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Porosity distribution in spherical activated carbon particles

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					           Porosity distribution in spherical activated carbon particles

Alejandro Robau-Sánchez, Alfredo Aguilar-Elguézabal, Obed Chimal-Valencia, Luis
de la Torre Saenz., Manuel Roman
         Laboratorio de Catálisis, División de Física y Química de los Materiales
               Centro de Investigación en Materiales Avanzados, CIMAV
 Miguel de Cervantes 120, Complejo Industrial Chihuahua, CP 31109, Chihuahua, México
                alejandro.robau@cimav.edu.mx www.cimav.edu.mx.

                                     Introduction
Controlled gasification of carbonaceous materials, as the fundament of char activation
process, has been an extensively studied field. Many authors have focused their work
to the synthesis of activated carbons from different raw materials, using both thermal
and chemical activation methods concerning thermal activation, influence of
carbonization parameters, applied activating agent, reaction temperature and retention
time has been described [1,2,3].

The objective of this work is to evaluate the characteristics of the porosity, at various
levels of both gasification temperature and conversion degree, following the radial
distribution of porosity in Quercus Agrifolia carbon spheres.

Char spheres were activated, using CO2 as activating agent (300 ml/min), at 820 and
8600C. For each one of these temperatures, the gasification reaction was run until
desired conversions (30, 50 and 70%) were reached. The activation process was
performed using TGA equipment (TA Instrument SDT 2950). In order to perform the
evaluation of the radial distribution of the porosity, two layers were removed from
particles, obtaining then three fractions for analysis: an outer layer, between 4 mm
and 3.5 mm diameter, a medium layer between 3.5 mm and 1.95 mm diameter and
the core of 1.95 mm diameter.

Porosity of obtained samples was measured according to physical characterization
performed using an automatic system (Quantachrome Autosorb 1), through physical
adsorption of nitrogen at –196ºC, obtaining the micropore volume (V0), total pore
volume (Vt) and the mesopore volume calculated as the difference between Vt and
V0.

                               Results and Discussion
Figure 1-a and 1-b depicts evolution of micropore volume and mesopore volume
respectively as a function of reaction conversion. From obtained results some points
could be highlighted:
                        a) Micropore volume vs conversion                                         c) Micropore volume vs conversion
            0.35                      820 oC                                          0.35                      860 oC

            0.30                                                                      0.30

            0.25                                                                      0.25




                                                                           Vo, cc/g
   Vo, cc/g




            0.20                                                                      0.20

            0.15                                                                      0.15

            0.10                                                                      0.10
                                                       External zone                                                               External zone
            0.05                                       Middle zone                    0.05                                         Middle zone
                                                       Core                                                                        Core
            0.00                                                                      0.00
                   30     40        50         60       70         80                        30       40         50           60         70
                                    Conversion, %                                                             Conversion, %

                        b) Mesopore volume vs conversion                                          d) Mesopore volume vs conversion
                                                                                      0.45                        o
            0.45                     820 oC                                                                    860 C
            0.40                                                                      0.40

            0.35                                                                      0.35
            0.30                                                                      0.30
Vme, cc/g




                                                                       Vme, cc/g




            0.25                                                                      0.25
            0.20                                                                      0.20
            0.15                                                                      0.15
            0.10                                       External zone                                                               External zone
                                                                                      0.10
                                                       Middle zone                                                                 Middle zone
            0.05                                                                      0.05
                                                       Core                                                                        Core
            0.00                                                                      0.00
                   30     40        50        60        70             80                    30       40        50          60          70
                                    Conversion, %                                                             Conversion, %

                    Fig.1. Pore volume as a function of reaction conversion.

                   1. For given experimental conditions, there were not significant differences in
                      observed porosity (Vmi + Vme) values or in its development between the three
                      analyzed particle zones at low temperature. The analysis should include the
                      macropore volume in order to find out the expected trend (higher pore volume
                      for the external layer and lower for the core).

                   2. Mesoporosity is enhanced at low activation temperatures, reaching the highest
                      values for high conversion levels. Under such circumstances, mesoporosity
                      appears to decrease toward the core of the particle.

                   3. For the high activation temperature, both micro and mesoporosity show the
                      same pattern in the middle zone and in the core of the particle as conversion is
                      increased. In the external layer the mesopore formation undergoes a slight
                      increment up to the greatest achieved conversion.


     1. F. Rodríguez-Reinoso, M. Molina-Sabio, M.T. González, Carbon 33 (1995) 1.
     2. J.C.González, M.T.González, M. Molina-Sabio, F. Rodríguez-Reinoso, A. Sepulveda-Escribano.
     Carbon 33 (1995) 8.
     3. K. Gergova, S. Eser. Carbon 34 (1996) 7.

				
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