Caustic Soda Scrubber

[padovano]

Hello All, I have a caustic soda scrubber (NaOH) to eliminate mercaptanes from butane. It is a batch. The concentration of NaOH at the begining of run is 30%. The solution usually gets spent when the concentration is around 10%. Recently, we discovered that, the NaOH solution gets spent when the concentration is still at above 20%. this has happened twice already.

I would like to know if any body has witness this phenomenon and how it was resolved.

 

[georgeverghese]

Criterion for spent NaOH is when mercaptans in exit sweet butane exceeds a certain limit?

If that is the case, then something has changed in the C4 sweetening contactor...

 

[padovano]

We carry out "Doctor Test" on butane at the exit of contactor. The test was positive. The problem is, the test use to be positive when the concentration of NaOH is around 11%. But presently the test is positive when the concentration is above 20%. The quality and flow rate of butane is the same.

 

[EdStainless]

Have you examined the internals? You may not be getting good flow distribution and poor scrubbing effectiveness.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Presume this is a once through loop from the NaOH side - ie no regeneration of caustic.

What about the H2S and RSH loading in the feed C4 ? Is there some residual water in the feed C4 which may be H2S saturated ?

 

[padovano]

we isolated the scrubber and discovered that it was filled with different crystal blocks. This created preferential passages in the drum. we washed the scrubber and it`s now back in service.

 

[georgeverghese]

That is good to hear. These crystals may be from residual water in the feed C4 that is reacting with caustic in this scrubber - if there is there a coalescer unit upstream of this scrubber to remove trace water from this C4 stream, check if this unit is working well.