Multi-objective Optimization of a Batch Transesterification Reactor Considering Reactor and Methanol Separation Unit Together

Abstract

Abstract Biodiesel, a mixture of fatty acid methyl esters (FAME), can be produced through transesterification of refined vegetable oils and is a potential alternate energy source. A higher concentration of FAME from a batch transesterification reactor can be obtained by maintaining a higher reactor temperature. This, however, increases the energy costs. Also, incomplete reactions influence the FAME separation during the downstream operations. Therefore, from an economical perspective, it is desirable to balance FAME yield, energy costs, and the cost of FAME separation. This study formulates and solves an energy cost minimization problem first, followed by a multi-objective optimization problem (MOOP) with objectives of- maximization of FAME concentration, minimization of reactor energy cost, and minimization of cost of methanol separation. The dynamic optimization problem is solved by utilizing orthogonal collocation on finite elements (OCFE) approach using the fmincon solver in MATLAB®. Optimal energy cost value of 2.0667 cents/kg batch is obtained corresponding to a FAME concentration of 0.75 mol/l. The MOOP portrays significant trade-off between the three objectives. For an increase in FAME concentration by 9.33 %, the optimal energy cost of the batch increases by 55 % which reduces the optimal cost of steam required in the methanol recovery unit by 12.7 %. The optimal energy cost of the reaction-separation process from the Pareto-optimal fronts is found to be 224.04 cents/ kg batch, at the FAME concentration of 0.82 mol/l. The proposed optimized strategy is found to achieve energy efficiency and thus, is economically superior.