October 12, 2006
Interest in simulating caustic treating processes for removal of acid gas from both vapor and liquid hydrocarbon streams has increased significantly. ProMax can accurately predict the absorption of acid gas compounds and sulfur species such as CO2, H2S, and mercaptans from liquid or vapor streams by caustic (NaOH) solutions. When modeling a system containing NaOH, an Electrolytic Property Package (i.e. Electrolytic ELR-SRK) should always be used to obtain accurate results. Sodium Hydroxide solution is a very effective but usually non-regenerable absorbent for CO2 and H2S, forming stable salts such as sodium carbonate and sodium sulfide. The actual salt compound is not shown in ProMax, however, the stream Ionic Information is used to determine the amount of salt formed. The moles of species [HS-] corresponds to the moles of NaHS formed, [S--] corresponds to Na2S, [CO3--] corresponds to Na2CO3, etc. Caustic solutions can be used to selectively remove H2S from vapor streams in the presence of CO2 provided that extremely short contact times of about 0.01 to 0.02 seconds are maintained as described by Hohlfeld (1979) and by Kent (1985). A caustic wash can be used to remove mercaptans from either natural gas or liquid hydrocarbon streams. When used to absorb mercaptans only, the caustic solution is usually regenerable either by thermal means or by oxidative regeneration. Thermal regeneration is achieved in a stripping column by the addition of open steam or by steam internally generated by a column heating element. About 6 lb steam per gal of NaOH solution is required for stripping. Although this steam rate seems high compared to typical amine unit stripping steam requirements, the amount of caustic solution required to pick up the mercaptans is much lower than amount of amine solution that would be used in a comparable amine sweetening system. Detailed information concerning caustic treating may be found in the GPSA Engineering Data Book, Section 21, and also in Kohl (1997).
Authored by Lili Lyddon - BR&E Technical Support / Help Author