(631c) Experimental Studies of the Bunsen Reaction in the Sulfur-Iodine Process

The Italian national agency for new technologies, energy and environment (ENEA), with the co-operation of the University of Rome ?La Sapienza?, is involved in a research investigation on thermochemical hydrogen production by means of the Sulfur-Iodine water splitting process (S-I). The aim of the research project is the production of hydrogen from solar energy with a higher yield than the traditional photovoltaic technology. This study reports the experimental work conducted on the reaction of sulfur dioxide with iodine and water to produce sulfuric and hydriodic acid in the S-I process:

2H₂O + SO₂ + I₂ ↔ H₂SO₄ + 2HI

According with the commonly adopted recipe [1], this reaction is carried out in liquid media under a certain excess of water (n) and iodine (x) with respect to the stoichiometric values. By this means it is possible to thermodynamically drive the reaction and enhance separation of the two products into two corresponding liquid phases, namely a sulfuric acid rich phase and a ?HIx phase?. Therefore, almost all excess iodine is finally dissolved in the heavier HIx phase, while water will split between the two contacted liquid phases.

Accordingly, the process can be schematized as follows:

(2+n)H₂O + SO₂ + (1+x)I₂ ↔ [H₂SO₄ + (n-m)H₂O]_{Sulf. phase} + [2HI + mH₂O + xI₂]_{HIx phase}

The performance of the whole S-I process is very sensitive to the composition of the two phases. Moreover, we recently highlighted that a non-negligible amount of sulfur-containing compounds are finally dissolved in the HIx phase [2]. These sulphates (and residual SO₂) must be quantitatively removed in a downstream purification unit before subsequent operations for HI decomposition.

Reaction experiments have been carried out using a batch jacketed stirred reactor (about 300 mL), loading some initial amount of iodine, water and HI and allowing the reaction to proceed with a fixed SO₂ flow rate at 1 atm until saturation. Afterwards, stirring and SO₂ feeding were stopped, and the obtained two phases separated.

The as-obtained HIx phase was analysed by ion chromatography and titration after samples dilution in NaOH and KI solutions, respectively.

A representative fraction of the lower HIx phase was then stripped with nitrogen to remove the sulfates. Reaction and stripping temperature conditions were in the range of 30-120°C. Side reaction occurrence, leading to sulfur formation [3], were monitored.

Results will be presented and discussed and compared with other results presented in the literature [4], [5]. These results will be helpful to get a better understanding of the mechanisms of this complex process, as well as to gather data useful for modelling and reactor design.

[1] Brown LC, Besenbruch GE, Lentsch RD, Schultz KR, Funk JF, Pickard PS, et al. High efficiency generation of hydrogen fuels using nuclear power. General Atomic Project 30047, June 2003.

[2] Giaconia A, Caputo G, Ceroli A, Diamanti M, Barbarossa V, Tarquini P, Sau S, Experimental study of two phase separation in the Bunsen section of the sulfur-iodine thermochemical cycle, Int J Hydrogen Energy 32 (2007), pp. 531?536.

[3] Sakurai M, Nakajima H, Amir R, Onuki K, Shimizu S, Experimental studies on side-reaction occurrence condition in the iodine?sulfur thermochemical hydrogen production process, Int J Hydrogen Energy 25 (2000), pp. 613?619.

[4] Giaconia A, Sau S, Caputo G, Tarquini P, Continuous Flow Operation of a Bunsen Reactorin the Iodine-Sulfur Thermochemical Water-Splitting Cycle, AIChE 2007 Annual Meeting, Salt Lake City (UT), November 2007.

[5] Byung Jin Lee, Hee Cheon NO, Ho Joon Yoon, Seung Jun Kim, Eung Soo Kim, An optimal operating window for the Bunsen process in the I?S thermochemical cycle, Int J Hydrogen Energy 33 (2008), pp. 2000?2010.