HCl manufacturing salt sulphuric acid process

1. HYDROCHLORIC ACID
INTRODUCTION
Hydrochloric acid (HCl), also known as muriatic acid, is a solution of hydrogen chloride in water. HCl
exists in solid, liquid, and gaseous states and is water soluble in all proportions.
The first hydrochloric acid was prepared through heating common salt and sulfuric acid by Benedictine
Monk and Basil Valentine in 15th century. Also, Libavius prepared free hydrochloric acid by heating salt
in clay crucibles in 16th century. In the 17th century, Johann Rudolf Glauber used NaCl and H2SO4 for
the preparation of sodium sulfate in the Mannheim process, releasing hydrogen chloride gas as a by-
product. Joseph Priestley prepared pure HCl in 1772, and chemical composition includes hydrogen and
chlorine was proven by Humphry Davy in 1818. Demand for alkaline substances increased during the
Industrial Revolution in Europe, Nicolas Leblanc developed cheap large-scale production of sodium
carbonate (soda ash). Using common salt, sulphuric acid, limestone and coal which release HCl as a by-
product. Until the British Alkali Act 1863 and similar legislation in other countries, the excess HCl was
vented to air. After the passage of the act, waste gas is absorbed in water, producing hydrochloric acid on
an industrial scale. In the twentieth century, the Leblanc process was effectively replaced by the Solvay
process without hydrochloric acid by-product. Since hydrochloric acid was already fully settled as an
important chemical in numerous applications, the commercial interest initiated other production methods,
some of which are still used today. After the year 2000, hydrochloric acid is mostly made by absorbing
by product hydrogen chloride during a chemical manufacturing process such as chlorination of
hydrocarbons.
MANUFACTURE
HCl is manufactured by various methods as follows-
1. From salt and sulfuric acid,
2. Synthesis from hydrogen and chlorine,
3. As by-product from chemical processes,
4. From incineration of waste organics,
5. Hydrochloric acid solutions
From salt and sulfuric acid
The sodium chloride is ground in a mill, mixed with current of hot compressed air to 50°C and liquid
sulphuric acid are charged through a feed inlet through the cover of the furnace. This is an externally
heated furnace in which the process stream is heated primarily by radiative and conductive heat transfer

2. from the flame and hot gases and is known as Continuous Mechanical Muffle furnace. This furnace as it
is referred to comprise the combustion chamber, the work space, the stationary circular muffle with a
bottom concave pan and a domed cover separated by a cylindrical mantle or steel column and the plough
mounted on rotating arms fixed to a central under driven shaft.
The combustion chamber is where the fuel/air combustion takes place to produce combustion gases, CO2
and H2. Hot flue gases are circulated around the muffle. The work space which is sufficiently tight to
keep out contaminants is where the actual decomposition of the reactants takes place to produce HCl gas
and solid Na2SO4. This is an externally heated furnace in which the process stream is heated primarily by
radiative and conductive heat transfer from the flames and hot gases (combustion gases) above the dome
and the pan transmit the required heat for the reaction by radiation from the cover and by conduction
through the pan (there is no direct contact between the combustion gases and the reactants/products). The
reaction mass is agitated by the ploughs. The rotating ploughs move the reacting mass toward the
periphery of the pan where the salt cake, sodium sulphate is discharged. Hydrogen chloride (30- 36% by
weight) and air are withdrawn from an outlet in the cover and transferred to coolers and absorbers.
Combustion chamber temperatures of about 1202°C (1475K) are used for heating. The reaction between
sodium chloride and sulphuric acid takes place at temperatures ranging 500 to 550°C. The product
hydrogen chloride gas is discharged at temperature 600°C and the by-product sodium sulphate is
discharged from the hearth at about 600°C.
Reactions
1. NaCl + H2SO4 → NaHSO4 + HCl (at temperature=150 °C)
2. NaCl + NaHSO4 → Na2SO4 + HCl (at temperature=550 -650°C)
Overall reaction
2NaCl + H2SO4 → Na2SO4 + 2HCl
CB CA
Since sulphuric acid is more expensive than sodium chloride (natural salt), it is taken as a limiting reagent
and the basis for the calculations in reactor kinetics, Let A, and B represent sulphuric acid and sodium
chloride respectively.
(-rA) = kCAC2
B
(-rA) = 0.005CAC2
B
Conversion of HCl is about 50-60%.Product gas temperatures from the reactor (furnace) exceed those
allowable for absorption. The method used for absorption varies with the temperature and volume of the

3. gas being processed. Some cooling is achieved in the pipeline carrying the gas from the generating unit to
the cooler or cooler-absorber. In the cast-iron or steel flue carrying the high-temperature gas from the
salt-sulphuric acid process, some heat is removed by radiation to the atmosphere. In synthesis plants using
impervious graphite or silica coolers, the pipe may be cooled with external water sprays.
Generally, the high-HCl low-volume gases are cooled in tubular equipment, and the low-HCl high-
volume gases by heat interchange with concentrated hydrochloric acid in packed towers. For this design
particular design the cooling was achieved by a tubular exchanger known as the Trombone Cooler. Other
names for the trombone cooler include trickle coolers or cascade coolers.
Trombone coolers are S-shaped bends, consisting of a bank of standard pipes one above the other in series
and over which water trickles downward, partly Tubes are made out of impervious ceramic material for
cooling corrosive gases at atmospheric pressure, such as HCl and NO2 that may be cooled by exterior
water or may be jacketed. Trombone coolers are also available in cross-flow types and banks of impervious
graphite tubes have been used which are submerged in running water. Packed columns may also be used
for the low volume gases.
The classical equipment for hydrogen chloride absorption was a system of cellarius focrills or woulfe
modern time’s use cooled – absorption towers. The cooling – absorber is essentially a vertical shell and
tube heat exchanger of impervious graphite or glass.
Production of hydrochloric acid in a concentration of 1 to 40 % HCl acid from chlorine and hydrogen,
using water or weak hydrochloric acid as the absorbing liquid. This is accomplished by increasing the
mass – transfer surface, by adding one or two absorbers and possibly increasing the length of the tubes.
The absorption of HCl in water however generates heat as the process is highly exothermic. This makes
the falling firm cooler – absorber ideal. However there is not enough information available for the design
of the type of absorber. Therefore a sieve plate water cooled tower will be opted for.