Development and design of a continuous laboratory-scale plant for hydrogen production by the Mark-13 cycle

The Mark-13 cycle, invented by Schütz and Fiebelmann in 1974, consists of the following three reactions:

$\text{SO}{}_2\text{+}\text{Br}{}_2\text{+2}\text{H}{}_2\text{O}\text{→}\text{H}{}_2\text{SO}{}_4\text{+2}\text{HBr}\text{,}\text{(320–370 K)}$

$\text{2}\text{HBr}\text{→}\text{H}{}_2\text{+}\text{Br}{}_2\text{,}\text{(electrochemical)}$

$\text{H}{}_2\text{SO}{}_4\text{→}\text{H}{}_2\text{O}\text{+}\text{SO}{}_2\text{+0.50}{}_2\text{.}\text{(1000–1100 K)}$

In the present paper the state of the art of the experimental development work carried out at JRC-Ispra is given. The results obtained have been considered a sound basis for the design and construction of a complete laboratory-scale cycle.The expected net hydrogen production rate of this plant is 4.0 gmol/h, i.e. about 100 l/h S.T.P.The aim of the work is the construction of a reliable small scale model of the process which should yield indispensable information for further scale-up. Therefore we applied design and/or simulation rules in the design calculations for the process.It is believed that this installation is the first demonstration of a complete thermochemical hydrogen production process.     

A nickel-iodine-sulfur process for hydrogen production

A thermochemical hydrogen production process which consists of the following reactions containing nickel, iodine and sulfur (NIS process) was studied.

(1.1)$\text{So}{}_2\text{(aq.)+I}{}_2\text{(aq.)+2H}{}_2\text{O(1)→2H}{}_2\text{SO}{}_4\text{(aq.)}$

(1.2)$\text{2HI(aq.)+H}{}_2\text{SO}{}_4\text{(aq.)+2Ni(c)→NiI}{}_2\text{+NiSO}{}_4\text{(aq.)+2H}{}_2\text{(g)}$

(2)$\text{NiI}{}_2\text{(c)→NI(c)+I}{}_2\text{(g)}$

(3.1)$\text{NiSO}{}_4\text{(c)→NiO(c)+SO}{}_3\text{(g)}$

(3.2)$\text{SO}{}_3\text{(g)→SO}{}_2\text{(g)+}\text{1}\text{2}\text{O}{}_2\text{(g)}$

(4)$\text{NiO(c)+H}{}_2\text{(g)→Ni(c)+H}{}_2\text{O(g)}$

This process is an improved iodine-sulfur process, and is characterized by the separation of the products of reaction (1.2) by solvent extraction, and by the absence of hydrogen or water in the high temperature reactions (3.2), (3.1) and (2). Experimental results of the main unit operations are described. The energy balance of the process is estimated, based on a simplified flow-sheet. A conceptual plant flow-sheet is discussed in connection with a VHTR.     

Reactor design for the UT-3 thermochemical hydrogen production process

The UT-3 thermochemical cycle for hydrogen production previously proposed consists of four reactions given by:

(1)$\text{CaBr}{}_2\text{+H}{}_2\text{O→CaO +2HBr}$

(2)$\text{CaO+Br}{}_2\text{→CaBr}{}_2\text{+}\text{1}\text{2}\text{O}{}_2$

(3)$\text{Fe}{}_3\text{O}{}_4\text{+8HBr→3FeBr}{}_2\text{+4H}{}_2\text{O+Br}{}_2$

(4)$\text{3FeBr}{}_2\text{+4H}{}_2\text{O→Fe}{}_3\text{O}{}_4\text{+6HBr+H}{}_2$

.At present, a series of both theoretical and experimental studies aiming at the industrialization of this process have been conducted. In this paper the measurements of the kinetic study for reaction (3) were first described. Based on experimental data and conceptual design of the whole process, then the simulation of the performance for the reactor using a honeycomb-shaped solid reactant was conducted. The result gives information on the reactor design.     

Heats of reaction of pyrotechnic compositions containing potassium chlorate

The heats of reaction of a number of mixtures containing potassium chlorate and lactose have been determined. It has been shown that when there is excess potassium chlorate in the mixture these results can be explained theoretically using Eqs. (1) and (2).

$\text{KC}\text{10}{}_3\text{+}\text{C}{}_{12}\text{H}{}_{22}\text{O}{}_{13}\text{?}\text{H}{}_2\text{O→8}\text{KC}\text{1+12}\text{CO}{}_2\text{+12}\text{H}{}_2\text{O,}$

$\text{KC}\text{10}{}_3\text{→}\text{KC}\text{1+}\text{3}\text{20}{}_2$

. When there is excess lactose present, the results can be correlated with theory using Eqs. (1), (3), and (4)

$\text{C}{}_{12}\text{H}{}_{22}\text{O}{}_{11}\text{·}\text{H}{}_2\text{O→12}\text{C}\text{+12}\text{H}{}_2\text{O}$

,

$\text{C}\text{+}\text{H}{}_2\text{O→}\text{CO}\text{+}\text{H}{}_2$

. Reaction (4) does not go to completion when there is less than 55% potassium chlorate in the mixture. The heats of reaction of mixtures of potassium chlorate with various organic compounds which can be used as fuels were also determined. It was found that, with two exceptions, there was little variation in the heat of reaction, so that this property cannot be the sole criterion when selecting a fuel. The heats of reaction of a few typical vaporizing compositions are given.     

Hydrogen production through microheterogeneous photocatalysis of hydrogen sulfide cleavage. The thiosulfate cycle

Cleavage of hydrogen sulfide into hydrogen and sulfur occurs in alkaline aqueous CdS dispersions under visible light illumination (?400 nm). Small quantities of a noble metal catalyst (RuO₂) loaded onto ‘naked’ CdS particles markedly improve the yield of hydrogen formation. The effect of RuO₂ is ascribed to catalysis of electron transfer to proton. Simultaneous and efficient photogeneration of hydrogen and thiosulfate occurs in CdS dispersions containing both sulfite and bisulfide (or sulfide) ions. Electron transfer from the conduction band of CdS to that of TiO₂ particles occurs in alkaline suspensions containing these HS^? ions and has been exploited to improve the performance of a system achieving decomposition of H₂S by visible light. Equally important is a recent finding that the performance of a system containing ‘naked’ CdS in combination with RuO₂-loaded TiO₂ particles is far better than that of CdS/RuO₂ alone. Additionally, conduction band electrons produced by bandgap excitation of TiO₂ particles efficiently reduce thiosulfate to sulfide and sulfite. The valence band process in alkaline TiO₂ dispersions is thought to involve oxidation of S₂ O₃^2? to tetrathionate, S₄O₆^2?, which quantitatively dismutates into sulfite and thiosulfate, the net reaction being

$\text{2h}{}_{\mathrm{vb}}{}^{\mathrm{+}}\text{(}\text{TiO}{}_2\text{)+0.5S}{}_2\text{O}{}_3{}^{\mathrm{2?}}\text{+1.5}\text{H}{}_2\text{O}\text{→}\text{SO}{}_3{}^{\mathrm{2?}}\text{+3}\text{H}{}^{\mathrm{+}}\text{.}$

The photodriven disproportionation of thiosulfate into sulfide and sulfite is of great interest in systems that photochemically cleave hydrogen sulfide into hydrogen and sulfur

$\text{1.5}\text{H}{}_2\text{O}\text{+1.5S}{}_2\text{O}{}_3{}^{\mathrm{2?}}\textcolor{red}{}\text{+2}\text{SO}{}_3{}^{\mathrm{2?}}\text{+}\text{SO}{}^{\mathrm{2?}}\text{+3}\text{H}{}^{\mathrm{+}}\text{.}$

     

Photochemical and thermoelectric utilization of solar energy in a hybrid water-splitting system

A hybrid thermochemical water-splitting cycle using solar energy is proposed and experimental results are presented. The cycle consists of a photochemical reaction conducted in a flat cell with a Fresnel lens and concentrating the remaining solar energy on a thermoelectric generator which produces electric power for the electrolysis steps. The photochemical reaction is:

$\text{2}\text{Fe}{}^{\mathrm{2+}}\text{+}\text{I}{}^{\mathrm{?}}\text{+}\text{light}\text{→2}\text{Fe}{}^{\mathrm{3+}}\text{+3}\text{I}{}^{\mathrm{?}}$

The overall efficiency is estimated to be as high as 15–25%.     

Blackbody radiation functions

The present paper reports an exact solution of the fractional function of blackbody radiation. The new equation is expressed as

$\text{F}{}_{\mathrm{0?\lambda T}}\text{=}\text{∫}\text{λT}\text{0}\text{E}{}_{\mathrm{b\lambda }}{}^{\mathrm{(T)}}\text{σT}{}^5\text{d (λT)=}\text{15}\text{π}{}^4\text{∑}\text{n=1}\text{∞}\text{e}{}^{\mathrm{?nx}}\text{n}\text{(x}{}^3\text{+}\text{3x}{}^2\text{n}\text{+}\text{6x}\text{n}{}^2\text{+}\text{6}\text{n}{}^3\text{)}$

     

Thermochemical hydrogen preparation—Part V. A feasibility study of the sulfur iodine cycle

A sulfur-iodine cycle consists of the following three reactions:

$\text{2H}{}_2\text{O + SO}{}_2\text{+ I}{}_2\text{→ H}{}_2\text{SO}{}_4\text{+ 2HI,}$

$\text{H}{}_2\text{SO}{}_4\text{→ H}{}_2\text{O + SO}{}_2\text{+}\text{1}\text{2O}{}_2\text{,}$

$\text{2 HI → H}{}_2\text{+ I}{}_2\text{.}$

It was found that the first reaction can be performed as a cell reaction without the addition of external energy. The sulfuric acid and the hydriodic acid are produced separately in the anode and cathode compartments, respectively. The second and third reactions can be carried out as catalytic thermal decompositions. A process flow sheet of this cycle and its mass balance was based on experimental results, and the heat balance for this cycle was made. It was found that internal heat exchange for this cycle was very large (about 2600 kcal/mol H₂), due mainly to the low yield of the decomposition reaction of hydrogen iodide. Theoretical and experimental studies were made to improve the yield of this reaction. The following three methods seem to be promising for this purpose: (1) continuous removal of the hydrogen produced in the reaction zone; (2) performance of the reaction at low temperature (185–250°C) and high pressure (100 atml; and (3) substitution of the benzene-cyclohexane cycle (6HI ? C₆H₆ → C₆H₁₂ + 3I₂; C₆H₁₂ → C₆H₆ ? 3H₂) for the hydrogen iodide decomposition step.     

Thermal analysis of pyrotechnic compositions containing potassium chlorate and lactose

Thermal analysis of a mixture containing potassium chlorate and lactose 50:50, which is commonly used in vaporizing pyrotechnic compositions, has revealed that there are two principal exothermic reactions in the absence of air. The first, coinciding with the fusion of lactose at approximately 200°C, is the oxidation of the lactose by the chlorate and may be represented by the stoichiometric reaction (1).

$\text{8}\text{KC}\text{10}{}_3\text{+}\text{C}{}_{12}\text{H}{}_{22}\text{O}{}_{11}\text{?}\text{H}{}_2\text{O→8}\text{KC}\text{1+12}\text{CO}{}_3\text{+12}\text{H}{}_2\text{O}$

, It is postulated that this reaction is initiated by the partial solution of the potassium chlorate in the lactose, and terminates when there is no liquid lactose in the mixture. This is followed immediately by the dehydration of lactose, represented by Eq. (2)

$\text{C}{}_{12}\text{H}{}_{22}\text{O}{}_{11}\text{?}\text{H}{}_2\text{O→12}\text{C}\text{+12}\text{H}{}_2\text{O}$

. This reaction terminates when approximately half the maximum possible amount of water is evolved, and does not contribute to the exothermicity at this stage. The second exothermic reaction occurs at about 340°C and is probably initiated by the fusion of the potassium chlorate. This reaction is the oxidation of the carbonaceous organic residues by the remaining potassium chlorate. Since there is an excess of fuel in this mixture other slow exothermic reactions occur in the presence of air due to further oxidation of the fuel. These reactions will not occur during combustion, when the ingress of air is prevented. An application of thermal analysis to a practical pyrotechnic composition is presented and discussed.     

Thermochemical hydrogen production via a cycle using barium and sulfur: Reaction between barium sulfide and water

The reaction between barium sulfide and water, a reaction found in several sulfur based thermochemical cycles, was investigated kinetically at 653–866°C. Gaseous products were hydrogen and hydrogen sulfide. The rate determining step for hydrogen formation was a surface reaction between barium sulfide and water. The rate of formation of hydrogen can be expressed as:

$\text{RH2}\text{= 1.07 × 10}{}^{\mathrm{?2}}\text{exp}\text{(}\text{?3180}\text{RT}\text{) (}\text{mol H}{}_2\text{/mol BaS s}$

.Hydrogen sulfide was produced during the initial period of reaction and the quantity of hydrogen sulfide formed during this period decreased as the temperature of reaction was increased.     

A kinetic model of hydrogen absorption in CeMg12

Comparing kinetic equations derived from a theoretical model with experimental data published, the kinetic mechanism of hydriding reaction of CeMg₁₂ was analysed.At the initial stage the reaction is controlled by chemisorption of hydrogen on the metal surface and the reacted fraction (F) is expressed as a function of time (t) and temperature (T)

$\text{F=(1.19 × 10}{}^2\text{)}\text{(P}{}_{\mathrm{o}}\text{? P}{}_{\mathrm{eq}}\text{)}\text{T}\text{1}\text{2}\text{exp}\text{(?3560}\text{cal/RT}\text{)t}$

in the range of 0 ? F ? 0.4. The later stage of the reaction is controlled by another mechanism of metal/hydride interface chemical reaction or hydrogen diffusion in the hydride phase which cannot be clearly distinguished at the moment.