hcl corrosion under control

7/29/2019 HCl Corrosion Under Control

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1s 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.

sHydrochloric acid results as

a by-product or waste prod-

uct in a gaseous or liquid form in

various processes of the chemical

industry and environmental engi-

neering (Fig.1s). But seldom in a

pure form it is usually aqueous

solutions with hydrochloric acid

concentrations of between 18 and

24%. In addition, there are conta-

minations caused through various

by-products, in particular salts

and organic compounds. In gener-

al, this hydrochloric acid cannot be

34 S U L Z E R T E C H N I C A L R E V I E W 4 / 2 0 0 0 3978

Hydrochloric Acid:Corrosion

under ControlURSULA LIECHTIURS HALLER

ULZER CHEMTECH

The distillation, concentration and purificationof hydrochloric acid (HCl) places stringentrequirements on engineering and process

engineering: As an aqueous solution, HCl ishighly corrosive; accidents or damage in plantscan have tremendous economic and ecologicalconsequences. The employment of suitablecorrosion-resistant materials is therefore vitallyimportant. With the experience that it hasacquired over many years, Sulzer Chemtech isnow a competent supplier of HCl distillation

plants.

Parentmaterial

Reaction

Product

HCl aqueous(waste product)

Pure H2O

Pure HCl gas

Distillation


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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. Neverthe-

less, it can be used again if the acid

is concentrated and purified. Vari-ous 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 SUCCESSWith the double-pressure process,

one each pressure and vacuum

columns are connected in series

(Figs.2s and 3s). The 1525%

hydrochloric acid is separated at a

pressure of 46 bar and a tempera-

ture 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 commer-cial 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 azeotrop-

ic point is displaced to such an

extent that highly concentrated

hydrochloric acid (about 22%) is

2s A plant designed according to the

double-pressure system with vacuum

and distillation columns.

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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 cir-

cuit.

This technology places enormous

demands on the process-engineer-

ing 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 sys-

tem, and therefore possesses the

know-how needed for the design

and construction of these plants.

Different materials are employed

depending on the field of applica-

tion. The corrosion-resistant reac-

tion of the materials differs with

the various pressures and temper-

atures. Since the stability of the

material cannot be calculated, a

wealth of practical experience is

needed to select the correct mate-

rials for the construction of such a

plant (Fig.4s).

This applies not only to the column

jacket and the pipework, but also

to the packings (Fig.5s) and the

column internals. Sulzer Chem-

tech has developed a broad spec-trum of diverse materials over the

years, which can be employed in a

goal-oriented manner. Depending

on the contaminations, tempera-

ture and required throughput, the

appropriate version can be select-

ed in graphite, various plastics,

ceramic, etc.

The requirements for the elements

of the measuring section, e.g. ther-

mocouples, pressure pick-ups and 0698 2511


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valves, are complex. Questions

concerning the thermal conductiv-

ity, permeation, sealing and safety

of operation have to be considered

during the selection of the appro-

priate components.

DIFFERENT MATERIALS,DIFFERENT PROBLEMS

The processing of hydrochloric acid

is always associated with the same

problems. The acid is very corro-

sive 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 prob-

lems of corrosion become more

serious with higher temperatures

and pressures. With normal high-

quality steel and a temperature of

160 C, for example, up to one

centimetre is etched away every

10 minutes.

Acid-resistant materials also reactto 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 plas-

tics. A high proportion of water in

the pressure stage (at 46 bar) of a

hydrochloric acid distillation plant

can destroy stoneware and porce-

lain, and even glass and enamel.

36 S U L Z E R T E C H N I C A L R E V I E W 4 / 2 0 0 0

4s Simulation of a typical case of damage

during the processing of hydrochloric acid:

structured packing made of apparently

corrosion-resistant steel, destroyed by corrosion.

3s The double-pressure distillation

system for hydrochloric acid

produces pure HCl gas without any

contamination through by-products.

THE AZEOTROPICPOINT OF

HYDROCHLORIC ACID

A mixture is azeotropic

when the composition of thevapour is the same as that

of the liquid. This mixture

behaves like a pure sub-

stance and cannot be sepa-

rated into its constituents by

means of conventional dis-

tillation.

With hydrochloric acid in

aqueous solution, the

azeotropic point is approxi-

mately 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 hydrochloricacid. The exact values can

only be determined in each

case by means of experi-

ment. Sulzer Chemtech has

acquired the respective

experience through its work

with hydrochloric acid

processes over many years.

HCl >22%

Vacuumcolumn

Pure H2O

HCl~28%

PureHCl gas

56bar, 160 C


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On the other hand, the same mate-

rials 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 resultin the most severe damage, with

the respective consequences for

human beings and the environ-

ment, as well as additional costs

for the loss of production. Opera-

tional safety and the reliability of

the distillation columns are there-

fore matters of great concern for

the operator. But the safety of the

plants can be guaranteed if the

materials are selected correctly.

SELECTION OF THEMATERIALS DECISIVE

The material can only be chosen in

intensive co-operation with cus-

tomers, suppliers and the plant

constructor. At the same time, all

the relevant aspects have to be con-

sidered it is here that reliability,

safety and the life cycle of the plant

are decided.

It is true that various special mate-rials 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 tanta-

lum, 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

5s The packing (photo: a Mellacarbon

packing from Sulzer Chemtech) is a

key component in the HCl distillation

column.

S U L Z E R T E C H N I C A L R E V I E W 4 / 2 0 0 0 37

F O R M O R E D E T A I L SSulzer Chemtech AG

Urs Haller, 0600

Postfach 65

CH-8404 Winterthur

Switzerland

Telephone +41 (0)52-262 37 98Fax +41 (0)52-262 00 76

E-mail [email protected]

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 diffi-

cult to process. When Teflon with

its low melting temperature iswelded together, it has to be made

with a fully protective mask,

because highly toxic vapours are

released during the welding opera-

tion.

In the last few years, Sulzer

Chemtech has made great efforts

to find materials that can with-

stand 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.

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 mate-

rials 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 PTFEliner)

Tantalum

Zirconium

Niobium

Graphite

Ceramic materials

Very suitable for high gas temperatures; with

liquids, however, only in relatively low tempera-

tures.

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hcl corrosion under control

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