Application of the carbon dioxide-barium hydroxide hydrate gas-solid reaction for the treatment of dilute carbon dioxide-bearing gas streams

Author

Gary L. Haag

Date of Award

8-1982

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Joseph J. Perona

Committee Members

J. S. Watson, E. S. Clark, M. H. Lietzke

Abstract

The removal of trace components from gas streams via irreversible gas-solid reactions is an area of interest to the chemical engineering profession. This research effort addresses the use of fixed beds of Ba(0H)₂ hydrate flakes for the removal of an acid gas, CO₂, from air that contains ~330 ppmv CO₂. Areas of investigation encompassed (1) an extensive literature review of Ba(0H)₂ hydrate chemistry, (2) microscale studies on 0.150-g samples to develop a better understanding of the reaction, (3) process studies at the macroscale level with 10.2—cm—ID fixed-bed reactors, and (4) the development of a model for predicting fixed-bed performance.

Experimental studies indicated fixed beds of commercial Ba(0H)₂ • 8H₂0 flakes at ambient temperatures to be capable of high CO₂ -removal efficiencies (effluent concentrations <100 ppb), high reactant utilization (>99%), and an acceptable pressure drop (1.8 kPa/m at a superficial gas velocity of 13 cm/s). Ba(OH)₂ • 8H₂0 was determined to be more reactive toward CO₂ than either Ba(OH)₂ • 3H₂0 or Ba(0H)₂ • 1H₂0. A key variable in the development of this fixed-bed process was relative humidity. Operation at conditions with effluent relative humidities >60% resulted in significant recrystallization and restructuring of the flake and subsequent pressure-drop problems. This phenomenon was attributed to capillary condensation of water at V-shaped contact points within the solid. For influent gas streams with water vapor pressures less than the dissociation vapor pressures of Ba(OH)₂ • 8H₂0, the activity of the bed decreased to essentially zero as the lower hydrates were formed. Based upon these constraints, an operating window for optimum bed operation for the isothermal treatment of an air-based (330-ppm_v CO₂) gas stream was determined.

Modeling studies indicated the rate of mass transfer to be limited by the gas-film mass transfer coefficient, K_F. The rate-limiting species was the reactant, C0₂. Because of nonuniformities in the thickness and size of the commercial Ba(OH)₂ • 8H₂0 flakes, the area available for mass transfer was modeled semiempirically as a function of reactant conversion, A = A₀ (I-X), where A₀ is the initial area and X is the extent of conversion. Numerical solutions to the controlling partial differential equations were obtained. Data analyses of the breakthrough profiles indicated excellent correlation between the data and model when the exponent, n, in the area correlation was equal to 1. Values for K_FA₀ obtained from the breakthrough profiles were correlated as a function of system parameters, most notably relative humidity, temperature, and gas velocity. Little correlation existed for either relative humidity or temperature, the latter case being indicative of gas-film control. K_FA₀ was found to be proportional to V₀^1.11 where V₀ is the superficial velocity at reference conditions (101.3 kPa, 294.3°K). Based upon published correlations for heat and mass transfer, one would expect K_F α V₀^0.3 → V₀^0.8 Thus the velocity exponent for K_FA₀ was greater than expected and was attributed to result from a functional dependency of the initial area available for mass transfer, A₀, upon gas velocity. For this system that is composed of randomly packed flakes, increases in gas velocity appear to reallign the flakes slightly, thus reducing the number of planar contact points and increasing the surface area available for mass transfer. Observed bed expansion was nominal. Pressure drop studies indicated the pressure drop or shear force to be a strong function of gas velocity, α V^1.4. Hence a correlation appears to exist between the shear force exerted upon the particles and the increase in surface area resulting from the slight realignment of the flakes and a reduction in the number of planar contact points.

Recommended Citation

Haag, Gary L., "Application of the carbon dioxide-barium hydroxide hydrate gas-solid reaction for the treatment of dilute carbon dioxide-bearing gas streams. " PhD diss., University of Tennessee, 1982.
https://trace.tennessee.edu/utk_graddiss/13246