Bunsen reaction

Here, reaction 4.9, known as "Bunsen reaction," is the low-temperature exothermic reaction, where the raw material, water, reacts with iodine and gaseous sulfur dioxide producing an aqueous solution of hydriodic acid and sulfuric acid. The acids are then separated and thermally decomposed to produce hydrogen and oxygen. The total reaction scheme... 

There are two main issues concerning the chemistry of the reaction and the separation. One is how to separate the hydriodic acid and sulfuric acid produced by the Bunsen reaction. The other is how to carry out the hydrogen iodide (HI) decomposition section, where the presence of azeotrope in the vapor-liquid equilibrium of the hydriodic acid makes the energy-efficient separation of HI from its aqueous solution difficult, and also, the unfavorable reaction equilibrium limits the attainable conversion ratio of HI to a low level, around 20%. 

As for the former problem, the researchers of GA found that the mixed acid solution produced by the Bunsen reaction separates spontaneously into two liquid phases in the presence of excess amount of iodine [17]. The heavier phase is mainly composed of HI, I2, and H20, and is called "Hix" solution. The main components of the lighter phase are H2S04 and H20. The phenomenon (liquid-liquid (LL)-phase separation) offered an easy way of separating the hydriodic acid and the sulfuric acid. As for the HI processing, some ideas have been proposed by GA [17], RWTH Aachen [18], and JAEA. JAEA studied the utilization of membrane technologies for concentrating the Hix solution to facilitate the HI separation and also for enhancing the one-pass conversion of HI decomposition [19,20]. 

Reaction (1), known as the Bunsen reaction, proceeds exothermically as the S02 gas absorption reaction by iodine-water mixture. The HI and H2S04 produced can be separated by liquid-liquid phase separation of sulphuric acid and HIx solution found by the researchers of General Atomics. Reaction (2) is slightly endothermic and can be carried out in gas phase using catalysts. Reaction (3) proceeds in two steps, i.e. ... 

A small scale sulphur iodine process loop made of Pyrex glass was built and operated. The sulphuric acid section and Bunsen reaction section was operated successfully in 2006. In 2008, hydrogen iodide decomposition aided by electro-dialysis (EED) (Hong, 2007) was demonstrated to produce 3.5 litres per... 

The Bunsen section is central to the cycle, not only because it objectively is the section which produces the two acids that are independently processed in the other two sections, but also because its optimisation is key for the whole cycle efficiency. Indeed, the Bunsen reaction does not actually proceed as written above, but requires large amounts of excess water and excess iodine (Norman, 1982) ... 

In order to take advantage of this purification effect and to optimise the water content of the iodine-rich phase, CEA has proposed to use a counter-current reactor to perform the Bunsen reaction. This idea arises from the consideration that, aside from S02, the chemical system includes two almost immiscible solvents (excess water and excess iodine) and two solutes (H2S04 and HI). Bunsen reactor... 

CEA s flow sheet calculations (Leybros, 2009) indicate no heat requirements in Bunsen section (the Bunsen reaction is actually quite exothermal), and a small electricity requirement of 4 kj/mol for S02/02 separation through compression. Due to the lack of adequate thermodynamic models, the effect of the presence of impurities in the acid phases is not taken into account. However, CEA, together with the University of Toulouse, has undertaken to build a model capable of describing hydroiodic and sulphuric acid mixtures (Hadj-Kali, 2009b), which will serve as the basis for future evaluations. 

Iodine and sulphur dioxide are combined with water in the Bunsen reaction to create two immiscible acid phases. The separation of these phases is facilitated by the presence of excess iodine. The lighter, sulphuric acid phase is decomposed first to S03, and then to S02. S02 formation typically occurs in the presence of a catalyst at temperatures above 800°C. The S02 and water are recycled back to the Bunsen reaction for reuse. 

The heavy phase, consisting of water, iodine and hydriodic acid (the mixture is also known as HIX), is treated to result in the decomposition of HI to produce the product hydrogen and iodine. The iodine and water are also recycled back to the Bunsen reaction for reuse. 

The Bunsen reaction proceeds at 120-130°C, under a pressure of 3 to 6 bars. For flow sheet calculations, product flows were supposed pure (i.e. no iodine trace in sulphuric acid, no sulphur trace in hydroiodic acid). Thermal balance of this section is summarised in Table 1, with values in kj/mol H2. 

GA 0 (exothermal reaction) 128 (Bunsen reaction) 0 (inside Bunsen section) 2 (pumps,...)... 

Common nomenclature is for the Bunsen reaction section to be called Section 1, for the sulphuric acid section to be known as Section 2, and for the HI decomposition section to be designated as Section 3. 

In the HyS cycle, the decomposition of sulphuric acid is the same as Section 2 of the S-I cycle. Thus, the difference between the S-I cycle and HyS cycle is the removal of the Bunsen reaction and the replacement of Section 3 with an electrolysis process. A simplified flow sheet for the HyS cycle is shown in Figure 3. 

Section 1 operates at 393 K, Section 2 operates at 1123 K, and Section 3 operates at 773 K. The section of the plant dedicated to the Bunsen reaction is dubbed Section 1, H2S04 decomposition is dubbed Section 2 and HI decomposition is dubbed Section 3. 

The thermochemical dissociation of water is initiated from the following Bunsen reaction. This reaction is an exothermic reaction, and therefore it proceeds between room temperature and 100°C ... 

Davis, M. E. and Conger, W. L., "An Entropy Production and Efficiency Analysis of the Bunsen Reaction in the General Atomic Thermochemical Hydorgen Production Cycle," Int. J. Hydorgen Energy, 5, 475 (1980). 

The sulfur-iodine (S-I) cycle is a thermochemical water-splitting process that utilizes thermal energy from a high-temperature heat source to produce hydrogen (H2). It is comprised of three coupled chemical reactions, as shown in figure 4.1. First, the central low-temperature Bunsen reaction (Section I) is employed to produce two... 

Other than the main Bunsen reaction, the other three steps in this section involve separation of gases and H2O from the flowing chemical stream. O2, which forms as a result of water splitting, is removed from the recycled stream to avoid formation of complexes in the rest of the section. SO2 is washed/separated from the Bunsen reactor output to prevent any side reaction downstream. The last step in Section I involves the extraction of H2O from the HI product stream before it is sent to Section III. This will reduce the energy requirement, as H2O evaporation processes impose a high heat demand based on the current flow sheet. Hence, any reduction in H2O content in the Bunsen reaction products will be beneficial to the overall cycle efficiency. 

Consequently, the HI at the bottom of the reactive column is I2 rich and is fed back to Section 1 as I2 supply for the Bunsen reaction. H2 is bled off the column top as a compressed gas for storage or use. The environment of HIx and high temperature and pressure required in the heat exchanger in the reactive distillation process make it potentially one of the most corrosive environments within the S-I cycle. 

HI is a strong reducing acid with a negative pH. Even though it is a common reagent in organic chemistry, corrosion data of materials in HI acid at elevated temperatures are limited. Table 4.6 shows a summary of the available data. Noble and refractory metals have shown low corrosion rates, but the temperatures at which the data were taken are lower than those in the Bunsen reaction environment. The corrosion mechanism of H2SO4 depends on temperature and concentration. Within the... 

---