1. Introduction

Hydrogen can be utilized as an energy carrier in various industrial applications. When it is produced from water, no environmental pollutants are emitted. Therefore, technologies for producing hydrogen from water have been investigated worldwide[1-3]. The thermochemical hydrogen production method proposed by Funk and Reinstrom can be performed at lower temperatures than that required to split water directly by combining several chemical reactions[4]. General Atomics (GA) proposed and investigated a thermochemical hydrogen production method known as the sulfur-iodine (SI) process[5]. The Japan Atomic Energy Agency (JAEA) demonstrated the continuous operation of the SI process at the bench scale with an output of 32 L/h[6]. It was reported that approximately 45-50% of the theoretical thermal efficiency of the SI process can be achieved under optimal operating conditions using the heat generated by a nuclear energy source[7-9]. The SI process is composed of the following three chemical reactions:

Sulfur dioxide (SO₂) reacts with water (H₂O) and iodine (I₂) to produce sulfuric acid (H₂SO₄) and hydrogen iodide (HI) in the Bunsen reaction (Eq. (1)). The Bunsen product solution spontaneously separates into two immiscible liquid phases (the H₂SO₄-rich phase is referred to as the SA phase, and the HI-rich phase is referred to as the HI_x phase) because the use of excess I₂ results in a density difference between the two phases. The H₂SO₄ and HI phases decompose as shown in eq. (2) and (3), respectively. The overall reaction leads to the decomposition of H₂O into H₂ and O2, and SO₂, I₂ and H₂O are recycled to the Bunsen reaction.

Side reactions in the Bunsen section should be minimized for the steady-state operation of the integrated SI process. It is also important to reduce the amount of impurities in each phase (HI and I₂ in the SA phase and H₂SO₄ in the HI_x phase). In previous study for characteristics of Bunsen products (quaternary mixture including H₂SO₄, HI, I₂ and H₂O), increasing the I₂ feed concentration enhances separation characteristics and reduces side reactions occurrence[10]. Since HI/H₂SO₄ molar ratio was fixed and continuous feeding of SO₂ was not considered, the study for quaternary mixture were insufficient to demonstrate Bunsen reaction system which carried out simultaneously Bunsen reaction, side reactions and phase separation. Therefore, we performed the Bunsen reaction with continuous SO₂ feeding and investigated the effects of I₂ feed concentration and temperature on the phase separation characteristics of Bunsen reaction and side reactions. The operating temperature was also varied because the I₂ feed concentration can be increased further with increasing temperature. Increasing the I₂ feed concentration results in a decrease in the impurities contents and inhibition of the side reactions occurrence[11-13].

In a typical SI process, all Bunsen reactants are recycled from the H₂SO₄ and HI decomposition sections. Specifically, I₂ and H₂O including HI are recycled to the Bunsen reactor as an HI_x solution (HI-I₂-H₂O)[14,15]. Therefore, we investigated the phase separation characteristics of the Bunsen reaction using the HI_x solution. The products separation was difficult due to the very low volume fraction of the SA phase. In our previous study, we suggested that the phase separation the Bunsen reaction could be enhanced when extra water was added to the Bunsen products[16]. We found that each mole of H₂SO₄ in the HI_x phase should be in contact with approximately 5-6 mol of H₂O to aid its transfer to the SA phase.

On the other hand, ultrasound irradiation can enhance the chemical and physical effects and increase the kinetic rates by inducing cavitation, which is the phenomenon of small bubbles expanding and contracting[17-19]. In this work, therefore, we performed the Bunsen reaction using the HI_x solution and ultrasound irradiation to improve its phase separation performance. The volume and composition of the Bunsen products were measured. The effects of ultrasound irradiation on the Bunsen reaction were investigated at various temperatures and I₂ and H₂O feed concentrations.

2. Experimental

Figure 1 shows the experimental apparatus for the Bunsen reaction with ultrasound irradiation. A mechanical stirrer was employed to dissolve the iodine quickly. The ultrasonic frequency was 28 kHz. The operating temperature was controlled by regulating the temperature of the water in the ultrasonic bath. A vent line was connected to a scrubber, which was filled with an aqueous NaOH solution, to trap unreacted SO₂ gas.

The concentration of HI was measured by chemical titration before use. The amounts of I₂ and H₂O were controlled within an I₂/HI molar ratio of 2.0-3.8 and a H₂O/HI molar ratio of 6.17 based on 0.5 mol of HI. The operating temperature was varied from 298 to 333 K. To prepare the HI_x solution, the reactants (HI, I₂ and H₂O) were introduced into the reactor and were mixed for 30 min at the desired temperature. Subsequently, the reaction began when SO₂ gas was fed at 110 mL/min with or without ultrasonic irradiation. The reaction proceeded until a quasi-steady state was achieved (i.e., 340 min). The Bunsen products were transferred to the liquid-liquid separator. Afterward, the amount and composition of the Bunsen products were measured.

The HI and I₂ concentrations were measured by titrating I- and I₂ with 0.1 N AgNO₃ and 0.1 N Na2S₂O₃ standard solutions (Samchun Chemical), respectively. The H₂SO₄ concentration was calculated by subtracting the amount of HI from the amount of H+ titrated with a 0.1 N NaOH standard solution (DC Chemical). The water concentration was calculated using a mass balance equation. The chemical titrations were performed using a potentiometric titrator (KEM, AT-510) and electrodes (acid-base titration electrode: KEM C-171, redox titration electrode: KEM C-272 and precipitation titration electrode: KEM C-373). Three samples for each ion were measured to minimize the errors resulted from sampling and analysis processes, and then the average concentration values were determined.

3. Results and Discussion

3.1. Effects of the temperature and I₂ concentration

The Bunsen reaction was performed using the HI_x solution with a 1/2/6.17 of HI/I₂/H₂O molar ratio at various temperatures between 298 and 333 K with or without ultrasound irradiation. As the temperature increased, the H₂SO₄ content (mol%) in the total product decreased (Figure 2a). This result was attributed to the decrease in the SO₂ conversion due to the increase in temperature[20]. The amount (vol%) of the SA phase increased gradually (Figure 2b), and the H₂SO₄ distribution ratio in the SA phase increased (Table 1) with increasing temperature. The total H₂SO₄ content decreased, and the amount of the SA phase increased when ultrasound was irradiated. Additionally, the H₂SO₄ distribution ratio in the SA phase increased under ultrasound irradiation. When ultrasound was irradiated, the increase in the amount of the SA phase was correlated to the H₂SO₄ distribution ratio in each phase. The effect of ultrasound irradiation on the Bunsen reaction was significant when temperature decreased.

A series of experiments were performed at 333 K with various I₂ concentrations with or without ultrasound irradiation. The H₂O/HI molar ratio was set to 6.17, and the I₂/HI molar ratio was varied from 2.0 to 2.9. The amount of the SA phase decreased from 9.1 to 3.9 vol% as the I₂ feed concentration increased (Figure 3a). In addition, the H₂SO₄ content distributed to the SA phase decreased from 47.8 to 30.0 mol%, whereas H₂SO₄ content distributed to the HI_x phase increased as the I₂ feed concentration increased (Figure 3b). The amount (vol%) of the SA phase was greater for the reaction with ultrasound irradiation than for the reaction without ultrasound irradiation. The use of ultrasound irradiation led to a slight increase in the H₂SO₄ content distributed to the SA phase. The effect of ultrasound irradiation on the phase separation of the Bunsen reaction was more significant for the lower I₂/HI molar ratios than for the I₂/HI molar ratio of 2.9 at 333 K.

Consequently, the H₂SO₄ content distributed to the SA phase increased when ultrasound was irradiated so that the amount of the SA phase increased. It was concluded that ultrasound irradiation improves the phase separation performance of Bunsen reaction products.

3.2. Effects of the H₂O concentrations

We proposed increasing the H₂O feed concentration to improve the poor phase separation due to a low volume ratio of the H₂SO₄ phase to the HI_x phase obtained from Bunsen reaction using the HI_x solution. We performed the Bunsen reaction at 298 K with various H₂O feed concentrations with or without ultrasound irradiation. The I₂/HI molar ratio was set to 2.0, and the H₂O/HI molar ratio was varied from 6.17 to 12.

The amount of the SA phase increased from 0.6 to 13.0 vol% as the H₂O feed concentration increased (Figure 4a) so that it was easier to separate the Bunsen products. The H₂SO₄ distribution ratio in the SA phase increased gradually with increasing H₂O feed concentration (Figure 4b). The amount of the SA phase increased when ultrasound was irradiated. The ultrasound irradiation led to an increase in the H₂SO₄ distribution ratio in the SA phase. Therefore, it was concluded that the amount of the SA phase increased as the H₂O feed concentration increased due to increasing the H₂SO₄ distribution ratio in the SA phase. In other words, ultrasound irradiation and a higher H₂O feed concentration led to better phase separation performance of the Bunsen products.

3.3. The specific role of ultrasound irradiation

The H₂SO₄ and HI content decreased, and the I₂ content in the Bunsen products increased after ultrasound irradiation. It was assumed that the H₂O generated by the microscopic shift in the reaction equilibrium formed a complex with the isolated H₂SO₄, and therefore, the volume of the SA phase increased when ultrasound was irradiated. The variation in the composition of products by ultrasound irradiation was investigated to clearly identify the effects of ultrasound irradiation on the Bunsen reaction. The Bunsen reaction was performed at 298 K using the HI_x solution with a 1/1.6/6.17 of HI/I₂/H₂O molar ratio, which resulted in a single-phase product (mainly the HI_x phase). After the Bunsen reaction was completed, the ultrasound was irradiated for 180 min with or without the SO₂ gas feed.

The molar fractions (mol%) of the product components before and after ultrasound irradiation and their variations are listed in Table 2. The I₂ and H₂O contents increased by approximately 0.03 and 0.06 mol%, and the H₂SO₄ and HI contents decreased by approximately 0.03 and 0.06 mol% when ultrasound was irradiated without the SO₂ feed. Here, the ratios of the variations in the I₂, HI and H₂O content to the variation in the H₂SO₄ content were approximately 1.1, 2.1 and 2.0, respectively. When the reverse Bunsen reaction (Eq. 4) occurs, these ratios are 1, 2 and 2 for I₂, HI and H₂O, respectively. Therefore, it appeared that the Bunsen reaction equilibrium shifted microscopically toward the SO₂, I₂ and H₂O when ultrasound was irradiated. This phenomenon is due to the creation of hot spots due to a localized high temperature associated with the transient collapse of cavitation voids[21,22].

Interestingly, the single-phase product was separated by producing the SA phase (approximately 3.7 vol%) during ultrasound irradiation.

When SO₂ gas was fed into the product solution during ultrasound irradiation, approximately 6.3 vol% of the SA phase was formed, and the H₂SO₄ distribution ratio in the SA phase increased to 17.0 mol%. The following steps were assumed to lead to these results: (a) The isolated H₂SO₄ formed a complex with the H₂O generated by the microscopic shift in the reaction equilibrium in the single-phase product, (b) When H₂SO₄ combined with a sufficient amount of H₂O, the SA phase formed due to the density differences.

Based on these results, a conceptual illustration of the effects of ultrasound irradiation on the behavior of the Bunsen products is depicted in Figure 5. Figure 5(a) shows the single-phase HI_x solution product. At low I₂ concentrations, the solution contains HI and HI₃^* (H+I₃ - ion pair) complexes and a small amount of H₂SO₄. The Bunsen products were not separated because the isolated H₂SO₄ molecules are not in contact with a sufficient amount of water (H₂O/H₂SO₄ molar ratio of approximately 5-6)[16]. When ultrasound was irradiated, the reaction equilibrium microscopically shifts toward the SO₂, I₂ and H₂O (Figure 5b). The generated H₂O forms a complex with the isolated H₂SO₄ molecules in the HI_x phase (Figure 5c). Consequently, the SA phase forms due to the number of H₂SO₄⋅xH₂O (x = 5-6) clusters that can be moved to the SA phase increases (Figure 5d).

4. Conclusions

To improve the phase separation performance of the Bunsen reaction using the HI_x solution, the Bunsen reaction was performed under ultrasound irradiation. The ultrasound irradiation increased the H₂SO₄ distribution ratio in the SA phase and the amount of the SA phase. The effect of ultrasound irradiation on the Bunsen reaction was significant when the operating temperature, I₂ and H₂O feed concentrations were decreased. It was assumed that ultrasound irradiation microscopically shifts the equilibrium toward the SO₂, I₂ and H₂O in the Bunsen reaction system. The H₂O which generated by ultrasound irradiation forms complex with the isolated H₂SO₄ in the HI_x phase, and the SA phase forms because the number of H₂SO₄⋅xH₂O (x = 5-6) clusters that can be moved to the SA phase increases. Therefore, the ultrasound irradiation effectively improves the phase separation performance of the Bunsen products.