Design of conventional batch distillation of anaerobic digestate to isolate ammonium bicarbonate considering non‐ideal mixture behavior

Abstract Anaerobic digestate is a byproduct of biogas production and it is suitable for use as fertilizer. Ammonia and carbon dioxide provide the anaerobic digestate with buffer capacity and these compounds are responsible for the regulation of the pH of this soil organic amendment. The present investigation used experimental and theoretical data collection to assess the feasibility of isolating ammonium bicarbonate from a complex organic matrix with 95% moisture by a conventional distillation procedure. The anaerobic digestate is a complex blend and similar behavior was expected to that of other non‐ideal mixtures, such as the well‐known ethanol–water solution. The design of the classical batch distillation setup to handle the anaerobic digestate relied on the modeling of the operation of the distillation column with an ethanol–water mixture, which was used to preliminarily elucidate the relations between heating power, boil‐up rate, reflux ratio and separation efficiency. Equilibrium data retrieved from Aspen Plus® v12 and McCabe–Thiele methods were successfully applied to determine the minimum number of stages to isolate ammonium bicarbonate and ethanol in their respective aqueous solutions. Operation at total reflux in the distillation collum with the ethanol–water mixture resulted in an increase in the separation efficiency as the heating rate was increased. Operation at maximum reflux was necessary to retrieve pure ammonium bicarbonate crystals out of the anaerobic digestate. In the case of an aqueous solution including ammonia and carbon dioxide, optimization of the dose of antifoam was required to successfully perform the operation, owing to excessive foam formation.