Deactivation Scheme Research Articles

Experiment and modeling of coke formation and catalyst deactivation in n-heptane catalytic cracking over HZSM-5 zeolites

Since paraffins catalytic cracking was of significant importance to light olefins and aromatics production, this work was intended to gain insights into the feature and model of coke formation and catalyst deactivation in n-heptane catalytic cracking over HZSM-5 zeolites. 18 tests of n-heptane catalytic cracking were designed and carried out over HZSM-5 zeolites in a wide range of operating conditions. A particular attention was paid to the measurement of the conversion, product distribution, coke content, and the porosity and acidity of the fresh and spent HZSM-5 zeolites. It was found that alkene and aromatic promoted coke formation, and it reduced the pore volume and acid site of HZSM-5 zeolites, tailoring its performance in n-heptane catalytic cracking. The specific relationship between HZSM-5 zeolites, n-heptane conversion, product distribution and coke formation was quantitively characterized by the exponential and linear function. Based on the reaction network, the coupled scheme of coke formation and catalyst deactivation were specified for n-heptane catalytic cracking. The dual-model was proposed for the process simulation of n-heptane catalytic cracking over HZSM-5 zeolites. It predicted not only the conversion and product distribution but also coke content with the acceptable errors.

Coupled simulation of thermal-metallurgical-mechanical behavior in laser keyhole welding of AH36 steel

A computational fluid dynamics (CFD) simulation of the molten pool in laser keyhole welding was utilized to acquire temperature data for further metallurgical and mechanical calculations. For the CFD simulation, the governing equations were solved, and the scattering and absorption of the laser beam in the plume were modeled at both the standard atmospheric condition (101,325 Pa) and a vacuum condition (3,000 Pa). A stochastic ray-tracing algorithm was adopted to effectively implement the transmission and scattering of laser bundles of rays. The temperature data from the CFD simulation were then imported to a finite element method (FEM)-based heat conduction analysis to simulate the thermal-metallurgical-mechanical behavior during the cooling phase of the weldment. The strain, residual stress, and distortion were calculated using an elastoplastic model based on the phase transformation-dependent material properties. An element deactivation scheme was used to take care of the zero-strength condition of the elements in the molten pool and keyhole region. The Vickers hardness and the residual stress were measured to verify the simulation model, and the experimental and simulation results had a similar tendency.

Investigation on the increased stability of the Ni–Co bi-metallic catalysts for the carbon dioxide reforming of methane

The carbon dioxide reforming of methane is one of the processes used to tackle global warming by reducing carbon dioxide, one of the most harmful greenhouse gases through the catalytic reaction with methane which produces syngas. Although the noble metals are good candidates for such a process, the cost and scarcity limit their uses. The non-noble metals on the other hand, with satisfactory activity and cost, are of interest except their stability against the coke formation, the main deactivation scheme during the dry reforming reaction. Therefore, the study of the enhanced stability found in the bi-metallic system of Ni catalysts via the introduction of Co was carried out in this work. The activity and coke resistance were studied on both mono and bi-metallic system of the Ni-based catalysts (10%wt) supported on γ-Al2O3-HY zeolite prepared via the sol-gel method. The activity testing was performed in a fixed-bed reactor with a unity feed volume ratio under atmospheric pressure at 973 K, where the Ni–Co bi-metallic system exhibits higher activity than both mono-metallic of either pure Ni or Co due to a higher number of active sites confirmed by the CO chemisorption. On the stability testing, the high stability was found on the bi-metallic system as verified by the time-on-stream testing. In addition, the higher reduction temperature for the bi-metallic system via the analysis using H2-TPR also suggested stronger metal-support interaction which may decrease the bonding strength of the coke on the surface leading to lower deactivation.

Deactivation of Spent Ion-exchange Resins Contaminated by Cesium and Cobalt Radionuclides

Simulation of iron oxide (magnetite and maghemite) and aluminosilicate (sillimanite and cyanite) deposits formed on the surface of spent ionexchange resins in the process of decontamination of liquid radioactive waste contaminated by cesium and cobalt radionuclides has been performed. A method of deep deactivation of spent ion-exchange resins contaminated by aluminosilicate and iron oxide deposits using alkaline and acidic solutions containing Zn-EDTA complexes has been suggested. The method of two-stage concentrating of cesium radionuclides using selective sorption materials (resorcinol-formaldehyde resin and Thermoxid-35 ferrocyanide sorbent) has been improved. The method advantage consists in using a solution containing EDTA complexes for elution of cesium radionuclides from the resorcinol-formaldehyde resin with their transition onto Thermoxid-35. High stability of the resorcinol-formaldehyde resin and Thermoxid-35 in the course of concentrating has been demonstrated. A scheme of deactivation of spent ion-exchange resins, which enables one to decrease the volume of secondary wastes due to utilization of a circulating water supply, has been suggested.

Adaptive deactivation and zero-forcing scheme for low-complexity LDPC decoders

A modified message propagation algorithm is proposed for a low-complexity decoder of low-density parity-check (LDPC) codes, which controls the information propagated from variable and check nodes.The proposed threshold-based node deactivation for variable nodes and zero-forcing scheme for check nodes remarkably reduce decoding complexity required for similar error performance. In the proposed scheme, different thresholds, which are determined from the base matrix of the LDPC codes, are applied for each type of variable node. In addition, thresholds for deactivating variable nodes are increased while the decoding process is operated for a reduction in decoding complexity without early error floor, which is a drawback of the conventional threshold-based deactivation scheme. Simulation results show that the proposed scheme enables normalized min-sum decoders to decode successfully with less complexity than the conventional threshold-based deactivating scheme.

Influence of simulation methods of temperature distribution on thermal and metallurgical characteristics in GMA welding

A thermal metallurgical CFD-based mass and heat transfer (MHT) analysis was compared with a FEM-based conductive heat transfer (CHT) analysis and a CHT with element activation and deactivation scheme (CHTE) analysis of gas metal arc welding process for the same process conditions. To evaluate the validity of each thermal analysis method, experimental results were also obtained. The CFD-based MHT analysis resulted in the best approximation in the validated thermal and metallurgical results with the vast computation time. The FEM-based CHT and CHTE analyses required very short computational time and showed appropriate agreement in results. The more highly concentrated martensite fraction in the fusion-zone of the CHT analysis result was influenced by the relatively lower temperature history and the faster cooling rate, while the more concentrated martensite fraction in the heat affected zone and the tempered zone of the CHTE analysis result was influenced by the relatively higher temperature history.

Adaptive Beam Deactivation for Coordinated Multi-cell System with Limited Feedback

In this paper, we propose an adaptive beam deactivation scheme to mitigate inter-cell interference for a coordinated single-cell beamforming downlink transmission with limited feedback in a multi-c...

Activity and coke formation of nickel and nickel carbide in dry reforming: A deactivation scheme from density functional theory

Dry reforming is a promising reaction to utilise the greenhouse gases CO2 and CH4. Nickel-based catalysts are the most popular catalysts for the reaction, and the coke formation on the catalysts is the main obstacle to the commercialisation of dry reforming. In this study, the whole reaction network of dry reformation on both flat and stepped nickel catalysts (Ni(111) and Ni(211)) as well as nickel carbide (flat: Ni3C(001); stepped: Ni3C(111)) is investigated using density functional theory calculations. The overall reaction energy profiles in the free energy landscape are obtained, and kinetic analyses are utilised to evaluate the activity of the four surfaces. By careful examination of our results, we find the following regarding the activity: (i) flat surfaces are more active than stepped surfaces for the dry reforming and (ii) metallic nickel catalysts are more active than those of nickel carbide, and therefore, the phase transformation from nickel to nickel carbide will reduce the activity. With respect to the coke formation, the following is found: (i) the coke formation probability can be measured by the rate ratio of CH oxidation pathway to C oxidation pathway (rCH/rC) and the barrier of CO dissociation, (ii) on Ni(111), the coke is unlikely to form, and (iii) the coke formations on the stepped surfaces of both nickel and nickel carbide can readily occur. A deactivation scheme, using which experimental results can be rationalised, is proposed.

Aggregation and Electrolyte Composition Effects on the Efficiency of Dye-Sensitized Solar Cells. A Case of a Near-Infrared Absorbing Dye for Tandem Cells

Time-resolved laser spectroscopy studies of complete solar cells sensitized with a near-infrared absorbing dye (HY103) and filled with different electrolytes are applied to explain their macroscopic parameters (efficiency and short-circuit current). Particular attention is paid to the effect of coadsorbent, size of cations in electrolyte (lithium vs guanidine ones), and addition of tert-butylpyridine. A complete deactivation scheme in the cell is revealed, and the rates of electron injection and all other processes are explored. For the most efficient electrolyte, the electron injection rate constants are 0.21 ps–1 from monomers and 0.07 ps–1 from H-aggregates. Moreover, two important and novel findings are revealed: energy transfer from the excited state of monomers to H-aggregates (with rate constants from 0.04 to 0.25 ps–1) and the decrease of internal conversion rate in HY103 attached to the nanoparticles (0.01 ps–1) with respect to that of free dye in solution (0.06 ps–1). Thus, our study gives more ...

Plasma Deactivation of Endotoxic Biomolecules: Vacuum Ultraviolet Photon and Radical Beam Effects on Lipid A

AbstractIt is widely accepted that plasma‐generated energetic and reactive species are responsible for plasma‐induced sterilization; however, how these species act alone or synergistically to deactivate endotoxic biomolecules is not completely understood. Using a vacuum beam system, we study the effects of vacuum ultraviolet (VUV) radiation, oxygen and deuterium radicals on lipid A, the immune‐stimulating region of lipopolysaccharide. VUV‐induced photolysis causes bulk modification of exposed lipid A film up to the penetration depth of VUV photons, ≈200 nm. Although radical‐induced etch yield of lipid A is lower than VUV‐induced photolysis, secondary ion mass spectrometry and human whole blood‐based assay suggest that radicals render a higher degree of modification at the film surface. This study contributes to the fundamental understanding of plasma effects on biomolecules for a better deactivation scheme and applications. magnified image

Consequences of Fast, Stochastic Rhodopsin Shutoff for a Model of Phototransduction in Rods

Rod photoreceptors signal the number and timing of photon absorption, a property that requires that each single photon response be of similar amplitude from trial to trial. How such reproducibility is achieved has been the subject of much experimental and theoretical work, which has demonstrated the importance of multiple steps in rhodopsin deactivation and diffusion of second messengers (Mendez et al., 2000; Bisegna et al., 2008). So far, all previous models have assumed that rhodopsin lifetime is significantly longer than recent measurements indicate (Krispel et al., 2006; Burns and Pugh, 2009). Additionally, recent biochemical studies have provided new details about the dependence of rhodopsin deactivation on phosphorylation level (Vishnivetskiy et al., 2007) that should inform a complete model of light response kinetics and reproducibility. We have implemented a spatio-temporal model of phototransduction in which the rhodopsin deactivation scheme is a stochastic multi-step process lasting no more than 50 ms. The parameters of this model were constrained using an extensive data set obtained from a variety of transgenic mouse lines, each developed to perturb rhodopsin activity, PDE deactivation, or Ca2+ feedback. Our simulations demonstrate the relative contributions of stochastic rhodopsin deactivation, Ca2+ feedback to guanylate cyclase, and second messenger diffusion to single photon response variability under biologically relevant constraints.

Characterization of aged hydrotreating catalysts. Part I: Coke depositions, study on the chemical nature and environment

In this work various spent Ni(Co)MoP/Al 2O 3 catalysts from different origins were selected. The coke deposits of the spent catalysts were characterized in order to compare their behaviour in terms of quantity, molecular arrangement and reactivity between a cobalt and a nickel promoted catalyst. A thermo gravimetric analysis (TGA) was employed to evaluate the reactivity of the coke. Raman and NMR 13C spectroscopies were used to characterize the chemical nature of coke. All the characteristics observed are relevant with a three-stage deactivation scheme. In the first stage, the alumina is assumed to be rapidly covered by coke, while the active sites remain protected by their high hydrogenation activity. The coke forms relatively small graphite-like planes (10 Å). In the second stage, the nickel segregated from MoS 2 particles during the aging and the graphite-like crystallite extends significantly (to 16 Å). In the final step of the coke evolution, it can be assumed that some organization occurs in the molecular structure of coke. The coke particles previously observed begin to stack, trapping the nickel segregated inside the coke matrix. Moreover, the coke deposit characterized on the spent CoMoP/Al 2O 3 is equally distributed between the active phase and the carrier whereas the coke seems to be preferentially deposited over the alumina surface on the spent NiMoP/Al 2O 3 catalysts. It suggests that the active sites on the CoMoP/Al 2O 3 catalysts are easily coked rather than the NiMoP/Al 2O 3 ones. It is also observed that the coke evolution is faster on the CoMoP/Al 2O 3 catalysts. Those differences, representing an important issue require solutions to prevent the coke deposit.

Characterization of aged hydrotreating catalysts. Part II: The evolution of the mixed phase. Effects of deactivation, activation and/or regeneration

The characterization of various spent Ni(Co)MoP/Al 2O 3 catalysts has been investigated in order to elucidate the modifications undergone on the catalysts in operating conditions. 12 catalysts coming either from industrial or pilot reactors were studied. The results are reported in two sections. In Part II, here presented, the active phase characteristics of spent catalysts are compared at different deactivation levels and after various treatments (sulfidation, regeneration and sulfidation) by means of different techniques (XPS, Extended X-ray Absorption Fine Structure, Transmission Electronic Microscopy/Energy Dispersive Spectroscopy). The influence of the treatment on the activity was considered by performing catalytic tests with model molecules (toluene hydrogenation). The analysis of the spent CoMo and NiMo catalysts straight after downloading and without any treatment, except for washing and drying, lead to a promotion rate decrease on both CoMo and NiMo. Nevertheless, the promotion rate decrease seems to origin from the nickel segregation on the NiMo catalysts and from coking over the MoS 2 active phase on the CoMo catalysts. Additionally, the spent catalysts exposed to the air are partially oxidized. To compensate the oxidation taking place while downloading the catalyst and to clearly compare the spent catalysts, a sulfiding treatment was performed. However the MoS 2 phase is deeply modified by the treatment and does not represents the original spent catalyst. As a consequence the spent sulfided catalysts could not be compared from the deactivation point of view; anyway this treatment has been used to describe the coke localization. For NiMo catalysts, the active phase remains free of coke during aging or at least for brief lifetimes. As a consequence, the promotion rate rises during the spent catalysts sulfidation followed by an activity increase. On the contrary, the CoMo catalysts exhibit a coked MoS 2 phase which prevents the promotion rate recovery and the activity augmentation. Finally, the spent CoMoP/Al 2O 3 catalysts can be totally regenerated by burning-off the coke, this air treatment leads to the same active phase as the fresh catalyst. By the way, it is not possible to reach the initial activity on the spent NiMoP/Al 2O 3 catalysts, and the total amount of ‘NiMoS’ active phase is lower than the amount on the fresh one. This study pointed out an important difference between the deactivation scheme of the NiMoP/Al 2O 3 and CoMoP/Al 2O 3 catalysts. Coke deposit is the main cause of CoMo catalyst deactivation. However, for NiMo catalysts, deactivation is due to the promoter segregation. Moreover, this study also highlighted that regeneration allows an easy coke removal, but does not allow the total recovery of the promotion rate for NiMo catalysts. It confirms the difference already noticed between the two types of catalysts presented in the first section of this study.

Deciphering Intrinsic Deactivation/Isomerization Routes in a Phytochrome Chromophore Model

High level ab initio correlated (CASPT2) computations have been used to elucidate the details of the photoinduced molecular motion and decay mechanisms of a realistic phytochrome chromophore model in vacuo and to explore the reasons underneath its photophysical/photochemical properties. Competitive deactivation routes emerge that unveil the primary photochemical event and the intrinsic photoisomerization ability of this system. The emerged in vacuo based static (i.e., nondynamical) reactivity model accounts for the formation of different excited state intermediates and suggests a qualitative rationale for the short (picosecond) excited state lifetime and ultrafast decay of the emission, its small quantum yield, and the multiexponential decay observed in both solvent and phytochromes. It is thus tentatively suggested that this is a more general deactivation scheme for photoexcited phytochrome chromophores that is independent of the surrounding environment. Spectroscopic properties have also been simulated in both isolated conditions and the protein that satisfactorily match experimental data. For this purpose, preliminary hybrid QM/MM computations at the correlated (CASPT2) level have been used in the protein and are reported here for the first time.

Time-Resolved Resonance Raman and Density Functional Theory Study of the Deprotonation Reaction of the Triplet State of p-Hydroxyacetophenone in Water Solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.

Study of l-aminoacylase deactivation in an ultrafiltration membrane reactor

The behaviour of an ultrafiltration membrane reactor (UFMR) (60 cm 3 of reactor volume) for the optical resolution of dl-butyrine catalysed by l-aminoacylase was studied, and the influence of substrate concentration (15–25 mmol dm −3 in N-acetyl- l-butyrine), temperature (30–50 °C) and the presence of CoCl 2 (0.5 mmol dm −3) on enzyme deactivation was analysed. Adsorption studies with polysulphone and regenerated cellulose membranes (30 cm 2 of filtration surface), as well as deactivation studies in the reaction conditions, were carried out to determine the causes of deactivation. A single-step deactivation scheme is proposed, and it was shown that this first-order model adequately describes enzyme deactivation. The dependence of K d on the enzyme concentration points to the enzyme deactivation, which is mainly caused by the adsorption phenomena on the membrane surface.

Excited state proton transfer and photochromism of an aromatic Schiff base. Pico- and femtosecond kinetics of the N, N′-bis(salicylidene)- p-phenylenediamine (BSP)

The results of time-resolved pico- and femtosecond absorption and emission study performed for the title photochromic Schiff base, (BSP) are presented. Transient absorption spectra of intermediates, appearing in the excited state intramolecular proton transfer (ESIPT) were identified. A full scheme of deactivation of the excited BSP molecule, including the enol-, keto- and photochromic tautomers, was proposed. In particular, the characteristic time of the ESIPT process was determined as <50 fs, and the time of the back proton transfer, as 0.5–1.5 ps. The photochromic transient in its ground state is created from the S 1 excited keto-tautomer with the rate constant k K− PC =2.8×10 10 s −1 .

Integrated approach for the analysis of FCC evaluation results

In this article we present an integrated approach for the treatment and analysis of FCC experimental data based on statistical techniques and the use of characteristic curves. The method involves a material balance reconciliation procedure that improves the reliability and precision of the experimental results. After data reconciliation, we propose the use of some characteristic curves that describe in a proper way the behavior of conversion and selectivity. The characteristic curves are used to reach three main objectives: detect suspicious points (outliers), verify the correct trend of a experimental set of results and determine the discrimination capacity of the results when comparing two sets of data. These curves are based on fundamental knowledge of the process and a five-lump kinetic scheme. The coke characteristic curve takes into account a selective deactivation scheme for this product. The curves have parameters that are estimated by fitting to the experimental data using non-linear least-square methods. The systematic utilization of this procedure is a powerful tool for a reliable analysis of FCC evaluation results.

Transformations of the chiral diphosphine rhodium catalyst [(1,5-COD)Rh(—)R,R-DIOP]+CF3SO3– under conditions of hydrogenation

Transformation products of the cationic rhodium complex [(1,5-COD)Rh(—)R,R-DIOP]+CF3SO3– (1) (COD is cycloocta-1,5-diene and DIOP is (±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), which were obtained in its reactions with molecular hydrogen, base (NEt3), and solvents in the absence of a substrate, were investigated by 1H and 31P NMR spectroscopy. The solvate complexes [(Solv)2Rh(—)R,R-DIOP]+CF3SO3–, which were generated from complex 1 in its reaction with molecular hydrogen, underwent destruction of the diphosphine ligand with elimination of benzene and were subjected to oxidation by traces of moisture and oxygen to form the DIOP dioxide complex with RhI. In the absence of hydrogen, complex 1 in solutions produced the diphosphine dioxide rhodium(i) complex and mono- and binuclear rhodium(i) solvate complexes. The scheme of deactivation of the complex in the absence of the substrate was proposed. The catalytic activity of the solvate complexes [(ArH)Rh(—)R,R-DIOP]+CF3SO3–, which contain benzene, p-xylene, and mesitylene in the coordination sphere, was studied in hydrogenation of Z-α-acetamidocinnamic acid.

Influence of the out-of-plane distortions of nuclear configurations of molecules with then-AO of the heteroatom directed along theC 2 symmetry axis on spin-orbit coupling between thenπ*- and ππ*-states

This paper studies changes in the matrix elements (Ump) of spin-orbit coupling between the nπ*- and ππ*-states, which are induced by the (“chair,” “bath”) distortions of the nuclear configurations of molecules. The analysis is performed for acridine molecules in which the n-pz atomic orbital (AO) of the heteroatom is directed along the C2 symmetry axis. Earlier, for molecules with a planar nuclear configuration of C2v symmetry and with the heteroatom lying on the C2 axis, we established the dependence of Ump on the symmetry of ππ*-states [Γ(ππ*)=A1 or B2]. The values of Ump differ by more than one order of magnitude; this is in line with the difference between the interconversion rate constants (Kisc; two or three orders). In this work we have found that this contrast in Ump (and, accordingly, in Kisc) is retained when the nuclear configuration of the acridine molecule is distorted to the “chair” (AC-A) configuration, although the individuality of both molecular orbital types (nσ-MO and π-MO) and states nπ* and ππ* is annihilated to a certain extent. For the “bath” (AC-B) conformation the difference in Ump considerably diminishes. Reasons for the changes in the matrix elements of spin-orbit coupling and rate constants of the S-T conversion are analyzed. The available energy level diagram is critically analyzed, and a slightly different diagam as well as a scheme of nonradiating deactivation of acridine are suggested.