Abstract

ETHANOL-WATER ADSORPTION ON COMMERCIAL 3A ZEOLITES:

KINETIC AND THERMODYNAMIC DATA

M.J. Carmo and J.C. Gubulin

Universidade Federal de São Carlos - Departamento de Engenharia Química - Rod. Washington Luiz, Km 235

CEP: 13565-905, São Carlos - SP - Brazil - Phone: (016) 274-8264; Fax: (016) 274-8266

(Received: March 5, 1997; Accepted: August 5, 1997)

Abstract: Dehydration of ethanol via adsorption using molecular sieves has recently been suggested as a promising alternative to the conventional separation methods for ethanol-water mixtures. 3A zeolites possess selective micropores whereon, due to the small size of their pores, the water molecules are adsorbed while the ethanol molecules are excluded. The scope of this work was, hence, the thermodynamic and kinetic study of ethanol-water adsorption on commercial zeolites of different origins, with the aim to select the best one. For the thermodynamic study, a thermostated bath was used at four different temperatures, where the data obtained by the static method could be correlated by means of a nonlinear isotherm. The kinetic data were obtained in a circulating finite liquid bath cell, where the effect of the temperature and of the mean diameter of the adsorbent particles on the rate of adsorption was studied. The results obtained in this way, expressed through uptake rate curves, showed that the adsorption rates were strongly dependent on the parameters studied. On comparing the adsorption rates among the adsorbents (commercial 3A zeolites), it could be concluded that, under the same operational conditions, exists a pronounced difference among them.

Keywords: Ethanol, molecular sieves, adsorption.

INTRODUCTION

Zeolite molecular sieves are adequate adsorbents for the removal of small amounts of water from organic solvents. In virtue of their small diameter (0.28 nm), the water molecules can easily penetrate the structural zeolite canals, while many organic molecules, such as ethanol (0.44 nm), are simultaneously excluded.

Large-scale production of anhydride ethanol from its azeotropic composition is usually done by extraction distillation processes. As this procedure is very onerous, alternatives such as liquid-liquid extraction, adsorption and separation through membranes are being developed.

Work like that of Carton et al. (1987) and Sowerby and Crittenden (1988) demonstrated that anhydride ethanol (>99.95 wt% ethanol) can be obtained by using adsorption on zeolites in the vapor phase. Studies prior to these on the liquid phase, especially the work of Ruthven (1984), proved the great capacity and selectivity of A zeolites in separating water from ethanol-water mixtures.

In the current work on the separation of ethanol-water, zeolites with the same chemical composition from different manufacturers, were studied with the aim to obtain thermodynamic and kinetic data in order to select the one which yields the best results.

MATERIALS AND METHODS

Zeolite pellets of two different shapes, spherical (produced by manufacturer 1) and cylindrical (produced by manufacturer 2), were used. For the spherical particles, the mean diameters, obtained by the Tyler/Mesh procedure, were in the range of from 2.38 to 4.76 mm. The cylindrical particles were quite uniform with a mean diameter of 1.65 mm and a mean length of 4.00 mm. These materials were physically characterized by water picnometry, which included real density, apparent density and porosity. Qualitative and quantitative chemical analysis of the zeolites was carried out , where the method used was alkaline fusion for the gravimetric determination of SiO2 and acid solubilization for the determination of the other elements. An inductively coupled plasma atomic emission spectrometer - AtomScan 25-Thermo Jarrel Ash was therefore used. The results are listed in Table 1.

The ethanol-water mixtures were prepared at the required weight concentrations (concentration range of from 0 to 100 wt% ethanol for the thermodynamic tests and around 90 wt% ethanol for the kinetic tests) from absolute ethanol of the Nuclear brand and distilled water with the aid of a scale with an accuracy of 0.0001g. The commercial ethanol was treated with previously activated 3A zeolite (300° C for 24h) in order to remove any remaining water. For measuring the concentration of the fluid phase (concentrations below 90 wt% ethanol), a Reichert-Jung Auto Abbé refractometer with automatic calibration was used and data reproducibility on the order of 0.5% was obtained in this concentration range. For concentrations above 90 wt% ethanol, an automatic Photovolt Aquatest titrator was used, based on the Karl Fischer method, where reproducibility was on the order of 0.05% in that concentration range. Before every test the zeolites were activated by placing them in an Edgcon 5P oven with programmable temperature (300° C for 24h) and stored in a desiccator in a vacuum until immediately prior to utilization.

EXPERIMENTAL PROCEDURE

The equilibrium data were obtained by the static method, which consists in placing inside hermetical 125 ml flasks a specific amount M_s of thermally treated (300° C for 24 h) adsorbent (» 10g) in contact with a specific mass M_f of aqueous ethanol solution (» 30g) of a well-defined initial concentration . Several initial concentrations were used in order to obtain a wide range of isotherms at four different temperatures (25, 40, 50 and 60° C). The flasks were maintained in a thermostated bath with an accuracy of ± 0.1° C, and gently shaken during approximately 7 days, after which the fluid phase concentration was measured and the end concentration of the liquid was determined. To obtain the amount of water adsorbed in the solid phase, a mass balance was used between the phases, where ethanol was considered the nonadsorbable component. The mass balance is described by equation 1, where at equilibrium when t® ¥ ,

(1)

To obtain the kinetic data, Azevedo (1992) developed an experimental circulating device (Figure 1) within which a finite volume of liquid (» 250 ml) circulates continuously in a closed loop and through a fixed bed of adsorbent particles (» 50g).

This device permits flow conditions at which the external resistances to mass transfer become negligible. The equipment possesses a centrifugal pump that continuously removes liquid at the bottom of the cell and replaces it in closed loop at the top. For a typical kinetic mass transfer test, ethanol-water solutions were prepared with mass M_f (» 200g) and initial concentration (» 10 wt% water) in a hermetic and thermostated cell. It was certified that the liquid filled the pump completely without the formation of bubbles. An acrylic support was used to accommodate the cell-pump setup in the thermostated bath, attaching the pump shaft to the variable rotational motor starting the liquid circulation. As soon as the system attained thermal equilibrium, a device in the lid was removed from the cell and at time t=0 a mass of thermally treated adsorbent Ms was introduced into the cell as quickly as possible. The system was then hermetically closed and at regular time intervals liquid samples were taken by puncturing a rubber septum with a long hypodermic needle. The sample concentrations were measured using the Karl Fischer procedure with a reproducibility of 0.05% in the experimental concentration range, where the concentration of the adsorbent phase is obtained by a simple mass balance, assuming that ethanol is a nonadsorbable component, according to equation 1.

RESULTS AND DISCUSSION

Equilibrium Data

The isotherms obtained, which relate the concentrations in the liquid and solid phases at the temperatures of 25, 40, 50 and 60° C for the spherical and cylindrical zeolites, can be seen in Figures 2 and 3, respectively.

A -Thermostated Bath

B -Variable Motor Rotation

C -Centrifugal Pump

D - Cell (particles and fluid)

E - Lid (with septum )

Figure 1: Experimental setup with circulation for immersion kinetic tests in a finite volume of liquid.

Figure 2: Adsorption isotherms for the system: ethanol-water/3A zeolite. (manufacturer 1)

Figure 3: Adsorption isotherms for the system: ethanol-water/3A zeolite. (manufacturer 2)

From the isotherms one may note that there is a decrease in the adsorbing capacity as the temperature increases for both types of zeolites. This may be explained by the increase in the vibrational energy of the molecules, which that at higher temperatures allows a smaller net number of molecules to be adsorbed at equilibrium since, due to the exothermic character of the adsorption process, an increase in temperature shifts the equilibrium towards the region unfavorable to adsorption. A nonlinear model was used, represented by the Langmuir isotherm defined by equation 2, which satisfactorily correlated the experimental data.

(2)

The constants obtained by this equation at the experimental temperatures and for the two types of zeolites are listed in Table 2.

The results obtained showed that the adsorption capacities for both zeolites are very similar, the difference being on the order of magnitude of the experimental error. This is explained by the very similar chemical compositions of the two types of zeolites. As expected, the Langmuir constant decreases as the temperature increases. The heat of adsorption (D H) could be estimated by the van ’t Hoff equation:

(3)

Figure 4 shows the fit obtained between the equilibrium constant and the temperature. The mean adsorption heat obtained at equilibrium conditions was equal to 42.58 kJ/mol and 43.23 kJ/mol for the spherical and cylindrical zeolites, respectively.

Kinetic Data

The kinetic results are presented by means of uptake rate curves, where the dimensionless concentration in the adsorbed phase is related to the adsorption time for the zeolites studied. The parameters like mean particle diameter and temperature were related to the adsorption rates. The interstitial velocity used eliminated the external resistance to mass transfer. The pore diffusion model was applied, with the diffusion resistance concentrated in the macropores, which satisfactorily represented the experimental data, making it possible to estimate the diffusivities for several temperatures and particle diameters.

The temperature is a feature of great interest in kinetic adsorption processes, as its influence on the diffusion process is very significant. From Figures 5 and 6 one may note that a temperature increase favors the kinetic process by increasing the adsorption rates. This fact may be explained by an increase in the degree of molecular agitation.

Figure 4: Effect of the temperature on Langmuir’s constant.

Figure 5: Uptake rate curves for several temperatures - D_p = 2.60mm and V_I>4cm/s.

For the specific case of spherical zeolites, the effect of the mean particle diameter on the uptake rates of the adsorbent was studied. From the results obtained and plotted in Figure 7, one may note that the adsorption rates increase as the mean particle size of the adsorbent decreases. This may be explained by the existence of a larger total area in agglomerates of smaller particles, where the area of contact between the particles decreases in relation to the same mass of larger particles, such that the diffusion path is smaller for the smaller size particles, diminishing the time it takes for the molecules to pass through the intricate network of meso and macropores on their way to the intracrystalline cavities. This indicates that the diffusion resistance inside the particle is predominantly located at the macropore level, without discarding the hypothesis that there might be a combined resistance effect of the micro and macropores.

Figure 6: Uptake rate curves for several temperatures - D_p = 1.65mm and L = 4mm.

Figure 7: Uptake rate curves for several particle sizes.

Figure 8: Simulation results.

The effective diffusivities for the spherical and cylindrical zeolites were calculated by means of the pore diffusion model, considering the cylindrical zeolites to be: equivalent spheres of the same area*, equivalent spheres of the same bulk** and infinite cylindrical pores***. The values obtained in this way are listed in Table 3 for several temperatures.

The pore diffusion model was applied for comparing the uptake rates of the zeolites from manufacturer 1, manufacturer 2 and manufacturer 3 (developed by Azevedo, (1992)), at a temperature of 25° C and a diameter of 1.65mm. The results of the simulation, according to Figure 8, showed that the zeolite from manufacturer 1 has the best uptake rates when the diameter is equivalent to the zeolite from manufacturer 2. The zeolite from manufacturer 3 has the worst uptake rates. In contrast, the zeolites from manufacturer 2, with a diameter of 1.65mm and zeolites from manufacturer 1, with a diameter of 2.60mm, verified a predominance of the material from manufacturer 2 for the various temperatures, as shown in Figures 5 and 6.

CONCLUSIONS

The thermodynamic results showed that the spherical and cylindrical zeolites have the same adsorbing capacity at the temperatures studied and that this capacity decreases with temperature increase. The Langmuir isotherm satisfactorily correlated the experimental data for the experimental temperatures and concentrations. The kinetic tests were shown to be dependent on the parameters studied, where a temperature increase resulted in an increase in the diffusivity values for both zeolites. The diameter increase of the spherical particles resulted in a decrease in the adsorption rates, but the diffusivity remained constant with a mean value of 1.67.10^-5 cm²/s. Comparatively, the zeolites from manufacturer 3 have lower uptake rates and the zeolites from manufacturer 1 show the best results.

NOMENCLATURE

c* Concentration of the liquid phase at equilibrium, weight % water

K Langmuir’s constant, g_sol/g_water

M_f Mass of liquid, g

M_s Mass of adsorbent, g

q_s Concentration of the adsorbed phase after a specific time interval, g/g_ads

q* Concentration of the adsorbed phase at equilibrium, g/g_ads

Q Capacity of the monolayer, g/g_ads

R Ideal gas constant, cal/molK

Initial mass fraction of ethanol in the solution, weight % ethanol

W_b Mass fraction of ethanol in the solution after a specific time interval, weight % ethanol

Mass fraction of ethanol in the solution at equilibrium, weight % ethanol

D H Heat of adsorption, kJ/mol

REFERENCES

Azevedo, D.C.S., Estudo Cinético e Termodinâmico da Adsorção para o Sistema Etanol-Água sobre Zeólita 3A, Ph.D. diss., Universidade Federal de São Carlos, 195 (1992).

Carton, A., González, G., Torre, A.I. and Cabezas, J.L., Separation of Ethanol-Water Mixtures Using 3A Molecular Sieve, J. Chem Tech. Biotechnol, 39, 125-132 (1987).

Ruthven, D.M., Principles of Adsorption and Adsorption Processes, John Wiley, New York (1984).

Sowerby, B and Crittenden, B.D., Scale-up of Vapor Phase Adsorption Columms for Breaking the Ethanol-Water Azeotrope, I. Chem. E. Symposium Series, 118 (1988).

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