| 1. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of individual colloidal aggregates in turbulent flow investigated by 3D particle tracking velocimetry
- 2018
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Ingår i: Multiphase Flow Phenomena and Applications: Memorial Volume in Honor of Gad Hetsroni. - : World Scientific Publishing Co. Pte. Ltd.. - 9789813227392 - 9789813227385 ; , s. 83-96
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence where their breakup is recorded by three-dimensional particle tracking velocimetry (3D-PTV). The aggregates have an open structure with fractal dimension around 2.2, and their size varies from 0.9 to 3.1 mm which is large compared to the Kolmogorov length scale η = 0.15 mm. 3D-PTV allows for the simultaneous measurement of aggregate trajectories and the full velocity gradient tensor along their pathlines which enables us to access the Lagrangian stress history of individual breakup events. The analysis suggests that aggregates are mostly broken due to accumulation of drag stress over a time interval of order Kolmogorov time scale, O(τη). This finding is explained by the fact that the aggregates are large, which gives their motion inertia and which increases the time for stress propagation inside the aggregate.
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| 2. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
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| 3. |
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| 4. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Rate of breakup of small inertial aggregates in homogeneous turbulence
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- The hydrodynamic breakup of small inertial aggregates in homogenous and isotropic turbulence is studied through numerical simulations. Small inertial aggregates are subject to shear stress caused by the local velocity gradient and drag stress caused by the relative velocity of the aggregate and the fluid flow. In our simulations, we follow aggregates moving through the flow and record the total stress acting on them. Breakup is assumed to occur when the total stress overcomes a predefined threshold representing the aggregate strength. By determining how long it takes for an aggregate to reach a stress exceeding its strength for the first time, we are able to derive a breakup rate. It is found that with increasing aggregate inertia, the drag stress rapidly becomes the dominant stress resulting in an increase of the breakup rate with increasing the aggregate inertia.
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| 5. |
- Frungieri, Graziano, et al.
(författare)
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Ductile Breakup of Tracer Aggregates in Homogenous Isotropic Turbulence
- 2023
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Ingår i: Chemical Engineering Transactions. - : Italian Association of Chemical Engineering - AIDIC. - 1974-9791 .- 2283-9216. ; 100, s. 373-378
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we study the ductile breakup of tracer aggregates in an incompressible, homogeneous, and isotropic three-dimensional turbulent flow. The flow dynamics is studied by means of a direct numerical simulation, whereas the Lagrangian velocities and stress statistics along trajectories are obtained by particle tracking. We investigate the breakup dynamics under the hypothesis that aggregates are able to deform and accumulate energy. Within this framework, breakup occurs when the energy transferred to the aggregate by the flow exceeds a critical value. We contrast our predictions for ductile breakup with those obtained for brittle breakup. We observe that turbulence intermittency is crucial for the breakup of brittle aggregates, while it becomes less relevant for ductile aggregates. In the limit of highly ductile aggregates the breakup rate is dictated by the mean properties of the flow. We propose a simple model to capture this behaviour.
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| 6. |
- Frungieri, Graziano, et al.
(författare)
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Heavy and light inertial particle aggregates in homogeneous isotropic turbulence : A study on breakup and stress statistics
- 2023
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Ingår i: Computers & Fluids. - : Elsevier BV. - 0045-7930 .- 1879-0747. ; 263
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Tidskriftsartikel (refereegranskat)abstract
- The breakup of inertial, solid aggregates in an incompressible, homogeneous and isotropic three-dimensional turbulent flow is studied by means of a direct numerical simulation, and by a Lagrangian tracking of the aggregates at varying Stokes number and fluid-to-particle density ratio. Within the point-particle approximation of the Maxey–Riley–Gatignol equations of motion, we analyze the statistics of the time series of shear and drag stresses, which are here both deemed as responsible for aggregate breakup. We observe that, regardless of the Stokes number, the shear stresses produced by the turbulent velocity gradients similarly impact the breakup statistics of inertial and neutrally buoyant aggregates, and dictate the breakup rate of loose aggregates. When the density ratio is different from unity, drag stresses become dominant and are seen to be able to cause to breakup of also the most resistant aggregates. A transition from a shear-dominated to a drag-dominated breakup regime is observed, and a power-law is seen to well describe the breakup rate of loose aggregates regardless of their inertia. The present work assesses the role of shear and drag stresses on aggregate breakup and computes breakup rates to be possibly used in population balance models.
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| 7. |
- Frungieri, Graziano, et al.
(författare)
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Shear-Induced Heteroaggregation of Oppositely Charged Colloidal Particles
- 2020
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 36:36, s. 10739-10749
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates numerically the shear-induced aggregation of mixed populations of colloidal particles leading to the formation of clusters. Suspensions with different amounts of positively and negatively charged colloidal particles are simulated. To resolve the aggregation kinetics and structural properties of the formed clusters, we resort to a mixed deterministic-stochastic simulation method. The method is built on a combination of a Monte Carlo algorithm to sample a statistically expected sequence of encounter events between the suspended particles and a discrete element method built in the framework of Stokesian dynamics to simulate the encounters in a fully predictive manner. Results reveal a strong influence of the composition of the population on both the aggregation kinetics and the aggregate structure. In particular, we observe a size-stabilization phenomenon taking place in the suspension when the relative concentration of the majority particles lies in the range 80-85%; i.e., starting from primary particles, after a short growth period, we observed a cessation of aggregation. Inspection of the aggregate morphology shows that the formed clusters are composed of few minority particles placed in the inner region, while the aggregate surface is covered by majority particles, acting to provide a shielding effect against further growth.
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| 8. |
- Icardi, M., et al.
(författare)
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Population Balance Models for Particulate Flows in Porous Media : Breakage and Shear-Induced Events
- 2023
-
Ingår i: Transport in Porous Media. - : Springer Nature. - 0169-3913 .- 1573-1634. ; 146:1-2, s. 197-222
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Tidskriftsartikel (refereegranskat)abstract
- Transport and particulate processes are ubiquitous in environmental, industrial and biological applications, often involving complex geometries and porous media. In this work we present a general population balance model for particle transport at the pore-scale, including aggregation, breakage and surface deposition. The various terms in the equations are analysed with a dimensional analysis, including a novel collision-induced breakage mechanism, and split into one- and two-particles processes. While the first are linear processes, they might both depend on local flow properties (e.g. shear). This means that the upscaling (via volume averaging and homogenisation) to a macroscopic (Darcy-scale) description requires closures assumptions. We discuss this problem and derive an effective macroscopic term for the shear-induced events, such as breakage caused by shear forces on the transported particles. We focus on breakage events as prototype for linear shear-induced events and derive upscaled breakage frequencies in periodic geometries, starting from nonlinear power-law dependence on the local fluid shear rate. Results are presented for a two-dimensional channel flow and a three dimensional regular arrangement of spheres, for arbitrarily fast (mixing-limited) events. Implications for linearised shear-induced collisions are also discussed. This work lays the foundations of a new general framework for multiscale modelling of particulate flows.
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| 9. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
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Effect of Stefan flow on drag coefficient of reactive spherical particles in gas flow
- 2018
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Ingår i: Turbulence Heat And Mass Transfer 9 (THMT-18). - : Begell House. ; , s. 1089-1092, s. 1089-1092
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Konferensbidrag (refereegranskat)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle or deposition at the surface can generate additional flow phenomena that affect the heat, mass and momentum transfer between the particle and bulk flow. This work aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow. Fully resolved 3D simulations were carried out for particle Reynolds numbers based on the free stream velocity ranging from 0.5 to 3. Simulations are carried out in foam-extend CFD software, using the Immersed Boundary(IB) method for treating fluid-solid interactions. The simulations were validated against data for particles without reactive flow, and against the analytical solution for Stefan flow around a particle in a quiescent fluid. We found that in the considered range of Reynolds number the drag coefficient decreases linearly with in increase in Stefan flow velocity.
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| 10. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
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The effect of Stefan flow on Nusselt number and drag coefficient of spherical particles in non-isothermal gas flow
- 2021
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 140
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Tidskriftsartikel (refereegranskat)abstract
- A Stefan flow can be generated during a phase change or reactions of a particle immersed in a fluid. This study investigates the effect of Stefan flow on the exchange of momentum (drag coefficient (CD)) and heat transfer (Nusselt number (Nu)) between the particle and bulk-fluid. Fully resolved simulations were carried out for a flow near a spherical particle immersed in a uniform bulk flow. The immersed boundary method is used for implementing fluid-solid interactions and the particle is considered as a static boundary with fixed boundary conditions. In a non-isothermal flow, the changes in thermophysical properties at the boundary layer played a role in the variation of CD and Nu by a Stefan flow further. The previously developed model for the drag coefficient of a spherical particle in a uniform isothermal flow was modified for a uniform non-isothermal flow. The model is developed based on physical interpretation. A new model is developed for the Nusselt number for a spherical particle with a uniform Stefan flow combining available models in literature. The models are validated for Stefan Reynolds number −8⩽Resf,p⩽25 and particle Reynolds number of 2⩽Ref⩽30 in gas flow (i.e. Pr≈0.7).
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| 11. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
-
The effect of Stefan flow on the drag coefficient of spherical particles in a gas flow
- 2019
-
Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 117, s. 130-137
-
Tidskriftsartikel (refereegranskat)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle can generate a Stefan flow that affects heat, mass and momentum transfer between the particle and the bulk flow. This study aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow under isothermal conditions. Fully resolved simulations were carried out for particle Reynolds numbers ranging from 0.2 to 14 and Stefan flow Reynolds numbers from (-1) to 3, using the immersed boundary method for treating fluid-solid interactions. Results showed that the drag coefficient decreased with an increase of the outward Stefan flow. The main reason was the change in viscous force by the expansion of the boundary layer surrounding the particle. A simple model was developed based on this physical interpretation. With only one fitting parameter, the performance of the model to describe the simulation data were comparable to previous empirical models.
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| 12. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
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The effects of Stefan flow on the flow surrounding two closely spaced particles
- 2023
-
Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 166
-
Tidskriftsartikel (refereegranskat)abstract
- The aim of the work was to study the effects of neighboring particles with uniform Stefan flow in particle–fluid flows. Particle-resolved numerical simulations were carried out for particles emitting a uniform Stefan flow into the bulk fluid. The bulk fluid was uniform and isothermal. The Stefan flow volume emitted from the two particles is equal, such that it represents idealized conditions of reacting particles. Particles were located in tandem arrangement and particle distances were varied between 1.1 and 10 particle diameters (). Three particle Reynolds numbers were considered during the simulations ( and 14), which is similar to our previous studies. Three Stefan flow velocities were also considered during simulations to represent inward, outward, and no Stefan flow. The drag coefficient of the particles without Stefan flow showed that the results fit with previous studies on neighbor particle effects. When the particle distance is greater than 2.5 diameters (), the effects of Stefan flow and neighboring particles are independent of each other. I.e. an outward Stefan flow decreases the drag coefficient () while an inward Stefan flow increases it and the upstream particle experience a higher than the downstream particle. When , the effect of Stefan flow is dominant, such that equal and opposite pressure forces act on the particles, resulting in a repelling force between the two neighboring particles. The pressure force showed a large increase compared to the viscous force at these distances. The effect of Stefan flow is weakened at higher Reynolds numbers. A model was developed for the calculation of the drag coefficient. The model, which reproduce the results from the numerical simulations presented above, is a product of independent models that describe the effects of both neighboring particles and two distinguished effects of the Stefan flow.
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| 13. |
- Kasedde, Hillary, et al.
(författare)
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Mineral recovery from Lake Katwe brines using isothermal evaporation
- 2013
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Ingår i: International Mine Water Association Annual Conference 2013. - : IMWA International Mine Water Association. - 9780615793856 ; , s. 855-860
-
Konferensbidrag (refereegranskat)abstract
- Lake Katwe is a saline lake within the East African Rift system in Western Uganda, with a rich source of mineral salts. The present work aims at evaluating possibilities of future salt extraction from the lake deposit. An isothermal evaporation experiment was conducted on the lake brines. The precipitated salts were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Various economic salts such as thenardite, gypsum, mirabilite, burkeite, hanksite, anhydrite, trona, halite, nahcolite, thermonatrite, and soda ash precipitate from the lake brines. The experiments also reveal the sequence of mineral salt precipitation in the order sulfates→chlorides→carbonates.
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| 14. |
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| 15. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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1D Finite Volume Scheme for Simulating Gas-Solid Reactions in Porous Spherical Particles with Application to Biomass Pyrolysis
- 2021
-
Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 60:29, s. 10603-10614
-
Tidskriftsartikel (refereegranskat)abstract
- Models for gas-solid reactions in porous particles typically consist of a set of mass and energy balances in the form of conservation equations. For spherical or close to spherical particles, these equations are formulated in ID spherical coordinates. In the case where accumulation of gas inside the particle is significant, the balance equations contain convective terms. The present work presents a simple numerical scheme based on flux limited finite volume methods for discretizing conservation equations for convective and diffusive transport of mass and energy in radial direction in a porous sphere. The velocity is governed by Darcy's law coupled to an equation of state. The proposed scheme is applied to a series of test problems that admit full or partial analytical solutions. For the cases where only partial analytical solutions are available, a Comsol model is adopted for comparison. It is found that the scheme is able to resolve step gradients without generating oscillations and that it properly handles changes in the sign of the convective velocity. Applying the scheme for solving a common model for biomass pyrolysis reveals the importance of convective gas transport in the pyrolysis of thermally thick biomass particles.
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| 16. |
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| 17. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Mechanisms behind overshoots in mean cluster size profiles in aggregation-breakup processes
- 2018
-
Ingår i: Journal of Colloid and Interface Science. - : Academic Press. - 0021-9797 .- 1095-7103. ; 528, s. 336-348
-
Tidskriftsartikel (refereegranskat)abstract
- Aggregation and breakup of small particles in stirred suspensions often shows an overshoot in the time evolution of the mean cluster size: Starting from a suspension of primary particles the mean cluster size first increases before going through a maximum beyond which a slow relaxation sets in. Such behavior was observed in various systems, including polymeric latices, inorganic colloids, asphaltenes, proteins, and, as shown by independent experiments in this work, in the flocculation of microalgae. This work aims at investigating possible mechanism to explain this phenomenon using detailed population balance modeling that incorporates refined rate models for aggregation and breakup of small particles in turbulence. Four mechanisms are considered: (1) restructuring, (2) decay of aggregate strength, (3) deposition of large clusters, and (4) primary particle aggregation where only aggregation events between clusters and primary particles are permitted. We show that all four mechanisms can lead to an overshoot in the mean size profile, while in contrast, aggregation and breakup alone lead to a monotonic, "S" shaped size evolution profile. In order to distinguish between the different mechanisms simple protocols based on variations of the shear rate during the aggregation-breakup process are proposed.
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| 18. |
- Sadegh-Vaziri, Ramiar
(författare)
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Modeling in Biomass Harvesting, Biomass Pyrolysis and Producer Gas Cleaning
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biomass is a viable alternative in order to mitigate the environmental effectscaused by the aggressive use of fossil feedstock during the last century.This thesis builds around the idea of a biofuel production process that iscomprised of biomass production, biomass gasication, gas cleaning andfuel production. Biomass production includes microalgae synthesis followedby harvesting to supply microalgae to the gasication process. In the gasi-cation process, the prepared microalgae is dried, pyrolyzed and gasiedto obtain a producer gas containing syngas and impurities. The producergas is cleaned from the impurities and fed to a fuel production unit, e.g. amethanation process.This thesis investigates three distinct aspects related to this processscheme, namely the occulation of microalgae as part of harvesting, biomasspyrolysis as a part of gasication, and sulfur removal from the producer gasas a part of gas cleaning.The investigation of occulation focuses on secondary phenomena thataccompany the aggregation and breakup of the suspended particles, namelyrestructuring, decay of oc strength and settling. For the study of theconsidered phenomena, a population balance model is developed.Slow pyrolysis of biomass is studied on both the reactor scale and thepellet scale. A model for a rotary drum reactor, using principles of isoconversionalanalysis, is developed for the study of dierent biomass feedstock.The proposed model allows for deriving a preliminary reactor design withminimal experimental input data. A one-dimensional nite volume schemeis developed for the investigation of pyrolysis on the pellet scale. The proposedscheme accounts for convective and diusive heat and mass transfer,and is tested against analytical solutions and commercial software packages.Sulfur removal by metal oxides in a packed bed is studied on both thesystem level and the process level. Criteria for the selection of metal oxidesand the design of packed bed units are derived. A detailed analysis isundertaken to study the reaction of H2S with ZnO in a packed bed, wherethe nano-particles of ZnO experience void formation and outward growth.
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| 19. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Modeling of slow pyrolysis of various biomass feedstock in a rotary drum using TGA data
- 2018
-
Ingår i: Chemical Engineering and Processing. - : Elsevier. - 0255-2701 .- 1873-3204. ; 129, s. 95-102
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Tidskriftsartikel (refereegranskat)abstract
- Design and optimization of biomass gasification faces the challenge of feedstock variation. Specifically, design calculations require kinetic rate expressions for the given feedstock, whose rigorous determination is demanding and often exceeds available recourses in an early development stage. In this work, we model the slow pyrolysis of biomass for the production of biochar. The aim is to predict the conversion of raw biomass to biochar as a function of the process conditions. Here, we will show that TGA data processed with an isoconversional method is enough to obtain an effective rate expression which allows for predicting the behavior of the biomass at an arbitrary temperature evolution. Such rate expressions can then be used in the process model to simulate conversion of raw biomass to biochar. To illustrate the feasibility of this approach we consider four vastly different biomass, namely spruce wood, pulp, lignin and xylan–lignin, undergoing slow pyrolysis in an indirectly heated rotary kiln reactor. The results of our modeling are compared to experimental data obtained from a 500 kW pilot plant pyrolyzer and to a more detailed process model.
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| 20. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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PBE Modeling of Flocculation of Microalgae : Investigating the Overshoot in Mean Size Profiles
- 2017
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Ingår i: Proceedings of the 9th International Conference on Applied Energy. - : Elsevier. ; , s. 507-512
-
Konferensbidrag (refereegranskat)abstract
- Microalgae is considered as a viable feedstock to biomass gasification. After synthesis in water medium, microalgae are separated and dried to a suitable degree to be fed to the gasification process. In order to achieve an efficient separation, a flocculation process is employed, in which microalgae primary particles aggregate and form larger clusters. Although flocculation is a well-established process, there are still some unknown issues related to it, that are worth further research. Experiments show that the mean size of clusters during flocculation goes through a maximum and then decreases with time. We refer to this pattern in the mean size profile as the overshoot. Studying this phenomenon is crucial since the size of clusters has a significant effect on the overall efficiency of the separation of microalgae from water. In this work, we aim at investigating the mechanisms behind the overshoot. The flocculation process is modeled as an aggregation-breakup system by using population balance equations (PBEs). The primary results show that the aggregation and breakup alone cannot lead to the overshoot in the mean size profile. Thus, we suggested three mechanisms that can lead to the overshoot: deposition of large clusters (DLC), restructuring of clusters (RC), and primary particle aggregation (PPA). These mechanisms were examined with numerical simulations and it was revealed that all three lead to the overshoot.
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| 21. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Providing sulfur free syngas to a fuel cell system
- 2019
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Ingår i: Energy Procedia. - : Elsevier Ltd. - 1876-6102. ; , s. 448-453
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Konferensbidrag (refereegranskat)abstract
- Fuel cells are viable alternatives as power backup systems for mini-grids. In this work a case is considered, where the hydrogen fuel to the fuel cells is supplied from biomass gasification. However, the producer gas obtained from biomass gasification needs to be cleaned of impurities and contaminants. In this work we examined the superiority of the hot producer gas cleaning, which results in a better thermal efficiency since the heat loss from the system is reduced. In order to have a viable hot cleaning process, sulfur should be removed at 800°C and this was shown possible by promising primary data from the experiments where H 2 S was removed down to an acceptable level.
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| 22. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Removal of hydrogen sulfide with metal oxides in packed bed reactors-A review from a modeling perspective with practical implications
- 2019
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Ingår i: Applied Sciences. - : MDPI AG. - 2076-3417. ; 9:24
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Forskningsöversikt (refereegranskat)abstract
- Sulfur, and in particular, H2S removal is of significant importance in gas cleaning processes in different applications, including biogas production and biomass gasification. H2S removal with metal oxides is one of the most viable alternatives to achieve deep desulfurization. This process is usually conducted in a packed bed configuration in order to provide a high solid surface area in contact with the gas stream per unit of volume. The operating temperature of the process could be as low as room temperature, which is the case in biogas production plants or as high as 900 °C suitable for gasification processes. Depending on the operating temperature and the cleaning requirement, different metal oxides can be used including oxides of Ca, Fe, Cu, Mn and Zn. In this review, the criteria for the design and scale-up of a packed bed units are reviewed and simple relations allowing for quick assessment of process designs and experimental data are presented. Furthermore, modeling methods for the numerical simulation of a packed bed adsorber are discussed.
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| 23. |
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| 24. |
- Suarez Corredor, Andres Felipe, 1988, et al.
(författare)
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Characterization Method for Gas Flow Reactor Experiments-NH3 Adsorption on Vanadium-Based SCR Catalysts
- 2021
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 60:30, s. 11399-11411
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Tidskriftsartikel (refereegranskat)abstract
- In this study, NH3 adsorption isotherms for a commercial vanadium-based SCR catalyst coated on a monolith substrate were obtained using a gas flow reactor over a wide range of parameters which have not been performed before in a single study. The isotherms were obtained under different conditions, where adsorption temperature, NH3 concentration, water concentration, washcoat loading, and catalyst oxidation state were varied. For this purpose, a systematic data processing method was developed, which characterizes the dispersion and delay effects in the experimental setup using a residence time distribution model, and artifacts such as NH3 adsorbed in the experimental setup and uncertainties in the washcoat loading were removed. As a result, data from different catalyst samples were integrated, and adsorption isotherms with low data spread and well-defined regions were obtained. This allows the identification of the complex nature of the catalyst and dynamics, where multiple types of adsorption sites are present. For instance, the oxidized catalyst has 50% higher NH3 storage capacity compared to the reduced state of the sample. Moreover, water reduces the NH3 storage capacity at high concentrations (5.0%), whereas at low concentration (0.5%), water increases the NH3 adsorption capacity for an oxidized catalyst. The proposed data processing method can be extended for the analysis of further phenomena in catalysts studied using gas flow reactors, complementing current methods and providing information for models with extended validity and lower parameter correlations.
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| 25. |
- Suarez Corredor, Andres Felipe, 1988, et al.
(författare)
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Understanding the NH3 adsorption mechanism on a vanadium-based SCR catalyst : A data-driven modeling approach
- 2022
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509 .- 1873-4405. ; 262
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Tidskriftsartikel (refereegranskat)abstract
- Ammonia adsorption is a precondition for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) to take place and it influences catalyst performance under transient conditions. For a vanadium-based SCR catalyst NH3 adsorption takes place on multiple adsorption sites over the catalyst surface with different behaviours depending on temperature, gas concentration and catalyst oxidation state. In this study, a mechanistic NH3 adsorption model within the framework of Langmuir adsorption models was developed for describing the NH3 adsorption isotherms obtained with a gas flow reactor for a vanadium-based SCR. The model was created by a data-driven modeling process, which involves different steps. First, a large set of candidate models was created systematically by combining multiple feasible adsorption mechanisms. Then, a parameter estimation workflow was performed using three different objective functions with increased complexity. Finally, a model reconciliation step was executed and a quality assessment was done for creating a unified robust model with a high degree of validity. As a result of this method, an NH3 adsorption model with five adsorption sites with different mechanisms was obtained that captures the main features from the experimental data. Furthermore, the model parameters have physical significance and relate to the adsorption strength and spatial arrangement for NH3 and water molecules. The proposed model can be used in the development of transient models with increased validity over a wide experimental region.
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| 26. |
- Tagliavini, Matteo, et al.
(författare)
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Low-Concentration Ozonation as a Feed Pretreatment Strategy to Reduce Organic Fouling in Pressure-Retarded Osmosis
- 2022
-
Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 61:43, s. 16317-16327
-
Tidskriftsartikel (refereegranskat)abstract
- Feed water pretreatment presents a significant issue in salinity-gradient energy production by pressure-retarded osmosis (PRO). Conventional pretreatment methods require an energy input comparable to the energy output of PRO, thus narrowing the feasibility window for power production. In this work, we investigate continuous low-dosage ozonation of the feedwater as a strategy to reduce organic membrane fouling in PRO. For this, we run laboratory experiments using humic acid, alginic acid, and natural surface water as foulants. For alginic acid and surface water, ozone at a concentration of 0.1 mg L-1substantially reduced membrane fouling and recovered the water flux of the fresh membrane. For humic acid, some residual fouling remained. Tests showed that the ozonation of humic acid leads to low-molecular-weight residues that might explain the persistent fouling. To assess the feasibility of the method, we tested the ozone compatibility of the membrane by means of Fourier transform infrared spectroscopy (FTIR) and estimated the power requirement for ozone production. FTIR showed some oxidation of the membrane material, but the membrane performance remained unaltered. The power requirement for ozonating the feed at 0.1 mg L-1is estimated to be 0.01 kWh/m3.
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| 27. |
- Tagliavini, Matteo, et al.
(författare)
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Simulation of spiral-wound pressure retarded osmosis for harvesting osmotic power : Module-level modeling and implications of feed pre-treatment
- 2024
-
Ingår i: Desalination. - : Elsevier BV. - 0011-9164 .- 1873-4464. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- A process-scale model for pressure retarded osmosis (PRO) using a 1-dimensional representation of the spiral-wound membrane is presented. Comparing a length-averaged modeling approach with a true counter-current approach reveals that the former holds only for relatively short membrane lengths while it over-predicts the net power output for longer membranes. For a membrane line with eight spiral-wound elements in series, the length-average model over-predicted the PRO performance by 20 %. The model was further used to assess the impact of feed pre-treatment on process performance and optimal process design. Feed pre-treatment was found to be a dominant factor affecting the optimal inlet feed flow rate and the power output. Our model suggests that for a moderate to high feed pre-treatment requirement the inlet feed flow rate is close to minimum feed flow rate of the membrane module. In the absence of feed pre-treatment a power density of 143 and 284 W m−2 for CTA and TFC membranes, respectively, was found. When feed pre-treatment with 100 Wh m−3 was employed these numbers dropped to 48 and 94 W m−2.
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