Instrumentation - Acid Mist/SO₃
November 30, 2023

Introduction

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Introduction

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 (SO₄^2-) 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)

Recent Developments

A recent offering for SO₃ analysis in stack gases comes from Opsis AB.  It is an in-situ, cross-stack analyzer based on Differential Optical Absorption Spectroscopy (DOAS).  SO₂ and SO₃ are components that provide strong signals in the UV (ultraviolet) and IR (infrared) spectral bands of light.  Measurement of SO₂ concentration using UV light is well established and proven.  In the concentration range typical in an acid plant stack, IR is the preferred method.  More details of the instrument are provided in the article titled "The SO₂ & SO₃ Continuous Monitoring Machine" presented in the 2013 Spring/Summer edition of Sulfuric Acid Today.