron (51262, M-cage), and hexakaidecahedron (51264, Lcage). The hydrate crystal structures are composed of a combination of a few types of cages. Structure-I (s-I) consists of two S-cages and six M-cages with 46 water molecules. Structure-II (s-II) consists of 16 S-cages and eight L-cages with 136 water molecules. The hydrate structure depends mainly on the size and shape of the guest molecules in addition to the temperature and pressure.1 CO2 hydrates have been applied in various fields, and many researchers have reported CO2 capture and sequestration as well as CO2 hydrate-based desalination processes.2−5 Tetrahydrofuran (THF) and tetra-n-butyl ammonium bromide (TBAB) have commonly been used as a thermodynamic promoter for CO2 hydrate.6,7 A previous study reported the effects of THF and TBAB additives on the CO2 separation and recovery from CO2 + H2 gas mixtures.8,9 Kang et al. showed that the dissociation pressures were also largely shifted to higher temperatures and lower pressure conditions by the addition of a small amount of THF from the CO2 + N2 mixtures.10 Nevertheless, THF and TBAB are toxic liquids and can contaminate the solution. Recently, CO2 mixture gas © XXXX American Chemical Society
studies using a gas instead of liquid attracted considerable attention. Kumar et al. examined the equilibrium hydrate conditions CO2 + H2 and CO2 + H2 + C3H8 mixtures. They reported that the addition of 3.2 mol % C3H8 into a CO2 + H2 mixture reduced the equilibrium pressure by 50%.11 Wang et al. examined the equilibrium hydrate dissociation conditions of CO2 + 1,1-dichloro-1-fluoroethane (HCFC 141b)/cyclopentane (CP). They reported that the addition of HCFC 141b or CP reduced the CO2−hydrate equilibrium pressures.12 Hydrofluorocarbons (HFCs), such as 1,1-difluoroethane (HFC-152a) and 1,1,1,2-tetrafluoroethane (HFC-134a), are well-known as alternative refrigerants for the air-conditioning system to replace chlorofluorocarbons (CFCs) and chlorohydrofluorocarbons (CHFCs). These gases are a promising option as a gas hydrate formation promoter because of the relatively lower equilibrium pressures of some HFCs hydrates (
