* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download This article was published in an Elsevier journal. The attached copy
Fluorochemical industry wikipedia , lookup
Acid dissociation constant wikipedia , lookup
Multi-state modeling of biomolecules wikipedia , lookup
Cracking (chemistry) wikipedia , lookup
Asymmetric induction wikipedia , lookup
Rate equation wikipedia , lookup
Photoredox catalysis wikipedia , lookup
Chemical equilibrium wikipedia , lookup
Nuclear chemistry wikipedia , lookup
Biochemistry wikipedia , lookup
Nucleophilic acyl substitution wikipedia , lookup
Marcus theory wikipedia , lookup
Electrochemistry wikipedia , lookup
Physical organic chemistry wikipedia , lookup
George S. Hammond wikipedia , lookup
Process chemistry wikipedia , lookup
Catalytic reforming wikipedia , lookup
Acid–base reaction wikipedia , lookup
Artificial photosynthesis wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Hydroformylation wikipedia , lookup
Photosynthetic reaction centre wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Transition state theory wikipedia , lookup
Stoichiometry wikipedia , lookup
Chemical reaction wikipedia , lookup
Bioorthogonal chemistry wikipedia , lookup
Click chemistry wikipedia , lookup
Citric acid cycle wikipedia , lookup
Electrolysis of water wikipedia , lookup
Hydrogen-bond catalysis wikipedia , lookup
This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author’s institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 www.elsevier.com/locate/ijhydene Hydrogen production from a chemical cycle of H2S splitting Hui Wang ∗ Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9 Received 22 August 2006; received in revised form 20 May 2007; accepted 23 May 2007 Available online 2 July 2007 Abstract The sulphur–iodine thermochemical water-splitting cycle (S–I cycle) developed for hydrogen production from water is fundamentally based on the following three chemical reactions: H2 SO4 → H2 O + SO2 + 0.5O2 , 2H2 O + SO2 + I2 → H2 SO4 + 2HI, 2HI → H2 + I2 . This paper proposes to replace the H2 SO4 decomposition with a reaction between H2 S and H2 SO4 and the replacement gives rise to a H2 S-splitting cycle that produces H2 and elemental S from H2 S, shown as follows: H2 S + H2 SO4 → S + SO2 + 2H2 O, 2H2 O + I2 + SO2 → H2 SO4 + 2HI, 2HI → H2 + I2 . Combined with the reactions such as O2 + S → SO2 , SO2 + 0.5O2 → SO3 , SO3 + H2 O → H2 SO4 , this new cycle cannot only produce more H2 and extra H2 SO4 but also facilitate flexible H2 to H2 SO4 production ratio. Thermodynamic analysis shows that the new cycle is more energy-efficient than the S–I cycle of water-splitting because a series of endothermic reactions in H2 SO4 decomposition have been replaced with an exothermic reaction between H2 S and H2 SO4 . Technologies to be developed from this chemistry will be able to convert H2 S from sour or acid gas into H2 and elemental S or H2 SO4 , which are more valuable than elemental S only, the product of the Claus process. In upgrading and refinery, the H2 produced can be returned to use in hydrotreating; and in gas plants, H2 from H2 S splitting is an alternative clean fuel. Environmentally, H2 production based on this H2 S-splitting cycle is carbon free. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen sulphide splitting; Hydrogen; S–I cycle; Sulphur removal; Elemental sulphur; Sulphuric acid 1. Introduction Bitumen and heavy oil upgrading and refining requires vast amount of hydrogen (H2 ) which is mainly converted into ∗ Tel.: +1 306 966 2685; fax: +1 306 966 4777. E-mail address: [email protected]. hydrogen sulphide (H2 S) in a hydrotreater. H2 S is then turned into elemental sulphur (S) and water (H2 O) in sulphur removal and recovery units such as the Claus plant. On the other hand, the hydrogen needed is produced from the reforming of hydrocarbons, mainly natural gas methane, where greenhouse gas carbon dioxide (CO2 ) is emitted. In addition, gas plants also 0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.05.030 Author's personal copy 3908 H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 produce a tremendous amount of H2 S as a by-product, of which the H element is now wasted in sulphur removal processes. This paper proposes a chemical route that converts H2 S into H2 and elemental S through the following chemical reactions: H2 S + H2 SO4 → S + SO2 + 2H2 O, (1) 2H2 O + I2 + SO2 → H2 SO4 + 2HI, (2) 2HI → H2 + I2 . (3) Given all other substances H2 SO4 , SO2 , H2 O, I2 , and HI are cycled in the system, the three reactions in fact form a novel chemical reaction cycle that splits H2 S into H2 and S: H2 S → H2 + S. (4) This cycle is denoted as the H2 S-splitting cycle. If elemental sulphur from reaction (1) can be further oxidized into SO2 : O2 + S → SO2 , (5) the subsequent reactions (2) and (3) would occur in doubled scale because there are two moles of SO2 for reaction (2). As a result, there must be one mole of water input to the cycle. With including reaction (5), the cycle, denoted as H2 S–H2 Osplitting cycle, eventually splits each mole of H2 S and H2 O, giving rise to two moles of H2 and one mole of sulphuric acid. The origin of this novel chemical route of H2 S splitting is based on the inspiration of a well-known sulphur–iodine thermochemical water-splitting cycle (S–I cycle). Although most of the hydrogen is currently produced from the reforming of natural gas and other hydrocarbons, people never stop pursuing hydrogen production from a water-splitting reaction, which is more environmentally friendly. Therefore, water electrolysis, thermal decomposition, and thermochemical water-splitting are intensively studied. It turns out that the chemical cycles of water-splitting are more technically viable and more energyefficient [1,2]. Among those cycles, one that attracts the most research interest is the S–I cycle that consists of also three reactions: H2 SO4 → H2 O + SO2 + 0.5O2 , (6) 2H2 O + SO2 + I2 → H2 SO4 + 2HI, (2) 2HI → H2 + I2 . (3) The overall reaction is H2 O → H2 + 0.5O2 . (7) Comparatively, the new cycle of H2 S splitting (reactions 1–3) is formed by replacing reaction (6) in the S–I cycle with reaction (1). This paper will first briefly review the research and development work that has been done for the S–I cycle of water-splitting and H2 production. Then it will present the framework of the new thermochemical cycle of H2 S splitting and the alternative route layouts. The economical and environmental benefits of H2 production from H2 S splitting will be discussed. Finally, a plan of the research and development of technologies based on this novel H2 S-splitting cycle will be proposed. 2. Brief review of the S–I cycle The S–I cycle has been considered the one of the most promising routes for hydrogen production from water splitting on a large scale [3,4]. Therefore, tremendous research has been done with this cycle, including experiments, process modelling, efficiency estimation, energy coupling, and chemical modification, etc. The S–I cycle was originally investigated by General Atomics Co. (GA) in the 1970s and the involved reactions with phase specification and reaction temperatures were presented in [3,4] as follows: (9I2 )l + (SO2 )g + (16H2 O)l → (2HI + 10H2 O + 8I2 )l−2 + (H2 SO4 + 4H2 O)l−1 (393 K), (8) L − 2 = (2HI + 10H2 O + 8I2 )l−2 → (2HI)g + (10H2 O + 8I2 )l (2HI)g → H2 + (I2 )l (500 K), (600 K), (9) (10) L − 1 = (H2 SO4 + 4H2 O)l−1 → (H2 SO4 )l + 4(H2 O)l (570 K), (11) (H2 SO4 )l → (H2 SO4 )g (630 K), (12) (H2 SO4 )g → (SO3 )g + (H2 O)g (SO3 ) → (SO2 )g + 0.5O2 (670 K), (1140 K). (13) (14) According to GA’s nomenclature, reaction (8), known as the Bunsen reaction, is Section 1 of the cycle. Compared with reaction (2) in Introduction, an excess of both H2 O and I2 allows the product to form two immiscible phases after this exothermic reaction occurring at a mild temperature. A light H2 SO4 solution phase and a heavy iodine/iodide–water phase enable an easy phase separation. In the subsequent step, Section 3 including reactions (9) and (10), the separation of HI from L − 2, the heavier iodine/iodide–water phase, is the most critical scenario of the cycle [4] and believed to be the most expensive and energy-consuming step [5]. After establishing the thermodynamic correlation of phase equilibrium for the quaternary mixture of H2 O/HI/I2 /H2 , Roth and Knoche [5] designed a reactive distillation column in which reactions (9) and (10) could be performed simultaneously under elevated pressure and temperature. And Section 2 is the H2 SO4 concentration and decomposition, also an energy-intensive phase in the cycle due to the heavy duty of water vaporization and endothermic reactions. Distillation was proposed to perform the concentration of dilute sulphuric acid (57 wt%) from the Bunsen reaction (reaction (8)) and several distillation flow sheets were simulated [4], including a series of flash evaporators [6]. The decomposition of H2 SO4 usually consists of two steps, reactions (13) and Author's personal copy H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 (14), both requiring high-temperature heat, either from nuclear reactors or from solar concentrators, to facilitate [6,7]. Suppiah et al. [8] developed a catalytic method allowing 100% conversion of H2 SO4 into SO3 (thermal decomposition at 773 K) and then from SO3 into SO2 (catalytic decomposition at 1173 K). In general, this cycle is an energy-intensive process. During the past two or three decades, a lot of research work focuses on coupling this cycle with possible energy sources, improving energy efficiency for separations and reaction processes, and modifying the cycle by involving more chemical species so as to lower the energy demand. Sadhankar [9] reported the work that the Atomic Energy of Canada Limited (AECL) engaged in for the development of a novel technology that integrates thermochemical cycles of hydrogen production with its Generation IV reactor system. Vitart et al. [3] suggested that the S–I cycle be coupled with a VHTR (very high-temperature reactor) in Commissariat à l’Energie Atomique (CEA) in France. Solar energy is another heat source. Giaconia and co-workers [10] developed a method of H2 /methanol production by the S–I cycle with the power of solar/fossil energy. In addition to the experimental work on individual reactions in the laboratory scale with batch reactors [3,8,11], mainly done by GA, the Japan Atomic Energy Agency (JAEA) has experimentally demonstrated the continuous hydrogen production in 32 L/h for 20 h in a closed-loop facility made of glass and fluorine [12]. In fact, in early 1990s in Japan, Mizuta and Kumagai built a continuous flow system of a modified Mg–S–I cycle which successfully demonstrated for 33 h a nearly constant generation of H2 and O2 in 0.5 and 0.25 L/h, respectively [13]. On the other hand, process modelling and simulation allows one to understand the phase equilibria and thermodynamic properties involved in the S–I cycle, and to conduct energy-efficiency analysis, process integration and design, and equipment sizing and cost analysis, either for individual sections or for the cycle as a whole [4–7,11,12,14]. All the attempts that have been made are to improve the technical viability and the energy efficiency of the cycle and push forward this conceptual chemical route to a commercial technology. However, all the work is still in research and development scenario. 3. Presentation of the H2 S-splitting cycle As discussed, the novel H2 S-splitting cycle that this paper proposes consists of three chemical reactions: H2 S + H2 SO4 → S + SO2 + 2H2 O, (1) 2H2 O + I2 + SO2 → H2 SO4 + 2HI, (2) 2HI → H2 + I2 . (3) The concept of this cycle is schematically represented in Fig. 1, which takes acid gas, a mixture of H2 S and CO2 from amine treatment train in a gas plant, as a feedstock. In principle, any H2 S-containing gas can be used as a feed to this cycle. However, the impact of the other components than H2 S, such as CO2 in acid gas or CH4 and other gases in sour natural gas, on the reactions and relevant separations must be taken into consideration. Compared with the illustration of 3909 the S–I cycle in a publication by Vitart et al. [3], the involvement of such “other components” adds more complexity to the H2 S-splitting cycle. To make this new cycle work, a process flow diagram is also proposed in this paper, shown in Fig. 2. The process can be seen as an integration of three sections, each including one chemical reaction and relevant separation processes: the oxidation of H2 S, or reaction between H2 S and concentrated sulphuric acid (Section 1), the Bunsen reaction, or the formation of hydrogen iodide solution (Section 2), and hydrogen iodide vaporization and decomposition, or hydrogen generation (Section 3). Although H2 SO4 , SO2 , I2 , and HI are working agents that are recycled within the cycle, separations or purifications are necessary to have them ready from products of one reaction to reactants of the other reaction. 3.1. Section 1—H2 S oxidation In this section, reaction (1) takes place between the H2 Scontaining gas and concentrated H2 SO4 solution, where H2 S is oxidized. In addition to reaction (1), another reaction that would occur simultaneously is [15] 2H2 S + SO2 → 3S + 2H2 O. (15) Reactions (1) and (15) occur in concentrated H2 SO4 solution (94–98 wt%) but the latter is much slower [16,17]. Overall, they give rise to SO2 in gas phase, less concentrated H2 SO4 aqueous solution due to the production of water, and elemental S in solid or immiscible liquid phase depending on the temperature of the reaction system. SO2 , rather than elemental S, is desired for the subsequent reaction, the Bunsen reaction; thus, reaction (15) becomes an undesired one that can be suppressed by using concentrated H2 SO4 (98–100 wt%) and high-reaction temperatures (∼ 150 ◦ C). Except H2 S, most components in sour gas streams in gas plants or oil upgrading and refinery plants, for example, CO2 and saturated low hydrocarbons, are inert to concentrated H2 SO4 [18]. Thus, these reactions can be conducted by passing the sour gas stream through concentrated H2 SO4 solution in a packed-bed column reactor. Necessary separations should follow so as to give relatively pure sweetened gas, elemental S, gas SO2 , and H2 SO4 solution. If more H2 is anticipated, then reaction (5) is carried out to convert elemental S generated in reaction (1) into SO2 . This reaction has been commonly used in sulphuric acid production and the technology to perform this reaction in engineering is mature. In this case, the following two reactions, (2) and (3), will be conducted in a twice larger scale. 3.2. Section 2—Bunsen reaction that may play role of SO2 scrubbing Depending on the feedstock of H2 S-containing gas for Section 1, the SO2 for the Bunsen reaction may be in a mixture with CO2 or CH4 and other low hydrocarbons. Whether this SO2 -containing mixture can be directly used for the Bunsen reaction is determined by whether there are reactions between Author's personal copy 3910 H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 S Acid gas (CO2 + H2S) H2 Reaction (1) Reaction (3) SO2 H2O CO2 H2SO4 H2SO4 Concentration I2 HI HI Vaporization Reaction (2) H2SO4+H2O HI+H2O+I2 Purified CO2 Fig. 1. Schematic representation of the H2 S-splitting cycle for hydrogen production. H2O I2 Reaction (2) Reactor Acid Gas H2S + CO2 Reaction (1) Reactor SO2+ CO2 H2SO4 HI Buffer CO2 out SO2 H2 out Gas-Liquid Separator Reaction (3) Reactor HI, I2 HIX Solution Distillation HIX HI Phase Purification Liquid Phase Separator S out Liquid Phase Separator H2SO4, S, SO2 H2SO4 H2SO4 H2SO4 Upgrading HI HI, I2 Fig. 2. Process flow diagram of H2 S-splitting cycle for hydrogen production. the species involved in the Bunsen reaction such as iodine and iodide on one hand and the components such as CO2 , CH4 and other hydrocarbons on the other. That is, if there are no such reactions, the SO2 mixture can be fed for the Bunsen reaction where SO2 is reacted and SO2 -free gas is released. Thus, the Bunsen reaction can play a role of SO2 scrubbing. If the species such as iodine or iodide would react with CO2 or hydrocarbons, for example, the addition of HI and olefin if the unsaturated hydrocarbons present, SO2 has to be sep- arated from the gas stream before it undergoes the Bunsen reaction. According to Vitart et al. [3], an excess of both water and iodine is provided in reactants such that the products of this reaction will form two immiscible liquid phases (reaction (8)), the light phase of H2 SO4 solution and the heavy phase of HI, I2 and water solution. The advantage of doing so is to allow easier phase separation to separate the two products, H2 SO4 and HI; the disadvantage is that it leads to less concentrated Author's personal copy H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 H2 SO4 solution and HI solution in products, which desire further concentration or purification before they are sent for the reactions they subsequently participate in. 3.3. Section 3—decomposition of HI to produce H2 Compared with the other two sections, Section 3 in the H2 Ssplitting cycle is exactly the same as it is in the S–I cycle of water-splitting. Any species that are brought into the system by sour natural gas or acid gas after amine treatment will not come to this section. Therefore, they will not have any deleterious effect on this section and its related separations. If the heavier aqueous solution of HI and I2 is used as a feedstock, reactions (9) and (10) will be conducted. The vaporization of HI from hydroiodic acid solution (reaction (9)) can also be expressed as the formation of gaseous molecular HI from ions of H+ and I− or I3− in the aqueous solution: + − H + I = HI(g) (16) or H+ + I3− = HI(g) + I2 . (17) Reaction (17) is likely the most reasonable because of the presence of I2 in excess. This section is known as the most costly and energy intensive step in the S–I H2 O-splitting cycle [5,11]. It is also the most expensive section in the H2 S-splitting cycles. The technical challenges that have been identified in the S–I cycle, for example, the separation and concentration of HI from the hydroiodic acid solution, the recycle of the unreacted HI and the separation of H2 , must be solved in the new cycles as well. In summary, the basic H2 S-splitting cycle includes three reactions: 3911 3.4. Other options in H2 SO4 concentration The mass concentration of the (H2 SO4 + 4H2 O) phase from reaction (8) is 57.65 wt%. For concentrating one mole H2 SO4 in this solution, either 3.8 mol of H2 O should be removed or 3.5 mol of SO3 should be added, so as to upgrade it to 98 wt%, the necessary concentration of the salable sulphuric acid or the acid for reaction (1). If sulphuric acid is more desired than H2 , it is suggested that part of the SO2 from reactions (1) or (5) be converted into SO3 : SO2 + 0.5O2 → SO3 . (21) Then the less concentrated H2 SO4 can be upgraded through the reaction SO3 + H2 O → H2 SO4 (22) another mature technology in sulphuric acid plants. Calculation using HSC Chemistry 5.11 shows that the heat of reaction (22) is −106.741 kJ at 298.15 K. Due to its exothermicity, to concentrate the dilute sulphuric acid, adding SO3 to the less concentrated H2 SO4 solution is more energy-efficient than removing H2 O utilizing either distillation or vaporization. Another advantage of involving reactions (21) and (22) is to allow the system to adjust the production ratio of H2 to H2 SO4 based on market demand. The other option is to import S to this cycle to generate SO3 for H2 SO4 concentration without compensating the H2 production, thus leading to more H2 SO4 production. This option is prompted by the fact that the production of sulphur from Canada’s gas and oil industries is excess to the world sulphur market that Canada can access. This market imbalance will persist well into the foreseen future so that elemental S is almost a waste and long-term sulphur storage methods are sought [19]. H2 S + H2 SO4 → S + SO2 + 2H2 O, (1) 4. Thermodynamic considerations 2H2 O + SO2 + I2 → H2 SO4 + 2HI, (2) 2HI → H2 + I2 , (3) It is understood that there is a lot of work to be done to make a chemically feasible route become an engineering process. In other words, an engineering process is more complicated than just a few reactors where the involved reactions are conducted. However, there has not been a satisfied design for the S–I water splitting process though some attempts have been made [11]. Therefore, it is expected that the thermodynamic analysis on both the novel H2 S-splitting cycle and the S–I water-splitting cycle can give a rough comparison between their energy efficiency. Let H , S and G be the changes in enthalpy, entropy and Gibbs free energy of a chemical reaction at a given temperature, and K be the equilibrium constant of the reaction, respectively. That is, for a chemical reaction, which overall lead to the splitting of one mole of H2 S into one mole of H2 and one mole of S. If this S is turned into SO2 via reaction (5), the cycle will be performed following another stoichiometry: H2 S + H2 SO4 → S + SO2 + 2H2 O, (1) O2 + S → SO2 , (5) 4H2 O + 2I2 + 2SO2 → 2H2 SO4 + 4HI, (18) 4HI → 2H2 + 2I2 . (19) aA + bB = cC + dD, (23) The overall reaction then appears to produce two moles of hydrogen and one mole of sulphuric acid from a feed of one mole of hydrogen sulphide, one mole of oxygen and two moles of water: H = cH C + dH D − aH A + bH B , (24) H2 S + O2 + 2H2 O → H2 SO4 + 2H2 . S = cS C + dS D − aS A + bS B , (25) (20) H , S, G and K are defined as follows [20]: Author's personal copy 3912 H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 Table 1 Thermodynamic data for involved reactions at 298.15 K Reactions H (kJ) S(J K−1 ) The S–I H2 O-splitting cycles H2 SO4 (ia) = H2 SO4 (g) H2 SO4 (g) = SO3 (g) + H2 O(g) SO3 (g) = SO2 (g) + 0.5O2 (g) 3I2 + SO2 (g) + 2H2 O = 2H(+a) + 2I3 (−a) + H2 SO4 (ia) 2H(+a) + 2I3 (−a) = 2HI(g) + 2I2 2HI(g) = H2 (g) + I2 For producing one mole of H2 from one mole of H2 O, 174.264 97.538 98.952 −143.846 155.645 −52.718 329.772 278.721 146.800 94.022 −237.804 166.807 −166.360 91.163 53.770 70.919 −72.945 105.911 −3.118 1.065 × 10−16 3.793 × 10−10 3.752 × 10−12 6.035 × 1012 2.775 × 10−19 3.518 The H2 S-splitting cycle H2 SO4 (ia) = H2 SO4 (l) H2 S(g) + H2 SO4 (l) = S + SO2 (g) + 2H2 O 3I2 + SO2 (g) + 2H2 O = 2H(+a) + 2I3 (−a) + H2 SO4 (ia) 2H(+a) + 2I3 (−a) = 2HI(g) + 2I2 2HI(g) = H2 (g) + I2 For producing one mole of H2 from one mole of H2 S, 121.057 −59.636 −143.846 155.645 −52.718 20.439 249.947 −55.593 −237.804 166.807 −166.360 46.535 −43.061 −72.945 105.911 −3.118 7.023 × 10−9 3.505 × 107 6.035 × 1012 2.775 × 10−19 3.518 The H2 S .H2 O-splitting cycle H2 SO4 (ia) = H2 SO4 (l) H2 S(g) + H2 SO4 (l) = S + SO2 (g) + 2H2 O S + O2 = SO2 (g) 6I2 + 2SO2 (g) + 4H2 O = 4H(+a) + 4I3 (−a) + 2H2 SO4 (ia) 4H(+a) + 4I3 (−a) = 4HI(g) + 4I2 4HI(g) = 2H2 (g) + 2I2 For producing two mole of H2 from each mole of H2 S and H2 O, 121.057 −59.636 −296.813 −287.692 311.290 −105.437 −317.231 249.947 −55.593 11.001 −475.608 333.614 −322.721 46.535 −43.061 −300.093 −145.889 211.823 −6.236 7.023 × 10−9 3.505 × 107 3.797 × 1052 3.642 × 1025 7.699 × 10−38 1.238 G(kJ) K Table 2 Thermodynamic data at the temperature at which G = 0 for those reactions with positive G at 298.15 K Reactions H (kJ) S (J K−1 ) Reactions in both H2 S- and H2 O-splitting cycles H(+a) + I3 (−a) = HI(g) + I2 155.888 241.934 643.88 94.630 97.267 141.132 92.268 670.51 1054.18 Reactions in S–I H2 O-splitting cycle H2 SO4 (g) = SO3 (g) + H2 O(g) SO3 (g) = SO2 (g) + 0.5O2 (g) G = cGC + dGD − aGA + bGB , (26) ln K = G/(−RT ), (27) where, a, b, c, and d are the stoichiometric coefficients of substances A, B, C and D, respectively; H, S and G are the enthalpy, entropy and Gibbs energy, respectively; R is gas constant (8.314 J K −1 mol−1 ); and T is temperature in K. The values of these thermodynamic properties of the involved reactions at 298.15 K were calculated using HSC Chemistry v5.11. It should be mentioned that HSC Chemistry requires specifying the physical states of the substances that are involved in a reaction. For a reaction, specifying different states of a substance will result in different calculation results. For instance, dilute sulphuric acid, H2 SO4 (ia), liquid sulphuric acid, H2 SO4 (l) and gas sulphuric acid, H2 SO4 (g) will give different values for a reaction that H2 SO4 is involved. Therefore, the reactions with the specified physical states of their reactants and products, together with values of the corresponding thermodynamic properties, are listed in Table 1. The reactions are grouped based on T (K) the three cycles they belong to, even though some of them may be presented in more than one cycles taking the same or different stoichiometric coefficients. First of all, comparison of the overall reaction heat of the cycles at 298.15 K indicates that the H2 S-splitting cycle needs much less energy than the S–I watersplitting cycle and that the H2 S.H2 O-splitting cycle becomes exothermic in overall. This is because the new H2 S-splitting cycles do not include the highly endothermic H2 SO4 decomposition but are joined with the exothermic reactions such as the oxidation of H2 S and elemental S. Secondly, comparison on G values shows that the endothermic, G-positive H2 SO4 decomposition reactions (13) and (14) in the original cycle have been replaced by the exothermic, G-negative reactions such as reaction (1) or reactions (1) and (5). Table 2 shows the values of H and S at the temperature at which G equals 0 for those reactions with a positive G at 298.15 K. Only one of such reactions is involved in the H2 S-splitting cycles, which is the concentration of HI from the hydroiodic acid solution. The reaction temperature has to be at least higher than 643.88 K Author's personal copy H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 such that the reaction occurs spontaneously. Nonetheless, the new H2 S-splitting cycles do not need high-temperature heat to heat a reactor at least to 1054 K to facilitate the SO3 decomposition for SO2 . It should be pointed out that the main purpose of this paper is to propose a novel H2 production route from a H2 S-splitting cycle with which the abundant H2 S resources in oil and gas industries can be made use of not only for the “cheap gold” (sulphur) but also for hydrogen; the latter being more economically and environmentally valuable. Additionally, the H2 S.H2 O-splitting cycle does split one mole of water, which means that simultaneous splitting of H2 S and H2 O would release the energy burden that the S–I water-splitting cycle is bearing. 5. Case study of H2 S-splitting cycle application in bitumen upgrading and refinery A bitumen (oil sand) upgrading plant with 120 kbbl/d (kilobarrels per day) capacity [21] will produce H2 S in 425 kta (kilotonnes per annum), that is 12.5 million kmol per year. Based on the stoichoimetry of the H2 S.H2 O-splitting cycle: H2 S + O2 + 2H2 O → H2 SO4 + 2H2 , (20) H2 production will be 25 million kmoles, i.e., 560 mcma (million cubic meters per annum) at the standard temperature and pressure (STP), and the H2 SO4 production will be 12.5 million kmoles, i.e., 1225 kta. For bitumen upgrading of such a capacity, the H2 demand for hydrotreating processes such as hydrocracking, hydrodesulphurization, hydrodenitrogenation, and hydrodearomatization is 250 mcma [21]. Thus, the H2 surplus will be 310 mcma. This surplus can be used as H2 energy, for instance, the feedstock for fuel cells. 6. Conclusions—plans for R and D It can be seen that the H2 S-splitting cycle to produce H2 is chemically feasible and thermodynamically more efficient than the S–I cycle. The cycle that is able to convert H2 S into H2 and elemental S or H2 SO4 has great market potential in gas and oil industry, especially oil sand bitumen upgrading and refinery to fuels and petrochemicals. Environmentally, H2 from H2 S splitting is greenhouse gas free. And its use will reduce the hydrogen reliance on natural gas reforming, and thus reduce CO2 emission from this sector. However, to make this economically and environmentally beneficial chemistry a real chemical engineering process, there are tremendous challenges that include old ones that have been identified in the S–I cycle research and the new ones that the particular reactions in the new H2 S-splitting cycle bring about. In addition to the to-beimproved areas that Goldstein et al. [4] recently identified, further R and D work in the H2 S-splitting cycle will be presented as follows: The separation of elemental sulphur from downgraded (diluted) sulphuric acid after the reaction between H2 S and H2 SO4 solution: Reaction (1) between H2 S-containing gas and concentrated sulphuric acid in liquid phase can be carried out in a trickle-bed reactor [18], giving rise to SO2 -containing 3913 gas and solid sulphur or liquid sulphur along with downgraded sulphuric acid. Whether the elemental sulphur is liquid or solid depends on whether the temperature is above the melting point of elemental sulphur (119 ◦ C). And how much the acid concentration is downgraded in one pass depends on the ratio of H2 S to H2 SO4 in feed. Moreover, the dissolution of SO2 in the acid solution is significant [22]. Therefore, the sulphuric acid solution has to be upgraded by removing elemental S, releasing SO2 , and concentrating. To develop engineering processes to conduct these separations constitutes the main tasks in R&D scenario. The use of I2 and water solution as solvent to absorb SO2 from gas stream: SO2 from the above section will come together with the other gaseous components that are present in the acid gas stream or sour natural gas but remain unchanged when undergoing the contact with sulphuric acid. Whether the SO2 should be separated from this gas stream before it is sent for the Bunsen reaction depends on whether there are interactions between these “other components” in the gas stream and the reactant and product species of the Bunsen reaction. This is another particular issue that needs to be studied to develop the H2 S-splitting process for H2 production. It is hoped that the research on both the H2 S-splitting cycle for H2 production from H2 S and the S–I cycle for H2 production from water be able to benefit each other’s achievements and speed up the commercialization of these technologies. References [1] Sata S. Thermochemical hydrogen production. In: Ohta T, editor. Solarhydrogen energy systems. Oxford: Pergamon Press; 1979. [2] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185–90. [3] Vitart X. et al. Hydrogen production using the sulphur–iodine cycle coupled to a VHTR: an overview. Energy Convers Manage 2006;47: 2740–7. [4] Goldstein S. et al. Upper bound and best estimate of the efficiency of the iodine sulphur cycle. Int J Hydrogen Energy 2005;30:619–26. [5] Roth M, Knoche KF. Thermochemical water splitting through direct HI-decomposition from H2 O/HI/I2 solutions. Int J Hydrogen Energy 1989;14:545–9. [6] Ozturk IT. et al. An improved process for H2 SO4 decomposition step of the sulphur–iodine cycle. Energy Convers Manage 1995;31:11–21. [7] Huang C, T-Raissi A. Analysis of sulphur–iodine thermochemical cycle for solar hydrogen production. Part I: decomposition of sulphuric acid. Solar Energy 2005;78:632–46. [8] Suppiah S, Li J, Deschenes L. Direct electro-thermal decomposition of H2 SO4 in S–I process for H2 production. In: Dincer I, Li X, editors. Proceedings of the second international green energy conference, Oshawa, Ontario, Canada, June 2006. p. 25–31. [9] Sadhankar RR. Leveraging nuclear research to support the hydrogen economy. In: Dincer I, Li X, editors. Proceedings of the second international green energy conference, Oshawa, Ontario, Canada, June 2006. p. 10–7. [10] Giaconia A. et al. H2 /methanol production by sulphur–iodine thermochemical cycle powered by combined solar/fossil energy. In: AIChE spring national meeting, Conference proceedings. 2005. p. 1587–602. [11] Brown LC, et al. Alternative flowsheets for the sulphur–iodine thermochemical hydrogen cycle. AIChE spring national meeting, Conference proceedings, 2003. Author's personal copy 3914 H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914 [12] Kubo S. et al. A demonstration study on a closed-loop hydrogen production by the thermochemical water-splitting iodine–sulphur process. Nucl Eng Des 2004;233:347–54. [13] Mizuta S, Kumagai T. Continuous flow system demonstration and evaluation of thermal efficiency for the magnesium–sulphur–iodine thermochemical water-splitting cycle. Ind Eng Chem Res 1990;29: 565–70. [14] Brown LC, Matias PM. Phase equilibria and thermodynamic properties of the sulphur–iodine cycle. AIChE spring national meeting, Conference proceedings, 2005. p. 3147. [15] Wang H. et al. Thermodynamics and stoichiometry of reactions between hydrogen sulfide and concentrated sulfuric acid. Can J Chem Eng 2003;81:80–5. [16] Wang H. et al. Kinetics of reaction between hydrogen sulphide and sulphur dioxide in sulphuric acid solutions. Ind Eng Chem Res 2002;41:4707–13. [17] Wang H. et al. Kinetics and mechanism of oxidation of hydrogen sulphide by concentrated sulphuric acid solutions. Ind Eng Chem Res 2002;41:6656–62. [18] Wang H. et al. Mass transfer characteristics for gas–liquid reaction of H2 S and sulphuric acid in a packed column. Ind Eng Chem Res 2004;43:5846–53. [19] Davis PM, et al. Laboratory and field studies concerning the long-term storage of elemental sulphur. 54th CSChE conference, Calgary, Canada, 2004. [20] Roine A. Outokumpu HSC chemistry for windows—user’s guide, version 5.1, Outokumpu Research, October 2002. [21] Laureshen CJ. From oil sand bitumen to petrochemical feedstock. Oil and gas processing review Touch Briefing; 2006. [22] Zhang Q. et al. Solubility of sulphur dioxide in sulphuric acid of high concentrations. Ind Eng Chem Res 1998;37:1167–72.