Gas-liquid mass transfer (with and without chemical reaction) in a frothing system

摘要

An investigation has been carried out on a pilot plant scale, into the factors affecting the hydrodynamic behaviour, the liquid residence time and the extent of liquid mixing, the mass transfer efficiency, the interfacial areas and the liquid phase mass transfer coefficients in a two sieve tray distillation column. The thesis is divided into two parts. Part I is mainly concerned with the hydrodynamic behaviour, the liquid residence time distribution functions and the degree of liquid mixing. The hydrodynamic behaviour was investigated as a function of the column operating conditions and the physical properties of the system; whenever possible the results have been reported in the form of correlation equations suitable for design purposes. The liquid residence time distribution functions were measured using a dye injection and photocell detection technique. The extent of liquid mixing on the tray was determined using the variance of the residence time distribution function, which were investigated as functions of column operating conditions and system geometry. In Part II of the thesis the problems involved in predicting and correlating efficiency data for sieve tray column are discussed, special attention being directed to the pre- diction of the liquid phase mass transfer coefficient. A survey was made of the methods used for determining the interfacial area obtained on sieve trays, special consideration being given to a method which involves comparing experimental with predicted absorption rates in a system where a first order or pseudo-first order reaction takes place between the solute and the solvent. This method was used in the present investigation. The Murphree tray efficiencies for the absorption system CO2-air-water and for the desorption system O[2]-aqueous glycerol-air have been reported. These systems were chosen because of the low solubilities of the gases into their respective liquids and the mass transfer may be regarded as being controlled by the liquid film. An accurate continuous sampling and analysing technique was employed and the efficiencies of both the systems were measured as functions of column operating conditions and CO2 composition in air phase (up to 12.5% by volume). Rates of absorption of CO[2] into sodium hydroxide solutions obtained under conditions such that the reaction between the CO[2] and NaOH is pseudo-first order, are also reported. The interfacial area 'a' per unit volume of the froth, calculated from these results was found to tary slightly with liquid flow rates. The values obtained were in the range of 1. 5 to 2. 4cm[-1]. The area was shown to increase when 'metal framework' was place over the tray. Values of k[L] were obtained by dividing the k[L] a values obtained from the CO[2]-air-water experiments with the values of 'a' obtained from the CO[2]-air-NaOH experiments. The values of obtained k[L] between 0.052 - 0.063cm/sec.