(198k) Investigating the Effects of Coil Diameters on the Heating Efficiency of a Twisted-Helix Preheater for Continuous API Synthesis through CFD Modeling

The importance of temperature control in continuous pharmaceutical manufacturing of drug substance (API) cannot be overstated. There are a wide variety of possible methods for achieving regulation and control of this critical parameter, and one such method involves manipulating the temperature of reactant streams prior to the reactor through the introduction of a preheater.

In the continuous manufacturing process, we consider the submerged-coil preheater design, which has been studied in literature using both experimental [1, 2] and modeling approaches [3] for a variety of setups and geometries. However, the majority of studies have been based around standard helical coil geometry, and one design less common in literature is the twisted helix and its variants. Consisting of a standard helix twisted into a secondary helix, there are three different diameters present that can influence the rate of heat transfer – the primary helix, secondary helix, and inner tube diameter. This gives us wider range of design parameters that could be used to fine-tune heat transfer for the reactant stream, but also increases the difficulty of accurately characterizing the system.

Heat transfer is difficult to characterize even in standard helices, much less the relatively complex geometry of a twisted helix. This is in part due to the Dean effect, which describes the phenomena of secondary flows developing perpendicular to primary flow which can form imbedded vortex structures based on flow region and regime [4, 5]. As a result, a large amount of data is desired at variety of flowrates and geometries to perform detailed analysis, which will be time, cost, and resource intensive. Mechanistic model such as computational fluid dynamics (CFD) model is an appealing tool in these scenarios to digitally examine heat transfer more closely and thereby to reduce the number of experimentations.

In this work, a submerged twisted-helix preheater used in a continuous API manufacturing process is modeled using computational fluid dynamics to determine the influence of the three diameters present in twisted-helix geometry on heat transfer efficiency. Experimental data was first used to calibrate the model, then the three diameters were varied utilizing a full factorial design.

The primary objective of this work is to increase understanding of which design parameters have the greatest influence on heat transfer within these twisted helix geometries. In standard helix studies, correlations for flow regime and heat transfer have been developed using the ratio of outer diameter to inner diameter, known as the curvature ratio [6]. However, in the case of the twisted helix, there are three diameters to consider, and thus three possible ratios that could influence system characterization. The dependent variable under investigation is heat transfer efficiency and determining its relationship to the three diameters.

Once all data had been collected, it was used to fit various empirical models using statistical software. In addition to the major diameter, secondary diameter, and inner diameter, all possible two-factor interactions between these variables are also considered. This is a major benefit of the developed CFD model; rather than running fractional factorials due to experimental limitations, CFD simulations can be used to generate enough data for a complete factorial. This allows for more in-depth analysis of interactions between various parameters which is not always possible when utilizing a fractional factorial approach.

Acknowledgement

References

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