Absorption and desorption mass transfer rates as a function of pressure and mixing in a simple hydrocarbon system

Determining the time required for degassing within a gas-liquid separator is usually accomplished by assuming thermodynamic equilibrium between the two phases and calculating the time required for entrained gas bubbles to rise out of the liquid. However, if the gas-liquid separation is being performed at elevated pressures, the inlet multiphase fluid may not be at thermodynamic equilibrium, thus the chance for solution gas to evolve out of the liquid must also be accounted for. There is, however, a paucity of mass transfer data in hydrocarbon systems available at elevated pressures. For the current experimental study, the rates of absorption and desorption of methane in a n-dodecane solution were measured in a batch stirred tank system at pressures ranging from 3.45 to 10.34 MPa and mixing speeds ranging from 100 to 250 rpm. All absorption and desorption experiments were performed using the same physical and hydrodynamic conditions. Additionally, care was taken in ensuring that the gas-liquid interface during mass transfer remained flat, allowing for the explicit calculation of the liquid-side mass transfer coefficient. Within the experimental variability, the rates of absorption and desorption were found to be the same in most cases. The mixing speed was found to be the main variable affecting the rate of mass transfer, while the saturation pressure was found to have little effect. The process was well described by the theoretically derived solid surface eddy cell model presented by Lamont and Scott, resulting in an average absolute error of 12.3% between the predicted and measured mass transfer coefficients. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.