In an absorption column, ammonia gas diffuses through a 1 mm thick layer of stagnant air and is instantaneously absorbed
Posted: Fri May 27, 2022 8:19 am
In an absorption column, ammonia gas diffuses through a 1 mm
thick layer of stagnant air and is instantaneously absorbed on
contact with a liquid water film (the resistance to mass transfer
is entirely gas-phase controlled, and the ammonia concentration at
the interface is negligible). The diffusivity of the ammonia-air
mixture is 1.8 × 10-5 m2 .s-1 at 295 K and 1 bar absolute pressure.
Consider a case where the bulk vapour contains 60 mol% ammonia.
Investigate the effect of temperature and pressure on the mass
transfer flux by considering temperatures from 295 to 1000 K and
pressures of 1.0, 2.0 and 3.0 bar absolute. The diffusivity of the
mixture is proportional to T1.75.P-1 , where T and P are absolute
temperature and pressure, respectively.
a) Plot the mass transfer flux against temperature for each
pressure (as separate series on one graph). What dependence of flux
on temperature and pressure do you find in (a)? Comment on your
results.
b) What happens if the ammonia concentration at the interface is
not negligible, but if the absorbing solution exerts a partial
pressure of ammonia of 20 kPa? Consider the mass transfer
resistance to remain on the gas-side only. Plot flux against
temperature for each pressure (as separate series on one graph). In
what ways does the plot differ to that of (a)?
thick layer of stagnant air and is instantaneously absorbed on
contact with a liquid water film (the resistance to mass transfer
is entirely gas-phase controlled, and the ammonia concentration at
the interface is negligible). The diffusivity of the ammonia-air
mixture is 1.8 × 10-5 m2 .s-1 at 295 K and 1 bar absolute pressure.
Consider a case where the bulk vapour contains 60 mol% ammonia.
Investigate the effect of temperature and pressure on the mass
transfer flux by considering temperatures from 295 to 1000 K and
pressures of 1.0, 2.0 and 3.0 bar absolute. The diffusivity of the
mixture is proportional to T1.75.P-1 , where T and P are absolute
temperature and pressure, respectively.
a) Plot the mass transfer flux against temperature for each
pressure (as separate series on one graph). What dependence of flux
on temperature and pressure do you find in (a)? Comment on your
results.
b) What happens if the ammonia concentration at the interface is
not negligible, but if the absorbing solution exerts a partial
pressure of ammonia of 20 kPa? Consider the mass transfer
resistance to remain on the gas-side only. Plot flux against
temperature for each pressure (as separate series on one graph). In
what ways does the plot differ to that of (a)?