CFD Simulation of Net Spacers in Membrane Modules for Carbon Dioxide Capture 2

Carbon dioxide (CO2) emissions from coal-fired power plants may lead to significant global climate change if not controlled. CO2 capture and sequestration (CCS) is one of many solutions being considered to mitigate emissions.

Membrane based processes potentially are a cost-effective option for CO2 capture. Spacers are a critical component of the flat sheet membrane modules being considered. A variety of forms exist, but net-type spacers consisting of two layers of cylindrical filaments are used commonly. Spacers create and maintain uniform flow channels during module fabrication and operation for the permeate and feed flows. Spacers also may be used to mix fluid within the channels to eliminate concentration polarization and increase mass transfer rates. Unfortunately, spacers increase pressure drop. This increase is strongly dependent on spacer geometry and is especially important in the membrane modules used for CO2 capture. Process economics require the pressure difference that drives transport across the membrane to be as small as possible due to flue gas compression costs. Therefore, the pressure drop that accompanies flow through a spacer filled channel will reduce this driving force and represents an additional parasitic energy load on the plant.

Computational fluid dynamics (CFD) is used to investigate the flow within a spacer-filled channel. While extensive simulations have been reported for nearly incompressible liquids, very little work is available for gas flows. Three dimensional simulations were performed for typical process conditions. The results are used to visualize flow within the channel and determine the relationship between pressure drop and flow rate expressed as the dependence of friction factor on Reynolds number. The simulation results are in good agreement with experimental measurements for nitrogen flows and help validate the use of CFD.

Given the good agreement between experiment and theory for one spacer geometry, simulations were performed to investigate the effect of spacer geometric parameters on velocity field and associated pressure drop. The parameters examined include spacer filament diameter, inter-filament spacing, and angle between filaments, and filament orientation to the flow. Additionally, the relative contributions of viscous and form drag to the overall flow resistance are determined as well as the uniformity of the velocity field in the flow channel.

The implications of these results on the selection of a spacer to use in membrane modules for CO2 capture are discussed. Of particular importance is how spacer design can affect overall module dimensions and concentration polarization in the module.

Key words: Computational fluid dynamics; Net-type spacer; membrane module; CO2 capture