Ph. Grenier

Frequency response for nonisothermal adsorption in biporous pellets

The frequency response for nonisothermal adsorption in a biporous pellet is analyzed theoretically, using a mathematical model which includes heat and mass transfer resistances in both micropores and macropores. It is confirmed that, when the heat effect is involved, the out-of-phase component may exhibit a bimodal form. Moreover, it is shown that when both macropore diffusion and micropore diffusion resistances are comparable, macropore diffusion behaves like a surface barrier and leads to an intersection of the in-phase and out-of-phase response functions. When either micropore diffusion or macropore diffusion alone is dominant, the frequency response is essentially the same and, therefore, provides no information concerning the nature of the controlling diffusional resistance. Experimental data for light linear paraffins-5A, reported by Yasuda (1991, J. phys. Chem. 95, 2486–2492), are reanalyzed by the present nonisothermal model. It turns out that the reported experimental response can be equally well represented by a nonisothermal model using several different combinations of mass transfer resistances. It, therefore, appears that the bimodal behavior of the experimental out-of-phase data is caused by the heat effect, thus contradicting the conclusion of Yasuda that there are two adspecies with different mobilities. With the reported data, it is, however, not possible to extract reliable values for the intracrystalline diffusion coefficient, and the nature of the controlling mass transfer resistances cannot be established with certainty.

Thermal Diffusivity and Adsorption Kinetics of Silica-Gel/Water

There have been performed experimental measurements of effective thermal conductivity of silica-gel for a stagnant cylindrical fixed bed under transient and steady state conditions in the presence of dry air at different pressures and for different amounts of adsorbed water. The Bauer-Schlünder and Kunii-Smith models have been used to identify the thermal solid conductivity of silica gel pellets from measurements of the conductivity in an adsorbent bed. Sorption rates of water vapor in silica gel were measured using a single-step thermal method by monitoring the sample surface temperature by infrared detection. In order to obtain the mass diffusivity it is necessary to match the numerical solution of the mathematical model to the experimental data.

Etude d'une machine solaire autonome à fabriquer de la glace

A solar ice maker with a 0.8 m2 collection surface was studied with a view to building an industrial machine. It is an adsorption machine with an intermittent daily cycle that uses the active carbon Ac 35-methanol pair. It is very simple in that the same exchanger can be used either as an evaporator or as a condenser. The system management consists simply of moving this exchanger between the thermally insulated ice bank and an ambient-temperature water tank, and in actuating the ventilation ports for the heating and cooling of the collector. No valve is required. The results show that, with a collection efficiency of 0.41 and a thermal COP of 0.40, it is possible to obtain a gross solar COP of 0.15 (as a function of the quantity of methanol). Because of insufficient insulation of the ice bank, the net COP obtained was twice as low. This machine produced 4 kg of ice per day, during summer, which corresponds to a daily output (non-optimized) of 5 kg ice per square metre of collection surface.

An apparatus for adsorption dynamics studies using infrared measurement of the adsorbent temperature

An apparatus has been built for kinetics measurement in adsorption processes. It consists of a variable volume chamber where the adsorbent sample is placed. During the adsorption process the pressure and the sample temperature are measured. The temperature is measured by infrared detection achieving fast, sensitive, and nonintrusive measurements. Two methods are used for adsorption kinetics measurement: the volume step and the thermal frequency response. In the volume step method the volume is suddenly reduced and the adsorption process occurs at constant volume. Due to the sorption heat released during adsorption, the adsorbed mass is closely related to the sample temperature. This method permits one to determine the relevant thermodynamic parameters of the system and the kinetics up to 20 ms. The different mass transfer modes may then be separated. The thermal frequency response (TFR) method consists of achieving a periodic volume of the chamber. The amplitude and phase lag of the temperature, taking th...

Influence of Traces of Water on Adsorption and Diffusion of Hydrocarbons in NaX Zeolite

Measurements have been performed on the influence of a small amount of water on adsorption properties of alkanes (C3 to C6) and olefins (C3 and C4) in NaX zeolite. Adsorption capacity and heat of adsorption have been measured by a Volume Step method. Kinetics has been measured by Thermal Frequency Response method and the results have been compared with the results obtained by PFG-NMR.It has been found that water reduces the adsorption capacity especially at low adsorbate concentration. The heat of adsorption is slightly reduced by water. The kinetics of alkanes is always reduced by the adsorbed water. On the contrary, the diffusion kinetics of olefins is improved or remains unchanged, depending of the sorbat concentration.

Nonisothermal adsorption of water by synthetic NaX zeolite pellets

Abstract Experimental measurements and theoretical analysis of the uptake and surface temperature are presented for adsorption of water by synthetic NaX zeolite pellets of different sizes. It is shown that, under the present experimental conditions, the heat dissipation effect plays a dominant role in the adsorption kinetics, after a short initial stage which is controlled essentially by both the macropore diffusion and the micropore diffusion. An excellent agreement is obtained between the experimental data and theoretical results given by a nonisothermal model for a bidisperse structure, both for the uptake and for the surface temperature curves. The values of diffusivities determined by the curve-fitting method over an initial stage are of the order of D p ∼ 3 × 10 −5 m 2 /s for the macropore diffusion and D ie / R i 2 ∼ 0.003 s −1 for the micropore diffusion, and these values exhibit an independence of the pellet size for all the tested pellets. Furthermore, it is demonstrated that the use of an effective model for the monodisperse structure may also give a good agreement, but the values of diffusivities determined by the best fit are not very meaningful because of their dependence on the pellet size.

A Temperature Frequency Response Method for Adsorption Kinetics Measurements

A Frequency Response Method based on the infrared measurement of the sample temperature has been developed for adsorption kinetics measurements. It consists in modulating the experimental chamber volume at constant frequency. The complex ratio of the temperature response over the pressure response is independent of time but is a function of the frequency depending on all the kinetics parameters of the system. This method is accurate and allows to measure very fast kinetics. Its major drawback is that a spurious signal is observed at high pressure in absence of adsorption. The results obtained with silicalite-propane and NaX-carbon dioxide are compared with results obtained from other techniques (NMR, permeation, etc.).

Un appareil pour la mesure thermique de la cinétique d'adsorption

Abstract An experimental set-up was developed to measure the mass diffusion kinetics in adsorbents. The mass and the temperature of a sample are measured simultaneously during a pressure step. The temperature is measured by an IR detector which yields very precise and very fast measurements without perturbation of the system. The kinetics parameters are identified by a numerical model. A qualitative approach permits mass diffusion effects to be separated from thermal relaxation, and a typical result obtained for NaX zeolite pellets in the presence of water vapour is presented.

Analysis of the temperature frequency response for diffusion in crystals and biporous pellets

Abstract The frequency response behaviour of the adsorbent temperature is analysed in detail for both diffusion in crystals and in biporous pellets. In the case of crystals, it is shown that the temperature frequency response may exhibit very different behaviour under effects of intracrystalline diffusion or surface barrier, making it easier to discriminate between the diffusion and surface barrier processes. For biporous pellets, the frequency response of the surface temperature proves to be very sensitive to the competition between micropore diffusion and macropore diffusion, due to finite-rate heat conduction inside pellets. This high sensitivity can lead to a much better estimation of the micropore diffusion coefficient in the presence of macropore diffusion than the estimation obtained with the gas pressure frequency response. Finally, it is shown that, in the case of high-pressure experiments, the effect of heating by gas compression may be significant if the gaseous adsorbate has a large specific heat ratio and a large heat capacity, and if the heat exchange at the chamber wall is slow enough. The effect of gas compression heating on the temperature frequency response is confirmed experimentally for diffusion of xenon in zeolite 5A crystals. The present paper shows, however, the effect of gas compression heating on the gas pressure can be efficiently eliminated using the frequency response data corrected with respect to blank experiments, as proposed by Yasuda.

On the Determination of Diffusion Coefficients by Uptake Rate Measurements

Abstract Diffusion coefficients are determined by the measurement of adsorbent surface temperatures using an infra-red detector. The advantages of the optical technique are high temperature sensitivity, fast response and the ability to focus on a precise position. This makes it possible to experiment on a single pellet or very few crystals, thus avoiding intrusion of external transfer resistances. The diffusivities obtained for several systems are in accordance with the n.m.r. data. It has been observed that regeneration conditions could have a large impact on the uptake rate and even probably create a surface barrier effect. The possibility to discriminate between the diffusion and surface barrier by the frequency response technique will be presented.