Study on Macro Kinetics of the Desulfurization Processes of Heteropoly Compounds in Ionic Liquids and Aqueous Solutions

Received: September 16, 2019
Revised: October 25, 2019
Accepted: November 14, 2019
Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2019.08.0423  

Cite this article:

Liu, X. and Wang, R. (2019). Study on Macro Kinetics of the Desulfurization Processes of Heteropoly Compounds in Ionic Liquids and Aqueous Solutions. Aerosol Air Qual. Res. 19: 2899-2907. https://doi.org/10.4209/aaqr.2019.08.0423

Highlights

• The macro kinetics of H₂S absorption in three HPC-based systems were investigated.
• The relevant parameters of absorption process were determined.
• The macro-kinetic equations of practical importance were obtained.

Keywords: H2S; Absorption; Macro kinetics; Heteropoly compound; Ionic liquid.

INTRODUCTION

As a common toxic, corrosive gas existing in natural gas, refined gas, biogas and other industrial gases, hydrogen sulfide (H₂S) has threatened the environmental protection and human health seriously (Kashfi and Olson, 2013; Wiheeb et al., 2013; Gupta et al., 2016). In recent years, a large number of works has been focused on the removal of H₂S, and various desulfurization methods have been developed (Lu et al., 2006; Ko and Hsueh, 2012; Wiheeb et al., 2013). Among these methods, the wet methods which use solutions as desulfurizers play an important position in the field of H₂S removal due to their high sulfur load bearing and high desulfurization efficiency (Wang, 2003; Dubois and Thomas, 2010).

Heteropoly compounds (HPCs) have been applied in many fields such as fuel oils desulfurization (Ding and Wang, 2016; Huang et al., 2019). The massive metal atoms existing in heteropoly anion lead to its unique reversible redox property (Pope and Müller, 1991; Kozhevnikov, 1998). As for the removal of H₂S, HPC solution has been proved as an excellent desulfurizer (Wang, 2003; Zou et al., 2013; Kim et al., 2014). Furthermore, studies reported that the oxygen atom in heteropoly anion could be replaced by O₂^2–, and then the peroxo heteropoly compound (PHPC) came into being (Ishii et al., 1988; Yadav and Mistry, 2001). Compared with HPC, the PHPC possesses higher catalytic activity and oxidation property. The PHPC has been applied commonly as catalyst for oxidative desulfurization of fuel oil (Wang et al., 2010a, b; Zhu et al., 2011). However, there are few works on the application of PHPC on H₂S removal.

Ionic liquid (IL) has brought about the widespread attention of researcher in related fields because of its unique properties such as low vapor pressure, good thermal stability and dissolving ability for numerous compounds (Bai et al., 2019). Compared with aqueous solution, the solution using IL as solvent could be operated at higher temperature which is higher than the boiling point of water without significant solvent loss. Hence, a series of researches on the desulfurization performance of IL have been reported (Pomelli et al., 2007; Jalili et al., 2009; Safavi et al., 2013). Among these researches, the introduction of active substance has been recognized as an excellent choice to enhance the H₂S removal ability of IL-based desulfurizer (Guo et al., 2011; Guo et al., 2015; Huang et al., 2016).

In our previous work, the H₂S removal performance of three HPC-based desulfurization systems, [Bmim]₃PMo₁₂O₄₀/BmimCl solution (Bmim: 1-n-butyl-3-methylimidazolium), aqueous solution of peroxo phosphomolybdic acid (PHPMo) and aqueous solution of CuH₂PMo₁₁VO₄₀ (CuPMoV), has been investigated (Ma et al., 2016; Liu and Wang, 2017; Liu et al., 2017; Ma et al., 2017). The results showed that all the three systems can remove H₂S with high efficiencies. In this work, to further understand the reaction characteristics of the desulfurization process of HPC-based system, the macro kinetic characteristics of H₂S absorption in three systems were investigated by a double stirred, concentration gradient-less gas liquid reaction cell. The [Bmim]₃PMo₁₂O₄₀/BmimCl system has high thermostability under 250°C. The three systems were all stable under the temperature of this experiment. The relevant parameters of absorption process were determined, and the macro kinetic equations of practical importance were obtained.

EXPERIMENTAL

Materials

Phosphomolybdic acid (H₃PMo₁₂O₄₀) was purchased from Sinopharm Chemical Reagent Co., Ltd., China; 1-n-butyl-3-methylimidazolium chloride (BmimCl) was supplied by Shanghai Cheng Jie Chemical Co. Ltd., China; hydrogen peroxide (H₂O₂, 30%) was purchased from Laiyang Kant Chemical Co., Ltd., China; phosphoric acid (H₃PO₄, 85%) was purchased from Laiyang Kant Chemical Co., Ltd., China; molybdenum trioxide (MoO₃) and copper oxide (CuO) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China; vanadium pentoxide (V₂O₅) was purchased from Tianjin Hongyan Chemical Reagents Factory, China.

The Preparation of Desulfurizers

The [Bmim]₃PMo₁₂O₄₀/BmimCl solution, aqueous PHPMo solution and aqueous CuPMoV solution were prepared according to our previous reports (Ma et al., 2016; Liu and Wang, 2017; Liu et al., 2017).

The Reaction Cell

As shown in Fig. 1, the double stirred, concentration gradient-less gas liquid reaction cell with an internal diameter of 3.82 cm and a height of 10 cm was used to investigate the macro kinetic characteristics of H₂S absorption in three systems. The temperature was controlled by thermostat water bath. The gas and liquid agitation was conducted by agitator and rotator, respectively. The stirring speed was adjusted to keep the interface smooth, so the area of gas-liquid interface could be identified as the area of absorption reaction. The specific stirring speed was identified by infrared digital tachometer. In our previous bubbling experiment, the slight foaming phenomenon will occur. However, in this work, the gas and liquid contact directly through the gas-liquid interface in the reaction cell, and no foaming phenomenon could be observed. In this experiment, the desulfurizer was used with relatively high concentration and large amount. As a result, the concentration of desulfurizer had little change in a certain period. A gas mixture (nitrogen as the carrier gas) containing a certain concentration of H₂S was passed to the reaction cell at a flow rate of 100 mL min^–1. The H₂S concentration of the outlet gas was detected by a TH-990s hydrogen sulfide gas analyzer. The residual H₂S gas was absorbed by aqueous NaOH solution. 

Fig. 1. The double-stirred, concentration gradient-less gas liquid reaction cell: 1. agitator; 2. sealing plug; 3. temperature probe; 4. gas inlet; 5. gas outlet; 6. reaction cell; 7. rotator; 8. heating ring.

Experimental Methods

The absorption processes of H₂S in the three systems are all mass transfer processes with chemical reaction, including the following three steps:

• The hydrogen sulfide diffused from gas into gas-liquid interface.
• The hydrogen sulfide dissolved and diffused into the liquid film at the interface, and reacted with the active component.
• The reaction products diffused into liquid phase.

In this experiment, the double stirred gas-liquid reaction cell was used to investigate the macro kinetic characteristics of H₂S absorption. The mass transfer coefficients of H₂S in gas and liquid phase, macro reaction orders and activation energy were determined.

Determination of Gas and Liquid Phase Mass Transfer Coefficients of H₂S

The absorption rate of H₂S was determined as follows:

N_H2S = V(C₁ – C₀)/S                                                                  (1)

where N_H2S is the absorption rate of H₂S, kmol m^–2 s^–1; C₁ and C₀ are the inlet and outlet concentrations of H₂S in the reaction cell, respectively, kmol m^–3; V is the gas flow rate, m³ s^–1; S is the area of gas-liquid interface, m².

The physical absorption rate of CO₂ in deionized water was determined using Eq. (1) under the same conditions. This process was limited by liquid phase mass transfer. The absorption rate can be expressed as: 

N_CO2 = k_LCO2(C^* – C_L)                                                               (2)

C^* = H∙p_CO2                                                                              (3)

where N_CO2 is the absorption rate of CO₂, kmol m^–2 s^–1; k_LCO2 is the liquid phase mass transfer coefficient of CO₂, m s^–1; p_CO2 is the gaseous phase partial pressure of CO₂, Pa; H is the solubility coefficient of CO₂, kmol m^–3 Pa^–1; C_L is the concentration of CO₂ in the main body of liquid phase. The k_LCO2 could be determined by Eq. (2), and then the liquid phase mass transfer coefficient of H₂S was determined as follows (Weisweiler and Blumhofer, 1984):

k_LH2S = k_LCO2(D_H2S/D_CO2)^2/3                                                                (4)

where k_LH2S is the liquid phase mass transfer coefficient of H₂S, m s^–1; D_H2S and D_CO2 are the liquid diffusion coefficient of H₂S and CO₂, respectively, m² s^–1.

The absorption rate of H₂S in NaOH solution (1 mol L^–1) under the same condition was measured using the same method. This process was limited by gas phase mass transfer. The absorption rate can be expressed as:

N_H2S = k_GH2S∙p_H2S                                                                                   (5)

where k_GH2S is the gas phase mass transfer coefficient, kmol m^–2 s^–1 Pa^–1; p_H2S is the gaseous phase partial pressure of H₂S, Pa. So, k_GH2S could be determined by measured N_H2S and p_H2S.

Determination of Macro Kinetic Equation of H₂S Absorption

The absorption rate of H₂S was correlated with the factors in the reaction by nonlinear fitting method, and then the kinetic reaction orders of the factors were determined. The activation energy of the overall reaction as well as the absorption rate equations were determined by the measured reaction rates at different temperatures as follows:

N_H2S = A∙[exp(–E/RT)]∙C_H2S^a∙C_absorbent^b                                      (6)

where A is the pre-exponential factor; E is the activation energy, KJ mol^–1; R is the molar gas constant, 8.314 J mol^–1 K^–1; T is the thermodynamic temperature, K; a and b are the reaction orders of the concentrations of H₂S and absorbent, respectively.

RESULTS AND DISCUSSION

The Gas and Liquid Phase Mass Transfer Coefficients of H₂S

The experimental results showed that the liquid phase mass transfer coefficient of CO₂ at 25°C is:

k_LCO2 = 3.41 × 10^–5 m s^–1

Hence, according to Eq. (4), the liquid phase mass transfer coefficient of H₂S at 25°C is:

k_LH2S = 3.26 × 10^–5 m s^–1

The measured gas phase mass transfer coefficient of H₂S at 25°C is:

k_GH2S = 1.46 × 10^–10 kmol m^–2 s^–1 Pa^–1

The Macro Kinetic Characteristics of H₂S Absorption in [Bmim]₃PMo₁₂O₄₀/BmimCl Solution

The effect of the concentration of [Bmim]₃PMo₁₂O₄₀ on the absorption rate was shown in Fig. 2, where T is the reaction temperature, r_G and r_L are the stirring rates in gas phase and liquid phase, respectively. It could be seen that the absorption rate increased with the increase of the concentration of [Bmim]₃PMo₁₂O₄₀. In our previous work (Liu et al., 2017; Ma et al., 2017), the removal of H₂S using [Bmim]₃PMo₁₂O₄₀/BmimCl solution has been proved to be achieved by the [Bmim]₃PMo₁₂O₄₀, and the BmimCl only played a role of solvent and reaction medium. The increase of the concentration of [Bmim]₃PMo₁₂O₄₀ is benefit to the decrease of liquid phase mass transfer resistance, thus promoting the absorption of H₂S. According to nonlinear fitting method, the relation between the concentration of [Bmim]₃PMo₁₂O₄₀ and absorption rate is: 

N_H2S = 7.29 × 10^–9 C_{[Bmim]3PMo12O40}^0.099, R² = 0.986                  (7)

Therefore, the H₂S absorption rate in [Bmim]₃PMo₁₂O₄₀/BmimCl solution is proportional to the 0.099 power of the concentration of [Bmim]₃PMo₁₂O₄₀. This means that the kinetic reaction order of [Bmim]₃PMo₁₂O₄₀ in the process of H₂S absorption is 0.099. 

Fig. 2. The effect of [Bmim]₃PMo₁₂O₄₀ concentration on the absorption rate: T = 80°C, C_H2S = 576 mg m⁻³, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The effect of the inlet concentration of H₂S on the absorption rate is shown in Fig. 3. The H₂S absorption rate increased with the increase of the concentration of H₂S, and the correlation is close to linear relation. The increase of the concentration of H₂S could decrease the gas phase mass transfer resistance significantly. According to nonlinear fitting method, the relation between the inlet concentration of H₂S and absorption rate is:

N_H2S = 7.07 × 10^–12C_H2S^1.120, R² = 0.992                                    (8)

The H₂S absorption rate in [Bmim]₃PMo₁₂O₄₀/BmimCl solution is proportional to the 1.120 power of the inlet concentration of H₂S. This means that the kinetic reaction order of H₂S is 1.120. Compared with the order of concentration of [Bmim]₃PMo₁₂O₄₀, it could be seen that the major limiting factor of H₂S absorption rate in [Bmim]₃PMo₁₂O₄₀/BmimCl solution is gas phase mass transfer resistance.

Fig. 3. The effect of the inlet concentration of H₂S on the absorption rate: T = 80°C, C_[Bmim]3PMo12O₄₀ = 0.005 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The effect of absorption temperature on the H₂S absorption rate is shown in Fig. 4. The increase of temperature was conductive to the absorption of H₂S in [Bmim]₃PMo₁₂O₄₀/BmimCl solution. A higher temperature could decrease the viscosity of ionic liquid, and increase the molecular activity of reactants. As a result, the absorption of H₂S was promoted at higher temperature. Hence, according to the results above as well as Eqs. (6), (7) and (8), the activation energy of the overall reaction was calculated as 8.84 KJ mol^–1, and the macro kinetic equation of H₂S absorption in [Bmim]₃PMo₁₂O₄₀/BmimCl solution is:

N_H2S = 6.6 × 10^–2∙[exp(–1064/T)]∙C_H2S^1.120∙ C_{[Bmim]3PMo12O40}^0.099      (9)

The relatively low activation energy demonstrated that the absorption reaction of H₂S in [Bmim]₃PMo₁₂O₄₀/BmimCl solution was a diffusion controlled process.

Fig. 4. The effect of temperature on the absorption rate: C_H2S = 576 mg m⁻³, C_[Bmim]3PMo12O₄₀ = 0.005 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The Macro Kinetic Characteristics of H₂S Absorption in Aqueous PHPMo Solution

The effect of the concentration of PHPMo on the absorption rate is shown in Fig. 5. A higher concentration of PHPMo is in favor of the liquid phase mass transfer process of H₂S molecule, which is in agreement with the result of our previous work. Hence, a higher concentration of PHPMo should be chosen in the practical application of H₂S removal. The relation between the concentration of PHPMo and absorption rate was determined using nonlinear fitting method as follows:

N_H2S = 5.033 × 10^–9C_PHPMo^0.131, R² = 0.962                                           (10)

Therefore, the H₂S absorption rate in aqueous PHPMo solution is proportional to the 0.131 power of the concentration of PHPMo. This means that the kinetic reaction order of PHPMo in the process of H₂S absorption is 0.131. 

Fig. 5. The effect of the concentration of PHPMo on the absorption rate: T = 25°C, C_H2S = 750 mg m⁻³, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

As shown in Fig. 6, the increase of the inlet concentration of H₂S showed a positive influence on the H₂S absorption rate due to the decreased gas phase mass transfer resistance. In our previous work, the results showed that the increase of H₂S concentration had little effect on the H₂S removal efficiency within a certain range, and the H₂S removal efficiency would decrease with further increase of H₂S concentration. Therefore, an appropriate inlet concentration of H₂S should be determined to achieve the highest utilization rate of desulfurizer without the decrease of H₂S removal efficiency. Based on nonlinear fitting method, the relation between the inlet concentration of H₂S and absorption rate was determined as follows:

N_H2S = 1.33 × 10^–9C_H2S^0.252, R² = 0.989                                               (11)

The H₂S absorption rate in aqueous PHPMo solution is proportional to the 0.252 power of the inlet concentration of H₂S.

Fig. 6. The effect of the inlet concentration of H₂S on the absorption rate: T = 25°C, C_PHPMo = 0.01 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The effect of temperature on the absorption rate is shown in Fig. 7. It could be seen that proper rise in temperature is beneficial to improve the H₂S absorption rate because of increased molecular activity at higher temperature. In addition, considering the volatilization of solvent, a moderate temperature should be chosen for practical application. According to the results above as well as Eqs. (6), (10) and (11), the activation energy of the overall reaction was calculated as 6.57 KJ mol^–1, demonstrating that the absorption reaction was a diffusion controlled process. The macro kinetic equation of H₂S absorption in aqueous PHPMo solution is:

N_H2S = 2.68 × 10^–6∙[exp(–790/T)]∙C_H2S^0.252∙C_PHPMo^0.131       (12)

Fig. 7. The effect of temperature on the absorption rate: C_H2S = 750 mg m⁻³, C_PHPMo = 0.01 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The Macro Kinetic Characteristics of H₂S Absorption in Aqueous CuPMoV Solution

The effect of the concentration of CuPMoV on the absorption rate is shown in Fig. 8. The H₂S absorption rate raise as the concentration of CuPMoV increased due to the decrease of liquid phase mass transfer resistance. Based on nonlinear fitting method, the relation between the concentration of CuPMoV and absorption rate was determined as follows:

N_H2S = 1.18 × 10^–8C_{CuH2PMo11VO40}^0.431, R² = 0.985                (13)

Hence, the kinetic reaction order of CuPMoV in the process of H₂S absorption is 0.431.

Fig. 8. The effect of the concentration of CuPMoV on the absorption rate: T = 25°C, C_H2S = 600 mg m⁻³, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

The effect of the inlet concentration of H₂S on the absorption rate is shown in Fig. 9. A higher H₂S concentration could promote the gas phase transfer process, and then increase the H₂S absorption rate. According to nonlinear fitting method, the relation between the inlet concentration of H₂S and absorption rate was determined as follows:

N_H2S = 4.63 × 10^–10C_H2S^0.510, R² = 0.989                               (14)

Therefore, the H₂S absorption rate in aqueous PHPMo solution is proportional to the 0.510 power of the inlet concentration of H₂S. 

Fig. 9. The effect of the inlet concentration of H₂S on the absorption rate: T = 25°C, C_CuPMoV = 0.001 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

As shown in Fig. 10, differing from the two systems above, the absorption temperature showed a negative effect on the H₂S absorption rate. Although the increase of temperature could promote the molecular motion, the increased temperature would hinder the reaction between CuPMoV and H₂S, which is exothermic reaction (Wang et al., 2006). Obviously, as for the reaction between CuPMoV and H₂S, the negative effect of increased temperature is stronger than its positive effect. According to the results above as well as Eqs. (6), (13) and (14), the activation energy of the overall reaction was calculated as –5.04 KJ mol^–1, and the macro kinetic equation of H₂S absorption in aqueous CuPMoV solution could be expressed as:

N_H2S = 1.02 × 10^–6∙[exp(607/T)]∙C_H2S^0.510∙C_{CuH2PMo11VO40}^0.431         (15)

Fig. 10. The effect of temperature on the absorption rate: C_H2S = 600 mg m⁻³, C_CuPMoV = 0.001 mol L⁻¹, r_G = 750 r min⁻¹, r_L = 220 r min⁻¹.

CONCLUSIONS

Ionic liquids and heteropoly compounds have been found to be effective systems for H₂S removal due to their unique properties. In our previous work, three systems for H₂S removal, a [Bmim]₃PMo₁₂O₄₀/BmimCl solution, an aqueous PHPMo solution and an aqueous CuPMoV solution, exhibited excellent desulfurization performance. This study continues our earlier research by investigating the macro kinetic characteristics of the three systems’ H₂S absorption using a double-stirred gas-liquid reaction cell. The gas and liquid phase mass transfer coefficients for H₂S in the reaction cell at 25°C were 1.46 × 10^–10 kmol m^–2 s^–1 Pa^–1 and 3.26 × 10^–5 m s^–1, respectively. For the [Bmim]₃PMo₁₂O₄₀/BmimCl solution, the orders of the reactions to [Bmim]₃PMo₁₂O₄₀ and H₂S were 0.099 and 1.120, respectively, and the activation energy of the overall reaction was identified as 8.84 KJ mol^–1; these macro kinetics can be expressed with the equation N_H2S = 6.6 × 10^–2∙[exp(–1064/T)]∙C_H2S^1.120∙C_{[Bmim]3PMo12O40}^0.099. For the aqueous PHPMo solution, the orders of the reactions to PHPMo and H₂S were 0.131 and 0.252, respectively, and the activation energy of the overall reaction was identified as 6.57 KJ mol^–1; these macro kinetics can be expressed with the equation N_H2S = 2.68 × 10^–6∙[exp(–790/T)]∙C_H2S^0.252∙C_PHPMo^0.131. For the aqueous CuPMoV solution, the orders of the reactions to PHPMo and H₂S were 0.131 and 0.252, and the activation energy of the overall reaction was identified as –5.04 KJ mol^–1; these macro kinetics can be expressed with the equation N_H2S = 1.02 × 10^–6∙[exp(607/T)]∙C_H2S^0.510∙C_{CuH2PMo11VO40}^0.431. The relatively low activation energy demonstrated that the absorption reactions of H₂S in all three systems were driven by diffusion.

ACKNOWLEDGMENTS

This work was supported by the Scientific Innovation Program of Shenzhen City, China, under Basic Research Program (JCYJ20170818102915033), the National Natural Science Foundation of China (Nos. 21276144 and 21511130021), and the Key Research and Development Program of Shandong Province, China (2017GSF217006).