|
Gas Cleaning System - Gas Cooling
June 10, 2003
Introduction
Cooling of the process
gas is required to remove excess water from the gas so that sulphuric acid of the desired
concentration can be produced. This is
commonly referred to as the plant water balance. Cooling of the gas can be done either by direct
contact or indirect cooling of the gas.
Direct cooling of the
gas involves contacting the gas directly with the cooling medium. This generally done in a packed cooling tower
with weak acid as the cooling medium.
Indirect gas cooling is
generally done in a some type of heat exchanger, the most common being vertical shell and
tube condensers cooled by cooling water.
Direct Contact
Coolers
There are basically two type of direct contact coolers in use
for cooling process gas; packed towers and tray towers. This type of cooling
equipment provides not only gas cooling but some degree of gas cleaning as well.
Generally, smaller temperature approaches can be achieved with a direct contact cooler
than with an indirect gas cooler.
Packed Towers
A colmns packed with
random or structured packing can be used as a direct contact cooler. The gas cooling
tower is a vertical cylindrical tower in which gas flows up through the packing against a
counter-current flow of weak acid. The weak
acid enters the top of the tower and is distributed by spray nozzles or distributor (i.e.
trough type) across the top of the packing. The
weak acid flows downward through the packing and collects either in the bottom of the
tower or in an external pump tank. Pumps
circulate the weak acid through weak acid coolers, typically plate and frame type, an back
to the top of the tower. Since water
condenses from the gas into the weak acid, the volume of the weak acid increases. Level in the reservoir is maintained by pumping
weak acid out of the system to the upstream system.
Gas is cooled by direct
contact with the re-circulated weak acid. Since
the gas enters the gas cooling tower saturated, water is condensed from the gas as it
cools. The cooling tower performs both
sensible cooling of the gas (i.e. heat transfer) as well as mass transfer (i.e.
condensation of water). The design of a gas
cooling tower must take into account both the temperature difference between the weak acid
and the gas which is the driving force available for heat transfer as well as the
difference in partial pressure of water vapour which is the driving force for mass
transfer. The packing in the tower provides
the contact surface area for mass and heat transfer.
Gas cooling towers can
achieve a minimum approach of 1.7°C (3°F) approach between the outlet gas and inlet weak
acid temperature. Approaches less than 1.7°C
are generally not used since they will result in excessively high packing depths. The temperature approach to be used for design
will also depend on the available cooling water temperature which determines the size of
the weak acid cooler. The gas to weak acid
temperature must be set to give a suitable LMTD for the design of the weak acid cooler.
Fibreglass reinforced plastic (FRP) and
thermoplastics are the usually materials of construction for a gas cooling system. The Gas Cooling Tower will be FRP construction
with a suitable corrosion layer. When
fluorides are present, a protective synthetic liner will be incorporated in the corrosion
layer. Since operating temperatures are
relatively low, polypropylene can be used for the packing material. Piping can be FRP with the appropriate corrosion
layer or of dual laminate construction.
Tray Towers
This type of scrubber/cooler is sometimes referred to as a
Peabody Scrubber, named after the company the designed them. A tray tower consists
of a number of trays or stages in which the gas flows upwards through a perforated plate
with the liquid flowing horizontally across the plate. There is turbulent mixing of
the gas and liquid mix at each hole in the tray which cools the gas and condenses water
from the gas.
Liquid is introduced to the top tray by a pipe distributor or
overflow weir. The liquid flows across the tray to the downcomer which brings the
liquid down to the next stage. A weir provides a liquid seal to prevent gas from
bypassing the tray and flowing up the downcomer. Small towers will be designed with
a single section of tray for each stage. In larger towers, the liquid may be
introduced in the middle of the tray and the liquid flows out across the tray in opposite
directions to separate downcomers located at opposite ends of the tray. Further
partitioning of the tray can be done with the use of multiple feed pipes.
Tray towers are ideal for plants that operate within a narrow
range of gas flows. If the gas flow is reduced too much, there is insufficient
pressure drop across the holes such that the gas is no longer able to support the liquid
head on top of the tray. Mixing and contact between the gas and liquid is reduced
and the liquid will then begin to weep down through the holes instead of across the tray.
Overall cooling efficiency is reduced to the point where the desired cooling cannot
be achieved.
Indirect
Coolers
Indirect coolers can be any type of heat
exchange equipment used to cool the gas. The gas generally flows down the tube side
of the exchanger with the cooling medium flowing counter-currently up the shell side.
The main difference between the different types of indirect coolers is the material
of construction.
Shell and Tube Condensers
Indirect cooling of the
gas stream can be done in vertical shell and tube condensers. Process gas generally enters the tube of the
exchanger and flow down through the tubes. Cooling
water generally flows up through the shell side of the unit.
As the gas cools, water
condenses in the tubes and flows down the inside of the tubes. Water or weak acid is often sprayed on to the
inlet tube sheet to prevent the build up of solids on the tubes sheet and to create a
liquid film down the inside of the tubes to keep the tubes clean.
Shell and tube
condensers are constructed of materials suitable for the service. In some cases, 316L stainless steel can be used
while in some cases more corrosion resistant materials such as Alloy C-276 must be used. In between, the two extremes are materials such as
904L stainless steel and 254 SMO. All wetted parts exposed to the process gas will
need to be corrosion resistant. The shell of
the condenser can be carbon steel since it is exposed to cooling water only. Tube sheets can be solid or carbon steel clad with
the corrosion resistant material.
The inlet or top tube
sheet is exposed to the most severe conditions from a corrosion point of view. If the unit is designed to be completely
symmetrical, then the unit can be rotated so that the outlet or bottom tube sheet becomes
the inlet tube sheet. This technique
essentially doubles the life of the unit.
Shell and tube
condensers can be economically attractive compared to other methods since there is not
circulating system, pumps or external cooler required.
However, the high cost of the material of construction, particularly alloy
C-276 may offset the cost advantage.
Karbate Coolers
Like shell and tube
exchangers, karbate coolers provide indirect cooling of the process gas. Process gas is cooled and water condenses as it
flows down the tubes. The resin impregnated
carbon offers excellent corrosion resistance to the weak acid.
The block style of
karbate coolers is the more common type used for gas cooling although there is no reason
why the shell and tube style cannot be used.
Star Coolers
Star coolers are
essentially shell and tube condensers but constructed out of lead. Lead offers excellent corrosion resistance against
the effect of weak acid. To enhance heat
transfer, the lead tubes are extruded with internal fins.
The main disadvantage of
lead is its limited mechanical properties compared to stainless steel. The shell side operating pressure is limited to
the point where a standard cooling water supply and return system cannot be utilized in
most cases. Cooling water supplied to the
star coolers cannot be return to the cooling tower directly since the resulting operating
pressure would exceed the limits of the lead tubes.
The solution is to have the water return to a ‘hot well’ where a pump
would return the cooling water to the cooling tower.
| 
