Mass transfer studies on gas absorption columns

MacKay, Donald (1961) Mass transfer studies on gas absorption columns. PhD thesis, University of Glasgow.

Abstract

The mass transfer and hydraulic characteristics of the Kuhni Distillation Plate have been studied using aqueous systems. The Kuhni Plate which is of Swiss origin, patented in 1937, has annular tunnel bubble caps with radial liquid flow from the periphery inwards to a central outlet well, from which the liquid is distributed to the periphery of the plate below by radial distribution tubes, giving co-ordinated reflux. Two sizes of plate were studied, 30 end 12 inches in diameter. Mass transfer cheracteristics were studied using two gas absorption systems, the humidification of air as a gas phase resistant system and the desorption of oxygen from water by air as a liquid phase resistant system. Air rates ranged from 100 to 500 pounds per square foot of column area per hour and water rates from 100 to 1,000 pounds per square foot of column area per hour. The plate efficiency was found to be relatively high and the relationship between the gas phase mass transfer coefficient and the gas contact time was of the form predicted by the Penetration Theory rather than the Two Film Theory of Mass Transfer. The Penetration Theory was further substantiated by consideration of the ratios of gas and liquid phase mass transfer coefficients and molecular diffusivities. In general the efficiency of the 12 inch plate was lower than that of the 30 inch plate owing to the lower weir height and the shorter liquid residence time on the plate. The throughput of the Kuhni Plate was found to be low in comparison with other plates. The hydraulic characteristics studied included plate pressure drop, clear liquid height, froth height, liquid hold-up, liquid residence time end the degree of liquid mixing. Plate pressure drops were found to be relatively high owing to the high "dry plate" pressure drop. It was found that the pressure drop characteristics could be improved by increasing the slot area although there was a corresponding reduction in efficiency. The effect of increasing the weir height was also studied and it is suggested that the performance of the plate can be improved by increasing both the slot area and the weir height. Froth heights were found to be low in comparison with other plates and it is probable that relatively little discrete bubbling takes place owing to the low weir height. Liquid hold-up, residence time and mixing were studied by a tracer injection technique, and a steady state dye injection technique was used to give a qualitative indication of the degree of lateral mixing. A discrepancy was observed between the liquid hold-up calculated from the tracer injection method and from the clear liquid height measurements. The degree of liquid mixing was characterised by the Peclet Number and the "eddy diffusivity". The relationship between the liquid residence time distribution function and the Peclet Number was derived for radial flow plates and the Peclet Number used in the deduction of the liquid phase mess transfer data. The effects of increasing the liquid viscosity on the gas and liquid phase mass transfer rates end on the liquid residence time, hold-up and degree of mixing were studied. The viscosities ranged from one to sixteen centiPoise and it was observed that the effect of increasing viscosity was complex, giving rise to maximum efficiencies in the four to eight centiPoise range. The density, viscosity, vapour pressure, oxygen solubility and oxygen diffusivity of aqueous solutions of the viscosity increasing agent ("Cellofas B") were studied to enable a more complete evaluation to be made of the effects of liquid viscosity on mass transfer rates.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: R G Gardner
• Keywords: Fluid mechanics, Hydraulic engineering
• Date of Award: 1961
• Depositing User: Enlighten Team
• Unique ID: glathesis:1961-73583
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 14 Jun 2019 08:56
• URI: https://theses.gla.ac.uk/id/eprint/73583