Modeling & Simulation of Small-scale Biodiesel Plant and Design of Excess Ethanol Recovery System
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Date
2019-03
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Addis Ababa University
Abstract
Rising world fuel prices, the growing demand for energy, depletion in fossil fuel, and concerns about global warming are the key factors driving the increasing interest in renewable energy sources and in biofuels in particular that can be used as an alternative fuel in CI engines. The aim of this study was modeling and simulation of the small-scale biodiesel plant under construction at Addis Ababa institute of technology and design of excess ethanol recovery mechanism for the plant.
Experimentally determined reaction conditions (Reaction temperature 80OC, Ethanol to Oil molar ratio 9:1, catalyst concentration 1% w/w, reaction time 3.5Hours) for optimal Biodiesel production were used for the design of the plant. Aspen Plus software version8.4 was used for the simulation of the Transesterification. The simulation was done by initially determining the reaction kinetics of the Transesterification process from a biodiesel yield vs time graph obtained from a defended thesis titled “Emission and Performance Characteristics of Jatropha Ethyl Ester and its blends with Diesel Fuel in a C.I. Engine” done by Haymanot Baynesagn at Addis Ababa institute of technology from which the reaction conditions for the plant design are taken. From the biodiesel yield vs time graph, the amount of biodiesel obtained at a point of 30 min interval time for temperatures of 60OC, 70OC, 80OC and 90OC was calculated. Then, the amount (in gram, mole & litter) and concentration of other components in the reaction process at every time interval for the specified temperatures were calculated. From the determined concentration values, the integrated form of the transesterification reaction was plotted so that from slope of the plot, the forward and reverse rate constants determined. The Arrhenius equation is an equation that shows the dependency of the rate constant on temperature. Thus, the natural logarithm of the rate constants determined for the specified temperatures above was plotted against the invers of the temperatures and from the slope of which the kinetic parameters; Activation Energy and Frequency factor (input parameters to the simulation) determined as; for forward reaction Eaf= 25.769KJ/mol and Frequency factor Af = 60.26/min and for the revers reaction Eab= 22.934KJ/mol and Frequency factor Ab=4.69/min.
Aspen software consists of property database for lots of materials. However, Triglyceride oils such as Jatropha oil have not been yet added to the database. Thus, it was a must to select a similar material from the existing once to represent Triglyceride oils. Triolein oil is the mostly selected model to represent Triglyceride oil. Accordingly, the transesterification reaction was modeled with Triolein in place of Triglyceride and the simulation done. The simulation result obtained was 91.4% yield of biodiesel which was higher than the experimentally determined value of 85.24%.
As the Transesterification reaction is a reversible, excess alcohol is normally added to force the reaction towards the forward. Thus, after the completion of the reaction the amount of alcohol being consumed in the reaction is much lower and much amount to remain unreacted that is termed as excess alcohol. The amount of ethanol that remains unreacted at the end of transesterification reaction was nearly 73%. If this excess amount of ethanol is left uncollected and reused, the process cannot be economical. Thus, inclusion of a mechanism for recovering of excess ethanol in a transesterification process is mandatory. For biodiesel production plant of higher capacity, distillation column is the mostly used alcohol recovery system. However, a reflux condenser is used for smaller production capacity of plant. In this paper, a design of vertical helical coil heat exchanger with dimensions of height 510mm, Shell Diameter 285mm, copper tube of Ø7.94mm (5/16 inch) with coil pitch of 17mm was designed for use as reflux condenser.
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Keywords
Modeling & Simulation, Biodiesel Plant, Design, Ethanol Recovery