Surfactant Surface Tension Effects on Promoting Hydrate Formation: An Experimental Study Using Fluorocarbon Surfactant (Intechem-01) + SDS Composite Surfactant ()
1. Introduction
Natural gas hydrate has huge gas storage capacity and the technology of natural gas hydrate storage and transportation offers a new method for energy storage and transportation. Compared with Liquefied Natural Gas (LNG) and Compressed Natural Gas (CNG) natural gas hydrate (NHG) has obvious advantages , which costs less and can be operated stably. Gudmundsson et al. [1] confirmed that methane gas can be stored long-term in hydrate above 258.15 K at atmospheric pressure, while LNG storage at T = 111.15 K, and the price of natural gas hydrate device is only 25% of LNG. Using Sodium dodecyl sulfate (SDS), tetradecyl sodium sulfate (STS) and sodium hexadecyl sulfate (SHS) as contrast experiments, Okutani K [2] discovered STS at concentration of 100 ppm had the same positive effect to 1000 ppm SDS. Jeffry Yoslim’s experiment [3] studied the effect of SDS on methane-propane gas hydrate and found that molar gas consumption increased 14 times in 242 - 2200 ppm SDS compared with pure water system. The results were caused by gas liquid contact area increasing since porous hydrate forming. H. Ganji [4] found that the hydrate formation rate increased by 35 times in 500 ppm SDS, gas storage ability increased by one time. At temperature of 274.15 K and pressure of 3.5 - 4.0 Mpa, Young-Ah Kwon [5] used various anionic surfactant with different length of carbon chain (C8,C10,C12) as hydrate promoter and found surfactant with shorter carbon chain had stronger promote ability. Multiple chain sulfate surfactant had had better gas storage capacity than SDS even at low concentrations, because of their lower critical micelle concentration (CMC) and special surface tension. Using drops method, Hu Luo et al. [6] measured the interfacial tension of ethylene surfactant solution in different temperature and pressure, and studied the influence of pressure for CMC, the results of the study showed that the CMC decreased with increasing pressure. Y. Zhong [7] investigated the effect of SDS on methane hydrate formation, they found CMC of SDS is 242 ppm.
As mentioned above, the effect of some surfactant has been studied, however, the current researches mostly limited to the influence of factors such as surfactant dosage and concentration. The promotion mechanism of additives is not very clear yet. The key effect of surfactant is that it reduces the gas-liquid interface surface tension so that the diffusion resistance between two phase is reduced and the gas molecules solubility is increased, as a consequence, better mass transfer effects is gained.
This study was performed based on previous study [8] which obtained the conclusion that using composite promoting agents of CP (chemically pure) + SDS and methylcyclohexane (MCH) + SDS can reduce the phase equilibrium temperature and shorten the induction time. The surface tension of Intechem-01 + SDS composite surfactants and its changes with temperature, concentration and proportion has been investigated. The aim of the study has been to reveal the essence of surfactant effects, which is significance to find the relationship between gas hydrate formation rate and surfactant kinds, dosage and concentration and establish new hydrate kinetics model containing surfactant factors.
2. Description of Experiments
2.1. Experimental Apparatus and Materials
The experimental apparatus produced by Shanghai Fangrui Inc is a QBZY series automatic surface tension determine instrument. Laboratory reagent are Perfluorinated alkyl ether glycol amine salt type anionic fluorocarbon surfactant (Intechem-01), industrial grade,active matter content ≥ 99% and sodium dodecyl sulfate (SDS), industrial grade, active matter content ≥ 86%.
2.2. Experimental Procedure
Composite Surfactants whose ratios (the proportion of fluorocarbon surfactant in the total active agent quality) are 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, respectively were prepared before experiment. 0 represents for pure SDS, 1 represents for pure fluorocarbon surfactant. Solution concentration is between 0.01% - 0.9%. 2. Surface tension values of measured samples at different concentrations and proportions, temperatures of 283.15 K, 288.15 K, 293.15 K, 298.15 K and 303.15 K, respectively, were measured by tensiometer. Record the experiment data.
3. Results and Discussion
3.1. Surface Tension Properties of Pure Surfactant Liquid
In Figure 1(a), is represented the surface tension changes of Sodium dodecyl sulfate along with the concentration at 298.15 K. As is shown, the critical micelle concentration (CMC) of sodium dodecyl sulfate is 1.5%, the lowest surface tension is 34 mN/m [9]. Figure 1(b), is represented the surface tension changes of pure fluorocarbon surfactant (Intechem-01) along with the concentration at 298.15 K, the critical micelle concentration is about 0.08%, and the lowest surface tension is 16.8 mN/m. By comparison, it is known that the fluorocarbon surfactant shows better surface activity in low concentration.
Figure 2 shows the surface tension changes of Intechem-01 solution along with concentration at different temperatures, and concentration between 0% - 1%, The higher temperature is, the lower surface tension, and the surface activity effects of solution are more evident The higher concentration is, the lower surface tension is, the surface tension has dropped sharply at concentration of 0% - 0.1% and did not reduced when the concentration exceeds 0.1%. The lowest surface tension is 16.2 mN/m and 17.1 mN/m at 303.15 K and 283.15 K. Fluorocarbon surfactant is better than the hydrocarbon surfactant in terms of amount and activity.