Land in Beijing, departure from Tianjin

Саша, you are definitely allowed to do that. Both Beijing and Tianjin are in the same, so called Beijing transit zone. So, if your itinerary is eligible, for example:

Russia - Beijing - Tianjin - South Korea

You are definitely eligible for 144 hour visa free transit in Beijing transit area. Also, you are allowed to depart from Tianjin by boat too.

Anyway, to be eligible for this visa free transit option, you need a ticket out of Tianjin even before you have departed to Beijing.

72 hour visa free transit has been replaced in Beijing transit area by 144 hour visa free transit a couple of years ago.