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Ind. Eng. Chem. Res. 2002, 41, 1914-1915
CORRESPONDENCE Comments on “Design of Entrained-Flow and Moving-, Packed-, and Fluidized-Bed Sorption Systems: Grain-Model Kinetics for Hot Coal-Gas Desulfurization with Limestone” Miloslav Hartman,* Karel Svoboda, and Otakar Trnka Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Suchdol, Czech Republic
Sir: In the above paper, Fenouil and Lynn1 also deal with the issue of the thermodynamic equilibria for the sorption of hydrogen sulfide by calcium oxide and/or calcium carbonate. Hydrogen sulfide is a weak-tomedium acidic gas that belongs to the repugnant compounds for its high toxicity, treacherous corrosiveness, and difficult odor. Therefore, we believe that some points in the paper of Fenouil and Lynn1 should be made more clear. The conditions for the efficient sorption of H2S by calcareous materials include the prerequisite that the carbonate particles are first decomposed to CaO. Our experiments with the 0.3-0.8 mm particles of limestones withdrawn from different quarries indicate that the solids already calcine very fast when the difference between the equilibrium dissociation pressure of calcium carbonate at a given temperature (P*CO2) and the actual partial pressure of CO2 in the gaseous phase (PCO2) amounts to 10-20 kPa. The equilibrium dissociation pressure of CaCO3 can be estimated with the aid of the following relationship:2,3
ln PCO2 ) -20007.43/T + 21.68602
(1)
Figure 1. Suggested temperature difference between temperature of sulfidation (t) and that of calcination (tc) for carbonate materials as a function of partial pressure of carbon dioxide (PCO2).
[PCO2] ) kPa which correlates Johnston’s experimental data (1910) taken from the handbook by Weast and Astle.4 Using eq 1, systematic computations were done to estimate the increases in temperature (∆t ) t - tc) which correspond to the driving forces of calcination P*CO2 - PCO2 ) 10 and 20 kPa. These results are plotted and compared with the recommendations of Fenouil and Lynn.1 As can be seen, the calcination is already very rapid, and hence it does not affect the sulfidation rate when the reaction temperature is just 4-14 °C above the calcination temperature (tc). Its value follows from eq 1 for a partial pressure of carbon dioxide of interest. In light of these results, an increase of 30-50 °C in temperature above the calcination point, as suggested by Fenouil and Lynn,1 seems to be somewhat conservative. Aside from the enthalpy of the sulfidation reaction, it is also the presence of water vapor in the gas phase that inherently influences the overall adverse effect of temperature on the equilibrium H2S level: * To whom correspondence should be addressed. Phone: +420 2 20390254. Fax: +420 2 20920661. E-mail: hartman@ icpf.cas.cz.
Figure 2. Influence of temperature and water vapor concentration on equilibrium level of H2S over CaO as predicted by eq 3.
d(ln PH2S - ln PH2O)/d(1/T) ) ∆H°(T)/R
(2)
The thermochemical data tabulated by Barin5 lead to formula (3) describing the equilibrium state of the
10.1021/ie0108797 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/03/2002
Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002 1915
reaction between CaO and H2S:
yH2S ) yH2O /exp(7258.68/T + 0.103379)
reaction in the range 600-1600 K (327-1327 °C):
(3)
in which the symbols yH2S and yH2O are the mole fractions of the respective species. As visualized in Figure 2, the adverse effect of temperature on the attainable level of H2S is significant, particularly at higher concentrations of water vapor. Thus, we believe that every care should be taken to maintain the temperature of sulfidation above the calcination point as closely as our results shown in Figure 1 suggest. The enthalpy of the sulfidation reaction reported by Fenouil and Lynn1 as -65 kJ/mol appears to be slightly overestimated. Barin’s data5 provide the standard enthalpy of the sulfidation reaction that amounts to ∆H°(298.15 K) ) -59.44 kJ/mol. The quantity ∆H° increases slowly with temperature, and it is as large as ∆H° ) -61.62 kJ/mol when T ) 1273.15 K. We have found that a simple quadratic equation can approximate the standard enthalpy of the sulfidation
∆H h °(T) ) -58.167 - 8.9745 × 10-4T 1.4256 × 10-6T 2 (4) Literature Cited (1) Fenouil, L. A.; Lynn, S. Design of Entrained-Flow and Moving-, Packed-, and Fluidized-Bed Sorption Systems: GrainModel Kinetics for Hot Coal-Gas Desulfurization with Limestone. Ind. Eng. Chem. Res. 1996, 35, 1024. (2) Hartman, M.; Martinovsky´, A. Thermal Stability of the Magnesian and Calcareous Compounds for Desulfurization Processes. Chem. Eng. Commun. 1992, 111, 149. (3) Hartman, M.; Svoboda, K.; Trnka, O. Effect of Water Vapor on the Equilibrium between CaO and COS in Coal Gas. Collect. Czech. Chem. Commun. 1999, 64, 157. (4) Weast, R. C.; Astle, J. M. CRC Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981; p F-76. (5) Barin, J. In Thermochemical Data of Pure Substances, 3rd ed.; Platzki, G., Collaborator; VCH: Weinheim, Germany, 1995; Vol. 1.
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