5 replies to this topic

[Mohamad Ali]

Dear sirs,

 

Kindly i want to ask about the limitations of sour gases in lpg to use the simple caustic wash treatment , as i know the sulfur content sholud be very low as spent caustic resulted from H2S is non regenrable , for that the caustic wash is used mainly to treat mercaptans ( spent caustic of it is regenrable) and amine unit can not deal with all mercaptans.

 

so, what is the limits of H2S in LPG to be treated with simple caustic wash unit , from econmic point of view as operating cost will be high i think i will lose all the caustic used in treatment .

i searched alot and i could not to reach to a fixed number or a design guide for this case , i appreiciate your kind help

 

Thanks

[P.K.Rao]

Caustic soda removes only hydrogen sulfide and light mercaptans ie, methyl and ethyl mercaptans. Higher mercaptans C3 are only partly soluble depending upon the caustic concentration. C4 and above mercaptans are not soluble in caustic soda solutions. They remain in LPG if present (though to a small extent). Merox/Merrichem treatment converts them to disulfides and separated as disulfide oils in the oxidizer (gasoline Merox). Solubility of mercaptans also depends upon the caustic concentration. Fresh caustic solution dissolves more mercaptans than spent caustic solution in the absence of hydrogen sulfide.

 

So a certain amount of caustic soda is invariably spent and lost in treating LPG. However, I understand Merrichem has a process to regenerate spent caustic soda solutions. You may contact Merrichem for details.

 

If mercaptans are only to be removed (after removing hydrogen sulfide), please contact me at  pkrao2012@yahoo.com.           

[Mohamad Ali]

thank you for your corporation,however my main enquiry is if i am going to design a new simple caustic wash unit what is the limits of H2S in the feed which make using this unit is economic otherwise i should using amine unit then caustic wash. or any alternative other method. i could not find such a guide for that.

[RockDock]

I would want the H2S at ppm levels before the caustic unit. However, if you have a fairly low flowrate of gas, it may not be worth having an amine unit and a caustic unit. What are your CO2 levels?

 

Whenever I face an issue like like this, I set up my ProMax model to report how much caustic is spent and make a calculation based on the cost of caustic. I then compare that to adding in an additional process, such as the amine unit. That is the easiest way to evaluate it.

[P.K.Rao]

Hydrogen sulfide is acidic and is highly soluble in caustic soda if given adequate contact time. Even dry caustic soda absorbs hydrogen sulfide. Na2S which forms also absorbs H2S forming NaHS (sodium hydrosulfide). The reactions are,

 

2NaOH + H2S = Na2S + H2O

Na2S + H2S = 2NaHS

[Mohamad Ali]

Thank you sirs