| 1. |
- Bergenholtz, Johan, 1964, et al.
(författare)
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On-off dissociation dynamics of colloidal doublets
- 2013
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 104:1
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Tidskriftsartikel (refereegranskat)abstract
- First-passage time theory is used to analyze the dissociation behavior of doublets of colloidal particles. The first-passage time distribution for particles interacting via a DLVO potential is determined numerically. For strongly attractive particles the distribution becomes broad such that the mean first-passage time becomes a poor measure of the dynamics. In spite of this, use can be made of the mean in a matching condition, which allows for reproducing distributions for strongly attractive doublets by a semi-analytical solution for particles interacting only through surface adhesion. The smallest eigenvalue in the analytical solution, which governs the long-time asymptotic behavior of the first-passage time distribution, is identified analytically for strongly attractive pairs of particles. In addition, in this limit the distribution is shown to asymptote to an exponential distribution, which means that the dissociation process can be simply captured by an on-off model, without sacrificing the effect of the surface chemistry, with a constant probability for dissociation. This probability is simply related to the surface-adhesive parameter and the separation distance at which the pair of particles ceases to be considered a doublet.
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| 2. |
- Kaunisto, Erik, 1982, et al.
(författare)
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A simple model for simulation of particle deaggregation of few-particle aggregates
- 2014
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Ingår i: AIChE Journal. - : Wiley. - 0001-1541 .- 1547-5905. ; 60:5, s. 1863-1869
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Tidskriftsartikel (refereegranskat)abstract
- A proper mechanistic understanding of the deaggregation process of small colloidal particle aggregates is of generic importance within many fields of science and engineering. The methodology for modeling colloidal deaggregation is currently limited to analytical solutions in the two-particle case and time consuming numerical algorithms, such as Brownian Dynamics (BD) simulations, for many-particle aggregates. To address this issue, a simplified alternative model that describes deaggregation of few-particle aggregates is presented. The model includes end-particle deaggregation and a particle reconfiguration mechanism, which are the two most important mechanisms for deaggregation. Comparison of the calculated first passage time distribution for various two-, three-, four-, and five-particle aggregates with the corresponding result using BD simulations confirms the validity of the model. It is concluded that the dominating mechanism behind deaggregation can be quantified using a deaggregation number, which reflects the time scale for reconfiguration relative to the time scale for end-particle deaggregation.
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| 3. |
- Vaitukaitis, Povilas, et al.
(författare)
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Water transport and absorption in pharmaceutical tablets – a numerical study
- 2020
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Ingår i: Meccanica. - : Springer Science and Business Media LLC. - 1572-9648 .- 0025-6455. ; 55:2, s. 421-433
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Tidskriftsartikel (refereegranskat)abstract
- The quality of a coated pharmaceutical tablet can be strongly affected by the interactions of water droplets with the porous substrate during processes such as coating process. Three different mechanisms co-exist in the coating process: water spreading, absorption and evaporation. Disentangling the fundamental understanding of these phenomena can therefore be crucial for achieving a higher quality of the products (e.g. a longer shelf-life of the tablets) and for controlling the efficiency of the process. This paper aims to investigate the spreading and absorption mechanisms after droplet impingement on a tablet using a Lattice-Boltzmann methodology. Our numerical results (droplet height and spreading, penetration depth and absorbed volume) are in a good agreement with experimental data and numerical simulations available in the literature. In particular, the spreading phase is characterised by the capillary spreading time scale, as confirmed by previous studies. In contrast to previous studies, we find that the absorption process begins at times shorter than the capillary spreading time but with a different power-law in the absorbed volume. We explain this behaviour through a modified Washburn law that takes into account three-dimensional effects. Our data can be used as a benchmark to test novel mathematical models.
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| 4. |
- Darelius, Anders, 1977, et al.
(författare)
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Fluid dynamics simulation of the high shear mixing process
- 2010
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 164:2-3, s. 418-424
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Tidskriftsartikel (refereegranskat)abstract
- The Eulerian-Eulerian two-fluid approach for modelling multiphase flows is used to simulate the flow in a high shear mixer. The results are compared with experimental velocity profiles for the solids phase at the wall in the mixer obtained using a high speed camera (Darelius et al. Chem. Eng. Sci. 62 (2007) 2366).The governing equations are closed using relations from the Kinetic Theory of Granular Flow (KTGF) combined with a frictional stress model due to Johnson and Jackson and Schaeffer and inter-phase drag due to Wen and Yu. In addition, calculations are presented for a model with a constant particle phase viscosity (CPV). Free slip and partial slip boundary conditions for the solid phase velocity at the vessel wall and the impeller have been utilized.The results show that the bed height could be well predicted by the partial slip model, whereas the free slip model could not capture the experimentally found bed height satisfactorily. For the KTGF model, the swirling motion of the rotating torus that is formed by the moving powder bed was over-predicted and the tangential wall velocity was under-predicted, probably due to the fact that the frictional stress model needs to be further developed, e.g. to tackle cohesive particles in dense flow. The CPV model gave predictions in good agreement with the experiments for a solids viscosity of 0.1. Pa. s and a wall slip parameter of 0.005. m/Pa. s. However, for a very low or very high value of the particle phase viscosity and for a high value of the wall slip parameter the agreement with experiments was poor. Interestingly, values of the viscosity that are commonly employed for fluidized beds seem applicable also in the present case. © 2009 Elsevier B.V.
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| 5. |
- Kaunisto, Erik, 1982, et al.
(författare)
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Fundamental mechanisms for tablet dissolution: Simulation of particle deaggregation via brownian dynamics
- 2013
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Ingår i: Journal of Pharmaceutical Sciences. - : Elsevier BV. - 0022-3549 .- 1520-6017. ; 102:5, s. 1569-1577
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Tidskriftsartikel (refereegranskat)abstract
- For disintegrating tablet formulations, deaggregation of small particles is sometimes one of the rate-limiting processes for drug release. Because the tablets contain particles that are in the colloidal size range, it may be assumed that the deaggregation process, at least qualitatively, is governed by Brownian motion and electrostatic and van der Waals interactions, where the latter two can be described by a Derjaguin–Landau–Verwey–Overbeek interaction potential. On the basis of this hypothesis, the present work investigates the applicability of Brownian dynamics (BD) simulations as a tool to understand the deaggregation mechanism on a fundamental level. BD simulations are therefore carried out to determine important deaggregation characteristics such as the so-called mean first passage time (MFPT) and first passage time distribution (FPTD) for various two-, three-, and four-particle aggregates. The BD algorithm is first validated and tuned by comparison with analytical expressions for the MFPT and FPTD in the two-particle case. It is then shown that the same algorithm can also be used for the three-particle case. Lastly, the simulations of three- and four-particle aggregates show that the initial shape of the aggregates may significantly affect the deaggregation time.
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| 6. |
- Li, Liang, 1987, et al.
(författare)
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Effect of drag models on residence time distributions of particles in a wurster fluidized bed: A DEM-CFD study
- 2016
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Ingår i: KONA Powder and Particle Journal. - : Hosokawa Powder Technology Foundation. - 0288-4534 .- 2187-5537. ; 2016:33, s. 264-277
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Tidskriftsartikel (refereegranskat)abstract
- Fluidized bed coating has been used to coat pellets or tablets with functional substances for a number of purposes. In this coating process, particle wetting, drying and film formation are coupled to particle motion. It is therefore of interest to study particle motion in such fluidized beds and to use the results to develop a model for predicting the quality of the final product. In this paper, we present results from DEM-CFD simulations, i.e. discrete element method and computational fluid dynamics simulations of particle motion in a laboratory-scale Wurster fluidized bed that was also employed in positron emission particle tracking (PEPT) experiments. As the drag force is the dominant interaction between the gas flow and the particle motion in this type of fluidized bed, the effect of drag models on the particle motion is investigated. More specifically, the particle velocity and residence time distributions of particles in different regions calculated from five different drag models are presented. It is found that the Gidaspow and Tang drag models predict both particle cycle and residence times well. The HKL and Beetstra drag models somewhat overestimate the particle velocity in the Wurster tube and therefore predict a reduced number of recirculations and a significantly shorter cycle time.
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| 7. |
- Li, Liang, 1987, et al.
(författare)
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PEPT Study of Particle Cycle and Residence Time Distributions in a Wurster Fluid Bed
- 2015
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Ingår i: AICHE Journal. - : Wiley. - 1547-5905 .- 0001-1541. ; 61:3, s. 756-768
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Tidskriftsartikel (refereegranskat)abstract
- Particle cycle and residence time distributions are critical factors in determining the coating quality in the Wurster process. Positron emission particle tracking experiments are performed to determine the cycle and residence times of particles in different regions of a Wurster fluid bed. The results show that particles tend to recirculate in and sneak out below from the Wurster tube. The experiments also show that a larger batch size leads to a shorter cycle time and a narrower cycle time distribution (CTD). It is possible to avoid recirculations and obtain a shorter cycle time and a narrower CTD by selecting the operating conditions appropriately or via equipment design. Experiments using binary mixtures of particles with a diameter ratio of 1.5 show that large particles have a longer cycle time than small particles and that the cycle time is shorter for mixtures with approximately equal amounts of small and large particles.
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| 8. |
- Li, Liang, 1987, et al.
(författare)
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Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD
- 2015
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 280, s. 124-134
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Tidskriftsartikel (refereegranskat)abstract
- Particle cycle and residence time distributions in different regions, particularly in the spray zone, play an important role in fluid bed coating. In this study, a DEM-CFD (discrete element method, computational fluid dynamics) model is employed to determine particle cycle and residence time distributions in a laboratory-scale Wurster fluid bed coater. The calculations show good agreement with data obtained using the positron emission particle tracking (PEPT) technique. The DEM-CFD simulations of different size particles show that large particles spend a longer time in the spray zone and in the Wurster tube than small particles. In addition, large particles are found on average to move closer to the spray nozzle than small particles, which implies that the large particles could shield small particles from the spray droplets. Both of these effects suggest that large particles receive a greater amount of coating solution per unit area per cycle than small particles. However, the simulations in combination with the PEPT experiments show that this is partly compensated for by a longer cycle time for large particles. Large particles thus receive more coating per unit area per pass through the spray zone, but also travel through the spray zone less frequently than small particles.
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| 9. |
- Remmelgas, Johan, et al.
(författare)
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A mechanistic model for the prediction of in-use moisture uptake by packaged dosage forms
- 2013
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Ingår i: International Journal of Pharmaceutics. - : Elsevier BV. - 0378-5173 .- 1873-3476. ; 441:1-2, s. 316-322
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Tidskriftsartikel (refereegranskat)abstract
- A mechanistic model for the prediction of in-use moisture uptake of solid dosage forms in bottles is developed. The model considers moisture transport into the bottle and moisture uptake by the dosage form both when the bottle is closed and when it is open. Experiments are carried out by placing tablets and desiccant canisters in bottles and monitoring their moisture content. Each bottle is opened once a day to remove one tablet or desiccant canister. Opening the bottle to remove a tablet or canister also causes some exchange of air between the bottle headspace and the environment. In order to ascertain how this air exchange might depend on the customer, tablets and desiccant canisters are removed from the bottles by either carefully removing only one or by pouring all of the tablets or desiccant canisters out of the bottle, removing one, and pouring the remaining ones back into the bottle. The predictions of the model are found to be in good agreement with experimental data for moisture sorption by desiccant tablets. Moreover, it is found experimentally that the manner in which the tablets or desiccant canisters were removed does not appreciably affect their moisture content.
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| 10. |
- Remmelgas, Johan, et al.
(författare)
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Characterization of microcrystalline cellulose spheres and prediction of hopper flow based on a μ(I)-rheology model
- 2020
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Ingår i: European Journal of Pharmaceutical Sciences. - : Elsevier BV. - 0928-0987 .- 1879-0720. ; 142
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this study was to characterize the rheology of a pharmaceutical material in the context of the µ(I)-rheology model and to use this model to predict powder flow in a manufacturing operation that is relevant to pharmaceutical manufacturing. The rheology of microcrystalline cellulose spheres was therefore characterized in terms of the μ(I)-rheology model using a modified Malvern Kinexus rheometer. As an example of an important problem in pharmaceutical manufacturing, the flow of these particles from a hopper was studied experimentally and numerically using a continuum Navier-Stokes solver based on the Volume-Of-Fluid (VOF) interface-capturing numerical method. The work shows that the rheology of this typical pharmaceutical material can be measured using a modified annular shear rheometer and that the results can be interpreted in terms of the μ(I)-rheology model. It is demonstrated that both the simulation results and the experimental data show a constant hopper discharge rate. It is noted that the model can suffer from ill-posedness and it is shown how an increasingly fine grid resolution can result in predictions that are not entirely physically realistic. This shortcoming of the numerical framework implies that caution is required when making a one-to-one comparison with experimental data.
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