Abstract
Sorption of hydrogen chloride gas by active soda and that of hydrogen sulfide gas by calcium oxide are explored by experiment as promising means of removing these detrimental contaminants from fuel gas: active Na2CO3 was prepared by the calcination of commercial NaHCO3 at 200 °C; reactive CaO was formed by decomposing a fine-grained, high-calcium limestone at 830 °C. Techniques with a differential reactor were employed to explore the rate of reaction of HCl with Na2CO3 at 500 °C and that of H2S with CaO at 800 °C. Time-resolved data on the sorbents’ conversion were collected as a function of mean particle size in the range between 0.285 and 1.12 mm. The surface reaction constants, deduced via the tractable model from the initial reaction rates of the two reactions, slightly increase with the increasing particle size. The proposed correlations enable to interpolate or cautiously extrapolate to other isotropic, irregularly shaped solids. The effective diffusivities educed by means of the model from the experimental curves decrease significantly with the increasing conversion and are affected by the particle size in both sorptions. The developed reaction rate equations can conveniently be applied to the design and simulation of the deep dechloridization and the bulk desulfurization of hot producer gas.
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Abbreviations
- D :
-
Effective (apparent) diffusion (transport) coefficient of reactant gas through the product shell, cm2 s−1
- d p :
-
Particle diameter, cm
- \( \bar{d}_{\text{p}} \) :
-
Mean particle size determined by sieving, cm
- (dr′/dτ′):
-
Dimensionless rate of reaction defined by Eq. (23)
- (dX/dτ):
-
Rate of reaction defined by Eq. (24), s−1
- (dX/dτ)in :
-
Initial rate of reaction (for τ → 0 and X → 0) (\( = \)3Kyρg /(r pρp)), s−1
- (dX/dτ′):
-
Dimensionless rate of reaction defined by Eq. (24)
- e :
-
Dimensionless fractional porosity
- e in :
-
Dimensionless initial fractional porosity of a particle
- e x :
-
Dimensionless fractional porosity of a reacted particle
- f :
-
Relative increase or decrease in the solid volume caused by chemical reaction
- F :
-
Specific reaction surface area (\( = \)3r′2/r p \( = \) 3(1 − X)2/3/r p), cm−1
- K :
-
Effective reaction rate constant per unit area of reaction interface (\( = \)[r pρp/(3yρg)]·(dX/dτ)in \( = \) (r pρp)/(τstd yρg)]), cm s−1
- K HCl :
-
Dimensionless equilibrium constant for reaction (1a)
- K H2S :
-
Equilibrium constant for reaction (1), kPa
- N :
-
Rate of disappearance of gas reactant in unit volume of sorbent particle volumetric rate of reaction defined by Eq. (32), mol cm−3 s−1
- P :
-
Total pressure in the main stream, kPa
- p :
-
Dimensionless auxiliary function defined by Eq. (15)
- p i :
-
Partial pressure of species, kPa
- q :
-
Dimensionless auxiliary function defined by Eq. (16)
- R 2 :
-
Dimensionless regression parameter
- r :
-
Radius of reaction interface within a spherical particle, cm
- r p :
-
Radius of a spherical particle, cm
- r′:
-
Relative radius of reaction interface within a spherical particle (\( = \) r/r p \( = \)(1 − X)1/3)
- t :
-
Celsius temperature, °C
- T :
-
Thermodynamic temperature, K
- V i :
-
Molar volume of solid component i, cm3 mol−1
- X :
-
Dimensionless fractional conversion of solid reactant (\( = \)1 − r′3)
- y i :
-
Mole fraction of species
- ΔH°:
-
Standard enthalpy change of reaction, kJ mol−1
- λ :
-
Dimensionless ratio of diffusional resistance in product shell to chemical reaction resistance at interface (\( = \) Kr p/D)
- π :
-
Dimensionless Ludolf constant (\( = \)3.141593)
- ρ g :
-
Molar density of gas at ambient pressure (101.325 kPa) (\( = \)0.012187/T), mol cm−3
- ρp :
-
Particle (apparent) molar density of solid, mol cm−3
- τ :
-
Elapsed time of exposure of solid to reactant gas, s
- τ std :
-
Standard time of exposure of solid to reactant gas (\( = \) r pρp /(Kyρg) \( = \) r 2p ρp /(λDyρg) \( = \) 3/(dX/dτ)in), s
- τ′:
-
Relative time of sorption / reaction (\( = \)τ/τstd \( = \) Kyρgτ /(r pρp) \( = \) λDyρgτ /(r 2p ρp) \( = \) τ d′ + τr′)
- τd′:
-
Relative time of exposure corresponding to diffusional resistance alone (\( = \) λ(2r′3 − 3r′2 + 1)/6)
- τr´:
-
Relative time of exposure corresponding to surface reaction alone (\( = \)1 − r′)
- cos−1 :
-
Anti-cosine; angle whose cosine is given, rad
- | |:
-
Absolute (positive) value
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Acknowledgements
This study was supported by the Grant Agency of the Czech Republic (GAČR) through the bilateral grant project of GAČR and National Science Council (NSC) Taiwan, Reg. No. (in ČR) 14-09692J (Reg. No. of foreign Project 102WBS0300011) and Reg. No. (in Taiwan) NSC 103-2923-E-042A-001-MY3. Special thanks are due to Mrs. Eva Fišerová for her dedicated assistance with the manuscript.
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Hartman, M., Svoboda, K., Pohořelý, M. et al. Reaction and transport effects in the heterogeneous systems for lean gas purification. Chem. Pap. 71, 563–577 (2017). https://doi.org/10.1007/s11696-016-0038-y
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DOI: https://doi.org/10.1007/s11696-016-0038-y