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Powder Technology 179

Influence of liquid binder dispersion on agglomeration in an intensive mixer

Karin Ax a,⁎, Hermann Feise a, Robert Sochon b, Michael Hounslow b, Agba Salman b

a BASF AG, Ludwigshafen, Germanyb Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield, UK

Available online 27 June 2007

Abstract

In industrial scale mixer granulation, liquid binder is usually sprayed onto the agitated powder bed by means of a nozzle in order to enhance theagglomeration process. The early stage of this process, where granule nuclei are formed and grow, is not well understood. As it is desirable tomodel the agglomeration state right from the beginning of the process for the purposes of control and modeling, this nucleation step is therefore animportant field of interest.

To investigate the influence of binder droplet size on the nucleation stage of the agglomeration process, experiments were carried out withlactose and water in an intensive mixer. Water was sprayed in to the mixer with different nozzles to vary the size of the produced droplets. As acomparison, water was also directly poured into the turning mixer. Samples of the produced granules were taken at specific time intervals andanalysed for size and water content. As the experiments were focused on examining short granulation times, the first samples were taken after onlyhalf of the water was added.

Particle size distribution and liquid distribution in the wet granule samples were analyzed. It was found, that the droplet size of the binder liquidhas great influence on agglomerate size and binder distribution at short mixing times, with increasing time, the mechanical stresses acting in themixer becomes more and more dominating in the process. Preliminary comparisons are also carried out with single drop penetration tests in anattempt to correlate drop size to penetration time and also to produced granule size.

In conclusion this paper studies the effect of different drop size conditions and subsequent spray flux on the behaviour of the nucleation and theearly stages of the agglomeration process. The context of these findings for agglomeration in an intensive mixer is examined.© 2007 Published by Elsevier B.V.

Keywords: Mixer granulation; Agglomeration nuclei; Binder distribution; Lactose; Droplet size

1. Introduction

The process of agglomeration is widely used to avoid un-desired properties of fine powders, such as dusting, caking orpoor flowability. Agglomeration is often carried out in intensivemixers, where the powder is wetted by a granulation liquid,which acts to bring the powder particles together by means ofliquid bridges and in turn leads to the build-up of larger granules.

Knight et al [1] studied the effect of binder addition andparticle size on the granulation of calcium carbonate powders ina high shear mixer. They concluded that the method of binderaddition and the powder size, were very important to the size ofproduced granules especially in the first stages of the granula-

⁎ Corresponding author.E-mail address: karin.ax@basf.com (K. Ax).

0032-5910/$ - see front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.powtec.2007.06.010

tion. However the importance of the lately discovered nucleationprocess [2] was not fully recognised out right within their work.

To study the influence of binder distribution on the formationof agglomeration nuclei, Litster et al. [3] carried out experi-ments where a moving powder bed surface passed through aspray zone. It was found, that spray flux controls the size andshape of the nuclei size distribution. The dimensionless sprayflux ψa was defined as the ratio of the wetted area covered bythe nozzle to the spray area in the nucleation zone. It was statedthat for a low dimensionless spray flux (ψab0.1), each droppenetrating the powder surface forms one agglomerate nucleiand a narrow size distribution is thus achieved. This operationalmode was named the “drop-controlled regime”.

The aim of this work is to investigate the influence of liquiddispersion on the early stage of agglomeration in an intensivemixer, allowing for greater insight in to the nucleation processand its effects on the size and binder distribution of the granulesso produced.

Table 1Liquid addition

Nozzle type Pressure[bar]

Liquid flowrate [l/min]

Mean dropletsize [μm]

Estimationof ψa

Aperture ø0.3 mm, 60°

4 0.035 57 0.06

Aperture ø0.5 mm, 70°

5 0.113 64 0.25

Aperture ø1.2 mm, 60°

6 0.72 125 N0.6

No nozzle _ 0.72 _ _

191K. Ax et al. / Powder Technology 179 (2008) 190–194

2. Experimental

To investigate the influence of binder droplet size on thenucleation stage of the agglomeration process, experimentswere carried out in the intensive mixer Eirich R02 (Maschi-nenfabrik Gustav Eirich, Hardheim, Germany). The mixerconsists of a rotating vessel, a scraper leading the product awayfrom the wall and a mixing tool, which is located eccentric tothe vessel. The mixing unit is arranged at a slope of 60°. Theexperimental setup is shown in Fig. 1.

The rotational speed of the outside of the vessel was 42 rpm.The mixing tool was operated with a speed of 1500 rpm,rotating in opposite direction to the vessel.

Each experiment was carried out with 1500 g of lactosemonohydrate. The powder (Granulac 230, Meggle, Germany)has a median particle size of 20 μm. The binder liquid, water,was sprayed in using 3 pressure nozzles (Schlick 121, Duesen-Schlick, Germany) of different size, and, as a comparison, waterwas also directly poured into the mixer. The complete amount ofwater added in each case was 225 g.

Nozzles and spray parameters for liquid injection wereselected to cover a broad range of wetting behavior. For liquidaddition with nozzles, ψa was roughly estimated using the tipspeed of the mixing tool as velocity of the powder bed. Thedifferent procedures of liquid distribution are summarized inTable 1.

Samples of the granulated product were taken at specifictime intervals. As the experiments were focused on examiningshort granulation times, the first samples were taken after half ofthe water was added. The next time step was immediately afterthe complete addition of the water. Further samples were taken2 minutes and 5 minutes after water addition finished. Owing tothe variation in liquid flow rate the time of water addition wasdifferent for each of the nozzles. When the water was poured in,the liquid flow rate was equal to that of the biggest nozzle.

After granulation, the product was dried and particle sizedistribution was analyzed by sieving. Liquid distribution in thewet granules was determined after freezing in liquid nitrogen, asdescribed by Mackaplow et al. [4]. Defining relative liquid load

Fig. 1. Experimental setup.

(XL/ XL, total) as the ratio of measured liquid load (XL), to thatexpected from the binder content for the batch(XL, total), basedon the powder and water mass.

3. Results and discussion

The principal behaviour in the change of granule sizedistribution with time was found to be similar for all methods ofbinder injection. The broadest particle size distribution and thehighest fraction of fines (b100 μm) were observed after half ofthe water was added in all cases. With increasing binder contentand agglomeration time, the fraction of fines decreases, andfurther mixing of the wetted product results in a more narrowsize distribution and a growth of median particle size. As anexample of this general behaviour, Fig. 2 shows the change ofparticle size distribution for liquid addition with the 0.5 mmnozzle.

The change of binder distribution with time shows that theliquid load in small particle size fractions decreases withincreasing granulation time (see Fig. 3). This behaviour wasalso observed by Scott et al. [5] and by Reynolds et al [6]. Thedifference noted here is that both their papers observed that atlong time the binder content throughout the batch became moreeven, indicating that the drop off in the binder content in finesmust be first noted, and then binder content must increase after aprocessing time in order to equilibrate over the whole batch.Indeed this is noticed in the results of Reynolds et al. after shortprocessing times, however it was not explained further.

This process of equilibration can be explained by agglom-eration of wetted fines, while those with less binding liquidremain in smaller size fractions. This can be accounted for by a

Fig. 2. Change of particle size distribution with time (nozzle 0.5 mm).Q3=Cumulative mass based distribution.

Fig. 5. Granule size distribution after complete liquid addition.Fig. 3. Change of liquid distribution with time (nozzle 0.5 mm).

192 K. Ax et al. / Powder Technology 179 (2008) 190–194

fragmentation process where wet granules break, leaving wetand dry fragments, from which the wet fragments preferentiallyagglomerate causing a measurable drying out of the fines at thelower end of the size classes measured.

The influence of liquid dispersion becomes clearer, whencomparing granule size distributions and binder distributions fordifferent methods of water dispersion at an early stage of thegranulation process, i.e. during wetting or immediatelyafterwards.

Pouring in the water seems to lead to a broader particle sizedistribution with a higher fraction of coarse granules comparedto distribution of the liquid with nozzles, even when the sameflow rate is applied (see Figs. 4, 5). This is probably due todifferent drop break-up mechanisms occurring for the small andlarge droplets as they enter the system. A comparison of resultsobtained with the three different nozzles shows that smallernozzles (and hence smaller drops) tend to lead to a higherfraction of fines, again likely due to smaller nuclei beingproduced as in Fig. 4. By the time all the binder is added,(Fig. 5) - and the particles have had time to grow and aggregatethis effect becomes masked in an aggregation effect, owing tothe different liquid addition times.

It is interesting to note that the longer wetting times in case ofthe two smallest nozzles (0.3 mm and 0.5 mm) do not lead to asubstantial growth of granules. This indicates that during thewetting and nucleation process, the granulating system isdominated by the binder content rather than the absolute processtimes, as is shown by the fact that the smallest nozzle has aflowrate that is approximately 1/3rd of the 2nd smallest nozzle.

The effect of nozzle size on binder distribution for the case ofhalf of the binder liquid added during wetting is shown in Fig. 6.

Fig. 4. Granule size distribution after half of liquid addition.

In which it is seen that based on the bigger droplet sizes (causedby increasing nozzle size) the difference in relative liquid load ismore pronounced. The small nozzles (0.3 mm and 0.5 mm)show a quite similar distribution of the water over differentgranule sizes. Owing to the low spray flux and long binderaddition time this can be related to a drop controlled nucleationregime. Each spray droplet forms a nucleus and binds powderparticles, until the excess of solid particles the solid-liquid ratiobecomes too high for further growth. Comparing the use of thebiggest nozzle (1.2 mm) to the measured water dispersion withthe fine nozzles, leads to a lower liquid load in small particlefractions and a higher liquid load for coarse granules. Thisindicates a less homogeneous binder distribution and localoverwetting of the powder bed.

The simple approximation used in the calculation of thedimensionless spray flux is, therefore shown to be in agreementwith the results and provides a method for estimating theboundaries of the spray controlled regime, especially during theearlier stages of granulation, when granules have not formed tosubstantially alter the powder bed speed of the processcompared to the raw powder.

Although wetting conditions strongly effect granule size andbinder distribution at the early stages of the granulation process,the influence of liquid addition become less important withfurther mixing of the wetted system. Five minutes after wettingis completed, the granule size and liquid distribution show nosignificant differences between the different nozzles any more(see Figs. 7, 8). This shows that after wetting, the granulationprocess is increasingly dominated by the mechanical impact ofthe mixer.

Fig. 6. Liquid distribution after half of liquid addition.

Fig. 9. Relationship between liquid dispersion and granule size.

Fig. 7. Cumulative granule size distribution 5 min after complete liquid addition.

193K. Ax et al. / Powder Technology 179 (2008) 190–194

The relationship between droplet size and granule size fordifferent stages of the granulation process is summarized inFig. 9. It shows the dependency of the median granule size onthe median droplet size at various time steps during the process.As no droplet size was defined for pouring in the liquid, theagglomerate sizes of these experiments are represented by ahorizontal line.

After half of the liquid is added, the granules were found tobe around 3 times the size of the droplet. It is interesting to notethat a similar relationship between drop and nuclei size wasfound by Litster et al. [3] in ex-granulator nucleationexperiments, where a powder on a variable speed riffler passedunder a flat spray of binder fluid. They observed, that when thepowder velocity was high enough that each drop formed asingle granule on the powder surface, the lactose granulesformed were three to four times the size of the water drops. Thisindicates that for the experiments in the intensive mixer withspray-in of the water, the nuclei formation can be described bythe model of the drop controlled regime according to Lister at al.[3] for low dimensionless spray flux.

To estimate the liquid saturation of the nuclei, the nucleiporosity ε=1 - Vsolid/Vnucleus was assumed equal to the bulkporosity of the lactose powder (0.68 kg/l). A drop with a size of⅓ to ½ of the nucleus size would lead to a liquid saturation ofthe pores between 0,05 and 0,2. According to Rumpf [7] andPietsch [8], this range of liquid saturation refers to a bridgemodel (“pendular”) state, where the strength of the granuleresults from liquid bridges of binder between solid particles.

Immediately after complete wetting, a correlation betweengranule size and droplet size can still be seen. Nucleations has

Fig. 8. Liquid distribution 5 min after complete liquid addition.

been followed by granule growth, and therefore a larger granuleto drop size ratio of about 5 was found. After an additionalmixing time of 2 min, the median granule sizes obtained withliquid injection by the different nozzles are very similar. 5 minafter complete liquid addition, the water dispersion method doesnot influence the median granule size any more. This shows theinfluence of breakage of big wet drops.

4. Single drop tests

As can be seen from Fig. 9 the spray drop size is related toproduced granule size. In an attempt to relate this using singledrops to give a more observable individual procedure ratherthan the chaotic behavior of the intensive mixer, it was decidedto perform tests in a static powder bed and observe the droppenetration time of the water and the size of the produced nucleiformed after the penetration was complete.

This method was achieved by sieving the lactose powderthrough a coarse 1 mm sieve and into a petri dish in order toaerate it, providing a recorded porosity of the powder bed ofaround 70%. Lactose was leveled horizontally by even sievingaround the dish. Drops of dyed water (Surface tension 62 mN/m) of different sizes where produced through a syringe anddropped on to the powder bed from a height of 3 cm thepenetration was recorded using high speed video imaging(Photron DVR, Photron USA). The produced “nuclei” were

Fig. 10. Drop penetration time on a packed powder bed (points mean of 15measurements).

Fig. 11. Produced nuclei size as a function of droplet size.

194 K. Ax et al. / Powder Technology 179 (2008) 190–194

quickly frozen and then sieved to separate them from theremaining powder in the Petri dish. These were then measuredoptically to provide a measure of produced nuclei size.

The penetration time of the drops was calculated from thenumber of high speed video frames that the drop appeared onbefore disappearing. The end point was taken as a change incolour of the drop as it was finally totally absorbed.

It is clearly shown from Fig. 10 that the drop penetration timehas a linear relationship to the produced drop size. As expectedbigger drops with a consequently bigger volume take longer topenetrate the lactose. Experiments in this area, however, requirecareful powder preparation and also care during collection ofdata and analysis to ensure that the correct measurements aremade without obfuscating the true parameters of interest.

Fig. 11 shows a linear relationship between the producednuclei size and the drop diameter recorded on the powder bed.For the intensive mixer discussed earlier, a similar linearrelationship was noticed in Fig. 9 between the droplet size andthe produced agglomerate size after both half and completewater addition. This emphasizes the correlation between thenuclei formation in a static powder bed by single granules andthe nucleation process in the intensive mixer.

These results need further study and a detailed examinationof more experimental data to further produce conclusions. Thestudy presented here is hampered by an inability to easilycontrol porosity of the powder bed, indeed this porosity is notrelatable to that found within an intensive mixer providing adriving force to aim for even higher porosities and simple intra-granulator experiments. The worth of such data however,

should not just be dismissed. It can provide a method ofshowing trends within data that is much more preparable andfunctional than intra-granulator porosity tests.

5. Conclusions

The studies presented here, show that both size of producedagglomerates and the liquid binder content therein vary withprocess time.

Spraying liquid onto the powder mass with the smallestnozzle and hence droplet size provided a better distribution ofliquid in the samples taken, but with smaller granules resultingafter binder addition, compared to that of bigger nozzles or thecomparative “pour in” experiment. The distinction betweenthese different methods of addition becomes less distinct asprocess time increases, probably owing to breakage and re-agglomeration processes that occur within the intensive mixer.

Ex-granulator studies serve to confirm linear relationshipsbetween penetration time and drop size onto a static powderbed, with again the produced nuclei size being a linear functionof the size of drops produced.

It is important that further studies are conducted in this areato elucidate precise mechanisms and allow for better control andmodelling of the process from mixer loading onwards.

References

[1] P.C. Knight, T. Instone, J.M.K. Pearson, M.J. Hounslow, An investigationinto the kinetics of liquid distribution and growth in high shear mixeragglomeration, Powder Technology 97 (1998) 246–257.

[2] T. Schaefer, C. Mathiesen, Melt pelletization in a high shear mixer. IX.Effects of binder particle size, International Journal of Pharmaceutics 139(1-2) (1996) 139–148.

[3] J.D. Litster, K.P. Hapgood, J.N. Michaels, A. Sims, M. Roberts, S.K.Kameneni, T. Hsu, Liquid distribution in wet granulation: dimensionlessspray flux, Powder Technology 114 (2001) 32–39.

[4] M.B. Mackaplow, L.A. Rosen, J.N. Michaels, Effect of primary particle sizeon granule growth and endpoint determination in high-shear wetgranulation, Powder Technology 108 (2000) 32–45.

[5] A.C. Scott, M.J. Hounslow, T. Instone, Direct evidence of heterogeneityduring high-shear granulation, Powder Technology 113 (2000) 205–213.

[6] G.K. Reynolds, C.A. Biggs, A.D. Salman, M.J. Hounslow, Non-uniformityof binder distribution in high-shear granulation, Powder Technology 140(2004) 203–208.

[7] H. Rumpf, Grundlagen und Methoden des Granulierens, Chemie-Ing.-Techn. 30 (3) (1958) 144–157.

[8] W. Pietsch, Agglomeration Processes - Phenomena, Technologies, Equip-ment, Wiley-VCH, Weinheim, 2002, pp. 61–70.