Heat transfer due to condensate dripping through an immiscible organic liquid has been modeled. This study was prompted by safety concerns about a potential runaway reaction between nitric acid and tributyl phosphate (TBP), which forms an immiscible organic layer above aqueous nitric acid solutions. TBP is used as a complexant to separate actinide elements dissolved in nitric acid. The reaction between nitric acid and TBP is highly exothermic and generates potentially explosive product gases. At elevated temperatures, this reaction undergoes a thermal excursion if there is not enough residual aqueous solution dissolved in the TBP to moderate the heat of reaction by its evaporation. Thus, the temperature, the pressure, and the degree of mixing of aqueous components in the TBP layer determine whether or not a given two-layer nitric acid-TBP mixture can be safely stored. Various mechanisms for heating the TBP layer have been considered. One of these mechanisms is steam condensate dripping onto and percolating through the TBP layer. With steam condensate heating, the nitric acid-TBP reaction, if it occurs, would proceed most rapidly at the top of the TBP layer. The bubbling reaction zone would not necessarily extend down to the TBP-nitric acid interface, so the acid layer might not replenish the water lost by evaporation. To address this concern, an experiment was conducted to study how steam condensate mixes with the TBP layer when steam passes over a TBP-nitric acid mixture.
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