Mass transfer studies on a sieve plate

McLean, Alan Y (1966) Mass transfer studies on a sieve plate. PhD thesis, University of Glasgow.

Abstract

The mass transfer and hydraulic characteristics of a 9 in diameter sieve plate have been studied using three gas phase controlled systems, (a) the adiabatic evaporation of water into air, (b) the non-adiabatic evaporation of carbon tetrachloride by decahydronaphthalene (decalin). The object was to investigate the variation of plate efficiency with gas and liquid rate, due to the effect of these variables on the hydraulic behaviour of the plate, i.e. on froth density, gas residence time etc., under these three different systems. It was also intended to investigate the correlation of the mass transfer data. The mass transfer behaviour of the plate during evaporation of water, was studied by examining the rate of change of a very dilute dye i.o. concentrations less than .01%. Gas rates varied from 1 to 4.5 ft/sec through the bubbling area and liquid rates from 70 to 800 gal/hr ft2 of column cross sectional area. It was found that the relationship between the Murphree plate efficiency, EXV and the gas rate was more complicated than has been previously reported for this system. Consideration of the hydraulic characteristics of the aqueous system studied, which included the froth density, fractional gas hold up and gas residence time, showed that the relationship found is to be expected under the conditions studied and a mechanism is proposed the explain the varioations in plate efficiency with gas rate. The mass transfer characteristics of the air/carbon tetrachloride evaporation system were studied by measuring the change in enthalpy of the air before and after mass transfer took place. The gas rates studied were in the range 1-3.5ft/sec and liquid rates from 80 to 310 gal/hr ft2 of column area. The Murphree plate efficiency, EMV, was found to fall with gas rate and rise with liquid rate as expected from the hydraulic characteristics of the system, but the magnitude of the efficiencies was found to be lower than expected. An explanation is suggested. The mass transfer characteristics of the absorption system were studied by measuring the changes in concentration of gas and liquid across the plate. The gas rates studied were from 1.4 to 3.0 ft/sec and the liquid rates studied varied from 110 to 560 gal/hr ft2. The Murphree plate efficiency, EMV, was found to fall with gas rate and rise with liquid rate approximately as predicted from the examination of the hydraulic behaviour of the system. Comparison of the hydraulic characteristics of the three systems showed that properties such as froth density and gas residence time, considered at similar gas and liquid rates, were not independent of the system properties. The fractional gas hold up could be predicted, for the three systems. Attempts were made to correlate the mass transfer results by Gerster's method i.e. by relating the numbers of transfer units for different systems by the ratio of the square roots of the respective Schmidt numbers, the comparison of transfer units being made at similar gas and liquid rates. No satisfactory correlation was obtained as was expected from the results of the hydraulic studies which showed that at similar gas and liquid rates, the factors which govern mass transfer such as gas residence time and froth density were different for different systems. When comparisons were made at similar gas residence times more satisfactory correlations were obtained and it was concluded that while Gerster's method is basically correct, more knowledge of the structure and properties of gas/liquid foams and froths on commercially operating sieve plates is necessary before the method can be applied with confidence in this field.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: R G Gardiner
• Keywords: Chemical engineering, Hydraulic engineering, Fluid mechanics
• Date of Award: 1966
• Depositing User: Enlighten Team
• Unique ID: glathesis:1966-72166
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 17 May 2019 12:42
• Last Modified: 17 May 2019 12:42
• URI: https://theses.gla.ac.uk/id/eprint/72166