01 volvo air system

I think it is a dedicated purge tank for regerating your air dryer. On my 2005 my wet actually has two sections, so I don't see a separated purge tank like yours. But my purge tank is built into my wet tank with separated section.

 

yes, it is purge tank. After 2002 they removed it from design, becouse they use dryers now, that use wet tank air to purge driyer (valve opens for some seconds to let air back to driyer)

 

On my 2005 wet tank actually has two separated sections. One section, may be a gallon size is for purge volume. The wet tank two drain valves also. One for purge tank in the front and the other one for regualr wet tank draining you pull the wire to drain from the back. On my tank I use wabco system saver 1200p, which is designed for this kind of air system.