Foam and drop penetration kinetics into loosely packed powder beds

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Abstract

Foam granulation is a new liquid delivery method for wet granulation, where the liquid binder is delivered as an aqueous foam, rather than as an atomised spray to the powder bed. This paper reports for the first time the similarities and differences between wet granulation via foamed and sprayed binder addition methods.

The kinetics of single foam and single drop (of HPC and HPMC solutions) penetrations into loosely packed powder beds (of glass ballotini and lactose powders) were studied. Specific penetration time (defined as penetration time per unit of binder mass) and nucleation ratio (defined as the ratio of nuclei granule mass to liquid binder mass) were compared between foam and drop nucleation methods. The impact of particle size and binder concentration on both parameters was also studied.

The results indicate that the foamed binder addition method allows a greater mass of binder fluid to be absorbed into the particle bed and uses less liquid binder to nucleate the same number of gram of powder, which indicates improved nucleation efficiency compared to the drop addition method.

Introduction

Wet granulation is a process of agglomerating fine primary particles into larger granules via a liquid phase. This process is traditionally achieved via atomised spray dispersed through the nozzle, and the finely spray droplets generate the initial granules. This process is very common in many industrial applications that deal with particulate or powdered ingredients to improve the properties of the powder mix. A wealth of literature concerning wet granulation has being considered experimentally with the role of material properties and process conditions on the properties of the granules. This understanding is vital to the performance of many industrial granulation processes.

Recently a new wet granulation approach—foam granulation has been developed (Keary and Sheskey, 2004), where the liquid binder is delivered to the powder bed as an aqueous foam, rather than an atomised spray. This novel method has been shown to produce granules and tablets with the required properties for pharmaceutical applications (Sheskey et al., 2007). Initial experiments indicate that foam granulation has improved binder dispersion and wetting throughout the powder mass; uses less binder; has better control of liquid addition without the need for nozzles; and reduces manufacturing and drying time (Keary and Sheskey, 2004). Successful scale-up (Sheskey et al., 2007) from laboratory scale to manufacturing scale has demonstrated the technical operating potential of foam granulation technology. Due to growing interest in the application of foam granulation in pharmaceutical production, it is essential to understand the fundamental principles underlying foam granulation. This study, for the first time, investigates foam-powder nucleation at experimental scale, where nucleus formation is observed by placing a small amount of foam on a loose powder bed and watching how it penetrates to form a nucleus. This work will be compared to drop nucleation and linked with the existing granulation and foam theories to determine the important properties of foam nucleation behavior.

Section snippets

Foam technology

Foam possesses several unique properties which make foam superior compared to the traditional application of water in a wide range of industrial processes. In the textile industry, the reduced liquid volume fraction in foam means that the water uptake by fabrics and the subsequent energy required to dry the fabrics are reduced, which makes foam a good replacement for water (Turner, 1981). As a fire-suppression agent in fire-fighting applications, foam has been widely used to extinguish fires

Spray granulation

The first stage in any wet granulation process is nucleation where particles are bound together by a fluid when the binder solution is sprayed onto the powder bed. The fluid penetrates into the powder pores forming the first particle agglomerate. This initial distribution of binder and subsequent wetting and nucleation are important in ensuring a controlled granulation in the later stage of granulation process. A good understanding of wetting and nucleation phenomena is therefore essential for

Experimental

In this paper, foam penetration time and foam nucleation ratio were investigated and compared to penetration time and nucleation ratio of a single drop of the same fluids. Both parameters are used to quantify the nuclei formation kinetics and liquid binder efficiency in the wetting and nucleation processes (Hapgood et al., 2002; Schaafsma et al., 1998). Penetration time and nucleation ratio were investigated in terms of the effects of powder and binder properties. Microscopy was also used to

Foam and drop specific penetration times

Fig. 2 compares the foam and drop specific penetration times for HPMC solutions on lactose powders. The trends for both foam and drop addition methods are similar when specific penetration time are plotted against binder mass. Despite the small ranges of binder mass for some binder–powder systems, it can be seen that specific penetration time increases with decreasing binder mass for both foams and drops. This may be attributed to the fact that liquid drained out from foam films will also be

Discussion

Foams are unstable systems as the foam structure changes irreversibly with time as a result of drainage, coarsening and coalescence (Maurdev et al., 2006). One common problem associated with the experimental work on foam is that results are often highly scattered unless conditions are very carefully controlled. In this study, aqueous foams were dispensed on the powder bed to create granules and the corresponding penetration time and nucleation ratio were measured to predict the effect of feed

Conclusions

This study is the first to present an overview of nucleation kinetics via foams and drops in wet granulation processes. The study of the effects of binder and powder properties on the penetration time and nucleation ratio can provide a basis guideline to the materials selection for future works on foam nucleation/granulation. Comparisons between foam and drop nucleation ratios have indicated that nucleation of powder via foams provides better liquid usage and improved liquid distribution

Role of the funding source

Financial support for the conduct of this research was provided by The Dow Chemical Company. The sponsor played no role in study design; data collection, analysis and interpretation of data or in the writing of the manuscript. The article was reviewed by the sponsors prior to submission.

Notation

d32equivalent spherical diameter with same surface to volume ratio
FQfoam quality
HPChydroxylpropyl cellulose
HPMChydroxylpropyl methylcellulose
Kmmass nucleation ratio
Kvvolume nucleation ratio
Mnnuclei mass
Mbbinder mass
Rporeeffective pore radius
tCDAdrop penetration time (for constant drawing area case)
tDDAdrop penetration time (for decreasing drawing area case)
tppenetration time
t^pspecific penetration time
Vbedvolume of powder bed
Vddrop volume
Vgasgas volume
Vliquidliquid volume
Vnnuclei volume
Vo

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