Membrane contactor for subsea natural gas dehydration: Model development and sensitivity study

A mathematical model of a membrane contactor using triethylene glycol (TEG) for dehydration of natural gas at subsea operation conditions has been developed. The goal for the membrane contactor is to dehydrate the natural gas to transport pipeline specifications, which are −19 °C at 69 barg. The membrane contactor model is based on a hollow fiber configuration with the gas in the shell side and the liquid absorbent inside the fiber. The shell side gas is modelled as one-dimensional on the assumption of turbulent flow, while the fiber/lumen side is modelled as two-dimensional flow due to the laminar flow. Orthogonal collocation is applied to solve the two-point boundary value problem. The developed model is validated against proprietary high pressure experimental data for dehydration of natural gas with TEG in a membrane contactor. The H2O molar flow removed from the gas phase is predicted by the model with a mean absolute error range of 3-7% compared to the experimental results, based on two methods of calculation due to uncertainty in experimental basis. A sensitivity study was performed to evaluate the effect of membrane properties and operation conditions on the dehydration performance. The main findings are that preferred membrane properties from a separation performance point of view are thin membrane, small fiber diameter, long membrane module, high porosity and high permeability. But, the production of the membrane, stability and long term operation also need to be considered in the selection of membrane material and module parameters. For high pressure subsea operation it is found that the use of a dense layer on the top of the porous support could be favourable to prevent wetting of the membrane. It is found that a wetting of 1% already provides a significant drop in separation performance. For the operation conditions the main findings are that high pressure and low temperature are favourable for the separation. Increasing the gas and liquid flow gives increased flux over the membrane, but also results in increased pressure drop in the module. However, as the membrane contactor will be placed in a system with regeneration of the solvent, the operation conditions should be optimized considering the whole system.