Subcritical Adsorption Research Articles

Perspectives of Gas Adsorption and Storage in Kerogens and Shales

Shale gas from unconventional resources will contribute to meeting the energy demand during the transition to a net-zero carbon economy. In this minireview, the current status of understanding methane adsorption on kerogens and shales is discussed in relation to methane storage capacity. Standard subcritical adsorption studies provide characterization data (micropore volumes, total pore volumes, surface areas, and pore size distributions) for the porous structures of shales and kerogens. However, supercritical methane adsorption measurements under simulated geological conditions are necessary to assess realistic methane adsorption storage capacities. Supercritical methane adsorption on shale and shale components (kerogens and clays) under high pressure and temperature conditions that simulated geological conditions is compared. Kerogen structural characteristics are discussed in relation to supercritical methane isotherms and isobars. The contribution of adsorbed methane gas relative to “free” or compressed gas to the total gas stored in shale is considered. The importance of kerogens in both storage of the adsorbed phase and as the source of methane is highlighted, and the areas where knowledge and understanding are deficient are identified, in particular, the relationship of kerogen type and maturity with supercritical methane adsorption under simulated geological conditions. The competitive adsorption of water on methane capacity is also an area where more detailed studies are necessary. These are challenges to address gaps in the current knowledge and understanding, which require future research.

Free-Energy-Averaged Potentials for Adsorption in Heterogeneous Slit Pores Using PC-SAFT Classical Density Functional Theory.

This study analyzes the adsorption behavior in two-dimensional heterogeneous slit pores using nonlocal density functional theory based on the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Both chemical heterogeneity and surface roughness on small atomistic scales are investigated. The solid structure is considered as individual solid interaction sites whereby chemical heterogeneity is introduced through the presence of different solid-fluid sites and molecular roughness by varying the position of the interaction sites in the first solid layers. The effect of both forms of heterogeneity on the adsorption behavior is assessed individually. Effective one-dimensional solid-fluid potentials provide a way to reduce the dimensionality and computational demand of the density functional theory (DFT) calculations. We determine one-dimensional free-energy-averaged (FEA) solid-fluid potentials of methane and n-butane in the low-density limit for solid systems with molecular roughness and chemical heterogeneity. Using this effective one-dimensional solid-fluid potential at any density, we find excellent agreement of adsorption isotherms for both solid descriptions in systems with homogeneous slit pores. Subcritical adsorption isotherms of n-butane in slit pores with surface roughness show deviations at higher pressures due to the formation of fluid layers in the one-dimensional FEA potential. Chemical heterogeneity introduces a shift of the capillary condensation pressure below the saturation pressure of the bulk liquid, which is well described by the free-energy-averaged system.

A novel process for CO2 purification and recycling based on subcritical adsorption in oat bran

A novel process for the purification and recycling of CO2 (PRCO2) in a pilot scale unit was implemented and validated. The system is a 4-cycle pseudocontinuous process composed of on-line supercritical fluid extraction integrated with fractionation of extracts in three separation vessels, followed by purification of CO2 in adsorption column at subcritical conditions, and subsequent recycling of the purified CO2 with the aid of a storage tank. Turmeric and annatto were the plant matrices used for extraction and oat bran was used as adsorbent. The PRCO2 reduced the consumption of CO2 in 85.24% for annatto and 74.27% for turmeric, representing monetary savings of USD 186.40 and USD 111.83, respectively. Economical evaluation showed that the increasing of the number of cycles decreased the cost of manufacture from USD 349.60/kg to 122.67/kg for the processing of annatto and USD 91.31/kg to USD 36.56/kg for the processing of turmeric.

CO2 Sorption Capacity in Clay-Rich Shales with Moisture Content

Injected CO2 in geological sites may contact the impermeable caprock/shales overlying the injection reservoirs. In such cases, the shales may provide additional storage capacity due to CO2 adsorption in the microstructures. Shales have heterogenous mineral compositions and nano-scale microstructures (pore width < 200 nm), which result in complex wetting properties and low permeability. In such nanoporous materials, a large percentage of gas in place is stored by adsorption to the high surface areas. The complex mineralogy of shales, such as clay, quartz, carbonate and organic matter, will control sorption properties in addition to environmental pressure and temperature effects. The goal of this study is to determine the mineralogy and moisture effect on CO2 accessible surface area using subcritical adsorption and low pressure adsorption. Our results indicate that the total organic content (TOC) is a controlling factor of micropore volume in shale. We find a positive correlation where the excess micropore volume increases with increasing TOC of the shale. The hydrophobic nature of organic matter (OM) shows preferential adsorption when tested with different fluids such as nitrogen, hexane and water. With the micropore volume estimated from CO2 subcritical adsorption, CO2 storage potential in Utica shale is estimated to be double of previous theoretical estimation by Godec et al (2014). Moisture effect on shale and zeolites is studied to image the effect of irreducible water saturation, where the CO2 adsorption is significantly reduced with the presence of moisture.

Construction and Validation of an Online Subcritical Adsorption-based Device for Assisting CO2 Recycling during a Supercritical Fluid Extraction Process

Background:An efficient process for extracting food ingredients from plant materials should demand the use of a reduced volume of extraction solvent to obtain a final product that is free of solvent and reduces both the processing time and the costs. In some cases, achieving a new efficient process requires the modification, reconfiguration or renewal of elements that are part of a processing unit.Objective:The goal of this work is to describe the development of a modification of a commercial supercritical fluid extraction pilot unit designed to assist CO2recycling based on subcritical adsorption on an adsorbent material. In addition to the construction and validation of the system, a cost survey was performed to estimate the cost of the homemade device developed to allow effective CO2recycling.Methods:The developed device was tested using cotton and Celite®as model adsorption materials and annatto seeds (Bixa orellana L.) as a model plant material. A 0.65 L adsorption column was installed with a set of connections and valves that complemented the unit’s recycle system. The validation procedure consisted of defatting annatto seeds.Results:The proposed online subcritical adsorption-based device was technically validated using cotton as an adsorbent material. The cost survey showed that an estimated total cost of USD 5731.36 would be required to install the developed device in a Supercritical Fluid Extraction (SFE) unit similar to the one coupled here (5 L).Conclusion:The proposed device was demonstrated to be very promising for application in the pseudocontinuous SFE, recirculating the same amount of CO2mass exceeding the S/F values by 14 times, when compared to a process without a CO2recycling system.

Saturation loadings on 13X (Faujasite) zeolite above and below the critical conditions. Part IV: inorganic multi-atomic species, halocarbons and oxygenated hydrocarbons data evaluation and modeling

The saturation loadings for subcritical adsorption of multi-atomic inorganic species, halocarbons and oxygenated hydrocarbons on 13X zeolite are modeled using the modified Rackett model of Spencer and Danner (J. Chem. Eng. Data 17(2):236–240, 1972) for the saturated liquid densities combined with crystallographic data for the 13X zeolite. A similar equation is used for supercritical adsorption involving supercritical adsorbate densities and crystallographic data for the 13X zeolite employing a different f(Tr) expression than used by Spencer and Danner. Adsorption data from the literature are first critically evaluated and then compared to the model. Log–log plots are used to determine whether each isotherm is near saturation; isotherms that exhibit a \(\left( {\partial \ln q} \right)/\left( {\partial \ln p} \right)\) slope of zero at the maximum pressure point are assumed to be saturated (capillary condensation points are deleted). The highest loading is used from each isotherm that approaches saturation. Unsaturated isotherms are not considered further. The theoretical equation satisfactorily models the available experimental data for the data that is subcritical except for water and methanol. However, steric factors are required in the model for tetrafluoromethane, sulfur hexafluoride and the aldehydes. The adsorption data for ethyl acetate is questionable. A significant amount of data in the supercritical region (tetrafluouromethane, and hexafluoroethane) revealed a decreasing trend with increasing Tr. For this data a f(Tr) is modeled using TCAR and the slope of the decreasing linear plot against Tr. The physical phenomenom causing this effect is attributed to increasing molecular vibration in the cavity reducing the total molecular loading with temperature rise.

Saturation loadings on 13X (faujasite) zeolite above and below the critical conditions. Part III: Inorganic monatomic and diatomic species data evaluation and modeling

The saturation loadings for subcritical adsorption of inorganic monatomic and diatomic species on 13X zeolite are modeled using the modified Rackett model of Spencer and Danner (J Chem Eng Data 17:236–240, 1972) for the saturated liquid densities combined with crystallographic data for the 13X zeolite. A similar equation is derived for supercritical adsorption involving supercritical adsorbate densities and crystallographic data for the 13X zeolite. Adsorption data from the literature are first critically evaluated and then compared to the model. Log–log plots are used to determine whether each isotherm is near saturation; isotherms that exhibit a $${{\left( {\partial \ln q} \right)} \mathord{\left/ {\vphantom {{\left( {\partial \ln q} \right)} {\left( {\partial \ln p} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {\partial \ln p} \right)}}$$ slope of zero at the maximum pressure point are assumed to be saturated. The highest loading is used from each isotherm that approaches saturation. Unsaturated isotherms are not considered further. The theoretical equation satisfactorily models the available experimental data for the data that are subcritical. However, a steric factor is required in the model for iodine. 80% of the data for helium, nitrogen and oxygen are in the supercritical region; this data reveals a linear decreasing trend with increasing reduced temperature Tr. For this data a new supercritical model is developed using the reduced critical adsorbate temperature, TCAR, and the slope of the decreasing linear plot against Tr. The physical phenomena causing this effect is considered to be increasing molecular vibration in the cavity reducing the total molecular loading with temperature rise.

Saturation loadings on 13X (faujasite) zeolite above and below the critical conditions. Part II: unsaturated and cyclic hydrocarbons data evaluation and modeling

The saturation loadings for subcritical adsorption of alkenes, aromatics and cyclic alkanes 13X zeolite are modeled using the modified Rackett model of Spencer and Danner (J Chem Eng Data 17(2):236–240, 1972) for the saturated liquid densities combined with crystallographic data for the 13X zeolite. Adsorption data from the literature is first critically evaluated and then compared to the model. Log–log plots are used to determine whether each isotherm is near saturation; isotherms that exhibit a (∂ ln q)/(∂ ln p) slope of zero at their maximum pressure point are assumed to be saturated. For subcritical isotherms, the reduced pressure at which all isotherms became saturated is found to be relatively constant below a molecular weight of 60 g/mol, and then to decrease exponentially with molecular weight above 60 g/mol. The highest loading is used from each isotherm that approaches saturation. Unsaturated isotherms are not considered further. The theoretical equation satisfactorily models the available experimental data for the alkenes and cyclic alkanes. However, steric factors are required in the model for the aromatics. For supercritical temperatures, no model presently exists to explain the data. However, the supercritical alkene data are satisfactorily modeled with an equation of the form qmax = 10 ± 2.0 g/100 g. There are no supercritical data for aromatic species or for cyclic alkanes.

Saturation loadings on 13X (faujasite) zeolite above and below the critical conditions. Part I: alkane data evaluation and modeling

The saturation loadings for subcritical adsorption of n-, iso- and neo alkanes C1–C8 in 13X zeolite are modeled using the modified Rackett model of Spencer and Danner (J Chem Eng Data 17:236–240, 1972) for the saturated liquid densities combined with crystallographic data for the 13X zeolite. For validation of this model, alkane adsorption data in the literature is first critically evaluated and then compared to the model. The saturation loading of each isotherm that approaches saturation is extracted from the data. Log–log plots are used to determine whether each isotherm is near saturation; isotherms that exhibit a \(({{\partial \text{ \ ln \ } q)} \mathord{\left/ ({\vphantom {{\partial { \ ln \ } q} {\partial { \ ln \ }p}}} \right. \kern-0pt} {\partial \text { \ ln \ }p}})\) slope of zero at their maximum pressure point are assumed to be saturated. Isotherms not fulfilling this criterion are deemed unsaturated and not considered further. The theoretical equation satisfactorily models the available experimental data for the n- alkanes. However, steric factors are required for the model to fit iso alkanes and neo-pentane. For supercritical temperatures, no model presently exists to explain the data. However, the data are satisfactorily modeled with an equation of the form qmax = 8.5 ± 2.5 g/100 g.

Methane Adsorption on Shale under Simulated Geological Temperature and Pressure Conditions

Shale gas is becoming an increasingly important energy resource. In this study, the adsorption of methane on a dry, organic-rich Alum shale sample was studied at pressures up to ∼14 MPa and temperatures in the range 300–473 K, which are relevant to gas storage under geological conditions. Maximum methane excess uptake was 0.176–0.042 mmol g–1 (125–30 scf t–1) for the temperature range of 300–473 K. The decrease in maximum methane surface excess with increasing temperature can be described with a linear model. An isosteric enthalpy of adsorption 19.2 ± 0.1 kJ mol–1 was determined at 0.025 mmol g–1 using the van’t Hoff equation. Supercritical adsorption was modeled using the modified Dubinin–Radushkevich and the Langmuir equations. The results are compared with absolute isotherms calculated from surface excess and the pore volumes obtained from subcritical gas adsorption (nitrogen (78 K), carbon dioxide (273 and 195 K), and CH4 (112 K)). The subcritical adsorption and the surface excess results allow an upp...

Nitrogen adsorption on carbon nanotube bundles: Role of the external surface

Nitrogen adsorption on two types of single-walled carbon nanotube bundles at both subcritical and supercritical temperatures is studied using Gibbs ensemble Monte Carlo simulation to understand the role of the external surface in the type of isotherm. On an infinite periodic hexagonal bundle without an external surface, which was mostly used in previous theoretical studies, subcritical adsorption is of type I with two steps in the adsorption isotherm and two maxima in the isosteric heat corresponding to adsorption, first forming annuli inside the nanotubes, and then with increased coverage at the centers of the nanotubes. Supercritical adsorption is also of type I, but with one step in the adsorption isotherm and a single maximum in the isosteric heat. Also, at high pressures the interstitial channels between the nanotubes become accessible to nitrogen molecules. On a small isolated hexagonal bundle with an external surface, subcritical adsorption is of type II, as has been observed in experiments, with two steps in the adsorption isotherm and two maxima in the isosteric heat. The first step corresponds to adsorption at the internal annuli and the grooves between the nanotubes, and the second step results from adsorption at the ridges on the external surface of the bundle and at the centers of the nanotubes. At higher coverages multilayer adsorption and wetting occur on the external surface as the bulk phase approaches saturation. Supercritical adsorption is of type I, without a step in the adsorption isotherm or a maximum in the isosteric heat, and there is no nitrogen adsorption in the interstitial channels even at high pressures. These results demonstrate the important role of the external surface of the nanotube bundle in the character of adsorption isotherm, and provide a substantial physical explanation for the difference between experimental observation and previous theoretical prediction.

Studies on the Transition Behavior of Physical Adsorption from the Sub- to the Supercritical Region: Experiments on Silica Gel

Adsorption equilibria of nitrogen and methane on a mesoporous silica gel were measured over a temperature range from the sub- to the supercritical region. Determination of the compressibility factor shows a significant effect on the experimentally measured isotherms under conditions ρ g/ρ c>0.3. The transition of adsorption mechanisms in passing over the critical temperature was proven by the transformation of isotherms from type II to type I. The characteristic feature of subcritical adsorption was still observed at a temperature 8 K higher than the critical and the critical depletion phenomenon was clearly shown on such an isotherm. The limit quantities defined in different ways for the supercritical adsorption are in reasonable agreement, indicating the existence of an upper limit state of the supercritical adsorbate.

Computer simulation of thin water films in nonwettable capillaries. The Laplace pressure and disjoining pressure

The behavior of water in flat unwettable microcapillaries was studied as a function of their width by the methods of Monte Carlo and molecular dynamics. The conditions of the coexistence of liquid–vapor phases in capillaries were found, and the values of the coefficient of isothermal compressibility were determined by the calculated supercritical and subcritical adsorption isotherms. The profiles of local density and energy were calculated. The Laplace pressure and disjoining pressure, whose values and signs manifest strong hydrophobic attraction of solid surfaces, were estimated.

A model for the prediction of the adsorption equilibria of arbitrary organic vapours on active carbon

From the basis of well-known molecular interaction potential equations, a model has been developed for calculating the micropore-size distribution of carbonaceous adsorbents from sub-critical adsorption equilibria of organic vapours. Only one simple iteration process and readily accessible physical properties are required for the calculation procedure. It is demonstrated that the resulting pore-size distributions computed from the characteristic curves of various sorbates agree well for a fixed sorbent. Thus, the model also allows the reliable prediction of the sub-critical adsorption equilibria of arbitrary organic vapours knowing the reference isotherm of another sorbate on the sorbent of interest. Even molecular sieve effects are reproduced; however, the assumption of spherical molecules only allows qualitative statements in respect of such effects.