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.