The present invention pertains generally to methods for growing algae. More particularly, the present invention pertains to a method for heterotrophically growing algae in a non-sterile environment. The present invention is particularly, but not exclusively, useful as a method for growing algae using a two-stage bioreactor wherein a first stage enables rapid algae cell growth under sterile conditions, and wherein a second stage stimulates rapid oil production in the algae cells under non-sterile conditions.
As worldwide petroleum deposits decrease, there is rising concern over petroleum shortages and the costs that are associated with the production of petroleum products. As a result, alternatives to energy products that are currently processed from petroleum are being investigated and commercially deployed. In this effort, biofuel has been identified as a viable alternative to petroleum-based transportation fuels.
One method of biofuel production currently in commercial use is the extraction of intracellular oil found within algae cells. Currently, the various processes used to create biofuel in this manner are quite expensive relative to the process of extracting and refining petroleum. Specifically, the conditions necessary to facilitate a rapid heterotrophic growth rate for algae cells in large-scale operations have been found to be expensive to create. In further detail, one reason for the high costs associated with the heterotrophic growth and harvesting of algae cells results from the need to maintain sterile conditions throughout the algae growth process. Sterile conditions are essential to heterotrophic algae growth to ensure that other microbes, specifically bacteria and fungi, are not present in the system to compete with algae cells for nutrients in an algal culture. Capital costs are high because of the cost of specialized equipment required to maintain a sterile growth environment. And, operational costs are high because of the significant amount of energy, usually in the form of heat, required to maintain a sterile growth environment. With costs associated with maintaining a sterile system being so high, algae cells cannot be produced on a large enough scale to make biofuel production cost-effective.
A further drawback of maintaining sterile growth conditions is that the system must be periodically shut down to sterilize all of the system components. While the system is shut down for sterilization, no algae cells are produced. Additionally, the need to periodically shut down the system means that only batch processing, as opposed to a continuous-flow method of processing, is available as an option for operation of the system. As understood in the trade, batch processing is less efficient than continuous-flow processing as only a finite amount of material can be processed by the system at one time.
In light of the above, it is an object of the present invention to provide a system and method for heterotrophically growing algae that is suitable for use in the production of biofuel. Another object of the present invention is to provide a system and method for heterotrophic algae growth that occurs in two stages with the second stage being conducted under non-sterile conditions. Still another object of the present invention is to provide a system and method for heterotrophically growing algae that reduces capital and operational costs by allowing the system to operate continuously. Yet another object of the present invention is to provide a system and method for heterotrophically growing algae in a non-sterile environment that is simple to implement, easy to use, and comparatively cost effective.
In accordance with the present invention, a system and method for implementing non-sterile heterotrophic algae growth is provided. For the present invention, a two-stage process is implemented. Importantly, the first stage is conducted under sterile conditions, while the second stage is conducted under non-sterile conditions. In the first stage, a full load nutrient is mixed with algae cells in a chamber of a Continuously Stirred Tank Reactor (CSTR) to form a culture and promote rapid growth rate of the algae cells. Then, the culture is transferred to a first Plug Flow Reactor (PFR) having a chamber where a selected nutrient is depleted from the culture in order to create an effluent. At this point, the first stage is complete, and the effluent is transferred to a chamber within a second PFR to begin the second stage. While in the chamber of the second PFR, organic carbon is added to the effluent to rapidly increase the lipid content of the algae cells in the effluent. As a final step, the effluent is sent to a processor to extract the oil from the algae cells.
Structurally, a first inlet pipe is connected to the CSTR to allow for the addition of algae cells to the chamber of the CSTR. Also, a second inlet pipe is provided for adding a full load nutrient to the chamber of the CSTR. In addition, the CSTR and the first PFR are in fluid communication to facilitate the transfer of the culture from the CSTR to the chamber of the first PFR after mixing. For the present invention, the first PFR and the second PFR are connected by a transfer pipe to advance the effluent through the system. Also, a conduit is connected between the second PFR and a source of organic carbon to allow for the addition of organic carbon to the effluent while it is in the chamber of the second PFR. Additionally, an outlet pipe is connected to the second PFR to remove the effluent to a processor that will extract the oil from the algae cells.
Operationally, each of the two stages will have a residence time, with the first stage having a first residence time and the second stage having a second residence time. During the first residence time, at least one key nutrient other than carbon (i.e. nitrogen or phosphorous) is depleted from the effluent. By depleting one of these nutrients, other microbial contaminants will be unable to reproduce or survive. When fewer microbial contaminants are present in the culture, algae growth is enhanced as the algae cells do not compete with other organisms for the nutrients added to the culture. For the purposes of the present invention, the first residence time is envisioned to be in a range of 4-10 hours. During the second residence time, the only nutrient source added to the effluent is organic carbon. By only adding organic carbon to the effluent, the carbon to nitrogen (C:N) ratio will increase to a predetermined level. This predetermined level is calculated to stimulate rapid lipid growth (in the form of oil) within the algae cells. The carbon to nitrogen (C:N) ratio should be greater than 10:1. Preferably, it will be greater than 14:1 and in a range between 14:1 and 25:1. As envisioned for the present invention, the second residence time is envisioned to be in a range of 12-120 hours to allow for maximum oil production within the algae cells. For a preferred embodiment, the first stage and the second stage comprise a bioreactor. Within the bioreactor, the first stage is significantly smaller than the second stage when comparing the overall volume of each stage. Energy and equipment costs are reduced because the size of the first stage is smaller than the types of sterile systems currently in commercial use. At the same time, for the second stage, minimal energy and inexpensive equipment can be used because less energy and less expensive equipment is needed to maintain the algae cells under non-sterile conditions. An additional benefit of this configuration is that the algal culture in the second stage can be maintained at a lower cell density than in a system that is sterile throughout. Further, the lower cell density means dehydration of the organic carbon will not be required before it is added to the effluent. As a consequence, energy is conserved by maintaining the carbon in a liquid state.
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:
Referring initially to
Still referring to
From an operational perspective,
Again referring to
As a last step, the effluent is moved through the outlet pipe 34 to the processor 36. At the processor 36, an initial step in converting the algae cells into biofuel takes place. More specifically, the intracellular oil (lipid) is extracted from the algae cells for further processing.
Now referring to
While the System and Method for Non-Sterile Heterotrophic Algae Growth 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.