Analytical Instrumentation

Industrial Gas analysis

What?
Cont.
 Oxygen (O2): Oxygen is valued, above all, for its reactivity. Oxygen enrichment of air is
used to increase the amount of oxygen available for combustion or biological activity.
Biological activity enhancement includes medical applications and environmental
applications such as industrial waste water and sanitary sewage treatment systems.
 Oxygen’s reactivity is commonly used in metals processing (steel, copper, lead, zinc),
glass furnaces, cement kilns, chemical manufacture, sewage treatment, pulp and paper
manufacture, welding and cutting of metals, and medical oxygen.
 Hydrogen (H2), methane (CH4), and carbon monoxide (CO): Hydrogen is used in
refineries to remove sulfur and to chemically restructure (reform) hydrocarbons. It is
used to hydrogenate unstable, unsaturated hydrocarbons and fatty acids in animal and
vegetable oils. It is also used as a reducing agent in steel and zinc manufacture –
removing oxygen that would react with and degrade the product.
 Methane (CH4): Methane is naturally produced by biological activity. It is the primary
component of “natural gas”, which is used as both a fuel and as a chemical raw
material.
 Carbon monoxide (CO) is co-produced with hydrogen by steam reforming plants using
methane or other hydrocarbons as feedstock. It is a raw material for making monomers
and other chemical products.
 Carbon Dioxide (CO2) does not react with oxygen, but will combine with other
elements and compounds. Thus its commercial uses include raw material for various
chemical processes and as a neutralizing agent for alkaline materials.
Basic concept
 All molecules, with the exception of the noble gases, consist of
several atoms which exhibit a regular three-dimensional structure by
chemical forces and whose valence electrons can attain defined
energy states.
 The molecules are primarily in rotational and translational motions,
relative to their surroundings, thus giving Brownian molecular motion.
 Also, the single atoms within a molecule can vibrate mutually and the
shells of the valence electrons within the molecule can reach
different states of energy.
 Eventually, analysers can be designed, so that the molecules of
measuring gases may be made to give physical or chemical reaction,
which may reveal their nature and extent.
 Three different interactions are generally utilised, which form the
basis of various gas analysers.
Types of Gas analysers
 PARAMAGNETIC OXYGEN ANALYSER
 Oxygen has the property of being paramagnetic in
nature (i.e. it does not have as strong magnetism as
permanent magnets, but at the same time it is
attracted into a magnetic field).
 Nitric oxide and nitrogen dioxide are other two gases
which are paramagnetic in nature.
 Most gases are, however, slightly diamagnetic (i.e.
they are repelled out of a magnetic field).
 The magnetic susceptibility of oxygen can be
regarded as a measure of the tendency of an oxygen
molecule to become temporarily magnetised when
placed in a magnetic field.
 Such magnetisation is analogous to that of a piece of
soft iron in a field of this type.
 Similarly, diamagnetic gases are comparable to non-
magnetic substances. The paramagnetic property of
oxygen has been utilised in constructing oxygen
analysers.
 The paramagnetic oxygen analyser was first described by
Pauling
 The arrangement incorporates a small glass dumb-bell
suspended from a quartz thread between the poles of a
permanent magnet.
 The pole pieces are wedge-shaped in order to produce a
non-uniform field.
 Referring to Figure, when a small sphere is suspended in a
strong non-uniform magnetic field, it is subject to a force
proportional to the difference between the magnetic
susceptibility of this sphere and that of the surrounding
gas. The magnitude of this force can be expressed as
follows

The forces exerted on the two spheres of the test body

are thus a measure of the magnetic susceptibility of the
sample and therefore of its oxygen content.
 The magnetic forces are measured by applying to one
sphere an electrostatic force equal and opposite to the
magnetic forces. The electrostatic force is exerted by
an electrostatic field established by two charged vanes
mounted adjacent to the sphere
 One vane is held at a higher potential than the test
body, the other at a lower potential.
 The test body is connected electrically to the slider of
Null Adjust potentiometer R20.
 This potentiometer is part of a voltage-dividing
resistor network connected between ground and B+ .
Potential to the test body can be adjusted over a large
range.
 An exciter lamp directs a light beam on to the small
mirror attached to the test body.
 From the mirror, the beam is reflected to a stationary
mirror and then on to a translucent screen mounted on
the front panel of the instrument. The geometry of the
optical system is
 When no oxygen is present, the magnetic forces exactly
balance the torque of the fibre.
 The resulting rotation of the suspension turns the small
mirror and deflects the beam of light over a scale of
the instrument.
 The scale is calibrated in percentages by volume of
oxygen or partial pressure of oxygen.
 Paramagnetic oxygen analysers are capable of sampling
 THE ELECTROCHEMICAL METHODS
 Analysers based on the electrochemical methods are mostly used for the
determination of oxygen content of a gas.
 They utilise an electrolytic cell and can be broadly classified as galvanic,
polarographic and conductometric methods
 Electrochemical sensors work by reacting with the gas of interest and producing an
electrical signal proportional to the gas concentration. Consisting of two
electrodes (a working electrode and a counter electrode), the sensor operates by
allowing charged molecules to pass through a thin layer of electrolyte.
 An electrochemical sensor consists of the following components:
 Gas permeable membrane – this material covers the sensing electrode and is used
to control the amount of gas molecules reaching the electrode surface. The
membrane also performs the important role of filtering unwanted particulates.
 Electrode (anode) – to create an effective reaction with gas molecules, the
electrode is typically made from metals such as platinum or gold and works as a
transducer. The anode is the point at which the current enters the electrode.
 Electrode (cathode) – this is the point where the current leaves the electrode.
 Electrolyte - the electrolyte facilitates the cell reaction and carries the ionic
charge across the electrodes.
 Galvanic Methods Cathode (reduction)
 Galvanometric methods are based on the fact that the electrical current of
a galvanic cell, which is equipped with appropriate electrodes and an
appropriate electrolyte, would depend upon the oxygen concentration, it
being related to rate of oxygen uptake by such a cell.
 These analysers are used for measurement of small oxygen concentrations Anode (oxidation)
 Analysers based on this principle are used to measure the content of
dissolved oxygen (DO) in boiler-feed water. For this purpose, the boiler-
feed water flows through the cell and acts as an electrolyte and the cell is
used for continuous monitoring.
 There is a need to control the oxygen uptake at the cathode. This is
generally done by having a porous carbon cathode and semi permeable
membranes. This is compensated using a combination of negative
temperature-coefficient (NTC) and positive temperature-coefficient (PTC)
thermistors.
 Current in the galvanic cell obeys Faraday’s law, which is given by the
following relations

 where I is the expected current in micro-amps, when a gaseous sample

containing C ppm of oxygen by volume passes through the cell at a flow
rate F cm3 /min measured at P atmosphere and T K. The expression
assumes that the perfect gas laws apply.
 Cont.
 Another type of electrochemical analyser employs the high temperature
galvanic cell.
 All of these cells consist of a calcium stabilised zirconium oxide electrolyte,
with platinum electrodes.
 At the operating temperature, oxygen molecules on the side of the cell
exposed to a high partial pressure of oxygen (the anode) gain electrons.
 Simultaneously, oxygen molecules are formed by the reverse action, at the
other electrode (the cathode). For cell operating at 850°C, the standard
Nernst equation for an oxygen cell is as follows:

 where Pa is the partial pressure of oxygen within the cell, and Pb is the
partial pressure of oxygen outside the cell.
 Since this effect is specific for oxygen, the instrument output is not affected
by the presence of water or CO2 .
 However, hydrocarbons, hydrogen and other combustible gases will burn at
the operating temperature and result in an indication of less oxygen than is
actually present. The response of analysers using such cells is very fast.
 Polarographic Cells
 Polarographic cells are generally used to measure the partial pressure or
percentage of oxygen from injected samples, continuous streams or in
static gas monitoring.
 They find maximum utility in the respiratory and metabolic laboratories.
Polarographic cells are based on the redox reactions, in a cell having
both the electrodes of noble metals.
 When a potential is applied, oxygen is reduced at the cathode in the
presence of KCl as the electrolyte and a current will flow.
 The cathode is protected by an oxygen permeable membrane, and the
rate a which oxygen reaches the cathode will be controlled by diffusion
through the membrane. The voltage-current curve will be a typical
polarogram.
 A residual current flows in the cell at the low voltages.
 The current rises with the increase in voltage, until it reaches a plateau
where it is limited by the diffusion rate of oxygen through the
membrane. For a given membrane and at a constant temperature, this
would be proportional to the partial pressure of oxygen across the
membrane.
 The oxygen analyser incorporates oxygen sensor, which contains gold
cathodes, silver anode, potassium chloride electrolyte gel and a thin
membrane. The membrane is precisely retained across the exposed face
of the gold cathode, compressing the electrolyte gel beneath, into a thin
film. The membrane, permeable to oxygen, prevents airborne solid or
liquid contaminants from reaching the electrolyte gel. The sensor is
insensitive to other common gases. A small electrical potential (750 mV)
is applied across the anode and cathode
 Conductometric Method
 The conductometric method is convenient and is the most widely
used method for trace gas analysis. In practice, the sample gas is
passed through a cell containing a liquid reagent, which can react
with the gas of interest. The conductivity of the liquid is
measured before and after the reaction with the gas. The
difference in conductivity is proportional to the gas concentration
 The metal oxide gas sensor works on chemi-resistance principle.
 When the gas molecule interacts with metal oxide surface, it acts
as either an acceptor or donor. This changes the resistivity or
electrical conductivity of thin film.
 The resistivity of the metal oxide semiconducting thin film
depends on the majority carrier in the film and also gas molecule
nature, i.e., whether it is oxidized or reduced in the ambient
temperature
 Surface adsorption sites ensure appropriate interaction of gas
molecules with the material. In the case of n-type (electron being
majority carrier), the surface is generally get depleted with
electrons by the appearance of oxygen ion species (O−, O2−,
etc.), and upon exposure to sensing gas, these species react with
gas molecules to revert back electron on the surface, thereby
increasing conductivity.
 The creation of these oxygen species on the surface is material
specific and broadly dependent of temperature. In the case of p-
type (hole being majority carrier), similar situation arises.
 THERMAL CONDUCTIVITY ANALYSERS
 The thermal conductivity of a gas is defined as the
quantity of heat (in calories) transferred in unit time
(seconds) in a gas between two surfaces 1 cm2 in area,
and 1 cm apart, when the temperature difference
between the surfaces is 1°C.
 The ability to conduct heat is possessed by all gases, but
in varying degrees.
 This difference in thermal conductivity can be employed
to determine quantitatively the composition of complex
gas mixtures.
 Changes in the composition of a gas stream may give
rise to a significant alteration in the thermal
conductivity of the stream.
 The changes in temperature can be detected by using
either platinum filament (hot wire) or thermistors.
 A gas analysis based on the thermal conductivity
procedure presupposes binary gas mixtures or such gas
mixtures, respectively, which include a measuring
component, whose thermal conductivity differs
sufficiently from the thermal conductivity to the carrier
gas.
 Examples: measurement of hydrogen in blast furnace
gases, the determination of argon in oxygen in the
process of air decomposition and of sulphur dioxide in
roasting gases in the production of sulphuric acid
 Cont.
 In a typical hot-wire cell thermal conductivity analyser, four platinum
filaments are employed as heat-sensing elements. They are arranged in a
constant current bridge circuit and each of them is placed in a separate
cavity in a brass or stainless steel block.
 The material used for construction of filaments must have a high
temperature-coefficient of resistance.
 The materials generally used for the purpose are tungsten, Kovar (alloy of
Co, Ni and Fe) or platinum
 A reference gas is made to flow through all the cells and the bridge is
balanced precisely with the help of potentiometer D. When the gas stream
passed through the measuring pair of filaments, the wires are cooled and
there is a corresponding change in resistance of the filaments.
 The higher the thermal conductivity of the gas, the lower would be the
resistance of the wire and vice versa.
 Greater the difference in thermal conductivities of the reference and
sample gas, the greater the unbalance of the Wheatstone bridge.
 The unbalance current can be measured on an indicating metre, or on a
strip-chart recorder.
 Thermistors can also be used as heat-sensing elements arranged in a
similar manner, as hotwire elements in a Wheatstone bridge configuration.
Thermistors possess the advantage of being extremely sensitive to
relatively minute changes in temperature, and have a high NTC. When
used in the gas analysers, they are encapsulated in glass. Thermistors are
available which are fairly fast in response.
 ANALYSERS BASED ON GAS DENSITY
 It is known that the density of an ideal gas has a direct linear relation with the
molecular weight of that gas. Fortunately, all real gases behave as ideal gases
at room temperature and normal atmospheric pressure.
 Figure illustrates the principle of operation of a gas-density balance. The
reference gas enters the balance at A, where it splits itself into two streams
and leaves the balance at D. Two detectors (B1 and B2 ), which may be either
hot wires or thermistors and mounted in the path of the two streams, are
connected as two arms of a Wheatstone bridge.
 When the reference gas flows such that the flow is balanced, the two
detectors are equally cooled and the recorder would indicate a zero base line.
The sample gas enters the balance at C and it also splits into two streams.
 It mixes with reference gas in the horizontal conduits and leaves at D. If the
sample gas has the same density as the reference gas, there will be no
unbalance of reference streams or of the detector elements.
 If the sample carries a gas having a higher density than the reference gas, it
will cause a net downward flow, partially obstructing the flow in the lower
path, like A-B2 -D. This would result in raising the temperature of the detector
element B2 .
 This in turn, increases the flow in the path A-B1 -D and causes more cooling of
the element B1 .
 This temperature differential causes an unbalance in the bridge, the
unbalance being linearly proportional to the gas-density difference between
the reference and the sample gas.