A mathematical model has been developed for absorption accompanied by chemical reaction of H2S and CO2 in alkanolamine solutions encountered in plants for natural gas sweetening in Libya. The model has been used to calculate actual number of plates in industrial columns. The model results errors in number of trays predictions due to the lack of procedures for estimating interfacial area for the in between flow regime that prevailed in two industrial plants. The developed correlation for interfacial area estimation for the in between flow regime gives zero error in number of trays prediction.