Two separate flights, 72 hours transit?

I will fly from Frankfurt to Shanghai, stay in Shanghai for a week with a valid visa (flight one, part one). Then, I will fly from Shanghai to Japan, stay there for a week and then back to Shanghai (flight 2). My flight back to Frankfurt (flight one, part two) is 24 hours later (I arrive from Sendai in Shanghai Sunday evening and will leave to Frankfurt Monday afternoon. For this second stay in Shanghai, can I use the 72 hours transit? I assume that my tourist L from my first stay will not be valid anymore, will it?