(283b) Thermo-Kinetic Modeling of Supercritical Processes for the Conversion of Algal Lipids to Biofuels

Biodiesel, derived from renewable feedstocks like algae, has the potential to replace traditional petroleum-based diesel.  However, numerous technical difficulties remain. The extraction of triglyceride oils from algae cells has the greatest economic variability while also being the most poorly understood operation [1].  Traditional hexane-extraction techniques create dirty feedstocks that must be cleaned and purified before proceeding to the transesterification process.  Among emerging technologies, supercritical processes appear promising for both the extraction and transesterification steps [2], but the high temperatures and pressures raise energy efficiency and safety concerns.  

Supercritical carbon dioxide is a non-toxic, environmentally benign solvent that has the potential to extract and convert algae-oils to biodiesel in a single step [3] at significantly lower temperatures than the supercritical methanol process. Supercritical carbon dioxide mixed with methanol forms a gas-expanded liquid (GEL), which drastically increases the solubility of the algal triglycerides.  To optimize conversion and minimize energy use, it is essential that a model be formulated which can accurately predict the reactor conditions in the GEL region. Therefore, an equation-of-state must be used, which can account for the effects of both long and short chain molecules and association, in both VLE and LLE.  For this purpose, the statistical associating fluid theory equations-of-state in ASPEN PLUS (PC-SAFT) and gPROMS (SAFT-γ Mie) are being used to calculate fluid-phase equilibria [4]. The necessary parameters are regressed from experimental data, and a thermo-kinetic reactor model is formulated to account for changing phase equilibria as the reaction proceeds.

[1] Silva, C.S.; Soliman, E.; Cameron, G.; Fabiano, L.A.; Seider, W.D.; Dunlop, E.H.; Coaldrake, A.K. Commercial-scale Biodiesel Production from Algae. Ind. Eng. Chem. Res. 2014, 53 (13), 5311-5324.

[2] Patil, P.D., Gude, V.D., Mannarswamy, A., Deng, S., Cooke, P., Munson-McGee, S., Rhodes, I., Lammers, P., Nirmalakhandan, N., 2011, Optimization of Direct Conversion of Wet Algae-to-biodiesel under Supercritical Methanol Conditions, Bioresource Technology, 102, 118-122.

[3] Soh, L., Zimmerman, J., 2011, Biodiesel production: The potential of algal lipids extracted with supercritical carbon dioxide, Green Chemistry, 13, 1422‒1428.

[4] Gross, J., Sadowski, G., 2001, Perturbed-Chain SAFT: An Equation-of-State Based on a Perturbation Theory for Chain Molecules, Ind. Eng. Chem. Res., 40, 1244‒1260.