Photobioreactor for Microalgae Cultivation

The goal of this project is to build a bench scale photo bioreactor for micro algae cultivation. Micro algae is an excellent resource with wide reaching applications, from waste water purification to biodiesel production. We aim to design a system capable of producing micro algae in a cost effective, reliable, and efficient manner by utilizing an airlift style reactor design.

Background
Photo Bioreactors are devices often used to produce micro algae commercially. Some of the challenges of photo bioreactor design are supplying sufficient light to the micro algae and preventing the micro algae from sticking to the sides of the bioreactor. There are a range of designs currently in use, including open ponds, water filled flexible plastic bags, and long glass tubes. Each of these have issues, either with light penetration, cost, or reliability. An airlift design that solves each of these issues effectively is the goal of this group, since they are inherently low power, low maintenance devices.

Design Task
Our task is to design a bench scale airlift photo bioreactor for research purposes. It should be designed to maximize efficiency and minimize cost, and thus should be composed of relatively inexpensive materials. The basic design should be such that it could be conceivably be scaled up and built of more durable materials such as stainless steel.

MicroAlgae Requirements
For our project, we needed to be very adaptable with the parameters microalgae need to survive. This is because the PBR needs to grow many different types of algae without major modifications to the physical design. To generalize our PBR’s capabilities, we decided to use a common microalga as a control for all the needed parameters. The Algae we chose to find these parameters is the Chlorella varieties. These species are very common green microalgae found worldwide in all types of water. Most grow best with high ambient light up to 2000 umol/m2s with wavelength of blue and red. They also thrive well in temperatures of 74-80 degrees Fahrenheit, and at a pH between 6.8-7.2. CO2 concentrations should be kept no less than 22 ppm, and greater values are acceptable.



Flow Considerations
Over the development of this project we found that it is hard to find fluid flow models around the sort of system we are working with, involving a bubbling gas flow inducing a liquid flow. We found empirical equations in a book titled "Air-Lift Bioreactors" by M. Y. Chisti. We have attempted to create a math model in TKsolver using these equations to optimize flow velocity and gas holdup. However, we have found for the most part due to the number of unknown variables we will need to actually construct the reactor before we can glean any meaningful data from it. Part of the purpose of this reactor is as an experimental prototype from which this sort of data can be taken.

Design Selection
The problem as it was given to us allowed a lot of room for creativity. There are a variety of possible designs for an air lift reactor, including external loop airlift, flat internal loop, and concentric internal loop. In selecting a basic design, we needed to consider a variety of factors, such as cost, flow, and ease of construction and maintenance. Below is a basic breakdown of the pros and cons of each basic design, and the rationale behind the one we chose. External Loop Flat Internal Loop Concentric Internal Loop
 * Pros: Easy to maintain.
 * Cons: Flow is less than optimal, the design is slightly more difficult to construct.
 * Pros: Easy to construct, maintain, and adjust.
 * Cons: Sub optimal flow (potential dead spots)
 * Pros: Easy to construct, excellent flow characteristics
 * Cons: Less flexible design.

We ultimately selected the concentric internal loop design as it both provided the features we deemed most important and fit well with the wishes of our client.

Alternative Designs
We initially considered the flat internal loop design for reasons of ease of construction and versatility. This design was composed of two transparent sheets of acrylic with an exchangeable flow guide placed between. The sheets would then be clamped down together, creating a box of sorts with the working fluid volume flowing between the two panels. This design was ultimately discarded due to concerns from our client that the rectangular cross-section of the flow channel would cause stagnation issues. Later on in the project, we considered adding baffles above the riser tube in order to improve fluid mixing, ultimately deciding not to in order to keep the design as simple as possible.



Our Design
Our design utilizes a concentric internal loop style airlift system for the circulation of the slurry in our photo bioreactor. It is composed of an outer cylinder made of 10" diameter clear acrylic tube and a 6" diameter inner tube. The suspension of algae and water flows up the inner cylinder, driven by the flow of CO2 bubbles from a gas diffuser (sparger) mounted in the riser tube, which is connected to a tank of CO2. The base of the bioreactor is made of a block of Nylon plastic which was machined to a contour that was designed to reduce flow stagnation (dead spots) and improve flow, as shown below. This block is sealed in place by the means of an aluminum flange which is attached to the outer tube of the reactor with silicone adhesive caulking. This allows the bioreactor to be partially disassembled for cleaning or later modification. Lighting is provided by waterproof, tunable RGB LED strips enclosed in 6 thin vertical tubes arranged in a radial pattern about the inside of the outer tube. The lighting system is controlled by infrared remote controls to allow for customization of the color and intensity of the light wirelessly. A CAD model of the system is shown to the right.



Concept Testing
We have created a small prototype of the basic concentric tube airlift design to perform some experimentation with. While it is not precise enough to collect meaningful quantitative data from, it does serve as a physical proof of concept to demonstrate the efficacy of the airlift system.

Testing
Leak Testing showed no detectable leaks. Initially the light tubes showed a tendency to float upward out of their place, however this was easily rectified by adding weight at their base. In order to check flow characteristics small beads at nearly neutral buoyancy were added and the bubbler was attached to a small air compressor. The flow pattern shown was the same as intended in the initial specifications, thus showing the success of the design. Initial flow speed testing showed particle movement speed of 4" per second at the lowest pressure setting, well under 20 psi.

Document Archive

 * [[Media:2017_autophyte_prelimdesignrev.pdf|Fall Semester Design Review (Fall 2017)]]
 * [[Media:2018_Autophyte_PhotoBioreactor_Design_Review.pdf|Spring Semester Design Review (Fall 2017)]]