IMPROVEMENT OF THE EFFICIENCY OF VACUUM EVAPORATION OF LIQUID SLURRY

Cilj magistrskega dela je bil raziskati možnost zmanjšanja hidravlične obremenitve bioplinske naprave s tekočim digestatom v zimskih mesecih s tehniko dvostopenjskega vakuumskega uparjanja. V prvi stopnji je bil cilj iz digestata v čim večji meri odstraniti vodno raztopino amonijevih ionov (NH4+) in slednjo nevtralizirati z žveplovo(VI) kislino (H2SO4). Amonijev sulfat ((NH4)2SO4), označen tudi kot AS, ki nastane kot produkt nevtralizacije, je anorganska sol, ki je uporabna kot umetno gnojilo. Ker je vsebnost amonijevih ionov v (NH4)2SO4 nizka in volumen raztopine velik, so stroški transporta visoki. Zato je bila potrebna tudi druga stopnja uparjanja, s katero smo zmanjšali volumen vode in sol koncentrirali do sprejemljive meje, določene z masnim deležem AS, to je, 35 %. S spreminjanjem parametrov (temperature, tlaka, vrednosti pH) smo si prizadevali najti najboljše pogoje uparjanja in v skladu z zakonodajo določiti, ali se lahko AS uspešno uporabi kot gnojilo. Na podlagi eksperimentalnega dela smo izračunali masne in energijske bilance vakuumskega uparjanja. Med karakterizacijo digestata, vzorčenega v različnih časovnih obdobjih, je bilo ugotovljeno, da fizikalne in kemijske vrednosti nihajo, predvsem vsebnost amonijaka. S tem smo potrdili visoko odvisnost digestata od pogojev fermentacije. Med prvo stopnjo uparjanja, ki je potekala pri 40 °C, vrednosti pH digestata nismo spreminjali, saj je ta znašala 7, kar pomeni, da se je ves amonijak nahajal v hlapni obliki in je med med uparjanjem prehajal v kondenzat. Ugotovili smo tudi odvisnost stopnje uparjanja od koncentracije topljenca, to je, amonijaka. Ta je višja pri nižji koncentraciji topljenca. Po nevtralizaciji kondenzata prve stopnje je delež AS znašal 0,61 %. Med drugo stopnjo uparjanja smo želeno mejo AS v koncentratu, to je 35,62 %, dosegli pri 40 °C in vrednosti pH 5. Delež AS v kondenzatu je znašal 0,12 %. Na osnovi laboratorijskih poskusov smo ugotovili, da je delež digestata, ki po prvi stopnji uparjanja prehaja v koncentrat, 24 %, in delež, ki prehaja v kondenzat, enak 76 %. Po drugi stopnji uparjanja je delež, ki preide v koncentrat, enak 8 %, in delež, ki preide v kondenzat, enak 92 %.The aim of this graduation thesis was to reduce the strain on the biogas plant by reducing the hydraulic pressure during winter by using the technique of two-stage vacuum evaporation. In the first stage the target was to remove as much of the aqueous ammonium ions (NH4+) as possible and neutralize it with sulfuric acid (H2SO4). Ammonium sulfate ((NH4)2SO4), or AS in short, resulting as a product from neutralization, is an inorganic salt that can be used as a fertilizer. Since the ammonium ion content in AS is low and the volume of solution is high, transport costs are considerably high. Therefore, a second stage was needed to reduce the volume of water and concentrate the salt to an acceptable limit, determined by the mass fraction of AS, i.e. 35 %. By changing the parameters of evaporation (temperature, pressure, pH values) we tried to find the best conditions for evaporation and, in accordance with the law, determined whether AS can be successfully used as a fertilizer. Based on experimental results we also calculated the mass and energy balances of vacuum evaporation. During the characterization of digestate sampled at different time periods it was found that the physical and chemical characteristics fluctuate, the ammonia content in particular. This confirmed the high dependence of the digestate on the conditions of anaerobic digestion. During the first stage of evaporation at 40 °C the pH value of the digestate was not changed as it was at the value 7, which means that all ammonia was in its volatile form and was converted to condensate during the process. We established the dependence of the evaporation rate on the concentration of the solute, i.e. ammonia. The rate is higher at lower solute concentrations. After the neutralization of the first stage condensate, the AS content was 0,61 %. During the second stage of evaporation the desired AS limit in the concentrate was reached at 40 °C and a pH value of 5, which is 35,62 %. The AS content in the second stage condensate was 0,12 %. On the basis of laboratory experiments we found that the fraction of first stage condensate based on the input of digestate was 76 % and the fraction of first stage concentrate was 24 %. After the second stage, the fraction that passes into the second stage concentrate, i.e. AS solution, is 8 %, while the fraction in the second stage condensate, i.e. clean water, is 92 %