Bubble Coalescence and Breakup in Gas-Liquid Stirred Tank Reactors

Transcription

1 Bubble Coalescence and Breakup in Gas-Liquid Stirred Tank Reactors Rahman Sudiyo

2 Background Stirred tank reactors are widely used in chemical, pharmaceutical and biochemical industries. Mass transfer between gas and liquid is controlled by, among others, number of bubbles and bubble size distributions. Bubbles break-up and coalescence occur leading to the change of bubble size distributions

3 Objective To study mechanisms of bubble coalescence and breakup To establish factors that need to be considered in modeling of bubble coalescence and break-up To develop population balance models that can be embedded into CFD

4 The System Studied Standard laboratory baffled stirred tank equipped with a six-blade Rushton disc turbine impeller was used in experiments. Continuous phase: deionised water Dispersed phase: air CFD simulations were conducted for the same tank as used in the experiments.

5 Shadowgraph Technique Determination of bubble shape and bubble size. Shadows of bubbles have a lower light intensity than the surrounding. By analyzing consecutive frames, bubble velocity and coalescence rate can be estimated

6 Experimental Set-up of PIV The existence of periodic velocity fluctuations was assessed using an external triggering system. The PIV measurements were made with no gas present in the tank. Measurement positions in PIV were the same as in shadowgraph.

7 Bubble Breakup and Gas Dispersion SELCHEM The high rotation of the vortices creates low pressure regions within the vortices. Gas bubbles can be drawn into the vortices forming gas cavities. Gas dispersion from the cavity tails is the main mechanism for gas phase mixing in the tank.

8 Bubble Coalescence Consecutive steps: - collision of bubbles - trapping and thinning of a thin liquid film - film rupture Time required by two bubbles from the first contact to complete coalescence (the two bubbles become a single entity) is mostly equal to or less than 2ms.

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10 Turbulent is not a totally random movement of fluid element, but has a structure, even if this structure change very fast in tine. 0.4 [m/s] 0.4 [m/s]

11 Bubble Coalescence SELCHEM Bubbles rotate and deform while they approach each other indicating they are trapped in a large turbulent eddy. The increase in bubble velocities as bubbles start to approach each other suggest the involvement of centrifugal force due to eddy rotation.

12 Bubble Coalescence SELCHEM In most of successful coalescence events, the interaction time is much shorter than the average eddy life time. Except for large bubbles, bouncing of bubbles was exceptional and film drainage is not the limiting step in turbulent driven coalescence.

13 Bubble Coalescence Probability of bubbles to collide depends on The size of bubbles The size and energy of eddies Viscosity through drag The lower probability for small bubbles is due too high drag resistance and for large bubbles is probably due too few large eddies that can trap large bubbles.

14 Flows around of a baffle (PIV) Instantaneous Velocity Fields behind a Baffle

15 Bubble Coalescence at the Stationary Vortex at leeward side of the baffle SELCHEM

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17 CFD simulation on the leeward side of the baffle Volume fraction of air Velocity vectors colored by total pressure

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19 3D simulation of rising bubble

20 Interaction between two rising bubbles

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