K.F. Knoche

Thermochemical water splitting through direct Hi-decomposition from H2O/HI/I2 solutions

Abstract Sulfur/iodine cycles have a considerable potential for thermochemical hydrogen production. GA Inc. have investigated these cycles for many years. They have been able to demonstrate the feasibility of such cycles and evaluated first chemical engineering flowsheets. The HI concentration and decomposition seems to be the most expensive and energy consuming step. Therefore, an alternative has been developed at the Institute for Technical Thermodynamics in Aachen, in which HI is decomposed directly from liquid H2O/HI/I2 solution under high pressure and temperature. After thermodynamic correlations for the phase equilibrium in the quaternary mixture of H2O/HI/I2/H2 have been established, a separation column could be designed and theoretically evaluated.

Vapor pressures of the system HI/H2O/I2 and H2

Abstract Vapor pressures of the system HI/H 2 O/I 2 and H 2 were determined with a static phase equilibrium apparatus. The selected temperatures were between 120 and 300°C. The measured liquid mole fractions were limited to 0 ⩽ x HI / x H 2 O ⩽ 0.19. For low iodine and high hydrogen iodide contents a significant dissociation of HI into hydrogen and iodine was observed. The positions of the pseudo-azeotropes and the miscibility gap are expressed analytically.

Experimental investigations of thermal HI decomposition from H2OHII2 solutions

Abstract The sulphur/iodine (S/I) cycle for thermochemical hydrogen production has been investigated by General Atomic, Inc. for many years. At the Institute for Technical Thermodynamics in Aachen an alternative to the step with the highest consumption of energy and cost of this cycle—i.e. the HI concentration and decomposition—has been developed. It was proposed to decompose HI directly from liquid H 2 O-HI-I 2 solutions. Theoretical investigations have shown that this process leads to higher HI dissociation rates. A batch separation column was designed to verify the direct HI decomposition in an experimental study. The results for different concentrations and temperatures up to 220°C are presented in this paper.

Thermochemical production of hydrogen by a vanadium/chlorine cycle. Part 1: An energy and exergy analysis of the process

Abstract A vanadium/chlorine water-splitting process for the thermochemical production of hydrogen was investigated both energetically (Part 1) and experimentally (Part 2). A detailed mass and energy balance is given and discussed, starting out from the process flowsheeting developed. Balancing and optimization of the total process followed, based on experimental results from the individual reactions. The total process includes a steam power plant for producing the required electrical power. This is integrated into the thermochemical process. Results of the mass and energy balances are shown and discussed in detail. The overall efficiency of the plant is 42.5%.

Direct dissociation of hydrogen iodide—An alternative to the General Atomic proposal

Abstract An exergetic analysis of the sulfur/iodine cycle according to General Atomic shows that the drying of hydrogen iodide with phosphoric acid is an important weakness of the flowsheet. This is caused by the high amount of required electrical energy for the reconcentration of the phosphoric acid. A preliminary flowsheet is presented, which needs very low amounts of electrical energy and works without any extraction agent. In this proposal the direct dissociation of HI and the liquid-liquid separation of HI/H2O mixtures with high HI contents are taken into account. A rough energy balance leads to a thermal efficiency of about 50%.

Thermochemical production of hydrogen by a vanadium/chlorine cycle. Part 2: Experimental investigation of the individual reactions

Abstract Chemical engineering aspects and the chemical practicability of a vanadium/chlorine hydrogen-producing cycle were investigated on a laboratory scale. The most significant results are presented and discussed. The total process was balanced and optimized using the experimental results as the basis. The overall thermal efficiency was calculated to be 42.5% (see Part 1).

Positioning heat exchangers in binary tray distillation using isoforce operation

The best way of adding two interstage heat exchangers to a binary distillation column is studied using irreversible thermodynamics. A distillation column is simulated with a computer program using the tray-to-tray calculation method. The purpose of the analysis is to find the locations of the two interstage heat exchangers which give the minimum entropy production rate in the column. According to the isoforce principle, minimum entropy production rate is obtained in distillation when the driving forces are uniformly distributed over the trays. This implies that the entropy production rate, in the optimum case, varies according to the value of the phenomenological coefficient. Therefore, locations with the largest deviations from this behavior are good locations for additional heat exchangers. A column separating n-pentane and n-heptane is used to demonstrate how the optimum variation in the coefficient may be used in practice.

Feasibility studies of chemical reactions for thermochemical water splitting cycles of the iron-chlorine, iron-sulfur and manganese-sulfur families

Abstract The number of chemical reactions in water splitting cycles of the iron-chlorine, iron-sulfur and manganese-sulfur families is quite high from a thermodynamic point of view. Applying chemical engineering criteria to the reactions allows the cancelling of unfavorable steps. The remaining reactions can be systemized to typical groups. Experimental results of these groups of reactions are presented, depending on temperature, ratio of reactants and reaction performance. The results are discussed with respect to the performance of the chemical reactions in water splitting cycles.

Pinch analysis for cooling towers

Abstract Optimizing the operation of a power plant with respect to the so-called “cold end” allows for a higher overall efficiency. Among other methods it also requires the proper adaptation of the mass flow rate of the cooling water. In this paper a pinch analysis with respect to heat and mass transfer has been applied to find the optimum mass flow rate for the cooling tower.

Experimental and theoretical investigation of thermochemical hydrogen production

Abstract Hydrolysis, chlorination and the reverse Deacon reaction as steps of thermochemical water splitting cycles have been investigated in a laboratory scale with respect to feasibility and chemical engineering aspects. The results are discussed. Some general theoretical considerations concerning the efficiency of thermochemical water splitting cycles are presented. Using general criteria, some chemical reactions of proposed schemes of water splitting processes are selected and compared.