Steady-state Flow Reactor Research Articles

(Invited) Electrifying Nitrogen Transformations: Decarbonizing Ammonia Using Plasma Catalysis and Electrochemical Reduction of Nitrate in Wastewaters

Nitrogen fixation in fertilizers forms the basis of modern agriculture and mediates global food insecurity. Energy-dense nitrogen-based compounds also represent promising carbon-free alternative fuels when synthesized using sustainable energy and materials. However, conventional thermally-activated nitrogen conversion processes consume substantial amounts of fossil fuels as materials and energy inputs, leading to an unsustainable energy and carbon footprint. Therefore, electron-driven approaches are needed to establish fossil-free nitrogen interconversions. Electrochemical and plasma-activated reactions operate under mild conditions, enabling facile coupling to intermittent renewable energy sources to reduce CO2 emissions and reactive nitrogen pollution, facilitating storage and transportation of renewable energy in nitrogen-based fuels.Non-thermal plasma has been used to synthesize ammonia under mild conditions, but the dearth of fundamental understanding of plasma catalytic reactions handicaps the development of efficient plasma catalytic N2 conversion. Therefore, an in situ FTIR reactor was employed in combination with steady-state flow reactor experiments and plasma kinetic modeling to elucidate the surface reaction mechanisms and plasma-catalyst interactions. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways. Furthermore, techno-economic analysis was employed to reveal the threshold efficiency required for a plasma process to become environmentally and economically competitive.Beyond electrification of N2 fixation for ammonia synthesis, existing waste streams containing fixed nitrogen as nitrate may be mined for fertilizer or carbon-free renewable energy storage while avoiding the need for H2 as a reagent, via selective nitrate electroreduction to ammonium. Nitrate electrochemical reduction can eliminate the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, stability, and mass transport of nitrate to electrocatalyst active sites. Therefore, electrochemical membranes were employed as porous flow-through electrodes to address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive electrified membranes, fabricated using inexpensive materials and green synthesis methods, showed significantly higher nitrate removal efficiency during electrified filtration compared to flow-by mode. The small pore sizes in the membrane reduced the diffusional boundary layer by several orders of magnitude with respect to flow-by mode, overcoming diffusional mass transport limitations. During electro-filtration, the nitrate conversion activity was easily controlled by tuning the permeate flow rate, regardless of applied potential. These multifunctional electrified membranes for reduction of nitrate in wastewaters have the potential to partially displace carbon-intensive industrially synthesized ammonia while simultaneously accomplishing water decontamination.

Optimizing Binding Energies of Key Intermediates for CO2 Hydrogenation to Methanol over Oxide-Supported Copper.

Rational optimization of catalytic performance has been one of the major challenges in catalysis. Here we report a bottom-up study on the ability of TiO2 and ZrO2 to optimize the CO2 conversion to methanol on Cu, using combined density functional theory (DFT) calculations, kinetic Monte Carlo (KMC) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, and steady-state flow reactor tests. The theoretical results from DFT and KMC agree with in situ DRIFTS measurements, showing that both TiO2 and ZrO2 help to promote methanol synthesis on Cu via carboxyl intermediates and the reverse water-gas-shift (RWGS) pathway; the formate intermediates, on the other hand, likely act as a spectator eventually. The origin of the superior promoting effect of ZrO2 is associated with the fine-tuning capability of reduced Zr(3+) at the interface, being able to bind the key reaction intermediates, e.g. *CO2, *CO, *HCO, and *H2CO, moderately to facilitate methanol formation. This study demonstrates the importance of synergy between theory and experiments to elucidate the complex reaction mechanisms of CO2 hydrogenation for the realization of a better catalyst by design.

Acido-basicity of lanthana/alumina catalysts and their activity in ethanol conversion

Lanthana on alumina samples (0.2, 0.8 and 4.7 theoretical monolayers) were prepared by incipient wetness impregnation using γ-Al2O3 as support. Characterization has been performed by BET, XRD, skeletal FT-IR, DR-UVvis, XPS, HR-TEM, IR spectra of the surface OH, adsorbed pyridine and CO2, and isopropanol TPD. Ethanol conversion was investigated both in temperature-programmed surface reaction (TPSR) dynamic conditions as well as in steady-state flow reactor tests. Lanthanum addition stabilizes alumina with respect to sintering and loss of surface area and La- containing phases are observed only for the high-La loaded catalyst. La-alumina catalysts are less active in ethanol dehydration than alumina but more selective to diethyl ether at partial conversion. 5%La2O3/Al2O3 is also equally or more selective than alumina to ethylene at high conversion, producing less carbonaceous material during reaction. Thus, this catalytic system might be a good candidate for (bio)ethylene production through (bio)ethanol dehydration.

A stochastic model of catalytic reaction networks in protocells

Protocells are supposed to have played a key role in the self-organizing processes leading to the emergence of life. Existing models either (i) describe protocell architecture and dynamics, given the existence of sets of collectively self-replicating molecules for granted, or (ii) describe the emergence of the aforementioned sets from an ensemble of random molecules in a simple experimental setting (e.g. a closed system or a steady-state flow reactor) that does not properly describe a protocell. In this paper we present a model that goes beyond these limitations by describing the dynamics of sets of replicating molecules within a lipid vesicle. We adopt the simplest possible protocell architecture, by considering a semi-permeable membrane that selects the molecular types that are allowed to enter or exit the protocell and by assuming that the reactions take place in the aqueous phase in the internal compartment. As a first approximation, we ignore the protocell growth and division dynamics. The behavior of catalytic reaction networks is then simulated by means of a stochastic model that accounts for the creation and the extinction of species and reactions. While this is not yet an exhaustive protocell model, it already provides clues regarding some processes that are relevant for understanding the conditions that can enable a population of protocells to undergo evolution and selection.

An in situ set up for the detection of CO2 from catalytic CO oxidation by using planar laser-induced fluorescence

We report the first experiment carried out on an in situ setup, which allows for detection of CO(2) from catalytic CO oxidation close to a model catalyst under realistic reaction conditions by the means of planar laser-induced fluorescence (PLIF) in the mid-infrared spectral range. The onset of the catalytic reaction as a function of temperature was followed by PLIF in a steady state flow reactor. After taking into account the self-absorption of CO(2), a good agreement between the detected CO(2) fluorescence signal and the CO(2) mass spectrometry signal was shown. The observed difference to previously measured onset temperatures for the catalytic ignition is discussed and the potential impact of IR-PLIF as a detection technique in catalysis is outlined.

Thermodynamic time invariances for dual kinetic experiments: Nonlinear single reactions and more

For dual kinetic experiments, i.e., experiments performed in batch or steady-state plug flow reactors from reciprocal initial conditions, thermodynamic (space-)time invariances for all reversible single reactions of the first and second order have been found explicitly.In all analyzed cases, quotient-like functions of concentrations can be defined which equal the equilibrium constant of the reaction during the whole course of the experiment, and not only at the end, i.e., under equilibrium conditions. The obtained invariances can be used as simple fingerprints for distinguishing the types of reactions.For multistep reactions: (a) a similar invariance was obtained for the two-step catalytic reaction (single route complex reaction) under pseudo-steady-state assumption; (b) for the two-step non-steady-state reaction, an approximation for the thermodynamic invariance was found to be valid in two domains, (1) at the very beginning of the reaction (2) at the end of the reaction, near equilibrium conditions. We hypothesize that such two-domain validity of the thermodynamic invariance is a general feature of dual kinetic experiments performed in complex chemical systems.

Effect of ethyleneglycol addition on the properties of P-doped NiMo/Al2O3 HDS catalysts: Part I. Materials preparation and characterization

Phosphorous-doped NiMo/Al2O3 hydrodesulfurization (HDS) catalysts (nominal Mo, Ni and P loadings of 12, 3, and 1.6wt%, respectively) were prepared using ethyleneglycol (EG) as additive. The organic agent was diluted in aqueous impregnating solutions obtained by MoO3 digestion in presence of H3PO4, followed by 2NiCO3·3Ni(OH)2·4H2O addition. EG/Ni molar ratio was varied (1, 2.5 and 7) to determine the influence of this parameter on the surface and structural properties of synthesized materials. As determined by temperature-programmed reduction, ethyleneglycol addition during impregnation resulted in decreased interaction between deposited phases (Mo and Ni) and the alumina carrier. Dispersion and sulfidability (as observed by X-ray photoelectron microscopy) of molybdenum and nickel showed opposite trends when incremental amounts of the organic were added during catalysts preparation. Meanwhile Mo sulfidation was progressively decreased by augmenting EG concentration in the impregnating solution, more dispersed sulfidic nickel was evidenced in materials synthesized at higher EG/Ni ratios. Also, enhanced formation of the so-called “NiMoS phase” was registered by increasing the amount of added ethyleneglycol during simultaneous Ni–Mo–P–EG deposition over the alumina carrier. That fact was reflected in enhanced activity in liquid-phase dibenzothiophene HDS (batch reactor, T=320°C, P=70kg/cm2) and straight-run gas oil desulfurization (steady-state flow reactor), the latter test carried out at conditions similar to those used in industrial hydrotreaters for the production of ultra-low sulfur diesel (T=350°C, P=70kg/cm2, LHSV=1.5h−1 and H2/oil=2500ft3/bbl).

Modeling residence time distribution in a twin-screw extruder as a series of ideal steady-state flow reactors

Knowledge about and control of residence time distribution (RTD) is critical for determining optimal conditions in an extrusion process. In this paper, previously proposed models for RTD have been summarized and compared with the experimental data. The best-fit model was found to be a plug flow in series with a finite number of constantly stirred tank reactors having dead volume fractions. Parameters of the best-fit model were predicted from input variables (moisture content, screw speed, nozzle diameter and barrel temperature). Predicted RTD from the input variables had a r 2 > 0.96. Experiments were conducted using a full factorial design with moisture contents of 16, 20, 24 and 28% (wb); screw speeds of 80, 120 and 160 rpm; nozzle diameters of 3, 4 and 5 mm; and barrel temperatures of 120 and 140 °C. Each experiment was replicated twice. It was found that, within the range of conditions studied, moisture content, screw speed, barrel temperature and nozzle diameter had significant effects on RTD.

Operando studies of desorption, reaction and carbonate formation during CO oxidation by Au/TiO 2 catalysts

Gas transient methods have been used to probe reaction pathways in catalytic CO oxidation under operando conditions. A low dead volume reactor is described, which permits monitoring of surface reaction species by repetitive FTIR while using mass spectrometry to continuously monitor reaction products and conversion throughout transient gas switching. CO desorption rates are measured from an active Au/TiO 2 catalyst under He and CO 2 and are consistent with literature values for CO desorption from Au single crystals and Au films on TiO 2. The rate of the reactive removal of adsorbed surface CO (a) was used to determine a turnover frequency of about 0.043 s −1 for CO oxidation on the Au particles, comparable to the rate obtained in a steady state flow reactor. Gas switching is used to investigate the storage of oxygen on the catalyst. It is found that following O 2 exposure, the amount of adsorbed active oxygen is either undetectably small, less than 3.1 μmoles O/g catal, or it desorbs very rapidly at 298 K. Although storage of oxygen is very ineffective on this catalyst, CO 2 is stored by its dynamic interaction with the support to form carbonates or bicarbonates causing slow removal of product CO 2 from the reactor bed. These results are summarized by a scheme describing the pathways for CO oxidation.

Population balance modeling of polymer branching and hyperbranching

Branched polymers have rheological and morphological properties that make them important subjects for investigation. We propose a chain polymerization model for branching kinetics that is not restricted to primary branching on the chain backbone, but incorporates branches on branches (hyperbranching). The proposed method involves multivariate distributions, a subject of considerable interest and activity in population balance modeling. For unsteady-state continuous-flow stirred reactors, the mathematical description is based on population balance equations for branch formation and growth by addition of chains of distributed molecular weight (MW). Computer algebra software facilitates the complex mathematical manipulations for the continuous MW and the discrete branch number. Key properties of branched macromolecules are presented, such as MW moments for various levels of branching, number of branches, and average MW of branches. A major conclusion is that hyperbranching presents distinctly different properties for unsteady batch and steady-state stirred flow reactors, whereas for primary branching, the results are similar.

A novel BN supported bi-metal catalyst for selective hydrogenation of crotonaldehyde

A series of boron nitride (BN) supported Pt-Sn catalysts was prepared with a co-incipient wetness method employing hexachloroplatinic acid and tin(II) chloride. Pt loading was fixed at 1.1 wt%; Sn loading varied from 0.25 to 0.75 wt% on BN support. Selective hydrogenation of gas-phase crotonaldehyde was conducted in a steady-state flow reactor with temperatures ranging from 40 to 100 °C. The selectivity of crotyl alcohol reached over 80%. An optimum yield of crotyl alcohol reached 38% at 60% conversion of crotonaldehyde at 80 °C using Pt-Sn(0.75)/BN catalyst, while Pt-Sn(0.75)/γ-Al 2O 3 yielded less crotyl alcohol and a lower rate of crotonaldehyde conversion. The maximum yield rate of crotyl alcohol was 2.4 mmol/(g-cat. h) at 80 °C. Negligible deactivation was found during reaction for 4–6 h. The crystalline phases of PtSn and SnPt 3 alloys were observed from the XRD spectra of Pt-Sn/BN catalysts with various Sn loadings. The selectivity of crotyl alcohol increased with Sn loadings but the activity values of the catalysts went through a maximum. The H 2 reduction at 300 °C gave an optimum Pt–Sn alloy particle size so that the selectivity of crotyl alcohol increased without losing catalyst activity. The C O bond of crotonaldehyde was preferentially hydrogenated and the hydrogenation of C C bond was suppressed, resulting in the increase of crotyl alcohol selectivity.

Effects of the radial distribution of platinum in spherical alumina catalysts on the oxidation of CO in air

The oxidation of CO in air has been studied using spherical alumina pellets with varying radial platinum distributions. Shell and homogeneous catalysts have been prepared with the same total amount of platinum. Co-impregnation of chloroplatinic acid and citric acid was used to obtain the homogeneous distribution. Temperature ramp experiments in a flow reactor showed significant differences in light-off behavior for the two catalysts. Compared to the shell catalyst, the conversion of CO started at lower temperatures when using the homogeneous catalyst, but higher temperatures were needed for total conversion. Steady-state experiments performed in a micro Berty tank reactor with the homogeneous catalyst were used to derive two different kinetic models for the oxidation. One of the kinetic expressions was used to simulate the catalytic activity in a steady-state flow reactor system. The maximum temperature gain which can be achieved by placing the active material at the optimum position in the pellet, in comparison to an infinitesimal shell distribution, was found to be 48°C at 95% conversion for 1.0% CO.

Methanation Activity in Relation to the Textural and Catalytic Parameters of Ni/Al2O3 Catalysts

A range of NiO/Al2O3, solids (4.0–12.0 wt% Ni) was prepared by the impregnation technique. The textural properties of these solids were determined from nitrogen adsorption measurements at 77 K. Reduction of the solids with hydrogen was carried out 650, 675, 700 and 725 K. Some catalytic parameters for these reduced materials were determined from studies of the adsorption of hydrogen at 400 K. The methanation of carbon monoxide over these reduced materials used as catalysts was followed between 550 K and 600 K using a steady-state flow reactor. The surface area of the NiO/Al2O3 solids decreased and their mean pore radius increased with increasing content of NiO. Increasing the temperature employed for their reduction led to increased diffusion of Ni atoms to the surface with a consequent increase in their catalytic activity. A direct relationship was found between % methanation and the surface area of the reduced catalytic material expressed as m2/g (catalyst).

Isomerization of 1-Butene over Silica-Supported Mo(VI), W(VI), and Cr(VI)

Isomerization of 1-butene on Mo(VI)/SiO2, W(VI)/SiO2, and Cr(VI)/SiO2was investigated by steady-state flow reactor studies and Fourier transform infrared spectroscopy. (η5-C5H5)2M2(CO)6or M(η3-C3H5)4(M=Mo, W) were used as precursors in preparing Mo(VI)/SiO2and W(VI)/SiO2catalysts. Cr(NO3)3was used to prepare Cr(VI)/SiO2. Isomerization proceeds by a Brønsted catalyzed pathway involving alkoxide intermediates and an oxidative dehydrogenation pathway involving allylic intermediates. Flow reactor experiments revealed that two pathways for isomerization could be observed with Mo(VI)/SiO2: through the alkoxide intermediate atT<523 K and through the allylic intermediate atT>573 K.break W(VI)/SiO2catalysts were more active for isomerization than Mo(VI)/SiO2catalysts, but the demarcation of the two pathways was not as clear. Only the oxidative dehydrogenation pathway for isomerization was observed for the Cr(VI)/SiO2catalysts atT>473 K. In IR studies, alkoxide intermediates were observed following adsorption of 1-butene on Mo(VI)/SiO2and W(VI)/SiO2catalysts at 300 K; complete desorption of the surface alkoxide occurred by 523 K. The surface species observed for adsorption of 1-butene on Cr(VI)/SiO2at 300 K were typical of allylic C–H abstraction; these surface species transform into deep oxidation products on heating, resulting in a surface that is poisoned. Pyridine adsorption studies reveal traces of Brønsted acidity on Mo(VI)/SiO2, W(VI)/SiO2, and Cr(VI)/SiO2. Brønsted acidity would account for the alkoxide mechanism of 1-butene isomerization. The reason that these Brønsted acid sites are most active for 1-butene isomerization on W(VI)/SiO2and inactive on Cr(VI)/SiO2is discussed.

Effect of Tetralin on the Degradation of Polymer in Solution

The effect of a hydrogen-donor solvent tetralin on thermal degradation of poly(styrene-allyl alcohol) in liquid solution was investigated in a steady-state tubular flow reactor at 1000 psig at various tetralin concentrations, polymer concentrations, and temperatures. The experimental data were interpreted with continuous- mixture kinetics, and rate coefficients determined for the specific and random degradation processes.

Boundary conditions of flow reactor: J. F. Wehner and R. H. Wilhelm, Chem. Engng Sci.6: 89–93, 1956

An analysis is presented of the boundary conditions for a steady state flow-reactor with axial diffusion and first order reaction. Conclusions regarding reactor properties are reached as the result of simultaneous solution of the three differential equations for the reaction section and fore and after sections. Axial diffusion, but no reaction, may occur in the latter two sections.

Boundary conditions of flow reactor

An analysis is presented of the boundary conditions for a steady-state flow reactor with axial diffusion and first-order reaction. Conclusions regarding reactor properties are reached as the result of simultaneous solution of the three differential equations for the reaction section, fore and after sections. Axial diffusion may occur in the latter two sections. The limits of behaviour, represented by a plug-flow reactor without diffusivity, and a fully-stirred reactor, are discussed. Contributions of the diffusivities in the three parts of the system to the course of reaction in space are considered.

Design, operation and performance of stabilization reservoirs for wastewater irrigation in Israel

Stabilization reservoirs are used for the seasonal storage and treatment of wastewater for irrigation. These reservoirs are not steady-state flow reactors, but rather accumulative batch ones with a relatively abrupt discharge. A new parameter, the percentage of fresh effluents within the reservoir (PFE) is proposed as a tool to analyze the age distribution of effluents in this kind of reactor. The equations to calculate the mean residence time (MRT) and the surface organic loading are also provided. The relatively small fraction of fresh effluents represented by PFE and not the whole age distribution of the effluents (MRT) determines the removal percentages of COD and BOD in the reservoir and the quality of the effluents released for irrigation. The effect of environmental parameters such as water temperature and solar radiation are overridden by the operational parameters (PFE). In steady-state flow reactors MRT can substitute PFE to evaluate the reactor performance because the ratio PFE/MRT is constant in this particular case. A value of 30–40 kg BOD/ha/day is recommended as the maximum allowable surface organic loading of stabilization reservoirs. The seasonal storage of wastewater in reservoirs reduces BOD, COD, TSS and detergents by 50–80%, and total coliforms by 90%, in terms of annual means. The quality of the effluents is maximal at the beginning of the irrigation season when the reservoir is full of old effluents, but sharply deteriorates when water level drops and new wastewater is pumped into the reservoir. Design and operational alternatives to overcome this problem can assure removals of at least I order of magnitude for chemical components, and of at least 3–4 orders of magnitude for total coliforms. These alternatives are based on a change from seasonal to multi-seasonal storage of wastewater, and/or a change from single reservoir to two or more reservoirs supplying effluents of different quality.

Polarization Enhanced Production in a Membrane Reactor

Abstract The membrane-based catalytic reactor with “free” catalyst has been modeled simply and studied by the authors on a theoretical basis. The hypothesis supporting an advantage of the membrane reactor is twofold: a) perfect rejection by the membrane and small reject fraction concentrates the catalyst highly in a steady-state continuous flow reactor (SSCFR) and b) the membrane reactor produces a filtered, high quality product. Of further advantage is the potential for concentrating the catalyst highly near the membrane in a thin diffusion dependent zone wherein the reacting substrate is also concentrated. The conjunction of concentrated catalyst and substrate leads to less inhibition of the reaction. The model involves conventional diffusional theory, simple membrane characterization of uniform flux, and Michaelis-Menten kinetics.

The effect of alumina structure on surface sites for alcohol dehydration

We have studied the effect of alumina form(-/, 5, and a), and impurities, on 2-propanol dehydration activity. In a steady-state flow reactor experiment, α-alumina was approximately 1000 times less reactive (in terms of moles propene formed/s/m 2) than reforming grade γ-alumina. The structure of the transitional alumina (γ vs δ) played a smaller role than Na contamination in determining alcohol dehydration activity. For instance, δ-alumina was ≈3–6 times less reactive than reforming grade γ-alumina in its activity for alcohol conversion, while Na-poisoned γ-alumina was ≈180 times less reactive. The number of surface sites on these aluminas were determined using temperature-programmed reaction (TPR) of 2-propanol. It was found that ≈1–2 × 10 18 alcohol molecules/m 2 reacted to form propene on all of the aluminas independent of alumina form or impurity concentration. The temperature at which the alcohol reacted to form propene in the TPR experiment correlated inversely with steady-state alcohol dehydration activity. The peak desorption temperature ( T P) during TPR ranged from 440 K for reforming grade γ-alumina to 600 K for α-alumina. These results are discussed in terms of a model for the nature of reactive sites on the surface of alumina.