Gas Stripping Tower Handbook

Design tool for packed bed gas stripping towers and degasifiers

Introduction

Gas stripping involves the mass transfer of a gas from the liquid phase to the gas phase. The transfer is accomplished by contacting the liquid containing the gas that is to be stripped with air that contains a much lower concentration of that specific compound. This process is commonly used for removing CO2, Ammonia, odors and VOCs.
Packed towers are columns containing packing that disrupts the flow of liquid, expanding and renewing the air–water interface and accelerating the natural migration of the volatile compounds or gases from the water to the air.

Design tool

Web Based Excel Interface

Diagram

Diagram

Quick calculation instructions

  1. Plant design inputs
    • Use the default safety factor for the tower height to account for the deviation of the Onda model.
    • Safety factor for the cross sectional area will only be used if the diameter was calculated based on the gas pressure drop.
  2. Economical input factors
    • Stripping factor defines the Air/Water ratio. Higher stripping factors, lower the packing height but the energy consumption will increase. Economical stripping factor are around 3.5.
    • Target air pressure drop will define the tower area. Higher pressure drops will reduce the area but increase the energy consumption. Economical values are between 50 an 100Pa/m.
    • Tower diameter: This parameter can be specified (enter a diameter) or type the word false for automatic calculation. It is recommended to automatically calculate and then recalculate with the final diameter selected.
  3. Contaminant properties
    • Specify the index (number) of the contaminant according to the database references. If you want to fill the chemical properties manually, type the word false in the index field.
    • The remaining fields will be filled automatically by the database data if you entered an index.
  4. Packing media properties
    • Specify the index (number) of the material according to the database references. If you want to fill the packing properties manually, type the word false in the index field.
    • The remaining fields will be filled automatically by the database data if you entered an index.

Calculation model description

  1. Absolute pressure at the plant altitude is calculated [2]
  2. Air density, Air viscosity, Water density and water viscosity are calculated at the design temperatures [3],[2],[4],[5]
  3. Henry constants from the database are corrected by the temperature using the data provided by NIST [11]
  4. Air/Water ratio is calculated from the stripping factor and the Henry constant [1].
  5. Limiting pressure drop is calculated using the Eckert linearized curves [7].
  6. Area and diameter are calculated from the limiting pressure drop.
  7. If the diameter is too small to prevent the wall effect or if the user defined the diameter, the minimum or user value will be adopted.
  8. If the diameter was different from the calculated from the limiting pressure drop, the pressure drop will be recalculated using the same Eckert curves [7].
  9. Final pressure drop is verified against the flooding limits [8].
  10. Liquid phase diffusion coefficients are calculated using the Hayduk-Laudie equations [9].
  11. Gas phase diffusion coefficients are calculated using the Wilke-Lee equations [10].
  12. Overall mass transfer coefficients are calculated using the updated Onda correlations [1].
  13. Height of the packing tower is calculated [1].
  14. Total pressure drops for the water and gas phases are calculated.
  15. Energy consumption is calculated [1].

Known limitations and important notes

  • The model assumes the concentration of the target compounds in the atmosphere is zero. For gases present at higher concentrations like CO2, N2 and O2, the effluent concentration will be affected by the equilibrium and the model will underestimate the required tower size at low product targets. For example, with CO2 concentrations lower than 5mg/L in the product, the calculated packing height tends to be lower than the actually required. CO2 concentrations lower than 0.5-0.7mg/L are impossible to get because this is lower than the equilibrium.
  • Packed towers are not good for aerating water for Iron and Manganese removal due fouling in the packing material and costs. Simple tray aerators are recommended in this case.
  • The diffusion and mass transfer coefficients calculated are valid for absorption and desorption of gases.

Literature references

[1] MWH’s water treatment : principles and design. – 3rd ed. / revised by John C. Crittenden et al.
[2] Metcalf & Eddy, AECOM - Wastewater Enginering: Treatment and Resource Recovery, 5th Edition, McGraw-Hill 2014
[3] McCutcheon, S.C., Martin, J.L, Barnwell, T.O. Jr. 1993. Water Quality in Maidment, D.R. (Editor). Handbook of Hydrology, McGraw-Hill, New York, NY
[4] Roberson, J.A., and C.T. Crowe, Engineering Fluid Mechanics, 4th edition, Houghton Mifflin, 1990
[5] R.C. Weast, 1983, CRC Handbook of Chemistry and Physics, 64th edition
[6] Weast, R.C., A.J. Melvin, and W.H. Beyer, CRC Handbook of Chemistry and Physics, 64th edition, CRC Press, Boca Raton, Florida, 1985
[7] Boadway, D.E., "Decision Analysis: Stripping and Adsorption to Treat Organic Hazardous Waste," M.S. Thesis, University of Texas at Austin, 1990
[8] Strigle, R. F., Jr., “Packed Tower Design and Applications”, 2 nd Ed., Gulf Publishing, Houston, TX , 1994
[9] Hayduk, W., and Laudie, H. (1974) ‘‘Prediction of Diffusion Coefficients for Nonelectrolytes in Dilute Aqueous Solutions,’’ AIChE J., 20, 3.
[10] Wilke, C. R., and Lee, C. Y. (1955) ‘‘Estimation of Diffusion Coefficients for Gases and Vapors,’’ Ind. Eng. Chem., 47, 6.
[11] NIST Standard Reference Database, https://webbook.nist.gov/chemistry/