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HIx processing section of Iodine-Sulphur (IS) thermochemical cycle dictates the overall efficiency of the cycle, which poses extremely corrosive HI–H2O–I2 environment, coupled with a very low equilibrium conversion (~22%) of HI to hydrogen at 450 °C. Here, we report the fabrication, characterization and operation of a 4-tube packed bed catalytic tantalum (Ta) membrane reactor (MR) for enhanced HI decomposition. Gamma coated clay-alumina tubes were used as supports for fabrication of Ta membranes. Clay-alumina base support was fabricated with 92% alumina (~8 μm particle size) and 8% clay (~10 μm particle size), sintered at a temperature of 1400 °C. An intermediate gamma alumina coating was provided with 4% polyvinyl butyral as binder for a 10% solid loading. Composite alumina tubes were coated with thin films of Ta metal of thickness 80% single-pass conversion of HI to hydrogen at 450 °C. The hydrogen throughput of the reactor was ~30 LPH at a 2 bar trans-membrane pressure, with >99.95% purity. This is the first time a muti-tube MR is reported for HIx processing section of IS process.

HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion as well as highly corrosive environment. Corrosion-resistant metal membrane reactor is proposed to be a process intensification tool, which can enable efficient HI decomposition by enhancing the equilibrium conversion value. Here we report corrosion resistance studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. Small pieces of the coated tubes were subject to immersion coupon tests in HI-water environment (57 wt% HI in water) at a temperature of 125–130 °C under reflux environment, and simulated HI decomposition environment at 450 °C. The unexposed and exposed cut pieces were analyzed using scanning electron microscope (SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium.

The third section of closed loop Iodine Sulphur (IS) thermochemical cycle, dealing with HI x processing, suffers from low equilibrium decomposition of HI to hydrogen with a conversion value of only ∼22% at 700 K. Here, we report a significant enhancement in conversion of HI into hydrogen (up to ∼95%) using a zeolite membrane reactor for the first time. The all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure was synthesized on the seeded clay alumina substrate by sonication mediated hydrothermal process. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX). Corrosion studies were carried out by exposing the membrane samples to simulated HI decomposition reaction environment (at 450 °C) for different durations of time upto 200 h. The FESEM, EDX and XRD analyses indicated that no significant changes occurred in the morphology, composition and structure of the membranes. Iodine adsorption on to the membrane surface was observed which got increased with the exposure duration as confirmed by secondary ion mass spectrometry studies. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of in-house synthesized zeolite membrane and Pt-alumina catalyst for carrying out HI decomposition studies. The tube side was chosen as reaction zone and the shell side as the permeation zone. The HI decomposition experiments were carried out for different values of temperature and feed flow rates. DDR zeolite based PBMR was found to enhance the single-pass conversion of HI up to ∼95%. The results indicate that for achieving optimal performance of PBMR, it should be operated with space velocities of 0.2–0.3 s −1 and temperature in the range of 650 K–700 K with permeate side vacuum of 0.12 kg/cm 2 . It is believed that the in-house developed zeolite PBMR shall be a potential technology augmentation in making the IS thermochemical cycle energy efficient.

HI decomposition reaction in Iodine–Sulphur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion. Metallic membrane reactor is proposed to be a process intensification tool that can enable efficient HI decomposition by overcoming the equilibrium limitation of the reaction. In this study, we report the fabrication and characterization of clay alumina supported tantalum (Ta) membrane and its application in packed bed catalytic membrane reactor for HI decomposition section of IS process for the first time. The alumina tubes were coated with thin film of Ta metal of thickness 2.5 μm using DC sputter deposition technique under the optimum set of parameters. The membranes were found to remain stable upto 200 Hrs of exposure under HI-H2O environment at 125 °C. Modeling of a packed bed Ta membrane reactor for HI decomposition was carried out to find out the optimum operating parameters of the membrane reactor to get a conversion of at least 95%. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of the in-house synthesized Ta membrane and Pt-alumina catalyst for carrying out HI decomposition studies at 450 °C. The tube side was chosen as permeation zone and the shell side as the reaction zone. A single-pass conversion of 94.7% of HI to hydrogen was obtained by employing Ta metal membrane reactor against the theoretical equilibrium conversion value of 22%. The findings showed that Ta-based PBMR shall offer a potential benefit to reach higher efficiency of IS thermochemical process.

HIx decomposition step of the Iodine-Sulphur (IS) thermochemical process for hydrogen production is the most critical step. The Coated Wall Membrane Reactor (CWMR) is an important configuration that can be used for carrying out HI decomposition. The present study uses Computational Fluid Dynamics (CFD) as a tool to assess the performance of a CWMR that ensures higher conversion for HI decomposition, which in turn positively affects the overall thermochemical efficiency of the IS process. The effects of various operating parameters, geometry and properties on conversion are studied. The results show that using a CWMR instead of a coated wall reactor (CWR) leads to significant process intensification by coupling reaction and separation in a single unit and enhancing conversion of the reaction. The operating parameters having significant impact on conversion are identified. Comparison of a CWMR with a Packed Bed Membrane Reactor (PBMR) shows that the PBMR gives higher conversion (at least 6%) than the CWMR for the entire range of parameters considered. For very low space velocity, high membrane permeability and low reactor diameters, conversions in PBMR and CWMR are almost the same.

Numerical simulations to develop an understanding of transport processes inside PBMRs (packed bed membrane reactors) and to evaluate effectiveness of PBMRs in increasing the conversion of HI (hydrogen iodide) decomposition reaction of IS (iodine–sulfur) thermochemical cycle are reported. The computational approach used in the simulations has been validated using the data reported for HI decomposition in a packed bed reactor (PBR). The validated computational approach has been used for parametric studies. Effects of different parameters (temperature, pressure, space velocity, membrane permeability, permselectivity, packed bed porosity and reactor diameter) on HI conversion are reported. The parameters having the maximum impact on the conversion are identified. The findings show that using a PBMR instead of a PBR leads to significant enhancement in conversion and the parameters having high impact on conversion are wall temperature, feed temperature, reactor diameter and packed bed porosity. Based on the findings of parametric studies, ranges of the parameters having maximum impact on conversion are suggested, e.g. the reactor wall temperature is recommended to be in the range of 690–700 K, the bed porosity is recommended to be in the range of 0.2–0.4.

Global warming and climate change necessitate a serious move away from fossil-based systems toward hydrogen economy. A study has been carried out on different thermochemical cycles for hydrogen production to bring about their important aspects. The study involves process description of different routes followed by thermochemical cycles. These include metal oxide processes, sulphur family processes like iodine sulphur cycle and Westinghouse cycle, halide family processes like UT-3 and Ispra Mark cycle, copper chlorine cycle and others. This paper gives an insight into the advantages and disadvantages associated with the processes. The review is done keeping in view the relevant and useful aspects in research to present information in terms of species, operating parameters, reactors, costs involved, safety, etc. This will provide a platform for further research to save effort by referring the basic information on thermochemical cycles, so that an appropriate and satisfactory cycle may be chosen.