3D DEM/CFD analysis of size-induced segregation during die filling

An experimental study on flow behaviour of pharmaceutical powders during suction filling

Die filling is a crucial step in the pharmaceutical tablet manufacturing process. For industrial-scale production using rotary presses, suction filling is typically employed due to its significant efficiency advantages over gravity filling. Despite its widespread use, our understanding of the suction filling process remains limited. Specifically, there is insufficient comprehension of how filling performance is influenced by factors such as suction velocity, filling velocity, and the properties of the powder materials. Building on our previous research, this study aims to further investigate the effects of powder properties and process parameters (e.g., filling velocity, suction velocity, fill depth) on suction filling behaviour. A systematic experimental investigation was conducted using a model suction filling system, considering both cohesive and free-flowing pharmaceutical powders. The effect of fill depth on suction filling of these powders was examined at different filling and suction velocities. The results demonstrate that two distinctive flow regimes for suction filling can be identified: slow filling and fast filling. These regimes are delineated by a critical filling-to-suction velocity ratio. In the slow filling regime, the filling-to-suction velocity ratio is lower than the critical ratio, meaning that the filling phase is slower than the suction phase. Conversely, the fast filling regime occurs when the filling-to-suction velocity ratio exceeds the critical ratio, implying that the filling phase is faster than the suction phase. This study reveals, for the first time, that when the powder flow pattern during suction filling is dominated by plug flow, full die fill (i.e., the fill ratio equals unity) is achieved in the slow filling regime. However, in the fast filling regime, incomplete die fill is obtained. It is also found that when plug flow prevails during fast filling, the fill ratio has an inverse correlation with the filling-to-suction velocity ratio. This study further reveals that when the plug flow assumption is valid, the filling ratio at various fill-to-suction velocity ratios can be well predicted mathematically. Furthermore, it is also found that once the powder flow pattern differs from the ideal plug flow, which could be induced by the filling conditions and powder cohesion, the fill ratio can be overpredicted.

Blend Segregation in Tablets Manufacturing and Its Effect on Drug Content Uniformity-A Review

Content uniformity (CU) of the active pharmaceutical ingredient is a critical quality attribute of tablets as a dosage form, ensuring reproducible drug potency. Failure to meet the accepted uniformity in the final product may be caused either by suboptimal mixing and insufficient initial blend homogeneity, or may result from further particle segregation during storage, transfer or the compression process itself. This review presents the most relevant powder segregation mechanisms in tablet manufacturing and summarizes the currently available, up-to-date research on segregation and uniformity loss at the various stages of production process—the blend transfer from the bulk container to the tablet press, filling and discharge from the feeding hopper, as well as die filling. Formulation and processing factors affecting the occurrence of segregation and tablets’ CU are reviewed and recommendations for minimizing the risk of content uniformity failure in tablets are considered herein, including the perspective of continuous manufacturing.

Powder flow during linear and rotary die filling

In the pharmaceutical industry, linear die filling is widely employed in R&D, while rotary die filling is very common in commercial production. It is not clear if powder die filling behaviour in a linear die filling system is representative of the flow performance in a rotary tablet press. In this study, a linear die filling system and a rotary die filling system were used to examine flow behaviours of both poor-flowing and free-flowing powders. It was found that the performance of poor-flowing powder in the linear die filling system is slightly better than that in the rotary die filling system, while the performance of free-flowing powders in the linear die filling system is similar to that in the rotary die filling system. Hence, it is suitable to use the linear die filling system to estimate the flow behaviour during rotary die filling with free-flowing powders, but caution needs to be taken when poor-flowing powders are used.

A brief review of (multi-scale) modelling approaches to segregation

Segregation in dense granular flows is a large problem in many areas of industry and the natural environment. In the last few years an advection-diffusion style framework has been shown to capture segregation in many geometries. Here, we review the different ways such a framework has been obtained by different authors, compare the forms and make recommendations for the best form to use. Finally, we briefly outline some of the remaining open-questions.

Modelling Complex Particle–Fluid Flow with a Discrete Element Method Coupled with Lattice Boltzmann Methods (DEM-LBM)

Particle–fluid flows are ubiquitous in nature and industry. Understanding the dynamic behaviour of these complex flows becomes a rapidly developing interdisciplinary research focus. In this work, a numerical modelling approach for complex particle–fluid flows using the discrete element method coupled with the lattice Boltzmann method (DEM-LBM) is presented. The discrete element method and the lattice Boltzmann method, as well as the coupling techniques, are discussed in detail. The DEM-LBM is thoroughly validated for typical benchmark cases: the single-phase Poiseuille flow, the gravitational settling and the drag force on a fixed particle. In order to demonstrate the potential and applicability of DEM-LBM, three case studies are performed, which include the inertial migration of dense particle suspensions, the agglomeration of adhesive particle flows in channel flow and the sedimentation of particles in cavity flow. It is shown that DEM-LBM is a robust numerical approach for analysing complex particle–fluid flows.

Scaling Tableting Processes from Compaction Simulator to Rotary Presses-Mind the Sub-Processes

Compaction simulators are frequently used in the formulation and process development of tablets, bringing about the advantages of flexibility, low material consumption, and high instrumentation to generate the most possible process understanding. However, their capability of resembling general aspects of rotary press compaction and their precision in simulating or mimicking sub-processes such as feeding and filling need to be systematically studied. The effect of material deformation behavior, blend composition, and feeding on tensile strength and simulation precision as compared with rotary presses of different scales is evaluated in this study. Generally, good simulation performance was found for the studied compaction simulator. Compaction profile-sensitivity was demonstrated for highly visco-plastic materials while shear-sensitivity in feeding was demonstrated for lubricated blends of ductile particles. Strategies for the compensation of both in compaction simulator experiments are presented by careful investigation of the compaction stress over time profiles and introduction of a compaction simulator-adapted shear number approach to account for differences in layout and operation mode between compaction simulator and rotary press, respectively. These approaches support the general aim of this study to provide a more straightforward determination of scaling process parameters between rotary press and compaction simulator and facilitate a quicker and more reliable process transfer.

Simulation of Unit Operations in Formulation Development of Tablets Using Computational Fluid Dynamics

Tablets are the most customarily used solid oral unit dosage form for its better patient compliance. Preparation of these tablets include granulation, granule drying, die filling, and tablet coating as few unit operations and evaluation tests like dissolution test and disintegration test. These are the most crucial segments influencing the quality of the tablet. Critical analysis of the impact of factors like flow pattern, temperature, velocity, and other properties of fluid affecting the unit operations is obligatory to enhance their efficiency. Computational fluid dynamics (CFD), a combined mathematical and numerical approach, is used to analyze the process parameters of fluid affecting the abovementioned processes during tablet formulation. The equations governing the laws of conservation of energy, mass, and momentum are solved numerically utilizing CFD software for better understanding of the role of fluids within the tablet processing steps. This review not only focuses on discrete explanations on how CFD is utilized in formulation and evaluation of tablet but it is also a compilation of multiple research works performed on each unit operation by applying CFD.

The Challenge of Die Filling in Rotary Presses-A Systematic Study of Material Properties and Process Parameters

For the efficient and safe production of pharmaceutical tablets, a deep process understanding is of high importance. An essential process step during tableting is the die filling, as it is responsible for a consistent tablet weight and drug content. Furthermore, it affects the results of subsequent process steps, compaction and ejection, and thus critical quality attributes. This study focuses on understanding the influences of process parameters and material properties on die filling on a rotary tablet press. By the systematic variation in process parameters as the turret and paddle speeds as well as the fill and dosing depths, five formulations with differing properties are processed. Analysis of the normalized tablet weight, called filling yield, revealed different limitation mechanisms of the filling process, i.e., incomplete filled dies for certain parameter settings. Kinetic limitations occur due to a short residence time under the feed frame (filling time) caused by high turret speeds, which additionally induce high tablet weight variation coefficients. Characteristic maximum turret speeds at certain paddle speeds can be found to still achieve complete filling. At low turret speeds, densification of the powder inside the dies takes place, induced by two mechanisms: either high paddle speeds or high overfill ratios, or a combination of both. The challenge to fill the dies completely as well as avoid densification is dependent on material properties as the flowability. The mass discharge rate from an orifice was found to be in a linear correlation to the filling results of different formulations below complete filling.

Size-induced segregation during die filling

The present study investigated particle size-induced segregation during die filling of binary pharmaceutical blends, consisting of fine and coarse particles in various fractions. Coarse fraction was made of milled and sieved acetylsalicylic acid, whereas the fine fraction was mannitol. The die filling process was carried out in gravity filling and suction filling. The segregation was assessed through determination of the coarse component concentration using UV–Visible spectrophotometry. The obtained values of concentration, determined for ten units of identical volume inside the die, were used to calculate the segregation index (SI), which was an indicator of uniformity of the powder blend deposited into the die. It was found that high segregation tendency was generally observed during gravity filling at a low velocity, due to the effect of air drag, and during gravity filling at a high velocity, as it was carried out through three consecutive filling steps. The lowest segregation tendency was generally observed during suction filling at a low velocity. The horizontal segregation was mostly observed in the top layers of the die, due to mainly two mechanisms: coarse particles cascading down the heap formed by the powder in the final steps of die filling, which produces higher coarse concentration at the near side of the die, observed at low coarse concentration; or coarse particle cascading down the top surface of the flowing powder stream into the die, which increases the coarse concentration at the far end of the die.

Feed frame: The last processing step before the tablet compaction in pharmaceutical manufacturing

The feed frame is a force-feeding device used in the die filling process. The die filling process is crucial within pharmaceutical manufacturing to guarantee the critical quality attributes of the tablets. In recent years, interest in this unit has increased because it can affect the properties of the powder blend and tablets, and because of the success in real time monitoring of powder blend uniformity potential for Process Analytical Technology as described in this review. The review focuses on the recent advances in understanding the powder flow behavior inside the feed frame and how the residence time distribution of the powder within the feed frame is affected by the operating conditions and design parameters. Furthermore, this review also highlights the effect of the paddle wheel design and feed frame process parameters on the tablet weight, the principal variable for measuring die filling performance.

Prediction of tablet weight variability in continuous manufacturing

This paper provides a method for prediction of weight variability of tablets made in rotary tablet presses as a function of material attributes and processing parameters. The goal was to be able to predict whether or not a formulation is suitable for direct compaction continuous manufacturing using the tablet weight variability as a criterion. The work focused on identifying the significant factors affecting the weight variability in tablets, within the design space studied. A wide range of blends with different powder properties were prepared. It was shown that among powder properties, cohesion, bulk density, and particle size were the most significant and sufficient material attributes to explain tablet weight variability. A response surface model was built and validated with three different blends. The model is not formulation dependent and can be expanded to include other blend properties or processing parameters effects.

Analysis of the origins of content non-uniformity in high-shear wet granulation

In this study, the origins of granule content non-uniformity in the high-shear wet granulation of a model two-component pharmaceutical blend were investigated. Using acetaminophen as the active pharmaceutical ingredient (API) and microcrystalline cellulose as the excipient, the distribution of the API across the granule size classes was measured for a range of conditions that differed in the duration of the initial dry mixing stage, the overall composition of the blend and the wet massing time. The coarse granule fractions were found to be systematically sub-potent, while the fines were enriched in the API. The extent of content non-uniformity was found to be dependent on two factors - powder segregation during dry mixing and redistribution of the API between the granule size fractions during the wet massing phase. The latter was demonstrated in an experiment where the excipient was pre-granulated, the API was added later and wet massed. The content non-uniformity in this case was comparable to that obtained when both components were present in the granulator from the beginning. With increasing wet massing time, the extent of content non-uniformity decreased, indicating that longer wet massing times might be a solution for systems with a natural tendency for component segregation.

Geometric effects in microfluidics on heterogeneous cell stress using an Eulerian-Lagrangian approach

The response of individual cells at the micro-scale in cell mechanics is important in understanding how they are affected by changing environments. To control cell stresses, microfluidics can be implemented since there is tremendous control over the geometry of the devices. Designing microfluidic devices to induce and manipulate stress levels on biological cells can be aided by computational modeling approaches. Such approaches serve as an efficient precursor to fabricating various microfluidic geometries that induce predictable levels of stress on biological cells, based on their mechanical properties. Here, a three-dimensional, multiphase computational fluid dynamics (CFD) modeling approach was implemented for soft biological materials. The computational model incorporates the physics of the particle dynamics, fluid dynamics and solid mechanics, which allows us to study how stresses affect the cells. By using an Eulerian-Lagrangian approach to treat the fluid domain as a continuum in the microfluidics, we are conducting studies of the cells' movement and the stresses applied to the cell. As a result of our studies, we were able to determine that a channel with periodically alternating columns of obstacles was capable of stressing cells at the highest rate, and that microfluidic systems can be engineered to impose heterogenous cell stresses through geometric configuring. We found that when using controlled geometries of the microfluidics channels with staggered obstructions, we could increase the maximum cell stress by nearly 200 times over cells flowing through microfluidic channels with no obstructions. Incorporating computational modeling in the design of microfluidic configurations for controllable cell stressing could help in the design of microfludic devices for stressing cells such as cell homogenizers.