Process design and simulation of an ammonia recovery plant from industrial sludge

Hydrothermal Carbonization (HTC) is an emerging thermochemical technology for the treatment of high-moisture biomass that, while effectively converting the solid fraction into a valuable carbonaceous material, generates a nitrogen-rich aqueous by-product whose direct discharge is environmentally prohibited. Within the framework of circular economy principles, the ammonia contained in this effluent represents a recoverable chemical resource rather than a waste stream, motivating the development of dedicated recovery processes. The present thesis, developed in collaboration with Biagini Impianti S.r.l. (Padova), addresses the detailed process engineering of a compact ammonia recovery unit treating HTC cooking sludge. Four process scenarios are examined, arising from the combination of two upstream feed configurations, direct HTC effluent and filter press pre-treated effluent, with two alternative products: a concentrated aqueous ammonia solution and crystalline ammonium sulphate. This comparative framework provides the industrial partner with a structured set of alternatives from which to select based on site-specific constraints and commercial valorisation opportunities. The process was modelled using a rigorous electrolytic thermodynamic framework, validated against experimental data independently obtained by the partner company's laboratory team. A systematic sensitivity analysis identified the key operational parameters governing recovery efficiency and product purity. The detailed design covers the packed stripping column, the spray scrubber of the absorption section, the heat exchange surfaces, and the vacuum system, with specific attention to the engineering challenges posed by the non-Newtonian rheology and foaming tendency of HTC sludge. The results demonstrate that vacuum thermal stripping combined with selective partial condensation represents a technically robust and energetically efficient solution for ammonia recovery from complex industrial matrices.