A novel process was developed for the biorefining of floatable wastewater scum and other waste oils from water treatment facilities into biodiesel and other value-added bio-products. To test the scalability and commercial potential of the technology, a 7,000 liter/year pilot-scale system was designed and built. Scum from a waste water treatment facility, located in St. Paul, Mn, was collected and converted into methyl esters (biodiesel) according to the process chemistry. All the incoming and outgoing process streams were sampled, tested, weighed and recorded to calculate both the process efficiency and product quality. Data from the pilot-scale systems operation was compared to laboratory results and the theoretically expected values for each individual unit operation. The product quality was tested using a third-party laboratory and confirmed the biodiesel produced during a single batch process met all of EPA’s test requirements for commercial-grade biodiesel. As a substrate for biodiesel, scum derived oil requires more pretreatment consideration than standard waste oils like used vegetable oil or brown grease. Combining acid hydrolysis and solvent extraction, a free fatty acid and acyl-glycerol rich product was produced from a highly impure source. Free fatty acids (FFA) present were converted to acyl-glycols via a high temperature (238°C) glycerin esterification process known as glycerolysis. The inorganic catalysts zinc aluminum oxide and sodium sulfate was tested during glycerolysis to compare the reaction kinetics of converting FFA to acyl-glycerols. It was concluded that the zinc-based catalyst increased the reaction rate significantly, from a “k” value of 2.57 (uncatalyzed) to 5.63, completing the reaction in 60 minutes, half the time it took the uncatalyzed reaction (120 min). Sodium sulfate’s presence however slowed the reaction, resulting in a “k” value of 1.45, completing the reaction in 180 minutes. Use of the external catalyst Zn-Al2O3 showed the greatest catalytic potential, but also assumes additional costs. In the U.S., the total amount of municipal solid waste is continuously rising each year. Millions of tons of solid waste and scum are produced annually that require safe and environmentally sound disposal. The availability of a zero-cost energy source like municipal waste scum is ideal for several types of renewable energy technologies. However, the way the energy is produced, distributed and valued also contributes to the overall process sustainability. An economic screening method was developed to compare the potential energy and economic value of three waste-to-energy technologies; incineration, anaerobic digestion, and biodiesel. A St. Paul, MN wastewater treatment facility producing 3,175 “wet” kilograms of scum per day was used as a basis of the comparison. After applying all theoretically available subsidies, scum to biodiesel was shown to have the greatest economic potential, valued between $491,949-$610,624/year. The incineration of scum yielded the greatest reclaimed energy potential at 29 billion kilojoules/year. The use of vacuum distillation for biodiesel production has become a reliable post-treatment method for removing multiple impurities, to consistently produce commercial-grade biodiesel. The waste produced from biodiesel distillation, vacuum distillation bottoms (VDB), is a mixture of higher molecular weight methyl esters (84%) and derivatives. Microwave-assisted pyrolysis (MAP) has been researched as a methyl ester recovery process for VDBs leaving vacuum distillation. Two types of MAP processing, dMAP and fMAP, were developed and tested to determine the optimal reaction conditions for producing a biodiesel analogue. The results indicate that after dMAP, 85.9% wt/wt of the VDBs were recovered as a transparent bio-oil then blended back into B100 biodiesel and certified for sale using ASTM D6751. Blending dMAP bio-oil (10% wt/wt) with B100 biodiesel met all certification requirements and demonstrated that MAP processing could be a significant yield improvement technology for any commercial biodiesel producer utilizing vacuum distillation.
University of Minnesota Ph.D. dissertation.December 2017. Major: Biosystems and Agricultural Engineering. Advisor: Roger Ruan. 1 computer file (PDF); xi, 134 pages.
New technologies for the complete rendering and economic conversion of waste oils to biofuels.
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