Can we do 72 hour free transit twice within 3 weeks?

We will be flying from Edmonton, Canada to Colombo, Sri Lanka in January 2018. We have a 21 hour layover in Shanghai on the way to Sri Lanka and an 18 hour layover in Shanghai on the way back home. Are we able to leave the airport to sightsee during both layovers? Or is there a limit to how many times you can do the 72 hour free transit in a certain time frame? Thanks!