Title

Supercritical fluid technology applied to the production and combustion of diesel and biodiesel fuels

Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Keywords

Supercritical combustion, Supercritical fluid, Diesel, Biodiesel, Fuels, Supercritical fuel-diluent mixtures

Subject Categories

Chemical Engineering | Engineering

Abstract

Biodiesel (BD) and diesel fuels (DF) in supercritical (SC) states exhibit enticing properties which could lead to superior performance upon combustion. Preparation and characterization of these SC fuels are the main goals of this project.

In the first part of the project, investigations were directed to obtain SC mixtures of fuels (e.g., DF and n -hexadecane (HD)) with an inert diluent (ID) such as carbon dioxide (CD), water, or exhaust gas (EG). Thermodynamic conditions of temperature and pressure were obtained for phase transitions of various fuel-ID compositions. For topographic distributions of the liquid, vapor, liquid-vapor, and SC states of interest, P-T fluid phase diagrams were constructed for HD-ID mixtures based on selected cubic equations of state. These diagrams were used as boundary frames for experimental planning to determine the suitable conditions of phase transition to SC states. To monitor these transitions, a view cell was designed and built to house SC fluids both in flow and batch modes up to 600 bar and 500 °C. The main properties of various fuel-ID mixtures, such as density and solubility, were obtained from laboratory experiments carried out with originally designed setups. Special emphasis was placed on thermal stability of the fuel during preparation process of SC fuel-ID mixtures since previous approaches to bring DF to SC states failed due to coking deposits. Our results showed that coking of the fuel was prevented in the presence of adequate ID. Oxidation with stoichiometric amount of air produced no harmful emissions under SC conditions at 500-550°C. Reactions carried out at lower temperatures provided intermediate product information which suggested reaction pathways and confirmed reported reaction mechanisms. Finally, a number of conceptual designs for fuel delivery systems to be used in a retrofitted diesel engine were studied by ChemCad ® simulations and mass/heat balances were performed.

In the second part of the research project, a SCF technology coupled with power cogeneration is proposed to produce BD fuels without the conventional complex separation and purification steps. The core of the integrated system consists of the transesterification (TE) of various triglyceride sources (i.e., vegetable oils and animal fats) with SC methanol/ethanol. Part of the reaction products can be combusted by a diesel engine integrated in the system which, in turn, provides the power to pressurize the system and the EG heat for the TE process. Laboratory investigations were directed to optimize system performance in regard to process operating conditions. The experiments have been conducted at 100-300 bar, 250-425 °C, and 0.73-8.2 minutes residence time with soybean and sunflower oils and SC methanol/ethanol at ratios of alcohol to oil from 3 (stoichiometric) to 24. Reactant phase transitions from liquid to SC states were monitored with a view cell. Complete oil conversion to BD has been achieved at selected parameters, with the glycerol decomposition products included in the fuel. For a plant capacity of 5 million gal/yr, processing costs of this method were estimated to be as low as $0.26/gal compared to ∼$0.51/gal of BD produced by conventional catalytic methods. A retail cost of BD produced by the proposed method is likely to be competitive with petroleum DF prices.

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