Agglomeration behaviour of powders in a Lödige mixer granulator

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ELSEVIER

Powder Technology 96 (1998) 116-128

Agglomeration behaviour of powders in a L6dige mixer granulator F~derik Hoornaert ~, Philippe A.L. Wauters b.,, Gabrie M.H. Meesters ~, Sotiris E. Pratsinis d Brian Scarlett b "bTuor Danief H. V., I'0 Box 4354, 2003 I~,I Haarlem. The Nedn,rlaml.~ h Purtk'l¢ Technology Group, Far'idly ol'Chemi('ui F.ngim,e,'i.g am/Mat(,rial,~ .5'('iem'(,,~)('!It University o/7"e(hnoh~.~y, PO Box 3034, 2fgXt GA ~)ell'l, The Neth('rhm
Received 8 June 1996, revised 29 Augu,q It)t)7

Al~Iraet

The agglomeration of a powder mixture which is commonly used to make granules containing enzyme was examined in a high shear mixer granulator of the L{klige type, The validity and exlension tff current granulation theory fiw practical high shear granulation was investigated. The effecls of process variables such as the amount of binder liquid, chopper impact, binder xiscosity and temperature on the granulation were in agreement with the theory. The onset of the different granulation mechanisms ( nucleation, contpactitm, coalescence, and crushing and layering) was demonstrated. © It.,'gXElsevier Science S.A. All rights reserved, Keywo~ls:

Granulation:Agglomenllitm; Mtxers;l.,6digemixergranulator

I, I n t r o d u c t i o n

The size enlargement of powder.,, hy granulation is a technique widely used in industry to improve important characteristics of the pr~luct powder, e,g, flowahility, bulk density,

apl~arance and dusting behaviour I 1,2 I. The last-mentioned is a prer~luisite for powders which contain enzymes G~r reasons of allergenicity, Powders can be agglomerated into granules by adding either a wet or dry hinder, i.e. a ~iscous liquid or a melted

wax, to a powder or powder mixture and by exerting a Ibrce on that mixture st) that the wetted panicles collide with each ~her, Various types el" granulation equipment are currentl) u ~ in industry, e.g, rotating drams or pans, fluidil.ed beds, and high shear mixer granulators, The latter are typically Ibund in the pharmaceutical and detergent industries and offer the benclits of short processing times and high granule

strength, in addition, they make the production ot" high density granules possible, in particular, compact detergents. High shear mixer granulators can be subdivided into those equipped with a horizontal impeller shaft, like the L0dige * C(~ding aulhor.Tel.: + 3 i 15 279 JA)95:fax: + 31 15 278 4452: e-maih wauters@Cl~6,stm.tudeU't.nl 0032-59101981519.00 © 1998 El.~vier Science S.A. All rights reserved. Pit $0032-5910 (97) 03 364-0

mixer gnu)ulator used in the present invesligalion, and those equipped with a vertical impeller shah, Although the general granulation regime5 are described extensively by Kapur 131, only limited research specilic to high speed mixer granulators has been published. This is in contrast to rotating pan and dram granulators 14,51. Thorough studies available on high shear granulation are mostly limited to particular applications in the pharmaceutical industry where batchwise production is dominant 16 ]. Studies on more general syslems focus only on one particular growth mechanism 17 I. As a result the over.'dl understanding ot" granulation in high shear mixer granulators is still not complete and coherent, and additional research is needed to iill in the gaps. In general, the trend is to describe granulation kinetics in a sen)i-empirical way 181. an approach which unh~rtunately limits the results to a specific case. The popularity of semi-empirical methods is not surprising since a detailed mechanistic approach would impose the huge task el' incorporating detailed knowledge about Ilow behaviour, forces, deformations and bonding characteristics of the thre~phase granulation system. Recently, Ennis et al. 191 proposed a theory capable of explaining granulation mechanics by paying close attention to the particle-particle interactions. It was stated that the viscous and elastic dissipation in both the binder layer and

l'. H o o n u w r t el al. / P o w d e r Technology 96 (! 998i !/6-12~

the particles relative to the kinetic energy of the colliding

panicles determines the actual chance of granulation. This theory is used here to provide a clarifying insight into granulation. The present research identifies the different granulation regimes wiach are encountered during the production of enzyme-containing granules in L0dige mixer granulators and points out the most important parameters and their effects. Furthermore, it gives an explanation of these results using the theory ofEnnis et al. ! 91 and ofSherington 110 I. The present results and explications offer a basis for optimization of this particular granulation process and are expected to be generally applicable to other granulation processes.

2. Granulation theory

2. I. Practical them3' The process t)l'grantdation can be subdivided into a number of regimes 131, based on different growth mechanisms ( Fig, I ). In general, a granulation process starts in the nucle-

ation regime where the primary particles form the first small, very porous, granules. The nucleation regime is characterized by rapid growth and. in general, it only lasts Ibr a short time. After nucleation, the coalescence regime appears. This regime is characterized by slower growth but because of its long dr, ration it is not unusual that most o1' granule growth takes place here. This regime persists as long as the granules are still deformable and not too large. Simultaneously with coalescence, the granules are continuously compacted by the external forces, bringing additional binder to the surface of the granules and thus making further coalescence possible.

;-+

0+(9 ~

Nucleation '+

"i

Coalescence ,. ;..,

""..~' +e;"

QO

.

Much research has addressed the influence of the surface tension of the binder on the wet granule strength I i I, 12]. If a static pendular bridge of binder liquid exists between two gn'anules, or between particles making up a granule, the surface tension result.,~in an attractive capillary force. This attractive force is connposed of the surface Ibrce, working along the solid-liquid line, and the negative Laplace-Young suction pressure which results l'rom the curvature of the airliquid meniscus. Indeed. the attractive lbn'ces due to these static pendular bridges are vital in maintaining the granule structure together and in stabilizing it. Ennis el al. 1131 pointed out. however, that during collision a static pendular bond will be del'ornled and will beconrlc a dynamic pendular bond. the strength of which is governed by the viscosity of the binder rather than by the surface tennion. They found thai if the capillary number. Ca. is larger then 10 ~ the nm.xilnum bridge strength of the dynamic bond exceeds the theoretical static bond strength because of the viscous force contribution. The capillary number is delined

e. •"-?5:.~ ,

= ~Tt..~!

(I)

0"

Here. r/is the dynamic viscosity of the binder liquid, u the

Snowballing : J .i ~ ,.]~' ;. +

2.2. Mechanistic them3'

Ca

g'",":~"~

o o

When the granules have grown larger and have become rigid, the third growth regime, ball growth, is entered. In this regime the growth mechanisms of snowballing, crushing and layering, and abrasion transfer can play a role, either simultaneously or as the sole mechanism. Snowballing is a mechanism which can be present when larger granules exist next to small panicles, often primary particles continuously fed to the system or newly formed nuclei. During snowballing a dense layer of small particles is deposited on the surface of the larger gra, ules. Crushing and layering is a growth mechanism where a certain class of granules, usually the smaller ones 18], are fractured upon collision with other panicles, with the wall, or with the agitating equipment. The remains of these crushed granules are then picked up by ille surrounding, intact granules by snowballing. Weak granules have a high chance ol" eventually growing according to the crushing and layering mechanism. Abrasion Iransl'er means that a snmll part of the surface of one granule is worn away and transported to another gramnle as the two granules roll against each othen'.

'IS

©, p

g

i !7

+

o

Crushing and layering

Abrasion transfer Fig. I. Possiblegrowthmechanisms.

relative velocity of the gnmules, and cr the surface tension of the binder liquid. For values of (:'a greater than one, which is the case in the majority of industrial granulation processes, Ennis et al. concluded that the viscous force dominates the capillary force and that, as a result, the tlynamic bridge strength approachc~ the value based on the theoretical viscous Ibrce. independently o1' the stu'face tension. It was stated that granules will stick Iogether upon collision if the vi.~cous Stokes number oflhe gramdes is smaller than a certain critical value, the critical viscous Stokes number. The viscous Stokes number is delined as

118

F. Hoormwrt el a L I Powder Tecimoh~gy 96 (19981 i 16-128

Table 1 Composition and amount of powder mixture agglomerated 1141 Compound

Purpose

Mass ( kg )

Conc. (wt.C,l- )

Na,SO~. Oaq CaSO.~ • !aq Potato protein Micro-crystalline potato cellulose PVP K-,~ (polyvinylpyrrolidone) Total

lil!er lil,.r dummy enzyme antilumping agent, tiller binder, anti-abrasion agent ( added as aqueous solution )

10.575 (I.450 0.750 3.000 0.225 15.000

70.5 3,0 5.0 20.0 1.5

Sty = 8p~(r)u.

(2)

91,/ Here, p~ is the granule density, (r) the mean radius of the granules, and uo the individual granule velocity, The relative vel~ity is the sum of the individual velocities of two granules. The critical viscous Stokes number is given by

•

"

" ""~~.~Nhirling bed

l

OI

•

Plouqhshare

," ~ (31

,.::.: '.'., Here, e is the coefficient ot" restitution of the granules, h the thickness of the binder layer on the surl~ce of the granules and h,° is a measure of the length of the asperities on the surface of the granules ! 91. IL on the other hand, the viscous Stokes number is larger then the critical viscous Stokes number, the granules will rebound, The viscous Stokes number gives the ratio of the kinetic energy of the two colliding granules and the viscous dissipation by the binder. The meaning of Eqs, (2) and (3) is that if the kinetic energy of two colliding granules (;an be dissipated by a combination of the viscous Ibrce exerted by the eft'active binder layers on the granule surfaces and the energy dissipation by granule defomlation, the two granules will coalesce. After successful coalescence the cohesive capillary t'orce kcep~ the two granules together,

3. Experimental All experiments were carried out batchwise in a L~idige mixer granulator, type FM 50, which has a total volume of 50 i, and was equipped with three ploughshare shovels attached to a rotating horizontal shah and a side-mounted spinning chopper (Fig. 2), The rotational speed of the ploughshares was variable and was set at 200 rpm in the standard experiments. A hammer type chopper ~as used with a speed lixed at 3(X)0 rpm, The action of the chopper and the ploughshares is different, as can be expected Item their size and, particularly, from their tip speeds of about 5 m/s fi)r the ploughshares and 25 rots tbr the chopper in the standard experiments, Roughly, it can be stated that the ploughshares ensure mixing while the chopper provides impact tortes on the granules, Cooling water flowing through the jacket prevented excessive temperature rise of the granulating mass. The dry. ~wder material was composed according to the recipe 1141 listed in Table I and consisted mainly of sodium

.

tql~l ,I'PI io qo

Chopper Fig. 2. Schematic repre.,,entation of the Liklige mixer granulator.

sulfate and cellulose. The binder was water with dissolved PVP (polyvinylpyrrolidone) K-30. In the standard experiments 14,775 kg of dry powder inixture was granulated with 3,525 kg of binder containing 0.225 kg of PVP. The initial viscosity of the standard binder was 3.9 mPa s, as determined at alnbient temperature. The initial mean Sauter diameter o1" the dry powder was 81) p,ln. with 95 wt/); of the particles being larger than 5(1 pun and 95 wt.~;~ being smaller than 300 Ixm in diameter. Prior to an experiment the dry powder mixture was homogenized in the L~idige mixer granulator. Then, all the binder was added through a strainer in one pulse ( t = 0). By adding all the binder at the start of an experiment, instead of the usual continuous addition, the granulation mechanisms could be studied at a constant moisture content. Samples of the granulating mass were taken at distincl time intervals, and were analyzed by sieving, after being dried at 80°C. The granule growth with time was examined primarily by observing consecutive cumulative granule size distributions and the time evolution of the mass mean granule diameter.

4. Results and discussion 4, !. Dymtmics o.l'the present gramdation process A typical time evolution is shown in Fig. 3 for the standard experiment with 19. I wt.% binder. The cumulative mass percentage undersize is shown as a function of the granulation

~: Hoornaerl el al. /Pow~h,r Techmdogy96 (1998~ 116-12~

119

100.0

A 80.0 o "o e~

60.0

E

40.0

i/1/~//l

E

20.0

o.o

.................................... 0,0

200,0

400.0

600.0

- ..... ,Z,_i2_800.0

.

I000.0

.

.

.

.

.

.

1200.0

.

.

.

.

.

.

.

.

.

.

.

1400.0

m,.j 1600.0

1flO0.O

2000.0

Particle sieve size (pro) Fig. 3. Cumulative undersize granule diameter distributions with time. on a ma.~s basis, for a typical experiment ( 19. I wt/~ hinder, 7/= 3.9 mPa .~),

time. Initially granulation is very rapid. After this fast growth the particle size distribution does not change for a substantial period, typically about 30 rain. Granule growth starts again after this 'standstill' in a second growing period which lasts for a relatively long time. Considerable differences are observed between the first and second growth period in speed, duration and the fact that the shape of the size distribution is broadened to a larger extent in the second growing stage, in contrast to the first. The granule morphology also changed with time. in the first growth stage the granules were irregularly shaped and were composed of primary particles ( Fig. 4(a) ). During the second growth stage the granules took on a s,l, ooth and rounded appearance (Fig. 4(b) ), and in some cases it could be observed that they were composed o1" two or three 'subgranules'. When the present results are plotted in the form of granule mass mean diameter against time, the different regimes become clearly visible (curve 3 in Fig. 5 ). The initial growth stage is identified as the nucleation regime (A), where the wetted primary particles stick together and form the lirst granules. During this stage the viscous Stokes number is smaller than the critical viscous Stokes number, since the granules are still small and the binder layer relatively thick. This means that every collision is successful and results in one larger granule. This explains the rapid growth character of this regime. Near to the end of the nucleation regime, the viscous Stokes number becomes equal to the critical viscous Stokes number. Then the kinetic energy ot" the granules can no !onger be dissipated upon collision and causes rebound. This is, in part, caused by the increased mass of the granules,

but at least as important is the tact that the binder has, by then, retracted from the surface into the interstitial spaces between the primary particles of the granule, thus resulting in the depletion of the surface liquid layer. During normal nucleation, the granule size has no effect on the rate of coalescence since the critical viscous Stokes number is not attained for a while. Thus the shape of the granule size distribution does not change ( Fig. 3, first two curves). Near to the end of the nucleation regime, however, both the smaller and the larger grallules continue to grow for a longer period than the average sized granules (Fig. 3, second and third curves). The smaller granules do so because ot' their smaller mass. and thus smaller viscous Stokes number. The larger granules do so because they are more deformable and consequently have a higher critical viscous Stokes number. The relatively small number of much larger than average granules during nucleation is probably a consequence of the applied binder addition method, which may result in some local overwetting 115l. Alter the nucleation regime, all growth temporarily stops and the compaction stage starts. This is ,~tage B ot' curve 3 in Fig. 5. During the compaction stage the in~ernal structure ot" the granules is continuously changing as a result of the collisions between the granules themselves, the granules and the wall, the ploughshares, and, in particular, the chopper. This constant deformation and rearranging leads to a cios~ packing with the smaller particles in the cavities created by the larger ones. The (:o,finuous decrease in porosity as a result of compaction, shown by Kapur 131. is illustrated by Fig. 4(a) and (b). At the end o1" the compaction state, the porosity has decreased to the point where the interstices in the granules

..............~: Hoo~aert et al. / Powder Tet'hnologv 90 tl998) 116-128

120

achieved a wet appearance just before the coal0scence regime started. In addition, the granules become more and more deformable, h is known that the compressive strength of granules dee,'eases with increasing degree of saturation and that granule,: can become very plastic when sufficient mobile liquid bonds,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . binder on the tution coefficient at me end ot the compaction stage, the critical viscous Stokes number increases significantly and granule growth starts again. Both effects are prerequisites for further growth. The second growth stage can be characterized as the coalescence regime. This is stage C of curve 3 in Fig. 5. Growth is slower in this regime compared to that in the nucleation regime since granule coalescence depends on the interplay of the binder layer thickness, granule mass and deformability. Compaction is still of major importance since it continuously forces binder out of the granules. After each successful collision, additional rearrangement is necessary m knead the granule into a round form and to create a binder layer at the granule surl~ce thick enough to make further coalescence possible. In contrast to the more random character of the nucleation regime, coalescence is distinctively favourable to larger granules because of their higher deformability. As a result the shape of the particle size distribution widens appreciably during this stage (Fig. 3, curves 4 and 5). In the literature, several experiments are reported in which the granule size distribution nam}ws during the coalescence state [ 7,16 ], in contrast to several other results [5,171 and the present results. In the first case. the smaller granules have a lower viscous Stokes number and grow preferentially. In the second case, the larger granules are more deformable, have a higher critical viscous Stokes number and hence grow faster. Appa,'eutly, which of these effects is dominant depends on the characteristics el' the system. At the end of the coalescence regime the crushing and h|yering mechanism gradually takes over. This stage can be

stage granules ( I t i n ) : (b) second growth slug,: gr~mules ( I0 mira-j. The granules are appmxin|alely f~W)I~m in diau|eler.

am gradually being lilled up with binder. When the interstitial volume becom0s almost equal to the volumetric hinder content, the binder is squeezed out onto the surface of the granules, It was observed experimentally that the granulating mass E

9OO

F 0oo

7OO

5

4

3 D

6OO

I

C

C

400

2

3oo 2o0 r A too o

.

.

.

.

B

T

0

20

40

60

60

Time (min)

100

'#20

140

- ' - 17.8 wt.% liquid -e- 18.4 wt,% liquid "-*- 19.1 wt% liquid -w.. 19.8 wt.% liquid -w--20.4 wL% liquid i:;,,..~.5. E,olution of the mas.,~mean granule diameter lbr different amounls of binder ( r/= 3.9 mPa s for all experiments). The various granulation regimes can he idemifi~l as: A. nucleation: B. compaction. C. coalescence: D. crushing and layering: E, equilibrium between coalescence and breakage: F, breakage.

121

F. Hoon~aert et aL / Powder Technology 96 fi998) !16-128 'r,1~

100.0

'•

90.0 ~ 80.0 70.0 {11

.o 60.0 (9

o. 50.0

1/)

40.0

--X-- 3 min I --e--4 min ! 7 min I -B--30 min J ~41 min I ..-er-. 56 rntnJ

•~ 30,0 ,.,=

E 20.0 :3

10.0 0'0 0,0

0.5

1.0 1.5 2.0 Reduced granule diameter ( )

2,5

3.1

Fig, 6, Cumulative undersize granule diameter distributions in time plotted against the reduced parti,:le diameter ( 19, I wl,C~ hinder, ,r/= i,0 mPa s). In this experiment° the coalescence regime lasted fronl I Io 13 mill (curves collapse onto a single curve ), while the transition regime lasted from 13 to 66 rain ( curves shilt in time ).

recognized in curves 2 and 3 of Fig. 5, stage D, and appears to last Ibr a long time. The cumulative reduced granule size distributions, defined as the cumulative distributions of the granule diameter divided by the mass mean granule diameter, shift with time during this stage (Fig. 6). From this figure it can be noted that the amount of smaller granules and particles increases, as well as the amount of larger granules. This shows that the dominant growth mechanism in this stage is crushing and layering, and 11o longer coalescence. The crushing and layering mechanism is the result of the large size of the granules. The ~e.~.ultinggrowth rates are low. As a result of continuous densilication, the granules become rigid, in combination with the fact that their size is larger than the clearance between the ploughshares and the hardened material on the wall, breakage occurs, as can be observed in stage F of curve 5 in Fig. 5.

4.2.

l~ffet't of amou,t of bimler

Sherington and Oliver I i 8 J pointed out that the amount of binder is the principal parameter to control granulation, it is not surprising that as a result much work has been done to pinpoint the binder requirement for granulation processes J 191. Fig. 5 shows the effect of the amount of binder (wt.%) on the time evolution of the granule mass mean diameter. It is evident that a relatively small change in the amount of binder can dramatically alter the granulation behaviour from practically no growth, at 17.8 wt.% of binder, to very rapid growth, at 20.4 wt.% of binder. Newitt and Conway-Jones J20] and Capes and Dankwerts [ Ill have shown that the minimum amoun: of hinder to make granulation possible varies between 90 and I 10% of the volume which is required to saturate the void space in the granules. The minimum amount of binder necessary to granulate the present powder mixture lies between 17.8 and 18.4 wt.%. It is not straightforward to relate this figure to a meaningful saturation per-

cemage since the packing and solubility are continuously changing, in contrast to other cases reported in the literature. If the solution phase ratio is calculated for the temperature at the onset of the coalescence stage (about 30°C), an estimated porosity of 0.4 is assumed J 18] and the minimum amount of binder is set as 18.2 wt.%, an estimated saturation of 100% follows. Although this figure cannot be used quantitatively, it does indicate that the granules are more or less saturated when the coalescence stage begins and that the minimum amount of binder for successful granulation is in agreement with Newitt and Conwav-Jones [201. Nucleation can be present, as Fig. 5 shows, even when the primary particles are wetted with substantially less binder than the minimum amount J 16 l. Coalescence, however, does not start below the minimum amount of binder. !!1 all cases shown in Fig. 5 the nucleation regime seems to be very short, i.e. less than one minute, which is the first justifiable sampling time because of the mixing time. Care should be taken in drawing conclusions about the nucleation stage from experiments reported in the literature, since the characteristic times of nucleation and mixing are often comparable. However, the rate of nucleation is not, in the first instance, influenced by a small inhomogeneity in binder distribution. The extent of granulation during a particular growth regime is detined as the increase in mass mean granule size relative to the mass mean granule size at the beginning of that regime. The extent of granulation during the nucleation stage increases with increasing amount of binder ( Fig. 7), in agreement with the increase in the critical viscous Stokes number (Eq. (3)). So, although no samples could be taken earlier than one minute, it can be concluded that the nucleation stage lasts longer if more binder is present since the growth rates are equal in the non-inertial regime. The duration ol'the compaclion stage is strongly influenced by the amount of binder. II takes a higher degree ot' compaction, that is more collisions and thus a longer time, in order

F. Hoornaer! et aL /Powder Technoh~gy 96 (1998) ; 16-128

122

1.8 1.6

1.4

1.2

i,

•

II

i~6 O,4

0,|

0,75

O,llS

0,05 l:)hm mlo, y Fig, 7. Extent of gnmulation ( relative gn~wlh of granule mas,,, mean di;lmetcr) of nucicatit)n and coalescence stage as a function of the solution phase ratio ( rt,~ 3,9 mPa s).

to force the binder out and to make the granules plastic when there is less binder present. The solution phase ratio, the volumetric ratio of binder liquid to solid material, y I 101, can be used to correct for dissolution of the dry material in the binder liquid: v~

•

,~' ( I + s)p, ( I

-,~,',,,,',,~')/~

(4)

Here, .~',~' is the mass ratio of hinder liquid to solid material, s is the solubility of the solid material in the binder, and #, a n d ~ are the densities of the solid material and the binder, respectively. It should be emphasized that all solution phase ratio figures presented are values at ambient temperature. During granulation it will increase as result of increasing temperature, Fig, 8 shows that a linear relationship holds hetwecn the duration of the compaction stage and the solution pha,~ ratio (for 18,4-20,4 wt,% of binder). Possibly, the presence of cellulose in the Ix)wder mixture enhances compaction, since cellulose is known to absorb water and release it again under pressure, if the initial particle size distribution can be compacted easily it is expected that granulation will be faster and yield larger granules, or that less binder is neces,,~ry, This has been verified experimentally by Adetayo et al, 1211, who found that the granulation g ~ s through a maximum, with respect to both rate and extent, if the number of fines in the initial particle size distribution is such that a minimum in the final porosity can be obtained, Coalescence rates are higher when more binder is present, as is shown in Fig, 5, This is in accordance with theory since a thicker binder layer on the surface makes successful colli-

sions with more and bigger granules possible, it also makes granules more deformable at an earlier stage. Hence, it is no surprise that the coalescence stage lasts longer with increasing amounts of binder, provided that no breakage is encountered. The transition stage, where both coalescence and crushing and layering coexist, appears to be absent if a large amount of water is present. This makes coalescence the dominant mechanism. As far as the growth rate is concerned, the coalescence plus crushing and layering stage goes faster il"more binder is present, a result of the larger contribution of coalescence. During this stage the mid-sized granules gradually disappear due to coalescence and the smallest granules grow in number by crushing as do the largest granules, mainly by coalescence. This results in a bimodal distribution. Finally, the last growth stage, crushing and layering, with its characteristic slow growth, becomes important when granules have grown really large, > 800 ixm, or at longer times when they become less deformable. Here it is only observed with the largest amount of water (the large granules in Fig. 5), although it is expected that crushing and layering would also have been encountered with lower bindercontents (rigid granules) at long times, typically longer than the 120 rain employed. On the other hand, it seems justifiable to consider the transition stage at low water contents, for example 18.4 wt.%+ as the crushing and layering stage since coalescence plays only a minor part in it. Granule breakage takes over alter crushing and layering, and was only observed with the highest amount of binder (20.4 wt.%). Only the largest granules, > 950 Ixm, were

123

F. Hoonlaert et al. /Powder Technology 96 (1998j 116-128 70

6O

•[

II

.

20

10

0.11

O,g

Solutionphau ratio, y Fig. 8. Durationof the compaction stage as a functionof the solution phase ratio ( 7I = 3.9 mPa s). broken, corresponding to the clearance between the ploughshares and the hard layer deposited on the wall. 4.3. Effect of binder viscosit3,

The effect of binder viscosity on the granulation process can be examined by performing experiments with different amounts of P~ P dissolved in the binder (Table 2). Fig. 9 shows the evolution of the mass mean diameter for binders having different PVP concentrations, it should be emphasized that the initial viscosity of the binder is not the actual viscosity of the binder in the process, since the dissolution of sodium sulfate in the binder also increases its viscosity (Table 3). Furthermore, the temperature rise during granulation also has an influence on the viscosity (Table 2). The effect of viscosity is best explained with curves B and C in Fig. 9. Comparison of these two curves shows that the nucleation stage lasts longer if the binder viscosity is increased. These results are in agreement with Eq. (2), where it is stated that growth will last longer, at constant growth rate, when the viscosity rises, on account of the lower viscous Stokes number. As a result, the extent of granulation during Table 2 Viscosity of PVP solutions ill water :is a funclion of concent~'ation and te:nperature 1221 PVP (K-30)

Table 3 Saturated concenlratitm of Na:SO.s in waler as a I'unetitmof tcmperalure J23 ! and the correspondingviscosity

1'/(mPa s)

( wt.~: )

5 !0 20

the nucleation stage increases considerably with viscosity. More specifically, the extent of granulation based on curves A, B arid C in Fig. 9 increases linearly by a factor of I 0 for a binder viscosity of I-9.3 mPa s, analogous to Fig. 7. The growth rates are equal since all collisions are successful in both cases, so the only difference is the duration of the nucleation stage. The compaction stage lasts considerably longer when viscosity is higher. This is due to the increased viscous dissipation in the granules, in Fig. 9, curves B and C illustrate this. The coalescence stage is characterized by faster growth when the binder viscosity is increased. Combined with the longer duration of the coalescence stage, the resulting granules are substantially larger, and the relative growth increases linearly by a factor of 2.5 with a binder viscosity of I-9.3 mPa s. Both results are in line with the theory as proposed by Ennis et al. 19 I, Eqs. (2) and (3). which states that with a higher viscosity less binder is required on the granule surface to give sufficient viscous dissipation tbr coalescence. At least as important is the decrease in the coefficient of restitution at higher viscosities, which is explained by the fact that more

20oc

30oc

40oC

3.1 7.0 29.7

2.2 5.8 21.7

1.7 4.7 17.1)

T ( "C )

r'"' Ha:SO4 ( wt/,~ )

;1 ( mPa s )

20 30

19.4 40.8 48.8

2.8 3.7 3.5

40

I': H¢umu~ert et aL / Powder Technology 96 (1998) ! 16-128

124

m

IOO0

B

I-

400

200

~ ~ 1 (mPu) ~*o 3,0 (mPu) --IF-9,3 (mPas) ~ . 16,4 (mPas) -'Jr"P.2,0(mPu) 1 m..J

0,0

20.0

40,0

60,0

80,0

100,0

120,0

Time (min) Fig. 9. Evolulio, o1' the ~ra,ule ma.~.~mean dia111eler fur dil'fi:ruuI iuitial hinder vi.~co.~itie~ ( Ig. I wt.~ ' hinder): curve A, Tt = 1.0, B. Tt= 3.9: C, T1=9.3, D, q - ,,,.0 mPa .~.

energy is dissipaled upon collision by the liquid which fills the voids between the particles within a granule, due to the higher liquid viscosity. This effect appears [o over-ride Ihe effect of slower compaction, although it is significant how the relative growth during nucleation increases more sharply than during coale,~ence. This is in contrast with the results of Fig. 7. For granulation with low viscosity binders, curve A il~ Fig. 9, il follows thai Ihe compaction singe is absenl. The primary panicles in the granules can be rearranged and con)pacIed easily, so nuclealion is inlmediately t'ullowed by coalescence, Coalescence does nol last very hmg, a consequence

of the low viscous dissipation of the hinder, and is followed by a very long Iransition, coalescence plus crushing and layering, singe, Evenlually granules break as they become large and rigid and the average particle size decreases, Curves D and E in Fig, 9 depict the behaviour of the granulation process al high viscosilies. The nucleation stage lasl~ signilk:antly longer than at lower viscosities, resulting in a high extent of granulation alter a few minutes, This is as exlx,'~ted, After nucleation, it seems as if the compaction stage is entered, However, examining the evolulion of the granule diameter distributions in lime, it seems Ihal this 'compaction stage" is not stalic ~i~:~11hut is an cquilibrium between breakage and growth (Fig, iO). II is concluded thai Ihe nucleation stage would have carried on fi)r a longer time, but the I'ast appearance (~t*large granules causes breakage along with this further gro~ th, At larger times, breakage becomes dominant, a t~ature w.hich would be hard to understand if the present no-growth ~tage is regarded as the compaction stage, it can be observ~ how Ix,th curves show the same growth rate during the~rsl minutes of nucleation, in accordance with the theory of ~ n i s et al. [91 which states that growth rates are still independent of the viscous Stokes number. The expected

35.0 30,0 A

~

25.0

I

15.0

--=- 13 mm ..-e- 21 mkn 30 rain

20.0

10.0 5.0 O0 00

500.0

1000 0 1500.0 2000.0 2500.0 Granule dlameler (pro) Fig. It). Gra,ule diameter dislril~utiu11.~ i11 time after 11ucleatiuu at high ~Lwo~iI~ (IL), I wtJ;; l)iudcr liquid, Tt= 22,0 InPa s). AIIhuugh lhe InaSs 1nea11dianleler is 111areur lass COllSta11Iin time. it ¢a11he COllcluded Ihal this is llUI a ¢Olllpactiu11 stage.

different behaviour at longer times does not occur, e.g. different duration of the nucleation and compaction stage and different growth rates during coalescence stage. Although the wettability of the particles is determined by the interracial energy of the binder and solid, problems may be encountered in spreading the binder unilbrmly over the particles if the viscosity is very high. With the present viscosities, however, this is not the case. Granulation experiments in high shear mixers with initial binder viscosities exceeding 80 or even 100 mPa s have been described [7,24 ], without report of wetting problems. The high shear forces applied ensure this, in contrast to granulation processes in fluidized beds [61.

4.4. Effect of chopper Fig. I I shows the effect of operating the chopper on the evolution of the average granule size for the standard exper-

F. Hoonmert et .I. / P~nvder TeHm.hlgy 06 (1998) 116-12~

125

800.0 700.0

f

J 600.0 ~ 500.0 |

i

o.o

j 200.0 200.0 100.0

[-,~ch0pl~er Off "~*"©hopperon ] 0.0

20.0

40.0

80.0

80.0

100.0

120.0

140.0

Time (rain) Fig. I I, Ew)lulion of the granule mass mean diameter for experiments wilh and wilhoul Ihe chopper w~rking ( 19. I wl.9; binder, ~ = 3.0 f,r both experimenl.~ ).

iment, if the chopper is not operated, it is clear that alter the nucleation stage the ploughshares by themselves cannot provide enough impact to compact the granuh.s and no coalescence stage is entered. The initial small rise in mass mean granule diameter at I rain is caused by the formation of a small number of lumps which are normally cut up by the chopper, but in this case they survived tbr a time. It should be emphasized that the shape of the chopper is most important, in the present experiments the blades of the 'Christmas tree' chopper used resemble hammers, it can be envisaged that these blades will have a densifying effect. In the literature, many experiments are described where 'tulip shaped' choppers with "knife-like' blades are used. These contribute more to comminution than to breakage. Knight ! 7 l, for example, l'ound a sharp reduction in the number of large granules when the chopper was switched on, whilst the median granule size growth rates for the 'chopper active' and stationary situations remain equal.

4.5. Eg'ect ql'tenq~erature Temperature is probably the most delicate parameter in high shear granulation since, on the one hand, it has a considerable effect on the process but on the other it can be controlled only in a very limited way due to the poor heat transfer between the granulating mass and cooling water. Fig. 12 shows the effect of cooling on the evolution of the mass mean granule diameter, in the absence of cooling the process temperature rises from 20 to 70°C, while in the standard experiment it ranges from 20 to 38°C (Fig. 9). It seems

that temperature affects the process since the dissolution of sodium sulfate in the binder increases with rising temperature (Table 3). As was shown in Fig. 5, granulation is very sensitive to minor variations in the solution phase ratio. A higher solution phase ratio leads to the disappearance of the compaction stage when there is abundant binder in the granules. As a consequence, granule growth is rapid in the coalescence regime, resulting in large particles. The increase in granule growth with increasing lemperature is. to a small extent, counteracted by the decrease in binder viscosity (Table 2). The similarities between higher temperatures and more binder are striking if Fig. 13 is considered. Here the results of an experiment without cooling and an experiment with a relatively high amount of binder are compared. De Smet [ 25 ] described how the temperature or power input can be used to monitor and control granulation processes. Fig. 12 shows that temperature indeed offers an oppoaunity for accurate monitoring of granulation, since it tbllows the same trend with the mass mean diameter. The increase in power input, and thus temperature, during the coalescence regime is in agreement with the plastic nature of the granules at this point J6 J.

4.6. Chemical changes hi the I~rmulation of detergents containing enzymes it is customary to use sodium sulfate in its anhydrous form, thenardite, since Ihe other decahydrate conligur:ltion. Glauher's salt, consists o1' hard lumps. However. it is possible that the anhydride takes up water during granulation and changes. partially, into decahydrate. Where the anhydride is stable with

126

F. Hot~rnaert et al. /Po+;" 'er Technology 96 ¢1998) i 16-128

1

LIB

c I +°

800 ++

+

7O

60.... u 6~ Q

"o

[

500"

5o &

200 30

I O0 I r ! 0

l,-,b-,

.......................................................

0

20

~.........................................................................................

20

40

60 80 100 120 Time (rain) Fig, 12, Bvolulion of the granule mass mean dialneter I'or experiments with and wilhou! c,mling I curves A and B, respectively ), combined with the evolution of the temperature ( curves C and D ) during these experiments ( 19. i wt.C/+ binds:r, v/= 3. ~) fi~r bo!h experiments ). 1000 ¸ r

9OO~ 800 i¸ i

?go,

|

5OO

i

4oo + 300,

100

High T -m- High y

~

"

0 ~............................................................................................................................................................................................................................ 0

10

20

30

40

50

,, . . . . . . . . . . . . . . . . . . . . . . .

70 Time (min) l~ig, 13, Evolutit)n t~t"the granule mas,,, mean diameter fi)r an experiment witlmut ~.'ooling [ Iq, I wt,~/r binder I and ~m e.xperimem with era)ling and a high content of hinder ! 20,4 wt,<;; ) ( ~ 3,9 mPa s fiw h~)lh experiments ),

rising temperature, decahydrate decomposes at 32.4°C into anhydride and water [261, An alternative way now of explaining the equilibrium phase, which in this work is identiffed as a compaction stage, is to state that during this stage the temperature gradually rises until the phase transition temperature of 32,4°C is reached and water is then released by the decomposition of the decahydrate which triggers the onset of the coalescence regime. Indeed, temperatures during the compaction stage of a typical experiment reported here were about 30°C. But a number of arguments can be raised which

60

make this hypothesis rather unlikely. The best way, however, to be sure of the chemical structure of the sodium sulfate during granulation was to look at the water of crystallization present at different stages by using thel'mogravimetry, It followed that the very small amount of water of crystallization present during nucleation and compaction was equal to the amount during coalescen;..,. Thus, it was concluded that the role of the chemical phase transition from sodium sulfate decahydrate to the anhydrous form is negligible during nucleation, compaction and coalescence.

F. Hoornaert et aL / Powder ~:,chn,il,~gy 96 (1991¢~ 116-128

5. Conclusions Agglomeration in a L~idige mixer granulator of a powder mixture with an aqueous PVP solution, used to make enzymecontaining granules was found to take place in subsequent, clearly distinguishable stages. Nucleation corresponds to the non-inertial regime as described by Ennis et al. [ 91. There, growth rates are fast and independent of the binder content and viscosity, since all the collisions are successful. The duration and the resulting extent of granulation of the nucleation stage increase with increasing binder content and viscosity because of the increasing viscous dissipation. The duration of the compaction stage decreases in a linear way when the solution phase ratio is increased, since the binder is squeezed out more easily and the granules become plastic sooner. Increasing the binder viscosity makes the compaction phase last longer due to more viscous dissipation. Without the chopper ( 'hammer lype' ), compaction occurs only to a limited extent and this results in a less thick binder liquid layer on the granule surface. This easily results in the retardation of granulation. Chemical changes which take place during nucleation and compaction can be neglected. During the coalescence stage the most growth is realized. Coalescence is preferential towards larger granules because o1"their higher deformability and increases with binder content or binder viscosity as a result of more viscous dissipation. Coalescence corresponds to the inertial regime as described by Ennis et al. 19 I. The transition stage, where both coalescence and crushing and layering are present, was observed to increase the growth rate with increasing binder content and decreasing binder viscosity due to changes in the relative contributions of both mechanisms. Breakage is caused by the crushing oi" the hu'gest granules between the ploughshares and the wall or wall cake. Modest temperature changes can be interpreted as a change in solution phase ratio and have an identical effect. The course of the temperature rise can be used to monitor granulation behaviour. All the regimes encountered and the effects t~l'varied parameters can be explained qu.'mtitatively by using the mechanistic model expressed by Eqs. ( 2 ) and ( 3 ). This knowledge enables educated optimization, modelling, and control o1"the process.

6. List of symbols

C,



h

concentration (wt.%) capillary number mean granule diameter ( Ixm ) coefficient of restitution of granules thickness of binder liquid layer on granule surface (m)

h,, (r} s Sty St*

u .r,,' v

127

characteristic length of surface asperities (m) mean particle radius ( m ) solubility (kg/kg) viscous Stokes number critical viscous Stokes number individual granule velocity, half of relative velocity at equal velocities (m/s) mass ratio of binder liquid and solid material (kg/kg) solution phase ratio ( m~/m 3)

Greek leners

r/ P O"

dynamic viscosity ( Pa s) density ( kg/m 3) surfi~ce tension, air-binder liquid ( N/m )

hufi~'es

g i s sat 0

grant;f:., solid matter liquid solid saturated initial, prior to contact of binder layers

Acknowledgements This work was sponsored by Royal Gist-Brocades N.V.. Dell't, The Netherlands.

Rcl'erences I IJC.F.. C~q'~c',. Pm'tich.' Size Enhlrgcmen! (it~mdbo.k ol I'owdth" Technoh~gy, Vol, I ). l~lsevit:r, Amsterdanl. 1980. 121 K.I.. Kudam. (h'anulatiotl Techtlology liar I]itlproduct..,. CRC Ih'es~. Boca Raton. H.. 1990. 131 P.C. Kapur. Adv. Chem. Eng.. I0 ( 1978 ) 55. 141 A.A. Adelayo. Moddling mid simulation of a fertilizer L.zranulalion circuit. Ph.D. Di.,,.~erlmion. Del~artment of Chemical Engineering. Univei'sil), of Queensland. Australia. 1993. 15 J A.A. Adetayo. J.D. Lilsler. I.T. Cameron. S.E. Pi'alsini~ and B. l:.rmi~. Pro¢. 6th Inl. Syrup. Agghmlel':lliorl. N~ig,ya. Jap~ln. 19q3. r), 105. I~1 H.G. Krislen.,.¢n and T. S¢lm¢l'~r. Drug I)cv. Ind. I~lmrm.. 13 (1~)87) 803. 171 P.C. KniLzlll. Powder "rechnol.. 77 ( 1993 ) 15¢L 181 N. Ouchiyanla and T. l"anak~l. Ind. Fng. Cllenl. Proc¢,,~, l)t...,,. Dev.. 14 (1075) 28(~. I~1 B.J. Eimi.,,. G.I. Tardo.,, and R. Pfeffcr. Powder ']'echnol.. (.~.,i ( 1091 ) 257. I IOI PJ. Sheringttm. Chem. Eng.. 220 (1908) CE201. I I I I C.E, Capes alld P.V. Dankwcns. Trans. In lt. Chem. Eng.. 4:; ( 1005 ) TI If~. 1121 W. Pielsch :rod H,K. Rumpf. Z, Tech. Chore,. 30 ( 1~}(~7) X85, 1131 B.J. Enni~. ().1. Tin'dim mid R, I~t~'l'fer. CI'J¢Ill Ell[=,,Sci,. 45 (I()~)()) 307 I, 1141 E.K. Markussen. Eur. P~ilcnl No, 0 304 331 ( It)89 ), 1151 J.T. Carsten.,,ttll. T. Lai. D,W. Flickncr. II I?,, Huber ~md M A , Zogli.. J. Pharm. Sci.. 65 ( 1970 ) 9t~2.

!

128

F. Hoonzaert et at. / Powder Technology 96 (1998) 116-128

! 161 H.G. Kristen~n. Proc. Ist Int. Particle Technology Forum, Denver, CO. USA. 1994. AIChE, New York, p. 214. 1171 P.C. Kaput and D.W. Fuerstenau, Trans. AIME, 229 ( 19641 348. 118J P.J. Shedngton and R. Oliver, Granulation, Heyden, London, 1981. I lt~I H. Leuenberger, H.P. Bier and H.B. Sucker, Pharm. Technol., 3 ( 19791 6. 12(11 D.M. Newitt and J.M. Conway-Jones, Trans. Inst. Chem. Eng., 36 (1958) ~2. 121 ! A.A. Adetayo, J.D. Litster and M. Desai, Chem, Eng, Sci., 48 (1993) 395 I.

1221 V. Biihler, Kollidon, Polyvinylpyrrolidone for the Pharmaceutical Industry, BASF, Ludwigshafen, Germany, 1992. 123J A. Seideli, Solubilities of Inorganic and Metal Compounds, Vol. I, Van Nostrand, New York, 3rd edn., 1940. J24 J M. Ritala, O. ]urgensen, P, Holm, T. Schaefer and H.G, Kristensen, Drug, Dev. Ind. Pharm., i 2 (1986) 1685. 125 J P, De Smet, Chemicaoggi, 3 ( 19851 63, 1261 O,A. Hougen, K,M. Watson and R.A. Ragatz, Chemical Process Principles, Part 1: Material and Energy Balances, Wiley, New York, 2nd edn., 1954, p, 141.