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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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As in the case of dust
measurement, the concentration of sulfuric acid mist can be detected photometrically by
scattered light measurement. In flue gas
desulphurization and in
the production of sulfuric acid, the catalytic oxidation of SO2
to SO3 is followed by the latter's absorption in water or
dilute acid. If the conditions are less than optimal, sulfuric acid can escape
through the stack in the form of an aerosol mist of finely dispersed droplets.
If this is not detected immediately the result is a visible stack and in the
extreme case, grave environmental pollution can occur. The measuring system used for this
purpose consists of a turbidimeter for detecting the light scattered by the droplets and a
sampling setup with an automatic temperature control system. The latter's job is to
cool the stack gas down to about 120°C. At this temperature the sulfuric acid
condenses, while the water remains in vapor form. This eliminates the problem of
falsification by water droplets, and the reading indicates the sulfuric acid concentration
selectively. In an optically homogeneous medium (with constant refractive
index and absorption), the light progresses in a straight line. Strictly speaking,
this is the case only in a vacuum. Any change in the optical properties will deflect
the light beam from its path. This physical process, which is referred to as the
scattering of light by particles, causes the phenomenon of turbidity. The phenomenon is not limited to
particles visible to the naked eye or in a microscope, however. Even in pure air or
pure water, some scatter occurs at the molecules. Though this molecular scatter is
very small, it can never be neglected entirely: for example, the sky looks blue because
the sunlight is being scattered by the air's molecules. One process that takes place when
light is scattered is diffraction, and another is the excitation of radiation.
Diffraction occurs because of the light's wave character: if a wave passes an obstacle in
the immediate vicinity, it will be deflected from its path. The deflection angle
depends on the relation between the wavelength and the size of the obstacle. The
second process occurs because the atoms are excited, i.e. raised to higher energy levels,
to radiate off the light that has struck them. This light will be radiated in
various directions, depending on the particle characteristics, in accordance with the laws
of light refraction, reflection, and dipole radiation. (A mirror's reflection is a special
case of this.) A practical application of scattered light measurement is
determination of the turbidity value, which provides information on the concentration of
solids in liquid and gaseous media. SO3 Analyzer An SO3 analyzer is offered by Pentol-Enviro AG (Switzerland). The analyzer was previously marketed by Severn Science Limited. The operating principle is that SO3 or H2SO4 in the gas is absorbed as sulphate ions (SO42-) in an aqueous solution of propan-2-ol in water. The solution is passed though a bed of barium chloranilate. The acid chloranilate ions created are measured in a continuous flow photometer. The concentration of acid chloranilate ion measured in the solution is directly proportional to the sulphate ion concentration in the absorbing solution and hence the SO3/H2SO4 concentration in the gas. This will be true as long as there is a constant ratio flow rate for the gas sample and propan-2-ol absorbing solution. The instrument has the following characteristics: Range: 0.05 to 5000 mg/m3 Accuracy: +/- 5% of reading (in calibrated range) Solution Consumption: 0.25 to 2 cm3/h (adjustable as required)
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
