Porous structure and morphology of granular chars from flash and conventional pyrolysis of grape seeds

Abstract

This work studies the influence of the operating conditions used in the pyrolysis of grape seeds on the morphology and textural properties of the chars resulting. Flash and conventional (283 Kmin-1 heating rate) pyrolysis have been used within a wide range of temperature (300-1000 ºC). The effect of a pretreatment for oil extraction has also been studied. The porous structure of the chars was characterized by adsorption of N2 at 77 K, Ar at 77 K and 87 K, and CO2 at 273 K and mercury intrusion porosimetry. The morphology was analyzed by scanning electron microscopy. All the materials prepared revealed an essentially microporous structure, with a poor or even negligible contribution of mesopores. Increasing pyrolysis temperature led to higher specific surface areas and lower pore size. The highest specific surface area values occurred within 700-800 ºC, reaching up to 500 m 2 g-1 with pore sizes in the 0.4-1.1 nm range. No significant morphological changes were observed upon carbonization so that the resulting chars were granular materials of similar size than the starting grape seeds. The hollow core structure of the chars, with most of the material allocated at the periphery of 2 the granules can help to overcome the mass transfer limitations of most common (solid or massive) granular activated carbons. The chars showed a good mechanical strength during attrition tests. These chars can be potential candidates for the preparation of granular carbons molecular sieve or activated carbons raw materials.

Development of porosity upon physical activation of grape seeds char by gas phase oxygen chemisorption–desorption cycles

“…The char yield was 34% of the extracted seeds weight on a dry basis. The elemental composition of char was 85.6%, 1.0%, 1.7% and 0.04% of C, H, N and S, respectively, and the ash content was 8% [6].…”

“…The seeds were washed, dried and then extracted with hexane to remove essential oil before pyrolysis [6]. The oil content of grape seeds depends on the grape variety, though usually ranges between 10% and 16% weight on a dry basis [2].…”

“…In our work it amounted 9%. The char used as starting material was obtained by flash pyrolysis of the extracted seeds at 800°C, according to the results of a previous work [6]. The char yield was 34% of the extracted seeds weight on a dry basis.…”

“…The starting char used in this work was mainly microporous with a S BET of 47 m 2 /g and a S DA of 505 m 2 /g [6]. Due to the narrow nature of the microporosity of the char, the porosity of the resulting activated carbons was analyzed by both the BET and DubininAstakhov methods.…”

“…In a former work the pyrolysis conditions were optimized in order to obtain a starting char maintaining the granular morphology of the grape seed and develop an incipient microporosity that can favor further activation [6]. The aim is to learn on the compromise between burn-off, porosity and particle integrity so that the resulting material can be used as a granular adsorbent, being the activation by chemisorption-desorption cycles characterized by low burn-off values.…”