and C0 ₂ Delivery to Photoautotrophs

Cross-Reference to Related Application

Priority is claimed to U.S. provisional application 61/267,968, filed December 9, 2009, which is incorporated by reference herein in its entirety.

Joint Research Agreement

This invention was made as a result of activities undertaken within the scope of a Joint Research Agreement between Algenol Biofuels, Inc. and Georgia Tech Research

Corporation.

Background Of The Invention

1 Field of the Invention

The invention relates to the fields of chemistry and biology. More specifically, the invention relates to capture of C0 ₂ from a gas phase and subsequent delivery of the carbon to a medium promoting the growth of photoautotrophic organisms and the production of biofuels therefrom.

2. Description of the Related Art

Global warming is largely attributed to an increase in atmospheric C0 ₂ , which results from the combustion of fossil fuels. In order to combat this phenomena, a great deal effort focuses on removal of C0 ₂ from the atmosphere, industrial sources and gas processing sources via various capture methodologies.

US Patent Application Publication No. 2008/0138265 [leading to U.S. Patent 7,699,909] discloses methods and systems for extracting, capturing, reducing, storing, sequestering, or disposing of carbon dioxide (C0 ₂ ), particularly from the air. US Patent Application Publication No. 2008/0087165 (leading to U.S. Patent 7,708,806 and in the family of PCT Application No. PCT/US2007/080229) discloses the extraction of C0 ₂ from air using conventional extraction methods or by using a humidity swing or electro dialysis method, and the subsequent delivery of C0 ₂ to a greenhouse or algal culture. US Patent Application Publication No. 2009/0120288 discloses the removal of carbon dioxide, from ambient air various sorbent technologies. In addition, there are numerous publications and patents describing capture and sourcing of C0 ₂ from anthropogenic streams (e.g., flue gas from coal fired power plants).

In adsorption-based C0 ₂ capture, the regeneration of the loaded adsorbent poses several potential hurdles. Of primary concern is the amount of energy required, i.e. steam generation, and chemical stability of the sorbent within the stripping environment, especially for amine-functionalized sorbents in a steam stripping environment. In addition the delivery of C0 ₂ to algal photobioreactors must also overcome several technical challenges including the need to deliver pure C0 ₂ to the reactor in order to prevent buildup of nitrogen or other inert species, the solubility limit of C0 ₂ in the aqueous phase if delivery through the water stream is preferred, or the energy penalties incurred with transport of large volumes of gas to many modular reactors if gas phase delivery is selected. In the present invention we present a process concept for integrated sorbent regeneration and C0 ₂ delivery that addresses these technical concerns.

Brief Summary Of The Invention

The utilization of low energy regeneration of adsorbent (no steam generation needed), simple low energy chemistry, and inexpensive active materials, e.g., soda ash, are utilized to provide for effective C0 ₂ loading in water or growth media for photoautotroph growth.

In a first aspect, the invention provides a system for delivery of carbon to a photoautotroph. The system comprises a stream containing C0 ₂ ; a solid adsorbent comprising an amine or other solid sorbent suitable for C0 ₂ capture; a carbonate-based stripping fluid; a device for washing the C0 ₂ loaded sorbent with the carbonate stripping solution, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution; a C0 ₂ selective membrane incorporated into a module allowing transfer of C0 ₂ between the bicarbonate rich contacting fluid and a photoautotroph culture medium; and a microfiltration membrane which prevents direct contact between the algae and the bicarbonate contacting solution, wherein the photoautotroph culture medium is enriched with bicarbonate providing carbon for algal growth.

In a second aspect, the invention provides a method for delivering carbon to a photoautotroph. The method comprises providing a stream containing C0 ₂ ; passing the C0 ₂ over a solid adsorbent comprising an amine or other solid sorbent suitable for C0 ₂ capture; passing a carbonate -based stripping fluid over the C0 ₂ loaded sorbent, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution; utilizing a C0 ₂ selective membrane that allows the transfer of C0 ₂ between the bicarbonate rich contacting fluid and a photoautotroph culture medium; and utilizing a microfiltration membrane to prevent direct contact between the algae and the bicarbonate contacting solution, wherein the photoautotroph culture medium is enriched with bicarbonate providing carbon for algal growth.

Brief Description of the Drawings

Figure 1 is a schematic of an integrated system illustrating elements of the first embodiment of the invention.

Figure 2 is a schematic of a system without the C0 ₂ selective membrane - example 2.

Figure 3 gives details of experiment for carbonate stripping.

Figure 4 gives the gas phase analysis demonstrating C0 ₂ capture with an APTES sorbent and quantifies the amount of C0 ₂ on sorbent to be stripped by carbonate solution.

Figure 5 gives the dissolved inorganic carbon (DIC) concentration of the aqueous solution as a function of time during the packed bed desorption experiment. Figure 6 gives the dissolved inorganic carbon (DIC) concentration in the C0 ₂ lean stream as a function a time which indicates C0 ₂ flux across the PDMS membrane.

Figure 7 gives real time pH measurements as a function of time in the C0 ₂ lean stream as a function a time indicating C0 ₂ flux across the PDMS membrane.

Figure 8 gives A) The dissolved inorganic carbon (DIC) concentration and B) the aqueous C0 ₂ concentration of each solution as a function of time during the liquid/liquid membrane contactor experiment (Example 3).

Detailed Description of the Invention

As used herein, "photoautotroph" refers to algae and cyanobacteria; more particularly it refers to macro algae or micro algae or cyanobacteria. As used herein, the term "macro algae" (seaweeds) refers to eukaryotic multicellular plants growing in salt, brackish or fresh water. They are classified into three broad groups based on their pigmentation: i) brown seaweed (Phaeophyceae); ii) red seaweed (Rhodophyceae) and iii) green seaweed iii) Chlorophyceae. Non-limiting examples include: Laminaria, Undaria, Gracilaria,

Ascophyllum, Eucheuma, Macrocystis, Lessonia, Chondrus, Sargassum, and Hizikia.

As used herein, the term "micro algae" refers to eukaryotic photoautotrophic organisms that may be unicellular or filamentous and found to be growing in salt, brackish, and fresh water or growing on land (terrestrial species). Non-limiting examples of eukaryotic micro algae include the diatoms (Bacillariophyceae), the green algae (Chlorophyceae), and the golden algae (Chrysophyceae) and all those described in Eukaryotic Microalgae

Genomics: The Essence of Being a Plant by Steven G. Ball ((2005) Plant Physiology 137: 397-398.

Cyanobacteria are photoautotrophic, prokaryotic organisms that may be unicellular or filamentous. Non-limiting examples include species of Chamaesiphon, Chroococcidiopsis, Arthrospira, Anabaena, Chlorogloeopsis, Chroococcus, Dermocarpella, Geitlerinema, Anabaenopsis, Fischerella Cyanothece, Myxosarcina Leptolyngbya Aphanizomenon, Dactylococcopsis, Pleurocapsa, Lyngbya, Calothrix, Gloeobacter, Stanieria, Microcoleus, Cylindrospermum, Gloeocapsa, Xenococcus, Oscillatoria, Microchaete, Gloeothece, Pseudanabaena , Nodularia, Microcystis, Spirulina, Nostoc, Synechococcus, Symploca, Scytonema, Synechocystis and Tolypothrix.

An important aspect of the present invention is a process concept for integrated sorbent regeneration and C0 ₂ delivery that addresses technical concerns found in the prior art. This concept is illustrated in Figure 1 which depicts a system for delivery of carbon species to a photoautotroph including the elements: a stream containing C0 ₂ [element (1)], a solid adsorbent suitable for C0 ₂ capture [element (2)], a carbonate-based stripping fluid [element (3)], a device for washing the C0 ₂ loaded sorbent with the carbonate stripping solution, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution [element (4)], a C0 ₂ selective membrane incorporated into a module allowing transfer of C0 ₂ between the bicarbonate rich contacting fluid and a photoautotroph culture medium [element (5)] and a microfiltration membrane which prevents direct contact between the algae and the bicarbonate contacting solution [element (6)].

Embodiments of the concept illustrated in Figure 1 could involve one sorbent bed, or more than one sorbent bed. The "one sorbent bed" embodiment could be done with the one bed sorbing C0 ₂ at night, from ambient air for example, and desorbing/stripping during the day as that would match the demand pattern of photoautotrophs. The two notations of element (2) in Figure 1 also illustrate that the point of entry of the stream containing C0 ₂ is distinct from the point of departure of the loaded stream.

This process concept is based on the chemical reaction of carbonate with C0 ₂ and water to form a bicarbonate. The reaction of sodium carbonate to form sodium bicarbonate is illustrated in equation 1 (computed using Heats of Formation of all species from Felder and Rousseau, Principles of Chemical Processes, 3 ^rd ed., John Wiley and Sons, 2005).

Na ₂ C0 ₃ + H ₂ 0 + C0 ₂ 2NaHC0 ₃

(eq. 1)

AH = -61.6kJ/mol

The complete regeneration from an amine-based based sorption system involves removal of C0 ₂ from the carbamate complex that is formed during capture. The carbamate formation reaction is shown in equation 2.

2RNH ₂ + C0 ₂ RNH ₃ ⁺ + RNHCOO

(eq. 2)

AH « -m lmol

It should be noted that the heat of reaction shown above is that for primary amines reacting with C0 ₂ (Kim, I, Hoff, K.A., Hessen E.T., Haug-Warberg, T., Svendsen, H.F., Enthalpy of absorption of C0 ₂ with alkanolamine solutions predicted from reaction equilibrium constants, Chemical Engineering Science, 64(9), 2009, 2027; Kim, I., and Svendsen, H.F., Heat of absorption of carbon dioxide (C0 ₂ ) in monoethanolamine (MEA) and 2-(aminoethyl) ethanolamine (AEEA) solutions, Industrial & Engineering Chemistry Research 46 (2007), pp. 5803; and Lee, L.L., 1996. Thermodynamic models for natural gas sweetening fluids. Report on GPI/GPA Project 5091-260-2288. University of Oklahoma, Norman, OK, USA, pp. 216). A typical solid adsorbent used for C0 ₂ capture may be comprised of primary, secondary and tertiary amines. As a result the effective heat of reaction is expected to be in the range of -50 to -85 kJ/mol C0 ₂ .

The net heat of reaction for removal of C0 ₂ from the loaded adsorbent into the aqueous phase is the difference in reaction energies from reaction 1 minus reaction 2, thereby producing a low regeneration energy requirement ranging from +20 to -10 kJ/mol. In various embodiments of the invention, the adsorbent may be a solid made of: porous inorganic, polymer with amine functionality (primary, secondary, tertiary or any combination) including ion exchange resins which form quaternary amine salts, or any other material which has a suitable binding energy for the physisorption or chemisorption of C0 ₂ . For example, see Youssef Belmabkhout and Abdelhamid Sayari, Adsorption (2009) 15: pp. 318-328 for suitable examples yielding exothermic enthalpies in the combination of reaction (1) minus reaction (2).

The utilization of this type of chemistry allows for the removal of C0 ₂ from the loaded adsorbent under milder conditions. This approach also utilizes the chemistry of bicarbonate conversion to drive C0 ₂ into the aqueous phase. The sodium bicarbonate can be delivered directly to the photobioreactors where the algae can use it as a carbon source. Although cyanobacteria fix carbon as C0 ₂ by the enzyme RubisCO, most of the carbon that is taken up by the cell can be HC0 ₃ ^~ (See PCT/EP2009/000892). The solubility of sodium bicarbonate in water (about 1M at room temperature) ensures adequate C0 ₂ delivery to the reactors.

Briefly, a sodium carbonate in water solution (bicarbonate lean) is employed to remove the C0 ₂ from the loaded adsorbent, thereby converting the carbonate to bicarbonate (Eq. 1). Two concepts are proposed. Figure 1 illustrates the first process concept. Here a closed system is used for the adsorbent stripping. The loaded adsorbent bed is contacted with a sodium carbonate solution. The C0 ₂ is removed through the formation of bicarbonate in the water phase. This bicarbonate rich solution is then passed over a C0 ₂ selective membrane which is swept with seawater broth (carbonate rich) from the photobioreactors. C0 ₂ moves across the membrane and the bicarbonate rich solution is returned to the bioreactor. In order to prevent biofilm formation on the permeation side of the perstraction membrane, the broth removed from the reactor is first passed through a micro/ultrafiltration membrane to remove the algae.

The device for washing the C0 ₂ loaded sorbent with the carbonate stripping solution, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution, may be designed and constructed utilizing Perry's Chemical Engineers' Handbook, Eighth Edition, ISBN: 0071422943, Authors: Green, Don W. and Perry, Robert H.

It should be noted that aqueous based carbonate scrubbing has been investigated for capture of C0 ₂ from flue gas and it compares favorably with liquid amines, but as with liquid amines, carbonate scrubbing of air for removal of C0 ₂ is not feasible due to solubility limits of (bi)carbonates in water and the low capacities at dilute C0 ₂ concentrations (Knuutila, H., Svendesen, H.F., Antilla, M., C0 ₂ capture from coal-fired power plants based on sodium carbonate slurry; a systems feasibility and sensitivity study, International Journal of

Greenhouse Gas Control, 3, 2009, 143). In a first aspect, the invention provides a system for delivery of carbon to a photoautotroph. The system comprises a stream containing C0 ₂ ; a solid adsorbent comprising an amine suitable for C0 ₂ capture; a carbonate-based stripping fluid; a device for washing the C0 ₂ loaded sorbent with the carbonate stripping solution, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution; a C0 ₂ selective membrane incorporated into a module allowing transfer of C0 ₂ between the bicarbonate rich contacting fluid and a photoautotroph culture medium; and a microfiltration membrane which prevents direct contact between the algae and the bicarbonate contacting solution, wherein the photoautotroph culture medium is enriched with bicarbonate providing carbon for algal growth.

In a second aspect, the invention provides a method for delivering carbon to a photoautotroph. The method comprises providing a stream containing C0 ₂ ; passing the C0 ₂ over a solid adsorbent comprising an amine suitable for C0 ₂ capture; passing a carbonate- based stripping fluid over the C0 ₂ loaded sorbent, thereby allowing the removal of C0 ₂ from the sorbent and formation of a bicarbonate contacting solution; utilizing a C0 ₂ selective membrane that allows the transfer of C0 ₂ between the bicarbonate rich contacting fluid and a photoautotroph culture medium; and utilizing a microfiltration membrane to prevent direct contact between the algae and the bicarbonate contacting solution, wherein the

photoautotroph culture medium is enriched with bicarbonate providing carbon for photoautotroph growth.

The second concept uses an open system where the C0 ₂ depleted broth is used as the C0 ₂ stripping medium. Here the broth is passed through a microfiltration membrane, and the permeate is fed directly into the C0 ₂ saturated adsorber. This concept is simpler in design, but is likely applicable only in a situation where volatile products, such as ethanol, have been removed from the reactor broth, as part of the C0 ₂ collection strategy.

Therefore, in various embodiments of the invention, the algal microfiltration membrane is not included in the systems and methods described herein.

The invention described herein may be utilized to provide C0 ₂ to open ponds or closed photobioreactors.

Examples

The following examples illustrate embodiments of the invention. Example 1 presents options for the C0 ₂ source, the adsorbent for C0 ₂ , the configuration of the contactor, the composition of the carbonate stripping fluid, the stripping device, the C0 ₂ selective membrane, and the microfiltration membrane. Example 2 presents an embodiment wherein the stripping fluid comprises a filtered bicarbonate lean stream from the photobioreactor or photobioreactors. Example 3 presents working embodiments in support of the invention.

Example 1. Regeneration of a C0 ₂ loaded adsorber with carbonate solution using a C0 ₂ - selective, membrane system

The source of C0 ₂ may be air, flue gas from power plants or industrial sources, or natural gas treating.

The adsorbent may be a solid made of: porous inorganic, polymer with amine functionality (primary, secondary, tertiary or any combination) including ion exchange resins which form quaternary amine salts, or any other material which has a suitable binding energy for the physisorption or chemisorption of C0 ₂ . These are well known in the art.

Contactor configuration may be a packed bed with low pressure drop adsorbent particles which may be mixed with inorganic components to increase porosity and limit pressure drop, a honeycomb monolithic ceramic supporting a suitable C0 ₂ adsorbent, or an inorganic-organic hybrid or polymer based fiber contactor made of functionalized amine, amine rich polymer or polymer loaded with ion exchange resin. These are well known in the art. Our intention here is to select from amongst the many C0 ₂ sorbent systems, those that are consistent with the extraction of C0 ₂ via carbonate contacting fluid and to design the sorbent module in a manner which optimizes the stripping process described below.

The stripping solution may be an aqueous carbonate preferentially sodium, but also potassium. The concentration of the fluid may range from 0.01 to 0.55M (1 mole goes to 2 moles of bicarbonate which has solubility limit of 1.19M in water). Solution may also contain other salts and metals such as found in seawater.

The stripping device is designed to allow the loaded adsorbent to be washed with stripping solution. The device will also allow for the heating of the stripping solution in the range of 10 to 50 degrees Celsius above ambient temperature. The device will include a means for delivery of the stripping solution such that the solution may have a residence time in the contactor from 30 seconds to 30 minutes. The stripping solution will be converted from carbonate rich to bicarbonate rich and leave the regenerated adsorber with a bicarbonate concentration from 0.02 to 1.1M with the balance of the carbonate from the lean solution being un-reacted carbonate. In an embodiment wherein photoautotrophs are present in a photobioreactor or photobioreactors, the C0 ₂ selective membrane takes a flow of the bicarbonate rich solution on one side (retentate) and is swept with filtered culture fluid (carbonate rich) from the photobioreactors on the other side (permeate). Such a system is not known in the art. The driving force for C0 ₂ transport across the membrane is the chemical potential gradient moving toward the carbonate/bicarbonate equilibrium of the two fluids. The membrane may be composed of a rubbery polymer such as polydimethylsiloxane or a glassy polymer chosen from a number of families such as cellulose acetate, polyimides or polyether sulfones, or the membrane may be of layered construction containing polymers of any or all of the classes mentioned above. The C0 ₂ permeability of the membrane may be in the range of 100— 10,000 barrers. In this device, C0 ₂ moves across the membrane converting the bicarbonate lean solution from the photobioreactor to a bicarbonate rich solution, which is then returned to the bioreactor.

The microfiltration membrane filters the photoautotrophs from the culture in order to prevent biofilm formation on the permeation side of the C0 ₂ selective membrane (due to the high concentration of bioavailable C0 ₂ ). The culture medium removed from the reactor is first passed through a micro/ultrafiltration membrane to remove the photoautotrophs. This membrane must have an effective pore size in the range of 0.05 - 5μιη and may be chosen from any number of commercially available microfiltration membranes made from ceramic or polymeric materials, which are all well known in the art.

Example 2. Regeneration of C0 ₂ loaded adsorber using a stripping solution comprising a bicarbonate lean broth from reactor as stripping solution.

This example uses the same elements as in example number 1 but comprises a filtered bicarbonate lean stream from the photobioreactors as the stripping solution. The same concentration ranges apply and same microfiltration membrane would be used.

Example 3. Demonstration of a C0 ₂ desorption from a C0 ₂ loaded sorbent using a carbonate solution and C0 ₂ transfer between aqueous solutions using a C0 ₂ selective polymer membrane.

Sorbing CO ₂ onto absorbent

A packed bed column with a 3-Aminopropyltriethoxysilane (APTES) sorbent was used to remove C0 ₂ from a gaseous stream. The loaded sorbent was then stripped an aqueous solution to assess the viability of desorbing C0 ₂ from the sorbent using an aqueous solution. A conceptual diagram of the setup is illustrated in Figure 3. A 7 mL column was first filled with 1.31 g of 3 mm of glass beads. Then 1 g of the amine grafted glass bead sorbent purchased from Polyscience, Inc. was placed into the column, then additional 1.73 g of 3 mm glass beads were packed on top of the sorbent. Column frits (10 μιη filters) at the top and bottom of the column prevent the movement of glass beads and sorbent. No C0 ₂ sorption was shown to occur onto the 3 mm glass beads. A 12.5% mol/mol C0 ₂ (the rest N ₂ ) was fed into the column at a rate of 0.05 SCFH (22.39 mL/min). The amount of C0 ₂ in the exit stream was measured as a function of a time (Figure 4). The amount of C0 ₂ adsorbed by the sorbent was calculated to be 0.156 mg as C which corresponds to a sorption capacity of 0.156 mg C0 ₂ as C/g of sorbent.

Valves were then changed to allow an aqueous solution to pass over the sorbent to desorb the sorbed C0 ₂ . The aqueous solution was prepared in a tedlar bag by weighing out 0.115 g NaOH and pumping in 115 mL of deionized water to eliminate C0 ₂ transfer to the solution similar to the preparation in the liquid/liquid membrane experiment. The solution was well mixed using a stir bar in the tedlar bag and 15 mL of solution was used to determine the initial pH and DIC concentrations giving a final water reservoir volume of 100 mL. The initial pH of the solution was 12.51 and the initial dissolved inorganic carbon (DIC) concentration was 1.073 mg/L. The initial DIC concentration in solution was attributed to C0 ₂ transfer from the air to the water before the water was pumped into the tedlar bags. The aqueous solution was pumped from the bottom of the column to top at a rate of 2 mL/min and returned back to the water reservoir. The change in DIC concentration of the aqueous solution increased as a function of time (Figure 5) with a final DIC concentration of 2.26 mg/L. The amount of C0 ₂ desorbed from the sorbent was 0.126 mg as C which corresponds to 80.8% of the total sorbed C0 ₂ .

Desorbing CO ₂ into stripping solution

The initial concentration of the bicarbonate solution 12.067 mg as C/L (=12.067 DIC/liter) and the aqueous C0 ₂ concentration was 1.91xl0 ^"2 M with an initial pH was 8.06. The initial concentration of C0 ₂ poor solution was 70.0 mg as C/L and the aqueous C0 ₂ concentration was 4.14xl0 ^"5 M with an initial pH of 8.38. Since the aqueous C0 ₂ concentration was greater in the bicarbonate solution, there was a driving force for C0 ₂ flux across the C0 ₂ selective membrane. The DIC concentration in the C0 ₂ poor solution increased with time (Figure 6) indicating C0 ₂ flux across the membrane. The pH of the C0 ₂ poor solution decreased with time (Figure 7). The pH change was due to the acidification of the solution with the addition of C0 ₂ and was used as an indirect indicator of C0 ₂ transfer in systems. The initial flux of C0 ₂ across the membrane was 3.55xl0 ^"6 kg/s m ² and the total amount of C0 ₂ transferred in this process was 135.4 mg as C.

CO ₂ transfer between aqueous solutions using a CO ₂ selective polymer membrane

A liquid/liquid membrane contactor experiment was used to determine the viability of a C0 ₂ selective membrane to transfer aqueous C0 ₂ between a bicarbonate (C0 ₂ rich) aqueous solution to a C0 ₂ poor solution. A conceptual diagram of the setup is illustrated in Figure 1. Solution 1 was a 1 M NaHCOs solution and Solution 2 is a 70 mg as C/L aqueous solution simulating the amount of dissolved inorganic carbon (DIC) found in a

photobioreactor solution. The container for each solution reservoir was a 1L tedlar bag with two polypropylene 2-in-l valves with a septum valve and a 1/8" fitting. The tedlar bags were used to eliminate C0 ₂ transfer between the atmosphere to solution. Each solution was made by weighing out the appropriate amount of sodium bicarbonate, NaHCC , (Solution 1 = 33.604 g and Solution 2 = 0.1960 g) then with the tedlar bag closed air was removed from each bag using the septum valves. Finally, 400 mL of deionized water was pumped into each tedlar bag to minimize the amount of gas in each bag. Each aqueous solution was well mixed using stir bars in each bag. During the experiment each solution was pumped at a flow rate of 2 mL/min. A pH probe was placed in the C0 ₂ poor solution to monitor the change in pH. The change in pH was due to the uptake of C0 ₂ into solution. At periodic intervals the aqueous solutions were sampled using a syringe to determine the dissolved inorganic carbon (DIC) concentration.

A polydimethylsiloxane (PDMS) membrane (92 μιη thick) was used as a

C0 ₂ selective membrane to transfer aqueous C0 ₂ between a bicarbonate (C0 ₂ rich) aqueous solution to a C0 ₂ poor solution. The PDMS membrane was synthesized by first weighing out 9:1 wt/wt ratio of silicone elastomer base to curing agent and dissolving in heptane to make a 20% wt/wt solution of PDMS in heptane. The solution of 20% wt/wt PDMS in heptane was then pipetted onto a Teflon plate and placed in a vacuum oven at 80°C for two hours. After boiling off the heptane, the PDMS film was left to cool at room temperature before removing the film and placed into a permeation cell to determine gas permeability and selectivity. The permeability [Barrer = 10 ^"10 cm ³ (STP) ^» cm / cm ^2» s ^» cm ^» Hg)] of the PDMS film were measured with pure gases at 22°C (C0 ₂ = 2170, 0 ₂ = 398, N ₂ = 206 Barrer). The gas selectivities were then calculated from the permeabilities (α0 ₂ /Ν ₂ = 1.9, aC0 ₂ /0 ₂ = 5.5, aC0 ₂ /N ₂ = 10.5), and confirmed the formation of a PDMS membrane.

Cyanobacteria have been shown to utilize aqueous C0 ₂ , HCO3 ^" and CO3 ^2" as the source of carbon for photosynthesis (to produce biofuels and other products). Dissolved inorganic carbon (DIC) concentration is defined as the total concentrations (grams

carbon/liter) of C0 ₂ , HCO3 ^" , and CO3 ^2" . The form of carbon used by the microalgae is not a critical issue because reactions that interconvert C0 ₂ ( _aq ), H2CO3, HCO3 ^" and CO3 ^2" are sufficiently fast as to not be limiting steps in carbon demand by the photoautotrophs. In the C0 ₂ selective membrane only C0 ₂ will pass through the membrane. When the C0 ₂ transfers to the C0 ₂ lean stream, it will disassociate quickly to another form of inorganic carbon dictated by the pH of the solution. The difference between aqueous concentrations of C0 ₂ on both sides of a membrane must be high enough to ensure C0 ₂ flux across the membrane.

Even though the difference between DIC concentrations of the two solutions remained large throughout the experiment (Figure 8A), the DIC concentration in the C0 ₂ lean solution reached a steady state concentration. The difference between the aqueous C0 ₂ concentration of the two solutions became small (Figure 8B) minimizing the concentration driving force between the two solutions. It is important to consider the difference in aqueous C0 ₂ concentrations when optimizing this process to other systems which allows the system to be flexible and may not require such a carbonate loaded stream in other processes. For example, to maintain the initial C0 ₂ flux in the experiment stated above, a DIC concentration of 12,000 mg/L was needed at pH of 8.0 but lower DIC concentrations would be needed at lower pH values (1430 mg/L and 273 mg/L of DIC at pH values 7.0 and 5.0 respectively). Operational pH values of the C0 ₂ stripping stream will be dependent on the sorbent type. The desired pH value could be as low as the sorbent can maintain its structural integrity but will lie within the pH range of 5.0 to 8.2. Stripping stream solutions with a pH greater than 8.2 with the desired C0 ₂ flux will have problems with the precipitating out of solids. The operational DIC concentrations of the C0 ₂ lean stream can vary from 0.31 mg/L (0.03 mM) to 3000 mg/L (250 mM) while the operational pH values of the C0 ₂ lean stream could vary from 7.0 to 9.0. A high C0 ₂ flux across the membrane is desired to minimize membrane area and to meet C0 ₂ delivery demand. In general to achieve a high C0 ₂ flux, a high pH ( > 8.0) and a low DIC ( < 70 mg/L) would be desired on the C0 ₂ lean stream while a low pH ( < 7.5) and a high DIC would be desired on the C0 ₂ stripping stream. The monitoring of aqueous C0 ₂ concentration in each stream is desired to ensure that the concentration in the C0 ₂ stripping stream is greater than the C0 ₂ lean stream at all times, and can be accomplished by an imbedded pH probes to provide real time pH readings, syringe pumps with a desired base (eg. sodium hydroxide) or a desired acid (eg. hydrochloric acid), and a direct measure of DIC using a TOC analyzer, infrared spectrometer, or Raman spectrometer.

Equivalents

Those of ordinary skill in the art will recognize, or be able to ascertain, using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. The invention is embodied in each novel characteristic and each combination of

characteristics, which particularly includes every combination of any features which are stated in the claims, even if this feature or this combination of features is not explicitly stated in the claims or in the examples.