Bubble shape, gas flow and gas–liquid mass transfer in pulp fibre suspensions

Abstract

Gas-liquid mass transfer in pulp fibre suspensions in a batch-operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical-capped/dimpled-elliptical bubbles. At relatively low mass concentrations (C m = 2-3% for the softwood and C m ∼ = 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas-liquid mass transfer rate, k L a, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in k L a occurred between C m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.le transfert de masse gaz-liquide dans des suspensions de fibres de pulpe, dans une colonneà bulles de traitement en lot, est expliqué par des observations faites dans une colonne 2D de la taille et de la forme des bulles. Deux suspensions de fibres de pulpe (pulpes kraft de bois dur et de bois tendre) ontétéétudiées sur un intervalle de concentrations en masse des suspensions et sur un intervalle de débits de gaz. Pour un débit de gaz donné, on a observé que la taille des bulles augmentait avec l'augmentation de la concentration de la suspension, passant de petites bulles sphériques/elliptiquesà des bulles plus grosses de forme quasi-sphérique avec capuchonà elliptique avec dépression.À des concentrations en masse relativement basses (C m = 2-3% pour le bois tendre et C m = ∼7% pour la pulpe de bois dur), des bulles distinctes n'étaient plus observées dans la suspension. Au lieu de cela, un réseau de canaux se formait, au travers duquel le gaz s'écoulait. Dans la colonneà bulles, le taux de transfert de masse volumétrique gaz-liquide, k L a, diminuait avec l'augmentation de la concentration de la suspension.À partir desétudes 2D, cela se produisait lorsque la taille des bulles et la vélocité ascendante augmentaient, ce qui devrait faire diminuer la surface d'ensemble des bulles et la retenue de gaz dans la colonne. Un minimum de k L a aété observé avec C m = 2% et 4% (selon le type de pulpe) etétait atteintà proximité de la concentration en masse pour laquelle les canaux d'écoulement commençaientà se former.

Local Distributions of Bubble Velocity and Interfacial Area in the Slender Particle-Containing Scrubbing–Cooling Chamber of an Entrained-Flow Gasifier

“…As shown in Figure 5a, the mean bubble velocities in 0.01 and 0.03% fiber volume fractions are greater than those in tap water in the three regions. This is consistent with the findings of Ishkintana and Bennington: 36 the rising bubble velocity is larger at a high fiber mass fraction in some cases. Because the bubbles push the fibers to the sides and a fiber-depleted region is formed because of the bubble wake.…”

“…The factors affecting bubble velocity distributions are mainly bubble characteristics (bubble size and shape), liquid properties (viscosity and surface tension), and operating conditions (bubble column structure, superficial gas velocity, solid concentration, pressure, and temperature) . Although many studies have been carried out on the bubble velocity distributions in the slurry bubble columns, − the studies on bubble velocity distributions in the fiber suspensions are still scarce and mainly focus on semi-dilute and concentrated fiber suspensions. − Moreover, bubble velocity distributions in an annular bubbling structure of the scrubbing–cooling chamber with diluted fiber suspensions have never been studied. There are mainly two techniques for bubble velocity measurement in a bubble column: nonintrusive and intrusive techniques.…”

Bubble formation regimes during gas injection into a liquid cross flow in a conduit

“…In gas‐liquid flows it is generally desired to produce gas bubbles (bubbly flow), because they provide a great dispersed interfacial area and a high mass transfer coefficient between the phases . Optimum gas‐liquid mass transfer, produced by the existence of a bubbly flow, is a key parameter during metallurgical processes, effervescent atomization, paper and pulp processing, chemical contactors, and many other applications. Therefore, the mechanics of bubbly flows, from generation to their evolution have received widespread attention.…”

Numerical Simulation of Mass-Transfer Characteristics of a Bubble Rising in Yield Stress Fluids

“…Therefore, many researchers attempted to study the mass-transfer behavior in gas–liquid phases experimentally. The focus of these investigations was primarily on the influencing factor of mass transfer, such as the gas nozzle configuration, gas flow rate, pH value of the liquid phase, bubble column diameter, liquid properties, − and so on.…”