2 replies to this topic

[uetian98]

Hi!

This is my first post here. I am a student and am working on a university project that involves Simulink Modelling of a pilot scale rig.

This pilot rig consists of a water filled glass walled Jar with an impeller (vertically mounted) in there. Water enters the Jar near the bottom and leaves (overflows) from a point near the top.

Nitrogen is bubbled via a sparger sitting on the botom of the jar to disturbed the already dissolved oxygen (resulting in desorbing the Oxygen out of water body). We have analysers installed that measure the oxygen contents in the water body.

Here, I am looking forward to a correlation between Liquid Mass transfer Coefficient K_L and agitation speed. Though its a direct correlation but have no knowledge of exact correlation.

I am stuck here.
Can anyone Help me please.

Thanks.
AHMED
awais_bhatti@hotmail.com

[ming]

I think you may search some articles on the agitator sizing and design, which will give the correlation formula between the Mass transfer coefficient and agitation speed.

For your question, here I give the following formula for your referrence.

KLa = k1*(P/V)^a *u^b

Here, a--specific surface area of gas / specific volume

P/V--agitator power per unit volume

u --- apparent gas velocity

a, b ---- dimensionless power number

Here, P is the agitator power with the gas bubbling. If no gas bubbling in the jar, the agitator power P0 will be larger than P. Based on my engineering experience, P/P0 locates is about 0.4 or so.

If you calculate P, you can get the agitator speed based on the agitator power number provided by the agitator's suppiliers.

Good luck!

Ming


ming.guo@iff.com

[uetian98]

let me be more specific,
Unsteady state mass transfer of Bubbling Nitrogen is given by

V(dC₀/dt)=F_W (C₁-C₀) - N_AV



Here
V.....................Volume
C₀.....................Outlet conc. of Oxygen in Water
C₁.....................Inlet conc. of Oxygen in Water
F_W.....................Volumetric Flow rate of Water
N_A.....................Mass Transfer rate (N_A = K_LA(C₀-C*)
K_L.....................Liquid Film Mass Transfer Coefficient
A.....................Surface Area of Bubbles / Unit Volume
C*.....................Equilibrium Conc. of Oxygen in Water



Now I will replace N_A with K_LA etc. Literature says that K_L.A can be lumped together to have Volumetric Film Coefficient and that is some function of agitation speed.

Now what is that relationship --- this is the mystery.