Separation of isoprene from biologically-derived gas streams

Most industrially relevant high temperature gas separations (≥150°C) of either carbon dioxide (flue gas) or hydrogen (syn-gas) must be performed in the presence of water vapor. At ambient temperatures, water vapor can permeate easily through most polymeric membranes and can influence the permeation of other gases through interaction with the polymer, such as swelling and clustering. At higher temperatures, water vapor can be destructive to polymer membranes by changing the polymer structure that can result in diminished gas separation performance. Little data has been reported on the influence of water vapor in gas separations at >100°C because most polymers are not stable at temperature. Many high performance (HP) polymers are able to endure high temperatures and aggressive chemical conditions. For example, polyimides are promising HP polymers that effectively separate permanent gases at temperatures higher than 150°C under dry conditions. In this report, the analysis of selected HP polymers in humidified gas streams (2–4 vol% water) shows that they can perform modest separations at ambient temperatures. In general, it was observed that water vapor permeability is greater than other tested gases. Additionally, the permeabilities of the analyte gases were somewhat influenced by the presence of humidity and their selectivities were significantly lower, as compared to corresponding experiments performed in the absence of water. To elucidate the role of water vapor in gas transport, energy of activation of permeation (Ep) values were obtained for Matrimid 5218 from 30–200°C in a humidified mixed gas stream, and it was found that the selectivities are nearly identical to dry gas streams at 150°C. This data suggests that water vapor functions as a gas and only slightly decreases selectivity of the other gases at elevated temperatures. As a result, economic wet gas separations may be possible using these materials if the gas stream is kept at higher temperature (≥150°C), which is assisted by the inherent stability of the membranes.

Considering that phenanthrene (Phe) is one of the main pollutants in the waste flue gases from organic material combustion, the aim of this paper is to examine the use of carbon materials to remove low concentrations of Phe from a hot exhaust gas stream as a function of its moisture content. Adsorption isotherms were measured with steam and Phe concentrations in the ranges of 0−20% and 0.02−3.2 ppmv, respectively. Three classic models [Langmuir, Freundlich, and Dubinin−Radushkevich (DR)] were applied and their parameters were then determined by regression analysis. It was found that all isotherms fitted the DR model. The determined parameters showed that, for a carbonaceous material, the higher the steam percentage (in volume) in the gas stream, the lower its Phe adsorption capacity. Results also showed that within the experimental conditions the Phe adsorption capacities of the carbonaceous materials in humid gas streams could be related to Phe concentration by the Langmuir model. The Freundlich model proved to be the most successful in describing the Phe adsorption behavior in dry gas streams. The effect of adsorbent characteristics on Phe adsorption from humid gas streams was determined from correlation coefficients between the adsorption capacities of sixteen carbonaceous materials and their textural parameters. The total micropore volume (pore size diameter < 2 nm) was the parameter that controlled the Phe adsorption both in humid and dry gas streams. Moreover, the difference, in percentage between adsorption capacities in dry and humid gas streams (10% steam percentage) was positively correlated with the narrow micropore volume (pore size diameter < 0.7 nm) with a statistical significance level higher than 99%.

Removal of VOCs from humidified gas streams using activated carbon cloth

The reprocessing of used nuclear fuel would release volatile radionuclides into the off-gas streams of a processing plant, including ¹²⁹I. The dissolver off-gas and more dilute vessel off-gas streams (VOG) must both be targeted when mitigating ¹²⁹I environmental release because each contains some amount of iodine. Iodine-129 will likely be found as elemental iodine (I₂) and methyl iodide (CH₃I) in these off-gas streams. Reduced silver-exchanged mordenite (AgZ or Ag⁰Z) has been investigated as a potential sorbent for iodine abatement and is studied here under prototypic VOG conditions. Because of the relatively low iodine abundance and high flow rates of the VOG, total iodine concentrations are expected to be in the parts per billion range. Thus, VOG experiments need to run for extended durations at low concentrations to reach sorbent saturation. Because the sorbent will be exposed to oxidizing gas streams for extended periods of time, aging effects and sorbent degradation need to be considered when designing a sorbent-based abatement system. Three sets of experiments were performed with the aim of determining how the adsorption of iodine by AgZ is affected by differing test durations and gas compositions. The first tested the capacity of sorbent aged for 8 months under a humid air stream. The second tested differences in sorbent capacity and mass transfer zone (MTZ) length during high-concentration (1200 ppbv) CH₃I loading in a humid nitrogen gas (N₂) stream and a humid air gas stream over 28 days. The third tested sorbent capacity and MTZ length during low concentration (< 200 ppbv) CH₃I and I₂ loading in a humid air stream over 9 months. The results of these tests suggest that AgZ capacity drops by ~50% after 1 month of aging, by ~60% after 2 months of aging, and then does not continue to decrease significantly at up to 8 months of aging. One-month aging tests that compared N₂ and air as the gas stream diluent resulted in similar maximum loading capacities and overall loading curves, indicating that the effects of aging cannot be mitigated by avoiding air as the balance gas. The 9-month extended VOG tests did not result in clear sorbent saturation at the inlet, but it can be inferred using an assumption of a maximum capacity of 45 mg I/g AgZ. Applying this assumption, then the MTZ of CH₃I is 12.8 cm, and the MTZ of I₂ is 12.4 cm. These results are similar to previous tests. Comparisons to tests completed at Idaho National Laboratory suggest that the CH₃I MTZ may be dependent on concentration. Future work should re-evaluate the design of the VOG iodine capture system with this updated data and should seek to understand fundamental characteristics of CH₃I adsorption by AgZ, such as the effect of concentration, gas velocity, and gas composition.

A flat-plate spiral-channeled membrane heat exchanger for methane dehumidification: Comparison of kraft paper and thin-film composite membrane

Supplying warmed saturated water vapour in anaesthetic gases during whole body hyperthermia (WBH) could potentially improve thermal uniformity in the trachea and esophagus. Four normal dogs were anaesthetized for WBH at 42 degrees C. A Puritan Bennett Cascade humidifier was used to supply anaesthetic gases saturated with water vapour at an average airway temperature of either 42 degrees C or 38 degrees C. Esophageal temperature was monitored at the thoracic inlet and 5 cm cephalad. Thermal dose was estimated by calculating equivalent minutes for an isoeffect at 43 degrees C (CEM 43 degrees Tx, where Tx is the site of temperature measurement). Endotracheal mucociliary transport velocity (MCTV) was determined before and 48 h following WBH by 99mTc-MAA scintigraphy. Compared to the 38 degrees C humidified gas group, dogs receiving 42 degrees C humidified gas reached 42 degrees C faster (p = 0.02) and had CEM 43 degrees T(esophageal) values equivalent to the target CEM 43 degrees T(rectal). Endotracheal MCTV with 42 degrees C humidified gas, however, was reduced 53% from baseline 48 h following WBH (p = 0.02). With 38 degrees C humidified gas, endotracheal mucociliary transport velocity was unchanged from baseline 48 h post WBH. Tracheal histology was examined using light and electron microscopy in four additional dogs euthanatized following 90 min of 42 degrees C humidified gas combined with WBH. There was no histological evidence of tracheal or lung thermal damage with 42 degrees C humidified gas in these four dogs. However, a moderate increase in tracheal goblet cell secretory granule staining was observed. This change could imply temporary heat-induced ciliary microtubule dysfunction, rather than decreased mucus production, as the likely mechanism of reduced mucociliary transport velocity 48 h following WBH. Administration of 42 degrees C humidified anaesthetic gases with WBH improves heating rate and esophageal thermal uniformity but temporarily depresses tracheal mucociliary transport velocity.

A hydrophobic hypercrosslinked polymer with poly (4-tert-butylstyrene-styrene-divinylbenzene) matrix (LC-1) was prepared as adsorbent for the removal of volatile organic compounds from gas streams. The content of oxygen-containing functional groups of LC-1 was about one-fourth that of commercial hypercrosslinked polymeric adsorbent (NDA-201). The results of the water vapor adsorption experiment indicated that LC-1 had a more hydrophobic surface than NDA-201. Three chlorinated volatile organic compounds (trichloroethylene, trichloromethane, and 1, 2-dichloroethane) were used to investigate the adsorption characteristics of LC-1 under dry and humid conditions. Equilibrium adsorption data in dry streams showed that LC-1 had good adsorption abilities for three chlorinated VOCs due to its abundant micropore structure. Moreover, the presence of water vapor in the gas stream had negligible effect on breakthrough time of three chlorinated VOCs adsorption onto LC-1 when values of relative humidity were equal to or below 50%; the breakthrough time of three chlorinated VOCs decreased less than 11% even if the relative humidity was 90%. Taken together, it is expected that LC-1 would be a promising adsorbent for the removal of VOCs vapor from the humid gas streams.

Long-term performance of highly selective carbon hollow fiber membranes for biogas upgrading in the presence of H2S and water vapor

The UK’s advanced gas-cooled (AGR) nuclear reactors have operated over the last 30 years, with the austenitic stainless steel sections primarily operating at temperatures ranging from 470 °C up to 650 °C. The coolant gases used in this type of nuclear system contain a mixture of carbon dioxide, carbon monoxide, hydrogen, methane and water vapor. A number of cracks have been reported in superheater boiler components made from 4 mm thick austenitic stainless steel tubes. The mechanism underlying the initiation of cracks is believed to be creep-fatigue which may be exacerbated by carburization of the metal surface, associated with the presence of a duplex oxide layer. In this paper, complementary microstructural characterization techniques have been used to investigate the oxidation behavior of Type UNS S31609 stainless steel in the simulated AGR environments. A primary focus was given to the effects of surface finish and the water vapor content on oxidation. The experimental results show that surface deformation promotes the formation of a thin oxide layer, whereas a deformation-free surface leads to formation of thick duplex oxide layers. Furthermore, the presence of water vapor in the mixed gas environment accelerated the growth of the oxides.

Highly efficient iodine capture by hydrophobic bismuth-based chrysotile membrane from humid gas streams

Sequential adsorption of water and organic vapor mixtures onto single-walled carbon nanotube (SWNT) bundles is studied experimentally and by grand canonical Monte Carlo (GCMC) simulation to elucidate the distinct interactions between select adsorbates and the nanoporous structure of SWNTs. Experimental adsorption isotherms on SWNT bundles for hexane, methyl ethyl ketone, cyclohexane, and toluene individually mixed in carrier gases that were nearly saturated with water vapor are compared with the GCMC-simulated isotherms for hexane, as a representative organic, on the external surface of the heterogeneous SWNT bundles. From the nearly perfect overlap between the experimental and simulated isotherms, it is concluded that until near saturation only the internal pore volume of pristine SWNT bundles fills with water. The adsorption of water vapor on the peripheral surface of the bundles remains insignificant, if not negligible, in comparison to the adsorption of water in the internal volume of the bundles. This is in contrast with the adsorption of pure hexane, which exhibits appreciable adsorption both inside the bundles and on their external surface. It is also suggested that during competitive adsorption, water molecules take precedence over small nonpolar and polar organic molecules for adsorption inside SWNTs and leave unoccupied the hydrophobic external surface of the bundles for other more compatible adsorbates.

Study on CO2 capture in humid flue gas using amine-modified ZIF-8

Porous electrolyte NOx sensors promise greater NOx sensitivity in comparison to conventional dense electrolyte NOx sensors, and they are capable of detecting single digit ppm levels of NOx. This is important for monitoring and regulating diesel engine operation as diesel engine technology is resulting in substantially lower emissions. However, cross-sensitivity to H2O limits the feasibility of porous electrolyte NOx sensors and further investigation is needed to understand factors affecting sensor selectivity. It has been suggested in other studies that the electrolyte material may influence water behavior. Therefore, this study examined H2O cross-sensitivity in sensors composed of partially-stabilized zirconia (PSZ) and fully-stabilized zirconia (FSZ) porous electrolyte microstructures accompanied by Au wire electrodes. The data, based on impedancemetric sensor operation, indicated significantly lower H2O cross-sensitivity in PSZ electrolyte based sensors, in comparison to sensors containing FSZ. Standard ceramic process methods were used to fabricate NOx sensors containing 4.7 mol% Y2O3 – ZrO2 (PSZ, Advanced Ceramics) and 8 mol% Y2O3 – ZrO2 (FSZ, Tosoh Corp.). Polyvinyl butyral (B-76 Butvar) and ethanol were added to the electrolyte samples to form a mixture that was ball milled into a uniform slurry of which a portion was air dried to form a dry powder. The powder was uniaxially pressed under a load of 200 MPa to form pellets. Au wires were positioned over the electrolyte pellets and the remaining electrolyte slurry was coated over the electrodes. The PSZ and FSZ based sensors were fired at 1050˚C for 1 hour. Impedance studies were conducted using a Gamry Reference 600 for sensor operation over a temperature range of 600-700°C where NO and NO2 concentrations varied from 0 to 100 ppm for dry and humidified (3, 5, and 10% H2O) gas environments. Based on analysis of the impedance data the sensitivity, selectivity, oxygen dependence, activation energy, and response rate of the sensors was determined. The microstructure and morphology of the electrolytes were studied using SEM, BET, and MIP. The impedance response indicated the PSZ based sensors demonstrated less cross-sensitivity to H2O in comparison to the FSZ based sensors. This was observed from the impedance data where the interfacial resistance decreased by 12% for FSZ based sensors when 10% water was added to the gas stream, whereas, a slight increase in interfacial resistance of approximately 2% occurred for PSZ based sensors under the same conditions. The addition of water likely resulted in hydroxyl groups that effected the interfacial conductivity of the sensors. It is possible that reactions involving hydroxyl groups were greater for the FSZ based sensors such that an increase in interfacial conductivity reduced the resistance at the Au/FSZ interface. The formation of hydroxyl groups can result from oxygen ions traveling through the electrolyte. As the oxygen ion conductivity is lower for PSZ in comparison to FSZ, it is possible that the difference in the water behavior for the PSZ versus the FSZ based sensors was due to a lower rate of hydroxyl formation at the PSZ/Au interface. The impedance data also indicated the NOx sensitivity was greater for FSZ versus PSZ based sensors for both dry and humidified gas conditions; and the FSZ based sensors had a more rapid response. The activation energy of the sensors was approximately 1.0 and 1.2 eV for the PSZ and FSZ based sensors, respectively. The activation energy decreased slightly when NOx was present in the gas stream, and when water was added. Oxygen partial pressure measurements indicated dissociative adsorption was the dominant rate limiting for both dry and humidified gas environments for both PSZ and FSZ based sensors. The effect of electrolyte particle size on water behavior was considered by studying the behavior of sensors composed of PSZ electrolytes containing 1 micron and 2 micron sized particles. The magnitude of the impedance response was larger for the sensors containing the 2 micron sized particles, however, the water behavior was similar for by types of PSZ based sensors. A slight increase in the impedance was observed when water was added to the gas stream for sensors containing PSZ irrespective of the electrolyte particle size. Overall, the morphology and microstructure of the sensors did not appear to influence the behavior of the sensors in the presence of water. Rather, it is likely that the formation of hydroxyl groups and related reactions contributed to cross-sensitivity that was primarily observed for the FSZ based sensors. It is possible that the limitation of hydroxyl group reactions at PSZ based sensors restricted water cross-sensitivity, thereby improving the selectivity of the sensors to NOx.

Evaluation of Microporous Biochars Produced by Single-step Oxidation for Postcombustion CO2 Capture under Humid Conditions

Selective water vapor permeation from steam/non-condensable gas mixtures via organosilica membranes at moderate-to-high temperatures