Bioreactors and processes for microbial gas fermentation.

Abstract

The invention relates to an aeration chamber (2) provided with means (3) for mixing the culture broth contained therein, at least a first gas supply chamber (4) and a second gas supply chamber (5) suitable for containing a first gas and a second gas, respectively, and at least a first gas supply chamber (5) adapted to connect an outlet terminal (7) of the aeration chamber (2) to at least one respective inlet (4a, 4b, 5a, 5b) of each of the first chamber (4) and the second chamber (5), and to introduce the culture broth coming from the aeration chamber (2) into the first gas supply chamber (4) and the second gas supply chamber (5), resulting in the culture broth being mixed with the first gas and the second gas, respectively. The present invention also relates to a process for producing one or more of a dry cell mass or single-cell proteins from CO2.

Description

[Technical Field]

[0001] The present invention relates to a Y-type bioreactor for microbial gas fermentation, in particular for CO2 fixation and conversion to valuable products such as dry cell mass and single cell protein (SCP). [Background technology]

[0002] Carbon dioxide (CO2) is the primary anthropogenic greenhouse gas (GHG) linked to climate change. The United Nations Paris Agreement set a global goal of keeping global temperature rise below 2°C above pre-industrial levels. In response, the EU has set a target of reducing GHG emissions by 40% below 1990 levels by 2030. Governments around the world are also introducing regulations to promote carbon capture, storage, and utilization (CCSU) technologies by providing carbon tax credits. Carbon capture and utilization (CCU) is considered an important CO2 mitigation strategy to support and complement carbon capture and storage (CCS) for CO2 reduction and capture. CCU represents a variety of pathways for utilizing CO2 as a feedstock for the production of value-added goods. It provides economic incentives to industries, which can diversify their revenue portfolios by benefiting from additional or new commercial CO2 uses or by selling captured CO2 to other interested users. Biological CCUs use biological pathways to convert CO2 into natural biomass and play a key role in the energy, water, and food nexus, which can be an important resource management tool for water, energy, and food security.

[0003] Hydrogen-oxidizing bacteria, such as Ralstonia eutropha, are a type of chemoautotrophic microorganism that can convert CO2 into desirable products by using hydrogen as the sole energy and reducing source and oxygen gas as the final electron acceptor [1]. Ralstonia eutropha is a representative chemoautotrophic bacterium that can grow using CO2 as a carbon source. Both H2 and O2 gas can be conveniently produced from water electrolysis using renewable energy sources, including solar, wind, hydroelectric, and geothermal [2]. Therefore, microbial CO2 fixation can play a role in sustainable carbon capture and utilization.

[0004] Microbial growth on gaseous substrates (CO2, H2, and O2) can produce valuable products from CO2, water, and renewable electricity, exceeding the capabilities of traditional green species such as green microalgae and plants that rely solely on sunlight. [2] Dry bacterial mass contains approximately 70 wt% protein, particularly single-cell proteins useful in aquaculture and animal feed. [3] The essential amino acid composition of microbial proteins is comparable to that of casein (a milk protein) and superior to that of soy protein (a common plant protein source). Food-grade proteins, peptides, and amino acids can also be recovered using proprietary downstream separation technologies.

[0005] Microbial gas fermentation can also operate continuously despite the intermittency of sunlight, which significantly increases process productivity. Carbon-to-products (C2P) technology can be applied to areas unsuitable for conventional agriculture. Specifically, large areas of arable land are required for crop cultivation, and large amounts of freshwater are consumed due to evaporation losses. Conventional agriculture also loses up to 70% of nitrogen fertilizer to the environment due to runoff. In contrast, microbial gas fermentation consumes nitrogen nutrients and water with extremely high efficiency, as shown in the following two stoichiometric equations [1]. Freshwater is converted primarily into hydrogen (H2) gas and oxygen (O2) gas by water electrolysis, and then converted into microbial biomass (CH 1.68 O 0.46 N 0.24Water evaporation losses from gas fermentation are very low (less than 0.001 kg / kg dry cell mass), and the water consumption of crop biomass production is less than 1%.

[0006] Water electrolysis:7.77H2O=7.77H2+3.88O2 Microbial growth: CO2 + 7.77H2 + 2.87O2 + 0.24NH3 = CH 1.68 O 0.46 N 0.24 +7.28H2O Nitrogen nutrients (NH3) are almost completely converted to organic nitrogen in the cell mass. Single-cell protein accounts for approximately 70% of the dry cell mass and can be a rich source of protein for aquaculture and animal feed [3].

[0007] The primary technological goal of industrial fermentation is high productivity of the desired product. Productivity is often expressed as the amount of product per volume of liquid per hour, which is equal to the product concentration per hour. It determines to a large extent the economic feasibility of a fermentation technology. For rapid CO2 fixation, microorganisms should be supplied with sufficient nutrients (nitrogen, phosphorus, minerals, etc.) and gaseous substrates. Providing sufficient O2 or H2 is the most difficult challenge due to their very low solubility in aqueous solutions.

[0008] Conventional gas fermentation uses a gas mixture, which is introduced into the bioreactor through a sparger at the bottom. Gas bubbles rise in the aqueous medium solution, and depending on the retention time of the bubbles, only a portion of the gas is dissolved in the solution and utilized by the microorganisms. As a result, gas recovery efficiency is low and a large amount of gas waste is generated.

[0009] A typical gas mixture of 70% H, 20% O, and 10% CO provides microbial cells with relatively abundant H and CO, but limited O. [6] In the absence of dissolved oxygen (DO), obligate aerobic microorganisms lose their metabolic activity and energy conversion efficiency.

[0010] Therefore, microbial gas fermentation in conventional bioreactors faces significant challenges in operational safety, gas recovery efficiency, and gas mass transfer for high fermentation productivity. First, a mixture of hydrogen and oxygen gases poses a high explosion risk unless the oxygen content is 6% v / v or less. A lack of dissolved oxygen leads to insufficient microbial growth and slow CO2 fixation. Second, low gas recovery efficiency means a significant amount of gas waste, including CO2 and H2. Discharging CO2 into the environment means insufficient performance for CO2 recovery and effective utilization. H2 is the most expensive gas substrate, and its waste not only increases fermentation costs but also generates potentially explosive mixtures of hydrogen and air. Finally, the gas mass transfer rate is limited by the partial pressure of each individual gas according to Henry's law. When gas bubbles expand in an aqueous solution, a moderate gas holdup results, resulting in a moderate volume transfer rate (k L a) is obtained.

[0011] Based on the above, it is very difficult to provide microorganisms with a constant environment and the balance of H2, O2 and CO2 required for continuous growth under optimal conditions. Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, with the above criticality in mind, it is an intent of the present invention to provide a bioreactor for microbial gas fermentation to maximize the efficiency of conversion of CO2 to desired products.

[0013] Another object of the present invention is to provide a process for producing one or more of dried cell mass and single cell proteins from CO2 with higher yield and efficiency than previously known processes.

[0014] It is a further object of the present invention to provide a process that can be operated in a continuous mode.

[0015] Another object of the present invention is to provide a process for the biological production of one or more of dry cell mass and single-cell proteins by absorption and biological conversion of carbon dioxide that is reliable and flexible in application, relatively easy to provide, has a competitive cost, and produces little process waste. [Means for solving the problem]

[0016] This intention, as well as these and other objects that will become more apparent hereinafter, is achieved by a Y-type bioreactor for microbial gas fermentation comprising an aeration chamber provided with means for mixing a culture broth contained therein, said bioreactor comprising: at least a first gas supply chamber and a second gas supply chamber suitable for containing a first gas and a second gas, respectively; at least a first duct configured to connect the outlet terminal of the aeration chamber to at least one respective inlet of each of the first and second chambers and to introduce the culture broth coming from the aeration chamber into the first and second gas supply chambers, so that the culture broth mixes with the first and second gases, respectively; at least a second duct configured to connect an outlet terminal of said first gas supply chamber to said aeration chamber; at least a third duct configured to connect an outlet terminal of said second gas supply chamber to said aeration chamber; at least one pump arranged blocking said first duct for conveying said culture broth; at least one outlet channel having an inlet located between the mid-height and the top of the aeration chamber and configured to continuously allow a portion of the culture to flow out;

[0017] The aims and objects of the present invention are also achieved by a process for producing one or more of dried cell mass and single cell proteins from CO, the process comprising: (i) continuously feeding an aqueous mineral solution to a culture of hydrogen-oxidizing microorganisms in an aeration chamber of a bioreactor according to the invention to obtain a suspension; (ii) supplying H gas to a first gas supply chamber of the bioreactor; (iii) supplying CO gas to a second gas supply chamber of the bioreactor; (iv) supplying O gas through an oxygen dispenser of the bioreactor; (v) supplying air or flue gas through a gas dispenser of the bioreactor; (vi) continuously recirculating the suspension between the aeration chamber and the first and second gas supply chambers, wherein the first gas supply chamber sprays droplets of culture into a stagnant H gas phase, and the second gas supply chamber sprays droplets of culture into a stagnant CO gas phase; (vii) continuously discharging the suspension of microorganisms in the aqueous mineral solution from the aeration chamber; (viii) recovering the dried cell mass or single cell proteins from the discharged suspension of step (vii).

[0018] Further features and advantages of the present invention will become more apparent from the description of preferred, but not exclusive, embodiments of a bioreactor for bacterial fermentation according to the invention, which are shown by way of non-limiting example in the accompanying drawings, in which: [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a front schematic elevation view of a first embodiment of a bioreactor for bacterial fermentation according to the present invention. [Figure 2] FIG. 2 is a plan view of the bioreactor of FIG. 1. [Figure 3]FIG. 2 is a front schematic elevation view of another embodiment of a bioreactor for bacterial fermentation according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The process according to the invention attempts to contribute to reducing atmospheric carbon dioxide concentrations while producing valuable products.

[0021] The present inventors have developed a novel Y-type bioreactor 1 that increases the gas transfer rate by using pure gas instead of the gas mixtures used in conventional gas fermentation.

[0022] In a first aspect, the present invention relates to a Y-shaped bioreactor 1 for microbial gas fermentation comprising an aeration chamber 2 provided with means 3 for mixing the culture broth contained therein. The bioreactor according to the invention comprises at least a first gas supply chamber 4 and a second gas supply chamber 5 suitable for containing a first gas and a second gas, for example H2 and CO2, respectively. The bioreactor further comprises: at least a first duct 6 configured to connect the outlet terminal 7 of the aeration chamber 2 with at least one respective inlet (4a, 4b, 5a, 5b) of each of the first and second chambers 5, and to introduce the culture broth coming from the aeration chamber 2 into the first gas supply chamber 4 and the second gas supply chamber 5, so as to mix the culture broth with the first gas and the second gas, respectively; at least a second duct 8 configured to connect the outlet terminal 4c of the first gas supply chamber 4 to the aeration chamber 2; at least a third duct 9 configured to connect the outlet terminal 5c of the second gas supply chamber 5 to the aeration chamber 2; at least one pump 10 arranged blocking the first duct 6 to convey the culture broth; at least one outlet channel 11 having an inlet 11a located between the mid-height and the top of the aeration chamber 2 and configured to continuously let out part of the culture;

[0023] In a preferred embodiment of the bioreactor according to the present invention, the at least one first gas supply chamber 4 is configured to receive hydrogen (H2) gas and mix it with the culture broth coming from the at least one first duct 6, and the second duct 8 is configured to transfer the mixture of hydrogen (H2) and culture broth from the outlet terminal 4c of the first chamber 4 to at least one respective inlet 12 of the aeration chamber 2.

[0024] In another preferred embodiment of the bioreactor according to the invention, the at least one second gas supply chamber 5 is configured to receive carbon dioxide (CO2) gas and mix it with the culture broth coming from the at least one first duct 6, and the third duct 9 is configured to transfer the mixture of dissolved carbon dioxide (CO2) and culture broth from the outlet terminal 5c of the second chamber 5 to at least one respective inlet 13 of the aeration chamber 2.

[0025] Preferably, the aeration chamber 2 comprises an oxygen dispenser 14 and a gas dispenser 15 configured to separately, independently and uniformly introduce oxygen and a gas selected from air and flue gas into the culture broth present inside the aeration chamber 2. Preferably, the oxygen dispenser 14 is located at a lower altitude than the gas dispenser 15 selected from air or flue gas.

[0026] Preferably, at least one first inlet 4a, 4b of the first gas supply chamber 4 is connected to at least one nozzle configured to spray the culture broth containing the microbial suspension and coming from the first duct 6 into the internal cavity delimited by the first gas supply chamber 4.

[0027] Preferably, at least one first inlet 5a, 5b of the second gas supply chamber 5 is connected to at least one nozzle configured to spray the culture broth containing the microbial suspension and coming from the first duct 6 into the internal cavity delimited by the second gas supply chamber 5.

[0028] Preferably, the aeration chamber 2 is equipped with at least one device selected from a probe, an indicator and a regulator that refers to at least one parameter selected from the fermentation temperature, pH, mixing rate of the culture broth, dissolved oxygen concentration, air flow rate, pressure in the corresponding chamber, liquid level, etc.

[0029] The bioreactor of the present invention allows fermentation to occur without mixing hydrogen and oxygen gases within the bioreactor, thus avoiding potentially explosive gas mixtures. Hydrogen gas is dissolved in an aqueous medium solution within a closed chamber, and the dissolved hydrogen molecules are delivered to the aeration chamber, where obligately aerobic microorganisms utilize the dissolved hydrogen molecules to fix CO2. The hydrogen gas recovery efficiency is nearly 100%. Furthermore, hydrogen gas pressure, and therefore hydrogen solubility, is significantly increased within the closed chamber due to Henry's law. The mineral aqueous solution or microbial slurry is spread into small droplets through multiple nozzles, creating a large gas-liquid contact area for rapid gas absorption. As a result, the mass transfer rate of hydrogen gas is significantly increased, resulting in high fermentation productivity. Furthermore, CO2 gas is dissolved in an aqueous medium solution within a separate gas chamber, and the dissolved CO2 molecules are delivered to the microorganisms in the aeration chamber. The CO2 gas recovery efficiency is nearly 100%. Finally, the aeration chamber is equipped with two gas spargers, one for air and the other for pure O2. The oxygen mass transfer rate of pure O2 is five times faster than that of air. It has been found that the dissolved oxygen concentration should be controlled within a range between a critical low level and an inhibitory high level. This dual oxygen supply design offers great flexibility for controlling the dissolved oxygen concentration in aqueous solutions.

[0030] Aqueous mineral solutions can be continuously added and removed from the bioreactor to allow the microorganisms to grow on the dissolved gases under optimal conditions. Dry cell mass or single-cell proteins are recovered from the slurry (suspension) discharged from the bioreactor.

[0031] 3 depicts one embodiment of a bioreactor according to the present invention with auxiliary equipment: a cooling water chiller, an air compressor, and a gas cylinder. The monitoring and control includes: a glass window (W), pressure indication (PI), pressure control (PC), pressure and control indication (PIC), flow indication (FI), flow control (FC), flow indication and control (FIC), temperature indication (TI), temperature control (TC), temperature indication and control (TIC), pH indication (pH), pH indication and control (pHC), level control (LC), agitation speed indication and control (RMP), and dissolved oxygen concentration indication (DO) and control (OC).

[0032] In a second aspect, the present invention relates to a process for producing one or more of dry mass or single cell proteins from CO, the process comprising: (i) continuously feeding an aqueous mineral solution to a culture of hydrogen-oxidizing microorganisms in the aeration chamber 2 of the bioreactor according to the invention to obtain a suspension; (ii) supplying H2 gas to the first gas supply chamber 4 of the bioreactor; (iii) supplying CO2 gas to the second gas supply chamber 5 of the bioreactor; (iv) supplying O2 gas through an oxygen dispenser 14 of the bioreactor; (v) supplying air or flue gas through a gas dispenser 15 of the bioreactor; (vi) continuously recirculating the suspension between the aeration chamber 2 and the first gas supply chamber 4 and the second gas supply chamber 5, in which the culture droplets are sprayed into a stagnant H gas phase in the first gas supply chamber 4 and into a stagnant CO gas phase in the second gas supply chamber 5; (vii) continuously discharging the suspension of microorganisms in the aqueous mineral solution from the aeration chamber 2; (viii) recovering the dried cell mass or single cell proteins from the discharged suspension of step (vii).

[0033] Preferably, throughout steps (i) to (vii), the culture in aeration chamber 2 is maintained at a temperature of 30±1° C. and a pH of 7±0.5.

[0034] Preferably, the hydrogen-oxidizing microorganism is a strain of Ralstona eutropha. There are two types of CO2 gas that can be used as a carbon source. In response to growing concerns about climate change and the expansion of the carbon credit market, carbon capture and biofuel production, such as ethanol fermentation, generate very pure CO2 streams (>95% CO2). This type of CO2 gas should be converted to products with very high efficiency (>90%), with little CO2 emitted into the air. This presents an engineering challenge due to the low solubility of CO2 in aqueous solutions. The second type of CO2 gas is flue gas emitted from various point sources, such as power plants, steel manufacturers, cement manufacturers, and chemical processing equipment. The carbon content of flue gas varies and is relatively low (<15% CO2) due to the presence of large amounts of inert gases, such as nitrogen. At point sources, flue gas is often vented to the air after scrubbing and dust removal. Reasonable carbon capture efficiency is also desirable for this type of gaseous waste required by process decarbonization. The novel Y-type bioreactor can handle both types of CO2 gas streams as carbon sources. The bioreactor 1 according to the present invention makes it possible to obtain conversion of CO with high productivity of the desired product by avoiding the problems associated with the low solubility of O and H in aqueous solutions and by achieving high gas mass transfer rates by using pure gas instead of the gas mixtures used in conventional gas fermentation. For example, replacing the gas mixture (20% O) with pure oxygen gas (100% O) can increase the oxygen mass transfer rate by a factor of 5.

[0035] The process according to the present invention provides very high productivity. When the cell density reaches a critical level determined by the gas mass transfer rate in the bioreactor, the microbial cells can be harvested for single-cell protein (SCP) production. This fermentation control strategy allows aerobic bacteria to avoid being limited by dissolved oxygen even at relatively high cell densities.

[0036] References: [1] Jian Yu, Yue Lu(2019). Carbon dioxide fixation by a hydrogen-oxidizing bacterium: biomass yield, reversal respiratory quotient, stoichiometric equations and bioenergetics, Biochemical Engineering Journal. [2]Jian Yu(2014).Bio-based products from solar energy and carbon dioxide,Trends in Biotechnology 32:5-10. [3]Jian Yu(2018)Fixation of carbon dioxide by a hydrogen-oxidizing bacterium for value-added products.World Journal of Microbiology and Biotechnology. [4]Yue Lu, Jian Yu(2017).Gas mass transfer with microbial CO2 fixation and poly(3-hydrobybutyarte)synthesis in a packed bed bioreactor,Biochemical Engineering Journal,122:13-21. [5]Shimin Kang,Jian Yu(2015).Reaction routes in catalytic reforming of poly(3-hydroxybutyrate)into renewable hydrocarbon oil,RSC Adv.,2015,5,30005-30013. [6]Yue Lu,Jian Yu(2017).Comparison analysis on the energy efficiencies and biomass yields in microbialCO2 fixation,Process Biochemistry,62:151-160.

Claims

1. A Y-type bioreactor for microbial gas fermentation, comprising an aeration chamber (2) provided with means (3) for mixing the culture broth contained therein, at least a first gas supply chamber (4) and a second gas supply chamber (5) suitable for containing a first gas and a second gas, respectively; at least a first duct (6) configured to connect the outlet port (7) of the aeration chamber (2) to at least one respective inlet (4a, 4b, 5a, 5b) of each of the first chamber (4) and the second chamber (5) and to introduce the culture broth coming from the aeration chamber (2) into the first gas supply chamber (4) and the second gas supply chamber (5), thereby mixing the culture broth with the first gas and the second gas, respectively; at least a second duct (8) configured to connect the outlet terminal (4c) of said first gas supply chamber (4) to said aeration chamber (2); at least a third duct (9) configured to connect the outlet terminal (5c) of said second gas supply chamber (5) to said aeration chamber (2); at least one pump (10) arranged in blocking the first duct (6) for conveying the culture broth; at least one outlet channel (11) having an inlet (11a) located between the mid-height and the top of the aeration chamber (2) and configured to continuously allow a portion of the culture to flow out; A Y-shaped bioreactor comprising:

2. The at least one first gas supply chamber (4) contains hydrogen (H ２ ) gas and mix it with the culture broth coming from said at least one first duct (6), and said second duct (8) is configured to receive hydrogen (H ２ 2. The bioreactor according to claim 1, characterized in that it is configured to transfer a mixture of the culture broth and the septic tank from the outlet port (4c) of the first chamber (4) to at least one respective inlet (12) of the aeration chamber (2).

3. The at least one second gas supply chamber (5) supplies carbon dioxide (CO ２ ) gas and mix it with the culture broth coming from said at least one first duct (6), and said third duct (9) is configured to receive dissolved carbon dioxide (CO ２ 10. A bioreactor according to any of the preceding claims, characterized in that it is configured to transfer a mixture of the culture broth and the septic tank (10) from the outlet port (5c) of the second chamber (5) to at least one respective inlet (13) of the aeration chamber (2).

4. 10. A bioreactor according to any of the preceding claims, characterized in that the aeration chamber (2) comprises an oxygen dispenser (14) and a gas dispenser (15) configured to introduce oxygen and a gas selected from air and flue gas separately, independently and uniformly into the culture broth present inside the aeration chamber (2).

5. 5. Bioreactor according to claim 4, characterized in that the oxygen dispenser (14) is located at a lower altitude than the gas dispenser (15) selected from air and / or flue gas.

6. 10. A bioreactor according to any of the preceding claims, characterized in that the at least one first inlet (4a, 4b) of the first gas supply chamber (4) is connected to at least one nozzle configured to spray the culture broth containing a microbial suspension coming from the first duct (6) into the internal cavity bounded by the first gas supply chamber (4).

7. 10. A bioreactor according to any of the preceding claims, characterized in that the at least one first inlet (5a, 5b) of the second gas supply chamber (5) is connected to at least one nozzle configured to spray the culture broth containing a microbial suspension coming from the first duct (6) into the internal cavity bounded by the second gas supply chamber (5).

8. 10. Bioreactor according to any of the preceding claims, characterized in that the aeration chamber (2) is equipped with at least one device selected from probes, indicators and regulators that refer to at least one parameter selected from the fermentation temperature, pH, mixing rate of the culture broth, dissolved oxygen concentration, air flow rate, pressure in the corresponding chamber, liquid level, etc.

9. CO ２ 1. A process for producing single-cell proteins from (i) continuously feeding an aqueous mineral solution to a culture of hydrogen-oxidizing microorganisms in the aeration chamber (2) of a bioreactor according to any one of the preceding claims to obtain a suspension; (ii) adding H to the first gas supply chamber (4) of the bioreactor ２ Supplying gas; (iii) adding CO to the second gas supply chamber (5) of the bioreactor ２ Supplying gas; (iv) supplying O 2 through the oxygen dispenser (14) of the bioreactor; ２ Supplying gas; (v) supplying air or flue gas through the gas dispenser (15) of the bioreactor; (vi) continuously recirculating the suspension between the aeration chamber (2) and the first gas supply chamber (4) and the second gas supply chamber (5), wherein the first gas supply chamber (4) is aerated with stagnant H ２ The culture droplets are sprayed into the gas phase and in the second gas supply chamber (5) CO ２ sprayed into a stagnant phase of the gas; (vii) continuously discharging said suspension of microorganisms in aqueous mineral solution from said aeration chamber (2); (viii) recovering the dried cell mass or single cell proteins from the discharged suspension of step (vii); The process includes:

10. 10. The process of claim 9, wherein throughout steps (i) to (vii), the culture in the aeration chamber (2) is maintained at a temperature of 30±1° C. and a pH of 7±0.

5.

11. 11. The process of claim 9 or 10, wherein the hydrogen-oxidizing microorganism is a strain of Ralstona eutropha.

Images