5 replies to this topic

[chemks2012]

All,

Once again with a query with the anticipation of comments/input from members!

Acid tanks containing 40% w/w nitric acid [HNO3] at 85 deg C are used for various metal finishing operations. Assume tanks are open in a plant building [building volume of 4000 m3]
and estimate the concentration in the plant building if scrubber does not exist.

1) If I change the number of tanks, of course this will change the number of moles of HNO3 and water in the liquid phase and hence the total number of moles [all highlighted in red text below] in liquid phase. However this does not affect the results i.e. concentration of vapour/gas in the building. Is this correct or am I missing anything here?

2) Data received from Perry's handbook [see attached file] , Table 2.16 states that Partial Pressure of HNO3 and H2O over Aqueous Solutions - And as per my understanding, this is the partial vapour pressure - please correct me if I am wrong.

Thanks
KS

Attached Files

•  Calculation - Rev.0.xlsx   1.71MB   65 downloads

Edited by chemks2012, 17 January 2013 - 06:17 AM.

[chemks2012]

any help on this please?

Thanks

[kkala]

Notes (probably useful) on post No 1 are as follows.
Q. no 2: Agreed that Perry's Table presents partial vapor pressures of HNO3 and H2O over nitric acid solutions.
Q. no 1: Assuming only equilibrium over HNO3 solution (and whole internal space "filled" with HNO3 vapors at equilibrium), HNO3 concentration in the building does not depend on the number of tanks or the storage volume.
In Calculation - Rev.0.xlsx, partial pressure over solution of 40% HNO3 is 0.00911 bara for HNO3 (not 0.0015 bara) and 0.3573 bara for H2O (not 0.3001 bara); for air 0.64684 bara. Water vapor pressure of 0.3573 bara is judged too much for whole building (air dew point would be 73 oC). But I do not have expertise to suggest proper concentration values of HNO3, H2O in the building.
Another issue is that HNO3 slightly decomposes http://en.wikipedia....iki/Nitric_acid '> http://en.wikipedia....iki/Nitric_acid , which may add NOx into the space, deserving further examination.
If H2 can remain in the HNO3 solution because of previous operations on "metal finishing", area hazardous classification has to be tried.
Regulations on buildings and their ventilation covering such nitric acid tanks for "metal finishing" could address queries as above, if there are.

[chemks2012]

Hi kkala



Thank you very much for your help.

However, I am not entirely clear what do you mean by ‘partial pressure over solution of 40% HNO3 is 0.00911 bara for HNO3 (not 0.0015 bara) and 0.3573 bara for H2O (not 0.3001 bara); for air 0.64684 bara as it contradicts with your agreement that Perry’s table shows partial vapour pressure. Please clarify.


Also, I believe that the number of storage tanks should affect in a way that surface area of liquid will be increased if we have more number of tanks. Any comment on this?


Your comments and/or revised calculation will be helpful.



Thanks.

KS

Edited by chemks2012, 23 January 2013 - 06:40 AM.

[kkala]

1. Let us look at Perry's table of nitric acid data. For 85 oC and 40% HNO3, equilibrium vapor pressure of HNO3 is 6.83 mm Hg = 0.00911 bara and of H2O 268 mm Hg = 0.3573 bara. I think these are the pressures in equilibrium over the acid aqueous solution, not their product by the corresponding molar fraction in liquid. Total pressure over the liquid is atmospheric, sum of partial vapor pressures of HNO3 + H2O as above, plus of air.
2. Surface area of aqueous HNO3 will normally increase by adding more tanks in the room. I think this will increase non equilibrium emissions due to slow decomposition of HNO3. On the other hand HNO3 equilibrium concentration over the liquid surface will not change.
Hope it clarifies the points.

[Bee5]

There are some nice calculations on equilibrium concentrations in 'enclosed' spaces described in: R.J. Watts, Hazardous Wastes: Sources, Pathways, Receptors. Look on pages 314 to 316 in Chapter 6. If you cannot get hold of the book, I could type out a few of the equations for you.