Bubble column reactors
Basic set up Structured catalytic bubble columns are new, very promising types of multiphase reactors. Their configuration lies basically between slurry reactors and trickle bed reactors. The solid phase, consisting of catalyst particles, is enclosed in fixed wire gauze wraps, which are mounted along the height of the column. The gas phase is dispersed into the liquid phase and it flows in the empty passages between adjacent envelopes. The liquid phase may be operated in a batch manner or it may also circulate in co-current or counter-current manner to the gas flow.
The main advantages no problems for separating catalyst from the liquid; improved conversion and selectivity due to staging of the liquid phase; no scale up problems because the hydrodynamics is dictated by the size of the open channels of the catalytic structure.
The main advantages over trickle beds lower pressure-drop even with 1 mm size particles; excellent radial dispersion possibility of counter-current operation without flooding.
USES Bubble column reactors are widely used as gasliquid and gas-liquid-solid contactors in many chemical, petrochemical and biochemical industries, such as absorption, oxidation, hydrogenation, catalytic slurry reaction, coal liquefaction, aerobic fermentation. The operation of these reactors is preferred because of their simple construction, ease of maintenance and low operating costs.
Characteristic structured parametersNumber of packing sections used in the column, N/ [-] Length of one packed element / [m] Diameter of one packed element / [m] Hydraulic diameter of the open channels, dh / [m] Inclination of corrugated sheets from vertical Solids hold up in the structured packed section, epsS / [-] Void fraction within "packed channels" / [-] Volume fraction of "packed channels" in the reactor, epsPC / [-] Specific surface for the gas flow (assuming the space between the glass spheres is completely filled with liquid), As / [m-1 ] Entrance length, He / [m] Height of the structured packed section, Hp / [m] Dispersion height, Hd / [m] Height between the pressure taps in the bubble column section, dH / [m] Distributor hole diameter, d0 / [m] Number of distributor holes
DT = 0.1 m
DT = 0.24 m
9 0.2 0.0935 0.007 45o 0.205 0.454 0.375 354.4
6 0.288 0.24 0.020 45o 0.198 0.505 0.400 122.3
0.07 1.8 1.68 0.9 0.0005 253
0.15 1.68 1.60 1.21 0.0005 1457
Modelling of bubble column reactors Modelling is classified according to the degree of mixing Perfect mixing CSTR Partial mixing No mixing PFR
Mixing The mixing in the liquid phase is more intense than with the gas phase due to the turbulent motion induced by the gas bubbles.
Design parameters Gas-liquid specific interfacial area, a. Individual mass transfer coefficient kla Flow regime Bubble size distribution Coalescence of bubbles the volumetric mass transfer coefficient, kLa, which depends fundamentally on the superficial gas velocity and on the physical properties of the absorption phase.
For fluids in motion the total pressure (also named impact pressure) exercised in a plane perpendicular to the direction of movement is given by the sum of static pressure and dynamic pressure. According to Bernoullis law, for a single steady state incompressible flow, the measured pressure difference is equal to: P = Lu2. he instantaneous fluid velocity is given by the difference between the two local instantaneous velocities at the two holes: P = L (uax 2 - uh 2 ) In turbulent flow, the velocity in one point of the fluid changes in magnitude and direction, oscillating around a mean value.
the axial velocity component is given by the sum of the steady flow, um and the fluctuating component u Uax = Um +u
The horizontal velocity component can also be split up in two components, the mean and the fluctuating terms: Uax = Uhm +u
1- the head hole facing upwards 2 hole facing downwards
The mean axial velocity measured in one position in the bubble column becomes:
The measured liquid velocity is :