Caustic Scrubber Design

[mcesm2]

Hi everyone,

I am a recent chem eng grad and I have recently been asked to design a scrubber for one of our plants, but I am running into a bit of trouble.

The scrubber is required to absorb some light organic vapours (methyl methacrylate, butyl methacrylate etc.) from a few vent streams we have on plant. We are planning to use 10%w/w caustic soln. to do the job. The scrubber sump will act as a 'reaction tank' providing the residence time for these organic species to react with the caustic and the packed section will act as a simple absorber, with no reaction assumed.

I have the vapour inlet concentration and flows and I know what the outlet must be. What I'm confused about, however, is how I design the scrubbers packed height and diameter, and the liquid flowrate. I feel like there's a cyclical problem here, such as, how do I design the packed height if i don't know the caustic flow or diam, and then how do I design the caustic flow if i don't know the packed height etc.

What I need to know is how do I begin this problem? I don't know where to start?

Any help on this would be excellent. I can provide any other info if required.

Cheers,

TwistedEcho

 

[25362]

Start with a book. For example: Chapters 1 and 2 of Packed Tower Design and Applications by Ralph F. Strigle, Jr (Gulf Publishing Company) ISBN 0-88415-179-4.

 

[siretb]

You have two major assumptions
the packing acts as a mere transfer device (no reaction)
the reaction is done in the sump

For the basic design, you will have to select you operating line (the L/G) from the performances required and the solubility data. If some pumparound (from sump back to top) is performed, then you must know how fast the destruction reaction takes place, because you need to know the concentration of your pollutants in the liquid fed at top.
25362 suggestion to use a book is excellent. Perry's handbook has a section on this topic, too.
So first step: define the concentrations at inlet, both liquid (will be zero is fresh liquid) and gaz, annd your requirements. If you recirculate fron sump to top, which is fairly common, the it may be a bit tricky to have the concentration.
Second step: from thermodynamical dat (solubility) define the operating line.
Third step: select a packing, and size it (H and diameter)

 

[kenvlach]

Hi scesm2,
Use an existing design as much as possible -- don't create problems that have been already solved!

Search the many past threads on caustic scrubbers:

http://www.eng-tips.com/search.cfm?...&action=search&CFID=18083531&CFTOKEN=85148184

Use Google on the Internet.
E.g.,

http://www.tri-mer.com/

seems to have some basic design info.

Also note: if the vent streams contain much air, you may mostly be scrubbing out the CO[sub]2[/sub], creating Na[sub]2[/sub]CO[sub]3[/sub], NaHCO[sub]3[/sub], etc. which can create solids buildup problems.

 

[mcesm2]

Thanks for the info provided guys, I appreciate it. I've got myself a design guide and I am working through my problem using that. With regards the CO2 issue kenvlach, there is no issue! Our vents dont contain much air at all as our vessels are N2 purged.

Cheers,

Shaun.