Using the developed gas-liquid mass transfer experiment, rates of gas evolution were measured in a reference methane-dodecane hydrocarbon system. The saturation pressure of the experiment was varied from 500 to 1,500 psia (3.45 to 10.3 MPa), while the mixing speed was varied from 100 to 250 rpm. The maximum mixing speed was limited to values that maintained a flat gas-liquid interfacial area, allowing the area available for mass transfer to be quantified for all trial conditions.
In order to ensure that the measured gas evolution rates were not significantly effected by bubble nucleation, both the rates of absorption and desorption were mea- sured for each trial condition. Within the measurement error, the absorption and desorption mass transfer coefficients were found to be symmetric. The symmetry between the two mass transfer coefficients confirms that bubble nucleation was not significantly affecting the measured gas evolution rates. All measured absorption and desorption mass transfer coefficients were within 17% of one another for the same trial conditions. The mixing speed was found to be the most significant variable affecting the rate of mass transfer while the saturation pressure within the range tested here had minimal effect.
In an attempt to generalize the mass transfer results beyond the stirred tank experimental setup, theoretically derived mass transfer expressions were evaluated for their ability to quantify the data. The surface renewal theory in the form of the small eddy model was found to be a good fit to the experimentally measured data. The solid and fluid surface eddy cell models were applied to the experimental conditions and resulted in a reasonable fit for both cases. The solid surface model was found to better fit the experimental results, yielding an averaged absolute error of 12.3%.
degassing model allows both the gas and liquid phases to be conserved. Using the solid surface mass transfer model validated for the reference hydrocarbon system at pressure, the degassing model was able to calculate gas carry-under due to entrained gas bubbles as well as excess solution gas in the liquid.
A horizontal separator’s ability to remove entrained gas was found to be driven primarily by the liquid viscosity as well as the overall size of the initial bubble dis- tribution. The liquid density, as well as the bubble density, within the liquid surface area were found to have little effect on the rate of entrained gas removal. From these results, new guidelines were developed in Table8.4based on the liquid residence times required to reach 1% gas volume remaining for a given bubble distribution mode and liquid viscosity. For gas-liquid separation involving supersaturated solutions, the re- sults highlight the need for large numbers of small bubbles required for adequate excess solution separation. These conditions are, however, in direct opposition to the conditions desired for good entrained gas separation. If an inlet stream contains a large amount of excess solution gas, it may be desirable to divert the stream to a gas-liquid contactor to remove the excess solution gas prior to sending the stream into a traditional gas-liquid separator.
CHAPTER X
RECOMMENDATIONS
10.1 Mass Transfer Experiment
The focus of the experimental work presented here was largely centered around validating the developed mass transfer experiment using a simple hydrocarbon sys- tems. Gas evolution should be further studied using gases and liquids that more closely approach real production systems. The saturation pressures investigated here did not have a strong effect on the rate of mass transfer. As the total pressure in- creases, the solute concentration within the solvent would begin to increase to the point were the physical properties of the system are significantly effected. Future gas evolution experiments should be conducted at higher saturation pressures to investi- gate if this trends continues to hold true.
The small eddy mass transfer model should also be tested against different combinations of gases and liquids. The model validation, however, requires knowl- edge of the solute diffusivity, liquid density, and liquid viscosity at the experimental pressure and solute concentration. The advantage of using a simple reference system is the availability of physical properties in the open literature. As different gas-liquid systems are tested, experimental capabilities should also be upgraded to measure the required physical properties at the conditions of interest. Once this mass trans- fer model is more thoroughly validated with well quantified systems, different crude oils can be explored where correlations are used to estimate the required physical properties.
transfer holds equally for the bulk gas-liquid interface as well as mass transfer oc- curring at the bubble interface within the liquid. Future studies should attempt to confirm whether the small eddy model holds true equally at the bulk and bubble interface. One of the drawbacks of the small eddy model framework is that interfacial forces are not considered. The presence of surface active components such as as- phaltenes could impact the rate of gas-liquid mass transfer. The effect of surfactants on gas evolution should also be studied going forward.