2 replies to this topic

[Francisco Angel]

Dear all:

I'm working at an explosive manufacturing plant, the spent nitric acid coming from the nitration operation, has dissolved gases (NO2), which is evidenced by its brown colour, this causes problems at the nitric acid distillation tower.

I was given the task of proposing a solution. Was I had researched until now is:

 

*In nitric acid manufacture, the acid at the outlet of the absorption tower (the tower absorbs NOx gases into water to produce nitric acid) has the same problem. Solutions are: heating the acid, at a temperature sufficient to  promote gas desorption; contacting with air inside bubbling or packing towers to strip the dissolved NOx gases. See for example patents: US2142646, US 4062928.

 

*Decomposition of nitrous acid: 3HNO2<->HNO3+2NO+H2O. See for example: "Role of nitrous acid decomposition in absorber and bleacher in nitric acid plant" (Ingale et al), "Accurate rate constants for decomposition of aqueous nitrous acid" (Rayson et al).

 

*Decomposition of nitric acid: 4HNO3<->4NO2+2H2O+O2. See for example: "Kinetics of the thermal decomposition in the liquid phase" (Robertson et al).

 

*Equilibrium pressures of the system NO2-H2O-HNO3, see for example "Phase relations in the nitric acid nitrogen dioxide water system at physicochemical equilibrium" (Webster et al).

 

My problems are:
*Patents treat the topic very lightly, just in a few lines. Does someone know about parameters, procedures, references for size/design of NOx degassing/nitric acid bleaching operations?

*I have problems putting together papers information. I have some familiarity with nitric acid absorption operations modelling, and is generally assumed that species (NO2,N2O3,N2O4) experience heterogeneous reactions to give HNO3 and HNO2 in solution, also that NO has a low solubility. How to consider "dissolved gases" then?. For example the model presented by Ingale is an adaptation of the typical absorber model, which doesnt account for "dissolved gases".

 

Any help will be greatly appreciated.

 

Best regards.

 

 

 

 

[Dazzler]

One initial avenue of investigation should be what was the plant original design (find the plant design specifications and manuals), particularly looking for what flow and quality and conditions of spent NA is expected to be produced for a given plant capacity, and what are the same parameters for the feed of this stream to the distillation section.  This might show up that your stream quality is running worse than design and you may be able to see for example that you are at higher temperatures or flows than the design allows for.  You could then also look for other factors that might be effecting the quality, such as any makeup or dilution process water quality/contaminants.  Therefore initial investigation this may show up why the problem exists, then there could be operating solutions to prevent it, before having to consider engineering modifications to the plant or process as a mitigation.

 

Dazzler

[Francisco Angel]

Dear Dazzler:
Thanks for your answer. As you said, documentation about the original design is a good starting point. The problem is that here the documentation system is terrible, it also happened to me in other projects that when I ask about documentation my boss are unable to provide any.

The reasons vary, like the plant being too old, or in the case of a new plant section, that the design was done in the central office, and they refuse to share the documents. I had also noticed that in general, the reasons why different plant section are designed in a way and not in another are unknown by the personnel (they only keep doing what was done before).

Being a novice here, I don't have a lot of influence to press for things to change (I don't neglect that maybe another attitude from my part could had given better results).

Long story short, lack of information is one of the reasons why I'm leaving in a couple of weeks, I only want to finish my projects the best I can.

Best regards.