Twenty-five years ago, many of us who intended to enter the feed industry were told that making pellets was a dead end. This is due to the energy crisis that enveloped the western world at that time. At the time, it was only thought that the energy prices in the future would be too expensive and making pellets would simply be a waste. Today, our annual production of pellets is higher than ever before, and the proportion of pellets in total feed production is higher than ever before. This is due in part to the reduction in the relative price of energy, but it is mainly because livestock breeders have realized that grain-fed diets have many benefits for livestock performance. With the improvement of the comprehensive management level of livestock and poultry husbandry, the husbandry industry will inevitably switch to the use of pellets. They believe that pellet production is a dynamic processing method that can be used to increase the nutritional value of feed. However, in many cases, the pellets are made very poor, but this processing method is still widely used.
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According to reports in the literature, many recent studies have proved that the performance of feed can be improved with the improvement of the quality of granular materials. In addition, many of us who work with feed processors have felt that the quality of pellets is a very important issue, and feed companies are willing to spend time and money to solve this problem. However, there is a great deal of misunderstanding among people about the importance of various factors affecting the quality of granular materials. The most important factors affecting the quality of the pellets are: feed formulation (40%), grinding (20%), conditioning (20%), compression moulding (15%) and cooling and drying (5%); The percentage after each factor represents the relative size of the factor in the overall mass of the pellets. It is important to note that the formula plus grinding can already determine 60% of the mass of the pellets before the powder reaches the granulator. Therefore, many pellet quality problems cannot be solved by improving the tempering method and selecting an appropriate stamper. This will never stop us from making quality improvement efforts, and it will never make particle processing systems and operators blamed for the poor quality of the pellets.
The purpose of this paper is not to elaborate on each of the factors that affect the quality of the pellets, but only to focus on one issue, that is, conditioning. Although tempering is far more important than the selection of a die or roller surface, this process is often overlooked and most people in the industry have a poor understanding of it, both equipment suppliers and feed processors.
The large number of changes that have taken place in recent years have affected our feelings on the issue of tempering. For example, compared to previous years, many of the current diets use less grain, but use more by-products. Many by-products, such as animal protein powder, corn gluten meal, bakery products, etc., and even poor quality cottonseed, have all been used in the current diet. The effect of conditioning on many of these materials is not as effective as grain flour and soy meal, so we often see machine overload, productivity loss, and/or poor pellet quality.
In addition, dietitians often add high levels (> 1.5%) of fat to the diet, but they have almost no idea what effect this will have on the quality of the pellets. Many feed mills have been forced to increase the production of pellets far beyond the rated design capacity of processing facilities to meet the demand for increased sales and animal feed requirements (for large integrated companies). In some cases, the amount of fat in the powder often exceeds the nutritional requirements of the animal but it is only to increase the production of the pellet production system. The harm to the quality of the pellets is obvious from this approach, but they have to be used to feed pigs and feed chickens.
It is important that many of our inputs will have a negative impact on the quality of the pellets, but the pellets we produce should be able to withstand any harsh conditions and remain as they were before reaching the animals. To have a good understanding of the refining process and to produce the best quality pellets at the required productivity, we still have a long way to go.
The definition of "conditioning" is tempered, at least from our point of view, including any processing measures or any added ingredients applied to the powder before it leaves the mixer and reaches the pelletizer's compression chamber. Therefore, conditioning includes adding water and/or steam, puffing, pressing, pre-cooking, “curingâ€, and the like. Each of the aforementioned measures will be discussed later, but we should be aware that anything that is done during the conditioning process should be intended to prepare the powders for the final processing (granulation) ). All of the conditioning measures used must be performed in an optimal manner so that the best quality pellets can be produced at the most reasonable productivity without significantly destroying the existing nutrients in the diet. As can be seen from the above discussion, this is a difficult task.
Despite the variety of methods of tempering, we should also recognize that each of these methods has its advantages, and each has its own disadvantages or negative effects that should be noted. Each conditioning method will be discussed below to discuss its advantages and disadvantages, and our recommendations on how to properly use these methods in order to produce the best quality pellets.
Introduction to various conditioning methods Atmosphere conditioners The typical conditioners commonly used in granulation systems are called atmospheric conditioners. As its name suggests, this type of conditioner operates at atmospheric pressure and is usually under ambient conditions. In general, an atmospheric conditioner is basically a single cylinder with an agitator shaft mounted thereon. The dimensions of the cylinder vary from design to design. Overall, the diameter is 15-30 inches and the length is 5-15 feet. The agitator shaft is usually drilled in order to install a number of adjustable or replaceable blades.
The function of the conditioner is to provide conditions for intimate contact between the steam and the raw material powder. The previous article has discussed the issues related to the quality of steam and how to manage the steam system, which will not be further discussed here. However, understanding the problem of how steam interacts with the raw material powder is extremely important for understanding and managing the problems of the granulation system.
Steam Conditioning: The application of steam during the granulation process is primarily due to the unique ability of steam to carry and transfer heat through condensation. If only for adding water, then using a watering pipe to connect the faucet is more economical than using steam; similarly, if only for the purpose of obtaining heat, using a direct gas stove is more expensive than using a boiler. However, we need a lot of heat and moisture in the refining process. The goal is a very fine place—the surface of each particle in the raw material powder. Therefore, steam is the only practical method that can take on this responsibility.
The relatively cool powder particles are in intimate contact with the steam, and the heat in the steam is transferred to the powder particles, causing the temperature of these particles to rise. Steam delivers 970 BTU of heat to the powder particles and 1 pound of water condenses on the surface of the powder particles. This phenomenon is like the phenomenon in which the steam in the moist air condenses on the cold beverage cans. If the reader can fully understand this concept, then his full understanding of the atmospheric refining process is also within reach. This is the most basic process that occurs during the refining process.
Once liquid condensation occurs on the surfaces of the raw powder particles, both heat and moisture begin to enter the interior of the particles because there is a temperature difference and humidity difference between the surface and the interior of the particles. Understand this, we can understand the "age-old principle of diffusion", according to this principle, the material (here is the heat and moisture) will move from the high concentration area to the low concentration area. The heat released when the steam condenses provides the energy to drive this movement.
Cereals, protein cakes and other commonly used raw materials usually have good thermal insulation properties (thermal conductivity is very low), so heat and moisture in the movement are relatively slow. In this way, the problem of optimization of atmospheric conditioning occurs. This mainly involves the problems of the size of raw material powder particles and the residence time of raw materials.
Particle size of raw material powder: If the above-mentioned view that the movement of heat and moisture is very slow is true, then it is natural that the smaller the particles, the more thorough the heat and moisture can penetrate the particles within a certain period of time. The core part. In contrast, if the particles are relatively large, heat and moisture cannot penetrate sufficiently into the interior of the particles that have a relatively hard and dry core, and such particles will not be sufficiently elastic to form good particles.
It is well known that the total surface area of ​​the powder particles increases with decreasing particle size. This concept is extremely important because steam condenses on this surface and it can be seen that the larger the total surface area, the greater the amount of condensed water per unit weight of powder.
The quality of the pellets is often improved by the finer grinding of the raw material. The main reason is that the particles of the fine grinding powder are smaller (heat and moisture move faster into the interior of the particles), and the total surface area of ​​the particles is larger (steam condensation More water). In order to optimize the atmospheric conditioning, we should try to grind raw materials as much as possible.
Residence time: As mentioned earlier, most of the raw materials we use have very high adiabatic values, so heat and moisture must pass through to the core of each particle after a certain amount of time. The amount of time that can be used is limited to the time it takes for a particle to pass through the conditioning chamber. This time is called the "residence time."
The determination of the residence time is not easy and it is not easy to measure accurately. Therefore, the residence time actually represents the average residence time of all particles in the conditioning room. You can close the hopper and start the stopwatch at the same time to roughly determine the residence time. The stopwatch reading was observed immediately when the amount of material in the granulation began to decrease. Using this method, you can get a certain impression of the residence time of the material in the conditioning room. Other assays include injecting dye into the hopper neck and then collecting samples from the conditioning room every 2 seconds. It can be seen that as time goes on, the depth of color in the sample is first deepened and then decreased. The time when the darkest color was seen was taken as the average residence time. Similar results can be obtained with iron particle tracers.
The purpose of determining the residence time is that, in order to obtain the best quenching and tempering effect, the residence time must be the best. Therefore, we must know where to start to solve this problem.
This raises the question: How long should the optimal residence time actually be? This issue has never been adequately studied, but the results of most studies indicate that the quality of the pellets is between 30 and 90 seconds. And the output will be improved. Therefore, if appropriate adjustments are made, there will be a real chance that the quality of the pellets will be improved. You must understand the situation before the adjustment in order to determine whether the effect of the change being made is favorable or unfavorable.
Several options to increase the residence time: The speed at which the powder passes through the conditioning chamber is affected by two factors: 1) the angle of the blade; 2) the rotational speed of the agitating shaft. The optimal residence time can be achieved by adjusting these two.
Blade angle: In general, the blade of the OEM conditioner has been set at a 30-45 degree angle forward in the manufacturing plant. In other words, as the agitator shaft rotates, all the blades drive the powder toward the discharge port. If the speed of the agitating shaft is faster (greater than 150 rotations per minute), the blade angle can be reduced to a relatively intermediate position (5-15 degrees). In other words, the angle of the blade can be set to a position almost perpendicular to the agitation axis. This attenuates the "pump-out" effect of each blade to extend the residence time.
At slow speed (80-100 rotations per minute) conditioner, the paddle can be set to a more parallel position with the agitation axis (5-15 degrees from the agitation axis). This angle can lift the powder. Get up to bring it around the conditioning tank.
Setting up the paddle in the best case is just a commissioning process. It should be noted that the paddle at the inlet (ie, in the first quarter of the quench cylinder) should be kept at the factory setting to ensure that the powder is quickly driven forward. The massifier thus forms an empty area for steam to enter the conditioning chamber. The blade angle should be adjusted about 50% of the center of the length of the conditioner. Our proposal is that the paddle should be set so that the powder in the conditioner only accounts for 70% of the total capacity of the conditioner. Too much powder in the conditioner will block the hopper and cause mechanical damage. In addition, the operator should realize that prolonging the residence time of the powder increases the load on the conditioner drive motor and causes overload. Measuring the current flowing out under the motor load condition and then adjusting it can solve this problem.
Stirring shaft speed: The second factor that can be optimized is the speed of the agitating shaft. Before talking about changing the rotational speed of the agitator shaft, two common practices should be explained. Some engineers advocate the use of a method called "stirred bed" conditioning, while others use "fluidized bed" conditioning. High-speed rotation of the agitator shaft (liquid bed method) causes the powder to be lifted during passage through the conditioning chamber in order to force the particles of powder into the top of the conditioning chamber and to be exposed to free or excess steam there. There will be more steam condensing on the powder particles, and the steam can be used more fully.
Slow rotation of the stirring shaft causes the powder to deposit on the bottom of the conditioning chamber and is pushed “gently†through the conditioning chamber. This will obviously increase the residence time of the powder, but it will make the steam in the upper part of the conditioning room flow freely and will not be used.
Many equipment suppliers gave up the slow method commonly used 30 years ago 10-15 years ago and adopted the fast method, but most of them now use the slow method again. The main point of the conditioner design is to allow the incoming steam to come into intimate contact with the cooler powder and immediately condense. This requires multiple air inlets on the conditioner housing or a long gap for steam entry. Regardless of the type of air intake used, these air inlets should remain unobstructed so that the steam enters at a lower speed and does not pass through the powder too quickly. There are no special rules for adjusting the rotational speed of the agitator shaft, but the rotational speed should not be so slow that the powder can be agitated well and the speed of passing through the conditioning chamber should not be too low. It should be avoided that the rotation speed is less than 80 rotations per minute to avoid poor stirring of the powder and that the passing speed is too slow.
The rotation speed of the agitator shaft can be changed by replacing the drive belt and the pulley or installing a VF controller on the drive motor. Since it is almost impossible to obtain the required rotation speed only once, it is the best choice to use variable frequency drive when possible. Another possibility is that different feeds require different speeds, and seasonal changes in raw materials may require different speeds. If there is more than one pelletizer in the factory, then only one inverter can be installed on the inverter controller. After the optimal speed is measured, the rotational speed of the agitator shaft on other machines can also be fixed at this speed.
In any case, it should be recognized that these two influencing factors - the angle of the blade and the rotational speed of the stirring shaft - are related to each other rather than to each other. Careful research and maintenance of detailed production records yield the most satisfying results. Addition of water during conditioning: It is now fully recognized that water is an important component of the binding process in the formation of granular materials. As mentioned earlier, in the typical granulation process, the only added water is added in the form of steam. The author feels that in the United States, the water content in the granulation process is insufficient in most areas at least 6-8 months per year. In areas where corn is the main grain, there is always excess moisture in the corn when it arrives each year. However, over time, the stored grains gradually reach the market and the grains received are relatively dry.
According to the different feed formulations, the moisture content of the best tempered powder is approximately 16.0-17.5%, of which 4-5% comes from the conditioning process. Sometimes, the target temperature cannot be reached without reaching the upper limit of humidity. In some cases, the grain is relatively dry and its temperature is relatively high. If the target temperature is not exceeded, sufficient steam cannot be introduced into the powder. Later in the grain harvest year, adding 1-2% of water during the conditioning process will improve the quality and productivity of the pellets. The question of how much water is added, when it is added, and how it is added has not been fully studied; however, we should consider whether the quality of the pellets produced later in the harvest year will be worse. We have several options for when to join, such as adding a mixer or adding a conditioning chamber. When to join is better, it should be determined through experiments in the local area.
Other types of conditioners Two-way conditioners or three-way conditioners: In order to extend and control the residence time, two-pass conditioners or three-way conditioners can sometimes be used, especially when producing aquatic animal feed. Use this method. Basically, two or three standard conditioners are stacked on top of the granulator. Variable speed drives, multiple steam injection ports, and steam heating jackets are all options available.
The unique advantage of a two-way or three-way conditioner relative to a single high-capacity conditioner is that it can also maintain a "first-in, first-out" order. Compared with the more novel conditioning method, this method also has the advantage of relatively low cost. There should be enough headroom above the granulator to facilitate installation.
Steam jacket conditioner: Many improvements have been made to the condition of the outer jacket, the conveyor or the conditioning room, resulting in varying degrees of effectiveness. The basic concept of using this tempering method is that the steam jacket can be used without adding too much moisture during heating. This is certainly a good idea, but it is difficult to do in practice. The reason for the failure is that the heat is only transferred to the powder through the surface of the conditioner wall. However, the ratio of surface area to capacity is usually so low that there is not much heat that can be transferred to the powder. The capacity of the conditioner is relatively low. This is especially true of big time.
Stress Conditioning: This is a new concept in granulation and is currently being field tested. The basic concept is to increase the working pressure in the conditioning room. When the pressure in the conditioning room is increased, a conditioning temperature of 212 degrees Fahrenheit or more can be obtained. The principle of this concept is the law of thermal balance. In simple terms, high pressure can force moisture and heat to enter the product faster and more thoroughly than at atmospheric pressure. The difficulty of getting the powder into and out of the high-pressure conditioning room is obvious. The problem of leaving can be solved by making the press chamber and the press roller also part of the high pressure zone. The inlet is covered with a spring-loaded pressure plate to maintain the pressure, and the feed is used to push it open.
The preliminary results of field studies are promising. There are still some technical issues that need to be addressed, but these problems are not very serious.
Novel conditioners Some of the more innovative conditioners currently on the market—compactors and expanders—have attracted a great deal of interest. Both machines use mechanical energy to increase the amount of heat that enters the powder prior to final granulation. With this method, it is possible not to introduce excessive moisture, so that there is no problem that the high-moisture brings about the slippage of the pressure roller and the excessive moisture of the granular material, and the advantages brought by the high temperature can be obtained, ie, the gelatinization rate of the starch is improved. . The following discussion is expected to help the reader understand the basic concepts of each of the relevant conditioners and their respective advantages and disadvantages.
Compactor: This is a relatively new concept of conditioning developed in North America, but it has had a significant impact on feed processors in Europe. The basic structure of the compactor is a standard conditioning room plus a compaction room. The compactor's conditioning section is more robust than conventional designs because the power to turn the compaction chamber roller assembly is transmitted from the conditioner shaft. In addition, the rest are applicable to the aforementioned discussion of atmospheric conditioners.
The “mystery†of this novel texturizer is the compaction and shearing that occurs when the tempered powder is forced through a narrow V-groove ring under the pressure of the press roll. This concept is difficult to describe in words, but a recent article in a trade magazine (Feed Management, Dec. 1996) shows a good picture of the various parts of the system.
There is a rotating platen assembly in the compaction chamber, which is mounted on the conditioner shaft. There is a large support system immediately before the press roller assembly to ensure the alignment and stability of the press roller assembly. The method of adjusting the gap between the pressure roller and the V-groove ring is the same as the conventional roller adjustment method in any granulator.
There are two large rings around the press roll. One ring is fixed and the other ring is movable and controlled by three hydraulic cylinders. The mating surface between the two rings is machined into a V-shape, the width of the inner side of the V-shape is approximately the same as the width of the press roll surface, and the width of the outward side gradually narrows to almost zero.
During operation, the gap between the two rings can be controlled to increase or decrease the pressure of the powder when it passes through the gap under the pressure of the pressure roller. In general, the size of this gap can be adjusted to a wide range, from zero to 1.25 inches. The control of gap size is extremely important and must be controlled with a dedicated controller. It is almost impossible to adjust it to the best condition manually.
The compactor has several advantages over standard conditioning, which may be of interest to people. Above the granulator only minimal headroom is required. Opening the gap, this system can be operated in the traditional way. This system requires a lot of electricity, but it consumes slightly less power than the extruder. The cost is also higher, but it is still slightly lower than a comparable bulking machine.
The author of this article has limited experience with compactors, but I have observed that a feed mill in Canada has significantly improved the quality and production of pellets after using a compactor. Ring and pressure roller wear is a problem, but it seems that the cost of maintenance is relatively low. An interesting finding was that cleaning after each batch of feed was extremely easy, and it took no more than 10-15 minutes.
Expanders: There are now many sources of information on the use of bulking technology in the refining process. Like the compactor, and in fact as in the granulator itself, the heat in the extruder is generated by the conversion of mechanical energy into friction. Most of the friction comes from the friction of the powder particles with each other, but there is also a considerable part of the friction between the powder particles and the screw and cylinder surface.
The bulking device, in its main part, is an improved extruder. The main difference is that the gap between the on-machine molds is variable. Careful control of the pressure applied to the cone part of the die can control the amount of energy and thus the amount of heat that can diffuse into the powder. The working conditions can be very gentle to quite intense, and generally can significantly improve the quality and yield of pellets. The degree of gelatinization of the starch and the solubility of the protein in the powder can be increased under a variety of factors (for example, tempering temperature, amount of water, diet formula, and fineness of feed particles). Ultimately, these factors can affect the adhesion of the powder particles to each other and thus affect the quality of the pellets. Due to the restriction of flow due to the reduction of the gap, the expander can generate pressures above 500 pounds per square inch and temperatures of 120-130°C. The residence time under these conditions is 3-5 seconds, so the physical change of the powder occurs very quickly.
In addition to many advantages (increasing the quality and yield of pellets, and in most cases improving the performance of animals), there are also some disadvantages that must be taken into account when using this method, such as reduced vitamin titers, feed additives ( Loss of activity of drugs as well as reduced utilization of certain proteins. None of these problems have yet been solved. Now we are actively studying it.
What is refreshing with other new technologies is that granulation processing technology has not changed for 50-60 years, but now new ideas and concepts are beginning to form a powerful challenge. The pressure of the granulation system is one example. One of the major extrusion equipment manufacturing companies in the United States has introduced a highly-improved system that takes advantage of the majority of extrusion processing while controlling or eliminating most of its shortcomings, such as high investment, high maintenance costs, and Low production capacity. This system is basically a combination of a highly complex atmospheric conditioner and an improved extruder with a short residence time. During operation, conditioners provide retention and contact time to optimize the quality of the pellets, while the improved extruder section provides the necessary pressure to force the powder through a die having appropriately sized holes to form pellets.
This concept has some very unique advantages compared to a comparable conventional granulation-puffing combination, and the quality of the pellets still has the same level of improvement. "UP/C" (manufactured by Wenger), as is well known, has the same size as a typical extruder without an expander, so most of its models have no problem of clogging. A very high level (>70%) of starch gelatinization can be achieved, so that good quality pellets can be produced and also the durability of the pellets can be improved. Even if high levels (>10%) of fat are added to the powder, the quality of the pellets produced is still acceptable. Perhaps the most attractive feature for commercial feed processors is their speed and ease of die change. In most cases, it takes only 10-15 minutes to change the size of the pellets. Therefore, as long as the time for cleaning the cooler can be used to complete the replacement of the die. This actually means that you don't need to spend time to change the size of the pellets.
This is a brand-new technology, and while it is clear that it needs to be further considered and studied to further improve its usefulness, this is a good example of how the feed industry is becoming more and more dynamic. specialty.
Conclusion There is no doubt that the conditioning process is the most important part of any feed granulation system, especially for the quality of the pellets. The author firmly believes that this may also be the worst part for the granulator operators, most feed mill managers, and equipment suppliers. The purpose of this paper is to discuss some of the issues that people understand poorly during the process of refining, and point out the advantages and disadvantages of each of these methods. No one method of conditioning can meet all requirements for all kinds of application purposes and application environments. In most cases, there is no need to replace the entire machine, as long as some methods are used to improve the current model of the machine, we can produce the best quality pellets with the highest possible productivity. However, it must be remembered that all factors related to the quality of the pellets are interrelated and must be considered together for success. 
