In this study, simulation of cyclic adsorption process for mercaptan removal from natural gas in non-isothermal and non-adiabatic conditions is presented. This process is used in mercaptan removal unit of South Pars Gas Refinery Phase 1. Six adsorption fixed beds used in this plant contain molecular sieve type zeolite 13X. Three beds are in the process for adsorption purposes and the other three beds are being used for regeneration simultaneously. Regeneration cycle involves two steps for heating and one step for cooling. In modeling of this process, linear driving force (LDF) is used for estimation of adsorption rate. For equilibrium relation between solid and gas phases, the extended Langmuir isotherm is used. The energy balance around the gas phase in the bed includes heat transfer to solid as well as axial heat dispersion. The set of partial differential equations is solved using implicit finite difference. Cyclic steady state (CSS) is obtained using cyclic simulation procedure and the variation of concentrations and temperatures along the bed and at different times.
A good agreement was obtained between the simulation results and those obtained from plant operational data. The effect of various operational parameters, such as regeneration steps, temperature and regeneration flow rate on process product was investigated. With increasing the first heating stage temperature, the concentration of water and mercaptan in the bed outlet decreases, but the decrease in mercaptan concentration is more significant. By increasing the second heating stage temperature, the water concentration in the bed outlet decreases significantly.