Mass Balance Of Species Research Articles

Mystery of the high chlorine consumption in disinfecting a chemically enhanced primary saline sewage

Stonecutters Island Sewage Treatment Works is one of the largest sewage treatment plants in the world and consists mainly of a chemically enhanced primary treatment (CEPT) unit and a disinfection unit. It has long been realized that most of the dosed chlorine (15 mg/L) is lost at the beginning part of the disinfection unit during disinfection of the CEPT effluent. Lab-scale tests were therefore conducted in this study to determine the causes. Because ferric chloride is used in CEPT, ferrous iron in the CEPT effluent (from the reduction of ferric iron) was initially thought to be the main chlorine consumer. However, the chlorine consumption by ferrous iron was found to be 1.2 mg/L at most. Suspended solids and sulfide also did not contribute significantly to the chlorine consumption. Batch tests were therefore conducted to evaluate the effects of mixing condition and chlorine stock solution concentration on the chlorine consumption. Less chlorine was consumed upon increased mixing. Using a high-concentration chlorine stock solution (25000 mg/L) resulted in a 3-times-higher chlorine consumption in the absence of mixing than using a low-concentration chlorine stock solution (2500 mg/L). By correlating the losses of ammonia and total nitrogen with the chlorine consumption, we hypothesized that the use of a high-concentration chlorine stock solution under poor mixing leads to a localized high ratio of chlorine to ammonia, resulting in breakpoint chlorination and an unusually excessive chlorine consumption. A novel apparatus was developed to quantify the nitrogen gas generated during chlorination of a simulated wastewater, and the mass balance of nitrogen-containing species (i.e., ammonia, nitrogen gas, nitrite and nitrate) during the chlorination was inspected. The good fit between the measured chlorine consumption and that back-calculated from nitrogen-containing species verified our hypothesis. Finally, it needs mentioning that the high chlorine consumption and the breakpoint chlorination may occur during chlorine disinfection of any sewage effluents with relatively high ammonia levels; thus it is suggested that either not-too-high concentrations of chlorine stock solutions or sufficient mixing should be applied during disinfection of the sewage effluents.

The interplay of methane and ammonia as key oxygen consuming constituents in early stage development of Base Mine Lake, the first demonstration oil sands pit lake

The efficacy of oil sands water capped tailings technology is currently being assessed in the first pilot pit lake in the Athabasca Oil Sands region (AOSR). Base Mine Lake (BML, Syncrude Canada Mildred Lake mine, Fort McMurray AB Canada) was commissioned in December of 2012 and consists of circa 40 m depth of fluid fine tailings (FFT) overlain by circa 10 m of a freshwater cap. As this is the first oil sands pit lake, it is unknown to what extent known oxygen consuming constituents (OCC) such as methane, hydrogen sulfide and ammonia will mobilize from the underlying FFT layer in BML and impact oxygen concentrations within the water cap. Thus, the field objectives here were to characterize the physico-chemistry and geochemistry of the BML water cap from the FFT water interface (FWI) to the surface over two summers (2015 and 2016). Results identify that the ∼10 m water cap thermally stratifies and that oxygen persists, albeit at low levels (i.e. <5% saturation) to the FWI during the summer season for both years. Consistent with the FFT acting as an OCC source, aqueous CH4, ∑H2S and NH4+ concentrations were highest closer to the FWI, decreasing upwards into the water cap. As O2 persists to the FWI, ∑H2S was rapidly removed within this region, with little mobilized to the overlying water as evidenced by nondetectable ∑H2S within 1 m of the FWI. In contrast, CH4 and NH4+ concentrations were detectable higher up into the water cap, indicating incomplete oxidation at the FWI most likely due to oxygen limitation. In 2015, CH4 was the only identified variable significantly negatively related to BML water cap oxygen concentrations. However in 2016, NH4+ emerged as an important OCC negatively related to water cap oxygen concentrations in addition to CH4. Mass balance of nitrogen redox species throughout the water column are consistent with active nitrification occurring in 2016; which was not evident in 2015 results. Thus, within 4 years of commissioning, nitrification has become an active and important process affecting water column oxygen concentrations within this pit lake. This is in contrast to an absence of detectable microbial nitrification in active oil sands tailing ponds, despite high levels of ammonia. While the absence of nitrification occurring in oil sands tailings ponds has previously been suggested to reflect naphthenic acids toxicity, results here identify that some lower limit of oxygen may also be required for this metabolism to flourish and that at very low oxygen levels (i.e. <5% saturation), nitrifying microbes appear to be outcompeted by methanotrophs, at least during early establishment stages within a pit lake.

Enhanced Fe(III)-mediated Fenton oxidation of atrazine in the presence of functionalized multi-walled carbon nanotubes

In this study we reported that the presence of functionalized multi-walled carbon nanotubes (FCNT-H) would greatly enhance the degradation of atrazine (ATZ), a model contaminant, in the Fe(III)-mediated Fenton-like system. Efficient ATZ degradation (>90%) was achieved within 30 min in the presence of 20 mg.L−1 FCNT-H, 2.0 mg.L−1 Fe(III), and 170 mg.L−1 H2O2, whereas negligible ATZ degradation occurred in FCNT-H free system. The structure and surface chemistry of FCNT-H and other CNTs were well characterized. The formed active species were determined based on ESR analysis, and the mass balance of Fe species during the reaction was monitored. In particular, a new method based on ferrozine complexation was proposed to track the formed Fe(II). The results indicated that ATZ was mainly degraded by the generated hydroxyl radical (HO·), and Fe(III)/Fe(II) cycling was still the rate-limiting step. Besides a small fraction of Fe(III) reduced by FCNT-H, a new pathway was revealed for fast reduction of most Fe(III), i.e., reaction of FCNT-H-Fe(III) complexes with H2O2. Comparison of different CNTs-mediated Fe(III)/H2O2 systems indicated that such enhanced effect of CNTs mainly resulted from the surface carboxyl group instead of hydroxyl and carbonyl group. Combined with X-ray photoelectron spectroscopy (XPS) analysis, the electron density migration from Fe(III) to FCNT-H possibly resulted in the fast reduction of FCNT-H-Fe(III) complexes by H2O2. This study enables better understanding the enhanced Fe(III)-mediated Fenton-like reaction in the presence of MWCNTs and thus, will shed new light on how to develop more efficient similar Fenton systems via Fe(III) complexation.

Unit Cell Modelling and Simulation of All Vanadium Redox Flow Battery

AbstractVanadium redox flow battery (VRFB) is one of the promising technologies for large scale renewable energy storage due to its long life time and flexible design. Electrolyte flow rate, electrolyte concentration, electrode porosity, temperature and applied magnitude of current have significant effect on the efficiency of VRFB. In the present work, a simplified dynamic lumped parameter model is developed in MATLAB to provide accurate results at unit cell level which can be used to control and optimize the parameters. The proposed model is based on the mass balance and charge conservation of various vanadium species and hydrogen ion concentration in the electrolyte. Major losses such as ohmic losses and activation losses are included in the cell potential. Recirculation of the electrolyte through external reservoirs is also considered. Numerical simulation demonstrates the effect of change in initial vanadium concentration and operating temperature on unit cell performance. It is shown that variations in electrolyte flow rate, magnitude of applied current substantially alter the charge/discharge characteristics and efficiency.

From MFA to FBA: Defining linear constraints accounting for overflow metabolism in a macroscopic FBA-based dynamical model of cell cultures in bioreactor

Macroscopic dynamical models of cell cultures in bioreactor are made of sets of ODEs representing the mass balances of the main macroscopic species (biomass, main substrates and metabolites). They can be coupled to a Flux Balance Analysis (FBA) linear program whose solution gives metabolic flux values at each time instant. The linear constraints used in this linear program are made of positivity constraints for the fluxes, equalities which link the fluxes to simple macroscopic models of the substrate uptake rates and a set of equalities and inequalities corresponding to some biological assumptions. In this paper, a methodology is proposed to help defining this latter set of linear constraints, with a special focus on overflow metabolism description. It is shown how these linear constraints, together with the objective cost function to be used in the linear program, can be derived from a preliminary Metabolic Flux Analysis (MFA) and a careful comparison of the admissible metabolic flux intervals obtained either using both input and output flux measurements or using only input flux measurements. Finally, a method is proposed for estimating the dynamical model output uncertainties resulting from the intervals of admissible fluxes. In a case study made of hybridoma cell batch and fed-batch cultures, it is shown how to model overflow metabolism on both glucose and glutamine thanks to inequality constraints. The linear program corresponding to a FBA problem is then used in a macroscopic dynamical model which is able to reproduce biomass, substrates and metabolites concentration time profiles in direct and cross validation.

Kinetics and modeling of iodoform degradation during UV/chlorine advanced oxidation process

Iodoform (CHI3) is an emerging disinfection by-product (DBP) that may be formed during pre-oxidation or disinfection processes in drinking water treatment. Degradation kinetics, modeling and mechanism of CHI3 by combined UV/chlorine advanced oxidation processes (AOPs) were studied in this manuscript. CHI3 was effectively removed by UV/chlorine process with the reactions followed pseudo-first order kinetics. The contributions of direct UV photolysis as well as indirect photolysis (hydroxyl radicals (OH) to CHI3 degradation during UV/chlorination under different experimental factors were investigated and determined as 20.3% and 79.7% at pH 5 to 97.1% and 2.9% at pH 9, respectively. Chlorine dosage and CHI3 concentration had slight effects on the contributions of different degradation pathways. NOM and bicarbonate have negative effects on CHI3 degradation. The degradation model of CHI3 during UV/chlorine processes was established, and the satisfactory match of the model calculation results and the experimental data were found. The reaction rate constant between CHI3 and UV light as well as CHI3 and OH were determined as 3.43×10−3s−1 and 7.7×109M−1s−1, respectively. On the basis of the iodine species (such as IO3−, HOI, I2 and I3−) mass balance analysis, the degradation pathways of CHI3 were proposed and IO3− contributed 13.7% of the total liberated iodine species during UV/chlorination. These results demonstrated that UV/chlorination process is a promising AOP technology for the treatment of water containing CHI3.

On modeling and optimization of micro-photosynthetic power cells

In this paper, we describe a model for a micro photosynthetic power cell (μPSC). μPSC is an electrochemical device which harnesses electrons and protons from the electron transport chains of photosynthesis (in the presence of light), and respiration (in the absence of light) occurring inside an oxygenic photosynthetic organism to generate electricity. μPSC has been identified as a potential power generating device for low power applications. A one dimensional model for a μPSC based on mass balances of species in the anode and cathode chambers considering unsteady one dimensional diffusion is developed. The governing equations are solved to obtain the v−i characteristics of a μPSC device. The unknown parameters in the model are estimated from experimental data. The model with its estimated parameters is validated using test v−i characteristic data. Simulation analysis is performed to understand performance improvement possibilities. Explicit optimization formulations to identify promising device designs are also explored.

A comprehensive dynamic model for downdraft gasifier using heat and mass transport coupled with reaction kinetics

A dynamic multiphase model combining mass and energy transport with kinetics for the gasification of wood in a downdraft gasifier is developed. The developed model takes into account the mass and energy balance of both solid and gas species. It also includes the reaction kinetics of various phenomena i.e. drying, pyrolysis, combustion and reduction occurring inside the gasifier. The global kinetics of pyrolysis, combustion kinetics of gaseous and solid species, homogenous and heterogeneous reduction kinetics and diffusion controlled moisture evaporation are incorporated in the model. The developed model is simulated to predict various variables such as biomass consumption rate, producer flow rate and its composition throughout the gasifier at various locations at different time. The model is validated with the experimental data reported in our earlier study. The simulation results are found to be well in agreement with the experimental data. The dynamic behavior of downdraft gasifier is studied by analyzing the axial profiles of temperature, concentration and density at different time.

Model-based evaluation of an integrated autothermal biomass gasification and solid oxide fuel cell combined heat and power system

Numerical analysis of combined heat and power plant consisting of a solid oxide fuel cell and autothermal gasification system has been made for several cases of different composition of fuel relevant to air and steam blown biomass gasification process. Wet wood is fed to the fixed-bed downdraft gasifier and gaseous fuel is produced then after gas cleaning and conditioning can be used in solid oxide fuel cells. The integrated plant is investigated by thermodynamic modeling combining a one-dimensional model of direct internal intermediate planar type solid oxide fuel cell which allows monitoring the temperature gradients along the cell length in different operating conditions and a zero-dimensional autothermal gasifier. The solid oxide fuel cell mathematical model is developed based on gas species mass balances, energy balance, and an electrochemical model beside the kinetics describing internal reforming and water-gas shift reactions. Such a model can be integrated with adiabatic gasification modeling which includes atom balance conservation for assumed gas species and a modified thermodynamic equilibrium analysis. Both gasifier and solid oxide fuel cell models are verified against experimental and previous numerical data available in the literature. Two main parameters, namely modified equivalence ratio and air-to-steam ratio are investigated and the most important cycle parameters such as power, electric and combined heat and power efficiencies, temperature gradients along the cell length, and mole fractions of gaseous species of the produced fuel are analyzed. It has been revealed that any increase in air-to-steam ratio at fixed modified equivalence ratio leads to penalty on cold gas efficiency of the gasifier and both solid oxide fuel cell and combined heat and power plant electric efficiencies. Increased air-to-steam ratio at constant modified equivalence ratio produces a mixture with lower low heating value, higher steam-to-carbon ratio, rich in CO and lower in CH4 content. Under this condition the operating temperature of the cycle and solid oxide fuel cell increases and consequently improves the operating voltage of the cell and combined heat and power efficiency of the plant. On the other hand, results show that gasification with increased modified equivalence ratio at constant air-to-steam ratio produces mixtures richer in CH4 and CO, poorer in H2 with higher low heating value and cold gas efficiency, and lower steam-to-carbon ratio. Such condition improves the electric efficiency of the solid oxide fuel cell and the integrated plant, but the combined heat and power efficiency of the cycle decreases due to decreased operating temperature of the solid oxide fuel cell and the cycle.

Insight into photochemical oxidation of Fenuron in water using iron oxide and oxalate: The roles of the dissolved oxygen

Abstract The role of dissolved oxygen in the degradation and mineralization of fenuron in the photo-Fenton-like system by using a natural iron oxide and oxalate was studied in detail during this study; the natural iron oxide (NIO) α-Fe2O3 was used as an iron source at acidic pH. The dissolved oxygen concentration variation was followed during the process. The dissolved oxygen effect was elucidated by carrying out experiments in the presence and absence of oxygen. It was found that the degradation and mineralization rate were strongly affected by changes in the dissolved oxygen concentration. In the absence of oxygen, we noticed a slight degradation of fenuron (22% in 360 min); this degradation was caused only by the photo-reduction of Fe(III). Ferrous ions were continually generated in this way. Ferrous irons were not oxidized to Fe (III) because of the lack of dissolved oxygen. Moreover, no production of hydrogen peroxide was noticed. The mineralization rate depended strongly on the abundance of dissolved oxygen; fenuron was totally degraded in 120 min in an oxygenated system, in 150 min in a forced aerated system and in 180 min in a free aerated system. Dissolved oxygen permitted the production of species that influenced the degradation efficiency; it improved hydrogen peroxide production under irradiation. The superoxide ion can react with organic radicals leading to an opening ring reaction. The mass balance of the reactive species showed that dissolved oxygen was the main reactive consumed during the process. Acute toxicity of the fenuron degradation by-products was measured by a bioluminescence test on a Microtox 500 analyzer and the results were expressed as the percentage of inhibition of the luminescence emitted by the bacteria Vibrio fisheri. Bio-assays showed that the toxicity decreased with irradiation time which proved that the intermediates, generated during photo-oxidation by the photo-Fenton-like system by using NIO, are less toxic than fenuron.

Implementation of chemical reaction equilibrium by Gibbs and Helmholtz energies in tubular reactor models: Application to the steam–methane reforming process

In the tubular reactor models the mass and heat source terms due to chemical reactions in the species mass balances and temperature equation are conventionally determined from kinetic rate expressions. In many cases a kinetic rate model is not available but the chemical equilibrium conversion can be determined from reaction equilibrium calculations minimizing the Gibbs or Helmholtz free energies. Although a process is believed to behave physically like a tubular reactor, previous feasibility and design studies have typically disregarded fluid flow and mass- and heat transfer limitations and performed a classical chemical equilibrium calculation. For non-adiabatic cases, valuable information on the heat transfer flux limitations of the chemical process at the wall (often provided by a specified axial heating/cooling media temperature profile) is lost when simplifying a model representing the physical tubular reactor process behavior by considering a classical thermodynamic system having uniform state properties. For this reason, in the present study, a new type of tubular reactor model, which we name the differential Gibbs (or Helmholtz) reactor model, is presented to improve on the conventional feasibility and design model. In the differential Gibbs (or Helmholtz) reactor model, the chemical conversion and the reaction heat are determined assuming chemical reaction equilibrium conditions along the axial flow direction by minimizing the Gibbs (or Helmholtz) free energy. The new model is verified through comparison with the conventional differential tubular model using the fast reaction kinetics of the steam–methane reforming process and neglected mass diffusion limitation of the catalyst.

MC-Smooth: A mass-conserving, smooth vorticity–velocity formulation for multi-dimensional flows

A novel vorticity–velocity formulation of the Navier–Stokes equations – the Mass-Conserving, Smooth (MC-Smooth) vorticity–velocity formulation – is developed in this work. The governing equations of the MC-Smooth formulation include a new second-order Poisson-like elliptic velocity equation, along with the vorticity transport equation, the energy conservation equation, and Nspec species mass balance equations. In this study, the MC-Smooth formulation is compared to two pre-existing vorticity–velocity formulations by applying each formulation to confined and unconfined axisymmetric laminar diffusion flame problems. For both applications, very good to excellent agreement for the simulation results of the three formulations has been obtained. The MC-Smooth formulation requires the least CPU time and can overcome the limitations of the other two pre-existing vorticity–velocity formulations by ensuring mass conservation and solution smoothness over a broader range of flow conditions. In addition to these benefits, other important features of the MC-Smooth formulation include: (1) it does not require the use of a staggered grid, and (2) it does not require excessive grid refinement to ensure mass conservation. The MC-Smooth formulation is a computationally attractive approach that can effectively extend the applicability of the vorticity–velocity formulation.

Hydrate phase equilibrium of H2/CH4/CO2 ternary gas mixtures and cage occupancy percentage of hydrogen molecules

Hydrogen, methane, and carbon dioxide are co-existence in many cases such as vent gas from the steam reforming of hydrocarbons. For the purpose of H2 recovery and/or CO2 sequestration via separation technology based on hydrate, the hydrate phase equilibrium of four groups of H2/CH4/CO2 ternary gas mixtures in pure water system were determined using the isothermal pressure-search method in a temperature range of (274.7–284.5)K. During modeling hydrate formation conditions, there is little difference with experimental data no matter whether H2 works as a hydrate former or not. Hydrogen can be regarded as a kind of diluent gas for convenience when presetting the operation conditions. To meet the mass balance of each gas species, the cage occupancy percentages of hydrogen in hydrate have been evaluated. The result illustrates that the occupancy percentage of hydrogen molecules ranges from 0.3 to 2.3 percent in small cage and (2.16–13.6)×10−7 percent in large cage, which deduces that H2 molecules are indeed trapped in hydrate solid.

A numerical study of fixed bed reactor modelling for steam methane reforming process

AbstractA numerical comparison of the pseudo‐homogeneous, conventional heterogeneous, and simplified heterogeneous reactor models is performed for the steam methane reforming process. The pseudo‐homogeneous reactor model consists of a set of partial differential equations, where the diffusional limitations are accounted for by considering the effectiveness factor. The heterogeneous model is divided into two categories: conventional and simplified heterogeneous models. In the conventional heterogeneous model, there are separate equations for the fluid phase species mass balance, the fluid inside the catalyst pores. This is type of reactor model is needed when there is a considerable amount of inter‐phase mass transfer resistance present in the process. However, in the simplified heterogeneous reactor model the mass transport phenomenon is accounted for by accounting the efficiency factors. The model is validated against literature data. Several closures for the intra‐particle mass diffusion fluxes, the Maxwell–Stefan, Wilke, dusty gas, and Wilke–Bosanquet models, have been compared on the level of the catalyst pellet and the impacts of the different particle flux closures on the reactor performance are investigated.The simulations show that the conventional heterogeneous reactor model is necessary for the SMR process, because the effectiveness factor values of different reactions of the SMR process vary along the reactor axis. The maximum deviation between the pseudo‐homogeneous and the conventional heterogeneous reactor model is less than 38 %, whereas between the conventional and simplified heterogeneous reactor models it is less than 21 %. A parametric study of the transport phenomena on the pellet level is recommended prior to any large‐scale reactor simulation to determine what the rate determining transport mechanisms are.

SOFC Stack Model for Integration Into a Hybrid System: Stack Response to Control Variables

Due to the tight coupling of physical processes inside solid oxide fuel cells (SOFCs), efficient control of these devices depends largely on the proper pairing of controlled and manipulated variables. The present study investigates the uncontrolled, dynamic behavior of an SOFC stack that is intended for use in a hybrid SOFC-gas turbine (GT) system. A numerical fuel cell model is developed based on charge, species mass, energy, and momentum balances, and an equivalent circuit is used to combine the fuel cell's irreversibilities. The model is then verified on electrochemical, mass, and thermal timescales. The open-loop response of the average positive electrode-electrolyte-negative electrode (PEN) temperature, fuel utilization, and SOFC power to step changes in the inlet fuel flow rate, current density, and inlet air flow rate is simulated on different timescales. Results indicate that manipulating the current density is the quickest and most efficient way to change the SOFC power. Meanwhile, manipulating the fuel flow is found to be the most efficient way to change the fuel utilization. In future work, it is recommended that such control strategies be further analyzed and compared in the context of a complete SOFC-GT system model.

Volatile organic compounds adsorption onto neat and hybrid bacterial cellulose

Adsorption dynamics of VOCs (volatile organic compounds) vapour from air streams onto fixed bed adsorbent were measured and simulated under various operation conditions. Isopropanol (IPA) and n-hexane (HEX) were selected as representatives of polar and nonpolar VOCs, whereas bacterial cellulose (BC) and BC incorporated with magnetite nanoparticles (M/BC), were tested as adsorbents. An experimental study emphasizing the influence of air superficial velocity (0.7cm/s and 1.7cm/s), operation temperature (30°C and 40°C), adsorbate and adsorbent type, on fixed bed saturation curves was conducted. Optimal adsorption performances evaluated in terms of saturation adsorption capacity were obtained for the adsorption of polar compound (IPA) onto M/BC composite (0.805g/g) and of nonpolar compound (HEX) onto neat BC (0.795g/g), respectively, at high values of air velocity and operation temperature. A mathematical model including mass balance of VOC species, whose parameters were fitted based on experimental data, was developed in order to predict the fixed bed saturation curves. A 23 statistical model indicating a significant increase in adsorption performances with process temperature was validated under the experimental conditions.

Numerical prediction of diffusion and electric field-induced iron nanoparticle transport

Zero valent iron nanoparticles (nZVI) are considered very promising for the remediation of contaminated soils and groundwaters. However, an important issue related to their limited mobility remains unsolved. Direct current can be used to enhance the nanoparticles transport, based on the same principles of electrokinetic remediation. In this work, a generalized physicochemical model was developed and solved numerically to describe the nZVI transport through porous media under electric field, and with different electrolytes (with different ionic strengths). The model consists of the Nernst–Planck coupled system of equations, which accounts for the mass balance of ionic species in a fluid medium, when both the diffusion and electromigration of the ions are considered. The diffusion and electrophoretic transport of the negatively charged nZVI particles were also considered in the system. The contribution of electroosmotic flow to the overall mass transport was included in the model for all cases. The nZVI effective mobility values in the porous medium are very low (10−7–10−4cm2V−1s−1), due to the counterbalance between the positive electroosmotic flow and the electrophoretic transport of the negatively charged nanoparticles. The higher the nZVI concentration is in the matrix, the higher the aggregation; therefore, low concentration of nZVI suspensions must be used for successful field application.

Identification of chlorinated oligomers formed during anodic oxidation of phenol in the presence of chloride

Chlorinated oligomer intermediates formed during the anodic electrochemical oxidation of phenol with a boron-doped diamond electrode were studied at two different concentrations of chloride (5mM and 50mM). Under the same ionic strength, with sodium sulfate being the make-up ion, a 10-fold increase in Cl− led to removal rates 10.8, 1.5, and 1.4 times higher for phenol, TOC, and COD, respectively. Mono-, di- and trichlorophenols resulting from electrophilic substitution were the identified by-products. Nevertheless, discrepancies between theoretical and measured TOC values along with gaps in the mass balance of chlorine-containing species indicated the formation of unaccounted-for chlorinated by-products. Accurate mass measurements by liquid chromatography quadrupole time-of-flight mass spectrometry and MS-MS fragmentation spectra showed that additional compounds formed were dimers and trimers of phenol with structures similar to triclosan and polychlorinated dibenzo-p-dioxins.

A general and robust approach for defining and solving microkinetic catalytic systems

Recent approaches for the rational design of heterogeneous catalysts have relied on first‐principles‐based microkinetic modeling to efficiently screen large phase spaces of catalytic materials for optimal activity and selectivity. Microkinetic modeling allows the calculation of catalytic rate and selectivity under a given set of conditions without a priori assumptions of rate or selectivity controlling steps by simultaneously solving nonlinear algebraic equations comprising species mass balances bound by the pseudo steady‐state approximation. We introduce a general approach to define and solve microkinetic systems that relies solely on its stoichiometric matrix and kinetic parameters of considered reaction steps. Our approach relies on linearization of the microkinetic system, enabling analytical calculation of system derivatives for use in quasi‐Newton solution schemes that exhibit excellent robustness and efficiency with minimal dependence on initial conditions. © 2014 American Institute of Chemical Engineers AIChE J, 61: 188–199, 2015

An efficient numerical model for multicomponent compressible flow in fractured porous media

An efficient and accurate numerical model for multicomponent compressible single-phase flow in fractured media is presented. The discrete-fracture approach is used to model the fractures where the fracture entities are described explicitly in the computational domain. We use the concept of cross flow equilibrium in the fractures. This will allow large matrix elements in the neighborhood of the fractures and considerable speed up of the algorithm. We use an implicit finite volume (FV) scheme to solve the species mass balance equation in the fractures. This step avoids the use of Courant–Freidricks–Levy (CFL) condition and contributes to significant speed up of the code. The hybrid mixed finite element method (MFE) is used to solve for the velocity in both the matrix and the fractures coupled with the discontinuous Galerkin (DG) method to solve the species transport equations in the matrix. Four numerical examples are presented to demonstrate the robustness and efficiency of the proposed model. We show that the combination of the fracture cross-flow equilibrium and the implicit composition calculation in the fractures increase the computational speed 20–130 times in 2D. In 3D, one may expect even a higher computational efficiency.