Octanol-water scrubber

[imatasb]

Hi,

I am designing a scrubber to absorb the octanol present in a hydrogen stream. I am planning to do this with water (solubility of octanol in water @25 C is 0.05 %). The amount of octanol to me removed is 0.3-0.8 g/m3.

Has anyone got experience in the system octanol-water? I haven't got any equilibrium data for this system.

Cheers

 

[sshep]

I have some experience with octanol and heavier alcohols and water- they are basically insoluble for your purposes. Conventional scrubbing with water seems impractical at first look- especially if the octanol is to be stripped from the water. You already know the solubility of octanol in water is very low- why not consider an alternate solvent (decanol or heavier alcohol, dodecane or heavier paraffin, NMP, etc). Almost any non-volatile hydrocarbon that won't freeze up at ambient temperature would be better than water. The solvent could be regenerated in a second tower and the octanol recovered.

Alternatively if the H2 flow is not too high, the alcohol could possibly be removed by a reasonably sized fixed bed arrangement that could be regenerated when spent.

just a few thoughts,
sshep

 

[imatasb]

Hi sshep,

yes, the solubility is the main problem but for for my apllication I would be speaking of around 1 m3/h of water which I can reuse from another process and in my case i don't to recover the octanol from the scrubber, h2 flowrate maximum is around 150 m3/h so the octanol amount is not very big either.

Something that sounds good is the fact you mention about using a heavier alcohol. I could use decanol (which I use for another process). The only problem I can see is if maybe I will be recovering to my system again byproducts of the main reaction like heptene and octenes which at the moments go to the vent (very small amount but if recycled to the reactor could decrease the yield)

But it's worth thinking about it.

Have you got any equilibrium data for these alohols?

Thanks for your comments.

 

[sshep]

Hello Imatasb,

Relative to VLE (or VLLE if using water) I pretty much use Aspen-plus for this type modeling, and I would think that a similar tool would be useful to you for this design. Because system data for heavier alcohols is hit and miss, the trick for me is to know how similar systems behave using whatever data can be obtained from Dechema, and then extropolate that behavior to systems where no data exists. With this approach in mind I can tell you:

1) Alcohol-Alcohol systems are basically ideal, and you can use pure component vapor pressure data available anywhere- check NIST, DIPPR or other easily accessible sources. An octanol-decanol scrubbing system could be designed with only this pure component vapor pressure data if no other components are of interest.
2) Similar boiling point Alcohol-Paraffin (or presumably Olefin) systems will form minimum boiling azeotropes, but structural prediction using UNIFAC will get you pretty close. For example in our process a minimum boiling hexanol-undecane (11wt% undecane) azeotrope is of frequent interest.
3) Water-Alcohol systems are basically immiscible above hexanol, but all normal alcohols up to and including decanol have been fitted, and NRTL parameters are readily available. You will need this data to analyze water scrubbing since you must keep enough water on your tower to avoid forming two liquid phases (heterogeneous azeotrope), otherwise your octanol removal will be limited at that point.
4) Hydrogen solubilities in alcohol and other hydrocarbons are an important consideration- hydrogen exihibits a reverse solubilty relation to most gases (increasingly soluble at increasing temperature). I have inherited some graphical data on this but am unsure of the source. This solubility could have safety implications if you are storing the solvent, sending off to sewer, or as additional load on vent systems for downstream towers. You may already have experience, otherwise H2 solubility may be worthy of some research. We have been using vacuum flashing or N2 stripping when needed deal with this safety consideration.

Anyway best wishes,
sshep