Gas absorption in foam reactors

Perry, David Charles

Title: Gas absorption in foam reactors
Creator: Perry, David Charles
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2003
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: This thesis is concerned with the absorption of oxygen into the liquid foam layers found in aerobic bioreactors and the adsorption of gas into foams of reactive liquids. Gas-liquid foams have the potential to facilitate mass transfer from gas bubbles. Foams containing small bubbles have a very large interfacial area per unit volume and provide a long contact time for mass transfer to take place. The literature contains relatively few investigations into mass transfer in foam reactors. Recent work has focused on gas absorption into foams with low liquid contents (<6% v/v). The liquid film that make up these foams may quickly become saturated with gas and cease contributing to mass transfer. The focus of this thesis is the investigation of the mass transfer characteristics of foams with high liquid holdups (15 to 35% v/v). Mass transfer investigations have been performed on a pilot-plant and bench-top scale. A pilot-plant bioreactor that incorporated a foam contact zone was investigated. The reactor biodegraded high-strength wastewater sludge using the autothermal thermophilic aerobic digestion process. The reactor was designed to accommodate a half-metre-deep layer of naturally occurring foam on the liquid surface, to increase the contract time of rising air bubbles with the process liquor. The foam extended the time available for mass transfer by 6-7 minutes. The digester performance was assessed by monitoring the wastewater characteristics, reactor temperature and the quantity of carbon dioxide in the exhaust gas. The thermophilic digestion rate increased with increasing temperature up to 67°C before rapidly declining. The oxygen transferred by the aeration system was estimated from the concentration of carbon dioxide in the exhaust gas by assuming a respiration quotient of one. Oxygen transfer increased at higher aeration rates. It is believed the higher transfer rates were a result of the higher liquid holdup and greater mixing in the foam contact zone at higher aeration rates. During the peak oxygen demand of the batch process, up to 60% of the supplied oxygen was transferred to the process liquid. Gas absorption was also investigated using a bench-scale foam contactor. Foams with high liquid contents were produced by distributing additional solution on the upper surface of the foam. This reflux liquid increased the liquid holdup and liquid drainage rate through the foam column. The hydrodynamic conditions of the foam contactor were established by measuring the liquid holdup, bubble size and liquid drainage rate. A simple two-phase flow model has been developed by assuming the rising bubbles act as rigid spheres. Use of the model enabled the bubble size and interfacial area of the foam to be calculated from the liquid holdup and foam drainage rate. The interfacial area within the foam was much higher than conventional gas-liquid contactors, ranging from 2,100 - 3,200 m²/m³. Mass transfer properties of the foam were evaluated by absorbing carbon dioxide into a sodium carbonate-bicarbonate buffer that was stabilised by the addition of 50ppm of polyglycol frother. The conversion of CO₂ in the column was measured and compared to mass transfer coefficient in the foam was estimated to be the same magnitude as that recorded in packed beds of spheres, approximately 5x10^-5 to 8x10^-5m/s. Reflux liquid addition increased the conversion rate considerably. Due to their high interfacial area, foam contactors are particularly suited to applications where the dissolved gas is rapidly removed from the liquid by reaction. In bioreactors, foam layers have the potential to improve the oxygen transfer efficiency by increasing the contact time of rising air bubbles with the liquid, concentrating hydrophobic cells at the interfacial area and by facilitating interfacial transfer. The addition of reflux liquid on the foam surface improves foam stability and increases the mass transfer performance of foams.
Subject: gas absorption; foam reactors; oxygen; liquid foam; mass transfer
Identifier: http://hdl.handle.net/1959.13/1312883
Identifier: uon:22477
Language: eng

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