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Acidity (pH) has been realized to be the most important soil characteristic that modulates bioavailability of heavy metals by affecting both the chemical speciation of metals in soil and the metal binding to the active sites on biota.In this work, we show that besides soil pH, metal bioavailability also depends to a certain extent on the type of soil. A better understanding of the role of soil type in regulating metal availability can be achieved with the analysis of soil composition and with calculations using chemical speciation models. Results of pot experiments, in which three different soils were spiked with nickel, show that the EC 50 of total nickel in decreasing the biomass production of oats varies widely (0.7-22.5 mmol kg -1 soil, more than 30 times). pH (4.7-7.0) is the most important factor, explaining up to a factor of 14 difference of nickel bioavailability in the soils. The remaining variation is caused by other differences in soil composition (soil type). The bioavailability and toxicity of nickel in the organic matter-rich soil studied is less than half of that in the sandy and clay soil studied at a similar pH. The chemical calculations using a multi-surface speciation model show that soil organic matter binds Ni much stronger than clay silicates and iron (hydr)-oxides within the acidic pH range, which supports the experimental findings. In all three soils, the EC 50 of Ni expressed in terms of Ni in 0.01 M CaCl 2 soil extraction is rather stable (24-58 µM), suggesting the possibility to use this extraction as an estimation of metal availability in soil.

The main objective of this study is to predict the performance of an industrial‐scale (ID = 5.8 m) slurry bubble column reactor (SBCR) operating with iron‐based catalyst for Fischer–Tropsch (FT) synthesis, with emphasis on catalyst deactivation. To achieve this objective, a comprehensive reactor model, incorporating the hydrodynamic and mass‐transfer parameters (gas holdup, εG, Sauter‐mean diameter of gas bubbles, d32, and volumetric liquid‐side mass‐transfer coefficients, kLa), and FT as well as water gas shift reaction kinetics, was developed. The hydrodynamic and mass‐transfer parameters for He/N2 gaseous mixtures, as surrogates for H2/CO, were obtained in an actual molten FT reactor wax produced from the same reactor. The data were measured in a pilot‐scale (0.29 m) SBCR under different pressures (4–31 bar), temperatures (380–500 K), superficial gas velocities (0.1–0.3 m/s), and iron‐based catalyst concentrations (0–45 wt %). The data were modeled and predictive correlations were incorporated into the reactor model. The reactor model was then used to study the effects of catalyst concentration and reactor length‐to‐diameter ratio (L/D) on the water partial pressure, which is mainly responsible for iron catalyst deactivation, the H2 and CO conversions and the C5+ product yields. The modeling results of the industrial SBCR investigated in this study showed that (1) the water partial pressure should be maintained under 3 bars to minimize deactivation of the iron‐based catalyst used; (2) the catalyst concentration has much more impact on the gas holdup and reactor performance than the reactor height; and (3) the reactor should be operated in the kinetically controlled regime with an L/D of 4.48 and a catalyst concentration of 22 wt % to maximize C5+ products yield, while minimizing the iron catalyst deactivation. Under such conditions, the H2 and CO conversions were 49.4% and 69.3%, respectively, and the C5+ products yield was 435.6 ton/day. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3838–3857, 2015

A multiphase-Eulerian, three-dimensional (3-D), computational fluid dynamics (CFD) model was built to investigate the local hydrodynamics of a pilot-scale (0.29 m ID, 3 m height) Slurry Bubble Column Reactor (SBCR). The model was first validated against the gas holdup radial profiles in an air-water-glass beads system obtained in a 0.254 m ID and 2.5 m height column under ambient conditions at various superficial gas velocities by Yu and Kim (Bubble characteristics in the radial direction of three-phase fluidized beds. AIChE Journal 34, 2069–2072, 1988). The model was next validated against the gas holdup radial profile data for N2-Drakeol-glass beads system obtained in a 0.44 m ID and 2.44 m height reactor, including internals, operating under ambient conditions at various superficial gas velocities by Chen et al. (Fluid dynamic parameters in bubble columns with internals. Chemical Engineering Science 54, 2187–2197, 1999). The model was also validated against experimental data obtained in our lab for N2-Fischer Tropsch (F-T) reactor wax-Fe catalyst system obtained in a pilot-scale, Slurry Bubble column Reactor, SBCR (0.29 m ID, 3 m height) under pressures and temperatures up to 25.9 bar and 490 K, respectively. These three validations led to the selection of the turbulence and interphase drag coefficient models, and the optimization of the solution method, mesh size and structure and the step size. Moreover, the inclusion of RNG k-ε turbulence model coupled with the Wen-Yu (Mechanics of Fluidization. Chemical Engineering Progress Symposium Series 62, 100–111, 1966) / Schiller-Naumann (A drag coefficient correlation. Zeitung Ver. Deutsch. Ing 77, 318–320, 1935) drag correlations, and the mass transfer coefficients were found to provide the most accurate predictions of the experimental data. The CFD model was then used to investigate local gas holdup, liquid recirculation, local turbulence intensities, bubble diameters, and solids distribution throughout our pilot-scale SBCR, operating under typical F-T process conditions. The model predictions showed strong liquid recirculation and backmixing near the walls of the reactor, and the solid-phase velocity vectors closely followed those of the liquid-phase. A relatively high liquid turbulence intensities were observed in the vicinity of the sparger upon startup, however, after reaching a steady state, the liquid turbulence intensities became more evenly distributed throughout the reactor. The liquid turbulence intensities were slightly higher near the center of the reactor, and closely resembled the velocity vectors. Also, the Sauter mean bubble diameters increased, whereas the solids distribution decreased with reactor height above the gas distributor.