A simple model for sizing hollow fiber membrane contactors in degassing applications, relationship between mass transfer efficiency, liquid flow rate, membrane area and pressure drop; part I: liquid-in-lumen configuration

• Mass transfer is governed by diffusion in the liquid. • Recovery rate and degassing efficiency are related, but not always identical. • Degassing efficiency depends on the ratio of liquid flow to membrane area. • Fiber diameter distribution decreases the degassing efficiency. • The smaller the fiber diameter, the shorter the fiber length to limit pressure drop. Liquid degassing in hollow fiber membrane contactors (HFMC) with lumen-side liquid flow and parallel gas flow is modelled by a one-dimensional approach. Due to the low solubility of gases, the dominant resistance to mass transfer is located in the liquid phase. The latter is assumed to be either in an ideal plug flow, or with a deviation from the ideal plug flow, linked to a Gaussian distribution of fiber diameters. Depending on the operational mode, vacuum or sweep gas stripping, the gas phase composition is considered to be uniform or spatially distributed. A mass transfer efficiency is defined. It estimates the approach to equilibrium at the liquid outlet and is generally not equal to the recovery rate of the target species. For a given species and a given internal fiber diameter, the mass transfer efficiency depends solely on the ratio between the liquid flow rate and the internal membrane area. For instance, for oxygen vacuum stripping using fibers with an internal diameter of 140 µm, an efficiency of 0.98 is attained for a liquid flow rate to membrane area ratio of 1.4 10 -5 m 3 L s −1 m 2 M int. The liquid side frictional pressure drop is predicted using Hagen-Poiseuille’s law. The fiber length leading to a pressure drop of 0.1 bar in the scenario mentioned above is 0.12 m. When scaling up the degassing operation, a large number of parallel fibers is required. The modelling approach was validated using experimental data reported in the literature on the stripping of oxygen and methane using commercial modules.