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Ind. Eng. Chem. Res. 2001, 40, 578-588
Activated Carbon from Macadamia Nut Shell by Air Oxidation in Boiling Water Man S. Tam,† Michael Jerry Antal, Jr.,*,† Emma Jakab,‡ and Ga´ bor Va´ rhegyi‡ Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, and Research Laboratory of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, Budapest 1525, Hungary
A high-yield activated carbon is produced from macadamia nut shell charcoal by (i) carbonization of the charcoal at 1173 K, (ii) air oxidation of the carbonized charcoal in boiling water (AOBW) at 503-553 K, and (iii) activation (a second carbonization) of the oxygenated carbon. In step ii, air is bubbled through a sparger to maintain a relatively high concentration of dissolved oxygen in the water, and the boiling water serves to control the temperature of the carbon during its gasification by the dissolved oxygen. Carbon dioxide is observed to be the only gaseous product of the oxidation chemistry. The oxidation results and the properties of the activated carbons from AOBW are similar to those obtained by controlled atmospheric air oxidation. However, the rate of CO2 formation is observed to increase with time to a plateau for AOBW, whereas the gasification rate decreases with time for atmospheric air oxidation. Multiple cycles, involving AOBW followed by activation, efficiently increase the specific surface area of the carbon to values approaching 1000 m2/g. Increases in the specific surface area occur by the removal of carbon during the AOBW step(s) and the activation step(s). Our findings indicate that carbon removal by desorption of chemisorbed oxygen during the activation step creates a specific surface area more efficiently than a prolonged, low-temperature gasification of the carbon during the AOBW step. If we assume a simple kinetic model in which the gasification reaction is first order with respect to dissolved oxygen and zero order in carbon, the activation energy for AOBW is estimated to be 108 kJ/mol between 513 and 533 K, according to measured CO2 evolution rates and dissolved oxygen concentrations. This value is near the range of activation energies observed in gaseous air oxidation at low temperatures. Introduction Activated carbon is widely used to remove impurities from gaseous and liquid streams.1-4 Conventional physical activation of a biomass to produce activated carbon involves carbonization (increasing carbon content) and activation (increasing specific surface area). The latter step is accomplished by steam or carbon dioxide gasification of the carbon at atmospheric pressure and temperatures above 1100 K.5-7 Gasification of the carbon by oxygen in air at lower temperatures could provide an attractive alternative to the conventional, high-temperature approach. However, experts discount this alternative. For example, in his article on activated carbons in Ullmann’s Encyclopedia of Industrial Chemistry,8 von Kienle states, “Oxygen or air are unsuitable as activating gases.” One explanation for this assumption is the difficulty in controlling the rapid, highly exothermic carbon-oxygen reactions. Also, the underlying chemistry of oxygen combustion is not as well understood as the chemistry of carbon gasification by steam or carbon dioxide.9-13 Recently, we described a novel process that demonstrated the possibility of producing moderate and high specific surface area activated carbons from macadamia nut shell (“macshell”) and coconut shell charcoal by air oxidation at temperatures below 673 K.14,15 Small * To whom correspondence should be addressed. Phone: 808/956-7267. Fax: 808/956-2336. E-mail: antal@ wiliki.eng.hawaii.edu. † University of Hawaii at Manoa. ‡ Hungarian Academy of Sciences.
samples of activated carbons were produced in 15-20 h with a specific surface area of 1000 m2/g and an overall yield of about 15 wt %. For the sake of comparison, the overall yield of an activated carbon with a specific surface area of 1000 m2/g, produced by the conventional physical activation of coconut shell, is approximately 8 wt %. Unfortunately, we were unable to control the temperature of the carbon during air oxidation of larger samples. To take control of the vigorous exotherm, we initiated this study of carbon oxidation in boiling water. This study builds upon the earlier research of Conesa et al.,16 who synthesized activated carbon by oxidizing carbonized charcoal in hot liquid water (373-473 K) containing dissolved oxygen. However, Conesa et al.16 used oxygen derived from the decomposition of hydrogen peroxide in liquid water at high pressure (10 MPa), whereas we employ air-derived oxygen dissolved in boiling water at much lower pressures (typically 3.4 MPa). Also, the low temperatures (503 K) used in this work. The reaction system for air oxidation in boiling water (AOBW) combines the functionality of a bubble column reactor17,18 and wet air oxidation (WAO).19,20 This process involves bubbling compressed air into a reactor that contains carbonized charcoal completely immersed in boiling water. Oxygen in the air bubbles dissolves in the water and subsequently oxidizes and gasifies the carbon. Heat produced by the oxidation chemistry is removed by the steady refluxing of the water. Even
10.1021/ie000461t CCC: $20.00 © 2001 American Chemical Society Published on Web 12/22/2000
Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 579
Figure 1. Schematic diagram of the apparatus used for AOBW of carbonized charcoal.
though this process is similar to the conventional WAO processes employed in the regeneration of spent activated carbons21-23 and in the destruction of organic contaminants in liquid streams,24-26 its main purpose is very different because our goal is to synthesize a valuable material in a controlled environment. Some studies of WAO of carbon have been reported,26-29 but most are concerned with the kinetics of the oxidation of adsorbed species on the carbon instead of the carbon itself. Charest and Chornet21 described a detailed kinetic study of WAO of virgin activated carbon. They employed a rate law with an activation energy of 33.5 kJ/mol and reaction orders of 0 in carbon and 1 in oxygen to fit their experimental data. They concluded that oxidation occurred as a free-radical process similar to the WAO of organics. This paper focuses on small batch syntheses of highquality (specific surface areas approaching 1000 m2/g) activated carbons from macshell-carbonized charcoal using AOBW at relatively low temperatures (
