Separation processes and mass transfer is so prevalent in the industry. Gas absorption is one of the most important separation processes. Numerous research have been carried out to increase the efficiency of absorption processes which include mechanical methods, adding surfactant, microparticles and nanoparticles to liquid phase, and using external fields. In this study, mass transfer coefficient of the carbon dioxide absorption in the Fe 3 O 4 / water nanofluid was investigated in presence of different magnetic fields at a falling film system. Vertical tubes with 8,10,14 mm inner diameter and 90 cm length were used to cause laminar, transition and turbulent flow respectively. Two parallel premanent magnets were used to apply perpendicular stationary magnetic field to flow direction. Also a solenoid with maximum 720G intensity was used to apply alternative and stationary field parallel to flow motion. Magnetic nanofluid in low concentration ( 0.05 Vol.%) was examined. The results show that mass transfer coefficient enhances with nanoparticle concentration up to 0.05 Vol.%. Mass transfer coefficient for Fe 3 O 4 / water in turbulent flow (211%) is higher than laminar flow (13.7%). Applying magnetic field slightly increases mass transfer coefficient of base fluid (water), while in magnetic nanofluid decrease mass transfer rate. Maximum reduction in mass transfer coefficient of nanofluid under perpendicular stationary magnetic field, parallel stationary magnetic field and alternative magnetic field was 62%, 49% and 42% respectively. Increasing in residence time of the nanofluid under magnetic field, results in further reduction in the mass transfer coefficient. In this study, two methods were employed for measuring mass transfer coefficient; the first method is calculation of the CO 2 bubble length variations in the falling film column and the second method is titration. Comparison of results show that the trend of variation of mass transfer coefficients are similar in both methods but in the titration method, mass transfer coefficient is higher than that of in the first method up to 11.78% in the laminar flow and 24.5% in turbulent flow. Keywords: Gas absorption, mass transfer coefficient, carbon dioxide, magnetic nanofluid, magnetic field, falling film system.
