19-08-2014, 10:44 AM
Stirred tanks are ubiquitous in chemical process industries for blending, gas-liquid, solid-liquid and gas-solid-liquid dispersion applications. The design procedures for these equipments have been largely empirical leading to significant over-designs and resulting into inflated fixed and operating costs as well as additional start-up times. Further, the empiricism does not give rational answers to the debottlenecking problems. Therefore, reliable procedures are needed for the design of chemical process equipment.
Stirred tanks are ubiquitous in chemical process industries for blending, gas-liquid, solid-liquid and gas-solid-liquid dispersion applications. The design procedures for these equipments have been largely empirical leading to significant over-designs and resulting into inflated fixed and operating costs as well as additional start-up times. Further, the empiricism does not give rational answers to the debottlenecking problems. Therefore, reliable procedures are needed for the design of chemical process equipment.
Accurate knowledge of the mean flow field and the turbulent characteristics is necessary in order to get reasonable estimates of mixing time, dispersion coefficients, mass and heat transfer coefficients, bubble break-up and coalescence, dispersion etc .and hence, enables to reduce the empiricism in the existing design procedures.
Fundamental understanding of turbulence, its contribution to transport phenomena and its modeling are necessary, therefore, initially assessment of different turbulence models i.e. std. k-e, RSM and LES have been carried out for the flows generated by various impeller designs. For the validation, LDA measurements have been carried out.
In case of two phase flows: (1) for the first time a detailed CFD study has been carried for hollow self inducing impeller system. Also, based on first principle, gas induction rate has been estimated, and the procedure is promised to hold for any new hollow impeller designs to carry out optimization studies. (2) a detailed investigation of solid-liquid dispersion has been performed using CFD, which includes predicting critical impeller speed, solid concentration profiles, mixing time over a wide range of design and operating conditions.
In three phase flows i.e. gas- solid- liquid dispersion: (1) we first studied sparged system: CFD methodology has been developed for the prediction of critical impeller speed for solid suspension.
It has been extended to investigate the solid suspension over a wide range of impeller design and operating parameters; (2) further, for hollow self inducing impeller system: we carried out experiments to measure critical impeller speed for solid suspension and corresponding gas induction rates. Also, we have developed the CFD model for the prediction of the same.
Horizontal two phase pipe flow contactor is now becoming popular as gas-liquid reactor because it ensures less back mixing in both the phases and good wall heat transfer coefficients. Important applications include sulfonations with SO3, ethoxylations, nitrations with NO2, neutralizations with NH3, CO2, etc, reactions with phosgene, ozonolysis and reactive removal of obnoxious gases using chemical solvents. Knowledge of liquid-phase axial mixing is important in addition to the kinetics and mass transfer coefficients. The degree of axial mixing decides the concentrations profile which ultimately affects the performance of reactors in terms of conversion and yield is thus essential for the modeling and design of reactors. In view of this, using CFD technique an attempt has been to study the axial dispersion in a two phase pipe flow for two regimes: Bubbly and Slug.