Natural Gas CO2 Research Articles

Supersonic separation benefiting the decarbonization of natural gas and flue gas

Supersonic separation utilizes the cryogenics created by gas expansion to purify the mixture. This method is a novel carbon capture scheme. In this work, the models of N2-CO2 (flue gas) and CH4-CO2 (natural gas) are established to calculate the condensation flow. The condensation behavior of CO2 in natural gas and flue gas is clarified respectively, and the difference between the two is compared for the first time. Under the same conditions, CO2 in flue gas reaches the limit non-equilibrium state first, forming non-equilibrium condensation with limit nucleation rate of 7.78 × 1020 m−3s−1 and maximum droplet growth rate of 8.19 × 10−3 m s−1. However, the maximum droplet growth rate in natural gas is only 2.22 × 10−3 m s−1, which implies that the interphase mass transfer rate in flue gas is larger during the CO2 condensation. In addition, the condensation behavior of CO2 in natural gas is more sensitive to the changes of inlet parameters. This work provides an invaluable contribution to the development of supersonic carbon capture.

Performance and cost potential for direct-fired supercritical CO2 natural gas power plants

Direct-fired supercritical CO2 (sCO2) power cycles are being explored as an attractive alternative to natural gas combined cycle (NGCC) plants with carbon capture and storage (CCS). Therefore, understanding their performance and cost potential is important for the commercialization of the technology. This study presents detailed techno-economic optimization results of natural gas-fired, utility-scale power plants based on the direct sCO2 power cycle, which is lacking in public literature. The study considered multiple plant configurations with varying levels of thermal integration with the plant air separation unit (ASU) to shows the systematic impact of thermal integration on plant performance and economics. Several design variables for each power cycle configuration were identified and optimized to minimize the levelized cost of electricity (LCOE). The optimized direct sCO2 power plants achieved ∼48 % plant efficiency (HHV basis) which is similar or slightly higher plant efficiencies than state-of-the-art NGCC plants based on the F-class gas turbine with carbon capture and storage (CCS). The LCOE of the optimized direct sCO2 plants is 13–17 % higher than the reference NGCC plants with CCS due to high capital costs associated with the ASU and sCO2 power block, though there is significant room for improvement due to high uncertainty in component capital costs for these new plants. Based on the results, direct sCO2 power cycles have the potential to achieve >50 % efficiency (HHV basis) with >98 % CO2 capture rate which can lead to significant efficiency improvement and CO2 emissions reductions compared to NGCC plants with CCS.

Study on separation of CO2 condensation from natural gas based on cellular automaton method

ABSTRACT The CO2-containing natural gas is increasing, and there is an urgent need for green treatment of natural gas containing CO2 to realize the utilization of natural gas. Conventional natural gas decarbonization methods have drawbacks such as non-environmental friendliness, and the low-temperature condensation separation of carbon dioxide (snowing-separation) method as a green method of carbon capture is proposed in this paper. Aiming at the characteristics of CO2 condensation in the snowing-separation process, the growth process of CO2 crystals is obtained by simulation using the CA (Cellular Automata) method and finds that the effect of supersaturation on the growth of CO2 crystals is exponential. The snowing-separation process at .5 MPa is analyzed, and it is found that the condensation range is 158.51 K–173.86 K, the upper limit of CO2 mole fraction in natural gas is 10.1%, and the minimum separation time is 8.65 × 10−8s. Analyzing the influencing factors of CO2 separation efficiency, combining the simulation results with the experimental results, the maximum relative error is 13.93%, which can better predict the snowing-separation technology of CO2 in natural gas, and then promote the development of efficient low-temperature carbon capture process.

Integrated ethane recovery and cryogenic carbon capture in a dual mixed refrigerant natural gas liquefaction process

Carbon capture by direct cryogenic distillation in conventional natural gas liquefaction plants is quite difficult due to the easy sublimation characteristics of CO2. The freeze–out temperature of CH4–CO2–C2H6 ternary mixtures is analyzed in this study, and the results show that with the azeotropic properties of ethane-CO2 mixtures, CO2 in natural gas can be reduced to below 50 ppm by cryogenic distillation. Based on thermodynamic analysis, a novel dual mixed refrigerant (DMR) liquefaction process integrated with ethane recovery and carbon capture is proposed for natural gas that is rich in ethane. CO2 is first captured from natural gas along with ethane by cryogenic distillation, and then further separated from the CO2–C2H6 mixture by extractive distillation to obtain ethane. The proposed process is simulated in HYSYS and optimized by a method combining a genetic algorithm (GA) and stepwise search. Energy analysis results show that when the ethane content is 2–20 mol%, the specific power consumption corresponding to the maximum allowable CO2 content with ethane recovery rate and purity both reaching 99.5 % is around 0.40 kWh/Nm3 (NG). In addition, the comparative analysis results show that the proposed process has better performance than chemical absorption in both energy consumption and cleaner production aspects.

Sulfone-functionalized metal–organic framework aerogels for selective CO2 capture from natural gas

The accumulation of CO2 in natural gas may reduce the heating value and even pose a risk of corrosion in transport pipelines, making the effective removal of CO2 highly essential. Metal–organic frameworks (MOFs), known for their high specific area and large porosity, show promising application in CO2 capture. However, a major obstacle is their intrinsic fragility and poor physicochemical stability. Herein, we present a sulfone-incorporated in-situ growth and fiber freeze-shaping strategy to develop lightweight, robust, and multiscale porous sulfone-functionalized UiO-67 (UiO-67-SO2) aerogels supported by aramid nanofibers. The resulting UiO-67-SO2 aerogels exhibit remarkable structural features, including a high MOF loading mass (64.39 wt%), low density (5.31 mg/cm3), high specific surface area (769.98 m2g−1), and large porosity (99.24 %). Moreover, owing to the introduction of CO2-philic sulfone groups, the UiO-67-SO2 aerogels exhibit high CO2 adsorption capacity (5.88 mmol/g), a high CO2/CH4 adsorption selectivity (43), and only a 2.77 % decline in their initial CO2 adsorption capacity after 10 cycles. The adsorption energy calculated by density functional theory method further confirms the strong interactions between UiO-67-SO2 aerogels and CO2 molecules. This work presents a promising direction for developing multiscale porous UiO-67-SO2 aerogels for the highly selective removal of CO2 from natural gas.

Tracking the origin of Ordovician natural gas in the Ordos basin using volatile sulfur compounds

Molecular and compound specific isotope analysis of volatile organic sulfur compounds (VOSC), alkanes and CO2 in natural gases from the Ordovician Majiagou Formation of the Daniudi field (Ordos basin, China) was performed. The analysis of VOSC demonstrated abundant thiols, sulfides and thiophenes in several samples while others had low concentrations of sulfides only. Molecular analysis of main alkanes and non-hydrocarbons showed the samples vary by the nitrogen and H2S concentrations. Thiols in the gas were at isotopic equilibrium with H2S bearing isotopic signature similar to that of Ordovician anhydrites within the reservoirs, hence the source of H2S was likely thermochemical sulfate reduction (TSR) of these anhydrites. In contrast, gas samples that lacked thiols also lacked H2S. Since all gases came from the same formation and similar thermal regime, the lack of thiols and H2S indicates that the occurrence of TSR in the field was limited, likely by contact between hydrocarbons to the anhydrites. The isotopic signature (δ34S) of thiophenes in the gas samples suggests the source of the gas is unlikely to be Carboniferous or Permian coals which were previously suggested as the source rocks for the gas. The δ13C and δ2H of alkanes further supports this suggestion as it demonstrates Carboniferous and Permian gases are unlikely to undergo TSR and generate the gas present in the Ordovician reservoirs. Instead, the δ13C and δ2H of alkanes hint that the gas source is shale rather than coal.This study demonstrates the applicability of compound specific sulfur isotope analysis of VOSC in identification of H2S sources, performing gas-source rock correlation and acting as a complementary proxy in case where routine analysis of hydrocarbons (δ13C and δ2H) is insufficient for identification of the gas source.

Current status and development trends of CO2 storage with enhanced natural gas recovery (CS-EGR)

CO2 storage with enhanced gas recovery (CSEGR) technology is a pivotal solution to mitigate the greenhouse effect and respond to national energy conservation and emission reduction policies. This involves injecting CO2 into gas reservoirs for storage and using it to displace gas into producing wells to enhance production. This paper provides a comprehensive analysis of the primary mechanisms of CO2 storage in gas reservoirs, quantifies the long-term contribution of these, and summarizes the methods used to evaluate CO2 storage potential. The mechanisms of the displacement of natural gas by CO2 are explained and two technical difficulties are addressed: the degree of CO2-natural gas mixing and competitive adsorption and displacement of CO2-CH4. Besides, the main factors affecting CSEGR are studied thoroughly, and the currently typical CSEGR field pilot tests across the word are considered along with CSEGR economic policy research. This paper then identifies the current challenges and future trends in CSEGR development. The results show, firstly, in the later stage of gas reservoir development, high rates of CO2 injection at the bottom of the gas reservoir and CH4 production at the top can significantly enhance gas recovery. Secondly, in previous studies, the degree of CO2-natural gas mixing has been studied more in sandstones and carbonates, while CO2-CH4 competitive adsorption has been studied more in shales and coal seams. The results of all these studies demonstrate the feasibility of CSEGR. Thirdly, Four major storage mechanisms interact to stabilize CO2 storage in gas reservoirs, but there is no systematic method to evaluate the CO2 storage potential. Finally, among recent experimental and simulation studies, CSEGR analytic techniques that consider complex formation conditions need to be further explored. Additionally, combining CSEGR with geothermal energy development in high temperature gas reservoirs presents a promising avenue for exploration.

Nucleation and condensation characteristics of carbon dioxide in natural gas: A molecular simulation perspective

The potential of supersonic separators to remove CO2 from natural gas has received extensive attention. However, microscopic understanding of the condensation mechanism of CO2 in natural gas and the applicability of classical nucleation theory (CNT) is unclear. In this study, we investigated the condensation characteristics of CO2 gas by using computational fluid dynamics (CFD) and molecular dynamics (MD) simulations with a CH4-CO2 mixture gas. The nucleation and growth pathways of CO2 are elucidated from the molecular scale. The results show that the Laval nozzle creates low-temperature conditions for CO2 liquefaction, and that reducing the inlet temperature and increasing the inlet pressure will create more favorable conditions for condensation. In the nucleation stage, CO2 gas molecules collide to form clusters, releasing large amounts of latent heat, leading to repeated condensation and evaporation of CO2 gas, and the stability of the clusters depends on the energy interactions with surrounding molecules. With higher initial pressure and lower cooling temperature, the nucleation phase duration is shortened, the nucleation rate increases, and the cluster growth is accelerated. In addition, CNT deviates from the MD simulation results by 3 ∼ 4 orders of magnitude, and reasonable corrections to the CNT will be made subsequently by combining the results of many MD simulations.

Title Supersonic Condensation and Separation Characteristics of CO2-Rich Natural Gas under Different Pressures

Supersonic separation technology is a new natural gas sweetening method for the treatment of natural gas with high CO2 (carbon dioxide) content. The structures of the Laval nozzle and the supersonic separator were designed, and the mathematical models of supersonic condensation and swirling separation for CO2-CH4 mixture gas were established. The supersonic condensation characteristics of CO2 in natural gas and the separation characteristics of condensed droplets under different inlet pressures were studied. The results show that higher inlet pressure results in a larger droplet radius and higher liquid phase mass fraction; additionally, the influence of centrifugal force is more pronounced, and the separation efficiency and removal efficiency of CO2 are higher. When the inlet pressure is 6 and 9 MPa, the liquefaction efficiency at the Laval nozzle outlet increases from 56.90% to 79.97%, and the outlet droplet radius increases from 0.39 to 0.72 μm, and the removal efficiency is 31.25% and 54.52%, respectively. The effects of inlet pressures on the removal efficiency of the supersonic separator are complicated and are controlled by the combined effects of liquefaction capacity of the nozzle and centrifugal separation capacity of the swirl vane.

Study on the mechanism of CO2 injection to improve tight sandstone gas recovery

Supercritical CO2 can improve gas recovery and achieve carbon sequestration in tight gas reservoir. It is a hot topic but not applied extensively. The main restricts include the unclear mechanism of gas-to-gas displacement and the reduction of the natural gas asset value due to CO2 breakthrough. In this paper, a novel experimental method is used, through constant-volume and constant-pressure experiments, to analyze the mixing process of CO2 and natural gas in different parts of vessel. Moreover, the mechanism and impact factors of natural gas displaced by supercritical CO2 are demonstrated in laboratory and numerical simulation. Based on the experiments, the diffusion coefficient of CO2 in natural gas (CH4) is measured and input into the numerical model. The results show that supercritical CO2 in the reservoir is ‘partially miscible’ with the natural gas. CO2 displaces CH4 to move upward due to the gravity differentiation, and CH4 is produced in the process of CO2-subsidence. CO2 flooding can enhance gas recovery by 18%, which verifies the technical feasibility of CO2 flooding in tight gas reservoirs.

A new approach of CO2 separation by applying rapid expansion of supercritical CO2 rich natural gas

The previous methods for CO2 separation from CO2 rich natural gas led to expensive production costs. This work was implemented to overcome the problems utilizing a new approach economically. The cooling and rapid expansion processes were integrated for the CO2 separation from CO2 rich natural gas on the supercritical condition. The experimental apparatus was newly constructed to perform the experiments, and the results were simulated using a various equation of state. The result reveals that the inlet temperature of supercritical expansion diminished the outlet temperature and the gas condensed easily. The simulation indicated that the 70% CO2 in natural gas was condensed easier than 45% CO2. We found that the outlet temperature of −42 °C and the vapor fraction of 0.69 was attained at the CO2 composition of 70%. Besides, the pressure drop change influences the vapor fraction at various CO2 compositions. The vapor fraction under supercritical diminished significantly compared with the non-supercritical condition. The expansion coefficient determined utilizing the equation of state escalates by the enhancement of expansion inlet temperature based on CO2 composition in natural gas. The acid gas equation of state was the perfect equation to estimate the expansion coefficient with the absolute average error of 4.83%. This work suggests that the CO2 separation from CO2 rich natural gas with the cooling and rapid expansion method promotes the new approach to overcome the disadvantages of previous methods.

China actively promotes CO2 capture, utilization and storage research to achieve carbon peak and carbon neutrality

China actively promotes CO2 capture, utilization and storage research to achieve carbon peak and carbon neutrality

Structural effects of microporous polymers on adsorption/separation of C1–C3 light hydrocarbons and CO2 in natural gas

Microporous polymers (PAN-T1 and PAN-T2) with the similar polymer skeleton but considerably different BET surface areas from 1106 to 1563 m2 g−1, micropore surface areas from 250 to 868 m2 g−1 and total pore volumes from 0.84 to 1.09 cm3 g−1 were synthesized utilizing two planar triangular-shaped hexamines to polymerize with para-phthalaldehyde, respectively. The pores in PAN-T2 are quite uniform centering at 1.50 nm, whereas PAN-T1 exhibits the hierarchical pores consisting of ultramicropore (0.48 nm), micropore (1.05 nm) and mesopore (4.71 nm). Of interest is the finding that the high micropore surface areas are more advantageous for the adsorption of C2H6 and C3H8 than CH4 and CO2. The introduction of methyl substituents brings about the increased selectivities for C3H8/CH4, C2H6/CH4 and CO2/CH4, but the selectivities of C3H8/CO2 and C2H6/CO2 exhibit an opposite trend. The effects of porosity parameters and chemical structure on the adsorption of CH4, C2H6, C3H8, CO2, and the separations of binary mixtures for C3H8/CH4, C2H6/CH4, C3H8/CO2, C2H6/CO2 and CO2/CH4 are interpreted in terms of the variations in adsorption enthalpies, Henry constants, first virial coefficients as well as the polarizabilities, critical temperatures and sizes of the adsorbates. The results are helpful to deeply understand the pore-modulation mechanism in microporous polymers and the correlations of porous/chemical structures to the adsorption/separation of CO2 and C1–C3 light hydrocarbons in natural gas.

Reservoir characterization and geological controls of the Lower Cretaceous sandstones in the northern Wuerxun Sag, Hailar Basin, China

The Lower Cretaceous sandstone reservoirs in the northern Wuerxun Sag host abundant hydrocarbon resources and natural CO2 gas. Reservoir quality and heterogeneity are regarded as significant factors affecting the hydrocarbon and CO2 distribution, which have not been comprehensively investigated. In this paper the characteristics of the reservoirs are studied using well log, core, thin section and petrophysical data, and related geological controls are discussed to analyse the factors responsible for the variable reservoir quality. Results show that the Lower Cretaceous sandstones mainly include common sandstones and dawsonite-bearing sandstones, characterized by variable grain size and poor sorting. That is probably caused by the rapidly variable sedimentary facies as a result of short multiple provenances and complex fault system. The reservoir quality is regarded as poor with general low-moderate porosity (ranging from 1.20% to 27.40%, with an average of 12.32%) and low permeability (ranging from 0.01 × 10−3μm2 to 1708.00 × 10−3μm2 with an average of 19.08 × 10−3μm2). The porosity and permeability generally decrease as the burial depth increases. However dawsonite cements have caused local heterogeneity as dawsonite-bearing sandstones are of lower porosity and permeability than comparatively deeper common sandstones. In general, tectonic and depositional evolution provided the heterogeneous lithologic distribution which is the material basis of the reservoir quality evolution. Diagenesis is proposed to be primarily responsible for the reduction of reservoir quality, with compaction being the most important mechanism volumetrically. Cementation also played a role in destroying reservoir quality, with dawsonite cements causing local heterogeneit and closely related to the presence of natural CO2 gas in the Surennuoer strike-slip fault zone. There is a genetic link among the dawsonite cements, inorganic CO2 gas degassed from the magmatic intrusion bodies and the Surennuoer strike-slip fault zone. Generally this study provides a better understanding of the reservoir quality in the northern Wuerxun Sag as well as related geological controls, which can guide subsequent hydrocarbon production activities in the study area.

CFD modelling of non-equilibrium condensation of CO2 within a supersonic nozzle using metastability approach

A high content of CO2 in natural gas reduces its heating value and causes operational problems. Hence, it must be removed in a cost-effective way. In this paper, the metastability concept was used to investigate the non-equilibrium condensation of CO2 in a supersonic nozzle using ANSYS CFX and RGP tables generated based on data from GERG-2008 equation of state. Once the simulation was completed, the pressure and temperature drops are analysed. It was found that the lowest pressure and temperature values correspond to the metastable region of CO2. Therefore, based on the simulation results it is concluded that non-equilibrium condensation of CO2 has been achieved in this study.

Recent Development of Enhanced Polymeric Blend Membranes in Gas Separation: A Review

Natural gas is the most rapid growing energy sources around the world. The presence of CO2 in natural gas lowers its calorific value and purification of a natural gas by removing CO2 is an essential process to increase its value. Several separation technologies are used to remove acidic gases like H2S and CO2 from natural gas. Among these technologies, membrane process is a feasible energy saving alternate to CO2 capture. The three types of membrane include polymeric, inorganic and mixed matrix membranes. Currently, polymer membranes and inorganic membranes were considered for gas separation, but inorganic membranes are too costly. Even mixed matrix membrane performance suffered defects caused by poor glassy polymer and particle interactions. Pure glassy and pure rubbery are problematic due to their instructive properties. The blending of glassy with rubbery polymers improve membrane properties for gas separation. To enhance the compatibility of the polymer blend, a third component is added such as alkanol amines. Although, the enhanced polymeric blend membranes have many advantages in terms of permeance, selectivity, thermal and chemical stability. Polymer blending also offers an effective technique to synthesize membranes with desirable properties.

Recent Development of Enhanced Polymeric Blend Membranes in Gas Separation: A Review

Natural gas is the most rapid growing energy sources around the world. The presence of CO2 in natural gas lowers its calorific value and purification of a natural gas by removing CO2 is an essential process to increase its value. Several separation technologies are used to remove acidic gases like H2S and CO2 from natural gas. Among these technologies, membrane process is a feasible energy saving alternate to CO2 capture. The three types of membrane include polymeric, inorganic and mixed matrix membranes. Currently, polymer membranes and inorganic membranes were considered for gas separation, but inorganic membranes are too costly. Even mixed matrix membrane performance suffered defects caused by poor glassy polymer and particle interactions. Pure glassy and pure rubbery are problematic due to their instructive properties. The blending of glassy with rubbery polymers improve membrane properties for gas separation. To enhance the compatibility of the polymer blend, a third component is added such as alkanol amines. Although, the enhanced polymeric blend membranes have many advantages in terms of permeance, selectivity, thermal and chemical stability. Polymer blending also offers an effective technique to synthesize membranes with desirable properties.

Turbulent flame structure characteristics of hydrogen enriched natural gas with CO2 dilution

This paper investigated the hydrogen enriched methane/air flames diluted with CO2. The turbulent premixed flame was stabilized on a Bunsen type burner and the two dimensional instantaneous OH profile was measured by Planar Laser Induced Fluorescence (PLIF). The flame front structure characteristics were obtained by extracting the flame front from OH-PLIF images. And the turbulence-flame interaction was analyzed through the statistic parameters. The role of hydrogen addition as well as CO2 dilution on the features of turbulent flame were revealed by those parameters. In this work, hydrogen fractions of 0, 0.2 and CO2 dilution ratios of 0, 0.05 and 0.1 were studied. Results showed that hydrogen addition can enhance turbulent burning velocity ST/SL through decreasing the scale of the finer structure of the wrinkled flame front, caused by the smaller flame instability scale. In contrast, CO2 dilution decreased turbulent burning velocity ST/SL due to its inactive response to turbulence perturbation and larger flame wrinkles. For all flames, the probability density function (PDF) profile of the local curvature radius R shows a bias to positive value, resulted from the flame intrinsic instability. The PDF profile of R decreases with CO2 dilution, while the value of local curvature radius corresponding to the peak PDF is larger. This indicates that larger wrinkles structure was generated due to CO2 dilution, which leads to the decrease in ST/SL as a consequence. Hydrogen addition increases the flame volume and results in more intense combustion. CO2 dilution has a decrease effect on flame volume for both XH2 = 0 and XH2 = 0.2 while the decrease is obvious at XH2 = 0.2, ZCO2 = 0.1. In all, hydrogen enrichment improves the combustion while CO2 can moderate combustion. Therefore, adding hydrogen and CO2 in natural gas can be a potential method for adjusting the combustion intensity in combustion chamber during the combustor design.

Composition and origin of natural gases accumulated in the Upper Triassic tight sandstone reservoirs in the Western Sichuan depression, China: Gaseous hydrocarbons, noble gases and carbon dioxide

In this paper, the gas composition and isotopic characteristics of the tight sandstone of the Upper Triassic Xujiahe Formation in the western Sichuan depression were analyzed, and the origin of hydrocarbon gas, rare gas and CO2 in natural gas were determined. The hydrocarbon gas content in the tight sandstone gas of the Xujiahe Formation is the highest, ranging from 96.43% to 99.44 vol.%, while the non-hydrocarbon gas content is only 0.34–2.73 vol.%. The hydrocarbon gas has the highest content of CH4, which is distributed from 88.52% to 97.86 vol.%, and the content of other hydrocarbon gases is relatively low. The studied natural gas is mainly dry gas, and a small part is wet gas. According to the hydrocarbon gas component and carbon isotope analysis results, it showed that the natural gas of the Xujiahe Formation was a thermogenic gas, and there was no mixing of biogenetic gas. The natural gas was derived from the thermal degradation of the humus kerogen. Moreover, the rare gas in the natural gas of the Xujiahe Formation is a crustal origin organic gas, and there is no mantle origin gas, which indicates the lack of inorganic components in the natural gas. We also found that the CO2 in the natural gas is mainly inorganic gas, which is derived from the dissolution of carbonate minerals; and only a small part is organic thermogenic gas, which is derived from the thermal maturation process of organic matter.

Effective stress constraints on vertical flow in fault zones: Learnings from natural CO2 reservoirs

Some fault zones leak vertically to the ground surface or seafloor, whereas most others remain naturally sealed. Understanding the factors that cause this leakage is essential for predicting and preventing such leakage for both conventional reservoir development and subsurface CO2 storage. This study, a comparison of leaking and nonleaking natural CO2 gas accumulations, provides such constraints. We compare and contrast trap configurations, fluid pressures, and stress states for several natural CO2 accumulations from the Colorado Plateau. Extensive surface geologic data are integrated with subsurface data from a large suite of groundwater and hydrocarbon wells. Leakage of CO2 is documented by geochemical surveys and the occurrence of extensive travertine deposits. The leakage occurs exclusively in fault fracture damage zones where the total fluid pressure reduces the minimum horizontal effective stress to approximately zero. These results are consistent with natural and accidentally induced fault seeps from some deep-water hydrocarbon reservoirs. These criteria can be used to evaluate the potential for fault zones to provide vertical leakage pathways and loss of fluid containment.