Flow accelerator for micro gas bubbles on air candles

Abstract

When an air mixture with CO2 is injected into a photobioreactor using air candles, gas bubbles with a diameter of 3-5 mm dissolve on the surface of the air candles once their buoyancy exceeds the holding forces on the substrate. However, from a process engineering perspective, it is highly advantageous if the gas bubbles are significantly smaller, as this creates a larger surface area for the exchange of CO2 molecules with the water, and the gas bubbles also remain in the reactor longer due to their lower buoyancy velocity. The device presented here describes a flow velocity accelerator consisting of a hopper and a jacket tube that partially or completely encloses the air chamber and ensures a higher flow velocity at the air chamber. Due to the increased flow, the resulting gas bubbles are separated from the substrate earlier and are therefore significantly smaller.

Description

[0001] Photobioreactors of various designs are used in the cultivation of photoautotrophic and mixotrophic microorganisms such as microalgae, diatoms, and bacteria. They provide the organisms with the necessary reactants—photons and carbon dioxide—for photosynthesis in the most appropriate dosage possible. While the exposure of photobioreactors to sunlight or artificial light, and thus the supply of photons, has received considerable attention and scientific studies in the literature, the supply of the equally necessary reactant, carbon dioxide, has received very little attention. Although stoichiometric calculations of the required amount are available, processes and equipment are lacking to deliver the CO2 to the microorganisms in the most economical form and according to their needs.

[0002] There is a great need to catch up in terms of considerations and technologies, as CO2 in the required sterile form, in combination with the residual gases that are also required in sterile form, is an expensive raw material that needs to be used in such a way that it dissolves as completely as possible in the suspension. A mixture of air and an added amount of CO2 is always used, as the microalgae often do not tolerate concentrations of more than 7% CO2 in the gas mixture. Concentrated CO2 cannot be blown in. The solubility of CO2 in water is 1.688 g / l, which is very high compared to the solubility of oxygen (0.043 g / l) and nitrogen (0.02 g / l) at a temperature of 20 °C. However, the actual solubility of CO2 in a suspension depends primarily on the direct contact of the CO2 molecules with the water, as the gas bubbles only remain in the water for a very short time due to their rapid buoyancy.

[0003] It is possible to introduce a very large amount of gas into the suspension in the form of larger or smaller gas bubbles in order to achieve the required CO2 concentration in the water. However, this is counteracted by the fact that air with a higher CO2 content is complex to produce, and in a perfect process to avoid contamination, not only the CO2 portion but the entire amount of injected gas must be sterilized. This incurs high costs, and so a way is sought to dissolve as much of the CO2 in the injected air as possible. A residual amount of CO2 in the gas bubbles after they have left the suspension by rising means an inefficient use of the expensive CO2 and also a discharge into the atmosphere, which contradicts the idea of CCS (CO2 Capture and Storage).Even if cheap sources are available for the discharge of CO2-containing exhaust gases from power plants or other industrial processes, the goal of any modern technology should be the almost complete processing of the contained CO2 into biomass.

[0004] If gas bubbles of a certain size are introduced into water, CO2 molecules are only absorbed from the contact area between the bubble and the water and go into solution. Gas bubbles typically detach from the surface of a filter fabric or capillary at a size of 3 - 5 mm when their buoyancy exceeds the holding forces on the substrate. The residence time of the gas bubble in the reactor is very short, as gas bubbles rise at a speed of around 0.1 m / s for very small bubbles of 1 mm in size and 0.25 m / s for bubbles of 3 - 5 mm in size, and in shallow reactors such as horizontal tubes and race ponds they often remain in suspension for far less than 1 second. Even in optimized reactor tanks with a height of more than 2 m, typical large gas bubbles require less than 10 seconds to emerge at the surface of the liquid.In such a short time, only limited convection occurs within the gas bubble itself, leaving a significant residual amount of CO2 in the gas bubble. A gas bubble detaches from the surface of the air candle or filter surface when the buoyancy force exceeds the adhesion force holding the gas bubble to the surface of the material. It has been proven that the size of the pores in the filter material and the geometry or nature of the air dispenser's surfaces are irrelevant in this process.

[0005] Gas bubbles typically dissolve at a size of 3.5 to 5 mm in aqueous solutions that lack active substances to reduce surface tension. These are soap-like substances that cannot be used in microalgae suspensions due to foaming and the associated problems. However, it would be particularly important to be able to significantly reduce the size of the rising gas bubbles for two reasons. The following comparison illustrates this: A very large gas bubble with a diameter of 5 mm has a volume of 0.0654 ml and a surface area of 78.5 mm 2 .

[0006] A typical gas bubble with a diameter of 3.5 mm has a volume of 0.0224 ml and a surface area of 38.48 mm 2 .

[0007] A very small gas bubble with a diameter of 1.0 mm has a volume of 0.0005 ml and a surface area of 3.14 mm 2 .

[0008] Consequently, for the same volumes, reducing the size of typical gas bubbles from 3.5 mm to 1.0 mm results in an increase in the total surface area by a factor of 3.5 or more. Since an increase in surface area is synonymous with a correspondingly larger contact area between water and gas bubbles, combined with the longer residence time of very small gas bubbles in the suspension, a significantly larger amount of CO2 can be dissolved, thus significantly better utilizing the CO2 present in the gas mixture.

[0009] Various devices are also known that use ultrasound or mechanical turbulence to generate micro gas bubbles with a diameter of only a few µm. This is used to purify particles from the water itself or for other applications. A significant disadvantage of such an enormous reduction in the size of the gas bubbles is that the water becomes cloudy due to the multiple refractive index, which hinders or even completely prevents the transport of photons in the liquid. However, this is undesirable in a photobioreactor, so such extreme reductions in the size of the air bubbles cannot be used here.

[0010] The author posed the question of how an effective but moderate reduction in the size of the rising gas bubbles could be achieved cost-effectively and using simple means, since this would significantly increase the reactive surface and more than double the residence time of the smaller gas bubbles in the suspension due to the reduced buoyancy. After careful consideration and a series of tests, the procedure described below was confirmed.

[0011] In a photobioreactor, integrated pumps maintain a certain process-dependent flow velocity (4) of the suspension in channels of any design in order to minimize the adhesion of algae to the vessel walls (1;2) and to prevent the algae themselves from clumping together. Furthermore, a certain flow and thus turbulence are necessary to supply all microorganisms with the necessary nutrients, trace elements, and vitamins as needed. This flow velocity is typically 0.25–0.5 m / s. However, if air bubbles (3) are integrated into the vertical side wall of the photobioreactor (1), or if filter fabric or porous elements are installed to inject gases from the outside (6), this low flow velocity (4) of the suspension is not sufficient to break off gas bubbles early and at a small size. Large gas bubbles (5) measuring 3.5–4.5 mm or even larger will therefore form.At a much higher flow velocity (4a) of the suspension, however, much smaller gas bubbles (5a) would be torn off from the ground due to the flow resistance.

[0012] However, the process-related flow velocity in photobioreactors cannot be increased arbitrarily, as this results in higher shear forces, particularly in the pump and at all flow constrictions and deflections. However, it is possible to use a special installation on the air chamber itself to ensure locally limited acceleration and thus an increased flow velocity of the suspension along the air chamber (3). This separates the gas bubbles from the substrate early in their formation and consequently results in a significantly smaller diameter. This can be easily and effectively installed, especially in photobioreactors with installations (2) for the meandering formation of flow channels, as can be seen from the attached drawings.

[0013] Such a flow accelerator consists of a hopper (7) that tapers towards the air chamber (3) and ends in a casing (8) surrounding the chamber, thus accelerating the suspension along the air chamber (3). It must be only long enough along the air chamber (3) to prevent rising air bubbles from collecting at the top of the channel. These would then coalesce into larger air bubbles, and the desired effect of forming very small air bubbles would no longer be achieved. The tubular channel (8) surrounding the air chamber (3) should be positioned close to the air chamber (3) itself to ensure a high flow velocity (4a) along the air chamber. The distance to the air chamber should be 5-10 mm, preferably 4-8 mm, and ideally 3-6 mm, and depends significantly on the size of the hopper, which can be installed in the photobioreactor depending on spatial constraints.Since the very small air bubbles (5a) in this process rise with a buoyancy of about 0.1 m / s, largely independent of their vertical movement, the length of the tube (8) surrounding the air candle (3) may not exceed the following formula:. L(mm)=Distance from pipe to air candle(mm)(top) divided by buoyancy velocity of the gas bubbles(mm / s) multiplied by flow velocity(mm / s)

[0014] Since measuring the flow velocity of the liquid along the air plug (3) is very difficult, a pragmatic approach is recommended: initially leaving the horizontal jacket tube (8) somewhat longer and observing whether larger gas bubbles emerge from its upper end. If this is the case, it can be concluded that small gas bubbles have clustered there. This tube (8) should then be shortened until this effect no longer occurs.

[0015] The retention funnel (7) can have various geometric shapes: round, oval, square, or rectangular, as shown in Figure 5. This depends on the installation situation in the photobioreactor channel. In general, the largest possible retention funnel (7) should be selected. The larger the retention funnel (7), the greater the back pressure and thus the achievable flow velocity in the reduced jacket tube (8) and along the entire air chamber (3). The adjoining centrally or eccentrically arranged jacket tube (8) can be round, oval, square, or rectangular, or have a different shape, as shown in Figure 5. In any case, it is advantageous if this jacket tube (8) is arranged so that a greater distance is left at the top to the air chamber (3), since according to the above equation, the jacket tube (8) can then be extended, and a greater length also increases the flow velocity of the water along the air chamber (3).Furthermore, a cutout (13) can be made at the upper end of the casing pipe to prevent small air bubbles from converging at the top of the pipe, but rather from escaping upwards, and to prevent larger air bubbles from forming and escaping as a result of small air bubbles converging. This allows the casing pipe to be significantly lengthened. The increase in flow velocity is proportional to the reduction in size of the gas bubbles then torn away from the ground. Increases in flow velocity along the air candle (3) by a factor of 3-5 can be achieved, depending on the geometry of the retention funnel (7) and the dimensions of the casing pipe (8). This is sufficient to achieve the desired reduction in the size of the gas bubbles to values of approximately 1 mm in diameter.

[0016] This flow accelerator can be made from various materials, such as metals or plastics. It is advantageous if the surfaces are very smooth and also consist of materials to which little biomass can adhere. They must also be corrosion-free, even when used in saline solutions. Otherwise, more durable materials must be used. If the flow accelerator is made of metal, it can be coated with PTFE to prevent biomass from adhering. Since the flow accelerator must maintain its position in the photobioreactor even during flow, it has a number of special features. It is inserted through a hole (12) in the bend of the pipe onto the air plug (3), and the retention funnel (7) is secured by a self-locking screw (10) through a tab (9) at the bottom of the reactor.To ensure that the positioning of the jacket tube (8) around the air candle (3) does not shift, 3 or 4 narrow guide plates or pins are attached as spacers (11) at the outlet end of the jacket tube (8), which touch the air candle (3) but do not noticeably impede the flow due to their low thickness. Legend to the reference numbers A hopper rectangular symmetrical B Reservoir funnel rectangular asymmetrical C Storage funnel oval / round D jacket pipe round E jacket pipe oval F casing pipe rectangular 1 vertical side wall photobioreactor 2 flow internals vertical 3 air candles 4 Flow suspension slow 4a Flow Suspension fast 5 large air bubbles 5a small air bubbles 6 air inlet air candles 7 storage funnels 8 jacket pipe 9 Mounting tab 10 self-locking screws 11 spacers 12 Hole for inserting the flow accelerator 13 Cutout at the top of the casing pipe

Claims

[1] Device for accelerating water or a suspension along an air candle (3) or a filter fabric to reduce the size of air bubbles, which are torn off from the ground at an early stage of their formation due to the higher flow velocity achieved and are therefore smaller in size. [2] Device for increasing the flow velocity designed in such a way that it consists of a hopper (7) and a suspended jacket pipe (8) which at least partially encloses an air candle (3). [3] Device for increasing the flow velocity of the water or the suspension designed in such a way that the retention funnel (7) and the casing pipe (8) are arranged in a line or the retention funnel (7) is arranged at a right angle or a more acute or obtuse angle to the casing pipe (8). [4] Device for increasing the flow velocity designed in such a way that the hopper (7) can be rectangular, polygonal, round or oval. [5] Device for increasing the flow velocity designed in such a way that the jacket tube (8) surrounding the air candle (3) can be rectangular, polygonal, round or oval. [6] Device for increasing the flow velocity designed in such a way that the distance between the jacket pipe (8) and the air candle (3) can be made higher at the top than at the sides and bottom. [7] Device for increasing the flow velocity designed in such a way that the respective distance between the jacket pipe (8) and the air candle (3) is secured with spacers (11). [8] Device for increasing the flow velocity designed in such a way that the length of the jacket pipe (8) is only so long that small air bubbles cannot combine to form larger air bubbles at the top of the pipe (8). [9] Device for increasing the flow rate designed in such a way that the casing tube (8) has an elongated cutout (13) at the top and towards the opening, through which small air bubbles can escape upwards in order to be able to make the casing tube (8) longer and nevertheless to avoid a merging of small air bubbles into larger air bubbles at the top of the tube (8).

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