Heat intensive processing has created a need to add ingredients downstream from the extrusion or drying process. Some of the necessary vitamins and nutrients are heat sensitive and lose much of their effect at the temperatures encountered in conditioning and extrusion of feeds.
The following information is an overview of methods used in the feed industry, as well as others, to apply micro ingredients after the extrusion process. For the purposes of this article, micro ingredients are defined as those ingredients that are added at a rate of less than 1%.
When using ingredients in any process it is necessary to examine the characteristics of the ingredient to insure that the method of controlling and metering the ingredient is appropriate. These characteristics include density, viscosity, pH, percent needed to apply, desired accuracy, and optimal temperature. For example, liquid animal digest is quite acidic and systems should be designed to withstand the corrosive nature of the ingredient.
Feeders and pumps should be sized for the appropriate flow for the desired percentage of ingredient being applied. When the ingredients are being applied after the cooling process, the application system should be placed as close as possible to the packaging or load-out system in order to minimize the build up of liquid ingredients on material handling equipment. In any coating system there are two issues that must be addressed: Control of metering and method of application.
Control of metering:
The preferred method for achieving accuracy in the proportioning of ingredients into a process is by weight. Batch processes are the most accurate weighing processes, but this becomes problematic in the addition of ingredients after extrusion or drying because it is a continuous process. The goal is to achieve a weight for the product without interrupting the process flow. There are three principle methods for measuring the flow of material in a continuous flow: Volumetric, mass flow, and loss in weight. In all of these methods, you must consider the flow of the carrier ingredients — in this case extruded product — to be the master flow. All other additives are saved from this master flow.
In volumetric metering (fig. 1) the master flow of material is sensed by the number of revolutions of a screw conveyor, rotary feeder, or belt conveyor. Calibrate the material flow by measuring how much product has run through the system in a given amount of time. It is important to have a method to divert material from the process flow so that check weights can be done to confirm calibration.
The flow of liquid additives is sensed by the number of pulses received from a positive displacement liquid meter (nutating disk, turbine, or piston) or the number of revolutions seen from a tachometer mounted on a positive displacement pump, or the number of cycles from a diaphragm type pump. If dry additives are to be added to the product stream then these are sensed in the same manner as the master flow.
The advantage of this type of system is that the up-front cost is low and that the overall system is simple. This type of system works quite well if the density of the products stays constant and if the formula of liquid to dry does not need to be frequently changed.
The disadvantage of this type of system is that it does not provide very good accuracy (1-2%). It also does not take into account changes in density or viscosity, so if the moisture or temperature of the product changes the system has to be re calibrated.
In mass-flow metering (fig. 2) the master flow of product is sensed with a weigh belt, weigh feeder, impact scale, or centriflow meter. This type of measurement integrates the weight of the product with a tach signal from a conveyor or over a specified time period.
The preferred liquid meter in this case is a mass flow coriolis type meter. This meter measures a shift in frequency and position of a tube that is proportional to the mass flowing through the tube. This in turn tells us the mass of product flowing through a tube in a specified period of time.