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This work presents a preliminary design of a two-stage vacuum evaporation process converting a diluted liquid digestate into concentrated liquid fertilizers. Digestate is produced in a 1 MW biogas plant during the anaerobic digestion of poultry manure and corn silage. Laboratory experiments showed that in the first evaporation stage, about three-quarters of input digestate can be stripped to a diluted ammonia solution, while the concentrate can be used as phosphorus-potassium PK-fertilizer. After neutralization with H2SO4, the ammonium sulphate solution is concentrated in the second evaporation stage. Feasible operating temperatures that allow heat integration between the two stages were determined in a laboratory environment at 40 oC for the first stage and 60 oC for the second. A preliminary process flow sheet was simulated in Aspen Plus to obtain data for heat integration and optimization of industrial-scale processes. The process was completely integrated by using the waste hot utility available at the site, while the external utilities demand was virtually zero. Optimizing the flow rate of the added sulphuric acid improved overall economic performance. The optimization and heat integration of the two-stage vacuum evaporation process within a biogas plant resulted in a circular and economically viable waste management technology.

Preliminary design of optimized heat integrated two-stage vacuum evaporation for processing digestate from biogas plant