Bubble generation for aeration and other purposes
Uses of a method of producing small bubbles of gas in a liquid include gas transfer in airlift bioreactors and anaerobic digesters, and particle separation. The method uses a source of the gas under pressure, a conduit opening into a liquid and oscillating the gas passing along the conduit. The oscillation is effected by fluidic oscillator, comprising a diverter that divides the supply into respect outputs, each output being controlled by a control port, wherein the control ports are interconnected by a closed loop. Separation of algae from water involves delivering a laminar flow of microbubbles in the range 10 to 100 μm diameter. Such bubbles also deliver a laminar flow in bioreactors that delivers enhanced liquid flow despite the small bubbles, which improves mixing and also enhances efficiency of gas exchange and retention of the bubbles in the reactor.
1. An anaerobic digester comprising:
a liquid fermenter tank for anaerobic microorganisms; and
a diffuser of a microbubble generator to introduce microbubbles of non-oxygen containing gas into liquid in the anaerobic digester so as to induce non-turbulent laminar flow of the liquid,
wherein:
the anaerobic digester is configured as an airlift bioreactor;
methane and acid gases produced by the anaerobic digester are exchanged across a surface of the microbubbles to strip the methane and acid gases from the liquid when said microbubbles connect with a header space of the liquid fermenter tank; and
the microbubbles of non-oxygen containing gas are generated at the diffuser with a fluidic oscillator.
2. The anaerobic digester of claim 1 , wherein, at a commencement phase of said digestion, said gas contained in the microbubbles is nitrogen or another inert gas that strips the fermenter of oxygen to promote the onset of anaerobic digestion.
3. The anaerobic digester of claim 1 , wherein during a growth phase of the microorganisms in the digester, said gas contained in the microbubbles comprises a biogas generated and released into the head space of the fermenter tank.
4. The anaerobic digester of claim 1 , wherein the gas contained in the microbubbles comprises approximately 60% methane and 40% carbon dioxide.
5. The anaerobic digester of claim 1 , wherein, during a growth phase of digester, said gas contained in the microbubbles comprises more than 90% carbon dioxide.
6. The anaerobic digester of claim 1 , wherein, at an endphase of said digestion, said gas contained in the microbubbles is nitrogen or another inert gas that serves to strip methane and hydrogen sulphide from the fermenter to neutralize an acidity of remaining liquid in the anaerobic digester.
7. The anaerobic digester of claim 1 , wherein the gas contained in the microbubbles comprises carbon dioxide, and the carbon dioxide is sourced from one of: power production from combustion of the methane produced and sequestered carbon dioxide from other sources.
8. The anaerobic digester of claim 1 , wherein the bubbles have a diameter in the range 10 to 100 μm.
9. The anaerobic digester of claim 1 , wherein at least a proportion of the bubbles have a diameter in the range 10 to 30 μm accounting for at least a gas holdup of 0.05%.
10. The anaerobic digester of claim 1 , wherein at least a proportion of the bubbles have a diameter in the range 10 to 30 μm accounting for at least a gas holdup of 0.5%.
11. The anaerobic digester of claim 1 , wherein:
the tank has sides and a base and the liquid in the tank has a top surface above which is the header space; and
the diffuser is disposed in the liquid at the base of the tank and is arranged to inject microbubbles of gas into the liquid in the tank whereby the apparent density of the liquid above the diffuser is reduced by the microbubbles thereby creating a flow of the liquid,
which flow is:
up the tank in a riser section thereof,
turned sideways at the surface of the liquid, where at least a proportion of the microbubbles either break at the surface, terminate passage through the liquid at the surface, or are reduced in diameter through dissolution of the gas in the microbubbles into the liquid during their passage up the riser section;
turned down at the sides of the tank into a downcomer section of the tank;
turned sideways back into the riser section; and
at least in the riser section, is non-turbulent laminar flow having a Reynolds number less than 2000, the Reynolds number being based on the liquid flow velocity, its constitutive properties, and the pore diameter of the diffuser.