Jefferson W. Tester

Correlating quartz dissolution kinetics in pure water from 25 to 625°C

Abstract Results for quartz dissolution kinetics in pure water from five different experimental apparatuses in this study were correlated with measurements from ten previous investigations. A simple global equation with an Arrhenius rate constant formulation satisfactorily represented experimental data spanning temperatures from 25 to 625°C obtained from crushed quartz crystals, quartz sand, and quartz slab samples. Geometric and BET-derived surface areas were successfully used to normalize dissolution rates. An Arrhenius expression describing a kinetic dissolution rate constant with an average activation energy of 89 ± 5 kJ/mol was regressed from the set of dissolution rate data covering eleven orders of magnitude in variation from about 4 × lO −14 mol/m 2 s at 25°C to 1 × 10 −3 mol/m 2 s at 625°C on a geometric area basis.

Quantitative Uncertainty Analysis of Life Cycle Assessment for Algal Biofuel Production

As a result of algaes promise as a renewable energy feedstock, numerous studies have used Life Cycle Assessment (LCA) to quantify the environmental performance of algal biofuels, yet there is no consensus of results among them. Our work, motivated by the lack of comprehensive uncertainty analysis in previous studies, uses a Monte Carlo approach to estimate ranges of expected values of LCA metrics by incorporating parameter variability with empirically specified distribution functions. Results show that large uncertainties exist at virtually all steps of the biofuel production process. Although our findings agree with a number of earlier studies on matters such as the need for wet lipid extraction, nutrients recovered from waste streams, and high energy coproducts, the ranges of reported LCA metrics show that uncertainty analysis is crucial for developing technologies, such as algal biofuels. In addition, the ranges of energy return on (energy) invested (EROI) values resulting from our analysis help explain the high variability in EROI values from earlier studies. Reporting results from LCA models as ranges, and not single values, will more reliably inform industry and policy makers on expected energetic and environmental performance of biofuels produced from microalgae.

Salt precipitation and scale control in supercritical water oxidation-Part A: fundamentals and research

Abstract Supercritical water oxidation (SCWO) is an effective technology for treatment of organics and organic components of aqueous wastes. Commercialization of SCWO processes has been hindered by concerns about corrosion and scale buildup/fouling which, when present, must be accommodated by system design and/or operational procedures. Salts are formed during SCWO when acidic solutions are neutralized to reduce corrosion and may also be present in the waste stream itself. Because salts have low solubility in supercritical water (SCW), they precipitate. Precipitated salts often form agglomerates and coat internal surfaces, thereby inhibiting heat transfer from/to exterior surfaces. When scale buildup is left uncontrolled, plugging of transport lines and/or the reactor can occur. The required cleaning can result in substantial and costly downtime in the SCWO process. General principles and research relevant to SCWO have been reviewed elsewhere. A review of the many technologies available to control scale during SCWO is given in the companion paper by Marrone et al. [J. Supercrit. Fluids (in press)]. Presented here is a review of fundamental principles and research pertinent to the precipitation of salts and scale control at the elevated temperatures and pressures found in an SCWO reactor. First, SCWO is introduced and the physics leading to scale buildup during SCWO is discussed. Next, the phase diagrams of model salt–water systems at relevant conditions are presented. Then, the many phenomena which complicate modeling of heat transfer in SCW (buoyancy, rapidly varying thermophysical properties, etc.) are reviewed and a set of correlations to calculate heat transfer coefficients is provided. Finally, the limited number of controlled experimental studies on scale buildup during SCWO are reviewed.

Salt precipitation and scale control in supercritical water oxidation—part B: commercial/full-scale applications

Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each companys SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.

Solubility of sodium chloride and sulfate in sub- and supercritical water vapor from 450–550°C and 100–250 bar

Abstract Armellini, F.J. and Tester,J.W., 1993. Solubility of sodium chloride and sulfate in sub- and supercritical water vapor from 450–550°C and 100–250 bar. Fluid Phase Equlibria, 84: 123-142. The solubility of sodium chloride in water vapor has been determined at supercritical temperatures ranging from 450–550°C and sub- and supercritical pressures varying from 100–250 bar. Measured sodium chloride concentrations ranged from 0.9–101 ppm (by weight). In the experiments, water vapor was saturated by continuously flowing it through a tube packed with solid salt. The results for sodium chloride agreed well with other studies which used continuous flow methods. Hydrolysis of the solid salt was found as a possible explanation for some of the reported discrepancies in the literature. Experiments with sodium sulfate at 500°C and 250 bar were also performed. Measured sodium sulfate concentrations were around 0.9 ppm, and exhibited unsteady behavior. Though only an estimate of Na2SO4 solubility could be obtained, this value was over two orders of magnitude lower than that for sodium chloride at identical conditions.

Incorporation of parametric uncertainty into complex kinetic mechanisms: Application to hydrogen oxidation in supercritical water

Abstract In this study, uncertainty analysis is applied to a supercritical water hydrogen oxidation mechanism to determine the effect of uncertainties in reaction rate constants and species thermochemistry on predicted species concentrations. Forward rate constants and species thermochemistry are assumed to be the sole contributors to uncertainty in the reaction model with all other model parameters and inputs treated as deterministic quantities. The analysis is conducted by treating the model parameters as random variables, assigning each a suitable probability density function, and propagating the parametric uncertainties through to the predicted species concentrations. Uncertainty propagation is performed using traditional Monte Carlo (MC) simulation and a new, more computationally efficient, probabilistic collocation method called the Deterministic Equivalent Modeling Method (DEMM). Both methods predict virtually identical probability distributions for the resulting species concentrations as a function of time, with DEMM requiring approximately two orders of magnitude less computation time than the corresponding MC simulation. The results of both analyses show that there is considerable uncertainty in all predicted species concentrations. The predicted H 2 and O 2 concentrations vary ± 70% from their median values. Similarly, the HO 2 concentration ranges of +90 to −70% of its median, while the H 2 O 2 concentration varies by + 180 to − 80%. In addition, the DEMM methodology identified two key model parameters, the standard-state heat of formation of HO 2 radical and the forward rate constant for H 2 O 2 dissociation, as the largest contributors to the uncertainty in the predicted hydrogen and oxygen species concentrations. The analyses further show that the change in model predictions due to the inclusion of real-gas effects, which are potentially important for SCWO process modeling, is small relative to the uncertainty introduced by the model parameters themselves.

Diffusional Effects in Simulated Localized Corrosion

The importance of diffusion was investigated under potentiostatic dissolution conditions with wire electrodes contained in inert supports. The artificial cavities created simulated localized corrosion conditions. Current-time behavior at voltages in excess of the critical pitting potential (>0.5V (SCE)) was examined for nickel and stainless steel specimens in concentrated chloride solutions. The effect of changing the concentration or activity gradient of the dissolving metal cations within the artificial cavity was studied by alternating the composition of the bulk solution. Solutions of FeCl/sub 2/, NiCl/sub 2/, CrCl/sub 3/, LiCl, NaCl, MgCl/sub 2/, and CeCl/sub 3/ ranging from 0.5 to 10M were used. Mass transfer models were developed for the observed transient and quasi steady-state periods of dissolution. (auth)

Rock failure mechanisms of flame-jet thermal spallation drilling—theory and experimental testing

Certain polycrystalline rocks will fracture into thin, disk-like fragments when exposed to rapid surface heating. Several current hard-rock drilling methods using supersonic flame-jets as heat sources exploit this behaviour for efficient granite quarrying or blasthole formation. The recent application of weibulls theory of rock failure to quantitatively analyze rock spallation has been extended to allow prediction of chip size distributions and rock surface temperatures at spallation under any intense heat source. Therefore, with measured values of the weibull distribution parameters, surface spallation temperatures can be estimated theoretically. However, finite-element analysis of intergranular stresses developed during transient heating prior to spall formation indicates that additional microcrack initiation will be favoured thereby altering weibull parameters measured at room temperature. Nonetheless, experimentally observed spallation characteristics of barre and westerly granites are consistent with theory when mechanically determined weibull parameters are substituted. Spallation temperatures induced by flame-jet heating were estimated to be below 520 deg C. In all cases, quartz or other mineral solid phase transitions are not expected at these conditions. Laser-induced spallation experiments were less successful, partially because extremely localized heating delivered by the beam greatly complicated the subsequent thermal stress analysis. (Author/TRRL)

Hydrolysis and Oxidation in Subcritical and Supercritical Water: Connecting Process Engineering Science to Molecular Interactions

Key engineering issues influencing the development of supercritical water oxidation (SCWO) for waste treatment were reviewed. Major chemical pathways and kinetics for hydrolysis and oxidation reactions of model organic wastes were discussed. In selective examples, results from extensive laboratory-scale measurements were compared with molecular simulations of solvation and reaction effects in supercritical water. Connections between reaction chemistry and observed corrosion in SCWO processing equipment were discussed to underscore the importance of understanding electrochemical phenomena over a wide range of temperature and density conditions. Research needs for improved understanding of physical and chemical effects in supercritical fluids were identified.

Precipitation of sodium chloride and sodium sulfate in water from sub- to supercritical conditions: 150 to 550 °C, 100 to 300 bar

Abstract Flow experiments simulating the rapid precipitation of salts during the supercritical water oxidation (SCWO) waste treatment process were performed. Aqueous salt solutions were injected into a coaxially flowing supercritical water stream at a constant pressure of 250 bar. Jet concentrations ranged from 0.1 to 10.0 wt % salt with a typical flow rate of 0.5 g min -1 and temperature of 150 °C. The flow rate of the pure supercritical water stream was typically 10.2 g min -1 with an initial temperature of 550 °C. Results from scanning electron microscopy of collected solids, in situ laser transmission measurements, and low-magnification microscopic or visual observation of the jets indicated that, at 250 bar, sodium chloride solutions first pass through a two-phase, vapor-liquid state before solid salt is formed, while sodium sulfate solutions nucleate solids directly from a homogeneous supercritical-fluid phase. Sodium sulfate solids appeared much finer and also more aggregated than sodium chloride solids. Primary sodium sulfate particle diameters were typically between 1 and 3 μm, while some aggregates reached diameters up to about 20 μm. In contrast, sodium chloride solids ranged from 5− to 25-μm shell-like particles for a 0.5 wt %NaCl jet and 20− to 100-μm semispherical particles for a 10.0 wt % NaCl jet. At a subcritical pressure of 200 bar, the average particle size increased dramatically for both salts. In mixed NaCI/Na 2 SO 4 solutions at 250 bar, the extent of small particle nucleation of sodium sulfate decreased with increasing sodium chloride concentration in the jet feed. Both the observed morphology and mixture effects were explained in terms of different isobaric phase behavior.