Reactors: Shaken, not Stirred

Professor R. Krishna
University of Amsterdam

Date: Thursday
April 6, 2006
Time: 4:00 p.m.
Place: Engineering II, Room 3361

ABSTRACT

Consider reactors in which gas bubbles are dispersed in the liquid phase, i.e. bubble columns. Such devices are widely used in the chemical process industries for carrying out a variety of liquid phase reactions, such as fermentation, oxidation, ozonization of drinking water, and hydrogenations. When the chemical reaction is fast, the overall process can be improved if the physical mass transfer process can be enhanced. Stirred vessels are commonly employed but this involves high energy inputs and introduces undesirable backmixing. A better way to improve gas-liquid contacting, at lower energy inputs and without backmixing, is to exploit bubble acoustics. Bubbles ring like bells and emit sound, caused by radial pulsations. The bubble size, its morphology and rise characteristics are influenced by sound waves. When a bubble column is shaken, i.e. vibrated at the bottom in a sinusoidal manner, pressure waves travel up the column and these are reflected downwards at the top. (I use vibrations frequencies of the order of 50-200 Hz, so I do not talk about ultrasonics). The bubbles experience a force, called the Bjerknes force (also called acoustic radiation force) that drives them towards the pressure anti-nodes in the column. As a result a bubble can be made to levitate, or move downwards, depending on the applied frequency. Just like a musical instrument, a bubble column can be made to operate under various harmonics; bubbles segregate into different layers within the column, with alternating high and low concentrations of bubbles. Under the influence of shaking, smaller bubbles (or drops in liquid-liquid systems) are generated at the distributor. Increases in gas holdups and mass transfer by a factor of about 2-4, are realized depending on the harmonic mode. Dramatic improvements due to vibrations are also realised in air-lifts, G-L monoliths, and liquid-liquid extractors. Vibrations facilitate fluidization of cohesive powders, and solids flow in silos. When a mixture of granular material of different sizes and densities is shaken, larger heavier particles can be made to rise (Brazil Nut Effect) or sink (Reverse Brazil Nut Effect); there are several potential applications in crystallizers and polymerization reactors.

My talk will focus on the underlying physics and I rely on 4500 fps video images to obtain mechanistic insights. You can preview these movies at:

Bubble levitation and downwards motion http://www.science.uva.nl/research/cr/BubbleMotionVibration/

Harmonic operation http://www.science.uva.nl/research/cr/GasHoldupSoundWaves/

Smaller bubbles and drops at distributor http://www.science.uva.nl/research/cr/vibrationvideo/

Airlift http://www.science.uva.nl/research/cr/AirliftVib/

Gas-liquid monolith http://www.science.uva.nl/research/cr/MonolithVib/

Brazil Nut and Reverse Brazil Nut Effects http://www.science.uva.nl/research/cr/GranularSegregation/