Patent Publication Number: US-2023159874-A1

Title: Systems and methods for recycling gas in reactors

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/281,550, filed on Nov. 19, 2021, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Bioreactors like fermentors and fermentation processes are widely used for cultivation of microorganisms and production of useful biomasses from the microorganisms. In many conventional fermentation processes, gaseous substrates, such as air, are introduced into a fermentor and mixed with microorganisms and other ingredients in a liquid medium in the fermentor. The fermentor is operated to maintain a reaction condition for the microorganisms to convert the nutrients and gaseous substrates into biomass. 
     There are a number of drawbacks associated with conventional stirred tank fermentors when gases such as hydrogen, oxygen, and carbon dioxide are used as the sole energy and/or carbon source. Most unconsumed gas within the fermentors is eventually released out of the fermentors as a waste without recovery or reuse. For example, for fermentation processes that involve use of hydrogen as an input gas, hydrogen is often from an expensive feedstock, and therefore direct relief of the unconsumed hydrogen out of the fermentors significantly reduces the yield and increases the cost. 
     In addition, due to the low solubility of many gas substrates in liquid, the fermentation process involves a heterogenous (multi-phase) mixture of gas and liquid in the fermentor. The gas must be dissolved in the water to pass across the cell membrane of the microorganism. Pressure and high agitation and shear forces are traditionally used to promote mass transfer efficiency of gas with liquid. However, use of high agitation may break the cell wall of the microorganism and undermine the fermentation productivity. Further, many fermentation processes generate substantial by-products, in particular carbon dioxide (CO 2 ) within the fermentor. The CO 2  by-product without proper handling may become a burden to the environment. 
     SUMMARY 
     In general terms, this disclosure is directed to a scalable gaseous fermentation system with high gas mass transfer including recycling of gases, reactors implementing the gas-recycling system, and processes for recycling gas in a reactor or a system, and processes for biomass production. 
     In one aspect, the present disclosure provides to a gas-recycling system for recycling gas in a reactor. In one example, a gas-recycling system comprises: a gas-recycling system for recycling gas in a reactor, the gas-recycling system comprising: a housing, a powered propeller, and a gas conduit. The housing encloses an internal space comprising a gas space and a liquid space, wherein the gas space is configured to collect gas separated from the liquid space within a reactor. The powered propeller comprises: a shaft having an upper end and a lower end, wherein the lower end extends into the liquid space; and a plurality of radial blades connected to the lower end of the shaft. The gas conduit extends from a first end to a second end, wherein the first end is in gas communication with the gas space, and wherein the second end is proximate the radial blades and extending into the liquid space. Upon rotation of the powered propeller, the powered propeller is configured to: generate a suction to cause the collected gas to flow from the gas space to the liquid space through the conduit; cause fluid of the reactor to flow in a direction from the upper end of the shaft to the lower end of the shaft; and mix the liquid and the collected gas proximate the radial blades. 
     In some embodiments, the reactor is a tank reactor. 
     In some embodiments, a lower portion of the housing is in connection with an upstream end of a loop section external to the housing, an upper portion of the housing is in connection with a downstream end of the loop section, and the reactor causes fluid therein to flow from the lower portion of the liquid space to the upper portion of the liquid space through the loop section. In some embodiments, the loop section further comprises a plurality of static mixers configured to promote the mixing of the gas into the liquid medium. 
     In some embodiments, the gas conduit is within the housing, and a part of the shaft extends through an interior space of the conduit. 
     In some embodiments, the gas conduit is external to the housing, and the gas-recycling system further comprises an gas compressor connected to the gas conduit, the gas compressor configured to promote gas flow from the gas space to the liquid space. 
     In some embodiments, the gas-recycling system further comprises a liquid pump placed in the liquid space, wherein the liquid pump is configured to increase or maintain pressure and flow direction of the liquid in the reactor. In some embodiments, the liquid pump comprises an a plurality of propeller blades connected to the powered propeller above the lower end of the shaft, wherein the propeller blades are configured to co-rotate conjunctively with the shaft during operation. 
     In some embodiments, the liquid space contains a reaction mixture comprising: a substrate comprising CO 2 , H 2 , O 2 , and optionally N 2 ; and a liquid medium comprising a microorganism capable of converting the substrate into biomass. 
     In some embodiments, the microorganism is a knallgas microorganism selected from the group consisting of:  Rhodopseudomonas  sp.,  Rhodospirillum  sp.,  Rhodococcus ; sp.,  Rhizobium  sp.:  Thiocapsa  sp.,  Pseudomonas  sp.,  Nocardia  sp.,  Hydrogenomonas  sp.,  Hydrogenobacter  sp.,  Hydrogenovibrio  sp.:  Helicobacter  sp.,  Xanthobacter  sp.,  Hydrogenophaga  sp.,  Bradyrhizobium  sp.,  Ralstonia  sp.,  Gordonia  sp.:  Mycobacteria  sp.,  Alcaligenes  sp.:  Cupriavidus  sp.:  Variovorax  sp.,  Acidovorax  sp.,  Anabaena  sp.:  Scenedesmus  sp.:  Chlamydomonas  sp.,  Ankistrodesmus  sp.,  Rhaphidium  sp.,  and combinations thereof.    
     In some embodiments, wherein the reactor further comprises one or more inlets configured to introduce CO 2 , H 2 , O 2 , N 2  or any combinations thereof into the reactor. 
     In some embodiments, the gas-recycling system further comprises a controller configured to adjust the ratio of the CO 2 /H 2 /O 2 /N 2  being introduced into the reactor. 
     In some embodiments, the reactor further comprises an exit port configured to harvest the produced biomass from the reactor. 
     In some embodiments, the external CO 2  gas being introduced into the reactor is from a source of biogenic CO 2 . In some embodiments, the sources of biogenic CO 2  comprise an aquatic farming plant, wherein the biogenic CO 2  is produced by living aquatic organisms therein. In some embodiments, the sources of biogenic CO 2  comprises a fermentation plant, wherein the biogenic CO 2  is produced by fermentation microorganisms therein. In some embodiments, the produced biomass from the reactor is supplied to the aquatic farming plant or the fermentation plant as feed or energy source to the living animals or microorganisms. 
     In some embodiments, the biogenic CO 2  recirculates in the reactor without substantial emission to the environment during operation. 
     In another aspect, the present disclosure provides a tank reactor comprising the gas-recycling system described herein. In one example, a tank reactor comprises: a housing, a powered propeller, and a gas conduit. The housing encloses an internal space comprising a gas space and a liquid space, wherein the gas space is configured to collect gas separated from the liquid space within a reactor. The powered propeller comprises: a shaft having an upper end and a lower end, wherein the lower end extends into the liquid space; and a plurality of radial blades connected to the lower end of the shaft. The gas conduit extends from a first end to a second end, wherein the first end is in gas communication with the gas space, and wherein the second end is proximate the radial blades and extending into the liquid space. Upon rotation of the powered propeller, the powered propeller is configured to: generate a suction to cause the collected gas to flow from the gas space to the liquid space through the conduit; cause fluid of the reactor to flow in a direction from the upper end of the shaft to the lower end of the shaft; and mix the liquid and the collected gas proximate the radial blades. 
     In yet another aspect, the present disclosure provides a loop reactor comprising the gas-recycling system described herein. In one example, a loop reactor comprises: a housing, a powered propeller, a gas conduit, and a loop section. The housing encloses an internal space comprising a gas space and a liquid space, wherein the gas space is configured to collect gas separated from the liquid space within a reactor. The powered propeller comprises: a shaft having an upper end and a lower end, wherein the lower end extends into the liquid space; and a plurality of radial blades connected to the lower end of the shaft. The gas conduit extends from a first end to a second end, wherein the first end is in gas communication with the gas space, and wherein the second end is proximate the radial blades and extending into the liquid space. The loop section is external to the housing, wherein the loop section comprises: an upstream end connected to a lower portion of the housing; and a downstream end connected to an upper portion of the housing. Upon rotation of the powered propeller, the powered propeller is configured to: generate a suction to cause the collected gas to flow from the gas space to the liquid space through the conduit; cause fluid of the reactor to flow in a direction from the upper end of the shaft to the lower end of the shaft; mix the liquid and the collected gas proximate the radial blades. The loop reactor causes fluid therein to flow from the lower portion of the liquid space to the upper portion of the liquid space through the loop section. 
     In a further aspect, the present disclosure provides a process for recycling gas in a reactor comprising the gas-recycling system described herein. In one example, a process comprises: collecting gas in the gas space within the housing of the reactor; rotating the powered propeller to: generate a suction to cause the collected gas to flow from the gas space to the liquid space through the conduit; cause fluid of the reactor to flow in a direction from the upper end of the shaft to the lower end of the shaft; and mix the liquid and the collected gas proximate the radial blades. 
     In another aspect, the present disclosure provides a process for producing biomass through use of the reactor described herein. In one example, a process comprises: (1) culturing a microorganism in a reaction mixture in a reactor, wherein the reaction mixture comprises a liquid medium, wherein the microorganism is capable of converting CO 2  into biomass; (2) introducing a substrate from an external source into the liquid medium and mixing the substrate with the liquid medium, wherein the substrate comprises CO 2 , H 2 , O 2 , and optionally N 2 ; (3) recycling gas separated from a liquid space back into the liquid space within the reactor, wherein the reactor comprises: a housing enclosing an internal space comprising a gas space and a liquid space; a powered propeller comprising: a shaft having an upper end and a lower end, wherein the lower end extends into the liquid space; and a plurality of radial blades connected to the lower end of the shaft; and a gas conduit extending from a first end to a second end, wherein the first end is in gas communication with the gas space, and wherein the second end is proximate the radial blades and extending into the liquid space; (4) collecting gas separated from the liquid space in the gas space within the housing; (5) rotating the powered propeller to: generate a Venturi suction to cause the collected gas to flow from the gas space to the liquid space through the conduit; cause fluid of the reactor to flow in a direction from the upper end of the shaft to the lower end of the shaft; and mix the liquid and the collected gas proximate the powered propeller; and (6) harvesting the biomass produced from the reactor. 
     In some embodiments, the CO 2  introduced into the reactor is recycled therein, without substantial emission to atmosphere during operation. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments of the present disclosure will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the present disclosure shown where illustration is not necessary to allow those of ordinary skill in the art to understand the teachings of the present disclosure. In the figures: 
         FIG.  1 A  illustrates a schematic view of one example gas-recycling system  100 . 
         FIG.  1 B  illustrates a schematic view of another example gas-recycling system  100 ′. 
         FIG.  1 C  illustrates an isometric view of at part of the gas-recycling system  100 ′ of  FIG.  1 B . 
         FIG.  2    illustrates a schematic view of a first example reactor comprising the gas-recycling system of  FIG.  1   . 
         FIG.  3    illustrates a schematic view of a second example reactor comprising the gas-recycling system of  FIG.  1   . 
         FIG.  4    illustrates a schematic view of a third example reactor comprising the gas-recycling system of  FIG.  1   . 
         FIG.  5    illustrates a schematic view of one example system comprising a reactor of any one of  FIGS.  2 - 4   . 
         FIG.  6    illustrates a flow diagram of one example method for recycling gas in a reactor. 
         FIG.  7    illustrates a flow diagram of one example method for producing biomass. 
         FIG.  8    illustrates a flow diagram of one example method  800  for recycling CO 2  and producing biomass from CO 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies. Reference to various embodiments does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the appended claims. 
     The present disclosure is generally related to reactors or reaction systems, in particular, fermentors and fermentation systems. Fermentors are generally defined as any vessel in which a fermentation process is carried out. Given the vast number of fermentation processes and the wide variety of fermentable substrates, fermentors can range from simple continuous stirred tank reactors to highly complex, specialized systems having gas distribution and internal structures tailored to a particular substrate and/or a particular biological species. Fermentors useful in converting carbon-containing gases to larger biomolecules generally disperse a gas substrate within a liquid medium containing one or more nutrients to provide a multi-phase reaction mixture. This multi-phase reaction mixture is fed to one or more microbiological organisms that convert the gas substrate to larger biomolecules through metabolism. The gas substrates, nutrients, and microbiological organisms comprising the colony (i.e., the biomass within the fermentor) can be variously adjusted or tailored to provide a desired product, which may be present as a liquid, gas, or intracellular material. Fermentors according to the present disclosure are particularly useful in converting a gas substrate containing CO 2  to protein-rich biomass product, e.g., biomass with relatively high proportions of proteins. 
     As used herein, the term “microorganism” refers to any microorganism or microbial culture having the ability to use one or more gaseous substrates as a source of carbon or energy or as its sole source of energy and carbon, and may or may not use other carbon substrates (such as sugars, proteins, lipids and complex carbohydrates) for energy and carbon. Preferably, the microorganisms used herein is capable of converting CO 2  into biomass and protein-rich biomass. Example of microorganism include at least one the following genera:  Acidithiobacillus  sp.;  Acidovorax  sp.;  Alcaligenes  sp.;  Anabaena  sp.;  Ankistrodesmus  sp.;  Aquificae  sp.;  Bradyrhizobium  sp.;  Chlamydomonas  sp.;  Cupriavidus  sp.;  Derxia  sp.;  Flavobacteriae  sp.;  Gordonia  sp.;  Helicobacter  sp.;  Hydrogenobacter  sp.;  Hydrogenomonas  sp.;  Hydrogenophaga  sp.;  Hydrogenothermaceae  sp.;  Hydrogenovibrio  sp.;  Mycobacteria  sp.;  Nocardia  sp.;  Pseudomonas  sp.;  Ralstonia  sp.;  Renobacter  sp.;  Rhaphidium  sp.;  Rhizobium  sp.;  Rhodococcus  sp.;  Thiocapsa  sp.;  Variovorax  sp.;  Xanthobacter  sp.; and combinations thereof. 
     The microorganisms or microbial cultures used herein may also include a chemo-autotrophic microorganism selected from the following genera:  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.;  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.;  Metallosphaera  sp.;  Methanobacterium  sp.;  Methanobrevibacter  sp.;  Methanococcus  sp.;  Methanosarcina  sp.;  Micrococcus  sp.;  Nitrobacter  sp.;  Nitrosococcus  sp.;  Nitrosolobus  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 combinations thereof.    
     If desired, the systems and processes described herein may be performed using microorganisms genetically modified so as to generate a desired chemical compound which can then be extracted from the intercellular fluid or the biomass harvested from the reactor. The scientific and patent literature contains numerous examples of such genetically modified microorganisms. 
     In particular, the present disclosure provides systems and methods for recycling gas in a reactor (or fermentor of fermentation system or the like). The gas can be a gas substrate introduced into the reactor, or a gas intermediate or product generated in the reaction or fermentation process within the reactor, or a gas separated from a liquid medium within the reactor, or a mixture thereof. The present gas-recycling systems and methods may be implemented in a reactor or fermentor. For example, the present gas-recycling system or components thereof may be installed on or integral with a reactor. Alternatively, the present-recycling system may itself provide a substantial part or framework for a reactor. 
     Some embodiments of the gas-recycling systems and methods described herein provide one or more possible advantages described below. Gas substrate that is introduced to the reactor that implements the present gas-recycling system is allowed to recirculate in the reactor without substantial release, thereby significantly improving efficiency of gas conversion and overall yield of biomass production. The gas-recycling system could improve the mixing efficiency of gas with the liquid medium; prolong the contact time of gas with the fermentation broth holding the microorganisms; increase the contact area of gas with the fermentation broth holding the microorganisms in the heterogeneous reaction mixture; improve mass transfer efficiency within the reactor; and/or maximize a pump efficiency of the reactor. Moreover, the gas-recycling system could substantially reduce waste and gas emission. In particular, the CO 2 , O 2  and hydrogen introduced to the reactor is recycled and reused without substantial emission, which could maximize the conversion yield of CO 2  and hydrogen thereby saving energy and benefit the environment. Further, reactors that employ the present gas-recycling system may advantageously allow users to avoid high agitation or shear forces applied to the reaction mixture in the reactors and to save energy and cost. Additionally, implementation of the present gas-recycling system may allow to keep the microorganisms intact or protect the microorganisms from being destroyed by high shear force, ease harvest of biomass during downstream processing, keep the biomass product intact, and/or enable production of single-cell protein. 
     Now referring to  FIGS.  1 A- 1 C , examples of the present gas-recycling system and various aspects thereof will be illustrated and described.  FIG.  1 A  is a schematic diagram of a general gas-recycling system  100 . In the illustrated example, the gas-recycling system  100  includes a housing  101 , a powered propeller  110 , and a gas conduit  120 . The housing  101  may be a housing of a reactor or fermentor and in a substantially vertical position relative to the ground level. The housing  101  may extend from a top end  109  downwardly to a bottom end  108  along a central axis  114 , and may further include a top wall  116 , a bottom wall  118 , and a continuous side wall  117  that circumstantially connects to the top wall  116  and the bottom wall  118 . The housing may be substantially closed and enclose an internal space  102  comprising a gas space  103  and a liquid space  104 . The gas space  103  takes an upper portion of the internal space  102  and is configured to collect and contain a gas  106  produced in the internal space  102 . The gas  106  may comprise a gas substrate that has been introduced into the housing  101  from an external source, or a gas by-product generated from a reaction or fermentation process in the housing  101 , or a gas that is separated from a liquid medium  107  within the housing  101 , or a mixture thereof. The liquid space  104  takes a lower portion of the internal space  102  and is configured to contain a liquid medium  107  within the housing  101 . The gas space  103  and the liquid space  104  are separated by a gas-liquid interface  105 . The volume of the gas space  103  and the liquid space  104  may be adjusted by the amount of liquid medium  107  and operating conditions of the gas-recycling system  100 . 
     The powered propeller  110  includes a rotatable shaft  111 , a motor  115 , and a suction-generating device  130 . The shaft  111  extends from an upper end  112  to a lower end  113  along the central axis  114  of the housing  101 . The upper end  112  may extend out of the housing  101 . The lower end  113  extends into the liquid space  104 . A distance between the lower end  113  of the shaft  111  and the bottom wall  118  of the housing  101  may be adjustable. The motor  115  is operably connected to the upper end  112  of the shaft  111  and provides power to drive rotation of the shaft  111  about the central axis  114 . 
     The gas conduit  120  extends from a first open end  121  to a second open end  122  along the central axis  114  within the housing  101 . The first open end  121  is within the gas space  103  and is in gas communication therewith, and the second open end  122  is within the liquid space  104  and is in gas communication therewith. In some embodiments, the gas conduit  120  comprises a plurality of openings  125  at the second open end  122 . The openings are proximate and slightly elevated in position relative to the lower end  113  of the shaft  111 . The gas conduit  120  includes an interior space  123  that allows a gas to pass therethrough in a downward direction from the first open end  121  to the second open end  122  without obstruction. The gas is allowed to exit the gas conduit  120  through the openings  125  and be reintroduced into the liquid space  104  and mixed with the liquid medium  107 . In a preferred embodiment, the gas conduit  120  and the shaft  111  are concentric, and the shaft  111  extends through the interior space  123  of the gas conduit  120 , forming a gap (G) therebetween. The diameter of the shaft  111 , the diameter of the gas conduit  120 , and the gap G are adjustable. 
     The suction-generating device  130  is operably connected to the lower end  113  of the shaft  111  and is proximate the openings  125  of the gas conduit  120 . The suction-generating device  130  is configured to generate a suction and apply the suction to the gas conduit  120  to cause the gas  106  to flow from the gas space  103  to the liquid space  104  through the gas conduit  120  within the housing  101 . In some embodiments, the suction-generating device  130  is a Venturi unit. The Venturi unit used herein is an apparatus or device that can cause a Venturi effect, which is the reduction in pressure that results when a fluid flows through a constricted section of the Venturi unit. Because of the proximity of the Venturi unit  130  with the second open end  122  of the gas conduit  120 , the reduced pressure induces a suction that may cause the gas  106  in the gas space  103  to flow through the gas conduit  120  and enter the liquid medium  107  in the liquid space  104 . In a particular embodiment, the Venturi unit comprises a plurality of blades  131 , each blade operably connected to the lower end  113  of the shaft  111 . The blades  131  may each extend in a radially outward direction and have a blade gap  132  between every two adjacent blades. The blades  131  upon rotation about the central axis  114  effectively shear the liquid medium  107 , which flows through the blade gap  132  and thereby generate a reduced pressure under the Venturi effect. In addition, the rotating blades  131  may further mix the liquid medium  107  and the gas  106  that is reintroduced into the liquid medium  107 . 
     In some embodiments, the gas-recycling system  100  optionally includes a liquid pump  160  operably connected to the powered propeller  110 . The liquid pump  160  is above the suction-generating device  130  and below the gas-liquid interface  105 . The liquid pump  160  is configured to increase the pressure and flow of the liquid medium  107  and maintain the downward flow direction thereof. In some embodiments, the liquid pump  160  comprises an impeller. The impeller may be an open impeller, a semi-open impeller, or a closed impeller. In some embodiments, the impeller is a part of the powered propeller and includes a plurality of propeller blades  161  and an adaptor  162 . The adaptor  162  is circumferentially connected to an exterior surface of the gas conduit  120  and fixed to the powered propeller  110  in relative position. The propeller blades  161  are each fixed onto the adaptor  162  and extend from the adaptor  162  outwardly to the side wall  117  in a radial direction relative to the central axis  114 . Advantageously, the pump capacity of the liquid pump  160  is maintained high because of the low gas holdup in the liquid space  104  between the gas-liquid interface  105  and the suction-generating device  130 . 
     During operation, the powered propeller  110  is driven by the motor  115  and controlled either automatedly or manually to rotate about the central axis  114  at a rotation speed. The shaft  111 , the suction-generating device  130 , and the liquid pump  160  may co-rotate about the central axis  114  conjunctively in the same direction and at the same rotation speed. Upon rotation, the powered propeller  110  is configured to perform at least one of the following functions: (1) generate a suction to cause the collected gas  106  to flow from the gas space  103  to the liquid space  104  through the gas conduit  120 ; (2) cause the liquid medium  107  to flow in a downward direction from the gas-liquid interface  105  to the bottom end  108 ; and (3) mix the liquid medium  107  and the gas  106  reintroduced into the liquid medium  107 . 
     The suction force (reduction of pressure) generated by the powered propeller  110  depends on several factors including the rotation speed, the type and configuration of the suction-generating device, and the physical properties (temperature, velocity, viscosity) of the liquid. These factors can be controlled coordinately to achieve a desired suction force. The suction force causes the gas  106  to flow at a flow rate into the liquid medium  107 . The flow rate is adjustable and depends on factors including the suction force, the size of gap G of the interior space  123 , the size of the open ends  121  and  122 . 
     The gas  106  reintroduced into the liquid medium  107  through the gas conduit  120  may form gas bubbles  124  below the lower end  113  of the shaft  111 . In some embodiments, the suction-generating device  130  may be further configured to shear or break apart the gas bubbles  124 , reduce the size of the gas bubbles  124 , prevent coalescence, and thereby promote mixing of the gas bubbles  124  with the liquid. As such, both contact area and contact time of gas-liquid interaction within the gas-recycling system  100  can be significantly improved. 
     In one example implementation of the gas-recycling system  100 , the rotating blades  131  are configured to produce Venturi effect and apply a suction to the gas conduit  120  to cause the gas  106  to flow from the gas space  103  to the liquid space  104  through the gas conduit  120 . The rotating blades  131  are further configured to shear or break apart the gas bubbles  124 , reduce the size of the gas bubbles  124 , prevent coalescence, and thereby promote mixing of the gas bubbles  124  with the liquid. The liquid pump  160  or the propeller blades  161  are configured to cause the liquid medium  107  to flow downwardly and prevent or reduce backflow of gas bubbles  124 . 
     Now referring to  FIGS.  1 B and  1 C , another example gas-recycling system  100 ′, as a variation of the of the gas-recycling system  100 , will be illustrated and described.  FIG.  1 B  is a schematic view of a gas-recycling system  100 ′;  FIG.  1 C  is an isometric view of a part of the gas-recycling system  100 ′ of  FIG.  1 B . In the illustrated example, the gas-recycling system  100 ′ includes a housing  101 , a powered propeller  110 , at least one a gas conduit  120 , a suction-generating device  130 ′, and optionally a fixture  140  and a liquid pump  160 . The housing  101 , the powered propeller  110 , the gas conduit  120 , and the liquid pump  160  are described above and will not be repeated here unless otherwise indicated. 
     In the illustrated example, the gas-recycling system  100 ′ includes at least one gas conduit  120  proximate and external to the shaft  111  along the central axis  114 . In some embodiments, the gas-recycling system  100 ′ includes a plurality of gas conduits  120  surrounding at least a portion of the shaft  111 , and each gas conduit  120  is proximate and external to the shaft  111  aligned with the central axis  114 . 
     In the illustrated example, the suction-generating device  130 ′ includes a base  132 ′ and a plurality of blades  131 ′. The base  132 ′ extends from a center  135 ′ to a circumference  136 ′ in a radial direction relative to the central axis  114 . The base  132 ′ has a top surface  133 ′ and a bottom surface  134 ′. The top surface  133 ′ is connected to the lower end  113  of the shaft  111  at or proximate the center  135 ′. The plurality of blades  131 ′ are connected to the bottom surface  134 ′ and aligned radially relative to the central axis  114 . Each blade  131 ′ extends in a direction from the center  135 ′ to the circumference  136 ′ and is apart from each other. In some embodiments, the suction-generating device  130 ′ may include at least 2, at least 3, at least 4, at least 5, at least 8, or at least 12 blades. Each blade  131 ′ may be a fluid shearing or mixing blade that is configured to generate Venturi effect to induce and apply a suction to the at least one gas conduit  120 , as described above. 
     The at least one gas conduit  120  may extend through the base  132 ′ between every two adjacent blades  131 ′. The at least one gas conduit  120  has a first open end located and in gas communication with the gas space  103  and a second open end  122  located and in gas communication with the liquid space  104 . The second open end  122  include at least one opening  125  allowed for the gas in the gas conduit  120  to enter into the liquid space  104 . The opening  125  of the at least one gas conduit  120  may be proximate the bottom surface  134 ′ of the base  132 ′. 
     In some embodiments, the gas-recycling system  100 ′ optionally includes a fixture  140  connected to the at least one gas conduit  120  and the shaft  111 . As illustrated, the at least one gas conduit  120  and the shaft  111  may each extend through the fixture  140  with their relative position fixed by the fixture  140 . During operation, the fixture  140  allows the at least one gas conduit  120  and the shaft  111  to co-rotate about the central axis  114  in a conjunctive manner in the same direction and at the same rotation speed. 
     In some embodiments, the gas-recycling system  100 ′ optionally includes a liquid pump  160  operably connected to the shaft  111 . Similar to the gas-recycling system  100 , the liquid pump  160  is below the gas-liquid interface  105  and configured to increase the pressure and flow of the liquid medium  107  and maintain the downward flow direction thereof. In some embodiments, the liquid pump  160  comprises an impeller having a plurality of propeller blades  161  and an adaptor  162 . The adaptor  162  is circumferentially connected to an exterior surface of the shaft  111  and fixed to the powered propeller  110 ′ in relative position. The propeller blades  161  are each fixed onto the adaptor  162  and extend from the adaptor  162  outwardly to the side wall  117  in a radial direction relative to the central axis  114 . 
     In one example implementation of the gas-recycling system  100 ′, the shaft  111 , the at least one gas conduit  120 , the fixture  140 , the suction-generating device  130 , and the liquid pump  160  co-rotate about the central axis  114  conjunctively in the same direction and at the same rotation speed. The suction-generating device  130 ′ is configured to produce Venturi effect and apply a suction to the at least one gas conduit  120  to cause the gas  106  to flow from the gas space  103  to the liquid space  104  through the at least one gas conduit  120 . The rotating blades  131 ′ are configured to shear or break apart the gas bubbles  124 , reduce the size of the gas bubbles  124 , prevent coalescence, and thereby promote mixing of the gas bubbles  124  with the liquid. The liquid pump  160  or the propeller blades  161  are configured to cause the liquid medium  107  to flow downwardly and prevent or reduce backflow of gas bubbles  124 . 
     The present disclosure also provides reactors (or fermentors or fermentation systems or the like) that implement any gas-recycling system described herein. The reactor provided herein may have various configurations, including but not limited to, tank or vessel reactor, or loop reactor. 
     Now referring to  FIGS.  2 - 4   , particular examples of the reactor will be illustrated and described.  FIG.  2    is a schematic view of a tank reactor. In the illustrated example, the tank reactor  200  includes the gas-recycling system  100  or  100 ′ as a substantial framework of the tank reactor  200 , at least one gas inlet  201 , at least one liquid medium inlet  203 , at least one outlet  206 , and optionally a gas controller  202  connected to the gas inlet  201 , at least one mixer  204 , and a pressure controller  205 . Various aspects of the gas-recycling system  100  or  100 ′ are described above and will not be repeated here unless otherwise indicated. 
     In some embodiments, the tank reactor  200  is configured to produce biomass via a fermentation process within the tank reactor. The liquid space  104  may include a reaction mixture comprising a gas substrate and the liquid medium  107 . The gas substrate may include CO 2 , H 2 , O 2 , and optionally N 2 . In some embodiments, the gas substrate further include a C 1  substrate such as CH 4 , CO, a syngas (CO+H 2 ), a natural gas, or combinations thereof. The liquid medium  107  includes water, one or more nutrients, and one or more microorganism or microbial culture described herein. The nutrients include ingredients capable of supporting or transporting dissolved or suspended sustenance to biomass forming microbiological organisms in the multi-phase reaction mixture within the tank reactor  200  and promoting microorganism growth. The nutrients may include ammonia or salts or derivatives thereof, phosphate (e.g. as phosphoric acid), minerals such as magnesium, calcium, potassium, iron, copper, zinc, manganese, nickel, cobalt and molybdenum, sulphates, chlorides, or nitrates. The microorganism may be of any type according to the present disclosure. In a particular embodiment, the microorganism or microbial culture in the liquid medium  107  is capable of converting CO 2  into biomass, in particular, protein-rich biomass. 
     The tank reactor  200  may include at least one gas inlet  201  connected to and in gas communication with the tank reactor  200 . In one embodiment, the gas inlet  201  is connected to the bottom wall  118  of the tank reactor  200 . In another embodiment, the gas inlet  201  is connected to the side wall  117  of the tank reactor  200  and is in gas communication with the gas space  103 . The at least one gas inlet  201  is configured to introduce at least one input gas substrate into the tank reactor  200 . The input gas substrate my include CO 2 , H 2 , O 2 , and optionally N 2 , a C 1  substrate such as CH 4 , CO, natural gas, a syngas (CO+H 2 ), or combinations thereof. In one particular embodiment, at least one gas inlet  201  is operably connected to a source of CO 2 , which is configured to supply CO 2  to the tank reactor  200 . The source of CO 2  may be from an industrial process such as a flue gas (exhaust gas or stack gas), a combustion plant that generates a CO 2 -rich gas, or a fermentation process that generates CO 2  from microbiol culture (e.g., yeasts,  coli, Bacillus, Streptococcus, Lactobacillu, Escherichia, Salmonella, Corynebacterium ), or an industrial farming process that generates biogenic CO 2  from metabolism of living aquatic animals (e.g., fish) in a farming plant, or a natural resource such as air or water. 
     In some embodiments, the tank reactor  200  further includes a gas controller  202  operably connected to the each of the gas inlet(s)  201 . The gas controller  202  is configured to adjust the ratio of the input gas substrates to a described level (e.g., in metabolic stoichiometry) before introduction to the tank reactor  200 . The input gas substrate(s) may be combined and mixed to form a homogenous gas mixture before introduction to the tank reactor  200 . Alternatively, the input gas substrate(s) may be separated introduced into the tank reactor  200  without mixing. 
     The tank reactor  200  may include a liquid medium inlet  203  configured to introduce the liquid medium  107  or any nutrients thereof to the tank reactor  200 . 
     The gas space  103  is configured to collect and contain the gas  106  introduced to and/or generated in the tank reactor  200 . The gas  106  may include water vapor, unconsumed gas substrates, gas by-products generated in the fermentation process, and combinations thereof. The gas  106  of the gas space  103  is recycled and reintroduced into the liquid medium  107  through use of the gas-recycling system  100 , and thereby recirculate in the tank reactor  200  as described above. In some embodiments, the tank reactor is closed during operation without gas exchange with the atmosphere and without substantially release of the gas  106  out of the tank reactor  200 . 
     The tank reactor  200  may further include at least one mixer  204  (e.g., an impeller) configured to stir the reaction mixture within the housing  101  and promote mixing of the gas, nutrient, and microorganism in the reaction mixture of the liquid medium  107 . In some embodiments, the mixer  204  is fixed on an interior surface of the bottom wall  118  and is configured to rotate about the central axis  114  at the same direction with the powered propeller  110  during operation. In some embodiments, the mixer  204  is fixed on an interior surface of the side wall  117 . In some embodiments, the tank reactor comprises at least one mixer  204  operably attached to the extended shaft  111  and configured to co-rotate with the shaft  111  conjunctively during operation. 
     The tank reactor  200  further includes an outlet  206  configured to allow the produced microorganisms from the fermentation process to be removed from the tank reactor  200 . The collected microorganisms can be further processed to recover desired biomass products. In some instances, the microorganisms collected via the outlet  206  can be introduced to a separation subsystem (not shown) for processing and recovery of desired products. The biomass products may be further processed to produce a feed product that can be supplied to aquatic organisms in industrial fermentation plants or aquatic farming plants. 
     The tank reactor  200  may optionally include a pressure controller  205  configured to control the pressure of the tank reactor for safety consideration and/or for improving gas mass transfer during operation. The pressure controller  205  may further comprise a gas relief unit common in the art. 
       FIG.  3    is a schematic view of one example loop reactor  300 . This reactor design allows gas mixture to be richer in O 2  with no risk in explosion, thus increasing productivity of biomass and conversion efficiency of gas substrates. In the illustrated example, the loop reactor  300  includes a loop section  301  and a gas-recycling system  100  or  100 ′ that is integral to the loop section  301 . Various aspects of the gas-recycling system  100  or  100 ′ are described above and will not be repeated unless otherwise indicated. 
     The loop section  301  includes an upstream end  302  and a downstream end  303 . The upstream end  302  is integrally connected to the housing  101  at the bottom end  108  thereof. The downstream end  303  is integrally connected to the side wall  117  of the housing  101 . The upstream end  302  is in fluid communication with the housing  101 , such that the liquid medium  107  can flow from the housing  101  to the loop section  301  through the upstream end  302  without physical obstruction. In some embodiments, the upstream end  302  is circumferentially connected to the side wall  117  of the housing  101  at the bottom end  108  thereof, and no bottom wall  118  is needed. The downstream end  303  is in fluid communication with the housing  101 , such that the liquid medium  107  flows from the loop section  301  into the housing  101  through the downstream end  303  without physical obstruction. The size (e.g., diameter of the housing  101 ) of the gas-recycling system  100  or  100 ′ may be substantially the same with the size (or diameter) of the loop section  301 , such that the gas-recycling system  100  or  100 ′ may be viewed as an integral segment of the loop reactor  300 . In such configuration, the gas space  103  may be viewed as a “headspace” of the loop reactor  300 . 
     During operation, the liquid medium  107  recirculates in the loop reactor in a flow direction from the liquid space  104  of the housing  101  downwardly to the upstream end  302 , through the loop section  301  to the downstream end  303 , and back to the housing  101 . The gas  106  also recirculates in the loop reactor  300  in the flow direction similar to the liquid medium  107 . In particular, the gas  106  collected in the gas space  103  flows downwardly into the liquid medium  107  through use of the powered propeller  110 . The liquid medium  107  carrying the gas  106  flows through the loop section  301  and back into the housing  101 , where a portion of the gas  106  is separated from the liquid medium  107  at the gas-liquid interface  105  and flows back into the gas space  103 . In some embodiments, the loop reactor  300  is substantially closed during operation, without substantial release of the gas  106  out of the loop reactor  300 . 
     As described above, the gas-recycling system  100  and  100 ′ may include a liquid pump  160  in the liquid space  104  between the gas-liquid interface  105  and the suction-generating device  130 . The liquid pump  160  is configured to maintain the downward flow direction of the liquid medium  107  in the liquid space  104 . The loop reactor  300  allows the gas to be enriched in the gas space (headspace)  103  and the loop section  301 , thereby minimizing gas holdup in the liquid medium within the liquid space  104 . As a result, the pump capacity of the liquid pump  160  may be improved and maintained high. 
     The loop section  301  may further include one or more of the following components: at least one gas inlet  201 , at least one gas controller  202 , a liquid medium inlet  203 , a pressure controller  205 , and an outlet  206 , the various aspects of which have been described above and will not be repeated here unless otherwise indicated. 
     The loop section  301  may further include at least one static mixer  304 , positioned along the length of loop section  301 . Benefits of the use of static mixers are described in U.S. Pat. No. 7,579,163 and include mixing of the gases into the multi-phase reaction mixture. Exemplary types of static mixers are also described in the &#39;163 patent. Static mixers that can be used in embodiments described are not limited to those described in the &#39;163 patent. Static mixers other than those described in the &#39;163 patent can be used in the embodiments described herein. For example, other types of static mixers are available from companies such as StaMixCo LLC of Brooklyn, N.Y. and Sulzer Management Ltd. of Winterthur, Switzerland. In some embodiments, as many as 100 static mixers  304  may be used in the loop reactor  300 . The static mixers  304  may be provided at a density of at least one mixer per three meters of the loop section  301  when the static mixer has a length of about 1 meter. In some embodiments, static mixers are spaced apart by a distance about equal to 1-3 times the length of one of the static mixers. In some embodiments, fewer or greater numbers of static mixers can be provided and the static mixers may be provided at a lesser or greater density. The particular number of static mixers used and the density at which they are deployed will be determined in part based upon their contribution to mass transfer of gas into the liquid and microorganisms and/or the pressure drop produced by the static mixers. In a particular embodiment, the loop section  301  comprises a plurality of static mixers  304  at a density of about one mixer per one meter distributed along a majority of the loop section  301 , with fewer or no static mixer in a segment closer to the downstream end  303  to allow the gas bubbles in the loop section  301  to coalesce, rise, and released into the gas space  103 . 
     In some embodiments, the loop reactor further include a heat transfer unit operation  305  operably connected to the loop section  301 . The heat transfer unit operation  305  is configured to introduce or remove thermal energy from the multi-phase reaction mixture in the loop section  301 . The heat transfer unit operation  305  can introduce thermal energy to or remove thermal energy from the multi-phase mixture in the loop section  301  at one or more locations along loop section  301 . In at least some instances, the microbiological activity that occurs within the loop reactor  300  generates heat as a byproduct. Left uncontrolled, such heat can adversely affect the metabolism or health of the microbiological organisms within the loop reactor  300 . Alternatively, microbiological organisms may also have a temperature below which the metabolism or health of the organism is adversely affected. As such, the biological organisms within the loop reactor  300  have a defined temperature range providing optimal growth and metabolic conditions. In at least some instances, the multi-phase reaction mixture within the loop reactor  300  can be maintained at a temperature of about 130° F. or less; about 120° F. or less; about 110° F. or less; about 100° F. or less; about 95° F. or less; about 90° F. or less; about 85° F. or less; or about 80° F. or less using the heat transfer unit operation  305 . In at least some instances, the multi-phase reaction mixture within the loop reactor  300  can be maintained at a temperature of from about 55° F. to about 120° F.; about 60° F. to about 110° F.; about 110° F. to about 120° F.; about 100° F. to about 120° F.; about 65° F. to about 100° F.; about 65° F. to about 95° F.; or about 70° F. to about 90° F., using heat transfer unit operation  305 . 
     In some embodiments, the loop section  301  may have a U-shaped configuration, including two elbow portions  306  that bend at about 90° angles when viewed from above. The loop section  301  may take other shapes. For example, loop section  301  may include more than the two 90° elbow portions  306  or may include more than one elbow portion  306  that is less than 90°. In other embodiments, loop section  301  can include a plurality of elbow portions  306  that are greater than 90° or less than 90°. 
     The loop section  301  may be substantially planar. In some embodiments, the loop section  301  is substantially co-planar with the housing  101  and is substantially vertical relative to the ground level. Alternatively, the loop section  301  may be substantially horizontal relative to the ground level. 
     In some embodiments, the loop reactor  300  does not have a pressure reduction device or a pressure reduction zone common in the art. The pressure of the liquid medium within the loop section  301  may be controlled at a constant profile, without an intentional reduction of pressure near the downstream end  303 . The gas-liquid separation occurs naturally within the housing  101  without external assistance (e.g. an intended reduction of pressure). With the contribution of the gas-recycling system  100 , the gas  106  is maximally remained in the liquid medium  107  and recirculates in the loop reactor  300 , which advantageously improves the mass transfer efficiency. 
       FIG.  4    is a schematic view of another example loop reactor  400 , which is a variation of the loop reactor  300 . Similar to the loop reactor  300 , the loop reactor  400  includes a loop section  301  and a gas-recycling system  100  or  100 ′ that is integral to the loop section  301 . Various aspects of the gas-recycling system  100  or  100 ′ and the loop section  301  are described above and will not be repeated here unless otherwise indicated. 
     In the illustrated example of  FIG.  4   , the housing  101  of the gas-recycling system  100  or  100 ′ further includes a bottom portion  150  that is in a “funnel-like” configuration with a sloped side wall  151  and a narrowed bottom end  152 . The upstream end  302  of the loop section  301  is integrally connected to the narrowed bottom end  152  and is in fluid communication with the housing  101 . In particular, the sloped side wall  151  of the bottom portion  150  is circumferentially connected to the side wall  117 , and is circumferentially connected to the loop section  301  at the upstream end  302 . The sloped side wall  151  may be sloped in an angle α from about 0 to about 80 degree relative to the ground level. In some embodiments, the narrowed bottom end  152  and the upstream end  302  may be substantially the same in size or diameter. The lower end  113  of the shaft  111  may extend into the bottom portion  150  or further into the loop section  301  at or proximate the upstream end  302  thereof. 
     In a preferred embodiment, the housing  101  has a diameter D 1  that is larger than the diameter D 2  of the bottom end  152  or the upstream end  302 . The ratio of D 1  to D 2  may be adjustable, e.g., at least about 1.2, at least about 1.5, at least about 2, at least about 3, at least about 4, at least about 5, or at least about 10. Such size restriction may advantageously allow the loop reactor  400  to generate a Venturi effect that may provide additional suction to recycle the gas  106 . In one embodiment, during operation, the liquid medium  107  flows from the housing  101  through the “funnel-like” bottom portion  150  into the loop section  301  that is restricted in size, thereby generating a reduced pressure under the Venturi effect that may cause the gas  106  to flow through the gas conduit  120  into the liquid medium  107 . 
     In another embodiment, the powered propeller  110  or  110 ′ upon rotation generates a suction to recycle the gas  106  in the gas space  103  into the liquid medium  107 , and the liquid medium  107  flows through the bottom portion  150  and the upstream end  302  generates an additional suction that further promotes recycling of the gas  106 . 
     In yet another embodiment, the loop reactor  400  further includes an external gas-recycling system  410  configured to provide an additional passageway to recycle the gas  106  and/or to improve the recycling efficiency. In some embodiments, the external gas-recycling system  410  includes an external gas conduit  401  and a gas compressor  402  operatively connected to the external gas conduit  401 . The external gas conduit  401  has a first end  403  and a second end  404 . The first end  403  is connected to the housing  101  and is in gas communication with the gas space  103 . The second end  404  is connected to the bottom portion  150  or the loop section  301  proximate the upstream end  302 , and is in gas communication with the liquid medium  107 . The gas compressor  402  is configured to pump the gas  106  to flow through the external gas conduit  401  into the liquid medium  107  proximate the upstream end  302 . When operating together with the powered propeller  110  or  110 ′, the loop reactor  400  may improve gas recycling efficiency and gas mass transfer efficiency. 
     The reactor described in the present disclosure may be operated with an adjustable flow control device (not shown) set for different flow rates of the gas and/or liquid medium through the adjustable flow control device. Steady state conditions, such as volumetric pump output, temperature of multi-phase reaction mixture, fluid and/or gas pressure, dissolved gas content of multi-phase mixture, volumetric flow rate of CO 2  into the loop section, volumetric flow rate of O 2  into the loop section, volumetric flow rate of H 2  into the loop section, volumetric flow rate of N 2  into the loop section, and/or pH of multi-phase reaction mixture within the reactor may vary. The liquid medium  107  may have a pH from about 4 to about 9, or from about 5 to about 8, or from about 6 to about 7. The liquid medium may have a density of the multi-phase mixture from about 1.0 kg/m 3  to about 2 kg/m 3 . 
     Now referring to  FIG.  5   , one example of a system for producing biomass will be illustrated and described. In the illustrated example, the system  500  includes a source of biogenic CO 2    501  and a reactor or fermentor  502 . The source  501  is in connection with the reactor  502  via a supply line  503  and a feed line  504 . The supply line  503  is configured to transport the biogenic CO 2  from the source  501  to the reactor  502 . The feed line  504  is configured to transport biomass or feed that is produced in the reactor  502  to the source  501 . The source of biogenic CO 2    501  may be an aquatic farming plant that cultivates aquatic animals such as fish in the plant or a fermentation plant that cultivates microorganisms as described above. The aquatic animals or fermentation microorganisms produce biogenic CO 2  through metabolism. The biogenic CO 2  may be collected and optionally processed (e.g., through purification, filtration, or condensation) before supplying to the reactor  502 . 
     The reactor or fermentor  502  may be any reactor described herein, including the tank reactor  200 , the loop reactor  300  or  400 , or a reactor comprising the gas-recycling system  100  or  100 ′ according to the present disclosure. The reactor  502  includes a liquid medium that has a microorganism capable of converting CO 2  into biomass via a fermentation process within the reactor, as described herein. The produced biomass product may be directly supplied to the source  501  via the feed line  504  as a feed for the living animals or fermentation microorganism therein. In some embodiments, the system  500  further includes a biomass processing unit  505  in connection with the reactor  502  and the source of biogenic CO 2    501  via a supply line  506  and a feed line  507  respectively. The biomass processing unit  505  is configured to receive the biomass product from the reactor  502 ; process and formulate the biomass product to produce a feed product; and supply the feed product to the source  501  for feeding the aquatic animals or fermentation microorganisms therein. 
     In some embodiments, the system  500  is substantially closed, and the biogenic CO 2  from the source  501  recirculates in the system  500 , and is at least partially reused and recycled through operation of the gas-recycling system  100  or  100 ′ of the reactor  502 , without substantial release to the environment. 
     Now referring to  FIGS.  6 - 8   , example methods for recycling gas in a reactor and/or for producing biomass in a reactor will be illustrated and described.  FIG.  6    is a flow diagram of one example method  600  for recycling gas in a reactor. The reactor of the method  600  may be a tank reactor  200 , a loop reactor  300  or  400 , or any other type of reactor that includes a gas-recycling system  100  or  100 ′ according to the present disclosure. The method  600  includes operation  610  and  620 . At  610 , gas is collected in the gas space  103  of the gas-recycling system  100  or  100 ′. The gas may include gas substrates introduced into the housing  101 , gas by-products generated in the housing  101 , or gas separated from the liquid medium  107 , or any combinations thereof. At  620 , the powered propeller  110  is operated to rotate about the central axis  114  to recycle at least a part of the gas  106  from the gas space  103  to the liquid medium  107  through the gas conduit  120 , according to the present disclosure. 
     In some embodiments, the reactor is a loop reactor  400  and further includes an external gas-recycling system  410  according to the present disclosure. The method  600  may further include activating the gas compressor  402  to cause at least a part of the gas to flow from the gas space  103  into the liquid medium  107  through the external gas conduit  401 . 
     The method  600  may optionally include operation  630 . At  630 , at least one gas substrate is introduced into the reactor via at least one gas inlet  201 . The gas substrate may include CO 2 , H 2 , O 2 , N 2 , or any mixture thereof. In some embodiments, the CO 2  is supplied from a source of biogenic CO 2  that is in direct or indirect connection with the reactor. 
     The method may optionally include operation  640 . At  640 , a liquid medium such as a microorganism culture medium is introduced into the reactor via at least one liquid medium inlet  203 . The liquid medium may include a microorganism capable of converting CO 2  into biomass. In some embodiments, a liquid is introduced into the reactor at  640  and allowed to slowly mix with the gas substrate in the reactor. Microorganisms are subsequently introduced into the reactor and combined with the liquid and gas to form a reaction mixture. The microorganism are allowed to grow in the reaction mixture by converting the gas substrate that is introduced to and recycled in the reactor into biomass. 
     The method may optionally include operation  650 . At  650 , the liquid medium is stirred by at least one static mixer (e.g., the mixer  204  or static mixer  304 ) to promote mixing of the reaction mixture. 
     The method may optionally include operation  660 . At  660 , the produced biomass within the reactor is collected and removed from the outlet  206 . The removed biomass may be supplied to an aquatic farming for feeding the aquatic animals therein. Alternatively, the biomass may be further processed and formulated to produce a feed product that is supplied to the aquatic farming plant. 
     In some embodiments, the method  600  allows to recycle the gas in the reactor, without substantial emission to atmosphere during operation. 
       FIG.  7    is a flow diagram of one example method  700  for producing biomass. In the illustrated example, the method  700  includes operations  710 ,  720 ,  730 ,  740 , and  750 . Operation  710  includes culturing a microorganism in a reaction mixture that recirculates in a reactor, wherein the reaction mixture comprises a liquid medium and gas, and wherein the microorganism is capable of converting the gas into biomass. The reactor may be a tank reactor or a loop reactor, according to the present disclosure. 
     Operation  720  includes introducing at least one gas substrate from an external source into the liquid medium. Operation  730  includes collecting gas in a gas space within the reactor. Operation  740  includes recycling the collected gas back to the liquid medium. In some embodiments, operations  730  and  740  are performed through use of a gas-recycling system  100  or  100 ′ and/or an external gas-recycling system  410  according to the present disclosure. Accordingly, operation  730  may include operation  610 , and operation  740  may include operation  620 , as described above. Operation  750  includes harvesting the biomass from the reactor. Operation  750  may include operation  660  as described above. 
     The method  700  may optionally include an operation  760 . At  760 , the gas within the reactor is prevented from substantially releasing into atmosphere during operation. As such, the method  700  allows the gas to recirculate in the reactor. 
       FIG.  8    is a flow diagram of one example method  800  for recycling CO 2  and producing biomass from CO 2 . In the illustrated example, the method  800  includes operations  810 ,  820 ,  830 ,  840 , and  850 . Operation  810  includes culturing a microorganism in a reaction mixture that recirculates in a reactor, wherein the reaction mixture comprises a liquid medium and a gas comprising CO 2 , and wherein the microorganism is capable of converting CO 2  into biomass. Operation  820  includes introducing a gas substrate comprising CO 2  from an external source into the liquid medium. In some embodiments, CO 2  gas is supplied from a source of biogenic CO 2  connected to the reactor via a supply line. 
     Operation  830  includes collecting at least a part of the CO 2  separated from the liquid medium within the reactor. Operation  840  includes recycling the collected CO 2  back to the liquid medium. Operations  830  and  840  may be performed through use of a gas-recycling system  100  or  100 ′ and/or an external gas-recycling system  410  according to the present disclosure. Operation  850  includes harvesting the biomass from the reactor. 
     The method  800  may optionally include operations  860  and  870 . At  860 , the produced biomass is supplied to the source of biogenic CO 2  for feeding the aquatic organisms therein. At  870 , the produced biomass is further processed and/or formulated to produce a feed product that is supplied to the source of biogenic CO 2  for feeding the aquatic organism, in accordance with the system  500  described above. 
     When introducing elements of the present disclosure or the various versions, embodiment(s) or aspects thereof, the articles “a”, “an,” “the,” and “said” are intended to mean that there are one or more elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The term “about” is used here in conjunction with numeric values to include normal variations in measurements as expected by persons skilled in the art, and is understood to have the same meaning as “approximately” and to cover a typical margin of error, such as 15% of the stated value. 
     “Fermentation” is defined as “a metabolic process that produces chemical changes in organic substrates through the action of enzymes. For the purposes of this disclosure, “fermentation” is a process for cultivating cells in a specialized containers, tanks, vessel, or reactors (made of glass, metal or plastic and known as a fermenter or fermentor or fermentation tank or bio-reactor) under controlled process conditions in order to optimize their growth and maximize efficiency. The controlled process conditions include sterility, temperature, pressure, agitation rate, pH, input gas composition and flow rate, nutrient composition, cell density, dissolved gas concentration, biomass removal rate (for continuous or semi-continuous harvesting) and the like. Fermentation may be aerobic or anaerobic. 
     “Gas fermentation” refers to a fermentation in a fermentation tank or a bioreactor, wherein the metabolic processes of microorganisms or microbes or cells extract energy and carbon from the gaseous inputs that are supplied to them. Gas fermentation can refer to anaerobic or aerobic process of microbe cultivation on gases. By combining these gas inputs with the simple inorganic salts in the medium, chemo-autotrophic cells convert these basic inputs into more complex biomass and other cellular products. Gas fermentation can be either aerobic or anaerobic, depending on the organism used and the feedstock gases available for fermentation. Gas fermentation is a particularly advantageous form of chemo-autotrophic fermentation because the key inputs are provided by widely available gases such as CO 2 , H 2 , O 2 , CH 4 , etc. 
     “Cultivating” is defined as meaning “the act or process of culturing living material (such as bacteria or yeasts) in a prepared nutrient medium.” “Nutrient” is defined as meaning “a substance or ingredient that promotes growth, provides energy, and maintains life.” “Medium” is defined as “a nutrient system for the artificial cultivation of cells or organisms and especially bacteria.” Media can be liquid, semi-solid or solid (e.g., agar, beads, or other scaffolding). Solid or semi-solid media can provide a growth support for the cells. 
     “Chemo-autotrophic” is defined as “being auto trophic and oxidizing an inorganic compound as a source of energy.” The inorganic compound as a source of energy may include H 2 , in the case of hydrogen-oxidizing microorganism, which can consume a combination of CO 2 , H 2 , and O 2 . Examples include anaerobic acetogens that consume CO 2  for carbon and H 2  for energy. Chemoautotrophic metabolism is known in bacteria and archaea, and may also exist as an undiscovered trait, or as a capability conferred by genetic modification, in some other organisms. Examples of chemoautotrophs are found across numerous bacterial genera including but not limited to  Cupriavidus, Rhodobacter, Methylobacterium, Methylococcus, Methylosinus, Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus, Paracoccus, Hydrogenothermus, Hydrogenovibrio, Clostridium, Rhodococcus, Rhodospirillum, Alcaligines, Rhodopseudomonas , and  Thiobacillus , as well as in a number of genera of the archaea, including methanogens. Specific examples of chemoautotrophs include  Cupriavidus necator, Cupriavidus basilensis, Rhodococcus opacus, Methylococcus capsulatus, Methylosinus trichosporium, Methylobacterium extorquens, Hydrogenothermus marinus, Rhodospirillium rubrum, Rhodopseudomonas palustrus, Paracoccus zeaxanthinifaciens, Rhodobacter sphaeroides, Rhodobacter capsulatus , and  Clostridium autoethanogenum.    
     The various embodiments described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the following claims.