Boiling nitric acid 2

[andmil]

I am designing a 130 litre cylindrical stainless vessel to contain boiling 70% nitric acid (BP=120ºC). The vessel is to be heated using heating pads (as opposed to immersion heaters).
The vent of the tank will be connected to existing scrubbers.
How do I calculate the amount of vapour that will be generated in say: 30 mins, and how do I size the vent line accordingly?
Also, I've been trying to calculate the length of time to heat from 20ºC to 120ºC, by looking at the energy needed to boil (H=mCpdeltaT) and dividing this by the (kW/m2 of the heating mats, multiplied by the external surface area of the reactor to obtain the power input). But I'm having my doubts about whether this is accurate, due to it not taking into account the shape of the vessel.
Thanks in advance

 

[kenvlach]

Is there some method in this? Sounds like an expensive way to get rid of nitric acid.

“ to calculate the length of time to heat from 20ºC to 120ºC” try
(mass_{nitric acid}x Cp_{nitric acid} + mass_stainlessx Cp_stainless)x 100ºC/heat input_{heating mats}

Upon reaching 120ºC, assuming perfect insulation,
boiling rate = [Δ]H_vaporization/heat input.

Of course, some heat will be lost, so heat-up time will be longer & boiling rate will be lower.

Also, insulate the vent line and slope it whichever way you want any condensate to flow. With a long enough cold line draining back to the Stainless vessel, you'd have refluxing.

Here are the links I have for nitric acid:

http://www.efma.org/publications/NitricAcid/Section15.asp

http://www.efma.org/publications/NitricAcid/Index.asp

http://www.cheresources.com/nitricacid.shtml

http://www.charis.wlc.edu/publications/symposium_spring02/hansen_a.pdf

With 70 wt% nitric acid, the phase diagrams indicate that the initial vapor will be richer in nitric acid, so the liquid will reach an azeotropic composition of ~67 wt%.

 

[andmil]

Thanks for the info kenvlach. Just to clarify, the aim is not to get rid of the nitric, but to maintain it at boiling point as it is used in-situ as a cleaning process. Therefore the less vapour generated the better, as lower load on the scrubbers.
Is the method shown above independant of vessel shape e.g,
surely heat-up time to a long thin vessel will be faster than to a "short fat" vessel even if they have the same external surface area? The heat input as given by suppliers is kW/m2.

 

[kenvlach]

Is this similar to a vapor degreasing system, only with nitric acid? I.e., it has a lid & the parts go inside, above the liquid? What is the vessel material (hot nitric is a bit corrosive, especially with iron or chloride cleaned off parts)?

Water cool the vent line & have it drain back into the vessel.

 

[andmil]

Yes, it does have a lid, but the items to be cleaned are immersed in the liquid. The vessel material will be stainless steel. Probably 304. Any ideas as to sizing the vent line?

 

[unclesyd]

This can be a very hazardous undertaking.

What is the soil your are trying to remove?

What is the material that the soil is attached?

Comeback with as much detail your proposed process as you can. Things like the volume of parts plus soil to the volume of acid, how are the parts to be introduced, etc.

Cleaning with HNO3 can be very hazardous especially at boiling and if organics are present.

As stated above there are some serious material considerations if this concept is at all feasible.

 

[Montemayor]

andmil:

UncleSyd is absolutely correct. This could be very hazardous. If you are not an experienced, professional engineer you shouldn't be messing with Nitric Acid.


In the past I have designed and led a project for the vacuum distillation of spent Nitric Acid and I would seriously caution you to heed what I have experienced and not attempt to do what you propose unless you have the credentials and the experience to back you up.

The plain and obvious important fact(s) about Nitric Acid in a distillation process has not been mentioned. It DECOMPOSES when heated - so look out with that "vent".

Nitric acid is subject to a certain degree of self-decomposition, depending on the concentration and temperature. The general rule is: the higher the concentration or the temperature, the faster the decomposition rate. During decomposition nitrogen dioxide and di-nitrogen tetroxide are formed in addition to water and oxygen and they are absorbed in the nitric acid. This leads to an orange-brown coloration and an increase in the di-nitrogen tetroxide concentration. These are nasty, toxic gases and can ruin your day(s) for you.

Forget about worrying over alloys and stainless steel construction. It takes a special grade of stainless to stand up to Nitric Acid. For my project, I required a special mill run – just to obtain the stainless alloy for the distillation column. The composition of the alloy was client’s proprietary knowledge. The real trouble and worry is how to handle and control the decomposition taking place.

Although my column was 13.5 feet in diameter by 50 feet tall with a 48" overhead vapor line, you can have the same difficult and hazardous process problems with your 130 liter vessel. The size of the equipment doesn't change the hazard.

 

[andmil]

Montemayor

Thanks for your input. This is not a first time ever done process, and I am fully aware of the hazards. This cleaning process is currently done in polyprop containers with a lid with nitric heated to 100ºC. The installation of stainless closed vessels is a vast improvement on the current setup. The operators are currently fully suited up and use breathing apparatus. The aim is to reduce the risk to them. I'm aware nitric decomposes to NO2 and N2O4. "The real trouble and worry is how to handle and control the decomposition taking place." How have you combatted these issues in your process?
If we refer back to the original question, I'm still trying to get a handle on how to calculate the required heat load, as the vessel shape impacts the requirement, and in my view, calculating the heat load in kJ is not sufficient. Also, do you have measures in place to reduce decomposition or how do you handle vapour formation. The aim of the process is not the vapourise the nitric, but purely to bring it to the point of incipient boiling.

 

[kenvlach]

The reddish NO_x fumes result from nitric reaction with metals.
The best way to control is to prevent them from ever leaving the liquid. Adding hydrogen peroxide converts them back to nitric acid while still in the liquid. Since hydrogen peroxide decomposes at high temperatures, you may need to monitor & perhaps even run a a continuous trickle feed. This is what the Stainless steel mfrs. do on their pickling lines, beginning in Sweden in the 1980s. See if you can find the article "Improved Technique for control of NO_x gas emissions from pickling of stainless steel' by I. Dalin et al. of Eka Nobel AB (Sweden), first published in Stainless Steel '87, pp. 531-534.

At ambient temperatures, as for stripping nickel plating, the hydrogen peroxide totally eliminates the NO_x fumes.

I agree that 304 may be very unsuitable, especially as the nitric picks creates ferric ions from parts cleaning -- there are ASTM corrosion tests for SS which are less severe!

 

[JStephen]

In regards to the heat load-
In a normal "heat transfer problem", you have the temperatures and wish to find the heat transfer. Here the heat transfer is fixed, to some extent, by the wattage of the heaters, which is why you don't need the shape of the container or any other information in your problem.

What you do not know is the operating temperature of those heaters to generate that heat flow. It could calculate out excessively high, or your heaters may have thermostats that cut off the power at a certain point.

 

[unclesyd]

At your concentration and temperature the reaction should be considered non-controllable other than rapid quenching or dumping.
The 70% put you above the constant boiling concentration of 68% so the MOC should be 304L SS with the Carbon content below 0.020%. We run 65% acid + organics at 115C-120C and have to use Ti as the material of construction.

As to heating once the reaction starts at your acid concentration the reaction will be essentially uncontrollable. The method of control is either water quench or dump it, we do both.

Personally I wouldn't have peroxides around HNO3 at your conditions.

What is the soil you are trying to remove?

 

[andmil]

unclesyd, the process is client-specified, so we have not looked at the nature of the soil. What impact could this have? As I mentioned earlier, this is an existing process that works, only the nitric is currently only heated to 100ºC. The reason for wanting to heat to BP is that the rate of cleaning doubles for every 10ºC rise in temperature.
Our intention is to dump the hot nitric to an intermediate "hot well" to allow it to cool, before either recycling it to the one of the vessels, or dumping to the treatment system. Just to clarify, the nitric will be at
its azeotrope of 68.4% and not 70% as I originally indicated.

 

[unclesyd]

I still think this could be a very hazardous undertaking under certain conditions. If the residue you are trying to remove is organic the reaction could get exponential instead of 10 x 10 as you state. At this concentration and temperature don't ever close this system in.

As you will be boiling the acid the condensed liquid will be extremely corrosive.

Again if the the soil is organic have you considered the use of controlled pyrolysis to remove the material.

http://www.pcpconline.com/pcpconline/index.php

 

[kenvlach]

Re “The reason for wanting to heat to BP is that the rate of cleaning doubles for every 10ºC rise in temperature.”
Any data showing the cleaning process has an activation energy of 86.7 kJ/mol?
The rate is perhaps limited by transport through a surface boundary layer which might decrease with bubbles on the part's surface. On the other hand, the rate of intergranular corrosion of the SS might well increase so with temperature. ASTM A262, Method C is the Boiling Nitric Acid test for IGC of stainless steels. Some differences are that a Pyrex glass vessel is used and high purity nitric acid is necessary (Reagent grade: [Cl^-]_max = 0.00007%, [Fe]_max = 0.0001%, with additional restrictions on fluorine (max 0.0001%) and phosphate (max 0.00002%).

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/A262.htm?L+mystore+sbvt8433

The corrosion rate of the SS will increase with Cl, Fl, most metals, organics....

The book, Materials Selection for the Chemical Process Industries, 2nd Edn., ISBN 1-57698-025-1, has a chapter on nitric acid. 304L stainless (UNS S30403) is frequently used. A nitric acid grade (NAG) with extremely low C, Si, P and S limits gives improved corrosion resistance, especially for the welded condition. Lots more info Here: 'Stainless Steels and Nickel-bearing Alloys in Nitric Acid' available from the Nickel Institute (free registration required):

http://www.nickelinstitute.org/index.cfm/ci_id/10626/la_id/1.htm

 

[BenThayer]

you might be using 304 SS but not for long, i am afraid.

no larger than your vessel is, you might consider zirconium.

boiling nitric acid will go after your heater sources rather quickly.

i have never boiled nitric acid tis true. but we made 100,000+ tons per year and condensed the vast majority of it prior to the absorber. and condensing it is much easier than boiling it.