Preliminary design of optimized heat integrated two-stage vacuum evaporation for processing digestate from biogas plant
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.
... PK-fertilizer. After neutralization with H2SO4, the ammonium sulfate 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 C for the first stage and 60 C for the second. A preliminary process flow sheet was simulated in Aspen Plus to obtain data for heat integration and optimization of industrialscale 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 sulfuric acid improved overall economic performance. The optimization and heat integration of the twostage vacuum evaporation process within a biogas plant resulted in a circular and economically viable waste management technology. neutralization producing ammonium salts, membrane processes, adsorption on zeolites, resins, and biochar, and the production of biomass  . The electrochemical recovery of phosphate as struvite, its electrochemical decomposition, and removal of the ammonia nitrogen by recycling the struvite product were tested to simultaneously recover phosphate and remove the ammonia nitrogen from swine wastewater  . Integrated membrane processes, including centrifugation, ultrafiltration, and reverse osmosis, can separate digestate into a solid N,P-fertiliser, a liquid N,K-fertiliser, and dischargeable water, but the high fouling potential of digestate prevents wider use of these processes  . One of the most promising technologies for nutrient recovery from digestate is ammonia stripping followed by absorption in sulfuric acid, which produces ammonium sulfate  . Alternatively, nitric acid can be used to produce ammonium nitrate  ; however, nitric acid is more expensive, and the supply of ammonium nitrate to private buyers is restricted because of its explosive properties  . Since the stripping process requires high energy consumption, alternatives have been studied, such as solar exploitation for ammonia stripping in a transparent greenhouse  , and vacuum thermal stripping and acid absorption  . In the vacuum process, ammonia is stripped from digestate at a temperature below boiling point  . This approach could be a feasible alternative in biogas plants where low-temperature waste heat is usually available and can be used for vacuum evaporation, thus improving the environmental sustainability of these plants  . Different pilot-scale vacuum evaporation systems for treating digestate from biogas plants were tested: single-stage and two-stage, with and without acidification. A two-stage configuration with acidification that transforms digestate into a solid product has proved the most efficient  . Among the commercially available industrial evaporators, the most efficient are multi-stage flash evaporators and falling-film evaporators, while forced-circulation evaporators have the highest utility consumption  . Vondra et al.  performed a techno-economic analysis of vacuum evaporation in combination with ammonia scrubbers, stripping and reverse osmosis, while utilizing waste heat from combined heat and power units in a biogas plant. They showed that the feasibility of an evaporation system within a biogas plant depends on transport cost, price of electricity, investment costs and government incentives. For thermal processes, it is essential to optimize utility consumption, which can be accomplished by using process systems engineering approaches, such as process integration, intensification and optimization  . Hernandez and Martin  applied a heat integration and optimization approach to waste processing into biodiesel, including nutrient recovery from digestate for algae growing. Ammonia stripping from liquid digestate can also be integrated with thermal processes for processing solid digestate, such as combustion, pyrolysis and gasification  . A literature review showed that vacuum evaporation could be a promising alternative for treating digestate and concentrating nutrients; however, the efficiency of this process needs to be maximized in order to use this technology in full scale systems. The aim of this study is to develop the preliminary design of an optimized heat-integrated plant for processing digestate from a 1 MW biogas plant, using two-stage vacuum evaporation technology. The goal was to produce high quality concentrated liquid fertilizers, one rich in phosphorus and potassium, and another rich in nitrogen in the form of a 36% ammonium sulfate solution. The first could be used as a basal dressing for winter crops, usually applied in autumn, while the other could be used as a top-dressing fertilizer to provide nitrogen for growth  . The production of liquid products instead of solids was chosen because of the limited available capacity and because the goal was to apply fertilizers locally within a radius of 10 to 15 km around biogas plant.