PROCESS
Application Note
PROCESS INSTRUMENTS
The Measurement of Hydrogen Chloride
and Chlorine
The considerations below cover both the
measurement of HCl and Cl2 and the
measurement of components such as
moisture where these are background
gases, since the issue is the same; the
resilience of the sample system and the
analyzer to corrosive attack.
(Please note that there are a very large
number of metal alloy variants (inconels,
hastalloys, monels etc) which all have
different corrosion resistant properties
under slightly different conditions The
perfect material may not be- in practice
or cost -useable to make analyzer
and sample system components, so our
first position is to avoid special materials if
possible).
Chlorine
Chlorine is a chemical element with symbol
Cl and atomic number 17. Chlorine is in the
halogen group (17), and is the second lightest
halogen after fluorine. The element is a
yellow-green gas under standard conditions,
where it forms diatomic molecules. It has the
highest electron affinity and the third highest
electronegativity of all the reactive elements; for
this reason, chlorine is a strong oxidizing agent.
This is the concern when we are measuring
chlorine itself or other chemicals, such as
H2O, in chlorine. The reaction of chlorine with
hydrogen or hydrogen containing compounds
can be considered an oxidation reaction and
is likely to produce hydrogen chloride, HCl.
The issues associated with HCl sampling and
measurements are also discussed below.
Chlorine gas is a toxic gas made on a large
scale from salt (sodium chloride). It is used to
sterilize drinking water, to disinfect swimming
pools and in the manufacture of many
consumer products such as paper, dyestuffs,
textiles, petroleum products, medicines,
antiseptics, insecticides, foodstuffs, solvents,
paints and plastics, especially PVC.
100% dry chlorine at temperatures up to 70ºC
may be used with stainless steels, but moist
chlorine is considered unsatisfactory as it
removes protective oxide layers from most
steels. Consequently, moisture measurement
is vital in chlorine plants.
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Even Titanium is not fully resistant to chlorine.
Hastalloy C-22 alloy has been found resistant
to a variety of concentrations of moist chlorine
and (the chromium nickel alloy) Inconel 601 has
been found effective for chlorine gas mixtures
at elevated temperatures. Monel (often Monel
400, a copper-nickel alloy) is not so resistant
to wet chlorine. There are no instant easy
answers to measuring in chlorine environments.
Each application has to be looked at on its own
merits. Special metals such as those above
are expensive especially if measuring cells
and sample components are both required to
be specially manufactured, so compromise
solutions where occasional replacement of less
resistant materials will be necessary may be more
customer acceptable.
Cl2 on metals), using a portable analyzer such as
a 5100P may be useful. After use, the portable
may be purged with air to remove chlorine
and extend the life of the analyzer. For a fixed
analyzer, such as a 5100HD, use PVDF sample
lines and non-steel components where possible.
Metal corrosion rates may be quite slow under
low moisture conditions and consequently use
of conventional carbon stainless steels for cells
and sample conditioning equipment may still be
the most cost effective solution. Where corrosion
resistant metals are preferred, Hastalloy C-22 is
often recommended.
Remember that, while humans can tolerate a few
Absolute solutions to sampling material questions
though will always be a question of degree, which
will be related to concentrations. For low ppm
moisture measurements in chlorine, it may take a
long time before any significant corrosion occurs.
Model 5100 Portable
ppm of Cl2 in drinking water and swimming pools,
where it is used to destroy harmful bacteria,
chlorine is toxic and care should be taken during
calibration procedures where it is present.
Summary
If measuring Chlorine, or other gases excluding
moisture, dry the sample gas by using a chiller
if the water concentrations are percent or high
1000s ppm level. For longer lifetimes of metal
sample system components, reduce moisture
even further by using chemical driers such as a
molecular sieve.
In chlorine plants, moisture measurements are
often made to ensure they do not rise above
(typically) 15ppmv. Significant corrosion/chemical
attack may occur above that level. In principal,
that is a useful guide to successful analyser use.
Where possible, use halogenated plastics such
as PVDF for sample tubing.
Avoid sample mixtures where there are significant
levels of ammonia, acetylene, hydrogen or fuel
gas as these may react explosively with chorine.
Model 5100HD
If measuring moisture in chlorine (a common site
requirement motivated by the adverse effect the
2
Upon contact, H2O and HCl combine to form
hydronium cations H3O+ and chloride anions Clthrough a reversible chemical reaction:
Chlorine, being a symmetric (homonuclear)
molecule, does not absorb in the infrared, but
may be measured in the UV spectrum where
AMETEK IPS-4s have been used for not only
measuring Cl2 in gas mixtures but also in
EDC (ethylene dichloride), a precursor to the
manufacture of PVC which is the major industrial
use of chlorine and for which processes there
are other gas analysis applications which may be
resolved with IPS-4s or 5100HDs. Fluorine, which
is also a halogen and oxidizing agent, may also
be measured using IPS-4 UVs.
HCl + H2O → H3O+ + ClThe resulting solution is called hydrochloric acid
and is a strong acid. The acid dissociation or
ionization constant, Ka, is large, which means
HCl dissociates or ionizes practically completely
in water. Even in the absence of water, hydrogen
chloride can still act as an acid. For example,
hydrogen chloride can dissolve in certain other
solvents such as methanol, protonate molecules
or ions, and serve as an acid-catalyst
for chemical reactions where anhydrous
(water-free) conditions are desired.
HCl + CH3OH → H2+Cl- +CH3O
Because of its acidic nature, hydrogen
chloride is corrosive in the presence of
moisture.
In the Hydrocarbon Processing Industry
it is often used in the manufacture of
halogenated plastics.
Dry HCl (or, to put it another way, HCl in the
gas phase –see acid dew point below) is not
generally a concern to us from a sampling
viewpoint. It will react easily with water however
to form hydrochloric acid.This is a problem when
it condenses, so a key issue here is to keep a
sample containing both HCl and H2O above the
acid dew point which means heating the sample
lines, components and in contact parts of the
analyser appropriately.
A complication when chlorine is involved in the
sample gas is its tendency to form hydrogen
chloride and protection against both of these
gases may be required for many applications.
Hydrogen Chloride
Hydrogen chloride is a diatomic molecule,
consisting of a hydrogen atom H and a chlorine
atom Cl connected by a covalent single
bond. Since the chlorine atom is much more
electronegative than the hydrogen atom, the
covalent bond between the two atoms is quite
polar. Consequently, the molecule has a large
dipole moment with a negative partial charge δ- at
the chlorine atom and a positive partial charge δ+
at the hydrogen atom. In part because of its high
polarity, HCl is very soluble in water (and in other
polar solvents).
It is possible to calculate an acid dew point
(T) from knowledge of the concentrations
(represented as partial pressures, P, below)
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Chlorine Measurement
PROCESS
Hydrochloric Acid (HCl) Dew Point
Hydrogen Chloride and Moisture Measurement
1000/T=3.7368−0.1591log e (P H2O)−0.0326log e
(P HCl)+0.00269log e (P H2O)log e (P HCl)
Both water and hydrogen chloride are dipolar
molecules having rotational fine structure which
enables measurement by tuneable diode laser
(TDL) in the near infrared. The AMETEK 5100HD
is capable of running at temperatures in excess of
100ºC to avoid acid dew point for HCl and, where
necessary, materials in contact can be modified to
reduce corrosion.
where:
T = The acid dew point temperature for
hydrochloric acid, in kelvins
P = Partial pressure in mmHg
(See reference 1)
A useful general guide however is (reference
2) that hydrochloric acid dew points at
concentrations of 1% (10,000ppm) or less HCl
and 30% or less water tend to plateau out at
about 80ºC (ref 2). Supporting data (ref 3) shows
dew points maximizing at less than 60ºC for
1000ppm HCl and less, 12.5% H2O and less).
For measurement of HCl, care should be taken
if the requested measuring ranges are low, less
than 50ppm. HCl will tend to absorb on sampling
components and both heat and special materials
are needed to minimize this effect.
Conclusion
Summary
The issue with these measurements is potential
corrosion of the analyzer and sample conditioning
system. Clearly, the lifetime of these components
is a primary customer question and difficult to
answer except at the extremes of concentration.
The possible concentration variations of three
different components and accuracy of customer
information regarding those may make it very
difficult for us to give firm advice and the customer
needs to be aware of this. We should also seek
their experience, where possible, regarding the
materials, temperatures used and consequent
maintenance issues. A conclusion may often be
that the use of special metal materials can be
avoided if the customer is prepared to replace
standard materials on a regular basis.
Where HCl and moisture are present, keep the
sample in the gas phase above the acid dew point
temperature. Temperatures above 100ºC will
usually be more than adequate.
If there is a risk of condensation, the materials
of construction need to be considered. Being an
acid, condensed HCl will react with most metals
forming hydrogen (and denaturing the metal
to a salt). Stainless steels are not resistant to
hydrochloric acid. Hastalloy C is however largely
resistant but for long term reliability, the lower the
concentration of HCl the better. As with chlorine,
plastics are much more resistant than metals
including PP, PVC, PTFE, PVDF, and PFA.
Note that relatively low levels of HCl and/ or Cl2,
less than 20ppmv, may be considered to have
enough of a minimal effect on corrosion even
when water is present that they can be safely
ignored.
4
Solution
OK - UV Photometer
Use corrosion resistant plastics or
metals where possible - UV
OK - IR or TDL Photometer
(Note that low ppm levels are difficult)
Either dry, or raise above acid dew point.
Corrosion resistant materials will extend
lifetime
IR or TDL Photometer
(Note that low ppm levels are difficult)
Sampling as for wet chlorine.
Use IR or TDL Photometer
Raise above acid dew point
Corrosion resistant materials will extend
lifetime
IR or TDL Photometer
Portable - Standard materials and air
purge after use.
Dry Chlorine
Wet (>15ppm H2O) Chlorine
Dry HCl
Wet (>15ppm H2O) HCl
Water in Chlorine
Water in HCl
5
PROCESS
Analyte
The Measurement of Chlorine and Hydrogen Chloride,
and Moisture
3) Huijbrechts and Leferink Anticorrosion Methods and Materials Vol 51, 3 (2004) pp 173-188
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2) Bahman Zarenezhad Oil and Gas Journal, Feb 2001
Application Note
1) Yen Hsiung Kiang (1981), “Predicting Dewpoints of Gases”, Chemical Engineering Vol. 88, Issue 3, p. 127