This is another Wrong Chemical - Wrong Tank incident in which Anhydrous Ammonia (NH3) was off-loaded to Propane Storage Tanks.  Two 30,000-gallon propane tanks, each at 70% capacity, were additionally filled to 85% with NH3. Similar plumbing on BOTH transport and storage tanks made the transfer possible, as both propane tanks shared common piping, had galvanized fittings, and were clearly labeled as “PROPANE”.

Chemical and Physical Properties

Propane and NH3 are immiscible liquids (liquids that don’t dissolve in each other). Although small amounts of NH3 will dissolve in large amounts of propane, and visa versa, a layering of liquids will occur if large quantities are involved. NH3 being heavier will form a liquid layer on the bottom with propane above.The vapor pressure of propane and ammonia in a closed container is NOT the average of, but the SUM of the partial pressures of each. Table 1 shows the relationship between temperature and vapor pressure of these two compounds in a closed container.

TABLE 1. Temperature versus vapor pressure of a large quantity MIX of NH3 and propane.

Temperature (ºF) Vapor Pressure (psi)
-30 5
-20 14
-10 26
0 39
10 56
20 75
30 97
40 122
50 152
60 186
70 224
80 268
90 317
100 371


Propane pressure relief valves will open when internal pressure reaches 250 psi. This corresponds to a propane/NH3 temperature of around 70ºF. Propane alone will pressurize to 250 psi at a temperature near 120ºF.

Reactivity
NH3 is corrosive to copper and galvanized fittings. Such fittings in contact with NH3 liquid or vapor will corrode and eventually fail, releasing NH3 and propane as vapor and liquid into the environment. Chemical reactions between NH3 and propane are not known to occur.

 

CLICK HERE for the Minnesota Department of Agriculture Fact Sheet (PDF: 47 KB / 2 pages)

 
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