Christian Missions in Persia. Christianity was introduced in Persia in the Parthian period, and several bishoprics were established there (Latourette, II, pp. 263ff.).
You will likely pay more for the exchange, even if your tank is completely empty. An empty 20 lb tank will hold about 4.7 gallons of propane. That is $18.47 at $3.93 per gallon, which is the low quantity price (I just paid $3.44 per gallon when filling several tanks). The exchange is about the same price. Keep in mind this comparison is as if the exchange gives you a FULL tank.
This way you can just buy the 15lb propane tank for around $16 ($1.07/lb) versus buying a 1lb tank at $3+. This is the video I watched on how to do this and the process that I used to fill these tanks: http://youtu.be/fD1CmorB_qM.