DOI: 10.1002/ente.201300064 Effects of Different Surfactants on the Kinetics of Ethane- Hydrate Formation: Experimental and Modeling Studies Reza Karimi, [a] Farshad Varaminian,* [a] Amir A. Izadpanah, [b] and Amir H. Mohammadi * [c, d] Introduction Gas hydrates, or clathrate hydrates, are ice-like compounds that form if certain molecules (such as methane, ethane, pro- pane, carbon dioxide, etc.) come into contact with water under appropriate thermodynamic conditions (high pressure and low temperature). Water molecules can form a lattice structure containing cavities, due to the hydrogen bonding between them. This structure is thermodynamically unstable. The inclusion of molecules with favorable shapes and sizes into cavities can stabilize this structure. In fact, due to the van der Waals forces between the encaged molecules and the water lattice, the hydrate structure is thermodynamically sta- bilized. Based on the size, shape, and number of the cavities in the unit cell, three different crystalline structures [structur- es I (sI), II (sII), and H (sH)] can be typically formed. [1] Natural-gas hydrates have been recently studied for uti- lization in industrial systems as a method to store and trans- port natural gas (e.g., 150–180 volumes of gas for each volume of hydrate). [2, 3] The idea of storing natural gas in the form of a hydrate is advantageous not only because of the high storage capacity but also for the preservation of the hy- drate itself. [4] The slow hydrate-formation rate has hindered the industrial applications of hydrate storage. Natural gas is slightly soluble in water, so the most straightforward method for increasing the hydrate-formation rate is the utilization of mixing processes (such as spraying water in a continuous gas phase, bubbling gas in water, and stirring) and these process- es can consume significant power. [1] Many investigations have been conducted to increase the hydrate formation rate by adding surfactants to liquid water at low stirring rates, particularly for the case of methane hy- drate. Zhong and Rogers [5] studied the effect of sodium do- decyl sulfate (SDS) on the ethane-hydrate-formation rate in a quiescent system. They found that SDS above its critical micelle concentration (CMC) increases the rate of ethane- hydrate formation by a factor of greater than 700. Han et al. [6] investigated the effect of SDS on natural-gas-hydrate formation containing 90 wt % methane. They reported that a concentration of 300 ppm marks the maximum in hydrate gas content. Karaaslan et al. [7] investigated the effect of linear alkyl benzene sulphonic acid in concentrations of 0, 0.01 and 0.05 wt % on a mixture of methane and gas (includ- ing 88.17 % propane). They found that the increase in the sI hydrate formation rate is higher than that of sII. Sun et al. [8] investigated the hydrate-formation rate and storage capacity of hydrates of synthetic natural gas (methane 92.05 mol %, ethane 4.96 mol %, propane 2.99 mol %) in the presence of anionic and nonionic surfactants . They found that the anion- ic surfactants are more effective than nonionic surfactants. Okutani et al. [9] surveyed the effect of the anionic surfactants SDS, sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS) on the methane-hydrate-formation rate. They reported that all of these surfactants increased the hydrate- formation rate. These materials differ in their alkyl chain length. Each of these surfactants increased the solubility of An experimental study of the ethane-hydrate-formation ki- netics has been performed. First, the effects of stirring veloci- ty and initial pressure on the hydrate-formation rate were in- vestigated. Second, the effects of additives on the ethane-hy- drate-formation rate were also studied. Three surfactants, namely sodium dodecylbenzenesulfonate (SDBS), dodecyl trimethyl ammonium bromide (DTAB), and TritonX-100 (TX-100) were tested. The results show that DTAB not only does not significantly promote the hydrate-formation rate at different concentrations, but actually inhibits the ethane-hy- drate-formation rate, whereas SDBS does increase the hy- drate-formation rate. Furthermore, TX-100 increases the ethane-hydrate-formation rate, but to a lesser degree than SDBS. [a] R. Karimi, F. Varaminian Department of Chemical Engineering, Oil and Gas, Semnan University Semnan (Iran) E-mail: fvaraminian@semnan.ac.ir [b] A. A. Izadpanah Gas and Petrochemical Engineering Faculty Persian Gulf University Bushehr (Iran) [c] Dr. A. H. Mohammadi Institut de Recherche en GØnie Chimique et PØtrolier (IRGCP) Paris Cedex (France) E-mail: a.h.m@irgcp.fr [d] Dr. A. H. Mohammadi Thermodynamics Research Unit, School of Chemical Engineering University of KwaZulu-Natal, Howard College Campus King George V Avenue, Durban 4041 (South Africa) 530 # 2013 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim Energy Technol. 2013, 1, 530 – 536