Optimal reflux splitting reactive distillation for algal biodiesel Production: Waste heat recovery through vapor recompression and organic Rankine cycle

Abstract

Algal biodiesel is widely accepted as a sustainable and alternative transportation fuel. This study aims at exploring the integration of multiple thermal intensification strategies to improve the energetic, economic and environmental performance of an optimal continuous flow algal biodiesel process. For this, a reflux splitting reactive distillation (RSRD) column is proposed for heterogeneous transesterification reaction. To enhance the biodiesel purity, the bottom stream of this RSRD scheme is subsequently proposed to treat in a decanter. A multi-objective optimization (MOO) technique, namely elitist non-dominated sorting genetic algorithm (NSGA-II) is employed to get the optimal configuration. The MOO problem is formulated with multiple conflicting objectives to optimize the biodiesel purity (maximize), total annual cost (minimize) and CO2 emissions (minimize). The technique for order of preference by similarity to ideal solution (TOPSIS) with entropy information as a weighting criterion is used to select the final optimal point. Then a vapor recompression scheme is introduced to enhance the energetic performance of the proposed RSRD column, which is here named as side vapor recompressed-reflux splitting RD (SVR-RSRD) configuration. Finally, the residual heat is proposed to recover by organic Rankine cycle (ORC) with selecting R245fa as a working fluid to further reduce the degree of heat irreversibility of the proposed configuration. Overall, 39.07% savings in energy consumption, 1.3% in total annual cost (TAC) within a payback period of 5 years (45.38% savings within a payback period of 10 years) and 40.01% reduction in CO2 emission with biodiesel purity of 93.5 mol% make this optimal integrated RSRD configuration attractive from energetic, economic and environmental perspective.