a chemical laboratory it was found that this dissociation temperature varied widely depending on the location of the ammonium carbonate in the process of fig2 for example the dissociation temperature of ammonium carbonate was affected substantially by the presence or absence of other chemical constituents present at a particular location in the process of fig2 accordingly it was found that the dissociation temperature range to be used in the practice of this invention is from about 120 to about 140 f the precise laboratory dissociation temperature of 136 f notwithstanding if one operated on the premise that ammonium carbonate dissociated essentially at 136 f this invention would not have been discovered for example one skilled in the art would not have expected dissociated ammonium carbonate to be present at the 125 f overhead temperature of tower 1 but it was it was further found that the operating temperature of the heat exchangers for each such tower i e exchangers 6 16 and 35 was such that the formation of ammonium carbonate from ammonia and carbon dioxide was promoted this caused a preferential deposition of ammonium carbonate solely in those heat exchangers and was found to be the proximate cause of the premature corrosion and catastrophic failures those exchangers were from time to time experiencing because towers 1 14 and 32 conventionally operate at a temperature that promotes the dissociation of ammonium carbonate to ammonia and carbon dioxide vapor any and all ammonia and carbon dioxide that reached the process in general and these towers in particular was preferentially forced into overhead streams 4 15 and 33 and therefore into their associated overhead heat exchangers 6 16 and 33 accordingly all fugitive ammonia.