FENCO ENGINEERS & CONSTRUCTORS INC by k5Hct74

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									                          FENCO ENGINEERS & CONSTRUCTORS INC.
                              SULPHURIC ACID TECHNOLOGY


Design:

   Reliability
   Cost efficient
   Meeting all environmental regulations
   Maximizing energy recovery

Advantages of Fenco design:

Fully converted Stainless Steel Converter:

Converter is fabricated throughout from stainless steel and is a completely welded construction.
Stainless steel is many times stronger than carbon steel at the operating temperatures of the
converter.

The first catalyst bed can be located at the bottom of the converter for easy access.

The converter beds can be arranged to minimize plant gas ducting.

The inherent high temperature resistance of stainless steel to oxidation eliminates the need for
metallizing parts of the converter internals.

Support posts above and below the catalyst beds makes safe access and working conditions for
catalyst changes.

The elimination of the brickwork and the reduced heat capacity of the converter shell and grids in
the Fenco’s designed converter enables the catalyst to be rapidly heated up to its firing
temperature.

Small converters can be shop fabricated and shipped to site as completed vessels.


Gas/Gas Heat Exchangers

The transfer of large quantities of heat from one gas stream to another using large       gas/gas
exchange equipment is an essential feature of the Contact process for the           manufacture of
sulphuric acid. Two different materials of construction are used:

1. The Hot and Intermediate Heat Exchangers

Stainless steel units are required to cool the hot gases (typically above 475ºC) leaving the first
and second catalyst bed.

2. The Reheat and Cold Heat Exchangers

Carbon steel units are used when inlet temperatures are less than 475ºC.



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The hot heat exchangers are required to operate at temperatures at which carbon steel is readily
oxidized. The oxidation products flake off and are then carried with the gas resulting in
contamination of the catalyst beds in the converter.

The reheat and cold heat exchangers can suffer from corrosion/erosion of the tube bundle at the
inlet to the shell as a consequence of impingement by the acid particles carried over from the
strong acid tower upstream. In the case of the reheat exchangers this carryover may be increased
by acid droplets condensed out in the inlet gas duct.

A disc and donut baffle arrangement is used to direct the gases over the tubes. For the hot heat
exchanger duties, the units are made completely from stainless steel.

The radial flow tube layout significantly improves the thermal efficiency of the heat exchanger,
providing a small heat exchanger surface for a given allowable gas pressure drop. The result is
that the radial hot heat exchanger units made from stainless steel are competitive with comparable
units made from carbon steel with Alonized tubing.

The radial gas flow pattern minimizes thermal stresses across the bundle and eliminates the
possibility of variations in temperature around the circumference of the heat exchanger.

The mechanical reliability of the hot heat exchanger is substantially enhanced by the use of
stainless steel and the welding of all tube/tubesheet joints.

The removal of most of the acid carryover from the gas, combined with the low gas velocity at
the inlet to the tube bundle, has substantially reduced the problems of corrosion in the cold and
cold reheat exchangers.


Acid Towers and Distributors

On all modern double absorption plants there are three acid towers; the drying tower, the
intermediate absorbing tower and the final absorbing tower.

Each of these towers is made of carbon steel with an internal lining of acid resistant brick. This
lining covers all the acid wetted surfaces. A membrane material or coating is applied between the
brick and the carbon steel wall of the tower to prevent acid from reaching the carbon steel. The
tower is filled with an acid resistant ceramic packing.

The gas enters at the bottom of the tower and flows upward through the packing, which is
completely wetted, by a downward flow of sulphuric acid. The gas leaving the top of the packing
passes through a demisting device before leaving the tower. In the drying tower, a mesh pad or
candle type mist eliminator may be chosen to remove the entrained acid mist droplets. In the
intermediate absorbing tower, where a high loading of fine mist particles is expected, a candle
type mist eliminator is specified. In the final absorbing tower, it is usual to install a very high
efficiency candle type filter which will remove virtually all the residual acid mist particles and
SO3. The gas leaving this tower is discharged to atmosphere.

The acid flow to the top of the tower is introduced via a trough type distributor.




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The iron sulphate formed has a volume seven times greater than that of the parent metal. The
result is that either the bricks or the steel shell must yield to accommodate this increased volume;
steel tower walls may bulge or bricks on a flat bottom may heave.

The acid flow evenly irrigates the top of the tower packing through a special proprietary
distributor.

For strong sulphuric acid services the distributor is fabricated from SX or Ze-cer austenitic
stainless steel. This material has demonstrated superior corrosion and erosion resistant properties
in strong sulphuric acid service.

The tower packing is held on a self-supporting brick dome or arch brick. The towers have a
dished bottom

The towers are elevated on legs to accommodate the dished bottom and provide access if
maintenance should ever be required.

Fenco’s alternative is to use a proprietary grid support that has all the elements arranged in
compression in a self-supporting dome. This dome forms a stable structure capable of bearing a
substantial load.

Towers designed with a brick system of arches or columns to support the packing grid create
local pressure points bearing on the base of the tower. These pressure points can ultimately
weaken the integrity of the membrane causing a leak path for the acid to reach the carbon steel
shell and initiate corrosion.

The elimination of the brick columns or arches allows a much simpler design for a tower bottom
with a dished base. With a dished bottom tower, the bricks are locked together in compression so
that if a leak were to occur through to the steel, then the ensuing ferrous sulphate formation could
not force the bricks to lift and further aggravate the problem.


Acid Coolers

When a metal contacts a chemical solution, the metal develops an electrical potential relative to a
reference electrode (an electrode whose own potential remains constant even though the
solution's chemical composition changes). This potential is defined as the free corroding potential
and is dependent upon both the metal alloy and the chemical solution which it is immersed. The
free corroding potential may be shifted by passing a DC current through the metal into the
solution. This process is called "polarizing" the metal.

A graphical plot of the current density (milliamperes/cm) against the potential required to achieve
this current density is called a polarization curve. In an electrochemical system involving two
electrodes immersed in a solution and an impressed current flowing from one electrode to the
other, the anodic electrode will tend to corrode. The rate of corrosion of the metal is directly
proportional to the current density. In most systems in which a metal is immersed in a chemical
solution, increases in applied potential cause corresponding increase in current density. Under
these circumstances, no corrosion protection is achieved by the impressed anodic current.




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Fortunately, there are a few metals and alloys, including the ferrous alloys which, when immersed
in strong sulphuric acid, exhibit a different phenomenon when an anodic potential is applied to
them. In these cases, as the potential is raised above the free corroding potential, there is an initial
increase in current density. During this period, a thin film begins to be formed on the metal
surface. The mechanism for the production of the film is complex, involving the oxidation of the
metal ions to higher valency states and the formation of a stable oxide product on the metal
surface. This oxide film is both inert to hot acid and resistant to the flow of electrical current. The
result is that, as this film builds up, the current flow actually decreases down to a very low level.
In this state, the metal corrosion has been reduced to virtually zero and the metal is said to be
passivated.

A metal will remain in this passive state, even in turbulent acid flows, while the anodic potential
on the metal surface is maintained. The range of potentials over which passivity may be achieved
is relatively large for the stainless steeVstrong sulphuric acid system.

These shell and tube heat exchangers are made from austenitic stainless steel. The hot
concentrated sulphuric acid flows on the shell side and water flows through the tubes.

Corrosion of the stainless steel by the hot acid is inhibited by anodic protection. A high water
velocity down the tubes ensures that potential problems of fouling are kept to a minimum.

The implications of a leak of acid into water or vice-versa demands high standards of design,
material quality and fabrication for this equipment duty. The hot exchangers have fixed
tubesheets, thus eliminating all mechanical connections between acid and water except for the
tube-tubesheet weld. This tube-to-tube sheet weld is made by a fully automatic two pass full
penetration welding technique that was specially developed to guarantee the integrity of this
critical joint.

The most reliable acid cooling system on the market today. The many units now in operation for
over 20 years clearly confirm that the projected life of this cooling equipment will far exceed the
life of the acid plant.

Using specially fabricated austenitic steels, these coolers have been built for direct seawater
cooling applications with the same proven reliability.

In areas where cooling water is scarce, water may be circulated in series through the acid coolers
and an air cooler to discharge the heat load into the atmosphere when necessary.

The acid coolers are ideally suited for energy recovery applications. Typical applications include
boiler feedwater heating, district heating of community housing, generation of LP steam. Other
applications considered include; heating for greenhouses, water heating for mineral leaching,
desalination and gypsum pond water heating to enhance evaporation.


Acid Piping Systems

The traditional material for strong sulphuric acid piping was grey cast iron and more recently,
ductile or nodular cast iron. Piping specified with these materials was usually designed for a
relatively low acid velocity of 3-4 ft./sec. (1-1.2 m/sec.) to prevent erosion/corrosion of the pipe
wall. Even at low design velocities cast iron piping systems have a limited life



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The use of SX or Ze-cor high silicon austenitic stainless steel in all acid piping to the towers
has significantly improved performance of acid circulating systems. The acid pipe diameters and
wall thicknesses can be reduced due to the excellent corrosion resistance of stainless steel. The
diameter of all fittings, valves, etc., can be reduced as a consequence of the reduced pipe size.

The materials offered by Fenco greatly improve the safety of operation for the plant operating
personnel. Stainless steel piping can be readily field assembled and installed. Prefabricated spool
pieces greatly simplify field assembly. Most flanged joints (flanges) are eliminated. Spare piping
inventory can be reduced to minimum levels.




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