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.
Some people have noticed they get more propane in the tank when they refill them rather than when using an exchange tank. Some note that it seems to be a policy to fill only 15 pounds in a 20-pound tank. Therefore, you get more propane if you fill it yourself to the 20-pound limit, which still allows for a 20 percent vapor area for safety.