Activated carbon from pecan shell: process description and economic analysis

Granular activated carbons derived from pecan shells have been shown to adsorb a variety of metal and organic species in various processing wastewaters. Their effectiveness is equivalent to or exceeds comparable commercial carbons in this regard. The objectives of this study were to develop process flow diagrams for the large-scale production of pecan shell-based carbons derived from steam or phosphoric acid activation and to carry out an economic evaluation to estimate the cost to manufacture these carbons. On the basis of laboratory investigations with pecan shell-based carbons, process flow diagrams were established for scale-up, and an economic evaluation of carbon production costs was determined. Process flow diagrams were developed for steam and phosphoric acid activation of pecan shells. Major unit operations included shell preparation for pyrolysis, pyrolysis/activation, washing/drying and collection of the final product. Process parameters were calculated for a production facility processing 10 000 kg/day of shells. Final product yields were 13.7% for steam activation and 29.6% for phosphoric acid activation. Therefore, 1370 kg of steam-activated and 2964 kg of acid-activated pecan shell carbon could be produced per day. Mass losses were incurred during milling (20%), during pyrolysis/activation (82% for steam and 55% of acid), during washing/drying (13%) and during sieving (5%) of the final product. Based on an annual production cost of $1.22 million and an annual production of 448 000 kg of carbon, steam-activated carbon would cost about $2.72 per kg. For the phosphoric acid activation process, annual production costs were estimated at $2.78 million and annual production at 960 000 kg. Therefore, the estimated cost for acid-activated carbon would be $2.89 per kg. (C) 2003 Elsevier Science B.V. All rights reserved.