| 1. |
- Darelius, Anders, 1977, et al.
(författare)
-
A Volume-Based Multi-Dimensional Population Balance Approach for Modelling High Shear Granulation
- 2006
-
Ingår i: Chemical Engineering Science. ; 61, s. 2482-2493
-
Tidskriftsartikel (refereegranskat)abstract
- A volume-based multi-dimensional population balance model based on the approach used by Verkoeijen et al. (2002) is further developed and applied to a wet granulation process of pharmaceutically relevant material, performed in a high shear mixer. The model is improved by a generalization that accounts for initial non-uniformly distributed liquid and air among the different particle size classes. Only the wet massing period of the granulation process has been modelled and it is experimentally found that the pores in the granules are fully saturated by liquid, i.e. no air is present in the granules during this period. Hence, an alternative model formulation is used as no model for the air in the granules is needed. Particle volume distribution, liquid saturation, liquid to solid ratio and porosity of the granules can all be modelled, as these properties can all be expressed as combinations of three model parameters, i.e. the volume fraction of solid material, total liquid fraction and the liquid fraction inside the granules. The model is also improved by introducing a new coalescence kernel and by increasing the number of size classes used. The simulated results are compared to measurements from a series of five designed experiments where impeller speed and water content are varied. It is found that the evolution of the volume, liquid saturation and porosity distributions could all be explained by fitting the compaction and coalescence rate constants.
|
|
| 2. |
|
|
| 3. |
- Darelius, Anders, 1977, et al.
(författare)
-
Fluid dynamics simulation of the high shear mixing process
- 2010
-
Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 164:2-3, s. 418-424
-
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.
|
|
| 4. |
- Darelius, Anders, 1977, et al.
(författare)
-
High shear wet granulation modelling - a mechanistic approach using population balances
- 2005
-
Ingår i: Powder Technology. ; 160, s. 209-218
-
Tidskriftsartikel (refereegranskat)abstract
- A population balance approach based on splitting the coalescence kernel into two factors, the first describing the collision frequency of particles and the second describing the collision efficiency, is applied to modelling wet granulation in a high shear mixer. Four different expressions for the collision frequency are compared and discussed. The kernels are the size independent kernel, the shear kernel proposed by Smoluchowski [1] and the two kernels proposed by Hounslow [2], i.e. the EKE kernel and the less used kernel based on equipartition of fluctuating translational momentum (ETM kernel). Microcrystalline cellulose (mcc) is granulated under different process conditions and it is found that the ETM kernel best describes the granulation at higher impeller speeds, whereas the EKE kernel gives better agreement at lower impeller speeds. The collision efficiency is assumed to be a function of the liquid saturation. By using this assumption, it was possible to detect similar trends for the remaining part of the collision efficiency regardless of process conditions.
|
|
| 5. |
- Darelius, Anders, 1977, et al.
(författare)
-
LDA measurements of near wall powder velocities in a high shear mixer
- 2007
-
Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 62:21, s. 5770-5776
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, a new method has been developed to measure particle velocity distributions in the near wall region of a high shear mixer by using Laser Doppler Anemometry (LDA). The velocities along the side of the granulator have been measured at different impeller speeds and it has been found that it is possible to obtain tangential and axial velocity data in the dense powder flow up to 4 mm in depth. Moreover, it has been found that the tangential velocity component increases slightly with distance from the wall in the near wall region, indicating a partial slip boundary condition for the solid phase at the vessel wall. It is also shown that the tangential velocity decreases with increased vertical distance to the impeller. The velocity fluctuations, represented by root mean square (rms) velocities, also decrease with increased vertical distance to the impeller and the tangential and axial rms components are found to be of the same order of magnitude.
|
|
| 6. |
- Darelius, Anders, 1977, et al.
(författare)
-
Measurement of the velocity field and frictional properties of wet masses in a high shear mixer
- 2007
-
Ingår i: Chemical Engineering Science. ; 62, s. 2366-2374
-
Tidskriftsartikel (refereegranskat)abstract
- In order to develop predictive process models and to enhance process understanding in high shear granulation, there is an ongoing search for non-intrusive methods for measuring the wet mass velocities in the mixer. In this study a high speed CCD camera is used in combination with software for Particle Image Velocimetry (PIV) calculations to obtain information about the wet mass velocities. The focus has been on obtaining good spatial and angular resolution for the velocities along the glass bowl wall. In a Jenike shear cell, both internal and wall frictional properties have been measured and together with velocity data, this information is used for prediction of the impeller torque. It has been shown that the near wall velocities are strongly dependent on the coefficient of wall friction, which decreases during liquid addition. The decrease in the coefficient of wall friction results in increased wet mass velocities close to the bowl wall. It is also found that the wet mass velocity has a strong angular dependence, resulting in a high frequency pulsing bed behaviour which cannot be detected by visual inspection. The predictive impeller torque model developed by Knight et al. (2001) has been generalized to account for cohesive materials and with frictional and velocity data, the level of the impeller torque is well predicted. However, the model is based on crude assumptions regarding the velocity distribution and hence, it cannot capture the dynamics in the measured torque curve satisfactorily.
|
|
| 7. |
- Darelius, Anders, 1977
(författare)
-
Fluid Dynamics and Granular Growth in High Shear Wet Granulation
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High shear wet granulation is a key step in the manufacturing of tablets in the pharmaceutical industry. From the pharmaceutical industry’s point of view, there is a desire for predictive quantitative process models to be able to make in-silico process and scale-up simulations, which would shorten and reduce the cost of technology transfer from laboratory scale to manufacturing scale in drug development. The primary objective of the thesis is to develop mathematical models for describing the high shear granulation process. Population balance models are found to describe granule coalescence and growth well and in this study, both one-dimensional and multi-dimensional population balances have been implemented. However, the applicability of the population balances is limited if the particle flow in the granulator is not known in detail. Thus, the secondary objective of the thesis focuses on Experimental Fluid Dynamics (EFD), i.e. to develop experimental techniques for measuring the powder flow pattern, and to model the flow using Computational Fluid Dynamics (CFD).In the one-dimensional population balance, the granule size distribution alone is modelled. By dividing the coalescence kernel into two factors, collision frequency and collision efficiency, respectively, it is found that the collision frequency expression derived from the assumption of equipartition of translational momentum between the colliding granules best describes the granulation process. In the multi-dimensional case, liquid saturation and porosity distributions for the granules are modelled as well. By generalizing the existing model, the temporal evolution of the distributions of particle size, liquid saturation and porosity can all be well modelled. Experimentally, a high speed camera is successfully used to obtain velocity data on the powder flow at the granulator wall and by using Laser Doppler Anemometry (LDA), the velocity field from the wall and up to 4 mm into the dense rotating powder mix can be measured. In the CFD flow simulations, the Kinetic Theory of Granular Flow combined with frictional stress models, is found to roughly describe the flow, and the experimentally found velocities act as validation of the flow model.
|
|
