Caustic Scrubbing in Ethylene Plant 1

[AndreChE]

Dear all

Is there any program or calculation model to simulate a caustic scrubber?

I don't know how to define the NaOH and the water flow? Ho do they relate each other in order to have a fixed caustic concentration in the spent effuent?

Regards
AndreChE

 

[Homayun]

I know in Aspen you can define the reactions that take place. But first you have to define which Ions exist. For example you should define NaOH as Na+ and OH-. So you must put on paper all possible reactions and then define them in Aspen. Then when you simulate a column, they program takes all these reactions into consideration.

 

[sshep]

This may seem simplistic, but if you are just interested in the material balance relationships across the tower, you can probably just use:

CO2 + 2 NAOH --> NA2CO3 + H2O
H2S + 2 NAOH --> NA2S + 2 H2O

and for the various circulating loops of the tower consider each section as a single CSTR with the caustic concentrations actually in use in the plant. This will tell you how much caustic make-up is needed, the exit acid gas composition from each stage, etc.

If you wish to get more rigorous you can model the following equilibrium and dissociation reactions. If you use the Aspen electrolytes model as Homayan suggests, use the wizard and select these reactions to show in your chemistry:

NAOH --> NA+ + OH-
H2O <--> H3O+ + OH-
H2S + H2O <--> H3O+ + HS-
HS- + H2O <--> H3O+ + S--
CO2 + 2 H2O <--> H3O+ + HCO3-
HCO3- + H2O <--> H3O+ + CO3--

An even more rigorous approach is to consider the kinetics. This is not something I have done much of for this system, and cannot give a specific approach, but because the reactions themselves are fast the diffusion of acid gas into the liquid can probably be considered the limiting step. As such it may be possible to model using whatever software you are using to do HETP estimations, but if you are actually designing the mass transfer equipment it would be wise to get technical assistance from Koch, etc.

hope this helps, sshep

p.s. Others please correct me if I have stated anything incorrect or unreasonable.

 

[AndreChE]

Thank you both Homayun and sshep

The balances that I was looking for is the degree of exploitation of the caustic in the tower.

The water and the fresh caustic flows are two variables that I was considering. The relation between both was my concern and now it's solved.

Take a look in the paper of Marcello Picciotti - "Optimize caustic scrubbing systems", Hydrocarbon Processing, May 1978.

Thank's again
AndreChE