Benfield Process - Minimum Flows for Wall Wetting and Vanadium Passivation

[Wayno78]

Hi, does anyone have any information or guidance to determine the minimum wall wetting hot potassium carbonate flows to a Benfield CO2 Absorber Column to ensure that the vanadium passivation (magnetite) remains effective (does not dry out)?

We are running a column installed in the early 1970's, 6'6" (1.98m) diameter with two sections of random packing of raschig #2 random packing top section is 21' (6.4m) and bottom section of packing is 20' (6.1m). Current minimum flows are 500T/hr to the column with a 60/40 bottom/top flow split.

I have discussed with UOP but they don't design for minimum flows because the turndown of the columns are considered much greater than the minimum wall wetting rates, but because the towers are so old we no longer have the design information available to see what the minimum design/turndown of the absorber towers are...... any guidance would be great!

 

[georgeverghese]

Minimum liquid wetting rates for random and structured packings are obtained by experiment and cannot be calculated (as far as I know). Raschig rings are generic manufacture - UOP would have obtained these values from the packing manufacturer, if they needed this information for a Benfield unit ( would agree that circulation rates in a CO2 removal unit even at min operational turndown would probably be in excess of min packing wetting rates). Talk to the people at Norton for instance. Whether these are ceramic or metallic packings may have some bearing on these rates.
However, I dont know whether these quoted rates you'd get from Norton are adequate to ensure vanadium passivation is effective.

 

[georgeverghese]

Looks like I've misinterpreted the problem here. What you're after here is whether solution circulation rate has an effect on corrosion on the absorber CS wall for potassium metavanadate doped Benfield solutions.

Did a bit more reading from what is available on the net, from which the following appears:
a)Wall corrosion seems to increase with increasing solution rate, indicating that surface erosion seems to be dominating, which is the opposite of what you're expecting
b)Corrosion rate increases with CO2 loading of the solution, which indicates high solution rates reduce wall corrosion. This effect in (b) seems to be more pronounced that that due to (a)
c)The galvanic cell set up between the metallic SS packing and the CS shell makes matters worse.

So this has nothing to do min wetting rates for the packing required for the absorption process.

My two cents worth on this, from a corrosion management perspective, if this is becoming a serious mechanical integrity issue, would be to (a) keep circulation rates high and (b) use inert ceramic raschig ring packing rather than SS packing.

 

[georgeverghese]

Whilst this is most probably information not relevant to your query, there is a formula to get min wetting rate relevant to hydraulic / mass transfer performance in the 6th edition of Perry Chem Engg Handbook with equation 14-69, for which information on surface area in m2 / m3 of packing is required.

 

[georgeverghese]

In any case, the following lower limits on circulation rate would apply, whichever of the two is higher:
a) The min wetting rate for the packing as defined in equation 14-69
b) The min loading point for the packing, which is recommended to correspond to a pressure drop of 0.5inH20/ft of packing height, as recommended on page 18-24 in the 6th edition of Perry. Operating below this loading point apparently results in a sharp reduction in mass transfer efficiency , i.e HETP or HTU increases.