May 15, 2014
Senior Design Night
May 10, 2014
The Diesel Crew presented their finsihed biodiesel reactor.
As the global demand for energy proliferates, especially as developing countries increase their use of technology, the need for improved and more sustainable energy resources increases. A large area of concern in sustainable energy is the transportation industry. Traditionally the energy resource for transportation has been petroleum based; however, petroleum is a not renewable over a reasonable time frame, and at some point in the future, petroleum may not be accessible in quantities to support the earth’s needs. A possible solution is to use a fuel that is renewable, such as biodiesel, a fuel so named because it can be harvested in a variety of ways from plants and animal fat. A particular attractive method is the conversion of waste cooking oil (WCO) to a usable biodiesel, as this not only reduces waste, but produces something that can be used. The Diesel Crew developed a system to perform this conversion for a home user or small institution that produces WCO, such as Calvin College.
The primary objective of this group was to design a quality alternative reactor to the commonly used batch-reactor-liquid-catalyst system. The team sought to build a prototype continuous flow reactor to convert WCO into a usable biodiesel fuel that meets the standards and qualities of ASTM for biodiesel fuel, a necessity for the commercial sale of the fuel. Originally, the team wanted to create a polished prototype that would be easily operable by a person of a non-technical background, but the team quickly realized this would take a budget and knowledge of control systems beyond what the team had. The team sought to design a prototype for a continuous system for biodiesel production that would be feasible for a home user or institution similar to Calvin College.
This group placed several limitations on this project. The team used a calcium oxide (CaO) catalyst which can be recycled, but the team left the catalyst recycling outside of the scope of this project, due to the already large nature of the project. Team one decided to use the WCO from Calvin College’s dining services (which is soybean oil) as a typical feed to the process, though different institutions may produce slightly different WCO. The goal for the system was to be able produce biodiesel at ASTM standard, but the team left full ASTM biodiesel testing out of scope due to the expenses and time associated with full testing. For design consideration, Team One defined a quality process as one that is safe, quick, small, inexpensive, and easily operable. A faster process results in a smaller size, which helps keep the system small enough to be transported to a variety of locations and not take up excess space. Also, due to the complexity in design of the individual systems and the intricacy of the control systems, a few sub units were investigated but left out of prototype integration. These systems were the catalyst feed system and automatic catalyst removal. For this reason, while running the reactor a person is required to feed the catalyst into the reactor.
Raw cooking oils should not be used directly as fuel in most engines as the higher viscosity of these oils results in carbon buildup and thus reduced engine life (US Department of Energy). Therefore, further processing is needed. The team chose to produce the popular product of cooking oil and methanol, fatty acid methyl esters, abbreviated FAME.
FAME, commonly called biodiesel, is typically used in a blend with traditional petroleum diesel. Biodiesel is made by converting oils or fats, which are triglycerides, and an alcohol (usually methanol) to fatty acid esters in a process known as transesterification (Figure 1-1 ). Ideally, this is done in a single step with feedstock oil that consists only of triglycerides. However, in practice, waste oils also contain water and free fatty acids (FFA), carbon chains not bound to a glycerol backbone. The presence of water and FFA is problematic as one of the preferred methods of enacting transesterification is by using a basic (alkali) catalyst such as sodium hydroxide (NaOH), which forms soap when added to FFA (Figure 1-3) (Suwannakarn). While substantial water in the final biodiesel product is not acceptable by itself, a large fraction of water also exacerbates the FFA to soap problem, converting biodiesel to FFAs (Figure 1-2) (Rinnova). Besides using oils that could be converted to biodiesel, impeding process flow, and deactivating heterogeneous catalysts, soap is also an emulsifier, making it more difficult to separate whatever water is present from the produced biodiesel, further reducing yield.
Figure 1-1: Biodiesel Reaction Chemistry
A common method to address these issues of water, fatty acid content, and soap formation is via a two-step batch reaction process. This process utilizes an acid catalyzed pretreatment to esterify FFA, removing methanol via vacuum before proceeding with a base-catalyzed transesterification using potassium hydroxide. The acid pre-treatment is simply the reaction shown in Figure 1-2 driven in reverse (from right to left).
Figure 1-2: Reversible reaction of FAME to FFA.
Note: This also illustrates another benefit of having an excess of methanol since existing FFAs will be better driven to biodiesel.
Figure 1-3: Soap Making Process (also known as saponification)
The reaction step is typically followed by vacuum vaporization of the excess methanol and using a water wash to remove any remaining FFA, glycerol, or soap that might have been dissolved in the biodiesel (the water is then also drained from the reactor).
The design of the Diesel Crew differed significantly in the type of reaction used. The team eliminated the two part reaction and water wash by using a solid catalyst, CaO. By eliminating the washing step and the liquid catalyst, the diesel crew was also able to make their system a continuous process. The steps after the reactor are very similar for both processes except that the continuous process includes a step to remove the catalyst. The final steps include the evaporation of methanol, separation of glycerol and final polishing of the fuel.
Any institution that produces WCO from their dining services that also has a moderate need for diesel fuel for transportation or lawn maintenance is a potential customer. Persons who make biodiesel at home as a hobby would also be potential customers for this type of system. The team will tailor the design to meet the needs of a home biodiesel producer or smaller institution such as Calvin College, which produces about 2000 pounds of WCO per month. Currently Calvin operates a shuttle van on biodiesel as well as a few lawnmowers. A reactor of this nature would help cut costs of purchasing fuel for this machinery.
A reactor of this type would also be useful to a larger chemical recycler that takes in WCO. Currently, many companies collect WCO and convert it to FAME in large batch processes with homogenous catalyst. One such company, collects and converts Calvin College’s WCO to FAME. A company such as this would benefit from a continuous process due to the lower amount of labor required for operation and less separation time.
For an institution to implement our reactor system, the design must meet a few requirements. First and foremost it must be completely safe to operate, not putting the operator or anyone else in danger. It is also desirable to have very little user interaction with the system and have the system be easily operable by a person of a non-technical background, but the team did not stress this in prototype construction and design due to limited control systems experience.
- Removes solid contaminants from the waste vegetable oil (WVO) feed
- Evoprates water from the WVO that would otherwise damage the reactor catalyst
- Provides initial heating of the reactor feed
- Conversion of WVO to biodiesel takes place in this vessel
- Microwave irradiation is used to efficiently heat the reaction mixture and induce higher conversion rates
- Methanol reacts with the WVO (fatty acid triglycerides) over a catalyst to form biodiesel (fatty acid methyl ethers)
Methanol Recovery System
- Recovers unreacted methanol from the reactor product stream
- Methanol is evoprated under vacuum, recondensed, and recycled to the methanol feed
- Reduces cost of system operation and helps with the separation
- Glycerol byproduct is separated from the biodiesel
- Secondary filtration removes any leached out reactor catalyst
- Further purification of biodiesel is performed to meet regulatory standards
*For more information on the details of this project, check out the final report found on the downloads page.