1 Korean J. Chem. Eng., 32(4), 1-11 (2015) DOI: 10.1007/s11814-016-0318-9 pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: mohebbi@put.ac.ir Copyright by The Korean Institute of Chemical Engineers. A new approach for modeling of multicomponent gas hydrate formation Vahid Mohebbi * ,† , Reza Mosayyebi Behbahani * , and Abbas Naderifar ** *Gas Engineering Department, Petroleum University of Technology, Iran **Chemical Engineering Department, Amirkabir University of Technology, Iran (Received 22 July 2016 • accepted 10 November 2016) Abstract-Several models have been proposed to investigate the kinetics of gas hydrate formation. The main differ- ences between the proposed models are the definition of the driving force, thermodynamics approach and the number of resistances to study the gas consumption by the hydrate phase. This paper concentrates on gas hydrate formation from multicomponent mixture, which has not been much studied before. In the present research, chemical potential has been considered as the driving force and, consequently, a new resistance coefficient was introduced. A complete discussion and reasonable assumptions has been provided to support this modelling. Keywords: Gas Hydrates, Natural Gas, Multicomponent, Driving Force, Kinetics, Thermodynamics INTRODUCTION Gas hydrates are a set of clathrates formed from the combina- tion of water and certain gases under conditions of high pressure and low temperature. The hydrate structure is stabilized when gas molecules occupy “cages” formed by hydrogen-bonded water mol- ecules. Interest in these compounds has risen in recent years due to the discovery of large deposits below the ocean floor and in permafrost regions [1]. Hydrates have important applications in many areas, including flow assurance of oil and gas lines, poten- tial sources of natural gas (mostly methane) from permafrost and deep-sea hydrate deposits, and energy storage and transportation [2]. Hydrate formation is a major issue as far as the flow assur- ance of oil and gas lines is concerned. The oil and gas industry spends over $200 million annually to prevent hydrate formation and maintain flow assurance [3]. Today, several applications have been proposed that have rendered the gas hydrate phenomenon as a novel technique. The application of gas hydrates in carbon diox- ide sequestration, separation processes, and natural-gas storage and transportation has intrigued many researchers over the past years [4]. Moreover, huge sources of natural-gas hydrate have been discovered, and considerable efforts have been made towards the production of this type of resource [5]. Understanding the kinet- ics of hydrate formation is necessary insofar as the mentioned applications are concerned. Several researchers have attempted to discover and predict hydrate formation. Vysniauskas and Bishnoi were the first to measure the rate of methane hydrate formation [6]. Based on crystallization kinetic and mass transfer effect, Englezos et al. developed a model to predict the formation kinetics of meth- ane and ethane hydrates [7]. Gillard et al. proposed an empirical correlation based on the work by Englezos et al [8]. Monfort et al. also proposed a semi-empirical model with fugacity and driving force taken from Englezos et al. and Vysniauskas and Bishnoi, respectively [9]. According to Clarke and Bishnoi, the model by Englezos et al. is only valid for low supersaturating systems due to the assumption of negligible primary crystallization after nucle- ation [10]. Zhang et al. used the proposed model by Englezos et al. and applied it for methane hydrate formation in the presence of sodium dodecyl sulfate [11]. Skovberg and Rasmussen simplified Englezos’ model to mass transfer limited model, where they assumed that the most important step in gas diffusion through water is gas- liquid interface mass transfer, which controls the hydrate forma- tion kinetics [12]. Most of the theoretical and experimental stud- ies were carried out to reveal that hydrate formation and growth mechanism were inherently system-dependent. Depending on the approach to model the behavior of the gas hydrate formation (or dissociation), the appropriate driving force has been selected by researchers. In this paper, the mass transfer approach was employed to model multicomponent gas hydrate formation. In this manner, chemical potential was selected as the driving force [13], and was implemented for the case of multicom- ponent mixture. A complete discussion and reasonable assump- tions has been provided for this purpose. APPROACHES TO PREDICT GAS HYDRATE KINETICS As the gas hydrate formation (or dissociation) process deals with heat and mass transfer, including several resistances, most research- ers prefer to emphasize one or some of these resistances and ap- proaches. As the temperature is kept constant in most of the exper- iments, thermal resistance is ignored in many studies, although there have been some considerable attempts to assume the gas hy- drate formation (or dissociation) as the thermal process. In the current study, an isothermal process was assumed. Consequently, thermal resistance was omitted. Three main approaches are avail- able in the literature as briefly described here.