Shun Wachi

MASS-TRANSFER WITH CHEMICAL-REACTION AND PRECIPITATION

Precipitation involving three steps, namely gas-liquid mass transfer, chemical reaction and crystallization, is analysed in terms of the coupled equations for the film theory of gas-liquid mass transfer with chemical reaction and the mass and population balances of crystallization. The equations are solved numerically using literature data for the absorption of carbon dioxide gas into lime water. Non-uniform spatial distributions of supersaturation and nucleated particles due to the mass transfer resistance are reflected in the predicted crystal size distributions: larger particles are formed under conditions of high mass transfer into a large liquid volume (small specific surface area), while small particles of low variance are produced when the mass transfer coefficient is low.

Flow dynamics in a draft-tube bubble column using various liquids

Abstract Gas hold-up and liquid circulation rates in a draft-tube bubble column (0.22 m diameter, 85 l capacity) fitted with various diameter draft-tubes are measured for water and aqueous solutions of ethanol, glycerol and carboxymethylcellulose (CMC), respectively. The data are analyzed in terms of a development of the simple energy balance incorporating flow contraction coefficients to quantify deviations from ideal flow. Ethanol and glycerol enhance the entrainment of gas bubbles into the downcomer compared with pure water and inhibit liquid circulation. In CMC solutions, however, large coalesced bubbles are easily disengaged at the top of the column with little entrainment into the downcomer and promote the highest liquid circulation rate. Bubble column geometry, i.e. draft-tube diameter, affects both bubble entrainment and the flow contraction coefficients, and interacts with the fluid properties to affect both gas hold-up and liquid circulation.

Gas-phase dispersion in bubble columns

Gas-phase dispersion in bubble columns of 0.5 and 0.2 m diameter was investigated experimentally by the impulse response method at superficial gas velocities in the range 0.029–0.456 m/s. Based on the recirculation theory of bubble columns, the expression for the axial dispersion coefficient of the gas phase was found to be EG = (180/α) UGD32T. The experimental results in water bubble columns were quantitatively described by this equation with the proportional constant α = 9. At gas velocities higher than 0.1 m/s, the gas-phase dispersion was suppressed by the insertion of perforated plates with a free area of 44%.

ASPECTS OF GAS-LIQUID REACTION SYSTEMS WITH PRECIPITATE PARTICLE FORMATION

Several examples of industrial systems for gas-liquid chemical reaction with crystal precipitation are reviewed together with a particular focus on the means of predicting and controlling the physical form of the product. Specifically, a recent population balance analysis of crystallization coupled with the gas-liquid mass transfer model to predict the concentration and mixing rate dependencies of precipitate crystal size distributions is described. Secondary processes of particle agglomeration and disruption are also examined in terms of the effect of reactor fluid dynamics. Finally, different types of gas-liquid reactor are compared and design considerations summarised.

Downward lean powder flow in a vertical tube

Abstract The flow dynamics of a gas—solid mixture were investigated for downward flow of polyvinyl chloride powder in a vertical tube fitted with a fluidized bed on the top. Powder was charged from the top bed to the vertical lean flow part through perforated plates. Radially non-uniform distributions of powder flux, solid holdup and particle velocity were determined by powder sampling and photographic observation of the tube. A prediction method for the flow dynamics, including the choking phenomenon and the effect of scale-up, is presented in terms of a development of two-phase turbulent circulation theory incorporated with the turbulent kinematic viscosity.