Analysis of a cathode catalyst layer model for a polymer electrolyte fuel cell

A macroscopic model for a non-isothermal cathode catalyst layer (CL) in a proton exchange membrane (PEM) fuel cell is presented, in which liquid water in the CL pores is neglected. The model couples three phases: an electrically conductive carbon/platinum phase, gas pores, and a proton-conducting Nafion phase. The reaction-diffusion dynamics are described by a nonlinear system of differential equations which, due to the wide range of physical parameters, is very stiff. To reduce the stiffness of the system, an appropriate scaled model is introduced. A numerical algorithm consisting of two embedded Newton loops, specifically developed to handle the stiffness, is presented. Several numerical results are used to investigate the dynamics of the layer and the validity of the interface reduction, or zero CL thickness limit, which is often used in large fuel cell computations. In the limit of infinite water adsorption rate between ionomer phase and pore, a boundary layer emerges at either side of the CL, in which the water sorption equilibrium [Zawodzinski, T., Derouin, C., Radzinski, S., Sherman, R., Smith, V., Springer T., Gottesfeld, S., 1993. A comparative study of water uptake by and transport through ionomeric fuel cell membranes. Journal of the Electrochemical Society 140, 1981-1985] between ionomer and pore is violated.     

Characterization of flow phenomena induced by ultrasonic horn

Mean flow and turbulence parameters have been measured using laser Doppler anemometer (LDA) in ultrasound reactor. The effects of the ultrasonic power have been investigated over a power density (P/V) range of 15-. The liquid circulation velocities are dominant in the zone nearer to the source of energy and are substantially low at the walls and at the bottom of the reactor. The levels of turbulence kinetic energy and dissipation rate are high near the horn and decrease rapidly with increasing distance from the horn. Average turbulent normal stresses are larger than the turbulent shear stresses. However, they are much lower than stirred reactors when compared at the same power consumption per unit mass. Comparisons of LDA measurements and computational fluid dynamics (CFD) predictions have been presented. The good agreement indicates the validity of the CFD model. The flow information has been extended for the prediction of mixing time. For uniform mixing in ultrasound-assisted reactors, optimum power density and diameter of the vessel is needed, yet it is far less effective than conventional stirred vessel. The possibility of optimization has been suggested in terms of power dissipation and the vessel size.     

Modeling of a 120 kW chemical looping combustion reactor system using a Ni-based oxygen carrier

A modeling tool for the investigation of chemical looping combustion (CLC) in a dual circulating fluidized bed (DCFB) reactor system is introduced. CLC is a novel combustion process with inherent CO₂ separation, consisting of two fluidized bed reactors, an air reactor (AR) and a fuel reactor (FR). A solid oxygen carrier (OC) that circulates between the reactors, transports the necessary oxygen for the combustion. In the DCFB concept both AR and FR are designed as circulating fluidized beds (CFBs). Each CFB is modeled using a very simple structure in which the reacting gas is only in contact with a defined fraction of the well mixed solids. The solids distribution along the height axis is defined by a void fraction profile. Different parameters that characterize the gas-solids contact are merged into only one parameter: the fraction of solids exposed to the gas passing in plug flow (φ_s,core). Using this model, the performance of the 120 kW DCFB chemical looping combustor at Vienna University of Technology is investigated. This pilot rig is designed for a Ni-based OC and natural gas as fuel. The influence of the reactor temperatures, solids circulation rate, air/fuel ratio and fuel power are determined. Furthermore, it is shown that with the applied kinetics data, the OC is only fully oxidized in the AR when the AR solids inventory is much larger than the FR solids inventory or when both reactors are very large. To compare different reactor systems, the effect of the solids distribution between AR and FR is studied and both gas and solids conversions are reported.     

Methanation of carbon dioxide by hydrogen reduction using the Sabatier process in microchannel reactors

This paper describes the development of a microchannel-based Sabatier reactor for applications such as propellant production on Mars or space habitat air revitalization. Microchannel designs offer advantages for a compact reactor with excellent thermal control. This paper discusses the development of a Ru-TiO₂-based catalyst using powdered form and its application and testing in a microchannel reactor. The resultant catalyst and microchannel reactor demonstrates good conversion, selectivity, and longevity in a compact device. A chemically reacting flow model is used to assist experimental interpretation and to suggest microchannel design approaches. A kinetic rate expression for the global Sabatier reaction is developed and validated using computational models to interpret packed-bed experiments with catalysts in powder form. The resulting global reaction is then incorporated into a reactive plug-flow model that represents a microchannel reactor.     

Numerical simulation of methane fuelled cogenerative SOFCs for the production of synthesis gas and electrical energy

Numerical simulation has been used to show the feasibility of the autothermal cogeneration of synthesis gas and electricity in a solid oxide fuel cell (SOFC) by the electrochemical partial oxidation of CH₄. Owing to the large positive entropy change of the CH₄ partial oxidation reaction and its low heating value, severe cooling effect is being induced in the SOFC due to heat absorbance by the reaction products. For this reason the autothermal operation of the SOFC reactor cannot be secured. As it is shown this can be overcome by combining the dynamic operation of the SOFC under forced periodic reversal of the flow and the bleeding of a small amount of CH₄(<2.5%) in the oxidant stream (cathode). In this respect the catalytic combustion of CH₄, on the perovskite cathodic electrode, provides the necessary energy demand so that in combination with flow reversal operation the SOFC is maintained ignited even at inlet temperature as low as 300 K. It is shown that the overall thermodynamic efficiency of the process can by far exceed unity (η^′>2), thus revealing the unique property of the SOFCs to produce high-quality energy and useful chemicals.     

Numerical simulation of temperature and concentration distributions in fluidized beds with liquid injection

In this paper a numerical simulation study of dynamic behavior of a fluidized bed with liquid injection is presented. A continuum model has been developed taking into account the mass and energy balances of solid, gas as well as liquid to describe the temperature and concentration distributions in gas-solid-fluidized beds. The model considers the deposition efficiency of the liquid droplets as well as the influence of the spray nozzle region. For solving the non-linear partial differential equations with discrete boundary conditions a finite element method is used. Numerical computations have been done with two different schemes of time integration, a fully implicit and a semi implicit scheme. The complex correlations of mass and liquid flow rates, mass and heat transfer, drying, and transient two-dimensional air humidity, air temperature, particle wetting, liquid film temperature and particle temperature were simulated. The model was validated with transient measurements of the air temperature and air humidity at the outlet of a fluidized bed with water injection.     

Theoretical and experimental study on hydrodynamic characteristics of fluidization in air-sand conical beds

This work was aimed at modeling hydrodynamic characteristics of fluidization in conical beds using quartz sand as the inert bed material and air as the fluidizing agent. The minimum fluidization velocity, u_mf, and the minimum velocity of full fluidization, u_mff, were determined by Peng and Fan's models modified for conical fluidized bed. Meanwhile, the pressure drop across a bed, Δp (including Δp_max and Δp_mff corresponding to u_mf and u_mff, respectively), was predicted by using modified Ergun's equations for variable superficial air velocity at an air distributor, u₀. The predicted results were validated by experimental data for some operating conditions. Effects of the sand particle size, cone angle and static bed height on the fluidization pattern and hydrodynamic characteristics are discussed. With the proposed models, the Δp-u₀ diagram were obtained with rather high accuracy for the conical air-sand beds of 30-45^° cone angles and 20-30 cm static bed heights, when using 300- sand particles. For the predicted u_mf and u_mff, the relative computational errors were found to be within 20% for wide ranges of operating variables, whereas Δp_max and Δp_mff could be predicted with lower (10-15%) relative errors. With higher cone angles and/or bed heights, the computational accuracy was found to deteriorate.     

Analytical solutions for the Colebrook and White equation and for pressure drop in ideal gas flow in pipes

The friction factor, evaluated from the Colebrook and White equation, is traditionally computed iteratively. An analytical solution of the Colebrook and White equation for the friction factor can be obtained, using the Lambert W function. Also, the equation relating the outlet pressure to the inlet pressure of an ideal gas flowing through a straight pipe under isothermal, steady state conditions has been hitherto considered in literature to be implicit in these variables, probably due to its inherent non-linear nature. However, it can be shown that an analytical solution to the above equation for the pressure drop (or alternatively, outlet pressure) can also be obtained using the Lambert W function.     

Systematics of salt precipitation in complexes of polyethylene oxide and alkali metal iodides

Conductivity measurements in PEO₃₀MI polymer electrolytes with M=Li, Na, K, Rb, or Cs over the temperature range from about 65 to 200 °C show an increasing tendency for salt precipitation with increasing cation size. The salt precipitation in these complexes upon heating is revealed by the decrease of the dc conductivity starting at a critical temperature T_c. Whereas LiI and NaI complexes do not show precipitation effects, T_c monotonically decreases from about 140 to 65 °C when changing the salt component from KI via RbI to CsI. For the PEO-RbI system, precipitation is further investigated by nuclear magnetic resonance (NMR) and tracer diffusion experiments. NMR analysis unambiguously demonstrates the onset of RbI salt precipitation and the increase of the precipitate fraction with increasing temperature. In diffusion experiments on PEO₃₀RbI with the radiotracers and , the precipitation effect is manifested by anomalous features in the penetration profiles, however, without noticeable changes in their depth range. Combining the resulting tracer diffusion coefficients with the dc conductivity data enables us to assess crucial parameters characterizing ionic transport in PEO₃₀RbI.     

Simulation of turbulent electrocoalescence

The combination of an electric field and a moderate turbulent flow is a promising technique for enhancing the separation of water from oil. In this work, a numerical framework based on the Eulerian-Lagrangian approach is presented, where the turbulent dispersion and the inter-droplet hydrodynamic and electrical forces are carefully handled. Water-in-oil emulsions are studied in a channel flow with almost isotropic, decaying turbulence. The results obtained agree qualitatively with experimentally data reported in the literature. Our simulations show that the collision frequency is mainly controlled by the turbulence, but strong electric fields may increase the collision rate at low turbulence levels. It is also observed that turbulent electrocoalescence works equally well for all simulated volume fractions of water droplets.     

Heat transfer in rotary kilns with interstitial gases

While slow granular flows have been an area of active research in recent years, heat transfer in flowing particulate systems has received relatively little attention. We employ a computational technique that couples the discrete element method (DEM), computational fluid dynamics (CFD), and heat transfer calculations to simulate realistic heat transfer in a rotary kiln. To maintain simplicity, while simulating the cylindrical kiln, we use a non-uniform grid in our code. Different materials, particle sizes, and rotation speeds are used to track the transition from convection-dominated heat transfer to conduction-dominated heat transfer. At low particle conductivities, the heat transfer is dominated by gas-solid conduction; however, at higher particle conductivities solid-solid conduction plays a more important role. Moreover, our results suggest that the rate of change of the average bed temperature can display a transition as the conductivity of the interstitial medium is increased. At low interstitial transport rates, such as in vacuum, high conductivity, high heat capacity particles get heated most rapidly, but with increased interstitial transport coefficients, lower heat capacity material may get heated faster despite lower values of conductivity.     

Self-diffusion versus ion conduction in polymer electrolytes: On the occurrence of pairs, triplets, and higher-order clusters

We show that the ion transport properties previously established for several amorphous polyether-based electrolytes by tracer diffusion and conductivity measurements cannot be exclusively explained by triple ions or higher-order ionic clusters as the most significant mobile species. A combined analysis of the tracer diffusion and conductivity data at different concentrations and temperatures rather indicates that charged single ions and neutral ion pairs play a prominent role. In particular, it is found that the single anion controls the transport of negative charge. Allowing for positive and negative charge carriers of various size, the analysis is based on upper and lower limits derived for the charge diffusivity D_σ with respect to the cation and anion tracer diffusivity. The derived expressions have a general validity and may be applied to any electrolyte system for which a complete set of mass and charge transport data is available.     

Multiscale optimization using hybrid PDE/kMC process systems with application to thin film growth

The problem of optimal time-constant and time-varying operation for transport-reaction processes is considered, when the cost functional and/or equality constraints necessitate the consideration of phenomena that occur over disparate length scales. Multiscale process models are initially developed, linking continuum conservation laws with microscopic scale simulators. Subsequently, order reduction techniques for dissipative partial-differential equations are combined with adaptive tabulation of microscopic simulation data to reduce the computational requirements of the optimization problem, which is then solved using standard search algorithms. The method is applied to a conceptual thin film deposition process to compute optimal substrate-surface temperature profiles that simultaneously maximize film-deposition-rate uniformity (macroscale objective) and minimize surface roughness (microscale objective) across the film surface for a steady-state process operation. Subsequently, optimal time-varying policies of substrate temperature and precursor inlet concentrations are computed under the assumption of quasi-steady-state process operation.     

Heterogeneous nucleation of clathrates from supercooled tetrahydrofuran (THF)/water mixtures, and the effect of an added catalyst

The statistics of liquid-to-crystal nucleation are measured for clathrate-forming mixtures of tetrahydrofuran (THF) and water using an automatic lag time apparatus (ALTA). We measure the nucleation temperature using this new apparatus in which a single sample is repeatedly cooled, nucleated and thawed. This is done for a series of tetrahydrofuran concentrations and in several different sample tubes since the nucleation is heterogeneous and so occurring on the tube wall. The measurements are also done at the same concentrations and tubes but with an added catalyst, a single crystal of silver iodide.     

Global and local mixing determinations for steel converter analysis

The steel converter is one of the major multiphase industrial reactors in which mixing plays an important role. The molten steel in the converter is mixed with the slag by introducing a gas that is blown over the liquid phases (called top blowing), under the liquid phases (bottom blowing) or using top and bottom blowing at the same time (combined blowing). Many chemical reactions that are controlled by the mixing and temperature magnitudes take place inside the converter. All the processes take about 14 min, the first 3 min being for the addition of materials. A 1/10 length scale down of a cold model of a 250-ton capacity industrial converter was employed to carry out the present study. A conductivimetric method was used to carry out the study of the mixing in the reactor. In this study, the mixing in the bath was determined for the three blowing types by conductimetry and the behaviour of the system was modelized.     

Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents

The steam gasification of biomass, in the presence of a calcium oxide (CaO) sorbent for carbon dioxide (CO₂) capture, is a promising pathway for the renewable and sustainable production of hydrogen (H₂). In this work, we demonstrate the potential of using a CaO sorbent to enhance hydrogen output from biomass gasifiers. In addition, we show that CaO materials are the most suitable sorbents reported in the literature for in situ CO₂ capture. A further advantage of the coupled gasification-CO₂ capture process is the production of a concentrated stream of CO₂ as a byproduct. The integration of CO₂ sequestration technology with H₂ production from biomass could potentially result in the net removal of CO₂ from the atmosphere.Maximum experimental H₂ concentrations reported for the steam gasification of biomass, without CO₂ capture, range between 40%-vol and 50%-vol. When CaO is used to remove CO₂ from the product gas, as soon as it is formed, we predict an increase in the H₂ concentrations from 40%-vol to 80%-vol (dry basis), based on thermodynamic modelling and previously published data.We examine the effect of key variables, with a specific focus on obtaining fundamental data relevant to the design and scale-up of novel biomass reactors. These include: (i) reaction temperature, (ii) pressure, (iii) steam-to-biomass ratio, (iv) residence time, and (v) CO₂ sorbent loading. We report on operational challenges related to in situ CO₂ capture using CaO-based sorbents. These include: (i) sorbent durability, (ii) limits to the maximum achievable conversion and (iii) decay in reactivity through multiple capture and release cycles. Strategies for enhancing the multicycle reactivity of CaO are reviewed, including: (i) optimized calcination conditions, and (ii) sorbent hydration procedures for reactivation of spent CaO. However, no CaO-based CO₂ sorbent, with demonstrated high reactivity, maintained through multiple CO₂ capture and release cycles, has been identified in the literature. Thus, we argue that the development of a CO₂ sorbent, which is resistant to physical deterioration and maintains high chemical reactivity through multiple CO₂ capture and release cycles, is the limiting step in the scale-up and commercial operation of the coupled gasification-CO₂ capture process.     

Diffusion of NaOH into a protein gel

The diffusivity of NaOH in a layer of protein gel on an inert surface is measured by combining experimental study of the reactive dissolution of the gel with simulation of transport and reaction within the gel. Pure β-lactoglobulin gels, formed under three different gelation conditions, were used as a model system. The alkaline solutions cause the gel to swell, and destroy the interprotein interactions in the gel matrix. The swelling and cleavage reactions depend on the local concentration of hydroxide and so are sensitive to the rate of transport of hydroxide through the gel. Experiments were performed to determine the effect of the NaOH concentration on the hydroxide penetration thickness and on the velocity of the penetration front marked by phenolthalein indicator. Simple simulations and a more advanced semi-theoretical analysis were performed, with proteins being treated as ideal polyelectrolyte polymers. Both yielded good agreement with the experimental results. The effective diffusivity of NaOH in the protein gels was found to be similar to that in water. The analytical procedure can be extended in principle to any protein gel.     

Optimal operation of enantioseparation by batch-wise preferential crystallization

This article describes batch-wise preferential crystallization separation of mixtures of L- and D-threonine. The use of online polarimetry combined with refractometry and microscopic investigation of the solid phase provides information on the crystallization kinetics. Results obtained for different crystallization conditions (supersaturation, temperature and enantiomeric excess) in a batch crystallizer are presented. Based on these results, a non-linear dynamic model has been developed. The control problem is to determine an optimal temperature profile which will result in a maximum amount of product with required quality. In this dynamic optimization problem B-splines have been used for interpolation of the temperature profile.     

Mass transfer across the falling film: Simulations and experiments

Mass transfer across the thin falling film gas-liquid interface is a very important process as in chemical engineering and other fields, and yet there is still a lack of general predictability of the transfer quantity based on basic hydrodynamic parameters and independent of the geometrical setup. In this work, a numerical simulation is carried out for a vertical falling film arrangement. The wave dynamics and the associated mass transfer phenomena are discussed and compared with previous experimental empirical relationships. Based on the validity of the simulated results for wave parameters, numerical experiments for mass transfer were carried out with the aim of comparing to the empirical relation based on a single hydrodynamic parameter β (the gradient of the vertical fluctuating velocity at the interface) established previously by Law and Khoo [2002. Transport across a turbulent gas-liquid interface. A.I.Ch.E. Journal 48(9), 1856-1868.] and Xu et al. [2006. Mass transfer across the turbulence gas-water interface. A.I.Ch.E. Journal 52, 3363-3374] with various non-falling film experiments. Separately, experiments in an inclined plate thin falling film apparatus were carried out to determine the β distribution and associated mass transfer. It is found that there is reasonable concurrence with the mentioned empirical relation, hence suggesting the general applicability of β characterizing the scalar transport across the gas-liquid interface independent of the means of turbulence generation.