Scrubber set-up (what chemicals needed/retention time)

[janecho]

Dear All,

Thanks for the reply.

I am using nitric acid for copper leaching.
2 NO3- + 4H+ + Cu :arrow: 2NO2 + H20 + Cu2+

Lou, I am not sure what is the concentration of the fumes. I am using 62% of nitric acid for the leaching process. How to calculate the concentration NO2 will be produce?

4metal, thanks for the calculation. I just confuse that 200ft3 is referring to packing volume or the scrubber column volume? If different type of packing being use, can help on reduce the packing height? By using the retention time of 8s, the packing height is calculate base on the empty surface area of the scrubber column. 1500CFM/7.5 =200 cu.ft. Dia= 1.5m, A = 19sq ft. So, the height = 3.2m. Since 3.2mHt is calculate base on the surface area without packing, It is means that NO2 need 3.2mHt to transfer along the scrubber when the column is without packing? If i am using the packing element as mention previously, the surface area will be increase, as well as the retention time. Therefore, the packing height will be much more lower that what have calculate? Am i correct? I am still confusing about it.

Can i calculate the actual packing height needed base on the packing factor or packing surface area of the packing element I use?

Thanks.

Your help is much appreciated

 

[butcher]

In concentrated nitric acid NO2 (nitrogen dioxide is formed) (red brown gas), this gas escaping is wasted nitric that could do more work if it was still in solution.
Cu + 4HNO3 --> Cu(NO3)2 + 2NO2(g) + 3H2O

(If a medium concentrated acid is used both NO and NO2 gas can be formed with the reaction.

In dilute nitric acid (formula for 3M HNO3), NO gas forms (nitrogen oxide gas) from the reaction.
this wastes less nitric acid and can put more copper into solution for the amount of acid used.
3Cu + 8HNO3 --> 3Cu(NO3)2 + 2NO(g) + 4H2O

NO in air (or with O2) forms NO2 gas:
2NO + O2 --> NO2
NO gas will not dissolve in water easily and can be hard to scrub, but if mixed with oxygen can form NO2 which is fairly easy to dissolve in water.

NO2 gas dissolved in water forms a dilute nitric acid
3NO2 + H2O --> 2HNO3 + NO

Hydrogen peroxide in the water can help to convert NO to NO2 in solution helping this reaction of dissolving the NOx gas into solution.

Hydrogen peroxide in the original reaction of nitric acid and copper (with water involved) can also help to keep these gases in solution where more copper can be dissolved.

(Actually there are several NOx gases formed in these reactions but for simplicity in discussion we just mention these).

These gases can be routed from the reaction vessel (distilled off), and bubbled into a small amount of water with H2O2 added, in this first receiver (chilled) vessel nitric acid will form, from this vessel the gases that escape go to a second receiver to try and capture more NOx (that escaped through the first receiver) this gas bubbled again through water and H2O2, the remaining gases are bubbled through a caustic solution like KOH, NaOH where any un-reacted gas can form KNO3 Or NaNO3 (which can be used later to make HNO3), this last receiver scrubbing the NOx before going to the fume hood or a small spray type scrubber system, before mixing with the fume hoods air and going through the fume hood exhaust.

 

[janecho]

1500 CFM will require a packed column of 200 cubic feet. In your example with a 1.5 meter diameter packed tower is about 18.6 cubic feet per foot of height. So a 10.75 foot packed section will handle the 1500 CFM of NOx. I would suggest adding 2 packed sections of equal height with a second spray head to assure complete reaction. I like to redistribute the fume every 1 to 1.5 column diameters of packing.

This is a pretty big scrubber, I've done bigger but for pretty big refiners. You can reduce the size considerably by using sealed reactors. What type of refining are you doing?

i) What you means by "adding 2 packed sections of equal height with a second spray head"? It is means that I need to have 2 unit of scrubber column of 1.5mDia x 3.3mHt in order to have complete reaction?

ii) What do you means by 1 to 1.5 column diameter? Can you explain in detail? Can you show me with some example?

Please kindly help.

Thanks.

 

[4metals]

1 to 1.5 column diameters in your case is 1.5 meters to 2.25 meters. So you should have a break because you need a packed section about 3.3 meters deep. The cubic foot area that is critical is the volume of the packing not the entire scrubber volume. We want the fume to be in the working area of the scrubber for 8 seconds and where the fume hits the chemistry is the working area.

Fumes tend to "channel" or develop flow patterns through packing material the rule of thumb is that whatever your diameter is of the scrubber, that same height in packing will effectively transfer fumes with minimal channeling. Depending on the packing type, some recommend 1 times the diameter and some up to 1.5 If your packed bed is deeper than that, you run the risk of channeling. For the packed depth you need you will exceed the rule of thumb. To be on the safe side, I would break the packed beds in half and have a foot of open space. In the open space I would run a second liquid distribution section. A packing support, redistribution manifold will aid in fume redistribution. (Google scrubber internals) The liquid (Caustic) would be pumped into the packing from the top of the packing and in the split halfway up the packing where there is an air break.

When a packed bed starts to channel, some areas will dry out from a lack of water flowing over them because the liquid flowing down can channel as easily as the air and fume flowing up. I have seen severe channeling where the caustic rich liquid (which has a high total dissolved solids) dries out and leaves a deposit of dry caustic which grows in the packing to the point the thing plugs solid.

It is easier to build it right than to fix it so I always follow the engineering principles. They have a nasty habit of always failing on the coldest of winter nights when you take shortcuts.

 

[the iron dwarf]

One of the best scrubbers I’ve seen to date is one comprising of a 6inch ID PVC pipe filled with cheap garden hose, that stuff you use for garden watering system, cut into 1 inch piece at a 45 degree angle. The stripping media (Alkali for acid) is fed through the top and off gas is fed through the bottom. The high surface area of the stripping medium exposed to the gas is enormous. Regulation of the stripping medium feed rate is vital, to much and you will stall the off gas feed, blowing nasty’s back at you.

This is the design I intend to build for by fume cupboard. Simple set up utilizing bits and pieces around the garage and based around commercially design systems. I’m a very visual learner, so those like me here it is, for those that aren’t I’m sorry.


A. Flow control valve
B. Stripping medium pump
C. From fume cupboard
D. Stripping column
E. Stripper medium input stripper
F. Sucker, blower backwards
G. Cleanly stripped gases
H. Stripping medium (mild alkali)

No need for filters and any goodies lost in the gas flow will be trapped in the stripping solution.

Retention time is governed by height and diameter of the column, flow rate of both mediums, and packing size. Too big reduces total surface area though increase flow rate(less friction), too small and gas flow will struggle. Sorry I should be able to do the maths, have done in the past, been a while, I find it easier using trial and error.


Deano

the above looks easy enough to make
I see that for NO you need to add an oxidizer first so 2 columns may be needed, first with H2O2 and NaOH in the second or is it possible to do it other ways, was thinking of using nitric to dissolve silver ( with some base metals probably mostly copper ) on a very small scale but the red gas is a problem
would I be making NO or NO2?
should I look to the 3 bottles I also seen in this thread?