Catalysis of Organic..

Kiss, Rothenberg and Dimian 411X (mole frac)0.25 0.5 0.75 1RDC: Liquid Composition ProfilesMETHANOLACIDESTERWATERTemperature C140 160 180 200 220RDC: Temperature Profile0 3 6 9 12 15Stage0 5 10 15StageFigure 7. Liquid composition profiles (left) and temperature profile (right) in RDC.The composition and temperature profiles in the RDC are shown in Figure 7.The ester product with traces of methanol is the bottom product, whereas a mixtureof water and fatty acid is the top product. This mixture is then separated in theadditional distillation column and the acid is refluxed back to the RDC. The ester isfurther purified in a small evaporator and methanol is recycled back to the RDC.In conclusion, the hydrophobicity of the catalyst surface and the density of theacid sites are of paramount importance in determining the activity and selectivity.The systematic study of reaction rates under controlled process conditions(temperature, pressure, reactants ratio) is indeed a suitable method for screeningcatalyst candidates for fatty acids esterification (21). Catalysts with small pores, suchas zeolites, are not suitable for making biodiesel because of diffusion limitations ofthe large fatty acid molecules. Ion-exchange resins are active strong acids, but havelow thermal stability, which is problematic as relatively high temperatures areneeded for increased reaction rates. Heteropolyacids are super-acidic compounds butdue to the high molecular weight, their activity per weight of catalyst is not sufficientfor industrial applications. However, of the mixed metal oxides family, sulfatedzirconia was found as a good candidate. It is active, selective, and stable under theprocess conditions hence suitable for industrial reactive distillation applications.Biodiesel can be produced by a sustainable continuous process based oncatalytic reactive distillation. The integrated design ensures the removal of water byproductthat shifts the chemical equilibrium to completion and preserves the catalystactivity. The novel alternative proposed here replaces the liquid catalysts with solidacids, thus dramatically improving the economics of current biodiesel synthesis andreducing the number of downstream steps. The key benefits of this approach are:1. High unit productivity, up to ~6-10 times higher than of the current process2. Lower excess alcohol requirements, with stoichiometric ratio at reactor inlet3. Reduced capital and operating costs, due to less units and lower energy use