Gas-phase HCl Research Articles

Polybenzimidazole Membranes for Waste Stream Remediation to High Value Products

Electrochemical remediation of industrial waste products represents an economically attractive method to continuously convert waste streams to high value products. Electrochemical remediation has been more limited for processes that are very dry, very acidic, or a combination of both, because the most widely used polymer membranes (for example, Nafion™) need external humidification to maintain conductivity. We show that para-polybenzimidazole (PBI) membranes can be used as an effective polymer membrane separator in these types of harsh environments, improving the performance of the current state-of-the-art aqueous HCl electrolyzer, enabling the first completely anhydrous gas-phase HCl electrolyzer, and pushing the electrolysis of SO2 in the Hybrid Sulfur (HyS) cycle to the point of economic viability. The discussion will also cover how PBI membrane preparation affects electrochemical performance and the development of a Computational Fluid Dynamics (CFD) model that both provides insight into the transport phenomena of the anhydrous electrolyzer and allows us to examine operating parameters that are dangerous experimentally.

Influence of nozzle structure on the combustion performance of a graphitic hydrogen-chlorine synthesis combustor

The growing semiconductor sector is driving up demand for high-purity gas-phase HCl. Large combustion zones and low HCl concentrations are problems for HCl manufacturing systems. Based on the realizable k-ε model, the eddy breakup combustion model, and the participating media radiation model, the computational fluid dynamics simulation was used as the primary means in this study. The influence of four structural factors, nozzle jet hole diameter, jet angle, and opening angle—another is nozzle mounting height, on the combustion effect of the synthesis combustor was investigated. The extent of the high-temperature zone in the synthesis combustor and the purity of the hydrogen chlorine at the outlet were used as the main evaluation criteria. The results of numerical simulations demonstrate that the better the gas mixing and combustion in the combustor, the lower the nozzle mounting height should be. Small nozzle diameters and large nozzle jet angle increase gas flow velocity in the recirculation zone close to the inner wall of the combustor. Additionally, the recirculation zone will appear in the combustor as a result of the nozzle's presence. The area of the high-temperature zone within the combustor decreases as internal gas velocity increases. On the other hand, the recirculation gas flow is not significantly impacted by the angle at the condition that the jet holes open. The results of the investigation provide a reference for the design of the miniaturization of the combustor in the non-premixed combustion synthesis of gases process.

Intracavity absorption spectroscopy of HCl isotopes, H2O, CH4, C2H4, and C2H6 in the 3.1-3.4 µm spectral range using a Cr:CdSe laser.

We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around λ = 3.1-3.4 µm. A detection scheme based on an integrated recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of Leff ≈ 850 m. Exemplary measurements of atmospheric H2O and CH4, and additionally introduced gas-phase HCl, C2H4, or C2H6 are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 104 are discussed.

The role of coarse aerosol particles as a sink of HNO<sub>3</sub> in wintertime pollution events in the Salt Lake Valley

Abstract. Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH3, HNO3, HCl, and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl−, and SO42- were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3-) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3- was nearly 3 times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3- and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse-mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO3- : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution.

Catalyst Effects on Gas Phase HCl Oxidation in PEM Electrolyzer

Electrolysis of HCl exhibits promise for large-scale production of hydrogen with the additional benefit of converting a low-value byproduct (HCl) into a more valuable feedstock (Cl2). However, most of the available publications in literature use aqueous phase HCl as the electrolyte [1-3]. Previously, gas phase hydrogen bromide (HBr) was shown to electrolyze at current densities an order of magnitude greater than that of aqueous HBr [4]. Aqueous HBr electrolysis was limited by slow diffusion in the liquid phase. We show here that gas phase HCl also exhibits higher reactions rates, as reflected by higher current densities, when compared to aqueous phase. The purpose of this work was to study the efficiency and durability of the anode catalyst (RuO2 and IrRuO2), the role of the polymer membrane (Nafion vs. polybenzimidazole), and the operating parameters (temperature, HCl flow rate, and current density) in a proton exchange membrane (PEM) electrolyzer. We found that crossing over of chorine from anode to cathode can poison the catalyst at the cathode, resulting in less durability and shorter lifetime of the electrolyzer. We also discuss how operating temperature, HCl flow rate, and hydration affect the performance of the electrolyzer.

Electrochemical gas phase oxidation of hydrogen chloride to chlorine: Model-based analysis of transport and reaction mechanisms

The electrochemical conversion of hydrogen chloride to chlorine plays a significant role within major industrial processes like the polyurethane or polycarbonate production. Our recent studies demonstrated that the direct electrolysis of gaseous HCl in combination with novel strategies for product purification leads to significant exergetic savings (36–38%) compared to the Bayer UHDENORA state-of-the art process, which employs aqueous hydrochloric acid as a feedstock. Furthermore, we showed that despite of the improvements in the efficiency of the electrochemical reactor, it still has by far the greatest exergy demand of all process units. While the oxygen depolarized cathode (ODC) utilized in both process variants has been investigated in great detail in the scientific literature, the oxidation reaction of gaseous HCl (HClOR) has not yet received much attention and is hence the major subject of this work. In earlier experimental investigations of the HClOR, a limiting current was observed that has the potential to critically reduce the performance of the overall reactor. In the past, membrane dehydration and mass transfer resistances on the anode side were suggested as possible reasons. In order to shed light on this phenomenon, a dynamic, one dimensional agglomerate model of the gas phase HCl oxidation half-cell, considering detailed micro kinetics, as well as the mass transfer of HCl in the different cell parts, is developed. The modeling results suggest that neither mass transfer resistance nor membrane dehydration but a kinetic limitation is the underlying reason for the observed limiting behavior. This has a significant impact on the strategy for further reactor optimizations and harbors the potential of major energy savings in an industrial application of this highly relevant process.

Spatial Distributions and Sources of Inorganic Chlorine in PM2.5 across China in Winter

Chlorine is an important atmospheric photochemical oxidant, but few studies have focused on atmospheric chlorine. In this study, PM2.5 samples were collected from urban and rural sites across China in January 2018, and concentrations of Cl− and other water-soluble ions in PM2.5 were analyzed. The size-segregated aerosol Cl− data measured across Chinese cities by other studies were compiled for comparison. The observed data demonstrated that the Cl− concentrations of PM2.5 in northern cities (5.0 ± 3.7 µg/m3) were higher than those in central (1.9 ± 1.2 µg/m3) and southern cities (0.84 ± 0.54 µg/m3), suggesting substantial chlorine emissions in northern cities during winter. The concentrations of Cl− in aerosol were significantly higher in urban regions (0.11–26.7 µg/m3) compared to than in rural regions (0.03–0.61 µg/m3) across China during winter, implying strong anthropogenic chlorine emission in cities. Based on the mole ratios of Cl−/Na+, Cl−/K+ and Cl−/ SO 4 2 − and the PMF model, Cl− in northern and central cities was mainly sourced from the coal combustion and biomass burning, but in southern cities, Cl− in PM2.5 was mainly affected by the equilibrium between gas-phase HCl and particulate Cl−. The size-segregated statistical data demonstrated that particulate Cl− had a bimodal pattern, and more Cl− was distributed in the fine model than that in the coarse mode in winter, with the opposite pattern was observed in summer. This may be attributed to both sources of atmospheric Cl− and Cl− involved in chemical processes. This study reports the concentrations of aerosol Cl− on a national scale, and provides important information for modeling the global atmospheric reactive chlorine distribution and the effects of chlorine on atmospheric photochemistry.

Adsorption and dissociation of gas-phase HCl molecules on Al17q (q = -2 - +3) ions.

Al17 clusters exhibit apparent changes in curvature, which resemble macroscopic metal tips. Here, we show, using the density functional theory method, how surface charges of Al17q (q = -2 to +3) ions affect the adsorption and dissociation behavior of HCl molecules. Geometries, adsorption energies, vibrational frequencies, Mulliken population analysis and transition states of (Al17HCl)q (q = -2 to +3) adsorption complexes were studied. The results revealed that HCl molecules tend to locate on tip sites of the Al17q (q = -2 to +3) ions. Anionic adsorption complexes are prone to H affinity adsorption, whereas cationic adsorption complexes favor Cl-affinity adsorptions. These adsorption behaviors look quite like macroscopic tip effects. H-Cl bonds of the adsorption complexes weaken with an increase in either positive or negative charge. Dissociation barriers of the H-Cl bonds exhibitbinding energies that are 2 orders of magnitude smaller than those of an isolated HCl molecule. Considering adsorption energies and dissociation barriers comprehensively, HCl molecules should dissociate spontaneously for all the models considered. Generally, the more negative charges the clusters carry, the more energy the reaction will release. Graphical abstract Dissociation barriers of the H-Cl bonds in Al17q (q = -2 - +3) cluster ions exhibit energy barriers ~2 orders of magnitude smaller than isolated HCI molecules.

Influence mechanism of the compositions in coal-fired flue gas on Hg0 oxidation over commercial SCR catalyst

Optimizing the performance of commercial SCR catalyst (i.e., V2O5–WO3/TiO2) for Hg0 oxidation remained stagnant as the influence mechanism of the compositions of flue gas on Hg0 oxidation was unclear. In this work, the mechanism of Hg0 oxidation and the influence mechanism of the compositions of flue gas on Hg0 oxidation over V2O5–WO3/TiO2 were investigated. The reaction orders of Hg0 oxidation over V2O5–WO3/TiO2 in regard to both the concentrations of Hg0 and HCl in gas phase were approximately 0. Hence, Hg0 oxidation over V2O5–WO3/TiO2 primarily followed the Langmuir–Hinshelwood mechanism, and the elementary reactions of Hg0 oxidation primarily involved the physical adsorption of Hg0, the formation of Cl* radial, and the reaction of physically adsorbed Hg0 and Cl* radial. SO2, NO, H2O, and NH3 not only restrained the Cl* radial formation but also disturbed the reaction of physically adsorbed Hg0 and Cl* radial. Meanwhile, the physical adsorption of Hg0 was restrained by both H2O and NH3. Hence, Hg0 oxidation over V2O5–WO3/TiO2 was obviously restrained when SO2, NO, H2O, and NH3 were present in flue gas.

Mechanism of Hg0 oxidation in the presence of HCl over a CuCl2-modified SCR catalyst

CuCl2-SCR catalysts prepared by an improved impregnation method were examined to evaluate the catalytic activity for gaseous elemental mercury (Hg0) oxidation in the presence of HCl at the typical SCR reaction temperature of 350 °C. It was found that Hg0 oxidation activity of commercial SCR catalyst was significantly improved by the introduction of CuCl2. The X-ray fluorescence and Hg0 temperature-programmed desorption (Hg0-TPD) methods were employed to characterize the catalysts. The results indicated that CuCl2 on CuCl2-SCR catalyst could release active Cl species in the presence of O2 at 350 °C, and the released active Cl species could be replenished in the presence of gas-phase HCl. CuCl2-SCR catalyst possessed the appropriate active sites for the adsorption of NH3 and HCl, which could scavenge the inhibiting effect of NH3 on Hg0 oxidation. Hg0-TPD results suggested that the oxidized mercury compounds mainly exited as HgCl2 once HCl was present. The Hg0 oxidation mechanism over CuCl2-SCR catalyst in the presence of HCl could be explained as follows: The adsorbed Hg0 reacted with active Cl species released by CuCl2 to form HgCl2. The reduced CuCl was re-chlorinated to CuCl2 via the intermediate copper oxychloride (Cu2OCl2) formation by being exposed to the gas-phase HCl.

Gas phase synthesis of isopropyl chloride from isopropanol and HCl over alumina and flexible 3-D carbon foam supported catalysts

Isopropyl chloride synthesis from isopropanol and HCl in gas phase over ZnCl2 catalysts supported on Al2O3 as well as flexible carbon foam was studied in a continuous reactor. A series of catalytic materials were synthesised and characterised by BET, XPS, SEM, TEM, XRD and NH3-TPD methods. Catalytic activity tests (product selectivity and conversion of reactants) were performed for all materials and optimal reaction conditions (temperature and feedstock flow rates) were found. The results indicate that the highest yield of isopropyl chloride was obtained over 5wt.% ZnCl2 on commercial Al2O3 (No. II) (95.3%). Determination of product mixture compositions and by-product identification were done using a GC-MS method. Carbon foam variant catalyst, 5wt.% ZnCl2/C, was found to perform best out of the carbon-supported materials, achieving ∼75% yield of isopropyl chloride. The kinetic model describing the process in a continuous packed bed reactor was proposed and kinetic parameters were calculated. The activation energy for the formation of isopropyl chloride reaction directly from isopropanol and HCl was found to be ∼58kJ/mol.

Activity of CuCl2-modified cobalt catalyst supported on Ti-Ce composite for simultaneous catalytic oxidation of Hg0 and NO in a simulated pre-sco process

To achieve the high Hg0 oxidation efficiency in the atmospheres with low concentration of HCl or in temporary absence of HCl and facilitate the following SCR process, CuCl2-CoOX/Ti-Ce catalyst for simultaneous catalytic oxidation of Hg0 and NO in a pre-sco process (300–420°C) was developed. N2 adsorption-desorption, XRD, TG and XPS techniques were used to characterize the catalyst and its catalytic performance was evaluated in a laboratory-scale fixed-bed system. Results indicated that the oxidation process occurred at CuCl2 sites and CoOx sites separately for Hg0 and NO over the catalyst. With an increasing CuCl2 loading, more active CuCl2 species would form and both amorphous and crystalline phase of CuCl2 contribute to the excellent mercury oxidation capacity, however, resulting in the decrease of NO conversion efficiency. CuCl2 might hinder the interaction between NO species and CoOx sites. Thus, well dispersion of CuCl2 and CoOx species over catalyst surface could relieve the suppression. In the presence of 5ppmHCl and 6%O2, a sustainable Hg0 oxidation capability (nearly 100% at 360°C) could be achieved over 7%CuCl2-CTC catalyst. Elemental mercury at gaseous state collided with the CuCl2 sites of catalyst and reacted with the active Cl to form HgCl2. The consumption of active Cl species could be compensated by the gas phase HCl with the aid of O2. SO2 did not show a significant negative effect on Hg0 oxidation but suppressed NO oxidation. SO2 with high concentration would speed up the formation of sulphates, resulting in the loss of activity.

Acidity of Strong Acids in Water and Dimethyl Sulfoxide.

Careful analysis and comparison of the available acidity data of HCl, HBr, HI, HClO4, and CF3SO3H in water, dimethyl sulfoxide (DMSO), and gas-phase has been carried out. The data include experimental and computational pKa and gas-phase acidity data from the literature, as well as high-level computations using different approaches (including the W1 theory) carried out in this work. As a result of the analysis, for every acid in every medium, a recommended acidity value is presented. In some cases, the currently accepted pKa values were revised by more than 10 orders of magnitude.

Thermodynamic Possibilities and Limits for Producer Gas Desulfurization and HCL Related Interferences for Zn, Mn, Ce and La Based Sorbents of Sulfur Compounds

The study is concentrated on thermodynamic analyses of gas desulfurization process (deep removal of H2S, COS, thiophene) by selected solid sorbents (ZnO, MnO, CexOy and La2O3) and on interferences caused by presence of hydrogen halides in a temperature range 500-1100 K. The results show that theoretically Ce2O3 and La2O3 are the best sorbents for sulfur compounds at temperatures over approx. 700 K. The CexOy, La2O3 and MnO based sorbents can suffer from significant interferences caused by higher concentrations of HCl and HF in gas phase. The thermodynamic equilibria suggest that removal of HCl (HF) by soda based sorbents at temperatures 650 – 850 K is practically without interferences from sulfur compounds. The common alkali carbonates are less suitable than the calcium based (Ca (OH)2, CaCO3) sorbents for deep removal of HF.

Efficient catalytic oxidation of HCl to recycle Cl2 over the CuO–CeO2 composite oxide supported on Y type zeolite

Abstract The CuO–CeO2 composite oxides supported on Y-zeolite were prepared by means of wet-impregnation method. The derived catalysts exhibited excellent activities and high stabilities for gas-phase HCl catalytic oxidation to Cl2. 90% conversion of HCl can be accomplished at a reaction temperature as low as 390 °C. Comparing with the results reported in literature, the CuO(12)–CeO2(13)/Y catalyst developed in the current study showed exceptionally high space time yield for Cl2 (62.7 mmol gcat−1 h−1 at 410 °C). A correlation between the characterization results and the activities of CuO–CeO2/Y suggested that higher content of well dispersed CuO species and intensive CuO–CeO2 association (the formed CuxCe1−xO2−y and surface oxygen spillover) are critical to determine high activity.

HCl Oxidation on IrO2-Based Catalysts: From Fundamentals to Scale-Up

IrO2 has been investigated as alternative rutile-type catalyst to RuO2 for gas-phase HCl oxidation. The HCl conversion level over IrO2 at 723 K was comparable to that of RuO2 at ca. 170 K lower temperature, in line with the higher computed energy barrier for chlorine evolution over the former oxide. Similarly to RuO2, chlorination took place only at the IrO2 surface, which is predicted to exhibit full occupation of the coordinatively unsaturated iridium sites and replacement of 50% of the oxygen bridge positions by chlorine. Advantageously, IrO2 is more resistant than RuO2 against oxidation, since the latter forms volatile RuO4 species at high temperatures. IrO2 (2 wt %) supported on TiO2-rutile displayed a 6 times higher activity than on TiO2-anatase. Although this corroborates the crucial role of the structural similarity between the carrier and active phase highlighted in the development of RuO2-based catalysts, some differences were uncovered. (i) Small and highly dispersed IrO2 clusters rather than t...

Study on the Reaction of CCl2F2with CH4in a Dielectric Barrier Discharge Nonequilibrium Plasma

The reaction of CCl2F2 (CFC-12) with methane (in an argon bath gas) in a dielectric barrier discharge (DBD) non-thermal plasma reactor at atmospheric pressure has been investigated. The reaction (where oxygen and nitrogen are excluded from the feed gas) produced a range of gas phase products, including H2, HCl, HF, CHF3, CF3CH2F, CHClF2, CH3Cl, CH2ClF, CH2Cl2, CHCl2F and CHCl3 as well as solid products which were primarily non-crosslinked polymers. The conversion of CCl2F2 increased, from 44 to 71%, when the input energy density of the plasma was raised from 3 to 13 kJ · L−1 (applied voltage range is 13.4–14.8 kV, peak-peak). The relatively low input feed concentration of CCl2F2 and methane in the argon bath gas modifies the strong filamentary appearance of the pure argon discharge to a more diffuse appearance.

Energy-efficient chlorine production by gas-phase HCl electrolysis with oxygen depolarized cathode

For the first time, gas-phase electrolysis of HCl to chlorine was conducted in a fuel cell type reactor with an oxygen depolarized cathode. Very low cell voltages below −1V at industrially relevant current densities of 4kAm−2 were obtained. Compared to technical HCl electrolysis processes, this leads to a reduction in electrical energy demand of approximately 30%. The temperature and the relative humidity (rH) of the cathode inlet gas were identified as key parameters controlling the overall performance of the system.

Environmental implications of the snow chemistry from Mt. Yulong, southeastern Tibetan Plateau

Snow chemical records were recovered from fresh and surface snow samples in the Mt. Yulong region. Striking seasonal differences were evident among the major ions. The non-monsoon snow samples show higher ionic concentrations than those of the monsoon season. The observations can be categorized into three groups based on their seasonal behavior and characteristic relationships that were explored using correlation and factor analyses. Calculations of backward trajectory modes suggest that the coarse mode ions (e.g. Ca2+, Mg2+) were mainly transported from Central Asia, through the south limb of the westerlies, in non-monsoon seasons. The Indian monsoon and southeast monsoon are the prevailing air masses in the Yulong region during the monsoon season, and can account for the extensive precipitation and lower snow ion concentrations in this season. Ion balance calculations indicate that there is an excess of cations in snow chemistry due to higher concentrations of Ca2+. In addition, the low snow Na+/Cl− ratio shows an obvious excess of Cl− concentrations possibly due to the precipitation scavenging of gas-phase HCl in the air and/or different sources between Na+ and Cl−. The snow chemical data presented here indicate that Mt. Yulong provides a probably unique record of atmospheric and environmental conditions in southwestern China.

Do observations on surface coverage-reactivity correlations always describe the true catalytic process? A case study on ceria

In situ (operando) investigations aim at establishing structure-function and/or coverage-reactivity correlations. Herein, we investigated the gas-phase HCl oxidation (4HCl+O2→2Cl2+2H2O) over ceria. Despite its remarkable performance, under low oxygen over-stoichiometry, this oxide is prone to a certain extent to subsurface/bulk chlorination, which leads to deactivation. In situ Prompt Gamma Activation Analysis (PGAA) studies evidenced that the chlorination rate is independent of the pre-chlorination degree but increases at lower oxygen over-stoichiometry, while dechlorination is effective in oxygen-rich feeds, and its rate is higher for a more extensively pre-chlorinated ceria. Even bulk CeCl3 could be transformed into CeO2 under oxygen excess. Electron Paramagnetic Resonance experiments strongly suggested that oxygen activation is inhibited by a high surface chlorination degree. The coverages of most abundant surface intermediates, OH and Cl, were monitored by in situ infrared spectroscopy and PGAA under various conditions. Higher temperature and p(O2) led to enhanced OH coverage, reduced Cl coverage, and increased reactivity. Variation of p(HCl) gave rise to opposite correlations, while raising p(Cl2) did not induce any measurable increase in the Cl coverage, despite the strong inhibition of the reaction rate. The results indicate that only a small fraction of surface sites is actively involved in the reaction, and most of the surface species probed in the in situ observation are spectators. Therefore, when performing in situ steady-state experiments, a large set of variables should be considered to obtain accurate conclusions.