Caustic Scrubbing in Ethylene Plant
Caustic Scrubbing in Ethylene Plant
(OP)
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
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





RE: Caustic Scrubbing in Ethylene Plant
RE: Caustic Scrubbing in Ethylene Plant
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.
RE: Caustic Scrubbing in Ethylene Plant
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