Membrane gas absorption and desorption (MGA/MGD) for the removal of CO2 in manned spacecraft or other enclosed environment is subject of study by Stork and TNO for many years. The system is based on the combination of membrane separation and gas absorption.
Advantage of this technology is that the system not only can be used to remove the carbon dioxide but also to control the relative humidity and temperature. Absorption of moisture and heat is achieved by cooling the absorption liquid below the dewpoint temperature of the gas stream.
From the start in 1995, the Crew Transfer Vehicle is used as a basis for the design (1,2). Compared to the planned air conditioning system, consisting of a condensing heat exchanger, LiOH cartridges and a water evaporator assembly, MGA/MGD shows advantage in volume, mass and power consumption. The absorption liquid circulates through the spacecraft thermal control loop, replacing the coolant water. The CO2 absorption capacity of the absorption liquid is restored in a desorption unit. This process is based on pervaporation. The absorption liquid is led through this membrane unit in which a reduced pressure is maintained using the space vacuum. Due to this pressure difference a driving force for water vapor and CO2 is created. The water evaporation and the CO2 desorption rate are controlled by a throttle valve in the venting duct to the vacuum source. Because the absorption liquid is used as coolant, temperature increases, a driving force for water vapor and CO2 mass transfer is created. Should additional heat rejection be required (off nominal case), it is established by dumping extra water, using the associated heat of vaporization.
For the initial studies an existing laboratory set-up has been applied for the practical work. In 1999 a project started on the development of a dedicated breadboard. In this paper, the test results of this breadboard assembly will be presented.