December 14, 2005
Hygienic feed production: new generation of conditioners for press meal and the pelleting process
While conditioning is a process that has kept the attention of the modern feed milling industry for some time now, the pelleting process of press meal is a subject that is no longer considered to be very important. It needs to be understood that feed milling is about the balance between the different processes being used. As one process is linked to another, it is difficult to discuss only one process without having to take another into consideration. The following is a discussion of two processes, which interact frequently with one another and also with all the other processes used in the feed milling industry.
Conditioning is a process in the feed milling industry, which takes place from intake to out-loading. However, in many cases of referrals to conditioning, it is the treatment of press meal prior to the pelleting process that is meant.
Conditioning of press meal is a process with time, humidity, temperature and pressure as variables. Pressure is only used in unconventional processes where expander or extruder techniques are being used. In the conventional conditioning process, only time, humidity and temperature are applicable.
While the aim of conditioning was to optimise the pelleting process in the past, today, it is much more the intention to optimise the nutritional and physical quality of the feed.
When using the variables in the conventional conditioning process, it should first be understood that temperature and humidity are related as steam is generally used to increase the temperature of the press meal.
Furthermore, there is also a maximum level of humidity of the press meal in order to avoid blockages of the pellet press. In general, it can be said that the maximum percentage of steam that can be added to the press meal is approximately a maximum of 5 percent and for each percentage of dry steam added, the temperature of the press meal will increase by approximately 15 degrees Celsius. Of course, the steam quality is influential and the values mentioned above are only applicable when good quality steam is being used. Besides steam quality, the influences of the humidity of raw materials as well as the feed composition are of great importance.
Steam quality and steam quantity control alone, are subjects enough to warrant further discussion. However, in many practical cases, steam quality and steam control are subjects that are not usually given the attention they need in order to optimise the conditioning process.
As temperature and humidity are strongly related, the actual variable available in a conventional conditioning process is time. It should, however, be kept in mind that the conditioning process is an optimisation and not a maximisation. While single conditioners were a common choice for quite a number of years, now, double and even triple conditioners are being used prior to the pelleting process. Conditioning times lasted, in most cases, less than a minute and more importantly, these types of conditioners did not guarantee the "first-in, first-out" principle. For many years, the only conventional conditioning process that could guarantee a given time and that extended a maximum retention time of one minute, while also guaranteeing "first-in, first-out", were ripeners. Ripeners were a kind of cooking vessel that have been in the feed milling industry for over 20 years and in many varieties. The main concern when using the ripeners in the feed milling industry is contamination but when much longer conditioning times (i.e. > 4 minutes) are required, the ripeners are still a valid option.
As already stated, the conditioning process is about optimisation and therefore, the conditioning temperature and the formulation mainly determine the time factor. The temperature level can be varied according to the retention time in order to avoid destructive effects on the nutrients in the feed (i.e. protein de-naturation). Whereas the formulation has an influence on the retention time, meaning that the optimum availability of nutrients in the feed can be reached as well as optimising the physical quality of the pellets.
Recently other aspects have had to be taken into account due to consumer concerns, for instance, Salmonella. Also this needs to be considered and treated in the conditioning process. Thus, the new generation of conventional conditioners (without using pressure) are focusing on the variables of time, temperature and humidity where time is maximized to 4 minutes at temperatures of approximately 85 degrees Celsius and capacities of up to 20 tph. These types of conditioners should guarantee a "first-in, first-out" while also guaranteeing retention time. The principle is rather simple, as generally, the feed is brought up to the required temperature by means of steam and subsequently transferred into a large screw conveyor in which the speed of the mash is determined by a frequency controller. The screw conveyor is steam or electrically heated and insulated.
So, by means of the filling degree and the frequency (at a certain pitch of the screw), retention time can be guaranteed. For those involved with feed milling practices, it is well known that there are a few concerns. Firstly, the height of installation for this new generation of conditioners is not easily available. For this reason, Van Aarsen International B.V in the Netherlands positions the conditioner beside the Long Time Conditioner (LTC). The second concern is control of the pelleting process because the long retention time leads to a delayed reaction of the pellet press on changed variables. This complicates the matter even further due to the fact that the pellet mill cannot be fed directly from the retention time screw. Therefore, some suppliers use small intermediate bins between the retention time screw and the pellet press with a feeding screw for the pellet press below the bin, leading possibly to contamination as well as loss of temperature. Once again, more height is required and control becomes even more difficult to maintain due to the use of buffer bins. In order to avoid this, Van Aarsen has integrated a feeding device in the Long Time Conditioner, facilitating the control and easily achieving the required height in a few meters less. All these features have been integrated into one conventional conditioner, which is called the "Conditioner LTC" (pictured below, left).
For a standard control, a PLC is integrated into the pelleting line control system and for a more sophisticated control,a PressM@nager is used, which is Van Aarsen pelleting line automation based on remote I/O with intelligence (meaning the system is capable of achieving even higher goals than a well-trained pellet mill operator).
_ Van Aarsen's PressM@nager screen with Long Time Conditioner (LTC)
After conditioning of the press meal, either the mash is cooled when heat-treated mash is required (for instance breeders) or, in most cases, the pelleting process starts. The pelleting process also has an effect on the nutritional as well as the physical quality of the feed. Experience has shown that it is not only the capacity of the pellet mill but also the formulation, die specifications as well as the required pellet quality that should be taken into account.
While a Van Aarsen C900 is producing 45 tph in South America, the same machine is capable of reaching a capacity of 18 tph in a Dutch environment. The big difference can be explained by formulation, die specifications and the required pellet quality. Also, a proper conditioning process influences the output of the pellet mill. However the figures above are based on the same conditioning process.
Of course, the pellet mill has a number of variables that can be used to optimise the process. These variables vary from speed of the die to hydraulic roller adjustment and should only be used when the influence of these options are thoroughly understood by the operating personnel. This practical know- how could also be delivered by an intelligent pelleting line automation system such as PressM@nager. This system is capable of combining the different variables in order to optimise the process.
However, before continuing the pelleting process, a small note needs to be made about the dies being used. As dies of reputable suppliers are generally all of the same material, which is completely hardened, the difference is generally found in the open area surface - OAS (i.e. the number of holes) that, of course, is related to the capacity of the pellet mill. Therefore when comparing dies, the aspects that need to be considered are the types of material to be used, the hardness and whether surface hardening or complete hardening will be used, the number of holes or the open area surface (OAS) and the wall thickness of the die including possible counter-drill.
When discussing capacity, using the same formulation and the required pellet quality, the die surface of the pellet mill (with the same OAS) and the die thickness are the major factors influencing capacity. These, together with the drive installed, determine the KW per cm2, which is a factor to be considered when researching the operational costs. Practical trials carried out under less than 100% identical circumstances, have indicated that differences in operational costs among different types of pellet mills can vary up to EUR 50, 000 per year which are, of course, significant figures, but realistic nonetheless.
Van Aarsen pellet mills are well known for their design criteria, huge die surface and low die speed, so that an acceptable capacity is reached while maintaining focus on pellet quality (which is also influenced by the retention time of the press meal in the die). This, combined with an intermediate drive, allows the possibility to change die speed relatively easily (changing a small pulley) without installing frequency controllers for the generally larger drives used on pellet mills. Furthermore, large roller diameters create a small angle between the die and the roller, which compacts the feed smoothly before forcing it into the die.
Of course, variables differ when using the pellet mill for traditional, non-traditional or aqua feed, but the principles remain the same. Because circumstances differ constantly, only general guidelines can be provided except when detailed information is available and a custom-made solution is given by those in the feed milling industry with first-hand practical knowledge.