Implementation of an online thermal imaging to study the effect of process parameters of roller compactor
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During roller compaction, not only the properties of the primary powder affect the product quality but also the process parameters. Any change in the process parameters during roller compaction will result in changing the properties of the ribbon. In this study, the temperature of the ribbon during production was monitored online using a thermal camera. The information from the thermal camera was used to explain the differences in ribbon properties at varying process parameters. Lactose powder was used as a primary powder, and ribbons were produced at different process parameters. The surface temperature of the ribbon during production was found to increase with increasing both the gap between the two rollers and the roller speed. This was attributed to the screw feeder speed, which increased to feed additional powder as required to adjust to the change in process parameters. Increasing the roller gap resulted in wider ribbons and decreased the percentage of fines in the product, which was a signature of better powder distribution across the roller width. The results were also supported by the uniform temperature distribution recorded across the ribbon width. It was found that increasing the roller speed during roller compaction decreased the width of the ribbon while increasing the percentage of fines in the product. The feeder screw speed was found to have a similar effect as the roller gap.
KeywordsRoller compaction Online thermal imaging Process parameters Lactose
Roller compaction is a dry granulation technology in which high pressure is used to compress the primary powder particles into ribbons, which are then milled to granules of different size classes. The roller compaction process consists of three units working simultaneously: feeding, compaction and milling unit. The feeding unit provides and transports the powder to the compaction unit using different methods (gravity or screw feeder system). The compaction unit consists of two counter-rotating rollers which apply high stress on the powder . The ribbon is then used to produce granules with the desired size after passing through a milling step.
During the roller compaction, not only the primary powder properties affect the product properties but also the process parameter during the ribbon production. There are few process parameters in the roller compactor which could be changed during production of compacts and granules. The compaction pressure, feeder screw speed, roller speed and the gap between the two rollers are the process parameters which affects the properties of ribbon and granules.
Compaction pressure is the most significant parameter that affects ribbon properties, as the compaction pressure determines the stress acting on the powder during compression. Increasing the compaction pressure increases the strength and the density of ribbon and granules [2, 3, 4, 5, 6, 7, 8]. Roller speed, screw speed and the gap between the rollers were also found to affect the quality of ribbon produced in the roller compactor [5, 6, 9, 10]. Roller speed, screw speed and the gap between the rollers were also found to affect the quality of ribbon produced in the roller compactor [5, 6, 9, 10].
The effect of process parameters on product quality has been investigated in the literature using different types of lactose , different types of MCC  and maize . Process parameters were represented by the compaction pressure and the speed of the horizontal and vertical screws, while the product quality was represented by the granule friability. Lower granules friability was suggested to represent better quality of the product. It was found that the best product quality (low friability) for all materials was achieved by operating the roller compactor at high compaction pressure. The effect of the horizontal screw speed was different for different powders; the best quality of the product was obtained using low horizontal screw speed for lactose  and high horizontal screw speed for both MCC and maize [6, 9]. This could be due to the difference in flow properties of the powders.
The speed of the roller, at constant feeder screw speed, affects the amount of powder between the rollers and also the dwell time of the powder in the compaction zone which may affect the strength of the ribbon. Increasing the roller speed, by keeping both horizontal and vertical screw speed constant, was found to decrease the force required to break the ribbon, consequently, decreasing ribbon tensile strength and density [3, 4, 10]. This was due to the fact that at constant feeder screw speed, an increase in the roller speed decreased the amount of powder in the compaction zone; therefore, ribbons were thinner, lesser in width and easier to break. However, operating the roller compactor at constant (roller speed/horizontal feed screw/vertical feed screw constant at 1:5:25) ratios was found to have no effect on ribbon thickness, width, strength and density . This was because the amount of powder between the rollers was always the same; therefore, ribbons with similar thickness and width were produced. An increase in the roller speed was also found to decrease the nip angle of MCC [11, 12] and Di-calcium phosphate dihydrate (DCPD) .
The gap between the two rollers is one of the process parameters which can be changed during the roller compaction. In the roller compactor, one of the rollers is movable while the other is stationary; the movable roller helps to fix the gap between the rollers. The gap determines the thickness of the produced ribbon, which may affect ribbon strength and porosity. Bindhumadhavan et al.  attempted to investigate the effect of the gap between the rollers on the behaviour of microcrystalline cellulose undergoing roller compaction using a gravity feeding system. They examined the effect of the roller gap on the maximum pressure applied to the powder during compaction. The pressure applied to the powder during roller compaction was found to decrease with increasing the gap between the two rollers. This could be due to the fact that increasing the gap between the rollers increases the amount of the powder in the gap region and reduces the applied stress on the powder (distributing the force acting on the powder over a wider thickness).
Miguélez-Morán et al.  investigated the effect of the gap between the rollers on the ribbon relative density. Microcrystalline cellulose MCC was used as a model powder, and ribbons were produced at two different gap settings. It was found that the average relative density of the ribbon was increasing with decreasing the gap between the rollers. This was due to the higher stress which was applied to the powder at lower roller gap; this is in agreement with the finding of Bindhumadhavan et al. .
During the roller compaction, the powder undergoes friction, deformation and fragmentation which then result in an increase in the product temperature. The product temperature was reported to increase with increasing the pressure during the roller compaction [2, 7, 16]. In these studies, a thermal camera was used to record the temperature of ribbon of different materials produced at different pressure. It was stated that different material exhibited varying temperature due to the differences in the behaviour of powders undergoing roller compaction. The higher temperature of ribbon at elevated pressure was attributed to the higher degree of deformation, fragmentation and friction between the particles. The online thermal imaging of ribbon was shown to be a useful technique to investigate the powder behaviour in the roller compaction . The fact that the thermal camera can be used as an online technique is useful in monitoring the behaviour of the powder while roller compacted in real time. This means that there is a potential of using the feedback from this technique to control the product quality in real time by altering the roller compaction process parameters.
Operating the roller compactor at different process parameters affects both the product properties and the production rate. Therefore, it is essential to investigate the effect of the process parameters during the roller compaction process. In the current work, a detailed study will be performed to investigate the effect of the roller compaction process parameters on the product properties using lactose as a model powder. An online thermal imaging will be implemented to further understand the behaviour of the powder during the roller compaction at different process parameters.
Material and methods
The lactose powder used in this study were equilibrated prior to compaction at 25 °C and 20% RH for 3 days using a Binder KMF 240 climatic chamber (Binder, Germany) . The equilibration of powder in the climatic chamber is faster than in a desiccator, due to the ability to control the air flow and temperature in the climatic chamber. Powders were spread in a thin layer in a plastic tray to increase the surface area of the powder.
An Alexanderwerk WP120 (Alexanderwerk, Germany) roller compactor was used for the compaction of the lactose powder in this study. The WP120 has three systems working simultaneously: the feeding system, compaction system and milling system. The feeding system consists of a hopper for the powder and a horizontal screw feeder, which transports the powder from the hopper to the compaction system. Before the compaction system, on the screw feeder, there is a de-aeration system which takes the air out of the powder and improves powder flowability. The compaction system consists of two counter-rotating rollers of 120 mm in diameter and 40 mm width with knurled surfaces. The two rollers apply high pressure on the powder using a hydraulic system, which gives a hydraulic pressure in the range of 18–230 bar. The time of compression can be controlled by changing the speed of the rollers from 3 to 13 rpm. The thickness of the ribbon can be defined by the gap between the two rollers which can be changed between 1 and 4 mm. There are two cheek plates on both sides of the rollers used to reduce the leakage of the powder during compression.
To eliminate the effect of humidity in the laboratory on the powder relative humidity, a GenRH-A (London, UK) humidity generator was connected to the top of the hopper on the roller compactor. The humidity generator was operated at the same conditions as the climatic chamber; this ensured a constant humidity condition of the powder during roller compaction same as that used for the storage of the powder prior to compaction.
During the roller compaction, an amount of fine (un-compacted powder) are produced, this is a main disadvantage of the roller compaction process. The amount of fines was defined as the amount of un-compacted powder (particles which are smaller than the d90 of the original powder) which is produced along with the ribbon. To determine the amount of fines, the entire product (ribbon and fines) was collected for a set period of time during the operation of the roller compactor. Then, the ribbons were separated from the fines by sieving (Retsch GmbH, Germany) at 0.2 mm amplitude for 1 min. From the total mass of the product and the mass of fines, the weight percentage of fines was determined.
The width of the ribbons was measured using a digital calliper. A total of ten ribbons were selected randomly, and the width was measured for each of them. The average of the ten values was then determined at each condition.
X-ray images of ribbons produced from the roller compaction were obtained using micro-CT 35 (Scanco Medical AG, Switzerland). Samples of ribbon of lactose were attached to a sample holder. The X-ray beam was operated at a voltage of 45 kV, a current of 177 μA and a power of 8 W. The voxel size used was 3.5 μm.
Images from the X-ray machine were then analysed using Image J software to determine the porosity of the ribbon. The black pixels in the X-ray images indicate the air (pores), and the white pixels indicate the powder. The number of black pixels (air) was determined and divided by the total number of pixels (air and powder); this ratio represents the porosity of the ribbon.
Results and discussion
Effect of the roller speed
The speed of the roller controls the dwell time of the powder between the two rollers and also the amount of powder between the rollers which may affect the strength of the ribbon. In order to examine the effect of the roller speed, the powder was compacted at a constant pressure of 50 bar and using different speeds of the roller (3, 4, 5 and 6) rpm. The gap between the rollers was kept constant at 3 mm by using the automatic feedback system which alters the speed of the feeder screw to maintain a constant gap.
Feeder screw speed at different roller speed
Roller speed (rpm)
Feeder screw speed (rpm)
Effect of the gap between the rollers
The gap between the two rollers is one of the process parameters which can be changed during the roller compaction. Increasing the gap during the production of ribbon increases the amount of powder between the rollers and results in thicker ribbon. The increase or decrease in the amount of powder between the two rollers may affect the stress applied to the powder at specific roller force. Previous work has been carried out to investigate the effect of the roller gap on the nip angle and the maximum pressure applied to the powder [14, 23]. It was found that the nip angle increased with increasing the roller gap during the roller compaction of MCC . This may affect the properties of the produced ribbon.
In this section, the effect of varying the roller gap on ribbon properties was studied using lactose. The roller compactor was operated at a fixed hydraulic pressure of 50 bar, the roller speed was set to 3 rpm and the gap between the rollers were (1.5, 2, 3 and 4) mm. To control the set gap, the feeder screw speed was varying automatically using the feedback system which adjusts the feeder speed to maintain the set gap.
The lower percentage of fines and the wider ribbon produced at high roller gap is due to a better powder distribution across the width of the roller. Increasing the gap size between the two rollers means more space for the powder to spread to the edges of the rollers. The better distribution of the power to the edges at larger roller gap could improve the stress distribution across the width of the roller. The stress across the roller width is more uniform when larger gap size is used during the compaction. This results in a wider ribbon and therefore a lower percentage of fines in the product.
Effect of the feeder screw speed
In roller compaction, a screw feeder is, sometimes, used to transport the powder from the hopper to the compaction zone. A change in the feeder screw speed (at constant pressure and roller speed) results in changing the gap between the rollers. In the previous sections, the gap between the rollers was controlled using the automatic feedback system which adjusts the feeder screw speed to maintain the gap constant; therefore, the screw speed will change automatically. In order to investigate the effect of the feeder screw speed, the powder was compacted using a fixed hydraulic pressure of 50 bar, a roller speed of 3 rpm and screw feeder speed of 20, 40, 60 and 80 rpm.
At a larger gap, the powder will have more space to distribute evenly across the width of the roller. The uniform distribution of the powder across the width of the roller is believed to result in a uniform stress being applied to the powder across the roller width. This will result in ribbons with a higher width (see Fig. 20) and reduce the percentage of the fines in the product (see Fig. 19).
The effect of the process parameters of the roller compaction was investigated. An online thermal imaging was employed to determine the maximum temperature of the ribbon during production at various process variables. The amount of fines, width, strength and porosity of ribbon were used to evaluate the product quality. It was found that increasing the roller speed during the roller compaction decreases the width of the ribbon, which consequently increases the fine percentage in the product. However, the speed of the rollers was found to have a minor effect on the tensile strength and porosity of the ribbon. This is an attractive finding, as it proposes the possibility of increasing the production rate of ribbon of lactose powders with increasing the roller speed, knowing that this will not affect the ribbon strength and porosity.
The gap between the two rollers and the screw feeder speed was also found to affect the powder compaction behaviour and ribbon properties. The temperature of the ribbon during production was found to increase with increasing both the gap size between the rollers and the screw feeder speed. An increase in the roller gap means more space for the powder to move and distribute across the width of the rollers. The better distribution of the powder across the width of the roller at larger gap size resulted in a uniform temperature distribution and, consequently, in wider ribbons and a lower percentage of fines. The tensile strength of ribbon decreased, and the porosity increased with increasing the roller gap and screw speed.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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