H2S Removal Research Articles

Ammonia-induced CuO/13X for H2S removal from simulated blast furnace gas at low temperature

Blast furnace gas (BFG) is an important by-product energy for the iron and steel industry and has been widely used for heating or electricity generation. However, the undesirable contaminants in BFG (especially H2S) generate harmful environmental emissions. The desulfurization of BFG is urgent for integrated steel plants due to the stringent ultra-low emission standards. Compared with other desulfurization materials, zeolite-based adsorbents represent a viable option with low costs and long service life. In this study, an ammonia-induced CuO modified 13X adsorbent (NH3–CuO/13X) was prepared for H2S removal from simulated BFG at low temperature. The XRD, H2-TPR and TEM analysis proved that smaller CuO particles were formed and the dispersion of Cu on the surface of 13X zeolite was improved via the induction of ammonia. Evaluation on H2S adsorption performance of the adsorbent was carried out using simulated BFG, and the results showed that NH3–CuO/13X-3 has better breakthrough sulfur capacity, which was more than twice the sulfur capacity of CuO/13X. It is proposed that the enhanced desulfurization performance of NH3–CuO/13X is attributed to an abundant pore of 13X, and combined action of 13X and CuO. This work provided an effective way to improve the sulfur capacity of zeolite-based adsorbents via impregnation method by ammonia induction.

A comparison between Claus and THIOPAQ sulfur recovery techniques in natural gas plants

A sulfur recovery process is one of the most important processes in the oil and gas industry to get rid of hydrogen sulfide (H2S) which is produced from the acid gas removal process of sour natural gas to convert it into sweet natural gas. Actual data from a gas field is used to obtain a realistic comparison between two sulfur recovery techniques, through which researchers and/or manufacturers can obtain information that will help them choose the most appropriate and cheapest method. A total feed acid gas flow rate of 5.1844 MMSCFD with an H2S concentration of 24.62% by mole percent was produced from amine acid gas removal units. Claus sulfur recovery technique is a traditional chemical process that uses thermal and catalytic reactors. Therefore, an acid gas enrichment unit is applied to increase the H2S concentration to approximately 50% mole to provide reliable and flexible operation in the thermal and catalytic reactors. Moreover, a tail gas treatment unit is applied to increase the overall conversion efficiency to 99.90% with the Claus technique instead of 95.08% without it to achieve high sulfur recovery and reliable operation through the conversion of carbonyl sulfide (COS) and mercaptans. Studies on the safety and simplicity of the Claus technique revealed many important hazards and a large number of transmitters (379) and control loops (128) in one Claus train. THIOPAQ sulfur recovery as a new technology is a biological desulfurization process that uses a natural mixture of sulfide-oxidizing bacteria. It is also a unique H2S removal process with an efficiency of 99.999%. In addition, studies on the safety and simplicity of the THIOPAQ technique have shown that the hazards, the number of transmitters (74), and the number of control loops (29) of a one THIOPAQ train are lower. The THIOPAQ technique showed higher efficiency, was safer, simpler, and had lower CAPEX and OPEX. This study was conducted using Aspen HYSYS V11 and actual data.

H2S removal at downhole conditions using iron oxide nanoparticles

The objective of the present work is the study of H2S removal from heavy oil, using iron oxide nanoparticles in a controlled environment that simulates the pressure and temperature conditions of a reservoir and the aqua-thermolysis process during enhanced oil recovery with steam injection. Since molecular diffusion of H2S plays an important role during the removal process, its measurement through experimental tests was also a major goal. The research divides into three stages: 1) preparation of nanoparticles; 2) diffusion tests, and, 3) H2S removal tests. The procedure for nanoparticle preparation from a microemulsion and a metal precursor salt was successful in yielding nanoparticle sizes less than 100 nm. The diffusion coefficient of H2S in heavy oil, measured in a stainless steel PVT cell, varied between 8.3 × 10–9 and 8.9 × 10–9 m2s–1 over the range of test temperatures. Finally, over 65% of the H2S was removed when 500 ppm of nanoparticles were used.

Preparation of nitrogen-doped activated carbon used for catalytic oxidation removal of H2S

In this study, nitrogen-doped modified activated carbons were synthesized for H2S removal from Zhuxi activated carbon and 4,4′-bipyridine as raw material and nitrogen source, respectively. The synthesis strategy was hydrothermal treatment and subsequent NH3 annealing, and the formation and conversion patterns of the different N configurations were investigated. When the annealing temperatures were 500 °C and 600 °C, N-5 account for the majority. As the annealing temperature increased, the proportion of N-6 gradually increased. After the temperature increased to 1000 °C, N-5 and N-6 were converted to N-Q to a certain degree, while the amount of nitrogen doping decreased significantly. The sample H160-0.2-800 exhibited excellent H2S removal with a high sulfur capacity of up to 206.89 mg/g, significantly higher than that of the original activated carbon ZX1200 (67.56 mg/g). The reason for this is that the micropores (Vmic = 0.5155 cm3/g) and specific surface area (SBET = 1369.5 m2/g) of the modified activated carbon are more developed than those of the original activated carbon. A high nitrogen content (3.14 wt%) and N-6 configuration proportion (73.56 %) are significant reasons for the excellent adsorption properties. The mechanism of the catalytic oxidation was investigated. The introduction of surface nitrogen-containing functional groups alkalizes the activated carbon surface, enhancing the adsorption and dissociation of H2S and O2 and facilitating the formation of sulfur radicals and elemental sulfur.

Importance of digester’s headspace geometry for the efficient H2S removal through microaeration; experimental and simulation study

Microaeration for biogas desulfurization occurs mainly in the headspace of anaerobic digesters where the biofilm growth is mainly reported; however, no study has been focused on the effect of headspace geometry on microaeration performance. This research evaluates the effect of headspace geometry on biogas flow, subsequent biofilm growth, and the effect of these parameters on microaeration efficiency. This effect was studied in an anaerobic digester with three different headspace configurations. Also, the computational fluid dynamics (CFD) model explained the experimental results. The CFD model developed described the biogas flow in all headspace configurations. It was proven that the modifications of headspace geometry altered biogas flow and avoided biofilm growth on the reactor walls leading to the reactor's protection from the adverse effects of biofilm growth. The mathematical model developed in this work can help design microaerobic digesters or find the best way to implement microaeration into the digesters already built.

In-situ regenerable Cu/Zeolite adsorbent with excellent H2S adsorption capacity for blast furnace gas

Desulfurization of blast furnace gas (BFG) is essential for steel companies to achieve its cleaner utilization. In this study, an in-situ regeneratable H2S adsorbent with a high sulfur capacity was developed. Zeolites with specific silica-aluminum ratios were selected, and hybrid Cu/zeolites adsorbents were synthesized using an impregnation method. The optimal adsorbent (DSZ-2) exhibited a high breakthrough sulfur capacity of 39.0 mg/g at 80 °C. Moreover, this adsorbent can also be in-situ regenerated at lower temperatures (280 ℃) and the regenerated adsorbent has even a higher breakthrough sulfur capacity of 49.0 mg/g. The possible desulfurization and regeneration mechanism of the DSZ-2 absorbent are discussed. Due to its high sulfur capacity and good regenerability, this absorbent has great application prospect in the removal of H2S from BFG.

Biodesulfurization of landfill biogas by a pilot-scale bioscrubber: Operational limits and microbial analysis.

Biogas serves as a crucial renewable energy vector to ensure a more sustainable energy future. However, the presence of hydrogen sulfide (H2S) limits its application in various sectors, emphasizing the importance of effective H2S removal techniques for maximizing its potential. In the present study, the limits of a pilot-scale bioscrubber for biogas desulfurization was study in a real scenario. An increase in the superficial liquid velocity resulted in significant improvements in the H2S removal efficiency, increasing from 76 ± 8% (elimination capacity of 6.2 ± 0.5 gS-H2S m−3 h−1) to 97.7 ± 0.5% (elimination capacity of 8 ± 1 gS-H2S m−3 h−1) as the superficial liquid velocity increased from 50 ± 3 m h−1 to 200 ± 8 m h−1. A USL of 161.4 ± 0.5 m h−1 was able to achieve outlet H2S concentrations as low as 3 ± 1 ppmv (H2S removal efficiency of 97 ± 1%) for 7 days. High superficial liquid velocity favoured the aerobic H2S oxidation reducing the nitrate demand. The maximum EC reached throughout the operation was 50.8 ± 0.6 gS-H2S m−3 h−1 (H2S removal efficiency of 96 ± 1%) and a sulfur production of 60%. Studies in batch flocculation experiments showed sulfur removal rates up to 97.6 ± 0.9% with a cationic flocculant dose of 75 mg L−1. Microbial analysis revealed that the predominant genus with sulfo-oxidant capacity during periods of low H2S inlet load was Thioalkalispira-sulfurivermis (61–69%), while in periods of higher H2S inlet load, family Arcobacteraceae was the most prevalent (11%).

Selenium‐Based Catalytic Scavengers for Concurrent Scavenging of H2S and Reactive Oxygen Species

AbstractHydrogen sulfide (H2S) is an endogenous gasotransmitter that plays important roles in redox signaling. H2S overproduction has been linked to a variety of disease states and therefore, H2S‐depleting agents, such as scavengers, are needed to understand the significance of H2S‐based therapy. It is known that elevated H2S can induce oxidative stress with elevated reactive oxygen species (ROS) formation, such as in H2S acute intoxication. We explored the possibility of developing catalytic scavengers to simultaneously remove H2S and ROS. Herein, we studied a series of selenium‐based molecules as catalytic H2S/H2O2 scavengers. Inspired by the high reactivity of selenoxide compounds towards H2S, 14 diselenide/monoselenide compounds were tested. Several promising candidates such as S6 were identified. Their activities in buffers, as well as in plasma‐ and cell lysate‐containing solutions were evaluated. We also studied the reaction mechanism of this scavenging process. Finally, the combination of the diselenide catalyst and photosensitizers was used to achieve light‐induced H2S removal. These Se‐based scavengers can be useful tools for understanding H2S/ROS regulations.

Remoção de H2S de biogas por adsorção em carvão ativado comercial e oxido de ferro granular

ABSTRACT Among the impurities that are harmful to the energy conversion of biogas generated in landfills is hydrogen sulfide (H2S). The present study compared the efficiency of H2S removal with the use of granular activated carbon (GAC) of two commercial brands, Norit GAC and Pelegrini GAC, and with the use of granular iron oxide from mineral residues. The tests were conducted on a laboratory scale, in a glass column, filled with filtering material, applying an inlet flow of gas and monitoring the outlet concentration. The best removal efficiencies occurred with the use of Norit GAC, which presented longer rupture times. Wet gas tests were also carried out, in which rupture and equilibrium times increased with the use of Norit GAC, while with the use of Pelegrini GAC the same adsorptive behavior was not observed. Tests with iron oxide showed low potential for H2S removal due to reduced specific surface values, although it provided removal of the contaminant. Knowledge of the saturation time and adsorption equilibrium data is essential for a better understanding and planning of adsorption systems, in addition to the conduction of studies on the influence of external factors and variations in initial concentration.

Hydrogen Sulfide Adsorption from Natural Gas Using Silver-Modified 13X Molecular Sieve.

The removal of hydrogen sulfide from natural gas and other gases such as biogas, refinery gases, and coal gas is required because it is toxic and corrosive, even in traces. Zeolites are widely used in the removal of H2S from the abovementioned gases. In this work, we prepared an Ag-exchanged 13X molecular sieve by using different concentrations of AgNO3 to increase its adsorption properties. XRD, SEM, and BET techniques were used to characterize samples. To determine the adsorption properties of each of the samples, a laboratory setup with a fixed-bed adsorber was utilized. The adsorption capacity of modified 13X increased when the molar concentration of AgNO3 increased from 0.02 M to 0.05 M. However, the breakthrough time was attained quicker at a high molar concentration of 0.1 M AgNO3, indicating a low adsorption capacity. When compared to unmodified 13X, the adsorption capacity of AgII-13X increased by about 50 times. The results of this study suggest that the silver-modified 13X molecular sieve is highly effective at extracting H2S from natural gas.

The crucial role of nMOFs in H2S absorption process using ionic liquid solution based nanofluid systems

Ionic liquid solutions based of polyamines were synthesized and used for H2S removal. The results of simulation calculation and characterization showed that ionic liquid trap H2S by forming hydrogen bonds. Five nanometer-sized MOFs (nMOFs) were synthesized and added into ionic liquid solutions to build nanofluid systems. The effects of types, particle size, concentration, and regeneration gas of nanoparticles on the desulfurization performance of different nanofluids were explored. The addition of nMOFs with appropriate types and concentrations showed significant enhancement effect on the H2S absorption of liquid. Among five nMOFs, the systems of HKUST-1 and MIL-101(Fe) had high enhancement as well as regeneration performance at low concentrations (0.05 %). Based on sweep effect, gas–liquid mass transfer was enhanced by nMOFs due to the quickly capture and release of H2S. Furthermore, in this work, an “enrichment effect” has been proposed, in which H2S molecular enrichment zones were formed around nanoparticles, promoting gas–liquid reactions. The nanoparticles at different stages were characterized using X-ray photoelectron spectroscopy, scanning electron microscope, transmission electron microscope and energy dispersive spectrum, and the results confirmed that nMOFs have the ability to enrich H2S, which play a catalyst-like and accelerator-like role in H2S absorption process.

A novel machine learning framework for predicting biogas desulfurization breakthrough curves in a fixed bed adsorption column

The corrosive and toxic hydrogen sulfide (H2S) in biogas is a pressing concern for researchers, spurring the development of predictive models for breakthrough curves in fixed-bed adsorption columns. This study utilizes Machine Learning (ML) to predict biochar H2S adsorption model parameters and establishes novel connections between contributing features and these parameters. The ML models could predict the process BC with an R2 score higher than 0.8 in 75 % of cases in the testing sets. A comprehensive features importance analysis reveals that Biochar's pH is the most influential factor, positively correlating with BC parameters and enabling more efficient H2S removal followed by H2S concentration, H/C molar ratio, and particle size. Understanding these relationships offers practical guidance for improving biogas purification, making this study valuable in addressing the H2S challenges and showcasing the potential of ML in predicting BC in fixed bed adsorption columns, providing a foundation for enhancing the adsorption process.

Highly efficient capture and removal of H2S by multi-amine functionalized ionic liquids

With the increasing awareness of environmental protection, H2S removal in natural gas has attracted more attention. In this study, three multi-amino functionalized ionic liquids (MFILs) were prepared with a one-step reaction. The solubility of H2S was measured at temperatures of 298.2 K to 328.2 K. Among them, diethylenetriamine imidazole ([DETAH][Im]) presented the unprecedented H2S solubility of 7.82 mol/kg (1.34 mol/mol) at 298.2 K and 0.1 bar H2S partial pressure, surpassing all the reported absorbent under the experimental condition. The H2S removal ability was determined with the interference of CO2, and the highest H2S removal efficiency reached 99.32% during four-hour absorption. The mechanism was elucidated by NMR, FT-IR, and DFT calculation. Moreover, five consecutive absorption–desorption cycles were performed to evaluate the regeneration efficiency. These MFILs represent apparent viscosity and absorption performance superiority over previously reported absorbents. These MFILs are a class of promising absorbents for sweetening natural gas.

Simulation study of hydrogen sulfide removal in underground gas storage converted from the multilayered sour gas field

A simulation study was carried out to investigate the temporal evolution of H2S in the Huangcaoxia underground gas storage (UGS), which is converted from a depleted sulfur-containing gas field. Based on the rock and fluid properties of the Huangcaoxia gas field, a multilayered model was built. The upper layer Jia-2 contains a high concentration of H2S (27.2 g/m3), and the lower layer Jia-1 contains a low concentration of H2S (14.0 mg/m3). There is also a low-permeability interlayer between Jia-1 and Jia-2. The multi-component fluid characterizations for Jia-1 and Jia-2 were implemented separately using the Peng-Robinson equation of state in order to perform the compositional simulation. The H2S concentration gradually increased in a single cycle and peaked at the end of the production season. The peak H2S concentration in each cycle showed a decreasing trend when the recovery factor (RF) of the gas field was lower than 70%. When the RF was above 70%, the peak H2S concentration increased first and then decreased. A higher reservoir RF, a higher maximum working pressure, and a higher working gas ratio will lead to a higher H2S removal efficiency. Similar to developing multi-layered petroleum fields, the operation of multilayered gas storage can also be divided into multi-layer commingled operation and independent operation for different layers. When the two layers are combined to build the storage, the sweet gas produced from Jia-1 can spontaneously mix with the sour gas produced from Jia-2 within the wellbore, which can significantly reduce the overall H2S concentration in the wellstream. When the working gas volume is set constant, the allocation ratio between the two layers has little effect on the H2S removal. After nine cycles, the produced gas’s H2S concentration can be lowered to 20 mg/m3. Our study recommends combining the Jia-2 and Jia-1 layers to build the Huangcaoxia underground gas storage. This plan can quickly reduce the H2S concentration of the produced gas to 20 mg/m3, thus meeting the gas export standards as well as the HSE (Health, Safety, and Environment) requirements in the field. This study helps the engineers understand the H2S removal for sulfur-containing UGS as well as provides technical guidelines for converting other multilayered sour gas fields into underground storage sites.

Benign-by-design N-doped activated carbon from wasted aqueous assisted hydrochar of leftover rice for efficient H2S removal

The N-doped activated carbon is a potential material for H2S removal, reusing N-rich waste aqueous as a nitrogen source is promising to prepare in-expensive N-doped activated carbon. In this study, N-rich waste aqueous assisted hydrothermal carbonization and KOH activation were used to prepare activated carbon from starchy waste, concentrating on the effects of the nitrogen species (inorganic-N and organic-N) on the physicochemical characteristics and desulfurization performance of carbon material. Introducing inorganic-N (i.e., NH3·H2O) can increase the defect degree and enhance the formation of micropores and mesopores for N-doped activated carbon, leading to high storage ability for desulfurization products and well oxidation performance. The organic-N (i.e., alanine) was conducive to forming pyridinic-N and quinonyl for N-doped activated carbon, resulting in a strong ability of adsorption, dissolution, and oxidation for H2S. The organic-N determined the high breakthrough sulfur capacity for hydrothermal aqueous doping activated carbon (110.7 mg/g), which was ∼ 4.08 times than undoped activated carbon. The primary product of the desulfurization process was elemental sulfur (74.08%), with sulfuric acid and sulfonic acid as the subsequent by-products. After nitrogen doping, the contents of elemental sulfur and sulfuric acid increased by 5.65 and 1.73 times, respectively. The results revealed the correlations between nitrogen species in hydrothermal aqueous and the desulfurization performance of activated carbon, which could guide the selection of nitrogen-rich aqueous for synthesizing H2S removal N-doped activated carbon via the above novel method.

Hydrogen Sulfide Removal via Sorption Process on Activated Carbon-Metal Oxide Composites Derived from Different Biomass Sources.

Biomass exploitation is a global trend due to the circular economy and the environmentally friendly spirit. Numerous applications are now based on the use of biomass-derived products. Hydrogen sulfide (H2S) is a highly toxic and environmentally hazardous gas which is emitted from various processes. Thus, the efficient removal of this toxic hazardous gas following cost-effective processes is an essential requirement. In this study, we present the synthesis and characterization of biomass-derived activated carbon/zinc oxide (ZnO@AC) composites from different biomass sources as potential candidates for H2S sorption. The synthesis involved a facile method for activated carbon production via pyrolysis and chemical activation of biomass precursors (spent coffee, Aloe-Vera waste leaves, and corncob). Activated carbon production was followed by the incorporation of zinc oxide nanoparticles into the porous carbon matrix using a simple melt impregnation method. The synthesized ZnO@AC composites were characterized using X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and nitrogen porosimetry. The H2S removal performance of the ZnO@AC composites was evaluated through sorption experiments using a handmade apparatus. Our findings demonstrate that the Aloe-Vera-, spent coffee-, and corncob-derived composites exhibit superior H2S sorption capacity up to 106 mgH2S/gads., 66 mgH2S/gads., and 47 mgH2S/gads., respectively.

Experimental study and DFT calculation of the role of Cu loaded on activated carbon in desulfurization under dry and moist atmospheres

The reactive adsorption of H2S under different atmospheres is of great significance to protect the staff on site from exposure risks of polluted gases. The Cu0-loaded activated carbon (AC@Cu0) adsorbent was synthesized and characterized, which was used to dynamic adsorption of H2S under ambient temperature and different relative humidity (RH). When RH was 60%, the highest adsorption capacity reached 77.7 mg/g. The desulfurization products comprised element sulfur, sulfide and sulfate by the catalytic oxidation and reactive adsorption of AC@Cu0. In addition to the chemical nature of AC@Cu0, the structural properties (surface area and pore volume) also play a crucial role in adsorption. DFT calculations were used to reveal the effect of Cu and H2O in the desulfurization process. The adsorption energy of H2S on activated carbon was improved, and the energy barrier of the dissociation of H2S was decreased when Cu was introduced under the moist atmosphere. Density of states and the interaction region indicator (IRI) analyses indicated that the strong attractive force was formed between Cu and S. According to the results of DFT calculations, the presence of Cu and H2O could promote the adsorption of H2S. Combined with the experimental and calculation results, possible desulfurization mechanisms were proposed, which provides novel insights for the efficient removal of H2S by adsorptive desulfurization, especially under moist conditions.

Optimization of effecting parameters for flue gas desulfurization in a self‐priming venturi scrubber using response surface methodology

AbstractHydrogen sulfide (H2S), a highly poisonous and corrosive gas, is regarded as a hazardous air pollutant with significant effects on human health. Hence, reduction of hydrogen sulfide (H2S) gas from the atmosphere is very essential. In the present work, self‐priming venturi scrubber is employed to extract H2S gas from the air. Response surface methodology (RSM) with a central composite design (CCD) has been selected to investigate maximizing the H2S removal efficiency by optimizing the process variables. Experiments were carried out by altering the throat gas velocity, outer cylinder liquid level and inlet concentration of H2S. The analysis of variance (ANOVA) test showed that the parameters had a significant effect on the efficiency of H2S removal. A quadratic equation has been developed that accurately predicts the percentage of removal efficiency. The appropriateness of the generated model has been verified by the value of higher R2 resulting from the regression analysis. According to the observations, the velocity of throat gas has the highest effect on the H2S removal efficiency, whereas inlet H2S concentration has the least effect.

Visible‐Light‐Driven Gas‐Phase Hydrogen Sulfide Decomposition using Organic‐Inorganic Hybrid Photocatalysts

AbstractHydrogen sulfide (H2S) is a toxic gas generated by chemical and geothermal power plants and has an unpleasant odor. Even low concentrations of H2S can cause odor problems upon release into the atmosphere; therefore, the efficient removal of H2S is required. Photocatalytic H2S decomposition methods have been extensively investigated because they are energy‐efficient; however, most methods involve the degradation of a sacrificial reagent after alkaline adsorption and gas‐phase decomposition at relatively high concentrations of 100 ppm or more. Although H2S decomposition by TiO2 under low‐concentration conditions has been reported, H2S is decomposed by ultraviolet (UV) light irradiation because TiO2 is unresponsive to visible light. In this study, a composite catalyst is synthesized to achieve the gas‐phase decomposition of low‐concentration hydrogen sulfide under visible light. By combining carbon nitride (g‐C3N4) with a cadmium sulfide (CdS) photocatalyst, a g‐C3N4/CdS catalyst is obtained, which reduces 10 ppm H2S to less than 1 ppm. This effective and simple photocatalyst is suitable for mitigating the odor issue related to H2S.

Digital twin-based optimization and demo-scale validation of absorption columns using sodium hydroxide/water mixtures for the purification of biogas streams subject to impurity fluctuations

This paper aims to validate a demo scale plant scrubber technology through experimental campaign and development of a digital twin. Thus, it is useful to evaluate the H2S absorption process in a biogas production plant for analysis and optimization purposes. The absorber unit removes H2S through the chemical absorption via sodium hydroxide (NaOH) as wet agent (30% w/w). The column treats 300 Nm3/h of biogas, whose inlet H2S concentration ranges from 1000 to 3000 ppm. Field measurements are conducted to investigate the H2S removal efficiency. An experimental dataset is collected, processed and used as input on Aspen PLUS suite to develop the digital twin.This model is helpful to generate a large dataset and simulate operating conditions different from the demo-scale plant. The process simulation is then exploited to perform a sensitivity analysis to figure out main variables influencing the H2S removal efficiency. Operating conditions such as H2S concentration, soda concentration and flowrate, temperature, and freshwater flowrate are perturbed in the sensitivity analysis. NaOH flowrate and its concentration are the variables with the biggest impact on the process. In detail, the highest efficiency performance was obtained using 50% NaOH solution with a flowrate higher than 8 kg/h.