Reactive Impurities Research Articles

A study of the influence of impurities when discriminating between the terminal and penultimate copolymerization models

We investigate the effect of the presence of reactive impurities during copolymerization on mathematical model discrimination and determine if impurities adversely affect the model discrimination process, depending on the criteria and response(s) used. Nonlinear mechanistic process models are employed in a multiresponse statistical discrimination scenario. The investigation also considers, via several case studies, the extremely interesting question of whether impurity effects could lead to incorrect choices about the copolymerization mechanism itself (terminal versus penultimate model). The answers to the above questions are that reactive impurities do affect the model discrimination process and, what is more, they may mislead researchers as to the choice of the copolymerization mechanism. The latter unexpected result is further illustrated with experimental data from the literature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2319–2332, 2000

Parallel-competitive reactions in a two-dimensional chaotic flow

A parallel-competitive reaction system, A+B1→2P and A+B2→2W, where A and B1 are the reactants, B2 is a reactive impurity, P is the desired product, and W is a byproduct, is simulated in a chaotic flow by solving the differential convection–diffusion–reaction equations. Three different flow conditions are investigated: a predominantly regular system, a globally chaotic flow, and an intermediate case that displays predominantly chaotic behavior but which has four small islands of regular flow. The time evolution and spatial distribution of species concentration depend strongly both on the nature of the flow and on the relative rates of the two reactions. Even in the globally chaotic flow, significant spatial heterogeneity exists throughout the duration of the reactive mixing simulation. Product and waste accumulate in different spatial regions, depending on the relative characteristic times of the two reactions. When the primary reaction is the faster one, waste tends to accumulate in local A-rich regions and the selectivity of product to waste is strongly affected by the degree of chaos in the system. On the other hand, when the side reaction is faster, waste accumulates in segregated regions which have local excesses of B2, but mixing has minimal effect on the quantity of waste produced.

Estimation of impurity and fouling in batch polymerisation reactors through the application of neural networks

The estimation of the amount of reactive impurities and the level of fouling in a batch polymerisation reactor is of strategic importance to the polymerisation industry. It is essential that the level of impurities and reactor fouling are known (estimated) in order to be able to develop robust and reliable monitoring and control strategies. This paper describes two approaches based upon stacked neural network representations. In the first approach, an inverse neural network model of the polymer process is constructed and the initial reaction conditions are predicted. The amount of impurities and reactor wall fouling are then calculated by comparing the predicted values with the nominal initial conditions. In the second approach, a neural network is used to model the dynamic behaviour of the polymer process. The predicted trajectories are then compared with the on-line measurements of conversion and coolant temperatures. The techniques are compared on a first-principles-based simulation of a pilot scale batch methyl methacrylate (MMA) polymerisation reactor.

Prediction of polymer quality in batch polymerisation reactors using robust neural networks

Abstract A technique for predicting polymer quality in batch polymerisation reactors using robust neural networks is proposed in this paper. Robust neural networks are used to learn the relationship between batch recipes and the trajectories of polymer quality variables in batch polymerisation reactors. The robust neural networks are obtained by stacking multiple nonperfect neural networks which are developed based on the bootstrap re-samples of the original training data. Neural network generalisation capability can be improved by combining several neural networks and neural network prediction confidence bounds can also be calculated based on the bootstrap technique. A main factor affecting prediction accuracy is reactive impurities which commonly exist in industrial polymerisation reactors. The amount of reactive impurities is estimated on-line during the initial stage of polymerisation using another neural network. From the estimated amount of reactive impurities, the effective batch initial condition can be worked out. Accurate predictions of polymer quality variables can then be obtained from the effective batch initial conditions. The technique can be used to design optimal batch recipes and to monitor polymerisation processes. The proposed techniques are applied to the simulation studies of a batch methylmethacrylate polymerisation reactor.

Diffusivity and defect reactions of lithium in GaAs

The diffusion of lithium in GaAs is shown to be trap limited at high doping levels. As in the case of other reactive impurity species in semiconductors, the intrinsic diffusivity of Li in GaAs can only be measured in lightly Li-doped material that has high purity or exhibits weak pairing interactions. From charge-density profiles in Zn-doped GaAs measured by the capacitance-voltage technique we determine the intrinsic diffusivity of Li to be ${D}_{i}{=D}_{0}\mathrm{exp}(\ensuremath{-}{E}_{m}{/k}_{B}T)$ with ${E}_{m}=0.67\ifmmode\pm\else\textpm\fi{}0.02 \mathrm{eV}$ and ${D}_{0}=(0.5--2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2} {\mathrm{cm}}^{2}/\mathrm{s}$. In Li-rich, nominally undoped starting material, however, the diffusion is limited by the formation of complexes involving several Li atoms and native defects. The most weakly bound Li atoms are released from the complexes above $100\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and a dissociation energy of ${E}_{d}=1.20\ifmmode\pm\else\textpm\fi{}0.03 \mathrm{eV}$ can be used to characterize the effective diffusion behavior. A previously reported 1.0-eV migration energy for lithium in Li-saturated GaAs at high temperatures is consistent with our observations under the model of trap-limited diffusion.

Water Calorimetry: The Heat Defect.

Domen developed a sealed water calorimeter at NIST to measure absorbed dose to water from ionizing radiation. This calorimeter exhibited anomalous behavior using water saturated with gas mixtures of H2 and O2. Using computer simulations of the radiolysis of water, we show that the observed behavior can be explained if, in the gas mixtures, the amount-of-substance of H2 and of O2 differed significantly from 50 %. We also report the results of simulations for other dilute aqueous solutions that are used for water calorimetry—pure water, air-saturated water, and H2-saturated water. The production of H2O2 was measured for these aqueous solutions and compared to simulations. The results indicate that water saturated with a gas mixture containing an amount-of-substance of H2 of 50 % and of O2 of 50 % is suitable for water calorimetry if the water is stirred and is in contact with a gas space of similar volume. H2-saturated water does not require a gas space but O2 contamination must be guarded against. The lack of a scavenger for OH radicals in “pure” water means that, depending on the water purity, some “pure” water might require a large priming dose to remove reactive impurities. The experimental and theoretical problems associated with air-saturated water and O2-saturated water in water calorimeters are discussed.

Tritium Purification via Zirconium-Manganese-Iron Alloy Getter St 909 in Flow Processes

A zirconium-manganese-iron alloy, St 909, was evaluated as a purifier in tritium handling, transport, and storage applications. High efficiency removal of CH{sub 4}, CO, CO{sub 2}, NH{sub 3}, and O{sub 2} was observed at concentrations of 0.1 to 1% in helium. Gas streams at 100 to 5000 sccm were passed through getters operated at 500-800{degree}C. On-getter residence times of two seconds were required to achieve > 99% removal of these reactive impurities. At this removal efficiency level, the individual impurity capacity of 100 g of St 909 purifier at 800{degree}C was 0.59, 0.28, 0.19, 0.14 and 0.12 moles of CH{sub 4}, CO, CO{sub 2}, O{sub 2} and NH{sub 3}, respectively. Hydrogen containing gases, CH{sub 4} and NH{sub 3}, were cracked on the purifier and the resultant elemental hydrogen was released. Only 8{+-}2 scc of H{sub 2} were retained on 100 g of St 909 at 800{degree}C. These features suggest that this alloy can be employed as an efficient purifier for hydrogen isotopes in inert gas, nitrogen, or perhaps even H, D, or T streams. 12 refs., 4 figs., 1 tab.

Generalized expressions for estimating the current efficiency of ion-exchange membrane and diaphragm chlor-alkali cells

Comprehensive chlorine and caustic current efficiency equations were derived for ion-exchange membrane and diaphragm chlor-alkali cells using material balances that account for all common and significant reactive brine impurities and their respective interactions with Cl2 and/or NaOH. These generalized equations, formulated in terms of molar flow rates of the species entering, exiting, generated in or consumed in the cells, require a knowledge of the feed and depleted brine compositions to calculate the current efficiency of either an individual electrolyser or an entire circuit. It was shown that the global current efficiency expression can be degenerated to the various currently employed equations in the chlor-alkali industry for estimating the current efficiency with certain assumptions or approximations. These simplified assumptions and approximations, usually involving omission of some brine components or anolyte reactions, were shown to result in differences in the published current efficiency expressions. Disparities between the caustic and chlorine current efficiency of membrane and diaphragm cells were quantified and approaches to achieve either high ηCl2 or ηOH were also outlined. Current efficiency equations, based on an analysis of anode gases and anolyte composition, require an accurate measure of all the components in the feed and depleted brine. A simplified method, which is analytically less cumbersome, is also presented to estimate the caustic current efficiency of ion-exchange membrane chlor-alkali cells.

Effect of impurities on continuous solution methyl methacrylate polymerization reactors—II. Closed-loop real-time control

Closed-loop experimental results for conversion control in the presence of reactive impurities in continuous solution methyl methacrylate polymerization reactors are presented. The desirable conversion level is maintained using initiator flow rate as the manipulated variable. During operation, known or unknown types and amounts of reactive impurities are injected into the reactor as disturbances. Conventional proportional-integral-derivative (PID) control algorithms (e.g. PID, Smith predictor and Dahlin's control) and stochastic control strategies [e.g. minimum variance control (MVC), constrained MVC and one-step optimal control] are evaluated first at the simulation level to identify promising control runs. Then, closed-loop experiments follow in a series of two- to three-day continuous runs to verify the simulation results. This study triggers many interesting ideas for practical polymer reactor control.

Tritium purification via zirconium–manganese–iron alloy getter St 909 in flow processes

A zirconium–manganese–iron alloy, St 909, was evaluated as a purifier in tritium handling, transport, and storage applications. High efficiency removal of CH4, CO, CO2, NH3, and O2 was observed at concentrations of 0.1% to 1% in helium. Gas streams at 100–5000 sccm were passed through getters operated at 600–800 °C. On-getter residence times of two seconds were required to achieve ≳99% removal of these reactive impurities. At this removal efficiency level, the individual impurity capacity of 100 g of St 909 purifier at 800 °C was 0.59, 0.28, 0.19, 0.14, and 0.12 moles of CH4, CO, CO2, O2, and NH3, respectively. Hydrogen containing gasses, CH4 and NH3 were cracked on the purifier and the resultant elemental hydrogen was released. Only 8±2 sccm of H2 were retained on 100 g of St 909 at 800 °C. These features suggest that this alloy can be employed as an efficient purifier for hydrogen isotopes in inert gas, nitrogen, or perhaps even H, D, or T streams.

Studies on the response of potentiometric sensors and the characterization of reactive impurities in propylene carbonate and gamma‐butyrolactone as solvents

AbstractThis article deals with an experimental protocol developed to characterize reactive impurities that seriously compromise the utility of relatively insert solvents such as those discussed here. It was found that potentiometric sensors for hydrogen, silver(I), and copper(II) ions give Nernstian response over a broad activity range in solutions of widely differing buffer capacity in γ‐butyrolactone (GBL), a promising but inadequately explored solvent for a variety of applications. Advantage was taken of the proper response of these sensors in applying the “ion probe method” described before to characterize impurities in both unpurified and purified γ‐butyrolactone on the basis of their reactivities rather than their concentrations alone. The same sensors were also used in conventional potentiometric titrations of impurities in propylene carbonate (PC), a solvent that has been extensively used in fundamental studies of solute‐solvent interactions as well as in the construction of commercial lithium batteries. The results of the potentiometric studies served as a guide in the design of appropriate gas chromatographic measurements. Three impurities not reported before were found in a reagent grade propylene carbonate: triethylamine and chloride ion, both at concentrations above 10−4 M, as well as 1,2‐dibromopropane. The effectiveness of different purification procedures was studied.

Kinetics of the reaction of OH radicals with t‐amyl methyl ether revisited

AbstractThe kinetics of the reaction of OH radicals with t‐amyl methyl ether (TAME) have been reinvestigated using both absolute (flash photolysis resonance fluorescence) and relative rate techniques. Relative rate experiments were conducted at 295 K in 99 kPa (740 torr) of synthetic air using ethyl t‐butyl ether, cyclohexane, and di‐isopropyl ether as reference compounds. Absolute rate experiments were performed over the temperature range 240–400 K at a total pressure of 4.7 kPa (35 torr) of argon. Rate constant determinations from both techniques are in good agreement and can be represented by k1=(6.32 ± 0.72) × 10−12 exp[(−40 ± 70)/T] cm3 molecule−1 s−1. equation image Quoted errors represent 2σ from the least squares analysis and do not include any estimate of systematic errors. We show that results from the previous kinetic study of reaction (1) are in error due to the presence of a reactive impurity. Results are discussed in terms of the atmospheric chemistry of TAME. © 1993 John Wiley & Sons, Inc.

Oxidation of CH3CCI3 and CH3CFCI2 in the atmosphere : kinetic study of OH reactions

The kinetics of the reactions OH + CH3CCl3 (1) and OH + CH3CFCl2 (2) have been studied using the EPR discharge-flow method. Highly purified reagents were used in order to avoid any overestimation of the measured rate constants due to the presence of reactive impurities. The results are : k1 = (1.2 ± 0.2) x 10-14 at 298 K and k2 = (2.4 ± 0.6) x 10-12 exp [-(1790 ± 100)/T] between 298 K and 479 K, in cm3 molecule-1S-1. These data are compared with earlier ones, and their atmospheric significance is discussed in relation with the calculation of the tropospheric lifetimes or ozone depletion potentials of man-made halogenated molecules.

On discrepancies between different laboratory measurements of kinetic parameters for the reaction of the hydroxyl radical with halocarbons

This paper discusses laboratory-measured rate coefficients for the reaction of hydroxyl radicals with halogenated organic compounds. A knowledge of such rate coefficients is vital in assessing the tropospheric lifetimes of the compounds. There appear to be significant differences between measurements from different laboratories. Possible sources of the discrepancies are discussed critically, and the possible influence of the technique employed (flash-photolysis or discharge-flow) is considered. Reactive impurities, secondary losses of OH and unanticipated heterogeneous processes might each invalidate the experimental measurements. The effects of these influences are analysed in turn. The conclusion is drawn that the most probable source of error derives from surface phenomena, and some experimental evidence is given that supports this idea. In these circumstances, it may turn out that the most reliable data are those obtained at high temperatures and that, contrary to received wisdom, it may be advantageous to extrapolate high-temperature data to calculate rate coefficients at tropospheric temperatures.

Heterogeneous processes of new phase growth in the system of various sinks: ion-beam synthesis of silicon oxynitride

In this paper we continue to model the formation of new phases in silicon after implantation of reactive ions. We generalize our foregoing investigation, and now two different reactive species of similar chemical characteristics are considered. New phase inclusions grow as a result of the impurity diffusion to a distributed sink. This consists of spherical nuclei which have a complex chemical composition and do not touch each other. The numerical results of this model agree with the experimental data, obtained by Auger depth profiling only if we introduce the dependence of the diffusion coefficients of the reactive impurities on the matrix imperfection.

Integrated processing for microelectronics science and technology

This paper is a review of integrated processing-an approach to microelectronics fabrication in which sequential processes are linked by wafer transfer through a clean, controlled environment (e.g., high vacuum or inert gas). The approach is rapidly becoming the state of the art in microelectronics research, development, and manufacturing. In microelectronics research, it provides a means for advancing mechanistic understanding and material quality through in situ fabrication of test structures and extensive in situ diagnostics. In microelectronics development and manufacturing, it promises process simplification, improved contamination control and yield, and potentially more flexible equipment utilization. With increasing emphasis on ultraclean processing, involving control of reactive impurities as well as particles, and on real-time process monitoring and control, applications of integrated processing are moving toward a common ground in which state-of-the-art research techniques can be used to address key issues in development and manufacturing, and provide in return substantive guidelines for manufacturing design and practice.

By-products in the derivatization of amines with the chiral reagent 2,3,4,6-tetra-O-acertyl-β- d-glucopyranosyl isothiocyanate and their elimination

The formation of the desired diastereomeric thiourea derivatives did not proceed quantitatively when β-adrenoceptor blocking drugs were allowed to react with 2,3,4,6-tetra-O-acetyl-β- d-glucopyranosyl isothiocyanate (GITC). A competing reaction caused losses, which increased with increasing excess of the reagent (decreasing relative amounts of amine), amounting to as much as 50%. Measurements indicate that the by-products were due to an impurity in the commercial reagent, but also in GITC produced in-house from the corresponding bromide. The side-reaction could be completely eliminated by pretreatment of the reagent solution with a limited amount of another amine. Similar observations of a reactive impurity were made with the chiral reagent α-methylbenzyl isothiocyanate.

Ion-beam synthesis of new phases in silicon interpreted as the decay of a supersaturated solid solution

An approach has been suggested for the study of substoichiometric ion-beam synthesis (IBS) as a process of decay of a supersaturated solid state solution. It is supposed in this approach that the possibilities for IBS can be considerably expanded with the use of this process based on the mass transport of reactive impurities to specially created centres of nucleation.

Ion-beam synthesis of new phases in silicon interpreted as the decay of a supersaturated solid solution

An approach has been suggested for the study of substoichiometric ion-beam synthesis (IBS) as a process of decay of a supersaturated solid state solution. It is supposed in this approach that the possibilities for IBS can be considerably expanded with the use of this process based on the mass transport of reactive impurities to specially created centres of nucleation.

SiO2 film decomposition reaction initiated by carbon impurities located at a Si-SiO2 interface

We have annealed Si-SiO2 structures at an elevated temperature in He(g) with SiO(g) added to the ambient. A SiO2 decomposition reaction that normally forms large voids in a thin SiO2 film during annealing in He(g) is not detected with the added SiO(g). Instead, chemically reactive SiC impurity sites which initiate the SiO2 decomposition reaction are found segregated along crystallographic planes in the substrate at the Si-SiO2 interface. The mechanism and technological importance of this interfacial reaction are discussed.