General Terminology for Light Ends

The simplest way of removing hydrogen sulfide is to scrub the hydrocarbon mixture with an aqueous solution of a strong base, such as sodium hydroxide. The hydroxide reacts with the HjS, which is a weak acid, and the hydrogen sulfide removal is quantitative even in a single contact stage. Spent caustic from this operation cannot be regenerated economically. 

Other regenerative methods are occasionally used to remove HjS from hydrocarbons, such as the tri- potassium phosphate (TPP) process. Other installations are DEA or ME A, and most TPP units have been converted to DEA since the latter consumes less steam for regeneration. 

Mercaptans are slightly acidic organic sulfur compounds. They can be removed from light ends mixtures by caustic washing in a Merox extraction 

When both HjS and mercaptans are to be removed in the light ends treating plant, the amine scrubbing tower precedes the caustic washing step. If the order were reversed, the large quantity of HjS present would fix the caustic. Also, the caustic will act to remove any final traces of HjS when it follows the scrubbing tower. 

Calcium chloride is the most common nonregenerative reagent used to dry low molecular weight refinery streams to moderately low dew points. Anhydrous potassium or sodium hydroxide have also been used at times to dry liquefied petroleum gas. Sodium chloride is used most commonly to remove entrained and some soluble water from middle distillate streams. 

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