Patent Application: US-201214232006-A

Abstract:
in the present invention , a photobioreactor and process for producing and harvesting microalgae involves a vessel for cultivating microalgae that is at least partially transparent to admit light into the vessel . at least a portion of the transparent part of the vessel is coated with a transparent conductive oxide layer . the tco layer is transparent to visible light necessary for algae growth , but is opaque to infrared light thereby reducing thermal heating load in the photobioreactor . the tco layer also acts as an electrode , which when combined with a counter - electrode can provide a potential difference across at least a portion of the interior of the vessel between the tco layer and the counter - electrode . the electrode arrangement can be utilized in an electrochemical process to dewater and harvest the microalgae in the same apparatus as the microalgae was cultivated .

Description:
a simplified microalgae value chain showing steps in a process for obtaining bio - product from microalgae cultivation is depicted in fig1 . in step # 1 , microalgae is cultivated by growing it on a cell culture medium in a photobioreactor . typically , the total solids content ( tss ) of the algal effluent created during cultivation is on the order of about 0 . 05 %. after cultivation to produce quantities of microalgae , the microalgae must be dewatered and harvested . dewatering typically takes place in a primary dewatering step ( step # 2 ) to produce an algal slurry having a tss in a range of from about 0 . 5 - 5 % followed by a secondary dewatering step ( step # 3 ) producing an algal sludge / cake having a tss in a range of from about 10 - 20 %. primary and secondary dewatering using electrochemical processes is primarily concerned with removing extracellular water . in the present process , cultivation , primary dewatering and secondary dewatering may all be accomplished in the same apparatus , i . e . the photobioreactor , and the algae harvested only at the end of the secondary dewatering step . the process can therefore be more efficient and cost effective . after harvesting the algae from the secondary dewatering step , the algae is further dried in step # 4 to provide dried algae having a tss of about 25 % or more . the drying step may further focus on removal of intracellular water . dried algae can then be processed to recover desired bio - products . referring to fig2 , a tubular photobioreactor comprises cylindrical vessel 1 having outer wall 2 made of a transparent plastic that permits solar energy to enter the interior of vessel 1 where the microalgae is being cultivated . outer wall 2 has a curved inside and outside surface and the inside surface is coated with transparent conducting oxide ( tco ) layer 3 comprising fluorine doped tin oxide ( fto ). the tco layer blocks infrared red light from entering the vessel while transmitting visible light . tco layer 3 also acts as an electrode in an electric circuit further comprising rod - like counter - electrode 5 made from ptfe - coated aluminum and power generator 9 for applying a voltage across the electrodes . applying low voltage and current across the electrodes after the microalgae production cycle is complete polarizes the electrodes , with tco layer 3 being a cathode ( negative ) and counter - electrode 5 being an anode ( positive ). since microalgae are slightly negatively charged , the microalgae produced during cultivation are repelled from negatively charged tco layer 3 on the outside wall of cylindrical vessel 1 and attracted to positively charged anode 5 suspended in the algae culture medium along the full length of and in the center of cylindrical vessel 1 . on applying a voltage and current sufficient to electrolyze water , aggregates 7 of microalgal cells are carried to the surface of the culture medium by hydrogen and oxygen gas bubbles formed during water electrolysis . for simplicity , standard photobioreactor accessories and connections are not shown in fig2 . referring to fig3 , a flat plate photobioreactor comprises vessel 11 having opposed first outer wall 14 and second outer wall 16 both made of a transparent plastic that permits solar energy to enter the interior of vessel 11 where the microalgae is being cultivated . the inside surfaces of outer walls 14 and 16 are coated with transparent conducting oxide ( tco ) layers 13 and 15 , respectively , each tco layer comprising fluorine doped tin oxide ( fto ). the tco layers block infrared red light from entering the vessel while transmitting visible light . tco layers 13 and 15 also act as electrodes in an electric circuit further comprising power generator 19 for applying a voltage across the electrodes . on applying a voltage and current sufficient to electrolyze water , aggregates 17 of microalgal cells are carried to the surface of the culture medium by hydrogen and oxygen gas bubbles formed during water electrolysis . for simplicity , standard photobioreactor accessories and connections are not shown in fig3 . aggregates of microalgae cells produced in photobioreactors , generally contain total solids content ( tss ) of about 20 % and may be collected in any one of a number of different ways . in a batch process , the anode having any aggregates of microalgal cells deposited thereon may be removed from the photobioreactor and the microalgae recovered from the anode either mechanically ( e . g . by scraping or skimming ) or chemically ( e . g . by dissolving in a solvent ( e . g . hexanes )). chemical recovery can further facilitate downstream bio - product extraction . in a continuous process , a skimmer and collection barrel may be added to the photobioreactor . the continuous process for microalgae harvesting is promoted by electroflotation in which the microalgae aggregates are moved toward the surface of the culture medium . the voltage and current across the electrodes is set to permit electrolysis of water so that oxygen formed at the anode will help flocculate the microalgae and float the flocculates to the surface . once at the surface , the flocculated microalgae is more easily collected by a skimmer into a barrel . electroflotation requires little energy and no chemical flocculants . since oxygen is formed at the anode and hydrogen is also formed at the cathode , the photobioreactor should be equipped with means to remove these gases , especially the oxygen , in order to increase yield of the microalgae . temperature , ph , current density and anode geometry may be adjusted to achieve a desired oxygen bubble size for more efficient flotation of the microalgae . typical operation for both batch and continuous processes is based on 24 hour cycles . during the day microalgae is grown , while at night an electrochemical process is applied to harvest the algae . thus electricity from off - peak power could be utilized , thereby reducing operating costs . other operations including changing water and other inputs may also done in the absence of solar radiation . shorter and longer cycle durations may also be used depending on the microalgae species and other considerations including solar irradiation and microalgae concentration . a transparent conducting oxide ( tco ) coating blocks the infrared ( ir ) portion of excitation lamps used as the light source for algae growth in the reactor . thus , the operating temperature of a tco - coated glass photobioreactor should be lower than that of a plain glass photobioreactor . further , because a plain glass photobioreactor is expected to operate at a higher temperature ( in the absence of additional cooling steps ), algae growth rate in the plain glass reactor should also be less than in the tco - coated glass bioreactor . two 9 l photobioreactors ( pbrs ) were constructed using a flat - plate design , one using plain glass walls ( glass - pbr ), and one using tco - coated glass walls ( tco - pbr ), where the tco layers coated on opposing glass walls act as electrodes for further harvesting of the algae . the tco layer comprised fluorine doped tin oxide ( fto ). a pavlova strain of algae obtained from mrs ( marine research station , nrc halifax ) was cultured in the bioreactors in an aqueous culture medium with carbon dioxide introduced into the culture medium by means of a conduit . the culture medium comprised f / 2 stock solution and tris ( hydroxymethyl ) aminomethane . ( tris ). the reactors were operated for an extended period of time using the same light source to supply light for algae growth . two sets of two 60 w g25 soft white bulbs were used . light was supplied under a normal daily photo - regime , and no additional cooling was supplied to either reactor . fig4 a shows the temperature in each reactor as a function of the time of day , and fig4 b shows the concentration of algae as a function of the length of time the photobioreactors are operated . fig4 a shows that culture temperature in the glass - pbr is about 2 ° c . higher for most of the photo - irradiation period than the temperature in the tco - pbr . further , the culture temperature in the glass - pbr exceeded 27 ° c . for much of the photo - irradiation period . for most algae strains , operation temperature above 27 ° c . is detrimental to algae growth , therefore additional cooling is normally required for a glass - pbr . however , the temperature in the tco - pbr never exceeded 27 ° c ., thereby reducing cooling requirements normally needed to sustain algae growth in a glass - pbr . fig4 b confirms that algae growth rate obtained using the tco - pbr is about 2 - times faster than what is obtained with the glass - pbr . harvesting of the algae in the tco - pbr was accomplished by electroflotation using the tco layers coated on opposing glass walls act as electrodes using a continuous power with 3 volts and 1 amp . electroflotation harvesting lead to algae concentration of 3 . 5 wt % ( or 35 g / l ), which is within the 2 - 5 wt % concentration range reported in the literature . concentration of the harvested algae was estimated using a freeze - dry process . the total electric power consumption of this electroflotation harvesting process was less than 0 . 3 kwh / m 3 . the low cost and high efficiency of this electroflotation harvesting process is a useful complement to more energy intensive centrifugation processes . the contents of the entirety of each of which are incorporated by this reference . au s h , shih s c c , wheeler a r . 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( 2003 ) plant - cell bioreactors with simultaneous electropermeabilization and electrophoresis . journal of biotechnology . 100 , 13 - 22 . other advantages that are inherent to the structure are obvious to one skilled in the art . the embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed . variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims .