Reaction Residence Time Research Articles

Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4-Activated Carbon

H3PO4-activated carbon was used as the catalyst in the pyrolysis of waste polyethylene (PE) to study its catalytic effect on the enrichment of the aromatics. The effect of the mass fraction ratio of phosphorus to wood chips (defined as the P/WC ratio) during impregnation and the residence time of the catalytic reaction on the product yield and the oil components were investigated, and the catalytic mechanism was discussed. The increase of the P/WC ratio from 10% to 50% and the residence time from 1 to 5 s would increase the oil yield from 23.2% to 41.8% and 40.8%, respectively. Alkanes, alkenes, and aromatics were the main components in the oils. The content of aromatics increased along with the residence time and could be up to 30.0% when the P/WC ratio was 40%. Monocyclic aromatics, mainly alkyl and alkenyl benzenes with carbon atom numbers between 7 and 10, were the main aromatic compositions, the highest content of which was up to 23.8% when the P/WC ratio was 40% and the residence time was 3 s. A hig...

High purity fructose from inulin with heterogeneous catalysis – from batch to continuous operation

AbstractBACKGROUNDInulin is a valuable source of high purity fructose. The conversion of inulin to fructose utilizing an advanced sulfonic ion‐exchange resin was previously investigated in detail in a batch reactor study, where a detailed mechanistic model for the reaction kinetics was demonstrated (J Chem Technol Biotechnol 2018;93:224–232). The same catalyst was employed in tailor‐made continuous fixed‐bed reactors in the current work in order to study the feasibility of continuous operation.RESULTSTwo types of reactors were utilized: a single‐bed reactor and a multibed reactor with sample withdrawal between catalyst beds. The results from the single‐bed reactor allowed optimal reaction conditions to be determined which were extrapolated and used for experiments in the multiple‐bed reactor. High yields of fructose (75%) were obtained with the multiple‐bed reactor without any degradation products. Detailed flow characterization was performed on the reactor system and residence time distribution results were merged with the detailed model for the chemical kinetics obtained from batch reactor studies taking into account mass transfer limitations, in order to obtain a rigorous model for the performance of the continuous reactor. The fit of the mathematical model to the experimental data was very good and this provides a useful tool for further development and scale‐up purposes.CONCLUSIONSThe results of the two related studies highlight the potential of new reactor design strategies towards the realization of more efficient, sustainable and green processes, which can easily be scaled‐up and implemented in bio‐based industry. © 2018 Society of Chemical Industry

Synergetic NO reduction by biomass pyrolysis products simulating their reburning in circulating fluidized bed decoupling combustion

Abstract The present work investigated the synergetic effect of pyrolysis-derived char, tar and gas (py-gas) on NO reduction, which may occur in circulating fluidized-bed decoupling combustion (CFBDC) system treating N-rich fuel. Experiments were carried out in a lab-scale drop-tube reactor for NO reduction by some binary mixtures of reagents including char/py-gas, tar/py-gas and tar/char. At a specified total mass rate of 0.15 g·min− 1 for NO-reduction reagent, the char/py-gas (binary reagent) enabled the best synergetic NO reduction in comparison with the others. There existed effective interactions between char and some species in py-gas (i.e., H2, CxHy) during NO reduction by pyrolysis products, meanwhile the tar/py-gas or tar/char mixture only caused a positive effect when tar proportion was necessarily lowered to about 26%. On the other hand, the synergetic effects were not improved for all tested binary reagents by increasing the reaction temperature and residence time.

Modeling Water Treatment Reactor Hydraulics Using Reactor Networks

Reactor hydraulics are integral to water treatment processes such as disinfection and chemical contaminant oxidation. This work uses reactor networks—conceptual reactors arranged in parallel and series combinations—to simplify the accurate modeling of residence time in water treatment reactors. Reactor networks were selected for 14 clearwells, ozone contactors, clarifiers, and filters using tracer data sets from literature. For seven of 14 full‐scale reactors, two parallel tanks‐in‐series reactors best balanced accuracy and simplicity. Reactor networks accurately represented tracer data using between two and eight fitting parameters, while segregated flow analysis required 32–164 inputs (two per tracer data point). The modeling work revealed that tracer data sets may have overestimated baffle factors by >10% in 10 of the 14 full‐scale reactors, potentially as a result of inaccurate measurement of flow rate or volume. Applications of reactor networks include more accurate approaches to disinfection regulation and modeling the degradation of emerging contaminants.

Oxidation of Dyeing Sludge in Supercritical Water

In this study, the effect of SCWO treatment, transformation characteristics of heavy metals and corrosion of Ni-based alloy 625 during dyeing sludge supercritical water oxidation (SCWO) were investigated. The experiment carried out at 580°C, 25MPa, oxidation coefficient of 1.3, reaction residence time 10min. The removal rate of COD reached 99.3% at the optimum reaction conditions which means the COD is almost completely destructed. Results of heavy metals determination showed that the concentration of Cu in the liquid products was 0.266mg/L, while Zn and Pb could not be detected under the experimental conditions. This implied that Cu, Zn and Pb in residues after SCWO treatment were more stable than that in raw sludge. The corrosion behaviour of Ni-based alloy 625 was discussed in different conditions. Consequently, it is believed that SCWO treatment is an effective alternative for safety disposal of dyeing sludge.

Process intensification of honeycomb fractal micro-reactor for the direct production of lower olefins from syngas

Inspired by the nature fractal structure, a novel honeycomb fractal micro-reactor was designed and applied to intensify both heat and mass transfer in the exothermic conversion of syngas to olefins. The results show that the honeycomb fractal micro-reactor provides more even distribution of temperature profiles, a narrower reactant residence time distribution, and relative smaller pressure drop per unit length, compared with parallel straight microchannel reactor and mini-fixed bed reactor under the same reaction conditions. Owning to the reinforcement of flow separation and convergence in the honeycomb structure, the contact between reactants and catalyst particles were enhanced, leading to a better heat and mass transfer. Under the current range of temperature, pressure and standard GHSV, the honeycomb fractal micro-reactor leads to supreme CO conversion and lower olefins yield.

Polymeric catalytic membrane for ozone treatment of DEET in water

The ozone treatment of N,N-diethyl-meta-toluamide (DEET) in water was carried out in a membrane reactor that uses ozone-resistant polymer membranes for ozone distribution, catalytic contactor and water separation. Iron oxide (ferrihydrite) nanoparticles supported on powdered activated carbon (CAT1) were used as catalyst, and were coated on the surface of the membrane contactor/distributor. Ferrihydrites promote ozone dissolution and transformation to reactive hydroxyl radicals, and accelerate the conversion and mineralization of DEET. The powdered activated carbon helped retain the organic pollutants in the reaction zone leading to greater conversion and deeper mineralization. Consistently, the catalytic membrane contactor/distributor displayed the highest DEET conversion and TOC reduction over the entire studied range of ozone dosage and reaction residence time. The selective removal water by the membrane separator had the desired effect of concentrating the organic pollutants resulting in their improved treatment. The compact membrane reactor unit equipped with catalytic membrane contactor/distributor and membrane water separator outperformed a semi-batch ozone reactor with 60% DEET conversion compared to 20%, and 30% TOC reduction versus 5%.

Uranium Removal from Groundwater by Permeable Reactive Barrier with Zero-Valent Iron and Organic Carbon Mixtures: Laboratory and Field Studies

Zhovty Vody city, located in south-central Ukraine, has long been an important center for the Ukrainian uranium and iron industries. Uranium and iron mining and processing activities during the Cold War resulted in poorly managed sources of radionuclides and heavy metals. Widespread groundwater and surface water contamination has occurred, which creates a significant risk to drinking water supplies. Hydrogeologic and geochemical conditions near large uranium mine tailings storage facility (TSF) were characterized to provide data to locate, design and install a permeable reactive barrier (PRB) to treat groundwater contaminated by leachate infiltrating from the TSF. The effectiveness of three different permeable reactive materials was investigated: zero-valent iron (ZVI) for reduction, sorption, and precipitation of redox-sensitive oxyanions; phosphate material to transform dissolved metals to less soluble phases; and organic carbon substrates to promote bioremediation processes. Batch and column experiments with Zhovty Vody site groundwater were conducted to evaluate reactivity of the materials. Reaction rates, residence time and comparison with site-specific clean-up standards were determined. Results of the study demonstrate the effectiveness of the use of the PRB for ground water protection near uranium mine TSF. The greatest decrease was obtained using ZVI-based reactive media and the combined media of ZVI/phosphate/organic carbon combinations.

Experimental and Kinetic Modeling Study of Cyclohexane Pyrolysis

The pyrolysis of undiluted cyclohexane has been studied in a continuous flow tubular reactor at temperatures from 913 to 1073 K and inlet feed flow rates in the range 288–304 g·h–1 at 0.17 MPa reactor pressure with average reactor residence time of 0.5 s calculated based on the pressure in the reactor, the temperature profile along the reactor, and the molar flow rate along the reactor estimated by the logarithmic average of the inlet and outlet molar flows. The reactions lead to conversions between 2% and 95%. Forty-nine products were identified and quantified using two-dimensional gas chromatography equipped with thermal conductivity and flame ionization detectors. The products with molecular weights between those of hydrogen and naphthalene constitute more than 99 mass % of the total products. A kinetic mechanism composed exclusively of elementary step reactions with high pressure limit rate coefficients has been generated with the automatic network generation tool “Genesys”. The kinetic parameters for the reactions originate either directly from high level ab initio calculations or from reported group additive values which were derived from ab initio calculations. The Genesys model performs well when compared to five models available in the literature, and its predictions agree well with the experimental data for 15 products without any adjustments of the kinetic parameters. Reaction path analysis shows that cyclohexane consumption is initiated by the unimolecular isomerization to 1-hexene but is overall dominated by hydrogen abstraction reactions by hydrogen atoms and methyl radicals. Dominant pathways to major products predicted with the new model are discussed and compared to other well performing models in the literature.

Characteristics of tar abatement by thermal cracking and char catalytic reforming in a fluidized bed two-stage reactor

With respect to the newly proposed fluidized bed two-stage (FBTS) gasification process, the researches on tar thermal cracking and catalytic removal by char bed layer were conducted on a FB two-stage reaction apparatus. It shows that the effect of char on tar removal was much related to the reaction temperature and residence time of tar-containing fuel gas in the tar reformer. Under the tested experimental conditions, increasing reaction temperature and residence time can effectively promote tar abatement and generate more H2, CO and CH4. For catalytic reforming by char, the higher reaction temperature and the longer residence time, the lower tar content in fuel gas. Compared to thermal cracking, catalytic reforming of tar by char was more effective in tar removal efficiency and inhibition of carbon deposition, upgrading the quality of fuel gas. The researches indicated that the suitable operational conditions for tar removal by char in the FB two-stage gasification process were at reforming temperature of 1000 °C, and residence time above 0.9 s.

Conversion of guaiacol in supercritical water gasification: Detailed effect of feedstock concentration

The effect of feedstock concentration on the gasification characteristics of guaiacol, a model compound for lignin, in supercritical water was investigated using a laboratory-scale continuous-flow reactor. The reaction temperature, pressure, and residence time used were 600 °C, 25 MPa, and 94 s, respectively. A reaction model was developed and kinetic parameters were determined so that experimental results could be reproduced. The carbon gasification efficiency (CGE) and TOC yield were not affected by the initial concentration of guaiacol. Solid yields increased with feedstock concentration, reflecting an order of reaction greater than one for solid production. A reaction model was developed and the reaction rate constants were determined successfully to reproduce experimental results. The decomposition rate constant was determined as 0.0718 ± 0.0403 /s and was found to be in good agreement with the previous study. The reaction order for the solid production was determined to be 1.25.

A Monte Carlo Method to Quantify the Effect of Reactor Residence Time Distribution on Polyolefins Made with Heterogeneous Catalysts: Part II ‐ Packing Density Effects

The residence time distribution of reactors used to produce polyolefins commercially has a significant impact on many of their properties. This effect often goes unnoticed, since the concept of reactor residence time is often poorly understood even among chemical engineers. In the first part of this series of articles, it is shown how the reactor residence time distribution has a marked impact on the particle size distribution of polyolefin particles using a new Monte Carlo model. In this article, the Monte Carlo model is combined with a drop and roll algorithm to predict how reactor residence time distribution affects the packing density of the polyolefin particles.

Effects of CO2 enriched atmosphere on chars from walnut shells pyrolysis in a drop tube reactor

A laminar drop tube reactor (DTR) was used to perform fast pyrolysis of walnut shells, a ligno-cellulosic biomass sample, in nitrogen and carbon dioxide atmospheres. The DTR reached the temperature of 1300 °C and the heating rate of 104–105 °C/s. Char samples collected at different residence times along the reactor were characterized by ultimate and proximate analysis and by SEM. Char combustion reactivity was then measured by non-isothermal thermogravimetric analysis (TGA) in air.The analyses show that at residence times of 66 ms pyrolysis in N2 is not complete, whereas it is complete in CO2. For residence times of 115 ms the differences between samples produced in N2 and CO2 atmospheres level off.The derivative thermogravimetric (DTG) curves of the char combustion show the existence of multiple peaks. Notably, early combustion peaks progressively fade in the chars collected at increasing reactor residence time, confirming the completion of pyrolysis. A kinetic model of char combustion is proposed which includes multiple parallel reactions.

Comparison of Numerical Combustion Models for Hydrogen and Hydrogen-Rich Syngas Applied for Dry-Low-Nox-Micromix-Combustion

The Dry-Low-NOx (DLN) Micromix combustion technology has been developed as low emission combustion principle for industrial gas turbines fueled with hydrogen or syngas. The combustion process is based on the phenomenon of jet-in-crossflow-mixing (JICF). Fuel is injected perpendicular into the air-cross-flow and burned in a multitude of miniaturized, diffusion-like flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. In the Micromix research approach, computational fluid dynamics (CFD) analyses are validated toward experimental results. The combination of numerical and experimental methods allows an efficient design and optimization of DLN Micromix combustors concerning combustion stability and low NOx emissions. The paper presents a comparison of several numerical combustion models for hydrogen and hydrogen-rich syngas. They differ in the complexity of the underlying reaction mechanism and the associated computational effort. The performance of a hybrid eddy-break-up (EBU) model with a one-step global reaction is compared to a complex chemistry model and a flamelet generated manifolds (FGM) model, both using detailed reaction schemes for hydrogen or syngas combustion. Validation of numerical results is based on exhaust gas compositions available from experimental investigation on DLN Micromix combustors. The conducted evaluation confirms that the applied detailed combustion mechanisms are able to predict the general physics of the DLN-Micromix combustion process accurately. The FGM method proved to be generally suitable to reduce the computational effort while maintaining the accuracy of detailed chemistry.

Kinetics and Mechanism Modeling of Liquid-Phase Toluene Oxidation to Benzaldehyde Catalyzed by Mn–Mo Oxide

In this study, liquid-phase aerobic oxidation of toluene catalyzed by Mn–Mo oxide was conducted in a 1.0 L batch reactor. The macroscopic kinetics of toluene consumption and benzaldehyde generation at 413–443 K were obtained from a combination of experimental observation and hypothetical models. The results clearly showed that both the oxidation rate of toluene and generation rate of the aromatic product were proportional to the concentration of the substrate, the partial pressure of oxygen and the surface area of the catalyst. The energy barrier of toluene oxidation to benzyl alcohol was the highest (≈ 81 kJ mol−1), while that of benzyl alcohol oxidation to benzaldehyde was the lowest (≈ 57 kJ mol−1). Moreover, the activation energy of further oxidation of benzaldehyde in an acetic acid solvent was only slightly lower (≈ 1.9 kJ mol−1) than that of toluene oxidation. Significantly, the transformation of benzyl alcohol indeed contributed to the generation of benzaldehyde and this step conformed to a first-order parallel-consecutive model. Increased reaction temperature and residence time favored the transformation of benzyl alcohol to benzaldehyde. In addition, doping with molybdenum at Mn/Mo = 3/1 enhanced the catalytic performance of the heterogeneous catalyst and was attributed to the presence of a synergetic effect between different metal cations. Regarding the microscopic kinetics, the LH-OS-ND mechanism (Langmuir–Hinshelwood adsorption of reagents on the same type of active sites and non-dissociative adsorption of oxygen) was verified as responsible for the heterogeneous oxidation of toluene. Oxygen and benzaldehyde were weakly adsorbed (ΔHads,Oxy ≈ − 15 kJ mol−1, ΔHads,Bald ≈ − 30 kJ mol−1), but showed strong mobility (ΔSads,Oxy ≈ − 22 J mol−1 K−1), ΔSads,Bald ≈ − 39 J mol−1 K−1). The fundamental intrinsic rates were deduced based on the LH-OS-ND mechanism and showed great consistency with the macroscopic results.

Supercritical water treatment of heavy metal and arsenic metalloid-bioaccumulating-biomass

Hyperaccumulator biomass, as a promising resource for renewable energy that can be converted into valuable fuel productions with high conversion efficiency, must be considered as hazardous materials and be carefully treated before further reuse due to the high contents of heavy metals. In this study, Pteris vittata L., an As-hyperaccumulator biomass was treated by an effective and environmental friendly method-supercritical water gasification (SCWG) using a bench-scale batch reactor. The contents of heavy metals (Cd, Pb and Zn) and arsenic metalloid in solid, liquid and gaseous products during SCWG process were thoroughly investigated. The speciation fractions including exchangeable, reducible, oxidizable and residual fractions of each heavy metal as the proportion of the total contents in solid residue were presented and the transformations trend of these heavy metals during the SCWG process was especially demonstrated. The significant operating parameters, including reaction temperature (395–445 °C), pressure (21–27 MPa) and residence time (0–40 min) were varied to explore their effects on the contents and forms. Moreover, the environmental risks of heavy metals in solid residues were evaluated based on risk assessment code, taking into consideration the speciation fractions and bioavailability. It was highlighted that although heavy metals particularly Pb and Zn tended to accumulate in solid residues with a maximum increment of about 50% in the total content, they were mostly converted to more stable oxidizable and residual fractions, and thus the ecotoxicity and bioavailability were greatly mitigated with no obvious increase in direct toxicity fractions. Each tested heavy metal presented no or low risk to the environments after SCWG treatments, meaning that the environmental pollution levels were markedly reduced with no or low risk to the environment. This study highlights the remarkable ability of SCWG for the heavy metal stabilization.

The Effect of Residence Time Distribution on the Slurry‐Phase Catalytic Ethylene Polymerization: An Experimental and Computational Study

AbstractThis work is focused on the development and validation of a model accounting for the impact of the reactor residence time distribution in well‐stirred slurry‐phase catalytic polymerization of ethylene. Particle growth and morphology are described through the Multigrain model, adopting a two‐site model for the catalyst and a conventional kinetic scheme. Particle size distribution and polymer properties (average molecular weights and polydispersity) are computed as a function of particle size through a segregated model, assuming that neither breakage nor aggregation occur. Reactors are modeled by means of fundamental mass conservation equations. The model is applied to a system constituted by a series of two ideal continuous stirred tank reactors, where the synthesis of polyethylene with bimodal molecular weight distribution is performed, employing the initial catalyst size distribution as the only adjustable parameter. The model provides insights at the single particle scale for each specific size, thus highlighting the inhomogeneity which arises from the synergic effects of chemical kinetics and residence time distributions in both reactors. The satisfactory agreement between model results and experimental data, in terms of particle size distribution and average molecular weights, confirmed the suitability of the model and underlying assumptions.

A Two-Stage System for the Large-Scale Cultivation of Biomass: a Design and Operation Analysis Based on a Simple Steady-State Model Tuned on Laboratory Measurements

The optimal design and operation at large scale of a continuous fermentation process including a biological reactor/photobioreactor and a gravity settler with partial recycle and purge of the biomass are addressed. The proposed method is developed with reference to microalgae (Scenedesmus obliquus) cultivation, but it can be applied to any fermentation process as well as to activated sludge wastewater treatment. A procedure is developed to predict the effect of process variables, mainly the recycle ratio (R), the solid retention time (θ c ), the reactor residence time (θ), and the ratio between feed and purge flow rates (F I /F W ). It includes a simple steady-state model of the two units coupled in the process and the experimental measurement of basic kinetic data, in both the bioreactor and the settler, for the tuning of model parameters. The bioreactor is assumed as perfectly mixed, and a rigorous gravity-flux approach is used for the settler. The process model is solved in terms of dimensionless variables, and plots are given to allow sensitivity analyses and optimization of operating conditions. A discussion about washout is presented, and a simple method is outlined for the calculation of the minimum values of residence time (θ min ) and recycle ratio (R min ) and of the maximum allowed recycle ratio (R max,operating ) and biomass purge rate (F Wmax ). In particular, it is shown that the system is sensitive to the concentration of biomass lost from the top of the settler (C X S ). The proposed method can be useful for the design and analysis of large-scale processes of this type.

Heterogeneous catalysis in microreactors with nanofluids for fine chemicals syntheses: Benzylation of toluene with benzyl chloride over silica-immobilized FeCl3 catalyst

A novel strategy for a heterogeneous benzylation reaction conducted in a capillary microreactor was proposed. Silica-immobilized FeCl3 nanocatalysts were dispersed in benzyl chloride to form a nanofluid, and then were applied for the benzylation of toluene with benzyl chloride to produce monobenzyl toluene and dibenzyl toluene. The effects of various factors, such as reaction temperature, catalyst composition and residence time on the reaction performance were systematically evaluated. The benzyl chloride conversion could reach 99.2% with the 10%FeCl3-SiO2 catalyst under optimal conditions. Both internal effectiveness factor and external effectiveness factor were estimated to explore the influence of internal diffusion and external diffusion on the reaction performance and to reveal the rate-controlling step during the continuous-flow synthesis. Furthermore, the carbenium ions mechanism was applied to elaborate this heterogeneous benzylation reaction. The nanocatalysts could be regenerated and reused for three times with a high catalytic activity (conversion of benzyl chloride >70%), showing great application potential of this nanofluid catalysis protocol on chemical transformations for fine chemicals syntheses.

A techno-economic assessment of the potential for combining supercritical water oxidation with ‘in-situ’ hydrothermal synthesis of nanocatalysts using a counter current mixing reactor

A combined process of supercritical water oxidation (SCWO) and supercritical water hydrothermal synthesis (SCWHS) in a continuous counter current reactor is reported. Acrylic acid was used as a model unsaturated carboxylic acid compound and the effects of the reaction temperature, residence time, oxidant ratio and acrylic acid concentration on chemical oxygen demand (COD) were all investigated. Two different experimental configurations for oxidant delivery were carried out in ‘pre-heated’ and ‘non-preheated’ oxidant configurations. With a stoichiometric excess of 100% oxygen, COD reduction levels of 80% (non-preheated) and 15% (preheated) were achieved with very short residence times. SCWHS was achieved through the addition of small amounts of various soluble metal salts in the cold upflow resulted in nanoparticles forming which increased the reaction rate and hydrothermal oxidation efficiency. The addition of small amounts of chromium nitrate (>5mM) results in nearly 100% COD reduction at 380 °C and residence times of 0.75 s. The potential economic benefits of combining the two processes together, in the different configurations, were also evaluated.