Close session on window refresh or tab close

I have a button that asks server that performs some heavy logic ( takes around 15min), but while vaadin is waiting for the answer, if i open new tab and try to go to the vaadin app, it does not load, it loads only after the first tab acquires its response from the request that took 15 min for the server to calculate. My question is how can i terminate the session of the window on refresh or close, or make vaadin accept more then 1 session)?