Modeling and Simulation of CO2 and H2S Absorption from Methane using Aqueous Methyldiethanolamine Solution

The acid gases (CO2 and H2S) removal process is very important in natural gas processing.
The acid gases absorption process using methyldiethanolamine (MDEA) solvent has
found increased application. Evaluation on the acid gases absorption process using
MDEA solvent can be done using modeling and simulation on the system. Several
attempts have been made on the modeling and simulation of the phase equilibrium and
also the absorption column system. Some limitations were found in the attempts.
The aim of this study is to construct a rigorous simulation procedure of the acid gases
absorption from methane using methyldiethanolamine solvent. The simulation of the
contactor column is based on equilibrium modeling. The methane solubility that was not
considered in the previous studies is accounted in this simulation. Part of the study is the
development of phase equilibrium model to determine the solubility of acid gases in
MDEA solvent. The ElecGC model is used for calculation of the activity coefficient of
the components in the liquid phase. The non-ideality of the components in the gas phase
is accounted using Peng-Robinson Equation of State. The Astarita representation
introduced by Hoff (2003) is used to solve the set of reaction equilibrium and component
balance equations and to calculate the liquid phase composition.
The study observed the solubility of CO2, H2S, CH4, and mixture of CO2 and H2S in
MDEA solvent. The predicted solubility was found to be in good agreement with
published experimental data. Relatively large error (84.7%) was found on CO2 partial
pressure prediction at loading lower than 0.1 mole CO2/mole MDEA. The absorption
column simulation compared the CO2 and H2S separation from methane, for the similar
specification of the feed gas (10 mole% acid gas) and solvent (45 wt% MDEA). The
simulation result shows that H2S composition in the product gas was lower than that of
the CO2 composition. For the 5 stages fixed to separate the acid gas, CO2 purity in gas
product is 0.085 mole%, while H2S purity is 0.059 mole%. Parametric analysis was
performed to evaluate the effect of changing the solvent flow rate and the solvent
concentration to the acid gas composition in the sweet gas.
Key words: Absorption, Acid gases, Methyldiethanolamine