A Comparative Study on the Coating Performance of Wurster, Bottom and Top Spray Fluidized Bed Coaters

The coating process is an important, sometimes even necessary, step in the manufacture of tablets or pellets. In the pharmaceutical engineering, basic purposes of the coating process include surface protection from dust, moisture or oxygen, odor or taste masking and hence improving the patient compliance, elongated release of active pharmaceutical ingredients (APIs), delayed release (also called enteric coating) and addition of special functional ingredients (Silva et al., 2014). The most widely used pharmaceutical coating techniques mainly include rotating drums (pan coaters) and fluidized beds (Salawi, 2022). The rotating drum is known for simple configuration and mild operation condition. However, it shows little resistance towards particle agglomeration due to the packing of wet particles over the drum (Suresh et al., 2017). By contrast, the fluidized bed coater shows superior resistance towards particle agglomeration because particles are suspended in the hot air flow rather than packing together. Moreover, the innate advantage of prominent heat and mass transfer performance is quite beneficial for the fast drying of particles. The fluidized bed coater has thus been showing wide application in the pharmaceutical coating process.

The fluidized bed coater can show various configurations. Fig. 1 shows several frequently used configurations in industrial processes, including the top spray coater, bottom spray coater, Wurster coater, rotor coater and spouted bed coater (Fries et al., 2011). These different configurations, although all named ‘coater’ herein, should show different applicability or performance towards the coating process. For example, the Wurster coater has been known for providing high coating quality, whilst the rotor coater is more appropriate for granulation rather than coating due to strong particle collision effect inside the bed (Song et al., 2023). Thus, it is of significance to study the coating characteristics of different configurations.

A special type of bottom spray coater named Wurster coater has been gaining popularity (Fig. 1(c)) for decades. The biggest feature of the Wurster coater is the insertion of a draft tube (generally cylindrical) which could direct the flow of gas and particles. The directed circulation of particles has been reported to be beneficial for high coating quality (Song et al., 2024a, Li et al., 2015, Zhou et al., 2016, Trogrlić et al., 2021). However, the number of operation parameters has been largely increased with the insertion of the draft tube and hence it is challenging to operate the Wurster coater well (Luštrik et al., 2012). Currently, engineers still rely heavily on trial and error to design and operate the Wurster coater in industrial processes. Mature theories concerning the Wurster coater have not been established yet although many researchers have been making efforts in this area.

Heinrich et al. (2015) constructed a multiscale simulation framework for the Wurster coater based on CFD-DEM and analyzed the influence of key process variables, such as gap height and atomization gas flow rate, on the hydrodynamics during the coating process. Jiang et al. (2018) investigated residence time, droplet deposition and collision dynamics of a binary particle mixture in a Wurster coater via CFD-DEM simulation. Madlmeir et al. (2022) coupled heat and mass transfer models with CFD-DEM simulation to estimate the coating efficiency of the Wurster coater. The results show that the loss of coating materials caused by the premature drying of droplets could be reduced from 28.2% to 6.1% by optimization of the operation parameters. Wang et al. (2016) studied the gas-solid flow characteristics in a lab-scale Wurster coater by electrical capacitance tomography (ECT) measurements, two-fluid model (TFM) and computational particle fluid dynamics (CPFD). Recently, Liu et al. (2023) conducted a series of experiments in a Wurster coater to investigate the effects of seed particle properties on the coating quality. Their results showed that increasing the size, density or bed inventory of the seed particles would lower the porosity of the coating shells. Studies on the Wurster coater have been booming and can be easily found in open literature.

The conventional bottom spray coater is another type of widely applied apparatus in spray coating. Theoretically, this type should be suitable for granulation because the number density of particles is high in the spray zone where frequent collisions of wet particles can be expected. This is innately beneficial for the generation of particle agglomerates. Hence, the operation conditions must be cautiously set if the purpose is particle coating. For example, the particle size should be large enough and the flow rate of coating liquids should be limited strictly to constrain the evolution rate of agglomerates. Researchers have studied this type of coaters for the purposes of both granulation and coating.

Flögel and Egermann (Flögel and Egermann, 1996) compared the granulation performance of top spray and bottom spray modes in a same granulator, using lactose as the primary particles and a 4% solution of Kollidon 90 (supplied by BASF) as the binder liquid. The growth rate of granules was found to be much slower in the bottom spray mode, which was particularly significant under a low fluidizing gas velocity. Figueroa and Bose (Figueroa and Bose, 2013) also made a comparative study on the granulation performance of top spray and bottom spray modes with drug nanoparticles being the primary particles. The spray modes were found to have a marked influence on the growth of granules. Orth et al. (2022) conducted coating experiments in a lab-scale bottom spray coater and investigated the influence of multiple process parameters on the surface roughness of the product particles. The results showed that low fluidizing gas temperature and large droplets with low velocities would induce rough and porous surface morphologies.

The top spray coater has been applied in both coating and granulation in the last several decades for the simple configuration. Generally, one or more spray guns are placed near the top of bed and fine droplets are sprayed towards the bed bottom. The fluidized particles periodically get wetted in the spray zone and then travel out to be dried. To enhance the coating efficiency, i.e., the ratio of coating liquids that can be sprayed onto the particles to the total amount consumed, the particles need to be fluidized high enough (close to or higher than the nozzle height) to receive the coating liquids. Besides the simple configuration, another prominent feature of this type of coaters is the flexibility of switching between granulators and coaters through tuning the properties of coating liquids or particles.

Based on the features above, researchers have been made efforts to study the coating and granulation characteristics in the top spray coater. Early to 2007, Ronsse et al. (2007) developed a numerical model to simulate the coating process in the top spray coater. The authors assessed the effect of the process parameters, such as the fluidizing air properties and the nozzle-relevant properties, on the coating growth rate uniformity and efficiency as well as the temperature and humidity distributions. Likewise, Fries et al. (2011) studied the gas and solid dynamics in a top spray granulation process using a CFD-DEM model. The particle wetting process was estimated based on a residence time distribution analysis inside an artificially set biconical spray zone. With this model, the effect of the operation parameters like apparatus geometry and air flow rate on the homogeneity of particle wetting was analyzed. Börner et al. (2014) analyzed the spray patterns inside a top spray coater using a self-constructed conductivity probe through which the spatial scopes of the spray zones and the drying zones were identified. By analyzing the residence time in the two zones, it was found that an increment of the injection flow rate of the coating liquids would accelerate the particle flow inside the spray zone noticeably. More recently, Schmidt et al. (2017) committed coating experiments with suspensions inside a top spray coater to study the dependence of the coating shell morphology on the process variables, such as fluidizing gas temperature, atomizing pressure and suspension composition. They found the gas temperature had little influence on the smoothness and porosity of the coating shell, which is contrary to the results of coating experiments with solutions of crystalline materials.

The multiphase flow and the heat and mass transfer on the microscale have brought significant complexity to spray fluidized bed coaters. To unravel the complexity, abundant research has been made on specific coater configurations. However, there still lacks a comprehensive comparative study on the three most widely applied configurations. Thus, the present study aims to compare the coating performance of three lab-scale fluidized bed coaters from three key aspects, i.e., coating variability, coating efficiency (also named coating yield in some literature) and particle agglomeration. The former two indices were estimated based on CFD-DEM simulations coupled with a ray tracing model (RTM) which has been developed and validated by many researchers (Toschkoff and Khinast, 2013, Toschkoff et al., 2013, Madlmeir et al., 2021, Mostafaei et al., 2023, Song et al., 2024b). The last one was analyzed indirectly from slip velocity, liquid bridge force and collision frequency of particles.

Section 2 introduces the key models used in this work, including CFD-DEM, the ray tracing model, the slip velocity model and the liquid bridge force model. On the meantime, the simulation conditions are given while introducing the specific models. Section 3 first analyzes the simulation results in the three configurations sequentially. The effects of important process parameters, such as the fluidizing gas velocity and the spray angle, are evaluated. Last in this section, the particle agglomeration problem is investigated qualitatively. Finally, specific conclusions are provided in Section 4. The present study would contribute to the deep understanding of the three types of coaters for the applicability of particle spray coating. This should also lay a reliable foundation for the selection and operation of appropriate coaters in industrial processes.

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Yinqiang Song, Rui Fang, Niannian Liu, Tong Wang, Tuo Zhou, Man Zhang, Hairui Yang, Comparative Study on the Coating Performance of Wurster, Bottom and Top Spray Fluidized Bed Coaters, Chemical Engineering Research and Design, 2025, ISSN 0263-8762, https://doi.org/10.1016/j.cherd.2025.06.028.