Alky Unit - KOH Neutralization with Lime - How to mix

[ClarkChe]

I am trying to find alternative solutions to this lime storage/metering/mixing problem. This occurs in a refinery HF alkylation unit.

Hydrated lime is used to regenerate liquid KOH that has become "spent" due to the KOH neutralizing the HF contained in vapor releases.

The problem is how to get the hydrated lime into the KOH neutralization tank.

We currently use 50# bags of hydrated lime, dumped directly into a vertical cone-shaped "eductor". Circulating KOH is used as the eduction fluid, so the lime is "sucked into" the circulating KOH. But the eductor frequently plugs and back-flows.

Improved solutions include these:

A) Storage of lime --
...1) 1 Ton SuperBags of Lime,
...2) 25 ton capacity Silo
B) Measurement of amount of lime
...1) Volumetric screw
...2) "Loss-in-weight" Feeder
C Injection of lime into the KOH
...1) eductor
...2) lime/KOH slurry mixing tank, with pump and controls

All of these solutions get expensive very fast. Are there other good possibilities ? Does anyone mix the lime directly into the KOH pit ? If so, how do you "suck" the regenerated KOH back out to go to the KOH scrubber ?

If you use an eductor, how do you keep the eductor from plugging ?

Does anyone dump 50# bags into a small (200# or so) hopper on top of an eductor, and then have some sort of "flour-sifter" to gradually drop the lime into the eductor ?

Or, what else ?

Thanks,

Clark

 

[Compositepro]

Well first of all you aren't regenerating KOH with lime. I guess at one time potassium hydroxide was used in your operation but you now use cheaper lime.

You can probably solve your problems by dumping the lime in a stirred tank to prewet the powder and break-up clumps before they get to to a small orifice like an eductor. Lime isn't very soluble so it is probably settling out in any unstirred areas. It can still do its job as more will dissolve as it is consumed. You could probably dump the lime straight into the "KOH" tank but for ergonomic reasons you are using the eductor to raise the lime to a higher level.

 

[ClarkChe]

I do appreciate your reply.

Below is a quote directly from

Handbook of Petroleum Processing, David S J Jones, Peter Pujado
P362

KOH Regeneration
The UOP Process minimizes chemical costs by regenerating the KOH used to treat products and all waste and storm water. This KOH regeneration is accomplished using lime. As the lime is mixed into the KOH, regeneration of the KOH takes place by the following reaction:

2KF + Ca(OH)2 ---> 2KOH + CaF2

The calcium fluoride forms a precipitate and can be easily separated from the regenerated KOH.

Yes, there needs to be better mixing of the lime into the circulating stream of KOH. I'm certain that other refineries have the same problem, and have found reasonable solutions. I would like to hear some of those solutions, if possible.

 

[Compositepro]

Thank you for the correction. That is a clever process and I learned something new.

 

[Latexman]

Take a look at the Quadro Ytron. We use something for emulsifying liquids that is somewhat similar.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[ClarkChe]

Latexman,

Thanks for the information. I do plan to follow-up with Quadro Ytron.

Clark