Algae Harvester and Dehydrator
|From left to right: |
Nicole Maas, Josh Nisson, Stefanie Marikis, Ryan Walters, Chance Scripter
|Duration||Fall 2019 - Spring 2020|
The goal of the project is to create an energy efficient device that will harvest algae from a photobioreactor, then dehydrate it to a 15% moisture content.
Biofuels have become more prevalent as scientists research alternatives to fossil fuels. Common feedstocks for biofuel such as soybean and canola oil have too low biofuel yields to sustain the amount of fuel used today. Algae oil has the potential to have much higher yield, making it a good feedstock option for biofuel. The problem with algae oil is it is difficult and expensive to grow, harvest, and dehydrate the algae so the lipids can be harvested and made into fuel. The goal of the Algae Harvester and Dehydrator project is to make a device that can harvest algae from a high water content, then dehydrate it to a 15% moisture content so it can be made into biofuel.
In the 2018-2019 school year, a team made a photo-bioreactor, (PBR), in which to grow algae. It is a 24.7 gallon cylindrical tank with six tubes for containing lights. It connects to an air pump which pumps air from the bottom of the tank, allowing for mixing and aeration. This photobioreactor is in an airlifter reactor configuration (shown to the left) so the air bubbles go up the center of the tank in a smaller cylinder and is released out of the top. The algae water then flows back down the outside of the cylinder, creating a current in the tank to cause mixing. At the bottom of the tank is a drain valve where the algae water can be collected. Some of the light tubes broken since it was made, which led to damage of the lights. These needed to be remade by sticking adhesive light strips to each side of a square dowel and connecting each strip with small wires.
The device has certain requirements we must meet. There are some design specifications that are desired by the client, but not required. We will accommodate the desired specifications if we are able.
Required design specifications:
-The device must retrofit onto the photobioreactor made by a previous senior design team.
-The device must dehydrate the algae to at least a 15% moisture content.
-The device must be as energy efficient as possible.
Desired design specifications
-The device should be able to fit on a table top.
-A continuous flow instead of batch flow is preferred.
-The device should recycle the water back into the photobioreactor.
The algae harvester and dehydrator is made of two components, the dewatering system and the dehydrating system. First, the algae is separated from the water and made into a sludge. Then, it is dehydrated to remove as much remaining water as possible to turn it into a powder.
Stage 1: Dewatering Systems
Our first design, shown on the left, for a dewatering system is a stack of filters going from larger to smaller hole sizes. Since the algae we are working with (Spirulina) is about 10 microns in length, we will be using a 5 micron filter so none of the algae gets through.
The filters will lay span the size of the square hole in the center of the system shown in the image. These will stack on top of each other to create a "filter tower" of sorts. The system will use gravity to separate the water from the algae. An issue with this system is potential clogging.
Another design we had for dewatering, shown on the right, is the three stage filter. In the center of the vertical tube is a mechanism with a filter chamber that can move up and down in the system. The cylindrical filter chamber is located on the bottom of the moving mechanism and has two holes, one on each side. One of these holes has a filter covering it and one is open. The filter chamber can rotate so the filter is on the far side when the algae water is pumped in, then it rotates and moves up to the second stage in the dewatering mechanism. This stage blows compressed air through the filtered algae to blow out any excess water. The filter mechanism then rotates again and moves up to the third and final stage. This stage blows compressed air through the filter to blow the algae off of the filter and into a catch basin. This design would be difficult to implement because it is so complicated. It would also use more energy than other designs since it has so many moving parts.
The design for the sixer filter is shown to the left. Each of the holes on this design is covered by a filter mesh. The way this dewatering system would work is algae water would flow in the thicker tube on the left and would filter through. Compressed air would blow through the smaller tube and push any standing water out through the filter to leave filtered algae sludge. The part of the mechanism with the holes would rotate to allow each hole to have algae water filter through, then have compressed air force any standing water out. The problem with this design is collecting the algae once it has been filtered. The device would need to be taken apart to collect the algae.
The rotary drum filter design is shown to the right. The wall of the drum is made out of a filter screen to allow water to go through, but to keep algae inside. An algae water stream flows perpendicular to the screen on one end of the drum. The drum rotates as the water filters out and causes the algae to tumble inside. This mechanism prevents clogging of the filter screen. The whole drum is set at an angle to cause the tumbling algae to eventually reach the other side of the drum and fall into a catch basin where it will then go to the dehydrator. This filter is the one that was almost chosen for the final design, but it had some issues. A scraping element on the inside would need to be added to scrape algae off of the filter screen and a spiral wall would need to be added to prevent standing water from flowing into the catch basin. These two things could not be implemented together, so this design had to be thrown out.
Stage 2: Dehydrating Systems
The design to the right is our design for a vertical cross flow dryer (VCFD) based off of a popcorn maker. This design would use the concept that wet algae has more mass than dry algae. Hot air would blow underneath the algae from the bottom of the dryer. As the algae became more dry, the air blowing from the bottom of the vertical cross flow dryer would blow it out the top to a collection bin. This design would require many complicated equations and calculations, so it was rejected.
To the left are the equations needed to determine the velocity of air needed to push dry particles out of the vertical cross flow dryer. There were too many unknown variables needed to determine this velocity for this design to be feasible. In the above center shows a diagram made of the vertical cross flow dryer to show how dry algae particles would flow out and what variable would need to be determined.
Another design for a dehydration system was the rotary drum dryer. This design is very similar to the rotary drum filter, except that it has caps on each end to prevent algae from spilling out the sides. Like the rotary drum filter, the wall of the drum is made of a filter screen to allow moisture to escape. This design was based off of clothes dryers and works in a similar way. Hot air is blown in while the drum rotates to create a tumbling effect to create a more even drying process. One of the caps would have to be removed to collect the algae once it has dried to the desired moisture content.
For our project, we decided to use Spirulina algae (Arthrospira platensis) as our algae source. Spirulina is non-toxic to humans and relatively easy to grow, so it was perfect for experimentation. To grow our algae, we set up a mini photobioreactor since the one previously made had never been tested. We used a 5-gallon tub with a light strip attached to the top and a fish tank air bubbler inside to provide the CO2 requirements for algae to grow. The algae growing process has proved to be more challenging than we initially anticipated. We thought it would take a couple of weeks for the algae kit we received to grow a sustainable amount of algae for the rest of the project. The first kit we bought did not grow a successful batch of algae. This could have been due to the algae in the kit arriving dead, us not allowing enough time for the algae to acclimate to the new environment, putting too many nutrients into the batch, keeping the algae culture too cold, or a combination of all of these things. Another problem could have been that we tried to use purple light to grow the algae. Spirulina algae grows best in white light with a wavelength of about 450-475 nm. On later batches, we set up a white LED lamp outside the tub, which was able to provide the correct light requirements. To keep the temperature at the desired level of 30 degrees Celsius, we set up a small, closed-off room with a heater inside to maintain the temperature.
The dewatering system we had decided to use for our final design was filtration so we needed to determine what size filter would work the best. We ordered a pack of filters ranging from 5-500 micron pore size. On the left is the comparison of a 5 micron filter versus a 43 micron filter. The 5 micron filter did very well at filtering the algae out. Once the water had been filtered, it was completely clear, as shown to the right. The problem with the 5 micron filter was that the filtration process was very slow. We decided to test the 43 micron filter as well since that was the next smallest size filter we had. Spirulina tends to form clumps, so we thought the 43 micron filter might be able to filter out these clumps. There were some small pieces of algae left on the filter once the water had been filtered through, but the majority of the algae passed through the filter. For our final design, we decided to use a 15 micron filter. Spirulina algae is about 6-16 microns long, so we knew a 15 micron filter would work to filter it while being faster than the 5 micron filter.
Initially, we had settled on the rotary drum filter and vertical cross flow dryer as the two components of our final design. After a meeting with our instructor, we learned about a filtering technique used in industry that involves the use of a conveyor belt made of filter material. The filter can be easily cleaned and the product can be easily directed to the desired location. The initial rotary drum filter design had two main problems, no way of cleaning the filter and direction of the algae to the dryer. We completely redesigned the filter into a conveyor belt design and implemented a modified rotary drum dryer shown on the right. We replaced the VCFD with a modified rotary drum dryer because we did not know how to transfer the algae sludge from the belt to a VCFD. In this design, water flows into the top right side of the belt while the belt rotates to the left on a decline. At one end of the belt is a scraper that removes the algae sludge from the belt and transfers it to the rotary drum dryer. Hot air is blown into the drum and tumble dries the algae. Once the algae has been dried to the 15% moisture content, it is collected from the drum. An issue we had with this design was how to keep the algae inside the drum as it is being dried. There had to be an opening for the algae sludge to get in, so the drum could not be completely closed off. We eventually decided to remove the modified rotary drum dryer and replace it with a cross flow dryer.
Shown on either side is the final, built design. We were only able to build the conveyor belt filter and scraper portions of the design since school was shut down. The cross flow dryer would have been built as a long tube coming off of the black scraper. Hot air would blow over the top of the algae and the dry algae particles would flow down and out the tube once they got dry enough.
In the conveyor belt are three rollers to rotate the filter and cause tension. Our design is adjustable so the rollers can be moved as needed to increase or reduce tension in the belt. The frame, rollers, and scraper are all 3D printed. The scraper was supposed to be made out of a cut piece of aluminum, but we were not able to use the shop to cut it. To move the conveyor belt, we used a stepper motor. Since the filtration process is so slow, the belt would rotate at about 0.1 rpm. From testing of the vertical cross flow dryer, we determined that the yield could be about 7.5g of dry algae for the 24.7 gallon photobioreactor. This number is likely much lower than the yield could be because there was loss of algae during the transferring process from filter to dryer and during the dehydration process.
A stepper motor was used to rotate the conveyor belt in the final design. We programmed it using arduino which could connect to Labview for the user interface. The motor can go clockwise or counter-clockwise and the speed can be controlled.
Once our design had been decided and built, we made a Design Failure Mode and Effects Analysis (DFMEA) sheet that is shown below. The three most left columns describe the part of the machine that could fail, how it could fail, and what effects failure could have on the whole system. The "sev" or severity column describes the severity of the failure should it happen. A score of 10 is the most severe. The next column shows a potential cause of each failure. The "prob" or probability column rates the probability each cause has to cause the failure. A score of 10 means it is the most likely to happen. The current design controls column describes what measures are already in place in the design to prevent that failure. The "det" detectability column rates how detectable the failure would be. A score of 10 means it is very hard to detect. The RPN column is the total of the severity, probability, and detectability columns multiplied together and shows how much of a risk the potential failure poses. The recommended actions columns describes what can be done to mitigate the failure. We were not able to test for any of the potential failures due to the school shut-down, so the remaining columns are blank.
Since we were unable to do any testing with the final design or dried algae, we could not verify that the design was successful in completing the required deliverables. We were able to make the final design within the size constraints.
Design Review Presentation
Release Review Presentation
Engineering EXPO Presentation