Mercaptan Removal from Natural Gas by Temperature-Pressure Swing Adsorption Process; Mathematical Modeling and Simulation

Simulation of a commercial Pressure-Temperature Swing Adsorption (PTSA) process was performed using six sequential steps of cyclic operation in two-layer adsorption columns consisting activated alumina and 13X zeolite to adsorb mercaptans from natural gas. The natural gas was a mixture of methane with water vapor, carbon dioxide, heavy hydrocarbons (C3+) and lumped mercaptan impurities. The process was working in cyclic operational mode consisting pressurization step, adsorption, depressurization, two stages regeneration by heating and cooling down steps to continuously reduce the mercaptan content from 134 to less than 10 ppmv as the standard level of environmental regulation. The mathematical model equations were constructed on the base of four mole balances of the mentioned impurities, one total mass balance, Ergun pressure drop equation and two energy balances for solid and gas phases in the adiabatic columns. The equilibrium adsorption conditions were evaluated by extended Langmuir isotherms for each component in the mixture. The mass and energy transfer zones and breakthrough curves were determined by numerical solution in ASPEN ADSM simulator. It was observed that the cyclic adsorption was approached to the steady conditions after seven cycles running the process. The predicted molar fractions out of the process were compared with the real results and good agreement was observed between the simulated results and real data