Contact Section - Stacks - Troubleshooting
January 18, 2003

Introduction Moisture in SO2 Gas or Air         Poor drying         Moist air leakage         Poor acid distribution         Insufficient acid circulation         Channelling in tower         Splash or spray from distributor         Failure or plugging of entrainment separators         Flooding of tower Mist Formed at Absorber Inlet         Cooling in SO3 cooler or economizer too great, too fast or localized Absorber Tower Operating Conditions         Acid temperature too high or too low         Acid strength too high or too low         Insufficient acid flow Oleum Tower Operation         Mixing of hot bypass gas and cold exit gas Sulphur Burning Plants        Steam or water leaks         Oxides of nitrogen in gas         Sulphur quality Metallurgical Plants         Mist leaving ESP's         Niter in the gas

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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

• moist air can be drawn into the system when the blower is located downstream of The dry tower (ie. 'sucker' arrangement).

• air leaks at the base of the stack can lead to poor stack appearance.

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

• sulphate formation and collection in the packing can cause acid and gas channelling.  High pressure drops across the tower as well as visual inspection are indicators of this problem.  Washing the tower may be necessary to clear tower.

Splash or spray from distributor

• check operation of distributor (trough or pipe) and eliminate source of splashing or spraying.

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

• flooding in the body of the packing or at the distributor level will cause problems with the operation of the tower.  High pressure drops across the tower as well as visual inspection are indicators of this problem.

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

• low acid flows can result in poor drying and absorption leading poor stack appearance.

Packing settled

• overtime packing in a tower may shift and settle resulting in channelling of gas and acid which will lead to poor tower performance.  This is generally one the last things to check once all other possible causes have been eliminated.

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

• gas leaving the ESP's should be optically clear

Niter in the gas

• arcing in the ESP's forms ozone and oxides of nitrogen. 

• arcing may be caused by broken or dirty wires.  Increasing the wash time and frequency may help.