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Rank: Newbie
Joined: 1/28/2010
Posts: 1
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Can anybody please tell me whether there should be any scrubber placed on the vent line to capture any gases that are coming out of 98% Sulfuric Acid Carbon Steel Storage Tanks. The tank is exposed to ambient conditions and the outside temperature can be as high as 105 F in summer days. The boiling point of 98% sulfuric acid is about 600 F. The gases that would release when concentrated sulfuric acid reacts with metal of storage tank is H2 gas, and is released when acid is first filled in the tank. I was told that as part of the permit the sulfuric acid tanks may require a scrubber to prevent sulfuric acid vapors from releasing to atmosphere. The PEL for sulfuric acid is 1 mg/m3. Under the above circumstances, do we really need a scrubber ? and will there be any additional gases that will be released ? I understand some of these tanks have a dessiccant dryer to prevent any ingress of moisture into the tank.
Please answer. Thank you in advance.
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Rank: Member
Joined: 11/8/2008
Posts: 18
Location: MUMBAI, INDIA
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Vent Scrubber is required for a tank storing 98% Sulphuric Acid. When it is filled, air inside the tank will go out and when it is emptied air will be sucked in at the same rate at which acid is being transferred. When outside air enters, the atmospheric moisture also enters. This will have the effect of dilution in the top layer and corrosion by dilute acid which can result in some H2 formation. If there is a vent scrubber irrigated by 98% sulphuric acid continuously, only dry air will remain inside the tank and no corrosion will take place if only 98% H2SO4 is added or removed from the tank; there will be no corrosion and no H2 generation as steel is suitable at room temperature and up to 50 C for 98% H2SO4. You should get a long life for your 98% H2SO4 tank with a vent scrubber irrigated by 98% H2SO4 supplied by a small pump to be operated 24 hours(from the tank itself ).
V.V.Rao
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Rank: Member
Joined: 10/21/2008
Posts: 13
Location: chesterfield, mo
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rnakka,
A need for acid tank vent scrubber will likely be determined by local statutory laws and rules. If the plant site is distant from major population, then company environmental safety and health regulations might have the only guiding statements on the need for vent scrubber. The vent dessicant dryer is needed for coastal or other ambient high humidity plant locations to prevent internal corrosion of acid storage tank walls.
The steel wall of sulfuric acid storage tank has a layer of iron sulfate that results to passivate the steel surface. The corrosion reaction of iron and acid results in hydrogen gas. The hydrogen gas bubbles float upward and cause grooves from removal of the passivating sulfate film. These grooves can be the failure point of the storage tank after severe grooving occurs in years of service. The hydrogen gas is also a hazard - some repairs attempted on acid storage tanks have had welder's torch ignite the hydrogen thru the tank vent or holes in the tank roof.
Some other gases possibly in the tank vent could be SO3 or SO2 - especially for 93% product acid taken from Dry tower fed with smelter gases. Typically those acid plants would include an SO2 stripper for the 93% product acid stream before going to storage tank.
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Rank: Member
Joined: 11/8/2008
Posts: 18
Location: MUMBAI, INDIA
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I think a vent scrubber irrigated by 98% H2SO4 will keep moisture out of the tank. It is really the moisture which can dilute the acid close to the walls and corrode Iron, forming more of /Iron Sulphate /Iron Oxide, Iron Sulphate and hydrogen.
Conc. H2SO4 attacks Steel very slowly and a layer of stable Iron Oxide is formed to protect the steel.This is what is written in Sulphuric Acid literature. High turbulence can disturb this layer and causes further corrosion,producing more FeO and FeSO4 and Hydrogen. If the acid is not diluted , corrosion of Fe will be minimum as the layer of FeO protects the surface and H+ ions are not produced due to mixing of H2O with Conc. H2SO4.
Whether the authority requires it by law or not, it is certainly a good thing to put a vent scrubber on a Sulphuric Acid storage tank to keep moisture out.
V.V.Rao
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