Powder compactor

The spray-dried flavours or powder blends are processed by a roller compactor into lumps (Fig. 21.12). These lumps are crushed into granules. This process cannot be categorised as a direct encapsulating technique, since the flavour-encapsulating effect of compacted flavours is based on the use of spray-dried raw material. 

Gereg and Capolla developed process parameters determined by a model laboratory bench scale Carver press, model C (Carver Inc. Savannah, Georgia, U.S.A.), which were translated to production scale compactor parameters (6). Their study provided a method to predict whether a material is suitable for roller compaction. Their study objectives were to characterize properties of the material to identify process parameters suitable to achieve the necessary particle size and density using the dry granulation process and then translate laboratory information to a production scale roller compactor. Actually, information developed from a Carver press was correlated and scaled-up to a production scale Fitzpatrick roller compactor. Model IR 520 (Fitzpatrick Co., Elmhurst, Illinois, U.S.A.) The compactor produced very similar powder granule characteristics as the Carver press. Various lactose materials, available as lactose monohydrate or spray dried lactose monohydrate, were used as the model compounds. Results indicated that a parametric correlation could be made between the laboratory bench Carver press and the production scale compactor, and that many process parameters can be transferred directly. 

Different spectral preprocessing and transformations available in SIMCA P-p (version 10.0, Umetrics, Sweden) were evaluated and the best approach for data handling and manipulation was determined. Data collected on the surrogate tablets were divided into a training set to generate the PLS models, and prediction set to test the PLS models. MCC powder, equilibrated at different RH, was also roller compacted at different roll speeds on a Fitzpatrick IR220 roller compactor fitted with smooth rolls. Powder feed rate and roll pressure were kept constant for all experiments. The key sample attributes measured on the surrogate tablets were also measured for the samples prepared by roller compaction. 

Sodium Borate Sulfur Powder Tungsten Carbide Zeolite Sulfuric Acid Clay Alcohol Clay-Water TUrbulatorVDisc Compactor Disc Pelletizer TurbulatorVDisc... 

Dehont et al. provided a simplified approach to roller compaction theory. They described that powder granules move through stages in the feed area. The material is drawn into the gap by rubbing against the roll surfaces. The densification that occurs in this area is particle rearrangement. At this stage, the speed of the powder is slower than the peripheral speed of the rollers. Fig. 1 represents compactor rolls in the horizontal plane powder is pushed vertically downward into the compaction area. 

Fig. 2 Front view of compactor rolls in horizontal plane depicting powder regions under different compaction forces. (From Ref. , courtesy of Transaction ASME.)...

A history of hopper and feed screw designs showed that each design evolved to facilitate and improve powder flow to the compactor feed screw conveyance system. Feed hopper designs depicted in Fig. 4 show designs incorporated in older compactor models [examples (A)-(C)]. Dehont et al. described the powder compaction feeding systems that were in use up to 1989. ... 

Compactor design features have evolved over the years. By the mid-1970s, research revealed a number of roll design improvements that increased compacting efficiency. Three key conditions were identified, at that time, which optimized the roll compact throughput and minimized leakage of non-compacted powder. ... 

Jerome et al. in the 1980s studied the effects of compactor adjustments on powder blend properties. His team s research showed that pressure applied by the movable roller is not a predominant factor. They found that the most important variable was the compactor feed screw in relation to the roll speed. They correlated improved tablet compression hardness to the feed screw speed in relation to the speed of the rolls.P ... 

Currently, one manufacturer of roller compactor equipment for the pharmaceutical industry has a fixed roll-pair system. This type of roll design system relies on powder gravity feed and a specially designed compression feed screw to continuously deliver powder feed stock to the fixed roll pair. Fig. 10. 

The chief objective of roller compaction is to consistently make an agglomerate of sufficient strength that meets required density, granulometry, and powder flow specifications. Operationally, the key goal of a compactor is to maintain a range of pressure on the feedstock, independent of the fluctuating powder granulometry and flow fed into the rolls, so that a consistent compact is made. 

Therefore, the delivery feed system plays a very important role in delivering poor flowing low density bulk powder materials to the roll compactor. 

Miller noted that the total compaction power requirement is the sum of the power required for the feed screw(s) and the roll drive. Feed screws not only convey the powder material from the compactor storage hopper but they also help deaerate the powder in the process. The deaeration of the powder acts as a mini compactor by precompacting the material just prior to roll compaction. Optimum compaction pressures and feed screw designs vary widely for different powder material properties. Changes in bulk powder density and feed screw speeds will affect the roll gap, the compaction pressure, the throughput, and the quality of the compact. ... 

Johanson predicted theoretical compactor operating conditions to handle air entrapment effects in materials. In general, he concluded that the critical compacting pressure level is dependent on a number of factors roll speed, roll diameter, powder permeability, compressibility, and compact strength. Johanson indicated, when applying these principles in commercial application, a compactor operator would have to operate the press at slightly less than maximum pressures to allow for material inconsistencies and variable in-feed flow rates. ... 

In several experiments, Miller studied the effects of using horizontal twin auger feed screws under partial vacuum (Alexanderwerk Inc., Horsham, Pennsylvania, model 50/75 compactor). The feed powder was vacuum deaerated just before roller compaction. The experimental design showed that the compactor s deaeration feed system significantly increased compaction output and minimized powder leakage when compacting very low density blends (<0.35 gcm ). ... 

The Fitzpatrick Co. roller compactor (Fig. 20) features two feed screws in series one horizontal and the second vertical. This system is designed to transfer powder bulk into a set of cantilevered rolls positioned in the horizontal mode one is fixed, the other floats. Operator interface allows for on-line monitoring and controlling feed screw speeds, roll pressure, and roll gap. Features such as on-line help and diagnostic functions, maintenance, and calibration screens are displayed. All functions are interfaced and adjustable by PLC for process control and report generating. The machines have vacuum deaeration and roll cooling capabilities. The sizing unit is separate from the compactor and is either a rotor bar or a hammer mill. 

Parrott, E.L. Densification of powders by concavo-convex roller compactor. J. Pharm. Sci. 1981, 70 (30), 288-291. 

The simplest approach during scale-up would be to maintain the applied roller compaction force as the roller compactor roll diameter and width increases on scale-up. This technique assumes that the applied force is linear at the point where the powder sees the maximum force. Dehont s team defined this as the neutral angle, gamma. This approach although simple to understand and theoretically correct has been shown to be successful only in a limited number of cases in the author s experience and is very much dependent on the formulation s composition. 

A second method of compression is to use a roller compactor. The powder mixture is passed between two contra-rotating cylindrical rollers to form a cake. 

---