Gas Hydrate Growth Kinetics ; Experimental Study Related to Effects of Heat Transfer

Abstract

Gas hydrate thermodynamics and phase equilibria is already well established. However, some knowledge gaps still need to be filled in gas hydrate growth kinetics, in relation to new gas hydrate based technologies in gas separation and storage; as well as in the modeling of gas hydrate growth from the mechanisms of intrinsic kinetics, mass transfer, and heat transfer. Our findings from this work contribute valuable insights to the ongoing discussion on gas hydrate growth kinetics. New technologies in gas separation and storage require fast and efficient gas hydrate formation rates. In line with this, we have investigated the effect of parameters that may be optimized to give rapid gas hydrate growth rates, such as; temperature, water content, stirring rate, and reactor size on gas hydrate growth kinetics. This was carried out in two studies, in the first one, the growth rate was estimated directly from gas consumption rates in normal milliliters per minute [NmL/min]; while the second study was an extension of the first with the growth rate normalized by the water content (volume of water) in the cell. In line with this investigation, we have employed the correlation for the average bubble diameter from literature, based on isotropic turbulence theory for estimating the average bubble size; for analysis of the dispersion parameters of the system. The results from these studies reveal the following:

1. For the temperature: increased subcooling increases gas hydrate growth rates. Increased subcooling in this case gives a direct reflection of the effect of increased driving force. 2. For the water: increased water content gave poorer gas-liquid dispersion and thus slower gas hydrate growth rates. 3. For stirring: increased stirring increased the growth rate up to a threshold stirring rate beyond which further increase in the stirring rate did not increase the gas hydrate growth rate. This was linked to negligible heat and mass transfer effects beyond the threshold stirring rate. 4. For reactor size (scale-up with geometric similarity): though more absolute volumes of gas hydrates was formed with increased reactor size, which is due to the increased volumes of reacting components, the growth rate per unit volume of water in the reactor decreased.

Furthermore, analyzing the effect of increased stirring in terms of power input per unit volume (P/V), increased power input per unit volume did not improve the gas-liquid dispersion parameters beyond the threshold stirring rate. With scale-up of reactor size, the results show that even at similar P/V and gas- liquid dispersion parameters, gas hydrate growth rate decreased. In addition we have performed studies on the effect of hydrate content on heat transfer using methane hydrate, Tetrahydrofuran (THF) and Ethylene oxide (EO) hydrates. The measurements from the heat transfer experiments were analyzed using a simple heat transfer model. These studies revealed important insights on hydrate plug deposition behavior on the reactor wall, as well as heat transfer through the hydrate slurry with increasing hydrate content. A solid hydrate mass formed at 40 – 60% hydrate content. Also, the heat transfer coefficient decreased with increasing hydrate content, but remained constant once a solid hydrate mass formed. The heat transfer coefficient would change as hydrate growth progresses. Finally methane hydrate growth was modeled based on heat transfer. The findings from this study confirmed the transient nature of the heat transfer coefficient during hydrate growth and that hydrate growth can be modeled based on heat transfer if the transient nature of the heat transfer coefficient is taken into account.