Laboratory Synthesis of Gas Hydrates: Growth Mechanism of Ice Seed Method and Catalytic Effect of Alcohol Vapor

This study has successfully set up Taiwan’s first gas hydrate synthesizing apparatus which has the capability for recording experimental temperature and pressure. A large (at least 100 grams) methane-saturated (more than 99%) solid hydrate sample can be produced by simply introducing highly pressurized methane gas into the reactor that contains an ice-seed sample (Otherwise called an Ice Seed Method experiment). The appearance and texture of manufactured hydrates remain the same as those of the original ice-seed samples. Also, by this system, we can prepare a variety of samples mixed with different sediments for meeting the needs of follow-up studies.

The process of the “Ice Seed Method” can be divided into two main stages, "Pressurization" and "Heating". Firstly, we investigated the influence of different experimental parameters (Initial temperature, pressure, and fabric of ice-seed sample, etc.) on ice-to-hydrate converting rates and total hydrate yields during the pressurization stage. This study has shown for the first time that before the second heating stage, a certain amount of gas hydrate (For example, 12% of ice can be converted to hydrate within 47 min from an ice-seed sample with a porosity of 66%) can be produced after introducing highly pressurized methane gas into the reactor while the texture of sample remains constant. We found that the amount of methane hydrates produced is inversely proportional to the initial temperature; the lower the temperature, the larger the amount of formed hydrate. The converting rates can be even faster in experiments with a slow pressurization procedure or samples mixed with sediments. A 25% conversion can be achieved in a slowly pressurized run within 1042 min.

We also found that if there are more hydrates formed during the pressurization stage, the cost of energy and time can be cut down dramatically during the succeeding heating stage for the purpose of making a completely saturated methane hydrate sample.

By analyzing experimental data along with some SEM work, this study proposed a simple schematic model to reasonably explain some unique characteristics of the “Ice Seed Method”, such as why hydrate samples can be manufactured with different degrees of cementation, or why the appearance and texture of samples can be maintained after the heating stage. By adjusting some experimental parameters properly, hydrate samples with different features can be manufactured by the guideline obtained from this study.

Although there were some previous researches concentrated on how to promote or retard the formation of gas hydrates in a liquid system by adding different additives, no report about how to improve the conversion efficiency of the “Ice Seed Method” can be found. This study found out that the total amount of methane or carbon dioxide hydrates formed during the “Pressurization” stage can substantially increase as some alcohol vapor (methanol, ethanol and 1-propanol) is added into the system. The one that has the best promoting effect is ethanol while the initial system temperature is set at 270.2K. Nearly 91% of ice seed can be converted into methane hydrates during pressurization stage. The discovery may have considerable practical value for the transportation and storage of natural gas.

The preliminary hypothesis for this promoting effect is that the presence of these trace gaseous alcohol is able to slow down the formation rate of hydrates and prevent the generation of impervious hydrate film covering the ice core in the early stage. Instead, the formed hydrates are permeable because of a different texture so the inner ice can keep converting into hydrates by continuously interacting with methane molecules. Maybe the catalytic efficiency has a relationship with the alcohol vapor pressure which changes with the temperature. According to our experimental data, the best catalytic efficiency for each kind of alcohol may be achieved at different temperatures when the vapor pressures are all around 10mmHg.