(131a) True Bridging Liquid-Solid Ratio (TBSR): Redefining a Critical Process Parameter in Spherical Agglomeration

Spherical agglomeration is a particulate process which allows the downstream processing of high value products to be improved through size enlargement. In the case of pharmaceuticals, spherical agglomeration is particularly well suited for problematic active pharmaceutical ingredients, especially those with needle-like morphologies. In spherical agglomeration, crystals are agglomerated through the use of a partially-immiscible bridging liquid into dense spherical agglomerates with superior mechanical properties. This includes their flowability, compressibility, and tabletability, making the product highly suitable for direct compression without subsequent processing steps. As such, downstream processing becomes less energy, time and cost intensive, a current drive within the industry.

Currently, the mechanistic understanding of the process is limited, and few studies specifically investigate and elucidate these mechanisms to further a wider understanding as a whole (Pitt et al., 2018). However, it is recognised that agglomeration is not only successful, but highly controllable if the ternary solvent system is within the immiscible region. Here, two distinct immiscible phases form. As the bridging liquid preferentially wets the particles of interest, agglomeration in these immiscible systems is highly successful. Conversely, the current literature provides a detailed account of the influence of process and formulation parameters on the final agglomerate properties.

One critical process parameter is the bridging liquid to solid ratio (BSR), which quantifies volumetrically the amount of both components within the process. This ratio must be accurately controlled as, ultimately, it directly dictates the performance of the process. Many studies recognise and define an optimal range for this parameter, within which dense, spherical agglomerates are formed. Outside this range, agglomeration is either inefficient as there is not enough bridging liquid, or, a paste-like substance is formed. Each of these scenarios is undesirable. Unfortunately, the critical range only is specific for each bulk solution-bridging liquid-solid system. If any of these three components are changed, the critical range no longer holds. This is, in part, because BSR fails to account for the influence of bridging liquid miscibility within the bulk solution.

Here, a new definition is introduced to standardise bridging liquid volume reporting as a dimensionless parameter: the true bridging liquid-solid ratio (TBSR), which incorporates the true volume of the bridging liquid rich phase. To evaluate solvent miscibility, the ternary phase diagram is pivotal. Through a combined approach of experimental and simulation work, these diagrams have been determined with a high degree of accuracy and agreement. The diagrams were used to evaluate bridging liquid miscibility in a variety of hypothetical agglomeration systems. As a result, a relationship BSR and TBSR has been identified. Critically, findings based upon these theoretical systems supported the rationale and need for the definition as a whole.

With the relationship between these parameters known, a target true bridging liquid-solid ratio can be easily identified. This includes calculation of the bridging liquid volume required to surpass solvent miscibility and form an immiscible agglomeration system. Experimental validation using water-acetone-bridging liquid systems produced highly similar agglomeration profiles for three different bridging liquids with varying miscibility with water-acetone mixtures. Changes in the bulk solution composition, or the bridging liquid, produced only small changes in these profiles. Interestingly, the percentage of solids not agglomerated remained consistent across all conditions. This demonstrates the suitability of the TBSR as a means of standardising bridging liquid volume reporting, whilst accounting for bridging liquid miscibility. As a dimensionless parameter, further mechanistic studies into spherical agglomeration should make full use of this new definition, especially those which are focussed on the wetting and nucleation rate processes. Ultimately, such studies will form the basis of regime maps for the development of robust spherical agglomeration processes.