FEED MANUFACTURING TECHNOLOGY - ISSUES AND CHALLENGES

Transcription

1 FEED MANUFACTURING TECHNOLOGY - ISSUES AND CHALLENGES DENNIS FORTE SUMMARY The handling of Feed Pellets during storage at the Mill, transport and transfer to bins on the farm has a direct impact upon the proportion of intact pellets that finally reach the Feeding Troughs. The degradation of the pellets leads to the generation of fines. Pellet Durability is therefore one of the primary Quality Attributes for manufactured feeds. There are numerous means that can be used to improve the pellet quality. These include Formulation Design (providing better binding ingredients), Equipment Design and Optimal Processing Parameters (for the chosen equipment). The following article will focus upon the design and performance characteristics of a range of alternative process technologies suitable for the manufacture of feeds. Dennis Forte & Associates Pty. Ltd, P.O. Box 8179, Wodonga, VIC 3690, Australia. Phone / Fax (02) ; forte1@iinet.net.au 88

2 INTRODUCTION Numerous alternative technologies are available for the preparation of manufactured feeds. The major factors to be considered (and benefits to be gained) when making the decision to manufacture feeds includes the following: o Improved Feed Efficiency (or Feed Conversion) o Reduced Segregation of Feed Ingredients o Reduced Losses (due to Fines etc.) o Improved Feed Handling o Increased Bulk Density o Improved Flowability (in Bins and Silos) The performance of the manufactured feed (with respect to both the Handling and the Conversion of the feed) is dependent upon the quality of the feed, which is predominantly controlled by the Pellet Durability. FACTORS INFLUENCING PELLET QUALITY Numerous studies have been completed to assess the impact of various factors upon the pellet durability. The measurement criteria have considered both the handling characteristics and also the feed conversion (in numerous species). A review of the published data indicates that the following rule of thumb can be applied when assessing pellet quality issues. Likely Source of Problems o Formulation 40% o Grinding 20% o Steam Conditioning 20% o Die Design (and Condition) 15% o Drying and Cooling 5% 89

3 THE FEED MANUFACTURING PROCESS The Feed Manufacturing Process is presented schematically in Figure 1. The key points of difference between the various alternative technologies that are available relate to the Conditioning, the Pressing and Cooling / Drying technologies that are employed. The following discussion will focus on a limited number of the steps in the process. Figure 1 - An overview of the Feed Manufacturing Process Each of the technologies that are on offer has particular features to offer the feed manufacturer. The major points of difference relate to: o Complexity of Design o Ease of Operation o Economics (Purchase Price and Operating Costs) o Pellet Quality Limitations Steam Preconditioning Technology The primary objectives of the Conditioning Process (or Steam Preconditioning) are to ensure uniform mixing of the Added Liquids (typically Water and Molasses) and the Steam with the Feed Mash, in order to initiate the Cooking Process. The cooking process relates to both the Gelatinization of the Starches and also the Denaturation of the Proteins. The 90

4 achievement of an appropriate Degree of Cook is dependent upon providing sufficient Hydration Time and also an Elevated Temperature. Numerous alternative design options are available for feed manufacturing and these include the following: Pressurized vs. Atmospheric - Use of an elevated pressure allows the process to operate at a temperature above 100 o C. Open vs. Weir Designs - The Residence Time within the Preconditioner is directly affected by the Paddle Configuration (and Paddle Speed) in open design systems. The provision of a Weir ensures an almost constant residence time regardless of the configuration and/or speed. Vertical vs. Horizontal - Most systems are horizontal, however, a vertical design will ensure more intimate contact between the mash and the steam. Single Shaft vs. Twin Shaft - The provision of multiple shafts seeks to enhance the Mixing Characteristics of the Preconditioner. Single Stage vs. Multiple Stages - The provision of multiple stages seeks to both enhance the residence time within the system and also to the mixing, since each of the stages may be individually optimized. Pellet Forming (or Pressing) Technologies Once again a significant number of alternative technologies are available for the Forming (or Pressing) duty. The options continue to grow as further refinements and improvements continue to be sought by the equipment manufacturers. Some of the more common technologies currently being used throughout the industry include the following: 91

5 Pellet Milling Technology - This is the traditional technology used for manufacturing feeds and continues to be the most popular. (Approximately 80% of the feed currently manufactured is prepared using this technology.) A typical pellet mill is shown in Figure 1 and the operating principle is presented in Figure 2. The preconditioned mash is fed into the centre of the Die. A series of rotating Rollers then both compress the mash and also drive it through the Die Holes. A combination of the Frictional Heat Generated due to the Shear (also referred to as Viscous Dissipation) by the rollers and the Residence Time in the die lead to the formation of a stable pellet. Figure 1 - A typical Pellet Mill Pellet Mill Figure 2 - Operating Principle of the For some applications (such as when durability and Water Stability are important) the use of Double Pelleting has become common practice. This involves taking the hot pellets from a pellet mill and feeding these directly into a second pellet mill. This results is a more compacted and durable pellet. 92

6 Compacting Mills - In this device the Preconditioning stage and the Compacting stage are combined into a single unit (see Figure 3). The primary Mixing Cylinder ensures uniform mixing of the mash with the added liquids. The heated mash then enters the compacting stage, where a number of rollers force the mash through a V-shaped Gap between two Friction Rings. One of the rings is fixed and the other may be moved in order to adjust the gap. The process is only suitable for the manufacture of Crumbles and Cooked Mash. A typical compacting mill is shown in Figure 3. Pellet Cookers - A hybrid of all of the above technologies the Pellet Cooker, wherein a well-conditioned mash is fed into a short High Speed Screw, which serves to both convey, shear and cook the mash (thereby Plasticizing it) prior to forcing it through the Die. The system may be characterized as a High Temperature Micro Time cooking technology. The residence time in the screw is very short (typically only 3 to 5 seconds). An example of this technology is presented in Figure 4. Figure 3 - The BOA Compacting Mill 93

7 Figure 4 - The Universal Pellet Cooker Extrusion Technology - The last refinement of the options for feed manufacturing is that of Extrusion Cooking technology. The key components of a typical system are shown in Figure 5. 94

8 Figure 5 - Key components of a typical Extrusion Cooking system This technology uses a combination of steam preconditioning and Frictional Shear to achieve a very degree of cook of the mash. Both the Screws and Barrels are typically of a modular design, thus the process can be optimized each different application. 95