Patent Application
20090305389
Embodiments of the present invention relate generally to permeable membranes in photobioreactors, and more specifically to integration of porous and non-porous membranes and other porous materials into bioreactors to transfer gases to and from the media used to grow organisms.
Producing biofuels, such as biodiesel, bioethanol, and/or biogasoline, from renewable energy sources provides numerous benefits. The increasing costs, increasing difficulty of extraction, and depletion of known fossil fuel reserves help to spur the development of such alternative fuel supplies. Efforts have been made to develop renewable energy fuels such as ethanol from corn grain or biodiesel from canola, rapeseed and other sources. The amount of biofuel that can be derived from food plant materials is often limited and the underlying increase in food commodity prices often negatively impacts food availability in developing countries and food prices in the developed world.
Efforts are underway to generate biofuels from non-food materials, such as cellulosic ethanol from wood pulp, corn stover or sugar cane bagasse. Algae and other photosynthetic microorganisms can provide feedstock for biofuel synthesis. Biofuel production from algae could permit productivities per unit of land area orders of magnitude higher than those of corn, rapeseed, canola, sugar cane, and other traditional crops.
Growing algae as a feedstock for biodiesel may involve growing the algae inside of closed bioreactors. Carbon, usually in the form of carbon dioxide (CO2), is often added to the bioreactor media to support photosynthesis. Similarly, the process of photosynthesis liberates oxygen (O2) which dissolves in the media. Relying on an open bioreactor exposed to ambient air in order to receive carbon dioxide from the air and vent the liberated oxygen to the air often does not yield enough carbon to support effective algae growth, due to the relatively low carbon dioxide content of air. Bubbling carbon dioxide directly into the bioreactor media may often involve a relatively low carbon dioxide absorption into the media, such that supplying the carbon dioxide often requires more energy than is produced by the algae growth. Using a complex membrane contactor to promote the absorption of carbon dioxide into the media often involves a relatively high expense, which also often requires a greater cost than the value of the energy produced through algae growth.
Embodiments of the present invention transfer CO2 to the bioreactor media molecularly in a highly cost-effective manner. According to some embodiments of the present invention, porous and non-porous membranes are incorporated into a film-based photobioreactor to create a continuous or distributed contactor. Such membranes used to transfer the CO2 into the media (e.g. water), or remove O2 from the media, may be integrated directly into a plastic film reactor structure, according to embodiments of the present invention. This reduces cost, reduces a need for pumping, and reduces the size of the reactor (compared to a less efficient reactor) according to embodiments of the present invention. Such a configuration also permits a double use of the contactor material, which can function as part of the photobioreactor structure in addition to its function as a gas exchange membrane. According to some embodiments of the present invention, the membranes may comprise one or more chambers filled with a gas, one or more valves, a pressure source, and/or a means to control pressure within the chambers.
Any known species of algae or photosynthetic microorganisms may be grown in a photobioreactor and utilize such integrated membranes, according to embodiments of the present invention. According to some embodiments of the present invention species such as, but not limited to, Nannochloropsis oculata, Nannochloropsis sp., Nannochloropsis salina, Nannochloropsis gaditana, Tetraselmis suecica, Tetraselmis chuii, Chlorella sp., Chlorella salina, Chlorella protothecoides, Chlorella ellipsoidea, Chlorella emersonii, Chlorella minutissima, Chlorella pyrenoidosa, Chlorella sorokiniana, Chlorella vulgaris, Chroomonas slaina, Cyclotella cryptic, Cyclotella sp., Dunaliella tertiolecta, Dunaliella salina, Dunaliella bardawil, Botryococcus braunii, Euglena gracilis, Gymnodimium nelsoni, Haematococcus pluvialis, Isochrysis galbana, Monoraphidium minutium, Monoraphidium sp., Nannochloris, Neochloris oleoabundans, Nitzschia laevis, Onoraphidium sp., Pavlova lutheri, Phaeodactylum tricornutum, Porphyridium cruentum, Scenedesmus obliquus, Scenedesmus quadricaula, Scenedesmus sp., Stichococcus bacillaris, Stichococcus minor, Spirulina platensis, Thalassiosira sp., Chlamydomonas reinhardtii, Chlamydomonas sp., Chlamydomonas acidophila, Isochrysis sp., Phaeocystis, Aureococcus, Prochlorococcus, Synechococcus, Synechococcus elongatus, Synechococcus sp., Anacystis nidulans, Anacystis sp., Picochlorum oklahomensis, Picocystis sp. may be grown either separately or as a combination of species.
While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.
While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
Researchers are exploring growing algae as a feedstock for biodiesel. In many designs the algae is grown inside closed reactors comprised of glass or plastic. Examples of closed system bioreactors suitable for growth of algae and other microorganisms are described in U.S. patent application Ser. No. 11/871,728, filed Oct. 12, 2007, which is incorporated by reference herein in its entirety.
One approach for introducing CO2 into the media, or the water in which the algae is grown, involves allowing the free surface of the media to be exposed to atmospheric air. Typical air contains approximately 0.038% CO2 by volume. While such a configuration is relatively easy to implement, it does not allow for much carbon to be added to the media/water and therefore the effectiveness of the algae growth may not be as high in such circumstances.
Another approach to increase the carbon content of the water is to bubble, or sparge, gaseous CO2 through the media. The CO2 may be sparged through the media either in pure form or mixed with other gases, such as, for example, air. Bubbles formed will rise through the media and a portion of the CO2 will be absorbed into the media, adding carbon and altering the pH content of the media. However, there is often not enough time for all of the CO2 in the bubbles to be absorbed before these bubbles reach the top surface of the media. In many cases little CO2 is absorbed and the non-absorbed CO2 is expelled with the vent air, resulting in low uptake efficiency. The cost to preprocess and pump the CO2 gas in such cases can be relatively high.
Other configurations increase the residence time of bubbles in the media before the bubbles reach the free surface. For example, CO2 as bubbles may be injected at the bottom of a pipe oriented vertically with the media flowing from top to bottom, such that the average velocity of the media in the pipe is approximately the same, or slightly slower than, the velocity at which the bubbles rise. While this increases the residence time of the bubbles in the media, energy is expended in continuously pumping the fluid.
An alternative to bubbling CO2 is to use porous or non-porous membranes or other materials that transfer the gas without bubbling, according to embodiments of the present invention. Two classes of materials can be used for this, for example. One class of such materials includes non-porous membranes that transfer CO2 molecularly into the media rather than by bubbling it through the media. Various different non-porous membranes successfully distribute CO2 into media for the purposes of growing algae. Such non-porous membranes may also be used in medical devices to oxygenate blood, or transfer other gases into liquids. Silicone rubber is one example of a non-porous membrane that has high permeability to CO2 and other gases yet is effectively waterproof in the sense that water or media does not permeate through the membrane.
Another class of materials capable of gas exchange with a photobioreactor media are porous membranes that have very small holes, according to embodiments of the present invention. Such holes may be large enough to allow CO2 molecules to penetrate through the membrane and form a gaseous CO2 skin, or attached bubble, in direct contact with the media, while not large enough to permit bubbles to form, detach, and rise in the media, according to embodiments of the present invention. Such holes may also be sufficiently small so as to not allow liquids to pass through and may be essentially “waterproof” as well, according to embodiments of the present invention. Beneficial characteristics of a gas transfer membrane according to various embodiments of the present invention include a high surface area available to transfer CO2 into the media, and a dimension to permit a sufficient time for the CO2 gas to be absorbed.
Membrane materials can be built into assemblies that are often referred to as “membrane contactors,” in which large amounts of such materials are folded and mounted in a container to provide a high surface-area-to-contactor volume ratio. Such contactor shells may be made of hard plastic, metal or other rigid material. Such contactors permit liquid to be circulated over the membrane at high flow rates while gas flows on the other side of such membrane to further increase gas transfer. Contactors provide a relatively compact passage for gas transfer, although they can often be expensive. Contactors are often localized, and a pump or similar device is used to move the media to the device. This can be expensive from a capital and operating cost standpoint, and many algae are sensitive to the shear caused by pumping.
Some embodiments of the present invention involve photobioreactors used to grow algae for the production of biodiesel. In some embodiments, the bioreactors may be used to grow algae or other photosynthetic microorganisms and the membranes may be optimized to efficiently introduce carbon dioxide (CO2), and/or remove dissolved oxygen (O2) from the media in which the algae or other microorganisms are grown. Based on the disclosure provided herein, one skilled in the art will recognize that similar configurations can also be used to grow algae for other purposes, and/or to grow other microorganisms.
The photobioreactor includes an integrated air tube 104 that is thermally welded into the film of the bioreactor 101, according to embodiments of the present invention. The integrated air tube 104 may be constructed with a 0.0035″ thick composite plastic also comprised of low density polyethylene, nylon and tie layers used to bond the nylon to the polyethylene, according to embodiments of the present invention. The integrated air tube 104 may be used for sparging the media 102; such sparging may be accomplished as the gas, usually air, leaves the air tube 104 through sparging holes 105. These holes are approximately 0.010 inches in diameter and are spaced approximately 0.5 inches apart, according to embodiments of the present invention. The holes 105 may be cut using a laser, or alternatively using mechanical punches or other hole creation methods.
The air tube 104 is fed from a fitting 106 that is connected to the air feed line 107, which in turn is connected to a source 108 of higher pressure air or gas mixture. Typical sparge pressures are two to three pounds per square inch gage (“psig”). The other (far) end of the air tube is sealed with another thermal weld 109, according to embodiments of the present invention. The sparge air rises as bubbles through the media 102 and leaves the photobioreactor 101 through the exhaust port 110, according to embodiments of the present invention. The exhaust port 110 may be thermally welded into the film of the bioreactor 101, according to embodiments of the present invention. The exhaust port 110 is in fluid communication with an exhaust line 111, and the exhaust line 111 is in fluid communication with a device 112 which regulates the backpressure in the photobioreactor bag 101.
The photobioreactor may also include an integrated membrane tube 113 used to allow dissolved oxygen in the media 102 to permeate through the media 102 and into the gas inside the tube 113, according to embodiments of the present invention. The tube 113 may be composed of 0.0015 inch thick composite film; in some cases the tube 113 may be thermally welded to the outer bag of the photobioreactor 101 such that the film of the tube 113 and the photobioreactor bag 101 form an integrated unit with the tube 113 being approximately one inch in diameter, according to embodiments of the present invention. The inside of the membrane tube 113 does not communicate with the media 102 inside the photobioreactor 101, other than to permit gas transfer, according to embodiments of the present invention. In one embodiment the tube 113 is made from a non-porous permeable membrane comprised of polyethylene and/or other plastics. Both non-porous and porous membranes may be used in the development of the photobioreactor 101 system, according to embodiments of the present invention. While the rates at which the different materials transfer the oxygen vary, satisfactory results are obtained with a variety of materials including numerous composite films both porous and nonporous, spun polyethylene (Tyvek), and/or silicone rubber, according to embodiments of the present invention. According to some embodiments of the present invention, non-porous membranes may be formed with a Sealed Air HP2700 (or 10K) film. According to some embodiments of the present invention, a porous membrane may be formed with an Aptra PP Microporous UV8 film manufactured by RKW US, a TYVEK 4058B and/or TYVEK 1025D film material manufactured by Dupont, a microporous film/non-woven laminate manufactured by Tredegar, a 4560-0400E-C microporous film and/or 2500-0400E-C microporous film manufactured by Celgard, and/or a UPHP000HC 0.45 UM UPE Membrane manufactured by Entegris.
As used herein, the phrase “membrane tube” is used in its broadest sense to refer to an enclosure and/or partial enclosure and/or liquid/gas interface comprised of a porous or nonporous membrane material which permits the transfer of one or more gases across the membrane, from an area containing liquid to an area containing gas or vice versa, according to embodiments of the present invention. A membrane tube need not be tubular, and need not include a cross section of uniform shape and/or diameter and/or dimension. A membrane tube need not have more than one opening. For example, a membrane tube may be a tube, a pocket, a line, a bag, a sleeve, and/or an enclosure formed at least partially of a gas permeable membrane, according to embodiments of the present invention.
The oxygen removal tube 113 is fed from a port 114 which is connected to (e.g. in fluid communication with) a feed line 115 which is fed by a supply of gas 116 used to strip the oxygen out of the media 102, according to embodiments of the present invention. The gas supply 116 may consist of air, air enriched with nitrogen, pure nitrogen, and/or other gases capable of drawing oxygen out of the media 102 and through tube 113. The stripping gas leaves the photobioreactor 101 through an exhaust port 117 in fluid communication with the membrane tube 113, through an exhaust tube 118, and into a backpressure control device 119, according to embodiments of the present invention. The dissolved oxygen leaving the media 102 and flowing into the membrane tube is depicted by arrows 120, according to embodiments of the present invention.
The photobioreactor 101 also includes a second membrane tube 121 constructed with a flexible film in a manner similar to tube 113, according to embodiments of the present invention. According to embodiments of the present invention, the fluid inside tube 121 does not communicate with the media 102 other than to permit gas transfer; in other words, the membrane tube 121 does not permit entry of the media 102 into the tube 121. Membrane tube 121 permits transfer of CO2 from inside the membrane tube 121 to the media 102, according to embodiments of the present invention. The CO2 membrane tube 121 is fed through a port 122 and a CO2 feed line 123, according to embodiments of the present invention. The flow to the line 123 and therefore the membrane tube 121 is controlled with a flow control valve 125, which in turn is fed by a source 124 of CO2, according to embodiments of the present invention. A pressure sensor 126 may be used to measure the pressure in the membrane tube 121. The other (far) end of the membrane tube 121 is sealed with a thermal weld 127, according to embodiments of the present invention. Arrows 128 illustrate the transfer of carbon dioxide from tube 121 into media 102, according to embodiments of the present invention.
The amount of CO2 transferred to the media 102 from inside the tube 121 is a function of the material properties, the surface area of the membrane tube 121 and the difference in the partial pressures in the gas inside the membrane tube 121 and the equivalent partial pressure in the media 102. Consequently, the amount of CO2 added to the media 102 can be controlled by adjusting the pressure inside the membrane tube 121, according to embodiments of the present invention. According to some embodiments of the present invention, pressures within tube 121 ranged from approximately one to ten psig.
As discussed above, tube 104 may include air or another sparge gas 250; membrane tube 113 may include a gas with a relatively low oxygen content 260 (e.g. air, air enriched with nitrogen, pure nitrogen, and/or other gases in which the partial pressure of oxygen is lower than the equivalent partial pressure of oxygen in the media, causing the oxygen to diffuse from the media 102 and through tube 113); and membrane tube 121 may include carbon dioxide or a carbon enriched gas 270, according to embodiments of the present invention.
According to some embodiments of the present invention, the photobioreactor also includes a membrane tube 513 comprised of flexible film comprised of a gas permeable membrane. The membrane is a non-porous plastic composite film, according to embodiments of the present invention. This integrated membrane tube 513 is constructed so that the fluid (e.g. gas) within the tube 513 does not communicate with the media 102. In other words, gas exchange occurs across the tube 513 but the media is not able to enter the tube 513, according to embodiments of the present invention. The membrane tube 513 is used to transfer CO2 from inside the membrane tube 513 to the media 502 and to remove dissolved oxygen from the media 502, according to embodiments of the present invention. Carbon dioxide is fed to the membrane tube 513 through a port 122 and a CO2 feed line 123. The flow to the line 123 and therefore the membrane tube 513 is controlled with a flow control valve 125 which is fed by a source 124 of CO2. A pressure sensor 126 is used to measure the pressure in the membrane tube, according to embodiments of the present invention. The far end of the membrane tube 513 is not closed but has a fitting 127 such that the gas inside the tube 513 can flow out of the membrane tube 513, according to embodiments of the present invention. The flow of CO2 into the media 102 is depicted with arrow 528 and the flow of O2 from the media is depicted with arrow 520, according to embodiments of the present invention.
When the mixture gas inside the membrane tube 513 leaves the photobioreactor 501 through fitting 127, it travels through a line 522 and into an oxygen/carbon dioxide separator 523 which separates the O2 gas from the CO2 gas, according to embodiments of the present invention. The O2 removed from the mixture gas is exhausted from the separator 523 as indicated by arrow 526 and the CO2 from the mixture gas is returned to the CO2 source 124 via CO2 recirculation line 525, as indicated by arrow 524, according to embodiments of the present invention. In this way, a single membrane tube 513, or multiple tubes with the same or a similar gas in them, simultaneously adds CO2 to the media 102 and removes O2 from the media 102, according to embodiments of the present invention. The oxygen removed as indicated by arrow 526 may be stored and/or used in other applications, as a byproduct of the photosynthesis process, according to embodiments of the present invention.
According to some embodiments of the present invention, two of the tubes 602, 604, 606 are used to remove O2 from the media 102 and one of the tubes 602, 604, 606 is used to add the CO2 to the media 102. According to some embodiments of the present invention, the membrane tubes 602, 604, 606 are constructed of similar material; according to other embodiments of the present invention, membrane tubes 602, 604, 606 may be constructed of different materials or a combination of materials. The fluids 620, 640, and 660 may be selected to be the same, or different, in order to remove oxygen from and/or add carbon dioxide to the media 102, as described above, according to embodiments of the present invention. For example, fluids 620 and 640 may be air or nitrogen enriched air to remove oxygen from the media 102, and fluid 660 may be carbon dioxide in order to add carbon dioxide to the media 102, according to embodiments of the present invention. Based on the disclosure herein, one of ordinary skill in the art will appreciate the number of different fluids 620, 640, 660 and the number of different permeable membrane materials and configurations that may be used for tubes 602, 604, 606 to achieve similar results, according to embodiments of the present invention.
According to some embodiments of the present invention, two of the tubes 702, 704, 706 are used to remove O2 from the media 102 and one of the tubes 702, 704, 706 is used to add the CO2 to the media 102. According to some embodiments of the present invention, the membrane tubes 702, 704, 706 are constructed of similar material; according to other embodiments of the present invention, membrane tubes 702, 704, 706 may be constructed of different materials or a combination of materials. The fluids 720, 740, and 760 may be selected to be the same, or different, in order to remove oxygen from and/or add carbon dioxide to the media 102, as described above, according to embodiments of the present invention. For example, fluids 720 and 740 may be air or nitrogen enriched air to remove oxygen from the media 102, and fluid 760 may be carbon dioxide in order to add carbon dioxide to the media 102, according to embodiments of the present invention. Based on the disclosure herein, one of ordinary skill in the art will appreciate the number of different fluids 720, 740, 760 and the number of different permeable membrane materials and configurations that may be used for tubes 702, 704, 706 to achieve similar results, according to embodiments of the present invention.
According to some embodiments of the present invention, all or a portion of the photobioreactor bag itself may include a higher permeability membrane in order to increase the amount of surface area available for gas transfer. Some higher permeability membranes do not pass light very well; this could affect the reactor performance in some cases, if the high permeability membrane is used for the exterior of the reactor. According to some embodiments, the reactor is configured such that the effect of the reduced light transmission is minimized.
Numerous methods may be used to manufacture membranes integrated into film bioreactors. One method of doing this is to weld the different layers of film together to form a film photobioreactor, where portions are made of regular film and other sections are made of porous membranes to permit gas transfer, according to embodiments of the present invention.
CO2 with pressures ranging from zero to as much as ten pounds per square inch or more may be used, according to embodiments of the present invention. In order to facilitate the welding, two outside pieces 1003, 1004 of thicker and more weldable film may be cut so that there is plastic at the desired locations of the welds. The outside pieces 1003, 1004 are configured to reinforce the weld locations of the membrane layers 1001, 1002 without adding too much plastic in the areas which do not need as much reinforcement. Adding too much heavier outer plastic can make the photobioreactors heavier for transport and may adversely affect the operation of the photobioreactor by reducing the amount of light that gets inside the reactor. Outside layers of film 1003, 1004 used to facilitate the welding may be made from 0.005 inch thick composite film that includes layers of polyethylene, nylon and “tie” layers between the polyethylene and nylon layers. The four layers 1001-1004 may be placed on top of each other as shown and welded using an impulse welder; during welding, the plastic from the outside welding layers 1003, 1004 melts and flows against the membrane layers 1001, 1002, according to embodiments of the present invention. This helps make a solid joint and reduces the possibility for holes or flaws in the weld joint, according to embodiments of the present invention. A similar method may also be used to join porous membranes consisting of spun polyethylene, and can be very effective with such materials as they tend to shrink and pull away from the weld area when exposed to heat. The outer layers 1003, 1004 also reduce the temperature experienced by the membranes 1001, 1002, further helping to control the welding process.
According to some embodiments of the present invention, the membrane tube includes a single opening in fluid communication with a gas source; for example, the membrane tube 121 of
According to some embodiments of the present invention, a photobioreactor operates in an open loop condition to provide stable pH control with accuracy, by employing a method to estimate the growth rate from the required pressure in the membrane bag if it were a stable membrane bag. According to such embodiments, a photobioreactor includes a membrane tube integrated into the reactor to introduce CO2 to the system. According to some embodiments of the present invention, the diffusion rate for CO2 into the media may be primarily driven by the difference in partial pressure (or equivalent partial pressure if in liquids) between the media and the gas inside the tube. At certain pH values, as CO2 is dissolved into the media the pH will become lower, all other things held constant, and it will rise as the CO2 is depleted from the media; this results in the diffusion of the CO2 through the membrane increasing as the pH rises, and decreasing as it lowers. According to embodiments of the present invention, a stable membrane may be included in the photobioreactor that will tend to automatically converge to a certain pH that would be a function of media content, cell growth rate, physical materials and configuration of the membrane (exposed surface area and permeability if the membrane material) and the pressure in the tube, according to embodiments of the present invention.
A membrane area can be selected such that the diffusion rate for a given partial pressure differential of CO2 across the membrane will provide the desired amount of carbon to the media to maintain the pH. According to some embodiments of the present invention, if the pH drops lower than the desired value the diffusion will be reduced and the pH will rise; conversely, if the pH becomes too high then the diffusion rate would increase and the pH would drop. These operating points would be stable for one growth rate and if the growth rate is higher the pressure may be raised to match the new growth rate, according to embodiments of the present invention.
A permeable membrane (integrated into the film bag) may be used to achieve stable pH without the use of buffers, according to embodiments of the present invention. Increasing pressure in the membrane tube provides more carbon to the media for growth, and a reduction in pressure lowers the pH if the pH is too high. If a pH sensor is used with a closed loop system to control the pH by increasing or decreasing the pressure in the membrane tube, the required pressure can be used to infer the growth rate, thus eliminating a need for an expensive turbidity meter, according to embodiments of the present invention.
Because the amount of gas transferred through the membrane is a function of the difference in partial pressures, the amount of CO2 transferred through the membrane into the media may be modulated by adjusting the pressure in the membrane tube. This provides an inexpensive and reliable way to control the delivery of CO2 or other gases to the media. Additional control can be achieved by selectively turning on and off different membrane tubes to adjust the total surface area of the membrane in active carbon transfer, or by controlling the concentrations of and constituents of the gases in the membrane tubes. The membrane tube may also include a gas mixture, in which the gas mixture includes carbon dioxide. The carbon dioxide delivery through the membrane tube may be controlled by changing the partial pressure of carbon dioxide within the gas mixture, and/or by changing the overall pressure of the gas mixture, and/or by changing the surface area of the membrane tube. These methods of control may be used in various combinations, along with other methods, to achieve various levels of control, according to embodiments of the present invention.
One or more membrane tubes may also be selectively permeable; in other words, a membrane tube may be configured for permeability to carbon dioxide but not to other gases, according to embodiments of the present invention. A membrane tube configured to remove dissolved oxygen from within the media solution may be permeable to oxygen but not other gases, according to embodiments of the present invention. For example, a stack gas or exhaust gas may be introduced into or circulated through a permeable membrane, which permits dissolved oxygen to enter membrane from the media, but which prohibits other gases (e.g. gases that may be toxic to algae or otherwise undesirable) from crossing into the media from within the permeable membrane, according to embodiments of the present invention. As another example, stack gas or exhaust gas may be introduced into or circulated through a permeable membrane within a photobioreactor, which permits carbon dioxide from within the permeable membrane to enter the media from within the membrane, but which prohibits other gases (e.g. gases that may be toxic to algae or otherwise undesirable) from crossing into the media from within the permeable membrane, according to embodiments of the present invention. According to some embodiments of the present invention, the photobioreactor bag itself is at least partially formed of a permeable membrane, is submerged in water, and stack gas and/or other exhaust gases are introduced directly into the water, such that the carbon dioxide equivalent partial pressure difference between the water and the media inside the photobioreactor bag causes the carbon dioxide from the water to cross into the photobioreactor bag.
Depending on the composition of the media used and desired setpoint for the pH of the media, the pH of the media may be increased by sparging air or other gases through it, according to embodiments of the present invention. This permits an inexpensive and robust way to raise the pH, according to embodiments of the present invention. Similarly, the pH can be reduced by sparging with a higher concentration of CO2 than is normally found in air. Based on the disclosure provided herein, one of ordinary skill in the art will appreciate the numerous compositions of sparging gas, and the various combinations of gas selection, membrane selection, membrane surface area, and other factors that can be used to affect the pH and/or the algae growth rate. According to some embodiments of the present invention, the membrane tube can be deflated and/or inflated over time to promote mixing.
Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/059,863, filed on Jun. 9, 2008, entitled, “Permeable Membranes in Film Photobioreactors,” which is incorporated herein by reference in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61059863 | Jun 2008 | US |
