Most gasoline and LP-gas streams contain sulfur in various forms and LP-gas and raw NGL streams also can contain carbon dioxide. Especially objectionable are hydrogen sulfide, mercaptans, and elemental sulfur. Gasoline containing hydrogen sulfide has objectionable odor and is corrosive. Mercaptans give an objectionable odor to gasoline and elemental sulfur makes the gasoline corrosive. Carbonyl sulfide in the LP-gas can hydrolyze and cause the product to become corrosive. A product containing the objectionable materials can be treated to remove the hydrogen sulfide, carbonyl sulfide, and elemental sulfur and to either remove the mercaptans or convert them to less objectionable compounds.
Hydrogen Sulfide and Carbon Dioxide Removal
Hydrogen sulfide and carbon dioxide can be removed from LP-gas and gasoline by liquid-liquid contacting processes using a caustic solution, aqueous alkanolamines, or solid KOH. The caustic wash processes are described later. The alkanolamine processes were described earlier in the gas treating section. For application, both a coalescer and a full flow carbon filter should be used to minimize the introduction of hydrocarbons into the regeneration section of the amine unit. When treating hydrocarbon liquids with amine, contacting is generally accomplished in a liquid-liquid contactor, though stirred reactors can be used. The tower should have a minimum of 20 feet of packing. The design flow rates for packed towers should not exceed 20 gpm liquid per square foot of cross sectional area.
Elemental sulfur is removed from the gasoline by contacting it with a polysulfide wash solution. The solution is made up by using the following amounts of chemicals per 1,000 gallons of water: 1,000 lb. of caustic (NaOH), 800 lb. of commercial Na2S, and 20 lb. of sulfur. The sodium sulfide (Na2S) is melted in a vat by use of a steam lance. Add the sulfur to the melted Na2S. The sulfur must be completely dissolved in the liquid sulfide, and then this mixture is added to the 10% (1,000 gal. water and 1,000 lb. NaOH) caustic solution. Protective clothing and goggles should be worn when handling these chemicals.
Mercaptans can be converted to disulfides or removed from liquid hydrocarbons by several methods. The method or combination of methods that can be used depends on the mercaptan content of the product to be treated and the specification that must be met.
Carbonyl Sulfide Removal
Because carbonyl sulfide (COS) can hydrolyze and cause sweet LP-gas to become corrosive, and as the concern for minimizing total sulfur emissions has increased, there has been a growth of interest in removing COS from propane and LP-gas streams. Several alternative processes can be used. COS can, of course, be removed sacrificially by MEA. Mick has reported successful use of a combination of potassium hydroxide and methyl alcohol. This process is also sacrificial. The Malaprop process uses diglycolamine but requires unspecified modifications in the process flow from that used for gas treating. The ADIP process utilizes aqueous diisopropanolamine. Molecular sieves can be used for removing COS. The Malaprop, ADIP and molecular sieve processes are regenerative.