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.     

Use of triboelectric probes for on-line monitoring of liquid concentration in wet gas-solid fluidized beds

The objective of this study was to validate the effectiveness of a novel method employing triboelectric probes for accurate on-line solid moisture measurements in fluidized beds. Liquid injections were conducted in a fluid bed of glass beads and the resulting solid moisture was monitored during the whole drying stage by acquiring triboelectric signals generated from several locations inside the bed. For various superficial gas velocities and amounts of injected liquid, the bed drying end point and the fraction of sprayed liquid involved in the formation of slow-vaporizing stable agglomerates were estimated performing fast signal analysis by means of the W statistic.     

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO₂ ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO₂ into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed for simulating dynamic process of CO₂ ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO₂ absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO₂ gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO₂. The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO₂ concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO₂ into seawater in the CO₂ ocean disposal.     

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.     

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.     

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.     

Gas mass transfer to AOT-based microemulsions

The present paper analyses the gas/liquid mass transfer process employing carbon dioxide as gas phase and ternary water in oil microemulsions as absorbent liquid phases. The liquid phases were obtained by a direct mixing of water, 2,2,4-trimethylpentane and sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT). The characteristics of the microemulsions employed as liquid phase have been analysed to interpret the experimental results observed in the absorption process. More specifically, they have been analysed in relation to the percolation phenomenon and the effects produced by this phenomenon upon the different physical properties. Characteristic results have been observed for the gas/liquid mass transfer using microemulsions, because ternary microemulsions with high viscosity values in relation to pure water show a faster absorption process than the carbon dioxide/water system. This characteristic behaviour has been explained on the basis of the microemulsions internal dynamics.     

Formation and behavior of composite CO2 hydrate particles in a high-pressure water tunnel facility

Sinking CO₂ composite particles consisting of seawater, liquid CO₂, and CO₂ hydrate were produced by a coaxial flow injector fed with liquid CO₂ and artificial seawater. The particles were injected into a high-pressure water tunnel facility to permit determination of their settling velocities and dissolution rates. Injections were performed at fixed pressures approximately equivalent to 1200-m, 1500-m, and 1800-m depths and at temperatures varying from approximately 2 to 5 °C. Immediately after injection, the cylindrical particles were observed to break away from the injector tip and often aggregated into sinking clusters. The seawater flow in the tunnel was then adjusted in a countercurrent flow mode to suspend the particles in an observation window so that images of the particles could be recorded for later analysis. The flow would often break or cause rearrangement of some of the clusters. Selected individual particles and some clusters were studied until they became too hydrodynamically unstable to follow. In general, the flow required to suspend clusters or individual particles decreased with time as the particles dissolved. For example, one particle was produced and observed for over 6 min at an average pressure of 15.022 MPa and an average temperature of 5.1 °C. Its sinking rate, determined from the flow required for stabilization, changed from 37.2 to 3.3 mm/s over this time. Particle sinking rates were compared to correlations from the literature for uniform cylindrical objects. Reasonable agreement was observed for short times; however, the observed decrease in sinking velocity with time was greater than that predicted by the correlations for longer times. Particle dissolution rates, based on changes in diameter, were also determined and varied from 5 to . A pseudo-homogeneous mass transfer model was used to predict single-particle dissolution rates. Good agreement was achieved between experimental dissolution data and the modeling results.     

Numerical simulation of the dynamic flow behavior in a bubble column: A study of closures for turbulence and interface forces

Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, C_S, as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737-1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79-95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965-2000] to account for the bubble-induced turbulence in the k-ε model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with C_S=0.08 and 0.10 agree well with the measurements. When C_S is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio (H/D=3 and H/D=6), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22-24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results.     

Optimization of the cathode geometry in polymer electrolyte membrane (PEM) fuel cells

A nonlinear constrained optimization procedure is used in the cathode design in order to maximize the average current density at a fixed voltage in a polymer electrolyte membrane (PEM) fuel cell with interdigitated fuel/air distributors. The operation of the PEM fuel cell is studied using a steady-state, two-phase, two-dimensional electro-chemical model. The following geometrical parameters of the cathode are considered: the thickness, and length per one shoulder of the interdigitated air distributor and the length of the shoulder. The optimization results obtained show that within manufacturability controlled lower and the space-limitation controlled upper bounds of these parameters, the optimal-cathode design corresponds to the lower bounds in the cathode length per one shoulder of the interdigitated air distributor and the fraction of the length associated with the shoulders and at a low (but larger than the lower bound) value of the cathode thickness. These findings are explained using an analogy with the effect of pipe dimensions on the fluid flow through a pipe and by considering the role of forced convection on the oxygen transport to the membrane/cathode interface.     

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.     

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.     

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.     

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.     

A greener, pressure intensified propylene epoxidation process with facile product separation

A novel biphasic process concept for the synthesis of propylene oxide (PO) from propylene is presented, using the long known catalyst, methyl trioxorhenium and aqueous hydrogen peroxide as the oxidant. Propylene is fed as gas, which is transported to the liquid phase containing the oxidant, catalyst and methanol as a cosolvent that improves propylene solubility. The selective oxidation produces PO which is distilled easily from the liquid phase taking advantage of the relatively low normal boiling point of PO compared to those of methanol and water. The process satisfies the sustainability principles of waste minimization, use of benign reagents and process intensification at mild conditions. The process produces PO in yields exceeding 98%. It operates at essentially ambient temperature and moderate pressures , using easily recyclable, low hazard aqueous methanol, as solvent. The catalyst, methyl trioxorhenium, is robust under the operating conditions. A key aspect of the innovation is the use of nitrogen gas pressure to enhance propylene availability in the liquid phase increasing its conversion from ∼80% (without N₂) to complete (with N₂) in a few hours. These findings pave the way for catalyst durability and recycle studies, aimed at demonstrating a continuous process that is economically and environmentally sustainable compared to existing processes.     

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.     

Robust control of a reverse-flow reactor

This paper is focused on the design of a robust controller for a catalytic fixed-bed reactor with periodical inversion of the flow direction (reverse-flow reactor, RFR). The analogy between the RFR operated at infinite switching frequency and the countercurrent reactor is the basis of the simplified mathematical model of the reactor.The control system uses dilution and internal electric heating to ensure complete conversion of the reactants and to prevent overheating of the catalyst. As the state of the system is not fully available, apart from some temperature measurements, an observer is designed and used in the control algorithm. This is a typical case of nonlinear system with uncertainties. Following the procedure described in detail by Fissore [2008. Robust control in presence of parametric uncertainties: observer-based feedback controller design. Chemical Engineering Science, in press, doi:10.1016/j.ces.2007.12.019.], the extended model for the process is setup, thus taking into account all the simplifications of the model and linking performance and robustness to the control law, which is a simple state feedback. Simulations with randomly varying feeding concentration have been carried out in order to demonstrate the effectiveness of the proposed control system.     

CO2 capture from syngas via cyclic carbonation/calcination for a naturally occurring limestone: Modelling and bench-scale testing

The intrinsic rate constants of the CaO-CO₂ reaction, in the presence of syngas, were studied using a grain model for a naturally occurring calcium oxide-based sorbent using a thermogravimetric analyzer. Over temperatures ranging from 580 to 700 °C, it was observed that the presence of CO and H₂ (with steam) during carbonation caused a significant increase in the initial rate of carbonation, which has been attributed to the CaO surface sites catalyzing the water-gas shift reaction, increasing the local CO₂ concentration. The water-gas shift reaction was assumed to be responsible for the increase in activation energy from 29.7 to 60.3 kJ/mol for limestone based on the formation of intermediate complexes. Changes in microporosity due to particle sintering during calcination have been credited with the rapid initial decrease in cyclic CaO maximum conversion for limestone particles, whereas the presence of steam during carbonation has been shown to improve the long-term maximum conversion in comparison to previous studies without steam present.     

COSMO-RS modeling on the extraction of stimulant drugs from urine sample by the double actions of supercritical carbon dioxide and ionic liquid

This work presents the first research linking chemical engineering and sport science as far as we know. The COSMO-RS (conductor-like screening model for real solvents) model was used to make a priori prediction for the extraction of stimulants from aqueous solution by the double action of supercritical carbon dioxide (SC CO₂) and ionic liquid. It was found that the suitable ionic liquids should have small molecular volume, unbranched group and no sterical shielding effect around anion charge center, and thus [C₂MIM]⁺[OAc]^- is the best among all the ionic liquids investigated. The calculated results from the COSMO-RS model were qualitatively consistent with those from experiments. On this basis, partition coefficients of amphetamine (C9N) and nikethamide (C10N) between aqueous phase and supercritical fluid (or MTBE) phase at different temperatures were calculated. It was shown that the separation efficiency of supercritical extraction with ionic liquid is generally higher than that of traditional liquid-liquid extraction. The modeling present can also be extended to the separation of trace amount of organic substances from aqueous solutions for other purposes.