Can I enter 2 times China with the L type ?

I am doing a semester abroad and my friends from Germany want to come and visit me.
For Christmas and new Year, we want to go to the beach in Thailand and then come back to visit China.
On there way to Thailand they have a 13 Hours stay in Beijing so now we are wondering, if the Visa already starts at the moment when they enter the country or can they use the 72 hours free transit time. Because they will need an entry permit for the 10 days stay after Thailand.