PRESS RELEASE
Chemical Reactions? No thanks!
Gerd Kieper, Michael Markus, SICK Maihak GmbH, Reute, Germany
Abstract
The safety of various chemical processes and the supply chain can only be secured if they are
kept under a controlled and modified gas atmosphere. Oxygen is often one of the unwanted gas
components to consider. For example, during storage of flammable liquids or gases in tanks or
their transport in pipelines, the removal of oxygen by inert gases is often needed to avoid
flammable or explosive gas mixtures. For safety reasons, the integrity of the so-called inertization
process must be insured at all times. The use of oxygen transmitters in or at the process devices
is a suitable means to increase safety and efficiency in inertization processes. Affordable TDLS
based oxygen transmitters have been recently introduced to the process market. They combine
the technological advantages of a tuneable diode laser sensor with a simple transmitter design for
a wide range of applications.
Introduction
To avoid any explosion, the operator of a process plant has the chance to eliminate at least one of the
three basic properties belonging to the so-called ignition triangle. These are basically a fuel, an ignition
source and an oxidizer. An explosion can only occur when all three items are coming together at the
same time. What can be done to avoid this occurrence? Very often, chemical substances to be stored or
transported for later use in a downstream process must be regarded as fuels. Ignition sources for
example, an electrical spark introduced by a current source or a hot surface in the surrounding area
cannot be easily eliminated without some effort and additional costs. Furthermore, the absence of any
ignition source at any time is hard to maintain across the whole production area and it’s supply chain. The
easiest way to reduce the risk of an explosion is through the reduction of the presence of any substance
in the environment which can act as an oxidizer like the oxygen molecules in the ambient air. This can be
achieved for example, by introducing a non-reactive gas, a so-called inert gas, to replace the air. Suitable
inert gases are nitrogen, carbon dioxide, flue gases, and all noble gases. Consequently, the process to
replace a potential oxidizing atmosphere by a non-reactive gas is called inertization.
Inertization Processes
The inertization of storage tanks, pipelines and/or process devices is not only used for preventing fire and
explosions. It can also be applied to avoid unwanted chemical reactions, like product degradation,
discoloration and undesired secondary reactions. Hence, the general target of inertization is either to
eliminate oxygen in the environment or at least to reduce its concentration to an acceptable and safe limit.
The safe area which needs to be established at any time to prevent a hazardous situation is defined as
the lower explosion limit (LEL) and the upper explosion limit (UEL) of the fuel to be considered. LEL and
UEL are the lower and upper limits of the fuel concentration range, which would allow flame propagation
in the presence of air. A flame propagation cannot take place below or above LEL and UEL, respectively.
In practical terms, a fuel mixture below the LEL is too lean to ignite or explode, whereas a mixture above
the UEL will be too rich. The required limits are typically given in percentage range of volume or in g/m³
units.
The maximum oxygen concentration which can be regarded as safe is the so called limiting oxygen
concentration (LOC). Therefore, the LOL value defines the maximum allowed oxygen level inside a
mixture of a flammable gas, air and any inert gas constituents in which an explosion will not occur. The
inert gas is either used to purge a vessel before filling it with the fuel or will be filled in as a safety or
protective atmosphere above the product. Such a vessel can be a storage tank or a degassing silo, but
also a chemical or pharmaceutical reactor, a centrifuge, a dryer, a mixer, a mill or any other equipment
containing flammable material.
Inertization in practice
There are different ways to achieve a safe atmosphere or an inertization inside a vessel.
Vacuum inerting:
This is a common method for the inertization of vessels meant for vacuum operation. This procedure is
normally not suitable for large storage tanks because they are usually not designed for vacuum conditions
and can only withstand a negative differential pressure of a few mbar. The single operating steps include:
 Create a vacuum in the vessel until a desired residual pressure is reached
 Re-pressurize the vessel to atmospheric pressure with an inert gas, such as nitrogen
The steps are repeated until the desired oxygen concentration is reached.
Pressure inerting:
Vessels may be pressurized by adding inert gas, followed by venting to the ambient environment. More
than one pressurization cycle might be necessary to reduce the oxygen content to the desired
concentration. An advantage over vacuum purging is that the pressurization process is much faster. The
disadvantage is that the process consumes more inert gas.
Flow through inerting:
In the flow-through purging process, a purge gas is introduced to a vessel at one inlet, and flushes the
mixed gas inside the vessel at another outlet. This purging process is commonly used when the vessels
or equipment are not rated for over- or under-pressure. The purge gas is added and vented at
atmospheric pressure. The process can be expensive for purging large storage tanks as it consumes a
large quantity of inert gas. Continuous inerting methods are used permanently or on demand,
respectively. Fixed-rate inertization requires a continuous feeding of the inert gas into the enclosure at a
constant rate and results in a corresponding release of a mixture of inert gas and flammable gas from the
vessel's head space. To ensure that the vessel is completely protected, the flow rate of the inert must be
sufficient to satisfy even peak-demand requirements.
Siphon inerting:
The siphon purging process starts with filling the vessel (e.g. a large storage tank) with a liquid, such as
water or any other non-flammable liquid compatible with the product. The purge gas is subsequently
added to the head space of the vessel and drains the liquid from the vessel. When using the siphon
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purging process, it may be desirable to fill the vessel with the liquid first and then use the flow-through
purge process to remove any oxygen from the residual head space.
Displacement inerting:
A method based on density differences between the inert gas and the gas which has to be replaced. Ideal
replacement happens when a stable horizontal interface exists between the lighter gas entering at the top
of the tank and the heavier gas at its bottom through some suitable piping arrangement. As it is important
to achieve the required degree of gas replacement throughout the full tank cross section this method
requires a relatively low entry velocity of the gases and more than one complete volume change to
ensure the inertization.
Inertization surveillance
Several methods exist to control these different inerting methods in a reliable way, including pressure,
flow and oxygen monitoring. In the flow monitoring mode, the flow rate of the inert gas supplied to the
vessel is regulated whereas in pressure monitoring mode the vessel pressure is controlled. Decreasing
vessel pressure will be compensated by increasing the amount of inert gas, whereas a pressure rise
inside the vessel might result in venting inert gas from the vessel into the ambient.
Oxygen monitoring is the only method of all the ones mentioned, where the gas composition of the inert
atmosphere is controlled in a proper way. With the help of oxygen monitoring, the accurate adjustment of
the allowable oxygen concentration can be made to the required level within the fuel-gas-mixture.
Therefore, oxygen monitoring provides a dynamic and flexible control of every inertization process.
Oxygen transmitters can be installed either directly into the inert gas generating system, e.g. nitrogen
generators, into the inert gas supply stream or directly into a vessel or pipeline, on the spot where
undesirable reactions should be prevented.
Because the integrity of the intertization must be secured along the whole process the use of oxygen
transmitters at any of these locations is recommended. This results in a cost efficient control of the
inerting process and speeds up the reaction time in case of any unforeseen changes or malfunctions of a
process device. The task for an oxygen measurement in the context of intertisation applications might be
also the safety of the staff, operating these processes. Potential leakages of the inert gas generator,
pipeline, or vessel during operation or maintenance could result in an oxygen deficiency in the
surrounding atmosphere and can in the worst case lead to suffocation. Hence, it is worth considering
oxygen monitoring also in the environment around the intertization processes.
How to find the right oxygen sensor?
Several oxygen sensor technologies are available in the market promising a reliable and safe solution for
the applications mentioned above. In general, if they are chosen with care all of them could be used at
some of the measurement points along the described inertization processes and in their periphery. Here,
the user’s expectations in terms of performance, reliability and lifetime cost of the sensor equipment will
make the difference. All types of oxygen sensors have their advantages and disadvantages. Limitations
for the individual sensor technology are shown as follows:
Oxygen analyzers based on electrochemical sensors are the most known technology to measure oxygen
in the ambient or in process gases. The sensor resembles an electric battery and consists of a cathode
and anode with an electrolyte (mostly acetic acid) in between. A red-ox reaction, initiated by the presence
of oxygen ions, results in an electric current which is measured to determine the oxygen concentration in
the surrounding gas. The big advantage of this measuring method is the wide range of concentrations
which can be measured (ppm-level … 100%O2). It is found very often in small portable analysers used for
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operator protection. The disadvantage of this measurement method is the finite lifetime of the sensor
which can vary between six months and three years depending on the actual measured oxygen levels.
These sensors tend to drift heavily. A frequent calibration - at least once a month - is needed to
compensate these drifts. Additionally, electrochemical sensors have only a very limited tolerance to harsh
chemical environments.
Zirconium Oxide (ZrO2) based sensors detect the difference in the oxygen content in a known gas
composition (e.g. reference air) and in the measuring gas by the resulting voltage drop according to the
Nernst equation. The voltage drop gets higher with increasing difference in the oxygen concentration in
the reference gases and in the measuring gas. This sensor is ideal for measurements in ppm-range but
can be also used up to 100% oxygen level. It has fast response times and good accuracy. Disadvantages
are the relative short lifetime of the sensor (approx. 18 months), the high power consumption and the
cross-sensitivity to hydrocarbons, CO, H2 and other combustible gases. Condensates will directly destroy
the sensor.
Measurement principles based on the paramagnetic properties of oxygen molecules are very often used,
especially in the context of inertization in the process industry. Applicable for relatively high oxygen
concentrations in the %-level, the sensor utilizes a strong magnetic field to detect the oxygen molecules
inside a sample gas. The analyzer works reliably and is stable only in connection with a proper sample
conditioning system. It has a good selectivity, but the sensor is quite expensive and sensitive to pressure
fluctuations, flow drifts, vibrations and gas impurities like humidity and dust. An in-situ design is not
available.
Tuneable Diode Laser Spectroscopy (TDLS) often used for oxygen detection is well known for its
selectivity and stability but also for its high cost. Until now, oxygen analyzers based on TDLS are mostly
used for rather difficult process measurements in the chemical and petrochemical industry or for
combustion control of incinerators. Typical installations include two flange-mounted units, one acting as a
receiver and the other as the transmitter. Therefore, beam alignment is often required as well as inert
gases e.g. nitrogen to purge the dead volume close to the mounting flanges. A big advantage of the
TDLS technology is the ability to measure under hard environmental conditions, e.g. high temperatures,
corrosive gases, etc. TDLS technology would be the optimum for all measurements, but have often been
disregarded as not affordable for standard applications.
A new TDL based oxygen transmitter on the market provides a reasonable and very cost efficient solution
which combines the unique properties of the TDLS technology with the simplicity of a process transmitter.
The product is designed for harsh ambient conditions and can be mounted either directly to a process
vessel or a gas pipe as an in-situ sensor or, if the process conditions do not allow this, in a simple bypass configuration using an optionally available sample cell. Also available as an option is a wall
mounting bracket which allows for installation close to the measuring point or as an ambient air monitor
for oxygen deficiency monitoring. Available is a general pupose version or an FM certified one for
hazardous areas. (ATEX version will follow).
For the inertization process described, the unique TDL based oxygen transmitter offers an excellent long
term stability (a calibration once a year is recommended) and the absence of any moving or wearing
parts. The sensor parts in contact with the process media are protected by a fine steel mash filter and by
an optional PTFE membrane. In any case, the optical parts can be easily cleaned, as well. The analyzer
tolerates aggressive chemicals and moisture. There is no need for an additional reference or purging gas.
Important for inertization processes: hydrocarbons do not interfere with the measurement. The product is
the first TDL based transmitter at a price which is reasonable for a wide range of oxygen monitoring
applications. As a consequence, TDL based oxygen transmitters can make inertization processes safe,
reliable and cost efficient at a minimum of investment and lifetime costs.
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Figures:
Fig 1. Ignition Triangle (Picture not licensed! Perhaps you can fill in something similar.)
Fig. 2. The TDL based oxygen transmitter TRANSIC100LP
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Fig.3 General pipeline picture
SICK is one of the world's leading manufacturers of sensors and sensor solutions for industrial
applications. Founded in 1946 by Dr.-Ing. e. h. Erwin Sick, the company is headquartered in the German
town of Waldkirch, in the Breisgau region near the city of Freiburg. It is a technology and market leader,
maintaining a global presence with more than 50 subsidiaries and equity investments as well as
numerous representative offices. In the 2014 fiscal year, SICK had around 7,000 employees worldwide
and generated Group revenues of €1,099.8 million.
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