Gas Cleaning System - Quench Systems
June 8, 2003

Introduction Liquid System Gas Retention Time Emergency Quench Water Materials of Construction

Associated Links Materials of Construction

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Introduction

The quench tower and system serves the following functions:

• Adiabatic saturation of the incoming gas
• Partial cleaning of the gas
• Provide residence for the condensation of metallic vapours to facilitate their subsequent removal in downstream equipment
• The tower may be designed to provide a liquid reservoir for operation of the pumps and holdup of the liquid on shutdown of the system

The traditional quench tower design consisted of an open vessel in which liquid is sprayed to contact the gas.  The gas typically enters the bottom of the tower through a side nozzle and flows upwards, counter-current to liquid that has been sprayed from the top of the tower.  By the time the gas has reached the top gas outlet, it has been cooled to its adiabatic saturation temperature.  A variation of this design is to have the gas entering the top of the tower and contacting the liquid co-currently as it travels down through the tower.

A packed tower has also be used as quench tower.  The packing provides additional gas-liquid contact by the film of liquid that forms on the surface of the packing.  The use of a packed tower may further enhance the removal of solids.

A variation of the co-current design is to design the tower as a low pressure drop venturi.  Liquid is fed into the tower above the throat of the venturi section and through sprays located below the throat.  The turbulent contact between the gas and liquid in the throat quenches and cools the gas and provides some degree of cleaning, however, the design is not meant to remove the vry fine sub-micron particles.

Quench towers have also been designed using the reverse jet scrubbing principle.  The gas enters the top of the reverse jet scrubber barrel and flows downwards where it contacts a liquid that has been sprayed from the bottom.  At some point in the barrel the momentum of the liquid is cancelled and a zone of intense mixing and turbulence is created.  Highly efficient cooling and cleaning of the gas is achieved.

Once the gas is cooled it proceeds into the next stages of the gas cleaning system.  One of the key functions of the quench system is the cooling of the gas which allows a change in the materials of construction from bricklined carbon steel to plastics and FRP.

Gas Retention Time

The gas retention time must be sufficient to allow the metallic vapours to be condensed into fine particles which will facilitate their removal in downstream equipment.  The gas retention time based on the inlet gas flow is typically a minimum of 3.0 seconds.

Liquid System

The liquid circulating system of the quench tower consists of one or more circulating pumps, a liquid reservoir, piping system, controls and spray nozzles.   Liquid is fed into the quench tower through a series of spray nozzles and/or feeder pipes.  The spray nozzles are designed to provide full coverage across the quench tower cross-section and to finely atomize the liquid to provide intimate contact between the gas and liquid.

The liquid fed into the tower falls to the bottom of the tower which is often used as the liquid reservoir.  Alternatively, the liquid may drain from the tower into a separate pump tank.  Liquid level in the reservoir is typically maintained by controlling the bleed stream from the quench system.  Liquid is pumped from the reservoir by one or more operating pumps.  The pumps are typically horizontal centrifugal pumps with materials of construction suitable for the liquid conditions.  One or more pumps are installed a standby pumps in case one of the duty pumps should fail. 

The quench process results in the net evaporation of liquid into the gas.  To maintain liquid in the system, liquid is added from the gas cooling system in which liquid is condensed from the gas.  The acid concentration and the levels of impurities is controlled by the amount of make-up water that is added to the gas cleaning system.  The more make-up water that is added, the lower the acid concentration and level of impurities in the quench system.  The bleed from the quench system is usually sent to the weak acid stripper before being sent to the effluent treatment syetem.

The quench system is usually operated with a weak acid concentration fo less than 10% H₂SO₄.  In some instances, the system is operated at higher acid concentrations up to 30% H₂SO₄.  This is done to reduce the weak acid effluent flow to the treatment system to reduce the size of the equipment.  The stronger weak acid also has the potential to be treated for reuse as dilution in the acid system.

Emergency Quench Water

In the event of a reduction or failure of the circulating liquid flow, the gas temperature exiting the Quench Tower will rise quickly.  The high temperature may result in damage to the downstream FRP ducting and equipment.  When the temperature exits the Quench Tower is too high, the flow of emergency water is started.  At the same time a plant shutdown is initiated.  The flow of emergency water will continue until the gas exit temperature drops to a safe level.

The source of emergency water must be an uninterruptable supply of water.   The supply can be from a water main such as a firemain.  An elevated head tank having sufficient volume for the expected duration of the high temperature can be used.

The flow of emergency water is dependent on the design of the spray system for the quench tower.  Emergency water may be fed into the quench tower through a set of dedicated nozzles or some of the spray nozzles may serve a dual function of emergency water and normal process spray nozzles.  The flow and pressure of emergency water should be sufficient to cool the gas and the nozzles selected to provide adequate coverage across the tower.

The duration that the emergency water is required to be on will depend on how long it will take to safety shutdown the plant to prevent the ingress of hot gas into the the quench tower.

Materials of Construction

The materials of construction for a quench system must be selected carefully to avoid operating and maintenance problems.  The system is subject to hot gas, weak acid and regions where the conditions vary from wet to dry and hot to cold.  The use of specialty alloys and brick linings are typical.  Click here for more details on materials of construction.

In a counter-current tower design, the area the sees the most severe conditions is the gas inlet.  Here the gas is still hot but is quickly quench as it enters the tower.  The design of the inlet nozzle should prevent the formation of eddies that would cause weak acid to be drawn up the gas inlet where it can attack the unprotected carbon steel ducting.  When gas flow is stopped, liquid circulation over the tower usually is left on.  Weak acid can migrate up the inlet gas duct and condense on the inside surface of the carbon steel duct.  Weak acid is extremely corrosive to carbon steel.  When the gas flow is resumed, the region is then exposed to higher temperatures which can further increase corrosion.

In a co-current tower design, the gas inlet is also the most vulnerable to corrosion and attack.  Spray nozzles are sometimes exposed directly to the hot gas.   Materials must be selected to withstand both high temperatures and corrosion from the weak acid.