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