Strong Acid System - Towers
February 5, 2003

Introduction Drying Towers Absorbing Towers Stripping Towers

Associated Links Pressure Grouting Erection Sequence Materials of Construction Acid Resistant Linings

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Introduction

There are three basic types of towers that can be found in a sulphuric acid plant:

• Drying
• Absorption
• Stripping

Drying Towers

Drying towers are designed to remove water from the incoming gas.  The gas can either be ambient air or water saturated gas leaving the gas cleaning section of a metallurgical or acid regeneration plant.  Typically, a tower is designed to dry the gas to a -40°C dewpoint.

The acid strength can varying from as low as 78% to as high as 98.5% H2SO4.  An acid plant designed to handle gas containing low concentrations of SO2 may be equipped with a pre-drying tower designed to operate at 78% H2SO4.   A metallurgical or acid regeneration plant will operate with an acid concentration of 93% or 96% H2SO4.  A sulphur burning acid plant may operate with an acid concentration of 98.5% H2SO4 to the tower.

The operating temperatures of a Drying Tower are low compared to those in an Absorber Tower and generally will not exceed 70°C (158°F).  Gas inlet temperatures are relatively low corresponding to the maximum ambient air temperature or the gas temperature leaving the gas cleaning section.  Acid temperatures are a function of the acid concentration.  As the concentration of the acid increases higher operating temperatures are generally used and are limited by the material of construction of the tower, piping and acid cooler.

The operating pressure of a Drying Tower will vary depending on the type of plant.  Towers operating under a high vacuum are typical of metallurgical and acid regeneration plants.  Towers in sulphur burning acid plants can operate either under a slight negative pressure or high positive pressure depending on the location of the blower.

Absorbing Towers

Absorbing towers are designed to remove sulphur trioxide from the gas.  In a single absorption acid plant there is only one tower located before the stack.  In a double absorption plant there are two Absorber Towers, the first generally located after beds 2 or 3 and the second tower located before the stack.   The absorption of SO3 by the circulating acid stream increases the acid concentration and through the process of dilution, the product acid is produced.

Intermediate or Interpass Absorbers refer to the abosrber located after bed 2 or 3 in the pocess.  The tower located before the stack is simply refer to as the Absorber Tower in a single absorption plant or as the Final Absorber in a double absorption plant.

Absorber towers generally operate in a narrow range of acid concentrations and temperatures of approximately 97.5% to 99.5% H2SO4 and 75°C (167°F) to 90°C (194°F) .  The vapour pressure of SO3 above the acid in this concentration and temperature range is at a minimum resulting in greater absorption efficiency.  In practice, the acid concentration and temperature are varied within the range to achieve the best tower performance.  Each tower seems to have its own unique operating point at which it performs best.

Virtually all Absorber Towers operate under a positive pressure.  Intermediate or Interpass Absorbers operate a much higher pressures than Final Absorbers which are located at the end of the process where the gas is being discharged to atmosphere.

Stripping Towers

Stripping tower are designed to remove dissolved SO2 from the acid entering the tower.  Anbient air is generally used as the stripping medium.  The air leaving the stripper enters the process as dilution air ahead of the Drying Tower or as quench air between one of the converter beds.  Strippers are feature of metallurgical or acid regeneration plants.  Sulphur burning acid plants do not require strippers.

Stripping towers can either be cross-flow strippers or product strippers depending on the acid stream being treated.  Cross-flow strippers remove SO2 from the drying acid cross-flow to the absorber to prevent SO2 transfer from one system to the other.  Product strippers remove SO2 from the product acid stream to ensure that product acid specification are met.