Elsevier

Powder Technology

Volume 316, 1 July 2017, Pages 171-180
Powder Technology

Investigation on drying kinetics and tribocharging behaviour of pharmaceutical granules in a fluidized bed dryer

https://doi.org/10.1016/j.powtec.2016.10.061Get rights and content

Highlights

  • The drying process was found to have two phases: constant rate and falling rate drying;

  • Arrhenius equation could be used to correlate drying rate and drying temperature;

  • Charges only started to build up when the drying process approaches the falling rate phase;

  • Drying air velocity increased specific charge of granules but drying temperature was observed to play a marginal role on the specific charge at equilibrium.

Abstract

Among various methods for drying of pharmaceutical granules in the pharmaceutical industry, fluidized bed drying is a frequently used method due to its high rate of moisture removal, excellent performance in solids mixing, and heat and mass transfer. As pharmaceutical powders are typically organic materials with high resistivity, they can easily be charged due to repeated collision and separation of particles along with particle-wall friction in a fluidized bed dryer. This phenomenon, also called “tribocharging”, could adversely affect the process performance. In this work, experiments were conducted to investigate the effects of drying air temperature and drying air velocity on drying performance, as well as electrostatic charges generated during the drying process. Experimental results showed that decreasing the drying air temperature increased the drying time, as expected. The drying rate was observed to be lower at lower operating temperatures. The specific charge of pharmaceutical granules was found to be a function of moisture content and drying air velocity, rather than drying air temperature. With a decrease in moisture content, the specific charge of pharmaceutical granules increased. There was a sudden increase in measured specific charge when the moisture content decreased to approximately 5 wt.%. In addition, regardless of operating conditions, the charge polarity of granules within the bed at the endpoint of the drying process was positive. The current findings indicate that the specific charge is directly indicative of moisture content in the fluidized bed dryer, and monitoring its dynamic changes could be used to monitor the drying process in the pharmaceutical industry.

Introduction

Tablets are the most applicable solid oral drug form in the pharmaceutical industry as they have high manufacturing efficiency and can carry a wide range of doses [1]. A typical tablet manufacturing process involves several steps, among which mixing and granulation have significant importance as having a homogenous mixture is equal to ending up with a high quality product with the correct amount of each component. In order to ensure that each tablet has the exact amount of each ingredient, pharmaceutical powders, which are combination of the active pharmaceutical ingredient (API) and excipients need to be mixed and granulated [2]. Granulation process, which can be carried out in either wet or dry form, offers several advantages: i) enhanced flowability and fluidity of granulated particles; ii) improved compressibility of granulated particles; iii) reduction in dust emission; iv) reduction in segregation; v) improved content uniformity of particulate substances [3], [4], [5]. In wet granulation, the most frequently used granulation process, a non-toxic and volatile liquid binder is added to the well-blended powders [6]. When the wet granulation method is chosen, a drying process is required to remove undesired moisture.

Among different conventional drying methods for particles in the range of 50–2000 μm, fluidized beds are considered to be one of the most suitable methods, especially for wet pharmaceutical granules [3], [7]. Compared to other drying methods, fluidized bed drying possesses many advantages, including a high rate of moisture removal, excellent mixing media, rapid heat and mass transfer between phases, large capacity of production and low capital cost [3], [7]. In order to determine suitable operation conditions and avoid production losses in a fluidized bed dryer, monitoring the drying process and more importantly, determining the drying endpoint, especially in the pharmaceutical industry, is of great importance. Therefore, some online measurement tools have been developed in attempts to meet stringent product quality assurance in fluidized bed drying. For example, Chaplin et al. [8] applied S-statistic analysis to detect the onset of entrainment when the granule moisture content was approximately 11 wt.%; however, this method was not able to measure the bed moisture content during drying. In a related study, Chaplin and Pugsley [9] applied the same analysis to electrical capacitance tomography (ECT) images in the presence of moisture in a fluidized bed dryer. However, this method is costly and difficult to implement in commercial practice. A technique developed by Tsujimoto et al. [10] employed an acoustic emission sensor in a fluidized bed granulator to detect unstable fluidization conditions, such as channeling and blocking, resulting from excessive increases in solids moisture content. However, significant changes in the mean acoustic emission amplitude would require moisture levels higher than 15%. Therefore, this method could not be used for adequate measurements of solids moisture with a desired endpoint of approximately 2 wt.% for typical pharmaceutical applications. Near-infrared (NIR) spectroscopy has been used to monitor drying by measuring the moisture content of wet granules in fluidized beds, but this method requires a clear path between NIR sensors and granules to be measured, as particulates can adhere to sensor tips installed in the bed [11]. Recently, Briens and Bojarra [3] used vibration and passive acoustic emission measurements to complement current methods to indicate an endpoint of drying. Again, this method does not directly monitor moisture content and drying profile. Acoustic emissions are also influenced by many other process-related sources. Given the widespread application of fluidized bed dryers in industry, new online monitoring of the drying process is needed to maintain the desired fluidization state and to control the endpoint of the drying process. Once such a new tool is available, fluidizing gas velocities can be optimized to shorten the drying time and product losses associated with attrition and entrainment can be minimized [3].

In gas-solid fluidized beds, charge generation is generally a concern, which usually has an adverse impact on process performance. In general, electrostatic charges are generated when two solids (of the same or different materials) are brought into contact and then separated [12]. In fluidized beds, repeatedly collision and separation of particles along with inter-particle and particle-wall friction will result in unavoidable electrostatic charge generation known as tribocharging [13], [14], [15]. Pharmaceutical powders by their very nature are mainly insulators with volume resistivity greater than 1013 Ω.m, which prevents the charge transferred in contact from leaking back [16], [17]. Notable changes in gas-solid fluidized bed hydrodynamics, segregation of particles, variability in the proportions of API and excipients and subsequent reduction in uniformity, adhesion of particles to the wall surface, and electrostatic discharge are generally the problems associated with tribocharging [1], [14], [15], [18].

In the literature, most studies on tribocharging of pharmaceutical powders have been conducted individually in their dry form and only a few works were carried out in a fluidized bed [1], [12], [19], [20], [21]. A comprehensive understanding of tribocharging behaviour of pharmaceutical granules in fluidized bed dryers is lacking, and this is demanding the development of a novel economic and robust technique utilizing this feature. Therefore, an advanced understanding of tribocharging behaviour in fluidized bed dryers is required to develop online monitoring of the drying process and to prevent any potential risk of static electrical discharges. In addition, limited drying data on fluidized bed drying having application to the pharmaceutical industry was available. Therefore, the objectives of the present study were to investigate tribocharging behaviour of pharmaceutical granules in a fluidized bed dryer, and to understand the drying kinetics under various operating conditions of relevance to current pharmaceutical industry operations.

Section snippets

Materials and wet granule preparation

Detailed properties along with the compositions of powders used in the wet granule formulation are shown in Table 1. The compositions are provided on their dry basis.

For preparing a batch of 1 kg of wet granules with a moisture content of approximately 30 wt.% (wet basis), dry ingredients were mixed in a 250 W low-shear granulator (Kitchen-Aid classic mixer) for 2 min at the lowest speed (setting 1) followed by continues water addition over a five-minute period at a constant rate of 59.5 ml/min [8].

Drying experiments

In order to operate in the bubbling fluidization regime and to prevent entrainment, three drying air velocities of 1.0, 1.4, and 1.8 m/s were chosen based on previous studies [27]. Channeling was observed at the beginning of the drying process due to the fact that the granules were wet and cohesive. When the drying process proceeded with time, channeling disappeared and smooth fluidization was observed.

It can be seen in Fig. 3 that when the temperature of the drying air increased, the moisture

Conclusions

In this work, drying performance and tribocharging behaviour of pharmaceutical granules in a conical fluidized bed dryer was studied under various operating conditions. Experimental results clearly showed that increasing the drying air temperature and velocity shortened the drying time, as expected. The effective diffusion coefficient of moisture in granules was evaluated and the effect of temperature was correlated by the classical Arrhenius equation. A marked increase in effective diffusion

Acknowledgments

The authors gratefully acknowledge financial support from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grant program and the University of Saskatchewan.

References (37)

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