Removal of Hydrogen Sulfide by Iron-Rich Soil : Application of the Deactivation Kinetic Model for Fitting Breakthrough Curve

In this study a deactivation kinetic model was used to predict the breakthrough curve in a noncatalytic gas-solid reaction. The iron-rich soil was tested to react with H2S under a reducing atmosphere at high temperature. The results indicated that the deactivation kinetic model can be well fitted to the breakthrough curve in the experimental range. The breakthrough curves were accurately predicted by the model, and provide useful information for the time to reload the solid materials in the reaction. The activation energy of the reaction of iron-rich soil and H2S was experimentally calculated to be about 34 kJ/mol and 131 kJ/mol, respectively for the deactivation kinetic model I (m = 0, n = 1) and model II (m = 1, n = 1). Both of the deactivation kinetic models can fit the experimental results. The order of H2S in the deactivation model probably ranged from zero to one.


INTRODUCTION
The gas-solid noncatalytic reaction is a very important process in chemical industrials, such as the reaction of CaO and SO 2 , and the sulfurization reaction between metal oxides and H 2 S, that have been widely applied in the coal gasification cleanup process.Many metal oxides, including ZnO, CuO, Fe 2 O 3 and MnO x have been widely used because of their thermodynamic superiority with H 2 S under high temperature (Slimane and Abbasian, 2000;Alonso and Palacios, 2004;Kim and Park, 2010;Cheah et al., 2011).The most attention was mainly aimed at the development of effective metal oxides for H 2 S removal.Discussion of kinetic analysis for gas-solid noncatalytic reaction is relatively lack.Most of kinetic analysis is built on the basis of the shrinking core model (Homma et al., 2005;Kar and Evans, 2008).Prior to fitting the shrinking core model, the experimental work has to be operated with a thermogravimetric analysis (TGA).Although the shrinking core model is appropriate to fit the kinetic results, a corrosion effect should be considered when the TGA is used to carry out.
During the reaction of H 2 S and metal oxides, a dense sulfide layer is expected to be formed on the reactive oxide.Diffusion resistance through this layer causes a significant decrease in the reactivity of the metal oxide (Kyotani et al., 1989;Kyotani et al., 1989).In addition, significant changes in pore structure, active surface area, and active site distribution during the sulfurization of the metal oxide will cause a significant deactivation of the solid.Deactivation models proposed in the literature for gas-solid reactions with significant changes of activity of the solid due to textural changes, as well as product layer diffusion resistance during reaction, were reported to be quite successful in predicting conversion time data (Dogu, 1981;Balci et al., 1993;Yasyerli et al., 1996;Bandyopadhyay et al., 1999;Yasyerli et al., 2003).
Our previous studies shown that the natural soils and contaminated soils have a highly reaction ability with H 2 S and thus be used for many times after regeneration process (Ko, 2008(Ko, , 2011)).The iron species is the major active metal to react with H 2 S. Unfortunately, the information of kinetic performance is lack due to instrument limitation.Therefore, the main objective was to evaluate the feasibility of the deactivation kinetic model and obtained a series of kinetic parameters to predict the breakthrough time.

MATERIALS AND METHODS
The tested soil was collected from the field of the campus of the National Pingtung University of Science and Technology.Soils were sampled at a depth of 0-15 cm from a site.After sampling, unwanted materials, such as leaves, tree root and blinding were removed from soil sample and then dried at room temperature for a week.The collected soils were ground with an agate mortar and sieved to pass through a 2-mm sieve.The physical and chemical property was tabulated in Table 1.
The experiment of this study was carried out using a fixedbed reactor near atmospheric pressure.The experimental system consisted of three parts: (i) a coal gasified gas simulation system; (ii) a desulfurization reactor system; and (iii) an exiting gas analyzing system.The composition of the simulation coal gas involved was 1 vol % H 2 S, 15 vol % H 2 , and balanced N 2 .To avoid the formation of by-products during the kinetic experiments, only H 2 and H 2 S were considered in this study.Gases were supplied from gas cylinders and flow rates were monitored through mass flow controllers.All mass flow controllers were monitored accurately by an IR soap bubble meter and the concentration of all species calculated at the condition of STP.Prior to entering the reactor the gases were conducted in a mixing pipe to confirm that the mixture gas was turbulent flow.The reactor consisted of a quartz tube, 1.6 cm i.d., 2.0 cm o.d., and 150 cm long, located inside an electric furnace.Quartz fibers were set in the reactor in order to support the soil samples.Weight hourly space velocity (WHSV) was controlled at 4000 mL/hr/g to avoid severe pressure drops and channeling flow effect, and provided enough retention time.Two K-type thermocouples were inserted exactly into the reactor near the positions on the top and bottom of the sorbent packing to measure and control the inlet and outlet temperatures.Before sorption proceeding, a pure nitrogen gas (purity 99.99%) was fed into the reactor for 30 minutes at 773 K in order to remove adsorbed water and impure materials, which coated on the surface of the sorbent.In addition, blank breakthrough experiments were also executed under the same conditions and verified that no reaction was taking place anywhere between H 2 S and the lines/reactor.The inlet and outlet concentration of H 2 S was analyzed by an on-line gas chromatograph (Shimadzu, GC-14B) equipped with a flame photometry detector (FPD) and fitted with a GS-Q capillary column.A six-port sampling with 0.5 mL sampling loop was used to sample the inlet and outlet concentration of H 2 S. The removal experiment was terminated when the outlet H 2 S concentration from the reactor approached the inlet concentration of H 2 S. In this study, the breakthrough time was defined as the time from the beginning of the sorption to the point outlet H 2 S concentration reached 100 ppm.

Deactivation Model for Kinetic Study
The formation of a dense product layer over the solid reactant creates an additional diffusion resistance and is expected to cause a drop in the reaction rate.One would also expect it to cause significant changes in the pore structure, active surface areas and activity per unit area of solid reactant with reaction extent.All of these changes cause a decrease in the activity of the solid reactant with time.As reported in the previous literatures, the deactivation model works well for gas-solid reactions (Suyadal et al., 2000;Yasyerli et al., 2002;Kopac and Kocabas, 2004).In this model, the effects of the factors on the diminishing rate of sulfur fixation were combined in a deactivation rate term.To simulate the removal of H 2 S by sorbents, the following assumptions were made: (i) The sulfidation is operated under isothermal conditions.(ii) The external mass-transfer limitations are neglected.(iii) The pseudo-steady state is assumed.(iv) The deactivation of the sorbent is first-order with respect to the solid active sites, while zero-order for the concentration of H 2 S and can be described as follows: With the pseudo-steady state assumption, the isothermal species conservation equation for the reactant gas H 2 S is expressed as follows: Integrating Eq. ( 2), the following equation can be obtained 0 0 0 Combining with Eq. ( 1) and Eq.(3), Eq. ( 4) can be obtained Arranging Eq (4), the following can be obtained: To obtain analytical solutions of Eq. (1) and Eq. ( 2) by taking n = m = 1, an iterative procedure was applied.In this procedure, the Eq. ( 4) is substituted into Eq.( 1), and the first correction for the activity is obtained by the integration of this equation.The following approximate expression was then derived (deactivation model type II, n = m = 1).
 

RESULTS AND DISCUSSION
Prior to investigating the feasibility of the deactivation kinetic model for the reaction of iron-rich soil and H 2 S, the effects of external and internal mass transfer resistances have to be considered in order to understand their influence.The performance of particle size and WHSV are the major indicators for determination of internal and external mass transfer in kinetic study.As shown in Fig. 1(a), the sulfur sorption capacity decreased with increasing particle size.Small particle size enhanced the sulfur sorption capacity compared to larger ones, implying that the internal mass transfer resistance should be considered if the particle size ranges from 10-60 meshes.On the other hand, the sulfur sorption capacity maintained constantly while the particle size ranged from 80-120 meshes, indicating that the internal mass transfer resistance could be ignored within this range.
To obtain an authentic experimental data, particle size samples ranged between 90-110 mesh were collected to carry out the deactivation kinetic model.For the external mass transfer experiments, as shown in Fig. 1(b), a remarkable decline was found when the WHSV was controlled at 15,000 mL/hr/g.The external mass transfer resistance heavily affected the overall reaction, thus the optimal WHSV should be controlled between the ranges of 1,000-10,000 mL/hr/g/.To minimize the external mass transfer resistance the kinetic study was conducted at a WHSV of 4,000 mL/hr/g.

Deactivation Kinetic Type I Model (m = 0, n = 1)
The temperature dependence of the H 2 S breakthrough curves for the iron-rich soil is presented in Fig. 2. According to the breakthrough curves, the relationship of time and ln(lnC o /C) could be easily obtained at various temperatures, as shown in Fig. 3.The R 2 values for all cases were better than 0.98.The initial reaction rate constants, k o , were calculated from intercept in Fig. 3.Meanwhile, a straight line was attained by plotting ln k o versus T −1 .Via its intercept and slope, the values of frequency factor and the apparent activation energy were calculated from Arrhenius relationship.The frequency factor and activation energy were 2.31 × 10 11 and 131.51 kJ/mol, respectively.

Deactivation kinetic type II model (m = 1, n = 1)
Unlike type I model, the relationship between k o and C o is a complex and nonlinear equation.To obtain parameters in Eq. ( 6), the regression fitting was performed using Eq. ( 6) in the Sigma Plot software.Fig. 4 shows the regression results for the experimental data by the deactivation type II model.The R 2 values for all cases were better than 0.99.Likewise, the activation energy could be obtained from Arrhenius relationship.The frequency factor and activation energy were 2.49 × 10 7 and, 34.02 kJ/mol, respectively.
To further establish the fitness of the deactivation type I and II models, three sets of reaction were performed at 598, Particle size (mesh) Fig. 2. Breakthrough curves of the reaction between iron-rich soil and H 2 S. Inlet H 2 S: 1%, H 2 : 15% and balanced N 2 , WHSV: 4,000 mL/hr/g with a flow of 100 mL/min.653, and 723 K as well as simulated the model to predict their breakthrough behaviors in Fig. 5.As can be seen, the R 2 values were higher than 0.99, indicating the deactivation kinetic type I and type II models could accurately predict the breakthrough behaviors for the reaction of iron-rich soil and H 2 S. In particular, the breakthrough times were accurately predicted, which provided useful information for the time to change loading materials in actual operating condition.
The reasonable values for the typical noncatalytic gassolid reaction of iron oxides and H 2 S were reported to be around 30-100 kJ/mol (Ranade and Harrison, 1981;Tamhankar et al., 1981;Sa et al., 1989;Pineda et al., 1995).The difference between the type I and type II was the order of outlet concentration of H 2 S (represent by C in the deactivation kinetic model, kmol/m 3 ).Although there was a significant change in activation energy for both two models, their activation energies appeared to be accepted because they ranged among the previous reported studies.In addition, the fitting results also presented well predictions for both two models.It was speculated that the actual order of C may be ranged between zero to one, which resulted in  the nearly perfective fitting results for both models.

CONCLUSIONS
The deactivation kinetic model was employed to fit the breakthrough curve for the reaction of H 2 S and iron-rich soil.Two types of deactivation kinetic model for describing the order of reactant and H 2 S concentration was used to predicted the breakthrough curve at various reaction temperatures.Results showed that the activation energy of the reaction of iron-rich soil and H 2 S was experimentally calculated about 34 kJ/mol and 131 kJ/mol, respectively for deactivation kinetic model I (m = 0, n = 1) and model II (m = 1, n = 1).The breakthrough curves were accurately predicted which provided useful information for the time to reload the solid materials in the reaction.
5) Thus, if ln[lnC o /C] is plotted versus time, a straight line should be obtained with a slope equal to -k d and intercept giving ln[k o W/Q o ], from which k o can be obtained.

Fig. 3 .
Fig. 3.The relationship of the time and ln(lnC o /C) at various temperatures by a deactivation model (m = 0, n = 1 ) and Arrhenius equation fitting result.

Fig. 5 .
Fig. 5. Regression fittings of the experimental with results predicted by deactivation type I and type II model under various temperatures.Inlet H 2 S: 1%, H 2 : 15% and balanced N 2 , WHSV: 4,000 mL/hr/g with a flow of 100 mL/min.

Table 1 .
Some chemical and physical properties of iron-rich soil