Fermentation methods and systems

Abstract

A fermentation reactor (200) including an interior volume (202, 218) for receiving a liquid medium (206), and a gas inlet (208) for introducing of one or more gases into the liquid medium to form gas bubbles (212). The interior volume forms an unvented closed loop (202, 218). The fermentation reactor includes a multi-phase pump (220) for circulating the liquid medium and the gas bubbles around the closed loop, and ensures any accumulations of gas in the interior volume have an individual volume of less than or equal to 600 mm3.

Description

174853 / 02Fermentation Methods and SystemsThis invention relates to methods and systems for producing biomass through fermentation of suitable micro-organisms.Growing hydrogen-oxidising bacteria (e.g. Knallgas bacteria) typically involves feeding substrate gases to the bacteria. The substrate gases may include carbon dioxide, hydrogen and oxygen. The reactor is typically filled with a liquid medium, with gas filled spaces in the reactor formed from gases produced by the microorganisms, and / or undissolved substrate gases. However, combining hydrogen and oxygen represents a highly volatile and explosive gas mixture. In order to prevent explosions in reactors, the conventional approach is to limit the level of either hydrogen or oxygen within the reactor to a level below safety margins, e.g. to keep the total concentration of either hydrogen or oxygen in any hydrogen oxygen mixtures within gas filled spaces in the reactor below their Low Explosion Limit (LEL), which for both hydrogen and oxygen is considered to be less than 5% v / v. Due to poor solubility and thus poor mass transfer, hydrogen is fed into the reactor at maximum level, while oxygen is kept low. However, in an oxygen-limited environment, the rate at which microorganisms grow is limited by the amount of oxygen available which means that the process is not as efficient as it could be.The present invention aims to address this.From a first aspect, the invention provides a fermentation reactor comprising an interior volume, said interior volume being substantially fully filled with a liquid medium comprising hydrogen oxidising bacteria and a plurality of gas bubbles for producing biomass;wherein the plurality of gas bubbles comprise carbon dioxide, hydrogen and oxygen or any mixture thereof; andwherein the concentration of oxygen in bubbles containing a mixture of hydrogen and oxygen is above 6% v / v.Thus, it will be seen that in accordance with the invention, the fermentation reactor is substantially fully filled with the liquid medium, i.e. there is no, or no significant, gaseous headspace above the liquid medium and no other substantial pockets of gas. As will be explained further below, this allows a higher concentration of oxygen to be used than that which would previously have been considered to be safe and hence allows for a higher growth rate of the microorganisms, thereby allowing yield within a given time to be increased.In accordance with the invention, hydrogen and oxygen gas are fed into the reactor (e.g. separately), where they form gas bubbles within the liquid medium. The gases are typically introduced at a higher pressure than the pressure within the reactor vessel at their entry point (e.g. at the bottom of the reactor). These gas bubbles dissolve in the liquid medium, such that they can be consumed by the bacteria. In a typical prior art fermentation reactor, undissolved gas bubbles will rise to the top of the reactor and accumulate to form a gaseous headspace above the liquid medium, containing a mixture of hydrogen and oxygen. It has therefore been important to ensure that the concentration of oxygen is kept well below the level at which there might be a risk of explosion - typically taken to be where the percentage of oxygen is above 5% v / v.However, the Applicant has appreciated that a mixture of hydrogen and oxygen is only volatile when present together in the gas phase, not when dissolved in for instance water. Therefore reducing or eliminating headspace in which explosive gases accumulate and as set out below keeping the size of bubbles containing such volatile gases small in preferred embodiments, a higher concentration of oxygen can be safely used whilst reducing the impact of any explosions. This is because if an explosion does occur the amounts of gases involved will be relatively small and the released energy and the resulting pressure wave would thus be absorbed by the surrounding liquid medium without causing serious damage to the equipment and its surroundings. Even a small increase (e.g. of a few percent) in the concentration of oxygen above the Lower Explosion Limit (LEL) in the reactor can approximately double the production rate of biomass. This therefore has the potential to bring significant benefits. In a set of embodiments the oxygen concentration is set to between 6% and 15% v / v, e.g. between 8% and 12% v / v. In such embodiments, hydrogen is introduced in excess in the reactor to maximise mass transfer to the bacteria.The bacteria may comprise a microorganism or microbial culture which use one or more gaseous substrates (when they are dissolved in the medium)) as a source of carbon and energy, to convert them into biomass. The liquid medium may comprise a culture medium, for example water enriched with nutrients (e.g. nitrogen and / or phosphorus) and / or supplements needed, to support the growth of the bacteria. For example, the bacteria may comprise a chemo-autotrophic microorganism selected from the group consisting of the following genera:

[0002] Acetoanaerobioum sp.; Acetobacerium sp.; Acetogenium sp.; Achronobacater sp., Acidianus sp.; Acinetobacer sp.; Actinomadura sp.; Aeromonas sp.; Alcaligenes sp.; Alcaligenes sp.; Arcobacter sp.; Aureobacterium sp.; Bacillus sp.; Beggiatoa sp.; Butyribacterium sp.; Carboxydothermus sp.; Clostridium sp.; Comamonas sp.;Cupriavidus sp. Dehalobacter sp.; Dehalococcoide sp.; Dehalospirillum sp.;Desulfobacterium sp.; Desulfomonile sp.; Desulfotomaculum sp.; Desulfovibrio sp.; Desulfursarcina sp.; Ectothiorhodospira sp.; Enterobacter sp.; Eubacterium sp.;Ferroplasma sp.; Halothibacillus sp.; Hydrogenobacter sp.; Hydrogenomonas sp.; Leptospirillum sp.; Metal losphaera sp.; Methanobacterium sp.; Methanobrevibacter sp.; Methanococcus sp.; Methanosarcina sp.; Micrococcus sp.; Nitrobacter sp.;Nitrosococcus sp.; NitrosoIobus sp.; Nitrosomonas sp.; Nitrosospira sp.; Nitrosovibrio sp. Nitrospina sp.; Oleomonas sp.; Paracoccus sp.; Peptostreptococcus sp.;Planctomycetes sp.; Pseudomonas sp.; RalsOntia sp.; Rhodobacter sp.; Rhodococcus sp.; Rhodocyclus sp.; Rhodomicrobium sp.; Rhodopseudomonas sp.; Rhodospirillum sp.; Shewanella sp.; Streptomyces sp.; Sulfobacillus sp.; Sulfolobus sp.; Thiobacillus sp.; Thiomicrospira sp.; Thioploca sp.; Thiosphaera sp.; Thiothrix sp.; and any combinations thereof..In a set of embodiments, the fermentation reactor is arranged to ensure that at least 90%, preferably 95%, preferably 100% of the plurality of gas bubbles have an individual volume of less than or equal to 600 mm3(equivalent to a radius of approximately 5 mm). This can provide the required level of safety since even if such small bubbles containing a mixture of hydrogen and oxygen were to explode, the pressure increase caused by the explosion would be absorbed by the surrounding liquid medium. Hence, even if an ignition source were present igniting a volume of a volatile gas mix below 600 mm3, it would not cause any damage or trigger further ignition Thus, by maintaining the bubble size below 600 mm3, an oxygen concentrationabove 6% v / v may be safely used. In conjunction with the substantially complete filling to leave no headspace in accordance with the invention, this means that there are no accumulations of gas in the vessel significantly greater than 600 mm3.The volume distribution of the gas bubbles may be between 200 mm3and 600 mm3, e.g. an average volume of 300 mm3. It will be appreciated that the smaller the bubble size, the safer the reaction. Additionally, small bubbles increase the rate of mass transfer of the gas dissolving in the liquid medium (due to the larger surface area to volume ratio of the bubbles), and thus can increase the rate of cell growth.There are a number of ways in which the fermentation reactor can be designed to ensure that there is no significant headspace formed during its use and / or maintaining a small average size of the bubbles. In a set of embodiments a fermentation reactor comprises an interior volume for receiving a liquid medium, the interior volume being arranged so as to form an unvented closed loop; and a multi-phase pump arranged to circulate the liquid medium and the gas bubbles around the closed loop.This is novel and inventive in its own right. Therefore, when viewed from another aspect, the invention provides a fermentation reactor comprising an interior volume for receiving a liquid medium, the interior volume being arranged so as to form an unvented closed loop;a gas inlet for the introduction of one or more gases into the liquid medium to form a plurality of gas bubbles, the fermentation reactor being arranged to ensure any accumulations of gas in the interior volume have an individual volume of less than or equal to 600 mm3; anda multi-phase pump arranged to circulate the liquid medium and the gas bubbles around the closed loop.Whilst gas bubbles may rise within the reactor and / or coalesce, circulating the liquid medium and gas bubbles around the interior volume of the reactor using the pump can prevent a headspace forming in the reactor, by moving any gas that would otherwise accumulate at the top of the reactor downwards. This may give the gas bubbles more time to dissolve within the liquid medium, thus increasing the amount of gas which dissolves. The circulation may also further reduce the size of undissolved bubbles by agitation, causing them to break up. As set out above, this can allow for a highpercentage of oxygen (e.g. above 6% v / v) to be used in the reactor, whilst avoiding the risk associated with explosions.The gas inlet introduces gases into the liquid medium as a plurality of gas bubbles. For example, the gas inlet may be arranged to introduce the substrate gases hydrogen and oxygen (e.g. separately) into the interior volume of the reactor.The closed loop is unvented. This in theory allows all of the gas introduced to the liquid medium to dissolve into the liquid medium and be used in the fermentation reaction -e.g. by being consumed by bacteria within the liquid medium for producing biomass. This is beneficial since hydrogen and oxygen, as typically used, are relatively expensive and thus it would be uneconomic to avoid a headspace forming by simply allowing them to escape. It also means that resources are used more efficiently.Having a closed loop system also relaxes the requirements for the environment in which it is housed as it does not need to ensure that vented gases do not accumulate in the immediate environment. Thus, in some embodiments it is important to tightly seal the reactor to prevent the unused substrate gases escaping.In a set of embodiments, the interior volume comprises a reactor vessel and a channel;wherein the channel is arranged to connect an upper part of the reactor vessel to a lower part of the reactor vessel; andwherein the multi-phase pump is arranged in the channel to transport a portion of the liquid medium and the gas bubbles from the upper part of the reactor vessel to the lower part of the reactor vessel.The channel may be used to divert the gas bubbles that have risen to the top of the reactor back to the bottom of the reactor. As well as avoiding an accumulation of gas to form a headspace, this can increase the total path length of the bubbles travelling through the reactor, giving more time for the bubbles to dissolve.Furthermore, by diverting the liquid / gas mixture from the top of the reactor via a separate channel to the bottom of the reactor vessel, a backflow of bubbles may be prevented and / or reduced, thus ensuring that gas bubbles that might have accumulated at the top of the reactor are recirculated to the bottom of the reactor. Forexample, the pump may be configured to cause the liquid medium and gas bubbles to flow from the top of the reactor vessel to the bottom of the reactor vessel. The multiphase pump may be arranged anywhere along the length of the channel.The multi-phase pump could comprise a discrete - e.g. off-the-shelf - component. However this is not essential. Equally it could comprise any mechanical arrangement in the reactor which has the effect of causing an adequate level of bulk fluid flow - e.g. it could be provided by one or more impellers, stirrers or blades arranged at one or more parts of the reactor. A multi-phase pump enables arrangements in which both liquid and gas bubbles are pumped through the pump arranged in the channel. In a set of embodiments, the channel has a substantially smaller cross-section than the reactor vessel. This may help the multi-phase pump to operate more efficiently.In some embodiments, the channel is arranged externally to the reactor vessel. For example, the channel may extend out of a top section of the reactor vessel, and connect back to the reactor vessel at a bottom section, causing the gas / liquid medium to travel out of the reactor vessel.In other embodiments, the channel is arranged within the reactor vessel, e.g. the channel may be arranged at the centre of the reactor vessel. This may be useful to set up effective recirculation currents within the reactor vessel, e.g. to improve the mixing of the liquid medium and gas bubbles for improved gas dissolution and / or helping to achieve more even circulation within the vessel. Such an internal channel would typically comprise a wall forming a conduit that is physically separate from the rest of the vessel.In a set of embodiments, the reactor comprises at least one static mixer. The static mixer will through turbulence encourage the breaking down of the gas bubbles in the liquid medium by mixing the liquid medium and gas bubbles together.In a set of embodiments, the reactor comprises at least one stirrer. One or more stirrers could be provided in the reactor vessel, in the channel or in both locations. . The stirrer(s) may comprise a rotating blade arranged to shear or break apart the gas bubbles, e.g. to reduce the size of the gas bubbles and / or to prevent coalescence of the gas bubbles.Both static mixers and agitation by a stirrer will promote the dissolution of gas bubbles within the liquid medium, as the smaller gas bubbles have a larger surface area to volume ratio and thus dissolve more easily. This may also improve the consumption of the gas by the bacteria and hence the rate of bacteria growth, as well as reduce and / or prevent the accumulation of any gas headspace.The multi-phase pump may comprise an axial-flow pump and / or a positive displacement pump, e.g. a twin screw pump. This may be used to shear or break apart the gas bubbles, e.g. in addition to, or instead of a stirrer in the channel.In embodiments where the channel is arranged within the reactor vessel, the reactor may comprise a channel stirrer in the channel and a vessel stirrer in the main body of the vessel. These may be arranged to co-rotate around a common axis, e.g. wherein the common axis is parallel to the direction of flow through the channel. For example, the channel may comprise a tube which also rotates in the same direction as the channel stirrer and the vessel stirrer, e.g. the channel may be connected to the channel and vessel stirrers.The vessel stirrer and the channel stirrer may comprise paddles mounted in opposite directions. This may enable the stirrers to encourage the liquid medium and gas bubbles to flow upwardly in the reactor vessel and downwardly in the channel to promote circulation, in addition to breaking up the gas bubbles.In another set of embodiments the reactor vessel comprises one or more sonic agitators or sonicator devices, that may serve to create turbulence and to break up and thus reduce the size of rising bubbles.In a set of embodiments, the reactor comprises a sparger arranged to introduce the gas bubbles into the liquid medium. The sparger may be connected to the gas inlet, such that the gas(es) is / are introduced into the interior volume of the reactor via the sparger. The sparger may be arranged to facilitate uniform dispersion of the gas into the interior volume. This may help the gas to be evenly distributed throughout the liquid medium and hence aid dissolution and reduce accumulation of gas. For example, the sparger may comprise a bubbling column sparger, a horizontal pipesparger (e.g. comprising a plurality of holes), a jet-nozzle, and / or a metallic membrane sparger.The reactor may comprise one or more spargers, e.g. the spargers may be arranged throughout the interior volume to introduce gas to the liquid medium more evenly. For example, there may be more than one gas inlet, and each gas inlet may comprise a respective sparger for introducing the gas to the liquid medium.In a set of embodiments, the reactor comprises an agitator arranged within the interior volume, wherein the agitator is arranged to agitate the liquid medium to break up the gas bubbles and promote dissolution of the gas bubbles in the liquid medium. For example, the agitator may comprise a vibrating surface. The reactor may comprise a plurality of agitators, e.g. there may be agitators arranged throughout the interior volume.In an alternative set of embodiments to those in which a reactor vessel and channel are provided, the interior volume is itself formed as a closed loop conduit for receiving a liquid medium, wherein the multi-phase pump is arranged in the conduit to circulate the liquid medium and the gas bubbles around the conduit. This can help prevent the liquid medium and / or gas bubbles stagnating in the interior volume, as the entire liquid medium / gas bubbles are circulated around the conduit in one direction. In addition, a plurality of static mixers may be arranged within the conduit, to break up any gas bubbles.In a set of embodiments, the conduit has a substantially constant cross-section. In some such embodiments, the conduit is substantially symmetrical. For example, the conduit may be substantially toroidal.In some embodiments, the conduit comprises an undulating portion. This may increase the length of the path travelled by the liquid medium and gas bubbles, thus help the gas bubbles to dissolve.The undulating portion may have a, zig-zag, sawtooth sinusoidal or other curved profile. The undulating portion may comprise at least two top portions (e.g. the ‘peaks’ of a sinusoidal curve), where gas may accumulate. Reducing the overall amount ofgas in a single region may help to prevent the formation of a volatile headspace, e.g. if there is ignition of a small amount of gas within a top portion, it may not propagate to other top portions within the conduit. Therefore, if an ignition were to occur within one of these accumulations, the other small gas accumulations can act to absorb the pressure increase. This may be safer than a typical tank reactor, where the gas may accumulate in one big headspace volume, and where any ignition would impose a large pressure increase in the tank.The multi-phase pump may act to circulate the liquid medium and gas bubbles such that gas may not accumulate in the top portions anyway (during normal operation), further reducing the risk of explosion and enabling a high (e.g. above 5% v / v) concentration of oxygen to be used in the reactor. The multiple top portions of the undulating shape may further prevent a dangerous explosion of the reactor in the event of a power failure, e.g. if the pump were to stop working.In some embodiments, the reactor is tilted at an angle relative to the horizontal. For example, the reactor vessel may be tilted between 5° and 15° from the horizontal, e.g. approximately 10°. This may help to reduce the tendency for gas bubbles to rise and form a headspace. The tilt of the reactor encourages undissolved bubbles to accumulate at an upper corner of the reactor, such that they can be more effectively removed and recycled by the external channel as they are accumulated in a small region. Where an agitator is provided this can continuously recycle rising bubbles from the higher parts of the reactor back to the lower parts, hence bubbles will move in a spiral fashion gradually progressing towards higher end of the tank.In some embodiments, the reactor comprises a controller arranged to control operation of the multi-phase pump. For example, the controller may act to increase or decrease the speed of the pump (e.g. increase or decrease the supplied , frequency, current or voltage), in order to control the rate of coalescence of gas bubbles into a headspace, dependent on operating conditions.In a set of embodiments, the reactor comprises a bubble sensor arranged to monitor the size of the gas bubbles - e.g. to determine an average gas bubble diameter. The bubble sensor may be arranged within the interior volume. For example, the bubble sensor may comprise a camera connected to a processor, wherein the camera isarranged to capture at least one image of the bubbles and the processor is arranged to determine the average bubble diameter based on the captured image(s). The bubble sensor may alternatively or in addition use laser, ultrasound, X-ray, magnetic resonance or any other measurements technique known to the skilled person for measuring bubble diameter, which can be connected to the processor for determining the average bubble diameter. As a coOmparison for air bubbles rising in water at 30°C, the maximum stable diameter before fragmentation is approximately 3-5 mm. Beyond this size, the bubble becomes unstable, deforms due to drag forces, and can break apart due to the Rayleigh-Taylor instability.In a set of embodiments, the reactor comprises a controller connected to the bubble sensor, the controller being arranged to operate one or more of the stirrer, agitator, and / or pump when the bubble sensor detects that a measure of bubble size is greater than a predetermined threshold. For example, the controller may increase the speed of the pump, agitator(s), and / or stirrer(s), in order to reduce the gas bubble size. In accordance with pre-set limits as outlined above. This may facilitate dynamic control to prevent the formation of a gas headspace.As the fermentation reaction progresses, the total liquid volume within the interior volume will reduce as the cells are harvested. To avoid the inadvertent formation of a headspace as a result, the liquid will typically need to be replenished. In set of embodiments, the reactor comprises a level sensor arranged to monitor the total volume of the liquid medium in the fermentation reactor. For example, the level sensor may be arranged near the top of the interior volume, to detect when the total working volume of the reactor falls beneath a given volume, such that more liquid can be introduced to prevent a headspace forming.As mentioned above, the Applicant has recognised that tight control of the volume of liquid in the fermentation reactor may be necessary to ensure that volatile accumulations of gas cannot take place. In a set of embodiments, the fermentation reactor comprises a monitoring volume arranged in fluid connection with the interior volume, wherein the monitoring volume has a substantially smaller internal diameter than the interior volume; and wherein the level sensor is arranged to monitor a level of liquid in the monitoring volume. It will be appreciated by the skilled person that as the monitoring volume has a much smaller diameter than the interior volume, a smallchange in the total volume of the liquid medium in the reactor will result in a large change in the level of liquid medium within the monitoring volume. Thus, the level sensor can more easily detect any small changes in the volume of the liquid medium, in order to provide tight volume control.In some such embodiments, a or the controller is connected to the level sensor, and is arranged to fill the fermentation reactor when the level sensor indicates that the volume of the liquid medium is less than the predetermined threshold. For example, the controller may be arranged to operate a liquid medium inlet valve, to add more liquid medium into the interior volume. This may prevent a headspace from forming within the interior volume by the reactor not being substantially fully filled. In some embodiments, the monitoring volume can be vented, such that gas that has accumulated in the monitoring volume can be released from the fermentation reactor.The reactor may comprise one or more sensors for monitoring the progress of the fermentation process, e.g. one or more of a pressure sensor, temperature sensor, a pH sensor, a bacteria cell density sensors, a foam formation sensor, a gas composition sensor, and / or a liquid medium composition sensor. The controller may respond based on the information from the one or more sensors, e.g. by increasing or decreasing the speed of the pump, or operating the sparger to introduce more gas. For example, the reactor may comprise a device / devices for heating and / or cooling, which may be operated by the controller when the temperature sensor indicates that the liquid medium is too cold or too hot respectively.The controller may be located externally to the reactor, e.g. it may be connected to the one or more sensors and reactor components via wired or wireless connections. The controller may include at least one processor and a memory, wherein the memory comprises instructions which, when executed by the at least one processing device, cause the controller to respond based on the information received form the one or more sensors.Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 is a cross-section of a typical conventional fermentation reactor;Figure 2 is a cross-section of a fermentation reactor in accordance with an embodiment of the invention including an exterior channel;Figure 3a is a cross-section of a fermentation reactor in accordance with a second embodiment of the invention, including a stirrer;Figure 3b is a cross-section of a fermentation reactor in accordance with another embodiment of the invention, including an ultrasonic stirrer;Figure 4 is a cross-section of a fermentation reactor in accordance with a third embodiment of the invention, including a controller;Figure 5 is a cross-section of a fermentation reactor in accordance with a fourth embodiment of the invention, including a sparger;Figure 6a is a cross-section of a fermentation reactor in accordance with a fifth embodiment of the invention, including an interior channel and a stirrer;Figure 6b is a top view of the stirrer shown in Figure 6a;Figure 7 is a cross-section of a tilted fermentation reactor in accordance with a sixth embodiment of the invention, including an exterior channel;Figure 8 is a cross-section of a closed loop fermentation reactor in accordance with a seventh embodiment of the invention;Figure 9 is a cross-section of an undulating closed loop fermentation reactor in accordance with an eighth embodiment of the invention;Figure 10 is a cross-section of a fermentation reactor in accordance with a ninth embodiment of the invention, including stirrers; andFigure 11 is a schematic flow diagram of an exemplary method for producing biomass using any of the reactors of Figures 2-10.Figure 1 shows a typical conventional fermentation reactor 100, including a reactor vessel 102, containing a liquid culture medium 106, gas bubbles 112 which are a mix of hydrogen, oxygen and carbon dioxide, and bacteria 114 (indicated schematically).The reactor vessel 102 is connected to a gas source 104, which feeds hydrogen, oxygen, carbon dioxide into the reactor via a gas inlet 108. These substrate gases areintroduced to the reactor vessel 102 from the gas source 104 to be used and consumed by the bacteria 114 in order to produce biomass, which is continuously extracted from the reactor vessel 102.Gases in the bubbles 112 which have not been consumed by the bacteria 114 rise to the top of the reactor vessel 102, where they form a gaseous headspace 116. As the headspace 116 includes a mixture of hydrogen and oxygen gas, it is important that the concentration of oxygen within the headspace is maintained at a concentration of less than 5% v / v, which is the accepted threshold below which a hydrogen / oxygen mix is not explosive (e.g. when hydrogen is added in excess). This is because a mixture of hydrogen and oxygen gas is considered non-explosive when the concentration of either hydrogen or oxygen gas is below 5% (v / v). Hydrogen is typically introduced in excess due to its poor solubility, which therefore limits the concentration of oxygen that can be safely introduced into the reactor vessel 102.The Applicant has recognised however that such a low oxygen concentration limits the reaction rate and thus the rate of production of biomass by the bacteria.Figure 2 shows a fermentation reactor 200 in accordance with a first embodiment of the present invention. The fermentation reactor includes a reactor vessel 202 which is fully filled with a liquid medium 206, which includes a culture medium and bacteria. In addition to the liquid medium, gas bubbles are fed into the reactor vessel 202 from a gas source 204 via a gas inlet 208.The gas source 204 includes individual sources of carbon dioxide, hydrogen and oxygen, which are each fed into the gas inlet 208. As carbon dioxide is readily dissolved into the liquid medium 206, it forms an equilibrium with dissolved carbonate within the liquid medium 206. However, the hydrogen and oxygen are less readily dissolved, such that the majority of the remaining gas bubbles within the reactor vessel 202 are made up of hydrogen and oxygen.Unlike the conventional reactor in Figure 1, there is no headspace in the reactor vessel 202. Instead, there is a recirculation channel formed by a narrow pipe 218 fluidically connecting the very top portion of the reactor vessel 202 to the bottom of the reactor vessel. A multi-phase pump 220 is arranged within the pipe 218, to pump the liquidmedium 206 and gas bubbles out from the top of the reactor vessel 202, and back into the bottom of the reactor vessel. This prevents gas from accumulating at the top of the reactor vessel 202 in a headspace.The mechanical agitation which arises from circulation of the liquid medium and gas bubbles - e.g. by being drawn through the narrow pipe 218 and the pump 220 breaks up the gas bubbles to be smaller and thus more easily dissolved into the liquid medium. For example, the gas bubbles can be maintained so that they have an individual diameter of less than approximately 10 mm and so a volume of less than approximately 600 mm3.As well as being small, the length of the path travelled by the gas bubbles prior to being dissolved in this embodiment is not limited by the height of the reactor vessel 202 and so they can dissolve completely. This, along with the lack of headspace means that the fermentation reactor 200 can be un-vented, such that any gas input to the reactor vessel 202 is eventually dissolved or consumed by the bacteria in the liquid medium 206 without being lost.The lack of headspace and small bubble size mean that the reactor is inherently safe from the risk of a pressure wave (e.g. during an explosion) without being limited to an oxygen concentration of below 5% v / v., This inherent safety arises from the fact that even if spontaneous ignition were to take place in one or more bubbles, there is a sufficiently small amount of energy released that it can be absorbed by the surrounding liquid medium without triggering an explosive reaction from the increase in pressure. Accordingly it is possible instead to use oxygen concentrations higher than this - e.g. between 6% and 15% - which give a dramatic increase in the rate of the fermentation reaction. For example it has been found that increasing the oxygen percentage just a few percent above 5% can lead to a doubling of the rate of microbial growth and thus the biomass yield rate. In order to take full advantage of this, the amount of hydrogen supplied is in excess.Figure 3a shows a second embodiment of a fermentation reactor 300, which also includes a reactor vessel 302 and a recirculation channel 318 as in the reactor shown in Figure 2. However, this embodiment further includes a stirrer 322. The stirrer 322 is made up of a motor 323 external to the reactor vessel 302, connected to a drive shaft134. The drive shaft 134 extends from the motor 323 into the reactor vessel to near the bottom thereof. Blades 326 are positioned along the length of the drive shaft 134 within the reactor vessel, such that when the motor 323 is operated, the drive shaft 134 and thus the blades 326 rotate within the reactor vessel 302 to stir the liquid medium 306. This, in addition to the circulation provided by the pipe 318 and multiphase pump 320, acts further to assist in breaking up the gas bubbles, to help the bubbles dissolve and maintain the beneficial small average diameter explained above.Figure 3b shows another embodiment of the fermentation reactor 1300, which also includes a reactor vessel 1302 and a recirculation channel 1318 as in the reactor shown in Figure 3a. However, this embodiment includes ultrasonic stirrers 1349 arranged within the reactor vessel 1302, and a channel ultrasonic stirrer 1343 arranged external to the recirculation channel 1318. As shown in the enlarged view of the channel portion 1341, the channel ultrasonic stirrer 1343 acts to vibrate the liquid in the channel portion 1341, to agitate the larger bubbles 1345 such that they are broken up into smaller bubbles 1347 as they pass through the channel portion 1341 past the channel ultrasonic stirrer 1343. The ultrasonic stirrers 1349, 1343 can break up bubbles rising within the reactor vessel 1302, which may be particularly useful if no mechanical stirrer is present.Figure 4 shows a third embodiment of a fermentation reactor 400, which includes a reactor vessel 402, recirculation channel 418 and stirrer 422. However, in this embodiment the reactor 400 further includes a range of sensors connected to a controller 428. The controller is connected to the multi-phase pump 420 and the stirrer 422, and adjusts their operational parameters - e.g. their speed, based on the output from the sensors. The controller 428 may further adjust the gas input volume at the gas input port 408 from the gas source(s) 404.The sensors include an agitation rate detector 424, a gas detector 432, a bubble size detector 430, a gas composition monitor 436, a culture medium composition monitor 428, a level sensor 435 and a microorganism density monitor 440.The bubble size detector 430 is disposed at the top of the reactor vessel 402 and monitors the sizes of the gas bubbles in the liquid medium 406. The bubble size detector 430, which may be a camera or a differential pressure (DP) cell, outputs areading to the controller 428. The controller 428 uses this to determine an average size for the bubbles and / or whether any or a proportion of the bubbles have exceeded a predetermined maximum volume. For example, where the bubble size detector 430 is a camera, it takes an image of bubbles in the reactor 402, and a processor within the controller 428 determines from the image the size of the bubbles. If a certain proportion of the bubbles are greater than a given diameter, e.g. more than 10% are greater than 10 mm in diameter, the controller 428 increases the agitation rate of the stirrer 422, the speed of the pump 420, and / or reduce the rate gas being fed into the reactor vessel from the gas source, in order to reduce the bubble size to the desired operating window.The gas detector 432 is arranged at the top of the reactor vessel 402 to detect any accumulation of gas at the top of the reactor, i.e. to monitor if a headspace begins to accumulate at the top of the reactor vessel 402. Similarly to the bubble size detector, if the gas detector 432 outputs a volume reading to the controller 428 that indicates that the volume of gas at the top of the reactor vessel 402 has exceeded a volume greater than a predetermined value, e.g. greater than 600 mm3, the controller 428 increases the agitation rate of the stirrer 422, the speed of the pump 420, and / or reduce the gas being fed into the reactor vessel from the gas source 404, in order to reduce or eliminate the accumulation.The agitation rate detector 424 is connected to the stirrer 422, external to the reactor vessel 402. For example, the agitation rate detector 424 could be connected to the drive shaft 424, to determine the rate of rotation of the stirrer and therefore the degree of agitation of the liquid medium. Alternatively, the agitation rate detector 424 could be connected to the power supply to the stirrer 422, e.g. to measure the voltage and / or current supplied to the stirrer 422. Either way, it allows the controller 428 to regulate the speed of the stirrer to a desired value.The flow rate detector 442 is disposed in the recirculation channel 418 of and measures the flow of liquid medium and gas bubbles passing through the channel 418. The measured flow rate is sent to the controller 428, so that it can operate the pump 420 to change its speed in response, e.g. the controller may increase the power supplied to the pump 420 if the flow rate is detected to be lower than a target value.The gas composition monitor 436 is located in the reactor vessel 402 to detect the composition of gases in the reactor vessel, e.g. by sampling the gas composition (e.g. the amount of oxygen and hydrogen) of gas bubbles rising to the top of the reactor vessel 402. The bubbles (e.g. the recycled bubbles from the loop formed by the reactor vessel 402 and channel 418) typically contain a mixture of gases (e.g. hydrogen and oxygen). For example, the gas detector may be positioned within the reactor vessel 402, or may be positioned in a chamber fluidically connected to the reactor vessel 402, arranged to collect a sample of the gas within the reactor vessel 402. The controller uses the information on the gas composition from the gas composition monitor 436 and determines whether the mix of gases fed into the reactor vessel from the gas source should be changed, e.g. if the concentration of oxygen is too high or whether the speed of the pump and / or stirrer should be changed e.g. to increase the rate of dissolution or absorption.The culture medium composition monitor 438 detects the composition of the culture medium, e.g. by detecting the pH of the culture medium, which is fed back to the controller 428. For example, the pH may be used to regulate the feeding of carbon dioxide into the reactor vessel. The controller operates a liquid valve (not shown) in response to admit more liquid medium, or components of the liquid medium, to the reactor vessel 402 when required to maintain the composition of the culture medium in the reactor vessel within a desired window.Similarly, the microorganism density monitor 440 detects the concentration of bacteria in the reactor vessel 402, which is fed back to the controller 428. If the microorganism density is not within the pre-set operating window, the controller adjusts the rate of harvesting bacteria from the reactor vessel 402 via operation of an associated valve (not shown), or stimulating cell growth by adjusting the media composition, temperature or feeding of substrate gases into the reactor vessel 402 by associated valves (not shown).The level sensor 435 detects the volume of the liquid medium 406 within the reactor vessel 402, which is fed back to the controller 428. The fermentation reactor 400 includes a monitoring chamber 433 fluidically connected to the top of the reactor vessel 402. The monitoring channel 433 has a much narrower diameter compared to the reactor vessel 402, such that a small change in the volume of the liquid medium406 within the fermentation reactor 400 will result in a relatively large change in the level of the liquid medium 406 within the monitoring chamber 433. Thus, the level sensor 435 can detect as soon as the liquid medium 406 drops below a predetermined volume, as a result of the harvesting of cells during the fermentation process, or if the gas input to the reactor vessel 402 changes. The controller operates a liquid valve (not shown) in response to the level sensor to admit more liquid medium 406 to the reactor vessel 402 when required to maintain the volume of the liquid medium 406 in the reactor vessel within a tight window. This prevents the volume of the liquid medium 406 falling to a point where a gas filled spaces is created in the reactor where ignition of the gas in such a gas filled space could cause a dangerous pressure wave. The monitoring channel 433 may be vented, such that the gas accumulated in the monitoring channel 433 can be released from the reactor vessel 402.Pressure sensors (not shown) may also be provided in the reactor vessel 402 (e.g. at the top and / or middle and / or bottom of the reactor vessel), to monitor the differential pressure inside the reactor vessel 402, which correlates with the hydrostatic pressure, and feed back to the controller 428 such that the controller 428 may act to change the volume of the combined gas bubbles and liquid medium 406 in the reactor vessel 402.As will be appreciated it is not necessary to include all of the detectors and monitors illustrated in this exemplary embodiment; any one or subset of them could be used depending on the particular application and required operating conditions.Figure 5 shows a fermentation reactor 500 of a fourth embodiment of the invention. In common with earlier embodiments, this includes a reactor vessel 502, recirculation channel 518 and stirrer 522. This embodiment further includes a sparger 544 connected to the end of the channel 518, such that the liquid / gas mixture that is recirculated from the top of the reactor vessel to the bottom of the reactor vessel is sparged back into the reactor vessel 502. This breaks up the gas bubbles and further promotes their dissolving into the liquid in the vessel 502, to increase the rate of mass transfer of the gas substrates and thus the rate of microbial growth.A sparger (not shown) can also be used to distribute the gas from the gas source 504 into the reactor vessel 502 via the gas inlet 508. This can help to provide smaller gas bubbles when they are initially introduced.Figures 6a and 6b show a fermentation reactor 600 of a fifth embodiment of the invention. This embodiment differs from those above in that it has an interior tubular channel 646 for recirculating the liquid / gas mixture from the top to the bottom of the reactor vessel 602, as indicated by the arrows 652. The liquid / gas mixture enters the channel 646 at the top of the reactor vessel 602, and exits at the bottom via a static mixer or sparger 650. As the interior channel 646 is located along the centre axis of the reactor vessel 602, the liquid / gas mixture is evenly circulated from top to bottom and from the outer portion to the inner portion of the reactor vessel 602. Gas from the gas source(s) 604 enters the reactor vessel 602 at the gas inlet 608, which is typically positioned in the lower corner of the reactor vessel 602.As is further evident from the isolated top view of Figure 6b, the interior channel 646 forms part of a rotating structure 622 connected to a motor 623 and which also comprises interior blades 648 which are connected to a drive shaft 624 extending down the central axis of the interior channel 646. Exterior blades 626 located outside of the interior channel 646 in the main body of the vessel 602 are connected to horizontal shafts which connect to the drive shaft 624 and to the tubular channel 646 so that the whole structure 622 is driven to rotate by the motor 623. The interior blades 648 and exterior blades 626 therefore rotate in the same direction 654 when the drive shaft 624 spins, to agitate the liquid medium 606. Since, the interior blades 648 and exterior blades 626 are oriented in opposite directions, whilst they spin in the same direction, the exterior blades 626 push the liquid / gas mixture outside of the interior channel 646 upwards, and the interior blades 648 draw the liquid / gas mixture inside the interior channel 646 downwards. This encourages the liquid medium 606 and gas bubbles to circulate through the interior channel 646.Figure 7 shows a fermentation reactor 700 of a sixth embodiment of the invention. This comprises a reactor vessel 702 tilted relative to the horizontal by an angle a. The angle a may typically be between 5° and 15°, e.g. 10°, to encourage the circulating gas bubbles within the reactor vessel 702 to reach the same region of the reactor vessel, i.e. to exit the reactor vessel 702 via the exterior channel 760. Gas from the gas source(s) 704 enters the reactor vessel 702 at the gas inlet 708, which is typically positioned in the lower corner of the reactor vessel 702, e.g. farthest from the exterior channel 760.The reactor 700 also includes a reactor stirrer 722 driven by a motor 723 extending across the width of the reactor vessel 702, and an exterior channel 760 connecting from the top to the bottom of the reactor vessel 702. Axial-flow pumps 720 (drive means not shown) are in this example positioned within the exterior channel 760 instead of a pump to help circulate the liquid medium 706 and gas bubbles from the top of the reactor vessel - i.e. at the top of the raised end - to the bottom surface, and to help break up gas bubbles.The liquid / gas mixture is introduced back into the reactor vessel 702 from the exterior channel 760 via a sparger 750. The sparger 750 is arranged at the bottom of the reactor in the corner furthest from the exterior channel 760 entrance, to increase the path length of the bubbles. The reactor stirrer 722 drives the rising bubbles downwards after they are emitted by the sparger, such that the bubbles migrate in a spiral fashion towards the upper right hand corner of the reactor and thus towards the external channel 760.Figure 8 shows a fermentation reactor 900 of an eighth embodiment of the present invention, which includes an interior volume formed as a closed loop conduit 902. As in earlier embodiments, the closed loop conduit 902 is fully filled with a liquid medium 906, which includes bacteria for consuming gas input to the reactor from the gas source 904 via the gas inlet 908. One or more multi-phase pump(s) 920 are positioned in the conduit to circulate the liquid / gas mixture around the conduit 902 in one flow direction as indicated by the arrows. Circulation of the whole of the liquid / gas mixture in one bulk flow prevents the formation of large gas bubbles and improve gas dissolution, as well avoiding a gas headspace forming as it prevents stagnation. Static flow disruptors 970 arranged along the conduit 902 break up bubbles further. This arrangement allows a small average diameter / volume of bubble to be maintained for the reasons set out earlier. The liquid medium 906 may include hydrogen oxidising bacteria which act to fully consume all of the gas input to the conduit 902 via the gas inlet 908, such that there is no need for a gas headspace and the conduit can be fully filled with the liquid medium 906.Figure 9 shows a fermentation reactor 1000 of a ninth embodiment, including an interior volume formed as an undulating closed loop conduit 1002. The undulatingconduit 1002 includes a liquid medium 1006, which includes bacteria for consuming gas input to the reactor from the gas source 1004 via the gas inlet 1008. One or more multi-phase pump(s) 1020 are connected to the conduit 1002, to circulate the liquid medium and gas bubbles.Whilst the operation principles of this are similar to the previous embodiment, undulating conduit 1002 is made up of top portions 1072 and bottom portions 1074, which increases the length of the path taken by the gas bubbles through the reactor 1000 for a given overall size of the reactor. Whilst small amounts of gas might accumulate in the top portions 1072 (e.g. in the event of a pump failure), since there is no single, large volume where such accumulation can occur the risk from an explosion is reduced, as the gas pocket volume is limited, and any other gas pockets in one of the other top portions 1072 can suppress any pressure wave generated from an explosion of one of the small gas pockets. Thus, oxygen concentrations of 6% or higher can safely be used.Figure 10 shows a fermentation reactor 1100 of a tenth embodiment similar to the reactor shown in Figure 9, including external channels 1178, connecting respective bottom portions 1174 to the corresponding top portions 1172. A multi-phase pump 1176 is arranged within each external channel 1178. These external channels 1178 further reduce or eliminate any pockets of gas that may accumulate in the top portions 1172.Figure 11 is a schematic system diagram of an overall system for producing biomass using a fermentation reactor 1200, which could be any of the reactors described with reference to Figures 2-10. The fermentation reactor 1200 is connected to a microorganism source 1280, a culture media source 1282 and a gas source 1284. The microorganism source 1280 provides bacteria, which are suspended in the liquid medium provided by the culture media source 1282 in the reactor vessel.The fermentation reactor 1200 produces biomass 1286 by the bacteria consuming the gas from the gas source 1284, which contains at least carbon dioxide, hydrogen and oxygen. The biomass 1286 is output from the reactor 1200 to a dryer 1288. The dryer 1288 receives, and dewaters or dries the biomass 1286 produced in the fermentationreactor 1200, and then outputs the dewatered or dried biomass to a formulation plant 1290.The formulation plant 1290 receives and processes the dewatered or dried biomass, which can be used as an ingredient produce feed 1292 (e.g. animal feed or fish feed). Alternatively, the biomass may be used in products for direct human consumption, or the biomass may be further processed to extract subcomponents from the cell mass.It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

CLAIMS1. A fermentation reactor comprising:an interior volume for receiving a liquid medium, the interior volume being arranged so as to form an unvented closed loop;a gas inlet for the introduction of one or more gases into the liquid medium to form a plurality of gas bubbles, the fermentation reactor being arranged to ensure any accumulations of gas in the interior volume have an individual volume of less than or equal to 600 mm3; anda multi-phase pump arranged to circulate the liquid medium and the gas bubbles around the closed loop.

2. The fermentation reactor claimed in claim 1, wherein the interior volume comprises a reactor vessel and a channel;wherein the channel is arranged to connect an upper part of the reactor vessel to a lower part of the reactor vessel; andwherein the multi-phase pump is arranged in the channel to transport a portion of the liquid medium and the gas bubbles from the upper part of the reactor vessel to the lower part of the reactor vessel.

3. The fermentation reactor as claimed in claim 2, wherein the channel has a substantially smaller cross-section than the reactor vessel.

4. The fermentation reactor as claimed in claim 2 or 3, wherein the multi-phase pump is arranged to cause the liquid medium and the gas bubbles to flow in substantially the opposite direction in the reactor vessel relative to the channel.

5. The fermentation reactor as claimed in any preceding claims, wherein the channel is arranged externally to the reactor vessel.

6. The fermentation reactor as claimed in any of claims 2-5, wherein the reactor vessel comprises at least one reactor vessel stirrer.

7. The fermentation reactor as claimed in any of claims 2-6, wherein the channel comprises at least one channel stirrer.

8. The fermentation reactor as claimed in any of claims 2-7, wherein the channel is arranged internally of the reactor vessel.

9. The fermentation reactor as claimed in claim 8, wherein the channel is provided by a rotating structure further comprising a reactor vessel stirrer and a channel stirrer arranged to co-rotate around a common axis parallel to a direction of flow through the channel.

10. The fermentation reactor as claimed in any of claims 6, 7 or 8, wherein the reactor stirrer and the channel stirrer comprise paddles mounted in opposite directions.

11. The fermentation reactor as claimed in any preceding claim, wherein the reactor is inclined at an acute angle relative to the horizontal.

12. The fermentation reactor claimed in claim 1, wherein the interior volume comprises a conduit formed as a closed loop for receiving a liquid medium, wherein the multi-phase pump is arranged in the conduit to circulate the liquid medium and the gas bubbles around the conduit.

13. The fermentation reactor as claimed in claim 1412 wherein the conduit has a substantially constant cross-section.

14. The fermentation reactor as claimed in claim 12 or 13, wherein the conduit is substantially symmetrical.

15. The fermentation reactor as claimed in any of claims 12-14, wherein the conduit comprises an undulating portion.

16. A fermentation reactor comprising an interior volume, said interior volume being substantially fully filled with a liquid medium comprising hydrogen oxidising bacteria and a plurality of gas bubbles for producing biomass;wherein the plurality of gas bubbles comprise hydrogen and oxygen; and wherein the concentration of oxygen in said bubbles is above 6% v / v.

17. The fermentation reactor as claimed in claim 16, wherein the gas bubbles have an individual volume of less than or equal to 600 mm3.

18. The fermentation reactor as claimed in claim 16 or 17, the interior volume being arranged so as to form an unvented closed loop; and further comprisinga gas inlet for the introduction of one or more gases into the liquid medium to form a plurality of gas bubbles, anda multi-phase pump arranged to circulate the liquid medium and the gas bubbles around the closed loop.

19. The fermentation reactor claimed in claim 16, wherein the interior volume comprises a reactor vessel and a channel;wherein the channel is arranged to connect an upper part of the reactor vessel to a lower part of the reactor vessel; andwherein the multi-phase pump is arranged in the channel to transport a portion of the liquid medium and the gas bubbles from the upper part of the reactor vessel to the lower part of the reactor vessel.

20. The fermentation reactor claimed in claim 16, wherein the interior volume comprises a conduit formed as a closed loop for receiving the liquid medium, wherein the multi-phase pump is arranged in the conduit to circulate the liquid medium and the gas bubbles around the conduit.

21. The fermentation reactor as claimed in any preceding claim, wherein the reactor comprises a controller arranged to control operation of the multi-phase pump.

22. The fermentation reactor as claimed in any preceding claim comprising at least one sparger arranged to introduce the gas bubbles into the liquid medium.

23. The fermentation reactor as claimed in any preceding claim comprising at least one of an agitator; a static mixer; or a stirrer.

24. The fermentation reactor as claimed in claim 23, wherein a or the controller is arranged to control operation of the stirrer and / or agitator.

25. The fermentation reactor as claimed in any preceding claim comprising one or more of:a bubble size sensor;a level sensor arranged to monitor a level of the liquid medium in the fermentation reactor;an agitation rate detector;a gas detector;a gas composition monitor;a culture medium composition monitor; ora microorganism density monitor.

26. The fermentation reactor as claimed in claim 25, wherein the fermentation reactor comprises a monitoring volume arranged in fluid connection with the interior volume, wherein the monitoring volume has a substantially smaller internal diameter than the interior volume; andwherein the level sensor is arranged to monitor a level of liquid in the monitoring volume.

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