Adding liquids to feed

by Fred Fairchild
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Liquids are added in the feed manufacturing process to enhance the quality and the animal’s growth performance and/or desire to consume the feed. A large number of liquid products, from oils and tallows, and molasses to very expensive additives are used in various formulations and finished product treatments. The location of the application of each liquid ingredient in the process is dependent on its effect on the process or the process’ effect on the performance of the ingredient.

For mash-type feeds, the ingredients are added in the mixer. For mashes that are to be further processed, the amount of oils or molasses that may be added at the mixer normally do not exceed 4% to 6% maximum. If higher levels of these ingredients are required, they are normally added post process just before bagging or bulk loadout. Here is a look at the various ways to add liquids at appropriate locations in the manufacturing process.

Liquid Addition at the Mixer

The most commonly added liquids at the mixer are oils and/or molasses. Whether it is a static or continuous mixer, these liquids are added into the mix through liquid meters while the mixing action is taking place. Care must be taken that the liquid addition is metered at a rate where it gets distributed throughout the material being mixed over a period of time. The liquid should be dispersed into the mixer through a liquid manifold that is aimed directly at the dry ingredients. This manifold is normally placed along the upsweep side of the mixer action.

In a batch mixing process, the liquids are normally metered into the mix, but in some cases, the system may have an actual tank on a scale into which the liquids are weighed for each batch and then added to the mix by gravity. The preferred way is to pump the liquids into the mix through the spray manifold system measuring the amount of liquid applied through a liquid meter. In former times the liquid meter used was a volumetric meter that had to be calibrated for the specific liquid including its viscosity. Any variation in the characteristics of the material would provide a decrease in the meter’s accuracy. Today, the liquid addition is added using mass flow meters or from a liquid scale using loss-in-weight recording. The mass flow meter is very accurate and records variances in the liquids as it measures by the mass of the liquid being applied, not the volume.

Post-Process Liquid Applications

Liquid application after product processing, such as pelleting or extrusion, is done for a number of reasons. These include liquids that are unstable to be applied during the initial process, accuracy of liquid addition, proper application procedures or to minimize system contamination. Liquids applied include oils, molasses, enzymes, flavorings, amino acids, vitamins and others. Many of these liquid ingredients are expensive or limited in the amount that may be applied due to concentration or expense.

Most post-process application systems require accurate measurement of both the dry product and liquid application rates. This is done using scales for batch systems or mass flow meters for continuous application of the liquid. Where the batch applications use specific ratios or amounts of dry and liquid material, both are weighed and mixed in batches. Continuous systems are tied together through a computer so the liquid application rate is controlled by the dry material flow rate.

Other things that are important in these systems are to have a uniform distribution of the dry material to which the liquid is being applied. Also important is the liquid droplet size so that it will disperse throughout the dry material. Temperature difference between the dry material and applied liquids may affect the mixing quality if the liquid solidifies or evaporates before it is thoroughly distributed in the mix. In some cases, the liquid needs to be atomized into smaller droplets as it is applied. This is done using compressed air in the liquid supply stream and special liquid nozzles designed for the application. Once the dry and liquid products are combined, the system should allow enough retention time and mixing to provide a thoroughly mixed final product.

Another challenge in adding liquids after processing is the relationship between the volume of product and the volume of liquid to be added to the product. An example of this is the volume of the product and the volume of liquid being applied. Assume we have a ton (2,000 pounds) of finished pelleted or kibble product with a density of 40 pounds per cubic foot. Dividing the amount of product to which the liquid is to be added by its density (2,000 pounds/40 pounds per cubic foot), we find the product occupies 50 cubic feet of volume. A cubic foot of volume holds 7.48 gallons of liquid. Multiplying 50 cubic feet times 7.48 gallons per cubic foot gives a volume of 374 gallons of initial product to which the liquid is to be applied.

Assume oil such as animal fat is to be added at a level of 12%. This would require 240 pounds of the animal fat to be added per ton of product. Animal tallow has a density of approximately 7.8 pounds per gallon. Dividing 240 pounds by 7.8 pounds per gallon, 30.8 gallons of animal fat is required. This would result in 30.8 gallons of animal fat being added to 374 gallons of product.

A second scenario is adding Xylonase when wheat is used in the dry product at a rate of 200 milliliters/ton. 200 ml of Xylonase results in having only 0.528 gallons of liquid to add to the 374 gallons of dry product. This presents another problem as this amount of liquid is inadequate to get properly dispersed in the mixture. Thus, it is necessary to add an additional amount of a carrier liquid such as water to have enough liquid to properly spread throughout the dry product and blend in.

Static Liquid Addition Systems

A static system is done by batch mixing the product and the liquid being applied. The dry product can be accurately weighed in a scale.

The liquid can be accurately weighed in a separate scale and then dispersed into the product during mixing (see Figure 1, page 82). The liquid can also be added by a liquid pumping system through a mass flow meter.

Sometimes it is desirable to add 30% or more liquids such as animal fat to extruded kibbles. This requires using a vacuum infusion technique in which the kibbles are put in a special batch mixer in which a vacuum can be created. After the kibbles are put in the mixer, a negative pressure (vacuum) is created inside the mixer before the liquid is added. The liquid is then sprayed onto the surface of the product and mixed. The negative pressure is released and atmospheric pressure forces liquid into the fissures (voids) in kibbles. The remaining liquid on the surface is dusted with a powder and the finished product is discharged from the mixer.

Continuous Liquid Addition Systems

Addition of liquids into a product being made on a continuous system process requires accurate measurement of the flow rates of the product and the liquid to be added or applied, and proper mixing action and retention time to produce the desired blended product. A diagram of a continuous addition system is shown in Figure 2 (page 82).

Figure 3 (page 84) shows one way to add liquid by gravity into a product stream using a Mistcoater. For this type of system, the liquid should be free flowing. Both the product and liquid flow rates should be metered and controlled using mass flow meters.

The product drops vertically on to spinning plates that fling the product into a circular curtain. The liquid is piped vertically into the center and hits another set of spinning plates that disperses it into a curtain that matches the diameter of the product curtain. The mixture then drops out of the liquid addition chamber into a mixing screw. The diameters of the product and liquid curtains are controlled by the speed of the turning plates for each.

Other systems spray the liquid into curtains or tumbling streams of the product to be coated. Mass flow meters and controllers should be used to monitor both flow rates and adjust the liquid rate to maintain the desired amount of liquid added to the product. Again, the spray pattern should touch the actual product, but not the chamber in which the addition is being done.

If the liquid droplets are too large, the liquid may be atomized by adding compressed air into the liquid stream prior to entering the spray addition chamber.

Care must be taken when atomizing the liquid that the some of the liquid ingredient does not flash off into the atmosphere when the liquid depressurizes in the spray chamber. In most cases, additional agitation is needed for the mixture as it exits the spray chamber in the system. There are many manufacturers of equipment for post-processing liquid addition. It is important to choose the correct system for the intended application.