pH control in ammonia scrubbers 3

[abutt116]

We are venting our excess ammonia from urea plant into two scrubbers those are not packed bed. We are using nitric acid to neutralize ammonia and in the end giving ammonium nitrate to UAN plant. We are facing two problems. (1)pH control is very eratic. Unable to control pH at 6 or 7. (2)Even at pH of 2 or 3 still getting ammonia emissions from the stacks.Are there any simple solutions to these problems?

Thanks.

 

[m777182]

1. Reaction among NH3 (weak base) and HNO3(strong acid) results in a solution that is a buffer with its maximum buffer capacity in acidic side of pH range.The buffer capacity is concentration dependant and not symetric around your operating point (pH=6 or pH=7). Therefore it is not a simple control problem; you need a more sophisticated contoller, perhaps adaptive or cascade type.The solution towards the alkaline side from the maximum of the buffer capacity curve you can consider as a mixture of 1:1 (on molar base) NH3NO3 + solution of dissolved NH3 in water.If your controller shifts pH towards alkaline, some of NH3 will leave the solution as a gas. The action is not immediate while contolled by hydrodynamics of the reactor.These effects causes what you call eratic contol action.
A solution to this problem seems to me a precipitation of NH3 in form of some salt and removal from the system.
2.Without a packed bed your scrubber actually works more or less as a homogeniously mixed reactor.In such a reactor there is always a part of not reacted input flow in the outflow.Try to rearange it as a pure counterflow reactor.

 

[abutt116]

Thanks for very knowledgeable reply.