Comparison of video analysis and simulations of a drum coating process

Tablet coating is a common unit operation in the pharmaceutical industry. To improve currently established processes, it is important to understand the influence of the process parameters on the coating quality. One of the critical parameters is the tablet velocity. In this work, numerical results are compared to results obtained experimentally. Tablet movement in the drums was simulated using the Discrete Element Method (DEM). The simulation parameters were adapted to fit the simulation to the experimental data. A comparison of the experimental and simulation results showed that the simulation correctly represents the real tablet velocity. A change in the velocity over time and its dependence on the rotation rates and the baffle position in the simulation were similar to the experimental results. In summary, simulations can improve the understanding of tablet coating processes and will thus provide insights into the underlying process mechanics, which cannot be obtained via ordinary experiments.

Simplified formulations with high drug loads for continuous twin-screw granulation

As different batches of the same excipients will be intermixed during continuous processes, the traceability of batches is complicated. Simplified formulations may help to reduce problems related to batch intermixing and traceability. Twin-screw granulation with subsequent tableting was used to produce granules and tablets, containing drug, disintegrant and binder (binary and ternary mixtures), only. Drug loads up to 90% were achieved and five different disintegrants were screened for keeping their disintegration suitability after wetting. Granule size distributions were consistently mono-modal and narrow. Granule strength reached higher values, using ternary mixtures. Tablets containing croscarmellose-Na as disintegrant displayed tensile strengths up to 3.1MPa and disintegration times from 400 to 466s, resulting in the most robust disintegrant. Dissolution was overall complete and above 96% within 30 min. Na-starch glycolate offers tensile strengths up to 2.8MPa at disintegration times from 25s to 1031s, providing the broadest application window, as it corresponds in some parts to different definitions of orodispersible tablets. Tablets containing micronized crospovidone are not suitable for immediate release, but showed possibilities to produce highly drug loaded, prolonged release tablets. Tablets and granules from simplified formulations offer great opportunities to improve continuous processes, present performances comparable to more complicated formulations and are able to correspond to requirements of the authorities.

Production of pellets via extrusion-spheronisation without the incorporation of microcrystalline cellulose: a critical review

Microcrystalline cellulose (MCC) is the golden standard to manufacture spherical particles (pellets) via extrusion-spheronisation since wetted microcrystalline cellulose has the proper rheological properties, cohesiveness and plasticity to yield strong and spherical particles. However, microcrystalline cellulose is not universally applicable due to a number of limitations: prolonged drug release of poorly soluble drugs, chemical incompatibility with specific drugs, drug adsorption onto MCC fibers. Hence, several products have been evaluated to explore their application as extrusion-spheronisation aid, aiming to avoid the disadvantages of MCC and to provide a broad application platform for extrusion-spheronisation: powdered cellulose, starch, chitosan, kappa-carrageenan, pectinic acid, hydroxypropylmethyl cellulose, hydroxyethyl cellulose, polyethylene oxide, cross-linked polyvinylpyrrolidone, glycerol monostearate. To determine the true potential of the proposed alternatives for MCC this review critically discusses the properties of the different materials and the quality of the resulting pellets in relation to the properties required for an ideal extrusion-spheronisation aid.

Comparison of two twin-screw extruders for continuous granulation

A comparison was made between two twin-screw extruders (APV Baker and Leistritz Micro) used for continuous wet granulation. Both extruders had similar screw configurations, based on the length-to-diameter ratio of the screws, existing out of a conveying zone of 20 D, i.e. 20 times the screw diameter and a granulation zone of 4D. The kneading blocks in the granulation zone were 2.2 and 2.5D for the Leistritz and APV extruders, respectively. An experimental design was used to investigate the influence of process parameters (total input rate and screw speed) and extruder type on granule and tablet quality. Dicalcium phosphate and alpha-lactose monohydrate were used as water-insoluble and water-soluble excipients, respectively. For dicalcium phosphate, the amount of fines (<125 microm), median granule size and granule friability were significantly influenced by extruder type and total input rate. For lactose, the amount of oversized agglomerates and median granule size were significantly affected by extruder type and total input rate. The granule formulations were properly agglomerated on both the extruders, although the extruder type had an important influence on the granule properties, which was more pronounced for dicalcium phosphate. This study shows that a given formulation cannot simply be interchanged between the two extruders without further work on the geometrics of the extruders.

Pretreatment of pigments to prepare liquids for enteric coating

Film coating fluids commonly contain different pigments. The objective of this work was a study of the distribution of these particles in the coating film. Different pretreatment forms (pigment suspension, pigment paste and untreated pigments) were applied. They were incorporated into a Eudragit L 30 D-55 dispersion. The surface roughness and the mechanical properties of the free films indicated, that the most homogeneous film was obtained with the pigment paste. The homogeneity of the film was investigated by mechanical testing. The protective effect of the coating did not vary with the application of pigments in different forms, but the appearance of the coated tablets underwent a considerable change.

Interaction of water with different cellulose ethers: a Raman spectroscopy and environmental scanning electron microscopy study

Different non-ionic cellulose ethers like methyl cellulose (MC), hydroxypropyl cellulose (HPC) and hydroxypropylmethyl cellulose (HPMC) were investigated. The characterization of the cellulose ethers was carried out by thermogravimetry and sorption/desorption isotherms. Differences in the properties of the cellulose ether films were described by time-dependent contact angle measurements. Changes in molecular structure of the raw materials, gels and films caused by water contact were studied using Raman spectroscopy. Differences between the substitution types and changes due to the gel or film formation were observed. An environmental scanning electron microscopy (ESEM) technique was used to distinguish the morphological behaviour of the cellulose ether films in contact with water. Based on in-situ ESEM experiments, the swelling and drying behaviour of the various stages of cellulose ether films (film-hydrated film-dried film) were quantified by using image analysis.

Polymer erosion in PLGA microparticles produced by phase separation method

This article deals with polymer erosion in biodegradable microparticles produced using the phase separation method. Poly(lactic-co-glycolic acid) copolymers with different compositions and molecular weights were employed. The microparticles were stored in phosphate buffer for 6 months. The molecular weight of the polymers was determined by size exclusion chromatography, and the weight loss was monitored gravimetrically. No weight loss was measured in the first weeks, although the molecular weight decreased significantly already from the start. After a certain storage period which was found to be specific for the type of polymer, the weight of the microparticles decreased rapidly. The start of this weight loss occurred when the molecular weight of the polymer in the degrading microparticles reached a threshold of approximately 15,000. This critical molecular weight was found to be identical for all investigated polymers, i.e. it was independent of the initial molecular weight of the polymer and of the lactic-glycolic ratio.

Extrusion/spheronization of pectin-based formulations. II. Effect of additive concentration in the granulation liquid

PURPOSE

The aim of this study was to improve the formation of spherical pectin pellets by investigating the effect of additive concentration in the granulation liquid on the shape and size of the products as well as by identifying an optimal additive concentration.

METHODS

High-methoxylated, low-methoxylated, and amidated low-methoxylated pectin types were evaluated in combination with different concentrations of methanol, ethanol, citric acid, lactic acid, and calcium chloride. Pellets were prepared in a power-consumption-controlled twin-screw extruder, then spheronized and dried. The moisture content of the extrudate was determined, and the final products were characterized by image analysis and sieving analysis. A cloud point test was employed for the identification of an optimal additive concentration.

RESULTS

The concentration of additive in the granulation liquid affected the moisture content of the extrudate and the shape, size, and mechanical stability of the pectin pellets. Improvements in the pellet characteristics are dependent on the pectin type employed. The 2 low-methoxylated pectins were more sensitive to concentration changes than was the high-methoxylated type. Above a certain threshold concentration, the quality of the pellets are improved. This additive concentration differs according to type of pectin and type of additive.

CONCLUSION

It was demonstrated that there is a concentration-dependent interaction between pectin and substances added to the granulation liquid that can be utilized to improve the formation of spherical pectin pellets.

Extrusion/spheronization of pectin-based formulations. I. Screening of important factors

This study investigated the possibility of producing pectin-based pellets by extrusion/spheronization. The study also identified factors influencing the process and the characteristics of the resulting product. Three types of pectin with different degrees of amid and methoxyl substitution were studied in combination with different granulation liquids (water, calcium chloride, citric acid, and ethanol) and/or microcrystalline cellulose. Pellets were prepared in a power-consumption-controlled, twin-screw extruder; then they were spheronized and dried. The products were characterized by image analysis, sieving analysis, and disintegration and dissolution tests. The results were evaluated by multivariate analysis. Different additives, either in the granulation liquid or in the powder mixture, influenced the ability of the extruded mass to form pellets (the processability) with this technique. However, the various pectin types responded to modifications to a different extent. Short, nearly spherical pellets are obtained with granulation liquids, such as ethanol, that reduce the swelling ability of pectin. Pellets produced with ethanol are, however, mechanically weak and tend to disintegrate. Pectin molecules with a high degree of free carboxylic acid groups seem to be more sensitive to changes in the granulation liquid. Addition of microcrystalline cellulose as an extrusion aid generally resulted in improvements in shape and size. It was demonstrated that the processability of pectin as well as the characteristics of the products can be influenced in different ways during the process (eg, adding substances to the granulation liquid or to the powder mixture).

Extrusion/spheronization of pectin-based formulations. I. Screening of important factors

This study investigated the possibility of producing pectin-based pellets by extrusion/spheronization. The study also identified factors influencing the process and the characteristics of the resulting product. Three types of pectin with different degrees of amid and methoxyl substitution were studied in combination with different granulation liquids (water, calcium chloride, citric acid, and ethanol) and/or microcrystalline cellulose. Pellets were prepared in a power-consumption-controlled, twin-screw extruder; then they were spheronized and dried. The products were characterized by image analysis, sieving analysis, and disintegration and dissolution tests. The results were evaluated by multivariate analysis. Different additives, either in the granulation liquid or in the powder mixture, influenced the ability of the extruded mass to form pellets (the processability) with this technique. However, the various pectin types responded to modifications to a different extent. Short, nearly spherical pellets are obtained with granulation liquids, such as ethanol, that reduce the swelling ability of pectin. Pellets produced with ethanol are, however, mechanically weak and tend to disintegrate. Pectin molecules with a high degree of free carboxylic acid groups seem to be more sensitive to changes in the granulation liquid. Addition of microcrystalline cellulose as an extrusion aid generally resulted in improvements in shape and size. It was demonstrated that the processability of pectin as well as the characteristics of the products can be influenced in different ways during the process (eg, adding substances to the granulation liquid or to the powder mixture).

A simple method for evaluating the mixing efficiency of a new type of pan coater

The objective of this study was to investigate some important process parameters on the mixing behavior of a new coater type. The coater used in this study, the Bohle BLC pan coater, differs from other coaters in its high length to diameter ratio. The pan coater can be divided into two zones: drying and spraying zone. The temperature difference between two points in the pan (one in each zone) was used to explore the influence of some process parameters, i.e. the pan speed and the inclination of the rotation axis on the mixing behavior. In addition, the effect of the spray rate on the temperature difference was studied. The results of the current study demonstrated the possibility to characterize the mixing behavior in a pan coater by a simple temperature measurement. The temperature difference method resulted in a model, which was comparable to the model from a conventional mixture experiment. It was shown that the pan speed and the spray rate influenced the temperature difference and, consequently, the mixing efficiency. However, the inclination of the rotation axis did not show an important effect.

Raman spectroscopic surface characterization of cellulose derivatives

First results of experiments on the surface characterization of cellulose derivatives are presented. Different water contents of the surface of microcrystalline cellulose have been investigated by means of Raman spectroscopy, SERS, and environmental scanning electron microscopy (ESEM).

Melt pelletization of a hygroscopic drug in a high shear mixer. Part 3. Effects of binder variation

Melt pelletization experiments with sodium valproate as a hygroscopic drug were performed in a laboratory scale high shear mixer. In the current part, the effect of altering the binder liquid properties (using different binders, varying the temperature, or adding highly dispersed silicon dioxide to the molten binder) on the pellet size, size distribution and the growth rate was studied. Three meltable binders, namely glycerol monostearate (GMS), hydrated castor oil (HCO), and polyethylene glycol (PEG), were included in the study. Two series of experiments with GMS or HCO showed a higher granule growth rate with decreasing binder viscosity. Also, increases in the granule growth rate were observed for all meltable binders tested, when the binder amount and the impeller speed were increased. Factorial designs with all three binders were performed under the same conditions. In these experiments, no correlation existed between the granule growth rate and the viscosity of the different binders. The different granule growth rate, however, was mainly attributed to the different solubility of sodium valproate in the binder liquid used. Higher solubility increased the volume of the binder liquid and, accordingly, the granule growth rate. Taking the amount of dissolved drug into account, the granule growth rates of GMS and PEG were comparable. However, HCO displayed a lower granule growth rate, which might be related to its low adhesion tension. During melt pelletization in a high shear mixer the solubility of the drug in the molten binder strongly influences the pelletization process.

Direct pelletization in a rotary processor controlled by torque measurements. II: effects of changes in the content of microcrystalline cellulose

In the present study we investigated the effect of changes in the content of microcrystalline cellulose (MCC) on a direct pelletization process in a rotary processor in which the liquid addition was terminated once a certain increase in torque was produced. Nine different mixtures of MCC and lactose with MCC contents varying from 10% to 100% (w/w) were pelletized using 6 different torque increase levels, and the changes in pellet characteristics were investigated. The pellet characteristics investigated were pellet shape, size, and size distribution as well as the water content of the pellets at the end of liquid addition. To produce spherical agglomerates with suitable characteristics in a reproducible way, a content of at least 20% (w/w) MCC was found necessary. Linear correlations were found between the MCC content and the water content and between the torque increase and the water content, showing that the torque increase is suitable to control the process. A higher torque increase or a higher MCC content was found to increase the water content independently of each other.

Direct pelletization in a rotary processor controlled by torque measurements. I. Influence of process variables

The aim of this study was to elucidate the feasibility of using torque measurement to control the end point of a wet pelletization process in a rotary processor at varying levels of friction plate rotation speed, air gap pressure difference, and batch size. A 1:1 mixture of lactose monohydrate (200 mesh) and microcrystalline cellulose (PH-101) was granulated into pellets in an instrumented laboratory scale rotary processor using water as aqueous binder liquid. A full factorial designed study was performed to investigate the influence of the friction plate rotation speed (600 and 1200 rpm), the air gap pressure difference (1 and 3 kPa), the torque increase (0.4 and 0.8 N.m) and the batch size (500 and 1000 g) on the pellet properties. All pellets produced were round and showed a narrow size distribution. The geometric mean diameter varied from 400 to 1900 microns with a good reproducibility. Increasing the batch size and the rotation speed led to smaller pellets, whereas a higher torque produced larger pellets. This study showed that the process can be controlled by means of the torque increase because it was possible to produce pellets of a reproducible size by stopping the liquid addition at a certain torque level.

Influence of the degree of polymerization on the behavior of cellulose during homogenization and extrusion/spheronization

The study objective was to investigate the influence of the degree of polymerization (DP) of cellulose materials (microcrystalline cellulose [MCC] and powder cellulose [PC]) on the behavior of these materials during homogenization and extrusion/spheronization processes. Suspensions of the cellulose types with different DP values were homogenized using a high-pressure homogenizer. The particle size, agglomeration index, and apparent viscosity of these suspensions was determined at different times after pouring. Additionally, these different cellulose types were processed into pellets using the extrusion/spheronization method, and the water content and power consumption as a function of the DP were determined. Cellulose types with a high DP value showed greater particle size after homogenization than the types with a low DP value. In contrast, no relevant relationship between the apparent viscosity and DP could be observed. During the extrusion process, water content in the extrudate and pellet porosity were increased as the DP was increased for the extrudates produced at the same level of power consumption. MCC types with various DPs compared with PC provided a novel way of understanding the role of cellulose in the extrusion process. The DP showed a remarkable influence on the physicochemical properties of the cellulose materials and, consequently, on the behavior of these materials during the extrusion/spheronization process. It is postulated that the sponge model is more appropriate for the cellulose type with high DP (PC), whereas the gel model is more applicable to cellulose types with lower DP (MCC).

Melt pelletisation of a hygroscopic drug in a high shear mixer. Part 2. Mutual compensation of influence variables

The process of melt pelletisation in a Diosna P10 high shear mixer was examined for sodium valproate and glycerol monostearate. The effects of binder concentration, impeller speed and massing time on mean granule size, size distribution and liquid saturation were investigated. Spherical pellets of almost similar size and size distribution were obtained after 20 min of massing time, with a binder content from 3.1 to 14.1% w/w by adjusting the impeller speed. Granule growth was observed at low levels of binder concentration and liquid saturation (<80%) which is untypical for melt granulation. The liquid saturation seemed to have no major influence on the final pellet size. Additional, mutually compensating effects on granule growth were found to be impeller speed and massing time for a fixed binder concentration. Low levels of both, binder concentration and impeller speed, allowed for good control of the process. The amount of water adsorbed by the hygroscopic drug was found to accelerate granule growth.

Melt pelletisation of a hygroscopic drug in a high shear mixer. Part 1. Influence of process variables

The applicability of a high shear mixer for melt pelletisation of binary mixtures of sodium valproate and glycerol monostearate was investigated. The effects of binder concentration, impeller speed, jacket temperature and massing time on mean pellet size and size distribution were examined in a 2(4)-factorial design. Binder concentration and impeller speed were found to be the most important variables influencing the mean granule size and size distribution. An increase in each of those accelerated the granule growth. Due to the solubility of the drug in the molten binder very low amounts of binder were necessary for the formation of pellets. The modified high shear mixer was found to be suitable for batch sizes of 1-4 kg; granule growth was delayed with increasing load. A common pellet growth pattern, which can be divided into three phases, was derived and confirmed from all trials. The process was monitored by means of a torque measuring system. The torque-time curve can be used to detect the beginning of the destruction phase.

Importance of the fraction of microcrystalline cellulose and spheronization speed on the properties of extruded pellets made from binary mixtures

The aim of the study was to prove the importance of the binary mixture composition and spheronization speed on pellet properties. Extrudates from different binary mixtures of microcrystalline cellulose (MCC) and dicalcium phosphate dihydrate were prepared with a power-consumption-controlled extruder and spheronized at different speeds. The water content of the extrudate for the production of spherical pellets was evaluated. The pellets were characterized in terms of size, shape, porosity, mechanical properties, and disintegration. The fraction of MCC in the binary mixtures had the highest impact on the pellet properties. With an increasing fraction of MCC more water was required for successful pelletization, size and porosity of the pellets decreased, and the surface tensile stress increased. These observations were evaluated using the "sponge" and the "crystallite--gel" models for MCC. The latter led to the conception that an extrudate consists of two phases: a percolating crystallite--gel phase formed by MCC and water during extrusion and a filler phase formed by the second component of the binary mixture. This two-phase concept provides explanations for the extent of shrinking during drying and for the disintegration behavior. The spheronization speed had an influence on the size but not on porosity or surface tensile stress of the pellets. The best results were obtained at intermediate spheronization velocities of 10 and 13.4 m/sec. Fundamental properties of extrudates and pellets can be described by applying a two-phase concept of the crystallite--gel model.

Mixture experiments with the oil phase of parenteral emulsions

The effects of the oil phase as a mixture (binary, ternary) on the emulsion droplet size were investigated. The binary trials were performed with the aid of simplex lattice design with constraints. Droplet diameter was evaluated in terms of the oil phase viscosity and the interfacial tension between oil phase and the aqueous phase. As a result it could be shown that increasing the oil phase viscosity as a function of castor oil concentration led to a greater increase in particle size. At the same time, decreasing the interfacial tension of the oil phase as a function of oleic acid or oleic alcohol was shown to have a negligible effect on the particle size of the dispersed phase. A further aim was to find out a formulation by using a ternary oil phase resulting in a stable emulsion which could pass the autoclaving process. It was ascertained that oleic acid as a part of the oil phase led to proper formulation showing a satisfactory stability.

Comparison between a twin-screw extruder and a rotary ring die press. Part II: influence of process variables

The influence of different processing steps on pellet quality was investigated: granulation/extrusion and spheronization. Pellets were produced at two levels of water content on two different types of extruders: a twin screw extruder and a rotary ring die press. In order to control the spheronization process each extrudate was rounded in two spheronizers using two radial velocities, respectively. Pellet shape and size were selected to describe the pellet quality. Under constant spheronization conditions the extrudates behaved dissimilar on the two spheronizers. This could be attributed to the geometry of the friction plates. The spheronizer with the rougher surface applied more mechanical energy to the extrudate and wet pellets which reduced the water content necessary for the formation of good pellets. Eliminating the influence of the spheronization process, high differences were observed in the quality of the extrudates produced by the two extruders. This confirmed the results from the first part of this study. Due to the crystallite-gel-model the different extruder types apply different mechanical stress on the extrudate which affect the network structure of the microcrystalline cellulose gel. The twin-screw extruder produced a more delicate network with a lower water movement. This led to a shift of the optimal moisture content towards higher values. Compared with spheronization, process changes in the granulation/extrusion process were more critical.

The crystallite-gel-model for microcrystalline cellulose in wet-granulation, extrusion, and spheronization

PURPOSE

A new model for the wet-extrusion/spheronization process with microcrystalline cellulose (MCC) is proposed. The crystallite-gel-model is able to elucidate the unique role of MCC in this process. Many other experimental results, which cannot be explained by the standard model of granulation, the liquid saturation model, give evidence for the crystallite-gel-model.

METHODS

Pellets were prepared from different types of MCC. Water content during extrusion, power consumption and aspect ratio were correlated. X-ray diffractograms of MCC powders, extrudates and pellets were taken in order to provide information on changes at the single crystallite level. SEM-photographs and leaching studies gave additional information on changes at the particulate level of MCC.

RESULTS

At the level of MCC powder particles, dramatic changes occurred during extrusion/spheronization. In contrast to this no changes could be observed at the level of individual crystallites.

CONCLUSIONS

During granulation and extrusion MCC-particles are thought to be broken down into smaller particles and possibly ultimate single crystallites in the presence of water. The crystallite-gel-model serves as the framework for a new interpretation of the wet-extrusion/spheronization process. Apart from the ability to explain experimental data published previously in the literature it can be used to develop new experimental plans for further research. Consequently, the crystallite-gel-model exhibits explanatory as well as predictive power.

Residual solvents in biodegradable microparticles. Influence of process parameters on the residual solvent in microparticles produced by the aerosol solvent extraction system (ASES) process

Different batches of microparticles were produced by the aerosol solvent extraction system (ASES) utilizing the extraction properties of supercritical carbon dioxide. Using a central composite design, the influence of production conditions such as spraying rate of the polymer solution and pump rate of the circulating carbon dioxide phase, on the characteristics of microparticles was studied. The experiments were carried out with regard to the residual solvent in the particles, the residual methylene chloride in the carbon dioxide flow of the ASES, the particle size, the morphology, and the yield. The results showed an influence of the pump rate of carbon dioxide on the yield, on the residual solvent in the microparticles, and on the circulating carbon dioxide. There was no major influence of the production conditions on the size and morphology of the microparticles, indicating that the ASES process seems to be a precipitation process.

Structure of disintegrating pellets with regard to fractal geometry

PURPOSE

The aim of this study was to investigate the influence of the granulation liquid on pellet properties.

METHODS

Pellets containing propyphenazone were obtained by extrusion/spheronization using different 2-propanol/water mixtures as granulation liquids. The pore structure of the pellets was determined by mercury porosimetry. The fractal dimension of the pore system was calculated according to the model of the Menger sponge. Further characterization included SEM-photographs, disintegration, dissolution and tensile strength.

RESULTS

Fractions exceeding 40% 2-propanol in the fluid resulted in rapid dissolution rates of the pellets. This effect was caused by the rapid and complete disintegration of the pellets as compared to those obtained with less 2-propanol in the mixture. These phenomena were interpreted as being due to a change in the particle bonding of the pellets at concentrations of 40% 2-propanol. Evidence for this hypothesis resulted from the spheronization process, the tensile strength measurements and from SEM-photographs. The analysis of the pore system in terms of fractal dimensions implied a dependency of the fraction of 2-propanol in the granulation liquid on the pore structure. High fractions of 2-propanol resulted in lower fractal dimensions close to the dimension of the Menger sponge (2.727).

CONCLUSIONS

The structure of pellets can be markedly influenced by the composition of the granulation liquid. Investigations of the pore system in terms of fractal geometry are more useful for the explanation of pharmaceutical properties than if the pure values for the porosity are taken.

Use of a power-consumption-controlled extruder in the development of pellet formulations

Shape is an important characteristic of pellets that is highly influenced by the water content of the extrudate during spheronization. The power-consumption-controlled extruder (pcc-extruder) is an appropriate tool to adjust the required water content to yield round pellets. For this purpose, it is necessary to run the extruder at a specific level of power consumption. In the first part of the study, this approach was employed to screen different drugs as well as excipients to prove the concept of the pcc-extruder. Usually round pellets were obtained but a few formulations did not produce round pellets. In the second part of the study, the influence of screw speed and powder feed rate on the required level of power consumption was investigated. A correlation between the water content and the shape of pellets was evident. To account for screw speed, it was necessary to adjust net power consumption instead of total power consumption. The influence of the screw speed on water content of extrudate and consequently on the shape of pellets was not significant in the studied range. Variations of the powder feed rate affected both the water content and the shape of pellets. However, the water content was not only dependent on the level of net power consumption. The specific power is the ratio of net power consumption to extrusion rate, and it correlates well with water content and pellet shape. Using the specific power, it is possible to adjust the net power consumption in response to changes in powder feed rate.

The power-consumption-controlled extruder: a tool for pellet production

Based on the assumption that there is a link between power consumption of an extruder and pellet properties, a control circuit for power consumption was developed. Powder and granulation liquid are fed separately into a twin-screw extruder. The power consumption is controlled by varying the pump rate at a given powder-feed rate; consequently each level of power consumption results in a specific water content of the extrudate for a particular formulation. The shape of pellets depends almost entirely on the level of power consumption irrespective of formulation. The size of dry pellets is additionally affected by a shrinking factor which depends on the water content. The power-consumption-controlled extruder is an appropriate tool for the production of pellets. The system is able to adapt the water content for a formulation automatically.

Use of powdered cellulose for the production of pellets by extrusion/spheronization

The use of powdered cellulose instead of microcrystalline cellulose in the extrusion/spheronization process was investigated. The aim of the study was to assess differences between two types of powdered cellulose using a 2(4-1) fractional factorial design. Water content and amount of binder were found to be most important while type of cellulose and screw speed had only negligible influence on the extrusion process and the resulting pellets. Pellets obtained with powdered celluloses showed higher porosities and faster dissolution rates compared with those made with microcrystalline cellulose. Image analysis was found to be an appropriate method for the simultaneous characterization of pellet size and shape.