You can exchange them at the main depot in Amposta.  Let me know from which direction you would approach the town and I can give you directions.

When you get to the rbt by the crem, take the first right off the rbt (almost back on yourself and to the right of the furniture store).  The depot is a few hundred metres further on the right.  I think they open more or less normal local shop hours.  You can also get them at the petrol station in Santa Barbara which is open all day.  Same price.

You should by rights have a gas certificate here.
Usually they do not ask for this, nor any questions asked when you go to exchange bottles.
Do you know anyone here, that can look out for, and buy a second hand, empty bottle, for you, beforehand.
Then it should be easy to just exchange it for a full one.
There is a petrol station, that supplies, along the dual carriageway, between the big roundabout, and Macdonalds,
near to where you will be staying.

Hi Patang,
I'm a bit confused as to where you will be living.  Is it Tortosa or Masdenverge.  Email me if you want to chat about gas bottles etc.