Round Trip Houston to Japan, Beijing Layover Transit Question

I booked a round trip tour from Houston to Japan but I have 2 layovers in Beijing, do I qualify for the 72 hour free transit separately both ways or only once? If I exit the first time, will I be able to return home or do I have to apply for a regular visa because I have already entered the country once? Does the Visa permit start when my flight arrives or if I exit the airport? Will the permit end if I return to the airport.

This is my itinerary
Houston to Japan with 9 hour Beijing layover.
I will return home 10 days later:
Japan to Houston with a 7 hour Beijing layover.