New apparatus for measuring surface areas as low as 0.1 cm2

An apparatus is described which permits accurate determination of gas adsorption of solids having lower adsorptive capacity. Conventional measurement of gas uptake by solids having surface areas less than 1 m² is not generally feasible at subatmospheric pressure. The small pressure drop due to adsorption is easily lost in the noise created by ambient temperature fluctuations. Possible experimental errors in the volumetric measurement of gas uptake due to temperature fluctuations were greatly reduced by using an apparatus whose reference and sample adsorption cells are disposed in a lateral symmetry. The apparatus consists of a differential micromanometer whose two arms are connected in a lateral symmetry to a pair of sample and reference burets of nearly equal volume for dosing gas into sample and reference cells also of nearly equal volume. When the two cells are immersed in the same temperature bath this design greatly reduces measurement uncertainties due to fluctuations in the temperature. Surface areas as small as 300, 1 and 0.1 cm² were measured through volumetric gas uptakes of nitrogen at 78 K, krypton at 78 K and xenon at 90 K, respectively.     

Adsorption micro calorimeter

In this work, it is described an innovative heat flux micro calorimeter Tian-Calvet type designed to measure adsorption heats and reactions as well as adsorption isotherms. It consists in an adsorption instrument for volumetric gases, which is coupled to the micro calorimeter. The changes in the pressure are monitored by means of high sensitivity and high precision pressure transducers. The micro calorimeter has thermo elements that work by a Seebeck effect, in a twin cells system. The cells are inside a box in which the temperature can be adjusted from 77 to 300 K. The sensitiveness of the calorimeter is established by applying a perfectly known electric work. The results corresponding to the electric calibration, the base line stability determination and the time constant in the equipment are shown.     

A new calorimeter for simultaneous measurement of isotherms and heats of adsorption

A new calorimeter designed for simultaneous measurements of heats and isotherms of gas adsorption and desorption systems is presented. It consists of a volumetric/manometric gas adsorption instrument, the adsorption vessel of which is placed within a second vessel filled with inert gas. This gas acts as a sensor, as not only its temperature but also its pressure is increased if heat is released from the adsorption vessel via the sensor gas to its thermostated surroundings. Indeed, the time integral of the sensor gas pressure signal turned out to be strongly related to the total heat released from the adsorption vessel.A basic theoretical equation of the measurement procedure is given. Results of numerous calibration measurements are presented. The question of what type and amount of sensor gas should be used to achieve high sensitivity of the instrument is discussed.Two examples of measurements of heats of adsorption and adsorption isotherms are given, namely adsorption of N₂ on alumina oxide (CRM-BAM-PM-104) at 77 K and CO₂ on zeolite Na13X and wessalite DAY both at 298 K.     

Ion Chromatography for Rapid and Sensitive Determination of Total Chlorine in Water

In this article, we report a new method that involves adsorption, oxygen bomb combustion, and ion chromatography for the preconcentration and determination of adsorbable organic halogen. This method can be divided into four steps, including: macroporous resin adsorption enrichment, optimization, oxygen bomb combustion, and ion chromatography determination. The project studies the macroporous resin adsorb organic halide pollutants and subsequent burn by an oxygen bomb combustion analyzer. The organic halogen in the aerobic environment combusts into hydrogen chloride. The water absorbing hydrogen chloride was then determined by ion chromatography. The influences of some adsorption and combustion parameters, such as the adsorption time, adsorption solution volume, solution pH, hexachlorobenzene acid concentration, combustion oxygen pressure, among others, on their respective adsorption efficiency were investigated. In the optimal condition, the adsorption time was 3.5 h; solution volume was 20 mL; pH was 4; combustion absorption time was 15 min; and the chloride calibration working range was from 1 ppm to 20 ppm (R ²  = 0.9998). Finally, the proposed method was successfully applied to the determination of organic halogen in water samples. The recoveries of chlorine (at spiked concentrations of 10 ppm and 20 ppm into water) in real samples ranged from 92.2% to 101.0%. Intra-day precision (N = 3) in terms of peak area, expressed as relative standard deviation, was found to be within the range of 0.68%–1.97%.     

In situ measurement of gas adsorption processes using Flux Response Technology

Flux Response Technology (FRT) is being developed as a powerful in situ perturbation technique to facilitate detailed characterisation of heterogeneous catalysts. FRT works by measuring minuscule changes in flowrate between two gas streams for potentially any gaseous process involving a change in volume (dV/dt). FRT functions analogous to an electrical Wheatstone bridge assembly whereby gas molecules represent electrons and flow capillaries represent resistors. A perturbation of pressure, temperature, but particularly of concentration causes an imbalance in the system, which is measured directly by a very sensitive differential pressure transducer (DPT). It is demonstrated how FRT can provide a simple, inexpensive, highly accurate means of quantifying the acidic sites of zeolites using ammonia ad/desorption as well as determining the dynamically available surface areas of catalysts using nitrogen adsorption at liquid nitrogen temperature. Temperature programmed experiments (temperature perturbations) can also be employed with FRT to rapidly compare catalyst activity directly (for catalyst development but also for quality control) without the need for much calibration.     

Determination of absolute gas adsorption isotherms: simple method based on the potential theory for buoyancy effect correction of pure gas and gas mixtures adsorption

The absolute adsorption isotherms are necessary to correctly evaluate the selectivity of the adsorbent material or to design adsorption processes at high pressure (e.g., H₂ purification from syngas processes, removal of acid gas from natural gas,…). The aim of this work is thus to propose an easy method to correct the buoyancy effect of the bulk phase on the adsorbed phase volume during both pure gas and gas mixtures adsorption for pressures up to 10 MPa. The potential theory of adsorption and the Dubinin–Radushkevich relation are adapted by introducing mixing parameters based on simple Berthelot rules. The concept of internal pressure used to characterize the adsorbed phase is also adapted for mixtures. The method is then improved on a commercial activated carbon (AC), when adsorbing pure H₂S and CH₄, and their mixtures up to 5 MPa. The study points out the importance to carefully consider the buoyancy effect of the bulk phase on the adsorbed phase volume. Its impact on the adsorbent material selectivity at high pressures could affect the design and the performances of PSA or TSA processes. For example, only considering the excess adsorption data leads to an apparent selectivity 13 % greater than the absolute one for a concentration of 6 ppm of H₂S in a CH₄ matrix at 5 MPa (298 K) on the AC.     

Development of a certified reference material of nitrous oxide in nitrogen for emission gas measurement

A reference gas mixture of nitrous oxide (N₂O) in nitrogen, filled in a 10-L high-pressure aluminum alloy gas cylinder, has been developed as a certified reference material for emission measurement of exhaust gases from automobiles. As an example of certified values, mole fraction of N₂O is 302.36 μmol/mol. An electronic mass comparator with a home-made automatic cylinder exchanger, gas-filling equipment, and a gas chromatograph with a thermal conductivity detector have been used for the production of this CRM. The gas chromatographic analysis has of sufficient precision. The mole fraction of N₂O has good long-term stability for 10 years and is independent of inner pressure in the gas cylinder. As these results, a relative expanded uncertainty (coverage factor is 2) of the certified value has become 0.28 %. This sufficiently small uncertainty of the N₂O mole fraction will be advantageous in the calibration of analytical instruments for emission gas analysis.     

Influence of acid treatment on structure of clinoptilolite tuff and its adsorption of methane

This study presents the results of the methane adsorption properties of clinoptilolite tuff from Bigadic, Turkey and that of acid treated forms at 273 and 293 K up to 100 kPa using volumetric apparatus. In order to assess changes in structural and gas adsorption properties of clinoptilolite, zeolite sample was treated with acid solutions of varying concentrations (0.1, 0.5, 1.0 and 2.0 M) at 70 °C during 3 h. Structural and thermal characterization of natural and acid treated clinoptilolite samples were carried out using a combination of techniques such as X-ray diffraction, X-ray fluorescence, thermogravimetric, differential thermal analysis and nitrogen adsorption methods. At both temperatures, uptake of methane (CH₄) increased in the following order: CLN < CLN-H2 < CLN-H1 < CLN-H05 < CLN-H01. CH₄ adsorption capacities of the original and acid treated clinoptilolites were found in the range of 0.476–0.910 mmol/g and 0.398–0.691 mmol/g at 273 and 293 K, respectively.     

Gas chromatography analysis: method validation and measurement uncertainty evaluation for volume fraction measurements of gases in simulated reformate gas stream

It is important to understand each analytical system and its limitations when performing any chromatographic measurements. In the present paper, a methodology for method validation and measurement uncertainty evaluation for the measurement of volume fractions of selected gases (CO₂, CO, CH₄, H₂) in simulated reformate gas streams by using gas chromatography was developed. A detailed procedure for in-house method validation based on a simple experimental design and consistent statistics is presented. The analytical protocol allowed us to quantify gases in volume fractions from 2.00 to 100.0 mL/(100 mL) with satisfactory recoveries. We proved that the method was selective for the measurement of gases in simulated reformate gas stream. In addition, a step-by-step illustration of modelling approach for measurement uncertainty evaluation of each component is also provided. Uncertainty arising from repeatability and trueness is relatively low, while the contribution from reproducibility is of higher level for all the analytes tested. The main reason for this is changes in atmospheric pressure that affect gas chromatographic measurements. Solution of this problem could be more frequent calibration of apparatus, yielding to higher costs and more time-consuming process, or by measuring the atmospheric pressure and using it to correct the response of the gas chromatograph for resulting variations in sample size. The obtained results confirm that it is imperative to fully characterize the analytical system before proceeding with an analysis.     

Adsorption and diffusion of nitrogen, methane and carbon dioxide in aluminophosphate molecular sieve AlPO4-11

The knowledge about the adsorption and diffusion properties (specially about diffusion) of aluminophosphate molecular sieves is very scarce in the literature. These materials offer interesting properties as adsorbents as they have a polar framework and do not contain charge-balancing cations. In this work, the adsorption isotherms of nitrogen, methane and carbon dioxide over an AlPO₄-11 sample synthesized in our laboratories have been measured with a volumetric method at 25, 35, 50 and 65 °C over a pressure range up to 110 kPa. The adsorption capacities of each gas are determined by the strength of interaction with the pore surface (carbon dioxide > methane > nitrogen). The equilibrium selectivity to carbon dioxide is quite high with respect to other adsorbents without cations due to the polarity of the aluminophosphate framework. The adsorption Henry’s law constants and diffusion time constants of nitrogen, methane and carbon dioxide in the synthesized AlPO₄-11 material have been measured from pulse experiments. A pressure swing adsorption (PSA) process for recovering methane from a carbon dioxide/methane mixture (resembling biogas) has been designed using a dynamic model where the measured adsorption equilibrium and kinetic information has been incorporated. The simulation results show that the proposed process could be simpler than other PSA processes for biogas upgrading based on cation-containing molecular sieves such as 13X zeolite, as it can treat the biogas at atmospheric pressure, and it requires a lower pressure ratio, to produce high purity methane with high recovery.     

Pure and binary adsorption isotherms of ethylene and ethane on zeolite 5A

Pure and binary adsorption equilibrium data of ethylene and ethane on zeolite 5A were collected with a volumetric method for the temperature range 283 K to 323 K and pressure up to 950 kPa. The applicability of the binary adsorption prediction by the vacancy solution theory (VST) was investigated. Further individual adsorption and selectivity were obtained by VST prediction. According to the experimental results, zeolite 5A has a high adsorption capacity and selectivity for ethylene in the ethylene/ethane system. VST predicts that ethylene selectivity increases with pressure; it also shows that the amount of ethylene separated by zeolite 5A increases as the temperature decreases at a specified pressure.     

Influence of free-space calibration using He on the measurement of adsorption isotherms

It is well known that helium (He) molecules that remain inside micropores after free-space calibration at a low temperature (77.4 K) affect the shape of an adsorption isotherm, especially in a very low relative pressure region. This negative effect of the remaining He leads to a misunderstanding of the porous characteristics, such as micropore size distribution and surface properties. However, it is still believed that such erroneous interpretations are limited to narrow microporous materials such as activated carbon and measurements at low temperatures, namely the measurement of the adsorption of N₂ and Ar at their boiling points. Here we report a systematic investigation of the influence of free-space calibration using He on microporous, mesoporous and non-porous materials. Zeolite Y, mesoporous silica, carbon black and aerosil 200 were used for the measurements. N₂, H₂O and CO₂ adsorption isotherms were measured at 77.4, 298 and 298 K, respectively. Free-space calibration was carried out before and after the isotherm measurement for each sample. Although the influence of the He that remained in the sample was small for the non-porous sample, the shape of the isotherms for the other samples in a low relative pressure region was rather affected by the timing of the free-space calibration even for the mesoporous sample, and at an ambient temperature.     

Use of strain gauge pressure transducer as working pressure standard up to 500 MPa

This paper describes the calibration of a strain gauge transducer (700 MPa full scale) against a primary pressure standard (500 MPa full scale) maintained at the National Physical Laboratory (NPL), India, over a pressure range of 500 MPa. The calibration data obtained at the NPL are also compared with the calibration data taken at Istituto Di Metrologia ‘G. Colonnetti’ (IMGC), Italy, using the primary pressure standard of IMGC in the possible overlapping pressure region of 158 – 457 MPa, with particular reference to reproducibility, hysteresis and the calibration equations.It is found that the reproducibility between the calibration factor of the standard at NPL and that at IMGC is typically ± 5 X 10⁻⁴ and the highest difference in the pressure values of NPL and IMGC calcualted using the second-order polynomial calibration equation is well below 2 X 10⁻³, the expected uncertainty for the transducer. This suggests a possible use of the strain gauge transducer as a working pressure standard in this pressure range if the accuracy required in the pressure measurement is of the order of 2 X 10.⁻³.     

Determination of rapeseed methyl ester oil volumetric properties in high pressure (0.1 to 350 MPa)

The isothermal compressibility coefficient, the bulk modulus, the cubic expansion coefficient, the density and the propagation speed of the pressure waves of rapeseed methyl ester oil (RMEO) are the thermophysical properties derived from the specific volume determined in this work and compared with the properties of diesel oil (DO). The temperature measurement interval ranges from 288.15 to 328.15 K and the pressure measurement interval from atmospheric pressure to 350 MPa. The experimental method used consisted of a volume change cell characterised by a linear variable differential transformer (LVDT) magnetic induction system adapted to a high-pressure vessel. To calculate the properties the modified Tait-Tammann equation was used and a high correlation coefficient was obtained with a 95% confidence level. The specific volume and compressibility coefficient were greater for DO than for RMEO; also, cubic expansion was greater for DO than for RMEO. These results pave the way for further practical application.     

测量多孔粉体扩散系数和孔分布的实验装置

多孔粉体在介孔和微孔范围内的表征需要专业的实验仪器,这限定了对该类材料全面的表征只能在大型实验室中进行.本文提出了一种改进的BET测量装置,其核心部件是高精度的压差传感器,这种设备让小型实验室也可以测量多孔粉体的内扩散系数和孔分布.使用数值解析方法和蒙特卡罗模拟方法分析了测量扩散系数实验的数据.     

Adsorption phenomena at high pressures and temperatures.

Equilibrium isotherms of adsorption of methane on crystalline Rho zeolite were measured with the use of a precision volumetric gravimetric setup that was developed for determining the equilibrium and Kinetic parameters of adsorption in the pressure range of 0.1–160 MPa and temperature range of 300–600 K. The method of determining the accessible volume of an adsorption system (free volume + micropore volume) and the micropore volume of a sorbent was used. Two independent methods for calculation of micropore volume on the basis of the isotherms of excess adsorption were used. The discrepancy in the results of the calculation of the micropore volume by three independent methods is within the limits of the calculation accuracy. An evaluation of a change in the isosteric heats of the excess and absolute adsorption of methane on Rho zeolite was carried out in relation to filling and temperature. An evaluaton of the adsorption volume above the outer surface of the zeolite crystals was performed. The results of experimental investigations of methane adsorption on Rho zeolite can be used to solve the problem of encapsulation of gases by solid sorbents. kg]Key words kw]adsorption kw]micropore volume kw]surface kw]isostere kw]heat kw]zeoliteFor Part 1 see Ref. I.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 568–573, March, 1996.     

Adsorption phenomena at high pressures and temperatures 1. Method of investigation; adsorption of nitrogen on NaA zeolite

A simple precision volumetric-gravimetric setup was elaborated and created for determining the equilibrium and kinetic parameters of adsorption in the 1–160 MPa pressure range and the 300–600 K temperature range. The isotherms of adsorption of nitrogen on crystalline NaA zeolite in the indicated temperature and pressure range were measured. A method was proposed for determining the pore volume of zeolites based on conducting adsorption measurements at high pressures of the gas phase. The isosteric heats of adsorption of nitrogen on NaA zeolite at 305, 334, and 373 K were estimated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1233–1238, June, 1990.     

Precision Evaluation in Kr Adsorption for Small BET Surface Area Measurements of Less Than 1 m2

A volumetric Kr-adsorption apparatus using a precise capacitance manometer has been developed. A specially designed adsorption cell (CVC: constant volume adsorption cell) utilizing a vacuum jacket (Joyner et al., 1949) is adopted to keep the adsorption cell volume constant regardless of the variation in liquid nitrogen level throughout the experiment. Using the CVC, the pressure change in accordance with liquid N₂ supply cycle has been minimized to less than 0.01 Pa compare to about 1 Pa for conventional cell. Time dependent change of the adsorption cell volume and repeatability in its measurements have been demonstrated in detail using helium gas. Good linearity of the BET plot of the Kr adsorption isotherm on several hundred cm² samples are demonstrated in the relative pressure range from 0.05 to 0.35.     

High pressure sorption isotherms via differential pressure measurements

A differential pressure adsorption unit (DPAU) has been constructed which is capable of accurately measuring isotherm data up to 2000 psia with as little as 100 mg of sample. This non-traditional adsorption/desorption method has been benchmarked by comparing hydrogen and methane isotherms measured with standard volumetric and gravimetric instruments on a NaA (4A) zeolite and an activated carbon at near ambient temperatures. The results from stability tests and well as the details of the mathematical analysis for this differential pressure method are also provided.