Process Intensification in Particle Technology: Intensive Granulation of

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Ind. Eng. Chem. Res. 2002, 41, 5436-5446

Process Intensification in Particle Technology: Intensive Granulation of Powders by Thermomechanically Induced Melt Fracture G. Akay,*,† L. Tong,† and R. Addleman‡ Process Intensification and Miniaturisation Centre, School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom, and Rosand Precision Ltd., 19 Oakland Rise, Welwyn AL6 0RN, United Kingdom

Process intensification in particle technology is illustrated by a novel continuous agglomeration and microencapsulation (granulation) technique based on the thermomechanically induced melt fracture. This intensive structuring method is based on the nonisothermal flow-induced phase inversion phenomenon in which the continuous phase of the fluid (poly(ethylene glycol) melt) undergoes fractional solidification, thus essentially increasing the volume fraction of the solid phase during passage between two disks. The polymer melt containing the filler particles (calcium carbonate) is fed centrally (using an extruder) into the gap between two disks, one of which is stationary (stator) while the other rotates (rotor) at a constant angular velocity. These disks have several sets of cavities formed in such a way that the upper and lower cavities never match during rotation and that they can achieve mixing as well as pumping. The rotor and stator are kept at different temperatures so as to allow the cooling of the melt as it is displaced outward in the radial direction, thus causing solidification while undergoing deformation. As the filled polymer melt travels between the heated stator and cooler rotor cavities, locally crystallized filled polymer particles are created (nucleation) which are subsequently re-dispersed into the polymer binder as a solid dispersed phase. This causes the fluid to (crumble) fracture and form granulated particles, the size of which approaches the clearance between the disks, and the size distribution narrows as the granulated particles move outward in the radial direction. The technique produces granulated particles with a narrow size distribution in which the concentration of the filler is constant (size independent), therefore giving a product which cannot be produced by any known granulation process. 1. Introduction Agglomeration and encapsulation of fine particles are encountered in several industrial sectors such as pharmaceuticals, detergents, fertilizers, and animal feedstocks. The volume throughput in these processes can vary enormously. Batch processing is suitable for pharmaceutical applications where the volume of production is quite low and product values are very high, while a continuous process is essential in other sectors where the volume throughput is very high. Currently available agglomeration techniques are based on either the provision of a suitable processing environment for primary particle collisions (growth agglomeration) or the compression of particles to form densified particles (pressure agglomeration) in the presence or absence of a binder. These techniques are wellknown and periodic reviews are available.1-6 The growth agglomeration technique is often conducted in batch mixers or fluidized beds in which particle break up and agglomeration occur simultaneously. The residence times are relatively long and the variety of possible primary particles for agglomeration and binders are restricted. In pressure agglomeration, pressurization is * To whom correspondence should be addressed. Tel.: +44191-222-7269. Fax: +44-191-222-5292. E-mail: Galip.Akay@ Newcastle.ac.uk. † University of Newcastle. ‡ Rosand Precision Ltd.

confined to a small volume and there are restrictions on the raw materials.5 Macroscopic rodlike pellets containing solid particles dispersed in a binder can also be prepared by the extrusion of such dispersions followed by cooling and a chopper using a die roller cutter.6 The so-called multinucleus-type capsules/microcapsules are essentially agglomerated particles with a prescribed release characteristics which can be controlled by the concentration and solubility of the binder. In agglomeration, the amount of binder is kept to a minimum as long as the agglomerates are sufficiently strong. Therefore, in principle, multinucleus microcapsules can be manufactured through an agglomeration process. However, currently available agglomeration processes do not have this flexibility and therefore such capsules/microcapsules are produced through specific techniques.7 A novel agglomeration/encapsulation technique known as “flow-induced phase inversion agglomeration/encapsulation” was recently disclosed by Akay.8-11 The technique is based on the phenomenon of flow-induced phase inversion (FIPI) observed in dispersed systems.8-14 The FIPI phenomenon has also been applied to the processing of more complicated (multiple) dispersed systems including the encapsulation of solids and liquids.8-12,15-18 In this process a concentrated suspension of powder in liquid is caused to crumble (phase invert) through further addition of a powder (a “crumbling agent”), that is, a phase inversion from a solid phase suspended in a

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liquid (a paste) to a liquid phase suspended in a solid (an agglomerated granular material). The process requires a degree of mechanical energy input to disperse the two phases and therefore must be carried out in some kind of mixing environment. The mixing requirement for FIPI is much less restrictive than it would be in conventional agglomeration since shear can be transmitted to a paste much more effectively than to an agglomerate. This mixing flexibility allows FIPI in either a batch or a continuous mode of operation. 2. Background Flow-Induced Phase Inversion and Its Applications in Process Intensification. As the name implies, flow-induced phase inversion is brought about by the application of deformation to multiphase structured fluids. This process essentially affects the thermodynamics of the system and therefore it is a generic process. Phase inversion in general can be achieved (in low-viscosity two-phase systems) by changing the thermodynamic state of the multiphase system through changes in the thermodynamic state variables (TSVs) such as temperature, phase volume, surface activity, type and degree of microstructure, and so forth.8-14,17-22 In FIPI, phase inversion is achieved without any change in TSVs. However, the critical deformation rate at which FIPI starts is dependent on the thermodynamic state of the structured fluid as well as the type of flow field, that is, shear or extensional or combined flows. It is therefore necessary, in certain applications, that the flow field should be uniform and the deformation rate should be very high.11-14 These two requirements result in small processing volume and high throughput, which are the conditions of process intensification. However, these restrictions on the size of the processing (flow) field and throughput are essential for the process to take place. Therefore, such processes are said to be inherently intensive. In addition to intensive agglomeration and microencapsulation, inherent process intensification based on FIPI has been applied to various intensive structuring of microstructured materials including polymer latexes and microporous polymeric particles,18-22 liquid detergents,23 and microporous polymeric materials through an emulsion polymerization route.24 In a typical isothermal FIPI-agglomeration/microencapsulation process,8-12 the phase inversion takes place at a critical dispersed phase concentration (crumbling concentration, Cc) when the primary particle size and mixing conditions are kept constant. It is known that8-12,15-18 Cc decreases with the logarithm of binder molecular weight, which implies that Cc may also depend on temperature. Due to the dependence of viscosity on molecular weight and temperature, there is considerable scope for using the FIPI process with a wide range of liquid phases ranging from aqueous/ nonaqueous solutions to molten polymers. As a result of this, the temperature becomes an important process variable since the rheology of the liquid phase is critical in determining the phase inversion behavior of the system. Elevating the temperature of the liquid phase increases the “crumbling concentration”, Cc, of the primary particles and allows higher solid loadings to be obtained while maintaining a pasty rheology. The final agglomerate size at a product particle concentration Cp is found to be a function of (Cp - Cc), which can be viewed as the degree of supersaturation of the system. The limiting extremes of this relationship are

of course when Cp ) Cc and the system forms a single agglomerate (the limit of paste behavior), and when Cp ) 1.0, and there is no binder phase, the agglomerate size is then identical to that of the primary particles. There is scope to control the final mean agglomerate size by a known temperature swing. This mechanism has not yet been investigated. When phase inversion occurs, the macroscopic domains of the suspension are surrounded by the crumbling agent. This means that the FIPI process can also be successfully adapted to coating processes. There is no requirement for the crumbling agent to be of the same material as that of the primary powder; therefore, thin even coatings are easily obtainable. In a microencapsulation process such as this an even coating of known thickness is extremely important in tailoring a desirable release profile for the active core ingredients. Since most release cases involve water, water-soluble polymers (such as poly(ethylene glycol) and poly(ethylene oxide)) and surfactants have recently been used as binders.11,15,16 In these studies, an interaction is observed between the binder and some crumbling agents (citric acid particles) create more flexibility in obtaining agglomerates/microcapsules with specific release characteristics. Previous studies8-12,15-18 of the FIPI agglomeration/ microencapsulation techniques have dealt primarily with the fundamental physical and chemical aspects of small-scale batch operations where the residence times were of the order of tens of minutes. A continuous FIPI agglomeration process8,11 has also been described with residence times of seconds. However, due to the complexity of the continuous process, unavailability of purpose built processing equipment and the large amounts of raw materials required, the continuous process has not yet been evaluated. In the limited trials carried out to date, the phase inversion stage of the process was not purpose built. This resulted in problems in controlling the product properties such as agglomerate shape, size, and size distribution. Until recently, FIPI-based process intensification studies have been conducted under isothermal conditions without a phase change in multiphase systems. Recently, Akay and Tong21 have shown that, in the presence of phase transformation (i.e., from melt to solid), flow (deformation) can also induce phase inversion in water/polymer/polymeric surfactant systems. The use of temperature and deformation as processing parameters can therefore promote phase inversion in solid particle/polymer systems and can be utilized in process intensification in particle technology. In this study, we describe a continuous intensive agglomeration/microencapsulation process using a purpose built agglomerator/microencapsulator, called ITIG. We also study the characteristics of the agglomerates and relate them to the processing characteristics. Because the present agglomeration and microencapsulation processes are achieved through a size reduction process (unlike the classical agglomeration method in which size enlargement takes place), we refer to the current agglomeration/microencapsulation technique as granulation. 3. Experimental Section 3.1. Materials. Binder: Poly(ethylene glycol) with an average molecular weight of 10 000 was supplied by Aldrich.

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Figure 1. Diagrammatic representation of the processing equipment including extruder, extruder drive unit, extruder control unit, and intensive agglomerator/microencapsulator processing and control units.

Filler: Calcium carbonate particles (coded MillicarbOG) with a mean particle size (D(50)) of 2.7 µm was supplied by Omya (U.K.) Ltd. 3.2. Equipment. (a) Differential Scanning Calorimetry (DSC). Mettler-Toledo Model FP90 with DSC cell unit FP85 were used to evaluate the thermal characteristics of the binder (poly(ethylene glycol)) and its mixtures with the filler (calcium carbonate) over a temperature range to characterize the full melting and crystallization behavior of these materials. Both heating and cooling cycles were used. The rate of heating or cooling was 2 °C/min. PEG was used as received while calcium carbonate filled PEG samples were obtained by mixing PEG and calcium carbonate in an extruder which is also used in feeding the rotating disk agglomerator/microencapsulator. The extrusion temperature was TE ) 70 °C and the samples were left to solidify at room temperature. (b) Haake Extruder and Haake High-Torque Rheometer. A Gebruder Haake GmbH mixer system was used in our experiments. This system consists of three units: extruder, drive unit, and control unit. The Haake extruder (Rheomex 252) is a single mixing screw extruder with a 19-mm-diameter screw and 480-mm screw length. The extruder barrel has three temperature control zones. This extruder is driven and controlled by the Haake high-torque rheometer (RC90/ 9000). The drive unit consists of a horizontally mounted heavy-duty motor with a torque sensor attached to the extruder screw. The temperatures of the extruder zones

are controlled by air cooling. The screw torque, screw speed, pressure at the extruder outlet, and temperatures along the barrel can be recorded continuously as a function of time. (c) ITIG: Intensive Agglomeration and Microencapsulation System. This intensive granulation technique and the equipment to achieve it (intensive agglomerator and microencapsulator (granulator)) were proposed and devised by one of us (G.A.) and the granulator was manufactured by Rosand Precision Ltd. ITIG’s granulation unit is connected to the extruder outlet through a heated section. ITIG has its own control system. The diagrammatic illustration of the full experimental system (extruder, extruder drive unit, extruder control unit, ITIG processing unit, and ITIG control unit) is shown in Figure 1. ITIG’s processing unit essentially consists of two rotating disks (rotors) and two stationary disks (stators) as shown in Figure 2. The diameters of the rotors and stators are 300 and 330 mm, respectively. All rotors and stators contain cavities which are designed to pump/ convey and/or achieve mixing. The rotor cavities are machined on the upper and lower surfaces of a stainless steel block and driven by a variable speed motor via a gear box. The rotational speed of the rotor assembly (i.e., upper and lower disks containing the rotor cavities) is ΩR ) 0-50 rpm or alternatively 0-500 rpm. The rotor assembly is sandwiched between the upper and lower stators. The upper stator and rotor contain a full set of cavities. The lower stator and rotor are in the form of a flat ring (outside diameter 300 mm, inner diameter 120 mm) to allow the rotor drive connection and to discharge the product. The upper and lower rotor/stator cavities are arranged in such a way that the cavities are offset by half a cavity length so that the rotor and stator cavities never exactly match. This arrangement is shown in Figure 3a-e. The separation of the upper (GU) and lower (GL) rotor and stator can be varied independently. These cavities are designed to function both when the particlefilled polymer melt is flowing through the ITIG processing unit as well as when the filled polymer melt is transformed into an agglomerated powder. Cavities with low aspect ratio (i.e., more circular) on the rotors and stators are for mixing while the highly elongated cavities pump (when liquid phase is present) or convey

Figure 2. Diagrammatic illustration of the intensive agglomerator/microencapsulator processing unit. Melt from the extruder can be fed from the melt feeding ports 1 or 2.


Figure 3. Arrangement of cavities on the rotors and stator: (a) upper rotor and the arrangement of cavities; (b) diagrammatic representation of the upper rotor and cavity numbers; (c) upper rotor after processing of 100 g of PEG (25%) and calcium carbonate (75%); (d) upper stator after processing of 100 g of PEG (25%) and calcium carbonate (75%); and (e) lower stator after processing 400 g of PEG (25%) and calcium carbonate (75%). The numbers indicate the cavity’s arrangement on the lower stator (e) and upper rotor (b).

(when particles are present). These pumping/conveying cavities make a relatively large angle with the radial direction. Such a design is necessary since the position of phase inversion from melt to granulated powder is

not known. Lower rotor and stator cavities are intended for particle conveying rather than mixing. The center of the upper stator has an inlet port from where the filled polymer melt is pumped from the

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Table 1. Distance of Each Ring on the Upper Rotor and Lower Stator from the Disk Center; Radius in the Table Indicates the Mean Radial Distance from the Center cavities ring no. radius (mm)

upper rotor 1 11.2

2 22.5

3 35.9

4 50.0

5 65.0

6 76.0

extruder directly on to the pumping cavities of the upper rotor. Due to the rotation of the upper rotor and the cavity arrangement, filled polymer melt follows a threedimensional flow path. Referred to a set of polar coordinates (r, θ, z), the radial motion (r-axis) is due to the applied pressure developed by the extruder (ignoring any centrifugal motion). The angular (θ-direction) motion is a result of rotor motion while the up-and-down (z-direction) motion between the rotor and stator cavities is a result of the radial, angular motions and the cavity arrangement. Temperatures of the upper and lower stators are controlled independently. Temperatures of the rotors are kept below that of the corresponding stator temperatures. The upper stator also contains three Auger powder feeders located symmetrically at various distances from the center. Additional powder can be added to the filled polymer melt after phase inversion. The addition of the extra powder is useful for increasing the particle content of the agglomerates. There are pressure transducers present at several locations on the upper stator to determine the location of the polymer melt and agglomerates. (d) Characterization of the ITIG Processing Unit. The processing unit of ITIG essentially consists of several unit stages connected to each other in three dimensions in parallel (in angular direction) and in series (in radial direction) in which the volume and shape of the stages change periodically with time as a result of the cavity arrangement and the rotor motion. Since our primary interest is in the agglomerates (final product and its transient development), and in the absence of any extra particle addition from the feeders located on the upper stator, we can number the product cavities from the center as shown in Figure 3a,b. Therefore, we assume that the cavities at a given angular position are identical. There are 11 rings of cavities on the upper rotor while the number of rings on the lower stator is 3 as illustrated in Figure 3e. Table 1 shows the radial location of the cavities on the upper rotor and lower stator where the products are collected from. The radius in Table 1 represents the mean radial distance from the disk center. (e) Filler Concentration Determination in Granules. The distribution of the filler in each size fraction is determined by burning of the binder at 600 °C. This is very important since all known granulation techniques yield size-dependent filler concentration, which include the isothermal FIPI agglomeration/microencapsulation process reported by Akay.8-12 3.3. Experimental Procedure and Evaluation of Process Dynamics. Poly(ethylene glycol) powder was dry-blended with calcium carbonate particles (filler) at a desired concentration (CP ) concentration of polymer in wt % and CF ) concentration of the filler in wt %) and fed into the extruder hopper. The extruder temperature along the barrel (three heating zones) as well as the extruder die was kept constant at TE ) 70 °C. The extruder screw speed was either ΩE ) 10 or 15 rpm. Polymer/calcium carbonate blend was conveyed along

lower stator 7 89.0

8 103.8

9 115

10 128

11 143

12 138

13 122

14 106

the extruder barrel when the polymer was melted and mixed with the filler. The filled polymer melt was then transferred to the ITIG intensive granulator via a heated section and was fed from the center of the upper stator. The upper stator (TUS) and rotor (TUR) temperatures were kept at 52 and 45 °C, respectively. The lower rotor temperature was 45 °C while that of the lower stator was 45 °C. ITIG processor rotor speed was kept at ΩR ) 30 rpm. The rotational speed needs to increase if the polymer melt is frozen rapidly in the ITIG processing unit, thus causing the blockage of the extruder. If the blockage happens, the extruder torque increases rapidly and, in response, the ITIG rotor speed is increased to 40 rpm until the blockage is cleared and the speed is restored back to 30 rpm. The variation of the extruder torque at a constant screw speed of 10 rpm is shown in Figures 4 and 5. After each experiment, the extruder and the ITIG were stopped and the disks were separated and allowed to cool to ambient temperature. The upper disks were photographed to determine the location of the phase inversion and to determine the nature of the structure change during phase transformation. The agglomerated material was removed from the upper rotor cavities and collected according to the diameter of a given cavity for particle size analysis. Figure 4 indicates that the extruder torque increases rapidly as the filled polymer melt starts melting and being transported along the extruder barrel. When the filled polymer enters into the ITIG processor unit, there is a sudden drop in torque if the temperature of the rotor and stator are constant at TE ) TUR ) TUS ) 70 °C (curve A in Figure 4). This is due to the pumping action of the ITIG processor which reduces the load on the extruder screw. However, as the filled polymer melt fills the space between the rotor and stator disks, torque starts increasing as a result of the increased flow path. If the temperature of the disks are low (TUR ) TUS ) 52 °C as indicated by curve B in Figure 4), there is no significant fall in the torque initially. But the torque continues to increase as the filled polymer melt continues to fill the space between the rotor and stator. If the filled polymer melt is allowed to form granules (phase inversion) by lowering the rotor temperature to TUR ) 45 °C (below the melting point of the filled polymer) and when TUS ) 52 °C, the extruder torque drops initially but does not recover subsequently as shown in Figure 4 (curve C). Large drops in the torque during the granulation is a result of increased rotor speed to stop large increases in the torque developing as a result of melt solidification in the ITIG processor. Large fluctuations in the extruder torque are also reflected in the pressure readings at the extruder exit. This is a common feature of the highly filled polymer melts. As the concentration of the filler is increased, the amplitude and the frequency of the fluctuations in the flow rate and pressure also increase.25-28 This situation is also reflected in the agglomeration process as shown in Figure 5. Here, the variation of the extruder torque is plotted against time as a function of polymer concentration. As the polymer concentration is decreased, the


Figure 4. Variation of extruder torque with time as a function of the rotor/stator temperature setting during the operation of the intensive granulator processing unit under different operating conditions. In all cases the polymer melt (27.5% PEG and 72.5% calcium carbonate) enters into the processing unit centrally at 70 °C. The extruder screw rotation is ΩS ) 10 rpm. The rotor speed is ΩR ) 30 rpm, which is increased to 40 rpm when the extruder torque exceeds a set value. Clearance between the rotor and stator is GU ) 1.5 mm. (a) Rotors and stators temperatures are TUS ) TUR ) TLR ) TLS ) 70 °C (no agglomeration). (b) Rotors and stators temperatures are set at TUS ) TUR ) TLR ) TLS ) 52 °C (no agglomeration). (c) Agglomeration is present when the upper stator temperature is TUS ) 52 °C and upper and lower rotor temperature and lower stator temperatures are TUR ) TLR ) TLS ) 45 °C. Typical changes in the processor rotor speeds are indicated to illustrate its effect on the extruder screw torque.

Figure 5. Variation of extruder torque with time as a function of PEG concentration in the melt during the agglomeration process. The temperatures are as follows: melt enters into the processor at TE ) 70 °C; upper stator temperature TUS ) 52 °C; upper and lower rotor temperature TLR ) 45 °C; lower stator temperature TLS ) 45 °C. Extruder screw speed ΩS ) 10 rpm; granulator rotor speed ΩR ) 30-40 rpm; clearance is GU )1.5 mm.

torque increases and the amplitude fluctuations in torque increase. Since the onset of crumbling is delayed when the concentration of the polymer is high, the fluctuations in the torque due to crumbling are also delayed compared with the case when the polymer concentration is low. 4. Results 4.1. Differential Scanning Calorimetry (DSC). The cooling curves for the unfilled poly(ethylene glycol), PEG 10 000, and calcium carbonate filled polymers are shown in Figure 6 after the complete melting of the

materials at 70 °C. The cooling rate was 2 °C/min. Tmp and Tcp are the peak melting and crystallization temperatures. The summary of the results is tabulated in Table 2 for heating and cooling cycles. As seen from Figure 6 and Table 2, PEG 10 000 is a crystalline polymer (heat of melting ∆Hm ) -171 J/g and heat of crystallization ∆Hc ) 93 J/g). As apparent from the reduction of the heat of crystallization and melting, the crystallization is greatly reduced when calcium carbonate is added. However, the presence of the filler does not appear to affect the peak melting temperature or the onset of melting (Tm (10%)) or completion of melting

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Figure 6. Differential scanning calorimetry curves for various PEG melts as a function of PEG concentration. These results represent cooling only with a cooling rate of 2 °C/min. Table 2. DSC Results of the PEG and CaCO3 Blends at Different Concentrations heating

cooling

melting range

solidification range

sample

melt peak Tmp, °C

Tm (10%), °C

Tm (95%), °C

∆Hm J/g

solidification peak Tcp, °C

Tc (10%) °C

Tc (95) °C

∆Hc, J/g

100% PEG 32.5% PEG 30% PEG 27.5% PEG 25% PEG

63.2 63.3 63.1 63.3 63.3

59.4 58.8 58.5 58.8 58.6

65 65.5 65.5 66 66

-171 -56.3 -51.9 -48.1 -48.9

51.8 47.6 46.2 46 46

52.9 49.4 48.9 48.7 48.4

50.3 45 44.1 43.4 43.3

93 36 30 34.2 32.1

(Tm (95%)) during heating. However, the peak crystallization temperature Tcp and the temperatures characterizing the onset of crystallization (Tc (10%)) and completion of crystallization (Tc (95%)) during solidification are lowered in the presence of filler. The thermodynamic data for cooling are used in the setting of the processing temperatures (based on the peak temperatures) in the ITIG intensive granulator (i.e., upper stator temperature is 52 °C and the lower stator temperature is 45 °C). 4.2. Visualization of the Process. After the phase inversion, it is possible to feed more filler (calcium carbonate) by using the powder feeders located on the upper stator. In the current experiments this facility was not used since the location of the phase inversion was not known. Our aim here is to evaluate the process characteristics including the location of crumbling. The typical appearances of the upper rotor and upper stator as well as the lower stator after an experiment are illustrated in Figure 7a-d. Here, the feed contains 32.5% PEG (10 000) and 67.5% calcium carbonate. The clearance between the rotor and stator disks is 1.5 mm and the extruder screw speed is 10 rpm while the rotor speed is 30 rpm which can be increased to 40 rpm if the extruder torque exceeds a prescribed value. When the torque is reduced below another set value, the rotor speed is reduced back to 30 rpm. Parts a and b of Figure 7 illustrate the appearance of the upper rotor and upper stator, respectively. It can be seen from these figures that there are three regions present on the upper rotor: (1) Solidified filled polymer (which represents the

filled polymer melt) with the imprint of the upper stator cavities; (2) transition region when the polymer melt is transformed from a paste to granulated powder; and (3) granulated powder which is only present in the rotor cavities. Figures 7a,b indicate that the filled polymer melt goes through the stator cavities, but after phase inversion, upper cavities are not fully filled by the agglomerated powder. This is because the ITIG operates at atmospheric pressure and there is no back pressure present. However, this does not mean that the upper stator or lower rotor cavities are not involved in the transportation of the powder. Powder is transported radial outward in the upper cavity and it is transported radially inward in the lower cavity. In separate experiments, we observed that the filling of the upper stator cavities require a certain amount of back pressure after phase inversion (i.e., restricted powder flow in the radial direction). However, operation at ambient pressure is more suitable since this enables us to feed additional powder after phase inversion. Furthermore, the existence of the stator cavities may be useful in the transportation of the agglomerates. Figure 7 c illustrates the state of the interface across which the phase inversion takes place. It can be seen that the phase inversion takes place over a short distance. In Figure 7d, the appearance of the lower stator is illustrated. Agglomerated particles from the upper rotor fall directly on to the lower stator and subsequently transported radially inward by the lower rotor. The lower rotor does not contain any particles, indicating the absence of any back pressure.


Figure 7. Visualization of the nonisothermal flow-induced phase inversion process and subsequent granulation. Melt entry temperature TE ) 70 °C; upper stator temperature TUS ) 52 °C; upper rotor temperature TUR ) 45 °C; lower rotor temperature TLR ) 45 °C; lower stator temperature TLS ) 45 °C. Upper rotor/stator clearance GU ) 1.5 mm and lower rotor/stator clearance GL ) 0.5 mm. Screw speed ΩS ) 10 rpm; rotor speed ΩR ) 30-40 rpm. (a) Upper rotor; (b) upper stator; (c) upper rotor illustrating phase inversion from melt to granules; (d) lower stator.

4.3. Location of the Phase Inversion. The location of the phase inversion is dictated by the temperature profile of the upper rotor and stator, thermal properties of the polymeric binder, and filler concentration as well as the temperature of the filled polymer melt entering the ITIG processing unit. In the current experiments, the temperature profile of the ITIG processing unit is fixed but the remaining parameters are changed. The phase inversion location is determined from the center of the upper rotor as illustrated in Table 1. A summary of the results is shown in Figure 8 which indicates that the phase inversion radius increases with increasing polymer concentration. 4.4. Development of Agglomerate Size Distribution. The particle size distribution after phase inversion is measured at various locations by collecting the samples from the cavities located at a fixed radius. The results are shown in Figure 9 which indicates that the particle size distribution becomes narrow as the agglomerates are transported in the radial direction after phase inversion. The particle size distribution is characterized by the particle size span, S, defined as

S ) [D(90) - D(10)]/D(50)

Figure 8. Variation of the phase inversion location with polymer concentration. TE ) 70 °C, TUS ) 52 °C, TUR ) 45 °C, GU ) 1.5 mm, ΩS ) 10 rpm, and ΩR ) 30 rpm.

where D(10), D(50), and D(90) are the diameters below which 10, 50, and 90% of the particles lie, respectively. The average particle size D(50) and the particle size span, S, corresponding to these cavities are shown in

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Figure 9. Histogram bar charts illustrating the development of agglomerate size distribution. The process conditions: TE ) 70 °C, TUS ) 52 °C, TUR ) 45 °C, TLR ) 45 °C, TLS ) 45 °C, ΩE ) 10 rpm, ΩR ) 30-40 rpm, GU ) 1.5 mm, GL ) 0.5 mm, CP ) 33.3 wt %, and CF ) 66.7 wt %. Table 3. Variation of the Average Particle Size D(50) as a Function of the Cavity Locations; Processing Conditions Are as in Figure 10 cavity number 1 2 3 4 5 6 7 8 9 10 11 Figure 10. Variation of the average particle size D(50) as a function of the cavity locations. Processing conditions were the as same as those in Figure 9.

Figure 10 and tabulated in Table 3. Before the phase inversion, D(50) f ∞; S f 0. It can be seen that the average particle size increases and the size distribution becomes narrow (S is low) as the granules are transported radially outward in the upper cavity. The average particle size approaches the disk clearance (1.5 mm) and S f 0.5, indicating a very narrow size distribution. As the granules are transferred to the lower disk cavities, the average particle size decreases due to the reduced clearance between the lower stator and rotor. The final granulate size distribution is also shown in Figures 9 and 11 for two different polymer concentrations when the particles are collected from the collection tray after passing through the cavities located on the lower stator. As seen from Figures 9 and 11, particle characteristics

12 13 14

cavity location (mm)

Cavities on Upper Rotor Surface 11.2-12.85 19.4-25.6 32.8-39.0 46.0-56.0 63.0-67.0 74.0-78.0 85.0-95.0 101.8-105.8 113.0-117.0 124.0-134.0 141.0-145.0 Cavities on Lower Stator Surface 131.0-145.0 115.0-129.0 99.0-113.0

D(50) (mm)

0.385 0.72 0.96 0.97 1.10 1.21 0.875 0.74 0.735

are not affected by the concentration of the polymer binder. The gap between the lower rotor and stator is 0.5 mm. Therefore, when the powder is transported through the lower cavities, the particle size is decreased and once again approaches that of the gap clearance and the particle size distribution becomes broad as a result of particle breakup at low temperature. 5. Discussion Process intensification in particle technology can be achieved through the application of flow-induced phase inversion (FIPI) phenomenon8-12 under isothermal conditions. Recently, we have investigated the applications of FIPI under nonisothermal conditions.21,22 In the


Figure 11. Particle size distribution (on mass basis) of the final product obtained from the collection tray after they emerge from ring 14 on the lower stator. Processing conditions: CP ) 25 wt %, CF ) 75 wt %, TE ) 70 °C, TUS ) 52°, TUR ) 45 °C, TLR ) 45 °C, LLS ) 45 °C, ΩE ) 10 rpm, ΩR ) 30-40 rpm, GU ) 1.5 mm, and GL ) 0.5 mm.

Figure 12. Variation of the normalized filler concentration (C/CF) as a function of particle size (D). Processing conditions: CP ) 25 wt %, CF ) 75 wt %, TE ) 70 °C, TUS ) 52°, TUR ) 45 °C, TLR ) 45 °C, LLS ) 45 °C, ΩE ) 10 rpm, ΩR ) 30-40 rpm, and GU ) 1.5 mm.

present technique, the nonisothermal FIPI is also used to obtain granulated particles from a melt filled with primary particles. However, when the phase inversion takes place from a binder continuous state to a particle continuous state, the particles are not the original primary particles but granules. The mechanism of the process indicates that the granulation occurs through a thermomechanically induced melt fracture process. In this process, the concentration range of the primary particles in the polymer binder can be extended to increase the concentration of the binder but still achieve a phase inversion, leading to granulation. Because of the nature of the process, the resulting granules have a constant concentration of the filler (within (1%), irrespective of the granule size as shown in Figure 12, where the normalized filler concentration C/CF is plotted against particle size, D. Here, C is the filler concentration in each size range and CF is the mean filler concentration (filler concentration in the melt). FIPI-based processes often require purpose built equipment to achieve intensification. In a nonisothermal FIPI, the temperature is used as another process parameter which can be used to control the binder

concentration to obtain microcapsules for controlled release. The present processing equipment has been designed, manufactured, and used to explore the use of nonisothermal FIPI in particle technology. Essentially, the granulator consists of two disks, upper stator and rotor. These disks contain several cavities to achieve mixing and conveying both in melt and powder stages. Each set of cavities located at a constant radius represents a unit operation stage. There are 11 such stages on the upper rotor/stator disks. To extend the number of stages, the bottom part of the upper rotor also contains cavities operating against a set of cavities located on the lower stator. Visualization and the process fingerprinting indicate that these cavities pump the material in the melt stage and transport the granulated powder after crumbling. As the purpose of this study is to describe the equipment and the technique, several process variables have been kept constant. In all FIPI processes, due to the interaction between the fluid microstructure and flow field, it is necessary to characterize well the relevant properties of the fluids, in this case the thermal properties. The thermal characteristics of the binder (PEG) and binder/filler (calcium carbonate) particle system were determined by using DSC. The thermal process profile was set up using the DSC data. To achieve effective temperature control, the upper stator was kept at 52 °C while the lower rotor temperature was 45 °C. The polymer melt enters into the gap between the upper rotor and stator. The material follows a 3-D path, the relative values of the radial, angular, and axial velocities are determined by the applied pressure (developed by the extruder and the rotor cavities), angular rotation, and the cavity design. The fingerprinting of the process can be obtained by recording the extruder torque during the process. It was shown that the granulator is capable of pumping the polymer melt and transporting the granules after the phase inversion. The location of the phase inversion from a melt to granulated powder (crumbling) is determined as a function of polymer binder concentration when the melt flow rate and temperature profile are fixed. As expected, the phase inversion occurs early (after the melt enters into the granulator) if the filler concentration is high. After the phase inversion, the granulator works at atmospheric pressure due to the powder conveying facility. Immediately after the phase inversion, the average granule size is small but the particle size distribution is broad. However, within 3-4 rings of the cavities, the granule size becomes narrow and the average granule size approaches the gap size (GU) between the disks. The presence of granules with a size greater than the gap size indicates that the granules are transported through a pathway involving the rotor and stator cavities. Nevertheless, the mean granular size can be controlled by the setting of the clearance between the rotor and stator. 6. Conclusions A novel intensive continuous agglomeration/microencapsulation (granulation) technique and the equipment to achieve the process are described. The method is based on the nonisothermal flow-induced phase inversion phenomenon in which solidification takes place under deformation (flow). This process results in the transformation of the melt into granulated powder

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through a thermomechanically induced melt fracture process. The design of the granulator processor unit is such that melt mixing and pumping as well as transfer of the granulated particles are achieved, and once the phase inversion took place, the processor operates under atmospheric pressure. The granular size and size distribution reach their equilibrium values within a short distance after crumbling (phase inversion). The average granule size can be controlled through the control of the clearance between the rotor and stator. The technique produces granulated particles with a narrow size distribution in which the filler concentration remains constant within each granule size range. Acknowledgment This research was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the U.K., AstraZeneca, Carl Stewart Ltd., Syngenta Thermo Haake, and Rosand Precision Ltd. Their support is gratefully acknowledged. List of Symbols C ) concentration of filler in each granule size, mm CP ) concentration of polymer (PEG 10 000) in the melt, wt % CF ) concentration of calcium carbonate in the melt, wt % GU ) upper rotor/stator clearance, mm GL ) lower rotor/stator clearance, mm ∆Hc ) heat of crystallization, J/g ∆Hm ) heat of melting, J/g ΩS ) extruder screw speed, rpm ΩR ) rotor speed, rpm Particle Size Characterization D(10), D(50), D(90) ) diameters below which 10, 50, and 90% of the particles lie, respectively S ) particle size span (S ) [D(90) - D(50)]/D(50)) Temperatures Used in Characterization by DSC Tc (10%) ) crystallization onset temperature (10% solidifies), °C Tc (95%) ) crystallization completion temperature (95% solidifies), °C Tm (10%) ) melting onset temperature (10% melts), °C Tm (95%) ) melting completion temperature (95% melts), °C Tcp ) peak crystallization temperature, °C Tmp ) peak melting temperature, °C Temperatures Used in Process Characterization TE ) extruder exit temperature, °C TLS ) set temperature of the lower stator, °C TLR ) set temperature of the lower rotor, °C TUS ) set temperature of the upper stator, °C TUR ) set temperature of the upper rotor, °C

Literature Cited (1) Capes, C. E. In Handbook of Powder Technology; Williams, J. C., Allen, T., Eds.; Elsevier: Amsterdam, 1980; Vol. 1. (2) Stanley-Wood, N. G. Enlargement and Compaction of Particulate Solids; Butterworth: London, 1983. (3) Stanley-Wood, N. G. In Principles of Powder Technology; Rhodes, M., Ed.; Wiley: New York, 1990. (4) Kristensen, H. G.; Schaefer, T. In Encyclopaedia of Pharmaceutical Technology; Dekker: New York, 1992; Vol. 7. (5) Pietsch, W. Agglomeration. Chem. Eng. Prog. 1996, April, p 30.

(6) Sekiguchi, I. Granulation. In Powder Technology Handbook; Gotoh, K., Masuda, H., Higashitani, K., Eds.; Marcel Dekker: New York, 1997; Vol. 7. (7) Hsieh, D. S. T., Ed.; Controlled Release Systems: Fabrication Technology; CRC Press: Boca Raton, FL, 1988; Vol. 2. (8) Akay, G. Agglomerated Abrasive Material. U.S. Patent, 4,988,369, 1991; European Patent 307278, 1992; Japanese Patent 940 37634, 1994. (9) Akay, G. Coating Process. European Patent 382 464, 1992; Australian Patent 633 299, 1993. (10) Akay, G. Flow induced phase inversion agglomeration: fundamentals and batch processing. Polym. Eng. Sci. 1994, 34, 865. (11) Akay, G. Flow-induced phase inversion in powder structuring by polymers. In Polymer Powder Technology; Narkis, M., Rosenzweig, N., Eds.; Wiley: New York, 1998; Chapter 20. (12) Akay, G. Flow induced polymer filler interactions: bound polymer properties and bound polymer-free polymer phase separation and subsequent phase inversion during flow. Polym. Eng. Sci. 1990, 30, 1361. (13) Akay, G. Stable oil-in-water emulsions. European Patent 0649 867, 2001. (14) Akay, G. Flow induced phase inversion in the intensive processing of concentrated emulsions. Chem. Eng. Sci. 1998, 53, 203. (15) Akay, G.; Garrett, P. R.; Yorke, J. W. H. Antifoam ingredient. European Patent 593441, 1994. (16) Akay, G.; Chapple, A.; Garrett, P. R.; Knight, P. C.; Yorke, J. W. H. Detergent powder and process for preparing them. Australian Patent, 651 732, 1994. (17) Chan, S. Y.; Burbidge, A. S.; Akay, G. Intensive agglomeration and microencapsulation of powders. In IChemE Research Event - 1998, Newcastle; IChemE CD-Rom; Institute of Chemical Engineers: Rugby, U.K., 1998. (18) Akay, G.; Price, V. J.; Chan, S. Y. Flow induced phase inversion in the intensive agglomeration/microencapsulation of powders. In Recent Advances in Particle Technology; IChemE CDRom; Institute of Chemical Engineers: Rugby, U.K., 1998. (19) Akay, G.; Tong, L. Intensive structuring: intensive processing of microstructured materials by flow induced phase inversion. Chem. Eng. Technol. 2000, 23, 285. (20) Akay, G.; Tong, L. Polymer dispersion preparation by flow induced phase inversion emulsification. J. Mater. Sci. 2000, 35, 3699. (21) Akay, G.; Tong, L. Preparation of colloidal low-density polyethylene latexes by flow-induced phase inversion emulsification of polymer melt in water. J. Colloid Interface Sci. 2001, 239, 342. (22) Akay, G.; Tong, L.; Hounslow, M. J.; Burbidge, A. S. Intensive agglomeration and microencapsulation of powders. Colloid Polym. Sci. 2001, 279, 1118. (23) Akay, G.; Irving, N.; Kowalski, A. J.; Machin, D. Process for the production of a liquid composition. European Patent Application, 94309 768.3, 1994. (24) Akay, G.; Downes, S.; Price, V. J. Microcellular polymers as cell growth media and novel polymers; PCT International Publication, WO, 00/34454, 2000. (25) Akay, G. Rheology of reinforced thermoplastics and its application to injection molding: IV. Transient injection capillary flow amd injection molding. Polym. Eng. Sci. 1982, 22, 1027. (26) Akay, G. Unsteady capillary flow of reinforced polymer melts. J. Non-Newtonian Fluid Mech. 1983, 13, 309. (27) Akay, G. The rheology and injection moulding of reinforced and liquid crystalline polymers. In Developments in Injection Moulding-3; Whelan, A., Goff, J. P., Eds.; Elsevier Applied Science Publishers: London, 1985; Chapter 7. (28) Akay, G. Rheological properties of thermoplastics. In Encyclopedia of Fluid Mechanics; Cheremisinoff, N. P., Ed.; Gulf publishing: Houston, 1986; Vol. 1, Chapter 35.

Received for review February 8, 2002 Revised manuscript received July 17, 2002 Accepted July 17, 2002 IE0201213

Process Intensification in Particle Technology: Intensive Granulation of

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