Decompression induced pool and flow liquid degassing

Liquid flow degassing

Contant Person: R. Oikonomidou [rania.oik@gmail.com]
 

The driving force of liquid degassing is the supersaturation of a liquid volume with a gas. Liquid supersaturation may be triggered either by pressure decrease or by temperature increase. A supersaturated liquid tends to reach equilibrium state due to gas desorption. Bubbles formation and growth is attributed to mass transfer phenomena. In case liquid is flowing while being supersaturated, process is called “flow” degassing.

The present project studies “flow” decompression type of degassing, the main application of which is Dissolved Air Flotation wastewater treatment process. Water saturated with air under pressure gets flashed inside the flotation tank through a nozzle, due to decompression. Water jet becomes air supersaturated and thus bubbles formation and growth is activated. These micro-scaled bubbles attach to solid particles suspended in wastewater and promote their separation.

 As DAF efficiency strongly depends on the small size of bubbles, several studies have been conducted on how experimental parameters affect bubble size. However, the effect of saturation pressure on bubbles size in not yet straightforward. Moreover little work has been done on dynamic bubble size evolution. This work aims to study the dynamics of bubble growth during “flow” decompression degassing under various saturation pressures. Mass transfer mathematical models will be further extended, considering the contribution of mass convection over diffusion in case of flow. Furthermore, experiments will be conducted under hypergravity conditions at the Large Diameter Centrifuge of European Space Agency. Thus, the effect of “g” value on existing degassing mathematic equations will be validated.

Bubbles size, trajectory and interactions are studied optically by means of a high resolution still digital camera. Electrical measurements are conducted as well using IVED (MDG) technique for gas fraction determination.

 

Figure 1: “Flow” decompression degassing flow diagram.

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Figure 2: “Flow” decompression degassing experimental apparatus. a) Optical measurements, b) Electrical measurements.

 

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Figure 3: Bubble Size Distributions under a) 220kPa, b) 300kPa, c) 400kPa and d) 500kPa saturation pressure. Results obtained by bubble image analysis.

 

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Figure 4: a) Gas fraction during the whole injection under 300kPa saturation pressure, b) Gas fraction values under various saturation pressures. Results obtained by electrical measurements.

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Figure 5: Flow conditions during the injection. Schematic description of a) Jet dispersion in the column, b) Bubble formation and growth

Liquid pool degassing

Contant Person: V. Papadopoulou [virginie.papadopoulou07@imperial.ac.uk]

 

Vascular gas bubbles are routinely observed after scuba dives using ultrasound imaging, however the precise formation mechanism and site of these bubbles are still debated and growth from decompression in vivo has not been extensively studied, due in part to imaging difficulties. An experimental set-up was developed for optical recording of bubble growth and density on tissue surface area during hyperbaric decompression. Muscle and fat tissues (rabbits, ex-vivo) were covered with nitrogen saturated distilled water and decompression experiments performed, from 3 to 0 bar, at a rate of 1bar/min. Pictures were automatically acquired every 5s from the start of the decompression for 1h with a resolution of 1.75 μm. A custom MatLab analysis code implementing a circular Hough transform was written and shown able to track bubble growth sequences including bubble center, radius, contact line and contact angles over time. Bubble density, nucleation threshold and detachment size, as well as coalescence behavior, were shown significantly different for muscle and fat tissues surfaces, whereas growth rates after a critical size were governed by diffusion as expected. Heterogeneous nucleation was observed from preferential sites on the tissue substrate, where the bubbles grow, detach and new bubbles form in turn. No new nucleation sites were observed after the first 10min post decompression start so bubble density did not vary after this point in the experiment. In addition, a competition for dissolved gas between adjacent multiple bubbles was demonstrated in increased delay times as well as slower growth rates for non-isolated bubbles.