Adsorção de metanol em aluminas ativadas.

Abstract

Methanol is a product with a large variety of industrial uses, such as formaldehyde synthesis, antifreeze in motors, as solvent in a great number of industries and production of MTBE (methyl terc-buthyl-ether). In some synthesis, as in the MTBE production, the unreacted methanol may be recycled after separation of the effluent by adsorption. The knowledge of equilibrium and kinetics is essential to project and optimize adsorption equipment as well as the correct choice of adsorbent. In order to compare the adsorption capacity of aluminas and other adsorbents suited for the adsorption of methanol, such as zeolites, in this work two types of activated aluminas were used: La-Roche 204- 4 spherical and Alcoa Selexsorb COS spherical. Their principal characteristics are: specific area of 326.53m2/g and 284.61 m2/g with pore volume of 0.64 cm3/g and 0.73 cm3/g respectively. This work deals with studies of equilibrium and kinetics in liquid phase. The adsorption isotherms were obtained in static medium at temperatures from 18°C to 35°C and concentration of the liquid phase varying from 0 to 16% by weight of methanol in toluene. The adsorption capacity determined for both types of aluminas from the experimental data, was of about 8g/100g (adsorbed methanol /adsorbent). This value is approximately one half of the adsorption capacity found in previous works for zeolites 4A. The Langmuir model was proposed to the isotherms, obtaining a good fitting of the experimental data. The qs (concentration of methanol in the adsorbent at saturation) parameter shows a slow decreasing with temperature. Nevertheless, the influence of temperature on the Henry constant, obtained from the Langmuir parameters, is well described by the Vant Hoff equation with correlation coefficient of 0.990. The calculated AHa d s is in the order of = 10 kcal/mol, greater than the 6.5 kcal/mol found for the zeolites 4A. The kinetics of adsorption was studied by mean of the finite bath method at the temperature of 30°C and initial concentration of methanol varying from 7.2% to 13.34% by weight. It was verified that the concentration as well as the pellet size influence the rate of adsorption. From this evidence it was assumed that the macropore diffusion is the controlling step. The "Shrinking Core" model was applied to the uptake curves with satisfactory correlation. The macropore diffusion coefficient of methanol in activated aluminas was estimated from this model with average value of 0.644E- 05 cm2/s at 30°C.