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Title: Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis

Abstract

The kinetics of xylan removal from quaking aspen, paper birch, American elm, and red maple by water prehydrolysis (autohydrolysis) was reevaluated, and additional data for the water prehydrolysis of southern red oak were obtained. Xylan removal from these hardwood species can be modeled kinetically as the sum of two parallel first-order reactions - one fast and one slow. The rate constant for the fast reaction is highly correlated with the rate constant for the slow reaction for all species studied. The rate constant for initial xylan removal usually reported in the literature is actually a complex function of the rate constants for both the fast and slow reactions and is based solely on the initial data points. This paper presents an improved method for modeling xylan removal that allows modeling throughout the course of its reactions. The reason there are two different rates of xylan removal can be more easily explained on the basis of accessibility rather than any variability in the polymeric structure of the xylan being removed. Thus, the slow rate may be due to a portion of the xylan being embedded within or attached to the lignin via lignin-carbohydrate bonds.

Authors:
Publication Date:
Research Org.:
Improved Chemical Utilization of Wood, Forest Products Lab, Forest Service, US Dept of Agriculture, Madison, WI 53705
OSTI Identifier:
6655128
Alternate Identifier(s):
OSTI ID: 6655128
Resource Type:
Journal Article
Resource Relation:
Journal Name: Wood Fiber; (United States); Journal Volume: 16:2
Country of Publication:
United States
Language:
English
Subject:
09 BIOMASS FUELS; WOOD; HYDROLYSIS; XYLANS; REMOVAL; CHEMICAL REACTION KINETICS; MATHEMATICAL MODELS; CARBOHYDRATES; CHEMICAL REACTIONS; DECOMPOSITION; HEMICELLULOSE; KINETICS; LYSIS; ORGANIC COMPOUNDS; POLYSACCHARIDES; REACTION KINETICS; SACCHARIDES; SOLVOLYSIS 090222* -- Alcohol Fuels-- Preparation from Wastes or Biomass-- (1976-1989); 140504 -- Solar Energy Conversion-- Biomass Production & Conversion-- (-1989)

Citation Formats

Conner, A.H.. Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis. United States: N. p., 1984. Web.
Conner, A.H.. Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis. United States.
Conner, A.H.. Sun . "Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis". United States. doi:.
@article{osti_6655128,
title = {Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis},
author = {Conner, A.H.},
abstractNote = {The kinetics of xylan removal from quaking aspen, paper birch, American elm, and red maple by water prehydrolysis (autohydrolysis) was reevaluated, and additional data for the water prehydrolysis of southern red oak were obtained. Xylan removal from these hardwood species can be modeled kinetically as the sum of two parallel first-order reactions - one fast and one slow. The rate constant for the fast reaction is highly correlated with the rate constant for the slow reaction for all species studied. The rate constant for initial xylan removal usually reported in the literature is actually a complex function of the rate constants for both the fast and slow reactions and is based solely on the initial data points. This paper presents an improved method for modeling xylan removal that allows modeling throughout the course of its reactions. The reason there are two different rates of xylan removal can be more easily explained on the basis of accessibility rather than any variability in the polymeric structure of the xylan being removed. Thus, the slow rate may be due to a portion of the xylan being embedded within or attached to the lignin via lignin-carbohydrate bonds.},
doi = {},
journal = {Wood Fiber; (United States)},
number = ,
volume = 16:2,
place = {United States},
year = {Sun Apr 01 00:00:00 EST 1984},
month = {Sun Apr 01 00:00:00 EST 1984}
}
  • A study was made of the kinetics of xylan hemicellulose removal with 0.10 M HCl at 120 degrees C from quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), American elm (Ulmus americana), red maple (Acer rubrum), and southern red oak (Quercus falcata). The mathematical model developed in Part I to describe the kinetics of xylan removal by water prehydrolysis of these species could be used to model xylan removal with dilute hydrochloric acid. Xylan removal could thus be modelled as the sum of two parallel first-order reactions - one fast and one slow. However, unlike the case with water prehydrolysismore » where the rate constants for the fast and slow reaction processes could be correlated with each other, they could not be correlated for HCl prehydrolysis. Instead, these constant values determined for each species clustered about average values for all the species as a whole. A single set of parameters determined from a nonlinear least squares fit of the experimental prehydrolysis data for all the species as a whole to the model could be used to describe the course of xylan removal from all the species. The fact that one set of parameters could be used suggests that the same reactions are taking place on prehydrolysis and the chemical structure and physical morphology of the xylan hemicellulose were essentially the same in the species studied and probably in all temperate hardwood species. The model thus provides a good approximation of xylan removal from any temperate hardwood with dilute hydrochloric acid at the reaction conditions studied. 20 references.« less
  • Most processes for utilizing wood as a source of chemicals and liquid fuels include a prehydrolysis step to remove the hemicellulose prior to the main hydrolysis of the cellulose to glucose. Two promising prehydrolysis methods, the Iotech steam explosion process and the Stake process, are based on water prehydrolysis (autohydrolysis). The kinetics of water and of dilute (5%) acetic acid prehydrolysis of southern red oak wood over the temperature range of 170 to 240 C were investigated. Kinetic parameters were determined that permitted modeling not only of xylan removal from the wood but also of the occurrence of xylan oligosaccharides,more » free xylose, furfural, and further degradation products in the prehydrolyzate. At lower temperatures (approximately 170 to 200 C), xylan removal could be modeled as the sum of two parallel reactions (one for an easily hydrolyzed portion and one for a more resistant portion of xylan) using the equation derived in Part I. At the highest temperature studied (236.9 C), the removal of xylan from the wood was best modeled as a single reaction with a small fraction of the xylan being essentially nonreactive. The occurrence of xylan oligosaccharides, xylose, furfural, and further degradation products in the prehydrolyzate was modeled as consecutive, irreversible pseudo first-order reactions. A timelag associated with the depolymerization of the xylan oligosaccharides to xylose was accounted for in the model by allowing the apparent rate constant for the formation of xylose to increase exponentially with time to a maximum value.« less
  • A novel two-temperature dilute-acid prehydrolysis of hybrid poplar xylan that exploits the xylan biphasic kinetics at moderate temperatures is described. A lower temperature (140[degrees]C) is applied to hydrolyze the easily hydrolyzable xylan, and a higher temperature (170[degrees]C) is subsequently applied to hydrolyze the remaining xylan. Using a bench-scale percolation reactor, yields of soluble xylose expressed in monomeric xylose equivalents as high as 92% of theoretical have been achieved with only 2% of the xylan being degraded to furfural. The lignocellulosic substrate produced from the pretreatment is readily converted to ethanol at a yield of 94% of theoretical via a simultaneousmore » saccharification and fermentation process in 48 h. In terms of both yield of xylose equivalents and ethanol product level and rate, these improvements are far superior to those previously reported using a single-temperature dilute-acid pretreatment. 36 refs., 5 figs., 1 tab.« less
  • Approximately 150 tons of green, hardwood whole-tree fuel chips were stored outdoors in a 20-foot-high, conical pile at Blacksburg, Va. The pile was monitored for one year to determine the effect of outside storage on the rate of fiber loss, the level of noncombustibles in the fuel, and the pH of water-soluble matter. Chip samples were extracted from within the pile on a regular schedule. Tests for specific gravity, ash content, and pH of water solubles were conducted. At ambient temperatures above 20 degrees C, wood substance loss occurred at a uniform rate 1.5 percent per month. Ash content increasedmore » by 0.23 percentage points per month after the first 4 months of storage. This increased ash content may burden fly ash and grate ash disposal systems unless they are properly designed. The adverse effects of fiber loss and ash content can be virtually eliminated by storage of frozen whole-tree chips during winter months in temperate climates. Water-soluble fractions of wood and bark were highly acidic (4.1 and 3.7 pH, respectively) and changed little over the year long study period. Uncontrolled, the leachate from storage piles may be harmful to the environment. Use of corrosion resistant materials is recommended to counter the effects of acids on conversion, storage, and handling equipment. 18 references.« less