A membrane process for separating CO 2 from diving atmospheres

Abstract

Computer simulations have shown that a membrane separation process can be used to remove CO 2 from exhaled air in an underwater breathing apparatus (UBA). Such a process could offer important advantages by comparison with the CO 2 removal technique used at present, which utilizes the chemical reaction of CO 2 with Ca(OH) 2 or LiOH. The advantages of the permeation process are (a) a substantial reduction in the size and weight of the CO 2 removal equipment, (b) increased safety, and (c) no limitations in diving time related to the equipment. The membrane process will use asymmetric or “composite” hollow fiber membranes made of either ethyl cellulose or an acrylonitrile/butadiene block copolymer in order to reduce the CO 2 concentration in the exhaled air from 4 mole % to 1 mole %. The simulations indicate that for a flow rate of exhaled air in the UBA of about 60 L(STP)/min, corresponding to a medium diver work load, the volume of a permeator module for CO 2 removal will be only about 10% of that of the Ca(OH) 2 or LiOH canisters used at present. The CO 2 removed during the process will be dissolved in sea water. A small loss of O 2 will also occur during the process; this loss will be on the order of 17% of the O 2 present in the exhaled air. The O 2 loss could be reduced by using membranes with higher CO 2/O 2 selectivity.