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Title: Membrane Processes for the Sulfur-Iodine Thermochemical Cycle

Abstract

Removal of water from aqueous hydriodic acid (HI) by pervaporation using Nafion-117® membranes has been studied. In this work, Nafion membranes have been used to separate water from HI at concentrations from 18 to 70 % and at temperatures ranging from 22 °C to 109 °C. Initial fluxes of the more dilute acid solutions were as high as 1500 g/m2h with a separation factor (á) of 139. Furthermore, separation factors as high as 500-700 were obtained for more concentrated samples. In general, increased temperatures yielded higher fluxes with lower separation factors and higher feed concentrations gave lower fluxes with higher separation factors. Activation energies of transport (EJ) values have been calculated for lower and higher concentration HI feeds and compared to pure water pervaporation. These data suggest that the degree of protonation and mole fraction water in the feed influence transport kinetics and that boundary layers issues become more prevalent at higher HI concentrations. Additionally, initial data using a sweep gas in place of the vacuum permeate trapping system was shown to provide similar flux magnitudes as the vacuum system providing versatility for potential plant applications.

Authors:
; ;
Publication Date:
Research Org.:
Idaho National Laboratory (INL)
Sponsoring Org.:
DOE - NE
OSTI Identifier:
912360
Report Number(s):
INL/JOU-05-00523
Journal ID: ISSN 0360-3199; IJHEDX; TRN: US200801%%794
DOE Contract Number:
DE-AC07-99ID-13727
Resource Type:
Journal Article
Resource Relation:
Journal Name: International Journal of Hydrogen Energy; Journal Volume: 32; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
08 - HYDROGEN; BOUNDARY LAYERS; HYDRIODIC ACID; KINETICS; MEMBRANES; REMOVAL; TRANSPORT; TRAPPING; VACUUM SYSTEMS; WATER

Citation Formats

Frederick F. Stewart, Christopher J. Orme, and Michael G. Jones. Membrane Processes for the Sulfur-Iodine Thermochemical Cycle. United States: N. p., 2007. Web. doi:10.1016/j.ijhydene.2006.06.055.
Frederick F. Stewart, Christopher J. Orme, & Michael G. Jones. Membrane Processes for the Sulfur-Iodine Thermochemical Cycle. United States. doi:10.1016/j.ijhydene.2006.06.055.
Frederick F. Stewart, Christopher J. Orme, and Michael G. Jones. Thu . "Membrane Processes for the Sulfur-Iodine Thermochemical Cycle". United States. doi:10.1016/j.ijhydene.2006.06.055.
@article{osti_912360,
title = {Membrane Processes for the Sulfur-Iodine Thermochemical Cycle},
author = {Frederick F. Stewart and Christopher J. Orme and Michael G. Jones},
abstractNote = {Removal of water from aqueous hydriodic acid (HI) by pervaporation using Nafion-117® membranes has been studied. In this work, Nafion membranes have been used to separate water from HI at concentrations from 18 to 70 % and at temperatures ranging from 22 °C to 109 °C. Initial fluxes of the more dilute acid solutions were as high as 1500 g/m2h with a separation factor (á) of 139. Furthermore, separation factors as high as 500-700 were obtained for more concentrated samples. In general, increased temperatures yielded higher fluxes with lower separation factors and higher feed concentrations gave lower fluxes with higher separation factors. Activation energies of transport (EJ) values have been calculated for lower and higher concentration HI feeds and compared to pure water pervaporation. These data suggest that the degree of protonation and mole fraction water in the feed influence transport kinetics and that boundary layers issues become more prevalent at higher HI concentrations. Additionally, initial data using a sweep gas in place of the vacuum permeate trapping system was shown to provide similar flux magnitudes as the vacuum system providing versatility for potential plant applications.},
doi = {10.1016/j.ijhydene.2006.06.055},
journal = {International Journal of Hydrogen Energy},
number = 4,
volume = 32,
place = {United States},
year = {Thu Mar 01 00:00:00 EST 2007},
month = {Thu Mar 01 00:00:00 EST 2007}
}
  • Thermochemical cycles have been proposed as processes for the manufacture of hydrogen from water in which the only effluent is oxygen. In this paper, membrane-based technologies are described that have the promise of enabling the further development of thermochemical cycle processes. In direct service of the sulfur-iodine (S-I) cycle, membranes have been studied for the concentration of HI and sulfuric acid using pervaporation. In this work, Nafion® and SPEEK membranes have effectively concentrated both acids at temperatures as high as 134 ºC without any significant degradation. Measured fluxes of water and separation factors are commercially competitive and have been characterizedmore » with respect to acid concentration in the feed streams. Further, hydrogen permeability is discussed at 300 ºC with the goal of providing a method for the removal of the product gas from HI in the decomposition step, thus increasing the productiveness of the equilibrium limited reaction.« less
  • Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydroiodic acid (HI) to produce hydrogen; a key reaction in the sulfur-iodine (S-I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. Within the group of ligno-cellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a ligno-cellulosic chemically-activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites,more » as determined by Bohem’s titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest, one more time, that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.« less
  • Eight activated carbon catalysts were examined for their catalytic activity to decompose hydroiodic acid (HI) to produce hydrogen; a key reaction in the sulfur-iodine (S-I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. No statistically significant correlation was found between catalyst sample properties and catalytic activity. Four of the eight samples were examined for one week of continuous operation at 723 K. All samples appeared to be stable over the period of examination.
  • One of the most promising cycles for the thermochemical generation of hydrogen is the Sulfur-Iodine (S-I) process, where aqueous HI is thermochemically decomposed into H2 and I2 at approximately 350 degrees Celsius. Regeneration of HI is accomplished by the Bunsen reaction (reaction of SO2, water, and iodine to generate H2SO4 and HI). Furthermore, SO2 is regenerated from the decomposition of H2SO4 at 850 degrees Celsius yielding the SO2 as well as O2. Thus, the cycle actually consists of two concurrent oxidation-reduction loops. As HI is regenerated, co-produced H2SO4 must be separated so that each may be decomposed. Current flowsheets employmore » a large amount (~83 mol% of the entire mixture) of elemental I2 to cause the HI and the H2SO4 to separate into two phases. To aid in the isolation of HI, which is directly decomposed into hydrogen, water and iodine must be removed. Separation of iodine is facilitated by removal of water. Sulfuric acid concentration is also required to facilitate feed recycling to the sulfuric acid decomposer. Decomposition of the sulfuric acid is an equilibrium limited process that leaves a substantial portion of the acid requiring recycle. Distillation of water from sulfuric acid involves significant corrosion issues at the liquid-vapor interface. Thus, it is desirable to concentrate the acid without boiling. Recent efforts at the INL have concentrated on applying pervaporation through Nafion-117, Nafion-112, and sulfonated poly(etheretherketone) (S-PEEK) membranes for the removal of water from HI/water and HI/Iodine/water feedstreams. In pervaporation, a feed is circulated at low pressure across the upstream side of the membrane, while a vacuum is applied downstream. Selected permeants sorb into the membrane, transport through it, and are vaporized from the backside. Thus, a concentration gradient is established, which provides the driving force for transport. In this work, membrane separations have been performed at temperatures as high as 134 degrees Celsius. Transmembrane fluxes of water are commercially competitive (~5000 g/m2h) and separation factors have been measured as high as 8000, depending on the membrane and the water content. For the Nafion-117 experiments, the common trade off in membrane performance is observed in that as flux is increased, separation factor decreases. Nafion-112, a thinner membrane, exhibited much higher fluxes than the Nafion-117; however without the expected loss in separation factor indicating that the permeability of iodine and HI through Nafion materials is low. Preliminary data for the sulfuric acid concentration suggests performance similar to the HI experiments. All membranes studied for the HI, HI/iodine and sulfuric acid feeds exhibited no degradation in membrane performance during use.« less
  • Nafion®-117 membranes have been successfully used to remove water from aqueous hydriodic acid (HI) by pervaporation. HI feeds were concentrated from approximately 1.7 M to 5 M, and permeate concentrations ranged from 10-3 M to 10-4 M, regardless of the feed HI concentration. Temperatures examined were 22 °C, 50 °C, 70 °C, and 100 °C. Using 180 ìm thick films, fluxes at 22 °C were 0.43 Kg/m2h, and increased with increasing system temperature to a maximum of 1.5 Kg/m2h at 100 °C. Durability studies over a period of three months operation revealed little membrane degradation and, in all cases, themore » membranes retained their bulk physical properties in that they remained flexible and plastic. More intensive thermomechanical testing revealed changes in the membrane morphology upon pervaporation of the HI feed at 100 °C, however these changes were not reflected in the observed water transport behavior.« less