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In the query below, gas also contains CO2 which can help to maintain an acidic environment. This query is regarding an upstream processing facility. In a Sour Water stripper, maintaining pH of the water phase is essential for stripping. H2S tends to ionize in a basic environment, an acid environment is most conducive to keep H2S as an un-ionized form good for stripping. Published literature suggests maintaining a pH of 5 to 6 for H2S stripping.
Please note refinery sour gas generally also contains NH3 along with H2S and a desirable pH for stripping H2S and NH3 is 8 and is therefore different from the sour water above which does not come from a refinery and does not contain any NH3. I have the following questions in this regards,
a) For sour water containing only H2S and no NH3, what chemical is added prior to stripper for maintaining pH for good stripping? Also, is the chemical injection system similar to other chemical injection systems, eg Corrosion Inhibitor, i.e tank and pump?
b) Does partly ionized H2S not maintain its own pH without addition of chemical?
c) What is the typical column top operating pressure and is it maintained by a PCV? Are higher operating pressures any good for stripping considering the fact that they will reduce the column diameter?
 
Answers
20/02/2008 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I've generally seen acetic acid used for lowering pH in a SWS. You want an acid that is stronger than H2S so the H2S will stay as H2S and be easier to strip. Injection systems are similar to other additive type systems.
Yes, the H2S by itself will go to some natural pH, but you want something that will drive the equilibrium back towards H2S rather than an ion.
Typically the column pressure is maintained with a PCV, but you could also let pressure float on the backpressure of the sulfur plant that it is presumably feeding. Higher pressure will reduce the diameter but reduce the energy efficiency and require a higher level heat source. Refinery sour water strippers are normally run at as low a pressure as possible to be able to use fairly low pressure steam as the heating medium for the reboiler. Sometimes steam is injected live into the column, but the condensate is not recovered and the volume of stripped sour water increases in this type of operation.