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?

 

[25362]

To 6748, if the formed acids (~20 kg/ton sulfur) were, for example, of polythionic (catenated sulfur) character, the moisture in sulfur as given, ~0.07%, would suffice stoichiometrically to form them.

Polythionic acids have the following formula:

HO[sub]3[/sub]SS[sub]n[/sub]SO[sub]3[/sub]H​

However, these acids are considered unstable, decomposing into S, SO[sub]2[/sub], and sometimes H[sub]2[/sub]SO[sub]4[/sub]. Please comment on your findings for us to learn from your experience.

 

[6748]

Hello 25362,
we are draining this acid from the economiser, gas cooler etc. But we have never seen the concentration of acid to be above 80% or so. It is a slightly ash colored free flowing liquid. We checked the acid for corrosion by dropping MS plates. No severe corrosion was noticed. We have sent the acid in MS tankers over a long distance -- (Spent acid). No noticeable corrosion at the tanker or at the purchasers end.
So we, till now presume that the acid is above 97% concentration. The fine ash mixed with the acid is adding weight to the acid, thereby yielding less concentration in our Lab analysis.
If it is an unstable polythionic acid, some yellow sulfur has to come out on exposure to air or if stored in open tank below the drain valve,
No SO2 smell, No sulfur precipitation. Still the puzzle is ON.
There could be some plants existing that do make SO2 gas or liquid by bringing down the furnace 14 % or less % SO2 gas to below 100 DEg C. Have they experienced condensation?

 

[ApC2Kp]

To 6748,
This thread seems to have run for some time, and yet the drip acid formation is continuing, so I will put my 2 cents in also for what it is worth. Your SO2 generator is different from the large 1,000 - 3,000 ton per day sulfuric acid plants, but there are similar points to consider. Most of those larger acid plants do daily drain a gallon or more drip acid at economizer. The smaller size of your SO2 lines probably have more "cold" surfaces such as support attachment points, and economizer casing surfaces where there will be condensation occurring. The inlet BFW temperature to economizer is probably less than 100 deg C, and some condensation would be expected there on tubes even with 200 deg C gas temperatures. The 35 deg C cooled SO2 would be expected to generate more drip acid.
That leaves the question of source of hydrogen or water to combine with the SO2. I would suggest checking three places. The alumina dessicant air dryer could be functioning well for most of the time, but does it have a purge or regenerating cycle? There could be intermittent high moisture levels in the air stream that would not be detected during spot tests. Another source of moisture has been mentioned as the sulfur gun. The air atomized spray nozzle is different from most larger sulfur guns. I wonder if there is a leak between the steam jacket and the air to the nozzle? I would like to see how well the steam and air sections are kept separate at the nozzle end of the sulfur gun. The temperature at the end of the sulfur gun could approach the decomposition temperature of water, and a leak could then occur as hydrogen and oxygen. The steam flow through the sulfur gun is actually cooling the sulfur gun during normal operation, and internal baffles in the sulfur gun must ensure cooling flow at the hot tip and nozzle of the sulfur gun. The steam outlet should be exhausted to atmosphere to keep steam pressure below the sulfur burner air pressure so any steam leaks would not go into SO2. The steam jacket pressure through out the molten sulfur system (jacketed piping and valves, sulfur filter jacket) should be lower pressure than the molten sulfur to ensure any leaks do not have moisture and steam entering the SO2 gas stream.
One more source of air into the process would be other air purge points such as the sulfur burner sight glass ports or at valve stem packing purges. If the connected air lines are not to the same dry air supply, but instead ordinary plant air, then that would be the entry point for moisture into your process.
The air compressor (6 psi) wouldn't be passing lube oil into air stream ? It would be difficult to imagine much of any lube oil passing through the alumina dessicant. Yet even compressed air systems with good filter / drier will end up with water and oil at the low point drains.

 

[6748]

Dear ApC2kp
01. Yes, the economizer does have cold surface. The Boiler feed water at the inlet of the Economiser is at around 90 Deg C. We do collect liquid acid from the economizer.

02. Source of Hydrogen: Intermittent high and low moisture level in the inlet to the dessicant is very very rare. Because this incoming air is coming out of freon chiller coils working at say +0 Deg C. The air coming to the dessicant bed attains a temp of +5 Deg C. Sometimes it may go to + 10 Deg C Max. All the physical water that gets condensed from the air is drained out/bleeded out thru drain valves.
03. The sulfur gun has jacketed steam passage for making sure that molten sulfur temp will not raise causing high viscosity. The steam jacket prevents the radiation heat from raising the temp of sulfur. Only the sulfur pipe is jacketed. The burner tip is cooled by the compressed air. The steam will not reach the burner tip.
04. The steam jacket pressure cannot be lower than the liquid sulfur. Liquid sulfur is pumped into the furnace volume which may be at 0.4 Kg/cm2g pressure. Whereas steam in the jacket has to be at at least 2 Kg/cm2 to ensure that molten sulfur temp does not go below 120 Deg C. This line is hardly 5 metres in length.
05. The steam traps of the pipe jackets and sulfur gun jacket are letting out condensate into ambient.
06. The sight glass ports are not cooled by air.
07. The compressor for atomizing air takes air from the combustion air that comes out after the dessicant. So no water or oil carryover.
This is not a sulfuric acid plant. The desired product from this unit is COLD sulfurdioxide gas.

6748

 

[ApC2Kp]

To 6748,
Yes, your SO2 process is somewhat different from a sulfuric acid plant. Most of your process conditions have similar parameters in the sulfur handling side of the process. There are some things that I have some doubts, as to whether the operation of the alumina dessicant beds is actually up to the required performance but that is not my area of expertise. I went back over your description (Feb 6) of your sulfur handling process. It has one thing missing from typical sulfur melter systems.
"The sulfur having rain water etc and atmospheric moisture is melted in melting pits to 120 Deg C."
Most users have their sulfur pile sitting outdoors exposed to rain and sand storms. The sand and dirt cause other problems for acid plants that would not be a problem for your SO2 plant. The water in the granular sulfur leads to the formation of acid that causes corrosion in equipment. While you believe that all the water is removed in the melters, that would eliminate water going into the process. That may or may not be totally true, but there still remains the presence of acid. Most melter systems include a lime dosing system to neutralize the acid. Does your sulfur system have lime addition for neutralizing the acids (into sulfates)? Otherwise the acids would only go downstream, condensing at the cold end of your process.
Some users have their sulfur pile in a wood or aluminum warehouse to keep it from rain. One user with a dust problem from his open sulfur pile thought it was OK to spray water on the pile and conveyor to hold down the dust. The lime dosing system was overwhelmed, and there was severe corrosion of his sulfur melting pit covers. There was also severe corrosion at the gas-gas heat exchangers from the drip acid.

 

[25362]

To 6748, it appears that ApC2Kp has got it right: the acid forms in the first pit exposed to the elements, and does not appear in the as bought sulfur analysed by a third party. Have you tried to run a similar check on the sulfur entering the reaction ?

 

[scorcher]

6748,
I know of a similar plant which generates SO2 (and acid as a byproduct). From talking to people at the plant I have learned 2 things.
1. They use liquid sulfur, stored above 135C but still "expect" to have some moisture in their sulphur.(the tank is open vented, I am not sure if they actually run a purchase spec on water content.
2. They too have had problems with acid in their SO2 stream (typically 7% v/v). However in this case, the SO2 stream from the burner was stripped of SO3 in a 98% acid tower. This acid carried over to some extent following corrosion of the tower. On tower replacement, which included a demister pad, the problem was dramatically reduced,however minor acids carry-over in the down stream SO2 is not a major issue on this plant, and they vave always collected small volumes of acid in the SO" lined which they assumed was minor carry-over.

How do you remove SO3 from your stream?,
Is the 14% w/w or v/v?
Is your SO2 compressed and stored or fed directally to a downstream process?

 

[scorcher]

6748,
I know of a similar plant which generates SO2 (and acid as a byproduct). From talking to people at the plant I have learned 2 things.
1. They use liquid sulfur, stored above 135C but still "expect" to have some moisture in their sulphur.(the tank is open vented, I am not sure if they actually run a purchase spec on water content.
2. They too have had problems with acid in their SO2 stream (typically 7% v/v). However in this case, the SO2 stream from the burner was stripped of SO3 in a 98% acid tower. This acid carried over to some extent following corrosion of the tower. On tower replacement, which included a demister pad, the problem was dramatically reduced,however minor acids carry-over in the down stream SO2 is not a major issue on this plant, and they vave always collected small volumes of acid in the SO2 lined which they assumed was minor carry-over.

How do you remove SO3 from your stream?,
Is the 14% w/w or v/v?
Is your SO2 compressed and stored or fed directally to a downstream process?
Does the acid cause problems or is it justan inconvenience?

 

[6748]

Scorcher,
01. Sulfur purchase specs do not have water content as a clause. Water usually gets into the system during storage and handling.
02. You have stated that “””still expect to have some moisture in the sulfur “””” .Do they expect because of their past/present experience? OR do they have any technical inputs for their expectation?
03. The SO3 is removed by acid wash. Acid mist is removed by mist eliminator that is installed at the top of the acid tower.
04. My gas composition is normally 12% v/v. But the system is designed for 14% v/v.
05. We are not compressing the gas. The 12 to 14% SO2 is directly used in the downstream process.
06. Acid formation is an inconvenience, because the pipeline, economizer body etc upto the acid tower is getting punctured now and then. Then it becomes a problem that can stop the complete plant at anytime of the day. Everytime you stop the plant, you are bringing in new cold spots in those pipeline etc that will fail the next day.

Right now we are draining acid from wherever it is formed. We conveniently bypassed the investigation by telling ourselves that our dew point measurement of the combustion air may not be accurate. We have declared “TRUCE” without knowing who the enemy is.
6748

 

[ApC2Kp]

To 6748,
There may only be a "TRUCE" in the war on corrosion. One has to live with a number of annoying intrusions on life. However, several items could influence the generation of acid in your SO2 gas process and reduce the volume of acid. The 80% acid concentration is like that of the old lead chamber acid manufacturing process, but there may be only few who know enough about it to relate to your process. The operating temperature of a sulfur burner could influence the proportion of SO3 /SO2 gases. A burner could be operating at the lower end temperatures and result in more than 3% SO3, or much less SO3 at the upper end of temperature range but generating more NOx. Also, now that it is understood that there is an acid absorbing tower in the process, then commonly known parameters about absorbing tower performance could also be addressed. The temperature in an absorbing tower will optimize absorbtion, or will result in carryover of acid fumes, or will generate sub-micron mist by shock cooling that will be difficult to remove even by fiber bed mist eliminators. (A mesh pad has practically no capability to remove 1-2 micron sized mist particles.) Some one knowledgable of mist eliminators and absorption towers (acid plants) could be of benefit to you.

 

[lairliq]

To 6748,

Have you thought about natural gas? Do you have a pilot on the burner? Is the alumina regenerated by natural gas combustion?

 

[6748]

To lairliq,

Let me place on record that natural gas is not used in any of the operations in this process. Sulfur + Oxygen in the air gives out SO2. There is no pilot flame/burner.
The alumina is regenerated by ambient air that is indirectly heated first by steam and subsequently by electrical heaters upto a temp of 150 Deg C.

All the three issues of your query: a)Natural gas burning for generating 10 to 14% SO2: b) Using a pilot burner for burning Sulfur with air: c) Alumina regeneration with natural gas combustion---> Are totally new to me.

6748

 

[JStephen]

Three thoughts:
You state that the air is "dry" based on the dewpoint, but even air at that dewpoint will have SOME water vapor. Have you calculated what the amount of water vapor actually is? IE, it may be small, but then again, the amount of acid seems to be small.

You state that the molten sulfur can't contain water. I am reminded of the carbon dioxide in water in soda pop. The CO2 isn't liquid at room temperatures, and you wouldn't expect to find it mixed with water, yet it does dissolve in the water. Could something similar happen with water vapor and sulfur?

Lastly, I have found, that when looking for something, when you exhaust all the places where you think it is, then you have to start looking in places where you think it isn't. While you may be sure no water is in the sulfur, no water is in the air, there are no leaks, etc., it is obvious that hydrogen has to come from somewhere, and therefore one of your observations has to be wrong.

 

[hacksaw]

if 80% acid is being collected, then a water leak is being dealt with. it is that simple...