Membrane biofilm reactors, systems, and methods for producing organic products

Abstract

The present disclosure is directed toward membrane biofilm reactors primarily comprising microorganisms that produce chemical fuel products or precursors thereof. Reactors of the present disclosure can primarily comprise acetogens, a methanotrophs, and/or Methanosarcina acetivorans.

Claims

1. A method of producing and extracting at least one of an alkane, an organic acid, or an alcohol comprising: providing a gas feedstock composition to at least one hollow fiber membrane of a membrane biofilm reactor, the gas feedstock composition alternating between a primary composition and a secondary composition, the membrane biofilm reactor comprising a biofilm disposed on the hollow fiber membrane in an aqueous medium; and extracting at least one of an alkane, an organic acid, or an alcohol from the aqueous medium; wherein the primary composition comprises CH.sub.4 and the secondary composition comprises CH.sub.4 and CO.

2. The method of claim 1, wherein the primary composition comprises a C-1 substrate and the secondary composition comprises one of N.sub.2, CO.sub.2, or 100% NH.sub.3.

3. The method of claim 2, wherein the secondary composition comprises CH.sub.4, CO, and N.sub.2.

4. The method of claim 2, wherein the secondary composition comprises CH.sub.4, CO, and CO.sub.2.

5. The method of claim 2, wherein the secondary composition comprises CH.sub.4, CO, and NH.sub.3.

6. The method of claim 1, wherein the primary composition comprises 5-15:5-15:70-90 of CH.sub.4, O.sub.2, and N.sub.2.

7. The method of claim 1, wherein the secondary composition comprises O.sub.2.

8. The method of claim 1, wherein the gas feedstock composition alternates between the primary composition and the secondary composition for pH adjustment of the aqueous medium.

9. The method of claim 1, wherein the primary composition and the secondary composition are both below a lower flammability limit.

10. The method of claim 1, wherein the primary composition is different from the secondary composition.

11. The method of claim 1, wherein the primary composition further comprises O.sub.2 and N.sub.2, and is substantially non-combustible.

12. The method of claim 1, wherein the biofilm consists essentially of thermophilic microorganisms.

13. The method of claim 1, wherein the biofilm comprises at least a majority of at least one of the following microorganisms: methanotrophs, and Methanosarcina acetivorans.

14. The method of claim 1, wherein the aqueous medium has a temperature of 50° C. to 70° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following drawings illustrate by way of example and not limitation. For the sake of brevity and clarity, every feature of a given structure may not be labeled in every figure in which that structure appears. Identical reference numbers do not necessarily indicate an identical structure. Rather, the same reference number may be used to indicate a similar feature or a feature with similar functionality, as may non-identical reference numbers.

(2) FIG. 1 illustrates a schematic showing gas transfer in conventional bubbled systems.

(3) FIG. 2 illustrates a schematic of a cross-section of a hollow-fiber membrane with a biofilm formed on the outer surface thereof, a liquid medium surrounding the biofilm, and gaseous feedstock delivered through the hollow fiber membrane to the biofilm.

(4) FIG. 3 illustrates a schematic of a membrane biofilm reactor.

(5) FIG. 4A illustrates a schematic of a membrane biofilm reactor and a separation unit with a circulation loop connecting the reactor to the separation unit.

(6) FIG. 4B illustrates a schematic of a membrane biofilm reactor and a vacuum distillation column disposed above vessel.

(7) FIGS. 5A-5C illustrate a schematic of a gas outlet pattern of a membrane module wherein a first gaseous feedstock composition is delivered to a hollow fiber membrane through the outlets represented by the black circles and a different gaseous feedstock composition is delivered to the outlets represented by the white circles.

(8) FIGS. 5D(i) and 5D(ii) illustrate a schematic of a cross-section of a hollow fiber membrane with a biofilm formed on the outer surface thereof.

(9) FIG. 6 illustrates a schematic of a sheet comprising a woven hollow fiber membrane with a partial cutaway and unwrapping to illustrate the layers of woven membrane sheets.

DETAILED DESCRIPTION OF THE INVENTION

(10) A Membrane Biofilm Reactor (MBfR) employs efficient, direct supply of gaseous substrates to microorganisms that form a biofilm on the outside of a gas-transfer membrane (see e.g., U.S. Pat. Nos. 6,387,262 and 7,618,537 incorporated by reference in their entirety). Major increases in the overall membrane volumetric mass transfer coefficient (K.sub.La) can be achieved through MBfRs. Direct delivery of a gaseous feedstock to a biofilm results in a reaction zone that has a continuous supply of the gaseous substrates without a diffusion limitation arising from a liquid film that forms in all bubbled gas transfer systems, such as CSTRs (compare, e.g., FIGS. 1 to 2). In an MBfR, the liquid film is replaced by the membrane wall; thus, as long as the resistance to mass transfer in membrane is lower than through a liquid film in a conventional system (i.e. 1/Hk.sub.M is lower than 1/k.sub.L), higher gas delivery rates will result.

(11) In accordance with the present invention, an MBfR is utilized to produce organic products, such as organic acids, polyhydroxyalkanoates, alcohols, and alkanes. The organic products are the metabolic byproducts of the microorganisms supported in the MBfR. These products can be utilized as liquid fuel or precursors to liquid fuels or other industrial chemical feedstocks. In some embodiments, the metabolic byproduct can be at least one of the following: methanol, ethanol, propanol, n-butanol, n-hexanol, formaldehyde, formic acid, acetic acid, lactic acid, succinic acid, butyric acid, or any combination thereof. Using an MBfR to achieve high-rate liquid biofuel production is a novel strategy that is targeted to overcoming the challenges of using gaseous substrates. Delivering gaseous feedstock or a combination of gaseous feedstock directly to a microbial biofilm MBfR provides superior gas transfer, even though all of the gases have very low water solubility.

(12) FIG. 2 schematically shows a cross-section of a hollow fiber membrane 110 with a biofilm layer 115 developed on the outer surface 111 of the membrane. FIG. 3 provides a schematic illustration of a MBfR 100 for providing gaseous feedstock for an active biofilm 115 consisting essentially of microorganisms that produce desired metabolic byproducts. Microorgansims that are suitable for the production of organic products include acetogens (including some homoacetogens), methanotrophs, and the methanogen, Methanosarcina acetivorans. These microorganisms have capabilities to convert gaseous substrates to value added chemical fuels, as shown in Table 2.

(13) TABLE-US-00002 TABLE 2 Examples of microorganisms that utilize gaseous substrates to produce valuable chemical products Microorganism Chemical products category Gases utilized formed Acetogens H.sub.2, CO.sub.2, CO Organic acids, alcohols Methanotrophs CH.sub.4, O.sub.2 (aerobic Methanol, members) Formaldehyde, polyhydroxybutyrate Methanosarcina CO Acetate, formate, some acetivorans methane

(14) A majority of biofilm 115 can comprise or biofilm 115 can consist essentially of acetogens, methanotrophs, Methanosarcina acetivorans, or a combination thereof. In some embodiments, biofilm 115 can consist essentially of a pure or mixed culture—such as Clostridium ragsdalei, Butyribacterium methylotrophicum, Clostridium ljungdahlii, Clostridium carboxidivorans, Acetobacterium woodii, Clostridium coskatii, Clostridium autoethanogenum, Acetobacterium bacchi, Methanosarcina acetivorans, Moorella thermoacetica, Thermoanaerobacter kivui, Methylosinus trichosporium (particularly, the OB3b strain), or combinations thereof—which can generate the organic products from the gaseous feedstock, depending on the gaseous stock and culture selected. Those skilled in the art will appreciate that numerous combinations of gaseous feedstock and culture can be selected as desired for generating a particular liquid product desired. In addition, in some embodiments, other microorganisms, which are not associated with the bio-production of organic products, can be present in a smaller percentage to facilitate a healthy ecology for efficient production of organic products, and these can include heterotrophic homoacetogens, such as from the Clostridium genera and Spirochaetes, Chlorojlexi phyla. MBfR 100 can comprise biofilm 115 that amounts to at least 2 g/L of vessel, 5 g/L of vessel, 10 g/L of vessel, 20 g/L of vessel, 25 g/L of vessel, 30 g/L of vessel, or more, or any range there between.

(15) The interface between biofilm 115 and membrane 110 keeps the liquid and gas phases separated from each other. As shown in FIG. 2, gaseous components 120 enter the system as gas stream and flow into a lumen 112 between a surrounding membrane wall. The gaseous feedstock diffuses through membrane 110 for consumption by the microbes in biofilm 115 that adhere to outer surface 111. Similarly, in lieu of a hollow fiber membrane configuration, a flat sheet membrane sealed and connected along the perimeter to another flat sheet membrane to form central space can also be used.

(16) With reference to FIG. 3, MBfR 100 can comprise a vessel 150 defining a volume and a plurality of hollow fiber membrane elements 110 disposed within the volume. The plurality of hollow fiber membranes 110 are coupled to one or more modules 170 defining at least one gas inlet and at least one conduit for delivering the gaseous substrate to lumen 112 of membranes 110. Vessel 150 surrounds the plurality of membrane elements 110 in MBfR 100 and retains an aqueous medium 120 for growth and maintenance of biofilm 115 on the outer surface of membrane 110.

(17) In some embodiments, medium 120 can be pressurized to increase the process gas transfer rate through the hollow fiber walls. In one embodiment, the membrane lumen is pressurized in the range of 2 to 1250 psig. Vessel 150 can be configured to a pressure vessel. Such vessels can be made of fiber reinforced plastic (FRP) composite materials for maximum operating pressures ranging from 150 psig to 1250 psig. Vessel 150 can comprise any suitable dimension to facilitate efficient production of the desired metabolic byproduct. In some embodiments, vessel 150 is configured to control temperature and pH of medium 120, which contains nutrients needed to sustain the activity of the microbial cells. Medium 120 can be stirred to provide adequate mixing and sparged with a suitable gas, if necessary, to maintain a suitable aqueous environment. A re-circulating liquid conduit 125, 126 can re-circulate medium 120 through vessel 150. To facilitate extraction of the metabolic byproducts, with reference to FIGS. 4A and 4B, medium 120 can flow from vessel 150 through outlet 151 into conduit 125 to a separation unit 130 to recover metabolic byproducts. Conduit 126 can return the remaining medium 120 from unit 130 to vessel 150 via inlet 152 with the aid of a pump at rate recorded by a flow meter. Separation unit 130 removes the desirable organic product from medium 120, while leaving a majority of the water and residual nutrients in the remaining medium 120. In some embodiments, a nutrient feed conduit 127 is connected to conduit 126 or vessel 150 to compensate for the amount of water removed and to replenish nutrients as needed. A mixing unit 160 can facilitate mixing and/or testing of returning medium 120 to vessel 150.

(18) The flow rates of medium 120 recirculated can be selected so that there is no significant liquid boundary layer that impedes mass transfer near the liquid-facing side of the membrane and there is no excessive shear that may severely limit the attachment and/or formation of biofilm 115 on membrane surface 111. The superficial linear velocity of the liquid tangential to the membrane should be in the range of 0.01 to 20 cm/s and preferably 0.05 to 5 cm/s. In addition to the liquid linear velocity, the thickness of biofilm 115 can be controlled by other means to create shear on the liquid-biofilm interface, including scouring of the external membrane surface with gas bubbles and free movement of the hollow fibers. Also, operating conditions that affect the metabolic activity of the microbial cells and the mass transfer rates of gases and nutrients can be manipulated to control the thickness of biofilm 115. The thickness of biofilm 115 can be in the range of 1-500 μm, preferably 5-200 μm.

(19) Upon the utilization of gaseous feedstock delivered through membrane, a gradient for their transport from the gas feed side is created due to biochemical reaction on the membrane liquid interface. This reaction creates liquid-fuel or chemical feedstock that diffuses into the liquid circulating past the biofilm. Thus, the very large surface areas of the membrane pores are usable for gas transfer to the biofilm and the product is recovered from the liquid side.

(20) Depending on the nature of the desired product, a number of technologies can be used for product recovery. For example, distillation (such a vacuum or fractional distillation), dephlegmation, pervaporation, and liquid-liquid extraction can be used for the recovery of alkanes and organic alcohols, such as methanol, ethanol, n-butanol, and the like, whereas electrodialysis and ion-exchange can be used for the recovery of organic acids in ionic form, such as acetate, butyrate, and other ionic products. In some embodiments, wherein the reactor is operated at elevated temperatures, a vacuum distillation column 130 can be connected to vessel 150, as illustrated in FIG. 4B.

(21) The membranes can be configured into typical modules 170 as shown in FIGS. 4A-4B for hollow fibers 110. The gas flows into the fine fibers that are bundled and potted inside a shell through which the gaseous feedstock is distributed to the lumen of the fibers 110. Very high surface area gaseous distribution in the range of 1000 m.sup.2 to 5000 m.sup.2 per m.sup.3 of the reactor can be achieved with such modules. The gaseous substrate fed into the lumen of hollow fiber membrane 110 passes to biofilm disposed on the outer surface of hollow fibers. In this manner, gaseous substrates are converted into a liquid product that mixes with the aqueous medium.

(22) In some embodiments, C-1 gases are provided to the biofilm, i.e., CH.sub.4, CO, and/or CO.sub.2. In further embodiments, depending on the type of microorganism, inorganic gases, such as H.sub.2, O.sub.2, and N.sub.2, also can be delivered to the biofilm. Table 2 provides examples of gaseous components that can be provided to a biofilm. In some embodiments, the gas feedstock comprises one of the following % mixtures: 75-85:15-25 of H.sub.2 and CO.sub.2, 55-65:25-35:5-15 of H.sub.2, CO, and CO.sub.2, 15-21:21-27:3-9:45-60 of H.sub.2, CO, CO.sub.2, and N.sub.2; and 5-15:5-15:70-90 of CH.sub.4, O.sub.2, and N.sub.2. In some embodiments, the feedstock components are fed through the lumen of the hollow fiber membrane as a mixture. In other embodiments, the feedstock components are fed to separate and distinct membranes. Delivering separately can reduce risk of combustion, as the case would be with a combined mixture of CH.sub.4 and O.sub.2.

(23) To facilitate delivery of feedstock components to separate and distinct membranes, the shell of the membrane module is configured to comprise at least two gas inlets and at least two distinct gas conduits, each of which are in fluid communication with the lumens of a separate plurality of membranes. The two gas conduits can branch to deliver respective feedstock components through gas outlets 171 to the plurality of hollow fiber membranes 110 in a certain pattern (e.g., as shown in FIGS. 5A-5C) and a particular spacing to ensure delivery of the different gaseous compositions to a biofilm 115 that is sandwiched between two neighboring hollow fiber elements 110 (e.g., as shown in FIGS. 5D(i) and 5D(ii)). For example, a hollow fiber membranes 110a receiving one gaseous composition 120a will have at least one neighboring hollow fiber membrane 110b receiving a different gaseous composition 120b. The distance between these two membranes 110a, 110b is such that biofilm 115 forms between them and receives gaseous feedstock from both. In some embodiments, the spacing between the neighboring membranes is less than 600 μm, less than 500 μm, less than 400 μm, or less than 100 μm.

(24) For example, again with reference to FIGS. 5D(i) and (ii), an MBfR can be configured to supply air 120a and CH.sub.4 120b in separate, alternating fibers 110a, 110b that are placed in sufficiently close proximity to each other. By separating the two gases 120a, 120b, a high concentration of both reactants in each fiber can be obtained. In addition, pure CH.sub.4 in one set of fibers and air in the other set of fibers can continuously flow through the respective hollow fibers at a rate that does not deplete the O.sub.2 concentration significantly. The high gas concentrations can translate into high fluxes, e.g., up to 20 times higher compared to a gas mix approach. In various embodiments, one type of hollow fiber 120a can be placed in close proximity (e.g., <500 μm) to the other type 120b to ensure that biofilm 115 grows in between the two types 120a, 120b. Counter diffusion of CH.sub.4 and O.sub.2 can occur within biofilm 115, where the gases can be quickly consumed. Because this configuration mixes the gases inside biofilm 115 (which contains mainly water), no explosive gas mixture is formed. Furthermore, CH.sub.4 off-gassing is minimized. Balancing the fluxes between the two streams can be achieved by controlling the gas pressure to each type of fiber.

(25) In various embodiments, with reference to FIG. 6, MBfR can comprise a membrane sheet 180 comprising a plurality of hollow fiber filaments woven into sheet 180 and wrapped around a perforated core 185 and an aqueous medium flows out radially. The lumen of each hollow fiber can be pressured with a gaseous feedstock for diffusion across the membrane wall.

(26) To facilitate efficient generation of organic products, flux rates of gaseous feedstock can be varied through the selection of the membrane material, thickness, and area; the gas pressure within the lumen of the membrane, and the temperature of the reactor. For example, H.sub.2 fluxes in an MBfR have been shown to depend on the fiber material and H.sub.2 pressure in the lumen of the fiber: composite (2.75 g of H.sub.2 per m.sup.2 per day at 2 bar), polypropylene (0.64 g of H.sub.2 per m.sup.2 per day at 3 bar), and polyester (0.31 g of H.sub.2 per m.sup.2 per day at 3.3 bar) at 25° C. (Tang et al, 2012). Note that these are not the maximum fluxes possible, as these measurements are made under non-limiting conditions. Fluxes are inversely proportional to the pressure in the lumen of the membrane and directly proportional to the diffusion coefficient and increases with temperature.

(27) In order to maintain a productive MBfR, the composition of the gas supply may be varied. Varying the gas supply can be useful for pH adjustment, for promotion of certain metabolic reaction pathways, or for promoting selective enrichment of desired microorganisms to inhibit undesired microorganisms. For example, gaseous substrates such as H.sub.2 and CO.sub.2 are consumed through autotrophic microbial reactions such as methanogenesis and homoacetogenesis, as shown below:
HCO.sub.3.sup.−+4H.sub.2+H.sup.+.fwdarw.4CH.sub.4+3H.sub.2O  (6)
2HCO.sub.3.sup.−+4H.sub.2+H.sup.+.fwdarw.4CH.sub.3COO.sup.−+4H.sub.2O  (7)

(28) As demonstrated above, for the same amount of bicarbonate, methanogenesis consumes twice as many protons as homoacetogenesis and raises the reactor pH more significantly than homoacetogens, thereby affecting the microbes and the corresponding volumetric production rates. Nonetheless, pH can be managed in the MBfR biofilms by alternating the gas supply in the fibers between the preferred gaseous substrates and 100% N.sub.2 gas, which enables the pH to re-adjust. Moreover, effective community management can be achieved by altering the gas mix to include CO, which promotes selective enrichment of desired partners that produce chemicals that inhibit methanogens. Specifically, CO has an inhibitory effect (only 50% of optimum growth) on methanogens (at headspace pressure of 0.4 atm) and sulfate reducers (at headspace pressure of 0.2 atm), while favoring optimal growth of acetogens at 1-2 atm headspace pressure of CO (See Sipma et al., 2006; Van Houten et al., 1996). Another example for varying the gaseous substrate includes supply of CO.sub.2 or NH.sub.3 gas to add the required acidity or alkalinity or C/N source, or both.

(29) In various embodiments, the MBfR can be configured to operate at higher temperatures (e.g., 50-70° C. or more specifically, 55-60° C.) to grow thermophilic microorganisms with sustained or even higher gas delivery fluxes. In addition, higher temperature further maximizes the liquid-gas transfer rate in an MBfR due to lower viscosity of the gas, although the gas is less soluble in this temperature range. However, since the gas is fed directly to the biofilm in an MBfR, the solubility is less important that the liquid-gas transfer rate, and a higher temperature increases the reaction rate. Moreover, the production of ethanol at higher temperature reduces the distillation overhead by a considerable amount than what occurs in the mesophilic temperature ranges. For an ethanol titer of 7% by wt, the typical distillation energy input is estimated to be 19,000 MJ/ton of ethanol. Aqueous ethanol volatilizes at a significantly higher rate above 50° C., facilitating membrane assisted vapor stripping or vacuum/continuous distillation, as opposed to conventional distillation, which results in significantly lower energy requirements.

(30) The above specification and examples provide a complete description of the structure and use of an exemplary embodiment. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the illustrative embodiment of the present membrane biofilm reactor, system, and method is not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, components may be combined as a unitary structure and/or connections may be substituted. Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments.

(31) The claims are not to be interpreted as including means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.

REFERENCES

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