Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Douglas P. Harrison

Abstract

The noncatalytic gas-solid reaction between CO$\sb2$(g) and CaO(s) to form CaCO$\sb3$(s) has been studied at high temperature and high pressure (HTHP) using a thermobalance reactor. This reaction could serve as the basis for a HTHP process for the separation of CO$\sb2$ from coal-derived gas. The kinetics of the calcination and carbonation reactions were studied as a function of temperature, pressure, CO$\sb2$ concentration, and background gas composition. Three sorbent precursors which produced CaO having a wide range of structural properties were selected for detailed kinetic studies. They were (i) reagent grade calcium carbonate, (ii) reagent grade calcium acetate, and (iii) commercial grade dolomite containing essentially equimolar quantities of CaCO$\sb3$ and MgCO$\sb3.$ Multicycle runs were conducted in order to have a better understanding of sorbent durability. Almost complete carbonation was possible using both calcium acetate and dolomite sorbent precursors; carbonation was incomplete when calcium carbonate precursor was used. The following operating conditions were found to be most appropriate: (UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\vbox{\halign{#\hfil&&\qquad#\hfil\cr &Calcination temperature: &750$\sp\circ$C\cr &Calcination pressure: &1--15 atm\cr &Calcination atmosphere: &any inert gas with low\cr &&CO$\sb2$ partial pressure\cr &Carbonation temperature: &650--750$\sp\circ$C\cr &Carbonation pressure: &15 atm\cr &Carbonation atmosphere: &any sulfur-free or\cr &&low-sulfur coal gas\cr}}$$. When sulfur-free simulated coal gas was tested, improved sorbent reactivity, capacity, and capacity maintenance were observed. The increase in reactivity was consistent with a higher concentration of CO$\sb2,$ possibly formed by the water-gas shift reaction. The distributed pore size model (Christman and Edgar, 1983) was used to analyze the carbonation results using the reagent grade calcium carbonate precursor. Good agreement between the model and experiment was achieved for runs at 650$\sp\circ$C with varying CO$\sb2$ mol fraction and reaction pressure. At different carbonation temperatures, however, it was necessary to assign zero activation energies to the intrinsic rate constant and product layer diffusion coefficient in order to match the experimental data. Both of these parameters should have quite large activation energies.

Pages

327

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