- Email: [email protected]
Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production Shaojie Xu, Yong He*, Guangshi Fu, Fanbo Dai, Yanwei Zhang, Zhihua Wang State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
article info
abstract
Article history:
As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration
Received 20 November 2017
and separation must cope with the pseudo-azeotropy of HIx (HI-I2-H2O) and excess iodine
Received in revised form
in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distilla-
15 April 2018
tion is a validated method of HI separation from HIx solution in the SI process, the iodine
Accepted 1 May 2018
deposition, resulting from changes in temperature and HI molality in HIx solution, can lead
Available online xxx
clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column.
Keywords:
The energy duty and required input flow rate per mole HI was investigated in this study
Sulfur-iodine cycle
using a process flowsheet simulation. A decrease in iodine concentration in the streams to
Hydrogen production
EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet
HI separation
resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar
Iodine precipitation
concentration in the feed of the distillation column exhibited an appreciable influence on
Heat duty
the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The utilization of fossil energy results in several major social and environmental problems, such as energy shortages, global warming, harmful gases, and particulate matter emissions. It is necessary to identify a realistic, viable, non-
pollution alternative to conventional energy sources. Hydrogen has attracted increasing global attention since the 1970s given distinct advantages such as renewability, cleanness, and zero carbon emissions [1]. Large-scale, high-efficiency, and low-cost hydrogen generation is the foundation for the realization of hydrogen
* Corresponding author. E-mail address: [email protected] (Y. He). https://doi.org/10.1016/j.ijhydene.2018.05.013 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
economy. Although several hydrogen production technologies have been proposed [2,3], the sulfur-iodine thermochemical water-splitting cycle originally developed by General Atomics presents excellent prospects for hydrogen production [4,5]. The SI cycle consists of the following three reactions: 290390K
1: Bunsen reaction: SO2 þ I2 þ2H2 O ! 2HI þ H2 SO4
(1)
9701270K
2: H2 SO4 decomposition reaction: H2 SO4 ! H2 O þ SO2 þ 1=2O2 570770K
3: HI decomposition reaction: 2HI ! I2 þ H2
(2)
(3)
Based on these essential reactions, the SI process can be logically divided into three sections: Bunsen section, H2SO4 section, and HIx section. Hydroiodic acid and sulfuric acid are produced by the three-phase reaction among sulfur dioxide (gas), water (liquid), and iodine (solid) in the Bunsen section [6]. With excess iodine, products can be separated into two immiscible liquid phases over a short time [7]. The upper solution, called the sulfuric acid phase, is rich in sulfuric acid and water. The bottom solution contains mainly hydroiodic acid, iodine, and water, referred to as the HIx phase. The H2SO4 phase and HIx phase each exhibit a few impurities because of the imperfect phase distribution equilibrium [8]. These two products are purified before being sent separately to the following sections. In the H2SO4 section, sulfuric acid is concentrated in a multi-stage flash distillation and decomposes at a high temperature (approximately 1123 K). The gaseous products of the H2SO4 decomposition reaction, SO2 and O2, are sent back to the Bunsen reactor, where SO2 can be absorbed by the HIx solution; then, the insoluble O2 is separated from SO2. The undecomposed sulfuric acid solution mixes with water from the top of the H2SO4 distillation column and is then recycled to the Bunsen reactor. Likewise, hydroiodic acid is also concentrated, separated, and decomposed into I2 and H2. Hydrogen is collected, and iodine is returned to the Bunsen section. Because of the recycling of iodine and sulfur dioxide in the total process, the SI cycle requires only heat and water. Due to the complex thermodynamic properties of the HI-I2H2O mixture, a high HI concentration is not easily obtained through a conventional distillation method [9]. Reports of three methods of HI concentration and separation in the HIx section have been published thus far, namely phosphoric acid extractive distillation [10], reactive distillation [11], and electro-electrodialysis coupled with conventional distillation [12]. In phosphoric acid extractive distillation, the recovery of phosphoric acid leads to high complexity and low efficiency in the SI process. Although reactive distillation has perfect integrity and high thermal efficiency, it has halted at the simulation stage given insufficient experimental study because of its harsh operational environment. Compared to the above two methods, EED coupled with distillation demonstrates more advantages, including simple performance and low heat duty [13e15]. In spite of positive effects in the Bunsen section, excess iodine poses two problems in the HIx section. One is that iodine may crystallize in the EED anode apartment when HI
molality in anolyte decreases gradually, leading to silting through the anode flow channel. The other is that excess iodine can cause the HI preconcentration duty to rise [16]. Therefore, retrieving excess iodine from HIx solution presents substantial benefits in the HIx section. Won-Chul Cho proposed the iodine separation process using H3PO4 as a solvent extraction in the conceptual design of the SI hydrogen production cycle [17]. Considering the disadvantages of introducing phosphoric acid into the SI process, extraction is not an appropriate technology for iodine separation. In practice, temperature exerts a strong influence on iodine solubility in the HIx solution. For example, when the HIx solution is maintained at a constant mole ratio of HI to H2O (1:6) and the temperature decreases from 363 K to 303 K, iodine solubility (i.e, mole fraction) declining from 0.4417 to 0.2287 [18]. Solid iodine precipitation by cooling the HIx solution appears to be a feasible approach to reducing iodine from the solution. Youngjoon Shin evaluated an SI flowsheet using precipitation, electrodialysis, and membrane separation to hydrogen production using a computer code simulation, Aspen Plus [19]. However, to the best of our knowledge, no detailed studies exist regarding the effects of iodine preseparation before EED cell on HI concentration and distillation in the SI process. To ensure EED operating temperature, the solution from the I2 precipitator outlet must be reheated to the design temperature; this additional energy should not be neglected. The purpose of the present work is to assess the effect of an I2 precipitator on the HI separation subsection, including the separation duty and required input flow rate. Three different flowsheets of HI separation are studied, and key parameters (i.e., HI molality in the EED outlet and distillation operational pressure) are also optimized to achieve the least duty in HI separation.
Theoretical analysis and thermodynamic model Model of iodine solubility in hydriodic acid A few reports have measured iodine solubility in hydroiodic acid solution [18,20,21]. However, data for the solid-liquid equilibrium of HI-I2-H2O ternary solution do not cover the scope of compositions in EED electrolytes. An adequate model of iodine solubility in hydriodic acid is essential to describe the solid-liquid equilibrium of the ternary mixture. Although Jayong Hur correlated data with an activity coefficient model (ELECNRTL), the modified model did not agree well with these data [20]. In fact, in addition to the activity coefficient method, some empirical models could also be used to correlate solubility data in general, such as the Apelblat model [22]. The empirical Apelblat model is derived from the Van's Hoff equation and can be expressed as follows at constant pressure: d ln lsat DHm 2 ¼ dT RT2
(4)
where lsat 2 is the saturation activity of solute, DHm is the molar enthalpy of the solution, and R is the gas constant. The Apelblat model can be obtained by simplifying and integrating
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Van's Hoff equation, which can be represented as a function of temperature T: lnðxÞ ¼ A þ B=T þ Cln ðTÞ
(5)
where coefficients A, B, and C are the constant parameters, and x is the saturation mole fraction of solute. Three model parameters of the empirical equation have been fitted. According to the R2 listed in Table 1, a strong correlation exists between solubility data and the Apelblat model. However, the considered range of the molar ratio of HI to H2O (1:5) exceeds the maximal experimental value (HI: H2O ¼ 1:6). Thus, the Apelblat model should be extrapolated to a higher molar ratio of HI to H2O (1:5), with iodine solubility (x) approximated by the temperature dependence equation with the parameters given in Table 1. The comparison of the curve of the Apelblat model equation at the molar ratio of HI to H2O (1:5) and results from Jayong Hur's measurement is shown in Fig. 1. The data trend is reproduced well, and the deviation is within the acceptable level.
Analysis of electrodialysis cell The HI concentration in the heavy phase product of the Bunsen reaction is often less than that in the pseudo-azeotropic HIx solution, which makes it difficult to distillate highly concentrated HI at the top of a conventional distillation column. EED was proposed by Onuki, K. to increase the HI molality of catholyte and decrease that of anolyte simultaneously with the help of electricity [23]. Given multiple advantages of simple operation, moderate reactions, and high concentration efficiency, EED has been studied by several research institutes [24e26]. According to the mass balance and principle of EED concentration, the flow rates and compositions of EED at two outlets can be calculated based on the known inlet streams and work current. The transport number of proton (tþ) and apparent electro-osmosis coefficient (b) were adopted to characterize the transport properties of the membrane, which constitute the key parameters in the EED calculation. These two parameters, tþ and b, are influenced by various factors, such as temperature, pressure, properties of the proton exchange membrane, and the composition of anolyte and catholyte. Dynamic interplays between these parameters render it difficult to determine relationships. In this work, tþ and b were treated as Hanfei Guo did in his study, keeping them constant: tþ ¼ 0.96 and b ¼ 1.65 [27]. According to the comparison between the calculations and experimental results in Onuki's report [28], iodine permeance through the
Fig. 1 e Iodine solubility as a function of temperature at different molar ratios of HI to H2O. proton exchange membrane was too small to warrant consideration. Besides temperature, pressure, and electrolyte composition, the flow ratio of anolyte to catholyte also affects the concentration performance of EED. The flow ratio of anolyte to catholyte is restricted by two factors: i) the HI molality difference between the catholyte and anolyte, which cannot exceed 5 mol/kg-H2O [29]; and ii) iodine in the anode compartment cannot precipitate out with a decrease in HI molality. Thus, the flow ratio of anolyte to catholyte must adapt to increasing HI molality in the EED cathode outlet. Table 2 lists the corresponding flow ratios of anolyte to catholyte, which are the minimum values in reasonable intervals. In addition to the preceding mass analysis, the EED energy requirement is another important factor for evaluation. The cell voltage mainly consists of open circuit voltage, electrode reaction overpotential, a potential drop resulting from the ohmic resistance of the solution, and a membrane voltage drop. The equation derived by Tanaka [30] was used to calculate the open circuit voltage, and the equation regressed by Hanfei Guo [31] was adopted to calculate the membrane voltage drop. According to M. Yoshida's report, the electrode reaction overpotential and potential drop due to the ohmic resistance of the HIx mixture were negligible [29]. Essentially, the voltage of the cell determines the duty of EED. The energy
Table 2 e The flow ratio of anolyte to catholyte. HI molality in EED cathode outlet (mol/kg-H2O)
Table 1 e Parameters for Apelblat model: ln(x) ¼ A þ B/ T þ Cln (T). n(HI):n(H2O) 1:5 1:6 1:7 1:8 1:9 1:10
A
B/K
C
R2
112.005 114.572 133.3809 126.363 120.152 117.878
4367.944 4487.539 5330.0666 4983.05 4657.549 4634.67
16.838 17.20024 19.97763 18.9247 17.99918 17.5935
0.99863 0.99862 0.9997 0.99906 0.99852 0.99769
11.5 12 12.5 13 13.5 14 14.5 a b
Flow (anolyte):flow (catholyte)a
Flow (anolyte):flow (catholyte)b
0.25 0.75 0.39:0.61 0.48:0.52 0.56:0.44 0.62:0.38 0.68:0.32 0.77:0.23
0.15:0.85 0.23:0.77 0.33:0.67 0.43:0.57 0.54:0.46 0.64:0.36 0.74:0.26
Without iodine recovery in SI process. With iodine recovery in SI process.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
demand of EED per mole hydrogen was calculated using the following equation:
Results and discussion Energy demand of EED
2FVEED Q¼ tþ h
(6)
where F is the faraday constant (96485 C/mol), VEED is the voltage of the EED cell, tþ is the transport number of the proton (0.96), and h is the electricity generation efficiency (0.45).
Thermodynamic properties model of HI-H2O-HI ternary solutions To ensure reliability of the simulation results, selecting an accurate thermodynamic model of the HI-I2-H2O mixture is paramount. In the Aspen Plus system, the ELECNRTL model is often used to predict the thermodynamic properties of aqueous electrolyte solutions. However, because of the chemically complex properties of the HIx mixture, the raw ELECNRTL model in Aspen Plus could not describe the phase equilibrium accurately. Therefore, Aspen Technology Inc. and General Atomics worked together to improve the accuracy of the SI process simulation by updating thermodynamic parameters [32]. With an increasing amount of phase equilibrium data from the HIx mixture having been published in recent years, Murphy and O'Connell developed a new thermodynamic properties model, the UVa model, to successfully describe nearly all the available phase equilibria [33]. Though the UVa model was originally developed for the reactive distillation of the HIx mixture, it can also apply to a conventional distillation simulation after removing the HI decomposition reaction. Notably, the UVa model is used primarily for the vapor-liquid equilibria of HI-I2-H2O, whereas the Apelblat model is intended for ternary solid-liquid equilibria.
Three different flowsheets in HI separation subsection As presented in Fig. 2, three flowsheets with and without an iodine precipitator were proposed to evaluate the energy demand in the HI separation subsection. The first flowsheet (Fig. 2a) displays the conventional version without an iodine precipitator. The second flowsheet (Fig. 2b), with an iodine precipitator, is designed to remove some iodine from the stream to the EED anode side only. The third flowsheet (Fig. 2c) is proposed for iodine removal from both streams introduced into the EED. The composition of the input stream corresponded to the HIx purification product in our previous research; the composition molar ration was H2O:I2:HI ¼ 0.5189:0.3931:0.0961 [34]. The operating temperatures of the iodine precipitator and EED cell were 308 K and 363 K, respectively. The HI distillation column was simulated by a rigorous distillation module (RadFrac) in Aspen Plus; it had nine theoretic stages, including the first-stage condenser and last-stage reboiler. The specified HI mole fraction in the distillate was fixed at 0.96. The other specified operating parameters, such as the distillate-to-feed ratio and reflux ratio, were determined via optimization using the design specify/vary form. Moreover, heat exchangers in the process were specified that the temperature approach was 15 K.
Two plans were designed to estimate the effect of iodine recovery from the streams before EED on the energy demand of the EED cell. Plan A was that iodine would be recovered only from the stream to the EED anode chamber, which would avoid blocking the EED anode flow channel as much as possible and retain the reactant concentration in the cathode chamber. Plan B was that iodine would be recovered from the input stream before splitting the two streams into a pair of EED chambers. In this case, the decrease in I2 concentration in the EED catholyte may have a positive impact on HI distillation in addition to the aforementioned advantage. These two plans shared the same the flow ratio of anolyte to catholyte, listed in Table 2. The effects of iodine precipitation on EED energy duties are shown in Fig. 3. As the iodine mole fraction in the EED inlet anolyte decreased from 0.39 to 0.23, the EED duty reduced by approximately 39% at 14.5 mol/kg-H2O HI molality in the EED outlet catholyte. The high molar concentration of iodine in anolyte prevented iodine desorption from the anode surface, resulting in a clear increase in the voltage of the EED cell. By comparing the two graphs in Fig. 3, a decrease of iodine concentration in the EED catholyte appears not to have affected the voltage of the EED cell significantly, consistent with the experimental results published by Pradeep Kumar Sow [35]. Though iodine is the reactant of a cathodic reduction, a decrease in the iodine concentration did not seem to change the cathode reaction. This finding suggests that the reaction on the cathode surface was controlled by the electrochemical reaction rate at this stage rather than the iodine molecule migration rate. The EED duty increased inappreciably as the HI molality in the EED outlet catholyte increased. However, in Hanfei Guo's report [27], the EED duty increased sharply as the HI molality increased. The flow ratio of anolyte to catholyte may be the primary cause of this distinction. The flow ratios of catholyte to anolyte were kept constant in Hanfei Guo's study, while the flow ratio in this study was regulated by the HI molality of the EED cathode outlet. With an increase in the flow ratio of anolyte to catholyte, the concentration difference between the EED outlet catholyte and anolyte declined, causing a reduction in the average voltage of the EED cell. For example, when the flow ratio of anolyte to catholyte was 0.5:0.5, the average voltage was 0.387 V with 12.5 mol/kg-H2O HI in the EED outlet catholyte. However, when the flow ratio was 0.7:0.3, the average voltage fell to 0.365 V.
Effect of iodine mole fraction in feed and operation pressure The effect of I2 mole fraction in feed on the distillation column duty is shown in Fig. 4. Here, the distillation column pressure was set at 0.5 Mpa, and the stage pressure drop was negligible. As is shown in the flowsheets, the iodine mole fraction in the distillation feed was bound by the operating temperature of the iodine precipitator and the HI molality in the EED outlet catholyte. The reboiler duty at the same HI
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
5
Fig. 2 e Flowsheets of three different designs in HI separation subsection. (a) Flowsheet 1: conventional flowsheet without iodine precipitation; (b) Flowsheet 2: iodine removal from anode side; (c) Flowsheet 3: iodine removal from anode and cathode side.
molality feed gradually decreased with a decline in the iodine mole fraction in feed due to the augment of the latent heat of solution vaporization as the concentration of iodine in the HIx mixture increased. The reboiler duty at the feed composition with an HI molality of 11.5 mol/kg-H2O and I2 mole fraction of 0.2194 was nearly the same as that at the feed composition with an HI molality of 14.5 mol/kg-H2O and I2 mole fraction of 0.3208. Thus, a lower iodine concentration in feed could create a positive effect on HI distillation as well as high HI molality. Besides the feed composition, operating pressure also plays an important role in the HI distillation duty. As
presented in Fig. 5, the distillation duty increaseds with increasing the operating pressure in the distillation column. High operating pressure led to an increase in the vaporization temperature of the HIx solution, and the required energy of HI distillation rose when the feed temperature remained constant. The iodine concentration in feed resulted in a significant effect on the distillation duty at a low operating pressure. However, the difference caused by varying iodine molar concentrations in the feed became smaller as the operating pressure grew. Clearly, a high operating pressure weakened the positive effect of the low iodine molar concentration on HI distillation.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Fig. 5 e Distillation duty as a function of distillation pressure at different I2 mole fractions.
Fig. 3 e Dependence of EED duties on HI molality in the EED outlet catholyte: (a) recovering iodine from the stream to the EED anode side only; (b) recovering iodine from the two streams to the EED anode and cathode sides.
Fig. 4 e Distillation duty as a function of I2 mole fraction in distillation feed at different HI molalities.
The ratio of HI mole rate in the distillate to that in feed (DHI/FHI), namely recovery of the light key component in distillate, was evaluated by the variation in operating pressure and iodine molar concentration. A large DHI/FHI contributed to a reduction in mass recycling of excessive materials and improved the total efficiency of the SI process. Fig. 6 demonstrates that increasing operating pressure promoted the growth of DHI/FHI. In fact, high operating pressure encouraged the HI mole fraction of rising vapor in the column [33]. Nevertheless, low iodine concentration had the opposite effect on the DHI/FHI. The divergence in DHI/FHI at five different iodine molar concentrations in feed became increasingly apparent at a high operating pressure; that is, the HI mole fraction in pseudoazeotropic compositions for the HIx solution increased as the iodine molar concentration diminished [33].
Fig. 6 e Effect of distillation pressure on the HI distillate mole ratio.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Comparison of three different flowsheets in HI separation subsection The HI separation energy demands for three different flowsheets are presented in Fig. 7. The results shown in Fig. 7a and c correspond with Flowsheets 1 to 3 in Fig. 2, respectively. The flowsheets demonstrate that the behavior of the plot curve of the conventional flowsheet differed from that of the other two
Fig. 7 e Effect of HI molality in distillation on HI separation duty.
7
flowsheets with iodine recovery. This significant difference may be attributed to the iodine precipitator and subsequent heat exchanger. As shown in Fig. 7a, the HI separation total duty decreased as HI molality in the distillation feed rose. An increase in HI molality in the feed led to two conflicting effects: slightly increasing energy demand of EED and clearly reduced duty of the distillation column. In general, high HI molality and low operating pressure were favorable for HI separation from an energy consumption perspective. However, the higher the HI molality in the distillation feed, the smaller the proportion of catholyte in the input, which required a greater amount of HIx input in the HIx section. For instance, when HI molality in feed and operating pressure were 14.5 mol/kg-H2O and 0.1 Mpa, respectively, the mole ratio of the input flow rate to HI distillate rate was 107.03. The effects of the HI molality in the distillation feed and the column pressure on the HI separation duty of the flowsheet with a precipitator in the stream to the EED anode side are shown in Fig. 7b. Besides the EED duty and the column duty, the heat duty of Heat exchanger_2 was also an important component of the total duty of HI separation. The HI separation total duty decreased with an increase of HI molality in the feed initially and then rose thereafter. This pattern suggests that a minimum HI separation total duty could be obtained. A higher operating pressure resulted in a larger minimum of HI separation total duty. However, at high HI molality, such as 14.5 mol/kg-H2O, the total duty varied inversely with the operating pressure, as the distillation duty was less affected by the operating pressure at higher HI molality in the distillation feed. While the effect of operating pressure on the energy demand of the EED cell was not considered in this work, the duty of Heat exchanger_2 reduced with a smaller amount of input at a high operating pressure. Fig. 7c shows that the Flowsheet 3 had a minimum HI separation total duty with HI molality of 12 mol/kg-H2O at an operating pressure of 0.3 Mpa. Flowsheet 3 demonstrates similar performance as in Flowsheet 2, especially at a high HI molality in the feed. Nevertheless, with a low HI molality in the feed, the behavior of HI separation duty as shown in Flowsheet 3 at different operating pressures did not resemble that of Flowsheet 2. Although the influence of operating pressure on the distillation duty was significant at low HI molality in feed, which could be counteracted by the effect of the heat input from the outside to Heat exchanger_2, Heat exchanger_2 in Flowsheet 3 needed to heat more HIx solution than in Flowsheet 2. Under the condition of minimum HI separation total duty, the input flow rates of three different flowsheets to rectify 1 mol HI are shown in Fig. 8. The required input flow rates were 107.03 mol, 44.92 mol, and 52.43 mol in Flowsheets 1, 2, and 3, respectively. The flow rate ratio (i.e., the input rate to the HI distillate rate) could be determined by HI molality in the EED cathode output, iodine concentration in the distillation feed, and operating pressure of the distillation column. In Flowsheet 1, high HI molality not only decreased the flow rate ratio of the EED catholyte to the total input but also reduced the iodine concentration in the distillation feed. The low operating pressure was not conducive to the HI distillate rate, resulting in the largest input flow rate. Contrary to Flowsheet
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Science Foundation of China (51621005) and the Program of Introducing Talents of Discipline to University (B08026).
Nomenclature FHI distillate Finput lsat 2 DHm R tþ b Q
Fig. 8 e The ratio of input flow rate to HI distillate rate at the minimum total duty for three flowsheets.
F VEED h
flow rate of HI in distillate, mol/h flow rate of input, mol/h saturation activity of solute molar enthalpy of solution kJ/mol gas constant, 8.314 J/(mol$K) transport number of proton electroosmosis coefficient energy demand of EED to generate 1 mol hydrogen, kJ/mol-H2 Faraday constant, 96485 C/mol voltage of EED cell, V electricity generation efficiency, 0.45
references 1, Flowsheets 2 and 3 demonstrated minimum total duty at a lower HI molality in feed. Additionally, Flowsheet 2 had a higher iodine concentration in the distillation feed than Flowsheet 3, which caused a smaller required input flow rate, indicating that Flowsheet 2 reflects the optimal subprocess.
Conclusions As excess iodine can cause several problems in the HIx section, a precipitator was used to recover some iodine from the HIx solution. The effect of iodine precipitation on the energy demand of EED and the distillation column duty was investigated first. Then, three flowsheets were proposed and compared based on the required input flow rate and separation duty per mole HI. Study findings revealed the following: (1) Recovering iodine from the stream to the EED anode side or the two streams to the anode and cathode sides reduced the energy demand of the EED cell. No significant differences appeared in the EED duty between these two cases. (2) For the HI distillation column, iodine concentration in the feed and the operating pressure were central parameters. The effect of iodine concentration in feed caused an HI distillate similar to that of the operating pressure; that is, high iodine concentration in feed and operating pressure improved the HI distillate rate and low iodine concentration in feed, and operating pressure attenuated the distillation column duty. (3) The flowsheet with an iodine precipitator in the stream to EED anode side was most efficient, requiring the least energy and the smallest input flow rate. The precipitation technology to recover excess iodine was especially useful and feasible.
Acknowledgements The authors gratefully acknowledge the financial support from the Innovative Research Groups of the National Natural
[1] Mazloomi K, Gomes C. Hydrogen as an energy carrier: prospects and challenges. Renew Sustain Energy Rev 2012;16:3024e33. [2] Wang H, Le Person A, Zhao X, Li J, Nuncio P, Yang L, et al. A low-temperature hydrogen production process based on H 2 S splitting cycle for sustainable oil sands bitumen upgrading. Fuel Process Technol 2013;108:55e62. [3] Kang K, Azargohar R, Dalai AK, Wang H. Hydrogen production from lignin, cellulose and waste biomass via supercritical water gasification: catalyst activity and process optimization study. Energy Convers Manag 2016;117:528e37. [4] Rosen MA. Advances in hydrogen production by thermochemical water decomposition: a review. Energy 2010;35:1068e76. [5] Norman JH, Mysels KJ, Sharp R, Williamson D. Studies of the sulfur-iodine thermochemical water-splitting cycle. Int J Hydrogen Energy 1982;7:545e56. [6] Ying Z, Zheng X, Zhang Y, Cui G. Kinetic analysis of Bunsen reaction with HI existence in the thermochemical sulfureiodine cycle for hydrogen production. Appl Therm Eng 2018;129:41e9. [7] Norman JH, Besenbruch GE, Brown LC, O'Keefe DR, Allen CL. Thermochemical water-splitting cycle, bench-scale investigations, and process engineering. Final report, February 1977-December 31, 1981. San Diego, CA (USA): GA Technologies, Inc; 1982. p. 294. Medium: ED; Size. [8] Ying Z, Zheng X, Zhang Y, Cui G. Development of a novel flowsheet for sulfureiodine cycle based on the electrochemical Bunsen reaction for hydrogen production. Int J Hydrogen Energy 2017;42:26586e96. [9] Engels H, Knoche KF. Vapor-pressures of the system Hi/H2o/ I2 and H-2. Int J Hydrogen Energy 1986;11:703e7. [10] Moore R, Parma E, Russ B, Sweet W, Helie M, Pons N, et al. An integrated laboratory-scale experiment on the sulfur-iodine thermochemical cycle for hydrogen production. In: Fourth International Topical Meeting on High Temperature Reactor Technology. American Society of Mechanical Engineers; 2008. p. 541e9. [11] Engels H, Knoche KF, Roth M. Direct dissociation of hydrogen iodidedan alternative to the General Atomic proposal. Int J Hydrogen Energy 1987;12:675e8. [12] Onuki K, Nakajima H, Shimizu S. Concentration of HIx solution by electrodialysis. Kagaku Kogaku Ronbun 1997;23:289e91.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
[13] Kasahara S, Kubo SJ, Onuki K, Nomura M. Thermal efficiency evaluation of HI synthesis/concentration procedures in the thermochemical water splitting IS process. Int J Hydrogen Energy 2004;29:579e87. [14] Kang K-S, Kim C-H, Cho W-C, Jeong S-U, Park C-S, Bae K-K. Demonstration of the HI decomposition section embedded with electrodialysis stack in the sulfureiodine thermochemical cycle for hydrogen production. Nucl Eng Des 2013;256:67e74. [15] Sow PK, Shukla A. Electro-electrodialysis for concentration of hydroiodic acid. Int J Hydrogen Energy 2012;37:3931e7. [16] Hong SD, Kim JK, Bae KK, Lee SH, Choi HS, Hwang GJ. Evaluation of the membrane properties with changing iodine molar ratio in HIx (HI-I-2-H2O mixture) solution to concentrate HI by electro-electrodialysis. J Membr Sci 2007;291:106e10. [17] Cho WC, Park CS, Kang KS, Kim CH, Bae KK. Conceptual design of sulfur-iodine hydrogen production cycle of Korea Institute of Energy Research. Nucl Eng Des 2009;239:501e7. [18] Yang J, Huang Z, Hu Y, Wang Z, Zhang Y, Zhou J, et al. Experimental investigation on two-phase separation ycharacteristics of bunsen reaction in sulfur-iodine cycle for hydrogen production. Acta Energiae Solaris Sin 2012;33:1289e94. [19] Shin Y, Lee K, Kim Y, Chang J, Cho W, Bae K. A sulfur-iodine flowsheet using precipitation, electrodialysis, and membrane separation to produce hydrogen. Int J Hydrogen Energy 2012;37:16604e14. [20] Hur J, O'Connell J, Bae K-K, Kang K-S, Kang JW. Measurements and correlation of solideliquid equilibria of the HIþ I 2þ H 2 O system. Int J Hydrogen Energy 2011;36:8187e91. [21] Powell C, Campbell I. The solubility of iodine in concentrated hydriodic acid solutions. J Am Chem Soc 1947;69:1227e8. [22] Apelblat A, Manzurola E. Solubilities of O-acetylsalicylic, 4aminosalicylic, 3, 5-dinitrosalicylic, andp-toluic acid, and magnesium-DL-aspartate in water fromT¼(278 to 348) K. J Chem Therm 1999;31:85e91. [23] Onuki K, Hwane GJ, Shimizu S. Electrodialysis of hydriodic acid in the presence of iodine. J Membr Sci 2000;175:171e9. [24] Wang ZL, Chen SZ, Zhang P, Wang LJ, Xu JM, Wang SM. Evaluation on the electro-electrodialysis stacks for hydrogen
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
9
iodide concentrating in iodine-sulphur cycle. Int J Hydrogen Energy 2014;39:13505e11. Sow PK, Shukla A. Electrochemical interaction of HIx solution at graphite electrode: an application towards EED process in I-S thermochemical cycle. Ionics 2015;21:175e84. Sow PK, Shukla A. A chronopotentiometry based identification of time-varying different transport resistances of electro-electrodialysis cell used for concentration of HIx solution. Int J Hydrogen Energy 2013;38:3154e65. Guo HF, Kasahara S, Tanaka N, Onuki K. Energy requirement of HI separation from HI-I-2-H2O mixture using electroelectrodialysis and distillation. Int J Hydrogen Energy 2012;37:13971e82. Onuki K, Hwang GJ, Arifal, Shimizu S. Electro-electrodialysis of hydriodic acid in the presence of iodine at elevated temperature. J Membr Sci 2001;192:193e9. Yoshida M, Tanaka N, Okuda H, Onuki K. Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process. Int J Hydrogen Energy 2008;33:6913e20. Tanaka N, Onuki K. Equilibrium potential across cation exchange membrane in HI-I-2-H2O solution. J Membr Sci 2010;357:73e9. Guo HF, Zhang P, Chen SZ, Wang LJ, Xu JM. Comparison of various circuit designs in the HI decomposition section of the iodine-sulfur process. Int J Hydrogen Energy 2012;37:12097e104. Brown LC, Besenbruch GE, Lentsch R, Schultz KR, Funk J, Pickard P, et al. High efficiency generation of hydrogen fuels using nuclear power. General Atomics Report GA-A24285. 2003. Q4. O'Connell JP, Murphy JE. A properties model of the HI-I(2)H(2)O-H(2) system in the sulfur-iodine cycle for hydrogen manufacture. Fluid Phase Equilibria 2010;288:99. 2010;293:266-. Maskalick NJ. Hydriodic acid-anode-depolarized hydrogen generator. Google Patents; 1984. Sow PK, Shukla A. Effect of asymmetric variation of operating parameters on EED cell for HI concentration in I-S cycle for hydrogen production. Int J Hydrogen Energy 2012;37:13958e70.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production Shaojie Xu, Yong He*, Guangshi Fu, Fanbo Dai, Yanwei Zhang, Zhihua Wang State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
article info
abstract
Article history:
As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration
Received 20 November 2017
and separation must cope with the pseudo-azeotropy of HIx (HI-I2-H2O) and excess iodine
Received in revised form
in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distilla-
15 April 2018
tion is a validated method of HI separation from HIx solution in the SI process, the iodine
Accepted 1 May 2018
deposition, resulting from changes in temperature and HI molality in HIx solution, can lead
Available online xxx
clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column.
Keywords:
The energy duty and required input flow rate per mole HI was investigated in this study
Sulfur-iodine cycle
using a process flowsheet simulation. A decrease in iodine concentration in the streams to
Hydrogen production
EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet
HI separation
resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar
Iodine precipitation
concentration in the feed of the distillation column exhibited an appreciable influence on
Heat duty
the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The utilization of fossil energy results in several major social and environmental problems, such as energy shortages, global warming, harmful gases, and particulate matter emissions. It is necessary to identify a realistic, viable, non-
pollution alternative to conventional energy sources. Hydrogen has attracted increasing global attention since the 1970s given distinct advantages such as renewability, cleanness, and zero carbon emissions [1]. Large-scale, high-efficiency, and low-cost hydrogen generation is the foundation for the realization of hydrogen
* Corresponding author. E-mail address: [email protected] (Y. He). https://doi.org/10.1016/j.ijhydene.2018.05.013 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
economy. Although several hydrogen production technologies have been proposed [2,3], the sulfur-iodine thermochemical water-splitting cycle originally developed by General Atomics presents excellent prospects for hydrogen production [4,5]. The SI cycle consists of the following three reactions: 290390K
1: Bunsen reaction: SO2 þ I2 þ2H2 O ! 2HI þ H2 SO4
(1)
9701270K
2: H2 SO4 decomposition reaction: H2 SO4 ! H2 O þ SO2 þ 1=2O2 570770K
3: HI decomposition reaction: 2HI ! I2 þ H2
(2)
(3)
Based on these essential reactions, the SI process can be logically divided into three sections: Bunsen section, H2SO4 section, and HIx section. Hydroiodic acid and sulfuric acid are produced by the three-phase reaction among sulfur dioxide (gas), water (liquid), and iodine (solid) in the Bunsen section [6]. With excess iodine, products can be separated into two immiscible liquid phases over a short time [7]. The upper solution, called the sulfuric acid phase, is rich in sulfuric acid and water. The bottom solution contains mainly hydroiodic acid, iodine, and water, referred to as the HIx phase. The H2SO4 phase and HIx phase each exhibit a few impurities because of the imperfect phase distribution equilibrium [8]. These two products are purified before being sent separately to the following sections. In the H2SO4 section, sulfuric acid is concentrated in a multi-stage flash distillation and decomposes at a high temperature (approximately 1123 K). The gaseous products of the H2SO4 decomposition reaction, SO2 and O2, are sent back to the Bunsen reactor, where SO2 can be absorbed by the HIx solution; then, the insoluble O2 is separated from SO2. The undecomposed sulfuric acid solution mixes with water from the top of the H2SO4 distillation column and is then recycled to the Bunsen reactor. Likewise, hydroiodic acid is also concentrated, separated, and decomposed into I2 and H2. Hydrogen is collected, and iodine is returned to the Bunsen section. Because of the recycling of iodine and sulfur dioxide in the total process, the SI cycle requires only heat and water. Due to the complex thermodynamic properties of the HI-I2H2O mixture, a high HI concentration is not easily obtained through a conventional distillation method [9]. Reports of three methods of HI concentration and separation in the HIx section have been published thus far, namely phosphoric acid extractive distillation [10], reactive distillation [11], and electro-electrodialysis coupled with conventional distillation [12]. In phosphoric acid extractive distillation, the recovery of phosphoric acid leads to high complexity and low efficiency in the SI process. Although reactive distillation has perfect integrity and high thermal efficiency, it has halted at the simulation stage given insufficient experimental study because of its harsh operational environment. Compared to the above two methods, EED coupled with distillation demonstrates more advantages, including simple performance and low heat duty [13e15]. In spite of positive effects in the Bunsen section, excess iodine poses two problems in the HIx section. One is that iodine may crystallize in the EED anode apartment when HI
molality in anolyte decreases gradually, leading to silting through the anode flow channel. The other is that excess iodine can cause the HI preconcentration duty to rise [16]. Therefore, retrieving excess iodine from HIx solution presents substantial benefits in the HIx section. Won-Chul Cho proposed the iodine separation process using H3PO4 as a solvent extraction in the conceptual design of the SI hydrogen production cycle [17]. Considering the disadvantages of introducing phosphoric acid into the SI process, extraction is not an appropriate technology for iodine separation. In practice, temperature exerts a strong influence on iodine solubility in the HIx solution. For example, when the HIx solution is maintained at a constant mole ratio of HI to H2O (1:6) and the temperature decreases from 363 K to 303 K, iodine solubility (i.e, mole fraction) declining from 0.4417 to 0.2287 [18]. Solid iodine precipitation by cooling the HIx solution appears to be a feasible approach to reducing iodine from the solution. Youngjoon Shin evaluated an SI flowsheet using precipitation, electrodialysis, and membrane separation to hydrogen production using a computer code simulation, Aspen Plus [19]. However, to the best of our knowledge, no detailed studies exist regarding the effects of iodine preseparation before EED cell on HI concentration and distillation in the SI process. To ensure EED operating temperature, the solution from the I2 precipitator outlet must be reheated to the design temperature; this additional energy should not be neglected. The purpose of the present work is to assess the effect of an I2 precipitator on the HI separation subsection, including the separation duty and required input flow rate. Three different flowsheets of HI separation are studied, and key parameters (i.e., HI molality in the EED outlet and distillation operational pressure) are also optimized to achieve the least duty in HI separation.
Theoretical analysis and thermodynamic model Model of iodine solubility in hydriodic acid A few reports have measured iodine solubility in hydroiodic acid solution [18,20,21]. However, data for the solid-liquid equilibrium of HI-I2-H2O ternary solution do not cover the scope of compositions in EED electrolytes. An adequate model of iodine solubility in hydriodic acid is essential to describe the solid-liquid equilibrium of the ternary mixture. Although Jayong Hur correlated data with an activity coefficient model (ELECNRTL), the modified model did not agree well with these data [20]. In fact, in addition to the activity coefficient method, some empirical models could also be used to correlate solubility data in general, such as the Apelblat model [22]. The empirical Apelblat model is derived from the Van's Hoff equation and can be expressed as follows at constant pressure: d ln lsat DHm 2 ¼ dT RT2
(4)
where lsat 2 is the saturation activity of solute, DHm is the molar enthalpy of the solution, and R is the gas constant. The Apelblat model can be obtained by simplifying and integrating
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Van's Hoff equation, which can be represented as a function of temperature T: lnðxÞ ¼ A þ B=T þ Cln ðTÞ
(5)
where coefficients A, B, and C are the constant parameters, and x is the saturation mole fraction of solute. Three model parameters of the empirical equation have been fitted. According to the R2 listed in Table 1, a strong correlation exists between solubility data and the Apelblat model. However, the considered range of the molar ratio of HI to H2O (1:5) exceeds the maximal experimental value (HI: H2O ¼ 1:6). Thus, the Apelblat model should be extrapolated to a higher molar ratio of HI to H2O (1:5), with iodine solubility (x) approximated by the temperature dependence equation with the parameters given in Table 1. The comparison of the curve of the Apelblat model equation at the molar ratio of HI to H2O (1:5) and results from Jayong Hur's measurement is shown in Fig. 1. The data trend is reproduced well, and the deviation is within the acceptable level.
Analysis of electrodialysis cell The HI concentration in the heavy phase product of the Bunsen reaction is often less than that in the pseudo-azeotropic HIx solution, which makes it difficult to distillate highly concentrated HI at the top of a conventional distillation column. EED was proposed by Onuki, K. to increase the HI molality of catholyte and decrease that of anolyte simultaneously with the help of electricity [23]. Given multiple advantages of simple operation, moderate reactions, and high concentration efficiency, EED has been studied by several research institutes [24e26]. According to the mass balance and principle of EED concentration, the flow rates and compositions of EED at two outlets can be calculated based on the known inlet streams and work current. The transport number of proton (tþ) and apparent electro-osmosis coefficient (b) were adopted to characterize the transport properties of the membrane, which constitute the key parameters in the EED calculation. These two parameters, tþ and b, are influenced by various factors, such as temperature, pressure, properties of the proton exchange membrane, and the composition of anolyte and catholyte. Dynamic interplays between these parameters render it difficult to determine relationships. In this work, tþ and b were treated as Hanfei Guo did in his study, keeping them constant: tþ ¼ 0.96 and b ¼ 1.65 [27]. According to the comparison between the calculations and experimental results in Onuki's report [28], iodine permeance through the
Fig. 1 e Iodine solubility as a function of temperature at different molar ratios of HI to H2O. proton exchange membrane was too small to warrant consideration. Besides temperature, pressure, and electrolyte composition, the flow ratio of anolyte to catholyte also affects the concentration performance of EED. The flow ratio of anolyte to catholyte is restricted by two factors: i) the HI molality difference between the catholyte and anolyte, which cannot exceed 5 mol/kg-H2O [29]; and ii) iodine in the anode compartment cannot precipitate out with a decrease in HI molality. Thus, the flow ratio of anolyte to catholyte must adapt to increasing HI molality in the EED cathode outlet. Table 2 lists the corresponding flow ratios of anolyte to catholyte, which are the minimum values in reasonable intervals. In addition to the preceding mass analysis, the EED energy requirement is another important factor for evaluation. The cell voltage mainly consists of open circuit voltage, electrode reaction overpotential, a potential drop resulting from the ohmic resistance of the solution, and a membrane voltage drop. The equation derived by Tanaka [30] was used to calculate the open circuit voltage, and the equation regressed by Hanfei Guo [31] was adopted to calculate the membrane voltage drop. According to M. Yoshida's report, the electrode reaction overpotential and potential drop due to the ohmic resistance of the HIx mixture were negligible [29]. Essentially, the voltage of the cell determines the duty of EED. The energy
Table 2 e The flow ratio of anolyte to catholyte. HI molality in EED cathode outlet (mol/kg-H2O)
Table 1 e Parameters for Apelblat model: ln(x) ¼ A þ B/ T þ Cln (T). n(HI):n(H2O) 1:5 1:6 1:7 1:8 1:9 1:10
A
B/K
C
R2
112.005 114.572 133.3809 126.363 120.152 117.878
4367.944 4487.539 5330.0666 4983.05 4657.549 4634.67
16.838 17.20024 19.97763 18.9247 17.99918 17.5935
0.99863 0.99862 0.9997 0.99906 0.99852 0.99769
11.5 12 12.5 13 13.5 14 14.5 a b
Flow (anolyte):flow (catholyte)a
Flow (anolyte):flow (catholyte)b
0.25 0.75 0.39:0.61 0.48:0.52 0.56:0.44 0.62:0.38 0.68:0.32 0.77:0.23
0.15:0.85 0.23:0.77 0.33:0.67 0.43:0.57 0.54:0.46 0.64:0.36 0.74:0.26
Without iodine recovery in SI process. With iodine recovery in SI process.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
demand of EED per mole hydrogen was calculated using the following equation:
Results and discussion Energy demand of EED
2FVEED Q¼ tþ h
(6)
where F is the faraday constant (96485 C/mol), VEED is the voltage of the EED cell, tþ is the transport number of the proton (0.96), and h is the electricity generation efficiency (0.45).
Thermodynamic properties model of HI-H2O-HI ternary solutions To ensure reliability of the simulation results, selecting an accurate thermodynamic model of the HI-I2-H2O mixture is paramount. In the Aspen Plus system, the ELECNRTL model is often used to predict the thermodynamic properties of aqueous electrolyte solutions. However, because of the chemically complex properties of the HIx mixture, the raw ELECNRTL model in Aspen Plus could not describe the phase equilibrium accurately. Therefore, Aspen Technology Inc. and General Atomics worked together to improve the accuracy of the SI process simulation by updating thermodynamic parameters [32]. With an increasing amount of phase equilibrium data from the HIx mixture having been published in recent years, Murphy and O'Connell developed a new thermodynamic properties model, the UVa model, to successfully describe nearly all the available phase equilibria [33]. Though the UVa model was originally developed for the reactive distillation of the HIx mixture, it can also apply to a conventional distillation simulation after removing the HI decomposition reaction. Notably, the UVa model is used primarily for the vapor-liquid equilibria of HI-I2-H2O, whereas the Apelblat model is intended for ternary solid-liquid equilibria.
Three different flowsheets in HI separation subsection As presented in Fig. 2, three flowsheets with and without an iodine precipitator were proposed to evaluate the energy demand in the HI separation subsection. The first flowsheet (Fig. 2a) displays the conventional version without an iodine precipitator. The second flowsheet (Fig. 2b), with an iodine precipitator, is designed to remove some iodine from the stream to the EED anode side only. The third flowsheet (Fig. 2c) is proposed for iodine removal from both streams introduced into the EED. The composition of the input stream corresponded to the HIx purification product in our previous research; the composition molar ration was H2O:I2:HI ¼ 0.5189:0.3931:0.0961 [34]. The operating temperatures of the iodine precipitator and EED cell were 308 K and 363 K, respectively. The HI distillation column was simulated by a rigorous distillation module (RadFrac) in Aspen Plus; it had nine theoretic stages, including the first-stage condenser and last-stage reboiler. The specified HI mole fraction in the distillate was fixed at 0.96. The other specified operating parameters, such as the distillate-to-feed ratio and reflux ratio, were determined via optimization using the design specify/vary form. Moreover, heat exchangers in the process were specified that the temperature approach was 15 K.
Two plans were designed to estimate the effect of iodine recovery from the streams before EED on the energy demand of the EED cell. Plan A was that iodine would be recovered only from the stream to the EED anode chamber, which would avoid blocking the EED anode flow channel as much as possible and retain the reactant concentration in the cathode chamber. Plan B was that iodine would be recovered from the input stream before splitting the two streams into a pair of EED chambers. In this case, the decrease in I2 concentration in the EED catholyte may have a positive impact on HI distillation in addition to the aforementioned advantage. These two plans shared the same the flow ratio of anolyte to catholyte, listed in Table 2. The effects of iodine precipitation on EED energy duties are shown in Fig. 3. As the iodine mole fraction in the EED inlet anolyte decreased from 0.39 to 0.23, the EED duty reduced by approximately 39% at 14.5 mol/kg-H2O HI molality in the EED outlet catholyte. The high molar concentration of iodine in anolyte prevented iodine desorption from the anode surface, resulting in a clear increase in the voltage of the EED cell. By comparing the two graphs in Fig. 3, a decrease of iodine concentration in the EED catholyte appears not to have affected the voltage of the EED cell significantly, consistent with the experimental results published by Pradeep Kumar Sow [35]. Though iodine is the reactant of a cathodic reduction, a decrease in the iodine concentration did not seem to change the cathode reaction. This finding suggests that the reaction on the cathode surface was controlled by the electrochemical reaction rate at this stage rather than the iodine molecule migration rate. The EED duty increased inappreciably as the HI molality in the EED outlet catholyte increased. However, in Hanfei Guo's report [27], the EED duty increased sharply as the HI molality increased. The flow ratio of anolyte to catholyte may be the primary cause of this distinction. The flow ratios of catholyte to anolyte were kept constant in Hanfei Guo's study, while the flow ratio in this study was regulated by the HI molality of the EED cathode outlet. With an increase in the flow ratio of anolyte to catholyte, the concentration difference between the EED outlet catholyte and anolyte declined, causing a reduction in the average voltage of the EED cell. For example, when the flow ratio of anolyte to catholyte was 0.5:0.5, the average voltage was 0.387 V with 12.5 mol/kg-H2O HI in the EED outlet catholyte. However, when the flow ratio was 0.7:0.3, the average voltage fell to 0.365 V.
Effect of iodine mole fraction in feed and operation pressure The effect of I2 mole fraction in feed on the distillation column duty is shown in Fig. 4. Here, the distillation column pressure was set at 0.5 Mpa, and the stage pressure drop was negligible. As is shown in the flowsheets, the iodine mole fraction in the distillation feed was bound by the operating temperature of the iodine precipitator and the HI molality in the EED outlet catholyte. The reboiler duty at the same HI
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
5
Fig. 2 e Flowsheets of three different designs in HI separation subsection. (a) Flowsheet 1: conventional flowsheet without iodine precipitation; (b) Flowsheet 2: iodine removal from anode side; (c) Flowsheet 3: iodine removal from anode and cathode side.
molality feed gradually decreased with a decline in the iodine mole fraction in feed due to the augment of the latent heat of solution vaporization as the concentration of iodine in the HIx mixture increased. The reboiler duty at the feed composition with an HI molality of 11.5 mol/kg-H2O and I2 mole fraction of 0.2194 was nearly the same as that at the feed composition with an HI molality of 14.5 mol/kg-H2O and I2 mole fraction of 0.3208. Thus, a lower iodine concentration in feed could create a positive effect on HI distillation as well as high HI molality. Besides the feed composition, operating pressure also plays an important role in the HI distillation duty. As
presented in Fig. 5, the distillation duty increaseds with increasing the operating pressure in the distillation column. High operating pressure led to an increase in the vaporization temperature of the HIx solution, and the required energy of HI distillation rose when the feed temperature remained constant. The iodine concentration in feed resulted in a significant effect on the distillation duty at a low operating pressure. However, the difference caused by varying iodine molar concentrations in the feed became smaller as the operating pressure grew. Clearly, a high operating pressure weakened the positive effect of the low iodine molar concentration on HI distillation.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Fig. 5 e Distillation duty as a function of distillation pressure at different I2 mole fractions.
Fig. 3 e Dependence of EED duties on HI molality in the EED outlet catholyte: (a) recovering iodine from the stream to the EED anode side only; (b) recovering iodine from the two streams to the EED anode and cathode sides.
Fig. 4 e Distillation duty as a function of I2 mole fraction in distillation feed at different HI molalities.
The ratio of HI mole rate in the distillate to that in feed (DHI/FHI), namely recovery of the light key component in distillate, was evaluated by the variation in operating pressure and iodine molar concentration. A large DHI/FHI contributed to a reduction in mass recycling of excessive materials and improved the total efficiency of the SI process. Fig. 6 demonstrates that increasing operating pressure promoted the growth of DHI/FHI. In fact, high operating pressure encouraged the HI mole fraction of rising vapor in the column [33]. Nevertheless, low iodine concentration had the opposite effect on the DHI/FHI. The divergence in DHI/FHI at five different iodine molar concentrations in feed became increasingly apparent at a high operating pressure; that is, the HI mole fraction in pseudoazeotropic compositions for the HIx solution increased as the iodine molar concentration diminished [33].
Fig. 6 e Effect of distillation pressure on the HI distillate mole ratio.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Comparison of three different flowsheets in HI separation subsection The HI separation energy demands for three different flowsheets are presented in Fig. 7. The results shown in Fig. 7a and c correspond with Flowsheets 1 to 3 in Fig. 2, respectively. The flowsheets demonstrate that the behavior of the plot curve of the conventional flowsheet differed from that of the other two
Fig. 7 e Effect of HI molality in distillation on HI separation duty.
7
flowsheets with iodine recovery. This significant difference may be attributed to the iodine precipitator and subsequent heat exchanger. As shown in Fig. 7a, the HI separation total duty decreased as HI molality in the distillation feed rose. An increase in HI molality in the feed led to two conflicting effects: slightly increasing energy demand of EED and clearly reduced duty of the distillation column. In general, high HI molality and low operating pressure were favorable for HI separation from an energy consumption perspective. However, the higher the HI molality in the distillation feed, the smaller the proportion of catholyte in the input, which required a greater amount of HIx input in the HIx section. For instance, when HI molality in feed and operating pressure were 14.5 mol/kg-H2O and 0.1 Mpa, respectively, the mole ratio of the input flow rate to HI distillate rate was 107.03. The effects of the HI molality in the distillation feed and the column pressure on the HI separation duty of the flowsheet with a precipitator in the stream to the EED anode side are shown in Fig. 7b. Besides the EED duty and the column duty, the heat duty of Heat exchanger_2 was also an important component of the total duty of HI separation. The HI separation total duty decreased with an increase of HI molality in the feed initially and then rose thereafter. This pattern suggests that a minimum HI separation total duty could be obtained. A higher operating pressure resulted in a larger minimum of HI separation total duty. However, at high HI molality, such as 14.5 mol/kg-H2O, the total duty varied inversely with the operating pressure, as the distillation duty was less affected by the operating pressure at higher HI molality in the distillation feed. While the effect of operating pressure on the energy demand of the EED cell was not considered in this work, the duty of Heat exchanger_2 reduced with a smaller amount of input at a high operating pressure. Fig. 7c shows that the Flowsheet 3 had a minimum HI separation total duty with HI molality of 12 mol/kg-H2O at an operating pressure of 0.3 Mpa. Flowsheet 3 demonstrates similar performance as in Flowsheet 2, especially at a high HI molality in the feed. Nevertheless, with a low HI molality in the feed, the behavior of HI separation duty as shown in Flowsheet 3 at different operating pressures did not resemble that of Flowsheet 2. Although the influence of operating pressure on the distillation duty was significant at low HI molality in feed, which could be counteracted by the effect of the heat input from the outside to Heat exchanger_2, Heat exchanger_2 in Flowsheet 3 needed to heat more HIx solution than in Flowsheet 2. Under the condition of minimum HI separation total duty, the input flow rates of three different flowsheets to rectify 1 mol HI are shown in Fig. 8. The required input flow rates were 107.03 mol, 44.92 mol, and 52.43 mol in Flowsheets 1, 2, and 3, respectively. The flow rate ratio (i.e., the input rate to the HI distillate rate) could be determined by HI molality in the EED cathode output, iodine concentration in the distillation feed, and operating pressure of the distillation column. In Flowsheet 1, high HI molality not only decreased the flow rate ratio of the EED catholyte to the total input but also reduced the iodine concentration in the distillation feed. The low operating pressure was not conducive to the HI distillate rate, resulting in the largest input flow rate. Contrary to Flowsheet
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
Science Foundation of China (51621005) and the Program of Introducing Talents of Discipline to University (B08026).
Nomenclature FHI distillate Finput lsat 2 DHm R tþ b Q
Fig. 8 e The ratio of input flow rate to HI distillate rate at the minimum total duty for three flowsheets.
F VEED h
flow rate of HI in distillate, mol/h flow rate of input, mol/h saturation activity of solute molar enthalpy of solution kJ/mol gas constant, 8.314 J/(mol$K) transport number of proton electroosmosis coefficient energy demand of EED to generate 1 mol hydrogen, kJ/mol-H2 Faraday constant, 96485 C/mol voltage of EED cell, V electricity generation efficiency, 0.45
references 1, Flowsheets 2 and 3 demonstrated minimum total duty at a lower HI molality in feed. Additionally, Flowsheet 2 had a higher iodine concentration in the distillation feed than Flowsheet 3, which caused a smaller required input flow rate, indicating that Flowsheet 2 reflects the optimal subprocess.
Conclusions As excess iodine can cause several problems in the HIx section, a precipitator was used to recover some iodine from the HIx solution. The effect of iodine precipitation on the energy demand of EED and the distillation column duty was investigated first. Then, three flowsheets were proposed and compared based on the required input flow rate and separation duty per mole HI. Study findings revealed the following: (1) Recovering iodine from the stream to the EED anode side or the two streams to the anode and cathode sides reduced the energy demand of the EED cell. No significant differences appeared in the EED duty between these two cases. (2) For the HI distillation column, iodine concentration in the feed and the operating pressure were central parameters. The effect of iodine concentration in feed caused an HI distillate similar to that of the operating pressure; that is, high iodine concentration in feed and operating pressure improved the HI distillate rate and low iodine concentration in feed, and operating pressure attenuated the distillation column duty. (3) The flowsheet with an iodine precipitator in the stream to EED anode side was most efficient, requiring the least energy and the smallest input flow rate. The precipitation technology to recover excess iodine was especially useful and feasible.
Acknowledgements The authors gratefully acknowledge the financial support from the Innovative Research Groups of the National Natural
[1] Mazloomi K, Gomes C. Hydrogen as an energy carrier: prospects and challenges. Renew Sustain Energy Rev 2012;16:3024e33. [2] Wang H, Le Person A, Zhao X, Li J, Nuncio P, Yang L, et al. A low-temperature hydrogen production process based on H 2 S splitting cycle for sustainable oil sands bitumen upgrading. Fuel Process Technol 2013;108:55e62. [3] Kang K, Azargohar R, Dalai AK, Wang H. Hydrogen production from lignin, cellulose and waste biomass via supercritical water gasification: catalyst activity and process optimization study. Energy Convers Manag 2016;117:528e37. [4] Rosen MA. Advances in hydrogen production by thermochemical water decomposition: a review. Energy 2010;35:1068e76. [5] Norman JH, Mysels KJ, Sharp R, Williamson D. Studies of the sulfur-iodine thermochemical water-splitting cycle. Int J Hydrogen Energy 1982;7:545e56. [6] Ying Z, Zheng X, Zhang Y, Cui G. Kinetic analysis of Bunsen reaction with HI existence in the thermochemical sulfureiodine cycle for hydrogen production. Appl Therm Eng 2018;129:41e9. [7] Norman JH, Besenbruch GE, Brown LC, O'Keefe DR, Allen CL. Thermochemical water-splitting cycle, bench-scale investigations, and process engineering. Final report, February 1977-December 31, 1981. San Diego, CA (USA): GA Technologies, Inc; 1982. p. 294. Medium: ED; Size. [8] Ying Z, Zheng X, Zhang Y, Cui G. Development of a novel flowsheet for sulfureiodine cycle based on the electrochemical Bunsen reaction for hydrogen production. Int J Hydrogen Energy 2017;42:26586e96. [9] Engels H, Knoche KF. Vapor-pressures of the system Hi/H2o/ I2 and H-2. Int J Hydrogen Energy 1986;11:703e7. [10] Moore R, Parma E, Russ B, Sweet W, Helie M, Pons N, et al. An integrated laboratory-scale experiment on the sulfur-iodine thermochemical cycle for hydrogen production. In: Fourth International Topical Meeting on High Temperature Reactor Technology. American Society of Mechanical Engineers; 2008. p. 541e9. [11] Engels H, Knoche KF, Roth M. Direct dissociation of hydrogen iodidedan alternative to the General Atomic proposal. Int J Hydrogen Energy 1987;12:675e8. [12] Onuki K, Nakajima H, Shimizu S. Concentration of HIx solution by electrodialysis. Kagaku Kogaku Ronbun 1997;23:289e91.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
[13] Kasahara S, Kubo SJ, Onuki K, Nomura M. Thermal efficiency evaluation of HI synthesis/concentration procedures in the thermochemical water splitting IS process. Int J Hydrogen Energy 2004;29:579e87. [14] Kang K-S, Kim C-H, Cho W-C, Jeong S-U, Park C-S, Bae K-K. Demonstration of the HI decomposition section embedded with electrodialysis stack in the sulfureiodine thermochemical cycle for hydrogen production. Nucl Eng Des 2013;256:67e74. [15] Sow PK, Shukla A. Electro-electrodialysis for concentration of hydroiodic acid. Int J Hydrogen Energy 2012;37:3931e7. [16] Hong SD, Kim JK, Bae KK, Lee SH, Choi HS, Hwang GJ. Evaluation of the membrane properties with changing iodine molar ratio in HIx (HI-I-2-H2O mixture) solution to concentrate HI by electro-electrodialysis. J Membr Sci 2007;291:106e10. [17] Cho WC, Park CS, Kang KS, Kim CH, Bae KK. Conceptual design of sulfur-iodine hydrogen production cycle of Korea Institute of Energy Research. Nucl Eng Des 2009;239:501e7. [18] Yang J, Huang Z, Hu Y, Wang Z, Zhang Y, Zhou J, et al. Experimental investigation on two-phase separation ycharacteristics of bunsen reaction in sulfur-iodine cycle for hydrogen production. Acta Energiae Solaris Sin 2012;33:1289e94. [19] Shin Y, Lee K, Kim Y, Chang J, Cho W, Bae K. A sulfur-iodine flowsheet using precipitation, electrodialysis, and membrane separation to produce hydrogen. Int J Hydrogen Energy 2012;37:16604e14. [20] Hur J, O'Connell J, Bae K-K, Kang K-S, Kang JW. Measurements and correlation of solideliquid equilibria of the HIþ I 2þ H 2 O system. Int J Hydrogen Energy 2011;36:8187e91. [21] Powell C, Campbell I. The solubility of iodine in concentrated hydriodic acid solutions. J Am Chem Soc 1947;69:1227e8. [22] Apelblat A, Manzurola E. Solubilities of O-acetylsalicylic, 4aminosalicylic, 3, 5-dinitrosalicylic, andp-toluic acid, and magnesium-DL-aspartate in water fromT¼(278 to 348) K. J Chem Therm 1999;31:85e91. [23] Onuki K, Hwane GJ, Shimizu S. Electrodialysis of hydriodic acid in the presence of iodine. J Membr Sci 2000;175:171e9. [24] Wang ZL, Chen SZ, Zhang P, Wang LJ, Xu JM, Wang SM. Evaluation on the electro-electrodialysis stacks for hydrogen
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
9
iodide concentrating in iodine-sulphur cycle. Int J Hydrogen Energy 2014;39:13505e11. Sow PK, Shukla A. Electrochemical interaction of HIx solution at graphite electrode: an application towards EED process in I-S thermochemical cycle. Ionics 2015;21:175e84. Sow PK, Shukla A. A chronopotentiometry based identification of time-varying different transport resistances of electro-electrodialysis cell used for concentration of HIx solution. Int J Hydrogen Energy 2013;38:3154e65. Guo HF, Kasahara S, Tanaka N, Onuki K. Energy requirement of HI separation from HI-I-2-H2O mixture using electroelectrodialysis and distillation. Int J Hydrogen Energy 2012;37:13971e82. Onuki K, Hwang GJ, Arifal, Shimizu S. Electro-electrodialysis of hydriodic acid in the presence of iodine at elevated temperature. J Membr Sci 2001;192:193e9. Yoshida M, Tanaka N, Okuda H, Onuki K. Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process. Int J Hydrogen Energy 2008;33:6913e20. Tanaka N, Onuki K. Equilibrium potential across cation exchange membrane in HI-I-2-H2O solution. J Membr Sci 2010;357:73e9. Guo HF, Zhang P, Chen SZ, Wang LJ, Xu JM. Comparison of various circuit designs in the HI decomposition section of the iodine-sulfur process. Int J Hydrogen Energy 2012;37:12097e104. Brown LC, Besenbruch GE, Lentsch R, Schultz KR, Funk J, Pickard P, et al. High efficiency generation of hydrogen fuels using nuclear power. General Atomics Report GA-A24285. 2003. Q4. O'Connell JP, Murphy JE. A properties model of the HI-I(2)H(2)O-H(2) system in the sulfur-iodine cycle for hydrogen manufacture. Fluid Phase Equilibria 2010;288:99. 2010;293:266-. Maskalick NJ. Hydriodic acid-anode-depolarized hydrogen generator. Google Patents; 1984. Sow PK, Shukla A. Effect of asymmetric variation of operating parameters on EED cell for HI concentration in I-S cycle for hydrogen production. Int J Hydrogen Energy 2012;37:13958e70.
Please cite this article in press as: Xu S, et al., Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.013