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 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.     

The effect of specific surface area on the activity of nano-scale ceria catalysts for methanol decomposition with and without steam at SOFC operating temperatures

Nano-particulate high surface area CeO₂ was found to have a useful methanol decomposition activity producing H₂, CO, CO₂, and a small amount of CH₄ without the presence of steam being required under solid oxide fuel cell temperatures, 700-1000 °C. The catalyst provides high resistance toward carbon deposition even when no steam is present in the feed. It was observed that the conversion of methanol was close to 100% at 850 °C, and no carbon deposition was detected from the temperature programmed oxidation measurement.The reactivity toward methanol decomposition for CeO₂ is due to the redox property of this material. During the decomposition process, the gas-solid reactions between the gaseous components, which are homogeneously generated from the methanol decomposition (i.e., CH₄, CO₂, CO, H₂O, and H₂), and the lattice oxygen on ceria surface take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen ( can produce synthesis gas (CO and H₂) and also prevent the formation of carbon species from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). V_O^·· denotes an oxygen vacancy with an effective charge 2⁺. Moreover, the formation of carbon via Boudouard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide with the lattice oxygen .At steady state, the rate of methanol decomposition over high surface area CeO₂ was considerably higher than that over low surface area CeO₂ due to the significantly higher oxygen storage capacity of high surface area CeO₂, which also results in the high resistance toward carbon deposition for this material. In particular, it was observed that the methanol decomposition rate is proportional to the methanol partial pressure but independent of the steam partial pressure at 700-800 °C. The addition of hydrogen to the inlet stream was found to have a significant inhibitory effect on the rate of methanol decomposition.     

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.     

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.     

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.     

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.     

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.     

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.     

Further work on cake filtration analysis

Analysis of cake filtration was made by the numerical solution of the appropriate equations of change based on the multiphase flow theory with the assumption that the cake properties are functions of the particle phase compressive stress, p_s. Unlike earlier studies which assume the relationship between p_s and the pressure of the fluid phase, p_l, to be ∇p_s+∇p_l=0, other possibilities were also considered in view of the recent work of Tien et al. (Chem. Eng. Sci. 56 (2001) 5361).In addition to investigating the effect of the p_s-p_l relationship, comparisons of predicted filtration performance with experiments made it possible to substantiate earlier findings that the p_s-p_l relationship is system specific. The results of the analysis were also used to test the parameter sensitivity of predictions, namely, values of the parameters of the constitutive relationships (i.e. ?_s vs. p_s and α vs. p_s, where ?_s and α are the cake solidosity and specific cake resistance). This information, in turn, can be used as a bench mark for improving existing and developing new procedures for determining cake solidosity and permeability.     

Non-uniqueness in capillary pressure-saturation-relative permeability relationships for two-phase flow in porous media: Interplay between intensity and distribution of random micro-heterogeneities

The two-phase flow behaviour in porous media is determined on the basis of capillary pressure-saturation-relative permeability relationships (P^c-S-K_r). These relationships are highly non-linear and obtained by laboratory experiments on porous samples, typically around 10-12 cm in length. It is normally assumed that these samples are homogeneous; however it is well-known that this is in fact not the case and that even at this scale micro-scale heterogeneities exist. Two-phase flow experiments on soils with different properties (e.g., particle and pore size distribution, permeabilities, etc) result in different P^c-S-K_r relationships implying that they cause non-uniqueness in these curves. Recent work has shown that the presence of the micro-heterogeneities has a significant effect on the measured P^c-S-K_r relationships and they cause non-uniqueness in these relationships. In the previous work in this area, the micro-heterogeneity effects on the P^c-S-K_r relationships have been analysed in a number of contexts, e.g., uniformly distributed heterogeneities (simplified cases), various binary sand combinations, hydraulic parameters (e.g., entry pressure, permeability), boundary conditions, etc. There is also some evidence that the intensity and distribution of the micro-heterogeneities affect the P^c-S-K_r relationships. In the present work we use numerical simulations to investigate further the nature of these effects, in particular how the interplay between the intensity and random distribution of micro-heterogeneities affect the P^c-S-K_r relationships. Seven randomly heterogeneous patterns have been defined. These domains represent coarse sand media with fine sand blocks embedded in them. The domain size () has been chosen so that it represents a typical laboratory scale device. The results of the simulations show that it is particularly important to take into account both the intensity and distribution of heterogeneity when determining the effective P^c-S-K_r relationships of a sample. Further, there is a complex interplay between the intensity and distribution of micro-scale heterogeneities which determines the P^c-S-K_r curves. This observation is in contrast to the results of domains with uniformly distributed heterogeneities. We have found that in general if the intensity of heterogeneity is high; the irreducible wetting phase saturation (S_iw) of the sample is also high. The direction of flow and the orientation of the samples also have significant effects. For example, the injection of an immiscible phase from the top (vertically downward) of water saturated porous domain leads to a lower S_iw than injecting on horizontal plane. On the other hand, injection from the bottom (vertically upwards) leads to a higher S_iw. As expected, the distribution of heterogeneity has a significant effect on the saturation distribution and the shape of the P^c-S-K_r curves. However, we show that if the heterogeneities are distributed in such a way that they are closer to the boundary of injection, the irreducible wetting phase saturation is higher.     

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.     

A multi-scale approach for CFD calculations of gas-liquid flow within large size column equipped with structured packing

This work has been carried out in the framework of post-combustion CO₂ capture process development. Considering the huge amount of gases to be treated and the constraints in terms of pressure drop, it appears that the absorption column will be equipped with high efficiency high capacity packings such as structured packings. The present paper focuses on the CFD modellisation of the two-phase flow within this complex geometry. For limited computational resources reasons, it is presently impossible to run computations at large scales taking into account the gas-liquid interaction and the real geometry of the packing and original approaches must be developed. In the present work, a multi-scale approach is proposed. It first considers liquid-wall and liquid-gas interaction at small scale via two-phase flow calculations using the VOF method. Second, the latter results are used in three-dimensional calculations run at a meso-scale corresponding to a periodic element representative of the real packing geometry. Last, those results are further used at large scale in three-dimensional calculations with a geometry corresponding to a complete column. Results are compared with experimental data and with other CFD simulations in terms of liquid hold-up, pressure drop and unit operation. Some suggestions are made for further development.     

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.     

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.     

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.     

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.     

Assessment of a redox alkaline/iron-chelate absorption process for the removal of dilute hydrogen sulfide in air emissions

The oxidative absorption of hydrogen sulfide (H₂S) into a solution of ferric chelate of trans-1,2- diaminocyclohexanetetraacetate (CDTA) was studied in a counter-current laboratory column randomly packed with 15 mm plastic Ralu rings. The present investigation takes concern about the Kraft pulping situation where dilute H₂S concentrations are omnipresent in large-volume gas effluents. A fractional two-level factorial approach was instigated to determine the significance of six operating variables, namely the solution's alkalinity (pH; 8.5-10.5), the liquid mass flow rate (L;1.73-), the solution's ionic strength (I_C;0.01-), the gas mass flow rate (G;0.19-), the inlet H₂S concentration (C_H2S,0;70-430 ppm) and the initial ferric CDTA concentration (C_Fe,0;100 -). Initially, a Plackett-Burman design matrix of seven duplicated experiments revealed that pH is the leading factor controlling the H₂S conversion rate while the ionic strength and ferric CDTA concentration effects remained negligible within the factorial domain. Surface response analysis based on 11 duplicated factorial experiments plus 10 central composite trials revealed that the H₂S conversion significantly increases with liquid flow rate but decreases with growing H₂S load up. Further examination about the influence of ferric CDTA on H₂S absorption rate was set up over a broader concentration range (C_Fe,0;0- at pH of 9.5 and 10.5. It showed good potential at as H₂S conversion increased by a significant 25% for both pH values in comparison to pure alkaline solutions containing no ferric CDTA.     

Hydrodynamics characterization of the Maxblend impeller

The hydrodynamic characteristics of the Maxblend^TM impeller have been investigated in the case of viscous Newtonian fluids. Both laboratory experiments and 3D finite element based computational fluid dynamics (CFD) simulations have been carried out. The power consumption, the mixing evolution yielding the mixing time, and the effect of baffles in the laminar and transition flow regimes have been determined. It was found that the limit Reynolds number between the laminar and transition regimes is approximately 25 and 38 for the unbaffled and baffled configurations, respectively. Based on the range of Reynolds numbers studied in this work, the best window performance of the Maxblend^TM mixer where fast and homogenous mixing is achieved is the end of the laminar regime and the early transition regime with baffles.