During different biobased process steps low molecular weight fatty acids may be formed, which are generally lost in the wastewater. For economic and environmental reasons these byproducts should be recovered for further usage. The pulping process as example produces an effluent with small amounts of formic acid and acetic acid. State of the art for separation of these valuable components is liquid-liquid extraction with reactive extractants followed by distillation for solvent recovery. The crude distillate then consists of formic acid, acetic acid and water from co-extraction. The isolation of these three components is challenging because of their vapor-liquid equilibrium behavior. In addition to a binary high boiling azeotrope between formic acid and water the mixture exhibits a ternary saddle point azeotrope. To avoid further energy consuming separation steps of the crude distillate into its pure components our approach is to replace the distillation for solvent recovery by a reactive distillation. The laden solvent is mixed with an alcohol, for example methanol, and is fed to a distillation column. The esterification of the low molecular weight fatty acids with methanol produces low boiling methyl esters which can be continuously removed in the distillate. Therefore the reaction equilibrium is forced to the product side and complete conversion is rendered feasible.
In the simultaneous esterification of both acids formic acid catalyzes acetic acid esterification because formic acid is the stronger acid. Combined with the elevated temperature in the reboiler the reaction rates for the esterification reactions are further enhanced and no catalyst is required.
The distillate of the reactive distillation contains methyl formate, methyl acetate and the excess of methanol. The bottom product contains the solvent and the reaction water which can easily be removed by phase separation. For further separation of the distillate into pure constituents, pervaporation is beneficial due to the low boiling azeotrope between methanol and methyl acetate.
Through the replacement of conventional distillation by reactive distillation the investment costs are reduced and the energy consumption for solvent recovery and isolation of pure constituents is decreased.