Catalysis of Organic..

410“Green” Catalysts for BiodieselReactive Distillation Design. In this part we present the design for fatty acidsesterification using SZ as ‘green solid acid catalyst’. Several secondary reactions arepossible (Figure 5, right). These can be avoided by using a selective solid catalystsuch as SZ. The following results were accomplished by rigorous simulations (usingAspenONE Aspen Plus) that integrate the experimental findings. The problem ishighly complex, as it involves chemical and phase equilibria, vapour-liquidequilibria and vapour-liquid-liquid equilibria, catalyst activity and kinetics, masstransfer in gas-liquid and liquid-solid, adsorption on the catalyst and desorption ofproducts (8, 18-20). The catalyst development was integrated in the process design atan early stage, by data mining and embedding of reaction kinetics in the processsimulation. The analysis of physico-chemical properties shows very high boilingpoints for dodecanoic acid and esters (Table 2).Table 2. Normal boiling points of chemical species involved in the process.Chemical nameChemical formula M w (g/mol) T b (K) T b (°C)Dodecanoic (lauric) acid C 12 H 24 O 2 200 571 298Methanol CH 4 O 32 338 652 Ehyl hexanol C 8 H 18 O 130 459 186Methyl dodecanoate C 13 H 26 O 2 214 540 2672 ethylhexyl dodecanoate C 20 H 40 O 2 312 607 334Water H 2 O 18 373 100Hence, the ester will be separated in the bottom of the RD column and water byproductis removed as top product. By removing water by-product the equilibrium isshifted towards ester formation. The fatty acid feed must be fed in the top of reactivezone to reduce the contamination of the final product. Figure 6 (left) presents theflowsheet. High purity final products are feasible. An additional evaporator is usedfor further ester purification. Alternatively, both reactions (trans-esterification &esterification) can be combined with separation, shifting the equilibrium towardsproducts by continuous removal of water and glycerol (Figure 6, right). In the firstcase, the RD column has 10 reactive stages and uses an acid reflux. For the optimalreflux ratio the maximum reaction rate is located in the centre of RDC, providingcomplete conversion of reactants at the ends of the column.Top: 373 KRDCDistillationcolumnMethanol recoveryDodecanoic acidWater ≥ 99.9 %Acid < 0.1 %TriglyceridesFatty acidsWaterFeed ratio 1:1MethanolEvaporatorMethanolRDCDistillationcolumnGlycerolBottom: 473 KEster ≥ 99.9 %FAMEBiodieselFigure 6. Flowsheet of FAME production by esterification (left) or combinedesterification + trans-esterification process (right).