An evaluation of polydimethylsiloxane hollow fibre gas separation membranes for carbon dioxide removal

With the introduction of mixed manning to the United Kingdom’s (UK) Royal Navy (RN) submarine fleet, QinetiQ was tasked to create a low carbon dioxide (CO2) refuge (the Protected Atmosphere Ventilated Environment (PAVE)) into which a pregnant submariner could retreat to avoid the elevated background CO2 concentration on-board. Once fitted with a QinetiQ designed and built PAVE Scrubber Assembly (PSA) the atmosphere in the PAVE could be maintained below 0.5 % CO2. However, the ExtendAir® LiOH Powercubes that are currently used in the PSA present a significant logistical burden (even unused Powercubes require replacing every 5-8 years). Additionally, operation of the PSA is labour intensive and the Powercubes represent a chemical hazard and occupy valuable storage space. In order to overcome these issues, QinetiQ was tasked to investigate the use of polydimethylsiloxane (PDMS) hollow fibre gas separation membranes as an alternative CO2 removal technique. A PSA based on membrane technology would have a number of advantages, including no consumables, a reduced storage volume, a reduced burden on the ships company and no hazardous chemicals. This work evaluated Permselect® PDMSXA 2.1 gas separation modules. Testing at an inlet CO2 concentration of 0.7 %, six membrane modules in series reduced the outlet concentration to below 0.2 %. This outlet concentration was determined, by computer modelling, to be the maximum permissible to maintain the equilibrium concentration of an occupied PAVE at <0.5 % CO2. Computer modelling using the experimental results indicated that 114 PDMSXA-2.1 modules would be needed to replicate the existing PSA. This prompted QinetiQ to recommend new Permselect® PDMSXA-25 modules for further investigation. Each of these modules would be equivalent to approximately 12 of the current PDMSXA-2.1 modules.