A Catalytic Combustion System Coupled with Adsorbents for Air Clean Up in Sealed Spacecraft Environment 2003-01-2624
Catalytic combustion coupled with activated carbon and molecular sieve adsorbents is applicable to all areas of air and gas clean up ranging from high to low levels of pollutants and trace contaminants control in a spacecraft environment is of no exception. In this study we propose a combined activated charcoal and catalytic combustion system based on a 70 watt power input achieving 350°C, operating on a 6 hour per 24 hour day catalytic cycle with an actual flow of 10.6 l min-1 in a residual free volume of 60 m3. The catalysts are optimally sized for keeping the contaminants within a specified limits whilst paying particular attention to the following design parameters in descending order of priority power consumption, catalyst mass and catalyst volume
We consider 2 conceptual catalytic systems namely:
A combined charcoal pre-filters and a catalyst operating at 350°C for 6 hours per earth day using an allocated power of 70 watts.
A single catalytic system operating at 350°C for 6 hours per 24 hour day.
The catalysts are precious metals on single or mixed base oxides to give the sustained low temperature performance. One of the catalyst is an extruded monolith of 200 cells per square inch and the other is a washcoated honeycomb substrate (cordierite) of 600 cpsi. The ceramic monolith has some significant advantages over the granular type catalyst namely
Low pressure drop
Large geometric area
Low coefficient of expansion
Good thermal shock resistance
Good vibration and crushing resistance
The two recommended catalysts of different formulations are:
Pt /Pd on extruded 200 cpsi titania monolith
Pt/Pd on tin IV oxide washcoated on 600 cpsi cordierite monolith
The Pt/TiO2 is a high destruction catalyst operating at high GHSV at low light off temperature. Our results show that it completely oxidises 300 ppm CH4 at 21000 h-1 in the temperature range 180-280°C. It oxidises 0.3% chloromethane (3000 ppmv) at GHSV of 21000 h-1 and 500°C. The combustion of halocarbons requires a post-charcoal filter to remove the haloacids formed. The versatility of Pt/TiO2 is shown in its ability to oxidise completely an aliphatic alcohol, an unsaturated aromatic, an aliphatic amine, and a straight chain aliphatic paraffin at 60000 h-1 in the temperature range 150-350°C with an inlet concentration of 500 ppmv.
The Pt/Pd on Sn(IV)O2 is an ambient temperature catalyst and will oxidise H2 and CO at room temperature. Our results show that this catalyst oxidises 300 ppmv CO saturated with water vapour at an inlet temperature of 24°C and operating at 72000 h-1 with a residual of 8 ppmv CO. Our study is based on an assumed list of likely contaminants, their generation rate, and a maximum allowable concentration, thence we recommend catalyst size and operating conditions based on experimental catalysts performance.