Article pubs.acs.org/IECR
Kinetic Analysis of Guaiacol Conversion in Sub- and Supercritical Water Tau Len-Kelly Yong† and Matsumura Yukihiko*,‡ †
Department of Mechanical Systems Engineering and ‡Division of Energy and Environmental Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527 Japan ABSTRACT: Hydrothermal conversion of guaiacol, which is used as a model compound to represent lignin, was conducted at temperatures of 300−450 °C for very short residence times of 0.5−40 s at a pressure of 25 MPa using a continuous-flow apparatus designed to rapidly heat the system to the desired temperature. The yields of char, gaseous products, phenolic compounds (phenol, guaiacol, catechol, o-cresol, and m-cresol), and benzene were determined. Guaiacol conversion under hydrothermal conditions occurred rapidly. The formation of compounds containing multiple benzene rings exclusively in the supercritical region indicated that extreme temperatures are needed to produce complex compounds. Under subcritical conditions, however, mainly phenolic and nonphenolic single-benzene-ring compounds were produced. The multiple-benzenering compounds were an important indication of cross-linking between the single-ring compounds to form higher-molecularweight fragments. This is consistent with our previous study of lignin conversion under hydrothermal conditions, which indicated that the formation of chars under supercritical conditions occurred because of the cross-linking reaction between the reactive degradation fragments, resulting in the production of higher-molecular-weight fragments. A reaction network model was proposed and the kinetic parameters for the pathways of guaiacol decomposition were determined by assuming a first-order reaction. It was observed that the rate constant of the overall guaiacol decomposition obeyed Arrhenius behavior, but some of the reactions in the proposed network deviated from this in the supercritical region. Thus, the individual rate constants of each reaction in the network were evaluated for the purpose of classification into ionic or radical reactions.
1. INTRODUCTION Most biomass contains a high percentage of water, which leads to high drying costs when it is to be used in a classical gasification process. Therefore, hydrothermal decomposition of biomass under sub- and supercritical conditions is a promising technology for materials with high moisture contents. Water in the sub- and supercritical states is a suitable medium for chemical reactions owing to large variations in its ionic product and dielectric constant with temperature and density. The ability to manipulate these properties increases its potential for enhancing decomposition of the biomass into its basic phenolic constituents. Lignin, a compound from biomass, is an amorphous threedimensional polymeric substance made up of phenylpropane units, which contain abundant aromatic rings with aliphatic as well as hydroxy and methoxy substituents. Among the compounds in biomass, lignin is the hardest to convert into gaseous or liquid compounds. Resende and Savage1 reported that lignin can be gasified to produce H2 and CH4 under suband supercritical conditions. However, char formation was significantly enhanced, especially under supercritical conditions, thus suppressing gaseous formation pathways. This is a major complication for biomass conversion in hydrothermal reactions because char formation is undesirable and the gaseous formation pathway is favored. In our previous study,2,3 the conversion of softwood lignin was conducted in sub- and supercritical water at 300−450 °C and 25 MPa for very short residence times (0.5−10 s). Under these conditions, we found that lignin was rapidly converted. Supercritical conditions resulted in a high yield of solids, and the formation of char occurred because of cross-linking © 2013 American Chemical Society
reactions between the reactive degradation fragments and residual lignin to produce higher-molecular-weight fragments. The constant formation of char during short and long residence times suggested that this cross-linking occurred instantaneously. The formation of phenolic compounds during short residence times indicated that ether bonds in lignin were easily degraded under supercritical conditions. In addition, gas was formed mainly from lignin, which suggests that it was produced during the early period of lignin depolymerization under supercritical conditions. The decrease in the rate of formation of aromatic hydrocarbons (benzene, naphthalene, and toluene) with temperature under supercritical conditions indicated that the formation of these compounds could be from ionic instead of free-radical reactions. However, previous studies have concluded that the formation of aromatic compounds is usually by pyrolysis via free-radical reactions and the formation of these compounds is enhanced at higher temperatures.3 Therefore, this study strives to investigate the formation of aromatic compounds and establish whether this behavior is similarly observed when guaiacol is used as the feedstock. The mechanisms of lignin conversion are extremely complicated because of its complex polymeric structure, and it is difficult to establish the primary decomposition pathways. Owing to the complexity of the lignin biopolymer, studying monomeric model compounds such as guaiacol is an essential Received: Revised: Accepted: Published: 9048
March 27, 2013 June 5, 2013 June 7, 2013 June 7, 2013 dx.doi.org/10.1021/ie4009748 | Ind. Eng. Chem. Res. 2013, 52, 9048−9059
Industrial & Engineering Chemistry Research
Article
water. This experimental setup avoided a slow heating rate, thus minimizing any intermediate reactions before the desired conditions were achieved. The guaiacol concentration used in the experimental work is limited to 0.1 wt % because of the tubing sizes. Higher concentrations of guaiacol caused plugging to occur in the tubing from char formation during the reaction. The liquid effluent was analyzed using a total organic carbon (TOC) analyzer to quantify the total carbon present in the liquid compounds (nonpurgeable organic carbon) and in the dissolved gaseous products (inorganic carbon). The liquid effluent was tentatively identified using liquid chromatography mass spectrometry (LC-MS), and the compounds were separated with an RSpak DE-413L column (Shodex). The operating conditions were as follows: flow rate, 1.0 mL/min; mobile phase, 50:50 0.1 wt % CH3COOH aqueous solution/ CH3CN (pH 3.5); column temperature, 40 °C. MS analysis was carried out using electrospray ionization with the following parameters: negative mode; probe voltage, 1.5 kV; curveddesolvation-line temperature, 300 °C; block heater temperature, 250 °C; nebulizing gas flow rate, 1.5 L/min. The liquid constituents were subsequently quantified by high-performance liquid chromatography (HPLC) using an RSpak DE-413L column (Shodex). The analytical conditions for the phenolic constituents (phenol, o- and m-cresol, guaiacol, and catechol) were as follows: flow rates, 0.7 and 0.4 mL/min; eluent, 50:50 0.005 M HClO4 aqueous solution/CH3CN; oven temperature, 40 °C; ultraviolet detection wavelength, 220 nm. For benzene, the following analytical conditions were used: flow rate, 0.7 mL/min; eluent, 50:50 0.005 M HClO4 aqueous solution/CH3CN; oven temperature, 30 °C; ultraviolet detection wavelength, 254 nm. The gaseous products were analyzed using a gas chromatograph (GC) equipped with both a thermal conductivity detector (TCD) and a flame ionization detector (FID). H2 was detected by a GC-TCD with N2 as the carrier gas. CO2 and CO were detected by a GC-TCD using He as the carrier gas, while CH4, C2H4, and C2H6 were detected by a GC-FID with He as the carrier gas. The solid product particles were trapped in the inline filter. These solid particles were subsequently removed using an ultrasonic cleaning device, placed onto a porcelain crucible, dried overnight in a desiccator, and then weighed until a constant weight was reached. Elemental analysis of the char obtained from the experimental work was conducted using a Perkin-Elmer series II CHNS/O analyzer 2400. The char particles were further analyzed to determine the functional groups present using a Shimadzu IRPrestige-21/FTIR-8400S. The spectral range was from 4000 to 600 cm−1 with the samples in the form of potassium bromide (KBr) pellets. The reaction pressure was set at 25 MPa with reaction temperatures of 300−450 °C. We also investigated the effects of changing the residence time (0.5, 2, 5, 10, and 40 s), which was varied by adjusting the feedstock flow rates. The total carbon balances (solid + liquid + gaseous) in this study were greater than 95% for all experimental results discussed below. Guaiacol (98.0%) obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), was used and was completely soluble in water. Deionized water (
