3 replies to this topic

[g3wtter]

Hello,

 

I am trying to simulate an formaldehyde absorption in water in aspen plus. My problem is that no matter what fluid package I use most of the formaldehyde leaves in the vapor phase and not in the aqueous phase. I have to use a ridiculously high water stream to absorb most of the formaldehyde and then I end up with a 5% solution.

 

Does anyone have an idea what fluid package to use or some other hint how to model an absorption column for this case?

 

kind regards,

steve

[ELVINA]

Hello, 

I work in UniSim Design and this problem is usually solved by changing binary coefficients in simulation basis. I don't know if this information is useful for aspen plus but maybe

[PingPong]

My problem is that no matter what fluid package I use most of the formaldehyde leaves in the vapor phase and not in the aqueous phase.

I am not familiar with this kind of process and I do not use Aspen Plus, but the formaldehyde equilibrium K-value depends on the temperature and pressure in your absorber.

 

You should also realize that the amount of absorption of any component always depends on the number of theoretical stages and the Absorption Factor A = L/(K.V)

See the famous Kremser Plot which you can find in various textbooks and websites.

 

For equilibrium K-value calculations of the formaldehyde/water system I would expect that one has to use an activity factor model like NRTL or UNIQUAC or UNIFAC.

[sifrmoro]

The problem is that in reality formaldehyde is not absorbed in water as CH₂O, but polymerize to form polyoxymethylene glycols having formula HO-(CH₂O)_n-H.

Just as example, in a 35% solution of formaldehyde at a temperature of 35°C you have 31 moles of HO-(CH₂O)-H, 21 moles of dimer HO-(CH₂O)₂-H, 17 as HO-(CH₂O)₃-H, 11 as HO-(CH₂O)₄-H, 8 as HO-(CH₂O)5-H, 5 as HO-(CH₂O)₆-H etc.

The polyoxymethylene glycols solubility decreases with increasing molecular weight up to precipitate as paraformaldehyde crystal.

Condensation reaction is exothermicand requires coolers to control column temperature.

You can simulate that but is alittle bit more complicated than a solution in water.