A rigorous rate-based model for acid gas absorption was developed and validated against mass-transfer data obtained from a 3-month campaign in a laboratory pilot-plant absorber in which the experimental gas-liquid material balance was within an average of 6%. The mass-transfer model is based on the penetration theory where the liquid film is discretized using an adaptive grid. The model was validated against all data and the deviation between simulated and averaged gas and liquid side experimental mass-transfer rates yielded a total variability of 6.26%, while the total average deviation was 6.16%. Simpler enhancement factor mass-transfer models were also tested, but showed slight over-prediction of mass-transfer rates. A sensitivity analysis shows that the accuracy of the equilibrium model is the single most important source of deviation between experiments and model, in particular at high loadings. Experimental data for the absorber in the integrated pilot plant are included. (c) 2007 American Institute of Chemical Engineers.