Radial liquid velocity distribution in an external‐loop airlift column with a tapered riser

Abstract

Radial and axial liquid velocity distributions in the tapered riser were investigated theoretically and experimentally. The liquid velocity distributions were computed by solving the NavierStokes equation numerically based on a modified mixing-length theory. Both radial and axial components of liquid velocity were taken into account. As a result, we found that the radial velocity component was much smaller than the axial velocity component. For a cylindrical column, which means no tapered section, a simplified solution was obtained. The simplified solution was found to agree well with the rigorous numerical solution even in the tapered riser. To confirm the validity of the present hydrodynamic model, the velocity distributions in the tapered riser were measured by an electric probe method using KCI solution as a tracer. The measured velocity distributions agreed with the computed ones, except in the vicinity of the bottom of the tapered riser at high gas flow rates.On a etudie de maniere theorique et experimentale les distributions de vitesse radiale et axiale du liquide dans une colonne montante conique. Les distributions de vitesse du liquide ont ete calculees par ordinateur en resolvant numeriquement I'equation de Navier-Stokes d'apres une theorie de longueur de melange modifiee. Les deux composantes radiale et axiale de la vitesse du liquide ont ete prises en consideration. C o m e consequence, nous avons trouve que la composante de vitesse radiale etait beaucoup plus petite que la composante de vitesse axiale. Pour une colonne cylindrique, c'est-a-dire sans section conique, on a obtenu une solution simplifiee. On a trouve que cette solution simplifiee concordait bien avec la solution numerique rigoureuse m&me dans la colonne montante conique. Afin de confirmer la validite du present modele hydrodynamique, les distributions de vitesse dans la colonne montante conique ont ete mesurees au moyen d'une sonde electrique et d'une solution de KCI comme traceur. Les distributions de vitesse mesurees concordent avec les distributions calculees par ordinateur, excepte dans le voisinage du fond de la colonne montante conique a de forts debits de gaz.

Liquid circulation hydrodynamics in an external loop airlift reactor

“…The inversion point (zero axial liquid velocity) for the air–water system in both flow regimes occurs at about 0.8–0.9 radius of the riser column. The existence of such internal liquid circulation in the riser of ELAR which is similar to that in a conventional bubble column has not been reported in literature, except Kawagoe et al (1999), who found downward liquid flow regions in the upper part of the tapered riser.…”

“…Utiger et al (1999) employed a constant temperature anemometer equipped with a hot‐film probe to measure local flow characteristics in the riser of a small size (riser diameter is smaller than 10 cm) ELAR with distilled water as the liquid phase. Kawagoe et al (1999) measured the local liquid velocity in a tapered riser (lower end diameter: 9 cm, upper end diameter: 27 cm) of an ELAR by an electro‐conductivity probe using KCl aqueous solution as a tracer. Vial et al (2002) investigated the local time‐averaged liquid velocity and root mean square velocity in the axial and tangential directions of the riser (15 cm in diameter) with a one‐component laser Doppler anemometer (LDA).…”

“…Although all the above‐mentioned few studies investigated the development of local liquid velocity profiles in the riser of a laboratory scale ELAR, they did not report the existence of liquid downward flow in the wall region of the riser, except Kawagoe et al (1999), who found downward liquid flow regions in the upper part of the tapered riser. In addition, it appears that ELARs have not been investigated using the recent improvements in the field of local measuring techniques, which have been applied only to conventional bubble columns, such as Pitot tube method (Hills, 1974; Krishna et al, 1999; Chilekar et al, 2007).…”