Hydrodynamics and axial mixing in a packed gas-liquid column

Abstract

The objective of this study was to investigate the pilot-plant gas absorption packed column hydrodynamics, as well as axial mixing in the system air-water. The pressure drop and the gas phase holdup data were determined in dependence on the flow rates of gas and liquid phases. The influence of superficial velocities of liquid and gas phases on the liquid axial dispersion in a gas-liquid packed bed column (ID 15 cm) consisting of Raschig rings (15x15x2 mm) were investigated. The pressure drop was measured with a U-type manometer, connected to the bottom and the top of the working part of the column. The gas phase holdup data in the air-water two-phase system was calculated as a ratio of the gas phase volume to the total volume of the two-phase system. Axial dispersion in the water phase has been determined by examining of the distribution of residence times of a salt tracer (NaCl) in the packed bed. The tracer was injected in the liquid flow above the packed bed; samples of liquid were simultaneously taken from two sites at 1 m distance along the bed. Salt concentrations in the samples were determined by conductivity measurements. The mean residence time and the axial dispersion number were calculated by the moment method. The axial dispersion increases with an increase of liquid flow velocities and decrease of superficial air velocities.U ovom radu su prikazani rezultati ispitivanja hidrodinamike i povratnog mesanja za dvofazni sistem vazduh-voda u poluindustrijskoj apsorpcionoj koloni sa punjenjem. Kao osnovne hidrodinamicke velicine odredjivani su pad pritiska kroz sloj punjenja, kao i srednji sadrzaj gasne faze u radnom delu apsorpcione kolone kao funkcije protoka gasne i tecne faze. Ispitivan je i uticaj prividnih brzina faza na povratno mesanje u tecnoj fazi za sistem vazduh-voda u apsorpcionoj koloni unutrasnjeg precnika 15 sm. Pad pritiska kroz kolonu napunjenu Rasigovim prstenovima (15h15h2 mm) meren je pomocu diferencijalnog manometra tipa U-cevi, srednji sadrzaj gasne faze u radnom delu kolone odredjen je zaustavnom metodom, dok je povratno mesanje u kontinualnoj fazi (voda) odredjeno pomocu srednjeg vremena zadrzavanja obelezene supstance (trasera) u radnom delu kolone. Srednje vreme zadrzavanja i maseni disperzioni broj su odredjivani metodom momenta. Utvrdjeno je da pad pritiska kroz kolonu raste s povecanjem prividne brzine gasa, pri konstantnoj vrednosti prividne brzine tecnosti. Na isti nacin, pri porastu prividne brzine tecne faze, pri konstantnoj brzini gasne faze, pad pritiska kroz kolonu raste. Srednji sadrzaj gasne faze u radnom delu kolone raste pri povisenju prividne brzine gasne faze. Srednje vreme zadrzavanja u radnom delu kolone raste s porastom protoka gasne faze, dok opada pri povecanju protoka tecne faze, kao sto se moglo i ocekivati. Medjutim, porast vrednosti masenog disperzionog broja s povisenjem protoka gasa, kao i njegovo smanjenje s porastom vrednosti protoka tecne faze nisu u skladu sa uobicajenim trendovima za ovakve vrste kolona. Ovakvi rezultati bi se mogli opravdati nedovoljnom osetljivoscu instrumenata za merenje protoka faza niskim brojcanim vrednostima dobijenim za maseni disperzioni broj, kao i nedostatkom konduktometrijskih merenja on-line metodom.