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Contact Section - Stacks
- Troubleshooting
January 18, 2003
Introduction
The appearance of an acid plant stack is an indication to the general
public of how well a plant is being operated. The expectation is that the stack
should be clear without any visible emissions. Anything being emitted from a stack
automatically, rightly or wrongly, gives the perception that a pollutant is being emitted
to the environment.
Therefore, the appearance of the acid plant stack is one of the key
factors in maintaining good public relations. The following is a general guide to
troubleshooting stack problems. As you can see, the cause of a poor stack may be
some problem in the operation of the plant well upstream in the process.
Moisture in SO2
Gas or Air
Poor drying
poor drying caused by a number of problems (ie. low acid flow, acid
temperature, acid concentration, etc.) can result in water in the gas leaving the drying
tower.
measure moisture content in gas check operation of dry tower.
Moist air leakage
Poor acid distribution
poor acid distribution caused by dirty distributor tubes or troughs,
improperly levelled troughs, plugged orifices, etc. will result in poor drying tower
performance.
Insufficient acid circulation
low acid flows can result in poor dry tower performance.
check acid flows, pump performance, temperature rise across the tower.
Channelling in tower
Splash or spray from distributor
Failure or plugging of entrainment separators
stick test in the duct leaving the dry tower will indicate if there is
excessive spray from the dry tower. A 2 minute stick test should be clean and dry.
inspection of the mesh pad or candles will determine if the is pluggage, gas
bypassing, damage to the fibre beds, etc. Replacement of the elements may be
necessary.
Flooding of tower
Mist Formed at
Absorber Inlet
Cooling in SO3 cooler or economizer too great,
too fast or localized
typically, a gas temperature of 232°C to 246°C (450°F to 475°F) entering
the tower is considered to be the recommended maximum, above which problems are likely to
occur.
sudden shock cooling of the SO3 entering the
absorber can result in generation of enormous amounts of mist. This becomes
drastically more pronounced as the difference between gas and acid temperatures at the
bottom of the tower is increased.
the appearance of drip acid in the economizer or SO3
cooler is a sign of cooling the gas below its dew point.
cooling of the duct going to the absorber tower may be the cause of a poor
stack. If the appearance of the stack is worse during rainstorms or sudden changes
in temperature and wind velocity then top shielding of the duct or insulation of the duct
may be required.
Absorber
Tower Operating Conditions
Acid temperature too high or too low
low acid temperatures has more of an effect on stack appearance then high
acid temperatures.
acid inlet temperatures can range from 50°C to 90°C. The optimum
temperature must be found by operating experience.
changing acid temperatures in a deliberate and controlled manner can be done
to try to improve the stack appearance. Sufficient time must be allowed for the
system to settle out before the next change.
Acid strength too high or too low
the optimum acid concentration must be found by operating experience.
changing acid concentrations in a deliberate and controlled manner can be
done to try to improve the stack appearance. Sufficient time must be allowed for the
system to settle out before the next change.
Insufficient acid flow
Packing settled
Oleum Tower Operation
Mixing of hot bypass gas and cold exit gas
mixing of the cold gas leaving an oleum tower and the hot gas bypassing the
tower will result in the formation of submicron particles which may overload mist
eliminators in the absorber tower.
avoid bypasses whenever possible.
tight shutoff dampers are required on bypasses. Blanking the bypass
temporarily will help to determine if this is the cause of the poor stack.
in a sidestream oleum tower application a mixed gas temperature of
approximately 150 to 160°C going to the absorber tower usually results in a better
stack appearance.
Sulphur Burning Plants
Steam or water leaks
steam or water may enter the sulphur system from a number of different
locations: sulphur line, sulphur gun, boiler, superheaters, economizers, etc.
shutting off steam to sulphur lines or sulphur gun will clear the stack
rapidly if the cause is a steam leak but care should be taken to avoid solidifying sulphur
in the lines.
steam equipment leaks are evident from and increase in drainings from SO3
coolers or economizers or decrease in strong acid dilution water requirements.
hydrostatic test on steam equipment may be required to locate leaks.
Oxides of nitrogen in gas
high sulphur furnace operating temperatures results in the formation of
oxides of nitrogen.
oxides of nitrogen will form sulphuric acid mist in the equipment between the
converter and the absorbing tower.
If drainings from SO3 coolers or economizers
are diluted and brown fumes are produced, than there is a considerable amount of niter
present.
reducing furnace operating temperature (ie. reducing gas strength) or
reducing air preheat will help to reduce the amount of oxides of nitrogen produced.
localized hot spots in the furnace may also be a source of oxides of
nitrogen. Improving the sulphur spray distribution or burning pattern will help.
Sulphur quality
Contaminants in sulphur such as acidity, hydrocarbons and nitrogen compounds
may be the cause of a poor stack. Analysis of the sulphur is required.
hydrocarbons may be reduced by filtering the acid.
acidity in the sulphur can be reduced by neutralizing with lime but
subsequent filtration of the sulphur is required.
Metallurgical Plants
Mist leaving ESP's
Niter in the gas
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