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 :

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