From Space Shuttle Reference Manual (available at NASA website),
The cabin air from the cabin fan is ducted to the two lithium hydroxide canisters, where carbon dioxide is removed and activated charcoal removes odors and trace contaminants. An orifice in the duct directs a specific amount of cabin air through each lithium hydroxide canister. The canisters are also located under the middeck floor. They are changed alternately every 12 hours through an access door in the floor. For a flight crew of seven, the lithium hydroxide canisters are changed alternately every 11 hours. Replacement canisters are stored under the middeck floor between the cabin heat exchanger and water tanks.
