Investigation of the mass transfer mechanism and impact of axial dispersion on the removal of Cd(II) and Zn(II) from binary aqueous solution onto fixed bed of natural zeolite : diploma thesis

Ispitan je proces uklanjanja kadmija i cinka postupkom u koloni iz binarne ekvimolarne vodene otopine ukupne početne koncentracije c o (Cd+Zn) ≈ 1 mmol/L pri protocima od Q = 1, 2 i 3 mL/min. Provedena su tri radna ciklusa i tri ciklusa regeneracije na istom uzorku zeolita visine sloja od 8 cm. Iz eksperimentalnih rezultata nacrtane su krivulje proboja i krivulje regeneracije, izračunati su karakteristični parametri te je utvrđena učinkovitost procesa. Povećanjem protoka otopine proboj i iscrpljenje se pojavljuju ranije, smanjuje se kapacitet zeolita i efikasnost kolone. Prirast ukupne količine (Cd+Zn) iona vezanih na zeolitu linearno raste od početka eksperimenta nakon čega se počinje smanjivati (točka infleksije) što ukazuje na promjenu mehanizma koji kontrolira ukupnu brzinu procesa. Primjenom Advekcijsko-disperzijsko-reakcijskog modela (ADR) izračunat je koeficijent aksijalne disperzije, D L , pomoću kojega je izračunat koeficijent prijenosa tvari, k f kroz difuzijski granični sloj čestice zeolita. Utvrđeno je da je utjecaj aksijalne disperzije izraženiji za veće protoke otopine kroz sloj jer otopina zaobilazi dio čestica u zeolitnom sloju zbog nedovoljnog kontakta između otopine i zeolita. Najveća vrijednost k f utvrđena je u točki proboja, a zatim opada prema točki iscrpljenja uslijed povećanja debljine difuzijskog graničnog sloja. Do područja infleksije brzinu procesa kontrolira prijenos tvari konvekcijom u tekućoj fazi, a nakon infleksije prijenos tvari difuzijom kroz česticu. Povećanjem protoka može se obraditi veći volumen otopine u kraćem vremenu, međutim protok postaje ograničavajući faktor jer uslijed njegova povećanja dolazi do pojave aksijalne disperzije koja dovodi do smanjenja efikasnosti kolone. Removal of cadmium and zinc from equimolar binary aqueous solution of total initial concentration of c o (Cd+Zn) ≈ 1 mmol/L has been investigated using the column process at the flow rates of Q = 1, 2 and 3 mL/min. Three service and three regeneration cycles were performed on the same zeolite bed with height of 8 cm. The experimental results are shown by breakthrough and regeneration curves, and the characteristic parameters have been calculated as well as removal efficiency. By increasing in flow rate, the breakthrough and exhaustion points occured earlier, while zeolite capacity and column efficiency decreased. The quantity of (Cd+Zn) ions increases linearly from the beginning of the experiment, after that decrease (inflection point) indicating a change in mechanism which controls the overall process rate. The Advection-Dispersion-Reaction (ADR) model was applied for calculation of the axial dispersion coefficient, D L from which the mass transfer coefficient, k f , through the diffusion boundary layer has been calculated. The impact of axial dispersion is more pronounced for higher flow rates because of the solution bypassing some of the zeolite particles due to insufficient contact between the solution and the zeolite layer. The highest value of k f is observed in the breakthrough point, and then decreases to the exhaustion point due to increase in thickness of diffusion boundary layer. Up to inflection, the mass transfer by convection through the liquid phase controls the overall procces rate, and after that the mass transfer by diffusion through the boundary layer of the zeolite particle. By increasing in flow rate a larger volume of the solution can be treated in a shorter time, but the flow of the solution becomes a limiting factor because with its increase, an axial dispersion occurs which consequently results in a reduction of column efficiency.