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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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Stacks are used to reduce the ground level concentration of a pollutant by
emitting the process gas at great heights. The ground level concentration depends on
the pollutant emitted, the gas temperature and velocity of emission, the actual stack
height, wind velocity, the terrain and the atmospheric conditions. At low wind
velocities, the rise of the gas due to its temperature is likely to carry the gas to great
heights. At high wind velocities, turbulence quickly disperses the gas. Thus
at some moderate wind speed the highest ground level concentrations are likely to occur. To achieve the requirements of gas dispersion stacks come in a variety of
types and materials of construction. The stacks can be either self-supporting or supported with the aid of steel
frames or guy wires. Self-supporting stacks are the simplest to deal with from the
point of view of plant layout. They will occupy the minimum plot area and can be
placed virtually anywhere on the site. Stacks designed to be supported by steel
structures or guy wires are more difficult to locate because of the greater area required
for the supporting structure and the interference presented by the guy wires.
However, foundation area for tall self-supporting stacks ca be large because of the
turning moment on the foundation from wind loads. Where the stack height is not excessive, the stack can be fitted on top of
the absorber tower to take advantage of the already high elevation of the top of the
absorber tower.
There is a definite distinction between stack opacity and its
appearance. Opacity is considered by the EPA and others to be the 'degree to which
emissions reduce the transmission of light and obscure the view of an object in the
background'. The stack appearance is dependent on the amount of light scattered by
the plume from the sun and its surroundings toward the viewer's eye relative to the
luminance of the background and the opacity of the plume. The opacity of a plume is an intrinsic physical property of the
particulate matter being emitted. The opacity can be measured by transissometer,
lidar, etc. or by trained observers. The EPA has developed Reference Method 9 for
determining the opacity of the plume by trained observers. Visual observation of the plume depends on a number of factors such as
location of the observer relative to the sun, environmental lighting and background
contrast. Factors such as geographical location, time of day and other illumination
variables do not significantly affect the ability of trained observers to accurately
evaluate plume opacities. There is often a requirement to design a plant to meet an effluent
which complies with both a weight limitation and a stack opacity requirement. The
process weight limitation is easy to design for since it is dependent on the design of the
absorption tower and mist eliminator. Designing to meet the opacity requirement is
more difficult because it relies more on experience to determine the relationship between
stack opacity and particulate concentration. A relationship between opacity and
particulate concentration was developed by Ensor and Pilat but relies on actual data to
accurately determine the value of a proportionality constant. The working equation
is as follows: W
= - [K p ln(I/Io)] / L where
The threshold of a visible stack corresponds to an opacity of
approximately 5%. The corresponding mist concentration is approximately 80 ppm SO3
or 0.087 grains H2SO4/(actual)
ft³. These values should only be used as a guideline as actual values may differ. The sample points for determination of plant emissions (SO2
and acid mist) are generally located on the stack. The sample point must be located
a minimum of 2 stack diameters from the top of the stack and a minimum of 8 stack
diameters from the stack breach. These criteria ensure that the gas flow past the
sample points is uniform. A minimum of two (2) sample nozzles are required and
should be located at 90° to each other. The nozzles should be at least 4"
diameter (full bore) and blinded when not in use. A means for the lifting of equipment up to the sample platform must be
provided. As well, an electrical power supply should also be provided at the sample
platform for use during stack sampling. The most common problem encountered with stacks is the emission of sulphate
deposits. During a prolonged stoppage the final inspection and cleaning of the stack
base should be left as late as practically possible before starting up. Some plant operators have found that cleaning the stack can be achieved by
introducing steam into the base during a plant stoppage. The steam is left on for
several hours and as it condenses it washes the sulphate off the walls. Some plants wash their stacks on a regular basis every 6 months.
Washing the stack will tend to shorten the life of the stack. Liquid will be present in the stack due to condensation of acid from the
process gas or from precipitation. The base of the stack must be equipped with a
sloped bottom and a low point drain to remove the liquid. Sulphate will also form in
the stack which will be present as a sludge at the bottom of the stack. An
inadequately sized drain will plug resulting in the accumulation of liquid at the bottom
of the stack. Provision must be made to adequately drain the stack and unplug the
drain line if required.
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
