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A ROBUST PROCESS FOR BIODIESEL PRODUCTION USING SUPERCRITICAL METHANOL
Impact/Purpose:
To evaluate the use of super critical methanol as a reaction medium in which both triglycerides and free fatty acids can be converted into methyl esters without the addition of acid or base catalysts.
Description:
A literature review was conducted in order to insure the feedstock choice had the potential to bring about positive impacts in making progress toward sustainability. The use of algae for a biodiesel feedstock was chosen for the many benefits it provides. Algae produced via the ATS system removes carbon dioxide from the atmosphere, removes nutrients from polluted water, provides opportunity for production of other bio-fuels (e.g., butanol or ethanol from the cellulosic component of the algae) and environmentally friendly fertilizers, and decreases the consumption of fossil fuel.
Experiments were performed to determine the most effective method for extracting algae oil through their virtually indestructible cell walls. The following methods were tested: mortar and pestle, Waring blender, hexane extraction alone, salt-water solution, glycerol solution, and urea solution. It was ultimately concluded that lysing the cells with an osmotic shock using glycerin would be the most effective method to release the oil for extraction with hexane. The fraction of oil extracted, on a dry solids basis ranged from 0.37% to 2.67%. The mortar and pestle method gave the highest yield of 2.67%. Unfortunately, the mortar and pestle method requires dried algae and grinding of dry algae; thus, costs on are prohibitive for a full scale plant. The least effective method, surprisingly, was using an 8 M urea solution to weaken the cell wall hydrogen bonds; we believe this is because there is a relatively low quantity of cellulose in the algae cell wall. Salt and glycerol solutions utilize osmotic shock as the cell disruption mechanism. The salt solution was actually more effective than the glycerol, yielding 2.53% vs. 2.2%. However, salt solution lysing is not practical for a full scale plant design because the salt must be recycled and the cost of removing water required for salt recycle is prohibitive.
Constructing a reactor that can run continuously is only a small part required to prove the viability for producing biodiesel at a larger level; however, the ability to create such a reactor provides evidence that the chosen technology has potential for being implemented on a full scale. Students at the University of Arkansas designed and built a continuous supercritical methanol reactor for the production of biodiesel from commercially available materials. The continuous supercritical methanol reactor is one of the first of its kind. The supercritical methanol reactor was tested with a variety of triglyceride and free fatty acid (FFA) feedstocks to demonstrate its robustness. The experimental runs proved that high conversion, ranging from 60% to 85%, could be obtained with feeds ranging from 100% FFA to 100% triglycerides.
A full scale simulation was designed using Pro II process simulation package. The material and energy balance information provided by the simulation allowed students to perform a streamlined life cycle analysis. Because glycerin is used to lyse the algae cell, and it is not economical to recover and recycle the glycerin, it is sent with the algae biomass to an anaerobic digester where methane is produced and used on site for heating needs; the excess is sold off site, and an avoided product credit is claimed for this natural gas. Due to the very heavy use of glycerin, this process does not yet make positive environmental impacts compared to alternative biodiesel production methods. If a source of waste glycerin can be found, it would not carry an environmental burden into this process, and the algae biodiesel becomes very favorable. This highlights the reason that algae have not yet become a viable alternative feedstock: it is extremely difficult to extract the oil. This also points clearly to the need for additional research in this area.
Although it is well known that a full scale process to produce biodiesel from algae is not economical as an industrial commercial venture with the current technology, the team evaluated the venture as a government funded project and determined that the project is revenue neutral; i.e., the venture does not cost the taxpayers. The proposed demonstration plant provides 71 permanent, high playing jobs in addition to removing fossil fuel from our fuel mix, which reduces CO2 emissions. The proposed capital estimate of the commercial scale plant is summarized below. The algal growing ponds cost $25,000,000 and the total installed cost of other equipment is $15,000,000, giving a total capital cost of $40,000,000. The project is essential to our economic and national security; thus, it should be financed with the sale of US Treasury Bonds bearing an interest rate of 4.00% p.a. for 30 years. After the bonds mature in 30 years, additional bonds will be sold to repay the original bondholders. The yearly interest on the bonds is $1,620,000 will be paid by the venture, resulting in no net cost to the US taxpayers.