Thermomechanical processes in drying of food materials in systems with air recirculation

Predmet rada u ovoj disertaciji je istraživanje termomehanckih procesa pri konvektivnom sušenju prehrambenih materijala vlažnim vazduhom kao agensom sušenja u sistemima sa recirkulacijom. Matemticko-numerickim metodama i regresionom analizom definisan je fizicko-matematicki model koji omogucava simulaciju promene vlažnosti materijala u bezdimenzionom obliku tokom procesa sušenja, a koji uzima u obzir uticaj termomehanickih osobina vlažnog vazduha (temperature, relativne vlažnosti i brzine strujanja) na proces sušenja. U cilju verifikacije postavljenog modela, osmišljena je i izraena laboratorijska instalacija za konvektivno sušenje prehrambenih materijala u tankom sloju, koja radi sa potpunom recirkulacijom vazduha. Odabrani materijali (jabuka, krompir i banana) su sušeni pod istim režimima, sa temperaturom sušenja u opsegu 35 ÷ 55°C , relativnom vlažnošcu u opsegu 10 ÷ 30% i brzinom strujaja vlažnog vazduha u opsegu 1÷ 2 m s . Jabuke su sušene u komadicima oblika kocke dimenzija 10 mm i 13 mm, krompir u listovima debljina 2 mm i 3 mm, a banana u kolutovima debljina 6 mm i 7 mm. Analiziran je uticaj termomehanickih osobina vlažnog vazduha na kinetiku procesa sušenja, na osnovu dobijenih vrednosti efektivnog koeficijenta difuzije. Rezultati istraživanja su potvrdili da termomehanicke osobine vlažnog vazduha znacajno uticu na proces sušenja, pri cemu se u slucajevima slabijeg uticaja na proces sušenja pojedine velicine mogu zanemariti. Analiziran je rad sistema sušenja koji je podržan radom toplotne pumpe, na osnovu izracunatih vrednosti osnovnih pokazatelja efikasnosti rada sistema. Ustanovljeno je da se pravilno utvrenim parametrima rada instalacije može uticati na povecanje energetske efikasnosti sistema i smanjenje specificne potrošnje energije po kolicini izdvojene vlage. Verifikacija dobijenih rezultata izvršena je poreenjem rezultata sopstvenog modela sa rezultatima sprovedenog eksperimentalnog istraživanja, a potom i sa modelima i eksperimentalnim istraživanjima drugih autora... The motivation of the research presented in this thesis was to investigate thermomechanical processes that appears during the convective drying of food materials in systems with air recirculation. The mathematical model was formed to describe the change of moisture ratio of the material during the time, by taking into account the influence of all relevant thermomechanical properties of drying air (temperature, relative humidity and velocity) on the drying process. Convective laboratory scale dryer with possibility of full drying air recirculation was designed and built in order to achieve and measure all relevant drying process parameters. Apples, potatoes and bananas were used in the experiments. The chosen materials were dried under the same drying conditions. Drying air temperature was in range 35-55oC, relative humidity in range 10-30% and velocity in range 1-2 m/s. The apples were cut into cubes with 10 mm and 13 mm side dimension, potatoes were sliced into chips of 2 mm and 3 mm thickness and bananas were sliced into the cylinders with 6 mm and 7 mm thickness. Analysis of error appearance probability was performed. The effective moisture diffusivity was used to describe drying process efficiency. The experimental results confirmed that thermomechanical properties of drying air have significant influence on drying process. It was discovered that drying air, relative humidity could be ignored in some occasions. The performance of heat pump drying system was analyzed in order to discover system efficiency. The system energy and mass flows, air circulation behavior, drying air properties and system components behavior were monitored in order to calculate the main parameters of system performance. It was discovered that appropriate system performance optimization could increase system energy efficiency and reduce specific energy consumption. The results of mathematical model equation were compared with the experimental results and with the models and with the experimental results of the other authors, in order to be verified...