Modeling Air Stripping of Ammonia in an Agitated Vessel Page: 4 of 23
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WSRC-MS-2005-00685
is accomplished with two flat-blade turbine impellers. Figure 1 is a schematic diagram of the
vessel under consideration.
Two mass balances are needed to develop the model: an overall NH3 balance on the
vessel, and an NH3 balance on a differential element of the vessel for the air stream. An overall
NH3 mass balance on the vessel is given by eq 1.
dCL dCG
VL L + EGVL dtG QGCGO - QGCGf (1)
VLdt dt
where VL is the liquid volume (m3), CL is the liquid-phase NH3 concentration (kmol/m3), t is
time (sec), EG is the gas (air) holdup or the volume fraction of the air bubbles entrained in the
liquid (dimensionless), CG is the NH3 concentration in the air bubbles (kmol/m3), QG is the air
sparge rate or air volumetric flow rate (m3/sec), and CGo and CGf are the NH3 concentration in the
air bubbles entering and leaving the vessel respectively (kmol/m3).
For dilute solutions, the accumulation of NH3 in the air is negligible. Hence, the second
term in eq 1 is zero. Note also that CGo = 0. This yields eq 2.
VL dCL =-QGCcf (2)
dt
The concentration of NH3 in the air bubbles leaving the vessel is needed to solve eq 2. It is
obtained by performing a mass balance on a differential element of the vessel for the air stream.
The mass balance is described by eq 3.
EdzA CG QGCG -QG (CG +dCG)+dzAkLa(CL CL) (3)
where dz is the height of the differential element (m), A is the vessel cross-sectional area (m2), kL
is the liquid-phase mass transfer coefficient (m/s), a is the specific interfacial area of bubbles per
unit volume of air-liquid mixture (m2/m3), and CL* is the NH3 concentration in the liquid in
equilibrium with the air bubbles (kmol/m3). The last term in the equation is the rate of mass
transfer across the air-liquid interface. For this air-liquid system, the rate of mass transfer isPage 3 of 22
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Kofi, Adu-Wusu; Martino, Christopher J.; Wilmarth, William R.; Bennett, William M. & Peters, Robert s. Modeling Air Stripping of Ammonia in an Agitated Vessel, article, November 29, 2005; Aiken, South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc873356/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.