If I am flying from Seoul to Beijing and back to Seoul?

No, you won’t be eligible for the 72 hour free transit because your layover in Beijing won’t be a transit if you travel like that. In principle, you should fly to a third country or region after transiting in Beijing.
If you can travel like Seoul->Beijing->HK->Seoul, or Seoul->HK->Beijing->Seoul, and transit in Beijing for 72 hour, you can enjoy the 72 hour free transit.