Raschig USA Inc.

CO2 degasification air stripping tower design - Case study


Courtesy of Raschig USA Inc.

Background - Water Chemistry Controls the Design
A common method to adjust pH of water - typically well water intended for potable use - is to pass the water through a packed tower. Such towers intended to degas the water was also commonly called 'air strippers'. This is because the water is passed down over a bed of 'packing1 as air is blown into the bottom of the tower to pass up and out of the tower.

Note: as a result the water and air pass each other 'counter currently'. This allows for the most efficient transfer of CO: from the water into the passing air that is possible.

But - it is also very important to consider the pH of the water being degassed. Both the initial pH of the water being treated, as well as the desired final pH of the degassed / treated water, will likely determine if a stripper tower / degassifier is practical. This is so because CCh when dissolved in water can assume 3 possible molecular forms, or mixture of these, dependent upon the pH of the water. The chemistry of CO2 dissolved in water is a little complicated:
C02 + H20 -* C02 (dissolved) in H20. forms H2CO3. predominates at pH - 4 H2CO3 -> H+ + HCO3': this is the predominate form of C02 in water at pH -8 HCO3' —► H* + CO}2: this is predominate form of CO2 in water at pH - 13

Please note: All of the above reactions are reversible.

Additional note: Only dissolved CO2 (or 'free' CO:) can be removed by air stripping the water!

Air Stripper / Degassifier Design Consequences
As a result of the pH dependence of which of the three forms CO2. or mixture of forms, will be found in water, an absolute rule for air stripper / degasification design is:

As only 'free' CO2 can be air stripped! Therefore, as a practical limit, the initial pH of the water to be treated must be no more than 6 and preferably less for this method of CO 2 removal from water to be practical!

And. as free C02 is removed from the water, the above chemistry lesson has the consequence that as the water passes down the tower, the pH of the water will rise. Therefore, another basic rule of water C02 degasification is:

As a practical consequence ofthepH dependence of the various forms 0fCO2 in water, thepH of the water leaving the tower can never be much greater than 7.

Water exiting a degassifier tower of pH slightly above 7 is sometimes achieved by adding several additional feet of packed depth in the tower. But to reach an exit pH of 7.5 is impractical.

Even a tower with a packed bed depth that would challenge Jack of 'Jack and the Beanstalk' fame the pH of the water will not adjust higher.

Note: This is due to the ionic nature of both Bicarbonate and Carbonate ions. The electrical forces holding the ions in solution are simply too great to be overcome by any amount of ait-passing through any depth of packing.

Result: Practical Air Stripper / Degassifier Design

Raschig-USA can assist in such tower design. Design data that must be answer are:

  • Inlet Water Flow Rate
  • Inlet Water Temperature
  • Inlet Water pH
  • Dissolved Mineral Content - typically expressed in terms of
  • hardness or Calcium and Magnesium content
  • Desired / Target Outlet Water pH
  • With the above data, Raschig-USA can advise:
  • Diameter of the tower required
  • Packing depth of the random packing required
  • Air flow required
  • Pressure drop across the tower

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