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DKL Engineering, Inc.
Handbook of Sulphuric Acid Manufacturing
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Introduction
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Preface
Contents
Instrumentation - SO2
Concentration
August 21, 2001
Introduction Principle of Operation Installation Instruments |
Associated Links |
SO2 concentration
measurements are often done in a number of locations throughout an acid plant.
The most common SO2 measurement is in the stack
allowing the plant operator to stay within emission the permitted limits.
The SO2
concentration can range from 0 to 250 ppm SO2 for
double absorption plants to as high as 0 to 3000 ppm SO2
in single absorption plants.
SO2 measurements may also be done at the inlet of
a gas cleaning system. This measurement tells the acid plant operator the
gas strength that is coming from the metallurgical operation.
Another common location for SO2 measurements is at
the blower discharge in metallurgical or regeneration acid plants. After
gas cleaning, the SO2 concentration is unknown as
well as the O2:SO2
ratio. Air dilution may be required to achieve the desired conversion in
the converter. On-line continuous measurement of SO2
and O2
may be required to control the amount of dilution air being added to the
process.
Ultraviolet Light Absorption - This technique is based on the differential
absorption of UV-visible light by the sample stream at two different
wavelengths. Radiation from a selected light source passes through the
sample and into the photometer, where a semi-transparent mirror splits it into
two beams. The measuring beam passes through an optical filter which
excludes all wavelengths except one absorbed strongly by the sample constituent
under analysis. The reference beam is filtered to a wavelength which the
sample absorbs weakly or not at all. Each beam then falls on a photodiode
which converts the light signal to an electrical current proportional to the
light intensity of the beam.
A logarithmic amplifier, in turn, provides a voltage proportional to the
negative logarithm of the photodiode currents. The differential of the
amplifier outputs provides a single output linearly proportional to the
concentration of the measured constituent in the sample, in accordance with
Beer's law. The split-beam technique is inherently accurate and
interference-free because the wavelengths are examined simultaneously.
Adverse sample conditions, such as particle matter turbidity and fluctuations in
light intensity, produce an equal effect on the intensity of the energy at both
wavelengths and, therefore, cancel out.
An instrument that draws a gas sample from the main process stream is preferred
over an in-line instrument because of the problem of maintaining the instrument.
If the instrument is in-line it often cannot be removed during operation for
maintenance, calibration or repair without a complete plant shutdown.
The experience at one plant with an SO2
analyzer installed at the discharge of the blower was that the vibration from
the blower caused the reflecting mirror at the opposite side of the duct to go
out of alignment such that the instrument failed to operate properly.
Manufacturer | Type | Range | Model No. | Remarks |
Lear Siegler | Optical Spectroscopy (Light Absorption) | SM8100 | Vibration free installation required | |
Rosemount | Ultraviolet Light Absorption | 0-30 ppm Autoranging up to 2000 ppm |
ETL 9100 | Quartz halogen source No moving parts |
Rosemount | UV 'Transflectance' | 0-50 ppm 0-5000 ppm |
Model 890 | Pulsed UV Lamp Operating Temp: 0 to 40°C Power: 350 W |
SICK | Light Absorption | GM 21 | ||
SICK | Light Absorption | GM 30 | ||
AMETEK | Ultraviolet Light Absorption | SO2:
0-100 ppm O2: 2-25% wet |
4600 | Meets all known EPA
requirements Sample Flow: 5 SCF/h Instr. Air: 2 SCFM field unit 2 SCFM probe Electrical: 2 kVa Sample taken to analyzer |
AMETEK | SO2:
0-500 ppm NOx: 0-200 ppm |
PDA-6010 | Instr. Air: 35 SCF/h
(60-100 psig) Electrical : 2 kVa Sample taken to analyzer |