Acid gas adsorption on zeolite SSZ-13: Equilibrium and dynamic behavior for natural gas applications

In developing new adsorption separation processes, it is necessary to study both the equilibrium and dynamic adsorption properties of potential materials. Experimental determination of isotherms and dynamic breakthrough properties aid in the development of modeling new adsorption systems toward process development. Here, the equilibrium adsorption properties of a small-pore zeolite, Na-SSZ-13, are studied for its natural gas separation potential. Using volumetric, gravimetric, and dynamic column breakthrough adsorption techniques, the adsorption properties of CO₂, CH₄, C₂H₆, H₂S, and H₂O are determined. High-pressure breakthrough experiments demonstrate the mixed gas separation performance of Na-SSZ-13 in mixtures containing CO₂, CH₄, C₂H₆, and H₂S. Simulations of these breakthrough experiments show that ideal adsorbed solution theory adequately describes the mixed gas adsorption modeling for this zeolite. In gas mixtures containing both CO₂ and H₂S, there is an observed acid gas reaction that results in elution of carbonyl sulfide, COS.     

Microbial treatment of sour gases for the removal and oxidation of hydrogen sulphide

It has been demonstrated that the bacterium Thiobacillus denitrificans may be readily cultured aerobically and anaerobically in batch and continuous cultures on hydrogen sulphide (H₂S) gas under sulphide-limiting conditions. Under these conditions sulphide concentrations in the culture medium were less than 1 μM resulting in very low concentrations of H₂S in the reactor outlet gas. The stoichiometries of both the aerobic and anaerobic reactions were determined and stable reactor operation demonstrated for up to five months. Maximum loading of the biomass was determined to be 5.4–7.6 mmoles H₂S/h-g biomass under anaerobic conditions and 15.1–20.9 mmoles H₂S/h-g biomass under aerobic conditions. Indicators of reactor upset were determined and recovery from upset demonstrated. Heterotrophic contamination was shown to have negligible effect on reactor performance with respect to hydrogen sulphide oxidation. In fact, growth of T. denitrificans in the presence of floc-forming heterotrophs produced a hydrogen sulphide active floc with excellent settling characteristics. A case study of the application of this technology to the removal and oxidation of H₂S from biogas is presented.     

Mesoporous-molecular-sieve-supported Polymer Sorbents for Removing H2S from Hydrogen Gas Streams

A series of sorbents with a linear polyethylenimine (PEI) supported on the mesoporous molecular sieves, including MCM-41, MCM-48 and SBA-15, have been prepared and used to remove H₂S from a model gas containing 0.40 v% of H₂S and 20 v% H₂ in N₂ gas. The sorption was conducted in a fixed-bed system at a temperature range of 22–75 °C, a GHSV range of 337–1,011 h^?1 and atmospheric pressure. The effects of the operating temperature, GHSV, the amount of PEI loading and the different molecular sieve supports were studied. A reduction in the temperature and GHSV improves the sorption performance of the supported PEI sorbents. A synergetic effect of the SBA?15 support and PEI on the H₂S sorption performance was observed. Loading 50 wt% PEI on SBA-15 gave the best breakthrough capacity, while loading 65 wt% PEI on SBA-15 had the highest saturation capacity. The mesoporous molecular sieve with large pore size and three-dimensional channel structure favors increasing the kinetic capacity of the supported PEI sorbent. In addition, the developed sorbents can be regenerated easily at mild conditions (temperature range of 75–100 °C) and have excellent regenerability and stability. The results indicate that the mesoporous-molecular-sieve-supported polymer sorbents are promising for removing H₂S from hydrogen gas streams.     

Breakthrough behaviour of a binary gas mixture with azeotropic adsorption equilibrium

This contribution reports on the breakthrough behaviour of binary gas mixtures with ideal and non-ideal multicomponent adsorption equilibria. Investigations were carried out on mixtures CO₂/C₂H₄ and C₂H₄/C₂H₆, both adsorbed on molecular sieve 5A (ms5A). The adsorption equilibrium of the system CO₂/C₂H₄/ms5A may exhibit azeotropic behaviour, which subsides with decreasing active pressure (= sum of partial pressures of adsorbable components) or on raising the temperature. In contrast, the system C₂H₄/C₂H₆/ms5A maintains its ideal behaviour also at higher active pressures or lower temperatures. Attempts to calculate the non-ideal adsorption equilibrium from measured single component isotherms have failed when known models were applied. The investigation of the effect of azeotropic equilibrium on the fixed bed adsorption led to intersecting breakthrough curves of the two components. This behaviour is due to a displacement of equilibrium caused by the change in the active pressure and partial pressures, and a superposed temperature effect. This can be shown by calculating the breakthrough curves with the equilibrium model.     

The performance of molecular sieve adsorption columns: systems with macropore diffusion control

Generalized saturation (adsorption) and regeneration (desorption) breakthrough curves are presented for molecular sieve adsorption columns operated isothermally under conditions of macropore diffusion control. The validity of the theory is confirmed by comparison of the theoretical and experimental breakthrough curves for sorption of C₃H₈ and 1-C₄H₈ in Davison 5 Å molecular sieve. For these systems kinetic and equilibrium data are available from gravimetric measurements and the theory is shown to provide a satisfactory a priori prediction of column performance.     

Adsorption of Off‐Gases from Steam Methane Reforming (H2, CO2, CH4, CO and N2) on Activated Carbon

Abstract

Hydrogen is the energy carrier of the future and could be employed in stationary sources for energy production. Commercial sources of hydrogen are actually operating employing the steam reforming of hydrocarbons, normally methane. Separation of hydrogen from other gases is performed by Pressure Swing Adsorption (PSA) units where recovery of high‐purity hydrogen does not exceed 80%.

In this work we report adsorption equilibrium and kinetics of five pure gases present in off‐gases from steam reforming of methane for hydrogen production (H₂, CO₂, CH₄, CO and N₂). Adsorption equilibrium data were collected in activated carbon at 303, 323, and 343 K between 0‐22 bar and was fitted to a Virial isotherm model. Carbon dioxide is the most adsorbed gas followed by methane, carbon monoxide, nitrogen, and hydrogen. This adsorbent is suitable for selective removal of CO₂ and CH₄. Diffusion of all the gases studied was controlled by micropore resistances. Binary (H₂‐CO₂) and ternary (H₂‐CO₂‐CH₄) breakthrough curves are also reported to describe the behavior of the mixtures in a fixed‐bed column. With the data reported it is possible to completely design a PSA unit for hydrogen purification from steam reforming natural gas in a wide range of pressures.     

Thermodynamic analysis of the thermochemical decomposition of H2S in the presence of iron sulphide

The decomposition of H₂S by a two-step closed loop thermochemical decomposition process was investigated. In the first step, H₂S decomposes by reacting with iron sulphide to produce hydrogen and a solid solution consisting of FeS and S₂. In the second step, the solid solution is decomposed at high temperature to recover sulphur and the iron sulphide which is reused in the first step. The thermodynamics of the first reaction were studied in detail by taking into account 12 gas phase chemical reactions and the phase equilibrium that is established between the sulphur in the gas and in the solid solution. The non-linear system of 13 equations, which relate the fugacities of the components at equilibrium to the equilibrium constants, were numerically solved by the Newton-Raphson iteration technique. The calculation results showed that the gas phase at equilibrium contains predominantly H₂S, H₂, and H₂S₂. The equilibrium conversion was found to decrease with increasing temperature; however, when the sulphur content of the solid solution was small, high hydrogen yields were possible even at high temperatures.     

Breakthrough Analysis for Adsorption of Phosphine on 5A Molecular Sieve

Adsorption experiments were carried out under dynamic conditions for the removal of phosphine (PH₃) in nitrogen on a fixed bed of 5A molecular sieve. The experiments were conducted to characterize the breakthrough characteristics of PH₃ in the fixed bed under different operating conditions, including bed temperature, bed mass, and initial PH₃ concentration. A mathematical model was developed to predict the breakthrough profiles of PH₃ during adsorption on the 5A molecular sieve. The model was successfully validated with the observed experimental breakthrough data, showing that the adsorption capacity increases with decreasing temperature and with rising phosphine concentration in the gas mixture. The study showed the potential application of 5A molecular sieve in controlling PH₃ emission.     

Mixed-amine Modified SBA-15 as Novel Adsorbent of CO2 Separation for Biogas Upgrading

A novel adsorbent of CO₂ from biogas was prepared by synthesizing and modifying the mesoporous molecular silica of SBA-15 with methyl-diethyl-amine (MDEA) and piperazine (PZ). The adsorbent showed good performance in separating CO₂ from biogas. The loaded amines did not change the ordered structure of SBA-15, but enhanced its adsorption of CO₂. The adsorbents were characterized by X-ray powder diffraction (XRD) and N₂ adsorption/desorption. With the increase in MDEA loading, the surface area, pore size, and pore volume of the MDEA-loaded SBA-15 decreased. The modification of amines enlarged the difference between the equilibrium adsorption of CO₂ and CH₄. Quantitatively evaluated on the basis of the breakthrough curves, the separation factors between CO₂ and CH₄, was increased more than seven fold due to the MDEA modification. With mixed-amine (MDEA + PZ) modification, the separation factors between CO₂ and CH₄ was further improved. In addition, not only the adsorbent was regenerable by purging with the purified gas, but also the adsorption performance is stable in adsorption cycles. Effect of moisture on adsorption of CO₂ is investigated and the results show the increase in the adsorption performance.     

Kinetic and thermodynamic analysis of the fate of sulphur compounds in gasification products

Synthesis gas produced from gasification of coal contains sulphur compounds that need to be removed before the gas can be further processed. For the sulphur removal process, it is normally assumed that all sulphur is present as hydrogen sulphide (H₂S) with minor amounts of carbonyl sulphide (OCS). This paper examines the equilibrium composition of a raw synthesis gas from a Texaco gasifier, and how the composition changes kinetically as the gas is cooled. It is confirmed that H₂S should always be the dominant sulphur species in cold syngas, with about 5% of sulphur appearing as OCS. Higher temperatures, and very high cooling rates can cause significant conversion of the sulphur to elemental S₂.     

Adsorption of carbon dioxide and water vapor on fly-ash based ETS-10

CO₂ capture from humid flue gas is always costly due to the irreplaceable pretreatment of dehydration in current processes, which creates an urgent demand for moisture-insensitive adsorbents with considerable CO₂ uptakes as well as remarkable H₂O tolerances. In the present work, the microporous titanium silicate molecular sieve ETS-10 was synthesized with coal fly ash as the only silica source. The as-synthesized ETS-10 was characterized by X-ray diffraction, scanning electronic microscopy and infrared spectroscopy to verify its crystal morphology, in which neither impurity nor aggregation was observed. The following CO₂ adsorption experiments on the thermal gravimetric analyzer demonstrated its similar CO₂ adsorption capacity yet dramatical adsorption kinetics among some other microporous materials, e.g., potassium chabazite. These specific properties consequently guaranteed its favorable CO₂ adsorption capacity even at high temperatures (1.35 mmol/g at 393 K) and shortened the breakthrough time of single CO₂ flow to less than 20 s. In CO₂/H₂O binary breakthrough experiments, the as-obtained ETS-10 still maintained excellent CO₂ uptake of 0.81 mmol/g at 323 K, regardless of the presence of water vapor, making it a promising substitute for direct CO₂ separation from humid flue gases at practical conditions of post-combustion adsorption.     

Single and multicomponent adsorption equilibria of carbon dioxide,nitrogen, carbon monoxide and methane in hydrogen purification processes

Single, binary, ternary and quaternary adsorption equilibria of CO₂, CO, CH₄ and N₂ on molecular sieve 5A and activated carbon were experimentally determined over a pressure range from 10^?4 to 10¹ MPa, a temperature range from 303 to 363 K and at various compositions. The adsorption equilibria of steam reformer gases as needed for the hydrogen purification in pressure swing adsorption units were measured by using a circulating volumetric method. For the temperature-dependent correlation of pure gas isotherm fields the Toth equation, which is a favorable model for heterogeneous adsorbents, was extended by two parameters accounting for the temperature-dependencies of the saturation loading and the heterogeneity parameter. Multicomponent equilibria were successfully predicted from single component isotherms by the Ideal Adsorbed Solution Theory based on the accurate representation of the pure component data by this temperature dependent Toth equation. Other thermodynamic models like the HIAS, the MIAS, the SPD or the VS theory and the Statistical Thermodynamics Model were also applied to the prediction of the adsorption equilibrium and the temperature and pressure dependence of the selectivity, with comparable success, which is due to the quasi-ideal adsorption behavior even at a high pressure.     

H2, CO2, and CH4 Adsorption Potential of Kerogen as a Function of Pressure,Temperature, and Maturity

We performed molecular dynamics simulation to elucidate the adsorption behavior of hydrogen (H₂), carbon dioxide (CO₂), and methane (CH₄) on four sub-models of type II kerogens (organic matter) of varying thermal maturities over a wide range of pressures (2.75 to 20 MPa) and temperatures (323 to 423 K). The adsorption capacity was directly correlated with pressure but indirectly correlated with temperature, regardless of the kerogen or gas type. The maximum adsorption capacity was 10.6 mmol/g for the CO₂, 7.5 mmol/g for CH₄, and 3.7 mmol/g for the H₂ in overmature kerogen at 20 MPa and 323 K. In all kerogens, adsorption followed the trend CO₂ > CH₄ > H₂ attributed to the larger molecular size of CO₂, which increased its affinity toward the kerogen. In addition, the adsorption capacity was directly associated with maturity and carbon content. This behavior can be attributed to a specific functional group, i.e., H, O, N, or S, and an increase in the effective pore volume, as both are correlated with organic matter maturity, which is directly proportional to the adsorption capacity. With the increase in carbon content from 40% to 80%, the adsorption capacity increased from 2.4 to 3.0 mmol/g for H₂, 7.7 to 9.5 mmol/g for CO₂, and 4.7 to 6.3 mmol/g for CH₄ at 15 MPa and 323 K. With the increase in micropores, the porosity increased, and thus II-D offered the maximum adsorption capacity and the minimum II-A kerogen. For example, at a fixed pressure (20 MPa) and temperature (373 K), the CO₂ adsorption capacity for type II-A kerogen was 7.3 mmol/g, while type II-D adsorbed 8.9 mmol/g at the same conditions. Kerogen porosity and the respective adsorption capacities of all gases followed the order II-D > II-C > II-B > II-A, suggesting a direct correlation between the adsorption capacity and kerogen porosity. These findings thus serve as a preliminary dataset on the gas adsorption affinity of the organic-rich shale reservoirs and have potential implications for CO₂ and H₂ storage in organic-rich formations.     

Prediction of breakthrough curves in the system of CH4, C2H6 and CO2 coadsorption on 4A molecular sieve

The coadsorption breakthrough curves of CH₄, C₂H₆ and CO₂ on 4A molecular sieve are predicted theoretically and verified experimentally in this paper. The theoretical results agree quite well with experimental results. In the prediction, only the experimental data of each single component are required, and the IAS (Ideal Adsorption Solution theory) method is used to calculate the coadsorption equilibrium. A new iterative procedure has been developed, which makes the iteration more stable.     

Removal of minor concentration of H2S on MDEA-modified SBA-15 for gas purification

A promising adsorbent for H₂S removal of minor concentration for gas purification was prepared by synthesizing and modifying the mesoporous molecular silica of SBA-15 with methyl-diethyl-amine (MDEA). Removal performance of minor concentration of H₂S on the adsorbent was experimentally studied in a dynamic setup. The adsorbents showed good performance in removing H₂S from gas steams. The loaded amines did not change the ordered structure of SBA-15, but enhanced its removing H₂S. The adsorbents were characterized by X-ray powder diffraction (XRD) and N₂ adsorption/desorption. Effect of the R (the loading ratio) of MDEA, effect of initial H₂S concentration and effect of moisture on removal performance of H₂S were studied respectively. With increase of the R of MDEA, the H₂S removal performance of the adsorbent was improved obviously. When R of MDEA was 0.6, the removal performance was optimum. Initial H₂S concentration had a large effect on the removal performance. Both breakthrough capacity and saturation capacity increased as the initial H₂S concentration increased. In the presence of moisture, the experimental results showed the improvement in the removal performance of H₂S. In addition, not only was the adsorbent regenerable by purging with the purified gas, but also the removal performance was stable in removal adsorption cycles.     

Precipitation of metal sulphides using gaseous hydrogen sulphide: mathematical modelling

A mathematical model has been developed that describes the precipitation of metal sulffides in an aqueous solution containing two different heavy metal ions. The solution is assumed to consist of a well-mixed bulk and a boundary layer that is contacted with hydrogen sulphide gas. The model makes use of Higbie's penetration model to calculate the transfer of gaseous hydrogen sulphide to this boundary layer. The conditions that have been used in the simulations resemble those of industrial wastewater from a zinc factory. The model predicts the rate of H₂S absorption, the size distribution of the metal sulphide crystals and the selectivity of precipitation. Higher precipitation rates are predicted at higher pH values and higher H₂S concentrations. In all cases considered, the rate of precipitation is fully controlled by mass transfer of H₂S, higher H₂S concentrations and higher specific surface areas yielding higher precipitation rates. The size of the obtained crystals is predicted to increase with H₂S concentration, but to decrease with specific surface area and liquid side mass transfer. These results illustrate the importance of reactor layout and operating conditions on the process of gas-liquid precipitation.     

Efficient Cycle Recovery of Hydrogen from a Low Concentration Pyrolysis Gas Stream by Pressure Swing Adsorption – An Experimental Evaluation

Breakthrough curves, cycle mass balances, and cycle bed productivities (mg H₂ per gram of adsorbent) on three dual adsorbent amounts (g) of 2,892, 1,963, and 1,013 respectively each filling 200 cm, 135 cm, and 70 cm of a 5.0 cm internal diameter stainless steel pipe were performed. The approximate optimum (sludge pyrolysis) synthesis gas with composition in volume % of 45% H₂/35% CO/20% CH₄ was used as the feed gas with molecular sieve 5 Å and activated carbon as adsorbents. Impurity breakthroughs occurred at ～14.9, 12.3, and 5.0 minutes respectively for % cycle recoveries of 72.2, 65.0, and 60.2 using 2,892, 1,962, and 1,013 g of adsorbent respectively. Our results indicated that basing % recycle recovery on cycle bed productivity can enable efficient hydrogen recovery with savings on adsorbent amount. An optimum cycle bed productivity of 2.3 mg H₂/g of adsorbent corresponded to a cycle recovery of 66.2% for 2,300 g of adsorbent used. Only 1.7 mg H₂/g of adsorbent was obtained for a cycle recovery of 72.2% requiring up to 2,800 g of adsorbent. This makes economic sense in the pressure swing adsorption separation of hydrogen from traditionally low hydrogen concentration biomass sources.     

On adsorption and electro-oxidation of some compounds on tungsten carbide; their effect on hydrogen electro-oxidation

Adsorption and electro-oxidation of carbon monoxide, ethylene, acetylene, and hydrogen sulphide on tungsten carbide, in solutions of these compounds in 1 N H₂SO₄, have been investigated. It was found that CO, C₂H₄, and C₂H₂ do not undergo adsorption and oxidation and do not affect adsorption and electro-oxidation of hydrogen. H₂S does not oxidise as well, and it does not displace adsorbed hydrogen in any measurable amounts, though it does inhibit electro-oxidation of molecular hydrogen. Methanol is inert on tungsten carbide like carbon monoxide and hydrocarbons. Electro-oxidation of formaldehyde and formic acid proceeds without apparent WC-surface coverage by the adsorbed compound.     

Selective nonanal molecular recognition with SnO2 nanosheets for lung cancer sensor

SnO₂ nanosheets were developed to detect nonanal gas in the order of ppb which was a marker of lung cancer. The nanosheets showed higher resistance change in nonanal gas than that in carbon monoxide (CO), nitrogen dioxide (NO₂), acetone (CH₃COCH₃), hydrogen (H₂), ethanol (C₂H₆O), ammonia (NH₃), hydrogen sulfide (H₂S), formaldehyde (HCHO), acetaldehyde (CH₃CHO), or butanal (C₄H₈O). Crystal surfaces of the nanosheets would be effective for adsorption of nonanal molecules. Furthermore, it was shown that resistance changed with an increase in carbon number in aldehyde. The nanosheets had molecular selectivity for a series of aldehyde molecules. Molecular recognition of the nanosheets gave us a great advantage to detect nonanal gas which was produced by lung cancer.     

Adsorption of hydrogen sulphide (H2S) by activated carbons derived from oil-palm shell

Adsorption of hydrogen sulphide (H₂S) onto activated carbons derived from oil palm shell, an abundant solid waste from palm oil processing mills, by thermal or chemical activation method was investigated in this paper. Dynamic adsorption in a fixed bed configuration showed that the palm-shell activated carbons prepared by chemical activation (KOH or H₂SO₄ impregnation) performed better than the palm-shell activated carbon by thermal activation and a coconut-shell-based commercial activated carbon. Static equilibrium adsorption studies confirmed this experimental result. An intra-particle Knudsen diffusion model based on a Freundlich isotherm was developed for predicting the amount of H₂S adsorbed. Desorption tests at the same temperature as adsorption (298 K) and at an elevated temperature (473 K) were carried out to confirm the occurrence of chemisorption and oxidation of H₂S on the activated carbon. Surface chemistries of the palm-shell activated carbons were characterized by Fourier transform infrared spectroscopy and Boehm titration. It was found that uptaking H₂S onto the palm-shell activated carbons was due to different mechanisms, e.g. physisorption, chemisorption and/or H₂S oxidation, depending on the activation agent and activation method.