(271a) Optimal Simultaneous Production of Hydrogen and Liquid Fuels From Glycerol: Integrating the Use of Biodiesel Byproducts

Biodiesel is one of the most important alternatives to fossil fuels due to its compatibility with the diesel engines and its supply chain. The economic feasibility and the net energy balance of the biodiesel are linked to the use of its byproducts. Among them, the most important one is glycerol. Glycerol has been used so far in cosmetics, drugs, personal care, detergents, food industry (Pagliaro & Rossi, 2010), and has kept its price at levels that can support the economy of the biodiesel. However, the expected increase in the production of biodiesel will eventually saturate the current glycerol market, and thus its price will drop affecting the production cost of biodiesel which can increase around 25%-50% with respect to the current production cost presented in previous papers (Martín & Grossmann 2012, Severson et al. 2012) for the expected future price of glycerol (Ahmed & Papadias 2010). Therefore, new markets for glycerol should be considered. Since the biodiesel production requires energy, around 2 to 4 MJ/gal (Martín & Grossmann 2012), there is an option to increase the production of fuels from glycerol and improve the energy balance of biodiesel.

 In this paper we present the optimization of the simultaneous production of hydrogen and liquid fuels from glycerol. We propose a limited superstructure embedding the various process units involved in the production of hydrogen and FT-fuels. The process starts with the reforming of the glycerol. Three different alternatives are evaluated: aqueous phase reforming, which uses liquid water in large quantities but operates at low temperature; steam reforming, which has a higher yield to hydrogen but is an endothermic process; and autoreforming which has a lower yield to hydrogen but operates close to adiabatic conditions. Next, the raw syngas obtained is cleaned up and its composition is adjusted in terms of the ratio CO / H₂ using three possible technologies (bypass, pressure swing adsorption, PSA, and water gas shift reaction, WGSR) allowing the production of pure hydrogen. Later, the removal of CO₂ is performed by means of PSA. Once the syngas is prepared, the Fischer - Tropsch reaction is conducted and the products separated. Hydrocracking of the heavy products is also considered to increase the yield towards diesel. Reduced order models for the key units such as reformer, FT reactor, WGSR or hydrocraking, are developed based on experimental data and/or rules of thumb from the literature (Choi et al, 2003; Shabaker et al, 2004; Douette et al, 2007; Adhikari et al, 2007; Jones & Pujado, 2006; Bezergianni et al, 2009). Our objective is to optimize the superstructure and the operating conditions minimizing the energy consumption while maximizing the yield and determining the optimal distribution of synthetic liquid fuels. The optimization of the process is formulated as a Mixed Integer Non-linear Programming (MINLP) problem in GAMS.

 The problem is solved first for the optimal production of Hydrogen alone and later for the simultaneous production of green gasoline, FT-diesel and hydrogen. It is shown that the production of hydrogen is competitive with that obtained from lignocellulosic biomass (Martín & Grossmann, 2011) as long as the glycerol price is below $0.06/lb, which above the target of the DOE (Ahmed & Papadias 2010), using aqueous phase reforming followed by WGSR. However, the production of hydrogen discards the use of glycerol as a carbon source. On the other hand, the simultaneous production of hydrogen and liquid fuels is attractive versus the use of switchgrass (Martín & Grossmann 2011b) only if the price of glycerol is below $0.05/lb, DOE target. For that option the selected technologies are auto reforming and bypassing the raw syngas partially to PSA for the purification of the hydrogen.

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