Adsorption of Helium on Carbons: Influence on Measurement of Density

Maggs, F. A. P.; Schwabe, Philipp; Williams, J. H.

doi:10.1038/186956b0

Cited by 75 publications

(40 citation statements)

References 2 publications

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“…This shows even further that the use of helium adsorption to define excess adsorbed amounts leads to ambiguity, because the apparent skeletal density will depend on the temperature at which the helium experiment is carried out. The temperature dependence of helium adsorption is an issue that is not new (Maggs et al 1960;Springer et al 1969). To correct for this in the determination of the skeletal density of microporous materials the initial approach was to assume zero adsorption at a high temperature and then determine iteratively the Henry law constant of helium as a function of temperature assuming a constant isosteric heat of adsorption (Suzuki et al 1987).…”

Section: Helium Adsorptionmentioning

confidence: 99%

Net, excess and absolute adsorption and adsorption of helium

Brandani¹,

Mangano²,

Sarkisov³

2016

Adsorption

105

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The definitions of absolute, excess and net adsorption in microporous materials are used to identify the correct limits at zero and infinite pressure. Absolute adsorption is shown to be the fundamental thermodynamic property and methods to determine the solid density that includes the micropore volume are discussed. A simple means to define when it is necessary to distinguish between the three definitions at low pressure is presented. To highlight the practical implications of the analysis the case of adsorption of helium is considered in detail and a combination of experiments and molecular simulations is used to clarify how to interpret adsorption measurements for weakly adsorbed components. List of symbolsGibbs energy of the solid without the adsorbate (J) DH Adsorption enthalpy (J mol -1 )Dimensionless (absolute) Henry law constant K ex Dimensionless excess Henry law constant K net Dimensionless net Henry law constant K P Henry law constant (absolute) (mol m -3 kPa) L Length of adsorption column (m) M Bu Mass of bucket (kg) M S Mass of solid (kg) Mw A Molecular weight of adsorbate (kg mol -1 ) n A Moles of adsorbate (mol) n abs Absolute adsorbed amount (mol) n ex Excess adsorbed amount (mol) n net Net adsorbed amount (mol) n S Moles of solid (mol) n TotTotal moles in the system (mol) PPressure (kPa) P 0 d Pressure in dosing cell before valve is opened (kPa) P 1 d Pressure in dosing cell after valve is opened (kPa) P 0 u Pressure in uptake cell before valve is opened (kPa) P 1 u Pressure in uptake cell after valve is opened (kPa) q A Adsorbed phase concentration (mol m -3 ) q abs Absolute adsorbed phase concentration (mol m -3 ) q ex Excess adsorbed phase concentration (mol m -3 ) q net Net adsorbed phase concentration (mol m -3 ) r Position (m) R Ideal gas constant (J mol -1 K -1 ) s A Molar entropy of adsorbed phase (J mol -1 K -1 ) s G Molar entropy of gas phase (J mol -1 K -1 ) T Temperature (K) U Interaction energy of atom (J atom -1 ) DU Adsorption energy (J mol -1 ) v G Molar volume of gas phase (m 3 mol -1 ) V Bu Volume of bucket (m 3 ) V d Volume of dosing cell (m 3 ) Volume of fluid phase (m 3 ) V NA Volume not accessible (m 3 ) V P Volume of pellet (m 3 ) V S Volume of solid, including micropores (m 3 ) V u Volume of uptake cell (m 3 ) z Compressibility factorGreek letters e m Porosity of microporous material e P Macroporosity of pellet / C Fraction of active material in a pellet l A Chemical potential of adsorbate (J mol -1 ) l S Chemical potential of solid (volume basis) (J m -3 ) l 0 S Chemical potential of solid without adsorbate (J m -3 ) g CP Reduced density at close packing g A CP Reduced density of adsorbed phase at close packing q Gas Density of gas phase (kg m -3 ) q S Solid density including micropores (kg m -3 ) q C S Density of active material (kg m -3 ) q SkSkeletal density (kg m -3 ) wGrand potential (J m -3 ) XSignal from microbalance (force) (N)

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Section: Helium Adsorptionmentioning

confidence: 99%

Net, excess and absolute adsorption and adsorption of helium

Brandani¹,

Mangano²,

Sarkisov³

2016

Adsorption

105

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“…with increasing surface area and •true• density of the sample. However, Maggs et ~· (1960) have reported that at room temperature helium is not adsorbed on coals appreciably enough to cause significant errors in the densities. As will be discussed shortly, helium, water and methanol densities of chars measured at room temperature are in close agreement ...…”

Section: Helium Densitymentioning

confidence: 99%

Porosity of Coals and Coal Products

Mahajan

Walker

1978

Analytical Methods for Coal and Coal Products

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“…Previously, Maggs et al and Suzuki et al have measured the adsorption of helium in carbons and zeolites, respectively, using differential analysis temperature in volumetric gas sorption systems [23, 24]. Sircar et al looked at correction to helium data from the Henry constant calculated over a large temperature range, assuming no adsorption at the highest temperature [17].…”

Section: Resultsmentioning

confidence: 99%

Experimental aspects of buoyancy correction in measuring reliable high-pressure excess adsorption isotherms using the gravimetric method

Nguyen

Horn

Thommes

et al. 2017

Meas. Sci. Technol.

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Addressing reproducibility issues in adsorption measurements is critical to accelerating the path to discovery of new industrial adsorbents and to understanding adsorption processes. A National Institute of Standards and Technology Reference Material, RM 8852 (ammonium ZSM-5 zeolite), and two gravimetric instruments with asymmetric two-beam balances were used to measure high-pressure adsorption isotherms. This work demonstrates how common approaches to buoyancy correction, a key factor in obtaining the mass change due to surface excess gas uptake from the apparent mass change, can impact the adsorption isotherm data. Three different approaches to buoyancy correction were investigated and applied to the subcritical CO2 and supercritical N2 adsorption isotherms at 293 K. It was observed that measuring a collective volume for all balance components for the buoyancy correction (helium method) introduces an inherent bias in temperature partition when there is a temperature gradient (i.e. analysis temperature is not equal to instrument air bath temperature). We demonstrate that a blank subtraction is effective in mitigating the biases associated with temperature partitioning, instrument calibration, and the determined volumes of the balance components. In general, the manual and subtraction methods allow for better treatment of the temperature gradient during buoyancy correction. From the study, best practices specific to asymmetric two-beam balances and more general recommendations for measuring isotherms far from critical temperatures using gravimetric instruments are offered.

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Adsorption of Helium on Carbons: Influence on Measurement of Density

Cited by 75 publications

References 2 publications

Net, excess and absolute adsorption and adsorption of helium

Net, excess and absolute adsorption and adsorption of helium

Porosity of Coals and Coal Products

Experimental aspects of buoyancy correction in measuring reliable high-pressure excess adsorption isotherms using the gravimetric method

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