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Gas Cleaning System - Weak Acid Cooling
September 7, 2001
Introduction
Plate heat exchangers are typically used for the majority of
all weak acid cooling duties.
The original idea for a plate and frame
heat exchanger was patented in the latter half of the 1800’s but the first
commercially successful design was introduced in 1923.
In the 1930’s plates pressed in thin gauge stainless steel were
introduced.
A plate and frame heat exchanger consists
of a frame in which closely spaced metal plates are clamped between a head and follower
plate. Fluid enters and leaves the plate pack
through ports located in the corner of the plates. Gaskets
are located around the ports and the plate edges which prevent the mixing of fluids and
the escape of liquid out of the plate pack.
Heat
exchanger plates can be pressed out of any material that is ductile enough to be formed
into a pressing. Plate materials are
typically 0.5 to 0.7 mm thick.
It is very important that the material
selected for a particular application be highly corrosion resistant to the process fluid
being handled. A corrosion rate that is
acceptable for a vessel which has relatively thick wall would not be acceptable for a
plate heat exchanger simply because the plate thickness is so much smaller.
Typical materials used in sulphuric acid
plant applications are:
316L SS
Weak acid applications
904L SS
Weak acid applications
254 SMO
Weak acid applications
Hastelloy® G-30
Weak
acid applications
Alloy C-276
Weak/strong acid applications
The selection of the proper gasket material
must take into account the fluids being handled, operating temperature, and the sealing
properties of the material itself. Typical
materials used in sulphuric acid plant applications are:
EPDM
Weak acid cooling applications
Viton
Weak/strong acid cooling applications
The plate heat exchanger should never be
opened unless absolutely necessary as damage to the gasket may occur.
After a few years of operation, the gasket
at the hottest points may slowly lose their elasticity and the glue will no longer retain
its full bonding strength. This can lead to a
section of the gasket loosening from the plate when the unit is open. In some cases the gasket can be simply be glued
back in place but in more severe cases the entire gasket may need to be replaced.
The normal life of Viton gaskets is about 5
to 6 years in normal service for strong acid cooling.
As a general rule of thumb, the life of the gasket will be reduced by half
for every 10°C above 90°C that the unit operates at for prolonged periods of time.
Plate heat exchangers constructed of Alloy C-276 plates and Viton gaskets
impose certain limitations on the process in order to achieve reliable and extended
operation of the heat exchangers. The maximum
temperature into a plate heat exchanger is a function of acid concentration. Exceed the maximum temperature will result in
higher corrosion rates and reduced life of the plate pack.
Concentration
Maximum Temperature
98%
90°C (200°F)
92%
70°C (160°F)
70%
60°C (120°F)
The acid concentration should never be
allowed to exceed 100% H2SO4
as the free SO3 will attack the gasket material causing it
to swell.
The maximum permitted acid velocity for 98%
H2SO4 is 3 m/s (9.8 ft/s) due to the risk of
increased erosion and corrosion.
The cooling water outlet temperature should
exceed 40°C (104°F). Carbonate fouling
increases significantly at temperatures above 40°C.
Bypassing fluid on the process side of the
heat exchanger should always be used to control the process temperature. Water flow should never be throttled or reduce as
increased fouling will occur on the cooling water side.
The inability to maintain the required
process temperature will generally indicate that the cooling water side of the heat
exchanger is fouled. This condition occurs
when the acid side bypass is fully closed.
The temperature difference between the
cooling water inlet and outlet will indicate the thermal performance of the unit since the
cooling water flow through the exchanger should be constant. Thus, higher cooling water outlet temperature
will indicate a higher heat load on the unit.
Every effort should be made to ensure that
the process side operating pressure is higher than the cooling water side of the
exchanger. If a leak occurs in the unit, acid
will leak into the cooling water and can be readily detected whereas a leak of cooling
water into the acid is more difficult to detect.
An instrument measuring pH or conductivity
of the cooling water leaving the exchanger is the simplest and method of detecting a leak. If a leak is suspected the heat exchanger should
be taken out of service, inspected and the necessary repairs performed.
Gasket leaks will always appear underneath
the heat exchanger. Tightening the plate pack
by a few millimetres can eliminate some leaks. If
this does not work the unit will need to be taken out of service and the gaskets
inspected.
A plate heat exchanger should always be
equipped with a plate pack cover which will prevent external leaks from spraying out and
direct the liquid down. A drip tray is often
provided underneath the exchanger to collect any liquid and direct it to a safe location. This is particularly necessary if the heat
exchangers are located above grade on a platform.
The most severe conditions in a plate heat
exchanger are at the acid inlet. Corrosion
rates cane expected to be highest in this region. The
temperature will decrease as the acid is cooled in the exchanger so the corrosion rate
will be less at the acid outlet. The
symmetry of the heat exchanger plates allows for the plates to be rotated so that the acid
inlet end becomes the acid outlet and vice versa. The
effect of doing this is to extend the life of the unit by exposing the less corroded end
of the plate to the more aggressive conditions.
The exact method of reversing the plates
will depend on the geometry of the plates and their arrangement in the frame. Consult the manufacturer’s instructions for
details.
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