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.
From October 2012 to March 2013, the average weekly price varied from from $2.195-$2.319, or about $880-$1,050 to fill a 500-gallon tank; between 2005 and 2010, prices averaged $1.73-$2.66 a gallon, or $690-$1,070 to fill a 500-gallon tank. With three to five fill-ups per six-month season, total winter costs could be $2,000-$5,500. Propane is a byproduct of oil and gas production, and in the past propane prices rose and fell in tandem with the cost of crude oil.