How acid is formed in dry 14% SO2 gas line ?

[6748]

Molten Sulfur is burnt with dry ( Dew Point below Minus 70 Deg C ) air to form 14 % SO2 gas in a furnace. When this 1350 to 1400 Deg C gas is cooled to 200 Deg C, liquid acid accumulates in the pipelines, gas coolers etc.
The quantity of acid relased is about 20 kgs per ton of sulfur burnt.
How Hydrogen based acid ( H2SO4) gets condensed with bone dry sulfur + almost bone dry air. From where this Hydrogen comes?

 

[saxon]

6748, What happens when a vapor phase is cooled below it's saturation temp? Right you are! Condensation! Now what happens when this comes into contact with the likes of Sulfur Dioxide? Right you are again, the production of Sulfurous Acid (H2SO3), also know as Sulfur Dioxide solution. To the Environmentalists and the world at large this is the evil substance know as "acid rain".

Just remember, any significantly advanced science/technology is indistingishable from "MAGIC"!

saxon

 

[6748]

*The saturation temp of 14% dry SO2 gas will be even less/lower than Minus 30 or 40 Deg C when the gas is at atmospheric pressure.
*Water vapor picking up SO2 gas to form acid rain is not possible, because the gas is in closed pipeline till it is consumed in the process.
* The question is how acid is formed in a closed system which does not have access to any Hydrogen atom, except the moisture content feasible in a Minus 70 Deg dew point air.

 

[KenA]

6748,
What is purity of sulphur? Does it contain significant levels of H2S? Where does the sulphur come from?

 

[scottengr]

If all you have is dry air (N2, O2, Ar etc) with no appreciable water where does the Hydrogen come from?

Are there sources of leakage into the system that contain hydrogen sources? Perhaps a leaking steam generator or superheater in the flue gas line.

I've seen this type of problem before in a refinery sulfur recovery unit before with a heat recovery system on the flue gas stack.

 

[6748]

The quality of as bought Sulfur, as analysed by an outside testing lab is as below:
Assay as purity(Dry basis) ---> 99.9%
Moisture ---> 0.07%. Ash -- > 0.04%.
Acidity ( H2SO4) ---> 0.002%
Once the sulfur is melted, the residual moisture in molten sulfur will be ZERO.
There is no leakage of water anywhere. The pipeline from the sulfur storage tank to the burner is just 6 mts length. This jacketted line has been checked thoroughly for leakge of steam.
Then the burner. then furnace. then waste heat boiler. The gas is at 250 deg c. And then it starts releasing muddy acid mixed with ash in the pipeline, economiser as it comes down to 200 Deg c.
From where this acidity or Hydrogen atoms come: This is my question.

 

[saxon]

6748, Let me see if I have this correct. You purchase sulfer in bulk then store it. After this is stored,it is then transported to furnaces where it is burned with natural gas/air or someother hydrocarbon to produce SO2 rich gas. This SO2 gas is then cooled, transported to the various units where it is consumed. In the cooling and transport phase the H2SO3 drops out as condensate.

The certified moisture content of the bought sulfur is one thing, what is the moisture content after storage/tansport/ storage? What are the by-products of the combustion of any hydrocarbon gas? H20 and CO2. Are you absolutely positve there are no other sources of water vapor into the system? Where is the hydrogen and water vapor coming from?

saxon

 

[6748]

Saxon,
The solid sulfur is melted and stored in molten sulfur feed tank at 120 Deg C. This sulfur at 120 Deg C cannot hold water. Molten sulfur is burnt with dry air to form SO2.
The only source of water is the moisture that can accompany the Minus 70 Deg c Dew point air.
Still that water in air is very low to cause condensing of SO2 in the form of H2SO3 at 200 Deg c.
Let me rephrase the problem.
How much water can go with the air having Minus 70 Deg C dew point? Let us assume 1000 M3/hr of air flow. At 14% SO2 volume/volume how much SO2 can mix with that water? Presuming that all water that accompanied the air is getting condensed as H2SO3 even at 200 Deg c, let us quantify the acid that can form in the pipeline. Let the quantity of acid be say X kgs.
The acid that gets drained is much much higher than this calculated quantity. ( multiple times X )

 

[hacksaw]

What color is your sulfur?

 

[6748]

Dear Hacksaw,
Are u asking a serious question ?
The sulfur used is from a reputed refinery. It is not a directly mined powdery sulfur.
It is as yellow as any sulfur could be. It is not a powder sulfur. It is granular in shape ( spherical ) having an average diameter 3 to 5 mm.

 

[hacksaw]

It is a serious question.

The color is an indication of purity and indirectly, water content.

Assuming that you are not experiencing equipment problems, I suspect that you've got an incomplete assay.

If it is equipment related, then you've got some testing to do.

Good luck.

 

[hacksaw]

After careful re-reading of your process description, even with chemically pure sulfur you can have acid formation under the conditions you describe.

Unfortunately those conditions are associated with incorrect equipment line-up, you process design, or failure of certain process equipment in your gas train.

It appears that you are dealing with a process design / warranty issue.

I can put u in contact with a fellow who has desifned many such plants and has the resources available to sort out the details, but i'll need your contact details.

 

[pvmohan]

Dear 6748,
I presume it is a sulfuric acid plant.

How sure are you about the moisture content of air from dryer?

Is air supply through a drying tower, using 98 % sulfuric acid for drying the air???

If you are sure about moisture content.... >>> then ... is there acid mist carry over???

Last chance is leak from a boiler or steam superheater... Not the type of of heavy leak but a small tiny pinhole


Last (again assuming sulfuric acid plant) .. why are you cooling SO2 to 200 C. Or is it a mixture of SO2 & SO3 which is being cooled?? in which case it may be SO3 condensed containing dissolved SO2.

Anyway,if you are sure about Hydrogen, source of hydrogen is either acid mist carry-over or a leak from Boiler or superheater or economiser ( I can go on assuming)
Do some moisture tests on the gas leaving each of the equipment... You'll know
By the way. when your problem is solved just say HellO

 

[pvmohan]

Dear 6748,
I forgot one more area:
How about you sulfur gun / nozzle assembly?? Isn't is steam jacketted? you can have a leak there??
Please RUN to the plant & find the real culprit
All the Best

 

[25362]

To 6748,

You assume -apparently off-hand- that molten sulfur at 120 celsius becomes bone dry. I beg to disagree.

The drying mechanism of any molten "wet" liquid is based on the fact that the air on top of the liquid is less humid than the liquid itself and depends on residence time, the surface of the pool exposed to air and the depth of the "drying" sulfur. It might, just might, happen that molten sulfur absorbs water from humid air up to a critical concentration.

In order to be sure that the acid or its precursors are not already present in the sulfur being burnt, lab checking of sulfur samples taken just ahead of the burner might shed some light to your problem.

The amount of water stoichiometrically needed to produce 20 kg of H2SO4 per ton of sulfur burnt is just 3.6 kg or 0.36%.

 

[hacksaw]

25362,

You are right on.

Even slight hydrocaron impurities decompose over time and release water.

The originator of the first question, appears to have dropped out.

His use of the term "bone dry" sulfur does not address chemically bound water and is hardly an true indication of water content as you indicate. Melting the sulfur does not remove the chemically bound water- so it is never truely "dry".

Also suspect that the claim of H2SO4 formation he reported, was mistaken for SO3 being formed and allowed to condense. At 14% gas strength, the equilibrium SO3 production is significant. This can occur without water in the system.

SO3 dropout (condensation) is indicative of equipment malfunction and or mis-operation of the plant. It quickly converts to H2SO4.

Drip acid that results from water leakage into the sysyem is usually an issue downstream.


Interesting issues,

 

[6748]

There is no equipment problem. The sequence is simple and visible.

Atmospheric air is compressed to 6 psi, chilled to Plus 10 Deg c, the condensed water is drained off. This 10 Deg Saturated air is sent to activated alumina dryers to get MINUS 70 Deg C dew point air. This is the air used in furnace for burning sulfur.
The sulfur having rain water etc and atmospheric moisture is melted in melting pits to 120 Deg C. The moisture comes out as steam at the first pit. And then sulfur gets melted. This molten sulfur goes as underpass to the 2nd 3rd pits. After the first pit, the molten sulfur does not get exposed to air. It always has sulfur vapour above the liquid surface. Sulfur at 120 Deg C cannot retain water. The only thing that goes with the sulfur is the ash. No hydrogen sulfide.

The Assay of Sulfur is correct. This has been confirmed by variuos reliable testing agencies. We pay premium price for this Sulfur.

The sulfur gun is a steam jacketted. And is compressed air atomised. This compressed air is sourced from the MINUS 70 Deg C dew point air.
so no water contamination in this atomisation air. Any leakage of steam in the burner will make the flame wobble and the flame color will change.

There is no leakage of steam in the pits, pump, pipeline, gun/burner, waste heat boiler, economiser.

This is not a sulfuric acid plant. There is no acid drying tower. So no acid mist carryover in the air.

There is no process design/warranty issue. It is only a technical puzzle. None of the acid plants could reply. In acid palnt the gas is not allowed to go below 500 Deg C. Hence nobody knows waht will happen when this 14% SO2 is cooled to below 200 Deg C. We are cooling to 35 Deg C.
Yes, SO3 can form. But it will not condense to Liquid at this condition. If you have handled Oleum 65% or 24%, you will know that one drop of liquid SO3 will release at least 1 Cubic metre of white fumes in ambient air. The acid containing 24% SO3 is called fuming sulfuric acid.

The acid that I am getting condensed in the pipelines and the economiser is totaly fumeless. This economiser is not a part of the WH Boiler. It is a stand alone economiser connected to the waste heat boiler with a pipeline.

No maloperation is feasible. This is a very small plant burning 12 Tons of sulfur per day. No superheaters. Just a Waste Heat Boiler generating saturated steam at 10 Kg/Cm2 pressure.

So we are back to same question. From where this Hydrogen Atom comes, when all the inputs are devoid of Hydrogen atoms.

 

[ggordonstewart]

Since you are using air as your oxygen source to combust with sulphur, you need to realize that air contains 5 ppm of hydrogen and 2 ppm methane (From analysis of Handbook of Physics and Chemistry). This is probably enough hydrogen to form the water and hence the acid that is causing the corrosion. G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant

http://www.ggordonstewart.com/

ggstewar@telusplanet.net

 

[hacksaw]

6748,

We appreciate the thouroughness you have given the condensation problem and I agree that it is a remarkably challenging problem.

The traditional view is that if all sources of water have been eliminated, the only other source of moisture is due to the "bituminus" content of the sulfur.

Industrially pure sulfur contains sources of moisture. It is not uncommon to maintain gas temperatures above 150~200 Deg C to avoid condensation.


A dewpoint of 200 Deg C suggest that the water content exceeds 500-1000 ppm.

The bitumin content of sulfur is not removed by melting and the the true water content can only be determined by ignition and collection of the products of combustion.
Heating sulfur to "dryness" normally does not reveal the chemically bound water.

Good luck,

 

[25362]

Eppur si muove... Molten "dry" sulfur can absorb moisture from the atmosphere physically as well as chemically.