I am to put in place a monitoring system on the NaOH solution been used in washing H2S from the LPG stream. I want to either check on the concentration of sulphur in the solution or analyse the pH of the solution. I would like to know which of the methods is best. I would really appreciate detail explanation of the chosen method.
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Caustic Scrubbers
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The caustic drum is not equiped with any online pH analyser. Analysis will involve sampling of the caustic solution for analysis to be done in the laboratory.when you talk of controlling Ph at 9-10,I think you mean the pH when the caustic solution is saturated or not?
Your scrubber will have a caustic solution and probably water feed near the top and gas stream at the bottom. The solution collects in the bottom and has some type of outlet (overflow or pump) put an on line pH meter somewhere that you are sure is liquid full and control the pH.
X Ray Fluorescence Sulphur analysers are available for testing hydrocarbon sulphur content but they are expensive. Don't know if they would be suitable in this case anyway.
JMW
JMW
You need to monitor the pH, but also the concentration of Sulfides in the liquids.
The pH needs to be monitored on-line, at any rate. Best location is the scrubber sump. Be careful about electrode specification. some glass electrodes can be a problem at pH>9. I have seen too often electrodes becoming very slow-responding due to a thin layer building up on the glass. This renders process control unstable.
The S-- SH- concentration is needed because you want to control the overall load of your liquid loops. So I'd say you need both.
on-line pH electrode
Sulfides load done on a regular basis, in lab
This way you can properly adjust the bleed (blow-off) rate.
Just one thing: I do not like much the caustic scrubbers for H2S, bleach is much better. H2S --> NaOH mass transfer is not efficient.
The pH needs to be monitored on-line, at any rate. Best location is the scrubber sump. Be careful about electrode specification. some glass electrodes can be a problem at pH>9. I have seen too often electrodes becoming very slow-responding due to a thin layer building up on the glass. This renders process control unstable.
The S-- SH- concentration is needed because you want to control the overall load of your liquid loops. So I'd say you need both.
on-line pH electrode
Sulfides load done on a regular basis, in lab
This way you can properly adjust the bleed (blow-off) rate.
Just one thing: I do not like much the caustic scrubbers for H2S, bleach is much better. H2S --> NaOH mass transfer is not efficient.
pH transmitters are low cost. The sample system is also minimal as compared to the online H2S analyzer sample system. Consult with the online analyzer manufacturers to verify that their more costly analyzers are suitable for your application. Several manufacturers make XRF analyzers used in the ULSD market. Examples include Antek, Thermo and XOS. Lead acetate paper tape H2S analyzer manufactures include Del-Mar, Galvanic and Thermo. These have tape and moisturizer consumables. Process gas chromatographs can also measure H2S in caustic.
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