3 replies to this topic

[siretb]

Dear Rosh,
There is no magical procedure for designing scrubbers, and for systems that have not yet been analyzed nothing will replace experiments.
You will find some elements in Perry"s handbook (Mr Graw Hill), Carl Branan Rules of thumb for chemical enginners. Useful data can be found on the Internet sites of packing vendors.

The important thing is that chemical reactions speeds up things by pulling the component(s) to be scrubbed into the liquid phase. Thus the kg.a (mass transfer coefficient) is multiplied by a factor known as the enhancement factor. Perry gives some formulas for estimating this enhancement (Hatta) factor.
One procedure is to estimate the kg.a / kl.a as if no reaction occurred. If the system is unknown, you may attempt to estimated it from available data on another system using:
kg.a (your system) = kg.a (available) * sqrt(Diffusivity(your system)/(Diffusivity(available system)) D is the diffusivity of the considered component in the vector gas. Diffusivity may be estimated from chemical scruture if not available.
Another approach may be to usin analogy between heat transfer and mass transfer if you have good heat transfer data for your system (unlikely in your case).
Then you apply the enhancement factor.

Then you must have VLE data (especially if you are scrubbing organics), or data like Henry's constant for each component/solvent pair
From this data, and the required performances you calculate the number of transfer units. I copy here the formula given by Branan
N=(Yi-Yo)/((Y-Y*)i-(Y-Y*)o)/Ln((Y-Y*)i/(Y-Y*o)) where Y is the mole fraction, Y* is the Mole fraction you would have, in equilibrium with the liquid, given by the VLE data. If the chemical reaction is fast, often , but not always Y* is close to zero. This also, like in distillation will give you the minimum operating condition L/V rate.
With the Kg.a Kl.a data (technology-dependant) and the N, you get the design.

There is not much more without becoming specific. Have a look at the Perry, it's worth it. Also, if the components are quite concentrated you may have heat effects.
I hope this helps.

[Art Montemayor]

siretb & rosh:

This is a First-Class, professional, and articulate response to a practical question. siretb, I tip my hat to you and I hope rosh sees the value of the well-communicated information you supplied all of us with.

There is very little that anyone can add to such a detailed and thorough response, except perhaps add to the reference list of reading material. I have found that Ralph F. Strigle Jr. has dealt with this subject in detail in his classic book, "Random Packings and Packed Towers - Design and Applications. It was last published by Gulf Publishing Co. (now defunct) in 1987. But I have a feeling that it is still available (maybe even used) and perhaps a Web search will surface a source.

Art Montemayor
Spring, TX