Sensitivity and uncertainty of reaction mechanisms for photochemical air pollution

A sensitivity/uncertainty analysis is performed on a mechanism describing the chemistry of the polluted troposphere. General features of the photochemical reaction system are outlined together with an assessment of the uncertainties associated with the formulations of mechanistic details and rate data. The combined effects of sensitivity and uncertainty are determined using the Fourier amplitude sensitivity test (FAST) method. The results of this analysis identify the key parameters influencing the chemistry of NO₂, O₃, and PAN. Based on these findings, a series of recommendations are made for future experimental kinetic studies.     

Oxidative Lime Pretreatment of Alamo Switchgrass

Previous studies have shown that oxidative lime pretreatment is an effective delignification method that improves the enzymatic digestibility of many biomass feedstocks. The purpose of this work is to determine the recommended oxidative lime pretreatment conditions (reaction temperature, time, pressure, and lime loading) for Alamo switchgrass (Panicum virgatum). Enzymatic hydrolysis of glucan and xylan was used to determine the performance of the 52 studied pretreatment conditions. The recommended condition (110°C, 6.89 bar O₂, 240 min, 0.248 g Ca(OH)₂/g biomass) achieved glucan and xylan overall yields (grams of sugar hydrolyzed/100 g sugar in raw biomass, 15 filter paper units (FPU)/g raw glucan) of 85.9 and 52.2, respectively. In addition, some glucan oligomers (2.6 g glucan recovered/100 g glucan in raw biomass) and significant levels of xylan oligomers (26.0 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor. Combining a decrystallization technique (ball milling) with oxidative lime pretreatment further improved the overall glucan yield to 90.0 (7 FPU/g raw glucan).     

Oxidative Lime Pretreatment of Dacotah Switchgrass

Oxidative lime pretreatment increases the enzymatic digestibility of lignocellulosic biomass primarily by removing lignin. In this study, recommended pretreatment conditions (reaction temperature, oxygen pressure, lime loading, and time) were determined for Dacotah switchgrass. Glucan and xylan overall hydrolysis yields (72 h, 15 FPU/g raw glucan) were measured for 105 different reaction conditions involving three different reactor configurations (very short term, short term, and long term). The short-term reactor was the most productive. At the recommended pretreatment condition (120 °C, 6.89 bar O₂, 240 min), it achieved an overall glucan hydrolysis yield of 85.2 g glucan hydrolyzed/100 g raw glucan and an overall xylan yield of 50.1 g xylan hydrolyzed/100 g raw xylan. At this condition, glucan oligomers (1.80 g glucan recovered/100 g glucan in raw biomass) and xylan oligomers (25.20 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor, which compensate for low pretreatment yields.     

Characterization and aging of aqueous vesicular dispersions of sodium 4-(1′-heptylnonyl)benzenesulfonate

Vesicles formed by sonication of aqueous dispersions of liquid crystals of the double-tailed surfactant sodium 4-(1′-heptylnonyl)benzenesulfonate (SHBS) are examined with several techniques. The average diameter of the vesicles prepared in water is about 450 Å. The average size decreases when prepared in NaCl or at higher surfactant concentrations. The presence of a few large liquid crystallites in the dispersion, as detected by fast-freeze cold-stage transmission electron microscopy, is shown to severely bias the measurement of vesicle sizes by quasi-elastic light-scattering techniques. The commonly used techniques of gel-permeation chromatography and ultrafiltration are shown to be ineffective in separating liquid crystals from SHBS vesicle dispersions. Vesicle preparation in the presence of uranyl acetate is shown to dramatically reduce the vesicle size. The spontaneous, irreversible reversion of vesicles to liquid crystallites as the dispersions age is documented and proves that SHBS vesicles are not equilibrium structures in water or brine.