Hydrochloric Acid:
Corrosion
under Control
URSULA LIECHTI
URS HALLER
SULZER CHEMTECH
■
The distillation, concentration and purification
of hydrochloric acid (HCl) places stringent
requirements on engineering and process
engineering: As an aqueous solution, HCl is
highly corrosive; accidents or damage in plants
can have tremendous economic and ecological
consequences. The employment of suitable
corrosion-resistant materials is therefore vitally
important. With the experience that it has
acquired over many years, Sulzer Chemtech is
now a competent supplier of HCl distillation
plants.
Hydrochloric acid results as
a by-product or waste product in a gaseous or liquid form in
various processes of the chemical
industry and environmental engineering (Fig. 1■). But seldom in a
pure form – it is usually aqueous
solutions with hydrochloric acid
concentrations of between 18 and
24%. In addition, there are contaminations caused through various
by-products, in particular salts
and organic compounds. In general, this hydrochloric acid cannot be
1■ Typical fields of application for the concentration of
hydrochloric acid are chemical reactions with chlorine,
where hydrochloric acid is produced as a by-product,
the chlorination of alcanes and the purification of
waste gases in environmental engineering.
Pure H2O
Parent
material
Reaction
Distillation
HCl aqueous
(waste product)
Product
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SULZER TECHNICAL REVIEW 4/2000
Pure HCl gas
3978
recycled as such any more and
therefore has no market value.
Furthermore, it cannot be disposed
by way of the waste-treatment
plant, because the concentration is
too high; the neutralization and
clarification of the hydrochloric
acid results in a high salt load and
also additional costs. Nevertheless, it can be used again if the acid
is concentrated and purified. Various techniques are applied for this,
depending on the desired yield of
hydrochloric acid or the required
degree of purity. Sulzer Chemtech
has experience with both the
calcium chloride (CaCl2) and
the double-pressure process. The
following describes the demanding
double-pressure process in detail.
TWO STAGES TO SUCCESS
With the double-pressure process,
one each pressure and vacuum
columns are connected in series
(Figs. 2■ and 3■). The 15–25%
hydrochloric acid is separated at a
pressure of 4–6 bar and a temperature of up to 160 °C in the pressure
column. The head product, HCl
gas, can be used further, namely
directly or dried and compressed
for reactions, or with the aid of an
absorber, processed to a commercial aqueous solution with water.
The bottom product, an azeotropic
hydrochloric acid mixture, cannot
be separated any further through
normal distillation (see box, p. 36).
This bottom product is now routed
to the vacuum column. As a result
of the underpressure, the azeotropic point is displaced to such an
extent that highly concentrated
hydrochloric acid (about 22%) is
obtained as bottom product. Water
with a low content of hydrochloric
acid in the ppm range is produced
as head product. The 22%
hydrochloric acid is returned to the
pressure column again via the circuit.
This technology places enormous
demands on the process-engineering system and also the employed
materials. In the last few years,
Sulzer Chemtech has concerned
itself intensively with the process
of hydrochloric acid distillation by
means of the double-pressure system, and therefore possesses the
know-how needed for the design
and construction of these plants.
Different materials are employed
depending on the field of application. The corrosion-resistant reaction of the materials differs with
the various pressures and temperatures. Since the stability of the
material cannot be calculated, a
wealth of practical experience is
needed to select the correct materials for the construction of such a
plant (Fig. 4■).
This applies not only to the column
jacket and the pipework, but also
to the packings (Fig. 5■) and the
column internals. Sulzer Chemtech has developed a broad spectrum of diverse materials over the
years, which can be employed in a
goal-oriented manner. Depending
on the contaminations, temperature and required throughput, the
appropriate version can be selected in graphite, various plastics,
ceramic, etc.
The requirements for the elements
of the measuring section, e.g. thermocouples, pressure pick-ups and
2■ A plant designed according to the
double-pressure system with vacuum
and distillation columns.
0698 2511
SULZER TECHNICAL REVIEW 4/2000
35
3■ The double-pressure distillation
system for hydrochloric acid
produces pure HCl gas without any
contamination through by-products.
Vacuum
column
Pure H2O
HCl >22%
Pure
HCl gas
5–6 bar, 160 °C
HCl
~28%
valves, are complex. Questions
concerning the thermal conductivity, permeation, sealing and safety
of operation have to be considered
during the selection of the appropriate components.
DIFFERENT MATERIALS,
DIFFERENT PROBLEMS
The processing of hydrochloric acid
is always associated with the same
problems. The acid is very corrosive even in the diluted form and
leads to severe corrosion of the
majority of materials employed in
chemical plants within a short
space of time. In general, the problems of corrosion become more
serious with higher temperatures
and pressures. With normal highquality steel and a temperature of
160 °C, for example, up to one
centimetre is etched away every
10 minutes.
Acid-resistant materials also react
to contaminants frequently in an
unforeseen manner. The chlorine
content of hydrochloric acid has a
devastating effect on graphite heat
exchangers, and even traces of
organic solvents can attack plastics. A high proportion of water in
the pressure stage (at 4–6 bar) of a
hydrochloric acid distillation plant
can destroy stoneware and porcelain, and even glass and enamel.
4■ Simulation of a typical case of damage
during the processing of hydrochloric acid:
structured packing made of apparently
corrosion-resistant steel, destroyed by corrosion.
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SULZER TECHNICAL REVIEW 4/2000
THE AZEOTROPIC
POINT OF
HYDROCHLORIC ACID
A mixture is azeotropic
when the composition of the
vapour is the same as that
of the liquid. This mixture
behaves like a pure substance and cannot be separated into its constituents by
means of conventional distillation.
With hydrochloric acid in
aqueous
solution,
the
azeotropic point is approximately 20%. It can be
reduced to a lower value
through overpressure and
brought to a higher value
with underpressure. There
is hardly any indication
of the existence of the
azeotropic point at higher
pressure in the literature
open to the public. Marked
deviations
can
occur
through contamination of
the aqueous hydrochloric
acid. The exact values can
only be determined in each
case by means of experiment. Sulzer Chemtech has
acquired the respective
experience through its work
with hydrochloric acid
processes over many years.
5■ The packing (photo: a Mellacarbon®
packing from Sulzer Chemtech) is a
key component in the HCl distillation
column.
0697 2522
On the other hand, the same materials have stood the test at a lower
temperature in the absorption
stage.
The high temperatures lead to
clearly faster corrosion rates, and
the resultant leakages can result
in the most severe damage, with
the respective consequences for
human beings and the environment, as well as additional costs
for the loss of production. Operational safety and the reliability of
the distillation columns are therefore matters of great concern for
the operator. But the safety of the
plants can be guaranteed if the
materials are selected correctly.
SELECTION OF THE
MATERIALS DECISIVE
The material can only be chosen in
intensive co-operation with customers, suppliers and the plant
constructor. At the same time, all
the relevant aspects have to be considered – it is here that reliability,
safety and the life cycle of the plant
are decided.
It is true that various special materials which have been employed
hitherto have a high corrosion
resistance, but they are also very
expensive and difficult to work.
For example, metals such as tantalum, niobium and zirconium are
just as expensive as gold; but they
are very difficult to process, and
there are only a few specialists in
the world who can master this.
Tantalum of a thickness of only a
few millimetres, for instance, is
used quite frequently as a lining in
the columns. In view of its high
melting point, it is very difficult to
weld together. Teflon is also difficult to process. When Teflon with
its low melting temperature is
welded together, it has to be made
with a fully protective mask,
because highly toxic vapours are
released during the welding operation.
In the last few years, Sulzer
Chemtech has made great efforts
to find materials that can withstand the chemical exposure and
physical stress, and are also more
economic than those employed
hitherto – with success; and the
developments continue. To ensure
that customers can concentrate
hydrochloric acid economically in
plants with high operational
safety.
Ω
FOR MORE DETAILS
Sulzer Chemtech AG
Urs Haller, 0600
Postfach 65
CH-8404 Winterthur
Switzerland
Telephone +41 (0)52-262 37 98
Fax
+41 (0)52-262 00 76
E-mail
urs.haller@sulzer.com
METAL, PLASTIC OR CERAMIC?
The material to be used depends on various
factors and is decided from case to case. The
selection can be made from three principal groups:
plastics, metals and ceramic.
Plastics
For low temperature:
• Polypropylene
• Glass-fibre reinforced plastics (GRP): new materials for this field of application
For higher temperatures:
• Various representatives of the Teflon® family, e.g.
PFA, PTFE, ICTFE, which differ, above all,
through their thermal stability
Metals
• Corrosion-resistant steel (possibly with PTFE
liner)
• Tantalum
• Zirconium
• Niobium
• Graphite
Ceramic materials
Very suitable for high gas temperatures; with
liquids, however, only in relatively low temperatures.
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