5 replies to this topic

[SarahChemmy]

Hi guys, first post so be kind. 

 

I'm currently studying Gas absorption and I'm stuck on a particular question. 

 

A gas stream containing 5 mol% Ammonia (NH3) in Air is to be passed to a packed absorption column at a rate of 10 kg.s-1 . The column is to use Water (H2O) as the solvent to reduce the ammonia content in the air leaving the column to 0.01 mol%. The gas and water streams can be assumed to be at 25 °C. The column operates at 1 bar pressure.

What is the minimum liquid flow rate and the liquid rate of solvent under normal operating conditions?

 

 

H = 1.3

Kg = 200 × 10^−6 kmol.m^-2 .s^-1

separation using 1 inch Berl Saddles

 

I assume I will have to make an initial guess for the flow rate of the solvent?

 

Can I get some guidance for this question please? I'm really stuck and there's not much study material for this topic online.

 

Thanks

Sarah

 

[SarahChemmy]

bump?

[Saml]

It would be unfair to give you an answer. I would do your work (I don't want to), and you would not learn (I hope you want to learn), and what is most important, if you don't grasp the concepts of absorption, you will have a very hard time understanding distillation later on.

 

But what can be done is to point you to different information.

Sometimes seeing the information expressed in other ways helps when you are stuck.

One clear explanation about this is in Treybal Mass Transfer Operations, chapter 8 in my edition (Gas Absorption). It even has a subchapter named "Minimum flow"

 

The things you need to consider when looking at "operating line" and such are:

- You want the water leaving the bottom of the absorber to carry all the ammonia you remove.

- You are feeding a certain concentration of water to the top (in your case, pure water)

- You cannot get any concentration in the liquid higher than the equilibrium with the gas at any point in the absorber, or any concentration in the gas below the equilibrium with the liquid.

 

Try to find where these considerations are implicit in the equations and the method and you will be 80% done.

 

Then go to the additional 20% about the details of calculating the internals, number of stages or transfer units, etc, etc...

[Francisco Angel]

I agree with saml. I recommend you to read Chapter 6 of Separation process principles, by Seader. The concepts are explained, followed by examples worked to great detail. I'm sure you will be able to solve this kind of problem with relative ease. Best regards.

[breizh]

Sarah ,

everything you need is there. As others said , take time to study and learn.

 

Breizh

[SarahChemmy]

Thanks guys, appreciate it. I didn't find distillation hard, nor the mcCabe methods but for some reason this process is doing my head it. 

 

Thanks Breizh

Edited by SarahChemmy, 10 November 2016 - 06:16 AM.