Short Contact Time Reactors Research Articles

Phosphorus uptake and accumulation in Chlamydomonas reinhardtii: Influence of biomass concentration, phosphate concentration, phosphorus depletion time, and light supply

The model microalga Chlamydomonas reinhardtii was cultivated under P depleted and P repleted conditions to test the influence of dissolved P concentration (0–20 mg-P·L−1), initial algal biomass concentration (0.17–0.57 g-DW·L−1), P-depletion time (1 h – 4 days) and light supply (continuous illumination versus darkness) on phosphate uptake rate (mg-P·h−1·L−1) and the cellular P content (as %P of dry cell mass). Dissolved P concentration positively influenced cellular P content, while biomass concentration and P-depletion time did not. Biomass concentration and P-depletion time instead positively influenced P uptake rates. Light supply neither influenced cellular P content nor P assimilation. These findings evidence that depleting the cells at high initial biomass concentration could enhance P removal during wastewater treatment in waste stabilization ponds. Practically, these findings imply that high P removal from wastewater could be achieved by exposing ‘concentrated’ P-depleted biomass to P-laden influent in a relatively short contact-time reactor and subsequently harvesting and removing the resultant P-rich biomass. This stage would be followed by normal biomass growth in larger ponds which depletes the remaining P, with this biomass being harvested and returned to the contact tank for P removal.

Catalytic Pt/Al2O3 Monolithic Foam for Ethanol Reforming Fabricated by the Competitive Impregnation Method.

Here, the synthesis of Pt/Al2O3 catalysts on a monolithic foam employing the competitive impregnation method is presented. NO3 - was used as a competitive adsorbate at different concentrations in order to delay the adsorption of Pt, minimizing the formation of Pt concentration gradients throughout the monolith. The catalysts' characterization includes the BET, H2-pulse titration, SEM, XRD and XPS techniques. The catalytic activity evaluation was performed under partial oxidation and autothermal reforming of ethanol in a short contact time reactor. The competitive impregnation method was able to produce better dispersion of the Pt particles through the Al2O3 foams. XPS analysis indicated the catalytic activity of the samples, by the presence of metallic Pt and Pt oxides (PtO and PtO2) in the internal regions of the monoliths. Compared to other Pt catalysts reported in the literature, the catalyst produced by the competitive impregnation method was revealed to be selective toward H2. Overall, the results showed that the competitive impregnation method employing NO3 - as the co-adsorbate is a promising technique to synthesize well dispersed Pt catalysts over α-Al2O3 foams.

Short contact time CH4 partial oxidation over Ni based catalyst at 1.5 MPa

Gas-to-liquid technologies to produce Fischer–Tropsch fuels are economically sustainable at very large scales— 30 000bbld−1. To achieve a viable process at a scale less than 100bbld−1 requires a compact design, like a short contact time reactor and mass manufacturing to reduce capital cost. We tested the activity of 2.25%Ni/0.1%Ru/CeO2 supported on FeCrAl gauze (Ni2510) to partially oxide methane at a contact time less than 0.050s. Besides, the very short contact time, an additional feature of this work is that the catalyst activated on-stream without a hydrogen pretreatment step. The reactor operated at 1.5MPa, 800°C to 950°C, and a CH4/O2 ratio varying from 1.6 to 1.8 v/v. Methane partially oxidized carbon monoxide (direct mechanism) rather than combusting to CO2 followed by steam reforming to CO (indirect mechanism). The following phenomena support the direct mechanism hypothesis: (i) the selectivity improved when reducing residence time, (ii) the mass spectrometer detected both O2 and CO at the effluent (simultaneously), (iii) metallic Ni clusters on the Ni2510 were absent under reaction conditions based on in situ X-ray absorption spectroscopy. Loading Ni/Al2O3 powder downstream of the Ni2510 increased syngas yield, as this catalyst promoted steam and dry reforming. Soot forms upstream of the Ni2510 catalyst via a retro-propagation mechanism in which methyl radicals produced on the catalyst surface react with the incoming feed gas.

Role of Potassium and Phosphorus in Catalytic Partial Oxidation in Short Contact Time Reactors

Potassium and phosphorus represent inorganics commonly found in biomass and can significantly affect catalytic gasification of biomass. The alkaline and acidic nature of potassium and phosphorus, respectively, can introduce different reaction chemistries. In this paper, the effects of potassium and phosphorus on Rh catalysts for catalytic gasification have been studied at different fuel/oxygen ratios and temperatures. Catalytic partial oxidation (CPO) of ethanol, a biomass surrogate, was carried out over potassium, phosphorus, and monobasic potassium-phosphate-doped catalysts at loadings of 0.1, 1, 10, and 100% of Rh atoms over a range of fuel/oxygen ratios. In addition, time-on-stream studies were also carried out with methane by predosing inorganics on the catalyst (10% loading) and ethanol by introducing inorganics in the feed (0.05 mol %) to understand their effects on the stability of autothermal operation. With ethanol CPO, synthesis gas selectivities did not change significantly with potassium-doped catalysts, whereas they decreased with phosphorus-doped catalysts. Monobasic potassium phosphate showed synergistic effects of potassium and phosphorus; however, at higher temperatures, the effect of phosphorus was predominantly observed as a result of volatilization of potassium. Similarly, with time-on-stream studies with methane CPO, potassium volatilized at higher temperatures, while phosphorus retained its catalytic activity at higher temperatures. With the introduction of potassium and phosphorus in ethanol feed, steady-state autothermal operation was possible without any deactivation of the catalyst. In all experiments, phosphorus exhibited a stronger poisoning effect on rhodium for catalytic partial oxidation than potassium at similar concentrations.

Autothermal reforming of butanol to butenes in a staged millisecond reactor: Effect of catalysts and isomers

Dehydration and isomerization of butanol is studied in an autothermal short contact-time reactor containing a 1wt% Pt stage followed by a zeolite or γ-Al2O3 stage downstream to convert butanol into butenes with up to 95% yield at residence times on the order of 100ms. CH4 is fed as a sacrificial fuel to the Pt stage and butanol is fed between the stages to avoid undesired oxidation and reforming reactions of butanol over Pt. The energy released by CH4 catalytic partial oxidation drives downstream butanol dehydration and isomerization.The effect of catalyst is studied by comparing the performance of HZSM-5, HFER, and γ-Al2O3 catalysts. Higher yields (20%) of butenes were obtained with γ-Al2O3 and HFER than with HZSM-5. The absence of Br∅nsted acid sites in γ-Al2O3 and the small pore structure of HFER lead to reduced yields of large side products such as higher hydrocarbons that promote oligomerization reactions. A 95% butene yield is obtained with HFER at temperatures ranging from 280–350°C and a 95% yield with γ-Al2O3 at temperatures between 320 and 350°C. Only a 75% butene yield was obtained with HZSM-5 at 230°C.The effect of hydrocarbon structure on product formation is studied by comparing conversions of each butanol isomer using a heated tube reactor at temperatures between 200 and 400°C. The reactivity of butanol follows as: t-butanol>2-butanol>iso-butanol>1-butanol. trans-2-Butene and cis-2-butene are primarily formed from linear butanol isomers, while isobutene forms from branched butanol isomers. Conversions and product distributions of butanol isomers formed over HZSM-5 in a staged reactor are comparable (<10% difference across all species) with data using a heated tube reactor at similar temperatures.We successfully demonstrate an alternative pathway to dehydrate butanol into butenes with an autothermal staged reactor compared to conventional methods for applications in small-scale biomass utilization. The largest advantage of this reactor is the integration of highly exothermic autothermal stage and endothermic alcohol dehydration stage which provides an alternative processing technique to maintain the bed temperature.

Investigation of Short Contact Time Reactors for Regeneratively-Cooled Hypersonic Vehicles

Investigation of Short Contact Time Reactors for Regeneratively-Cooled Hypersonic Vehicles

Investigation of Short Contact Time Reactors for Regeneratively Cooled Hypersonic Vehicles

To counter the extreme heat loads experienced by hypersonic engine structures, it is imperative that fuels double as coolants. However, the use of logistical fuels (e.g., JP-7 and JP-10) in hypersonic applications is becoming more accepted in near-term flight engine test applications. In order for hydrocarbon based fuels to be viable as scramjet fuels they must be able to absorb heat in the form of sensible, latent, and chemical enthalpies, while attempting to minimize coke formation. In this study, we attempt to achieve these ends using a short contact time (SCT) catalytic reactor. This type of reactor has the advantages of short residence times, enhanced wake mixing, and heat conduction into the core flow of the process stream, while minimizing pressure losses. In this investigation, we have performed experiments using a series of different catalysts (Pt- αAl2O3, Pd-αAl2O3, and zeolite) on a set of logistical fuels (JP-7, JP-8, JP-10 and S-8, a synthetic hydrocarbon fuel currently being investigated by the USAF). Experiments were performed at low (1-3 atmospheres) and elevated (40-50 atmospheres) pressures, at temperatures expected in a hypersonic engine heat exchanger. Experiments measured the production of various gas-phase and liquidphase species in the reactors as a function of pressure, temperature, residence time, and catalyst formulation. Results show the production of small (H2, C1 – C3) species in the gas phase, with a shift from hydrogen and ethylene formation at low pressures towards methane and ethane formation at elevated pressures. In addition, experiments have shown significant coke formation at elevated pressures for all of the fuels investigated. Detailed kinetic modeling has identified shortcomings in models used for describing the pyrolysis of these fuels.

Catalytic partial oxidation of CH 4 with nickel–lanthanum-based catalysts

The aim of this work, concerning with the partial oxidation of methane to syngas in a self-sustained short contact time reactor using pure oxygen as oxidant, was the development of perovskite-based nickel–lanthanum (LaNiO 3 with Ni partially substituted by Co and Pt) catalysts and the comparison of their performance with that of 5% nickel on alumina catalyst. The prepared catalytic materials were arranged in a fixed bed and the activity tests were carried out by increasing the WHSV from ∼130 to ∼560 N l h −1 g cat −1. Perovskite-based catalysts were characterized by performances lower than that of 5% Ni/Al 2O 3. The very low perovskites catalytic performance towards the partial oxidation of methane was mainly due to the high reducibility of their structures into La 2O 3, metallic Ni, and Ni oxides; moreover, La and Ni species further reacted with the CPO reaction products CO 2 and H 2O leading thus to the formation of the poorly active species La(OH) 3, La 2NiO 4 and La 2O 2CO 3. On the contrary, 5% Ni/Al 2O 3 catalyst showed a very satisfactory CH 4 conversion always above 85%, remaining stable by varying WHSV. The same trend was noticed for H 2 and CO selectivity, both around 90%. Furthermore, as concerns the bed temperature by increasing WHSV, 5% Ni/Al 2O 3 catalyst presented quite stable T in and T out (the latter around 1200–1250 °C), whereas the perovskite-based catalysts showed a quick T in decrease and T out increase, anyway not exceeding 1200 °C. The good performance and high thermal stability of 5% Ni/Al 2O 3 catalyst was probably related to the arrangement of the nickel particles, which resulted somehow thermally protected as partially embedded in the alumina carrier.

Alumina-supported nickel catalysts for catalytic partial oxidation of methane in short-contact time reactors

The aim of this work was the development and performance evaluation of alumina supported nickel catalysts for partial oxidation of methane to syngas in a self-sustained short contact time fixed bed reactor using pure oxygen or air. It was also verified the attainment of relative constant temperatures in an appropriate range (750–1050 °C) along the catalytic fixed bed independently of the operative weight hourly space velocity (WHSV); thus sintering and coking problems and the consequent performance decay were avoided. The activity tests with a Ni load of 5% and 10% by weight were carried out increasing the WHSV from 150 to 1440 Nl h −1 g cat −1. The results obtained with 10% Ni/Al 2O 3 catalyst when working with pure O 2 showed an excellent performance towards CH 4 conversion (>90%) and H 2 selectivity (>95%), comparable to the best rhodium based catalysts tested at the same operating condition [29]. When working with air, 10% Ni/Al 2O 3 was able to maintain the reactor thermally self-sustained using air rather than pure oxygen until WHSV of 1440 Nl h −1 g cat −1 (GHSV ≈ 750,000 h −1) and gave a performance similar to that with pure O 2 (but producing a syngas diluted by N 2) till a WHSV value of about 600 Nl h −1 g cat −1.

Hydrogen production by catalytic partial oxidation of iso-octane at varying flow rate and fuel/oxygen ratio: From detailed kinetics to reactor behavior

Hydrogen production by catalytic partial oxidation of iso-octane is experimentally and numerically studied over a rhodium/alumina coated honeycomb monolith at millisecond contact times by varying both fuel-to-oxygen ratio and flow rates and at varying flow rates. At fuel rich conditions, the formation of by-products potentially serving as coke precursors is observed. The quantity of by-products strongly depends on the flow rate. Both fuel conversion and hydrogen yield increase with increasing flow rate, i.e., decreasing residence time. This extraordinary behavior of autothermally operated short-contact time reactors can be understood by the interaction of mass and heat transport and chemical reactions. Therefore, an elementary-step-like heterogeneous reaction mechanism is implemented into a two-dimensional flow field description of a single monolith channel, coupled with a heat balance of the entire monolithic structure.

Experimental and Modeling Analysis of Methane Partial Oxidation: Transient and Steady-State Behavior of Rh-Coated Honeycomb Monoliths

The present study consists of an experimental and theoretical study of the performance of Rh-coated honeycomb monoliths for methane partial oxidation. The thermal behavior of Rh-coated honeycomb monoliths was studied under representative operating conditions, at steady state and during light-off. Model analysis (based on a dynamic heterogeneous reactor model that incorporates a kinetic scheme of the process independently developed, and well-assessed correlations for heat and mass transfer) provided a key for interpreting the observed effects. The comprehension of how transport phenomena and surface kinetics affect the reactor behavior leads to the conclusion that the feasibility of small-scale production of syngas via CH4 catalytic partial oxidation relies on thermal management of the short contact time reactor and not the obtainment of high syngas yields (which is not a challenging task). Severe operating conditions (and high surface temperatures) can deplete the catalyst activity and cause unstable reactor operation. Guidelines for optimal reactor design are proposed.

Conversion of hydrocarbon fuels to syngas in a short contact time catalytic reactor

Some results of the theoretical and experimental research on the oxidative production of syngas from hydrocarbon fuels in catalytic reactors which operate at high temperatures and short contact times are presented. Pilot scale tests of the partial oxidation of methane, isooctane and gasoline have been carried out in nearly adiabatic conditions on structured catalysts developed at the Boreskov Institute of Catalysis and characterized by a low (≤0.5 wt./wt.%) content of noble metals. High yield of syngas and stable performance of the catalysts were revealed in the experiments. The details of interaction between chemical and physical processes inside adiabatic monolith reactor have been elucidated by mathematical modeling of the partial oxidation reaction on the base of catalyst detailed chemistry. The problems that emerged from the short contact time reactor operating on a pilot scale are also discussed.

The chemistry of ethane dehydrogenation over a supported platinum catalyst

An investigation of the production of C2H4 through the selective dehydrogenation of C2H6 in short contact time reactors featuring a ceramic-foam catalyst coated with platinum is reported. The surface chemistry was derived on the basis of four reaction classes (direct adsorption, adsorption on an adsorbate, surface reactions with adsorbed reactants and unimolecular surface reactions including desorption) using collision theory with energy barriers obtained from DFT studies and the UBI–QEP method. The derived mechanism accounts for differences in site occupation and surface bonding types and includes 35 adsorbed chemical species and 283 reversible reactions. The study also features a comprehensively validated C1–C2 mechanism for the gas phase consisting of 44 chemical species and 271 reversible reactions. The complete chemistry was evaluated through extensive sensitivity analysis with reference to 16 sets of new experimental data. The reaction dynamics were studied using inlet velocities in the range 1.34 to 6.72 m/s and the impact on selectivities of variations in the oxygen to carbon weight ratio between 0.45 and 0.75 assessed. It is shown that the simulations capture all experimental trends with reasonable quantitative accuracy. The study suggests that the catalytic dehydrogenation of C2H6 initially contributes to the net production of C2H4 and that changing conditions in the gas phase adjacent to the catalyst can result in the consumption of ethylene principally leading to methane and carbon deposition. It is shown that the consumption of H2 and O2 proceeds predominantly through catalytic conversion under all conditions and that OH(s) provides a dominant channel for the removal of surface carbon. It is also shown that the consumption of oxygenated species features surface specific channels.

Transient behavior of the methane partial oxidation in a short contact time reactor: Modeling on the base of catalyst detailed chemistry

Abstract The objective of the paper is a numerical study of the partial oxidation of methane to synthesis gas in a monolith reactor with Pt/Ce-Zr-La catalyst. A dynamic one-dimensional two-phase reactor model of the processes with accounting for both transport limitations in the boundary layer of a fluid near the catalyst surface and detailed molecular unsteady-state kinetic model for surface reactions have been developed and verified with the transient experiments data. The mathematical model was used to explain a transient behavior of the process in the monolith reactor during start-up (ignition). Also studied is the influence on the process dynamics such parameters as linear velocity, equivalent diameter of triangular channel and effective thermal conductivity of the monolith. It was found that higher linear velocity and equivalent channel diameter as well as the worse axial conductivity of solid phase favor decreasing a time delay in syngas production in the Pt/Ce-Zr-La/α-Al2O3 honeycomb monolith with a triangular shape channels.

Hydrogen-rich gas production from gasoline in a short contact time catalytic reactor

Presented is a study of hydrogen-rich gas generation by selective catalytic oxidation of gasoline in a nearly adiabatic monolith reactor. Experiments were conducted using a gasoline mixture containing 191 hydrocarbon species, while corresponding thermodynamic models employed a surrogate mixture of 29 organics with well-defined properties. Based on the data obtained, it was demonstrated that a nearly equilibrium synthesis gas could be produced using catalytic monoliths. Also discussed are problems encountered with a short contact time reactor operating at a pilot-scale, issues that are largely independent of the catalysts employed. It was found that this particular reactor design is prone to pre-reformation of the fuel prior to catalytic reaction. In addition, the breakthrough of the feed mixture was found to arise near the reactor wall under certain circumstances. Feed composition, superficial velocity, and various reactor design factors were all found to affect these phenomena.

Experimental and numerical investigation of the catalytic partial oxidation of [formula omitted] mixtures diluted with [formula omitted] and [formula omitted] in a short contact time reactor

The catalytic partial oxidation (CPO) of fuel-rich CH 4 / O 2 mixtures, heavily diluted with H 2 O and CO 2 (46.3% and 23.1% volumetric feed composition, respectively), was investigated experimentally and numerically at 5 bar. Experiments were carried out in an ∼ 8 ms residence time prototype gas-turbine honeycomb reactor coated with a Rh/ ZrO 2 catalyst and included temperature measurements along the reactor and exhaust gas analysis. Simulations with detailed hetero-/homogeneous chemical reaction schemes were performed using a steady, full elliptic 2D code for both the gas and solid phases. The employed catalytic reaction scheme overpredicted mildly the impact of the total over the partial oxidation route and this effect was more pronounced at lower reactor inlet temperatures ( < 523 K ). The contribution of the various chemical pathways to the synthesis gas yields and selectivities has been elucidated. It was shown that the addition of water provided a source of surface oxygen and hydroxyl radicals, which in turn enhanced the CH 4 conversion and H 2 selectivity and reduced the CO selectivity. On the other hand, the addition of CO 2 had a minor impact on the aforementioned parameters. The increase in the H 2 / CO product ratio with water dilution is highly desirable in new power generation processes with large exhaust gas recycle, which utilize the “catalytic rich combustion” methodology (the partial oxidation products stabilize a post-catalyst flame). At steady operation the catalyst surface temperatures exceeded by 200 K the adiabatic equilibrium temperature and standard (passive) heat transfer mechanisms in the solid were shown to be ineffective in providing proper reactor thermal management. Catalytic ignition was achieved at 670 K, however, the strong ignition/extinction hysterisis allowed for sustained steady CPO at inlet temperatures as low as 473 K.

Crossing the breakthrough line of ethylene production by short contact time catalytic partial oxidation

A novel catalyst (Pt-LaMnO 3) for the oxidative dehydrogenation of ethane to ethylene in short contact time reactors was developed by Istituto di Ricerche sulla Combustione (CNR) in cooperation with Snamprogetti, with a multi-layered structure consisting of a thin layer of Pt deposited onto a layer of LaMnO 3 perovskite supported over monolithic carriers. Such structured catalyst was tested in a head-to-head comparison with state-of-the-art Pt and Pt-Sn systems, under autothermal process conditions, showing enhanced performances both in terms of high temperature stability and yields to ethylene. The development of a durable, high-performance catalyst undertaken in this work may be beneficial to making catalytic ethane ODH more economically viable and represents a further step towards its pre-commercial implementation.

Partial oxidation of alcohols to produce hydrogen and chemicals in millisecond-contact time reactors

We compare the autothermal partial oxidation of 1-propanol and 2-propanol with methanol and ethanol on Rh with several additives in short-contact time reactors. All alcohols could produce H 2 at 70–90% selectivity, and Rh–Ce was superior to Rh in producing H 2. Methanol produced high conversion and high selectivity to H 2 even at high C/O where temperatures fell to <600 °C. 2-Propanol gave lower conversions and less H 2 and CO than the other alcohols, but produced the most chemicals. Above C/O = 1.5, ∼ 70 % of 2-propanol was converted into acetone or propylene. Up to 20% propylene was formed at C/O = 1.5. In contrast, 1-propanol gave <8% propylene and <15% propanal at any C/O and produced more ethylene than propylene. Much more oxygenates and olefins were formed on Rh than on Rh–Ce. These results show that different alcohols have very different selectivity in catalytic partial oxidation at short contact times even at high temperatures. Rapid adsorption of alcohols as alkoxy species leads to complete dissociation to H 2 and CO. Our results suggest that acetone and olefins likely were produced primarily by homogeneous reactions after all O 2 had been consumed in the catalyst. Although alkanes do not form significant oxygenates by partial oxidation at short contact times, alcohols can be made to produce predominantly oxygenates through suitable adjustments of C/O and catalyst.

The effect of support morphology on the reaction of oxidative dehydrogenation of ethane to ethylene at short contact times

The oxidative dehydrogenation of ethane is carried out in short contact time reactors over Pt and LaMnO 3 based catalysts supported on a large number of different ceramic substrates (45, 60 and 80 ppi foam monoliths and 200, 400, 600, 900 and 1200 cpsi honeycomb monoliths). Experimental results, obtained under the same conditions at varying the C 2H 6/O 2 ratio, showed that the highest performance in terms of ethylene selectivity and yield is always attained on LaMnO 3 catalysts. Furthermore, the results are significantly influenced by the morphology and cell density of the support, with 45 and 60 ppi foams and 400 and 600 cpsi honeycombs giving the best performance. The experimental results are explained by means of geometrical and fluid dynamic considerations on the support, and by means of a 2D mathematical model, which clearly indicates an optimal intermediate cell density for maximising ethylene selectivity and yield.

05/01455 Catalyst investigation for applications ofoxidative dehydrogenation of ethane in short contact time reactors: Donsì, F. et al. Catalysis Today, 2004, 91–92, (15), 285–288

05/01455 Catalyst investigation for applications ofoxidative dehydrogenation of ethane in short contact time reactors: Donsì, F. et al. Catalysis Today, 2004, 91–92, (15), 285–288