More than just kitty litter
Soda lime is a Carbon Dioxide absorbent used for removal of CO2 mainly from breathable gases, in medical, military, safety and recreational applications. Each of these applications has differing parameters for use and therefore slightly different product make-ups. For example, medical gas is moisturized to a degree to aid in patient comfort, therefore soda lime used in anesthesia machines has a low moisture content. Additionally, the granules are lower on a hardness scale. Alternately, the granules of soda lime used in diving have a higher moisture content and hardness. The moisture content is important as a catalyst in the process.
Soda lime itself is not a catalyst; it is an absorbent which converts the carbon dioxide from a gas to a stable solid which is retained within the absorbent canister. It is possible to predict the capacity of a scrubber canister and therefore the life expectancy for a given set of conditions. Variations in the conditions can significantly affect the predicted life expectancy. Flow rate, CO2 concentration, temperature, pressure, humidity etc. are all examples of conditions that affect the life expectancy of soda lime.
Soda lime is contained in a scrubber canister, appropriately sized for the application. The gas is passed through the scrubber and the CO2 is removed by a base catalyzed chemical reaction, converting the CO2 to Calcium Carbonate. Once the soda lime is consumed, CO2 breakthrough will occur and the CO2 level in the exiting gas stream begins to increase.
Particle size and shape are particularly important in terms of the performance of scrubber material in its intended application. Most soda lime grades are now produced with shape that offers a high surface to volume ratio. These carefully engineered shapes ensure maximum CO2 penetration into the particles, by minimizing the distance to the centre of the particle, thereby increasing the CO2 capacity of the product. The hardness of the material is optimized to offer low-dust content, even after shipping and travel.
Another key factor in performance is the particle size distribution. Particle size has an effect on the speed of reaction, which in turn affects the volume of the reaction zone and the capacity of the scrubber. In general terms, the smaller the particle size, the faster the reaction and the smaller the reaction zone volume. However, the smaller the particle size, the greater the pressure-drop across the scrubber, which may affect work of breathing and other characteristics.
--Jill Heinerth
