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A three dimensional CFD modeling of hydrodynamics and mass transfer in a gas-liquid impeller stirred tank reactor

Gorji, M ; Sharif University of Technology | 2006

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  1. Type of Document: Article
  2. Publisher: 2006
  3. Abstract:
  4. Multiphase impeller stirred tank reactors enhance mixing of reacting species used in a variety of chemical industries. Gas-liquid mixing and mass transfer between these phases is an important application in reactor design. One such an example is hydrogenation reaction of aqueous NDMA (Nitroso dimethyl amine) solution in which one of the parameters having a significant effect on the reaction rate is mass transfer. This parameter in turn is a function of impeller shape and rotational speed, gas flow rate and reactor scale. Computational Fluid Dynamics (CFD) is a useful tool in the analysis, design and scale up of stirred tank reactors via investigating hydrodynamics of gas and liquid flow, mixing and dispersion of gas in liquid and its mass transfer. However, most simulations reported in the literature consider only the hydrodynamics of single or two phase systems. In this work a 3D CFD model is used to investigate both hydrodynamics and mass transfer in a fully baffled cylindrical stirred tank for hydrogen-water system with a standard Rushton turbine impeller utilizing sliding mesh method to account for the relative movement of impeller and stationary baffles. For this purpose, two fluid (Eulerian-Eulerian) model along with standards - S model to address turbulent behavior of liquid phase, have been used. The equations corresponding to mass transfer have been solved along with the equations governing the hydrodynamics of the system. The tank geometry employed in this work is a flat bottom fully baffled cylindrical tank, T=0.20 m, H=0.20 m. Four equally spaced baffles, b=T/10, have been mounted perpendicular to the vessel wall. A standard (six blade) Rushton turbine impeller with diameter, D=T/3, impeller blade height, B=D/4, and impeller blade width, W=D/5, have been used with an off-bottom clearance of, C=T/3, (from disk center of the impeller to vessel bottom) on a shaft. Hydrogen was sparged into the vessel through a ring sparger of inner and outer diameter 30, 34 mm, respectively. The tank was modeled by a finite volume grid in cylindrical coordinate. The quadratic upstream interaction for convective kinetics (QUICK) has been used for discretization. In this work, the upper side of sparger from which hydrogen is sparged, has been modeled as an inlet velocity boundary condition. The pure hydrogen has been assumed at this boundary. The height of 0.195 m of the vessel contains water where the rest of the volume is occupied by hydrogen. Standard wall function has been used for all wall boundary conditions. Mass transfer flux of components has been considered to be zero at these wall boundaries. The bubble size has been assumed at 4 mm. It is assumed that no coalescence and break up of bubbles exists. Fluid properties have been set to those of water and hydrogen for primary and secondary phases, respectively. The interaction between hydrodynamics and mass transfer in this system has been studied. The results show that due to high relative volatility of the hydrogen with respect to water, the effect of mass transfer on the hydrodynamic behavior of the system is negligible. However, the hydrodynamics has a large effect on the amount of hydrogen dissolved in liquid. In order to study the effect of mass transfer on the hydrodynamics, hydrogen-water system has been replaced with a binary mixture in which the relative volatility is smaller. For this system, the effect of mass transfer on hydrodynamics at various impeller speeds has been studied. The results show that at low impeller speed, the effect of mass transfer on velocity field is higher and as the impeller speed increases, the effect of mass transfer on the hydrogen hydrodynamics gets less
  5. Keywords:
  6. Amines ; Chemical industry ; Finite volume method ; Hydrodynamics ; Hydrogenation ; Mass transfer ; Gas-liquid mixing ; Hydrogenation reactions ; Reactor design ; Stirred tank reactors ; Chemical reactors
  7. Source: CHISA 2006 - 17th International Congress of Chemical and Process Engineering, Prague, 27 August 2006 through 31 August 2006 ; 2006 ; 8086059456 (ISBN); 9788086059457 (ISBN)