WO2010056458A2

WO2010056458A2 - Hydrogen production by biological water-gas shift reaction using carbon monoxide

Info

Publication number: WO2010056458A2
Application number: PCT/US2009/060798
Authority: WO
Inventors: Jens Wiik Jensen; Eric Dickman
Original assignee: UNI-CONTROL LLC
Current assignee: UNI-CONTROL LLC
Priority date: 2008-11-12
Filing date: 2009-10-15
Publication date: 2010-05-20
Anticipated expiration: 2011-05-12
Also published as: WO2010056458A3

Abstract

A hydrogen production process is disclosed that includes (a) mixing together at least one species of microorganism and a liquid nutrient medium lacking a carbon source, to form a mixed liquor solids suspension (MLSS); (b) photoactivating a metabolic pathway in the microorganism; (c) incubating a stream of the resulting activated MLSS in the absence of light, and without a carbon source, to obtain a first stream comprising a CO-starved activated MLSS flowing at a first flow rate; (d) saturating a liquid stream with a CO-containing gas, to form a CO-saturated stream; (e) incubating, in the absence of light, a second stream comprising the CO-saturated stream and at least a main portion of the first stream; and (f) forming from the second stream H₂ and CO₂ products and a stream of CO-depleted MLSS.

Description

HYDROGEN PRODUCTION BY BIOLOGICAL WATER-GAS SHIFT REACTION USING CARBON MONOXIDE

FIELD OF THE INVENTION

[0001] This invention generally relates to the microbial production of hydrogen gas by fermentation of carbon monoxide, and more particularly to biological processes that include a water-gas shift reaction.

BACKGROUND

[0002] Hydrogen (H₂) is an attractive alternative to fossil fuels as a portable, non-polluting source of energy. Today, hydrogen gas is usually produced by reforming fossil sources (petroleum, natural gas and coal) to produce synthesis gas (syngas), which is a mixture of hydrogen and carbon monoxide (CO). Syngas is customarily generated by steam or dry reforming or partial oxidation of natural gas or liquid hydrocarbons, by gasification of coal, or by waste-to-energy gasification processes (e.g., biomass gasification). The relative amounts of CO and H₂ in a syngas product varies depending upon the way it is generated. Existing technologies for separating and purifying the hydrogen component of syngas usually involve pressure swing adsorption (PSA), membrane separation, or chemical reaction on solid iron oxide and calcium oxide beds, with regeneration of the solids. [0003] Different technologies such as electrolysis and thermolysis have been investigated for producing hydrogen from water. Still other technologies used for hydrogen production include inorganic chemical reduction and various biological reactions. The biological production of hydrogen using photosynthetic and fermentative microorganisms has been described. Among these microorganisms are certain photosynthetic bacteria that contain a carbon monoxide oxidation pathway in which the water-gas shift reaction occurs, according to the stoichiometry:

CO + H₂O → CO₂ + H₂ (1)

This reaction has been reported for Rubrivivax gelatinosus, Rhodospirillum rubrum, Rhodopseudomonas palustris, and others. The feeding of synthesis gas to certain microbial cultures capable of carrying out the water-gas shift reaction to convert the CO component of the syngas into additional H₂ has been described. Carbon monoxide reportedly induces the de novo synthesis of a hydrogenase enzyme linking to the CO oxidation pathway in Rhodospirillum rubrum. [0004] As the CO substrate is of low solubility in an aqueous solution, mass transfer of CO into the microbial culture medium is likely the rate-limiting step for the biological water-gas shift reaction. A challenge in developing synthesis gas fermentation processes is providing for efficient gas mass transfer, and resolving microbial toxicity issues with respect to CO and CO₂ gases. There is continuing interest in the development of practical ways to produce bulk quantities of hydrogen.

SUMMARY

[0005] In accordance with certain embodiments of the invention, a process for production of hydrogen by a biological water-gas shift reaction is provided that comprises: (a) mixing together at least one species of microorganism and a liquid nutrient medium lacking a carbon source, to form a mixed liquor solids suspension (MLSS), wherein the microorganism has a metabolic pathway that includes a water-gas shift reaction; (b) photoactivating the metabolic pathway in the microorganism; (c) incubating a stream of the resulting activated MLSS in the absence of light, and without a carbon source, to obtain a first stream comprising a CO- starved activated MLSS flowing at a first flow rate; (d) saturating a liquid stream with a CO- containing gas, to form a CO-saturated stream; (e) incubating, in the absence of light, a second stream comprising the CO-saturated stream and at least a main portion of the first stream, wherein the second stream flows at a second flow rate that is faster than the first flow rate; and (f) forming from the second stream H₂ and CO₂ products and a stream of CO- depleted MLSS.

[0006] In some embodiments, at least steps (e) and (f) are carried out at superatmospheric pressure. In some embodiments, step (e) comprises injecting the CO-saturated stream into the at least a main portion of the first stream. In some embodiments, multiple streams of the CO-saturated liquid are injected. Injecting in this manner creates a gas-lift mixture comprising the CO-saturated liquid and undissolved carbon monoxide in combination with the main portion of the first stream, in some embodiments.

[0007] In some embodiments, step (g) includes removing a portion of the CO-depleted MLSS and subjecting the portion to liquid/solids separation, to obtain a recycled liquid stream and a sludge fraction. In step (d), saturating a liquid stream with a CO-containing gas comprises saturating the recycled liquid stream with a stream of carbon monoxide to form the CO-saturated stream, in certain embodiments.

[0008] In some embodiments, an above-described process comprising step (f), which includes combining at least a portion of the CO-depleted MLSS from (f) with the activated MLSS in step (c). In some embodiments, step (d) includes removing a portion of the CO- starved activated incubation mixture to form the liquid stream. In some embodiments, the pH of the MLSS at any or all stages of the process is regulated at about pH 9. [0009] In some embodiments, in step (d), the liquid stream is passed into a CO-saturation vessel, in the absence of light, together with a stream of carbon monoxide-containing gas, to form the CO-saturated liquid. In step (b), photoactivating includes exposing the microorganisms to visible light sufficient to initiate the metabolic pathway in a first reactor chamber. In some embodiments, step (c), during the incubation period the activated MLSS is passed into a second reactor chamber, in the absence of light.

[0010] In step (b), photoactivating includes exposing the microorganisms to visible light, sufficient to initiate the metabolic pathway in a first reactor chamber, in some embodiments. Some embodiments of an above-described process also includes step (f '), collecting the H₂- containing off-gas in the first reactor chamber, for subsequent vacuum withdrawal, for example.

[0011] In some embodiments, step (d) includes varying the concentration of CO in the CO- containing gas inversely to the CO concentration, if any, in the H₂-containing off-gas in the first reactor chamber. In some embodiments, an above-described process excludes atmospheric oxygen during operation of the fermentation system. In some embodiments, an above-described process includes regulating the temperature of the MLSS in the range of about 20-65⁰C, and in some cases the temperature is in the range of about 37-50⁰C. [0012] In some embodiments, the process comprises regulating the pH of the MLSS to about pH 7.5 - 9.5. In some embodiments, the process further comprises the addition of an alkaline pH adjustment agent to the MLSS, whereby a portion of the CO₂ product is converted to bicarbonate. In some embodiments, the conversion of a portion of CO₂ product reduces CO2-related toxic effects on the microorganism in the MLSS, and reduces the amount of CO₂ product released into the air by the process.

[0013] These and other embodiments of the present invention, and various features and potential advantages will be apparent with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Fig. 1 is a schematic illustration of a system for carrying out a process according to an embodiment of the invention. [0015] Fig. 2 is a schematic illustration of a system for carrying out a process according to another embodiment of the invention.

[0016] Fig. 3 is a schematic illustration of a system for carrying out a process according to another embodiment of the invention.

[0017] Fig. 4 is a schematic illustration of a system for carrying out a process according to another embodiment of the invention.

[0018] Fig. 5 is a schematic illustration of a system for carrying out a process according to another embodiment of the invention.

[0019] Figs. 6A and 6B are horizontal cross sections of respective embodiments of a bioreactor as employed in the system of either Fig. 1 or 2. Fig. 6A shows an embodiment with one slow-flow zone and one fast-flow zone. Fig. 6B shows an embodiment with one slow-flow zone and two fast-flow zones.

[0020] Fig. 7 is a schematic block flow diagram illustrating a process for generating molecular hydrogen (H₂) by microbial fermentation of the CO component of synthesis gas, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0021] A method of producing hydrogen via biological fermentation of carbon monoxide (CO) as the food source for selected microorganisms has been discovered. For the purposes of this disclosure, the terms "CO-fermentation" and "fermentation of carbon monoxide" refer to the production of energy by microorganisms by metabolic pathway(s) that include a water gas shift reaction (reaction (1), above), to produce hydrogen and carbon dioxide products, using carbon monoxide as the carbon-containing food source. The CO-fermentation microorganisms are suspended in a nutrient medium initially lacking a carbon source, exposed to a light source to activate a metabolic pathway that includes a water-gas shift reaction, and then incubated anaerobically in a slow-flowing, carbon monoxide-depleted stream. The stream of CO-starved microorganisms is then combined with a stream of CO- saturated liquid to provide a CO-enriched mixed liquor solids suspension (MLSS), which is then rapidly ejected to create a fast- flowing, carbon monoxide -rich stream. The metabolically activated microorganisms metabolize the dissolved CO to form hydrogen (H₂) and carbon dioxide (CO₂) products. The gas coming out of solution from the ejected CO- saturated liquid creates a gas-lift pump that facilitates circulation of the microorganisms and nutrients from the fast-flowing phase back to the slow-flowing phase. Product H₂ and CO₂, and other undissolved gases, are removed from the fast-flowing phase prior to return of the MLSS to the slow-flowing stream.

[0022] A system suitable for carrying out an exemplary fermentative hydrogen production process is illustrated schematically in Fig. 1. System 10 includes reactor 30, one or more inlet lines 20 for feeding nutrients, water and microorganisms, carbon monoxide (CO)- saturation chamber 80, MLSS recycle unit 90, CO feed unit 100 and H₂ recovery unit 110. Reactor 30 includes head 40, slow-flow zone 50, injection assembly 60 and fast-flow zone 70. Injection assembly 60 is coupled to CO-saturation chamber 80. Head 40 is coupled to fast- flow zone 70, H₂ recovery unit 110, and MLSS recycle unit 90. Recycle unit 90 is also coupled to carbon monoxide (CO feed unit 100 and CO saturation chamber 80, and also includes a sludge outlet line 94. For the purposes of this disclosure, the term "coupled to" includes fluid communication (i.e., flow of gas, liquid or both) between the coupled components, either directly or indirectly, unless specified. Indirect fluid communication means that there may be one or more intervening components between the coupled components. The reactor, and all parts exposed to the MLSS, are made of stainless steel or are clad with stainless steel, or other suitable material.

[0023] In Fig. 2, a variation of system 10' is shown. System 10' includes reactor 30' one or more inlets 20' for introducing nutrients, water and microorganisms into reactor 30', MLSS CO-saturation chamber 80, MLSS recycle unit 90, CO feed unit 100' and H₂ recovery unit 110'. Reactor 30' includes head 40', slow-flow zone 50', injection assembly 60' and fast- flow zone 70'. Slow-flow zone 50' and injection assembly 60' are separately coupled to MLSS CO-saturation chamber 80'. Head 40' is coupled to fast-flow zone 70', H₂ recovery unit 110', and MLSS recycle unit 90'. Recycle unit 90' is also coupled to an inlet line 20' and also includes a sludge outlet line 94'. A carbon monoxide feed gas unit 100' is coupled to carbon monoxide-saturation chamber 80'. In some embodiments, unit 100' is coupled directly to chamber 80', and in other embodiments is coupled indirectly via line 52' connecting zone 50' to chamber 80', as shown in Fig. 2. In some embodiments, the reactor assembly is a more basic configuration that does not require either H₂ recovery unit 110 or MLSS recycle unit 90, or both. Instead, the reactor assembly has respective outlets for H₂ and MLSS. [0024] In an exemplary embodiment depicted in Fig. 3, system 200 includes reactor 230, one or more inlet lines 241 for feeding nutrients, water and microorganisms, carbon monoxide (CO)-saturation chamber 280, MLSS recycle unit 290, CO feed unit 300 and H₂ recovery unit 310. Reactor 230 includes head 240, slow-flow zone 250, injection assembly 260 having one or more nozzles 262 and fast-flow zone 270. Injection assembly 260 is coupled to CO- saturation chamber 280. Head 40 is coupled to fast-flow zone 70, H₂ recovery unit 110, and MLSS recycle unit 90. MLSS recycle unit 290 is also coupled to carbon monoxide (CO) feed unit 300 and CO saturation chamber 280, and also includes a sludge outlet line 294. [0025] Reactor 230 is a vertical in-ground reactor (also sometimes referred to as a deep shaft reactor) with a head 240 and attached shaft 232. Head 240 includes a lower basin 242, an upper headspace 244, and illumination assembly 246. The illumination assembly 246 may be concealed into the roof (i.e., head 240) of the reactor to avoid chemical and humidity damage during operation, and to ensure that the appropriate light wavelength enters the basin portion 242. In some embodiments, the head 240 is made to be detachable from shaft 332. In some embodiments head 240 has a larger diameter than the diameter of shaft 332, and therefore includes an annular extension 48', as illustrated in Fig. 5, for example. In some embodiments, head 240 has a smaller diameter than that of shaft 332. Head 240' includes one or more inlets 241 for introducing nutrients, microorganisms, sterile fresh water injection, pH adjusting agents, and other materials into the reactor. H₂ recovery unit 310 is coupled to headspace 244 via line 312. MLSS recycle unit 290 is coupled to basin 242 of head 240 via line 296, and includes filtrate outlet line 292 and sludge outlet line 294. To help ensure that no contaminants are introduced into the process, MLSS recycle unit 290 may contain one or more centrifuge and ultrafiltration membrane, for example. In some embodiments, the shaft diameter is up to 6 meters (20 ft), and the depth of the shaft is up to 600 meters (2000 ft) deep. In some embodiments, the depth of the shaft to be used for a particular application is optimized based on the pressure sensitivity of the microorganism(s) to be used in the reactor, and such other factors as CO or CO₂ toxicity at various pressures. For instance, the depth of the shaft is about 150 meters (492 ft) in some cases. In some embodiments, the reactor has a large diameter deep shaft with a partition or separation wall 234 dividing the shaft 232 vertically into a slow-flow zone 250 and a narrower fast-flow zone 270. As shown in Fig. 3, the slow- and fast-flow zones are in indirect fluid communication at their upper ends 254 and 274, respectively, through the lower portion of head 240, also referred to as basin 242. The slow- and fast-flow zones are in direct fluid communication at their lower ends 252 and 272, respectively.

[0026] CO-saturation chamber 280 may be located either above ground or below ground, and is coupled to water outlet line 292. Line 292 is coupled to CO saturation chamber via line 282. In some embodiments, MLSS inlet line 286 is also coupled to line 282. Line 286 is made to be removable in some embodiments. Line 282 includes a pump and venturi, in some embodiments. Known pumps and venturi devices may be employed for this purpose. In embodiments in which line 282 is present, it is positioned at lower end 252 of zone 250 to withdraw a stream of MLSS. Carbon monoxide feed gas unit 300 is coupled to chamber 280 via line 302 and line 282, as shown in Fig. 3. Alternatively, in some system configurations, the CO feed gas unit 300 is coupled directly to chamber 280 so that the carbon monoxide and MLSS may be fed separately into chamber 280 and dispersed inside chamber 280. [0027] Injection assembly 260 is coupled to saturation chamber 280 via line 284 and includes at least one injection nozzle 262, the first of which is located at the lower end 272 of zone 270. In some embodiments, assembly 260 includes a pump. System 200 may also include a temperature control unit in thermal communication with reactor 230 and chamber 280. Dashed arrows indicate the primary direction of fluid circulation during operation of reactor 230.

[0028] In the embodiment shown in Fig. 4, the reactor system 400 is like that shown in Fig. 3, except that instead of providing for the use of water from the recycle unit for CO saturation, it provides for return of the recycled water directly into the reactor basin. Accordingly, reactor 430 includes a head 440 and attached shaft 432. Head 440 includes a lower basin 442, an upper headspace 444, and illumination assembly 446. In some embodiments, head 440 detachable from shaft 432. Head 440 includes one or more inlets

441 for introducing nutrients, microorganisms, sterile fresh water injection, sterile recycled MLSS filtrate, pH adjusting agents, and other materials into the reactor. H₂ recovery unit 510' is coupled to headspace 444' via line 512'. MLSS recycle unit 490 is coupled to basin

442 of head 440 via line 496, and includes water (filtrate) outlet line 492 and sludge outlet line 494. The slow- and fast-flow zones 450, 470 are in indirect fluid communication at their upper ends 454 and 474, respectively, through basin 442. The slow- and fast-flow zones are in direct fluid communication at their lower ends 452 and 472, respectively. The slow-flow zone 450 is coupled to CO-saturation chamber 480 via an line 486 and line 482. In some embodiments, line 482 includes a pump and venturi where line 502 couples to line 482. Line 486 is positioned to withdraw a MLSS stream at lower end 452 of zone 450. Carbon monoxide feed gas unit 500 is coupled to chamber 480 via line 502 and line 482, as shown in Fig. 3. Injection assembly 460 is coupled to saturation chamber 480 via line 484 and includes at least one injection nozzle 462, the first of which is located at the lower end 472 of zone 470.

[0029] Referring now to Fig. 5, in a variation of the system shown in Fig. 3, system 200' includes an injection assembly 260' that includes a plurality of conduits and sprayers or nozzles 262' distributed along the length of fast- flow zone 270'. In some embodiments, assembly 260' also includes one or more pumps in electrical communication with a controller for regulating the injection characteristics of each sprayer. For example, in some applications the volume of gas/liquid dispersion per unit of time from different nozzles 262' varies along the length of zone 70. In this exemplary embodiment, head 240' has a larger diameter than the diameter of shaft 332', and therefore includes an annular extension 248', which may serve as an overflow for basin 242'.

[0030] A schematic horizontal cross section view of an embodiment of shaft 532 of a bioreactor 530, configured substantially as described in any of Figs. 3-5, is shown in Fig. 4A. Shaft 532 includes one slow-flow zone 550 and one fast-flow zone 570. The slow- and fast- flow zones are divided by a partition or wall 534, which is positioned so that the volume of slow-flow zone 550 is greater than the volume of fast- flow zone 570 to facilitate a faster flow rate of MLSS in zone 570 during operation.

[0031] In some embodiments, the bioreactor shaft of any of Figs. 3-5 is configured substantially as illustrated schematically in Fig. 6B. A horizontal cross section of the bioreactor shaft 632 of reactor 630 is shown that includes two fast-flow zones 670a, 670b located on opposite sides of the reactor shaft 632. One slow-flow zone 650 is disposed between the two fast- flow zones 670a, 670b.

[0032] Alternatively, any other suitable reactor system design may be employed for carrying out the disclosed CO-fermentation process, provided that it is able to handle large volumes of CO gas (e.g., above 5000 kg CO/hr), promotes excellent gas to liquid transfer rates, and allows for effective gas injection control. In some embodiments, the reactor assembly also permits cooling control, is able to run at high pressures, and allows for effective cleaning to avoid contamination (e.g., contamination with undesirable microorganisms). For some applications, it is desirable to construct the reactor using common, standard parts for easy servicing and economy of construction. Durable, explosion-proof construction, and ability to run continuously and to produce bulk quantities of hydrogen are still other desirable features of some embodiments of the reactor assembly. Fermentative Hydrogen Production Process

[0033] An exemplary fermentative hydrogen production process generates molecular hydrogen (H₂) by anaerobic microbial fermentation of carbon monoxide gas (CO) as the sole carbon source for the microorganisms by a biological pathway that include the water-gas shift reaction. In brief, the process, as schematically shown in the box flow diagram of Fig. 7, includes the steps of combining water, microorganisms and nutrients in the absence of a carbon-containing food source, photoactivating the microorganisms, incubating a slowly- flowing stream of the resulting mixed liquor solids suspension ("MLSS") under carbon- monoxide depleted conditions, incubating the resulting MLSS under highly CO-enriched conditions in a faster-flowing stream, where the water-gas shift reaction occurs using the dissolved CO as the food source for the microorganisms, and the H₂-containing product gas is removed by vacuum withdrawal. More specifically, an exemplary process includes the following stages:

I. MLSS Preparation.

[0034] Water, selected microorganisms, nutrients, and any other desired components, are combined to prepare a mixed liquor solid suspension ("MLSS"). Any suitable growth medium for the selected microorganisms may be used, provided that it lacks a carbon source. [0035] Examples of potentially suitable microorganisms for production of hydrogen from carbon monoxide feed are represented by a group of photosynthetic bacteria that consume carbon monoxide under aqueous anaerobic conditions and release H₂, including thermophilic Carboxydothermus hydrogenoformans Z-2901; Rubrivivax gelatinosus; and Rhodospirillum rubrum. Some additional bacteria that are potentially suitable for production of hydrogen according to the disclosed process include Bdellovibrio sp., Rhodopseudomonas palustris, Rhodobacter sphaeroides, Citrobacter sp. Y19, Methanosarcina acetivorans c2A, and Bacillus smithii. These bacteria are available from the American Type Culture Collection (ATCC), for example. This list is intended to be merely exemplary of potentially suitable microorganisms, and should not be considered limiting. Additional microorganisms are also expected to be useful for generating hydrogen in the CO fermentation process described herein. II. Photo-activation Stage.

[0036] The selected microorganisms are photoactivated to initiate a biological pathway that includes the water-gas shift reaction. After an initial exposure of the MLSS to illumination effective to stimulate the photosynthetic CO oxidation pathway in the microorganisms, the activated microorganisms are then grown in darkness. No carbon-containing nutrients are provided to the activated microorganisms except for the gaseous CO that is provided in stage (4). Photoactivation is further described below under the subheading "Photo- Activation."

III. Dark Fermentation Stage.

[0037] A. Slow-flow Stage. The activated microorganisms in the MLSS are then incubated in the dark under anaerobic, slow-flow, carbon-monoxide-depleted conditions ("CO-depleted conditions") to produce CO-starved MLSS in a slow-flow zone. [0038] B. CO Saturation. Recycled effluent water and/or a portion of the CO- starved MLSS, is/are circulated into a CO-saturation vessel, in the absence of light. A stream of carbon monoxide gas is fed into the stream of water and/or portion of the CO-starved MLSS, to dissolve sufficient carbon monoxide to saturate or supersaturate the water and/or portion of MLSS, with CO. The result is a stream of CO-saturated water or MLSS. [0039] C. Fermentation/Fast-flow Stage. The fast-flow stage takes place in a fast- flow zone. "Fast-flow" means that the flow rate is faster than that of the slow-flow stage. The CO-saturated water and/or portion of MLSS and undissolved CO gas are rapidly injected or sprayed into a fast-flow zone and rapidly circulated to stage IH(A)-(B) and IV of the process. This circulation is facilitated by gas-lift created by undissolved CO gas and the injected CO-saturated stream. At the same time, a stream of CO-starved MLSS mixes with the injected gas/liquid stream(s). The resulting fast- flowing mixture is enriched in dissolved CO, and is referred to as CO-enriched MLSS. The activated microorganisms in the CO- enriched form H₂ and CO₂ from the dissolved carbon monoxide. This metabolic process by which H₂ is generated occurs primarily during circulation through the fast-flow zone. The production of H₂ may also occur as the MLSS circulates through the lower portion (basin) of the reactor head. IV. H₂ Collection and Removal.

[0040] Rapid removal of the off-gases from the MLSS deters potential toxic effects of the gases on the microorganisms, and reduces the occurrence of undesirable side reaction. The evolved gases from stage HI(C) are collected and extracted from the reactor via vacuum. The vacuum is applied to the MLSS as it circulates through the lower portion (basin) of the reactor head. H₂ product is recovered from the vacuum extracted gas.

V. Biomass Removal.

[0041] The microorganisms metabolize and proliferate, in some cases doubling in population after two hours of processing. To maintain a desired concentration of microorganisms, all or a portion of the circulating MLSS is removed from the reactor and treated to remove excess or spent microorganisms. Such treatment may comprise filtration, centrifugation, or a combination of those, for example. If desired, the resulting sludge may be recycled as combustible biomass, for further production of carbon monoxide feed gas. The withdrawal of MLSS may be periodic, continuous, or semi-continuous, or may vary depending on measured microorganism concentration.

[0042] The temperature of the MLSS is maintained in the range of about 37-70⁰C, depending upon the particular growth requirements of the selected strain(s) of microorganism(s). In some cases the process is operated in the range of about 37-53°C. Any suitable carbon monoxide-containing gas may serve as the feed for saturating the MLSS in stage III (B), provided that the concentration of other gaseous components of the feed are not prohibitively toxic to the selected microorganisms. Accordingly, for some applications it may be desirable to include a feed gas pre-cleaning step to remove any components that are potentially detrimental to the selected microorganisms. For example, a synthesis gas cleanup technique as is known in the art may be used.

[0043] Exemplary embodiments of the process are described as follows: [0044] Referring to the CO fermentation systems shown schematically in Figs. 1 and 2, the MLSS is circulated repeatedly between a down flow chamber ("downcomer"), which comprises the CO-depleted slow-flow zone 50, 50' and the up flow chamber, ("riser"), which comprises the fast flowing, CO-rich fast-flow zone 70, 70'. The circulation of MLSS through fast-flow zone 70, 70' is driven primarily by injection of a CO-saturated liquid stream into the rapid flow zone 70, 70'. The undissolved gas, including primarily CO, H₂ and CO₂, in fast- flow zone 70, 70' provides gas lift to drive circulation of the MLSS from fast-flow zone 70, 70' into head 40, 40'. The MLSS in the downcomer (i.e., zone 50, 50') has a higher density than the liquid-bubble mixture in the riser (i.e., zone 70, 70'). This density differential also promotes circulation from the riser to the downcomer.

[0045] Referring to the embodiment shown in Fig. 1, recovered filtrate from attached recycle unit 90 is introduced into CO saturation chamber 80, together with carbon monoxide gas from CO feed unit 100. Alternatively, as shown in Fig. 2, in some applications, recovered water/filtrate from recycle unit 90' is returned to the process via head 40', and a portion of the MLSS is withdrawn from slow-flow zone 50' via line 52 to serve as the "water" that is fed to CO saturation chamber 80', together with carbon monoxide from CO feed unit 100'. In some applications in which a high pressure pump and a venturi are used to combine the withdrawn portion of MLSS with the CO gas stream, it may be desirable to use microorganisms that are more tolerant of high pressures. Alternatively, or additionally, in some applications the quantity of microorganisms in the portion of MLSS withdrawn at line 52' that are killed or are rendered less capable of producing H₂ and CO₂ due to elevated pressure and sheer conditions of the pump and venturi, are offset by the proliferation of microorganisms that occurs in fast- flow zone 70'. The CO-saturated or supersaturated portion of MLSS is injected via assembly 60' into zone 70', and the resulting MLSS mixture is enriched in carbon monoxide. This promotes rapid water gas shift reaction, in some applications occurring in 10 seconds or less, and also promotes multiplication of the microorganisms .

[0046] Referring to Figs. 1 and 2, the injected CO-containing gas dissolves in the MLSS as the gas is dispersed in the liquid phase. This dissolved carbon monoxide provides the necessary carbon source for the microorganisms to carry out the water-gas shift reaction. The products formed include H₂ and CO₂. As the circulating MLSS and product gases ascend in the riser (fast- flow zone 70, 70') to regions of lower hydrostatic pressure (e.g., in the upper end 274 of zone 270 and basin 242, as shown in Fig. 3) the dissolved off-gas separates as bubbles. When the liquid/bubble mixture from the fast- flow zone enters the basin portion of head 40, 40', gas disengagement occurs. For example, in some embodiments the disengaged gas accumulates in headspace 244 until withdrawn via line 312, as shown in Fig. 3. [0047] The biological reaction between the microorganisms and the carbon monoxide may take place at any point during circulation where the reaction conditions are favorable. Most of the water-gas shift reaction and production of H₂ and CO₂ will typically occur in fast-flow zone 70, 70'. The major products of the biological water-gas shift reaction are hydrogen and carbon dioxide. Product gases, including H₂, exit head 40, 40' and are withdrawn into H₂ recovery unit 110, 110', where they may be subjected to additional processing and separation. In some applications, the product gases accumulate in headspace 244 and are withdrawn by H₂ recovery unit 310 via line 312, as shown in Fig. 3. For example, the evolved gases may be collected and vacuum extracted from headspace 244 into recovery unit 310. In some cases, as mentioned above, a portion of the recovered H₂ may be combined with the CO feed in line 302 to dilute the CO concentration.

[0048] In some embodiments of the process, excess and/or spent microorganisms in the circulating MLSS are either continuously or periodically withdrawn as a stream of MLSS to MLSS recycle unit 90, 90', which it is subjected to liquid/solids separation and the biomass is removed as sludge via line 94, 94'. If desired, a portion of the resulting sludge may be recycled as combustible biomass, for combining the combusted gas with the syngas feed to the front of the fermentation process. Alternatively, or additionally, a portion of the sludge may be disposed of as waste, in accordance with applicable regulations. Still another possible use for the recovered biomass is as a protein source for animal feed. In some embodiments, the effluent, which contains nutrient medium, is recycled into reactor 30, 30'. In some applications, the liquid is ultrafiltered to ensure sterility prior to being returned to the reactor. The recycled effluent may be supplemented with fresh nutrients and sterile water as needed.

[0049] Photo activation (Stage II). An aqueous medium containing suitable H₂-producing microorganisms is introduced into the slow-flow zone of an in-ground reactor such as that shown in Figs. 3 and 5, for example. The reactor is filled with MLSS to a predetermined fill line or level 243, 243' in basin 242, 242'. Basin 242, 242 refers to the lower region of head 240, 240' between the fill line 243, 243' and the top ends 254, 254' and 274' of zones 250, 250' and 270, 270'. The upper region of head 240, 240' between fill level 243, 243' and the light unit 246, 246' is referred to as headspace 244, 244'. The microorganisms are activated (i.e., the CO-oxidation pathway including the water-gas shift reaction is induced) by exposing the MLSS in basin 242, 242' to light in the visible wavelength range. Illumination assembly 246, 246' may be configured within reactor 230, 230' to provide light of an optimum wavelength to microorganisms within basin 242, 242'. Illumination of the MLSS is provided in some cases by incandescent lamps, for example. The length of time of exposure to light is determined based on the parameters of a given application, such as flow rate of the MLSS, the intensity of the light, the exposed surface area of the MLSS in head 40. In some embodiments of the process, the reactor includes a head 240' having a diameter greater than that of shaft 232' (as illustrated in Fig. 5). Such a configuration enhances exposure of the circulating MLSS to light, prior to its flowing downward into the darker slow-flow zone 250' of reactor 230'. Although fermentation of CO by the water-gas shift reaction, would continue in the presence of light, endogenous hydrogenases would potentially oxidize the produced H₂ to support light-dependent CO₂ fixation. Therefore, after activation, it is desirable in some applications to perform the subsequent stages of the fermentation process in the absence of light to enhance the yield of H₂ product. The desired biochemical reaction (i.e., the water-gas shift reaction) benefits from the CO feed gas starvation stage of the process, as the food-deprived microorganisms are primed for enhanced metabolic activity upon restoration of a food source (i.e., carbon monoxide). The same benefit would not be achieved by merely injecting carbon monoxide feed gas into a conventional batch fermentation mixture.

[0050] In a variation of the above-described photo activation procedure, the light activation step is performed prior to introduction of the microorganisms into the reactor, in which case the illumination assembly 246, 246' shown in Figs. 3 and 5 may be omitted from reactor 230, 230', and an alternative illumination means may be employed external to the reactor. In some instances, during operation of the process, the MLSS circulating from the fast flow zone 270, 270', through head 240, 240', and downward into the slow-flow zone 250, 250', is again exposed to visible light as it passes through head 240, 240', to ensure activation of the CO oxidation pathway in the recirculated microorganisms. This secondary exposure to light may be brief. Following such exposure, the flowing MLSS is again exposed, in the absence of light, to CO-depleted conditions within zone 250, 250'.

[0051] Dark Fermentation (Stage III). Referring to Figs. 3 and 5, the light-activated MLSS initially incubates in a relatively slow-flowing downward stream in the slow-flow zone 250, 250' of reactor 230, 230'. In slow-flow zone 250, 250', the microorganisms remain or become depleted in the CO carbon source. In some applications, during operation of the fermentation process, the withdrawal of MLSS from reactor head 240, 240' is controlled in response to changes in the level of MLSS in the basin portion 242, 242'. Similarly, the flow of water, nutrients and fresh microorganisms into reactor head 240, 240' may be controlled in response to deviations from a predetermined fill level 243, 243', or in response to measurement of total dissolved salts, or other process parameter. Effluent or filtrate from recycling unit 290, 290' and line 292, 292' is combined with a carbon monoxide feed from CO feed unit 300, 300' and line 302, 302' to form a CO-saturated aqueous mixture in saturation chamber 280, 280'. For ease of reference, the effluent or filtrate from MLSS recycle unit 290, 290' is sometimes referred to herein simply as "water." For example, in some cases the water is fed into saturation chamber 280, 280' via line 282, 282' using a high-pressure pump and a venturi device (not shown). The high pressure pump and venturi device may combine the liquid stream and the CO feed gas, thereby saturating or supersaturating the MLSS with carbon monoxide. Pumps and venturi devices as are known in the art may be satisfactorily employed for this purpose. Depending on the reactor depth, diameter, volume of gas-water mixture, nature of microorganisms used, and the desired result, the system may be designed to achieve CO saturation of the water.

[0052] In some variations of the above-described process, as illustrated in Figs. 2, 3 and 4, the liquid stream that is saturated with CO includes a portion of the CO-starved MLSS, which is transferred from slow-flow zone 50', 250, 450, combined with carbon monoxide gas from CO feed unit 100, 300, 500, to MLSS CO saturation chamber 80', 280, 480. As shown in Figs. 3 and 4, this portion of MLSS is transferred via line 282, 482 via line 286, 486, which may be either fixed or removable. In some embodiments, it may be desirable to install line 286 only periodically, to facilitate cleaning of the reactor, for example.

[0053] Mass transfer of CO into the liquid phase of the MLSS is enhanced by carrying out the process in a deep shaft reactor, by forming a fine dispersion of the gases in the liquid phase, and by application of pressure and maintenance of a temperature at which production Of H₂ is promoted. Any suitable means for regulating the temperature of the MLSS may be used, such as employing heating and cooling techniques that are known in the art. Referring now to Figs. 3-5, in the faster-flowing gas-lift stage, the CO-saturated water is injected via an injection assembly 260, 460, 260' (e.g., a single nozzle 262, 462, or an array of sprayer nozzles 262') into the fast-flow zone. The circulation flow pattern is thus propagated by injection of CO-saturated liquid on one side of the reactor (i.e., zone 270, 470, 270'), forcing the MLSS into motion between the fast-flow (270, 470, 270') and slow-flow zone (250, 450, 250'). As the CO-depleted MLSS circulates from the lower end 252, 252, 252' of the slow- flow zone, through end 272, 472, 272', and upward in fast-flow zone 270, 470, 270', it becomes enriched in CO due to mixing of the injected gas/liquid with the circulating MLSS. In some embodiments, reactor head 240, 440, 240' is constructed for vacuum degasification. This promotes disengagement and removal of hydrogen gas from the MLSS in basin 242, 442, 242' prior to reentry of the MLSS into slow-flow zone. In some embodiments, the basin portion of head 240, 440, 240' may be fitted with a vacuum system combined with baffles (not shown) to encourage the MLSS emerging from the riser to traverse a major part of basin portion 242, 442, 242' to facilitate release of H₂ product into the headspace 244, 444, 244' before the MLSS again descends more slowly in the downcomer (i.e., slow-flow zone 250, 450, 250') and again becomes CO depleted.

[0054] For every mole of hydrogen produced, a mole Of CO₂ is also produced. Production of acid gas, in some cases, may lead to an undesirable shift in pH. This pH shift may affect microbial toxicity limitations with respect to CO₂, H₂ or CO, for example. In some applications, the pH is controlled using a NaOH injection system to increase the alkalinity of the mixture to a desired pH range by converting excess CO₂ to soluble bicarbonate. CO₂ conversion to bicarbonate within a deep shaft bioreactor, for example, reduces CO₂-related microbial toxic effects, and also potentially reduces CO₂ greenhouse gas release from the process. A desired pH may be from about 7.5 to about 9.5, in some applications. In some cases, pH is controlled to about pH 9 by injecting aliquots of a NaOH solution into the reactor head 240, 440, 240'. In many applications, the selected microorganism(s) are relatively insensitive to the CO concentration in the MLSS. Therefore, the amount of CO gas that is fed to the process (via line 302) may be adjusted such that substantially all the CO feed is consumed by the microorganisms. If, in some instances, the CO concentration becomes too high, leading to excess unconverted CO in headspace 244, 444, 244' a portion of the product H₂, or an inert gas may be injected with the CO feed gas to provide the desired gas-lift and dilution. The produced gas is continuously removed by a constant vacuum system in the head of the reactor. In some cases, an engineered defoaming system (not shown) is incorporated into head 240, 440, 240' to avoid liquid overflow. A defoaming system as known in the art may be suitably employed for this purpose.

[0055] Referring to Figs. 3 and 5, a portion of the circulating MLSS is continuously withdrawn into MLSS recycle unit 290, 292', which may include a centrifuge for continuous water/solids separation, and/or may include an ultrafiltration membrane to separate residual solid components of the MLSS from the aqueous phase. The separated liquid, and dissolved components of the MLSS, are returned to the CO fermentation process via lines 292 and 282 to saturation chamber 280. Optionally, the separated liquid (filtrate) is diluted with fresh sterile water, as needed. In For example, in order to reduce the bicarbonate concentration. Therefore, in these embodiments, the recycle water (filtrate) serves as the sole or primary source of liquid for the CO-saturated or supersaturated stream(s) injected from nozzle assembly 260, 260'. During continued operation, the microorganism population in the MLSS will expand sufficiently that it will be desirable to remove the excess biomass from reactor 230, 230'. Microbial biomass removed from the recycle unit 290, 290' as sludge may be transferred to an incinerator for carbon recycling and generation of synthesis gas for feeding to the microbial hydrogen generation process. Alternatively, the excess biomass may be disposed of as a protein food source for farm animals. Still another alternative use of the excess biomass is for digestion with suitable added substrates to produce methane gas butanol, ethanol or acetone.

[0056] Referring now to Fig. 4, an embodiment of the CO fermentation process is similar to that described above with respect to Figs. 1, 3 and 5, except that instead of using the water that is recycled from the liquid/solids separation (recycle) unit, a portion of the CO-starved MLSS circulation from end 452 of zone 450 is withdrawn via line 486 into line 482, to MLSS CO saturation chamber 480. A stream of carbon monoxide gas is fed into the stream of withdrawn CO-starved MLSS, to dissolve sufficient carbon monoxide to saturate or supersaturate the aqueous phase of the withdrawn portion of MLSS with CO. The result is a stream of CO-saturated MLSS. In some applications the pressure and sheer conditions of the pump and venturi may cause a reduction in the number of microorganisms in the portion of MLSS that is used to generate the saturated injection liquid, or might cause them to be less capable of producing H₂ and CO₂. In some cases, those effects are offset by the proliferation of the larger portion of microorganisms that continue to circulate into end 472 of fast-flow zone 470. The CO-saturated or supersaturated portion of MLSS is injected via injection assembly 460 into zone 470, and the resulting upwardly flowing MLSS mixture is enriched in carbon monoxide and utilized the dissolved carbon monoxide as a food source for the biological water-gas shift reaction. In some embodiments, the CO-saturated MLSS is injected using an injection assembly 460 having an plurality nozzles arranged along the length of the fast-flow zone of shaft 432, similar in appearance to the nozzle assembly 262' shown in Fig. 5. Due to the variation in pressure at different depths of the MLSS in reactor 430, it may be desirable in some applications to vary the force and amount of CO-saturated water or MLSS that is injected by different nozzles or groups of nozzles along the length of the fast- flow zone. In this embodiment, instead of using water from the recycle unit 490 for CO saturation, the recycled water is returned directly into the reactor via line 492 and one or more feed line 441. A stream of reacted MLSS is withdrawn via line 496 from basin 442, adjacent to end 474 of fast-flow zone 470, into recycle unit 490 where it is subjected to liquid/solids separation, as described above. Sludge is removed via outlet 494. [0057] Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the preferred embodiments of the invention have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein.

References:

Maness, Pin-Ching, Jie Huang, Sharon Smolinski, Vekalet Tek, and Gary Vanzin, "Energy generation from the CO oxidation-hydrogen production pathway in Rubrivivax gelatinosus," June 2005 Applied and Environ. Microbiol. 71 :2870-2874.

Merida, Walter, Pin-Ching Maness, Robert C. Brown and David B. Levin, "Enhanced hydrogen production from indirectly heated, gasified biomass, and removal of carbon gas emissions using a novel biological gas reformer," 2004 Int. J. Hydrogen Energy 29:283-290.

Maness, Pin-Ching, Sharon Smolinski, Anne C. Dillon, Michael J. Heven, and Paul F. Weaver, "Characterization of the oxygen tolerance of a hydrogenase linked to a carbon monoxide oxidation pathway in Rubrivivax gelatinosus," June 2002 68:2633-2636.

Maness, Pin-Ching and Paul F. Weaver, "Hydrogen production from a carbon-monoxide oxidation pathway in Rubrivivax gelatinosus " 2002 Int. J. Hydrogen Energy 27: 1707-1411.

Claims

CLAIMS What is claimed is:

1. A process for production of hydrogen by a biological water-gas shift reaction, comprising:

(a) mixing together at least one species of microorganism and a liquid nutrient medium lacking a carbon source, to form a mixed liquor solids suspension (MLSS), wherein the microorganism has a metabolic pathway that includes a water-gas shift reaction;

(b) photoactivating the metabolic pathway in the microorganism;

(c) incubating a stream of the resulting activated MLSS in the absence of light, and without a carbon source, to obtain a first stream comprising a CO-starved activated MLSS flowing at a first flow rate;

(d) saturating a liquid stream with a CO-containing gas, to form a CO-saturated stream;

(e) incubating, in the absence of light, a second stream comprising said CO-saturated stream and at least a main portion of said first stream, wherein the second stream flows at a second flow rate that is faster than the first flow rate; and

(f) forming from the second stream H₂ and CO₂ products and a stream of CO- depleted MLSS.

2. The process of claim 1, wherein at least (e) and (f) are carried out at superatmospheric pressure.

3. The process of claim 1, wherein (e) comprises injecting said CO-saturated stream into said at least a main portion of said first stream.

4. The process of claim 3, wherein said injecting comprises injecting multiple streams of said CO-saturated liquid.

5. The process of claim 4, wherein said injecting creates a gas-lift mixture comprising said CO-saturated liquid and undissolved carbon monoxide in combination with said main portion of said first stream.

6. The process of claim 1, comprising (g) removing a portion of said CO-depleted MLSS and subjecting said portion to liquid/solids separation, to obtain a recycled liquid stream and a sludge fraction.

7. The process of claim 4, wherein in (d), saturating a liquid stream with a CO- containing gas comprises saturating said recycled liquid stream with a stream of carbon monoxide to form said CO-saturated stream.

8. The process of claim 1, comprising (f) combining at least a portion of said CO- depleted MLSS from (f) with said activated MLSS in (c).

9. The process of claim 1, wherein (d) comprises removing a portion of said CO-starved activated incubation mixture to form said liquid stream.

10. The process of claim 1, wherein in (d), said saturating includes passing said liquid stream into a CO-saturation vessel, in the absence of light, together with a stream of carbon monoxide-containing gas.

11. The process of claim 1, wherein in (b), said photoactivating comprises: in a first reactor chamber, exposing said microorganism to visible light sufficient to initiate said metabolic pathway.

12. The process of claim 11, wherein in (c) said incubating includes passing said activated MLSS into a second reactor chamber, in the absence of light.

13. The process of claim 11, wherein in (b), said photoactivating comprises: in a first reactor chamber, exposing said incubation mixture to visible light, sufficient to initiate said metabolic pathway; and said process further comprises (f ') collecting said H2-containing off- gas in said first reactor chamber.

14. The process of claim 13, wherein (d) comprises varying the concentration of CO in said CO-containing gas inversely to the CO concentration, if any, in said H₂-containing off- gas in said first reactor chamber.

15. The process of claim 1, comprising excluding atmospheric oxygen from said process.

16. The process of claim 1, comprising regulating the temperature of said MLSS in the range of about 20-65⁰C.

17. The process of claim 1, wherein said temperature is in the range of about 37-50⁰C.

18. The process of claim 1, comprising regulating the pH of said MLSS to about pH 7.5 - 9.5

19. The process of claim 18, further comprising addition of an alkaline pH adjustment agent to said MLSS, whereby a portion of said CO₂ product is converted to bicarbonate.

20. The process of claim 19, wherein said conversion of said portion of CO₂ product reduces CO₂-related toxic effects on said microorganism in said MLSS, and reduces the amount of CO₂ product released into the air by said process.