Gamma ray tomography and solid phase hold-up measurements in a gas-solid circulating fluidized bed

Chemical industry has numerous cases wherein reactions are carried between reactants that exist in different phases. Equipment designed for such a reaction is called multiphase reactor and intimate contact between the phases in any multiphase reactor is a must to achieve high efficiencies. When the reactants are in gas and solid phases or when the solid phase is a catalyst or a product to be dried in a gas stream, better contact efficiency can be achieved by suspending fine solids in the high velocity of fluid streams. A number of configurations are possible for carrying out such operations, such as rotary bed, fixed bed, moving bed, and fluidized bed and its several sub-configurations. Circulating fluidized bed (Cf) is a special mode of fluidization, and comprises of a tall riser column in which the solids are fluidized at high carrier phase velocity (gas and/or liquid). The entrained solids are captured in a cyclone or bag filters and returned back to the bottom of the riser continuously. CFB has found important applications in petroleum and power industry. Fluidized catalytic cracking of crude naphtha to petroleum products is carried out in a CFB system wherein the catalyst particles are suspended and entrained at high temperature in gasified naphtha in the riser column. Naphtha cracking is a multi-million dollar application of CFB and requires close study for efficiency and performance. Optimized operation of a CFB requires knowledge of solid phase holdup profiles and residence time distribution in the riser column. Attempts, direct and indirect, have been made to accurately measure these quantities. Use of radioisotopes offers a powerful and cost effective method for estimating both solid phase holdup profiles as well as residence time distribution in continuous multiphase flow. Efforts were therefore made through this investigation to determine solid phase axial and radial holdup profiles in the riser section of a 'cold' gas-solid circulating fluidized bed using fixed radioactive sources