Experimental Study of the Characteristics of HI Distillation in the Thermochemical Iodine–Sulfur Cycle for Hydrogen Production

Abstract

Hydrogen energy, as a clean, renewable, and high-calorific energy carrier, has garnered significant attention globally. Among various hydrogen production methods, the thermochemical iodine–sulfur (I-S) cycle is considered the most promising due to its high efficiency and adaptability for large-scale industrial applications. This study focuses on the distillation characteristics of the HI_x (HI–I₂–H₂O) solution within the I-S cycle, which is crucial for achieving the high-concentration HI necessary for efficient hydrogen production. Previous methods, including phosphoric acid extraction–distillation and reactive distillation, have addressed azeotrope issues but introduced complexities and equipment demands. This research constructs a hypo-azeotropic HI_x solution distillation experimental system and uses the Aspen Plus v14 software to optimize distillation parameters. By analyzing the effects of feed stage, reflux ratio, and feed temperature, the study provides essential data for improving distillation efficiency and supports the scale-up of I-S cycle technology. The findings indicate that optimal distillation is achieved with a feed position at 1/3 column height, a reflux ratio of 1.4, and a feed temperature near the boiling point, enhancing the feasibility of industrial hydrogen production via the I-S cycle.

1. Introduction

Hydrogen energy, as a form of energy carrier [1], has drawn continuous attention worldwide in recent years due to its characteristics of being green, clean, zero carbon, and renewable, and having a high calorific value [2]. It has been widely used in transportation, manufacturing, construction, and power sectors. Free hydrogen is extremely rare on Earth, with most of it existing in combined chemical forms and most of it existing in water. Worldwide, scholars have continuously explored and proposed different types of techniques for hydrogen production, including fossil fuel gasification/reforming [3], water electrolysis/photocatalysis [4,5], etc. As one type of H₂ production technology, the thermochemical cycle, using heat from solar or nuclear to split water into H₂ and O₂ through several cycled chemical reactions, is considered one of the amazing H₂ production methods. Without the conversion of heat into electricity, it is more efficient and economical compared with typical water electrolysis technology. More than 100 types of thermochemical cycles have been proposed for generating H₂. Considering the efficiency, engineering feasibility, economics, etc., the Iodine–Sulfur (I-S) or the called Sulfur–Iodine (S-I) cycle was considered one of the most promising thermochemical cycles [6,7].

The thermochemical I-S cycle was first proposed by General Atomics in the United States in the 1970s [8]. This cycle consists of three fundamental reactions:

$$\mathrm{Bunsen} \mathrm{reaction}:2{\mathrm{H}}_2\mathrm{O}+{\mathrm{I}}_2+\mathrm{S}{\mathrm{O}}_2\stackrel{290-390\mathrm{K}}{\to }{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4+2\mathrm{H}\mathrm{I}$$

(1)

$$\mathrm{HI} \mathrm{decomposition} \mathrm{reaction}: 2\mathrm{H}\mathrm{I}\stackrel{570-770\mathrm{K}}{\to }{\mathrm{H}}_2+{\mathrm{I}}_2$$

(2)

$${\mathrm{H}}_2{\mathrm{SO}}_4 \mathrm{decomposition} \mathrm{reaction}: {\mathrm{H}}_2{\mathrm{SO}}_4\stackrel{970-1270\mathrm{K}}{\to }{\mathrm{H}}_2\mathrm{O}+\mathrm{S}{\mathrm{O}}_2+1/2{\mathrm{O}}_2$$

(3)

The thermochemical I-S cycle for water-splitting hydrogen production involves the Bunsen reaction, the decomposition reaction of HI, and the decomposition reaction of H₂SO₄. The conceptual diagram is shown in Figure 1. Intermediate products such as SO₂ and I₂ are recycled, and the entire process results in water being decomposed into hydrogen and oxygen. As the core of the I-S cycle, the HI_x (HI-I₂-H₂O) solution has a significant impact on the entire cycle. Low concentrations of HI_x solution lead to high energy consumption and low yield in the HI decomposition reaction. Therefore, obtaining a high-concentration HI_x solution is key to the efficient operation of the I-S cycle for hydrogen production.

In terms of phase equilibrium properties, when the temperature is above the melting point of iodine, two immiscible liquid phases exist in the I₂–H₂O mixture. Additionally, at 0.1 MPa, the HI–H₂O mixture forms an azeotrope with a mole fraction of HI of around 0.19. At 25 °C, when the mole fraction of HI exceeds 0.346, a new liquid phase forms, and the HI–I₂–H₂O ternary mixture exhibits pseudo-azeotropic behavior [9,10]. Unlike normal azeotropes, the compositions of the liquid and vapor phases in a pseudo-azeotropic mixture under vapor–liquid equilibrium (VLE) are not identical [11]. Moreover, at high HI mole concentrations, due to the existence of a miscibility gap in the HI–H₂O binary mixture, the ternary system also shows liquid–liquid equilibrium. Therefore, the pseudo-azeotropic nature poses an azeotropic distillation challenge for the separation of HI from HI_x mixtures.

Figure 2 shows the residue curve map of the HI–I₂–H₂O system. Residue curve maps are widely used in azeotropic distillation analysis. They record the change in the composition of the residue over time during simple distillation. The arrows on the residue curve and the three sides of the triangle indicate the direction of this change. The marked temperatures show the bubble points. The curve connecting the vertices I₂ and the HI–H₂O azeotropic point, where the HI mole fraction is 0.19 at 0.1 MPa, is known as the distillation boundary. It divides the map into two distinct regions: the hypo-azeotrope region and the hyper-azeotrope region.

Previous research has mainly focused on addressing the azeotropic issue between HI and water. In the 1990s, General Atomics (GA) developed a method using phosphoric acid to disrupt the HI–water azeotrope [12,13]. When the HI–H₂O–I₂ mixture is combined with phosphoric acid, iodine precipitates out of the solution. In the remaining H₃PO₄–HI–H₂O system, phosphoric acid binds more strongly with water, preventing it from forming an azeotrope with HI. Distillation of this system yields high-concentration HI at the top of the distillation column. The dilute phosphoric acid at the bottom of the column can be concentrated and reused. This phosphoric acid extraction–distillation technique was implemented by GA in a 100–200 L/h iodine–sulfur cycle platform, funded by the French Atomic Energy Commission and the U.S. Department of Energy [14]. While phosphoric acid extraction–distillation resolves the separation issue caused by the HI–H₂O azeotrope, it introduces new complexities by adding phosphoric acid and the corresponding steps for its concentration and reuse, thus complicating the hydrogen production process and reducing energy utilization efficiency.

In 1987, the Technical University of Aachen in Germany proposed a method called reactive distillation [15,16], which was improved by CEA in 2009 [17,18,19,20]. In the reactive distillation vessel, the HI_x solution is first concentrated through distillation. The evaporated HI gas containing H₂O undergoes a decomposition reaction at the top of the reactor, with hydrogen (H₂) being extracted from the top of the equipment after cooling and gas–liquid separation. The advantage of reactive distillation is that it combines the separation and decomposition of HI in the same vessel, significantly reducing the complexity of the process compared to phosphoric acid extraction–distillation. Additionally, continuously removing the H₂ product from the mixture in the reactor shifts the chemical equilibrium towards favoring the decomposition of HI, thereby improving the conversion efficiency of HI. However, this technique requires operation under high temperature and high pressure. Given the highly corrosive and volatile nature of HI_x, the demands on reactors, pipelines, and pumps are significantly higher. Consequently, research on reactive distillation processes remains mostly at the stage of process simulation and mathematical modeling, with no specific experimental equipment or research results reported.

In 1997, the Japan Atomic Energy Agency (JAEA) proposed using the electro-electrodialysis process (also known as membrane electrolysis, abbreviated as EED) for the pre-concentration of HI from the HI–H₂O–I₂ mixture before distillation [21,22,23,24]. This EED distillation scheme offered a novel approach for HI concentration and extraction. In 2003, Hwang et al. from Korea conducted the first EED study using DuPont’s Nafion 117 proton exchange membrane [25], with an effective exchange area of 5.06 cm² [26]. Since 2005, the Institute of Nuclear and New Energy Technology (INET) at Tsinghua University has been conducting research on EED for hydrogen production via the iodine–sulfur cycle [27,28,29]. The EED distillation method has good application prospects due to its simplicity, mild conditions, and high efficiency in HI concentration [30]. However, during long-term operations, the presence of elemental iodine can block the flow channels, and current research on EED concentration distillation remains at the laboratory scale, unable to achieve stable long-term operation.

These studies have effectively addressed the purification challenges posed by the azeotropic properties of the HI–water mixture. However, each method has its limitations. The phosphoric acid recovery and concentration process is complex, the reactive distillation method requires high-performance equipment and remains mostly at the simulation stage, and the EED distillation method faces issues with I₂ clogging the flow channels and has not yet been scaled up for industrial hydrogen production.

Previous research has mostly focused on solving the HI_x azeotrope problem, but there have been few studies on the distillation of hypo-azeotropic HI_x. There is a lack of guiding data for industrial production. Therefore, in order to accelerate the industrial production of hydrogen via thermochemical I-S cycle for water-splitting hydrogen production technology, based on pilot-scale continuous stable operation and future factory-scale production, this paper constructs a hypo-azeotropic HI_x solution distillation experimental system and uses Aspen Plus v14 software to verify the distillation of HI_x in order to seek effective improvement of the distillation parameters of hypo-azeotropic HI_x solution and guide the scale development of iodine–sulfur thermochemical cycle hydrogen production technology.

2. HI_x Distillation Experimental System and Analysis Method

2.1. Preparation of Initial Ternary Mixed Solution

The feed solution ratio for this experiment was based on the pilot test data from the Qingshan Lake Energy Research Base at Zhejiang University in 2021 [31] using the molar ratio of purified HI as n(HI):n(I₂):n(H₂O) = 1:1.6:5.8. The raw materials used for preparation were HI solution (55%wt, the solvent is water, (Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China)), high-purity iodine (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), and deionized water (Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China).

2.2. HI Distillation Equipment

The distillation column used in this experiment is a packed column, as shown in Figure 3. It mainly consists of the distillation column reboiler, stripping section, feed port, rectification section, column top, condenser, condensate reflux device, distillate outlet, and reboiler outlet. Additionally, the experiment is equipped with a reboiler heater, oil bath, and reflux ratio control box. The distillation column is primarily made of GG-17 high borosilicate glass. The evaporator is a 500 mL round-bottom flask, including a connector, discharge port, and a thermocouple temperature measuring sleeve. The distillation column body is composed of three sections, with lengths of 300 mm, 500 mm, and 700 mm, respectively, and an inner diameter of 20 mm. The number of separation stages of the experimental setup is determined from the simulation results of the experiment. Short feed sections can be installed at each connection point, and each section is connected using ground glass joints sealed with silicone grease. The condenser’s cooling medium is room-temperature tap water. The reflux ratio is controlled using an electromagnetic valve connected to a control cabinet panel. The distillation column packing consists of glass springs with a specification of 4 mm. The distillation column is wrapped with an externally insulated fiberglass heating tape to ensure the column’s body temperature.

2.3. Operation of the Distillation Column

Before the distillation experiment, connect the feed port to high-purity nitrogen and check the gas tightness of the entire experimental setup. Introduce the original solution into the reboiler up to 2/3 of its volume. Turn on the control cabinet power supply, start the condenser water circulation, and then turn on the reboiler power while simultaneously switching on the insulation tape to preheat the column, ensuring that the temperature inside the column and the column body are roughly equal. Due to the high volatility of iodine, purple iodine vapor will first reach the condenser as the reboiler temperature rises. At this point, the iodine content in the top product is high, so it is necessary to perform pre-flooding operations. In a full reflux state, increase the reboiler heating temperature and the condenser water flow until a local flooding phenomenon occurs, repeating two to three times until a stable gas–liquid equilibrium is established in the distillation column. The iodine in the condenser returns to the reboiler, ensuring the packing is fully wetted and the separation efficiency is guaranteed. Then, introduce the original solution at a flow rate of 1.2 mL/min, set the appropriate reflux ratio, and take samples from the reboiler and column top every 15 min to analyze the component content until the composition of the last two samples is nearly identical.

2.4. Analysis Method

In this experiment, the components of the solutions at the top and bottom of the column were determined using an automatic potentiometric titrator(Mettler Toledo Technology Co., Ltd., Shanghai, China). The main components of the solution to be tested are HI, I₂, and H₂O; therefore, the concentrations of H⁺ and I₂ were measured by titration. To minimize measurement errors, each sample was tested multiple times and the average value was taken.

(1)

Determination of H⁺ Concentration

In the determination of H⁺ concentration, a 0.5 mol/L NaOH standard solution is used for acid–base titration. The specific reaction equation is as follows:

$${\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{H}}_2\mathrm{O}$$

(4)

The mass molar concentration of H⁺ is calculated based on the volume and concentration of the NaOH standard solution consumed.

$${\mathrm{M}}_{\mathrm{H}+}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}{\mathrm{V}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}}{\mathrm{m}}}$$

(5)

In the formula, ${\mathrm{M}}_{\mathrm{H}+}$ is the mass molar concentration of hydrogen ions in the sample (mol/kg), ${\mathrm{C}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}$ is the molar concentration of the NaOH standard solution (mol/L), ${\mathrm{V}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}$ is the volume of the standard solution consumed during titration (L), and m is the weighed mass of the test solution (kg).

(2)

Determination of I₂ Concentration

In the determination of I₂ concentration, a small amount of KI is first added to the sample to ensure complete dissolution of solid iodine. Then, a 0.1 mol/L Na₂S₂O₃ standard solution is used for redox titration. The reaction equation is as follows:

$${\mathrm{I}}_2+2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}\to 2{\mathrm{I}}^{-}+{\mathrm{S}}_4{\mathrm{O}}_6^{2-}$$

(6)

The mass molar concentration of I₂ is calculated based on the volume and concentration of the Na₂S₂O₃ standard solution consumed.

$${\mathrm{M}}_{{\mathrm{I}}_2}={\displaystyle \frac{{\mathrm{C}}_{{\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3}{\mathrm{V}}_{{\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3}}{2\mathrm{m}}}$$

(7)

In the formula, ${\mathrm{M}}_{{\mathrm{I}}_2}$ is the mass molar concentration of elemental iodine in the sample (mol/kg), ${\mathrm{C}}_{{\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3}$ is the molar concentration of the Na₂S₂O₃ standard solution (mol/L), ${\mathrm{V}}_{{\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3}$ is the volume of the standard solution consumed during titration (L), and m is the weighed mass of the test solution (kg).

(3)

Determination of H₂O Concentration

The concentration of H₂O is calculated using the principle of mass conservation. The specific calculation equation is as follows:

$${\mathrm{M}}_{{\mathrm{H}}_2\mathrm{O}}=(1000-{\mathrm{M}}_{{\mathrm{I}}^{-}}\times 127.9-{\mathrm{M}}_{{\mathrm{I}}_2}\times 253.8)/18)$$

(8)

In the formula, ${\mathrm{M}}_{{\mathrm{H}}_2\mathrm{O}}$ represents the mass molar concentration of H₂O in the test solution (mol/kg), and 127.9, 253.8, and 18 are the molar masses of HI, I₂, and H₂O, respectively (g/mol).

3. Simulation Model

3.1. Thermodynamic Model

The essence of the distillation process is the mass transfer between the light and heavy components through vapor–liquid equilibrium (VLE) within the distillation column. Therefore, obtaining the thermodynamic data and models required for the entire distillation process through the study of the phase equilibrium of mixtures is crucial for the analysis and simulation of all distillation operations. According to Guo’s research, the UVA model developed by Murphy and O’Connell can well describe the vapor–liquid equilibrium of HI_x solutions. Hence, in the subsequent distillation simulation, the UVA model is adopted as the basic model [32]. The UVA model uses the typical electrolyte NRTL model, and its binary interaction parameter expression is as follows:

$${\tau }_{ij}=a_{ij}+{\displaystyle \frac{b_{ij}}{T}}+e_{ij}\ln \left(T\right)+f_{ij}T$$

(9)

where $ij$ represent different components; the binary interaction parameters can be obtained by regression fitting of existing phase equilibrium experimental data, yielding the energy parameters ${\tau }_{ij}$ in the activity coefficient equation.

The standard state reference fugacity of pure components is calculated using the symmetric Lewis–Randall ideal solution method:

$$f_i^l(T,P,\left\{x\right\})=a_i(T,\{x\})f_i^0(T)\exp \left[{\int }_{P_i^0}^P{\displaystyle \frac{\overline{V_i}(T,P)}{RT}}\right]dP$$

(10)

In the formula, $a_i$ is the activity, $f_i^0$ is the standard state fugacity, and $\overline{V_i}$ is the partial molar volume of component $i$.

3.2. Design of HI_x Distillation Column

Figure 4 shows the flow diagram of this distillation simulation. The HI_x solution obtained from HI purification (S1) first enters the MIXER, and then Stream FEED enters the heat exchanger, where it is heated to its boiling point. Stream F1 then enters the distillation column. The column top stream F2 yields a high-purity HI solution, and the column bottom stream F3 returns to the MIXTER to mix with stream S1. The feed molar ratio designed in this study is the same as that in the experiment: (n(HI):n(I₂):n(H₂O) = 1:1.6:5.8). The feed temperature is at the boiling point, the pressure is atmospheric, and the feed rate is the same as in the experiment, at 1.2 mL/min. The entire distillation column is considered an ideal column, meaning that the pressure drop is zero and the tray efficiency is 100%. The condenser is set for partial condensation. According to the quick simulation calculation and material balance calculation of the distillation tower, the theoretical number of trays is temporarily set to 15 and the top product-to-feed ratio is set to 0.1. This study mainly investigates the effects of the reflux ratio and feed stage on the distillation efficiency. The variations in the reflux ratio and feed stage are shown in

Table 1. Simulated experimental design values.

Parameter	Design Values
Reflux ratio	0.5, 0.7, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2
Feed stage	3, 4, 5, 6, 7

.

4. Results and Discussion

In the simulation part, the impact of the feed stage and reflux ratio on the mass fraction of the top product HI was studied. In the distillation experiments, due to the difficulty of achieving the bubble point temperature in actual operations, this study mainly investigated the effects of three variables (reflux ratio, feed temperature, and feed stage) on the molar ratio of each component in the top and bottom products. Given that HI is the key product required for distillation, the separation performance of the top and bottom products was characterized using the molar ratios of I₂ to HI (I₂/HI) and H₂O to HI (H₂O/HI).

4.1. The Impact of Reflux Ratio on Distillation

The reflux ratio has a very important impact on the distillation operation. The operating line equation is shown in Figure 5. According to the distillation operating line equation

$$y={\displaystyle \frac{R}{R+1}}x+{\displaystyle \frac{x_D}{R+1}}$$

(11)

$y$ is the concentration of the component in the vapor phase, $x$ is the concentration of the component in the liquid phase, $x_D$ is the concentration of the component in the top product, and $R$ is the reflux ratio. It can be seen that increasing the reflux ratio makes the slope of the equation closer to 1, enhancing the separation capability of the rectification section.

As shown in Figure 6, with the increase in reflux ratio, the H₂O/HI ratio in the top product increases while the I₂/HI ratio decreases, indicating that the separation ability of the light components is strengthened. For the ternary mixture system of HI–I₂–H₂O, in a hypo-azeotropic state, H₂O, with a boiling point lower than HI, belongs to the light component, while I₂, with a boiling point higher than HI, belongs to the heavy component. Although a higher reflux ratio can significantly reduce the I₂ content in the top product, it also results in a lower concentration of the HI solution. Therefore, our goal is to increase the concentration of HI while ensuring the basic separation of I₂. At a reflux ratio of 1.5, the I₂/HI ratio is 0.02613 and the H₂O/HI ratio is 7.95617. At this point, it can be considered that I₂ is basically separated from the HI solution, and the concentration of the HI solution is 46.07%.

Figure 7 shows the impact of the reflux ratio on the molar ratio of each component in the bottom product. It can be observed from the figure that as the reflux ratio increases, the concentration of heavy components in the bottom product increases, with the I₂/HI ratio rising from 2.93636 to 3.11843. This is due to the increase in reflux ratio leading to a higher concentration of the vapor phase at the top of the column according to the stripping section operating line equation:

$$y={\displaystyle \frac{L^{\prime }}{L^{\prime }-W}}x-{\displaystyle \frac{W}{L^{\prime }-W}}{x}_w$$

(12)

$$y={\displaystyle \frac{L^{\prime }}{V^{\prime }}}x-{\displaystyle \frac{W}{V^{\prime }}}{x}_w$$

(13)

where $y$ is the concentration of the component in the vapor phase, $x$ is the concentration of the component in the liquid phase, $L^{\prime }$ is the liquid descent rate in the stripping section, $W$ is the bottom product rate, $V^{\prime }$ is the steam rising rate in the stripping section, and $x_W$ is the concentration of the component in the bottom product.

A high reflux ratio improves separation efficiency by increasing liquid volume, turbulence effect, and mass transfer surface. Due to the improved mass transfer effect within the column, heavy components in the bottom product can be more effectively separated, with more light components being separated at the top. As a result, the bottom product mainly enriches heavy components. At a reflux ratio of 1.5, the I₂/HI ratio in the bottom liquid is 3.10215, and the H₂O/HI ratio is 5.54153. It can be clearly seen that after this point, the change in molar ratio in the bottom liquid becomes gradually slower.

4.2. The Impact of Feed Temperature on Distillation

Besides the reflux ratio, the thermodynamic state of the feed significantly affects the vapor–liquid distribution within the distillation column, thereby influencing the separation efficiency of the column. According to the thermodynamic state of the feed, it is usually classified into five forms: subcooled liquid feed, saturated liquid feed, vapor–liquid mixed feed, saturated vapor feed, and superheated vapor feed. Due to the limitations of experimental equipment, the feed temperatures studied in this paper are 323.15 K, 353.15 K, 383.15 K, and 393.15 K, with the first two being subcooled states and the last two approaching the saturated liquid state of the HI_x solution.

Figure 8 shows the variation in the molar ratio of each component in the top product with changes in feed temperature. As shown in the figure, with the increase in feed temperature, the H₂O/HI ratio in the top product continuously rises, from 6.77679 to 7.81096, while the I₂/HI ratio gradually decreases, from 0.06661 to 0.03047. This is because at lower feed temperatures, the material needs to absorb more heat from within the column to vaporize, increasing the load on the reboiler at the bottom of the column, thereby lowering the top temperature. This leads to a decrease in the purity of the top product because, with an unchanged reflux ratio, the slope of the operating line in the rectification section remains unchanged. The intersection of the rectification operating line and the feed line equation decreases, causing the rectification operating line to shift downward. Consequently, the intersection point with the diagonal line reduces, thereby affecting the effective separation of light components.

When the feed temperature is relatively high, the material entering the column carries more heat, which may cause certain parts of the column to experience a temperature rise. If the feed temperature exceeds the operating temperature within the column, the load on the top condenser will increase, requiring more cooling energy to condense the rising vapor, thus affecting the quality of the top product. In practice, distillation operations typically use feed at its bubble point, i.e., saturated liquid feed. As shown in the figure, when approaching the saturated liquid phase, the increase in the H₂O/HI ratio at the top of the column slows down. At this point, the I₂/HI ratio is 0.3047 and the concentration of the HI solution is 46.32%.

For the stripping section, when the feed temperature is in a subcooled state, the molar amount of descending liquid in the stripping section increases, absorbing more surrounding heat. If the amount of rising vapor from the reboiler remains unchanged, the liquid-to-vapor ratio in the stripping section will increase, causing the position of the stripping operating line on the McCabe–Thiele diagram to shift downward, reducing the separation ability of the stripping section. As shown in Figure 9, when the feed temperature decreases, the I₂/HI ratio in the bottom liquid decreases from 3.09409 to 2.72346, indicating a weakened separation ability of the heavy component phase in the stripping section. On the other hand, the H₂O/HI ratio decreases with the increase in temperature, from 5.79345 to 5.67432. The higher the feed temperature, the closer it gets to the bubble point temperature, making the light components more likely to vaporize and rise to the top of the column for separation. Therefore, the concentration of light components in the bottom liquid decreases, and the purity of the bottom liquid improves. Clearly, the separation ability of the stripping section improves with the increase in temperature.

In summary, increasing the feed temperature to approach the bubble point temperature helps improve the overall separation efficiency within the column. This is because the mass and heat transfer processes within the column become more effective, allowing for better separation of light and heavy components.

4.3. The Impact of Feed Stage on Distillation

The change in feed stage directly affects the length of the rectification and stripping sections. As shown in Figure 10, the ratio of H₂O/HI in the top product decreases with the increase in feed stage, while the ratio of I₂/HI increases with the increase in feed stage. When the feed stage is increased from 300 mm to 1000 mm, the length of the rectification section in the distillation column decreases and the length of the stripping section increases, which is not conducive to the separation of light components. Therefore, as the feed stage increases, the separation ability of the distillation column for light components weakens, resulting in a decrease in the proportion of H₂O at the top of the column and an increase in the proportion of I₂. Although the ratio of I₂/HI increases with the feed stage, the value remains relatively low. To obtain a high concentration of HI solution, it is necessary to ensure a smaller H₂O/HI ratio. At the feed stage of 500 mm, the ratio of I₂/HI is 0.03486, which basically achieves the separation of I₂ and HI. At this point, the ratio of H₂O/HI is 6.63297, with a HI mass fraction of 49.95%, ensuring that the low content of I₂ does not affect the decomposition of HI and also maintaining a high concentration of HI solution, thereby reducing the energy consumption for decomposition.

Figure 11 shows the molar ratio of each component in the bottom product as a function of the feed stages. The results indicate that as the feed stage increases, the I₂/HI ratio increases from 3.05753 to 3.28173, while the H₂O/HI ratio decreases from 5.69633 to 5.5786. This change pattern is the same as that of the top product. The increase in the stripping section results in a higher concentration of heavy components in the bottom product. However, after the rectification operation, the HI concentration in the bottom product is higher than that in the feed solution, which is detrimental to improving the system’s thermal efficiency. Therefore, it is necessary to lower the feed location to improve the system’s thermal efficiency. At a feed location of 500 mm, the I₂/HI ratio in the bottom liquid is 3.10537 and the H₂O/HI ratio is 5.64506.

4.4. Aspen Plus Simulation

In the Aspen Plus simulation, we use a tray-type distillation column with 15 trays, studying the mass fractions of each component in the distillate when the feed stage varies from 3 to 7. As shown in Figure 12, with the increase in feed position, the ratio of H₂O/HI decreases from 6.07765 to 4.297715, while the ratio of I₂/HI increases from 0.079872 to 0.123027. In the solution, the mass fraction of HI rises from 49.67% to 54.1%.

The primary objective of HI_x distillation is to separate I₂ from the ternary solution, followed by increasing the concentration of HI solution. The presence of I₂ inhibits the decomposition reaction of HI. The chemical equation for the decomposition of HI is shown in Equation (2). In this reaction, if a significant amount of I₂ is present in the system, according to Le Chatelier’s principle, the reaction equilibrium will shift to the left, favoring the formation of HI. Thus, the presence of I₂ reduces the decomposition rate of hydroiodic acid, thereby stabilizing its existence.

From Figure 12, it can be seen that the ratio of I₂/HI increases significantly starting from tray 5, while the ratio of H₂O/HI gradually slows down in its decrease. Therefore, it can be concluded that the distillation effect is optimal when the feed stage is at tray 5. At this point, the mass fraction of HI in the solution is 52.3% and the mass fraction of I₂ is 10.14%.

In addition to studying the feed stage, this paper also conducted Aspen Plus simulation studies on another important parameter affecting the top product of the column, the reflux ratio. By maintaining the feed position at the third tray and varying the reflux ratio from 0.5 to 2, the impact of the reflux ratio on the mass fractions of each component in the top product was investigated, as shown in Figure 13.

As the reflux ratio increases, the ratio of H₂O/HI increases from 6.07765 to 7.50785, while the mass fraction of HI decreases from 49.67% to 47.68%, showing minimal overall change. Conversely, the ratio of I₂/HI significantly decreases from 0.07987 to 0.02099 with the increase in the reflux ratio, indicating that a higher reflux ratio helps reduce the I₂ content. Therefore, appropriately increasing the reflux ratio can effectively lower the mass fraction of I₂ in the solution, promoting the subsequent HI decomposition reaction. When the reflux ratio is around 1.4, the ratio of H₂O/HI is 7.05711 and the mass fraction of HI remains above 49%. Moreover, the ratio of I₂/HI is relatively low, at about 0.03 or less. This point represents a compromise that significantly reduces the mass fraction of I₂ while ensuring that the mass fraction of HI remains relatively high and stable.

The reboiler heat duty is a crucial operational parameter in the distillation process. It significantly impacts the separation efficiency of the distillation column and is a primary factor influencing the energy consumption and economic efficiency of the distillation process. This paper studies the relationship between reboiler heat duty and reflux ratio while keeping other parameters constant, as shown in Figure 14.

The figure indicates that an increase in the reflux ratio leads to an increase in the reboiler heat duty. When the reflux ratio increases from 0.5 to 2, the reboiler heat duty rises from 4.36144 kW to 8.7902 kW. Increasing the reflux ratio effectively reduces the I₂ content in the solution but also results in higher heat duty, which negatively affects economic efficiency. Therefore, it is essential to reasonably control the reflux ratio to achieve good distillation performance while keeping the heat duty within an acceptable range. At a reflux ratio of 1.4, which is mentioned above, the reboiler heat duty is 6.925 kW.

4.5. Error Analysis

Figure 15a shows a comparison of the HI mass fraction at the top of the column with the variation in feed position in both the experiment and the simulation. In the experiment, the feed height was used, while in the simulation, the feed plate number was used. For comparison purposes, we normalized these to the ratio of the feed position to the overall height of the distillation column. HI-s represents the HI mass fraction obtained from the simulation, and HI-t represents the HI mass fraction obtained from the experiment. With the same reflux ratio and feed rate, the HI mass fraction obtained from the simulation is slightly higher than the experimental values, with an error ranging from 4.48% to 6.60%. When the feed position moves to 5/15, the HI-s mass fraction increases to approximately 0.523, while the HI-t mass fraction increases to approximately 0.499, reducing the gap between them. Both HI-s and HI-t show a trend of slow growth in their HI mass fractions when the feed positions are at 6/15 and 7/15, and their trends are consistent.

Figure 15b shows a comparison of the HI mass fraction at the top of the column with the variation in the reflux ratio in both the experiment and the simulation. It can be seen from the figure that both HI-s and HI-t decrease with the increase in the reflux ratio, and their trends are consistent. The decrease in HI-s is 4.02% while the decrease in HI-t is 2.68%, indicating that the variation in the simulation is greater than that in the experiment, with a discrepancy of 1.34% between the two.

From the above analysis, it can be concluded that the experimental results and the simulation results show a certain degree of similarity, and the error is relatively small, indicating that this experiment has a certain level of reliability.

5. Conclusions

This study conducted distillation experiments on the hypo-azeotropic HI–I₂–H₂O ternary mixed solution. By analyzing the molar ratios of I₂ to HI (I₂/HI) and H₂O to HI (H₂O/HI), the effects of feed position, reflux ratio, and feed temperature on the separation of light and heavy components were analyzed. The hypo-azeotropic ternary mixed solution was also simulated using Aspen Plus v14 software to verify the experimental results. The relationships between the mass fractions of each component in the top product, the reflux ratio, and the feed height were obtained, as well as the relationship between the reboiler heat load and the reflux ratio. Finally, the simulation results were compared with the experimental results, the error range was analyzed, and the reliability of the distillation experiment was ultimately validated.

Through experimental and simulation analysis, the following conclusions were obtained in this study:

• As the feed position increases, the separation effect of the light components at the top of the column worsens while the separation effect of the heavy components at the bottom of the column improves. Consequently, the HI concentration in the top product gradually increases; however, the I₂ concentration also increases.
• As the reflux ratio increases, the separation effect of the light components at the top of the column improves, and the separation effect of the heavy components at the bottom of the column also improves. Consequently, the I₂ concentration in the top product decreases; however, the HI concentration also decreases.
• As the feed temperature approaches the boiling point temperature, the overall separation efficiency within the column increases.
• As the reboiler heat load increases with the reflux ratio, an excessively high reflux ratio will affect the overall distillation thermal efficiency.
• To obtain a high-concentration HI solution with a lower I₂ content and a lower reboiler heat load, this study concludes that the optimal distillation effect can be achieved with a feed position at 1/3 of the column height, a reflux ratio of 1.4, and a feed temperature at the boiling point temperature.