(335aa) Useful Remarks to Reduce Experimental Information Required to Determine Water Content of Gas in Equilibrium with Gas Hydrate, Ice or Liquid Water
Natural gases normally contain significant quantities of water. During production, transportation and processing operations, undesired amounts of dissolved water may condense altering the physical state from vapor to condensate, gas hydrate and/or ice. Condensate formation may lead to corrosion and /or two-phase flow problems. The formation of gas hydrate/ice could result in pipelines blockage and shutdown. Accurate knowledge of phase behavior in water + natural gas system, especially near and inside the gas hydrate or ice formation regions, is therefore of interest to avoid these problems. A gas phase (with dissolved water) can form condensate, gas hydrate or ice at the liquid water ? gas, hydrate ? gas or ice ? gas boundaries, respectively, without a free water phase, from a strict thermodynamic standpoint. The question of the accumulation of gas hydrate/ice phase is a question of kinetics, dependent upon the time necessary for nuclei to attain a critical size. This time may be in excess of that available for laboratory study but may occur in processes, which operate over extended periods of days, months or years. The experimental water content data at low temperatures, especially near and inside the gas hydrate or ice formation regions, are normally scarce and often rather dispersed. The works done to describe the water content of gas in equilibrium with gas hydrate/ice in the two-phase region are limited in accuracy mainly due to two factors: the fact that meta-stable liquid water may extend well into the gas hydrate/ice region and the experimental restraint that the existing analysis methods require large amounts of gas in equilibrium with gas hydrate. The establishment of hydrate-gas or ice-gas equilibrium constitutes the principle problems associated with properly conducting the tests. It is necessary to decompose and re-crystallize the gas hydrate phase to ensure that the hydrate crystal is indeed in equilibrium with the gas phase. Therefore, there is a paucity of accurate equilibrium data in the hydrate - gas and ice - gas regions. Most of the available experimental data inside the hydrate and ice regions do not warrant real thermodynamic equilibrium, i.e. these data are not in equilibrium with gas hydrate or ice. Literature survey reveals the availability of few sets of experimental data for water content of gases in equilibrium with gas hydrate/ice and all other data represent meta-stable liquid water ? gas equilibrium. Therefore, few predictive methods for the water content of gases in equilibrium with gas hydrate/ice have been recommended in the literature as these methods are generally based on experimental data. The main aim of this work is to present useful remarks to reduce experimental information required to determine water content of gas in equilibrium with condensate, gas hydrate or ice. A thermodynamic model based on uniformity of fugacity of each component throughout all the phases is employed to model the liquid water ? gas equilibrium. The Valderrama modification of the Patel - Teja equation of state with the non density dependent mixing rules is used for calculating fugacities of components in fluid phases. The binary interaction parameters are tuned using a Simplex algorithm. The model is extended to methane (the major component of natural gases) + water system. It is shown that using only experimental data on solubility of gas in liquid water for tuning thermodynamic model can lead to acceptable predictions of water content of gas phase. Predictive tools based on equality of water fugacity in equilibrium phases for estimating the water content of gas in equilibrium with gas hydrate or ice and relating it to the corresponding water content in equilibrium with meta-stable liquid water are then presented. The capabilities of these tools are investigated for estimating the water content of methane in equilibrium with gas hydrate or ice by comparing their predictions with some experimental data reported in the literature. It is shown that the results are in relatively acceptable agreement demonstrating the capability of the methods reported in this work for estimating the water content of gas being in equilibrium with gas hydrate/ ice.
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