Onset Temperature Of Reaction Research Articles

Formation of charcoal from biomass in a sealed reactor

In this paper, samples o cellulose, hemicellulose, lignin, and nine species of whole biomass are pyrolyzed in sealed reactors. Very high charcoal yields (e.g., 40% from cellulose, 48% from Eucalyptus gummifera) were obtained. Higher sample loading (sample mass per unit reactor volume) increased charcoal yield and the associated exothermic heat release and lowered the reaction onset temperature. These effects were induced by the vapor-phase concentrations of the volatile products, and not the system pressure. Addition of water catalyzed the reaction and increased the char yield. These observations suggest that charcoal formation is autocatalyzed by water, an initial pyrolysis product. When whole biomass was used as a feedstock, higher charcoal yields were obtained from species with high lignin and/or low hemicellulose content.

On the Kinetics of Nb5Si3 Compound Formation

The kinetics of intermetallic Nb 5 Si 3 compound formation via the mechanical alloying process was investigated. Interrupted milling process, X-ray diffraction, SEM examination and TEM imaging and diffraction were utilized to characterize changes in the milled powders, while DTA analyses were used to determine the critical and onset temperatures of reaction as a function of milling time. On the basis of experimental results, a kinetic model was proposed for formation of Nb 5 Si 3 via the interrupted MA process. It is suggested that precipitation of Nb 5 Si 3 particles during cooling in the interrupted milling process is responsible for the exothermic reaction after resumption of milling.

Runaway reactions, their courses and the methods to establish safe process conditions

Much of the literature on runaway reactions deals with the consequences such as mechanical damage toxic and flammable release. The DIERS literature provides effective methods for vent sizing where experimental information is requested. Thermal stability measurements provide information on the onset temperature and kinetic data for chemical reactions. There is less information on the way the runaway reactions occur whereas the runaway reactions may have different causes. The purpose of this paper is to describe the various process deviations which can cause a runaway reaction to occur and to discuss the experimental information necessary for risk assessment, the choice of a safe process and the mitigation of the consequences of the runaway reaction. Each possible hazardous process deviation is illustrated by examples from the process industry and/or relevant information obtained from laboratory experiments. The typical hazardous situations to be considered are the following: 1) The homogeneous thermal runaway due to too high a temperature. 2) The homogeneous runaway reaction by unintended introduction of additional reactants or catalyst. 3) The heterogeneous runaway reaction due to too high a local temperature. 4) The heterogeneous runaway reaction caused by slow heat conduction to the outside. 5) The runaway reaction caused by excess residence time at the process temperature (autocatalytic reactions). 6) The runaway reaction caused by reactant accumulation. The controling reactant feed rate is higher than the consumption rate perhaps because the temperature is too low, or the catalyst is absent. 7) The runaway reaction due to the pressurization of the enclosure by gaseous oxidizing intermediates (typical of nitric oxidations). 8) The runaway reaction due to phase separation of unstable species (liquids, solids) by loss of mixing or an cooling. 9) The runaway reaction on mixing of fast reacting chemicals in separate phases. 10) The runaway reaction due to fire or external heating. Considering the various runaway situations, the effectiveness of the following approaches is discussed: -Theoretical and experimental information required for hazard assessment. -Choice of adequate process conditions. -Choice of adequate methods for process control. -Experimental information required for vent sizing

Adiabatic calorimetry for evaluation of thermally induced reactions in liquid propellants

The gun propellants that are aqueous mixtures of the nitrate salts of hydroxylamine (HAN) and triethanolamine (TEAN) are homogeneous, ionic liquids and are susceptible to damage by a variety of contaminants. Reaction initiation that results from the absorption of thermal energy is a method for determination of propellant stability. An adiabatic calorimetry based, diagnostic system utilizes a commercially available accelerating rate calorimeter (ARC) that has been modified to make it suitable for use with liquid gun propellants. The effects of contaminants and impurities on reaction onset temperature which is related to safe storage temperature are readily determined using the ARC. In addition to propellant safety studies, the ARC data also provide insight into propellant efficacy, since ignition and combustion are observed. Reaction onset temperatures, as determined with the ARC, are independent of the quantity of material being stored, thereby eliminating the extrapolation from small laboratory samples that is a common uncertainty in many stability evaluation test methods. The presence of nitric acid, a common propellant contaminant, affects both storage stability and propellant efficacy in the gun. The data indicate that storage stability is adversely affected to a much greater extent than efficacy.

Influence of changes in fine structure on thermal properties of cotton fibres

Examinations of the structural changes of mercerized cottons in sodium hydroxide or liquid ammonia were followed by determination of the degree of polymerization, sorption ratio and X-ray percentage crystallinity. Comparisons were made of the thermal stability, as indicated by differential thermal analysis and thermogravimetric analysis of the mercerized cottons. A decrease in the percentage crystallinity was found to lower then onset and peak temperatures of the major decomposition reaction. It was also found that the temperature where the major weight loss began increased in the sequence: cellulose III, cellulose II, and cellulose I. Samples in cellulose III crystals form were less thermally stable than samples of cellulose I and cellulose II. It will be noted however that the difference between cellulose II and cellulose III was small. Cotton cellulose II gave a lower yield of levoglucosan than either cotton cellulose I or II.