Modelling of dehydration adsorption column at Statoil Kårstø
MetadataVis full innførsel
In the master thesis project of spring 2013, the adsorption dehydration of natural gas on a bed of SylobeadTM Molecular Sieve Grade 564 adsorbent pellets was to be modelled in MATLAB 7.12.0 (R2011a). This process is a part of the Åsgard section at the Kårstø processing plant. The main goal of the project was to establish a dynamic rate based model for the adsorption column and to implement realistic equilibrium and mass transfer relationships for the Kårstø process conditions.The modelling was divided into two parts; the reactor model and the intrapellet model. The reactor model described the adsorption process from bulk gas phase to the surface of the adsorbent pellets, while the intrapellet model examined the adsorption behaviour of water within a single pellet. Several assumptions were made to simplify the problem. The models describe water being adsorbed from inert methane in an isothermal and isobaric adsorption column. The conditions of the system were provided by Statoil, while the kinetics and certain parameters of the process were determined by literature correlations or estimations.The resulting models seem to give appropriate presentations of the adsorption behaviour. Due to the many assumptions made, there may be inconsistencies compared to the real process at Kårstø. As there is little information on the adsorption behaviour inside the columns at Kårstø to compare the models to, there was conducted a parameter study to verify the expected adsorption behaviour and stability of the models. The problem that was to be solved was stiff and showed some numerical instability. The general trend in the parameter study seemed to be that the stability increased with slower adsorption mechanisms, either it be a reduction in the Langmuir constant, the mass transfer coefficient or increase axial dispersion. With the initial conditions set for the process, it seems like the adsorption column at Kårstø may be underutilised. Only 40% of the column is saturated with water for the given cycle time.An approximate effectiveness factor, which describes the ratio between the actual adsorption rate occurring in the pellet and the adsorption rate that would occur if the entire pellet were at the surface conditions, was implemented in the reactor model. The implemented function was found from the intrapellet model with reduced mass transfer coefficient and pore dispersion, slowing down the adsorption process inside the pellet. The function resulted in an apparent effect on reactor model concentration profiles, indicating a reduced efficiency of the column.