Abstract:
A method and system are provided for supporting the growth of algae cells. Initially, an inoculum of algae cells are grown in a closed bioreactor. Thereafter, the inoculum is passed into an open Expanding Plug Flow Reactor (EPFR). Growth medium is added at a plurality of locations along the EPFR. This addition is controlled in response to the growth rate of the algae cells to maintain a substantially same concentration of cells at each location in the EPFR. At all times, the medium provides sufficient nutrients to support growth and maintain a high concentration of algae cells, i.e., at least 0.5 grams per liter of medium, in the EPFR. After the desired level of growth is reached, the algae cells are transferred from the EPFR to a standard plug flow reactor wherein oil production is activated in the algae cells.

Description:
[0001]    This application is a continuation of application Ser. No. 12/821,943, filed Jun. 23, 2010, which is currently pending. The contents of application Ser. No. 12/821,943 are incorporated herein by reference. 
     
    
       [0002]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. HR0011-09-C-0034 awarded by DARPA. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention pertains generally to methods for growing algae. More particularly, the present invention pertains to the use of an expanding plug flow reactor to reduce the requirement of using expensive closed system bioreactors for growing algae. The present invention is particularly, but not exclusively, useful as a method for growing algae in an open system comprising an expanding plug flow reactor fed with a medium to maintain a high concentration of algae cells. 
       BACKGROUND OF THE INVENTION 
       [0004]    As worldwide petroleum deposits decrease, there is rising concern over shortages and the costs that are associated with the production of hydrocarbon products. As a result, alternatives to products that are currently processed from petroleum are being investigated. In this effort, biofuel such as biodiesel has been identified as a possible alternative to petroleum-based transportation fuels. In general, a biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from plant oils or animal fats. In industrial practice, biodiesel is created when plant oils or animal fats are reacted with an alcohol, such as methanol. 
         [0005]    For plant-derived biofuel, solar energy is first transformed into chemical energy through photosynthesis. The chemical energy is then refined into a usable fuel. Currently, the process involved in creating biofuel from plant oils is expensive relative to the process of extracting and refining petroleum. It is possible, however, that the cost of processing a plant-derived biofuel could be reduced by maximizing the rate of growth of the plant source. Because algae is known to be one of the most efficient plants for converting solar energy into cell growth, it is of particular interest as a biofuel source. Importantly, the use of algae as a biofuel source presents no exceptional problems, i.e., biofuel can be processed from oil in algae as easily as from oils in land-based plants. 
         [0006]    While algae can efficiently transform solar energy into chemical energy via a high rate of cell growth, it has been difficult to create environments in which algae cell growth rates are optimized. Currently, the production of biofuel from algae is limited by a failure to maximize algae cell growth. Specifically, the conditions necessary to facilitate a fast growth rate for algae cells in large-scale operations have been found to be expensive to create. For instance, while providing high rates of algae cell growth, closed sterile environments such as inoculant tanks and controlled bioreactors are expensive to maintain and limited in scale. On the other hand, outdoor large-scale open systems, such as open runways, are plagued by contaminant organisms which fight the selected algae cells for nutrients and sunlight and reduce the rate of algae cell growth. Specifically, these contaminants include non-selected, i.e., “weed”, algae, viruses, bacteria, and grazers. Until now, it has been virtually impossible to prevent contaminant organisms from causing microbial instability and reducing selected algae cell growth rates in open systems. In fact, standard open systems typically provide only one to two days of microbial stability. 
         [0007]    In light of the above, it is an object of the present invention to provide a method for minimizing the need for closed system inoculation of algae cells in a biofuel production system. Another object of the present invention is to maximize the cell growth rate of selected algae cells in an open system. Another object of the present invention is to provide an expanding plug flow reactor for supporting logarithmic growth of algae cells. Another object of the present invention is to selectively pump medium into the expanding plug flow reactor to maintain a high concentration of algae and a selected shallow depth of medium. Still another object of the present invention is to provide a method and system for growing selected algae cells in an open system in which contaminants cannot compete with the selected algae cells. Yet another object of the present invention is to provide a system and method for growing selected algae cells that is simple to implement, easy to use, and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention, a system is provided for growing selected algae cells in a medium and for preventing the growth of contaminants in the medium. In this endeavor, the system relies on the initial use of a closed reactor to grow an inoculum of microalgae. Importantly, the closed reactor is five times smaller than those used in known algae production systems. Specifically, the closed reactor comprises 0.4% of the present system while closed reactors typically comprise about 2% of known systems. For purposes of the present invention, the closed reactor is a continuous flow reactor such as a photobioreactor. Further, the closed reactor is designed to grow the inoculum of microalgae to a full concentration. 
         [0009]    After the closed reactor grows microalgae to full concentration, the inoculum of microalgae is passed in an effluence to an open system. Specifically, the open system comprises an expanding plug flow reactor and a standard plug flow reactor. For the present invention, the expanding plug flow reactor continuously receives the effluence containing the inoculum of algae cells from the closed reactor. Further, the expanding plug flow reactor includes a conduit for continuously moving the effluence downstream under the influence of gravity with little back mixing. Preferably, the expanding plug flow reactor is an open raceway. 
         [0010]    Structurally, the expanding plug flow reactor increases in width from its first end to its second end. Also, the expanding plug flow reactor is provided with a plurality of pumps along its length for introducing a growth medium to the conduit. Initially, the pumps dilute the effluence until the algae reaches a high concentration. For purposes of the present invention, “high concentration” is defined as at least about 0.5 grams per liter of fluid. Thereafter, as fluid evaporates and the algae cells grow, the pumps add growth medium to maintain the high concentration of algae. Further, the growth medium includes the nutrients necessary to support the desired growth of the algae cells. 
         [0011]    Importantly, the pumps are controlled in response to the growth rate of the algae cells. For instance, the algae growth rate may decrease due to a reduction in the amount of sunlight received and lower air temperatures. As a result, in order to ensure a high concentration of algae as the expanding plug flow reactor widens, the pumps will provide less medium. Therefore, the depth of the medium will decrease slightly, and the flow rate of the algae cells will decrease due to the viscosity of the algae cells. With the reduced flow rate, the algae cells are provided with enough time to grow sufficiently to remain at a high concentration as the expanding plug flow reactor widens. Because the selected algae is maintained at a high concentration, the nutrients provided in the growth medium are rapidly consumed by the selected algae. As a result, the time available for growth of contaminants is limited. 
         [0012]    When the selected algae cells reach the end of the expanding plug flow reactor, they have reached the desired level of growth. Thereafter, the algae cells are transferred to a standard plug flow reactor. Typically, the standard plug flow reactor will have the same width as the downstream end of the expanding plug flow reactor. Further, a trigger medium may be fed into the standard plug flow reactor to activate production of oil in the algae cells. Alternatively, no medium may be fed into the standard plug flow reactor. This alternative method is effective to trigger oil production because algae cells will convert stored energy to oil when being starved of certain, or all, nutrients. Further, as the medium evaporates in the standard plug flow reactor, the depth of the medium will be reduced until the algae naturally flocculates. In this manner, the standard plug flow reactor may be designed to self-flocculate when optimal oil production has been achieved. 
         [0013]    For an alternate embodiment of the present invention, a system for growing algae cells includes a plurality of open ponds. In combination, open ponds in this plurality are connected for selective fluid communication with each other, and they are arranged in sequence from a first upstream pond to a last downstream pond. In a variation from the expanded plug flow reactor (EPFR) described above, this alternate embodiment of the invention establishes each downstream pond with an exponentially greater surface area relative to its adjacent upstream pond. 
         [0014]    Structurally, the alternate embodiment of the present invention includes a first transfer conduit for transferring inoculum from an inoculum source into the first upstream pond. A culture is thereby created for algae growth in the first upstream pond. A subsequent transfer of the culture can then be made from the first upstream pond to successive downstream ponds for further algae growth. For the present invention, such transfers are periodically accomplished in a controlled manner, and algae is allowed to grow for a predetermined time in each of the successive ponds. Eventually, fully grown algae cells are transferred from the last downstream pond to an oil formation pond via a last transfer conduit. 
         [0015]    Each open pond in the system, regardless of its relative size, will preferably have a fluid circulating device, such as a paddle wheel or circulation pump, that can be used to establish liquid flow in the pond. 
         [0016]    Preferably, each pond will also have a medium addition conduit for adding medium into the culture in the pond. Further, as envisioned for the present invention, the transfer of culture from an upstream pond to its adjacent downstream pond can be accomplished in either of two ways. For one, each pond may include a transfer pump for transferring the culture downstream from the pond to its adjacent downstream pond. For another, the ponds can be terraced so that a gravity flow can be established from an upstream pond to a downstream pond. 
         [0017]    As implied above, a fixed multiplier is determined to establish a ratio of the surface areas for adjacent ponds. More specifically, the surface area of each pond relative to the surface area of an adjacent upstream or downstream pond will be established by this multiplier. In practice, the value of the multiplier may vary from system to system. Specifically, in each case the multiplier will be determined by the growth rate of the algae that is being used for cultivation in the particular system. 
         [0018]    In an operation for the alternate embodiment of the present invention, a transfer sequence is periodically performed in accordance with a set procedure. Specifically, the transfer sequence is initiated by first transferring fully grown algae from the last downstream pond to an oil formation pond. Once this is done, and the last downstream pond has been emptied, culture from the adjacent upstream pond is then transferred into the now-empty, last downstream pond. As the culture is transferred, additional medium can also be transferred into the last downstream pond for further algae growth in the last downstream pond. The now-empty, immediately upstream pond can then receive culture transferred from its respective adjacent upstream pond. This process of transfer from an upstream pond to an emptied adjacent downstream pond continues until the first upstream pond has been emptied and subsequently refilled with inoculum from the source of inoculum. After an entire transfer sequence has been completed, the cultures in all of the open ponds are individually circulated to promote algae growth. Once algae growth in the respective ponds has been completed, the entire transfer sequence can then be repeated. Preferably, transfer sequences for the alternate embodiment of the present invention are accomplished during the nighttime. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0020]      FIG. 1  is a schematic view of the system of the present invention, illustrating the flow of algae from the closed reactor, through the expanding plug flow reactor, and to the standard plug flow reactor in accordance with the present invention; 
           [0021]      FIG. 2  is an overhead view, not to scale, of the expanding plug flow reactor shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a longitudinal cross-sectional view of the expanding plug flow reactor of  FIG. 2 , showing the depth of the medium in the conduit; and 
           [0023]      FIG. 4  is a schematic view for an alternate embodiment of a system in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring initially to  FIG. 1 , a system for growing selected algae cells is shown, and is generally designated  10 . As shown in  FIG. 1 , the system  10  includes a closed reactor  12 , such as a continuous flow photobioreactor. As shown in  FIG. 1 , the closed reactor  12  is fed with an inoculum medium  14  and continuously grows an inoculum of algae  16 . As the inoculum of algae  16  reaches the end  18  of the closed reactor  12 , it is at full concentration. Then, the inoculum of algae  16  passes out of the closed reactor  12  in an effluence (arrow  20 ). 
         [0025]    As shown in  FIG. 1 , the effluence  20  containing the inoculum of algae  16  passes from the closed reactor  12  to an open system  22 , such as an open raceway. In  FIG. 1 , it can be seen that the open system  22  comprises an expanding plug flow reactor (EPFR)  24  and a standard plug flow reactor (SPFR)  26 . Structurally, the EPFR  24  includes a conduit  28  with a first end  30  for receiving the effluence  20  and a second end  32 . Further, the open system  22  includes a pump  34 . As the effluence  20  enters the EPFR  24 , the pump  34  adds a growth medium (arrow  36 ) to the EPFR  24  to dilute the concentration of algae  38  within the EPFR  24  to about 0.5 grams per liter of fluid. Further, the growth medium  36  includes the nutrients necessary to support the desired growth of the algae  38 . As shown in  FIG. 1 , the open system  22  may include a plurality of pumps  34  for feeding the growth medium  36  at locations  40  along the length of the EPFR  24 . 
         [0026]    Referring now to  FIG. 2 , the structure and operation of the EPFR  24  may be understood. As shown, the first end  30  of the EPFR  24  has a width W 1  and the second end  32  of the EPFR  24  has a width W 2  that is substantially greater than W 1 . In  FIG. 2 , the EPFR  24  is not drawn to scale. In certain embodiments, W 1  will equal ten feet, while W 2  will equal 300 feet. Further, the EPFR  24  can be seen to include a plurality of sections  42 . Further each section  42  expands in width from its proximal end  44  to its distal end  46 . As shown, the width of each section  42  doubles from its proximal end  44  to its distal end  46 . As a result, the EPFR  24  has a substantially logarithmic increase in width. While  FIG. 2  illustrates an increase in width for each successive section, it is envisioned that sections  42  having a constant width could be interspersed among the widening sections  42 . 
         [0027]    Importantly, the fluid growth medium  36  and algae  38  flow through the EPFR  24  under the influence of gravity. For purposes of the present invention, this gravity flow is accomplished using a structured gradient. A preferred embodiment of a structured gradient for use with the EPFR  24  is shown in  FIG. 3 . There it will be seen that the floor  48  of the conduit  28  is formed with a plurality of steps  50 . In detail, the steps  50  are defined by a height “h” of approximately 3 centimeters, with a distance “s” between the steps  50  being preferably on the order of approximately 100 meters. Typically, the EPFR  24  may be over 1000 meters long and the algae  38  may have a residence time of about thirty days in the EPFR  24 . 
         [0028]    An important aspect of the EPFR  24  for the present invention will be appreciated with reference to  FIG. 3 . This aspect is that the depth “d” of the fluid growth medium  36  in the conduit  28  needs to be rather shallow (i.e. less than about 15 cm, and preferably around 7.5 cm). To maintain this depth “d”, however, it is necessary to add the fluid growth medium  36  along the length of the EPFR  24  as the EPFR  24  widens. Importantly, the increase in width among EPFR sections  42  allows for logarithmic growth of the algae  38  while the concentration of the algae  38  is maintained at the high concentration of at least 0.5 grams per liter. 
         [0029]    In cross-reference to  FIGS. 1 and 2 , as the growth medium  36  and algae  38  reach the second end  32  of the EPFR  24 , they are transferred to the SPFR  26 . At this stage, the algae  38  stops growing and, instead, begins to produce oils to store energy. In order to instigate oil production in the algae  38 , a pump  52  may introduce a trigger medium  54  into the SPFR  26 . Specifically, the trigger medium  54  may lack a desired nutrient, such as nitrogen or phosphorus, which causes the algae  38  to produce oil. Alternatively, the SPFR  26  may receive only the growth medium  36  and algae  38  from the EPFR  24 , without any additional trigger medium  54 . In either case, oil production in the algae  38  is triggered by the lack of nutrients to support growth. 
         [0030]    In  FIG. 4 , an alternate embodiment for the present invention is shown and is generally designated  60 . As shown, the system  60  includes an “n” number of open ponds  62  with the smallest open pond  62   (1)  being designated as the “first upstream pond”, and the largest open pond  62   (n)  being designated as the “last downstream pond”. Intermediate open ponds  62  are arranged in order, according to size, with an exponentially increasing surface area in a downstream direction. In this case, the downstream direction extends from the first upstream pond  62   (1)  to the last downstream pond  62   (n) . For the system  60 , the ratio between adjacent surface areas of respective open ponds  62  is established by a fixed multiplier. Importantly, this fixed multiplier is determined by the growth rate of the particular algae  38  that are to be cultivated in the system  60 . 
         [0031]    For the present invention, it is to be appreciated that all of the open ponds  62  in the system  60  are substantially similar to each other. The exception here is only in the size of their respective surface areas. Accordingly, each pond  62  will have a fluid circulating device  64  that is provided for moving (stirring) algae  38  around in the pond  62 . Functionally, this is done to promote the growth of algae  38  while there is a culture of the algae  38  in the particular open pond  62 . Examples for a suitable fluid circulating device  64  would be a standard circulation pump or a paddle wheel. Both of these types of devices are well known in the pertinent art. 
         [0032]    It will also be seen in  FIG. 4  that each open pond  62  has a medium addition conduit (represented by arrow  66 ) which is provided to add medium into the respective open pond  62 , as needed. Further, the open ponds  62  are connected via respective transfer conduits for selective communication with each other. For example, the upstream open pond  62   (n-1)  is connected in fluid communication via a transfer conduit with its adjacent downstream open pond  62   (n) . Preferably, the transfer conduits are transfer pumps  68 . As shown in  FIG. 4 , the transfer conduit between open pond  62   (n-1)  and open pond  62   (n)  is a transfer pump  68   (n-1) . As implied above, however, this particular structure is only exemplary. As an alternative to using transfer pumps  68 , the open ponds  62  in system  60  can be terraced to provide for a gravity flow of liquid between the various pairs of upstream and downstream open ponds  62 . 
         [0033]    In addition to the specific structural components of the system  60  described above, inoculum algae  16  in an inoculum medium  14  can be fed into the first upstream open pond  62   (1)  via a first transfer conduit (represented by the arrow  70 ). At the downstream end of the system  60 , after traversing the system  60 , the now fully grown algae  38  can be removed from the last downstream open pond  62   (n)  via a last transfer conduit (e.g. transfer pump  68   (n) . 
         [0034]    In the operation of the system  60 , algae  38  are progressively grown as they are selectively passed from one open pond  62  to another. The actual time spent by the algae  38  in each open pond  62  in the series will be substantially the same, and will depend on the type of algae  38  that is being cultivated. As a practical matter, the time spent by algae  38  in a particular open pond  62  can be as much as several (e.g. 3) days. In the event, the transfer of algae  38  through the system  60  is done methodically. And preferably, the transfer will be accomplished at nighttime when the growth of algae  38  is delayed due to a lack of sun light. 
         [0035]    A transfer sequence for moving algae  38  through the system  60  begins by first emptying the last downstream pond  62   (n) . To do this, the fully grown algae  38  therein are transferred through a transfer conduit (e.g. transfer pump  68   (n) ) to an oil formation pond (i.e. SPFR  26 ). Next, the contents of the adjacent upstream open pond  62   (n-1)  are then emptied into the now-empty last downstream open pond  62   (n) . At this time, additional medium can be added to the last downstream open pond  62   (n)  via the medium addition conduit  66   (n) . Specifically, this is done to establish proper conditions for further growth of algae  38  in the open pond  62   (n) . In turn, the contents of open pond  62   (n-2)  (not shown) are emptied into open pond  62   (n-1) , and an appropriate amount of medium is added. This continues, in sequence, with the contents of each upstream open pond (e.g. pond  62   (2) ) being transferred into the just-emptied adjacent downstream open pond (e.g. pond  62   (3) ). The transfer sequence finally ends when the contents of the first upstream open pond  62   (1)  have been emptied into open pond  62   (2)  and the now-empty upstream open pond  62   (1)  has been refilled with inoculum of algae  16 . The system  60  then continues to grow algae  38  in respective open ponds  62  until another transfer sequence is initiated. 
         [0036]    While the particular Method and System for Growing Microalgae in an Expanding Plug Flow Reactor as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.