Bubble Column Reactors
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Transcript of Bubble Column Reactors
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 gas-liquid 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 parameters DT = 0.1 m DT = 0.24 m
Number of packing sections used in the column, N/ [-] 9 6
Length of one packed element / [m] 0.2 0.288
Diameter of one packed element / [m] 0.0935 0.24
Hydraulic diameter of the open channels, dh / [m] 0.007 0.020
Inclination of corrugated sheets from vertical 45o 45o
Solids hold up in the structured packed section, epsS / [-] 0.205 0.198
Void fraction within "packed channels" / [-] 0.454 0.505
Volume fraction of "packed channels" in the reactor, epsPC / [-] 0.375 0.400
Specific surface for the gas flow (assuming the space between the glass spheres is completely filled with liquid), As / [m
-1]354.4 122.3
Entrance length, He / [m] 0.07 0.15
Height of the structured packed section, Hp / [m] 1.8 1.68
Dispersion height, Hd / [m] 1.68 1.60
Height between the pressure taps in the bubble column section, dH / [m] 0.9 1.21
Distributor hole diameter, d0 / [m] 0.0005 0.0005
Number of distributor holes 253 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 Bernoulli’s law, for a single steady state incompressible flow, the measured pressure difference is equal to:
• ΔP = ½ ρLu2.
• he instantaneous fluid velocity is given by the difference between the two local instantaneous velocities at the two holes:
• ΔP = ½ ρL (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 upwards2 – hole facing downwards
The mean axial velocity measured in one
position in the bubble column becomes:
• The measured liquid velocity is :
• http://ct-cr4.chem.uva.nl/bc/lit_radi.html\\