Inherently Safety Quenching of HCN Polymerization with Phosphoric Acid

Hydrogen Cyanide (HCN) product is stored in tanks at many production facilities.  It is capable of self-reaction that could end in explosive polymerization if safeguards are not maintained.  An explosion of a large storage tank could cause damage in a radius of several hundred meters, potentially causing fatalities, equipment damage, and significant restriction to the industry’s right to operate.  One of the last lines of defense against such an event is the addition of acid to “quench” the polymerization reaction.  Many sites that store HCN use sulfuric acid to quench polymerization, but over-addition of this acid has the additional hazard of explosion by a hydrolysis reaction. 

Phosphoric acid is presented as an inherently safer additive for emergency quenching of HCN polymerization.  One of the industry’s objections to using phosphoric acid is that it is believed that it does not mix with HCN as well as sulfuric acid.  This work is intended to determine if phosphoric acid can be practically mixed with HCN to quench a polymerization event. 

Two methods of mixing are investigated.  The first is addition of a small quantity of acid into a tank.  The velocity of the HCN in the tank in this case is very small (on the order of centimeters per second).  The second method is to inject a small flow of acid into the circulating line on the tank.  In this case, the velocities are much higher (on the order of meters per second).

The first part of the work involves modeling to compare mixing of phosphoric and sulfuric acid.  This will provide a fundamental comparison between the two acids, and also between the two mixing methods.  The second part will involve experimental comparisons of mixing of the two acids with acetonitrile.  The use of acetonitrile as a less-toxic substitute for HCN will greatly simplify experimentation while providing a liquid with similar density, viscosity, and chemical properties as HCN.  Both mixing methods will be tested experimentally as well.

Mixing equipment used in this work should be relatively easy to scale up to industrial facilities.  Mixing data provided by this study could be used for the design of new HCN storage tanks or new quenching systems on existing HCN storage tanks.