November 9, 1995 ISSUE #35
Modern broiler plants will process 200,000 to 1,000,000 birds per day. With average live bird weights running from four to six pounds, the tonnage of waste material is enormous. All of this waste material (feathers, offal, trims, blood, and DOA's) goes to the By-Products operation. The by-products are mostly animal feed, which is produced by rendering (cooking) the waste materials.
A most important by-product at a rendering plant is feather meal. Feathers constitute 9% by weight of the live bird, and they are rich in protein content. This protein is in a form, like hair, that is not digestible. To make it digestible it must first be converted through a hydrolysis process. Hydrolyzation is accomplished by cooking the feathers with steam. This was previously done in batch cookers; the current technology is to use continuous hydrolyzers.
Hydrolyzed feathers are produced with moisture contents in the range of 45% to 65%. This moisture must be reduced to 8% for the product to have adequate shelf life as an animal feed. Vincent rotating drum dehydrators are used to remove this excess moisture.
In the Spring of 1995 we worked with a rendering plant that had reached the capacity limit of their feather meal dryer. Immediate relief was required because of the economic opportunity being lost.
Trials were run and it was found that their continuous hydrolyzers were producing feather meal at 55% to 56% moisture. This contrasts to another renderer with whom we have worked where the meal leaves their homemade continuous hydrolyzer at 47% to 48% moisture. There is a third western firm with whom we are in contact where the meal has 60% moisture prior to drying.
A series of tests were run using a rental VP-16 screw press. Running the press at 60 Hz corresponds to the standard 13 rpm screw speed. At this speed the press did very little dewatering. It reduced the moisture content of the feather meal by only one or two percentage points.
However when the press speed was reduced to half speed performance improved. Running at 30 Hz, readings of 49% to 50% press cake moisture were obtained. This does not sound like much, but the impact on dryer capacity is surprisingly large.
Here is how we calculate the impact: If we put 2,000 pounds of feather meal at 56% moisture into the dryer, and dry it down to 10% moisture, we end up with 978 pounds of finished meal. This is calculated by taking 44% of 2,000 (which is the pounds of dry solids in the 2,000 pounds), and dividing those solids by 0.9 (which is 1.0 minus the final 10% moisture). To check that answer we can multiply the 978 figure by 90% to see if that is indeed all of the solids in the 2,000 pounds we started with.
We can see that to produce 978 pounds of meal, it was necessary for the dryer to evaporate (2000 - 978) or 1,022 pounds of water.
For the alternative case, let us assume the screw press takes the moisture down to 50%. If we put 2,000 pounds of that material into the dryer, we will be putting in 1000 pounds of dry solids. If this is dried down to the same 10% moisture, we end up with 1,111 pounds of finished feather meal. To produce this meal the dryer had to remove (2000 - 1111) or 889 pounds of water.
We can compare these two results to determine the factor by which dryer capacity is affected. In the first case the dryer must remove 1,022 pounds of water to produce 978 pounds of meal. This calculates to removing 2,090 pounds of water per ton of 10% feather meal.
In contrast, in the second case the dryer needs to remove only 889 pounds of water to produce 1,111 pounds of meal. This works out to evaporating 1,600 pounds of water to produce one ton of 10% feather meal.
Since the dryer is running flat out, it will evaporate the same number of pounds of water per hour in both cases. Thus we can calculate that with the press in operation the production of feather meal increases by 30%!
This is figured by dividing the capacity of the dryer (in pounds of water evaporation per hour) by 2,090. Since the dryer in question has been evaporating 14,100 pounds of water per hour, the production calculates at 6.7 tons of 10% meal per hour. Under the second operating condition, 14,100 divided by 1,600 gives 8.8 tons per hour. This is a production increase of 30% with the same dryer.