Probing Differences in Mass-Transfer Coefficients in Beaker and Stirrer Digestion Systems and the USP Dissolution Apparatus 2 Using Benzoic Acid Tablets

Timothy A G Langrish,Chao Zhong,Lizhe Sun

doi:10.3390/pr9122168

Abstract

Measurements of external mass-transfer coefficients for dissolution have been made with benzoic acid tablets with a diameter of 13 mm and approximately 3 mm thick, using two different dissolution systems. One system has been a beaker with a platform for the tablet and either 80 mL or 120 mL of water, with three different types of stirrers, and the other has been a USP dissolution apparatus 2 (paddle) with either 200 mL or 900 mL water. Various stirring speeds have also been used in the different pieces of equipment. The same mass-transfer coefficient may potentially be obtained from the same tablet by adjusting the operating conditions in the two different devices. The ranges of the external mass-transfer coefficients measured in both devices overlapped significantly, with the range being 0.193–4.48 × 10−5 m s−1 in the beaker and stirrer system and 0.222–3.45 × 10−5 m s−1 in the USP dissolution apparatus 2. Dimensional analysis of the results, using Sherwood and Reynolds numbers, shows that the Ranz–Marshall correlation provides a lower bound for estimates of the Sherwood numbers measured experimentally. Calculations of time constants for mass transfer suggest that mass transfer may be a rate-limiting step for dissolution and food digestion under some circumstances. The range of mass-transfer coefficients measured here is representative of other measurements from the literature, and the use of the Ranz–Marshall correlation supports the suggestion that this range of values should be generally expected in most situations.

Highlights

The qualitative links between mass transfer theory and human digestion have been understood for a long time
The range of mass-transfer coefficients measured in the beaker and stirrer system was 0.193–4.48 × 10−5 m s−1, while the range measured in the USP dissolution apparatus 2 was 0.222–3.45 × 10 −5 m s−1, so there is significant overlap between the ranges of mass-transfer coefficients found in the two devices
With the external mass-transfer coefficients ranging from 0.193–4.48 × 10−5 m s−1 in the beaker and stirrer system and 0.222–3.45 × 10−5 m s−1 in the USP dissolution apparatus 2, there was considerable overlap between the coefficients measured in both devices

Summary

Introduction

The qualitative links between mass transfer theory and human digestion have been understood for a long time. The qualitative links and connections between digestion, mass transfer theory and momentum transfer, fluid mechanics, rheology, materials engineering and stress–strain analysis are clear from this monograph [1]. The quantitative understanding of the links between mass transfer theory and human digestion has not been explored in any detail, and Cussler [1] does not do any quantitative calculations for these links. Where K is the overall mass-transfer coefficient, A is the interfacial area (between the two phases) for mass transfer, Cs is the saturation concentration of the solute (just outside the solid, above the solid surface), and Cb is the bulk concentration of the solute in the solution. The saturation concentration for a solute in a solvent is usually a function of the solution temperature

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Conclusion